Electrochemical studies of capacitance in cerium oxide thin films and its relationship to anionic and electronic defect densities

Small polaron carrier density in epitaxial, doped CeO_2 thin films under low oxygen partial pressure was determined from electrochemically-measured capacitance after accounting for interfacial effects and shown to agree well with bulk values

Modulating metal-oxygen bonding in lithiated metal oxides with point defects

The strength of the metal-oxygen (M-O) bond in oxides principally determines the band structure and the stability of oxygen relative to O2 gas. Accordingly, such bonding is central to energy storage (for determining the redox potential) and electrocatalysis (for determining adsorbate bonding strength at the electrochemical interface). Traditionally, the M-O bond strength is tuned by changing the metal. We have recently discovered another important knob in LiXMO2: metal vacancies and antisite defects. In these materials, which are ubiquitous as positive electrodes in lithium-ion batteries, metal vacancies can form by moving a metal into the Li van der Waals gap. X-ray and neutron scattering measurements confirmed that introducing metal vacancies can substantially contract neighboring M-O bond length, transforming single bonds to double bonds (i.e., terminal metal oxo ligand). In select local configurations, even the peroxo species (O-O)2- can form. These variations of oxygen bonding leads to dramatic variation in the energetics of the bonding and antibonding states as well as the stability of oxygen relative O2 gas. In this talk, I will discuss the connection between local defect configurations and the M-O and O-O bonding in LiXMO2, where M spans 3d, 4d and 5d transition metals

High electrochemical activity of the oxide phase in model ceria–Pt and ceria–Ni composite anodes

Fuel cells, and in particular solid-oxide fuel cells (SOFCs), enable high-efficiency conversion of chemical fuels into useful electrical energy and, as such, are expected to play a major role in a sustainable-energy future. A key step in the fuel-cell energy-conversion process is the electro-oxidation of the fuel at the anode. There has been increasing evidence in recent years that the presence of CeO_2-based oxides (ceria) in the anodes of SOFCs with oxygen-ion-conducting electrolytes significantly lowers the activation overpotential for hydrogen oxidation. Most of these studies, however, employ porous, composite electrode structures with ill-defined geometry and uncontrolled interfacial properties. Accordingly, the means by which electrocatalysis is enhanced has remained unclear. Here we demonstrate unambiguously, through the use of ceria–metal structures with well-defined geometries and interfaces, that the near-equilibrium H_2 oxidation reaction pathway is dominated by electrocatalysis at the oxide/gas interface with minimal contributions from the oxide/metal/gas triple-phase boundaries, even for structures with reaction-site densities approaching those of commercial SOFCs. This insight points towards ceria nanostructuring as a route to enhanced activity, rather than the traditional paradigm of metal-catalyst nanostructuring

Ceria as a Thermochemical Reaction Medium for Selectively Generating Syngas or Methane from H_2O and CO_2

Doped CeO_2 with a low specific surface area is thermochemically cycled between MO_2 and MO_(2-δ) using H_2O and CO_2 as oxidants. The system rapidly and selectively produces syngas in the absence of a metal catalyst, and CH_4 in the presence of Ni. The Ni catalyst, which permits intermediate C to form on its surface, is proposed to shift the product from syngas to CH_4

Inverse opal ceria–zirconia: architectural engineering for heterogeneous catalysis

The application of inverse opal structured materials is extended to the ceria–zirconia (Ce_(0.5)Zr_(0.5)O_2) system and the significance of material architecture on heterogeneous catalysis, specifically, chemical oxidation, is examined

Electrochemical kinetics of SEI growth on carbon black, II: Modeling

Mathematical models of capacity fade can reduce the time and cost of lithium-ion battery development and deployment, and growth of the solid-electrolyte interphase (SEI) is a major source of capacity fade. Experiments in Part I reveal nonlinear voltage dependence and strong charge-discharge asymmetry in SEI growth on carbon black negative electrodes, which is not captured by previous models. Here, we present a theoretical model for the electrochemical kinetics of SEI growth coupled to lithium intercalation, which accurately predicts experimental results with few adjustable parameters. The key hypothesis is that the initial SEI is a mixed ion-electron conductor, and its electronic conductivity varies approximately with the square of the local lithium concentration, consistent with hopping conduction of electrons along percolating networks. By including a lithium-ion concentration dependence for the electronic conductivity in the SEI, the bulk SEI thus modulates the overpotential and exchange current of the electrolyte reduction reaction. As a result, SEI growth is promoted during lithiation but suppressed during delithiation. This new insight establishes the fundamental electrochemistry of SEI growth kinetics. Our model improves upon existing models by introducing the effects of electrochemical SEI growth and its dependence on potential, current magnitude, and current direction in predicting capacity fade.Comment: 1 manuscript, 7 main text figures, 2 supplementary information figure

