Outstanding room-temperature capacitance of biomass-derived microporous carbons in ionic liquid electrolyte

A remarkable capacitance of 180 F·g−1 (at 5 mV·s−1) in solvent-free room-temperature ionic liquid electrolyte, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, was achieved in symmetric supercapacitors using microporous carbons with a specific surface area of ca. 2000 m2·g−1 calculated from gas sorption by the 2D-NLDFT method. The efficient capacitive charge storage was ascribed to textural properties: unlike most activated carbons, high specific surface area was made accessible to the bulky ions of the ionic liquid electrolyte thanks to micropores (1–2 nm) enabled by fine-tuning chemical activation. From the industrial perspective, a high volumetric capacitance of ca. 80 F·cm−3 was reached in neat ionic liquid due to the absence of mesopores. The use of microporous carbons from biomass waste represents an important advantage for large-scale production of high energy density supercapacitors

Understanding enhanced charge storage of phosphorus-functionalized graphene in aqueous acidic electrolytes

The mechanisms behind enhanced charge storage of P-functionalized carbons are unraveled for the first time using non-porous graphene oxide treated with phosphoric acid and annealed at either 400 or 800 degrees C. The electrochemical study in 1 M H2SO4 reveals that phosphorus groups boost charge storage and electrochemical stability, with more effect for the higher annealing temperature. Annealing at 800 degrees C also leads to the material withstanding 60,000 charge-discharge cycles with no capacitance loss at 1.5 V. The improvement in the electrochemical performance is shown to be mainly governed by the change in surface chemistry comprehensively studied with NMR, FTIR and XPS characterization techniques. The collective analysis of electrochemical response and surface chemistry demonstrates that enhanced charge storage by phosphorus-functionalized graphene materials is made possible due to the following synergistic mechanisms: i) non-Faradaic charging; ii) nascent hydrogen storage in the interlayer; iii) benzoquinoneto-hydroquinone redox processes; iv) phosphate-to-phosphonate like transformation. From the practical perspective, the stored charge can be boosted due to the higher capacitance upon prior electrochemical activation in the vicinity of oxygen evolution potential and the wider usable electrochemical window enabled by phosphorus-related groups. (C) 2020 The Author(s). Published by Elsevier Ltd.The authors thank the European Union (Graphene Flagship, Core 2, Grant number 785219) and the Spanish Ministry of Science and Innovation (MICINN/FEDER) (RTI2018-096199-B-I00) for the financial support of this work. J. L. G. U. is very thankful to the Spanish Ministry of Education, Science and Universities (MICINN) for the FPU grant (16/03498). We also want to acknowledge the company GRAPHENEA for supplying the graphene oxide used in this work and Yan Zhang from CIC Energigune for collecting FTIR spectra

Effect of cation (Li+, Na+, K+, Rb+, Cs+) in aqueous electrolyte on the electrochemical redox of Prussian blue analogue (PBA) cathodes

International audiencePrussian blue analogue (PBA) material is a promising cathode for applications in Na-ion and K-ion batteries which can support high c-rates for charge and discharge. In this study, the material of composition [K2CuIIFeII(CN)6] was synthesized and its structural and electrochemical redox behavior was investigated with 5 different alkali insertion cations (Li+, Na+, K+, Rb+, Cs+). Galvanostatic measurements indicate that the redox potential strongly depends on the ionic radius of the inserted cation. The redox potential varies by ∼400 mV between using Li+ (0.79 Å) or Cs+ (1.73 Å) in the electrolyte. The underlying modification of the Fe2+/Fe3+ redox potential in PBA is proposed to be due to the weakening of the Fe–C bond in the material. This hypothesis is supported by XRD measurements which reveal that the lattice parameter of the de-intercalated host structure follows the same trend of monotonic increase with the cation size. The relatively minor volume changes accompanying the redox (1.2%–2.4%) allow the PBA to accommodate differently sized cations, although the structural hindrances are quite pronounced at high c-rates for the larger ones (Rb+ and Cs+). Cycle aging studies indicate that the minimum capacity fade rate is observed in case of K+ and Rb+ containing electrolyte. The peak intensity corresponding to the [220] crystallographic plane varies depending on the state of charge of PBA, since this plane contains the insertion cations. Owing to the sensitivity of the redox potential to the insertion cation coupled with the observed fast ion-exchange ability, the PBA material may find additional analytical applications such as ion sensing or filtration devices

Review on supercapacitors: Technologies and materials

info:eu-repo/semantics/publishe

Highly packed graphene-CNT films as electrodes for aqueous supercapacitors with high volumetric performance

