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\begin{document}

\title{Self-Powered Piezo-Supercapacitors Based on ZnO@Mo-Fe-MnO2 Nanoarrays}



\author[1]{Luo  Sun}%
\author[1]{Zhiguo  Ye}%
\author[1]{Xinyuan  Peng}%
\author[1]{Shaojie  Zhuang}%
\author[1]{Duosheng  Li}%
\author[2]{Zhong  Jin }%
\affil[1]{Nanchang Hangkong University}%
\affil[2]{Nanjing University}%


\vspace{-1em}



  
  \date{May 27, 2024}


\begingroup
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\selectlanguage{english}
\begin{abstract}
The development of self-charging supercapacitor power cells (SCSPCs) has
profound implications for smart electronic devices used in different
fields. Here, we epitaxially electrodeposited Mo- and Fe-codoped MnO2
films on piezoelectric ZnO nanoarrays (NAs) grown on the flexible carbon
cloth (denoted ZnO@Mo-Fe-MnO2 NAs). An SCSPC device was assembled with
the ZnO@Mo-Fe-MnO2 NA electrode and
poly(vinylidenefluoride-co-trifluoroethylene) (PVDF-Trfe) piezoelectric
film doped with BaTiO3 (BTO) and carbon nanotubes (CNTs) (denoted
PVDF-Trfe/CNTs/BTO). The SCSPC device exhibited an energy density of 30
\selectlanguage{greek}μ\selectlanguage{english}Wh cm-2 with a high-power density of 40 mW cm-2, and delivered an
excellent self-charging performance of 363 mV (10 N) driven by both the
piezoelectric ZnO NAs and the PVDF-Trfe/CNTs/BTO films. More
intriguingly, the device also could also be self-charged by 184 mV due
to residual stress alone, and showed excellent energy conversion
efficiency and low self-discharge rate. This work illustrates for the
first time the self-charging mechanism involving electrolyte ion
migration driven by both electrodes and films. A comprehensive analysis
strongly confirmed the important contribution of the piezoelectric ZnO
NAs in the self-charging process of the SCSPC device. This work provides
novel directions and insights for the development of SCSPCs.%
\end{abstract}%



\sloppy


\textbf{Self-Powered Piezo-Supercapacitors Based on
ZnO@Mo-Fe-MnO\textsubscript{2} Nanoarrays}

Luo Sun,\textsuperscript{1,\#} Zhiguo Ye,\textsuperscript{*,1} Xinyuan
Peng,\textsuperscript{1} Shaojie Zhuang,\textsuperscript{1} Duosheng
Li,\textsuperscript{1} Zhong Jin\textsuperscript{*,2}

\emph{\textsuperscript{1} School of Materials Science and Engineering,
Nanchang Hangkong University, 696\#, FengHeNan Road, Nanchang 330063,
China}

\emph{\textsuperscript{2} State Key Laboratory of Coordination
Chemistry, MOE Key Laboratory of Mesoscopic Chemistry, MOE Key
Laboratory of High Performance Polymer Materials and Technology, Jiangsu
Key Laboratory of Advanced Organic Materials, School of Chemistry and
Chemical Engineering, Nanjing University, Nanjing, Jiangsu 210023,
China.}

*Correspondence and requests for materials should be addressed to Z. G.
Ye (email:\emph{yezhiguo2008@163.com}) or to Z. Jin
(email:\emph{zhongjin@nju.edu.cn}).

\textbf{Acknowledgements}

The authors are grateful to the supports by the Key Research and
Development Program of Jiangxi Province (20203BBE53069), the National
Key R\&D Program of China (2017YFA0208200), the National Natural Science
Foundation of China (51862026, 22022505 and 21872069), the Fundamental
Research Funds for the Central Universities of China (02051438026,
020514380272 and 020514380274), the Scientific and Technological
Innovation Special Fund for Carbon Peak and Carbon Neutrality of Jiangsu
Province (BK20220008), the Natural Science Foundation of Jiangxi
Province (20192ACBL21048), the Nanjing International Collaboration
Research Program (202201007 and 2022SX00000955), and the Suzhou Gusu
Leading Talent Program of Science and Technology Innovation and
Entrepreneurship in Wujiang District (ZXL2021273).

\textbf{Abstract}

The development of self-charging supercapacitor power cells (SCSPCs) has
profound implications for smart electronic devices used in different
fields. Here, we epitaxially electrodeposited Mo- and Fe-codoped
MnO\textsubscript{2} films on piezoelectric ZnO nanoarrays (NAs) grown
on the flexible carbon cloth (denoted ZnO@Mo-Fe-MnO\textsubscript{2}
NAs). An SCSPC device was assembled with the
ZnO@Mo-Fe-MnO\textsubscript{2} NA electrode and
poly(vinylidenefluoride-co-trifluoroethylene) (PVDF-Trfe) piezoelectric
film doped with BaTiO\textsubscript{3} (BTO) and carbon nanotubes (CNTs)
(denoted PVDF-Trfe/CNTs/BTO). The SCSPC device exhibited an energy
density of 30 \selectlanguage{greek}μ\selectlanguage{english}Wh cm\textsuperscript{-2} with a high-power density of 40
mW cm\textsuperscript{-2}, and delivered an excellent self-charging
performance of 363 mV (10 N) driven by both the piezoelectric ZnO NAs
and the PVDF-Trfe/CNTs/BTO films. More intriguingly, the device also
could also be self-charged by 184 mV due to residual stress alone, and
showed excellent energy conversion efficiency and low self-discharge
rate. This work illustrates for the first time the self-charging
mechanism involving electrolyte ion migration driven by both electrodes
and films. A comprehensive analysis strongly confirmed the important
contribution of the piezoelectric ZnO NAs in the self-charging process
of the SCSPC device. This work provides novel directions and insights
for the development of SCSPCs.

\textbf{KEYWORDS:} Self-charging; Supercapacitors; Piezoelectric; ZnO
nanoarray; MnO\textsubscript{2}

\textbf{Introduction}

With the advent of the Internet of Things and the artificial
intelligence era, the rapid development of microelectromechanical
systems has brought great convenience to peoples'
lives\textsuperscript{{[}1{]}}. These energy devices have received much
attention because they are well suited for implantable medical devices
(\emph{e.g.} , pacemakers), wearable electronic devices, high-precision
sensors, and other equipment\textsuperscript{{[}2, 3{]}}. However,
frequent charging and replacement of the conventional batteries and
their inability to provide a continuous power supply has drastically
hindered commercial applications\textsuperscript{{[}4, 5{]}}. To address
this issue, it is intriguing to develop self-charging supercapacitor
power cells that can directly convert mechanical energy into
electrochemical energy by integrating the energy harvesting devices and
storage devices internally\textsuperscript{{[}6, 7{]}}. The SCSPCs hold
great potential for both micro (\emph{e.g.} , electronic skin and
biosensors) and macro (\emph{e.g.} , electric transport vehicles and
health monitoring systems) device applications\textsuperscript{{[}8,
9{]}}. Supercapacitors (SCs) are considered to be potential
next-generation candidates for energy storage devices because of their
fast charge/discharge rates, long lifespans, high power densities, low
costs, and safer operability compared to other
batteries\textsuperscript{{[}10{]}}. In particular, in previous studies
of SCSPCs, supercapacitors and nanogenerators were successfully
integrated to charge the supercapacitors directly from external forces
without rectifiers\textsuperscript{{[}11{]}}. However, the integrated
SCSPCs usually suffer from low self-charging voltages and low energy
conversion efficiencies. Moreover, the problem of self-discharge, which
usually exists in ordinary SCs, is still of significant concern with the
SCSPCs\textsuperscript{{[}12{]}}. These problems limit the application
of SCSPCs, and must be addressed carefully.

