All-solid-state direct carbon fuel cells with thin yttrium-stabilized-zirconia electrolyte supported on nickel and iron bimetal-based anodes

https://doi.org/10.1016/j.ijhydene.2016.04.063Get rights and content

Highlights

  • Ni1-xFexOδ-YSZ anode-supported DC-SOFCs are systematically investigated.

  • Anode of DC-SOFCs can be reduced in situ by CO generated from Boudouard reaction.

  • A proper amount of Fe addition in anode can improve DC-SOFC performance.

  • A DC-SOFC with optimal anode of x = 0.1 gives MPD of 529 mW cm−2 at 800 °C.

  • DC-SOFC of x = 0.1 run at 0.1 A cm−2 shows a stable plateau of 0.86 V of 15 h.

Abstract

Solid oxide fuel cells (SOFCs) with thin yttrium-stabilized-zirconia (YSZ) electrolyte supported on composite anode of nickel-iron bimetal and YSZ are prepared and operated directly on Fe-loaded activated carbon fuel, without any liquid medium or purging gas. The composition of the anode is represented as Ni1-xFexOδ (x = 0, 0.05, 0.1, 0.2, 0.3)-YSZ. Experiment result shows that such kind of all-solid-state direct carbon solid oxide fuel cells (DC-SOFCs) perform well at 800 °C, giving maximum output power densities in the range of 425–529 mW cm−2. Similar to the case of a SOFC operated on hydrogen fuel, a small amount of Fe addition into the Ni-based anode can improve the performance of a DC-SOFC and the optimum composition is x = 0.1. A DC-SOFC with Ni0.9Fe0.1Oδ-YSZ anode, loaded with 2.5 g Fe-loaded activated carbon fuel, is steadily operated at a constant current density of 0.1 A cm−2 for 15 h at 800 °C. The anodes and the DC-SOFCs are characterized through XRD, AC impedance spectroscopy, and SEM measurements. The superior performance of the Fe-added anode is analyzed accordingly.

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 isC+2O2=CO2+4e

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 anodeCO+O2=CO2+2e

The produced CO2 molecules diffuse to the carbon fuel to perform the reverse Boudouard reactionCO2+C=2CO

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.

References (64)

  • A.C. Chien et al.

    Molten salts chemistry: from lab to applications

    (2013)
  • B. Cantero-Tubilla et al.

    Investigation of anode configurations and fuel mixtures on the performance of direct carbon fuel cells (DCFCs)

    J Power Sources

    (2013)
  • A.D. Bonaccorso et al.

    Development of tubular hybrid direct carbon fuel cell

    Int J Hydrogen Energy

    (2012)
  • M. Dudek et al.

    Feasibility of direct carbon solid oxide fuels cell (DC-SOFC) fabrication by inkjet printing technology

    Electrochim Acta

    (2013)
  • A.C. Rady et al.

    Direct carbon fuel cell operation on brown coal

    Appl Energy

    (2014)
  • X. Yu et al.

    Using potassium catalytic gasification to improve the performance of solid oxide direct carbon fuel cells: expherimental characterization and elementary reaction modeling

    J Power Sources

    (2014)
  • T.M. Gür et al.

    High performance solid oxide fuel cell operating on dry gasified coal

    J Power Sources

    (2010)
  • S. Li et al.

    Direct carbon conversion in a helium fluidized bed fuel cell

    Solid State Ionics

    (2008)
  • Y.B. Tang et al.

    Effect of anode and Boudouard reaction catalysts on the performance of direct carbon solid oxide fuel cells

    Int J Hydrogen Energy

    (2010)
  • Y.H. Bai et al.

    Direct carbon solid oxide fuel cell—a potential high performance battery

    Int J Hydrogen Energy

    (2011)
  • L. Zhang et al.

    Behavior of strontium-and magnesium-doped gallate electrolyte in direct carbon solid oxide fuel cells

    J Alloys Compd

    (2014)
  • W.Z. Cai et al.

    A facile method of preparing Fe-loaded activated carbon fuel for direct carbon solid oxide fuel cells

    Fuel

    (2015)
  • Y.M. Xie et al.

    Electrochemical gas–electricity cogeneration through direct carbon solid oxide fuel cells

    J Power Sources

    (2015)
  • R.Z. Liu et al.

    A novel direct carbon fuel cell by approach of tubular solid oxide fuel cells

    J Power Sources

    (2010)
  • M. Konsolakis et al.

    Carbon to electricity in a solid oxide fuel cell combined with an internal catalytic gasification process

    Chin J Catal

    (2015)
  • C. Li et al.

    Performance improvement of direct carbon fuel cell by introducing catalytic gasification process

    J Power Sources

    (2010)
  • Y.Z. Wu et al.

    A new carbon fuel cell with high power output by integrating with in situ catalytic reverse Boudouard reaction

    Electrochem Commun

    (2009)
  • Y. Jiao et al.

    Structurally modified coal char as a fuel for solid oxide-based carbon fuel cells with improved performance

    J Power Sources

    (2015)
  • Y. Jiao et al.

    In situ catalyzed Boudouard reaction of coal char for solid oxide-based carbon fuel cells with improved performance

    Appl Energy

    (2015)
  • J. Jewulski et al.

    Lignite as a fuel for direct carbon fuel cell system

    Int J Hydrogen Energy

    (2014)
  • R. Antunes et al.

    Chronoamperometric investigations of electro-oxidation of lignite in direct carbon bed solid oxide fuel cell

    Int J Hydrogen Energy

    (2015)
  • K. Xu et al.

    Effect of coal based pyrolysis gases on the performance of solid oxide direct carbon fuel cells

    Int J Hydrogen Energy

    (2014)
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