Efficient and stable photocatalytic hydrogen production from water splitting over ZnxCd1–xS solid solutions under visible light irradiation

A simple co-precipitation method was employed to synthesize a series of cubic zinc-blende phase of Zn_xCd_1−xS photocatalysts using Na₂S as the S source. Structural, morphological and optical properties of the samples have been investigated by XRD, SEM, EDS, XRF, ICP, N₂ physisorption and UV–vis diffuse reflectance techniques. The Zn_xCd_1−xS solid solution is not a simple compound mixture of ZnS and CdS, its XRD patterns show new structural peaks instead of mixture of original peaks. The lattice parameter a measured from the XRD patterns of the Zn_xCd_1−xS samples exhibits a slightly nonlinear relationship with the Zn mole fraction, which is slightly inconsistent with Vegard's law, thus suggesting that a nonhomogeneous alloy structure exists in Zn_xCd_1−xS solid solution. The photocatalytic H₂ evolution from water splitting in the sacrificial reagents of 0.25 M Na₂S/0.35 M K₂SO₃ under visible light at 30 °C and 55 °C were also examined in the study. It is found that Zn_xCd_1−xS solid solution with composition x = 0.4–0.5 shows the highest photocatalytic H₂ production performance. The studied Zn_xCd_1−xS exhibits at least 50 h stable photocatalytic activity under outdoor sunlight irradiation.     

Structural and electrochemical characterization of La2−xSrxNiTiO6−δ

Materials of the series La_2−xSr_xNiTiO_6−δ (0 ≤ x ≤ 0.5) have been characterized by both structural and electrochemical methods in order to assess their possible use as electrodes for SOFCs. Neutron and X-ray powder diffraction experiments have shown that they are stable under both oxidizing and reducing conditions while chemically compatible with YSZ at SOFC operating temperatures. Moreover, the thermal expansion has been determined to be isotropic with α_L = 10.0(3) × 10⁻⁶ K⁻¹. This value is similar to that found for other perovskite materials used in SOFCs. However, polarization resistances reveal modest values of electrochemical response under oxygen (1.5 Ω cm² at 900 °C) and are quite poor in 5%H₂/N₂ mixtures (15 Ω cm²) at the same temperature. Nevertheless, microstructure has not been optimized fully enough to discard the material as a potential SOFC cathode.     

SrCo1−xSbxO3−δ perovskite oxides as cathode materials in solid oxide fuel cells

The SrCo_1−xSb_xO_3−δ (x = 0.05, 0.1, 0.15 and 0.2) system was tested as possible cathode for solid oxide fuel cells (SOFCs). X-ray diffraction results show the stabilization of a tetragonal P4/mmm structure with Sb contents between x = 0.05 and x = 0.15. At x = 0.2 a phase transition takes place and the material is defined in the cubic Pm-3m space group. In comparison with the undoped hexagonal SrCoO₃ phase, the obtained compounds present high thermal stability without abrupt changes in the expansion coefficient. In addition, a great enhancement of the electrical conductivity was observed at low and intermediate temperatures (T ≤ 800 °C). The sample with x = 0.05 displays the highest conductivity value that reaches 500 S cm⁻¹ at 400 °C and is over 160 S cm⁻¹ in the usual working conditions of a cathode in SOFC (650-900 °C). Moreover, the impedance spectra of the SrCo_1−xSb_xO_3−δ/Ce_0.8Nd_0.2O_2−δ/SrCo_1−xSb_xO_3−δ (x ≥ 0.05) symmetrical cells reveal polarization resistances below 0.09 Ω cm² at 750 °C which are much smaller than that displayed by the pristine SrCoO_3−δ sample. The composition with x = 0.05 shows the lowest ASR values ranging from 0.009 to 0.23 Ω cm² in the 900-600 °C temperature interval with an activation energy of 0.82 eV.     

