Cathode reaction models and performance analysis of Sm0.5Sr0.5CoO3−δ-BaCe0.8Sm0.2O3−δ composite cathode for solid oxide fuel cells with proton conducting electrolyte

Cathode reaction models for solid oxide fuel cells with proton conducting electrolyte (H-SOFC) are proposed, and the reacting orders for each elementary step with respect to oxygen and water vapor partial pressure are calculated. The limiting steps of cathode reactions are investigated with Sm_0.5Sr_0.5CoO_3−δ (SSC)-BaCe_0.8Sm_0.2O_3−δ (BCS) composite cathodes. The results suggest that the migration of protons to TPBs and the surface diffusion of might be the limiting reactions for SSC-BCS composite cathodes in wet atmosphere, while the oxygen ions transferring into electrolyte, the reducing of O_ad to , and surface diffusion of might be the limiting reactions for SSC-BCS composite cathode in dry atmosphere.     

Fabrication and characterization of Sm0.2Ce0.8O2−δ-Sm0.5Sr0.5CoO3−δ composite cathode for anode supported solid oxide fuel cell

The Sm_0.5Sr_0.5CoO_3−δ (SSC) with perovskite structure is synthesized by the glycine nitrate process (GNP). The phase evolution of SSC powder with different calcination temperatures is investigated by X-ray diffraction and thermogravimetric analyses. The XRD results show that the single perovskite phase of the SSC is completely formed above 1100 °C. The anode-supported single cell is constructed with a porous Ni-yttria-stabilized zirconia (YSZ) anode substrate, an airtight YSZ electrolyte, a Sm_0.2Ce_0.8O_2−δ (SDC) barrier layer, and a screen-printed SSC-SDC composite cathode. The SEM results show that the dense YSZ electrolyte layer exhibits the good interfacial contact with both the Ni-YSZ and the SDC barrier layer. The porous SSC-SDC cathode shows an excellent adhesion with the SDC barrier layer. For the performance test, the maximum power densities are 464, 351 and 243 mW cm⁻² at 800, 750 and 700 °C, respectively. According to the results of the electrochemical impedance spectroscopy (EIS), the charge-transfer resistances of the electrodes are 0.49 and 1.24 Ω cm², and the non charge-transfer resistances are 0.48 and 0.51 Ω cm² at 800 and 700 °C, respectively. The cathode material of SSC is compatible with the YSZ electrolyte via a delicate scheme employed in the fabrication process of unit cell.     

(La,Pr)0.8Sr0.2FeO3−δ–Sm0.2Ce0.8O2−δ composite cathode for proton-conducting solid oxide fuel cells

Mixed rare-earth (La, Pr)_0.8Sr_0.2FeO_3−δ–Sm_0.2Ce_0.8O_2−δ (LPSF–SDC) composite cathode was investigated for proton-conducting solid oxide fuel cells based on protonic BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY) electrolyte. The powders of La_0.8−xPr_xSr_0.2FeO_3−δ (x = 0, 0.2, 0.4, 0.6), Sm_0.2Ce_0.8O_2−δ (SDC) and BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY) were synthesized by a citric acid-nitrates self-propagating combustion method. The XRD results indicate that La_0.8−xPr_xSr_0.2FeO_3−δ samples calcined at 950 °C exhibit perovskite structure and there are no interactions between LPSF0.2 and SDC at 1100 °C. The average thermal expansion coefficient (TEC) of LPSF0.2–SDC, BZCY and NiO-BZCY is 12.50 × 10⁻⁶ K⁻¹, 13.51 × 10⁻⁶ K⁻¹ and 13.47 × 10⁻⁶ K⁻¹, respectively, which can provide good thermal compatibility between electrodes and electrolyte. An anode-supported single cell of NiO-BZCY|BZCY|LPSF0.2–SDC was successfully fabricated and operated from 700 °C to 550 °C with humidified hydrogen (∼3% H₂O) as fuel and the static air as oxidant. A high maximum power density of 488 mW cm⁻², an open-circuit potential of 0.95 V, and a low electrode polarization resistance of 0.071 Ω cm² were achieved at 700 °C. Preliminary results demonstrate that LPSF0.2–SDC composite is a promising cathode material for proton-conducting solid oxide fuel cells.     

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.     

