Optimal Rule-of-Thumb Design of Nickel-Vanadium Oxides as an Electrochromic Electrode with Ultrahigh Capacity and Ultrafast Color Tunability

The use of electrodes capable of functioning as both electrochromic windows and energy storage devices has been extended from green building development to various electronics and displays to promote more efficient energy consumption. Herein, we report the electrochromic energy storage of bimetallic NiV oxide (NiVO) thin films fabricated using chemical bath deposition. The best optimized NiVO electrode with a Ni/V ratio of 3 exhibits superior electronic conductivity and a large electrochemical surface area, which are beneficial for enhancing electrochemical performance. The color switches between semitransparent (a discharged state) and dark brown (a charged state) with excellent reproducibility because of the intercalation and deintercalation of OH^- ions in an alkaline KOH electrolyte. A specific capacity of 2403 F g^-1, a coloration efficiency of 63.18 cm² C^-1, and an outstanding optical modulation of 68% are achieved. The NiVO electrode also demonstrates ultrafast coloration and bleaching behavior (1.52 and 4.79 s, respectively), which are considerably faster than those demonstrated by the NiO electrode (9.03 and 38.87 s). It retains 91.95% capacity after 2000 charge-discharge cycles, much higher than that of the NiO electrode (83.47%), indicating that it has significant potential for use in smart energy storage applications. The superior electrochemical performance of the best NiVO compound electrode with an optimum Ni/V compositional ratio is due to the synergetic effect between the high electrochemically active surface area induced by V-doping-improved redox kinetics (low charge-transfer resistance) and fast ion diffusion, which provides a facile charge transport pathway at the electrolyte/electrode interface.

A Robust Nonprecious CuFe Composite as a Highly Efficient Bifunctional Catalyst for Overall Electrochemical Water Splitting

To generate hydrogen, which is a clean energy carrier, a combination of electrolysis and renewable energy sources is desirable. In particular, for both the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in electrolysis, it is necessary to develop nonprecious, efficient, and durable catalysts. A robust nonprecious copper-iron (CuFe) bimetallic composite is reported that can be used as a highly efficient bifunctional catalyst for overall water splitting in an alkaline medium. The catalyst exhibits outstanding OER and HER activity, and very low OER and HER overpotentials (218 and 158 mV, respectively) are necessary to attain a current density of 10 mA cm^-2 . When used in a two-electrode water electrolyzer system for overall water splitting, it not only achieves high durability (even at a very high current density of 100 mA cm^-2 ) but also reduces the potential required to split water into oxygen and hydrogen at 10 mA cm^-2 to 1.64 V for 100 h of continuous operation.

Two-Dimensional Layered Hydroxide Nanoporous Nanohybrids Pillared with Zero-Dimensional Polyoxovanadate Nanoclusters for Enhanced Water Oxidation Catalysis

The oxygen-evolution reaction (OER) is critical in electrochemical water splitting and requires an efficient, sustainable, and cheap catalyst for successful practical applications. A common development strategy for OER catalysts is to search for facile routes for the synthesis of new catalytic materials with optimized chemical compositions and structures. Here, nickel hydroxide Ni(OH)₂ 2D nanosheets pillared with 0D polyoxovanadate (POV) nanoclusters as an OER catalyst that can operate in alkaline media are reported. The intercalation of POV nanoclusters into Ni(OH)₂ induces the formation of a nanoporous layer-by-layer stacking architecture of 2D Ni(OH)₂ nanosheets and 0D POV with a tunable chemical composition. The nanohybrid catalysts remarkably enhance the OER activity of pristine Ni(OH)₂ . The present findings demonstrate that the intercalation of 0D POV nanoclusters into Ni(OH)₂ is effective for improving water oxidation catalysis and represents a potential method to synthesize novel, porous hydroxide-based nanohybrid materials with superior electrochemical activities.

Self-Assembled Nanostructured CuCo2 O4 for Electrochemical Energy Storage and the Oxygen Evolution Reaction via Morphology Engineering

CuCo₂ O₄ films with different morphologies of either mesoporous nanosheets, cubic, compact-granular, or agglomerated embossing structures are fabricated via a hydrothermal growth technique using various solvents, and their bifunctional activities, electrochemical energy storage and oxygen evolution reaction (OER) for water splitting catalysis in strong alkaline KOH media, are investigated. It is observed that the solvents play an important role in setting the surface morphology and size of the crystallites by controlling nucleation and growth rate. An optimized mesoporous CuCo₂ O₄ nanosheet electrode shows a high specific capacitance of 1658 F g^-1 at 1 A g^-1 with excellent restoring capability of ≈99% at 2 A g^-1 and superior energy density of 132.64 Wh kg^-1 at a power density of 0.72 kW kg^-1 . The CuCo₂ O₄ electrode also exhibits excellent endurance performance with capacity retention of 90% and coulombic efficiency of ≈99% after 5000 charge/discharge cycles. The best OER activity is obtained from the CuCo₂ O₄ nanosheet sample with the lowest overpotential of ≈290 mV at 20 mA cm^-2 and a Tafel slope of 117 mV dec^-1 . The superior bifunctional electrochemical activity of the mesoporous CuCo₂ O₄ nanosheet is a result of electrochemically favorable 2D morphology, which leads to the formation of a very large electrochemically active surface area.

Direct growth of 2D nickel hydroxide nanosheets intercalated with polyoxovanadate anions as a binder-free supercapacitor electrode

A mesoporous nanoplate network of two-dimensional (2D) layered nickel hydroxide Ni(OH)2 intercalated with polyoxovanadate anions (Ni(OH)2-POV) was built using a chemical solution deposition method. This approach will provide high flexibility for controlling the chemical composition and the pore structure of the resulting Ni(OH)2-POV nanohybrids. The layer-by-layer ordered growth of the Ni(OH)2-POV is demonstrated by powder X-ray diffraction and cross-sectional high-resolution transmission electron microscopy. The random growth of the intercalated Ni(OH)2-POV nanohybrids leads to the formation of an interconnected network morphology with a highly porous stacking structure whose porosity is controlled by changing the ratio of Ni(OH)2 and POV. The lateral size and thickness of the Ni(OH)2-POV nanoplates are ∼400 nm and from ∼5 nm to 7 nm, respectively. The obtained thin films are highly active electrochemical capacitor electrodes with a maximum specific capacity of 1440 F g-1 at a current density of 1 A g-1, and they withstand up to 2000 cycles with a capacity retention of 85%. The superior electrochemical performance of the Ni(OH)2-POV nanohybrids is attributed to the expanded mesoporous surface area and the intercalation of the POV anions. The experimental findings highlight the outstanding electrochemical functionality of the 2D Ni(OH)2-POV nanoplate network that will provide a facile route for the synthesis of low-dimensional hybrid nanomaterials for a highly active supercapacitor electrode.