A Continuous Electrochemical Process to Convert Lignin to Low Molecular Weight

Home , Concentration cell

Aromatic Compounds and Cogeneration of Hydrogen

A dissertation presented to

the faculty of

the Russ College of Engineering and Technology of Ohio University

In partial fulfillment

of the requirements for the degree

Doctor of philosophy

Mahtab Naderinasrabadi

May 2020

This dissertation titled

A Continuous Electrochemical Process to Convert Lignin to Low Molecular Weight

Aromatic Compounds and Cogeneration of Hydrogen

MAHTAB NADERINASRABADI

has been approved for

the Department of Chemical and Biomolecular Engineering

and the Russ College of Engineering and Technology by

John Staser

Associate Professor of Chemical and Biomolecular Engineering

Mei Wei

Dean, Russ College of Engineering and Technology 3

ABSTRACT

NADERINASRABADI, MAHTAB, Ph.D., May 2020, Chemical Engineering

A Continuous Electrochemical Process to Convert Lignin to Low Molecular Weight

Aromatic Compounds and Cogeneration of Hydrogen

Director of Dissertation: John Staser

Lignin is one of the main byproducts of pulp and paper industry and biorefineries. Depolymerization of lignin can lead to producing valuable low molecular weight compounds with different functional groups, which are mainly achieved from crude oil sources.

Lignin electrolysis could address issues of other lignin depolymerization methods such as complexity, lignin combustion, and low selectivity. On the other hand, lignin electrolysis can occur at significantly lower overpotentials than those required for water electrolysis, which leads to lower-voltage electrolyzer operation and as a result lower energy consumption for hydrogen production.

This study includes research and experimental works on developing a continuous electrochemical process for both lignin electrolysis and hydrogen production in an electrolyzer.

At the first step of this project, high surface area TiO2 or carbon-supported

NiCo electrocatalysts were synthesized and applied for lignin depolymerization at room temperature and pressure. The electrocatalysts were characterized by Brunauer-

Emmett-Teller (BET), X-Ray Diffraction (XRD), Scanning Electron Microscopy

(SEM), and Energy-dispersive X-ray spectroscopy (EDS) techniques. In addition, a three-electrode rotating disc electrode (RDE) system was used to test the performance 4 and durability of 6 electrocatalysts individually and among them 1:3NiCo/TiO2 was selected as the most effective catalyst for lignin depolymerization.

In the second step, a continuous electrochemical cell with 10 cm2 electrodes, separated by an anion exchange membrane (AEM), was applied for lignin electrolysis in the anode and hydrogen generation in the cathode. The effects of temperature, lignin concentration, cell voltage, and electrolysis time on hydrogen production, oxygen evolution, lignin conversion, products with different functional groups, and energy efficiency of the electrochemical reactor were investigated. Although applying high cell voltages increases the rate of electrochemical reactions and lignin conversion, it produces inefficiencies in energy consumption by enhancing oxygen evolution reaction (OER).

Several techniques including gas chromatography-mass spectroscopy

(GC/MS), ultraviolet-visible (UV-Vis) spectroscopy, Fourier-transform infrared

(FTIR) spectroscopy, gel permeation chromatography (GPC), and Raman spectroscopy were applied to analyze the lignin samples. In addition, the generalized standard addition method (GSAM) for the first time was applied for measuring lignin conversion after electrolysis.

The third step of this project included scaling-up the 10 cm2 continuous

2 electrochemical cell to a 200 cm reactor and utilizing it for lignin electrolysis and H2 production on a larger scale. The results indicate that the performance of the scaled-up reactor is extremely analogous to the 10 cm2 cell.

DEDICATION

To my mother and father, who always had confidence in me and encouraged me to go on every adventure in my

life

and my adviser, Dr. John Staser,

for all of his guidance and supports during my Ph.D. study

ACKNOWLEDGMENTS

We appreciatively acknowledge the financial support of this project by the US

Department of Energy (DOE) under award DE-EE0007105.

The project team is also thankfully acknowledging the contribution of the

Center for Intelligent Chemical Instrumentation, directed by Professor Harrington, for all of their helpful guidance on lignin characterization and introducing Generalized

Standard Addition Method for measuring lignin conversion.

In addition, the collaboration of Hexion Company1 in collecting gel permeation chromatography (GPC) data and OH number determination is gratefully acknowledged.

1 Address: 6200 Camp Ground Rd, Louisville, KY 40216 7

TABLE OF CONTENTS

Page

Abstract ...... 3 Dedication ...... 5 Acknowledgments...... 6 List of Tables ...... 10 List of Figures ...... 12 Chapter 1: Introduction ...... 15 Chapter 2: Literature Review ...... 18 2-1- Biomass Conversion ...... 18 2-1-1- Lignocellulosic Biomass ...... 18 2-1-2- Bonds Within Lignocellulose Biomass and Building Units of Polymers 19 2-1-3- Functional Groups of Lignocellulose Biomass Components ...... 21 2-1-4- Lignin Depolymerization ...... 22 2-1-5- Electrochemical Oxidation of Lignin...... 23 2-2-Hydrogen Production ...... 27 2-2-1-Hydrogen Generation by Water Electrolysis ...... 27 2-2-2- Anode Electrocatalysts for Water Electrolyzers ...... 30 2-3- Hydrogen Production by Biomass Electrolysis ...... 32 2-3-2 Electrolysis of Alcohol Solutions for Hydrogen Production ...... 33 2-3-3- Lignin Electrolysis for Hydrogen Production ...... 35 2-3-4-Influence of Different Parameters on Biomass Electrolysis for Hydrogen Production ...... 37 Chapter 3: Synthesis and Characterization of Nickel Cobalt Electrocatalysts ...... 40 Objectives ...... 40 3-1-Introduction ...... 40 3-2-Materials and Methods ...... 42 3-2-1-Electrocatalysts Synthesis ...... 42 3-2-2- Electrocatalysts Physical Characterization ...... 43 3-2-2-1-Brunauer-Emmett-Teller (BET) Method ...... 43 3-2-2-2-X-Ray Diffraction Spectroscopy (XRD) ...... 43 3-2-2-3-Scanning Electron Microscopy (SEM) and Energy-Dispersive X-Ray (EDS) Spectroscopy ...... 44 3-2-3-Electrochemical Characterization of Electrocatalysts ...... 44 8

3-2-3-1-Three Electrode RDE Electrochemical Cell ...... 44 3-2-3-2-Lignin Solution Preparation...... 45 3-3-Results and Discussion ...... 46 3-3-1- Physical Characterization of Electrocatalysts ...... 46 3-3-2- Electrochemical Characterization of Electrocatalysts ...... 52 3-4-Conclusion ...... 55 Chapter 4: Hydrogen ProductIon and Energy Efficiency ...... 56 Objectives ...... 56 4-1-Introduction ...... 56 4-2-Materials and Methods ...... 58 4-2-1-Lignin Solution ...... 58 4-2-2-Electrode Preparation ...... 58 4-2-3-Continuous Electrochemical Reactor ...... 58

4-2-4-H2 Detection ...... 60 4-3-Results and Discussion ...... 60 4-3-1-The Effect of Temperature on Current Density and Reaction Kinetics .60 4-4-2-The Effect of Lignin Concentration ...... 66

4-3-3-H2 Purity and Energy Consumption ...... 69 4-4-Conclusion ...... 73 Chapter 5: Biomass Depolymerization ...... 74 Objectives ...... 74 5-1-Introduction ...... 74 5-2-Materials and Methods ...... 76 5-2-1-Gas Chromatography/Mass Spectroscopy (GC/MS) ...... 76 5-2-2- Fourier-Transform Infrared (FTIR) Spectroscopy ...... 78 5-2-3-Raman Spectroscopy ...... 78 5-2-4- Hydroxyl (OH) Numbers Measurement ...... 79 5-2-5- Gel Permeation Chromatography (GPC) ...... 79 5-2-6- Lignin Conversion Measurement ...... 80 5-3-Results and Discussion ...... 84 5-3-1- The Effect of Cell Voltage and Electrolysis Time on Biomass Conversion ...... 84 5-3-2-Detection of Lignin Oxidation Products and Functional Groups ...... 97 5-3-2-1- The Effect of Cell Voltage and Electrolysis Time on LMWACs% ..97 5-3-2-2-Comparing Chloroform and Diethyl Ether in the Extraction of Organic Compounds for GC/MS Test ...... 99 9

5-3-2-3- GC/MS Detection of Products and Functional Groups in Lignin Solutions ...... 100 5-3-2-4- Detection of Functional Groups in the Solid Phase by FTIR ...... 103 5-3-2-5- Hydroxyl (OH) Number Determination ...... 108 5-3-2-6- Detection of Functional Groups in the Neat and Oxidized Lignin Solutions by Raman Spectroscopy...... 109 5-3-3- Gel Permeation Chromatography (GPC) Analysis for Molecular Weight Measurement ...... 111 5-4- Conclusion ...... 115 Chapter 6: Scaled-Up Process ...... 116 6-1- Design of 200 cm2 Electrochemical Cell ...... 116 6-2- Results and Discussion ...... 117 6-2-1- The Effect of Cell Voltage and Temperature on Cell Current and Oxygen Evolution ...... 117 2 6-2-2- H2 Production and Energy Efficiency of the 200 cm Cell ...... 121 6-2-3- Analysis of Lignin Oxidation Products...... 122 6-3-Conclusion ...... 127 Chapter 7: Conclusion...... 128 References ...... 130 Appendix: Matlab Code for GSAM Calculations ...... 147

LIST OF TABLES

Page

Table 2-1- Average distribution of the three main components of Lignocellulosic biomass in softwood, hardwood, and switchgrass [10]...... 19 Table 2-2- Bonds between building units that form individual polymers: lignin, cellulose, and hemicellulose and lignocellulose biomass components [11]...... 20 Table 2-3- Functional groups of the components of lignocellulose biomass [11] ...... 22 Table 2-4- Advantages and disadvantages of alkaline, PEM, and AEM water electrolysis [32] ...... 30 Table 3-1- Electrocatalysts BET surface areas and crystallite sizes...... 48 Table 3-2- Energy-dispersive X-ray spectroscopy elemental analysis and nominal composition of 1:3 NiCo/carbon...... 51 Table 3-3- Energy-dispersive X-ray spectroscopy elemental analysis and nominal composition of 1:3 NiCo/TiO2 ...... 51 Table 3-4- LMWACs% of the neat and oxidized lignin at 400 mV (vs. Hg/HgO) for 32 h ...... 54 Table 4-1- Faradaic efficiency for oxygen evolution from alkaline water electrolysis and biomass-depolarized electrolysis at different cell voltages and temperatures...... 65

Table 4-2- Rate of H2 generation from 50 g/L lignin electrolysis and energy consumption of the continuous electrochemical reactor with 10 cm2 cell working at room temperature and pressure. The data was collected after 24 h 2 2 of the reactor operation using 8 mg/cm 1:3 NiCo/TiO2 and 2 mg/cm Pt as anode and cathode electrocatalyst, respectively...... 71 Table 5-1- The concentration of neat lignin in the diluted oxidized samples (c0) estimated by the classical and BLUE calculations ...... 92 Table 5-2- Percentage of LMWA compounds extracted by chloroform from neat lignin and electro-oxidized lignin at different cell voltages and electrolysis times ...... 98 Table 5-3- The concentration and percentage of LMWACs and total compounds extracted by diethyl ether and chloroform from a lignin sample electrolyzed at 1.4 V for 19.5 min ...... 100 Table 5-4- LMWACs detected by GC/MS in neat and oxidized lignin samples, electrolyzed at 1.4 V for 19.5 min, extracted by diethyl ether...... 102 Table 5-5- FTIR peak assignment...... 107 Table 5-6- Hydroxyl number of lignin samples before and after electrolysis, achieved by ASTM D 4274-99 Test Method A...... 108 11

Table 5-7- Average molar mass data and polydispersity (PDI) of neat and oxidized lignin samples at different cell voltages and electrolysis times...... 113 Table 5-8- Percentage of lignin fractions from selected samples ...... 114 Table 6-1- Rate of hydrogen evolution, faradaic efficiency, and energy efficiency of the 200 cm2 electrochemical cell ...... 121 Table 6-2- LMWACs% in neat and oxidized lignin samples, electrolyzed in 200 cm2 cell at different cell voltages and electrolysis times...... 125

LIST OF FIGURES

Page

Figure 2-1- Lignocellulosic biomass components [9]...... 18 Figure 2-2- Structure of lignin and the main phenylpropane units present in lignin [4]...... 19 Figure 2-3- The structure of lignin β-O-4 bond and bond dissociation energies [13]...... 21 Figure 2-4- Main technologies for lignin conversion [4]...... 23 Figure 2-5- Schematic of the electrochemical membrane reactor with in-situ lignin electrolyzed product removal [25]...... 26 Figure 2-6- The schematic of AEM, PEM, and solid oxide water electrolysis (modified from [31])...... 28 Figure 2-7- The dependence of current and current density on applied cell overpotential using methanol, ethanol and isopropanol solutions as electrolytes [52]...... 34 Figure 2-8- Schematic illustration of the lignin electrolysis cell. 1-Calomel reference electrode, 2-Working electrode, 3-Condenser, 4-Cathode compartment, 5-Thermometer,6-Counter electrode, 7-Glass frit [53]...... 36 Figure 2-9- The effect of temperature on lignin electrolysis, using Pt/C electrode. a) I-V curves at different operating temperatures; b) H2 evolution rate as a function of current density [55]...... 39 Figure 3-1- The schematic of a three-electrode batch system for electrochemical lignin oxidization at room temperature and ambient pressure ...... 45 Figure 3-2- XRD profiles of a) 1:1 Ni-Co/carbon, b) 1:3 Ni-Co/carbon, c) 3:1 Ni-Co/carbon, d) 1:1 Ni-Co/TiO2, e) 1:3 Ni-Co/TiO2, f) 3:1 Ni-Co/TiO2 ...... 47

Figure 3-3- Typical SEM images of a)1:3 NiCo/carbon and b)1:3 NiCo/TiO2 ...... 49 Figure 3-4- Energy-dispersive X-ray spectroscopy (EDS) spectra of 1:3 NiCo/ carbon and 1:3 NiCo/TiO2 electrocatalysts...... 50 Figure 3-5- Chronoamperometric curves of NiCo electrocatalysts at a constant working electrode potential of 400 mV vs. Hg/HgO ...... 53 Figure 4-1- Schematic representation of the continuous electrochemical process for lignin depolymerization with cogeneration of H2 ...... 59 Figure 4-2- The effect of temperature and the presence of 50 g lignin/L on current/energy density, applying continuous electrochemical reactor using. Linear scan voltammetry from 0.6 V to 1.8 V; scan rate (0.5 mV/s). Anode: 1:3 NiCo/TiO2 on carbon paper and cathode: Pt-loaded carbon cloth...... 61 Figure 4-3- OER faradaic efficiency as a function of cell voltage for water electrolysis (solid triangle markers) and biomass-depolarized electrolysis (solid circle markers) at 25°C (˗˗˗ ⸳⸳ ˗˗˗), 40°C (solid line) and 60°C (- - -)...... 63 13

Figure 4-4- Current density versus cell voltage at different lignin concentrations in 1 M NaOH, electro-oxidized on 1:3 NiCo/TiO2 in the continuous electrochemical reactor, using AEM between electrodes at room temperature...... 67 Figure 4-5- Current density as a function of lignin concentration at 1.3 V, 1.4 V, 1.5 V, and 1.6 V...... 68 Figure 4-6- GC-TCD chromatogram of the generated gas in the cathode during oxidation of 50 g lignin /L NaOH in the anode...... 72 Figure 5-1- Three phases formed after lignin sample extraction with chloroform and acidification with sulfuric acid...... 78 Figure 5-2- UV-Vis spectra of neat lignin and oxidized lignin at different residence times at a) 1.4 V, b) 1.6 V, c) 1.8 V (a.u. stands for arbitrary unit) ...... 86 Figure 5-3- Panel a represents the UV spectral responses from the standard additions with the incremented concentrations; panel b corresponds the second drivative of UV spectral responses. mAU stands for milli-absorbance unit ...... 88 Figure 5-4- Typical Spectra from the normalized neat lignin (blue line) and oxidized lignin (red line) ...... 89 Figure 5-5- Panel a represents principal component scores from the standard addition of neat lignin to the oxidized lignin; panel b corresponds to variable loadings of the first principal component that spans 100% of the variance ...... 91 Figure 5-6- Panels a and b correspond to Best Linear Unbiased Estimates (BLUE) least squares and classical GSAM sensitivities; panels c and d refer to inverse least squares regression coefficients of BLUE and classical GSAM...... 93 Figure 5-7- The conversion of neat lignin to products at different electrolysis times based on the neat lignin concentration in the oxidized samples, calculated from BLUE k approach...... 94 Figure 5-8- Lignin conversion based on BLUE k versus total potentiostatic charge transferred at the cell voltages of 1.4 V and 1.6 V...... 96 Figure 5-9- The percentage of linear (left) and aromatic (right) compounds with different functional groups detected by GC/MS in neat and oxidized lignin samples from two experiments. Panels a and b correspond to lignin electrolyzed at 1.4 V for 19.5 min; panels c and d refer to lignin electrolyzed at 1.6 V for 4 min...... 103 Figure 5-10- FTIR spectra of neat and electro-oxidized lignin in the 10 cm2 cell at different electrolysis times under constant cell voltage of 1.4 V ...... 105 Figure 5-11- FTIR spectra of neat and electro-oxidized lignin in the 10 cm2 cell at different electrolysis times under constant cell voltage of 1.6 V ...... 106 Figure 5-12- Raman spectra of neat and electro-oxidized lignin, electrolyzed at a constant cell voltage of 1.4 V for 19.5 min...... 110 Figure 5-13- Raman spectra of neat and electro-oxidized lignin, electrolyzed at a constant cell voltage of 1.6 V for 4 min...... 111 14

