Nano-Metal Oxide Based Supercapacitor via Electrochemical Deposition

Saima G Sayyed; Mahadeo A Mahadik; Arif V Shaikh; Jum Suk Jang; Habib M Pathan

ES Energy & Environment

ISSN: 2578-0646 (Print Version) ISSN: 2578-0654 (Online Version)

Review Paper

Citation: ES Energy & Environment, 2019, 3, 25-44

DOI:10.30919/esee8c211

Received: 30 Nov 2018
Revised: 05 Feb 2019
Accepted: 06 Feb 2019
Published online: 07 Feb 2019

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Nano-Metal Oxide Based Supercapacitor via Electrochemical Deposition

Saima G Sayyed,^1,* Mahadeo A Mahadik,² Arif V Shaikh,¹ Jum Suk Jang² and Habib M Pathan³

¹ Department of Electronic Science & PG Center, Poona College of Arts, Science and Commerce, Camp, Pune, India.
² Division of Biotechnology, Chonbuk National University, Iskan 570-752, Republic of Korea.
³ Advanced Physics Laboratory, Department of Physics, Savitribai Phule Pune University, Pune – 07

* Email: sayyed.saima26@gmail.com

Abstract

In this rapid growing world, the demand of alternate or non-conventional energy sources with high density and power has been tremendously increased. Supercapacitor is one of the promising energy storage devices which possess high specific capacitance, high power density and long life cycle. The performance of supercapacitors is evaluated by its electrode materials. Among the various supercapacitor electrode materials, recent research focused on synthesis of transition metal oxides/ hydroxides, carbon metals and polymers. Transition metal oxides such as manganese oxide (MnO₂), ruthenium oxide (RuO₂), cobalt oxide (Co₃O₂), nickel oxides (NiO) etc. have been widely used as supercapacitor electrode materials for storing the potential energy. In this paper, we explored the details of metal oxide material based supercapacitor electrodes and their composition via electrochemical deposition technique. We also discussed the basic parameters involved in supercapacitor studies and advantages of electrochemical deposition technique through analysis of the literature.

Keywords: Supercapacitors; Metal oxide; Electrodeposition technique.

1. Introduction:

Owing to the environmental issues like global warming, pollution, fuel problems, etc, it has become necessary to develop clean, efficient and sustainable energy sources for storing energy.^1-2 Also many applications such as stand-by power systems, cell phones and electric hybrid vehicles require energy storage.³ In 1978 a device called supercapacitor was introduced by NCE which was used to provide the power backup for computers.⁴ Further it was found that these supercapacitors could be used to boost the fuel cell or battery in an electric hybrid vehicle to provide the necessary power.^5-10 Recent developments have made supercapacitor as a complement of fuel cells or batteries to store the energy.¹¹ Also it can be used in laptops, mobile phones, digital cameras, etc.³ Hence tremendous theoretical and practical research work is going on for development of supercapacitor as it can be an environmental friendly and low cost storage device.^12-19

Supercapacitor consists of two identical electrodes with a separator immersed in an electrolyte.²⁰ Electrode material plays an important role in supercapacitor.^21,22 There should be an effective contact between the electrode materials and the electrolyte to achieve excellent supercapacitive properties. Transition metal oxides (2 to 50 nm) such as RuO₂, NiO, Co₃O₄, MnO₂, In₂O₃, Fe₃O₄, V₂O₅, IrO₂, Bi₂O₃, NiFe₂O₄, BiFeO₃ etc are promising materials for the fabrication of supercapacitor with high energy density due to their exceptional physic-chemical properties, suitable pore size and high specific surface area.^23-28

One of the most promising techniques for fabrication of transition metal oxide is electrochemical deposition. Because of its versatility it leads to increase the specific capacitance of the supercapacitors.³⁰ This technique is widely used as it is cheap, easy and one step technique to synthesis metal oxides, polymers and its composite. The morphology and the chemical composition of the deposited film can be easily controlled by optimizing electrochemical parameters to achieve adherent film.

There are many published review article on the supercapacitors. In literature [1, 34, 37, and [97] metal oxides-based materials, conducting polymers and carbon based materials for supercapacitor electrodes are reviewed in detail. According to authors in article,¹²¹ supercapacitive performance can be enhanced by developing the nanostructure and composite materials. R.C. Ambare et al., has presented a brief review on electrode materials, also discussed their charge transport and configurations of supercapacitors.⁶⁴ Evaluation of charge capacity for both negative as well as positive electrode materials was demonstrated in ref. [66].

