Carbon is the most widely used material in MSCs, with many advantages such as low price, good electrical conductivity, abundant yield, large specific surface area, stable physicochemical properties, stable structure, light weight, and highly controllable. When carbon materials are used as electrode materials, the main EDLC storage mechanism occurs, and its high specific surface area facilitates the storage of a large amount of charge on the surface of the electrode material, while the good electrical conductivity is conducive to the rapid transport of electrons. Carbon materials that are widely studied and used today include activated carbon (AC), onion carbon (OLC), carbon nanotubes (CNT), graphene, reduced graphene oxide (rGO), and carbide-derived carbon.

Graphene (GR), as a representative carbon-based material, is capable of providing a large number of active sites for fast charge transfer while maintaining charge and discharge cycling stability over long periods of time, making it an ideal choice as an electrode material. As early as 2013, Zhong-Shuai Wu et al. reported reduced graphene films prepared using the method of graphene oxide (GO) spin coating on oxygen plasma-treated silicon wafers [19], with which MSCs were constructed with power densities of up to 495 W cm^–3 and a capacity retention of 98.3% after 100,000 cycles. However, the limited number of effective storage sites in graphene materials results in energy and power densities that are not sufficient for practical applications. The charge storage performance of MSCs can be further enhanced by constructing graphene materials with different microstructures to increase the porosity or specific surface area of the electrode active material or by process improvement [32–35]. As shown in Fig. 3a, Ray et al. obtained laser-induced interdigital structured graphene electrodes (LIG) [32]. This LIG-based MSC demonstrated a stabilized potential window of up to 2.0 V, an energy density of 0.256 µWh cm^–2 and a power density of 0.11 mW cm^–2. Ravichandran et al. grafted electrodes with p-phenylenediamine (PPD) using a solvent–thermal grafting process [33], as shown in Fig. 3b. The electrode was obtained by mixing reduced graphene oxide and PPD through hydrothermal reaction, which has a higher specific capacitance of 573.7 F g^–1 than that of the pristine graphene oxide electrode, provides a wide operating voltage (1.2 V) and a superior capacitance of 38.4 mF cm^–2 in a hydrogel electrolyte, and energy density of 7.6 μWh cm^–2 and power density of 4.46 mW cm^–2.

Carbon nanotubes (CNTs) have the advantages of high electrical conductivity and large specific surface area, and their tubular structure can provide favorable transport channels for electrolyte ions, which is conducive to enhancing the power density and charge/discharge rate in MSCs. However, because the van der Waals force between the CNTs and the substrate is a weak physical interaction force, the contact internal resistance between the electrode material and the substrate may increase during use, and the carbon nanotubes may even completely detach from the substrate, which ultimately leads to a shorter cycle life [36]. Therefore, researchers have mostly improved the cycling life by constructing multidimensional electrode structures [20,37,38]. As shown in Fig. 4a, He et al. formed a three-dimensional hybrid electrode structure by selectively growing carbon nanosheets (CNs) on CNTs arrays via the microwave plasma chemical vapor deposition technique. The combination of which can effectively enhance the mechanical connection between the CNTs and the substrate, indirectly solving the problem that the CNTs may detach from the substrate owing to weak physical adsorption, thus improving the cycling stability and lifetime of the electrode [20]. Du et al. designed a three-dimensional MSC [37] based on a mortise-and-tenon structure shown in Fig. 4b–d. This structural design not only improves the contact area between the electrode material and the substrate and reduces the contact internal resistance, but also avoids the problem that the CNT may be detached from the substrate due to the weak physical adsorption by the precise assembly of the mortise and tenon structure, and the capacitance retention of the MSCs after 10,000 times of charging and discharging is 85%.

