Enhanced synthesis of nanoporous carbons from lignocellulosic sources for energy storage applications

A multistep-wash hydrothermal process for lignocellulosic carbon sources was designed in order to synthesise activated carbons (ACs) for energy storage applications. The process was carried out on Cannabis Sativa fibres, Tilia Cordata tree leaves and common grass. The process involved a series of mild washes of the raw biomass, followed by hydrothermal carbonization in a stainless-steel autoclave to obtain biochar. Subsequent chemical washing steps were carried out before and after chemical activation, which was performed with potassium hydroxide in an inert atmosphere at 800°C. A final annealing was performed in order to assess the effect of a heat treatment on the conductivity and the capacitance of the activated carbon grades. The raw biomass, the biochar and the activated carbons were studied by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS), while the ACs were also analysed using X-ray diffractometry (XRD) to assess crystallinity. Results showed that the multistep wash reduces the mineral content naturally present in the biomass, and that the ACs were predominantly amorphous. The electrochemical characterization consisted in the production of an AC-based electroactive paste by rolling, which was subsequently tested in a three-electrode configuration in sodium sulphate 1M, and in potassium hydroxide 2M. Electrochemical impedance spectroscopy and cyclic voltammetry tests were performed in various potential windows and scan rates to measure gravimetric capacitance, charge transfer and pore diffusion impedances. The best performing grade was subsequently used as a benchmark to optimise the biomass to KOH ratio of the recipe. At a later stage, the electroactive material of every grade was used to set up a symmetric packed cell to measure the specific energy and specific power densities of the system to assess its behaviour as a supercapacitor. Results showed in most cases that gravimetric capacitances were in the range from 100 to 200 F/g at slow scan rates (1 or 5 mV/s), while at a fast scan rate (100 mV/s) several grades showed big drops in capacitance due to the uncontrolled pore structure of the ACs. Finally, it was found that, despite the nonideality of some AC grades in the electrochemical tests, all the cells built with the electroactive material grades lay in the supercapacitor region of the Ragone plot.

E' stato proposto un metodo idrotermico di sintesi di carboni attivi a partire da fibre di canapa, foglie di Tilia Cordata, ed erba comune. Successivamente, i carboni attivi sono stati utilizzati per la produzione di un film elettroattivo capace di immagazzinare cariche elettriche. La caratterizzazione del materiale è stata eseguita nelle configurazioni di due e tre elettrodi in sulfato di sodio 1M e in idrossio di potassio 2M.