From Green to Black Gold: Highly Microporous Carbons from Pistachio Shells by a Controlled Physical Activation Process

Highly microporous carbons with BET surface areas of up to ca. 3300 m² g^-1 and pore volumes of up to 1.6 cm³ g^-1 have been successfully synthesized from pistachio shells, a waste whose generation is growing on account of the nutritive value of pistachios and the resilience of this crop to climate change. Such a high pore development has been achieved by a simple and benign CO₂ physical activation process assisted by a custom pre-treatment of the biomass. Different approaches have been explored in this work for the transformation of pistachio shells into carbon materials with diverse microstructures, mineral matter content and particle size/morphology, tuning thereby their reactivities towards CO₂ and diffusion kinetics and, in this way, pore development. In particular, the most efficient route for the production of highly microporous carbons from pistachio shells involves a hydrothermal carbonization process which increases the degree of aromatization and effectively removes the mineral matter, enhancing thereby the efficiency of both carbon production and porosity generation. By increasing the activation temperature, substantial shortening of the operation time can be achieved without compromising pore development. This work provides new integral strategies towards the production of biomass-based, CO₂-activated carbons with a focus on optimizing pore structure, minimizing energy consumption and maximizing product yield.