The increasing complexity of portable electronics demands the development of energy storage devices with higher volumetric energy and power densities. In this work we report a simple strategy for the preparation of partially reduced graphene oxide/carbon nanotube composites (prGO-CNT) as highly packed self-standing binder-free films suitable as electrodes for supercapacitors. These carbon-based films are easily obtained by the hydrothermal treatment of an aqueous suspension of graphene oxide and CNTs at 210 °C and then compacted under pressure. The prGO-CNT films, which had an apparent density as high as 1.5 g cm-3, were investigated as binder-free electrodes for aqueous supercapacitors using 6 M KOH solution as the electrolyte. The results show that the presence of merely 2 wt% of CNTs produces a significant enhancement of the capacitance retention at high current densities compared to the CNT-free samples, and this improvement is especially relevant in systems formed using electrodes with high mass loadings. Volumetric capacitance values of 250 F cm-3 at 1 A g-1 with outstanding capacitance retention (200 F cm-3 at 10 A g-1) were achieved using the prGO-CNT electrodes with an areal mass loading above 12 mg cm-2.This work was financially supported by the European Union (Graphene Flagship, Core 1) and the Spanish Ministry of Economy and Competitiveness (MINECO/FEDER) (MAT2015-64617-C2-2-R). The authors also thank GRAPHENEA Company for supplying the graphene oxide used in this study. We also thank Dr Oleksandr Bondarchuk for his fruitful help on the XPS data acquisition.Peer reviewe

Thin films of pure vanadium nitride: Evidence for anomalous non-faradaic capacitance

International audienc

IDENTIFICATION OF THE LENGTH DISTRIBUTION OF LUMBER DEFECT-FREE AREAS

Presented here are the results of statistical analysis of length distribution of defect-free areas (DFA) of pine, beech and oak blanks. The investigated empirical distributions of the lengths of defect-free areas nearly always exhibit right-side asymmetry and “heavy tails”, with high coefficients of variation (30% c 110%). Therefore, the arithmetic mean of these lengths is not an appropriate measure for description of any of the investigated samples, and to describe the dimensional and qualitative characteristics of blanks, not only characteristics of location must be used, but also the relative characteristics of the dispersion. It is proposed that rather than use estimates of variability, to apply an assessment of stability – the value inverse to the squared coefficient of variation, which allows, with minimal computational cost, to correctly compare the lengths of DFA obtained from different lumber and in different operating conditions. It is shown that the distribution of lengths of DFA for pine, oak and beech blanks can be only described entirely by two theoretical distributions – the Burr and log-logistic, with different parameters for different wood species and various sizes of defect-free areas

Ball-milling-enhanced capacitive charge storage of activated graphene in aqueous, organic and ionic liquid electrolytes

Ball-milling under either air or argon is explored as a facile way to adjust the textural properties, surface chemistry and morphology of activated reduced graphene oxide to the requirements for optimum electrode materials in electrical double layer capacitors operating in aqueous (KOH), organic (tetraethylammonium tetrafluoroborate in acetonitrile) and ionic liquid (1-Ethyl-3-methylimidazolium bis(trifluoromethyls​ulfonyl)​imide) electrolytes. Ball-milling is evidenced to specifically remove excessively large pores through the collapse of mesoporosity with no negative effect on in-pore resistance in any electrolyte, a concomitant improvement in volume-based storage and no loss of gravimetric performance. Ball-milling under air results in high oxygen content in the materials, which brings about performance deterioration in tetraethylammonium tetrafluoroborate in acetonitrile, but not significantly in aqueous and ionic liquid electrolytes. The best performance is achieved using activated graphene ball-milling under argon, with a volumetric capacitance of 90, 60, 70 F.cm−3 and a characteristic cell response time of 0.28, 1.3 and 8 s for aqueous, organic and ionic liquid electrolyte. While the highest energy density of 25 Wh.L−1 is reached in ionic liquid electrolyte at a cell potential of 3 V, the highest practical power density of 15 kW.L−1 is measured in tetraethylammonium tetrafluoroborate in acetonitrile at the energy density of 10 Wh.L−1. Our study underscores that simple ball-milling can provide the best trade-off among multiple performance parameters, resulting in sufficiently high volumetric capacitance with no detriment in high-rate response and cycle stability

Using Graphite Intercalation Compounds for Producing Exfoliated Graphite-Amorphous Carbon-TiO₂ Composites

A new method for producing the TiO2-exfoliated graphite-amorphous carbon composites was developed. It involves anatase-type TiO2, residual graphite hydrosulfate and sucrose as starting materials and makes it easier to produce such floating composites on a large scale. The composites are promising for water purification as they couple the buoyancy of exfoliated graphite with the sorption capacity of amorphous carbon and the photocatalytic activity of anatase