Preparation of Sc-doped Lanthanum Strontium Manganite (LSM) Materials

Introduction

Lanthanum strontium manganate (LSM) chalcogenide-type composite oxides are the most commonly used cathode materials for anode-supported medium-temperature solid oxide fuel cells due to their high conductivity and good oxygen reduction reaction activity, as well as their good thermal and chemical compatibility with Y₂O₃-doped ZrO₂ (YSZ) electrolyte materials.

The YSZ electrolyte material is usually introduced into the cathode to form an LSM-YSZ composite cathode in the study. The composite cathode combines the advantages of better oxygen reduction activity of LSM and higher ionic conductivity of YSZ. Currently, the LSM-YSZ composite cathode is the most classical cathode material in IT-SOFC.

Fig.1 Strontium-doped lanthanum manganites with a cubic perovskite structure show excellent photocatalytic properties in generating high oxygen amounts.

Currently, a considerable number of researchers are focusing on the modification of LSM-based cathodes. The low ionic conductivity of LSM materials and their low activity towards oxygen reduction reaction at low temperatures limit the low-temperature performance of the batteries. In the LSM-like chalcogenide structure, the B-site cation plays an important role in the catalytic activity of the material, and the B-site doping of chalcogenide is an effective means to improve the cathode activity of the material.

In short, the introduction of variable-valent or low-valent transition metal ions into the B-site of LSM can improve the activity of LSM-based cathode materials through the synergistic effect of the B-site ion valency. However, the examined cathode materials containing elements such as Co, Fe, and Ni suffer from poor chemical compatibility with the YSZ electrolyte and a mismatch in thermal expansion, which affects the stability of the long-term operation of IT-SOFC.

Sc, as the first transition metal element in the periodic table, has its special properties: a large ionic radius and a stable valence (+3).

Preparation Protocol

Materials Preparation and Characterization

• Synthesis of La_0.8Sr_0.2Mn_1-xSc_xO_3±δ (LSMS_x, x = 0, 0.02, 0.05, and 0.1) powders using the modified citric acid method:
  
  • Dissolve La₂O₃ in dilute nitric acid and Sr(NO₃)₂, Mn(NO₃)₂ and Sc(NO₃)₃·6H₂O in deionized water separately.
  • Mix the metal nitrate solutions according to the stoichiometric ratio, and add ammonium citrate while heating and stirring. Adjust the solution pH to 1-2 to form a gel-like solution.
  • Heat the gel solution to initiate a self-combustion reaction, obtaining the LSMS_x precursor powders.
  • Calcine the precursor powders at 900-1200 ℃ in air for 3 hours to obtain the final LSMS_x powders.
• Structural characterization of the LSMS_x samples:
  
  • Perform X-ray diffraction (XRD) analysis using Cu Kα radiation, Ni filter, 40 kV, 200 mA, and a scanning rate of 5°/min.
• H2-Temperature-Programmed Reduction (H2-TPR) characterization:
  
  • Press the LSMS_x powders into pellets, granulate, and select the 80-110 mesh fraction for H2-TPR analysis.
  • Place 50 mg of the sample in a U-shaped quartz reactor. Pre-treat the sample in high-purity He (30 ml/min) at 30 ℃ for 30 min, then switch to a 10% H₂/90% Ar (50 ml/min) mixture and heat the sample at a rate of 10 ℃/min from 100 to 950 ℃, using a TCD detector.

Single Cell Fabrication

• Anode substrate preparation:
  
  • Mix NiO and YSZ in a mass ratio of 40:60 in anhydrous ethanol, grind thoroughly, and add suitable binders, dispersants, and strengthening agents to prepare the initial slurry.
  • Use tape casting to fabricate the anode substrate.
• Anode/electrolyte bi-layer fabrication:
  
  • Coat a YSZ slurry on the anode substrate using the doctor-blade method, dry, and co-sinter at 1400 ℃ in air for 2 hours.
  • The final anode/electrolyte bi-layer has a diameter of ~21 mm and a thickness of ~500 μm, with the YSZ electrolyte being ~15 μm thick.
• Cathode fabrication:
  
  • Mix LSMS_x and YSZ in a 1:1 mass ratio, grind in anhydrous ethanol to form a paste-like mixture, and then add suitable binders, plasticizers, and n-butanol to prepare the cathode slurry.
  • Apply the cathode slurry on the pre-sintered anode/electrolyte bi-layer using the coating method, and sinter at 1100 and 1200 ℃ in air for 2 hours, respectively, to obtain the coin-shaped single cells.

Single Cell Performance Evaluation

1. Apply silver paste on the cathode and place the single cell in the test setup.
2. Feed O₂ to the cathode and H₂ to the anode, maintaining a constant gas flow rate (10 ml/min for both).
3. At 800 ℃, reduce the NiO in the anode, then measure the cell's current-voltage (I-V) characteristics.
4. Perform electrochemical impedance spectroscopy (EIS) measurements in the open-circuit state, with a frequency range of 1×10⁵ to 0.1 Hz and an AC voltage amplitude of 10 mV.

Reference

1. Strontium Doped Lanthanum Manganites for Efficient and Robust Photocatalytic Water Oxidation Coupled with Graphene Oxide. Materials Letters (2014).