Figures (9)  Tables (3)

    • Figure 1. 

      Schematic diagram of the formation mechanism and influencing factors of biochar pore structure.

    • Figure 2. 

      Correlation analysis of thermal conductivity[64].

    • Figure 3. 

      (a) Effect of process conditions on biochar properties[73]. (b), (c) Volatile solids contents and fixed carbon contents after pyrolysis of biomass and biochar[74]. (d) Relationship between volatile matter content and pyrolysis temperature[75].

    • Figure 4. 

      Conductivity test device diagram: (a) Two-probe technique[81]. (b) Four-probe technique[82]. (c) Modified two-probe technique[76]. (d) Setup for compression test to measure conductivity[78].

    • Figure 5. 

      (a) SEM image of sugar maple biochar and the plot of bulk conductivity against porosity[76]. (b) Electrical conductivity of biochar produced from lignin and other biomass[80]. (c) Electrical conductivity of biochar composites prepared under different pressures[79].

    • Figure 6. 

      Dielectric constant and loss factor measurements as a function of biochar content at 2.45 GHz for each biomass[84].

    • Figure 7. 

      (a) Relationship between dielectric properties and heating temperature[87]. (b) Microwaves in biomass pyrolysis[88].

    • Figure 8. 

      (a) UV–vis diffuse reflectance spectra of TiO2 and biochar/ TiO2 composites[93]. (b) UV–vis diffuse reflectance spectra of biochar, TiO2, and biochar/TiO2 composites[94]. (c) Absorption coefficient (α) vs photon energy (hν) of PEO/PVP/biochar composite[95]. (d) UV-vis spectra of TiO2, BC/TiO2, and N-doped TiO2/biochar powder[96].

    • Figure 9. 

      Roadmap for the development of biochar.

    • Feedstock
      type      		 Synthesis strategy Specific conditions Pore volume (cm3/g) Pore size Specific surface area (m2/g) Ref.
      Poplar wood Medium-temperature pyrolysis → High-temperature annealing → Air oxidation activation N2, 500 °C, 1 h, 1,000 °C, 2 h. Air, 450 °C, 1 h / Channel:
      10–80 μm      		 663 [23]
      Inert atmosphere high-temperature pyrolysis → Metal-induced activation FeCl3, CoCl2
      Ar, 800 °C, 1 h      		 / Average pore size: 0.5 nm 334 [24]
      Low-temperature pre-carbonization → High-temperature pyrolysis under inert atmosphere → CO2 physical activation → Salt-assisted structure regulation 240 °C, 6 h, Ar, 1,000 °C, 6 h. CO2, 800 °C, 10 h. Salt impregnation, 150 °C, 2 h / 8–13 μm,
      30–50 μm      		 810 [25]
      Balsa wood Inert atmosphere high-temperature pyrolysis → Alkali impregnation and chemical activation Ar, 1,000 °C, 6 h, Ultrasonic.
      Alkali impregnation, N2, 700 °C,
      2 h      		 / Average pore size: 40 μm 809 [11]
      Hydrothermal precursor construction → Oxidative modification → High-temperature pyrolysis under inert atmosphere Impregnation dopamine, Hydrothermal,
      Co(NO3)2, 60 °C, 2 h, H2O2, 2h. N2, 800 °C, 2 h      		 0.163 Average pore size: 2.43 nm 110 [26]
      Inert atmosphere pyrolysis → Metal-induced activation FeCl3 impregnation, N2, 600 °C,
      2 h      		 0.118 Average pore size: 3.88 nm 275 [27]
      Pine wood Inert atmosphere high-temperature pyrolysis → Doping/metal-induced activation NH4Cl impregnation, Ar, 1,000 °C, 3 h. CuCl2 impregnation, Ar, 1,000 °C, 3h 0.250


