A Discussion of Fuel Cells with particular reference to Direct Methanol Fuel Cells (DMFC’s) Outline Fuel Cell Definition Principle of operation Components: cell, stack, system Types Fuel-oxidant combinations Performance E fficiencies Applications Issue: methanol as a high-purity cost-effective “direct” fuel cell feed - specifications versus current commercial standards - “benchmark” a distillation-based purification technology Direct Methanol Fuel Cell (DMFC) Effect of Methanol impurities on cell performance

R H 2 → 2e - + 2H + + 2e - → H 2 O 2H + + ½O 2 AnodeCathode Proton flow Membrane Principle of Fuel Cell Operation Consider a fuel cell reaction in which the fuel-oxidant combination is hydrogen (H 2 ) and oxygen (O 2 ) - the reversal of water electrolysis – in a solid polymer membrane-partitioned cell Electrodes Cell potentials Electrolyte Electrocatalysis Electrical charge transfer Key factors governing the operation of a fuel cell Fuel cells are steady-state Galvanic reactors to which reactants are continuously supplied and from which products are continuously withdrawn

Fuel Cell Components Flow field plate and gas porous anode substrate Bipolarity: the substrate layer may be linked to adjacent cells Electrolyte: materials, structures and thickness balance high conductivity against low porosity Thin gas porous catalyst layer - good ionic contact with the electrolyte is essential electrons Ohmic losses occur during transport of electrons and ions Stack components Bipolar plates Membrane Exchange Assembly (MEA) Current collector plates End plates Key design concerns: Mass transfer effects Heat management

Fuel CellAcronym Temp. range (°C) Anode Reaction (1) Cathode Reaction (1) AlkalineAFC60 – 90 Solid Polymer SPFC, PEMFC (2 70 – 90 Phosphoric acid PAFC~220 Molten Carbonate MCFC~650 Solid OxideSOFC~1000 Types of Fuel Cells defined by: a) electrolyte, as this defines chemical environment; and, b) by temperature of operation (1) The charge carrier in the case of each of the fuel cell types is shown in bold letters. (2) Proton Exchange Membrane Fuel Cell

Fuel – Oxidant Combinations Oxidant: Oxygen from air for economic reasons Fuels Hydrogen: generated from fuels such as natural gas, propane, methanol, petrochemicals - typically reformed gas contains approximately 80% hydrogen, 20% CO 2 in high temperature cells, internal steam reforming of (for example) methane and methanol can take place by the injection of the fuel with steam storage technologies: gas cylinders; cryogenic liquid, metal hydride matrix “renewable” hydrogen from water electrolysis the demand for hydrogen purity decreases with increasing operating temperature Methanol: reforming takes place at 250°C “direct” feed to the cell in water mixture

Fuel Cell Performance Energy generation by electrochemical reaction : dW e = - Vdq = - V[nΓdε] Reversible potential - maximum cell potential : E o rev = ΔG o /nΓ for hydrogen oxidation E o rev = 1.23 v the equilibrium oxidation and reduction rates of reaction at the electrode defines the exchange current density – a strong measure of the facility of the overall electrochemistry Overpotential = f(T, exchange current density) Heat generation = f(overpotential) E 0 mf Voltage Current VcE 0 mf - V Characteristic Performance Curve kinetic effects slope reflects ohmic resistance mass transfer effects = overpotential

Fuel CellTemp. °C Pressure atm (kPa) Current density A/cm 2 Voltage V Alkaline701 (101) Phosphoric acid 1901 (101) Phosphoric acid 2058 (808) Molten carbonate 6501 (101) Solid oxide10001 (101) Fuel Cell performance A high performance cell: 1 Acm -2 at 1 Volt potential (1 Wcm -2 power density)

Thermal Energy Conversion Mechanical Energy Conversion Chemical Energy of the FuelsElectrical Energy Conversion Electrochemical reaction Heat Engine: Power Generating Fuel Cell Efficiency efficiency at a given current density: E = 0.675V H 2 /O 2 cell: theoretical maximum thermodynamic efficiency: Eth = 83% at an open-cell voltage of 1 Volt ( let us say ), the max. electrochemical efficiency is 80% corresponding to an open-circuit fuel-cell efficiency of approximately 65% The theoretical maximum thermodynamic efficiency of a heat engine is: E carnot = 1 – T L /T H The Carnot cycle must draw its energy from a heat source at 1480°K in order to match the theoretical maximum thermal efficiency of the H 2 /O 2 fuel cell

Fuel CellFuelElectrolyte Electric Efficiency (system) (%) Power Range and Application AlkalinePure H 2 35 – 50% KOH < 5 kW military, space Proton Exchange Membrane Pure H 2 Methanol (e.g.,) NAFION® –250 kW portable, CHP, transportation Phosphoric acidPure H 2 Concentrated phosphoric acid kW CHP Molten carbonate H 2, CO, CH 4, other hydrocarbons Lithium and potassium carbonate > kW-MW CHP, grid- independent power Solid oxideH 2, CO, CH 4, other hydrocarbons Yttrium- stabilized zirconium dioxide > 50 2 kW – MW CHP, grid- independent power Currently Developed Types of Fuel Cells - after Gregor Hoogers, (ed.,) Fuel Cell Technology Handbook, CRC Press, 2002 CHP: combined heat and power generation More Power for less Fuel

Applications Smart Fuel Cell A Portable market: recreation, remote industrial ~12 V 1.5 L Methanol/ KWh 2.5 L plastic container Siemens-Westinghouse Stationary Power Generation Unit

Direct Methanol Fuel Cell (DMFC) Potential benefits Liquid fuel - high energy density/unit volume Current distribution network No need for fuel reforming Technological Limitations Poor electrode kinetics - anode andcathode Mass transport effects - CO 2 and water rejection Methanol crossover Anode: dilute methanol/water feed CO2 rejection Pt-based catalyst system PEM membrane

carbon monoxidewt %, max methanewt %, max acetone + aldehydeswt %, max acetonewt %, max ethanolwt %, max aciditywt %, max0.003 waterwt %, max Methanol Purity Requirements ASTM Fuel Cell (ppm) Published allowable impurity limits in commodity methanol not directly applicable CO as an inert adsorbate on Pt surface - at 10 ppm reduces H 2 /PEMFC cell voltage by 50% at 0.5 Acm -2 CO 2 effect is modest compared with CO ethanol and aldehydes are electrochemical fuels

Methanol as a Direct Feed to Fuel Cells - Issues What is the commercial value of ultra-pure liquid methanol in direct methanol electro-oxidation? Can the ultra-pure methanol be produced at commodity prices - without necessarily having the benefit of economy of scale - using distillation as the primary purification technology? This project serves to establish an important technological and economic “benchmark”: - the “distillation + recycle” case What is the relationship between purity and energy requirement? Is there a need and opportunity to make some of the energy versus buying all of the requirement? (Are there special storage requirements for ultra-pure methanol?)