A Nickel Hexacyanoferrate Based Thermo-Electrochemical Device For Efficient Heat-to-Electricity Conversion

by

Guang Wen (Jame) Sun

B.A.Sc. Mechanical Engineering University of Waterloo, 2017

SUBMITTED TO THE DEPARTMENT OF MECHANICAL ENGINEERING IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF

MASTER OF SCIENCE IN MECHANICAL ENGINEERING

AT THE

MASSACHUSETTS INSTITUTE OF TECHNOLOGY

JUNE 2019

2019 Massachusetts Institute of Technology All rights reserved Signature redacted Signature of Author: Departmert of Mechanical Engineering Signat ire redacted May 7,2019 Certified by: / Yang Shao-Horn W.M.Keck Professor of Energy Depart cal Engineering sis Supervisor

Accepted by: Signature redacted_ NicolaWsTIadjiconstantinou MASSACHUSETTS INSTITUTE Profesr of Mechanical Engineering OF TECHNOLOGY Chairman, Committee of Graduate Students

JUN 13 2019

LIBRARIES ARCHivEs

A Nickel Hexacyanoferrate Based Thermo-Electrochemical Device For Efficient Heat-to-Electricity Conversion

by

Guang Wen (Jame) Sun

Submitted to the Department of Mechanical Engineering On May 7, 2019 in Partial Fulfillment of the Requirements for the Degree of Master of Science in Mechanical Engineering

Abstract

Effective and reliable ways to generate renewable energy is crucial for reducing global carbon emission in the ongoing battle against the climate crisis. Currently, low-temperature waste heat accounts for more than half of the rejected waste thermal energy produced in the United States. Traditional waste heat recovery methods such as steam cycle and thermoelectrics fall short at low temperatures due to uneconomically low conversion efficiency.

The electrochemical conversion of heat to electricity, or thermogalvanic energy conversion, had been investigated for decentralized low-temperature applications. Traditional thermogalvanic cells were capable of harvesting thermal energy from spatial temperature gradients similar to thermoelectric plates. Lately, novel thermogalvanic devices had also been devised to harvest energy from cyclical temperature fluctuations through a technique known as Thermally Regenerative Electrochemical Cycle (TREC). In particular, the charging-free TREC cell could passively generate energy through no other external input than ambient temperature fluctuations. Thermogalvanic cells typically suffered low conversion efficiency and low open-circuit voltage due to a plethora of limitations. The motivation of this work was therefore to construct a highly-efficient thermogalvanic cell that could also produce high potential for practical applications.

In this work, a charging-free TREC thermogalvanic cell based on Nickel Hexacyanoferrate was conceptualized, designed, and built. Owing to NiHCF's competitive temperature coefficient and gravimetric capacity of -1.0 mV/K and 60 mAh/g, the resultant charging-free cell achieved a full-cell temperature coefficient of -2.0 mV/K and a conversion efficiency of 9.33% relative to the Carnot limit. Furthermore, the practicality and manufacturability of the cell was verified through electronic integration testing and flexible cell fabrication.

Thesis Supervisor: Yang Shao-Horn Title: W.M. Keck Professor of Energy

3 I Acknowledgement

First and foremost, I want to thank my advisor Professor Yang Shao-Horn for her guidance. I have learned many valuable lessons under her tutelage.

Next, I would like to thank my family for their unwavering support. I thank my fiancee

Julia for her understanding and encouragement through the last 15 months. Time and time again you have pushed me to new heights. My achievements are as much yours as they are mine. I would also like to thank my parents for providing me with the series of opportunities that culminated in my accomplishments today. I can't even begin to appreciate the sacrifices they have made to move to Canada.

I would like to thank my funding sources, the HKUST MIT Research Alliance

Consortium, Texas Instruments, and Intel for their financial support.

Special thanks goes to Dr. Botao Huang for starting me off on the thermogalvanic project and continuing to provide his experimental expertise. Much of this thesis wouldn't have been possible without him.

Last but not least, my gratitude goes out to my labmates. I have never worked with a more passionate and supportive group of people banded under a common cause. Shout out to the occupants of office 3 1-183. You guys were the true heroes in this journey.

5 6 Table of Contents

1 Introduction ...... 15

1 .1 M o tiv a tio n ...... 15 1.2 Thermogalvanic Energy Conversion ...... 17 1.3 Prussian Blue Analogue (PBA) M aterials for Thermogalvanic Conversion...... 21 1.4 Research Approach...... 21 2 Experim ent Procedures...... 22

2.1 Nickel Hexacyanoferrate Electrode ...... 22 2.2 Iron Chloride Electrode...... 24 2.3 Electrochemical Cell Design and Assem bly ...... 25 2.4 Electrochemical M easurement Setup ...... 33 2.5 Heat-to-Electricity Conversion Efficiency M odeling...... 40 2.6 Integration Setup...... 42 2.7 Flexible Charging-Free Cell Fabrication ...... 45 3 R esults and D iscussion ...... 47

3 2 3 3.1 Tem perature Coefficients of Fe(CN) /4- and Fe +/ " Redox Couples...... 47 3.2 NiHCF Half-Cell M easurements ...... 49 3.3 Full-Cell Tem perature Coefficient and Prelim inary Cycling...... 52 3.4 Cycling Performance Im provements ...... 55 3.5 Fe2+/3+ Concentration Optimization for M axim um Conversion Efficiency...... 62 3.6 Energy Harvester Integration Results...... 70 3.7 Flexible Charging-Cell Cycling Performance ...... 72 4 Conclusion and Suggestions for Future Work...... 75

4.1 C o n c lu sio n ...... 7 5 4.2 Suggestions for Future W ork...... 76 R eferences...... 77

7 8 Figure 1: Estimated 2017 U.S. Energy Consumption Flow Chart published by Lawrence Liverm ore N ational Laboratory (LLNL) [1]...... 15 3 Figure 2: Schematic for thermogalvanic cell operation using Fe(CN)6 '-/4 redox couple [1. The redox activities at both electrodes are maintained by the inter-electrode mass transfer. The efficiency is calculated as power output over the heat flux between the two electrodes...... 17 2 Figure 3: Schematic a Thermally Rechargeable Electrochemical Cycle using a Cu/Cu 'redox couple and a CuHCF, a solid Sodium Ion intercalation material. The TREC has a negative temperature coefficient and is therefore charged at a high temperature and discharged at a lower temperature [15,24]. The conversion efficiency is defined as energy output over heat input each c y c le ...... 1 9 Figure 4: Operation schematics for (a) Electrically-assisted Thermally Regenerative Electrochemical Cycle (TREC) (b) and charging-free TREC as well as its (c) operating scheme. While both cells recover energy through temperature cycling, TREC requires energy input from an external power source to operate. Figures taken from Yang et al [25]. The conversion efficiency calculation for the two also differ slightly as seen in figure. The heat input has an active material com ponent and a physical cell component ...... 20 Figure 5: (a) As synthesized NiHCF powder after drying. A typical synthesis session yields from 3 to 5 grams of NiHCF material. The material was verified through XRD. The spectra of (b) NiHCF used in this work and the (c) reference spectra are shown [30...... 22 Figure 6: Discharge curve of Nickel Hexacyanoferrate for Potassium Ion intercalation in 1 M

KNO 3 electrolyte [30]. The potential range of NiHCF is shown to be between 0.6 V and 0.9 V vs. the Standard Hydrogen Electrode (SHE). The plateau of the charging curve occurs between 0.7 V and 0.75 V vs. SHE. The theoretical capacity of the material is 60 mAh/g...... 24 Figure 7: A qualitative, side-section view schematic for Cell A. The fluid channel is filled with an electrolyte containing aqueous redox couples. The temperature coefficient of the couple is measured at the two Gold or Platinum electrodes held at two prescribed temperatures. The fluid channel is sealed using 0-rings. The assembly also include two Peltier temperature controllers each containing a Peltier heater and a RTD temperature sensor...... 26 Figure 8: (a) SolidWorks rendering of the Teflon cell design for Cell A. (b) An image of the temperature coefficient measurement assembly using Cell A...... 27 Figure 9: Simplified thermal circuit for the symmetrical temperature coefficient measurement setup. The thermal resistances are in the form of resistance per unit area. The thermal conductivity of the electrolyte is approximated using that of liquid water at room temperature. The total areal resistance of the electrolyte is at least four orders of magnitudes larger than the rest of the thermal resistances combined, making the electrolyte the dominant source of thermal re sista n ce ...... 2 8 Figure 10: A qualitative, side-section view schematic for Cell B assembly. The electrode temperatures are raised and lowered simultaneously so that the cell is approximately isothermal at all times. The electrolytes are injected using needle and syringe. The fluid channels are sealed using 0-rings. The assembly also include two Peltier temperature controllers each containing a Peltier heater and a RTD temperature sensor ...... 29 Figure 11: An image of the Cell B assembly - complete with Teflon cells, temperature sensors, an d P eltier h eaters...... 30

9 Figure 12: A qualitative rendering of the Cell C assembly. Functionally, Cell C serves the same purpose as Cell B albeit with a few geometrical modification...... 31 Figure 13: SolidWorks rendition of the semi-transparent Cell C designs. The cell consists of a male (top right) and a female (bottom left) components that are held together using the six side extensions during assem bly...... 32 Figure 14: (a) Cell C assembly mechanism. (b) Image of the standalone Cell-C. (c) Image of Cell-C assembly during integration testing ...... 32 Figure 15: Beaker cell testing schematic for NiHCF half-cell cycling. A controlled mass of NiHCF is deposited on the working electrode. A large mass of activated carbon is used as the counter electrode as a capacitive ion sink. The reference electrode is a commercial Mercury Sulfate reference electrode ...... 35 Figure 16: Full-cell schematic including all electrochemically active components. An anion exchange membrane (typically Fumasep FAB-PK-130) is used to prevent active species mixing and to transport Chloride ions between half-cells...... 36 Figure 17: Thermal cycling procedure diagram indicated by transient full-cell and half-cell potentials. The cell rotates between 25'C and 45'C. Each temperature is held for 2 hours, during which the charging-free cell is discharged, shorted, and allowed to rest. Approximately 15 minutes is needed for the cell to entirely transition from one temperature to the next...... 37 Figure 18: High-level schematic for integration testing. The Peltier heaters control the temperature of the charging-free cells, which transfers energy to the flyback energy harvester circuit. The charging-free cell potential and the temperature reading from the circuit are m onitored through a PC and a potentiostat...... 42 Figure 19: (a) Physical layout of circuit components on the printed circuit board. (b) Electrical schematic of the energy harvesting mechanism (right) consisting of a MOS rectifier to correct the polarity of the cell potential, and a flyback converter to boost the extracted voltage. Credit to Mr. Zheyuan L iu for diagram s ...... 43 Figure 20: A captured image of the integration testing setup. The charging-free cell is outlined in red. Heater and heater connections are outlined in yellow. Flyback circuit is outlined in blue. Electrical connections are highlighted in green...... 44 Figure 21: Side-section exploded view of the flexible charging-free cell. Black strips represent hydrophobic carbon paper current collectors. Eggshell white blocks represent non- electrochemically active support components. Yellow strip in the middle is the anion exchange membrane. Grey blocks electrolyte soaked fiber separators. The figure is not to scale...... 45 Figure 22: Design schematic depicting the hydrophobic flexible cell components pushing the electrolyte to be in contact with the ion exchange membrane...... 46 Figure 23: (a) Image of single flexible cell and (b) 2x2 patterned flexible cells. The cell is observed to be optically clear aside from the electrochemically active components and the carbon cu rrent co llectors...... 4 6 3 Figure 24: (a) Potential vs. time plot from the 2 mM equimolar Fe(CN)6 '-4- temperature coefficient measurement experiment. The hot and cold electrodes are initially steadily held at 25'C. The hot electrode temperature is increased to 35'C and lowered to 25'C in 2.5'C increments every 10 minutes. (b) The averaged potential is plotted against the hot electrode

