Joint research project

Clean and economic energy through development of advanced metallic interconnects for solide oxide fuel cells

Project leaders
Massimo Viviani, Taha Mattar
Agreement
EGITTO - ASRT - Academy of Scientific Research and Technology
Call
CNR-ASRT 2016-2017
Department
Chemical sciences and materials technology
Thematic area
Chemical sciences and materials technology
Status of the project
New

Research proposal

Fuel cells are an energy user's dream: an efficient, combustion-less, virtually pollution-free power source, capable of being sited in downtown urban areas or in remote regions that runs almost silently and has few moving parts.
A solid oxide fuel cell (SOFC) is an electrochemical conversion device that produces electricity directly from oxidizing a fuel. Fuel cells are characterized by their electrolyte material; the SOFC has a solid oxide or ceramic electrolyte. Advantages of this class of fuel cells include high efficiency, long-term stability, fuel flexibility, low emissions, and relatively low cost. The largest disadvantage is the high operating temperature which results in longer start-up times and mechanical and chemical compatibility issues. A solid oxide fuel cell is made up of five layers, three of which are ceramics (hence the name) an electrolyte, an anode, a cathode, and two interconnect wires.
Interconnect is a critical component of the planar SOFCs stack that separates the oxidant and fuel gases. Interconnects (ICs) are responsible for the physical separation of anode and cathode and for electrical contact between individual cells in a stack, as cell stacking is essential for increasing power density. The IC materials must comply with strict requirements:
- Complete density to block gas crossover between anodic and cathodic layers;
- Chemical stability at high temperatures and reduced chemical reactions with the materials comprising the cathode (oxidizing atmosphere) and the anode (reducing atmosphere);
- Low area specific resistance (ASR), which should not increase during operation;
- Coefficient of thermal expansion (CTE) matching the other cell components;
- Adequate mechanical strength and creep resistance;
- Good thermal conductivity.
- Easy fabrication, low material cost.
IC can be either a ceramic or metallic material between each individual cell to connect cells in series, so that the electricity generated by each cell can be combined. Because the interconnect is simultaneously exposed to both the oxidizing side and reducing side of the cell at high temperatures, it must be extremely stable. For this reason, ceramics have been more successful in the long term use than metals as interconnect materials. Nevertheless, these ceramic interconnecting materials are very expensive and difficult to process as compared with metals. Metallic materials are becoming more promising as lower temperature (600oc-800 oC) SOFCs have been developed. The most common metallic materials used today are Cr2O3-forming alloys such as ferritic stainless steel (FSS) that has demonstrated to be thermally stable at high temperatures and have the ability to match the coefficient of thermal expansion (CTE) of adjacent cell/stack components.
Recent trends in new electrolytes for Solid Oxide Fuel Cells (SOFCs) have shown that it is possible to reduce the operating temperatures into the 600-800oC range . Thus, some ceramic based materials such as conventional doped lanthanum chromite ceramics (LaCrO3) can be replaced by cheaper metallic components, For example, Crofer22 APU, E-Brite, and SUS430 ferritic stainless steels.
The most suitable metallic alloys for many purposes are stainless steels Cr or Ni based alloys or Ferritic stainless steels (FSSs), so far with best compromise regarding the required properties. Regarding the CTE, FSSs vary in the range of 9 to 11 *10-6 K-1, which is a near perfect match with the ceramic components of a SOFC, namely, lanthanum and strontium doped manganite perovskites (LSM) cathodes and the yttria stabilized zirconia (YSZ) electrolytes. Ni based alloys and other types of stainless steels present unacceptably high CTE values, even though a slightly higher CTE of the IC is desirable to maintain ceramic components under compressive stress.
Innovated Chromium steel alloys for metallic interconnects in SOFCs:
Designing Chromium steel alloys that satisfy the needs and requirements for SOFCs working conditions takes in consideration both the chemical stability of the alloy and alloy/coat during operation at working temperature to avoide chromium poisoning and the dimension stability and keeping the CTE value in the accepted range. Alloying elements like niobium, aluminum, silicon, molybdenum, titanium and/ or vanadium were added in small amounts to control the coefficient of thermal expansion in the accepted range as well as to stabilize chromium and inhibit the chromium poisoning phenomenon.
Advanced coatings of metallic interconnects in SOFCs
The coatings on material (FSS) particularly at the cathode side of SOFC. Accordingly, the candidate materials for the coatings should be an effective barrier to the oxygen inward diffusion and/or the chromium outward diffusion in order to reduce the growth rate and evaporation of chromia formed on FSS, and be also required to have a high electrical conductivity as well as good thermal expansion match with the substrate steel and be thermal chemically stable and compatible to adjacent stack components in SOFC stack. Thus, an effective diffusion barrier coating is required. With this in mind, a variety of coatings such as conductive perovskites and spinels have been explored as barrier to hinder the evaporation of chromia. This type of coating is intended to increase anti-oxidative ability of the metallic substrate, thereby extending SOFCs lifetime.
The typical techniques used for coatings preparation are thermal spraying, slurry spraying, screen printing and sol-gel processing.
Recently, electrodeposition of metal or alloy coatings followed by subsequent thermal exposure to form coatings directly on the substrates has been considered as an economical and simple technique due to its advantages such as better coating-to-substrate adhesion, denser, less porous spinel coatings and good coverage of the electrodeposited coating on almost any substrate surfaces especially for complex-shaped interconnects.

Research goals

The major drawback for the application of FSSs or Cr based alloys as ICs is the volatilization and release of Cr compounds from chromia scales. In the presence of water vapor, Cr oxy-hydroxides species such as CrO2(OH)2 can be formed (although the exact composition depends on temperature and vapor pressure) that have the capability of migrating through several layers of the cell. This mechanism is known as Cr poisoning and it is felt mostly on the cathode/electrolyte interface (vapor pressures are higher in the air electrode), where these species are eventually reduced and create deposits that deteriorate cell performance by blocking ion flow and reducing electroactive area of the electrode.
There are two strategies put forward in literature as possible solutions addressing the problem of Cr poisoning; they are alloy development strategy and interconnects coating strategy.

The proposed project aims at:
- Designing of innovated steel metallic interconnects for solid oxide fuel cells
- Development of production technology of the steel metallic interconnects for SOFCs
- Improvement the economy of electricity production via using of metallic interconnects in SOFCs
- Clean and eco friendly electricity production
- Rising awareness and knowledge of clean energy and environment impact of traditional production techniques

Last update: 28/03/2024