Polymer electrolyte membrane (PEM) fuel cell technology is gaining recognition as a promising zero-emission energy source for portable electronic products and automotive applications. For it to become commercially viable, its cost per unit power output ratio must be reduced in comparison with other alternative energy sources. To achieve this, a number of performance-limiting parameters need to be addressed.
This thesis investigates the issue of cathode-side water management in PEM fuel cell stacks. It is widely known that a fuel cell stack can only reach optimum performance if its membrane is well-hydrated. During operation, water is formed at the cathode of each cell as a by-product of an electrochemical reaction. Ideally, this water keeps the electrolyte membrane uniformly at the correct level of hydration. Unfortunately, under certain operating conditions, evaporation can exceed the rate of water production, leading to membrane dehydration. Conversely, if the hydration level is too high, flooding may occur, inhibiting the delivery of reactants to the electrodes.
In order to recommend water management strategies for PEM fuel cell stacks, a onedimensional, isothermal, steady-state model is developed and used to calculate relative humidity at the cathode outlet under specified design parameters and operating conditions. To ensure that the electrolyte membrane remains correctly hydrated, it is recommended that relative humidity at the cathode outlet be maintained at 100% under all operating conditions. Model simulation results show that, at high operating temperatures and low reactant pressures, this cannot be achieved without a source of external humidification. For typical “heavy use” operating conditions of 85 °C and 1.2 atm, analysis concludes that 6.74 × 10-4 kg.s-1 of water must be supplied to the reactant stream at the cathode inlet of the fuel cell stack.
With humidification requirements known, an analytical solution is used to assess the suitability of a range of commercially-available shell-and-tube type gas-to-gas membrane humidifiers for fuel cell use. The FC-series humidifiers, manufactured by Perma Pure LLC, are found to have remarkable mass-transfer performance, exceeding the requirements of the fuel cell stack in question.