Poison-Resistant Electrocatalysis Enabled By Encapsulation of Platinum with Silicon Oxide Nanomembranes

Abstract

Improvement in the efficiency and lifetime of electrochemical technologies such electrolyzers and photoelectrochemical cells are critically important for the realization of storable chemical fuels.[1-5] Typically, the device lifetime relies on the durability of electrocatalyst nanoparticles. In an effort to reduce costs and catalyst loading, catalyst particle sizes are kept small (< 5 nm). However, decreasing particle size often leads to increased rates degradation mechanisms that generally reduce the electrochemically active surface area (ECSA).[1,6,7] To prevent this degradation, previous studies have encapsulated the active electrocatalyst material with an ultrathin, permeable or porous silica layer.[8,9] These encapsulated electrocatalysts exhibited greatly enhanced stability compared to silica-free electrocatalysts while still maintaining high electrochemical activity. It was hypothesized that transport of reactant and product species occurred through the silica layers, although direct evidence and detailed understanding of transport through the silica was lacking. This understanding is complicated by the complex electrode geometries studied in both reports, where the silica coatings had varying thickness and did not uniformly coat the electroactive Pt. This study removes this complexity by investigating well-defined Pt thin film electrodes that are encapsulated with silicon oxide (SiOx) nanomembranes. By systematically changing the SiOx thickness and evaluating hydrogen evolution reaction (HER) performance, we seek to gain deeper understanding of the structure-property relationships that affect the transport properties through SiOx nanomembranes. This membrane coated electrocatalyst (MCEC) architecture provides a promising approach to enhance electrocatalyst stability, improve poison resistance, and/or tune reaction selectivity. We use a room-temperature UV ozone synthesis process to systematically control the thickness of SiOx overlayers with nanoscale precision and evaluate the effects on the ECSA and HER performance of the underlying Pt thin films. Through detailed characterization of the SiOx overlayers this study shows that proton and H2 transport occur primarily through the SiOx coating. Notably, the SiOx nano-membranes exhibit high selectivity for proton and H2 transport compared to a HER poison species such as copper ions. These results demonstrate that MCECs are capable of multifunctional catalysis with poisoning resistance, still a more complete understanding of the structure-property-performance relationships will enable design improvements to further minimize efficiency losses due to mass-transport overpotential losses.