Engineering aspects for the electrochemical CO₂ reduction to formate

To fight global warming, researchers have developed a wide variety of carbon capture and utilization techniques. Specifically, due to the increase in renewable electricity, the study on electrochemical CO2 reduction has received a lot of attention from the scientific community during the past decade. One of its most promising products is formate, which is mainly due to three reasons: (i) cheap earth abundant metal catalysts, (ii) easy and cheap storage of product and (iii) a simple two electron reaction mechanism. In most of these studies the focus was primarily on the optimization of the cathode catalyst material, as it makes a big impact on the product selectivity, overpotential and stability of the process. As a result significant progress has been made in the field of catalyst development. However, while the understanding of the molecular details at the catalyst surface is ever more growing, so far little attention has been given to the upscaling and studying of electrochemical CO2 reduction in a continuous flow cell. Nevertheless extensive understanding of the behavior of continuous flow cell electrolyzers is necessary in order to achieve an industrial feasible process with a high current density (>200 mA/cm²), high Faradaic efficiency (80 – 90%) and low cell voltage (< 3.5V), which will be the scope of this thesis. In the first part of the thesis the fundamentals of electrochemical CO2 reduction as well as electrochemical reactor engineering are discussed. Additionally a literature review on gas diffusion electrode based CO2 electrolyzers is provided. Next a continuous flow-by electrolyzer is constructed and benchmarked for the electrochemical reduction of CO2 towards formate. Once the baseline behavior of the system is well understood several process parameters are adjusted in order to optimize the reactor performance. In Chapter 5 to 7 the gas diffusion electrode based electrolyzers were further optimized. First it was found that flooding of the electrode can be averted when the differential pressure across the electrode was maintained at 0 mbar. Next the influence of the membrane type (bipolar membrane versus cation exchange membrane) was investigated. Here we showed how the use of a bipolar membrane can shift the product selectivity from formate towards the more desirable formic acid. Finally a novel reactor set-up in which direct water injection is used as a method to humidify zero-gap electrolyzers is discussed. The final experimental chapter of this thesis, Chapter 8, explores the direct conversion of bicarbonate instead of gaseous CO2. This alternative makes the need for energy intensive CO2 recovery processes unnecessary. A proof of concept and a limited optimization study is provided in this thesis. Overall we have significantly reduced the knowledge gap between academic research and industrial scale electrochemical CO2 reduction and have thereby accomplished the goals of this thesis. The novel electrolyzer system reached excellent performance: a high current density (100 mA/cm²), high Faradaic efficiency (85%) and low cell voltage (< 2.5V) in combination with high product concentrations (<60 g/l). These results approach the values set-out at the beginning of this thesis as required for industrial scale. However the stability of the final system is still limited and therefore additional research is required. Future scientist are advised to continue studying these systems in order to better understand and solve the degradation mechanisms at play. Additionally, future research should also focus on the implementation towards other reaction products such as CO and ethylene.

Jaar van publicatie:2022

Trefwoorden:Doctoral thesis

Toegankelijkheid:Open