Advantages and disadvantages of various MSC device configurations.
Owing to their unique features such as high surface area, rich electroactive sites, ultrathin thickness, excellent flexibility and mechanical stability and multiple surface functionalities enables outstanding electrochemical response which provides high energy and power density supercapacitors based on them. Also, the Van der Waals gap between layered 2D materials encourages the fast ion transport with shorter ion diffusion path. 2D materials such as MXenes, graphene, TMDs, and 2D metal–organic frame work, TMOs/TMHs materials, have been described with regard to their electrochemical properties for MSCs. We have summarized the recent progress in MSC based on well-developed 2D materials-based electrodes and its potential outcomes with different architectures including interdigitated pattern, stacked MSC and 3D geometries for on-chip electronics. This chapter provides a brief overview of the recent developments in the field of 2D material based all-solid-state microsupercapacitors (MSCs). A brief note on the MSC device configuration and microfabrication methods for the microelectrodes have been discussed. Taking advantage of certain 2D materials such as 2D MXenes, TMDs, TMOs/TMHs that provide good surface chemistry, tunable chemical and physical properties, intercalation, surface modification (functionalization), heterostructures, phase transformations, defect engineering etc. are beneficial for enhancement in pseudocapacitance as it promotes the redox activity.
- solid-state supercapacitors
- two-dimensional materials
- energy storage
The popularization of portable electronic equipment has concentrated heavily on miniaturization and convergence of different technologies. While technologies such as wearable sensors and flexible displays has progressed, advances in energy storage are still lagging behind innovations in other electronic devices. Miniaturization of energy sources is also essential for environmental, medical, biological and other applications. Consequently, the reduction in size and integration of micro-power systems such as micro-batteries, micro-fuel cells, micro-supercapacitors (MSCs) and piezoelectric power harvesters are essential for the future growth of portable electronic devices . MSCs have gained considerable attention among these micro-power systems due to its high power densities, fast rate capabilities, ultra-long – cycle life and simple integration into the micro-nano electronic system as energy sources [2, 3]. Three main types of device configurations for MSCs have been developed to date: in-plane architecture, fiber shape and three-dimensional (3D) type (Figure 1). The advantages and disadvantages of these device configurations are given in Table 1
|Three- dimensional type|
The choice of electrode materials and electrolytes is the two critical parameters influencing the electrochemical performance of the MSCs. Two-dimensional (2D) materials with unusual properties such as ultra-thin thickness, large lateral size, excellent flexibility and tunable physicochemical properties are currently the perfect choice for MCS as an electrode material. A large number of 2D materials have been developed to date, including graphene and analog nanosheets such as transition metal dichalcogenides (TMDs), transition metal oxides/hydroxides (TMOs/TMHs), metal carbides and nitrides (MXens), boron nitride (BN), phosphorene, and so on. In addition to electrode material selection, the electrolyte selection also plays a crucial role in the performance of MSC and these electrolytes can be classified into two types (i) conventional liquid electrolytes and (ii)solid-state electrolytes . Conventional liquid electrolytes have a common disadvantage in terms of their liquid nature; therefore, a strict encapsulation process is required to avoid electrolyte leakage. However, when the device is damaged, electrolyte leakage remains unavoidable. Accordingly, to overcome this disadvantage, a solid-state electrolyte was developed by blending the acids, ionic liquids and salts into a polymer matrix. Several polymer matrixes have been used in solid-state electrolytes, including poly-(vinylidene-fluoride) (PVP), polyacrylonitrile (PAN) and poly-(vinyl-alcohol) (PVA) . These electrolytes can provide long cycle-life, low leakage current, high ionic conductivity and high mechanical flexibility. For example, the ionic conductivity of PVA/H3PO4 is about 10−3 Scm−1, while the ionic conductivity PVA/H2SO4 can be even higher, about 7 x 10−3 Scm−1. However, aqueous solid-state electrolytes suffer from a low voltage window at about 1 V due to the electrolysis voltage of water similar to aqueous electrolytes. A high operating voltage of 2.5 V can be achieved for micro-supercapacitors through ionic liquid solid-state electrolytes, resulting in a high energy density in sequence . They also allow additional functionality, such as flexibility and stretchability, in addition to easy encapsulation. Considering these advantages, the choice of solid-state electrolytes in micro-supercapacitors is more reasonable.
