In a series of experiments, parameters governing the CO2 feed, such as flow rate and partial pressure, were varied systematically and analysed for their effect on CO2 neutralization and reduction. Using these results and multiphysics simulations, a descriptor for CO2 reduction performance named ‘surface-accessible CO2 concentration’ ([CO2]SA) is determined. [CO2]SA represents the transient ratio of local concentration of species at the electrode surface ([CO2]/[OH–]). Ultimately, maximizing [CO2]SA can be achieved by increasing [CO2] and decreasing [OH–] (pictured). A straightforward but effective approach to increase [CO2] is to increase the partial pressure of CO2 supplied to the cell. On the other hand, since OH– is generated in situ through activity at the cathode, strategies to minimize [OH–] necessarily involve controlling operation of the cell, two of which strategies are presented in the study. First, lowering the charge passed per active site reduces [OH–] and is practically achieved by increasing the catalyst layer thickness or surface area which increases the number of active sites per geometric area. Second, the use of a pulsed electrochemical (EC) method (1 s at operating voltage, 0.5 s at recovery voltage (zero current)) maintains [CO2]SA at a high level through re-equilibration of the dissolved species.
This study reveals many possibilities to improve CO2 reduction performance by bringing the issue of local CO2 availability to the fore. High-pressure CO2 electrolysis and pulsed electrochemical methods offer two such high-potential routes. With critical and creative thinking, many other strategies to maximize [CO2]SA may rise to the surface.
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