The effect of Cr deposition and poisoning on BaZr0.1Ce0.7Y0.2O3−δ proton conducting electrolyte

With the development of chromium tolerant electrode materials, the evaluation of the chromium deposition and poisoning on electrolyte is critical significance for the commercial and widespread application of solid oxide fuel cell stacks (SOFCs). The Cr deposition and poisoning on BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY) proton conducting electrolyte are initially studied, in order to understand and develop the compatibility for proton conducting SOFC (H-SOFCs). The XRD results imply that Cr₂O₃ is not chemically compatible with BZCY and BaCrO₄ is formed at high temperature above 600 °C. To simulate the Cr volatilization from interconnect and poisoning on BZCY surface, the BZCY bar sample is heat-treated in the presence of Cr₂O₃ at 600 °C, 700 °C, and 800 °C for 50 h. It is clear that Cr deposition occurs even at 600 °C by SEM examination. The XPS results indicate the chemical deposition of BaCrO₄ and physical deposition of Cr₂O₃ on BZCY surface at 600 °C but only chemical deposition at 700 °C and 800 °C. The content of Cr deposition increases with the increase of poisoning temperature. Moreover, the proton conductivity of BZCY after Cr deposition reduces after Cr deposition, indicating the Cr poisoning effect of the electrochemical performance of BZCY electrolyte.     

Electrical conductivity and electrochemical performance of cobalt-doped BaZr0.1Ce0.7Y0.2O3−δ cathode

To develop efficient cathode materials for solid oxide fuel cells (SOFCs) based on Ba(Zr_0.1Ce_0.7Y_0.2)O_3−δ (BZCY) electrolyte, we have examined a series of cobalt-doped BZCY samples with the intended composition of BaZr_0.1Ce_0.7Y_0.2−xCo_xO_3−δ (where x = 0, 0.02, 0.05, 0.075, 0.1, 0.2). It is found that the solubility of cobalt is less than 10 mol% and the electrical conductivity of BaZr_0.1Ce_0.7Y_0.2−xCo_xO_3−δ decreased with the content of cobalt within this solubility. When the content of cobalt is greater than its solubility, a BaCoO₃-based phase forms, which markedly increases the conductivity of the sample (e.g., 2.48 S/cm for a composite material with an intended composition of BaZr_0.1Ce_0.7Co_0.2O_3−δ at 700 ^°C). Typical cells with the cobalt-doped BZCY cathode display much-improved performance than cells with other transition metal doped barium cerate ever reported, yielding a polarization resistance of 0.085 Ω cm² at 750 ^°C.     

Promotion effect of proton-conducting oxide BaZr0.1Ce0.7Y0.2O3−δ on the catalytic activity of Ni towards ammonia synthesis from hydrogen and nitrogen

In this report, for the first time, it has been observed that proton-conducting oxide BaZr_0.1Ce_0.7Y_0.2O_3?δ (BZCY) has significant promotion effect on the catalytic activity of Ni towards ammonia synthesis from hydrogen and nitrogen. Renewable hydrogen can be used for ammonia synthesis to save CO₂ emission. By investigating the operating parameters of the reaction the optimal conditions for this catalyst were identified. It was found that at 620 °C with a total flow rate of 200 mL min^?1 and a H₂/N₂ mol ratio of 3, an activity of approximately 250 μmol g^?1 h^?1 can be achieved. This is ten times larger than that for the unpromoted Ni catalyst under the same conditions although the stability of both catalysts in the presence of steam was not good. The specific activity of Ni supported on proton-conducting oxide BZCY is approximately 72 times higher than that of Ni supported on non-proton conductor MgOCeO₂. These promotion effects were suspected to be due to the proton conducting nature of the support. Therefore it is proposed that the use of proton conducting support materials with highly active ammonia synthesis catalysts such as Ru and Fe will provide improved activity of at lower temperatures.     

