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

Stanford University, Stanford CA

Investigators

Abstract

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.