Recently, supercapacitors have been widely studied through all over the world as a new class of energy storage devices to fulfill the need of increasing clean and renewable energy demand. Over many energy storage devices (batteries, commercial capacitors etc) supercapacitors have their remarkable advantages such as fast charge-discharge ability, long cycling stability and comparatively high power density which enable them to be used as a promising energy sources for various portable electric vehicles and electronic gadgets. According to the charge storage mechanism the supercapacitors can be categorized into two distinct groups. One is the electrochemical double layer capacitors (EDLCs) which can store charge by non-faradic process i.e. there is no charge transfer occurs at the interface of electrode material and electrolyte solution and the other mechanism is pseudocapacitors in which charge can be stored within the electrode material by the redox reaction or Faradic process. In the both charge storage mechanisms, surface of the electrode material takes a leading role. Porous, interconnected, conducting materials with high surface to volume ratio such as nanocrystal can offer high specific capacitance value and can act as high performance supercapacitors. Generally, carbon based materials like carbon nano tube (CNT), carbon aerogel, activated carbon etc. are used as electrode materials for EDLCs whereas various metal oxides, conducting polymers are used as materials for pseudocapacitor electrode^1,2. From last few decades’ various metal oxides (RuO₂, NiO, Fe₂O₃, MnO₂, V₂O₅, Co₂O₃ etc) have drawn considerable attention as promising electrode materials due to their excellent theoretical capacitance values. But their low electrical conductivity and poor stability are the main problem which are still yet to be resolved. Thus, people are working to search new advance electrode materials to improve the low electrochemical conductivity as well as poor cycling stability. It was observed that many binary and ternary metal oxides and combination of metal oxides with Carbon based materials or conducting polymers can improve electrochemical performance. Among many binary and ternary metal oxides Ni-M-oxide (M= Mn, Co, Fe, Zn etc.) have attracted great attentions due to their natural abundance, low cost, easy synthesis technique, environmental friendliness and excellent electrochemical performance.

In this work, we have synthesized Ni-Mn-Oxides with different molar ratios (Ni:Mn= 1:1, 1:2, 1:3 and 1:4) using easy sol-gel route and studied the overall electrochemical properties of these electrode materials. We have achieved a high specific capacitance value (1215 Fg^-1 at scan rate 2mVs^-1) along with low charge transfer resistance and long cycle life for the composite having Ni:Mn =1:3. This composite shows high surface area (127 m² g^-1) and large pore size (8.2 nm) distribution along with high electrical conductivity which can provide easy path for charge transport.

RESULTS AND DISCUSSION

Nanostructure studies of Ni-Mn-Oxides with different molar ratios (Ni:Mn = 1:1, 1:2, 1:3 and 1:4) were performed using XRD and FTIR spectroscopy. The morphological and surface areas with pore size studies have also been performed by field emission scanning electron microscopy (FESEM) and Brunauer–Emmett–Teller (BET) method. The electrochemical performance like CV, galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) of all electrode materials were carried out using three electrodes set up in 1 M Na₂SO₄ solution. The specific capacitance (C_m) for all electrode materials are calculated from the CV profiles, following the equation C_m = i/ 2mν, where m and ν are the mass of the electroactive material and potential scan rate and current (i) is obtained by integrating the area of the curves as it is defined by eq.(1), i=∫i(V)dv/(V_c -V_a) , where v_a and v_c are the lowest and highest voltage of the potential range. The electrochemical studies were shown in fig. 1. It was observed that the surface area as well as specific capacitance values of the materials increase with increase of Mn molar ratio upto (Ni:Mn= 1:3) and next decreased with increase of Mn. Morphological changes also observed with change of Fe-molar ratio. The maximum specific capacitance 1215 Fg^-1 is achieved in 1M Na₂SO₄ solution in 2mVs^-1 scan rate with molar ratio of Ni:Mn= 1:3.

CONCLUSION

The synthesized Ni-Mn-Oxides exhibit excellent electrochemical performance with high surface area and good stability. The overall studies on the electrochemical properties of the materials suggest that it can be used as energy storage application in future.

ACKNOWLEDGMENTS

Author A. Ray wish to thank Department of Science and Technology (D.S.T), INSPIRE, Govt. of India for financial support.

REFERENCES

1. S. Periyasamy et al., ACS Appl. Mater. Interfaces, 8, 12176–12185 (2016).
2. B. E. Conway, J. Electrochem. Soc., 138, 1539 (1991) http://jes.ecsdl.org/cgi/doi/10.1149/1.2085829.