Abstract

Ex situ transmission X-ray microscopy reveals micrometer-scale state-of-charge heterogeneity in solid-solution Li1- x Ni1/3 Co1/3 Mn1/3 O2 secondary particles even after extensive relaxation. The heterogeneity generates overcharged domains at the cutoff voltage, which may accelerate capacity fading and increase impedance with extended cycling. It is proposed that optimized secondary structures can minimize the state-of-charge heterogeneity by mitigating the buildup of nonuniform internal stresses associated with volume changes during charge.

Forecasting the health of a battery is a modeling effort that is critical to driving improvements in and adoption of electric vehicles. Purely physics-based models and purely data-driven models have advantages and limitations of their own. Considering the nature of battery data and end-user applications, we outline several architectures for integrating physics-based and machine learning models that can improve our ability to forecast battery lifetime. We discuss the ease of implementation, advantages, limitations, and viability of each architecture, given the state of the art in the battery and machine learning fields.

Ion insertion at the interfaces of batteries, fuel cells, and catalysts constitutes an important class of technologically relevant, charge-transfer reactions. However, the molecular nature of charge separation at the adsorbate/solid interface remains elusive. It has been hypothesized that electrostatic dipoles at the adsorbate/solid interface could result from adsorption-induced charge redistribution, preferential segregation of charged point defects in the solid, and/or intrinsic dipoles of adsorbates. Using operando ambient-pressure X-ray photoelectron spectroscopy, we elucidate the coupling between electrostatics and adsorbate chemistry on the surface of CeO2–x , an excellent electrocatalyst and a model system for studying oxygen-ion insertion reactions. Three adsorbate chemistries were studiedOH–/CeO2–x (polar adsorbate), CO3 2–/CeO2–x (nonpolar adsorbate), and Ar/CeO2–x (no adsorbate)under several hundred mTorr of gas pressure relevant to electrochemical H2/CO oxidation and H2O/CO2 reduction. By integrating core-level spectroscopy and contact-potential difference measurements, we simultaneously determine the chemistry and coverage of adsorbates, Ce oxidation state, and the surface potential at the gas/solid interface over a wide range of overpotentials. We directly observe an overpotential-dependent surface potential, which is moreover sensitive to the polarity of the adsorbates. In the case of CeO2–x covered with polar OH–, we observe a surface potential that increases linearly with OH– coverage and with overpotential. On the other hand, for CeO2–x covered with nonpolar CO3 2– and free of adsorbates, the surface potential is independent of overpotential. The adsorbate binding energy does not change systematically with overpotential. From these observations, we conclude that the electrostatic dipole at the adsorbate/CeO2–x interface is dominated by the intrinsic dipoles of the adsorbates, with the solid contributing minimally. These results provide an atomistic picture of the gas/solid double layer and the experimental methodology to directly study and quantify the surface dipole.

The efficacy of the electrical conductivity relaxation (ECR) technique for investigating the oxygen transport properties of mixed conducting oxides has been evaluated. Fifteen mol% samarium doped ceria (SDC15), for which approximate values of the two principal transport properties, bulk oxygen diffusivity, D Chem , and surface reaction rate constant, k S , can be found in the literature, was chosen as the benchmark material against which to validate the methodology. Measurements were carried out at temperatures between 750 • C and 850 • C and over a wide range of oxygen partial pressures. An unexpectedly high p-type electronic transference number enabled ECR measurements under oxidizing conditions. A systematic data analysis procedure was developed to permit reliable extraction of the kinetic parameters even in the general case of simultaneous bulk and surface limitation. The D Chem from this study showed excellent qualitative and quantitative agreement with expected values, falling in the range from ∼ 2 × 10 −5 to 2 × 10 −4 cm 2 /s. The surface reaction constant under H 2 /H 2 O mixtures also showed good agreement with literature results. Remarkably, this value increased by a factor of 40 under mixtures of CO/CO 2 or O 2 /Ar. This observation suggests kinetic advantages for production of CO rather than H 2 in a two-step solar-driven thermochemical process based on samarium doped ceria.

