SOFC Cathode Research Articles

Oxygen Non-Stoichiometry and Point Defect Equilibria in (La1/6Pr1/6Nd1/6Gd1/6Ba1/6Sr1/6)MnO3- δ

In this work, the A-site of the common SOFC cathode La0.8Sr0.2MnO3-δ (LSM) is modified with additional lanthanide and alkali elements, resulting in the composition (La1/6Pr1/6Nd1/6Gd1/6Ba1/6Sr1/6)MnO3- δ. We show that the amount of oxygen released during reduction measured by thermo-gravimetric analysis is significantly greater for this “high entropy” perovskite oxide (HEPO) in comparison with La2/3Sr1/3MnO3-δ, the latter having the same amount of total acceptor dopant and with a similar average A-site radius. The origins of enhanced reduction will be discussed.

Influence of Temperature and Water Vapour on the High-Temperature Oxidation and Cr Evaporation Behaviour of High-Cr Alloys for SOFC Cathode Air Pre-Heaters

The oxidation and Cr2O3 evaporation mechanisms of the stainless steels Inconel 625, SS309 and AluChrom 318 have been investigated by varying temperatures and water content of the exposure atmosphere. The finding of this research demonstrates that Cr2O3 evaporation and oxidation rates were significantly lowered with decreasing temperature for Inconel 625 and SS309. AluChrom 318 showed a decreased oxidation rate with decreasing temperature, but it demonstrated a reverse trend to the temperature-dependent Cr2O3 evaporation compared to Inconel 625 and SS309. The major influence of water vapour on the three tested materials appeared to be the further increase of Cr2O3 evaporation.

Enhancing ORR Catalytic Activity and Electrochemical Investigation of La1−2xBaxBixFeO3 Cathode for Low-Temperature Solid Oxide Fuel Cell

Perovskite cathodes have emerged as a promising alternative to traditional cathode materials in low-temperature solid oxide fuel cells (LT-SOFCs) due to their exceptional catalytic properties and high oxygen reduction reaction (ORR) activity. Their fast catalytic activity and chemical stability have drawn significant attention to lowering the operating temperature of SOFCs. In this study, Ba2+ and Bi3+ are doped into LaFeO3. The aim is to investigate the catalytic activity and electrochemical performance of LT-SOFCs. The presented cathode material is characterized in terms of phase structure, surface morphology, and interface studies before being applied as a cathode in SOFCs to measure electrochemical performance. The XPS study revealed that La1−2xBaxBixFeO3 (x = 0.1) exhibits enriched surface oxygen vacancies compared to La1−2xBaxBixFeO3 (x = 0.2). La1−2xBaxBixFeO3 with (x = 0.1 and 0.2) delivers a peak power density of 665 and 545 mW cm−2 at 550 °C, respectively. Moreover, impedance spectra confirmed that La1−2xBaxBixFeO3 with x = 0.1 exhibits lower electrode polarization resistance (0.33 Ω cm2) compared to La1−2xBaxBixFeO3 with x = 0.2 (0.57 Ω cm2) at 550 °C. Our findings thus confirm that LBBF cathode-based SOFCs can be considered a potential cathode to operate fuel cells at low temperatures, and it will open up another horizon in the subject of research.

Adjusting electrochemical properties of PrBaCo2O6–δ as SOFC cathode by controllable Ca3Co4O9 additions

Composite electrode materials were successfully obtained by mixing of PrBaCo2O6–δ (PBCO) and Ca3Co4O9±δ (CCO) oxides. No interactions between the components under long term sintering and SOFC's operating conditions were evidenced by means of x-ray diffraction analysis. Increase of CCO content is shown to contribute to the perceptible decline in thermal expansion coefficients in the background of a negligible decrease in electrical conductivity of ceramics. The obtained powdered composites are shown to possess a developed microstructure and an excellent adhesion with Ce0.8Sm0.2O1.9 dense support. Low values of polarization resistances equal 0.28 and 0.69 Ω∙cm2 at 873 K were achieved for compositions with 70% and 50% of the PBCO phase, respectively. The distribution relaxation times (DRT) method was implemented to reveal the rate-limiting steps of the electrochemical reaction. The applying of CCO as TEC lowering admixture to PBCO instead ceria-based oxides was demonstrated to be more effective approach.

