Vanadyl phosphate/reduced graphene oxide nanosheet hybrid material and its capacitance

Diffusion-Mediated Morphological Transformation in Bifunctional Mn2O3/CuO-(VO)3(PO4)2·6H2O for Enhanced Electrochemical Water Splitting

A strategical approach for morphological transformation and heterojunction formation was utilized to suppress the shortcomings of uni-metal oxide electrocatalysts and enhance their bifunctionality. In situ generation of copper oxide (CuO) over the surface of manganese oxide (Mn₂O₃) resulted in a morphological transformation from solid spheres to hollow spherical structures due to the ion-exchange diffusion (Kirkendall effect) of Cu ions into Mn₂O₃ particles. This hollowness resulted in the advancement of the bifunctional electrocatalytic behavior of Mn₂O₃/CuO (overpotential (η₁₀) of 280 mV for an OER and 310 mV for an HER at a current density of 10 mA/cm²) by virtue of increased exposed surface active sites aiding the adsorption of water molecules on the surface. The increased electrochemical active surface area (ECSA/C_dl = 34 mF/cm²) and reduced charge transfer resistance resulted in the formation of Mn₂O₃/CuO hollow spheres to achieve an approximately threefold enhancement in the turnover frequency (TOF) compared to the bare Mn₂O₃. The electrocatalytic efficiency of Mn₂O₃/CuO was further enhanced by virtue of the faster charge transfer coefficient of two-dimensional (2D) vanadyl phosphate hexahydrate (VOP) sheets deposited over its surface. This boosted the overall water splitting with attained overpotential (η₁₀) values of 190 and 220 mV with Tafel slopes of 60 and 105 mV/decade for an OER and HER, respectively. The morphological transformation and formation of an n-p heterojunction between Mn₂O₃ and CuO based on their work function (φ) values evaluated from the density functional theory (DFT) calculation and the effect of the VOP overlayer for faster reaction kinetics at the electrolyte interface resulted in an ∼10-fold increment in TOF values compared to the bare counterpart.

Pseudocapacitance multiporous vanadyl phosphate/graphene thin film electrode for high performance electrochemical capacitors

Supercapacitors are being considered as promising electricity storage devices with green sustainable energy conversion. To efficiently develop and optimize pseudocapacitive material of vanadyl phosphate, herein, multiporous vanadyl phosphate/graphene (denoted as MP-VOPO₄@rGO) is fabricated for the first time with phytic acid as a phosphorus source by extremely simple sol-gel and drop coating methods, and used as the free binder thin film electrode of supercapacitors. The smart combination of honeycomb-like architecture and graphene incorporation results in more active sites and low internal resistance, significantly improving energy storage performance. The effect of introducting polystyrene (denoted as PS) template and rGO on the performance of the nanocomposite is systematically analyzed by comparing the performance of the corresponding thin film electrodes. The MP-VOPO₄@rGO thin film electrode delivers superior pseudocapacitive performance of 672 F g^-1 at 1 A g^-1 as well as a remarkable rate capability of 552 F g^-1 at 5 A g^-1, and it presents a remarkable longterm cycling stability, with a capacitance retention of 83.5% after 5000 cycles. Very interestingly, the results of surface capacitance contribution dominance clearly demonstrates its rapid capacitive response. In addition, based on MP-VOPO₄@rGO thin film as positive and negative electrodes, the corresponding assembled symmetric supercapacitors exihibits outstanding energy density of 26.3 Wh kg^-1 at power density of 249.9 W kg^-1. This investigation can not only provide a versatile strategy to design other thin film electrode materials but also open up a new insight into the development of polyanion phosphate composites for next-generation high performance energy storage systems.

