Microporous Layer-Supported Catalyst-Coated Membrane for PEM Water Electrolysis
Authors: Hansen Kentaro Uzuka , Jiao Feng 
Published: December 2024

Innovative approaches are necessary for thrifting iridium (in the form of IrO2) from PEM water electrolysis (PEMWE) anodes without compromising catalyst utilization. Adding a microporous layer (MPL) to the anode can realize high performance with low IrO2 loadings (<0.1 mg cm-2) by facilitating the efficient conduction of electrons from catalyst particles to the current collector.
As an alternative to fabricating a monolithic, sintered MPL on the porous transport layer (PTL), we demonstrate how a loose-particle microporous layer (MPL) can be fabricated directly on the CCM via decal transfer with a single added step of particle ink deposition on a PTFE decal. Benchmarking tests confirm that when low-loaded IrO2 catalyst layers (0.05-0.2 mg cm-2) are supported with 0.4 mg cm-2 platinum black (PtB) in a bilayer configuration, kinetic and mass transport performance at parity or better than unsupported 1.0 mg cm-2 IrO2 anodes can be achieved for PEMWE.
First, an idealized in-plane catalyst utilization model is developed to estimate the requisite sheet conductivity for the MPL to minimize electronic conduction losses assuming in-plane electronic resistance is dominant. Then, several materials were screened for sufficient conductivity as loose-particle MPLs on a fluorinated ethylene propylene sheet under realistic confinement conditions using the Van der Pauw method using a gasket with conductive edges. Due to non-ideal airbrushing and agglomerates, the actual amount of PtB required to achieve requisite conductivity is roughly four times higher than that expected assuming sheet conductivity is proportional to the areal mass loading.
An initial formulation for the MPL particle ink using an ionomer binder was then reformulated to use an insulating polymer binder (PVDF) to avoid a reversible >1 Ω cm2 contact resistance present under anodic conditions at current densities as low as 0.01 A cm-2. Since tests used pure water and the contact resistance vanishes under cathodic conditions, it is likely that the loose PtB MPL reversibly oxidizes near the onset of the oxygen evolution reaction due to the ionically conductive binder.
Then, MPL-supported CCMs (0.4 mg cm-2 PtB, PVDF binder) were benchmarked against bare CCMs operating as a PEMWE cell at 80C with pure water fed to the anode. A five-electrode method was utilized to extract informative anode electrodynamics such as catalyst layer resistance, charge transfer resistance, and conduction resistance via impedance spectroscopy to enhance the data quality of the benchmark tests. Finally, the stability and reusability of the MPL-supported CCM was checked by cycling between a galvanostatic hold at 4 A cm-2 and a short series of impedance measurements at 0.1 A cm-2. Break-in effects were observed for the membrane and cathode (directly measured) while anode-specific hardware and MPL issues such as the passivation of the titanium endplate and MPL delamination were observed.
The results of these experiments for the flexible MPL-supported CCM for PEMWE along with the generalizability of the Van der Pauw method, and five-electrode method for other membrane-based cells will be discussed.

Figure 1