Silver incorporation into cathodes for solid oxide fuel cells operating at intermediate temperature
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- Uhlenbruck, S., Tietz, F., Haanappel, V. et al. J Solid State Electrochem (2004) 8: 923. doi:10.1007/s10008-004-0510-4
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Silver (Ag) at 0.1–2.0 wt% was incorporated into cathodes for solid oxide fuel cells as a catalyst for oxygen reduction. A novel processing route for Ag incorporation ensuring a very homogeneous Ag ion distribution is presented. From the results of X-ray powder diffraction it can be concluded that the La0.65Sr0.3MnO3−δ perovskite phase is already formed at 900 °C.
The solubility of Ag in the crystal lattice in this type of perovskite was below 1 wt%. The electrochemical tests of these materials show that there is only a slight catalytic effect of Ag. Scanning electron microscopy reveals a low mechanical contact of the cathode grains to the electrolyte due to the low cathode sintering temperature that was chosen.
KeywordsSolid oxide fuel cellCathodeSilver
Fuel cells can convert chemical energy into electricity without the intermediate step of heat generation. Therefore the efficiency of such a device is not limited by the Carnot efficiency of heat engines. The development of the well-known polymer electrolyte (or proton exchange) membrane (PEM) fuel cells suffers from the high cost of platinum catalysts and the need to be operated with high-purity hydrogen without carbon monoxide (catalyst poisoning). Solid oxide fuels cells (SOFCs) overcome these problems as they are operated at 800–1,000 °C where the electrode materials have sufficient catalytic activity and carbon monoxide in the fuel gas can be easily oxidized and desorbed. The SOFC can even be powered with hydrocarbons. Nevertheless, the high operating temperature causes problems, too. High temperature leads to enhanced aging and requires heat-resistant system components (e.g. interconnect plates for connecting single fuel cells; heat exchangers). Lowering the temperature, however, decreases the power density of SOFCs due to the decreasing catalytic activity.
Silver (Ag) is known to improve the oxygen exchange reaction activity [1, 2]. Therefore, attempts were made to mix or to impregnate SOFC cathodes (e.g. LaxSryMnO3−δ) with Ag [3, 4]. Problems can arise during impregnation due to low adhesion forces between the Ag precursor solutions and the ceramic cathode material. On the other hand, mechanical mixing of metals and ceramics always suffers from the ductility of the metal. Metal particles cannot be milled to the desired particle size, and if milled the particles are rolled to irregular flat bands which would destroy the microstructure of the cathode. Using Ag salts instead of solid Ag metal has proved to be detrimental to the power density of fuel cells with such cathodes .
In this paper an alternative method for incorporating Ag into ceramic cathodes is presented that is suitable for producing a very homogeneous distribution of Ag on an atomic scale. These cathode materials were characterized by X-ray powder diffraction (XRD), surface area [according to Brunauer, Emmett and Teller (BET)] and scanning electron microscopy (SEM). The influence of Ag on the electrochemical performance of the cathode material was evaluated by single fuel cell tests.
Materials and methods
The ceramic cathode material with the nominal composition La0.65Sr0.3MnO3−δ along with Ag was prepared by a citrate complexation method (Pechini method) : nitrate salts of La, Sr, Mn and Ag (Merck, Germany, quality: analytical pure) were dissolved in de-ionized water and stirred well. This method has the advantage that, in contrast to earlier experiments using oxides, carbonates, hydroxides and nitrates simultaneously [1, 2], the solution contained—besides the water as the solvent—solely the desired cations and (NO3)− ions. Thus, no reaction between the set of cations and different types of anions can take place, e.g. the cations Ag+ and Mn2+ may react with (CO3)2− ions and precipitate as Ag2CO3 and (MnCO3), which are almost insoluble in water.
An excess of citric acid (>2 moles per mole cation) was added to this solution in order to obtain a complexation of the cations. Ethylene glycol was added in excess (>5 moles per mole citric acid) to polymerize the solution. The temperature of this gel was gently increased to 700 °C to combust all organic compounds and to decompose the (NO3)− ions. The residue was a fine dark black powder.
The pure La0.65Sr0.3MnO3−δ without Ag which was used as a reference was a spray-pyrolyzed powder (for details see ).
Chemical analysis was performed using an inductively coupled plasma optical emission spectroscope (ICP-OES), XRD using a Siemens D 500 X-ray diffractometer and surface area determination by means of BET Areameter II (Ströhlein). SEM imaging was carried out with a LEO 1530 electron microscope.
