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. 1 2017/7/11 Electricity Generation When the magnetic field around a conductor changes, a current is induced in the conductor Fleming’s right-hand rule Electricity Generation 2 2017/7/11 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 3 2017/7/11 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. 4 2017/7/11 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, 5 2017/7/11 Acid Rain Environmental Pollutions 6 2017/7/11 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 7 2017/7/11 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 8 2017/7/11 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 9 2017/7/11 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 10 2017/7/11 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. 11 2017/7/11 Challenges Other electrode materials Al, Li, … Rechargeable Enhanced capacity Tesla Roadster Electricity Free electrons/ions How many? Atomic structure Atoms/Molecules 12 2017/7/11 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) 13 2017/7/11 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 14 2017/7/11 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. 15 2017/7/11 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 16 2017/7/11 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. 17 2017/7/11 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) 18 2017/7/11 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 19 2017/7/11 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. 20 2017/7/11 Plasma Reforming Electrolysis for Hydrogen H2O H2 + ½O2 21 2017/7/11 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.