Boosting the electrocatalytic performance of Fe-based perovskite cathode electrocatalyst for solid oxide fuel cells

The development of cathode materials with excellent electrocatalytic activity and CO₂ tolerance is an important direction for the wide application of solid oxide fuel cells. Herein, the cobalt-free perovskite oxides Bi_0.5Sr_0.5Fe_1-xV_xO_3-δ (BSFVx, x = 0.025, 0.05 and 0.075) are developed as the efficient cathode electrocatalysts for SOFCs. The V-doping strategy is beneficial to improve the thermal stability, CO₂ tolerance and electrochemical performance of undoped Bi_0.5Sr_0.5FeO_3-δ. Among all samples, Bi_0.5Si_0.5Fe_0.95V_0.05O_3-δ (BSFV0.05) cathode presents excellent oxygen reduction reaction activity, achieving a lower polarization resistance of 0.076 Ω cm² and the peak power density of the single cell with the BSFV0.05 cathode reaches to 1.16 W cm⁻² at 700 °C, which can be comparable to those of the representative cobalt-based cathodes. Furthermore, the improved CO₂ tolerance of the BSFV0.05 cathode can be ascribed to the high acidity of the V⁵⁺ and the larger average bonding energy in the oxide.     

Impedance studies of cathode/electrolyte behaviour in SOFC

This paper reports impedance studies of the cathode/electrolyte behaviour in solid oxide fuel cells (SOFC), based on comparative investigation of half-cells with yttria stabilized zirconia (YSZ) electrolyte and different cathode materials: lanthanum strontium manganite (LSM), and composite LSM/YSZ with low ionic conductivity as well as the electron conducting Ag, Pt and Au. For improved impedance data analysis the technique of the differential impedance analysis is applied. It ensures structural and parametric identification without preliminary assumptions about the working model. It is found that despite the low ionic conductivity of LSM, the cathode reaction of the oxide cathode materials is a two-step process including: (i) charge transfer with activation energy of the resistivity E_a increasing with the temperature and (ii) transport of oxygen ions through the bulk of the electrode (rate-limiting stage) with E_a independent on the temperature. For the metal (electron conducting) electrodes, the reaction behaviour is described with one step process with higher E_a at higher temperatures. The activation energy of the electrolyte conductivity decreases with the increase of the temperature. The observed changes in E_a for the electrolyte and the cathode reaction (the charge transfer step for the LSM-based electrodes) appear in the same temperature interval. This interesting coincidence suggests for correlation between the bulk (electrolyte) and surface conduction properties. Approaches for improvement of both the ionic conductivity and the supply with electrons in LSM should be also searched.     

Improved electrochemical activity of Bi0.5Sr0.5FeO3-δ–Ce0.9Gd0.1O1.95composite cathode electrocatalyst for solid oxide fuel cells

Developing MIEC materials with high electrocatalytic performance for the ORR and good thermal/chemical/structural stability is of paramount importance to the success of solid oxide fuel cells (SOFCs). In this work, high-activity Bi_0.5Sr_0.5FeO_3-δ-xCe_0.9Gd_0.1O_1.95 (BSFO-xGDC, x = 10, 20, 30 and 40 wt%) oxygen electrodes are synthesized, and confirmed by XRD, SEM and EIS, respectively. The crystal structure, microstructure, electrochemical property and performance stability of the promising BSFO-xGDC composite cathodes are systematically evaluated. It is found that introducing GDC nanoparticles can obviously improve the electrochemical property of the porous composite electrode. Among all these composite cathodes, BSFO-30GDC composite cathode shows the best ORR activity. The peak power density of anode supported single cells employing BSFO-30GDC composite cathode reaches 709 mW cm^?2 and the electrode polarization resistance (R_p) of the BSFO-30GDC is about 0.14 Ω cm² at 700 °C. The analysis of the oxygen reduction kinetic indicates that the major electrochemical process of the GDC-decorated composite cathode is oxygen adsorption-dissociation. These preliminary results demonstrated that BSFO-30GDC is a prospective composite cathode catalyst for SOFCs because of its outstanding ORR activity.     

