WooChul Jung

High electrochemical activity of the oxide phase in model ceria–Pt and ceria–Ni composite anodes

Fuel cells, and in particular solid-oxide fuel cells (SOFCs), enable high-efficiency conversion of chemical fuels into useful electrical energy and, as such, are expected to play a major role in a sustainable-energy future. A key step in the fuel-cell energy-conversion process is the electro-oxidation of the fuel at the anode. There has been increasing evidence in recent years that the presence of CeO(2)-based oxides (ceria) in the anodes of SOFCs with oxygen-ion-conducting electrolytes significantly lowers the activation overpotential for hydrogen oxidation. Most of these studies, however, employ porous, composite electrode structures with ill-defined geometry and uncontrolled interfacial properties. Accordingly, the means by which electrocatalysis is enhanced has remained unclear. Here we demonstrate unambiguously, through the use of ceria-metal structures with well-defined geometries and interfaces, that the near-equilibrium H(2) oxidation reaction pathway is dominated by electrocatalysis at the oxide/gas interface with minimal contributions from the oxide/metal/gas triple-phase boundaries, even for structures with reaction-site densities approaching those of commercial SOFCs. This insight points towards ceria nanostructuring as a route to enhanced activity, rather than the traditional paradigm of metal-catalyst nanostructuring.

Nanowire Conductive Polymer Gas Sensor Patterned Using Self-Assembled Block Copolymer Lithography

Nanostructured conjugated organic thin films are essential building blocks for highly integrated organic devices. We demonstrate the large-area fabrication of an array of well-ordered 15 nm wide conducting polymer nanowires by using an etch mask consisting of self-assembled patterns of cylinder-forming poly(styrene-b-dimethylsiloxane) diblock copolymer confined in topographic templates. The poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) nanowires operated as an ethanol vapor sensor, suggesting that the electronic properties of the organic film were preserved during the patterning processes. The higher sensitivity to ethanol vapor, compared to an unpatterned film with the same thickness, was attributed to the enhanced surface-to-volume ratio of the nanowire array.

Investigation of surface Sr segregation in model thin film solid oxide fuel cell perovskite electrodes

While SOFC perovskite oxide cathodes have been the subject of numerous studies, the critical factors governing their kinetic behavior have remained poorly understood. This has been due to a number of factors including the morphological complexity of the electrode and the electrode- electrolyte interface as well as the evolution of the surface chemistry with varying operating conditions. In this work, the surface chemical composition of dense thin film SrTi1−xFexO3-δelectrodes, with considerably simplified and well-defined electrode geometry, was investigated by means of XPS, focusing on surface cation segregation. An appreciable degree of Sr-excess was found at the surface of STF specimens over the wide composition range studied. The detailed nature of the Sr-excess is discussed by means of depth and take-off angle dependent XPS spectra, in combination with chemical and thermal treatments. Furthermore, the degree of surface segregation was successfully controlled by etching the films, and/or preparing intentionally Sr deficient films. Electrochemical Impedance Spectroscopy studies, under circumstances where surface chemistry was controlled, were used to examine and characterize the blocking effect of Sr segregation on the surface oxygen exchange rate.

Impact of Sr segregation on the electronic structure and oxygen reduction activity of SrTi1−xFexO3 surfaces

The correlation between the surface chemistry and electronic structure is studied for SrTi1−xFexO3 (STF), as a model perovskite system, to explain the impact of Sr segregation on the oxygen reduction activity of cathodes in solid oxide fuel cells. Dense thin films of SrTi0.95Fe0.05O3 (STF5), SrTi0.65Fe0.35O3 (STF35) and SrFeO3 (STF100) were investigated using a coordinated combination of surface probes. Composition, chemical binding, and valence band structure analysis using angle-resolved X-ray photoelectron spectroscopy showed that Sr enrichment increases on the STF film surfaces with increasing Fe content. In situ scanning tunnelling microscopy/spectroscopy results proved the important and detrimental impact of this cation segregation on the surface electronic structure at high temperature and in an oxygen environment. While no apparent band gap was found on the STF5 surface due to defect states at 345 °C and 10−3 mbar of oxygen, the surface band gap increased with Fe content, 2.5 ± 0.5 eV for STF35 and 3.6 ± 0.6 eV for STF100, driven by a down-shift in energy of the valence band. This trend is opposite to the dependence of the bulk STF band gap on the Fe fraction, and is attributed to the formation of a Sr-rich surface phase in the form of SrOx on the basis of the measured surface band structure. The results demonstrate that Sr segregation on STF can deteriorate oxygen reduction kinetics through two mechanisms – inhibition of electron transfer from bulk STF to oxygen species adsorbing onto the surface and the smaller concentration of oxygen vacancies available on the surface for incorporating oxygen into the lattice.