2. Microfabrication technologies for microelectrodes of MCSs
This technology used for the fabrication of microelectrodes of MCSs can be grouped into two categories. The first categories include direct electrode material synthesis on the patterned current collectors using laser scribing, CVD, electrolytic deposition and pyrolysis. The second category consists of indirect manufacturing using existing electrode materials in powder or solution form. The advantages and disadvantages of various techniques developed are explained below in Table 2.
|Chemical vapor deposition (CVD)||Controlled design and structure ||Expensive, time-consuming process, low mass loading and vigorous reaction condition (2).|
|Electrolytic deposition||Simple, efficient, cost-effective, environmentally friendly and large scale-production [2, 11]||Uncontrolled lateral direction growth .|
|Electrophoretic deposition||Cost-effective, simple, thickness can be controlled [2, 11].||Restricted by the species charged .|
|Inkjet printing||Cost-effective, fast process, low mass loading, precise thickness control, direct patterning, large scale production, fair resolution (around 50 μm) enhance the resolution and scalability because no manual assembly is required during device manufacturing [2, 12].||Ink preparation is a complicated process, limited by resolution, jam of nozzle [2, 10].|
|Screen printing||Low-cost, scalable and fast process||Low resolution|
|Photolithography (UV lithography)||Cost-effective with high control precision , simple fabrication process , can produce uniform and accurate large-area samples .||A sacrificial template is required; hence it’s a complicated process, long preparation time .|
|Drop, spin and spray coating||Facile, simple, thickness-control, large-scale fabrication, time and energy saving .||Low-production efficiency and heterogeneous .|
|Vacuum filtration||Low- cost, simple, convenient, thickness can be controlled .||Shape and size is limited|
|Laser scribing||Cost-effective, scalable, simple, gives high throughput .||Non-universal .|
|Layer-by layer assembly||Multilayer films can be easily prepared, cost-effective and straightforward method, high resolution .||Time-consuming process|
|Pyrolysis||Single-step synthesis .||Complex and high-temperature process .|
No strategy in the fabrication of MSC microelectrodes is yet dominant over the others. Therefore, improving existing assembly strategies and exploring new manufacturing methods to overcome those limitations has become essential. In the meantime, to select appropriate assembly strategies to achieve high-performance MSCs, consideration should be given to overall factors such as active electrode materials, electrolytes, and interface between electrolytes and micro-electrodes .
3. Performance metrics of MCSs
The parameters used generally to assess supercapacitors’ performance against volume and weight units are gravimetric capacitance, energy, and power. It is important to note that the supercapacitors gravimetric capacitance varies according to total density, mass and thickness of the electrode, and other components’ weight. So it’s hard to compare the various MSC based on gravimetric capacitance . But this parameter is not suitable for planar MSC where electrode material’s weight is insignificant and the device’s volume and surface area are always limited. Since the overall mass load of active materials in MSCs is small, the volumetric performance and, more significantly, the areal performance are more adapted for electrochemical performance .
Since equipment need to be integrated with miniaturized electronic devices with limited area, a performance assessment against the footprint area of MSCs is essential. Therefore areal capacitance, power density and energy density are the more reliable parameters for MSC performance monitoring. These parameters can be calculated using the equations given below 
Where Cs is the areal capacitance in F/cm2, ‘s’ is the total area of microelectrode array and is the voltage range. Ps and Es is the maximum power and energy density. The essential parameter to detect the electrode’s areal performance is to measure the total area accurately.
4. Two- dimensional materials for MCSs
A promising material for the production of MCSs is Planar 2D molecules of atomic thickness with a large specific surface area. The reduced dimension of these materials also satisfies the miniaturization requirements of device size, offering new possibilities for high-performance MSC development [18, 19]. The material properties of this rich family consisting of graphene, transition metal oxides (TMOs), transition metal chalcogenides (TMDs), metal carbides and nitrides (MXene), black phosphorus (BP), etc., range from superconducting, metallic, semiconducting and insulating behavior due to its diverse electronic structure, offering a wide range of material solutions to achieve high-performance. The essential reasons why 2D material based solid-state MSC are essentials is enlisted below
Excellent electrical conductivity
The essential qualification for high-performance MCS electrode materials is the excellent electrical conductivity, which can accelerate the adsorption and desorption of the charges and increase the diffusion rate of ions. This electron transport behavior is very closely linked to the electronic crystal structure, resulting in three typical insulating, semiconducting, and metallic transport behaviors. In general, metallic 2D materials, such as TMDs, have good electrical conductivity and several other 2D semiconductors like graphenes and BPs also offer favorable electron transport characteristics .
The electrochemical activity of some 2D material would be useful for the redox reaction to further increase the pseudocapacitance. The promising electrode material of microsized pseudocapacitors, which generally display high capacitance performance, is proven to be 2D MXenes, layered double hydroxides (LDHs), metal oxides and hydroxides with excellent electrochemical properties .
