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Abstract A microwave sintering technique is reported for fabricating co-sintered proton-conducting solid oxide fuel cells. With this method, high-quality ceramic electrolyte membranes can be prepared at 1100 °C, thus enabling the fabrication of entire cells in a single step. The microwave sintering method not only enhances electrolyte densification but also improves the cathode/electrolyte interface, which is critical for improving fuel cell performance. The power output of the co-sintered cell prepared under the microwave conditions (up to 449 mW cm−2 at 700 °C) was significantly higher than that of the cell fabricated using the traditional co-sintering method (approximately 292 mW cm−2 at the same temperature). Electrochemical analysis revealed that the enhanced electrolyte density and the improved cathode/electrolyte interface achieved by using the microwave sintering technique decrease both the ohmic resistance and the polarisation resistance of the cell, leading to good fuel cell performance. [ABSTRACT FROM AUTHOR]

Abstract NiO is used as a sintering aid to modify BaZr 0.1 Ce 0.7 Y 0.2 O 3-δ by an external addition method and by an internal doping strategy to improve the sinterability of BaZr 0.1 Ce 0.7 Y 0.2 O 3-δ. In both cases, the modified BaZr 0.1 Ce 0.7 Y 0.2 O 3-δ materials show an improved sinterability compared with the original BaZr 0.1 Ce 0.7 Y 0.2 O 3-δ. However, doping BaZr 0.1 Ce 0.7 Y 0.2 O 3-δ with NiO to form BaZr 0.1 Ce 0.66 Ni 0.04 Y 0.2 O 3-δ is found to be an effective strategy to significantly improve the electrolyte properties. The BaZr 0.1 Ce 0.66 Ni 0.04 Y 0.2 O 3-δ sample shows a high density and large grain size after sintering at a relatively low temperature (1400 °C). Electrochemical studies reveal that the doping strategy offers a high proton conductivity in both the bulk and across grain boundaries. The conductivity of BaZr 0.1 Ce 0.66 Ni 0.04 Y 0.2 O 3-δ sintered at 1400 °C is observed to be higher than that of BaZr 0.1 Ce 0.7 Y 0.2 O 3-δ sintered at 1600 °C. With BaZr 0.1 Ce 0.66 Ni 0.04 Y 0.2 O 3-δ as the electrolyte, a proton-conducting solid oxide fuel cell displays a large peak power density of 477 mW cm-2 at 600 °C and a high electrolyte membrane conductivity of 6.3 × 10−3 S cm−1. Highlights • NiO was used as the dopant for BaZr 0.1 Ce 0.7 Y 0.2 O 3-δ proton-conducting oxide. • High sinterability with improved conductivity was achieved. • The doping strategy showed advantages over the traditional approach of using NiO. • The cell with BaZr 0.1 Ce 0.66 Ni 0.04 Y 0.2 O 3-δ electrolyte showed a high performance. [ABSTRACT FROM AUTHOR]

The microwave sintering strategy was for the first time adopted to prepare proton-conducting electrolyte membranes for solid oxide fuel cells. The preparation of a dense proton-conducting BaCe 0.7 Zr 0.1 Y 0.2 O 3-δ (BCZY) electrolyte membrane can be achieved at 1200 °C with the microwave sintering method. In sharp contrast, a BCZY sample prepared at 1200 °C using the conventional thermal sintering method was found to be porous. In comparison with a dense BCZY sample prepared at 1400 °C using the conventional sintering method, the microwave-sintered BCZY electrolyte showed an improved proton conductivity, which is beneficial for fuel cell applications. Experimental results showed that the microwave sintering method enabled a homogenous elemental distribution and a suppression of barium evaporation, leading to the conductivity improvement in both bulk and grain boundaries. With the microwave sintered BCZY film as the electrolyte, a proton-conducting solid oxide fuel cell delivered a maximum power density of 838 mW cm −2 at 700 °C with an electrolyte film conductivity as high as 1.4 × 10 −2 S cm −1 . This study suggests that the microwave sintering method is a promising strategy to prepare electrolyte membranes at a relatively low temperature with high conductivity, which could advance the development of proton-conducting solid oxide fuel cells. [ABSTRACT FROM AUTHOR]

