Dong-Hyun Peck

Chromium Vapor Species over Solid Oxide Fuel Cell Interconnect Materials and Their Potential for Degradation Processes

Alloys protected from corrosion by a chromia scale and LaCrO{sub 3}-based perovskites are used as materials for the interconnect of solid oxide fuel cells (SOFCs). The chromium vaporization of these materials was studied by thermochemical modeling. Partial pressures of the vaporizing species were determined for different O{sub 2} and H{sub 2}O concentrations in the oxidizing gas. CrO{sub 2}(OH){sub 2}(g) and CrO{sub 3}(g) are the most abundant species in air with and without humidity, respectively. The potential of the Cr-containing vapor species for the degradation of the electrical properties of an SOFC was analyzed by thermodynamic computations. The electrochemical reduction of the Cr-containing vapor species at the cathode/electrolyte/gas phase boundary can lead to polarization losses.

Performance of a polymer electrolyte membrane fuel cell with thin film catalyst electrodes

In order to develop a kW-class polymer electrolyte membrane fuel cell (PEMFC), several electrodes have been fabricated by different catalyst layer preparation procedures and evaluated based on the cell performance. Conventional carbon paper and carbon cloth electrodes were fabricated using a ptfe-bonded Pt/C electrocatalyst by coating and rolling methods. Thin-film catalyst/ionomer composite layers were also formed on the membrane by direct coating and transfer printing techniques. The performance evaluation with catalyst layer preparation methods was carried out using a large or small electrode single cell. Conventional and thin film membrane and electrode assemblies (MEAs) with small electrode area showed a performance of 350 and 650 mA/cm2 at 0.6 V, respectively. The performance of direct coated thin film catalyst layer with 300 cm2 MEAs was higher than those of the conventional and transfer printing technique MEAs. The influence of some characteristic parameters of the thin film electrode on electrochemical performance was examined. Various other aspects of overall operation of PEMFC stacks were also discussed.

Water transport in polymer membranes for PEMFC

To determine the net electro-osmotic drag coefficient of Nafion 115 and Hanwha membrane, we measured the fluxes of water discharged from anode and cathode at different current densities. Also, we investigated the contribution of water supply for membrane from anode and cathode. When the cathode was humidified, water supply for membrane at low current densities was achieved via the cathode, but the contribution of the anode became more important as current density gradually increased. The net electro-osmotic drag coefficient decreased sharply with current density, but it had a nearly constant value over 200 mA cm−2. When the cathode was not humidified, at low current densities, most of water generated at cathode was supplied for membrane, but water supply from cathode at high current densities decreased proportionately, and the net electro-osmotic drag coefficient showed larger value.

A novel process to fabricate membrane electrode assemblies for proton exchange membrane fuel cells

A new fabrication method of membrane electrode assembly (MEA) for proton exchange membrane fuel cells is developed by using perfluorosulfonyl fluoride copolymer powder and Pt/C catalyst. The perfluorosulfonyl fluoride copolymer powder is pressed into a sheet at 230°C by hot pressing. The Pt/C catalyst is then coated on to either side of the sheet by screen printing, followed by hot pressing. During this process, due to the melt-fabricable property of the pre-formed sheet, the coated catalyst layer is embedded into the membrane. The resultant MEA is converted into perfluorosulfonate polymer by hydrolysis of NaOH solution. The thermal property of the copolymer powder has been analyzed by DTA-TGA, and the interfacial contact of the catalyst with the membrane has been also investigated by SEM. The performance characteristics of the MEA have been evaluated in a single cell.

Phase diagram studies in the SrO-Cr2O3-La2O3 system in air and under low oxygen pressure

Abstract Phase equilibria were determined in the complete composition range of the CaO–Cr 2 O 3 –La 2 O 3 system in air at 1223 K and under low oxygen pressure (high vacuum, p (O 2 )≈10 −9 bar). XRD, DTA and electron probe micro analysis of quenched samples were used. The (La, Ca)CrO 3 perovskite phase is the only ternary phase stable in air. The Ruddlesden-Popper phases (La, Ca) n +1 Cr n O n +1 ( n =1, 2, 3) were identified in addition to the perovskite phase in the samples annealed under low oxygen pressure. The solubility limit of Ca in the La 1− x Ca x CrO 3 phase was determined in air as x =0.31±0.01 at 1223 K and under low oxygen pressure as x =0.22±0.02 at 1873 K. In addition, investigations of the pseudo binary CaO–Cr 2 O 3 phase diagram were performed which clarify contradictions in the literature.