In this review article, we have presented synthesis of different metal oxides via electrochemical deposition used by various research groups as supercapacitor electrode material. We focused on fabrication of transition metal oxide by electrochemical deposition technique only. The main aim of this review is to give detailed information on metal oxide based supercapacitors, parameters and performance of supercapacitors.

1.1Fundamentals of Supercapacitors:

Supercapacitors are also known as Ultracapacitors, double layer capacitors or electrochemical capacitors. They utilize large surface area and thinner dielectrics to achieve greater power density than that of batteries and greater capacitances with higher energy density than that of conventional capacitors.^31-37 Supercapacitor reaches 20 times higher power density (>10 kW/kg) and better life cycle (>10⁵cycles) than that of batteries, also it can be charged/discharged rapidly.^{29, 38, 39} It can be used in various energy storage devices, either in combination with batteries or stand-alone. Fig. 1 shows the comparison between specific energy and specific power for different electrical energy storage devices.^{40, 41}

Fig. 1 Ragone plot: Specific Energy Vs Specific Power Plot

This Ragone plot indicates that supercapacitors occupy a region between batteries and conventional capacitors. Supercapacitors are driven by the basic principle of conventional capacitors but the difference is that they have electrode material with higher surface area and have thinner dielectrics which decrease the distance between the electrodes. The capacitance ‘C’ is directly proportional to the surface area ‘A’ and inversely proportional to the distance ‘D’ between the electrodes:

C=εoεrAD (1)

where, ε_r is the electrolyte dielectric constant, ε₀ is the permittivity of a vacuum.⁹ The stored energy E in a supercapacitor depends upon specific capacitance (C) and the operating voltage (V):⁴²

E=12CV2 (2)

The maximum power (Pmax) depends upon operating voltage (V) and the internal resistance (R) as follows:

Pmax=V²4R (3)

Generally, the mechanism of the supercapacitors categorizes into three types based on energy storage and cell configuration: (i) Electric Double- Layer Capacitors (EDLC’s), (ii) Pseudocapacitors and (iii) Hybrid capacitors ⁴³as shown in Fig. 2.

Fig. 2 Classification of Supercapacitors

Electric double-layer capacitors (EDLCs):

EDLCs are made up of two carbon based porous electrode material which are separated by an insulator. A basic configuration of EDLC is shown in Fig. 3.The energy charge is stored in a non-faradaic manner; the charge storage mechanism is based on the electrostatic charge accumulation at the electrode-electrolyte interface.^{44, 45} The most common electrode material is activated carbon. Carbon nano materials are having unique structures with large surface area, better electrical conductivity and high chemical & mechanical stability. They require wide potential window, high conductivity, fast charge/discharge rate and large surface area.^{46, 47} The specific capacitance in carbon-based electrode materials is less and hence achieving a high energy density has become a difficult task in EDLC’s.

Fig. 3 Schematic diagram of EDLC

Pseudocapacitors:

Pseudo-capacitors electrostatically store the charge as compared to EDLC’s. The faradaic charge transfer in Pseudocapacitors takes place at electrode-electrolyte interface.^48-52 It exhibits high energy density and high specific capacitance than that of electrical double layer capacitance due to Faradic process.⁵³ Transition metal oxides⁵⁴ and conducting polymers⁵⁵ are mainly used as pseudocapacitor electrodes. It requires high surface area, large potential window, doping of the conducting polymer and fast charge/discharge rate.^{56, 57} The main disadvantage of the pseudocapacitors is low power density.⁵⁸

Hybrid capacitors:

EDLC’s offers large power performance and good cyclic stability while pseudocapacitors possess greater specific capacitance and energy densities. Hybrid supercapacitors are combination of both EDLC and Pseudocapacitors which offer a high energy density and fast charging rate in the same cell.^54-62 The combination of two different electrodes typically results in more energy storage due to the wider operating voltage of an organic electrolyte and the good specific capacity of the battery type electrode. Hybrid capacitors have been tested with both negative and positive electrodes in aqueous electrolytes solution to improve the performance.⁶³ There are three types of hybrid capacitors based on configurations of electrodes (a) composite, (b) asymmetric and (c) battery-types.

