Mechanistic analysis of the electro-reduction of oxygen to water on metal catalyst surfaces partially inhibited by electro-oxidation

JOSE LUIS FERNANDEZ; FERNANDEZ, WANDA V.; MAURICIO D. ARCE

Mar del Plata

Congreso; 34th Topical Meeting of the International Society of Electrochemistry; 2023

International Society of Electrochemistry (ISE)

One of the biggest challenges for the development of widely accessible fuel cells is the slow kinetics of the cathodic reaction, the oxygen reduction reaction (ORR). The sluggish ORR kinetics results in an important loss of power density as well as in the use of large amounts of costly precious metal catalysts. Thus, understanding the ORR mechanism is crucial for defining rational guidelines toward the development of improved electrocatalysts for this reaction. This is a quite challenging goal, not only due to the intrinsically complex sequence of elementary steps (involving several adsorbed intermediates) but also due to the potential occurrence of parallel processes, such as the metal electro-oxidation proceeding at anodic potentials, which may inhibit active sites. Complete kinetic models that precisely describe all these processes are necessary in order to obtain reliable and meaningful kinetic information from experimental data. In this context, this work reports equations for describing steady state polarization curves for the ORR on noble metals, which were derived on the basis of a mechanistic analysis that takes into account the potential-dependent inhibition of active sites due to their electro-oxidation. It is proposed that the electro-oxidation of the metallic surface sites occurs in parallel to the ORR, causing the formation of subsurface oxides that block the active sites capable to adsorb ORR intermediates. Thus, the kinetic regime and adsorption isotherm of this electro-oxidation reaction play important roles, together with the ORR mechanism, in defining the current-potential dependence at lower overpotentials. On the one hand, complete kinetic equations without rate-determining approximations were derived for the ORR operating through the simultaneous occurrence of two routes, so-called the direct and the indirect (or associative) routes, which allowed to describe the current-potential dependence on an electrode surface that could be partially inhibited. Moreover, for accounting the potential-dependent inhibition, metal electro-oxidation was described through a single-step reaction, either at equilibrium or operating through a Butler-Volmer scheme, including a Frumkin-type interaction term for the oxidized metal sites. An aprioristic analysis of the effects of elementary kinetic and adsorption parameters on the polarization curves and on the theoretical dependences of adsorbed intermediate coverages was carried out. This analysis allowed to visualize the strong effects that the kinetic and interaction parameters of the electro-oxidation reaction have on the slopes and half-wave potential of the ORR polarization curve. Moreover, experimental curves measured on Ni-supported polycrystalline Pt and Pd rotating disk electrodes in alkaline media (0.1 M KOH) were properly correlated with this model.