10 temperature. The temperature coefficient of the 2 mM equimolar Fe(CN) 3 '-/4- couple is calculated to be -1.75 m V /K through linear fitting...... 47 2 Figure 25: (a) Potential vs. time plot from the 2 mM equimolar Fe +/3+ temperature coefficient measurement experiment. The hot and cold electrodes are initially steadily held at 25'C. The hot electrode temperature is increased to 35'C and lowered to 25'C in 2.5'C increments every 10 minutes. (b) The averaged potential is plotted against the hot electrode temperature. The 2 temperature coefficient of the 2 mM equimolar Fe +/3+ couple is calculated to be 1.31 mV/K through linear fitting...... 48 Figure 26: (a) Potential vs. time plot from the 400 mM total Fe2'+/3+ temperature coefficient measurement experiment. The hot and cold electrodes are initially steadily held at 25'C. The hot electrode temperature is increased to 35'C and lowered to 25'C in 2.5'C increments every 10 minutes. (b) The averaged potential is plotted against the hot electrode temperature. The 2 temperature coefficient of the 200 mM equimolar (400 mM total) Fe +3+ couple is calculated to be 1.22 m V /K through linear fitting...... 49 Figure 27: NiHCF cycling plot using the three-electrode beaker cell setup. The gravimetric capacity of NiHCF plateaus at just over 50 mAh/g after 5 full-cycles. The C rate used is C/2 based on a theoretical gravimetric capacity of 60 mAh/g...... 50 Figure 28: Charging and discharging curve for NiHCF. A linear slope is fitted at the plateau of the NiHCF charging curve near 750 mV vs. SHE. The slope of the plateau based on experimentally obtained 50.1 mAh/g gravimetric capacity is 2.6 mV/mAh g 1 . The slope of the plateau based on a theoretical 60 mAh/g is 2.17 mV/mAh g-. Experiment was performed under ro om tem p eratu re...... 5 1 2 Figure 29: Charging curve of NiHCF against an excess volume of 200 mM Fe +, 200 mM Fe3+ electrolyte (400 mM total). The charging curve at 25'C is experimentally obtained. The charging curve at 45'C is simulated based on an assumed 40 mV of thermally recovered potential, and superimposed on top of the 25'C charging curve. The operating range of the charging-free cell lies near the NiHCF plateau allowing maximum capacity to be extracted. The membrane used was Selem ion A M V N ...... 52 Figure 30: Transient potential plot for the first full-cycle of charging-free cell operation. The thermal recovery of cell potential is indicated by a steep increase or decrease at the beginning of each temperature step. Selemion AMVN membrane was used and 0.6 mL of solutions were used...... 5 3 Figure 31: Potential vs. time plot for the first iteration of charging-free cell over 16 cycles. The recovered potential is shown to decrease with each cycle due to self-discharge. The cell's self- discharge is likely as a result of active species crossing over the anion exchange membrane. The Teflon Cell B as well its associated cycling regime described earlier in text were used...... 54 Figure 32: Potential vs. time plot for the charging-free cell using Selemion AAV membrane over 16 cycles. Though the self-discharge seems to be eliminated based on recovered potential each cycle, the source of peak potential fluctuation from cycle to cycle is unknown. Cell B and its associated cycling regim e w as used...... 56 Figure 33: Potential vs. time plot for the charging-free cell using Fumasep FAB-PK-130 membrane over 16 cycles. Each cycle recovers to approximately the same potential since self- discharge has largely been limited. Cell B and its associated cycling regime was used ...... 56

11 Figure 34: Normalized total discharge capacity vs. cycle from the charging-free cell cycling data using three different membranes. The total capacity is the sum of discharged capacities from the hot and cold half-cycles. Each set of cycling data is normalized to the highest value in the set... 57 Figure 35: Normalized total discharge energy vs. cycle from the charging-free cell cycling data using three different membranes. The total energy is the sum of discharged energy from the hot and cold half-cycles. Each set of cycling data is normalized to the highest value in the set...... 57 Figure 36: Normalized average voltage vs. cycle from the charging-free cell cycling data using three different membranes. The average voltage is calculated as the sum of discharged energy from the two half cycles over the sum of discharged capacity from the two half cycles...... 58 Figure 37: (a) Normalized total charge capacity and (b) total discharged energy of each cycle extracted from two charging-free cell. The two cells are identical with the exception of electrode substrate pre-butane torch treatment. The membrane used is Fumasep FAB-PK-130. Cell B and its associated cycling regim e was used...... 60 Figure 38: (a) The normalized charge capacity and (b) discharged energy from an elongated cycling study of a charging-free cell using a 400 mM combined FeCl 2/ 3 , 100 mM HCl half-cell. Electrolytes in both half-cells are replaced entirely with pristine electrolytes on the 2 2 "d cycle.. 61 Figure 39: (a) The coulombic efficiency and (b) energy efficiency from an elongated cycling study of a charging-free cell using a 400 mM combined FeC2/3, 100 mM HCl half-cell. Electrolytes in both half-cells are replaced entirely with pristine electrolytes on the 2 2 "d cycle.. 61 2 Figure 40: Computed theoretical heat-to-electricity conversion efficiency for a 400 mM Fe +/3+ 2 (combined) as a function of Fe +/3+ electrolyte volume with respect to NiHCF mass. The maximum calculated efficiency is 0.355% when 1.17 mL of electrolyte is used for every gram of N iH C F ...... 6 2 2 Figure 41: Fe +13+half-cell reaction temperature coefficient (black, left) and modeled full-cell 2 3 maximum conversion efficiency (red, right) as a function of the total concentration of Fe + + sp e c ie s ...... 6 3 Figure 42: Average internal resistance each cycle for charging-free cells acidified with 100 mM HCI, 200 mM HCI, and 500 mM HCL. Average internal resistance is calculated from the average overpotential at the start of the hot and cold half-cycles...... 64 Figure 43: (a) The normalized charge capacity and (b) normalized discharged energy from the cycling study of a charging-free cell using 1000 mM combined FeCl 2/3 , 200 mM HCl half-cell. 65 Figure 44: (a) The normalized coulombic efficiency and (b) energy efficiency from the cycling study of a charging-free cell using 1000 mM combined FeCl 23/ , 200 mM HCl half-cell...... 65 Figure 45: (a) The normalized charge capacity and (b) energy from the cycling study of a charging-free cell using 1300 mM combined FeCl 23/ , 260 mM HCl half-cell...... 66 Figure 46: (a) The normalized coulombic efficiency and (b) energy efficiency from the cycling study of a charging-free cell using 1300 mM combined FeCl 2/3 , 260 mM HCl half-cell...... 66 Figure 47: Potential vs. time plot for the charging-free cell using Fumasep FAB-PK-130 2 membrane and a pre-treated NiHCF electrode over 16 cycles. The concentration of the Fe +/3+ half-cell electrolyte is 1000 mM combined FeCl 2/3, 200 mM HC...... 67 Figure 48: Potential vs. time plot for the charging-free cell using Fumasep FAB-PK-130 membrane and a pre-treated NiHCF electrode over 11 cycles. The concentration of the Fe2 +/3 + half-cell electrolyte is 1300 mM combined FeCl 23 , 260 mM HCI...... 67

12 2 Figure 49: Normalized average voltage plotted against cycle count for three Fe +/3+ concentrations. Lower voltage efficiencies can be associated with elevated Fe2'+/3+ concentrations as a result of overpotential effects. Teflon cell B was used for cycling as well as its associated cy clin g reg im e ...... 6 8 Figure 50: Theoretical conversion efficiency relative to Carnot efficiency as a function of heat recuperation efficiency up to 50% for this work, Yang et al [" (based on modeled energy output), and Linford et al [361 (based on experimental energy output). Heat capacity calculation for all three works are based only on electrochemically active materials...... 69 Figure 51: Transient response of the charging-free cell potential when connected to the flyback circuit. The flyback circuit is designed to extract a set quantity of energy when the source voltage reaches 59 mV. The increasing idle time between extractions indicates depletion of cell energy. 70 Figure 52: Snapshots from an integration demonstration video. (a) A hairdryer is used to raise the PCB temperature, which is then (b) captured through the embedded temperature sensor. Link for video: https://youtu.be/u7syY G ecJ3E ...... 71 Figure 53: PCB temperature readings reported by the embedded temperature sensor...... 71 Figure 54: Transient potential plot of the flexible single cell's first two thermal cycles. The flexible cell performance in the first two cycles is functionally identical to its rigid counterpart. 72 Figure 55: The normalized charge capacity vs. cycle from flexible single cell cycling. A longer start-up period is observed compared to previous experiments...... 73 Figure 56: The normalized charge capacity vs. cycle from flexible quad cell cycling. Cell capacity drops dramatically after the first 5 cycles due to electrolyte loss...... 73 Figure 57: Transient potential plot of the flexible quad cell's entire cycling experiment. The effect of electrolyte loss leads to high ionic resistivity, which results in large overpotentials tow ards the end of the experim ent...... 74

13 14 1 Introduction

1.1 Motivation

In 2017, an annual energy review conducted by the Lawrence Livermore National

Laboratory concluded that up to 68.2% of energy generated domestically in the United

States was rejected as waste heat in the process of producing useful energy (Figure 1) [.

If utilized efficiently, this significant source of thermal energy can be one of the key methods to alleviate the ever-rising energy demand E2, as well as play a critical role in relieving the impending climate crisis [31. For instance, if the entirety of the U.S. annual waste heat is harvested at a mere 0.1% conversion efficiency, the regenerated electricity suffices to provide the utility needs of more than two million U.S. households for a year according to a 2019 Monthly Energy Review compiled by the Department of Energy .

EtImated UJ Energy Consumption In 2017: 07.7 Quada

IN,

Figure 1: Estimated 2017 U.S. Energy Consumption Flow Chart published by Lawrence Livermore National Laboratory (LLNL) [1].

15 Historically, most methods of waste heat recovery are powered by the thermodynamic

driving force in a spatial temperature gradient [5,6. In other words, most heat engines operate between a hot reservoir and a cold reservoir. Popular techniques for harvesting

industrial and residential waste heat include Steam or Organic Ranking Cycle, Heat

Exchangers, or Thermoelectric devices. These energy recovery techniques typically perform well against a hot reservoir with temperature in excess of 300'C. In particular,

Rankine cycles can achieve conversion efficiencies as high as 35% based on an inlet temperature of 500'C, room temperature outlet, and using water as the working fluid E71.

Thermoelectric devices are also capable of reaching conversion efficiency above 10% based on a temperature difference between hot and cold reservoir of at least 2750C [8].

Despite excellent performance with high-grade waste heat over 300'C, the traditional heat recovery methods suffer uneconomically low conversion efficiencies using temperature inlets lower than 3000C as energy source E,7,9]. Waste heat rejected below

100 0 C, commonly defined as low-grade waste heat, is an especially challenging space for waste heat recovery where little to no commercial solutions exist [10,11].

Predominately generated as a bi-product in the transportation and power generation industries, low-grade waste heat make up more than three-quarters of all rejected thermal energy generated in the U.S. E12]. The lack of accountability for low-grade waste heat presents an enormous opportunity for research effort, and motivates the development of an economical technique to efficiently recover low-grade waste heat as electricity.

