Electrochemical properties of typical m-SC microelectrodes in literature .
Micro–supercapacitors (m–SC) arise from the demand of developing micro–power system for MEMS devices, attracting much research interest in recent years. As m–SC has to achieve high areal energy and power densities, the volumetric capacitance and the rate capability of the electrode materials have become the most important concern. This review compares the intrinsic electrochemical properties of the state–of–art electrode materials for m–SC, reporting the recent advances in the three types of electrode materials. For carbon electrode materials, two developing trends are identified: one is to enhance volumetric capacitance through a proper film fabrication process, while the other one is to further promote its fast response rate by making open–structured devices. For pseudocapacitive oxides, in order to achieve better rate capability and cyclability, the relationship between the electrochemical property and the structure is worth further exploration. As an example, the composition, microstructure, and morphology of the molybdenum oxide film were optimized to realize superior electrochemical performance as an electrode material for m–SC. Architecture design is another important factor for m–SC. In–plane interdigital architectures have proven its success to fabricate fast response devices. Further study on the interplay effect between such architecture and pseudocapacitive materials is in need.
- electrode materials
Since the microelectromechanical systems (MEMS) develop rapidly toward standalone microsensors, actuators, and various functional devices, the design of power supply has received more and more research interests . The conventional bulky batteries severely limit the advantages of these smart systems, a micropower system, i.e., generating power directly from microstructures, thus has to be developed . After the earliest explorations on the miro-internal-combustion engine which requires complicated micromachining processes and high manufacturing cost [2–4], researchers turn to microbattery system which is potentially of low cost and high capacity and more desirable for MEMS devices . A complete micropower system should consist of energy conversion and storage units integrated on chip. The energy conversion devices include microscaled fuel cells and solar cells, while the energy storage devices mainly refer to rechargeable microbatteries, which have been remarkably advanced under many research efforts . Microbatteries, or thin-film batteries, have become commercially available with a rapid expanding market. Nevertheless, similar to the features of macro-scaled batteries, the shortages of microbatteries are limited lifetime and low power density, which bring economic and environmental challenges to systems that they power.
Micro-supercapacitors (m-SC) appeared later as another important energy storage unit. Also known as an electrochemical capacitor (EC), a supercapacitor works through the accumulation of the electrostatic charge within an electrochemical double layer at the electrode/electrolyte interface. It could present high specific capacitance, which mainly depends on the high surface area of the electrode materials, or some pseudo-Faradaic charge transfer process. Compared with the batteries, supercapacitors possess inferior energy density but superior power density, i.e., they can be fully charged/discharged in seconds or minutes. Another prominent advantage of supercapacitors is the long cycle life, which is rather comparable with that of the functional devices. For the large-scale application system, supercapacitors are usually used combined with batteries so that both the high energy density of the batteries and the high power density of superapacitors could be utilized to ensure sufficient power supply. Similarly, m-SC is also complementary but indispensable when high power density is required to support the MEMS devices. More importantly, it could even replace the microbatteries when the cycle life of the device is preferred over the energy density for the whole system. As a matter of fact, with the great development of energy harvesters and nanogenerators, i.e., microscaled energy conversion devices that harvest energy from the ambient environment such as solar power, wind, water flow, vibrational energy, and thermal energy from waste heat, m-SC has been much more competitive as an alternative to batteries to play the role of energy storage in the self-powered micro/nanosystems . Figure 1 illustrates such a sustainable self-powered system, which consists of five different modules, namely energy harvester, energy storage, sensor, data processor/controller, and data transmitter/receiver .
M-SC is originally targeted at high power delivery and robust cyclability, and thus, carbon-based electrode materials are first employed to design on-chip electrochemical double-layer capacitors (EDLC), but the capacitance is relatively low [8–10]. The research interest grows quickly since 2010, when several works were reported on the improved design of the carbon electrode materials, especially the carbide-derived carbon film that possesses high volumetric capacitance and compatible with microfabrication [11–13]. Thanks to the fruitful development of the conventional supercapacitors and nanomaterials, m-SCs also received rapid advances when more research groups turn their interest to the on-chip devices, employing various nanomaterials and designing different fabrication protocols even without being limited to conventional MEMS fabrication routes [14–19].
