1 Fuel Cells ME 252 Thermal-Fluid Systems G. Kallio

2 Introduction Fuel cells directly convert chemical energy into electricity Battery: fixed quantity of fuel used as a energy source or storage device (may be recharged) Fuel Cell: continuous supply of fuel and outflow of products, used solely as an energy source (no storage or recharging function) concept first demonstrated 1950s - developed as a useful device

3 Characteristics of Fuel Cells Oxidation-Reduction reaction: No thermal-to-mechanical energy conversion; therefore, operation is not limited by Second Law or Carnot thermal efficiency

4 Applications of Fuel Cells Stationary power –Large systems: hospitals, schools, hotels, office buildings, airports –Residential Transportation –Major auto manufacturers –Trains, buses,boats, planes, scooters, bicycles Portable power –Small electronics, power tools Landfill/Wastewater Treatment –Methane  H 2  electricity

5 Characteristics of Fuel Cells, cont. Fuel cell power plants produce very low levels of environmental pollution Pollution estimated to be a factor of 10 less than fossil-fuel plants with best available pollution control Hydrogen fuel can be extracted from fossil-fuels by reforming Off-peak electricity can be “stored” by electrolysis, generating H 2 and O 2 that are recombined by fuel cells during peak hours

6 Characteristics of Fuel Cells, cont. Fuel cells are built from a large number of unit cells, forming a stack Electrode characteristics –catalytic: platinum or sintered nickel used to accelerate ion formation –high porosity: allows fuel and electrolyte to penetrate, presenting large surface area for catalytic reaction –conductive: allow efficient electron migration to terminal Electrolyte - provide high rate of ion transport between anode to cathode

7 The H 2 -O 2 Fuel Cell Overall reaction: H O 2  H 2 O Acidic electrolyte reactions H 2  2H + + 2e - (anode) 0.5O 2 + 2H + + 2e -  H 2 O (cathode) Alkaline electrolyte reactions H 2 + 2OH -  2H e - (anode) H 2 O + 0.5O 2 + 2e -  2OH - (cathode)

8 Fuel Cell Thermodynamics Recall energy balance for chemical reaction: Theoretically, all of the energy due to enthalpy change could be converted into electrical energy if the process were adiabatic, i.e.,

9 Fuel Cell Thermodynamics, cont. However, some of the energy is converted to heat in an oxidation reaction; the minimum amount of heat generated is the reversible heat transfer: –if the reaction is isothermal, then –and so the “real” maximum work is

10 Fuel Cell Thermodynamics, cont. The quantity, h - Ts, is a thermo- dynamic property known as the specific Gibbs free energy, g The maximum fuel cell conversion efficiency is defined as

11 Maximum Efficiency for the H 2 -O 2 Fuel Cell Assuming 25°C isothermal conditions, Actual fuel cells have efficiencies of 40-60% due to various losses and inefficiencies

12 Fuel Cell Electrical Inefficiencies Ohmic polarization - internal resistance to motion of electrons through electrodes and ions through electrolyte Concentration polarization - mass transfer rate losses related to diffusion through porous electrodes and solubility of reactants/products Activation polarization - activation energy barriers in the oxidation- reduction reaction

13 Fuel Cell Performance Trade-offs Theoretical conversion efficiency of fuel cells increases with decreasing temperature Low temperature fuel cells require expensive catalysts to maintain high reaction rates High temperature fuel cells can achieve high reaction rates w/o catalysts but typically have reduced lifetimes due to corrosion and other effects Cogeneration or a combined cycle can improve the overall efficiency of high temperature fuel cells

14 Fuel Cell Types Classified by electrolytes: –PEFC and AFC: space/military applications with limited life –PAFC and MCFC: large-scale, commercial power plants –SOFC: high-temp, non-stack geometry offers less expensive design and combined cycle operation

15 Fuel Cell Types: New Developments Proton Exchange Membrane (PEM): low- temperature (80°C), platinum-coated, plastic membrane electrode and solid organic polymer electrolyte; high power density, suitable for light-duty vehicles Direct Methanol Fuel Cells (DMFC): similar to PEM where H 2 is drawn directly from liquid methanol which eliminates reformer; operates at °C; suitable for small applications such as cell phones, laptops, and military electronics. Protonic Ceramic Fuel Cell (PCFC): high- temperature (700°C), high-protonic conductivity ceramic electrolyte cell; hydrocarbon fuel directly oxidizes at anode to produce H 2, eliminating need for reformer.

16 Hydrogen Fuel Reforming Natural gas partial oxidation reaction: CH 4 + aO 2  bH 2 + cCO +dCO 2 + eH 2 O + fC Shift reaction: CO + H 2 O  CO 2 + H 2