Characterization of nanosized Ce0.8Sm0.2O1.9-infiltrated Sm0.5Sr0.5Co0.8Cu0.2O3−δ cathodes for solid oxide fuel cells

This study investigates the microstructure and electrochemical properties of Sm_0.5Sr_0.5Co_0.8Cu_0.2O_3−δ (SSC-Cu) cathode infiltrated with Ce_0.8Sm_0.2O_1.9 (SDC). The newly formed nanosized electrolyte material on the cathode surface, leading the increase in electrochemical performances is mainly attributed to the creation of electrolyte/cathode phase boundaries, which considerably increases the electrochemical sites for oxygen reduction reaction. Based on the experiment results, the 0.4 M SDC infiltration reveals the lowest cathode polarization resistance (R_P), the cathode polarization resistances (R_p) are 0.117, 0.033, and 0.011 Ω cm² at 650, 750, and 850 °C, and the highest peak power density, are 439, 659, and 532 mW cm⁻² at 600, 700, and 800 °C, respectively. The cathode performance in SOFCs can be significantly improved by infiltrating nanoparticles of SDC into an SSC-Cu porous backbone. This study reveals that the infiltration approach may apply in SOFCs to improve their electrochemical properties.     

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.     

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.     

Investigation of Sm0.5Sr0.5CoO3−δ/Co3O4 composite cathode for intermediate-temperature solid oxide fuel cells

The electrochemical properties of an Sm_0.5Sr_0.5CoO_3−δ/Co₃O₄ (SSC/Co₃O₄) composite cathode were investigated as a function of the cathode-firing temperature, SSC/Co₃O₄ composition, oxygen partial pressure and CO₂ treatment. The results showed that the composite cathodes had an optimal microstructure at a firing temperature of about 1100 °C, while the optimum Co₃O₄ content in the composite cathode was about 40 wt.%. A single cell with this optimized C₄₀-1100 cathode exhibited a very low polarization resistance of 0.058 Ω cm², and yielded a maximum power density of 1092 mW cm⁻² with humidified hydrogen fuel and air oxidant at 600 °C. The maximum power density reached 1452 mW cm⁻² when pure oxygen was used as the oxidant for a cell with a C₃₀-1100 cathode operating at 600 °C due to the enhanced open-circuit voltage and accelerated oxygen surface-exchange rate. X-ray diffraction and thermogravimetric analyses, as well as the electrochemical properties of a CO₂-treated cathode, also implied promising applications of such highly efficient SSC/Co₃O₄ composite cathodes in single-chamber fuel cells with direct hydrocarbon fuels operating at temperatures below 500 °C.     

Nd0.5Sr0.5Fe0.8Cu0.2O3−δ–xSm0.2Ce0.8O1.9 cobalt-free composite cathodes for intermediate temperature solid oxide fuel cells

Cobalt-free composites Nd_0.5Sr_0.5Fe_0.8Cu_0.2O_3−δ (NSFCu)–xSm_0.2Ce_0.8O_1.9 (SDC) (x = 0–60 wt%) are investigated as IT-SOFC cathodes. The characteristic properties of cobalt-free composite cathodes comparing to cobalt-based composites are revealed. The DC conductivity and thermal expansion coefficient of the composite cathodes decrease with the content of SDC x, while the polarization resistance R_p shows the least value with addition of 40 wt% of SDC. The power density of the single cell with NSFCu-40% SDC composite cathode improved significantly compared with that of undoped NSFCu cathode, with peak values of 488, 623, 849 and 1052 mW cm⁻² at 600, 650, 700, and 750 °C, respectively. Moreover, the performance of the composite cathode is stable within testing period of 370 h at 700 °C, indicating that the NSFCu-40% SDC is an excellent cobalt-free composite cathode applied in IT-SOFC.     

Preparation and characterization of Ba0.5Sr0.5Fe0.9Ni0.1O3−δ–Sm0.2Ce0.8O1.9 compose cathode for proton-conducting solid oxide fuel cells

A cobalt-free Ba_0.5Sr_0.5Fe_0.9Ni_0.1O_3−δ–Sm_0.2Ce_0.8O_1.9 (BSFN–SDC) composite was employed as a cathode for proton-conducting solid oxide fuel cells (H-SOFCs) using BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY) as the electrolyte. The chemical compatibility between BSFN and SDC was evaluated. The XRD results showed that BSFN was chemically compatible with SDC after co-fired at 1100 °C for 5 h. The thermal expansion coefficient (TEC) of BSFN–SDC, which showed a reasonably reduced value (16.08 × 10⁻⁶ K⁻¹), was effectively decreased due to Ce_0.8Sm_0.2O_1.9 (SDC) added. A single cell of Ni–BZCY/Ni–BZCY/BZCY/BSFN–SDC with a 25-μm-thick BZCY electrolyte membrane exhibited excellent power densities as high as 361.8 mW cm⁻² at 700 °C with a low polarization resistance of 0.174 Ω cm². The excellent performance implied that the cobalt-free BSFN–SDC composite was a promising alternative cathode for H-SOFCs.     

