Activated carbons from KOH and H3PO4-activation of olive residues and its application as supercapacitor electrodes

Abstract

Activated carbons from a mixture of olive-residue, oil and vegetable water, known as alperujo, OR, were prepared by KOH and H3PO4 activation. KOH activations were made following two different methods (carbonization-activation and impregnation-activation) with two different KOH/carbon ratios. Textural characterization with N2 and CO2 reveals that a prior carbonization of samples followed by the activation with KOH, produces samples with similar textural and chemical properties to the impregnated ones at low KOH ratio, while at high KOH ratio; more porous samples are obtained by carbonization. However, no differences are observed increasing the KOH ratio for the impregnated samples. As a consequence, similar or even more porous samples are obtained by using an easier preparation method such as carbonization-activation. On the other side, H3PO4 activation leads to samples with less surface area but with a more developed mesoporosity. In addition, the chemical surfaces of KOH and H3PO4 activated samples are very different as shown by XPS and TPD results. XPS data denote a similar chemical texture in KOH activated samples in spite of the different preparation methods and KOH ratio, whereas the H3PO4 method fixed different phosphorus surface groups. Analysing TPD data while on KOH activated samples, oxygenated surface groups (OSG) decompose mainly as CO2 (carboxylic acid, anhydrides, lactones, etc.) during the TPD experiments, on H3PO4 activated one those OSG ones evolve as CO (semiquinone, carbonyl, etc.) which has been attributed to the presence of COP species that decompose in the form of CO at around 860°C. As a consequence, similar electrochemical capacitances are obtained using KOH activated samples, which depend linearly on the surface area while the H3PO4 activated one presents higher capacitance value and capacitance stability in spite of its smaller surface area. This fact was related with the presence and nature of phosphorus surface groups.