  We present a controller for a power electronic system composed of a fuel cell
(FC) connected to a boost converter which feeds a resistive load. The
controller aims to regulate the output voltage of the converter regardless of
the uncertainty of the load. Leveraging the monotonicity feature of the fuel
cell polarization curve we prove that the nonlinear system can be controlled by
means of a passivity-based proportional-integral approach. We afterward extend
the result to an adaptive version, allowing the controller to deal with
parameter uncertainties, such as inductor parasitic resistance, load, and FC
polarization curve parameters. This adaptive design is based on an indirect
control approach with online parameter identification performed by a ``hybrid''
estimator which combines two techniques: the gradient-descent and
immersion-and-invariance algorithms. The overall system is proved to be stable
with the output voltage regulated to its reference. Experimental results
validate our proposal under two real-life scenarios: pulsating load and output
voltage reference changes.
