2017/7/11
Energy Storage An Electrochemical Perspective
Shaowei Chen Department of Chemistry and Biochemistry University of California Santa Cruz, CA 95064
http://chemistry.ucsc.edu/~schen
Electricity
A general term that encompasses a variety of phenomena resulting from the presence and flow of electrical charge
~30,000 ºC
In 1750 he published a proposal for an experiment to prove that lightning is electricity by flying a kite in a storm which appeared capable of becoming a lightning storm.
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Electricity Generation
When the magnetic field around a conductor changes, a current is induced in the conductor Fleming’s right-hand rule
Electricity Generation
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Electro-Mechanical Generators
Steam turbine Invented by Sir Charles Parsons in 1884 Accounts for ~80% of the electric power in the world by using a variety of heat sources Energy sources Fossil fuel (e.g., coal, gas) Nuclear reactions Wind or flowing water
Our Lifestyle
Asia Africa + Europe North America
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Energy Consumption
Growing competition and anxiety over access to energy resources Limited reserves of fossil fuels US became a net oil importer in the 1940s Million barrels per day
Fossil Fuels
Fossil fuels are fuels formed by natural resources such as anaerobic decomposition of buried dead organisms. The age of the organisms and their resulting fossil fuels is typically millions of years, and sometimes exceeds 650 million years. The fossil fuels include coal, petroleum, and natural gas which contain high percentages of carbon Fossil fuels are non-renewable resources because they take millions of years to form, and reserves are being depleted much faster than new ones are being made.
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Environmental Impacts
Combustion products globe CO2, SO2, NO2, etc Risk of adverse climate change (green house effect)
Northern hemisphere
Southern hemisphere
If the world follows a “business-as-usual” path,
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Acid Rain
Environmental Pollutions
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No problem, no job Challenge and Opportunity
Science has unambiguously shown that we are altering the destiny of our planet Science and technology have given us solutions in the past and it will come to our aid in the future “To protect our planet, now is the time to change the way we use energy. Together, we must confront climate change by ending the world’s dependence on fossil fuels, by tapping the power of new sources of energy like the wind and sun, and calling upon all nations to do their part.”
Barack Obama, in Prague, Czech Rep, 5 April 2009
Renewable and Sustainable Energy
Renewable energy sources Solar: conversion of sun light into electricity Wind: conversion of mechanical energy into electricity Low efficiency and limited accessibility Fuel cells A kind of battery with green energy sources High-efficiency conversion of chemical energy to electricity
Wide accessibility of energy sources: H2, methanol, ethanol, formic acid, etc
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Battery
Primary battery Disposable battery, designed to be used once and discarded when they are exhausted
Secondary battery Rechargeable battery
Alkaline Battery
Electrolyte: KOH, NH4Cl, ZnCl2, etc A type of disposable battery dependent upon the reaction between zinc (anode) and manganese (IV) oxide (cathode) − − Zn(s) + 2OH (aq) → ZnO(s) + H2O(l) + 2e (1.26 V) − − 2MnO2(s) + H2O(l) + 2e →Mn2O3(s) + 2OH (aq) (0.135 V) Overall Reaction
Zn(s) + 2MnO2(s) → ZnO(s) + Mn2O3(s) (1.395 V)
Leaking of KOH
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Lead Acid Battery
Electrolyte is fairly concentrated sulfuric acid (H2SO4, about 4M). Anode a thick, porous plate of metallic lead (Pb) − + − Pb + HSO4 →PbSO4 + H + 2e Cathode
a plate consisting mostly of porous lead dioxide (PbO2) paste, supported on a thin metal grid + − − PbO2 + 3H + HSO4 + 2e →PbSO4 +2H2O Overall Reactions
Pb + PbO2 + 2H2SO4 → 2PbSO4 + 2H2O
Lithium Ion Battery
Electrolyte is a lithium salt in an organic solvent Cathode contains lithium The anode is generally made of a type of porous carbon Overall Reaction
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Lemon Battery
H2 H+
Zinc-Air Battery
Zinc-air batteries (non-rechargeable), and zinc-air fuel cells, (mechanically-rechargeable) are electro-chemical batteries powered by oxidizing zinc with oxygen from the air. These batteries have high energy densities and are relatively inexpensive to produce. Sizes range from very small button cells for hearing aids, larger batteries used in film cameras that previously used mercury batteries, to very large batteries used for electric vehicle propulsion.
