An Investigation into the Deposition Mechanism and Capacitive Behaviour of Thin Films of Manganese Oxides Deposited from KMnO4

Manganese oxides (MnO₂, MnOOH) are promising materials for electrochemical capacitors due to their low cost and toxicity, coupled with their attractive energy storage properties, and are hence well represented in the literature [1].  Thin films of MnO₂ are capable of achieving high values for gravimetric specific capacitance (>2000 F/g) [2].  Material activity, as well as material morphology play an important role in determining the capacitive behaviour of a material [3].  It is important to understand the electrodeposition mechanism of MnO₂from a variety of starting reagents, so that the morphology and hence capacitance of thin films can be intelligently designed.

In this presentation, the electrodeposition mechanisms possible from KMnO₄and the effect these different mechanisms have on the activity of the resultant material will be discussed.  The electrodeposition mechanisms have been elucidated from data collected on the rotating ring disc electrode (RRDE) and the electronic quartz crystal microbalance.  The morphology of the resultant deposit was investigated using a scanning electron microscope (SEM).  The capacitance of the materials has been investigated using cyclic voltammetry on the EQCM, coupled with SPECS experiments on a platinum disc electrode.

Linear sweep voltammetry and RDE experiments in solutions containing between 0.001 and 0.1 M KMnO₄ with a supporting electrolyte of 1 M Na₂SO₄show an inverse relationship between the concentration of the solution and the current seen at the disc.  The current obtained at the ring for the lowest concentration solution is half that of the solutions with concentrations 10x and 100x higher.  This suggests that most of the soluble species produced in the higher concentration solutions are retained at the surface.  This is supported by results obtained on the EQCM.  There is a peak in all of the data obtained at ~-0.2 V.  From these data, we propose that three mechanisms are occurring, each at different potentials.

The first mechanism for electrodeposition of MnO₂ proposed is the formation of Mn²⁺ at the disc, which subsequently reacts with the MnO₄^- ions in solution to form MnO₂:

MnO4- + 8 H+ + 5 e- → Mn2+ + 4 H2O	(1)
3 Mn2+ + 2 MnO4- + 2 H2O → 5 MnO2 + 4H+	(2)

A peak at ~0.2 V has been attributed to the further reduction of this deposited product to form MnOOH, which can be reoxidised by MnO₄^- to form MnO₂in a sort of chemical looping process, i.e.,

MnO2 + H+ + e- → MnOOH	(3)
3 MnOOH + MnO4- + → 4 MnO2 + H2O + OH-	(4)

As there is continued collection at the ring throughout the course of the experiment, a third possible mechanism has been proposed, where either solid MnO₂ or MnOOH is further reduced to form soluble Mn²⁺, which then reacts with MnO₄^-in solution; another chemical looping style process; i.e.,

MnO2 + 4 H+ + 2 e- → Mn2+ + 2 H2O	(5)
MnOOH + 3 H+ + e- → Mn2+ + 2 H2O	(6)
3 Mn2+ + 2 MnO4- + 2 H2O → 5 MnO2 + 4 H+	(7)

SEM images show a sort of bladed and rippled structure, with what appears to be a high macroscopic surface area.  Other materials exhibit MnOOH urchins and finely bladed MnOOH tips overlay a bumpy MnO₂ substrate.  These have been determined to be MnOOH purely by way of their apparent conductivity, as indicated by their charging behaviour, given MnOOH is less conductive than MnO₂.

Bibliography

1. Wei, W., et al., Manganese oxide-based materials as electrochemical supercapacitor electrodes.Chemical Society Reviews, 2011(40): p. 1697 - 1721.

2. Andrew Cross, A.M., Ariana Cormie, Tony Hollenkamp, Scott Donne, Enhanced Manganese Dioxide Supercapacitor Electrodes Produced by Electrodeposition. Journal of Power Sources, 2011. 196: p. 7847 - 7853.

3. Conway, B.E., Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications. 1999, New York: Springer.