A review on the different types of electrode materials for aqueous supercapacitor applications

Electrical double-layer capacitors or EDLCs are the class of supercapacitors where a charge is stored electrostatically. The oppositely charged double layers are accumulated at the junction of the electrode and electrolyte as a result of a reversible accumulation of electrolyte ions onto the surface of active electrodes. The storage mechanism is non-faradaic i.e. no charge is transferred between the electrodes chemically. Since EDLC is a surface storage process, therefore the surface properties of electrode materials such as surface area and porosity determine the optimum performance of this type of supercapacitor [9].

Carbon-based active materials such as activated carbon (AC), carbon quantum dots (CQDs), graphene, graphene oxide (GO), reduced graphene oxide (rGO), carbon nanotubes (CNTs), etc follow this kind of charge storage mechanisms. The high electrochemical stability, conductivity, porosity, and high surface area of carbon-based materials play a key role in achieving high capacitance in EDLCs. Since it was observed that higher capacitance values can be obtained from mesoporous carbon structures containing a large number of small micropores due to the partial desolvation of ions [11].

EDLCs have longer cycle life than its counterpart like pseudocapacitors due to the physical transportation of charges between the electrodes. Also, the surface charge storage mechanism facilitates fast energy input and throughput, which results in high power density but lower energy density.

Pseudocapacitors belong to that type of supercapacitors where a charge is stored electrochemically through the reversible redox reactions of electroactive species of electrode materials at the surface of active material. The charge storage mechanism is faradaic i.e. charge is transferred between the electrodes. Metal oxides (such as zinc oxide, manganese oxide, ruthenium oxide, nickel oxide, titanium oxide, etc) and conducting polymers (namely polyaniline, polypyrrole, polythionine) undergo pseudocapacitance mechanism to store the charge. The redox reactions of transition metal oxides and doping/dedoping of CPs initiate the charge storing process in pseudocapacitors.

The performance of pseudocapacitors relies on many factors intrinsically and extrinsically. Intrinsically, it depends on the size of the particle and the architect of electrode materials. While extrinsically, pseudocapacitance depends on the accessibility of the electrode surface area. The fast redox reactions of metal oxides and conducting polymers result in the high capacitance of pseudocapacitors in comparison with EDLCs [1]. But due to the redox reactions, pseudocapacitors suffer from a lack of stability during the long charging-discharging cycles because the possibilities of swelling and shrinking of polymeric long chains resulted in lower cyclic stability and power density of pseudocapacitors than EDLCs [5].

Since the value of energy density depends mainly on two factors, which are capacitance and voltage (according to the formula, E = 1/2CV²). Therefore, an alternative approach is to combine the EDLC electrode with the pseudocapacitance electrode to develop a hybrid supercapacitor to give high capacitance and a wide operating range for increasing the cell total voltage. Thus, hybrid supercapacitors belong to that class of supercapacitors where simultaneous incorporation of both EDLC and pseudocapacitance mechanisms takes place. In other words, in hybrid supercapacitors, the charge is stored both electrostatically and electrochemically.

Hybrid capacitors are further divided into three categories, (i) asymmetric pseudotype electrodes in which both the electrodes are coated with completely different active materials, for example one electrode is coated with carbon material while the another with either metal oxide or conducting polymer; (ii) composite type electrodes in which carbon-based material combines with metal oxide/conducting polymers, thus enabling a single electrode to have both electrostatic and electrochemical charge storage mechanisms, both the electrodes are coated with the same active material and (iii) rechargeable battery type electrodes in which battery type electrode combines with supercapacitor electrode. In general, the faradaic electrode of the hybrid supercapacitors will result in increased energy density at the cost of low cyclic stability. Therefore, current research has been carried onto the above-mentioned types of hybrid supercapacitors to give high energy and power density with high cyclic stability.

