Tae Hoon Lim

Properties of the TiO2 membranes prepared by CVD of titanium tetraisopropoxide

Abstract Thin films of TiO 2 were deposited on the inner surface of porous Vycor glass tubes by atmospheric pressure chemical vapor deposition (APCVD) at temperatures ranging from 200 to 400°C using titanium isopropoxide as a precursor. Dense and hydrogen-permselective membranes were formed only in the temperature range between 230 and 300°C. Gas permeation through the dense membrane was governed by an activated diffusion mechanism and the H 2 :N 2 permeation ratio was far beyond the range of Knudsen diffusion. At 200 and 400°C, however, highly porous films were formed and the permselectivity of the membrane was not improved at all, because gas permeation through the porous membrane was in the Knudsen diffusion regime. Selectivity (H 2 :N 2 permeation ratio) profile showed a maximum value of 57 at a deposition temperature of 250°C. The oxygen added to the reaction environment appeared to have some effects on the stability and the permselectivity of the membrane formed. The permeation properties of the TiO 2 membranes were not so appreciable and the stability of the membrane was poor when compared with dense SiO 2 membranes. The characterization of the TiO 2 films by SEM and TEM reveals that phase change from amorphous to crystalline state occurs at a temperature about 300°C.

Electrodeposited Ni dendrites with high activity and durability for hydrogen evolution reaction in alkaline water electrolysis

Different shapes of various nickel structures, including dendrite, particle and film are fabricated by electrodeposition under various conditions. The shape of nickel structures is definitely dependent on the deposition potential, leading to different electrochemical surface area and edge facets. The nickel particle which has a polycrystalline center and edge is obtained at high negative potential. On the other hand, the nickel dendrite deposited by relatively low negative potential exhibits large electrochemical surface area and a particularly active facet for hydrogen evolution reaction (HER) in alkaline water electrolysis. In fact the nickel dendrite shows the highest catalytic activity and stability for HER among the various nickel structures.

Carbon dioxide mediated, reversible chemical hydrogen storage using a Pd nanocatalyst supported on mesoporous graphitic carbon nitride

Reversible, carbon dioxide mediated chemical hydrogen storage was first demonstrated using a heterogeneous Pd catalyst supported on mesoporous graphitic carbon nitride (Pd/mpg-C3N4). The Pd nanoparticles were found to be uniformly dispersed onto mpg-C3N4 with an average size of 1.7 nm without any agglomeration and further exhibit superior activity for the dehydrogenation of formic acid with a turnover frequency of 144 h−1 even in the absence of external bases at room temperature. Initial DFT studies suggest that basic sites located at the mpg-C3N4 support play synergetic roles in stabilizing reduced Pd nanoparticles without any surfactant as well as in initiating H2-release by deprotonation of formic acid, and these potential interactions were further confirmed by X-ray absorption near edge structure (XANES). Along with dehydrogenation, Pd/mpg-C3N4 also proves to catalyze the regeneration of formic acid via CO2 hydrogenation. The governing factors of CO2 hydrogenation are further elucidated to increase the quantity of the desired formic acid with high selectivity.

Supported core@shell electrocatalysts for fuel cells: close encounter with reality.

Core@shell electrocatalysts for fuel cells have the advantages of a high utilization of Pt and the modification of its electronic structures toward enhancement of the activities. In this study, we suggest both a theoretical background for the design of highly active and stable core@shell/C and a novel facile synthetic strategy for their preparation. Using density functional theory calculations guided by the oxygen adsorption energy and vacancy formation energy, Pd3Cu1@Pt/C was selected as the most suitable candidate for the oxygen reduction reaction in terms of its activity and stability. These predictions were experimentally verified by the surfactant-free synthesis of Pd3Cu1/C cores and the selective Pt shell formation using a Hantzsch ester as a reducing agent. In a similar fashion, Pd@Pd4Ir6/C catalyst was also designed and synthesized for the hydrogen oxidation reaction. The developed catalysts exhibited high activity, high selectivity, and 4,000 h of long-term durability at the single-cell level.

Effects of Cathode Inlet Relative Humidity on PEMFC Durability during Startup–Shutdown Cycling I. Electrochemical Study

This work investigated the effect of cathode inlet relative humidity (RH) on the durability of proton exchange membrane fuel cells (PEMFCs) during startup-shutdown cycling via single-cell experiments. Electrochemical techniques, including measurements of polarization curves, electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and linear sweep voltammetry, were performed to examine the effect of cathode inlet RH on the degradation of PEMFCs. The performance was better for PEMFCs cycled at a lower cathode inlet RH than for those cycled at a higher cathode inlet RH on the order of 0 > 50 > 100%. The CV and EIS results showed that as the cathode inlet RH increased, the loss of electrochemically active surface area and the increase in the charge-transfer resistance (R ct ) were faster during the startup-shutdown cycling. However, changes in ohmic resistance (R ohm ) and hydrogen crossover current density were not detectable, revealing that severe membrane degradation did not occur regardless of the cathode inlet RH during startup-shutdown cycles.

