Investigation of Cathode Kinetics in SOFC: Model Thin Film SrTi_(1-x)Fe_xO_(3-δ) Mixed Conducting Oxides

To understand the kinetics controlling the SOFC cathode processes, a model mixed conducting perovskite materials system, SrTi_(1-x)Fe_xO_(3-δ), was selected, offering the ability to systematically control both the levels of electronic and ionic electrical conductivity as well as the energy band structure. This, in combination with considerably simplified electrode geometry, served to demonstrate that the rate of oxygen exchange at the surface of SrTi_(1-x)Fe_xO_(3-δ) is only weakly correlated with either high electronic or ionic conductivity, in apparent contradiction with common expectations. On the other hand, evidence was found suggesting the importance of minority electronic species in determining the rate of oxygen exchange. Furthermore, the enrichment of Sr to the surface of the electrodes was found to reduce the oxygen exchange rate constant; this effect becoming more evident with increasing values of x. The observed trends are discussed in relation to the cathodic behavior of MIEC electrodes

Solar to fuels conversion technologies: a perspective

To meet increasing energy needs, while limiting greenhouse gas emissions over the coming decades, power capacity on a large scale will need to be provided from renewable sources, with solar expected to play a central role. While the focus to date has been on electricity generation via photovoltaic (PV) cells, electricity production currently accounts for only about one-third of total primary energy consumption. As a consequence, solar-to-fuel conversion will need to play an increasingly important role and, thereby, satisfy the need to replace high energy density fossil fuels with cleaner alternatives that remain easy to transport and store. The solar refinery concept (Herron et al. in Energy Environ Sci 8:126–157, 2015), in which captured solar radiation provides energy in the form of heat, electricity or photons, used to convert the basic chemical feedstocks CO[subscript 2] and H[subscript 2]O into fuels, is reviewed as are the key conversion processes based on (1) combined PV and electrolysis, (2) photoelectrochemically driven electrolysis and (3) thermochemical processes, all focused on initially converting H[subscript 2]O and CO[subscript 2] to H[subscript 2] and CO. Recent advances, as well as remaining challenges, associated with solar-to-fuel conversion are discussed, as is the need for an intensive research and development effort to bring such processes to scale.United States. Dept. of Energy. Office of Basic Energy Sciences (Award# DE SC0002633)National Science Foundation (U.S.) (Award# DMR-1507047)MIT Skoltech Initiativ

Engineering electrochemical nanoscale oxides

Oxides are playing an increasingly critical role as functional components in the fields of energy conversion/storage, microelectronics, sensors/actuators and catalysis. In turn, their electrical (ionic & electronic), optical and catalytic properties depend sensitively on their defect structure and oxygen nonstoichiometry, typically frozen in during processing, and rarely well defined. This is particularly true for thin films and nanoparticles/wires, where conventional methods, appropriate to bulk materials, do not apply. In this presentation, we review in-situ optical, electrochemical and dilatometric methods, developed or refined in our laboratory, to monitor, analyze and control nonstoichiometry, defect equilibria, transport and optical properties of oxide thin films and nano-sized particles. Examples include materials of interest as electrodes in fuel cells, and as components of sensors, catalysts and memory devices

Photo-Activated Low Temperature, Micro Fuel Cell Power Source

A Key objective of this program is to identify electrodes that will make it possible to significantly reduce the operating temperature of micro-SOFC and thin film-based SOFCs. Towards this end, efforts are directed towards: (a) identifying the key rate limiting steps which limit presently utilized electrodes from performing at reduced temperatures, as well as, (b) investigating the use of optical, as opposed to thermal energy, as a means for photocatalyzing electrode reactions and enabling reduced operating temperatures. During Phase I, the following objectives were achieved: (a) assembly and testing of our unique Microprobe Thin Film Characterization System; (b) fabrication of the model cathode materials system in thin film form by both PLD and ink jet printing; and (c) the successful configuration and testing of the model materials as cathodes in electrochemical cells. A further key objective (d) to test the potential of illumination in enhancing electrode performance was also achieved

Study of orientation effect on nanoscale polarization in BaTiO3 thin films using piezoresponse force microscopy

We have investigated the effect of texture on in-plane (IPP) and out- of plane (OPP) polarizations of pulsed-laser-deposited BaTiO3 thin films grown on Pt and La0.5Sr0.5CoO3 (LSCO) buffered Pt electrodes. The OPP and IPP polarizations were observed by piezoresponse force microscopy (PFM) for three-dimensional polarization analyses in conjunction with conventional diffraction methods using x-ray diffraction and reflection high energy electron diffraction measurements. BaTiO3 films grown on Pt electrodes exhibited highly (101) preferred orientation with higher IPP component whereas BaTiO3 film grown on LSCO/Pt electrodes showed (001) and (101) orientations with higher OPP component. Measured effective d(33) values of BaTiO3 films deposited on Pt and LSCO/ Pt electrodes were 14.3 and 54.0 pm/ V, respectively. Local piezoelectric strain loops obtained by OPP and IPP-PFM showed that piezoelectric properties were strongly related to film orientation

Photoconductivity analyzed in the frequency domain - an introductory case study of strontium titanate

