Chi Chen

Nonstoichiometric Oxides as Low-Cost and Highly-Efficient Oxygen Reduction/Evolution Catalysts for Low-Temperature Electrochemical Devices

Reduction/Evolution Catalysts for Low-Temperature Electrochemical Devices Dengjie Chen,†,⊥,∇ Chi Chen,†,⊥ Zarah Medina Baiyee,† Zongping Shao,‡,§ and Francesco Ciucci*,†,∥ †Department of Mechanical and Aerospace Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China ‡State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry & Chemical Engineering, Nanjing Tech University, No. 5 Xin Mofan Road, Nanjing 210009, China Department of Chemical Engineering, Curtin University, Perth, Western Australia 6845, Australia Department of Chemical and Biomolecular Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China

Compositional Engineering of Perovskite Oxides for Highly Efficient Oxygen Reduction Reactions

Mixed conducting perovskite oxides are promising catalysts for high-temperature oxygen reduction reaction. Pristine SrCoO(3-δ) is a widely used parent oxide for the development of highly active mixed conductors. Doping a small amount of redox-inactive cation into the B site (Co site) of SrCoO(3-δ) has been applied as an effective way to improve physicochemical properties and electrochemical performance. Most findings however are obtained only from experimental observations, and no universal guidelines have been proposed. In this article, combined experimental and theoretical studies are conducted to obtain fundamental understanding of the effect of B-site doping concentration with redox-inactive cation (Sc) on the properties and performance of the perovskite oxides. The phase structure, electronic conductivity, defect chemistry, oxygen reduction kinetics, oxygen ion transport, and electrochemical reactivity are experimentally characterized. In-depth analysis of doping level effect is also undertaken by first-principles calculations. Among the compositions, SrCo0.95Sc0.05O(3-δ) shows the best oxygen kinetics and corresponds to the minimum fraction of Sc for stabilization of the oxygen-vacancy-disordered structure. The results strongly support that B-site doping of SrCoO(3-δ) with a small amount of redox-inactive cation is an effective strategy toward the development of highly active mixed conducting perovskites for efficient solid oxide fuel cells and oxygen transport membranes.

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.

Data Mining of Molecular Dynamics Data Reveals Li Diffusion Characteristics in Garnet Li 7 La 3 Zr 2 O 12

Understanding Li diffusion in solid conductors is essential for the next generation Li batteries. Here we show that density-based clustering of the trajectories computed using molecular dynamics simulations helps elucidate the Li diffusion mechanism within the Li7La3Zr2O12 (LLZO) crystal lattice. This unsupervised learning method recognizes lattice sites, is able to give the site type, and can identify Li hopping events. Results show that, while the cubic LLZO has a much higher hopping rate compared to its tetragonal counterpart, most of the Li hops in the cubic LLZO do not contribute to the diffusivity due to the dominance of back-and-forth type jumps. The hopping analysis and local Li configuration statistics give evidence that Li diffusivity in cubic LLZO is limited by the low vacancy concentration. The hopping statistics also shows uncorrelated Poisson-like diffusion for Li in the cubic LLZO, and correlated diffusion for Li in the tetragonal LLZO in the temporal scale. Further analysis of the spatio-temporal correlation using site-to-site mutual information confirms the weak site dependence of Li diffusion in the cubic LLZO as the origin for the uncorrelated diffusion. This work puts forward a perspective on combining machine learning and information theory to interpret results of molecular dynamics simulations.

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.

Frequency spectroscopy of irreversible electrochemical nucleation kinetics on the nanoscale

An approach is developed for probing the thermodynamics and kinetics of irreversible electrochemical reactions on solid surfaces based on local frequency-voltage spectroscopy. For a model Li-ion conductor surface, two regimes for bias-controlled behavior are demonstrated and ascribed to the difference in the critical nucleus size. The electrostatic and electrochemical phenomena at the tip-surface junction are analyzed. These studies suggest an experimental pathway for exploring local electrochemical activity in solids.

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.

In situ preparation of Ca0.5Mn0.5O/C as a novel high-activity catalyst for the oxygen reduction reaction

Rock-salt-type MnO is rarely used as an electrocatalyst because of its relatively poor activity. Herein, through an in situ preparation and Ca-substitution of MnO/C, we were able to obtain a novel composite, i.e., Ca0.5Mn0.5O/C, as a highly active, stable, and cost-effective oxygen reduction reaction (ORR) catalyst in alkaline media. Ca0.5Mn0.5O/C and MnO/C share a similar rock-salt phase. In comparison to MnO/C, Ca0.5Mn0.5O/C follows a more effective four-electron pathway (versus a two-electron pathway) and displays higher ORR activity, including a more positive onset potential (by 0.05 V), a more positive half-wave potential (by 0.04 V), and a higher current density (by 1.48 mA cm−2). The Ca0.5Mn0.5O/C also shows comparable mass activity, higher activity per material cost, and superior stability in alkaline media in comparison to commercial Pt/C. Additionally, the as-prepared Ca0.5Mn0.5O/C exhibits higher ORR activity than the physical mixture of Ca0.5Mn0.5O and carbon. The enhanced ORR performance of Ca0.5Mn0.5O/C is likely due to (1) the presence of the divalent redox pair MnII/MnIII; (2) the formation of MnOOH on the Ca0.5Mn0.5O surface; and (3) a stronger synergistic interaction between Ca0.5Mn0.5O and C resulting from the in situ preparation method. This work provides new routes to develop advanced electrocatalysts using transition-metal-oxide/carbon composites.