After several years of active research, the term of micro-supercapacitor is formally defined and the performance metrics are well recognized . According to the definition given by Beidaghi and Gogotsi, micro-supercapacitors, or electrochemical micro-capacitors, refer to miniaturized supercapacitors that are designed and fabricated to serve as power sources or energy storage units in microelectronic devices. Due to the purpose of being specifically assembled to microelectronic devices, there comes the confinement in the fabrication methods that should be compatible with the current techniques in the semiconductor industry. Hence, the general appearance of m-SCs is a device taking up a footprint area in the millimeter or centimeter scale and a thickness of less than 10 μm. The configurations include sandwiched assemblies consisting of thin-film electrodes, planar arrays of microelectrodes like interdigital electrodes, and three-dimensional (3D) architectures of nanoscale building blocks [6, 12]. The former two configurations are commonly adopted in the present laboratory prototypes, while the latter one, 3D architecture, is a composed idea of the next generation whose realization still requires innovations.
2. Performance metrics for micro-supercapacitors
As perceived to power the microelectronic devices in a self-powered system, sufficient and energy have to be delivered by the m-SCs. The specific requirement of the power depends on the functional devices varying from environmental sensors to personal electronics, which may be in the range of 1–100 μW. Meanwhile, the duration of the power delivery is also required, meaning certain energy has to be supplied. Thus, power and energy together determine the suitability of an m-SC to power a microelectronic system. It should be mentioned that there is another direction emerging for the application of m-SCs other than micropower units, namely replacing electrolytic capacitors in electronic circuits such as alternating current line filtering. In order to utilize the advantage of miniaturized size of m-SCs to replace the conventional bulky electrolytic capacitors, the m-SCs have to response fast with a relatively large capacitance, i.e., ideal capacitive behavior with a small resistor-capacitor (RC) time constant (e.g., <8.3 ms for ac line filtering). In such case, the capacitance is a critical parameter for the evaluation, and it is also important whether the capacitance is well kept under faster charge/discharge conditions, which may be termed as rate capability, a terminology from the battery field. In addition, good reversibility (usually assessed by the Coulombic efficiency) and long cycle life should persist for the m-SCs.
As a matter of fact, assuming that an m-SC with a constant capacitance of C is charged from 0 V to an ultimate voltage of U in a duration of
3. Electrode materials for micro-supercapacitors
Selection of suitable electrode materials is the kernel in the design of m-SC for certain application. According to the electrochemical energy storage mechanism, the electrode materials currently studied could be divided into three types: carbon-based materials, pseudocapacitive oxides, and conducting polymers. The main purpose of this review is to give a clear comparison among the intrinsic electrochemical properties of the state-of-art electrode materials for m-SC, expecting to open up helpful strategies on developing advanced m-SCs in future.
3.1. Carbon-based materials
Carbon is a typical electrode material for EDLC, which possesses good reversibility and cyclability but limited specific capacitance. For conventional supercapacitors, carbon materials are usually fabricated as powders with a loose structure to facilitate the ions diffusion, so as to acquire a high gravimetric capacitance and fast response rate, but the volumetric capacitance is sacrificed. In addition, it is difficult to prepare uniform and qualified thin films from powders, which hinders the direct application of carbonaceous powders in the microelectrodes. Aiming at carbon-based m-SCs, Chimiola et al. tackle the problem by embracing the technological hurdles . They found the carbide-derived carbon (CDC) films produced by selective etching from metal carbides exhibiting an unprecedentedly high volumetric capacity, holding the promise of developing advanced m-SCs. As a first step, they demonstrated that a thin CDC film on a bulk TiC ceramic plate with strong interface adherence was fabricated by direct chlorination at elevated temperatures. Ti is extracted from TiC as TiClO4, forming a porous carbon film, while the conductive TiC plays as both the substrate and the current collector. Both the porosity of the CDC film and the etching speed are closely related to the chlorination temperature . After 15 s etching at 500°C, a CDC film of 2 μm thick, with a pore size of about 0.7 nm, was synthesized. Its volumetric capacitance reaches 180 and 160 F/cm3 in an acetonitrile solution dissolving 1 M tetraethylammonium tetrafluoroborate (TEABF4) and an aqueous solution of 1 M H2SO4, respectively, which are contributed by almost pure EDLC. They proposed the fabrication processes for m-SCs based on CDC films, which is schematically shown in Figure 2. The deposition of the precursor carbides and gold collectors could be conducted by well-known chemical and physical vapor depositions (CDC and PVD). The chlorination and the plasma etching of the photolithography are well-established techniques. Thus, the good compatibility with the semiconductor industry highlighted the promise of CDC-based m-SC devices.
