Supercapacitors using binderless composite monolith electrodes from carbon nanotubes and pre-carbonized biomass residues

Binderless composite monolith (BCM) electrodes prepared from carbon nanotubes (CNTs) and self-adhesive carbon grains (SACGs) were used in a symmetrical supercapacitor. The SACGs were prepared from fibers of oil palm empty fruit bunches (EFBs) from oil palm tree (Elaeis guineensis), Heliotropium dasycarpum (H. dasycarpum) and Guaiacum officinale (G. officinale). For each biomass, the BCMs were prepared by the carbonization and activation of green monoliths (GMs) containing SACGs treated with KOH and a mixture of SACGs and CNTs treated with KOH. Thermal decomposition behavior of all SACGs was found to be slightly different because of the difference in their compositions. In addition, BCMs from H. dasycarpum and G. officinale were found to have SiO2. The BET surface areas were 1656, 1031 and 532 m2 g−1 for the BCMs from EFB, H. dasycarpum and G. officinale, respectively, and these values decreased by 40, 50 and 31% upon CNTs addition. Consequently, the specific capacitance decreased from ∼124 to ∼104 and ∼49 F g−1 to ∼111, ∼87 and ∼31 F g−1, respectively. However, addition of CNTs reduced the equivalent series resistance (ESR) by a factor of 83.9 (EFB), 90.6 (H. dasycarpum) and 38.8 (G. officinale) %. It was also found that CNTs addition contributed to improving the decay of Csp with increasing scan rate if the electrode surface area was sufficiently high. •Oil palm empty fruit bunches, Heliotropium dasicarpum and Guaiacum officinale were processed into supercapacitor electrodes.•Activation with small quantities of KOH and CO2 gas produced highly porous, binderless carbon electrodes.•Small quantities of CNTs reduced the ESR for supercapacitor cells.•This novel method using biomass residues for energy applications offers significant technical and economic advantages.