High performance proton-conducting solid oxide fuel cells with a stable Sm0.5Sr0.5Co3−δ-Ce0.8Sm0.2O2−δ composite cathode

A Sm_0.5Sr_0.5CoO_3−δ-Ce_0.8Sm_0.2O_2−δ (SSC-SDC) composite is employed as a cathode for proton-conducting solid oxide fuel cells (H-SOFCs). BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY) is used as the electrolyte, and the system exhibits a relatively high performance. An extremely low electrode polarization resistance of 0.066 Ω cm² is achieved at 700 °C. The maximum power densities are: 665, 504, 344, 214, and 118 mW cm⁻² at 700, 650, 600, 550, and 500 °C, respectively. Moreover, the SSC-SDC cathode shows an essentially stable performance for 25 h at 600 °C with a constant output voltage of 0.5 V. This excellent performance implies that SSC-SDC, which is a typical cathode material for SOFCs based on oxide ionic conductor, is also a promising alternative cathode for H-SOFCs.     

Silver-modified Ba0.5Sr0.5Co0.8Fe0.2O3−δ as cathodes for a proton conducting solid-oxide fuel cell

Electrochemical performance of silver-modified Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF-Ag) as oxygen reduction electrodes for a protonic intermediate-temperature solid-oxide fuel cell (SOFC-H⁺) with BaZr_0.1Ce_0.8Y_0.1O₃ (BZCY) electrolyte was investigated. The BSCF-Ag electrodes were prepared by impregnating the porous BSCF electrode with AgNO₃ solution followed by reducing with hydrazine and then firing at 850 °C for 1 h. The 3 wt.% silver-modified BSCF (BSCF-3Ag) electrode showed an area specific resistance of 0.25 Ω cm² at 650 °C in dry air, compared to around 0.55 Ω cm² for a pure BSCF electrode. The activation energy was also reduced from 119 kJ mol⁻¹ for BSCF to only 84 kJ mol⁻¹ for BSCF-3Ag. Anode-supported SOFC-H⁺ with a BZCY electrolyte and a BSCF-3Ag cathode was fabricated. Peak power density up to 595 mW cm⁻² was achieved at 750 °C for a cell with 35 μm thick electrolyte operating on hydrogen fuel, higher than around 485 mW cm⁻² for a similar cell with BSCF cathode. However, at reduced temperatures, water had a negative effect on the oxygen reduction over BSCF-Ag electrode, as a result, a worse cell performance was observed for the cell with BSCF-3Ag electrode than that with pure BSCF electrode at 600 °C.     

Study on BaCo0.7Fe0.2Nb0.1O3−δ—SDC composite cathodes for intermediate temperature solid oxide fuel cell

BaCo_0.7Fe_0.2Nb_0.1O_3−δ(BCFN)/Ce_0.8Sm_0.2O_1.9(SDC) composite material was prepared and characterized as cathode for intermediate temperature solid oxide fuel cells. The X-ray diffraction result proved that there was no obvious reaction between the BCFN and SDC after calcination at 1000 °C for 10 h. AC impedance spectra based on La_0.9Sr_0.1Ga_0.8Mg_0.2O_3−δ(LSGM) electrolyte measured at intermediate temperatures showed that a cathode with 30 wt% SDC exhibited the best electrochemical performance among the electrodes studied. The interfacial resistance value for BCFN/30SDC was as low as 0.0104, 0.017, 0.029, and 0.062 Ω cm² at 800, 750, 700 and 650 °C, respectively. The maximum power density of a single cell with BCFN/30SDC cathode, Ni_0.9Cu_0.1-SDC anode, and LSGM/SDC electrolyte was 209.7, 298.2, 407.1, 543.4 and 697.9 mW cm⁻² at 600, 650, 700, 750 and 800 °C.     