Development of a Durable PEMFC Start-Up Process by Applying a Dummy Load II. Diagnostic Study

To investigate the effect of an application of a dummy load during the start-up procedure on the degradations of a membrane electrode assembly (MEA) exposed to the 1200 repetitive start-up-shutdown cycling, this paper uses a variety of physicochemical methods such as online CO 2 analysis, field-emission-scanning electron microscopy, energy dispersive X-ray, electron probe microanalysis, X-ray diffraction, field-emission transmission electron microscopy, selected-area electron diffraction, X-ray photoelectron spectroscopy, Fourier transformation infrared, and inductively coupled plasma. After the 1200 start-up-shutdown cycles, a pronounced particle size growth/agglomeration/oxidation/dissolution of the Pt catalysts and corrosion of the carbon support were observed at the cathode catalyst layer when starting up a proton exchange membrane fuel cell (PEMFC) without a dummy load, which significantly contributed to the losses of active Pt surface area and Pt mass for electrochemical reactions, and thus the degradation of cell performance. However, applying a dummy load during the start-up procedure remarkably mitigated such severe degradations and should be used to improve the durability of MEAs during start-up-shutdown cycling of PEMFCs. Degradations at the anode and membrane were not as severe as those observed at the cathode.

Effects of temperature and partial pressure of CO2/O2 on corrosion behaviour of stainless-steel in molten Li/Na carbonate salt

Abstract The corrosion tests with AISI-type 316L and 310S stainless steels are carried out to understand the abnormal corrosion behaviour observed in a molten 52 m/o Li 2 CO 3 –48 m/o Na 2 CO 3 salt in the temperature range of 520°C to 580°C, particularly in the presence of CO 2 and O 2 . Two experimental methods, namely, an out-of-cell test and an electrochemical method, were employed to analyze the corrosion behaviour with varying gas composition as well as temperature. The samples tested in the temperature range of 520°C to 580°C suffer more corrosion attack than those tested in the temperature range of 600°C to 650°C. Optical microscope analysis of samples from out-of-cell tests for 100 h show that the surfaces of the samples, regardless of the type of stainless-steel, were corroded severely by pitting when the temperature is below 580°C. Samples tested above 600°C, however, do not suffer significant corrosion attack. This is also confirmed by potentiodynamic results. The polarization curves of 316L stainless-steel samples measured above 600°C exhibit the typical active–passive behaviour, but the passive region disappears when the temperature is below 580°C. This is attributed to the formation of a porous LiFe 5 O 8 passive film. By contrast, the formation of a LiFeO 2 passive film, dense enough to provide protection, is observed with increasing temperature over 600°C. It is also found that the partial pressure of CO 2 affects markedly the corrosion rate, but the partial pressure of O 2 does not.

Metal-free, polyether-mediated H2-release from ammonia borane: roles of hydrogen bonding interactions in promoting dehydrogenation.

Polyetheral additives were found to be efficient promoters to enhance the rate of H2-release from ammonia borane (AB) at various temperatures. In particular, tetraethylene glycol dimethyl ether (T4EGDE, 29 wt% relative to AB + T4EGDE) exhibited significantly improved activities for AB dehydrogenation, with the material-based hydrogen storage capacity of 10.3 wt% at 125 °C within 40 min. In situ FT-IR spectroscopy indicated the formation of B-(cyclodiborazanyl)amino-borohydride (BCDB), borazine, and μ-aminodiborane as gaseous byproducts. In addition, (11)B nuclear magnetic resonance (NMR) spectroscopy further revealed that diammoniate of diborane (DADB) was initially formed to give polyaminoborane as liquid and/or solid spent-fuel, consistent with previous reports. Density Functional Theory (DFT) calculations suggested that hydrogen bonding interactions between AB and a polyetheral promoter initially played an important role in increasing the reactivity of B-H bonds of AB by transferring electron density from oxygen atoms of the promoter into B-H bonds of AB. These partially activated, hydridic B-H bonds were proposed to help promote the formation of diammoniate of diborane (DADB), which is considered as a reactive intermediate, eventually enhancing the rate of H2-release from AB. In addition, our in situ solid state (11)B magic angle spinning (MAS) NMR measurements further confirmed that the rate of DADB formation from AB with a small quantity of T4EGDE was found to be much faster than that of pristine AB even at 50 °C. This metal-free method for H2-release from AB with an added, small quantity of polyethers would be helpful to develop feasible hydrogen storage systems for long-term fuel cell applications.

Partial oxidation of methane over nickel-added strontium phosphate

Abstract It was found that nickel-added strontium phosphate exhibited high activity and selectivity in partial oxidation of methane. The optimum nickel content could be determined. Over the optimum catalyst, methane conversions and H 2 and CO concentrations in excess of those predicted by the thermodynamic equilibrium were observed. It is believed that the catalytically active species is metallic nickel. This metallic nickel is considered to come from nickel-substituted strontium phosphate under reducing environment, giving highly dispersed nickel metal particles.

Nickel-calcium phosphate/hydroxyapatite catalysts for partial oxidation of methane to syngas: Effect of composition

Nickel-calcium phosphate/hydroxyapatite catalysts have recently been reported to exhibit high activity and selectivity in partial oxidation of methane (POM). In this work the optimum composition was determined. The optimum mole ratio of Ca/PO4 was around 10/6 and that of Ni/PO4 was in a range from 1.0/6 to 3.0/6 with the optimum Ca/PO4, and the activity could be related with the amount of metallic nickel. In a temperature range from ca. 400 to 700 K, an apparent autothermal reaction was observed to occur in some cases. This is due to the fact that the actual catalyst temperature is higher than the measured temperature, which comes from the exothermic nature of the reaction. The mixing sequence of the precursors during the catalyst preparation does little affect the catalyst activity and characteristics. Deactivation of the catalyst occurred slowly, but the catalyst could easily be regenerated. Moreover, the nickel-calcium phosphate/hydroxyapatite catalyst showed higher activity than the nickel-strontium phosphate catalyst.