Electrochemical evaluation of Sr2Fe1.5Mo0.5O6-δ/Ce0.9Gd0.1O1.95 cathode of SOFCs by EIS and DRT analysis

Employing potent advanced materials to perform as the cathode of SOFC requires a profound investigation on the material’s electrochemical response within the oxygen reduction reaction. Accordingly, in this research, electrochemical characteristics of Sr2Fe1.5Mo0.5O6-δ/Gd0.1Ce0.9O1.5 (SFM/GDC) air electrode was studied using electrochemical impedance spectroscopy (EIS) and distribution of relaxation time (DRT) technique. SFM/GDC composite electrode sintered at 1050 °C shows the lowest polarization resistance of 0.23 Ω.cm2 at 800 °C. Three DRT peaks determined, and an LRΩ(QRH)G equivalent circuit is proposed for the EIS response, in which the Gerischer element was related to the low-frequency part. Two of the peaks indicated by DRT were associated to the Gerischer response, which reveals simultaneous occurrence of oxygen surface reaction and diffusion sub-processes during ORR. By temperature rising from 650 °C to 800 °C, Gerischer resistance (RG) reduces by 91%. Introduction of Pd nanoparticles into the electrode reduces the RG by 48% and 10% at 650 °C and 800 °C, respectively. The results demonstrate that at the intermediate temperature range, the SFM/GDC composite electrode requires improvement by surface decoration due to its insufficient surface exchange kinetics; however, significantly improved surface exchange kinetics is observed at higher temperature range without any surface modification.

Doping strategy on improving the overall cathodic performance of double perovskite LnBaCo2O5+δ (Ln=Pr, Gd) as potential SOFC cathode materials

A series of single-phase double perovskite Pr1-xGdxBaCo2-yFeyO5+δ (x = 0, 0.5 and 1, 0 ≤ y ≤ 1) materials were engineered through A/B site co-doping strategy to improve the mechanical, electrical and electrochemical properties as potential cathode materials for the application of intermediate solid oxide fuel cells (IT-SOFCs). The corresponding thermochemical stability, thermal expansion behavior, electrical conductivity and cathodic polarization resistance of the materials were systematically investigated. It was found that the A-site dual lanthanide doped Pr0.5Gd0.5BaCo2O5+δ (PGBCO) exhibits improved electrical conductivity, reduced thermal expansion, and comparatively low electrochemical polarization resistance versus single lanthanide double perovskite, PrBaCo2O5+δ (PBCO) and GdBaCo2O5+δ (GBCO) materials. Further investigation on the effect of B-site Fe-doping on Pr0.5Gd0.5BaCo2-yFeyO5+δ (PGBCF-y, 0 ≤ y ≤ 1) reveals that all the PGBCF-y compositions exhibit excellent chemical stability with Gd-doped ceria (GDC) at operating temperatures not higher than 1 100 °C. Besides, doping of Fe in B-site can effectively reduce the thermal expansion coefficients (TECs) of the Pr0.5Gd0.5BaCo2O5+δ ceramics at 30–1 000 °C. And the electrochemical impedance spectra (EIS) results show that the PGBCF-y|GDC| PGBCF-y symmetric cells have acceptable low area specific polarization resistances. Further examination of the cathodic polarization and characteristic capacitance from the AC impedance spectra by employing the relaxation time distribution (DRT) method demonstrated that charge transfer is the dominating sub-process for the oxygen transport through the materials.

