Ranran Peng

Sintering and electrical properties of (CeO2)0.8(Sm2O3)0.1 powders prepared by glycine–nitrate process

Abstract (CeO2)0.8(Sm2O3)0.2 (SDC) powders were prepared using the glycine–nitrate process (GNP) with different glycine/metal ratios. The phase identification, morphology and electrical properties of SDC powders were investigated by XRD, TEM and the AC impedance spectroscopy, respectively. It was found that the ratio of glycine/metal had a great effect on both the morphology and the sinterability of the powders as well as the conductivity of the sintered pellets. When the ratio is around the stoichiometric value (about 1.6), the loose powders possessed a foam-like structure. The bulk density of SDC pellets sintered at 1500 °C could reach 95% only when the ratio of glycine to metal was in the range of 1.3–2.0. The conductivity increased with the ratio of glycine to metal till at the ratio of 1.7, and then decreased. The maximum conductivity of the sintered specimens was 0.082 Scm−1 at 800 °C.

Cathode processes and materials for solid oxide fuel cells with proton conductors as electrolytes

This article provides a brief review of the cathode reaction mechanisms in proton-conducting solid oxide fuel cells (H-SOFCs) and a comparative analysis of electrochemical performance and the ionic and electronic transport in cathode materials. The transfer of protons from electrolyte to triple phase boundaries (TPBs) and the diffusion of O−ad from catalytic sites to TPBs are proposed to be the rate-limiting steps for H-SOFCs, of which the latter is more related to the cathode components, microstructure, conducting species and electrical conductivity. The experimental and theoretical analyses also suggest that cathode materials with electron and proton conductivity present smaller polarisation resistances due to the ability of protons to transfer from the electrolyte into the bulk, which extends the reaction areas for protons and oxygen species to the entire gas/cathode surface.

SDC-Carbonate Composite Electrolytes for Low-Temperature SOFCs

Novel composite materials based on mixtures of samarium-doped ceria SDC -carbonate were examined for use as electrolyte materials in low-temperature solid oxide fuel cells LTSOFCs operating at 400-600°C. The composite electrolytes showed high ion conductivity at evaluated temperatures. Composition and calcination temperature were found to affect the morphology and conductivity of the composite electrolytes greatly. During fuel cell operation, water was observed at both electrodes, indicating that the new electrolyte materials conduct both oxygen ions and proton simultaneously. According to fuel cell performance, these composite electrolytes are chemically stable, which is an attractive prospect in LTSOFC applications.

Preparation of yttria stabilized zirconia membranes on porous substrates by a dip-coating process

Abstract Gas-tight yttria stabilized zirconia (YSZ, 8 mol.% Y 2 O 3 ) membranes with expected composition were coated on porous ceramic substrates by a dip-coating process with YSZ sol, which was prepared by ultrasonic dispersion of YSZ powder in ethanol. The YSZ powder was synthesized by co-precipitation from inorganic aqueous solutions, followed by azeotropic distillation and sintering at 600°C for 2 h. The crystalline structure of YSZ powder was characterized by X-ray diffraction. Microstructures of YSZ membranes were analyzed by scanning electron microscopy and nitrogen permeation testing. The investigation showed that defects in the membranes could be gradually removed by repeating the dip-coating procedures. The electrical transport properties of the YSZ membrane were analyzed by AC impedance spectroscopy. The electrical conductivity of the membrane was 0.001 s cm −1 at 900°C and the activation energy of electrical conductivity was found to be 87.5 kJ mol −1 .

Intermediate-temperature SOFCs with thin Ce0.8Y0.2O1.9 films prepared by screen-printing

Thin electrolyte films of yttria-doped ceria (YDC, Ce0.8Y0.2O1.9) were fabricated on green substrates of NiO–YDC by a screen-printing technique. Dense YDC thin layers were subsequently formed on the porous NiO–YDC substrates after the green bi-layers were co-sintered at 1350 °C. The YDC films were about 15 μm as characterized by scanning electron microscope. With Sm0.5Sr0.5CoO2.75 as cathodes, single cells were tested from 450 to 650 °C with humidified (3% H2O) hydrogen as fuel and air as oxidant. Open circuit voltage of 0.97 V was obtained at 450 °C, indicating negligible gas permeation through the YDC thin films. Maximum power density of about 360 mW/cm2 at 650 °C was achieved with current density of about 800 mA/cm2. Electrochemical characterizations of the single cells indicate that it is critical to reduce the electrode polarization by developing new electrode materials as well as fabrication processes for solid oxide fuel cells that are operated at intermediate temperature.

