Experimental Requirements for High-Temperature Solid-State Electrochemical TEM Experiments.

The ability to perform both electrochemical and structural/elemental characterization in the same experiment and at the nanoscale allows to directly link electrochemical performance to the material properties and their evolution over time and operating conditions. Such experiments can be important for the further development of solid oxide cells, solid-state batteries, thermal electrical devices, and other solid-state electrochemical devices. The experimental requirements for conducting solid-state electrochemical TEM experiments in general, including sample preparation, electrochemical measurements, failure factors, and possibilities for optimization, are presented and discussed. Particularly, the methodology of performing reliable electrochemical impedance spectroscopy measurements in reactive gases and at elevated temperatures for both single materials and solid oxide cells is described. The presented results include impedance measurements of electronic conductors, an ionic conductor, and a mixed ionic and electronic conductor, all materials typically applied in solid oxide fuel and electrolysis cells. It is shown that how TEM and impedance spectroscopy can be synergically integrated to measure the transport and surface exchange properties of materials with nanoscale dimensions and to visualize their structural and elemental evolution via TEM/STEM imaging and spectroscopy.

Ion-beam deposited platinum as electrical contacting material in operando electron microscopy experiments at elevated temperatures.

Establishing a stable and well conducting contacting material is critical for operando electron microscopy experiments of electrical and electrochemical devices at elevated temperatures. In this contribution, the nanostructure and electrical conductivity of ion beam deposited Pt are investigated both in vacuum and in oxygen as a function of temperature. Its microstructure is relatively stable up to a temperature of approx. 800°C and up to an applied current density of approx. 100 kA/cm2 . Its conductivity increases with temperature, attributed to densification, with changes in the hydrocarbon matrix being less important. Recommendations are provided with respect to the Pt deposition parameters in terms of maximizing stability and minimizing electrical resistance. RESEARCH HIGHLIGHTS: It is feasible to use ion beam deposited Pt as electrical contacting material in operando electron microscopy. The deposited Pt is relatively stable up to 800°C and approx. 100 kA/cm2 . The resistivity can be reduced by increasing the applied ion current during deposition and by thermal annealing at a temperature of 500°C in a few mbar of oxygen.

Electrochemical Impedance Spectroscopy Integrated with Environmental Transmission Electron Microscopy.

The concept of combining electrical impedance spectroscopy (EIS) with environmental transmission electron microscopy (ETEM) is demonstrated by testing a specially designed micro gadolinia-doped ceria (CGO) sample in reactive gasses (O2 and H2 /H2 O), at elevated temperatures (room temperature-800 °C) and with applied electrical potentials. The EIS-TEM method provides structural and compositional information with direct correlation to the electrochemical performance. It is demonstrated that reliable EIS measurements can be achieved in the TEM for a sample with nanoscale dimensions. Specifically, the ionic and electronic conductivity, the surface exchange resistivity, and the volume-specific chemical capacitance are in good agreement with results from more standardized electrochemical tests on macroscopic samples. CGO is chosen as a test material due to its relevance for solid oxide electrochemical reactions where its electrochemical performance depends on temperature and gas environment. As expected, the results show increased conductivity and lower surface exchange resistance in H2 /H2 O gas mixtures where the oxygen partial pressure is low compared to experiments in pure O2 . The developed EIS-TEM platform is an important tool in promoting the understanding of nanoscale processes for green energy technologies, e.g., solid oxide electrolysis/fuel cells, batteries, thermoelectric devices, etc.

Unravelling the role of dopants in the electrocatalytic activity of ceria towards CO2 reduction in solid oxide electrolysis cells.

CO2 reduction in Solid Oxide Electrolysis Cells (SOECs) is a key-technology for the transition to a sustainable energy infrastructure and chemical industry. Ceria (CeO2) holds great promise in developing highly efficient, cost-effective and durable fuel electrodes, due to its promising electrocatalytic properties, and proven ability to suppress carbon deposition and to tolerate high concentrations of impurities. In the present work, we investigate the intrinsic electrocatalytic activity of ceria towards CO2 reduction by means of electrochemical impedance spectroscopy (EIS) on model systems with well-defined geometry, composition and surface area. Aiming at the optimization of the intrinsic catalytic properties of the material, we systematically study the effect of different dopants (Zr, Gd, Pr and Bi) on the reaction rate under varying operating conditions (temperature, gas composition and applied polarization) relevant for SOECs. The electrochemical measurements reveal the dominant role of the surface defect chemistry of the material in the reaction rate, with doping having only a mild effect on the rate and activation energy of the reaction. By analyzing the pO2 and overpotential dependence of the reaction rate with a general micro-kinetic model, we are able to identify the second electron transfer as the rate limiting step of the process, highlighting the dominant role of surface polarons in the energy landscape. These insights on the correlation between the surface defects and the electrocatalytic activity of ceria open new directions for the development of highly performing ceria-based technological electrodes.

