Supercapacitive Behavior of Two Glucose-Derived Microporous Carbons: Direct Pyrolysis versus Hydrothermal Carbonization

This is the accepted version of the following article: Sevilla, M., Yu, L., Ania, C. O. and Titirici, M.-M. (2014), Supercapacitive Behavior of Two Glucose-Derived Microporous Carbons: Direct Pyrolysis versus Hydrothermal Carbonization. CHEMELECTROCHEM, 1: 2138–2145. doi: 10.1002/celc.201402233, which has been published in final form at http://dx.doi.org/10.1002/celc.201402233. This article may be used for non-commercial purposes in accordance with the Wiley Self-Archiving PolicyThe physical and chemical characteristics of activated carbons produced from glucose and hydrothermally carbonized glucose are compared for the first time, as well as their performance as electrodes in supercapacitors with aqueous electrolyte (H2SO4). Both KOH-activated carbons exhibit similar textural properties, with Brunauer–Emmett–Teller surface areas of ≈1400–1500 m2 g−1 and a pore volume of ≈0.70 cm3 g−1, with the pore size distribution centered in the micropore range. When tested as supercapacitor electrodes, the activated carbon produced from hydrothermally carbonized glucose exhibits a superior rate capability, owing to lower equivalent distributed resistance (being able to work at an ultrahigh discharge current of 90 A g−1), as well as higher specific capacitance (≈240 F g−1 vs. ≈220 F g−1 for the glucose-derived activated carbon at 0.1 A g−1). Both supercapacitors have excellent robustness, even for a large cell voltage of 1.2 V in 1 M H2SO4.This research work was supported by Spanish MINECO (MAT2012-31651 and CTM2011-23378). M.S. acknowledges the award of the Ramón y Cajal contract. M.M. Titirici and Linghui Yu are grateful to the Max-Planck Society for financial support for this project.Peer reviewe

High-performance CO2 sorbents from algae

[EN] Highly porous N-doped carbon materials with apparent surface areas in the 1300–2400 m2 g−1 range and pore volumes up to 1.2 cm3 g−1 have been synthesized from hydrothermal carbons obtained from mixtures of algae and glucose. The porosity of these materials is made up of uniform micropores, most of them having sizes <1 nm. Moreover, they have N contents in the 1.1–4.7 wt% range, and the heteroatom is mainly a pyridone-type structure. These microporous carbons present unprecedented large CO2 capture capacities, up to 7.4 mmol g−1 (1 bar, 0 °C). The importance of the pore size on the CO2 capture capacity of microporous carbon materials is clearly demonstrated. Indeed, a good correlation between the CO2 capture capacity at sub-atmospheric pressure and the volume of narrow micropores is observed. The results suggest that pyridinic-N, pyridonic/pyrrolic-N and quaternary-N do not contribute significantly to the CO2 adsorption capacity, owing probably to their low basicity in comparison with amines. These findings will help the design of high-performance CO2 capture sorbents.The financial support for this research work provided by the Spanish MCyT (CQT2011-24776) is gratefully acknowledged. M.S. acknowledges the assistance of the Spanish MCINN for its award of a Ramón y Cajal contract.Peer reviewe

Surface Modification of CNTs with N-Doped Carbon: An Effective Way of Enhancing Their Performance in Supercapacitors

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 http://dx.doi.org/10.1021/sc500069hCarbon nanotubes have been successfully coated with a N-doped carbon layer via hydrothermal carbonization in the presence of a nitrogen-containing carbohydrate, i.e., glucosamine hydrochloride. By controlling the amount of glucosamine added, it was possible to tune the N content of the composites N-doped carbon/CNT between 1.8–2.5 wt %. The prepared composites exhibited superior supercapacitor performance in comparison to bare CNTs even though they possess lower textural properties. Thus, a 2- to 4-fold increase in specific capacitance per surface area was registered at low current densities and sweep rates and a 2-fold increase in energy density, while keeping the power density. Besides, the composites possess superb long-term stability, losing only 4–6% of specific capacitance after 10,000 cycles at 10 A g–1.This research work was supported by Spanish MINECO (MAT2012-31651 and CTM2011- 23378). M.S. acknowledges the award of the Ramón y Cajal contract.Peer reviewe

