The Role of Catalyst Microstructure in Gas Diffusion Electrosynthesis of C2+ Products

斯坦福大学的Matthew Kanan教授得到化学系化学催化（CAT）项目的支持，研究铜催化剂的结构与其对一氧化碳（CO）电化学转化的催化性能之间的关系。该项目涉及使用采用气体扩散电极（GDE）的电化学电池，其最大限度地将气体输送到催化剂材料和从催化剂材料输送气体，从而实现高产品合成速率。 最终目标是将二氧化碳（CO2）转化为燃料和有用的化学品。 将CO2转化为CO的技术已经存在，但随后通过电解将CO转化为更有用的化学品和燃料的技术还没有很好地建立起来。 该项目的重点是提高阴极铜催化剂材料性能的方法，阴极是CO与电子和质子结合形成碳氢化合物，醇或羧酸等产品的电极。 使用铜催化剂用于所需产物的规模化生产的可行性取决于调整其结构以使活性、选择性和耐久性最大化的能力。 可再生电力的快速增长和成本下降增加了利用电力为从二氧化碳和水合成燃料和化学品提供动力的吸引力。 微观结构如何影响电化学CO转化性能的研究可能会揭示在其他催化过程中利用微观结构的策略，这些催化过程对能量转换和工业化学合成很重要。为了配合研究活动，该小组通过电化学转化演示和实验室模块向当地高中生进行科学推广，以说明核心化学概念，并激励学生追求科学事业。基于低电流密度下的溶液相研究，提出了许多铜催化二氧化碳和一氧化碳还原的结构-活性模型。然而，实际的电合成将需要使用GDE在高得多的电流密度下操作。尚不清楚在溶液相电解中阐明的结构-活性关系是否适用于在高电流密度下在GDE中操作的Cu材料。此外，可能影响Cu材料的固有催化性能的微观结构特征与GDE的催化剂层的复杂的非均质结构相交织，所述结构通常由催化剂颗粒、碳颗粒、氟化聚合物和离聚物组成。本项目的研究旨在建立一种方法，用于阐明微观结构对GDE中运行的Cu催化剂的影响，并将这些影响与依赖于GDE催化剂层结构的形态学和传质现象分离开来。开发了新的合成方法来靶向具有可控晶界密度和几何形状的定义明确的Cu纳米颗粒。使用透射菊池衍射和其他电子衍射和X射线衍射技术绘制晶粒结构。接下来，使用定制的电化学电池在GDE中全面评估微观结构变体样品的催化活性。这些研究旨在确定晶界与特定C2+产物的活性或选择性之间的相关性，并评估这些相关性如何取决于电流密度/过电位、电解质和其他参数。平行地，开发喷涂程序以改变GDE的催化剂层的厚度、孔隙率和润湿性。这些程序用于优化GDE的架构，以在最小过压下获得高电流密度，并评估架构与催化剂微观结构在此制度中的相对重要性。该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Professor Matthew Kanan of Stanford University is supported by the Chemical Catalysis (CAT) Program of the Division of Chemistry to investigate the relationship between the structure of copper catalysts and their catalytic properties for the electrochemical conversion of carbon monoxide (CO). The project involves the use of electrochemical cells employing gas diffusion electrodes (GDEs), which maximize the transport of gases to and from the catalyst material and hence enable high product synthesis rates. The ultimate goal is to convert carbon dioxide (CO2) to fuels and useful chemicals. The technology to convert CO2 to CO is already available, but the subsequent conversion of CO into more useful chemicals and fuels via electrolysis is not well established. The project focuses on means to enhance the performance of the copper catalyst material on the cathode, the electrode where CO is combined with electrons and protons to form products such as hydrocarbons, alcohols, or carboxylic acids. The viability of using copper catalysts for scalable production of the desired product hinges on the ability to tailor its structure to maximize activity, selectivity, and durability. The rapid growth and falling costs of renewable electricity have increased the attraction of using electricity to power the synthesis of fuels and chemicals from carbon dioxide and water. The study of how microstructure affects performance in electrochemical CO conversion may unveil strategies for exploiting microstructure in other catalytic processes that are important to energy conversion and industrial chemical synthesis. To complement the research activities, the group engages in scientific outreach to local high school students by using electrochemical conversion demonstrations and lab modules to illustrate core chemical concepts and inspire students to pursue scientific careers.Numerous structure - activity models have been proposed for Cu-catalyzed carbon dioxide and carbon monoxide reduction based on solution-phase studies at low current density. However, practical electrosynthesis will require operating at much higher current density using a GDE. It is unclear if the structure - activity relationships that have been elucidated in solution-phase electrolyses are applicable to Cu materials operating in GDEs at high current densities. Moreover, microstructural features that may influence the intrinsic catalytic properties of Cu materials are convoluted with the complex, heterogeneous architecture of the catalyst layer of GDEs, which typically is composed of catalyst particles, carbon particles, fluorinated polymers, and ionomers. The research in this project seeks to establish a methodology for elucidating microstructural effects on Cu catalysts operating in GDEs and disentangle these effects from morphological and mass transport phenomena that depend on the architecture of the GDE catalyst layer. New synthetic methods are developed to target well-defined Cu nanoparticles with controllable grain boundary density and geometry. The grain structures are mapped using transmission Kikuchi diffraction and other electron diffraction and x-ray diffraction techniques. Next, the catalytic activity of microstructurally variant samples is comprehensively evaluated in GDEs using custom electrochemical cells. These studies seek to identify correlations between grain boundaries and the activity or selectivity for specific C2+ products and assess how these correlations depend on current density/overpotential, electrolyte, and other parameters. In parallel, spray-coating procedures are developed to vary the thickness, porosity, and wettability of the catalyst layer of a GDE. These procedures are used to optimize the architecture of the GDE for high current density at minimal overvoltage and assess the relative importance of architecture vs catalyst microstructure in this regime.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.