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ARTICLE
Nanoelectrode design from microminiaturized
honeycomb monolith with ultrathin and stiff
nanoscaffold for high-energy micro-
supercapacitors
Zhendong Lei1,2,5, Long Liu1,5, Huaping Zhao1*, Feng Liang 3*, Shilei Chang3, Lei Li4, Yong Zhang 2,
Zhan Lin4*, Jörg Kröger 1& Yong Lei 1*
Downsizing the cell size of honeycomb monoliths to nanoscale would offer high freedom of
nanostructure design beyond their capability for broad applications in different fields. How-
ever, the microminiaturization of honeycomb monoliths remains a challenge. Here, we report
the fabrication of microminiaturized honeycomb monoliths—honeycomb alumina nanoscaf-
fold—and thus as a robust nanostructuring platform to assemble active materials for micro-
supercapacitors. The representative honeycomb alumina nanoscaffold with hexagonal cell
arrangement and 400 nm inter-cell spacing has an ultrathin but stiff nanoscaffold with only
16 ± 2 nm cell-wall-thickness, resulting in a cell density of 4.65 × 109cells per square inch, a
surface area enhancement factor of 240, and a relative density of 0.0784. These features
allow nanoelectrodes based on honeycomb alumina nanoscaffold synergizing both effective
ion migration and ample electroactive surface area within limited footprint. A micro-
supercapacitor is finally constructed and exhibits record high performance, suggesting the
feasibility of the current design for energy storage devices.
https://doi.org/10.1038/s41467-019-14170-6 OPEN
1Institute of Physics and IMN MacroNano®, Ilmenau University of Technology, Ilmenau, Germany. 2NUS Graduate School for Integrative Sciences and
Engineering, National University of Singapore, Singapore, Singapore. 3Faculty of Metallurgical and Energy Engineering, Kunming University of Science and
Technology, Kunming, China. 4School of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou, China.
5
These authors
contributed equally: Zhendong Lei, Long Liu. *email: huaping.zhao@tu-ilmenau.de;liangfeng@kust.edu.cn;zhanlin@gdut.edu.cn;yong.lei@tu-ilmenau.de
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Honeycomb monoliths (HMs) are a kind of cellular mate-
rials with two-dimensional cell configuration consisting of
parallel and straight channels extended throughout the
body1. HMs have been widely exploited as catalyst supporters in
different gaseous reactor applications such as chemical and
refining processes, catalytic combustion, ozone abatement, and
photocatalytic air purification2–7. As catalyst supporters, HMs
have higher surface-to-volume ratio comparing to that of planar
substrates for providing high contact efficiencies between the
supported catalysts and the gaseous reactants, meanwhile their
parallel and straight channels facilitate mass transport of gas with
low diffusion resistance. Besides these gas-phase catalysis appli-
cations, the cellular and homogeneous structure of the HM pre-
sents potential opportunities for other functional applications.
For example, it could be a desirable platform to assemble an
electroactive electrode for electrochemical devices (e.g., batteries,
supercapacitors, fuel cells, water electrolyzer, and electrochemical
sensors), in which an efficient electrochemical reaction requires
both high specific surface area of the electrode and favorable ionic
transport within the electrode8–11. Despite these merits, so far
HMs have not been widely utilized for electrochemical applica-
tions mainly due to the bulky structure of the existing HMs. The
present electrode design based on HMs makes electrochemical
devices cumbersome (especially the device volume), and mean-
while the macrostructural channels of HMs require an imprac-
tically large amount of electrolytes to fill up all the channels in
order to ensure a sufficient contact between the supported elec-
troactive materials and the electrolyte, which is different from
gas-phase catalysis applications. Moreover, the specific surface
area of the conventional HMs, although it is higher than that of
planar substrate counterparts, is far from sufficient to satisfy the
requirements of electrochemical applications. Currently, the best
reported HM can only attain 1.6 × 104cells per square inch (cpsi)
and about 3 μm in cell wall thickness12, with a specific surface
area not being comparable with that of widely used nanoelec-
trodes (i.e., mainly arrays of nanowires or nanotubes). However,
most of those nanoelectrodes suffer from the limit of aspect ratio
to maintain their highly oriented nature by avoiding the
agglomeration since nanowires and nanotubes with high aspect
ratio would prefer to form into many dense clusters, and con-
sequently are difficult to simultaneously satisfy both high specific
surface area and low ion transport resistance. To this end,
downsizing the cell size (e.g., channel diameter and cell wall
thickness) of the conventional HMs to nanoscale shall be a
solution to efficiently address the challenges for extending the
application potentials of HMs. Nevertheless, the micro-
miniaturization of the HMs encounters technological difficulties
in creating exactly parallel and straight nanoscale channels over a
large area by all reported HM fabrication approaches whatever in
industry or in laboratory, and meanwhile faces the challenge to
guarantee that the microminiaturized HM would hold the similar
excellent mechanical stabilities as the conventional HM.
In this article, microminiaturized HMs with ultrathin and stiff
nanoscaffold—honeycomb alumina nanoscaffold (HAN)—is
realized for the first time. Specifically, a large-scale HAN is fab-
ricated by using a nanoindentation-anodization-etching process
with a high-purity aluminum foil. A representative HAN is
actually a microminiaturized HM with hexagonal cell arrange-
ment, 400 nm inter-cell spacing and only 16 ± 2 nm cell wall
thickness. Impressively, the cell density of the HAN reaches
4.65 × 109cpsi that is five orders of magnitude higher than that of
the reported highest value. Meanwhile, the HAN with such
ultrathin cell wall is stiff and can sustain surprisingly high
mechanical stability identified by an experimental nanoindenta-
tion. Such robust HAN offers a stable nanostructuring platform
to assemble electroactive materials for micro-supercapacitors
(MSCs). The insulating HAN is retained in the nanoelectrodes
and thus endows the nanoelectrodes with vertically aligned and
robustly stable nanoporous structure to simultaneously achieve
both effective ion migration and ample electroactive surface area
within limited footprint. As a result, a MSC constructed with the
HAN-based nanoelectrodes exhibits record high-energy perfor-
mance among the reported micro-supercapacitors. The max-
imum capacitance of the MSC reaches 128 mF cm−2at a current
density of 0.5 mA cm−2, and the peak energy and power densities
are 160 μWh cm−2and 40 mW cm−2, respectively.
