Elsevier

Electrochimica Acta

Volume 50, Issue 24, 30 August 2005, Pages 4814-4819
Electrochimica Acta

Variations in MnO2 electrodeposition for electrochemical capacitors

https://doi.org/10.1016/j.electacta.2005.03.006Get rights and content

Abstract

Results are presented from an electrochemical investigation into properties of hydrous MnO2 grown by electrodeposition from aqueous solution. MnO2 grown from MnSO4 solutions mixed with acetate based electrolytes has been studied for electrochemical capacitor applications. The addition of acetates to the electrodeposition solution permits a controllable reduction in the deposition potential from roughly 0.95 to 0.55 V under galvanostatic conditions. We can observe some morphological changes in the material under SEM examination when different acetate solutions are used but the capacitance appears to be insensitive to this variable. We have grown material under both potentiostatic and galvanostatic conditions for comparison. We have observed a consistent insensitivity in specific capacitance for material deposited under potentiostatic conditions at 1 V in comparison to galvanostatic depositions occurring in the 0.55 V range. We have also observed a consistent decrease in specific capacitance from 260 to 50 F/g as material thickness increases.

Introduction

Electrochemical capacitors [1] are very interesting as charge storage devices for electrical energy due to their ability to deliver high power and survive high cycle counts. The range of potential practical applications extends from cellphones and other types of personal electronics to hybrid vehicles. The application in hybrid vehicles is particularly interesting because successful development of cost effective charge storage could have a beneficial impact on oil consumption patterns and help to mitigate their contribution to climate change. In a broader sense, any application where load levelling of electrical power or rapid charge/discharge is needed could be addressed by electrochemical capacitors.

Electrochemical capacitance is observed in two forms. The first is based on the formation of electrical double layers (EDL) in high surface area materials such as activated carbons which have surface areas in excess of 1000 m2/g. Capacitors based on activated carbon materials have been successfully commercialized using nonaqueous electrolytes to increase the operating potential of the devices. The second form of electrochemical capacitance is based on a faradaic charge exchange mechanism which allows motion of ions and electrons into the electrode material itself. This is called pseudocapacitance because it shows a capacitive behaviour rather than the distinctly peaked redox behaviour normally associated with intercalation. Early reports on the pseudocapacitance of ruthenium oxide has stimulated substantial research into the properties and mechanisms of pseudocapacitive charge storage [2], [3], [4], [5], [6], [7], [8]. RuO2 possesses a specific capacitance of >700 F/g when measured using a 2 mV/s scan rate in sulphuric acid electrolyte [2]. Ruthenium oxide has been developed for military capacitors, but it has a relatively high cost and this will prevent broader commercial applications where cost is an important factor.

This cost factor has motivated many researchers to look at alternative materials which also display this pseudocapacitive behaviour. Manganese, nickel and iron oxides are prominent among the transition metal oxides being studied for pseudocapacitor applications. Numerous reports in the literature can be found based on sol–gel, electrodeposition or other aqueous chemistry approaches [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19]. Manganese oxide is used in a variety of different types of batteries and is less harmful than some compounds used in batteries such as cadmium. Manganese oxide exists in a number of stable valence states and crystal structures which makes it worthwhile to study it's properties through variation in preparation techniques. In 2000, Pang et al. [9], [10] reported a specific capacitance of 700 F/g at 50 mV/s in very thin MnO2 films prepared by sol–gel techniques. These reports have sparked strong interest in MnO2 as a pseudocapacitive material. Hu et al. [12], [13] have shown that electrodeposited MnO2 in thicker layers provides more than 265 F/g. We have shown that 500 nm Mn films deposited by sputtering and anodically oxidized yielded a specific capacitance of 600 F/g [20]. Our intent with this work is to explore some aspects of electrodeposition chemistry of MnO2. The earliest report of using acetates as precursors for MnO2 deposition that we have found is from Tench and Warren [21]. More recently, Chang et al. [22] and Wu et al.[23] have reported on the use of manganese acetate as a source material for electrodeposition of MnO2. Chen et al. [24] have reported on the use of various Mn precursors in the electrodeposition of MnO2 and found that solutions of manganese acetate (MnAc) have an interesting property of reducing the potential at which film deposition occurs. We were interested in this effect and experimented with MnAc solutions as a starting point for our own work in electrodeposition. We found, however, that solutions of MnAc self oxidized within days and became cloudy and brown with MnO2 precipitating out of solution. We subsequently observed that by mixing a solution of MnSO4 and sodium acetate (NaAc) we could obtain a stable solution and provide a variable decrease in the deposition potential depending on the ratio of solutions. We used this as starting point for exploring a variety of admixtures of MnSO4 and various other additive electrolytes to look for similar interesting effects on the electrodeposition of MnO2 and subsequent effects on the pseudocapacitive behaviour of the films deposited.

Section snippets

Experimental

The electrochemical measurements were conducted with a CHI 660a potentiostat/galvanostat. The deposition cell was a glass cylinder with a tightly fitted polypropylene insert in the bottom which has a O-ring sealed hole (1.3 cm inner diameter (i.d.)) and a steel spring to clamp the substrate against the O-ring. Two types of substrates were used for these experiments. Polished silicon wafers coated with titanium as an adhesion layer and then with 300 nm of platinum as a standard base coating for

Results and discussion

Our first series of experiments examined the impact of varying the acetate content by mixing MnSO4, Na2SO4 and NaAc together and changing the ratio of Na2SO4 and NaAc. This experiment consisted of mixing 20 ml of 1 M MnSO4 with 40 ml of mixed 1 M Na2SO4 and NaAc in the ratios shown in Fig. 1. This figure shows the impact of increasing the acetate content on the LSV. As the portion of NaAc is increased, the onset of deposition shifts to lower values of potential. This can be exploited in two ways:

Conclusions

We have investigated the impact of varying cations and anions on the efficiency of depositing MnO2 for the purposes of developing electrochemical capacitors. We found that the acetate ion had a significant and controllable effect on the deposition potential. The addition of NaAc to solutions of MnSO4 allowed the deposition solutions to remain stable for weeks whereas solutions of MnAc alone tended to begin self oxidizing within days. Although the impact of the acetate ion was interesting from

Acknowledgements

We are grateful for funding from Micralyne Inc. and the NSERC, iCORE and MSTRI organizations.

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