Figure 6-1- The shape of the microchannels of the 200 cm2 electrochemical flow reactor...... 117 Figure 6-2- Cell current versus cell voltage for water electrolysis (solid circle markers) and biomass-depolarized electrolysis (solid triangle markers) at 25°C (….), 40°C (- - - ), and 60°C (solid line) in 200 cm2 cell...... 118 Figure 6-3- Rate of oxygen production in 200 cm2 electrochemical cell as a function of cell voltage for water electrolysis (solid circle markers) and biomass-depolarized electrolysis (solid triangle markers) at 25°C (….), 40°C (- - -), and 60°C (solid line)...... 119 Figure 6-4- OER faradaic efficiency in 200 cm2 electrochemical cell as a function of cell voltage for water electrolysis (solid circle markers) and biomass-depolarized electrolysis (solid triangle markers) at 25°C (….), 40°C (- - -), and 60°C (solid line)...... 120 Figure 6-5- Lignin conversion percentage (up) and UV-Vis spectra of neat and oxidized lignin (down), electrolyzed at different cell voltages and electrolysis times in 200 cm2 cell ...... 123 Figure 6-6- Lignin conversion versus total potentiostatic charge transferred at the cell voltages of 1.5 V and 1.6 V, applying 200 cm2 cell ...... 124 Figure 6-7- The percentage of linear (right) and aromatic (left) compounds with different functional groups in neat and oxidized lignin samples, electrolyzed in 200 cm2 cell at different cell voltages and electrolysis times. Panels a and b corresponds to lignin electrolysis at 1.5 V; panels c and d refers to lignin electrolysis at 1.6 V...... 126

CHAPTER 1: INTRODUCTION

Lignocellulosic biomass is a major part of bio-resources and consists of three main components: cellulose, hemicelluloses, and lignin. Lignin is an important biopolymer in plants cell walls that constitutes about 20 and 30% of softwood and hardwood, respectively [2]. In fact, lignin is the main source of renewable aromatics in the world, which represents a great potential as an alternative to crude oil for the production of valuable aromatic compounds applied in the production of pharmaceuticals, detergents, cosmetics, and food flavors [3]. Lignin mostly is produced as a waste by-product in huge quantities during processing wood into paper pulp at paper mills in the form of black liquor [2], [4],[5]. Nevertheless, it is also considered as the main by-product in growing industries associated with biofuels and bioproducts. The global production of Kraft lignin was about 50 million tons/year during 2013 [6]. It is also estimated an additional 62 million tons/year lignin will be produced from the second generation of biorefineries [6]. The depletion of fossil fuels and growing concern about climate change encourages governments to focus on having a carbon-neutral green economy. For example, the U.S. Department of Energy is expected to derive 20% of transportation fuels and 25% of U.S chemical merchandise from biomass [6]. In Europe, the Dutch Ministry of Economics Affair set the goals of replacing 30% of transportation fuels by biofuels and 20-40% of fossil- based chemicals by biomass-based chemicals by 2040 [6].

One way to add economic value to waste biomass is to transform its components, such as lignin, into valuable chemical products as precursors for other end-use chemicals. However, because of the stable and rigid structure of lignin, which is composed of cross-linked phenylpropane (C6–C3) units and also the lack of 16 effective methods for lignin depolymerization, almost 95% of the produced lignin from paper and pulp industry is burned, despite the fact it is a low-grade fuel [7].

Today, lignin conversion into industrial chemicals presents a challenge in sustainability, economy, and protecting the environment. To this end, many different technologies have been developed in the past; however, most of them are expensive processes, have low selectivity towards aromatic compounds, and require drastic operation conditions for lignin depolymerization. In contrast, the electrochemical depolymerization of lignin is an efficient method to achieve value-added chemicals from biorefinery waste. It has the advantage of working under ambient pressure and room temperature, which results in low energy consumption. In addition, the yield of lignin electrochemical conversion to useful chemicals can be increased by elimination of the char production, which occurs at high temperatures in thermochemical pyrolysis processes of biomass upgrading. Since electrochemical processes are essentially surface-catalyzed, high selectivity and conversion can be obtained by applying appropriate electrocatalysts. The phenol derivatives, vanillin, and other low molecular weight aromatics (LMWA) from lignin depolymerization can be used in industrial processes, for example producing resin and other binder precursors.

The main purpose of this research is developing a continuous electrochemical process to convert lignin-rich waste biomass (from the Department of Energy) to

LMWA compounds (LMWACs) at the anode of an electrochemical reactor with cogeneration of hydrogen at the cathode. To this end, in the first part of this research, an effective low-cost NiCo electrocatalyst was developed and used as the anode electrocatalyst in a continuous 10 cm2 electrochemical cell. 17

The effects of several parameters such as cell voltage, electrolysis time, lignin concentration, and temperature on hydrogen production, energy efficiency, lignin depolymerization, oxygen evolution, and functional groups and LMWACs% in the products were studied. Several characterization techniques such as gas chromatography-mass spectroscopy (GC/MS), ultraviolet-visible (UV-Vis) spectroscopy, Fourier-transform infrared (FTIR) spectroscopy, Raman spectroscopy, and gel permeation chromatography (GPC) were applied to analyze the product stream.

After optimization of the main parameters of the electrochemical process, a

200 cm2 lab-scale continuous electrochemical reactor was developed and applied for lignin oxidation with cogeneration of hydrogen in a lab-scale. The developed lab- scale electrochemical and the results of the experiments performed in this research can be used in future studies on scaling up this process to be applied in industry.

CHAPTER 2: LITERATURE REVIEW

2-1- Biomass Conversion

2-1-1- Lignocellulosic Biomass

Lignocellulosic biomass is a predominant part of bio-resources and consists of three main components: cellulose, hemicelluloses, and lignin (Figure 2-1). The fraction of each component in the lignocellulosic biomass depends on the source of the plant (Table 2-1). For example, lignin constitutes about 20-30% of the weight of all woody plants [8]. Lignin is a three-dimensional amorphous polymer with a complex structure, which is built up from three main phenylpropane units: synapyl alcohol, coniferyl alcohol, and p-coumaryl alcohol (Figure 2-2). These units are bonded together by several C−O (mostly β–O–4, α–O–4, 4–O–5) and C−C (e.g., β–5,

β−β, 5−5’, β–1) inter-unit linkages.

Figure 2‎ -1- Lignocellulosic biomass components [9].

Table 2-1-

Average distribution of the three main components of Lignocellulosic biomass in softwood, hardwood, and switchgrass [10].

Lignocellulosic biomass Softwood Hardwood Switchgrass components

Lignin 27-30% 20-25% 5-20%

Cellulose 35-40% 45-50% 30-50%

Hemicelluloses 25-30% 20-25% 10-40%

Figure 2‎ -2- Structure of lignin and the main phenylpropane units present in lignin [4].

2-1-2- Bonds Within Lignocellulose Biomass and Building Units of Polymers

Table 2-2 summarizes the linkages between lignocellulose biomass components and the bonds between building units that form lignin, cellulose, and hemicellulose individually. Ether bonds and carbon-to-carbon bonds are the main types of bonds that link the building units of lignin [11]. Compared to other linkages within lignin molecule the ether bonds are more dominant and constitute 70% of the total bonds between the monomer units [11]. Based on theoretical calculation C−O ether bonds are significantly more fragile than C−C bonds [12]. One of the dominant etheric bonds within lignin is the β-O-4 ether bond (Figure 2-3), which is comprised 20 of 40-60% interunit linkages in lignin, and it is mainly the targeted bond to breakdown during lignin degradation [13].

Table 2-2-

Bonds between building units that form individual polymers: lignin, cellulose, and hemicellulose and lignocellulose biomass components [11].

Bonds within different components (interpolymer linkages)

Ether bond Lignin, (hemi)cellulose

Carbon to carbon Lignin

Hydrogen bond Cellulose

Ester bond Hemicellulose

Bonds connecting different components (interpolymer linkages)

Cellulose-Lignin Ether bond Hemicellulose lignin

Ester bond Hemicellulose-lignin

Cellulose-hemicellulose

Hydrogen bond Hemicellulose-Lignin

Cellulose-Lignin 21

Figure ‎2-3- The structure of lignin β-O-4 bond and bond dissociation energies [13].

2-1-3- Functional Groups of Lignocellulose Biomass Components

The functional groups of the three main components of lignocellulose biomass are summarized in Table 2-3. Among these components, lignin has the most dissimilar functional groups, which are involved in lignin depolymerization and eventually degradation to derivatives [11]. Besides the fact that the ether bond is the predominant linkage in lignin, it also holds the glucose monomers in a polymer chain.

Therefore, the separation of lignin from the polysaccharide matrix occurs by cleavage of the ether bonds. The cleavage of ether bonds occurs under alkaline or acidic conditions with different mechanisms [11]. Aromatic rings of lignin can be oxidized to form a cyclic structure and eventually to smaller molecules such as mono- and dicarboxylic acids [11].

Table 2-3-

Functional groups of the components of lignocellulose biomass [11]

Functional groups Lignin Cellulose Hemicellulose

Aromatic ring X

Hydroxyl group X

Carbon to carbon linkage X

Ether (glycosidic) linkage X X X

Ester bond X

Hydrogen bond X X

2-1-4- Lignin Depolymerization

Lignin can be depolymerized and serve as a renewable source of valuable aromatic chemicals. Lignin is typically depolymerized by hydrolysis reactions, catalytic reductions, or catalytic oxidation reactions (Figure 2-4). Generally, hydrolysis and catalytic reduction approaches disturb the lignin molecule structure and eliminate functional groups from lignin to produce simple phenols. However, oxidation reactions can form more complex aromatic compounds with additional functionality such as aldehydes, acids, and other low molecular weight products.

Therefore there are several studies on lignin oxidation as an effective method of lignin depolymerization [4]. However, most of these studies were performed on lignin model compounds due to the complexity of the lignin molecule and its difficult characterization. 23

Figure 2-4- Main technologies for lignin conversion [4].

Conventionally, metal oxides, nitrobenzenes, or CoCrO4 were used for catalytic oxidation of native lignin and lignin model compounds [4]. However, in the past few years, some new approaches such as mechanochemistry, sonochemistry, photocatalysis, and electrochemistry were used in lignin oxidation [10]. Among these strategies, electrochemistry could be considered as an efficient, low-cost, and eco- friendly method for lignin oxidation [10], [14], [15], while it is in the early stages of development. Under electrochemical oxidation conditions, depolymerization of lignin can be achieved under ambient pressure and room temperature.

2-1-5- Electrochemical Oxidation of Lignin

Electrochemical depolymerization of waste lignin has recently been developed as a potentially sustainable and clean procedure to produce valuable chemicals as a replacement for depleting crude oil sources. Several processes for lignin electrolysis have been developed in the last few years [16]–[21], [14]. Most of the studies on the electrochemical oxidative lignin depolymerization in alkaline medium included two or three electrodes in batch systems. Since electrochemical processes are essentially surface-catalyzed, the electrode structure and the applied electrocatalyst have a substantial effect on the selective and efficient oxidation of lignin [15]. While many 24 of the research efforts to date have focused on large metal electrodes, some recent studies have been focusing on applying nanoparticle electrocatalysts with high surface areas on electrodes for lignin oxidation [19], [20], [22]. For example, Chen et al. [19] used lead dioxide (PbO2) nanoparticles supported on TiO2 nanotubes for the electrochemical decomposition of waste lignin, which led to a 13% reduction in

C−O−C group content. Although PbO2 is a strong oxidant [20], [22], it is a toxic agent. Precious metal catalysts such as platinum, palladium, and rhenium generally showed higher activity than non-noble metal catalysts, nevertheless, lignin oxidation catalysts are typically derived from less precious transition metals such as titanium, nickel, cobalt, manganese, cerium, and cupper due to their abundance and lower cost.

Among several transition metal, Ni-based catalysts exhibited the highest activity and product selectivity towards the depolymerization of lignin to produce value-added chemicals [23].

One major challenge in lignin electro-oxidation is to avoid over-oxidation of the depolymerized products. One way to prevent over oxidation and cleavage of aromatic monomers to saturated compounds is to separate the lignin oxidation products throughout the process. For example, Wessling et.al. [24] used emulsion electro-oxidation of lignin approach that allowed in situ lignin depolymerized product recovery during lignin electrolysis, which led to the prevention of over-oxidation of smaller molecules. However, this method was associated with complex problems and requirement of a large amount of solvent due to low lignin concentration (i.e. 5 and 10 g/L). In addition, lignin electrolysis in batch operation mode is another obstacle for practical large-scale purposes. 25

Some researchers applied continuous systems for electro-oxidation of lignin under constant currents [15], [25]. Wessling et.al. [25] applied a membrane-based continuous reactor to separate the low molecular weight depolymerization products simultaneously with electro-oxidation of lignin (Figure 2-5). Applying this method led to a decrease in the average molecular weight of lignin from 5625 Da to 342.8 Da by lignin electrolysis; however, the process was complicated and expensive to be used on a large industrial scale. In addition, the very low concentration of lignin in sodium hydroxide (NaOH), 0.005 g/L in this process leads to the high amount production of waste NaOH that is required to be separated from the desired product.

In another study, Wessling et.al. [15] applied a simpler semi-continuous system for electro-oxidation of 5 g/L lignin in a continuous cell followed by a membrane for separation of lower molecular weight compounds. In this process, the membrane is permeated by NaOH carrying the low molecular weight depolymerized products but recycled the high molecular weight lignin for further oxidation. Although using the membrane reactor increased the selectivity of lignin oxidation products to lower molecular weight compounds, the long electrolysis time, 4 h, and the low concentration of lignin (which leads to the high amount of waste NaOH production) make this process economically inefficient to be used in the industry.

. 26

Figure ‎2-5- Schematic of the electrochemical membrane reactor with in-situ lignin electrolyzed product removal [25].

Another problem associated with lignin-rich biomass oxidation is producing a variety of compounds (byproduct compounds), which varies based on the source of the waste lignin. This issue leads to very complicated and expensive processes for separations of products. Oxidation of lignin also can increase the reactivity of lignin by depolymerization of lignin to smaller molecules with several functional groups.

One way to avoid the expensive separation processes is to produce lignin-based phenolic resins from the value-added low molecular weight electro-oxidized lignin.

These resins have several applications in producing molded products such as laboratory countertops, billiard balls, and as coatings and adhesives. Recently, several studies have been done on the production of lignin-based phenolic resins, using depolymerized Kraft lignin [26]–[28]. In addition, depolymerization of lignin to produce phenolic resins is close to commercialization by some companies such as 27

VTT Company, Finland. For resin production, lignin suggests a low-cost, non-toxic, bio-based, and sustainable raw material as a replacement for phenol.

2-2-Hydrogen Production

Hydrogen is the most abundant element in the universe and is considered as a renewable, clean energy carrier, which can store or deliver a great amount of energy for domestic, transportation, and industrial usage. About 61 to 65 million metric tons

(MMT) of hydrogen are produced annually in the world and 10 MMT of this amount is produced in the United State [29]. Currently, most of the hydrogen produced in the

United State is used by industry for oil refining, treating metals, and processing foods.

Hydrogen can also be used as a clean fuel in fuel cells to produce electricity with high efficiencies for different transportation and domestic purposes. The majority of hydrogen is currently produced from fossil fuels. Steam reforming or partial oxidation of methane and coal gasification offer the least expensive technique of hydrogen production and accounts for approximately 95% of the hydrogen generated in the

United States. However, the steam reforming process is not considered a sustainable process for hydrogen production and it can produce a large volume of greenhouse gases that are linked to global warming [30].

One method to produce hydrogen from a sustainable source with zero production of greenhouse gases is water electrolysis. Currently one of the main challenges in hydrogen production by water electrolysis is the high cost of the electrolysis process, which makes this process economically undesirable [30].

2-2-1-Hydrogen Generation by Water Electrolysis

Water electrolyzers depending on the type of their electrolyte can be classified into three main groups of proton exchange membrane (PEM), anion exchange 28 membrane (AEM), and solid oxide electrolyzers (SOE) (Figure 2-6). The overall reaction in all three types of electrolyzers is:

1 H O → H + O 2 2 2 2

Figure ‎2-6- The schematic of AEM, PEM, and solid oxide water electrolysis

(modified from [31]).

Solid oxide electrolyzer (SOEs) typically work under high temperatures, in the range of 500-1000oC, using a ceramic O2- ion conductors. The safety issues, improper sealing, and mechanically unstable electrodes are the main problems associated with

SOEs [31].

Currently, conventional alkaline and PEM electrolyzers have a dominant market share. In conventional alkaline electrolyzers, nickel (Ni) and cobalt (Co) electrodes are the most common electrodes, which are separated by a ceramic oxide diaphragm. Low current densities and gas crossover are the main problems associated 29 with this type of alkaline electrolyzers. Unlike the conventional alkaline electrolysis,

PEM electrolysis in acidic media work at lower temperatures (mainly bellow 100oC).

In PEM electrolyzers, acidic PEM membranes (Nafion, DuPont) are used as the electrolyte, which conducts H+ ions from the anode to the cathode. The main drawbacks associated with PEM electrolyzers are the high cost of PEM membranes and the requirement of expensive noble metal electrocatalysts to be used in their acidic media. To reduce the problems associated with PEM water electrolysis, considerable research efforts in the past few years have focused on the development of anion exchange membrane (AEM) water electrolysis. In contrast to PEM electrolyzers, AEM electrolyzers operate under alkaline media. The alkaline environment provides an enhancement in OER kinetic, which allows the utilization of low-cost, non-noble-metal electrocatalysts. In addition, it can reduce the risk of the corrosion of the electrodes in the cell compartments. In AEM electrolyzers, the OH– ions, which are generated from oxygen reduction reaction in the cathode, pass through the AEM membrane toward the anode to be used in OER. Table 2-4 shows the main advantages and drawbacks of alkaline, PEM, and AEM water electrolysis.

