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Thermodynamics and Electrochemistry of Low Temperature Fuel Cells

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Thermodynamics and Electrochemistry of Low Temperature Fuel Cells Thermodynamics and Electrochemistry Of Low Temperature Fuel Cells Frano Barbir Professor, FESB, Split, Croatia Joint European Summer School for Fuel Cell and Hydrogen Technology Prof. Dr. Frano Barbir energy partners What is fuel cell? Fuel cell is an electrochemical energy converter. It converts chemical energy of fuel (H 2) directly into electricity. Fuel cell is like a battery but with constant fuel and oxidant supply. heat _ hydrogen FUELBATTERY CELL DC electricity oxygen + water Fuel cell history Christian Schönbein Invention of fuel cell (1799-1868): Prof. of Phys. and Chem. in Basel, first observation of fuel cell effect (Dec. 1838, Phil. Mag.) William Robert Grove (1811 –1896): Welsh lawyer turned scientist, demonstration of fuel cell (Jan. 1839, Phil. Mag.) 1839 Fuel cell history Invention of fuel cell Scientific curiosity Friedrich Wilhelm Ostwald (1853-1932, Nobel prize 1909): founder of “Physical Chemistry”, provided much of the theoretical understanding of how fuel cells operate 1839 Friedrich Wilhelm Ostwald – visionary ideas: “I do not know whether all of us realise fully what an imperfect thing is the most essential source of power which we are using in our highly developed engineering – the steam engine” Energy conversion in combustion engines limited by Carnot efficiency unacceptable levels of atmospheric pollution vs. Energy conversion in fuel cells direct generation of electricity highly efficient, silent, no pollution Transition would be technical revolution Prediction: practical realization could take a long time! Source: Z. Elektrochemie, Vol. 1, p. 122-125, 1894. Fuel cell history Invention of fuel cell Scientific curiosity Reinvention 1839 1939 Fuel cell history Invention of fuel cell Scientific curiosity Reinvention Industrial curiosity 1839 1939 1960’s Fuel cell history Invention of fuel cell Scientific curiosity Reinvention Industrial curiosity Space program 1839 1939 1960’s PEM Fuel Cells used in Gemini program Apollo program used alkaline fuel cells Space Shuttle uses alkaline fuel cells Renewed interest in PEM fuel cells First PEM Fuel Cell 1960s Gemini Space Program Grubb and Niedrach (GE) Courtesy of UTC Fuel Cells Fuel cell history Invention of fuel cell Scientific curiosity Reinvention Industrial curiosity Space program Entrepreneural phase 1839 1939 1960’s 1990’s Fuel cell history Invention of fuel cell Scientific curiosity Reinvention Industrial curiosity Space program Entrepreneural phase Birth of new industry 1839 1939 1960’s 1990’s Fuel Cell Applications --DemonstrationsDemonstrations Automobiles Buses Scooters Bicycles Golf-carts Distributed power generation Cogeneration Back -up power Portable power Space Airplanes Locomotives Boats Underwater vehicles Why fuel cells Promise of high efficiency Promise of low or zero emissions Run on hydrogen/fuel may be produced from indigenous sources/issue of national security Simple/promise of low cost No moving parts/promise of long life Modular Quiet What is fuel cell? Fuel cell is an electrochemical energy converter. It converts chemical energy of fuel (H 2) directly into electricity. Fuel cell is like a battery but with constant fuel and oxidant supply. electrode hydrogen electrolyteFUEL CELL oxygen electrode water What is fuel cell? Fuel cell is an electrochemical energy converter. It converts chemical energy of fuel (H 2) directly into electricity. electrode hydrogen electrolyte oxygen electrode water Types of fuel cells (based on electrolyte) Alkaline Acid polymer membrane or Polymer Electrolyte Membrane PEM or Proton Exchange Membrane Phosphoric acid Molten carbonate Solid oxide - Zinc/Air fuel cell is not a fuel cell - Direct methanol fuel cell is PEM fuel cell Reactions in fuel cells load e- depleted fuel and depleted oxidant and product gases out product gases out H - O 2 OH 2 AFC 65-220 °C H2O H2O H2 H+ O2 PEMFC 60 -80 °C PAFC 205 °C H2O H2 = O2 (CO) CO 3 MCFC CO 2 650 °C CO 2 H2O (CO) H = O 2 O 2 SOFC (CH ) 600-1000 °C 4 H2O fuel in oxidant in anodeelectrolyte cathode Types of fuel cells (based on electrolyte) Alkaline Acid polymer membrane or Polymer Electrolyte Membrane PEM or Proton Exchange Membrane Phosphoric acid Molten carbonate Solid oxide Why PEM Fuel Cells? Simple Quick start-up Fast response High efficiency High power density (kW/kg and kW/l) Zero emissions PEM Fuel Cell Basic Components porous electrode proton exchange membrane catalyst layer porous electrode Porous electrode structure Gas diffusion layer surface (carbon fiber paper) 80 µµµm PEM Fuel Cell Basic Components porous electrode proton exchange membrane catalyst porous layer electrode MEA – Membrane Electrode Assembly MEA Cross-Section cell frame / bipolar