Surface Decorations on Mixed Ionic and Electronic Conductors: Effects on Surface Potential, Defects, and the Oxygen Exchange Kinetics

The oxygen exchange kinetics of epitaxial Pr_0.1Ce_0.9O_2-δ electrodes was modified by decoration with submonolayer amounts of different basic (SrO, CaO) and acidic (SnO₂, TiO₂) binary oxides. The oxygen exchange reaction (OER) rate and the total conductivity were measured by in situ PLD impedance spectroscopy (i-PLD), which allows to directly track changes of electrochemical properties after each deposited pulse of surface decoration. The surface chemistry of the electrodes was investigated by near-ambient pressure XPS measurements (NAP-XPS) at elevated temperatures and by low-energy ion scattering (LEIS). While a significant alteration of the OER rate was observed after decoration with binary oxides, the pO₂ dependence of the surface exchange resistance and its activation energy were not affected, suggesting that surface decorations do not alter the fundamental OER mechanism. Furthermore, the total conductivity of the thin films does not change upon decoration, indicating that defect concentration changes are limited to the surface layer. This is confirmed by NAP-XPS measurements which find only minor changes of the Pr-oxidation state upon decoration. NAP-XPS was further employed to investigate changes of the surface potential step on decorated surfaces. From a mechanistic point of view, our results indicate a correlation between the surface potential and the altered oxygen exchange activity. Oxidic decorations induce a surface charge which depends on their acidity (acidic oxides lead to a negative surface charge), affecting surface defect concentrations, any existing surface potential step, potentially adsorption dynamics, and consequently also the OER kinetics.

The analysis of charge transport mechanism in mixed ionic electronic conductor composite of Sr2TiCoO6double perovskite with yttria stabilized zirconia

In this investigation, the ionic conduction mechanism in mixed ionic electronic conductors composites of Sr₂TiCoO₆/YSZ has been studied with the help of universal dynamic response. 3 mol% and 8 mol% yttria stabilized ZrO₂have been mixed with Sr₂TiCoO₆(STC) double perovskite in 1:1 ratio to prepare STC/3YSZ and STC/8YSZ composites via solid-state reaction route. AC Impedance spectroscopy has been carried out to examine the charge transport mechanism, which has been modeled using the microstructural networks of resistors and capacitors. Grain boundaries are more resistive and capacitive compared to the bulk. Modulus spectroscopy analysis demonstrates the non-Debye character of conductivity relaxation with frequency. Complex frequency-dependent AC conductivity is found to obey Almond West power law and reveals that ion migration occurs through the correlated hopping mechanism. Further, the DC conductivity and relaxation time have been found to follow the Barton Nakajima and Namikawa relation, which is correlated with AC to DC conduction. The time-temperature superposition principle has been used to explain the conductivity scaling in the intermediate frequency range. At low temperatures, the ions are localized in the asymmetric potential well, while at high temperatures, hopping behavior starts dominating. Further Kramers-Kronig transformation connects the dielectric strength with conductivity relaxation and verifies the impedance data.

Mixed Ionic-Electronic Conductors Based on PEDOT:PolyDADMA and Organic Ionic Plastic Crystals

Mixed ionic-electronic conductors, such as poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) are postulated to be the next generation materials in energy storage and electronic devices. Although many studies have aimed to enhance the electronic conductivity and mechanical properties of these materials, there has been little focus on ionic conductivity. In this work, blends based on PEDOT stabilized by the polyelectrolyte poly(diallyldimethylammonium) (PolyDADMA X) are reported, where the X anion is either chloride (Cl), bis(fluorosulfonyl)imide (FSI), bis(trifluoromethylsulfonyl)imide (TFSI), triflate (CF3SO3) or tosylate (Tos). Electronic conductivity values of 0.6 S cm-1 were achieved in films of PEDOT:PolyDADMA FSI (without any post-treatment), with an ionic conductivity of 5 × 10-6 S cm-1 at 70 °C. Organic ionic plastic crystals (OIPCs) based on the cation N-ethyl-N-methylpyrrolidinium (C2mpyr+) with similar anions were added to synergistically enhance both electronic and ionic conductivities. PEDOT:PolyDADMA X / [C2mpyr][X] composites (80/20 wt%) resulted in higher ionic conductivity values (e.g., 2 × 10-5 S cm-1 at 70 °C for PEDOT:PolyDADMA FSI/[C2mpyr][FSI]) and improved electrochemical performance versus the neat PEDOT:PolyDADMA X with no OIPC. Herein, new materials are presented and discussed including new PEDOT:PolyDADMA and organic ionic plastic crystal blends highlighting their promising properties for energy storage applications.

Tailoring Nonstoichiometry and Mixed Ionic Electronic Conductivity in Pr0.1Ce0.9O2-δ/SrTiO3 Heterostructures

The oxygen deficiency or excess, as reflected in the nonstoichiometry of oxide films, plays a crucial role in their functional properties for applications such as micro solid oxide fuel cells, catalysis, sensors, ferroelectrics, and memristors. High concentrations of oxygen vacancies may be beneficial or detrimental according to the application, and hence there is interest in controlling the oxygen content of films without resorting to compositional changes. Here, we demonstrate that substantial changes in the nonstoichiometry of Pr_0.1Ce_0.9O_2-δ (PCO), a model mixed ionic electronic conductor, can be achieved by fabricating multilayers with an inert material, SrTiO₃ (STO). We fabricated heterostructures using pulsed laser deposition, keeping the total thickness of PCO and STO constant while varying the number of layers and thickness of each individual layer, to probe the effects of the PCO/STO interfaces. Conductivity measurements as a function of oxygen partial pressure (P_O2) and temperature showed a significant weakening of the P_O2 dependence compared to bulk PCO, which scaled with the density of interfaces. We confirmed that this change was due to variations in nonstoichiometry, by optical transmission measurements, and show that the lower oxygen content is consistent with a decrease in the effective oxygen reduction enthalpy of PCO. These results exemplify the dramatic differences in properties between films and their bulk counterparts, achievable by interface engineering, and provide generalized insight into tailoring the properties of mixed ionic electronic conductors at the nanoscale.

Praseodymium Cuprate Thin Film Cathodes for Intermediate Temperature Solid Oxide Fuel Cells: Roles of Doping, Orientation, and Crystal Structure

Highly textured thin films of undoped, Ce-doped, and Sr-doped Pr₂CuO₄ were synthesized on single crystal YSZ substrates using pulsed laser deposition to investigate their area-specific resistance (ASR) as cathodes in solid-oxide fuel cells (SOFCs). The effects of T' and T* crystal structures, donor and acceptor doping, and a-axis and c-axis orientation on ASR were systematically studied using electrochemical impedance spectroscopy on half cells. The addition of both Ce and Sr dopants resulted in improvements in ASR in c-axis oriented films, as did the T* crystal structure with the a-axis orientation. Pr_1.6Sr_0.4CuO₄ is identified as a potential cathode material with nearly an order of magnitude faster oxygen reduction reaction kinetics at 600 °C compared to thin films of the commonly studied cathode material La_0.6Sr_0.4Co_0.8Fe_0.2O_3-δ. Orientation control of the cuprate films on YSZ was achieved using seed layers, and the anisotropy in the ASR was found to be less than an order of magnitude. The rare-earth doped cuprate was found to be a versatile system for study of relationships between bulk properties and the oxygen reduction reaction, critical for improving SOFC performance.