BENCHMARKING METAL-ORGANIC FRAMEWORKS FOR ELECTROCHEMICAL CARBON DIOXIDE REDUCTION

Abstract

The electrochemical carbon dioxide reduction reaction (CO2R) can serve as a strategy to utilize anthropogenic CO2 emissions as a C1 feedstock to produce a diverse array of carbon-based products from renewable energy inputs. However, fundamental kinetic challenges confer large energy requirements and poor product selectivity for electrochemical CO2R on traditional metal electrocatalysts, motivating the exploration for more advanced electrocatalyst design strategies.Metal-organic frameworks (MOFs) are a class of porous, crystalline material that—because of their unique structure and material properties—have demonstrated promise as next-generation CO2R electrocatalyst candidates capable of assuming many different material identities. For example, the porous structure of MOFs can be used to expose a high number of active sites or support CO2R-active metals. MOFs can also be used as precursors to form MOF-derived (MOF-d) nanostructured materials under thermal or (electro)chemical treatments. This wide scope of available materials underscores the importance of developing activity descriptors for benchmarking MOF and MOF-d materials for electrochemical CO2R. In this dissertation, a portfolio of in situ characterization techniques and fundamental electrochemical CO2R kinetic experiments are used to benchmark MOF and MOF-d materials against related, state-of-the-art materials. A Cu-based MOF (HKUST-1) showed electrochemical CO2R activity similar to that of metallic Cu materials, owing to the electrochemically induced restructuring of HKUST-1 into nanostructured Cu films. Conversely, the Zr-based PCN-222(Fe) exhibited structural stability under applied potential and demonstrated an ideal turnover frequency (TOF) for electrochemical CO2R at low MOF wt. loadings, enabling fundamental mechanistic studies that elucidated a potential-dependent change in the electrochemical CO2R mechanism. These mechanistic insights motivated investigations into design strategies for developing Zr-based MOF (UiO-66) overlayers to augment CO2R on metal electrodes by modulating the local CO2 concentration and stabilizing reaction intermediates. Both top-down and bottom-up strategies were explored for depositing UiO-66 on Ag electrodes, highlighting challenges associated with stable UiO-66 film deposition due to film agglomeration and structural instabilities under CO2R-relevant conditions. The investigations presented in this dissertation highlight both the opportunities and challenges that MOFs enable for electrochemical CO2R and can inform the exploration for advanced electrocatalyst design strategies.