Comparative study of Pd and PdO as cathodes for oxygen reduction reaction in intermediate temperature solid oxide fuel cells

Metallic Pd and PdO electrodes were prepared by using Pd and PdCl₂ slurries, respectively, and their electrochemical performance as a cathode for oxygen reduction reaction in intermediate temperature solid oxide fuel cells was evaluated by electrochemical impedance spectroscopy (EIS) and direct current polarization (DC polarization). The electrochemical activity of metallic Pd was much higher than that of PdO for the reaction of oxygen reduction; below the decomposition temperature, a thin layer of PdO formed on the surface of metallic Pd electrode, which increased its polarization resistance. The decomposition temperature of PdO decreased from 810 to 750 °C as oxygen partial pressure decreased from 20 to 5 kPa, and was further lowered under the influence of the applied current during DC polarization test. The charge transfer resistance of PdO increased by decreasing oxygen partial pressure, while that of metallic Pd was less sensitive to it.     

Deposition mechanism of convex YSZ particles and effect of electrolyte/cathode interface structure on cathode performance of solid oxide fuel cell

A convex-structured electrolyte surface will lead to the formation of a high specific surface area which could be applied in many fields. Thermal spraying process is promising to fabricate such surface through particle deposition. However, the locally flat surfaces will be formed by using completely molten particles due to well spreading of liquid droplets on impact. In this study, a flame spraying method was employed to deposit convex yttria-stabilized zirconia (YSZ) particles on YSZ substrate for solid oxide fuel cell application. The convex structure was formed by the deposition of surface-melted particles, which were created due to the low particle temperature and velocity of the spray process. The effect of acetylene flow rate on the particle deposition behavior was examined. The surface morphology and surface area of YSZ particles were characterized by scanning electron microscope (SEM) and a 3D laser scanning microscope, respectively. The bonding between deposited YSZ particles and YSZ substrate was examined from cross section. The electrochemical behavior of single cell with the structured cathode was characterized by the electrochemical impedance spectroscopy. The results indicate that spray parameters have significant influence on the surface morphology of deposited YSZ particles and the surface area is increased up to a factor of 3.45. The cathode polarization with a structured cathode is approximately one-third of that with a flat cathode.     

Novel gradient porous cathode for intermediate temperature solid oxide fuel cell

As a mixed ion electronic conducting oxide, PrBaCo₂O_5+δ is regarded as a promising solid oxide fuel cell cathode. To further improve PrBaCo₂O_5+δ cathode's oxygen reduction reaction activity, porosity graded PrBaCo₂O_5+δ-based cathode is prepared by screen printing technology. With a porous top and a comparative denser base, oxygen ions concentration and oxygen gas concentration in the cathode can be graded distributed. This concentration gradient works as driven force which can enhance the cathode catalytic activity. And distribution of relaxation time analysis is carried out to investigate cathodes performance optimization mechanism, the result shows that gradient porous PrBaCo₂O_5+δ cathode's area specific resistance value is much lower than the traditional homogeneous porosity PrBaCo₂O_5+δ cathode. The scaffold porosity modification promotes the cathode oxygen ions transfer processes without obvious impact on the cathode oxygen surface processes.     

Enhancing the oxygen reduction reaction activity and durability of a solid oxide fuel cell cathode by surface modification of a hybrid coating

While (La_0.6Sr_0.4)_0.95Co_0.2Fe_0.8O_3-δ (LSCF) has been one of the most investigated materials for a long time, its relatively insufficient oxygen reduction reaction (ORR) activity and inherent performance degradation are still two main obstacles to its massive application on oxygen-ion conducting solid oxide fuel cell (SOFC). To solve those issues, a composite of Pr₆O₁₁ and NiO has been deposited on LSCF successfully via a facile infiltration method in this study. The modified LSCF cathode exhibits ∼30% lower polarization resistance than LSCF. The excellent performance promotion may be due to the synergistic effect of Pr₆O₁₁ and NiO on the LSCF surface. The distribution of relaxation time (DRT) analyses of electrochemical impedance spectra (EIS) in different oxygen partial pressure and long-term operation indicate that the performance enhancement is caused by the facilitated oxygen surface adsorption-dissociation process and suppression of Sr segregation on modified LSCF cathode, thus achieving a higher peak power density of 1.40 W cm⁻² at 800 °C and better long-term operation stability of only 3% voltage decline rate after 80 h operation. These results indicate that Pr₆O₁₁ and NiO composite modification is a promising method for improving the electrochemical performance of LSCF.     

