Evidence for the Lack of Caffeine Specific Adsorption and Its Impact on Water Structure to Increase HOR/HER Activity on Pt

Abstract

As the cornerstone for much of our electrochemical knowledge, the inexplicable 2 orders of magnitude decrease in the hydrogen oxidation and evolution reactions (HOR/HER) in moving from pH 1 to 14 on precious metals continues to undermine our understanding of and ability to better design electrochemical systems.1 Several theories have been proposed for this anomalous change, from electric field effects, the influence of cations, co-adsorption of H and OH, and the orientation of interfacial water, driving innovations in in situ electrochemical techniques, such as potential changes with Fourier Transform Infrared Spectroscopy (FTIR) or X-ray Absorption Spectroscopy (XAS).2–6 Recently, it was found that including small concentrations of caffeine in pH 13 was able to drastically increase the HOR/HER catalytic activity of platinum through what was postulated as surface specific adsorption.7 In this work, we challenge this notion through an in situ surface sensitive technique, termed Attenuated Total Reflection-Surface Enhanced Infrared Reflection Absorption Spectroscopy (ATR-SEIRAS) while simultaneously analyzing the underlying theory of water reorganization. Due to SEIRAS’ s surface sensitivity, we can distinguish between specific electrochemical surface adsorption and molecules merely located in the outer Helmholtz plane (OHP) through stark tuning, where species located in the inner Helmholtz plane (IHP) experience the strongest electric fields, and demonstrate a potential dependence of their vibrational frequency.8 Further, trends in caffeine induced water structure changes support our hypothesis that surface bound water layer changes observed in our in situ spectra are responsible for the “apparent pH dependence” of HOR/HER.The electrochemical interface is traditionally viewed as a double layer model, with specific electrochemical adsorbates existing in the Inner Helmholtz Plane and the first layer of non-adsorbates at the Outer Helmholtz Plane. ATR-SEIRAS enhanced surface sensitivity allows for strong signals to be obtained within 5 nm of the surface, selectively capturing both the IHP and OHP with decreasing strength into the diffuse layer and bulk electrolyte. In situ surface sensitive IR shows strong peaks corresponding to caffeine on addition of 10-4 M to basic electrolyte, but a lack of stark tuning suggests no specific adsorption of caffeine on the Pt surface. Furthermore, SEIRAS spectra reveal subtle changes to the interfacial water with addition of caffeine, namely losses of certain types of interfacial water and changes in the water-water hydrogen bonding network. These spectra compared with similar samples collected in traditional 0.1 M HClO4 electrolyte point toward the “apparent pH dependence” being a function of pH induced changes to the interfacial water structure. 1. Durst, J., Siebel, A., Simon, C., Hasche, f., Herranz, J., & Gasteiger, H. A. New insights into the electrochemical hydrogen oxidation and evolution reaction mechanism. Energy Environ. Sci. 7, 2255–2260 (2014). Chen, X., McCrum, I. T., Schwarz, K. A., Janik, M. J. & Koper, M. T. M. Co-adsorption of Cations as the Cause of the Apparent pH Dependence of Hydrogen Adsorption on a Stepped Platinum Single-Crystal Electrode. Angew. Chemie - Int. Ed. 56, 15025–15029 (2017).Yang, X., Nash, J., Oliveira, N. J., Yan, Y. & Xu, B. Understanding the pH Dependence of Underpotential Deposited Hydrogen on Platinum. Angew. Chemie - Int. Ed. 58, 17718–17723 (2019).Sarabia, F. J., Sebastián-Pascual, P., Koper, M. T. M., Climent, V. & Feliu, J. M. Effect of the Interfacial Water Structure on the Hydrogen Evolution Reaction on Pt(111) Modified with Different Nickel Hydroxide Coverages in Alkaline Media. ACS Appl. Mater. Interfaces 11, 613–623 (2019).Liu, E. Jiao, L., Li, J., Stracensky, T., Sun, Q., Mukerjee, S., & Jia, Q. Interfacial water shuffling the intermediates of hydrogen oxidation and evolution reactions in aqueous media. Energy Environ. Sci. 13, 3064–3074 (2020).McCrum, I. T. & Koper, M. T. M. The role of adsorbed hydroxide in hydrogen evolution reaction kinetics on modified platinum. Nat. Energy 5, 891–899 (2020).Intikhab, S. et al. Caffeinated Interfaces Enhance Alkaline Hydrogen Electrocatalysis. ACS Catal. 10, 6798–6802 (2020).Lambert, D. K. Vibrational Stark effect of CO on Ni(100), and CO in the aqueous double layer: Experiment, theory, and models. J. Chem. Phys. 89, 3847–3860 (1988). Figure 1