Low-temperature solid oxide fuel cells with La1−xSrxMnO3 as the cathodes

La_1−xSr_xMnO₃ (LSM) has been widely developed as the cathode material for high-temperature solid oxide fuel cells (SOFCs) due to its chemical and mechanical compatibilities with the electrolyte materials. However, its application to low-temperature SOFCs is limited since its electrochemical activity decreases substantially when the temperature is reduced. In this work, low-temperature SOFCs based on LSM cathodes are developed by coating nanoscale samaria-doped ceria (SDC) onto the porous electrodes to significantly increase the electrode activity of both cathodes and anodes. A peak power density of 0.46 W cm⁻² and area specific interfacial polarization resistance of 0.36 Ω cm² are achieved at 600 °C for single cells consisting of Ni-SDC anodes, LSM cathodes, and SDC electrolytes. The cell performances are comparable with those obtained with cobalt-based cathodes such as Sm_0.5Sr_0.5CoO₃, and therefore encouraging in the development of low-temperature SOFCs with high reliability and durability.     

Low-temperature solid oxide fuel cells with novel La0.6Sr0.4Co0.8Cu0.2O3−δ perovskite cathode and functional graded anode

The perovskite La_0.6Sr_0.4Co_0.8Cu_0.2O_3−δ (LSCCu) oxide is synthesized by a modified Pechini method and examined as a novel cathode material for low-temperature solid oxide fuel cells (LT-SOFCs) based upon functional graded anode. The perovskite LSCCu exhibits excellent ionic and electronic conductivities in the intermediate-to-low-temperature range (400-800 °C). Thin Sm_0.2Ce_0.8O_1.9 (SDC) electrolyte and NiO-SDC anode functional layer are prepared over macroporous anode substrates composed of NiO-SDC by a one-step dry-pressing/co-firing process. A single cell with 20 μm thick SDC electrolyte on a porous anode support and LSCCu-SDC cathode shows peak power densities of only 583.2 mW cm⁻² at 650 °C and 309.4 mW cm⁻² for 550 °C. While a cell with 20 μm thick SDC electrolyte and an anode functional layer on the macroporous anode substrate shows peak power densities of 867.3 and 490.3 mW cm⁻² at 650 and 550 °C, respectively. The dramatic improvement of cell performance is attributed to the much improved anode microstructure that is confirmed by both SEM observation and impedance spectroscopy. The results indicate that LSCCu is a very promising cathode material for LT-SOFCs and the one-step dry-pressing/co-firing process is a suitable technique to fabricate high performance SOFCs.     

Infiltrated multiscale porous cathode for proton-conducting solid oxide fuel cells

A Sm_0.5Sr_0.5CoO_3−δ (SSC)-BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY) composite cathode with multiscale porous structure was successfully fabricated through infiltration for proton-conducting solid oxide fuel cells (SOFCs). The multiscale porous SSC catalyst was coated on the BZCY cathode backbones. Single cells with such composite cathode demonstrated peak power densities of 0.289, 0.383, and 0.491 W cm⁻² at 600, 650, 700 °C, respectively. Cell polarization resistances were found to be as low as 0.388, 0.162, and 0.064 Ω cm² at 600, 650 and 700 °C, respectively. Compared with the infiltrated multiscale porous cathode, cells with screen-printed SSC-BZCY composite cathode showed much higher polarization resistance of 0.912 Ω cm² at 600 °C. This work has demonstrated a promising approach in fabricating high performance proton-conducting SOFCs.     

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.     

High-performance PrBaCo2O5+δ-Ce0.8Sm0.2O1.9 composite cathodes for intermediate temperature solid oxide fuel cell

