Mixed Metal Phosphide Chainmail Catalysts Confined in N-Doped Porous Carbon Nanoboxes as Highly Efficient Water-Oxidation Electrocatalysts with Ultralow Overpotentials and Tafel Slopes

Electrocatalytic hydrogen production driven by surplus electric energies is considered a promising sustainable process for hydrogen supply. The high overpotential and low energy-conversion efficiency caused by the slow kinetics of the four-electron transfer oxygen-evolution reaction (OER), however, hamper its competitiveness. Herein, a highly stable, efficient OER catalyst was developed, taking the effects of both composition and nanostructure into account for the catalyst design. N-doped carbon-armored mixed metal phosphide nanoparticles confined in N-doped porous carbon nanoboxes, a particle-in-box nanostructure, were synthesized from monodisperse Ni3[Fe(CN)6]2·H2O nanocubes through sequential conformal polydopamine coating, ammonia etching, and thermal phosphorization. The product exhibited outstanding catalytic abilities for the OER in 1.0 M KOH, delivering 10, 100, and 250 mA/cm2 at ultrasmall overpotentials of 203, 242, and 254 mV, respectively, with an ultrasmall Tafel slope of 38 mV/dec, outperforming most recently reported top-notch iron-group-based OER catalysts. The long-term stability was also excellent, showing a small chronopotentiometric decay of 2.5% over a 24 h operation at 50 mA/cm2. The enhanced catalytic efficiency and stability may be attributable to the unique particle-in-box structure as a nanoreactor offering a local, fast reaction environment, the conductive N-doped porous carbon shell for fast charge and mass transport, the synergistic effect between multicomponent metal phosphides for enhanced intrinsic activities, and the carbon protection layer to prevent/delay the catalyst core from being deactivated. This combined particle-in-box and chainmail design concept for electrocatalysts is unique and advantageous and may be readily applied to the general field of heterogeneous reactions.