Synthesis and properties of Sm-deficient Sm1−xBaCo2O5+δ perovskite oxides as cathode materials

Double-layered perovskite oxides of Sm_1−xBaCo₂O_5+δ (S_1−xBCO) with various A-site Sm³⁺-deficiencies (x = 0.00–0.08) were synthesized and evaluated as cathode materials of intermediate-temperature solid oxide fuel cells (IT-SOFCs). The Sm³⁺-deficiency content in S_1−xBCO was limited up to x = 0.05, and higher content x = 0.08 caused impurity phase. S_1−xBCO oxides were chemically stable with GDC electrolyte at 1050 °C and below. Introduction of Sm³⁺-deficiency caused decreased oxygen content and increased concentration of oxygen vacancy in S_1−xBCO. Electrical conductivities of S_1−xBCO decreased with increasing temperature in air, and also changed with the Sm³⁺-deficiency content. Electrochemical performance of S_1−xBCO cathodes were characterized by impedance spectra measurement based on symmetric cells. Higher Sm³⁺ deficiency content has resulted in decreased area specific resistances (ASRs) and activation energy (Ea), i.e. enhanced electrochemical reaction reactivity for the S_1−xBCO cathodes. Among the studied samples, the S_0.95BCO (x = 0.05) oxide showed the best electrochemical performance with ASR values of 0.316 Ω cm² at 600 °C, 0.137 Ω cm² at 650 °C, 0.068 Ω cm² at 700 °C and 0.038 Ω cm² at 750 °C respectively, thus it's a promising cathode material of IT-SOFCs.     

Electrical conductivity and cell performance of La0.3Sr0.7Ti1−xCrxO3−δ perovskite oxides used as anode and interconnect material for SOFCs

The anode materials La_0.3Sr_0.7Ti_1−xCr_xO_3−δ (LSTC, x = 0, 0.1, 0.2) with cubic structure were prepared via solid state reaction route. The influence of Cr content on the properties of LSTC as anode and interconnect materials for solid oxide fuel cells (SOFCs) was investigated. The Cr-doping decreased the lattice parameter while increased the sinterability of LSTC materials. The total electrical conductivity decreased with Cr doping level, from 230 S cm⁻¹ for x = 0 to 53 S cm⁻¹ for x = 0.2. The total electrical conductivity exhibited good stability and recoverability in alternative atmospheres of air and 5% H₂/Ar, showing excellent redox stability. The cell testing showed that the anode performance of LSTC was enhanced somewhat by Cr doping. The present results indicated that the prepared La_0.3Sr_0.7Ti_1−xCr_xO_3−δ can be potential anode and interconnect materials for SOFCs.     

Preparation and characterization of La1−xSrxNiyFe1−yO3−δ cathodes for low-temperature solid oxide fuel cells

Perovskite-type La_1−xSr_xNi_yFe_1−yO_3−δ (x = 0.3, 0.4, 0.5, 0.6, y = 0.2; x = 0.3, y = 0.2, 0.3, 0.4) oxides have been synthesized and employed as cathodes for low-temperature solid oxide fuel cells (SOFCs) with composite electrolyte. The segregation of La₂NiO_4±δ is observed to increase with the increasing Sr²⁺ incorporation content according to X-ray diffraction (XRD) results. The as-prepared powders appear porous foam-like agglomeration with particle size less than 1 μm. Maximum power densities yield as high as 725 mW cm⁻² and 671 mW cm⁻² at 600 °C for fuel cells with the LSNF4628 and LSNF7337 composite cathodes. The maximum power densities continuously increase with the increasing Sr²⁺ content in LSNF cathodes, which can be mainly ascribed to the possible charge compensating mechanism. The maximum power densities first increase with the Ni ion incorporation content up to y = 0.3 due to the increased oxygen vacancy, ionic conductivity and oxygen permeability. Further increase in Ni ion content results in a further lowering of fuel cell performance, which can be explained by the association of oxygen vacancies and divalent B-site cations in the cathode.     

Preparation and characterization of Nd2−xSrxCoO4+δ cathodes for intermediate-temperature solid oxide fuel cell

K₂NiF₄-type structural Nd_2−xSr_xCoO_4+δ (x = 0.8, 1.0, 1.2) was synthesized and evaluated as cathodes for intermediate-temperature solid oxide fuel cell (IT-SOFC). The crystal structure, thermal expansion, electrical conductivity and electrochemical properties were investigated by X-ray diffraction, dilatometry, DC four-probe method, AC impedance and polarization techniques. It is found that the electrochemical properties were remarkably improved with the increasing of Sr in the experiment range. Nd_0.8Sr_1.2CoO_4+δ showed the highest electrical conductivity of 212 S cm⁻¹ at 800 °C, the lowest polarization resistance and cathodic overpotential, 0.40 Ωcm² at 700 °C and 35.6 mV at a current density of 0.1 A cm⁻² at 700 °C, respectively. The chemical compatibility experiment revealed that Nd_0.8Sr_1.2CoO_4+δ cathode was chemically stable with the SDC electrolyte. The thermal expansion coefficient also increased with the Sr content.     

