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.