Synthesis,phase stability and conduction behavior of rare earth and transition elements doped barium cerates

BaCeO₃ exhibits high protonic conductivity, however, it suffers from low chemical stability. In this study, BaCeO₃ was doped with erbia (BCE) and BCE was co-doped with ytterbia (Yb-BCE) and scandia (Sc-BCE) with a view to improve the chemical stability. The undoped sample (BaCeO₃) disintegrated into BaCO₃ and CeO₂ on exposure to CO₂ while doped (BCE) and co-doped compositions (Yb-BCE and Sc-BCE) remained almost unaffected indicating effectiveness of the dopant Er³⁺ and the co-dopants (Yb³⁺, Sc³⁺) in improving the chemical stability. The Sc-BCE composition showed better stability. Exposure to water atmosphere led to formation of Ba(OH)₂ in BCE and Yb-BCE samples while the Sc-BCE sample exhibited much better stability against water vapor as well. A modified solution combustion synthesis was found to be better for phase formation by calcinations at comparatively lower temperatures. The conduction mode was found to be ionic in dry air and mixed protonic–ionic in wet air atmosphere. In wet Argon–Hydrogen mix (AHM), the conduction mode was primarily protonic. The co-doped compositions, Yb-BCE and Sc-BCE showed higher conductivity compared to BCE and Yb-BCE exhibited the best conductivity in wet AHM.     

Yttrium dependent space charge effect on modulating the conductivity of barium zirconate electrolyte for solid oxide fuel cell

Development of high proton conducting, chemically stable electrolyte for solid oxide fuel cell application still remains as a major challenge. In this work, yttrium (0, 5, 10, 15 and 20 mol%) doped barium zirconate synthesised by hydrothermal assisted coprecipitation exhibited highly crystalline cubic perovskite. The results demonstrate that the proton conductivity is higher than oxygen ion conductivity measured in the temperature range of 200–600 °C. The 20 mol% Y doped BaZrO₃ exhibited higher protonic conductivity (6.1 mScm^?1) with an activation energy 0.64 eV under the reducing atmosphere. The Mott–Schottky analysis carried out in hydrogen atmosphere at 200 °C revealed that the barrier height of doped BaZrO₃ reduced from 0.6 to 0.2 V. The Schottky depletion layer width also decreased from 4 to 2 nm with the increase in yttrium concentration and the boiling water test showed good phase stability. Our study highlights the critical role of space charge in the grain boundary and its suppression with the increase in dopant concentration. The results demonstrate that Y doped BaZrO₃ sintered at low temperature is a promising candidate as the electrolyte material for the intermediate temperature proton conducting solid oxide fuel cells.     

Electrochemical impedance spectroscopy of BaCeO3 modified by Ti and Y

Barium cerate exhibits high protonic conductivity, especially when modified by trivalent dopant such as Y, Yb, Nd, Sm or Dy. Unfortunately, the poor chemical stability in the presence of CO₂ is the main disadvantage of this material. One of the possible approach to get the stable protonic conductor is the preparation of solid solutions. For example, doping of BaCeO₃ with Zr leads to the improvement of the chemical stability, but the electrical properties are simultaneously corrupted.In the present work the influence of Ti, per analogy to Zr, and Y dopants on electrical properties of BaCeO₃ was investigated using the electrochemical impedance spectroscopy (EIS) technique. BaCe_1−xTi_xO_3−δ (0 ≤ x ≤ 0.3) and Ba(Ce_0.95Ti_0.05)_0.95Y_0.05O_3−δ solid electrolytes were prepared by solid-state reaction method. It was found that the changes of electrical properties due to the introduction of Ti into the BaCeO₃ lattice is caused predominantly by the modification of the grain boundary properties. The Ti doping leads to the substantial decrease of grain boundary electrical conductivity, comparing to undoped material. The introduction of yttrium dopant to the BaCe_0.95Ti_0.05O₃ lattice has the opposite effect. The total electrical conductivity increases, due to significant modification of grain boundary electrical properties.     