PrBaCo₂O_5+δ-Ce_0.8Sm_0.2O_1.9 (PBCO-SDC) composite material are prepared and characterized as cathode for intermediate temperature solid oxide fuel cells (IT-SOFCs). The powder X-ray diffraction result proves that there are no obvious reaction between the PBCO and SDC after calcination at 1100 °C for 3 h. AC impedance spectra based on SDC electrolyte measured at intermediate temperatures shows that the addition of SDC to PBCO improved remarkably the electrochemical performance of a PBCO cathode, and that a PBCO-30SDC cathode exhibits the best electrochemical performance in the PBCO-xSDC system. The total interfacial resistances R_p is the smallest when the content of SDC is 30 wt%, where the value is 0.035 Ω cm² at 750 °C, 0.072 Ω cm² at 700 °C, and 0.148 Ω cm² at 650 °C, much lower than the corresponding interfacial resistance for pure PBCO. The maximum power density of an anode-supported single cell with PBCO-30SDC cathode, Ni-SDC anode, and dense thin SDC/LSGM (La_0.9Sr_0.1Ga_0.8Mg_0.2O_3−δ)/SDC tri-layer electrolyte are 364, 521 and 741 mW cm⁻² at 700, 750 and 800 °C, respectively.     

Bi0.5Sr0.5MnO3 as cathode material for intermediate-temperature solid oxide fuel cells

Bi_0.5Sr_0.5MnO₃ (BSM), a manganite-based perovskite, has been investigated as a new cathode material for intermediate-temperature solid oxide fuel cells (SOFCs). The average thermal-expansion coefficient of BSM is 14 × 10⁻⁶ K⁻¹, close to that of the typical electrolyte material. Its electrical conductivity is 82-200 S cm⁻¹ over the temperature range of 600-800 °C, and the oxygen ionic conductivity is about 2.0 × 10⁻⁴ S cm⁻¹ at 800 °C. Although the cathodic polarization behavior of BSM is similar to that of lanthanum strontium manganite (LSM), the interfacial polarization resistance of BSM is substantially lower than that of LSM. The cathode polarization resistance of BSM is only 0.4 Ω cm² at 700 °C and it decreases to 0.17 Ω cm² when SDC is added to form a BSM-SDC composite cathode. Peak power densities of single cells using a pure BSM cathode and a BSM-SDC composite electrode are 277 and 349 mW cm² at 600 °C, respectively, which are much higher than those obtained with LSM-based cathode. The high electrochemical performance indicates that BSM can be a promising cathode material for intermediate-temperature SOFCs.     

Electrochemical performance of (Ba0.5Sr0.5)0.9Sm0.1Co0.8Fe0.2O3−δ as an intermediate temperature solid oxide fuel cell cathode

This study presents the electrochemical performance of (Ba_0.5Sr_0.5)_0.9Sm_0.1Co_0.8Fe_0.2O_3−δ (BSSCF) as a cathode material for intermediate temperature solid oxide fuel cells (IT-SOFC). AC-impedance analyses were carried on an electrolyte supported BSSCF/Sm_0.2Ce_0.8O_1.9 (SDC)/Ag half-cell and a Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF)/SDC/Ag half-cell. In contrast to the BSCF cathode half-cell, the total resistance of the BSSCF cathode half-cell was lower, e.g., at 550 °C; the values for the BSSCF and BSCF were 1.54 and 2.33 Ω cm², respectively. The cell performance measurements were conducted on a Ni-SDC anode supported single cell using a SDC thin film as electrolyte, and BSSCF layer as cathode. The maximum power densities were 681 mW cm⁻² at 600 °C and 820 mW cm⁻² at 650 °C.     

In situ drop-coated BaZr0.1Ce0.7Y0.2O3−δ electrolyte-based proton-conductor solid oxide fuel cells with a novel layered PrBaCuFeO5+δ cathode

In order to develop a simple and cost-effective route to fabricate proton-conductor intermediate-temperature SOFCs, a dense BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY) electrolyte was fabricated on a porous anode by in situ drop-coating. The PrBaCuFeO_5+δ (PBCF) composite oxide with layered perovskite structure was synthesized by auto ignition process and examined as a novel cathode for proton-conductor IT-SOFCs. The single cell, consisting of PBCF/BZCY/NiO-BZCY structure, was assembled and tested from 600 to 700 °C with humidified hydrogen (∼3% H₂O) as the fuel and the static air as the oxidant. An open-circuit potential of 1.01 V and a maximum power density of 445 mW cm⁻² at 700 °C were obtained for the single cell. A relatively low interfacial polarization resistance of 0.15 Ω cm² at 700 °C indicated that the PBCF is a promising cathode for proton-conductor IT-SOFCs.     

