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Exsolution is a technique to create metal nanoparticles embedded within a matrix. The phenomenon has previously predominantly been studied in A-site deficient and stoichiometric perovskite powders. Here, we present a systematic study of an A-site excess perovskite oxide based on SrTiO 3 thin films, doped with nickel and exsolved under different conditions. The study aims to shed light on particle formation in these novel systems, including the effects of (i) the thin film thickness, (ii) pre-exsolution annealing in an oxidative atmosphere, (iii) a reductive atmosphere during the exsolution step, and (iv) exsolution time on the particle size and particle density. Our results indicate that exsolution occurs quickly, forming nanoparticles both on the surface and in the bulk of the host perovskite. The findings indicate that pre-annealing in an ambient atmosphere leads to fewer but larger exsolved particles compared to samples without pre-annealing. Consequently, while crystallization of the thin film occurs in both atmospheres, the simultaneous crystallization of the thin film and formation of the nanoparticles leads to a smaller apparent average radius. Moreover, we present evidence that metal particles can be found beyond the originally doped region. These findings are a step towards realizing tunable functional materials using exsolution to create metallic nanostructures within a thin film in a predictable manner., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Perovskite oxides have great flexibility in their elemental composition, which is accompanied by large adjustability in their electronic properties. Herein, we synthesized twelve perovskite oxide-based catalysts for the oxygen evolution reaction (OER) in alkaline media. The catalysts are based on the parent oxide perovskite Ba 0.5 Gd 0.8 La 0.7 Co 2 O 6-δ (BGLC587) and are synthesized through the sol-gel citrate synthesis route. To reduce the demand on cobalt (Co), but also increase the intrinsic catalytic activity of BGLC587 for the OER, we substitute Co on the B-site with certain amounts of Fe and Ni, synthesizing catalysts of the general formula Ba 0.5 Gd 0.8 La 0.7 Co 2-x-y Fe x Ni y O 6-δ . A plethora of physicochemical and electrochemical methods suggest that an Fe content between 30 % and 70 % increases the intrinsic catalytic activity of BGLC587, while Tafel slopes in combination with in-situ Raman spectroscopy suggest the rate determining step is likely a proton-exchange reaction, progressing possibly through the lattice oxygen mechanism (LOM). We apply one of the optimized, Co-substituted perovskites in a monolithic, photovoltaic (PV)-driven electrolysis cell and we achieve an initial solar-to-hydrogen (STH) conversion efficiency of 10.5 % under one sun solar simulated illumination., (© 2024 The Authors. ChemSusChem published by Wiley-VCH GmbH.)

Proton conducting acceptor-doped SrZrO3 has a history as long as that of BaZrO3, but has attracted less interest. Inspired by its higher transport number of ionic conduction in wet oxygen, we here explore further aspects of doped SrZrO3 as electrolyte in proton ceramic electrochemical cells. In-situ HT-XRD analysis of SrZr0.9Y0.1O3-δ (SZY10) indicated an anisotropic chemical expansion of hydration, stronger along the b than the a direction, and negative in the c direction. A systematic EMF and impedance spectroscopy study allowed determination of partial conductivities of electron holes and ions (mainly protons) in bulk and grain boundaries. Enthalpies and preexponentials were determined and interpreted for bulk and grain boundary partial conductivities based on defect chemistry. The hole conductivity in bulk is modest and ensures high ionic transport numbers in oxidizing atmospheres, while grain boundaries exhibit lower ionic transport numbers from a relatively higher hole conductivity attributed to tunneling past the deepest part of the space charge region. Y-doped SrZrO3 (SZY) materials exhibit lower proton conductivities, but excel over Y-doped BaZrO3 (BZY) in terms of thermal expansion compatibility with electrode materials and higher ionic transport numbers in oxidizing atmospheres, and may hence be candidates for functional layers between BZY-based electrolytes and positrodes., (© 2023 Wiley-VCH GmbH.)

Ba 1- x Gd 1- y La x + y Co 2 O 6- δ (BGLC) compositions with large compositional ranges of Ba, Gd, and La have been characterised with respect to phase compositions, structure, and thermal and chemical expansion. The results show a system with large compositional flexibility, enabling tuning of functional properties and thermal and chemical expansion. We show anisotropic chemical expansion and detailed refinements of emerging phases as La is substituted for Ba and Gd. The dominating phase is the double perovskite structure Pmmm , which is A-site ordered along the c -axes and with O vacancy ordering along the b -axis in the Ln-layer. Phases emerging when substituting La for Ba are orthorhombic Ba-deficient Pbnm and cubic LaCoO 3 -based R 3̄ c . When La is almost completely substituted for Gd, the material can be stabilised in Pmmm , or cubic Pm 3̄ m , depending on thermal and atmospheric history. We list thermal expansion coefficients for x = 0-0.3, y = 0.2.

There is a growing need to control and tune nanoparticles (NPs) to increase their stability and effectiveness, especially for photo- and electrochemical energy conversion applications. Exsolved particles are well anchored and can be re-shaped without changing their initial location and structural arrangement. However, this usually involves lengthy treatments and use of toxic gases. Here, the galvanic replacement/deposition method is used, which is simpler, safer, and leads to a wealth of new hybrid nanostructures with a higher degree of tailorability. The produced NiAu bimetallic nanostructures supported on SrTiO 3 display exceptional activity in plasmon-assisted photoelectrochemical (PEC) water oxidation reactions. In situ scanning transmission electron microscopy is used to visualize the structural evolution of the plasmonic bimetallic structures, while theoretical simulations provide mechanistic insight and correlate the surface plasmon resonance effects with structural features and enhanced PEC performance. The versatility of this concept in shifting catalytic modes to the hydrogen evolution reaction is demonstrated by preparing hybrid NiPt bimetallic NPs of low Pt loadings on highly reduced SrTiO 3 supports. This powerful methodology enables the design of supported bimetallic nanomaterials with tunable morphology and catalytic functionalities through minimal engineering., (© 2022 The Authors. Small published by Wiley-VCH GmbH.)

