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Mass transport losses ultimately suppress gas evolving electrochemical energy conversion technologies, such as fuel cells and carbon dioxide electrolyzers, from reaching the high current densities needed to realize commercial success. In this work, we reach ultrahigh current densities up to 9 A/cm 2 in a polymer electrolyte membrane (PEM) water electrolyzer with the application of custom porous transport layers (PTLs) with patterned through-pores (PTPs), and we reduce the mass transport overpotentials of the electrolyzer by up to 76.7 %. This dramatic performance improvement stems from the 43.5 % reduction in gas saturation at the catalyst layer-PTL interface region. Moreover, the presence of PTPs leads to more rapid bubble coalescence and subsequently more frequent bubble snap-off (∼3.3 Hz), thereby enhancing the rate of gas removal and liquid water reactant delivery to the reaction sites. This work is highly informative for designing PTLs for optimal gas removal for a wide range of gas evolving electrochemical energy conversion technologies.

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