T. Maiyalagan

Spinel-type lithium cobalt oxide as a bifunctional electrocatalyst for the oxygen evolution and oxygen reduction reactions

Development of efficient, affordable electrocatalysts for the oxygen evolution reaction and the oxygen reduction reaction is critical for rechargeable metal-air batteries. Here we present lithium cobalt oxide, synthesized at 400 °C (designated as LT-LiCoO2) that adopts a lithiated spinel structure, as an inexpensive, efficient electrocatalyst for the oxygen evolution reaction. The catalytic activity of LT-LiCoO2 is higher than that of both spinel cobalt oxide and layered lithium cobalt oxide synthesized at 800 °C (designated as HT-LiCoO2) for the oxygen evolution reaction. Although LT-LiCoO2 exhibits poor activity for the oxygen reduction reaction, the chemically delithiated LT-Li1-xCoO2 samples exhibit a combination of high oxygen reduction reaction and oxygen evolution reaction activities, making the spinel-type LT-Li0,5CoO2 a potential bifunctional electrocatalyst for rechargeable metal-air batteries. The high activities of these delithiated compositions are attributed to the Co4O4 cubane subunits and a pinning of the Co(3+/4+):3d energy with the top of the O(2-):2p band.

Electrodeposited Pt on three-dimensional interconnected graphene as a free-standing electrode for fuel cell application†

A three-dimensional interconnected graphene monolith was used as an electrode support for pulsed electrochemical deposition of platinum (Pt) nanoparticles. Pt nanoparticles with well-defined morphology and small size can be obtained by controlling electrodeposition potential and time. Electrochemical characterization was carried out to examine the electrocatalytic activity of this monolithic electrode towards methanol oxidation in acidic media. The results show that the carbon material surface and structure have a strong influence on the Pt particle size and morphology. Compared with the three-dimensional scaffold of carbon fibers, the three-dimensional graphene when used as a free-standing electrode support resulted in much improved catalytic activity for methanol oxidation in fuel cells due to its three-dimensionally interconnected seamless porous structure, high surface area and high conductivity.

Cobalt oxide-coated N- and B-doped graphene hollow spheres as bifunctional electrocatalysts for oxygen reduction and oxygen evolution reactions

A simple and scalable method has been developed for the synthesis of Co3O4-coated N- and B-doped graphene hollow spheres (Co3O4/NBGHSs). These Co3O4/NBGHSs are highly active for both oxygen reduction and evolution reactions and can exhibit higher electrocatalytic activities and better durability than commercial Pt/C and RuO2/C, respectively, demonstrating them to be efficient bi-functional electrocatalysts. In-depth analysis shows that the coupling between Co3O4 and NBGHSs, strong interaction with adsorbed O2, high electric conductivity, and the specific hollow structure play important roles in imparting the higher electrocatalytic activities to the Co3O4/NBGHSs. When tested as cathode catalysts for Zn–air batteries, the Co3O4/NBGHSs exhibit better performance and higher stability than the Pt/C catalyst and other catalysts reported previously. This strongly suggests that the Co3O4/NBGHSs could be used as efficient electrocatalysts for metal–air batteries with great potential to replace precious metal/carbon based materials.

Nitrogen doped graphene nanosheet supported platinum nanoparticles as high performance electrochemical homocysteine biosensors

Functional carbon nanomaterials are significantly important for the development of high performance sensitive and selective electrochemical biosensors. In this study, graphene supported platinum nanoparticles (GN-PtNPs) and nitrogen doped graphene supported platinum nanoparticles (N-GN-PtNPs) were synthesized by a simple chemical reduction method and explored as high performance nanocatalyst supports, as well as doped nanocatalyst supports, toward electrochemical oxidation of homocysteine (HCY) for the first the time. Our studies demonstrate that N-doped graphene supported PtNPs show higher electrocatalytic activity for HCY with an experimental detection limit of 200 pM. Moreover, N-doped graphene supported Pt was demonstrated to have excellent selectivity in the electrochemical oxidation of HCY i.e., the detection of HCY is successful in the presence of a 20-fold excess of ascorbic acid (AA). The practical application of N-doped graphene supported PtNP materials is effectively shown for the determination of HCY in both human blood serum and urine samples, by differential pulse voltammetry under optimized conditions. Our findings conclude that N-doped graphene supported PtNPs can be developed as a high performance and versatile nano-electrocatalyst for electrochemical biosensor applications.