The cyclic voltammetry (CV) curve of EDLC supposed to be rectangular in shape, but in pseudo-capacitance the shape of the curve will become non-rectangular due to faradaic process as shown in Fig. 4. Hence overall shape of the CV curve in hybrid type capacitors is a non-rectangular as it is combination of both EDLC and Pseudocapacitors.

Fig. 4 Typical CV curve of EDLC and Pseudocapacitors

1.2 Supercapacitive parameters:

There are various significant parameters to evaluate the performance of as prepared electrode materials for supercapacitive application as shown in Table 1.^64-66

1.3 Background of Electrodeposition technique for synthesis of Nanostructure electrode materials:

Thin films play an important role in the electrochemical studies and applications. The behavior of the thin film typically < 1 μm depends upon the properties of the electrode surface. There are many synthesis technique used to produce electrodes for supercapacitors such as Chemical bath Deposition,^{67, 68} Chemical vapor deposition (CVD),^{69, 70} Spray pyrolysis,⁷¹ SILLAR method,⁷² sol-gel method,^73,74 hydrothermal technique⁷⁵ and Electrochemical deposition etc. Among various methods, electrochemical deposition is an attractive and well known technique due to its inexpensive, simple and effective process of fabrication of the metallic coatings under ambient temperature. It is a versatile technique used for deposition of the metals,^{76, 77} metal alloys,^{78, 79} metal oxides⁸⁰ and hybrid materials.⁸¹The technique involves the movement of metallic ions towards a cathode in the solution driven by an electric field. The ions either accept the electron and get deposited on the cathode or lose electron and get deposited on anode in the form of atom or molecule. The general setup of electrochemical deposition is shown in Fig. 5.⁸² It involve the following "electrical" terms.

Fig. 5 Electrochemical deposition setup

a. Electrolyte- The electrolyte is a conducting medium through which the flow of electric current takes place by movement of ions. It can be aqueous, non-aqueous or molten, in presence of suitable metal and chalcogenide salts.

b. Electrode- An electrode is a conductor through which an electric current enters or leaves an electrolyte. When electrode connected to positive terminal then it is referred as an anode where other electrode is known as cathode. At anode, positive ions are formed or negative ions are discharged or oxidizing reactions occur. At cathode, positive ions are discharged or negative ions are formed or reducing reactions occur.

c. Electrode potential- An electrode potential is the difference in potential between an electrode and the electrolyte, measured against or referred to, an arbitrary zero of potential.

d. Equilibrium electrode potential- It is a static electrode potential when the electrode and electrolyte are in equilibrium with respect to a specified electrochemical reaction.

e. Standard electrode potential- A standard electrode potential is the equilibrium potential, for an electrode in contact with an electrolyte, in which all the components of a specified electrochemical reaction are in their standard state.

f. Reference electrode- A reference electrode is defined as an electrode on which the state of equilibrium of a given reversible electrochemical reaction is permanently secured under constant physicochemical conditions. Equilibrium potential of standard hydrogen electrode is 0 V, whereas, it is + 0.2415 V for saturated calomel electrode (SCE).

Electrodeposition method is an isothermal process in which, the thickness, crystallographic orientation,⁸⁰ morphology,^{83, 84} and dopant density⁸⁵of the films can be easily controlled by electrochemical parameters such as electrode potential or current (charge),⁸⁶ time, deposition temperature, electrolyte composition,⁸⁷ concentration,⁸⁹ pH of the bath,⁹⁰ etc. Thus, electrodeposition allows obtaining uniform films grown on substrate of complex shapes and areas which is not possible by other methods. One disadvantage of electrodeposition is that, it requires a conducting substrate such as glassy carbon, metals (Au, Pt, Ti, Ni, and Cu), oxides (ITO, FTO) or alloys (stainless steel).

2. Electrochemical deposition of metal oxides:

To deposit metal oxides mostly alkaline solutions with metal complex are used as an aqueous solution. Electrochemical deposition of metal oxides can be carried out under both oxidizing and reducing conditions from alkaline solutions. In both conditions, the metal ions are directly deposited on the electrode as an oxide. Deposition under oxidation condition includes the deposition of MnOx from Mn(II) ammine complex,⁹¹ CuO from Cu(II)-tartrate,⁸⁶CeO₂ from Ce(III)-acetate,⁹² NiOx from Ni(II) ammine complex⁹³ and Co₃O₄from Co(II) glycine in alkaline solutions.⁹⁴ Deposition of metal oxides under reduction conditions includes deposition of ZnO,^{88, 95} CdO⁹⁶ and Cu₂O from alkaline Cu(II) solution etc.

