It is an immense challenge to develop bifunctional electrocatalysts for oxygen reduction reactions (ORR) and oxygen evolution reactions (OER) in low temperature fuel cells and rechargeable metal-air batteries. Herein, a simple and cost-effective approach is developed to prepare novel materials based on carbon nanotubes (CNTs) and a hexagonal boron nitride (h-BN) nanocomposite (CNT/BN) through a one-step hydrothermal method. The structural analysis and morphology study confirms the formation of a homogeneous composite and merging of few exfoliated graphene layers of CNTs on the graphitic planes of h-BN, respectively. Moreover, the electrochemical study implies that CNT/BN nanocomposite shows a significantly higher ORR activity with a single step 4-electron transfer pathway and an improved onset potential of +0.86 V versus RHE and a current density of 5.78 mA cm in alkaline conditions. Interestingly, it exhibits appreciably better catalytic activity towards OER at low overpotential (η=0.38 V) under similar conditions. Moreover, this bifunctional catalyst shows substantially higher stability than a commercial Pt/C catalyst even after 5000 cycles. Additionally, this composite catalyst does not show any methanol oxidation reactions that nullify the issues due to fuel cross-over effects in direct methanol fuel cell applications.

applied in commercial electric and hybrid electric vehicles. However, the increasing market demand for rechargeable LIBs has increased the concerns regarding the economic sustainability of the LIB technology owing to uneven geographical distribution and relatively low amount of available lithium resources. [1-3] Thus, battery technologies based on abundant and inexpensive resources shall be an important option for large-scale energy storage devices. In this context, sodium-ion battery (SIB) technologies have emerged as a promising and low-cost alternative to LIBs. An intercalation-based chemical mechanism, similar to that of their LIB counterparts, wide sodium resource availability, and excellent electrochemical performance make SIBs an attractive candidate technology for large-scale storage devices. A variety of electrode materials such as layered oxides, polyanions, and fluorophosphates were successfully studied as cathodes for SIBs. [4,5] Although graphite does not offer a favorable Na + intercalation chemistry, several metal sulfides, metals, phosphides, alloys, and hard carbon were investigated as anodes for SIBs. [6-8] The limited capacity of metal oxides, poor cycle life, high cost of hard carbon, and large volume expansion of alloy/metal type anodes limit their practical application. [9,10] Transferring the experience gained from current LIBs to SIBs may lead to the rapid commercialization of the SIB technology. The most studied electrode materials in both LIBs and SIBs are based on transition metal-based chemistries using metal elements that are generally nonrenewable resources. Furthermore, the highly toxic nature of transition metals and high energy consumption involved in metal mining must be addressed to increase the sustainability and environment friendliness of energy storage devices. [11-13] Organic materials are promising candidates for further advancing the sustainability and ecofriendliness of energy storage devices. Organic electrode materials offer many advantages because they mostly consist of light elements such as C, H, O, N, and S. First, organic electrode materials are directly available from natural resources or can be prepared from natural derivatives. [14,15] Organic electrodes have good structural flexibility and wide chemical diversity; further, they can provide a high specific capacity and high voltage at low Sodium-ion batteries (SIBs) have become increasingly important as next-generation energy storage systems for application in large-scale energy storage. It is very crucial to develop an eco-friendly and green SIB technique with superior performance for sustainable future use. Replacing the conventional inorganic electrode materials with green and safe organic electrodes will be a promising approach. However, the poor electrochemical kinetics, unstable electrode-electrolyte interface, high solubility of the electrodes in the electrolyte, and large amount of conductive carbon present great challenges for organic SIBs. In this study, the issues of organic electrodes are addressed ...

The demand for energy storage is exponentially increasing with growth of the human population, which is highly energy intensive. Batteries, supercapacitors, and hybrid capacitors are key energy storage technologies, and lithium and sodium ions are critical influencers in redefining the performances of such devices. Batteries can store energy with high density, and capacitors can deliver a high power density. In addition, hybrid capacitors bridge the energy and power gap between a battery and supercapacitor by combining reactions from a battery-type electrode and a capacitor-type electrode. Sodium-ion hybrid capacitors (NICs) can combine the benefits of high power capacitors and high energy batteries at a cost potentially lower than that of Li analogues. However, research on NICs is in the nascent stage and requires significant attention to enable their use in practical applications. This review presents a comprehensive summary of the development of Na-ion hybrid capacitors based on carbon materials, a sodium superionic conductor NASICON, and metal oxide or sulfide-type anodes, with a particular emphasis on the performance metrics. Furthermore, design strategies and unsolved issues in emerging capacitor systems, such as pseudocapacitive electrodes, organic electrodes, MXenes, and flexible capacitors, which could be trend setters for next-generation applications, are the focus. The revolving issues with each system and the strategies to overcome such issues are also briefly discussed. A perspective and outlook on the future of NICs will help the scientific community direct their future studies.