Large surface area
The extra-large surface area offers an energy storage platform with huge active sites to increase the electrochemical activity and charge adsorption, thus making 2D metal–organic frameworks (MOFs) and covalent organic frameworks (COFs) with inherent porosity a promising MSC functional electrode material .
Superior mechanical flexibility at the atomic level and a diverse array of various 2D nanosheets provide desirable flexibility and multiple functionalities .
Graphene is the most widely studied electrode material for MSCs due to its excellent electrical conductivity, large specific area, chemical stability, excellent intrinsic double-layer capacitance of approximately 21 μF/cm2 and theoretical capacitance of around 550 F/g [20, 21, 22]. Several reviews of graphene-based MSCs have been published. Zhang
Several strategies were employed to enhance the electrochemical performance of graphene-based MSCs. The first approach was to improve the charge storage capacity of electrode materials by preparing graphene composites with other pseudocapacitive materials [11, 24, 25] or doping graphene with heteroatoms like boron  and fluorine . The second approach consists of constructing an asymmetric structure [11, 25] and the third approach was to increase the loading quantity of active electrode materials by 3D electrode construction on the confined area of MSC . To realize this 3D electrode construction, Wang
4.2 Transition metal dichalcogenids (TMDs)
Single or few layers TMDs have attracted considerable attention because of their tunable band gaps and extensive natural reserves . These compounds show a typical MX2 formula, where M is an element in Group IV-VI metal, and X is a chalcogen (S, Se, or Te). In this case, Stoichiometry relies on the process and the strategy of producing a compound made up of transition metal and chalcogen elements. The layered TMDs are typically 6 to 7 Å thick and consist of an X-M-X hexagonal sandwich with a metal-atomic layer separated by both layers of chalcogen . The physical properties of bulk TMD’s vary from true metals such as VSe2 and TaS2, semi-metals such as WTe2 and TiS2, semiconductors such as MoS2 and SnS2 and insulators such as HfS2. Suitable electrode materials for MSCs are among these metallic TMDs with large surface area and high conductivity .
MoS2 can effectively store charges over a single atomic layer utilizing an inter and intrasheet double layers. Here the central atom Mo shows an oxidation state ranging from +2 to +6 and shows a pseudocapacitive behavior with a theoretical capacitance of about 1000 F/g. But aggregation and low electrical conductivity between the atomic layers of MoS2 hinder their extensive use in MSCs. Hybridization of TMDs with carbon material, which provides quick-electron transport and more active-sites, is one approach to solve these problems. Hence Yang
The fabrication of supercapacitors with excellent energy storage capacity and flexibility in wearable smart electronics has recently attracted significant attention. Thus, a fabric supercapacitor using low-cost textile fabrics with good mechanical properties and biocompatibility coated with ternary composite poly(3,4ethylenedioxythiophene): poly(styrenesulfonate)/MoS2/poly3,4ethylenedioxythiophene) (PEDOT: PSS/MoS2/PEDOT) is manufactured by Chen and the group. This all-solid-state fabric MCS was fabricated by vapor phase polymerization (VPP) and the vapor phase deposition method exhibits an energy density of around 1.81 mWh/cm3 and power density of around 0.82 W/cm3. The fabric coated with this ternary composite has a 3D configuration with interconnected structure and exhibits a large surface area that enables fast electrolyte transport and provides active electrolyte accessibility. This MSC assembled in a belt-shaped device was also used by the group as transient power sources to operate the light-emitting diodes . Very recently, Li
4.3 MXenes (Ti3C2Tx)
The overall performance of MSCs is based on the intrinsic properties of electrode materials. In many cases, carbonaceous materials such as graphene [49, 50], graphene oxide , CNTs [52, 53], carbide-derived carbon [54, 55] and their hybrids [56, 57] with charge storage via electric double layer, were reported in MSCs. Later, high capacity MSCs based on pseudocapacitive materials such as conductive polymers 