N 2 O, and mixtures of N 2 and O 2 , dissolved in water-both in the presence and absence of added noble gases-have been subjected to ultrasonication with quantification of nitrite and nitrate products. Significant increase in product formation upon adding noble gas for both reactant systems is observed, with the reactivity order Ne < Ar < Kr < Xe. These observations lend support to the idea that extraordinarily high electronic and vibrational temperatures arise under these conditions. This is based on recent observations of sonoluminescence in the presence of noble gases and is inconsistent with the classical picture of adiabatic bubble collapse upon acoustic cavitation. The reaction mechanisms of the first few reaction steps necessary for the critical formation of NO are discussed, illustrated by quantum chemical calculations. The role of intermediate N 2 O in this series of elementary steps is also discussed to better understand the difference between the two reactant sources (N 2 O and 2 : 1 N 2  : O 2 ; same stoichiometry).

The surface protonic conductivity of porous monoclinic ZrO 2 sintered at temperatures in the range 700-1100 °C yielding relative densities of around 60% and grain sizes of approximately 160 nm has been studied using impedance spectroscopy as a function of temperature well below the sintering temperature in wet atmospheres ( p H 2 O = 0.03 bar). The sum of two high-frequency impedance responses is argued to represent surface conductance according to a new model of impedance over curved surfaces. A simple brick layer model is applied to compare the measured macroscopic conductivities with predicted surface conductances. The well-faceted samples sintered at the highest temperatures exhibited activation enthalpies up to 58 kJ mol -1 of surface protonic conduction in wet atmospheres at temperatures above 300 °C. We attribute this to the mobility of dissociated protons over surface oxide ions, and the high preexponential is in good agreement with a model comprising relatively strong dissociative chemisorption. With decreasing sintering temperature, the particles appear more rounded, with less developed facets, and we obtain activation enthalpies of surface protonic conduction in the chemisorbed layer down to around 30 kJ mol -1 , with correspondingly smaller preexponentials and an observed dependency. Supported by the thermogravimetry of adsorption, we attribute this to weaker and more molecular chemisorption on the more randomly terminated less faceted surfaces, providing water layers with fewer dissociated charge carrying protons, but also smaller activation enthalpies of mobility. Below 200 °C, all samples exhibit a strongly inverse temperature dependency characteristic of conduction in the 1st physisorbed layer with increasing coverage. The preexponentials correspond well to the models of physisorption, with dissociation to and proton migration between physisorbed water molecules. The enthalpies fit well to physisorption and with enthalpies of dissociation and proton mobility close to those of liquid water. We have by this introduced models for proton conduction in chemisorbed and physisorbed water on ZrO 2 , applicable to other oxides as well, and shown that preexponentials are quantitatively assessable in the order-of-magnitude level to discriminate models via a simple brick layer model based topographical analysis of the ceramic microstructure.

Proton ceramic reactors offer efficient extraction of hydrogen from ammonia, methane, and biogas by coupling endothermic reforming reactions with heat from electrochemical gas separation and compression. Preserving this efficiency in scale-up from cell to stack level poses challenges to the distribution of heat and gas flows and electric current throughout a robust functional design. Here, we demonstrate a 36-cell well-balanced reactor stack enabled by a new interconnect that achieves complete conversion of methane with more than 99% recovery to pressurized hydrogen, leaving a concentrated stream of carbon dioxide. Comparable cell performance was also achieved with ammonia, and the operation was confirmed at pressures exceeding 140 bars. The stacking of proton ceramic reactors into practical thermo-electrochemical devices demonstrates their potential in efficient hydrogen production.

In this study, polyol-made CdS and CdSe crystalline nanoparticles (NPs) are loaded by impregnation on TiO 2 nanotube arrays (TNTAs) for solar-simulated light-driven photoelectrochemical (PEC) water vapor splitting. For the first time, we introduce a safe way to utilize toxic, yet efficient photocatalysts by integration in solid-state PEC (SSPEC) cells. The enabling features of SSPEC cells are the surface protonic conduction mechanism on TiO 2 and the use of polymeric electrolytes, such as Nafion instead of liquid ones, for operation with gaseous reactants, like water vapor from ambient humidity. Herein, we studied the effects of both the operating conditions in gaseous ambient atmospheres and the surface modifications of TNTAs-based photoanodes with well-crystallized CdS and CdSe NPs. We showed 3.6 and 2.5 times increase in the photocurrent density of defective TNTAs modified with CdS and CdSe, respectively, compared to the pristine TNTAs. Electrochemical impedance spectroscopy and structural characterizations attributed the improved performance to the higher conductivity induced by intrinsic defects as well as to the enhanced electron/hole separation at the TiO 2 /CdS heterojunction under gaseous operating conditions. The SSPEC cells were evaluated by cycling between high relative humidity (RH) (80%) and low RH levels (40%), providing direct evidence of the effect of RH and, in turn, adsorbed water, on the cell performance. Online mass spectrometry indicated the corresponding difference in the H 2 production rate. In addition, a complete restoration of the SSPEC cell performance from low to high RH levels was also achieved. The presented system can be employed in off-grid, water depleted, and air-polluted areas for the production of hydrogen from renewable energy and provides a solution for the safe use of toxic, yet efficient photocatalysts.

It has over the last few years been reported that the application of a DC electric field and resulting current over a bed of certain catalyst-support systems enhances catalytic activity for several reactions involving hydrogen-containing reactants, and the effect has been attributed to surface protonic conductivity on the porous ceramic support (typically ZrO2, CeO2, SrZrO3). Models for the nature of the interaction between the protonic current, the catalyst particle (typically Ru, Ni, Co, Fe), and adsorbed reactants such as NH3 and CH4 have developed as experimental evidence has emerged. Here, we summarize the electrical enhancement and how it enhances yield and lowers reaction temperatures of industrially important chemical processes. We also review the nature of the relevant catalysts, support materials, as well as essentials and recent progress in surface protonics. It is easily suspected that the effect is merely an increase in local vs. nominal set temperature due to the ohmic heating of the electrical field and current. We address this and add data from recent studies of ours that indicate that the heating effect is minor, and that the novel catalytic effect of a surface protonic current must have additional causes.