Heon et al.  continued the exploration of the electrochemical property of the CDC films, aiming at the CDC-based m-SCs. The synthesis of uniform and adherent porous CDC films on various substrates by reactive DC magnetron sputtering and chlorination was realized, and the high volumetric capacitance of ∼180 F/cm3 in 1.5 M TEABF4/acetronitrile electrolyte was achieved. Later, the on-chip m-SC from CDC films was fabricated and tested by Huang et al. . The preparation process is generally similar to that proposed in Figure 2a–d except that Photolithography step is applied on the TiC film to produce interdigitated electrodes before the chlorination and the current collectors deposited on the CDC electrodes are Ti/Au layers instead of the single Au layer. The active material, i.e., TiC-CDC film, was 1.6 μm thick. The device was dipped into 1 M NEt4BF4 in propylene carbonate (PC) electrolyte in glove box under Ar atmosphere for electrochemical tests, exhibiting the EDLC behavior over a potential window of 2 V with an areal capacitance of 1.5 mF/cm2, a maximum energy density of 3.0 mJ/cm2, and a maximum power density of 84 mW/cm2. At this point, the EDLC property of TiC-CDC film and the feasibility of manufacturing on-chip m-SCs that are to be integrated into MEMS and electronics have been fully demonstrated, which should be recognized as a representative research on the development of carbon-based m-SC device. Nevertheless, the energy and power performance of the TiC-CDC m-SC are within the range of values reported for other carbon-based micro-supercapacitors, in spite of the high volumetric capacitance of the TiC-CDC active material [10, 22–24]. This is fundamentally attributed to the intrinsic limitation of the capacitance inhibited from the EDLC behavior, which only involves the variation of the ion concentrations in the electric double layer, and the theoretical capacitance is 16–50 μF/cm2.
To acquire high surface area within the limited electrode volume is still the most effective way to obtain high capacitance for the carbonaceous m-SCs. In this case, activated carbon is still a good choice. However, many well-developed fabrication routes of activated carbon powders are not easily applicable for synthesis of thin activated carbon films (ACFs) which is necessary for m-SCs. The challenges include the formation of cracks in the carbon film due to the shrink of polymers during heating and carbonization, the weak interface between the polymer and the substrate resulted from the large stress produced under the harsh synthesis and cooling conditions, and the possible damage to the brittle film in the photolithographical process. Wei et al reported an effective way to minimize some of the interface stresses in order to fabricate ACFs, namely catalyst-assisted low-temperature carbonization of an organic compound solution . The sucrose and H2SO4 (as a catalyst) aqueous solution was spin-coated onto a silicon wafer, dried at room temperature, carbonized, annealed at 700°C in vacuum to remove decomposition products of the carbohydrate and catalyst residues, and activated by annealing at 900°C in CO2 to induce open porosity within carbon. ACFs of 1–2 μm thick were thus produced free of microcracks, while the interface adhesion to the SiO2/Si wafer could be reinforced by further annealing at 1100°C in Ar, which could survive lithographical patterning as evidenced experimentally. The electrochemical property of such ACF film electrodes was tested in a symmetric sandwich-type configuration with 1 M H2SO4 as the electrolyte solution. Exhibiting typical EDLC property by the rectangular cyclic voltammetry (CV) curves, an extremely high volumetric capacitance of 390 F/cm3 was obtained under the slow scan rate of 1 mV/s, which is the highest value reported for carbon film electrode materials at present (see Figure 3). Moreover, Figure 3 shows clearly that the performance was strongly affected by the activation time. As the film becomes more porous after longer activation time, the capacitance increases and the rate capability improves as well, which should be due to easier accessibility of ions into the films. It means that the volumetric metrics of the carbonaceous electrode materials could be greatly enhanced when the materials structure is carefully optimized with proper fabrication techniques. But it is also worth noting that the fabrication process is still quite harsh as several times of high temperature annealing are required, which probably causes difficulties for the practical manufacturing of the devices integrated on the chips, although the realization of uniform and robust ACF films represents a great progress itself.