16 1.2 Thermogalvanic Energy Conversion

Electrochemical waste heat recovery uses thermogalvanic cells to convert the temperature differences between a hot inlet and a cold outlet or between the peak and trough in a thermal cycle to stored chemical energy. The thermogalvanic effect exploits the electrode potentials' temperature dependence in the thermogalvanic cell, which is derived from the entropy of the reactions of the redox couples [13].

dE ASrxn (1) dT nF

Historically, the first type of thermogalvanic cell utilize the temperature coefficient of a single redox couple. These cells are built symmetrically, where two identical electrodes held at two different temperatures share a common electrolyte (Figure 2). A popular redox

3 couple used in these thermogalvanic cells is Fe(CN)6 /4-, which has a temperature

3 4 coefficient of -1.4 mV/K based on a 200 equimolar mM Fe(CN)6 - electrolyte [14].

Fe(CN)i 4 *=Fe(CN) 3 Redox reaction -- + Mass transfer

co~thdl"U " Hot anode

3 Figure 2: Schematic for thermogalvanic cell operation using Fe(CN)6 44- redox couple [15. The redox activities at both electrodes are maintained by the inter-electrode mass transfer. The efficiency is calculated as power output over the heat flux between the two electrodes.

17 AN=- ,

Similar to solid thermoelectric devices, the performance of liquid thermogalvanic cells can also be characterized with a figure of merit ZT based on its temperature coefficient a, ionic conductivity a, and thermal conductivity K:

ZT = a2 (2) K

The thermal and ionic conductivities of the electrolyte are typically tightly coupled.

Thus, the figure of merit ZT for thermogalvanic cells largely hinges on the temperature coefficient of the redox couple. Most aqueous redox couples display temperature coefficients in the range of 0.6 to 2.0 mV/K [16-18], which generally limits the efficiency for thermogalvanic energy conversion to below 0.1% due to limited power output [19,20]

Recent advancements have improved the thermogalvanic cell conversion efficiency beyond 0.5%. Notably, Im et al have used carbon nanotube aerogel as a high-surface area electrode to increase the cell power output and achieved a conversion efficiency of 0.55%, or 3.95% relative to the Carnot limit [21]. Duan et al have raised the temperature coefficient

3 of the Fe(CN)6 /4- couple from -1.4 mV/K to -4.2 mV/K by the additions of Urea and

Guanidinium Chloride, resulting in a five-fold increase in conversion efficiency [22]

Thermally Rechargeable Electrochemical Cycle (TREC) is a technique for electrochemically recovering waste heat from ambient temperature fluctuations [231. TREC requires two redox couples with high temperature coefficients of opposite signs in order to form a full-cell. The full-cell would then have a temperature coefficient equal to the sum of the magnitudes of the half-cell temperature coefficients

afull-cell = apositive - anegative (3)

18 A TREC recovers heat by charging the cell at one temperature and discharging it at

2 another. For instance, a TREC devised by Lee et al [241 using a Cu/Cu + redox couple and a solid CuHCF Sodium-Ion intercalation material has a negative full-cell temperature coefficient of -1.20 mV/K (Figure 3). The Cu/CuHCF TREC has achieved an estimated conversion efficiency of 4% when cycling between 10*C and 60*C - more than 25% of the

Carnot limit. Though TREC displays a tremendous improvement from traditional single redox couple thermogalvanic cells, its practical usefulness is equally hindered by requirement for an external power source and its convoluted operating procedures.

Discharge j Charge C 2 0 CU2 * CUO V1 Wa NO- Redox reaction 4 -"*Masstransfer

0 Eout ?lconh, = Q

CuHCF anion exchong@ Cu CuHCF anion exchange Cu cathode melbrwn anode cathode membmne anode

Figure 3: Schematic a Thermally Rechargeable Electrochemical Cycle using a Cu/Cu" redox couple and a CuHCF, a solid Sodium Ion intercalation material. The TREC has a negative temperature coefficient and is therefore charged at a high temperature and discharged at a lower temperature [15,241. The conversion efficiency is defined as energy output over heat input each cycle.

More recently, a charging-free TREC designed by Yang et al [25] is able to circumvent

TREC's need for an external power source by using two redox couples with nearly identical redox potentials. Taking the example of a charging-free TREC cell with a positive temperature coefficient: The cell potential begins operation at 0 V, and is lowered to a negative value through raising the temperature. The cell is discharged to 0 V by an

19 oxidizing current at the working electrode. The temperature is lowered to the original temperature which raises the cell potential to a positive value. The cell then is able to be discharged to 0 V via a reducing current at the working electrode. Contrary to the electrically-assisted TREC where energy is consumed at one temperature and produced at another temperature, charging-free TREC generates energy at both temperatures. Unlike

TREC, the charging-free TREC's capacity is bounded by the recovered potential, which results in lower conversion efficiencies in comparison. Furthermore, since the conversion efficiency of both TREC are limited by the required heat input. Strategies to decrease cells' heat capacity will be successful in improving conversion efficiency.

a) Z"FlC41 Voft Electrically Assisted: M VDitchrge (T,) It11 S Discharge (TI) =QAM + Q~I ( hag2 T, t~hiEliIi~I~iCharge Charging-Fr..: Discharge (T,) Charge IT) MB Cpaciy

ctrIcly-assltad Thermally Ragenerativ. Elsctrochamlcal Cycle Ecold + Eht QAM + Qcll b) F C,, ,,,,,c)

i' II, D charg ccharge (TiJ

charglng-fve Ilecrchemflcal CA Dlisharge (T) 2 Cherging-fre. cell operating scheme

Figure 4: Operation schematics for (a) Electrically-assisted Thermally Regenerative Electrochemical Cycle (TREC) (b) and charging-free TREC as well as its (c) operating scheme. While both cells recover energy through temperature cycling, TREC requires energy input from an external power source to operate. Figures taken from Yang et al [25]. The conversion efficiency calculation for the two also differ slightly as seen in figure. The heat input has an active material component and a physical cell component.

20 1.3 Prussian Blue Analogue (PBA) Materials for Thermogalvanic Conversion

Prussian Blue Analogue (PBA) materials like Prussian Blue (FeFe(CN)6) [25], Copper

Hexacyanoferrate (CuHCF, CuFe(CN)6) [24], and Nickel Hexacyanoferrate (NiHCF,

KNiFe(CN)6) [26] have seen extensive use in the development of TREC and charging-free

TREC devices. Much like Lithium-Ion battery cathode materials, PBA materials store electrochemical energy through intercalation reactions of aqueous Sodium and Potassium

Ions. PBA has high charge capacity retention [27], low voltage hysteresis E29], and good gravimetric capacity in the neighborhood of 60 mAh/g [27-29]. Notably, Nickel

Hexacyanoferrate (NiHCF) has also been reported to have a temperature coefficient of around -1.0 mV/K [26]

1.4 Research Approach

The goal of this work is to develop an efficient and durable charging-free TREC electrochemical device to harvest thermal energy from near-room temperature thermal fluctuations. Nickel Hexacyanoferrate (NiHCF) is chosen to form the basis of the charging- free cell owing to its excellent cycling performance and a high negative temperature coefficient of -1.0 mV/K [26]. Using NiHCF as one of the half-cells requires the screening for another redox couple with high positive temperature coefficient that also has a redox potential within the potential window of NiHCF.

In the end, this work aims to construct a charging-free cell with better conversion performance and efficiency than previous devices in literature. The charging-free cell will be subject to electrochemical cycling to validate its durability, and integration testing to demonstrate the its capability to power small electronics.

21 2 Experiment Procedures

2.1 Nickel Hexacyanoferrate Electrode

Nickel Hexacyanoferrate (KNiFe(CN)6, NiHCF) powder was synthesized using a simple co-precipitation method [261. 250 mL of 50 mM K3Fe(CN)6 solution (Potassium

Ferricyanide (III), 99%, Sigma-Aldrich) and 250 mL of 25 mM Ni(N0 3)2 solution (Nickel

Nitrate Hexahydrate, Sigma-Aldrich) were slowly added to a beaker initially containing

100 mL of deionized water (Type-1 Water, 18.2 Mfl -cm, Milli-Q) at a rate of approximately 1 to 2 drops per second. The solution was rigorously stirred at 400 RPM.

After both precursors were transferred entirely to the beaker, the solution was left stirring for another 60 minutes. The precipitation was filtered and washed with deionized water at least five times. The resultant, mud-like substance was dried covered overnight on a hot plate maintained at 60'C. The final product was a brown powder-like substance identified to be NiHCF through an X-ray powder diffraction (XRD) measurement (Figure 5).

a) b)

C)

10 20 30 40 50 60 70 Dffrhci* &46(2 Owha)

Figure 5: (a) As synthesized NiHCF powder after drying. A typical synthesis session yields from 3 to 5 grams ofNiHCF material. The material was verified through XRD. The spectra of (b) NiHCF used in this work and the (c) reference spectra are shown [30.

22 As-synthesized NiHCF powder was mixed with conductive carbon black (Super P

Conductive Carbon Black, TIMCAL) and PVDF (Polyvinylidene Fluoride, Sigma-

Aldrich) binder according to a weight ratio of 80:10:10 (NiHCF:CB:PVDF) in NMP solvent (N-Methyl-2-Pyrrolidone, Sigma-Aldrich) to form a thick slurry. The slurry was casted onto a carbon cloth (EC-CC 1-060, Electrochem.Inc) using doctor blade. The doctor blade gap height was set to 0.015 inches. In the later segments of this work, the carbon cloth was pre-treated directly using a butane torch for approximately 10 seconds to improve casting quality. The composite NiHCF electrode was dried in a vacuum oven maintained at 70'C for 48 hours. The typical loading of NiHCF on the 16 mm diameter electrode was

5 mg - 10 mg.

The Potassium-Ion containing electrolyte for NiHCF electrode was made by adding suitable amounts of KCl (Potassium Chloride, Sigma-Aldrich), NiCl 2 (Nickel (II) Chloride,

Sigma Aldrich), and HCl (Hydrochloric Acid 37%, Sigma-Aldrich) to deionized water.

2 The concentration of KCl and NiCl 2 were set to be 1 M and 0.2 M respectively. The Ni + ions had been reported to have a stabilizing effect on NiHCF which prevented its dissolution. While it had been reported that NiHCF best operates at a pH of 2 [26], the pH of the NiHCF-side electrolyte was instead adjusted to match the pH of the electrolyte for the other half-cell in the charging-free cell, typically in the pH range of 1 to 2.

23 U1.0 In 1 M KN03 - 0.83C 8.3C ---16.7C 0....- 1 --41.7C 0.6-

C -O0.4

0 20 40 60 Specific Capacity (mAh/g)

Figure 6: Discharge curve of Nickel Hexacyanoferrate for Potassium Ion intercalation in 1 M KNO 3 electrolyte [30]. The potential range of NiHCF is shown to be between 0.6 V and 0.9 V vs. the Standard Hydrogen Electrode (SHE). The plateau of the charging curve occurs between 0.7 V and 0.75 V vs. SHE. The theoretical capacity of the material is 60 mAh/g.

