Highly Stable and Efficient Perovskite Ferrite Electrode for Symmetrical Solid Oxide Fuel Cells

Exsolution on perovskite oxides: morphology and anchorage of nanoparticles

Perovskites are very promising materials for a wide range of applications (such as catalysis, solid oxide fuel cells…) due to beneficial general properties (e.g. stability at high temperatures) and tunability - doping both A- and B-site cations opens the path to a materials design approach that allows specific properties to be finely tuned towards applications. A major asset of perovskites is the ability to form nanoparticles on the surface under certain conditions in a process called "exsolution". Exsolution leads to the decoration of the material's surface with finely dispersed nanoparticles (which can be metallic or oxidic - depending on the experimental conditions) made from B-site cations of the perovskite lattice (here, doping comes into play, as B-site doping allows control over the constitution of the nanoparticles). In fact, the ability to undergo exsolution is one of the main reasons that perovskites are currently a hot topic of intensive research in catalysis and related fields. Exsolution on perovskites has been heavily researched in the last couple of years: various potential catalysts have been tested with different reactions, the oxide backbone materials and the exsolved nanoparticles have been investigated with a multitude of different methods, and the effect of different exsolution parameters on the resulting nanoparticles has been studied. Despite all this, to our knowledge no comprehensive effort was made so far to evaluate these studies with respect to the effect that the exsolution conditions have on anchorage and morphology of the nanoparticles. Therefore, this highlight aims to provide an overview of nanoparticles exsolved from oxide-based perovskites with a focus on the conditions leading to nanoparticle exsolution.

Pr-Doped SrTi0.5Mn0.5O3-δ as an Electrode Material for a Quasi-Symmetrical Solid Oxide Fuel Cell Using Methane and Propane Fuel

The use of identical electrodes for both the cathode and the anode in a symmetrical solid oxide fuel cell (SSOFC) can simplify the preparation process and increase the durability of the cell, but it is also demanding on the properties of the electrode including stability, electric conductivity, and electrocatalysis. The doping of variable-valence Mn^4+/3+2+ on the B site of stable SrTiO₃ is explored in this study as both the cathode and the anode for an SSOFC. Though the limit of Mn doping in SrTiO₃ is generally low, the additional Pr^3+/4+ donor on the Sr site of SrTi_0.5Mn_0.5O₃ was found to enhance the structure stability, electric conductivity, and electrocatalysis. The cell with Pr_0.5Sr_0.5Ti_0.5Mn_0.5O₃ electrodes excels under H₂, propane, or CH₄/H₂ fuel, providing the cocatalyst was infiltrated on the anode side. The polarization resistance value of Pr_0.5Sr_0.5Ti_0.5Mn_0.5O₃ was 0.27 Ω·cm² as the cathode and 0.33 Ω·cm² for the SSOFC using H₂ fuel. The performance under CH₄/H₂ fuel can be boosted to above 0.9 W cm^-2 if Ni/ceria was loaded onto the anode to enhance the methane reforming. This work contributes to a perovskite anode with high Mn doping in SrTiO₃ for application in SSOFC for natural gas with renewable H₂ injection.

Ni/NiO Exsolved Perovskite La0.2Sr0.7Ti0.9Ni0.1O3-δ for Semiconductor-Ionic Fuel Cells: Roles of Electrocatalytic Activity and Physical Junctions

A semiconductor-ionic fuel cell (SIFC) is recognized as a promising technology and an alternative approach to reduce the operating temperature of solid oxide fuel cells. The development of alternative semiconductors substituting easily reduced transition metal oxide is a great challenge as high activity and durability should be satisfied simultaneously. In this study, the B-site Ni-doped La_0.2Sr_0.7Ti_0.9Ni_0.1O_3-δ (LSTN) perovskite is synthesized and used as a potential semiconductor for SIFC. The in situ exsolution and A-site deficiency strategy enable the homogeneous decoration of Ni/NiO nanoparticles as reactive sites to improve the electrode reaction kinetics. It also supports the formation of basic ingredient of the Schottky junction to improve the charge separation efficiency. Furthermore, additional symmetric Ni_0.8Co_0.15Al_0.05LiO_2-δ (NCAL) electrocatalytic electrode layers significantly enhance the electrode reaction activity and cells' charge separation efficiency, as confirmed by the superior open circuit voltage of 1.13 V (close to Nernst's theoretical value) and peak power density of 650 mW cm^-2 at 550 °C, where the latter is one order of magnitude higher than NCAL electrode-free SIFC. Additionally, a bulk heterojunction effect is proposed to illustrate the electron-blocking and ion-promoting processes of the semiconductor-ionic composite electrolyte in SIFCs, based on the energy band values of the applied materials. Overall, we found that the energy conversion efficiency of novel SIFC can be remarkably improved through in situ exsolution and intentional introduction of the catalytic functionality.

