Solid oxide fuel cells fueled by simulated biogas: Comparison of anode modification by infiltration and reforming catalytic layer

https://doi.org/10.1016/j.cej.2020.124755Get rights and content

Highlights

  • Conventional Ni-YSZ anodes were modified by infiltration and a catalytic layer.

  • Detailed comparison between the modified cells and the bare cell was made.

  • The GDC infiltration on anode is beneficial only in certain cases.

  • Enhancement of cell performance and stability was achieved by the catalytic layer.

Abstract

Solid oxide fuel cell (SOFC) provides a new method for clean and efficient conversion and utilization of hydrocarbon fuels due to its high fuel flexibility. The utilization of biogas fuel through direct internal reforming (DIR) allows for a simplification of the SOFC system since no external reformer appears strictly necessary. Despite its considerable convenience and potential, at present, direct internal dry reforming is still not considered as a competitive process for commercial application, which is limited by the reaction difficulty and high risk of carbon deposition. To address the problem, in this study, conventional Ni-yttrium stabilized zirconia (YSZ) anodes of SOFCs are modified by two methods, i.e. Ce0.9Gd0.1O2-δ (GDC) infiltration and a Ni-GDC reforming catalytic layer. A comparative study between the modified cells and the bare cell is made under operation with humidified H2 and simulated biogas. Both of these two modification methods lead to a slight but acceptable decrease of the cell performance under humidified H2. In the case of simulated biogas, results show that GDC infiltration is only beneficial to the cell performance with very low or very high CO2/CH4 molar ratio. While with a moderate CO2/CH4 ratio of 0.5–1.5, GDC infiltration instead results in a decrease of cell performance. However, the cell modified by a Ni-GDC catalytic layer shows superior performance towards the direct utilization of simulated biogas than the bare one and the one modified by GDC infiltration. The anode modification by a Ni-GDC catalytic layer can also improve the stability of the cell under simulated biogas, which is verified under different operating currents, although serious carbon deposition is detected in the catalytic layer afterwards.

Introduction

In the present transitional phase of energy structure based on fossil fuels to one based on large-scale utilization of renewable fuels, the solid oxide fuel cell (SOFC) with high electrical efficiency, low pollutant emission and good fuel adaptability can act as an important role [1], [2]. SOFC systems with hundreds of watts to hundreds of kilowatts output power have been commercialized around the world, with continuous operation time of thousands of hours and maximum electrical efficiency of more than 60% [3], [4], [5]. At present, there is an urgent need to further improve the lifetime of SOFC systems and reduce costs to meet the demand for further large-scale applications.

The most common structure of state-of-the-art SOFCs is based on yttrium stabilized zirconia (YSZ) electrolyte, Ni-YSZ cermet anode and La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) cathode. In principle, SOFCs can directly use not only H2 fuel, but also hydrocarbon and alcohol fuels because of O2− conduction through YSZ electrolyte and the high temperature (>600 °C) electrochemical oxidation on Ni-YSZ anode [6], [7], [8]. However, in practice, one of the main risks related to the direct utilization of hydrocarbon fuels is the undesirable carbon deposition on the Ni-YSZ anode, which may reduce the catalytic activity and even damage the fuel cell [9], [10]. The carbon deposition is mainly caused by the faster dissociative hydrocarbon adsorption compared to the carbon oxidation, and Ni has a very strong catalytic activity for the dehydrogenation of hydrocarbon fuels [11], [12]. To address this problem, low Ni content anodes such as ceria-based anodes [13], [14] and perovskite anodes with metal/alloy exsolution [15], [16] have been investigated and confirmed to be effective to increase carbon resistance. However, their chemical- and electro-catalytic activities are significantly lower than those of Ni-based anodes, resulting in a lower output performance of the fuel cell, which hinders the further application of these materials.