2.2 Iron Chloride Electrode

A second half-cell reaction was required to form a charging-free cell along with the

NiHCF electrode. Since NiHCF had been reported to have a temperature coefficient of a =

-1.0 mV/K, and operated in the potential range of 0.65 V and 0.85 V vs. SHE [26,30] the

complementing redox couple should have the following electrochemical characteristics:

1) A sizeable, positive temperature coefficient (a > 0.5 mV/K)

2) Reduction potential between 0.65 V and 0.85 V vs. SHE, ideally between 0.70 V

and 0.75 V vs. SHE where the plateau of the NiHCF charging curve lies (Figure 6)

Through intensive literature review, several water-stable redox couples were screened

according these two criteria. Ultimately, the reaction:

24 Fe3 + e~ -> Fe'z (4) was selected to be the complementing reaction to NiHCF in order to form a charging-free cell. The Fe2+/3+ redox couple had been documented to have a positive temperature coefficient of around a = 1.0 mV/K, and a standard reduction potential of E' = 0.77 V vs.

SHE [31,32]

The Fe2+/3+ electrolyte used in this study was made from FeCl2 (Iron (II) Chloride,

Sigma-Aldrich), FeCl3 (Iron (III) Chloride, Sigma-Aldrich), and Hydrochloric Acid. The concentration of the Iron Chloride species varied throughout this study. The pH of the Iron

Chloride solution was adjusted to between 1 and 2 to stabilize the Fe3 * species [33]

2.3 Electrochemical Cell Design and Assembly

Two custom electrochemical cells, each with its own purpose and specifications, were designed and fabricated for this work.

Cell A: Temperature coefficient measurement for half-cell reactions of aqueous redox

3 4 2 3 couples such as Fe(CN)6 ~ and Fe +/ +. Temperature coefficient was defined as change

in potential over change in temperature.

Cell B: Full-cell characterization and cycling for charging-free cells consisting of two

half-cell reactions.

Cell C: 3D printed transparent cell for demonstrating the charging-free cell

25 Au/Pt Electrode

Teflon Cell 0-Rings in,

Figure 7: A qualitative, side-section view schematic for Cell A. The fluid channel is filled with an electrolyte containing aqueous redox couples. The temperature coefficient of the couple is measured at the two Gold or Platinum electrodes held at two prescribed temperatures. The fluid channel is sealed using O-rings. The assembly also include two Peltier temperature controllers each containing a Peltier heater and a RTD temperature sensor.

Cell A was a symmetrical cell designed to allow the reduction and oxidation reactions of one redox couple to occur simultaneously at both electrodes. The two electrodes were held at two different temperatures (Figure 7).

The body of the cell was milled from a piece of 3 inches wide, 3 inches long, and 1 inch thick Teflon bar (Teflon@ PTFE, McMaster-Carr). The cell body consisted of a centered circular electrolyte channel, 8 clearance holes, as well as O-ring grooves and additional removed features to accommodate the Peltier temperature controller assembly

(Figure 8). This design aimed to minimize the heat losses from the electrolyte channel to the environment using thick Teflon side walls, and to maximize the inter-electrode thermal resistance by minimizing the radius of the channel and elongates the channel by increasing the thickness of the cell. The ideal diameter for the channel was found to be 5 mm. Smaller

26 channels were found to be difficult to fill due to the surface tension of the electrolyte, which in turn ionically insulated the two electrodes and render measurement impossible.

a) b)

I

*

Figure 8: (a) SolidWorks rendering of the Teflon cell design for Cell A. (b) An image of the temperature coefficient measurement assembly using Cell A.

An O-ring, a platinum electrode, a RTD temperature sensor embedded in a copper current collector, and a Peltier plate were bolted into each side of the Teflon cell through a stainless steel backing plate.

Assuming that contact resistance and heat losses through side walls were negligible, a one-dimensional thermal circuit approximation revealed that the temperature difference between the RTD temperature sensor and the surface of the platinum was negligible (Figure

9). In other words, the temperature sensor reading accurately matched the temperature at which reactions are occurring on the electrode surface.

27 upt TCu/PtTpR|flectrolyte TELectrolyte/pt Tptlcu TCold

6 2 7 2 1 2 Rc" - 3x 10- Km W- R', -3,5x1O Km W- R, 0-0.042Km2W-1 R", -3,5x10-7Km -I R",- 3X 10-GK N' W-1

Figure 9: Simplified thermal circuit for the symmetrical temperature coefficient measurement setup. The thermal resistances are in the form of resistance per unit area. The thermal conductivity of the electrolyte is approximated using that of liquid water at room temperature. The total areal resistance of the electrolyte is at least four orders of magnitudes larger than the rest of the thermal resistances combined, making the electrolyte the dominant source of thermal resistance.

3 Cell A was experimentally validated using the model Fe(CN)6 44- redox couple. At 2 mM concentration for both species, Cell A measured the temperature coefficient of the couple to be a = -1.75 mV/K, which benchmarked well against reported values around -

1.72 mV/K [18,34].

Cell B was designed for the purpose of charging-free full-cell long term cycling (>48 hours). Contrary to Cell A, Cell B should be as thin as physically possible to expedite heat

conduction between the Peltier heater/cooler and the electrodes. Cell B must be assembled

from two half-cell components in order to incorporate an anion exchange membrane

(AEM) (Figure 10), which was a critical component of the proposed charging-free cell that prevented cross-over of active species in the electrolyte solutions.

28 I H I IE I

Teflon Cells

Electrode SO-rings Current Collector

Figure 10: A qualitative, side-section view schematic for Cell B assembly. The electrode temperatures are raised and lowered simultaneously so that the cell is approximately isothermal at all times. The electrolytes are injected using needle and syringe. The fluid channels are sealed using O-rings. The assembly also include two Peltier temperature controllers each containing a Peltier heater and a RTD temperature sensor.

Similar to Cell A, the two components of Cell B were also fabricated from Teflon blocks. The two Teflon pieces were identical, each featuring a circular electrode/electrolyte channel, O-ring grooves, and two small injection channels for electrolyte injection. The full assembly consisted of two pieces of Teflon cells, two Peltier temperature controller assemblies, and stainless steel clamps (Figure 11). The cell was sealed using Viton rubber

O-Rings (Viton@ Fluoroelastomer, McMaster-Carr).

29 1'

Figure 11: An image of the Cell B assembly - complete with Teflon cells, temperature sensors, and Peltier heaters.

The need for a third cell, Cell C, arose along with the need to demonstrate the charging- free cell outside of a laboratory setting. Cell B was deemed too bulky as all of the components in the cell assembly, including Peltier heater, temperature sensor, and controller, were required to demonstrate the full functionality of the cell. Cell C was designed to be easy to transport and assemble, as well as being able to demonstrate a few cycles of the charging-free cell operation using an external heat source. A qualitative rendering of Cell C is shown in Figure 12.

30 Carbon Cloth/Paper NIHCF

Anion 3D-Prlnted Exchange Cells flembrane

Figure 12: A qualitative rendering of the Cell C assembly. Functionally, Cell C serves the same purpose as Cell B albeit with a few geometrical modification.

Cell C was designed for 3D printing due to difficulty in machining the designed shape and need for see-through cell walls. A stereolithography (SLA) 3D Printer (Form 2,

Formlabs) and a clear resin (Clear Resin V4, Formlabs) were used to manufacture Cell C.

A scrap piece printed using the clear resin was submerged in both 1 M KC1, 200 mM NiCl 2 ,

100 mM KC1 electrolyte and 200 mM FeCl3, 200 mM FeC12, 100 mM HCl electrolyte for over 72 hours. No noticeable degradation of the 3D printed piece was observed. The finished cell components featured rectangular slits to house electrodes and electrolytes, 0- ring grooves, transparent cell bodies that allowed for electrolyte level to be conveniently monitored (Figure 13).

31 Figure 13: SolidWorks rendition of the semi-transparent Cell C designs. The cell consists of a male (top right) and a female (bottom left) components that are held together using the six side extensions during assembly.

Instead of fasteners, the two Cell C components used hook-like mechanisms to tightly assemble (Figure 14). This design allowed ease of assembly and monitoring, especially for off-site demonstrations.

a) b) C)

kehilew

w

Figure 14: (a) Cell C assembly mechanism. (b) Image of the standalone Cell-C. (c) Image of Cell- C assembly during integration testing.

32 2.4 Electrochemical Measurement Setup

In all of the temperature-dependent measurements, Peltier plates (Peltier Module, CUI

Inc.), RTD temperature sensors (HEL-700, Honeywell) embedded in copper current collectors, and a Peltier temperature controller (PTC 10, Stanford Research Systems) were used to monitor and control the electrode temperatures using a built-in PID controller. After a target temperature was set, the electrodes typically reached that temperature within 10 -

20 minutes depending on the geometry and material of the cell. The electrochemical experiment inputs and data collections were completed using a Bio-Logic potentiostat

(VSP-300, Bio-Logic).

Temperature Coefficient Measurement

The temperature coefficient of aqueous redox couples, such as the Fe2 /3 and

Fe(CN)63-/4-, were measured using in a geometrically symmetrical setting using cell A.

Approximately 0.35 mL of electrolyte was used to fill the circular channel of the cell. The cell was then assembled with Peltier temperature controller, temperature sensor, and stainless steel clamping plates.

Both Platinum electrodes were initially heated to 25'C and allowed to rest, at which point the potentiostat reported an open circuit voltage (OCV) of<1 mV. In increments of

2.5'C, the temperature of the working electrode (WE) was slowly raised to 35'C then lowered back to 25'C, while the temperature of the counter electrode (CE) was steadily held at 25'C. 30 minutes was typically allocated for each increment in order to allow the cell to reach thermal equilibrium. The 100 C WE temperature window was chosen to

33 minimize the effect of the thermal expansion of the electrolyte, which at higher temperature

(>45'C) had been found to cause more than 5 mV potential drifts over time.

The potential at each temperature was calculated by averaging ascending potential and the descending potential. The averaged potential was plotted against the electrode temperature, which allowed the temperature coefficient to be calculated through line fitting.

NiHCF Half-Cell Characterization

The charging profile of the NiHCF composite electrode was obtained in a beaker cell.

The working electrode (WE) was a NiHCF composite electrode (80%/10%/10% wt.

NiHCF/CB/PVDF) deposited on carbon cloth. The counter electrode (CE) was a large mass of activated carbon (80%/20% wt. activated carbon/PVDF) deposited on carbon cloth that acted as an ion sink owing to its large capacitance. The reference electrode (RE) used was a commercial Hg/HgSO4 reference electrode (Double Junction Mercury Sulfate Reference

Electrode, Pine Research Instrumentation). A 1 M KCl, 200 mM NiCl 2, 100 mM HCl solution was used as electrolyte.

The NiHCF electrodes used in this experiment were stamped using a 2" oblong stamp

(Master Tools Oblong Punch, Weaver Leathercraft), which enabled accurate measurement of the NiHCF mass. The resultant electrode had a long and slender shape, with a small amount (<10 mg) of NiHCF deposited at the tip of the electrode. The NiHCF was completely discharged and charged 3 times before the charging curve was recorded. The temperature coefficient of NiHCF was measured in full-cell tests.

34 CE:

WE: NINCF Carbon RE: Hg/HgSO 4

Figure 15: Beaker cell testing schematic for NiHCF half-cell cycling. A controlled mass of NiHCF is deposited on the working electrode. A large mass of activated carbon is used as the counter electrode as a capacitive ion sink. The reference electrode is a commercial Mercury Sulfate reference electrode.

Full-Cell Characterization and Thermal Cycling

Full cell testing was carried out using Cell B. Round electrodes used in this study were created using a 16 mm circular punch from sheets of NiHCF composite electrode and butane torch-treated carbon cloth (EC-CC 1-060, Electrochem.Inc). The typical loading of

NiHCF on the 16 mm diameter electrode was 5 mg - 10 mg. A qualitative schematic of the assembled cell is show below in Figure 16. The cell was further inserted between two

Peltier plates to regulate the cell temperature.