plate current collector / gas diffusion layer Pt-catalyst polymer membrane Pt-catalyst current collector / gas diffusion layer cell frame / bipolar plate PEM Fuel Cell: How does it work? electrode membrane electrode Carbon support Porous Membrane electrode structure Platinum How does it work? external load collector collector electrode membrane electrode plate plate oxygen feed O2 2e - 2H + H → 2H + + 2e - + - 2 1/2O 2 + 2H + 2e → H2O Carbon support Porous Membrane H H O electrode 2 2 structure hydrogen feed Platinum PEM Fuel Cell - Basic Principle external load collector electrode membrane electrode electrode membrane electrode plate oxygen feed oxygen feed O2 O2 - 2e 2e - 2H + 2H + bi-polar collector H → 2H + + 2e – 2 plate + – ½O 2 + 2H + 2e → H2O H2 H2O H2 H2O hydrogen feed hydrogen feed FUEL CELL STACK end plate bus plate bi-polar collector plates coolant in oxidant in hydrogen out anode membrane cathode hydrogen in oxidant + water out coolant out tie rod Actual fuel cell stacks Teledyne Ballard Honda Hydrogenics UTC Fuel Cells Fuel cell needs supporting equipment/balanceequipment/balance--ofof--plantplant load DC/ DC pressure hydrogen tank regulator gorivni članak exhaust compressor pressure regulator heat exchanger/ M humidifier water pump heat exchanger battery fan M M ©2002 by Frano Barbir. Actual fuel cell system pressure regulator DC/DC inverter hydrogen tank heat exchangers compressor bat tery el. motor fuel cell stack water tank humidifier water pump ©2002 by Frano Barbir. Understand the difference between: Single cell Stack load DC/ DC pressure hydrogen tank regulator System gorivni članak exhaust compressor pressure regulator heat exchanger/ M humidifier water pump heat exchanger battery fan M M Fuel Cell Theory --ThermodynamicsThermodynamics Basic reactions Hydrogen side: anode – hydrogen oxidation Oxygen side: cathode – oxygen reduction Mnemonic: red uction of oxygen on cat hode: red-cat – + Anode reaction: H2 →→→ 2e + 2H oxidation – + Cathode reaction: ½O 2 + 2e + 2H →→→ H2O reduction Overall reaction: H2 + ½O 2 →→→ H2O Same as combustion of hydrogen Heat of Formation Combustion of hydrogen is an exothermic reaction – heat is generated H2 + ½O 2 →→→ H2O + heat How much heat? From the tables: Heat of formation: ∆∆∆H = (h f)H2O – (h f)H2 – ½(h f)O2 = – 286 – 0 – 0 = – 286 kJ/mol ∆∆∆ means difference between products and reactants mol is a measure of quantity 1 mol of hydrogen contains 2 grams of hydrogen (2.0159 to be more precise) 1 mol of oxygen contains 32 grams of oxygen 1 mol contains 6.022 x 10 23 molecules (Avogadro’s number) Chemistry can be confusing!!! Don’t forget – fuel cell is an electro chemical device Enthalpy = heat of formation Negative heat means heat is released →→→ exothermic reaction Forget about plus and minus →→→ use common sense Of course that combustion of hydrogen and oxygen produces heat. How much heat? →→→ 286 kJ/mol This enthalpy of hydrogen combustion reaction is also called: Hydrogen Heating Value (or to be more precise Higher Heating Value) obviously – how much heat we can get from hydrogen Important note: all chemical tables refer to 25ºC. That means that both reactants and products are at 25 ºC. Heat of combustion – heating value 1 mol of H O (l) 1 mol of H 2 + ½ mol of O 2 2 18 g of H O (l) 2 g of H 2 + 16 g of O 2 2 T = 25 °C (298.15 K) T = 25 °C (298.15 K) PV = GRT or PV = N RT Heat = 286 kJ Heat of combustion – heating value 1 mol H 2 + x mol O 2 1 mol of H 2O (g) + (x- ½) mol O 2 2 g of H 2 + 32x g of O 2 18 g of H 2O (g) T = 25 °C (298.15 K) T = 25 °C (298.15 K) PV = GRT or PV = N RT Heat = 241 kJ Heat of combustion – heating value Saturation pressure at 25°C: Psat = 3.169 kPa Maximum water content in air(oxygen) - at saturation: mol H 2O/mol O 2 = P H2O /P O2 = 3.169/(101.3 – 3.169) = 0,0323 1 mol of H 2O (g) + (x- ½) mol O 2 x – ½ >>>= 1/0.0323 = 31 18 g of H 2O (g) T = 25 °C (298.15 K) X >>> = 31.5 1 mol H2 + 32 mol O2 Heat = 241 kJ Higher and lower heating value Liquid water produced: 286 kJ/mol Higher heating value Water vapor produced: 241 kJ/mol Lower heating value The difference is: 45 kJ/mol Heat of evaporation 45 kJ/mol = 2.5 kJ/g = 2500 kJ/kg 18 g/mol Why do we care about heating value? There is no combustion in fuel cell! We use heating value as a measure of energy going into the fuel cell: mol/s x kJ/mol = kJ/s = kW Beauty of SI system! In a fuel cell both heat and electricity are generated. But how much electricity? Can all heat be converted into useful work (electricity)? Remember 2 nd law of thermodynamics? Remember entropy? For mechanical processes: W = Q - T∆∆∆S For chemical reactions: ∆∆∆G = ∆∆∆H - T ∆∆∆S ∆∆∆G = Gibbs free energy ∆∆∆ means difference between products and reactants ∆∆∆G = 237.2 kJ/mol (at 25ºC) Theoretical cell voltage Electrical work = charge x voltage Electrical power = current x voltage Mechanical work = distance
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