Overview of representative Ni and Co based material and their performance in PCs.
The recent advanced electronic appliances demand special high power devices with lightweight, flexible, inexpensive, and environment friendly in nature. In addition, for many industrial and automotive applications, we need energy storage systems that can store energy in a short time and deliver an intense pulse of energy for long duration. Till date the Li-ion battery is the only choice for fulfilling all our energy storage demands. However, the high cost, limited availability and non-environmental nature of electrodes and electrolyte material of Li-ion battery limits its applicability. Hence, the world demands an alternative replacement for the Li-ion battery. In this regard, the supercapacitor is one of the most emerging and potential energy storage devices. The electrode plays an important role in supercapacitors. The nickel and cobalt based oxide, hydroxides, and their composites with conducting polymer are promising and highly appreciated electrode materials for supercapacitors. This chapter covers the recent advances in supercapacitors supported by nickel, cobalt and conducting polymer based materials and their applications predominantly described in the recent literature. Recent advances are reviewed including new methods of synthesis, nanostructuring, and self-assembly using surfactant and modifiers. This chapter also covered the applications of supercapacitors in powering the light weight, flexible and wearable electronics.
- mixed ternary metal oxides
- energy density
Supercapacitor (SCp) is also known as ultracapacitor. SCP is the advanced electrochemical energy storage device. At present, the lithium ion battery (LiBs), lead acid battery and SCP are the major available energy storage systems. Over the other energy storage systems, the SCP is stands out to be a promising energy storage device with very attractive properties such as high specific capacitance, high power density, moderate energy density, good cyclic stability, low cost, environmental friendly nature, etc. SCP has been utilized in various electrical applications viz. hybrid vehicles, power backup, military services, and portable electronic devices like laptops, mobile phones, roll-up displays, electronic papers, etc. [1, 2, 3].
The performance of SCP is strongly depends on types electrode materials i.e. active material used in supercapacitor. Based on the type of active material used and process energy storage, the SCP can be divided into three main categories, including pseudocapacitors (PCs), electric double-layer capacitors (EDLCs) and hybrid capacitors [4, 5, 6]. In PCs metal oxides, metal hydroxides and conducting polymers are employed as active material. On the other hand, carbon base materials such carbon nanotubes, graphene and carbon black, etc. are used as active material in EDLCs employed. Likewise, the used of combination of metal oxide, conducting polymer and carbon based material as active material results hybrid capacitor. The PCs delivered high specific capacitance, high energy density than EDLCs, but demonstrates poor power density and cycle stability. Nevertheless, owing to the high active surface area, the EDLCs delivered high specific capacitance but suffer poor energy density than PCs . The hybrid capacitors retain the advantage of both PCs and EDLCs and hence they delivered high specific capacitance, high energy density, and large cycle life [7, 8]. Moreover, the performance of supercapacitors is equally rely on different aspect of active material viz. quality, electric conductivity, material, size, porosity, synthesis method, etc. More specifically, the synthesis method can bring many attractive advantages in active material for extraordinary electrochemical performance of supercapacitor. Therefore, synthesis of active materials with high porosity, stable performance and good electrical conductivity has a very wide research potential.
