磷化镍@镍-氮-碳双功能催化剂电还原CO2-H2O制合成气

电催化还原CO₂由于具有温和的反应条件、反应产物组成可调、环境友好等优点，是CO₂转化技术中最有前景的方法之一，然而目前发展的电催化CO₂还原技术仍未达到工业化盈利所需的技术经济指标。因此，通过简单的两电子转移，将CO₂-H₂O电还原为合成气通常被认为是电化学还原CO₂过程中较有前景的实现盈利的途径之一，因此研究能够精确调控合成气比例的非贵金属电催化剂至关重要。在本文中，我们提出了通过三元纳米复合材料的分子工程学设计的进行了高活性、可实现特定比例合成气制备的双功能电催化剂的合成策略。将三聚氰胺、三苯基膦(TPP)和乙酸镍研磨并溶解在乙醇中，通过旋蒸得到三元纳米复合材料，在850 ℃下热解2 h得到催化剂。该方法简单、易操作并且易于放大。该系列双功能电催化剂的比表面积和孔体积均随着三苯基膦量的增加而增加，且分级孔结构有利于提供活性位以及促进物质传输。此外，拉曼图谱表明材料的缺陷度由于前驱体中三苯基膦的增加而增加。另外，X射线光电子能谱验证了析氢反应活性位点Ni-P和CO₂电催化活性为位点Ni-N的存在。因此，该系列电催化剂的性能具有较大的调控空间，可从CO产物主导调控至H₂产物主导。电化学性能通过在CO₂饱和0.5 mol·L^-1 KHCO₃溶液中进行线性扫描伏安法以及恒电位电解进行评价。结果表明催化剂的活性受材料表面P/Ni-N_x比例的影响。最高的CO法拉第效率为91%，由不含磷化镍的纯Ni-N-C材料在-0.8 V (相对于可逆氢电极(RHE))电位下实现。在-0.7至-1.1 V (vs RHE)电位区间内，合成气流中CO/H₂的摩尔比可从2 : 5调整至10 : 1，并能与磷化镍和镍-氮-炭组分的摩尔比关联。此外，我们还对优化的催化剂在-0.7 V电位下的稳定性进行了测试，合成气摩尔比例在8 h内保持在1.2-1.3，说明材料具有良好的稳定性。本工作将温室气体CO₂和H₂O转化成比例可调的合成气，为双功能电催化剂的工程设计提供了新的方向。

Engineering of Bifunctional Nickel Phosphide@Ni-N-C Catalysts for Selective Electroreduction of CO2-H2O to Syngas

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.