Oxygen vacancies endow atomic cobalt-palladium oxide clusters with outstanding oxygen reduction reaction activity.

ORR pathways on the surface of (a) Co@Pd and (b) CPCo-3 NCs at an open-circuit voltage (OCV) and under potential driven conditions from 1.0 to 0.7 V. In Co@Pd NC, the O 2 splitting to 2Oads occurs in the Pd surface the Oads is relocated to the neighbouring oxygen vacancies for conducting the reduction reaction with H 2 O in CoO X V sites. In CPCo-03, the hydration reaction removes the amorphous CoO X and thus expose the CoPdO X V to the electrolyte from OCV to 1.0V. The PdCoO X V sites are reaction centre for the O 2 splitting to 2Oads and collaborate with the neighbouring Pd atoms for completing the ORR. [Display omitted] • A novel catalyst of oxygen vacancies enriched atomic CoPdO x clusters is developed. • It delivers a mass activity of 426 mAmg Co −1 at 0.90 V vs RHE in alkaline ORR. • It increases the mass activity by 40 % at 20 k potential cycles in degradation test. Considering the technological importance of fuel cells, developing highly efficacious, durable, and Platinum (Pt)-free catalysts are crucial. In this work, we propose a novel nanocatalyst (NC) comprising oxygen vacancies (OV) enriched atomic CoPdO x clusters (CoPdO x V) anchored Pd nanoparticles (NP)s on cobalt-oxide support (denoted as CPCo). As-prepared CPCo NC with an additional 3 wt% of Co decoration (denoted as CPCo-3) delivers an exceptionally high mass activity (MA) of 4394 mAmg Co −1 at 0.85 V vs RHE and 426 mAmg Co −1 at 0.90 V vs RHE in alkaline oxygen reduction reaction (ORR) (0.1 M KOH), which surpasses the commercial J.M.-Pt/C (20 wt%) catalyst by 65-times. More importantly, the CPCo-3 NC exhibits outstanding durability in an accelerated durability test (ADT) with a progressively increased MA by 40 % (6,140 mAmg Co −1) as that of the initial condition after 20 k cycles. Through in-depth physical characterization, electrochemical analysis, and in-situ X-ray absorption spectroscopy (XAS), we demonstrated the conceptual framework of potential synergism between the CoPdO x V and neighbouring metallic Pd-sites. In this event, the surface-anchored CoPdO x V species coupling with OV promotes the O 2 splitting, while the neighbouring Pd-sites simultaneously trigger the Oads relocation (i.e. OH− desorption) step. In addition, the cobalt oxide support underneath assists the electron injection to surface Pd-sites. This work not only marks a step ahead for designing high-performance transition metal oxide catalysts for fuel cells but also uncovers the material's aspects of cobalt that shall spark motivation for the other catalytic applications. [ABSTRACT FROM AUTHOR]