Unraveling the effect of anode catalyst layer thickness on the performance of proton-exchange membrane water electrolyzers by numerical modeling

Abstract

Lowering the Iridium (Ir) loading in the anode catalyst layer (CL) while preserving high performance and durability plays a pivotal role for promoting an affordable adoption of proton-exchange membrane water electrolyzers (PEMWEs). A reduction of the Ir loading is typically accompanied by a decrease of the CL thickness, thus complicating the analysis of the effects originated from each one. Modeling offers a tool to isolate phenomena and perform extensive parametric analyses that would be difficult to accomplish experimentally. In this work, a 3D multiphase, non-isothermal model is presented to systematically analyze the effect of anode CL thickness as a function of key performance variables: (𝑖) electrochemical specific surface area, (𝑖𝑖) ionomer sorption/desorption rate, (𝑖𝑖𝑖) membrane thickness, and (𝑖𝑣) anode PTL/CL interfacial pore size. The results show that there is an optimal anode CL thickness around 5 − 20 μm, regardless of the values adopted for other variables. The optimum is reached at relatively low anode kinetic overpotentials, moderate ionic voltage losses, and low electrical and mass transport voltage losses. Ensuring a relatively thick anode CL is particularly important to reduce the possibility of anode dry-out by electro-osmotic drag, and mitigate the adverse effect of in-plane electrical voltage losses and gas accumulation at the porous transport layer (PTL) interface. The performance at low Ir loading, while preserving CL thickness, can be boosted by maximizing the electrochemical surface area per unit of catalyst mass and/or the exchange current density, enhancing the ionomer water sorption rate, reducing the membrane thickness, and decreasing the PTL/CL pore size.