Proton-conducting Oxide Research Articles

Density functional theory calculations for cathode materials of proton-conducting solid oxide fuel cells: A mini-review

Density functional theory (DFT) calculations have been widely used to investigate insights for electrochemical materials, but its application in proton-conducting solid oxide fuel cells has just started a few years ago. Despite the limited time, the DFT calculation is regarded as a powerful tool to anticipate the properties of the proton-conducting oxides with success, bring insights into the material design that promotes the electrochemical performance of the cells. Furthermore, the DFT calculation provides a solution to reveal the potential proton conduction in the cathode materials for proton-conducting solid oxide fuel cells that is difficult to achieve by experimental approaches. However, some limitation for the current DFT application has also been observed. This mini-review briefly documents the application of DFT calculations for the cathodes of proton-conducting solid oxide fuel cells, focusing on the exploration of new cathode materials with high electrochemical performance. Both success stories and the current limitations have been presented, aiming to arouse the attention of the community for this interesting method.

Exploring the Effect of NiO Addition to La0.99Ca0.01NbO4 Proton-Conducting Ceramic Oxides

To improve the performance and overcome the processing difficulties of La0.99Ca0.01NbO4 proton-conducting ceramic oxide, external and internal strategies were used, respectively, to modify La0.99Ca0.01NbO4 with NiO. The external strategy refers to the use of the NiO as a sintering aid. The NiO was added to the synthesized La0.99Ca0.01NbO4 powder as a secondary phase, which is the traditional way of using the NiO sintering aid. The internal strategy refers to the use of NiO as a dopant for the La0.99Ca0.01NbO4. Both strategies improve the sinterability and conductivity, but the effect of internal doping is more significant in enhancing both grain growth and conductivity, making it more desirable for practical applications. Subsequently, the influences of different concentrations of NiO were compared to explore the optimal ratio of the NiO as the dopant. It was found that the sample with 1 or 2 wt.% NiO had similar performance, while with 5 wt.%, NiO doping content hampered the grain growth. In addition, the inhomogeneous distribution of the element in the high-NiO content sample was found to be detrimental to the electrochemical performance, suggesting that the moderate doping strategy is suitable for La0.99Ca0.01NbO4 proton-conducting electrolyte with improved performance. Furthermore, first-principle calculations indicate the origin of the enhanced performance of the internally modified sample, as it lowers both oxygen formation energy and hydration energy compared with the un-modified one, facilitating proton migration.

Direct-Hydrocarbon Proton-Conducting Solid Oxide Fuel Cells

Solid oxide fuel cells (SOFCs) are promising and rugged solid-state power sources that can directly and electrochemically convert the chemical energy into electric power. Direct-hydrocarbon SOFCs eliminate the external reformers; thus, the system is significantly simplified and the capital cost is reduced. SOFCs comprise the cathode, electrolyte, and anode, of which the anode is of paramount importance as its catalytic activity and chemical stability are key to direct-hydrocarbon SOFCs. The conventional SOFC anode is composed of a Ni-based metallic phase that conducts electrons, and an oxygen-ion conducting oxide, such as yttria-stabilized zirconia (YSZ), which exhibits an ionic conductivity of 10−3–10−2 S cm−1 at 700 °C. Although YSZ-based SOFCs are being commercialized, YSZ-Ni anodes are still suffering from carbon deposition (coking) and sulfur poisoning, ensuing performance degradation. Furthermore, the high operating temperatures (>700 °C) also pose challenges to the system compatibility, leading to poor long-term durability. To reduce operating temperatures of SOFCs, intermediate-temperature proton-conducting SOFCs (P-SOFCs) are being developed as alternatives, which give rise to superior power densities, coking and sulfur tolerance, and durability. Due to these advances, there are growing efforts to implement proton-conducting oxides to improve durability of direct-hydrocarbon SOFCs. However, so far, there is no review article that focuses on direct-hydrocarbon P-SOFCs. This concise review aims to first introduce the fundamentals of direct-hydrocarbon P-SOFCs and unique surface properties of proton-conducting oxides, then summarize the most up-to-date achievements as well as current challenges of P-SOFCs. Finally, strategies to overcome those challenges are suggested to advance the development of direct-hydrocarbon SOFCs.

