Coulostatic Potential Transients Induced by Laser Heating of a Pt(111) Single-Crystal Electrode in Aqueous Acid Solutions. Rate of Hydrogen Adsorption and Potential of Maximum Entropy

Abstract

Short pulses (∼10 ns) of high-power laser light produce a sudden increase in the surface temperature of a Pt(111) single-crystal electrode in acidified potassium perchlorate or sulfate solutions. The change of the electrode potential at open circuit was monitored during the relaxation of the temperature. At low pH, the potential transients in the hydrogen adsorption region exhibit a bipolar shape, which can be explained considering that the relaxation is influenced by the rate of hydrogen adsorption. With this assumption, we have estimated a value of the rate constant for hydrogen adsorption around 8 × 10 6 M -1 s -1 . By increasing the pH, the rate of hydrogen adsorption is reduced, making it possible to decouple the double-layer response from the hydrogen adsorption process. In this case, the potential where the double-layer response to the temperature increase is zero can be identified with the potential of maximum entropy of formation of the double layer. This potential is located in the double-layer region (ca. 0.43 V vs Pd/H 2 ) for a solution of pH 3. The relevance of this measurement in terms of the location of the potential of zero charge is discussed.

Towards the understanding of the interfacial pH scale at Pt(1 1 1) electrodes

“…With this information it could be concluded that pzfc and pztc almost coincided in the case of Pt(111) with a value of 0.32 V SHE. This value agrees reasonable well with the position where water dipole reorientation is inferred from IR measurements [33] and laser induced temperature jump experiments [34,40]. A maximum in the differential capacity has been indeed interpreted as a consequence of dipole reorientation [32], although in this case, it was proposed that the pzfc does not coincide with the peak but it is located at higher potentials, in the OH adsorption region.…”

“…Alternatively, the position of the pzfc can be inferred from indirect measurements, like the reorientation of water dipoles as monitored with infrared spectroscopy [33] or laser inducted temperature jump measurements [34][35][36][37][38][39][40][41]. It is also worth mentioning in this context the studies in UHV of model double layer systems created by sequential dosing of the same components of the true double layer in electrochemical environment [42,43].…”

Investigating interfacial parameters with platinum single crystal electrodes

“…In addition to the previous description, thermodynamic considerations demonstrate that the thermal coefficient of the potential drop is linked with the entropy of double layer formation through [32,33,35,36]:…”

“…The change in the open circuit potential during a change of temperature induced by pulsed laser irradiation can be rationalised considering that the main contribution comes from the perturbation of the dipolar part of the potential drop [32][33][34][35]. Such dipolar contribution will be negative for positive electrode charges, since, in this case, the water dipoles will be oriented with the oxygen towards the metal adding a negative term to the potential difference  M - S .…”

“…Other contributions to the potential change come from the variation of the spillover of electrons during temperature change and from the thermodiffusion potential in the solutions. It has been shown that the latter contributions can be considered negligible [32], therefore the sign of the laser induced potential transients are indicative of the orientation of the dipoles at the interfase. Then, the potential of zero transient coincides with the turnover of the direction of the dipoles induced by the potential.…”