Hard carbon is the most promising anode material for sodium‐ion batteries and potassium‐ion batteries owing to its high stability, widespread availability, low‐cost, and excellent performance. Understanding the carrier‐ion storage mechanism is a prerequisite for developing high‐performance electrode materials; however, the underlying ion storage mechanism in hard carbon has been a topic of debate because of its complex structure. Herein, it is demonstrated that the Li+‐, Na+‐, and K+‐ion storage mechanisms in hard carbon are based on the adsorption of ions on the surface of active sites (e.g., defects, edges, and residual heteroatoms) in the sloping voltage region, followed by intercalation into the graphitic layers in the low‐voltage plateau region. At a low current density of 3 mA g–1, the graphitic layers of hard carbon are unlocked to permit Li+‐ion intercalation, resulting in a plateau region in the lithium‐ion batteries. To gain insights into the ion storage mechanism, experimental observations including various ex situ techniques, a constant‐current constant‐voltage method, and diffusivity measurements are correlated with the theoretical estimation of changes in carbon structures and insertion voltages during ion insertion obtained using the density functional theory.

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The reductive catalytic fractionation (RCF) of lignocellulosic and herbaceous biomass over heterogeneous catalysts has been demonstrated to recover high-yield phenolic monomers and holocellulose-rich solids effectively, and these products could be further used to produce value-added chemicals and second-generation biofuel. Catalyst selection plays a critical role in the performance of the RCF process, and noble metal catalysts (e.g., Pt, Pd, and Ru) with a high loading of 5 wt % have been extensively used to obtain high-yield phenolic monomers and delignified holocellulose-rich solids. In this study, we demonstrated that the RCF of biomass over extremely low Pd loaded on N-doped carbon (CN x ) support catalysts could produce phenolic monomers at approximately theoretical maximum yield and presented high holocellulose-rich solid recovery. When birch wood was converted over the catalyst with 0.25 wt % Pd loaded on CN x (Pd0.25/CN x ) at 250 °C and an initial H2 pressure of 3.0 MPa for 3 h, a lignin-derived phenolic monomer carbon yield and highly delignified holocellulose recovery of 52.7 C % and 84.2 wt %, respectively, were achieved. The Pd0.25/CN x catalyst contained both ultrasmall Pd nanoclusters and single Pd atoms, which were stabilized on the N-functionalized carbon support. The highly activated hydrogenolysis and double-bond saturation that occurred over the Pd0.25/CN x catalyst dominantly produced 4-n-propyl guaiacol/syringol. In contrast, 4-n-propanol guaiacol/syringol with residual −OH groups was the major species obtained over the typical 5 wt % Pd/activated carbon catalyst. The plausible reaction pathways for the production of different types of phenolic monomers were discussed using density functional theory calculations. The excellent RCF performance of the Pd0.25/CN x catalyst was demonstrated using other types of biomass, such as oak, pine, and miscanthus. The successful use of extremely low-Pd-loaded catalysts is advantageous for implementing economically viable RCF techniques.

The development of an efficient non-sulfided and non-precious catalyst for selective hydrogenation (HD) and hydrodeoxygenation (HDO) of biomass-derived feedstocks to produce fuels and chemicals is of great interest.