Micro-ionics: next generation power sources

The desire for ever smarter systems-on-a-chip and plug-free portable electronics with longer operating times between recharge has stimulated growing interest in micro-ionic systems. The use of thin film and photolithographic processing techniques, commonly at temperatures considerably below those utilized in conventional ceramics processing methods, leads to ionic or mixed ionic-electronic materials with nanosized dimensions. The implications for nanosized grains on the conductivity of thin film solid oxide electrolytes are examined. Grain boundary engineering, as a means of controlling and ultimately enhancing transport along and across grain boundaries, becomes essential given that such boundaries often dominate the transport properties of such nano-dimensioned materials. Heterogeneous doping by selective in-diffusion along grain boundaries was introduced as a potentially powerful means of achieving this. This is coupled with the modeling of space charge distributions at such boundaries, taking into account possible dopant segregation to the boundaries. The use of lithographic methods for generating geometrically well defined structures is used to illustrate how one can achieve a much improved understanding of electrode processes in SOFC structures. Indeed, the more idealized structures achievable by application of microelectronic processing provide a marvelous opportunity to uncover the science underlying the technology of micro- and ultimately macro-ionics.

High electrode activity of nanostructured, columnar ceria films for solid oxide fuel cells

Highly porous oxide structures are of significant importance for a wide variety of applications in fuel cells, chemical sensors, and catalysis, due to their high surface-to-volume ratio, gas permeability, and possible unique chemical or catalytic properties. Here we fabricated and characterized Sm0.2Ce0.8O1.9−δ films with highly porous and vertically oriented morphology as a high performance solid oxide fuel cell anode as well as a model system for exploring the impact of electrode architecture on the electrochemical reaction impedance for hydrogen oxidation. Films are grown on single crystal YSZ substrates by means of pulsed laser deposition. Resulting structures are examined by SEM and BET, and are robust up to post-deposition processing temperatures as high as 900 °C. Electrochemical properties are investigated by impedance spectroscopy under H2–H2O–Ar atmospheres in the temperature regime 450–650 °C. Quantitative connections between architecture and reaction impedance and the role of ceria nanostructuring for achieving enhanced electrode activity are presented. At 650 °C, pH2O = 0.02 atm, and pH2 = 0.98 atm, the interfacial reaction resistance attains an unprecedented value of 0.21 to 0.23 Ω cm2 for porous films 4.40 μm in thickness.

Robust nanostructures with exceptionally high electrochemical reaction activity for high temperature fuel cell electrodes

Metal nanoparticles are of significant importance for chemical and electrochemical transformations due to their high surface-to-volume ratio and possible unique catalytic properties. However, the poor thermal stability of nano-sized particles typically limits their use to low temperature conditions (<500 °C). Furthermore, for electrocatalytic applications they must be placed in simultaneous contact with percolating ionic and electronic current transport pathways. These factors have limited the application of nanoscale metal catalysts (diameter <5 nm) in solid oxide fuel cell (SOFC) electrodes. Here we overcome these challenges of thermal stability and microstructural design by stabilizing metal nanoparticles on a scaffold of Sm_(0.2)Ce_(0.8)O_(2−δ) (SDC) films with highly porous and vertically-oriented morphology, where the oxide serves as a support, as a mixed conducting transport layer for fuel electro-oxidation reactions, and as an inherently active partner in catalysis. The SDC films are grown on single crystal YSZ electrolyte substrates by means of pulsed-laser deposition, and the metals (11 μg cm^(−2) of Pt, Ni, Co, or Pd) are subsequently applied by D.C. sputtering. The resulting structures are examined by TEM, SIMS, and electron diffraction, and metal nanoparticles are found to be stabilized on the porous SDC structure even after exposure to 650 °C under humidified H_2 for 100 h. A.C. impedance spectroscopy of the metal-decorated porous SDC films reveals exceptionally high electrochemical reaction activity toward hydrogen electro-oxidation, as well as, in the particular case of Pt, coking resistance when CH_4 is supplied as the fuel. The implications of these results for scalable and high performance thin-film-based SOFCs at reduced operating temperature are discussed.