Isotopic effect of proton conductivity in gadolinium sesquioxide

Due to the higher hydrogen solubility and diffusivity, gadolinium sesquioxide (Gd2O3) could be expected as a proton conductive material. In this study, monoclinic Gd2O3 was used to evaluate the possible proton conductivity in Ar (σAr), Ar+4% H2 (σH2), Ar+4% D2 (σD2), Ar+H2O (σH2O), Ar+D2O (σD2O), and O2 (σO2) atmospheres in the temperature range from 500°C to 1000°C, to understand its possible application in the DT fuel system of a nuclear fusion reactor. At all temperatures, the conductivity σD2 was lower than that of the σH2. Similarly, the σD2O was always lower than that of the σH2O. The isotope dependence was appreciable at higher temperatures, indicating that Gd2O3 is a proton-conductive oxide at elevated temperatures. However, much improvement is required for the use of Gd2O3 in the DT fuel recovery, analysis, and purification system of fusion reactors.

Enhanced La0.6Sr0.4Co0.2Fe0.8O3–δ-based cathode performance by modification of BaZr0.1Ce0.7Y0.1Yb0.1O3–δ electrolyte surface in protonic ceramic fuel cells

Effect of the cathode/electrolyte interface modification by applying a porous BaZr0.1Ce0.7Y0.1Yb0.1O3–δ (BZCYYb) interlayer (PBI) on cathode performance in protonic ceramic fuel cells (PCFCs) is investigated to achieve a lower temperature operation with high power density. Here, in a PCFC with anode-supported configuration, a proton-conducting oxide of BZCYYb is used as the material for both the interlayer and electrolyte, and demonstrates that this interlayer improves the cathode/electrolyte interfacial structure by increasing the electrochemically active triple-phase boundary (TPB). This BZCYYb interlayer has a porous structure formed on the dense BZCYYb electrolyte. A cathode material that is a composite of La0.6Sr0.4Co0.2Fe0.8O3–δ (LSCF) and BZCYYb fine particulates is impregnated into the submicrometer to single-micrometer pores within the PBI, yielding a three-dimensionally extended TPB region. This structure effectively enhances the cathode performance, and the resulting maximum power density of an anode-supported PCFC with the PBI reaches 1.05, 0.81, 0.57, 0.38, and 0.24 W cm−2 at 700, 650, 600, 550, and 550 °C, respectively. In conclusion, the modification of the cathode/electrolyte structure by applying a PBI is a simple, yet very effective approach to improve PCFC performance even when the cathode is a conventional material such as LSCF.

Determination of Oxide Ion Conductivity in Ba-Doped LaYbO3 Proton-Conducting Perovskites via an Oxygen Isotope Exchange Method

Proton-conducting oxides have garnered much attention as electrolyte materials of electrochemical devices operating at intermediate temperatures ranging 400–700 °C. For the implementation, quantifying the transport number of oxygen ions is crucial to predict the fuel utilization efficiency of solid oxide fuel cells (SOFCs). The acceptor-doped LaYbO3 proton conductors are one of the candidates for the electrolyte material of electrochemical devices such as SOFCs because of their high pure proton conduction. However, the oxide ion conductivity in this material is not quantified yet. Here, we evaluate the tracer diffusion coefficient (D*) of 5 mol % Ba-doped LaYbO3 by the oxygen isotope exchange and depth profile technique using secondary ion mass spectrometry. It is found that the oxygen ion conductivities in this material are lower than 10–4 S cm–1 below 800 °C, less than 2% to the total conductivity. We also visualize the domain of dominant conduction in this material. It reveals that proton conduction is dominant in fairly wide areas of the atmospheric condition, in particular below 600 °C, suggesting that the Ba-doped LaYbO3 is suitable for the SOFC electrolyte in terms of transport number.

