Electrode materials and reaction mechanisms in solid oxide fuel cells: a brief review

This work is focused on the comparative analysis of electrochemical and transport properties in the major families of cathode and anode compositions for intermediate-temperature solid oxide fuel cells (SOFCs) and materials science-related factors affecting electrode performance. The first part presents a brief overview of the electrochemical and chemical reactions in SOFCs, specific rate-determining steps of the electrode processes, solid oxide electrolyte ceramics, and effects of partial oxygen ionic and electronic conductivities in the SOFC components. The aspects associated with materials compatibility, thermal expansion, stability, and electrocatalytic behavior are also briefly discussed. Primary attention is centered on the experimental data and approaches reported during the last 10–15 years, reflecting the main challenges in the field of materials development for the ceramic fuel cells.     

Mixed conductivity, oxygen permeability and redox behavior of K2NiF4-type La2Ni0.9Fe0.1O4+δ

The total conductivity and Seebeck coefficient of La₂Ni_0.9Fe_0.1O_4+δ with K₂NiF₄-type structure, studied in the oxygen partial pressure range from 10⁻⁵ to 0.5 atm at 973-1223 K, were analyzed in combination with the steady-state oxygen permeability, oxygen non-stoichiometry and Mössbauer spectroscopy data in order to examine the electronic and ionic transport mechanisms. Doping of La₂NiO_4+δ with iron was found to promote hole localization on nickel cations due to the formation of stable Fe³⁺ states, although the electrical properties dominated by p-type electronic conduction under oxidizing conditions exhibit trends typical for both itinerant and localized behavior of the electronic sublattice. The segregation of metallic Ni on reduction, which occurs at oxygen chemical potentials close to the low-p(O₂) stability boundary of undoped lanthanum nickelate, is responsible for the high catalytic activity towards partial oxidation of methane by the lattice oxygen of La₂Ni_0.9Fe_0.1O_4+δ as revealed by thermogravimetry and temperature-programmed reduction in dry CH₄-He flow at 573-1173 K. A model for the oxygen permeation fluxes through dense La₂Ni_0.9Fe_0.1O_4+δ ceramics, limited by both bulk ionic conduction and surface exchange kinetics, was proposed and validated.     

The role of the 5f orbitals in bonding, aromaticity, and reactivity of planar isocyclic and heterocyclic uranium clusters

The molecular and electronic structures, stabilities, bonding features and magnetic properties of prototypical planar isocyclic cyclo-U n X n ( n = 3, 4; X = O, NH) and heterocyclic cyclo-U n (mu 2-X) n ( n = 3, 4; X = C, CH, NH) clusters as well as the E@[ c-U 4(mu 2-C) 4], (E = H (+), C, Si, Ge) and U@[ c-U 5(mu 2-C) 5] molecules including a planar tetracoordinate element E (ptE) and pentacoordinate U (ppU) at the ring centers, respectively, have been thoroughly investigated by means of electronic structure calculation methods at the DFT level. It was shown that 5f orbitals play a key role in the bonding of these f-block metal systems significantly contributing to the cyclic electron delocalization and the associated magnetic diatropic (magnetic aromaticity) response. The aromaticity of the perfectly planar cyclo-U n X n ( n = 3, 4; X = O, NH), cyclo-U n (mu 2-X) n ( n = 3, 4; X = C, CH, NH), E@[ c-U 4(mu 2-C) 4], (E = H (+), C, Si, Ge) and U@[ c-U 5(mu 2-C) 5] clusters was verified by an efficient and simple criterion in probing the aromaticity/antiaromaticity of a molecule, that of the nucleus-independent chemical shift, NICS(0), NICS(1), NICS zz (0) and the most refined NICS zz (1) index in conjunction with the NICS scan profiles. Natural bond orbital analyses provided a clear picture of the bonding pattern in the planar isocyclic and heterocyclic uranium clusters and revealed the features that stabilize the ptE's inside the six- and eight-member uranacycle rings. The ptE's benefit from a considerable electron transfer from the surrounding uranium atoms in the E@[ c-U 4(mu 2-C) 4], (E = H (+), C, Si, Ge) and U@[ c-U 5(mu 2-C) 5] clusters justifying the high occupancy of the np orbitals of the central atom E.     

