The Cycling Behaviour of MnO2 as Investigated By Synchrotron XRD, Stepped Potential Electrochemical Spectroscopy (SPECS) and Cycling on the Electrochemical Quartz Crystal Microbalance (EQCM)

Manganese oxides are promising materials for supercapacitor applications due to their low cost and relatively low toxicity, coupled with their high electrochemical activity [1].  MnO₂ comes in many structural forms, with two main structural types to be discussed in this presentation.  The first being the layer structure which is favoured by many researchers as it is believed to be able to undergo repeated cycling. Layer structures involve the arrangement of component [MnO₆] octahedra such that they form a coherent flat plane.  These flat planes are separated by charged species, with the number being dependent on the average oxidation state of the Mn in the layer.  Birnessite is a layered form of MnO₂ with either sodium, potassium or water in the interlayer spacing.  Tunnel structures involve the arrangement of component [MnO₆] octahedra in such a manner as to generate tunnels that run in one direction through the structure, with width and height equal to between one and three octahedra; the width and height do not need to be equal.  Ramsdellite and Pyrolusite are both tunnel structures, with tunnel dimensions being 2 x 1 and 1 x 1 respectively [2]. 

One possible problem with the long term utilisation of pseudo capacitors, such as metal oxides, is that they can undergo structural change during cycling.  This presentation will cover the cycling behaviour of Birnessite, Ramsdellite and Pyrolusite as examined by a number of different techniques.  SPECS experiments performed in-situ at the Australian Synchrotron to map structural changes with respect to the state of charge of a manganese oxide material.  In addition, data obtained on the mass changes during SPECS cycling of the three phases on the EQCM will be presented.  The mass change, when corroborated with the amount of charge passed, gives information about the type of ions being inserted into the structure.  In the case of these experiments, % values of K⁺ and H⁺ insertion have been obtained.  In addition, values for retention of these ions during repeated cycling have been elucidated, which will provide valuable information for researchers engaging in the intelligent design of electrochemical capacitors utilising MnO₂.

References

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

2. Post, J.E., Manganese oxide minerals: crystal structures and economic and environmental significance. Proceedings of the National Academy of Science, USA, 1999: p. 3477.