Indium as an ideal functional dopant for a proton-conducting solid oxide fuel cell

A high In-dopant level BaCeO₃ material was used as an electrolyte for a proton-conducting solid oxide fuel cell (SOFC). Indium behaved as an ideal dopant for BaCeO₃, which improved both the chemical stability and sinterability for BaCeO₃ greatly. The anode supported BaCe_0.7In_0.3O_3−δ (BCI30) membrane reached dense after sintering at 1100 °C, much lower than the sintering temperature for other BaCeO₃-based materials. Additionally, the BCI30 membrane showed adequate chemical stability against CO₂ compared with the traditional rare earth doped BaCeO₃. The BCI30-based fuel cell also showed a reasonable cell performance and a good long-term stability under the operating condition. Besides, the LaSr₃Co_1.5Fe_1.5O_10−δ (LSCF) was also evaluated as a potential cathode candidate for a proton-conducting SOFC.     

Thermionic emission of protons across a grain boundary in 5 mol% Y-doped SrZrO3, a hydrogen pump

Given the fact that polycrystalline SrZrO₃ doped with 5 mol% Y³⁺ (p-SZY5), a proton-conducting solid electrolyte with superior chemical stability, has attracted considerable attention over the past years due to its potential for various applications including hydrogen pumps, it is rather unusual that such little information is available in literature regarding the electrical nature of the grain boundary in this material, although the grain boundary may determine its proton conductivity. Here we report the results of our investigation on the conduction mechanism at the grain boundary in p-SZY5. We clearly demonstrate that the current–voltage (I–V) characteristic of the grain boundary is consistent with thermionic emission current over a back-to-back Schottky barrier, suggesting that such a potential barrier is present at the grain boundary in p-SZY5 to limit the internal proton current. More importantly, based on the capacitance–voltage relation, we were able to deduce the values of both the potential-barrier height and the dopant concentration in p-SZY5.     

Proton-conducting barium stannates: Doping strategies and transport properties

High temperature proton conductors (HTPCs) find their applications in steam electrolysis, gas sensing, and most importantly, fuel cells. In this work, proton conductivities and transport properties of doped BaSnO₃ are investigated. Samples of BaSn_0.9D_0.1O_2.95 (D = In, Lu, Er, Y, and Gd) were prepared by solid-state reaction and relative densities >93% were achieved after sintering at 1600 °C or lower. Although Y doping is commonly known to yield the highest conductivities for BaCeO₃ and BaZrO₃, In-doped BaSnO₃ exhibits the highest conductivity and conductivities decrease in the order In > Lu > Er > Y > Gd. Ionic radius and electronegativity matching between dopants and host Sn⁴⁺ is shown to be an important doping strategy for enhancing conductivities of BaSnO₃. Measurements of H/D isotope effect and electromotive force (EMF) were performed on BaSnO₃ to give direct evidence for proton conduction and to examine transport properties. The ratio between conductivities in H- and D-atmosphere (σ_H/σ_D) is 2.62 in reducing conditions, indicating protons transfer via the Grötthus mechanism. Proton transport numbers reached above 0.75 at 450 °C, and n- and p-type electronic conduction is identified to be secondary in reducing and oxidizing atmospheres, respectively. Electronic contribution to conductivity is found to increase with temperatures. Conductivities of BaSnO₃ are seen to be comparable to Y-doped BaZrO₃ (e.g., grain conductivities for both Y-doped BaSnO₃ and BaZrO₃ are ∼1.5 × 10⁻⁴ S cm⁻¹ at 350 °C), and are much higher than other common HTPCs, such as LaPO₄ and LaNbO₄. The high conductivity and good sinterability make BaSnO₃ a promising HTPC.     

