Showing papers by "Thomas E. Rufford published in 2021"

The role of electrode wettability in electrochemical reduction of carbon dioxide

Abstract: The electrochemical reduction of carbon dioxide (CO2RR) requires access to ample gaseous CO2 and liquid water to fuel reactions at high current densities for industrial-scale applications. Substantial improvement of the CO2RR rate has largely arisen from positioning the catalyst close to gas–liquid interfaces, such as in gas-diffusion electrodes. These requirements add complexity to an electrode design that no longer consists of only a catalyst but also a microporous and nanoporous network of gas–liquid–solid interfaces of the electrode. In this three-dimensional structure, electrode wettability plays a pivotal role in the CO2RR because the affinity of the electrode surface by water impacts the observed electrode reactivity, product selectivity, and long-term stability. All these performance metrics are critical in an industrial electrochemical process. This review provides an in-depth analysis of electrode wettability's role in achieving an efficient, selective, and stable CO2RR performance. We first discuss the underlying mechanisms of electrode wetting phenomena and the foreseen ideal wetting conditions for the CO2RR. Then we summarize recent advances in improving cathode performance by altering the wettability of the catalyst layer of gas-diffusion electrodes. We conclude the review by discussing the current challenges and opportunities to develop efficient and selective cathodes for CO2RR at industrially relevant rates. The insights generated from this review could also benefit the advancement of other critical electrochemical processes that involve multiple complex flows in porous electrodes, such as electrochemical reduction of carbon monoxide, oxygen, and nitrogen.

Unveiling the effects of dimensionality of tin oxide-derived catalysts on CO2 reduction by using gas-diffusion electrodes

Abstract: We report the effects of catalyst dimensionality on electrochemical CO2 conversion to formate by comparing the performance of tin oxide-derived 3D nanoparticles and tin oxide-derived 2D nanosheets deposited on gas-diffusion electrodes. Our results indicated that an extensive interface between the catalyst and the electrode substrate could lower the surface tin oxidation states and the hydrophobicity of the catalyst layer during CO2 electrolysis at a current density over 100 mA cm−2. This catalyst–substrate interfacial effect provides the nanosheets with a large interface area to become more selective for CO2 electrochemical reduction but with a higher overpotential requirement as compared to the 3D nanoparticle catalysts with limited interfacial area. Consequently, the electrode with nanosheets as the catalyst achieved a partial current density of formate at 116 mA cm−2 at a cathode potential of −1.03 V versus reversible hydrogen electrode, which is equivalent to a formate production rate of 36 μmol min−1 cm−2. Our work here demonstrates the importance of the catalyst–substrate interface in determining the oxidation states and wettability at the catalyst surface and the ultimate performance of the gas-diffusion electrode. These findings also have potential to guide the design of a catalyst–substrate to advance other important electrochemical processes such as fuel cells and water splitting.

Cobalt Electrochemical Recovery from Lithium Cobalt Oxides in Deep Eutectic Choline Chloride+Urea Solvents.

Abstract: Electrochemical recovery of the cobalt in deep eutectic solvent shows its promise in recycling and recovery of valuable elements from the spent lithium-ion battery due to its high selectivity and minimal environmental impacts. This work unveiled the roles of the substrates, applied potentials and operating temperatures on the performance of cobalt electrochemical recovery in a deep eutectic choline chloride + urea solvent. The solvent contains cobalt and lithium ions extracted from lithium cobalt oxides - an essential lithium-ion battery cathode material. Our results highlight that the substrate predetermines the cobalt recovery modes via substrate-cobalt interactions, which could be predicted by the cobalt surface segregation energies and crystallographic misfits. We also show that a moderate cathode potential under -1.0 V vs. silver quasi-reference electrode at 94 °C - 104 °C is essential to ensure a selective cobalt recovery at an optimal rate. We also found that the stainless-steel mesh as an optimal substrate for cobalt recovery due to its relatively high selectivity, fast recovery rate, and easy cobalt collection and substrate regeneration. Our work provides new insights on metal recovery in deep eutectic solvents and offers a new avenue to control the metal electrodeposition modes via modulation of substrate compositions and crystal structures.

Understanding the Effects of Anion Interactions with Ag Electrodes on Electrochemical CO2 Reduction in Choline Halide Electrolytes

Abstract: Interactions of electrolyte ions at electrocatalyst surfaces influence the selectivity of electrochemical CO2 reduction (CO2R) to chemical feedstocks like CO. We investigated the effects of anion type in aqueous choline halide solutions (ChCl, ChBr, and ChI) on the selectivity of CO2R to CO over an Ag foil cathode. Using an H-type cell, we observed that halide-specific adsorption at the Ag surface limits CO faradaic efficiency (FECO) at potentials more positive than −1.0 V vs. reversible hydrogen electrode (RHE). At these conditions, FECO increased from I−−−, that is, in the opposite order to the strength of specific adsorption of the halide ions (Cl−−−). At potentials of −1.0 to −1.3 V vs. RHE, restructuring of the Ag surface in ChI and ChCl via dissolution and re-electrodeposition led to more CO-selective Ag facets ([220], [311], and [222]) than in ChBr. This mechanism allowed very high faradaic efficiencies for CO of 97±2 % in ChI and 94±2 % in ChCl to be achieved simultaneously with high current densities at −1.3 V vs. RHE. We also demonstrate that high selectivity to CO (FECO>90 %) in ChCl (at −0.75±0.06 Vvs. RHE) and ChI (at −0.78±0.17 V vs. RHE) could be achieved at a current density of 150 mA cm−2 in a continuous flow-cell electrolyser with Ag nanoparticles on a commercial gas diffusion electrode. This study provides new insights to understand the interactions of anions with catalysts and offers a new method to modify electrocatalyst surfaces.

Evaluation of Flowsheet Design Approaches to Improve Energy Efficiency in Multistage Membrane Processes to Recover Helium

Abstract: In pressure-driven membrane-based gas separation processes, the driving force-related cost (capital and operating costs) is one of the most significant cost components. To reduce this driving force...