Investigation of electrocatalytic activity of a new mononuclear Mn(II) complex for water oxidation in alkaline media

Engineering active sites in ternary CeO2-CuO-Mn3O4 heterointerface embedded in reduced graphene oxide for boosting water splitting activity

The rational design of highly efficient and stable bifunctional catalysts for overall water splitting is vitally important. In this study, to increase the active catalytic sites of CeO2 for electrochemical water splitting, a ternary CeO2-CuO-Mn3O4 heterostructure, synthesized by coprecipitation method, is loaded on reduced graphene oxide (rGO) nanosheets in different amounts to produce CeO2-CuO-Mn3O4@rGO nanocomposites. It is found that CeO2-CuO-Mn3O4@rGO nanocomposites show higher electrocatalytic activity than unsupported samples, and the best activity is observed when the wieght ratio of CeO2-CuO-Mn3O4 is three times that of rGO. The CeO2-CuO-Mn3O4@rGO(3:1) requires low overpotentials of 130 and 270 mV for hydrogen and oxygen evolution reactions (HER and OER) at a current density of 10 mA cm-2. Furthermore, this material demonstrates a large electrochemically active surface area, low charge transfer resistance, suitable kintics, and high long-term stability for both OER and HER. Additionally, when CeO2-CuO-Mn3O4@rGO(3:1) is used as self-supported electrodes for the overall water splitting reaction, a low cell voltage of 1.68 V is obtained. This superior performance is due to: (i) active multi-metal sites that produce strong synergistic effects; (ii) the high conductivity of rGO, which faciliate favorable electron transfer; and (iii) the homogenous anchoring of CeO2-CuO-Mn3O4 on rGO, which increases the number of active sites available on the catalyst surface.

Formation and Reactivity of a MnIV(O)(μ-O)CeIV Species: A Closest Mimic of Photosystem II

Understanding the basic structure of the oxygen-evolving complex (OEC) in photosystem II (PS-II) and the water oxidation mechanism can aid in the discovery of more efficient and sustainable catalysts for water oxidation. In this context, we present evidence of the formation of a [(TPA)MnIV(O)(μ-O)CeIV(NO3)3]+ (2) complex (TPA = tris(pyridyl-2-methyl)amine) by adding aqueous ceric ammonium nitrate to an acetonitrile solution of the [(TPA)MnII]2+ (1) complex. This unique intermediate (2) was analyzed by using various spectroscopic techniques and electrospray ionization mass spectrometry. Remarkably, 2 closely mimics the structure of MnV(O)(μ-O)CaII(OH2) proposed in the OEC of PS-II. Notably, 2 reacts effectively with ferrocene derivatives, indicating that redox-active CeIV binding enhances the electron transfer efficiency. Additionally, 2 demonstrated the ability to perform oxygen atom transfer and hydrogen atom abstraction reactions. The discovery of this reactive [(TPA)MnIV(O)(μ-O)CeIV(NO3)3]+ species provides exciting opportunities for investigating the structure of the MnV(O)(μ-O)CaII(OH2) unit in the OEC.

Phytochemical-enhanced NiO nanostructures for superior oxygen evolution and asymmetric supercapacitor applications

The development of new energy conversion and storage technologies has contributed to the widespread use of renewable energy. However, new methodologies for electrochemical energy storage systems remain to be developed. This study presents a facile, low-cost, scalable, and environmentally friendly method for the synthesis of nickel oxide (NiO) nanostructures by hydrothermal methods using lotus root extract. The different volumes of lotus root extract were tested on NiO nanostructures (sample 1, sample 2) using 1 ml and 2 ml amounts of the extract, respectively. Therefore, phytochemicals from lotus extract have influenced the surface morphology, crystal quality, optical band gap, electrical conductivity, and surface active sites of NiO nanostructures. Sample 2 of the NiO nanostructures was found to be highly active for oxygen evolution reaction (OER) and showed an overpotential of 380 mV at 10 mA cm−2 and a durability of 30 h at 10 mA cm−2. Furthermore, sample 2 of NiO has shown specific capacitance of 1503.84 F g−1 at 2 A g−1 as well as cycling stability over a period of forty thousand GCD cycles. The percentage specific capacitance retention were highly improved up to 100.6%. An asymmetric energy storage device has been constructed using NiO sample 2 as the anode electrode material, demonstrating excellent specific capacity of about 1113 C g−1 at 5 A g−1. For the asymmetric supercapacitor device, a power density of 20000 W kg−1 and an energy density of 245 Wh kg−1 were obtained. In a study of cycling stability for 40000 GCD cycles, it was observed that the asymmetric device retained 96.86% of its specific capacitance. A significant contribution was made to the electrochemical performance of sample 2 of NiO by phytochemicals derived from lotus extract.