Dat T. Tran

Oxygen reduction reaction catalyst on lithium/air battery discharge performance

Lithium/air batteries have the potential to substantially outperform the best battery system nowadays on the market. Oxygen reduction reaction (ORR) at the cathode in an aprotic organic lithium electrolyte is well-known to limit the discharge rate and capacity of the lithium/air batteries. In this study, the discharge characteristics of Li/air cells with cathodes made of different carbon materials were examined. The results showed that the ORR kinetics in the lithium/air batteries can be drastically improved by using an effective catalyst, achieving higher discharge voltage and rate. The discharge capacity of the lithium/air battery was found to be correlated to the cathode pore volume, to which the mesopore volume of the carbon material has a large contribution. An ORR mechanistic model involving a reaction product deactivating the catalytic sites on the carbon surface is proposed to explain the experimental results.

Hydrogel, aerogel and film of cellulose nanofibrils functionalized with silver nanoparticles.

In this work, we describe hydrogels, aerogels and films of nanofibrillated cellulose (NFC) functionalized with metal nanoparticles using silver as an example. The TEMPO process used to produce NFC generates negatively charged surface carboxylate groups that provide high binding capability to transition metal species such as Ag(+). The gelation of NFC triggered by transition monovalent metal ions was revealed for the first time. The interaction was utilized to bind Ag(+) on the NFC surface and simultaneously induce formation of NFC-Ag(+) hydrogels, where Ag(+) was slowly reduced to Ag nanoparticles by hydroxyl groups on NFC without additional reducing agent. The NFC-Ag(+) hydrogel was initiated by strong association of carboxylate groups on NFC with Ag(+) and sufficient NFC surface charge reduction. The stiff hydrogel has a storage modulus leveled off at a plateau value of ~6800Pa. Porous aerogels and flat thin films comprising a continuous matrix of NFC were decorated with Ag nanoparticles through freeze-drying or solution-casting of NFC-Ag(+) dispersions with low contents of Ag(+), respectively, followed by UV reduction. The presence of Ag species on NFC reduced coalescence of nanofibrils in the film formation as revealed from AFM phase images.

Pyrite FeS2 as an efficient adsorbent of lithium polysulphide for improved lithium–sulphur batteries

Pyrite FeS2 chemically combines lithium polysulphide (Li2Sn) to form active Li2FeS2+n complexes that substantially reduce the out-diffusion of dissolved Li2Sn from sulphur cathodes and consequently improve the cycling performance of lithium–sulphur batteries. The same functions are also applicable to other transition metal sulphides, which opens a new direction for polysulphide sequestration in lithium–sulphur batteries.

Mesoporous silica nanoparticles for high capacity adsorptive desulfurization

Desulfurized JP-8 fuel is of great interest for use as the hydrogen feedstock of fuel cells. The refractory aromatic sulfur species present, however, are particularly challenging to remove through traditional methods. We report the first use of mesoporous silica nanoparticles (MSN) for desulfurization and the material displays a four-fold greater performance towards JP-8 fuel over previous sorbents. The bulk form of mesoporous silica shows three-fold greater level of desulfurization. Silver-impregnated nanoparticle and bulk MCM-41 were found to have saturation adsorption capacities of 32.6 mgS g−1 and 25.4 mgS g−1, respectively. MSN display a high capacity for the notoriously difficult to remove 4,6-dimethyldibenzothiophene along with a two-fold improvement in breakthrough capacity over previously published materials, of 0.98 mgS g−1 at 10 ppmwS.

Poly(acrylic acid) gel as a polysulphide blocking layer for high-performance lithium/sulphur battery

The lithium/sulphur (Li/S) battery is one of the most promising electrochemical energy storage systems after the Li ion battery, due to the high theoretical energy density and low cost of elemental sulphur. However, its development has been hindered by many problems in relation to the out-diffusion of dissolved lithium polysulphide (PS, Li2Sn with 4 ≤ n ≤ 8), the series of sulphur reduction intermediates. In this paper we demonstrate a proof of concept for blocking the out-diffusion of the dissolved PS by employing a dual-layer structural sulphur cathode with a porous poly(acrylic acid) (PAA) membrane coated on the top surface. Upon activation with the liquid electrolyte, the porous PAA membrane becomes a gel and the resulting gel chemically blocks the out-diffusion of PS anions by forming hydrogen bonds between the COOH groups in the gelled PAA and the negatively charged PS anions. Verified visually by a potentiostatic polarization experiment at 1.7 V vs. Li/Li+, the out-diffusion of PS in an electrolyte-flooded Li/S cell is effectively blocked by the dual-layer structural sulphur cathode. As a result, the Li/S cell consisting of a dual-layer structural sulphur cathode exhibits much improved capacity retention while still providing a similar specific capacity, as compared with the cell using the conventional sulphur cathode.

A simple approach for superior performance of lithium/sulphur batteries modified with a gel polymer electrolyte

A gel polymer electrolyte (GPE) is superior to a liquid electrolyte in reducing electrolyte leakage and flammability for the safety of rechargeable batteries. However, using GPE in lithium/sulphur (Li/S) batteries reduces the capacity because the highly viscous GPE traps lithium polysulfide (PS) within the electrolyte and makes the PS electrochemically inactive. In order to compensate for the loss of capacity, a porous poly(ethylene oxide)–sulphur composite has been selected to modify the commercial separator. It is shown that elemental sulphur in the composite layer not only serves as the pore-making agent (or called wetting agent) to facilitate filling of the liquid electrolyte in the cell assembly, but also provides additional sulphur to increase the cells capacity. As a result, the Li/S cell with a GPE-modified separator has even higher capacity than the liquid electrolyte cell while still retaining the advantages of GPE. In this paper we discuss the effect of the composites composition on the morphology, electrolyte wettability and cells performance.

