Arjun Kumar Thapa

Pd / MnO2 Air Electrode Catalyst for Rechargeable Lithium/Air Battery

The oxidation of a carbon binder that occurred during a charge in a carbon-MnO 2 air electrode for a Li-air battery resulted in an excessively high charge potential of 4.2 V. The air electrode activity for the Li-air battery was studied on various metals or metal oxides and the mixture of Pd and MnO 2 shows the high activity to oxidation and reduction of Li to form Li 2 O 2 or Li 2 0, respectively. Although the discharge capacity decreased, the application of Pd/MnO 2 without a carbon binder for the air electrode is effective to decrease the charge potential and to improve the energy efficiency from 60 to 89%.

Spectroscopic Investigation of Photoinduced Charge-Transfer Processes in FTO/TiO2/N719 Photoanodes with and without Covalent Attachment through Silane-Based Linkers

Understanding electron-transfer (ET) processes in dye-sensitized solar cells (DSSCs) is crucial to improving their device performance. Recently, covalent attachment of dye molecules to mesoporous semiconductor nanoparticle films via molecular linkers has been employed to increase the stability of DSSC photoanodes. The power conversion efficiency (PCE) of these DSSCs, however, is lower than DSSCs with conventional unmodified photoanodes in this study. Ultrafast transient absorption pump-probe spectroscopy (TAPPS) has been used to study the electron injection process from N719 dye molecules to TiO2 nanoparticles (NPs) in DSSC photoanodes with and without the presence of two silane-based linker molecules: 3-aminopropyltriethoxysilane (APTES) and p-aminophenyltrimethoxysilane (APhS). Ultrafast biphasic electron injection kinetics were observed in all three photoanodes using a 530 nm pump wavelength and 860 nm probe wavelength. Both the slow and fast decay components, attributed to electron injection from singlet and triplet excited states, respectively, of the N719 dye to the TiO2 conduction band, are hindered by the molecular linkers. The hindering effect is less significant with the APhS linker than the APTES linker and is more significant for the singlet-state channel than the triplet-state one. Electron injection from the vibrationally excited states is less affected by the linkers. The spectroscopic results are interpreted on the basis of the standard ET theory and can be used to guide selection of molecular linkers for DSSCs with better device performance. Other factors that affect the efficiency and stability of the DSSCs are also discussed. The relatively lower PCE of the covalently attached photoanodes is attributed to the multilayer and aggregation of the dye molecules as well as the linkers.

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.

High rate and durable, binder free anode based on silicon loaded MoO3 nanoplatelets.

In order to make fast-charging batteries a reality for electric vehicles, durable, more energy dense and high-current density resistant anodes need to be developed. With such purpose, a low lithiation potential of 0.2 V vs. Li/Li+ for MoO3 nanoplatelet arrays is reported here for anodes in a lithium ion battery. The composite material here presented affords elevated charge capacity while at the same time withstands rapid cycling for longer periods of time. Li2MoO4 and Li1.333Mo0.666O2 were identified as the products of lithiation of pristine MoO3 nanoplatelets and silicon-decorated MoO3, respectively, accounting for lower than previously reported lithiation potentials. MoO3 nanoplatelet arrays were deposited using hot-wire chemical vapor deposition. Due to excellent voltage compatibility, composite lithium ion battery anodes comprising molybdenum oxide nanoplatelets decorated with silicon nanoparticles (0.3% by wt.) were prepared using an ultrasonic spray. Silicon decorated MoO3 nanoplatelets exhibited enhanced capacity of 1037 mAh g−1 with exceptional cyclablity when charged/discharged at high current densities of 10 A g−1.

Incommensurate Graphene Foam as a High Capacity Lithium Intercalation Anode

Graphite’s capacity of intercalating lithium in rechargeable batteries is limited (theoretically, 372 mAh g−1) due to low diffusion within commensurately-stacked graphene layers. Graphene foam with highly enriched incommensurately-stacked layers was grown and applied as an active electrode in rechargeable batteries. A 93% incommensurate graphene foam demonstrated a reversible specific capacity of 1,540 mAh g−1 with a 75% coulombic efficiency, and an 86% incommensurate sample achieves above 99% coulombic efficiency exhibiting 930 mAh g−1 specific capacity. The structural and binding analysis of graphene show that lithium atoms highly intercalate within weakly interacting incommensurately-stacked graphene network, followed by a further flexible rearrangement of layers for a long-term stable cycling. We consider lithium intercalation model for multilayer graphene where capacity varies with N number of layers resulting LiN+1C2N stoichiometry. The effective capacity of commonly used carbon-based rechargeable batteries can be significantly improved using incommensurate graphene as an anode material.

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.

Mesoporous TiO2 coating on carbon–sulfur cathode for high capacity Li–sulfur battery

In this paper, a meso-porous TiO2 (titania) coating is shown to effectively protect a carbon–sulfur composite cathode from polysulfide dissolution. The cathode consisted of a sulfur impregnated carbon support coated with a few microns thick mesoporous titania layer. The carbon–sulfur cathode is made using activated carbon powder (ACP) derived from biomass. The mesoporous titania coated carbon–sulfur cathodes exhibit a retention capacity after 100 cycles at C/3 rate (433 mA g −1) and stabilized at a capacity around 980 mA h g−1. The electrochemical impedance spectroscopy (EIS) of the sulfur cathodes suggests that the charge transfer resistance at the anode, (Ract) is stable for the titania coated sulfur electrode in comparison to a continuous increase in Ract for the uncoated electrode implying mitigation of polysulfide shuttling for the protected cathode. Stability in the cyclic voltammetry (CV) data for the first 5 cycles further confirms the polysulfide containment in the titania coated cathode while the uncoated sulfur electrode shows significant irreversibility in the CV with considerable shifting of the voltage peak positions. Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) studies confirm the adsorption of soluble polysulfides by mesoporous titania.