Highly active Pd and Pd–Au nanoparticles supported on functionalized graphene nanoplatelets for enhanced formic acid oxidation

Pd and Pd–Au nanoparticles supported on poly(diallyldimethylammonium chloride) (PDDA) functionalized graphene nanoplatelets (GNP) have been synthesized by the ethylene glycol reduction method and characterized by transmission electron microscopy (TEM) and electrochemical measurements for formic acid oxidation. TEM analysis shows that the Pd–Au nanoparticles are uniformly distributed on the surface of graphene nanoplatelets with an average particle size of 6.8 nm. The Pd–Au nanoparticles supported on PDDA–xGNP show higher activity for formic acid electro-oxidation than Pd nanoparticles supported on PDDA–xGNP and Pd or Pd–Au supported on traditional Vulcan XC-72 carbon. The higher catalytic activity of Pd–Au/PDDA–xGNP is mainly due to the alloying of Pd with Au. The promotional effect of Au and the absence of continuous Pd sites significantly suppress the poisoning effects of CO, enhancing the catalytic activity for formic acid oxidation and making them promising for direct formic acid fuel cells (DFAFC).

Microwave-assisted synthesis of porous Mn2O3 nanoballs as bifunctional electrocatalyst for oxygen reduction and evolution reaction

Technological hurdles that still prevent the commercialization of fuel cell technologies necessitate designing low-cost, efficient and non-precious metals. These could serve as alternatives to high-cost Pt-based materials. Herein, a facile and effective microwave-assisted route has been developed to synthesize structurally uniform and electrochemically active pure and transition metal-doped manganese oxide nanoballs (Mn2O3 NBs) for fuel cell applications. The average diameter of pure and doped Mn2O3 NBs was found to be ~610 nm and ~650 nm, respectively, as estimated using transmission electron microscopy (TEM). The nanoparticles possess a good degree of crystallinity as evident from the lattice fringes in high-resolution transmission electron microscopy (HRTEM). The cubic crystal phase was ascertained using X-ray diffraction (XRD). The energy dispersive spectroscopic (EDS) elemental mapping confirms the formation of copper-doped Mn2O3 NBs. The experimental parameter using trioctylphosphine oxide (TOPO) as the chelating agent to control the nanostructure growth has been adequately addressed using scanning electron microscopy (SEM). The solid NBs were formed by the self-assembly of very small Mn2O3 nanoparticles as evident from the SEM image. Moreover, the concentration of TOPO was found to be the key factor whose subtle variation can effectively control the size of the as-prepared Mn2O3 NBs. The cyclic voltammetry and galvanostatic charge/discharge studies demonstrated enhanced electrochemical performance for copper-doped Mn2O3 NBs which is supported by a 5.2 times higher electrochemically active surface area (EASA) in comparison with pure Mn2O3 NBs. Electrochemical investigations indicate that both pure and copper-doped Mn2O3 NBs exhibit a bifunctional catalytic activity toward the four-electron electrochemical reduction as well the evolution of oxygen in alkaline media. Copper doping in Mn2O3 NBs revealed its pronounced impact on the electrocatalytic activity with a high current density for the electrochemical oxygen reduction and evolution reaction. The synthetic approach provides a general platform for fabricating well-defined porous metal oxide nanostructures with prospective applications as low-cost catalysts for alkaline fuel cells.

Components for PEM Fuel Cells: An Overview

Fuel cells, as devices for direct conversion of the chemical energy of a fuel into electricity by electrochemical reactions, are among the key enabling technologies for the transition to a hydrogen-based economy. Among the various types of fuel cells, polymer electrolyte membrane fuel cells (PEMFCs) are considered to be at the forefront for commercialization for portable and transportation applications because of their high energy conversion efficiency and low pollutant emission. Cost and durability of PEMFCs are the two major challenges that need to be addressed to facilitate their commercialization. The properties of the membrane electrode assembly (MEA) have a direct impact on both cost and durability of a PEMFC. An overview is presented on the key components of the PEMFC MEA. The success of the MEA and thereby PEMFC technology is believed to depend largely on two key materials: the membrane and the electro-catalyst. These two key materials are directly linked to the major challenges faced in PEMFC, namely, the performance, and cost. Concerted efforts are conducted globally for the past couple of decades to address these challenges. This chapter aims to provide the reader an overview of the major research findings to date on the key components of a PEMFC MEA.