For supercapacitor application, metal oxide required some properties includes: (i) It should be electronically conductive. (ii) It must exist in two or more oxidation states which coexist in the continuous range without changing the phase. (iii) The protons should be freely intercalated into the oxide lattice and out of the lattice for reduction and oxidation states respectively. Till date above mentioned properties are explore for metal oxide such as manganese oxide, ruthenium oxide, nickel oxide and cobalt oxide.

2.1 Ruthenium oxide/hydroxide and their composition:

Among the various metal oxides, both crystalline and amorphous RuO₂ are promising electrode material because of excellent electrochemical capacitance (~2000 F/g), high electrical conductivity, good thermal & chemical stability, large potential window, long life cycle and good electrochemical reversibility.^{97, 98} It has various forms for example nano-porous film⁶⁷ nanoneedles,⁹⁹ and nanoparticles.¹⁰⁰ Ruthenium Oxide formed by various techniques including CBD,⁶⁷ CVD,⁶⁹ Sol-gel method,¹⁰¹ Polyolmethod¹⁰³Hydrothermal,⁷⁵ electrodeposition method etc. Also lots of research carried out on the combination of RuO₂ with other oxides or polymers such as NiO, TiO₂, VOx, SnO₂, RuO₂/ CNT, RuO₂/ PPy, PANi etc. Table 2 represents the synthesis conditions with details of deposition used by various researchers to obtain the electrodeposited ruthenium oxide/hydroxide and their composition thin films.

Amorphous ruthenium oxide electrode shows different reaction in alkaline and acidic electrolyte solution for example in KOH electrolyte the electrode exhibited specific capacitance of 710 F/g when calcinated at 200°C while in H₂SO₄aqueous electrolyte it showed capacitance of 720 F/g when heated at 150°C.¹⁰¹ In acidic electrolyte solution, RuO₂ obeys following rapid faradaic reaction:^{118, 119}

RuO₂ + nH⁺ + ne^-↔RuO_2-n(OH)_n(4)

Where 0≤ n≤ 2, according to above eq (16) in acidic solution, oxidation states of Ru can change from Ru(II) to Ru(IV). But in an alkaline solution, changes of oxidation states for RuO₂ are different. It has been reported that RuO₂ composite with carbon electrode will be oxidized to RuO42- , RuO4- and RuO₄and reduced to RuO₂.^{120, 121}

The performance of RuO₂ depends upon crystallinity, surface area, combination of water, temperature and size of particle. The crystallinity depends on the synthesis technique which affects the supercapacitive performance of RuO₂. Amorphous RuO₂ thin films formed by anodic deposition showed maximum capacitance of 1190 F/g in H₂SO₄electrolyte for 10 cycles, as number of cycles increases capacitance decrease upto ~800 F/g for 1000 cycles.¹¹⁰ To increase capacitance value one of the most effective way is to increase the surface area.¹⁰¹ The unique electrochemical features result in CV curve as shown in Fig. 6. The figure indicates an ideal capacitive behavior.¹⁰⁴ As reported in ref [105] the specific capacitance and energy efficiency decreases with increasing the film thickness. Combination of water with RuO₂ is used to enrich the diffusion of cations inside the electrode layer. As reported in ref [111] hydrous ruthenium oxide (RuO₂.nH₂O) formed by cathodic electrodeposition showed a capacitance of 786 F/g. Whereas RuO₂.nH₂O formed by anodic deposition showed specific capacitance of 552 F/g when heated at 150 ˚C for 2 hr.¹¹²The RuO₂·nH₂O nanotubular array electrode formed by using anodic deposition exhibits SC as high as 1300 F/g along with an energy density of 7.5 Wh/kg.¹⁰² Annealing Temperature is another most important factor for electrochemical performance, RuO₂electrode was prepared by cathodic electrodeposition on Ti substrate exhibits maximum capacitance of 788 F/g when calcinated at 100˚C.¹⁰⁵ Kim et al., has reported that the electrochemically prepared composition of RuO₂ with carbon nanotube film exhibits much higher capacitance of 1170 F/g.¹¹⁵