Renewable Microalgae-derived Nitrogen Doped Hydrothermal Carbons

Nitrogen-doped carbon materials are synthesized via an effective, sustainable, and green one-step route based on the hydrothermal carbonization of microalgae with high nitrogen content (ca. 11 wt %). The addition of the monosaccharide glucose to the reaction mixture is found to be advantageous, enhancing the fixation of nitrogen in the synthesized carbons, resulting in materials possessing nitrogen content in excess of 7 wt %, and leading to promising reaction yields. Increasing the amount of glucose leads to a higher nitrogen retention in the carbons, which suggests co-condensation of the microalgae and glucose-derived degradation/hydrolysis products via Maillard-type cascade reactions, yielding nitrogen-containing aromatic heterocycles (e.g., pyrroles) as confirmed by several analytical techniques. Increasing the HTC processing temperature leads to a further aromatization of the chemical structure of the HTC carbon and the formation of increasingly more condensed nitrogen-containing functional motifs (i.e., pyridinic and quaternary nitrogen).Peer reviewe

From Waste to Wealth: From Kraft Lignin to Free-standing Supercapacitors

Pure eucalyptus Kraft lignin derived carbon fiber mats were produced based on a model workflow. It covers the preparation and characterization of the lignin precursor and the carbon materials and its testing in the final application (supercapacitor). Sequential solvent extraction was employed to produce a eucalyptus Kraft lignin precursor which could be electrospun into lignin fibers without any additives. The fiber formation from low molecular weight lignin is assigned to strong intermolecular interactions via hydrogen bonding and π-π-stacking between individual lignin macromolecules which gives rise to association complexes in the electrospinning solution. By stabilization in air, carbonization in N2 and an activation step in CO2, free-standing microporous carbon fiber mats could be produced. These fiber mats possess mainly basic oxygen functional groups which proved to be beneficial when tested as free-standing electrodes in symmetric supercapacitors. Consequently, the CO2-activated fiber mats showed a high specific gravimetric capacitance of 155 F/g at 0.1 A/g, excellent rate capability with 113 F/g at 250 A/g and good capacitance retention of 94% after 6000 cycles when tested in 6 M KOH electrolyte. Therefore, we conclude that lignin itself is a promising precursor to produce microporous, oxygen functionalized carbon fibers serving as free-standing electrodes in aqueous supercapacitors.We would like to thank EPSRC (EP/R021554/1, EP/N509899/1, EP/P031323/1) for the financial support

Carbonización hidrotermal: una ruta "verde" de síntesis de materiales de carbono avanzados

[EN] This review p aper provides an overview of the hydrothermal carbonisation (HTC) technology, a “green” and versatile strategy for the synthesis of advanced carbon materials suitable for a wide variety of applications of high impact in the current society. We will focus on the carbon formation mechanism, chemical and structural properties of hydrothermal carbons, porosity development, nanostructuring, functionalisation and applications.[ES] Este artículo de revisión proporciona una visión general de la tecnología de carbonización hidrotermal (HTC), estrategia “verde” y de gran versatilidad de síntesis de materiales de carbono útiles en una gran variedad de aplicaciones de gran imp acto en la sociedad actual. Nos centraremos en el mecanismo de formación del material carbonoso, sus propiedades químicas y estructurales, desarrollo de porosidad, nanoestructuración, funcionalización y aplicaciones.Peer reviewe

Hydrothermal synthesis of microalgae-derived microporous carbons for electrochemical capacitors

N-doped highly microporous carbons have been successfully fabricated from N-rich microalgae by the combination of low-cost hydrothermal carbonization and industry-adopted KOH activation processes. The hydrothermal carbonization process was found to be an essential step for the successful conversion of microalgae into a carbon material. The materials thus synthesized showed BET surface areas in the range ∼1800–2200 m2 g−1 exclusively ascribed to micropores. The carbons showed N contents in the 0.7–2.7 wt.%, owing to the use of N-rich microalgae as a carbon precursor. When tested in symmetric double layer capacitors (occasionally called supercapacitors) based on aqueous LiCl electrolytes, pseudocapacitance was only observable for the sample synthesized at the lowest temperature, 650 °C, which is the one exhibiting the largest amount of N- and O-containing groups. The samples synthesized at 700–750 °C exhibited excellent rate capability (only 20% of capacitance loose at 20 A g−1), with specific capacitances of 170–200 F g−1 at 0.1 A g−1. These materials showed excellent long-term cycling stability under high current densities.The financial support for this research work provided by the US Army Research Office (grant W911NF-12-1-0259) and by the Spanish MINECO (MAT2012-31651) is gratefully acknowledged. M.S. thanks the Spanish MINECO for the award of a Ramón y Cajal contract.Peer reviewe