As a matter of fact, there have also been great advances in the research of carbon-based electrode materials for macro-scaled supercapacitors that pursue higher volumetric property in recent years. For example, liquid-mediated dense graphenes  and nitrogen-doped mesoporous carbon  were reported to have unprecedentedly high volumetric capacitance that is comparable with pseudocapacitive materials. Pseudocapacitance has been induced on these modified carbon materials actually . Yang et al. tackle the problem by considering the paradox between gravimetric capacitance
Besides the attempts to enhance the volumetric capacitance for energy storage through a proper film fabrication process, there is another important direction for carbon-based m-SC, i.e., to utilize its fast response rate to replace electrolytic capacitors by making open-structured devices [29, 30]. Pech et al. produced m-SCs through electrophoretic deposition of a several-micrometre-thick layer of nanostructured carbon onions (OLC) with diameters of 6–7 nm onto the interdigital Au current collectors patterned on silicon wafers. The OLC particles were prepared by annealing nanodiamond powder at 1800°C. A stable capacitive behavior was obtained for the microdevice over a 3 V potential window in a 1 M solution of tetraethylammonium tetrafluoroborate in PC, with a linear dependence of the discharge current on the scan rate and low resistive contributions up to 100 V/s, which is about three orders of magnitude higher than conventional SCs. Such a microdevice preserves an areal capacitance of 0.9 mF/cm2 at 100 V/s, which is comparable to values usually reported at much lower scan rates (1–100 mV/s) for microscaled EDLC devices (0.4–2 mF/cm2) [9, 11, 31]. The most appealing feature of such OLC m-SC is the extremely small characteristic relaxation time constant
Thereafter, several research papers reported high-power m-SCs fabricated from graphene, whose performance reaches a similar level with that of the OLC-based device (see Figure 4). For example, interdigitated graphene m-SCs were produced through laser burning along designed patterns on a graphene oxide (GO) film supported on a PET sheet which was inserted into a LightScribe DVD drive . Due to the photo-thermal effect under laser radiation, the exposed GO was converted into graphene, constructing the positive and negative electrodes, while the unexposed GO remained insulating and served as a separator. A hydrogel-polymer electrolyte, poly(vinyl alcohol) (PVA)-H2SO4, was then drop-cast on the patterned area to create a planar m-SC. Such a device using reduced GO (rGO) as electrode materials exhibits an areal capacitance of 2.32 mF/cm2 and a volumetric capacitance of 3.05 F/cm3, with a characteristic relaxation time of only 19 ms and a high power density of nearly 200 W/cm3. There is another planar device using rGO and carbon nanotube (CNT) composites as the electrode material, which is prepared by combining electrostatic spray deposition (ESD) and photolithography lift-off methods . The m-SC delivers an areal capacitance of 6.1 mF/cm2 at 10 mV/s, and a value of 2.8 mF/cm2 is still preserved at 50 V/s, corresponding to 3.1 F/cm3. Its characteristic time constant is only 4.8 ms. An even faster m-SC device is made from graphene films of only 6–100 nm thick, whose maximum capacitance is 0.807 mF/cm2 and 17.9 F/cm3 (specific values of 0.323 mF/cm2 and 71.6 F/cm3 for the electrode material), with the maximum power density reaching 495 W/cm3, the maximum energy density 2.5 mWh/cm3, and the characteristic time constant as short as 0.28 ms . It is concluded from these researches that the electronic conductivity of the electrodes has to be enhanced in order to acquire fast response performance. The most straightforward route is to reduce micropores within the electrodes and enlarge the open area in direct contact with the electrolyte, as the outmost surface is the most easily accessible with the ions for charge/discharge processes. However, such a design is usually at the cost of volumetric capacitance, and the film thickness should be thin as well, which thus limits the areal capacitance of the device.
