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\begin{document}

\title{Surface Microenvironment Optimization Induced Robust Oxygen Reduction
for Neutral Zinc-Air Batteries}



\author[1]{Si Liu}%
\author[1]{Han Cheng}%
\author[1]{Jun Xia}%
\author[1]{Chun Wang}%
\author[1]{Renjie Gui}%
\author[1]{Tianpei Zhou}%
\author[1]{Hongfei Liu}%
\author[1]{Jing Peng}%
\author[1]{Nan Zhang}%
\author[1]{Wenjie Wang}%
\author[1]{Wangsheng Chu}%
\author[1]{Hengan Wu}%
\author[1]{Changzheng Wu}%
\author[1]{Yi Xie}%
\affil[1]{University of Science and Technology of China}%


\vspace{-1em}



  
  \date{May 19, 2024}


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\selectlanguage{english}
\begin{abstract}
Neutral zinc-air batteries (ZABs) are promising candidates for the
next-generation power devices with considerably elongated lifetime
comparing to conventional alkaline ZABs. However, neutral cathodic
oxygen reduction reaction is seriously limited by the mass transfer
efficiency of hydroxyl due to insufficient interfacial chemical
potential-gradient between catalytic layer and electrolyte. Herein, we
highlight that electrochemical oxidation induced surface
microenvironment optimization could realize optimal chemical
potential-gradient around catalytic sites and bring outstanding neutral
ORR activity. The electro-deposited sub-nano Pt decorated
surface-microenvironment-optimized Co2N samples (denoted as Pt-SMO-Co2N
NWs) possessed 92 mV and 365 mV lower overpotential than commercial Pt/C
and pristine Co2N in 0.2 M PBS. As for neutral ZABs, Pt-SMO-Co2N NWs
cathode delivers a power density of 67.9 mW*cm-2 and displays negligible
decay after nearly 80 hours stability test at 20 mA*cm-2. In depth
characterization proposes that remarkable performance improvement
originates from optimized microenvironment which increases the surface
chemical potential gradient and facilitates proton coupled electron
transfer during ORR. We anticipated that such synergetic optimization of
microenvironment and intrinsic activity of active sites is an effective
strategy which may be extended the catalytic reactions beyond ORR.%
\end{abstract}%



\sloppy


\textbf{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
sustainability\textsuperscript{1-5}. 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 friendliness\textsuperscript{6-10}.
Comparing with conventional alkaline ZABs, neutral ZABs involve the same
electrochemical reactions (air-cathode: O\textsubscript{2} +
2H\textsubscript{2}O + 4e\textsuperscript{-} = 4OH\textsuperscript{-};
zinc anode: Zn + 2OH\textsuperscript{-} = ZnO + H\textsubscript{2}O +
2e\textsuperscript{-}) but are much more resistant to self-discharging
induced zinc electrode corrosion and ambient
CO\textsubscript{2}absorption resulted electrolyte
carbonation\textsuperscript{11-13}. However, the neutral media normally
suffers from the insufficient ionic conductivity in electrolyte and the
low chemical potential-gradient across the electrolyte/electrode
interface\textsuperscript{14}. 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 solution\textsuperscript{15}. 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 on\textsuperscript{16-18}.
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
performance\textsuperscript{19,20}. 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 kinetics\textsuperscript{21,22}. 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 sites\textsuperscript{23,24}. 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-Co\textsubscript{2}N NWs presented superior ORR activity in a 0.2
M phosphate buffer solution at pH = 7.0 to pristine Co\textsubscript{2}N
NWs and commercial Pt/C. Moreover, the rechargeable ZABs based on
Pt-SMO-Co\textsubscript{2}N NWs and neutral electrolyte reached a power
density of 67.9 mW*cm\textsuperscript{2} 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 H\textsubscript{2}O and facilitating proton transfer process
in ORR catalysis.

