This research is focused in the missing link between the specific surface area of carbons surface and their electrochemical capacitance. Current protocols used for the characterization of carbons ...applied in supercapacitors electrodes induce inconsistencies in the values of the interfacial capacitance (in F m−2), which is hindering the optimization of supercapacitors. The constraints of both the physisorption of N2 at 77 K and the standard methods used for the isotherm analysis frequently lead to a misleading picture of the porosity. Moreover, the specific surface area of carbons loses their meaning when the supercapacitor operates with organic electrolytes and ionic liquids and the actual surface involved in charge storage has to be assessed by molecular probes suiting the critical dimensions of the ions. In the case of certain carbons such as graphene type-materials, the voltage-driven mechanism may facilitate the access of electrolyte ions to spaces between carbon layers, providing a larger area than that estimated by gas adsorption. Finally, the morphological and porous features of carbons can be extremely modified when they are processed in electrodes. Due to their impact, all these issues should not be neglected and the characterization protocols must be adapted for this specific application of carbons.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
Graphene shows substantial promise in improving the technical performance of a range of applications. For its development and before its potential mass adoption, it is critical to understand the ...associated cradle-to-grave life cycle environmental impacts. Previous studies on graphene environmental performance do not include end of life and the potential environmental credits generated by graphene reuse. This study undertakes a cradle-to-grave approach to evaluating graphene applications, considering a case study of supercapacitors manufactured with graphene and activated carbon active materials. The analysis includes active materials commercial-size production, supercapacitors production, supercapacitors use phase in an automotive application, and their end of life in which both devices are recycled. With current material performance and energy mixes, the graphene-based supercapacitor would increase impacts in all environmental categories analysed, ranging from 27% higher human toxicity to 213% greater ozone depletion and showing a 48% increase in GHG emissions. This unfavourable result arises due to the tested graphene material exhibiting inferior specific capacitance to the activated carbon comparator, as well as a more energy-intensive production process. Prospective analysis considers hypothetical performance where both active materials reach their theoretical specific capacitance, and the decarbonisation of electricity generation. The environmental impacts of both the activated carbon and graphene supercapacitors are reduced in these prospective scenarios, with the graphene based supercapacitor becoming the least impacting solution due to its lower active material requirements. The graphene-based device shows 36% lower GHG emission and overall shows lower impacts ranging from 14% less mineral, fossil and renewable resource depletion to 43% less photochemical ozone formation. These results support graphene as a valid candidate material for substituting activated carbon in supercapacitors provided graphene technical performance is improved, production optimised, and recycling developed.
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•Activated carbon supercapacitors show the lowest environmental impact.•Recycling of graphene is key to reduce its environmental impacts.•Prospective LCA indicates graphene supercapacitors as the least impacting.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
The study of 28 porous carbons shows that the specific capacitance in the electrolyte (C(2)H(5))(4)NBF(4)/acetonitrile is relatively constant between 0.7 and 15 nm (0.094 ± 0.011 F m(-2)). The ...increase in pores below 1 nm and the lower values between 1 and 2 nm reported earlier are not observed in the present work.
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A comprehensive comparison of symmetrical supercapacitors assembling carbon electrodes with exclusively microporous, mesoporous or combined micro-mesoporous networks provides a ...critical outlook on the influence of pores size on the performance with ionic liquid-based electrolyte 1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIm-TFSI) dissolved in acetonitrile. Contrary to widespread claims, the results for an electrodes set involving carbons of different origin indicate that the presence of large pores does not ensure a better supercapacitor performance. At low current density, the capacitance is basically determined by the surface in pores above 0.8 nm, regardless of the pore size distribution. In addition, the beneficial effect of large pores on the response rate of the supercapacitor cannot be concluded in a straightforward manner. On the contrary, wide porosity in electrodes has detrimental effects that should not be underestimated as far as the competitiveness of the final device is concerned. The greater amount of electrolyte required by larger pores will increase both the weight and the cost of the cell. More importantly, the widening of carbon pores (even in the range of micropores) notably reduces the density of the corresponding electrodes and, consequently, the supercapacitor performance in volumetric terms may not be suitable for practical applications.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
An efficient alternative for the management and valorization of the huge amount of seasonal wastes generated by winery industries is shown. Sustainable porous carbons were successfully prepared by ...one-pot activation of grape bagasse, stalks and oil free-seeds. Benefiting from the high moisture of these residues, a prior hydrothermal carbonization increases the activation yield and generates carbons with less oxygen content and inorganic impurities. The mild physical or chemical activation of winery wastes resulted in highly microporous carbons with specific surface SBET up to 2053 m2/g and great potential for environmental protection and electrical energy storage. They are excellent sorbents for CO2 capture under post- and pre-combustion conditions (16 and 69 wt%, respectively) and achieve superior electrochemical capacitance of nearly 300 F/g in aqueous electrolyte and 180 F/g in ionic liquid based-medium.
