3D ternary NixCo2−xP/C nanoflower/nanourchin arrays grown on HCNs: a highly efficient bi-functional electrocatalyst for boosting hydrogen production via the urea electro-oxidation reaction

Abstract

Over the last few years, substantial efforts have been made to develop earth-abundant bi-functional catalysts for urea oxidation and energy-saving electrolytic hydrogen production due to their low cost and the potential to replace traditional noble-metal-based catalysts. Nevertheless, finding a straightforward and effective route to prepare efficient catalysts with unique structural features and optimal supports still is a big challenge. Among the various candidates, metal–organic framework (MOF)-derived materials show great advantages as new kinds of active non-precious catalysts. On the other hand, the controllable integration of MOFs and carbon-based nanomaterials leads to further enhancement in terms of the stability and electrical conductivity of catalysts. In this communication, we develop an MOF-carbon-based composite to synthesize a transition metal phosphide (TMP) catalyst for the electrocatalytic oxidation of urea. First, poly(pyrrole-co-aniline) (PPCA) hollow nanospheres were fabricated via the in situ emulsion polymerization of a mixture of aniline and pyrrole in the presence of Triton X-100. Then, the simple carbonization treatment of these PPCA hollow spheres led to the carbonized hollow carbon nanospheres (HCNs) with ultrahigh surface areas and uniform nano-morphologies. After that, bimetallic MM′/MOFs (M/M′ = Ni, Co) were uniformly grown around the HCNs via a simple hydrothermal reaction (NiCo/MOF@HCNs). During the synthesis process, by adjusting Ni/Co ratios, the MOF morphology can be engineered so that by reducing the Ni/Co ratio, the flower-like structures change into urchin-like structures. Finally, this NiCo/MOF@HCNs precursor with different Ni/Co ratios during the in situ carbonization/phosphorization was chemically converted into Ni–Co mixed-metal phosphides (Ni_xCo_2−xP/C@HCNs). Finally, the electrocatalytic activity of the prepared catalysts was tested for the urea oxidation reaction (UOR) and hydrogen evolution reaction (HER).