Electrochemical Impedance Spectroscopy Study Of Hydrogen And Oxygen-containing Species Adsorption On Ir(111) Electrode

Electrochemical reactions often occur in the double layer,accompanied by the specific adsorption of some species,such as water,cations,anions,reactants,reaction intermediates or other species in the solution.The most simple adsorption process is the adsorption of hydrogen,since the process only involves one proton and one electron,and thus it is very suitable for studying electrochemical mechanism as a model reaction.At the same time,the adsorption of hydrogen is the basic process of hydrogen electocatalytic system?hydrogen evolution reaction and hydrogen oxidation reaction?.Oxygen-containing species adsorption is an indispensable process in the oxygen electrocatalytic system?oxygen reduction reaction and oxygen evolution reaction?,the behavior of which is widely studied.Studying the adsorption behavior of hydrogen and oxygen-containing species can help us understand the mechanism of hydrogen/oxygen electocatalytic system,thus providing theoretical support for us to solve the basic problem of hydrogen fuel cell and water electrolysis technology,which is how to design more effective and cheaper catalyst.At present,Pt is the best catalyst of hydrogen/oxygen electrocatalytic system,and we try to study the influence of different substrate on the adsorption of hydrogen and oxygen-containing species from the point of structure-activity relationship.Since Ir is very similar to Pt in terms of physical and chemical properties,we choose well-defined Pt?111?and Ir?111?as the model electrode to study the adsorption of hydrogen and oxygen-containing species.We mainly carried out the following studies:Electrochemical impedance study of hydrogen adsorption on Pt?111?and Ir?111?electrode:We used cyclic voltammetry and electrochemical impedance spectroscopy to study the adsorption behavior of hydrogen on Pt?111?and Ir?111?electrode in perchloric acid and sulfuric acid solution.In 0.1 mol/L HClO4 solution,the kinetics of hydrogen adsorption on Pt?111?is extremely fast and is almost independent on the potential.Compared with Pt?111?,the hydrogen adsorption rate on Ir?111?is slower and decreases with the increase of potential.At 0.2 V,the adsorption rate of hydrogen on Ir?111?in 0.1 mol/L HClO4 solution(1.74×10-8 mol^-2s^-1)is there orders of magnitude slower than that on Pt?111?(8.06×10-6 mol^-2s^-1).The slower kinetics of hydrogen adsorption on Ir?111?is due to the strong interaction between Ir?111?electrode and water,which means the water network on Ir?111?surface is so stable that the hydrogen species must overcome much higher energy barrier to break the stable water structure and absorb on the Ir?111?surface.In 0.5 mol/L H2SO4 solution,the adsorption rate of hydrogen species on Ir?111?is decreasing with the increase of potential.At 0.2 V,the adsorption rate of hydrogen on Ir?111?in 0.5 mol/L H2SO4 solution(2.04×10-9 mol^-2s^-1)is one order of magnitude slower than that in 0.1 mol/L HClO4 solution(1.74×10-8 mol^-2s^-1),and this is caused by the strong adsorption of sulfate species,that is to say,sulfate species occupy the active sites and inhibit the adsorption of hydrogen species.Electrochemical impedance study of oxygen-containing species adsorption on Pt/Rh/Ir?111?electrode:We used cyclic voltammetry and electrochemical impedance spectroscopy to study the behavior of oxygen-containing species adsorption on Pt/Rh/Ir?111?electrode in perchloric acid solution under the atmosphere of nitrogen and oxygen.In N2-saturated-0.1 mol/L HClO4,the kinetics of oxygen-containing species adsorption on Pt?111?is extremely fast and is almost independent on the potential,and the kinetics on Rh?111?reaches maximum at the peak potential?0.65 V?,while the kinetics on Ir?111?is increasing with the increase of potential,due to the increasing of thermodynamic force.In O2-saturated-0.1 mol/L HCIO4,the kinetics of ORR on Pt?111?is increasing with the increase of potential,and the kinetics on Rh?111?reaches maximum at the peak potential?0.65 V?,while the kinetics on Ir?111?is decreasing with the increase of potential,which is because the coverage of oxygen-containing species is increasing with the increase of potential,thus causing the reduced number of active sites on the electrode surface.