Zinc-air hearing aid batteries
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Zinc-Air Battery
Reactions
– 2– – Anode (Zn particles): Zn + 4OH → Zn(OH)4 + 2e (E0 = －1.25 V) 2– – Fluid: Zn(OH)4 → ZnO + H2O + 2OH – – Cathode: 1/2 O2 + H2O + 2e → 2OH (E0 = 0.34 V, pH＝11)
Overall: 2Zn + O2 → 2ZnO (E0 = 1.59 V)
Zinc-air batteries have some properties of fuel cells as well as batteries: the zinc is the fuel, the reaction rate can be controlled by varying the air flow, and oxidized zinc/electrolyte paste can be replaced with fresh paste. A future possibility is to power electric vehicles.
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Challenges
Other electrode materials Al, Li, … Rechargeable Enhanced capacity
Tesla Roadster
Electricity
Free electrons/ions
How many? Atomic structure Atoms/Molecules
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Atomic Structure
Atoms are most stable when they have filled the outer shell of electrons which normally holds a max of 8. If an atom has 1 electron in its outer layer getting rid of it will give it stability, in much the same way having 7 electrons will mean gaining one electron will give it stability. Now lets say the two meet, they react with each other and everybody is happy.
Reactivity
Electrochemical series Li > K > Sr > Ca > Na > Mg > Al > Zn > Cr > Fe > Cd > Co > Ni > Sn > Pb > H > Cu > Ag > Hg > Pt > Au Electrochemical potential (º) Anode: oxidation (electron donating) Cathode: reduction (electron accepting)
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Zn Ag Cell Notation
AgCl Cell notation Zn2+, Cl- Zn|Zn2+, Cl-|AgCl|Ag Procedure Identify electrodes (anode and
cathode) Anode: Zn2+ + 2e Zn Starting from anode to Cathode: AgCl + e Ag + Cl- cathode Vertical bars represent phase boundaries Two half cells: Half reactions in reduction format
Exercises
Zn|ZnCl2 (aq)||CuCl2(aq)|Cu
Pt|FeSO4(aq), Fe2(SO4)3 (aq)||K2Cr2O7 (aq), H2SO4(aq)|Pt
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Transition-State Theory
Transition state theory is also known as activated-complex theory or theory of absolute reaction rates. In chemistry, transition state theory is a conception of chemical reactions or other processes involving rearrangement of matter as proceeding through a continuous change or "transition state" in the relative positions and potential energies of the constituent atoms and molecules.
Svante August Arrhenius
Svante August Arrhenius (19 February 1859 – 2 October 1927) was a Swedish scientist, originally a physicist, but often referred to as a chemist, and one of the founders of the science of physical chemistry. The Arrhenius equation, lunar crater Arrhenius and the Arrhenius Labs at Stockholm University are named after him.
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Arrhenius Equation
EA k Ae RT
G RT k Ae‛ Gº Gº = -nFEº
Standard Electrode Potential
Cathode (Reduction) Standard Potential Half-Reaction E° (volts vs NHE) Li+(aq) + e Li(s) -3.04 K+(aq) + e K(s) -2.92 Ca2+(aq) + 2e Ca(s) -2.76 Zn2+(aq) + 2e Zn(s) -0.76
+ 2H (aq) + 2e H2(g) 0.0 Cu2+(aq) + 2e Cu(s) +0.34
+ O3(g) + 2H (aq) + 2e O2(g) + H2O(l) +2.07
- F2(g) + 2e 2F (aq) +2.87
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Electrochemical Cells
Cell voltage = cathode – anode Energy output Reactions at both cathode and anode Reaction kinetics Capacity (energy density) – amount of consumables Performance factors Materials Packaging
Fuel Cell
Alternative Energy
A fuel cell is an electrochemical cell that converts a source fuel into an electrical current. It generates electricity inside a cell through reactions between a fuel and an oxidant, triggered in the presence of an electrolyte. The reactants flow into the cell, and the reaction products flow out of it, while the electrolyte remains within it. Fuel cells can operate continuously as long as the necessary reactant and oxidant flows are maintained.
