Based on electronic structure and atomic size considerations, nitrogen has been regarded as the most suitable impurity for p-type doping in ZnO. However, numerous experimental efforts by many different groups have not resulted in stable and reproducible p-type material, casting doubt on the efficacy of nitrogen as a shallow acceptor. Based on advanced first-principles calculations we find that nitrogen is actually a deep acceptor, with an exceedingly high ionization energy of 1.3 eV, and hence cannot lead to hole conductivity in ZnO. In light of this result, we reexamine prior experiments on nitrogen doping of ZnO.

http://absimage.aps.org/image/MAR10/MWS_MAR10-2009-001476.pdf

Why nitrogen cannot lead to p-type conductivity in ZnO

Presently, supercapacitors have gained an important space in energy storage modules due to their extraordinarily high power density, although they lag behind the energy density of batteries and fuel cells. This review covers recent approaches to not only increase the power density, rate capability, cyclic stability, etc. of supercapacitors, but also to increase their energy density using hybrid architectures. Electrodes are the most important component of a supercapacitor cell, and thus this review primarily deals with the design of hybrid supercapacitor electrodes offering a high specific capacitance, together with the elucidation of the mechanisms involved therein. The electrode performance significantly depends on the available surface area, porosity and conductivity of the component materials, and thus nano-structuring of the electrode is an elegant approach, which is discussed in the subsections for 0-, 1-, 2-, 3-dimensional hybrid materials, including some miscellaneous hybrids. The fabrication of different hybrid materials using metal oxides, metal sulfides, carbon materials, etc. with conducting polymers such as polyaniline and polypyrrole and their characterization are delineated from the literature data. Here, we primarily focus on the mechanism of energy storage by non-faradic electrical double-layer capacitance and faradaic pseudo-capacitance, discussing the contributions of different component mechanisms towards the total capacitance. In the hybrids, the impact of the component concentration operating via different mechanisms for charge storage on their final electrochemical performance is discussed. The specific capacitance, volumetric capacitance, charge–discharge cycles, Ragone plot, etc. of hybrid supercapacitors are described. Besides household and heavy-duty applications, the state-of-the-art future applications of supercapacitors in robotics, renewable and sustainable energy devices, wearable and self-healing supercapacitors, and biotechnology and their challenges in real-world applications with the scope of future work are elucidated.

A review on the recent advances in hybrid supercapacitors

Metal oxide nanoparticles (MONPs) have enormous applications such as in optical devices, purification systems, biomedical systems, photocatalysis, photovoltaics etc. In this review, we have explored a stable and efficient synthesis protocol of particularly four MONPs: titanium dioxide (TiO2), tin oxide (SnO2), tungsten oxide (WO3) and zinc oxide (ZnO) for getting desired chemical composition, nanostructure, and surface properties. The selection of an efficient synthesis process is a key factor that significantly influences the efficacy of the MONPs. The chemical synthesis of nanoparticles (NPs) via sol–gel route is an effective method to produce high-quality MONPs in comparison to other physical and chemical methods. Sol–gel synthesis is one of the simple, fastest and economically less expensive method, and has its own advantages like low processing temperature, homogeneity of the produced material and formation of the complex structures or composite materials. We believe that this detailed review will provide an insight into sol–gel synthesis of MONPs along with their characterization and diverse applications.

Metal oxides nanoparticles via sol–gel method: a review on synthesis, characterization and applications

In this work, a rational design and construction of porous spherical NiO@NiMoO4 wrapped with PPy was reported for the application of high-performance supercapacitor (SC). The results show that the NiMoO4 modification changes the morphology of NiO, and the hollow internal morphology combined with porous outer shell of NiO@NiMoO4 and NiO@NiMoO4@PPy hybrids shows an increased specific surface area (SSA), and then promotes the transfer of ions and electrons. The shell of NiMoO4 and PPy with high electronic conductivity decreases the charge-transfer reaction resistance of NiO, and then improves the electrochemical kinetics of NiO. At 20 A g−1, the initial capacitances of NiO, NiMoO4, NiO@NiMoO4 and NiO@NiMoO4@PPy are 456.0, 803.2, 764.4 and 941.6 F g−1, respectively. After 10,000 cycles, the corresponding capacitances are 346.8, 510.8, 641.2 and 904.8 F g−1, respectively. Especially, the initial capacitance of NiO@NiMoO4@PPy is 850.2 F g−1, and remains 655.2 F g−1 with a high retention of 77.1% at 30 A g−1 even after 30,000 cycles. The calculation result based on density function theory shows that the much stronger Mo-O bonds are crucial for stabilizing the NiO@NiMoO4 composite, resulting in a good cycling stability of these materials.

