Advances on three‐dimensional electrodes for micro‐supercapacitors: A mini‐review

Abstract

Owing to the high power density, long cycle life and maintain‐free, micro‐supercapacitors (MSCs) stand out as preferred miniaturized energy source for the miscellaneous autonomous electronic components. However, the shortage of energy density is the main stumbling block for their practical applications. To solve this energy issue, constructing a three‐dimensional (3D) electrode within the limited footprint area is proposed as a new solution for improving the energy storage capacity of MSCs. In the last few years, extensive efforts have been devoted to developing 3D electrodes for MSCs, and significant progress and breakthrough have been achieved. While, there is still lack of systematic summary on the 3D electrode design strategies. To this end, it is imperative to outline the basic design conception, summarize the current states, and discuss the future research about 3D electrodes in MSCs based on the latest development.

Full text

REVIEW ARTICLE
Advances on three-dimensional electrodes for
micro-supercapacitors: A mini-review
Long Liu | Huaping Zhao | Yong Lei
Fachgebiet Angewandte Nanophysik,
Institut für Physik & IMN MacroNano
®
(ZIK), Technische Universität Ilmenau,
Ilmenau, Germany
Correspondence
Yong Lei, Fachgebiet Angewandte
Nanophysik, Institut für Physik & IMN
MacroNano
®
(ZIK), Technische Universität
Ilmenau, Ilmenau 98693, Germany.
Email: yong.lei@tu-ilmenau.de
Funding information
China Scholarship Council; Deutsche
Forschungsgemeinschaft, Grant/Award
Numbers: LE 2249/4-1, LE 2249/5-1;
Bundesministerium für Bildung und
Forschung, Grant/Award Number:
03Z1MN11
Abstract
Owing to the high power density, long cycle life and maintain-free, micro-supercapacitors
(MSCs) stand out as preferred miniaturized energy source for the miscellaneous auton-
omous electronic components. However, the shortage of energy density is the main
stumbling block for their practical applications. To solve this energy issue, constructing
a three-dimensional (3D) electrode within the limited footprint area is proposed as a new
solution for improving the energy storage capacity of MSCs. In the last few years, exten-
sive efforts have been devoted to developing 3D electrodes for MSCs, and significant
progress and breakthrough have been achieved. While, there is still lack of systematic
summary on the 3D electrode design strategies. To this end, it is imperative to outline the
basic design conception, summarize the current states, and discuss the future research
about 3D electrodes in MSCs based on the latest development.
KEYWORDS
in-plane configuration, Internet of Things, micro-supercapacitors, sandwich configuration, three-
dimensional electrodes
1|INTRODUCTION
Evolving in leaps and bounds in recent years, the Internet of
Things (IoT), which refers to the cross-linked network of
autonomous electronic devices, greatly pervades the area we
are living in and facilitates our daily life.
1
As key technolo-
gies of IoT to drive the autonomous and wireless systems,
like the wireless sensor nodes, electronic surveillance, data
logger, radio frequency identification devices, and health
control, the miniaturized electronic components require the
energy harvesting technologies to realize their own energy
autonomy.
2,3
Wireless sensors, for instance, have started to
power themselves by scavenging from the renewable sources
in the environment, such as light, wind, wave motion, or
temperature variations.
4
Whereas, the renewable energy
sources from nature usually have a certain intermittency or
randomicity, which need to be supplemented by the energy
storage components. For this reason, it is an ever-rising
demand for designing miniaturized electrochemical energy
storage (EES) units as energy buffer to support these elec-
tronic components.
Among the available EES technologies, the rechargeable
lithium-ion microbatteries (MBs) have exhibited remarkable
energy (1 mWh cm
−2
), but they suffer from moderately low
power density (<5 mW cm
−2
) and limited lifetime
(<10 000 cycles), thus tedious and/or even unrealistic peri-
odic replacements are necessary, resulting in the system
maintenance difficulties. Attributing to ultrahigh power den-
sity (>10 mW cm
−2
), long cycle life (≥100 000 cycles) and
maintenance-free feature, micro-supercapacitors (MSCs) are
attractive as potential alternatives to MBs.
