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.