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Proton-conducting oxides offer a promising way of lowering the working temperature of solid oxide cells to the intermediate temperate range (500 to 700 °C) due to their better ionic conductivity. In addition, the application of proton-conducting oxides in both solid oxide fuel cells (SOFCs) and sold oxide electrolysis cells (SOECs) provides unique advantages compared with the use of conventional oxygen-ion conducting conductors, including the formation of water at the air electrode site. Since the discovery of proton conduction in some oxides about 30 years ago, the development of proton-conducting oxides in SOFCs and SOECs (the reverse mode of SOFCs) has gained increased attention. This paper briefly summarizes the development in the recent years of R-SOFCs with proton-conducting electrolytes, focusing on discussing the importance of adopting chemically stable materials in both fuel cell and electrolysis modes. The development of electrode materials for proton-conducting R-SOFCs is also discussed. [ABSTRACT FROM AUTHOR]

A chemically stable electrolyte structure was developed for proton-conducting SOFCs by using two layers of stable BaZr0.7Pr0.1Y0.2O3−δ to sandwich a highly-conductive but unstable BaCe0.8Y0.2O3−δ electrolyte layer. The sandwiched electrolyte structure showed good chemical stability in both CO2 and H2O atmosphere, indicating that the BZPY layers effectively protect the inner BCY electrolyte, while the BCY electrolyte alone decomposed completely under the same conditions. Fuel cell prototypes fabricated with the sandwiched electrolyte achieved a relatively high performance of 185mWcm−2 at 700°C, with a high electrolyte film conductivity of 4×10−3 Scm−1 at 600°C. [ABSTRACT FROM AUTHOR]

Abstract: Ba0.5Sr0.5(Co0.8Fe0.2)1−xTixO3− δ (x=0, 0.05, and 0.1) materials were successfully prepared via an improved solid-state reaction route in an attempt to get better chemical stability for Ba0.5Sr0.5Co0.8Fe0.2O3− δ (BSCF). Stability tests showed that the novel Ti-doping strategy can effectively increase the chemical stability for Ba0.5Sr0.5Co0.8Fe0.2O3− δ in CO2-containing environments. The larger the Ti doping amount, the better the chemical stability. Ti-doped samples showed only a slight increase in area specific resistance (ASR) values, as shown from electrochemical tests performed on symmetrical cells. Therefore, anode-supported single fuel cells using BaZr0.4Ce0.4Y0.2O3− δ (BZCY) as the electrolyte and BZCY-Ba0.5Sr0.5(Co0.8Fe0.2)0.9Ti0.1O3− δ as the composite cathode, were fabricated and tested. The measured maximum power density values were 181, 116, and 49mWcm−2 at 700, 600, and 500°C, respectively. [Copyright &y& Elsevier]

Abstract: The La1.95Ca0.05Ce2O7−δ (LCCO) material is successfully synthesized using the Pechini method. The synthesized powders are exposed to atmospheric CO2 and H2 with 3% H2O at 700°C. The treated LCCO powders are investigated using X-ray diffraction (XRD) to study the chemical stability. According to the XRD results, LCCO is very stable and shows no reactions with CO2 or H2O. A fuel cell with the LCCO electrolyte is prepared using the suspension spray method and is tested in the range from 600°C to 700°C using humidified hydrogen (∼3% H2O) as the fuel and static air as the oxidant. An open-circuit potential of 0.832V and a maximum power density of 259mWcm−2 are obtained for a single cell with an interface resistance of 0.23Ωcm2 at 700°C. [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: 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]

Abstract: A novel composition of BaCe0.7Ta0.1Y0.2O3− δ (BCTY10) electrolyte membrane was successfully fabricated on porous NiO-BCTY10 anode substrate. The anode was prepared through a route combining a solid state reaction and a wet chemical method. After sintering at 1450°C for 5h, the BCTY10 membrane showed adequate chemical stability against CO2 and H2O. With a mixture of La0.7Sr0.3FeO3− δ (LSF) and BaCe0.7Zr0.1Y0.2O3− δ (BZCY7) as cathode, a single fuel cell with 25μm thick BCTY10 electrolyte generated maximum power densities of 195, 137, 84, 44mW/cm2 at 700, 650, 600 and 550°C, respectively. The interface resistance of the cell under open circuit condition was also investigated. [Copyright &y& Elsevier]

Abstract: 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–BaZr0.1Ce0.7Y0.2O3−δ was measured in an H2S-containing atmosphere at 900°C. The hydrogen permeation flux began to degrade in 60ppm H2S and decreased by about 45% in 300ppm 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–BaZr0.1Ce0.7Y0.2O3−δ . [Copyright &y& Elsevier]