Y-doped BaZrO 3 (BZY) electrolyte films are successfully fabricated by utilizing the driving force from the anode substrate, aiming to circumvent the refractory nature of BZY materials. The BZY electrolyte film on the high shrinkage anode becomes dense after sintering even though no sintering aid is added, while the BZY electrolyte remains porous on the conventional anode substrate after the same treatment. The resulting BZY electrolyte shows a high conductivity of 4.5 × 10 − 3 S cm − 1 at 600 °C, which is 2 to 20 times higher than that for most of BZY electrolyte films in previous reports. In addition, the fuel cell with this BZY electrolyte generates a high power output of 267 mW cm − 2 at 600 °C. These results suggest the strategy presented in this study provides a promising way to prepare BZY electrolyte films for fuel cell applications. [ABSTRACT FROM AUTHOR]

Abstract: In3+ was used as dopant for BaZrO3 proton conductor and 30 at%-doped BaZrO3 samples (BaZr0.7In0.3O3-δ , BZI) were prepared as electrolyte materials for proton-conducting solid oxide fuel cells (SOFCs). The BZI material showed a much improved sinteractivity compared with the conventional Y-doped BaZrO3. The BZI pellets reached almost full density after sintering at 1600°C for 10h, whereas the Y-doped BaZrO3 samples still remained porous under the same sintering conditions. The conductivity measurements indicated that BZI pellets showed smaller bulk but improved grain boundary proton conductivity, when compared with Y-doped BaZrO3 samples. A total proton conductivity of 1.7×10−3 Scm−1 was obtained for the BZI sample at 700°C in wet 10% H2 atmosphere. The BZI electrolyte material also showed adequate chemical stability against CO2 and H2O, which is promising for application in fuel cells. [Copyright &y& Elsevier]

Abstract: We describe a solid state reaction for the preparation of both NiO–BaCe0.7Ta0.1Y0.2O3−δ anode substrates and BaCe0.7Ta0.1Y0.2O3−δ (BCTY10) electrolyte membranes on porous NiO–BCTY10 anode substrates. The amounts of the pore forming additive in the substrates showed a significant influence on the densification of the BCTY10 membranes. After sintering at 1450°C for 5h, the BCTY10 membrane on a NiO–BCTY10 anode containing 30wt.% starch achieved a high density and showed adequate chemical stability against H2O and CO2. The chemical stability of BCTY10 was even better than that of BaCe0.7Zr0.1Y0.2O3−δ . With a mixture of BaCe0.7Zr0.1Y0.2O3−δ (BZCY7) and La0.7Sr0.3FeO3−δ (LSF) as a cathode, a single fuel cell with 12μm thick BCTY10 electrolyte generated maximum power densities of 142, 93, 29mW/cm2 at 700, 600 and 500°C, respectively. The electrolyte resistance and interfacial polarization resistance of the cell under open circuit conditions were also investigated. [Copyright &y& Elsevier]

Abstract: Proton-conducting BaCeO3 membranes with different In-doping levels (from 10 to 30%) were fabricated on NiO-based anode substrates. Indium, which was only used as a trivalent element to create oxygen vacancies in BaCeO3 previously, was found to have the function of stabilizing BaCeO3 in this study. The In-doped BaCeO3 showed improved chemical stability against CO2, while even the traditional BaCeO3 substituting with a small amount of Zr decomposed in the same environment. Furthermore, unlike other strategies for stabilizing BaCeO3, the supported In-doped BaCeO3 membrane became dense after firing at relatively low temperatures. We also investigated the influences of the sintering temperatures and the In-doping levels on the densification and the electrical properties of the supported BaCeO3 membranes, which revealed that the In-doping strategy increased both the chemical stability and sinterability for BaCeO3 with little loss of electrical performance. [Copyright &y& Elsevier]