Recently, to enhance the energy density, cycle life and electrochemical performance of the supercapacitors, the use of electrode material with desired structure with uniform porosity is one of the appealing strategies. From many decades, the nanostructured single transition metal oxides such as RuO2 , MnO2 , CeO2 , Fe2O3 , Fe3O4 , Co3O4,, Mn3O4,, etc., and the nanostructured mixed ternary metal oxide (TMOs) such as ZnFe2O4, NiFe2O4 CuFe2O4, CoFe2O4, MnCo2O4 ZnCo2O4, NiCo2O4, and etc., and conducting polymers such as, polyaniline (PANi), polypyrrole (Ppy), polythiophene (Pth), etc. has been extensively used as active material for all types of supercapacitors. Out of the different materials used as the electrodes for SCp applications. The TMOs are highly studied and excessively used as active material in all types of supercapacitors. The mixed TMOs are also called the spinel metal oxide. The spinel TMOs have the general formula AB2O4. In AB2O4, the cubic crystal structure of TMOs consists of closely packed O2− anions and an octahedral and tetrahedral space of the lattice occupied by the transition metal cations A and B, respectively. Due to this closed packed structure, the mixed TMOs show the extraordinary characteristics over single metal oxides, such as two order higher electrical conductivity, superior electrochemical performance and excellent stability over single metal oxides. Moreover, the recent research reports shows the TMOs have better structural advantages and higher surface area and porosity . More specifically, the TMOs show low cost, natural abundance, low toxicity and environmental friendly nature. Hence, TMOs have drowned more research attention in recent years. In addition, the extraordinary electrochemical performance of TMOs in solid as well liquid electrolyte makes it a promising and potential candidate as electrode material for PCs. The various mixed TMOs viz. ZnFe2O4, NiFe2O4 CuFe2O4, CoFe2O4, MnCo2O4 ZnCo2O4, NiCo2O4, etc. has been utilized as electrode material for PCs. Out of the different TMOs the nickel and cobalt based TMOs have gained more research attention as electrode material in supercapacitors due to their attracting properties such as low cost, natural abundance, low toxicity and environmental friendly nature. More specifically, these materials show variable structures, diverse morphologies, high specific surface and uniform porosity and outstanding electrical conductivity. The NiCo2O4 demonstrated high electrical conductivity due to the presence of Ni in it. Whereas, Co enhances the electrochemical activity of oxides, further, the synergistic effect among Ni in Co offers high electrical conductivity with an excellent electrochemical behavior in supercapacitors . The NiCo2O4 demonstrated a high theoretical capacity . The nickel and cobalt based TMOs show diverse morphologies, this includes various nanostructures ranging from 0 to 3 D architectures viz. quantum dots, nanowires, nanosheets, platelets like nanoparticles, porous network like framework, coral- like porous crystals, ordered mesoporous particles, urchin-like microstructures and urchin-like nanostructures. Till date many recent attractive reviews have presented recent development in mixed TMOs as electrode material for SCs [7, 16, 17, 18, 19]. We recommend few of them for readers who are new to this field of energy research.
In the present chapter we provide the recent advancements in synthesis of nanostructured nickel and cobalt base mixed TMOs and their composites with conducting polymer based materials as electrode material for supercapacitors predominantly described in the recent literature. Moreover, here our special emphasis will be on new methods of synthesis, nanostructuring, and self-assembly using surfactant and modifiers. In addition, we provide a summary of structural and morphological advancements regarding the electrochemical properties of supercapacitors. Finally, we link our discussion to the recent applications in powering the light weight, flexible and wearable electronics real world applications.