By modifying the proton conductor BaTbO 3 with the element Co, it is discovered that the solubility of Co in BaTbO 3 is restricted to 20 mol.%, and phase segregations happen for high Co doping concentrations in BaTbO 3. The fabrication of nominal BaTb 0.5 Co 0.5 O 3 results in the in-situ development of the heterostructure cathode composed of BaTb 0.8 Co 0.2 O 3 +BaCoO 3. The critical role of the interface between BaTb 0.8 Co 0.2 O 3 and BaCoO 3 in enhancing fuel cell performance is to accelerate charge transfer kinetics, thereby enabling proton-conducting solid oxide fuel cells (H-SOFCs) to operate at higher performance levels than cells employing BaTb 0.8 Co 0.2 O 3 or BaCoO 3 cathodes alone. Furthermore, the favorable chemical stability of the BaTb 0.8 Co 0.2 O 3 +BaCoO 3 nanocomposite is maintained, thereby enhancing the fuel cell's operational stability. This research suggests that unanticipated phase segregation may occasionally improve the performance of the cathode, thereby presenting an interesting approach to cathode design in H-SOFCs. [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]

Improving chemical stability and performance is desirable for protonic ceramic cathodes (PCFCs). In this study, the proton conductor SrSn 0.8 Sc 0.2 O 3-δ (SSSc) is used to couple the classically unstable Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3-δ (BSCF) cathode to increase the chemical stability of the composite cathode. Compared to the conventional BSCF + BaCe 0.7 Zr 0.1 Y 0.2 O 3-δ (BCZY) composite cathode, the new BSCF + SSSc cathode demonstrates enhanced chemical stability. This is a result of the superior chemical stability of SSSc, which protects BSCF. Moreover, the formation of oxygen vacancies is easier at the BSCF/SSSc interface than at the BSCF/BCZY interface, thereby enhancing the cathode oxygen reduction reaction (ORR) activity. The closer proximity of the O p-band center to the Fermi level in BSCF + SSSc compared to BSCF + BCZY further validates the higher ORR activity of the BSCF + SSSc cathode. The peak power density of the PCFC with BSCF + SSSc cathode, which reaches 1404 mW cm−2 at 700 °C, is significantly greater than that with BSCF + BCZY cathode. The protection of SSSc to BSCF is also reflected in the fuel cell's long-term operation, as the BSCF + SSSc cell operates without detectable degradations for more than 150 h, whereas the BSCF + BCZY cell exhibits observable degradation. These findings indicate that utilizing the SSSc proton conductor to couple the cathode material is a feasible and effective strategy for producing the composite cathode with high chemical stability and superior electrochemical performance for PCFCs. [ABSTRACT FROM AUTHOR]

By synthesizing the nominal Pr x Sr 0.5 MnO 3-δ materials (x = 0.5, 0.6, 0.7, 0.8), new Pr 0.5 Sr 0.5 MnO 3-δ (PSM50)+PrO 2−x composite cathodes for proton-conducting solid oxide fuel cells (SOFCs) were developed. The structure analysis and morphology observations verified the exsolution of PrO 2−x particles, and the amount of exsolved PrO 2−x increased with the amount of Pr in Pr x Sr 0.5 MnO 3-δ. An H-SOFC with a Pr 0.7 Sr 0.5 MnO 3-δ (PSM70) cathode enabled the highest reported fuel cell output for H-SOFCs with manganate cathodes. The construction of a PSM50/PrO 2 heterostructure interface can reduce the formation energy of oxygen vacancies, hence accelerating the cathode oxygen reduction reaction (ORR) kinetics, as confirmed by oxygen diffusion and surface exchange experiments. The excellent electrochemical performance was combined with its good chemical stability against CO 2 and H 2 O, allowing a stable operation of the cell for over 100 h, indicating that PSM70, which was in fact PSM50 +PrO 2−x , was a highly efficient and durable cathode material for H-SOFCs. [ABSTRACT FROM AUTHOR]

By screening a series of La 0.5 Sr 0.5 MnO 3-δ (LSM)-La 0.5 Sr 0.5 FeO 3-δ (LSF) solid solutions using first-principles calculations, an optimal solid solution of LSM and LSF was chosen as the cathode for proton-conducting solid oxide fuel cells (H–SOFCs). In terms of oxygen vacancy formation energy, O 2 adsorption energy, bond length of adsorbed O 2 , and O p-band center, La 0.5 Sr 0.5 Mn 0.875 Fe 0.125 O 3 (LSMF125) and La 0.5 Sr 0.5 Mn 25 Fe 0.75 O 3 (LSMF75) exhibited superior properties. Subsequent experimental investigations revealed that the H–SOFC with the LSMF75 cathode performed better than the cell with the LSMF125 cathode. The structure analysis revealed that LSMF75 has both Mn and Fe cations on its surface, similar to the designs of several naturally occurring enzymes, hence the increased cathode activity. By improving the LSMF75 cathode further, the cell utilizing the LSMF75 cathode attained a peak power density of 1238 mW cm-2 at 700 °C, which was much greater than that of the cells using the LSM or LSF cathodes, illustrating the benefits of using the LSM-LSF solid solution. Additionally, the good chemical stability of the LSMF75 cathode was maintained, allowing for 150 h of stable operation under operational conditions. [ABSTRACT FROM AUTHOR]