Water photoelectrolysis has the potential to produce renewable hydrogen fuel, therefore addressing the intermittent nature of sunlight. Herein, a monolithic, photovoltaic (PV)-assisted water electrolysis device of minimal engineering and of low (in the μg range) noble-metal-free catalysts loading is presented for unassisted water splitting in alkaline media. An efficient double perovskite cobaltite catalyst, originally developed for high-temperature proton-conducting ceramic electrolyzers, possesses high activity for the oxygen evolution reaction in alkaline media at room temperatures too. Ba 1- x Gd 1- y La x + y Co 2 O 6-δ (BGLC) is combined with a NiMo cathode, and a solar-to-hydrogen efficiency of 6.6% in 1.0 M NaOH, under 1 sun simulated illumination for 71 h, is demonstrated. This work highlights how readily available earth-abundant materials and established PV methods can achieve high performance and stable and monolithic photoelectrolysis devices with potential for full-scale applications.

A high temperature thermoelectric test setup for the NORECS ProboStat™ sample holder cell has been designed, constructed, and tested. It holds four thermoelectric legs of up to 5 × 5 mm 2 area each and flexible height, allows various interconnects to be tested, and utilizes the spring-load system of the ProboStat for fixation and contact. A custom stainless steel support tube flushed with water provides the cold sink, enabling large temperature gradients. Thermocouples and electrodes as well as the gas supply and outer tube use standard ProboStat base unit feedthroughs and dimensions. The setup allows for testing in controlled atmospheres with the hot side temperature of up to around 1000 °C and a temperature gradient of up to 600 °C. We demonstrate the test setup on a four-leg Li-NiO/Al-ZnO module with gold interconnects. The comparison between the predicted performance based on individual material parameters and the experimentally obtained module performance underlines the necessity for testing materials in combination, including interconnects. The four-leg setup allows versatile match-screening, performance evaluation, and long-term stability studies of thermoelectric materials in combination with hot and cold side interconnects under realistic operational conditions.

We report a near-broken-gap alignment between p-type FeWO 4 and n-type Fe 2 WO 6 , a model pair for the realization of Ohmic direct junction thermoelectrics. Both undoped materials have a large Seebeck coefficient and high electrical conductivity at elevated temperatures, due to inherent electronic defects. A band-alignment diagram is proposed based on X-ray photoelectron and ultraviolet-visible light reflectance spectroscopy. Experimentally acquired nonrectifying I - V characteristics and the constructed band-alignment diagram support the proposed formation of a near-broken-gap junction. We have additionally performed computational modeling based on density functional theory (DFT) on bulk models of the individual compounds to rationalize the experimental band-alignment diagram and to provide deeper insight into the relevant band characteristics. The DFT calculations confirm an Fe-3d character of the involved band edges, which we suggest is a decisive feature for the unusual band overlap.

The application of double perovskite cobaltites Ba Ln Co 2 O 6-δ ( Ln = lanthanide element) in electrochemical devices for energy conversion requires control of their properties at operating conditions. This work presents a study of a series of Ba Ln Co 2 O 6-δ ( Ln = La, Pr, Nd) with a focus on the evolution of structural and electrical properties with temperature. Symmetry, oxygen non-stoichiometry, and cobalt valence state have been examined by means of Synchrotron Radiation Powder X-ray Diffraction (SR-PXD), thermogravimetry (TG), and X-ray Absorption Spectroscopy (XAS). The results indicate that all three compositions maintain mainly orthorhombic structure from RT to 1000 °C. Chemical expansion from Co reduction and formation of oxygen vacancies is observed and characterized above 350 °C. Following XAS experiments, the high spin of Co was ascertained in the whole range of temperatures for BLC, BPC, and BNC.

We report the synthesis of phase pure Fe 2 WO 6 and its structural characterization by high quality synchrotron X-ray powder diffraction, followed by studies of electric and thermoelectric properties as a function of temperature (200-950 °C) and pO 2 (1-10 -3 bar). The results are shown to be in accordance with a defect chemical model comprising formation of oxygen vacancies and charge compensating electrons at high temperatures. The standard enthalpy and entropy of formation of an oxygen vacancy and two electrons in Fe 2 WO 6 are found to be 113(5) kJ mol -1 and 41(5) J mol -1 K -1 , respectively. Electrons residing as Fe 2+ in the Fe 3+ host structure act as charge carriers in a small polaron conducting manner. A freezing-in of oxygen vacancies below approximately 650 °C results in a region of constant charge carrier concentration, corresponding to an iron site fraction of X Fe 2+ ≅ 0.03. By decoupling of mobility from conductivity, we find a polaron hopping activation energy of 0.34(1) eV and a charge mobility pre-exponential u 0 = 400(50) cm 2 kV -1 s -1 . We report thermal conductivity for the first time for Fe 2 WO 6 . The relatively high conductivity, large negative Seebeck coefficient and low thermal conductivity make Fe 2 WO 6 an interesting candidate as an n-type thermoelectric in air, for which we report a maximum zT of 0.027 at 900 °C.

