ICMCTF2010 Session TS2: Coatings for Fuel Cells and Batteries

In the planar design of a solid oxide fuel cell (SOFC) stack, the interconnect acts not only as electrical connection between the various cells but also as the mechanical support for the thin electroactive ceramic parts and as gas-proof separation of air and fuel gas. With the reduction of the SOFC operating temperatures to 600-800°C, chromia-forming ferritic steels are widely used as interconnect materials in the planar-type SOFC stacks currently under development. One of the most serious problems for these ferritic steels such as Crofer 22 APU is the Cr volatility and associated “poisoning” of the cathode under the operating environments of SOFC. Co plating is widely used for protecting these interconnect alloys, with the Co layer subsequently thermally converted into a Co-containing spinel coating at the SOFC operating temperatures; however, the effect of the substrate alloy composition on the microstructure, composition and the electrical conductivity of the converted spinel coating has not been systematically evaluated. In this presentation, six Fe-Cr alloy substrates with different compositions including three relatively simple model alloys and three commercial alloys were electrodeposited with a 5-mm Co layer, followed by thermal oxidization at 800°C in air for 100 h. The surface morphology and scale microstructure of the oxide layer formed on different substrates were studied with X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy. It was demonstrated that the addition of La and Mn into the alloys appeared to alter scale growth behavior of the plated Co layer, with La leading to a more adherent scale/metal interface and Mn leading to a thinner Cr₂O₃ layer. In addition, diffusion of Co into the substrates was observed except for the SS430 substrate. Outward diffusion of Mn was restricted to the Cr-rich scale layer, leading to formation of a Co₃O₄ spinel outer layer essentially free of Mn. For the three commercial alloy substrates (Crofer 22 APU, SS430, and ZMG 232), the thickness of the Cr-rich oxide scale was much thinner and their area-specific resistance (ASR) was lower than that of three home-made alloys. The scale ASR of the different alloys could be correlated with the thickness of the Cr-rich oxide scale formed on them.

Despite of their widely use in mobile communication and portable electronic equipment there is a strong need for further development and optimization of the components for lithium-ion batteries, especially for the positive electrode (cathode). Nanocrystalline thin film cathode materials are of increasing interest due to their potential to provide both high power and high energy density.

Li-Co-O thin film cathodes have been deposited onto Si and stainless steel substrates by RF magnetron sputtering from a ceramic LiCoO₂ target at various working gas pressures ranging from 0.15 to 25 Pa. The composition, crystal structure and thin film morphology were examined using inductive coupled plasma optical emission spectroscopy (ICP-OES), carrier gas hot extraction (CGHE), X-ray diffraction (XRD), Raman spectroscopy (RS), atomic force microscopy (AFM) and scanning electron microscopy (SEM). Thin film properties such as intrinsic stress, conductivity and film density were determined. As deposited films at 0.15 Pa as well as in the range between 5 Pa and 10 Pa working gas pressure showed a nanocrystalline metastable rocksalt structure with an unordered cation arrangement and were nearly stoichiometric.

With increasing annealing temperature a cation ordering process was observed by XRD. Heat treatment of the films deposited at 10 Pa Argon gas pressure at 600 °C leads to the formation of the hexagonal high temperature phase HT-LiCoO₂ with a layered structure. The Raman spectrum of the films deposited at 0.15 Pa and annealed at 400°C indicates the formation of the low temperature phase LT-LiCoO₂ with a cubic spinel-related structure, which is assumed to be stabilized due to high compressive stress in the film.

In this presentation some of our efforts to date on different anode coatings for application in a direct glucose fuel cell will be reviewed. Our initial research in this area focused on gold and gold surfaces coated by self-assembled monolayer (SAM) [1]. We have shown that the activity of gold for glucose oxidation is strongly influenced by temperature and an applied potential window. The rough gold undergoes structural changes at elevated temperature, which leads to loss of activity for glucose oxidation and lower affinity for the self-assembly. The transformation of "active" into "inactive" surface can be prevented by SAM formation. The SAM modified gold exhibits high activity for glucose oxidation.

The SAM modified gold surface was further used for enzyme (glucose-oxidase (GOx)) immobilization. For this purpose the pyrroloquinoline quinine (PQQ) mediator was covalently attached, followed by an attachment of a modified enzyme cofactor flavin adenine dinucleotide (FAD) and a reconstitution of an apo-GOx. Our results showed that the activity for glucose oxidation decreases with an increase of number of layers and the activity of enzyme modified gold electrode can not be assigned to enzyme itself.

Our recent approach concentrates on use of conductive organic salts (charge transfer complexes (CTC)) as an active surface for enzyme immobilization [2]. This enzymatic electrode exhibits high stability and activity for glucose oxidation and significant oxygen tolerance. The influence of different parameters, including CTC loading and morphology, GOx loading, membrane thickness, etc. on the activity of the bioanode has been investigated.

[1] Ivanov, I.; Vidaković, T.; Sundmacher, K. The influence of a self-assembled monolayer on the activity of rough gold for glucose oxidation, Electrochemistry Communications 2008, 10, 1307

[2] Ivanov, I.; Vidaković, T.; Sundmacher, K. Glucose Electrooxidation for Biofuel Cell Applications. Chemical and Biochemical Engineering Quarterly 2009, 23, 77.

Deposition of Pt particles on carboneous materials using wet impregnation, physicalevaporation, electrodeposition, microwave-assisted polyol strategy, supercritical fluid method, and microemulsion have been reported. These methods, however, are often complex and not environmentally friendly, and often require expensive equipments. In this paper, we report the dposition of Pt particles on carbon nanofibers (CNFs) using a convenient hydrothermal method. The sizes and the distribution of the resulting Pt particles can be effectively controlled using this method. The obtained Pt modified CNFs were characterized using transmission electron microscopy, X-ray diffractometry, and X-ray photoelectron spectroscopy. Electrodes were also fabricated using selected Pt modified CNFs for using in direct methanol fuel cells. The electrodes were examined for the methanol electro-oxidation using cyclic voltammetry.