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.     

GdBaCo2O5+x layered perovskite as an intermediate temperature solid oxide fuel cell cathode

GdBaCo₂O_5+x (GBCO) was evaluated as a cathode for intermediate-temperature solid oxide fuel cells. A porous layer of GBCO was deposited on an anode-supported fuel cell consisting of a 15 μm thick electrolyte of yttria-stabilized zirconia (YSZ) prepared by dense screen-printing and a Ni–YSZ cermet as an anode (Ni–YSZ/YSZ/GBCO). Values of power density of 150 mW cm⁻² at 700 °C and ca. 250 mW cm⁻² at 800 °C are reported for this standard configuration using 5% of H₂ in nitrogen as fuel. An intermediate porous layer of YSZ was introduced between the electrolyte and the cathode improving the performance of the cell. Values for power density of 300 mW cm⁻² at 700 °C and ca. 500 mW cm⁻² at 800 °C in this configuration were achieved.     

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.     

Effects of a current treatment for an in-situ sintered cathode in a Ni-supported solid oxide fuel cell

Metal-supported solid oxide fuel cells (SOFCs) are one of the most promising candidates for applications in power plants as well as in portable applications due to their good mechanical and thermal properties. A Ni-supported SOFC that consists of a metal support (Ni, ∼180 μm), an anode functional layer (Ni-yttrium stabilized zirconia YSZ, ∼15 μm), an electrolyte (YSZ, ∼5 μm), and a nanocrystalline La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF) cathode is prepared. A nanocrystalline LSCF synthesized with ethylenediaminetetraacetic acid, citric acid, and inorganic nanodispersants, is used as an in situ sinterable cathode. The Ni-supported SOFC with nanocrystalline LSCFs is operated without a high temperature treatment for cathode sintering. The cell exhibits the maximum power density of 580 mW cm⁻² at 780 °C. A current treatment for 8 h (0.5 A cm⁻² at 780 °C) enhances the interfacial contact between the cathode and the electrolyte. After the current treatment, the maximum power density at 730 °C increase by 1.6 times from 260 mW cm⁻² to 390 mW cm⁻². The ohmic resistance (R_ohm) at 730 °C decreases from 0.43 Ω cm² to 0.21 Ω cm² and the charge transfer polarization at 0.7 V decreases from 0.42 Ω cm² to 0.30 Ω cm² due to lowered interfacial resistance between the cathode and the electrolyte. However, the mass transfer polarization increases from 0.09 Ω cm² to 0.17 Ω cm², which may result from the morphological change in the porous microstructure of the Ni support. The current treatment of the Ni-supported SOFC with in situ sintered LSCFs exhibit an increment in fuel cell performance due to the lowered ohmic resistance, which is beneficial for simple and mechanically improved fabrication and operation of metal-supported SOFCs.     

Evaluation of Ba0.6Sr0.4Co0.9Nb0.1O3−δ mixed conductor as a cathode for intermediate-temperature oxygen-ionic solid-oxide fuel cells

A perovskite-type Ba_0.6Sr_0.4Co_0.9Nb_0.1O_3−δ (BSCN) oxide is investigated as the cathode material of oxygen-ionic solid-oxide fuel cells (SOFCs) with Sm_0.2Ce_0.8O_3−δ (SDC) electrolyte. Powder X-ray diffraction and SEM characterization demonstrate that solid phase reactions between BSCN and SDC are negligible at temperatures up to 1100 °C. The results of thermal-expansion and electrical conductivity measurements indicate the introduction of Ba²⁺ into the A-site of SrCo_0.9N_0.1O_3−δ (SCN) led to a decrease in the thermal-expansion coefficient (TEC) and electrical conductivity of the compound. A TEC of 14.4 × 10⁻⁶ K⁻¹ is observed for BSCN within a temperature range of 200-500 °C. The chemical diffusion coefficient (D_chem) and surface exchange constant (k_ex) of BSCN and SCN are obtained using an electrical conductivity relaxation technique and BSCN prove to have higher D_chem and k_ex than SCN. An area-specific resistance of 0.1 Ω cm⁻² is achieved for BSCN cathodes at 600 °C based on symmetric cells test. Peak power density of ∼1150 mW cm⁻² is reached for a thin-film electrolyte cell with BSCN cathode at 600 °C, which is higher than a similar cell with SCN cathode (∼1008 mW cm⁻²). BSCN is a promising cathode material for oxygen-ionic IT-SOFCs.     

