Molecular Tuning of Electrocatalysts for Generation of Commodity Chemicals

Citation

Heim, Gavin (2024) Molecular Tuning of Electrocatalysts for Generation of Commodity Chemicals. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/dm95-6856. https://resolver.caltech.edu/CaltechTHESIS:03302024-040656026

Abstract

Improving our understanding of electrocatalytic transformations is envisioned to facilitate society’s implementation of technologies that achieve a net zero carbon footprint. Carbon dioxide is one of the most emitted greenhouse gases, and improvement in CO₂ capture technologies along with decreasing costs of renewable energy provide an opportunity to convert this species to value-added chemicals using electrochemical processes. Tuning homogeneous and heterogeneous electrocatalyst performance with well-defined molecular species can render systems more selective and active while also allowing us to readily predict variables crucial in achieving these transformations. This thesis investigates 1) molecular and polymeric species as electrode coatings for enhanced generation of carbon-coupled products and 2) discrete electrocatalyst active sites for formation CO₂ reduction products at low overpotentials; generation of highly reduced liquid fuels is observed with molecular electrocatalysts supported on electrodes. Chapter I provides context and background to the contents of this thesis.

Chapter II discusses novel, polyaromatic molecular additives utilized for low pH CO₂ reduction on Cu electrodes. N-phenyl isoquinolinium triflate film facilitates high selectivity for C₂+ products in 0.1 M H₃PO₄/KH₂PO₄, suggesting enhancement in CO₂ mass transport rather than limiting proton carrier diffusion. Improvement in long-term stability and tolerance to lower pH compared to previous films is observed.

Chapter III reports on a series of polystyrene-based ionomers to probe the effect of local [K⁺] in the Cu electrode microenvironment on CO₂R performance. Partial current density towards C₂₊ products (|jC₂₊|) increases monotonically with [K⁺] in ionomer, up to 225 mA cm⁻². Replacing K⁺ with [Me4N]⁺ lowers performance to the level of bare Cu, highlighting the crucial role of K⁺ in improving C₂₊ product selectivity. Molecular dynamics simulations and partial pressure CO₂ electrolysis experiments are consistent with enhanced CO₂ mass transport due to K⁺ in the film.

Chapter IV discusses variation of ionomer/polymer structures to maximize CO₂R performance. Incorporation of neutral comonomers bearing cross-linking units rich in biphenyl and terphenyl motifs result in high current densities (~270 mA cm⁻²) towards C₂₊ products with 82% Faradaic efficiency. The analogous neutral variants (i.e., those lacking the charged comonomer) show comparable |jC₂₊| to the K⁺-containing polymers, suggesting a non-innocent role of the aryl-rich polymers in boosting performance.

Chapter V presents novel four-coordinate, dicationic Co complexes supported on carbon nanotubes capable of generating MeOH from CO₂. Electrolysis with CO also leads to formation of MeOH, suggesting a CO-bound complex to be a crucial intermediate in CO₂R to MeOH. This work highlights rare examples of molecular systems facilitating multi-electron electrochemical transformations to highly demanded commodity chemicals.

Chapter VI presents work on molecular electrocatalysts bearing novel polyaromatic ligands that lower the electrocatalytic potential (Ecat) of CO₂R by ~310 mV compared to state-of-the-art complexes as determined via cyclic voltammetry. The extended π system motif is more proximal to the metal center relative to previously reported nanographene-containing electrocatalysts. Well-defined characterization was obtained via single-crystal X-ray diffraction in addition to solution-state techniques. Density functional theory calculations reveal significant ligand contributions in the frontier orbitals of relevant CO₂R intermediates.

Chapter VII highlights a polycyclic aromatic hydrocarbon (PAH) bearing twelve edge nitrogen atoms. Spectroscopy, electrochemistry, and computational results suggest a significant narrowing of the HOMO-LUMO gap compared to the N-free analogue owing to the electron-deficient extended π system imposed by the nitrogen dopants. Changes to absorption and emission spectra from titration of the PAH with metal salts suggest that coordination chemistry provides an additional degree of freedom towards tuning electronic structure. Dramatic changes from addition of trifluoromethanesulfonic acid suggest this material to be a possible pH sensor. This approach in judiciously tuning the band gap of bulk graphene materials via saturation of the nanographene edge sites with nitrogen atoms gives rise to a novel compound with intriguing electronic properties.

Appendix A describes systematic attempts in demonstrating cascade electrocatalysis between molecular CO₂-to-CO complexes and pyridinium film-modified Cu towards enhanced rates of C₂₊ products formation.

Appendix B provides results coupling electrodeposited imidazolium-derived films with pyridinium towards enhanced CO₂R to C₂₊ on Cu. While promising performance is achieved, the difficulty in characterizing the films limits the tractability of these systems with respect to their impacts on the microenvironment.

Appendix C discusses developing coordination complexes of heteroatom containing polyaromatic hydrocarbons. Several examples characterized via X-ray crystallography are reported.

Appendix D shows CO₂R data on K⁺ ionomer-coated Au. Elevation in |jCO| is demonstrated as a function of potassium content in the electrode-electrolyte interface provided by the film.

Appendix E discusses attempts to determine and CO₂ uptake by K⁺ ionomers via solid state NMR spectroscopy.

Thesis Files