Activated carbon (AC), as a typical carbon-based material, has a rich pore structure, which provides an effective surface for charge accumulation and thus enhances the energy storage capacity. In addition, the relatively low cost and environmentally friendly preparation of AC has led to its widespread use in MSCs, which are usually prepared by heat treatment or chemical activation [39]. Nevertheless, activated carbon has a difficult pore size control and low specific surface area utilization, which may limit its application in high-performance supercapacitors. Researchers usually improve these drawbacks by compositing with other materials and optimization of techniques [40–42]. Shen et al. improved pore size control by using composites of AC with PVDF and graphite powder. The design of this composite allowed the pore size distribution to be adjusted to optimize the electrochemical properties of the electrode materials. The results showed that the MSC has a large capacitance of 90.7 mF cm^–2 and a fast power of 51.5 mW cm^–2, with high stability and high charging/discharging efficiency [40]. fan et al. made a hybrid electrode material by combining AC with a conductive polymer, PEDOT:PSS, and prepared MSCs by using the Microplotter technique to load the electrode material (AC or ACpedot:PSS) loaded onto Au IDEs, as shown in Fig. 5. This approach effectively improves the surface area utilization of activated carbon and enhances its charge storage capacity. The MSCs significantly improved energy density while maintaining high power density. The measured area capacitance of the AC-PEDOT:PSS is 29.5 mF cm^–2 (11.8 mF cm^–2) at 1 mA cm^–2, and the area energy and power is 2.79 µWh cm^–2 at 0.8 mW cm^–2. The ACPEDOT:PSS SMSCs exhibit stable long-term capacitance, maintaining 85% capacitance even after 5000 cycles. maintaining 85% capacitance even after 5000 cycles [41].

Depiste the mentioned carbon-based materials above, carbide-derived carbon (CDC) and onion carbon (OLC), are also more widely used at present [43–46]. CDCs have high electrical conductivity, high hardness, and good chemical stability, which make them an ideal material for MSC electrodes. However, their high stress levels limit the thickness of the film, which in turn affects the capacitive performance. Some studies have successfully reduced the film stress by finely tuning the sputtering deposition parameters, such as increasing the deposition temperature and pressure, to prepare thick and low-stress titanium carbide films in Fig. 6, which were then derived into carbide-derived carbon electrodes with high specific surface area (a volume specific capacitance of up to 350 F cm^–3) [43]. OLC, on the other hand, is an ideal electrode material of its unique layered structure, high power density, and fast charging and discharging ability. However, the energy density is relatively low, main causing by impurities generated during the preparation process. Pech et al. prepared highquality OLC by high-temperature annealing of nano-diamond powder and applied it to MSCs by electrophoretic deposition. The results show that the optimized OLC electrodes have improved the energy density by about 50% while maintaining high power density, and the capacitance retention rate is more than 85% after 10,000 cycles [44].

Conducting polymers, as a class of materials with potential for a wide range of applications in energy storage, are known for their unique electrochemical activity, good environmental friendliness, and excellent electrical conductivity. Through chemical modification and doping, the electrochemical properties (e.g., conductivity, capacitance, and stability) of conducting polymers can be tuned to suit different applications [47]. Polyaniline (PANI), polypyrrole (PPy) and poly(3,4-ethylenedioxythiophene) (PEDOT) as several typical conducting polymers have attracted much attention due to their significant advantages in electrode materials for MSCs.

The relatively simple and inexpensive synthesis process of PANI facilitates large-scale production. However, in current study, its capacitance is still far below the theoretical value [48]. Therefore, enhancing the energy density of PANI as a MSC electrode material remains a challenge. Researchers usually use doping as well as improvement of preparation methods to enhance its performance [21,49–52]. For example, Wu et al. found that the specific capacitance of capacitors prepared from doped electrodes was enhanced to 364.3 F g^–1 by doping Ni²⁺ into PANI electrodes [21]. Xiang Chu et al. developed a PANI ink for wearable, flexible and printable high power density MSCs [49], as shown in Fig. 7. This ink significantly improved the electron transfer kinetics by embedding a network of carboxylated multi-walled carbon nanotubes (C-MWCNTs) into PANI nanosheets. MSCs constructed with this electrode exhibited an excellent energy density of 2.6 mWh cm^–3 and a large area capacitance of 45.4 mF cm^–2.

