This paper is focused on the control of air-feed system of Polymer Electrolyte Membrane Fuel Cell (PEM-FC). This system regulates the air entering in the cathode side of the fuel cell. The control objective is to maintain optimum net power output by regulating the oxygen excess ratio in its operating range, through the air compressor. This requires controllers with a fast response time in order to avoid oxygen starvation during load changes. The problem is addressed using a robust nonlinear second order sliding mode controller in cascaded structure. The controller is based on sub-optimal algorithm, which is known for its robustness under disturbances and uncertainties. The controller performance is validated through Hardware-In-Loop (HIL) simulation based on a commercial twin screw air compressor and a real time fuel cell emulation system. The simulation results show that the controller is robust and has a good transient performance under load variations and parametric uncertainties. (C) 2012 Elsevier Ltd. All rights reserved.