Table 2‎ -4-

Advantages and disadvantages of alkaline, PEM, and AEM water electrolysis [32]

Alkaline PEM AEM

Advantages

Mature technology Higher performance Non-noble metal catalyst

Non-PGM catalyst Higher voltage efficiencies Noncorrosive electrolyte

Long term stability Good partial load Compact cell design

Low cost Rapid system response Low cost

Megawatt range Compact cell design Absence of leaking

Cost-effective Dynamic operation High operating pressure

Disadvantages

Low current densities High cost of components Laboratory stage

Crossover of gas Acidic corrosive components Low current densities

Low dynamic Possible low durability Durability

High ohmic resistance Noble metal catalyst Membrane degradation

Corrosive liquid Stack below Megawatt range Excessive catalyst

electrolyte loading

2-2-2- Anode Electrocatalysts for Water Electrolyzers

In SOE electrolyzers, solid oxide ceramics are typically used as electrolytes, which have the ability to conduct oxygen ions (O2-). While high operating temperatures in these electrolyzers provide the desired kinetics, it causes a 31 number of degradation and stability issues, which creates difficulties in the selection of appropriate electrodes and electrocatalysts. In order to increase the electrocatalytic activity and mechanical stability of these electrolyzers, the electrocatalytic active phase is combined with the electrolyte ceramic phase. The main electrocatalysts used for oxygen electrodes in SOE electrolyzers are perovskite oxides, which have high catalytic activity, proper thermal adaptability with the ceramic oxide electrolyte, and great electronic conductivity. However, there is still on-going research to find an alternative perovskites electrocatalysts with better chemical adaptability with the electrolyte and with mixed ionic-electronic conductivity [33].

In the water electrolysis process, the oxygen evolution reaction is the rate- determining step. The dominant energy losses in both AEM and PEM electrolyzers arise from the high overpotentials occurring at the anode due to the slow kinetics of

OER. In this regard, several studies were performed on developing electrodes and electrocatalysts to reduce the anodes overpotential and consequently increase the rate of OER in electrolyzers.

In PEM electrolyzers, the acidic environment hinders the redox reactions kinetics and cause corrosion of most transition metal-based electrocatalysts. Therefore expensive noble metal electrocatalysts are typically required to be used in PEM electrolyzers, which results in preventing the large scale commercialization of this technology. Among several noble metals of palladium, rhenium, iridium, rubidium, platinum, gold, niobium, both rubidium oxide and iridium oxide show high activity toward OER in PEM electrolysis; however, RuO2 suffers from corrosion during oxygen evolution in acidic media [34], [35]. 32

In conventional alkaline and AEM electrolyzers, low-cost transition metal electrocatalysts can be used in the anodic compartment. While noble metals have a great activity toward OER, transition metal oxides are typically used in alkaline and

AEM electrolysis, due to their abundance and relatively high activity and stability in alkaline media. Therefore, recent studies were done on developing Ni, Co, Fe, and

Mn-based electrodes for alkaline electrolysis. Ni-based material has shown a notable improvement in the increasing OER rate in alkaline electrolysis [36]. In addition, Ni- based oxides show a proper activity and high resistance to corrosion in alkaline media

[37]. When Ni is exposed to the air, several layers of nickel oxide (NiO) form on the metal surface; however, in alkaline medium, a compact layer of Ni(OH)2 is in contact with the solution [37], [38]. Besides NiO, both cobalt oxide and iron oxide are promising electrocatalysts for OER in alkaline electrolysis. A comparative study of

Atanass et.al. [39] on several transition metals supported by carbon nanotubes (CNT) anodes, MOx/CNT (M: Co, Ni, Mn, Fe, and Cu), showed nickel oxide and cobalt oxide have the highest activity toward OER in alkaline medium.

2-3- Hydrogen Production by Biomass Electrolysis

Water electrolysis is a clean and promising method to produce highly pure hydrogen; however, it is economically undesirable mainly due to the high overpotential of OER in the anode, which results in the high-energy consumption.

−1 Theoretically, the energy consumption for water electrolysis is 39.4 kW.h.kg H2 or

-3 −1 3.54 kW.h.Nm H2 and in commercial electrolyzers, it is around 50–55 kW.h.kg H2

-3 or 4.5–5 kW.h.Nm H2 [40].

The high amount of required energy for water splitting arises from thermodynamic driving force limits. About 68% of the energy input in both alkaline 33 and PEM electrolyzers is consumed by thermodynamic, whilst kinetic accounts for only about 32% of this energy [41]. One way to overcome this thermodynamic limitation is to replace anodic OER with the oxidation of more readily oxidizable species such as biomass, which leads to significantly lower energy consumption for hydrogen production.

2-3-2 Electrolysis of Alcohol Solutions for Hydrogen Production

Recent studies have shown that electrolysis of water-alcohol mixtures such as methanol [42]–[45], glycerol [46], [47], ethanol [48], [49], bio-ethanol [1], and ethylene glycol [50] has a great potential for pure hydrogen generation due to their remarkable lower thermodynamic potential for electrolysis compared to water electrolysis. The utilization of such compounds in electrolyzers leads to electrical power savings in comparison with conventional electrolytic water splitting [51]; however, most of these alcohols are already considered as bio-fuels and further transformation to H2 is not attractive in the industry.

Short-chain alcohols and their mixtures can readily be oxidized in electrolyzers. Sapountzi et.al. in a study compared the hydrogen production from methanol, ethanol, iso-propanol electrolysis [52]. They reported that the anodic overpotential is higher while the number of C-atoms in the alcohol increases (Figure

2-7).

Figure ‎2-7- The dependence of current and current density on applied cell overpotential using methanol, ethanol and isopropanol solutions as electrolytes [52].

In 2013, Lucas-Consuegra et.al. [1] produced pure hydrogen by PEM electrolysis of bio-ethanol obtained from a winery waste stream. They reported that cell deactivation occurs after 4 h system operation as a result of bio-ethanol crossover through the PEM membrane from anode to cathode and the poisoning of the anodic electrocatalysts due to the adsorption of some reaction intermediates produced by bio- ethanol electrolysis. In addition, Katsaounis et.al [40] observed both CO2 and methanol cross over through a commercial Nafion 117 membrane during methanol electrolysis.

Lucas-Consuegra et.al. [51] in another study on PEM electrolysis of biomass- derived organic short-chain alcohols reported about a 48% decrease in electrical energy consumption for producing hydrogen compared to commercial water electrolyzers. However, the high cost of the Pt-based electrocatalyst that was applied 35 as the anode electrode and the prospect of direct usage of bio-alcohols to produce bio- fuel makes this process unattractive for practical purposes.

2-3-3- Lignin Electrolysis for Hydrogen Production

In contrast to alcohols, lignin is not considered as bio-fuel due to its chemical characteristics and rigidity of the structure. In addition, the minimum thermodynamic potential for lignin electrolysis is 0.21 V [53], which is about one-sixth of water electrolysis thermodynamic potential, i.e. 1.23 V. Due to these unique characteristics of lignin, it can be considered as a great candidate for electrolysis to produce hydrogen. In addition to hydrogen production at the cathode, lignin electrolysis can lead to the simultaneous generation of valuable chemicals at the anode.

Although in 1982 Michael R. St. John for the first time introduced the idea of hydrogen production by direct lignin electrolysis [54], only a few studies have been conducted on this subject to this date, [54]–[56]. In most of these studies, lignin was electrolyzed in a conventional 3-electrode electrochemical cell in batch operation mode. Different Pt-based, mild steel, and NiCo-based electrodes were applied in these studies. For example, Lalvani et.al. in 1993 applied a three-electrode cell to produce hydrogen by alkaline lignin electrolysis at mild temperatures [53]. They used Pt electrodes for both anode and cathode compartments and applied a glass frit to separate two half-cells (Figure 2-8). While their system was able to produce pure hydrogen, its faradaic efficiency for hydrogen production was around 90%, which is lower than that of a more recently evolved membrane-based electrolyzer with a zero- gap arrangement of electrodes.

Figure 2-8- Schematic illustration of the lignin electrolysis cell. 1-Calomel reference electrode, 2-Working electrode, 3-Condenser, 4-Cathode compartment, 5-

Thermometer,6-Counter electrode, 7-Glass frit [53].

Hibino et.al. in 2017 reported the direct electrolysis of lignin to hydrogen at temperatures range of 100-150oC [55]. For this purpose, they impregnated lignin with

85% H3PO4 and set at the Pt/C anode in a batch electrochemical cell. Despite hydrogen production with a faradaic efficiency of 100%, this configuration met significant limitations in the aspect of industrial applications. Once the pre-deposited lignin on the electrode is deactivated, the system should be disassembled to deposit a 37 new layer of lignin on the electrode and reassembled again. To address this issue,

Caravaca et.al. [56] introduced a continuous-flow polymer electrolyte membrane reactor for electrolysis of standard kraft lignin (obtained from Sigma Aldrich) to generate hydrogen. Although the continuous-flow processes are highly favorable for industrial applications, the mentioned system meets two significant problems in the sense of practical implication. First, the usage of expensive Pt-Ru as anode electrocatalyst leads to the high cost of electrolyzer assembly. In addition, Pt is highly prone to be poisoned and deactivated by lignin oxidation product intermediates, which requires a high cost of electrode replacement. Second, the low lignin concentration in

NaOH solution, i.e. 10 g.L-1, was used in their experiments, which leads to producing a high amount of wasted NaOH in this process.

2-3-4-Influence of Different Parameters on Biomass Electrolysis for Hydrogen

Production

In the process of hydrogen production by biomass electrolysis, various parameters influence electrodes, energy consumption, and membranes in PEM and

AEM electrolyzers. Among them, the applied cell voltage, temperature, and biomass concentration have the most significant impact on the process.

The applied cell voltage determined the required energy for hydrogen production. Therefore, several studies evaluated the electrolyzer performance at various voltages by measuring the current density and hydrogen production rate. In this regard, Caravaca et.al. [48] performed studies on bio-ethanol and ethanol electrolysis and they showed the applied cell voltage determines the reaction regime, whether it is in kinetic control or diffusion control. They also investigated the effect of ethanol concentration on electrolyzer performance and showed that at high cell 38 voltages and current densities, at which point the system is limited by ohmic losses and surface phenomenon (mass transfer), the ethanol concentration has a significant effect on system performance. In addition to cell voltage and biomass concentration, the efficiency of biomass electrolyzers shows a great dependence on the operational temperature [40]. Increasing temperature leads to enhancement of reactions kinetic and electrolyte conductivity as well as decreasing the diffusion resistance of reactant [57].

Hibino et.al. investigated the effect of temperature on both current density and hydrogen production rate in a lignin electrolyzer working at intermediate temperatures (100–200oC) [55]. The result of this study showed the current density and as a result hydrogen production rate increases at elevated temperatures particularly at higher cell voltages (Figure 2-9). Similar results were observed in the case of methanol and ethanol electrolysis [48], [57]. However, one of the significant issues regarding small-chain alcohols electrolysis is their volatility, which leads to their evaporation at elevated temperatures. This issue is even more important in the case of methanol, which is highly toxic.

Figure ‎2-9- The effect of temperature on lignin electrolysis, using Pt/C electrode. a) I-

V curves at different operating temperatures; b) H2 evolution rate as a function of current density [55].

CHAPTER 3: SYNTHESIS AND CHARACTERIZATION OF NICKEL COBALT

ELECTROCATALYSTS

Objectives

The purpose of chapter 3 is discussing the physical and electrochemical characterization of NiCo electrocatalysts and selecting the most effective electrocatalyst for biomass depolymerization. To achieve this goal the following objectives were accomplished.

1. Synthesis of carbon and TiO2 supported NiCo electrocatalysts with three

different Ni:Co mass ratios and characterization of the electrocatalysts by

BET, XRD, SEM, EDS to measure their surface area, crystalline structure

and sizes, morphology, and chemical composition.

2. Investigate the ability of electrocatalysts to convert lignin to LMWACs

and electrocatalysts stability, using a three-electrode batch electrochemical

cell, to select the most effective electrocatalyst for future experiments in a

continuous electrochemical cell

3-1-Introduction

Electrochemical depolymerization of lignin has been studied in many research works, in the last few decades. However, most of the efforts have proposed novel electrode materials for effective lignin depolymerization [21]. Some recent studies revealed that the utilization of electrocatalysts in the electrochemical cells leads to increasing lignin conversion, decreasing energy consumption, and improving the selectivity of products toward desired compounds [10]. One of the main challenges in lignin depolymerization is selective cleavage of the weak C–C and C–O linkages between subunits of lignin to produce LMWACs rather than breaking aromatic bonds 41

[21]. Zhu et.al. [16] proposed a pathway for lignin depolymerization in alkaline medium. In this pathway, the C–O–C bonds between lignin units can be cleaved in an alkaline medium to form phenolic hydroxyl groups. The produced phenolic groups can be oxidized into phenoxyl radicals through a single electron transfer by the anode or by previously generated OH• radicals, which eventually can be converted into benzoquinonyl radical by disproportionation, which is a redox reaction The resultant benzoquinonyl radicals can be further oxidized in two competitive paths. In one path, the C–C bonds cleavage can occur by transferring electrons to the anode, which results in turning the hydroxyl groups into aldehydes or ketones. In the other path,

− • • benzoquinonyl radicals are prone to O2 or HOO radicals leading to lignin depolymerization into hydrophilic groups [16].

Lignin electrolysis in alkaline medium provides a suitable environment for utilization of low-cost, non-noble-metal electrocatalysts such as Ni and Co; since alkaline medium can enhance oxidation reactions and diminish the risk of the corrosion of the electrodes cell compartments. The results of Song et.al. [58] study on lignin depolymerization revealed that nickel has a high potential for converting lignin to phenolic compounds. Based on their results, nickel has a high tendency for breaking aryl–aryl C–O–C linkages and C–OH bonds in side chains containing methyl or methylene functions, typically generating C1–C3 alkane-substituted guaiacols as the main product. Although nickel has the capability for aromatic C–O and arene bonds cleavage, it is much more active for breaking C–O aliphatic bonds

[58]. He et al. [59] performed a study on the selective breakage of aromatic ether bonds using a nickel catalyst in lignin model compounds that had β–O–4, α–O–4, and

4–O–5 bonds. The results show these linkages can be selectively broken down over 42 the Ni catalyst and producing monomeric aromatic compounds. Additionally, the oxide of transition metals, such as cobalt oxide, are known to be effective catalysts for oxidation reactions in the liquid phase. Cobalt oxide nanoparticles serve as the active sites of redox reactions resulting in high activity for oxidation reactions [60]. In addition, cobalt is one of the most attractive elements for the oxidation reaction due to its strong capability to activate molecular oxygen. It is also more cost-effective than noble metals [61].

Therefore, the synergistic effect of Co and Ni can provide an effective cleavage of C–C and C–O bonds in alkaline medium, which can lead to depolymerization of lignin into LMWACs with different functional groups, such as aldehydes, ketones, and phenols.

3-2-Materials and Methods

3-2-1-Electrocatalysts Synthesis

Carbon and TiO2-supported Ni-Co electrocatalysts were synthesized by a wet impregnation method. For this purpose, 60 wt% of Ni-Co with 1:1, 3:1, and 1:3 mass ratios were deposited on either carbon or TiO2 nanoparticles. To prepare the 1 g of each electrocatalyst, NiCl.6H2O (purchased from Sigma-Aldrich), CoCl2 (obtained from Fisher Scientific), and TiO2 nanopowder (nanoparticles with an average particle size of 21 nm, purchased from Sigma-Aldrich))/carbon black (from Fisher Scientific) with different stoichiometric ratios were added to 75 ml ethylene glycol. The mixture was heated to 80 oC under 300 rpm stirring (with a magnetic stir bar) for 2 h.

Subsequently, the pH of the mixture was adjusted to 10 by addition of 1.6 g NaOH.

Approximately 0.25 g sodium citrate dihydrate as a reducing agent was added to the mixture. Afterward, the temperature was raised to 180 oC and was held under reflux 43 for 2 h. Once the mixture cooled down to room temperature, the nanoparticles were separated from the solution with a paper filter. The nanoparticles were then washed with deionized water several times in order to eliminate the chloride ions.

Subsequently, the samples were air-dried at 80oC for 24 h and ground into a powder.

3-2-2- Electrocatalysts Physical Characterization

Physical characterization of the electrocatalysts was performed by BET, XRD,

SEM, and EDS spectroscopy.

3-2-2-1-Brunauer-Emmett-Teller (BET) Method

The specific surface area of the electrocatalyst was measured by N2 adsorption at 77 K based on Brunauer-Emmett- Teller (BET) method using a ASAP 2010 instrument (from Micromeritics). The average size of nanoparticles was measured from the BET surface area by assuming the spherical shape of nanoparticles from

2 dBET= 6000/ρSBET, in which, SBET is BET surface area (m /g), dBET is the nanoparticles average size (nm), and ρ is the skeletal density (g/cm3).