An alternative solution to this problem in the short and medium term is represented by the utilization of an external fuel processor for the reforming of hydrocarbon fuels to syngas [17], [18]. This method is commonly used in the current commercial SOFC systems. However, the pre-reformer inevitably causes an increase of the complexity and cost of the SOFC system. Another possibility is the utilization of a direct internal reforming (DIR) process, i.e. adding H2O, CO2 or O2 into the fuel, so that the fuel can be in situ reformed to H2 and CO at the Ni-YSZ anode [19], [20], [21]. The DIR process allows for a simplification of the SOFC system since no external reformer appears strictly necessary. Besides, endothermic reforming reaction and exothermic electrochemical oxidation reaction are integrated into the anode, which is conducive to internal heat management and overall efficiency improvement. Our previous studies have shown that for both direct internal steam reforming and dry reforming, the cells can reach thermal neutrality at a certain operating current (or fuel utilization) [22].

Among all kinds of DIR processes, the direct internal dry reforming of methane has attracted more and more attention in recent years because it can achieve simultaneous conversion of CH4 and CO2, which are identified as the most abundant greenhouse gases (GHG) on earth [23], [24], [25].CH4+CO2=2CO+2H2(ΔH0=261kJ/mol)

On this basis, some studies have focused on SOFCs fueled by biogas, which is intrinsically rich in CH4 and CO2 and can greatly facilitate the direct internal dry reforming process [26], [27], [28], [29]. However, the presence of CO2 dilutes the CH4 fuel and causes the reduction of electromotive force, and may even adversely affect the output performance of the cell. Lanzini et al. [30] investigated the effect of CO2 to CH4 ratio on the stability of a single planar fuel cell, and they found that excessive CO2 may not help reduce the degradation rate. In our previous study [31], detailed experimental research was carried out to investigate the influence of CO2 to CH4 ratio on fuel cell’s electrochemical performance and stability, and the optimum ratio was obtained as 1.5–1.7.

Despite its considerable convenience and potentials, the relatively long reaction time and high risk of carbon deposition render direct internal dry reforming a temporarily unpractical process for commercial application which requires long-term stable operation for thousands of hours [32]. On the one hand, it has been shown that the performance of the fuel cell operated with direct internal dry reforming is significantly lower than that with direct internal steam reforming at the same CO2/CH4 and H2O/CH4 ratios [22]. The EIS analysis shows that the polarization resistance of the cell under direct internal dry reforming is significantly larger than that under direct internal steam reforming, which indicates that it is more difficult to generate H2 through internal dry reforming. On the other hand, the carbon deposition tendency of the cell operated with direct internal dry reforming is higher (with the thermodynamic threshold CO2/CH4 ratio as 1.5 at 750 °C), so larger CO2 content is required to ensure carbon-free operation. However, excessive CO2 can reduce the output performance of the cell, even lead to the re-oxidation of Ni catalyst at the anode, which is not conducive to the stable operation [30]. Therefore, the rational design of electrode catalysts at the nanoscale is of vital importance for the realization of higher output performance and more stable operation of the cell with lower amount of reforming agent [33].

In recent years, many studies have been focused on the modification of traditional Ni-YSZ cermet anode by a variety of materials via wet impregnation [34], such as inert metals (Cu, Sn, etc.) [35], [36], basic oxides (BaO, CaO, etc.) [37], doped ceria [38], [39] and perovskites [40]. Wang et al. [41] evaluated the effect of Al2O3 and SnO2 additives introduced into the Ni-YSZ cermet anode on fuel cell performance and stability operated with simulated biogas. For the conventional Ni-YSZ anode, it has been proven that the infiltration of CeO2 [42], [43] or doped ceria [39] can help enhance the in-situ conversion of hydrocarbon fuels, thus improving the fuel cell performance and stability. In our previous studies [40], [44], the effects of Ce0.9Gd0.1O2-δ (GDC), LSCF and La0.8Sr0.2FeO3-δ (LSF) infiltration into Ni-YSZ anode were investigated and compared. It was found that the infiltration of GDC, LSCF and LSF can all efficiently enhance the cell durability in wet CH4, whereas only the infiltration of GDC could help improve output performance. Dogdibegovic et al. [45] studied the effect of Ni, Ru and Cu infiltration on direct internal ethanol reforming in metal-supported SOFCs, thus greatly improving the output performance and decreasing the operating temperature of ethanol-fueled SOFCs.