Structural and electric properties of Ag modified (La0.8Sr0.2)1−yAgyMnO3{y = 0.05 and 0.10} perovskite cathode of SOFCs

Ag modified (La0.8Sr0.2)1-yAgyMnO3{y = 0.05 and 0.10} perovskite was prepared with solid state reaction route. Samples were sintered at 1400 °C for 6 h, and after that characterized to obtain conductive as well as structural behaviour. X- ray diffraction (XRD) showed the single phase of the material and crystalline size was found in nanometer. Material showed tensile strain which increased with Silver doping. Morphology of the prepared samples was studied by means of scanning electron microscope which confirmed increase in grain size with dopant. Material’s density has been found to be fall on Ag substitution. Conductivity of the prepared ceramic is best enough required to be it act as cathode electrode of SOFCs. Conductivity increases where as activation energy decreased with Ag doping. Fall in activation energy confirm the increases in electronic conductivity value on Ag doping.

In-situ self-assembly nano-fibrous perovskite cathode excluding Sr and Co with superior performance for intermediate-temperature solid oxide fuel cells

The development of high-performance cathodes is crucial for intermediate temperature SOFCs. Thereinto, morphology regulation is an effective way to improve the electrocatalytic activity of SOFC cathodes. Nanofibers have been confirmed with the advantages of large specific surface area and high porosity over traditional nanoparticles, possessing a significant promotion effect on oxygen reduction reaction (ORR) activity. However, the traditional cell fabrication with high-temperature sintering will break the fiber structure and cause agglomeration. Herein, we adopted a sintering-free method to in-situ self-assemble La0.6Ca0.4Fe0.8Ni0.2O3-δ (LCaFN) nanofibers onto the YSZ electrolyte surface by polarization operation, which showed impressive performance compared to our previous work. After polarization for 21 h, the peak power density of the cell at 700 °C increased from 0.36 W·cm−2 to 0.60 W·cm−2, and the highest peak power density at 750 °C could approach 0.91 W·cm−2. Meanwhile, the cell showed good long-term durability with a current density of about 0.35 A·cm−2 at 0.7 V and 650 °C for 90 h. This work manifests that the combination of fiber cathodes and in-situ self-assembly techniques is a promising strategy for high-performance SOFCs.

Study of the efficiency of composite LaNi0.6Fe0.4O3-based cathodes in intermediate-temperature anode-supported SOFCs

The paper focuses on the performance comparison of LaNi0.6Fe0.4O3-δ (LNF) composite cathodes comprising Ce0.8Sm0.2O1.9 (SDC) and Bi1.5Y0.5O3 (YDB) electrolytes and La0.6Sr0.4Fe0.8Co0.2O3-δ (LSFC)-SDC cathodes in anode-supported SOFCs with YSZ/GDC electrolyte films obtained by magnetron sputtering. Cathodes with LNF-SDC and LNF-YDB functional layers and the LNF-YDB-CuO oxide collector show a sufficient thermo-mechanical compatibility with the electrolyte. The performance of the anode-supported SOFC with the LNF-YDB/LNF-YDB-CuO cathode, reaches 650 and 1050 mW/cm2 at 700 and 800 °C, respectively, which is significantly higher than that obtained in other works for anode-supported cells with LNF cathodes. The initial total polarization resistance of the NiO-YSZ/YSZ/GDC/LNF-YDB/LNF-YDB-CuO cell, is 0.53 Ω cm2, which is lower than the initial resistance of the similar anode-supported cell with the LNF-SDC/LNF-YDB-CuO cathode (1.35 Ω cm2) and LSFC-SDC cathode with LNF-YDB-CuO (1.71 Ω cm2) and La0.6Sr0.4CoO3 (1.17 Ω cm2) collectors. The most probable reason for the LNF-YDB electrode aging is the growth of Bi-containing particles. Experimental results show that LNF-based composite cathodes are competitive with cobalt-containing cathodes and can be promising for anode-supported SOFCs with decreased operating temperature, that allows extending the material choice for both functional and collector cathode layers.

Electronic and ionic effects of sulphur and other acidic adsorbates on the surface of an SOFC cathode material.