Strontium titanate (STO, SrTiO3) has been used for many applications in solid state electrochemistry and is considered a standard and model material. Its characteristics, and those of its derivatives such as STF (SrTi0.65Fe0.35O3-x), have been characterized by many groups on various aspects, such as electronic/ionic conductivity, oxygen exchange kinetics and the impact of doping. Recently, the interaction of light with STO/STF has been of increased interest. A persistent photoconductivity has been observed [1] and enhanced oxygen exchange kinetics have been detected, opening up new fields of application, such as a light-driven fuel cell [2]. The reasons behind these effects remain subject to discussion or even speculation as the relation to the relatively large bandgap and the photoresponse at long wavelengths remains unclear. What makes the analysis of these effects difficult is the interplay of many electrochemical and photoelectrochemical processes that contribute to the photoresponse including the electronic and ionic conductivity, the number and nature of charge carriers, charge traps, phonon related effects, and surface reactions. With electrochemical impedance spectroscopy (EIS), one can distinguish diverse processes on the basis of their time constants and how they evolve as a function of operating conditions, such as temperature, atmosphere (leading to stoichiometry changes) and illumination. However, the impact of light can only be characterized implicitly as a change in other processes that also prevail in the dark. Intensity modulated photocurrent/-voltage spectroscopy (IMPS/IMVS) have been shown to reveal valuable information about charge carrier dynamics for photoelectrodes and photovoltaic cells [3]. To the best of our knowledge, these techniques have never been applied to devices or materials that are not photoactive, or in other words, that do not show a photovoltage, such as a symmetrical model cells based on STO or STF. However, with the small signal light perturbation that is the key element of IMPS and IMVS, we can trigger the light effect directly and analyze the system response by its current and voltage signals. In this contribution, we will begin with a brief introduction into IMPS and IMVS and show how these techniques can be applied to model electrodes consisting of STO and STF. The results are compared to EIS under different illumination and we will show how to extract the relevant information about the photoresponse. By evaluating the activation energies of the different electrochemical and photoelectrochemical processes, we can attribute those to physical effects and clarify some of the previously unknown processes that lead to anomalies observed in STO/STF under illumination. The capacity of IMPS and IMVS have been underestimated so far and in this contribution, we will conclude with an outlook for their potential to other fields of application, such as ionic motion in perovskite solar cells that are thought to be responsible for their accelerated degradation under illumination. This work was supported by JSPS Core-to-Core Program, A. Advanced Research Networks: “Solid Oxide Interfaces for Faster Ion Transport”. References [1] M. C. Tarun et al., Phys. Rev. Lett. 111, 187403, 2013. [2] G. C. Bunauer, Adv. Funct. Mater. 26, 120, 2016. [3] D. Klotz et al., Phys. Chem. Chem. Phys. 18, 23438, 2016

Investigation of surface Sr segregation in model thin film solid oxide fuel cell perovskite electrodes

While SOFC perovskite oxide cathodes have been the subject of numerous studies, the critical factors governing their kinetic behavior have remained poorly understood. This has been due to a number of factors including the morphological complexity of the electrode and the electrode- electrolyte interface as well as the evolution of the surface chemistry with varying operating conditions. In this work, the surface chemical composition of dense thin film SrTi_(1−x)Fe_xO_(3-δ) electrodes, with considerably simplified and well-defined electrode geometry, was investigated by means of XPS, focusing on surface cation segregation. An appreciable degree of Sr-excess was found at the surface of STF specimens over the wide composition range studied. The detailed nature of the Sr-excess is discussed by means of depth and take-off angle dependent XPS spectra, in combination with chemical and thermal treatments. Furthermore, the degree of surface segregation was successfully controlled by etching the films, and/or preparing intentionally Sr deficient films. Electrochemical Impedance Spectroscopy studies, under circumstances where surface chemistry was controlled, were used to examine and characterize the blocking effect of Sr segregation on the surface oxygen exchange rate

Thermal conductivity control by oxygen defect concentration modification in reducible oxides: The case of Pr0.1Ce0.9O2−δ thin films

We demonstrate the impact on thermal conductivity of varying the concentration of oxygen vacancies and reduced cations in Pr[subscript 0.1]Ce[subscript 0.9]O[subscript 2−δ] thin films prepared by pulsed laser deposition. The oxygen vacancy concentration is controlled by varying the oxygen partial pressure between 1 × 10[superscript −4] and 1 atm at 650  °C. Corresponding changes in the oxygen non-stoichiometry (δ) are monitored by detecting the lattice parameters of the films with high-resolution X-ray diffraction, while the thermal properties are characterized by time-domain thermoreflectance measurements. The films are shown to exhibit a variation in oxygen vacancy content, and in the Pr[superscript 3+]/Pr[superscript 4+] ratio, corresponding to changes in δ from 0.0027 to 0.0364, leading to a reduction in the thermal conductivity from k = 6.62 ± 0.61 to 3.82 ± 0.51 W/m-K, respectively. These values agree well with those predicted by the Callaway and von Baeyer model for thermal conductivity in the presence of point imperfections. These results demonstrate the capability of controlling thermal conductivity via control of anion and cation defect concentrations in a given reducible oxide.National Science Foundation (U.S.). Materials Research Science and Engineering Centers (MRSEC Program, Award No. DMR-0819762

Tailoring non-stoichiometry and mixed ionic-electronic conductivity in nanostructured Pr-substituted ceria

High concentrations of mobile oxygen vacancies are crucial for devices such as SOFCs, SOECs, gas permeation membranes, and sensors, while for other applications such as ferroelectrics and piezoelectrics, oxygen vacancies are detrimental. Hence there is great interest in tailoring the oxygen vacancy concentration and mobility for given materials. Changes in oxygen non-stoichiometry also result in dilation of the crystal lattice, known as chemical expansion, and therefore there is a coupling between the electrical, chemical, and mechanical properties known as electro-chemo-mechanical coupling. Confined systems, such as thin films, are being investigated as a way to tailor the non-stoichiometry and transport properties of materials, shifting the paradigm away from searching for new materials or compositions. Please click Additional Files below to see the full abstract