All-solid-state direct carbon fuel cells with thin yttrium-stabilized-zirconia electrolyte supported on nickel and iron bimetal-based anodes
Introduction
With increasing concern on the global climate change caused by overconsumption of fossil fuels, there is an urgent need for reducing carbon emission. As fossil fuels, especially coal, will still be the major energy source in the foreseeable future, it is desirable to develop novel technologies which can convert chemical energy of fossil fuels to electricity with high efficiency and low emission. Therefore, the direct carbon fuel cell (DCFC), which has been proposed since one and half centuries ago [1], has regained attention due to its high conversion efficiency and concentrated CO2 product which can be sequestrated with low cost [2], [3], [4], [5]. The ideal overall electrochemical reaction in a DCFC is
The theoretical electrical conversion efficiency of this reaction is the Gibbs energy change ΔG divided by its enthalpy change ΔH. As ΔG is slightly larger than ΔH in a wide range of temperature, the conversion efficiency is over 100%, meaning some heat from external sources may be directly converted into electricity along with the DCFC operation. In a DCFC, carbon is electrochemically oxidized at the anode chamber. Compared to carbon oxidation through combustion in air, high concentrated CO2 (without N2) is produced. Such CO2 can be further sequestrated or utilized in a much lower cost. A report from EPRI of U.S. evaluating the feasibility of DCFC in electricity generation from coal has confirmed the advantages of DCFC over conventional coal fired power plant [6].
DCFCs can be classified into three types, according to different electrolytes: molten hydroxide [7], [8], [9], molten carbonate [10], [11], [12], [13], and oxygen ion conducting solid oxide [14], [15], [16], [17], [18], [19], [20], [21], [22] among which the last one is the only solid state electrolyte. As the activation energy of carbon oxidation is high, a DCFC needs to operate at relatively high temperatures. Any liquid with high temperature is dangerous as it may cause leaking or corrosion destroying the cell system. In this respect, solid state electrolyte is superior to the liquid electrolyte. However, liquid metals [14], [15], [16] or molten salts [17], [18], [19] have been added to the anode chamber for carbon delivering in solid electrolyte DCFCs. While the liquid material helps in solving the problem of carbon refilling to DCFCs, it diminishes the merit of solid electrolyte. Gür et al. have proposed DCFCs with solid oxide electrolyte, in which coal is provided with Ar or CO2 as carrying or reforming gas [23], [24], [25]. In recent years, our group have been focusing on developing an all-solid-state direct carbon solid oxide fuel cell (DC-SOFC), which is a SOFC operated on solid carbon directly filled in the cell as fuel, without any liquid medium or purging gas [26], [27], [28], [29], [30], [31], [32], [33], [34] (Fig. 1). The reaction mechanism of such DC-SOFCs was first proposed by Nakagawa and Ishida [35] and was lately verified by Xie et al. [31]. According to the mechanism (Fig. 1a), the DC-SOFC operation is driven by the electrochemical oxidation of CO at the anode
The produced CO2 molecules diffuse to the carbon fuel to perform the reverse Boudouard reaction
This reaction is favored at high temperatures because CO dominates the equilibrium gas composition in a C–O system that contains excess C. For example, the molar fraction of CO and CO2 is 89% and 11%, respectively, at 800 °C. Through reaction (3), more CO is produced for reaction (2). In such way, carbon fuel is continuously delivered to the anode for the DC-SOFC operation. There is no need for chemical or even physical contact between carbon and the anode. While such DC-SOFC is still at an early stage of development and there are very few researchers reporting on it, there is an accelerated progress. In 2009, Tang et al. reported a tubular electrolyte-supporting DC-SOFC operated on graphite fuel which gave a peak power density of 9.2 mW cm−2 at 800 °C, but the performance dropped rapidly during a stability test [26]. Later on, by utilizing activated carbon instead of graphite as the fuel, a similar DC-SOFC had steadily operated at a current density of 12.5 mA cm−2 for 37 h, verifying the self-sustainability of a DC-SOFC [27]. The performance of the DC-SOFCs were significantly improved by loading Fe on activated carbon fuel and applying Ag-GDC (gadolinium doped