Double functionalization of N-doped carbon carved hollow nanocubes with mixed metal phosphides as efficient bifunctional catalysts for electrochemical overall water splitting

A double functionalization approach was developed to create efficient bifunctional electrocatalysts for electrochemical overall water splitting. Uniform Ni3[Co(CN)6]2·12H2O nanocubes were taken as the starting material to fabricate the bifunctional catalysts through ammonia carving, conformal polydopamine coating, precursor metal ion coordination, and thermal phosphorization. N-doped carbon carved hollow nanocubes were loaded with hydrogen evolution reaction (HER) active (Ni-Co) phosphides in the interior cyanide-derived carbon layers and oxygen evolution reaction (OER) active (Ni-Fe) phosphides on the exterior polydopaimne-derived carbon layers. The product catalyst exhibited excellent electrolytic performances, achieving overpotentials of 251 and 142 mV for the OER and HER, respectively and a working cell voltage of 1.63 V for the overall water splitting all at 10 mA/cm2. Holes of several tens nanometers in size located at the eight vertices of the hollow cube allow easy access of the electrolyte to and fast rejection of evolving gas bubbles from the interior of the cube. The N-doped carbon nanocubes, providing fast charge transport pathway, and the positive synergistic effects arising from the mixed metal phosphides, significantly enhanced catalytic performances of the product catalyst. Both quantity and quality of the active sites are largely improved with the doubly functionalized N-doped carbon carved hollow nanocubes, which is a superior catalyst design for all electrocatalytic applications.