J.N. Broughton, M.J. Brett / Electrochimica Acta 50 (2005) 4814–4819
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chemistry approaches [9–19]. 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 inter-
est 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 elec-
trodeposition. 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 ex-
ploring 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.
deionized water and the various reagents as received. The
counter electrode (CE) is a 10 turn coil of platinum wire and
the reference electrode (RE) is an Ag/AgCl electrode. We
have used linear sweep voltammetry (LSV) to characterize
the changes in behaviour between the different admixtures
of deposition solutions. Depositions for capacitance testing
were conducted in galvanostatic mode using a constant cur-
rent of 1 mA. After the deposition is complete the electrolyte
is exchanged for 1 M Na2SO4 and the film is ramped from the
open circuit voltage (OCV) to 1.0 V for 5 min to provide for
complete oxidation of the deposited material. Potentiostatic
depositions are conducted by preparing a fresh substrate and
ramping from the OCV to 1 V at 10 mV/s and then holding
for the requisite time period. The same electrolyte exchange
is made after deposition but no extra oxidation step is used
for the potentiostatic experiments. Cyclic voltammetry (CV)
was conducted at a sweep rate of 10 mV/s. The O-ring used to
clamp the samples defines the working electrode (WE) area.
For these experiments we used a 1.3 cm (i.d.) O-ring which
2
presents an area of 1.27 cm working electrode (WE) area.
The measurement of deposited film mass was conducted us-
ing a Sartorius MC5 microbalance. For the post-deposition
mass measurements the samples are rinsed gently in 18 Mꢀ
deionized water and then dried in an N2 flushed oven at 100 C
for 5 min. The film is then stripped with nitric acid to deter-
mine the net mass of the active material. Film morphology
was studied with a Hitachi field emission scanning electron
microscope (FE-SEM).
Our experimental approach was to start from stock so-
lutions of MnSO4, Na2SO4, NaAc and the other mixtures
described in the results section. We initially experimented
with a variety of additives to look for novel behaviours sim-
ilar to the acetate effect of lower deposition potentials. The
list of additives which we explored included: tartrate, phos-
phate, sulfate, chloride, lactate, carbonate, bicarbonate and
various acetates. Our observation was that none of these ad-
ditives had an effect that was comparable to that of the acetate
ion which could be observed with a variety of acetate salts.
Since the acetate salts showed the interesting behaviour we
have focused on them for this work. Our exploration of the
acetate additive is presented in two basic thrusts. First, we
examined the effect of varying the acetate additive content
on the deposition and capacitance of the MnO2. Secondly,
we looked at the effect of varying the deposition conditions
using galvanostatic versus potentiostatic techniques.
2
. 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 electrochemical measurements. Electrical contact
was made directly with the platinum using a simple spring
clip. We have also used graphite substrate material (Poco
DFP-1) for thicker deposition experiments to compare results
to the depositions onto platinum. The graphite was scoured
using fine grit SiC sandpaper and then cleaned with deion-
ized water. Deposition solutions were prepared with 18 Mꢀ
3. 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