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

Abstract

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 circuit (EEC) 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 ( φ)) plots are well differentiated for the two faces, and co-adsorption of OH − was evaluated from resistance parameters. Characteristic facespecific 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).