Phase transformations driven by compositional change require mass flux across a phase boundary. In some anisotropic solids, however, the phase boundary moves along a non-conductive crystallographic direction. One such material is LiFePO, an electrode for lithium-ion batteries. With poor bulk ionic transport along the direction of phase separation, it is unclear how lithium migrates during phase transformations. Here, we show that lithium migrates along the solid/liquid interface without leaving the particle, whereby charge carriers do not cross the double layer. X-ray diffraction and microscopy experiments as well as ab initio molecular dynamics simulations show that organic solvent and water molecules promote this surface ion diffusion, effectively rendering LiFePO a three-dimensional lithium-ion conductor. Phase-field simulations capture the effects of surface diffusion on phase transformation. Lowering surface diffusivity is crucial towards supressing phase separation. This work establishes fluid-enhanced surface diffusion as a key dial for tuning phase transformation in anisotropic solids.

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.

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.

In interfacial charge-transfer reactions, the complexity of the reaction pathway increases with the number of charges transferred, and becomes even greater when the reaction involves both electrons (charge) and ions (mass). These so-called mixed ion electron transfer (MIET) reactions are crucial in intercalation/insertion electrochemistry, such as those occurring in oxygen reduction/evolution electrocatalysts and lithium-ion battery electrodes. Understanding MIET reaction pathways, particularly identifying the rate-determining step (RDS), is crucial for engineering interfaces at the molecular, electronic, and point defect levels. Here we develop a generalizable experimental and analysis framework for constructing the O 2 (g) incorporation reaction pathway in Pr 0.1 Ce 0.9 O 2-x. We converge on four candidate RDS (dissociation of neutral 31 oxygen adsorbate) out of more than 100 possibilities by measuring the current density-32 overpotential curves while controlling both oxygen activity in the solid and the oxygen gas 33 partial pressure, and quantifying the chemical and electrostatic driving forces using operando 34 ambient pressure X-ray photoelectron spectroscopy. 35 36 3 Mixed ion and electron transfer (MIET) reactions involve the transfer of both ionic and electronic charges across interfaces. They are substantially more complex than electron transfer and proton-coupled electron transfer reactions because the ionic charge also crosses the electrochemical double layer. 1 The net reactions are usually chemical in nature (i.e., no net charge transfer). Examples include H + intercalation in layered hydroxides and Li + insertion in 41 metal oxides (Fig. 1a,b). 2 Another ubiquitous example is the oxygen incorporation reaction (OIR) 42 occurring at the solid/gas interface (Fig. 1c). It is rate-determining for many energy-and 43 environment-related technologies, including oxygen storage materials for emission control, 3 44 solid oxide fuel cells (SOFCs), 4 electrolysis cells, 5 thermochemical water splitting cycles, 6 and oxygen permeation membranes. 7 The OIR is expressed as − − + → 2 2 O 4e 2O. (1) 47 Understanding the OIR reaction pathway is crucial for engineering and discovering catalysts, 48 typically oxides, with high activity and stability. 8,9 Mixed ionic-electronic conductors (MIECs) 49 have received widespread interests because they expand the effective OIR site to the gas/solid 50 double-phase boundary beyond the traditional triple phase boundary between gas, ionic and 51 electronic conductors. 10,11 There, oxygen ions and electrons react with oxygen adsorbates at the same active site, resulting in a reaction that involves the transfer of two oxygen ions and four electrons. The number of charges transferred during OIR has made it challenging to isolate the ratedetermining step (RDS). Most experimental work has focused on measuring the exchange coefficients 12 using tracer diffusion, 13 conductivity and mass relaxation, 14 and impedance spectroscopy, 15 as well as their reaction order with respect to oxyge...