Synergistically enhancing CO2-tolerance and oxygen reduction reaction activity of cobalt-free dual-phase cathode for solid oxide fuel cells

High-performance cathodes with adequate CO₂ tolerance are vital for further development of intermediate-temperature solid oxide fuel cells (IT-SOFCs). However, there is always a trade-off between CO₂ tolerance and oxygen reduction reaction (ORR) performance for single-phase cathodes. Here, we report a cobalt-free Ba_0.6La_0.4FeO_3-δ-Ce_0.8Sm_0.2O_2-δ (BLF-SDC) dual-phase cathode with excellent ORR activity and CO₂ tolerance. Introducing ionic conductor Ce_0.8Sm_0.2O_2-δ (SDC) into the Ba_0.6La_0.4FeO_3-δ (BLF) phase can boost ORR activity due to the extended active sites and enhanced oxygen surface exchange process with a polarization resistance of 0.121 Ω cm² for the BLF-30% SDC (weight ratio, BLF-30SDC) cathode at 700 °C. The CO₂ resistance of the BLF-30SDC composite cathode outperforms BLF cathode by three times at 600 °C. This stability enhancement is owing to low CO₂ adsorption of SDC, which is confirmed from thermodynamic calculation. This work indicates that dual-phase mixed conductors can be developed as highly active and stable cathodes for IT-SOFCs.     

PbO effect on the oxygen reduction reaction in intermediate-temperature solid oxide fuel cell

Ag paste is often used in solid oxide fuel cells (SOFCs) as the current collector. This study investigates the effect of lead oxide (PbO), which is usually present in Ag paste, on the oxygen reduction reaction (ORR) at the typical electrocatalysts, lanthanum strontium cobalt ferrite (LSCF) and lanthanum strontium ferrite (LSF), for intermediate-temperature SOFCs. The chemical oxygen surface exchange coefficients are greatly increased by the deposition of PbO particles on the LSCF and LSF bar surfaces as demonstrated by the electrical conductivity relaxation technique. For example, at 800 °C, the deposition of 0.074 mg cm⁻² PbO particles increases the coefficients from 4.68 × 10⁻⁵ to 7.2 × 10⁻⁴ cm s⁻¹ and from 2.62 × 10⁻⁵ to 1.60 × 10⁻⁴ cm s⁻¹ for LSCF and LSF, respectively. Furthermore, the infiltration of PbO into porous LSCF and LSF electrodes significantly improves the ORR as determined with AC impedance spectroscopy using symmetrical cells comprising of samaria-doped ceria as the electrolyte. When 5.88 and 5.67 wt % PbO is infiltrated into the porous backbone of LSCF and LSF electrodes, at 650 °C, the area specific interfacial polarization resistance (ASR) is reduced from 0.26 to 0.14 Ω cm² and from 1.34 to 0.37 Ω cm², respectively. The distribution of relaxation time (DRT) analysis suggests that PbO efficiently accelerate the charge transfer and oxygen incorporation processes.     

Pd-YSZ composite cathodes for oxygen reduction reaction of intermediate-temperature solid oxide fuel cells

Pd-Y₂O₃ stabilized ZrO₂ (YSZ) composite cathodes are prepared by conventional mechanical mixing and infiltration methods. In the case of infiltration, thermal decomposition and chemical reduction processes are used to form Pd particles on the YSZ scaffold. The phase structure, morphology and electrochemical performance of the Pd-YSZ composite cathodes are characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) and electrochemical impedance spectroscopy (EIS). The performance of mechanically mixed Pd-YSZ composite cathodes is inadequate due to significant growth and sporadical distribution of Pd particles. The 5 wt.% Pd-loaded cathode prepared by infiltration-thermal decomposition process shows the lowest polarization resistance, i.e. between 0.042 Ω cm² and 1.5 Ω cm² in the temperature range of 850-600 °C, benefited from the formation of nano-sized Pd particles and the presence of well connected Pd network. The effect of Pd loading on the performance of the infiltrated-thermal decomposed Pd-YSZ composite cathodes is also evaluated, 5 wt.% Pd loading results in the lowest polarization resistances.     