3-2-2-2-X-Ray Diffraction Spectroscopy (XRD)

The powder X-ray diffraction (XRD) pattern of the electrocatalyst was obtained on an Ultima IV device (from Rigaku) for identifying the approximate crystallite sizes and phase structure of the nanoparticles. The XRD diffraction pattern was obtained in the 2θ range of 20-80o at a scan rate of 3o/min. The average crystallite size of the nanoparticles was estimated using Scherrer’s equation: dXRD= Kλ/(βcosθ) (3-1) where K is a dimensionless shape factor, which is about 0.9, λ is the X-ray wavelength in radian, and β is full width at half-maximum (FWHM) in radian. 44

3-2-2-3-Scanning Electron Microscopy (SEM) and Energy-Dispersive X-Ray (EDS)

Spectroscopy

The SEM analysis was performed to obtain the morphology of the electrocatalysts and the EDS elemental analysis was applied to find the chemical composition of the electrocatalysts. Both SEM and EDS analysis were performed by a

JSM-6390, scanning electron microscope (from JEOL). To prepare the electrocatalyst powder for SEM analysis, the electrocatalyst powder was dispersed on a plate and an ultrathin layer of Pd was deposited on the sample by low-vacuum sputter coating.

3-2-3-Electrochemical Characterization of Electrocatalysts

Electrochemical characterization of the electrocatalyst was performed by a three-electrode RDE electrochemical cell using a biomass lignin-rich solution. The parameters such as electrocatalysts stability, steady-state current density, and selectivity of lignin cleavage to LMWACs were considered to select the best electrocatalyst for future experiments in continuous electrochemical rectors.

3-2-3-1-Three Electrode RDE Electrochemical Cell

A three-electrode batch electrochemical cell was applied to examine the ability of electrocatalysts to convert lignin to LMWACs. The three-electrode batch cell consisted of a Pt rotating disk electrode (working electrode or WE), a Pt ring electrode (counter electrode or CE), and an Hg/HgO reference electrode (Figure 3-1).

For the electrocatalyst deposition on the platinum disk (WE), 2 mg of the electrocatalyst powder was added to a 1:1 ethanol: water mixture. Subsequently, the mixture was kept under ultrasonic mixing for 5 min for the effective dispersion of the electrocatalyst in the solution. Then 2.5 µL of the mixture was dispersed on the Pt disk and kept in an oven at 75oC for 5 min to be dried. This procedure was repeated 45 four more times. Finally, 2 µL of 5 wt% anion exchange ionomer (FAA-3, FUMA-

Tech) in ethanol, which acts as a binding agent, was dispersed on top of the electrocatalyst layer. All of the electrochemical experiments were carried out at ambient pressure and room temperature.

Figure 3‎ -1- The schematic of a three-electrode batch system for electrochemical lignin oxidization at room temperature and ambient pressure

3-2-3-2-Lignin Solution Preparation

A corn stover lignin-rich solid samples, obtained from DOE, was used for electrochemical experiments in this project. To prepare the lignin solution for experiments the solid biomass waste was firstly dispersed in distilled water and kept under stirring for 2 days. Then the slurry was filtered with a 25-micron sieve to separate the large particles. The remaining mixture was dried at room temperature for several days and further dried at 75oC in the oven. Afterward, a different amount of the solid sample was added to 1 M NaOH solution to reach various concentrations of 46 lignin. After stirring the solution for a day, the solution was centrifuged to separate the solid insoluble part, which was mostly composed of cellulose and hemicellulose.

3-3-Results and Discussion

To select an effective electrocatalyst for lignin depolymerization, some basic physical and electrochemical characterization tests on six different electrocatalysts were performed.

3-3-1- Physical Characterization of Electrocatalysts

Typical XRD profiles of 1:1, 3:1, and 1:3 Ni:Co on carbon and TiO2 are shown in Figure 3-2. The XRD patterns show the carbon-supported electrocatalysts have broader and blunter peaks compared to TiO2-supported electrocatalysts. This suggests the lower crystallinity and crystallite sizes of carbon-supported electrocatalysts. In addition, all the notable peaks of the XRD patterns in Figure

Figure 3-2 are indexed based on Joint Committee on Powder Diffraction Standards

(JCPDS) reference numbers. The notable peaks of TiO2-supported electrocatalysts correspond to both anatase and rutile phases of TiO2. The peaks corresponding to Ni and Co have lower intensities compared to the TiO2 peaks. This can be due to the higher crystallinity of TiO2 comparing to Ni and Co and the overlapping of peaks to some degrees. In addition, some peaks corresponding to nickel and cobalt hydroxide in both carbon and TiO2-supported electrocatalysts can be observed. The presence of cobalt hydroxide could be a result of a partial reduction of Co(OH)2 during the synthesis process [62], [63].

Table 3-1 summarizes the average crystallite sizes and the BET surface areas of the six electrocatalysts. For both the carbon and TiO2-supported electrocatalysts, the average crystallite sizes decrease as the mass ratio of Co to Ni increases. The BET 47 results show carbon-supported electrocatalysts have higher surface areas comparing to

TiO2-supported electrocatalysts. Furthermore, increasing the Ni content of the electrocatalysts results in a minor increase in the electrocatalyst surface areas.

Figure 3‎ -2- XRD profiles of a) 1:1 Ni-Co/carbon, b) 1:3 Ni-Co/carbon, c) 3:1 Ni-

Co/carbon, d) 1:1 Ni-Co/TiO2, e) 1:3 Ni-Co/TiO2, f) 3:1 Ni-Co/TiO2 48

Table 3-1-

Electrocatalysts BET surface areas and crystallite sizes.

2 Catalyst BET surface area (m /g) dXRD (nm) dBET (nm)

1:1 Ni:Co/carbon 139 12.1 6.9

1:3 Ni:Co/carbon 141 8.7 6.8

3:1 Ni:Co/carbon 148 14.1 6.5

1:1 Ni:Co/TiO2 129 18.7 6.6

1:3 Ni:Co/TiO2 130 15.7 6.5

3:1 Ni:Co/TiO2 144 20.7 5.9

Figure 3-3 shows the typical SEM images of both 1:3 Ni:Co/carbon and 1:3

Ni:Co/TiO2. Both electrocatalysts are in the form of stacks of aggregated particles.

These particles are seen as a single, large particle instead of separate individual entities especially in the case of 1:3 Ni:Co/carbon. The observation of the 1:3

Ni:Co/carbon revealed that its particles have an angular shape with a rough surface; however, 1:3 Ni:Co/TiO2 sample have a near-sphere shape with a smooth surface.

Figure 3-4 illustrates typical EDS spectra of both 1:3 Ni:Co/carbon and 1:3

Ni:Co/TiO2 electrocatalysts and the results of the elemental analysis of the samples obtained by EDS are summarized in Tables 3-2 and 3-3. The results indicate the experimental mass percentage of components agree with their nominal composition except oxygen. The presence of extra oxygen in the samples can be due to the high reactivity of the nanoparticles with oxygen.

Figure ‎3-3- Typical SEM images of a)1:3 NiCo/carbon and b)1:3 NiCo/TiO2

1:3NiCo/Carbon

1:3 NiCo/TiO2

Figure ‎3-4- Energy-dispersive X-ray spectroscopy (EDS) spectra of 1:3 NiCo/ carbon and 1:3 NiCo/TiO2 electrocatalysts.

Table 3-2-

Energy-dispersive X-ray spectroscopy elemental analysis and nominal composition of

1:3 NiCo/carbon

Experimental Nominal Experimental Element Relative Error (%) Mass (%) Mass (%) Atom (%)

Carbon 37.6 40 65.8 4.53

Oxygen 12.4 - 16.3 1.65

Nickel 12.3 15 4.5 0.43

Cobalt 37.7 45 13.4 1.18

Sum 100 100 100

Table 3-3-

Energy-dispersive X-ray spectroscopy elemental analysis and nominal composition of

1:3 NiCo/TiO2

Experimental Nominal Experimental Element Relative Error (%) Mass (%) Mass (%) Atom (%)

Titanium 23.1 24 18.2 0.78

Oxygen 19.0 16 44.9 2.69

Nickel 12.5 15 7.9 0.51

Cobalt 45.4 45 29.0 1.47

Sum 100 100 100

3-3-2- Electrochemical Characterization of Electrocatalysts

Figure 3-5 shows graphs of the current as function of time graph for each electrocatalyst during 48 h of lignin electro-oxidation at 400 mV vs. Hg/HgO. The lignin concentration in 1 M NaOH was 10 g/L in all experiments. Figure 3-5 shows that the current density decreases to reach a steady-state value over 48 h. The results indicate the steady-state current density of TiO2-supported electrocatalysts is about

1.5 to 2.5 times higher than that of carbon-supported electrocatalysts. The lower steady-state current density for carbon-supported electrocatalysts can be due to the carbon corrosion in the alkaline medium, which accelerates the deactivation of the carbon-supported electrocatalyst [64]. In addition, data in Figure 3-5 demonstrate that the high mass ratio of Co to Ni in the electrocatalysts enhances their stability and increases their steady-state current density. A similar result has been observed for

NiCo electrocatalysts in another study [65]. The maximum steady-state current density of 448 mA/g is observed for 1:3-Ni:Co/TiO2.

In addition, chronoamperometric curves of the TiO2-supported electrocatalysts have an oxidation peak at the first hours of the reaction. The oxidation peak time of

3:1 NiCo/TiO2 is shorter than that of 1:3-NiCo/TiO2. This shows a faster oxidation kinetics at 3:1 NiCo/TiO2 [66]. However, 3:1 NiCo/TiO2 has lower stability, and as a result, its steady-state current density is about 50% lower than that of 1:3-NiCo/TiO2. 53

Figure ‎3-5- Chronoamperometric curves of NiCo electrocatalysts at a constant working electrode potential of 400 mV vs. Hg/HgO

To compare the selectivity of the electrocatalysts toward LMWACs, the three- electrode batch cell was applied for lignin electro-oxidation on each electrocatalyst at a constant working electrode potential of 400 mV (vs. Hg/HgO) for 32 h. The

LMWACs% of the neat and oxidized lignin for each experiment was achieved by

GC/MS analysis (Table 3-4). The GC/MS results indicate the high ratio of Co to Ni

(i.e. 1:3 NiCo) in the electrocatalysts results in the selectivity toward LMWACs.

Cobalt serves as active sites of oxidation reactions [60] resulting in high lignin 54 oxidation activity at lower cell voltages and cleavage of lignin inter-unit C–C and C–

O linkages. The maximum selectivity toward LMWACs corresponds to 1:3

NiCo/TiO2, which causes about 2 times increase in the LMWACs% of the lignin sample by oxidation.

Table 3-4-

LMWACs% of the neat and oxidized lignin at 400 mV (vs. Hg/HgO) for 32 h

LMWACs % Electrocatalyst neat lignin oxidized lignin

1:1 NiCo/carbon 25 20

1:3 NiCo/carbon 15 25

3:1 NiCo/carbon 43 25

1:1 NiCo/TiO2 48 30

1:3 NiCo/TiO2 9 18

3:1 NiCo/TiO2 36 28

3-4-Conclusion

The effective cleavage of C–C and C–O bonds under NiCo electrocatalysts results in depolymerization of lignin into low molecular weight aromatic compounds with different functional groups such as aldehydes, ketones, and phenols. NiCo electrocatalysts were synthesized with various Ni to Co mass ratios on carbon and

TiO2 supports. In addition to the physical characterization of six different electrocatalysts; Chapter 3 includes the results of the electrochemical experiments that were performed in the three-electrode RDE cell with applying the electrocatalysts.

These results were used to select the electrocatalyst with the highest stability and selectivity toward LMWACs for future experiments in the continuous flow electrochemical reactors.

According to XRD and BET results, the carbon-supported electrocatalysts have a lower crystallinity but higher surface areas compared to the TiO2-supported electrocatalysts. However, the TiO2-supported electrocatalysts have higher stability and as a result, have greater steady-state current density values. In addition, the

GC/MS results show the electrocatalysts with a higher content of Co have high selectivity toward LMWACs. Altogether, the results indicate the 1:3 Ni-Co/TiO2 is the most selective and stable electrocatalyst among the six tested electrocatalysts for lignin conversion to LMWACs.

CHAPTER 4: HYDROGEN PRODUCTION AND ENERGY EFFICIENCY

Objectives

Chapter 4 is aimed at investigating the effect of cell voltage, lignin concentration, and temperature on hydrogen production, oxygen evolution, and energy efficiency of the electrochemical reactor. To fulfill this aim, the following objectives were performed:

1. Investigate the effect of cell voltage on current density and faradaic

efficiency of hydrogen and oxygen evolution

2. Investigate the effect of the lignin concentration on the cell current density

and oxygen evolution

3. Investigate the electrochemical reactor energy efficiency and oxygen

evolution at different temperatures

4-1-Introduction

Hydrogen is known as a promising energy carrier for domestic, industrial, and automotive purposes. The energy content of hydrogen is 118 MJ/kg, at 25 oC, which is about 3 times higher than that of gasoline (44 MJ/kg, at 25 oC). In addition, using hydrogen as an energy carrier may result in little to know greenhouse gases emissions if renewable energy can be used to generate hydrogen [67], [68]. Currently, about

95% of the total hydrogen used in the United States is produced by the steam reforming of natural gas or methane. Water electrolysis is one of the cleanest methods for hydrogen production from a sustainable source.

The main drawback of water electrolysis is that the high anodic overpotential, caused by slow OER kinetics, can lead to significant energy losses and consequently higher hydrogen production costs compared to other methods. Several methods 57 including coal-depolarized electrolysis [69], SO2-depolarized electrolysis [70], and biomass-depolarized electrolysis [54], [55], [71] have been applied to decrease the anode overpotential. Among these methods, lignin depolarization with cogeneration of hydrogen has attracted more attention recently [72], [73] due to the abundance of lignin and additional possible revenue from lignin oxidation products.

Lalvani and Rajagopal showed lignin electrolysis in the anode can significantly decrease the anode overpotential [72]. They calculated the equilibrium or minimum cell potential, 퐸0, required for lignin electrolysis by the following reaction:

∆퐺푐푒푙푙 퐸0 = 푛퐹 where ∆퐺푐푒푙푙 is Gibbs free energy change, n is the number of moles of transferred electrons and F is Faraday constant. Based on their result, 퐸0 for lignin electrolysis is equal to 0.21 V, which is one-sixth of the minimum cell potential of water electrolysis. They also observed hydrogen evolution at 1.24 V vs. standard hydrogen electrode (SHE), with hydrogen production rate increasing with temperature.

In this chapter, a continuous electrochemical system was applied for lignin oxidation at the anode and hydrogen production in the cathode of a 10 cm2 cell. The effect of cell voltage, lignin concentration, and operating temperature on hydrogen production at the cathode and oxygen evolution, as a competing reaction to lignin oxidation, at the anode was investigated and discussed. In addition, energy consumption at different cell voltages and temperatures is calculated and reported. 58

4-2-Materials and Methods

4-2-1-Lignin Solution

The lignin purification process and solution preparation were described in detail in chapter 3. The same procedure for lignin solution preparation with different concentrations was followed for all the experiments in this chapter.

4-2-2-Electrode Preparation

The anode was prepared in the following way: 1:3 NiCo/TiO2 electrocatalyst powder was synthesized with the same procedure explained in chapter 3. After synthesis, a mass of 0.5 g electrocatalyst was added to 65 mL ethanol solution containing 0.16 wt% fumion ionomer, purchased from Fuma Tech., as a binder. The mixture was kept under ultrasonic agitation for an hour to uniformly disperse the electrocatalyst powder in the ethanol/fumion solution. Then the electrocatalyst was spray-coated onto the carbon paper GDL by means of a hobby spray gun. Spray- coating of the electrocatalyst continued until a loading of 8 mg/cm2 was achieved.

In the cathode compartment, a 2 mg/cm2 Pt-loaded carbon cloth, purchased from the Fuel Cell Store, was applied as the cathode electrode.

4-2-3-Continuous Electrochemical Reactor

The 10 cm2 electrochemical reactor used for all the experiments reported in this chapter was a single cell purchased from Fuel Cell Technologies, Inc. Figure 4-1 shows the schematic diagram of the continuous electrochemical system including the electrochemical cell, pumps, power supply, and hydrogen collector. The cell was constructed in the typical way, such that a symmetric 0.68 mm-wide flow channel with a length of 0.66 m was machined into the graphite flow blocks on either side of the cell. 59

Lignin dissolved in NaOH was introduced into the anode side channel and

NaOH solution was fed into the cathode side channel with the flowrates of 1 and 2 mL/min, respectively, using peristaltic pumps. The H2 generated at the cathode was separated from the liquid and the NaOH solution was recycled. H2 production was measured at the cathode with a bubble flow meter and the volume of O2 generated at the anode was measured by water displacement. The cell voltage was controlled by a power supply (Agilent E3631) and the current at each applied cell voltage was monitored by the device. The temperature of the cell was controlled by a built-in heating rod and a thermocouple inside the cell endplate, which were attached to a PID temperature controller.

Figure ‎4-1- Schematic representation of the continuous electrochemical process for lignin depolymerization with cogeneration of H2 60

4-2-4-H2 Detection

The H2 generated at the cathode was detected by a gas chromatograph (Agilent

Technology 7890B) equipped with an NGA column set with a thermal conductivity detector (TCD). Helium with a flow rate of 0.5 mL/min was used as the carrier gas.

The column temperature was kept at 60oC for 11 min and then raised to 200oC at the rate of 36oC/min, and remained at 200oC for 5.1 min. The inlet and detector temperatures were kept at 250oC.

4-3-Results and Discussion

4-3-1-The Effect of Temperature on Current Density and Reaction Kinetics

Figure 4-2 shows the effect of temperature on current density and energy consumption of the 10 cm2 continuous electrolyzer during lignin-rich biomass or alkaline water electrolysis in the cell voltage range of 0.6 to 1.8 V. The results shows an approximately 4 times increase in the current density occurs at the cell voltage range of 1.4 V to 1.6 V by increasing the temperature from 25oC to 60oC during lignin electrolysis. This dramatic increase in the current densities by increasing the temperature can be due to an enhancement in the kinetics of both lignin electro- oxidation and OER at the anode and HER at the cathode [41], [56], while the activation energy for the reactions is provided by heating rather than higher overpotentials [74]. In addition, the ionic conductivity of the membrane can be enhanced at higher temperatures, which leads to lower ohmic losses and higher current densities [51], [74].

