High performance electrolyte-coated anodes for low-temperature solid oxide fuel cells: Model and Experiments
Introduction
Recently, there have been lots of interests in improving the performance of electrodes for solid state fuel cells (SOFCs) to lower their operating temperatures [1]. In the case of the anodes, most researches were focused on the micro-structure optimization via catalyst surface modification [2], [3], [4], [5]. One of the practices is to coat fine particles of electrolyte such as yttria-stabilized zirconia (YSZ) and doped ceria on an electron conducting matrix phase of Ni-based porous materials. One advantage of the electrolyte-coated anodes is that they are capable of effectively suppressing carbon deposition when hydrocarbon is directly used as the fuel [4], [6], [7], [8]. The other advantage is that the anodic performance is significantly improved by coating the fine electrolyte particles, which are usually introduced using an ion impregnation process [3]. Jiang et al. have shown that the performance of Ni-based anodes could be substantially improved by coating nano-sized electrolyte particles of YSZ and gadolinia-doped ceria (GDC). At 700 °C, the interfacial polarization resistance of an uncoated anode was 10 Ω cm2. It was reduced to 3.1 Ω cm2 when 4.0 mg cm−2 of YSZ was impregnated [9]. The resistance was further reduced to 0.71 Ω cm2 when the electrode was coated with 1.7 mg cm−2 of GDC [10]. Zhu et al. [7] have also demonstrated that a significant reduction of the interfacial polarization resistance was achieved when 20 mg cm−2 of samaria-doped ceria (SDC) was coated into NiO-based anode substrate. In addition, distinct improvement in performance was observed for single cells with the electrolyte-coated anodes, compared with that of the cells without impregnation treatment. The improved performances are usually attributed to an effective extension of the triple-phase-boundary (TPB) area, in addition to significant electro-catalytic effect of coating nano-sized electrolyte particles on the electrochemical activity of the anode [3]. It is well known that TPB in a composite electrode is the active site for electrode reaction, where electron conducting phase (Ni), ion conducting phase (YSZ) and gas phase (H2 and product water vapor) meet. The TPB length plays an important role in determining the electrode performance. As demonstrated by Mizusaki et al. [11] and Bieberle and Gauckler [12], the electrode performance of a Ni-based anode was directly related to the TPB length, which has been shown to be determined by micro-structural parameters such as grain size, pore diameter and porosity [13] as well as the size ratio of electronic and ionic phase particles [14]. From this point of view, the surface modification is to achieve the TPB length per unit volume as large as possible by optimizing one or more parameters. However, no model is available to predict the modification effect with regards to TPB up to now.
Anyway, several models have been proposed to investigate the correlation between the micro-structural parameters and the performance of conventional composite electrodes in order to provide fundamental information for micro-structure design [15], [16], [17]. In a random packing micro-model, TPBs are formed by the contacts of percolated electronic and ionic phase particles and thus depend not only on micro-structure characteristics of the particles such as the number of each particle per unit volume and the co-ordination number but also on the probability of each phase particle belonging to the percolated cluster [18], [19]. Therefore, the determination of TPB length is quite complex. In Deng's geometrical model [13], only the relative value of TPB length was calculated. For an electrolyte-coated anode, this issue seems to be somewhat different and can probably be simplified. The reason lies on the following facts: (1) the fine coated particles do not belong to the random packing system in a three dimensional space. Instead, the particles form a continuous phase for oxygen-ion conduction on the surface of the particles belonging to the porous matrix based on Ni; (2) the difference of the size of the two phase particles is in the level of magnitude, which is much larger than the difference for a conventional composite anode. Further, there exists an important fact that the exposed surface of matrix phase particles per unit volume, i.e. the available surface for coating, is determined by the average number of matrix phase particles and the average co-ordination number between matrix phase particles, which both are dependent on the porosity. Therefore, it is necessary to develop a new model and correlative experiment to investigate the micro-structure effect on the electrode performance and optimize the fabrication parameters for the electrolyte-coated anode.
In this work, a geometrical micro-model was developed for electrolyte-coated anodes. The TPB length per unit volume was predicted as a function of the porosity and the amount of coated electrolyte. In addition, the performance of single cells based on the electrolyte-coated anodes was experimentally investigated concerning the porosity and electrolyte loadings. The anode was based on an electron conducting Ni framework that was coated with ion conducting phase of SDC. The experimental results were in good agreement with the model predictions.
Section snippets
Geometrical micro-model for an electrolyte-coated anode
As shown in Fig. 1a, an electrolyte-coated anode is composed of an electron conducting framework and coating layer of oxygen-ion conducting particles (hereafter they will be referred to as i-particles). The framework is fabricated with a conventional ceramic processing method such as dry-pressing and tape-casting. Those i-particles are subsequently deposited on the surface of the particles belonging to the framework using an ion-impregnation technique. Since the framework is formed before the
Experimental
Ni was used as the electron conducting framework and Sm0.2Ce0.8O1.9 (SDC) as the ionic conductor that was coated on the framework using an ion impregnation process. The preparation process of single cells, which were consisted of NiO–SDC anode substrates, SDC electrolytes, and Sm0.5Sr0.5CoO3 (SSC)–SDC cathodes, and the impregnation method were described in detail in previous work of our laboratory [7].
All of the powders involved in this experiment, such as NiO, SDC and SSC were synthesized
Model results and analysis
To conduct the model calculation, the radius of the e-particle (Ni) and i-particle (SDC) is assumed to be 0.5 and 0.05 μm, respectively. This assumption was made based on the micro-structure observation of the coated-anode, which is discussed in the following section. In the previous reports on micro-models of SOFC electrodes, a relative low contact angle, 2θ = 30°, is generally assumed [25], [26], [27] since it is described as the contact angle between two particles of heterogeneous phases. The
Conclusions
We demonstrate a strategy to achieve the anode with high performance via not only a micro-model but also experiments. A geometric micro-model for the electrolyte-coated anode is developed according to a random packing system and an assumption of monolayer coverage. The model shows that the length of triple-phase-boundary depends on the porosity and the impregnation loading when an ion impregnation method is used to fabricate the anode. In the porosity range of 0.3–0.5, the maximum loading
Acknowledgement
This work was supported by the Natural Science Foundation of China (50672096 and 50730002).
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