In addition, the choice of SCSPC electrode materials has a significant
impact on the performances of both energy storage and self-charging.
Previous study has shown that the choice of SCSPC electrode materials is
basically the same as that of ordinary SCs, and the goal is to obtain
excellent energy storage performance with minimal to zero impact on the
self-charging process\textsuperscript{{[}13{]}}. To address this
problem, Iqra Rabani et al\textsuperscript{{[}14{]}}. prepared a
BNNT-CNF/ZnO electrode that was a composite of three materials:
cellulose nanofibers (CNFs), boron nitride nanotubes (BNNTs), and ZnO
nanoparticles. It had a specific capacitance of 300 F
g\textsuperscript{-1} (1 A g\textsuperscript{-1}) and a piezoelectric
coefficient of -12.6 pC N\textsuperscript{-1}. Bhavya Padha et
al\textsuperscript{{[}15{]}}. assembled an SCSPC with perovskite
NiSnO\textsubscript{3} as the positive electrode,
FeSnO\textsubscript{3}as the negative electrode, and a PVA-KOH
electrolyte. The device could be self-charged to 266 mV at an applied
force of 20 N, and showed an energy density of 45 Wh
kg\textsuperscript{-1} at a power density of 1.25 kW
kg\textsuperscript{-1}. Overall, it is crucial to design new
nanocomposite electrode materials that can simultaneously provide a good
piezoelectric response and excellent energy storage performance.
MnO\textsubscript{2} is widely used as an electrode of supercapacitors
because of its abundance of resources, lower cost, environmental
soundness, and high theoretical specific capacitance of 1370 F
g\textsuperscript{-1{[}16, 17{]}}. However, the inferior conductivity
and inefficiency of MnO\textsubscript{2} hamper its use in
supercapacitors\textsuperscript{{[}18{]}}. Previous studies have found
that the use of metal ion doping (\emph{e.g.} , Mo, Fe, Au, Ag) to
adjust the crystal electron cloud orbitals, narrow the forbidden band
width, constitute nonintrinsic semiconductors or nanocarbon material
dopants (\emph{e.g.} , carbon nanotubes, graphene) to form conducting
channels and heterogeneous structures could effectively tackle the issue
of low conductivity for MnO\textsubscript{2}\textsuperscript{{[}19,
20{]}}. The inefficiency of MnO\textsubscript{2} utilization could also
be effectively solved by increasing its specific surface
area\textsuperscript{{[}21{]}}. ZnO is a hemimorphite piezoelectric
material with high stability, a high mechanical quality factor and a
considerable specific surface area, which facilitates the transformation
of mechanical energy towards electrochemical energy to a certain
extent\textsuperscript{{[}22, 23{]}}. ZnO has been used as a filler
compounded with PVDF to form a piezoelectric film that utilizes the
piezoelectric properties of ZnO, but the excellent nanostructure of ZnO
was not effectively utilized\textsuperscript{{[}24, 25{]}}. If ZnO and
ion-doped MnO\textsubscript{2} can be effectively combined as composite
electrode materials for SCSPCs via a simple method, the surface area of
ion-doped MnO\textsubscript{2} will be increased by using ZnO
nanostructures. In this way, the special morphologies of ZnO nanoarrays
and their piezoelectric properties will be fully utilized, and the
problems of inferior conductivity and inefficiency for
MnO\textsubscript{2} will be effectively solved.

In this study, we report the development of an SCSPC with a high
charging voltage and excellent energy conversion efficiency. ZnO
nanoarrays were used as the current collectors to expand the effective
surface area of the MnO\textsubscript{2} layer doped with Mo and Fe
ions\textsuperscript{{[}26{]}}. Additionally, the utilization of ZnO and
piezoelectric films to drive the migration of electrolyte ions gave the
SCSPC a larger self-charging voltage and higher energy conversion
efficiency. Interestingly, the presence of ZnO enabled the SCSPC to
self-charging by residual stress\textsuperscript{{[}27{]}}, and the
self-discharge rate was significantly reduced, which provides useful
support for practical applications of SCSPCs. Moreover, self-charging
via electrolyte ion migration driven by both the electrode and the film
is demonstrated for the first time in this work, and it provides strong
evidence for the great contributions of ZnO.

\textbf{Results and discussion}

Figure 1a-c shows a summary of the process used to prepare
ZnO@Mo-Fe-MnO\textsubscript{2} NAs on the flexible carbon cloth (CC),
PVDF-Trfe/CNTs/BTO piezoelectric film and SCSPC devices. Here, the
ZnO@Mo-Fe-MnO\textsubscript{2} NA electrode was prepared with a two-step
electrodeposition method (Figure 1a). First, the ZnO nanoarrays with a
mass of approximately 1.1 mg were grown on the CC via the cathodic
electrodeposition method. After that, a Mo and Fe ion-doped
MnO\textsubscript{2} film with a load of 1.1 mg was epitaxially
codeposited on the ZnO nanoarrays via anodic electrodeposition, and
ZnO@Mo-Fe-MnO\textsubscript{2} NAs were formed on the flexible CC.
PVDF-Trfe/CNTs/BTO piezoelectric films with arranged micropores were
prepared by the casting method (see Figure 1b) \textsuperscript{{[}28,
29{]}}. The piezoelectric film functioned as both a separator and an
energy harvester\textsuperscript{{[}30{]}}. Figure 1c shows the entire
structure of the SCSPC device assembled from
ZnO@Mo-Fe-MnO\textsubscript{2} NA electrodes and PVDF-Trfe/CNTs/BTO
piezoelectric thin films. The electrode and separator were filled with
the H\textsubscript{3}PO\textsubscript{4} electrolyte and bonded.

\textbf{Characterization of electrode materials}

Figure 1d shows the X-ray diffraction (XRD) patterns for
CC@Mo-Fe-MnO\textsubscript{2} (electrodeposited Mo- and Fe-codoped
MnO\textsubscript{2} films on CC), ZnO@Mo-Fe-MnO\textsubscript{2} NAs,
ZnO@Mo-MnO\textsubscript{2} NAs (epitaxially electrodeposited Mo-doped
MnO\textsubscript{2} films on ZnO NAs), ZnO@MnO\textsubscript{2} NAs
(epitaxially electrodeposited MnO\textsubscript{2} films on ZnO NAs),
and CC/ZnO NAs (electrodeposited ZnO NAs on CC). As shown in the figure,
the diffraction peaks seen at 25.8\selectlanguage{ngerman}° and 43.4° for all of the as-prepared
samples were those of the CC (JCPDS No.
75-1621)\textsuperscript{{[}31{]}}. The weak diffraction peaks located
at 31.8° and 34.4° corresponded to the (100) and (002) crystal planes of
ZnO, respectively, owing to overlap with the MnO\textsubscript{2} film
on the surface (JCPDS No. 99-0111). The remaining peaks at 35.2°, 38.6°,
53.0°, 60.5°, and 65.4° corresponded to the (310), (111), (121), (321)
and (002) crystal planes of MnO\textsubscript{2}, respectively (JCPDS
No. 72-1983). Obviously, doping of the Mo and Fe ions into the bulk
MnO\textsubscript{2} did not change the structure owing to the small
doping levels.