Porous spherical NiO@NiMoO4@PPy nanoarchitectures as advanced electrochemical pseudocapacitor materials

The advanced electrochemical properties, such as high energy density, fast charge–discharge rates, excellent cyclic stability, and specific capacitance, make supercapacitor a fascinating electronic device. During recent decades, a significant amount of research has been dedicated to enhancing the electrochemical performance of the supercapacitors through the development of novel electrode materials. In addition to highlighting the charge storage mechanism of the three main categories of supercapacitors, including the electric double-layer capacitors (EDLCs), pseudocapacitors, and the hybrid supercapacitors, this review describes the insights of the recent electrode materials (including, carbon-based materials, metal oxide/hydroxide-based materials, and conducting polymer-based materials, 2D materials). The nanocomposites offer larger SSA, shorter ion/electron diffusion paths, thus improving the specific capacitance of supercapacitors (SCs). Besides, the incorporation of the redox-active small molecules and bio-derived functional groups displayed a significant effect on the electrochemical properties of electrode materials. These advanced properties provide a vast range of potential for the electrode materials to be utilized in different applications such as in wearable/portable/electronic devices such as all-solid-state supercapacitors, transparent/flexible supercapacitors, and asymmetric hybrid supercapacitors.

Electrode Materials for Supercapacitors: A Review of Recent Advances

NiO-CNT composite for high performance supercapacitor electrode and oxygen evolution reaction

Study on charge storage mechanism in working electrodes fabricated by sol-gel derived spinel NiMn2O4 nanoparticles for supercapacitor application

Polyaniline-multiwalled carbon nanotube (PANI-MWCNT): Room temperature resistive carbon monoxide (CO) sensor

Electrochemical properties of TiO2-V2O5 nanocomposites as a high performance supercapacitors electrode material

In this work, we report a facile one-spot synthesis process and the influence of compositional variation on the electrochemical performance of Ni-Mn-oxides (Ni:Mn = 1:1, 1:2, 1:3, and 1:4) for high-performance advanced energy storage applications. The crystalline structure and the morphology of these synthesized nanocomposites have been demonstrated using X-ray diffraction, field emission scanning electron microscopy, and transmission electron Microscopy. Among these materials, Ni-Mn-oxide with Ni:Mn = 1:3 possesses a large Brunauer?Emmett?Teller specific surface area (127 m2 g?1) with pore size 8.2 nm and exhibits the highest specific capacitance of 1215.5 F g?1 at a scan rate 2 mV s?1 with an excellent long-term cycling stability (?87.2% capacitance retention at 10 A g?1 over 5000 cycles). This work also gives a comparison and explains the influence of different compositional ratios on the electrochemical properties of Ni-Mn-oxides. To demonstrate the possibility of commercial application, an asymmetric supercapacitor device has been constructed by using Ni-Mn-oxide (Ni:Mn = 1:3) as a positive electrode and activated carbon (AC) as a negative electrode. This battery-like device achieves a maximum energy density of 132.3 W h kg?1 at a power density of 1651 W kg?1 and excellent coulombic efficiency of 97% over 3000 cycles at 10 A g?1.

/pdf/electrochemical-energy-storage-properties-of-ni-mn-oxide-3rlgf033v2.pdf

Atanu Roy

Papers

NiO-CNT composite for high performance supercapacitor electrode and oxygen evolution reaction

Study on charge storage mechanism in working electrodes fabricated by sol-gel derived spinel NiMn2O4 nanoparticles for supercapacitor application

Polyaniline-multiwalled carbon nanotube (PANI-MWCNT): Room temperature resistive carbon monoxide (CO) sensor

Electrochemical properties of TiO2-V2O5 nanocomposites as a high performance supercapacitors electrode material

Electrochemical Energy Storage Properties of Ni-Mn-Oxide Electrodes for Advance Asymmetric Supercapacitor Application.