Sourav Bag

Facile Single-Step Synthesis of Nitrogen-Doped Reduced Graphene Oxide-Mn3O4 Hybrid Functional Material for the Electrocatalytic Reduction of Oxygen

Development of efficient electrocatalyst based on non-precious metal that favors the four-electron pathway for the reduction of oxygen in alkaline fuel cell is a challenging task. Herein, we demonstrate a new facile route for the synthesis of hybrid functional electrocatalyst based on nitrogen-doped reduced graphene oxide (N-rGO) and Mn3O4 with pronounced electrocatalytic activity towards oxygen reduction reaction (ORR) in alkaline solution. The synthesis involves one-step in situ reduction of both graphene oxide (GO) and Mn(VII), growth of Mn3O4 nanocrystals and nitrogen doping onto the carbon framework using a single reducing agent, hydrazine. The X-ray photoelectron (XPS), Raman and FTIR spectral, and X-ray diffraction measurements confirm the reduction of GO and growth of nanosized Mn3O4. The XPS profile reveals that N-rGO has pyridinic (40%), pyrrolic (53%), and pyridine N oxide (7%) types of nitrogen. The Mn3O4 nanoparticles are single crystalline and randomly distributed over the wrinkled N-rGO sheets. The hybrid material has excellent ORR activity and it favors the 4-electron pathway for the reduction of oxygen. The electrocatalytic performance of the hybrid catalyst is superior to the N-rGO, free Mn3O4 and their physical mixture. The hybrid material shows an onset potential of -0.075 V, which is 60-225 mV less negative than that of the other catalyst tested. It has excellent methanol tolerance and high durability. The catalytic current density achieved with the hybrid material at 0.1 mg cm(-2) is almost equivalent to that of the commercial Pt/C (10%). The synergistic effect of N-rGO and Mn3O4 enhances the overall performance of the hybrid catalyst. The nitrogen in N-rGO is considered to be at the interface to bridge the rGO framework and Mn3O4 nanoparticles and facilitates the electron transfer.

Hierarchical three-dimensional mesoporous MnO2 nanostructures for high performance aqueous asymmetric supercapacitors

We describe a new facile chemical route for the synthesis of hierarchical mesoporous α-MnO2 and the development of an aqueous asymmetric supercapacitor. The hierarchical α-MnO2 is synthesized by the thermodynamically favorable redox reaction between metallic Zn and MnO4− in mild acidic solutions without any template. Zn and the in situ generated nascent hydrogen efficiently reduce MnO4− to MnO2. The growth mechanism is studied with time-dependent electron microscopic measurements. The α-MnO2 has a three-dimensional (3D) flower-like mesoporous hierarchical structure with an average size of 500 nm and a large surface area (206 m2 g−1) with a pore size of 48.34 A and a pore volume of 0.543 cm3 g−1. It has significantly high electronic conductivity with respect to traditional/commercial MnO2. A specific capacitance as high as 322 F g−1 at a current density of 1 A g−1 with excellent cycling stability (90% after 8000 cycles) is achieved. An aqueous asymmetric supercapacitor (ASC) is developed by pairing α-MnO2 and reduced graphene oxide-based electrodes. An ASC could deliver a specific capacitance of 63.5 F g−1 at 2 A g−1 with a potential window of 0–2 V. The ASC retains 100% initial specific capacitance even after 3000 continuous charge–discharge cycles. It has an energy density of 35.28 W h kg−1 at a power density of 2.0 kW kg−1 and could retain 27.78 W h kg−1 at a power density of 16.67 kW kg−1. The three-dimensional mesoporous structure favors the facilitated transport of the electrolyte across the electrode. The post-mortem XRD analysis shows that the MnO2 nanostructure retains its initial α phase even after 3000 charge–discharge cycles, though a partial disintegration of the hierarchical structure was observed.

Layered inorganic–organic hybrid material based on reduced graphene oxide and α-Ni(OH)2 for high performance supercapacitor electrodes