Figure 1e shows field emission scanning electron microscopy (SEM) images
of the flexible CC and CC/ZnO NAs. The ZnO NAs grew uniformly on the CC
surface and formed an ordered three-dimensional (3D) NA structure.
Figure 1f shows the surface morphologies of the
ZnO@Mo-Fe-MnO\textsubscript{2} NAs. The Mo and Fe ion-doped
MnO\textsubscript{2} tightly wrapped the ZnO NAs and formed a hexagonal
prism-shaped ZnO@Mo-Fe-MnO\textsubscript{2} NAs. The ZnO nanoarrays
provided a basis for the formation of ZnO@Mo-Fe-MnO\textsubscript{2} NA.
Figure S1 demonstrates the homogenous distribution of the ZnO nanoarrays
and ZnO@Mo-Fe-MnO\textsubscript{2} NAs throughout the CC as well as the
hexagonal prism shapes of the ZnO@Mo-Fe-MnO\textsubscript{2} NAs. Mo ion
doping formed the 3D nanoarray with the tubular structure, as shown in
Figure S2-4. Fe ion doping transformed the original tubular structure to
a hexagonal prismatic structure.

X-ray photoelectron spectroscopy (XPS) analyses were used to identify
the elemental compositions and valence states of the
ZnO@Mo-Fe-MnO\textsubscript{2} NAs. Figure S5 indicates the presence of
C, O, Mn, Mo, Fe and Zn in the ZnO@Mo-Fe-MnO\textsubscript{2} NAs.
Figure 2a shows the Mn 2p XPS data for the
ZnO@Mo-Fe-MnO\textsubscript{2} NA electrode. The high-resolution Mn 2p
spectrum contained two peaks at 653.9 and 642.7 eV, which corresponded
to the binding energies of the Mn 2p\textsubscript{1/2} and Mn
2p\textsubscript{3/2} states, respectively\textsuperscript{{[}32,
33{]}}. The spin orbit separation was 11.2 eV, and the signals
corresponded to the Mn\textsuperscript{4+}species in
MnO\textsubscript{2}, which was consistent with the phases indicated by
XRD. Figure 2b shows the high-resolution Mo 3d spectrum. As displayed in
the figure, the peaks located at 235.6 and 232.4 eV corresponded to the
Mo 3d\textsubscript{3/2} and Mo 3d\textsubscript{5/2} states with a
splitting of 3.15 eV, respectively, which was assigned to
Mo\textsuperscript{6+{[}34{]}}. In Figure 2c, the high-resolution Fe 2p
spectrum contained two peaks exhibiting spin-orbit splitting and two
accompanying satellite (Sat) peaks. Among these, the peaks located at
725.6 and 712.5 eV corresponded to the Fe 2p\textsubscript{1/2} and Fe
2p\textsubscript{3/2} states, respectively, and exhibited a spin-orbit
splitting energy of 13.1 eV. The peaks located at 730.4 and 719.0 eV
were identified as the accompanying satellite
peaks\textsuperscript{{[}35{]}}. This proved that Fe was present in the
form of Fe\textsuperscript{3+} ions in the electrode. Figure 2d shows
that the peaks located at 1044.8 and 1021.8 eV corresponded to the Zn
2p\textsubscript{1/2} and Zn 2p\textsubscript{3/2} states, respectively.
The spin-orbit splitting energy of 23 eV corresponded to the
Zn\textsuperscript{2+} in ZnO\textsuperscript{{[}36{]}}, which was
consistent with the XRD results. The high-resolution O 1s spectrum shown
in Figure 2e was deconvoluted into four peaks (O1, O2, O3, O4). They
were derived from Mn-O-Mn bonds (530.2 eV, O1), Zn-O bonds (531.1 eV,
O2), Mo-O bonds (529.7 eV, O3), and Fe-O bonds (532.3 eV, O4).
Interestingly, the peak areas of O1 and O2 were larger than those of O3
and O4, which perfectly matched the component contents in the
ZnO@Mo-Fe-MnO\textsubscript{2} NA electrode.

Figure 2f-g shows transmission electron microscopy (TEM) and scanning
TEM (STEM) images of the ZnO@Mo-Fe-MnO\textsubscript{2} NA electrode.
The hexagonal prismatic structure of the
ZnO@Mo-Fe-MnO\textsubscript{2}NAs can be seen in the figure. The
hexagonal ZnO prisms were surrounded by the Mo- and Fe-ion-doped
MnO\textsubscript{2}. Figure 2h-i shows a high-resolution TEM (HRTEM)
image of the ZnO@Mo-Fe-MnO\textsubscript{2}NA electrode. The analysis
showed that the interplanar distances of 0.23 and 0.25 nm corresponded
to the (111) and (310) crystal planes of MnO\textsubscript{2},
respectively. The interplanar distances of 0.28 and 0.26 nm corresponded
to the (100) and (002) crystal planes of ZnO, respectively. This once
again confirmed the successful synthesis of the
ZnO@Mo-Fe-MnO\textsubscript{2} NA heterostructure, and the conclusions
were entirely consistent with the XRD analyses. Selected area electron
diffraction (SAED) images are shown in Figure S6. Most of the regions in
the high-resolution TEM images were amorphous, which indicated the
excellent electrochemical performance of the
electrode\textsuperscript{{[}37{]}}. Figure 2j-n shows the energy
dispersive X-ray (EDX) element maps for the selected area of the
ZnO@Mo-Fe-MnO\textsubscript{2} NA electrode. This figure illustrates the
evenly distributed of Mn, Mo, Fe, Zn and O in the
ZnO@Mo-Fe-MnO\textsubscript{2} NA electrode.

\textbf{Electrochemical performances of electrode materials}

Cyclic voltammetry (CV) measurements were used to evaluate the
electrochemical properties of all as-prepared electrodes. Figure 3a
shows the CV curves generated for all of the electrodes with scan rates
of 100 mV s\textsuperscript{-1}. It is clear that the
ZnO@Mo-Fe-MnO\textsubscript{2} NA electrode had the largest CV-active
area and exhibited the highest specific capacitance at an ultrahigh scan
rate. In the galvanostatic charge\selectlanguage{english}-discharge (GCD) curves for the
electrodes (Figure 3b), the discharge time for the
ZnO@Mo-Fe-MnO\textsubscript{2} NA (332.5 s) electrode was far longer
than those of the ZnO@MnO\textsubscript{2} NA (156.3 s),
ZnO@Mo-Mn\textsubscript{2} NA (267.6 s) and
CC@Mo-Fe-MnO\textsubscript{2} (199.3 s) electrodes, which indicated the
largest specific capacitance. Electrochemical impedance measurements
(EIS) analyses were performed for evaluated the electrochemical kinetics
from all as-prepared electrodes. In the high-frequency region, the
Nyquist plot intercepted with the real axis was the equivalent series
resistances (\emph{R\textsubscript{s}} ) for the electrodes, which arose
from the internal resistances of the electrolyte and the electrodes
itself\textsuperscript{{[}38{]}}. The half-circle in the high- to
mid-frequency region represented the charge transfer resistance
(\emph{R} \textsubscript{ct}). A smaller semicircle diameter indicates a
lower resistance to charge transfer from the electrolyte to the
electrode surface\textsuperscript{{[}39{]}}. The slope of the straight
line in the low frequency region indicates the Warburg impedance
(\emph{R\textsubscript{w}} ) of the electrode, and the greater the
slope, the less resistance of electrolyte ions diffusion into the
electrode material\textsuperscript{{[}40{]}}. As shown in Figure 3c, the
ZnO@Mo-Fe-MnO\textsubscript{2} NAs exhibited the smallest semicircle
diameter in the mid-high frequency region. This indicated that the
ZnO@Mo-Fe-MnO\textsubscript{2} NAs had the smallest resistance to ion
diffusion from the electrolyte to the electrode
material\textsuperscript{{[}41{]}}. Moreover, the
ZnO@Mo-Fe-MnO\textsubscript{2} NA electrode had the smallest\emph{R}
\textsubscript{s} and \emph{R} \textsubscript{w}. This indicated that
doping with the Mo and Fe ions improved the electrical conductivity of
MnO\textsubscript{2} and reduced the internal resistance of the
electrode and the resistance to diffusion of the electrolyte ions into
MnO\textsubscript{2}\textsuperscript{{[}42{]}}. In summary, the
ZnO@Mo-Fe-MnO\textsubscript{2} NA electrode showed the largest specific
capacitance and much better electrochemical performance than other
electrodes. The reasons for this can be attributed to the following: (1)
the ZnO nanoarrays provided a larger surface area for the growth of
MnO\textsubscript{2} and formed strongly bound
ZnO@Mo-Fe-MnO\textsubscript{2} NAs. This special micromorphology
withstood the volume changes occurring during long-term charging and
discharging and improved the effective area of the active material
(MnO\textsubscript{2}) and shortened the ion diffusion paths between the
electrode and the electrolyte\textsuperscript{{[}43, 44{]}}. (2) Doping
of the Mo and Fe ions improved the electrical conductivity of
MnO\textsubscript{2}, accelerated charge transfer in the electrode and
thus increased the specific capacitance of the
electrode\textsuperscript{{[}45, 46{]}}. (3) The combined crystalline
and amorphous nanostructures in the ZnO@Mo-Fe-MnO\textsubscript{2} NA
electrode exposed many defects, provided more active sites for the
faradic processes and thus increased the specific capacitance of the
electrode\textsuperscript{{[}44, 47{]}}.

