Tu Quang Nguyen

Reduced SnO2 Porous Nanowires with a High Density of Grain Boundaries as Catalysts for Efficient Electrochemical CO2-into-HCOOH Conversion

Electrochemical conversion of CO2 into energy-dense liquids, such as formic acid, is desirable as a hydrogen carrier and a chemical feedstock. SnOx is one of the few catalysts that reduce CO2 into formic acid with high selectivity but at high overpotential and low current density. We show that an electrochemically reduced SnO2 porous nanowire catalyst (Sn-pNWs) with a high density of grain boundaries (GBs) exhibits an energy conversion efficiency of CO2 -into-HCOOH higher than analogous catalysts. HCOOH formation begins at lower overpotential (350 mV) and reaches a steady Faradaic efficiency of ca. 80 % at only -0.8 V vs. RHE. A comparison with commercial SnO2 nanoparticles confirms that the improved CO2 reduction performance of Sn-pNWs is due to the density of GBs within the porous structure, which introduce new catalytically active sites. Produced with a scalable plasma synthesis technology, the catalysts have potential for application in the CO2 conversion industry.

High rate capacity retention of binder-free, tin oxide nanowire arrays using thin titania and alumina coatings

This paper reports the use of thin titania or alumina coatings on tin oxide nanowire arrays for high cyclability electrodes for lithium-ion batteries. We demonstrate that such coatings can significantly reduce irreversible capacity loss associated with the formation of a solid electrolyte interface and improve the capacity retention at high rates. Specifically, tin oxide nanowires grown on stainless steel substrates were conformally coated with thin films of either titania or alumina using atomic layer deposition and were tested as anodes in coin cells. Both titania and alumina coatings resulted in no initial capacity loss due to solid electrolyte interface formation in the first cycle. Tin oxide nanowire array electrodes coated with 5 nm thick titania layer and 1 nm thick alumina layer retained capacities of 767 and 725 mA h g−1 after 30 cycles using current density of 700 mA g−1. Both electrodes retained capacity around 664 mA h g−1 after 30 cycles using a current density of 1500 mA g−1, respectively. The results indicate that thin coatings acted as mechanical shells preserving the electrode nanostructure morphology necessary for high capacity retention. The study also showed that within the first two cycles, tin migrates out forming nanoclusters on the surface of nanowires due to both stress enhanced diffusion and the Kirkendall effect. The presence of tin nanoclusters on the surface of protective layers further enhances high rate capability.

Scalable synthesis and surface stabilization of Li2MnO3 NWs as high rate cathode materials for Li-ion batteries

Li2MnO3 nanowires (NWs) are synthesized using a scalable two-step process involving a solvo-plasma technique, utilizing inexpensive precursors such as commercially available MnO2 microparticle powders and KCl, followed by a solid state lithiation process. Lithium manganese oxide (Li2MnO3) nanowires exhibited high capacity retention of 120 mA h g−1 in the 2–4.5 V voltage window even at high C-rates such as 20 C. The specific capacity of the Li2MnO3 NWs gradually increased with cycling and subsequently stabilized. Further, the Li2MnO3 NW cathodes exhibited no loss in the capacity for 100 cycles with close to 100% coulombic efficiency. Most importantly, single crystalline Li2MnO3 nanowires with short transport length scales for Li, O and Mn atoms along the radial direction allow for the formation of a thick and conformal LiMn2O4 shell resulting in increased capacity, excellent capacity retention and high coulombic efficiencies.

Nanowire architectures for iodide free dye-sensitized solar cells

In this study, we show that the performance of iodide free redox couples in dye-sensitized solar cells could be significantly improved by engineering the electron transport and surface properties of the electrode materials. Specifically, tin oxide nanowires electrophoretically coated with titania nanoparticles and subsequently passivated with a submonolayer of alumina by atomic layer deposition show a remarkable ten-fold increase in short-circuit current densities over those obtained with titania nanoparticles, even when a typical N-719 dye is used for sensitization. Comparison of the performance of different electrode materials such as nanowires, nanoparticles and nanowire–nanoparticle hybrid architectures of tin oxide and titania suggests that fast electron transport helps in improving the short-circuit current density with ferrocene/ferrocenium and TEMPO redox couples. The nature of the surface trap states and their passivation have a significant effect on the electron lifetimes in the semiconductor and the resulting open-circuit voltage with these redox couples. The higher electron diffusion lengths with the tin oxide nanowire based architectures allow for thicker electrodes with enhanced dye loading. The analysis of literature data on DSCs made using different dyes and alternate redox couples suggests smaller delta G or reorganization energy for non-ruthenium based dyes.