Results
Formation and structure of HAN. A schematic of the farbication
process of HAN, and the corresponding scanning electron
microscope (SEM) images, are shown in Fig. 1a–h. Briefly, the
fabrication of HAN includes nanoindentation, anodization, and a
two-step etching process (Fig. 1a). The fabrication process begins
with the anodization of a surface-nanopatterned aluminum sub-
strate to obtain nanoporous alumina (Al
2
O
3
) with a hexagonal
cell arrangement and a cell periodicity of 400 nm. Notablely, the
as-prepared nanoporous alumina has a double-layer cell structure
(Fig. 1b), with an acid acid anion-contaminated Al
2
O
3
thick layer
(in dark-gray color with the thickness of 90 ± 2 nm) adjacent to
the cell center and a relatively pure Al
2
O
3
thin layer (in light-gray
color with the thickness of 21 ± 2 nm) remote from the cell
center12. By applying a precisely controlled two-step etching in
aqueous H
3
PO
4
solutions, the acid anion-contaminated Al
2
O
3
thick layer are easy to be totally dissolved and meanwhile the pure
Al
2
O
3
thin layer is partially etched. Finally, a honeycomb
nanoscaffold consisting of only an ultrathin layer of pure Al
2
O
3
is
left with the cell wall thickness of 16 ± 2 nm (Fig. 1c), which is a
HAN (i.e., a microminiaturized HM). Figure 1d schematically
shows the evolution of the HAN through the second etching
process. In the case of the HAN with hexagonal cell arrangement
and 400 nm inter-cell spacing, the cell density is calculated to be
4.65 × 109cpsi that is five orders of magnitude higher than that of
the reported highest value13. The cross-sectional SEM image of a
HAN with cell depth of about 25 μm confirms the straight and
stable nanoporous structure of the HAN (Fig. 1e). Noting that the
cell depth of the HAN depends on the anodization time, by
tuning the anodization time, the cell depth of the HAN can be
adjusted from hundreds of nanometers to hundreds of micro-
meters. The surface area enhancement factor for an above-
mentioned HAN with the 25-μm-deep cell is calculated to be 240.
And, importantly, the relative density of HAN is calculated to be
only 0.0784, corresponding to its porosity of 0.9216. When
applying such HAN as a scaffold for designing electrodes and/or
devices, the high specific surface area and the lower “dead
volume”(i.e., the proportion of the insulating Al
2
O
3
to the whole
electrode) in electrodes/devices will be simultaneously satisfied,
benefiting to accomplishing high device performance. Not limited
to hexagonal cell arrangement with 400 nm inter-cell spacing, two
other HANs also have been produced through tuning the nano-
patterns on the surface of aluminum foils generated by nanoin-
dentation, one with hexagonal cell arrangement but 800 nm inter-
cell spacing (Fig. 1f) and the other with 400 nm inter-cell spacing
but square cell arrangement (Fig. 1g). Note that the HAN can be
fabricated over large area with almost no structural defects
(Fig. 1h and Supplementary Fig. 1a–b), which is also an impor-
tant advantage of the HAN for electrode designing.
Generally, a compressive stack pressure typically in the range of
0.1–10 MPa is applied during the assembly of batteries (coin- and
punch-cell) and supercapacitors for the purpose of maintaining
intimate contact between the electrodes and the current collectors as
well as preventing active materials delamination and deformation in
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the electrodes during operation14. Obviously, such high extrusion
pressure would bring forward the critical challenges and require-
ments to the mechanical stabilities of the nanoelectrodes for
supercapacitors and batteries. As known, the cellular solids with
honeycombs are much stiffer and stronger to manifest high
compressive strength when loaded at the out-of-plane direction (i.e.,
along the cell axis) than at the in-plane direction15,16,thus
nanoindentation experiments have been further conducted for
characterizing the nanomechanical properties of the HAN. The
Young’s modulus of the HAN with hexagonal cell arrangement,
400 nm inter-cell spacing and only 16 nm cell wall thickness are
obtained in the range of 1.41–3.22 GPa measured at five different
positions (Supplementary Fig. 1c). The excellent mechanical
performance of the HAN should be attributed to not only the
high hardness of the pure Al
2
O
3
and also the anisotropic
honeycomb microarchitectures of the HAN17–20, and it would be
asignificant advantage when applying as nanostructuring platform
to assemble electroactive materials especially for supercapacitors
and batteries, because the HAN can play the part of a rigid keel in
the nanoelectrodes to sustain the mechanical extrusion during the
device assembly but maintain the structural features at the
nanoscale of the electrodes, which are inherited from the HAN,
for finally accomplishing nanoelectrodes with both high specific
surface area and low ion transport resistance for efficient
electrochemical energy storage.
Fabrication and characterizations of HAN@SnO
2
as nanos-
tructured current collectors. As aforementioned, the HAN is a
kind of microminiaturized HMs with much higher cell density
than the conventional HMs, and the cellular and homogeneous
structure of the HAN presents opportunities as nanostructuring
platform to assemble electroactive materials for electrochemical
devices. Given the insulating nature of the HAN, the first step to
design nanoelectrodes with the HAN for electrochemical appli-
cations is to convert the insulating HAN to be a nanostructured
current collector. As shown in Fig. 2a, a 12-nm-thick tin oxide
(SnO
2
) layer was conformally coated onto the HAN with hex-
agonal cell arrangement and 400 nm inter-cell spacing (Fig. 1c)
through atomic layer deposition (ALD). Note that a thick layer of
nickel (~10 μm) as the supporting substrate was electrochemically
deposited onto the top of the HAN before the conduction of the
ALD process. The SnO
2
-coated HAN (denoted as HAN@SnO
2
)
preserves the original honeycomb features with the cell wall
thickness increasing from 16 ± 2 to 40 ± 2 nm. To verify the
possibility of HAN@SnO
2
as nanostructured current collectors, a
Nanoindentation
a
d
b
e
h
c
fg
Anodization 1st etching 2nd etching
2nd etching
Relatively
pure Al2O3
Acid anion
contaminated
Al2O3
Fig. 1 Fabrication and structure of HAN. a Illustration of the HAN fabrication process. bSEM image of nanoporous alumina after anodization process
revealing the double-layer cell structure. cSEM image of HAN with hexagonal cell arrangement and 400 nm inter-cell spacing showing the structure of
HAN with ultrathin wall of 16 ± 2 nm. dSchematic of the evolution of the HAN by the second etching process. eCross-sectional view SEM image of an
HAN indicating the cell depth to be about 25 μm. fSEM image of HAN with hexagonal cell arrangement and 800 nm inter-cell spacing. The wall thickness
of the cell is 18 ± 2 nm. gSEM image of HAN with square cell arrangement and 400 nm inter-cell spacing. The wall thickness of the cell is 12 ± 2 nm. h
Large-scale SEM image showing the uniform, stable and defect-free structure of the HAN with hexagonal cell arrangement and 400 nm inter-cell spacing.