Modulating cobalt-iron electron transfer via encapsulated structure for enhanced catalytic activity in photo-peroxymonosulfate coupling system

In recent years, many efforts have been made to modulate the interaction between carriers and nanoparticles under the integrity of the active site structure. Herein, SrFeO₃ @CoSe₂ nanocomposite was fabricated by loading CoSe₂ onto SrFeO₃ particles with a perovskite structure in the form of an encapsulation. The optimized SFO@CS-0.3 catalyst exhibited high catalytic activity in photo-peroxymonosulfate-based reaction and the catalyst was structurally stable over a wide temperature range. Characterization and theoretical results demonstrated that the charge in the SrFeO₃ was transferred from Fe to Co cation of the CoSe₂ via the interfacial oxygen atom. Moreover, the newly established oxygen-metal structure (Fe-O_v-Co) acted as a catalytic site, accelerating the cleavage of the peroxymonosulfate bond to generate radicals, which were desorbed into solution to attack the contaminant. Simultaneously, the heterojunction constructed by the catalyst underwent internal electron transfer under visible light, creating a field in which multiple reactive oxygen species co-oxidized organic contaminant.

A Flexible Method to Fabricate Exsolution-Based Nanoparticle-Decorated Materials in Seconds

Decorating metallic nanoparticles on the surface of oxide support is a promising approach to tailor the catalytic activity of perovskite. Here, for the first time using thermal shock to rapidly fabricate nanoparticle-decorated materials (NDMs) is proposed. Low-cost and size-tailorable carbon paper is used as the heating source during the thermal shock. It is reported that by thermal shock technique, only ≈13 s including heating and treating time is needed to fabricate the exsolution-based NDMs (the fastest method to date). Benefitted by the sufficiently provided driving force and the short treating time, as compared to the product prepared by the conventionally furnace-based method, higher particle density and smaller particle size of the exsolved catalysts are acquired for the thermal shock fabricated NDM, giving rise to a fascinating improvement (12-fold) of the electrochemical performance. This work develops a new technique to rapidly fabricate NDMs in an economic and high-throughput manner, profoundly improving the flexibility of the application of exsolution-based materials in electrochemical devices.

Manipulating Electrocatalytic Activity of Perovskite Oxide Through Electrochemical Treatment

Perovskite oxides are widely used in electrochemical cells, profiting from their excellent accommodation of different elements and structure stability. Here, it is reported that when rapidly exceeding the electrochemical stability window of a perovskite oxide through electrochemical treatment, nanoparticles can dynamically exsolve from the perovskite lattice, yielding a nanoparticle decorated material (NDM) with fascinating particle population and distribution. It is reported that as compared to the NDM produced by chemical gas reduction, electrochemical treatment fabricated NDM shows much better electrochemical performance. At 900 °C, a peak power density (PPD) of 896 mW cm^-2 (more than tenfold enhancement) is obtained for a yttrium stabilized zirconia (YSZ) electrolyte-supported symmetrical cell with La_0.43 Ca_0.37 Ti_0.8 Co_0.1 Fe_0.1 O_{3- δ} (LCTCF) electrode after electrochemical treatment for several minutes, while it only reaches to 210 mW cm^-2 after chemical gas treatment for tens of hours using humidified hydrogen as fuel. The study establishes a new fairyland for tuning the performance of-but not limited to-the electrochemical cells.

Redox-Reversible Electrode Material for Direct Hydrocarbon Solid Oxide Fuel Cells

Solid oxide fuel cells (SOFCs) can directly operate on hydrocarbon fuels such as natural gas; however, the widely used nickel-based anodes face grand challenges such as coking, sulfur poisoning, and redox instability. We report a novel double perovskite oxide Sr₂Co_0.4Fe_1.2Mo_0.4O_6-δ (SCFM) that possesses excellent redox reversibility and can be used as both the cathode and the anode. When heat-treated at 900 °C in a reducing environment, double perovskite phase SCFM transforms into a composite of the Ruddlesden-Popper structured oxide Sr₃Co_0.1Fe_1.3Mo_0.6O_7-δ (RP-SCFM) with the Co-Fe alloy nanoparticles homogeneously distributed on the surface of RP-SCFM. At 900 °C in an oxidizing atmosphere, the composite transforms back into the double perovskite phase SCFM. The excellent oxygen reduction reaction catalytic activity and mixed ionic-electronic conductivity make SCFM an excellent cathode material for SOFCs. When SCFM is used as the anode, excellent performance and stability are achieved upon either direct oxidation of methane as a fuel or operation with sulfur-containing fuels. The excellent redox reversibility coupled with outstanding electrical and catalytic properties manifested by SCFM will enable a broad application in energy conversion applications.