Capacitor starts its journey from 1745, and still moves forward in form of supercapacitor. Supercapacitor is one of the advanced forms of capacitor with higher energy density that bridges between...

Hybrid supercapacitors are the most desirable electrochemical energy storage devices, owing to their versatile and tunable performance characteristics, specifically in energy and power densities, towards applications in research and development. Construction‐wise, optimized assembly of batteries (energy devices) and supercapacitors (power devices) are the key for hybrid supercapacitors. Based on scientific advancements and technological achievements, hybrid ion capacitors are the most important segments in hybrid supercapacitors, as well as in the overall energy storage arena. Herein, opportunities and challenges of hybrid ion capacitors are intensively addressed in light of lithium‐ion, sodium‐ion, potassium‐ion, magnesium‐ion, calcium‐ion, zinc‐ion, and aluminum‐ion capacitors. The historical origins and their developmental pathways are identified for each type of capacitor. Possible classes of materials for every hybrid ion capacitor are discussed, and relevant mechanisms are demonstrated. These discussions reveal that a rich materials bank exists for lithium‐ion, sodium‐ion, and zinc‐ion capacitors, but the same is not applicable for potassium‐ion, magnesium‐ion, calcium‐ion, and aluminum‐ion capacitors. Consequently, such hybrid ion capacitors have not yet reached the level of commercial benchmarks like lithium‐ion, sodium‐ion, and zinc‐ion capacitors. However, this Review focuses on mostly full‐cell device data that synchronize the performances of practical scaled‐up systems. Several electrolytes based on solvent media (aqueous, organic, and ionic liquid), phase (liquid, gel, and solid), and redox activity (active and passive) are exemplified in different sections of hybrid ion capacitors. Various device constructions are elaborated upon, such as liquid‐electrolyte devices, polymeric gel devices, all‐solid‐state devices, flexible‐cum‐wearable devices, microdevices, solar‐charging devices, and so forth. The Review culminates with feasible future directions for the commercial success of hybrid ion capacitors, which are in the nascent stages of developments. To the best of our knowledge, it is the first holistic account of hybrid ion capacitors from their historical perspectives to present developments.

Recent developments in supercapacitor technology in terms of materials and devices are reviewed herein. Beyond the conventional materials (i. e., carbonaceous matters, metallic compounds and conducting polymers), various multifunctional materials are reported in literature as future supercapacitive materials. A comprehensive account on such materials is lacking due to the diversified electrochemical characteristics of these materials. In this review, we bring all such non‐conventional multifunctional energy storage materials under a same umbrella for summarizing the recent advancements in supercapacitors. The envisaged multifunctional materials include metal‐organic‐frameworks (MOFs), covalent‐organic‐frameworks (COFs), heteroatom‐doped carbonaceous materials, biomass‐derived porous carbons, black phosphorous, mixed conductors, perovskite nanoparticles, polyoxometalates (POMs), redox active electrolytes, slurry materials for flow supercapacitors, thermal self‐charging materials, thermal self‐protective materials, piezoelectric materials and electrochromic materials. Inherent pros and cons of each class of material are discussed, and materials modifications towards the successful device fabrications are highlighted herewith. While the MOF‐based supercapacitors are drawing some attentions, other non‐conventional energy storage materials are truly in the nascent stage of developments. This review culminates with summary and proposed future directions for product developments. In brief, this article provides a holistic view regarding all non‐conventional multifunctional energy storage materials for future supercapacitor technology.