Figure 3d shows that CV curves of the ZnO@Mo-Fe-MnO\textsubscript{2} NA
electrode obtained at different scan rates of 2--100 mV
s\textsuperscript{-1}. All of the voltammograms were rectangular in
shape and highly symmetric. This showed that the electrode was mainly
contributed by the bilayer capacitance and had a reversible Faraday
effect and high Coulombic efficiency\textsuperscript{{[}48{]}}. Figure
3e shows that GCD curves for the ZnO@Mo-Fe-MnO\textsubscript{2} NA
electrode with different current densities from 1--100 A
g\textsuperscript{-1}. All of the curves were symmetrical and triangular
in shape and exhibited no significant voltage drops, which meant
excellent specific capacitance and high transfer rates. The CV and GCD
curves for the other electrodes are shown in Figure S7-9. Figure 3f
compares the capacitances of all electrodes at different current
densities from 1--100 A g\textsuperscript{-1}. The specific values are
given in Table S1. The ZnO@Mo-Fe-MnO\textsubscript{2} NA electrode
showed a high specific capacitance of 415.6 F g\textsuperscript{-1} at a
current density of 1 A g\textsuperscript{-1} and still exhibited a
specific capacitance of 250 F g\textsuperscript{-1} with an extremely
high current density of 100 A g\textsuperscript{-1}. The
ZnO@Mo-Fe-MnO\textsubscript{2} NA electrode had the largest mass (415.6
F g\textsuperscript{-1}), area (457.2 mF cm\textsuperscript{-2}) and
volume (14.3 F cm\textsuperscript{-3}) specific capacitance among the
prepared electrodes (Figure 3g). In addition, it had the highest
capacitance retention rate (60.2\%), which is detailed in Figure S10.
The electrodes exhibited both double layer capacitance and Faraday
capacitance during energy storage. Among them, the Faraday capacitance
is controlled by ion diffusion\textsuperscript{{[}49{]}}. Therefore, its
capacitive contribution was calculated (Figure 3h and Figure S11). Since
doping with Mo and Fe enhanced the conductivity of the electrode, it
showed a higher capacitance contribution. The
ZnO@Mo-Fe-MnO\textsubscript{2} NA electrode contributed up to 87.4\% at
a scan rate of 100 mV s\textsuperscript{-1} and still had a contribution
of 61.2\% at a scan rate of 5 mV s\textsuperscript{-1}. This proved that
the capacitance of the electrodes was mainly dominated by the bilayer
capacitance, which was consistent with the above analysis. Figure 3i
displays the stability of the ZnO@Mo-Fe-MnO\textsubscript{2} NA
electrode after 10,000 cycles at a current density of 10 A
g\textsuperscript{-1}. The results showed that the electrodes still
retained 83.6\% of the original capacitance and exhibited approximately
100\% Coulombic efficiency after 10,000 cycles. Interestingly, the
electrode still retained the original NA morphology after cycling
(Figure S12). The remarkably long cycling stability was attributed to
strong binding of the ZnO@Mo-Fe-MnO\textsubscript{2} NAs and the special
microscopic morphologies of the electrodes. This morphology was
instrumental in reducing the internal stresses generated during cycling
and the resulting volume changes.

\textbf{Energy storage performances of SCSPC}

To probe the electrochemical performance further, a symmetric
ZnO@Mo-Fe-MnO\textsubscript{2} NA SCSPC was generated with
ZnO@Mo-Fe-MnO\textsubscript{2} NA electrodes, the
H\textsubscript{3}PO\textsubscript{4}/PVA gel electrolyte and a
PVDF-Trfe/CNTs/BTO piezoelectric separator. For comparison,
CC@Mo-Fe-MnO\textsubscript{2} SCSPC devices without the ZnO NAs were
also assembled. Since the ionic conductivity of PVDF-Trfe
(3.10x10\textsuperscript{-4} S
cm\textsuperscript{-1})\textsuperscript{{[}50{]}} was much lower than
that of PVA (1.1x10\textsuperscript{-2} S
cm\textsuperscript{-1})\textsuperscript{{[}51{]}}, the PVDF-Trfe/CNT/BTO
piezoelectric separator required pore formation before assembly. When
assembled, the micropores of the PVDF-Trfe/CNTs/BTO piezoelectric
separator were completely filled with the
H\textsubscript{3}PO\textsubscript{4}/PVA gel electrolyte, which
significantly improved the ionic conductivity of the piezoelectric film
and the electrochemical performance of the SCSPC. Figure 4a-b displays
the CV and GCD curves for the ZnO@Mo-Fe-MnO\textsubscript{2} NA SCSPC
with different scan rates and current densities. It is noteworthy that
none of the CV curves had obvious redox peaks, the GCD curves were
nearly symmetric and triangular, and all of the curve shapes were
observed with different scan rates and current densities. These results
displayed the high reversibility of the assembled SCSPC and were still
dominated by the contribution of the typical bilayer capacitance.
Obviously, the SCSPC had a larger CV curve area and a longer
charge/discharge time than the CC@Mo-Fe-MnO\textsubscript{2} SCSPC
(Figure S13) at the same scan rate and current density, which again
confirmed the great role played by the ZnO nanoarrays in the electrodes.
Moreover, the ZnO@Mo-Fe-MnO\textsubscript{2} NA SCSPC exhibited a
remarkably large specific capacitance of 810 \selectlanguage{greek}μ\selectlanguage{english}F cm\textsuperscript{-2}at
a current density of 30 \selectlanguage{greek}μ\selectlanguage{english}A cm\textsuperscript{-2} and still showed a
specific capacitance of 337.5 \selectlanguage{greek}μ\selectlanguage{english}F cm\textsuperscript{-2} with a high
current density of 100 \selectlanguage{greek}μ\selectlanguage{english}A cm\textsuperscript{-2} (Figure S14). It is
worth noting that three ZnO@Mo-Fe-MnO\textsubscript{2} NA SCSPCs
combined in series lit up a red LED (Figure 4c). This indicated that the
ZnO@Mo-Fe-MnO\textsubscript{2} NA SCSPC has excellent electrochemical
properties. On the other hand, the long-term cycling stabilization of
the ZnO@Mo-Fe-MnO\textsubscript{2} NA SCSPC was evaluated with a current
density of 30 \selectlanguage{greek}μ\selectlanguage{english}A cm\textsuperscript{-2}. The results showed 94.3\%
capacitance retention after 2000 cycles (Figure S15). This proved that
the ZnO@Mo-Fe-MnO\textsubscript{2} NA SCSPC had excellent recyclability.
In addition, the ZnO@Mo-Fe-MnO\textsubscript{2} NA SCSPC exhibited a
better energy storage capability than the
CC@Mo-Fe-MnO\textsubscript{2}SCSPC (Figure 4d). The Ragone plot in
Figure 4d shows that the ZnO@Mo-Fe-MnO\textsubscript{2} NA SCSPC had a
high energy density of 72 \selectlanguage{greek}μ\selectlanguage{english}Wh cm\textsuperscript{-2} with a
corresponding power density of 12 mW cm\textsuperscript{-2} at a current
density of 30 \selectlanguage{greek}μ\selectlanguage{english}A cm\textsuperscript{-2}. At a high current density of
100 \selectlanguage{greek}μ\selectlanguage{english}A cm\textsuperscript{-2}, it showed an energy density of 30 \selectlanguage{greek}μ\selectlanguage{english}Wh
cm\textsuperscript{-2} and a corresponding power density of 40 mW
cm\textsuperscript{-2}. This was far more useful than the previously
reported SCSPC devices, including a graphene SCSPC (3 \selectlanguage{greek}μ\selectlanguage{english}Wh
cm\textsuperscript{-2} at 1.77 mW
cm\textsuperscript{-2})\textsuperscript{{[}52{]}}, a graphene PI-SCSPC
(6.27 \selectlanguage{greek}μ\selectlanguage{english}Wh cm\textsuperscript{-2} at 0.0178 mW
cm\textsuperscript{-2})\textsuperscript{{[}53{]}}, a
MoSe\textsubscript{2} SCSPC (10 \selectlanguage{greek}μ\selectlanguage{english}Wh cm\textsuperscript{-2} at 0.269 mW
cm\textsuperscript{-2})\textsuperscript{{[}54{]}}, a Co-NPC/LIG/Cu SCSPC
(27.8 \selectlanguage{greek}μ\selectlanguage{english}Wh cm\textsuperscript{-2} at 0.089 mW
cm\textsuperscript{-2})\textsuperscript{{[}55{]}} and a
MoS\textsubscript{2}-Nafion-MoS\textsubscript{2} SCSPC (0.019 \selectlanguage{greek}μ\selectlanguage{english}Wh
cm\textsuperscript{-2} at 0.04 mW
cm\textsuperscript{-2})\textsuperscript{{[}56{]}}.

