The synthesis of functional carbon materials with controllable morphology and structure using a simple, effective, and green process starting from biomass has been an attractive and challenging topic in recent years. After decades of technological development, high value-added biomass-derived carbon nanomaterials with different morphologies and structures prepared by low-temperature hydrothermal carbonization (HTC) have been gradually developed into a huge system covering different series in different dimensions, and are widely used in the fields of adsorption, electrochemical energy storage, and catalysis. However, due to a vague understanding of the fundamental structure–performance correlation and the absence of customized material design strategies, the diverse needs in practical applications cannot be well met. Herein, we reviewed the mechanism, modifications, and applications of the low-temperature HTC method for biomass. The synthesis mechanisms, structural designs strategies, and related applications of biomass-derived hydrochar are highlighted and summarized in different dimensions, including six major categories: zero-dimensional spherical structure, one-dimensional fibrous and tubular structure, two-dimensional lamellar structure, three-dimensional hierarchical porous structure, and special-shaped asymmetric structure. Then a sustainability assessment is conducted on the hydrothermal carbonization process. Finally, the controllable preparation of biomass-derived hydrochar is summarized and prospected for the application requirements in different fields.

Hydrothermal synthesis and structural design of zero- to three-dimensional biomass-derived carbon nanomaterials
    Abstract
    1 Biomass-derived carbon nanomaterials prepared by HTC
        1.1 HTC mechanism
        1.2 Activation and application of biomass-derived hydrochar
    2 Structural design of biomass-derived hydrochar
        2.1 Zero-dimensional (0D) spherical structure
            2.1.1 Effect of biomass feedstock
            2.1.2 Effect of temperature
            2.1.3 Effect of pressure
            2.1.4 Effect of residence time
        2.2 One-dimensional (1D) nanofiber structure
        2.3 One-dimensional (1D) nanotubular structure
        2.4 Two-dimensional (2D) lamellar structure
        2.5 Three-dimensional (3D) hierarchical porous structure
        2.6 Asymmetric hollow structures (asymmetric spheresbottlesbowls)
    3 Sustainability assessments of HTC carbon materials
        3.1 Energy efficiency
        3.2 Carbon emissions
        3.3 Environmental impact
        3.4 Economic analysis
    4 Summary and outlook
    References

• Guo Cheng(Department of Chemistry and Chemical Engineering, Taiyuan Institute of Technology, Taiyuan 030008, China)
• Hui‑jia Li(Department of Chemistry and Chemical Engineering, Taiyuan Institute of Technology, Taiyuan 030008, China)
• Jing‑hua Fang(Department of Chemistry and Chemical Engineering, Taiyuan Institute of Technology, Taiyuan 030008, China)
• Xue‑qing Wang(Department of Chemistry and Chemical Engineering, Taiyuan Institute of Technology, Taiyuan 030008, China)
• Jia‑qi Li(Department of Chemistry and Chemical Engineering, Taiyuan Institute of Technology, Taiyuan 030008, China)
• Jun‑feng Chang(Department of Chemistry and Chemical Engineering, Taiyuan Institute of Technology, Taiyuan 030008, China)
• Na Teng(National Engineering Laboratory for Carbon Fiber Technology, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China)
• Ji‑tong Wang(State Key Laboratory of Chemical Engineering, School of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China) Corresponding author