Fabrication and characterization of a Ba0.5Sr0.5Co0.8Fe0.2O3−δ—Gadolinia-doped ceria cathode for an anode-supported solid-oxide fuel cell

Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) and gadolinia-doped ceria (GDC) were synthesized via a glycine-nitrate process (GNP). A cubic perovskite of BSCF was observed by X-ray diffraction (XRD) at a calcination temperature above 950 °C. An anode-supported solid-oxide fuel cell was constructed from the porous NiO + YSZ as the anode substrate, the yittria-stabilized zirconia (YSZ) as the electrolyte, and the porous BSCF-GDC layer as the cathode with a GDC barrier layer. For the performance test, the maximum power density was 191.3 mW cm⁻² at a temperature of 750 °C with H₂ fuel and air at flow rates of 335 and 670 sccm, respectively. According to the AC-impedance data, the charge-transfer resistances of the electrodes were 0.10 and 1.59 Ω cm², and the oxygen-reduction and oxygen-ion diffusion resistances were 0.69 and 0.98 Ω cm² at 750 and 600 °C, respectively. SEM microstructural characterization indicated that the fuel cell as fabricated exhibited good compatibility between cathode and electrolyte layers.     

A high-performance Gd0.8Sr0.2CoO3–Ce0.9Gd0.1O1.95 composite cathode for intermediate temperature solid oxide fuel cell

Powders of Gd_0.8Sr_0.2CoO₃ (GSC) were prepared by a glycine–nitrate process. Symmetrical cathodes of GSC–50Ce_0.9Gd_0.1O_1.95 (GDC) (50:50 by volume) powders were deposited on GDC electrolyte pellets, and the electrochemical properties of the interfaces between the porous cathode and the electrolyte were investigated at intermediate temperature (500–750 °C) using electrochemical impedance spectroscopy. The addition of 50 vol.% GDC to GSC resulted in an additional factor ≈3 decrease in the area-specific resistance (ASR). The ASR values for the GSC–50GDC cathodes were as low as 0.064 Ω cm² at 700 °C and 0.16 Ω cm² at 600 °C, respectively. The maximum power density of a cell using the GSC–50GDC cathode was 356 mW cm⁻² at 700 °C.     

Intermediate-temperature electrochemical performance of a polycrystalline PrBaCo2O5+δ cathode on samarium-doped ceria electrolyte

A-site cation-ordered PrBaCo₂O_5+δ (PrBC) double perovskite oxide was synthesized and evaluated as the cathode of an intermediate-temperature solid-oxide fuel cell (IT-SOFC) on a samarium-doped ceria (SDC) electrolyte. The phase reaction between PrBC and SDC was weak even at 1100 °C. The oxygen reduction mechanism was investigated by electrochemical impedance spectroscopy characterization. Over the intermediate-temperature range of 450–700 °C, the electrode polarization resistance was mainly contributed from oxygen-ion transfer through the electrode–electrolyte interface and electron charge transfer over the electrode surface. An area-specific resistance as low as ∼0.4 Ω cm² was measured at 600 °C in air, based on symmetric cell test. A thin-film SDC electrolyte fuel cell with PrBC cathode was fabricated which delivered attractive peak power densities of 620 and 165 mW cm⁻² at 600 and 450 °C, respectively.     

