Development of Highly Robust, Volume-Manufacturable Metal-Supported SOFCs for Operation Below 600°

Abstract

Ceres Power has been developing fuel cell stacks and mCHP systems based on its unique metal-supported SOFC design incorporating a gadolinia-doped ceria (CGO) electrolyte operating in the 450-600°C temperature range. The metal supports are lowcost ferritic stainless steel foils which are perforated using a laser to create a porous region for gas diffusion on the anode side, and an impermeable region around the perimeter for sealing. Conventional volume ceramic processing techniques are used for the deposition of the anode, bulk electrolyte, cathode active and cathode current-collector layers, which are all sintered in a nonreducing atmosphere. A proprietary technique has been developed for the deposition of a dense stabilized zirconia layer and a CGO barrier layer between the bulk CGO electrolyte and the cathode active layer, largely blocking the internal short-circuiting currents inherent in the use of ceria-based electrolytes due to the mixedconductivity of CGO when exposed to an anodic atmosphere. The use of this layer enables close-to-theoretical cell efficiencies to be obtained, whilst still maintaining the advantages of high ionic conductivity and sinterability which make CGO ideal for lowtemperature processing and operation. The present generation of cells being manufactured are designed to meet the commercial requirements for a wall mounted, natural gas fuelled mCHP product, producing an average of 140 mWcm -2 at 0.75 V/cell at 570°C and 60% fuel utilization, operating on steam-reformed natural gas. Open-circuit voltages are around 1.10V operating on hydrogen/nitrogen; close to theoretical values indicating the effective suppression of internal short-circuits. The unique metal supported design facilitates simple and robust hermetic sealing of the cell to the interconnect by welding, and layer-to layer sealing and isolation is achieved using a commercial gasket material, thus largely eliminating degradation due to thermal stress induced seal leakage found in conventional planar SOFC designs. The extreme robustness of the metal foil cell design has been demonstrated by thermally cycling welded cell/interconnect assemblies from 100-600°C (<5 min total cycle time) using an infrared heater and forced air cooling. After 100 cycles, there was no degradation of the gas tightness of the assembly (pressure-decay leak tester). These tested cells were then assembled into a short stack and tested, with electronic performance indistinguishable from a short stack of new cells.

Recent developments in metal‐supported solid oxide fuel cells

“…Ceres Power, which was spun‐off from Imperial College, UK, became the first company to effectively make an MSC stack product . Other major organizations who have been involved in the development of metal‐supported stacks, since have included—Topsoe Fuel Cells (TFC), which is no longer operational as an independent entity, Danish Technological University (DTU), Riso National Laboratory (Denmark), Forschungszentrum Julich (Fz‐J) from Germany, Plansee (Austria), DLR (Germany), University of Toronto (as part of NRC's initiative on SOFCs), and several other groups in Europe and in other countries, e.g., universities in Korea and China have also been reviewed in this article.…”

“…Most developers continue to use Ferritic steels for IC applications . As the IC development is ‘across the board’ extending to all kinds of cells, developments in ICs and coatings, have not been discussed very much in this review.…”

“…The cell substrate is a ferritic stainless steel foil, perforated to create a gas permeable central region surrounded by an impermeable outer region (where there are no perforations). [6][7][8][9] A thick-film cermet anode is deposited over the perforated region of the substrate and the deposition of the electrolyte occurs all around-over the anode and overlaps onto the surrounding steel, forming a seal around the edge of the anode. Ceres' design ensures that the porous anode is completely sealed by the nonporous electrolyte.…”

“…Most developers continue to use Ferritic steels for IC applications. [5][6][7][8][9][10][11][12][13][15][16][17][18][19] As the IC development is 'across the board' extending to all kinds of cells, developments in ICs and coatings, have not been discussed very much in this review. Only coatings on porous metallic substrates are discussed further, as part of manufacturing operations.…”

Trends in the processing and manufacture of solid oxide fuel cells

“…This offers the prospect of reduced materials costs, especially if common engineering alloys such as ferritic stainless steel or Hasteloy can be utilized, along with improved robustness and ease of manufacture via welded cell components. Both tubular and planar geometries have been proposed with the latter being more common …”