High electrode activity of nanostructured, columnar ceria films for solid oxide fuel cells

Highly porous oxide structures are of significant importance for a wide variety of applications in fuel cells, chemical sensors, and catalysis, due to their high surface-to-volume ratio, gas permeability, and possible unique chemical or catalytic properties. Here we fabricated and characterized Sm_(0.2)Ce_(0.8)O_(1.9−δ) films with highly porous and vertically oriented morphology as a high performance solid oxide fuel cell anode as well as a model system for exploring the impact of electrode architecture on the electrochemical reaction impedance for hydrogen oxidation. Films are grown on single crystal YSZ substrates by means of pulsed laser deposition. Resulting structures are examined by SEM and BET, and are robust up to post-deposition processing temperatures as high as 900 °C. Electrochemical properties are investigated by impedance spectroscopy under H_2–H_2O–Ar atmospheres in the temperature regime 450–650 °C. Quantitative connections between architecture and reaction impedance and the role of ceria nanostructuring for achieving enhanced electrode activity are presented. At 650 °C, _pH_2O = 0.02 atm, and _pH_2 = 0.98 atm, the interfacial reaction resistance attains an unprecedented value of 0.21 to 0.23 Ω cm^2 for porous films 4.40 μm in thickness

Unusual decrease in conductivity upon hydration in acceptor doped, microcrystalline ceria

The impact of hydration on the transport properties of microcrystalline Sm_(0.15)Ce_(0.85)O_(1.925) has been examined. Dense, polycrystalline samples were obtained by conventional ceramic processing and the grain boundary regions were found, by high resolution transmission electron microscopy, to be free of impurity phases. Impedance spectroscopy measurements were performed over the temperature range 250 to 650 °C under dry, H_2O-saturated, and D_2O-saturated synthetic air; and over the temperature range 575 to 650 °C under H_2–H_2O atmospheres. Under oxidizing conditions humidification by either H_2O or D_2O caused a substantial increase in the grain boundary resistivity, while leaving the bulk (or grain interior) properties unchanged. This unusual behavior, which was found to be both reversible and reproducible, is interpreted in terms of the space-charge model, which adequately explains all the features of the measured data. It is found that the space-charge potential increases by 5–7 mV under humidification, in turn, exacerbating oxygen vacancy depletion in the space-charge regions and leading to the observed reduction in grain boundary conductivity. It is proposed that the heightened space-charge potential reflects a change in the relative energetics of vacancy creation in the bulk and at the grain boundary interfaces as a result of water uptake into the grain boundary core. Negligible bulk water uptake is detected under both oxidizing and reducing conditions

Highly Enhanced Concentration and Stability of Reactive Ce^3+ on Doped CeO_2 Surface Revealed In Operando

Trivalent cerium ions in CeO_2 are the key active species in a wide range of catalytic and electro-catalytic reactions. We employed ambient pressure X-ray photoelectron spectroscopy and electrochemical impedance spectroscopy to quantify simultaneously the concentration of the reactive Ce^3+ species on the surface and in the bulk of Sm-doped CeO_2(100) in hundreds of millitorr of H2–H2O gas mixtures. Under relatively oxidizing conditions, when the bulk cerium is almost entirely in the 4+ oxidation state, the surface concentration of the reduced Ce^3+ species can be over 180 times the bulk concentration. Furthermore, in stark contrast to the bulk, the surface’s 3+ oxidation state is also highly stable, with concentration almost independent of temperature and oxygen partial pressure. Our thermodynamic measurements reveal that the difference between the bulk and surface partial molar entropies plays a key role in this stabilization. The high concentration and stability of reactive surface Ce^3+ over wide ranges of temperature and oxygen partial pressure may be responsible for the high activity of doped ceria in many pollution-control and energy-conversion reactions, under conditions at which Ce^3+ is not abundant in the bulk