Specific adsorption of phosphate species on Ag (111) and Ag (100) electrodes and their effect at low overpotentials of the hydrogen evolution reaction

Investigation of phosphate species adsorption/desorption processes was performed on Ag(100) and Ag(111) electrodes in H$_{3}$PO$_{4}$, KH$_{2}$PO$_{4}$ and K$_{3}$PO$_{4}$ solutions by Current-Potential ($j-V$) profiles and Electrochemical Impedance Spectroscopy ($EIS$). We used the equivalent circuit method to fit the impedance spectra. Different electrical equivalent circuits ($EECs$) were employed depending on the potential region that was analyzed. For potentials more negative than the onset of the hydrogen evolution reaction ($her$), a charge transfer resistance (R$_{ct}$) in parallel to the $(RC)$ branches was included. Peaks from $j-V$ profiles were integrated to estimate surface coverage. A reversible process was observed for Ag(hkl)/KH$_{2}$PO$_{4}$ systems, where a value of 0.07 ML was obtained. For Ag(111)/H$_{3}$PO$_{4}$, a coverage of about 0.024 ML was calculated from anodic/cathodic $j-V$ profiles, whereas for Ag(hkl)/K$_{3}$PO$_{4}$ systems different values were obtained from integration of anodic/cathodic peaks due to highly irreversible processes were observed. In the case of Ag(hkl)/K$_{3}$PO$_{4}$, the capacitance (C$_{(phi)}$) plots are well differentiated for the two faces, and co-adsorption of OH$^{-}$ was evaluated from resistance parameters. Characteristic face-specific relaxation times are obtained for each electrode. In addition, it was found that the onset potential of $her$ for Ag(111) at pH=1.60 was about 100 mV more negative compare to Ag(100).