Enhanced sinterability of BaZr0.1Ce0.7Y0.1Yb0.1O3−δ by addition of nickel oxide

The effect of nickel oxide addition on the sintering behavior and electrical properties of BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3−δ (BZCYYb) as an electrolyte for solid oxide fuel cells was systematically studied. Results suggest that the addition of a small amount (∼1 wt%) of NiO to BZCYYb greatly promoted densification, achieving ∼96% of the theoretical density after sintering at 1350 °C in air for 3 h (reducing the sintering temperature by ∼200 °C). Further, a sample sintered at 1450 °C for 3 h showed high open circuit voltages (OCVs) when used as the electrolyte membrane to separate the two electrodes under typical SOFC operating conditions, indicating that the electrical conductivity of the electrical conductivity of the BZCYYb was not adversely affected by the addition of ∼1 wt% NiO.     

Preparation of BaZr0.1Ce0.7Y0.2O3−δ thin membrane based on a novel method-drop coating

The thin membrane of a BaZr_0.1Ce_0.7Y_0.2O_3−δ (BYCZ) was fabricated on the porous NiO-BYCZ anode substrate through a novel method-drop coating. The intact cell was assembled with the cathode of La_0.7Sr_0.3FeO_3−δ/BYCZ and tested from 600 to 700 ^°C with humidified hydrogen (∼3% H₂O) as the fuel and the static air as the oxidant. We investigated the influence of different contents of BYCZ in the slurry and different pre-fired temperatures to the open-circuit potential and performance of the prepared cells. The results showed that the cell with 5% (wt) BYCZ in the slurry and pre-fired temperature of 500 °C had the highest open-circuit potential which indicated the densest electrolyte and the highest power density. Double-layer-drop-coating with the anode functional layer was also adopted and improved the cell performance to 377 mW cm⁻² at 700 °C.     

Intermediate-to-low temperature protonic ceramic membrane fuel cells with Ba0.5Sr0.5Co0.8Fe0.2O3-δ–BaZr0.1Ce0.7Y0.2O3-δ composite cathode

The perovskite-type Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ–BaZr_0.1Ce_0.7Y_0.2O_3-δ (BSCF–BZCY) composite oxides were synthesized by a modified Pechini method and examined as a novel composite cathode for intermediate-to-low temperature protonic ceramic membrane fuel cells (ILT-PCMFCs). Thin proton-conducting BaZr_0.1Ce_0.7Y_0.2O_3-δ (BZCY) electrolyte and NiO–BaZr_0.1Ce_0.7Y_0.2O_3-δ (NiO–BZCY) anode functional layer were prepared over porous anode substrates composed of NiO–BaZr_0.1Ce_0.7Y_0.2O_3-δ by a one-step dry-pressing/co-firing process. A laboratory-sized quad-layer cell of NiO–BZCY/NiO–BZCY(∼50 μm)/BZCY(∼20 μm)/BSCF–BZCY(∼50 μm) was operated from 550 to 700 °C with humidified hydrogen (∼3% H₂O) as fuel and the static air as oxidant. A high open-circuit potential of 1.009 V, a maximum power density of 418 mW cm⁻², and a low polarization resistance of the electrodes of 0.10 Ω cm² was achieved at 700 °C. These investigations have indicated that proton-conducting BZCY electrolyte with BSCF perovskite cathode is a promising material system for the next generation solid oxide fuel cells (SOFCs).     

Enhancing sinterability and electrochemical properties of Ba(Zr0.1Ce0.7Y0.2)O3-δ proton conducting electrolyte for solid oxide fuel cells by addition of NiO

The influence of NiO on the sintering behavior and electrical properties of proton conducting Ba(Zr_0.1Ce_0.7Y_0.2)O_3-δ (BZCY7) as an electrolyte supporter for solid oxide fuel cells is systematically investigated. SEM images and shrinkage curve demonstrate that the sinterability of the electrolyte pellets is dramatically improved by doping NiO as a sintering aid. The sintering aid amount and sintering temperature are optimized by analyzing the linear shrinkage, grain size and morphology for a series of sintered BZCY7 electrolyte pellets. Almost full dense electrolyte pellets are successfully formed by using 0.5–1.0 wt% NiO loading after sintering at 1400 °C for 6 h. The linear shrinkage of 0.5 wt% NiO modified BZCY7 sample is about 14.25% higher than that without NiO addition (4.81%). Energy dispersive X-ray spectroscopy analysis indicate that partial NiO might dissolve into the perovskite lattice structure and the other NiO react with BZCY7 to form BaY₂NiO₅ secondary phase as a sintering aid. Excessive NiO is especially detrimental to the electrical properties of BZCY7 and thus lower the open circuit voltage. The electrochemical performance for a series of single cells with different concentration NiO modified BZCY7 electrolyte are measured and analyzed. The optimized composition of 0.5 wt% NiO modified BZCY7 as an electrolyte support for solid oxide fuel cell demonstrates a high electrochemical performance.     