Tungsten carbide nanotubes supported platinum nanoparticles as a potential sensing platform for oxalic acid.

Supported tungsten carbide is an efficient and vital nanomaterial for the development of high-performance, sensitive, and selective electrochemical sensors. In this work, tungsten carbide with tube-like nanostructures (WC NTs) supported platinum nanoparticles (PtNPs) are synthesized and explored as an efficient catalyst toward electrochemical oxidation of oxalic acid for the first the time. The WC NTs supported PtNPs modified glassy carbon (GC) electrode is highly sensitive toward the electrochemical oxidation of oxalic acid. A large decrease in the oxidation overpotential (220 mV) and significant enhancement in the peak current compared to unmodified and Pt/C modified GC electrodes have been observed without using any redox mediator. Moreover, WC NTs supported PtNPs modified electrode possessed wide linear concentration ranges from 0 to 125 nM and a higher sensitivity toward the oxidation of oxalic acid (80 nA/nM) achieved by the amperometry method. The present modified electrode showed an experimentally determined lowest detection limit (LOD) of 12 nM (S/N = 3). Further, WC NTs supported PtNPs electrode can be demonstrated to have an excellent selectivity toward the detection of oxalic acid in the presence of a 200-fold excess of major important interferents. The practical application of WC NTs supported PtNPs has also been demonstrated in the detection of oxalic acid in tomato fruit sample, by differential pulse voltammetry under optimized conditions.

Synthesis and optimisation of IrO2 electrocatalysts by Adams fusion method for solid polymer electrolyte electrolysers

IrO 2 as an anodic electrocatalyst for the oxygen evolution reaction (OER) in solid polymer electrolyte (SPE) electrolysers was synthesised by adapting the Adams fusion method. Optimisation of the IrO 2 electrocatalyst was achieved by varying the synthesis duration (0.5 - 4 hours) and temperature (250 - 500°C). The physical properties of the electrocatalysts were characterised by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and x-ray diffraction (XRD). Electrochemical characterisation of the electrocatalysts toward the OER was evaluated by chronoamperometry (CA). CA analysis revealed the best electrocatalytic activity towards the OER for IrO 2 synthesised for 2 hours at 350 o C which displayed a better electrocatalytic activity than the commercial IrO 2 electrocatalyst used in this study. XRD and TEM analyses revealed an increase in crystallinity and average particle size with increasing synthesis duration and temperature which accounted for the decreasing electrocatalytic activity. At 250°C the formation of an active IrO 2 electrocatalyst was not favoured.

Morphology and phase tuning of α- and β-MnO2 nanocacti evolved at varying modes of acid count for their well-coordinated energy storage and visible-light-driven photocatalytic behaviour

A simple hydrothermal method is developed to synthesize two different phases, α and β of MnO2 nanocacti (comprising nanowires with 1–10 nm diameter self assembled by ultrathin sheets) as well as MnO2 nanorods (10–40 nm diameter) without any seed or template. Sudden addition of concentrated H2SO4 (0.3–0.4 μL) results in the formation of nanocacti while gradual addition (dropwise) of H2SO4 solution (0.3–0.4 M) results in nanorods. Besides, the α phase of MnO2 exists at relatively high acidic strength (4 pH) compared to the β phase, which is consistent at 5 pH. Thus this could be the first report exploring the possibilities of tuning morphology as well as the phase of MnO2 through simple optimizations in acidic content. We find that polymorphic MnO2 nanocacti exhibit superior photocatalytic activity and high energy capacity as an anode in Li-ion batteries than polymorphic MnO2 nanorods. The α phase of MnO2 performs better than the β phase. α-MnO2 nanocacti demonstrate high visible light driven photocatalytic activity by degrading >90% of congo red and methyl orange dyes in 40 mg L−1 organic dye aqueous solution with 0.1 g of the as-prepared sample within 25 and 70 min, respectively. We highlight the differences between the photocatalytic activities of different phases, α and β of MnO2 nanostructures, depending on the charge transport through different dimensions of the same pristine MnO2. The constant cycling stability of α-MnO2 nanocacti with capacities as low as 300 mA h g−1 at 1C rate after 50 cycles as an anode makes it a promising material for energy storage applications. We attribute the high electro- and photo-chemical activity for α-MnO2 nanocacti to their highly mesoporous structure making this one of the highest specific surface areas (271 m2 g−1) possibly ever reported for pristine MnO2.