\textbf{Characterization and energy-harvesting performances of
Piezoelectric films}

Figure 5a shows the XRD patterns of the PVDF-Trfe, PVDF-Trfe doped with
CNTs (PVDF-Trfe/CNTs) and PVDF-Trfe/CNT/BTO piezoelectric films. The
diffraction peaks at 18.6\selectlanguage{ngerman}° and 20.3° corresponded to the (202) crystal
plane of the \selectlanguage{greek}α-\selectlanguage{english}phase and the (110) crystal plane of the \selectlanguage{greek}β-\selectlanguage{english}phase,
respectively\textsuperscript{{[}57, 58{]}}. The \selectlanguage{greek}β-\selectlanguage{english}phase exhibited
piezoelectric and ferroelectric properties with the highest potential
energy. The \selectlanguage{greek}α-\selectlanguage{english}phase has neither piezoelectric nor ferroelectric
properties and has the lowest potential energy. All of the remaining
peaks were the diffraction peaks of BaTiO\textsubscript{3} (JCPDS No.
79-2263). Nano-BaTiO\textsubscript{3} was successfully doped into
PVDF-Trfe. The peaks corresponding to the CNTs were not found because of
the extremely low CNT content. However, the optical image (Figure S16)
shows that doping of the CNTs changed the PVDF-Trfe film colour from
slightly yellow to black (the same colour as the CNTs). This
demonstrated successful doping of the CNTs into PVDF-Trfe. Figure 5b
shows an SEM image of the PVDF-Trfe/CNT/BTO piezoelectric film, and this
analysis combined with Figure S17-19 showed that the
nano-BaTiO\textsubscript{3} and CNTs were uniformly dispersed in
PVDF-Trfe. Figure 5c-d displays the open-circuit voltages and
short-circuit currents at a pressure of 50 N for all piezoelectric
films. The specific values are given in Table S2. The PVDF-Trfe/CNT/BTO
piezoelectric films showed the largest open-circuit voltage of 71.6 V
and the largest short-circuit current of 0.65 \selectlanguage{greek}μ\selectlanguage{english}A. The analysis showed
that the improved performance of the PVDF-Trfe/CNT piezoelectric films
was attributable to doping with the carbon nanotubes, which increased
the content of the \selectlanguage{greek}β-\selectlanguage{english}phase in the PVDF-Trfe and improved the energy
harvesting efficiency and conduction paths of the ions and resulted in
improved piezoelectric properties\textsuperscript{{[}59, 60{]}}. The
piezoelectric properties of
BaTiO\textsubscript{3}(d\textsubscript{33}=190 pC
N\textsuperscript{-1})\textsuperscript{{[}61{]}} are much better than
those of PVDF-Trfe (d\textsubscript{33}=-38 pm
V\textsuperscript{-1})\textsuperscript{{[}62{]}}. Therefore, the
improved performance of PVDF-Trfe/CNTs/BTO piezoelectric films was
mainly attributed to doping of the nano-BaTiO\textsubscript{3}. The
SCSPC devices were therefore assembled with the PVDF-Trfe/CNTs/BTO
piezoelectric films.

\textbf{Self-powered performances of SCSPCs}

To investigate the self-charging performance of the
ZnO@Mo-Fe-MnO\textsubscript{2} NA SCSPC device, the pressure of 10 N and
3 Hz was used to test the self-charging performance (Figure 6a). The
device's voltage increased from 58 mV to 409 mV (self-charging of 351
mV) after continuous pressure for 1800 s. It is noteworthy that the
voltage of the device was still increasing after withdrawal of the
pressure (yellow area in Figure 6a), and it increased from 409 mV to 421
mV (12 mV self-charged) in 1800--2200 s (400 s). This was attributed to
self-charging due to the release of residual
stress\textsuperscript{{[}63{]}}. The self-discharge potential of the
device was only 9 mV over 2200--3000 s (800 s) (blue area in Figure 6a)
in the absence of a continuously applied pressure. Magnified images of
the 1800-3000 s self-charging period are shown in Figure S20. To
investigate the self-charging mechanism of the
ZnO@Mo-Fe-MnO\textsubscript{2} NA SCSPC device more deeply, the
self-charging capabilities of CC@Mo-Fe-MnO\textsubscript{2} SCSPC and
the CC@Mo-Fe-MnO\textsubscript{2} SC (assembled with a
CC@Mo-Fe-MnO\textsubscript{2} electrode and a commercial separator)
devices were tested with the same pressure and frequency. As shown in
Figure 6b, the CC@Mo-Fe-MnO\textsubscript{2} SCSPC device self-charged
by 121 mV in 330 s and immediately self-discharged by 112 mV in 330--440
s (a self-discharge of 9 mV). As shown in Figure S21, the
CC@Mo-Fe-MnO\textsubscript{2} SC device did not exhibit self-charging
under a continuous pressure lasting for approximately 550 s. It is
significant that the ZnO@Mo-Fe-MnO\textsubscript{2} NA SCSPC device had
the largest self-charging voltage (363 mV), and there was a
self-charging phase generated by the residual stress in the
ZnO@Mo-Fe-MnO\textsubscript{2} NA SCSPC but not in the
CC@Mo-Fe-MnO\textsubscript{2} SCSPC. In addition, the self-discharge
rate of the ZnO@Mo-Fe-MnO\textsubscript{2} NA SCSPC was much lower. The
analysis showed that the ZnO@Mo-Fe-MnO\textsubscript{2} NA SCSPC device
had the largest self-charging voltage, which was attributable to the
synergistic effect of the piezoelectric material ZnO and the
PVDF-Trfe/CNTs/BTO piezoelectric film, which together generated more
charge distribution on the electrode surface and eventually increased
the voltage of the device again. In addition, the residual
stress-induced self-charging phenomenon and the lower self-discharge
rate were attributed to the presence of ZnO and the special morphology
of the ZnO@Mo-Fe-MnO\textsubscript{2} NA heterostructure. Even under
residual stress, a potential difference was generated on the ZnO
surface, which drove the mobility of electrolyte ions to the
MnO\textsubscript{2} surface and generated self-charging. Thus, the
self-charging caused by the residual stress partially offset the
self-discharge process of the device, so the result was a device with a
low self-discharge rate. Collectively, ZnO improved the efficiency of
converting mechanical energy into chemical energy.

