Voltammetric response of hydrogen adsorbates on platinum in acid solutions: a possible H-adatom subsurface state

Journal of the Brazilian Chemical Society

Zinola, C.F¹  Arvia, A.J¹  Facultad de Ciencias, Montevideo, Uruguay¹  Martins, M.E¹  Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas (INIFTA), La Plata, Argentina¹ 

关键词: platinum electrodes;    hydrogen evolution;    H-adatoms;    third H-adatom current peak     Introduction The hydrogen evolution reaction (HER);    one of the most important processes in electrocatalysis;    has been used as a prototype for several electrode reactions1-7. Depending on the nature of the metal/solution interface;    the kinetics of the HER has been explained through different reaction pathways involving the participation of H-adatoms as reaction intermediates. Accordingly;    depending on the type of adsorption isotherm which H-adatoms obey;    kinetic equations derived from different reaction pathways account for specific potential and time dependencies ofq;    the degree of electrode surface coverage by reaction intermediates1-3. However;    in conventional mechanistic approaches to the HER under steady-state conditions;    little attention has been paid to the influence of the characteristics of adsorption sites on the kinetics of the reaction. In fact;    as far as we know;    there is only one example in which the possibility of the surface diffusion of H-adatoms occupying different adsorption sites of Pt has been considered8. Since most of the work done at the metal/aqueous solution interface has focused on the evaluation of the isotherm for H-adatoms and chemisorption energy2;    3;    7-10;    non-equilibrium effects due to surface diffusion processes have been disregarded. Valuable information about the mechanism of the HER has been obtained from the underpotential deposition of H-adatoms at noble and semi-noble metals;    but little has been gained from those species which might act as actual precursors to H2 molecule formation at overpotential conditions11-14. This drawback is caused by the fact that the steady H2 evolution current greatly exceeds those transient current contributions associated with H-adatom reactions. In fact;    AC impedance spectroscopy data have made the analysis of H intermediates possible in a potential range close to the HER threshold potential15;    16. On the other hand;    the application of different potential routines to noble metal electrodes produces changes in the surface morphology;    including crystallographic orientation17-19. These changes involve modifications in the adsorbate characteristics of the metal electrode;    as has been well established for H-adatoms on Pt19. In this case;    the H-adatom electrosorption voltammogram acts as a sort of fingerprint of the adsorption energy characteristics of the electrode surface;    thus offering the possibility of determining the influence of a specific metal topography on a particular reaction;    as in the case of the H-adatom electrosorption reaction on Pt substrates. This paper provides experimental data on the possible existence of a H-adatom state acting as a precursor in the HER;    yielding species which appear to be closely related to the so-called "third" anodic voltammetric peak for Pt in acid solutions. In situ infrared data20 and visible-infrared sum frequency generation vibrational spectroscopy21;    22 indicated the presence of these species;    which was supported by quantum chemistry calculations23.   Experimental Electrochemical runs were made in different conventional electrochemical cells made of either glass or Teflon;    with a Pt working electrode;    a large Pt gauze counterelectrode;    and a reversible hydrogen reference electrode. The following aqueous solutions were employed: x M H2SO4 (0.05 £ x £ 1.0);    0.5 M HClO4;    0.3 M HF;    and 1 M HF. Solutions were prepared from a.r. chemicals and Millipore-MilliQ* water. All runs were performed under continuous oxygen-free nitrogen saturation at 25 °C. Potentials in the text are given in the reversible hydrogen electrode (RHE) scale. Three types of Pt working electrodes were used. i) Smooth polycrystalline (pc) Pt electrodes;    which were first immersed in a hot H2SO4-HNO3 1:1 mixture;    subsequently rinsed with Millipore-MilliQ* water;    and finally placed in the working solution. ii) Smooth pc Pt electrodes;    which were subjected to repetitive triangular potential cycling (routine I) at 0.1 V s-1 between Esc = 0.05 V and Esa = 1.45 V until a stable voltammogram (blank) was achieved. They were then subjected to preset repetitive symmetric square wave potential cycling (routine II) to develop a certain preferred crystallographic orientation (pco). This potential routine was characterized by an upper (Eu) and a lower switching potential (El) and a frequency (f). Details of these techniques have been given elsewhere17-19. Two types of pco Pt electrodes (pco I and pco II) were employed. The preparation conditions under routine II were -0.2 V £ El £ 0.05 V;    Eu = 1.5 V;    4 kHz £ f £ 7 kHz for 120 s £ t £ 3600 s for pco I Pt;    and 0.60 £ El £ 0.70 V;    Eu = 1.5 V;    4 kHz £ f £ 7 kHz for 120 s £ t £ 3600 s for pco II Pt. The cyclicvoltammogram of pco I Pt run in 1 M H2SO4 at 0.1 V s-1 was similar to that of a Pt(100) surface after it has been cycled a few times through the O-adatom potential region24-26. However;    in contrast to previous results18;    the electroreduction sweep for pco I Pt exhibited a current hump very near the HER threshold potential where the Pt surface was almost completely covered by H-adatoms. On the other hand;    the cyclicvoltammograms of pco II Pt closely resembled the characteristics described in the literature for Pt(111) after it has been potential cycled in the O-adatom potential range27. iii) Columnar structured (cs) Pt electrodes involving roughness factors in the range of 10 to 150. The preparation technique of this type of electrode has been described elsewhere28. Voltammograms obtained for cs Pt electrodes in 1 M H2SO4 showed the same pair of conjugated current peaks for the strongly and weakly bound H-adatoms. A combined cyclic voltammetry technique was employed to follow the various H-adatom electrosorption processes. The first step consisted of the application of a repetitive triangular potential sweep covering the range Esc = 0.02 V to 0.05 V £ Esa £ 0.25 V (routine III);    and the second step involved the application of triangular potential sweeps;    including a potential holding at Eh (0.02 V £ Eh £ 0.25 V) in the potential sweep run in the negative potential direction;    for time th (10 s £ th £ 20 min) (routine IV). Both routines III and IV were followed by cyclicvoltammetry run in the range from Esc = 0.05 V to either Esa = 0.50 V or 1.45 V. Changes in the successive voltammograms allowed us to infer possible rearrangements of the H-adatom configuration on Pt promoted by the application of the potential routines. Finally;    the potential cycling was continued until the stable cyclicvoltammogram was attained. The same routine was applied to all working solutions. The real surface area of pc-Pt;    pco-Pt;    and cs-Pt electrodes was measured through the change of the H-adatom electrosorption read from the voltammogram run at 0.1 V s-1;    covering the range from Esc = 0.05 V to Esa = 0.50 V in aqueous 0.5 M H2SO4. For this purpose;    the H-adatom monolayer charge density for each Pt surface was taken from the literature as 210 mC cm-2;    242 mC cm-2;    and 210 mC cm-2 for Pt (100);    Pt (111);    and pc-Pt;    respectively29.   ResultsGeneral featuresCyclic voltammograms run at 0.1 V s-1 for pco I-Pt and pco II-Pt in aqueous 0.5 M H2SO4 and 1 aqueous M HF (Fig. 1);    as well as those run in aqueous 0.5 M HClO4 in the range of Esc = 0.05 to Esa = 1.45 V;    exhibit qualitatively comparable features. Thus;    the H-adatom electrosorption potential range extends from 0.05 V to 0.45 V;    and the O-atom electrosorption and oxide formation potential range covers from ca. 0.70 V to 1.50 V. The charging-discharging double layer region is located in between. These results confirm the influence of the Pt surface morphology on the H- and O-adatom electrosorption voltammograms in those electrolytes. ;   

