Dengjie Chen

Computational and experimental analysis of Ba0.95La0.05FeO3−δ as a cathode material for solid oxide fuel cells

Solid oxide fuel cells (SOFCs) may play a crucial role in solving the energy crisis because they are clean and energy efficient. Finding suitable cathode materials for SOFCs is key to facilitating their widespread use. Besides developing high performance materials, understanding the stability and intrinsic properties of a material is equally important. Herein, Ba0.95La0.05FeO3−δ (BLF) is studied combining molecular simulations and experiments on single crystal thin films. Lattice dynamics simulations are applied to study the stabilization of barium orthoferrate BaFeO3−δ upon doping with La3+. Simulation results reveal the defect energy for substituting one Ba2+ with La3+ in the cubic phase to be lower than that in the monoclinic phase, contributing to its stabilization. Analogous results are also found by doping the Ba site with Sm3+, Gd3+ and Y3+. In addition, the simulation results suggest that the charge compensation mechanism upon doping is filling oxygen vacancies and La3+ tends to trap the mobile oxygen anions. In turn, as the doping level increases the oxygen anion diffusivity decreases, as is also supported by molecular dynamics simulations. In light of this conclusion, single crystal thin films of La3+ slightly doped BaFeO3−δ, BLF, are grown on yttria-stabilized zirconia substrates using pulsed laser deposition. The polarization resistance of the dense film is 0.07 Ω cm2 at 700 °C in an ambient atmosphere, which is comparable to state-of-the-art Co-based materials.

A CO2-tolerant nanostructured layer for oxygen transport membranes

Dual-layer membranes with enhanced CO2 tolerance and unprecedented oxygen permeability under CO2-containing sweep gas are reported. Specifically, a SrFe0.8Nb0.2O3−δ/Ba0.5Sr0.5Co0.8Fe0.2O3−δ (SFN/BSCF) dual-layer membrane structure has been successfully prepared by pulsed laser deposition of SFN thin layer onto polished BSCF membranes. The phase structure and microstructure of the SFN/BSCF membrane are characterized by XRD and TEM, respectively. Two distinct phases originated from SFN and BSCF are both obtained, which suggests that the SFN is in high crystallinity under the as-deposited condition and BSCF maintains its original status. TEM images clearly show that SFN nanostructured layer is compactly coating on the BSCF substrate. Oxygen permeation fluxes of 2.721, 2.276, 1.809 and 1.303 mL cm−2 min−1 at 900, 850, 800 and 750 °C are attained for a ∼45 nm nanostructured SFN layer decorated on a 1 mm thick BSCF membrane using air as the feed and He as the sweep gas. These high oxygen permeation fluxes are comparable with the pristine BSCF membrane since SFN membrane is also a promising mixed conductor and the coated layer is extremely thin. Under He sweep gas with 10% CO2, a stable oxygen permeation flux of ∼2.25 mL cm−2 min−1 at 850 °C is maintained for ∼550 min with the SFN/BSCF membrane, while it is only lower than 0.4 mL cm−2 min−1 with the uncoated membrane. The results indicate that both high oxygen flux and stability can be simultaneously achieved with adoption a nanostructured protective layer.

Insight into an unusual lanthanum effect on the oxygen reduction reaction activity of Ruddlesden-Popper-type cation-nonstoichiometric La2−xNiO4+δ (x = 0–0.1) oxides

The creation of a cation deficiency in the Ln-site of some Ruddlesden-Popper-type Ln2NiO4+δ oxides (Ln = Pr, Nd) can promote the oxygen reduction activity of these materials at elevated temperatures; however, La-nonstoichiometric La2NiO4+δ materials have been reported to behave differently. In this study, a series of La2−xNiO4+δ (x = 0–0.1) materials was synthesized and systematically characterized to obtain information regarding the origin of their different electrochemical behaviors. Based on XRD and HR-TEM characterizations, a high-order La3Ni2O7 phase was detected, even in the case of a slight La deficiency (x = 0.02), and its content increased with the increasing degree of La nonstoichiometry, suggesting the unfavorable formation of a cation deficiency in La2−xNiO4+δ. Such a high-order Ruddlesden-Popper-type oxide typically improves the oxygen reduction activity; however, reduced activity with the increasing La nonstoichiometry of the samples was actually detected based on symmetrical cell tests. In-depth impedance analysis revealed that the deteriorated electrochemical performance was mainly due to the inferior charge transfer process occurring at the electrode/gas interface and/or oxygen ion transfer through the electrolyte/electrode interface. The microstructures of the La-deficient La2−xNiO4+δ electrodes were then carefully observed using SEM. Both the electrode particle interconnection and the adhesion of the electrode layer to the electrolyte were poor, which were related to the poor sintering ability of the high-order La3Ni2O7 phase that deteriorated the electrode performance.

A bi-functional catalyst for oxygen reduction and oxygen evolution reactions from used baby diapers: α-Fe2O3 wrapped in P and S dual doped graphitic carbon

The development of energy conversion and storage devices and the disposal of solid waste represent two major challenges for environmental sustainability. The development of many key sustainable energy technologies relies on fast oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) kinetics. However, both reactions are sluggish. In the present work, we address these two issues synergistically by fabricating Fe2O3 wrapped in P and S dual-doped graphitized carbon (Fe2O3/P–S-CG) from soiled baby diapers, whose disposal and recycling are beneficial to the environment. Electrochemical tests revealed that Fe2O3/P–S-GC has a significant catalytic activity towards both the ORR and OER in 0.1 M KOH. In particular, the difference between the ORR potential at 3 mA cm−2 and the OER potential at 10 mA cm−2 is as small as ∼0.86 V. This value is comparable to that of commercial precious metal-based catalysts. In addition, Fe2O3/P–S-GC exhibits a favorable catalytic durability, making it a promising bi-functional non-precious catalyst for the ORR and OER.