ICMCTF2011 Session TS2: Coatings and Materials for Fuel Cells and Batteries

High Temperature Vapor Electrolysis (HTVE) has been studied since several years as a promising solution for future hydrogen massive production plant. HTVE device is similar to SOFC system and consists in several stacks. Each stack is composed of a pile of ceramics cells working at a temperature range about 700°C to 900°C . Cells are connected together via metallic interconnects that separate anodic and cathodic parts, and permit the current supply of the cells for the electrolysis reaction. To guarantee the HTVE lifetime with a good efficiency, interconnects have to resist to high temperature environment. Chromia former alloys are good candidates for interconnects because it forms at high temperature a compact and dense Cr₂O₃ scale that protects the alloy from the oxidant atmosphere as a barrier. However, the volatility of chromium oxides and hydroxydes may cause degradation of cells efficiency, particularly in presence of water vapor.

Increase in the performance of the alloys can be achieved using MOCVD (Metal-Organic Chemical Vapor Deposition) coatings. In a previous study performed for SOFC application, La₂O₃ coating applied by MOCVD improved kinetics oxidation rate in SOFC type atmospheres^1,2 and also modified the electronical properties of the oxide layer by increasing its conductivity^2,3,4.

Hence, in the present work, the same La₂O₃ coating has been applied to several alloys, such as Haynes^®230, Haynes^®242 and K41X (AISI441). The alloys were oxidized at 800°C in the both atmospheres representative of the HTVE operating conditions: ie Ar-1%H₂-9%H₂O (on cathodic side) and air (on anodic side). Oxidation kinetics were measured with a thermobalance and the Area Specific Resistance (ASR) was evaluated via contact resistance measurements. XRD, SEM coupled with EDS analyses were used to characterize the thermally grown oxide scale. Results are discussed through the comparison of the behaviors in HTVE type atmospheres of uncoated and La₂O₃ coated samples.

Beyond the possible coating beneficial influence, several additional experiments was carried out to understand the mechanism involved in presence of the coating or/and the oxide scale. ¹⁸O tracer experiments and PhotoElectroChemistry characterizations were used in order to identify the diffusion mechanism. The possible role played by the protons on the conductivity and the oxidation kinetics was investigated using atmospheres enriched in deuterium.

[1]S. Fontana and al, J. Power Sources 193 Issue 1 (2009) 136-145.

[2]S. Fontana and al, J. Power Sources 171 (2007) 652-662.

[3]W.Z. Zhu, S.C. Deevi, Mater. Res. Bull. 38 (2003) 957-972.

[4]Chandra-Amborn, PhD report, Grenoble Institute of Technology, (2006)

The development of IT-SOFCs will also require electrolyte materials with ionic conductivity higher than the conventional yttria-stabilised zirconia (YSZ) at moderate temperatures. Recently, lanthanum silicates materials (La_9.33Si₆O₂₆) with an apatite-like structure have attracted considerable interest as potential low cost electrolyte materials. Some of these materials show conductivities comparable to, or better than, YSZ at 875 K, and are thus potential electrolytes for economic feasible fuel cells. Their high level of oxide ion mobility is related to the presence of oxygen channels along the c axis which facilitate the diffusion of anionic species (O^2- for SOFC applications).

Magnetron sputtering has already been used to synthesize thin film electrolytes for SOFCs owing to its versatility as well as the ability to control composition and morphology. Most of the reported work focuses on the deposition of thin dense yttria-stabilised zirconia (YSZ) gadolinium doped ceria (GDC) and lanthanum gallate electrolyte layers. The main objective of this work is the production of apatite-like lanthanum silicates thin films by magnetron sputtering. La-Si-O films with the appropriate La/Si atomic ratios were deposited by reactive magnetron sputtering from La-Si and Si targets and subsequently annealed in controlled atmosphere to obtain the targeted lanthanum silicate oxide.

The chemical composition of the coatings was determined by electron probe microanalysis (EPMA). The structure of the coatings was studied by X-ray diffraction (XRD) using a Phillips diffractometer operated in Bragg-Brentano configuration with Co(Kα) radiation. The cross section and surface topography of the La-Si films were examined on a JEOL scanning electron microscope (SEM) equipped with an EDAX energy dispersive spectrometer (EDS). The electrical properties of the films were measured by AC impedance spectroscopy (HP4284A precision LCR meter, 20 Hz – 1 MHz).

A novel process to synthesize the (Mn,Co)₃O₄ spinel coating for protecting ferritic interconnect alloys has been developed, which is based on thermal conversion of an electrolytically co-deposited composite layer. Simultaneous electrolytic deposition of Co and Mn₃O₄ onto a Crofer 22 APU substrate resulted in a composite coating layer consisting of a Co matrix embedded with the Mn₃O₄ particles. Thermal conversion of the as-deposited layer in air at 850ºC led to the formation of a dense and adherent (Mn,Co)₃O₄ spinel coating on the substrate. The spinel coating was effective in blocking Cr migration and improving the electrical performance of the interconnect alloy.