Investigations into the supercapacitor activity of bisphosphonate-polyoxovanadate compounds

Abstract

Electrochemical properties of two different bisphosphonate-polyoxovanadate derivatives have been studied. Both the compounds are oxovanadate polyanion derivatives having formula (NH4)4[H6(VIVO2)(VV2O5)2{O3P–C(O)(CH2-2-C5NH4)-PO3}2]•9H2O (1) and (NH4)4[H6(VIVO2)(VV2O5)2{O3P–C(O)(CH2-3-C5NH4)-PO3}2]•8H2O (2). Compounds 1 and 2 are isostructural mixed-valent polyoxovanadate structures containing 1-hydroxy-2-(2-pyridyl) ethylidene-1,1-bisphosphonate and 1-hydroxy-2-(3-pyridyl) ethylidene-1,1-bisphosphonate, respectively. Composites of these compounds have been prepared with activated carbon (AC), viz.AC-1 and AC-2 for compounds 1 and 2, respectively. In order to investigate the interaction between the parent compounds with activated carbon, the as prepared composites were further characterized using various analytical techniques, such as Infrared spectroscopy, thermal stability analysis, powder X-ray diffraction, FE-SEM, EDX, and surface adsorption studies. Electrodes from these composites have been prepared by deposition upon carbon cloth, and their electrochemical properties, with respect to the supercapacitance behavior, have been extensively studied. The electrochemical performance of both the composite materials have been tested in an aqueous 0.1 ​M ​H2SO4 electrolytic solution. AC-2 displays the highest specific capacitance of 313 ​F ​g−1 at a current density of 1 ​A ​g−1, along with incredible specific energy and power of 43.56 ​Wh kg−1 and 1999.72 ​W kg-1, respectively. Likewise, AC-1 exhibits a specific capacitance of 212 ​F ​g−1 also at the current density of 1 ​A ​g−1 with a specific energy of 29.45 ​Wh kg−1. The AC-2 electrode, therefore, shows more conducting and lower resistivity. As such, composite AC-2 was observed to light up red, green, and blue color LED bulbs for 71, 70, and 15 ​seconds, respectively, with only 42 ​mg of sample coated on carbon cloth, suggesting a remarkable specific power of that material. Comparatively, the composite AC-1 electrode could light up the red LED bulbs for about 190 ​seconds, albeit with 143 ​mg of sample coated on carbon cloth. As such, AC-2 was also observed to retain 98.71% of the electrode stability after 4500 cycles.