ELECTROCHEMICAL SYSTEMS FOR ELECTRIC POWER GENERATION Dzmitry Malevich Depatrment of Chemistry and Biochemistry University of Guelph

Electric power conversion in electrochemistry Electrolysis / Power consumption Electric Power Chemical Reactions Electrochemical battery / Power generation

Volta’s battery (1800) Paper moisturized with NaCl solution Cu Zn Alessandro Volta 1745 - 1827 Paper moisturized with NaCl solution Cu Zn

Principles of power generation in the electrochemical systems Me2n+ - ne- = Me20 CATHODE Me20 - ne- = Me2n+ ANODE Me2 Me1 Salt Bridge Me1n+ SO42- Me2n+ SO42-

IMPORTANT NOTICE ! Electrolysis ANODE + CATHODE - Battery ANODE - System consumes energy G>0 ANODE + CATHODE - Battery System releases energy G<0 ANODE - CATHODE + (oxidation process) (oxidation process) (reduction process) (reduction process)

Principles of power generation in the electrochemical systems Me2n+ - ne- = Me20 CATHODE Me20 - ne- = Me2n+ ANODE Me2 Me1 Diaphragm Membrane or Me1n+ SO42- Me2n+ SO42-

Primary batteries Modern Zinc-Manganese battery Zn-container Carbon rod MnO2 paste (cathode) Gas space Gel electrolyte Leclanché’s battery (1866) Anode: Zn  Zn2+ + 2e- Cathode: 2MnO2 + 2H2O +2e-  2MnOOH + 2OH- Electrolyte: Zn2+ 2NH4Cl +2OH-  Zn(NH3)Cl2 + 2H2O 2MnO2 + Zn + 2NH4Cl  2MnOOH + Zn(NH3)Cl2 Georges Leclanché (1839-1882) Seal Zn-container MnO2 paste (cathode) Carbon rod NH4OH electrolyte

Primary batteries Zinc-Manganese alkaline battery MnO2 paste (cathode) Gel electrolyte Porous Zn (anode) Zinc-Manganese alkaline battery Anode: Zn + 2OH- - 2e-  Zn(OH)2 Cathode: MnO2 + H2O +1e-  MnOOH + OH- aaaaaaaaa MnOOH + H2O +e-  Mn(OH)2 + OH- Zinc-Air battery Anode: Zn + 2OH- - 2e-  Zn(OH)2 Cathode: 1/2 O2 + H2O + 2e-  Zn(OH)2

Secondary (rechargeable) batteries Lead-acid battery Lead paste in Pb-mesh (anode) Lead dioxide paste in Pb-mesh (cathode) Porous separator Safety valve Lead-acid battery Pb PbO2 E=2.06 V 36% H2SO4 discharge charge PbSO4+H2O PbO2+(2H++SO42-)+2H++2e- PbSO4+ 2H+ Pb+(2H++SO42-)-2e- PbSO4 PbO2 + Pb + H2SO4 2PbSO4 + 2H2O discharge

Secondary (rechargeable) batteries Lithium-ion battery Cathode: LiMeO2 - xe- Li1-xMeO2 + xLi+ Anode: C + xLi+ + xe- CLix CHARGE DISCHARGE Discharge Charge Anode (CLix) Cathode (LiMexOy) LiCoO2 -utilized for commercial batteries LiNiO2, LiMn2O4-prospective Separator Aluminum can Positive terminal Negative terminal

Secondary (rechargeable) batteries Nickel-Metal Hydride battery Cathode: NiOOH + H2O - e- Ni(OH)2 + OH- Anode: Me + OH- + e- Me + H2O CHARGE DISCHARGE CHARGE DISCHARGE Picture from: T. Takamura / Solid State Ionics 152-153(2002)19

Types of the electrochemical system for electric power generation Fuel cells Reaction products (exhaust) Reductant (fuel) Oxidant POWER Primary batteries POWER Secondary batteries Recharge POWER

Grove’s fuel cell (1839) O2 H2 4H+ + 4e- 2H2 2H2O - 4e- O2 + 4H+ Sir William Grove 1811–1896 4H+ + 4e- 2H2 2H2O - 4e- O2 + 4H+

Fuel Cells performance improving Raising the voltage: Raising the current: • Increasing the temperature • Increasing the area of eelectrode electrolyte interface • The use of catalyst Cell stack ANODE CATHODE ELECTROLYTE Bipolar electrode ANODE CATHODE ELECTROLYTE Connection of cells in series Anode catalyst Cathode catalyst Electrolyte frame Bipolar plate H2 O2 Stack of several hundred

Phosphoric Acid Fuel Cell (PAFC) Electrolyte in SiC porous matrix O2 Pt-particles catalysts (anode or cathode) Gas (H2 or O2) PACF parameters: current density - 200- 400 mA cm-2 single cell voltage - 600-800 mV temperature - 220 oC At atmospheric pressure H2

Gas Diffusion Electrode Reaction zone Dry zone (no reaction) H2 Electrode Gas e- e- Electrolyte Reaction zone Dip zone (reaction is slow because diffusion limitation)

