Degradation of PGM and PGM-free Coatings on PEMWE Porous Transport Layers

A good and long-term stable electrical contact between the porous anode transport layer (PTL) and the adjacent catalyst layer is essential for efficient polymer electrolyte membrane water electrolyzers. This study describes the extensive comparison of seven titanium passivation-protecting coatings using short- and long-term measurements for at least 2000 h. The measurements are supported by before and after scanning electron microscope investigations of cross sections, energy-dispersive X-ray spectroscopy, X-ray diffractometry of the coatings, contact resistance measurements, and ex situ rapid aging tests. Overall, iridium and platinum PTL coatings offer outstanding contact and excellent corrosion protection. Compared to the uncoated reference sample, platinum shows a 93% reduction in the overall degradation rate to 7 μV h^-1 (at a current density of 3 A cm^-2) over 5000 h and even reduces ohmic overvoltages over time in the first 2000 h. Interestingly, the interface to the flow field does not appear to be influenced by precious metal coatings and, hence, does not need to be coated. In contrast, niobium and titanium nitride PTL coatings under investigation do not provide an improvement compared to the uncoated reference but show dissolution and oxidation phenomena, respectively. Titanium hydride produced by hydrochloric acid improves the electrical contact and reduces degradation by 49% overall and 62% in terms of ohmic overvoltages compared to the uncoated reference. It also shows a saturation behavior in degradation with a stable rate of 23 μV h^-1 in the second 1000 h of the measurement. Ex situ rapid aging tests additionally support the main trends. For all surface treatments, more detailed information about the occurring aging mechanisms and reversible overvoltages is obtained by separating the degradation rate into partial rates of the overvoltage mechanisms.