DOI  :  10.1590/S0103-50531997000400008

学科分类：化学（综合）

来源: SciELO

【 摘 要 】

The hydrogen evolution reaction (HER), one of the most important processes in electrocatalysis, has been used as a prototype for several electrode reactions1-7. Depending on the nature of the metal/solution interface, the kinetics of the HER has been explained through different reaction pathways involving the participation of H-adatoms as reaction intermediates. Accordingly, depending on the type of adsorption isotherm which H-adatoms obey, kinetic equations derived from different reaction pathways account for specific potential and time dependencies ofq, the degree of electrode surface coverage by reaction intermediates1-3. However, in conventional mechanistic approaches to the HER under steady-state conditions, little attention has been paid to the influence of the characteristics of adsorption sites on the kinetics of the reaction. In fact, as far as we know, there is only one example in which the possibility of the surface diffusion of H-adatoms occupying different adsorption sites of Pt has been considered8. Since most of the work done at the metal/aqueous solution interface has focused on the evaluation of the isotherm for H-adatoms and chemisorption energy2,3,7-10, non-equilibrium effects due to surface diffusion processes have been disregarded. Valuable information about the mechanism of the HER has been obtained from the underpotential deposition of H-adatoms at noble and semi-noble metals, but little has been gained from those species which might act as actual precursors to H2 molecule formation at overpotential conditions11-14. This drawback is caused by the fact that the steady H2 evolution current greatly exceeds those transient current contributions associated with H-adatom reactions. In fact, AC impedance spectroscopy data have made the analysis of H intermediates possible in a potential range close to the HER threshold potential15,16. On the other hand, the application of different potential routines to noble metal electrodes produces changes in the surface morphology, including crystallographic orientation17-19. These changes involve modifications in the adsorbate characteristics of the metal electrode, as has been well established for H-adatoms on Pt19. In this case, the H-adatom electrosorption voltammogram acts as a sort of fingerprint of the adsorption energy characteristics of the electrode surface, thus offering the possibility of determining the influence of a specific metal topography on a particular reaction, as in the

【 授权许可】

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