Disadvantages of liquid electrolyte fuel cell Low operation temperature ! (reaction is slow, expensive catalysts are needed to produce valuable current) Difficulties in three-phase interface maintaining ! Strong fuel crossover! Recombination (no electron transfer through outer socket - energy loss) H2 O2 Anode Liquid electrolyte Cathode

+ - Proton Exchange Membrane Fuel Cell (PEMFC) H2 H+ Nafion® membrane H2O +Air (O2) H+ Nafion® membrane Catalyst support (carbon cloth) Current collector / gas distributor H2 crossover H2 Air (O2) + -

Proton Exchange Membrane (PEM) Polymerization Polyethylene Ethylene C H S F C O Grafting Nafion® (DuPont) - H+ C F Fluorination Polytetrafluoroethylene (PTFE, Teflon®)

Fuel reforming CnHm + nH2O = nCO + (m/2 + n)H2 CH4 + H2O = CO + 3H2 CO + H2O = CO2 + H2 CH3OH + H2O = 3 H2 + CO2 T~ 500 oC, Ni-catalyst T~ 250 oC, Ni-catalyst no CO Stainless still Catalyst CH4 + H2O H2 + COx CH4 + O2 CO2 + H2O HEAT

+ - Direct Methanol Fuel Cell (DMFC) CH3OH + H2O + CO2 H+ Nafion® H2O +Air (O2) H+ Nafion® membrane Catalyst support (carbon cloth) Current collector / fuel distributor CH3OH crossover CH3OH + H2O Air (O2) + -

Methanol oxidation mechanism + ê + ê + ê + ê + ê + ê carbon oxygen hydrogen Pt Pt

Direct Methanol Fuel Cell (DMFC) Theoretical voltage = 1.182 V Real voltage Current 0.046 1.23 Potential vs. HRE, V CH3OH + H2O = CO2 + 6H+ + 6e- 3/2O2 + 6H+ + 6e- = 3H2O

Carbon monoxide tolerant anode oxygen hydrogen Ru Pt

Methanol crossover through Nafion From M.P. Hogharth and G.A. Hards, Platinum Metals Rev. 40 (1996) 150 Temperature oC Current density, A cm-2 Crossover rate, A cm-2 90 0.1 0 .32 90 0.2 0.30 90 0.3 0.27 S. R. Narayanan, DOE/ONR Fuel Cell Workshop, Baltimore, MD, Oct 6-8 1999 Number of methanol moles (Nm) transported by crossover can be calculated by Faraday low: Nm = jc·S·t/n·F, where j - current density(crossover rate) , S - membrane area, t - time, n-number of electrons (n=6 for methanol oxidation), F - Faraday constant

Catalysts for fuel cells with polymer electrolyte PEMFC DMFC Anode: Pt or PtRu (~50% Pt) black 1-10 nm Cathode: Pt (~50% Pt) black 1-10 nm Anode: usually PtRu (~50% Pt) black 1-10 nm Cathode: Pt (~50% Pt) black 1-10 nm Catalysts are supported on carbon nanoparticles (50-200 nm, for example Vulcan XC72) Catalysts are usually unsupported Precious metals load is 0.2 - 0.5 mg cm-2 for both electrodes Precious metals load is 1.0 - 10.0 mg cm-2 for both electrodes Power density - 500 mW cm -2 at cell voltage 0.5 V (t=80 oC, CO-free hydrogen) Power density - 100 mW cm -2 at cell voltage 0.5 V (t=90 oC, CH3OH concentration - 0.75 M) Catalysts cost ~ 0.8 g per kW ( ~140 CAN$ per kW) Catalysts cost ~ 10 g per kW ( ~1750 CAN$ per kW)

Molten Carbonate Fuel Cell (MCFC) Anode Porous electrolyte support Cathode LiNiO2 or LiCoO2 NiCr alloy Alkali metal carbonates in LiAlO2 matrix H2 +CO2 + H2O O2 +CO2 CO32- H2 O2 +CO2 0.2 - 1.5 mm 0.5 - 1.0 mm 0.5 - 1.0 mm T= 600-700 oC O2 + 2CO2 + 4e - = 2CO32- 2H2 + 2CO32- - 4e- = 2H2O + 2CO2

Solid Oxide Fuel Cell (SOFC) Anode Electrolyte Cathode Sr doped La-manganite H2 + H2O O2 O2- H2 O2 YSZ 2H2 + 2O2- - 4e - = 2H2O O2 + 4e - = O2- Ni+YSZ T= 800-1100 oC Electrolyte Anode Air Air Fuel Cathode

Types of Fuel Cells Phosphoric Acid Fuel Cell (PAFC) Mobile ion Operating temperature Power range Phosphoric Acid Fuel Cell (PAFC) H+ ~220 oC 10 - 1000 kW H + 50 - 100 oC 1 - 100 kW H + 50 - 100 oC 1 - 100 kW CO32- ~650 oC 0.1 - 10 MW O2- 500 - 1000 oC 0.01 - 10 MW Proton Exchange Membrane Fuel Cell (PEMFC) Direct Methanol Fuel Cell (DMFC) Molten Carbonate Fuel Cell (MCFC) Solid Oxide Fuel Cell (SOFC)