Designing Three-Dimensional Nanoporous Metal Alloys for Selective Electrochemical Conversion Catalysis

Download or read book Designing Three-Dimensional Nanoporous Metal Alloys for Selective Electrochemical Conversion Catalysis written by Swarnendu Chatterjee and published by . This book was released on 2020 with total page 149 pages. Available in PDF, EPUB and Kindle. Book excerpt: The rising demands of clean energy owing to a burgeoning global population and deteriorating climate has given rise to new avenues of research in electrocatalysis focusing on extraction and storage of energy through electrochemical reactions. In contrast to heterogeneous catalysts, electrochemical catalysts often need to withstand harsh reaction environments with respect to electrolyte pH and applied overpotentials. The stability requirements constrain the breadth of applicable materials, limiting the viable catalysts to those composed of more noble metals, which are invariably more costly. The design of next generation electrocatalyst materials requires strategies to balance activity and stability while at the same time minimizing the utilization of expensive materials to limit costs. Open-framework nanocatalyst architectures show promise as they maximize surface area to volume ratios and their morphology and surface chemistry are readily tuned through controlled processing methodologies. Among the high aspect ratio, open-framework nanostructures, nanoporous metals obtained through dealloying offer a unique class of three dimensional electrode materials that are useful for a number of electrolytic processes owing to their conductive high surface area structure and tunable near surface composition. Herein, we study the porosity evolution processes in multimetallic alloys through classical dealloying and alternative methods, in pursuit of creating optimal bicontinuous nanoporous architectures for two important electrochemical reactions, central to the carbon and water cycles: CO2 reduction reaction (CO2RR) and oxygen evolution reaction (OER). To address the limitations of electrochemical dealloying for nanoporous metal synthesis, we first develop a new alternative method where we thermally decompose readily available transition metal dichalcogenides to create bicontinuous three dimensional metallic nanostructures. We show that spinodal decomposition with a proper balance of removal of the chalcogen component and surface diffusion of metal is possible that gives rise to uniform porosity length scales below 100 nm. Our new method is applicable to a broader range of materials including the refractory metals which are difficult to obtain in nanoporous bicontinuous form through conventional dealloying techniques. For CO2RR, we demonstrate favorable tuning of the near surface composition of core shell nanoporous alloys to mitigate a common problem in CO2 electrolysis which is the poisoning of electrocatalytic surface during long term reaction. CO2RR shows great promise as a remediation strategy to convert and store anthropogenic CO2. To realize its practical integration in industries with high CO2 emissions, uninterrupted activity of electrocatalyst is important at reasonable reaction timescales. Among the materials capable of electrochemically converting CO2 to formate which is a high energy density product, Palladium is unique in that it has shown substantial faradaic efficiency at minimal overpotential. Limiting its implementation, however, is its gradual deactivation through CO poisoning during constant potential CO2RR. Here, we show synthesis of core-shell nanoporous multi-metallic Pd alloys that display suppressed CO deactivation during formate production based on suitable choice of alloying component. The improvement in deactivation tolerance has been attributed to a combination of electronic impacts of subsurface alloying components as well as the composition dependent hydricity of the Pd alloys. The Pd skinned nanoporous alloys have been obtained by electrochemical dealloying where the high surface area electrode structure provides high formate partial current densities with minimal CO poisoning while not altering the formate selectivity at low CO2RR overpotentials. Aqueous CO2RR system also requires a stable electrocatalyst at the anode for the OER which requires stricter stability constraints for the electrocatalysts. OER is also the performance limiting component in the water splitting reactions of PEM electrolyzers. The oxidative potential of OER is difficult for many active materials to survive. In addition to the sluggish kinetics of the anodic OER, low catalyst stability and electrode conductivity lead to process inefficiencies. Higher valent oxidation states of Ir have been identified as the only materials that demonstrate a reasonable balance of activity and durability for acidic OER. Attempts to make nanoporous Ir employing dealloying for high surface area electrodes are limited owing to its strong tendency to make immobile oxides that defy morphology evolution through dealloying. Here we design a dealloying protocol to create unique nanoporous Ir morphologies, including porous nanosheets that exhibit sufficient activity and durability while displaying higher lateral and through-plane conductivity when compared to standard IrO2 catalysts. The metallic core of the nanoporous metal ligaments and absence of any binder/support result in low electrode and charge transfer resistances; ultimately giving rise to lower overpotential and improved electrochemically active surface area (ECSA) normalized current densities compared to IrO2. This thesis outlines the analysis of design of nanoporous core shell bicontinuous alloys and porous nanosheets through top down techniques for wide combinations of metals including the refractory metals which are difficult to obtain through existing dealloying methods. Nanoporous metal based electrodes show promise for utilization in high throughput CO2RR systems and PEM water electrolyzers both of which are important parts of renewable energy technologies.