Introduction
With the depletion of non-renewable energy sources, the development of efficient energy storage technologies with low environmental impact becomes essential to realize energy and environment sustainability1-5. Zinc-air batteries (ZABs) is considered as one of the most promising candidates for the next-generation power devices due to their high theoretical energy density, low cost and environmental friendliness6-10. Comparing with conventional alkaline ZABs, neutral ZABs involve the same electrochemical reactions (air-cathode: O2 + 2H2O + 4e- ⇌ 4OH; zinc anode: Zn + 2OH- ⇌ ZnO + H2O + 2e) but are much more resistant to self-discharging induced zinc electrode corrosion and ambient CO2absorption resulted electrolyte carbonation11-13. However, the neutral media normally suffers from the insufficient ionic conductivity in electrolyte and the low chemical potential-gradient across the electrolyte/electrode interface14. Consequently, the oxygen reduction reaction (ORR), key electrochemical reactions at the air-cathode, are supposed to be kinetically more sluggish and possesses higher reaction barrier in neutral media than alkaline solution15. In this regard, it is crucial to develop highly active catalysts which can work efficiently and robustly in neutral environment to maximize the performance of ZABs.
During the last decade, plenty of efforts have been devoted to seeking various catalyst modification strategies for enhancing their ORR performance, including such as heteroatoms doping, molecular engineering, morphology regulation and so on16-18. Despite the regulation of intrinsic activity of catalytic materials, designing the local microenvironment around catalytic sites also plays a key role to realize enhanced catalytic performance19,20. Optimal microenvironment could realize the enrichment of intermediates across the electrolyte/electrode interface and further promote the interfacial chemical potential-gradient. With satisfied chemical potential-gradient, the intermediate energy states or even the reaction pathways can be tailored, which will decrease the reaction energy barrier and promote the reaction kinetics21,22. However, most of current work were focused on elevating the intrinsic activity of catalytic sites, yet there still lacks effective strategy to optimize the surface microenvironment around active sites23,24. Therefore, it remains a challenging but perspective route to improve the ZABs performance in neutral media through local microenvironment design of ORR catalysts.
Herein, we highlight that surface microenvironment optimization via electrochemical oxidation could serve as an effective way to design highly active ORR catalysts for air electrode of neutral ZABs. Owing to the synthetically tuning of both intrinsic catalytic activity and local microenvironment of the active sites, the prepared Pt-SMO-Co2N NWs presented superior ORR activity in a 0.2 M phosphate buffer solution at pH = 7.0 to pristine Co2N NWs and commercial Pt/C. Moreover, the rechargeable ZABs based on Pt-SMO-Co2N NWs and neutral electrolyte reached a power density of 67.9 mW*cm2 and showed negligible decay during nearly 80 hours’ stability test. Our work suggests that surface microenvironment optimization would be a new strategy to design advanced electrocatalysts for neutral ZABs, disclosing the pivotal mechanism of activating H2O and facilitating proton transfer process in ORR catalysis.