Low temperature (<500 K) methane steam reforming in an electric field was investigated over various catalysts. To elucidate the factors governing catalytic activity, activity tests and various characterization methods were conducted over various oxides including CeO 2 , Nb 2 O 5 , and Ta 2 O 5 as supports. Activities of Pd catalysts loaded on these oxides showed the order of CeO 2 > Nb 2 O 5 > Ta 2 O 5. Surface proton conductivity has a key role for the activation of methane in an electric field. Proton hopping ability on the oxide surface was estimated using electrochemical impedance measurements. Proton transport ability on the oxide surface at 473 K was in the order of CeO 2 > Nb 2 O 5 > Ta 2 O 5. The OH group amounts on the oxide surface were evaluated by measuring pyridine adsorption with and without H 2 O pretreatment. Results indicate that the surface OH group concentrations on the oxide surface were in the order of CeO 2 > Nb 2 O 5 > Ta 2 O 5. These results demonstrate that the surface concentrations of OH groups are related to the proton hopping ability on the oxide surface. The concentrations reflect the catalytic activity of low-temperature methane steam reforming in the electric field., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

We apply density functional theory to estimate the energetics and charge carrier concentrations and, in turn, the resistance across the (210)[001] and (111)[11[combining macron]0] grain boundaries (GBs) in proton conducting Y-doped BaZrO 3 , assessing four commonly used approximations in space charge modelling. The abrupt core approximation, which models the GB core as a single atomic plane rather than a set of multiple atomic planes, gives an underestimation of the GB resistance with around one order of magnitude for both GBs. The full depletion approximation, which assumes full depletion of effectively positive charge carriers in the space charge layers, has negligible effect on the GB resistance compared to a more accurate model with decaying depletion. Letting protons redistribute in the continuity between atomic planes gives a GB resistance up to 5 times higher than the case where protons are restricted to be located at atomic planes. Finally, neglecting trapping effects between the acceptor doping and the defect charge carriers gives a higher GB resistance with a factor of roughly 2.

Catalytic methane steam reforming was conducted at low temperature using a Pd catalyst supported on Ce 1- x M x O 2 ( x = 0 or 0.1, M = Ca, Ba, La, Y or Al) oxides with or without an electric field (EF). The effects of the catalyst support on catalytic activity and surface proton hopping were investigated. Results show that Pd/Al-CeO 2 (Pd/Ce 0.9 Al 0.1 O 2 ) showed higher activity than Pd/CeO 2 with EF, although their activity was identical without EF. Thermogravimetry revealed a larger amount of H 2 O adsorbed onto Pd/Al-CeO 2 than onto Pd/CeO 2 , so Al doping to CeO 2 contributes to greater H 2 O adsorption. Furthermore, electrochemical conduction measurements of Pd/Al-CeO 2 revealed a larger contribution of surface proton hopping than that for Pd/CeO 2 . This promotes the surface proton conductivity and catalytic activity during EF application., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

This is the first direct observation that surface proton hopping occurs on SrZrO 3 perovskite even under a H 2 (i.e. dry) atmosphere. Understanding proton conduction mechanisms on ceramic surfaces under a H 2 atmosphere is necessary to investigate the role of proton hopping on the surface of heterogeneous catalysts in an electric field. In this work, surface protonics was investigated using electrochemical impedance spectroscopy (EIS). To extract the surface proton conduction, two pellets of different relative densities were prepared: a porous sample (R.D. = 60%) and a dense sample (R.D. = 90%). Comparison of conductivities with and without H 2 revealed that only the porous sample showed a decrease in the apparent activation energy of conductivity by supplying H 2 . H/D isotope exchange tests revealed that the surface proton is the dominant conductive species over the porous sample with H 2 supply. Such identification of a dominant conductive carrier facilitates consideration of the role of surface protonics in chemical reactions.

An all-oxide thermoelectric generator for high-temperature operation depends on a low electrical resistance of the direct p-n junction. Ca 3 Co 4- x O 9+ δ and CaMnO 3- δ exhibit p-type and n-type electronic conductivity, respectively, and the interface between these compounds is the material system investigated here. The effect of heat treatment (at 900 °C for 10 h in air) on the phase and element distribution within this p-n junction was characterized using advanced transmission electron microscopy combined with X-ray diffraction. The heat treatment resulted in counter diffusion of Ca, Mn and Co cations across the junction, and subsequent formation of a Ca 3 Co 1+ y Mn 1- y O 6 interlayer, in addition to precipitation of Co-oxide, and accompanying diffusion and redistribution of Ca across the junction. The Co/Mn ratio in Ca 3 Co 1+ y Mn 1- y O 6 varies and is close to 1 ( y = 0) at the Ca 3 Co 1+ y Mn 1- y O 6 -CaMnO 3- δ boundary. The existence of a wide homogeneity range of 0 ≤ y ≤ 1 for Ca 3 Co 1+ y Mn 1- y O 6 is corroborated with density functional theory (DFT) calculations showing a small negative mixing energy in the whole range., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

We apply inline electron holography to investigate the electrostatic potential across an individual BaZr0.9Y0.1O3 grain boundary. With holography, we measure a grain boundary potential of -1.3 V. Electron energy loss spectroscopy analyses indicate that barium vacancies at the grain boundary are the main contributors to the potential well in this sample. Furthermore, geometric phase analysis and density functional theory calculations suggest that reduced atomic density at the grain boundary also contributes to the experimentally measured potential well.

Polymer electrolyte membrane (PEM) fuel cells and electrolysers offer efficient use and production of hydrogen for emission-free transport and sustainable energy systems. Perfluorosulfonic acid (PFSA) membranes like Nafion ® and Aquivion ® are the state-of-the-art PEMs, but there is a need to increase the operating temperature to improve mass transport, avoid catalyst poisoning and electrode flooding, increase efficiency, and reduce the cost and complexity of the system. However, PSFAs-based membranes exhibit lower mechanical and chemical stability, as well as proton conductivity at lower relative humidities and temperatures above 80 °C. One approach to sustain performance is to introduce inorganic fillers and improve water retention due to their hydrophilicity. Alternatively, polymers where protons are not conducted as hydrated H 3 O + ions through liquid-like water channels as in the PSFAs, but as free protons (H + ) via Brønsted acid sites on the polymer backbone, can be developed. Polybenzimidazole (PBI) and sulfonated polyetheretherketone (SPEEK) are such materials, but need considerable acid doping. Different composites are being investigated to solve some of the accompanying problems and reach sufficient conductivities. Herein, we critically discuss a few representative investigations of composite PEMs and evaluate their significance. Moreover, we present advances in introducing electronic conductivity in the polymer binder in the catalyst layers.

