The electrochemical behavior of hydrogen peroxide, which is a possible intermediate of the oxygen reduction reaction (ORR), has been investigated on platinum single crystal electrodes containing terraces with (111) symmetry separated by monoatomic steps with either (110) or (100) symmetry in different pH solutions [1]. On these electrodes, a decrease, in absolute value, of the reduction current at low potentials is observed. The potential at which the decrease is observed, can be correlated with the potential of maximum entropy (pme) of the double layer formation on the terraces. In addition, it is possible to relate the peak appearing at potentials below 0.3 V (RHE) to the local pme in the steps.

By performing experiments at different pH values, up to pH ≈ 5, it is observed that the inhibition potential on the terraces shifts 59 mV per pH unit towards more positive potentials, in line with what was observed for the pme in laser-induced temperature jump experiments. For the possible explanation of this phenomenon, it must be taken into account that OH adsorption may influence this trend when the pme values are close to the potentials corresponding to OH adsorption. From results obtained in alkaline medium, it is clear that the surface charge plays an important role in this inhibition of the limiting diffusion current.

Finally, the comparison of the results obtained under similar conditions for the hydrogen peroxide and oxygen reduction reactions suggests that the formation of hydrogen peroxide as an ORR intermediate is less favored with increasing pH. As a consequence, there must be a bifurcation in the ORR mechanism prior to peroxide formation. These results agree with previous results [2], which indicate that ORR involves different steps in which several bifurcation points may exist. Depending on the experimental conditions, the reaction may follow a different reaction pathway [3].

References.

(1) Briega-Martos, V.; Herrero, E.; Feliu, J.M. Electrochimica Acta 2020, 334, 135452.

(2) Gomez-Marin, A.M.; Feliu, J.M. ChemSusChem 2013, 6, 172-176.

(3) Briega-Martos, V.; Herrero, E.; Feliu, J.M. Chinese J. Catal. 2020, 41, 732-738.