Figure 4-2 also shows that the current density of biomass electrolysis is higher than that of water electrolysis at cell voltages below about 1.45 V – 1.5 V. Current density is an indicator of the rate of electrochemical reactions. Higher current density 61 shows higher reaction rates. However, at cell voltages higher than about 1.5 V, the water electrolysis current density is higher than that of biomass electrolysis.

Figure ‎4-2- The effect of temperature and the presence of 50 g lignin/L on current/energy density, applying continuous electrochemical reactor using. Linear scan voltammetry from 0.6 V to 1.8 V; scan rate (0.5 mV/s). Anode: 1:3 NiCo/TiO2 on carbon paper and cathode: Pt-loaded carbon cloth.

This observation can be explained by two possible major reactions, which may occur at the anode of the biomass electrolyzer: 1) lignin oxidation, which is the desired reaction and 2) the OER, which is an undesired reaction [22], [75]. At the cell voltage lower than about 1.4-1.5 V, lignin oxidation is more likely to occur compared to OER, since lignin oxidation is thermodynamically more favorable than oxygen evolution [14], [16]. Therefore, at lower cell voltages lignin oxidation results in higher current densities compared to water electrolysis. However, OER is more kinetically facile than lignin oxidation at higher cell voltages (more than 1.5 V).

Therefore, at higher cell voltages (>1.5 V) water electrolysis leads to higher current densities and a high amount of O2 production. High-rate O2 generation at the electrolyzer anode would consume energy that would better be used for lignin oxidation and represents an energy-intensive process that should be avoided.

However, it may not be possible to completely eliminate the OER at all cell voltages, because as the anode overpotential increases, the OER may compete with lignin oxidation.

This trend can be further explained by way of Figure 4-3, which presents OER faradaic efficiencies at three cell voltages and temperatures outlined in Table 4-1. To measure the Faradaic efficiency of oxygen evolution, the flowrate of the O2 generated at the anode was measured (Table 4-1) and divided by the theoretical flow rate of O2

calculated based on the four-electron reaction from Faraday’s law: 휐푂2/퐶푂2 = 퐼 4퐹, where I is the current attained under different onset potentials, and F is the Faraday constant.

At any cell voltages shown in Figure 4-3, the OER faradaic efficiency of lignin electrolysis (solid circle marked graphs), are lower than the OER faradaic 63 efficiency of water electrolysis (solid triangle marked graphs). OER faradaic efficiency represents the percentage of the anodic current consumed for O2 evolution rather than lignin oxidation; therefore, higher OER faradaic efficiencies are undesirable for this system.

Figure ‎4-3- OER faradaic efficiency as a function of cell voltage for water electrolysis (solid triangle markers) and biomass-depolarized electrolysis (solid circle markers) at 25°C (˗˗˗ ⸳⸳ ˗˗˗), 40°C (solid line) and 60°C (- - -).

At a lower cell voltage of 1.4 V, the OER faradaic efficiency for water electrolysis is about 50% and 61% at 25 oC and 60 oC, respectively. However, the

OER faradaic efficiencies of lignin electrolysis at 1.4 V at both 25 oC and 60 oC are 64 essentially 0%, which confirms that lignin oxidation more likely occurs at lower potentials than the OER. By increasing cell voltage to 1.5 V and 1.6 V at any temperature, the OER faradaic efficiency of lignin electrolysis increases to 5-15% and

50%, respectively. This observation indicates oxygen evolution becomes increasingly dominant at higher cell voltages, resulting in lower efficiency toward lignin oxidation at these cell voltages [27]. However, even at high cell voltages of 1.6 V, the OER faradaic efficiency of the biomass electrolyzer is 50%, which is still about half that of the water electrolyzer.

As it is shown in Table 4-1, the current density increases in both water and biomass electrolysis as the cell voltage is increased. In the case of water electrolysis, this increase can be due to the increase in the OER rate. In contrast, in lignin electrolysis, both the rates of lignin oxidation and the OER increase with increasing cell voltage. Therefore, even though increasing the cell voltage increases the rate of lignin oxidation, it also increases the rate of the OER, which leads to decreasing energy efficiency of the overall process.

Table 4-1-

Faradaic efficiency for oxygen evolution from alkaline water electrolysis and biomass-depolarized electrolysis at different cell voltages and temperatures.

Alkaline water 50 g/L lignin in NaOH Cell electrolysis oxidation

Temperature voltage Current O2 Current O2 (oC) (V) density production density production (mA/cm2) (mL/min) (mA/cm2) (mL/min) 1.4 1.0 0.019 3.5 0 25 1.5 2.8 0.100 4.8 0.010 1.6 17.0 0.620 10.3 0.200 1.4 2.5 0.058 4.5 0 40 1.5 10.0 0.362 9.5 0.050 1.6 34.3 1.280 31.0 0.592 1.4 4.2 0.091 14.2 0 60 1.5 26.0 0.97 20.0 0.101 1.6 67.5 2.512 40.0 0.750

In addition, the results shown in both Table 4-1 and Figure 4-3 indicate that increasing the temperature leads to increasing the current density (or reaction rate).

However, the OER faradaic efficiency at each voltage remains constant by increasing temperature. For example, at 1.4 V as the temperature increases from 25 oC to 60 oC, the current density of the lignin electrolyzer increases from 3.5 mA/cm2 to 14.2 mA/cm2 but the OER faradaic efficiency remains constant at 0%. A similar trend for higher cell voltages can be observed. At 1.6 V the current density increases from 10.3 mA/cm2 to 40.0 mA/cm2 as the temperature increases from 25 oC to 60 oC. However, the OER faradaic efficiency remains constant, about 50%, at all three temperatures. 66

These results have potential implications in the operation of biomass- depolarized electrolyzers, because at many practical operating voltages, some measurable fraction of the overall current (the reaction rate) may be the result of the

OER.

4-4-2-The Effect of Lignin Concentration

The lignin concentration is another important parameter in biomass electrolysis. Figure 4-4 presents cell current densities of biomass electrolysis at different operating cell voltages from 1.2 V to 1.6 V and lignin concentrations of 0,

10, 50, 100 g lignin/L in 1 M NaOH. The addition of lignin to NaOH with any concentration leads to an increase in the current density, particularly at lower cell voltages. For example, introducing 50 g lignin/L solution into the electrolyzer results in about 5.7 and 3.6 times increase in the current density at 1.3 V and 1.4 V, respectively, compared to NaOH solution alon. The increase in the current density by the addition of lignin to NaOH is due to the lower minimum thermodynamic potential required for lignin oxidation compared to the OER [14], [16]. However, lower lignin concentrations results in an increase in the current density at higher cell voltages (>1.5

V) so that at 1.6 V NaOH (no lignin present) has the highest current density among all other solutions. At higher cell voltages, oxygen evolution becomes increasingly dominant, resulting in lower efficiency toward lignin oxidation at these voltages [27], as has been discussed and confirmed above detailing the results outlined in Table 4-2 and Figure 4-4.

Figure ‎4-4- Current density versus cell voltage at different lignin concentrations in 1

M NaOH, electro-oxidized on 1:3 NiCo/TiO2 in the continuous electrochemical reactor, using AEM between electrodes at room temperature.

For more clear observation of the effect of lignin concentrations on the electrochemical reactor performance, the current density as a function of lignin concentration is shown in Figure 4-5.

Figure ‎4-5- Current density as a function of lignin concentration at 1.3 V, 1.4 V, 1.5

V, and 1.6 V.

At the lower cell voltage (about 1.3 V), the addition of any amount of lignin

(from 10 to 100 g/L) to 1 M NaOH, leads to about the same, 4 times, increase in the current density. By the addition of lignin, the current density is increased due to the lower required thermodynamic potential required for lignin oxidation compared to

OER [28]. In addition, the kinetics of the reactions mainly control the overall reaction rate at the low cell voltages (below 1.4 V). However, at the cell voltages range between 1.4 and 1.5 V the current density as a function of concentration exhibits a 69 maximum around 50 g/L lignin solution. At this voltage range, lignin oxidation is still dominant, compared to OER, so the observed current density is mainly due to this reaction. Increasing the lignin concentration increases the rate of reaction, as expected. However, concurrently, by increasing the cell voltage (>1.4 V), the reaction rate becomes more limited to the surface phenomena, i.e. mass transfer and ohmic losses, than reaction kinetics [29], [30]. Higher lignin concentration may decrease the mass transfer of reactants to the surface of the electrode, where the steric effects are noticeable for large organic molecules [31]. In addition, at higher lignin concentrations (above 50 g/L), lignin may block active electrode sites, reducing the reaction rate. Therefore, a combination of the mentioned effects results in an optimum lignin concentration at each cell voltage, which leads to a maximum current density.

By increasing the cell voltage to 1.6 V, a constant decrease in current density by increasing lignin concentration is observed. At higher cell voltage (above 1.5 V),

OER becomes dominant, as discussed and confirmed in the previous section. As a result, any lignin in solution reduces the reaction rate because it represents a competing reaction that possibly occurs at a lower rate than OER. In addition, at higher cell voltages, reaction rates can be mainly controlled by mass transfer and the presence of lignin in the solution decreases the rate of mass transfer of the reactants to the surface of the electrode. The combination of these two negative factors, i.e. OER dominance and mass transfer limitation at 1.6 V, leads to a constant decrease in the current density by introducing 10 to 100 g/L into the electrochemical reactor.

4-3-3-H2 Purity and Energy Consumption

The flow rate of H2 generated at the cathode was measured over the current density range of 3.5 to 30 mA/cm2 in the 10 cm2 cell after 24 h of reactor operation 70 under 1.4 V, using 50 g lignin/L NaOH in the anode and NaOH in the cathode (Table

4-2). For each current density outlined in Table 4-2 the faradaic efficiency of H2 evolution was calculated based on the two-electron reaction from Faraday’s law.

Average faradaic efficiency of about 99% for HER was observed. In addition, the electrical energy consumed by the electrolyzer and energy efficiency for H2 generation based on the electrical energy stored as H2, in terms of higher heating value (HHV) at each current density is shown in Table 4-2. The results indicate that the electrolyzer operates at room temperature with an energy efficiency of higher than

90% at the current densities of 15 mA/cm2 and lower, which is significantly higher than the energy efficiency of conventional alkaline electrolyzers, in which a diaphragm is placed between electrodes [76]. Applying a thin anion exchange membrane (thickness: 45-50 µm), which provides a greater ion conductivity and negligible gap between electrodes, leads to minimizing the ohmic losses in the system

[76]–[78] and consequently enhances the energy efficiency of the electrolyzer.

Table 4-2 also depicts that increase in the cell voltage from 1.4 V to 1.72 V leads to an increase in the energy requirements for H2 production from 39.5 to 45.7 kWh/kg H2, which correspond to a 14% decrease in the electrolyzer energy efficiency.

The increase in the energy requirement could be due to the competing OER, which introduces inefficiencies.

Table 4-2-

Rate of H2 generation from 50 g/L lignin electrolysis and energy consumption of the continuous electrochemical reactor with 10 cm2 cell working at room temperature and pressure. The data was collected after 24 h of the reactor operation using 8 mg/cm2

2 1:3 NiCo/TiO2 and 2 mg/cm Pt as anode and cathode electrocatalyst, respectively.

Current Measured Faradaic Energy Average cell Energy density density flowrate of efficiency of efficiency voltage (V) (kWh/kg H2) 2 (mA/cm ) H2 (mL/min) H2 evolution (HHV)* (%)

1.40 3.5 0.25 0.95 39.5 100

1.51 5.0 0.37 0.98 41.3 95

1.60 10.0 0.74 0.99 43.4 91

1.64 15.0 1.12 1.00 44.0 90

1.67 20.0 1.49 1.00 44.8 88

1.72 30.0 2.26 1.01 45.7 86

*The energy efficiency was calculated as follow:

(푚퐻 )(퐻퐻푉퐻 ) 퐸푛푒푟푔푦 푒푓푓푖푐푖푒푛푐푦 = 2 2 × 100 퐸

where 푚퐻2 푘푔 is the mass of H2 produced in one hour at each current density and

o 퐻퐻푉퐻2 is the higher heat value of H2 gas, which is 141.79 MJ/kgH2 at 25 C and 1 atm and E (MW.h) is the electrical energy consumed at each current density

Although biomass electrolysis leads to relatively lower energy requirements compared to water electrolysis at lower cell voltage (less than 1.5 V), the current density is relatively small, leading to a low rate of hydrogen production. However, on a larger scale, the valuable product stream from the lignin oxidation reaction at the anode [79], which is extra revenue for this process, would be needed to offset the capital cost.

Furthermore, the hydrogen that is produced by the biomass-depolarized electrolyzer is highly pure. The GC chromatogram of the gas collected from the electrolyzer cathode during biomass-depolarized electrolysis is shown in Figure 4-6.

The H2 peak occurs at 1.5 min and two minor peaks, corresponding to O2 and N2, are detected at 7.6 and 8.2 min, respectively. Analysis indicates an H2 purity of about

97.6%. The N2 and O2 impurities were air-like mixtures, which likely come from sample collection.

Figure 4‎ -6- GC-TCD chromatogram of the generated gas in the cathode during oxidation of 50 g lignin /L NaOH in the anode.

4-4-Conclusion

In this chapter, the functionality of biomass electrolysis in hydrogen production at various cell voltages, temperatures, and lignin concentration was investigated. This electrolyzer can produce hydrogen at voltages lower than those typically required for water electrolysis, however, at relatively low current densities.

Higher current densities are achievable at higher cell voltages, at which the rate of the

OER is increased, which is an undesired reaction in this system. However, at lower cell voltages, the electrolyzer operates with an energy efficiency higher than 90% for

H2 production. In addition, this biomass electrolyzer can generate H2 at room temperature and atmospheric pressure. Increasing the temperature leads to an increase in the rate of both OER and lignin oxidation reactions in anode and hydrogen evolution reaction in the cathode. On top of electrolytic production of H2 in the cathode, the biomass oxidation products in the anode of this electrolyzer could provide additional revenue for the overall process. Chapter 5 will focus on biomass conversion and the anode product stream.

CHAPTER 5: BIOMASS DEPOLYMERIZATION

Objectives

The main goal of chapter 5 is to investigate the effect of cell voltage and biomass electrolysis time on biomass conversion, the selectivity of products toward

LMWACs, functional groups in the product stream, and the average molecular weight of biomass after depolymerization. To meet this goal, the main objectives are:

3. Apply the generalized standard addition method (GSAM) to investigate

the effect of cell voltage and biomass electrolysis time on biomass

conversion

4. Investigate the effect of cell voltage and biomass electrolysis time on the

selectivity of biomass oxidation products toward LMWACs and functional

groups by mean of GC/MS, FTIR spectroscopy, and Raman spectroscopy

5. Analyze the change in the average molecular weight of the anode product

stream by using GPC

5-1-Introduction

From the middle of the 1900’s, several studies have been carried out on lignin depolymerization to obtain industrial chemical compounds. Various thermochemical methods such as pyrolysis (thermolysis), hydrogenolysis, hydrolysis, and chemical oxidation applied for lignin depolymerization [80]–[83]. The most significant problems related to these methods are their low selectivity, the requirement of drastic operation conditions, and employment of toxic or expensive catalysts [14], [84]. One of the promising approaches for selective depolymerization of lignin to LMWACs under ambient pressure and room temperature is electrochemical oxidation of lignin

[16]. 75

To date, most of the studies on the electrochemical oxidation of lignin in alkaline medium included two or three electrodes in batch systems for lignin depolymerization [16]–[21], [14]. Despite the fact they have made some progress in converting lignin to value-added compounds, they are not economically viable methods for depolymerization of lignin on a large industrial scale.

Applying the biomass electrolyzer for lignin depolymerization over high surface area 1:3 Ni-Co/TiO electrocatalyst in this study can provide the effective cleavage of C–C and C–O bonds under NiCo electrocatalysts. This process can result in depolymerization of lignin into low molecular weight aromatic compounds with different functional groups such as aldehydes, ketones, and phenols depending on the electrolyzer onset voltage and the biomass electrolysis time. The low-cost NiCo electrocatalyst, the uncomplicated continuous system operating under room temperature and pressure, and cogeneration of hydrogen at the cathode make this process highly desirable to apply in the industry.

Besides the challenges associated with lignin depolymerization methods, the lack of quantitative methods for the characterization of lignin before and after depolymerization is another problem in understanding and modifying lignin depolymerization processes.

Lignin is a 3-dimensional macromolecule with an undefined structure, which is soluble in a highly caustic solution of pH 13. These characteristics of lignin make the analysis of lignin very challenging. This problem is even more substantial in the case of depolymerization of kraft lignin (i.e. industrial waste lignin), which is a mixture of various large and small molecules with inconsistent compositions. While a considerable number of studies have been devoted to the analytical characterization of 76 lignin [85], a few studies have been done on a precise quantitative analytical method for lignin characterization [86].

In this chapter, the attempts in applying the biomass electrolyzer, which is defined in chapter 4, for lignin depolymerization over the high surface area 1:3

NiCo/TiO2 electrocatalyst are detailed. Several analytical techniques including

GC/MS, FTIR spectroscopy, Raman spectroscopy, UV-Vis, and GPC were applied for the analysis of lignin samples before and after oxidization in the electrochemical reactor. In addition, the generalized standard addition method (GSAM), as a precise quantitative method for calculating the percentage of lignin conversion after electrochemical depolymerization, is well defined and discussed in this chapter.

