Critical flow rate of anode fuel exhaust in a PEM fuel cell system

A manual purge line was added into the exterior fuel exhaust stream of a Ballard PEM stack in a Nexa™ power module. With the addition of manual exhaust purge, high levels of inert gases were intentionally added to the anode feed without changing normal operational procedures. A new method of determining the critical minimum flow rate in the anode exhaust stream was given by an anode mass balance. This type of operation makes dual use of membranes in the MEAs as both gas purifiers and as solid electrolytes. The PEM stack was successfully operated with up to ca. 7% nitrogen or carbon dioxide in the absence of a palladium-based hydrogen separator at ca. 200 W power level. Nitrogen in the anode stream was concentrated from 7.5% to 91.6%. The system maintained a fuel efficiency of 99% at a manual purge rate of 2.22 ml s−1 and no auto purge. The fuel cell stack efficiency was 64% and the stack output efficiency was 75%. The overall system efficiency was 39%. After troublesome CO and H2S poisons were removed, a hydrocarbon reformate containing high levels of CO2 and H2O was further used in the Nexa™ stack. The size and complexity of the fuel processing system may be reduced at a specified power level by using this operational method.