To investigate further the self-charging performance of the
ZnO@Mo-Fe-MnO\textsubscript{2} NA SCSPC resulting from the residual
stress, the pressure was withdrawn after a period of continuous force to
ensure that the device only self-charged under the residual stress
(Figure 6c). The device voltage increased from 100 mV to 284 mV
(self-charging of 184 mV) when subjected to a residual stress for 2500
s. This again indicated that the ZnO@Mo-Fe-MnO\textsubscript{2} NA SCSPC
had a very high energy conversion efficiency. To examine the
self-discharge rate of the ZnO@Mo-Fe-MnO\textsubscript{2} NA SCSPC
further, a leakage current test was performed for 3000 s (Figure S22).
At the beginning stage, the current of the device dropped rapidly from
0.1 mA to 1.2 \selectlanguage{greek}μ\selectlanguage{english}A within 200 s. This was followed by a slow decrease to
0.3 \selectlanguage{greek}μ\selectlanguage{english}A over 1200 s (200--1400 s) and the current remained stable. This
again demonstrated that the ZnO@Mo-Fe-MnO\textsubscript{2} NA SCSPC had
an extremely low self-discharge rate. In summary, the
ZnO@Mo-Fe-MnO\textsubscript{2} NA SCSPC device showed efficient
self-charging, a high energy conversion efficiency and a very low
self-discharge rate, as seen in Table S3.

Forces of 1 N and 10 N and 0.1--3 Hz frequencies were used in
self-charging tests of the ZnO@Mo-Fe-MnO\textsubscript{2} NA SCSPC
devices. Figure 6d displays the self-charging curves of the device
subjected to a 1 N force and different frequencies ranging from 0.1--3
Hz for 180 s. At 0.1, 0.5, 1 and 3 Hz, the device voltage increased from
40 mV to 55, 61, 67 and 69 mV, respectively, which constituted increases
of 15, 21, 27 and 29 mV, respectively. In the same environment, the 10 N
force raised the device voltage from 54 mV to 81, 94, 107 and 112 mV,
which were increases of 27, 40, 53 and 58 mV, respectively (Figure 6e).
The self-charging voltage of the device increased with increasing
applied stress and frequency, indicating excellent self-charging at high
and low potentials due to the residual stress (see Video S1 and Video
S2). Interestingly, the device exhibited excellent self-charging even
with a small force of 1 N at 0.1 Hz, which indicated excellent
sensitivity. Even with small force changes, the
ZnO@Mo-Fe-MnO\textsubscript{2} NA SCSPC still gave immediate feedback
(see Figure S23). Figure 6f shows the stepwise self-charging curve of
the ZnO@Mo-Fe-MnO\textsubscript{2} NA SCSPC. Under a continuous
pressure, the voltage of the device increased rapidly, and when the
pressure was removed, the voltage remained essentially constant. This
demonstrated that the ZnO@Mo-Fe-MnO\textsubscript{2} NA SCSPC devices
converted mechanical energy into chemical energy with excellent
reproducibility\textsuperscript{{[}64{]}}.

\textbf{Working mechanism of ZnO@Mo-Fe-MnO\textsubscript{2} NA SCSPC}

To understand the role of the ZnO NAs in the self-charging process of
the ZnO@Mo-Fe-MnO\textsubscript{2} NA SCSPC, the self-charging mechanism
was analysed. As shown in Figure 7a, the device was in electrochemical
equilibrium without any external force applied. When an external force
was applied to the device, the ZnO and PVDF-Trfe/CNTs/BTO films were
polarized due to the piezoelectric effect. Polarization of the ions
produced a potential difference on the surface of the
ZnO@Mo-Fe-MnO\textsubscript{2} NA electrode owing to the presence of the
ZnO NAs and the PVDF-Trfe/CNTs/BTO film. It is assumed that positive and
negative piezoelectric potentials were generated at the upper and lower
surfaces of the NA electrodes and PVDF-Trfe/CNTs/BTO films,
respectively\textsuperscript{{[}6{]}}. In this case, the electrochemical
equilibrium of the device was disrupted, and the piezoelectric potential
generated by the electrodes and the film caused the electrolyte ions to
migrate to the positive and negative electrodes, respectively. As shown
in Figure 7b, the positive potential generated above the NA electrode
and PVDF-Trfe/CNTs/BTO film drove the migration of cations to the upper
electrode, while the negative potential drove the reverse process. The
electrolyte ions were simultaneously driven by the PVDF-Trfe/CNTs/BTO
film and the NA electrode, so more ions migrated to the electrode
surface, which in turn generated a higher self-charging voltage.
Although the external stress was removed, the ion migration processes
continued due to the residual stress of the device, which meant that the
self-charging process continued, as shown in Figure 7c. Figure 7d shows
that a new electrochemical equilibrium was finally reached as the
residual stress was released. Finally, the potential difference
generated by the PVDF-Trfe/CNTs/BTO films and NA electrodes was a
consequence of relaxation of the residual stresses. The ions in the
electrolyte reversed their migrations and returned to their original
states (Figure 7e). It is evident that ZnO increased the driving force
for ion migration during self-charging of the device so that the
ZnO@Mo-Fe-MnO\textsubscript{2} NA SCSPC developed a higher self-charging
voltage. Moreover, the ZnO@Mo-Fe-MnO\textsubscript{2} NA SCSPC had
higher sensitivity and a better energy conversion efficiency because it
achieved the same electrolyte ion driving force as the
CC@Mo-Fe-MnO\textsubscript{2} SCSPC with a smaller external force
(Figure S24).

\textbf{Conclusions}

In summary, we successfully electrodeposited Mo- and Fe-codoped
MnO\textsubscript{2} films on ZnO NAs to improve their specific
capacitance. The SCSPC device assembled with the
ZnO@Mo-Fe-MnO\textsubscript{2} NA electrode and the PVDF-Trfe/CNTs/BTO
piezoelectric film has demonstrated an energy density of 30 \selectlanguage{greek}μ\selectlanguage{english}Wh
cm\textsuperscript{-2} at a high power density of 40 mW
cm\textsuperscript{-2}. The device realized self-charging voltage of 363
mV under the action of a 10 N external force. Intrugingly, a
self-charging voltage of 184 mV was also achieved by the residual stress
as the ZnO and piezoelectric films drove the migration of electrolyte
ions. Importantly, this work demonstrates for the first time the
self-charging mechanism involving electrolyte ion migration driven by
both electrodes and films, which provided strong evidence for the
crucial contribution of ZnO to the self-charging process. Additionaly,
three SCSPCs connected in series successfully lit up a red LED,
confirming the great potential for smart electronics applications. The
development of high-performance SCSPC devices provides important
insights for their future use in industry and life activities.