Composite cathodes based on Sm0.5Sr0.5CoO3−δ with porous Gd-doped ceria barrier layers for solid oxide fuel cells

Cobaltite-based perovskites based on Sm_0.5Sr_0.5CoO_3−δ (SSC) are attractive as a cathode material with a barrier layer for solid oxide fuel cells (SOFC) due to their high electrochemical activity and electrical conductivity. SSC, synthesized by a complex method, is used as a cathode material in a composite cathode with Gd-doped ceria (GDC). A porous GDC layer is fabricated as a barrier to resist reactions of SSC with yttria-stabilized zirconia (YSZ). The effects of the ratio of SSC on GDC in composite cathodes and the thickness of the GDC barrier are characterized in this study. An SOFC with an SSC7–GDC3 composite cathode on a 4 μm GDC layer at 0.8 V yields the highest fuel cell performance: 1.24 W cm⁻² and 0.61 W cm⁻² at 780 °C and 680 °C, respectively. Impedance analysis indicates that the ohmic resistances are more dependent upon the GDC barrier thickness than the cathode composition. The polarization resistances at 780 °C and 730 °C exhibit similar values, but with decreasing temperature, the polarization resistances change dramatically according to the composition and barrier thickness. The ohmic and polarization resistances show different trends in different temperature ranges, due to the different charge transfer mechanisms of SSC and GDC within those temperature ranges. To obtain higher fuel cell performance, the addition of GDC into the porous SSC is effective, and the compositions of the composite cathode as well as the thickness of the barrier layer need to be optimized.     

Sm0.5Sr0.5Co0.4Ni0.6O3−δ–Sm0.2Ce0.8O1.9 as a potential cathode for intermediate-temperature solid oxide fuel cells

The mixed ionic and electronic conductors (MIECs) of Sm_0.5Sr_0.5Co_0.4Ni_0.6O_3−δ (SSCN)–Sm_0.2Ce_0.8O_1.9 (SDC) were investigated for potential application as a cathode material for intermediate-temperature solid oxide fuel cells (IT-SOFCs) based on an SDC electrolyte. Electrochemical impedance spectroscopy (EIS) technique was performed over the temperature range of 600–850 °C to determine the cathode polarization resistance which is represented by area specific resistance (ASR). To investigate the ORR mechanism, the impedance diagram for 70SSCN–30SDC was measured under applied cathodic voltage from E = 0.0 to E = −0.3 V. It indicated that the charge transfer dominated the rate-determining step at the temperature of 600 °C; whereas the diffusion or dissociative adsorption of oxygen dominated the rate-determining step at the temperature of 800 °C. In this study, the exchange current density (i₀) for oxygen reduction reaction (ORR) was determined from the EIS data. The i₀ value of 70SSCN–30SDC/SDC was 187.6 mA cm⁻² which is larger than the i₀ value of 160 mA cm⁻² for traditional cathode/electrolyte, i.e. LSM/YSZ at 800 °C, indicating that the 70SSCN–30SDC composite cathode with a high catalytically active surface area could provide the oxygen reduction reaction areas not only at the triple-phase boundaries but also in the whole composite cathode.     

Electrochemical performance of La0.7Sr0.3CuO3−δ–Sm0.2Ce0.8O2−δ functional graded composite cathode for intermediate temperature solid oxide fuel cells

The fabrication and electrochemical properties of graded La_0.7Sr_0.3CuO_3−δ–Sm_0.2Ce_0.8O_2−δ (LSCu–SDC) composite cathodes were investigated in this paper. The phase composition, microstructure and electrochemical properties of the electrodes were characterized using X-ray diffraction (XRD), electron microscopy, electrochemical impedance spectroscopy (EIS) and cathodic polarization examinations. The results showed that the triple-layer graded cathode had super electrochemical performance comparing with the monolayer cathode. The graded LSCu–SDC cathode showed a polarization resistance of 0.094 Ωcm², a value much lower than the monolayer LSCu cathode of 0.234 Ωcm² at 800 °C in air. The current density of the graded cathode was 0.341 A cm⁻², more than double higher than monolayer LSCu of 0.146 A cm⁻² at an overpotential of 30 mV. The improved electrochemical performance could be attributed to the improved physical and chemical compatibility of the cathode layers in graded compositions with SDC electrolyte as well as the enlargement of triple-phase boundary for oxygen reduction.     

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.     

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.     