5-2-Materials and Methods

The procedures for lignin purification, lignin solution preparation, and electrocatalyst synthesis were explained in Chapter 3. In addition, the detail of the electrochemical reactor and electrocatalyst deposition on the electrode were described in chapter 4. In this section, the purpose of using each spectroscopic method for lignin characterization and the detail of sample preparation for different spectroscopic analysis is described.

5-2-1-Gas Chromatography/Mass Spectroscopy (GC/MS)

GC/MS provides information regarding the relative concentrations and mass percentage of different components in the neat and oxidized lignin samples. For this study, the target products were low molecular weight aromatic compounds

(LMWACs). To prepare lignin samples for GC/MS analysis, neat and oxidized lignin solutions were extracted by chloroform or ether and then the extract samples were injected into a GC column. For extraction, 2 ml chloroform or ether was added to 77 every 2 ml or 1 ml of lignin sample, respectively, and the mixture was shaken vigorously to extract the LMW organic compounds from it. Then the samples were acidified by adding 0.86 M H2SO4 to the mixture to reach the pH of 2 or 3. By acidification, lignin residua and heavier molecules precipitated in a solid form. After centrifuging at the rate of 5000 rpm for 5 min, three phases of aqueous, organic, and solid was observed (Figure 5-1). The organic phase was separated and collected in another container. Then the same procedure of extraction (i.e. addition of 2 mL chloroform/ether to the reminder, vigorously mixing, and then centrifuging) was repeated until the extract organic phase became clear (i.e. had no color). After extracting the samples several times, the collected organic solutions were mixed together. To reduce the errors of analyte losses during sample preparation, the internal standard method was used. For this purpose, 2-fluorophenol, as an internal standard, was added to the collected organic phase after extraction. The resulting organic phase was analyzed using a ThermoScientific ISQ LT Single Quadrupols Mass

Spectrometer (MS) connected to a Thermo Scientific Trace 1310 Gas Chromatograph

(GC) with a 60 m TraceGOLD GC column (0.25 mm ID and 0.25 µm film thickness) and a flame ionization/thermal conductivity detector FID detector. 1 µL of samples were injected into the GC column with a split ratio of 33. The oven temperature was kept at 50oC for 3 min followed by heating to 270oC at the rate of 5oC/min and maintained at this temperature for 5 min. Highly pure helium (purity of 99.99%) with the flow rate of 38.7 ml/min was used as the carrier gas. Finally, the concentration of the LMWACs in the lignin solution was measured by comparing the peak area of each compound as calculated by the Chromeleon software including the incorporated 78 standard NIST database, to the 2-fluoro phenol peak area multiplied by the response factor, calculated by internal standard method.

Figure ‎5-1- Three phases formed after lignin sample extraction with chloroform and acidification with sulfuric acid.

5-2-2- Fourier-Transform Infrared (FTIR) Spectroscopy

FTIR spectroscopy was applied to detect the functional groups of lignin residue after extraction. To prepare solid samples for FTIR analysis, after extraction of the lignin samples with chloroform, the solid lignin residue was washed and dried at room temperature. After drying, 3 mg of the lignin sample was added to 0.3 g KBr powder and very well grind to completely disperse the lignin sample in KBr powder.

Then 0.05 g of the powdered mixture was very well dispersed in 0.05 g KBr to reach the final concentration of 0.005 g lignin in KBr. The prepared powder was pressed in the shape of pellets. Subsequently, the FTIR spectra were recorded by mean of a

Bruker Vertex 80 device in the wavenumber range of 400 to 4000 cm-1.

5-2-3-Raman Spectroscopy

Raman spectroscopy, as a complementary analysis to FTIR spectroscopy, was performed to detect the functional groups in lignin samples. However, lignin auto- 79 fluorescence is an obstacle to obtain Raman spectra with an acceptable quality [87].

To overcome lignin auto-fluorescence, a SENTERRA Raman microscope (from

Bruker co.), with the ability of fluorescence removal was applied to perform Raman tests. For this purpose, Raman spectra of neat and oxidized lignin samples in the liquid form were recorded in the wavenumber range of 500 to 3500 cm-1.

5-2-4- Hydroxyl (OH) Numbers Measurement

Hydroxyl numbers of solid lignin samples were measured using ASTM D

4274-99 Test Method A. According to this method, the solid lignin samples were acetylated with a solution of acetic anhydride in pyridine in a pressure bottle at 98°C.

The excess reagent was hydrolyzed with deionized water and the acetic acid was titrated with standard sodium hydroxide solution. The hydroxyl number was calculated from the difference in the titration of the blank and sample solutions.

5-2-5- Gel Permeation Chromatography (GPC)

The molecular weight distribution of the neat and oxidized lignin samples was determined by GPC method. For this aim, lignin samples neutralized with an H2SO4 solution and the precipitates were washed with deionized water. The precipitates were suspended in a 9:1 dioxane: water solution and stirred for an hour for purification.

The resulting mixture was centrifuged and the supernatants were collected. The supernatants were filtered on a medium sintered glass funnel and the dioxane was removed from them by rotary evaporation. After purification, the samples were acetylated by adding a 1:1 volume ratio of acetic anhydride:pyridine to them. The solution was maintained at room temperature for 24 h. To remove acetic acid and pyridine from the samples, 30 mL ethanol was added to the solution. After stirring for

30 min, the solvents were removed from the solution by rotary evaporation. The 80 residue was dissolved in chloroform and the solution was added drop-wise to anhydrous ether. Then after the precipitates were collected by centrifugation, washed two times with ether, and dried under vacuum at 40°C for 24 h. 2 mg of the resulting lignin acetate was dissolved in 1 ml of tetrahydrofuran THF and filtered through a

0.45μm PTFE filter. Finally, the sample placed in a 2 mL autosampler vial for GPC test on Waters e2695 separations module with Waters temperature control module II and Waters 2489 UV/VIS detector.

5-2-6- Lignin Conversion Measurement

Quantifying lignin conversion is critical to determine the efficiency of the electrochemical process. The typical method to measure the percentage of lignin conversion is measuring the lignin loss by weighing the solid lignin before and after the depolymerization process [88]. However, this method involves different steps of lignin-rich biomass treatment and extraction before the depolymerization processes. It also includes extraction of the products, precipitation, and drying the solid lignin after the depolymerization process. These treatment and extraction steps make lignin conversion quantification a complex and difficult process.

UV-Vis is an appealing quantitative spectroscopic method for lignin conversion measurement, due to the presence of chromophores in lignin. Decreases in the UV-Vis absorbance intensity of lignin after oxidation is evidence of lignin molecule breakage [89]. Since UV-Vis absorbance can be measured across a broad range (240-400 nm), a multivariate calibration method such as the generalized standard addition method (GSAM) is applicable to measure the unknown concentration of lignin residue in the oxidized samples precisely. In this study, GSAM 81 was used to measure the percentage of lignin conversion in the electrochemical process.

5-2-6-1-UV-Vis Spectroscopy

To prepare samples for UV-Vis spectroscopy, 465 µL of oxidized lignin sample was added to 450 mL 1 M NaOH solution, which we called the diluted oxidized lignin solution. Then 10 µL of neat lignin solution (i.e., lignin solution before oxidation), was added to 15, 30, 45, 60, and 90 mL of the diluted oxidized lignin solution, which provides the neat lignin concentrations of 33.3, 16.7, 11.1, 8.3, and 5.5 mg/L, respectively.

The UV-Vis spectrum of each solution was obtained with a Hewlett Packard

(Palo Alto, CA) 8452A Diode-Array Spectrophotometer in the wavelengths between

200 nm to 800 nm, using 1 M NaOH as the blank sample. A Fisherbrand™ (Fisher

Scientific, Hampton, NH) standard quartz cuvette with 1.0 cm pathlength was used for the UV measurements. The purpose of applying GSAM in this study is to find the concentration of neat lignin in the diluted oxidized lignin solution and consequently calculate the extent of lignin conversion.

5-2-6-2- GSAM Theory

In contrast to the normal standard addition method, GSAM is a promising approach to measure an unknown quantity in multi-component samples. The unique characteristics of GSAM i.e. the correction of matrix effects as well as spectral interferences, and using all the measurement variables make this method suitable for measuring unknown concentration [90], [91]. GSAM has the capability to detect interference effects, quantifying the magnitude of the interferences, and concurrently determining analyte concentrations. 82

To develop GSAM firstly the matrix of response R is achieved experimentally.

This matrix is further applied in classical and inverse GSAMs and modified methods discussed in this section. For this purpose, given a set of n responses (in this study

UV-Vis absorbances) on a sample of the unknown quantity (in this study oxidized lignin), a matrix matched standard (in this study neat lignin) is added and the new concentrations are calculated correcting for dilution as Δc. This procedure is repeated m-1 times and put together into matrix R with m-1 rows and n columns.

To develop classical GSAM [90], the unknown object is subtracted from matrix R as given by

′ 푹 = 푹 − 풋풓ퟎ (1) in which j is a column vector of m-1 values of unity, and r0 is the UV spectrum as a row vector of the reactor effluent (oxidized lignin) without any standard (neat lignin) added. In the next step, the sensitivity matrix 푲 is estimated by using classical least squares regression.

푲 = ∆풄푻푹′ /(∆풄푻∆풄) (2)

After achieving 푲 , the unknown concentrations 풄ퟎ can be obtained from the

푲 푻 ∆풄 augmented matrixes of R and by a regression coefficient i.e. 푲 푻푲 .

For a single analyte, the average concentration 풄 ퟎ with m element column vector can be obtained by

풓 ퟎ 푲 푻 푹 0 풄 = − (3) ퟎ 푲 푻푲 ∆풄

Another approach to estimate regression coefficients is to use regularized inverse least squares (ILS).

+ 푩 = 푹′푹′푻 푹′푻∆풄 (4) 83 in which 푩 is the average regression coefficient. The concentrations 풄ퟎ are obtained in a similar way as before.

풓 0 풄 = ퟎ 푩 − (5) ퟎ 푹 ∆풄

The inverse GSAM is used since the corrected response matrix R’ is typically singular and the threshold for selecting the singular values is a significant parameter.

However, it is only useful when the CLS and ILS give similar results.

The problem associated with the classical GSAM is that it assumes the multivariate regression line must fit the unknown sample [92]. To eliminate this problem, a modified method that offers the best linear unbiased estimate (BLUE) can be applied. In this method, the intercept is modeled in the regression by centering the responses and concentrations about their means instead of subtracting the response of the sample. The corrected augmented response matrix R’ is obtained by the following equation.

풓 푹′ = ퟎ − 풋풓 (6) 푹 in which j is an m element column vector of values of unity multiplied by the average of the augmented response matrix 풓 , which is defined as

풓 풓 = ퟎ (7) 푹

The added concentrations are adjusted with the same method.

0 (8) ∆ 푐 = ∆풄

0 ∆풄′ = − 풋 ∆ 푐 (9) ∆풄

The estimated BLUE sensitivity matrix 푲 is calculated using CLS by

푻 푲 = ∆풄′ 푹′ /(∆풄′푻∆풄′) (10) and the unknown BLUE estimated concentration is achieved by 84

풓 푲 푻 (11) 푐 = − ∆ 푐 0 푲 푻 푲

For a single analyte, the concentration of the unknown 푐 0 is a scalar quantity so the goodness of fit estimate is not possible. The regression matrix 푩 is calculated using regularized ILS the same as before.

+ 푩 = 푹′푹′푻 푹′푻∆풄′ (12) and the estimated concentration of the unknown 푐 0 by

푐0 = 풓 푩 − ∆ 푐 (13)

The CLS and ILS models are complementary. CLS minimizes the least squared error with respect to the signals (i.e., absorbances) and ILS minimizes the least squared error of the concentrations [93]. However, the BLUE calculations are most accurate compared to classical calculations, but it is supportive to see conformity between the CLS and ILS results.

5-3-Results and Discussion

5-3-1- The Effect of Cell Voltage and Electrolysis Time on Biomass Conversion

Figure 5-2 shows typical UV-Vis spectra of neat and oxidized lignin samples with different electrolysis times from 3.5 to 19.5 min. The experiments were performed at 3 different cell voltages of 1.4 V, 1.6 V, and 1.8 V. The dominant UV-

Vis absorbance peaks are observed at the wavelength of 235, 300, and 330 nm for the neat lignin samples. Lignin electrolysis for 3.5 to 19.5 min results in an apparent diminishing in the intensity of the peaks particularly around the wavelength of 300 to

330 nm, which correspond to complicated aromatic molecules with several conjugated units [94], [95]. Based on Woodward’s rule, the larger conjugated systems, like lignin molecule, require a smaller amount of energy for the transition of 85 electrons from one state to another state; therefore, they are absorbed in higher wavelengths [5].

Data shown in Figure 5-2-a indicate that the absorption intensity of the oxidized samples at 270 nm increases at the lowest cell voltage (1.4 V). The absorption around 270 nm corresponds to aromatic rings and unsaturated substituents such as carbonyl groups and double bonds on α-carbon of a side chain [22], [95], [98].

The suggesr the break down of the large lignin molecule having a high content of conjugated units to smaller fragments containing fewer conjugated units, which mostly absorb lower wavelengths, namely 235 nm and 270 nm [99]. On the other hand, at the higher cell voltage of 1.6 V and 1.8 V a diminishing of the absorption intensity around 270 nm occurs at the electrolysis times higher than 11.5 min and3.5 min, respectively (Figure 5-2-b and 5-2-c). This observation indicates that the aromatic ring structures of lignin were significantly destroyed, which means the over oxidation of lignin occurs at higher residence times [22].

Figure 5-2- UV-Vis spectra of neat lignin and oxidized lignin at different residence times at a) 1.4 V, b) 1.6 V, c) 1.8 V (a.u. stands for arbitrary unit) 87

Figure 5-2 (Continued)- UV-Vis spectra of neat lignin and oxidized lignin at different residence times at a) 1.4 V, b) 1.6 V, c) 1.8 V (a.u. stands for arbitrary unit)

5-3-1-1- Applying GSAM to Measure Lignin Conversion

To calculate lignin conversion by GSAM, after achieving the UV-Vis spectrum for oxidized samples by the method explained in section 5-2-6-1, the spectral range of the samples was reduced to 240-400 nm to characterize the aromatic range of the oxidized versus neat samples. Then second derivative spectra were calculated to remove the sloping baseline and level the absorbances (Figure 5-3).

Figure ‎5-3- Panel a represents the UV spectral responses from the standard additions with the incremented concentrations; panel b corresponds the second drivative of UV spectral responses. mAU stands for milli-absorbance unit

Since in the standard addition method the oxidized lignin concentration is kept constant, it can be modeled by the average spectrum. For comparison, the second derivative spectra of all the concentrations of added neat lignin were normalized to unit vector length (i.e., L2 norm). For example, Figure 5-4 shows the normalized spectrum of the highest concentration of added neat lignin minus the mean spectrum, so that it models both the neat lignin spectral features and the normalized mean spectrum that models the oxidized lignin spectral features.

Figure ‎5-4- Typical Spectra from the normalized neat lignin (blue line) and oxidized lignin (red line)

As indicated in Figure 5-4, there is an increase in the absorbance at 250 nm and a concomitant loss of absorbance at 360 nm for the oxidized normalized spectrum.

Hypothetically, these changes would correspond to the loss of conjugation as the lignin molecules are cleaved during electrolysis. The absorbance at 285 nm and 325 nm are symmetrical for the spectrum from the oxidized lignin sample and skewed for the neat lignin with the higher absorbance at 285 nm. The decrease in the absorbance at these two wavelengths could be due to the loss of conjugation units during the electrolytic oxidation.

Figure 5-5 shows the principal component scores from the standard addition and the variable loading of the first principal component that covers 100% of the 90 variance. It can be observed that the dominant variance (100%) correlates with the neat lignin concentration. Since the oxidized lignin concentration is fixed in the standard addition method, it was removed during the subtraction of the mean spectrum prior to the principal component analysis.

Table 5-1 shows the concentration of neat lignin in the diluted oxidized solution from the reactor effluent, which is obtained by both classical GSAM and

BLUE approach. As it is expected both CLS and regularized ILS give similar results because there is one component. There are small differences between the BLUE calculation and the original classical calculation that represents the significance of the more modern BLUE approach.

Figure 5-6 gives comparisons of the estimated sensitivity 풌 and the estimated regression coefficients 풃 for both classical and BLUE calculations in the wavelength range of 240 nm to 400 nm. A very good agreement between both calculations for sensitivity and coefficients can be observed and the maximum sensitivity occurs at

270 nm, 315 nm, and 375 nm. In addition, the sensitivity and coefficient values agree well with the variable loadings of the first principal component (Figure 5-5-b).

91 a

Figure ‎5-5- Panel a represents principal component scores from the standard addition of

neat lignin to the oxidized lignin; panel b corresponds to variable loadings of the first

principal component that spans 100% of the variance

Table 5-1-

The concentration of neat lignin in the diluted oxidized samples (c0) estimated by the classical and BLUE calculations

Cell Experiment BLUE k BLUE b Class k Class b voltage Electrolysis mg/L mg/L mg/L mg/L (V) time (min)

3.50 32.4 32.2 31.4±0.8 31.3±0.8

A 1.6 11.50 14.3 14.3 14.3±0.3 14.3±0.3

19.50 7.0 7.0 7.3±0.1 7.3±0.1

3.50 42.2 42.0 39±1 39±1

B 1.4 11.50 37.6 37.3 35±2 34±2

19.50 33.5 33.5 32.6±0.7 32.5±0.7

1.28 42.4 42.3 41.6±0.8 41.5±0.8

C 1.6 2.88 34.8 34.8 35.3±0.3 35.3±0.3

4.00 30.8 30.8 30.7±0.4 30.7±0.4

3.00 35.3 35.3 36.1±0.4 36.0±0.4 D 1.6 7.00 28.8 28.8 29.5±0.3 29.5±0.3

a b

c d

Figure ‎5-6- Panels a and b correspond to Best Linear Unbiased Estimates (BLUE) least squares and classical GSAM sensitivities; panels c and d refer to inverse least squares regression coefficients of BLUE and classical GSAM.

