Disclosed is a fuel cell system wherein the flow of fuel and oxygen is optimized thereby improving the thermal efficiency of the entire system. The fuel cell system comprises at least one stack for generating electrical energy by an electrochemical reaction between hydrogen gas and oxygen, a fuel supply portion for supplying fuel to the stack, and an oxygen supply portion for supplying oxygen to the stack. The stack is formed in a stacked configuration with MEAs and separators. The separators are positioned on either surface of the MEAs. The separators have a plurality of ribs proximate to the MEAs which define a plurality of channels wherein the ratio of a width of the channels to the width of the ribs is from about 0.8 to 1.5.