Understanding water management in platinum group metal-free electrodes using neutron imaging.

Platinum group metal-free (PGM-free) catalysts are a low-cost alternative to expensive PGM catalysts for polymer electrolyte fuel cells. However, due to the low volumetric activity of PGM-free catalysts, the catalyst layer thickness of the PGM-free catalyst electrode is an order of magnitude higher than PGM based electrodes. The thick PGM-free electrodes suffer from increased transport resistance and poor water management, which ultimately limits the fuel cell performance. This manuscript presents the study of water management in the PGM-free electrodes to understand the transport limitations and improve fuel cell performance. In-operando neutron imaging is performed to estimate the water content in different components across the fuel cell thickness. Water saturation in thick PGM electrodes, with similar catalyst layer thickness to PGM-free electrodes, is lower than in the PGM-free electrodes irrespective of the operating conditions, due to high water retention by PGM-free catalysts. Improvements in fuel cell performance are accomplished by enhancing water removal from the flooded PGM-free electrode in three ways: (i) enhanced water removal with a novel microporous layer with hydrophilic pathways incorporated through hydrophilic additives, (ii) water removal through anode via novel GDL in the anode, and (iii) lower water saturation in PGM-free electrode structures with increased catalyst porosity.

Effects of Cathode Corrosion on Through-Plane Water Transport in Proton Exchange Membrane Fuel Cells.

The corrosion of carbon in the cathodes of proton-exchange-membrane fuel cells leads to electrode collapse, reduced active catalyst area, and increased surface hydrophilicity. While these effects have been linked to performance degradation over cell lifetime, the role of corrosion in the evolving water balance has not been clear. In this study, neutron imaging was used to evaluate the through-plane water distribution within several cells over the course of accelerated stress testing using potential holds and square-wave cycling. A dramatic decrease in water retention was observed in each cell after the cathode was severely corroded. The increasing hydrophilic effect of carbon surface oxidation (quantified by ex situ X-ray photoelectron spectroscopy) was overwhelmed by the drying effect of increased internal heat generation. To evaluate this mechanism, the various observed electrode changes are included in a multiphase, non-isothermal one-dimensional cell model, and the simulated alterations to cell performance and water content are compared with those observed experimentally. Simulation results are consistent with the idea that collapse and compaction of the catalyst layer is the dominant limitation to cell performance and not the lower amounts of active Pt surface area, and that higher temperature gradients result in drying out of the cell.