Proton conducting solid oxide fuel cells with layered PrBa0.5Sr0.5Co2O5+δ perovskite cathode

BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY7) exhibits adequate protonic conductivity as well as sufficient chemical and thermal stability over a wide range of SOFC operating conditions, while layered perovskite PrBa_0.5Sr_0.5Co₂O_5+δ (PBSC) has advanced electrochemical properties. This research fully takes advantage of these advanced properties and develops a novel protonic ceramic membrane fuel cell (PCMFC) of Ni–BZCY7|BZCY7|PBSC. Experimental results show that the cell may achieve the open-circuit potential of 1.005 V, the maximal power density of 520 mW cm⁻², and a low electrode polarization resistance of 0.12 Ωcm² at 700 °C. Increasing operating temperature leads to the decrease of total cell resistance, among which electrolyte resistance becomes increasingly dominant over polarization resistance. The results also indicate that PBSC perovskite cathode is a good candidate for intermediate temperature PCMFC development, while the developed Ni–BZCY7|BZCY7|PBSC cell is a promising functional material system for SOFCs.     

Novel layered perovskite GdBaCoFeO5+δ as a potential cathode for proton-conducting solid oxide fuel cells

While cobalt-containing perovskite-type cathode materials facilitate the activation of oxygen reduction, they also suffer from problems like poor chemical stability in CO₂, high thermal expansion coefficients, etc. Partial B site substitution with Fe element is expected to be able to mitigate these problems while keeping high catalyst performance. In this paper, a layered perovskite GdBaCoFeO_5+δ (GBCF) was developed as a cathode material for protonic ceramic membrane fuel cells (PCMFCs) based on proton-conducting electrolyte of stable BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY7). The button cells of Ni-BZCY7|BZCY7|GBCF were fabricated and tested from 600 to 700 °C with humidified H₂ (∼3% H₂O) as a fuel and ambient oxygen as oxidant. An open-circuit potential of 1.002 V, maximum power density of 482 mW cm⁻², and a low electrode polarization resistance of 0.11 Ωcm² were achieved at 700 °C. The experimental results indicated that the layered perovskite GBCF is a good candidate for cathode material, while the developed Ni-BZCY7|BZCY7|GBCF cell is a promising functional material system for intermediate temperature solid oxide fuel cells.     

Novel layered perovskite oxide PrBaCuCoO5+δ as a potential cathode for intermediate-temperature solid oxide fuel cells

A novel layered perovskite oxide PrBaCuCoO_5+δ (PBCCO) is employed as a potential cathode for intermediate-temperature solid oxide fuel cells (IT-SOFCs). Thermal expansion and electrochemical performance on samarium-doped ceria (SDC) electrolyte are evaluated. The thermal expansion coefficient (TEC) of PrBaCuCoO_5+δ (PBCCO) is close to that of SDC electrolyte and electrical conductivity of PrBaCuCoO_5+δ (PBCCO) reaches the general required value of cathode material. Symmetrical electrochemical cell with the configuration of PrBaCuCoO_5+δ (PBCCO)/SDC/PrBaCuCoO_5+δ (PBCCO) applied for the impedance studies, the area specific resistance of PrBaCuCoO_5+δ (PBCCO) cathode is as low as 0.047 Ω cm² at 700 °C. A maximum power density of 791 mW cm⁻² is obtained at 700 °C for the single cell consisting of PrBaCuCoO_5+δ (PBCCO)/SDC/NiO-SDC. Preliminary results indicate that PrBaCuCoO_5+δ (PBCCO) is especially promising as a cathode for IT-SOFCs.     