Highly methanol-tolerant non-precious metal cathode catalysts for direct methanol fuel cell

This work reports on the oxygen reduction activity of several non-precious metal (non-PGM) catalysts for oxygen reduction reaction (ORR) at the fuel cell cathode, including pyrolyzed CoTPP, FeTPP, H₂TMPP, and CoTMPP. Of the studied catalysts, pyrolyzed CoTMPP (Co-tetramethoxyphenylporphyrin) was found to perform significantly better than other materials. The catalyst underwent a thorough testing in both hydrogen-air polymer electrolyte fuel cell (PEFC) and direct methanol fuel cell (DMFC). It was found that CoTMPP cathode can sustain currents that are only 2-3 times lower than those obtained with a conventional Pt-black cathode in an H₂-air PEFC. DMFC experiments, including methanol crossover and methanol tolerance measurements, indicate high ORR selectivity of the CoTMPP catalyst. Based on results obtained to date, the CoTMPP-based catalyst offers promise for the use in conventional and mixed-reactant DMFCs operating with concentrated methanol feeds. However, hydrogen-air fuel cell life data, consisting of over 800 h of continuous cell operation, indicate that improvement to long-term stability of the CoTMPP catalyst will be required to make it practical.     

Highly active and robust biomimetic ceramic catalyst for oxygen reduction reaction: Inspired by plant leaves

Structural instability under working conditions is critical issue that restricts applications of perovskite O₂ catalysts in the field of solid oxide fuel cells. Inspired by plant leaves, a biomimetic ceramic catalyst with PrBaCo₂O_5+δ ‘mesophyll’ and Gd_0.1Ce_0.9O_1.95 ‘epidermis’ and ‘vein’ was successfully engineered in this work. The ‘epidermis’ reduces the polarization resistance of O₂ reduction reaction on catalyst surface by ∼24%, and the ‘vein’ reduces resistance of O²⁻ transport through cathode layer by ∼65%. Moreover, this biomimetic catalyst increases output power density of the cell by 79% and reverses rapid decay trend of the cell with a 23% increase in power density during first 20 h followed by stabilization at 0.91 W cm⁻² (at 750 °C and 0.7 V). This discovery provides new avenue for the development of high-performance O₂ catalysts with practical applications and enriches the scientific understanding of catalysis.     

Effects of fluorine doping on the electrochemical properties of LiV3O8 cathode material

A series of fluorine-doped lithium trivanadates LiV₃O_8−yF_z (z = 0, 0.03, 0.05, 0.1, 0.15, 0.2 and 0.5) were synthesized by the solid-state reaction. X-ray diffraction (XRD), Fourier transform infrared (FTIR) and scanning electron microscope (SEM) tests show that a proper amount of fluorine substituting for oxygen in LiV₃O₈ can modify its structure and surface morphology. Charge-discharge tests show that the doped samples with a proper amount of fluorine display good cycling stability, high coulombic efficiency and good rate capability, compared with undoped sample. The cyclic voltammetry (CV), area-specific impedance (ASI) and electrochemical impedance spectroscopy (EIS) tests indicate that the doped samples with a low fluorine content can stabilize the interface between the surface layer of the active particles and the electrolyte after cycling, while a high fluorine content form an unstable interface. The fluorine substitution is a convenient and effective method for improving the electrochemical performances of LiV₃O₈.     

Non-uniform polarizations in SOFC oxygen electrodes

The polarization resistance (R_p) of Ag/(ZrO₂)_0.9(Ln₂O₃)_0.1 (Ln = lanthanides) oxygen electrodes was measured as a function of the resistivity (ρ) of the electrolyte, which was systematically changed by using different Ln dopants. It was found that R_p increases with ρ in contrast with the expectation from the usual electrode theory assuming the adsorption and diffusion of O₂ molecules on the electrode surface. We attempt to explain the ρ-dependence of R_p based on a non-uniform electrode model which assumes locally variable polarization resistances in combination with the interfacial electrolyte resistance. According to this model, as ρ is higher, the current distribution becomes spatially more uniform, therefore making the polarizations more non-uniform. The more non-uniform is the polarization, the larger is R_p, because it is determined by the largest local polarization. The model can thus explain why R_p increases with ρ, without assuming any dependence of the local polarization resistance (r_p) on ρ. If this mechanism dominates R_p, a negative slope of t₀ (time constant of decay curve) versus R_p plots is expected while a positive slope from the conventional electrode theory. In fact, R_p decreases with t₀ in a high ρ region, thus proving that R_p is controlled by the non-uniform polarization mechanism proposed. R_p versus ρ experiments were also carried out for Au/(ZrO₂)_0.9(Ln₂O₃)_0.1 electrodes. It was found that R_p decreases with ρ in contradiction to the Ag electrodes. This ρ-dependence of R_p was explained in terms of the unusual decrease of the interfacial electrolyte resistance, which might be due to a highly concentrated current density near the 3 phase boundary.     

