Nanoscopic Plugs Block Hydrogen Crossover in Submicron Thick Proton-Conducting SiO ₂ Membranes for Water Electrolysis

Zero-gap electrolyzers based on submicron thick proton-conducting oxide membranes (POMs) represent a promising approach to increasing the efficiency of H₂ production from water electrolysis while moving away from conventional perfluorosulfonic acid (PFSA) membranes. A critical barrier to the commercialization of such electrolyzers is that the ultrathin nature of POMs, which is necessary to achieve low cell resistance, makes them more susceptible to defects that can lead to unacceptably high rates of H₂ crossover. Herein, we demonstrate an approach to mitigate this problem through selective deposition of carbon-containing silicon oxide (SiO_<i>x</i>C_<i>y</i>) "nanoplugs" into the defects of submicron thick SiO₂ membranes using a facile electrochemically mediated deposition process. Selective deposition of nanoplugs within the defects was verified by multiple characterization techniques, while scanning electrochemical microscopy (SECM) was used to confirm selective plugging of H₂-crossover hotspots associated with defects at identical locations. Thanks to the use of nanoplugs, the H₂ permeance of 250 nm thick SiO₂ membranes was reduced by 5 to 6 orders of magnitude compared to the unmodified atomic layer deposition (ALD) SiO₂ membranes while having negligible impact on the ionic resistance of the membrane. These plug-modified membranes also enabled safe and stable operation of a zero-gap full cell electrolysis cell, in contrast to cells lacking nanoplugs that produced anode effluent streams having H₂ concentrations near or exceeding the lower flammability limit (LFL) of H₂. Beyond water electrolysis, this defect-sealing strategy has the potential to be broadly implemented in other applications, such as fuel cells and flow batteries, offering a versatile solution to mitigate crossover-related performance losses.