Activated carbon is one of the most used carbon-based electrode materials for aqueous supercapacitors due to their low cost and large surface area due to porous structure. AC can be prepared from various natural carbon precursors such as wood, coconut shell, coal, sugarcane bagasse, corn grain, coffee ground, etc either by the process of activation (physically or chemically) in the controlled oxidative atmosphere. Physical activation refers to the heating of carbon precursors at high temperatures (700 °C–1200 °C) in the atmosphere of CO₂ and H₂O while chemical activation refers to heating amorphous carbon at low temperatures (400 °C–700 °C) in combination with chemicals such as chlorides, alkalis, oxyacids, etc. The process of activation results in the formation of amorphous and porous carbon in large amounts with a high surface area. Furthermore, the obtained AC structure can be divided based on pore size as follows: (i) micropores (less than 2 nm); (ii) mesopores (2 to 50 nm) and (iii) macropores (greater than 50 nm) [19].

However, the activation for longer durations deteriorates the density of electrode material, thus leads to a lower power delivery rate. Therefore, research focusses on using AC in combination with different MOs, CPs, binders, and electrolytes so that all of the surface areas can be utilised to determine the capacitance. One of the many approaches to fabricate a high-performance asymmetric supercapacitor is to choose electrode materials with different potential windows for both the electrodes. AC, because of its ability to form a doubly charged layer efficiently to facilitate the transport of ions for storing charge owing to its significant electrical conductivity and high surface area, is employed as a negative electrode in combination with MO or CP as a positive electrode. An asymmetric nanocomposite using GO/PPY hybrid nanocomposite as positive and AC as a negative electrode was fabricated. The device yields a maximum energy density of 21.4 Wh kg^–1 and power density of 453.9 W kg^–1 with cyclic stability during 5000 long cycles [20]. Also, an asymmetric supercapacitor of MnO₂/carbon nanofiber hybrid composite and activated carbon nanofibers as positive and negative electrodes was also developed. The device exhibits good long cyclic stability with high retention of 94% of its initial capacitance after 5000 long cycles [21].

The wrapping-up of graphene sheets in which carbon atoms are sp² hybridised covalently connected leads to the formation of CNTs. These nanotubes based on graphene layers are further classified as single-walled CNTs or SWCNTs and multi-walled CNTs or MWCNTs. CNTs offer high electrical (≈5000 S cm⁻¹) and mechanical conductivity along with high thermal stability, unique porosity due to which electrolyte ions diffuse easily and contribute to low internal resistance and large surface area (SWCNT > 1600 m² g⁻¹, MWCNT > 430 m² g⁻¹). Also, CNTs are low-cost, easy to handle, and abundantly available. These superior properties enable CNTs to be a potent electrode material candidate for aqueous supercapacitor applications [22]. CNTs generally provide high power density owing to high electrical conductivity but lower energy density as compared to AC due to the low specific surface area of the former comparatively. Furthermore, it is not easy to hold on to the innate properties of individual CNT on a visual scale. Therefore, compositing CNTs with other materials and synthesising CNTs in different morphologies resulted in the enhanced performance of CNTs for supercapacitors [23].

Since AC offers a higher surface area than CNTs, therefore a composite of CNT with AC was reported in which initially a CNT and polymer composite was prepared, and then in the controlled oxidative environment, carbonisation was formed. The AC matrix over which the charge storage process occurred was strongly supported by the CNT mechanically and electrically. The enhanced performance of the prepared nanocomposite was analysed during the 50,000 long charging-discharging cycles in comparison to 30,000 cycles performed without CNT [24]. Also, combining CNT with either conducting polymers (such as PPY, PANI) or metal oxides resulted in the formation of hybrid composites with high capacitance and cyclic stability. In these hybrid composites, the high specific capacitance of CPs or MOs contributes to the overall high capacitance of nanocomposites while CNT due to its high conductivity provides a solid backbone to the charge storage mechanisms. In a comparison of randomly tangled CNTs, vertically aligned counterparts proved to be more advantageous in terms of accessibility of electrolyte ions which results in improved charge storage and delivery rates for supercapacitors [25, 26].

CQDs are carbon nanomaterials with zero dimensions with a size of less than 10 nm. CQDs have high aqueous, chemical, and biological stability, ease of functionalisation, less toxicity, and fine electrical conductivity. Also, CQDs show the property of fluorescence and have high inherent capacitance due to superior electronic properties. These characteristics indicate the strong ability of CQDs to act as an electrode material for energy storage applications. However, CQDs have some limitations such as low productivity, costly instrumentation, and complex synthesis procedures. Thus, the need for an economical and highly productive synthesis method for CQDs has attracted the researcher's attention to preparing CQDs from a natural source. Several research works were reported in which CQDs were prepared from chitosan; fruit and vegetable peels such as watermelon and capsicum; and plant and grass leaves via a variety of synthesis methods such as microwave-assisted, hydrothermal, pyrolysis, electrochemical oxidation, annealing, and carbonisation [27].

