The present invention relates to hydrogen/oxygen fuel cells having a solid-oxide electrolytic layer separating an anode layer from a cathode layer; more particularly, to fuel cell stack assemblies and systems comprising a nickel-based anode; and most particularly, to such fuel cell assemblies and systems wherein the anode is protected from oxidation, especially during cool-down after the assembly has been shut down.
Fuel cells which generate electric current by the electrochemical combination of hydrogen and oxygen are well known. In one form of such a fuel cell, an anodic layer and a cathodic layer are separated by an electrolyte formed of a ceramic solid oxide. Such a fuel cell is known in the art as a xe2x80x9csolid oxide fuel cellxe2x80x9d (SOFC). Hydrogen, either pure or reformed from hydrocarbons, is flowed along the outer surface of the anode and diffuses into the anode. Oxygen, typically from air, is flowed along the outer surface of the cathode and diffuses into the cathode. Each O2 molecule is split and reduced to two Oxe2x88x922 anions catalytically by the cathode. The oxygen anions transport through the electrolyte and combine at the anode/electrolyte interface with four hydrogen ions to form two molecules of water. The anode and the cathode are connected externally through a load to complete the circuit whereby four electrons are transferred from the anode to the cathode. When hydrogen is derived from xe2x80x9creformedxe2x80x9d hydrocarbons, the xe2x80x9creformatexe2x80x9d gas includes CO which is converted to CO2 at the anode via an oxidation process similar to that performed on the hydrogen. Reformed gasoline is a commonly used fuel in automotive fuel cell applications.
A single cell is capable of generating a relatively small voltage and wattage, typically between about 0.5 volt and about 1.0 volt, depending upon load, and less than about 2 watts per cm2 of cell surface. Therefore, in practice it is usual to stack together, in electrical series, a plurality of cells. Because each anode and cathode must have a free space for passage of gas over its surface, the cells are separated by perimeter spacers which are vented to permit flow of gas to the anodes and cathodes as desired but which form seals on their axial surfaces to prevent gas leakage from the sides of the stack. The perimeter spacers include dielectric layers to insulate the interconnects from each other. Adjacent cells are connected electrically by xe2x80x9cinterconnectxe2x80x9d elements in the stack, the outer surfaces of the anodes and cathodes being electrically connected to their respective interconnects by electrical contacts disposed within the gas-flow space, typically by a metallic foam which is readily gas-permeable or by conductive filaments. The outermost, or end, interconnects of the stack define electric terminals, or xe2x80x9ccurrent collectors,xe2x80x9d which may be connected across a load.
A complete SOFC system typically includes auxiliary subsystems for, among other requirements, generating fuel by reforming hydrocarbons; tempering the reformate fuel and air entering the stack; providing air to the hydrocarbon reformer; providing air to the cathodes for reaction with hydrogen in the fuel cell stack; providing air for cooling the fuel cell stack; providing combustion air to an afterburner for unspent fuel exiting the stack; and providing cooling air to the afterburner and the stack. A complete SOFC assembly also includes appropriate piping and valving, as well as a programmable electronic control unit (ECU) for managing the activities of the subsystems simultaneously.
The anodes of cells in a fuel cell assembly typically include metallic nickel and/or a nickel cermet (Nixe2x80x94YSZ) which are readily oxidized. During operation of an assembly, the anodes are in a reduced state. A problem exists in that the anodes are vulnerable to oxidation by atmospheric oxygen which can enter the stacks via the reformate passageways during cool-down of the assembly. The anodes are still hot enough that oxidation of the nickel can occur readily, and gas compositions (reformate) which can prevent such oxidation are no longer flowing through the stack. The grain growth of the nickel in the anode cermet during operation can lead to severe stresses caused by the volume changes associated with the oxidation/reduction cycles experienced by the nickel in the cermet. Repeated oxidation and reduction of nickel in the cermet anodes can lead to severe mechanical stresses because of volume differences between metallic nickel and nickel oxide, and can result in catastrophic cracking of the anodes.
It is a principal object of the present invention to protect the nickel anodes of a fuel cell from structural degradation by periodic oxidation and reduction of the nickel.
It is a further object of the present invention, through such prevention, to improve the reliability and extend the lifetime of solid oxide fuel cells.
Briefly described, in a fuel cell assembly, for example, a solid-oxide fuel cell assembly, metallic nickel in a Nixe2x80x94YSZ anode is readily oxidized when exposed to oxygen as may happen through atmospheric invasion of the assembly during cool-down following shutdown of the assembly. Anodes are in an oxidized equilibrium state when the assembly is fabricated and are then reduced by fuel such as reformate when the assembly is first turned on. Repeated anode oxidation and reduction can affect the structure of the anodes and can lead to cracking and failure of the anodes and thus the entire assembly. To prevent such oxygen migration and re-oxidation, a set of passive devices are employed which are low in cost, simple to implement, and do not require any electronic controls or power.
First, oxygen intrusion is minimized by installation of check valves in the fuel flow passages upstream and down-stream of the anodes. The check valves include balls formed of high-temperature materials and are held in place on a valve seat by gravity. The weight of the ball and the size of the seat determine the pressure drop across the valve.
Second, because oxygen is bound to eventually leak around the check valves and into the anode passages given sufficient time, oxygen getter devices, containing oxygen-gettering material such as metallic nickel, are provided in the fuel passageways leading to and from the anodes. Oxidation of the oxygen-gettering material is readily reversed through reduction by fuel when the assembly is restarted.