Development of electrochemical reactors for CO2 electrolysis

Removal of atmospheric carbon dioxide or its capture from point sources and subsequent utilisation might play a relevant role in the mitigation of climate risks generated by increasingly impactful anthropogenic activity. The electrochemical reduction of captured CO2 allows to store electrical energy in the form of carbon-based products, substituting petrochemical processes. Furthermore, it offers the possibility of tuning the reaction for several products, from simple C1 compounds such as carbon monoxide and formic acid to more complex C2+ chemicals such as ethylene, ethanol or propanol. The production of carbon monoxide and formic acid is the closest to industrial implementation due to their lower energy requirements and wide demand. This dissertation aims at advancing the knowledge on the scalability of CO2 electrolysis towards these two products. With that goal, the focus is centred on the challenges hampering its industrial implementation, such as the poor stability of the gas diffusion electrodes commonly employed in CO2 electrolysers and the prohibitive cost of state-of-the-art catalysts. Concerning reactor engineering, the two most relevant electrolyser classifications – three-compartment flow reactors and zero-gap reactors – are scrutinized. While the former allows for the possibility of accurate cathodic potential control for electrocatalytic study, the latter minimises ohmic losses and optimises its energy efficiency due to its compact structure. The research conducted in this PhD fine-tunes several critical reaction parameters such as catalyst loading, Nafion® ionomer content, working potential and CO2 supply. Furthermore, several ion exchange membranes and separators are tested as well as different electrolytes. The results achieved utilizing the three-compartment cells show a preference for flow-by mode with catalyst loadings above 0.75 mg cm-2 and Nafion® wt% between 10 and 20%. Regarding the zero-gap system, it is concluded that employing an anion exchange membrane allows to drive carbonates away from the cathode, which is beneficial for long-term operation. Additionally, whereas water injection is disadvantageous in terms of CO2 reduction selectivity for the CO reaction pathway, it has a beneficial effect on the production of formate, minimizing the formation of precipitates inside the pores of the gas diffusion layer. Regarding catalyst development, this dissertation analyses the performance of alternative noble metal-free catalysts based on cheap and abundant materials, at industrially relevant current densities. Nickel-nitrogen-doped carbons (NiNC) are shown to be suitable catalysts for the production CO as well as tin-nitrogen-doped carbon catalysts for the production of formate. Both are competitive with the commercial alternatives (i.e. Ag and Sn nanoparticles) in terms of Faradaic efficiency and stability at current densities as high as 200 mA cm-2 in a three-electrode flow-by cell, with NiNC even outperforming state-of-the-art commercial Ag nanoparticles in terms of overpotential required. Its performance in zero-gap conditions reveals again high electrochemical performance, although exposing stability weaknesses that should be subject to further research. These considerations are paramount for successful upscaling of CO2 electrolysis and build upon the results of hundreds of other studies on this rapidly advancing field.

Publication year:2022

Keywords:Doctoral thesis

Accessibility:Open