Scale bar: 500 nm (b); 500 nm (c); 5 μm(e); 1 μm(f); 500 nm (g); 10 μm(h).
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Content courtesy of Springer Nature, terms of use apply. Rights reserved
symmetric supercapacitor based on two HAN@SnO
2
electrodes,
HAN@SnO
2
//HAN@SnO
2
, were assembled and characterized
since SnO
2
is also one kind of electroactive materials for super-
capacitors. Rate performance of supercapacitors, especially under
high scan rate, could indirectly reflect the ionic and electronic
transport behavior of electrodes. The cyclic voltammetry (CV)
curves of the device at different scan rates are shown in Fig. 2b,
and all have a quasi-rectangular shape even at a high scan rate of
20 V s−1, which indicates the typical double-layer capacitive
behavior of the device. Figure 2c reveals the corresponding var-
iation of the discharge current density at different scan rates.
Clearly, the discharge currents keep a linear relationship upon
scan rates nearly until 20 V s−1, suggesting the high instantaneous
power characteristics of the device. The representative Nyquist
plot of the device, and the corresponding expanded view at the
high-frequency region, again reveals the capacitive behavior even
at high frequencies that are attributed to the fully accessible
surface area of the electrodes for electrolyte ions adsorption/
desorption (Supplementary Fig. 2). Moreover, the high-frequency
semicircle reflects the small charge transfer resistance (R
ct
) of only
1.8 Ωin HAN@SnO
2
//HAN@SnO
2
device, also contributing to
realize high-rate performance. Therefore, high-rate performance
should be attributed not only to the rapid mass transport of ions
but also to the efficient charge transport and collection in
HAN@SnO
2
nanoelectrodes during the ultrafast charge-
discharge process21.
In addition, a nearly 98% of the initial performance was
retained after continued 30,000 charge/discharge cycles at a scan
rate of 1 V s−1(Fig. 2d) and no obvious structural changes and/or
damages can be found for the electrodes when comparing the
SEM images of the HAN@SnO
2
electrodes before and after long-
termed cycling (Fig. 2a, e, respectively). The high stability in both
electrochemical performance and electrode structure should be
attributed to the existence of the HAN in the nanoelectrodes to
functionalize as mechanically robust keel. On the other hand, it is
well-known that the mechanical extrusion is an essential step
during the device assembly of supercapacitors and batteries14,
however, at the same time it would generate a destructive impact
on nanoelectrodes. For example, the collapse of nanostructures
and then the block of ionic transport pathway will subsequently
result in unexpectedly device performance degradation compared
to the result of three-electrode configuration. Attributing to the
high stiffness of Al
2
O
3
, the monolithic structure of HAN without
structural defects over a large area and the high Young’s modulus
a
d
g
e
h
f
i
b60
100
80 2
1
0
–1
–2
0.0 0.2 0.4 0.6 0.8 1.0
60
40
20
0
12
8
4
0
0
0
0.0 0.2 0.4 0.6
Potential (V)
Potential (V)
@ 500 mV s–1
@1 V s–1
0.8 1.0
4 8 12 16
1.5
0.5
–0.5
5000 10,000 15,000
Cycle number
Time (min)
20,000 25,000 30,000
40
30
20
10
0
0 5 10 15
6 MPa
10 MPa
8 MPa
Glass fiber
20
3 V s–1
20 V s–1 15 V s–1
1st
500th
1000th
5000th
10,000th
15,000th
20,000th
25,000th
30,000th
10 V s–1 5 V s–1
1 V s–1
2 V s–1
40
20
0
–20
Current (mA cm–2)
Capacitance retention (%)
Pressure (MPa)
Current density (mA cm–2)
Current density (mA cm–2)
Discharge density (mA cm–2)
–40
0 0.2 0.4 0.6
Potential (V) Scan rate (V s–1)
0.8 1.0
c
Fig. 2 Characterizations of HAN@SnO
2
as nanostructured current collectors. a Top-view SEM image of HAN@SnO
2
.bCV curves of HAN@SnO
2
//
HAN@SnO
2
device at different scan rates. cThe discharge current as function of the scan rates based on the CV curves of HAN@SnO
2
//HAN@SnO
2
device at different scan rates. dCycling stability of HAN@SnO
2
//HAN@SnO
2
device tested at a scan rate of 1 V s−1.eTop-view SEM image of HAN@SnO
2
electrode after 30,000 CV cycles. fPhotographs of HAN@SnO
2
//HAN@SnO
2
device tested under different mechanical extrusion pressure. gCV profiles
of HAN@SnO
2
//HAN@SnO
2
device measured at a scan rate of 500 mV s−1under different mechanical extrusion pressure. hTop-view, and icross-
sectional SEM image of HAN@SnO
2
electrode after CV test under a mechanical extrusion pressure of 10 MPa, respectively. Scale bar: 500 nm (a,e); 2 μm
(h); 500 nm (i).
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of the HAN, HAN@SnO
2
electrodes are endowed with high
compression strength to sufficiently withstand the mechanical
extrusion during the device assembly without collapse or
deformation of the nanoporous structure. As a result,
HAN@SnO
2
electrodes could remain their initial integrated
structure with vertically aligned nanoporous structure and finally,
the HAN@SnO
2
//HAN@SnO
2
device still works very well
without deteriorating the capacitive behavior even under an
applied mechanical extrusion pressure up to 10 MPa (Fig. 2f, g).
The constant CV profiles indicate that there is nearly no change
in the ion-accessible surface area and the ion transport resistance
under the continuous mechanical extrusion. Furthermore, the
SEM images (Fig. 2h, i) reveal that the HAN@SnO
2
electrodes
still remain the original nanoporous features after the electro-
chemical test under the continuous mechanical extrusion. Note
that the residues in Fig. 2h, i is the residual glass fibers from a
glass microfiber filter that was used as separator, and partials of
the glass microfiber filter were damaged and fallen off during the
disassembly of the HAN@SnO
2
//HAN@SnO
2
device. This result
further reveals the excellent mechanical stability of the HAN that
can efficiently sustain the mechanical extrusion pressure during
the device assembly process to maintain the integrality of the
nanoelectrodes. Overall, the excellent electrochemical perfor-
mance and the superior structural stability verify the great
capability of HAN@SnO
2
as desirable nanostructured current
collectors to design nanoelectrodes for batteries and
supercapacitors.