Solid oxide proton conductors beyond perovskites

Solid oxide proton conductors have application in hydrogen-based energy technologies. This perspective presents an overview of the structural and mechanistic aspects of proton conduction in oxide systems outside conventional perovskite conductors.

Catalytic methane activation over La1−xSrxScO3−α proton-conducting oxide surface: A comprehensive study

The detailed investigation of the influence of strontium on the methane activation over the state-of-the-art proton-conducting oxide catalyst La1−xSrxScO3−α was carried out using a combination of the novel H/D isotopic exchange method, Raman Spectroscopy, 1H NMR and DFT studies in the temperature range 673 – 973 K, at 10 mbar of CH4 + H2 (95%+5%) mixture. It was found that methane activation occurs via the four parallel channels of the hydrogen adsorption and incorporation mechanism, followed by the formation of methane intermediates with different hydrogen content (CHx species). The dominant type of adsorbed intermediate is governed by the temperature and the surface defect structure, which is in close correlation with the strontium content. The increase of strontium content in La1−xSrxScO3−α drastically increases the catalytic activity due to the surface modification with oxygen deficiencies in the ScO6 octahedron, and stabilisation of the CHx species with a low amount of hydrogen.

Evaluation of the hydrogen solubility and diffusivity in proton-conducting oxides by converting the PSL values of a tritium imaging plate

Proton-conducting oxides have potential applications in hydrogen sensors, hydrogen pumps, and other electrochemical devices including the tritium purification and recovery systems of nuclear fusion reactors. Although the distribution of hydrogen (H) in such oxide materials is an important aspect, its precise measurement is difficult. In the present study, the hydrogen solubility and diffusivity behavior of BaZr0.9Y0.1O2.95 (BZY), BaZr0.955Y0.03Co0.015O2.97 (BZYC), and CaZr0.9In0.1O2.95 (CZI) were studied using tritiated heavy water vapor i.e., DTO (~2 kPa, tritium (T) = 0.1%) by converting the photostimulated luminescence (PSL) values of the imaging plate (IP). The samples were exposed to DTO vapor at 673 K for 2 h or at 873 K for 1 h. The disc-shaped oxide specimens (diameter ~7.5 mm; thickness ~2.3 mm; theoretical density (TD) > 98 %) were prepared by conventional powder metallurgy. The IP images of the specimen surfaces of all the three materials T-exposed revealed that BZY showed the most uniform T distribution with the highest tritium activity. The cross-sectional T concentration profiles of the cut specimens showed that T diffused deeper into BZY and BZYC than into CZI. The hydrogen solubility and diffusivity in the CZI specimen were lower than that in the BZY and BZYC specimens. This suggested that barium zirconates were more favorable proton conductors than calcium zirconates.

Hydration-induced chemical expansion of BaCa(1+y)/3Nb(2−y)/3O3−δ∙xH2O (BCN) and other proton-conducting perovskite oxides

Thermal expansion and chemical strain caused by water uptake were measured for BaCa(1+y)/3Nb(2−y)/3O3−δ∙xH2O (BCN) perovskite-type oxides by dilatometric and X-ray diffraction methods. To describe the total and chemical expansion, we proposed a simple geometric chemical strain model based on the relative change in ionic radii upon hydration. The model calculations agree well with the experimental data for BCN oxides, for which the values of the linear chemical expansion coefficient, β, were found to be around 0.038 per mole of water-filled oxygen vacancies. Next, the model was applied to different perovskite-type proton-conducting oxides with existing literature data on chemical expansion. For example, the values of β, estimated using the model, are around 0.036 and 0.031 for BaZr1−yMyO3−δ and BaCe1−yMyO3−δ, respectively (M = rare-earth metal, Y or Sc). These values are close to those obtained by others using either density functional theory (DFT) calculations or direct experimental measurements, especially given the rather large scatter in the data reported in the literature. The latter is most probably related to the kinetic limitations since, unlike the most oxygen exchange phenomena, humidification of oxides happens at lower temperatures where the slow kinetics of water uptake plays an important role.