Compositing CQDs with either MOs or CPs results in the formation of hybrid nanocomposites with improved electrochemical properties such as high capacitance and cyclic stability. A hybrid nanocomposite of Fe₃O₄ and CQD was prepared via cost-effective hydrothermal and green solvothermal methods. The achieved capacitance of the CQDs decorated Fe₃O₄ composite was 208 F g⁻¹ with 200 long cycles [28]. Also, combining CQDs with both EDLC and pseudo material resulted in the increased interaction which leads to effective charge transportation between the components and thus a high value of capacitance. In this regard, a ternary nanocomposite of CQDs with PPY and GO was prepared by the in-situ polymerisation method. Here, CQDs intermediated the PPY and GO and reduced the internal resistance, thus promoted the effective electron transfer between PPY and GO for storing charge. A high capacitance of 576 F g⁻¹ was obtained with the 5000 long cyclic stability for the prepared nanocomposite [29].

Graphene is a single layer 2D honeycomb lattice arrangement of sp² hybridised carbon atoms. Graphene possesses high mechanical strength (≈1TPa), thermal conductivity (2000–5000 Wm K⁻¹), electrical conductivity (electron mobility 2.5 × 10⁵ cm² V⁻¹ s⁻¹), and specific surface area (2620 m² g⁻¹). Also, it offers a tunable bandgap and low cost. These excellent electrical, mechanical, and chemical properties enable graphene for applications including energy generation as well as a storage [30]. Furthermore, it can enhance the overall performance of resulting nanocomposites on combining with different nanomaterials. Graphene acts as the conductive matrix for different MOs and CPs onto which the charge transport mechanism occurs [31]. These hybrid nanocomposites utilise the synergistic effect of EDLC and pseudocapacitance electrode materials in such a way that dispersion of nanoparticles of MOs and CPs takes place over the conductive network of graphene resulting in the high capacitance achieved with long cyclic stability of hybrid nanocomposites [32].

A hybrid nanocomposite of graphene with CoNi₂S₄ synthesised via physical processes of centrifugation and ultrasonication was reported. The exceptional electrochemical performance of this nanocomposite was analysed as the capacitance achieved was 2009.1 F g⁻¹ during 2000 long cycles [33]. Graphene-based ternary nanocomposites also showed superior electrochemical performance such that a hybrid ternary nanocomposite of graphene, PPY, and Cu₂O-Cu(OH)₂ was prepared by the economic electrodeposition technique. The electrochemical contributions of each of these three composites resulted in the large value of capacitance (i.e., 997 F g⁻¹ and cyclic stability of 2000 long cycles). Similarly, a nanocomposite of graphene CNT and PPY was also prepared in which pyrrole was electropolymerised in the presence of CNT and PPY. The capacitance of the ternary nanocomposite achieved was 453 F g⁻¹ [34].

As mentioned in the previous section, graphene exhibits excellent and flexible electronic, mechanical, and optical properties which make graphene an efficient building block for carbon-based electrode materials in energy storage applications. Graphene can be synthesised by a variety of techniques. Chemical oxidation followed by reduction is one of the most suitable techniques which results in the mass production of graphene at a durable cost. The process of chemical oxidation results in the formation of graphene oxide (GO) which can be further reduced to graphene and used for aqueous supercapacitors. GO is the single-layered derivative of graphene which can be synthesised in bulk directly from the oxidation of graphite or exfoliation of graphite oxide at low cost. Based on the synthesis method, different oxygen-containing functional groups such as hydroxyl, carboxyl, and epoxy are present around the edges of graphene sheets. Generally, the structure of GO consists of epoxy and epoxide groups at the basal plane while the ionisable carboxyl groups are present on the edges. The presence of ionisable groups in GO enhanced the dispersibility of GO in an aqueous medium via weak Vander Waals and dipole interactions [35].

