Sodium-ion hybrid capacitors are attracting great attention and are emerging as promising energy storage devices, with their remarkable footsteps in energy and power densities. However, the development of efficient electrode materials that result in a minimum trade-off between their energy and power densities, and that allow long-term cycling stability, still remains a challenge for realizing their full potential as an alternate energy storage system for commercial applications. Herein, for the first time, we study the sodium-ion intercalation pseudocapacitance behavior of brown TiO2 nanotubes for their application as an efficient anode material for Na-ion hybrid capacitors. We synthesized semicrystalline and crystalline anatase brown TiO2 nanotubes, aggregated in a flowerlike morphology, through a hydrothermal route, and performed detailed electrochemical studies. The kinetic studies reveal that semicrystalline brown TiO2 exhibits a Na-ion intercalation pseudocapacitive behavior with 57% of capacitive storage at 1.0 mV s–1, whereas crystalline brown TiO2 is more faradaic in nature. Further, hybrid Na-ion capacitors are fabricated with brown TiO2 materials as an anode and activated carbon as a cathode, and the fabricated device showed an excellent electrochemical performance with a high energy density of ∼68 Wh kg–1 and a high power density of ∼12.5 kW kg–1 and with a good cycling stability up to 10 000 cycles with ∼80% capacitive retention. The obtained results represent a promising approach toward developing efficient electrodes for hybrid Na-ion capacitors.

Phosphorene has attracted great interest in the rapidly emerging field of two-dimensional layered nanomaterials. Recent studies show promising electrocatalytic activity of few-layered phosphorene sheets toward the oxygen evolution reaction (OER). However, controllable synthesis of mono/few-layered phosphorene nanostructures with a large number of electrocatalytically active sites and exposed surface area is important to achieve significant enhancement in OER activity. Here, a novel strategy for controlled synthesis and in situ surface functionalization of phosphorene quantum dots (PQDs) using a single-step electrochemical exfoliation process is demonstrated. Phosphorene quantum dots functionalized with nitrogen-containing groups (FPQDs) exhibit efficient and stable electrocatalytic activity for OER with an overpotential of 1.66 V @ 10 mA cm–2, a low Tafel slope of 48 mV dec–1, and excellent stability. Further, we observe enhanced electron transfer kinetics for FPQDs toward the Fe2+/Fe3+ redox probe in comparison with pristine PQDs. The results demonstrate the promising potential of phosphorene as technologically viable OER electrodes for water-splitting devices.

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Transition metal oxysulfides (TMOS) exhibit promising catalytic properties for hydrogen evolution reactions (HER). However, the development of facile and controllable routes for obtaining nanostructured TMOS under ambient conditions still remains a significant challenge. Here we report a simple and controllable route to synthesize nanoparticles of tungsten oxysulfides (WO x S y ) that exhibit enhanced electrocatalytic activity toward HER with outstanding stability. The sulfur-rich tungsten oxysulfides with engineered anionic species can offer multiple functionalities, including abundant active sites and improved conductivity that synergistically contribute to enhanced electrocatalytic activity for HER. The optimized WO x S y electrocatalyst shows low overpotential of 103 mV at a current density of 10 mA cm–2, along with a Tafel slope of 54 mV decade–1 and 5.89 × 10–2 mA cm–2 exchange current density. Density functional theory (DFT) based calculations further establish the improved catalytic activity of tungsten oxysulfide (WO x S y ), compared to the pristine 1T-WS2, based on the free energy calculations. The present work demonstrates a highly promising approach toward the development of cost-effective, efficient, and durable electrocatalysts to replace precious metals for electrocatalytic hydrogen generation.

The development of earth-abundant and highly efficient electrocatalysts for hydrogen evolution reaction (HER) in alkaline media is essential for practical alkaline water electrolysis. The possibility of tuning the electrocatalytic activity of alkaline HER electrocatalysts through various approaches, such as interfacial engineering or doping, has been recently explored. In this work, electrochemically exfoliated Co­(OH)2 and chemically derived 1T-MoS2 nanostructures are electrostatically coupled to form a synergistic nanostructured two-dimensional heterostructure, which is shown to remarkably improve the HER activity in an alkaline medium. The Co­(OH)2/1T-MoS2 heterostructure with an optimal 1:5 ratio (Co1Mo5) showed a low overpotential of 151 mV at a current density of 10 mA cm–2 and a Tafel slope of 94 mV dec–1 in alkaline media. The shift in the overpotential achieved for the heterostructure (>250 mV) compared to the individual MoS2 component is remarkably high, as per the earlier reports.