Despite highly promising characteristics of three-dimensionally (3D) nanostructured catalysts for the oxygen evolution reaction (OER) in polymer electrolyte membrane water electrolyzers (PEMWEs), universal design rules for maximizing their performance have not been explored. Here we show that woodpile (WP)-structured Ir, consisting of 3D-printed, highly-ordered Ir nanowire building blocks, improve OER mass activity markedly. The WP structure secures the electrochemically active surface area (ECSA) through enhanced utilization efficiency of the extended surface area of 3D WP catalysts. Moreover, systematic control of the 3D geometry combined with theoretical calculations and various electrochemical analyses reveals that facile transport of evolved O2 gas bubbles is an important contributor to the improved ECSA-specific activity. The 3D nanostructuring-based improvement of ECSA and ECSA-specific activity enables our well-controlled geometry to afford a 30-fold higher mass activity of the OER catalyst when used in a single-cell PEMWE than conventional nanoparticle-based catalysts.

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Scanning electrochemical microscopy (SECM) is a powerful and suitable tool for visualizing local electrochemical activity as well as topography of surfaces. Due to the development of electrode manufacturing technology, high spatial resolution can be achieved, and various applications are possible by combining SECM with other technologies. Many studies have applied SECM to bioanalytical chemistry at the single‐cell level. The purpose of this review is to introduce various advanced nanoelectrode fabrication techniques and their subsequent applications in bioanalytical research using SECM.