Abstract: Proton-conducting solid oxide fuel cells (SOFCs), consisting of BaCe0.7In0.3O3−δ (BCI30)-NiO anode substrates, BCI30 anode functional layers, BCI30 electrolyte membranes and BCI30-LaSr3Co1.5Fe1.5O10−δ (LSCF) composite cathode layers, were successfully fabricated at 1150°C, 1250°C and 1350°C respectively by a single step co-firing process. The fuel cells were tested with humidified hydrogen (∼3%H2O) as the fuel and static air as the oxidant. The single cell co-fired at 1250°C showed the highest cell performance. The impedance studies revealed that the co-firing temperature affected the interfacial polarization resistance of a single cell as well as its overall electrolyte resistance. [Copyright &y& Elsevier]

Abstract: Screen-printing technology was employed to fabricate BaCe0.8Sm0.2O3−δ (BCS) electrolyte membranes on porous NiO–BCS anode substrates. With a mixture of BCS and La0.7Sr0.3FeO3−σ as cathode, a single cell with 16μm thick BCS electrolyte generated maximum power densities of 222, 141, 72mW/cm2 at 700, 650 and 600°C, respectively. After the electrolyte surface was modified by spray coating, the single cell with 16μm thick BCS electrolyte generated maximum power densities of 352, 241, 125mW/cm2 at 700, 650 and 600°C, respectively. The improved performance of the fuel cell was attributed to the surface modification by spray coating promoting the adherence of the cathode to electrolyte, which was confirmed by both scanning electron microscope (SEM) observation and impedance measurement. The results illustrate that screen-printing technology with spray coating is a simple and low cost method for preparing thin electrolyte membranes of protonic ceramic fuel cells. [Copyright &y& Elsevier]

Abstract: 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 BaCe0.7In0.3O3−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 LaSr3Co1.5Fe1.5O10−δ (LSCF) was also evaluated as a potential cathode candidate for a proton-conducting SOFC. [Copyright &y& Elsevier]

A thin Ba3Ca1.18Nb1.82O9−δ (BCN18) membrane electrolyte was prepared by a novel and facile method on a NiO–BCN18 anode substrate. The interesting aspect of the method was the adoption of a in situ reaction of BaCO3, CaCO3, and Nb2O5 on the anode substrate to prepare a dense BCN18 membrane. The BCN18 membrane obtained, which was about 15 μm in thickness and showed high chemical stability against H2O and CO2, attained a high density after sintering at 1400°C. The elemental composition of the BCN18 membrane prepared was identified by energy-dispersive X-ray spectroscopy analysis. With La0.7Sr0.3FeO3−σ (LSF) as the cathode, the fuel cell performance with the structure of Ni–BCN18/BCN18/LSF was studied. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Uranium cations were immobilized in the proton-conducting oxide BaCe 0.8 Y 0.2 O 3 (BCY) by partially replacing Ce with U. Uranium cations can be incorporated into the BCY lattice to form the new BaCe 0.7 Y 0.2 U 0.1 O 3 (BCUY) compound. This new BCUY material shows an improved chemical stability against CO 2 compared with the traditional BCY material. The first-principles calculations reveal that the doping of U elevates the energy barrier for the interaction between BCUY and CO 2 , which is the reason for the improved chemical stability. Despite the improved chemical stability, U-doping inhibits the grain growth for the BaCeO 3 proton-conducting oxide, making the grain size of the sintered BCUY reach only 0.5 μm, which is approximately one-tenth of that for BCY sintered at the same temperature. As a result, the conductivity of BCUY is 2.8 × 10−3 S cm−1 at 700 °C. The H-SOFC using a BCUY electrolyte reaches a peak power density of 237 mW cm−2 at 700 °C, which is lower than that for BCY cells, but comparable to that using other proton-conducting electrolytes. The current study demonstrates that U cations can be immobilized in BCY proton-conducting oxide, and that U-doping improves the stability of the material at the cost of reducing the sinterability, and thus conductivity. [ABSTRACT FROM AUTHOR]