Electrochemical performance of PrBaCo2O5+δ layered perovskite as an intermediate-temperature solid oxide fuel cell cathode

PrBaCo₂O_5+δ (PBCO) powder was prepared by a combined EDTA and citrate complexing method. The electrochemical performance of PBCO as a cathode for intermediate-temperature solid oxide fuel cells (IT-SOFCs) was evaluated. A porous layer of PBCO was deposited on a 42 μm thick electrolyte consisting of Ce_0.8Sm_0.2O_1.9 (SDC), prepared by a dry-pressing process. A fuel cell with a structure PBCO/SDC/Ni-SDC provides a maximum power density of 866, 583, 313 and 115 mW cm⁻² at 650, 600, 550 and 500 °C, respectively, using hydrogen as the fuel and stationary air as the oxidant. The total resistance of the cell was about 0.41, 0.51, 0.57 and 0.77 Ω cm², respectively. This encouraging data identifies PBCO as a potential cathode material for IT-SOFCs.     

Characterization and evaluation of NdBaCo2O5+δ cathode for proton-conducting solid oxide fuel cells

The layered perovskite structure oxide NdBaCo₂O_5+δ (NBCO) with rapid oxygen ion diffusion and surface exchange kinetics was synthesized by auto ignition process and initially examined as a cathode for proton-conducting fuel cells (H-SOFCs). The single cell, consisting of NdBaCo₂O_5+δ (NBCO)/BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY)/NiO–BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY) structure, was assembled and tested from 650 to 700 °C with humidified hydrogen (∼3% H₂O) as the fuel and air as the oxidant. A maximum power density of 438 mW cm⁻² at 700 °C was obtained for the single cell and electrochemical performance of the cell was studied.     

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.     

Enhanced electrochemical performance of the solid oxide fuel cell cathode using Ca3Co4O9+δ

This paper reports on the electrochemical performance of an SOFC cathode for potential use in intermediate-temperature solid oxide fuel cells (IT-SOFCs) using the oxygen non-stoichiometric misfit-layered cobaltite Ca₃Co₄O_9+δ or composites of Ca₃Co₄O_9+δ with Ce_0.9Gd_0.1O_1.95 (CGO/Ca₃Co₄O_9+δ). Electrochemical impedance spectroscopy revealed that symmetric cells with an electrode of pure Ca₃Co₄O_9+δ exhibit a cathode polarization resistance (R_p) of 12.4 Ω cm², at 600 °C in air. Strikingly, R_p of the composite CGO/Ca₃Co₄O_9+δ with 50 vol.% CGO was reduced by a factor of 19 (i.e. R_p = 0.64 Ω cm²), the lowest value reported so far for the Ca₃Co₄O₉ family of compounds. These findings together with the reported thermal expansion coefficient, good compatibility with CGO and chemical durability of this material suggest that it is a promising candidate cathode for IT-SOFCs.     

Potentiality of cobalt-free perovskite Ba0.5Sr0.5Fe0.9Mo0.1O3−δ as a single-phase cathode for intermediate-to-low-temperature solid oxide fuel cells

Cobalt-free perovskite Ba_0.5Sr_0.5Fe_0.9Mo_0.1O_3−δ (BSFMo) was investigated as a single-phase cathode for intermediate-to-low-temperature solid oxide fuel cells (IL-SOFCs). The X-ray diffraction (XRD) Rietveld refinement, electrical conductivity, thermogravimetric (TG) measurements, the phase reaction were investigated. The doping of high-valence Mo cations into Fe-site obviously enhanced the electrical conductivity of BSFMo sample with the maximum value of 174 S cm⁻¹. XRD results showed that BSFMo cathode was chemically compatible with the BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3−δ (BZCYYb) electrolyte for temperatures up to 1000 °C. Laboratory-sized tri-layer cells of NiO-BZCYYb/BZCYYb/BSFMo were operated from 550 to 700 °C with humidified hydrogen (～3% H₂O) as fuel and the static air as oxidant, respectively. An open-circuit potential of 1.001 V, the maximum power density of 428 mW cm⁻², and a low electrode polarization resistance of 0.148 Ω cm² were achieved at 700 °C. The experimental results indicated that the single-phase BSFMo is a promising candidate as cathode material for IL-SOFCs.     