Effect of powder preparation on (CeO2)0.8(Sm2O3)0.1 thin film properties by screen-printing

Chemical co-precipitation process and glycine-nitrate process (GNP) were used to synthesize (CeO2)0.8(Sm2O3)0.1(SDC) powders for the preparation of thin dense film by screen-printing. XRD, transmission electron microscope (TEM), scanning electron microscope (SEM) and the AC impedance spectroscopy were used to investigate the phase identification, morphology and electrical properties of SDC powders, respectively. It was found that the powders prepared by both processes possessed similar sinterability and conductivity. To both synthesized powders, the SDC pellets sintered at 1350 °C can reach 96% in relative density and 0.08 S cm−1 in conductivity at 800 °C in air. Thin electrolyte films about 30 μm have been successfully achieved by screen-printing with both powders. While the thin electrolyte film by GNP powders possessed better sinterability and higher conductivity, which was attributed to its high pack density during the green film preparation.

Low magnetic field response single-phase multiferroics under high temperature

A single-phase material where ferroelectricity and ferromagnetism coexist at room temperature (RT) is hardly available at present, and it is even more rare for such a material to further have an intrinsic and low magnetic field response magnetoelectric (ME) coupling at temperatures higher than RT. In this communication, a new single-phase Aurivillius compound, SrBi5Fe0.5Co0.5Ti4O18 has been discovered that exhibits a plausible intrinsic ME coupling. Remarkably, this property appears at a high temperature of 100 °C, surpassing all single-phase multiferroic materials currently under investigation. With a magnetocapacitance effect detectable at 100 °C and under a low response magnetic field, a RT functioning device was demonstrated to convert an external magnetic field variation directly into an electric voltage output. The availability of such a single-phase material with an intrinsic and low magnetic field response that is multiferroic at high temperature is important to the fundamental understanding of physics and to potential applications in sensing, memory devices, quantum control, etc.

A high performance cathode for proton conducting solid oxide fuel cells

Intermediate temperature solid-oxide fuel cells (IT-SOFCs) ), as one of the energy conversion devices, have attracted worldwide interest for their great fuel efficiency, low air pollution, much reduced cost and excellent longtime stability. In the intermediate temperature range (500–700 °C), SOFCs based on proton conducting electrolytes (PSOFCs) display unique advantages over those based on oxygen ion conducting electrolytes. A key obstacle to the practical operation of past P-SOFCs is the poor stability of the traditionally used composite cathode materials in the steam-containing atmosphere and their low contribution to proton conduction. Here we report the identification of a new Ruddlesden–Popper-type oxide Sr3Fe2O7−δ that meets the requirements for much improved long-term stability and shows a superior single-cell performance. With a Sr3Fe2O7−δ-5 wt% BaZr0.3Ce0.5Y0.2O3−δ cathode, the P-SOFC exhibits high power densities (683 and 583 mW cm−2 at 700 °C and 650 °C, respectively) when operated with humidified hydrogen as the fuel and air as the cathode gas. More importantly, no decay in discharging was observed within a 100 hour test.

Oxygen reduction and transport on the La1−xSrxCo1−yFeyO3−δ cathode in solid oxide fuel cells: a first-principles study

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−yFeyO3 (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 (O2−) 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.

Synthesis of Ni-substituted Bi7Fe3Ti3O21 ceramics and their superior room temperature multiferroic properties

Layer-structured bismuth complex oxides Bi7Fe3−xNixTi3O21 (0 ≤ x ≤ 2) (BFNT) were synthesized using a low-temperature combustion synthesis method. X-ray diffraction patterns and high-resolution transmission electron microscopy analysis indicated that the samples presented a six-layer Aurivillius structure. Substituting Fe sites by Ni ions inside the lattice was found to be effective in enhancing the multiferroic properties at or above the room-temperature. The sample with a composition of x = 1 exhibited a large remnant magnetization (2Mr = 1.32 emu g−1) that is about five hundred times higher than that in un-substituted Bi7Fe3Ti3O21 ceramics. The work is an important step in the effort to find a single phase and a fully functioning multiferroic material.