Atomistic insight into the hydration and proton conducting mechanisms of the cobalt doped Ruddlesden-Popper structure Sr3Fe2O7-δ

Sr₃Fe₂O_7-δ (SFO) with two-layer Ruddlesden-Popper (R–P) structure has recently been proved to be a promising material for the single phase cathode in proton conducting solid oxide fuel cells (P–SOFCs). To investigate the hydration reactions and proton conducting mechanisms of SFO and cobalt doped SFO (SFCO), both bulk and surface properties were calculated. We conclude that R–P structures have advantages in P–SOFCs. The unique Sr–O–M layer can facilitate the hydration process. Although in Sr–O–F and Sr–O–N layers, it is difficult for the formation and migration of oxygen vacancies, protons are most stable. Furthermore, cobalt doping can not only improve the electronic conductivity but also enhance surface properties of SFCO. The easily exposed Co–Fe–O surface can also facilitate the hydration reactions on the surface. Our work could give an informative insight into the relationships among the doped elements, the R–P structures, the hydration process and the proton conducting properties.     

Physicochemical properties of Ba(Zr,Ce)O3-δ-based proton-conducting electrolytes for solid oxide fuel cells in terms of chemical stability and electrochemical performance

To enhance the power generation efficiency of solid oxide fuel cells (SOFCs), the use of proton-conducting solid solutions of doped BaCeO₃ and doped BaZrO₃, with formulas of the Ba(Zr_0.1Ce_0.7Y_0.1X_0.1)O_3-δ (X = Ga, Sc, In, Yb, Gd), was investigated as SOFCs electrolyte materials with respect to both chemical stability and electrical conductivity. Regarding chemical stability, the weight changes of each material were measured under a CO₂ atmosphere in a temperature range of 1200 °C–600 °C.Higher chemical stability was observed for dopant ions with smaller radii. Regarding conductivity, the dependences of the total conductivities on the oxygen partial pressure and temperature were measured in the temperature range of 600 °C–900 °C. In each material, the total conductivity was proportional to the oxygen partial pressure to the 1/4 power at high oxygen partial pressures, as previously observed for accepter-doped proton-conducting perovskite-type oxides. The derived conductivities for each type of charge carrier showed that the hole conductivity increased with the ionic conductivity. Based on the measured data, the leakage current densities were calculated for SOFCs with each of the investigated electrolyte materials and an area-specific resistance of 0.383 Ωcm². BZCYSc showed the minimum leakage current density, with a value of 3.7% of the external current density at 600 °C. Therefore, this study indicates that BZCYSc is the most desirable among the materials investigated for use as SOFCs electrolyte. However, for BZCYSc to be used as SOFCs electrolyte material, a protective layer is needed to ensure its chemical stability.     

Doping effects on complex perovskite Ba3Ca1.18Nb1.82O9−δ intermediate temperature proton conductor

In this work, the effects of Ce doping on the Ca and Nb ions in complex perovskite Ba₃Ca_1.18Nb_1.82O_9−δ (BCN18) proton conductor have been evaluated. It has been found that cerium ions can be doped into both the Ca and Nb sites to form a single-phase complex perovskite structure when the sintering temperature is 1550 °C. Ce ions substituted with Nb ions enhances the electrical conductivity, especially the grain boundary conductivity. The highest conductivity has been obtained for a composition of Ba₃Ca_1.18Nb_1.62Ce_0.2O_9−δ, possessing a conductivity of 2.69 × 10⁻³ S cm⁻¹ at 550 °C in wet H₂, a 78% enhancement compared with BCN18 (1.51 × 10⁻³ S cm⁻¹). The chemical stability tests show that Ce-doped BCN18 samples remain single phase after treated either in boiling water for 7 h or in pure CO₂ for 4 h at 700 °C. This work has demonstrated a new direction in developing intermediate temperature proton conducting materials that possess both high conductivity and good stability.     

