Spinel oxides are promising electrocatalysts for the oxygen reduction reaction (ORR) in alkaline fuel cells. However, conventional theoretical models often fail to capture experimental performance, because they overlook the critical role of spectator species under realistic working conditions. In this work, we constructed thermodynamic phase diagrams to identify the realistic active configurations at ORR working potentials and elucidated the origin of activity on Mn-, Fe-, and Ni-doped as well as pure ZnCo2O4. Our findings revealed two distinct enhancement mechanisms corresponding to shifting and climbing the activity volcano. On the (100) surface, a H* spectator stabilizes the OOH* intermediate via hydrogen bonding. This geometric effect breaks the linear scaling relations and effectively shifts the volcano to overcome the intrinsic theoretical limitations. Conversely, on the (110) surface, OH* or OH* + H* spectators primarily induce an electronic effect that optimizes the adsorption energies. Consequently, the catalyst climbs the volcano toward the summit of the conventional activity plot. Notably, for Ni-doped ZnCo2O4(110), a dynamic ORR mechanism driven by the reversible switching between OH* + H* and sole OH* spectators was identified. Furthermore, we proposed multiple general strategies to decouple the scaling relation between OOH* and OH* binding energies. These findings provide new insights into the accurate theoretical modeling of oxide-based catalysts and highlight spectator engineering as an effective pathway to surpass the intrinsic performance boundaries of conventional activity volcano.
Zhang et al. (Thu,) studied this question.