High performance Pd promoted Sm0.5Sr0.5CoO3–La0.8Sr0.2Ga0.8Mg0.15Co0.05O3−δ composite cathodes for intermediate temperature solid oxide fuel cells

Pd promoted Sm_0.5Sr_0.5CoO₃ (SSC)–La_0.8Sr_0.2Ga_0.8Mg_0.15Co_0.05O_3−δ (LSGMC5) composite cathodes for intermediate temperature solid oxide fuel cells (ITSOFC) were prepared using the wet impregnation method. XRD analyses demonstrated that the Pd in the electrode was in the form of PdO. The activity for oxygen reduction of the electrode increased with the increase in the concentration of Pd in the electrode and with the decrease in the electrode sintering temperature. The electrode containing 2.4 wt% Pd sintered at 1123 K showed an electrode resistance about 0.12 Ω cm² at near equilibrium conditions in oxygen at 873 K, which was only about one fourth of the electrode resistance without Pd addition. The addition of Pd species in the electrode showed no obvious effect on the mechanism of the oxygen reduction reaction.     

Ruddlesden-Popper-based lanthanum cuprate thin film cathodes for solid oxide fuel cells: Effects of doping and structural transformation on the oxygen reduction reaction

Highly textured, c-axis oriented thin films of undoped, Ba-doped, and Sr-doped La₂CuO₄ are successfully prepared on YSZ (100) substrates by using pulsed laser deposition. Intriguingly, the films undergo a structural transformation from T′(square-planar) to T (octahedral) crystal structure with partial substitution of Ba and Sr ions on La site. Meanwhile, the addition of both Ba and Sr dopants lead to reducing in polarization resistance and alternative kinetics of oxygen reduction reaction (ORR), hereinto La_1·8Ba_0·2CuO₄ film presents superior electrochemical properties because it can accommodate much more oxygen vacancies than the other two films. First-principles calculations reveal that much lower O-defect formation energy originate fundamentally from the increase of Cu–O bond length along c-axis orientation after structural transformation from T′-phase to T-phase. These findings unveil the relationship between the structural transformation and ORR activity, and provide a novel approach to rational design film cathodes for higher electrochemical performance.     

CaO effect on the electrochemical performance of lanthanum strontium cobalt ferrite cathode for intermediate-temperature solid oxide fuel cell

The oxygen reduction reaction (ORR) on lanthanum strontium cobalt ferrite (LSCF) catalyst is critical for intermediate temperature solid oxide fuel cells (SOFCs). The reaction rate can be effectively improved by addition various nanoparticles including electrocatalysts such as Pd, Ag and mixed electronic-ionic conductors and electrolytes like samaria doped ceria (SDC). This work shows that ORR rate can also be improved by CaO, which is neither catalyst nor conductor. The CaO nanoparticles are deposited to porous LSCF electrodes using the infiltrating technique. No obvious reaction between CaO and LSCF is detected with X-ray diffraction analysis, indicating that CaO is chemically compatible with LSCF in the intermediate-temperature SOFC operation conditions. Impedance spectrum analysis demonstrates that the CaO nanoparticles can effectively reduce the interfacial polarization resistances for both single phase LSCF electrodes and LSCF-SDC composite electrodes. In addition, CaO nanoparticles can improve the peak power densities and reduce the total electrode resistances of single cells consisting of NiO-SDC anodes, SDC electrolytes, and LSCF based cathodes. Further, CaO can increase the oxygen surface exchange coefficient as demonstrated with electrical conductivity relaxation measurement. The improving factor is comparable to those for Rh and Pd catalysts, suggesting it is effective to increase ORR rate by infiltrating CaO nanoparticles.     

