Chemically stable perovskites as cathode materials for solid oxide fuel cells: La-doped Ba0.5Sr0.5Co0.8Fe0.2O(3-δ)

Cu-doped Nd0.6Sr0.4Co1-xCuxO3-δ (x = 0, 0.05, 0.1, 0.15, 0.2) as the cathode for intermediate-temperature solid oxide fuel cells

Nd_0.6Sr_0.4Co_1-xCu_xO_3-δ (x = 0, 0.05, 0.1, 0.15, 0.2) (NSCC_x) was prepared by replacing Co with Cu. Its chemical compatibility, electrical conductivity, and electrochemical properties were studied by X-ray powder diffractometry, scanning electron microscopy, and X-ray photoelectron spectroscopy. The conductivity, AC impedance spectra, and output power of the single cell were tested in an electrochemical workstation. Results showed that the thermal expansion coefficient (TEC) and electrical conductivity of the sample decreased with the increase in Cu content. The TEC of NSCC_0.1 decreased by 16.28% in the temperature range of 35 °C-800 °C, and its conductivity was 541 S cm^-1 at 800 °C. Furthermore, a single cell was constructed with NSCC_x as the cathode, NiO-GDC as the anode, and GDC as the electrolyte. The peak power of the cell at 800 °C was 444.87 mW·cm^-2, which was similar to that of the undoped sample. Compared with the undoped NSCC, NSCC_0.1 showed lower TEC while maintaining its output power. Therefore, this material can be used as a cathode for solid oxide fuel cells.

Peculiar Properties of the La0.25Ba0.25Sr0.5Co0.8Fe0.2O3-δ Perovskite as Oxygen Reduction Electrocatalyst

The electrochemical reduction of molecular oxygen is a fundamental process in Solid Oxide Fuel Cells and requires high efficiency cathode materials. Two La0.25Ba0.25Sr0.5Co0.8Fe0.2O3-δ-based perovskite compounds were prepared by solution combustion synthesis, and characterized for their structural, microstructural, surface, redox and electrochemical properties as potential cathodes in comparison with Ba0.5Sr0.5Co0.8Fe0.2O3-δ and La0.5Sr0.5Co0.8Fe0.2O3-δ perovskites. Results highlighted that calcination at 900 °C led to a "bi-perovskite heterostructure", where two different perovskite structures coexist, whereas at higher calcination temperatures a single-phase perovskite was formed. The results showed the effectiveness of the preparation procedures in co-doping the A-site of perovskites with barium and lanthanum as a strategy to optimize the cathode's properties. The formation of nanometric heterostructure co-doped in the A-site evidenced an improvement in oxygen vacancies' availability and in the redox properties, which promoted both processes: oxygen adsorption and oxygen ions drift, through the cathode material, to the electrolyte. A reduction in the total resistance was observed in the case of heterostructured material.

Electrokinetic Proton Transport in Triple (H+ /O2- /e- ) Conducting Oxides as a Key Descriptor for Highly Efficient Protonic Ceramic Fuel Cells

Recently, triple (H+ /O2- /e- ) conducting oxides (TCOs) have shown tremendous potential to improve the performance of various types of energy conversion and storage applications. The systematic understanding of the TCO is limited by the difficulty of properly identifying the proton movement in the TCO. Herein, the isotope exchange diffusion profile (IEDP) method is employed via time-of-flight secondary ion mass spectrometry to evaluate kinetic properties of proton in the layered perovskite-type TCOs, PrBa0.5 Sr0.5 Co1.5 Fe0.5 O5+ δ (PBSCF).Within the strategy, the PBSCF shows two orders of magnitude higher proton tracer diffusion coefficient (D* H , 1.04 × 10-6  cm2 s-1 at 550 °C) than its oxygen tracer diffusion coefficient at even higher temperature range (D* O, 1.9 × 10-8  cm2 s-1 at 590 °C). Also, the surface exchange coefficient of a proton (k*H ) is successfully obtained in the value of 2.60 × 10-7  cm s-1 at 550 °C. In this research, an innovative way is provided to quantify the proton kinetic properties (D* H and k*H ) of TCOs being a crucial indicator for characterizing the electrochemical behavior of proton and the mechanism of electrode reactions.