Hydrogen production from water electrolysis is a key enabling energy storage technology for the large-scale deployment of intermittent renewable energy sources. Proton ceramic electrolysers (PCEs) can produce dry pressurized hydrogen directly from steam, avoiding major parts of cost-driving downstream separation and compression. However, the development of PCEs has suffered from limited electrical efficiency due to electronic leakage and poor electrode kinetics. Here, we present the first fully operational BaZrO 3 -based tubular PCE, with 10 cm 2 active area and a hydrogen production rate above 15 Nml min -1 . The novel steam anode Ba 1-x Gd 0.8 La 0.2+x Co 2 O 6-δ exhibits mixed p-type electronic and protonic conduction and low activation energy for water splitting, enabling total polarization resistances below 1 Ω cm 2 at 600 °C and Faradaic efficiencies close to 100% at high steam pressures. These tubular PCEs are mechanically robust, tolerate high pressures, allow improved process integration and offer scale-up modularity.

Ta3N5 nanotubes (NTs) were obtained from nitridation of Ta2O5 NTs, which were grown directly on Ta foil through a 2-step anodization procedure. With Co(OH)x decoration, a photocurrent density as high as 2.3 mA cm-2 (1.23 V vs. NHE) was reached under AM1.5G simulated solar light; however, the electrode suffered from photocorrosion. More stable photoelectrochemical (PEC) performance was achieved by first loading Co(OH)x, followed by loading cobalt phosphate (Co-Pi) as double co-catalysts. The Co(OH)x/Co-Pi double co-catalysts may act as a hole storage layer that slows down the photocorrosion caused by the accumulated holes on the surface of the electrode. A "waggling" appearance close to the "mouth" of Ta2O5 NTs was observed, and may indicate structural instability of the "mouth" region, which breaks into segments after nitridation and forms a top layer of broken Ta3N5 NTs. A unique mesoporous structure of the walls of the Ta3N5 NTs, which is reported here the first time, is also a result of the nitridation process. We believe that the mesoporous structure makes it difficult for the nanotubes to be fully covered by the co-catalyst layer, hence rationalizing the remaining degradation by photocorrosion.

Mixed oxide hydride anion systems constitute a novel class of materials exhibiting intriguing properties such as solid-state hydride ion conduction and fast anion exchange. In this contribution we derive the kinetics of hydride ion transport in a mixed oxide-hydride system, SrTiO 3- x H x , through isotope exchange and depth profiling. Density functional theory (DFT) calculations indicate that migration of H - to neighboring vacant oxygen lattice sites is fast, but that long-range transport is impeded by slow reorganization of the oxygen sublattice. From measured hydride tracer-diffusion coefficients and the correlation factors derived from DFT, we are able to derive the hydrogen self-diffusion coefficients in SrTiO 3- x H x . More generally, the explicit description of hydride ion transport in SrTiO 3- x H x through combination of experimental and computational methods reported in this work can be applied to explore anion diffusion in other mixed anion systems.

High-temperature instability of the Ca₃Co 4- y O 9+ δ and CaMnO 3- δ direct p-n junction causing the formation of Ca₃Co 2- x Mn x O₆ has motivated the investigation of the thermoelectric performance of this intermediate phase. Here, the thermoelectric properties comprising Seebeck coefficient, electrical conductivity, and thermal conductivity of Ca₃Co 2- x Mn x O₆ with x = 0.05, 0.2, 0.5, 0.75, and 1 are reported. Powders of the materials were synthesized by the solid-state method, followed by conventional sintering. The material Ca₃CoMnO₆ ( x = 1) demonstrated a large positive Seebeck coefficient of 668 μV/K at 900 °C, but very low electrical conductivity. Materials with compositions with x < 1 had lower Seebeck coefficients and higher electrical conductivity, consistent with small polaron hopping with an activation energy for mobility of 44 ± 6 kJ/mol and where both the concentration and mobility of hole charge carriers were proportional to 1- x . The conductivity reached about 11 S·cm -1 at 900 °C for x = 0.05. The material Ca₃Co 1.8 Mn 0.2 O₆ ( x = 0.2) yielded a maximum zT of 0.021 at 900 °C. While this value in itself is not high, the thermodynamic stability and self-assembly of Ca₃Co 2- x Mn x O₆ layers between Ca₃Co 4- y O 9+ δ and CaMnO 3- δ open for new geometries and designs of oxide-based thermoelectric generators.

All-oxide thermoelectric modules for energy harvesting are attractive because of high-temperature stability, low cost, and the potential to use nonscarce and nontoxic elements. Thermoelectric modules are mostly fabricated in the conventional π-design, associated with the challenge of unstable metallic interconnects at high temperature. Here, we report on a novel approach for fabrication of a thermoelectric module with an in situ formed p-p-n junction made of state-of-the-art oxides Ca 3 Co 4- x O 9+δ (p-type) and CaMnO 3 -CaMn 2 O 4 composite (n-type). The module was fabricated by spark plasma co-sintering of p- and n-type powders partly separated by insulating LaAlO 3 . Where the n- and p-type materials originally were in contact, a layer of p-type Ca 3 CoMnO 6 was formed in situ. The hence formed p-p-n junction exhibited Ohmic behavior and a transverse thermoelectric effect, boosting the open-circuit voltage of the module. The performance of the module was characterized at 700-900 °C, with the highest power output of 5.7 mW (around 23 mW/cm 2 ) at 900 °C and a temperature difference of 160 K. The thermoelectric properties of the p- and n-type materials were measured in the temperature range 100-900 °C, where the highest zT of 0.39 and 0.05 were obtained at 700 and 800 °C, respectively, for Ca 3 Co 4- x O 9+δ and the CaMnO 3 -CaMn 2 O 4 composite., Competing Interests: The authors declare no competing financial interest.