Assembly and electrochemical performance of symmetric
MSCs. Following the electrochemical characterization of
HAN@SnO
2
as nanostructured current collectors, a layer of
pseudocapacitive materials was further electrochemically depos-
ited with HAN@SnO
2
as nanostructured current collectors to
produce nanoelectrodes for MSCs, which are magnesium oxide
(MnO
2
) coated HAN@SnO
2
(HAN@SnO
2
@MnO
2
) and poly-
pyrrole (PPy) coated HAN@SnO
2
(HAN@SnO
2
@PPy) nanoelec-
trodes, respectively. The pore depth of HAN in both two
nanoelectrodes was 25 μm, and the mass loading of MnO
2
and
PPy was 0.65 and 1.32 mg cm−2, respectively. The detailed cross-
sectional SEM images of different sections (Supplementary Fig. 3)
and the corresponding energy dispersive X-ray spectroscopy
(EDX) elemental mapping results (Supplementary Figs. 4–5)
clarify that the cell wall of HAN@SnO
2
have been coated with PPy
and MnO
2
, respectively. Thereafter, MSCs with symmetric device
configuration were assembled through stacking two same HAN-
based nanoelectrodes separated by a glass microfiber filter and
with 1.0 M Na
2
SO
4
aqueous solution as electrolyte. Note that the
footprint of all stacked MSCs is 0.5 cm2
,
which is equal to that of
single electrode. Figure 3a is the CV curves of the assembled
HAN@SnO
2
@MnO
2
//HAN@SnO
2
@MnO
2
MSCs over a wide
range of scan rates from 2 to 500 mV s−1in a potential range of
0–0.8 V. Obviously, the CV curves at different scan rates exhibit a
symmetrically rectangular shape, indicating an ideally capacitive
behavior and fast charge–discharge characteristics of MSCs. The
galvanostatic charge–discharge (GCD) curves of HAN@S-
nO
2
@MnO
2
//HAN@SnO
2
@MnO
2
MSCs at different current
densities are shown in Fig. 3b. These charging curves are very
symmetric to the discharge counterparts, again indicating the
excellent capacitive behaviors in MSCs. When depositing MnO
2
,
the layer thickness of MnO
2
was kept the same in all HAN@S-
nO
2
@MnO
2
electrodes, thus electrodes with deeper HAN have
higher MnO
2
mass loading. Figure 3c illustrates the device areal
capacitance of MSCs with different pore depth of original HAN
and MnO
2
mass loading as a function of scan rates. The device
capacitance depends strongly on the pore depth of original HAN
and the subsequent changes of MnO
2
mass loading of HAN@S-
nO
2
@MnO
2
electrodes. For HAN@SnO
2
@MnO
2
electrodes con-
sisting of 25-μm-pore-deep HAN, the MnO
2
mass loading was
0.65 mg cm−2, consequently resulting in highest device capaci-
tance of 137 mF cm−2at a scan rate of 2 mV s−1(121 mF cm−2at
a current density of 0.2 mA cm−2, Supplementary Fig. 6). Bene-
fiting from the vertically aligned nanoporous structure features,
these symmetric MSCs have good rate performance, i.e., the
capacitance of MSCs based on HAN@SnO
2
@MnO
2
electrodes
with 25-μm-deep HAN remains to be 47 mF cm−2when the scan
rate increasing from 10 to 2,000 mV s−1. Additionally, the
HAN@SnO
2
@PPy//HAN@SnO
2
@PPy MSCs have the similar
electrochemical performance to that of HAN@SnO
2
@MnO
2
//
HAN@SnO
2
@MnO
2
MSCs except the different operating poten-
tial window that is from −0.8 to 0 V. Figure 3d, e is the CV and
GCD curves of HAN@SnO
2
@PPy//HAN@SnO
2
@PPy MSCs,
respectively. The pore depth of original HAN in electrodes was 25
μm and the corresponding PPy mass loading was 1.32 mg cm−2,
and in this case, the maximum device capacitance of HAN@S-
nO
2
@PPy//HAN@SnO
2
@PPy MSCs reaches 124 mF cm−2at a
scan rate of 10 mV s−1(158 mF cm−2at a current density of 0.2
mA cm−2, Supplementary Fig. 6).
