Abstract
Introduction
Mixed-potential gas sensors based on solid electrolyte have attracted much attention due to their simple structure, robust stability, and high sensitivity [1]. As the sensor performance is mainly dominated by the (electro-)catalytic activity of the sensing materials [2], materials screening has been adopted as the most important approach to developing high performance sensors. Unfortunately, the criteria for the materials design remain still unclear. One major reason for this is that for usual materials the compositions cannot be finely varied, which prevents a clear understanding how the composition governs the sensing property as well as the (electro)catalytic activities.
Perovskite oxides with a general formula ABO3 can accommodate large composition variations, which renders them abundant electrocatalytic properties and thus possibility to tackle the above problem [3]. In this work, La0.8Sr0.2CrxB1-xO3-δ (x=0, 0.5, 1; B = Fe, Mn) perovskite series was selected as a model system to investigate the composition - activity - performance relationship of the sensing electrode. The perovskite powders were synthesized via a sol-gel method. YSZ was used as the solid electrolyte and Pt as the reference electrode. The H2 response as a function of composition will be discussed in combination with results of polarization curves, impedance spectroscopy, and catalytic conversion.
Results and Conclusions
Fig. 1a shows the dependence of response values on H2 concentrations at 450 ᵒC, indicating an order of LSFe > LSCr > LSCrFe > LSCrMn > LSMn. The sensor response increased linearly with logarithmic H2 concentration in the range of 100-1000 ppm. The sensitivities, i.e. the slopes of the linear fittings, followed an order similar to the response. These results showed clearly that variation of the B-site composition in the perovskite indeed has a great impact on the sensing properties. Fig. 1b compares the selectivity of the devices. All the sensors showed the largest response to H2. The corresponding selectivity, defined as the response ratio for H2 over the second most responsive gas, presented an order of LSCrMn > LSCrFe ~LSFe > LSMn > LSCr.
Fig. 2a displays the tafel curves of the sensors tested in air at 450 ᵒC. The exchange current density, which is indicative of the electrocatalytic activity to the O2 reduction reaction (ORR), was found to be LSFe > LSMn >> LSCr~ LSCrFe >LSCrMn. Impedance spectroscopy also presented a similar order of the polarization resistance. Fig. 2b compares the heterogeneous catalytic conversion rate (normalized to the BET surface area) of H2 for the materials, revealing LSFe and LSMn as the most active ones while LSCr and LSCrFe as the least active ones. Apparently, the most responsive composition, LSFe, exhibited the highest ORR and catalytic activities, whilst the second most responsive composition, LSCr, presented low ORR and catalytic activities.
Our results showed that LSFe exhibited the best overall performance in the series, including high response, large sensitivity, fast speed, and good selectivity, which is also among the best of state-of-the-art mixed potential H2 sensors. The sensing performance as a function of the perovskite composition and the governing factors will be discussed in terms of the mixed-potential theory.
References
[1] N. Miura, T. Sato, S. A. Anggraini, H. Ikeda, S. Zhuiykov, A Review of Mixed-Potential Type Zirconia-Based Gas Sensors, Ionics. 20 (2014) 901-925. doi: 10. 1007/s11581-014-1140-1.
[2] Y. X. Liu, J. Parisi, X. C. Sun, Y. Lei, Solid-State Gas Sensors for High Temperature Applications – A Review, J. Mater. Chem. A, 2 (2014) 9919-9943. doi: 10.1039/c3ta15008a.
[3] H. Zhang, J. X. Yi, X. Jiang, Fast Response, High Sensitive and Selective Mixed-Potential H2 Sensor Based on (La, Sr)(Cr, Fe)O3-δ Perovskite Sensing Electrode, ACS Appl. Mater. Interfaces, 9 (2017) 17218-17225. doi: 10.102/acs ami.7b01901.
Figure 1