High performance of anode supported BaZr0.1Ce0.7Y0.2O3−δ(BZCY) electrolyte cell for IT-SOFC

BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY) electrolyte has been extensively studied as novel electrolyte for intermediate temperature solid oxide fuel cells (IT-SOFCs). In this short communication, we report our progress on the fabrication and characterization of the high performance anode supported BZCY cell. Maximum power density of 650 mW/cm² at 700 °C is obtained for the single cell with configuration of Ni-BZCY/BZCY (20 um)/SSC-BZCY. Further, significant performance enhancement is observed during long-term stability test at 600 °C under constant voltage of 0.5 V, indicating excellent stability of BZCY under fuel cell operating condition. Those results indicate that BZCY based cell is very promising for practical applications.     

Carbon dioxide reforming of methane in a SrCe0.7Zr0.2Eu0.1O3−δ proton conducting membrane reactor

Utilizing CO₂ for fuel production holds the promise for reduced carbon energy cycles. In this paper we demonstrate a membrane reactor, integrating catalytic CO₂ reforming of methane with in-situ H₂ separation, that results in increased CO₂ and CH₄ conversion and H₂ production compared to a Ni catalyst alone. The tubular proton-conducting SrCe_0.7Zr_0.2Eu_0.1O_3−δ membrane reactor demonstrates that the addition of the membrane improves CO₂ conversion, due to in-situ H₂ removal, by 10% and 30% at 900 °C for CH₄/CO₂ = 1/1 and CH₄/CO₂/H₂O = 2/1/1 feed ratios, respectively. It also improves total H₂ production at 900 °C by 15% and 18% for CH₄/CO₂ = 1/1 and CH₄/CO₂/H₂O = 2/1/1, respectively. Further, the H₂/CO in the reactor side effluent can be adjusted for subsequent desired Fischer-Tropsch products by combining CO₂ reforming and steam reforming of methane.     

Fabrication and characterization of anode-supported micro-tubular solid oxide fuel cell based on BaZr0.1Ce0.7Y0.1Yb0.1O3−δ electrolyte

Anode-supported micro-tubular solid oxide fuel cells (SOFCs) based on a proton and oxide ion mixed conductor electrolyte, BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3−δ (BZCYYb), have been fabricated using phase inversion and dip-coating techniques with a co-firing process. The single cell is composed of NiO-BZCYYb anode, BZCYYb electrolyte and La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF)-BZCYYb cathode. Maximum power densities of 0.08, 0.15, and 0.26 W cm⁻² have been obtained at 500, 550 and 600 °C, respectively, using H₂ as fuel and ambient air as oxidant.     

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.     

Electrode performance and analysis of reversible solid oxide fuel cells with proton conducting electrolyte of BaCe0.5Zr0.3Y0.2O3−δ

Reversible solid oxide fuel cells (R-SOFCs) are regarded as a promising solution to the discontinuity in electric energy, since they can generate electric powder as solid oxide fuel cells (SOFCs) at the time of electricity shortage, and store the electrical power as solid oxide electrolysis cells (SOECs) at the time of electricity over-plus. In this work, R-SOFCs with thin proton conducting electrolyte films of BaCe_0.5Zr_0.3Y_0.2O_3−δ were fabricated and their electro-performance was characterized with various reacting atmospheres. At 700 °C, the charging current (in SOFC mode) is 251 mA cm⁻² at 0.7 V, and the electrolysis current densities (in SOEC mode) reaches −830 mA cm⁻² at 1.5 V with 50% H₂O-air and H₂ as reacting gases, respectively. Their electrode performance was investigated by impedance spectra in discharging mode (SOFC mode), electrolysis mode (SOEC mode) and open circuit mode (OCV mode). The results show that impedance spectra have different shapes in all the three modes, implying different rate-limiting steps. In SOFC mode, the high frequency resistance (R_H) is 0.07 Ωcm² and low frequency resistances (R_L) are 0.37 Ωcm². While in SOEC mode, R_H is 0.15 Ωcm², twice of that in SOFC mode, and R_L is only 0.07 Ωcm², about 19% of that in SOFC mode. Moreover, the spectra under OCV conditions seems like a combination of those in SOEC mode and SOFC mode, since that R_H in OCV mode is about 0.13 Ωcm², close to R_H in SOEC mode, while R_L in OCV mode is 0.39 Ωcm², close to R_L in SOFC mode. The elementary steps for SOEC with proton conducting electrolyte were proposed to account for this phenomenon.     

