Modeling Water Management in Polymer-Electrolyte Fuel Cells
Fuel cells may become the energy-delivery devices of the 21st century with realization of a carbon-neutral energy economy. Although there are many types of fuel cells, polymerelectrolyte fuel cells (PEFCs) are receiving the most attention for automotive and small stationary applications. In a PEFC, hydrogen and oxygen are combined electrochemically to produce water, electricity, and waste heat. During the operation of a PEFC, many interrelated and complex phenomena occur. These processes include mass and heat transfer, electrochemical reactions, and ionic and electronic transport. Most of these processes occur in the through-plane direction in what we term the PEFC sandwich as shown in Figure 1. This sandwich comprises multiple layers including diffusion media that can be composite structures containing a macroporous gas-diffusion layer (GDL) and microporous layer (MPL), catalyst layers (CLs), flow fields or bipolar plates, and a membrane. During operation fuel is fed into the anode flow field, moves through the diffusion medium, and reacts electrochemically at the anode CL to form hydrogen ions and electrons. The oxidant, usually oxygen in air, is fed into the cathode flow field, moves through the diffusion medium, and is electrochemically reduced at the cathode CL by combination with the generated protons and electrons. The water, either liquid or vapor, produced by the reduction of oxygen at the cathode exits the PEFC through either the cathode or anode flow field. The electrons generated at the anode pass through an external circuit and may be used to perform work before they are consumed at the cathode. The performance of a PEFC is most often reported in the form of a polarization curve, as shown in Figure 2. Roughly speaking, the polarization curve can be broken down into various regions. First, it should be noted that the equilibrium potential differs from the open-circuit voltage due mainly to hydrogen crossover through the membrane (i.e., a mixed potential on the cathode) and the resulting effects of the kinetic reactions. Next, at low currents, the behavior of a PEFC is dominated by kinetic losses. These losses mainly stem from the high overpotential of the oxygen-reduction reaction (ORR). As the current is increased, ohmic losses become a factor in lowering the overall cell potential. These ohmic losses are mainly from ionic losses in the electrodes and separator. At high currents, mass-transport limitations become increasingly important. These losses are due to reactants not being able to reach the electrocatalytic sites. Key among the issues facing PEFCs today is water management. Due to their low operating temperature (< 100 C), water exists in both liquid and vapor phases. Furthermore, state-of-the-art membranes require the use of water to provide high conductivity and fast proton transport. Thus, there is a tradeoff between having enough water for proton conduction (ohmic losses), but not too much or else the buildup of liquid water will cause a situation in which the reactant-gas-transport pathways are flooded (mass-transfer limitations). Figure 3 displays experimental evidence of the effects of water management on performance. In Figure 3(a), a neutron image of water content displays flooding near the outlet of the cell due to accumulation of liquid water and a decrease in the gas flowrates. The serpentine flow field is clearly visible with the water mainly underneath the ribs. Figure 3(b) shows polarization performance at 0.4 and 0.8 V and high-frequency resistance at 0.8 V as a function of cathode humidification temperature. At low current densities, as the inlet air becomes more humid, the membrane resistance decreases, and the performance increases. At higher current densities, the same effect occurs; however, the higher temperatures and more humid air also results in a lower inlet oxygen partial pressure.
- Research Organization:
- Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
- Sponsoring Organization:
- Environmental Energy Technologies Division
- DOE Contract Number:
- DE-AC02-05CH11231
- OSTI ID:
- 929427
- Report Number(s):
- LBNL-316E; TRN: US200814%%153
- Country of Publication:
- United States
- Language:
- English
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