The synthesis and characterization of the Sm0.5Sr0.5Co1−xCuxO3−δ cathode by the glycine-nitrate process

In this study, a new oxygen-deficient cathode material, Sm_0.5Sr_0.5Co_1−xCu_xO_3−δ (SSCCu) was developed. It is expected to enhance the efficiency of intermediate-temperature solid oxide fuel cells (IT-SOFCs). The structure, conductivity and electrochemical performance of SSCCu were examined as a function of copper content. The structure of Sm_0.5Sr_0.5Co_0.9Cu_0.1O_3−δ and Sm_0.5Sr_0.5Co_0.8Cu_0.2O_3−δ samples was a single orthorhombic perovskite phase. Second phase SrCoO_2.8, however, formed in the Sm_0.5Sr_0.5Co_0.7Cu_0.3O_3−δ and Sm_0.5Sr_0.5Co_0.6Cu_0.4O_3−δ samples. The conductivity of the Sm_0.5Sr_0.5Co_0.7Cu_0.3O_3−δ cathode was higher than that of other samples. However, the Sm_0.5Sr_0.5Co_0.8Cu_0.2O_3−δ electrode exhibited the lowest overpotential of 25 mV at 400 mA cm⁻² and the lowest area special resistance of 0.2 Ω cm² at 700 °C.     

Low thermal expansion material Bi0·5Ba0·5FeO3-δ in application for proton-conducting ceramic fuel cells cathode

The cobalt-free material of Bi³⁺-doped BaFeO_3-δ (BBFO) is synthesized and applied as a cathode material for intermediate-temperature solid oxide fuel cells (IT-SOFCs) with proton conducting electrolyte Ba(Zr_0·1Ce_0·7Y_0.2)O₃ (BZCY). The as-prepared BBFO demonstrates a tetragonal structure with sufficient chemical compatibility, high thermal stability and low thermal expansion coefficient. BBFO exhibits higher electrical conductivity of 4.1 S cm⁻¹compared to the parent material BaFeO_3-δ (BFO) of 3.5 S cm⁻¹. The composite cathode BBFO-BZCY with the mass ratio of 7:3 presents a relatively low R_p of 0.128 Ω cm² at 700 °C in air. According to the oxygen reduction reaction process, the rate-determining step is transformed from charge transfer to oxygen gas (O₂) adsorption-dissociation with the rising temperature. In addition, the performance of the anode-supported cell NiO-BZCY∣BZCY∣BBFO-BZCY is 120 mW cm⁻² as the thickness of electrolyte is 150 μm. It thus promises BBFO as a novel cathode for proton-conducting ceramic fuel cells.     

Fabrication and characterization of a co-fired La0.6Sr0.4Co0.2Fe0.8O3−δ cathode-supported Ce0.9Gd0.1O1.95 thin-film for IT-SOFCs

A dense membrane of Ce_0.9Gd_0.1O_1.95 on a porous cathode based on a mixed conducting La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ was fabricated via a slurry coating/co-firing process. With the purpose of matching of shrinkage between the support cathode and the supported membrane, nano-Ce_0.9Gd_0.1O_1.95 powder with specific surface area of 30 m² g⁻¹ was synthesized by a newly devised coprecipitation to make the low-temperature sinterable electrolyte, whereas 39 m² g⁻¹ nano-Ce_0.9Gd_0.1O_1.95 prepared from citrate method was added to the cathode to favor the shrinkage for the La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ. Bi-layers consisting of <20 μm dense ceria film on 2 mm thick porous cathode were successfully fabricated at 1200 °C. This was followed by co-firing with NiO–Ce_0.9Gd_0.1O_1.95 at 1100 °C to form a thin, porous, and well-adherent anode. The laboratory-sized cathode-supported cell was shown to operate below 600 °C, and the maximum power density obtained was 35 mW cm⁻² at 550 °C, 60 mW cm⁻² at 600 °C.     

Synthesis,electrical and electrochemical properties of Ba0.5Sr0.5Zn0.2Fe0.8O3−δ perovskite oxide for IT-SOFC cathode

The Ba_0.5Sr_0.5Zn_0.2Fe_0.8O_3−δ (BSZF) complex oxide with cubic perovskite structure was synthesized and examined as a new cobalt-free cathode for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The electrical conductivity was relatively low with a peak value of 9.4 S cm⁻¹ at about 590 °C, which was mainly caused by the high concentration of oxygen vacancy and the doping of bivalent zinc in B-sites. At 650 °C and under open circuit condition, symmetrical BSZF cathode on Sm-doped ceria (SDC) electrolyte showed polarization resistances (R_p) of 0.48 Ω cm² and 0.35 Ω cm² in air and oxygen, respectively. The dependence of R_p with oxygen partial pressure indicated that the rate-limiting step for oxygen reduction was oxygen adsorption/desorption kinetics. Using BSZF as the cathode, the wet hydrogen fueled Ni + SDC anode-supported single cell exhibited peak power densities of 392 mW cm⁻² and 626 mW cm⁻² at 650 °C when stationary air and oxygen flux were used as oxidants, respectively.     

