A. Bhardwaj, U. Okoroanyanwu, J.J. Watkins
UmassAmherst,
United States
Keywords: graphene composite, photothermal processing, micro-supercapacitor
Summary:
Micro-supercapacitors with exceptional capacitance of 300 mF/cm2 at 1mA current density and more than 88% capacitance retention after 2000 charge/discharge cycles have been fabricated using millisecond photothermal pyrolysis of polybenzoxazine polymers. The porous graphene composite framework is prepared by coating benzoxazine monomer on a carbon fiber substrate, crosslinking it at nominal temperatures, converting it photothermally (in milliseconds) to graphene composites, and subsequent electrochemical deposition of MnO2 on the porous structure. A high intensity xenon flash lamp with a wide emission spectrum (300 nm –1100 nm) was utilized to convert the polymeric precursor directly into graphene upon millisecond exposure. The absorption spectrum of the precursor overlaps well with the maxima of xenon flash lamp emission spectrum. Consequently, the precursor material is heated to extremely high temperatures in a fraction of a second – a duration that is much shorter than the timescale for thermal equilibrium. This enables the conversion of the precursor material to graphene in air and at room temperature, and without thermally damaging the substrate. Characterization of these graphene composites revealed high porosity, excellent conductivity, and good adhesion to the carbon fiber, all of which facilitate the transport of ions through the composite and consequently enabled very high supercapacitance. The preparation of graphene via polymer precursor coating and photothermal pyrolysis of appropriate precursors is amenable to roll-to-roll processing, and thus large-scale production of flexible supercapacitors.