Solid-state electrochemical devices are normally cells that include two porous electrodes, the anode and the cathode, and a dense solid electrolyte membrane disposed between the electrodes. In the case of a typical solid oxide fuel cell, for example, the anode is exposed to fuel and the cathode is exposed to an oxidant in separate closed systems to avoid any mixing of the fuel and oxidants.
The electrolyte membrane is normally composed of a ceramic oxygen ion conductor in solid oxide fuel cell applications. In other implementations, such as gas separation devices, the solid membrane may be composed of a mixed ionic electronic conducting material (“MIEC”). The porous anode may be a layer of a ceramic, a metal or a ceramic-metal composite (“cermet”) that is in contact with the electrolyte membrane on the fuel side of the cell. The porous cathode is typically a layer of a mixed ionically and electronically-conductive (MIEC) metal oxide or a mixture of an electronically conductive metal oxide (or MIEC metal oxide) and an ionically conductive metal oxide.
Solid oxide fuel cells normally operate at temperatures between about 650° C. and about 1000° C. to maximize the ionic conductivity of the electrolyte membrane. At appropriate temperatures, the oxygen ions easily migrate through the crystal lattice of the electrolyte.
Since each fuel cell generates a relatively small voltage, several fuel cells may be associated to increase the power output of the system. Such arrays or stacks generally have a tubular or planar design. Planar designs typically have a planar anode-electrolyte-cathode deposited on a conductive interconnect and stacked in series. However, planar designs are generally recognized as having significant safety and reliability concerns due to the complexity of sealing of the units and manifolding a planar stack. Tubular designs utilizing long porous support tubes with electrodes and electrolyte layers disposed on the support tube reduce the number of seals that are required in the system. Fuel or oxidants are directed through the channels in the tube or around the exterior of the tube.
The manufacture of concentric tubular structures with multiple layers that display varying properties to accomplish such tubular fuel cell designs is routine, especially in the field of high temperature electrochemical devices. Bonding between the layers is typically achieved through chemical or sinter bonding. This limits the types of materials that can be bonded to one another. For example, a ceramic layer and metal layer generally will not bond to each other easily by chemical or sintering means. Additionally, the desirable opportunity to inspect the outside of an internal concentric layer before applying an external concentric layer is not available in conventional manufacturing schemes where all of the layers are produced as a single green body and subsequently co-sintered.
Thus, improved techniques for joining concentric tubular structures suitable for use in devices operating at high temperatures are needed.