Electrical conduction behavior of proton conductor BaCe1-xSmxO3-δ in the intermediate temperature range

Determining the relationship between electrical conductivity and doping level in high temperature proton conductors is of great significance to accelerate their process of practicality and develop new applications. In this work, BaCe_1-xSm_xO_3-δ (x = 0.01-0.50) was synthesized via citric-nitrate method and their electrical conduction behavior in 5% H₂/Ar in the intermediate temperature range was investigated. The solubility of Sm in BaCeO₃ was between 0.30 and 0.40. Both the bulk and the grain boundary conductivity increased with Sm content up to x = 0.20 and then decreased. The infrared spectra results indicated that the degree of hydrogen bonding decreased with Sm content (x = 0.20-0.30), which should be responsible for the descending bulk conductivity of samples with x = 0.25 and 0.30. BaCe_0.80Sm_0.20O_3-δ, with electrical conductivity of 0.017 S cm⁻¹ at 600 °C, was a promising electrolyte for intermediate temperature solid oxide fuel cells.     

Dense Y-doped ion conducting perovskite films of BaZrO3, BaSnO3, and BaCeO3 for SOFC applications produced by powder aerosol deposition at room temperature

State-of-the-art solid oxide fuel cells (SOFC) are based on oxide ion conducting zirconia electrolytes, typically doped by yttria or scandia. Major drawback of these systems are their high operation temperatures of 800 °C and above. These are necessary for a sufficient ionic conductivity. Instead of oxide ions, also protonic charge carriers could be used in SOFC. The material classes of barium zirconates (BaZrO₃), barium stannates (BaSnO₃), and barium cerates (BaCeO₃) are described as good proton conductors, especially when the B-site of the ABO₃ perovskite structure is aliovalently doped by yttria. Their protonic conductivity values in the moderate temperature regime up to 600 °C are comparable to YSZ at 800 °C, making these compounds ideal candidates for a usage in future SOFC. Unfortunately, very high sintering temperatures up to 1800 °C are required to process dense and therefore gas-tight solid electrolyte membranes. However, a novel spray coating method called powder aerosol deposition (PAD, also known as AD) enables to form fully dense ceramic films directly at room temperature without any necessary sintering processes. Films are deposited from ceramic powders that are accelerated by a dry carrier gas flow under vacuum conditions.In this work, we investigated the film formation of three different barium based perovskite ceramics, namely yttria doped barium zirconate, barium stannate, and barium cerate by powder aerosol deposition. The optical and mechanical quality of films was evaluated using scanning electron microscopy and microhardness indentation and their crystallographic properties were characterized by X-ray diffraction. The electrical behavior was analyzed by electrochemical impedance spectroscopy and DC polarization methods up to temperatures of 1000 °C and 800 °C, respectively. Furthermore, a preliminary study about the film formation on porous electrodes was conducted.     

Chemical stability and hydrogen permeation performance of Ni–BaZr0.1Ce0.7Y0.2O3−δ in an H2S-containing atmosphere

Composite membranes based on Ni and Zr-doped BaCeO₃ are promising for hydrogen separation. Such composites show high proton conductivity and adequate chemical stability in H₂O and CO₂, but may be unstable in H₂S. In this work, the hydrogen permeation performance of Ni–BaZr_0.1Ce_0.7Y_0.2O_3−δ was measured in an H₂S-containing atmosphere at 900 °C. The hydrogen permeation flux began to degrade in 60 ppm H₂S and decreased by about 45% in 300 ppm H₂S. After hydrogen permeation tests, X-ray diffraction analysis revealed the formation of BaS, doped CeO₂, Ni₃S₂ and Ce₂O₂S. Analysis of the microstructure and phase composition, and results of thermodynamic calculations suggest that reaction between H₂S and doped BaCeO₃ caused the performance loss of the Ni–BaZr_0.1Ce_0.7Y_0.2O_3−δ.     

