Hydrogen Ion Conductivity Research Articles

Physicochemical and FT-IR analyses of some freshwater and inflow wastewater-treated effluents in the Eastern Cape Province of South Africa

Monitoring changes occurring in water bodies is a tool to make a sound decision on water quality management. This work investigates water quality parameters and metabolites in the Bloukrans, Buffalo, Swartkops and Tyhume Rivers in the Eastern Cape Province of South Africa. The water physicochemical parameters assessed include the dissolved oxygen, oxidation-reduction potentials, temperature, Hydrogen ion concentration, salinity, conductivity, turbidity and total dissolved solids measured with a pre-calibrated Hanna Multiparameter instrument on-site. Chemical oxygen demand, phosphate, chloride, ammonium, sulphate and nitrate ions were measured in the laboratory with a UVmini spectrophotometer. FT-IR analysis of extracted metabolites was done on a Perkin Elmer FT-IR spectrometer. The result shows that upstream river samples have lower COD values and below the detection level in some. The concentrations of phosphate, chloride, ammonium, sulphate and nitrate ions were above the recommended values in the wastewater effluents, midstream and downstream river samples. FT-IR peaks similar to vinylidene, saturated aliphatic aldehydes, aryl aldehydes, primary amines, hydantoin amides, halogen, and organofluorides were common in the wastewater effluent samples and the receiving rivers. Grahamstown treated wastewater effluents and its receiving Bloukrans River were distinguished with absorption peaks of urethane and aromatic aldehydes. Absorption peaks of sulphonyl halides were present only on the spectra of treated wastewater effluents of Alice and the receiving downstream Tyhume River. Multivariate analysis shows correlations between the water samples from different geographical locations and treated wastewater effluents. It is concluded that the quality of the rivers was very poor and more efforts are needed to make the treated wastewater conform to the acceptable standard for a sustainable aquatic ecosystem.

Proton-Conducting Ceramic Membranes for the Production of Hydrogen via Decarbonized Heat: Overview and Prospects

Proton-conducting ceramic membranes show high hydrogen ion conductivity in the temperature range of 300–700 °C. They are attracting significant attention due to their relevant characteristics compared to both higher-temperature oxygen ion-conducting ceramic membranes and lower-temperature proton-conducting polymers. The aim of this review is to integrate the fundamentals of proton-conducting ceramic membranes with two of their relevant applications, i.e., membrane reactors (PCMRs) for methane steam reforming (SMR) and electrolysis (PCEC). Both applications facilitate the production of pure H2 in the logic of process intensification via decarbonized heat. Firstly, an overview of various types of hydrogen production is given. The fundamentals of proton-conducting ceramic membranes and their applications in PCMRs for SMR and reversible PCEC (RePCEC), respectively, are given. In particular, RePCECs are of particular interest when renewable power generation exceeds demand because the excess electrical energy is converted to chemical energy in the electrolysis cell mode, therefore representing an appealing solution for energy conversion and grid-scale storage.

Defect-engineered hexagonal boron nitride nanosheets as a new atomic-level directional ion conductor

With electron-conducting information technology, ion-conducting energy and environmental technology are promising for wide applications in a carbon-neutral world, but it has been challenging to achieve directional ion conduction in fast ion conductors, especially at the microscopic or even atomic scale. Here, the authors report a new atomic-level directional ion conductor of defect-engineered hexagonal boron nitride (h-BN) nanosheets for bioinspired nanofluidic iontronic devices, and propose a vacancy-defect-controlled strategy to achieve atomic-level directional hydrogen ion conduction in h-BN nanosheets. The adsorption and migration behaviors of hydrogen ions in h-BN nanosheets with both B and N vacancy defects were systematically investigated through the first-principle method. The best hydrogen adsorption sites are directly above N and B atoms in the B-vacancy and N-vacancy h-BN, respectively. It is found that the migration barriers of hydrogen ion in B-vacancy and N-vacancy h-BN are as high as 1.83–2.47 eV and 1.71–2.30 eV, respectively, which are much higher than the 0.72 eV of intrinsic h-BN nanosheets. This means that hydrogen ions can conduct in defect-free h-BN, while they cannot conduct in B/N-vacancy defects. Accordingly, hydrogen ion conduction in h-BN nanosheets can be directionally controlled by controlling the position of vacancy defects, which can be artificially constructed by femtosecond laser plasma lithography. This work provides a new technology of vacancy-defect-controlled atomic-level directional ions conduction in 2D materials for ion-conductor integrated circuit, and opens the door for constructing bioinspired nanofluidic iontronic devices in future neuronal-computer interfaces.