Fig. 6 The CV curves of RuO₂ electrode at different scanning rates in 0.5 M H₂SO₄ electrolyte. Reproduce from ref [104]

In summary, the composition of RuO₂ with carbon nanotube based electrode improves the supercapacitive performance. Even though RuO₂ showed extremely high specific capacitance but it is not suitable for commercial application due to its relatively high cost and environmental harmfulness. There are two ways to reduce the cost: (i) by composing RuO₂ with other metal oxide. (ii) Depositing RuO₂ on low cost substrate.

2.2 Manganese oxide/hydroxide and their composition:

Manganese oxide (MnO₂) shows all over good electrochemical performance that why it has been widely used as an electrode materials for supercapacitor applications. Manganese oxides found to be an alternative of RuO₂ because of their relatively low toxicity, low cost, and high theoretical capacitances value between 1100 to 1300F/g and long cycle life ~10,000 cycles.^122–131 Many efforts have been made to obtain mesoporous MnO₂ by using different synthesis techniques includes CBD, SILAR method, template method, hydrothermal, ultrasound irradiation and electrodeposition method.^132-134 Table 3 presents the summary of synthesis condition with deposition details electrodeposited manganese oxide /hydroxide and their composition thin films.

MnO₂ has various oxidation states, such as Mn(0), Mn(II), Mn(III), Mn(IV), Mn(V), Mn(VI), and Mn(VII)¹⁷¹ with several crystal structures like α, β, γ, δ.¹⁷² Following two mechanisms shows the charge behavior of MnO₂which involve a redox reaction of oxidation states between the III and IV of Mn.

MnO₂ + C⁺ + e⁻ ↔ MnOOC (5)

(MnO₂)_surface + C⁺ + e⁻ ↔ (MnOOC)_surface (6)

Equ (17) indicates the insertion of electrolyte cations like H⁺=C⁺, Na⁺, Li⁺, and K⁺ in the bulk material and equ (18) implies that the surface adsorption of electrolyte cations on the MnO₂ electrode.^{173, 174}

Electrochemical performance of MnO₂ depends upon some important factors includes Morphology, Crystallinity, Crystal Structure, Thickness of the electrode and Surface area.

Morphology: The Morphology of film depends upon synthesis process and conditions. Dubal et al., ¹³⁸ has prepared the MnO₂ thin film by different modes of electrochemical deposition, it was found that significant change in the surface morphologies due to different modes. Four different morphologies i.e. nanonods, nanospheres, nanosheets and nanoflowers of MnO₂ were demonstrated by varying current density and concentration of H₂SO₄. It was found that highest capacitance of 362.5 F/g for nanonode electrode at 0.5 A/g.¹⁴⁷
Crystallinity: Alike to Ru_O2cystallinity depends upon synthesis process. High crystallinity gives increase in conductivity but decrease in surface area. To achieve greater conductivity annealing temperature plays an important role. Chang et al.,¹³⁵ has investigated effect of heat treatment on material. Amorphous oxide film converted into fibrous shape with nanocrystalline when calcinated at 200°C for 2 h. However at high temp (400°C) formed films indexed to Mn₃O₄ and Mn₂O₃.
Crystal structure: The performance of electrode also depends upon crystal structure. It is observed that the various synthesis conditions can results in the different structures of MnO_x. Three types of crystal structures for MnO₂ were demonstrated, ε-MnO₂prepared without any complex agent, defective rock salt MnO₂from EDTA containing solutions, and defective antifluorite MnO₂ from citrate containing solutions. It was found that the defective rock salt and antifluorite structures of MnO₂exhibit better capacitive properties than that off ε-MnO₂. Whereas in ref [141] GS deposition mode indexed to tetragonal phase of Mn₃O₄ while pulse current mode indexed to two structures i.e. tetragonal Mn₃O₄ and orthorhombic MnOOH as shown in Fig. 7 (XRD patterns). It was observed that the pulse current deposition mode showed better capacitive properties.
Thickness and surface area: As thickness increase the specific capacitance decreases. Qiu et al.,¹⁴⁰has reported that the formation of Mn_O2film with thickness of 0.58 ~ 1.25 μm and the specific surface area of as-prepared sample was 7.7^m2/g. Wanchaem et al., has prepared MnOx by two precursors i.e. MnS_O4and KMn_O4. The highest specific capacitance was found by using MnS_O4 as a precursor due to its nanosheet structure with a large surface area.¹⁴²

