Experimental assessment and analysis of mass transport limiting current density in water vapor-fed polymer electrolyte membrane electrolyzers

Polymer electrolyte membrane water electrolyzers (PEMWEs) are a critical technology for efficient hydrogen production to decarbonize fuels and industrial feedstocks. To make hydrogen cost-effective, the overpotentials across the cell need to be decreased and platinum-group metal loading reduced. One overpotential that needs to be better understood is due to mass transport limitations from bubble formation within the porous transport layer (PTL) and anode catalyst layer (ACL), which can lead to a reduction in performance at typical operating current densities. When operating at ultra-high current densities (UHCD), the rate of the OER may reach a critical point at which oxygen gas bubbles fill the pores of the ACL and PTL, completely blocking access of liquid water to the ACL. Because of this, there is a possibility that the cell will rely on water vapor diffusion through the evolving oxygen gas to deliver the water reactant to the OER catalyst. To assess the operational limitation of a PEMWE while relying on water vapor diffusion, a commercially manufactured membrane electrode assembly (MEA) was tested by flowing water vapor with an inert carrier gas into the anode as the reactant. To identify a limiting current density (i_lim) of the electrolyzer under these conditions, potentiostatic polarization curves were obtained for a range of relative humidity (RH) and backpressures. The RH was varied to assess the impact of reactant concentration on the catalyst mass activity at low current and on the i_lim, while the backpressure was varied to isolate the impact of the molecular gas diffusion coefficient on the i_lim. Our findings highlight that water vapor diffusion through evolved oxygen is readily able to support the OER without notable mass transport overpotentials. However, our results show that water vapor feed inhibits high current density through reduced catalyst specific activity and polymer electrolyte membrane dry-out.