Solid solution limits and electrical properties of scheelite SryLa1-yNb1-xVxO4-δ materials for x = 0.25 and 0.30 as potential proton conducting ceramic electrolytes

The proton conductivity and solid solubility limits of acceptor strontium doped vanadium stabilised lanthanum niobate (Sr_yLa_1?yNb_1-xV_xO_4-δ, x = 0.25, 0.30 and y = 0 to 0.10) were explored as potential proton conducting ceramic electrolytes. All samples were synthesized via a solid-state method. The phase purity, microstructure and thermal expansion behaviour of the materials were studied using powder X-ray diffraction, scanning electron microscopy and dilatometry, respectively. A maximum solid solution limit of 5% Sr in the A-site of Sr_yLa_1?yNb_1-xV_xO_4-δ samples is observed for a vanadium content of x = 0.25, while further increases in the Sr or vanadium contents lead to the presence of Sr₃(VO₄)₂ as a secondary phase. This acceptor dopant content of 5%Sr in the current scheelite material exceeds that possible in the parent vanadium-free fergusonite Sr_yLa_1?yNbO_4-δ material by a factor of 5. All Sr doped scheelite materials show linear thermal expansion behaviour, successfully avoiding the scheelite to fergusonite structural phase change during thermal cycling. The average grain size is shown to be increased by increasing vanadium content. In humid conditions, all phase pure samples show predominantly proton conductivity at lower temperatures, while p-type conductivity is noted at higher temperatures under dry oxidising conditions. In the low temperature range, the Sr_0.05La_0.95Nb_0.75V_0.25O_4-δ sample, containing the largest acceptor dopant concentration, exhibits slightly higher bulk and specific grain boundary conductivities in comparison to other phase pure compositions.     

Effect of strontium and zirconium doped barium cerate on the performance of proton ceramic electrolyser cell for syngas production from carbon dioxide and steam

Syngas has been produced from carbon dioxide (CO₂) and steam using a proton ceramic electrolyser cell. Proton-conducting electrolytes which exhibit high conductivity can suffer from low chemical stability. In this study, to optimize both proton conductivity and chemical stability, barium cerate and doped barium cerate are synthesized using solid state reaction method: BaCeO₃ (BC), Ba_0.6Sr_0.4CeO_3-α (BSC), Ba_0.6Sr_0.4Ce_0.9Y_0.1O_3-α (BSCY), and BaCe_0.6Zr_0.4O_3-α (BCZ). The BC, BSC, and BSCY are calcined at 1100 °C for 2 h and BCZ is calcined at 1300 °C for 12 h, respectively. All samples exhibit 100% perovskite and crystallite sizes equal 37.05, 28.46, 23.65 and 17.46 nm for BC, BSC, BSCY and BCZ, respectively. Proton conductivity during steam electrolysis as well as catalytic activity toward the reverse water gas shift reaction (RWGS) is tested between 400 and 800 °C. The conductivity increases with temperature and the values of activation energy of conduction are 64.69, 100.80, 103.78 and 108.12 kJ mol⁻¹ for BSCY, BC, BSC, and BCZ, respectively. It is found that although BCZ exhibits relatively low conductivity, the material provides the highest CO yield at 550–800 °C, followed by BSCY, BSC, and BC, correlating to the crystallite size and BET surface area of the samples. Catalytic activity toward RWGS of composited Cu and electrolytes is also measured. Additional Cu (60 wt%) significantly increases catalytic activity. The CO yield increases from 3.01% (BCZ) to 43.60% (Cu/BCZ) at 600 °C and CO can be produced at temperature below 400 °C. There is no impurity phase detected in BCZ sample after exposure to CO₂-containing gas mixture (600 °C for 5 h) while CeO₂ phase is detected in BSC and BSCY and both CeO₂ and BaO are observed in BC sample.     

