An FeNbO4-based oxide anode for a solid oxide fuel cell (SOFC)

Abstract

The advantage of n-type semiconductor for an anode of solid oxide fuel cells (SOFCs) lies in its higher electronic conductivity in reducing atmosphere than in air. In this study, n-type FeNbO4-based oxides that can be reduced at temperatures below 700 o C for a conductivity above 1 S cm -1 are explored as anode materials for a ceria-based SOFC utilizing liquefied-petroleum-gas (LPG) fuel apart from pure H2. Fe0.8Nb1.2O4 with 20 at.% Fe deficiency was founded in the sample sintered at 1250 o C. The structure stability of FeNbO4 under reducing atmosphere can be improved by its solid solution with a lessreducible TiO2 that also stabilizes the high-temperature -PbO2 type structure with mixed Fe 3+ and Nb 5+ cation. In particular, a full cell employing Ti0.36(Fe0.985Nb1.015)0.84O4, a stable and electrically conductive (1 S cm -1 ) oxide in 5% H2, as anode shows a powder density of 180 mW cm -2 at 700 o C if 0.5 wt.% Pd is impregnated to increase the electrocatalysis and the electric loss is mostly from the electrolyte. The oxide anode showed a degradation (20% during the 5 to 26 hours aging) and the carbon deposition is slight after 5-hour operation under an LPG fuel.[1] and are not restrained by efficiency limitations applicable to heat engines (i.e. the Carnot cycle). The potential widespread use of SOFCs also depends on the availability of fuel[2]. Comparing to the low-temperature proton-exchange-membrane fuel cell (PEMFC), the advantage of an all-ceramic SOFC device could be the ability to use carbonaceous fuel, i.e. carbon monoxide, methane et al., other than pure H2 fuel[3-6]. The fuel versatility of an SOFC increases the fuel availability since the infrastructure for carbonaceous fuel is mature, which will facilitate the commercialization of this technique and reduce the additional cost on H2 storage and transport[2]. The conventional nickelbased anode catalyzing the oxidation of the H2 fuel on GDC electrolyte is Ni(O)-GDC cermet and a high performance (e.g. 2 W cm 2 at 550 o C)[7] has been achieved via the optimization of the cathode materials and the thinning of the electrolyte[8]. However, the carbon deposition or coking process of the Ni under a carbonaceous fuel would prohibit the long-term operation for continuous power supply[9] and thus novel oxide perovskites, such as (La, Sr)(Cr, Mn)O3, Sr2MoMO6 (M=Mg, Cr or Mn) and (La, Sr)TiO3, has been developed for the operation under hydrocarbon fuels[10-20].Generally, both nand p-type semiconducting oxides can be used as the anode materials in an SOFC. La0.75Sr0.25Cr0.5Mn0.5O3 is a paradigmatic p-type semiconducting oxide showing a decreased conductivity under reducing atmosphere of fuel than in air, while (La, Sr)TiO3 is an n-type semiconductor showing superior stability and conductivity if is fully reduced[21] at elevated temperatures [22] (above 750 o C under H2). However, an SOFC under the intense in-situ reduction at elevated temperature for anodes can possibly induce the reduction of the GDC electrolyte, causing the increase of electronic conduction and i...

Optimization of La0.2Sr0.7–xCaxTi0.95Fe0.05O3−δ Fuel Electrode Stoichiometry for Solid Oxide Fuel-Cell Application

“…A typical SOFC fuel electrode material that displays outstanding catalytic performance in hydrogen is Ni-cermet. − However, this electrode is not very stable and attractive in hydrocarbon fuels because of its quick carbon deposition at operating temperatures and the necessity for a high water concentration in the fuel stream to suppress the carbon deposition (which leads to decrease of the SOFC power characteristics). − Additionally, Ni-based fuel electrodes are very sensitive against impurities like H 2 S in natural gas and HCl, as well as KCl in syngas, ,,− and tend to re-oxidize at higher cell loads. A large number of studies has been devoted to diminishing these drawbacks by exploring new materials to replace the Ni-cermet fuel electrode. ,, One attractive branch of novel electrode materials is perovskite-type complex oxides (ABO 3 ) with mixed ionic–electronic conductivity (MIEC). These materials have some advantages compared to Ni-cermet, such as a broader electrochemically active region compared to cermets, higher carbon as well as sulfur tolerance, and good redox stability.…”

“…These materials have some advantages compared to Ni-cermet, such as a broader electrochemically active region compared to cermets, higher carbon as well as sulfur tolerance, and good redox stability. Several properties of the ABO 3 perovskites result from the transition metal B-site cations in the octahedral sites, but electrodes are also significantly influenced by the A-site cations’ characteristics, typically used in combination with rare-earth and alkaline-earth metals. , The MIEC materials used in the SOFC anode could be p-type conductors such as doped LaCrO 3 or n-type semiconducting perovskites such as doped SrTiO 3 . (La, Sr)­TiO 3 has attracted specific attention over the past years because of its superior dimensional stability, high conductivity, and significant tolerance against sulfur poisoning and carbon depositions. , To increase the catalytic activity and chemical stability, many different doping levels and stoichiometries for La x Sr 1– x TiO 3−δ (LST) have been investigated …”

Density Functional Theory Study of Monoclinic FeNbO4: Bulk Properties and Water Dissociation at the (010), (011), (110), and (111) Surfaces

“…In recent years, research has intensified into the development of ABO 4 -type oxides, including iron niobate (FeNbO 4 ). − So far, three different FeNbO 4 phases have been reported, i.e., the monoclinic wolframite-type structure (m-FeNbO 4 , space group P 2/c), the orthorhombic α-PbO 2 -type structure (o-FeNbO 4 , space group P 2/c), and the rutile-type structure (r-FeNbO 4 , space group P 4 2 /mnm) . Under ambient conditions, m-FeNbO 4 has the most stable structure, , where both Fe 3+ and Nb 5+ are coordinated by six oxygen ions, forming stable octahedra.…”

“…Out of the three iron niobate phases, m- and o-FeNbO 4 are promising materials for many applications. For example, m-FeNbO 4 has excellent potential as a gas sensor material, − owing to its good electrical properties due to similar radii but different valence states of the Fe and Nb cations, whereas o-FeNbO 4 has been investigated as a photodetector for solar energy conversion devices − and an anode material in solid oxide fuel cells (SOFC). , Conventionally, nickel metal particle-based materials have been used widely in the cathodes of solid oxide electrolysis cells (SOEC) owing to their significant advantages, i.e., low cost, good catalytic, and high electronic conductivity, − although they also have disadvantages, such as tendency to coarsening and depletion of Ni particles. , To improve the stability of the cathode, suitable ABO 3 perovskites are being explored as replacements for the Ni metal, or other metals are introduced to form more stable alloys with Ni. − Interestingly, both m- and o-FeNbO 4 have shown potential as replacement materials for the Ni metal, owing to their proven catalytic conductivity and properties as cathodes in SOEC, where the splitting of water vapor takes place.…”

Recent Progress on Defect‐rich Transition Metal Oxides and Their Energy‐Related Applications

“…In recent decades, various clean energy conversion devices have been developed rapidly. Lithium ion batteries (LIBs), [2][3][4] solid oxide fuel cells (SOFCs), [5][6][7] proton exchange membrane fuel cells (PEMFCs), [8][9][10] metal-air batteries [11][12][13] and other clean and efficient devices have been continuously studied, fast developed and commercialized. For PEMFCs and metal-air batteries, the oxygen evolution reaction (OER) at the anode and oxygen reduction reaction (ORR) at the cathode are the main reactions of electrochemical conversion.…”