Phase- and Crystal Structure-Controlled Synthesis of Bi₂O₃, Fe₂O₃, and BiFeO₃ Nanomaterials for Energy Storage Devices

Controlling the phase and crystal structure of nanomaterials is a challenging mission in a wet chemical method and has remarkable importance to the materials properties. Herein, we demonstrate a facile sol–gel method to synthesize Bi2O3, Fe2O3, BiFeO3, Bi36Fe2O57, secondary phase, and mixed phase of BiFeO3 (Bi25FeO40 and Bi2Fe4O9) by tailoring the parameters such as molar concentration, calcination temperature, and duration. Further, all the electrode materials were demonstrated for supercapacitor (SC) application. The pure-phase BiFeO3 nanoparticles show a highest specific capacitance of 253 F/g at a current density of 1 A/g compared to all other electrodes under a 3 M KOH electrolyte. The higher specific capacitance of BiFeO3 nanoparticles is ascribed to their higher surface area, pure ABO3 structure, and lower charge-transfer resistance. Moreover, the BiFeO3 nanoparticles were also tested under a neutral electrolyte (1 M Na2SO4) and found to have 3.7 times lower specific capacitance compared to the alkaline electrolyte (3 M KOH). The electrokinetic study of the as-synthesized active electrodes illustrates the maximum capacitive involvement to store the overall charge. The BiFeO3 nanoparticles display outstanding stability with a retention rate of 99.02% after 1100 consecutive galvanostatic charge–discharge cycles at various current densities. Moreover, a solid-state symmetric SC device (SSD) was fabricated using BiFeO3 nanoparticles. The device delivered a maximum energy density of 17.01 W h/kg at a current density of 1 A/g and a power density of 7.2 kW/kg at a current density of 10 A/g. The BiFeO3 SSD showed an excellent capacitive retention rate of 88% after 5000 cycles, suggesting that it could be a promising electrode material for practical application in energy storage devices

Microwave-Assisted Solvothermal Synthesis of Cupric Oxide Nanostructures for High-Performance Supercapacitor

Enhancing the performance and stability of the low-cost materials for electrochemical energy storage device is an important aspect. Herein, we report microwave-assisted solvothermal synthesis of three-dimensional (3D) spherical CuO structures composed of either one-dimensional (rod-like) or two-dimensional (2D) flake-like building blocks by varying the reaction medium, i.e., water and ethylene glycol (EG). A higher EG in the reaction medium facilitates formation of the flake-like structures. A specific surface area of 168.47 m² g^–1 is achieved with the 3D flower-like CuO, synthesized using copper acetate precursor in 1:3 water/EG solvent ratio. The same sample delivers a specific capacitance of 612 F g^–1 at an applied current density of 1 A g^–1 and shows high stability with capacity retention of 98% after 4000 galvanostatic charge–discharge cycles. The high specific capacitance of flower-shaped CuO architecture is attributed to large surface area and availability of sufficient pores for ions diffusion. Furthermore, two-electrode asymmetric supercapacitor device is fabricated using the 3D flower-shaped CuO as positive electrode and activated carbon as negative electrode, which shows an energy density of 27.27 Wh kg^–1 at a power density of 800 W kg^–1. This underlines the potential of inexpensive CuO architecture as an active material for energy storage devices

Bond-Energy-Driven, Low- or High-Angle-Grain-Boundary-Movement-Mediated Synthesis of Porous Se–Te for Use in Water-Splitting Reactions

Herein, for the first time, we applied the metal–metal-bond-energy factor to the evolution of a porous Se–Te alloy. The porous Se–Te material has been prepared from the constituents’ elemental states, through only a heating–cooling process in silicone oil without the use of any reagent, surfactant, or capping agent. Surprisingly, the reaction occurred at a much lower temperature (240 °C) than the mp (450 °C) of Te⁰. The reaction’s nucleation and growth by means of varied bond energy have been clarified for the first time. A difference in the bond energies of a hetero metal–metal bond (Se–Te) and a homo metal–metal bond (Se–Se) directs nucleation and growth toward the fabrication of a porous structure, even from the constituents’ elemental states, in which low-angle-grain-boundary (LAGB) and high-angle-grain-boundary (HAGB) movements play governing roles. Proper band-gap alignment of Se and Te makes the alloy composite applicable to water-splitting reactions under Xe-arc-lamp illumination. PEC efficiency of Se–Te was found to be higher than those reported for Se and other composite materials

Nitrogen-Enriched Nanoporous Polytriazine for High-Performance Supercapacitor Application

Polytriazine with high nitrogen content (c.a. 50.5 wt %) has been synthesized by an ultrafast microwave-assisted method using melamine and cyanuric chloride. The nitrogen-enriched nanoporous polytriazine (NENP-1) has exhibited high specific surface area (maximum SA_BET of 838 m² g^–1) and narrow pore size distribution. The NENP-1 has been employed as electrode material for supercapacitor application. A maximum specific capacitance (C_sp) of 1256 F g^–1 @1 mV s^–1 and 656 F g^–1 @1 A g^–1 are estimated from the cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD) measurements, respectively, in a three-electrodes configuration. This C_sp value is considered as very high for a nonmetallic system (organic polymer). Superior capacitance retention of 87.4% of its initial C_sp was observed after 5000 cycles at a current density of 5 A g^–1 and demonstrates its potential as an efficient electrode material for practical applications. To test this claim, an asymmetric supercapacitor device (ASCD) was fabricated. The C_sp values of the device in the two-electrode configuration are 567 F g^–1 @5 mV s^–1 and 287 F g^–1 @4 A g^–1 in the CV and GCD measurements, respectively. The ASCD has shown superior energy density and power density of 102 Wh kg⁻¹ and 1.6 kW kg^–1, respectively, at the current density of 4 A g^–1. The energy density is much higher than the best reported supercapacitors and also close to the commercial batteries. This indicates the material could bridge the gap between the commercial batteries and supercapacitors