Characterization of nitrogen gas crossover through the membrane in proton-exchange membrane fuel cells

摘要

Gas crossover phenomenon through a membrane is inevitable in a proton-exchange membrane fuel cell (PEMFC). For nitrogen, the concentration at the cathode side is usually higher than that at the anode side, so N_2 permeates to the anode side. Nitrogen gas crossover (NGC) may cause fuel starvation, if N_2 gas accumulates in the hydrogen recir-culation loop. Thus, it is important to determine the NGC under various PEMFC operating conditions. In this study, characterization of NGC under both open circuit voltage (OCV) and power generation conditions is investigated using a mass spectrometer. Under OCV conditions with the PEMFC membrane fully hydrated, N_2 concentration in the anode exit stream increases as cell temperature increases. Nitrogen permeability coefficients (NPC) are calculated based on the obtained N_2 concentration data. The results show that NPC exhibits an Arrhenius type relationship. Under OCV conditions, the maximum NPC is 5.14 x 10~(-13) mol m~(-1) s~(-1) Pa~(-1) with an N_2 activation energy of 19.83 kj. Under power generation conditions, the NGC increases with increasing current density, which is the result of elevated membrane temperature and increased water content. When the anode stoichiometric ratio (SR_A) is lowered, the N_2 concentration increases under all tested current densities. A low hydrogen flow rate, along with a low SR_A at low current density, significantly increases N_2 concentration at the anode outlet.