Short communicationHigh performance Ni–Sm2O3 cermet anodes for intermediate-temperature solid oxide fuel cells
Introduction
Solid oxide fuel cells (SOFCs) have the potential to produce electricity with high efficiency [1]. The fuel is electrochemically oxidized at the anode according to the following reaction:H2 + O2− → H2O + 2e−
Therefore, the catalytic properties of the anode to fuel oxidation reaction are of great importance [2], [3]. In addition, the anode must provide paths for the transport of oxygen ions, electron, and gas since the reaction takes place where the three species are available [4]. Beside its low cost, nickel is a very good electronic conductor and an excellent electrocatalyst for the electrochemical oxidation for hydrogen and hydrocarbons. So, the anodes are usually based on porous composites of Ni with oxygen-ion conducting ceramics such as yttria-stablized zirconia (YSZ) or doped ceria (DCO) [1]. The use of YSZ in the anode serves several purposes, one of which is to supply a conductive network for oxygen ions. In this way an extended region of triple-phase-boundary (TPB) is formed, where electrons, oxygen ions, and the gas phase can meet and react. The electrochemically active zone thus expands up to 10–20 μm from the physical electrolyte–anode interface due to the ionic transport in the YSZ network [5]. As a result, the fuel cell performance is greatly improved and the interfacial polarization resistance is substantially reduced by YSZ incorporation [6]. The involvement of DCO can further enhance the performance of the Ni anodes due to the reason that DCO has higher oxygen ion conductivity and higher catalytic activity than YSZ [7]. In principle, the performance improvement is more significant for an oxygen-ion conductor with higher conductivity [8]. Comparative studies of the anode materials with different levels of the oxygen ionic transport, such as Ni–Zr0.85Y0.15O1.93, Ni–Ce0.9Gd0.1O2−δ, Ni–Ce0.6Gd0.4O2−δ, Ni–La0.75Sr0.25Cr0.97V0.03O3−δ, and Zr0.71Y0.12Ti0.17O2−δ, confirmed that increasing ionic conductivity leads to lower values of the total polarization resistance [9].
However, as sharp contrast to the principle that high performance is directly associated with high ionic conductivity, this work reports a composite anode consisting of Ni and Sm2O3, which is a non-conductive oxide. Although the conductivity of Sm2O3 is negligible compared with that of DCO, the interfacial polarization resistance of Ni–Sm2O3 is even lower than that of Ni-samaria doped ceria (SDC) when the fuel cells are operated at 600 °C. This result suggests new materials and/or alternative viability for the development of SOFCs.
Section snippets
Experimental
The powders involved in this work including Sm2O3, Sm0.2Ce0.8O1.9 (SDC), NiO and Sm0.5Sr0.5CoO3 (SSC) were synthesized via a glycine nitrate method. Taking the synthesis of Sm2O3 as an example, a solution containing Sm3+ (1 mol L−1) was prepared by dissolving Sm(NO3)3 in distilled water. After adding glycine, the solution was heated till self-combustion occurred. The resulted ashes were calcined at 800 °C for 2 h to form Sm2O3 powders. The synthesis of other powders was described in detail in our
Results and discussion
Fig. 1 shows typical current–voltage curves for single cells with Ni–Sm2O3 anodes. At 600 °C, the cell exhibited open-circuit voltage (Voc) of 0.853 V, which is close to those reported for H2/air fuel cells with SDC electrolytes and Ni–SDC anodes. The reported values were 0.850 V [12], 0.88 V [13], and 0.85 V [15] when SSC–SDC cathodes were fabricated with various methods. The closeness in Voc suggests that Ni–Sm2O3 anodes exhibit electrochemical activity as high as those of Ni–SDC composites. Low
Conclusions
We have demonstrated that the single cells with the Ni–Sm2O3 anode exhibited high performance when hydrogen was used as the fuel. Comparing with Ni–SDC anodes which have large TPB length, the electrochemical reaction in the Ni–Sm2O3 anode should be constricted to the physical interface between the electrolyte and anode since Sm2O3 has extremely low conductivity. The high performance might be due to the electrocatalytic activity of Sm2O3 and the unique microstructure of the anodes. Although the
Acknowledgements
This work was supported by the Natural Science Foundation of China (50672096 and 50730002) and the Ministry of Science and Technology of China (2007AA05Z151).
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