Engineering Carbon Materials from the Hydrothermal Carbonization Process of Biomass By Bo Hu, Kan Wang, Liheng Wu, Shu-Hong Yu,* Markus Antonietti, and Maria-Magdalena Titirici* 1. Introduction Synthesis and application of carbon materials have a long history and carbon black, fabricated from fuel-rich partial combustion, has been used for ink, pigments, and tattoos for more than 3000 years. [1] Starting with the discovery of fullerenes [2] and carbon nanotubes, [3] the material science related to valuable carbon materials has become a hot area, motivated by its potential applications in carbon fixation, catalyst supports, adsorbents, gas storage, electrode, carbon fuel cells and cell biology. [4–8] Many synthetic methods, such as carbonization, high-voltage-arc electricity, laser ablation, or hydrothermal carbonization have been reported for the preparation of amorphous, carbonaceous, porous, or crystalline carbon materials with different size, shape, and chemical compo- sitions. [9–13] In this Review, we will focus on a more sustainable approach, which relies on low specific energy input and replaces fossil-fuel-based starting products with biomass. Biomass is a qualified carbon raw material for the synthesis of valuable carbon materials because it is available in high quality (e.g., as pure saccharose) and huge amount, and is a environmental friendly renewable resource. An illustration of its potential is the production of bioethanol, which has emerged as a new fuel for vehicles (usually by mixing gasoline with alcohol). In the United States, more than 7 billion gallons bioalcohol were produced in 2007. In Brazil, almost all the light automobiles are running on the blend of gasoline and bioalcohol, and similar scales can be easily envisaged for materials, appropriate carbon products assumed. Even more abundant, waste biomass derived from agricultural resides and forest byproducts has drawn little attention as a raw material, since only simple combustion has been used to elevate the value of waste biomass. Carbon materials fabricated from waste biomass have shown promising applications as sorption materials, hydrogen storage, biochemicals, and others. [14–17] The problem is that there is still no general and satisfactory process for the production of valuable carbon materials from crude biomass to date. In this respect, a hydrothermal carbonization (HTC) process might have the opportunity to turn into a powerful technique for the synthesis of valuable carbon materials from biomass, especially crude biomass (Scheme 1). According to different experimental conditions and reaction mechanisms, two HTC processes can be classified. Based on the pyrolysis of biomass, a high-temperature HTC process is apt to synthesize carbon nanotubes, graphite, and activated carbon materials under high temperature and high pressure. [18,19] A low-temperature HTC process is carried out up to 250 8C, employing several chemical transformation cascades, and is a more environmentally friendly route. [20,21] Various carbonac- eous materials with different sizes, shapes, and surface functional groups have been synthesized by this process. Furthermore, these REVIEW www.MaterialsViews.com www.advmat.de [*] Prof. Dr. S. H. Yu, Dr. B. Hu, K. Wang, L. H. Wu Division of Nanomaterials and Chemistry Hefei National Laboratory for Physical Sciences at Microscale Department of Chemistry University of Science and Technology of China Hefei, Anhui 230026 (P. R. China) E-mail: shyu@ustc.edu.cn Dr. M.-M. Titirici, Prof. Dr. M. Antonietti Department of Colloid Chemistry Max Planck Institute of Colloids and Interfaces MPI Research Campus Golm 14424 Potsdam (Germany) E-mail: magdalena.titirici@mpikg.mpg.de DOI: 10.1002/adma.200902812 Energy shortage, environmental crisis, and developing customer demands have driven people to find facile, low-cost, environmentally friendly, and nontoxic routes to produce novel functional materials that can be com- mercialized in the near future. Amongst various techniques, the hydrothermal carbonization (HTC) process of biomass (either of isolated carbohydrates or crude plants) is a promising candidate for the synthesis of novel carbon-based materials with a wide variety of potential applications. In this Review, we will discuss various synthetic routes towards such novel carbon-based materials or composites via the HTC process of biomass. Furthermore, factors that influence the carbonization process will be analyzed and the special chemical/ physical properties of the final products will be discussed. Despite the lack of a clear mechanism, these novel carbonaceous materials have already shown promising applications in many fields such as carbon fixation, water puri- fication, fuel cell catalysis, energy storage, CO 2 sequestration, bioimaging, drug delivery, and gas sensors. Some of the most promising examples will also be discussed here, demonstrating that the HTC process can rationally design a rich family of carbonaceous and hybrid functional carbon materials with important applications in a sustainable fashion. Adv. Mater. 2010, 22, 1–16 ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 Final page numbers not assigned