\textbf{Experimental section}

\textbf{Chemicals and Materials}

HCP 330 N CC was sourced from Guangdong Canrd New Energy Technology Co.,
Ltd (Guangdong, China). Hexamethylene tetramine
(C\textsubscript{6}H\textsubscript{12}N\textsubscript{4}) obtained from
DAMAO Co., Ltd (Tianjin, China). Manganese acetate tetrahydrate
(MnC\textsubscript{4}H\textsubscript{6}O\textsubscript{4}\selectlanguage{ngerman}·4H\textsubscript{2}O)
was purchased from MACKLIN Co., Ltd (Shanghai, China). Zinc nitrate
hexahydrate
(Zn(NO\textsubscript{3})\textsubscript{2}·6H\textsubscript{2}O),
phosphoric acid (H\textsubscript{3}PO\textsubscript{4}), sodium sulfate
anhydrous (Na\textsubscript{2}SO\textsubscript{4}), ethanol
(CH\textsubscript{3}CH\textsubscript{2}OH) and ammonium iron(II) sulfate
hexahydrate
((NH\textsubscript{4})\textsubscript{2}Fe(SO\textsubscript{4})\textsubscript{2}·6H\textsubscript{2}O)
obtained from Xilong Scientific Co., Ltd (Guangdong, China). Sodium
molybdate (Na\textsubscript{2}MoO\textsubscript{4}·2H\textsubscript{2}O)
was purchased from Tianjing No. 4 Chemical Reagent Factory (Tianjin,
China). The multiwalled CNTs and dispersants (TNWDIS) obtained from
Chengdu Organic Chemical Co., Ltd., Chinese Academy of Sciences
(Chengdu, China). PVDF-Trfe and poly(vinyl alcohol) (PVA) obtained from
Shanghai Aladdin Biochemical Technology Co., Ltd (Shanghai, China).
N,N-dimethylformamide (DMF) obtained from Tianjin Kemiou Chemical
Reagent Co., Ltd (Tianjin, China). Nanometre barium titanate obtained
from Sinopharm Chemical Reagent Co., Ltd (Beijing, China). All reagents
and materials were of analytical grade and were used without further
purification.

\textbf{Preparation of ZnO nanoarrays}

The flexible carbon cloth was ultrasonically cleaned in ethanol, a 0.5
mol L\textsuperscript{-1} H\textsubscript{2}SO\textsubscript{4} solution
and distilled water for 30 min. It was then put into a drying oven at a
temperature of 60 °C and dried for 12 h. The nanoarray was fabricated
via facile electrodeposition with a three-electrode system. The carbon
cloth or ZnO nanoarrays served as the working electrode, a carbon rod
served as the counter electrode and a saturated calomel electrode (SCE)
served as the reference electrode.

The ZnO nanoarrays were synthesized on a carbon cloth with a mixed
solution of 0.01 M Zn(NO\textsubscript{3})\textsubscript{2} and 0.01 M
C\textsubscript{6}H\textsubscript{12}N\textsubscript{4} and a simple
cathodic electrodeposition method. The electrodeposition process was
performed for 60 min used a current density of 0.8 mA
cm\textsuperscript{-2}. The electrodeposition process was carried out at
70 °C and 125 r min\textsuperscript{-1} stirring speed.

\textbf{Preparation of ZnO@Mo-Fe-MnO\textsubscript{2} NA}

MnC\textsubscript{4}H\textsubscript{6}O\textsubscript{4}·4H\textsubscript{2}O
(0.021 mol L\textsuperscript{-1}),
(NH\textsubscript{4})\textsubscript{2}Fe(SO\textsubscript{4})\textsubscript{2}·6H\textsubscript{2}O
(0.2 mmol L\textsuperscript{-1}) and
Na\textsubscript{2}SO\textsubscript{4} (0.05 mol L\textsuperscript{-1})
were dissolved in distilled water.
Na\textsubscript{2}MoO\textsubscript{4}·2H\textsubscript{2}O (0.003 mol
L\textsuperscript{-1}) was then dissolved in a 0.5 mol
L\textsuperscript{-1} H\textsubscript{2}SO\textsubscript{4} (5 mL)
solution. Finally, they were mixed and placed in a 500 mL volumetric
flask and served as the electrodeposition solution. The ZnO nanoarrays
were the working electrode. The electrodeposition process was carried
out in a water bath heated to 70 °C and with stirring at 125 r
min\textsuperscript{-1}. The electrodeposition time was approximately 80
min, and the current density was 0.5 mA cm\textsuperscript{-2}. For
comparison, the remaining nanoarrays were synthesized via the same
method. ZnO@Mo-MnO\textsubscript{2} NA were synthesized by omitting
(NH\textsubscript{4})\textsubscript{2}Fe(SO\textsubscript{4})\textsubscript{2}·6H\textsubscript{2}O
from the composition, and the electrodeposition process took 55 min.
ZnO@MnO\textsubscript{2} NA were synthesized by omitting
(NH\textsubscript{4})\textsubscript{2}Fe(SO\textsubscript{4})\textsubscript{2}·6H\textsubscript{2}O
and Na\textsubscript{2}MoO\textsubscript{4}·2H\textsubscript{2}O from
the composition, and the electrodeposition process took 65 min. The
carrier used for the electrodeposition process was the CC, and the
electrodeposition process took 40 min. The synthesized active substance
was identified as CC@Mo-Fe-MnO\textsubscript{2}.

\textbf{Preparation of piezoelectric films}

PVDF-Trfe/CNT/BTO piezoelectric films were prepared with a simple
casting method. 10 mg of CNTs, 20 mg of TNWDIS and 9 mL of DMF solvent
were mixed in a beaker. After ultrasonication for 30 min to disperse the
CNTs evenly, 1 g of PVDF-Trfe was added and stirred vigorously for 12 h.
Then, 0.25 g of BTO was added and stirred vigorously again for 12 h. All
stirring processes were carried out on a thermostatic table heated at 40
°C. The resulting viscous solution was cast on a 70 °C constant
temperature heated table with glass plates and spread evenly. After the
solvent volatilized, PVDF-Trfe/CNT/BTO piezoelectric films were
obtained. Finally, the PVDF-Trfe/CNT/BTO piezoelectric film was heated
in an oil bath at 60 °C and poled under a 2000 V DC electric field for 2
h. The preparation method of PVDF-Trfe andPVDF-Trfe/CNTs films are the
same as described above. The difference is was the CNTs, TNWDIS, BTO and
BTO reagents were omitted from the components.

\textbf{Preparation of H\textsubscript{3}PO\textsubscript{4} gel
electrolyte}

PVA (4 g) was mixed with 40 mL of distilled water and heated and stirred
until the PVA was completely dissolved and a homogeneous viscous
solution was formed. Then, 4 mL of
H\textsubscript{3}PO\textsubscript{4}was added to the viscous solution
and stirred until the solution cooled to room temperature.

\textbf{Preparation of the SCSPC device}

The SCSPC device was assembled with ZnO@Mo-Fe-MnO\textsubscript{2} NAs
or CC@Mo-Fe-MnO\textsubscript{2} (1 cm×1 cm) as both the positive and
negative electrodes. PVDF-Trfe/CNT/BTO piezoelectric films with ordered
micropores were used as separators. A
H\textsubscript{3}PO\textsubscript{4} gel solution was used as the
electrolyte in the assembled SCSPC device. The electrode was immersed in
the H\textsubscript{3}PO\textsubscript{4} gel electrolyte for 2 h under
vacuum to completely contact the electrolyte with the surface of the
electrode. The piezoelectric film was then sandwiched between the two
electrodes and encapsulated with PI tape. The assembled SCSPC device was
allowed to dry at 60 °C for 6 h. During the drying process, the SCSPC
device was maintained under pressure.

