Biopolymer Films of Metal-Coordinated Chlorophyll-a Derivatives for “green” Supercapacitor Electrodes

Chlorophylls (Chls) are the most naturally abundant organic dye pigments on earth. However, there is little research on Chl polymers so far. In this paper, we demonstrate the fabrication of a series of polymerized chlorophyll-a derivatives with different central metals (PolyMChls, M = Mg, Co, Ni, Cu, Zn, and Ga) via the electrochemical coupling of a peripheral vinyl group at the C3-position on the chlorin macrocycle. The formation of PolyMChls is confirmed by the broadening and slight red-shift of the Soret band in their ultraviolet-visible absorption spectra. PolyMChl films yield a morphology of uniform stacked nano-spheres (PolyCuChl similar to 237 nm), while the original MChl particles are irregular with a size of about 10 mu m by scanning electron microscopy. When these PolyMChl films are employed as the working electrodes in a three-electrode system, the highest capacitance of 334 F g-1 at 5 mV s-1 is obtained for PolyCuChl. Basing on the widely existing Chl derivatives, we find a new biopolymer family. The great potential of biopolymer PolyMChl is also exhibited in the energy storage area.