Enhanced hydrogen permeability and reverse water–gas shift reaction activity via magneli Ti4O7 doping into SrCe0.9Y0.1O3−δ hollow fiber membrane

Hydrogen proton conducting perovskite-based hollow fiber membrane is an attractive hydrogen separation technology that shows higher stability relative to Pd-based membranes above 800 °C. One of the challenges towards high hydrogen (H₂) permeability on such proton conducting membrane is enabling simultaneously high proton and electronic conductivities to be achieved in single phase membrane. This has been addressed by developing dual-phase membrane. Here, we showed another promising approach, i.e., exploitation of beneficial phase reactions to create new conductive phases along the grain boundaries. By doping up to 8 wt. % magneli Ti₄O₇ into SrCe_0.9Y_0.1O_3?δ (SCY), Ce-doped SrTiO₃ and Y-doped CeO₂ were created in-between SCY grains. Electrical conductivity tests confirmed higher conductivities for 5 and 8 wt. % Ti₄O₇-doped SCY relative to SCY between 750 and 950 °C. These higher conductivities manifested into higher H₂ permeation fluxes for the doped SCY membranes. The highest flux of 0.17 mL min^?1 cm^?2 was observed for 5 wt. % Ti₄O₇-doped SCY at 900 °C when 50 vol. % H₂/He and 100 vol. % N₂ were used in the feed side and the permeate side, respectively. This is much higher than the flux of 0.05 mL min^?1 cm^?2 obtained from SrCe_0.9Y_0.1O₃ membrane at identical condition. More essential is the fact that the doped SCY membranes displayed catalytic activity for the reverse water-gas shift (RWGS) reaction which consumed H₂ in the permeate side; increasing the H₂ flux up to 0.57 mL min^?1 cm^?2 at 900 °C. The 5 wt. % Ti₄O₇-doped SCY furthermore showed stable flux for more than 140 h at 850 °C despite the formation of minor amount of SrCO₃ in H₂-CO₂-containing atmosphere; highlighting its potential application as membrane reactor for RWGS or dehydrogenation reaction.     

Chemical stability and electrical and mechanical properties of BaZrxCe0.8-xY0.2O3 with CeO2 protection method

This study investigated the decline in the conductivity and mechanical strength after CO₂ poisoning and found a new protective method for BaZr_xCe_0.8-xY_0.2O₃ proton-conducting electrolyte. The high temperature solid state reaction (SSR) was used in synthesizing electrolyte to naturally generate CeO₂ on the surface. A comparison of the oxides in the conductivity decline test revealed that the sample with CeO₂ on the surface substantially improved the stability of conductivity, reducing the decline ratio from 56% to 7% for BCY electrolyte and 50% to 7% for BCZY sample. Raman mapping results indicate the naturally generated CeO₂ on electrolyte surface can considerably reduce impurity formation and maintain the microstructure of electrolyte. This work demonstrates that samples with CeO₂ on the surface effectively protect the BaCeO₃-based proton-conducting electrolyte from CO₂ poisoning. This method may be applied to similar BaCeO₃-based perovskite materials as a new protective method.     

A comparative study of surface segregation and interface of La0·6Sr0·4Co0·2Fe0·8O3-δ electrode on GDC and YSZ electrolytes of solid oxide fuel cells

Electrode/electrolyte interface plays a critical role in the performance and stability of solid oxide fuel cells (SOFCs). In the case of La_0·6Sr_0·4Co_0·2Fe_0·8O_3-δ (LSCF) cathode, it is well known that cathodic polarization promotes the Sr segregation and diffusion towards the LSCF electrode and Y₂O₃–ZrO₂ (YSZ) electrolyte interface, leading to the formation of SrZrO₃ secondary phase and the disintegration of LSCF structure at the interface. On the other hand, LSCF is chemically stable with doped ceria electrolytes such as Gd-doped CeO₂ (GDC). However, there appears no comparative studies on the intrinsic relationship between the surface segregation, interface reaction and stability of LSCF in YSZ and GDC electrolytes. Here, a comparative study has been carried out on the segregation and interface formation of LSCF on GDC and YSZ electrolyte under identical cathodic polarization conditions at 750 °C and 1000 mAcm^?2 using focused ion beam and scanning transmission electron microscopy (FIB-STEM) techniques. Segregation of Sr occurs in the LSCF-GDC system, however, the inertness of GDC electrolyte suppresses the segregation process of Sr species. Instead, surface segregation of B-site Co cation becomes dominant under the cathodic polarization, forming isolated CoO_x particles. The results indicate that the existence of chemical catchers such as Zr in the case of YSZ electrolyte for the segregated Sr species is kinetically the driving force for the Sr segregation and stability of LSCF electrodes under SOFC operation conditions.     

