面向电催化CO2还原的双金属氮掺杂碳材料研究

为了缓解大气中过高二氧化碳（CO₂）浓度诱发的全球变暖等问题，实现CO₂的净零排放，急需开展以风电、光电等为主要来源的可再生能源高效转化和CO₂资源化利用过程研究。CO₂资源化利用技术中的电化学还原方法因其能在温和条件下生产高附加值燃料及工业化学品的优势而受到关注。然而，产物选择性低、稳定性差、过电位较高等问题限制了技术应用。开发制备工艺简单和成本低廉的高性能非贵金属催化剂是解决上述问题的一种合适策略。本论文以金属有机骨架（MOFs）衍生多孔碳基材料为研究对象，制备了具有高稳定性特征的双金属氮掺杂碳催化剂，通过对照实验和系统的结构表征等手段初步探究材料中催化活性位点的作用机制。主要内容如下：

通过溶剂热法制备了前驱体Co/Ni-ZIF-8（ZIF 沸石咪唑酯骨架），并采用高温金属替代策略得到多孔碳材料Co/Ni-N-C。探究双金属掺杂对ZIF-8前驱体成型的干扰及材料电催化性能。结果显示Co/Ni-N-C相对可逆氢电极（vs. RHE）在−1.0 V至−1.2 V高还原电势区间保持CO/H₂比在7:1附近，并于−1.2 V（vs. RHE）取得最高电流密度（23 mA cm⁻²），经10 h极化后电流密度和产物法拉第效率无明显衰减。这项工作验证了引入Co和Ni双金属对提升氮掺杂碳ECO₂RR稳定性和催化活性存在积极作用。

以3-巯基丙酸为S供体，使用金属乙酸盐和2-甲基咪唑在水溶液中一步合成类MOF配位的双金属掺杂多层片状前驱体Co/Ni-ZIF-8-S。通过高温处理得到具备CoNi合金纳米颗粒和原子级Co和Ni分散位点的Co/Ni-N-S-C催化剂，探究其电催化性能和体系中S的作用机制。Co/Ni-N-S-C在−0.5 V至−1.1 V（vs. RHE）宽电势范围内能基本保持产物CO/H₂比为1:2，在−0.9 V（vs. RHE）持续极化9 h后仍能保持25 mA cm⁻²的电流密度且产物法拉第效率基本不变。电催化过程金属-S的脱落将形成空穴结构并使体系中吡啶N的含量上升。这项工作为理解双金属N、S共掺杂碳结构中S的作用提供补充。

本文提供了一种构建高稳定性的碳基催化剂新途径，有助于加深对N和S双非金属共掺杂碳材料活性位点的认识，为后续高性能双金属氮掺杂碳催化剂的开发提供借鉴。

In order to alleviate environmental problems (e.g., global warming) caused by excessive carbon dioxide (CO₂) concentrations in the atmosphere and achieve net-zero CO₂ emissions, it is an urgent need for research on the efficient conversion and utilization of CO₂ from the renewable energy sources, such as wind power and photovoltaics. At present, the electrochemical reduction method in CO₂ resource utilization technology has attracted attention because of its advantages in producing high-value-added fuels and industrial chemicals under mild conditions. However, some problems such as low product selectivity, poor stability, and high overpotential limit the application of the technology. Designing an easy preparation and inexpensive non-noble metal catalyst with high catalytic activity and high stability is a suitable strategy to solve the above problems. In this dissertation, two highly stable bimetallic nitrogen-doped carbon catalysts were prepared by metal organic framework (MOFs) as the precursors, and the mechanism of catalytic active sites in the materials was preliminarily explored through control experiments and systematic structural characterization. The main works are as follows:

The precursor Co/Ni-ZIF-8 was prepared by solvothermal method. By high temperature metal substitution strategy, the porous carbon material Co/Ni-N-C was successfully obtained. Then, the effects of bimetallic doping on the precursor molding process and electrochemical CO₂ reduction performance were studied. The results showed that the bimetallic doped Co/Ni-N-C maintained the ratio of CO/H₂ fluctuating around 7 in a high reduction potential window of −1.0 V to −1.2 V (vs. RHE), and achieved a current density up to 23 mA cm⁻² at −1.2 V (vs. RHE). Besides, the current density and product Faraday efficiency were not significantly declined after 10 h polarization process. This work verifies the positive effect of doping Co and Ni on improving the stability and catalytic activity of nitrogen-doped carbon for ECO₂RR, and provides reference for developing subsequent high-performance bimetallic nitrogen-doped carbon catalysts.

With 3-mercaptopropionic acid as the S donor agent, MOF-coordinated bimetallic doped multilayer sheet precursor Co/Ni-ZIF-8-S was synthesized in one step by using metal acetate and 2-methylimidazole in aqueous solution. Through high temperature calcination, the material named as Co/Ni-N-S-C with CoNi alloy nanoparticles and atomic-level Co and Ni dispersion sites was obtained. Then, the electrocatalytic properties of Co/Ni-N-S-C and the mechanism of S in the system were explored. Co/Ni-N-S-C basically maintained the ratio of CO/H₂ as 1:2 in the wide potential range of −0.5 V to −1.1 V (vs. RHE), and maintained the Faraday efficiency of the product and the current density of 25 mA cm⁻² after continuous polarization at −0.9 V (vs. RHE) for 9 h. The hollow structure generated by the detachment of the metal-S bond in the electrocatalytic process increases the content of pyridine N in the catalyst. This work complements the understanding of the role of S in bimetallic N and S co-doped carbon structures and provides a new way to build high-stability carbon-based catalysts.

This dissertation provides a new way to construct a highly stable carbon-based catalyst, which is helpful to deepen the understanding of the active sites of N and S bimetallic co-doped carbon materials, and provide reference for the subsequent development of high-performance bimetallic nitrogen-doped carbon catalysts.

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