5,6
Particularly for
some wireless sensors or communication transmitters which
need to be frequently switched between stand-by and activa-
tion status, MSCs are able to meet their requirements on
Received: 31 March 2019 Revised and accepted: 7 April 2019
DOI: 10.1002/inf2.12007
This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original
work is properly cited.
© 2019 The Authors. InfoMat published by John Wiley & Sons Australia, Ltd on behalf of UESTC.
74 wileyonlinelibrary.com/journal/inf2 InfoMat. 2019;1:74–84.

high-power operation. In such case, the MSCs can replace
MBs and deliver the moderate energy but high power per
unit area to satisfy these applications (Figure 1).
7,8
According to the charge storage mechanisms, MSCs
based on electrochemical double layer capacitors (EDLCs)
using various forms of carbonaceous materials tend to elec-
trostatically store the charges by adsorption of ions at the
electrode/electrolyte interface. Similarly, MSCs based on
transition metal oxides or conductive polymers store charges
in a faradaic manner with fast and reversible redox reactions
and finally exhibit pseudocapacitive behavior. Generally,
EDLCs offer better rate capability and superior cycle life,
whereas pseudocapacitors exhibit optimum specific capaci-
tances and high energy density. At present, the key consider-
ation for MSCs is the inferior energy density (<0.1 mWh
cm
−2
), which hinders their further promotion in practical
applications. One promising approach to increase the surface
energy storage capability for MSCs is to build 3D electrodes
in a small footprint area. To date, there are some inspiring
work emerged about 3D electrode design for MSCs, of
which their energy and power densities have dramatically
improved. In this mini review, we specially focus on the
research status and progress of 3D electrodes in MSCs,
including the basic design conception of 3D electrodes, their
contribution in performance optimization, and the future
direction of development. The first section will outline the
main topology structures of MSCs and highlight the advan-
tages of their 3D electrodes construction, followed by their
general concept of design. Next, we analyze the recent
advances in 3D electrodes design for MSCs. The future chal-
lenges and outlook for 3D electrode based MSCs are dis-
cussed in the end.
2|THREE-DIMENSIONAL
ELECTRODE DESIGN FOR MSCs
2.1 |The main topologies of MSCs
As known, the basic components for a MSC are the elec-
trodes materials, electrolyte, current collectors, and separator
in some cases. The performance achieved by MSCs depends
not only on the electrode or electrolyte materials, but also
tightly on the configuration which is the arrangement or
geometry of the positive and negative electrodes. Currently,
there are two basic topologies for MSCs affecting their
energy storage performance. One is sandwich topology, in
which the positive and negative electrodes are parallel to the
substrate with electrolyte and separator in the middle. Differ-
ent from traditional supercapacitors with same stacked con-
figuration, MSCs with such electrode arrangements should
limit their device thickness within several hundred microme-
ters. The other is the interdigital planar configuration, of
which the positive and negative electrodes arranged in the
interdigitated fingers with width of several hundred micro-
meters on an insulating substrate. Particularly, the in-plane
electrodes configuration not only eliminates the separator
and thus significantly reducing the size of MSC unit, but
also narrows interspace between positive and negative elec-
trodes to shorten the ionic diffusion distance.
9,10
Moreover,
its intriguing features of straightforward integratability,
11-14
out-
standing mechanical flexibility,
14-18
good scalability,
19-22
and
ingenious deployments
23
as well as precise tunability of elec-
trodes pattern enable it to gain much popularity recently.
24-27
Alternative to gravimetric normalization of
performance,
28-30
the large areal/volumetric capacities are
long-term scientific pursuits for MSCs, although high volu-
metric energy and power densities are readily achieved for
the in-plane MSCs with thin films at present,
31-33
they still
suffer from severely low areal energy density compared with
the sandwich counterparts.