The transport of protonic charge carriers along and within the pore surfaces of porous oxide matrices is of significant importance for many catalytic and electrochemical applications, with porous TiO2 being a candidate material both for photocatalytic applications and as an electronically conducting support for polymer-based electrochemical cells. This work investigates the effect of acceptor (Cr and Fe) and donor (Nb) doping on protonic surface conduction in porous TiO2 over a wide range of relative humidity, temperature and oxygen activity. Generally, we find that acceptor dopants on the surface counteract dissociation and reduce the mobility of protons, while donor dopants give rise to enhanced dissociation making protonic surface conduction the highest for donor-doped samples, contrary to conventional bulk proton conductors. Moreover, protonic surface conduction in Cr-doped TiO2 is significantly higher under oxidising conditions compared to reducing conditions, which we relate to the presence of a higher valent species such as Cr6+ on the surface under oxidising conditions, again emphasising that protonic surface conduction increases with higher-valent (donor) and more acidic cations present on the surface.

Density-functional-theory (DFT) calculations within the generalised gradient approximation (GGA) were used to examine the behaviour of point defects in the cubic B VI O 3 perovskite-type oxide, ReO 3 . Energies of reduction and of hydration were calculated, and the results are compared with literature data for ABO 3 perovskite oxides. The activation energies of migration were determined for O 2- , H + , Li + , Na + , K + and H 3 O + . An occupied A site in ReO 3 is found to be beneficial to oxide-ion migration by a vacancy mechanism as well as to proton migration by a Grotthuss mechanism. Na + , K + and H 3 O + exhibit activation energies of migration higher than 2 eV, whereas Li + is characterised by a very low migration barrier of 0.1 eV. Reasons for this behaviour are discussed. Our results suggest that H + , O 2- , and especially Li + , are highly mobile ions in ReO 3 .

Diffusivity on the A- and B-site of polycrystalline perovskite CaMnO 3-δ with Ca deficiency and spinel CaMn 2 O 4 (marokite) as a secondary phase was studied using chemical tracers and secondary ion mass spectrometry (SIMS) complemented by electron probe microanalysis (EPMA). Thin films containing Sr and Co chemical tracers were deposited on the polished surface of the polycrystalline composite sample followed by annealing at 800-1200 °C for 96 h. Diffusion profiles for each tracer were determined with SIMS, followed by calculation of diffusion coefficients by fitting to appropriate models. The Sr tracer showed mainly lattice diffusion, with an activation energy of 210 ± 30 kJ mol -1 , whereas the Co tracer showed a combination of lattice and enhanced grain-boundary diffusion, with activation energies of 270 ± 30 kJ mol -1 and 380 ± 40 kJ mol -1 , respectively. The diffusivities may be used to predict interdiffusion and lifetime of junctions between n-type CaMnO 3-δ or CaMnO 3-δ /CaMn 2 O 4 composites and metallization interlayers or p-type leg materials in oxide thermoelectrics. In particular, the relatively high effective diffusivity of Co in polycrystalline CaMnO 3-δ may play a role in the reported fast formation of the secondary phase (Ca 3 Co 2-y Mn y O 6 ) between p-type Ca 3 Co 3.92 O 9+δ and n-type CaMnO 3-δ in a direct p-n thermoelectric junction.

We have fabricated and tested a photoelectrochemical (PEC) cell where the aqueous electrolyte has been replaced by a proton conducting hydrated Nafion® polymer membrane. The membrane was sandwiched between a TiO 2 -based photoanode and a Pt/C-based cathode. The performance was tested with two types of photoanode electrodes, a thermally prepared TiO 2 film on Ti foil (T-TiO 2 ) and a nanostructured TiO 2 films in the form of highly ordered nanotubes (TNT) of different lengths. Firstly, photovoltammetry experiments were conducted under asymmetric conditions, where the anode was immersed in deionized water, while the cathode was kept in ambient air. The results showed a high incident photon-to-current efficiency (IPCE) of 19% under unassisted conditions (short-circuit, 0 V vs. cathode) with short TNT (ca. 1 μm) under 4 mW cm -2 illumination with UV-A rich light. Secondly, the deionized water was replaced by 0.5 M Na 2 SO 4 and now the performance was higher with longer nanotubes, assigned to increased ionic conductivity inside the tubes. An unassisted (0 V) IPCE of 33% was achieved with nanotubes of ca. 8 μm. The presented solid-state PEC cell minimizes the electrode distance and volume of the device, and provides a way towards compact practical applications in solar water splitting.

This work presents a structural investigation of La2-xNdxCe2O7 (x = 0.0, 0.5, 1.0, 1.5, 2.0) using X-ray powder diffraction and total scattering neutron powder diffraction, analysed using Rietveld and the reverse Monte Carlo method (RMC). Ab initio molecular dynamics (MD) modelling is also performed for further investigations of the local order. The main intensities in the neutron diffraction data for the La2-xNdxCe2O7 series correspond to the fluorite structure. However, additional C-type superlattice peaks are visible for x > 0 and increase in intensity with increasing x. The Nd-containing compositions (x > 0) are best fitted with Rietveld analysis by using a combination of oxygen deficient fluorite and oxygen excess C-type structures. No indications of cation order are found in the RMC or Rietveld analysis, and the absence of cation order is supported by the MD modelling. We argue that the superlattice peaks originate from oxygen vacancy ordering and associated shift in the cation position away from the ideal fluorite site similar to that in the C-type structure, which is seen from the Rietveld refinements and the observed ordering in the MD modelling. The vacancies favour alignments in the 〈110〉, 〈111〉 and especially the 〈210〉 direction. Moreover, we find that such ordering might also be found to a small extent in La2Ce2O7, explaining the discernible modulated background between the fluorite peaks. The observed overlap of the main Bragg peaks between the fluorite and C-type phase supports the co-existence of vacancy ordered and more disordered domains. This is further supported by the observed similarity of the radial distribution functions as modelled with MD. The increase in long range oxygen vacancy order with increasing Nd-content in La2-xNdxCe2O7 corresponds well with the lower oxide ion conductivity in Nd2Ce2O7 compared to La2Ce2O7 reported earlier.

