Mechanistic investigation of hydrogen evolution reaction from multiple proton donors: The case of mildly acidic solutions containing weak acids

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© The Author(s) 2019. The present study provides a theoretical framework for mechanistic investigation of the hydrogen evolution reaction (HER) in acidic solutions containing a secondary proton donor in the form of a weak acid. The mechanistic, thermodynamic, and kinetic implications associated with the presence of a weak acid are discussed. The presence of a weak acid was shown to be able to significantly influence the polarization behavior of the HER from H+ through its corresponding chemical dissociation reaction. This effect could lead to an increased limiting current, increased apparent Tafel slope, or even appearance of a secondary limiting current. The theoretical discussions were then applied to the case study of the HER in mildly acidic solutions containing acetic acid, on gold surface. The polarization data showed two Tafel slopes of 65mV at lower current densities and 120mV at higher current densities. A mechanistic mathematical model based on the initial theoretical discussions was developed and used to analyze and quantify the polarization behavior of this system. It was shown that, while in low Tafel slope range the presence of acetic acid has no effect on the HER, at 120 mV Tafel slope range the HER from acetic acid is significant, and it is occurring through a Heyrovsky type electro-desorption reaction.