Clarifying the Role of the Reducers-to-Oxidizers Ratio in the Solution Combustion Synthesis of Ba0.5Sr0.5Co0.8Fe0.2O3-δ Oxygen Electrocatalysts

Ba0.5Sr0.5Co0.8Fe0.2O3-δ perovskite-type compounds are well-known mixed ionic-electronic conductors for oxygen electrocatalytic applications, although their performance is strictly dependent on the selected preparation methodology and processing parameters. The reducers-to-oxidizers ratio (Φ) is a very important parameter in the solution combustion synthesis of mixed ionic-electronic conductors. Selection of Φ is not trivial and it strongly depends on the type of fuel used, the chemical composition and the specific application of the material. This work clarifies the role of Φ in the solution combustion synthesis of Ba0.5Sr0.5Co0.8Fe0.2O3-δ for application as oxygen electrocatalysts. Ba0.5Sr0.5Co0.8Fe0.2O3-δ powders were synthesized by solution combustion synthesis using sucrose-polyethylene glycol fuel mixtures with reducers-to-oxidizers ratio values between 1 (stoichiometric) and 3 (over-stoichiometric). Chemical-physical properties were studied by X-ray diffraction, scanning electron microscopy, N2 adsorption at −196 °C, H2-temperature programmed reduction and thermogravimetric analysis. The results evidenced the direct role of Φ on the intensity and redox environment of the combustion process, and its indirect influence on the Ba0.5Sr0.5Co0.8Fe0.2O3-δ electrode materials properties. Taking into account the general picture, the highly over-stoichiometric Φ was selected as the optimal one and the electrochemical activity of the corresponding powder was tested by electrochemical impedance spectroscopy on electrolyte-supported half-cells employing a Ce0.8Sm0.2O2-x electrolyte.

Platinum-Decorated Ceria Enhances CO2 Electroreduction in Solid Oxide Electrolysis Cells

CO₂ electroreduction by solid oxide electrolysis cells (SOECs) can not only attenuate the greenhouse effect, but also convert surplus electrical energy into chemical energy. The adsorption and activation of CO₂ on the cathode play an important role in the SOEC performance. La_0.6 Sr_0.4 Co_0.2 Fe_0.8 O_3-δ -Ce_0.8 Sm_0.2 O_2-δ (LSCF-SDC; SDC=samarium-doped ceria) is a promising SOEC cathode. However, its electrocatalytic activity still needs to be improved. In this study, Pt/SDC interfaces are constructed by decorating Pt nanoparticles onto the SDC surface. Electrochemical measurements indicate that the polarization resistance of the SOEC is decreased from 0.308 to 0.120 Ω cm² , and the current density is improved from 0.913 to 1.420 A cm^-2 at 1.6 V and 800 °C. Physicochemical characterizations suggest that construction of the Pt/SDC interfaces increases the oxygen vacancy concentration on the cathode and boosts CO₂ adsorption and dissociation, which leads to enhanced CO₂ electroreduction performance in SOECs.

Oxygen-deficient perovskites for oxygen evolution reaction in alkaline media: a review

Oxygen vacancies in complex metal oxides and specifically in perovskites are demonstrated to significantly enhance their electrocatalytic activities due to facilitating a degree of control in the material’s intrinsic properties. The reported enhancement in intrinsic OER activity of oxygen-deficient perovskites surfaces has inspired their fabrication via a myriad of schemes. Oxygen vacancies in perovskites are amongst the most favorable anionic or Schottky defects to be induced due to their low formation energies. This review discusses recent efforts for inducing oxygen vacancies in a multitude of perovskites, including facile and environmentally benign synthesis strategies, characterization techniques, and detailed insight into the intrinsic mechanistic modulation of perovskite electrocatalysts. Experimental, analytical, and computational techniques dedicated to the understanding of the improvement of OER activities upon oxygen vacancy induction are summarized in this work. The identification and utilization of intrinsic activity descriptors for the modulation of configurational structure, improvement in bulk charge transport, and favorable inflection of the electronic structure are also discussed. It is our foresight that the approaches, challenges, and prospects discussed herein will aid researchers in rationally designing highly active and stable perovskites that can outperform noble metal-based OER electrocatalysts.

A Zn-Doped Ba0.5Sr0.5Co0.8Fe0.2O3-δ Perovskite Cathode with Enhanced ORR Catalytic Activity for SOFCs

The insufficient oxygen reduction reaction activity of cathode materials is one of the main obstacles to decreasing the operating temperature of solid oxide fuel cells (SOFCs). Here, we report a Zn-doped perovskite oxide Ba0.5Sr0.5(Co0.8Fe0.2)0.96Zn0.04O3-δ (BSCFZ) as the SOFC cathode, which exhibits much higher electrocatalytical activity than Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) for the oxygen reduction reaction (ORR). The BSCFZ cathode exhibited a polarization resistance of only 0.23 and 0.03 Ω·cm2 on a symmetrical cell at 600 and 750 °C, respectively. The corresponding maximum power density of 0.58 W·cm−2 was obtained in the yittria-stabilized zirconia (YSZ)-based anode-supported single cell at 750 °C, an increase by 35% in comparison to the BSCF cathode. The enhanced performance can be attributed to a better balance of oxygen vacancies, surface electron transfer and ionic mobility as promoted by the low valence Zn2+ doping. This work proves that Zn-doping is a highly effective strategy to further enhance the ORR electrocatalytic activity of state-of-the-art Ba0.5Sr0.5Co0.8Fe0.2O3-δ cathode material for intermediate temperature SOFCs.