Fig. 7 XRD patterns of (a) g-MnOx and (b) p-MnOx. Reproduced from Ref. [141]

In literature¹⁴⁶ Amorphous MnOx.nH₂O fabricated by three different modes i.e. potentiostatic, galvanostatic, and potentiodynamics. It was observed that all deposits showed similar capacitive properties because of similar oxidation states.¹⁵⁴ Rusi et al., has fabricated composition of MnO₂with NiO by three different modes of electrochemical deposition. The best electrochemical performance of CV mode was found in mixed KOH/K₃Fe (CN)₆ electrolyte in comparison with Na₂SO₄ electrolyte. The maximum specific capacitance of 3509 F/g was found.¹⁵⁸ Whereas in same electrolyte (mixed KOH/K₃Fe (CN)₆) electrodeposited rGO/MnO₂nanocomposite electrode exhibits specific capacitance as high as 13,333 F/g with power density of 68.35 kW/kg and energy density of 1851 Wh/kg.¹⁵⁹ Fig. 8 and 9 represent a typical charge discharge, CV curve and cyclic stability of MnO₂-NiO composite and rGO/MnO₂ nanocomposites respectively.^158,159

Fig. 8 (A) CV curve and (B) Charge-discharge curve of MnO₂-NiO Composite electrode. Reproduced from ref. [158]

Fig. 9 Charge-discharge curve and (b) Cyclability of rGO/MnO₂ nanocomposites electrode in three different electrolytes. Reproduced from ref. [159]

In conclusion, one can increase the specific capacitance, energy and power densities by depositing MnO₂onto carbon material with large surface area and high conductivity. The composition of NiO with MnO₂ is versatile, cost efficient and scalable for supercapacitor applications. Addition of glucose with MnO₂can give rise in specific capacitance, energy and power densities.

2.3 Nickel oxide/hydroxide and their composition:

Nickel oxide/hydroxide electrode plays an important role in fabrication of supercapacitors because of its high specific capacitance (theoretically ~3750 F/g), easy synthesis, high chemical and thermal stabilities, environment friendliness and low cost.^175-179 NiO has several nanostructures such as nanorods, nanowires, nanobelts, and nanoflowers. Literature analysis for synthesis conditions with deposition details of Nickel oxide/hydroxide and their composition thin films via electrodeposition technique shown in Table 4.

The redox reaction of NiO in an alkaline electrolyte can be described as follows:^186-188

NiO + OH⁻↔ NiOOH⁻+ e⁻ (7)

The electrochemical performance of NiO totally depends upon Crystallinity which affects by heating treatment. Wu et al., has reported that the nickel hydroxide electrodeposited on nickel substrate was transformed into the nickel oxide when calcinating at 250˚C, which exhibits high SC of 1478.¹⁸⁰ In literature ¹⁸² NiO electrode obtained from three precursors i.e. nitrate, chloride and sulphate. It was observed that the NiO electrode prepared from sulphate solution showed all over good electrochemical performance. Particle-like nickel hydroxide prepared by electrodeposition technique exhibits the maximum specific capacitance of 2595 F/g.¹⁸⁴

Because of high specific capacitance and low cost of Ni/ Ni(OH)₂, it should be promising electrode materials for supercapacitor applications. But there are two main disadvantages of using NiO for supercapacitor electrode (i) it has poor cyclic stability. (ii) Low electric conductivity. To overcome these drawbacks, composing NiO with other materials and fabricating nanostructured NiO are advisable.

2.4 Cobalt oxide/hydroxide and their composition:

Cobalt oxide (Co₃O₄) has a cubic structure and most studied material due to their high electrical conductivity, large surface area, excellent reversible redox behavior and long-term stability with high theoretical capacitances value (~3560 F/g).^189-196 Table 5 presents the summary of synthesis conditions with deposition details used by various researchers for obtaining electrodeposited Cobalt oxide /hydroxide and their composition thin films.