Influence of rare-earth doping on the microstructure and conductivity of BaCe0.9Ln0.1O3−δ proton conductors

Doped barium cerates BaCe_0.9Ln_0.1O_3−δ containing earth-rare dopants with different ionic radii, Ln = La, Nd, Sm, Gd, Yb, Tb and Y, have been investigated as candidate materials for fuel cells and other electrochemical applications. The synthesis of these materials was performed using a precursor method based on freeze-drying, which allows a precise control of the homogeneity of the ceramic powders. Dense ceramic pellets were obtained at 1400 °C under identical sintering conditions. The microstructure of the ceramics exhibits similar features with relative density higher than 95% and the grain size decreasing as the ionic radius of the dopant decreases. Impedance spectroscopy measurements were performed to study separately the different contributions to the total conductivity. The bulk, grain boundary and total conductivities depend on the ionic radius of the dopant, reaching a maximum for Gd-doped samples with a value of 0.02 S cm⁻¹ for the total conductivity at 600 °C.     

Ceria-based nanocomposite with simultaneous proton and oxygen ion conductivity for low-temperature solid oxide fuel cells

The samarium doped ceria-carbonate (SDC/Na₂CO₃) nanocomposite systems have shown to be excellent electrolyte materials for low-temperature SOFCs, yet, the conduction mechanism is not well understood. In this study, a four-probe d.c. technique has been successfully employed to study the conduction behavior of proton and oxygen ion in SDC/Na₂CO₃ nanocomposite electrolyte. The results demonstrated that the SDC/Na₂CO₃ nanocomposite electrolyte possesses unique simultaneous proton and oxygen ion conduction property, with the proton conductivity 1-2 orders of magnitude higher than the oxygen ion conductivity in the temperature range of 200-600 °C, indicating the proton conduction in the nanocomposite mainly accounts for the enhanced total ionic conductivity. It is suggested that the interface in composite electrolyte supplies high conductive path for proton, while oxygen ions are probably transported by the SDC grain interiors. An empirical “Swing Model” has been proposed as a possible mechanism of superior proton conduction.     

Grain boundary space charge modulation in BaZr0.8Y0.2-xMxO3−δ with transition metal (M= Ni,Co, Fe,and Zn) co-doping

The higher grain boundary resistance of proton-conducting ceramic electrolytes such as BaZrO₃ drops the total conductivity. In the present study, BaZr_0.8Y_0.2-xM_xO_3−δ electrolyte with various transition metals (M= Ni, Co, Fe, and Zn) with molar concentrations (x = 0.01, 0.03 & 0.05) were prepared by hydrothermal assisted precipitation method. The structural analysis, such as XRD and Raman, revealed the presence of the cubic perovskite phase of barium zirconate. The existence of space charge layer was confirmed by nonlinear behaviour of the grain boundary resistance on applying DC bias. The Mott–Schottky analysis performed at 200 °C in reducing atmosphere showed a reduction in barrier potential from 0.24 to 0.13 V upon the addition of the secondary dopants. The elastic energy minimization by dopant segregation and electrostatic interaction between the charged defects and dopants suppress the space charge layer. The transition metal dopant, especially Ni and Fe, segregates along the grain boundary and acts as the counterbalance for charged defects. Besides, Fe act as perfect sintering aid which leads to higher densification (>99%) as well as higher protonic conductivity of 21.2 mScm⁻¹ at 600 °C. These results imply that the transition metal ions as co-dopant influence the space charge effect and the density of the barium zirconate electrolyte.