Layered perovskite PrBa0.5Sr0.5Co2O5+δ as high performance cathode for solid oxide fuel cells using oxide proton-conducting electrolyte

The layered perovskite PrBa_0.5Sr_0.5Co₂O_5+δ (PBSC) was investigated as a cathode material for a solid oxide fuel cell using an oxide proton conductor based on BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY). The sintering conditions for the PBSC-BZCY composite cathode were optimized, resulting in the lowest area-specific resistance and apparent activation energy obtained with the cathode sintered at 1200 °C for 2 h. The maximum power densities of the PBSC-BZCY/BZCY/NiO-BZCY cell were 0.179, 0.274, 0.395, and 0.522 W cm⁻² at 550, 600, 650, and 700 °C, respectively with a 15 μm thick electrolyte. A relatively low cell interfacial polarization resistance of 0.132 Ω cm² at 700 °C indicated that the PBSC-BZCY could be a good cathode candidate for intermediate temperature SOFCs with BZCY electrolyte.     

Fabrication and evaluation of Ag-impregnated BaCe0.8Sm0.2O2.9 composite cathodes for proton conducting solid oxide fuel cells

Ag-BaCe_0.8Sm_0.2O_2.9 (BCS) composite cathodes are fabricated by an ion impregnation technique in this work, and the effect of fabrication details on their electro-performance is studied. The firing temperature of impregnated Ag has little effect on Ag loading but has a great impact on the polarization resistances. When fired at 400 °C, the minimum polarization resistance for symmetric cells reaches 0.11 Ω cm² measured at 600 °C with an Ag loading of 0.40 mg cm⁻². When fired at 600 °C, the minimum polarization resistance is 29.73 Ω cm² at 600 °C with 0.24 mg cm⁻² Ag-impregnated cathodes due to severe aggregation. The performance of Ag-impregnated cathodes is also compared with that of a Sm_0.5Sr_0.5CoO_3−δ (SSC) impregnated cathode. With the same volume ratio of 57%, the polarization resistance of an Ag-impregnated cathode is only about half of that for a SSC-impregnated cathode. Resistance simulation suggests that the reduction of low frequency resistances is the main reason for the decrease in polarization resistances in Ag-impregnated cathodes, which is consistent with its high oxygen diffusion coefficient. With a 57 vol.% Ag-impregnated cathode fired at 400 °C, the maximum power density of single cells is 283 mW cm⁻² at 600 °C, about 16% larger than that for a 57 vol.% SSC-impregnated cathode.     

Electrochemical performance of a solid oxide fuel cell based on Ce0.8Sm0.2O2−δ electrolyte synthesized by a polymer assisted combustion method

A polyvinyl alcohol assisted combustion synthesis method was used to prepare Ce_0.8Sm_0.2O_2−δ (SDC) powders for an intermediate temperature solid oxide fuel cell (IT-SOFC). The XRD results showed that this combustion synthesis route could yield phase-pure SDC powders at a relatively low calcination temperature. A thin SDC electrolyte film with thickness control was produced by a dry pressing method at a lower sintering temperature of 1250 °C. With Sm_0.5Sr_0.5Co₃-SDC as the composite cathode, a single cell based on this thin SDC electrolyte was tested from 550 to 650 °C. The maximum power density of 936 mW cm⁻² was achieved at 650 °C using humidified hydrogen as the fuel and stationary air as the oxidant.     

Micro-tubular solid oxide fuel cells with graded anodes fabricated with a phase inversion method

Micro-tubular proton-conducting solid oxide fuel cells (SOFCs) are developed with thin film BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3−δ (BZCYYb) electrolytes supported on Ni-BZCYYb anodes. The substrates, NiO-BZCYYb hollow fibers, are prepared by an immersion induced phase inversion technique. The resulted fibers have a special asymmetrical structure consisting of a sponge-like layer and a finger-like porous layer, which is propitious to serving as the anode supports for micro-tubular SOFCs. The fibers are characterized in terms of porosity, mechanical strength, and electrical conductivity regarding their sintering temperatures. To make a single cell, a dense BZCYYb electrolyte membrane about 20 μm thick is deposited on the hollow fiber by a suspension-coating process and a porous Sm_0.5Sr_0.5CoO₃ (SSC)-BZCYYb cathode is subsequently fabricated by a slurry coating technique. The micro-tubular proton-conducting SOFC generates a peak power density of 254 mW cm⁻² at 650 °C when humidified hydrogen is used as the fuel and ambient air as the oxidant.