Cobalt Phosphide Composite Encapsulated within N,P-Doped Carbon Nanotubes for Synergistic Oxygen Evolution.

Exploring highly efficient and stable oxygen evolution reaction (OER) electrocatalysts such as transition-metal phosphides (TMPs) is critical to advancing renewable hydrogen fuel. TMP nanostructures typically involving binary or ternary TMPs tuned by cation or anion doping are suggested to be promising low-cost and durable OER catalysts. Herein, the preparation of CoP/CoP2 composite nanoparticles encapsulated within N,P-doped carbon nanotubes (CoP/CoP2 @NPCNTs) is demonstrated as a synergistic electrocatalyst for OER via the calcination of a CoAl-layered double hydroxide/melamine mixture and subsequent phosphorization. Facile visualization by scanning electron microscopy in conjunction with electron backscatter diffraction demonstrates the encapsulation of the CoP/CoP2 nanoparticles within the N,P-codoped CNTs. Electrocatalytic evaluation shows that the composite electrode requires a low overpotential of 300 mV for the OER at 10 mA cm-2 in a 1.0 m KOH solution and, in particular, exhibits an excellent long-term durability of ≈100 h, which is superior to that of the state-of-the-art RuO2 electrocatalyst. Density functional theory calculations reveal that the synergistic effect of CoP and CoP2 can enhance the electrocatalytic performance. In addition, molecular dynamics simulations demonstrate that the generated O2 molecules can readily diffuse out of the CNTs. Both the effects give rise to the observed OER enhancement.