A solid hydrogel electrolyte-based micro fuel cell integrated with fuel/electrolyte storage and supply function

<p indent="0mm">The development of modern electronic equipment has promoted the requirement of portable energy conversion devices. Fuel cell technology provides clean energy, which is pollution-free and environmentally friendly. It has several advantages, including instant charge and discharge, high energy conversion efficiency, and easy integration. However, some disadvantages, such as high membrane cost, membrane aging degradation, water management, and extra ohmic resistance, always accompany the conventional proton exchange membrane fuel cells. In addition, the need for fuel tanks increases the system cost and complexity and is unfavorable for miniaturization. Meanwhile, the consumption of additional pump power to supply fuel can reduce the net output power. Hydrogel is a solid-state electrolyte crosslinked via a three-dimensional (3D) polymer network. The porous structure possesses the abilities of water absorption and retention and enables fast mass transport on a large surface. To simplify the fuel cell system and improve its portability, a miniature fuel cell integrated with the fuel/electrolyte is proposed, complemented by the water retention and ionic conductivity of the hydrogel. Polyvinyl alcohol (PVA) hydrogel is prepared via cyclic freezing and thawing processes. The effects of the fabrication parameters, such as swelling and conductivity of the hydrogel electrolyte, are discussed, and the fuel cell equipped with hydrogel is set up. The micromorphology and element distribution of PVA hydrogel are evaluated via physical characterization. PVA hydrogel has an abundant 3D-network porous structure. PVA polymer chain is uniformly crosslinked, and C and O elements are evenly distributed on the hydrogel skeleton. Fourier transform infrared results reveal numerous hydrophilic groups in PVA hydrogel, including –OH and C–O, contributing to the high aqueous electrolyte absorption ability. The effect of fabrication parameters on the swelling properties of PVA hydrogel is studied. The high concentration and efficient repetitive freezing-thawing can improve the crosslinking density of the PVA hydrogel, firmness of the hydrogel skeleton, and swelling equilibrium. The PVA hydrogel in 5% can reach the maximum swelling ratio of 45%. The ionic conductivity of the PVA hydrogel after swelling the electrolyte is studied via the two-probe AC impedance method. The various freezing-thawing cycles have an evident influence on the crosslinking density and pore size of the hydrogel, contributing to the absorption ability of the sulfuric acid solution and hydrogel conductivity. The ionic conductivity of the PVA hydrogel in 5% by three freeze-haw cycles is up to 173.61 mS/cm, facilitating the hydrogen ion conduction in the fuel cell. Moreover, the effects of hydrogel thickness, fuel concentration, and electrolyte concentration on the fuel cell performance are studied. Consequently, when the hydrogel electrolyte thickness is <sc>6 mm</sc> using <sc>6 mol/L</sc> HCOOH and <sc>1 mol/L</sc> H₂SO₄, the maximum power density is <sc>2.5 mW/cm²</sc>, and the limiting current density is <sc>15.2 mA/cm².</sc> The results suggest that the fuel cell can be used to power miniature portable electronic devices.

Protonic ceramic electrolysis cells for fuel production: a brief review

Proton-conducting oxides exhibit significant hydrogen ion (proton) conductivity at intermediate temperatures around 300–600 °C. Owing to their distinguished features compared to high-temperature oxygen ion-conducting oxide electrolytes and low-temperature proton-conducting polymer electrolytes, diverse electrochemical applications based on the proton-conducting oxides have attracted great attention for efficient energy conversions. This review particularly aims to introduce protonic ceramic electrolysis cells (PCECs) and their extended applications. The constituent materials, recent developments, remaining issues in PCECs as well as the application integrated with PCEC for electrochemical ammonia synthesis will be presented. In addition, for each section, the relevant prospects and recommendations for future research directions will be discussed.

