Supercapacitors based on Partial pyrolysis of Polyacrylonitrile

1. Supercapacitors based on Partial pyrolysis of Polyacrylonitrile M.Brittin,(1), F.J. Davis,(1), E. Eweka(2) (the late) A. Gilmour,(3), C.O. Giwa(3) G.R. Mitchell (2) and A.G. Ritchie(3) (1) Polymer Science Centre, University of Reading, RG6 6AD, (2) QinetiQ Limited, Haslar, Gosport, Hampshire, PO12 2AU, (3) Lexcel Technology Limited, Henley-on-Thames, Oxon., RG9 1LU 4. Surface Area 1. Introduction 3. Nitrogen content Conducting polymers are versatile materials that offer promise in a range of applications[1]. Previously we have been interested in the electro-chemical production of such materials since the properties may be controlled by control of electrochemical parameters (voltage, charge passed etc.)[2], or in some cases by simply the appropriate choice of counter-ion [3]; more complex properties may be introduced by chemical substitution [4]. Recently, as part of the UK vehicle foresight programme we have been involved in turning our experiences to the development of electrochemical capacitors or super-capacitors These are novel power devices with long cycle life (typically > 100,000 cycles) and energy storage capabilities, which are higher than those of conventional double layer capacitors. Super-capacitors are of particular application in electric vehicles, where they can be used to supply peak loads or to absorb electrical charge, e.g. during regenerative braking. Most super-capacitors use a high surface area carbon with a sulphuric acid electrolyte. Though materials such as polypyrrole-based composites [5] offer some advantages, from a commercial point of view a particularly attractive route to provide high capacitance materials is to use a system based on complexation between nitrogen atoms in nitrogen - containing polymers with sulphur trioxide [6]. [1] D. Kumar and R. C. Sharma European Polymer J., 1998, 34(8),105. [2] F.J. Davis, H. Block, and R.G. Compton, J. Chem. Soc. Chem. Commun., 1984, 890. [3] A. Kassim, H. Block, F.J. Davis, and G.R. Mitchell, J. Mater. Chem.., 1992, 2, 987. [4] P.J. Langley, F.J. Davis, and G.R. Mitchell, J.Chem. Soc, Perkin Transactions, 1997,2229. [5] J.H. Kim, A.K. Sharma, and Y.S. Lee, Materials Letters, 2006, 60, 1697. [6] R. A. A. Gilmour , Non-aqueous electrochemical cell containing conjugated polyimine functionality coupled to sulphur trioxide, Patent no GB2335073, 8th September 1999. CHN ANALYSIS SO3 DOPING To increase the surface area, PAN was pyrolysed in the presence of a series of compounds. These included sodium chloride, sodium carbonate oxalic acid, polyethylene glycol, polyethylene oxide, carbazole, and tetra-methylammonium iodide Relative proportions of carbon, nitrogen and hydrogen determined by combustion of samples at high temperatures. Tested small sample (~2 g): immersed in liquid SO3 for 1 day. Combustion analysis carried out on doped and undoped samples Undoped Doped % C 71.80 54.98 % H 1.50 2.12 % N 18.05 16.34 % S 0.00 3.75 Additive PAN: Pyrolysis Surface Additive temperature/ oC area /m2 g-1 Carbazole 1:2 500 1.2 TMAI 1:2 500 1.3 PEG 1:3 750 72 C black 1:1 750 480 Na2CO3 1:3 750 930 As the temperature increases, gradually more nitrogen and hydrogen are lost, with the proportion of carbon present increasing as a result 2.Thermal Restructuring of Polyacrylonitrile XPS DATA 5. Devices During restructuring, nitrogen is lost from the polymer Nitrogen can exist in various forms; however, only pyridinic nitrogen is suitable for chemical doping XPS has been used to determine how much is present • Polyacrylonitrile (PAN) is a low cost, readily available commercial material • When heated to high temperature, a chemical reaction takes place which causes a structural rearrangement in the polymer. As a result, the polymer becomes conducting: • SUPER C PRODUCTION • PAN, binder, carbon, lithium sulphite • Binder dissolved in solvent slurry coated by doctor blade process in inert atmosphere • Propylene carbonate / LiBF4 electrolyte, tin / copper oxide negative electrode Pyridinic nitrogen atoms in the polymer can be used as sites for chemical doping (e.g. using lithium sulphite, Li2SO3): “N-5” Under suitable pyrolysis conditions and doping with sulphite, we observed a specific capacitance of > 300 F/g pyrolysed PAN and an energy density > 50 Wh/kg. The cycle life of the PAN electrodes was evaluated using conventional cycling techniques. “N-6” “N-Q” Before pyrolysis, samples are white discs After pyrolysis at 200 oC, samples have turned yellow After pyrolysis at 300 oC, samples have turned black and undergone a small decrease in volume After pyrolysis at temperatures of 400 oC and above, samples become gradually smaller and increasingly misshapen CONCLUSION High surface area and high nitrogen content have been identified as being necessary to achieve high capacitance A high surface area material has been produced This shows high levels of capacitance, particularly when doped with Li2SO3. 6. Acknowledgements We thank the EPSRC and the DTI for funding this programme (GR/M86613b) . We should also like to acknowledge additional contributions from Hawker Batteries and HILTech Developments Limited.