Over the past three decades, intensive research activities have focused on the development of electrochemical energy storage devices, particularly exploiting the concept of flow batteries. Amongst these, vanadium redox flow batteries (VRFB) are an attractive option, which have been studied extensively and are now being commercialized around the world. The performance of the VRFB system is governed by several critical components namely the electrolyte, the electrode, the ion‐exchange membrane and the flow field design. Here, the focus is mainly on recent research activities relating to the development and modification of electrode materials and new ion‐exchange membranes. The feasibility of novel flow field designs for high energy density VRFB systems and their future prospects are also discussed in detail.

lithium with cheaper alternatives might make future batteries less susceptible to price fluctuations when the market expands. [5] This concern has in turn spawned a flurry of research activity to look beyond LIB technology. In view of the various rechargeable batteries, sodiumion batteries (SIBs) that consist of a main element of seawater made their presence felt in the battery industry due to their chemical similarity (e.g., only 0.3 V more positive than lithium) but higher abundance compared to lithium, [6] as illustrated in Table 1. SIBs work in the same way as LIBs in that they involve the reversible migration of cations/anions across a separator toward the electrodes to realize voltage-driven electrochemical reactions. [7] The upward trend for sodium-ion battery research is driven by the concern over the scarcity and price rise of lithium. Hence, these major merits, as well as the suitable redox potential (E = 0.3 V vs Li) make the SIB an appropriate choice as a rechargeable battery system.In fact, SIBs started almost in parallel with LIBs in the 1980s but the development of LIBs eventually eclipsed SIBs due to the electrochemical performances of SIBs. [8,9] A major obstacle is that it is difficult to get a sodium host material with comparable operating voltage and capacity as the LIB analogs. Firstly, the larger Na + radius (0.98 Å) than that of Li + (0.69 Å) leads to the kinetically sluggish Na + insertion/extraction and transport cross the host-material framework, which will always make the specific capacity and rate capacity largely degraded. [10] Secondly, the larger volume expansion caused by Na + insertion will also bring a change in the phase and lattice of the host materials, making it difficult to achieve a favorable electrochemical stability compared with the LIB counterparts. [11] In addition, they also suffer from a lower specific energy than LIBs, due to the lower potential and larger atomic weight of sodium. It is still a challenge to build an affordable Na-host materials endowed with a high specific energy (e.g., 500 W h kg −1 in LIBs). [12] Recently, the topic of SIBs was revisited in the hope of exploring possible ways to overcome the concerns about the low energy density and poor cycling life of SIBs.In spite of the above distinct drawbacks, SIBs can actually behave slightly differently in some chemical aspects. For instance, sodium does not react with aluminum, which is contrary to the alloying tendency of lithium. In the commercial market, manufacturers could appreciate this "inertness" to reduce the production cost of batteries by substituting the Among the various energy solutions, lithium-ion batteries (LIBs) play an important role in the process of the transition from fossil fuels to renewables. However, the necessity to replace lithium with cheaper alternatives due to its scarcity has recently attracted great interest to developing sodium-ion batteries (SIBs). Hence, the discovery and development of suitable cathode materials that exhibit high specific capacity, good cycling stabil...

Research progress in the development of various battery type and supercapacitor type materials is discussed from the point of view of Na-ion capacitors.

Two-dimensional (2-D) polymer has properties that are attractive for energy storage applications because of its combination of heteroatoms, porosities and layered structure, which provides redox chemistry and ion diffusion routes through the 2-D planes and 1-D channels. Here, conjugated aromatic polymers (CAPs) were synthesized in quantitative yield via solid-state polymerization of phenazine-based precursor crystals. By choosing flat molecules (2-TBTBP and 3-TBQP) with different positions of bromine substituents on a phenazine-derived scaffold, C-C cross coupling was induced following thermal debromination. CAP-2 is polymerized from monomers that have been prepacked into layered structure (3-TBQP). It can be mechanically exfoliated into micrometer-sized ultrathin sheets that show sharp Raman peaks which reflect conformational ordering. CAP-2 has a dominant pore size of ∼0.8 nm; when applied as an asymmetric supercapacitor, it delivers a specific capacitance of 233 F g at a current density of 1.0 A g, and shows outstanding cycle performance.

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Modern wearable technologies require highly efficient energy storage devices for improving their performance attributing a controlled shape and flexibility. Here, the present study is focused on the fabrication of a highly improved V2O5 thin film based supercapacitor. Symmetric device of V2O5||PVA-KOH||V2O5 was fabricated using thin flexible electrodes prepared by a thermal evaporation technique. The symmetric supercapacitor devices were fabricated using both annealed and as-prepared films separately, and their performance characteristics were compared. The V2O5 symmetric device having pseudocapacitive behavior delivered a maximum specific areal capacitance of 9.7 mF cm–2 at a scan rate of 10 mV s–1. The symmetric microcapacitor also showed promising performance even after being bent at 60 and 120°. This indicates the reliability of the fabricated devices for flexible electronic applications. Moreover, the symmetric capacitor showed excellent capacitance retention (95%) even after 30 000 cycles with the coulombic efficiency of 99%. Furthermore, practical feasibility of the as fabricated devices was demonstrated by lighting blue light-emitting diodes by connecting them in series. On the other hand, characteristics of the V2O5 thin films were also studied using X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, field emission scanning electron microscopy, and atomic force microscopy analysis.