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.