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Enhanced efficiency (and lowered costs) of energy consumption Motivations Reduced pollution to environments
Alternative energy sources (small organic molecules, such Primary Goals as H2, methanol, ethanol, formic acid, etc)
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Steam Reforming
Fossil fuel currently is the main source of hydrogen production
At high temperatures (700–1100 °C), steam (H2O) reacts with methane (CH4) to yield syngas
CH4 + H2O → CO + 3H2 + 191.7 kJ/mol In a second stage, hydrogen is generated through the lower-temperature water gas shift reaction, performed at about 130 °C:
CO + H2O → CO2 + H2 - 40.4 kJ/mol
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Partial Oxidation
The partial oxidation reaction occurs when a substoichiometric fuel-air mixture is partially combusted in a reformer, creating a hydrogen-rich syngas General Reaction equation By heating oil By coal
Syntrolysis
Syntrolysis is a procesess for synthetic fuels from CO2, electricity and steam Below is a syntrolysis cell operating at 830°C (1525°F). The cell consists of a sandwich of exotic metals and ceramic materials that simultaneously electrolyze carbon dioxide and steam. The resulting synthesis gas is the precursor to synthetic fuels.
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Plasma Reforming
Electrolysis for Hydrogen
H2O H2 + ½O2
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Biohydrogen Routes
From urine
To break the molecule down, a voltage of 0.37 V needs to be applied across the cell - much less than the 1.23 V needed to split water.
Fermentation for Hydrogen
Biohydrogen can be produced in bioreactors that utilize feedstocks, the most common feedstock being waste streams. The process involves bacteria feeding on hydrocarbons and exhaling hydrogen and CO2 Two major barriers the cost of the feedstock: sugar-rich feedstocks the yield of hydrogen In 2000 it was discovered that if C. reinhardtii algae are deprived of sulfur they will switch from the production of An algae bioreactor for oxygen, as in normal photosynthesis, to hydrogen production. the production of hydrogen.
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Microbial Electrolysis Cell
Water Splitting
Photoelectrolysis
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Metal Hydrides
A hydride is the anion of hydrogen, H−
Silica Encapsulated Metal Hydride
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MeOH Production
Synthesis gas (syngas) production
Steam-methane reforming (SMR) CH4 + H2O → CO + 3H2
Partial oxidation of methane 2CH4 + O2 → 2CO + 4H2 Methanol production
CO + 2H2 → CH3OH
The most widely used catalyst is a mixture of Cu, ZnO, and Al2O3 first used by ICI in 1966. At 5–10 MPa (50–100 atm) and 250 °C, it can
catalyze the production of methanol from CO and H2 with high selectivity Feedstocks for methanol production,
Natural gas (CH4): the most economical and widely used Coal: particularly in China Biomass gasification: woody biomass
2C16H23O11 + 19 H2O + O2 → 42H2 + 21CO + 11CO2 → 21CH3OH + 11CO2
Ethanol (CH3CH2OH) Production
Bio-ethanol is usually obtained from the conversion of carbon based feedstock. Agricultural feedstocks are considered renewable because they get energy from the sun using photosynthesis, provided that all minerals required for growth (such as nitrogen and phosphorus) are returned to the land. Ethanol can be produced from a variety of feedstocks such as sugar cane, bagasse, miscanthus, sugar beet, sorghum, grain sorghum, switchgrass, barley, hemp, kenaf, potatoes, sweet potatoes, cassava, sunflower, fruit, molasses, corn, stover, grain, wheat, straw, cotton, other biomass, as well as many types of cellulose waste and harvestings, whichever has the best well-to-wheel assessment.
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Ethanol Production - Algae
An alternative process to produce bio-ethanol from algae is being developed by the company Algenol. Rather than grow din mor algae and then harvest and ferment it the algae grow in sunlight and produce ethanol directly which is removed without killing the algae. It is claimed the process can produce 6000 gallons per acre per year compared with 400 gallons for corn production.
seaweeds
Ethanol Production - Corn
Currently, the first generation processes for the production of ethanol from corn use only a small part of the corn plant: the corn kernels are taken from the corn plant and only the starch, which represents about 50% of the dry kernel mass, is transformed into ethanol. Two types of second generation processes are under development. The first type uses enzymes and yeast to convert the plant cellulose into ethanol while the second type uses pyrolysis to convert the whole plant to either a liquid bio-oil or a syngas. Second generation processes can also be used with plants such as grasses, wood or agricultural waste material such as straw.
