Abstract:

Electroreduction of CO₂ is one of the most promising CO₂ conversion pathways because of its moderate reaction conditions, controllable product composition, and environment-friendliness. However, most of the current CO₂ 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 CO₂ to CO, which is further processed into syngas with the reduction of H₂O 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 ℃ 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-P_x and Ni-N_x functionalities, which were the active sites for hydrogen evolution and CO₂ 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 CO₂-saturated 0.5 mol·L^-1 KHCO₃ 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-N_x 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 CO₂ and water into syngas with tailorable CO/H₂ ratios.

Key words: Nonprecious catalyst, CO₂ electroreduction, Syngas, Porous carbon, Nickel phosphide