5,6
Additionally, in contrast to
MBs, there is a considerable gap in energy storage capability
for MSCs whatever their configurations are to fail to meet
the growing demand for energy consumption of electronic
components. This is mainly due to the fact that MSCs with a
footprint area no larger than several square millimeters usu-
ally have thin-layer-thickness electrode, leading to the low
mass loading of active materials for charge storage. To store
more energy by increasing the mass loading to obtain thick-
layer electrode is also not a sensible choice owning to the
reduction of mechanical integrity of the electrode caused by
the increased film thickness.
34
Worse still, the utilization
efficiency of active materials in thick-layer electrode based
MSC is too low to play a significant role, especially for the
pseudocapacitive materials with poor electrical conductivity.
In most cases, the thick-layer electrodes in MSCs require
conductive additives or polymer binders to ensure a good
FIGURE 1 Area-normalized Ragone plot of energy and power
density showing the estimated performance of MSCs (marked by the
violet rectangle), MBs (marked by the orange rectangle), and some of
typical electronic components such as sensors and communication
transmitters. MBs, microbatteries; MSCs, micro-supercapacitors
LIU ET AL.75

electrical conductivity or mechanical stability, this will in
turn take up dead volume of MSCs and inevitably compro-
mise to their power density.
2.2 |The advantages of three-dimensional
electrodes in MSCs and their basic design
concept
Given this, it is desirable to adopt 3D electrodes taking
advantage of the third dimension, height, to unblock the
“dead surface”of MSCs electrodes while maintaining high
mass loading in the given footprint area of device.
35
Figure 2A,B shows the typical 3D electrodes arranged in
sandwich and in-plane MSCs, respectively. In these topolo-
gies, the 3D electrode architecture facilitates fast mass trans-
port kinetics and shortens ionic diffusion distances thanks to
the large interface between active materials and electrolyte.
36
Furthermore, these configuration designs free the device
from the utilization of the polymer binders or conductive
additives, thereby offering more room for the electrolyte
infiltration in. Besides, unlike the thick-layer electrode that
is susceptible to “fragile”film, the 3D electrode shaped by
the robust nano-building-blocks guarantees the mechanically
stable of the electrode structure and a long cycle life.
One common strategy for building 3D electrodes is to in
situ “grow”the free-standing electrode materials usually
with high electrical conductivity on the current collector to
straightforwardly achieve homogeneous electrodes.
37-39
The
arrays of electrode materials with high electrical conductiv-
ity can greatly improve the longitudinal charge transfer effi-
ciency in 3D electrodes. In the fabrication approach, ordered
nanoarchitectures of electrode materials are either self-
assembled spontaneously through an elaborate gas/liquid
phase reaction or fabricated by a sophisticated rigid or
dynamic template method.
40-42
Another particularly
promising avenue is to fabricate the bulky current collector
into nanostructured one first and then conformally deposit a
thin layer of pseudocapacitive materials that have high theo-
retical specific capacitance but low electrical conductivity on
it to form heterogeneous electrode (Figure 2C).
43
In this fab-
rication system, the design core lies in the nanostructure of
the conductive substrate, which can refer to the preparation
techniques of the 3D homogeneous electrode just mentioned.
It is noted that the conductive scaffolds employed in MSCs
are not restricted to metallic or carbonaceous materials, but
could be silica-based or any other conductive oxides, sul-
fides, carbides, and nitrides (MXenes)
44-46
materials and so
forth. Both of these preparation strategies can create large
specific surface area for electrodes to improve the utilization
efficiency of active materials as the charge storage process is
only occurred on their surface or near-surface, and finally
contribute to improving areal capacitance and energy den-
sity. It is worth mentioning that the nanowires shown in
Figure 2 is just one representative but not limited to it, there
are varieties of nanostructured electrodes with high specific
surface area springing up, such as nanotubes,
47
nanocones,
48
nanosheets,
49
and nanopores
50-52
and being widely investi-
gated in the field of 3D electrode based MSCs (3D MSCs).