Review of composite cathodes for intermediate-temperature solid oxide fuel cell applications

A composite cathode exhibits low activation polarisation by spreading its electrochemically active area within its volume. Composite cathodes enable the development of high-performance electrodes for solid oxide fuel cells (SOFCs) at intermediate temperatures (600 °C – 800 °C) because of their significant role in determining the kinetics of oxygen reduction reaction (ORR). Few anions O²⁻ are transferred through the electrolyte component when the ORR is low, thereby lowering the reaction with cation H⁺ from an anode side to transfer electrons along the outer circuit to the cathode side to participate in ORR. The resistance to the ORR at the cathode is minimised, thereby contributing to performance degradation and efficiency loss in existing SOFCs, especially at intermediate temperatures. The suitability and compatibility of the cathode and electrolyte are crucial in the development of cathodes and electrochemical reactions. The intercomponent compatibility is important to ensure the robustness and durability of SOFCs, especially at an operating temperature around 800 °C, at which the components experience extreme thermal and mechanical stresses. Composite cathodes are used to improve cathode performance. These composite cathodes help enhance the properties of mixed electronic–ionic conductors and the intercomponent compatibility. Herein, we reviewed historical data of composite-cathode development for SOFCs, including its basic principle and criteria. The overall performance of as-synthesised composite cathodes in terms of microstructure, electrochemical reaction and intercomponent compatibility is briefly discussed.     

LaNi0.9Ru0.1O3 as a cathode catalyst for a direct borohydride fuel cell

LaNi_0.9Ru_0.1O₃ as cathode catalyst for a direct borohydride fuel cell (DBFC) was synthesized and investigated for the first time. The electrochemical experiments indicated that perovskite-type oxide LaNi_0.9Ru_0.1O₃ exhibited higher electrochemical performance compared with LaNiO₃, which suggested incorporation of element Ru into LaNiO₃ could further improve the catalytic ability for oxygen reduction reaction (ORR) in alkaline solution. LaNi_0.9Ru_0.1O₃ catalyst was found to have good tolerance of BH₄⁻. Meanwhile the maximum power density of 171 mW cm⁻² was obtained at 65 °C without using any precious ion exchange membrane. A life test indicated that the DBFC displayed no significant degradation for about 70 h testing. The electrochemical data suggested that LaNi_0.9Ru_0.1O₃, which provided a simple way to construct DBFCs without using any ion exchange membrane, might be promising cathode catalyst with high performance and low cost for DBFCs.     

Influence of copper oxide modification of a platinum cathode on the activity of direct methanol fuel cell

To determine whether a copper oxide modified Pt cathode (PtCuO_m) improves a performance of direct methanol fuel cells (DMFC), we performed structural and morphological analysis of the cathode and measured current-potential profile and impedance spectroscopy. Comparing with an unmodified Pt cathode, we found that PtCuO_m prepared by rf sputtering techniques induced higher oxygen reduction reaction rate and suppressed electrocatalytic oxidation of methanol, which is the main reason of the mixed potential occurred at a cathode. Therefore, PtCuO_m increased the power performance of DMFC applying both oxygen and air and electrochemical impedance spectra clearly supported the difference of the performance between unmodified and modified Pt electrodes. These results may play a role in better long-term stability of DMFC systems.     

Partially oxidized niobium carbonitride as a non-platinum catalyst for the reduction of oxygen in acidic medium

Partially oxidized NbC_0.5N_0.5 has been evaluated as a non-platinum catalyst for the reduction of oxygen in acidic medium. NbC_0.5N_0.5 powder was partially oxidized in N₂ gas containing O₂ of 10⁻⁴ atm at the temperature range of 700-1000 °C. Partially oxidized NbC_0.5N_0.5 had a definite oxygen reduction reaction (ORR) activity, while as-prepared NbC_0.5N_0.5 and completely oxidized Nb₂O₅ had a poor catalytic activity for ORR. The onset potential of the partially oxidized NbC_0.5N_0.5 for the ORR achieved 0.92 V vs. RHE in 0.1 M H₂SO₄ at 30 °C. The results of X-ray absorption spectroscopy and ionization potential measurements suggested that oxygen-vacancy defects might be responsible for the oxygen reduction capability by creating electronically favorable oxygen adsorption sites.     