Figure 5-7 illustrates the percent of neat lignin converted to products after electro-oxidation at two cell voltages of 1.4 V and 1.6 V and different electrolysis times from 1.28 min to 19.5 min. The calculation of the percent of neat lignin converted to products, Conv., is based on the neat lignin concentration remaining in the samples after electrochemical oxidation. The Conv. determined from BLUE 풌 calculation is introduced in equation (14). 94

푐푖 − 푐0 × 450.5 mL × 1 g/(0.465 mL × 1000 mg) (14) 푐표푛푣. % = 100 × 푐푖 for which 푐푖 is the initial concentration of neat lignin solution before electro- oxidation, i.e., 50 g/L and 푐0 is the concentration of lignin in the diluted oxidized lignin solution, determined from BLUE 풌 calculation.

Figure ‎5-7- The conversion of neat lignin to products at different electrolysis times based on the neat lignin concentration in the oxidized samples, calculated from BLUE k approach.

A significant increase in the lignin conversion by increasing the cell voltage from 1.4 V to 1.6 V is observed. In general, higher cell voltages and longer residence 95 times result in increased conversion of lignin. In addition, Figure 5-7 indicates a linear increase in lignin conversion with electrolysis time up to 4 min and 19.5 min for the cell voltages of 1.6 V and 1.4 V, respectively. It is clear from this result that high cell voltages, i.e. higher energy input, and increased reaction times leads to a significant extent of lignin macromolecule conversion, which is expected. At 1.6 V, for example, electrochemical depolymerization achieves nearly 90% conversion of lignin, according to the GSAM-based analytical procedure developed here. As it is discussed in Chapter 4, increasing energy input to the system (the cell voltage) drives the electrochemical reactions at a higher rate. From these results, it is apparent that electrochemical techniques represent a possible method for lignin depolymerization processes.

Figure 5-8 demonstrates lignin conversion vs. charge transferred at 1.4 V and

1.6 V. A linear trend between the total charge transferred and lignin conversion up to

40 % can be observed for both cell voltages in Figure 5-8. However, when 1.6 V is applied, about 50% more total charges, i.e. input energy, is required to achieve a conversion similar to that of 1.4 V. The reason for this observation can be attributed to the oxygen evolution reaction (OER) as a competitive side reaction in the reactor.

As discussed in Chapter 4, the rate of oxygen evolution is enhanced at higher cell voltages as the kinetic limitations of the OER are dominant. The faradaic efficiency of oxygen evolution in the reactor at the cell voltages of 1.4 V and 1.6 V are 0% and

52%, respectively, which is in agreement with the lignin conversion results presented in Figure 5-7. Higher faradaic efficiency of OER at 1.6 V, i.e. 53%, leads to lower lignin conversion efficiency because half of the total charge transferred is consumed in OER instead of the lignin conversion. These results have implications for potential 96 commercial systems since it is clear that as the cell voltage is increased, the OER begins to dominate and the system operates more similar to a water electrolyzer than a lignin conversion system.

Figure ‎5-8- Lignin conversion based on BLUE k versus total potentiostatic charge transferred at the cell voltages of 1.4 V and 1.6 V.

5-3-2-Detection of Lignin Oxidation Products and Functional Groups

5-3-2-1- The Effect of Cell Voltage and Electrolysis Time on LMWACs%

Table 5-2 shows the percentage of LMWACs with the molecular weights lower than 280 g/mole in neat and oxidized lignin samples detected by GC/MS. Since the neat lignin used in these experiments (industrial waste biomass) has an inconsistent composition, the LMWACs% in the electro-oxidized lignin were compared with the LMWACs% in its corresponded neat lignin. The LMWACs% can be an indicator of the selectivity of product in the organic phase toward aromatic/aliphatic compounds. The results in Table 5-2 indicates that lignin oxidation at a lower cell voltage of 1.4 V leads to a high selectivity toward LMWACs.

However, the selectivity towards LMWACs dramatically decreased by applying a high cell voltage of 1.6 and 1.8 V. This decrease in the selectivity could arise from the breakdown of aromatic bonds as well as aliphatic bonds at high applied voltage, i.e. high energy. In addition, high cell voltage can lead to producing a high amount of

• OH radicals in the solution, which can cause total oxidation of lignin to CO2 instead of partial lignin oxidation to other LMW products [75].

While high cell voltage provides a higher electron transfer, which results in increasing the rate of electrochemical reactions, it yields a high concentration of OH• radicals both on the surface of the electrode and in the solution than breakdown both aromatic and aliphatic bonds of lignin with no selectivity, which is called lignin combustion [75]. On the other hand, high cell voltage can increase the rate of electrocatalyst deactivation over time.

Table 5-2-

Percentage of LMWA compounds extracted by chloroform from neat lignin and electro-oxidized lignin at different cell voltages and electrolysis times

LMWAC% Cell voltage Neat Oxidized lignin (V) lignin 3.5 min 7.5 min 11.5 min 15.5 min 19.5 min

1.4 55 63 64 60 57 56

1.6 55 57 55 45 45 36

1.8 55 43 30 25 - -

By applying 1.6 V, the percentage of LMWACs increases during the first 3.5 min of electrolysis time. However, the selectivity toward LMWACs decreases for electrolysis times longer than 3.5 min. This decrease in the selectivity can be due to the over oxidation of lignin to carboxylic acids such as hexadecanoic and octadecanoic acids and linear compounds by increasing electrolysis time [11].

It should be noted that the detection limit of GC/MS device applied for detecting of compounds is lower than 350 g/mol (MW <350 g/mole). The probability of the presence of some large compounds (MW>350 g/mole) and compounds with high boiling point (>270 oC) in the solution, which cannot be detected by this instrument, as well as incompletely extraction of compounds into chloroform phase, may cause some limitations and errors in a precise measurement of LMWACs% in the solutions. However, the GC/MS can be used as a reliable method to track and compare the changes in LMWACs% in the solutions by applying different cell voltages and electrolysis times. 99

5-3-2-2-Comparing Chloroform and Diethyl Ether in the Extraction of Organic

Compounds for GC/MS Test

To compare the capability of diethyl ether and chloroform in the extraction of

LMWACs from lignin solution, an oxidized lignin sample (electrolyzed at 1.4 V for

19.5 min) was acidified and then extracted by each solvent, separately. After extraction, the concentration of extracted compounds by each solvent was measured by GC/MS (Table 5-3). The results indicate diethyl ether has a dramatically higher capacity in the extraction of LMWACs particularly p-coumaric acid, from the acidified lignin solution. One of the reasons for this observation can be attributed to the limitation in the solubility of some of the oxidized lignin products in organic solvents and their preference to remain in the aqueous phase rather than transferring into the organic phase during the extraction. For instance, p-coumaric acid is highly soluble in ether [100] but slightly soluble in chloroform [101]; therefore it would not be extracted very well into the chloroform phase in the first place and therefore it is not correctly detected by GC/MS.

In addition, diethyl ether, similar to other ethers, such as ethyl acetate and ketones, is a hydrogen-bond acceptor molecule and as a result, extracts electron donor solutes more readily than chloroform [102]. However, the lower relative polarity of diethyl ether, i.e. 0.117, compared to that of many other organic solvents, can diminish the extraction of compounds that have polar functional groups such as alcohols, phenols, carboxylic acids, and esters. The mentioned compounds require more polar solvents for efficient extraction from aqueous solutions [102].

100

Table 5-3-

The concentration and percentage of LMWACs and total compounds extracted by diethyl ether and chloroform from a lignin sample electrolyzed at 1.4 V for 19.5 min

Solvent Compounds detected by GC/MS Diethyl ether Chloroform LMWACs (ppm) 9419.9 221.6 Total detected compounds (ppm) 11135.8 420.4 LMWAC (%) 85 53

5-3-2-3- GC/MS Detection of Products and Functional Groups in Lignin Solutions

A variety of different LMWACs are detected by GC/MS at each experiment.

Table 5-4 lists compounds that are typically detected in neat and oxidized lignin samples at 1.4 V for 19.5 min, extracted by diethyl ether. The results show p- coumaric acid is the dominant aromatic product of lignin oxidation, while it has the highest concentration in the aromatic compounds of the neat lignin sample as well. An average of 1.5 to 3 fold rise in the concentration of all aromatic compounds after lignin oxidation can be observed; however, this increase in the concentration is noticeable for linear compounds as well (Figure 5-9). The over oxidation of

LMWACs such as aldehydes, p-coumaric acid, phenolic compounds, etc. leads to breakdown of the aromatic ring and production of other substances such as organic acids (for example methyl formate and n-Hexadecanoic acid) and other saturated linear compounds. At high cell voltages and electrolysis times, these products can also be broken down in to carbon dioxide. In other words, the quantity of aromatic 101 compounds generated in the oxidation reactions is balanced with the formation and consumption of the aromatics during the process [81], [103], [104].

It should be noted that the lignin-rich waste biomass samples (neat lignin) that are used in these experiments contained a considerable amount of LMWACs in the first place (before oxidation), which are prone to further oxidation to linear compounds during the process. Considering this problem, a perfect selectivity toward

LMWACs is not achievable for this lignin sample though the yield of LMWACs production can be increased during lignin depolymerization by optimization of cell voltage and electrolysis time.

102

Table 5-4-

LMWACs detected by GC/MS in neat and oxidized lignin samples, electrolyzed at

1.4 V for 19.5 min, extracted by diethyl ether.

Neat lignin Oxidized lignin Typical compounds detected by GC/MS (ppm) (ppm)

Methyl formate 96.5 163.9 Linear 2,4-Dimethyl-1-heptene 25.9 156.8 compounds n-Hexadecanoic acid 380.4 1158.1

Benzaldehyde, 4-methyl- 88.9 156.7

Benzaldehyde, 4-hydroxy- 117.0 307.7

Methyl salicylate 49.4 61.8

Vanillin 323.7 481.2

4-Acetoxyacetophenone 30.8 87.4

Ethanone, 1,1'-(1,4-phenylene)bis- 100.2 151.2

Propofol 48.7 121.8

Aromatic Benzoic acid, 4-hydroxy-3-methoxy- 85.8 212.1 compounds p-coumaric acid 3946.2 5346.6

Benzoic acid, 4-hydroxy-3,5-dimethoxy- 110.2 220.3

Benzene, 1,3-bis(1,1-dimethylethyl)- 170.6 246.9

Phenol, 2,4-bis(1,1-dimethylethyl)- 122.7 641.1 Ethanone, 1-(4-hydroxy-3,5- 157.6 278.5 dimethoxyphenyl)-

2-Propenoic acid, 3-(4-hydroxy-3- 239.4 257.2 methoxyphenyl)-

103

Figure ‎5-9- The percentage of linear (left) and aromatic (right) compounds with different functional groups detected by GC/MS in neat and oxidized lignin samples from two experiments. Panels a and b correspond to lignin electrolyzed at 1.4 V for

19.5 min; panels c and d refer to lignin electrolyzed at 1.6 V for 4 min.

5-3-2-4- Detection of Functional Groups in the Solid Phase by FTIR

Figures 5-10 and 5-11 show the FTIR spectra of neat and oxidized lignin samples, electrolyzed at different electrolysis times at constant cell voltages of 1.4 V and 1.6 V, respectively. The FTIR spectra notable peak assignment for these lignin 104 samples are shown in Table 5-5. In both cell voltages, there is a noticeable increase in the intensity of peaks around 1720 cm-1, which is attributed to C=O stretching in unconjugated ketone, carbonyl and ester groups [60], [105] after lignin oxidation, which suggests the oxidization occurring during electrolysis converting hydroxyl groups into carbonyl groups. The intense peaks between 1400 to 1600 cm-1 are assigned to C=C stretch in aromatics [89]. At 1.6 V a detectable diminish at the intensity of peaks around 1128 cm-1 and between 1400 to 1600 cm-1 can be observed at 19.5 min electrolysis time, which can be due to breakage of aromatic bonds and lignin combustion under high cell voltage and electrolysis time. This is in agreement with GC/MS results. The observance of a broad strong peak around 3400 cm-1 is due to O–H peak stretching vibration and water adsorption. Despite the expectation of a decrease in intensity of O–H peak in the lignin sample after oxidation, the FTIR spectra show an opposite result. The reason for the increase in the intensity of the peak around 3400 cm-1 after lignin oxidation can be attributed to water adsorption on the active sides of lignin samples [106]. Lignin is essentially a hygroscopic compound because it has different hydrophilic groups, such as carbonyl, carboxyl, methoxyl, and hydroxyl (phenolic or alcoholic) groups [107]. Therefore oxidized lignin samples

FTIR spectra show a higher peak intensity around 3400 cm-1 compared to neat lignin samples (Figures 5-10 and 5-11), which further suggests a higher number of active sides in the oxidized lignin samples. Altogether, the FTIR spectra show an increase in the aromatic functional groups of oxidized lignin, which is in confirmation with the

GC/MS result. 105

Figure ‎5-10- FTIR spectra of neat and electro-oxidized lignin in the 10 cm2 cell at different electrolysis times under constant cell voltage of 1.4 V 106

2 Figure ‎5-11- FTIR spectra of neat and electro-oxidized lignin in the 10 cm cell at different electrolysis times under constant cell voltage of 1.6 V

107

Table 5-5-

FTIR peak assignment

Wavenumber Peak assignment

(cm-1)

3460-3412 O-H Stretching

2924 C-H stretch in alkanes/alkyl

C-H stretch in aldehyde group, and also C-H stretch in 2840 methyl/methylene group

C=O stretching in unconjugated 1720 Ketone, carbonyl and ester groups.

1600 Aromatic skeletal vibration (S>G)

1500 Aromatic skeletal vibration (G>S)

1460 C-H deformation in CH3 and CH2

1400 Aromatic skeletal vibrations

1365 Aliphatic G-H stretch in CH3

1267 G ring plus C=C stretch

Aromatic C-H in-plane deformation plus secondary alcohols plus C=O 1128 stretch

1035 Aromatic CH in-plane deformation G > S

834 Aromatic C-H out-of-plane deformation (S+H)

108

5-3-2-5- Hydroxyl (OH) Number Determination

Table 5-6 shows the results of hydroxyl number measurements of neat and oxidized lignin samples, using ASTM D 4274-99 Test Method A. The decrease of the

OH number of lignin samples after oxidation, shown in Table 5-6, implies that oxidation reaction occurs during electrolysis, which thereby turns OH into carbonyl groups. Table 5-6 also shows lignin oxidation at higher cell voltages leads to a higher decrease in OH number, which can be due to the higher rate of lignin oxidation at higher cell voltages.

Table 5-6-

Hydroxyl number of lignin samples before and after electrolysis, achieved by ASTM

D 4274-99 Test Method A.

Equivalent Hydroxyl Cell Electrolysis Lignin Percentage of weight number voltage (V) time (min) sample decrease in (g/OH) (mg KOH/g) OH number %

Neat 274.2 204.6 1.4 19.5 23 Oxidized 356.0 157.6

Neat 265.2 211.5 1.6 19.5 29.5 Oxidized 376.8 148.9

Neat 296.8 189.0 1.8 7.5 28.5 Oxidized 414.7 135.3

109

5-3-2-6- Detection of Functional Groups in the Neat and Oxidized Lignin Solutions by

Raman Spectroscopy

Figures 5-12 and 5-13 show, respectively, the Raman spectra of neat and oxidized lignin samples, electrolyzed at constant cell voltages of 1.4 V and 1.6 V. In the Raman spectrum of the oxidized lignin at 1.4 V, there is a prevalent broad peak around 1000-1200 (maximum at 1170) and two minor peaks around 1350 and 1500 cm-1. The occurrence of the broad peak at 1000-2000 can be from aromatic C–H in- plane deformation and C–C stretching band for n-alkyl compounds. The peak around

1350 can be due to C–H deformation, phenols, and C–H rocking vibration of aldehydes and the peaks around 1500 cm-1 could be from aliphatic C–H deformation.

On the other side, the Raman spectrum of the oxidized lignin at 1.6 V has two significant broad peaks around 1450-1720 and 2550-2950 (maximum at 2750) cm-1.

The observance of peak around 1450-1720 can be attributed to C=C conjugated with

C=C or C=O or carbonyl groups, which is in confirmation with the FTIR results. The broad peak around 2550-2950 in this sample can partially be attributed to the background and also it can arise from C–H stretching in aldehydes.

It should be noted that the Raman spectrum of the neat lignin in Figure 5-12 is different from the neat lignin solution in Figure 5-13, which shows the inconsistency of the lignin used in the experiments and might affect the product of lignin oxidation.

110

Figure 5‎ -12- Raman spectra of neat and electro-oxidized lignin, electrolyzed at a constant cell voltage of 1.4 V for 19.5 min.

111

Figure ‎5-13- Raman spectra of neat and electro-oxidized lignin, electrolyzed at a constant cell voltage of 1.6 V for 4 min.

5-3-3- Gel Permeation Chromatography (GPC) Analysis for Molecular Weight

Measurement

Table 5-7 summarizes GPC analysis result of the number average molecular weights (Mn), weight average molecular weights (Mw), higher average molecular weights (Mz) and polydispersity index (PDI=Mw/Mn) obtained for each lignin 112 sample. In addition, the fraction percentage of each lignin sample with three molecular weight markers of 500, 1000, and 3000 is shown in Table 5-8.