In the next section, we will specially focus on the recent pro-
gress on 3D electrodes in MSC devices.
3|RECENT PROGRESS IN
THREE-DIMENSIONAL ELECTRODE
ARCHITECTURE FOR MSCs
As demonstrated in previous section, the 3D electrodes offer
many benefits: (a) providing large effective surface area at a
limited footprint area, (b) shortening ion diffusion length
between electrodes, (c) facilitating the ion and electron
FIGURE 2 Schematic of two basic
configurations of 3D MSCs: A, Sandwich and B,
in-plane topology, respectively, based on
nanostructured electrodes. C, Schematic of the
cross-section of the electrodes highlights the
nanoarchitecture of homogeneous or
heterogeneous electrode unit. More active
materials can be grown along the longitude
direction in the limited footprint area leading to
improved areal capacitance. MSCs, micro-
supercapacitors
76 LIU ET AL.

TABLE 1 Summary of the MSC devices based on 3D electrodes reported recently and their corresponding electrochemical performances
MSCs based on homogeneous nanoelectrodes
Electrode materials
Main
techniques
MSC
topology
Electrolyte/
device voltage (V)
Device energy
(μWh cm
−2
)
Device power
(mW cm
−2
) Cyclic stability Ref.
CPHs EP In-plane PVA/H
2
SO
4
gel/0.8 49.9 0.4 76% (10 000) 53
Porous carbon ZnO template
method
In-plane LiTFSi/2.4 1.53 7.92 105% (10 000) 54
B-3D-PCP 3D lithography In-plane PVA-H
3
PO
4
/1.0 7.1 (mWh cm
−3
)66(Wcm
−3
) 98% (30 000) 55
HPC 3D lithography In-plane PVA-H
3
PO
4
/1.0 1.4 (mW h cm
−3
) 1.7 (W cm
−3
) 95% (30 000) 56
3D graphene 3D printing In-plane PVA/H
2
SO
4
/1.0 - - 100% (10 000) 57
VG PVT Sandwich EMIMBF
4
/3.0 0.8 (mWh cm
−3
)61 (W cm
−3
) 83% (10 000) 58
3D G/CNTCs CVD In-plane H
2
SO
4
/1.0 0.16 (mWh cm
−3
) 115 (W cm
−3
)- 59
VN NSAs Solvothermal In-plane CMC/Na
2
SO
4
/2.4 87.62 1.2 92% (5000) 60
PANI nanowires PPT In-plane PVA/H
2
SO
4
/0.8 0.78 (mWh cm
−3
) 8 (W cm
−3
) 88% (3000) 61
G-MDHA 3D printing In-plane KOH/0.8 50 1.5 90% (10 000) 62
Cu(OH)
2
@FeOOH NTs ECD In-plane PDMS/EMIMBF
4
/1.5 18 <1 82% (10 000) 63
MSCs based on heterogeneous nanoelectrodes
Electrodes
Active
materials 3D scaffold
Main
techniques
MSC
topology
Electrolyte/device
voltage (V)
Device energy
(μWh cm
−2
)
Device power
(mW cm
−2
)
Cyclic
stability Ref.
