Single Ni Atoms and Clusters Embedded in N-Doped Carbon "Tubes on Fibers" Matrix with Bifunctional Activity for Water Splitting at High Current Densities.

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Among the bifunctional catalysts for water splitting, recently emerged transition-metal single-atom catalysts are theoretically considered to possess high potential, while the experimental activity is not satisfactory yet. Herein, an exceptionally efficient trifunctional metal-nitrogen-carbon (M-N-C) catalyst electrode, composed of a hierarchical carbon matrix embedding isolated nickel atoms with nickel-iron (NiFe) clusters, is presented. 1D microfibers and nanotubes grow sequentially from 2D nanosheets as sacrificial templates via two stages of solution- and solid-phase reactions to form a 1D hierarchy. Exceptionally efficient bifunctional activity with an overpotential of only 13 mV at 10 mA cm-2 toward hydrogen evolution reaction (HER) and an overpotential of 210 mV at 30 mA cm-2 toward oxygen evolution reaction (OER) is obtained, surpassing each monofunctional activity ever reported. More importantly, an overpotential of only 126 and 326 mV is required to drive 500 mA cm-2 toward the HER and OER, respectively. For the first time, industrial-scale water splitting with two bifunctional catalyst electrodes with a current density of 500 mA cm-2 at a potential of 1.71 V is demonstrated. Lastly, trifunctional catalytic activity including oxygen reduction reaction is also proven with a half-wave potential at 0.848 V.