Evaluation of hydrogen permeation properties of Ni–Ba(Zr0.7Pr0.1Y0.2)O3−δ cermet membranes

In order to obtain chemically stable hydrogen-permeable cermet membranes against CO₂ and H₂O, the composite membranes consisting of Ni and Ba(Zr_0.7Pr_0.1Y_0.2)O_3−δ (BZPY) are fabricated by the dry-press technique and reducing atmosphere sintering process. SEM results show that the cermet membrane is extremely dense and metal nickel is randomly distributed in BZPY oxide matrix. Hydrogen permeation properties of the Ni-BZPY membranes are systemically studied including the influence of the operating temperature, H₂ concentration in feed stream, humidification degree and membrane thickness. The Ni-BZPY membrane presents good chemical stability in humid condition or CO₂-containing environments and is potential candidates for hydrogen separation.     

High-performance Ni–BaZr0.1Ce0.7Y0.1Yb0.1O3−δ (BZCYYb) membranes for hydrogen separation

Cermet membranes composited of Ni and doped barium cerate have been widely studied for hydrogen separation; however, their practical application is limited primarily by the relatively low permeation rate and instability of doped barium cerate in H₂O and CO₂ containing gases. Here we report our findings on the development of a thin-film cermet membrane consisting of Ni and BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3−δ (BZCYYb), supported on a porous Ni–BZCYYb substrate. High fluxes of 1.12 and 0.49 ml min⁻¹ cm⁻² have been demonstrated at 900 °C and 700 °C, respectively, when hydrogen was used as the feed gas on one side and N₂ as the sweep gas on the other side. Most importantly, the high-performance membrane can be easily fabricated by a cost-effective particle-suspension coating/co-firing process, offering great promise for large scale hydrogen separation applications.     

BaZr0.1Ce0.7Y0.1Yb0.1O3−δ as highly active and carbon tolerant anode for direct hydrocarbon solid oxide fuel cells

BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3−δ (BZCYYb) perovskite is synthesized and examined as an alternative anode material for intermediate temperature solid oxide fuel cells (IT-SOFCs) based on direct hydrocarbon fuels, using polarization and electrochemical impedance spectroscopy techniques. Single-phased BZCYYb anode shows an excellent activity for both hydrogen and methane oxidation reactions, achieving a polarization resistance of 0.25 and 0.93 Ω cm², and overpotential of 20 and 202 mV at 100 mA cm⁻² and 750 °C in wet H₂ (3% H₂O/97% H₂) and wet CH₄ (3% H₂O/97% CH₄), respectively. The electrocatalytic activity of BZCYYb anodes is significantly higher than that of the (La,Sr)(Cr,Mn)O₃ anodes as reported in the literature. Furthermore, BZCYYb exhibits excellent resistance to carbon deposition. The present study demonstrates that BZCYYb perovskite is a promising alternative anode material for direct hydrocarbon fuels based SOFCs.     

BaZr0.1Ce0.7Y0.1Yb0.1O3−δ electrolyte-based solid oxide fuel cells with cobalt-free PrBaFe2O5+δ layered perovskite cathode

A new anode-supported SOFC material system Ni-BZCYYb|BZCYYb|PBFO is investigated, in which a cobalt-free layered perovskite oxide, PrBaFe₂O_5+δ (PBFO), is synthesized and employed as a novel cathode while the synthesized BZCYYb is used as an electrolyte. The cell is fabricated by a simple dry-pressing/co-sintering process. The cell is tested and characterized under intermediate temperature range from 600 to 700 °C with humified H₂ (∼3% H₂O) as fuel, ambient air as oxidant. The results show that the open-circuit potential of 1.006 V and maximal power density of 452 mW cm⁻² are achieved at 700 °C. The polarization resistance of the electrodes is 0.18 Ω cm² at 700 °C. Compared to BaZr_0.1Ce_0.7Y_0.1O_3−δ, the conductivity of co-doped barium zirconate-cerate BZCYYb is significantly improved. The ohmic resistance of single cell is 0.37 Ω cm² at 700 °C. The results indicate that the developed Ni-BZCYYb|BZCYYb|PBFO cell is a promising functional material system for SOFCs.     

