Carbons activated with potassium hydroxide and plant extracts: micromorphology and infrared evidence of sulfur oxygen functionalization

Abstract

Converting woody residues into conductive carbons is attractive for low cost electrodes, but claims of graphite or graphene formation from biomass at modest temperatures are often made on thin evidence. This study examined carbons prepared from Moringa oleifera stems that were carbonised at 500 to 600 °C, activated at 700 °C with potassium hydroxide, Moringa leaf extract, lemon juice, lime juice or watermelon juice, and then heated in concentrated sulfuric acid for 4 h at either 100 °C
(A100) or 150 °C (A150). Scanning electron microscopy at 50 and 200 μm scale bars showed angular fragments that preserved the vessels, fibres and honeycomb parenchyma of the parent stem in every sample; no feature at this scale distinguished A100 from A150 products or identified exfoliated sheets. Infrared spectra were dominated by a band at 1021 to 1029 cm-1 with companions at 1101 to 1109, 1142 to 1156, about 1314, 865 to 893 and 660 to 671 cm-1, a pattern consistent with sulfonic acid and bisulfate groups introduced by the acid. Normalised to the aromatic C=C band near 1620 cm-1, this band gave ratios of 1.45 to 3.31, highest for the lemon juice samples and lowest for KOH treated at 100 °C, while carbonyl ratios remained between 0.85 and 0.98. For the plant activated carbons the two acid temperatures gave ratios within 8 % of each other. The products are therefore best described as sulfonated, acid treated graphitic carbons; Raman spectroscopy, X ray diffraction and sulfur analysis are required before any claim of graphitic ordering or exfoliation can be sustained.