Optimization of in Situ pH Detection

Abstract

High selectivity and electrode efficiency are essential for practical CO2 reduction [1-2]. However, the simultaneous hydrogen evolution reaction in aqueous solutions lowers process efficiency. Both reactions change the local pH, which in turn, has been shown to have an impact on CO2 reduction reactions [2-5]. In situ detection of pH using ultramicroelectrodes can help elucidate the pH dependency of these reactions [5-6]. However, the presence of the ultramicroelectrode tip can significantly influence conditions near the surface due to transport restrictions. Using a numerical model that accounts for the presence of the tip, the operando environment near the surface can be elucidated.In this work, a 2D-axisymmetric finite element (FEM) simulation was developed to predict concentration fields of reacting species near the electrode surface and in the presence of the ultramicroeletrode tip. The simulation can be used to identify optimal geometric and spatial parameters for experimental surface characterization. Using the FEM model, the operando pH within 100 µm of the surface electrode varies by up to 6 pH units within a 0.5 - 0.9 V vs. Ag/AgCl potential window. This wide variation is due to a diffusion boundary layer that grows with decreasing potential. The near-surface concentration of buffer species (Figure 1) was found to vary, along with pH, as a function of electrode potential and time.Lee, C. W.; Cho, N. H.; Im, S. W.; Jee, M. S.; Hwang, Y. J.; Min, B. K.; Nam, K. T. New Challenges of Electrokinetic Studies in Investigating the Reaction Mechanism of Electrochemical CO2 Reduction. J. Mater. Chem. A 2018, 6 (29), 14043–14057. https://doi.org/10.1039/C8TA03480J.Goyal, A.; Marcandalli, G.; Mints, V. A.; Koper, M. T. M. Competition between CO2 Reduction and Hydrogen Evolution on a Gold Electrode under Well-Defined Mass Transport Conditions. J Am Chem Soc 2020, 142 (9), 4154–4161. https://doi.org/10/gg84dq.Zhang, B. A.; Ozel, T.; Elias, J. S.; Costentin, C.; Nocera, D. G. Interplay of Homogeneous Reactions, Mass Transport, and Kinetics in Determining Selectivity of the Reduction of CO2 on Gold Electrodes. ACS Cent. Sci. 2019, 5 (6), 1097–1105. https://doi.org/10/ggbvcs.Seifitokaldani, A.; Gabardo, C. M.; Burdyny, T.; Dinh, C.-T.; Edwards, J. P.; Kibria, M. G.; Bushuyev, O. S.; Kelley, S. O.; Sinton, D.; Sargent, E. H. Hydronium-Induced Switching between CO2 Electroreduction Pathways. J. Am. Chem. Soc. 2018, 140 (11), 3833–3837. https://doi.org/10.1021/jacs.7b13542.Monteiro, M. C. O.; Jacobse, L.; Touzalin, T.; Koper, M. T. M. Mediator-Free SECM for Probing the Diffusion Layer PH with Functionalized Gold Ultramicroelectrodes. Analytical Chemistry 2020, 92 (2), 2237–2243. https://doi.org/10/gg77vg.Carneiro-Neto, E. B.; Lopes, M. C.; Pereira, E. C. Simulation of Interfacial PH Changes during Hydrogen Evolution Reaction. Journal of Electroanalytical Chemistry 2016, 765, 92–99. https://doi.org/10/f8pbnv. Figure 1