Solid-state protonic conductors: principles, properties, progress and prospects

This critical review presents an overview of the various classes of oxide materials exhibiting fast oxide-ion or proton conductivity for use as solid electrolytes in clean energy applications such as solid oxide fuel cells. Emphasis is placed on the relationship between structural and mechanistic features of the crystalline materials and their ion conduction properties. After describing well-established classes such as fluorite- and perovskite-based oxides, new materials and structure-types are presented. These include a variety of molybdate, gallate, apatite silicate/germanate and niobate systems, many of which contain flexible structural networks, and exhibit different defect properties and transport mechanisms to the conventional materials. It is concluded that the rich chemistry of these important systems provides diverse possibilities for developing superior ionic conductors for use as solid electrolytes in fuel cells and related applications. In most cases, a greater atomic-level understanding of the structures, defects and conduction mechanisms is achieved through a combination of experimental and computational techniques (217 references).

Oxide-ion and proton conducting electrolyte materials for clean energy applications: structural and mechanistic features

Combustion control and sensors: a review

Global ammonia production reached 175 million metric tons in 2016, 90% of which is produced from high purity N2 and H2 gases at high temperatures and pressures via the Haber-Bosch process. Reliance on natural gas for H2 production results in large energy consumption and CO2 emissions. Concerns of human-induced climate change are spurring an international scientific effort to explore new approaches to ammonia production and reduce its carbon footprint. Electrocatalytic N2 reduction to ammonia is an attractive alternative that can potentially enable ammonia synthesis under milder conditions in small-scale, distributed, and on-site electrolysis cells powered by renewable electricity generated from solar or wind sources. This review provides a comprehensive account of theoretical and experimental studies on electrochemical nitrogen fixation with a focus on the low selectivity for reduction of N2 to ammonia versus protons to H2. A detailed introduction to ammonia detection methods and the execution of control experiments is given as they are crucial to the accurate reporting of experimental findings. The main part of this review focuses on theoretical and experimental progress that has been achieved under a range of conditions. Finally, comments on current challenges and potential opportunities in this field are provided.

Recent Advances and Challenges of Electrocatalytic N2 Reduction to Ammonia.

Recent progress towards the electrosynthesis of ammonia from sustainable resources

The causes of the decrease in electrical conductivity with aging in Y{sub 2}O{sub 3}-stabilized zirconia, an oxygen-ion conductor, were studied. This study was carried out using the dc four-probe technique for measuring electrical conductivity and the activation energy for the migration of oxygen ions. The results show that conductivity decreased with aging below certain temperatures in all specimens. Moreover, it was found that conductivity decreases significantly as the temperature decreases. Samples that were aged at relatively low temperatures exhibited a decrease in conductivity and an increase in activation energy. It was concluded that short range ordering of oxygen ion vacancies toward the zirconium to relax the anisotropy of the lattice distortion is the cause of the decrease in electrical conductivity and the increase in activation energy. When aging was carried out at a relatively high temperature, fully stabilized zirconia showed no change in activation energy and only a slight increase in conductivity. This is because oxygen ion vacancies are in the disordered state and the cubic phase is the only phase at this temperature. Short range ordering of oxygen ion vacancies takes such a long time presumably because these oxygen ion vacancies are still able to move even after aging.more » This was explained using the concept of mean first passage time.« less

Effect of Aging on Yttria‐Stabilized Zirconia I. A Study of Its Electrochemical Properties

Stress on a reaction vessel by the swelling of a hydrogen absorbing alloy

Measurements of the thermoelectric power were applied to investigate the electrical conductivity in 5 mol % Sr-substituted LaPO 4 as a function of temperature (600 to 900°C), water vapor partial pressure (0.2 to 6 kPa), and oxygen partial pressure (4 to 100 kPa). Expressions of the thermoelectric power for materials conducting protons, oxide ions, and electronic defects were derived, based on a thermodynamic treatment of entropy production by heat and charge transfer. The experimental data for 5 mol % Sr-substituted LaPO 4 were interpreted in terms of protonic conduction, and some additional conduction, possibly by electron holes. In air, the proton transport number is unity at 600°C when p H2 O > 1 kPa, and at 700°C when p H2 O > 3 kPa. In wet atmospheres, the transported entropy of protons in this material was found to be nearly constant, 112 ± 2 J mol -1 K -1 at 600 to 800°C in wet atmospheres.

Protonic and Native Conduction in Sr‐Substituted LaPO4 Studied by Thermoelectric Power Measurements

Changes in local structures with aging in yttria-stabilized zirconia were examined by extended x-ray absorption fine structure (EXAFS) and internal friction measurement. An analysis of EXAFS shows that a decrease in the first nearest neighbor coordination number of Zr ions, which means short range ordering of oxygen ion vacancies, a decrease in distance between a Zr ion and its first nearest atoms, and an increase in distance between a Zr ion and its second nearest atoms occurred in aged 8YSZ. Aging caused a noticeable decrease in the relaxation peak of internal friction in 8YSZ. These phenomena are attributed to the short range ordering of oxygen ion vacancies around a Zr ion that results from relaxation in the anisotropy of periodical lattice distortion. Therefore, it is concluded that the trapping of oxygen ion vacancies at Zr ions in order to relax such distortion is responsible for a decrease in conductivity with aging. The relaxation peak of internal friction before aging changed with increasing dopant concentrations, and the behavior of this change from 8YSZ to 10YSZ was very similar to that when 8YSZ was aged, and that from 10YSZ to 15YSZ was similar to that when a single crystal of 10YSZ was aged. This marked similarity indicates that the changes in crystal structure which cause the decrease in conductivity with increasing dopant concentration are caused by the short range ordering of oxygen ion vacancies brought about by relaxation of the distortion, similar to those when conductivity decreases with aging.

Effect of Aging on Yttria‐Stabilized Zirconia III. A Study of the Effect of Local Structures on Conductivity

Microstructural changes in yttria‐stabilized zirconia with aging were investigated by X‐ray diffraction, high resolution transmission electron microscopy (TEM), energy dispersive X‐ray spectroscopy under TEM, and electron diffraction. First, various microstructural changes reported in the literature as the causes of the decrease in conductivity were carefully examined. As a result, almost all of them proved not to be the direct cause. Therefore, the changes with aging were analyzed mainly by calling attention to a characteristically broad area that appears on the left shoulder of the X‐ray diffraction peaks as well as the superlattice reflections and diffuse scattering seen on electron diffraction patterns, and the relationship between microstructural changes and conductivity was clarified. It is concluded that various phenomena due to aging at relatively lower temperatures are caused by short range ordering of oxygen ion vacancies, which occurs in order to relax the anisotropy of periodical lattice distortion. We also studied changes in microstructures occurring with changing dopant concentrations and concluded that the changes in microstructures responsible for the decrease in conductivity associated with increasing dopant concentration are caused by short range ordering of oxygen ion vacancies in order to relax the anisotropy in strain fields, similarly to those with aging.

Yasuhiko Ito

Papers

Effect of Aging on Yttria‐Stabilized Zirconia I. A Study of Its Electrochemical Properties

Stress on a reaction vessel by the swelling of a hydrogen absorbing alloy

Protonic and Native Conduction in Sr‐Substituted LaPO4 Studied by Thermoelectric Power Measurements

Effect of Aging on Yttria‐Stabilized Zirconia III. A Study of the Effect of Local Structures on Conductivity

Effect of Aging on Yttria‐Stabilized Zirconia II. A Study of the Effect of the Microstructure on Conductivity