2. Synthesis of nickel and cobalt base mixed TMOs
Compared to the micro sized the nanostructured cobalt and nickel based TMOs show higher specific capacitance and long cycle life. Therefore many recent research strategies have drowned to synthesized nanosize mixed cobalt and nickel based TMOs. The various synthesis methods for synthesis of nanostructured cobalt and nickel based mixed TMOs viz., hydrothermal method sol–gel, thermal evaporation method, chemical bath deposition, electrodeposition, oil/water interfacial self-assembly strategy, etc. have been extensively reported in literature. Hydrothermal method is one of the excessively adopted synthesis methods for hierarchical nanostructure synthesis. This method is cost effective, simple, and easy to scale-up at room temperature. This method is mostly used for fine tuning the morphology and controlling the size of nanostructures. Agglomeration of NiCo2O4 results in low electrical conductivity and decreases the specific capacitance and cycle life of SCp . Therefore, to enhance the electrical conductivity the use of high surface area with high porosity conductive substrates are highly recommended. These substrates enhance the contact between electrode and electrolyte and allowed more electrolyte ions penetration in active material. The various conductive substrates such as textiles, sponges, carbon clothes, carbon fibers, conventional paper, cables, etc. are used as substrates to fabricate SCs. Such conductive substrates are advantageous for enhancing the electrochemical performance via providing short diffusion path, high electrical conductivity, ample electroactive sites . In this regard, Yang
The morphology of NiCo2O4 is reported to be nanoflower-like. The fabricated 3D electrode provides fast ions and electrons transfer rate and enhances the electroactive surface area of the NiCo2O4 via forming a complex 3D network. In addition, the nickel foam as substrate adds the electric conductivity whereas the gelatin based carbon on the nickel foam provides high surface area for uniform growth of NiCo2O4 during synthesis. In our previous study, we have reported the synthesis of nanostructured NiCo2O4 via surfactant assisted hydrothermal method and studied the effect surfactant and reaction parameters on the morphology of nanostructured NiCo2O4. From this synthesis, we got two distinct morphologies viz. platelet-like and nanorod-like using surfactants TEA ethoxylate and polyethylene glycol (PEG), respectively. We further used this nanostructured NiCo2O4 for SCp applications .
In addition to the hydrothermal method, the combustion method is one of the simple and easy to scalable synthesis methods. Over the hydrothermal method the combustion method does not requires Teflon-lined stainless steel autoclaves and centrifuge for product washing, is less time consuming and provides high phase purities. This regard, Kumar
For enhancing the surface area and porosity and electrochemical activities of NiCo2O4, the formation of composites of NiCo2O4 with carbon based material is one of the appealing strategies, for example, NiCo2O4/CNT, NiCo2O4/MWCNT, NiCo2O4/graphene, NiCo2O4/reduced graphene oxides (r-GO), etc. demonstrated to be a potential candidates for SCp applications. Carbon base material viz. CNT, MWCNT, graphene, r-GO, etc. provide excellent flexibility, high specific surface areas, remarkable electrical conductivity, good thermal and chemical stability [5, 25, 26, 27]. For example, Li
3. Applications of Ni and Co based metal oxides and their composites
3.1 Pseudocapacitor (PCs)
Recently, PCs received considerable attention due to the one order higher capacitance, higher volumetric capacitance, higher energy density and use of low cost and easily synthesized active material than EDLCs. [28, 29, 30]. For example, Eskandari
|Sr. No.||Material||Method of synthesis||Electrolyte||Voltage window (V)||Specific capacitance (Fg−1) at current density-scan rate||Energy density Whkg−1||Retention of capacitance at (current density) (cycle numbers)||Reference|
|1.||NiCo2O4||Hydrothermal||1 M Na2SO4||0–0.6 V||479/ 5 mVs−1||21.3||87.21% (5000)|||
|2.||NiCo2O4||Hydrothermal||1 M Na2SO4||0–0.4 V||320 0.1 mVs−1||16.1||95.34%, (1000)|||
|3.||NiCo2O4 /NiCo2S4||Molecular design||—||0.1–0.6 V||1296 1 Ag−1||44.8||93.2%|
|4.||NiCo2O4||Hydrothermal||3 M KOH||0.0–0.6 V||3143|
|5.||NiCo2O4 @α-Co(OH)2 nanowires||Hydrothermal||2 M KOH||−0.2 -0.5 V||1298-1 Ag−1||39.7||83%|
|6.||Mesoporous NiCo2O4 nano-needles||Hydrothermal||—||0.0–0.5 V||1410 Fg−1-1 Ag−1||94.7% (3000)|
|7.||NiCo2O4 nanosheets||Solvothermal||—||—||2690 Fg−1||52.6||80.9% (3,000)|
20 mA cm−2.
