Study of hydrogen evolution reaction in acid medium on Pt micro electrodes

This work describes the utilization of a Pt UME in the study of the hydrogen evolution reaction in 0.5 M H2SO4. A non-linear fitting procedure was employed in order to analyze polarization curves obtained at several temperatures (25-75 degreesC). The results revealed that the traditionally accepted model described by a Volmer-Tafel route fails to fit the obtained experimental data. In this sense, a new model was proposed involving the Volmer-Heyrovsky mechanism, being the Heyrovsky reaction rate determining step. To achieve the best fit between experimental and calculated data, the kinetic equations had to be proposed with a small value of the transfer coefficient (beta < 0.2). This unusual value was associated with an activationless process, which can also justify the limiting kinetic current (not diffusional) observed. Trying to get further insight into this possibility, the polarization studies were also performed on a surface modified by underpotentially deposited copper. With a degree of coverage as high as 0.8, the only observed effect on the polarization curves was a shift towards minor current values. This shift can be completely justified by the blocking of surface area. A change in mechanism was not observed albeit the Cu UPD eliminated the pairs of neighbor active sites necessary to the Volmer-Tafel pathway. (C) 2001 Elsevier Science Ltd. All rights reserved.