Oxygen Affinity: The Missing Link Enabling Prediction of Proton Conductivities in Doped Barium Zirconates

Proton-conducting oxides, specifically doped barium zirconates, have garnered much attention as electrolytes for solid-state electrochemical devices operable at intermediate temperatures (400–600 °...

Novel Proton-Conducting Oxygen-Deficient Complex Oxides: Synthesis, Hydration Processes, Transport Properties

The complex oxide BaLaIn0.9Nb0.1O4.1 with Ruddlesden-Popper structure was obtained for the first time. It was found that the introduction of niobium into indium sublattice leads to the increase in the cell volume. Hydration processes and electrical properties have been investigated. For BaLaIn0.9Nb0.1O4.1 it was proved the capability for water uptake and the appearance of proton current carriers. It was established that niobium doping leads to the increase of conductivity compared to undoped composition BaLaInO4 at ~1 order of magnitude in whole temperature range.

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.

A Computational Study of Fast Proton Diffusion in Brownmillerite Sr2Co2O5

Solid oxide proton conductor materials are crucial for enabling hydrogen-based energy applications such as solid oxide fuel cells and solid oxide membranes. The improvement in the performance and e...

LaScO3-based electrolyte for protonic ceramic fuel cells: Influence of sintering additives on the transport properties and electrochemical performance

In this work, the effect of the content of CuO, ZnO, Al2O3, NiO, Fe2O3, Co3O4 and MoO3 additives on the sinterability, microstructure, and electrical properties of La0.9Sr0.1ScO3-α proton-conducting oxide in a form of tubes produced by slip casting is studied. It is shown that the relative density of La0.9Sr0.1ScO3-α tubes without sintering aids is ~86%. The introduction of 1.0 wt % ZnO, Al2O3, NiO or 0.5 ÷ 1.0 wt % Co3O4 leads to a relative density of ceramics higher than 95%. Among dense ceramic tubes, LSS +0.5 wt % Co3O4 has the highest electrical conductivity. The introduction of 1 wt % Fe2O3 additive does not affect the sinterability of the LSS, and 1 wt % CuO and MoO3 inhibits LSS sintering. Single protonic ceramic fuel cells based on LSS +0.5 wt % Co3O4 and LSS +1.0 wt % NiO supporting electrolyte tubes with a thickness of 300 μm are studied. The maximum power density of about 30 mW/cm2 at 800 °C was obtained for a cell based on LSS +0.5 wt % Co3O4 using wet hydrogen as fuel and air as an oxidizing gas.

Sintering and electrochemical performance of Nd2Ce2O7 electrolyte with Bi7WO13.5 sintering aid for proton conductor solid oxide fuel cells

