This paper reports the performance of hydrogen evolution reaction (HER) electrocatalysts based on Pt thin film electrodes that are encapsulated by silicon oxide (SiO x ) nanomembranes. This membrane-coated electrocatalyst (MCEC) architecture offers a promising approach to enhancing electrocatalyst stability while incorporating advanced catalytic functionalities such as poison resistance and tunable reaction selectivity. Herein, a roomtemperature ultraviolet (UV) ozone synthesis process was used to systematically control the thickness of SiO x overlayers with nanoscale precision and evaluate their influence on the electrochemically active surface area (ECSA) and HER performance of the underlying Pt thin films. Through detailed characterization of the physical and electrochemical properties of the SiO x -encapsulated electrodes, it is shown that proton and H 2 transport occur primarily through the SiO x coating such that the HER takes place at the buried Pt|SiO x interface. Increasing the thickness of the SiO x overlayers results in monotonic increases in the overpotential losses of the MCEC electrodes. These overpotential losses were fit using a one-dimensional diffusion model, from which the H + and H 2 permeabilities through SiO x were obtained. Importantly, the SiO x nanomembranes were found to exhibit high selectivity for proton and H 2 transport in comparison to Cu 2+ , a model HER poison. Leveraging this property, we show that SiO x encapsulation can enable copperresistant operation of Pt HER electrocatalysts. It is expected that a more complete understanding of the structure−property− performance relationships of the SiO x overlayers will enable the design of stable MCECs capable of multifunctional catalysis with minimal loss in efficiency from concentration overpotential losses associated with mass transport through SiO x .

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. References: [1] Shao-Horn, Y.; Sheng, W. C.; Chen, S.; Ferreira, P. J.; Holby, E. F.; Morgan, D. Top. Catal. 2007, 46(3–4), 285. [2] Wang, Y.; Chen, K. S.; Mishler, J.; Cho, S. C.; Adroher, X. C. Appl. Energy 2011, 88(4), 981. [3] Paidar, M.; Fateev, V.; Bouzek, K. Electrochimica Acta. 2016, pp 737–756. [4] Debe, M. K. Nature 2012, 486(7401), 43. [5] Pinaud, B. A.; Benck, J. D.; Seitz, L. C.; Forman, A. J.; Chen, Z.; Deutsch, T. G.; James, B. D.; Baum, K. N.; Baum, G. N.; Ardo, S.; Wang, H.; Miller, E.; Jaramillo, T. F. Energy Environ. Sci. 2013, 6(7), 1983. [6] Ferreira, P. J.; la O’, G. J.; Shao-Horn, Y.; Morgan, D.; Makharia, R.; Kocha, S.; Gasteiger, H. A. J. Electrochem. Soc. 2005, 152(11), A2256.  [7] Pavlišič, A.; Jovanovič, P.; Šelih, V. S.; Šala, M.; Hodnik, N.; Hočevar, S.; Gaberšček, M. Chem. Commun. (Camb). 2014, 50(28), 3732. [8] Takenaka, S.; Miyamoto, H.; Utsunomiya, Y.; Matsune, H.; Kishida, M. J. Phys. Chem. 2014, 118(2), 774. [9] Labrador, N. Y.; Li, X.; Liu, Y.; Tan, H.; Wang, R.; Koberstein, J. T.; Moffat, T. P.; Esposito, D. V. Nano Lett. 2016, 16 (10), 6452.