ceria) to the anode to catalyze the Boudouard reaction and the electrochemical oxidation of CO, respectively. A peak power density of 45 mW cm−2 was obtained at 800 °C [28]. Meanwhile, a Ni-ScSZ anode-supported SOFC operated directly on carbon fuel was reported to produce a peak power density of 75 mW cm−2 at 800 °C [36]. Our group prepared a cone-shaped NiO-YSZ anode-supported three-cell-in-series stack and operated it on Fe-loaded activated carbon fuel and obtained a peak power densities of 465 mW cm−2 at 850 °C. The stack was with a total effective area of 5.2 cm2 releasing a total power of 2.4 W [29]. As the energy density of carbon is high (8935 mAh g−1), DCFCs are recommended to be a potential high performance battery [30]. Very recently, a variety of efforts have been offered on DC-SOFCs including verifying the mechanism [31], applying different electrolyte materials [32], improving the fuel process [37], [38], [39], [40], [41], and operating with coal-based fuels [42], [43], [44], [45], [46], [47]. As CO is the favored product in C–O system with excess C at high temperatures, a DC-SOFC is also developed for gas-electricity co-generation [34]. Nevertheless, the DC-SOFC is still at a very early stage of research and development. There are many aspects to be explored including materials, operating conditions, kinetics, etc. It is not clear if the materials of SOFCs operated on hydrogen are proper for DC-SOFCs. Most of the reported DC-SOFCs are electrolyte-supporting. Making the electrolyte thin will improve the performance. While there has been research on DC-SOFCs with conventional NiO-YSZ supported SOFCs showing good output performance, the operation stability has not been well studied [29].
Many studies have shown that adding a small amount of Fe into the Ni-based anode can improve the SOFC performance operated on hydrogen and hydrocarbon fuels [48], [49]. This improvement in performance might be related to the increased porosity and triple phase boundary (TPB) of the anode. Ishihara et al. [50] reported that Fe addition in Ni-based anode decreases the activation energy of the anode reaction and changes the H2 oxidation kinetics. However, if the performance of a DC-SOFC with Ni-based anode can be enhanced in this way is still unknown.
In this paper, we report our work on DC-SOFCs with thin YSZ electrolyte supported on nickel and iron bimetal-based anodes. The composition of the anode is optimized. The operation stability and the starting process are also investigated.
Section snippets
Anode-supported SOFC preparation
Nickel and iron bimetal anode-supported SOFCs were prepared through the following steps in sequence: synthesizing the mixed powder of nickel and iron oxides, fabricating anode substrates, coating YSZ electrolyte on the anode substrates, applying cathode on the electrolyte layer, and assembling SOFCs for tests. The details of the process are described as following.
Mixture of nickel oxide and iron oxide, with a composition represented by Ni1-xFexOδ (x = 0, 0.05, 0.1, 0.2, 0.3, 1), was synthesized
Phase identification of the Ni–Fe oxide powders
Fig. 2a shows the XRD patterns of Ni–Fe oxide powders with different composition. The X-ray spectrum for pure NiO clearly shows a face-centered cubic (FCC) structure, with no second phase detected. Similarly, only hematite (Fe2O3), in a rhombohedral structure, presents in the pure ferric oxide. When a small amount (5–30 mol%) of ferric oxide is mixed with NiO, the corresponding XRD patterns indicate the coexistence of NiO and a compound of Ni and Fe (NiFe2O4) in a spinel-type structure.
Conclusions
In summary, Ni1-xFexOδ-YSZ anode-supported thin YSZ electrolyte SOFCs can be directly operated on solid carbon fuel, without any liquid medium and purging gas. The electrochemical performance of such kind of all-solid-state DC-SOFCs increases with Fe addition when x ≤ 0.1 while it decreases when x ≥ 0.1. The DC-SOFC with Ni0.9Fe0.1Oδ-YSZ anode reveals the highest MPD of 529 mW cm−2 and the lowest polarization resistance of 0.173 Ω cm2 at 800 °C. As a comparison, the DC-SOFC with traditional
Acknowledgment
This work was supported by the National Science Foundation of China (NSFC, No. 21276097), the Special Funds of Guangdong Province Public Research and Ability Construction (No. 2014A010106008), Guangdong Innovative and Entrepreneurial Research Team Program (2014ZT05N200), and Program of Excellent Ph.D Thesis Authors of Guangdong Province.
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