Tantalum oxide-based compounds as new non-noble cathodes for polymer electrolyte fuel cell

Tantalum oxide-based compounds were examined as new non-noble cathodes for polymer electrolyte fuel cell. Tantalum carbonitride powder was partially oxidized under a trace amount of oxygen gas at 900 °C for 4 or 8 h. Onset potential for oxygen reduction reaction (ORR) of the specimen heat-treated for 8 h was 0.94 V vs. reversible hydrogen electrode in 0.1 mol dm⁻³ sulfuric acid at 30 °C. The partial oxidation of tantalum carboniride was effective to enhance the catalytic activity for the ORR. The partially oxidized specimen with highest catalytic activity had ca. 5.25 eV of ionization potential, indicating that there was most suitable strength of the interaction of oxygen and tantalum on the catalyst surface.     

Catalytic activity of titanium oxide for oxygen reduction reaction as a non-platinum catalyst for PEFC

A non-platinum cathode electrocatalyst must have the stability and catalytic activity for the oxygen reduction reaction (ORR) in order to be used in polymer electrolyte fuel cells (PEFCs). Titanium oxide catalysts as the non-platinum catalyst were prepared by the heat treatment of titanium sheets in the temperature range from 600 to 1000 °C. The prepared catalysts were chemically and electrochemically stable in 0.1 mol dm⁻³ H₂SO₄. The titanium oxide catalysts showed different catalytic activities for the ORR. The ORR of the catalysts heat-treated at around 900 °C occurred at the potential of about 0.65 V versus RHE. It is considered that the deference in the catalytic activity for the ORR of the heat-treated titanium oxide catalysts was due to the fact that the heat-treatment condition changed the material property of the catalyst surface. In particular, it was found that the catalytic activity for the ORR of the Ti oxide catalysts increased with the increase in the specific crystalline structure, such as the TiO₂ (rutile) (1 1 0) plane and the work function. It is considered that a surface state change, such as the crystalline structure and work function, might affect the catalytic activity for the ORR.     

Progress in non-precious metal oxide-based cathode for polymer electrolyte fuel cells

The cathode catalysts for polymer electrolyte fuel cells should have high stability as well as excellent catalytic activity for oxygen reduction reaction (ORR). Group 4 and 5 metal oxide-based compounds have been evaluated as a cathode from the viewpoint of their high catalytic activity and high stability. Although group 4 and 5 metal oxides have high stability even in acidic and oxidative atmosphere, they are almost insulator and have poor ORR activity because they have a large band-gap. It is necessary to modify the surface of the oxides to improve the ORR activity. We have tried the surface modification methods of oxides into four methods: (1) formation of complex oxide layer containing active sites, (2) substitutional doping of nitrogen, (3) introduction of surface oxygen defect and (4) partial oxidation of carbonitrides. These modifications were effective to improve the ORR activity of the oxides. The solubility of the oxide-based catalysts in 0.1 mol dm⁻³ at 30 °C under atmospheric condition was mostly smaller than that of platinum black, indicating that the oxide-based catalysts had sufficient stability compare to the platinum. The onset potential of various oxide-based cathodes for the ORR in 0.1 mol dm⁻³ at 30 °C achieved over 0.9 V vs. a reversible hydrogen electrode.     

Enhanced electrocatalytic activity and CO2 tolerant Bi0.5Sr0.5Fe1-xTaxO3-δ as cobalt-free cathode for intermediate-temperature solid oxide fuel cells

One of the significant motivations in developing intermediate-temperature solid oxide fuel cells (IT-SOFCs) is to design cobalt-free cathodes with high electrocatalytic activity and CO₂ tolerance ability. In this work, iron-based perovskite materials Bi_0.5Sr_0.5Fe_1-xTa_xO_3-δ are investigated as potential cathodes for IT-SOFCs. The effects of Ta doping on crystal structure, thermal expansion coefficients and electrocatalytic activities are systematically evaluated. Among the Ta-doped oxides, Bi_0.5Sr_0.5Fe_0.9Ta_0.1O_3-δ exhibits the highest electrochemical performance with the lowest polarization resistance (R_p) of 0.124 Ω cm² at 700 °C in air. The peak power density of the single cell with Bi_0.5Sr_0.5Fe_0.9Ta_0.1O_3-δ cathode reaches 1.36 W cm⁻² at 700 °C. Compared to Bi_0.5Sr_0.5FeO_3-δ, the improved CO₂ tolerance of Ta-doped oxides can be attributed to the high acidity of Ta⁵⁺ cations and the increased average metal bond energy (ABE) within the material. Further study proves that the adsorption-dissociation process of molecular oxygen is the limiting step for oxygen reduction reaction (ORR) on Bi_0.5Sr_0.5Fe_0.9Ta_0.1O_3-δ cathode.     