BaSrInFeO 5 (BSIF), a new cathode material for proton-conducting solid oxide fuel cells (SOFCs), is designed based on the modification of the Ba 2 In 2 O 5 proton conductor with Sr and Fe cations. Compared with the Ba 2 In 2 O 5 proton conductor tailored with only Fe cations (Ba 2 InFeO 5 , BIF), doping Sr can improve the chemical stability and also benefit the formation of oxygen vacancies. The proton mobility is also improved with Sr-doping, which is confirmed by first-principles calculations and experimental studies. An H-SOFC using the BSIF cathode generates a relatively high peak power density of 1192 mW cm-2 at 700 oC, which is superior to many cells in previous reports. First-principles calculations find that the cathode oxygen reduction reaction (ORR) energy barrier for BSIF is significantly lower than that for BIF. Although Ba 2 In 2 O 5 is less studied, the derived cathode materials can still present decent performance, probably offering new material selections for H-SOFCs. [ABSTRACT FROM AUTHOR]

U cations were immobilized in the traditional Sr 2 Fe 2 O 6-δ (SFO) oxide, which was subsequently evaluated as a cathode for proton-conducting solid oxide fuel cells (H–SOFCs). Experimental studies indicated that the U cations could be incorporated into the SFO lattice, forming the new Sr 2 Fe 1.5 U 0.5 O 6-δ (SFUO) oxide material. The SFUO material exhibited improved chemical stability than the SFO material, preventing the formation of carbonates after treatment in a CO 2 atmosphere. Although the total electrical conductivity was decreased with the U-doping, the surface catalytic activity of SFUO was improved compared with SFO, which was demonstrated by first-principles calculations. The subsequent electrochemical studies indicated that an H–SOFC using the SFUO cathode showed higher fuel cell output and smaller polarization resistance than the SFO cathode. The present study demonstrated that SFO could locate U cations in the lattice, and the produced SFUO oxide exhibited improved performance, suggesting the utilization of SFO cathode in the harsh environment (such as U-containing conditions) would be possible, and the electrochemical performance of the fuel cell is not reduced. [ABSTRACT FROM AUTHOR]

PrBaCo 2 O 5+δ (PBC) with high catalytic activity is identified as a prospective cathode material for intermediate temperature solid oxide fuel cells (IT-SOFCs). However, its poor chemical stability hinders its application. To address this problem, a Ta-doping strategy was presented in this study. The cathode with Ta-doping PBC was applied in proton conducting SOFCs. And the influence of Ta-doping on the crystal structure, electrochemical performance, structure stability and electrical conductivity of PBC was investigated. The resistance to CO 2 of PBC at elevated temperature is significantly improved with Ta-doping. The electrochemical performance measurements indicated that a low Ta-doping concentration did not change the performance of the cells obviously, while large Ta-doping concentration could lower the fuel cell performance. • Tantalum element was successfully doped into the crystal lattice of PBC. • Ta-doped PBC was successfully applied in H–SOFC. • The resistance to CO 2 of PBC at elevated temperature is significantly improved with Ta-doping. [ABSTRACT FROM AUTHOR]

• A novel strategy was proposed to efficiently and irreversibly remove Cd2+. • A new mechanism for the removal of Cd2+ with ZnS was confirmed. • Formation of a solid solution instead of CdS plays a predominant role in the removal process. • Solid solution possesses better thermodynamic stability than CdS. Sulfur-containing materials are very attractive for the efficient decontamination of some heavy metals. However, the effective and irreversible removal of Cd2+, coupled with a high uptake efficiency, remains a great challenge due to the relatively low bond dissociation energy of CdS. Herein, we propose a new strategy to overcome this challenge, by the incorporation of Cd2+ into a stable Zn x Cd 1-x S solid solution, rather than into CdS. This can be realised through the adsorption of Cd2+ by ZnS nanoparticles, which have exhibited a Cd2+ uptake capacity of approximate 400 mg g−1. Through this adsorption mechanism, the Cd2+ concentration in a contaminated solution could effectively be reduced from 50 ppb to <3 ppb, a WHO limit acceptable for drinking water. In addition, ZnS continued to exhibit this noteworthy uptake capacity even in the presence of Cu2+, Pb2+, and Hg2+. ZnS displayed high chemical stability. Particles aged in air for 3 months still retained a> 80% uptake capacity for Cd2+, compared with only 9% uptake capacity for similarly-aged FeS particles. This work reveals a new mechanism for Cd2+ removal with ZnS and establishes a valuable starting point for further studies into the formation of solid solutions for hazardous heavy metal removal applications. [ABSTRACT FROM AUTHOR]