35 Anion Exchange Membrane

Carbon Cloth

s , Fe H( KCI, NiC12, HCI

Figure 16: Full-cell schematic including all electrochemically active components. An anion exchange membrane (typically Fumasep FAB-PK-130) is used to prevent active species mixing and to transport Chloride ions between half-cells.

Approximately 0.6 mL of electrolyte were injected into each compartment of the cell.

Since NiHCF was synthesized in its completely oxidized state [30], the cell OCV upon assembly was typically in the realm of 120 mV - 150 mV. The cell was discharged to a potential of 0 mV while the cell temperature was maintained at 25 0C before any electrochemical measurements and cycling. While the initial discharge also converted some of the Fe2+ species to Fe3+ species in the FeCl2/3 compartment, at 200 mM FeCl2 and

200 mM FeCl3, the Fe2+/3+ half-cell's capacity was 11.58 mAh when all iron species were fully oxidized. At the same time, the NiHCF consumed at most 0.3 mAh to be fully discharged to 0 mV. Since the Fe 2+/3+ half-cell's capacity was more than 30 times larger than that of the NiHCF half-cell, it was simply assumed that the concentration of the Fe 2+ and Fe31 stayed constant during the course of the cycling experiments.

36 250 C 450C 250C 450C

Short & Rests Short & Rest >Short & Rest: Short Rest'---

Time

Figure 17: Thermal cycling procedure diagram indicated by transient full-cell and half-cell potentials. The cell rotates between 25'C and 45'C. Each temperature is held for 2 hours, during which the charging-free cell is discharged, shorted, and allowed to rest. Approximately 15 minutes is needed for the cell to entirely transition from one temperature to the next.

After the cell was discharged to 0 mV, the cell temperature was increased from 25*C to 45*C. The cell reached thermal equilibrium after 30 minutes of waiting. Due

NiHCF/Fe2+/3+ cell's negative temperature coefficient, a rise in temperature resulted in a negative cell potential. The cell was then discharged to 0 mV by applying a positive current, followed by a 30 minute period during which the cell was held at 0 mV. The discharge rate was set such that the cell could be completely discharged during thermal cycling in under an hour. Similarly, after the cell was discharged completely at 45*C, the temperature was lowered to 25*C which enabled the cell to regenerate to a positive potential. The cell was discharged at 25*C through an applied negative current, which was again followed by a 30 minute short. Discharging the cell to 0 mV at 25'C returned it to its initial state, allowing for continuous cycling. The thermal cycling procedure was found in Figure 17.

37 For all cycling experiments, the concentrations of the NiHCF side electrolyte were 1

M KCI, 200 mM NiCl 2, 100 mM HCl. For the iron-side electrolyte, the concentrations were

initially 200 mM FeCl 3, 200 mM FeCl2, and 100 mM HCl. These were the concentrations

used unless an experiment stated otherwise.

The ion exchange membrane used was initially a standard Selemion anion exchange

membrane (Selemion AMVN, AGC Engineering Co.). In a series of membrane-dependent

cycling studies to improve cell cycling, the following membranes, Selemion AAV and

AHO (AGC Engineering Co.), Fumasep FAB-PK-130 (Fumatech), and a surface-treated

Nafion 115 (DuPont) were also used.

The Nafion membrane was treated using a Layer-by-Layer (LbL) treatment that had been documented to prevent the crossover of multivalent cations [351. The membrane was treated alternately in 20 mM polycyclic aromatic hydrocarbon cation solution (PAH',

Sigma Aldrich) and 20 mM poly(sodium styrene sulfonate) anion solution (PSS-, Sigma

Aldrich). The treatment began and ended with PAH solution after 3 full cycles between the two solutions, each treatment lasting 5 minutes. In theory, the treated Nafion membrane allowed transport of H' ions to maintain electro-neutrality while blocking out iron cations.

A concentration dependent Electrochemical Impedance Spectroscopy (EIS) study was

2 performed to gauge the cycling performance of the cell the following Fe +/3+ concentrations: 500 mM FeCl3, 500 mM FeCl2, and 650 mM FeCl3, 650 mM FeCl2. The concentration of HCl in both electrolytes is 100 mM. Further studies were conducted on

2 the effect of HCl concentration on cycling life. The Fe +/3+ concentrations were held constant at 500 mM FeCl3 and 500 mM FeCl2 The HCl concentrations used were 100 mM,

200 mM, and 500 mM.

38 39 U -~ --- ~-- - -~

2.5 Heat-to-Electricity Conversion Efficiency Modeling

The heat-to-electricity conversion efficiency of the charging-free cell was calculated in order to gauge the cell's effectiveness for recycling waste heat. The efficiency modeling was largely built upon the theoretical foundations from the works of Lee et al [24]

The conversion efficiency was defined as the ratio between the net work done and the thermal energy input:

W WH +W (5)

QH + QHC THAS + (1 - 7HR) (TH - TC) iC

W was the work done by the charging free in a full thermal cycle between the hot and cold temperatures, TH and Tc. QH was the thermal energy available at TH. QHC denoted the thermal energy required to physically heat the charging-free cell from Tc to TH. The JHR term indicated the heat recuperation efficiency of the system, where high 7JHR could offset the thermal energy required to heat up the system.

Similar to Lee et al [241, only the heat capacities of the electrochemical active components and electrolytes were considered in this analysis. Furthermore, the electrical losses in operation were also ignored. This modeling served only as a thermodynamic analysis of the NiHCF/Fe2+/3+ charging-free system.

The efficiency expression could be further expanded by recognizing that the entropy of reaction could be defined as:

ASmotar _ LSsystem _ (6) nF Qc

Qc was the charge capacity of the system.

40 Moreover, by approximating the work done by the charging free cell as:

W =WH +WC ~ aQC(TH - TC) (7)

The efficiency could be re-expressed as:

aQC(TH - TC) (8)

aQCTH + (1 - HR) (TH - TC) Z2imCpJ

When T1HR, the heat recuperation efficiency was 100%, the conversion efficiency of the charging-free became the Carnot efficiency:

aQC(TH -Tc) H -TC (9) 77Carnot - CTH TH

It then became evident that the key to maximizing the conversion efficiency was to minimize the heat capacity of the system. Adjusting the amount of NiHCF active material,

NiHCF electrolyte, and Fe2+/3+ electrolyte could alter both the charge capacity and the heat capacity of the system, which affected the conversion efficiency.

A MATLAB script was written to optimize the ratio between the mass of NiHCF and volume of Fe2+/3+ electrolyte to maximize the charging-free cell's conversion efficiency.

The discharge curve of NiHCF was approximated as a straight line with a slope of 2.6 mV/mAh g'. The discharge behavior of Fe2+/3+ was assumed to follow the Nernst equation.

The volume of the NiHCF electrolyte was calculated from the simulated capacity of the cell. The temperature coefficient of the cell was obtained experimentally.

41 2.6 Integration Setup

To validate the charging-free cell's capability for potential applications outside of the laboratory, such as powering autonomous sensors in remote locations, an integration experiment was devised to power a temperature sensor using two serially-connected charging-free cells. The integration setup used a custom designed energy harvesting PCB

(Designed and manufactured by Zheyuan Liu, Chinese University of Hong Kong, Hong

Kong) to extract thermally generated energy from the charging-free cells, which was then used to power a temperature sensor (Figure 18).

Peltier Heaters

Controls Cell Temperature

Charging-Free Monitors Cell Thermogalvanic Cells Potential

Harvests Energy Receives Sensor Flyback Computer & Circuit Potentlostat

Figure 18: High-level schematic for integration testing. The Peltier heaters control the temperature of the charging-free cells, which transfers energy to the flyback energy harvester circuit. The charging-free cell potential and the temperature reading from the circuit are monitored through a PC and a potentiostat.

42 The PCB had three main components: MOS rectifier, flyback converter circuit, and a microcontroller (Figure 19). The rectifier corrected the polarity of the source input, since the charging-free cell had a positive potential in cold environments and a negative potential in hot environments. The flyback converter stepped-up the potential of the two serially- connected charging-free cells, which was on the order of tens of mV, to a higher potential on the order of single volts in order to power the temperature sensor. The microcontroller unit (MCU) read the temperature reading from the embedded sensor and passed it through a USB connection. In the current iteration of the circuit, the MCU and the MOS rectifier were powered through an external USB power source. A future version of the circuit is in the works that uses a temperature-controlled mechanical rectifier, and uses a higher source

voltage to power the circuit entirely through charging-free cells.

Tompersture sensor 8) /Tom 1SMF b) OPAmp

Ts-C

Moe ANONt~ PF~mConv4Is

Use OMc noiflon STM Contral'ir

Figure 19: (a) Physical layout of circuit components on the printed circuit board. (b) Electrical schematic of the energy harvesting mechanism (right) consisting of a MOS rectifier to correct the polarity of the cell potential, and a flyback converter to boost the extracted voltage. Credit to Mr. Zheyuan Liu for diagrams.

43 The integration experiment went much like the charging-free full-cell cycling

experiments outlined in the previous subsection. The cell temperatures were controlled

using the Peltier plates. However, instead of discharging the cells through a potentiostat,

the cells were connected to the PCB and discharged through the flyback circuit instead.

The potential of the cell during discharge as well as the reported temperature were

recorded. The experimental setup is shown in Figure 20. When the cells were completely

discharged and thermally regenerating, the cells were disconnected from the PCB to

prevent further discharging.

C Free Cel

Fly k C ult

Connection between Cell, Potentiostat and Circuit

Figure 20: A captured image of the integration testing setup. The charging-free cell is outlined in red. Heater and heater connections are outlined in yellow. Flyback circuit is outlined in blue. Electrical connections are highlighted in green.

44 2.7 Flexible Charging-Free Cell Fabrication

The charging-free cell concept was further put to the test through the construction of a thin, flexible cell out of inexpensively, readily available commercial materials.

The NiHCF and carbon cloth electrodes, anion exchange membrane, and the electrolytes were identical to material used in the cycling experiments studies. The body of the cell was constructed out of transparent acrylic foam double-sided tape (3M VHB 4905,

McMaster-Carr) that functioned as cell's physical support, as well as doubled as spacer that created a volume in which electrolyte and electrodes were contained. Transparent, chemically inert PVC backing layer (Clear Chemical-Resistant PVC Film, McMaster-Carr) was used to enclose the cell. Hydrophobic PTFE-treated carbon paper current collectors

(Sigracet 29 BC, SGL Carbon) were used for electrical interconnects between cells and the potentiostat. An exploded view of the tape cell is shown below in Figure 21.

Transparent Acrylic Foam Tape (Support/Spacer) ------L Hydrophobic Carbon Anion Exchange Membrane Paper Current Collector

Electrolyte Soaked =wpm Transparent, Chemically Inert Microglass fiber separator PVC acking/Support Layer

NIHCF Composite Electrode

Figure 21: Side-section exploded view of the flexible charging-free cell. Black strips represent hydrophobic carbon paper current collectors. Eggshell white blocks represent non- electrochemically active support components. Yellow strip in the middle is the anion exchange membrane. Grey blocks electrolyte soaked fiber separators. The figure is not to scale.

45 Hydrophobic components were intentionally chosen to reinforce contact between the electrolyte, electrode, and the anion exchange membrane. Furthermore, hydrophobic supports and current collectors were used to divert electrolyte leakage due to thermal expansion (Figure 22).

Transparent Acrylic Foam Hydrophobic Carbon Tape (Support/Spacer) Paper Current Collector

Figure 22: Design schematic depicting the hydrophobic flexible cell components pushing the electrolyte to be in contact with the ion exchange membrane.

