Electrocatalysis of CO2/CO interconversion and hydrogen evolution in bicarbonate buffers: from solution to interfacial reactions

Bicarbonate buffer is largely found in nature due to its ability to regulate pH variations around neutral values. As the pH changes, so does the speciation of the buffer. At low pH the main buffer species is carbon dioxide, which for increasing pH transforms into bicarbonate, and then into carbonate. Besides the acid-base reactions in solution, the buffer species may react at the electrified interface to be reduced or oxidized.Given the importance of storing intermittent renewable electricity into chemical bonds using abundant reactants, such as carbon dioxide, it is crucial to understand the interplay of electrochemical and homogeneous reactions. Hence, in this PhD thesis we studied how the presence of a buffer in the electrolytes affects relevant electrocatalytic reactions, i.e. CO2 electrochemical reduction (Chapter 2 and 4), hydrogen evolution reaction (Chapter 4 and 5) and CO electrooxidation (Chapter 3). A special focus is placed on the employment of well-defined mass transport techniques in combination with digital simulations.Catalysis and Surface Chemistr

Understanding hydrogen evolution reaction in bicarbonate buffer

Catalysis and Surface Chemistr

Electrolyte buffering species as oxygen donor shuttles in CO electrooxidation

Electrolyte buffering species have been shown to act as proton donors in the hydrogen evolution reaction (HER). Analogously, we study here whether these electrolyte species may participate in other reactions by investigating CO electrooxidation (COOR) on a gold rotating disk electrode. This model system, characterized by fast kinetics, exhibits a dffusion-limited regime, which helps in the identification of the species dictating the diffusion-limited current. Through a systematic concentration dependence study in a variety of buffers, we show that electrolyte buffering species act as oxygen donor shuttles in COOR, lowering the reaction overpotential. A similar correlation between electrolyte and electrocatalytic activity was observed for COOR on a different electrode material (Pt). Probing the electrode-electrolyte interface by attenuated total reflection infrared spectroscopy (ATR-FTIR) and modelling the surface speciation to include the effect of the solution reactions, we propose that the buffer conjugated base generates the oxygen donor (i.e. OH-) through its acid-base reaction with water.Horizon 2020(H2020)722614-ELCORELCatalysis and Surface Chemistr

Electrolyte effects on CO2 electrochemical reduction to CO

The electrochemical reduction of CO2 (CO2RR) constitutes an alternative to fossil fuel-based technologies for the production of fuels and commodity chemicals. Yet the application of CO2RR electrolyzers is hampered by low energy and Faradaic efficiencies. Concomitant electrochemical reactions, like hydrogen evolution (HER), lower the selectivity, while the conversion of CO2 into (bi)carbonate through solution acid−base reactions induces an additional concentration overpotential. During CO2RR in aqueous media, the local pH becomes more alkaline than the bulk causing an additional consumption of CO2 by the homogeneous reactions. The latter effect, in combination with the low solubility of CO2 in aqueous electrolytes (33 mM), leads to a significant depletion in CO2 concentration at the electrode surface. The nature of the electrolyte, in terms of pH and cation identity, has recently emerged as an important factor to tune both the energy and Faradaic efficiency. In this Account, we summarize the recent advances in understanding electrolyte effects on CO2RR to CO in aqueous solutions, which is the first, and crucial, step to further reduced products. To compare literature findings in a meaningful way, we focus on results reported under well-defined mass transport conditions and using online analytical techniques. The discussion covers the molecular-level understanding of the effects of the proton donor, in terms of the suppression of the CO2 gradient vs enhancement of HER at a given mass transport rate and of the cation, which is crucial in enabling both CO2RR and HER. These mechanistic insights are then translated into possible implications for industrially relevant cell geometries and current densities.Catalysis and Surface Chemistr

Electrolyte effects on the faradaic efficiency of CO2 reduction to CO on a gold electrode

The electrochemical reduction of CO2 aims to be a central technology to store excess electricity generated by wind and solar energy. However, the reaction is hindered by the competition with the hydrogen evolution reaction. In this paper, we present a detailed quantitative study of the Faradaic efficiency (FE) to CO on a gold electrode under well-defined mass-transport conditions using rotating ring-disk electrode voltammetry. Varying the concentration of the bicarbonate and the electrolyte cation employing different rotation rates, we map out how these parameters affect the FE(CO). We identify two different potential regimes for the electrolyte effects, characterized by a different dependence on the cation and bicarbonate concentrations. For hydrogen evolution, we analyze the nature of the proton donor for an increasingly negative potential, showing how it changes from carbonic acid to bicarbonate and to water. Our study gives detailed insights into the role of electrolyte composition and mass transport, and helps defining optimized electrolyte conditions for a high FE(CO).Catalysis and Surface Chemistr