Junliang Zhang

In situ grown nanoscale platinum on carbon powder as catalyst layer in proton exchange membrane fuel cells (PEMFCs)

An extensive study has been conducted on the proton exchange membrane fuel cells (PEMFCs) with reducing Pt loading. This is commonly achieved by developing methods to increase the utilization of the platinum in the catalyst layer of the electrodes. In this paper, a novel process of the catalyst layers was introduced and investigated. A mixture of carbon powder and Nafion solution was sprayed on the glassy carbon electrode (GCE) to form a thin carbon layer. Then Pt particles were deposited on the surface by reducing hexachloroplatinic (IV) acid hexahydrate with methanoic acid. SEM images showed a continuous Pt gradient profile among the thickness direction of the catalytic layer by the novel method. The Pt nanowires grown are in the size of 3 nm (diameter)×10 nm (length) by high solution TEM image. The novel catalyst layer was characterized by cyclic voltammetry (CV) and scanning electron microscope (SEM) as compared with commercial Pt/C black and Pt catalyst layer obtained from sputtering. The results showed that the platinum nanoparticles deposited on the carbon powder were highly utilized as they directly faced the gas diffusion layer and offered easy access to reactants (oxygen or hydrogen).

The respective effect of under-rib convection and pressure drop of flow fields on the performance of PEM fuel cells

The flow field configuration plays an important role on the performance of proton exchange membrane fuel cells (PEMFCs). For instance, channel/rib width and total channel cross-sectional area determine the under-rib convection and pressure drop respectively, both of which directly influence the water removal, in turn affecting the oxygen supply and cathodic oxygen reduction reaction. In this study, effects of under-rib convection and pressure drop on cell performance are investigated experimentally and numerically by adjusting the channel/rib width and channel cross-sectional area of flow fields. The results show that the performance differences with various flow field configurations mainly derive from the oxygen transport resistance which is determined by the water accumulation degree, and the cell performance would benefit from the narrower channels and smaller cross sections. It reveals that at low current densities when water starts to accumulate in GDL at under-rib regions, the under-rib convection plays a more important role in water removal than pressure drop does; in contrast, at high current densities when water starts to accumulate in channels, the pressure drop dominates the water removal to facilitate the oxygen transport to the catalyst layer.

Facile preparation of unique three-dimensional (3D) α-MnO2/MWCNTs macroporous hybrid as the high-performance cathode of rechargeable Li-O2 batteries

Undoubtedly, there remains an urgent prerequisite to achieve significant advances in both the specific capacity and cyclability of Li-O2 batteries for their practical application. In this work, a series of unique three-dimensional (3D) α-MnO2/MWCNTs hybrids are successfully prepared using a facile lyophilization method and investigated as the cathode of Li-O2 batteries. Thereinto, cross-linked α-MnO2/MWCNTs nanocomposites are first synthesized via a modified chemical route. Results demonstrate that MnO2 nanorods in the nanocomposites have a length of 100–400 nm and a diameter ranging from 5 to 10 nm, and more attractively, the as-lyophilized 3D MnO2/MWCNTs hybrids is uniquely constructed with large amounts of interconnected macroporous channels. The Li-O2 battery with the 3D macroporous hybrid cathode that has a mass percentage of 50% of α-MnO2 delivers a high discharge specific capacity of 8,643 mAh·g−1 at 100 mA·g−1, and maintains over 90 cycles before the discharge voltage drops to 2.0 V under a controlled specific capacity of 1,000 mAh·g−1. It is observed that when being recharged, the product of toroidal Li2O2 particles disappears and electrode surfaces are well recovered, thus confirming a good reversibility. The excellent performance of Li-O2 battery with the 3D α-MnO2/MWCNTs macroporous hybrid cathode is ascribed to a synergistic combination between the unique macroporous architecture and highly efficient bi-functional α-MnO2/MWCNTs electrocatalyst.

Icosahedral Pt-Ni Nanocrystalline Electrocatalyst: Growth Mechanism and Oxygen Reduction Activity

Engineering the structure of Pt alloy offers an effective way to the design of high performance electrocatalysts. Herein, we synthesize a sandwich-structured, icosahedral Pt2.1 Ni catalyst through a hot injection method. Its growth involves three steps: 1)u2005burst nucleation of Pt atoms to form a Pt-enriched core, 2)u2005heterogeneous nucleation of Ni atoms onto the Pt core to form a Ni-enriched interlayer, and 3)u2005kinetic controlled growth of a Pt-enriched shell. The Pt-enriched core protects the nanostructure from collapse and mitigates the strain change caused by lattice mismatch, and thus enhances the stability of the structure. The Ni-enriched interlayer induces the electronic modification of the outermost Pt shell, and in turn tunes the activity. The Pt-enriched shell provides more active sites through the exposure of (1u20091u20091) facets and retards the dissolution of Ni atoms. As a result, this sandwich-structure enables impressive electrocatalytic activity (0.91u2005mAu2009cm-2 and 0.32u2005AmgPt-1 @ 0.9u2005V) and duability.

An exploration of the use of Au submonolayer decorated Pd7Ir nanoparticles as a highly active electrocatalyst for the ethanol oxidation reaction in alkaline media

Although very promising in its stabilizing effect, an inevitable loss on activity mostly exists when combining gold (Au) with other noble metals. Contrary to this knowledge, in this research an unexpected promotion of the Au submonolayer on palladium-iridium/carbon (Pd7Ir/C) nanoparticles towards the ethanol oxidation reaction in alkaline media was explored and the corresponding mechanism was clarified.

The Experimental Measurement of Local and Bulk Oxygen Transport Resistances in the Catalyst Layer of Proton Exchange Membrane Fuel Cells

Remarkable progress has been made in reducing the cathodic Pt loading of PEMFCs; however, a huge performance loss appears at high current densities, indicating the existence of a large oxygen transport resistance associated with the ultralow Pt loading catalyst layer. To reduce the Pt loading without sacrificing cell performance, it is essential to illuminate the oxygen transport mechanism in the catalyst layer. Toward this goal, an experimental approach to measure the oxygen transport resistance in catalyst layers is proposed and realized for the first time in this study. The measuring approach involves a dual-layer catalyst layer design, which consists of a dummy catalyst layer and a practical catalyst layer, followed by changing the thickness of dummy layer to respectively quantify the local and bulk resistances via limiting current measurements combined with linear extrapolation. The experimental results clearly reveal that the local resistance dominates the total resistance in the catalyst layer.