Clear, flexible, and mechanically stable charging-free cells were created using this method. On top of single cells, serially-connected cells in 2x2 and 3x3 patterns were also constructed (Figure 23). The cells were subject to cycling testing.

b)

Figure 23: (a) Image of single flexible cell and (b) 2x2 patterned flexible cells. The cell is observed to be optically clear aside from the electrochemically active components and the carbon current collectors.

46 - - -- I

3 Results and Discussion

3.1 Temperature Coefficients of Fe(CN)6 3 '4- and Fe2"'3- Redox Couples

Temperature coefficients for aqueous redox couples were measured using Cell A and the symmetrical non-isothermal electrochemical measurement setup outlined in the 3 previous section. The reaction temperature coefficient of a model compound Fe(CN6 ) '~4- was first measured to validate the accuracy of the homemade measurement setup.

a) 2 2 0 0 -2 -2 -4 27.5*C

E .4

-10 30'C -12 I -12 -14 32.5*C -16 a. -1.75 mV/K -14 -18 35'c -20 -12 I I ii - 0 20 40 60 60 26 28 30 32 34 36 Time / min Tempersture / C

3 Figure 24: (a) Potential vs. time plot from the 2 mM equimolar Fe(CN) 6 -4 temperature coefficient measurement experiment. The hot and cold electrodes are initially steadily held at 25'C. The hot electrode temperature is increased to 35*C and lowered to 25'C in 2.5*C increments every 10 minutes. (b) The averaged potential is plotted against the hot electrode temperature. The temperature coefficient of the 2 mM equimolar Fe(CN) 3 4-' couple is calculated to be -1.75 mV/K through linear fitting.

The temperature coefficient of a 2 mM equimolar electrolyte Fe(CN)63-/4- was measured to be -1.75 mV/K (Figure 24), which corresponded well to reported values in literature of around -1.72 mV/K [18,34]

47 14 35C14

12 12 32.5C 10 10

8 WC

4 27.5*C E p - 'ppp - - a 1 8 m / 2 -2

0 01

0 20 40 80 80 100 120 24 26 28 30 32 34 38 Time / min Temperature / C

2 3 Figure 25: (a) Potential vs. time plot from the 2 mM equimolar Fe +' + temperature coefficient measurement experiment. The hot and cold electrodes are initially steadily held at 25'C. The hot electrode temperature is increased to 35'C and lowered to 25'C in 2.5'C increments every 10 minutes. (b) The averaged potential is plotted against the hot electrode temperature. The 2 temperature coefficient of the 2 mM equimolar Fe +/3+ couple is calculated to be 1.31 mV/K through linear fitting.

The same setup was used to measure the temperature coefficient of the Fe2+/'+ half-cell reaction - the same reaction to be used in the proposed charging-free cell. The temperature coefficient of a 2 mM FeCl2, 2mM FeCl3, 15 mM HCl electrolyte, and a 200 mM FeCl2,

200 mM FeC3, 15 mM HCl electrolyte were measured. The temperature coefficient of the

2 mM equimolar Fe2+/3+ electrolyte is 1.31 mV/K (Figure 25), while the 200 mM equimolar

(400 mM total) temperature coefficient was measured to be 1.22 mV/K (Figure 26).

48 a) U4 I -) 35C 12 12 10 10 32. 5C 8- Es I - 30'C

44 27.5*C 2 2 2 1.a.i22 mV/K 0 0

-2 a - -2 0 20 40 60 80 100 24 26 26 30 32 34 36 Time mIn Temperature / C

Figure 26: (a) Potential vs. time plot from the 400 mM total Fe2+11+ temperature coefficient measurement experiment. The hot and cold electrodes are initially steadily held at 25'C. The hot electrode temperature is increased to 35'C and lowered to 25*C in 2.5*C increments every 10 minutes. (b) The averaged potential is plotted against the hot electrode temperature. The 2 3 temperature coefficient of the 200 mM equimolar (400 mM total) Fe +/ + couple is calculated to be 1.22 mV/K through linear fitting.

3.2 NiHCF Half-Cell Measurements

The charging curve of Nickel Hexacyanoferrate (NiHCF) was measured using a previously outlined three electrode beaker cell measurement setup. The reference electrode was a saturated Hg/HgSO4 electrode that has reduction potential of 0.68 V vs. SHE. The measured NiHCF potential was therefore corrected to Standard Hydrogen Potential according an Eref = 0.68 V. Since the as-synthesized NiHCF was in a near-fully oxidized state, the initial potential was between 0.25 V to 0.3 V vs. Hg/HgSO4, or between 0.93 V and 0.98 V vs. SHE. The charge and discharge current was chosen to be C/2 based on

NiHCF's theoretical gravimetric capacity of 60 mAh/g, which is 30 mA/g.

49 The NiHCF composite electrode was cycled through its full capacity twelve times to verify the integrity of the material, and to also obtain its charging and discharging curve.

Although no extensive half-cell cycling testing has been done on NiHCF, this limited cycling test inadvertently verified the short term stability of the material (Figure 27). In cycling, the NiHCF electrode exhibited a "ramp-up" behavior in the first few cycles where the measured capacity slowly increased to a steady-state value. This was presumably due to that the electrolyte required a certain amount of time to penetrate the porous carbon- cloth based composite electrode and to fully wet the active materials.

I I I I I 60 I

50.1 mAh g- 50 0*..* # 0 - 0 -

40

20 z 10

0 - I . I . I 0 2 4 6 8 10 12 Cycle

Figure 27: NiHCF cycling plot using the three-electrode beaker cell setup. The gravimetric capacity of NiHCF plateaus at just over 50 mAh/g after 5 full-cycles. The C rate used is C/2 based on a theoretical gravimetric capacity of 60 mAh/g.

50 NiHCF charged/discharged at C/2 1000 -

g00 -

2.6 mV/ mAh g 700

600

400

0 20 40 80 GravImetric Capacity / mAh g

Figure 28: Charging and discharging curve for NiHCF. A linear slope is fitted at the plateau of the NiHCF charging curve near 750 mV vs. SHE. The slope of the plateau based on experimentally obtained 50.1 mAh/g gravimetric capacity is 2.6 mV/mAh g'. The slope of the plateau based on a theoretical 60 mAh/g is 2.17 mV/mAh g'. Experiment was performed under room temperature.

The charge/discharge curve of Nickel Hexacyanoferrate (NiHCF) was captured

between approximately 0.4 V and 1.0 V vs. SHE (Figure 28). The charging curve plot

showed symmetry between charging and discharging curves. The plateau of NiHCF was

found between approximately 0.7 V and 0.8 V vs. SHE. At peak performance, the

synthesized NiHCF had a gravimetric capacity of 50.1 mAh/g, comparable to the

theoretical 60 mAh/g. For the purpose of conversion efficiency modeling in the next

sections, the slope of the NiHCF was calculated to be 2.6 mV/mAh g-1.

51 3.3 Full-Cell Temperature Coefficient and Preliminary Cycling

Full-cell testing of the charging-free cell comprising the NiHCF and the Fe2+/3 + half- cells were carried out using Teflon Cell B and the temperature controlled isothermal cycling regime outlined in the previous sections. In the charging-free cell's first iteration, the Selemion AMVN was used as the anion exchange membrane. At room temperature, the cell's assembled potential was found to be in the range of 130 mV to 160 mV depending on the concentrations of the Fe2+/3+ electrolyte, which was initially 400 mM summed across both Iron species, or 200 mM Fe2+ and 200 mM Fe3+. The cell was discharged to a potential of -200 mV to map out the discharge curve of NiHCF with respect to a 400 mM Fe2+/3+ electrolyte at room temperature.

250 0.2 M Fe* 200 0.2 M Fel* E 150 100 LL 5Z C (full call) *-500

0 -

-150 -200

-250

2 3 Figure 29: Charging curve of NiHCF against an excess volume of 200 mM Fe +, 200 mM Fe electrolyte (400 mM total). The charging curve at 25'C is experimentally obtained. The charging curve at 45'C is simulated based on an assumed 40 mV of thermally recovered potential, and superimposed on top of the 25'C charging curve. The operating range of the charging-free cell lies near the NiHCF plateau allowing maximum capacity to be extracted. The membrane used was Selemion AMVN.

52 Based on a speculated full-cell temperature coefficient of -2 mV/K and a temperature difference of 20'C for temperature cycling, the operating potential range for NiHCF was therefore between 0 mV and 40 mV with respect to the Fe2+/3+ redox potential. The concentration of Fe2+/3+ electrolyte was assumed to be constant since an excess volume of the electrolyte was used in the cell compared to the NiHCF solid loading. The exact ratio

2 between the charge capacity of the Fe +/3+ half-cell and NiHCF was at least 10:1 depending on the Fe2+/3+ concentrations and the NiHCF solid loading.

The full-cell charge curve between 150 mV and -200 mV showed that the 40 mV operating potential range of the charging free cell was well within the plateau of NiHCF

(Figure 29). The cell was then fully discharged to 0 mV and rested for 30 minutes at room temperature before thermal cycling.

40 25*C 30

20

10 0 jo 10--.

-20 -30 .45*C -40 0 1 2 3 4 Mime /hrs

Figure 30: Transient potential plot for the first full-cycle of charging-free cell operation. The thermal recovery of cell potential is indicated by a steep increase or decrease at the beginning of each temperature step. Selemion AMVN membrane was used and 0.6 mL of solutions were used.

53 Thermal cycling between 25 C and 45'C showed that the first iteration of the charging- free cell had temperature coefficient between -1.8 mV/K and 2.0 mV/K (Figure 30). The cell also showed a stable potential upon thermally recharging, and a voltage hysteresis of under 3 mV after shorting for 30 minutes. Although temperature fluctuation could be observed after the cell reaches a new thermal equilibrium due to artifacts from the PID heater controller.

Beyond the first cycle, further attempt to harvest waste heat using the charging-free cell showed that the thermally recovered potential, charge capacity, and energy decrease drastically after each cycle (Figure 31).

40 25'C 30 20 E 10 I 0 .10

.20

-30

-40 45*C 0 10 20 30 40 50 60 Time / hr

Figure 31: Potential vs. time plot for the first iteration of charging-free cell over 16 cycles. The recovered potential is shown to decrease with each cycle due to self-discharge. The cell's self- discharge is likely as a result of active species crossing over the anion exchange membrane. The Teflon Cell B as well its associated cycling regime described earlier in text were used.

54 3.4 Cycling Performance Improvements

It was speculated that the declining performance of the first charging-free cell was due to two reasons. A likely cause is chemically active Fe2+/3+ cations crossing over to the

NiHCF side due damaged or ineffective anion exchange membrane, leading to the observed self-discharge. Alternatively, NiHCF active material physically could be delaminating from the composite electrode during elongated cycling between two temperatures.

A variety of ion exchange membranes were tested in a full-cell setting to gauge their capability to prevent aqueous active species cross-over. The Selemion AMVN (Figure 31),

Selemion AAV (Figure 32), and Fumasep FAB (Figure 33) demonstrated cycling capability. On the other hand, the Selemion AHO and surface-treated Nafion 115 membranes could not maintain a steady potential upon assembly due to rapid self- discharging, and therefore did not produce any semblance of thermally recovered energy.

The Nafion 115 membrane was treated using a Layer-by-Layer surface treatment to prevent the cross-over of multivalent cation [351. Had the Nafion membrane experiment been successful, Had the Nafion experiment been successful, the intended function of the membrane would have been to shuttle protons as opposed to chloride ions between the two half-cells to maintain electro-neutrality.