Nonstoichiometric ceria(CeO 2−δ ) is a candidate reaction medium to facilitate two step water splitting cycles and generate hydrogen. Improving upon its thermodynamic suitability through doping requires an understanding of its vacancy thermodynamics. Using density functional theory(DFT) calculations and a cluster expansion based Monte Carlo simulations, we have studied the high temperature thermodynamics of intrinsic oxygen vacancies in ceria. The DFT+U approach was used to get the ground state energies of various vacancy configurations in ceria, which were subsequently fit to a cluster expansion Hamiltonian to efficiently model the configurational dependence of energy. The effect of lattice vibrations was incorporated through a temperature dependent cluster expansion. Lattice Monte Carlo simulations using the cluster expansion Hamiltonian were able to detect the miscibility gap in the phase diagram of ceria. The inclusion of vibrational and electronic entropy effects made the agreement with experiments quantitative. The deviation from an ideal solution model was quantified by calculating as a function of nonstoichiometry, a) the solid state entropy from Monte Carlo simulations and b) Warren-Cowley short range order parameters of various pair clusters.

High-temperature CO2 electrolyzers offer exceptionally efficient storage of renewable electricity in the form of CO and other chemical fuels, but conventional electrodes catalyze destructive carbon deposition. Ceria catalysts are known carbon inhibitors for fuel cell (oxidation) reactions, however for the more severe electrolysis (reduction) conditions, catalyst design strategies remain unclear. Here we establish the inhibition mechanism on ceria and show selective CO2 to CO conversion well beyond the thermodynamic carbon deposition threshold. Operando X-ray photoelectron spectroscopy during CO2 electrolysis -using thin-film model electrodes consisting of samarium-doped ceria, nickel, and/or yttria-stabilized zirconia -together with density functional theory modeling reveal the crucial role of oxidized carbon intermediates in preventing carbon buildup. Using these insights, we demonstrate stable electrochemical CO2 reduction with a scaledup 16 cm 2 ceria-based solid oxide cell under conditions that rapidly destroy a nickel-based cell, leading to substantially improved device lifetime.Main Text: CO2 utilization is expected to play a key role in achieving a carbon-neutral sustainable energy economy. Electrochemical CO2 reduction, in particular, is a promising way to store intermittent electricity derived from solar and wind in the form of chemicals, such as synthetic hydrocarbons compatible with the existing energy infrastructure, and is therefore an essential technology in decarbonization strategies [1][2][3][4] . Currently, the most efficient CO2 electrolysis technology is the elevated-temperature solid oxide electrochemical cell (SOC), which utilizes O 2as the mobile ion. SOCs produce CO and O2 at the thermoneutral voltage of ~1.46 V with current densities exceeding 1 A/cm 2 -similar to steam electrolysis, which can be carried out simultaneously in the same cell to produce syngas or methane 1,2,5,6 . The same SOC can be operated in reverse as a fuel cell to re-oxidize the fuel products, thereby enabling operation as a flow battery 6,7 . Another important application is O2 (and CO) production from the CO2-rich atmosphere of Mars for rocket propulsion and life support, which will be demonstrated on the NASA Mars 2020 rover 8 .

Elastic strain is being increasingly employed to enhance the catalytic properties of mixed ion–electron conducting oxides. However, its effect on oxygen storage capacity is not well established. Here, we fabricate ultrathin, coherently strained films of CeO2-δ between 5.6% biaxial compression and 2.1% tension. In situ ambient pressure X-ray photoelectron spectroscopy reveals up to a fourfold enhancement in equilibrium oxygen storage capacity under both compression and tension. This non-monotonic variation with strain departs from the conventional wisdom based on a chemical expansion dominated behaviour. Through depth profiling, film thickness variations and a coupled photoemission–thermodynamic analysis of space-charge effects, we show that the enhanced reducibility is not dominated by interfacial effects. On the basis of ab initio calculations of oxygen vacancy formation incorporating defect interactions and vibrational contributions, we suggest that the non-monotonicity arises from the tetragonal distortion under large biaxial strain. These results may guide the rational engineering of multilayer and core–shell oxide nanomaterials.