Unveiling the origin of catalytic sites of Pt nanoparticles decorated on oxygen-deficient vanadium-doped cobalt hydroxide nanosheet for hybrid sodium-air batteries

Highly active bifunctional electrocatalysts are crucial for improving the performance of rechargeable metal-air batteries. However, most reported bifunctional electrocatalysts feature poor electrocatalytic activity and stability toward oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Here, we have reported the first-ever study of an effective one-step reduction-assisted exfoliation method to exfoliate bulk vanadium-doped cobalt hydroxide (V-doped Co(OH)2, denoted as V-Co(OH)2) into ultrathin nanosheets with abundant oxygen vacancies (V-Co(OH)2-Ov) and simultaneously anchor them with highly dispersed ultrafine Pt nanoparticles (NPs) with a nominal size of 0.8-2.4 nm (denoted as Pt/V-Co(OH)2-Ov). The Pt/V-Co(OH)2-Ov catalyst exhibits improved catalytic performance in ORR/OER. X-ray absorption spectroscopy analysis and theoretical calculations reveal the strong interfacial electronic interactions between Pt NPs and V-Co(OH)2-Ov, which synergistically improves oxygen intermediates' adsorption/desorption, enhancing the ORR and OER performance. Using Pt/V-Co(OH)2-Ov as a catalyst in the air cathode, a hybrid sodium-air battery displays a record value of an ultralow charging-discharging voltage gap of 0.07 V at a current density of 0.01 mA cm-2 with remarkable stability of up to 1000 cycles. This reduction-assisted exfoliation approach provides a new strategy to generate oxygen vacancies in metal hydroxides, which act as anchoring sites for deposition of sub-nanometal NPs via a strong interfacial effect.