\textbf{Results and discussion}

To realize the synergetic optimization of intrinsic activity and
microenvironment for active sites, electrochemical oxidation was
conducted by applying a voltage from 0V to 1.6V on Co\textsubscript{2}N
nanowires in a Pt precursor contained solution \textbf{(Figure 1a)} . To
follow the surface evolution of Co\textsubscript{2}N nanowires, HRTEM
was operated to study the surface microstructure of Co\textsubscript{2}N
before and after surface microenvironment optimization. \textbf{Figure
S3} shows the HRTEM image of pristine Co\textsubscript{2}N nanowires
with a lattice spacing value of 0.196 nm which could be ascribed to
(120) plane. After the microenvironment optimization, a rough surface
layer with small Pt clusters could be clearly observed in \textbf{Figure
1b} . High-angle annular dark-field scanning transmission electron
microscopy (HAADF) further confirmed the formation of sub-size Pt
clusters with a diameter of 1\textasciitilde{}2 nm (the bright areas
in\textbf{Figure 1c} ). The \emph{in-situ} X-ray photoelectron spectra
(XPS) equipped with Ar ions etching was operated to investigate the
composition and electronic structure of the oxidized surface layer as a
function of depth. As shown in \textbf{Figure 1d} , the fine-scanned Co
2p XPS spectra of the pristine Co\textsubscript{2}N nanowires and the
Pt-SMO-Co\textsubscript{2}N NWs can be well fitted with three different
valence states (i.e. Co\textsuperscript{3+} at 795.2 eV,
Co\textsuperscript{2+} at 797.0 eV, and Co\textsuperscript{0} at 794.5
eV)\textsuperscript{25,26} The filled area under each fitting curves
represents the abundance of each valence state. The increased surface
Co\textsuperscript{3+}/Co\textsuperscript{0} ratio after surface
microenvironment optimization suggests the formation of more
Co\textsuperscript{3+} species through the oxidation of surface
Co\textsuperscript{0} of Co\textsubscript{2}N nanowires, which is
probably corresponding to high valence CoOOH layer according to previous
studies\textsuperscript{26}. With 3 keV Ar etching, the ratio of
Co\textsuperscript{3+}/Co\textsuperscript{0} was decreased to nearly the
same level with pristine Co\textsubscript{2}N nanowires, indicating that
the internal phase of Co\textsubscript{2}N was maintained. Moreover, as
shown in \textbf{Figure 1e} , two fitted peaks at 70.8 eV and 74.1 eV
could be ascribed as the Pt 4f\textsubscript{5/2} and
4f\textsubscript{7/2} bands of Pt\textsuperscript{027}. With 3 keV Ar
etching of surface layer, the peak intensity of Pt 4f\textsubscript{5/2}
and 4f\textsubscript{7/2} was greatly reduced, suggesting that sub-nano
Pt sites were deposited on the most surface layer after microenvironment
optimization.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image1/image1}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image3/image3}

\end{center}
\end{figure}

\textbf{Figure 1} (a) Schematic illustration of synergistic surface
optimization and proposed catalysis function. (b) HRTEM image of
Pt-SMO-Co\textsubscript{2}N NWs. (c) HADDF image of
Pt-SMO-Co\textsubscript{2}N NWs. (d) Co 2p XPS pattern of
Co\textsubscript{2}N NWs, Pt-SMO-Co\textsubscript{2}N NWs and
Ar-Pt-SMO-Co\textsubscript{2}N NWs. (e) Pt 4f XPS pattern of
Pt-SMO-Co\textsubscript{2}N NWs and Ar-Pt-SMO-Co\textsubscript{2}N NWs.