Inhibition of Redox Behaviors in Hierarchically Structured Manganese Cobalt Phosphate Supercapacitor Performance by Surface Trivalent Cations

The stability and performance of supercapacitor devices are limited by the diffusion-controlled redox process occurring at materials' surfaces. Phosphate-based metal oxides could be effectively used as pseudocapacitors because of their polar nature. However, electrochemical energy storage applications of Mn-Co-based phosphate materials and their related kinetics studies have been rarely reported. In this work, we have reported a morphology-tuned Mn _x Co_3-x (PO₄)₂·8H₂O (MCP) spinel compound synthesized by a one-step hydrothermal method. Detailed physical and chemical insights of the active material coated on the nickel substrate are examined by X-ray diffraction, field-emission scanning electron microscopy, field-emission transmission electron microscopy, and high-resolution X-ray photoelectron spectroscopy analyses. Physiochemical studies reveal that the well-defined redox behavior usually observed in Co²⁺/Ni²⁺ surface-terminated compounds is suppressed by reducing the divalent cation density with an increased Co³⁺ and Mn³⁺ surface states. A uniform and dense leaflike morphology observed in the MnCo₂ phosphate compound with an increased surface area enhances the electrochemical energy storage performance. The high polar nature of P-O bonding formed at the surface leads to a higher rate of polarization and a very low relaxation time, resulting in a perfect square-shaped cyclic voltagram and triangular-shaped galvanostatic charge and discharge curve. We have achieved a highly pseudocapacitive MCP, and it can be used as a vital candidate in supercapacitor energy storage applications.

A high-performance asymmetric supercapacitor based on vanadyl phosphate/carbon nanocomposites and polypyrrole-derived carbon nanowires

A novel asymmetric supercapacitor device in an aqueous electrolyte is fabricated using a vanadyl phosphate/carbon nanocomposite as the positive electrode and a polypyrrole-derived carbon nanowire as the negative electrode. The vanadyl phosphate/carbon nanocomposites are synthesized by a simple two-step approach in which layered VOPO₄·2H₂O is first intercalated by dodecylamine and then annealed at high temperature, leading to the in situ carbonization of the intercalated dodecylamine. It is found that the sample in which the incorporated carbon with a high degree of graphitization exhibits a high specific capacitance of 469 F g^-1 at a current density of 1 A g^-1 and excellent rate performance (retained 77% capacitance at 10 A g^-1). A polypyrrole-derived carbon nanowire is synthesized by the direct carbonization of nanowire-shaped polypyrrole, revealing a rough surface of nanowire-like frameworks and good electrochemical behavior. Taking advantage of both positive and negative materials, the assembled asymmetric supercapacitor device exhibits a high energy density of 30.6 W h kg^-1 at a high power density of 813 W kg^-1 in a wide voltage region of 0-1.6 V, as well as a good electrochemical stability (84.3% capacitance retention after 5000 cycles). The present work can shed light on the fabrication of novel asymmetric supercapacitors with high-performance.

Metal Phosphides and Phosphates-based Electrodes for Electrochemical Supercapacitors

Phosphorus compounds, such as metal phosphides and phosphates have shown excellent performances and great potential in electrochemical energy storage, which are demonstrated by research works published in recent years. Some of these metal phosphides and phosphates and their hybrids compare favorably with transition metal oxides/hydroxides, which have been studied extensively as a class of electrode materials for supercapacitor applications, where they have limitations in terms of electrical and ion conductivity and device stability. To be specific, metal phosphides have both metalloid characteristics and good electric conductivity. For metal phosphates, the open-framework structures with large channels and cavities endow them with good ion conductivity and charge storage capacity. In this review, we present the recent progress on metal phosphides and phosphates, by focusing on their advantages/disadvantages and potential applications as a new class of electrode materials in supercapacitors. The synthesis methods to prepare these metal phosphides/phosphates are looked into, together with the scientific insights involved, as they strongly affect the electrochemical energy storage performance. Particular attentions are paid to those hybrid-type materials, where strong synergistic effects exist. In the summary, the future perspectives and challenges for the metal phosphides, phosphates and hybrid-types are proposed and discussed.

Ultrathin 2D VOPO4nanosheets: a novel reinforcing agent in polymeric composites

In the present work, ultrathin VOPO₄nanosheets were prepared through simple ultrasonication in 2-propanol, starting from bulk VOPO₄·2H₂O chunks.