Another practical modification method is to apply a catalytic layer on the conventional Ni-YSZ anode [33]. Such a catalytic layer should be able to enhance the CH4 reforming reaction and mitigate the carbon deposition reactions such as CH4 cracking and CO disproportionation. Zhan et al. [7], [46] integrated a Ru-CeO2 catalytic layer on the NiO-YSZ anode (for high-temperature application) and the NiO-SDC anode (for low-temperature application), both of which showed good output performance and stability under iso-octane fuel. Steil et al. [47], [48] investigated the effect of Ir-GDC catalytic layer on the ethanol reforming reaction, and almost no attenuation caused by carbon deposition was observed after 600 h of continuous operation in dry ethanol. In order to reduce the cost, non-noble metal catalysts such as modified Ni-Al2O3 [49] and La2NiO4 [50] were bonded to the Ni-YSZ anode and tested under methane fuel, showing competitive performance at high temperature. Chen et al. applied a porous core-shell catalyst layer (Ni-BaO-CeO2@SiO2) on the surface of Ni-YSZ anode, and galvanostatic operation under low concentration coal bed CH4 fuel for over 150 h was achieved [51], [52]. Besides, in-situ exsolution of metal alloy nanoparticles has been investigated and regarded as a time-saving and convenient method to form an evenly distributed reforming catalytic layer on the anode surface [16], [53]. In addition, some other Ni- or alloy-based cermet catalytic layers have also been investigated to promote the in-situ conversion of liquid fuels such as ethanol [54], [55], [56] and gasoline [57] at conventional SOFC anodes.

However, most of the above cells with anode modification were tested under a certain component of hydrocarbon fuel, such as dry CH4 or humidified CH4. There is a lack of clear understanding of the specific reforming mechanism of hydrocarbon fuels at the anode, which requires more systematic experiments. Besides, few studies have focused on the comparison of above mentioned two anode modification methods, i.e. infiltration and reforming catalytic layer. Therefore, the basic idea of this study is to modify the conventional Ni-YSZ anodes of commercially available SOFCs by means of infiltration or a catalytic layer, and to evaluate and compare the effectiveness under operation in the presence of simulated biogas. The GDC and Ni-GDC were selected as the material for infiltration process and a thin-film catalytic layer respectively, due to the high ionic conductivity and mobility of GDC and excellent electronic conductivity and catalytic activity of Ni. Besides, it has been confirmed that the existence of metal support interaction (MSI) between Ni and CeO2 particles is beneficial to the uniform dispersion of Ni particles and to prevent their aggregation [58], [59]. To assess and compare the feasibility of the bare and modified SOFCs operated with biogas, conditions simulating operation with a mixture of CH4 and CO2 were investigated.

Section snippets

Fabrication of fuel cells

The fabrication procedure for the bare SOFC (Cell #1) has been reported in our previous work [60]. The prepared cell consists of porous NiO-YSZ anode support (with the weight ratio of 1:1, and the thickness of 500–600 μm), dense and thin YSZ electrolyte (8 mol% yttria stabilized zirconia, with the thickness of 10–20 μm), a GDC barrier layer (Gd0.1Ce0.9O2-δ, with the thickness of 10 μm) and LSCF cathode (La0.6Sr0.4Co0.2Fe0.8O3-δ, with the thickness of 30–40 μm). Based on the bare Cell #1, the

Electrochemical properties under humidified H2

Firstly, the electrochemical performances of Cell #1, #2 and #3 under humidified H2 were tested, including I-V-P curves and EIS plots under 700, 750, 800 °C, as shown in Fig. 2. Some key performance indicators including peak power density (PPD), series ohmic resistance (Rs) and polarization resistance (Rp) are summarized in Table 1. The I-V-P curves, shown in Fig. 2(a), (c) and (e), reflect that the difference between the output performance of three cells under humidified H2 is not very large.

Conclusions

In this study, conventional Ni-YSZ anodes of commercially available SOFCs were modified by two methods, i.e. GDC infiltration and a Ni-GDC reforming catalytic layer. The effectiveness of these two methods was evaluated and compared under cell operation in the presence of humidified H2 and simulated biogas. Both of these two modification methods lead to a slight but acceptable decrease of the cell performance under humidified H2. The decrease of cell performance by GDC infiltration should be

Declaration of competing interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by the Ministry of Science and Technology, China (2018YFB1502203, 2017YFB0601903); Dongguan Science and Technology Bureau, Guangdong (201460720100025); Tsinghua University Initiative Scientific Research Program (20193080046).

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