Defeng Zhou

Electrical conductivity optimization in electrolyte-free fuel cells by single-component Ce0.8Sm0.2O2-δ–Li0.15Ni0.45Zn0.4 layer

Single-component electrolyte-free fuel cells possess a similar function to the traditional fuel cells with a complex three-component structure. However, how to enhance their electrical properties for practical industrial applications remains a timely and important issue. Here, we report the manipulation of concentration ratios of ionic to electronic conductors in an electrolyte-free Ce0.8Sm0.2O2-δ–Li0.15Ni0.45Zn0.4 by adjusting the relative weight between its two inside compositions. Our systematic investigations reveal that the fuel cell with 30% in weight of Li0.15Ni0.45Zn0.4 exhibits an almost uniform distribution of the two compositions and has a total conductivity as high as 10 × 10−2 S cm−1 at 600 °C. Such an enhancement is found to be attributed to the established balance between the numbers of its inside ionic and electronic conductors. These findings are relevant for the technological improvement of this new species of electrolyte-free fuel cell and represent an important step toward commercialization of this single-component fuel cell.

Trends in elasticity and electronic structure of 5d transition metal diborides: first-principles calculations

We investigate the cohesive energy, heat of formation, elastic constant and electronic band structure of transition metal diborides TMB2 (TM = Hf, Ta, W, Re, Os and Ir, Pt) in the Pmmn space group using the ab initio pseudopotential total energy method. Our calculations indicate that there is a relationship between elastic constant and valence electron concentration (VEC): the bulk modulus and shear modulus achieve their maximum when the VEC is in the range of 6.8–7.2. In addition, trends in the elastic constant are well explained in terms of electronic band structure analysis, e.g., occupation of valence electrons in states near the Fermi level, which determines the cohesive energy and elastic properties. The maximum in bulk modulus and shear modulus is attributed to the nearly complete filling of TM d–B p bonding states without filling the antibonding states. On the basis of the observed relationship, we predict that alloying W and Re in the orthorhombic structure OsB2 might be harder than alloying the Ir element. Indeed, the further calculations confirmed this expectation.

First-principles study of La2CoMnO6: a promising cathode material for intermediate-temperature solid oxide fuel cells due to intrinsic Co-Mn cation disorder

AbstractLa2CoMnO6 has attracted intensive research interest because of its prospect for novel technological applications and rich fundamental physics. And, due to the similar radii size as well as small covalent difference (as large as 2) between Co and Mn, cation disorder should be intrinsic within this perovskite. We performed comprehensive first-principles calculations on both La2CoMnO6 (LCMO) and LCMO with CoMn-MnCo antisite defects (AD:LCMO), focusing on the formation of bulk oxygen vacancies, which plays a key role in oxygen ion diffusion process in solid oxide fuel cell (SOFC) electrodes. First, it is found that the covalent states are 2 and +4 for Co and Mn at their regular sites while they are both prone to be +3 in the antisites. The formation energies for oxygen vacancies are predicted to follow the trend Co2+-O-Mn4+ > Co2+-O-Co3+ > Mn3+-O-Mn4+, and the underlying microscopic mechanism is attributed to the more electron delocalization between mixed-covalent transition metals (Co2+-O-Co3+ and Mn3+-O-Mn4+), which is beneficial to diminish the electronic repulsion and help to stabilize the vacancy. Therefore, we could conclude that oxygen ionic conductivity should be enhanced in the compounds with higher degree of cation disorder. Our results indicate that AD:LCMO should be a promising intermediate-temperature solid oxide fuel cell cathode material. Graphical AbstractBased on first-principles calculations, we studied ordered LCMO as well as LCMO with CoMn-MnCo antisite defects (AD:LCMO). First, it was found that the covalent states are +2 and +4 for Co and Mn at regular sites while they are both prone to be +3 in the antisites. The formation energies for oxygen vacancies are predicted to follow the trend Co2+-O-Mn4+ > Co2+-O-Co3+ > Mn3+-O-Mn4+, and the underlying microscopic mechanism for this trend is attributed to the more delocalization of the electrons within AD:LCMO due to the mixed-covalent transition-metal pairs.

Charge, orbital, and magnetic ordering in YBaFe2O5 from first-principles calculations.

First principles calculations using the augmented plane wave plus local orbitals method, as implemented in the WIEN2k code, have been used to investigate the electronic and magnetic properties of YBaFe2O5, especially as regards the charge-orbital ordering. Although the total 3d charge disproportion is rather small, an orbital order parameter defined as the difference between t2g orbital occupations of Fe2+ and Fe3+ cations is large (0.73) and gives unambiguous evidence for charge and orbital ordering. Strong hybridization between O2p and Fe e g states results in the nearly complete loss of the separation between the total charges at the Fe2+ and Fe3+ atoms. Furthermore, the relationship between the orbital ordering and charge ordering is also discussed. The dxz orbital ordering is responsible for the stability of the G-type antiferromagnetic spin ordering and the charge ordering pattern.