MXenes are promising layered materials derived from the precursors with the general formulae Mn + 1AXn (M refers to Ti, Sc, Nb
Flexible MSCs are highly demanding to manufacture portable and on-chip energy storage devices because of their high security, lightweight and miniaturization . Direct printing of functional inks is crucial for various applications such as smart electronic devices, healthcare, and energy storage. Nevertheless, currently available inks are distant from ideality. A low concentration of ink or the additives/surfactants are contained, which put on complexity to the fabrication and affects the printing resolution. Based on these facts, Zhang
4.4 Other important 2D materials
Transition metal oxides/hydroxides (TMOs/TMHs) are electrochemical pseudocapacitor materials and widely used as electrode materials in supercapacitor applications due to their high energy density, abundance and high capacitance . But their performance as supercapacitor electrode materials limited because of low intrinsic conductivity. So, 2D TMOs/TMHs have been explored in supercapacitors owing to their enhanced electronic conductivity and high specific surface area . Recently, research has been put into the fabrication of 2D TMOs/TMHs for MSC electrodes, limitations remain when using electrode based on a single material. The major disadvantages mainly rely on poor rate capacity caused by low electrical conductivity, restricted enhancement of energy density, and low capacitance, limiting their practical implementations . To surpass the challenges of using single electrode materials, it is appropriate to fabricate nanoarchitectures based on composites of TMOs/TMHs. This can hone the configuration to avoid the agglomeration of 2D nanosheets and raise the performance level of various electrode materials to execute effective enhancement of supercapacitor performance . Inspired from these findings, Wang
|MSC||Electrolyte||Voltage window (V)||Device performance||Specific capacitance||Cycling stability||Ref|
|Power density||Energy density||Areal/mF cm−2||Volumetric/ F cm−3|
|NOG-X-Y||PVA-H3PO4||0 to 1||0.23 mW cm−2||2.59 μWh cm − 2||18.70||—||93% after 10,000 cycles|||
|GMP microflakes based 3D MSC||PVA/H2SO4 gel||0 to 1||10 mW cm−2||1 μWh cm − 2||11||—||80% after 2000 cycles|||
|Silicon nanowire-Graphene- PANI||PVA/H2SO4 gel||−0.2 to 1||0.78 mW cm−2||10.8 μWh cm −2||77.7||—||75% after 2000 cycles|||
|G/CA/MnO2 based ID patterned MSC||PVA/H2SO4 gel||0 to 1||43.2 μW cm −2||1.2 μWh cm− 2||8.7||—||85% after 5000 cycles|||
|M-PBV-RGO||PVA/H2SO4 gel||0 to 1||5 mW cm−2||2.49 μWh cm−2||21.86||—||99% after 10,000 cycles|||
|Graphene based integrated planar on-chip MCS||PVA/H2SO4 gel||0 to 2||68.268 mW cm−||3.792 mWh cm−3||—||27.30||89% after 10,000 cycles|||
|MoS2@S/rGO||KOH-PVA gel||13.4 mWcm−3||0.58 mWh cm−3||6.56||—||91% after 1000 cycles|||
|MoS2@rGO/CNT||PVA/H2SO4 gel||0–1||—||5.6 mWh cm−3||13.7||—||96.6% after 10,000 cycles|||
|C/VS2||0–1.2||2.88 Wcm−3||15.6 mWh cm−3||—||86.4||97.7% after 10,000 cycles|||
|1 T MoS2 (t-lf laser)||PVA/H2SO4 gel||0–0.5||14 kW cm−3||15.6 mWh cm−3||36||93% after 5000 cycles|||
|PEDOT: PSS / MoS2 / PEDOT||PVA/H3PO4 gel||−0.2 to 1||0.82 W/cm3||1.81 mWh/cm3||1.43||—||93.6% after 5000 cycles|||
|Inkjet printed MSC based on MoS2||PVA/H2SO4 gel||0 to 0.6||0.079 W cm−3||0.215 mWh cm−3||175 μF/cm3||85.6% after 10,000 cycles|||
|2D Ti3C2Tx/PDMS||PVA-H2SO4||0 to 0.6||189.9 mW cm−3||1.48 mW h cm−3||23.4||—||92.4%|
After 5000 cycles
|PANI/EG/ Ti3C2Tx||PVA-H3PO4||0 to 0.7||159.6 mW cm−3|
2015 mW cm−3
|2.3 mWh cm−3|
1.3 mWh cm−3
|Ti3C2Tx-PET||PVA-H3PO4||0 to 0.6||225 mW cm−3|
744 mW cm−3
|2.8 mWhh cm−3|
2.3 mWh cm−3
|Ti3C2Tx/polymer electrolyte (PE)||PVA-H3PO4||0 to 0.8||8 mW cm−2||28 μWh cm−2||276||—||95%|
|Extrusion printed MXene|
Inkjet printed MXene
|0 to 0.5|
0 to 0.5
|Ti3C2Tx||PVA-H2SO4||0 to 0.6||0.7–15 W cm−3||11–18 mWh cm−3||27||357||100%|
|3DMXene-r-GO composite aerogel||PVA-H2SO4||0 to 0.6||180 |
|MXene/BC composite paper electrodes||PVA-H2SO4||0 to 0.6||—||0.00552 mWh cm−2||111.5||—||5000 (72.2%)|||
|Ti3C2Tx@Silver-plated Nylon Fiber Electrodes||PVA-H2SO4||0 to 0.4||132 μW cm−2||7.3 μWh cm−2||328||—||10,000(80%)|||
|r-GO/MnO2/Ag NW-PET||SiO2–1-butyl-3-methylimidazoliumbis(trifluoromethylsulfonyl)imide||0 to 2.5||162.0 mW. cm−3||2.3 mWh. cm−3||—||2.72||90.3%|
|VN// Co(OH)2||KOH/PVA||0 to 1.6||1750 mW cm−3||12.4 mWh cm−3||21||39.7||84%|