PPy is also a common electrode material for MSCs for good electrical conductivity, and PPy can store charges stably over a wide potential window, which helps to increase the operating voltage and energy density for MSCs. However, MSCs with PPy as electrode material generally suffer from insufficient thermal stability and poor cycling stability [53]. Compounding with other materials and optimization of processes and structures are often used to improve these drawbacks [22,54–56]. For example, Peng Zhao et al. utilized the electrical conductivity of PPy and the electrochemical activity of MnO₂ to construct composites, and the MSCs assembled with this material showed good thermal stability and had a long lifetime of 92.6% of initial capacitance after 10,000 cycles [22]. Muhammad et al. significantly improved the cycling stability of PPy by electrochemically depositing multilayers of reduced graphene oxide (rGO) on micropatterned Au and growing porous PPy nanostructures on top of it, MSCs constructed with PPy@rGO electrodes demonstrated excellent cycling stability after 10,000 charge/discharge cycles. The capacitance retention rate was 82% [54]. Tahir et al. optimized the structure of PPy by adding CNTs, further optimized the performance of the electrode material by changing the structure of the collector to inhibit structural crushing of the PPy and enhance the adhesion of the PPy, and the machining process diagrams are shown in Fig. 8. The MSCs constructed from this material have high area capacitance and good cycling stability over a wide potential window (0–1 V). A capacitance of 79% was maintained after 10,000 charge/discharge cycles [55].

PEDOT is a conductive polymer with excellent transparency and high electrical conductivity, which is well suited for electrodes in MSCs. Compared to PANI and PPy, PEDOT has superior chemical stability and processing properties, but its mechanical properties are relatively poor. Researchers usually optimize its performance through process improvement, compounding with other materials, and gel improvement [23,57,58]. Kurra et al. successfully improved the performance of PEDOT through an innovative surfactant-mediated electrochemical deposition process as shown in Fig. 9a–f. This improvement enabled the PEDOT to exhibit a scan rate of up to 500 V s^–1 and a crossover frequency of 400 Hz, as well as a maximum area capacitance of 9 mF cm^–2 and a maximum volume capacitance of 50 F cm^–3. In addition, these capacitors exhibit excellent flexibility and stability in hydrogel electrolytes with 80% capacitance retention after 10,000 cycles and 100% Coulombic efficiency. The solid-state ionogel-based PEDOT MSCs achieved an energy density of 7.7 mWh cm^–3, which is comparable to that of lithium-based thin-film batteries and superior to existing carbon- and metal-oxide-based microsupercapacitors [23]. PEDOT has also been compounded with AC, the prepared AC-PEDOT:PSS MSC exhibited an area capacitance of 29.5 mF cm^–2 at a current density of 1 mA cm^–2, much higher than that of the pure AC micro-supercapacitor of 15.7 mF cm^–2. In addition, by introducing lithium bis(trifluoromethane)sulfonamide salt and polyvinyl alcohol into the PEDOT:PSS hydrogel, the obtained hydrogel maintains 93% of the original capacitance under 200% stretching, demonstrating excellent mechanical stability [57].