Proton-conducting oxides for energy conversion and storage

Proton-conducting oxides are a class of solid-state ion-conducting ceramic materials that demonstrate significant hydrogen ion (proton) conductivity at intermediate temperatures (e.g., 300–700 °C). They are garnering significant attention due to several unique characteristics that distinguish them from both higher temperature oxygen ion conducting oxides and lower temperature proton-conducting polymers. By enabling proton-mediated electrochemistry under both dry and wet environments at moderate temperatures, protonic ceramics provide unique opportunities to enhance or synergize a diverse range of complementary electrochemical and thermochemical processes. Because of this potential, significant efforts have been devoted to advancing numerous energy-related applications using these materials. This review aims to comprehensively summarize these applications and analyze the most up-to-date and future developments of proton-conducting oxides. We aim to bring together this diverse subject matter by integrating the fundamentals of proton-conducting oxides with application-oriented insights. We begin with a historical roadmap, followed by a basic overview of the materials, theories and fundamentals, and fabrication and processing technologies underlying the field. The central section of our review summarizes major applications and developments of proton-conducting ceramics, ranging from maturing applications approaching commercialization to embryonic technologies just now emerging from the lab. These include protonic ceramic fuel cells, protonic ceramic electrolysis cells, reversible protonic ceramic electrochemical cells, protonic ceramic membrane reactors, and protonic ceramic electrochemical reactors. For each application, we analyze both the prospects and challenges and offer recommendations for future research directions so that tomorrow's researchers can continue to advance the development and commercialization of these fascinating materials.

ASSESSMENT OF WATER QUALITY IN THE EUPHERTES RIVER, SOUTHERN IRAQ

This study was carried out on Euphrates River, which is one of the main sources of water in Iraq to assess the water quality of Southeast of DhiQar province. Water samples were collected monthly and seasonally at three stations from October 2016 to September 2017. Eight parameters were analyzed, water temperature, dissolved oxygen, total hardness, total dissolved solid, hydrogen ion, turbidity, chloride and electrical conductivity. Monthly variations of water quality index were differing among months in three stations of study area in Euphrates River. The lowest value of WQI in station 1 was poor (1.32) in June, and the highest (2.30) in November, the value was indicated a good water. At station 2, WQI values ranged from 1.28 in August to 2.15 in February as good. The lowest of WQI (1.29) in June and the highest (1.58) in February was recorded at station 3. The ANOVA for water quality index was found statically significant (F= 0.008, P<0.05) of three stations and showed statistically significant seasonal variations of water quality index among study stations.

PEM 단위 연료전지 가시화 셀을 이용한 당량비 변화에 따른 플러딩 현상에 관한 연구

본 논문은 고분자 전해질 연료전지 공기극 유로 내부에서 당량비에 따라 발생하는 플러딩 현상을 가시화를 통해 확인하고, 전류 변화에 따른 물의 운송 특성에 관한 연구를 수행하였다. 공기극 당량비는 1.5, 2.0, 연료극 당량비는 1.5로 고정하여 실험을 수행하였다. 연료전지 공기측 당량비 2.0로 공급하였을 때 1.5와 비교하여 짧은 시간에 물이 생성되기 시작하였으며, 플러딩 영역이 빠르게 생성되는 결과가 나타났다. 또한, 공기극 당량비 1.5로 유지하는 경우 플러딩 영역 이후에 건조화 7.8A 이후 구간에서 건조화가 진행되며, 8A 이후구간에서 건조화가 시작되는 공기극 당량비 2.0에서 작동하는 연료전지와 비교하여 넓은 영역에서 물 생성이 활발하게 이루어져 MEA의 내구성과 수소이온전도도가 우수한 결과를 확인하였다.

Optimizing the Manufacturing Method of Half-Cell Fuel Cell Based on Solid Electrolyte with Hydrogen Ion Conductivity

Barium cerate-based perovskite oxides are protonic conductor candidates for intermediate temperature solid oxide fuel cells due to their high ionic conductivity and good sinterability. The aim of the present study is to fabricate a half-cell single-cell includes substrate, anode and electrol yte layers. The exact composition of BaZr0.1Ce0.7Y0.2O3─δ (BZCY7) has been selected as a proton conducting electrolyte. The fabrication process of a dense electrolyte membrane on a NiO- BaZr0.1Ce0.7Y0.2O3─δ (NiO-BZCY7) anode substrate has been studied by a co- pressing process after co -firing at 1400oC. BZCY7 powders were synthesized by solid-state reaction method after calcinations at 1150oC. A single phase was obtained at this low temperature. The phase composition of the resulting specimens was investigated using X-ray diffraction (XRD) analysis. Scanning el ectron microscope (SEM) was used to evaluate the features of the synthesized powders and also the condition of connected layers in half -cell.

