Figure 3 (a) ORR polarization curves of Pt-SMO-Co2N NWs and reference catalysts in 0.2 M PBS. (b) Corresponding mass activity comparison of Pt-SMO-Co2N NWs and reference catalysts. (c) Tafel plots of Pt-SMO-Co2N NWs and reference catalysts. (d) ORR polarization curves for Pt-SMO-Co2N NWs at different rotation speeds. (e) Polarization and power density curves of neutral ZABs with Pt-SMO-Co2N NWs and reference catalysts as air-cathodes. (f) Galvanostatic discharge-charge cycling curves of the RZABs at 2 mA*cm−2 with Pt-SMO-Co2N NWs as air-cathodes.
To further understand the impact of surface microenvironment optimization for ORR, we constructed a Pt-CoOOH@Co2N model to simulate the composite structure of Pt-SMO-Co2N NWs for thorectical study. DFT calculation was implemented to evaluate their surface energetics30-32. As shown inFigure 4a , the Pt sites are highly active for binding O2 and the Pt-O2 interaction is much stronger than Pt-H2O, indicating that the Pt sites prefer to bind O2 rather than H2O. In contrast, CoOOH sites possess a notable higher affinity for H2O instead of O2 (Figure 4b) . Thus, it is supposed that Pt sites and CoOOH sites on the surface are responsible for binding different adsorbates, preferentially yielding Pt-O2 and Co-OH2 species respectively. On the basis of above adsorption model, DFT calculations further suggested the capability of breaking the O-O bond and O-H bond at catalytic sites and the dissociation of H2O is energetically highly favorable on the CoOOH sites while the O2 dissociation is favored on the Pt sites. Consequently, it is suggested that the constructed model of surface CoOOH layer with deposited Pt cluster possess synergistic surface for ORR catalysis. With the Pt sites binding and cleaving O2and the CoOOH sites enriching and activating H2O, the proton-coupled electron transfer process of oxygen reduction could be significantly facilitated. Based on the above results, the proposed synergistic mechanism of the Pt-SMO-Co2N NWs catalyzed ORR is illustrated stepwise in Figure 4c . Considering Pt-OH and Co-OH as the initial states, the O2 would preferentially binds to the Pt site after the detachment of an OH-group, yielding a Pt-O2 superoxide intermediate. Meanwhile, a H2O molecule attaches to the Co site to generate yield a Co-OH2 species. Afterwards, surface proton transfer would occur between Co-OH2 and Pt-O2, leading to the formation of Co-OH and Pt-OOH peroxide intermediates. Followed by two further electron reduction, Pt-OOH species would turn into Pt-O by releasing an OH- group. The second proton transfer process from Co-OH2 to the generated Pt-O would then proceed to regenerate the Pt-OH. Overall, the proposed mechanism is mainly constituted by two aspects: the proton mediation rising from turnover of Co-OH/ Co-OH2 and the proton transfer between contiguous Co and Pt sites. The above theoretical observations clearly demonstrated that synergistic active sites and optimized surface microenvironment could mediate the transportation of intermediate species to accelerate the reaction kinetics.