Fast fabrication of self-supported porous nickel phosphide foam for efficient, durable oxygen evolution and overall water splitting

Self-supported three-dimensional porous nickel phosphide (Ni-P) foam has been fabricated by exposing commercially available Ni foam in phosphorus vapor at an elevated temperature for a short period of time. The as-fabricated Ni-P foam consists of Ni2P skeletons covered with vertically aligned Ni5P4-NiP2 nanosheets. When used as a self-supported anode to catalyze the oxygen evolution reaction (OER), it exhibits exceptionally high catalytic current density (191.0 mA cm(-2) at an overpotential of eta = 0.35 V) and outstanding long-term stability and durability (affording 10 mA cm(-2) at 1.45 V vs. RHE for 26 h without degradation). Scanning electron microscopy, transmission electron microscopy, and X-ray diffractometry analyses show that during the galvanostatic OER electrolysis the surface Ni-P nanosheets are transformed to nickel oxide/hydroxide (NiO/Ni(OH)(x)), forming a Ni-P/NiO(Ni(OH)(x)) heterojunction on ligament surfaces of the porous Ni-P foam which would enhance the OER performance. The synergistic effect between Ni-P and NiO/Ni(OH)(x) is also confirmed by control experiments with self-supported NiO and Ni(OH)(2) nanosheet electrodes. An alkaline electrolyzer has been built using two identical self-supported porous Ni-P foams as the anode and cathode, respectively. The electrolyzer exhibits superior electrolysis efficiency of 90.2% at 10 mA cm(-2), and can maintain sufficiently high efficiency of 72.2% even at 100 mA cm(-2). Moreover, the electrolyzer is substantially durable when working at 10 and 20 mA cm(-2), splitting water constantly up to 1000 h with only a minor variation in the cell's voltage. The self-supported porous Ni-P foam holds substantial promise for use as both cathode and anode in industrial alkaline water electrolyzers.