The effects of sulphur adsorbates and other typical solid oxide fuel cell (SOFC) poisons on the electronic and ionic properties of an SrO-terminated (La,Sr)CoO3 (LSC) surface and on its oxygen exchange kinetics have been investigated experimentally with near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS), low energy ion scattering (LEIS) and impedance spectroscopy as well as computationally with density functional theory (DFT). The experiment shows that trace amounts of sulphur in measurement atmospheres form SO2- 4 adsorbates and strongly deactivate a pristine LSC surface. They induce a work function increase, indicating a changing surface potential and a surface dipole. DFT calculations reveal that the main participants in these charge transfer processes are not sub-surface transition metals, but surface oxygen atoms. The study further shows that sulphate adsorbates strongly affect oxygen vacancy formation energies in the LSC (sub-)surface, thus affecting defect concentrations and oxygen transport properties. To generalize these results, the investigation was extended to other acidic oxides which are technologically relevant as SOFC cathode poisons, such as CO2 and CrO3. The results unveil a clear correlation of work function changes and redistributed charge with the Smith acidity of the adsorbed oxide and clarify fundamental mechanistic details of atomic surface modifications. The impact of acidic adsorbates on various aspects of the oxygen exchange reaction rate is discussed in detail.

A heuristic approach to boost the performance and Cr poisoning tolerance of solid oxide fuel cell cathode by robust multi-doped ceria coating

Herein, highly conductive, robust, and innovative Gd and Pr multi-doped ceria (GPDC) coating, to boost the ORR kinetics and stability against Cr poisoning of the state-of-the-art La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) cathode, is reported. The GPDC coating significantly enhanced the ORR kinetics by ameliorating the surface of the LSCF cathode and showed a two-fold increment in the electrochemical performance compared to the cell with a bare LSCF cathode. The SOFC with GPDC-coated LSCF cathode showed outstanding stability at 700 °C for 500 h with a degradation rate of 0.001% h−1 in an accelerated Cr poisoning test which is attributed to the alleviation of the formation in SrCrO4. Both distributions of relaxation time-based analysis and experimental results indicate that the GPDC coating improves the ORR activity and Cr tolerance of the LSCF cathode. This study provides a comprehensive and smart strategy to tailor the surface of SOFC cathode materials to enhance performance and durability.

SOFC cathode material of La1.2Sr0.8CoO4±δ with a fibrous morphology: Preparation and electrochemical performance

Using soluble salts as metal-ion sources and polyacrylonitrile (PAN) as a polymer matrix, La1.2Sr0.8CoO4±δ cathode material with a fibrous morphology is prepared by electrostatic spinning, and microstructural characteristic of this material is investigated by field-emission scanning microscopy and X-ray diffraction. Electrochemical performance of the material in solid-oxide fuel cells is then tested. The results demonstrate that phase-pure La1.2Sr0.8CoO4±δ fibrils with tetragonal structure can be prepared from fresh silky precursors using electrospinning after annealing at high temperature. Compared to the conventional cathode material that possesses a plain granular structure, La1.2Sr0.8CoO4±δ fibrils exhibit superior electrochemical performance. At a temperature of 800 °C, the area specific resistance with this fibrous cathode is as low as 0.043 Ω cm2, and maximum power density with the corresponding single-cell is 716 mW cm−2, demonstrating the fast electrode kinetics in the O2 reduction reaction. Comparatively, the area specific resistance with the plain cathode is 0.062 Ω cm2, and the maximum power density with the corresponding single-cell is only 642 mW cm−2. Under a constant voltage load of 0.6 V at a fixed temperature of 750 °C, the power output from a single-cell with the fiber-structured cathode maintains between 615 mW cm−2 and 585 mW cm−2 even after 15 h of running time, showing a slower fading rate and a more stable electrochemical performance than the plain cathode.

