Francesco Ciucci

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

Small-Signal Apparent Diffusion Impedance of Intercalation Battery Electrodes

The small-signal apparent diffusion impedance of intercalation battery electrodes is studied on the basis of generalized Poisson-Nernst-Planck equations for planar, cylindrical, and spherical geometry. Compared with conventional treatment in tracking the concentration of neutral intercalation species as chemical diffusion, the present work focuses on the voltage and current terms of intercalating ions and electrons and discusses explicit physical meanings of these electrical terms, in the form of equivalent circuits. It can be shown that the present treatment generates the same results as those from conventional chemical diffusion when the electronic transference number is close to I. Finally, the correlation between the frequency domain impedance methods and the time domain methods, such as chronoamperometry and chronopotentiometry, is discussed.

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.

3D core–shell architecture from infiltration and beneficial reactive sintering as highly efficient and thermally stable oxygen reduction electrode

Solid oxide fuel cells (SOFCs) as alternatives for energy conversion have the capacity to overcome low energy conversion efficiency, highly detrimental emissions from traditional fuel utilization and the limited reserves of fossil fuels crisis. Herein, a 3D core–shell architecture has been fabricated from solution infiltration in combination with high-temperature reactive sintering and evaluated as the oxygen reduction electrode for SOFCs. The resultant electrode is composed of a stable porous Sm0.2Ce0.8O1.9 scaffold as the core for bulk oxygen ion diffusion, and a connective Sm,Ce-doped SrCoO3−δ perovskite film as the shell for efficient oxygen reduction reaction and partial current collection. The significant enhancement in conductivity, chemical and thermal compatibility with such core–shell structured electrodes can deliver promising and stable power outputs. An anode-supported solid oxide fuel cell with such a core–shell structured cathode exhibits a peak power density of 1746 mW cm−2 at 750 °C, which is comparable to the most promising cathodes ever developed. In addition, both a symmetrical cell and a fuel cell demonstrate favourable short-term stability during 200 h operation at 700 °C. The combined strategy involving infiltration and high-temperature reactive sintering (accompanied by ion diffusion) appears to be a promising approach to fabricate cathodes with high electrochemical performance and stability.

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.

Frequency dependent dynamical electromechanical response of mixed ionic-electronic conductors

Frequency dependent dynamic electromechanical response of the mixed ionic-electronic conductor film to a periodic electric bias is analyzed for different electronic and ionic boundary conditions. Dynamic effects of mobile ions concentration (stoichiometry contribution), charge state of acceptors (donors), electron concentration (electron-phonon coupling via the deformation potential), and flexoelectric effect contribution are discussed. A variety of possible nonlinear dynamic electromechanical responses of mixed electronic ionic conductors (MIEC) films including quasi-elliptic curves, asymmetric hysteresis-like loops with pronounced memory window, and butterfly-like curves are calculated. The electromechanical response of ionic semiconductor is predicted to be a powerful descriptor of local valence states, band structure and electron-phonon correlations that can be readily measured in the nanoscale volumes and in the presence of strong electronic conductivity. V C 2012 American

Spatially resolved mapping of oxygen reduction/evolution reaction on solid-oxide fuel cell cathodes with sub-10 nm resolution.

Spatial localization of the oxygen reduction/evolution reactions on lanthanum strontium cobaltite (LSCO) surfaces with perovskite and layered perovskite structures is studied at the sub-10 nm level. Comparison between electrochemical strain microscopy (ESM) and structural imaging by scanning transmission electron microscopy (STEM) suggests that small-angle grain boundaries act as regions with enhanced electrochemical activity. The ESM activity is compared across a family of LSCO samples, demonstrating excellent agreement with macroscopic behaviors. This study potentially paves the way for deciphering the mechanisms of electrochemical activity of solids on the level of single extended structural defects such as grain boundaries and dislocations.

Impedance spectra of mixed conductors: a 2D study of ceria

In this paper we develop an analytical framework for the study of electrochemical impedance of mixed ionic and electronic conductors (MIEC). The framework is based on non-equilibrium thermodynamics and it features the coupling of electrochemical reactions, surface transport and bulk transport processes. We utilize this work to analyze two-dimensional systems relevant for fuel cell science via the finite element method (FEM). Alternate current impedance spectroscopy (AC-IS or IS) of a ceria symmetric cell is simulated near equilibrium conditions (zero bias) for a wide array of working conditions including variations of temperature and H(2) partial pressure on a two-dimensional doped ceria sample with patterned metal electrodes. The model shows agreement between computed IS curves and the experimental literature where the relative error on the impedance is consistently below 2%. Important two-dimensional effects such as the impact of thickness decrease and the influence of variable electronic and ionic diffusivities on the impedance spectra are also explored.

Structural origin of the superionic Na conduction in Na2B10H10closo-borates and enhanced conductivity by Na deficiency for high performance solid electrolytes

Recently reported superionic Na closo-borates have drawn considerable attention due to their potential as solid-state electrolytes for Na ion batteries. However, a fundamental understanding of the ion transport mechanism in these materials is still missing. We studied Na conduction in Na2B10H10, a model material, using first-principles calculations. We found that the superior Na diffusivity is closely linked to the behavior of the large B10H102− anionic groups. The reorientations and disorder of these groups facilitate the Na+ hopping to the octahedral (Oh) sites, which link the tetrahedral (Td) sites to form a connected diffusion network. We also found that, in spite of the frequent Na hopping events, the diffusional paths are often blocked, thereby hindering Na transport. Such a mechanism suggests that the Na conductivity can be improved by introducing extrinsic vacancies. By simulating Na-deficient systems, we found that at high temperatures, even a minimal amount of additional Na vacancies significantly increases the ionic diffusivity.

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.