Haaring Robert, Kang Phil Woong, Guo Zunmin, Lee Jae Won, Lee Hyunjoo
Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology, Daejeon 34141, Republic of Korea.
Acc Chem Res. 2023 Oct 3;56(19):2595-2605. doi: 10.1021/acs.accounts.3c00349. Epub 2023 Sep 12.
ConspectusAs the demand for a carbon-neutral society grows rapidly, research on CO electrolysis has become very active. Many catalysts are reported for converting CO into value-added products by electrochemical reactions, which have to perform at high Faradaic and energy efficiency to become commercially viable. Various types of CO electrolyzers have been used in this effort, such as the H-cell, flow cell, and zero-gap membrane-electrode assembly (MEA) cell. H-cell studies are conducted with electrodes immersed in CO-saturated electrolyte and have been used to elucidate reaction pathways and kinetic parameters of electrochemical CO reduction on many types of catalytic surfaces. From a transport phenomenological perspective, the low solubility and diffusion of CO to the electrode surface severely limit the maximum attainable current density, and this metric has been shown to have significant influence on the product spectrum. Flow and MEA cells provide a solution in the form of gas-diffusion electrodes (GDEs) that enable gaseous CO to closely reach the catalyst layer and yield record-high current densities. Because GDEs involve a complicated interface consisting of the catalyst surface, gaseous CO, polymer overlayers, and liquid electrolyte, catalysts with high intrinsic activity might not show high performance in these GDE-based electrolyzers. Catalysts showing low overpotentials at low current densities may suffer from poor electron conductivity and mass transfer limitations at high current densities. Furthermore, the stability of the GDE-based catalysts is often compromised as CO electrolysis is pursued with high activity, most notoriously by electrolyte flooding.In this Account, we introduce recent examples where the electrocatalysts were integrated in GDEs, achieving high production rates. The performance of such systems is contingent on both GDE and cell design, and various parameters that affect the cell performance are discussed. Gaseous products, such as carbon monoxide, methane, and ethylene, and liquid products, such as formate and ethanol, have been mainly reported with high partial current density using the flow or MEA cells. Different strategies to this end are described, such as controlling microenvironments by the use of polymers mixed within the catalyst layer or the functionalization of catalyst surfaces with ligands to increase local concentrations of intermediates. Unique CO electrolyzer designs are also treated, including the incorporation of light-responsive plasmonic catalysts in the GDE, and combining the electrolyzer with a fermenter utilizing a microbial biocatalyst to synthesize complex multicarbon products. Basic conditions which the catalyst should satisfy to be adapted in the GDEs are listed, and our perspective is provided.
综述
随着对碳中和社会的需求迅速增长，CO电解研究变得非常活跃。有许多关于通过电化学反应将CO转化为高附加值产品的催化剂的报道，这些催化剂必须具有高法拉第效率和能量效率才能在商业上可行。在这项工作中使用了各种类型的CO电解槽，如H型电解槽、流动电解槽和零间隙膜电极组件（MEA）电解槽。H型电解槽研究是将电极浸入CO饱和电解质中进行的，已用于阐明多种催化表面上电化学CO还原的反应途径和动力学参数。从传输现象学的角度来看，CO在电极表面的低溶解度和扩散严重限制了可达到的最大电流密度，并且该指标已被证明对产物谱有重大影响。流动电解槽和MEA电解槽以气体扩散电极（GDE）的形式提供了一种解决方案，使气态CO能够紧密到达催化剂层并产生创纪录的高电流密度。由于GDE涉及由催化剂表面、气态CO、聚合物覆盖层和液体电解质组成的复杂界面，具有高本征活性的催化剂在这些基于GDE的电解槽中可能不会表现出高性能。在低电流密度下显示低过电位的催化剂在高电流密度下可能会受到电子传导性差和传质限制的影响。此外，随着以高活性进行CO电解，基于GDE的催化剂的稳定性常常受到损害，最明显的是受到电解液泛滥的影响。
在本综述中，我们介绍了最近将电催化剂集成到GDE中并实现高产率的实例。此类系统的性能取决于GDE和电解槽设计，并讨论了影响电解槽性能的各种参数。使用流动电解槽或MEA电解槽主要报道了具有高部分电流密度的气态产物，如一氧化碳、甲烷和乙烯，以及液态产物，如甲酸盐和乙醇。为此描述了不同的策略，例如通过在催化剂层中混合聚合物来控制微环境，或用配体对催化剂表面进行功能化以增加中间体的局部浓度。还讨论了独特的CO电解槽设计，包括在GDE中引入光响应等离子体催化剂，以及将电解槽与利用微生物生物催化剂合成复杂多碳产物的发酵罐相结合。列出了催化剂在GDE中适用应满足的基本条件，并给出了我们的观点。