MnO
2
,Fe
2
O
3
AgNPS Laser-induced In-plane PVA/LiClO
4
/1.4 16.3 (mWh cm
−3
) 3.54 (W cm
−3
) 94% (5000) 51
MnO
2
NCAs ED Sandwich FS/EMIMBF
4
/2.5 2.7 (mWh cm
−3
) 3.5 (mW cm
−3
) 95.3% (20 000) 48
MnO
2
Si pillar PL In-plane EMITFSI/1.5 6.5 8 >80% (15 000) 47
MnO
2
SiNWs CVD Sandwich LiClO4-PMPyrrBTA/2.0 9 <1 91% (5000) 64
δ-MnO
2
AuNPS Chemical dealloying In-plane Na
2
SO
4
/0.8 24.3 (mWh cm
−3
) 295 (W cm
−3
) 88% (20 000) 65
Ni(OH)
2
CuSe NS ED In-plane PVA/LiCl/1.0 5.4 (mWh cm
−3
) 0.8 (W cm
−3
) 100% (10 000) 66
PPy SiNTrs CVD Sandwich PYR13 TFSI/1.5 ~4 0.8 100% (10 000) 67
PANI nanofiber Microcavity array RIE In-plane PVA/H
2
SO
4
/0.8 0.01 2.7 85.7% (1000) 68
MnO
2
3D graphene Laser scribing Sandwich Na
2
SO
4
/2.0 ~40 - 96% (10 000) 69
MnO
2
Si pillars RIE Sandwich Na
2
SO
4
/0.8 - - 85% (15 000) 70
RuO
2
3D CNT ICP, FIB In-plane Na
2
SO
4
/1.0 7 19 98.6% (6000) 71
RuO
2
Porous Au HBDT Sandwich PVA/H
4
SiW
12
O
40
/0.9 126 8 95% (2000) 50
RuO
2
CNW CVD Sandwich PVA/H
4
SiW
12
O
40
/0.9 49 ~28 >90% (2000) 72
(Continues)
LIU ET AL.77

transportation, and (d) improving the electrode cyclic
stability. To drive the inferior energy storage capability per
unit area of MSCs, designing and developing suitable 3D
electrode to give full play of above advantages are of cru-
cial importance. Recently developed nanoarchitectured
electrodes based MSCs are summarized in Table 1, includ-
ing the electrodes morphology, main fabrication tech-
niques, and the electrochemical performances regarding the
energy/power densities, and cyclic stability. In this table,
the 3D MSCs were classified into two categories from a
design perspective, namely homogeneous- and heteroge-
neous nanoelectrodes based MSCs. As expected, the elec-
trochemical performances of 3D electrodes based MSCs
are found to be greatly improved and promoted. To clarify
this point further, three representative results reported
recently are specially chosen in Figure 3 to highlight the
unique nanostructure of 3D electrodes and corresponding
contribution to the MSCs.
In terms of homogeneous electrodes based MSC devices,
achieving high energy density by nanostructuring of high-
loading carbonaceous materials is highly practical and feasible
(Table 1).
77
Some key fabrication approaches such as 3D
printing,
78
3D lithography, and laser-etching techniques
79
ensure the production of carbonaceous materials with 3D archi-
tecture. Typically, Kim and co-workers reported the MSCs
based on micrometer-thick boron-doped 3D porous carbon pat-
tern (B-3D-PCP) electrodes, which were fabricated by 3D refer-
ence lithography and carbonization with B doping process.
55
It
is found that the B-doping content in electrodes increased along
with the decrease amount of N when the temperature goes up,
which will dramatically increase the electrochemically active
sites and electrical conductivity as a whole. Besides, the B-3D-
PCP has a unique monolithic architecture with electrically con-
nected skeleton and numerous pore channels for the electrolyte
ions infiltration in (Figure 3A). Eventually, this solid-state B-
3D-PCP MSC exhibits an outstanding area-normalized capaci-
tance of 7 mF cm
−2
at 100 mV s
−1
and an extremely high rate
capability of 81% at 2000 mV s
−1
. In addition, the B-3D-PCP
remains a constant volumetric capacitance and energy density,
despite the elevated thickness of electrode materials, indicating
its excellent electrical conductive of porous architecture.
On the other hand, the heterogeneous electrode is a preferred
approach if high theoretical pseudocapacitive materials are effi-
ciently utilized for the high-performance MSCs. As shown in
Table 1, most heterogeneous electrodes are comprised of
pseudocapacitive materials shell and 3D conductive scaffold
core. Among them, the 3D conductive scaffold facilitating elec-
tron transport and meanwhile supporting pseudocapacitive
materials is the key in shaping the ultimate electrodes prototype
and determining the overall performance of MSCs.
