CO2的电化学还原被视为解决碳排放、达到碳中和的理想途径之一。为了实现其应用，寻找高活性的电催化材料以提高还原效率至关重要。碳基单原子催化剂（Single-Atom Catalyst，SAC）因其高活性、高选择性、高原子利用率等优点被认为是最具潜力的CO2还原催化剂之一。碳基SAC明确的活性中心使得基于密度泛函理论的计算方法能够在催化剂的本征性质与催化活性之间建立联系，从而在催化剂研究中发挥重要作用。然而碳基SAC在电催化过程中的实际行为不仅由其本征性质决定，其电子结构特点也导致极易受到溶液、电势等反应环境因素的影响。本论文旨在讨论碳基SAC在真实反应条件下催化CO2还原的反应机理，并探讨了催化剂的本征性质和环境因素对反应活性与选择性的共同作用。首先，本论文讨论了在反应条件下，对M-N4-G（M = Fe，Co，Ni）催化剂的石墨烯载体进行掺杂对催化剂活性的影响。研究发现，掺杂可以通过改变M-N4-G催化剂的功函数来影响其在反应电势下的电荷量，从而影响反应决速步的质子转移。针对酸性条件下CO2吸附成为决速步的情况，论文基于分子轨道理论提出了氮掺杂对CO2吸附能的两种影响机制，适用于不同的M-N4-G催化剂。针对M-N4-G催化剂上CO2还原反应与析氢反应的竞争关系，本文通过巨正则热力学推导证明：优势反应的转变电势与催化剂的量子电容正相关、与双电层电容负相关，为选择性转变的根源提供了新的理解。为了研究分子类SAC在反应电势下的催化行为，本论文建立了一套半经验方法，以解决电子数随电势离散变化的问题、精确计算分子类SAC催化CO2二电子还原过程中所有反应步骤的自由能变化。基于该方法，论文对酞菁钴催化CO2二电子还原的反应机理进行了定量阐释，并讨论了对酞菁钴的改性方法。最后，论文还研究了碳基SAC在催化CO2二电子还原和多电子还原之间的选择性问题。通过对溶液环境的分子动力学模拟，发现了一种存在于*CO的C原子和水分子的H原子之间的新型氢键，并阐明了其形成机制。该氢键只出现在以酞菁钴为催化剂的系统中，对*CO的加氢过程有着极大的促进作用，是理解酞菁钴具有独特的CO2多电子还原选择性的关键。

Electrochemical reduction of CO2 is an ideal solution for the carbon emission problem and achieving carbon neutrality. To effectively apply this method, identifying highly active electrocatalytic materials is crucial for enhancing the reduction efficiency. carbon-based Single-Atom Catalyst (SAC) is considered to be one of the most promising catalysts for CO2 reduction due to its high activity, selectivity and atom utilization. The precisely defined activity centers of carbon-based SACs enable the establishment of a connection between the intrinsic properties of the catalysts and their catalytic activity, making computational studies based on density functional theories instrumental in catalyst research. Nevertheless, the electrocatalytic performance of carbon-based SACs is not solely dictated by their intrinsic properties. The electronic structure characteristics of these SACs make them highly susceptible to environmental factors such as solvation effect and applied potential. In this study, we delve into the reaction mechanism of carbon-based SACs catalyzing CO2 reduction under operational conditions, exploring the combined influence of intrinsic catalyst properties and environmental factors on catalyst activity and selectivity.We first discussed the role of doping in the graphene support of M-N4-G (M = Fe, Co, Ni) catalysts on the catalyst performance under reaction conditions. Our investigation revealed that doping can induce changes in the work functions of M-N4-G catalysts, consequently influencing their charge state at the working potential. This alteration, in turn, affects the proton affinity of adsorbates on these catalysts. This discovery introduces a new approach to enhance the proton transfer process in CO2 reduction. In instances where CO2 adsorption becomes a reaction-determining step in acidic environments, we also proposed two mechanisms elucidating the influence of nitrogen doping on CO2 adsorption energy. These mechanisms, grounded in molecular orbital theory, offer insights applicable to diverse M-N4-G catalysts.Furthermore, we investigated the competition between CO2 reduction and hydrogen precipitation reactions on M-N4-G catalysts. We demonstrate by giant canonical thermodynamic derivation that the transition potential for the dominant reaction is positively correlated with the quantum capacitance of the catalyst and negatively correlated with the electrical double layer capacitance, providing a new understanding of the origin of the selectivity transition.In order to study the catalytic behaviors of molecular SACs under working potentials, we developed a semi-empirical protocol to address the problem of discrete changes in the number of electrons with the potential. This protocol features its accuracy in computing the free energy changes associated with all reaction steps in the two-electron reduction of CO2. Leveraging this protocol, we quantitatively elucidated the reaction mechanism involved in the cobalt-phthalocyanine-catalyzed two-electron CO2 reduction and explored strategies for the modification of cobalt phthalocyanine to enhance its reactivity.Finally, our investigation delved into the selectivity of carbon-based SACs in catalyzing the multi-electron reduction versus two-electron reduction of CO2. Based on AIMD simulations of the solution environment, we unveiled a novel hydrogen bond forming between the carbon atom of *CO and the hydrogen atom of the water molecule. This hydrogen bond, observed exclusively in systems with cobalt phthalocyanine as the catalyst, significantly facilitates the hydrogenation process of *CO, playing a pivotal role in understanding the distinctive multi-electron reduction selectivity of CO2 by cobalt phthalocyanine.