To further investigate the local structure of cobalt and platinum in
above catalysts, \emph{in-situ} X-ray absorption fine structure (XAFS)
was then employed to monitor the structure evolution during surface
microenvironment optimization. Both Co K-edge and Pt
L\textsubscript{3}-edge spectra were recorded for the pristine
electrocatalyst, and the electrocatalysts after surface microenvironment
optimization for 40 min, 80 min and 120 min. As shown in \textbf{Figure
2a} , despite a slight right shift toward higher absorption energy, the
Co K-edge XANES spectra of the Pt-SMO-Co\textsubscript{2}N NWs were
still similar to pristine Co\textsubscript{2}N NWs, suggesting that the
bulk lattice framework was maintained. From the magnified spectra, a
continuous right shift could be observed as the time increased from 0 to
120 min, indicating the surface was gradually oxidized to higher valence
during the electrochemical surface optimization. The Fourier transformed
spectra of Co K-edge were shown in \textbf{Figure 2b} . During surface
microenvironment optimization, EXAFS spectrums underwent a continuous
decrease on Co-N bonds, while an increase of Co-O bonds with the
elongated electrochemical treatment, indicating the formation of
oxyhydroxide layer\textsuperscript{26}. \emph{In-situ} XAFS was also
operated to investigate local structure of Pt during surface
microenvironment optimization at different stages \textbf{(Figure 2c and
2d)} . The peak at 2.41\selectlanguage{ngerman}Å could be ascribed to Pt-Pt bonds in
Pt-SMO-Co\textsubscript{2}N NWs, of which intensity increased with
extended electrochemical oxidation duration. To illustrate the evolution
of Pt local structure more straightforwardly, the wavelet transform (WT)
analysis was performed \textbf{(Figure 2e and 2f)} \textsuperscript{28}.
The WT contour plots also showed the same trend as that of Fourier
transformed Pt L-edge spectra. All these results clearly indicate the
formation of sub-nano Pt cluster deposited on generated oxyhydroxide
layer on the surface of Co\textsubscript{2}N NWs.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image7/image7}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image11/image11}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image19/image19}

\end{center}
\end{figure}

\textbf{Figure 2} \emph{In-situ} Co K near edge XANE spectra (a) and Pt
L near edge XANE spectra (b) of Pt-SMO-Co\textsubscript{2}N NWs during
synergistic surface microenvironment optimization process. FT-EXAFS
spectra at the (c)Co K-edge and (d) Pt L-edge of
Pt-SMO-Co\textsubscript{2}N NWs. (e) Co K-edge WT-EXAFS contour plots
and (f) Pt L-edge WT-EXAFS contour plots of Pt-SMO-Co\textsubscript{2}N
NWs during synergistic surface microenvironment optimization process.