Double-perovskites YBaCo2−xFexO5+δ cathodes for intermediate-temperature solid oxide fuel cells

Double-perovskites YBaCo_2−xFe_xO_5+δ (YBCF, x = 0.0, 0.2, 0.4 and 0.6) are synthesized with a solid-state reaction and are assessed as potential cathode materials for utilization in intermediate-temperature solid oxide fuel cells (IT-SOFCs) on the La_0.9Sr_0.1Ga_0.8Mg_0.115Co_0.085O_2.85 (LSGMC) electrolyte. The YBCF materials exhibit chemical compatibility with the LSGMC electrolyte up to a temperature of 950 °C. The conductivity of the YBCF samples decreases with increasing Fe content, and the maximum conductivity of YBCF is 315 S cm⁻¹ at 325 °C for the x = 0.0 sample. A semiconductor-metal transition is observed at about 300-400 °C. The thermal expansion coefficient of the YBCF samples increases from 16.3 to 18.0 × 10⁻⁶ K⁻¹ in air at temperatures between 30 and 900 °C with increase in Fe content. The area-specific resistances of YBCF cathodes at x = 0.0, 0.2 and 0.4 on the LSGMC electrolyte are 0.11, 0.13 and 0.15 Ω cm² at a temperature of 700 °C, respectively. The maximum power densities of the single cells fabricated with the LSGMC electrolyte, Ce_0.8Sm_0.2O_1.9 (SDC) interlayer, NiO/SDC anode and YBCF cathodes at x = 0.0, 0.2 and 0.4 reach 873, 768 and 706 mW cm⁻², respectively. This study suggests that the double-perovskites YBCF (0 ≤ x ≤ 0.4) can be potential candidates for utilization as IT-SOFC cathodes.     

Scandium-doped PrBaCo2−xScxO6−δ oxides as cathode material for intermediate-temperature solid oxide fuel cells

Scandium-doped PrBaCo_2−xSc_xO_6−δ(PBCS-x, x = 0.00–1.00) oxides have been evaluated as cathode materials of intermediate-temperature solid oxide fuel cells (IT-SOFCs) with respect to phase structure, oxygen content, thermal expansion behavior and electrical and electrochemical properties. The XRD results have demonstrated a phase transition in PBCS-x due to Sc³⁺ doping from tetragonal double-layered perovskite structure at x = 0.00–0.20, bi-phase mixtures at x = 0.30–0.40, to cubic perovskite structure at x = 0.50–0.90. The oxygen contents (6-δ) and average valences of cobalt ions in PBCS-x decrease with the higher Sc³⁺ content and increasing temperatures in air. Sc³⁺ doping has also led to decreased thermal expansion coefficients, lowered electrical conductivities and enhanced electrochemical reaction activities for PBCS-x characterized by decreased area-specific resistances (ASRs) and smaller reaction activation energies. Among the studied samples, the PBCS-0.50 oxide with Sc³⁺-doping content of x = 0.50 exhibits the best electrochemical performance on Ce_0.9Gd_0.1O_1.95 electrolyte. Its ASR values range from 0.123 Ω cm² at 600 °C to 0.022 Ω cm² at 750 °C, which are much lower than the related cathode materials. These results have demonstrated that the PBCS-0.50 oxide is a promising cathode material for IT-SOFCs.     

Thermogravimetric relaxation study of the proton conductor lanthanum tungstate,La28−xW4+xO54+δv2−δ, x = 0.85

Diffusion- and surface exchange coefficients for incorporation and transport of protons and water in lanthanum tungstate, La_27.15W_4.85O_55.28v_0.73 have been determined from thermogravimetric relaxation. Tracer diffusion of protons has proved to be considerably faster than chemical diffusion of water, whereas the tracer surface exchange process is somewhat slower than its chemical counterpart. Consequently, tracer relaxations display larger critical thicknesses than the chemical ones and are moreover found to be predominantly surface controlled. The activation energy of the chemical diffusion coefficient changes above 700 °C and this transition and a corresponding change in its water vapor dependency are discussed in light of chemical diffusion of water. The activation energy of the chemical surface exchange coefficient under reducing conditions is the half of that under oxidizing. Platinum nano particles have proved to increase the rate of water incorporation under reducing conditions.     