The redox reaction of Co₃O₄ in alkaline electrolyte can be expressed as follows:^{202, 203}

Co₃O₄ + OH⁻ + H₂O ↔ 3CoOOH + e⁻ (8)

CoOOH + OH⁻ ↔ CoO₂ + H₂O + e⁻(9)

Nanocrystalline Co₃O₄ film was formed by electrodeposition method exhibits specific power and energy of 1.33kW/kg and 4.0Wh/kg respectively.¹⁹⁷ Jagadale et al., has prepared cobalt oxide by three different modes of electrodeposition technique. Film deposited by PS mode showed maximum values of specific capacitance, specific energy and specific power as compare to PD and GS modes.¹⁹⁹ Aghazadeh et al., has prepared β- cobalt hydroxide with flake-like morphology by green electrochemical synthesis as shown in Fig. 10 (TEM image) exhibits the specific capacitance of 1288.1 F/g.²⁰⁰ Rajeswari et al., has prepared cobalt hydroxide nanoplates on cadmium oxide (CdO) as conducting base electrode exhibits high capacitance value of 1119 F/g.²⁰¹

Fig. 10 TEM image of β- cobalt hydroxide with flake-like morphology.

Reproduced from ref. [200]

In conclusion, Co(OH)₂ electrodes showed good performance as compare to Co₃O₄. However, both NiO/Ni(OH)₂ and Co₃O₄/Co(OH)₂ have same drawbacks, which limits their practical use.

2.5 Other metal oxides:

Other than RuO₂, MnO, NiO and Co₃O₄ electrodes, copper oxide (CuO),^204-207 Vanadium oxide (V₂O₅),²⁰⁸^{, 209} Molybdenum oxide (MoO_x),^{210, 211}Titanium oxide (TiO₂),^{212, 213}Tin oxide (SnO₂),²¹⁴ Bi₂O₃,²¹⁵ Iron oxides (Fe₂O₃ / Fe₃O₄)²¹⁶ and Indium Oxide (In₂O₃) have been studied for supercapacitor electrode materials.

Amorphous copper oxide thin films have been synthesized by electrodeposition on different substrate for example copper oxide grown on copper foam exhibits maximum capacitance of 212 F/g²⁰⁶ while on stainless steel substrates showed specific capacitance of 36 and 179 F/g in 1 M Na₂SO₄electrolyte.^{204, 205} Ghadge et al., has reported the copper hydroxide thin film electrode formed by anodization method exhibits maximum specific capacitance of 6000 F/g.²⁰⁷ TiO₂has been deposited via electrochemical anodization technique on Titanium metal foil showed specific capacitance of 1300 μF/cm².²¹⁷ Lee et al., has reported that the amorphous V₂O₅ exhibits a maximum specific capacitance of 350 F/g.²¹⁸Amorphous MoOx film formed by electrodeposition technique showed capacitance as high as 507 F/g in 1 M H₂SO₄electrolyte.²¹⁹ ElectrodepositedBi₂O₃ thin film on copper substrate exhibits specific capacitance of 98 F/g.²²⁰ Amorphous SnO₂ exhibits the maximum specific capacitance of 285 F/g synthesized by electrochemical deposition method.²²¹ Prasad et al., has prepared In₂O₃ film via electrochemical deposition method which exhibited a specific capacitance of 190 F/g.²²²

3. Conclusion:

Supercapacitors have emerged as an alternative solution to energy technology with higher energy density, excellent electrochemical properties and good cyclic stability. Due to its large surface area thinner dielectric and higher thermal & electrochemical conductivity it can be used in many application such as emergency power supplies, specific power systems, back-up and pulse power applications. Also there has been great interest in developing supercapacitors for electric hybrid vehicles power systems. Supercapacitor can be easily fabricated using various transition metal oxides/hydroxides due to their high conductivity, larger surface area, and better stability. We have reviewed the various transition metal oxides/hydroxides used for supercapacitor via electrochemical deposition. The high performance of supercapacitors could be achieved by metal oxide/hydroxides electrodes with composite materials. Key challenges for supercapacitor are their limited energy and power densities. To overcome this problem, researchers should focus to develop new electrode materials with high capacitance, energy and power densities and wider potential range. For these electrode materials should have low internal resistance, suitable pore size, high surface area and better electrochemical & mechanical stability. Hence this area requires further research and development for supercapacitors to become a realistic power solution.

References:

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