Electrodeposited Manganese Oxide for Electrochemical Capacitor: Effect of Substrates on the Capacitive Behavior of Electrodes

Electrochemical capacitor (EC), also named as supercapacitor or ultracapacitor have been known as one of the potential energy storage systems. In recent years, many research focus on two modes of electrochemical capacitor: the pseudocapacitor and the electrochemical double layer capacitor [1]. The types of electrode materials are carbon, transition metal oxides and conducting polymers. Manganese oxides have attracted increasing interest in the field of active electrode material for ECs due to their low cost, abundance, environmental friendly nature and potentially replacing ruthenium oxide in the future [2]. However, these materials exhibit the inherent disadvantage of low conductivity and low specific capacitance. The improvement pursed in manganese oxides mainly concerns electronic conductivity, the specific capacitance and rate capability. Several methods have been developed for this improvement, consisting of nanostrutured material, deposited thin film, Ni or Co doped materials and composite material (with carbon, other metal oxides such FeO_x, NiO…) [2-4]. In order to improve its electrochemical properties (cycling stability, charge/discharge capacity), all the factors related to the method synthesis such as stoichiometry, crystal structure and morphology of the active materials need to be wholely investigated. 

In this work, thin films of manganese oxide were synthesized by electrochemical deposition on three substrates such as: graphite (EMD), modified graphite by two layers of hydrophobic layer and hydrophilic layer (EMD2L) and stainless steel (EMDSS). The initial solution was MnSO₄ aqueous solution and the electrodeposition were carried by using Cyclic Voltammetry (CV) and Chronoamperometry (CA).

The results shows that the specific capacitances (C_sp) of the prepared electrodes were determined by cyclic voltammetry measured in 1 M Na₂SO₄ at different potential scan rates: 1, 5, 10, 25 and 50 mV/s. The deposited film on graphite substrate exhibited C_sp of 250 – 99 F/g. The hydrophilic layer improves the specific capacitance of the modified electrode that was about 405 – 108 F/g. By contrast, thin film on staineless steel shows the C_sp value between 258 -102 F/g. The highest value of EMD2L may be attributed to the porous surface of the hydrophilic layer modified electrode providing a larger surface area, enhancing the interface between the aqueous electrolyte and the electrode materials. Electrochemical impedance spectroscopy (EIS) measurements were also carried 100 kHz-0.1 Hz at open circuit potential (OCP) with an AC perturbation of 10 mV. With graphite and modified graphite substrate, EIS shows the charge transfer resistance (R_ct) about 52 mΩ and 144 mΩ. With stainless steel,  manganese oxide films were deposited by CV in changing time deposition. The EIS spectra of these films shows R_ct from 20 to 40 Ω corresponding to the variation of time deposition (5, 7, 9, 11, 13 min, respectively). 

The C_sp of EMD 2L obtained values of 51 F.g^-1, 74 F.g^-1, 118 F.g^-1, 218 F.g^-1 with good stability upon cycling. This result can be explained by the hydrophilic property of electrode which improved the ionic transport in aqueous electrolyte. And high porosity of electrodeposited surface increased the conduction pathway of deposited MnO₂.

Keywords:electrochemical capacitor, EMD, electrocatalyst

Acknowledgement:

The authors gratefully acknowledge Office of Naval Research Global (ONRG) and VNU-HCM for financial support throught grants N62909-13-1-N235 and HS2013-76-01.

References:               

[1] J. R. Miller, P. Simon, Science, 321 (2008).

[2] W. Wei, X. Cui, W. Chen, D. G. Ivey, Chem. Soc. Rev. 40 (2011).

[3] P. Simon, Y. Gogotsi, Nature Materials, 7 (2008).

[4] A. T. Ha, M. L. P. Le, T. P. T. Nguyen, V. M. Tran, ECS Transactions, 45 (20) (2013).