Synthesis and characterization of aluminum-doped perovskites as cathode materials for intermediate temperature solid oxide fuel cells

(Ba_0.5Sr_0.5)(Fe_1-xAl_x)O_3-δ (BSFAx, x = 0–0.2) oxides have been synthesized as novel cobalt-free cathode materials for intermediate temperature solid oxide fuel cells (IT-SOFCs) using a sol-gel method. The BSFAx (x = 0–0.2) materials have been characterized by X-ray diffraction and scanning electron microscopy. The electrical conductivities and electrochemical properties of the prepared samples have also been measured. At 800 °C, the conductivity drops from 15 S cm⁻¹ to 5 S cm⁻¹ when the doping level of aluminum is increased to 20%. The aluminum-doping concentration has important impacts on the electrochemical properties of BSFAx materials. The BSFA0.09 cathode shows a significantly lower polarization resistance (0.26 Ω cm²) and cathodic overpotential value (55 mV at the current density of 0.1 A cm⁻²) at 800 °C. Furthermore, an anode-supported single cell with BSFA0.09 cathode has been fabricated and operated at a temperature range from 650 to 800 °C with humidified hydrogen (∼3vol% H₂O) as the fuel and the static air as the oxidant. A maximum power density of 676 mWcm⁻² has been achieved at 800 °C for the single cell. We believe that BSFA0.09 is a promising cathode material for future IT-SOFCs application.     

Effect of NiO addition on oxygen reduction reaction at lanthanum strontium cobalt ferrite cathode for solid oxide fuel cell

Lanthanum strontium cobalt ferrite (LSCF), a perovskite's family member, has gained much attention as the electrocatalyst for the oxygen reduction reaction (ORR) in intermediate temperature solid oxide fuel cells. However, it still needs a strategy to improve its catalytic activity. In this work, NiO is primarily investigated as a possible synergistic catalyst for ORR on the LSCF surface. The effect of NiO particles on the effective oxygen chemical surface exchange coefficient is revealed with the electrical conductivity relaxation (ECR) technique. At 800 °C, the coefficient is increased from 3.48 × 10^?5 to 6.65 × 10^?5 cm s^?1 and 6.9 × 10^?4 cm s^?1 when NiO particles are deposited using the sputter and drop coating methods, respectively. Adding 5 wt % NiO to LSCF reduces the area specific interfacial polarization resistance, for example from 0.108 to 0.082 Ω cm² at 700 °C, as demonstrated by impedance spectroscopy on symmetric cells using samaria-doped ceria as the electrolyte. Adding NiO can also improve the performance of anode supported button cells, increasing the peak power density from 0.731 to 1.031 W cm^?2 at 800 °C. On the whole, the increased oxygen surface exchange rate together with the reduced electrode resistance and improved power density, exhibit that NiO is a potential additive to enhance the LSCF catalytic activity.     

Boosting electrocatalytic performance of Ruddlesden-Popper type Fe-based cathode catalysts by La and Ni co-doping for solid oxide fuel cells

Exploring advanced electrode materials with high electrochemical performance and sufficient durability is crucial to the commercialization of solid oxide fuel cells (SOFCs). Herein, a Ruddlesden-Popper Sr_2·9La_0·1Fe_1·9Ni_0·1O_7?δ (SLFN) oxide is systematically evaluated as efficient oxygen electrode material. La and Ni co-doping strategy demonstrates improved oxygen desorption ability and promoted electrochemical activity of pristine Sr₃Fe₂O_7?δ (SF) toward oxygen reduction react (ORR). Further, the ORR process of the SLFN electrode is probed by electrochemical impedance spectroscopy (EIS) and distribution of relaxation time (DRT) technique. The button cell with the SLFN cathode delivers a peak power density of 1.01 W cm^?2 at 700 °C, along with desirable stability over a period of 60 h. This study offers a feasible strategy for developing Ruddlesden-Popper type cathode candidates for SOFCs.     