Pathways of diffusion mixed with subsequent reactions with examples of hydrogen extraction from hydride-forming electrode and oxygen reduction at gas diffusion electrode

This paper covers the role of the rate-determining step (RDS) in anodic hydrogen extraction from hydride-forming electrode. In general, hydrogen extraction from the electrode proceeds through the following steps: (1) hydrogen diffusion within the electrode, (2) hydrogen transfer from absorbed state to adsorbed state, (3) electrochemical oxidation of hydrogen to hydrogen ion involving charge transfer, and (4) hydrogen ion conduction through the electrolyte. In most theoretical and experimental investigations, it has been assumed that the RDS of anodic hydrogen extraction is hydrogen diffusion through the electrode. In real situation, however, the overall rate of hydrogen extraction is simultaneously determined by the rates of two or more reaction steps including hydrogen diffusion. The present work provides the overview of anodic hydrogen extraction in case that diffusion is coupled with interfacial charge transfer, interfacial hydrogen transfer, and hydrogen ion conduction through the electrolyte as well as the purely diffusion-controlled hydrogen extraction. In addition, the mixed controlled diffusion model was also exemplified with oxygen reduction at gas diffusion electrode of fuel cell system.

The Mechanism of the Hydrogen Ion Conduction in Liquid Light and Heavy Water Derived from the Temperature Dependence of Their Limiting Conductivities

The anomalous behavior of liquid water is apparent from the temperature dependence of many experimental properties. Among these are the abnormal high limiting ionic conductivities λ(0) of protons and hydroxyl ions, which in a previous paper we found to depend linearly on the square root of the absolute temperature T(1/2). Here we describe a further study of these conductivities in both light and heavy water, together with the remarkable transition temperature T(0) [242.7 K in H(2)O and 250.5 K in D(2)O], where the supercooled liquid becomes inert and, for example, restricted water molecule rotation is arrested. From T(0) the rotation barrier heights for the two solvents are determined. The conductivity data enable to obtain experimentally the zero point energies of H(+) and D(+) in liquid water, with results in the right order of magnitude as compared to values estimated along quantum mechanical routes. Contrary to general opinion, the isotope effect λ(0)(H(+))/λ(0)(D(+)) is temperature dependent, its value being close to 2(1/2) only near 20 °C. The isotope effect λ(0)(OH(-))/λ(0)(OD(-)) also takes temperature dependent values, substantially higher than 2(1/2). Still, the linear relationships with T(1/2) sustain a model based on water rotation control for the conductivity mechanism. For a quantitative analysis, the rotation frequency ω is expressed by a simple linear function of T(1/2) in terms of the moment of inertia I and the quantity T(0)(1/2). This "ab initio" calculation is found to be in perfect agreement with theoretical and experimental data in the literature, when ω is identified with the reciprocal of the so-called single molecule relaxation time τ(s). The data analysis eventually leads to the conclusion that the hydrogen ion transfer between water molecules proceeds via two parallel pathways. Next to the rotation controlled hopping mechanism there exists a temperature independent transfer controlled by tunnelling, occurring at all relative orientations of the participating water molecules. The applicability of the excess conductivity concept for hydrogen and hydroxyl ions is critically discussed.

Transport of hydrogen species in a single crystal SrTiO 3

The total conductivity and the partial hydrogen ion conductivity of a single crystal of SrTiO 3 have been investigated by transport number measurements using a hydrogen activity concentration cell/EMF method in wet atmospheres as a function of pO 2 (10 − 20 –1 atm) at 1000 °C and as a function of temperature (350–1050 °C) in wet hydrogen. The single crystal exhibits a small but readily measurable proton conductivity contribution in wet oxidising atmospheres. Earlier indications of apparent – but unlikely – transport by negatively charged hydrogen ions under reducing conditions and high temperatures in polycrystalline SrTiO 3 have been reproduced for the single crystal, thus eliminating grain boundaries and porosity as a cause of such findings. The paper proposes transport of neutral hydrogen species through the oxide as an alternative cause: This results in polarization of the hydrogen activity at the interface between the electrodes and the electrolyte, in turn giving rise to an unintended gradient in oxygen activity and an EMF resulting from the relatively high oxygen ion transport number of the material.

