Exploring the mechanism for the electrocatalytic reduction of CO2 to CO and the industrial integration of a bismuth-based electrocatalyst
Date
2017
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
University of Delaware
Abstract
Global energy demand has historically been provided for by fossil fuels. As energy demand continues to rise in response to increasing population and industrialization, the use in fossil fuels will also rise. This rise is anticipated to have negative impacts on our global environment in the form of environmental risk factors. Therefore, a switch in energy dependency from fossil fuels to clean renewable energy is necessary to mitigate these risk factors. To make this transition, we envision a zero-emission pathway by harnessing energy from the sun in the form of solar fuels. By harnessing solar energy via a photovoltaic, electrons will be provided to reduce CO2 to CO. The well-known Fischer-Tropsch process will then use CO as the C1 feedstock to produce the solar fuels. ☐ For the initial transformation, the Rosenthal Research Lab developed a bismuth carbon monoxide evolving catalyst (Bi-CMEC), prepared by an acid aqueous electrodeposition bath. Its ability to selectively (>90 %) and efficiently (83 %) electrocatalytically transform CO2 to CO with the incorporation of imidazolium-based ionic liquids (ILs) was demonstrated. Bi relates to the historic Ag and Au cathodes in the ability to selectively evolve CO, but is not impeded by the cost of material when used on an industrial scale. ☐ To further encourage industrial integration of Bi-CMEC and to enhance these impressive metrics, I helped with the development of a 3D-printed flow electrolysis assembly for the electrocatalytic reduction of CO2 to CO. 3D printing allows for rapid and cost effective prototyping with high precision and accuracy. The flow cell is a sealed system which does not require constant operation or system re-optimization. Importantly, the flow cell recycles electrolyte and encourages faster mass transport from the constant flow of fresh electrolyte across the electrode surface. Here, we have modified existing drop casting techniques to screen a slew of inexpensive and commercially available Bi3+ salt precursors. We also took advantage of inexpensive carbon supports to further the likelihood of transitioning this chemistry into industry. Our progress with Bi-CMEC preparation and industrial cell design truly highlights the feasibility of integrating a Bi-based CO2 electrolyzer into industry. ☐ Mechanistic insights for Bi-CMEC to facilitate this transformation were studied with Tafel analysis and MD calculations performed in a collaborative effort with the University of Minnesota and co-workers of the Rosenthal Research Lab. Specifically, Tafel analysis suggested that the rate determining step (RDS) for Bi-CMEC is the first electron transfer (ET) to yield the surface bound CO2•–. MD simulations agreed with the first ET being rate limiting and highlighted the role of surface bound IL and IL in the solution bulk. Upon developing an organic plating procedure, in situ studies were performed which offered additional insight for the catalytic behavior of Bi-CMEC. The organic electrodeposition procedure was general and allowed for materials other than Bi (Sn, Pb, and Sb) to be studied. The ability of these p-block metals to reduce CO2 to CO was explored and ultimately revealed unique electrochemical behaviors. Despite Sb showing poor activity for CO2 reduction, Bi, Sn, and Pb demonstrated high CO selectivity (~80 %) with current densities for CO generation ranging from 5–8 mA/cm2. While Bi and Sn both showed impressive electrochemical behaviors, Pb demonstrated film passivation and CO2 activation occurred at a larger overpotential. These initial studies inspired curiosity as to why these materials, when prepared in a similar manner, demonstrated unique catalysis. ☐ To understand why Pb becomes passivated, I performed extensive studies on the RDS, electrochemical events, and surface adsorbate interactions for the Bi, Sn, and Pb cathodes. Tafel studies performed on the Pb cathode suggested that an adsorbate forms on the surface to block charge transfer events. Surface studies were subsequently performed to identify the evolution of adsorbates for the Bi, Sn, and Pb cathodes. While Bi and Sn were nearly identical, Pb demonstrated an additional impurity adsorption component. This component was later identified as an imidazolium-carboxylate adduct. This adduct was found to exist both on the surface and in solution for Pb, but was not identified in either medium for Bi or Sn. ☐ Imidazolium-carboxylate is highlighted in the literature to be an essential intermediate for the stabilization of CO2•– to lower the activation energy of this redox reaction. Because this theory disagrees with our previous experimental results, I performed thorough studies on the adduct itself. Here, direct information on its electrochemical behavior and participation in catalysis were observed. It was revealed that the adduct formation is irreversible, such that it creates a thermodynamic sink upon formation during electrolysis. When the adduct exists in solution, it was found to interfere with the diffusion of reactants and products migrating to and from the electrode surface to negatively impact the current density. For Bi and Sn specifically, the adduct interferes with the cathode/IL interaction at the surface without formally adsorbing. The IL monolayer becomes loosely adsorbed and no longer suppresses hydrogen production to selectively favor CO evolution. ☐ IL serves multiple purposes during catalysis including stabilizing the electrode bias, providing protons for the 2H+/2e– redox reaction, and functioning as electrolyte. Therefore, I was interested in observing how neat IL systems would compare to the organic system with dilute IL. Additionally, ILs are a greener solvent compared to organics and would further help with industrial integration. For these studies, the essential cathode/IL interface remained, but the inherent physical properties of the IL electrolyte became more prevalent, i.e. viscosity, symmetry, intermolecular interactions. The higher solution viscosity was shown to drastically reduce mass transport. Poor diffusion was remedied with temperature enhancement. Here, it was found that Sn-CMEC was more durable than Bi-CMEC for extreme reaction conditions. Thus, temperature studies were continued with Sn-CMEC. Temperature studies were performed with 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM]PF6) and 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([BMIM]OTf). Interestingly, the durability of these ILs were found to be highly dependent on the anion. [BMIM]OTf demonstrated high selectivity and current density up to 100 ⁰C, whereas [BMIM]PF6 showed no positive correlation to any temperature range. These differences were due to the durability (decomposition temperature) of each IL. ☐ To enhance mass transport for [BMIM]PF6 without sacrificing the durability of the IL during catalysis, admixtures of acetonitrile were added. This revealed fascinating chemistry, such that there is now a switch in catalytic dependency. Remarkably, the electronic behavior became less resistive to facilitate charge transfer, mass transport, and CO selectivity. Importantly, the IL did not show evidence for significant decomposition. These IL studies highlight electrochemical behavior of IL during electrolysis.
Description
Keywords
Pure sciences, Bismuth, Electrochemical reduction, Electrochemistry, Energy, Ionic liquid