Effects of La0.9Sr0.1Ga0.9Mg0.1O3−δ on the microstructure and ionic conductivity of purity/impure Ce0.8Nd0.2O1.9 electrolytes

Five electrolytes based on Ce0.8Nd0.2O1.9 with or without 1000 ppm SiO2 (NDC and NDCSi) are synthesised via a sol–gel process. Composite electrolytes are prepared by mixing NDC and NDCSi sols with 10 wt% precalcined La0.9Sr0.1Ga0.9Mg0.1O3−δ (LSGM) powders. The structure and ionic conductivity of these compounds are investigated by X-ray diffraction, field-emission scanning electron microscopy, and alternating current impedance spectroscopy. The result indicated that the LSGM additives can significantly promoted the grain boundary conductivity of the composite electrolytes. The total conductivity of the composite electrolytes significantly increased with increasing grain boundary conductivity. This paper thereby proposes LSGM as new scavenger materials able to mitigate the harmful effects of SiO2 impurities on grain boundary conduction in NDCSi. Indeed, these additives substantially increase both the grain boundary and the total conductivity of NDCSi.

Synergistic effects of intrinsic cation disorder and electron-deficient substitution on ion and electron conductivity in La1-xSrxCo0.5Mn0.5O3-δ (x = 0, 0.5, and 0.75).

The effects of intrinsic cation disorder and electron-deficient substitution for La1-xSrxCo0.5Mn0.5O3-δ (LSCM, x = 0, 0.5, and 0.75) on oxygen vacancy formation, and their influence on the electrochemical properties, were revealed through a combination of computer simulation and experimental study. First-principles calculations were first performed and found that the tendency of the oxygen vacancy formation energy was Mn(3+)-O*-Mn(4+) < Co(2+)-O*-Co(3+) < Co(2+)-O*-Mn(4+), meaning that antisite defects not only facilitate the formation of oxygen vacancy but introduce the mixed-valent transition-metal pairs for high electrical conductivity. Detailed partial density of states (PDOS) analysis for Mn on Co sites (MnCo) and Co on Mn sites (CoMn) indicate that Co(2+) is prone to being Co(3+) while Mn(4+) is prone to being Mn(3+) when they are on antisites, respectively. Also it was found that the holes introduced by Sr tend to enter the Co sublattice for x = 0.5 and then the O sublattice when x = 0.75, which further promotes oxygen vacancy formation, and these results are confirmed by both the calculated PDOS results and charge-density difference. On the basis of microscopic predictions, we intentionally synthesized a series of pure LSCM compounds and carried out comprehensive characterization. The crystal structures and their stability were characterized via powder X-ray Rietveld refinements and in situ high-temperature X-ray diffraction. X-ray photoelectron spectroscopy testified to the mixed oxidation states of Co(2+)/Co(3+) and Mn(3+)/Mn(4+). The thermal expansion coefficients were found to match the Ce0.8Sm0.2O2-δ electrolyte well. The electrical conductivities were about 41.4, 140.5, and 204.2 S cm(-1) at doping levels of x = 0, 0.5, and 0.75, and the corresponding impedances were 0.041, 0.027, and 0.022 Ω cm(2) at 850 °C, respectively. All of the measured results testify that Sr-doped LaCo0.5Mn0.5O3 compounds are promising cathode materials for intermediate-temperature solid oxide fuel cells.

Magnetic structure and orbital ordering in tetragonal and monoclinic KCrF3 from first-principles calculations

KCrF(3) has been systematically investigated by using the full-potential linearized augmented plane wave plus local orbital method within the generalized gradient approximation and the local spin density approximation plus the on-site Coulomb repulsion approach. The total energies for ferromagnetic and three different antiferromagnetic configurations are calculated in the high-temperature tetragonal and low-temperature monoclinic phases, respectively. It reveals that the ground state is the A-type antiferromagnetic in both phases. Furthermore, the ground states of the two phases are found to be Mott-Hubbard insulators with the G-type orbital ordering pattern. In addition, our calculations show the staggered orbital ordering of the 3d(x(2) ) and 3d(y(2) ) orbitals for the tetragonal phase and the 3d(z(2) ) and 3d(x(2) ) orbitals for the monoclinic phase, which is in agreement with the available data. More importantly, the relationship between magnetic structure and orbital ordering as well as the origin of the orbital ordering are analyzed in detail.

Effect of Nd/Mg co-doping on the electrical properties of ceria-based electrolyte materials

A series of Nd/Mg co-doped ceria electrolytes, Ce0.8Nd0.2−x MgxO2−δ (x = 0, 0.05, 0.10 and 0.15), were synthesised through a facile solgel method. The structure and electrical properties were characterised by X-ray diffraction, scanning electron microscopy and AC impedance spectroscopy. An appropriate elemental ratio of Nd/Mg in the co-doped experiment was found to be essential in improving the electrical performance of the ceria-based electrolytes. The optimum grain-boundary conductivity (2.28 × 10−2 S cm−1) and total conductivity (2.68 × 10−3 S cm−1) were obtained at the doping 10 at.% Nd and 10 at.% Mg.