Investigation of Cathode Behavior of Model Thin-Film SrTi1 − x Fe x O3 − δ (x = 0.35 and 0.5) Mixed Ionic-Electronic Conducting Electrodes

SrTi 1-x Fe x O 3-δ (STF) model cathodes, with compositions x = 0.35 and 0.5, prepared as dense films with a well-defined area and thickness on top of a single-crystal yttria stabilized zirconia substrate by pulsed layer deposition at 700°C were investigated by electrochemical impedance spectroscopy as a function of electrode geometry, temperature, and oxygen partial pressure. The STF cathode was observed to exhibit typical mixed ionic-electronic behavior with the electrode reaction occurring over the full electrode surface area rather than being limited to the triple-phase boundary. The electrode impedance was observed to be independent of electrode thickness and inversely proportional to the square of the electrode diameter pointing to surface exchange limited kinetics. Furthermore, a Ce 0.9 Gd 0.1 O 2-δ interlayer was found to have no effect on the electrode impedance. Values for the surface exchange coefficient, k, were calculated and found to be comparable in magnitude to those exhibited by other popular mixed ionic-electronic conductors, such as (La,Sr)(Co,Fe)Ο 3 , thereby, confirming the suitability of STF as a model-mixed conducting cathode material. The observed trends are discussed in relation to the known defect and transport properties of STF.

Oxygen diffusion and surface exchange in the mixed conducting oxides SrTi1−yFeyO3−δ

Oxygen transport in the mixed ionic-electronic conducting perovskite-oxides SrTi1-yFeyO3-δ (with y = 0.5 and y = 1.0) was studied by oxygen isotope exchange measurements. Experiments were performed on thin-film samples that were grown by Pulsed Laser Deposition (PLD) on MgO substrates. Isotope penetration profiles were introduced by 18O2/16O2 exchanges into the plane of the films at various temperatures in the range 773 < T/K < 973 at an oxygen activity aO2 = 0.5. Isotope profiles were determined subsequently by Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS), and their analysis yielded tracer diffusion coefficients D* and oxygen surface exchange coefficients k*. Activation energies for oxygen diffusion ΔHD* and surface exchange ΔHk* were obtained. Isothermal values of D* and values of ΔHD* are compared with literature data as a function of Fe content. D* is seen to increase monotonically with Fe content; ΔHD* shows more complex behaviour. D* and ΔHD* are also compared with the predictions of defect-chemical models. Analogous comparisons with literature data for k* and ΔHk* indicate, in contrast to prior studies, no mechanistic difference between electron-poor and electron-rich materials. It is concluded that the single operative mechanism of surface exchange for the entire series of STF compositions requires conduction-band electrons (minority electronic charge-carriers).

Determination of optical and microstructural parameters of ceria films

Light-matter interactions are of tremendous importance in a wide range of fields from solar energy conversion to photonics. Here the optical dispersion behavior of undoped and 20 mol. % Sm doped ceria thin films, both dense and porous, were evaluated by UV-Vis optical transmission measurements, with the objective of determining both intrinsic and microstructural properties of the films. Films, ranging from 14 to 2300 nm in thickness, were grown on single crystal YSZ(100) and MgO(100) using pulsed laser deposition (both dense and porous films) and chemical vapor deposition (porous films only). The transmittance spectra were analyzed using an in-house developed methodology combining full spectrum fitting and envelope treatment. The index of refraction of ceria was found to fall between 2.65 at a wavelength of 400 nm and 2.25 at 800 nm, typical of literature values, and was relatively unchanged by doping. Reliable determination of film thickness, porosity, and roughness was possible for films with thickness ranging from 500 to 2500 nm. Physically meaningful microstructural parameters were extracted even for films so thin as to show no interference fringes at all.