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•Simultaneous carbonization/activation for valorization of winery wastes.•Hydrothermal carbonization: efficient alternative for the management of winery wastes.•Bagasse, stalks and oil free-seeds are excellent precursors of sustainable carbons.•Activated carbons offer great potential for CO2 capture.•Activated carbons achieve superior electrochemical capacitance.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
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.
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•Integral and profitable management of apple waste by hydrothermal carbonization.•Optical microscopy to evaluate successfully the transformation of biomass by HTC.•Solid by-products ...are CO2 neutral fuels and show potential to enhance soil quality.•Liquid by-products contain valuable phenolic compounds and saturated fatty acids.•By-products properties are suited to final applications by selecting HTC conditions.
Hydrothermal carbonization makes feasible the integral and profitable recovery of industrial apple waste within a zero-residue bio-economy. 82–96% of the energy and 80–93% of the C in the apple bagasse are retained in the solids generated by hydrothermal treatment at 180 and 230 °C for 2 and 4 h. Such processes stabilize the apple waste and lead to CO2 neutral solid fuels with calorific value close to 30 MJ/kg. The agrochemical properties of the solid by-products suggest their potential to improve soil quality. Aqueous streams containing valuable phenolic compounds and saturated fatty acids are generated simultaneously, which provide additional cost-effectiveness. The by-products characteristics can be suited to the final application by selecting the reaction temperature, whereas the process duration has less impact. Optical microscopy and reflectance measurements are presented, for the first time, as powerful tools for assessing the biomass transformation when subjected to hydrothermal treatment under different conditions.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
This study analyses and compares the behaviour of 5 commercial porous carbons in the ionic liquid N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR14TFSI) and its mixture with ...propylene carbonate (PC) as electrolytes. The results of this investigation show that the existence of a distribution of pore sizes and/or constrictions at the entrance of the pores leads to significant changes in the specific capacitance of the investigated materials. The use of PYR14TFSI as an electrolyte has a positive effect on the EDLC energy storage, but its high viscosity limits the power density. The mixture 50 : 50 wt% propylene carbonate-PYR14TFSI provides high operative voltage as well as low viscosity and thus notably enhances EDLC operation.
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.
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•Optimization of electrodes based on low-cost carbons for high-power supercapacitors.•The combination with rGO greatly improves the operation of microporous carbons in SC.•Composite electrodes reach capacitance as high as 260 F g−1 and 200 F cm−3.•Cells with composite electrodes store 8 times more energy with a power 4 times higher.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The optimization of carbon-based supercapacitors is of great technological importance for electrical energy storage. This challenge stresses the relevance of current attempts to increase the ...surface-related capacitance C/S(F m super(-2)) of carbons. We show that for microporous carbons this property is practically constant for pore widths between 0.7 and 1.8 nm. The study is based on two independent approaches: (i) the effective surface area accessible to the ions, which may differ considerably from the BET-surface and (ii) the volumetric capacitance in the micropores. One obtains 0.094 F m super(-2) in (C sub(2)H sub(5)) sub(4)NBF sub(4)/acetonitrile and approximately 0.100 F m super(-2) in aqueous H sub(2)SO sub(4) after correcting for pseudo-capacitance effects. This pattern is supported by recent modelling which takes into account the solvent. It is also suggested that constant values of the surface based-capacitance may reflect a gradual decrease of the dielectric constant of the electrolytes in smaller micropores due to desolvation.
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