A system for modulating a temperature of one or more fuel cells in a fuel cell stack includes a catalytic combustor in heat exchange relationship with the fuel cell stack. The catalytic combustor promotes an exothermic reaction. A hydrogen source selectively supplies hydrogen (H.sub.2) to the catalytic combustor. The H.sub.2 reacts with oxygen (O.sub.2) in the exothermic reaction. In one feature, the catalytic combustor lies adjacent to the fuel cell stack and includes a series of catalyst coated flow channels. In another feature, the catalytic combustor includes a plate having a catalyst layer and that is offset from the fuel cell stack. Heat to radiates from the catalytic combustor to the fuel cell stack. In still another feature, a jacket encloses the fuel cell stack to form a gap between the jacket and the fuel cell stack through which hot exhaust from the catalytic combustor circulates.