Cobalt-free oxide Ba0.5Sr0.5Fe0.8Cu0.2O3−δ for proton-conducting solid oxide fuel cell cathode

A cobalt-free perovskite oxide Ba_0.5Sr_0.5Fe_0.8Cu_0.2O_3−δ (BSFC) is employed as a cathode material for intermediate-temperature proton-conducting solid oxide fuel cells. Symmetrical electrochemical cell with the configuration of BSFC-BZCY/BZCY/BSFC-BZCY is applied for the impedance study. The single cell, consisting of BSFC-BZCY/BZCY/NiO-BZCY structure, is assembled and tested from 600 to 700 °C with humidified hydrogen (∼3% H₂O) as the fuel and the static air as the oxidant. A maximum power density of 430 mW cm⁻² is obtained at 700 °C for the single cell. Long-term stability of the BSFC-BZCY/BZCY/NiO-BZCY single cell at 600 °C for 40 h has also been studied. Preliminary results demonstrat that cobalt-free oxide BSFC is a very promising cathode for application in proton-conducting solid oxide fuel cells.     

Layered perovskite LnBa0.5Sr0.5Cu2O5+δ (Ln = Pr and Nd) as cobalt-free cathode materials for solid oxide fuel cells

Cobalt-free layered perovskite LnBa_0.5Sr_0.5Cu₂O_5+δ (Ln = Pr and Nd, PBSC and NBSC) powders are prepared using combined citrate and EDTA complexing method. The performance of PBSC and NBSC cathode materials are evaluated for solid oxide fuel cells (SOFCs). Two oxidation states (Cu²⁺/Cu⁺) for Cu ions exist in LnBa_0.5Sr_0.5Cu₂O_5+δ oxides. The main valence of Pr ions in PBSC is 3+. The average thermal expansion coefficients (TECs) of PBSC and NBSC are 14.2 and 14.6 × 10^?6 K^?1 between 30 and 950 °C, which are similar to the TECs of La_0.9Sr_0.1Ga_0.8Mg_0.2O_3?δ (LSGM) intermediate-temperature electrolyte. The electrical conductivity of PBSC is slightly higher than that of NBSC. At 800 °C, the polarization resistance (R_p) values of the PBSC and NBSC cathodes on the LSGM electrolyte are 0.043 and 0.057 Ω cm², respectively. The electrolyte-supported single cells were prepared by using PBSC and NBSC as cathode, LSGM as electrolyte (300 μm thickness), Ce_0.9Sm_0.1O_1.95 (SDC) as interlayer and Ni/SDC as anode. At 850 °C, the maximal power densities are obtained as 681 and 651 mW cm^?2 for PBSC and NBSC cathodes.     

Characterization of cation-ordered perovskite oxide LaBaCo2O5+δ as cathode of intermediate-temperature solid oxide fuel cells

A-site cation-ordered perovskite oxide LaBaCo₂O_5+δ (LBCO) was synthesized and evaluated as a cathode material of intermediate-temperature solid oxide fuel cells (IT-SOFCs). LBCO was structurally stable when calcined at 850 °C in air but transformed into cation-disordered structure at 1050 °C. LBCO showed chemical compatibility with Gd_0.1Ce_0.9O_1.95 (GDC) electrolyte at 850 °C and 1000 °C in air. Conductivity of LBCO firstly increased slightly with higher temperature to a maximum of 470 S cm⁻¹ at ∼250 °C and then decreased gradually with further increase in temperature. Electrochemical impedance spectra of the LBCO/GDC/LBCO symmetric cell were measured, and electrode reaction mechanism for the LBCO cathode was analyzed. The electrode polarization resistance of LBCO was mainly contributed by oxygen ionic transfer across the cathode/electrolyte interface and oxygen atom diffusion-electronic charge transfer process. Low area-specific resistances with values ranging from 0.15 Ω cm² at 650 °C to 0.0086 Ω cm² at 800 °C were obtained. These results have demonstrated that the A-site cation-ordered perovskite oxide LBCO is a promising cathode material for IT-SOFCs.     