Transition metal-based electrode materials show great potential in the field of MSCs, mainly due to the ability of transition metal compounds to undergo rapid redox reactions on their surfaces, providing high pseudocapacitance capacity and energy density. These compounds typically include compounds such as transition metal oxides (e.g., MnO₂, Fe₂O₃, CuO, etc.) and hydroxides (e.g., Ni(OH)₂). However, poor electrical conductivity and cycling stability have been the main issues limiting their application in MSCs [59,60]. In order to improve these drawbacks, researchers have actively explored them by means of preparation method optimization, process improvement, and doping [24,25,61–66]. Wang et al. significantly enhanced the electrical conductivity and cycling stability of MnO₂ nanosheets through atomic-level doping modification [65]. In particular, Fe-doped MnO₂ exhibited the best performance in terms of specific area capacitance and volume capacitance (as shown in Fig. 10a–d), and micro-supercapacitors with high volumetric energy density (1.13×10^–3 Wh cm^–3) and power density (0.11 W cm^–3) were prepared. The Fe–MnO₂-based MSCs were able to achieve a capacitance retention of 86.7% after 300 bending cycles (bending radius of about 2 cm), and a capacitance retention of 78.7% after 5,200 charging and discharging cycles without bending. In addition, Xia et al. optimized the preparation process of Fe₂O₃ thin films (as shown in Fig. 10e) [24] using pulsed-current deposition (PCD) technique and combined it with MnO₂ to prepare a Fe₂O₃/MnO₂ micro-supercapacitor. The capacitor exhibited a high volumetric capacitance of 110.6 F cm^-3 and a wide operating voltage range of 1.6 V. The capacitance of the capacitor was measured by the cycling test. After 10,000 cycle tests, its capacitance retention remained as high as 95.7%, while it also showed good flexibility in mechanical bending tests. In addition, Dey et al. also focused on improving the conductivity and cycling stability of CuO, and successfully prepared CuO ink for screen printing by optimizing the preparation process [25]. The MSCs constructed with this electrode exhibited a high energy density of 10.1 mWh cm^–3 and a capacity retention of 99.5% after 3,000 cycles. For hydroxide electrode materials, Kurra et al. deposited a layer of Ni on Pt/Ti metal fingers to promote the uniform growth of Ni(OH)₂, which successfully improved its cycling stability [66]. Their prepared Ni(OH)₂-based micro-pseudocapacitors exhibited excellent performance, including an area specific capacitance of up to 16 mF cm^–2, a volumetric stacking specific capacitance of 325 F cm^–3, and an energy density of 21 mWh cm^–3. In addition, the capacitor achieved 80% capacity retention after 1,000 cycle tests.

MXene, metal-organic frameworks (MOFs), and quantum dots (QDs), as emerging electrode materials for MSCs, have unique advantages and disadvantages respectively, demonstrated significant contributions to the performance enhancement of MSCs [26–31,67–70]. MXene, with its high conductivity, hydrophilicity, and good mechanical flexibility, has becomes an ideal choice of electrode material for MSCs. The enhancement of its capacitance and rate capability has been challenging. as shown in Fig. 11a–c, MXene was significantly enhanced the electrical capacity by introducing hydrated lithium ions to expand the layer spacing. The electrical capacity was still maintained at 200 mF cm^–2 with a rate capability of up to 80% at a current density of 0.4 mA cm^–2 [26]. In addition, Huang et al. further enhanced the capacitance by adjusting the formulation of MXene ink, utilizing acetone to regulate the viscosity and surface tension of the ink, and employing a natural settling strategy to reduce the stacking of MXene lamellae. The prepared MSCs exhibited a volumetric energy density of up to 75 mWh cm^–3 at an operating voltage of 1.2 V and an electric capacity of 39.6 mF cm^–2 at a scan rate of 10 mV s^–1 [27]. MOFs, with their ultra-high specific surface area and tunable pore structure, provide favorable conditions for the rapid transport of electrolyte ions. However, poor electrical conductivity is the main factor limiting the application of MOFs in supercapacitors. To overcome this challenge, a study was conducted to prepare flexible solid-state MSCs with high capacitance by alternately depositing conductive polymers and MOFs (ZIF-67) on laser-induced graphene (LIG) via electrochemical deposition. The capacitance reaches 719.2 mF cm^–2 at a current density of 0.5 mA cm^–2. This strategy not only improves the electrode conductivity, it also enhances the capacitance performance through the pseudocapacitance effect of MOFs, which opens a new way for the performance enhancement of MSCs [28]. In addition, the study by Wu et al also focused on improving the conductivity of MOFs with a different strategy. They directly grew conductive MOFs (e.g., Ni-CAT) on LIG by electrochemical deposition to avoid the problem of poor conductivity of conventional MOFs. The prepared MSCs achieved an area capacitance of 15.2 mF cm^–2, an energy density of 4.1 µWh cm^–2, and a power density of 7 mW cm^–2 under a wide voltage window of 1.4 V [29]. QDs with unique quantum size effect and tunable optoelectronic properties, show a wide range of applications in optoelectronics and biomedical fields. In the field of supercapacitors, although the applications of quantum dots are relatively few, studies have been conducted to enhance the specific capacitance and multiplicity performance of MSCs by modifying the activated carbon electrodes with quantum dots and utilizing the pseudo-capacitance effect of quantum dots, which still retains more than 87.6% of the capacitance after 6,000 charging and discharging cycles, demonstrating good cycling stability [30]. In addition, the development of microfluidic synthesis technology provides the possibility of large-scale production of high-quality quantum dots, which have great potential in future electrode materials for MSCs. It is worth noting that, in addition to the aforementioned properties of the materials themselves, the structural design of the electrode materials and the preparation process also have an important impact on the performance of MSCs [31]. For example, the advantages of different materials can be fully utilized to optimize the performance of electrode materials through rational material composite and structural design. Meanwhile, advanced preparation processes such as electrochemical deposition and screen printing also provide technical support for the large-scale production of high-performance MSCs.