By contrast, HAN@SnO
2
//HAN@SnO
2
MSCs with the same
footprint (0.5 cm2) were also assembled and characterized to
further understand the role of SnO
2
layer in HAN@SnO
2
based
pseudocapacitive electrodes. It can be concluded from Fig. 3c, f
that the negligible capacitance (<4 mF cm−2) of HAN@SnO
2
//
HAN@SnO
2
MSCs, in which the pore depth of HAN in all
electrodes was 25 μm, suggests the function of SnO
2
layer in
HAN@SnO
2
based pseudocapacitive electrodes mostly for charge
transport rather than charge storage. Furthermore, electrochemi-
cal impendence spectroscopy (EIS) was performed in order to
clearly understand the role of HAN@SnO
2
as nanostructured
current collectors. Figure 4a is the Nyquist plots of HAN@SnO
2
//
HAN@SnO
2
MSCs with different pore depth of original HAN
(i.e., 5, 16 and 25 μm, respectively). All MSCs have very similar
equivalent series resistances (ESR), however, there is a significant
raise in R
ct
. The gradually increased R
ct
should be the inevitable
result arising from the limited electrical conductivity of SnO
2
,
which is much lower than that of metals. The intrinsic electrical
resistance of SnO
2
leads the total resistance of HAN@SnO
2
//
HAN@SnO
2
MSCs to be progressively increased accompanying
with HAN pore depth increase, and then consequently results in
an increase in R
ct
. With respect to HAN@SnO
2
@MnO
2
//
HAN@SnO
2
@MnO
2
and HAN@SnO
2
@PPy//HAN@SnO
2
@PPy
MSCs (Fig. 4b, c, respectively), all exhibit similar Nyquist curves,
including a semicircle in the high-frequency region and a nearly
vertical line in the low-frequency region. In the high-frequency
region, the diameters of the semicircle for HAN@SnO
2
@MnO
2
//
HAN@SnO
2
@MnO
2
MSCs are bigger than those of HAN@S-
nO
2
@PPy//HAN@SnO
2
@PPy MSCs, indicating a higher R
ct
of
HAN@SnO
2
@MnO
2
//HAN@SnO
2
@MnO
2
MSCs. Additionally,
the higher R
ct
of HAN@SnO
2
@MnO
2
//HAN@SnO
2
@MnO
2
MSCs are also evidenced by the more obvious potential drops
of GCD curves than those of HAN@SnO
2
@PPy//HAN@S-
nO
2
@PPy MSCs (Fig. 3b, e). In addition, both HAN@S-
nO
2
@MnO
2
//HAN@SnO
2
@MnO
2
and HAN@SnO
2
@PPy//
HAN@SnO
2
@PPy MSCs have the similar ESR as those of
HAN@SnO
2
//HAN@SnO
2
MSCs. The similar ESR suggests the
negligible influence on the series bulk resistance with the use of
HAN@SnO
2
as nanostructured current collectors. In addition, the
smaller ESR in HAN@SnO
2
@PPy//HAN@SnO
2
@PPy MSCs
(Fig. 4c) could be attributed to the higher electrical conductivity
of PPy than that of MnO
2
. Figure 4d summarizes the R
ct
of all
HAN-based MSCs. For HAN@SnO
2
@MnO
2
//HAN@S-
nO
2
@MnO
2
MSCs, the R
ct
are all higher than those of
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HAN@SnO
2
//HAN@SnO
2
MSCs. Besides the increased electrical
resistance in HAN@SnO
2
current collectors, the gradual raise in
R
ct
of HAN@SnO
2
@MnO
2
//HAN@SnO
2
@MnO
2
MSCs should
be due to the low intrinsic conductivity of MnO
2
. The low
conductive MnO
2
layer makes the charge transport in HAN@S-
nO
2
@MnO
2
electrodes mainly rely on the conductive SnO
2
layer,
which results in the exclusive pathway for charge transportation.
On the contrary, the highly conductive PPy layer could not only
store charges but also partially contribute to the charge transport
for improving the charge transport efficiency in HAN@S-
nO
2
@PPy electrodes, resulting in lower R
ct
of HAN@S-
nO
2
@PPy//HAN@SnO
2
@PPy MSCs than those in both
HAN@SnO
2
//HAN@SnO
2
and HAN@SnO
2
@MnO
2
//HAN@S-
nO
2
@MnO
2
MSCs. Furthermore, fast ion transport at the
interface between electrode and electrolyte could be evident by
the inconspicuous Warburg region. In the low-frequency region
(Supplementary Fig. 7), all HAN-based MSCs exhibit an almost
vertical line, representing the promising permeability for electro-
lyte infiltration and ion diffusion to access the surface of
pseudocapacitive materials, and to create more active sites for
electrochemical reactions to store more charges.
Construction and performance evaluation of asymmetric
MSCs. The areal energy density is directly proportional to the
areal capacitance value and the square of the cell voltage.
Nevertheless, the energy density of supercapacitors is restricted by
the use of pseudocapacitive materials with a narrow potential
window. An asymmetric device configuration affords the
opportunity for the expansion of the operating potential window
of pseudocapacitors, and subsequently accomplishes the
enhanced energy and power densities. Asymmetric MSCs were
therefore assembled by using HAN@SnO
2
@MnO
2
as positive
electrode and HAN@SnO
2
@PPy as negative electrode. The two
electrodes were stacked together with a glass microfiber filter as
separator and 1.0 M Na
2
SO
4
aqueous solution as electrolyte. As
the aforementioned symmetric MSCs, the footprint of HAN@S-
nO
2
@MnO
2
//HAN@SnO
2
@PPy MSCs was also 0.5 cm2. Note
that a charge balance between the two electrodes was accom-
plished by controlling the deposition time of MnO
2
at the positive
electrode and the thickness of the PPy film at the negative elec-
trode. Figure 5a outlines the working potential windows of each
symmetric MSCs individually, −0.8–0 V for HAN@SnO
2
@PPy//
HAN@SnO
2
@PPy MSCs and 0–0.8 V for HAN@SnO
2
@MnO
2
//
HAN@SnO
2
@MnO
2
MSCs, so that the asymmetric MSCs by
integrating HAN@SnO
2
@PPy and HAN@SnO
2
@MnO
2
into
single MSCs will result in an increased operating cell voltage up to
1.6 V. As shown in Fig. 5b, the asymmetric MSCs with 16-μm-
pore-deep HAN in all electrodes work efficiently in the range
from 0.8 to 1.6 V. Figure 5c, d is the corresponding CV and GCD
curves of asymmetric MSCs in a potential range of 0–1.6 V,
respectively. The nearly rectangular CV curves and the highly
triangular charge–discharge profiles demonstrate an ideally
capacitive behavior and the fast charge–discharge characteristics
of the asymmetric MSCs. Particularly, the CV curves retain their
rectangular shape without apparent distortions when increasing
scan rates up to a high rate of 1000 mV s−1, indicating the
extraordinary high-rate performance of asymmetric MSCs. And a
highest device capacitance of 147 mF cm−2has been achieved at a
scan rate of 10 mV s−1and still remain 80 mF cm−2at the scan
rate of 1000 mV s−1(Supplementary Fig. 6). The maximum
capacity of this device could reach 186 mF cm−2at a current
density of 0.2 mA cm−2(Supplementary Fig. 8), and is one of the
highest values among the reported MSCs (Supplementary
Table 1). Moreover, the asymmetric MSCs have an excellent
cyclic capability of 87% of its original capacity after continued
30,000 charge/discharge cycles tested at a current density of 20
mA cm−2and with nearly 100% Coulombic efficiency (Fig. 5f).
The slight performance degradation should be mainly due to
MnO
2
, as evidenced by the obvious morphological changes after
cycling (Supplementary Fig. 9).
