Electrochemical deposition of manganese oxide-phosphate-reduced graphene oxide composite and electrocatalysis of the oxygen evolution reaction

A composite of manganese oxide and reduced graphene oxide (rGO) is prepared in a single step electrochemical reduction process in a phosphate buffer solution for studying as an electrocatalyst for the oxygen evolution reaction (OER). The novel composite catalyst, namely, MnOx-Pi-rGO, is electrodeposited from a suspension of graphene oxide (GO) in a neutral phosphate buffer solution containing KMnO4. The manganese oxide incorporates phosphate ions and deposits on the rGO sheet, which in turn is formed on the substrate electrode by electrochemical reduction of GO in the suspension. The OER is studied with the MnOx-Pi-rGO catalyst in a neutral phosphate electrolyte by linear sweep voltammetry. The results indicate a positive influence of rGO in the catalyst. By varying the ratio of KMnO4 and GO in the deposition medium and performing linear sweep voltammetry for the OER, the optimum composition of the deposition medium is obtained as 20 mM KMnO4 + 6.5% GO in 0.1 M phosphate buffer solution of pH 7. Under identical conditions, the MnOx-Pi-rGO catalyst exhibits 6.2 mA cm(-2) OER current against 2.9 mA cm(-2) by MnOx-Pi catalyst at 2.05 V in neutral phosphate solution. The Tafel slopes measured for OER at MnOx-Pi and MnOx-Pi-rGO are similar in magnitude at about 0.180 V decade(-1). The high Tafel slopes are attributed to partial dissolution of the catalyst during oxygen evolution. The O-2 evolved at the catalyst is measured by the water displacement method and the positive role of rGO on catalytic activity of MnOx-Pi is demonstrated

An oxygen evolution Co-Ac catalyst - the synergistic effect of phosphate ions

Formation of an amorphous cobalt based oxygen evolution catalyst called Co-Pi has been recently reported from a neutral phosphate buffer solution containing Co2+. But the concentration of Co2+ is as low as 0.5 mM due to poor solubility of a cobalt salt in phosphate medium. In the present study, a cobalt acetate based oxygen evolution catalyst (Co-Ac) is prepared from a neutral acetate buffer solution, where the solubility of Co2+ is very high (>100 times in comparison with phosphate buffer solution). The Co-Ac possesses better catalytic activity than the Co-Pi with an additional advantage of easy bulk scale preparation. The comparative studies on the oxygen evolution reaction (OER) activity of Co-Ac and Co-Pi in phosphate and acetate buffer electrolytes reveal that the Co-Ac exhibits enhanced synergistic catalytic activity in phosphate solution, probably due to partial substitution of acetate in the catalyst layer by phosphate, resulting in the formation of a Co-Ac-Pi catalyst

Electrochemical impedance studies of Na/MnO2 primary cells

Primary Na/MnO2 cells are expected to gain importance for replacement of primary Li/MnO2 cells in future akin to the Na-ion cells as alternative for cells. Amorphous MnO2 is prepared by a drop-wise addition of KMnO4 solution to MnSO4 solution, and coin cells of Na/MnO2 are fabricated in a non-aqueous electrolyte of Na salt. The electrochemical impedance spectroscopy (EIS) data provides a high resistance of Na metal due to the surface passive film. On subjecting the cell for discharge, the surface film causes a delay response of the cell voltage and the closed circuit voltage reaches the normal discharge level following dielectric breakdown of the film. The EIS data measured at different stages of cell discharge are subjected to non-linear least square fitting with the aid of an appropriate equivalent circuit. The impedance parameters are examined to throw light on state-of-charge of Na/MnO2 primary cells

Ir-phosphate cocatalyst for photoelectrochemical water oxidation using alpha-Fe2O3

alpha-Fe2O3 is an ideal photoanode for solar water oxidation owing to its visible light absorbing capability, suitable valence band position, low cost, high abundance, non-toxicity and eco-friendliness. However, the reported efficiencies are very low due to the poor kinetics of water oxidation by photogenerated holes on alpha-Fe2O3. In the present study, an alpha-Fe2O3 electrode is obtained by heating a film of Fe which is prepared by the electrochemical reduction of Fe2+ ions. Film thickness and calcination temperature are carefully optimized to get a maximum photoresponse in neutral phosphate solution. In order to improve the water oxidation kinetics and reduce the charge carrier recombination, an iridium-phosphate (Ir-Pi) catalyst is electrodeposited on the surface of alpha-Fe2O3. Ir-Pi is found to reduce the OER onset potential by 350 mV, and enhances the photocurrent density by 3 times at 1.23 V vs. RHE

Investigation of in situ grown and carbon-free copper sulfide electrode for rechargeable lithium battery