Vaporization and thermodynamics of La1−xCaxCrO3−δ investigated by Knudsen effusion mass spectrometry

Abstract Vaporization of the La 1− x Ca x CrO 3− δ perovskite phase, x =0.047–0.21, was investigated by the use of Knudsen effusion mass spectrometry in the temperature range of 1950–2050 K. Ion intensities were assigned to the neutral precursors by the isothermal evaporation method in addition to literature data. The partial pressures of Cr(g), CrO(g), CrO 2 (g), CaO(g), and LaO(g) were determined for all the samples investigated. The partial pressure of O 2 (g) was evaluated from the gaseous equilibria. The experimental data were interpreted using the phase equilibria in the CaO–Cr 2 O 3 –La 2 O 3 system under low oxygen pressure we previously studied. The ion intensities were used to calculate the thermodynamic activities of the system components at 2000 K. The Gibbs energy of formation of La 0.79 Ca 0.21 CrO 3 at 2000 K was estimated from the thermodynamic activities as −81.5 kJ mol −1 . The results were compared with thermodynamic data of the undoped LaCrO 3 (s). Implications of the present data for the potential use of the material in SOFC technology were discussed as well.

Electrocatalytic effects of carbon dissolution in Pd nanoparticles.

Highly dispersed Pd nanoparticles were prepared by borohydride reduction of Pd(acac)(2) in 1,2-propanediol at an elevated temperature. They were uniformly dispersed on carbon black without significant aggregation. X-ray diffraction showed that carbons from the Pd precursor dissolved in Pd, increasing its lattice parameter. A modified reduction process was tested to remove the carbon impurities. Carbon removal greatly enhanced catalytic activity toward the oxygen reduction reaction. It also generated an inconsistency between the electronic modifications obtained from X-ray photoelectron spectroscopy and the electrochemical method. CO displacement measurements showed that the formation of Pd-C bonds decreased the work function of the surface Pd atoms.

Periodic fuel supply to a micro-DMFC using a piezoelectric linear actuator

A reciprocating pump using a piezoelectric linear actuator is introduced as a fuel supply module for a direct methanol fuel cell (DMFC) stack. The results show that the key advantage of the pump with the linear actuator is its ability to operate at a low frequency, which enables high flow rectification performance. At the end of this study, the pump is applied to a DMFC system to demonstrate that the output voltage of a DMFC stack remains stable with time even when the pumping pressure oscillates periodically at frequencies of a few Hz. It is also demonstrated that the maximum output power of the stack exceeds 6 W with power consuming 3.5% of the stack power for the fuel pump.

Preparation and application of reduced graphene oxide as the conductive material for capacitive deionization

This paper reports the effect of adding reduced graphene oxide (RGO) as a conductive material to the composition of an electrode for capacitive deionization (CDI), a process to remove salt from water using ionic adsorption and desorption driven by external applied voltage. RGO can be synthesized in an inexpensive way by the reduction and exfoliation of GO, and removing the oxygen-containing groups and recovering a conjugated structure. GO powder can be obtained from the modification of Hummers method and reduced into RGO using a thermal method. The physical and electrochemical characteristics of RGO material were evaluated and its desalination performance was tested with a CDI unit cell with a potentiostat and conductivity meter, by varying the applied voltage and feed rate of the salt solution. The performance of RGO was compared to graphite as a conductive material in a CDI electrode. The result showed RGO can increase the capacitance, reduce the equivalent series resistance, and improve the electrosorption capacity of CDI electrode.

Preparation and Characteristics of SiOx Coated Carbon Nanotubes with High Surface Area.

An easy method to synthesize SiOx coated carbon nanotubes (SiOx-CNT) through thermal decomposition of polycarbomethylsilane adsorbed on the surface of CNTs is reported. Physical properties of SiOx-CNT samples depending on various Si contents and synthesis conditions are examined by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), nitrogen isotherm, scanning electron microscope (SEM), and transmission electron microscope (TEM). Morphology of the SiOx-CNT appears to be perfectly identical to that of the pristine CNT. It is confirmed that SiOx is formed in a thin layer of approximately 1 nm thickness over the surface of CNTs. The specific surface area is significantly increased by the coating, because thin layer of SiOx is highly porous. The surface properties such as porosity and thickness of SiOx layers are found to be controlled by SiOx contents and heat treatment conditions. The preparation method in this study is to provide useful nano-hybrid composite materials with multi-functional surface properties.