Metallic interconnects for proton ceramic fuel cells : oxidation behavior under simulated fuel cell conditions

Abstract

Fuel cells are expected to serve as a contribution to meet the demand for clean energy. High temperature fuel cells such as solid oxide fuel cells (SOFC) and proton ceramic fuel cells (PCFC) are developed for use as environment friendly energy conversion devices. However, the successful implementation of such devices in practical applications relies on series connections of multiple cells by so-called interconnects. During operation at high temperatures (600 – 850 °C) facing both air and fuel, oxidation of these metallic interconnect materials is inevitable. Formation of oxide scales will result in a reduced overall performance of the fuel cell stack. It is therefore crucial to investigate the oxidation behavior and the mechanisms responsible for the oxide growth on the interconnect.

This thesis consists of six chapters where the first five chapters give the basis for the work presented in five articles. Chapter six presents a summarizing discussion which links the results from PAPER I – IV and discusses them further, and to some extent in more speculative terms than found suitable in the individual papers.

The applicability of a material as an interconnect for SOFC and PCFC rely on several high temperature materials’ properties. Some of the most essential properties were investigated in this study for the Sandvik Sanergy HT. It was found that the thermal expansion was ∼12.5×10-6 /°C, a value that is regarded as suitable for SOFC and PCFC application. Further, it was found that due to the limited formation of electrical resistive oxide scales, the area specific resistance (ASR) measured at 700 °C was as low as ∼6 mOhm×cm2 after 500 h in wet air. This is below the generally regarded threshold value of 10 mOhm×cm2 for interconnect materials.

The oxidation behavior of Sanergy HT was thoroughly investigated throughout this thesis. Up to 900 °C the oxidation behavior showed parabolic kinetics, whereas at 1000 °C the oxidation process was accelerated after ∼300 h. The activation energy for oxidation (800 – 900 °C) was found to be 272±20 kJ/mol. The oxide scales formed during oxidation in air comprised an inner layer of Cr2O3 and an outer layer of (Cr,Mn)3O4-spinel.

Two-stage oxidation experiments were performed where the first stage of oxidation was in 18,18O2 (g) and the second stage was in 16,16O2 (g) in order to elucidate the oxide growth mechanisms. SIMS profiles revealed that the governing transport mechanism responsible for oxide growth was outward cation diffusion. Oxygen tracer diffusion experiments showed that inward diffusion of oxygen was significant in the outer region of the oxide scale. As a result of outward cation transport through the inner layer of Cr2O3 and inward oxygen diffusion through the outer layer comprising (Cr,Mn)3O4 it was suggested that the oxide growth takes place within the scale, likely near the Cr2O3 - (Cr,Mn)3O4 interface. However, the diffusion of cations through the inner chromia layer is still regarded as the rate limiting mechanism for the oxidation process.

During operation in a fuel cell the interconnect is facing air on the cathode side and fuel on the anode side simultaneously. Such dual atmosphere exposures have been found to significantly alter the oxidation behavior of the interconnect on the cathode side as a result of transport of hydrogen species through the alloy. This was identified by an extensive formation of Fe-rich oxide nodules, accompanied by localized internal oxidation and metal loss. The influence of dual atmosphere was further enhanced by increasing the water vapor content in the air on the cathode side. Introducing water vapor on the anode side gave however the opposite effect; less extensive nodule formation and metal attack. Further, it was observed that the preferred location of nodule formation and internal oxidation was related to surface deformations of the as received samples left by cold work during fabrication, e.g. the rolling process. Interestingly, dual atmosphere conditions was not found to have any significant effect on the oxidation of samples coated with a metallic layer of Ce (10 nm) with Co (800 nm) on top.

The anomalous oxidation behavior of uncoated samples encountered under dual atmosphere conditions was suggested to be a breakaway type of oxidation. The transport of hydrogen through the alloy increases the H2O (g)/O2 (g) ratio near the metal – oxide interface and triggers breakaway oxidation, identified by internal oxidation, metal attack and formation of Fe-rich oxide phases observed as nodules. The reduced effect of dual atmosphere conditions on coated samples was suggested to be due to a combination of reduced hydrogen transport through the coated samples, and a decreased susceptibility towards breakaway oxidation as a result of a reduction in the chromium evaporation.

Chromium nitrides are known to improve mechanical and chemical properties of alloys. It has therefore been suggested that the formation of CrNx on interconnects could also improve the high temperature performance of these materials. The literature on thermal nitridation of chromium bearing alloys is limited, and in order to contribute to a more fundamental understanding of this subject ten Fe-Cr, Ni-Cr and Fe-Ni-Cr model alloys and two ferritic interconnect materials were treated at high temperatures in an atmosphere containing a mixture of nitrogen and hydrogen. It was found that the extent of internal precipitation of Cr2N increased with increasing chromium content, except for the ternary Fe-Ni-Cr alloys. It was also found that the nitridation kinetics were generally slower for the nickel bearing alloys. Chromium nitrides were formed on the surface of the ferritic interconnects proving that thermal nitridation is a possible technique to form an external Cr2N layer on commercial interconnect alloys. The potential effect on the high temperature properties of these materials was not further investigated.

On the basis of the investigations presented in this thesis the Sandvik Sanergy HT may be a good candidate interconnect material for PCFC. The TEC of the alloy is regarded suitable for PCFC, however this depends on the other materials used in the fuel cell assembly. At lower temperatures (700 - 800 °C) the alloy proves good oxidation resistance and the oxide scales formed holds rather good electrical conductivity (∼6 mΩcm2 after 500 h at 700 °C). The predicted lifetime of the interconnect far exceeds the expected lifetime of the fuel cell (>50 000 h). At higher temperatures (>800 °C) the effect of dual atmosphere exposures is significant, and is likely to accelerate the degradation of the performance of the interconnect if used in SOFC. However, the temperature regime of PCFC (600 – 700 °C) is regarded to result in slow oxide growth kinetics, and dual atmosphere environments may therefore not significantly affect the performance of the material. Any effects of dual atmosphere conditions is likely to be reduced by application of metallic Ce/Co coatings, also improving the overall performance of the fuel cell stack.

The application of metallic Ce and Co coatings is regarded as a beneficial and likely improvement of performance of the Sanergy HT, both with respect to limited chromium evaporation and reduced the effects of dual atmosphere.