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This document is the Accepted Manuscript version of a Published Work that appeared in final form in ACS Sustainable Chemistry and Engineering, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see https://doi.org/10.1021/acssuschemeng.9b04266, The implementation of sustainable strategies based on the integral valorization of residues is the most efficient way to achieve a profitable circular economy. This comprehensive study highlights the potential of apple waste from juice and cider production as a precursor of porous carbons and provides guidelines to achieve a wide spectrum of physicochemical properties. Hydrothermal carbonization (HTC) of apple bagasse is proposed as a feasible integrated process with zero waste that allows stabilizing this highly pollutant residue in the form of a carbon-enriched solid while extracting valuable compounds in the aqueous phase. The liquid fraction resulting from HTC at 200 °C contains a high total phenolic content and antioxidant activity, the major products being catechol (1,2-dihydroxybenzene) and 5-hydroxymethyl-2-furfural (HMF). The successful upgrading of the solid byproduct into porous carbons provides additional advantage for a more cost-effective waste management. It is reported that a simple one-step activation leads to apple-derived carbons with specific surface areas up to 2000 m2 g–1 and electrochemical capacitances as high as 260–290 F g–1. Their excellent performance as supercapacitor electrodes make them very promising for the storage of electrical energy from renewable sources., The funding from INTERREG V- SUDOE 2017 through project CEMOWAS2 (SOE2/P5/F0505) and from Plan de Ciencia, Tecnología e Innovación (PCTI) 2018-2020 del Principado de Asturias and the European Regional Development Fund (ERDF) through project IDI/2018/000233 is acknowledged. The SERIDA financial support for this work was managed by the National Institute of Research Agro-Food Technology (INIA) and co-financed with ERDF and ESF funds (RTA 2015-00060-CO4-03).

This work shows relevant inconsistencies found in a paper recently published in Electrochim. Acta and how the conclusions are challenged by a more detailed textural characterization of a microporous carbon monolith.

With the awareness of the need for low-cost carbons for supercapacitors, the recycling of biomass wastes as precursors of activated carbons is focusing great interest. The comparison of a diversity of carbons derived from residues of industrial processing of grapes, apples and cherries reveals the utility of physical activation to achieve better volumetric performance in aqueous supercapacitors. Whereas the high porosity development of the materials obtained by KOH activation (mostly around 1500–2000 m2 g-1) provides a superior gravimetric capacitance up to nearly 300 F g-1 in 2 M H2SO4, the low density of the electrodes reduces the volumetric capacitance to 60–130 Fcm−3. Porous carbons produced by steam or CO2 reaches higher efficiency for compact devices. The physically activated materials display an ultramicroporous structure with standard surface areas of around 900 m2 g−1 and, therefore, their gravimetric capacitance is limited to 100–150 F g-1. However, harnessing the balance of porosity, density and good packaging, the corresponding electrodes reach 80–170 F cm−3, without sacrificing power delivery by the supercapacitor. Higher carbon yield and lower environmental impact represent additional advantages of the physical activation to produce biomass wastes derived-carbons., The funding from European Regional Development Fund (ERDF) through project CEMOWAS2 (SOE2/P5/F0505)-INTERREG V-SUDOE 2017 and Plan de Ciencia, Tecnología e Innovación (PCTI) 2018–2020 del Principado de Asturias (Spain) and the European Regional Development Fund (ERDF) through project IDI/2018/000233 is acknowledged.

We present a simple and effective alternative which optimizes electrodes based on low-cost carbons for high-performance supercapacitors. The combination with reduced graphene oxide (rGO) greatly improves the operation of microporous carbons easily produced by one-pot activation of grape seeds. The use of composite electrodes with rGO lowers the supercapacitor resistance and enables a much higher rate capability. The mixture of rGO flakes and particles of a highly porous carbon obtained by KOH activation allows retaining the high capacitance of 260 F g−1 of the standard electrodes at 1 mA cm−2 in aqueous H2SO4 whereas the value at 200 mA cm−2 is increased by around 2.4 times. Consequently, at high current density, the capacitor assembled with these composites stores eight times more energy and the power density is multiplied by four. The synergy between rGO and an ultramicroporous carbon produced by CO2-activation results extremely profitable, the cell assembled with composite electrodes reaching three times more energy and power at 200 mA cm−2 than the best performance of the standard counterpart. More importantly, the higher density of the composite electrodes leads to a capacitance of around 200 F cm−3 which translates into a remarkable improvement in the supercapacitor operation normalized to volume., T.A.C. gratefully acknowledges the funding from CSIC [Proyecto Intramural Especial 2016 8 0E035]. V.V. and V.S. thank the support by Slovak Research and Development Agency under the contract APVV-16-0319.