High total-electrode and mass-specific capacitance cellulose nanocrystal-polypyrrole nanocomposites for supercapacitors

For practical applications, new supercapacitor electrode materials need to exhibit a high mass-specific capacitance (CM/F g−1), a high total-electrode capacitance (CE/F cm−2), and high stability during charge–discharge cycling. Very often, newly developed materials display high CM values for thin films (nm or μm thickness) but these rapidly drop off in the thicker electrode structures needed for commercial devices. In this work, we describe the fabrication of thick nanocomposites of polypyrrole (PPY) and cellulose nanocrystals (CNXLs) with consistently high capacitance (CM = 240 F g−1) and performance. CE of the PPY-CNXL nanocomposite increased linearly with increasing film thickness up to a value of 1.24 F cm−2 and this increased to a maximum of 1.54 F cm−2 for even thicker films where non-linear CE increases were due to electrolyte diffusion limitations. Testing of a symmetric supercapacitor with these high CE electrodes showed that it retained half of its initial capacity after 50 000 charge–discharge cycles, demonstrating the excellent stability of PPY-CNXL supercapacitor electrode materials.