3.2. Pseudocapacitive oxides
For the pseudocapacitive materials, Faradaic charge transfer occurs on the electrode/electrolyte interface during charge/discharge processes, giving much higher areal capacitance than EDLC does. Many transition metal oxides exhibit pseudocapacitive behavior in certain aqueous electrolyte. RuO2 is the first discovered pseudocapacitive oxide and still the most ideal candidate [35, 36]. Within the potential range of 0–1.4 V vs. SHE, RuO2 continuously changes its valence from Ru2+ to Ru4+, following the reaction of , where 0 ≤
There have been researches on RuO2 thin-film electrodes ever since 1990s. Jow and Zheng coated an amorphous RuO2 film onto Ti substrate through sol-gel method. In spite of inferior uniformity and many cracks, the film still shows a capacitance of 40 mF/cm2 as tested at 50 mV/s in 0.5 M H2SO4, which decreases by only 10% at 500 mV/s . Zheng et al. further employed pulsed laser deposition (PLD) to prepare the RuO2 films . The amorphous film deposited at room temperature possessed the highest capacitance (6.3 mF/cm2). As the deposition temperature increased to above 200°C, the capacitance reduced to only 0.3 mF/cm2. The diffusion length of protons in amorphous RuO2 film was estimated to be about 5.8 nm, while it reached >11.1 nm in RuO2·
The development of RuO2-based m-SCs emerged in recent years. Liu et al.  fabricated the planar device through depositing RuO2 nanorods onto the patterned stack layer of Ru/Au/Ti/SiO2 on silicon wafer, which was subjected to electrodeposition for another layer of hydrous RuO2 on top. It worked well in 0.5 M H2SO4 electrolyte, providing a capacitance of 21.4 mF/cm2 at 50 mV/s and 14.9 mF/cm2 at 500 mV/s. Makino et al.  reported the fabrication of an m-SC with ordered mesoporous RuO
In spite of the ideal pseudocapacitve property, RuO2 is too expensive for large-scale application. Cheaper oxides have been widely researched, such as MnO2 and NiO, wherein MnO2 attracts the most attention [44, 45]. MnO2 works in neutral aqueous solutions, with the potential window within 0.8 V and above 0 V vs. Ag/AgCl, and thus is suitable to serve as a positive electrode in asymmetric devices. Its working mechanism is the surface adsorption/desorption of the electrolytic cations and protons in the solution, which is described as the reaction . [46, 47]. It is obvious from the CV curve on Figure 6 that the charge/discharge behavior of MnO2 is similar to that of EDLC. Besides, the abundant resource and the safe working condition of neutral solutions also boost the wide research on MnO2, although the specific capacitance of MnO2 powders or micrometer-thick films is only 150 F/g.