Every material has its own characteristics but to become an extraordinary choice for electrodes it needs to have high activity surface area, good conductivity, and a suitable structure that can be beneficial for ion diffusion. In order to be able to visualize the electrochemical performance of MSCs prepared from various materials as well as the advantages and disadvantages of electrodes prepared from various materials, we list the studies with more excellent performance in this paper in Table 1.

Microsupercapacitors, combining a high power density with compact dimensions to serve a wide range of contemporary electronic applications, represent a state-of-the-art development in energy storage technology. Energy storage and release are vitally handled by the electrode materials. Hence, the choice of electrode materials is critical in MSCs. In the present work, we have summarized the recent development and merits on the various materials. Carbon-based electrode materials are sought for their electrical conductivity and surface area. Conducting polymers and transition metal-based electrodes, undergo rapid redox reactions on their surfaces, provide high pseudocapacitance capacity and energy density. Some emerging electrode materials also have been explored in MSCs. MOFs, making up of metal ions coupled with organic ligands, have a large surface area and customizable characteristics that improve energy storage capacities. Quantum dots, nanoscale semiconductor particles, contribute to increased energy density due to their size-dependent electrical properties. MXenes, two-dimensional materials consisting of transition metal carbides, nitrides, or carbonitrides, provide strong conductivity and enhanced ion transport.

It is believed that electrode materials, due to their vast surface area, strong stability, good aspect ratio and layered structure, are thought to play a significant role in MSCs. Moreover, their various properties such as chemical composition, porosity, pore-size distribution and morphology also can be beneficial for ion diffusion.

The advancement of electrodes in MSCs faces numerous research challenges that need to be addressed to unlock their full potential. These challenges span across material development, performance optimization, fabrication techniques and large-scale production. The following insights for electrodes can enhance MSCs to sustainable energy storage solutions:

Material development: A significant challenge in material development involves finding advanced electrode materials that strike a balance between high conductivity and large surface area. While materials like graphene, CNTs, and MOFs have shown promise, researchers need to discover and develop new materials that can enhance performance while remaining cost-effective for large-scale production.

Performance optimization: Hybrid electrodes that combine multiple materials, aiming to optimize performance by leveraging the strengths of each component, is Is an effective solution for electrode performance optimization. such as graphene with CNTs or MOFs. Therefore, exploring the advantages of various combinations, utilizing their respective advantages such as high energy density, excellent electrical conductivity, and good mechanical flexibility. Hybrid electrodes can realize synergistic enhancement of performance in MSCs.

Fabrication techniques: The micostructure of electrode directly affects its electron transport efficiency. The improvement of the micostructure is a challenge for material synthesis technology, for example, MOF materials have great potential to be used as electrodes for MSCs, however, the synthesis of MOFs can be challenging. Therefore, researchers need to discover new and controllable methods for the preparation of nanostructures, in order to efficiently prepare the ideal electrode structures in MSCs. electrode structures efficiently.

Large-scale production: At present, the vast majority researches on electrode materials focuses on electrochemical properties, while the synthesis methods of electrode materials are neglected. In fact, to realize the application of MSCs, efficient large-scale production is an issue that must be considered. The cost of materials, production conditions and other factors need to be taken into account in mass production.