GO exhibits a large surface area, high mechanical strength, and better electrolytic ion facilitation owing to the presence of hydrophilic oxygen-containing functional groups. Also, the oxygen-containing functionalities impart the additional pseudocapacitance to GO which results in higher capacitance and cyclic stability as compared to graphene [36]. Furthermore, GO on combining with MOs or CPs enhanced the electrochemical performance of these pseudocapacitance electrode materials by acting as a conducting platform for them such that hybrid nanocomposite of GO and PANI was prepared by the electrochemical process of co-deposition method. The SEM results indicated that PANI nanofibers were intercalated into the GO sheets. The higher capacitance achieved is 1136.4 F g⁻¹ with 1000 long cycles in comparison to 484.5 F g⁻¹ capacitance obtained for PANI [37]. Similarly, 728 F g⁻¹ capacitance was achieved for GO/PPY nanocomposite prepared via chemical polymerisation method [20].

As discussed in the previous section about GO, the partial reduction of GO leads to the formation of rGO. Like GO, rGO also serves as the porous and conducting platform with a large surface area for carrying out the effective electron transportation mechanism for storing charge. However, the partial reduction of GO leads to the formation of structural defects such as reduced conjugation, the formation of holes, and the presence of heteroatoms in rGO. The one and most prominent way to compensate for these structural defects are to combine this carbon material with pseudocapacitance electrode materials such as CPs or MOs. The resulting hybrid composites utilise the synergistic effect of these two types of materials (i.e., carbon-based and either CPs or MOs) which leads to the enhanced electrochemical activity of nanocomposites [38].

A hybrid nanocomposite of rGO with PANI was synthesised via diffusion-driven layer by layer assembly in which rGO acts as a scaffold onto which the nanoparticles of PANI were electropolymerised. The highest capacitance achieved was 438.8 F g⁻¹ with the 2000 long cycles [39]. A ternary nanocomposite of rGO, PANI, and PPY was also prepared by the physical process of blending. The π-π stacking of aromatic moieties of both rGO and CPs leads to improved electron transportation for storing charge and the highest capacitance obtained for nanocomposite was 317.5 F g⁻¹ with 4000 long cycles [40]. Furthermore, rGO, CNT, and Co₃O₄ based ternary nanocomposite was synthesised hydrothermally. The maximum capacitance achieved was 790 F g⁻¹ and 1000 cycle stability [41].

The above-discussed section indicated that each of the carbon-based electrode materials has its advantages and disadvantages. Furthermore, their properties can be tuned by combining these materials with another type of electrode materials in which carbon-based materials act as a conductive platform with a large surface area over which redox reactions via EDLC mechanism for storing charge will take place.

Among metal oxides, ZnO is the wide bandgap (3.37 eV) semiconducting metal oxide with high binding energy (60 meV). It acts as a suitable electrode material for aqueous supercapacitor applications due to its good electrochemical activity, low cost, easy processing, non-toxicity, and environment-friendly nature. Furthermore, ZnO nanostructures have different morphologies such as nanoparticles, nanorods, nanotubes, nanospheres, nanowires, and its morphology plays a crucial role in determining its properties. However, it lacks cyclic stability and shows slow redox kinetics. This can be overpowered by combining ZnO with carbon-based materials that have high cyclic stability to form hybrid nanocomposites [44, 45].

The nanocomposites of rGO and ZnO were prepared in four different molar ratios using a one-step thermal reaction method. The resulting nanocomposite consisted of rGO as a conductive matrix onto which ZnO nanorods were decorated and the highest capacitance achieved for 1:2 molar ratios of rGO and ZnO respectively was 472 F g⁻¹ with 750 long cyclic stability indicating the high conductivity of ZnO [46]. In addition, a hybrid nanocomposite of rGO, ZnO and Cu nanoparticles was also synthesised using one-pot synthesis technique. The prepared nanocomposite interpreted the homogeneous distribution of ZnO nanorods and Cu nanoparticles over the conductive platform of rGO and the capacitance achieved was 365.5 F g⁻¹ with high cyclic stability of 3000 long cycles [47].

