Abstract: As electrochemical CO2 reduction studies progress from beaker or H-cell devices operating at low current densities to gas diffusion electrode (GDE)-based devices that sustain high reaction rates and provide an avenue toward commercialization, the overall system becomes significantly more complex. While the current densities may vary for the different approaches, it is essential to maintain the same scientific rigor when analyzing these systems. The mass transfer optimizations used in GDE based approaches necessarily add complexity and provide new challenges that need to be analyzed and overcome in terms of both engineering as well as analysis techniques. This Account puts into perspective our recent works analyzing high current density CO2 electrolysis performance via a comprehensive investigation of the entire system.
In particular, we show the importance of monitoring (i) the gas flow rates at the outlet of the cathodic compartment, (ii) the anodic gas composition for CO2/O2 ratio, and (iii) pH variations in the electrolyte. A rigorous analysis of these parameters allows us to achieve a complete carbon balance, in addition to accounting for a total of 100% Faradaic efficiency. By analyzing both the cathode outlet and anodic CO2:O2 ratio, we demonstrate that these methods can be used to self-validate results providing robustness. We show that this analysis approach holds for both a zero-gap membrane electrode assembly device and a flowing-catholyte device. In addition, a comprehensive monitoring approach reveals that having an alkaline environment in the vicinity of the cathode can absorb substantial amounts of CO2, which may greatly distort Faradaic efficiencies if not accounted for. While monitoring the outlet flow rate of a reactor appears a simple task, the mixed gases and small flow rates in lab-scale reactors can add challenges and we discuss various methods to measure these flow rates.
While pH is well-known to play a role in the activity and selectivity of CO2 reduction, we demonstrate that (i) the operational pH is not necessarily the pH of the initial electrolyte, (ii) there are long transients in pH before steady state is reached (on the order of hours), and (iii) the pH of the anolyte and catholyte can be significantly different over the duration of the electrolysis.
By varying the membrane type in a flowing-catholyte reactor (anion exchange, cation exchange, or bipolar membrane), we can use this monitoring approach to quantitatively identify the major differences in CO2 reduction performance related to these distinct membrane types. The overall conclusion is that complex engineering processes entail that a thorough monitoring of parameters is necessary to accurately analyze the performance of high current density electrochemical CO2 reduction devices.
Abstract: In this work, the effect of ion-selective membranes on the detailed carbon balance was systematically analyzed for high-rate CO2 reduction in GDE-type flow electrolyzers. By using different ion-selective membranes, we show nearly identical catalytic selectivity for CO2 reduction, which is primarily due to a similar local reaction environment created at the cathode/electrolyte interface via the introduction of a catholyte layer. In addition, based on a systematic exploration of gases released from electrolytes and the dynamic change of electrolyte speciation, we demonstrate the explicit discrepancy in carbon balance paths for the captured CO2 at the cathode/catholyte interface via reaction with OH when using different ion-selective membranes: (i) the captured CO2 could be transported through an anion exchange membrane in the form of CO32-, subsequently releasing CO2 along with O2 in the anolyte, and (ii) with a cation exchange membrane, the captured CO2 would be accumulated in the catholyte in the form of CO32-, while (iii) with the use of a bipolar membrane, the captured CO2 could be released at the catholyte/membrane interface in the form of gaseous CO2. The unique carbon balance path for each type of membrane is linked to ion species transported through the membranes.