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Herein, the peculiarities of complex phosphate formation in self-fluxes of Cs–MII–Bi–P–O (MII = Ca, Sr, Ba and Pb) systems with Cs/P = 0.7–1.3 at fixed ratios of Bi/P = 0.2 and Bi/MII = 1.0 were studied and discussed. Three novel isostructural diphosphates with the general composition Cs3MIIBi(P2O7)2 (MII = Ca, Sr and Pb) and the original framework topology were synthesized and characterized via single-crystal and powder X-ray diffraction, SEM, DTA, and FTIR- and UV-VIS-spectroscopy. In addition, electronic structure (DFT) and Voronoi–Dirichlet polyhedra (VDP) characteristics calculations and crystallochemical analysis were performed. In the structure of the new compounds, the MIIO7 and BiO6 polyhedra are connected via common oxygen vertices forming infinite helical-like chains, which are linked by P2O7 groups into a 3D-framework with pentagonal tunnels, where the Cs+ cations are located. The structural peculiarities are discussed considering perspectives for the creation of new luminescent materials. The dielectric bandgaps, Eg, of the Cs3MIIBi(P2O7)2 crystals reveal an ∼0.2 eV low-energy shift in the Ca–Sr–Pb sequence of MII cations, which reveals the possibility to tune the optical absorbance spectra of the crystals via the synthesis of solid solutions with various contents of MII cations. The glass-ceramic synthetic approach is also proposed as a convenient method for the creation of new diphosphates, and the applicability of this method is verified for Cs3CaBi(P2O7)2. [ABSTRACT FROM AUTHOR]

The evolution of high-performance and stable electrode materials for supercapacitors plays a vital role in the next generation of energy storage devices. In this work we present a simple method for preparing Bi(nanoparticles)/CNx(nanosheets) nanocomposites as electrode materials for supercapacitors, which were synthesized by thermally treating bismuth citrate and urea at 550–700 °C under an Ar atmosphere. According to physicochemical studies (XRD, SEM, TG-DTA, XPS, FTIR, and BET), a "smeared" bismuth formation or the formation of nanoparticles on the CNx surface of interwoven 2D-nanosheets at different calcination temperatures was observed. Electrochemical measurements show that the specific capacity of the composites can reach 1251 F g−1 (more than 90% of the theoretical value) at a current density of 500 mA g−1 in a 6 M KOH electrolyte, and most two-dimensional CNx-based nanostructures remain intact after multiple galvanostatic charge–discharge processes, which is promising for the development of highly efficient supercapacitors. A supercapacitor composed of Bi/CNx nanocomposites for the negative electrode and Ni-layered hydroxide for the positive electrode demonstrates a high energy density of 58 W h kg−1 with a power density of 800 W kg−1 accompanied by a good cycle life (the parameters decreased down to only 78% after 1000 charge–discharge cycles). Our current results indicate that the addition of urea not only determines the morphology of the composites, but also lays the foundation for the development of new types of nanocomposites for the power industry. [ABSTRACT FROM AUTHOR]

The great interest in developing efficient and stable bifunctional electrodes for electrocatalytic water splitting is due to the rapid growth in demand for sustainable and renewable energy sources. Here, we present an original electrode design strategy which can be used for the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER). This strategy is based on the direct formation of active catalytic material on the electrode surface during electrolysis. In our case, cobalt hydroxides (β-Co(OH)2 and CoO(OH) for HER and OER, respectively) in a carbon matrix are produced by the chemical and electrochemical transformation of various forms (glass, glass-ceramic and polycrystalline) of Na4Co3(PO4)2P2O7 during electrolysis in 1 M NaOH solution. The CoOx(OH)y/C composites demonstrate remarkable stability over time (more than 50 h), and bifunctional catalytic activity with an overall water splitting potential close to 1.8 V at a current density of 10 mA cm−2. The range of overpotentials for OER and HER is 346–365 and 326–369 mV, respectively. These electrodes can invert the OER and HER processes with a total overpotential of 0.6 V in the cell. This remarkable CoO(OH) ↑ Co(OH)2 inversion in the carbon matrix its further utility for creating bifunctional electrodes for long-life water splitting cells. [ABSTRACT FROM AUTHOR]