Preparing Ni3S2 composite with neural network-like structure for high-performance flexible asymmetric supercapacitors

Mo-doped Co3O4 ultrathin nanosheet arrays anchored on nickel foam as a bi-functional electrode for supercapacitor and overall water splitting

Simple preparation, favorable price and environmental protection have been a long-term challenge in the field of electrochemistry. Herein, we studied and prepared a bifunctional Mo-doped Co₃O₄ ultrathin nanosheets, which has been validated as an effective binder-free electrode material for electrocatalytic water splitting and supercapacitors. The material has a large specific surface area, high electrical conductivity and exposure to more active sites, breaking down the limited performance and range of use of transition metal oxides. Benefiting from intriguing ultrathin property and conductivity, OER and HER process of 0.4Mo-Co₃O₄ have a small Tafel slope of 83.7 and 98 mV dec^-1, respectively. The current density at 10 mA cm^-2 show a low overpotential of 315 and 79 mV and significant stability. The water electrolytic device requires a potential of 1.64 V to reach 10 mA cm^-2, and the potential change is negligible after 12 h of continuous electrolysis. In addition, the manifest improved electrochemical performance of 0.3Mo-Co₃O₄ as supercapacitor electrode material shows high areal capacitance 2815 mF cm^-2 at 1 mA cm^-2, excellent rate performance (85% at 10 mA cm^-2) and retains 90% of the initial capacitance by cycling 5000 at a current density of 10 mA cm^-2. Moreover, 0.3Mo-Co₃O₄||0.3Mo-Co₃O₄ symmetrical supercapacitor has a maximum volumetric energy density of 1.25 mW h cm^-3 at a power density of 7.1 mW cm^-3 and superior cycle life. The influence of doping on electrochemical performance was studied by changing the content of doped metal ions, which is of great significance for the exploration of supercapacitor and electrocatalytic hydrolysis of bifunctional electrode materials.

Superior Pseudocapacitive Storage of a Novel Ni3Si2/NiOOH/Graphene Nanostructure for an All-Solid-State Supercapacitor

Recent developments in the synthesis of graphene-based structures focus on continuous improvement of porous nanostructures, doping of thin films, and mechanisms for the construction of three-dimensional architectures. Herein, we synthesize creeper-like Ni₃Si₂/NiOOH/graphene nanostructures via low-pressure all-solid melting-reconstruction chemical vapor deposition. In a carbon-rich atmosphere, high-energy atoms bombard the Ni and Si surface, and reduce the free energy in the thermodynamic equilibrium of solid Ni-Si particles, considerably catalyzing the growth of Ni-Si nanocrystals. By controlling the carbon source content, a Ni₃Si₂ single crystal with high crystallinity and good homogeneity is stably synthesized. Electrochemical measurements indicate that the nanostructures exhibit an ultrahigh specific capacity of 835.3 C g^-1 (1193.28 F g^-1) at 1 A g^-1; when integrated as an all-solid-state supercapacitor, it provides a remarkable energy density as high as 25.9 Wh kg^-1 at 750 W kg^-1, which can be attributed to the free-standing Ni₃Si₂/graphene skeleton providing a large specific area and NiOOH inhibits insulation on the electrode surface in an alkaline solution, thereby accelerating the electron exchange rate. The growth of the high-performance composite nanostructure is simple and controllable, enabling the large-scale production and application of microenergy storage devices.

Nanostructured Carbon-Nitrogen-Sulfur-Nickel Networks Derived From Polyaniline as Bifunctional Catalysts for Water Splitting

The development of reliable production routes for sustainable hydrogen (H₂), which is an essential feedstock for industrial processes and energy carrier for fuel cells, is needed. It appears to be an unavoidable alternative to significantly reduce the dependence on conventional energy sources based on fossil fuels without increasing the atmospheric CO₂ levels. Among the different power-to-X scenarios to access high purity H₂, the electrochemical approach based on electrolysis looks to be a promising sustainable solution at both the small and large industrial scales. However, the practical realization of this important opportunity faces several challenges, including the efficient design of cost-effective catalytic materials to be used as a cathode with improved intrinsic and durable activity. In this contribution, we report the design and development of efficient nanostructured catalysts for the electrocatalytic hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in aqueous media, whereby noble metal-free elements are embedded in a matrix of a conducting polymer, polyaniline (PANI). To increase the electrical conductivity and further the electrocatalytic ability toward HER of the chemically polymerized PANI in the presence of nickel (II) salt (nitrate), the PANI-based materials have first been stabilized at a mild temperature of 250-350°C in air and then carbonized at 800-1,000°C under nitrogen gas to convert the chemical species into nitrogen, sulfur, nickel, and carbon nanostructured networks (CNNs). Different physicochemical (TGA-DSC, Raman spectroscopy, XRD, SEM, EDX, ICP, CHNS, BET, and XPS) and electrochemical (voltammetry and electrochemical impedance spectrometry) methods have been integrated to characterize the as-synthesized CNNs materials and interrogate the relationship of material-to-performance. It has been found that those synthesis conditions allow for the substantial increase of the electrocatalytic performance toward HER and OER in alkaline media in terms of the onset potential and charge transfer resistance and overpotential at the specific activity of 10 milliamps per square centimeter, thus ranking the present materials among the most efficient noble metal-free catalysts and making them possible candidates for integration in practical low-energy consumption alkaline electrolyzers.

Three dimensional Ni3S2 nanorod arrays as multifunctional electrodes for electrochemical energy storage and conversion applications

The increasing demand for energy and environmental protection has stimulated intensive interest in fundamental research and practical applications. Nickel dichalcogenides (Ni3S2, NiS, Ni3Se2, NiSe, etc.) are promising materials for high-performance electrochemical energy storage and conversion applications. Herein, 3D Ni3S2 nanorod arrays are fabricated on Ni foam by a facile solvothermal route. The optimized Ni3S2/Ni foam electrode displays an areal capacity of 1602 µA h cm-2 at 5 mA cm-2, excellent rate capability and cycling stability. Besides, 3D Ni3S2 nanorod arrays as electrode materials exhibit outstanding performances for the overall water splitting reaction. In particular, the 3D Ni3S2 nanorod array electrode is shown to be a high-performance water electrolyzer with a cell voltage of 1.63 V at a current density of 10 mA cm-2 for overall water splitting. Therefore, the results demonstrate a promising multifunctional 3D electrode material for electrochemical energy storage and conversion applications.