X-ray photoelectron spectroscopic studies of Ba0.5Sr0.5Co0.8Fe0.2O3−δ cathode for solid oxide fuel cells

The Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) cathode for solid oxide fuel cell has been prepared by glycine–nitrate combustion process. Crystal structure and chemical state of BSCF have been studied by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). XRD pattern indicates that a single cubic perovskite phase of BSCF oxide is successfully obtained after calcination at 850 °C for 2 h. XPS results show there exists a little amount of SrCO₃ in the surface of BSCF. Co2p spectra indicate that some Co³⁺ ions have changed into Co⁴⁺ ions to maintain the electrical neutrality. O1s spectra present that adsorbed oxygen species appear in the surface BSCF oxide.     

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.     

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.     

Structure-property relationship in layered perovskite cathode LnBa0.5Sr0.5Co2O5+δ (Ln = Pr, Nd) for solid oxide fuel cells

The layered cobaltites LnBa_0.5Sr_0.5Co₂O_5+δ (Ln = Pr, Nd) have been prepared by solid state reaction technique and structure-property relationships were investigated by means of neutron diffraction, ac impedance and dc conductivity measurements. Room temperature neutron diffraction shows the ordered distribution of oxygen vacancies in [PrO_δ] planes which doubles the lattice parameters from the perovskite cell parameter as a = b ≈ 2a_p, and c ≈ 2a_p (a_p is the cell parameter of the simple perovskite) yielding tetragonal symmetry in the P4/mmm space group. On heating, the oxygen vacancy ordering disappears and the structure can be defined as a = b ≈ a_p and c ≈ 2a_p in the same space group. Oxygen occupancies have been determined as a function of temperature from neutron diffraction. It was found that from 573 K to 973 K the total oxygen loss is about 0.265 O/formula unit and 0.366 O/formula unit for Pr and Nd containing materials, respectively. The oxygen occupancy decreases and cell volume increases with increasing temperature. Electrical conductivity measurements in air show that conductivity decreases with temperature, and at 873 K the conductivity is 493 S cm⁻¹ and 255 S cm⁻¹ for Pr and Nd containing samples, respectively. AC impedance measurements in symmetrical cell arrangement with CGO electrolyte shows that area specific resistance decreases with increasing temperature. At 873 K the ASR is 0.286 Ω cm² and 1.15 Ω cm² for Pr and Nd containing samples, respectively.     

Electrochemical performance of BaZr0.1Ce0.7Y0.1Yb0.1O3−δ electrolyte based proton-conducting SOFC solid oxide fuel cell with layered perovskite PrBaCo2O5+δ cathode

BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3−δ (BZCYYb) exhibits adequate protonic conductivity as well as sufficient chemical and thermal stability over a wide range of SOFC operating conditions, while layered perovskite PrBaCo₂O_5+δ (PBCO) has advanced electrochemical properties. This research fully takes advantage of these advanced properties and develops a novel protonic ceramic membrane fuel cell (PCMFC) of Ni-BZCYYb|BZCYYb|PBCO. The performance of the button cell was tested under intermediate-temperature range from 600 to 700 °C with humified H₂ (∼3% H₂O) as fuel and ambient air as oxidant. The results show that the open circuit potential of 0.983 V and the maximal power density of 490 mW cm⁻² were achieved at 700 °C. By co-doping barium zirconate-cerate with Y and Yb, the conductivity of electrolyte was significantly improved. The polarization processes of the button cell were characterized using the complicated electrochemical impedance spectroscopy technique. The results indicate that the polarization resistances contributed from both charge migration processes and mass transfer processes increase with decreasing cell voltage loads. However the polarization resistance induced by mass transfer processes is negligible in the studied button cell.     

