Shi-Chern Yen

Novel Iron Oxyhydroxide Lepidocrocite Nanosheet as Ultrahigh Power Density Anode Material for Asymmetric Supercapacitors

A simple one-step electroplating route is proposed for the synthesis of novel iron oxyhydroxide lepidocrocite (γ-FeOOH) nanosheet anodes with distinct layered channels, and the microstructural influence on the pseudocapacitive performance of the obtained γ-FeOOH nanosheets is investigated via in situ X-ray absorption spectroscopy (XAS) and electrochemical measurement. The in situ XAS results regarding charge storage mechanisms of electrodeposited γ-FeOOH nanosheets show that a Li(+) can reversibly insert/desert into/from the 2D channels between the [FeO6 ] octahedral subunits depending on the applied potential. This process charge compensates the Fe(2+) /Fe(3+) redox transition upon charging-discharging and thus contributes to an ideal pseudocapacitive behavior of the γ-FeOOH electrode. Electrochemical results indicate that the γ-FeOOH nanosheet shows the outstanding pseudocapacitive performance, which achieves the extraordinary power density of 9000 W kg(-1) with good rate performance. Most importantly, the asymmetric supercapacitors with excellent electrochemical performance are further realized by using 2D MnO2 and γ-FeOOH nanosheets as cathode and anode materials, respectively. The obtained device can be cycled reversibly at a maximum cell voltage of 1.85 V in a mild aqueous electrolyte, further delivering a maximum power density of 16 000 W kg(-1) at an energy density of 37.4 Wh kg(-1).

High performance of catalysts supported by directly grown PTFE-free micro-porous CNT layer in a proton exchange membrane fuel cell

A proton exchange membrane fuel cell (PEMFC) uses a solid polymer electrolyte, viz. Nafion®, sandwiched between the two electrodes. Nafion® not only plays the role as an electronic insulator and gas barrier but also allows rapid proton transport and supports high current densities. In order to maintain the high proton conductivity of Nafion®, humidified H2 and O2 are passed through the two electrodes. However, water gets easily condensed in the electrodes. This process, called water-flooding, degrades the performance of PEMFC. Hence, a hydrophobic agent, viz. polytetrafluoroethylene (PTFE), is normally incorporated into the electrodes to prevent this phenomenon. Since it is electrically insulating, the incorporation of PTFE increases the internal resistance of the fuel cell. In this study, we successfully demonstrate a PEMFC with catalyst layer comprising of low loading of platinum nanoparticles (0.05 mg cm−2) supported by a directly grown micro-porous carbon nanotube (CNT) layer without incorporation of PTFE, (Pt/MPL-CNT). This cell performs well without exhibiting water-flooding. A commercial electrode, the catalyst layer of which was supported by a conventional micro-porous layer of carbon black mixed with 30 w.t.% PTFE, was used as a reference (Pt/PTFE-MPL-CB). In the single cell tests, PEMFCs with 0.05 mg cm−2Pt/MPL-CNT and 0.25 mg cm−2Pt/PTFE-MPL-CB were used at the cathodes. These cells yielded maximum power densities of 902 mW cm−2 and 824 mW cm−2, respectively, at 70 °C when operated with H2/O2. Notably, the Pt-loading of Pt/MPL-CNT cell is one-fifth of that of Pt/PTFE-MPL-CB, but the former still outperforms the latter. It is shown that the directly grown micro-porous CNT layer has low electronic resistance and is intrinsically hydrophobic, which are the properties responsible for the high performance obtained here.

Experimental Mass Transfer at a Forced‐Convective Rotating‐Disk Electrode

The mass transfer at a rotating-disk electrode with an external forced convection has been studied in this paper. The mass-transfer factor of the forced-convective rotating disk suitable for Schmidt numbers greater than 1 is presented. The limiting currents of the ferricyanide reduction at various forced-flow strength and rotating speeds were measured experimentally and used to estimate the mass transfer at the forced-convective rotating disk. It is found that the experimental results agree very well with the values calculated theoretically