|8.||NiCo2O4 PANi||Hydrothermal and in-situ polymerization||6 M KOH||0–0.5 V||3108|
|9.||NiCo2O4 nanoneedles||Hydrothermal via annealing approach||1 M KOH||0–0.7 V||1076 0.5 Ag−1||30.5||14%|
|10.||NiCo2O4 nanoneedles||Pulsed laser ablation||3 M KOH||0–0.6 V||1650 Fg−1|
3.2 Hybrid capacitors
Even if the Ni and Co based TMOs are advantageous for SCp applications, however, in the long cycling process the rapid degradation of NiCo2O4 electrode materials is the major obstacle among the commercialization of NiCo2O4 based SCp. By increasing the electrical conductivity of NiCo2O4 this hurdle can be minimized and the higher rate capabilities can be attained. Therefore, from the last two decades, researchers devoted more efforts to enhance the electrical conductivity of NiCo2O4, this includes fabrication of hybrid composite with other conducting electrode materials, viz. carbon based material (CNts, SWCNts, MWCNts, activated carbon, doped and undoped reduced graphene oxides, etc.), conducting polymers, etc. In addition, recent formation of composite of NiCo2O4 with other mixed TMOs has gain enormous attention. For example, Mary
|Sr.No.||Material||Method of synthesis||Electrolyte||Voltage window (V)||Specific capacitance (Fg−1) at current density/ scan rate||Energy density Whkg−1||Retention of capacitance (cycle numbers) at current density||Reference|
|NiCo2O4 CNT||Hydrothermal||2 M KOH||−0.1- 0.5||574.3 0.5 Ag−1||—||111.5% (1000)|||
|2||NiCo2O4 @MWCNT||Hydrothermal||0.5 M K2SO4||—||374|
|3||3D NiCo2O4 /MWCNT||Sol–gel||2 M KOH||0–0.5 V||1010 0.1 Ag−1||37.7||83.4% (2000)|
|4||Ordered Mesoporous Carbon/NiCo2O4||co-precipitation||6 mol·|
|0–0.6 V||577.0 1 Ag−1||—||92.7%. (2000)|
|5||NiCo2O4- nanoporous carbon.||Chemical||1 M KOH||−0.2-0.6 V||89 0.1 - Ag−1||28||85%|
|6||Mesoporous carbon - NiCo2O4||hydrothermal|
followed by calcination
|3 M KOH||−0.45-0.45||204.28 1 - Ag− 1||5.75||90.35% (3000) 20 Ag−1|||
|7||Hallow bamboo-shaped NiCo2O4||Template||6 M KOH||0.0–0.6 V||680.1C g−1|
|8.||rGO- NiCo2O4 quantum dots||Chemical||1 M Na2SO4||0.0–1.6 V||265 0.73 Ag−1||47||69% (1000)|||
|9.||Oxygen-vacancy-rich NiCo2O4/nitrogen-deficient graphitic carbon nitride hybrids||Chemical||6 M KOH||0.0–0.6 V||1998|
|10||NiCo2O4@Ppy/CC||Hydrothermal||2 M KOH||0.0–0.5 V||155.4 mAh g−1 1 mA cm−1||22.3||71%|
(8000) 10 mA cm−2
3.3 Asymmetric capacitors
The symmetrical SCp limits their specific capacitance due to narrow potential windows. Moreover, the use of aqueous base liquid electrolyte in symmetrical SCp decreases the specific capacitance energy density and cycle life. To overcome such drawback the fabrication of SCp with two different kinds of active material based electrode is demonstrated to be an effective strategy. The SCp fabricated using two different electrodes is termed an asymmetric supercapacitor. In asymmetric SCp positive electrode is fabricated using metal oxide base material, while negative electrode is fabricated by carbon based material. The combination of different active materials in a single device with higher operating potential result in higher the specific capacitance and energy density . However, aqueous-based symmetric supercapacitors suffer from narrow potential windows, due to the limitation of the water decomposition. Therefore, an effective way is to construct asymmetric supercapacitor, which consists of two kinds of electrode materials, for instance positive electrode having pseudocapacitive nature and negative electrode having electric double layer capacitance with higher operating potential, for obtaining higher energy density [19, 20]. In the case of positive electrode materials, transition metal oxide based nanoparticles, conducting polymers based materials have been widely utilized, which exhibits pseudocapacitance as well as reversible redox Faradaic reaction. As negative electrode materials, carbon based materials like carbon nanotubes (CNT), graphene oxide (GO), activated carbon, and mesoporous carbon materials displaying electric double layer capacitance have been used. Among the carbon allotropes, mesoporous carbons have been extensively used as negative electrode material due to its high surface area and good electrical conductivity. For more understanding the recent advancements in NiCo2O4 and their composites and their performance in asymmetric SCp are summarized in Table 3.
