The optimal co-doping of SrFe1-xCoxO3-δ oxygen carriers in redox applications

Fe-Doped SrCoOx FET Sensors for Extreme Alkaline pH Sensing

The accurate measurement of pH in highly alkaline environments is critical for various industrial applications but remains a complex task. This paper discusses the development of novel Fe-doped SrCoOx-based FET sensors for the detection of extreme alkaline pH levels. Through a comprehensive investigation of the effects of Fe doping on the structure, electrical properties, and sensing performance of SrCoOx, we have identified the optimal doping level that significantly enhances the sensor's performance in highly alkaline conditions. With a Fe doping level of 5 mol %, the sensitivity of the sensor improves to 0.86 lg(Ω)/pH while maintaining the response rate. Further increasing the Fe doping to 10 mol % results in a sensor that demonstrates favorable response time, a suitable pH range, and a linear correlation between lg(R) and pH. The combination of X-ray photoelectron spectroscopy and X-ray diffraction analysis provides insight into the regulation mechanisms of Fe doping on the crystal structure, electronic structure, and oxygen vacancy concentration of SrCoOx. Our findings indicate that Fe doping leads to an increase in oxygen vacancy concentration and a decrease in the energy barrier for oxygen ion migration, which contributes to the improved sensing performance of the Fe-doped SrCoOx sensors. Additionally, the study highlights the influence of oxygen vacancy concentration on the electrical properties of SrCoOx. Precise control over the concentration of oxygen vacancies is crucial for optimizing the sensitivity and response speed of SrCoOx FET sensors under extreme alkalinity conditions.

Ni-doping effects on formation and migration of oxygen vacancies in SrFe1-xNixO3-δ oxygen carriers

Ni is a promising B-site doping element capable of improving the oxygen carrier performance of SrFeO3 perovskite. In this work, the effect of Ni doping on the formation and migration of oxygen vacancies in SrFe1-xNixO3-δ (x = 0, 0.0625, 0.125, 0.1875, and 0.25) is investigated using density functional theory calculations. Our results show that the oxygen vacancies formed from Ni-O-Fe chains exhibit lower formation energy (Ef) compared to those from Fe-O-Fe chains in each doping system. Additionally, Ef generally decreases with an increase of Ni content. This Ni-promoted formation of VO is attributed to three factors: weakened Ni-O bonding, the closure of O-2p states to the Fermi level by Ni-O hybridization, and Ni3+ decreasing the positive charges to be compensated by VO formation. Due to these multiple advantages, a modest Ni doping of x = 0.25 can induce a higher PO2 and a lower T comparted to the relatively larger Co doping of x = 0.5, thermodynamically. Kinetically, Ni-doping appears to be a disadvantage as it hinders oxygen migration, due to a higher oxygen migration barrier through SrSrNi compared to the SrSrFe pathway. However, the overall oxygen ion conduction would not be significantly influenced by hopping through a nearby pathway of SrSrFe with a low migration barrier in a system doped with a small amount of Ni. In a word, a small amount of Ni doping has an advantage over Co doping in terms of enhancing the oxygen carrier performance of the parent SrFeO3 system.

First-principles studies of oxygen ion migration behavior for different valence B-site ion doped SrFeO3-δ ceramic membranes

Density functional theory calculations were performed to investigate the structural, electronic, and oxygen ion migration properties of B-site ion doped SrFeO_3-δ perovskite (B = Al, Zr, Nb, and W) materials, which were used as oxygen transport membranes (OTMs) for pure oxygen output and catalytic reactions. The results of our calculations indicate that the Fe-O bond length increased and the M-O bond length decreased with the doping of Zr, Nb, and W. And the doping of Al caused the valence state of Fe ions to increase. The states near the Fermi level were mainly contributed by Fe atoms and O atoms. The strength of the Fe-O bond gradually weakened with the increase in the valence of the doped ions. Through studying the oxygen vacancy defect and the mechanism of oxygen ion migration, it was found that the doping of Al promoted the migration of oxygen ions, while the doping of Zr, Nb, and W limited the migration of oxygen ions. This study provides important insights into the behavior of oxygen ion migration in doped SrFeO_3-δ perovskite materials.

Investigation of Sr0.7 Ca0.3 FeO3 Oxygen Carriers with Variable Cobalt B-Site Substitution

A-site and B-site substitutions are effective methods towards improving well-studied oxygen carrier materials that are vital for emerging gasification technologies. Such materials include SrFeO₃ , which greatly benefits from the inclusion of calcium and/or cobalt, and Sr_0.8 Ca_0.2 Fe_0.4 Co_0.6 O₃ has been regarded as the best-performing composition. In this study, systems with higher calcium and lower cobalt contents are investigated with a view to lessening the societal and economic burdens of these dual-doped carriers. Density functional theory calculations are performed to illustrate the Fe-O bonding and relaxation contributions to the oxygen vacancy formation energy in Sr_1-x Ca_x Fe_1-y Co_y O₃ systems (x=0.1875, 0.25, 0.3125; y=0.125, 0.25, 0.375, 0.5) and determine that increased calcium A-site substitution requires the use of less cobalt B-site doping to reach the same oxygen vacancy formation. These findings are experimentally validated in situ and ex situ characterization of bulk Sr_0.7 Ca_0.3 Fe_1-y Co_y O₃ materials. Sr_0.7 Ca_0.3 Fe_0.7 Co_0.3 O₃ is found to have similar O₂ adsorption/desorption rates and storage capacity to Sr_0.8 Ca_0.2 Fe_0.4 Co_0.6 O₃ in air/N₂ cycling experiments. Additionally, both materials are outperformed by Sr_0.7 Ca_0.3 Fe_1-y Co_y O₃ systems with y=0-0.10 at 400-500 °C, which cycle 1.5 wt% O₂ in under ten minutes.