A cobalt-free Sm0.5Sr0.5FeO3−δ–BaZr0.1Ce0.7Y0.2O3−δ composite cathode for proton-conducting solid oxide fuel cells

A cobalt-free Sm_0.5Sr_0.5FeO_3−δ–BaZr_0.1Ce_0.7Y_0.2O_3−δ (SSF–BZCY) was developed as a composite cathode material for proton-conducting solid oxide fuel cells (H-SOFC) based on proton-conducting electrolyte of stable BZCY. The button cells of Ni-BZCY/BZCY/SSF–BZCY were fabricated and tested from 550 to 700 °C with humidified H₂ (～3% H₂O) as a fuel and ambient oxygen as oxidant. An open-circuit potential of 1.024 V, maximum power density of 341 mW cm⁻², and a low electrode polarization resistance of 0.1 Ω cm² were achieved at 700 °C. The experimental results indicated that the SSF–BZCY composite cathode is a good candidate for cathode material.     

Oxygen reduction mechanism at Ba0.5Sr0.5Co0.8Fe0.2O3−δ cathode for solid oxide fuel cell

Perovskite structure Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) and La_0.9Sr_0.1Ga_0.8Mg_0.2O_3−δ (LSGM) powders have been successfully synthesized by glycine–nitrate combustion process. A porous and crack-free BSCF cathode is obtained by spraying the slurry of BSCF powders and terpineol onto LSGM pellet. The oxygen reduction reaction mechanism has been investigated by AC impedance spectroscopy and cyclic voltammetry method. AC impedance spectroscopy analysis shows that there are two different processes in the cathode reaction which are related to oxygen dissociation/adsorption and bulk oxygen diffusion. And the molecular oxygen is involved in the rate-determining step. The polarization resistance decreases with an increase of temperature and the oxygen partial pressure. With an increase of the applied DC bias, the logarithm of the polarization resistance decreases linearly due to additional oxygen vacancies and the lowered chemical potential of oxygen at the BSCF/LSGM interface by the applied voltage. The exchange current density reaches to 182 mA cm⁻² at 700 °C, suggesting that the ORR kinetics at the BSCF/LSGM interface is high due to the excellent mixed ionic and electronic conductivity of BSCF.     

A cobalt-free Sm0.5Sr0.5Fe0.8Cu0.2O3−δ-Ce0.8Sm0.2O2−δ composite cathode for proton-conducting solid oxide fuel cells

A cobalt-free composite Sm_0.5Sr_0.5Fe_0.8Cu_0.2O_3−δ-Ce_0.8Sm_0.2O_2−δ (SSFCu-SDC) is investigated as a cathode for proton-conducting solid oxide fuel cells (H-SOFCs) in intermediate temperature range, with BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3−δ (BZCYYb) as the electrolyte. The XRD results show that SSFCu is chemically compatible with SDC at temperatures up to 1100 °C. The quad-layer single cells of NiO-BZCYYb/NiO-BZCYYb/BZCYYb/SSFCu-SDC are operated from 500 to 700 °C with humidified hydrogen (∼3% H₂O) as fuel and the static air as oxidant. It shows an excellent power density of 505 mW cm⁻² at 700 °C. Moreover, a low electrode polarization resistance of 0.138 Ω cm² is achieved at 700 °C. Preliminary results demonstrate that the cobalt-free SSFCu-SDC composite is a promising cathode material for H-SOFCs.     

A novel cobalt-free layered GdBaFe2O5+δ cathode for proton conducting solid oxide fuel cells

While cobalt-containing perovskite-type cathode materials facilitate the activation of oxygen reduction, they also suffer from problems like poor chemical stability in CO₂ and high thermal expansion coefficients. In this research, a cobalt-free layered GdBaFe₂O_5+δ (GBF) perovskite was developed as a cathode material for protonic ceramic membrane fuel cells (PCMFCs) based on proton conducting electrolyte of stable BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY7). The button cells of Ni-BZCY7|BZCY7|GBF were fabricated and characterized using complex impedance technique from 600 to 700 °C. An open-circuit potential of 1.007 V, maximum power density of 417 mW cm⁻², and a low electrode polarization resistance of 0.18 Ω cm² were achieved at 700 °C. The results indicate that layered GBF perovskite is a good candidate for cobalt-free cathode material, while the developed Ni-BZCY7|BZCY7|GBF cell is a promising functional material system for solid oxide fuel cells.