43,80
For
instance, an in-plane MSC with bicontinuous nanoporous Au
architecture (Au NPs) as current collectors and MnO
2
as
TABLE 1 (Continued)
MnO
2
ITO NWs CVD In-plane Na
2
SO
4
/1.0 27 15 61.1% (20 000) 73
RuO
2
Pt NTs AAO template Sandwich H
2
SO
4
/1.2 - - - 74
PEDOT Si NWs CVD Sandwich Na
2
SO
4
/1.2 3 4 95% (500 000) 75
RuO
x
GNW RIE, sputtering In-plane PVA-H
3
PO
4
/1.0 15 2.5 90% (3000) 76
Abbreviations: AAO, anodic aluminum oxide; B-3D-PCP, boron-doped 3D porous carbon pattern; CMC, carboxymethyl cellulose; CNW, vertically aligned carbon nanowalls; CPHs, conducting polymer hydrogels; CVD,
chemical vapor deposition; ECD, electroless copper deposition; ED, electrodeposition; EMIMBF4, 1-ethyl-3-methylimidazolium tetrafluoroborate; EMITFSI, 1-ethyl-3-methylimidazolium-bis (trifluoromethylsulfonyl) imide;
EP, electrochemical polymerization; FIB, focused-ion-beam; FS, fumed silica; G/CNTCs, graphene/carbon nanotube carpets; G-MDHA, graphene-based mixed-dimensional hybrid aerogels; GNW, graphene nanowalls; HBDT,
hydrogen bubble dynamic template; HPC, hierarchical pore-patterned carbon; ICP, inductively coupled plasma; ITO NWs, indium-tin oxide nanowires; LiTFSi, lithium bis (trifluoromethane sulfonyl) imide; MSCs, micro-
supercapacitors; NCAs, nickel nanocone arrays; NPS, nanoporous scaffold; NS, vertically oriented nanosheet; PDMS, polydimethylsiloxane; PEDOT, poly(3,4-ethylenedioxythiophene; PL, photolithography; PMPyrrBTA, 1-
methyl-1-propylpyrrolidinium bis(trifluromethylsulfonyl)imide; PPT, potentiodynamical polymerization technique; PPy, polypyrrole; PVA, polyvinyl alcohol; PVT, physical vapor transport; RIE, reactive ion etching; SiNTrs,
silicon nanotrees; VG, unidirectional arrays of vertically aligned graphene nanosheets; VN NSAs, VN nanosheet arrays.
78 LIU ET AL.

pseudocapacitive material was reported by Li et al.
65
Through
further regulating the crystallographic structures of MnO
2
,the
internal resistance of the MSC device will be significantly
reduced. This is due to the fact that the layered crystalline
δ-MnO
2
offers highly matched lattice with that of Au, which
will greatly relieve the fairly high contact resistances in the
interface between pseudocapacitive materials and current col-
lectors (Figure 3B). Moreover, benefiting from the Au NPs, the
heterogeneous electrode in MSCs possesses nanoscale pores for
the rapid mass transfer. Its cross-linked conductive substrate has
ultrahigh electronic conductivity and short electron-transport
length as well as robust mechanical strength that ensuring a long
cycle life. These enlist the MSC to deliver a high volumetric
capacitance up to 922 F cm
−3
and peak power density of
295 W cm
−3
while keeping a high volumetric energy density.
However, it should be noted that the costliness of the Au NPs
hinders its wide application in industry. Most recently, Xie's
group reported a high-performance pseudocapacitive MSCs by
employing the ITO nanowire arrays (NWs) as 3D current col-
lectors.
73
The as-prepared ITO NWs have a large aspect ratio of
~120, with diameter of 100 nm and length of 12 μm (Figure 3C).
