Buildup of nitrogen in direct hydrogen polymer-electrolyte fuel cell stacks

Hydrogen-fueled polymer-electrolyte fuel cell stacks (PEFC) operate at less than 100% fuel utilization per pass, with the anode exit gas being recycled to the anode inlet. Any inert gases present in the anode gas then increase in concentration as the hydrogen is consumed. A portion of the recirculating anode gas is purged to prevent excessive buildup of the inert gases. It has been observed that N2 diffuses across the polymer-electrolyte membrane from the cathode side to the anode side, adding to the inert gases in the anode channels. This paper discusses the results of a study to model and analyze the buildup of N2 in the recirculating anode gas, and the impact of this N2 on the performance of an automotive 90 kWe PEFC stack. Results show that N2 crossover from the cathode air to the anode gas depends on a number of parameters, including the power level, N2 concentration in feed hydrogen (if any), purge rate, and membrane thickness. The buildup of N2 is mainly a function of the degree of purge, defined as the average fraction of the anode exit gas that is vented. Even with pure fuel H2 and 90% hydrogen consumption per pass, N2 concentrations can reach 50–70% at low purge rates, and 5–20% at a 2% purge rate. As a result of this N2, the cell voltage decreases by 10–18 mV if the N2 concentration in anode channels is allowed to reach 25–60%, but by <5 mV if the N2 concentration is limited to 2–25% by purging. There is an optimum level of purge for which the overall degradation in cell performance is the smallest. The optimum purge level is about 2% with pure H2 feed, but increases to about 9% if the fuel hydrogen contains 2% N2. The allowable level of N2 impurity in the fuel gas depends on the acceptable loss in stack efficiency. For a 25 μm thick membrane, 0.08% N2 in feed can be tolerated if the acceptable loss in efficiency is 0.1 percentage point, 0.5% N2 in the H2 for a 0.5 percentage point loss in efficiency, and 1.5% N2 in the H2 if a 1 percentage point loss in system efficiency is acceptable.