Different molar ratios Bi 7 WO 13.5 (BWO) as sintering aids are used to modify Nd 2 Ce 2 O 7 (NCO) material by mechanical method. The influence of Bi 7 WO 13.5 additions on the phase structure, linear shrinkage, micromorphology and electrical conduction behaviors of Nd 2 Ce 2 O 7 are characterized by X-Ray diffraction(XRD), dilatometry technique, Raman spectroscopy, Scanning electron microscopy(SEM) and Electrochemical impedance spectroscopy(EIS). All modified samples still retain the fluorite-type structure. BWO additives in all molar ratios greatly improve the sintering ability. It is found that 5mol% BWO doped NCO(NCO-5) promote the linear shrinkage from 6.60% to 21.23% at 1300 °C. Electrochemical characterizations reveal that high total conductivities of all modified ceramics are obtained through this modified method. The conductivity of modified NCO sample sintered at 1300 °C is higher than that of unmodified NCO pellet sintered at 1500 °C. Especially the NCO-5 pellet offers 1.38 × 10 −2 and 2.26 × 10 −2 Scm −1 at 750 °C in dry air and 5% H 2 -Ar conditions, respectively. Furthermore, an anode-supported single cell with NCO-5 electrolyte is manufactured and test, the cell displays a peak power density of 613 mWcm −2 at 750 °C. All these results indicate the BWO can be used as sintering aids for NCO electrolyte to improve the sintering ability and electrochemical performance. • Bi 7 WO 13.5 was used as the sintering aids for Nd 2 Ce 2 O 7 proton-conducting oxide. • High sinterability and improved electrical conductivity were achieved at lower sintered temperatures. • A single cell with NCO-5 electrolyte is successfully fabricated with higher performance.

Electrochemical Analysis of Anode Supported Pcfc with Pseudo Symmetrical Cell

Protonic Ceramic Fuel Cell (PCFC) is expected to be the next-generation fuel cell for its higher energy conversion efficiency and lower operation temperature. BaZrO3 based oxide are potential materials as PCFC electrolyte because they have a high chemical stability under CO2 condition and high proton conduction. [1] Recently, we have developed PCFC using BaZr0.8Yb0.2O3-δ (BZYb) as the electrolyte. The proton conductivity of the BZYb pellet was 1.1×10-2 S/cm at 600℃ and did not form the complex oxide with NiO. The power density of the developed cell (Ni-BaZr0.8Yb0.2O3-δ|BaZr0.8Yb0.2O3-δ|La0.6Sr0.4CoO3-δ) achieved 0.49 W/cm2 at 600℃.To improve the performance of anode supported PCFC, anode supported pseudo symmetrical cell:Ni-BaZr0.8Yb0.2O3-δ|BaZr0.8Yb0.2O3-δ|Niwas manufactured and analyzed by impedance spectroscopy under the various conditions and temperatures (Figure).The thickness dependence of the ohmic resistance was measured for three types of electrolyte (thickness: 8 μm, 16 μm and 25 μm) under 48%H2-49%N2-3%H2O gas condition at 300-700℃. The conductivity was calculated from the relation between electrolyte thickness and the ohmic resistance, which was 0.35×10-2 S/cm at 600℃. The degradation of proton conductivity occurs due to Ni dissolution. The performance of anode supported PCFC might be improved further if the Ni dissolution to electrolyte can be inhibited.Reference[1] K. D. Kreuer, Proton-conducting oxides, Annu. Rev. Mater. Res., 33 (2003) 333–359. Figure 1