Normally, coating with ultrathin thickness of manganese oxide
film will inevitably lead to insufficient energy storage capability,
a preferable measure is to deposit high mass loading of MnO
2
with thick layer on ITO NWs to prominently improve the areal
energy density of MSCs. Different from the conventional thick-
layer electrodes that have varieties of issues in mechanical struc-
ture and utilization efficiency as we mentioned in the previous
section, thick-layer electrodes with the help of 3D current collec-
tors will greatly address the above deficiencies. For instance, in
contrast to the MSCs electrode without ITO NWs that suffering
from the crack and peeling in the thick-film state, the MnO
2
MSCs based on ITO NWs show improved mechanically struc-
tural stability. Ultimately, the hierarchical electrode based MSC
device delivers a peak areal energy density of 27 μWh cm
−2
at
the current density of 0.3 mA cm
−2
and exhibits considerable
cycling life span.
4|SUMMARY AND PERSPECTIVES
Given the present progress, one can readily discover the huge
potential of 3D electrodes based MSCs toward high-performance
FIGURE 3 A, Boron-doped 3D porous carbon pattern (B-3D-PCP) electrodes based MSCs and their corresponding electrochemical performances.
Reproduced with permission.
55
Copyright 2018, Elsevier Ltd. B, MSCs based on 3D Au nanoporous scaffold (AuNPS)/MnO
2
electrodes and their
corresponding electrochemical performances. Reproduced with permission.
65
Copyright 2016, Wiley-VCH. C, ITO NWs@MnO
2
based MSC and its
electrochemical performances. Reproduced with permission.
73
Copyright 2019, the Royal Society of Chemistry. MSCs, micro-supercapacitors
LIU ET AL.79

energy-storage technologies. While a few 3D electrode based
MSCs (3D MSCs) have been demonstrated at present, it
should be pointed out that they are still in their infancy stage
and no specific 3D architecture electrodes or their arrange-
ments have been considered as faultless. From this point of
view, future development of 3D electrodes based MSCs can
borrow ideas from the leading-edge 3D MBs to some extent,
of which their research and development have started rela-
tively early and their micro-fabrication technology have
already been highly advanced.
36,81-85
Figure 4 exhibits some
representative 3D electrodes based MSCs, which are of great
potential to become promising 3D MSCs in the future. It is
known that to maximize the energy density of MSCs, it is
highly effective to increase the loading of electrode mate-
rials while reducing as much as possible the volumetric or
areal capacity in the micro-device. Following this idea, each
nanorod or nanoplate electrodes distributed and interdigi-
tated along the longitude direction with solid electrolyte fill-
ing between them can be designed (Figure 4A,B). This
design approach ensures the full utilization of device volume
while greatly shortening the electron/ion diffusion length.
86
If
developing a step further, the solid electrolyte can be con-
formally deposited on the electrodes leaving more room to load
counter electrode materials by forming the concentric nanopores
or aperiodic “sponge”configuration (Figure 4C,D).
87-89
In these
3D architecture blueprints, the volume of electrolyte utilized is
further reduced, leading to the enhanced volumetric/areal energy
density. Unfortunately, far from what we naively expected, the
technologies of fabricating and assembling 3D MSCs still lag far
behind the conceive and are facing unprecedented challenges.
These comprise some aspects, such as cumbersome fabrication
technology, packaging issues, electrolyte adoption, and practical
reliability.
1. Fabrication technology: Currently, the 3D MSCs can-
not do what thin-film electrode based MSCs did in mass-
production but require a series of complex micro-fabrication
techniques. Particularly, the fabrication of nanostructured
electrodes is usually independent from the overall device
manufacture for 3D MSCs. Thus, technological operation
needs further to be simplified to achieve accurate, efficient,
cost-effective, and high-yield production. For instance, recent
emerged 3D-printing technology offers new hope in realizing
3D electrodes with more miniaturization, large-scale, and effi-
ciency in a highly accurate controlled way.
90-97
2. Packaging considerations: Packaging plays a role in
sealing and well conserving the electrolyte in practically
usage process. It is the final step toward end product, but
many researchers have ignored its significance. To cater the
practical application, the encapsulation should not ruin the
tiny volumetric content of device, nor should it affect the
electrode materials or their 3D architecture. Hence, the pack-
aging of 3D MSCs goes much beyond those designs devel-
oped for conventional supercapacitors.