Sintering aids are commonly used to improve the sinterability of proton-conducting oxides, whereas the sintering aid elements are being used alone in previous research. In this study, a high-entropy design method for the BaCe 0.4 Zr 0.4 Y 0.2 O 3 (BCZY) proton-conducting oxides sintering aid is proposed. In contrast to the use of sintering aids such as Ni, Fe, Cu, Co, and Zn alone, the high-entropy design allows for the simultaneous use of all five elements, resulting in the new composition BaCe 0.4 Zr 0.4 Y 0.15 Ni 0.01 Cu 0.01 Co 0.01 Fe 0.01 Zn 0.01 O 3 (HE-BCZY). Compared to conventional BaCe 0.4 Zr 0.4 Y 0.15 M 0.05 O 3 (BCZYM, M = Ni, Cu, Co, Fe, Zn) materials, the sinterability of the new HE-BCZY material is significantly greater than that of BCZYM, despite the fact that the concentration of sintering aids is the same. Following sintering at 1300 °C, the HE-BCZY electrolyte membrane is completely dense. In addition, the HE-BCZY electrolyte has a higher conductivity than BCZYM. The low sintering temperature allows the successful preparation of proton ceramic fuel cells (PCFCs) with the HE-BCZY electrolyte, and the fuel cell reaches a high peak power density of 1218 mW cm−2 at 700 °C, which is much higher than that of PCFCs using BCZY-based electrolytes. In addition, the excellent chemical stability of HE-BCZY is maintained under both H 2 O and CO 2 conditions. Consequently, a fuel cell employing the HE-BCZY electrolyte operates without detectable degradation for 200 h. This study demonstrates that although the high-entropy design is seldom used for sintering aids of proton-conducting oxides, it is an intriguing and promising strategy for creating high-performance electrolyte materials with good sinterability, high conductivity, and excellent stability. [ABSTRACT FROM AUTHOR]

Abstract The microwave sintering method is found to have advantages over the conventional thermal treatment method for preparing BaZr 0.1 Ce 0.7 Y 0.2 O 3-δ powder. Comparing with the conventional thermal treatment, the microwave sintering method allows the solid material to be self-heated, enabling the formation of pure phase BaZr 0.1 Ce 0.7 Y 0.2 O 3-δ powder at a relatively low temperature (900 °C) with a short dwell time of 1 h, while the same pure phase can only be formed at 1000 °C in an electric furnace. Importantly, the grain size for the microwave prepared BaZr 0.1 Ce 0.7 Y 0.2 O 3-δ powder is much smaller than that of the conventionally thermal treated BaZr 0.1 Ce 0.7 Y 0.2 O 3-δ powder. The small powder grain size is found to be beneficial for the densification and grain growth of the resultant electrolyte membrane during the electrolyte sintering procedure. A fuel cell fabricated using this electrolyte membrane with a conductivity of 7 × 10−3 S cm−1 delivers a power output of 791 mW cm−2 at 700 °C. Highlights • Microwave sintering was used to prepare BaZr 0.1 Ce 0.7 Y 0.2 O 3-δ powder at 900 °C. • Microwave sintering allowed a small grain size of 25 nm for the as-prepared powder. • Microwave prepared BaZr 0.1 Ce 0.7 Y 0.2 O 3-δ powder benefited the electrolyte preparation. • High electrolyte conductivity and desirable fuel cell performance were achieved. [ABSTRACT FROM AUTHOR]

