Layer‐by‐Layer Assembly‐Based Electrocatalytic Fibril Electrodes Enabling Extremely Low Overpotentials and Stable Operation at 1 A cm −2

For the practical use of water electrolyzers using non-noble metal catalysts, it is crucial to minimize the overpotentials for the hydrogen and oxygen evolution reactions. Here, cotton-based, highly porous electrocatalytic electrodes are introduced with extremely low overpotentials and fast reaction kinetics using metal nanoparticle assembly-driven electroplating. Hydrophobic metal nanoparticles are layer-by-layer assembled with small-molecule linkers onto cotton fibrils to form the conductive seeds for effective electroplating of non-noble metal electrocatalysts. This approach converts insulating cottons to highly electrocatalytic textiles while maintaining their intrinsic 3D porous structure with extremely large surface area without metal agglomerations. To prepare hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) electrodes, Ni is first electroplated onto the conductive cotton textile (HER electrode), and NiFe is subsequently electroplated onto the Ni–electroplated textile (OER electrode). The resulting HER and OER electrodes exhibit remarkably low overpotentials of 12 mV at 10 mA cm−2 and 214 mV at 50 mA cm−2, respectively. The two-electrode water electrolyzer exhibits a current density of 10 mA cm−2 at a low cell voltage of 1.39 V. Additionally, the operational stability of the device is well maintained even at an extremely high current density of 1 A cm−2 for at least 100 h. © 2021 Wiley-VCH GmbH1