Hardwood versus softwood Kraft lignin – precursor-product relationships in the manufacture of porous carbon nanofibers for supercapacitors

The process of stabilization is essential in the production of carbon fibers from lignins. During stabilization, the initially thermoplastic lignin polymer is converted to a thermoset polymer allowing for high-temperature treatment without a change in shape. In this work, hardwood (HKL) and softwood (SKL) Kraft lignins were stabilized in air at temperatures between 190 and 340 °C before carbonization at 800 °C in a nitrogen atmosphere. Due to the differences in side-chain linkages, functional groups and molar mass, the lignins exhibit different structural changes upon stabilization and hence develop different porosities upon carbonization. Both lignins undergo major crosslinking reactions in the side chains at low temperatures and degradation reactions at high temperatures during stabilization. Crosslinking gives rise to narrow pore size distributions with mainly (sub-) nanometer pores, whereas degradation reactions lead to a more open pore structure with additional mesoporosity (>2 nm). When both types of reactions take place simultaneously, highly accessible (sub-) nanoporosity can be effectively created, which boosts the performance of supercapacitors operating in 6 M KOH(aq). This effect terminates when the crosslinking reactions cease and mainly degradation reactions take place, which occurs in HKL at 340 °C. SKL shows both a lower degree of crosslinking and degradation and hence develops less specific surface area. The optimum performance in an aqueous alkaline supercapacitor is achieved with HKL stabilized at 310 °C. It shows a specific gravimetric capacitance of 164 F g−1 at 0.1 A g−1 and 119 F g−1 at 250 A g−1 with a capacitance retention of more than 90% after 10 000 cycles.M. J. M. L. and D. C. A. thank Spanish Ministry of Science, Innovation and Universities and FEDER (project RTI2018-095291-B-I00) for financial support. C. P. G. and C. O'K. thank Shell. MMT and PS thank RISE AB for co-funding Philipp Schlee's PhD position

Boosting the Oxygen Reduction Electrocatalytic Performance of Nonprecious Metal Nanocarbons via Triple Boundary Engineering Using Protic Ionic Liquids

The oxygen reduction reaction (ORR) in aqueous media plays a critical role in sustainable and clean energy technologies such as polymer electrolyte membrane and alkaline fuel cells. In this work, we present a new concept to improve the ORR performance by engineering the interface reaction at the electrocatalyst/electrolyte/oxygen triple-phase boundary using a protic and hydrophobic ionic liquid and demonstrate the wide and general applicability of this concept to several Pt-free catalysts. Two catalysts, Fe–N codoped and metal-free N-doped carbon electrocatalysts, are used as a proof of concept. The ionic liquid layer grafted at the nanocarbon surface creates a water-equilibrated secondary reaction medium with a higher O2 affinity toward oxygen adsorption, promoting the diffusion toward the catalytic active site, while its protic character provides sufficient H+/H3O+ conductivity, and the hydrophobic nature prevents the resulting reaction product water from accumulating and blocking the interface. Our strategy brings obvious improvements in the ORR performance in both acid and alkaline electrolytes, while the catalytic activity of FeNC-nanocarbon outperforms commercial Pt–C in alkaline electrolytes. We believe that this research will pave new routes toward the development of high-performance ORR catalysts free of noble metals via careful interface engineering at the triple point.M.-M.T. would like to acknowledge EPSRC grants EP/R021554/1 and EP/N509899/1 and the EU CIG 631092. M.Q., Y.Y., H.L. acknowledge the CSC for a PhD scholarship. Professor Maria-Magdalena Titirici was founded by: EPSRC EP/S018204/1, EPSRC EP/R021554/1, and EPSRC EP/N509899/1