Development of highly efficient and durable large-area solid oxide fuel cell by a direct-ink-writing three-dimensional printer

The commercialization of efficient and durable solid oxide fuel cells is hugely hindered due to the expensive and complex fabrication process. In this regard, the additive manufacturing technique, i.e., three-dimensional printing (3-D printing), has gained tremendous attention because of its ability to fabricate tunable functional ceramic layers with cost-effectiveness and mass customization. Therefore, in this work, the anode (NiO-ScSZ) and cathode (LSM) of the large-area SOFC (5 × 5 cm2) are printed using a direct-ink-writing (DIW) printer. The anode-functional and electrolyte layers are coated by spray and spin coating. The viscosity of the optimized anode and cathode inks are 5.85 Pa s and 0.97 Pa s, respectively. The electrochemical impedance and the performance of 3D-SOFC are investigated by supplying hydrogen and air. The maximum power density of the cell is 368 mW cm−2 at 800 °C. However, by inserting a hybrid scandia stabilized zirconia layer by spraying followed by magnetron sputtering onto the AFL, the electrochemical performance of the cell is significantly (21%) enhanced; the peak power density is 442 mW cm−2, and the corresponding polarization resistance is 0.267 Ω cm2 at 800 °C. Furthermore, the long-term cell test under galvanostatic mode (j = 0.5 A cm−2) at 700 °C for 100 h and thermal cycling between 400 and 700 °C concludes that the 3D-SOFC exhibits exceptional stability with a voltage loss of 0.845% h−1 and thermomechanical durability.

Ba0.5Sr0.5Fe0.8Sb0.2O3-δ- Sm0.2Ce0.8O2-δ bulk heterostructure composite: A cobalt free Oxygen Reduction Electrocatalyst for low-temperature SOFCs

Low-temperature operated ceramic fuel cells (LT-CFCs 350° to 550 °C) hold a great promise than high-temperature solid oxide fuel cells (HT-SOFCs ≥ 800 °C) for numerous large-scale real-application. If a suitable cathode should be developed to overcome the sluggish oxygen reduction activity at low temperatures, the low-temperature operation of ceramic fuel cells could be possible. In this study, we have developed a cobalt-free Ba0.5Sr0.5Fe0.8Sb0.2O3-δ- Sm0.2Ce0.8O2-δ (BSFSb- SDC) a bulk heterostructure composite for efficient ORR electrocatalyst for LT-SOFC cathode. The established BSFSb-SDC bulk heterostructure composite exhibits large lattice parameters, very low-area-specific resistance, and high oxygen reduction reaction (ORR) activity at low operating temperatures. The prepared fuel cell device has demonstrated high-power densities of 890 mW-cm-2 at 550 °C for button-sized SOFC on H2 and even possible operation at 400 °C. It is also found that BSFSb-SDC effectively facilitates small polaron hopping of valence electrons and diffusion of oxygen ions. Various spectroscopic measurements such as X-ray photoelectron, UV–visible, Raman, and Density Functional Theory (DFT) calculations were employed to understand the improved ORR electrocatalyst function of BSFSb-SDC bulk heterostructure composite SOFC cathode. The results can further help to develop functional cobalt-free electro-catalysts for LT-SOFCs and other related applications.

Cobalt-free fluorine doped Bi0.7Sr0.3FeO3-δ oxides for energetic cathodes of low-temperature solid oxide fuel cells

Cobalt-free fluorine doped Bi0.7Sr0.3FeO3-δ-xFx oxides are presented here as cathode catalysts of low-temperature solid oxide fuel cells (LT-SOFCs). Their crystal structure, thermal behaviors, catalytic activity, oxygen transport properties, oxygen reduction kinetics, long-term stability and CO2 tolerance are systematically exploited. At 600 °C, the optimized Bi0.7Sr0.3FeO2.9-δF0.10 (BSFF0.10) cathode possesses outstanding ORR activity with a low polarization resistance of 0.12 Ω cm2. When tested under realistic conditions, the anode-supported fuel cell with BSFF0.10 cathode delivers a high output of 837 mW cm−2 with stable operation for 110 h. Such outstanding performance can be ascribed to the improved oxygen transport kinetics, larger oxygen vacancy concentration, increased oxygen surface adsorption energy, and O 2p-band centers close to the Fermi level. In addition, the oxygen reduction process on BSFF0.10 electrode interface is mainly determined to be the dissociation of adsorbed molecule oxygen processes. The BSFF0.10 cathode exhibits superior CO2 tolerance because of the lower basicity resulting from the presence of Bi3+, Fe2+/Fe3+/Fe4+ and F−. Although the BSFF0.10 cathode is not reactive with CO2 to form carbonate species, post-characterizations reveal that the CO2 atmosphere can induce the migration of Bi ions to the surface, which in turn affects the performance of the cathode. These results show that fluorine doping is an effective approach to develop highly active and robust cathode for SOFCs.