Dielectric properties of a hydrated sulfonated poly(styrene–ethylene/butylenes–styrene) triblock copolymer

The present study showed that sulfonated poly(styrene–ethylene/butylenes–styrene) (S-SEBS) triblock copolymer ionomers can be made to exhibit dielectric constants on the order of a hundreds thousand. Although they are too lossy at this point to use as dielectric materials in capacitors or as electrostrictive Maxwell effect transducer materials because of their high hydrogen ion conductivity, the results of these initial dielectric studies as a function of ion content were used to try to understand the effects of a polar plasticizer, water, on dielectric properties of the acid form of this ionomer. This was done before moving on to more tightly bound ions (rather than the hydrogen ions of the sulfonic acid groups used here) and to other polar, less mobile plasticizers (which also interact strongly with the ionic dipoles). The discovery of such high dielectric constants suggested the possibility that low dielectric loss versions of this type of polymer, as well as other members of the class known as ionomers, might find future applications as extremely high dielectric constant materials in capacitors or transducers. Experimental results for films with degrees of sulfonation on the order of 10% or more showed dielectric constants on the order of ɛ′ ∼ 100,000 but dielectric loss tangents near D = tan δ ∼ 0.3, when the materials were exposed to high humidity conditions. Experiments to determine the effects of water content on the material's dielectric response showed that water can easily move into and out of the films studied and that this transport behavior is strongly correlated to the relative humidity of the environment and to the degree of sulfonation. Water content, in this case, was thus the primary consideration when attempting to understand the observed high dielectric constants in films with degree of sulfonation greater than 5.5%. However, vacuum-dried films were, also, examined and observed to exhibit a dielectric constant on the order of 2 until the degree of sulfonation was greater than 11%. Above this value, the dielectric constant increased by approximately 100% to a value on the order of 4.

Proton NMR relaxation study of the CsHSO 4 solid acid system

At 141 °C the solid acid CsHSO 4 is known to undergo transition to a superprotonic phase that is characterized by dramatic (several-order-of-magnitude) increases in hydrogen ion conductivity. Proton NMR spin–spin relaxation time T 2 measurements reported here for CsHSO 4 also reveal substantial increases (factors of 20–30) in the vicinity of the transition temperature. In the temperature range just below the transition (70–136 °C), T 2 increases by a factor of order 10 relative to the rigid-lattice regime, suggesting motional narrowing of the NMR resonance line. In the regime of motional narrowing, the activation energy barrier to diffusion is 0.40 eV, as determined from the present T 2 results. NMR spin–lattice relaxation T 1 measurements also show behavior consistent with transition to a regime of rapid hydrogen motion. In particular, proton T 1's decrease with temperature (from 80 to 120 °C), and then drop sharply near the transition temperature. Above the transition temperature, T 1 exhibits a minimum in which the correlation time is found to be ∼2 ns.

A novel approach to assess the impact of landuse activity on chemical and biological parameters in river catchments

Summary1. Natural variation in environmental parameters, as well as practical constraints in study design and sampling methodology, often pose difficulties in treating impact assessments in river catchments as controlled field experiments. It is frequently impossible to develop robust relationships between reference and test stations prior to the onset of an impact and the range of statistical tools which can be adopted in data analysis to detect a change or disturbance is limited.2. In an attempt to overcome these problems we introduce a novel disturbance index to assess the impact of landuse activities on river systems. The index identifies differences in hydrochemical parameters and macroinvertebrate community metrics between reference and test stations (at a set level of significance). This approach allows for objective assessment of the occurrence and direction of change as well as the duration of an impact. The disturbance index can be applied at different scales – for a single stream, a catchment or a region.3. In this paper we describe the derivation of the index and illustrate its utility through worked examples. We use the index to assess impact of clearfelling on hydrochemical parameters such as hydrogen ion concentration, total hardness, suspended solids, conductivity and nitrate concentration as well as on macroinvertebrate parameters including abundance, richness, reciprocal of Simpson's diversity index, evenness, Ephemeroptera/Plecoptera/Trichoptera (EPT) richness and percentage of EPT taxa.4. The sensitivity of the disturbance index changes with scale of application however, and the clearfelling (CF) index has proven sensitive to the detection of even quite small changes, although in these cases ecological significance should be considered. We show that the CF index, particularly when derived from a regional scale, is a conservative index but is very robust to variation in the number of samples used in its derivation. The application of the index corresponded very well with the application of more standard statistical approaches. We believe that the index can thus be applied to other impact studies with similar project design.