Structural, electronic, bonding, magnetic, and optical properties of bimetallic [Ru(n)Au(m)](0/+) (n + m ≤ 3) clusters

The structural, electronic, bonding, magnetic, and optical properties of bimetallic [Ru(n)Au(m)](0/+) (n + m ≤ 3; n, m = 0-3) clusters were computed in the framework of the density functional theory (DFT) and time-dependent DFT (TD-DFT) using the full-range PBE0 non local hybrid GGA functional combined with the Def2-QZVPP basis sets. Several low-lying states have been investigated and the stability of the ground state spinomers was estimated with respect to all possible fragmentation schemes. Molecular orbital and population analysis schemes along with computed electronic parameters illustrated the details of the bonding mechanisms in the [Ru(n)Au(m)](0/+) clusters. The TD-DFT computed UV-visible absorption spectra of the bimetallic clusters have been fully analyzed and compared to those of pure gold and ruthenium clusters. Assignments of all principal electronic transitions are given and interpreted in terms of contribution from specific molecular orbital excitations.     

Magnetization, Mössbauer and isothermal dilatometric behavior of oxidized YBa(Co,Fe)4O(7+δ)

M?ssbauer spectroscopy and magnetization studies of YBaCo(4-x)Fe(x)O(7+δ) (x = 0-0.8) oxidized at 0.21 and 100 atm O(2), indicate an increasing role of penta-coordinated Co(3+) states when the oxygen content approaches 8-8.5 atoms per formula unit. Strong magnetic correlations are observed in YBaCo(4-x)Fe(x)O(8.5) from 2 K up to 55-70 K, whilst the average magnetic moment of Co(3+) is lower than that for δ ≤ 0.2, in correlation with the lower (57)Fe(3+) isomer shifts determined from M?ssbauer spectra. The hypothesis on dominant five-fold coordination of cobalt cations was validated by molecular dynamics modeling of YBaCo(4)O(8.5). The iron solubility limit in YBaCo(4-x)Fe(x)O(7+δ) corresponds to approximately x ≈ 0.7. The oxygen intercalation processes in YBaCo(4)O(7+δ) at 470-700 K, analyzed by X-ray diffraction, thermogravimetry and controlled-atmosphere dilatometry, lead to unusual volume expansion opposing to the cobalt cation radius variations. This behavior is associated with increasing cobalt coordination numbers and with rising local distortions and disorder in the crystal lattice on oxidation, predicted by the computer simulations. When the oxygen partial pressure increases from 4 × 10(-5) to 1 atm, the linear strain in YBaCo(4)O(7+δ) ceramics at 598 K is as high as 0.33%.     

Mixed conductivity of zircon-type Ce1−xAxVO4±δ (A=Ca, Sr)

Incorporation of alkaline-earth cations into the zircon-type lattice of Ce_1−xA_xVO_4+δ (A=Ca, Sr; x=0−0.2) was found to significantly increase the p-type electronic conductivity and to decrease the Seebeck coefficient, which becomes negative at x≥0.1. The oxygen ionic conductivity is essentially unaffected by doping. The ion transference numbers of Ce_1−xA_xVO_4+δ in air, determined by the faradaic efficiency measurements, are in the range from 2×10⁻⁴ to 6×10⁻³ at 973–1223 K, increasing when temperature increases or alkaline-earth cation content decreases. The results on the partial conductivities and Seebeck coefficient suggest the presence of hyperstoichiometric oxygen, responsible for ionic transport, in the lattice of doped cerium vanadates. The activation energies for the electron-hole and ionic conduction both decrease on doping and vary in the ranges 39–45 kJ/mol and 87–112 kJ/mol, respectively. Paper presented at the 9th EuroConference on Ionics, Ixia, Rhodes, Greece, Sept. 15–21, 2002.     

Spin-glass, antiferromagnetism and kondo behavior in Ce2Au1−x Co x Si3 alloys

Recently, the solid solution Ce₂Au_{1− x Co} xSi₃ has been shown to exhibit many magnetic anomalies associated with the competition between magnetic ordering and the Kondo effect. Here we report high pressure electrical resistivity of Ce₂AuSi₃, ac susceptibility (X) and magnetoresistance of various alloys of this solid solution in order to gain better knowledge of the magnetism of these alloys. High pressure resistivity behavior is consistent with the proposal that Ce₂AuSi₃ lies at the left-hand side of the maximum in Doniach’s magnetic phase diagram. The ac X data reveal that there are in fact two magnetic transitions, one at 2 K and the other at 3 K for this compound, both of which are spin-glass-like. However, as the Co concentration is increased, antiferromagnetism is stabilized for intermediate compositions before attaining non-magnetism for the Co end member.     