In all conditions, the results show lignin electrolysis leads to an increase in the portions of smaller lignin fractions, which thereby enhances their reactivity compared to neat lignin. However, lignin electrolysis at a lower cell voltage of 1.4 V provides a narrower molecular weight distribution in the product and a higher decrease in the

Mw, which is more sensitive to molecules of high molar mass compared to Mn.

Lignin oxidation at 1.6 V for 4 min results in about the same conversion of 35-

40 %, compared to lignin electrolyzed at 1.4 V for 19.5 min (in Table 5-7). However, it leads to an increase in the PDI of the lignin after electrolysis, which indicates, the breakdown of small molecules at higher cell voltages is more likely to occur due to the high concentration of OH• radicals. Table 5-7-

Average molar mass data and polydispersity (PDI) of neat and oxidized lignin samples at different cell voltages and electrolysis times.

Cell Decrease in Lignin Electrolysis Lignin Mw Mn Mz conversion voltage PDI from GSAM time (min) sample (Dalton) (Dalton) (Dalton) Mw Mn (V) (%) (%) (%)

Neat 3461 589 15220 5.876 4 18 21 40 Oxidized 2818 463 14061 6.083 1.6 Neat 3254 544 14898 5.983 11 35 19 72 Oxidized 2102 443 8656 4.749

Neat 3387 554 14372 6.111 11 18 15 27 Oxidized 2784 470 13747 5.919 1.4 Neat 3306 571 15263 5.791 19.5 31 26 35 Oxidized 2269 418 12262 5.422

114

Table 5-8-

Percentage of lignin fractions from selected samples

Cell Lignin MW % poly > MW % poly > MW % poly > Electrolysis voltage Sample Marker 1 MWM1 Marker 2 MWM2 Marker 3 MWM3 time (min) (V) (Daltons) (Daltons) (Daltons)

Neat 31.386 56.724 74.428 4 3000 1000 500 Oxidized 26.091 51.069 68.544 1.6 Neat 29.673 54.357 71.720 11 3000 1000 500 Oxidized 21.092 47.968 67.075 Neat 30.877 55.598 72.709 11 3000 1000 500 Oxidized 25.221 50.557 68.554 1.4 Neat 30.124 55.498 73.249 19.5 3000 1000 500 Oxidized 21.413 45.456 62.944

5-4- Conclusion

In this chapter, the functionality of biomass electrolyzer in biomass conversion, the selectivity of products toward LMWACs, the generation of functional groups in the product stream, and decreasing average molecular weight of biomass after depolymerization was discussed. The overall result indicates biomass electrolysis leads to lignin breakage to lower molecular weight fractions with different functional groups. However, the selectivity of products toward LMWACs and the Mw distribution of the lignin oxidation products mainly depends on the applied cell voltage and electrolysis time.

The optimization of the cell voltage and electrolysis time to achieve about

40% lignin conversion, as well as having a selectivity toward lignin breakage to

LMWACs, rather than linear compounds, is one of the main goals of this project. The combination of the results of lignin conversion, in section 5-3-1, and LMWAC% in

Table 5-2 show that 40% lignin conversion with selectivity to LMWACs in the product stream are achievable at both 19.5 min electrolysis at 1.4 V and 4 min electrolysis at 1.6 V. However, the GPC results in Table 5-7 shows a higher decrease in the Mw and a more uniform molecular weight distribution for the lignin sample oxidized for 19.5 min at 1.4 V compared to the lignin sample oxidized for 4 min at 1.6

V. It should be noted while lower cell voltage leads to a lower rate of lignin conversion and consequently lower yield of the reaction products (please see section

5-3-1), it results in lower energy consumption by making the electrochemical reactions selective toward lignin oxidation rather oxygen evolution as it discussed in

Chapter 4.

116

CHAPTER 6: SCALED-UP PROCESS

The objective of Chapter 6 is to investigate of the effect of scaling-up the electrochemical cell on different operating parameters such as voltage-current density at different temperatures, hydrogen production and efficiency, and lignin depolymerization.

6-1- Design of 200 cm2 Electrochemical Cell

The scaled-up 200 cm2 cell was generally similar to the 10 cm2 reactor with some modifications at the flow channels. Figure 6-1 shows the modified flow channels of the 200 cm2 reactor. To reduce the pressure drop and the lignin concentration gradient inside the cell, 16 parallel channels with the cross-sectional area of 0.687 × 0.635 mm2 were applied. Furthermore, several T-micromixers were built inside each channel to generate turbulent flow and as result increase mass transfer. The length of each channel was 893.5 mm and the total volume of the reactor was 6.756 cm3. 117

2 Figure ‎6-1- The shape of the microchannels of the 200 cm electrochemical flow reactor.

6-2- Results and Discussion

6-2-1- The Effect of Cell Voltage and Temperature on Cell Current and Oxygen

Evolution

Figure 6-2 presents the cell current as a function of cell voltage in the 200 cm2 electrochemical reactor for both water and biomass electrolysis at 25°C, 40°C, and

60°C. Similar to the 10 cm2 cell in the 200 cm2 cell, increasing the cell voltage at all operating temperatures causes an increase in the cell current, and the cell current is higher at elevated temperatures. In addition, Figures 6-3 and 6-4 show oxygen evolution rate and OER faradaic efficiencies for both water and biomass-depolarized electrolysis at the three temperatures outlined in Figure 6-2. The results are analogous to the 10 cm2 results. As the cell voltage is increased at all temperatures, the rate of oxygen evolution and OER faradaic efficiency is increased. However at the higher 118 cell voltage of 1.6 V, the biomass-depolarized electrolyzer OER faradaic efficiency is significantly increased compared to that observed at 1.4 V, but still about half that of the water electrolyzer. The reader is referred to chapter 4 for the discussion of the section results.

Figure ‎6-2- Cell current versus cell voltage for water electrolysis (solid circle markers) and biomass-depolarized electrolysis (solid triangle markers) at 25°C (….),

40°C (- - - ), and 60°C (solid line) in 200 cm2 cell.

119

2 Figure ‎6-3- Rate of oxygen production in 200 cm electrochemical cell as a function of cell voltage for water electrolysis (solid circle markers) and biomass-depolarized electrolysis (solid triangle markers) at 25°C (….), 40°C (- - -), and 60°C (solid line). 120

2 Figure ‎6-4- OER faradaic efficiency in 200 cm electrochemical cell as a function of cell voltage for water electrolysis (solid circle markers) and biomass-depolarized electrolysis (solid triangle markers) at 25°C (….), 40°C (- - -), and 60°C (solid line).

121

2 6-2-2- H2 Production and Energy Efficiency of the 200 cm Cell

Table 6-1 summarizes the result of H2 production measurement and energy efficiency of the 200 cm2 cell at different cell voltages. The average faradaic

2 efficiency of H2 evolution in this cell is 99%, similar to 10 cm cell. In addition, increasing the cell voltage from 1.4 V to 2 V leads to about a 30% decrease in energy efficiency. The decrease in energy efficiency can be due to the enhancement of OER at higher cell voltages. The detailed discussion of these results can be found in

Chapter 4.

Table 6-1-

Rate of hydrogen evolution, faradaic efficiency, and energy efficiency of the 200 cm2 electrochemical cell

Measured Faraday Energy Current Energy Potential Current volume of efficiency efficiency density density (V) (mA) H2 of H2 (HHV) 2 (mA/cm ) (kWh/kg H2) (ml/min) production (%)

2 6000 30 45 1.01 53.4 74

1.91 5000 25 37.3 1.00 51.26 77

1.82 4200 21 31.3 1.00 48.89 80

1.69 3000 15 22.3 1.00 45.52 86

1.58 2000 10 14.9 1.00 42.46 93

1.54 1000 5 7.4 0.99 41.67 94

1.4 600 3 4.3 0.96 39.11 100

122

6-2-3- Analysis of Lignin Oxidation Products

Figure 6-5 represents the percentage of lignin conversion and UV-Vis spectra of lignin before and after electrolysis in the 200 cm2 cell at two different cell voltages of 1.5 V and 1.6 V. About 2-fold increase in the lignin conversion occurs by applying the higher cell voltage of 1.6 V. However, when 1.6 V is applied, about 35% more total charge, i.e. input energy, is consumed to achieve a conversion similar to that of

1.5 V (Figure 6-6). This observation is in agreement with the faradaic efficiency of oxygen evolution in the 200 cm2 at the cell voltages of 1.5 V and 1.6 V, which was about 13 and 50%, respectively. As it is very well discussed in chapter 5 (section 5-3-

1) and in chapter 4 (section 4-3-1), the rate of oxygen evolution is enhanced at higher cell voltages, which leads to lower lignin conversion efficiency by consuming input energy on OER instead of the lignin conversion.

123

1.6 V 1.5 V

Figure 6‎ -5- Lignin conversion percentage (up) and UV-Vis spectra of neat and oxidized lignin (down), electrolyzed at different cell voltages and electrolysis times in

200 cm2 cell 124

Figure ‎6-6- Lignin conversion versus total potentiostatic charge transferred at the cell voltages of 1.5 V and 1.6 V, applying 200 cm2 cell

Table 6-2 summarized the LMWACs% in neat and oxidized lignin, electrolyzed at 1.5 V and 1.6 V in the 200 cm2 reactor. Lignin electrolysis at a lower cell voltage of 1.5 V results in increasing the LMWAC%; however, a high decrease in the LMWACs% is observed when lignin oxidation occurs at 1.6 V. Lignin electrolysis at higher cell voltage results in lignin combustion and decrease the selectivity toward breakage of aliphatic linkages than aromatic bonds. The decrease in the percentage of aromatic carboxylic acid, mainly p-coumaric acid when lignin is electrolyzed at 1.6 V

(Figure 6-7) is an example of the cleavage of aromatic bonds at high cell voltage. large lignin molecules can be depolymerized to form small aromatic compounds and eventually to smaller molecules such as mono- and dicarboxylic acids [11]. 125

Figure 6-7 also exhibits a maximum in the percentage of aromatic carboxylic acids, mostly p-coumaric acid, v.s. electrolysis time for lignin electrolysis at 1.5 V for

7.8 min. Lignin electrolysis for more than 7.8 min at 1.5 V leads to a decrease in

LMWACs percentage. Although higher electrolysis times result in a higher degree of lignin conversion (Figure 6-5), it can lead to over oxidation of products to saturated compounds.

Table 6-2-

LMWACs% in neat and oxidized lignin samples, electrolyzed in 200 cm2 cell at different cell voltages and electrolysis times.

LMWAC% Cell voltage Oxidized lignin (V) Neat lignin 7.8 min 15.6 min 1.5 69 76 64 1.6 83 79 -

126

Figure ‎6-7- The percentage of linear (right) and aromatic (left) compounds with different functional groups in neat and oxidized lignin samples, electrolyzed in 200 cm2 cell at different cell voltages and electrolysis times. Panels a and b corresponds to lignin electrolysis at 1.5 V; panels c and d refers to lignin electrolysis at 1.6 V.

127

6-3-Conclusion

Chapter 6 includes the efforts of scaling-up the 10 cm2 electrolyzer to a 200 cm2 electrochemical cell and investigating the functionality of the larger reactor in biomass electrolysis and hydrogen production. The results of lignin electrolysis in the

200 cm2 cell show about 30% lignin conversion with selectivity to LMWACs is achievable when lignin is electrolyzed under the cell voltage of 1.5 V for 7.8 min. In addition, the faradaic efficiency of hydrogen production in the 200 cm2 cell is 99% and the faradaic efficiency of OER varies between 0 and 50% at the cell voltage range of 1.4 V and 1.6 V, respectively.

Altogether, the attained results of biomass electrolysis and hydrogen production from the experiments performed in the 200 cm2 cell are comparable to the results that were achieved from 10 cm2 cell experiments. This accomplishment, i.e. development of the lab-scale 200 cm2 cell for efficient and low-cost lignin conversion is an essential step, which can increase the prospect for scaling up this process to be applied in industry.

128

CHAPTER 7: CONCLUSION

In this study, a continuous electrochemical process has been developed and applied for lignin depolymerization and H2 production.

In the initial stage, carbon and TiO2 supported NiCo electrocatalysts with different Ni:Co ratios were synthesized and their performance and durability were tested for lignin oxidation. The results indicated while carbon-supported electrocatalysts have higher surface areas, TiO2-supported electrocatalysts offer higher stability and have greater steady-state current density values. In addition, higher Co content in the electrocatalysts can increase the rate and selectivity of lignin conversion to LMWACs.

After selecting the most effective electrocatalyst, i.e. 1:3NiCo/TiO2, a continuous electrochemical reactor with 10 cm2 electrodes was utilized for lignin electrolysis in anode and hydrogen generation in the cathode. Applied cell voltage, lignin electrolysis time, temperature, and lignin concentration were the most important parameters on lignin conversion, the selectivity of products to LMWACs,

H2 production, O2 evolution, and reactor energy efficiency.

While higher current densities and consequently higher H2 production rates are achievable at elevated cell voltages, the reactor energy efficiency is lower at these cell voltages (>1.5 V). This inefficiency arises from OER, as a competitive reaction to lignin oxidation in the anode, which is enhanced at high cell voltages.

In addition, increasing the temperature causes an increase in both OER and lignin oxidation reaction rates in the anode and hydrogen production reaction in the cathode. Therefore, by increasing the temperature the faradaic efficiencies of OER and HER do not change. 129

Lignin electrolysis time is another crucial parameter, which determines the percentage of lignin conversion and selectivity toward LMWACs. While high electrolysis times suggest higher lignin conversion, it can also lead to over oxidation of depolymerized products and producing linear compounds.

In addition, the optimization of lignin concentration in NaOH is one of the important objectives of this work. While lignin electrolysis can occur at lower overpotentials than alkaline water electrolysis, high concentrations of lignin in NaOH has a negative effect on reactor performance by increasing diffusion resistance and blockage of electrocatalysts active sites in the anode.

After the optimization of reaction parameters in the 10 cm2, a 200 cm2 lab- scale reactor was developed and examined for lignin electrolysis and H2 generation.

The results showed that the performance of the 200 cm2 cell was greatly comparable to the 10 cm2 cell.

In general, the uncomplicated continuous electrochemical reactor, working under room temperature and pressure, low-cost NiCo electrocatalyst, and the less than

20 min reaction times can make this process desirable to be applied in the industry. In addition, these results of this research are valuable in understanding the operation of biomass electrolyzers for future studies.

130

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APPENDIX: MATLAB CODE FOR GSAM CALCULATIONS

%%------GSAM------% %author Peter Harrington, [email protected] close all; clear variables; clc; der = 1; npts = 11; porder = 3; wlmin =270; wlmax = 330; ifile = 'file name'; %% the increment of standard contraction (ppm): y = [0,2,77,4.16,5.55,8.33,16.66]'; %% read data here [x1, t] = xlsread(ifile); wl = x1(:, 1); [n, m] = size(x1); icnt = 0; x = zeros(n, m/3); for j = 2:3:m if ~isnan(x1(1, j)) icnt = icnt + 1; x(:, icnt) = x1(:, j); end end x = x'; m = icnt; x = svgol(x, npts, porder, der); ind = wl >= wlmin & wl <= wlmax; wl = wl(ind); x = x(1:m, ind); figure; plot(wl, x'); ylabel('Absorbance (AU)'); xlabel('Wavelength (nm)'); title('First Derivative Spectra'); ltext = cell(m, 1); for i = 1:m ltext{i} = sprintf('Added Conc. %3.2f ppm\n', y(i)); 148 end legend(ltext); axis tight;

%% classical GSAM------dx = x-x(1, :); K = y'*dx/(y'*y); c0 = x/K-y; [mean(c0), std(c0)*tinv(0.05, m-1)/sqrt(m)] figure; plot(wl, K); xlabel('Wavelength (nm)'); ylabel('Sensitivity (AU/ppm)'); title('Analytical Sensitivity'); axis tight; figure; x1 = x-mean(x); s = x1'*x1; s = sign(s).*sqrt(abs(s)); imagesc(wl, wl, s); h = gca; set(h, 'YDir', 'normal'); h = colorbar; set(get(h,'label'),'string','AU'); title('Crossed Deviations Plot'); xlabel('Wavelength (nm)'); ylabel('Wavelength (nm)');

%% inverted GSAM------dx = x(2:end, :)-x(1, :); dy = y(2:end); B = pinv(dx'*dx, 1e-4)*dx'*dy; c02 = x*B-y figure; plot(wl, B); title('Inverse Least Squares Regression Coefficients'); xlabel('Wavelength (nm)'); ylabel('Coefficients (ppm/AU)'); [mean(c02), std(c02)*tinv(0.05, m-1)/sqrt(m)]

149

%%------function------function [y, coef] = svgol(x, npts, order, der);

% [y, coef] = svgol(x, npts, order, der); % y = filtered data % coef = digital filter points % x = data with objects to be smoothed as rows % npts = number of points in the filter odd points are better than even % order = order of the polynomial % der = derivative of the polynomial % 0 = smooth % 1 = first derivative % 2 = second derivative % returns Savitzy-Golay Filter of npts to x % version 1 % author Peter Harrington, [email protected] istrt = -floor(npts/2); ifin = floor(npts/2); iseq = [istrt:ifin]'; npts = length(iseq); if order < der 'Error: Derivative must be less than polynomial order!' return; end p = []; for i = 0:order p = [p, iseq.^i]; end coef = inv(p'*p)*p'; t = coef(der+1,:); coef = t.*factorial(der);

[m, n] = size(x); y = zeros(m, n); iib = -istrt+1; iif = n-ifin;

150 for ii = 1:m for i = iib:iif ib = i+istrt; ie = i+ifin; y(ii, i) = coef*x(ii, ib:ie)'; end end

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