We describe a facile one-step strategy for the synthesis of a novel layered hybrid material of reduced graphene oxide (rGO) and α-Ni(OH)2 by non-hydrothermal route and the supercapacitive performance of the material. The hybrid material rGO/α-Ni(OH)2 was synthesized using glucose as a templating agent for the growth of layered α-Ni(OH)2 and a reducing agent for the reduction of graphene oxide (GO). The templating agent partially reduces GO to rGO and assists the growth of α-Ni(OH)2 layers in between the rGO sheets. The electron microscopy measurements show the stacking of layered α-Ni(OH)2 over rGO sheets. The activity of the hybrid material was evaluated by voltammetric, electrochemical impedance and charge–discharge measurements in alkaline pH in terms of specific capacitance, internal resistance and capacitance retention. The hybrid material has superior performance, which is comparable to rGO, free α-Ni(OH)2 and the physical mixture of rGO and free α-Ni(OH)2. A high specific capacitance of 1671.67 F g−1 was obtained at a current density of 1 A g−1. The hybrid material retains 81% of its initial capacitance after 2000 continuous charge–discharge cycles. The large surface area and high electronic conductivity of the hybrid material favor a facile charge transport, whereas the layer structure ensures the easy diffusion of electrolytes ions and enhances the overall performance. An asymmetric supercapacitor device was made by pairing the hybrid material with rGO, and it delivers a high energy density of 42.67 W h kg−1 at a power density of 0.4 kW kg−1.

Facile shape-controlled growth of hierarchical mesoporous δ-MnO2 for the development of asymmetric supercapacitors

Synthesis of pseudocapacitive mesoporous transition metal oxides with hierarchical structures is of great interest in the development of high performance energy storage devices. Herein, we demonstrate a facile single-step, template-free chemical route for the synthesis of hierarchical mesoporous δ-MnO2 and its supercapacitive performance. The mesoporous δ-MnO2 is synthesized by the thermodynamically favourable redox reaction of MnO4− with HBr. The growth of δ-MnO2 involves the facile reduction of MnO4− to Mn2+ and the subsequent reaction of in situ generated Mn2+ with unreacted MnO4− in one pot at room temperature. Br− has dual roles of reducing MnO4− and controlling the growth of MnO2 by surface etching. The possible Ostwald ripening and self-assembling of the nanoseeds formed at the initial stage of the reaction and the ensuing surface etching of the urchin-like MnO2 by Br− produce hierarchical flower-like δ-MnO2 of 300 nm size. It has a three-dimensional mesoporous structure with a large surface area of 238 m2 g−1. It has an average pore size and pore volume of 36.14 A and 0.567 cc g−1, respectively. The concentration of Br− controls the growth of δ-MnO2 and a large excess of Br− completely reduces MnO2 to Mn2+. The δ-MnO2 nanostructure shows excellent supercapacitive performance with a specific capacitance of 364 F g−1 at a current density of 1 A g−1. An aqueous asymmetric supercapacitor (ASC) is developed by pairing the δ-MnO2-based cathode with an activated carbon anode. ASC delivers a specific capacitance of 86.5 F g−1 at 1 A g−1 with a wide potential window of 0–2 V. It retains 100% initial specific capacitance even after 3000 continuous charge–discharge cycles. The device has an energy density of 48.06 W h kg−1 at the power density of 1.0 kW kg−1 and it retains 24.44 W h kg−1 at a power density of 20 kW kg−1. The favourable access of the electrode material to the electrolyte due to the mesoporous structure enhances the overall performance of the device.

On the electrocatalytic activity of nitrogen-doped reduced graphene Oxide: Does the nature of nitrogen really control the activity towards oxygen reduction?

AbstractSynthesis of metal-free electrocatalyst for the cathodic reduction of oxygen is of great interest for fuel cell and metal-air battery applications. The heteroatom-doped graphene/reduced graphene oxide (rGO) is very promising and the nitrogen-doped rGO (N-rGO) is emerging as a new inexpensive electrocatalyst for oxygen reduction reaction (ORR). Herein, we describe the effect of the chemical nature and amount of nitrogen in N-rGO towards ORR in acidic solution. Four different samples of N-rGO with different nitrogen content were synthesized by simple chemical route. The chemical nature and nitrogen content were analyzed with X-ray photoelectron spectroscopic measurements. The electrocatalytic performance of the catalyst was examined by cyclic and hydrodynamic voltammetric studies. All the N-rGO samples favor 4-electron pathway for the reduction of oxygen in acidic solution. The onset potential and kinetic current density depends on the nature of the doped nitrogen. It is demonstrated that the chemical nature and the amount of nitrogen actually control the ORR activity. The N-rGO which contains a large amount of pyridinic nitrogen with N/C ratio of 0.074 has high catalytic activity. The carbon bonded to pyridinic nitrogen could be a possible catalytic site in ORR. Our studies suggest that the graphitic nitrogen does not significantly influence the electrocatalytic activity of N-rGO. Graphical AbstractIt is demonstrated that the chemical nature and the amount of nitrogen atom substitutionally doped onto the carbon framework of the N-doped reduced graphene oxide controls the electrocatalytic performance towards oxygen reduction reaction.