In order to judge the performance between difference anion exchange membranes, the normalized charge capacity, averaged between the hot and cold half-cycles, were shown in

Figure 34 for all three membranes. The charge capacities were normalized to the first cycle.

In the same vein, the normalized thermally recovered energy, averaged between hot and cold half-cycles, were also plotted for all three membranes (Figure 35).

55 s0

40 - 25*C 30 20 10 I 0 I -10 'I -20 -30 -40 450C -50 0 10 20 30 40 50 60 lime / hrs

Figure 32: Potential vs. time plot for the charging-free cell using Selemion AAV membrane over 16 cycles. Though the self-discharge seems to be eliminated based on recovered potential each cycle, the source of peak potential fluctuation from cycle to cycle is unknown. Cell B and its associated cycling regime was used.

50 40 25*C 30 20 20 10 E 0 -10 -20 -30 -40 450C -50 0 10 20 30 40 50 60 Time / hs

Figure 33: Potential vs. time plot for the charging-free cell using Fumasep FAB-PK-130 membrane over 16 cycles. Each cycle recovers to approximately the same potential since self-discharge has largely been limited. Cell B and its associated cycling regime was used

56 1.0 0. Fumasep FAB 0 0.9 * 04DO Selemion AAV V 0 @0 * 0 0 e *0 0.8 *0 e 0.. 0 . 0 10.8 Selemlon AMVN a0 A. 20.5

0.4

0 2 4 6 8 10 12 14 18 18 Cycle

Figure 34: Normalized total discharge capacity vs. cycle from the charging-free cell cycling data using three different membranes. The total capacity is the sum of discharged capacities from the hot and cold half-cycles. Each set of cycling data is normalized to the highest value in the set.

I . I I I I I . I I I . f I . 1 1. 1.0 - 060 000. Selemion AAV 0.8 Fumasep FAB 0 00 -I 0 0.4 0 Selemlon AMVN

0.2 I- 000... 0.01 0 2 4 8 8 10 12 14 16 18 Cycle

Figure 35: Normalized total discharge energy vs. cycle from the charging-free cell cycling data using three different membranes. The total energy is the sum of discharged energy from the hot and cold half-cycles. Each set of cycling data is normalized to the highest value in the set.

57 Selemion AMVN displayed inferior performance to Selemion AAV and Fumasep FAB in terms of capacity and energy retention. The Selemion AAV and Fumasep FAB showed comparable performance. An average voltage, calculated as the discharged energy divided by discharge capacity, was defined in order to measure the cell's capacity to thermally recover and maintain potential after reaching a new thermal equilibrium. After normalizing to the highest value, the average voltage evaluated the impact of overpotential and self- discharge on the energy inefficiency of the cell by decoupling the effect of capacity loss via the physical delamination of NiHCF. In doing so, the two top-performing membranes could be objectively compared (Figure 36). The Fumasep FAB cell had a higher "voltage efficiency" than the Selemion AAV cell, and was therefore used for the remainder of this work.

1,2

Fumasep FAB 1,0 .

10,s - 0 Selemion AAV

10,60

0,4 Selemlon AMVN 0,4 0 1

0 2 4 8 8 10 12 14 18 18 Cycle

Figure 36: Normalized average voltage vs. cycle from the charging-free cell cycling data using three different membranes. The average voltage is calculated as the sum of discharged energy from the two half cycles over the sum of discharged capacity from the two half cycles.

58 Although the full cell with Fumasep FAB membrane could cycle up to 15 cycles with reduced active species crossover, the cell still experienced as much as 30% capacity loss after 15 cycles. Since contamination across the membrane was no longer a major hindrance to cycling performance, the sources of cell capacity loss then might be attributed to the delamination of electrochemically-active NiHCF particles during cycling, resulting in a proportional decrease in accessible charge capacity.

The carbon cloth was treated using a butane torch to burn off the hydrophobic surface coating and reduce the surface tension of the electrode precursor slurry on the carbon cloth.

This enabled the slurry to deeper penetrate into the carbon cloth as opposed to remain on the surface, and promoted improved bonding between the electrode materials and the carbon cloth. Experiments shown prior to this point did not undergo the same treatment.

A new composite NiHCF composite electrode was fabricated using the same method and composition as the previously made composite electrodes, with the exception of a butane torch treatment before casting the electrode slurry. A new charging-free cell was

2 assembled using an identical cell setup and electrolyte as the 400 mM Fe +/3+ cell shown in the previous section with the pre-treated NiHCF composite electrode. The cycling performance between the two otherwise identical cells were compared based on the normalized charge capacity and discharged energy (Figure 37).

The new pre-treated electrode showed less average charge capacity and energy depreciation each cycle. The average charge capacity and energy were defined as the average between the discharged capacity and energy between the hot and cold half-cycles.

The treated electrode showed greater superior retention compared to the untreated electrode, while the self-discharge in both cells were contained using the Fumasep FAB

59 membrane. The cell using the pre-treated electrode was further cycled to 35 cycles. The cell potential vs. time cycling plot for charging-free cell using the pre-treated electrode is not shown, since it would be largely similar to its untreated counterpart (Figure 33) as the capacity fade in the untreated cell only manifested in shorter discharge times and steeper discharge curves.

Figure 38 showed the charge capacity and energy for both hot and cold half-cycles for the charging-free cell using the treated electrode. A warm-up period was observed due to incomplete wetting of the electrode in the first few cycles. Otherwise, the cell exhibited excellent coulombic efficiency and energy efficiency (Figure 39).

I), . -,., ., 1 b) ,I ,I ., . 1.0 *$se* Treated - 044-eee Treated C **eeSee eege 0.9 . * 08Trete & Untreated *S* Untreated 00

0.4 0.4

0.2 I-0.2

0.01 0.0 0 6 10 16 0 6 10 15 Cycle Cycle Figure 37: (a) Normalized total charge capacity and (b) total discharged energy of each cycle extracted from two charging-free cell. The two cells are identical with the exception of electrode substrate pre-butane torch treatment. The membrane used is Fumasep FAB-PK- 130. Cell B and its associated cycling regime was used.

60 -

a)42 b) 1.

S 45*C 1.4 1.0 - 1.2 I 0.8 - 2 45*C Is I1.0 0.6 - S25*C 6 0.8 0 I 0.4 [ I0.6 0.4 I 0.2 , 0.2

0.0 0.0 0 10 20 30 40 0 10 20 30 40 Cycle Cycle

Figure 38: (a) The normalized charge capacity and (b) discharged energy from an elongated cycling study of a charging-free cell using a 400 mM combined FeCl 2/3 , 100 mM HCl half-cell. Electrolytes in both half-cells are replaced entirely with pristine electrolytes on the 2 2 nd cycle.

a)2 b) 120 P_ 120

115 116

110 I 110 I 106 i 106 I1- 99% 0 ato I'U100 100 96 06

90 90 0 10 20 30 0 10 20 30 Cycle Cycle

Figure 39: (a) The coulombic efficiency and (b) energy efficiency from an elongated cycling study of a charging-free cell using a 400 mM combined FeC2/3, 100 mM HCl half-cell. Electrolytes in both half-cells are replaced entirely with pristine electrolytes on the 2 2nd cycle.

61

- 3 3.5 Fe2+1 + Concentration Optimization for Maximum Conversion Efficiency

Following previously outlined efficiency calculation method, the theoretical maximum

conversion efficiency for a cell using 400 mM Fe2+/3+ electrolyte was 0.355% based on a

2 3 temperature difference between 25'C and 45'C, as well as a ratio between 400 mM Fe +/ +

solution volume to NiHCF mass of 1.17 mL to 1 g (Figure 40).

However, the ratio between electrolyte and active materials in experiment typically

exceeded 10 mL to 1 g, which rendered the estimated maximum efficiency difficult to achieve. Furthermore, this conversion efficiency calculation only took into account the heat capacity of electroactive components, which were NiHCF active species, electrolyte, and

2 the Fe +/3+ electrolyte. After all, the computed efficiency served little practical purpose other than a metric to compare the performance of similar electrochemical devices.

0.4

X 1.17mi g 1 0.3 -I (,3Y%

0.2

0.1 - - 0.0 0.6 1.0 1.6 2.0 2.6 3.0 Volume of Fe'.* soluUon per gram of NIHCF / mL gr

2 Figure 40: Computed theoretical heat-to-electricity conversion efficiency for a 400 mM Fe +/3+ (combined) as a function of Fe2+/3* electrolyte volume with respect to NiHCF mass. The maximum calculated efficiency is 0.355% when 1.17 mL of electrolyte is used for every gram of NiHCF.

62 Besides the electrolyte containing 400 mM of Fe2+/3+ combined, 1000 mM and 1300 mM combined electrolytes were electrochemically characterized in a full-cell setting. As is the case with most aqueous redox couples [18], the temperature coefficient of the redox reaction decreased as the concentrations of ionic species increase. Increased ion concentration lowered the heat capacity of the cell, which in return increased the conversion efficiency. On the other hand, lower temperature coefficients also impacted the thermally recovered energy by both lowering the recovered potential and capacity.

The conversion efficiency peaked at 0.587% using the 1000 mM combined Fe2+/+

electrolyte, which had a temperature coefficient of 1.145 mV/K (Figure 41). The

corresponding Carnot limit for the modeled temperature range was 6.29 %.

0.6 1.5 -

-0. o .. Toa1.0F C E 0

C4C 0< _ S0.5

Figure 41: Fe2+13+ half-cell reaction temperature coefficient (black, left) and modeled full-cell concentration of Fe2+/3+ maximum conversion efficiency (red, right) as a function of the total species.

63 At first, the concentration of HCI in the Fe2+/3+ electrolytes was maintained at 100 mM.

However, 1000 mM Fe2+/3+ cell cycling testing revealed sizeable increases in overpotential each cycle. Therefore, H+ concentration dependent experiments were carried out using electrolytes with varying concentration of HCl and constant concentrations for Fe2+/3 +

The internal resistances at the start of each hot and cold half-cycle were calculated from the cell overpotential. Over time, the 100 mM HC and 500 mM HCl cycling tests showed moderate and steep increase in internal resistance respectively, whereas the internal resistance of the charging-free cell using 200 mM HCl began to plateau after the 10* cycle

(Figure 42). Further cycling experiments were carried out at higher Fe2+/3+ concentrations while maintaining a Fe2+/3+ to H+ ratio of 5:1 (Figures 43, 44, 45, 46). The voltage vs. time cycling diagram for 1000 mM and 1300 mM concentration cells are listed in Figures 47 and 48.

Soo -

600

400 500 mM

300 --

200 -100 mM

1200 -

0 0 6 10 1 Cycle

Figure 42: Average internal resistance each cycle for charging-free cells acidified with 100 mM HCll 200 mM HCI, and 500 mM HCL. Average internal resistance is calculated from the average overpotential at the start of the hot and cold half-cycles.

64 a)1.2 b)1.2

* 25*C 1.0 . 25* - 1.0

0. , S * 45*C 0.6 45*C I . 0 0 0.6 0.6 LI 0 . 0 I I 0.4 0.4 I 0.2 I 0.2 0.0 0.0 0 10 20 0 10 20 Cycle Cycle

Figure 43: (a) The normalized charge capacity and (b) normalized discharged energy from the cycling study of a charging-free cell using 1000 mM combined FeCl 23/ , 200 mM HCl half-cell.

b) 120

f 116 1 JD P

110 110 0 6 106 105

0. . 100 100

16

90 90 0 10 20 0 10 20 Cycle Cycle

Figure 44: (a) The normalized coulombic efficiency and (b) energy efficiency from the cycling study of a charging-free cell using 1000 mM combined FeC 2/3 , 200 mM HCl half-cell.