Electrochemical performance of a Ni0.8Co0.15Al0.05LiO2 cathode for a low temperature solid oxide fuel cell

The electrochemical performance of the Ni_0.8Co_0.15Al_0.05LiO₂ (NCAL) cathode was investigated by comparing it with the traditional La_0.4Sr_0.6Co_0.2Fe_0.8O_3-δ (LSCF) and LSCF/Ce_0.9Gd_0.1O_2-δ (GDC) cathodes with a GDC electrolyte-supported solid oxide fuel cell (SOFC). It is found that the electrochemical performance of the cells with the NCAL and NCAL/GDC cathode is better than that of the cells with the LSCF and LSCF/GDC cathode at 550 °C. The results of the electrochemical performance tests of the cells with different NCAL/GDC mass ratios (10/0, 9/1, 8/2, 7/3 and 6/4) show that the NCAL/GDC composite cathode with the mass ratio of 8/2 has the best electrochemical performance. XRD results show that when the sintering temperature is higher than 700 °C, the NCAL/GDC composite will undergo chemical reactions and generate new phases, reducing the performance of the composite cathode. XPS results show that a small amount of Li₂CO₃ was formed on the surface of NCAL during cathode preparation, forming a special interface between NCAL, Li₂CO₃ and GDC. At the NCAL-Li₂CO₃/GDC interfaces, due to the migration and aggregation of Li⁺ to the interface, a space charge region may be formed in which the Li⁺ enrichment may lead to the formation of the region with a high oxygen vacancy concentration. A very high oxygen vacancy concentration at the NCAL-Li₂CO₃/GDC interfaces will provide sufficient oxygen ion conductivity for oxygen reduction reaction (ORR) and reduce the activation energy of the reaction. NCAL will be a potential cathode material that can reduce the operating temperature of the traditional SOFC to 550 °C or lower.     

Electrochemical performances of spinel oxides as cathodes for intermediate temperature solid oxide fuel cells

The spinel-type oxides of (Mn, Co, Cu)₃O₄ prepared via a citric–EDTA acid process were investigated as candidate cathodes of intermediate temperature solid oxide fuel cells (IT-SOFCs). (Mn, Co)₃O₄ spinel oxide shows a phase transition from tetragonal to cubic when the doping amount of cobalt element increases. Their electric conductivities increase with the cobalt content and are enough high for them used as cathodes of IT-SOFCs. A fuel cell with (Mn, Co)₃O₄ spinel cathode was successfully evaluated based on YSZ electrolyte. (Mn, Co)₃O₄ spinel cathodes show good electrochemical activities, demonstrating the feasibility of the spinel oxide being a cathode of IT-SOFC. As copper doped into (Mn, Co)₃O₄ spinel, the P_peak for Cu_0.5MnCo_1.5O₄ cathode rise to 343, 474 and 506 mW cm⁻² at 700, 750 and 800 °C, respectively. The results reveal that the spinel-type oxides are promising cathodes for IT-SOFCs, especially for Cu_0.5MnCo_1.5O₄.     

Non-Pt catalyst as oxygen reduction reaction in microbial fuel cells: A review

Oxygen Reduction Reactions (ORR) are one of the main factors of major potential loss in low temperature fuel cells, such as microbial fuel cells and proton exchange membrane fuel cells. Various studies in the past decade have focused on determining a method to reduce the over potential of ORR and to replace the conventional costly Pt catalyst in both types of fuel cells. This review outlines important classes of abiotic catalysts and biocatalysts as electrochemical oxygen reduction reaction catalysts in microbial fuel cells. It was shown that manganese oxide and metal macrocycle compounds are good candidates for Pt catalyst replacements due to their high catalytic activity. Moreover, nitrogen doped nanocarbon material and electroconductive polymers are proven to have electrocatalytic activity, but further optimization is required if they are to replace Pt catalysts. A more interesting alternative is the use of bacteria as a biocatalyst in biocathodes, where the ORR is facilitated by bacterial metabolism within the biofilm formed on the cathode. More fundamental work is needed to understand the factors affecting the performance of the biocathode in order to improve the performance of the microbial fuel cells.     

Advanced electrocatalytic activity of praseodymium-deficient copper-based oxygen electrodes for solid oxide fuel cells

Herein praseodymium-deficient Pr_1.90−xCe_0.1CuO₄ oxides are evaluated as potential oxygen electrodes for solid oxide fuel cells (SOFCs). Introducing Pr-deficiency promotes the oxygen vacancy concentration, further improving electrocatalytic activity of the Pr_1.90−xCe_0.1CuO₄ electrodes towards oxygen reduction reaction (ORR). The Pr_1.75Ce_0·.1CuO₄ (P_1·75CC) component exhibits outstanding electrode performance, as supported by a polarization resistance as low as 0.025 Ω cm² and high peak power density of the single cell (1.11 W cm⁻²) at 700 °C. In addition, the rate-limiting steps for ORR kinetics are determined to be the charge transfer reaction and oxygen adsorption/diffusion process on the electrode surface. This work highlights an effective way for developing the cathode candidates with high electrocatalytic activity and superior stability.     