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Formic Acid Production (HCOOH)
Systematically called methanoic acid, which may occur naturally, most notably in the venom of bee and ant stings From methyl formate and formamide When methanol and carbon monoxide are combined in the presence of a strong base, the formic acid derivative methyl formate results, CH3OH + CO → HCOOCH3 In industry, this reaction is performed in the liquid phase at elevated pressure. Typical reaction conditions are 80 °C and 40 atm. The most widely- used base is sodium methoxide. Hydrolysis of the methyl formate produces formic acid
HCOOCH3 + H2O → HCOOH + CH3OH
Formic Acid Production
By-product of acetic acid production A significant amount of formic acid is produced as a byproduct in the manufacture of other chemicals. Acetic acid once was produced on a large scale by oxidation of alkanes, via a process that cogenerates significant formic acid. This oxidative route to acetic acid is declining in importance, so that the dedicated routes to formic acid have become more important. Hydrogenation of carbon dioxide --- “burning backwards” The catalytic hydrogenation of CO2 has long been studied. This reaction can be conducted homogeneously. Laboratory methods In the laboratory, formic acid can be obtained by heating oxalic acid in anhydrous glycerol and extraction by steam distillation. Another preparation (which must be performed under a fume hood) is the acid hydrolysis of ethyl isonitrile (C2H5NC) using HCl solution. C2H5NC + 2 H2O → C2H5NH2 + HCOOH The isonitrile can be obtained by reacting ethyl amine with chloroform (note that the fume hood is required because of the overpoweringly objectionable odor of the isonitrile).
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Varied Options of Fuels
H2 HCOOH CH3OH CH3CH2OH
Production + ++ ++ +++ storage +++ ++ + transportation +++ ++ +
Oxidation (fuel) +++ ++ +
Environmental none CO2/CO CO2/CO CO2/CO Impacts
Storage and Transportation Media for Hydrogen
Liquid/slush hydrogen (e.g., in space shuttles) Cryogenic storage (boiling at 20.268K) that imposes a large energy cost . Insulation of storage tanks Compressed hydrogen High-pressure gas
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Chemical / Physical Storage
Chemical storage
Metal hydride (LiH, LiAlH4, TiFeH2)
Carbohydrates (C6H10O5) Synthesized hydrocarbons: hydrogen reformer Physical Storage Carbon nanotubes Metal-organic frameworks ……
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Fuel Cell Reactions
Hydrogen Fuel Cell + Anode: H2 2H + 2e + Cathode: O2 + 4H + 4e 2H2O
Overall: 2H2 + O2 2H2O Methanol Fuel Cell + Anode: CH3OH + H2O 6H + CO2 + 6e + Cathode: O2 + 4H + 4e 2H2O
Overall: 2CH3OH + 3O2 2CO2 + 4H2O
Fuel Cell Reactions
Formic Acid Fuel Cell + Anode: HCOOH 2H + CO2 + 2e + Cathode: O2 + 4H + 4e 2H2O
Overall: 2HCOOH + O2 2CO2 + 2H2O Electrocatalysts
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Catalysis
Catalysis is the change in rate of a chemical reaction due to the participation of a substance called a catalyst, which works by providing an (alternative) mechanism involving a different transition state and lower activation energy. Unlike other reagents that participate in the chemical reaction, a catalyst is not consumed by the reaction itself.
Great Challenge
+ Cathode: O2 + 4H + 4e 2H2O
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Common Catalysts – Platinum Group Metals
Platinum Group Metals
High prices Limited reserves The largest known reserves are in South Africa $ per oz, as of Aug 3, 2011 In 2005, South Africa was the top producer of platinum with an almost 80% share followed by Au Ag Pt Pd Russia and Canada. 1653 41 1792 823 239 tons of platinum sold in 2006 54% for vehicle emissions control devices 20% used for jewelry 6% used in electronics 5% used by the chemical industry as a catalyst The remaining 15% produced were used in various other minor applications, such as electrodes, anticancer drugs, oxygen sensors, spark plugs and turbine engines
A vehicle catalytic converter typically contains 1 to 3g of platinum, according to the World Health Organization.
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FePt Electrocatalysts
Size Nanoparticles with large surface to volume ratio Shape Reaction active sites Support Metal-support interactions
Pt Nanocatalysts
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Pt Nanoparticles
24-facet nanocrystals whose catalytic activity per unit area can be as much as four times higher than existing commercial platinum catalysts.
