Rong Kou

Self-assembled TiO2-Graphene Hybrid Nanostructures for Enhanced Li-ion Insertion

We used anionic sulfate surfactants to assist the stabilization of graphene in aqueous solutions and facilitate the self-assembly of in situ grown nanocrystalline TiO2, rutile and anatase, with graphene. These nanostructured TiO2-graphene hybrid materials were used for investigation of Li-ion insertion properties. The hybrid materials showed significantly enhanced Li-ion insertion/extraction in TiO2. The specific capacity was more than doubled at high charge rates, as compared with the pure TiO2 phase. The improved capacity at high charge-discharge rate may be attributed to increased electrode conductivity in the presence of a percolated graphene network embedded into the metal oxide electrodes.

Ternary self-assembly of ordered metal oxide-graphene nanocomposites for electrochemical energy storage.

Surfactant or polymer directed self-assembly has been widely investigated to prepare nanostructured metal oxides, semiconductors, and polymers, but this approach is mostly limited to two-phase materials, organic/inorganic hybrids, and nanoparticle or polymer-based nanocomposites. Self-assembled nanostructures from more complex, multiscale, and multiphase building blocks have been investigated with limited success. Here, we demonstrate a ternary self-assembly approach using graphene as fundamental building blocks to construct ordered metal oxide-graphene nanocomposites. A new class of layered nanocomposites is formed containing stable, ordered alternating layers of nanocrystalline metal oxides with graphene or graphene stacks. Alternatively, the graphene or graphene stacks can be incorporated into liquid-crystal-templated nanoporous structures to form high surface area, conductive networks. The self-assembly method can also be used to fabricate free-standing, flexible metal oxide-graphene nanocomposite films and electrodes. We have investigated the Li-ion insertion properties of the self-assembled electrodes for energy storage and show that the SnO2-graphene nanocomposite films can achieve near theoretical specific energy density without significant charge/discharge degradation.

Stabilization of Electrocatalytic Metal Nanoparticles at Metal−Metal Oxide−Graphene Triple Junction Points

Carbon-supported precious metal catalysts are widely used in heterogeneous catalysis and electrocatalysis, and enhancement of catalyst dispersion and stability by controlling the interfacial structure is highly desired. Here we report a new method to deposit metal oxides and metal nanoparticles on graphene and form stable metal-metal oxide-graphene triple junctions for electrocatalysis applications. We first synthesize indium tin oxide (ITO) nanocrystals directly on functionalized graphene sheets, forming an ITO-graphene hybrid. Platinum nanoparticles are then deposited, forming a unique triple-junction structure (Pt-ITO-graphene). Our experimental work and periodic density functional theory (DFT) calculations show that the supported Pt nanoparticles are more stable at the Pt-ITO-graphene triple junctions. Furthermore, DFT calculations suggest that the defects and functional groups on graphene also play an important role in stabilizing the catalysts. These new catalyst materials were tested for oxygen reduction for potential applications in polymer electrolyte membrane fuel cells, and they exhibited greatly enhanced stability and activity.

Mesoporous carbon/silica nanocomposite through multi-component assembly

Ordered mesoporous carbon/silica nanocomposites were synthesized through a novel multi-component molecular assembly and show promising potential as corrosion-resisted electrocatalyst supports.

Polarization Losses under Accelerated Stress Test Using Multiwalled Carbon Nanotube Supported Pt Catalyst in PEM Fuel Cells

The electrochemical behavior for Pt catalysts supported on multiwalled carbon nanotubes and Vulcan XC-72 in proton exchange membrane (PEM) fuel cells under accelerated stress test at 1.2 V was examined by cyclic voltammetry, electrochemical impedance spectroscopy, and polarization technique. Pt catalyst supported on multiwalled carbon nanotubes exhibited a highly stable electrochemical surface area, oxygen reduction kinetics, and fuel cell performance under highly oxidizing conditions, indicating multiwalled carbon nanotubes have a high corrosion resistance and a strong interaction with Pt nanoparticles. Further analyses were conducted using Tafel slope, ohmic resistances, and limiting current density were conducted for the multiwalled carbon nanotube supported Pt catalyst from the actual polarization curve to differentiate kinetic, ohmic, and mass-transfer polarization losses. It was found that kinetic contribution to the total overpotential was the largest throughout the stress test. However, during accelerated stress test, the fraction of kinetic overpotential decreased, the fraction of ohmic overpotential increased, and the fraction of mass-transfer overpotential remained relatively constant. The increased fraction of ohmic overpotential suggests increased proton transport limitation in the catalyst layer.

Hierachical mesoporous silica wires by confined assembly

The assembly of silicate and surfactant confined within cylindrical alumina pore channels results in circular hexagonal, concentric lamellar and other unique mesostructures.

Fast test for the durability of PEM fuel cell catalysts

ETek Pt/C catalyst was used as standard materials to develop a new test protocol for fast screening durable catalyst for PEM fuel cells. Potential step (Pstep) method with the upper potential of 1.4V and the potential-static (Pstat) holding at 1.4 V or 1.2V are used to degrade the catalyst. The degradation in the electrochemical surface area (ESA) for Pt/C under Pstep conditions is greatly accelerated as compared with other conditions. The durability of Pt/Vulcan and Pt/CNT were studied using the new protocol with the electrochemical stressing of Pstep(1.4V/0.6V), which provided the same results as those tested using conventional protocols: Pt/CNT is more durable than Pt/Vulcan. This confirms that the new protocol works well in screening catalyst in terms of durability. The new protocol can differentiate the durability of electrocatalysts by shortening the test time to several hours. It is reliable and time-efficient.