Understanding Interfaces of PEM Electrolyzers with Operando Synchrotron X-Ray Computed Tomography and Radiography

Abstract

Polymer electrolyte membrane (PEM) electrolyzers are electrochemical energy-conversion devices that convert electricity into hydrogen fuel at high efficiencies. The state-of-the-art systems use high IrOx loading (~2-3 mg/cm2), largely due to a lack of stable electrocatalyst supports that can effectively disperse the catalyst particles. Currently, the catalyst layers are made of IrOx and ionomer without an additional support. Without micro-porous layers (MPLs), the rough contact between the porous transport layer (PTL) and catalyst layer presents a challenge. In this study we use operando x-ray computed tomography (CT) and x-ray radiography to visualize operation of PEM electrolyzer under two current densities: 500 and 800 mA/cm2. First, we compare performance of electrolyzer with catalyst-coated membrane (CCM) and that with porous transport electrode (PTE) and correlate polarization curves to morphology of the interfaces observed with x-ray CT. At 1 A/cm2, the micro-CT CCM electrolyzer showed 200 mV improvement in potential primarily due to better contact between the electrocatalyst, membrane, and PTL. From the nano-CT imaging we discovered non-homogeneous distribution of IrOx electrocatalsyt and from the images it is also not clear whether the larger IrOx agglomerates are electrically connected via a percolating conductive network of nanoparticles. The modeling study shows that the primary reason for performance loss in PTE configuration is low connectivity of catalyst particles with membrane. This causes bottlenecks in proton transport and results in high potential losses in anode. We also compared the polarization behavior and morphology of cells with CCMs but two types of PTLs. One was made with sintered Ti and the other with Ti fiber. The Ti fiber PTL showed higher porosity and lower tortuosities, both in-plane and thru-plane. However these better morphological properties did not necessarily translate into lower potentials, as 1 A/cm2 was not a high enough current density to resolve differences attributable to improved two phase flow. This was also confirmed by radiography study, where oxygen residence fraction in the channels showed similar fractions for both types of the PTLs.