Kinetics of CO/CO2 and H2/H2O reactions at Ni-based and ceria-based solid-oxide-cell electrodes.

The solid oxide electrochemical cell (SOC) is an energy conversion technology that can be operated reversibly, to efficiently convert chemical fuels to electricity (fuel cell mode) as well as to store electricity as chemical fuels (electrolysis mode). The SOC fuel-electrode carries out the electrochemical reactions CO2 + 2e(-) ↔ CO + O(2-) and H2O + 2e(-) ↔ H2 + O(2-), for which the electrocatalytic activities of different electrodes differ considerably. The relative activities in CO/CO2 and H2/H2O and the nature of the differences are not well studied, even for the most common fuel-electrode material, a composite of nickel and yttria/scandia stabilized zirconia (Ni-SZ). Ni-SZ is known to be more active for H2/H2O than for CO/CO2 reactions, but the reported relative activity varies widely. Here we compare AC impedance and DC current-overpotential data measured in the two gas environments for several different electrodes comprised of Ni-SZ, Gd-doped CeO2 (CGO), and CGO nanoparticles coating Nb-doped SrTiO3 backbones (CGOn/STN). 2D model and 3D porous electrode geometries are employed to investigate the influence of microstructure, gas diffusion and impurities.Comparing model and porous Ni-SZ electrodes, the ratio of electrode polarization resistance in CO/CO2vs. H2/H2O decreases from 33 to 2. Experiments and modelling suggest that the ratio decreases due to a lower concentration of impurities blocking the three phase boundary and due to the nature of the reaction zone extension into the porous electrode thickness. Besides showing higher activity for H2/H2O reactions than CO/CO2 reactions, the Ni/SZ interface is more active for oxidation than reduction. On the other hand, we find the opposite behaviour in both cases for CGOn/STN model electrodes, reporting for the first time a higher electrocatalytic activity of CGO nanoparticles for CO/CO2 than for H2/H2O reactions in the absence of gas diffusion limitations. We propose that enhanced surface reduction at the CGOn/gas two phase boundary in CO/CO2 and in cathodic polarization can explain why the highest reaction rate is obtained for CO2 electrolysis. Large differences observed between model electrode kinetics and porous electrode kinetics are discussed.

TOF-SIMS characterization of impurity enrichment and redistribution in solid oxide electrolysis cells during operation.

TOF-SIMS analyses of state-of-the-art high temperature solid oxide electrolysis cells before and after testing under different operating conditions were performed. The investigated cells consist of an yttria stabilized zirconia (YSZ) electrolyte, a La1-xSrxMnO3-δ composite anode and a Ni-YSZ cermet cathode. The surfaces and cross-sections of the cells were analyzed, and several elemental impurities like Si, Ca and Na were identified and spatially mapped and their enrichment and migration during operation is reported. With advancing operation time, the concentration of these elements, especially Na and Ca, increases. For Si, a concentration gradient is found from the gas inlet to the gas outlet. Additionally, a loss of Ni percolation in the active cathode is observed in the same area where the Si enrichment is found. Based on the obtained TOF-SIMS results, the influence of the operating conditions on degradation is discussed.

Defining chemical expansion: the choice of units for the stoichiometric expansion coefficient.

Chemical expansion refers to the spatial dilation of a material that occurs upon changes in its composition. When this dilation is caused by a gradual, iso-structural increase in the lattice parameter with composition, it is related to the composition change by the stoichiometric expansion coefficient. In this work, three different approaches to defining the stoichiometric expansion coefficient (αS) are discussed. While all three definitions of αS given here are legitimate, we show that there are advantages to selecting certain ones for comparison across different crystal structures. Examples are provided for changes in oxygen content in fluorite, perovskite, and Ruddlesden-Popper (K2NiF4) phase materials used in solid oxide fuel cells.

Experimental determination of the Onsager coefficients of transport for Ce(0.8)Pr(0.2)O(2-delta).

For a mixed oxide-ion and electron conducting oxide, with oxygen vacancies (V(O)) and electrons (e') or holes (h ) as charge carriers, a flux of (V(O)) (J(i)) can in principle be driven, not only directly by its own electrochemical potential gradient (inverted Delta eta(i)), but also indirectly by that of electrons (inverted Delta eta(e)), and vice versa for the flux of electrons (J(e)). It is common practice to assume that electrons and mobile ions migrate independently, despite the lack of experimental evidence in support of this. Here, all the Onsager coefficients, including the cross coefficients, have been measured for Ce(0.8)Pr(0.2)O(2-delta) within the a(O(2)) range 10(-21)-1 at 800 degrees C, using local ionic and electronic probes in a four-probe configuration. The cross coefficients of transport were found to be negligible in comparison to the direct coefficients in the a(O(2)) range 10(-21)-10(-4), but of the same order of magnitude as the direct coefficients for high a(O(2)) values (10(-2)-1). This is in contrast to the commonly used assumption that the two types of carriers migrate independently, i.e. that L(ie) = 0.