Nonoxidative methane dehydroaromatization (MDA: 6CH4 ↔ C6H6 + 9H2) using shape-selective Mo/zeolite catalysts is a key technology for exploitation of stranded natural gas reserves by direct conversion into transportable liquids. However, this reaction faces two major issues: The one-pass conversion is limited by thermodynamics, and the catalyst deactivates quickly through kinetically favored formation of coke. We show that integration of an electrochemical BaZrO3-based membrane exhibiting both proton and oxide ion conductivity into an MDA reactor gives rise to high aromatic yields and improved catalyst stability. These effects originate from the simultaneous extraction of hydrogen and distributed injection of oxide ions along the reactor length. Further, we demonstrate that the electrochemical co-ionic membrane reactor enables high carbon efficiencies (up to 80%) that improve the technoeconomic process viability., (Copyright © 2016, American Association for the Advancement of Science.)

The rising importance of converting high peak electricity from renewables to fuels has urged field specialists to organize this Faraday Discussion on Solid Oxide Electrolysis. The topic is of essential interest in order to achieve a greater utilization of renewable energy and storage at higher densities.

An apparatus for measuring the Seebeck coefficient and electrical conductivity is presented and characterized. The device can be used in a wide temperature range from room temperature to 1050 °C and in all common atmospheres, including oxidizing, reducing, humid, and inert. The apparatus is suitable for samples with different geometries (disk-, bar-shaped), allowing a complete thermoelectric characterization (including thermal conductivity) on a single sample. The Seebeck coefficient α can be measured in both sample directions (in-plane and cross-plane) simultaneously. Electrical conductivity is measured via the van der Pauw method. Perovskite-type CaMnO3 and the misfit cobalt oxide (Ca2CoO3)q(CoO2) are studied to demonstrate the temperature range and to investigate the variation of the electrical properties as a function of the measurement atmosphere.

Crystal structures of Sr4(Sr2Ta2)O11 and Sr4(Sr1.92Ta2.08)O11.12, synthesized by solid state reaction technique in dry and hydrated state have been studied mainly using Transmission Electron Microscopy. Due to the lesser ability of X-rays to probe details in oxygen sublattice, the change in crystal symmetry due to ordering of oxygen vacancies could be detected better using Transmission Electron Microscopy. After detailed analysis through TEM, it was observed that no major change occurs in the cation sublattice. The TEM observations are compared with XRD data and discussed. The crystal symmetries and corresponding unit cells of all the perovskites based on the ordering of oxygen vacancies is deduced. Crystal unit cells based on the observations are proposed with ideal atomic coordinates. Finally an attempt is made to explain the water uptake behaviour of these perovskites based on the proposed crystal structure., (Copyright © 2014 Elsevier Ltd. All rights reserved.)

Sodium cobaltate with high Na content is a promising thermoelectric material. It has recently been reported that oxygen vacancies can alter the material properties, reducing its figure of merit. However, experimental data concerning the oxygen stoichiometry are contradictory. We therefore studied the formation of oxygen vacancies in Na(x)CoO(2) with first principles calculations, focusing on x = 0.75. We show that a very low oxygen vacancy concentration is expected at the temperatures and partial pressures relevant for applications.

Various inorganic membranes have demonstrated good capability to separate hydrogen from other gases at elevated temperatures. Hydrogen-permeable, dense, mixed proton-electron conducting ceramic oxides offer superior selectivity and thermal stability, but chemically robust candidates with higher ambipolar protonic and electronic conductivity are needed. In this work, we present for the first time the results of various investigations of La1-xSrxCrO3-∂ membranes for hydrogen production. We aim in particular to elucidate the material's complex transport properties, involving co-ionic transport of oxide ions and protons, in addition to electron holes. This opens some new possibilities for efficient heat and mass transfer management in the production of hydrogen. Conductivity measurements as a function of pH2 at constant pO2 exhibit changes that reveal a significant hydration and presence of protons. The flux and production of hydrogen have been measured under different chemical gradients. In particular, the effect of water vapor in the feed and permeate gas stream sides was investigated with the aim of quantifying the ratio of hydrogen production by hydrogen flux from feed to permeate and oxygen flux the opposite way ("water splitting"). Deuterium labeling was used to unambiguously prove flux of hydrogen species.

The nitrogen related defect chemistry and electronic structure of wide band gap oxides are investigated by density functional theory defect calculations of N(O)(q), NH(O)(×), and (NH2)(O)(·) as well as V(O)(··) and OH(O)(·) in MgO, CaO, SrO, Al(2)O(3), In(2)O(3), Sc(2)O(3), Y(2)O(3), La(2)O(3), TiO(2), SnO(2), ZrO(2), BaZrO(3), and SrZrO(3). The N(O)(q) acceptor level is found to be deep and the binding energy of NH(O)(×) with respect to N(O)' and (OH(O)(·) is found to be significantly negative, i.e. binding, in all of the investigated oxides. The defect structure of the oxides was found to be remarkably similar under reducing and nitriding conditions (1 bar N(2), 1 bar H(2) and 1 × 10(-7) bar H(2)O): NH(O)(×) predominates at low temperatures and [N(O)'] = 2[V(O)(··) predominates at higher temperatures (>900 K for most of the oxides). Furthermore, we evaluate how the defect structure is affected by non-equilibrium conditions such as doping and quenching. In terms of electronic structure, N(O)' is found to introduce isolated N-2p states within the band gap, while the N-2p states of NH(O)(×) are shifted towards, or overlap with the VBM. Finally, we assess the effect of nitrogen incorporation on the proton conducting properties of oxides and comment on their corrosion resistance in nitriding atmospheres in light of the calculated defect structures.

