This study investigated a novel electrocatalyst with a core–shell structure of CoNiP@N,P-C. The unique carbon shell of this catalyst serves a dual purpose: exposing numerous active sites and safeguarding CoNiP nanoparticles from dissolution caused by the electrolyte. As a result, the CoNiP@N,P-C nanoparticles exhibit exceptional electrochemical properties. The CoNiP@N,P-C catalyst displays overpotentials of 234 and 314 mV for the HER and OER, respectively, within a simulated seawater solution (1 M KOH + 0.5 M NaCl), indicating its outstanding catalytic performance. Moreover, when subjected to full seawater splitting, the CoNiP@N,P-C catalyst exhibited high activity, achieving a 1.71 V cell voltage at a current density of 10 mA cm−2. As revealed by density functional theory (DFT) calculations, the CoNiP@N,P-C catalyst exhibits Gibbs free energy for hydrogen adsorption (ΔGH* = −0.19 eV) that is decreased in comparison with CoP@N,P-C, NiP@N,P-C, and N,P-C (−0.321 eV, −0.434 eV, and 0.723 eV, respectively). This finding confirms that the core–shell structure plays a role in enhancing the HER kinetics and improving the catalytic performance, which is consistent with the experimental observations. Consequently, this study introduces the concept of utilizing bimetal phosphide core–shell structures for overall seawater splitting, offering a novel approach in this field of research.