Co and Hf co-doped BaFeO3 cathode with obviously enhanced catalytic activity and CO2 tolerance for solid oxide fuel cell

It is essential to develop efficient and stable cathode materials operated at intermediate temperature for the application of solid oxide fuel cell. In this work, BaFeO3–δ based perovskite oxide was used to explore the influence of Co and Hf co-doping on the catalytic activity and CO2 tolerance of SOFC cathode materials. Among the series materials, BaCo0·2Fe0.7Hf0.1O3–δ (BC2FHO) shows excellent catalytic activity and CO2 stability. The EIS measurement shows that the polarization resistance (Rp) of BC2FHO is ∼0.04 Ω cm2 at 800 °C. In the CO2 tolerance test, the Rp decreases to ∼0.0435 Ω cm2 within 1 h after removing 10% CO2. The peak power density of the single cell with BC2FHO cathode are 1010.4, 649.4, and 256.6 mW cm−2 at 800, 700, and 600 °C, respectively. And the total degradation rate of the output voltage at 700 °C is only 0.009% h−1. The significantly enhanced activity and stability are attributed to Co and Hf co-doping. The incorporation of Hf allows BaFeO3 to obtain a highly symmetrical cubic structure with increased oxygen ion transport channels, which is conducive to promote the oxygen reduction reaction. In addition, Hf-doping can also increase the acidity to make the material resistant to CO2. The incorporation of Co improves the concentration of oxygen vacancies, which makes BC2FHO show excellent electrochemical performance.

Mathematical and Experimental Analysis of an Internally Finned Double Pipe Heat Exchanger for Coupling of Solid Oxide Fuel Cell Cathode Exhaust Heat and Vapor Absorption Refrigeration System on Refrigerated Trucks

Abstract Solid oxide fuel cells (SOFCs) generate electricity with high quality waste heat which if harnessed and used as energy source for vapor absorption refrigeration systems (VARS) will address the emission issues related to refrigerated transport infrastructure. The temperature range of the heat source required at the desorber of the VARS is between 120 °C and 200 °C, while SOFCs cathode exhaust heat temperatures are 600 °C and above. Therefore, an internally finned double pipe heat exchanger (DPHX) was used in this study with thermal oil as the coupling fluid to experimentally couple the SOFC cathode exhaust heat with the VARS indirectly. The experimental setup mimics a 5 kWe SOFC stack. Results showed that 1.84 kW of heat was recovered at a cathode exhaust flowrate of 24.64 L s−1 resulting in a heat exchanger effectiveness of 12.22% and overall heat transfer coefficient of 60.15 W m−2 K−1. The recovered 1.84 kW of heat are able to power a single effect VARS on board a small, refrigerated truck to supply 1 kW of cooling load. Results also revealed an exchanger effectiveness and overall heat transfer coefficient increase by 70.2% and 19.4%, respectively, at a reduced exhaust flowrate of 7.347 L s−1. Further improvement of 81.1% and 39.22%, respectively, of exchanger effectiveness and overall heat transfer coefficient was achieved at 4.653 L s−1 exhaust flowrate.