To examine the catalytic effect of surface microenvironment
optimization, the Pt-SMO-Co\textsubscript{2}N NWs was used as working
electrodes in O\textsubscript{2}-saturated 0.2 M PBS (pH =7.0) to
evaluate its electrocatalytic performance toward ORR. As illustrated
in\textbf{Figure 3a} , the linear scan sweep voltammetry (LSV) curve
exhibits the catalytic performance of Pt-SMO-Co\textsubscript{2}N 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 \textbf{Figure 3b} , the mass activity of Pt-SMO-Co\textsubscript{2}N
NWs was almost doubled in comparison to commercial Pt/C. Moreover, the
Pt mass activity of Pt-SMO-Co\textsubscript{2}N NWs is nearly 30 times
higher than that of commercial Pt/C, excluding the contribution of
Co\textsubscript{2}N. As shown in \textbf{Figure 3c} ,
Pt-SMO-Co\textsubscript{2}N NWs possessed the lowest Tafel value,
suggesting that the ORR kinetics on Pt-SMO-Co\textsubscript{2}N NWs were
obviously enhanced comparing to commercial Pt/C. To quantitatively
understand the ORR activity of Pt-SMO-Co\textsubscript{2}N NWs, detailed
LSV tests were operated at different rotating speeds from 600 to 1600
rpm \textbf{(Figure 3d)} . The Koutecky--Levich (K--L) equation was used
to calculate number of electrons transferred during the ORR
process\textsuperscript{29}. The electron-transfer number of
Pt-SMO-Co\textsubscript{2}N NWs is about 3.9 from 0.2--0.6 V, suggesting
the Pt-SMO-Co\textsubscript{2}N 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-Co\textsubscript{2}N NWs recorded after 1000 cycles show no
shift\textbf{(Figure S4)} , indicating the excellent stability.
Moreover, the electrocatalytic performance of OER was also assessed, as
shown in\textbf{Figure S5} , Pt-SMO-Co\textsubscript{2}N NWs showed
smallest overpotential at a current density of 10
mA*cm\textsuperscript{-2}, comparing to the Co\textsubscript{2}N NWs and
commercial Pt/C. In the light of the excellent bifunctional oxygen
electrocatalytic property, Pt-SMO-Co\textsubscript{2}N NWs could serve
as an ideal air-electrode of rechargeable zinc-air batteries (RZABs).
Zn-Air battery was constructed by loading Pt-SMO-Co\textsubscript{2}N
NWs onto a gas diffusion layer (GDL) air electrode with a spray gun. A
mixture aqueous solution of 4.0 M NH\textsubscript{4}Cl and 2.0 M KCl
(pH = 7.0) was used as the electrolyte. For performance comparison,
reference tests with equal loading of Co\textsubscript{2}N and
commercial Pt/C catalysts on GDL were also conducted in the same manner.
The polarization and power density curves are presented in
\textbf{Figure 3e} , showing that Pt-SMO-Co\textsubscript{2}N NWs
possessed a comparable open-circle voltage with commercial Pt/C. The
power density of Pt-SMO-Co\textsubscript{2}N NWs eventually peaks at
67.9 mW *cm\textsuperscript{-2}, which is superior that of commercial
Pt/C (50 mW *cm\textsuperscript{-2}) and pristine Co\textsubscript{2}N
NWs (22.5 mW *cm\textsuperscript{-2}). Chronoamperometric tests were
then performed to evaluate the stability of the constructed RZABs. As
shown in Figure \textbf{S7} , under the current density of 20
mA*cm\textsuperscript{-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-Co\textsubscript{2}N NWs on GDL as air
electrode was obtained under the current density of 2 mA
*cm\textsuperscript{-2}. As expected, the cell voltage overpotential
showed negligible change after 80 h cycles \textbf{(Figure 3f)} .
Significantly, these results demonstrate that
Pt-SMO-Co\textsubscript{2}N NWs obtained by surface microenvironment
optimization could serve as an excellent air electrode for RZABs which
could deliver a high power density sustainably.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image27/image27}

\end{center}
\end{figure}

\textbf{Figure 3} (a) ORR polarization curves of
Pt-SMO-Co\textsubscript{2}N NWs and reference catalysts in 0.2 M PBS.
(b) Corresponding mass activity comparison of
Pt-SMO-Co\textsubscript{2}N NWs and reference catalysts. (c) Tafel plots
of Pt-SMO-Co\textsubscript{2}N NWs and reference catalysts. (d) ORR
polarization curves for Pt-SMO-Co\textsubscript{2}N NWs at different
rotation speeds. (e) Polarization and power density curves of neutral
ZABs with Pt-SMO-Co\textsubscript{2}N NWs and reference catalysts as
air-cathodes. (f) Galvanostatic discharge-charge cycling curves of the
RZABs at 2 mA*cm\textsuperscript{-2} with Pt-SMO-Co\textsubscript{2}N
NWs as air-cathodes.

To further understand the impact of surface microenvironment
optimization for ORR, we constructed a Pt-CoOOH@Co\textsubscript{2}N
model to simulate the composite structure of Pt-SMO-Co\textsubscript{2}N
NWs for thorectical study. DFT calculation was implemented to evaluate
their surface energetics\textsuperscript{30-32}. As shown
in\textbf{Figure 4a} , the Pt sites are highly active for binding
O\textsubscript{2} and the Pt-O\textsubscript{2} interaction is much
stronger than Pt-H\textsubscript{2}O, indicating that the Pt sites
prefer to bind O\textsubscript{2} rather than H\textsubscript{2}O. In
contrast, CoOOH sites possess a notable higher affinity for
H\textsubscript{2}O instead of O\textsubscript{2} (\textbf{Figure 4b)} .
Thus, it is supposed that Pt sites and CoOOH sites on the surface are
responsible for binding different adsorbates, preferentially yielding
Pt-O\textsubscript{2} and Co-OH\textsubscript{2} 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 H\textsubscript{2}O is
energetically highly favorable on the CoOOH sites while the
O\textsubscript{2} 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
O\textsubscript{2}and the CoOOH sites enriching and activating
H\textsubscript{2}O, 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-Co\textsubscript{2}N NWs catalyzed ORR is illustrated stepwise in
\textbf{Figure 4c} . Considering Pt-OH and Co-OH as the initial states,
the O\textsubscript{2} would preferentially binds to the Pt site after
the detachment of an OH\textsuperscript{-}group, yielding a
Pt-O\textsubscript{2} superoxide intermediate. Meanwhile, a
H\textsubscript{2}O molecule attaches to the Co site to generate yield a
Co-OH\textsubscript{2} species. Afterwards, surface proton transfer
would occur between Co-OH\textsubscript{2} and Pt-O\textsubscript{2},
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\textsuperscript{-} group. The second proton
transfer process from Co-OH\textsubscript{2} 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-OH\textsubscript{2} 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.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image33/image33}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image35/image35}