Performance of double-perovskite Sr2−xSmxMgMoO6−δ as solid-oxide fuel-cell anodes

Double-perovskite Sr_2−xSm_xMgMoO_6−δ (SSMM, 0 ≤ x ≤ 0.8) is investigated as a possible anode material for solid-oxide fuel cells on La_0.9Sr_0.1Ga_0.8Mg_0.2O_3−δ (LSGM) electrolytes. Single-phase SSMM samples with 0 ≤ x ≤ 0.4 are prepared. At x ≥ 0.6, a small amount of SrMoO₄ and Sm₂O₃ impurities are observed. The Mg/Mo ordering in SSMM decreases with increasing Sm content. Substitution of Sm for Sr significantly improves the electrical conductivity of SSMM. At x = 0.6, the sample yields the highest conductivity, with values reaching 16 S cm⁻¹ in H₂ at 800 °C. The maximum power densities of single cells achieved with x = 0.0, 0.2, 0.4, 0.6, and 0.8 anodes on a 300 μm-thick LSGM electrolyte are 693, 770, 860, 907, and 672 mW cm⁻², respectively, in H₂ at 850 °C. The SSMM sample with x = 0.4 is considered as the best anode candidate because of the impurity formation seen in x ≥ 0.6 samples. The x = 0.4 sample not only has a thermal-expansion coefficient closer to that of the LSGM electrolyte but also exhibits good electrochemical performance and stability in commercial city gas containing H₂S, where the maximum power density achieved is 726 mW cm⁻² at 850 °C.     

Hydrogen production from water splitting under visible light irradiation using sensitized mesoporous-assembled TiO2–SiO2 mixed oxide photocatalysts

This work focused on hydrogen production from the photocatalytic water splitting under visible light irradiation using Eosin Y-sensitized mesoporous-assembled TiO₂–SiO₂ mixed oxide photocatalysts, of which the mesoporous-assembled TiO₂–SiO₂ mixed oxides with various TiO₂-to-SiO₂ molar ratios were synthesized by a sol–gel process with the aid of a structure-directing surfactant. The effects of SiO₂ content, calcination temperature, and phase composition of the mixed oxide photocatalysts were investigated. The experimental results showed that the TiO₂–SiO₂ mixed oxide photocatalyst with the TiO₂-to-SiO₂ molar ratio of 97:3 and calcined at 500 °C provided the maximum photocatalytic hydrogen production activity. The characterization results supported that the 0.97TiO₂–0.03SiO₂ mixed oxide photocatalyst (with the suitable SiO₂ content of 3 mol%) possessed superior physicochemical properties for the photocatalytic reaction as compared to the pure TiO₂, particularly higher specific surface area, lower mean mesopore diameter, higher total pore volume, and lower crystallite size, which led to an enhanced photocatalytic activity.     

New Nb-doped SrCo1−xNbxO3−δ perovskites performing as cathodes in solid-oxide fuel cells

The high-temperature cubic phase of SrCoO_3−δ is a promising cathode material for solid oxide fuel cells (SOFC) due to its high electrical conductivity and oxygen permeation flux. However, this phase is not stable below 900 °C where a 3C-cubic to 2H-hexagonal phase transition takes place when the sample is slowly cooled down. In this work we have stabilized a 3C-tetragonal P4/mmm structure for SrCo_1−xNb_xO_3−δ with x = 0.05. We have followed the strategy consisting of introducing a highly-charged cation at the Co sublattice, in order to avoid the stabilization of the unwanted 2H structure containing face-sharing octahedra. The characterization of this oxide included X-ray (XRD) and neutron powder diffraction (NPD) experiments. SrCo_0.95Nb_0.05O_3−δ adopts a tetragonal superstructure of perovskite with a = a₀, c = 2a₀ (a₀ ≈ 3.9 Å) defined in the P4/mmm space group containing two inequivalent Co positions. Flattened and elongated (Co,Nb)O₆ octahedra alternate along the c axis sharing corners in a three-dimensional array (3C-like structure). In the test cell, the electrodes were supported on a 300-μm-thick pellet of the electrolyte La_0.8Sr_0.2Ga_0.83Mg_0.17O_3−δ (LSGM). The test cells gave a maximum power density of 0.4 and 0.6 W/cm² for temperatures of 800 and 850 °C, respectively, with pure H₂ as fuel and air as oxidant. The good performance of this material as a cathode is related to its mixed electronic-ionic conduction (MIEC) properties, which can be correlated to the investigated structural features: the Co³⁺/Co⁴⁺ redox energy at the top of the O-2p bands accounts for the excellent electronic conductivity, which is favored by the corner-linked perovskite network. The considerable number of oxygen vacancies, with the oxygen atoms showing high displacement factors suggests a significant ionic mobility.     