The intrinsic electrochemical properties of MnO2 films produced by different preparation conditions and of different morphologies and structures have been explored [48–51]. For example, the MnO2 film deposited by cathodic electrodeposition is poorly crystalline and porous, whose specific capacitance strongly decreases with the scan rate and the film thickness . The specific capacitance of a film bearing a deposited mass of 45 μg/cm2 is tested to be 353 F/g (15.9 mF/cm2) at 2 mV/s and 135 F/g (6.1 mF/cm2) at 100 mV/s. For films annealed under 200 °C, whose porosity is reduced and crystallinity increased, the maximum specific capacitance is decreased, while rate capability is improved more or less. PLD has also been utilized to prepare manganese oxide films, with amorphous MnO
Because of the insulating property of MnO2 and the increased difficulty for ions to access into a thicker film, the rate capability of MnO2 film is always unsatisfying. Research efforts have been devoted to enhance the electronic conductivity so as to improve the rate capability. Si et al.  deposited the MnO
Other pseudocapacitve oxides include Co3O4, NiO, NiCo2O4, and so on [56–58]. They could reversibly form hydroxides in alkaline solutions, the process of which could be represented as , wherein M refers to elements such as Ni and Co. Their specific capacitances are reported to be very high (382–1400 F/g); however, phase transformations are always involved during the reaction processes, and the charge/discharge behaviors are more like that of batteries, for example, with potential plateaus, short potential windows (0.4–0.5 V), limited rate capabilities .
Overall, pseudocapacitive oxides could exhibit high specific capacitance, but poor rate capability and cycling stability hinder their application, which is mainly attributed to their insulating property. For the film electrodes, when thickness increases, the areal capacitance rarely scales up as expected. To disperse the active materials onto a 3D current collector is a common way to acquire certain areal capacitance with acceptable rate capability, which relies on the innovation of nanofabrication techniques. Nevertheless, the intrinsic relationship between the comprehensive electrochemical property and the microstructure of the oxides is still worth studying, which may help to optimize the intrinsic electrochemical performance of the film electrodes, contributing to the development of m-SCs compatible with conventional microelectronics manufacturing techniques.
Our group has systematically studied the fabrication and electrochemical property of molybdenum oxide thin film, which has various chemical valences with quite different properties . The composition, microstructure, and morphology were controlled to enhance the electrochemical performance of the molybdenum oxide film, and its potential to be applied as a superior electrode material in m-SC is evaluated. We fabricated electronically conductive MoO2+
assembly in 1 M H2SO4
|1.5 μm||390 (1 mV s−1)||1111||95% after 10,000 cycles at 10 A g−1|||
|TiC-CDC nanofelts||Filled into a micro
cavity electrode tested
in 1 M H2SO4
|125 μm in diameter, 35 μm in depth||12 (10 mV s−1)||379||Stable over 10,000 cycles|||
device with H2SO4
|400 nm||588 (0.1 mA cm−2)||–||96% after 1000
device with 3 M KCl
|6 μm||37.5 (10 mV s−1)||4.8||Slight decline over 1000 cycles|||
device with H3PO4
|25 nm||359 (1 A g−1)||–||90% after 1000
|Photoresist-derived porous carbon||Three-electrode assembly in 3.5 M KCl||1 μm||35 (10 mV s−1)||–||No loss after 10,000 cycles at 100 mV s−1|||
|TiC-CDC||Three-electrode assembly in 1 M H2SO4 or 1 M TEABF4||∼2 μm||160 (in H2SO4); 180 (in TEABF4)||–||–|||
device with H2SO4-PVA gel or 1 M Li2SO4
|50 nm||78.6 (10 mV s−1, H2SO4-PVA gel); ∼400 (10 mV s−1, Li2SO4)||5||74.1% retained after 15,000 cycles at
1 V s−1
|rGO||Interdigital microdevice with H2SO4/PVA gel||15 nm||71.6 (10 mV s−1)||0.28||98.3% retained after 105 cycles at 50 V s−1|||
|CNT film||Three-electrode assembly in 1 M H2SO4||30–250 nm||132 ± 8 (50
|Laser-scribed graphene||Interdigital microdevice with H2SO4-PVA gel||7.6 μm||∼12.2 (10
|19||94% retained after 10,000 cycles|||
|Onion-like carbon||Interdigital microdevice in 1 M Et4NBF4/PC||7 μm||∼9.7 (1 V s−1)||26||Stable over 10,000 cycles at 10 V s−1|||
|Activated carbon||Interdigital microdevice in 1 M Et4NBF4/PC||5 μm||∼92.8 (500
||Three-electrode assembly in 2 M Li2SO4||120 nm||1162 (5 mV s−1)||1381||88% retained after 4000 cycles at 100 mV s−1|||
annealing at 350°C for 1.5 h
