Electrochemical properties and thermal neutral state of solid oxide fuel cells with direct internal reforming of methane
Introduction
At present, the development of global economy is still based on the predominant use of fossil fuels and such a situation will last for a few decades. The solid oxide fuel cell (SOFC) has always been considered as a low-carbon and environmentally-friendly solution for fossil energy conversion and utilization [1]. It is advantageous for its high efficiency, low pollutant emission and it requires no precious metals. Besides, SOFCs have the potential to directly use hydrocarbon fuels and are gaining attention from both academia and industry in the past few decades [2], [3], [4]. SOFC power systems operated with natural gas/biogas are already present in commercial applications, such as Bloom Energy in the US, and the “ENE-FARM” project in Japan [5]. However, further promotion and commercialization of SOFC power systems still face challenges in both cost and lifespan.
Direct internal reforming (DIR) is a promising method to reduce the total cost of SOFC power systems [6]. In order to avoid the carbon deposition at the anode and improve the electrochemical performance of SOFCs, it is necessary to reform the hydrocarbon fuel (such as CH4) entering the fuel cell into CO and H2. Since the commonly used Ni-based anode materials are highly active to catalyze CH4 reforming reactions. When mixtures of CH4 and H2O (or CO2) enter the Ni-based anode channel, the direct internal steam reforming [7], [8] or dry reforming [9], [10] can be well catalyzed. Direct internal reforming can eliminate the need for an external reformer, which simplifies the overall system and reduces the cost, thus attracting the attention of many researchers in recent years [11].
When SOFCs operate under DIR condition, the gas mixture within anodes containing at least CH4, CO, H2, H2O and CO2, can lead to complex heterogeneous reaction kinetics, which are difficult to rationalize with the addition of electrochemical reactions (see Table 1). Hecht et al. [12] proposed a heterogeneous reaction mechanism for CH4 reforming on Ni-based porous catalysts, which consists of 42 reactions among 18 species including gas-phase and surface adsorbed species. However, participation of electrochemical reactions can further complicate the mechanisms. Thattai et al. [13] performed experimental studies on CH4 direct internal steam reforming kinetics with operating SOFCs. Their results show a large dependency of the reaction orders on the current density, indicating that the previously proposed rate expressions for methane steam reforming (MSR) on Ni catalysts may not be suitable for operating SOFCs. The interaction between electrical current and chemical reactions on anodes must be considered, thus further detailed analysis of the elementary reactions and experimental investigations are required.
The complexity of reaction mechanism has significant effect on modelling research for DIR-SOFCs, including the computational fluid dynamic (CFD) models, system-level simulations and micro-models. Most previous simulations are based on simplified kinetic models due to the large number of involved species and aforementioned complexity to incorporate electrochemistry. Comprehensive study on multi-physics simulation of SOFCs have been done by Ni et al. [14]. The mathematical models were well developed and successfully adopted in both HSOFCs and OSOFCs, which were operated with many kinds of hydrocarbon fuels including CH4, C2H5OH and direct carbon [15], [16], [17]. D.L. Tran et al. [18] established a mathematical model considering the concurrent effects of steam and dry reforming, or so-called methane multiple-reforming (MMR). This mathematical model was then applied and verified in a CFD model. The commonly used semi-empirical models ignore the specific reaction pathway, reducing the difficulty of mathematical modelling. However, the semi-empirical models need calibration using experimental data to obtain empirical parameters. Therefore, if the model is calibrated with different experimental data, considerable prediction bias may be present [13]. Recently, studies are also focused on the 3D reconstruction of anodes using focused ion beam (FIB)-SEM tomography, with which key parameters such as phase volumes, TPB length and tortuosity can be quantified more accurately [19], [20]. Bertei et al. [21] built a physically-based model assisted by 3D reconstruction. With this model, they successfully decoupled different processes and reproduced the electrochemical impedance of different types of fuel cells. However, researches on multi-physics simulation still faces the lack of adequate and systematic experimental data on fuel cells and stacks, especially for direct internal dry reforming. Therefore, more detailed experimental studies are required to further study the reaction mechanisms in operating SOFCs.
