Hard carbon (HC) is the most promising candidate for sodium‐ion battery anode materials. Several material properties such as intensity ratio of the Raman spectrum, lateral size of HC crystallite (La), and interlayer distance (d002) have been discussed as factors affecting anode performance. However, these factors do not reflect the bulk property of the Na+ intercalation reaction directly, since Raman analysis has high surface sensitivity and La and d002 provide only one‐dimensional crystalline information. Herein, it was proposed that the crystallite interlayer area (Ai) defined using La, d002, and stacking height (Lc) governs Na+ intercalation behavior of various HCs. It was revealed that various wood‐derived HCs exhibited the similar total capacity of approximately 250 mAh g−1, whereas the Na+ intercalation capacity (Ci) was proportional to Ai with the correlation coefficient of R2=0.94. The evaluation factor of Ai was also adaptable to previous reports and strongly correlated with their Ci, indicating that Ai is more widely adaptable than the conventional evaluation methods.

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There is an urgent need to develop renewable sources of energy and use existing resources in an efficient manner. In this study, in order to improve the utilization of unused biomass and develop green processes and sustainable technologies for energy production and storage, unused Douglas fir sawdust (SD) was transformed into catalysts for the oxygen reduction reaction. Fe and N were doped into SD during hydrothermal carbonization, and the N- and Fe-doped wood-derived carbon (Fe/N/SD) was carbonized in a nitrogen atmosphere. After the catalyst had been calcined at 800°C, its showed the highest current density (−5.86 mAcm −2 at 0.5 V versus reversible hydrogen electrode or RHE) and E onset value (0.913 V versus RHE). Furthermore, its current density was higher than that of Pt/C (20 wt% Pt) (−5.66 mA cm −2 @0.5 V versus RHE). Finally, after 50 000 s, the current density of sample Fe/N/SD (2 : 10 : 10) remained at 79.3% of the initial value. Thus, the synthesized catalysts, which can be produced readily at a low cost, are suitable for use in various types of energy generation and storage devices, such as fuel cells and air batteries. This article is part of the theme issue ‘Bio-derived and bioinspired sustainable advanced materials for emerging technologies (part 2)’.

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Liquid fuels from biomass and up-conversion of biomass in advanced supercritical fluid are reviewed in this chapter. Lignin can be converted into heavy hydrocarbons in subcritical water extraction. Lipid, which is triglyceride, is catalytically converted into straight-chain hydrocarbons of free fatty acid (decarboxylation) formed by hydrolysis. Carbohydrate is also hydrothermally converted into furan ring compound and fatty acids. Protein is converted into amino acids in hydrothermal water and depolymerization of protein is favored with rapid heating and denaturation agency such as alkaline earth metals. Free amino acids are further decomposed into carboxylic acid through deamination and into amine through decarboxylation. To inhibit Maillard reactions, which result in polymerization, the deamination of amino acid at low temperature was favored and a solid catalyst was quite active for deamination of free amino acids at quite low temperature hydrothermal water. Cellulose was dissolved in some ionic liquids with high mass percentages (5-20 wt%) and converted into monomers and useful components such as furan ring compounds and supercritical fluid cosolvent such as hydrothermal water in ionic liquids supported improvement of reaction efficiency. For hydrogenation of biomass, it was confirmed that hydrogen solubility was enhanced with supercritical carbon dioxide and it must be helpful for hydrogen reaction with biomass molecule.