|Three-electrode assembly in 2 M Li2SO4||63 nm||700 (5 mV s−1)||11||99% retained after 4000 cycles at 100 mV s−1|||
The pseudocapacitive MoO
3.3. Conducting polymers
Conducting polymers are another group of pseudocapacitive materials, which work through the fast redox reaction of ion doping [68, 69]. Currently studied conducting polymers mainly include P-type doping materials such as polypyrrole (PPy) and polyaniline (PANI), and N-type doping materials such as polythiophene (PTH). They are electronically conductive, leading to low ESR, and their capacitance is generally 2–3 times as high as that of activated carbon materials. However, the cycling stability is always unsatisfying due to the large volume expansion and shrink during the charge/discharge processes. The application of conducting polymers in m-SCs is relatively pioneering, i.e., developed in 3D structures [16, 70, 71]. Beidaghi and Wang  fabricated such a 3D-structured interdigital m-SC through carbon-microelectrochemical system (C-MEMS) technology. Briefly, the carbonization of patterned photoresist pillars produced microarrays of carbon pillars on the interdigital carbon layer supported on silicon wafer. The carbon arrays served as the C-MEMS current collectors, on which PPy was coated by electrodeposition. The resulted PPy/C-MEMS electrode presented a high areal capacitance of 162 mF/cm2 (volumetric capacitance estimated to be 11.6 F/cm3) and a power density of 1.62 mW/cm2 at 20 mV/s scan rate. Correspondingly, the entire symmetric m-SC device exhibited an average capacitance of 78 mF/cm2 and a power density of 0.63 mW/cm2. In spite of the high areal capacitance, the electrodes were not robust for consecutive cycling, as only 56% of the capacitance is retained after 1000 cycles.
Architecture design is especially important for m-SC, which significantly affects the comprehensive device performance. Structure of a separator layer sandwiched by thin-film electrodes is the traditional architecture, but the performance is closely related to the thickness of the electrode layer, which hinders it from scaling up to acquire higher areal energy density . In-plane interdigital architecture is often employed in recent years, especially for fast response devices [13, 17]. Besides, integrated 3D architectures have also been proposed for m-SCs . The microelectrodes with the structure of micro- or nanoarrays as cited above [16, 63, 70, 71] could be considered as ordered 3D structure, which is already realized by microfabrication techniques. However, a “true” 3D device is composed to consist of interpenetrating electrodes that are separated by a very thin layer of electrolyte, which is proposed by Long and coworkers . Although several possible strategies for fabrication of electrochemical energy storage devices with 3D architectures have been proposed , there has been no work fully realized the concept of 3D m-SC .
Above all, structure control is still the most important factor in exploring the property limits of different kinds of electrode materials. Although excellent efforts have been made on developing carbon film electrode with better volumetric and areal capacitances, the value is still lower than that of pseudocapacitive materials, which is limited by the theoretical limits of EDLC. For the pseudocapacitive materials such as molybdenum oxide, it is possible to optimize the intrinsic electrochemical property by structure design, i.e., to obtain both good rate capability and cyclability as well as keep relatively high capacitance. However, the application of the oxides is seriously limited by the electrolyte condition, which is always a disadvantage compared with carbon materials. Pseudocapacitive carbon materials represent another promising trend to achieve balanced performance. In-plane m-SCs with interdigital architectures have proven its success to fabricate fast response devices with carbon electrodes. Thus, further study on the interplay effect between such architecture and pseudocapacitive materials is in need. Exploring 3D architecture for m-SC is a difficult but attracting challenge.
The authors are grateful to the financial support by the National Basic Research Program of China (973 program, Grant No. 2013CB934301), the National Natural Science Foundation of China (Grant No. 51531006 and No. 51572148), the Research Project of Chinese Ministry of Education (Grant No. 113007A), and the Tsinghua University Initiative Scientific Research Program.
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