60
a
def
bc
0.8
0.6
0.4
0.2
0.0
–0.8
–0.6
–0.4
–0.2
0.0
0 100 200 300
Time (s) Scan rate (mV s–1)
400 500
0 100 200 300
Time (s)
400 500
160
120
200
150
100
50
0
80
40
0
0 500 1000 1500 2000
Scan rate (mV s–1)
0 500 1000 1500 2000
500 mV s–1 200 mV s–1 100 mV s–1
0.5 mA cm–2
25 μm@0.65 mg cm–2
25 μm@1.32 mg cm–2
16 μm@0.83 mg cm–2
5 μm@0.21 mg cm–2
25 μm@0 mg cm–2
16 μm@0.43 mg cm–2
25 μm@0 mg cm–2
MnO2 mass loading
PPy mass loading
Pore depth
Pore depth
5 μm@0.11 mg cm–2
1 mA cm–2
2 mA cm–2
5 mA cm–2
10 mA cm–2
0.5 mA cm–2
1.0 mA cm–2
2.0 mA cm–2
5.0 mA cm–2
10 mA cm–2
10 mV s–1
100 mV s–1
10 mV s–1
20 mV s–1
2 mV s–1
200 mV s–1
20 mV s–1
2 mV s–1
50 mV s–1
5 mV s–1
500 mV s–1
50 mV s–1
5 mV s–1
40
20
Current (mA cm–2)Current (mA cm–2)
Potential (V)
Potential (V)
Areal capacitance (mF cm–2)Areal capacitance (mF cm–2)
–20
–40
–40
–20
0
20
40
60
80
–60
0 0.2 0.4
Potential (V)
Potential (V)
0.6 0.8
0.0–0.2–0.4–0.6–0.8
0
Fig. 3 Electrochemical performance of symmetric MSCs based on HAN-based nanoelectrodes. a CV curves at different scan rates, bGCD profiles at
different current densities, and cdevice areal capacitance as a function of scan rates of HAN@SnO
2
@MnO
2
//HAN@SnO
2
@MnO
2
MSCs, respectively. d
CV curves at different scan rates, eGCD profiles at different current densities, and fdevice areal capacitance as a function of scan rates of
HAN@SnO
2
@PPy//HAN@SnO
2
@PPy MSCs, respectively.
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When further replacing aqueous electrolyte with the ionic
liquid electrolyte (1-ethyl-3-methylimidazolium bis(trifluoro-
methylsulfonyl)imide, EMIM-TFSI), the asymmetric MSCs can
work efficiently in an enlarged potential range of 0–3.0 V (Fig. 5g,
h). The distorted CV and GCD curves arise from the high
viscosity and large charge transfer resistance of the electrolytes
because of the large ion size of the ionic liquids22,23. The device
capacitance reaches 128 mF cm−2at a current density of 0.5 mA
cm−2(or 162 mF cm−2at a scan rate of 10 mV s−1). When the
current density is increased from 0.5 to 5 mA cm−2, the
capacitance drops to 93 mF cm−2(72% retention) and it recovers
to 123 mF cm−2(96% retention) with the current density again
being decreased to 0.5 mA cm−2(Fig. 5i), revealing the excellent
rate performance of the asymmetric MSCs. In addition, the
asymmetric MSCs with ionic liquid electrolyte still exhibit
remarkable cycling stability with 82.5% of initial capacitance at
20 mA cm−2over a potential window of 0–3.0 V withstanding
continued 10,000 cycles with high Coulombic efficiency of nearly
100% and slight decay in CV curves at 100 mV s−1before and
after cycling (Supplementary Fig. 10). Figure 6a presents the areal
energy performance metrics of one representative HAN@S-
nO
2
@MnO
2
//HAN@SnO
2
@PPy asymmetric MSCs with EMIM-
TFSI ionic liquid electrolyte. The maximum capacitance of the
MSCs reaches 128 mF cm−2at a current density of 0.5 mA cm−2,
and the peak energy and power densities are 160 μWh cm−2and
40 mW cm−2, respectively. Remarkably, the energy storage
performance of the asymmetric MSCs with HAN-based nanoe-
lectrodes is among the best comprehensive performance of the
reported stacked MSCs (Fig. 6b)21,24–34. In particular, the peak
energy density of the stacked asymmetric MSCs in this work is
roughly fourfold that of the carbide-derived-carbons (CDC)
based stacked MSCs (~40 μWh cm−2) but with a similar peak
power density24. Additionally, the areal energy density of the
asymmetric MSCs with EMIM-TFSI electrolyte is even compar-
able with that of some state-of-the-art 3D micro-batteries but
with much higher areal power density35,36.
Discussion
The insufficient energy of MSCs is their crucial flaw compared
with micro-batteries and remains a major challenge to overcome.
This work signifies the capability of the HAN as a promising
nanostructuring platform to assemble pseudocapacitive materials
toward rationally designing nanoelectrodes for MSCs with high-
energy performance. In comparison with the conventional HMs,
the HAN as a kind of microminiaturized HMs has the highest cell
density, lower relative density, higher surface area enhancement
factor, and excellent compressive performance. With such robust
HAN as the keel, the as-prepared nanoelectrodes could possess
vertically aligned and robustly stable nanoporous structure. And
in contrast to nanoelectrodes design based on nanowires and
nanotubes, there is no limit about the aspect ratio to sustain such
vertically aligned nanoporous structure. Besides high surface area
to provide abundant electroactive sites for surface Faradaic
reactions, the vertically aligned nanoporous structure affords the
smooth channels to favor the permeability for electrolyte
10
a
c
b
d
10
0.11 mg cm–2
0.21 mg cm–2 0.83 mg cm–2
1.32 mg cm–2
0.43 mg cm–2
0.65 mg cm–2
88
66
–Z′′ (Ω)
–Z′′ (Ω)
–Z′′ (Ω)
Z′ (Ω)
Z′ (Ω)
44
22
0
0
1
2
3
4
5
0
8
6
4
Rct (Ω)
2
0
HAN@SnO2
HAN@SnO2@MnO2
HAN@SnO2@PPy
5 μm5 μm
5 μm
16 μm16 μm
16 μm
25 μm
5 μm
16 μm
25 μm
25 μm
25 μm
0
2
3
4
5
1
0
2
4
6
8
10
Z′ (Ω)
0
2
4
6
8
10
Fig. 4 Electrochemical impedance properties comparison. Nyquist plots of aHAN@SnO
2
//HAN@SnO
2
,bHAN@SnO
2
@MnO
2
//HAN@SnO
2
@MnO
2
,
and cHAN@SnO
2
@PPy//HAN@SnO
2
@PPy MSCs with different pore depths of HAN. dComparison of the charge transport resistance (R
ct
) of different
HAN-based MSCs.
