Evaluation, Investigation, and Mitigation of CO2-Crossover in Electrochemical CO2 Reduction Cells

The Carbon Dioxide Reduction Reaction (CO2RR) is a promising pathway for converting CO2 and electrical energy into valuable feedstock for the chemical industry, thereby aiding in the challenging defossilization of this sector. Three decades of research and a recent surge in scientific interest have led to significant advances, particularly in the development of highly selective and stable electrocatalysts. These catalysts, especially those used for the reduction to CO, now meet the criteria for industrial application regarding stability and selectivity. The presence of CO2 at the cathode leads to unique requirements and challenges that are specific to CO2RR. This work focuses on CO2-crossover, where the educt CO2 is transported within the cell and eventually mixes with the anode product (O2), leading to a loss of purified CO2. CO2-crossover has long been overlooked but poses a significant obstacle to the successful scaling of CO2RR. In this thesis, the challenges that CO2-crossover presents to CO2RR are investigated, and potential solutions are explored. A high-level overview of configurations for CO2RR cells, defined by their anode and cathode environments and their separators, is presented, and their suitability for CO2RR is evaluated. The economic implications of CO2-crossover are considered, and an “equivalent overpotential”, 𝜂𝑒𝑞, and an “equivalent cell voltage”, 𝑈𝑐𝑒𝑙𝑙,𝑒𝑞, are introduced as tools to frame CO2-crossover in an electrochemical way and balance out the optimization of cell voltage and CO2-crossover. Through the introduction of 𝑈𝑐𝑒𝑙𝑙,𝑒𝑞, it becomes apparent that CO2-crossover is a major economic obstacle and that a high CO2-crossover outweighs the low cell voltages seen in Anion Exchange Membrane (AEM)-based cells. This first part of the thesis demonstrates that it is paramount to reduce and mitigate CO2-crossover. After the introduction of CO2-crossover and analysis of its impact, two approaches for mitigating CO2-crossover are presented. The first approach is based on an established Gas Diffusion Electrode (GDE) and uses a “mixed electrolyte” configuration, which offers high electrolyte stability. The initial configuration with a KHCO3 electrolyte shows a high CO2-crossover. By changing to a “carbonate-free” mixed electrolyte (K2SO4), the transport mechanism via carbonates is avoided, leading to a reduction of CO2-crossover. However, CO2-crossover is not completely eliminated. Different potential transportation pathways are investigated and it is demonstrated that CO2 is transported as a physically dissolved gas. Two methods — degassing the electrolyte and using a Gas Diffusion Anode (GDA) — are proposed and successfully tested to further reduce the CO2-crossover. These methods can reduce CO2-crossover to an acceptable limit: Compared to a K2SO4 reference case, they reduce CO2-crossover by 80 % and 90 %, respectively. With these methods, high selectivities of FECO ≥ 90 % at200 mA cm−2 can be achieved with CO2-crossover below 𝜒 ≤ 0.1 (CO2 lost per CO2 converted). In the second approach, CO2RR is facilitated on an Ag-coated Cation Exchange Membrane (CEM) by ammonium ions. The introduction of a CEM between the cathode and anode can prevent CO2-crossover via chemically and physically dissolved gases.However, the use of a proton-conducting CEM can suppress the CO2RR due to the high acidity at the cathode catalyst interface. First, a mechanism of how ammonium can locally buffer the cathode catalyst is proposed. Afterwards, the electrolysis cell is sequentially eveloped to prove that ammonium can enable CO2RR on a CEM and then to identify and eliminate potential hurdles for stable and continuous operation of the cell. A Gas Diffusion Electrode (GDE)-based setup is used to prove the feasibility of CO2RR in the presence of NH4+. Then, an Ag-coated CEM is introduced and first tested in a double-CEM configuration, with an isolated, NH4+-based catholyte ((NH4)2SO4). Here, it is established that NH3 is released at the cathode and needs to be recaptured for an economically viable operation. Recapturing NH3 was successfully proven for a runtime of more than 100 h at 50 mA cm−2. In a second step, the cell is changed to a single-CEM configuration, which eliminates one CEM and reduces ohmic losses in the cell. It is shown, that, unlike NH3, NH4 ions are not electrochemically oxidized at the anode. 𝐹 𝐸CO ≥ 90 % was observed at 50 mA cm−2, and at 200 mA cm−2, 𝐹 𝐸CO ≥ 50 % was measured at 3.6 V. In another experiment, the NH4+-based electrolyte was replaced by H2SO4 and NH3 was supplied via the educt gas feed. The gaseous NH3 supply was sufficient to enable CO2RR, and using H+ as charge carrier in the CEM reduced the ohmic losses. However, 𝐹𝐸CO was lower than for the comparable single-CEM experiment with a NH4+-based electrolyte.