A/B-site co-doping enabled fast oxygen reduction reaction and promoted CO2 tolerance of perovskite cathode for solid oxide fuel cells

The significant challenges facing the development of the cathode for intermediate-temperature solid oxide fuel cells (IT-SOFCs) are the poor oxygen reduction reaction (ORR) activity as well as the CO2 poisoning issues. Herein, an La/Nb co-doped perovskite Ba0.9La0.1Co0.7Fe0.2Nb0.1O3-δ (BLCFN) has been developed as a promising cathode for IT-SOFCs. The polarization resistance of BLCFN cathode is 0.073 Ω cm2 at 700 °C air. This remarkable improved ORR activity can be attributed to the increased oxygen vacancies caused by the introduction of La3+ and Nb5+ ions. Furthermore, the collaboration introduction of La3+ and Nb5+ ions greatly reduce the alkalinity of the BLCFN electrode, bringing excellent CO2-tolerance for BLCFN cathode. No carbonate is yielded on the surface of the BLCFN cathode. The improvement on CO2-tolerance for these cathodes is largely attributed to the introduction of Nb5+ ions, which have higher acidity and more powerful M − O bond, and the introduction of La3+ can improve the ORR activity of the material on this basis. The co-doping of La/Nb elements furtherly stabilizes the lattice structure. Overall, the high ORR activity and CO2-tolerance of the cathode manifest a bright application prospect for BLCFN cathode in SOFCs and other membrane reactors.

Sr-doped BaZr0.5Fe0.5O3-δ cathode with improved chemical stability and higher performance for proton-conducting solid oxide fuel cells

A new Ba0.5Sr0.5Zr0.5Fe0.5O3-δ (BSZF) material was proposed as a cathode for proton-conducting solid oxide fuel cells (H–SOFCs). Compared with the traditional Fe-doped BaZrO3 (BZF) cathode, the incorporation of Sr into the lattice decreased the lattice volume of the material but enhanced the chemical stability against CO2 that has been confirmed by both experiments and first-principles calculations. In addition, doping Sr allowed larger oxygen vacancy content and enhanced interaction with O2, facilitating the oxygen reduction reaction. The fuel cell with the BSZF showed an encouraging performance of 1048 mW cm−2 at 700 °C, much larger than that of the traditional BZF cell. The analysis for cell resistances revealed that the Sr-doping strategy reduced the cell polarization resistance, and the reduction was more profound under the loading condition, providing an interesting strategy for designing H–SOFC cathodes.

A-site doping enabled higher-oxygen-vacancy cobalt-free layered perovskite cathode for higher-performing protonic ceramic fuel cells

Cobalt-free layered perovskite as a promising mixed ionic electronic conductor cathode has shown outstanding performance in conventional oxygen-ion conducting solid oxide fuel cells (O–SOFCs), but not in next-generation proton-conducting solid oxide fuel cells (H–SOFCs) due to insufficient oxygen vacancy so far. We propose a facile A-site doping strategy to enable higher-oxygen-vacancy cobalt-free layered perovskite cathode for higher-performing protonic ceramic fuel cells at reduced temperatures. As proof of concept, two different typical A-site doping of mixed-rare-earth Nd and alkali-metal K in PrBaFe1.9Zn0.1O5+δ are chosen as potential H–SOFC cathode materials. Pr0.8Nd0.2BaFe1.9Zn0.1O5+δ (PNBFZ) and PrBa0.9K0.1Fe1.9Zn0.1O5+δ (PBKFZ) layered perovskite cathodes were synthesized using a nitrate-gel combustion method. Both PNBFZ and PBKFZ have a layered perovskite structure and are chemically compatible with protonic BaZr0.1Ce0.7Y0.2O3−δ (BZCY) electrolyte. Both Nd and K doping dramatically improve the electrical conductivities under operation temperatures. Compared with PBKFZ, PNBFZ shows a lower average thermal expansion coefficient (TEC) of 13.9✕10−6 K−1, being thermally compatible with the BZCY electrolyte without cracks. A-site doping in Fe-based layered perovskite cathodes promotes the single-cell output performance by around 100-26% at 550–700 °C, especially for the mixed-rare-earth doped PNBFZ with high peak power densities of 401–650 mW cm−2.