65 a)

1.0 45*C

0 I 0.9 - 0 10, 0 0 0.9 I, --.. 00 0,6 25*C 6 25*C 01 I 0.7 0- 060

0 5 10 0 6C 10 Cycle Cycle

Figure 45: (a) The normalized charge capacity and (b) energy from the cycling study of a charging- free cell using 1300 mM combined FeC2/3, 260 mM HCl half-cell.

a) 120 b) 140 .0 I ~.30 --10- 1120 105 - 9 110 I 100- , 0 0 95 vu F a 90 80 5 10 6 10 Cycle Cycle

Figure 46: (a) The normalized coulombic efficiency and (b) energy efficiency from the cycling study of a charging-free cell using 1300 mM combined FeC2/3, 260 mM HCl half-cell.

66 I I 50 I 40 25*C 30 20 10 0 -10 I -20 -30 -40 450C

-60 - 0 10 20 30 40 50 60 Tme I hm

Figure 47: Potential vs. time plot for the charging-free cell using Fumasep FAB-PK-130 membrane 2 and a pre-treated NiHCF electrode over 16 cycles. The concentration of the Fe +/3+ half-cell electrolyte is 1000 mM combined FeCl 2/3 , 200 mM HCl.

40 25*C

20

I i 0 I -20

45C -40

0 5 10 IS 20 2 5 30 35 40 lime / hro

Figure 48: Potential vs. time plot for the charging-free cell using Fumasep FAB-PK-130 membrane 2 and a pre-treated NiHCF electrode over 11 cycles. The concentration of the Fe +/3+ half-cell electrolyte is 1300 mM combined FeC2/3, 260 mM HCl.

67 The 1000 mM total Fe2+/3+ charging-free cell required a longer warm up period than its

400 mM counterpart. The coulombic efficiency and energy efficiency of the 1000 mM cells

exceeded 99% and 96% respectively. The difference between the two efficiencies might

be attributed to energy loss from overpotential.

Even larger overpotentials were observed in the 1300 mM charging-free cell, which

also featured a longer warm up period that spans the whole cycling experiment. As a result,

the thermally recovered energy decreased each cycle despite the steadily increasing charge

capacity.

The average voltage was utilized again to visualize the impact of overpotential on

energy efficiency for all three Fe2+/3+ concentrations (Figure 49). The impact of increased

Fe2+/3+ concentration on cycling performance was evident.

1.06

*0. C,* t.00 V9

0.96

1300mM 1000 mM * 1 ~ 96.5% * r;~99% j0.90

0 p 0.86 p p p 0 6 10 16 20 Cycle

2 Figure 49: Normalized average voltage plotted against cycle count for three Fe +/3+ concentrations. Lower voltage efficiencies can be associated with elevated Fe2+/3+ concentrations as a result of overpotential effects. Teflon cell B was used for cycling as well as its associated cycling regime.

68 While the 1000 mM Fe2+/3+ version of the charging-free cell could theoretically reach higher heat-to-electricity conversion efficiency than its 400 mM counterpart, the 1000 mM cell would eventually be weighed down by the growing overpotential in long-term cycling.

Moreover, the maximum conversion efficiency for the 1000 mM cell occured at 0.645 mL of Fe2+/3+ electrolyte to every I g of NiHCF. In practice, enforcing such a low liquid to solid mass ratio would inevitably hinder the wetting of electrode active material, or the migration of ionic species in the electrolyte.

The 1000 mM iteration of the charging-free cell featured higher theoretical conversion

efficiency compared to other works on charging-free cell in literature (Figure 50).

However, in the end, less thermodynamically efficient yet more functional cells could be

constructed using a lower concentration Fe2+/3+ electrolyte, such as a 400 mM or lower.

20

16

10 PB/Fe(CN)b'J 4

LIxCoO 2/LxKV20

0 1 0 20 40 Heat Rocuperaton Efficiency / %

Figure 50: Theoretical conversion efficiency relative to Carnot efficiency as a function of heat recuperation efficiency up to 50% for this work, Yang et al (2 (based on modeled energy output), three and Linford et al 1361 (based on experimental energy output). Heat capacity calculation for all works are based only on electrochemically active materials.

69 3.6 Energy Harvester Integration Results

The practicality of NiHCF/Fe2+/3+ charging-free cell was put to the test through an

integration setup described in the experiment methods section. Two charging-free cells

were built and connected serially. After the cells were thermally recharged, they were

connected to an energy harvester circuit which extracted the converted energy to power a

temperature sensor.

During integration testing, the cell potential was monitored through the potentiostat

(Figure 51). The two serially-connected cells initially began with a potential around 90

mV. The cell immediately transferred energy to the circuit immediately upon connection

to power the temperature sensor, which was evident from the large initial potential drop.

The circuit then ceased to draw energy from the cells until a potential of 59 mV was reached

across the two cells. The extraction frequency slows as cell energy was eventually depleted.

100 I .

80

60 E

.~40

0 0- 20

0

-20 -

0 2 4 6 8 Time min

Figure 51: Transient response of the charging-free cell potential when connected to the flyback circuit. The flyback circuit is designed to extract a set quantity of energy when the source voltage reaches 59 mV. The increasing idle time between extractions indicates depletion of cell energy.

70 a)

Figure 52: Snapshots from an integration demonstration video. (a) A hairdryer is used to raise the PCB temperature, which is then (b) captured through the embedded temperature sensor. Link for video: https.//youtu. be/u 7syYGecJ3E.

A common hair dryer was used to heat up the temperature sensor embedded on the energy harvester/management circuit. A demonstration clip was filmed that shows a full cycle of the charging free cell. A snapshot of the clip was shown in Figure 52. The change in temperature of the circuit was captured by the cell-powered temperature sensor and transferred to a PC through an USB connection (Figure 53).

I I I I I 45 0S 40

Heating Onset

25 -41

0 10 20 30 40 60 Time / sec

Figure 53: PCB temperature readings reported by the embedded temperature sensor.

71 3.7 Flexible Charging-Cell Cycling Performance

Flexible charging-free cells were constructed using commercially available materials

using a simple tape-based assembly method. These flexible cells were not meant for long

term cycling, but rather a proof-of-concept of a manufactured charging-free cell. A single

cell and a 2x2 patterned cell were constructed and put through cycling. The flexible cell

showed no difference from that of a rigid Teflon cell in the first two cycles (Figure 54).

40 250C

20 E

0

-20

45'C -40

i 0 2 4 6 9 Time / hours

Figure 54: Transient potential plot of the flexible single cell's first two thermal cycles. The flexible cell performance in the first two cycles is functionally identical to its rigid counterpart.

In the limited number of cycles, the single flexible cell showed reasonable charge retention. Although the flexible cell apparently required a significantly longer warm up period, likely due to a longer electrode wetting period (Figure 55).

72 1.1

1.0 -2S*C

0.9

0.8 45*C

10.7

0.8 I i - - 0 2 4 8 8 10 12 14 Cycle

Figure 55: The normalized charge capacity vs. cycle from flexible single cell cycling. A longer start-up period is observed compared to previous experiments.

0 1.0 45 C - - I 0.S 2SOS 0.6 I, S- 0 0.4

I 0.2

010 o 5 10 16 20 26 Cycle

Figure 56: The normalized charge capacity vs. cycle from flexible quad cell cycling. Cell capacity drops dramatically after the first 5 cycles due to electrolyte loss.

73 The 2x2 serially-connected flexible cell recovered approximately 130 mV over a 20'C

temperature difference (Figure 57), lower than the expected 160 mV. This discrepancy

could be accounted for by the fact that the temperature sensor was no longer directly in

contact with a single cell, but rather was surrounded by 4 cells. So while the temperature

sensor may read 45*C, the actual temperatures at the cell electrodes were more likely to be

in the range of 35*C to 45'C.

The 2x2 patterned flexible cell showed good charge retention in the first few cycles

(Figure 56). After the 7t cycle, the charge capacity declined sharply primarily due to

electrolyte drying out in the electrolyte-soaked microfiber spacer. The cells fabricated

using this method might be expected to yield higher life-time if the electrolyte drying issue

could be resolved through improved sealing or using spacer with better liquid retention.

300 .

200 Increasing overpotential du 25*C to electrolyte drying out E 100-

II

-100 -

-200 -45C

0 20 40 60 80 100 Time /hrs

Figure 57: Transient potential plot of the flexible quad cell's entire cycling experiment. The effect of electrolyte loss leads to high ionic resistivity, which results in large overpotentials towards the end of the experiment.

74 4 Conclusion and Suggestions for Future Work

4.1 Conclusion

In summary, a charging-free thermogalvanic cell based the working principles of

Thermally Regenerative Electrochemical Cycle (TREC) was created to harvest thermal

energy from ambient temperature fluctuations. The charging-free cell comprised a solid

Nickel Hexacyanoferrate (NiHCF) Potassium Ion intercalation electrode and an aqueous

Ferrous/Ferric Chloride electrode as half-cells. At peak performance, the charging-free cell

achieved a theoretical heat-to-electricity conversion efficiency of 9.33% relative to the

Carnot efficiency. The theoretical conversion efficiency of this work showed improvement

over 5.6% relative to Carnot efficiency reported by Yang et al [25], as well as over 2.3%

relative to Carnot efficiency reported by Linford et al [36]. In realistic operating conditions,

the cell was capable of producing 2 mV open circuit voltage for every degree change in

temperature, higher than 1.4 mV/K reported by Yang et al [25] and -0.5 mV/K per cell

reported by Linford et al [361. The cell could maintain cycle-to-cycle coulombic efficiency

and energy efficiency of at least 99.5% and 99.0% respectively for at least 35 cycles,

comparable to the cycling performance of the Prussian Blue/Fe(CN)6 3-4 ~charging-free cell

showcased by the work of Yang et al [25.

Two serially connected charging-free cells were used to power a temperature sensor to

demonstrate thermal energy recovery towards practical applications. A thin, flexible, and

portable version of the cell was also fabricated as a possible prototype for manufacturing to moderate success. At a glance, the performance of the NiHCF/Fe2+/3+ charging-free cell

documented in this thesis had exceeded its predecessors from both a theoretical and a practical standpoint.

75 4.2 Suggestions for Future Work

While this version of the thermogalvanic cell has been shown to be technologically suitable for industrial applications, its lack of economic feasibility likely still remains the largest obstacle before commercialization. A detailed economic analysis of the

NiHCF/Fe2+/3+ charging-free cell is out of the scope of this work. However, any engineer who is serious about commercializing a charging-free TREC systems should note the following opportunities for improvements and pitfalls.

The quest for theoretical conversion efficiency is largely a fool's errand. M\ATLAB models can generate attract figures on the order of magnitude of the Carnot efficiency, but such model systems are rarely - if ever realizable. Optimal efficiencies demand unrealistic designs, such as pairing minimal amounts of electrolyte to massive solid electrodes which inevitably leads to poor electrode wetting and ion conduction.

On the other hand, the truly important metric for a charging-free cell is its volumetric or gravimetric energy density. Now, it goes without saying that the cell in consideration should contain a healthy amount of electrolyte among other critical components. The author believes that charging-free device's path of commercializing lie beyond none other than improving electrode capacities and temperature coefficients.

Last, even if the perfect charging-free thermogalvanic cell (think a = 5.0 mV/K, capacity comparable to Lithium-Ion battery cathodes) is built, it may still have a difficult time competing with solar panels for a vast majority of commercial applications. A tailored application where charging-free thermogalvanic cell outshines other energy recovery technologies still remains to be found.

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