EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Nanoporous activated carbons (ACs) are widely used in different energy storage systems, e.g. as electrodes for supercapacitors and batteries. ACs also show great potential as adsorbents for hydrogen (Η2) storage tanks operating at cryogenic temperatures. These applications require large specific surface areas (SSAs) and micropore volumes to achieve the desired adsorption capacities. ACs may be synthesized from biological waste, which would otherwise only be used for incineration, and can thus be brought back into sustainable use. Every Austrian consumes on average about 6.6 kg of coffee per year, therefore leaving behind a lot of unused waste. Spent coffee grounds (SCG) and coffee silver skin (CSS), the latter being a by-product of the roasting process, can be used as precursors for the synthesis of ACs. Carbonization of the SCG and CSS materials under constant nitrogen flow and elevated temperatures followed by activation using solid potassium hydroxide results in ACs with high SSAs (up to ~2600 m²/g) and micropore volumes (up to ~1.6 cm³/g) along with mean pore widths <1 nm. Gas sorption analysis (N2 at 77 K and CO2 at 273 K) was used to identify the porosity-related properties (surface area, pore volume and pore size distribution). Raman spectroscopy and X-ray scattering methods revealed the turbostratic nature of the ACs and their structural characteristics. The Η2 uptake was measured at different cryogenic temperatures (77-97 K) under a low-pressure regime (0-1 bar). Fully reversible gravimetric H2 capacities of up to 2.81 wt% were recorded at 77 K and 1 bar while the isosteric enthalpies of adsorption at zero coverage were estimated around ~7.7 kJ/mol. The studied bio-waste-derived ACs show a huge potential as possible Η2 storage materials.