Figure 2 In-situ Co K near edge XANE spectra (a) and Pt
L near edge XANE spectra (b) of Pt-SMO-Co2N NWs during
synergistic surface microenvironment optimization process. FT-EXAFS
spectra at the (c)Co K-edge and (d) Pt L-edge of
Pt-SMO-Co2N NWs. (e) Co K-edge WT-EXAFS contour plots
and (f) Pt L-edge WT-EXAFS contour plots of Pt-SMO-Co2N
NWs during synergistic surface microenvironment optimization process.
To examine the catalytic effect of surface microenvironment
optimization, the Pt-SMO-Co2N NWs was used as working
electrodes in O2-saturated 0.2 M PBS (pH =7.0) to
evaluate its electrocatalytic performance toward ORR. As illustrated inFigure 3a , the linear scan sweep voltammetry (LSV) curve
exhibits the catalytic performance of Pt-SMO-Co2N NWs
with a positive onset potential of 0.960V and a half-wave potential of
0.812 V, much better than that of commercial Pt/C. Furthermore, the mass
activity at 0.7, 0.8 and 0.9 V vs. RHE was calculated. As demonstrated
in Figure 3b , the mass activity of Pt-SMO-Co2N
NWs was almost doubled in comparison to commercial Pt/C. Moreover, the
Pt mass activity of Pt-SMO-Co2N NWs is nearly 30 times
higher than that of commercial Pt/C, excluding the contribution of
Co2N. As shown in Figure 3c ,
Pt-SMO-Co2N NWs possessed the lowest Tafel value,
suggesting that the ORR kinetics on Pt-SMO-Co2N NWs were
obviously enhanced comparing to commercial Pt/C. To quantitatively
understand the ORR activity of Pt-SMO-Co2N NWs, detailed
LSV tests were operated at different rotating speeds from 600 to 1600
rpm (Figure 3d) . The Koutecky–Levich (K–L) equation was used
to calculate number of electrons transferred during the ORR
process29. The electron-transfer number of
Pt-SMO-Co2N NWs is about 3.9 from 0.2–0.6 V, suggesting
the Pt-SMO-Co2N NWs could catalyze oxygen reduction in a
direct four-electron pathway under neutral conditions. Except for
electrocatalytic activity, stability is also a key index to assess the
performance of a given electrocatalyst. The polarization curves of
Pt-SMO-Co2N NWs recorded after 1000 cycles show no shift(Figure S4) , indicating the excellent stability. Moreover, the
electrocatalytic performance of OER was also assessed, as shown inFigure S5 , Pt-SMO-Co2N NWs showed smallest
overpotential at a current density of 10 mA*cm−2,
comparing to the Co2N NWs and commercial Pt/C. In the
light of the excellent bifunctional oxygen electrocatalytic property,
Pt-SMO-Co2N NWs could serve as an ideal air-electrode of
rechargeable zinc-air batteries (RZABs). Zn-Air battery was constructed
by loading Pt-SMO-Co2N NWs onto a gas diffusion layer
(GDL) air electrode with a spray gun. A mixture aqueous solution of 4.0
M NH4Cl and 2.0 M KCl (pH = 7.0) was used as the
electrolyte. For performance comparison, reference tests with equal
loading of Co2N and commercial Pt/C catalysts on GDL
were also conducted in the same manner. The polarization and power
density curves are presented in Figure 3e , showing that
Pt-SMO-Co2N NWs possessed a comparable open-circle
voltage with commercial Pt/C. The power density of
Pt-SMO-Co2N NWs eventually peaks at 67.9 mW
*cm-2, which is superior that of commercial Pt/C (50
mW *cm-2) and pristine Co2N NWs (22.5
mW *cm-2). Chronoamperometric tests were then
performed to evaluate the stability of the constructed RZABs. As shown
in Figure S7 , under the current density of 20
mA*cm-2, it could maintain a stable voltage for nearly
80 hours, indicating the mitigated zinc corrosion in neutral
environment. Moreover, the galvanostatic discharge-charge profile of
Zn-air battery using Pt-SMO-Co2N NWs on GDL as air
electrode was obtained under the current density of 2 mA
*cm-2. As expected, the cell voltage overpotential
showed negligible change after 80 h cycles (Figure 3f) .
Significantly, these results demonstrate that
Pt-SMO-Co2N NWs obtained by surface microenvironment
optimization could serve as an excellent air electrode for RZABs which
could deliver a high power density sustainably.