Engineering of Bifunctional Nickel Phosphide@Ni-N-C Catalysts for Selective Electroreduction of CO_2-H_2O to Syngas

Electroreduction of CO_2 is one of the most promising CO_2 conversion pathways because of its moderate reaction conditions, controllable product composition, and environment-friendliness. However, most of the current CO_2 electroreduction technologies have not reached the techno-economic threshold for a competitively profitable electrochemical process. Based on a simple two-electron transfer process, the electroreduction of CO_2 to CO, which is further processed into syngas with the reduction of H_2O to H2, is postulated to be the most promising pathway for a profitable electrochemical process. Such a process urgently requires nonprecious electrocatalysts that can precisely control the CO/H2 ratio. Herein, we present a tailored synthesis of bifunctional electrocatalysts with high activity, which can realize the preparation of syngas with controlled compositions via molecular engineering of a ternary nanocomposite. Specifically, a mixture of melamine, triphenylphosphine, and nickel acetate was milled and dissolved in ethanol; the ternary nanocomposite was obtained after rotary evaporation of the mixture. We prepared the catalysts by pyrolyzing the obtained composites at 850 °C for 2 h. The synthesis strategy was facile and easy to scale. The specific surface area and pore volume of the bifunctional electrocatalyst were both significantly enhanced upon increasing the concentration of the phosphorus source, triphenylphosphine, during the precursor preparation. The obtained bifunctional electrocatalysts had hierarchically porous structures, which had well-dispersed active sites and could promote mass transport. Raman spectra revealed higher degrees of disorder with higher P/Ni ratios in the precursor. X-ray photoelectron spectroscopy verified the presence of Ni-Px and Ni-Nx functionalities, which were the active sites for hydrogen evolution and CO_2 reduction, respectively. Hence, the electrocatalytic performance of this series of bifunctional electrocatalysts can be tuned from CO-dominant to H2-dominant. The electrochemical performance was evaluated using a CO_2-saturated 0.5 mol·L~(-1) KHCO_3 aqueous solution at ambient temperature by linear sweep voltammetry and potentiostatic electrolysis. Through these experiments, we determined that the activity of the catalysts was influenced by the surface phosphorus/Ni-Nx site ratio. The highest CO faradaic efficiency (91%) was achieved at-0.8 V (vs a reversible hydrogen electrode, RHE) with Ni-N-C in the absence of Ni-P. The CO/H2 molar ratio in the syngas stream was tunable from 2: 5 to 10: 1 in the potential range from-0.7 to-1.1 V (vs RHE) with a total faradic efficiency of 100%. The syngas composition directly links to the molar ratio of the two integrated components, nickel phosphide and Ni-N-C. Additionally, the stability of the optimized bifunctional electrocatalyst at-0.7 V for 8 h was tested, in which the CO/H2 ratio was maintained between 1.2 and 1.3, indicating excellent stability. This study provides a new perspective for the engineering of bifunctional electrocatalysts for the conversion of abundant CO_2 and water into syngas with tailorable CO/H2 ratios.