Shut-down procedure for hydrogen-air fuel cell system

A procedure for shutting down an operating fuel cell system, wherein during operation of the fuel cell system a continuous flow of air is fed into a cathode flow field into contact with a cathode electrode disposed on one side of an electrolyte, and a continuous flow of hydrogen containing fuel is fed into an anode flow field into contact with an anode electrode disposed on the other side of the electrolyte, and an electric current is being generated by the fuel cell within an external circuit, wherein the current is operating a primary load connected to the external circuit, the shut-down procedure comprising:

• disconnecting the primary load from the external circuit;
• shutting off the air flow into the cathode flow field and then reducing the concentration of oxygen remaining within the cathode flow field and increasing the concentration of hydrogen within the fuel cells by a) controlling the amount of hydrogen entering the fuel cell system, b) adding air directly into the anode flow field, as necessary, and c) controlling the flow rate of the gases leaving the fuel cell system, such that the gases within the anode flow field and cathode flow field come to equilibrium with a gas composition of between 0.0001% and less than 4.0% hydrogen, balance fuel cell inert gases, including nitrogen, wherein all the nitrogen in the equilibrium gas composition is from air introduced into the fuel cell system, including from air, if any, mixed into the fuel prior to feeding the fuel into the anode flow field;
• and,
• once the gas composition has reached a gas composition with a selected hydrogen concentration within said range, balance fuel cell inert gases, maintaining an equilibrium gas composition having a hydrogen concentration within said range, balance fuel cell inert gases, during the entire time the primary load is disconnected by adding hydrogen, as needed.