|LSG/Ni-Catecholate-MOF||LiCl/PVA||0 to 1.6||7 mWcm−2||4.1 Wh cm−2||15.2||—||87%|
|FGO//FrGO||PVA/Na2SO4||28.3 μW cm−2||2.52 μWh cm−2||7.3||—||100% over 500 cycles|||
|MXene//MXene-MoO2-AMSCs||PVA-H3PO4||0 to 1.2||0.8 W cm−3||9.7 mWh cm−3||19||63.3||88%|
|Co(OH)2//erGO||PVA-KOH-KI||0 to 1.4||100.38μWh cm−2||0.35μWh cm−2||2.28||—||89%|
MSCs as an energy storage devices attract considerable attention due to their notable characteristics such as smaller volume and high electrochemical performance. This chapter provides a brief overview of the recent developments in the field of 2D material-based all-solid-state MSCs. A brief note on the MSC device configuration and microfabrication methods for the microelectrodes has been illustrated. 2D materials based MSCs open up new avenues for the technologically relevant real-world applications. 2D materials such as MXenes, graphene, TMDs, and 2D metal–organic framework, TMOs/TMHs materials, have been described with regard to their electrochemical properties for MSCs. It is reported that the one issue faced by 2D materials is their unavoidable aggregation or restacking owing to their intense van der Waals interactions. To overcome this, there are approaches available like expansion of interlayer space with regard to enhanced storage ability or intercalation of guest molecules to increase the active sites. Moreover, for MSCs, 2D materials with vertical orientation grown on interdigitated current collectors is favorable to attain enhanced charge transport and low interfacial resistance. Additionally, to achieve higher conductivity and large specific surface area, combining various materials with 2D hybrids is a practical approach to surpass each component material’s challenges. Precisely, novel 2D materials with fascinating electrochemical properties are highly required. For example, 2D materials such as borophene, tellurene, silicene, phosphorene and germanene with higher electrical conductivity and enhanced specific surface area can be suitable candidates for high-performance MSCs. However, the coatings or surface functionalization of these 2D materials will be needed due to their chemical degradation and intrinsic surface instability under surrounding conditions. Above all, the processibility, reliability, and scalability of high-quality 2D materials are necessary not only for basic research but also for the real-world technological applications that need improved microfabrication methods such as screen printing and inkjet printing and 3D printing,
Despite the recent advances in the design and fabrication of MSCs, MSC is still imperfect and require more developments. Some challenges limit practical implementation such as sustaining stable output voltage for wearable devices (microsystem and MSCs array just ignore these issues), current and output voltage are yet not pleased and more attempts should be assigned to design MSCs with a wider potential window. Moreover, various features, such as self-healing, hydrophobia, and stretchability, could be more developed to improve MSC performance. The device fabrication holds a significant role in technological innovation that, successively, affects the large scale production and the complexity of MSCs. It is expected that the integration of microdevices and smart functions into systems is unavoidable for facilitating the fast growth of smart electronic devices. MSCs based on 2D materials are focused on the powering of energy-consuming microdevices. Because of the complication in the smart systems’ fabrication process, only limited works have been reported. So, innovative self-powered smart systems, including energy storage units, constitute a highly emerging research direction. Besides, the fabrication of a smart system with flexible, biodegradable and washable features can open the way for future independent, continuous, and intelligent daily electronics functioning. Moreover, these all-in-one self-powered systems can be used for health care applications in the future.
This work was financially supported by the Department of Science and Technology (DST)-SERB Early Career Research project (Grant No. ECR/2017/001850), DST-Nanomission (DST/NM/NT/2019/205(G), DST/TDT/SHRI-34/2018), Karnataka Science and Technology Promotion Society (KSTePS/VGST-RGS-F/2018-2019/GRD NO. 829/315).
Conflict of interest
The authors declare no conflict of interest.