Effect of Anodic Deposition Parameters on Electrochemical Behavior and Microstructure of Mn-Ni Oxide As a Pseudocapacitive Electrode

Hybrid supercapacitors, or briefly pseudocapacitors, are an emerging class of energy storage devices, with the capability of providing both high power and energy density. In contrast to conventional electric double layer capacitors, which accumulate charge mostly through an electrostatic mechanism, pseudocapacitors utilize the pseudocapacitance arising from reversible Faradic reactions occurring at the electrode surface [1]. Although various metal oxides have been investigated as active material for pseudocapacitors, greater attention has been given to manganese oxides. In recent years, also Mn-based binary oxides have been studied for pseudocapacitance, in particular, mixed oxides comprising Ni or Co as the other component [2-4]

In the present work, Mn-Ni oxide thin films were deposited potentiodynamically at scan rate of 100 mVs^-1, pH=7 and room temperature, on a stainless steel substrate. Undoubtedly, a survey of the literature shows that a systematic study on the effect of operating parameters on the properties of the mixed oxide thin films is still lacking. Accordingly, this work was undertaken with the aim of exploring the processing space for these oxides studying in particular the effects of the electrolyte Ni to Mn molar ratio, the deposition peak potential and the deposited mass on chemical composition, microstructure and capacitance behavior of Mn-Ni oxide thin films.

The binary oxide deposits were characterized by energy dispersive X-ray spectroscopy (EDX) and scanning electron microscopy (SEM). The electrochemical behavior of the Mn-Ni oxide electrodes was studied by cyclic voltammetry (CV) at 5, 20, 50 and 100 mVs^-1 and electrochemical impedance spectroscopy (EIS) in 1 M Na₂SO₄electrolyte.

Expectedly, with increasing the Ni to Mn molar ratio in solution (from 0.1 to 10), the molar fraction of Ni in the oxide film was found to increase, resulting, on the one hand, in smoothing of the surface morphology and, on the other hand, in a change of the specific capacitance through a maximum of 145 Fg^-1 at approximately 12% at Ni fraction and CV scan rate of 20 mVs^-1. With further increase of the Ni fraction in the oxide to about 19% at, the specific capacitance started to decrease gradually. Changing the peak potential in the potentiodynamic deposition from +1.2 to +1.6 V vs. Ag/AgCl, the CV curves revealed a box-like shape for all values of the peak potential, while the capacitance showed a decreasing trend. EIS results revealed that charge transfer resistance increased with increasing peak potential and the lowest R_ct was found for sample prepared at peak potential of 1.2 V vs. Ag/AgCl. The mass load per unit area of the film was controlled by the number of cycles. Finally, specific capacitance of 196 Fg^-1 was obtained at scan rate of 5 mVs^-1 for the Mn-Ni oxide film deposited potentiodynamically (5 cycles, 100 mVs^-1, pH=7 and RT) in solution with Ni to Mn molar ratio of 2. At the high scan rate of 100 mVs^-1, the specific capacitance was 135 Fg^-1 showing good retention of the capacitance at high rate.

References

[1] B.E. Conway, Electrochemical Supercapacitors, Kluwer Academic/Plenum Press, New York (1999).

[2] Kalakodimi Rajendra Prasad, Norio Miura, Electrochemically synthesized MnO₂-based mixed oxides for high performance redox supercapacitors, Electrochemistry Communications 6 (2004) 1004–1008.

[3] Banafsheh Babakhani, Douglas G. Ivey, Investigation of electrochemical behavior of Mn–Co doped oxide electrodes for electrochemical capacitors, Electrochimica Acta 56 (2011) 4753–4762.

[4] Rusi, S.R. Majid, High performance super-capacitive behaviour of deposited manganese oxide/nickel oxide binary electrode system, Electrochimica Acta 138 (2014) 1–8.