NH4Cl辅助热解制备镍-氮-碳纳米管催化剂及其电还原CO2性能

二氧化碳（CO₂）的资源化利用是实现“碳达峰，碳中和”的重要手段。在众多CO₂转化技术当中，电催化CO₂还原反应因反应条件温和、工艺过程简单等优点，被认为是极具应用前景的减碳技术之一，其关键在于高效、高稳定性电催化剂的开发。过渡金属-氮-碳（M-N-C）材料是电还原CO₂生成CO的有效催化剂，针对其高温热解制备过程中活性金属原子容易聚集且氮原子流失严重，进而使得活性位密度降低，催化性能下降等问题，本文提出以双氰胺（DCDA）为碳源和氮源，以乙酰丙酮镍（Ni（acac）₂）为金属源，以氯化铵（NH₄Cl）为第二氮源和造孔剂，采用简单的NH₄Cl辅助热解-酸刻蚀的方法制备得到镍-氮-碳纳米管（Ni-N-CNTs）电还原CO₂催化剂，并详细考察NH₄Cl添加量对催化剂结构和催化性能的影响。表征结果表明：NH₄Cl的加入有利于催化剂纳米管状形貌和多级孔结构的生成，同时有利于催化剂中Ni-Nx （1.6%，摩尔分数）和pyridinic-N （1.75%，摩尔分数）物种含量的增加。一系列性能测试结果表明：催化剂的活性中心为Ni-Nx，同时pyridinic-N的存在也有利于催化性能的提高，当前体中NH₄Cl加入量与氮源和金属源总质量比为1∶1时，所得Ni-N-CNTs-1催化剂催化性能最好，在电压为-0.65 V （vs RHE）时，CO法拉第效率最高达92%，此时CO部分电流密度为8 mA·cm^-2。此外，该催化剂还表现出良好的催化稳定性，连续恒电位电解12 h，催化性能基本不变。该催化剂制备工艺简单，制备条件可控，研究结果可为高效M-N-C电还原CO₂催化剂的设计和制备提供一种切实有效的研究思路和方法。

NH4Cl assisted preparation of Ni-N-CNTs catalyst and its performance for electrochemical CO2 reduction

The resource utilization of carbon dioxide (CO₂) is an important means to achieve “carbon peak, carbon neutrality”. Among various CO₂ conversion technologies, electrochemical CO₂ reduction is considered to be one of the most promising carbon reduction technologies due to its mild reaction conditions and simple process. The key lies in the development of high-efficiency and high-stability electrocatalysts. Transition metal-nitrogen-carbon (M-N-C) material is an effective catalyst for electrochemical CO₂ reduction to CO. However, the inevitable aggregation of active metal atoms and the serious N loss during the pyrolysis preparation usually lead to the reduction of active sites density and then the degradation of catalytic performance. Herein, this paper proposes the construction of Ni-N-carbon nanotubes (Ni-N-CNTs) catalysts for electrochemical CO₂ reduction via NH₄Cl-assisted pyrolysis and associated acid leaching method. The catalysts are prepared by using dicyandiamide as carbon and nitrogen sources, the nickel acetylacetonate as metal source and the NH₄Cl as additional nitrogen source and pore-forming agent. The influence of NH₄Cl addition on the structure and catalytic performance of catalysts is investigated in detail. The characterization results show that the addition of NH₄Cl is beneficial to the formation of nanotube-like morphology and hierarchical pore structure of the catalyst. At the same time, it is beneficial to increase the content of Ni-Nx (1.6%, mole fraction) and pyridinic-N (1.75%, mole fraction) species in the catalyst. A series of catalytic performance results indicate that the catalyst active site is the atomic Ni-Nx, and the presence of pyridinic-N species also contribute to an enhanced catalytic performance. When the mass ratio of NH₄Cl to the total mass of nitrogen and metal sources is 1∶1, the derived Ni-N-CNTs-1 catalyst exhibits the best catalytic performance. The CO Faradaic efficiency is as high as 92% with a large CO partial current density of 8 mA·cm^-2 at a low potential of -0.65 V (vs RHE). In addition, this catalyst also shows good operation stability for 12 h without obvious performance decay. The catalyst preparation process is simple and the preparation conditions are controllable. This work will provide a practical and feasible approach to build highly efficient M-N-C catalyst for CO₂ electroreduction.