Interfacial analysis of a PEM electrolyzer using X-ray computed tomography†
Polymer electrolyte membrane (PEM) electrolyzers are electrochemical energy-conversion devices that convert electricity into hydrogen fuel at high efficiencies. The interface between the porous transport layer (PTL) and catalyst layer is of interest from a transport perspective, as this interface is rough, reducing contact between the layers. Electrons, protons, water and oxygen have to simultaneously meet at this interface and there is a need to understand the optimal morphology. In this study we use operando X-ray computed tomography (CT) and X-ray radiography to visualize the operation of PEM electrolyzers under two current densities: 500 and 800 mA cm−2. First, we compare the performance of catalyst-coated membrane (CCM) electrolyzers with porous transport electrode (PTE) electrolyzers by correlating polarization curves with interface morphology observed with X-ray CT. At 1 A cm−2, 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 an IrOx electrocatalyst. The modeling study shows that the primary reason for performance loss in the PTE configuration is due to the low connectivity of catalyst particles with the membrane. This causes bottlenecks in proton transport and results in high ionic potential losses in the anode. Then, we compared the polarization behavior and morphology of cells with CCMs but two types of PTLs. One was made with sintered titanium and the other with titanium fiber. The Ti fiber PTL showed higher porosity and lower tortuosities, however these better morphological properties did not necessarily translate into significantly lower potentials (45 mV difference), as the electrolyzer was not operated above 1 A cm−2. This was also confirmed by radiography study, where oxygen residence time in the channels showed similar fractions for both types of PTLs.