Transport numbers from hydrogen concentration cells over different oxides under oxidising and reducing 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).

Zeolite–Nafion composites as ion conducting membrane materials

Composite membranes formed of zeolitic fillers embedded in Nafion can be made by evaporating the solvents from a suspension of small zeolite crystals in a Nafion solution. Two natural zeolites were selected as fillers: chabazite and clinoptilolite. Membranes with various zeolite content were obtained. Composite membranes with zeolite content up to 40 vol.% exhibited uniform distribution of the zeolite fillers throughout the thickness, as ascertained by scanning electron microscopy (SEM), energy dispersive X-rays absorption (EDX) and ion conductivity measurements. Although more brittle than Nafion, these composite sheets retain—when hydrated—reasonable flexibility below 40 vol.% zeolite content. We have determined the hydrogen ion conductivity and the permeability of methanol molecules through these membranes (in the H +-form as well as in the Na +-form) in the temperature range 22–60 °C. We found that the presence of the zeolitic fillers in the membranes can bear notable changes of conductivity, permeability and selectivity with respect to membranes made out of sole Nafion.

Proton and apparent hydride ion conduction in Al-substituted SrTiO 3

Hydrogen ion conductivity in 2% and 10% Al-substituted SrTiO 3 has been investigated by transport number measurements using the concentration cell/emf method in wet atmospheres as a function of pO 2 (10 −20–1 atm) and temperature (350–1000 °C). Earlier indications of apparent negative charge transport by hydrogen under reducing conditions and high temperatures have been confirmed. By the present measurements, possible artefacts from the type of acceptor-dopant, gas buffer, electrode material, and porosity of the sample appear to have been ruled out. Electrochemical pumping experiments with gas chromatography were inconclusive with respect to hydride ion transport. Thermogravimetry as a function of hydrogen activity did not show evidence of hydride ion incorporation, but indicated uptake of neutral hydrogen under reducing conditions and high temperatures. Quantum molecular dynamics simulations indicate the existence of defect species or clusters that may be reminiscent of interstitial hydrogen with a tendency to associate with effective negative charge, e.g., on neighbouring titanium ions, under simulated reducing conditions.

Polymer electrolyte membranes formed of sulfonated polyethylene

Sulfonated polyethylene membranes have been obtained by reacting flat sheets of commercial polyethylene with a gaseous mixture of chlorine and sulfur dioxide with subsequent hydrolysis of the sulfonyl chloride groups. Polyethylene materials with various densities have been utilized. Depending on the polyethylene density and on the operating conditions of the chlorosulfonation reaction, membranes with various contents of acidic sulfonate groups have been produced. These membranes have been characterized for the ion-conduction and also for the permeation properties of methanol molecules at various temperatures between 25 and 60 °C. Some of these membranes showed hydrogen ion conductivity near 1×10 −1 Ω −1 cm −1 at room temperature.

Hydrogen ion conduction in iron-substituted strontium titanate, SrTi 1− xFe xO 3− x/2 (0≤ x≤0.8)

Conduction of hydrogen species in selected compositions of SrTi 1− x Fe x O 3− x/2 (0≤ x≤0.8) has been investigated by ac conductivity and concentration cell emf transport number measurements, as a function of p O 2 , p H 2O and temperature (500–1100°C). The main contributions to the total conductivity are n-type electronic, dominating at low p O 2 , ionic (oxygen vacancies) at intermediate p O 2 , and p-type electronic at high p O 2 . Hydrogen ion conduction is measurable, but the hydrogen containing species are minority defects in this system at the temperatures investigated, both in terms of concentration and conductivity. When the conditions are not too reducing, hydrogen ions are positive, assumed to be protons, H +. At lower p O 2 , however, there is a change in the sign of the emf of the water vapour concentration cell, and thus apparently of the charge of the hydrogen-containing charge carrying species. This can be rationalised by the presence and migration of substitutionally dissolved hydride ions, H −, but it is questioned whether these can be stable under the conditions used.