Effects of nitrogen defects on the electrical properties of RE(2)O(3) (RE = Nd, Gd, Er, Y) and ZrO(2) have been investigated by equilibration in ammonia (NH(3)) atmospheres in the temperature range 1000-1200 °C. The electrical conductivity in ammonia corresponded to that in H(2)-Ar mixtures of similar pO(2). However, upon replacing ammonia with an inert gas, the conductivity increases abruptly, typically one order of magnitude, before gradually returning to its equilibrium value. A defect model based on dissolution and dissociation of effectively neutral imide defects substituting oxide ions, NH, is proposed to describe this behavior. Conductivity measurements are interpreted in terms of nitrogen acceptors which are passivated by protons in the presence of H(2)(g), and subsequently compensated by positive charge carriers in an inert atmosphere as out-diffusion of hydrogen leaves an effective acceptor, N. In the case of Y(2)O(3), a NH concentration of 0.7 mol% was estimated from quantification of the nitrogen and hydrogen contents of a sample quenched in NH(3).

The gradual change in the crystal structure of the high temperature proton conductor LaNbO(4) through a second order phase transition and its relation to the activation enthalpy of mobility of protons have been studied by means of first principles calculations and conductivity measurements. The computations have revealed that protons diffuse by an inter-tetrahedral mechanism where the activation enthalpies of mobility are 39 and 60 kJ mol(-1) in tetragonal and monoclinic LaNbO(4), respectively. The activation enthalpy of mobility of protons for tetragonal LaNbO(4), determined from the conductivity curve, is 35 kJ mol(-1). Below the transition temperature the conductivity curve bends; initially dropping off steeply, followed by a less steep decrease towards lower temperatures. The bend in the conductivity curve at the onset of the phase transition in LaNbO(4) should not be given the traditional interpretation as an abrupt change in the activation enthalpy of mobility. After application of the proper analysis of the conductivity data, which takes the second order transition into account, the activation enthalpy of mobility of protons is found to continuously increase with increasing monoclinic angle at decreasing temperature, reaching approximately 57 kJ mol(-1) at 205 degrees C for the end monoclinic phase.

The behaviour of hydrogen in both defect free and acceptor doped bulk rutile TiO2 is investigated through defect calculations performed within the density functional theory formalism. Both interstitial and substitutional hydrogen defects are shown to behave as shallow donors in the material, thereby existing as effectively positive hydroxide defects, OH*(O), and substitutional hydrogen defects, H*O. Of the investigated isolated hydrogen defects, the OH*(O) defect is shown to have the lowest formation energy over the whole Fermi level range under both oxidising and reducing conditions. However, the formation energy of H*O is only 0.35 eV higher under conditions where TiO2 is in equilibrium with Ti2O3, indicating that it exists as a minority defect in rutile TiO2 under highly reducing conditions such as under growth of TiO2 on a Ti metal surface. The enthalpy of hydration of oxygen vacancies is calculated to be -1.64 and -1.60 eV for the 2 x 2 x 3 and 3 x 3 x 3 supercells, indicating that OH*(O) defects will prevail in wet atmospheres, even at high temperatures. Hydrogen incorporated in the material can furthermore be expected to associate significantly with various acceptor centres (e.g., substitutional N and Al acceptors and Ti vacancies). The binding energies of the substitutional NH(x)O (imide) defect and of the association complexes between H and fully ionised substitutional Al, (Al'Ti x OH*(O)), and Ti vacancies, (V(Ti)x OH*(O)and (V(Ti) x 4OH*(O)) (i.e., Ruetschi type defects), are calculated to be -0.36, -0.12, -0.47 and -0.82 eV, respectively.

In this work, the correlation between the characteristic green emissions and specific defects of ZnO was investigated through a series of experiments that were designed to separate the subtle interplays among the various types of specific defects. With physical analysis and multimode Brownian oscillator modeling, the underlying mechanisms of the variant effects on green emission were revealed. The results demonstrate that the observed green emissions can be identified as two types of individual emissions, namely high energy and low energy, that are associated with specific defects and their locations. The surface modification that leads to downwards band bending was found to be responsible for the high-energy green emission. The relationship between the intensity of the low- energy green emission and the crystallographic lattice contraction indicates that oxygen vacancy is the dominant cause of such an emission that resides within the bulk of ZnO.

First principles calculations reveal that formation of a fully ionized oxide vacancy leads to local condensation of coordination polyhedra forming Nb3O11(7-) in t-LaNbO4.

The paper reviews the history and present understanding of protons in oxides; their defect chemistry, thermodynamics, and transport. Focus is put on correlations between hydration thermodynamics and other materials properties which may help to predict proton uptake and proton conduction in oxides. Also effects of defect association and the particular problem of high grain boundary resistance in high temperature proton conductors are addressed. In the second part, a number of experimental observations attributed to the presence of hydride ions under mildly reducing conditions are discussed in relation to the unlikelihood that general thermodynamic considerations predict of finding these species under such conditions.

Partial hydrogen ion conductivity in acceptor doped CaTiO(3), BaCeO(3) and ZrO(2) and nominally undoped BaTiO(3) and TiO(2) has been investigated by transport number measurements using the hydrogen concentration cell/EMF method in wet atmospheres as a function of pO(2)(10(-20)-1 atm) at 800 or 1000 [degree]C. All oxides investigated, except ZrO(2), show minor proton conductivity in oxidising atmospheres. Earlier indications of apparent negative charge transport by hydrogen under reducing conditions and high temperatures in SrTiO(3) samples have been reproduced for all the investigated oxides including ZrO(2).