Abstract
Recently, a lot of attention has been dedicated to double- or triple-atom catalysts (DACs/TACs) as promising alternatives to platinum-based catalysts for the oxygen reduction reaction (ORR) in fuel cell applications. However, the ORR activity of DACs/TACs is usually theoretically understood or predicted using the single-site association pathway (O(2) → OOH* → O* → OH* → H(2)O) proposed from Pt-based alloy and single-atom catalysts (SACs). Here, we investigate the ORR process on a series of graphene-supported Fe-Co DACs/TACs by means of first-principles calculation and an electrode microkinetic model. We propose that a dual channel for electron acceptance-backdonation on adjacent metal sites of DACs/TACs efficiently promotes O-O bond breakage compared with SACs, which makes ORR switch to proceed through dual-site dissociation pathways (O(2) → O* + OH* → 2OH* → OH* → H(2)O) from the traditional single-site association pathway. Following this revised ORR network, a complete reaction phase diagram of DACs/TACs is established, where the preferential ORR pathways and activity can be described by a three-dimensional volcano plot spanned by the adsorption free energies of ΔG(O*) and ΔG(OH*). Besides, the kinetics preferability of dual-site dissociation pathways is also appropriate for other graphene- or oxide-supported DACs/TACs. The contribution of dual-site dissociation pathways, rather than the traditional single-site association pathway, makes the theoretical ORR activity of DACs/TACs in better agreement with available experiments, rationalizing the superior kinetic behavior of DACs/TACs to that of SACs. This work reveals the origin of ORR pathway switching from SACs to DACs/TACs, which broadens the ideas and lays the theoretical foundation for the rational design of DACs/TACs and may also be heuristic for other reactions catalyzed by DACs/TACs.