A cobalt-free SrFe0.9Sb0.1O3−δ cathode material for proton-conducting solid oxide fuel cells with stable BaZr0.1Ce0.7Y0.1Yb0.1O3−δ electrolyte

A cobalt-free cubic perovskite oxide SrFe_0.9Sb_0.1O_3−δ (SFSb) is investigated as a novel cathode for proton-conducting solid oxide fuel cells (H-SOFCs). XRD results show that SFSb cathode is chemically compatible with the electrolyte BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3−δ (BZCYYb) for temperatures up to 1000 °C. Thin proton-conducting BZCYYb electrolyte and NiO-BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3−δ (NiO-BZCYYb) anode functional layer are prepared over porous anode substrates composed of NiO-BZCYYb by a one-step dry-pressing/co-firing process. Laboratory-sized quad-layer cells of NiO-BZCYYb/NiO-BZCYYb/BZCYYb/SFSb are operated from 550 to 700 °C with humidified hydrogen (∼3% H₂O) as fuel and the static air as oxidant. An open-circuit potential of 0.996 V, maximum power density of 428 mW cm⁻², and a low electrode polarization resistance of 0.154 Ω cm² are achieved at 700 °C. The experimental results indicate that the cobalt-free SFSb is a promising candidate for cathode material for H-SOFCs.     

Partial conductivities of mixed conducting BaCe0.65Zr0.2Y0.15O3–δ

The electrical properties of BaCe_0.65Zr_0.2Y_0.15O_3–δ (BCZY) were studied as a function of both oxygen partial pressure and water vapor partial pressure in the temperature range of 500–800 °C, and the partial conductivities of protons, holes, and oxygen vacancies were calculated from the defect model. P-type conduction was dominant in an oxidative atmosphere. In a wet atmosphere, BCZY was a mixed conductor of protons, holes, and oxygen ions. A conduction transition from protons to holes and/or oxygen ions was found with increasing temperature. The calculated activation energy of oxygen ion transport was 0.71 eV. The standard solution enthalpy for water dissolution was best fitted with a slope of −120.19 kJ/mol, which is somewhat smaller in absolute terms than that of BaCe_0.9Y_0.1O_3–δ and of BaCe_0.8Y_0.2O_3–δ. This result also agrees well with the literature reports that the Ba, rather than Sr, occupation of A-site and the Ce, rather than Zr, occupation of B-site in perovskite proton conductors induce more negative hydration enthalpies due to the increasing basicity of the corresponding oxides.     

Investigation of A-site deficient Ba0.9Co0.7Fe0.2Nb0.1O3−δ cathode for proton conducting electrolyte based solid oxide fuel cells

A novel cathode material for proton conducting electrolyte based solid oxide fuel cells (SOFCs) of A-site deficient Ba_0.9Co_0.7Fe_0.2Nb_0.1O_3−δ (BCFN) has been synthesized and characterized. The A-site deficient BCFN has demonstrated a thermal expansion coefficient (TEC) similar to that of BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY) electrolyte, enhanced oxygen reduction activity and chemical stability. The application of BCFN in proton-conducting electrolyte-based SOFCs has been investigated, and the SOFCs with Ni-BZCY|BZCY|BCFN configuration demonstrate maximum cell power outputs of 180, 240 and 300 mW cm⁻² at 600, 650 and 700 °C, respectively. The cell polarization resistance is as low as 0.951, 0.387 and 0.201 Ω cm² under open circuit voltage (OCV) at 600, 650 and 700 °C, respectively. The cell maximum power density increases from 300 to 356 mW cm⁻² at 700 °C after over 118 h operating under a constant current of 300 mA cm⁻².