A novel dual phase BaCe0.5Fe0.5O3-δ cathode with high oxygen electrocatalysis activity for intermediate temperature solid oxide fuel cells

A novel iron-based perovskite BaCe_0.5Fe_0.5O_3-δ (BCF) powders were successfully fabricated and the phase composition, lattice structure, oxygen surface exchange coefficient and electrochemical performance were investigated. The ultrafine BCF powder with grain size of about 200 nm consisted of dual phase BaCe_0.15Fe_0.85O_3-δ and BaCe_0.85Fe_0.15O_3-δ. Electrical conductivity relaxation measurement illustrates the low conductivity activation energy and the high oxygen surface exchange kinetics with oxygen surface exchange coefficient of 3.8 × 10⁻⁵ cms⁻¹ at 600 °C. BCF cathode exhibits 1.04 Ωcm² on doped ceria electrolyte and remains stable in 400 h long term test at 600 °C. A single cell based on doped ceria electrolyte with BCF cathode shows a maximum power density of 228 mW cm⁻² at 650 °C. The preliminary results indicate that the dual phase BCF can be applied as cathode material for oxygen ion conductive solid oxide fuel cells.     

Enhancing oxygen reduction reaction activity of perovskite oxides cathode for solid oxide fuel cells using a novel anion doping strategy

Possessing a high oxygen reduction reaction (ORR) activity is one of the most important prerequisites for the cathode to ensure an efficient solid oxide fuel cell. Herein, a highly active cathode is developed by doping the fluorine anion in anion sites of perovskite oxides (ABO₃). The electrocatalytic activities of three different cathode samples including the original perovskite La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF), the doped La_0.6Sr_0.4Co_0.2Fe_0.8O_2.95-δF_0.05 (LSCFF0.05) and La_0.6Sr_0.4Co_0.2Fe_0.8O_2.9-δF_0.1 (LSCFF0.1) are comparatively investigated. The fluorine doped perovskites reveal higher electrochemical performance than the original perovskite. Based on three cathodes of LSCF, LSCFF0.05 and LSCFF0.1 operated at 850 °C, the measured area specific resistance was 0.018, 0.017 and 0.91 Ω cm², respectively; and the respective maximum power density of the single fuel cell using the 9-μm-thick YSZ electrolyte was 754, 1005, and 737 mW cm^?2. Such performance results vividly indicate that, the obtained perovskite oxyfluoride by doping an optimum amount of F ions can efficiently improve ORR activity and thus is a feasible strategy to develop cathode for high-performance solid oxide fuel cells.     

An oxygen reduction reaction active and durable SOFC cathode/electrolyte interface achieved via a cost-effective spray-coating

One of the critical obstacles for commercialization of solid oxide fuel cells (SOFCs) technology is to develop efficient interfaces between cathode and electrolyte that enable high activity toward oxygen reduction reaction (ORR) while maintain long-term durability. Here, we report a cost-effective spray-coating process that applied in the building of an ORR active and durable cathode/electrolyte interface. When tested at 750 °C, such spray-coated cathodes show a typical interfacial polarization resistance of ~0.059 Ωcm², much lower than that of ~0.10 Ωcm² for screen-printed cathodes. Detailed distribution of relaxation time analyses of the impedance spectra over time indicates that the capability of mass transfer and surface exchange process in the spray-coated cathode/electrolyte interface has been enhanced and maintained in the testing periods of ~100 h. As a result, a Ni-based anode supported cell with thin electrolyte and spray-coated cathodes shows an excellent peak power density of 1.012 Wcm⁻², much higher than that of 0.712 Wcm⁻² for cells with screen-printed cathodes, when tested at 750 °C using wet H₂ as fuel and ambient air as oxidant. It is demonstrated that ORR activity and durability of the SOFC cathodes can be dramatically enhanced via a cost-effective spray-coating process.