\textbf{Material characterizations}

Phase analyses of the electrodes and piezoelectric films were performed
with an XRD (D8ADVANCE-A25). The micromorphologies of the electrodes and
piezoelectric films were detected with field emission SEM (Nova Nano SEM
450). The surface topographies and structures of the electrodes were
analysed with TEM (FEIG2-20-TWIN). The elemental compositions and
distributions of the electrodes were obtained with EDX spectrometry. The
functional groups of the electrodes were determined with XPS (XPSINCA
250 X-max 50). The load of the electrode was weighed with a
microelectronic balance (Mettler Toledo, XS205 Dual Range) of accuracy
0.1 mg. A digital multimeter (DMM 6500) by used to measure the
open-circuit voltages of the piezoelectric films and the self-charging
capabilities of the SCSPC devices. The piezoelectric film was poled with
an ET2671A withstand voltage tester. An electrochemical workstation
(CHI660e, Chenhua Shanghai) was used to determine the short-circuit
currents of the piezoelectric films and the electrochemical capabilities
of the electrodes. These included CV, GCD, and EIS. The experimental
electrodeposition process was performed with an Advanced Electrochemical
System (PARSTAT 2273). Long cycling lives of the assembled SCSPC devices
were tested with an Arbin BT-2000 (\#169483) system. The linear
reciprocating motor was used to apply pressure.

\textbf{Electrochemical measurements}

The measurements for all as-prepared electrodes were performed with a
three-electrode system (electrochemical workstation, CHI660e). The
as-prepared electrode served as a working electrode, an SCE served as
the reference electrode. A platinum metal sheet was used as the counter
electrode. The measurements were performed in a 0.5 mol
L\textsuperscript{-1} Na\textsubscript{2}SO\textsubscript{4} solution
and at room temperature. The single electrode CV scan rates ranged from
2--100 mV s\textsuperscript{-1}, and the GCD measurement current
densities were 1--100 A g\textsuperscript{-1}. The EIS measurements were
performed over the frequency range 0.01--100000 Hz. The SCSPC device
used a CV scan speed range of 10--200 mV s\textsuperscript{-1} and GCD
current densities of 15--100 \selectlanguage{greek}μ\selectlanguage{english}A cm\textsuperscript{-2}.

\textbf{Supporting Information}

Supporting Information is available from the Wiley Online Library or
from the author.\textbf{Author Contributions}

Z.G.Y. conceived the idea of this study and designed the experiments.
L.S. performed the sample synthesis. L.S. performed the material
characterizations and electrochemical measurements. Z.G.Y., L.S.,
X.Y.P., S.J.Z., D.S.L. and Z.J. analyzed the data and discussed the
results. Z.G.Y., Z.J. and L.S. co-wrote and revised the manuscript.
Z.G.Y. supervised the project.

\textbf{References}

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Wang,\emph{Adv. Energy Mater.} \textbf{2019} , 9, 25.

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\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image1/image1}

\end{center}
\end{figure}

\textbf{Figure 1.} Schematic diagram of the preparation procedures
of\textbf{(a)} the ZnO@Mo-Fe-MnO\textsubscript{2} NA electrode
and\textbf{(b)} the PVDF-Trfe/CNTs/BTO piezoelectric film, and
\textbf{(c)}the SCSPC device. \textbf{(d)} XRD diffraction patterns of
the prepared electrodes and current collectors. SEM images of
\textbf{(e)} CC and CC/ZnO NAs and \textbf{(f)} the
ZnO@Mo-Fe-MnO\textsubscript{2} NAs.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image2/image2}

\end{center}
\end{figure}

\textbf{Figure 2. (a)} Mn 2p, \textbf{(b)} Mo 3d, \textbf{(c)} Fe 2p,
(d) Zn 2p and \textbf{(e)} O 1s XPS data of the
ZnO@Mo-Fe-MnO\textsubscript{2} NAs. \textbf{(f)} TEM, \textbf{(g)}
STEM,\textbf{(h, i)} HRTEM images, \textbf{(j-n)} corresponding EDX
elemental mappings at Mn, Mo, Fe, Zn, O regions of the
ZnO@Mo-Fe-MnO\textsubscript{2} NAs, respectively.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image3/image3}

\end{center}
\end{figure}

\textbf{Figure 3.} Electrochemical performances of the as-prepared
electrodes. \textbf{(a)} CV curves of all as-prepared electrodes at a
scan rate of 100 mV s\textsuperscript{-1}. \textbf{(b)} GCD curves of
all as-prepared electrodes at a current density of 1 A
g\textsuperscript{-1}. \textbf{(c)} Nyquist plots measured in the
frequency range 10\textsuperscript{5}-0.01 Hz for all as-prepared
electrodes. \textbf{(d)} CV curves generated at the scan rates ranged in
2-100 mV s\textsuperscript{-1}. \textbf{(e)} GCD curves with different
current densities of the ZnO@Mo-Fe-MnO\textsubscript{2} NA
electrode.\textbf{(f)} Specific capacitance comparison of the
as-prepared electrodes over a current density range of 1--100 A
g\textsuperscript{-1}. \textbf{(g)} The mass, area and volume
capacitances of the as-prepared electrodes at a current density of 1 A
g\textsuperscript{-1}. \textbf{(h)} Capacitance
contributions,\textbf{(i)} cycling performance and Coulombic efficiency
of the ZnO@Mo-Fe-MnO\textsubscript{2} NA electrode.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image4/image4}

\end{center}
\end{figure}

\textbf{Figure 4. (a)} CV curves and \textbf{(b)} GCD curves of the
ZnO@Mo-Fe-MnO\textsubscript{2} NA based SCSPC. \textbf{(c)} Three
series-connected ZnO@Mo-Fe-MnO\textsubscript{2} NA based SCSPC devices
lit up a red LED. \textbf{(d)} Ragone plots of the SCSPC device in this
study compared with previous works.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image5/image5}

\end{center}
\end{figure}

\textbf{Figure 5. (a)} XRD patterns of the PVDF-Trfe, PVDF-Trfe/CNT and
PVDF-Trfe/CNT/BTO piezoelectric films. \textbf{(b)} Surface SEM image of
the PVDF-Trfe/CNTs/BTO piezoelectric film. \textbf{(c)} Open-circuit
voltage and \textbf{(d)} short-circuit current of the PVDF-Trfe,
PVDF-Trfe/CNT and PVDF-Trfe/CNT/BTO piezoelectric films, respectively.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image6/image6}

\end{center}
\end{figure}

\textbf{Figure 6. (a)}Self-charging performance of the
ZnO@Mo-Fe-MnO\textsubscript{2} NA based SCSPC devices.
\textbf{(b)}Self-charging performance of the
CC@Mo-Fe-MnO\textsubscript{2} based SCSPC devices.
\textbf{(c)}Self-charging curves resulting from residual stress,
\textbf{(d)}self-charging performance with a 1 N force, \textbf{(e)}
self-charging performance with a 10 N force, and \textbf{(f)} stepwise
self-charging curve of a ZnO@Mo-Fe-MnO\textsubscript{2} NA based SCSPC
device.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image7/image7}

\end{center}
\end{figure}

\textbf{Figure 7.} Self-charging mechanism of the
ZnO@Mo-Fe-MnO\textsubscript{2} NA based SCSPC. \textbf{(a)} The original
state of the device without external forces. \textbf{(b)} The potential
difference created by the external force drove the mobility of the
electrolyte ions. \textbf{(c)} The potential difference resulting from
the residual stress caused the electrolyte ions to migrate
again.\textbf{(d)} A new electrochemical equilibrium was reached.
\textbf{(e)}The positive and negative potentials disappeared, and the
counter migration of electrolyte ions occurred and returned to the
original states.

\selectlanguage{english}
\FloatBarrier
\end{document}