The BaCe 0 · 4 Zr 0 · 4 Y 0 · 2 O 3 (BCZY) was modified with the NiO sintering aid and prepared by a microwave sintering (MVS) method as the electrolyte for proton-conducting solid oxide fuel cells (H–SOFCs). The incorporation of NiO into the BCZY lattice improves the sinterability of the material, leading to the increase of the shrinkage from 3% to 26% after the high-temperature sintering. The dense BaCe 0 · 4 Zr 0 · 4 Y 0 · 15 Ni 0 · 05 O 3 (BCZYN) pellet can be obtained after sintering at 1500 °C, whereas the Ni-free BCZY is porous after sintering at the same condition. By further using the MVS method, the BCZYN can be densified at 1300 °C, which is 200 °C lower than conventional sintering. Furthermore, the microwave-sintered BCZYN has a higher conductivity than the conventionally sintered one due to the larger grain size. Using the MVS method in the H–SOFCs fabrication, the high porosity of the anode substrate can be retained. In contrast, the cell prepared by the conventional sintering (CS) method has a relatively dense anode, which is detrimental to gas fuel transportation. As a result, the microwave sintered cell exhibits an output of 1010 mW cm−2 at 700 °C, which is significantly larger than that for the conventionally sintered cell, being 535 mW cm−2 at the same testing condition, suggesting the advantage of using the MVS method for the fuel cell fabrication. [ABSTRACT FROM AUTHOR]

CeO 2 nanocubes with (100) surface orientation are successfully synthesized and found to facilitate the sinterability of CeO 2 material. The CeO 2 nanocubes show a much-improved sinterability relative to CeO 2 nanoparticles prepared by the conventional citric-nitrate method. The nanocubes can be successfully sintered at a relatively low temperature of 1200 °C without using any sintering aids. In contrast, a pellet using conventional CeO 2 nano-powder obtained from the conventional citric-nitrate method can be densified only after sintering at 1400 °C, which is 200 °C higher than that for the CeO 2 nanocube sintering, although the starting particle size of both CeO 2 samples is similar. Density functional theory indicates that the surface energy of the (100) plane is significantly higher than that of the (111) plane, which is the more typical surface presentation of conventional CeO 2 particles. This high surface energy allows fast growth of the CeO 2 nanocubes during sintering, contributing to their improved sinterability. [ABSTRACT FROM AUTHOR]

A step sintering strategy is proposed to sinter the solid oxide fuel cell fabricated by the tri-layer co-firing method. Compared with the conventional sintering strategy for the tri-layer co-firing method which has to compromise the cathode performance with electrolyte densification, tailoring sintering steps allows the densification of electrolyte and the formation of an optimal cathode microstructure simultaneously, leading to a much enhanced electrochemical performance. The impedance analysis suggests that the desirable cathode microstructure achieved through the step sintering favors the cathode reaction, attaining a polarization resistance as low as 0.022 Ω cm 2 at 700 °C which is about one third of that for the cell sintered by a conventional method. The cell performance reaches 455 mW cm −2 at 700 °C by employing the step sintering strategy, while the cell performance measured is only 289 mW cm −2 with the conventional sintering approach. [ABSTRACT FROM AUTHOR]

Abstract: A dense hydrogen permeable composite membranes consisting of Ni and BaCe0.7Zr0.1Y0.2O3−δ (BCZY) were fabricated by a packing method at a lower temperature than the powder-mixing method. The results of backscattered electron microscope (BSEM) and scanning electron microscope (SEM)-energy dispersive analysis system of X-ray (EDS) revealed that the membrane derived from the packing method showed the most uniform mixture of the Ni phase and the BCZY phase. The hydrogen permeation measurement showed that a more uniform microstructure would result in higher hydrogen permeation. [ABSTRACT FROM AUTHOR]

• A brief history of using sintering aids for proton-conducting oxides is presented. • Whether the electrochemical performance is influenced by sintering aids is discussed. • Prospects and possible solutions are discussed. Sintering aids are often used in the fabrication of proton-conducting oxides to facilitate their application in solid oxide cells, but the detrimental effects of sintering aids on electrochemical performance have been noted in early studies. However, the landscape has changed as some recent studies have suggested that overall electrochemical performance is actually enhanced by using sintering aids, making this an important topic for debate. This mini review presents a discussion of the influence of sintering aids on the electrochemical performance of proton-conducting oxides, considering older studies as well as more recent results. The merits and challenges of sintering-aid-modified proton-conducting oxides are discussed. [ABSTRACT FROM AUTHOR]