Hydrogen Evolution Reaction by Atomic Layer-Deposited MoNx on Porous Carbon Substrates: The Effects of Porosity and Annealing on Catalyst Activity and Stability.

Molybdenum-based compounds are considered as a potential replacement for expensive precious-metal electrocatalysts for the hydrogen evolution reaction (HER) in acid electrolytes. However, coating of thin films of molybdenum nitride or carbide on a large-area self-standing substrate with high precision is still challenging. Here, MoNx is uniformly coated on carbon cloth (CC) and nitrogen-doped carbon (NC)-modified CC (NCCC) substrates by atomic layer deposition (ALD). The as-deposited film has a nanocrystalline character close to amorphous and a composition of approximately Mo2 N with significant oxygen contamination, mainly at the surface. Among the as-prepared ALD-MoNx electrodes, the MoNx /NCCC has the highest HER activity (overpotential η≈236 mV to achieve 10 mA cm-2 ) owing to the high surface area and porosity of the NCCC substrate. However, the durability of the electrode is poor, owing to the poor adhesion of NC powder on CC. Annealing MoNx /NCCC in H2 atmosphere at 400 °C improves both the activity and durability of the electrode without significant change in the phase or porosity. Annealing at an elevated temperature of 600 °C results in formation of a Mo2 C phase that further enhances the activity (η≈196 mV to achieve 10 mA cm-2 ), although there is a huge reduction in the porosity of the electrode as a consequence of the annealing. The structure of the electrode is also systematically investigated by electrochemical impedance spectroscopy (EIS). A deviation in the conventional Warburg impedance is observed in EIS of the NCCC-based electrode and is ascribed to the change in the H+ ion diffusion characteristics, owing to the geometry of the pores. The change in porous nature with annealing and the loss in porosity are reflected in the EIS of H+ ion diffusion observed at high-frequency. The current work establishes a better understanding of the importance of various parameters for a highly active HER electrode and will help the development of a commercial electrode for HER using the ALD technique.

Hierarchically three-dimensional (3D) nanotubular sea urchin-shaped iron oxide and its application in heavy metal removal and solar-induced photocatalytic degradation.

In this study, hierarchically three-dimensional (3D) nanotubular sea urchin-shaped iron oxide nanostructures (3D-Fe2O3) were synthesized by a facile and rapid ultrasound irradiation method. Additives, templates, inert gas atmosphere, pH regulation, and other complicated procedures were not required. Dense 3D-Fe2O3 with a relatively large Brunauer-Emmett-Teller (BET) surface area of 129.4 m2/g was synthesized within 23 min, and the BET surface area was further improved to 282.7 m2/g by a post heat-treatment process. In addition, this post processing led to phase changes from maghemite (γ phase) to hematite (α phase) Fe2O3. Subsequent characterization suggested that the growth mechanism of the 3D-Fe2O3 follows self-assembly and oriented attachment. The prepared 3D-Fe2O3 was applied to wastewater purification. Ultrasound-irradiated 3D-Fe2O3 can eliminate a As(V) and Cr(VI) from water with 25 times faster removal rate by using a one third smaller amount than commercial α-Fe2O3. This was attributed to the inter-particle pores and relatively positively charged surface of the nanostructure. In addition, post heat treatment on ultrasound-irradiated 3D-Fe2O3 significantly influenced the photocatalytic degradation of methylene blue and phenol, with a 25 times higher removal efficiency than that of commercial α-Fe2O3, because of both high BET surface area and good crystallization of the prepared samples.

Chromium halides mediated production of hydroxymethylfurfural from starch-rich acorn biomass in an acidic ionic liquid.

Chromium halides were introduced for the sustainable production of hydroxymethylfurfural (HMF) from raw acorn biomass using an acidic ionic liquid. The free sugars (glucose and maltose) released by the acidic hydrolysis of the biomass were confirmed by the FT-IR absorption bands around 995-1014cm(-1) and HPLC. FESEM analysis showed that the acorn biomass contains various sizes of starch granules and their structures were severely changed by the acidic hydrolysis. An optimal concentration of HCl for the HMF yields was 0.3M. The highest HMF yield (58.7+1.3dwt%) was achieved in the reaction mixture of 40% [OMIM]Cl+10% ethyl acetate+50% 0.3M HCl extract containing a mix of CrBr(3)/CrF(3). The combined addition of two halide catalysts was more effective in the synthesis of HMF (1.2-fold higher on average) than their single addition. The best productivity of HMF was found at 15% concentration of the biomass and at 50%, its relative productivity declined down to ca. 0.4-fold.

Photocatalytic performance of nanocrystalline Bi5Ti3FeO15 layered perovskite under visible light.

Nanocrystalline Bi5Ti3FeO15 layered perovskite exhibiting Aurivillius phase was synthesized by polymerized complex (PC) method and investigated for its physico-chemical as well as optical properties. The crystallization of Bi5Ti3FeO15 synthesized by PC method was found to occur in the temperature range of 800-1050 degress C, whereas the single crystalline Bi5Ti3FeO15 formed at 1030 degrees C by solid state reaction (SSR) method. The observation of highly pure phase and such lower crystallization temperature in Bi5Ti3FeO15 prepared by PC method, is in total contrast to that observed in Bi5Ti3FeO15 prepared by the conventional solid-state reaction (SSR) method. The band gap of nanocrystalline Bi5Ti3FeO15 determined from UV-Vis diffusion reflectance spectrometer was 2.38 eV (525 nm). The photocatalytic activity of Pt/Bi5Ti3FeO15 prepared by PC method was investigated with the photodecomposition of isopropyl alcohol (IPA) and hydrogen production from water-methanol mixed solution under visible light (lambda > or = 420 nm). The respective activities for PC sample were higher than that of Pt/Bi5Ti3FeO15 prepared by SSR as well as Pt/TiO(2-x)N(x).

Synthesis and electrochemical studies of nano-scale carbon-coated LiFePO4 electrodes for Li-ion batteries.

Regardless of high capacity and stability during lithium extraction, LiFePO4 materials have difficulty in the applications for high electrical density because of low electrical conductivities. In order to optimize this problem, we synthesized carbon coated LiFePO4 by adding humic acid using solid state reaction method. We characterized the synthesized compounds via the crystallinity, the valence states of Fe ions, and their shapes. We found the carbon coating using X-ray photoelectron spectroscopy (XPS) and transmission electron microscope (TEM). We also found that the iron ion is substituted from 3+ to 2+ through XPS measurement. We showed that the carbon coating increased the electrochemical behavior by measuring the charge-discharge characteristics.