Transport, thermomechanical, and electrode properties of perovskite-type {\left( {{\hbox{L}}{{\hbox{a}}_{0.{75} - x}}{\hbox{S}}{{\hbox{r}}_{0.{25} + x}}} \right)_{0.{95}}}{\hbox{M}}{{\hbox{n}}_{0.{5}}}{\hbox{C}}{{\hbox{r}}_{0.{5} - x}}{\hbox{T}}{{\hbox{i}}_x}{{\hbox{O}}_{{3} - }}_\delta \left( {x = 0 - 0.{5}} \right)

Increasing Sr²⁺ and Ti⁴⁺ concentrations in perovskite-type $ {\left( {{\hbox{L}}{{\hbox{a}}_{0.{75} - x}}{\hbox{S}}{{\hbox{r}}_{0.{25} + x}}} \right)_{0.{95}}}{\hbox{M}}{{\hbox{n}}_{0.{5}}}{\hbox{C}}{{\hbox{r}}_{0.{5} - x}}{\hbox{T}}{{\hbox{i}}_x}{{\hbox{O}}_{{3} - }}_\delta \left( {x = 0 - 0.{5}} \right) $ results in slightly higher thermal and chemical expansion, whereas the total conductivity activation energy tends to decrease. The average thermal expansion coefficients determined by controlled-atmosphere dilatometry vary in the range (10.8?C14.5)?×?10^?6?K^?1 at 373?C1,373?K, being almost independent of the oxygen partial pressure. Variations of the conductivity and Seebeck coefficient, studied in the oxygen pressure range 10^?18?C0.5?atm, suggest that the electronic transport under oxidizing and moderately reducing conditions is dominated by p-type charge carriers and occurs via a small-polaron mechanism. Contrary to the hole concentration changes, the hole mobility decreases with increasing x. The oxygen permeation fluxes through dense ceramic membranes are quite similar for all compositions due to very low level of oxygen nonstoichiometry and are strongly affected by the grain-boundary diffusion and surface exchange kinetics. The porous electrodes applied onto lanthanum gallate-based solid electrolyte exhibit a considerably better electrochemical performance compared to the apatite-type La₁₀Si₅AlO_26.5 electrolyte at atmospheric oxygen pressure, while Sr²⁺ and Ti⁴⁺ additions have no essential influence on the polarization resistance. In H₂-containing gases where the electronic transport in $ {\left( {{\hbox{L}}{{\hbox{a}}_{0.{75} - x}}{\hbox{S}}{{\hbox{r}}_{0.{25} + x}}} \right)_{0.{95}}}{\hbox{M}}{{\hbox{n}}_{0.{5}}}{\hbox{C}}{{\hbox{r}}_{0.{5} - x}}{\hbox{T}}{{\hbox{i}}_x}{{\hbox{O}}_{{3} - }}_\delta $ perovskites becomes low, co-doping deteriorates the anode performance, which can be however improved by infiltrating Ni and $ {\hbox{Ce}}{{\hbox{O}}_{{\rm{2}} - }}_\delta $ v into the porous oxide electrode matrix.     

Mössbauer spectroscopy analysis of Fe-doped YBaCo4O7+δ: Effects of oxygen intercalation

Mössbauer spectroscopy of layered YBaCo_3.96Fe_0.04O_7+δ (δ=0.02 and 0.80), where 1% cobalt is substituted with ⁵⁷Fe isotope, revealed no evidence of charge ordering at 4-293 K. The predominant state of iron cations was found trivalent, irrespective of their coordination and oxygen stoichiometry variations determined by thermogravimetric analysis. The extremely slow kinetics of isothermal oxidation at 598 K in air, and the changes of Fe³⁺ fractions in the alternating triangular and Kagomé layers in oxidized YBaCo_3.96Fe_0.04O_7.80, may suggest that oxygen intercalation is accompanied with a substantial structural reconstruction stagnated due to sluggish cation diffusion. Decreasing temperature below 75-80 K leads to gradual freezing of the iron magnetic moments in inverse correlation with the content of extra oxygen. The formation of metal-oxygen octahedra and resultant structural distortions extend the temperature range where the paramagnetic and frozen states co-exist, down to 45-50 K.