Sulfur and metal sulfides have been the materials of choice for the cathodes to achieve high energy density from lithium based batteries. Among several metal sulfides, copper sulfide has been widely investigated as a positive electrode material for Li-ion batteries. In the present study, in situ preparation of copper sulfide by reaction of sulfur coated on copper foil current collector is investigated. Sulfur is blended with either acetylene black as a conducting agent or TiO2 nanotubes as porous and non-conducting additive and electrodes are prepared on Cu foil. Electrodes are also prepared without any additive. Copper reacts with sulfur forming copper sulfide which has been confirmed by X-ray diffraction studies. Formation of sulfides with platelet like morphology has been observed in all the coatings. Specific discharge capacity of 282 mAh g(-1) has been obtained from S-TiO2 electrode based cell as compared to 317 mAh g(-1) from S-C electrode based cell. Discharge capacity of 230 mAh g(-1) has been obtained from the cells with S electrode without any additive. This is attributed to the higher conductivity of copper sulfide than sulfur and also to the formation of conducting copper particles on discharge. On subjecting the cells to 100 discharge-charge cycles, almost stable performance has been observed with S-TiO2 electrode based cell. Furthermore, the discharge capacity of S-TiO2 based cell is greater than S-C based cell and it increases on repeated cycling. (C) 2017 Elsevier B.V. All rights reserved

Electrooxidation of Ascorbic Acid on a Polyaniline-Deposited Nickel Electrode: Surface Modification of a Non-Platinum Metal for an Electrooxidative Analysis

The electrooxidation of ascorbic acid $(H_2A)$, which does not occur on a bare Ni electrode, has been shown to take place on a polyaniline (PANI)-coated Ni electrode in aqueous electrolytes of a wide pH range. The characteristic voltammetric peak of PANI in $0.1 M H_2SO_4$ at 0.2 V vs SCE corresponding to the transformation of leucoemeraldine to emeraldine gradually diminishes with an increase in concentration of $H_2A$ as a result of adsorption. This peak disappears before the appearance of another peak corresponding to the oxidation of $H_2A$ at a concentration of 1 mM. The irreversible oxidation current of $H_2A$ exhibits a linear dependence on the concentration. The effect of adsorption of $H_2A$ on PANI has been shown to increase the voltammetric peak current. A study on the variation of the PANI thickness and its influence on the voltammetric oxidation of $H_2A$ has led to an optimum thickness of $1.6 \mu m$. The oxidation currents on the porous PANI/Ni electrode have been found to be several times higher at lower potentials in comparison with the data of a Pt electrode. The reaction has also been studied by ac impedance spectroscopy. In alkaline electrolytes, the Nyquist impedance plot is characterized by two semicircles instead of a single semicircle in acidic electrolytes. Thus, Ni, which is a non-platinum metal, has been found to be useful, by surface modification with PANI, for electrooxidation of $H_2A$. The data are reproducible in the electrolytes of a wide pH range, thus suggesting a good stability, reusability and a long life for the PANI/Ni electrodes

Electrodeposited Nickel–Cobalt–Sulfide Catalyst for the Hydrogen Evolution Reaction

A novel Ni–Co–S-based material prepared by the potentiodynamic deposition from an aqueous solution containing Ni²⁺, Co²⁺, and thiourea is studied as an electrocatalyst for the hydrogen evolution reaction (HER) in a neutral phosphate solution. The composition of the catalyst and the HER activity are tuned by varying the ratio of the concentrations of Ni²⁺ and Co²⁺ ions in the electrolytes. Under optimized deposition conditions, the bimetallic Ni–Co–S exhibits higher electrocatalytic activity than its monometallic counterparts. The Ni–Co–S catalyst requires an overpotential of 150 mV for the HER onset, and 10 mA cm^–2 current density is obtained at 280 mV overpotential. The catalyst exhibits two different Tafel slopes (93 and 70 mV dec^–1) indicating two dissimilar mechanisms. It is proposed that the catalyst comprises two types of catalytic active sites, and they contribute selectively toward HER in different potential regions

High Catalytic Activity of Amorphous Ir-Pi for Oxygen Evolution Reaction

Large-scale production of hydrogen gas by water electrolysis is hindered by the sluggish kinetics of oxygen evolution reaction (OER) at the anode. The development of a highly active and stable catalyst for OER is a challenging task. Electrochemically prepared amorphous metal-based catalysts have gained wide attention after the recent discovery of a cobalt-phosphate (Co-Pi) catalyst. Herein, an amorphous iridium-phosphate (Ir-Pi) is investigated as an oxygen evolution catalyst. The catalyst is prepared by the anodic polarization of carbon paper electrodes in neutral phosphate buffer solutions containing IrCl₃. The Ir-Pi film deposited on the substrate has significant amounts of phosphate and Ir centers in an oxidation state higher than +4. Phosphate plays a significant role in the deposition of the catalyst and also in its activity toward OER. The onset potential of OER on the Ir-Pi is about 150 mV lower in comparison with the Co-Pi under identical experimental conditions. Thus, Ir-Pi is a promising catalyst for electrochemical oxidation of water

Hierarchically porous Li1.2Mn0.6Ni0.2O2 as a high capacity and high rate capability positive electrode material

Layered composite samples of lithium-rich manganese oxide (Li1.2Mn0.6Ni0.2O2) are prepared by a reverse microemutsion route employing a soft polymer template and studied as a positive electrode material. The product samples possess dual porosity with distribution of pores at 3.5 and 60 nm. Pore volume and surface area decrease on increasing the temperature of preparation. Nevertheless, the electrochemical activity of the composite increases with an increase in temperature. The discharge capacity value of the samples prepared at 800 and 900 degrees C is about 240 mA h g(-1) at a specific current of 25 mA g(-1) with a good cycling stability. The composite sample heated at 900 degrees C possesses a high rate capability with a discharge capacity of 100 mA h g(-1) at a specific current of 500 mA g(-1). The high rate capability is attributed to porous nature of the composite sample