La0.9−xCaxCe0.1CrO3−δ as potential anode materials for solid oxide fuel cells

LaCrO₃ doped with calcium and cerium on the A-site in the series of La_0.9−xCa_xCe_0.1CrO_3−δ (LCCC3060, LCCC4050, LCCC5040, LCCC6030 corresponding to x = 0.6, 0.5, 0.4, and 0.3 respectively), is synthesized by a sol–gel combustion method and evaluated as anode material for solid oxide fuel cells (SOFCs). Relatively higher Ca-doping on La in LaCrO₃ is found to improve both electronic and ionic conductivity. LCCC compositions have demonstrated good chemical stability in reducing atmospheres. Evaluation of the LCCC material as anode in symmetrical cell configuration shows that the highest Ca-doping composition results in the lowest activation energy and the lowest polarization resistance. La_0.8Sr_0.2Ga_0.83Mg_0.17O_3−δ (LSGM) electrolyte-supported single cells with LCCC3060 as the anode and La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF) as the cathode show that LCCC3060 can be a potential anode material for H₂, but not for CH₄.     

Structure, chemical stability and mixed proton-electron conductivity in BaZr0.9−xPrxGd0.1O3−δ

BaZr_0.9−xPr_xGd_0.1O_3−δ (x = 0.3 and 0.6) was prepared by combustion synthesis and characterised with respect to conductivity and stability in an attempt to combine the desirable properties of the end members. The polycrystalline materials exhibit a cubic or pseudo-cubic structure as determined by X-ray synchrotron radiation and transmission electron microscopy. The chemical stability of the compositions is strongly dependent on the praseodymium content, the materials with more Pr present lower stability. Electron holes dominate the conductivity under oxidising atmospheres in BaZr_0.3Pr_0.6Gd_0.1O_3−δ, while BaZr_0.6Pr_0.3Gd_0.1O_3−δ exhibits a mixed electron hole-proton conducting behaviour. Substitution of Zr by Pr in acceptor-doped BaZrO₃ decreases the sintering temperature and increases the grain growth rate.     

Investigation of cobalt-free perovskite Ba0.95La0.05FeO3−δ as a cathode for proton-conducting solid oxide fuel cells

Cobalt-free perovskite Ba_0.95La_0.05FeO_3−δ (BLF) was synthesized. The conductivity of BLF was measured with a DC four-point technique. The thermal expansion coefficient of the BLF was measured using a dilatometer. The BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY7) electrolyte based proton conducting solid oxide fuel cells (SOFCs) were fabricated. A composite cathode with BLF + BZCY7 was used to mitigate the thermal expansion mismatch with the BZCY7 electrolyte. The polarization processes of the button cell NiO-BZCY7/BZCY7/BLF + BZCY7 were characterized using the complicated electrochemical impedance spectroscopy technique. The open circuit voltage of 0.982 V, 1.004 V, and 1.027 V was obtained at 700 °C, 650 °C, and 600 °C, respectively, while the peak power density of 325 mW cm⁻², 240 mW cm⁻², and 152 mW cm⁻², was achieved accordingly.     

Synthesis and electrochemical properties of (Pr–Nd)1−ySryMnO3−δ and (Pr1−xNdx)0.7Sr0.3MnO3−δ as cathode materials for IT-SOFC

(Pr–Nd)_1−ySr_yMnO_3−δ (P-NSM, y = 0.2, 0.25, 0.3, 0.35) powders made from commercial Pr–Nd mixed oxide, as well as (Pr_1−xNd_x)_0.7Sr_0.3MnO_3−δ (PN3SM, x = 0, 0.5, 0.7, 1) were synthesized by a glycine-nitrate process and characterized as cathode materials for intermediate temperature solid oxide fuel cell (IT-SOFC). XRD patterns showed the powders had formed pure perovskite phase after being calcined at 800 °C for 2 h. (Pr–Nd)_0.7Sr_0.3MnO_3−δ (P-N3SM) achieved a high conductivity of 194 S cm⁻¹ at 500 °C and showed a good chemical stability against YSZ at 1150 °C. And the thermal expansion coefficient of P-N3SM/YSZ cathode was 11.1 × 10⁻⁶ K⁻¹, which well matched YSZ electrolyte film. The tubular SOFC with P-N3SM/YSZ cathode exhibited the maximum power densities of 415, 367, 327 and 282 mW cm⁻² at 850, 800, 750 and 700 °C, respectively, which indicated P-N3SM was potentially applied in SOFC for low cost.     