\end{center}
\end{figure}

\textbf{Figure 4} (a) Density functional theory (DFT) calculated
adsorption energies for H\textsubscript{2}O and O\textsubscript{2} on
the Pt sites of (100), (110) and (111) surface. (b) Density functional
theory (DFT) calculated adsorption energies for H\textsubscript{2}O and
O\textsubscript{2} on the CoOOH sites of (003), (012) and (101) surface.
(c) Schematic illustration of the proposed synergistic mechanism of ORR
on Pt-SMO-Co\textsubscript{2}N NWs, including the dissociative reduction
of O\textsubscript{2} at the Pt site, the proton mediation by the CoOOH
site, and the surface proton transfer in between (reactions I and II).

\textbf{Conclusions}

In summary, we have demonstrated surface microenvironment optimization
as an effective way to design highly active ORR catalyst, serving as air
electrode for neutral ZABs. Benefiting from the optimal surface
microenvironment induced synergistic effects between different active
sites, the achieved Pt-SMO-Co\textsubscript{2}N NWs presented
extraordinary ORR activity. In 2.0 M PBS (pH=7.0),
Pt-SMO-Co\textsubscript{2}N NWs showed a positive onset potential of
0.960V and a half-wave potential of 0.812 V, which is 92mV higher than
that of commercial Pt/C. The power density of neutral ZABs taking
Pt-SMO-Co\textsubscript{2}N NWs as cathode catalyst could reach 67.9
mW*cm\textsuperscript{-2}, outperforming commercial Pt/C under the same
circumstance and displayed barely decay after 80 hours' discharge-charge
test. Moreover, based on the ideal material platform built on
Pt-SMO-Co\textsubscript{2}N NWs, in-depth characterization and
mechanistic understanding of ORR was disclosed. Our work reveals a new
strategy for the ORR catalyst design through the construction of optimal
surface microenvironment and offers new insights toward the key role of
activating H\textsubscript{2}O and facilitating proton transfer process
in ORR catalysis.

ASSOCIATED CONTENT

XRD patterns, TEM images and additional electrochemical date are
included in the supporting information. This material is available free
of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author

czwu@ustc.edu.cn;

yxie@ustxc.edu.cn

Author Contributions

\selectlanguage{ngerman}\textsuperscript{§}These authors contributed equally.

\textbf{Notes} The authors declare no competing financial interests.

ACKNOWLEDGMENT

This work was financially supported by the National Basic Research
Program of China (2017YFA0206702), Natural Science Foundation of China
(No. 21925110, 21890751), China Postdoctoral Science Foundation
(2019TQ0299) and Fundamental Research Funds for the Central Universities
(No. WK 2060190084, No. WK 5290000001). The authors thank Dr Jie Tian
and Dr Huijuan Wang at Engineering and Materials Science Experiment
Centre for the help of HRTEM experiments. The authors also appreciate
the support from the Major/Innovative Program of Development Foundation
of Hefei Center for Physical Science and Technology. This work was
partially carried out at the USTC Center for Micro and Nanoscale
Research and Fabrication.

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