A unique configuration of aqueous Na-ion batteries is investigated for solar energy storage, where single-wall carbon nanotube (SWCNT)-coated stainless steel (SS304), Co-Prussian blue analogue (Co-PBA/Na 2 CoFe(CN) 6 ), and sodium vanadate (NVO/NaV 3 O 8 ) nanorods are employed as a current collector, positive active material, and negative active material, respectively. The SWCNT coverage on SS radically obstructs the metallic corrosion under anodic polarization and also enhances the electrolyte stability window by preventing direct contact between the metal substrate and electrolyte. Both the positive and negative materials are structurally analyzed by Rietveld refinement of powder X-ray diffraction data. The Co-PBA framework structure demonstrates one-dimensional channels with ∼5.3 and 5.1 Å widths along [100] and [011], respectively, whereas the layered NVO depicts an interlayer spacing of ∼4.2 Å for facile Na-ion transportations. Resultantly, the high diffusion coefficients of Na-ions in Co-PBA and NVO are achieved as 1.6 × 10 −13 and 2.0 × 10 −11 cm 2 s −1 , respectively. Both Co-PBA and NVO exhibit a diffusioncontrolled Faradaic charge storage mechanism, which has been demonstrated by cyclic voltammetry. The Co-PBA provides 122 mAh g −1 specific capacity at 1C rate, which is the highest reported value in aqueous medium with low-cost current collectors. The electrochemical performance testing of NaV 3 O 8 is first described by us, explicitly for the negative electrode, and it delivers 83 mAh g −1 specific capacity at 1C. The 1.5 V silica gel-based Co-PBA//NVO full cell is fabricated with mass balancing, which shows higher cell voltage by maximizing the water splitting window. The full cell delivers a specific capacity of 141 mAh g −1 (@ 1C), an energy density of 211 Wh kg −1 (@ 250 W kg −1 ), a power density of 2466 W kg −1 (@ 94 Wh kg −1 ), and good durability (80% capacity retention @ 5C) up to 500 cycles. A 3 V/5 mAh rated prototype device is assembled, and it delivers satisfactory solar energy storage performances under 1 week of continuous operation.

Herein, we have explored low cost Prussian blue analogues (PBA), namely Na2MnFe(CN)6 (Mn-PBA) and Na2Mn0.5Ni0.5Fe(CN)6 (MnNi-PBA), as positive electrodes and iron oxides, namely Fe2O3 and Na-incorporated Fe2O3 (NaxFe2O3), as negative...

Herein, we have reviewed the recent developments of rechargeable manganese dioxide−zinc (MnO2−Zn) batteries under both alkaline and mild acidic electrolyte systems. The evolution pathway of MnO2−Zn system from Leclanché cell to alkaline primary batteries and from primary to secondary batteries is chronologically depicted. Several adverse phenomena are associated with the reversibility of metallic zinc negative electrode under alkaline (pH 14) electrolyte mediums, and these may include zinc dendrite formation, passivation of electrode surface, shape change of the electrode, zincate crossover through separator and hydrogen evolution upon charging. The MnO2 positive electrode also experiences few performance degrading issues under alkaline mediums; like generation of electrochemically inert phases (Mn3O4 and ZnMn2O4) in the electrode upon deep‐discharge and Mn‐dissolution in the electrolyte. The mitigation measures of these challenges are well documented and systematically analysed. On the invention of zinc‐ion batteries, the MnO2−Zn secondary batteries are assembled under mild acidic (pH 4–6) electrolytes, and eventually, several adverse effects of alkaline systems are drastically nullified. However, recent scientific and technical efforts are coined to address the challenges of large‐scale MnO2−Zn batteries in mild acidic mediums, and formulate the optimization strategies. This review culminates with a few smart designs of MnO2−Zn batteries, whereas, truly path‐breaking concepts are associated with. To the best of our knowledge, it is the first review that covers the entire spectrum of MnO2−Zn system in both alkaline and mild acidic mediums, along with evolution pathways.

Herein, we have developed a sodium ion based aqueous energy storage device with nickel prussian‐blue‐analogue (Ni‐PBA) positive and functionalized carbon‐black negative electrodes in 1 M Na2SO4 electrolyte solution. The components required to develop the device, i. e., stainless steel (SS) current‐collectors, absorbent‐glass‐mat separator, electrolyte, carbon‐black, and precursors of Ni‐PBA, are all environmentally benign and inexpensive. To minimize the corrosion of pristine‐SS, polyaniline coating on the SS surface is applied by in situ electrodeposition method. The full cell exhibits a specific capacity of 28 mAh g−1 with 90 % Coulomb efficiency (@0.2C), an energy density of 34 Wh kg−1 (@20 W kg−1), a power density of 100 W kg−1 (@18 Wh kg−1) and a good life cycle (70 % capacity‐retention over 500 cycles @1.0C rate) within the 0–1.2 V window. The cell performance is further tested under variable temperatures, and 0–50 °C range is reported to be the working window for this cell.

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