High-temperature proton exchange membrane fuel cells (HT-PEMFCs) based on phosphoric-acid-doped polybenzimidazole (PBI) membranes operate at 140–180 °C without external humidification, yet product water and any inlet humidity drive a substantial diffusive flux of water across the membrane electrode assembly (MEA). The source study (Bezmalinović et al., 2014) measured a membrane water permeability coefficient of 2.4×10−132.4\times10^{-13} mol s−1^{-1} cm−1^{-1} Pa−1^{-1} at 160 °C for a commercial Celtec-P 1000 stack — more than an order of magnitude above earlier PBI values — while its 2D model needed a four-times-larger value to match in-operando data, overestimated water transport at low current density, and underestimated it at high current density. The open problem is to establish a validated, mechanism-resolved quantitative description of water transport across the PBI/H3_3PO4_4 MEA: the dependence of the water transport (permeability) coefficient on relative humidity and temperature (in particular at higher dew points, where a slight increase with cathode humidification at 140 °C could not be distinguished from experimental uncertainty), the possible contribution of electro-osmotic drag for high acid-doping membranes, possible variation of the coefficient along the flow channel, and the extension of the validated description to counterflow or dead-ended-anode operation in combination with reformate feed for system water management.

Background

A proton exchange membrane fuel cell (PEMFC) converts hydrogen and oxygen to electricity with a polymer electrolyte conducting protons between electrodes. In high-temperature PEMFCs (HT-PEMFCs), the electrolyte is a polybenzimidazole (PBI) membrane — an aromatic basic polymer — doped with phosphoric acid (H3_3PO4_4); proton conduction proceeds through the acid via Grotthuss hopping and vehicle mechanisms at 140–180 °C, so no external humidification of the reactant gases is required. The membrane electrode assembly (MEA) is the membrane sandwiched between catalyst layers and porous gas diffusion layers (GDLs). Water is produced at the cathode by the oxygen reduction reaction; because water vapor dissolves in the hygroscopic phosphoric acid and its partial pressure generally differs between anode and cathode channels, water diffuses through the MEA — predominantly from cathode to anode at the inlet end, with the transport direction able to reverse along a long channel. This matters for system design: water accumulating at the anode dilutes hydrogen (critical for dead-ended anodes, where the anode outlet is closed except for periodic purges, and for operation on reformate gas, the hydrogen-rich but diluted mixture produced by reforming a fuel such as methanol or natural gas). The water transport coefficient (or permeability coefficient) characterizes this flux as proportional to the water vapor partial-pressure difference across the MEA; electro-osmotic drag (EOD) — water molecules carried from anode to cathode by migrating protons — is a second transport mechanism whose coefficient is the number of water molecules dragged per proton. For PBI membranes with low acid doping levels (about 5 H3_3PO4_4 per polymer repeat unit), EOD was measured to be essentially negligible (Weng et al., 1996; Li et al., 2001), and most HT-PEMFC water models accordingly include only Fickian diffusion. Before the source study, the only in-operando quantification (Galbiati et al., 2012, on a Celtec-P 2100 MEA) found up to 18% of product water at the anode with dry feeds, and ex-situ permeability coefficients for non-commercial PBI membranes were on the order of 10−1410^{-14}–10−1510^{-15} mol s−1^{-1} cm−1^{-1} Pa−1^{-1} (Weng et al., 1996; Daletou et al., 2009). Bezmalinović et al. (2014) measured water transport in a 12-cell stack with Celtec-P 1000 MEAs at 160 °C: net cathode-to-anode flux rose with current density (0.1–0.7 A cm−2^{-2}), fell with cathode stoichiometry, and reversed sign under strong anode humidification (dew point 45 °C). From ex-situ tests with dry and humidified nitrogen they obtained a membrane permeability of 2.4×10−132.4\times10^{-13} mol s−1^{-1} cm−1^{-1} Pa−1^{-1} (membrane contributing >95% of MEA resistance), while a 2D along-channel CFD model had to use 9.6×10−139.6\times10^{-13} to reproduce the operating data and still overestimated water transport at low current densities and underestimated it at high ones. At 140 °C the ex-situ coefficient rose slightly with cathode humidification, but the differences were too small to claim a trend; they proposed that the coefficient depends on relative humidity and could be higher than the ex-situ value under water-rich operating conditions, that unmodeled EOD (possibly non-negligible for the higher doping level of Celtec-P 1000) or a permeability coefficient that is not constant along the channel could explain the model bias, and that counterflow or dead-ended-anode operation effects on water transport in combination with reformate composition remained to be investigated.

Problem Statement

Establish a validated, mechanism-resolved quantitative description of water vapor transport across the membrane electrode assembly of phosphoric-acid-doped polybenzimidazole high-temperature PEM fuel cells, in the sense left unresolved by Bezmalinović et al. (2014) for commercial Celtec-P 1000 MEAs. Concretely: (1) determine the MEA/membrane water transport (permeability) coefficient and its dependence on water vapor partial pressure (relative humidity) and temperature — in particular whether, at dew points higher than the ~32–50 °C range accessible in the source experiments, the coefficient rises systematically (as hinted by the slight increase with cathode humidification observed at 140 °C) and whether it indeed exceeds the ex-situ value of 2.4×10−132.4\times10^{-13} mol s−1^{-1} cm−1^{-1} Pa−1^{-1} under operating conditions, with uncertainty bounds that distinguish a trend from experimental scatter; (2) determine whether a nonzero electro-osmotic drag of water for high acid-doping membranes and/or a permeability coefficient that varies along the flow channel are required to explain the systematic overestimation of water transport at low current densities and underestimation at high current densities obtained with constant-coefficient diffusion models; and (3) use the validated transport description to quantify water transport under counterflow or dead-ended-anode operation in combination with different reactant compositions (e.g. reformed hydrogen), so that water accumulation, hydrogen dilution, and purge requirements can be predicted for system water management. Answers of type (1) or (2) alone constitute partial progress; the complete objective is the combination of (1), (2), and (3) — a transport model whose coefficients are measured functions of humidity (and, if needed, position) and which reproduces measured water transport across current densities, stoichiometries, and humidification levels, and predicts water transport for counterflow, dead-ended-anode and reformate operation.

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