Catalysis Theory Center, Department of Physics, Technical University of Denmark, 2800 Kgs. Lyngby, Denmark.
Acc Chem Res. 2022 Feb 15;55(4):495-503. doi: 10.1021/acs.accounts.1c00679. Epub 2022 Feb 2.
Electrochemical CO reduction (eCOR) enables the conversion of waste CO to high-value fuels and commodity chemicals powered by renewable electricity, thereby offering a viable strategy for reaching the goal of net-zero carbon emissions. Research in the past few decades has focused both on the optimization of the catalyst (electrode) and the electrolyte environment. Surface-area normalized current densities show that the latter can affect the CO reduction activity by up to a few orders of magnitude.In this Account, we review theories of the mechanisms behind the effects of the electrolyte (cations, anions, and the electrolyte pH) on eCOR. As summarized in the conspectus graphic, the electrolyte influences eCOR activity via a field (ε) effect on dipolar (μ) reaction intermediates, changing the proton donor for the multi-step proton-electron transfer reaction, specifically adsorbed anions on the catalyst surface to block active sites, and tuning the local environment by homogeneous reactions. To be specific, alkali metal cations (M) can stabilize reaction intermediates via electrostatic interactions with dipolar intermediates or buffer the interfacial pH via hydrolysis reactions, thereby promoting the eCOR activity with the following trend in hydrated size (corresponding to the local field strength ε)/hydrolysis ability: Cs > K > Na > Li. The effect of the electrolyte pH can give a change in eCOR activity of up to several orders of magnitude, arising from linearly shifting the absolute interfacial field via the relationship = - (2.3)pH, homogeneous reactions between OH and desorbed intermediates, or changing the proton donor from hydronium to water along with increasing pH. Anions have been suggested to affect the eCOR reaction process by solution-phase reactions (e.g., buffer reactions to tune local pH), acting as proton donors or as a surface poison.So far, the existing models of electrolyte effects have been used to rationalize various experimentally observed trends, having yet to demonstrate general predictive capabilities. The major challenges in our understanding of the electrolyte effect in eCOR are (i) the long time scale associated with a dynamic picture of the catalyst|electrolyte interface and (ii) the overall activity determined by the length-scale interplay of intrinsic microkinetics, homogeneous reactions, and mass transport limitations. New developments in dynamic models and coupling the effects of mass transport can provide a more accurate view of the structure and intrinsic functions of the electrode-electrolyte interface and the corresponding reaction energetics toward comprehensive and predictive models for electrolyte design.
电化学 CO 还原（eCOR）使将废 CO 转化为高附加值燃料和商品化学品成为可能，该过程由可再生电力驱动，从而为实现净零碳排放目标提供了可行的策略。过去几十年的研究集中在催化剂（电极）和电解质环境的优化上。表面积归一化电流密度表明，后者可以使 CO 还原活性提高几个数量级。
在本综述中，我们回顾了电解质（阳离子、阴离子和电解质 pH 值）对 eCOR 影响背后的机制理论。正如综述图总结的那样，电解质通过对偶极（μ）反应中间体的电场（ε）效应来影响 eCOR 活性，改变多步质子-电子转移反应的质子供体，具体来说，是催化剂表面上吸附的阴离子会阻塞活性位点，通过均相反应来调节局部环境。具体来说，碱金属阳离子（M）可以通过静电相互作用与偶极中间体稳定反应中间体，或通过水解反应缓冲界面 pH 值，从而促进 eCOR 活性，其水化尺寸（对应局部电场强度 ε）/水解能力的顺序为：Cs > K > Na > Li。电解质 pH 值的影响可使 eCOR 活性发生高达几个数量级的变化，这是由于通过关系 = - (2.3)pH 线性移动绝对界面场、OH 与脱附中间体之间的均相反应，或随着 pH 值的增加，质子供体从水合氢离子变为水。阴离子被认为通过溶液相反应（例如，缓冲反应来调节局部 pH 值）、作为质子供体或表面毒物来影响 eCOR 反应过程。到目前为止，电解质效应的现有模型已被用于合理化各种实验观察到的趋势，但尚未证明具有普遍的预测能力。我们在理解 eCOR 中电解质效应方面面临的主要挑战是：（i）与催化剂-电解质界面动态图像相关的长时间尺度，以及（ii）由固有微观动力学、均相反应和传质限制的长度尺度相互作用决定的整体活性。动态模型的新发展和质量传输效应的耦合可以为电极-电解质界面的结构和内在功能提供更准确的视图，以及对电解质设计的综合和预测模型的相应反应能学。