Three-Dimensionally Micro-Architected Protonic Ceramic Fuel Cells

The solid oxide fuel cells (SOFCs) have been considered as important alternative energy technology, due to a variety of advantages, such as their high energy conversion efficiency, free noxious emissions, and direct use of a wide range of hydrocarbon fuels. Especially, the reverse operation of SOFCs, which is called solid oxide electrolysis cell (SOEC), is currently one of the most promising technologies for fuel production. However, their high-temperature operation (typically >700°C) leads to the rapid degradation of electrodes and necessitates a high balance of plant costs. To make SOFCs more economically feasible and viable, further reduction of operating temperature is inevitable. To this end, the utilization of proton-conducting oxides, which exhibit relatively high ionic conductivity with the low activation energy (<0.5eV), enables protonic ceramic fuel cells (PCFCs) to operate at an intermediate temperature regime (IT, 400–600°C), where the aforementioned benefits of SOFCs can also be accomplished. Moreover, PCFCs prevents the dilution of the fuel during the operation and provides higher resistance of carbon cocking on hydrocarbon fuels.Although the PCFCs have distinct advantages against traditional oxygen conducting SOFCs, the lower-than-predicted performance of PCFCs and scalability issues caused the considerable skepticism of the successful applicability of PCFCs. Most importantly, unlike traditional SOFCs, the development of mixed oxide ion and electron conductors (MIECs) for oxygen reduction reaction (ORR) at the cathode of PCFCs has shown marginal success due to the absence of protonic conduction and still needs to be improved. At the cathode, the ORR is limited to triple phase boundary (TPB), where the electrolyte, gas, and cathode mutually contact, and its geometrical enhancement at the interface area between cathode and electrolyte layers is crucial to improve the performance of PCFCs. Therefore, electrode-electrolyte interface architecting via micro-patterning is one of the most attractive approaches, since it significantly expands the catalytic active sites of TPB near the interface, where ORR preferentially occur. Moreover, this architecting approach also facilitates the use of currently viable state-of-art cathode materials of SOFCs; Lanthanum Strontium Cobalt Ferrite (LSCF), Barium Strontium Cobalt Ferrite (BSCF). Therefore, patterns with a high aspect ratio, which maximize the interface area, are required for significant enhancement of electrochemical performance.Here, for the first time, we describe a three-dimensionally micro-architecting process for anode-supported PCFCs, demonstrating dramatic performance enhancement from the expanded interface area between cathode and electrolyte. The various patterns with high ratios were exactly transferred from the original PUA (polyurethane acrylate) molds to the thin and dense protonic conducting electrolyte layer. The patterns maintained the same aspect ratios during the sintering process and finally increased the interface area of more than 30%. The morphology of the patterned surface was characterized by scanning electron microscope (SEM) and atomic force microscopy (AFM). The AC electrochemical impedance spectroscopy was utilized to examine the electrochemical performances of patterned cells. Our unique patterning method opens the new pathway for further performance enhancement of PCFCs. Figure 1

Fast Magneto-Ionic Switching of Interface Anisotropy Using Yttria-Stabilized Zirconia Gate Oxide.

Voltage control of interfacial magnetism has been greatly highlighted in spintronics research for many years, as it might enable ultralow power technologies. Among a few suggested approaches, magneto-ionic control of magnetism has demonstrated large modulation of magnetic anisotropy. Moreover, the recent demonstration of magneto-ionic devices using hydrogen ions presented relatively fast magnetization toggle switching, tsw ∼ 100 ms, at room temperature. However, the operation speed may need to be significantly improved to be used for modern electronic devices. Here, we demonstrate that the speed of proton-induced magnetization toggle switching largely depends on proton-conducting oxides. We achieve ∼1 ms reliable (>103 cycles) switching using yttria-stabilized zirconia (YSZ), which is ∼100 times faster than the state-of-the-art magneto-ionic devices reported to date at room temperature. Our results suggest that further engineering of the proton-conducting materials could bring substantial improvement that may enable new low-power computing scheme based on magneto-ionics.

Self-sustainable protonic ceramic electrochemical cells using a triple conducting electrode for hydrogen and power production

The protonic ceramic electrochemical cell (PCEC) is an emerging and attractive technology that converts energy between power and hydrogen using solid oxide proton conductors at intermediate temperatures. To achieve efficient electrochemical hydrogen and power production with stable operation, highly robust and durable electrodes are urgently desired to facilitate water oxidation and oxygen reduction reactions, which are the critical steps for both electrolysis and fuel cell operation, especially at reduced temperatures. In this study, a triple conducting oxide of PrNi0.5Co0.5O3-δ perovskite is developed as an oxygen electrode, presenting superior electrochemical performance at 400~600 °C. More importantly, the self-sustainable and reversible operation is successfully demonstrated by converting the generated hydrogen in electrolysis mode to electricity without any hydrogen addition. The excellent electrocatalytic activity is attributed to the considerable proton conduction, as confirmed by hydrogen permeation experiment, remarkable hydration behavior and computations.