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6
a
d
ghi
ef
bc
15
10
5
0
–5
–10 –80
–40
0
40
80
120
0 0.4 0.8 1.2 1.6
3
Current (mA cm–2)
Current (mA cm–2)
Current (mA cm–2)
Current (mA cm–2)
0
–3
–6
–0.8
1.6
1.2
0.8
0.4
0
0
20
40
–20
–40
0 200 400
Time (s)
Potential (V)
Scan rate (m V s–1)
600
160
120
80
40
0
0
2
3
1
0 200 400 600 800
800600400
Time (s)
20000123
1000
100
150
100
50
0
0 1020304050
100
80
60
Coulombic efficiency (%)
40
20
0
80 1.6
1.2
0.8
0.4
0.0
0.00 0.01 0.02 73.68
Time (h)
73.69 73.70
60
40
20
0
0 5000 10,000 15,000
Cycle number
Cycle number
20,000 25,000 30,000
–0.4 0
Potential (V)
Potential (V)
Potential (V)
Potential (V)
Areal capacitance (mF cm–2)
Areal capacitance (mF cm–2)Capacitance retention (%)
Potential (V)
0 0.4 0.8 1.2 1.6
Potential (V)
0.4
0–0.8 V
–0.8–0 V
HAN@SnO2@MnO2
HAN@SnO2@PPy
0.8
@ 100 mV s–1
@ 20 mA cm–2
0.5 mA cm–2
1 mA cm–2
2 mA cm–2
5 mA cm–2
10 mA cm–2
15 mA cm–2
25 mA cm–2
5 mA cm–2
0.5 mA cm–2
@ 100 mV s–1
1000 mV s–1
200 mV s–1 150 mV s–1
20 mV s–1 10 mV s–1
100 mV s–1
50 mV s–1
0.5 mA cm–2 25 μm
16 μm
5 μm
1.0 mA cm–2
1 mA cm–2
2 mA cm–2
5 mA cm–2
10 mA cm–2
15 mA cm–2
20 mA cm–2
2.0 mA cm–2
5.0 mA cm–2
10 mA cm–2
500 mV s–1 200 mV s–1
20 mV s–1
50 mV s–1
5 mV s–1
100 mV s–1
10 mV s–1
1.6 V 1.4 V 1.2 V 1.0 V
0.8 V
Fig. 5 Electrochemical performance of HAN@SnO
2
@MnO
2
//HAN@SnO
2
@PPy asymmetric MSCs. a Typical CV curves of HAN@SnO
2
@MnO
2
and
HAN@SnO
2
@PPy based symmetric MSCs, respectively, with 1.0 M Na
2
SO
4
electrolyte. bCV curves within different potential ranges. cCV curves at
different scan rates. dGCD profiles at different current densities. eDevice areal capacitance as a function of current densities. fCycling stability test at a
scan rate of 20 mA cm−2.gCV curves at different scan rates, hGCD profiles at different current densities, and irate performance of asymmetric MSCs
with EMIM-TFSI electrolyte, respectively.
120
ab
1020.36 s
10 h
This work, MnO2//PPy
(in Na2SO4)
This work, MnO2//PPy
(in EMIMTFSI)
This work, MnO2//MnO2
LWG, ref. 28
rGO MSCs, ref. 29
PG-MSCs, ref. 30
MPG-MSCs, ref. 32
SG-MSCs, ref. 31
G-CNTCs, ref. 21
Textile MSCs, ref. 34
OLC, ref. 25
LSG-MSCs,
ref. 27
CDC, ref. 24
LSG/MnO2, ref. 26
This work
PPy//PPy
1 h 360 s 36 s 3.6 s
36 ms
3.6 ms
0.36 ms
36 μs
100
10–2
Energy density (μ Wh cm–2)
Power density (mW cm–2)
10–4
10–3 10–1 101103
Capacitance (mF cm
–2
)
Power density (mW cm
–2
)
Energy density (μwh cm
–2
)
80
40
0
40
40
80
120
160
30
20
10
0
Fig. 6 Energy performance of HAN@SnO
2
@MnO
2
//HAN@SnO
2
@PPy asymmetric MSCs. a Areal performance metrics of HAN@SnO
2
@MnO
2
//
HAN@SnO
2
@PPy asymmetric MSCs with EMIM-TFSI ionic liquid electrolyte. bRagone plots of MSCs with HAN-based nanoelectrodes compared with
some reported MSCs21,24–34.
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infiltration and ion diffusion to access the surface of pseudoca-
pacitive materials, guaranteeing a high utilization efficiency of the
pseudocapacitive materials. Taken together, the HAN-based
nanoelectrodes design synergizes the effects of both effective
ion migration and high electroactive surface area, thus enabling
high and reversible capacitive behavior even at high
charge–discharge rates. In the current HAN-based nanoelec-
trodes design, four strategies have been combined together into
single MSCs aiming to achieve high-energy storage capability: (i)
the utilization of pseudocapacitive materials with high specific
capacitance in both positive and negative electrodes; (ii) the
adoption of asymmetric device configuration to extend the
operating potential window of MSCs; (iii) the application of ionic
liquid electrolyte to further widen the operating potential window
of MSCs; and more importantly, (iv) the prominent nanoelec-
trode architecture design based on HAN for both positive and
negative electrodes. The combination of pseudocapacitive mate-
rials with unique nanoelectrodes architecture results in the
increased areal capacitance with reduced footprint, meanwhile
the adoption of asymmetric device configuration coupled with
ionic liquid electrolyte helps to overcome the narrow operating
potential window of pseudocapacitive materials. As a result, the
asymmetric MSCs constructed with the HAN-based nanoelec-
trodes exhibit one of the most remarkable comprehensive areal
device performance metrics.
We emphasize that nanoelectrode design with the robust HAN
as the keel to assemble electroactive materials inherits the unique
structural characteristics of the HMs and can be manipulated to
achieve desired application requirements for electrochemical
devices beyond MSCs. Our work opens up the ample opportu-
nities for further expanding the application range of the HMs and
also provides a paradigm about rationally designing nanos-
tructures for various functional devices with new features and
high performance.
