Negative impact of sulfur doping on ORR/OER performance in dual–active–site Co–MOF/MXene electrocatalysts

Designing efficient, sustainable, and cost–effective electrocatalysts for oxygen reduction (ORR) and oxygen evolution (OER) reactions is essential for advancing fuel cells and metal–air batteries as clean energy technologies. Despite significant progress in developing non–precious electrocatalysts, the nature of their active sites and interface chemistry remains unclear due to the complexity of the reaction mechanisms. To address these challenges, this study presents a Co–MOF/MXene hybrid electrocatalyst engineered to optimize synergy between dual–metal active sites, achieving high bifunctional activity for both ORR and OER. Notably, our comparative analysis provides clear experimental evidence that sulfur doping disrupts this balance in the reported catalyst. Structural analysis reveals that sulfur substitutes for carbon atoms within the framework, altering the local coordination environment and reducing catalytic performance, contrary to the common assumption that heteroatom doping is universally beneficial. The sulfur–free Co–MOF/MX–1 catalyst exhibits a high ORR onset/half–wave potential of 0.90/0.81 V (vs. RHE), low hydrogen peroxide yield (2.95 %), 90 % current retention, and an ultralow bifunctional gap (ΔE ORR–OER  = 0.78 V). We attribute this superior performance to the interplay of Co–N₃–Ti₃C and Ti N active sites, which synergistically optimize oxygen adsorption and electron transfer, aligning with the metaphorical Janus principle of functional balance. This work challenges prevailing views on heteroatom doping and establishes new design principles for non–precious bifunctional electrocatalysts in fuel cells and metal–batteries.