Various examples are provided for operational control of fuel cells. In one example, among others, a system for controlling a fuel cell includes a stack temperature controller in cascade with a liquid level controller. The liquid level controller can provide a control output based at least in part upon an indication of a liquid level of a liquid fuel tank and a level reference. The stack temperature controller can provide a fan speed control output based at least in part upon an indication of a stack temperature of the fuel cell and the control output of the liquid level controller. In another example, a system for estimating methanol concentration of a fuel cell system includes a state observer that generates an estimate of the methanol concentration of fuel provided to a direct methanol fuel cell based upon a plurality of states of the fuel cell system.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
 This invention was made with government support under agreement no. 000 85 243 awarded by the Department of Energy. The Government has certain rights in the invention.