      		 Average pore size: 2.55 nm 582 [28]
      Inert atmosphere high-temperature pyrolysis → Metal-induced structure regulation 800 °C, 0.5 h, N2. Ni(NO3)2·6H2O, 800 °C, 1 h, N2 0.804 Average pore size: 3.96 nm 813 [29]
      Pyrolysis and annealing in an inert atmosphere 500 °C, 3 h and 450 °C, 4 h, N2 0.197 Average pore size: 2.01 nm 393 [19]
      Bamboo Hydrothermal carbonization → Chemical activation → Medium-temperature pyrolysis activation Hydrothermal, 200 °C, 6 h. H3PO4 impregnation, 600 °C, 2 h 1.09 Average pore size: 2.42 nm 1,798 [30]
      Low-temperature pre-carbonization in an inert atmosphere → High-temperature primary carbonization in an inert atmosphere N2, 200 °C, 1 h, 700 °C, 3 h 1.51 Average pore size: 2.28 nm 2,715 [20]
      Introduction of nitrogen source and gaseous foaming agent → Alkali impregnation and chemical activation → Microwave rapid pyrolysis/activation Urea, urea nitrate, KOH, microwave, 460 W, 30 min 0.66 1–5 nm 1,195 [31]
      Rice husk Medium and high temperature carbonization 395–618 °C, 4 h 0.255 / / [32]
      Corn cob 0.243 / /
      Bamboo Intermediate-temperature pyrolysis under an inert atmosphere 500 °C, 1 h, N2 0.099 6.24 nm 71 [21]
      Rice husk 0.039 3.42 nm 29
      Corn cob 0.023 2.39 nm 10
      Sewage sludge Medium temperature carbonization 400 °C, 1 h / 10.6 nm 1 [33]
      Pine needles / 2.16 nm 430
      Pineapple leaves Medium and high temperature carbonization 300–700 °C, 2 h 0.01–0.1 1–9 nm 1–215 [18]
      Banana stems 0.01–0.18 2–8 nm 3–335
      Sugarcane bagasse 0.01–0.1 2–6.5 nm 2–195
      Horticultural substrate 0.01–0.09 3–16 nm 2–120
      Chicken manure Calcination at 550–950 °C for 4 h / 15.4–15.7 nm 11 [34]
      Swine manure Intermediate-temperature pyrolysis under an inert atmosphere Pyrolysis at 500–650 °C for 2 h
      in N2      		 0.032–0.038 15.4–26.0 nm 6–8 [35]
      Pyrolysis at 500 °C for 4 h in Ar 0.021 6.76 nm 13 [36]
      Cow manure Intermediate-temperature pyrolysis under an inert atmosphere → Metal-induced structure regulation Impregnation with CuSO4, Pyrolysis at 500 °C for 4 h in Ar 0.031 4.85 nm 26

      Table 1. 

      Pore structure characteristics of various biochars

    • Feedstock type Synthesis strategy Specific conditions Hardness (GPa) Modulus (GPa) Ref.
      Pinewood Medium and low temperature carbonization 450 °C, 10 min 0.43 4.9 [51]
      Birch wood 300 °C, 1 h 0.27 3.9 [52]
      Chicken litter 450 °C, 20 min ~0.75 ~5 [38]
      Spruce wood Medium and high temperature carbonization 700–2,000 °C, 2h 4–5 30–40 [50]
      Sewage sludge 680 °C, 10 min ~2.5 ~10 [38]
      90% softwood and 10% hardwood − − 0.28 5.1 [53]
      Fruit pit 0.22 3.4
      Pine bark High temperature carbonization 800 °C, 1 h ~0.47 ~4.5 [54]
      Gluten 0.5 7.8
      Pine sawdust 900 °C, 1 h 4.29 25 [38]

      Table 2. 

      Hardness and modulus of different types of biochar measured by nanoindentation

    • Feedstock type Synthesis strategy Specific conditions Thermal conductivity (W/m∙K) Ref.
      Lemon peel Medium and low temperature carbonization 180 °C, 1 h 0.84 [65]
      Wood offcuts 350–400 °C 0.079–0.132 [63]
      Phoenix leaf 450–600 °C, 2 h 0.056–0.06 [59]
      Garlic stem 700 °C, 2 h 0.141 [66]
      Pine wood High temperature carbonization 1,000 °C, 1 h 0.222 [58]
      Copper-based preservatives, 1,000 °C, 1 h 0.395
      Bamboo 1,000 °C, 6 h 0.3–0.7 [61]
      Peanut shell 900 °C, 2 h combined with stearic acid (SA) 0.53 [67]
      Poplar wood 0.38
      Corn straw 0.32

      Table 3. 

      Thermal conductivity of different types of biochar