Nanocatalyst Synthesis
Chemical Reduction Electrolysis Laser Ablation Photoreduction ……
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Grand Challenges
Limited knowledge about the electrode reaction mechanisms Optimal electrocatalysts Maximal performance Lowest costs
Catalyst Poisoning
Catalyst poisoning refers to the effect that a catalyst can be 'poisoned' if it reacts with another compound that bonds chemically (similar to an inhibitor) but does not release, or chemically alters the catalyst. This effectively reduces the usefulness of the catalyst, (i. e. the number of active sites) as it cannot participate in the reaction with which it was supposed to catalyze. Common cause Residual or intermediate CO
hemogloblin
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Nanoscale Engineering
Highly Stable and CO-Tolerant Pt/Ti0.7W0.3O2 Electrocatalyst for Proton-Exchange Membrane Fuel Cells
J. Am. Chem. Soc., 2010, 132 (30), pp 10218–10220
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Fuel Cell Vehicles An essential and critical component is electrocatalysts for fuel cell anodes and cathodes. Of these, platinum-based alloys (PtM) have been found to be the most promising candidates, because of the bifunctional mechanism and ligand-field effects.
Microbial Fuel Cell - History
A bio-electrochemical system that drives a current by mimicking bacterial interactions found in nature. The idea of using microbial cells in an attempt to produce electricity was first conceived by M. C. Potter in 1911, a professor of botany at the University of Durham. Potter managed to generate electricity from E. coli. In May 2007, the University of Queensland, Australia, completed its prototype MFC, as a cooperative effort with Fosters Brewing Company. The prototype, a 10 liter design, converts the brewery waste water into carbon dioxide, clean water, and electricity (and to produce 2 KW of power.
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Microbial Fuel Cell
A typical microbial fuel cell consists of anode and cathode compartments separated by a cation (positively charged ion) specific membrane. In the anode compartment, fuel is oxidized anaerobically by microorganisms, generating electrons and protons. Electrons are transferred to the cathode compartment through an external electric circuit, and the protons are transferred to the cathode compartment through the membrane. Electrons and protons are consumed in the cathode compartment, combining with oxygen to form water.
Mediator Microbial Fuel Cell
Most of the microbial cells are electrochemically inactive. The electron transfer from microbial cells to the electrode is facilitated by mediators such as thionine, methyl viologen, methyl blue, humic acid, neutral red and so on. Most of the mediators available are expensive and toxic.
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Mediator-less Microbial Fuel Cell
A mediator-less microbial fuel cell does not require a mediator but uses electrochemically active bacteria to transfer electrons to the electrode (electrons are carried directly from the bacterial respiratory enzyme to the electrode). Among the electrochemically active bacteria are, Shewanella putrefaciens, Aeromonas hydrophila, and others. Some bacteria, which have pili (hair-like appendages) on their external membrane, are able to transfer their electron production via these pili. Mediator-less microbial fuel cells can, besides running on wastewater, also derive energy directly from certain aquatic plants. These include reed sweetgrass, cordgrass, rice, tomatoes, lupines, and algae. These microbial fuel cells are called (Plant-MFC). Given that the power is thus derived from a living plant (in situ-energy production), this variant can provide extra ecological advantages.
Electricity Generation
When micro-organisms consume a substrate such as sugar in aerobic conditions they produce carbon dioxide and water. When oxygen is not present they produce carbon dioxide, protons and electrons + C12H22O11 + 13H2O 12CO2 + 48H + 48e
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Applications
Virtually any organic material could be used to ‘feed’ the fuel cell MFCs could be installed to wastewater treatment plants. Use of a renewable form of energy and would not need to be recharged like a standard battery would
Microbial Fuel Cells
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Summary
New generation of energy storage devices and systems Sustainability Interdisciplinary research Materials chemistry Nanoscience and nanotechnology
Electroreduction of CO2
The electrochemical reduction of carbon dioxide (ERC) is the conversion of carbon dioxide to more reduced chemical species using electricity as the energy source.
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History
The first examples of electrochemical reduction of carbon dioxide are from the 19th century, when carbon dioxide was reduced to formic acid using a zinc cathode. Research in the space intensified in the 1980s following the oil embargoes of the 1970s. Electrochemical reduction of carbon dioxide is currently considered a possible means of producing chemicals or
fuels from carbon dioxide (CO2), making it a feedstock for the chemical industry
State of the Art
Current challenges poor thermodynamic efficiency low current efficiency low selectivity slow kinetics poor stability Catalysts Cu Zn
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Nanostructure-Based Capacitors
Supercapacitors Porous carbons Nanostructured Pseudo-capacitors RuO2
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