Palladium and ceria infiltrated La0.8Sr0.2Co0.5Fe0.5O3−δ cathodes of solid oxide fuel cells

La_0.8Sr_0.2Co_0.5Fe_0.5O_3−δ (LSCF) cathodes infiltrated with electrocatalytically active Pd and (Gd,Ce)O₂ (GDC) nanoparticles are investigated as high performance cathodes for the O₂ reduction reaction in intermediate temperature solid oxide fuel cells (IT-SOFCs). Incorporation of nano-sized Pd and GDC particles significantly reduces the electrode area specific resistance (ASR) as compared to the pure LSCF cathode; ASR is 0.1 Ω cm² for the reaction on a LSCF cathode infiltrated with 1.2 mg cm⁻² Pd and 0.06 Ω cm² on a LSCF cathode infiltrated with 1.5 mg cm⁻² GDC at 750 °C, which are all significantly smaller than 0.22 Ω cm² obtained for the reaction on a conventional LSCF cathode. The activation energy of GDC- and Pd-impregnated LSCF cathodes is 157 and 176 kJ mol⁻¹, respectively. The GDC-infiltrated LSCF cathode has a lower activation energy and higher electrocatalytic activity for the O₂ reduction reaction, showing promising potential for applications in IT-SOFCs.     

Investigation of cobalt-free cathode material Sm0.5Sr0.5Fe0.8Cu0.2O3−δ for intermediate temperature solid oxide fuel cell

A cobalt-free cubic perovskite oxide Sm_0.5Sr_0.5Fe_0.8Cu_0.2O_3−δ (SSFCu) was investigated as a novel cathode for intermediate temperature solid oxide fuel cells (IT-SOFCs). The thermal expansion coefficient (TEC) of SSFCu was close to that of Sm_0.2Ce_0.8O_1.9(SDC) electrolyte and the electrical conductivity of SSFCu sample reached 72–82 S cm⁻¹ in the commonly operated temperatures of IT-SOFCs (400–600 °C). Symmetrical electrochemical cell with the configuration of SSFCu/SDC/SSFCu was applied for the impedance study and area specific resistance (ASR) of SSFCu cathode material was as low as 0.085 Ω cm² at 700 °C. Laboratory-sized tri-layer cells of NiO-SDC/SDC/SSFCu were operated from 450 to 700 °C with humidified hydrogen (∼3% H₂O) as fuel and the static air as oxidant. A maximum power density of 808 mW cm² was obtained at 700 °C for the single cell.     

Sr0.92Y0.08TiO3−δ/Sm0.2Ce0.8O2−δ anode for solid oxide fuel cells running on methane

Yttria-doped strontium titanium oxide (Sr_0.92Y_0.08TiO_3−δ; SYT) was investigated as an alternative anode material for solid oxide fuel cells (SOFCs). The SYT synthesized by the Pechini method exhibits excellent phase stability during the cell fabrication processes and SOFC operation and good electrical conductivity (about 0.85 S/cm, porosity 30%) in reducing atmosphere. The performance of SYT anode is characterized by slow electrochemical reactions except for the gas-phase diffusion reactions. The cell performance with the SYT anode running on methane fuel was improved about 5 times by SDC film coating, which increased the number of reaction sites and also accelerated electrochemical reaction kinetics of the anode. In addition, the SDC-coated SYT anode cell was stably operated for 900 h with methane. These results show that the SDC-coated SYT anode can be a promising anode material for high temperature SOFCs running directly on hydrocarbon fuels.     

Solid oxide fuel cells with Sm0.2Ce0.8O2−δ electrolyte film deposited by novel aerosol deposition method

In this study, dense electrolyte ceramic Sm_0.2Ce_0.8O_2−δ (SDC) thin films are successfully deposited on NiO-SDC anode substrate by aerosol deposition (AD) with oxygen as the carrier gas at the substrate temperature ranging from room temperature to 300 °C. To remove the effect of humidity on the starting powders, this study found that, in depositing SDC films, having the starting powders preheat-treated at 200 °C helped generate a smooth and dense layer, though a lower deposition rate was achieved. At a deposition time of 22 min, SDC films with a uniform thickness of 1.5 μm and grain sizes of ≈67 nm are obtained. SOFC single cells are then built by screen printing a LSCF cathode on the anode-supported substrates with SDC electrolyte. The cross-sectional SEM micrographs exhibit highly dense, granular, and crack-free microstructures. The open circuit voltages (OCV) of the single cells decrease with the rise in temperature, dropping from 0.81 V at 500 °C to 0.59 V at 700 °C. Maximum power densities (MPD) decline with decreasing operating temperature from 0.34 to 0.01 W cm⁻² due to the increase of the R₀ and R_P of the single cells. The electrochemical results testify to the fine quality of SDC films as well as illustrate the electrolyte thickness effect and the effect of mixed ionic and electronic conduction of the SDC electrolyte in the reducing atmosphere.     

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.