The effect of doping in the electrochemical performance of (Ln1−xMx)FeO3−δ SOFC cathodes

A family of iron perovskites with the general formula AFeO_3−δ (A = Ln_1−xM_x; Ln = La, Nd and/or Pr; M = Sr or/and Ca) has been prepared keeping fixed the A cation radius 〈r_A〉 and cation size mismatch to isolate the effect of divalent dopant concentration from the A-cation steric effects. The electrochemical behaviour of these compounds for their application as SOFC cathodes was evaluated by using I-V curve measurements and ac impedance spectroscopy over three electrodes electrolyte supported cells processed under identical conditions. In contrast with the bulk behaviour, trends are more difficult to observe due to microstructural effects, but results seem to indicate that the doping level, x, does not influence in a significant way the electrochemical performance of iron perovskites with identical 〈r_A〉 and σ²(r_A).     

Phase stability and electrical conductivity of Ca-doped LaNb1−xTaxO4−δ high temperature proton conductors

The electrical conductivity, crystal structure and phase stability of La_0.99Ca_0.01Nb_1−xTa_xO_4−δ (x = 0, 0.1, 0.2, 0.3, 0.4 and 0.5, δ = 0.005), a potential candidate for proton conductor for solid oxide fuel cells (SOFCs), have been investigated using AC impedance technique and in situ X-ray powder diffraction. Partially substituting Nb with Ta elevates the phase transition temperature (from a monoclinic to a tetragonal structure) from ∼520 °C for x = 0 to above 800 °C for x = 0.4. AC conductivity of the La_0.99Ca_0.01Nb_1−xTa_xO_4−δ both in dry and wet air decreased slightly with increasing Ta content above 750 °C, while below 500 °C, it decreased by nearly one order of magnitude for x = 0.4. It was also determined that the activation energy for the total conductivity increases with increasing Ta content from 0.50 eV (x = 0) to 0.58 eV (x = 0.3) for the tetragonal phase, while it decreases with increasing Ta content from 1.18 eV (x = 0) to 1.08 eV (x = 0.4) for the monoclinic phase. By removing the detrimental structural phase transition from the intermediate-temperature range, consequently avoiding the severe thermal expansion problem up to 800 °C, partial substitution of Nb with Ta brings this class of material closer to its application in electrode-supported thin-film intermediate-temperature SOFCs.     

Phase stability and conductivity of Ba1−ySryCe1−xYxO3−δ solid oxide fuel cell electrolyte

The structure, phase stability, and electrical properties of BaCe_1−xY_xO_3−δ (x = 0-0.4) in humidity air and CO₂ atmosphere are investigated. XRD results indicate that the BaCe_0.9Y_0.1O_3−δ sample has a symmetric cubic structure, and its phase changes to tetragonal as the Y³⁺ doping amount increases to 20 mol%. The conductivity of BaCe_1−xY_xO_3−δ increases with temperature, and it depends on the amount of yttrium doping and the atmosphere. BaCe_0.8Y_0.2O_3−δ exhibits the highest conductivity of 0.026 S cm⁻¹ at 750 °C. The activation energy for conductivity depends on yttrium doping amount and temperature. The conductivity of BaCe_0.8Y_0.2O_3−δ is 0.025 S cm⁻¹ in CO₂ atmosphere at 750 °C which is 3.8% lower than that in air due to reactions with CO₂ and BaCO₃ and the CeO₂ impure phases formed. The structure of BaCe_0.8Y_0.2O_3−δ is unstable in water and decomposes to Ba(OH)₂ and CeO₂ phases. It is found that the activation energy of samples in CO₂ atmosphere is higher than that of sample in air. Sr-doped Ba_1−ySr_yCe_0.8Y_0.2O_3−δ (y = 0-0.2) is prepared to improve the phase stability of BaCe_0.8Y_0.2O_3−δ in water. The conductivity of Ba_0.9Sr_0.1Ce_0.8Y_0.2O_3−δ is 0.023 S cm⁻¹ at 750 °C which was 11% lower than that of BaCe_0.8Y_0.2O_3−δ, however, the phase stability of Ba_0.9Sr_0.1Ce_0.8Y_0.2O_3−δ is much better than that of BaCe_0.8Y_0.2O_3−δ in water.