Methods
Preparation of HAN. The HAN was fabricated through a nanoindentation-
anodization process, followed by a precisely controlled two-step chemical etching
process. Firstly, a hexagonal patterns was generated onto the surface of aluminum foil
by nanoindentation, then the aluminium foil was anodized in a 0.4 M phosphoric acid
(H
3
PO
4
) aqueous solution at 160 V for different time. The temperature ofthe solution
and the aluminium foil was kept at 10°C during the anodization process. After
anodization, the unanodized aluminum was etched by a mixture aqueous solution of
CuCl
2
(85 wt%) and HCl (15wt%). Thereafter a two-step chemical etching process
was applied to obtain the HAN: (1) etching with a 5wt% H
3
PO
4
solution at 60 °C to
achieve an open-end membrane; (2) further etching with a 5 wt% H
3
PO
4
solution at
30 °C to finally obtain HAN. The surface area enhancement factor (A)forHANwas
calculated by Eq. 1:
A¼
6
ffiffi
3
p´d´h
s2´sin θð1Þ
where dand hare corresponding to the cell diameter and depth of the cell, respec-
tively, sis the intercell distances, measured between the centers of the cells, and θis
the angle of the pattern in which the cells are positioned. For the typical HAN
structure with cell depth of 25 μm, the specific values are d=384 nm, h=25 μm, s=
400 nm and θ=60°, respectively. Accordingly, the Ais calculated to be 240.
Fabrication of HAN-based nanoelectrodes. The HAN@SnO
2
substrate was
produced by conformally depositing SnO
2
on the surface of the as-prepared HAN
at 250 °C using an ALD system (Picosun, SUNA LE R-150). The precursors were tin
(IV) chloride (SnCl
4
) for tin and ultrapure water for oxygen, respectively, and high-
purity nitrogen was the carrier and purging gas. Specifically, each ALD cycle
consisted of a 0.2 s pulse of SnCl
4
and a 4 s purge of N
2
, followed by a 1 s pulse of
H
2
O and an 8 s purge of N
2
, and this procedure was repeated 1500 times. The
growth rate of SnO
2
is estimated to be about 0.16 Å per cycle. With the as-obtained
HAN@SnO
2
as the working electrodes, the HAN-based nanoelectrodes were fab-
ricated through electrochemical deposition method. The HAN@SnO
2
@PPy elec-
trode was prepared by electrochemical polymerization of PPy onto HAN@SnO
2
substrate. The plating solution consisted of 0.1 M pyrrole monomer (98%) and 0.2
M oxalic acid, and the applied potential was 0.8 V (vs. Ag/AgCl)37. Likewise, the
HAN@SnO
2
@MnO
2
electrode was fabricated by electrochemical deposition of
MnO
2
onto the HAN@SnO
2
substrate by using an electrolyte consisting of 50 mM
manganese acetate and 100 mM sodium acetate with a constant current density of
1mAcm
−2. The active mass of the electrodes were determined according to
Faraday’s law and 100% charge efficiency was assumed, according to Eq. 2:
m¼QM
zF ð2Þ
where Qis the charge passed during the electrochemical deposition process of
active materials (i.e., PPy/MnO
2
). And it is worth noting that the charge is precisely
adjust here in order to achieve the same layer thickness of active materials for
HAN@SnO
2
with different cell depth. Mis the molar mass of the active electrode
material (M
MnO2
=86.9 g mol−1;M
py
=65.09 g mol−1), Fthe Faraday constant
and zthe number of transferred electrons per active electrode atom (z=2 for
MnO
2
and PPy deposition).
Apparatus for characterizations. The surface morphologies and microstructures
of the electrodes were characterized using scanning electron microscopy (ZEISS
AURIGA) equipped with an EDX detector. Cyclic voltammetry (CV), galvanostatic
charge–discharge (GCD), electrochemical impedance spectroscopy (EIS) mea-
surements, and cycling performance measurements were conducted with a
Potentiostat (BioLogic, VSP). Nanoindentation was conducted with the Tri-
boindenter (Hysitron) equipped with a Berkovich diamond indenter with an
approximately 600-nm-radius.
Electrochemical measurements and calculation of the electrochemical per-
formances. All the electrochemical measurements were carried outin a two-electrode
configuration with 1.0 M Na
2
SO
4
aqueous electrolyte and a glass microfiber filter
(Whatman, GF/B) as separator as well as 1-ethyl-3-methylimidazolium bis(tri-
fluoromethylsulfonyl)imide (EMIM-TFSI) ionic liquid electrolyte (1M in acetoni-
trile). The CV curves were collected with various scan rates of 5 to 2000 mVs−1,and
the GCD curves were obtained at current densities of 0.2, 0.5, 1, 2, 5, 10, 15, 20 and
25 mA cm−2. The EIS measurement was performed with a frequency range from
100 MHz to 10 mHz and a 5 mV AC amplitude.
The capacitance of the device C
GCD
(mF cm−2) is determined from the GCD
profiles based on Eq. 3:
CGCD ¼I´Δt
A´ΔVð3Þ
where Δt (s) is the discharge time, I(mA) is the discharge current, A(cm2) is total
footprint area of device, and ΔV (V) is the voltage window.
The device capacitance C
CV
(mF cm−2) can also be calculated from CV curves
based on Eq. 4:
CCV ¼RIdV
υ´ΔV´Að4Þ
where I(A) is the response current and ν(V s−1) is the potential scan rate.
With GCD plots, the energy density (E, unit: μWh cm−2) and power density (P,
unit: mW cm−2) of the device were calculated by Eqs. 5and 6:
E¼0:5´C´ΔV2ð5Þ
P¼E
Δtð6Þ
where C(mF cm−2) is the areal capacitance of the device, Δt (s) is the
discharge time.
Date availability
The data that support the findings of this study are available from the corresponding
authors on reasonable request.
Received: 22 April 2019; Accepted: 13 December 2019;
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Acknowledgements
The authors gratefully acknowledge the support from German Research Foundation
(DFG: LE 2249/4-1 and LE 2249/5-1), as well as financial support for Long Liu from the
China Scholarship Council (CSC).
Author contributions
Zh. L and Lo. L contributed equally to this work. Y.L. supervised the project. The concept
was conceived by H.Z. and Y.L. Zh. L, Lo. L and H.Z. were involved in the fabrication and
related characterizations. Zh. L, Lo. L, S.C. and Le. L performed the electrochemical
experiments and data analysis under the guidance of H.Z., F.L., Zh. L, J.K. and Y.L. H.Z.
and Y.L. wrote the paper with the assistance of F.L., Y.Z. and J.K.
Competing interests
The authors declare no competing interests.
Additional information
Supplementary information is available for this paper at https://doi.org/10.1038/s41467-
019-14170-6.
Correspondence and requests for materials should be addressed to H.Z., F.L., Z.L. or Y.L.
Peer review information Nature Communications thanks the anonymous reviewer(s) for
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ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-019-14170-6
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