(Invited) Fundamental Aspects of Electrocatalysis at Work in Water Electrolysis

Abstract

Text: A general aspect of electrocatalysis described most recently in [1,2] , is the involvement of the electrode (over)potential in generation of catalytically active sites. It has been demonstrated that , in a wide variety of ORR processes taking place in low temperature fuel cells, the electrocatalytic process is "ignited" by a potential-driven change in steady state catalyst surface composition . The common involvement of such potential-driven, surface composition change in the mechanism of electrocatalytic processes , has led to the conclusion that fuel cell electrocatalytic processes , particularly the ORR, are, in general, "redox mediated" , including so at metal electrocatalysts such as Pt [1,2]. In this paper, the applicability of the same type mechanism to electrocatalytic processes in water electrolysis, particularly the OER, is elucidated. Much the same way as in the case of ORR at a variety of electrocatalyst surfaces, the OER also exhibits generally a dependence on overpotential of: 'low Tafel slope of 60 mV/decade ,or lower, at low currents, followed by higher Tafel slope of around 120mV/decade at higher currents '. This general pattern is fully expected for a mechanism involving potential activation of surface sites. Inclusion of the change of active site population with potential in the expression for the current potential dependence, results in potential-dependence of the pre-exponential factor, with the consequence of a low Tafel slope around the standard potential of the redox couple mediator [1,2]. Recently reported polarization curves for the OER ,to be discussed in this talk (3), exhibit the typical behavior described above. As commonly done, the interpretation was given in the form of change in RDS with overpotential. The insight proposed here, sugegsts that the RDS may very well remain the same throughout the relevant potential range and the observed change in Tafel slope is fully explained by change with overpoential in steady state, active surface site population. Much the same way as in the case of fuel cell electrocatalysis [1,2], the important consequence of such general mechanism is that the effectiveness of the electrocatalyst depends on the magnitude of the overpotential required to activate the surface-redox-mediating system. Proximity of E0,surf.redox to E0,cell process should enable, in principle, high voltage efficiency of the electrolyzer. (1) S. Gottesfeld, ECS Trans. 2014 61(31): 1-13 (2) S. Gottesfeld, ECS Trans. 2015 66(29): 13-30 (3) R.I. Doyle and E.G. Lyons. JECS, 160, H42, 2013