Apart from ZnO, MnO is also one among the widely studied metal oxides as electrode material for aqueous supercapacitor applications owing to its low cost, ease of availability, high capacitive performance (1370 F g⁻¹) and eco-friendliness. The high pseudocapacitance of MnO resulted from its reversible redox reactions between Mn⁺³/Mn⁺², Mn⁺⁴/Mn⁺³ and Mn⁺⁶/Mn⁺⁴. Like ZnO, MnO also exists in different morphologies such as nanorods, nanobundles, nanowires, nanoblets, flower like microspheres and nanowhiskers. Apart from many advantages, MnO suffers from some drawbacks such as limited surface area, low electronic conductivity and partial dissolution of MnO₂ in the electrolyte during charging-discharging which lead to reduction in capacitance [43].

However, these limitations can be combated by compositing these oxides with carbon materials with high surface area. A sandwich type hybrid nanocomposite of MnO₂ nanospheres and rGO nanosheets was synthesised by filtration-directed self-assembly method. It was observed that MnO₂ nanospheres were uniformly embedded in the rGO matrix. The highest capacitance achieved is 446 F g⁻¹ during 1000 long cycles [48]. A ternary hybrid nanocomposite of MnO₂ was prepared with NiO and nitrogen-doped GO hydrothermally. The prepared nanocomposite exhibited a high capacitance of 1490 F g⁻¹ with 2000 long cycle stability [49].

Among all the transition metal oxides, RuO₂ is the most widely studied pseudocapacitive electrode material for aqueous supercapacitors due to its numerous advantages. These include high capacitance (≈1000 F g⁻¹), high cyclic stability, high redox reaction ability, high metallic conductivity, high thermal stability, and wide potential window (1.2 V). Despite these advantages, the high cost of RuO₂ limits its applications. Therefore, various approaches have been used to compensate for its high costs such as synthesis of mixed oxide composites and synthesis of ruthenium-based composites with low-cost materials such as carbon materials and CPs [9, 50].

A binary nanocomposite of RuO₂ and PANI was prepared in which RuO₂ nanoparticles were embedded onto the PANI layer electrochemically deposited on a tantalum sheet. The highest capacitance achieved is 474 F g⁻¹ with 1000 long cycle stability [51]. Furthermore, the same composition (i.e., PANI / RuO₂) was also reported where composite thin films were prepared using chemical bath deposition technique and the maximum capacitance achieved is quite high (830 F g⁻¹) [52].

NiO is also one of the promising metal oxide electrode materials that can be used for aqueous supercapacitors as it offers a simplified synthesis procedure, low cost, non-toxicity, and high capacitance (≈3750 F g⁻¹). The electrochemical activity of NiO relies strongly on its crystal structure which depends on the synthesis temperature. During synthesis, the calcination temperature affects the crystallinity of NiO and thus changes the capacitance value. Like other metal oxides, NiO also has some drawbacks such as low cyclic stability and low electrical conductivity. These sufferings can be overcome by combining NiO with conductive and high surface area carbon-based materials such that the conductive network of carbon materials enhances the electrical conductivity of NiO and the high surface area provides the more active sites for redox reactions of NiO to take place [5, 8].

A binary hybrid nanocomposite of NiO with rGO was synthesised using an eco-friendly hydrogen reduction method. The zest of this prepared nanocomposite is that apart from acting as an additive, NiO also facilitates the reduction by acting as a catalyst. The capacitance achieved is 428 F g⁻¹ and the cyclic stability is analysed through 5000 long cycles [53]. A hybrid supercapacitor using NiO/Graphene nanocomposite was fabricated. The NiO nanoflakes were grown in-situ using the chemical bath deposition technique on a graphene scaffold supported on Ni foam. The highest capacitance achieved for the nanocomposite is 425 F g⁻¹ and the cycle life was analysed during 2000 long cycles [54].

The above-discussed section interpreted that each of the metal oxides has its own merits and demerits. While their merits can be availed and demerits can be eradicated by combining these metal oxides with carbon-based materials where the redox reactions will be performed by metal oxides for achieving high capacitance values via pseudocapacitance mechanism for storing charge.

Among all the CPs, PANI is the most promising electrode material for aqueous supercapacitor applications owing to its better electrical conductivity, bulk production, low cost, ease of synthesis, and redox ability. Furthermore, PANI exhibits three interconvertible oxidation states; (a) Leucoemaraldine salt (O.S.= +1) is the complete reduced state and on oxidation get converted to (b) emaraldine salt (O.S.= +0.5) which further oxidised to (c) pernigraniline salt (O.S.= 0) which is the fully oxidised state. PANI is the p-doped CP owing to the lower n-doping potential in comparison to the reduction potential of the electrolytic solution. A protic solvent is generally preferred to PANI due to the requirement of proton for charging and discharging [57, 58].