On the other hand, in order to meet the requirements of commercial applications, the performance of DIR-SOFCs needs to be further improved. Researchers have proposed numerous methods to improve the performance of SOFCs with direct utilization of hydrocarbon fuels, including the adjustment of fuel gas composition [22], [23], modification of anode materials [24], [25] and the addition of an anode catalytic layer [26]. Hua et al. [27] employed a Ni0.8Cu0.2 (NiCu)Ce0.8Zr0.2O2 (ZDC) layer on the top of conventional NiY2O3 stabilized ZrO2 (YSZ) anode support to enhance the dry reforming of CH4. The multi-layer fuel cell showed a peak power density (PPD) of about 1.05 W cm−2 at 800 °C, fueled by CH4CO2 gas mixture containing 50 ppm H2S. Chen et al. [28]integrated a layer of Ce0.90Ni0.05Ru0.05O2 (CNR) coating on the surface of anode supporting layer to internally catalyze steam reforming of CH4. The fuel cell demonstrated a peak power density of 0.37 W cm−2 at a very low temperature of 500 °C fueled with low content of steam. There was no evidence of coking after ∼550 h of continuous operation. Besides, some other materials such as CuCeO2 cermet and La0.4Sr0.6Co0.2Fe0.7Nb0.1O3-δ (LSCFN) perovskite [29], have also been studied in recent years to provide alternatives for DIR-SOFC anodes.
However, it should also be considered that the endothermic reforming reactions could give rise to large thermal stress within the fuel cell, especially in the gas inlet region [3], which is one of the reasons why external reformer (ER) is now preferred in most commercial SOFC systems. Therefore, reacting rate of reforming reaction should be properly controlled [30], [31]. With the lower operation temperature of SOFCs and the advent of better anode preparation process, the problem of excessive temperature gradient is promised to be well solved and DIR can be universally adopted in commercial fuel cell systems [11]. From another perspective, the endothermic reforming reactions (eqs. (1)–(3)) and the exothermic electrochemical oxidation reactions (eqs. (8)–(11)) occur simultaneously at the anode, making the thermal neutral state achievable inside a fuel cell. The energy required for the reforming of CH4 can be well compensated due to the direct and efficient heat exchange within anode substrate, contributing to the improvement of system efficiency. Although this idea was proposed years ago, few experiments or simulations verifying this issue can be found so far, mainly due to the sensitivity of real stacks operated with hydrocarbon fuels and the aforementioned large uncertainty in simulation results caused by unclear reaction mechanism.
In this study, a calculation model combining experimental data and thermodynamic results was established, which validate the possibility of achieving thermal neutral state in DIR-SOFCs. In the process of modeling, the electrochemical and thermodynamic characteristics with direct internal steam and dry reforming were elaborately compared. Detailed experimental investigation was carried out to determine the influence of H2O/CO2 on the electrochemical properties of DIR-SOFCs. Besides, analysis of distribution of relaxation times (DRT) combined with elementary reactions in CH4H2O and CH4CO2 atmospheres was proposed to distinguish different physical and chemical processes within anodes. The results of this study can be meaningful for practical application of DIR-SOFCs and conducive to a more precise understanding of reaction mechanism on SOFC anodes.
Section snippets
Material and methods
The schematic diagram of the experimental set-up is shown in Fig. 1. The commercially available fuel cell (Suzhou Huatsing Jingkun New Energy Technology Co., Ltd), consisting of a NiO-YSZ (8 mol% Y2O3-stabilized ZrO2) anode, a thin YSZ electrolyte, a GDC (Gd0.1Ce0.9O2-δ) barrier layer and a LSCF (La0.6Sr0.4Co0.2Fe0.8O3-δ) cathode, was sealed on the top of an alumina tube. Detailed parameters of the fuel cell and fabrication steps can be found in our previous study [32]. Both the anode and the
Comparison in thermodynamics
Carbon deposition is one of the issues that adversely affect the stable operation of hydrocarbon-fueled SOFCs and can be partially relieved by DIR [35], [36]. The variation of carbon amount in thermodynamic-equilibrium state with temperature in CH4H2O and CH4CO2 mixtures is shown in Fig. 3(a) and Fig. 3(b), respectively. Thermodynamic calculations in this study were performed by the software HSC chemistry 6.0, based on the minimization of system Gibbs free energy [37]. Although the results of
Conclusions
In this study, detailed experimental and thermodynamic investigation was carried out to investigate the influences of H2O/CO2 content on the electrochemical properties of DIR-SOFCs. The addition of H2O and CO2 to CH4 improves the fuel cell performance. The maximum PPDs were obtained at S/C and CO2 to CH4 ratios of 1 and 1.5 for CH4H2O and CH4CO2 mixtures, respectively. While excessive addition conversely affects the performance due to the increase of concentration polarization. Thermodynamic
Acknowledgments
This work was supported by National Key R&D Program of China (2017YFB0601903, 2017YFB0601901 and 2018YFB1502203); Dongguan Science and Technology Bureau, Guangdong (201460720100025); Tsinghua University State Key Lab Program (SKLD18M13).
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