|Sr. No.||Material||Method of synthesis||Electrolyte||Voltage window (V)||Specific capacitance (Fg−1) at current density/ scan rate||Energy density Whkg−1||Retention of capacitance (cycle numbers) at current density||Reference|
|NiCo2O4/C composites // activated carbon (AC)||Hydrothermal||6 M KOH||0–1.4 V||995.2|
|2||NiCo2O4 quantum dots // reduced graphene oxide (rGO)||Hydrothermal||1 M Na2SO4||0–2.4 V||362, 0.5 Ag−1.||69.5||86% (1000)|||
|3||NiCo2O4 //AC||Hydrothermal||—||0–0.5 V||153.2|
|4||NiCo2O4/ gelatin-based carbonenickel foam //AC||Hydrothermal||0–0.5 V||1416|
|5||NiCo2O4@Ni foam// sugar-derived carbon (SC)||Combustion||6 M KOH||0–0.5 V||169, 1.5 Ag−1||48||96.5%|
|6.||MWCNTs intermingled NiCo2O4// Cu2WS4||Hydrothermal||3 M KOH||0–0.5 V||116 mAh g−1,|
|7.||NiCo2O4 nanoparticles and nanowires||Hydrothermal and wet chemical||1 M Na2SO4||0–1.6 V||1066.03 Ag−1||59.56||77%|
(5000) 6.66 Ag−1
|8||NiCo2O4- carbon nanofiber||—||2 M KOH||—||991.96 5 Ag−1||37.23||97.02% (3000) 30 Ag−1|||
|9||NiCo2O4/ r-GO||Hydrothermal||1 M KOH||0.0–1.6 V||702|
4 mA cm−1
|10||NiCo2O4/ rGO||Co-precipitation||1 M|
|0.0–0.6 V||1380 1 Ag−1||—||90% (1000) 5 Ag−1 after|||
4. Conclusions and outlooks
With ever increasing energy demands, day by day the SCp gaining much interest as an energy storage device. From a many years, the nickel and cobalt based TMOs and their composites have been studied and successively employed as an active material in all types of SCp. The nanostructured NiCo2O4 is low cost, in abundance, environmentally friendly in nature and has high electrical conductivity. In addition, due to the enhanced mobility of charge carriers the nanostructured NiCo2O4 demonstrated to be higher electrochemical performance than the single metal oxides. In this regard, the recent advances in synthesis of pristine NiCo2O4 and their composites with diverse morphologies and their applications for electrochemical performance in all types of SCp have been summarized in this chapter. Out of the different synthesized methods used for synthesis nanostructured NiCo2O4, the hydrothermal method is found to be excessively used. Moreover, the hydrothermal method is demonstrated to be more advantageous for the synthesis of diverse morphologies ranging from 0 D to 3 D, and resulted in high specific surface area and uniform porosity. The pristine nanostructured NiCo2O4 has many limitations for its commercial supercapacitor applications. Therefore, advanced strategies like synthesis of hierarchical nanostructures of NiCo2O4 and the fabrication of composite with other mixed TMOs, carbon based material and conducting polymers can enhance the specific capacitance, energy density and rate capability of Ni and Co based supercapacitors.
Conflict of interest
The authors declare no conflict of interest.