Chemical stability and electrochemical characteristics of FeS microcrystals as the cathode material of rechargeable lithium batteries

Carbon precursor coated iron monosulphide (CP@FeS) microcrystals are synthesized by a facile one-pot solvothermal reaction of FeSO4 and two moles of Na2S with sucrose as the carbon source, and evaluated as the cathode material of rechargeable lithium batteries. The results show that the obtained CP@FeS microcrystals consist of two FeS phases and contain a small amount of sulphur. In storage, FeS transforms into Fe3S4, which is further oxidized by oxygen in air to release elemental sulphur. Interestingly, such transformation and oxidation are found to affect only the first discharge voltage profile with negligible impact on the specific capacity and cycling performance of a Li/FeS battery. It is shown that the cyclability of the Li/FeS battery is greatly affected by the electrolyte solvent and charging cutoff voltage. In this paper, we discuss the chemical stability and redox mechanism of the FeS cathode material, and investigate the factors that affect the cycling performance of Li/FeS batteries.

Pyrite FeS2–C composite as a high capacity cathode material of rechargeable lithium batteries

Pyrite FeS2 is a promising cathode material for rechargeable lithium batteries because of its high theoretical capacity (894 mA h g−1), low cost and near-infinite earth abundance. However, the progress in developing viable Li/FeS2 batteries has been hampered by the poor cyclability of the FeS2 cathode. Aiming to improve the cyclability of the FeS2 cathode, we here report a facile method for the synthesis of FeS2–C composites by a one-pot hydrothermal reaction of FeSO4 and Na2S2 in the presence of carbon black, and examine the effect of composition on the structure of FeS2–C composites and the cycling performance of Li/FeS2 cells. It is shown that the added carbon not only surrounds the FeS2 surface but also penetrates into the entire FeS2 particle, forming continuously conductive networks throughout the FeS2 particle. However, introduction of carbon meanwhile increases the particle size of the FeS2 active material. These two factors lead to an improvement in the rate capability of Li/FeS2 cells while having little effect on the specific capacity and capacity retention of the FeS2 cathode. On the other hand, we show that the electrolyte plays an important role in affecting the cyclability of Li/FeS2 cells, and that the ether- and carbonate-based electrolytes affect the cycling performance of Li/FeS2 cells in their unique manners.

Synthesis of porous ZnO based materials using an agarose gel template for H2S desulfurization

This work describes the synthesis and characterization of pure ZnO and Ni-doped ZnO materials using agarose gel as a template for H2S desulfurization. H2S gas is not only harmful to the environment, but also corrosive to metals and poisonous to fuel reformer and fuel cell catalysts. Removal of H2S is an important step in fuel processing technology. This synthetic approach resulted in highly porous network ZnO based sorbent materials. The phase structure and morphology of these two materials were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Brunauer, Emmett and Teller (BET) surface area measurements, and mercury porosimetry. The H2S desulfurization performance of the as-synthesized ZnO and Ni-doped ZnO materials versus commercial ZnO was studied in a simulated fuel processing operation at 400 °C with an initial H2S concentration of 400 ppmv. It is shown that the sulfur adsorption capacity is greatly affected by the size and morphology of the particles. Desulfurization analysis revealed that commercial ZnO exhibits a low saturation capacity of 245 mg S per g while as-synthesized ZnO has a capacity of 457 mg S per g. Notably, when ZnO was doped with 4 wt% Ni, the sorbent capacity increased still further to 730 mg S per g.

External surface and pore mouth catalysis in hydrolysis of inulin over zeolites with different micropore topologies and mesoporosities

Hydrolysis of inulin over zeolite catalysts with various micropore topologies (FER, MFI, MOR, BEA, MWW and FAU) and mesoporosities (pillared MFI (PMFI) and pillared MWW (PMWW)) was studied. The reaction includes cleavage of four types of glycosidic bonds: the terminal glucosyl to fructosyl bond to produce glucose, the terminal sucrosyl to fructosyl bond to form sucrose, and the terminal fructosyl to fructosyl bond and internal fructosyl bonds within the polymer chain to generate fructose. Inulin conversion has shown an initially slow rate followed by pseudo first-order kinetics. Fructose production occurred at a much faster rate than that of sucrose followed by glucose. Rigorous kinetic data analysis showed that the reaction was inclined to proceed on the external surface acid sites of zeolites with cleavage of terminal sucrosyl to fructosyl and terminal fructosyl to fructosyl bonds. The increase in the micropore size in zeolites promoted pore mouth catalysis for the cleavage of the terminal fructosyl to fructosyl bond and terminal glucosyl to fructosyl bond. The mesoporosity in PMFI and PMWW zeolites enhanced external surface and pore mouth catalysis compared to those of their microporous analogues, but did not enable new types of catalytic events. The measured kinetic data were interpreted using a mathematical model based on a network involving parallel and series reactions. Inulin hydrolysis was probed in the transition from external surface to pore mouth catalysis depending on the zeolite topology and mesoporosity in bulky biomass processing. The present study provides guidelines for the utilization of zeolites with variable topologies and porosities for processing inulin and other biomass feedstocks for food and energy applications.