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Wang Z.,CAS Hefei Key Laboratory of Materials for Energy Conversion | Peng R.,CAS Hefei Key Laboratory of Materials for Energy Conversion | Zhang W.,CAS Hefei Key Laboratory of Materials for Energy Conversion | Wu X.,CAS Hefei Key Laboratory of Materials for Energy Conversion | And 4 more authors.
Journal of Materials Chemistry A | Year: 2013

Oxygen reduction and successive migration on a cathode are key steps in solid oxide fuel cells. In this work, we have systematically studied the adsorption, dissociation, incorporation, and successive diffusion of oxygen species on the La1-xSrxCo1-yFe yO3 (LSCF) cathode on the basis of density-functional theory calculation. We found that the O2 molecule prefers to be adsorbed on the transition metal atoms at the B site (Fe or Co) than those at the A site (La or Sr). The oxygen molecule forms either superoxide (O 2-) or peroxide (O22-) species on the surface transition metal atoms, and the isomerisation energy barrier energies between them are less than 0.14 eV. The SrCo-terminated surface has the smallest oxygen vacancy formation energy, and the existence of surface oxygen vacancy promotes the oxygen dissociation on the B-site atom without an energy barrier. Instead, without the surface oxygen vacancy, the oxygen dissociation on the Co site needs to overcome an energy barrier of 0.30 eV, while that on the Fe site is about 0.14 eV. The calculated minimum energy pathways indicate that the energy barrier of oxygen migration on the surface is much higher than that in the bulk which contains the oxygen vacancy. Moreover, increasing the concentration of Co will effectively facilitate the formation of oxygen vacancy, greatly enhancing the oxygen bulk transport. Our study presents a comprehensive understanding of the mechanism of oxygen reduction and migration on the LSCF cathode. © 2013 The Royal Society of Chemistry. Source


Wang Z.,CAS Hefei Key Laboratory of Materials for Energy Conversion | Yang W.,CAS Hefei Key Laboratory of Materials for Energy Conversion | Zhu Z.,CAS Hefei Key Laboratory of Materials for Energy Conversion | Peng R.,CAS Hefei Key Laboratory of Materials for Energy Conversion | And 8 more authors.
Journal of Materials Chemistry A | Year: 2014

Bulk proton mobility and catalytic activity to surface oxygen reduction are the two factors for determining the effectiveness of cathode materials for protonic-solid oxide fuel cells (p-SOFCs). In this work, a mixed protonic/electronic conductor (MPEC) of BaZr0.75Co0.25O3(BZCO) was selected as a potential cathode for p-SOFCs, and its bulk proton transporting and oxygen reduction behaviours at the microscopic level were investigated using the first-principles approach. Two plausible proton migration pathways in BZCO were examined, and the highest proton migration barrier was calculated to be 0.63 eV, which agrees remarkably well with the experimental findings. Compared with the weak adsorption of oxygen on BaZrO3(100) surface, the BZCO(100) surface provides a relatively large adsorption energy of -0.64 eV, indicating that Co doping enhances the oxygen adsorption on the surface. Furthermore, an oxygen reduction reaction over the MPEC cathode surface was explored using a hydrogenated BZCO(100) surface model, where four protons are located to react with one O2 molecule to generate two water molecules. For the formation and desorption of the first water molecule on the BZCO surface, four possible reaction pathways have been mapped. The potential energy profiles indicate that the reaction with two protons simultaneously migrating to the adsorbed oxygen molecule to break the O-O bond (path-4) is the most feasible process for the formation of the first water molecule. Our study presents an atomistic level understanding of oxygen reduction and proton migration over or inside the MPEC cathode for the first time. This journal is © the Partner Organisations 2014. Source


Yang W.,CAS Hefei Key Laboratory of Materials for Energy Conversion | Wang Z.,CAS Hefei Key Laboratory of Materials for Energy Conversion | Yang Z.,CAS Hefei Key Laboratory of Materials for Energy Conversion | Xia C.,CAS Hefei Key Laboratory of Materials for Energy Conversion | And 7 more authors.
ACS Applied Materials and Interfaces | Year: 2014

Using the first-principles calculation and the electronic conductivity relaxation (ECR) experimental technique, we investigated the adsorption and dissociation behaviors of O2 on Pt-modified La0.625Sr0.375Co0.25Fe0.75O3-δ (LSCF) surface. Toward the O2 reduction, the calculation results show that the perfect LSCF (100) surface is catalytically less active than both the defective (100) surface and the perfect (110) surface. O2 molecule can weakly adsorb on the perfect LSCF (100) surface with a small adsorption energy of about -0.30 eV, but the dissociation energy barrier of the O2 molecule is about 1.33-1.43 eV. Doping of Pt cluster on the LSCF (100) surface can remarkably enhance its catalytic activity. The adsorption energies of O2 molecules become -1.16 and -1.89 eV for the interfacial Feint site and the Ptbri bridge site of Pt4-cluster, respectively. Meanwhile, the dissociation energy barriers are reduced to 0.37 and 0.53 eV, respectively. The migration energy barrier of the dissociated oxygen from the interfacial Pt to the LSCF surface is 0.66 eV, and it is 2.58 eV from the top site of the Pt cluster to the interfacial Pt site, suggesting that it is extremely difficult for oxygen to migrate over the Pt cluster. The Bader charge analysis results further indicate that the charges transferring from Pt cluster to LSCF surface promote the adsorption and dissociation of O2 molecules. Experimentally, a dramatic decrease of the surface oxygen exchange relaxation time was observed on Pt-modified LSCF cathode, with a chemical surface exchange coefficient increased from 6.05 × 10-5 cm/s of the bare LSCF cathode to 4.04 × 10-4 cm/s of the Pt-modified LSCF cathode, agreeing very well with our theoretical predictions. © 2014 American Chemical Society. Source

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