Direct observations of liquid water formation at nano- and micro-scale in platinum group metal-free electrodes by operando X-ray computed tomography

Platinum group metal (PGM)-free catalyst materials are promising alternatives to Platinum-based catalysts for use in polymer electrolyte fuel cells (PEFCs). The reduced cost and abundancy of the PGM-free catalyst materials can be potentially transformative. Catalytic activity, associated with atomically dispersed transition metal active sites, is intimately linked with the emerging morphology of the carbonaceous graphene-like material at nano-scale. Optimizing the morphology of these PGM-free catalyst layers for enhanced transport properties at the nano and micro-scales is critical for achieving overall performance, expressed as power-density, needed to make such materials technologically attractive. Unfortunately, the current understandings of both morphology and transport processes are very limited due to the novelty of the materials and the challenges in designing operando characterization techniques. In order to bridge these gaps in understanding, we used operando synchrotron X-ray computed tomography to visualize water transport in operating PEFCs. We found that liquid water pooled at the components interfaces and within larger catalyst layer voids. In the smaller macro-pores, ionomer swelling was observed under humidified conditions with nano X-ray computed tomography. These previously unknown insights will provide guidance on electrode and interface design to improve water management and power density of the PEFC with cost-effective PGM-free electrocatalysts.