La substituted Sr2MnO4 as a possible cathode material in SOFC

Sr_2−xLa_xMnO_4+δ (x = 0.4, 0.5, 0.6) oxides were studied as the cathode material for solid oxide fuel cells (SOFC). The reactivity tests indicated that no reaction occurred between Sr_2−xLa_xMnO_4+δ and CGO at annealing temperature of 1000 °C, and the electrode formed good contact with the electrolyte after being sintered at 1000 °C for 4 h. The total electrical conductivity, which has strong effect on the electrode properties, was determined in a temperature range from 100 to 800 °C. The maximum value of 5.7 S cm⁻¹ was found for the x = 0.6 phase at 800 °C in air. The cathode polarization and AC impedance results showed that Sr_1.4La_0.6MnO_4+δ exhibited the lowest cathode overpotential. The area specific resistance (ASR) was 0.39 Ω cm² at 800 °C in air. The charge transfer process is the rate-limiting step for oxygen reduction reaction on Sr_1.4La_0.6MnO_4+δ electrode.     

Novel nano-network cathodes for solid oxide fuel cells

A novel nano-network of Sm_0.5Sr_0.5CoO_3−δ (SSC) is successfully fabricated as the cathodes for intermediate-temperature solid oxide fuel cells (SOFCs) operated at 500–600 °C. The cathode is composed of SSC nanowires formed from nanobeads of less than 50 nm thus exhibiting high surface area and porosity, forming straight path for oxygen ion and electron transportation, resulting in high three-phase boundaries, and consequently showing remarkably high electrode performance. An anode-supported cell with the nano-network cathode demonstrates a peak power density of 0.44 W cm⁻² at 500 °C and displays exceptional performance with cell operating time. The result suggests a new direction to significantly improve the SOFC performance.     

La0.6Sr0.4Co0.8Ni0.2O3−δ hollow fiber membrane reactor: Integrated oxygen separation – CO2 reforming of methane reaction for hydrogen production

An integrated reactor system which combines oxygen permeable La_0.6Sr_0.4Co_0.8Ni_0.2O_3−δ (LSCN) perovskite ceramic hollow fiber membrane with Ni based catalyst has been successfully developed to produce hydrogen through oxy-CO₂ reforming of methane (OCRM). Dense La_0.6Sr_0.4Co_0.8Ni_0.2O_3−δ hollow fiber membrane was prepared using phase inversion-sintering method. OCRM reaction was tested from 650 °C to 800 °C with a quartz reactor packed with 0.5 g Ni/Al₂O₃ catalyst around the LSCN hollow fiber membrane. CH₄ and CO₂ were used as reactants and air as the oxygen source was fed through the bore side of the hollow fiber membrane. In order to gauge the effectiveness of this membrane reactor system, air flow was closed at 800 °C and dry reforming of methane (DRM) was tested for comparison. The results show that the oxygen fluxes of LSCN membrane swept by helium are nearly 3 times less than those swept by OCRM reactants. With increasing temperature and oxygen supply, methane conversion in the OCRM reactor reaches 100%, but CO₂ conversion decreases from 87% to 72% due to the competition reaction with POM. CO selectivity is as high as nearly 100% at reaction temperatures of 700 °C–800 °C while H₂ selectivity reaches a maximum of 88% at 700 °C. At 800 °C, when air supply was closed and DRM was conducted for comparison, CO selectivity decreased to 91%, resulting in carbon deposition which was around 4 times more than those obtained under OCRM reaction and H₂/CO ratio decreased from 0.93 to 0.74, showing better carbon resistance and higher H₂ selectivity of the Ni-based catalyst over the integrated oxygen separation-OCRM reaction across the LSCN hollow fiber membrane reactor.