PANI has some limitations also. During the long cycles of charging-discharging, it is highly susceptible to degradation due to the swelling-shrinkage property of the polymeric chain and has limited available surface area also. These limitations can be overcome by combining PANI with high surface area carbon-based materials which result in high capacitance with an improved electrochemical performance of PANI, for example a binary hybrid composite of PANI with rGO was prepared and the highest capacitance achieved is 715 F g⁻¹ in the 1:6 ratios of rGO and PANI respectively [59]. Also, a hybrid nanocomposite of PANI with binary nanocomposite of nickel hydroxide Ni(OH)₂ and iron oxide (FeO) doped rGO was synthesised by the process of in-situ oxidative polymerisation. The maximum capacitance achieved for this nanocomposite was quite high 2714 F g⁻¹ and cyclic stability of 2000 long cycles [60].

Apart from PANI, PPY is also among the promising CP based electrode material candidate for energy storage applications. PPY possesses fast reversible faradaic redox reaction ability, high energy density, wide operating thermal range, high conductivity, and reasonable cost. Its monomer, pyrrole is water-soluble and can be easily oxidised making PPY the fast-developing CP of all. Furthermore, the appropriate use of aromatic dopants enhances the electron transportation property of PPY resulting in high values of pseudocapacitance. However, sometimes the use of anionic dopants destabilises its polymeric structure which leads to the deteriorated electrochemical performance of PPY. The prime advantages of PPY as an electrode material for aqueous supercapacitor include its fast charging-discharging ability and high energy density [6, 8].

Owing to fast redox reaction capability, PPY on combining with carbon-based materials gives high pseudocapacitance, for example a hybrid ternary nanocomposite of rGO, PPY, and Cu₂O-Cu(OH)₂ was prepared by the low-cost electrodeposition method. This hybrid nanocomposite utilises the synergistic effect of both EDLC and pseudocapacitance electrode materials as the highest capacitance achieved for nanocomposite is 997 F g⁻¹ in comparison to binary GO/PPY (500 F g⁻¹) and rGO/PPY (685.5 F g⁻¹). The cyclic stability of the nanocomposite was analysed via 2000 long cycles [61]. Similarly, a hybrid ternary nanocomposite of PPY was also synthesised with graphene and CNT via in-situ polymerisation [25]. The capacitance achieved for this nanocomposite was not that much (i.e., 453 F g⁻¹) due to the pseudocapacitance contribution of only one component (i.e., PPY) in comparison to the previous reference where there are two pseudocapacitance components (PPY and Cu₂O-Cu(OH)₂).

Like PANI and PPY, PTh is also one of the CP that offers high charge carrier mobility, oxidative stability, tunable doping/dedoping ability, tunable electrochemical activity, ease of synthesis, and economy. PTh can be easily prepared by a variety of synthesis methods such as chemical, photochemical, electrochemical, template-assisted, and ultrasonic-assisted methods. However, it suffers from a sudden decrease in capacitance due to the structural degradation during long charging-discharging cycles. This demerit can be overcome somehow by forming the hybrid composites of PTh with carbon materials as these carbon materials offer a large surface area for carrying out the redox reactions of PTh [62, 63].

A binary hybrid nanocomposite of PTh and MWCNT was prepared by the electropolymerisation method and the capacitance achieved for this nanocomposite was 110 F g⁻¹ with the cyclic stability of 1000 long cycles [64]. A binary nanocomposite of PTh was also prepared with different ratios of TiO₂ to analyse the performance of PTh after the incorporation of TiO₂ via oxidative polymerisation method. The maximum capacitance achieved was 250 F g⁻¹ with 10:2 ratios of PTh and TiO₂ respectively [65].

Like metal oxides, CPs also suffer from pros and cons which can be utilised and compensated on combining CPs with carbon-based materials to form composites where CPs due to their tendency to show variable oxidation states undergo faster redox reactions through pseudocapacitance mechanism for energy storage.