One-step, integrated fabrication of Co₂P nanoparticles encapsulated N, P dual-doped CNTs for highly advanced total water splitting

Highlights: • One-step strategy to synthesize phase pure Co₂P encapsulated N,P dual-doped carbon nanotubes (denoted as Co₂P/CNT). • Co₂P/CNT acts as a highly efficient bifunctional electrocatalyst for full water splitting. • Overall activity is either superior or comparable to recent bifunctional catalysts reported in the literature. A one-step/one-pot strategy to synthesize phase pure Co₂P nanoparticles encapsulated N, P dual-doped carbon nanotubes (denoted as Co₂P/CNT) is developed. The method is free of toxic, pyrophoric alkylphosphine as the phosphorus source, does not involve the use of sophisticated instrumentation or expensive precursors and may be extended to other transition-metal phosphides. When the as prepared Co₂P/CNTs are applied as an anode for OER in 1 M KOH, a current density of 10 mA/cm² is achieved at an overpotential of 292 mV which is 36 mV less than that required for the state-of-art OER catalyst RuO₂ with a small Tafel slope of ∼68 mV/decade. While applied as a cathode towards HER, Co₂P/CNTs exhibit a current density of 10 mA/cm² at an overpotential of 132 mV with a Tafel slope of 103 mV/dec that compares favourably with the state-of-the art HER catalyst, Pt/C. After 15 h of continuous electrolysis for both HER and OER, the electrode material preserves its structure along with its robust catalytic activity which points out to their excellent stability. A total alkaline water electrolyzer constructed by employing Co₂P/CNT as catalyst on both anode and cathode delivered a current density of 10 mA/cm² at around 1.53 V over an extended operational period rivalling the state-of-art combination of Pt/C and RuO₂ and is among the best of the bi-functional total-water splitting electrocatalysts reported till date. This remarkable performance of Co₂P/CNTs can be attributed to the intrinsic catalytic activity of Co₂P nanoparticles fortified with heteroatom doped few layered graphene which results in enhanced electrical conductivity besides providing long-term stability.