98,99
3. Electrolyte adoption: The design and optimization of
electrolytes for MSCs have always been a hot topic for a
long time.
100,101
Suitable electrolytes for 3D MSCs are not
well-defined largely because each and every one of electrode
materials needs to well match with a certain electrolyte.
102
Furthermore, the challenge toward designing of 3D MSCs
with leakage-free electrolytes is still remained. This will call
for solid or gel polymer electrolytes with high ionic conduc-
tivities to maintain an appropriate power density.
103
Cur-
rently, one of widely used electrolytes in 3D MSCs is the
ionic liquid-polymer gel electrolytes with low flammability
or vapor tension.
5,104,105
4. Practical reliability: Reliability assessment of 3D
MSCs is an important concern for the practical application.
It means that MSCs who can bear relatively harsh condition
or have other special functional performance will win. Espe-
cially, the 3D electrodes based MSCs which can tolerate
external forces like extrusion, bending, scratching or
stretching, and exhibit mechanical shape memory are highly
promising.
106,107
The development of other special-purpose
of MSCs like thermal resistance/sensitivity
23,108,109
or light
transparency/translucency
110,111
that can well meet some
specific applications are also hotspots of future research.
This review proposes some general ideas of design for
3D electrodes in MSCs and summarizes current advances in
terms of the fabrication strategies and their corresponding
FIGURE 4 Schematic of some promising 3D MSCs based on
nanoarchitectured electrodes. The design models were inspired by 3D
MBs reported in previous literatures. MBs, microbatteries; MSCs,
micro-supercapacitors
80 LIU ET AL.

performance contribution and finally identifies the major
development challenges and future trends in this field. It is
expected that high-performance MSCs based on novel 3D
electrodes will be further developed in the near future to
shorten the energy density gap with MBs and further pro-
mote the development of IoT system.
CONFLICT OF INTEREST
The authors declare no conflict of interest.
ORCID
Yong Lei https://orcid.org/0000-0001-5048-7433
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AUTHOR BIOGRAPHIES
LONG LIU received his Bachelor degree
(2013) and Master degree (2016) in
Material Science from the Qingdao Uni-
versity. He is now a PhD student under
the supervision of Prof. Yong Lei in the
Ilmenau University of Technology. His
research interests focus on functional
nanostructures for supercapacitors and batteries.
HUAPING ZHAO obtained his PhD in
Materials Science from the State Key
Laboratory of Crystal Materials, Shan-
dong University in 2007. He worked as
a postdoctoral fellow successively at
the Institute of Chemistry, Chinese
Academy of Sciences, and the
University of Muenster from 2007 to 2011. Since 2012,
he works as a senior scientist and a subgroup leader in
Prof. Yong Lei's group at the Ilmenau University of
Technology. His current research focuses on the design
and fabrication of functional nanostructures for energy
storage and conversion.
YONG LEI received his PhD at Chinese
Academy of Sciences in 2001. After 2-
year postdoc research at Singapore-
MIT Alliance, he worked as an Alexan-
der von Humboldt Fellow at Karlsruhe
Institute of Technology (2003-2006).
Later, he worked at University of
Muenster as a group leader (2006-2009) and a Junior
Professor (2009-2011). In 2011, he joined Ilmenau Uni-
versity of Technology as a Chair Professor and the Head
of Group of Applied Nano-physics (Fachgebiet
Angewandte Nanophysik). His research focuses on tem-
plate-based nanostructuring, energy-related and optoelec-
tronic applications of functional nanostructures. He
received a few prestigious funding including European
Research Council Grant and BMBF (Federal Ministry of
Education and Research of Germany) project.
How to cite this article: Liu L, Zhao H, Lei Y.
Advances on three-dimensional electrodes for micro-
supercapacitors: A mini-review. InfoMat. 2019;1:
74–84. https://doi.org/10.1002/inf2.12007
84 LIU ET AL.