Ditty Dixon

In this study, we have prepared N and O co‐doped porous carbon from walnut shells and significantly improved its physico‐chemical properties by a two‐step chemical activation approach using ZnCl ₂ and KOH. The synergistic effect of ZnCl ₂ and KOH activation resulted in a well‐developed hierarchical porous architecture with a high specific surface area (901 m ² .g ⁻¹ ) and abundant O and N functional groups, as confirmed by XPS analysis. These features facilitated efficient ion transport and enhanced charge storage capacity. Tested in a three‐electrode supercapacitor configuration using a 1 M H ₂ SO ₄ electrolyte, the optimized carbon electrode exhibited a high specific capacitance of 520 F·g ⁻¹ at 1 A·g ⁻¹ , and outstanding cycling stability over 5000 cycles with a capacity retention greater than 99%. This work presents a sustainable and efficient route to designing high‐performance electrode materials from biowaste for next‐generation energy storage devices.

Li‐rich cathodes possess high capacity and are promising candidates in next‐generation high‐energy density Li‐ion batteries. This high capacity is partly attributed to its poorly understood oxygen‐redox activity. The present Li‐rich cathodes contain expensive and environmentally‐incompatible cobalt as a main transition metal. In this work, cobalt‐free, iron‐containing Li‐rich cathode material (nominal composition Li_1.2Mn_0.56Ni_0.16Fe_0.08O₂) is synthesized, which exhibits excellent discharge capacity (≈250 mAh g⁻¹) and cycling stability. In operando, X‐ray absorption spectroscopy at Mn, Fe, and Ni K edges reveals its electrochemical mechanism. X‐ray absorption near edge structure (XANES) features of Fe and Ni K edges show unusual behavior: when an electrode is charged to 4.5 V, Fe and Ni K edges’ XANES features shift to higher energies, evidence for Fe³⁺→Fe⁴⁺ and Ni²⁺→Ni⁴⁺ oxidation. However, when charged above 4.5 V, XANES features of Fe and Ni K edges shift back to lower energies, indicating Fe⁴⁺→Fe³⁺ and Ni⁴⁺→Ni³⁺ reduction. This behavior can be linked to a reductive coupling mechanism between oxygen and Fe/Ni. Though this mechanism is observed in Fe‐containing Li‐rich materials, the only electrochemically active metal in such cases is Fe. Li_1.2Mn_0.56Ni_0.16Fe_0.08O₂ has multiple electrochemically active metal ions; Fe and Ni, which are investigated simultaneously and the obtained results will assist tailoring of cost‐effective Li‐rich materials.

Polyacrylonitrile (PAN)-based carbon felt was subjected to N₂-plasma treatment to increase the heteroatom defects and reactive edge sites as a method to increase the performance in vanadium redox flow batteries (VRFBs). N-doping in the felt was mainly in the form of pyrrolic and pyridinic nitrogen. Even though the amount of oxygen functional groups on the N₂-plasma-treated sample was very low, the felt showed enhanced electrochemical performance for both V³⁺/V²⁺ as well as V⁵⁺/V⁴⁺ redox reactions. The result is highly significant as the pristine electrode with the same amount of oxygen functional groups showed significantly less activity for the V³⁺/V²⁺ redox reaction. Overall, the single-flow cell experiments with N₂-plasma-treated felt showed superior performance compared to the pristine sample. Therefore, the enhanced performance observed for the N₂-plasma-treated sample should be attributed to the increase in defects and edge sites. Thus, from the present study, it can be concluded that an alternate way to increase the performance of the VRFBs is to introduce specific defects such as N-doping/substitution or to increase the edge sites. In other words, defects induced in the carbon felt such as heteroatom doping are as beneficial as the presence of oxygen functional groups for the improved performance of VRFBs. Therefore, for an optimum performance of VRFBs, defects such as N-substitution as well as oxygen functionality should be tuned.

Conversion and alloying type negative electrodes attracted huge attention in the present research on lithium/sodium-ion batteries (LIBs/SIBs) due to the high capacity delivered. Among these, SnO₂ is investigated intensively in LIBs due to high cyclability, low reaction potential, cost-effectiveness, and environmental friendliness. Most of the LIB electrodes are explored in SIBs too due to expected similar electrochemical performance. Though several LIB negative electrode materials successfully worked in SIBs, bare SnO₂ shows very poor electrochemical performance in SIB. The reason for this difference is investigated here through combined in operando and ex situ X-ray absorption spectroscopy (XAS). For this, the electrodes of SnO₂ (space group P4₂/mnm synthesized via one-pot hydrothermal method) were cycled in Na-ion and Li-ion half-cells. The Na/SnO₂ half-cell delivered a much lower discharge capacity than the Li/SnO₂ half-cell. In addition, higher irreversibility was observed for Na/SnO₂ half-cell during electrochemical investigations compared to that for Li/SnO₂ half-cell. In operando XAS investigations on the Na/SnO₂ half-cell confirms incomplete conversion and alloying reactions in the Na/SnO₂ half-cell, resulting in poor electrochemical performance. The difference in the lithiation and sodiation mechanisms of SnO₂ is discussed in detail.