Electrochemically Assisted Deposition of Nanoscopic SiO₂ Plugs Reduces Hydrogen Crossover in Sub-Micron Thick Proton-Conducting Oxide Membranes

Production of green H₂ via polymer electrolyte membrane (PEM) electrolyzers (~ $3.4 - $12 per kg) remains significantly expensive compared to the blue H₂ produced by steam methane reforming process (~ $1 - $3 per kg). Per- and polyfluoroalkyl substances (PFAS) membranes also known as nafion membranes (nafion-117 is ~178 µm-thick) are crucial for polymer electrolyte membrane (PEM) electrolyzers, because they provide high stability and proton (H⁺) conductivity (0.08 − 0.12 S cm⁻¹ at 50 °C).¹ However, PFAS materials have significant environmental impact and are listed as forever chemicals. Alternatively, a sub-micron or an ultrathin SiO₂ based proton-conducting oxide membranes (POM) can be deposited on electrodes encapsulating the catalytic interface using techniques like atomic layer deposition (ALD) and spin-coating.² Sub-micron POM are 2 to 4 orders thinner in magnitude compared to conventional nafion membranes. This enables a drastic decrease in membrane resistance, which allows for electrolyzer operations at much higher current densities and consequently with high efficiency.^1,2 However, occurrence of nanoscopic defects in the form of pinholes or cracks on the sub-micron thick POM can lead to undesirable permeation of H₂ across the membrane, which is also a safety issue. Unfortunately, submicron thick SiO₂ based POM fabrication are prone to formation of these defects.³ In this work, we demonstrate electrochemically assisted deposition of nanoscopic SiO₂ plugs into the defects of atomic layer deposition (ALD) sub-micron thick (250 nm) SiO₂ POM overlayer. The reported process effectively plugs the pinholes and cracks that are acting as highways of H₂ crossover. Selective deposition of plugs at defective locations is verified by Scanning electron microscopy (SEM) images, Raman spectroscopy, Energy-dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). It was further electro-analytically supported by SECM measurements at identical defect locations before and after administering nanoscopic plugs. ALD SiO₂ Membranes modified with SiO₂ plugs decrease H₂ permeation by &gt; 99% compared to unmodified membranes. Additionally, electrochemical impedance spectroscopy (EIS) measurements showed that the “plugged” membranes still possess the desirable H⁺ conductivity (&gt; 0.12 S cm^-1) and electronic resistivity (~120 mΩ cm^-2).<br/><b>References:</b><br/>1. Cohen, L. A., Weimer, M. S., Yim, K., Jin, J., Alvarez, D. V. F., Dameron, A. A., Capuano, C. B., Ouimet, R. J., Fortiner, S., Esposito, D. V. (2024) How Low Can You Go? Nanoscale Membranes for Efficient Water Electrolysis. <i>ACS Energy Letters</i>, 9 (4), 1624-1632.<br/>2. Beatty, M. E., Gillette, E. I., Haley, A. T., Esposito, D. V. (2020) Controlling the Relative Fluxes of Protons and Oxygen to Electrocatalytic Buried Interfaces with Tunable Silicon Oxide Overlayers. <i>ACS Applied Energy Materials</i>, 3, 12338-12350.<br/>3. Stinson, W. D. H., Brayton, K. M., Ardo, S., Talin, A. A., Esposito, D. V. (2022) Quantifying the Influence of Defects on Selectivity of Electrodes Encapsulated by Nanoscopic Silicon Oxide Overlayers. <i>ACS Applied Materials & Interfaces</i>, 14 (50), 55480-55490.