Sodium metal (Na) anodes are considered the most promising anode for high‐energy‐density sodium batteries because of their high capacity and low electrochemical potential. However, Na metal anode undergoes uncontrolled Na dendrite growth, and unstable solid electrolyte interphase layer (SEI) formation during cycling, leading to poor coulombic efficiency, and shorter lifespan. Herein, a series of Na‐ion conductive alloy‐type protective interface (Na‐In, Na‐Bi, Na‐Zn, Na‐Sn) is studied as an artificial SEI layer to address the issues. The hybrid Na‐ion conducting SEI components over the Na‐alloy can facilitate uniform Na deposition by regulating Na‐ion flux with low overpotential. Furthermore, density functional study reveals that the lower surface energy of protective alloys relative to bare Na is the key factor for facilitating facile ion diffusion across the interface. Na metal with interface layer facilitates a highly reversible Na plating/stripping for ≈790 h, higher than pristine Na metal (100 h). The hybrid self‐regulating protective layers exhibit a high mechanical flexibility to promote dendrite free Na plating even at high current density (5 mA cm−2), high capacity (10 mAh cm−2), and good performance with Na3V2(PO4)3 cathode. The current study opens a new insight for designing dendrite Na metal anode for next generation energy storage devices.

Sodium‐ion batteries (SIBs) hold great potential for use in large‐scale grid storage applications owing to their low energy cost compared to lithium analogs. The symmetrical SIBs employing Na3V2(PO4)3 (NVP) as both the cathode and anode are considered very promising due to negligible volume changes and longer cycle life. However, the structural changes associated with the electrochemical reactions of symmetrical SIBs employing NVP have not been widely studied. Previous studies on symmetrical SIBs employing NVP are believed to undergo one mole of Na+ storage during the electrochemical reaction. However, in this study, it is shown that there are significant differences during the electrochemical reaction of the symmetrical NVP system. The symmetrical sodium‐ion cell undergoes ≈2 moles of Na+ reaction (intercalation and deintercalation) instead of 1 mole of Na+. A simultaneous formation of Na5V2(PO4)3 phase in the anode and NaV2(PO4)3 phase in the cathode is revealed by synchrotron‐based X‐ray diffraction and X‐ray absorption spectroscopy. A symmetrical NVP cell can deliver a stable capacity of ≈99 mAh g‐1, (based on the mass of the cathode) by simultaneously utilizing V3+/V2+ redox in anode and V3+/V4+ redox in cathode. The current study provides new insights for the development of high‐energy symmetrical NIBs for future use.

Increasing capacity and cycle stability is key to successfully commercializing sodium battery technologies. Currently, cation-rich cathodes are well suited for this trend owing to superior overall performance through fluorine substitution in the cation. However, the exact effects and synergy due to fluorine substitution as anions are still unknown. Understanding such synergistic effects can significantly facilitate increasing the capacity, leading to maximum performance at optimum conditions. In this work, the systematic addition of F into sodium-layered oxide and its corresponding changes in the crystal structure or phase formation are extensively studied. More specifically, the effect of fluorine substitution in the anion site on Na diffusion and the Na−Tm polyhedral are extensively studied using X-ray diffraction and subsequent Rietveld analysis. In addition, the effects of F on transition metals (i.e., Mn) and its valence states are also analyzed using X-ray absorption spectroscopy. Na 1.2 Mn 0.8 O 1.5 F 0.5 not only showed superior capacity of approximately 172 mAh g −1 and capacity retention but also remarkable cycling stability for up to 300 cycles at higher current densities. Improvements in the performance due to distortion induced by F substitution are also presented. This study provides insight into the transition of cathode material properties from F doping (y = 0−0.2) to F substitution (y ≥ 0.3) and the associated structural changes and capacity improvements.

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