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This perspective review paper provides an overview on recently developed carbon material technology synthesised from the hydrothermal carbonisation (HTC) approach, with a particular focus on the carbon formation mechanism, perspectives on large scale production, nanostructuring, functionalisation and applications. Perceptions on how this technology will be developed especially with regard to application fields where the use of HTC-derived materials could be extended will also be introduced and discussed.

The objective of this paper is to better describe the structure of the hydrothermal carbon (HTC) process and put it in relationship with the more classical pyrolytic carbons. Indeed, despite the low energetic impact and the number of applications described so far for HTC, very little is known about the structure, reaction mechanism, and the way these materials relate to coals. Are HTC and calcination processes equivalent? Are the structures of the processed materials related to each other in any way? Which is the extent of polyaromatic hydrocarbons (PAH) inside HTC? In this work, the effect of hydrothermal treatment and pyrolysis are compared on glucose, a good model carbohydrate; a detailed single-quantum double-quantum (SQ-DQ) solid state (13)C NMR study of the HTC and calcined HTC is used to interpret the spectral region corresponding to the signal of furanic and arene groups. These data are compared to the spectroscopic signatures of calcined glucose, starch, and xylose. A semiquantitative analysis of the (13)C NMR spectra provides an estimation of the furanic-to-arene ratio which varies from 1:1 to 4:1 according to the processing conditions and carbohydrate employed. In addition, we formulate some hypothesis, validated by DFT (density functional theory) modeling associated with (13)C NMR chemical shifts calculations, about the possible furan-rich structural intermediates that occur in the coalification process leading to condensed polyaromatic structures. In combination with a broad parallel study on the HTC processing conditions effect on glucose, cellulose, and raw biomass (Falco, C.; Baccile, N.; Titirici, M.-M. Green Chem., 2011, DOI: 10.1039/C1GC15742F), we propose a broad reaction scheme and in which we show that, through HTC, it is possible to tune the furan-to-arene ratio composing the aromatic core of the produced HTC carbons, which is not possible if calcination is used alone, in the temperature range below 350 °C.

Hydrothermal carbonization (HTC) is an aqueous-phase route to produce carbon materials using biomass or biomass-derived precursors. In this paper, a comprehensive physicochemical and textural characterization of HTC materials obtained using four different precursors, namely, xylose, glucose, sucrose, and starch, is presented. The development of porosity in the prepared HTC materials as a function of thermal treatment (under an inert atmosphere) was specifically monitored using N(2) and CO(2) sorption analysis. The events taking place during the thermal treatment process were studied by a combined thermogravimetric/infrared (TGA-IR) measurement. Interestingly, these inexpensive biomass-derived carbon materials show good selectivity for CO(2) adsorption over N(2) (CO(2)/N(2) selectivity of 20 at 273 K, 1 bar and 1:1 gas composition). Furthermore, the elemental composition, morphologies, degree of structural order, surface charge, and functional groups are also investigated.

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Advanced porous materials with tailored porosity (extremely high development of microporosity together with a narrow micropore size distribution (MPSD)) are required in energy and environmental related applications. Lignocellulosic biomass derived HTC carbons are good precursors for the synthesis of activated carbons (ACs) via KOH chemical activation. However, more research is needed in order to tailor the microporosity for those specific applications. In the present work, the influence of the precursor and HTC temperature on the porous properties of the resulting ACs is analyzed, remarking that, regardless of the precursor, highly microporous ACs could be generated. The HTC temperature was found to be an extremely influential parameter affecting the porosity development and the MPSD of the ACs. Tuning of the MPSD of the ACs was achived by modification of the HTC temperature. Promising preliminary results in gas storage (i.e. CO 2 capture and high pressure CH 4 storage) were obtained with these materials, showing the effectiveness of this synthesis strategy in converting a low value lignocellulosic biomass into a functional carbon material with high performance in gas storage applications. IntroductionHydrothermal Carbonization (HTC) is now a well-established thermochemical synthesis alternative to produce functional carbon materials with a tunable chemical structure from pure carbohydrates or lignocellulosic biomass [1][2][3].During HTC, biomass-derived precursors are converted into valuable carbon materials using water as reaction medium at mild temperatures (< 200°C) under self-generated pressures [4]. Even though this methodology has been known for almost 100 years [5], its full potential, as a synthetic route for carbon materials having important applications in several fields such as catalysis, energy storage, CO 2 sequestration, water purification, soil remediation, has been revealed only recently, mainly via the work of Dr. Titirici and coworkers [6].Under hydrothermal conditions monosaccharides are dehydrated to 5-hydroxymethylfurfural (5-HMF) via the well-known Lobry de Bruyn-Alberta van Ekstein rearrangement [7]. Once 5-HMF is formed, it is in situ "polymerised" yielding the HTC carbon product [8]. While most of the research efforts focus on the exploitation of HMF for the production of chemicals, bioplastics and biofuels [9], the Titirici´s group rediscovered these processes for the production of green and valuable carbon and carbon-hybrid materials [1,4].One of the main limiting factors, hindering the effective and straightforward exploitation of HTC carbons for several end-applications (eg. catalysis, separation science, energy production and storage), is their low surface area and porosity [10]. In the case of monosaccharide derived HTC carbons, this problem has been elegantly overcome by using hard\soft-templating strategies or by addition of structural directing agents [11][12][13]. Such synthetic routes are effective because of the homogeneous nature of the pre-HTC aqueous reaction mixt...

Acid pretreatment of lignocellulosic biomass, required for bioethanol production, generates large amounts of by-products, such as lignin and hydrolyzed hemicellulose fractions, which have found so far very limited applications. In this work, we demonstrate how the recovered hemicellulose hydrolysis products can be effectively utilized as a precursor for the synthesis of functional carbon materials through hydrothermal carbonization (HTC). The morphology and chemical structure of the synthesized HTC carbons are thoroughly characterized to highlight their similarities with glucose-derived HTC carbons. Furthermore, two routes for introducing porosity within the HTC carbon structure are presented: i) silica nanoparticle hard-templating, which is shown to be a viable method for the synthesis of carbonaceous hollow spheres; and ii) KOH chemical activation. The synthesized activated carbons (ACs) show an extremely high porosity (pore volume≈1.0 cm(3) g(-1)) mostly composed of micropores (90 % of total pore volume). Because of their favorable textural properties, the ACs are further tested as electrodes for supercapacitors, yielding very promising results (300 F g(-1) at 250 mA g(-1)) and confirming the high suitability of KOH-activated HTC carbons derived from spruce and corncob hydrolysis products as materials for electric double layer supercapacitors.

N-doped highly microporous carbons have been successfully fabricated from N-rich microalgae by the combination of low-cost hydrothermal carbonization and industryadopted 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 m 2 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 ºC, which is the one exhibiting the largest amount of N-and O-containing groups. The samples synthesized at 700-750ºC 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.