Carbon Dioxide Electroreduction on Gas Diffusion Electrodes

Abstract

Rapid industrialization and use of carbon based fuels has caused a drastic increase in the atmospheric CO₂ levels in the last few decades. The rising anthropogenic CO₂ levels pose a significant threat to the environment as evidenced by the increase in the mean global temperature levels, and the rising ocean levels. To mitigate the challenges associated with rising CO₂ levels, there is an urgent need to move towards carbon neutral sources of energy and to curb carbon emission from large scale point emitters such as industries. Additionally, emitted CO₂ could be converted into energy dense organic fuels using carbon-neutral forms of energy. This not only helps in reducing the carbon emissions but also balances the intermittent nature of renewable energy supply. CO₂ could also be converted into platform chemicals such as ethylene/CO, which can be further up-converted or directly used in industry. Ethylene is particularly interesting due to its high
energy density and wide industrial usage as a precursor in the polymer industry. Electroreduction of CO₂ provides one such approach to electrochemically convert CO₂ produced at large scale emitters to useful organic compounds. Different metallic catalysts are known to catalyse the electrochemical reduction towards different products, which follows from Sabatier’s principle. In this study copper is used as the model catalyst due to its unique ability to electrochemically convert CO₂ to multi-carbon products, such as ethylene. From a cell design perspective conventional electrochemical reduction of CO₂ in aq. media suffers from low production rates due to the low solubility of CO₂ in aq. electrolytes which makes it not feasible from an industrial standpoint. To overcome the low production rates, this study was carried out on novel gas diffusion electrodes. Another factor limiting the implementation of CO₂ electrolysers on an industrial scale, is the scalability of the catalyst synthesis. To improve this, electrodeposition of copper catalysts was employed. Electrodeposition is a well-established industrial technique and integrable within the existing infrastructure. Electrodeposition facilitates in-situ growth of the catalyst on gas diffusion layers, thereby providing a facile alternative to the conventional multi-step process for catalyst synthesis. Different morphologies of copper were synthesized by varying the electrodeposition process parameters. Copper nanowires were also synthesized by using templated electro-deposition techniques. The catalysts were characterised before and after the CO₂ reduction experiments by Scanning ElectronMicroscopy, and X- Ray Diffraction. CO₂ reduction experiments using the synthesised copper catalysts were carried out over a range of potentials. A peak Faradaic Efficiency (FE%) of 15% was measured at -1.5 V vs RHE (uncompensated) for ethylene, 19% FE at -1.1 V vs RHE for formic acid, and 13% FE at -1.5 V vs RHE for methane. It was also seen that the catalyst suffered from stability issues which were overcome by using pulsed electrolysis. Using pulsed electrolysis the lifetime of the catalyst was increased from 30 minutes to 15 hours.