Electrochemical properties of LaBaCo2O5+δ-Sm0.2Ce0.8O1.9 composite cathodes for intermediate-temperature solid oxide fuel cells

The electrochemical properties of LaBaCo₂O_5+δ-xSm_0.2Ce_0.8O_1.9 (LBCO-xSDC, x = 20, 30, 40, 50, 60, wt%) were investigated for the potential application in intermediate-temperature solid oxide fuel cells (IT-SOFCs). The LBCO-50SDC composite cathode exhibited the best electrochemical performance in the LBCO-xSDC cathodes. With x = 50 wt%, the ASR was 1.308 Ω cm² at 500 °C (0.267 Ω cm² at 600 °C and 0.052 Ω cm² at 700 °C). The maximum of exchange current density i₀ was 0.5630 A cm⁻² at 700 °C. The improved electrochemical properties of LBCO-50SDC were ascribed to the porous structures of the cathode and more cathode/electrolyte/gas triple phase boundary (TPB) areas.     

Investigation of LSM/8YSZ cathode within an all-ceramic SOFC,Part II: Optimization of performance and co-sinterability

This paper focuses on the cathode and current collector layers of a co-sintered, all-ceramic solid oxide fuel cell (SOFC) concept. Challenges to reach good electrochemical performance have to be overcome, due to more demanding manufacturing conditions, including a relatively high co-sintering temperature. Master sintering curves show that the sintering activity of lanthanum strontium manganite (LSM) is significantly higher than that of 8-mol% yttria stabilized zirconia (8YSZ). By applying a double-layered cathode and a current collector with optimized microstructures the best electrochemical performance of the cathode is 0.26 $\mathrm{\Omega }$cm${}^2$ at 800 $°$C, evaluated from polarization resistances of 8YSZ electrolyte-supported symmetric cells post-sintered at 1150 $°$C $<T<1250$ $°$C. The cathode and current collector materials are adapted to fit the co-sintering process by adjustment of the paste compositions. Half-cells consisting of silicate mechanical support, LSM current collector, LSM mixed with 8YSZ composite cathode and 8YSZ electrolyte are co-sintered porous and defect-free at 1150 $°$C $<T<1250$ $°$C.     

Characterization of NdSrCo1−xFexO4+δ (0 ≤ x ≤ 1.0) intergrowth oxide cathode materials for intermediate temperature solid oxide fuel cells

NdSrCo_1−xFe_xO_4+δ (0 ≤ x ≤ 1.0) intergrowth oxides have been investigated as cathode materials for intermediate temperature solid oxide fuel cells (IT-SOFCs). All the cathodes prepared by a glycine nitrate process (GNP) indicated single phase intergrowth oxides. The introduction of Fe for Co leads to decrease TEC values and electrical conductivity, and increase polarization resistance and oxygen content. The polarization resistance of NdSrCoO_4+δ composition is 0.16 Ω cm² at 800 °C in air atmosphere, which is the best electrochemical performance compared with other compositions.     

Kinetic study on the role of an LSGMC5 interlayer in improving the performance of Sm0.5Sr0.5CoO3-La0.8Sr0.2Ga0.8Mg0.15Co0.05O3 (LSGMC5)/LSGMC5 interface

The kinetics of oxygen reduction over various Sm_0.5Sr_0.5CoO₃(SSC)-La_0.8Sr_0.2Ga_0.8Mg_0.15Co_0.05O₃(LSGMC5)/LSGMC5 (interlayer)/LSGMC5 (electrolyte) assemblies were studied, which were essential to find the role of an interlayer in improving the performance of an electrode/electrolyte interface. Two major arcs were identified in the impedance spectra at near equilibrium conditions. The reciprocal of the electrode resistance corresponding to the high frequency arc showed a PO₂ dependency about 0.5 at 1073 K and decreased to one-fourth at 873 K, suggesting that the rate-determining step (rds) changed from the dissociative adsorption of oxygen or diffusion of adsorbed oxygen atoms to charge transfer. The reciprocal of the electrode resistance corresponding to the low frequency arc showed a PO₂ dependency about 1, suggesting an rds involving the gas diffusion of oxygen. DC polarization curves of various assemblies agreed well with the Butler-Volmer equation. Both the cathodic and anodic charge transfer coefficients were about 1, and the PO₂ dependencies of the exchange current densities were about 0.25, especially at low temperatures. The characteristics under polarization corresponded to a charge transfer process. The introduction of an LSGMC5 interlayer between the SSC-LSGMC5 electrode and LSGMC5 electrolyte did not change the reaction mechanism, and the role of the interlayer was to increase the number of active sites for oxygen reduction.