Electrocatalytic reduction of CO₂ has the potential to convert CO₂ into carbon-based fuels by using renewable energy. While a wide range of electrocatalytic materials have been investigated, the process is still limited by poor reaction selectivity or large overpotentials. Hence many have studied the effects of surface oxides, crystal facets, and nanoparticles on the activity and selectivity of CO₂ reduction. Others have also shown that the reaction is sensitive to conditions such as temperature, CO₂ pressure, electrolyte buffer concentration and electrode porosity. These effects mean that it can be challenging to determine the underlying cause for differences in intrinsic catalytic behaviour [1,2], and thus it is important to understand the effect of experimental conditions on CO₂reduction.

Firstly, we will review the literature and our own experimental work which clearly highlight the importance of interfacial pH on the reaction [2-4]. It is well known that in weak buffers such as KHCO₃, the pH at the surface of the cathode is significantly higher than the bulk due to the hydrogen evolution and CO₂ reduction reactions. By conducting experiments over a range of KHCO₃ concentrations (and thus different interfacial pH values), it is clear that this alters the selectivity and activity of CO₂ reduction. As the hydrodynamics at the cathode surface will also alter interfacial pH, we have examined the effect of hydrodynamics on the CO₂ reduction reaction using a Cu rotating cylinder electrode [5]. Given that the enhanced mass transport will also increase the CO₂ concentration at the electrode surface [6], it seems clear that mass transfer effects should influence the reaction selectivity. We confirm that this is indeed an important factor and suggest that increasing mass transport not only decreases the interfacial pH (closer to bulk values) but also decreases the surface coverage of CO on the cathode (a key immediate during CO₂ reduction), which ultimately lowers the current going to the CO₂reduction reaction.

As these experiments revealed the importance of both KHCO₃ concentration and mass transport, we developed a numerical model to predict how the bulk electrolyte composition changes during long term electrolysis experiments. This model was validated against experiment data and shows that changes in the bulk electrolyte (ionic resistance, pH, CO₂-electrolyte equilibria) can occur over the course of long-term electrolysis experiments. These changes can complicate the interpretation of long-term electrode behaviour as well as the control of the electrochemical process. From these findings, we suggest a range of strategies to improve the experimental aspects of electrochemical CO₂reduction investigations.

[1] A.S. Hall, Y. Yoon, A. Wuttig, Y. Surendranath, Mesostructure-Induced Selectivity in CO₂Reduction Catalysis, Journal of the American Chemical Society 137 (2015) 14834-14837.

[2] R. Kas, R. Kortlever, H. Yılmaz, M.T.M. Koper, G. Mul, Manipulating the Hydrocarbon Selectivity of Copper Nanoparticles in CO₂Electroreduction by Process Conditions, ChemElectroChem 2 (2015) 354-358.

[3] K.J.P. Schouten, E. Pérez Gallent, M.T.M. Koper, The influence of pH on the reduction of CO and to hydrocarbons on copper electrodes, Journal of Electroanalytical Chemistry 716 (2014) 53-57.

[4] A.S. Varela, M. Kroschel, T. Reier, P. Strasser, Controlling the selectivity of CO₂electroreduction on copper: The effect of the electrolyte concentration and the importance of the local pH, Catalysis Today 260 (2016) 8-13.

[5] C.F.C. Lim, D.A. Harrington, A.T. Marshall, Effects of mass transfer on the electrocatalytic CO₂ reduction on Cu, Electrochimica Acta (2017) in press.

[6] N. Gupta, M. Gattrell, B. MacDougall, Calculation for the cathode surface concentrations in the electrochemical reduction of CO₂ in KHCO₃solutions, Journal of Applied Electrochemistry 36 (2006) 161-172.

[7] K. Hara, A. Tsuneto, A. Kudo, T. Sakata, Electrochemical Reduction of CO₂ on a Cu Electrode under High Pressure: Factors that Determine the Product Selectivity, Journal of The Electrochemical Society 141 (1994) 2097-2103.