The hexagonal perovskite Ba0.5Sr0.5Co0.8Fe0.2O3−δ as an efficient electrocatalyst for the oxygen evolution reaction

Hexagonal structure perovskite Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ was sucessfully synthesized as an OER electrocatalyst, exhibiting excellent electrochemical performance in 0.1 M KOH.

The p(O2)-T stability domain of cubic perovskite Ba0.5Sr0.5Co0.8Fe0.2O3-δ

Cubic perovskite-type Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF) is one of the mixed ionic-electronic conductors with the highest oxygen permeability known to date. It serves as a parent material for the development of functional derivatives for electrochemical applications including oxygen separation membranes, solid electrolyte cell electrodes and electrocatalysts for the oxygen evolution reaction. The present study is focused on the determination of the precise stability boundaries of cubic perovskite BSCF employing a coulometric titration technique in combination with thermogravimetric analysis, X-ray and neutron diffraction, and molecular dynamics simulations. Both the low-p(O₂) and high-p(O₂) stability boundaries at 700-950 °C were found to correspond to a fixed value of oxygen content in the perovskite lattice of 3 - δ = ∼2.13 and ∼2.515, respectively. The stability limits in this temperature range are expressed by the following equations: high-p(O₂) boundary: log p(O₂) (atm) (±0.1) = -10 150/T (K) + 8.055; low-p(O₂) boundary: log p(O₂) (atm) (±0.03) = -20 750/T (K) + 4.681. The p(O₂)-T phase diagram of the BSCF system under oxidizing conditions is addressed in a wider temperature range and is shown to include a region of precipitation of a "low-temperature" phase occurring at 400-500 °C. The fraction of the low-temperature precipitate, which co-exists with the cubic perovskite phase and is stable up to 790-820 °C, increases upon increasing p(O₂) in the range 0.21-1.0 atm.

A Highly Efficient and Robust Cation Ordered Perovskite Oxide as a Bifunctional Catalyst for Rechargeable Zinc-Air Batteries

Of the various catalysts that have been developed to date for high performance and low cost, perovskite oxides have attracted attention due to their inherent catalytic activity as well as structural flexibility. In particular, high amounts of Pr substitution of the cation ordered perovskite oxide originating from the state-of-the-art Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF) electrode could be a good electrode or catalyst because of its high oxygen kinetics, electrical conductivity, oxygen capacity, and structural stability. However, even though it has many favorable intrinsic properties, the conventional high-temperature treatment for perovskite synthesis, such as solid-state reaction and combustion process, leads to the particle size increase which gives rise to the decrease in surface area and the mass activity. Therefore, we prepared mesoporous nanofibers of various cation-ordered PrBa_0.5Sr_0.5Co_2-xFe_xO_5+δ (x = 0, 0.5, 1, 1.5, and 2) perovskites via electrospinning. The well-controlled B-site metal ratio and large surface area (∼20 m² g^-1) of mesoporous nanofiber result in high performance of the oxygen reduction reaction and oxygen evolution reaction and stability in zinc-air battery.

Triple-conducting layered perovskites as cathode materials for proton-conducting solid oxide fuel cells

We report on an excellent anode-supported H(+) -SOFC material system using a triple conducting (H(+) /O(2-) /e(-) ) oxide (TCO) as a cathode material for H(+) -SOFCs. Generally, mixed ionic (O(2-) ) and electronic conductors (MIECs) have been selected as the cathode material of H(+) -SOFCs. In an H(+) -SOFC system, however, MIEC cathodes limit the electrochemically active sites to the interface between the proton conducting electrolyte and the cathode. New approaches to the tailoring of cathode materials for H(+) -SOFCs should therefore be considered. TCOs can effectively extend the electrochemically active sites from the interface between the cathode and the electrolyte to the entire surface of the cathode. The electrochemical performance of NBSCF/BZCYYb/BZCYYb-NiO shows excellent long term stability for 500 h at 1023 K with high power density of 1.61 W cm(-2) .