Evaluation and improvement of oxygen reduction kinetics in proton exchange membrane fuel cells

Research was conducted to investigate and improve oxygen reduction kinetics in PEM fuel cells through understanding the effects of temperature and relative humidity (RH) on the Tafel slope, reaction order, and catalytic activity. The operating conditions ranged from 60 to 120°C in cell temperature and 20 to 100% in RH. ^ The effects of elevated temperature and reduced RH on the oxygen reduction kinetic parameters were studied. Tafel slopes were found to be higher than 2.303RT/F at low RH. The kinetic reaction order with respect to oxygen partial pressure was less than one at low RH. The change of these kinetic parameters with the RH was found to be associated with the rate-determining step of oxygen reduction pathways. ^ Polarization losses including cell resistance loss, oxygen diffusion loss, and cathode activation loss at elevated temperature and low RH were analyzed. The catalytic activity was found to increase with an increase in RH from 0-60∼70%RH; there was no apparent RH effect on the catalytic activity when the RH was above 70%. Dependence of catalytic activity on the RH was explained by changed proton activity, oxygen permeability, and platinum surface conditions with the RH. ^ Ionomers with equivalent weight (EW) lower than 1100 and Pt-Co/C catalysts were introduced to electrodes to improve cell performance. Lower EW ionomer (800EW) was found to improve the cell performance more (45mV) at low RH than at high RH (15mV) whereas Pt-Co/C improved the cell performance more (55mV) at high RH than at low RH (17mV). The decay of 36mV in cell voltage was observed in a 450-hour cell durability test and the catalyst degradation was found to be a major factor for this voltage loss. ^ The effect of anode pressurization on the cell resistance and performance using an ambient pressure cathode was investigated. The results show that high RH from anode pressurization at both 120°C and 150°C can decrease the membrane resistance and therefore increase the cell voltage. The results presented here provide a concept for the application of a coupled steam reformer and PEM fuel cell system.