In order to obtain fuel cells with these Ag-containing cathodes the synthesized powders were first mixed with terpineol and ethyl cellulose to a paste and then screen-printed on top of an 8 mol% yttria-stabilized zirconia (8YSZ) electrolyte of about 5 μm in thickness that was mechanically supported by a NiO/8YSZ anode substrate of 1.5 mm in thickness. Details of the processing of the anode and the electrolyte can be found in [7, 8, 9]. The sintering of the cathode was carried out at 920 °C for 3 h in air. A significant increase in sintering temperature of the Ag-containing cathode materials was avoided due to the low melting point of Ag (961 °C).
The screen printing of the cathode was carried out in two steps: firstly, a “functional layer” of 15 μm in thickness consisting of a mixture of 8YSZ and La0.65Sr0.3MnO3−δ (+Ag), and secondly, a 30 μm thick layer of pure La0.65Sr0.3MnO3. The introduction of such a functional layer has three advantages: this layer reduces the mismatch in the thermal expansion coefficient between La0.65Sr0.3MnO3−δ (12.5×10−6 1/K) and 8YSZ (10.5×10−6 1/K); the three-phase boundary of the electrode/electrolyte/gas where the reduction of oxygen takes place is increased so that the effective electrochemical active area is enhanced and more oxygen ions per time period are fed to the electrolyte; and the grain growth of La0.65Sr0.3MnO3−δ during sintering and operation is (at least partially) suppressed and thus the surface area where oxygen can react is not decreased.
The electrochemical cell tests were performed in an alumina housing using gold seals, air on the cathode side (1,000 ml min−1) and hydrogen (1,000 ml min−1) containing 3 vol.% water vapor on the anode side. The electrical contact was made by means of a fine platinum mesh on the air side of the fuel cell and a nickel mesh on the fuel side .
Results and discussion
Chemical analysis of the synthesized ceramic powders calcined at 900 °C for 5 h
BET surface (m2/g)
La0.65Sr0.3MnO3−δ+0.1 wt% Ag
La0.66Sr0.30MnO3−δ+0.11 wt% Ag
La0.65Sr0.3MnO3−δ+0.5 wt% Ag
La0.66Sr0.30MnO3−δ+0.50 wt% Ag
La0.65Sr0.3MnO3−δ+1.0 wt% Ag
La0.66Sr0.30MnO3−δ+0.92 wt% Ag
La0.65Sr0.3MnO3−δ+2.0 wt% Ag
La0.66Sr0.30MnO3−δ+1.96 wt% Ag
The BET surface area was around 5 m2/g for all samples, which is in the same range as for the Ag-free powder (4.7±0.5 m2/g), so no influence of Ag on the BET surface area was found.
Moreover, a peak that corresponds to solid Ag metal is found (referring to JCPDS No. 4–0783). That means that the solubility limit for Ag in this type of perovskite seems to be significantly lower than about 1 wt% Ag—a fact that was not observed previously . At lower Ag concentrations, Ag peaks could not be detected in the diffraction patterns because of the detection limit of XRD for proportions lower than about 1 vol.%. Even at 1 wt% and 2 wt% it is difficult to identify the solid Ag metal since the relative intensity of the Ag diffraction pattern is low and the Ag peaks are rather close to the perovskite and La(OH)3 and Mn3O4 peaks. Nevertheless, these peaks could not be accounted for by either the La(OH)3, Mn3O4 or the perovskite peaks.
Electrochemical performance of SOFCs sintered at 1,100°C for 3 h
Cathodes with 2 wt% Ag: current density (A/cm2)
Cathodes without Ag: current density (A/cm2)
To conclude the SOFC cathode material La0.65Sr0.3MnO3−δ with small amounts of very finely and homogeneously distributed Ag (0.1 to 2.0 wt% Ag) was prepared by the Pechini method. The solubility of Ag in the La0.65Sr0.3MnO3−δ perovskite was lower than about 1 wt%. Higher amounts of Ag were segregated as solid metal.
SOFCs made with this type of cathode produced a current density of about 0.5 A/cm2 at 800 °C and 0.7 V cell voltage. The low sintering temperature of 920 °C, which was chosen due to the low melting point of Ag, led to limited sintering of the functional layer and adhesion to the electrolyte. This poor contact is thought to be the main reason for power densities of these cells being only moderate.
The authors thank the Central Department of Analytical Chemistry of Forschungszentrum Jülich for ICP-OES measurements. Thanks are also due to D. Rutenbeck for the preparation of the Ag-free samples sintered at 920 °C, P. Lersch for XRD measurements and C. Tropartz, B. Roewekamp and H. Wesemeyer for performing the electro-chemical tests. Financial support from the Federal Ministry of Research and Education, under contract no. 01SF0039, is gratefully acknowledged.