A fuel cell assembly comprising: (a) a first fuel cell comprising: (i) a first anode; (ii) a first cathode; (iii) a first proton exchange membrane electrolyte interposed between said first anode and said first cathode; (iv) a first anode flow field plate adjacent said first anode, said first anode flow field plate comprising a first anode flow channel for directing a fuel to said first anode; and (v) a first cathode flow field plate adjacent said first cathode, said first cathode flow field plate comprising a first cathode flow channel for directing an oxidant to said first cathode, wherein the cross-sectional area of said first cathode flow channel decreases in the oxidant flow direction along at least a portion of the length of said first cathode flow channel; (b) a second fuel cell comprising: (i) a second anode; (ii) a second cathode; (iii) a second proton exchange membrane electrolyte interposed between said second anode and said second cathode; (iv) a second anode flow field plate adjacent said second anode, said second anode flow field plate comprising a second anode flow channel for directing said fuel to said second anode; and (v) a second cathode flow field plate adjacent said second cathode, said second cathode flow field plate comprising a second cathode flow channel for directing said oxidant to said second cathode; and (c) a coolant channel interposed between said first cathode flow field plate and said second anode flow field plate, for directing a coolant in a heat transfer relationship with at least one of said first cathode flow field plate and said second anode flow field plate, said coolant channel having an inlet and an outlet, characterized in that said coolant channel is a thermally compensated coolant channel having a cross-sectional area that decreases monotonically in the coolant flow direction from said inlet to said outlet of said thermally compensated coolant channel, such that the width of said coolant channel nearer said inlet is greater than the width of said coolant channel nearer said outlet, or such that the width of said coolant channel at said inlet is greater than the width of said coolant channel at said outlet. (a) a first fuel cell comprising: (i) a first anode; (ii) a first cathode; (iii) a first proton exchange membrane electrolyte interposed between said first anode and said first cathode; (iv) a first anode flow field plate adjacent said first anode, said first anode flow field plate comprising a first anode flow channel for directing a fuel to said first anode; and (v) a first cathode flow field plate adjacent said first cathode, said first cathode flow field plate comprising a first cathode flow channel for directing an oxidant to said first cathode, wherein the cross-sectional area of said first cathode flow channel decreases in the oxidant flow direction along at least a portion of the length of said first cathode flow channel; (i) a first anode; (ii) a first cathode; (iii) a first proton exchange membrane electrolyte interposed between said first anode and said first cathode; (iv) a first anode flow field plate adjacent said first anode, said first anode flow field plate comprising a first anode flow channel for directing a fuel to said first anode; and (v) a first cathode flow field plate adjacent said first cathode, said first cathode flow field plate comprising a first cathode flow channel for directing an oxidant to said first cathode, wherein the cross-sectional area of said first cathode flow channel decreases in the oxidant flow direction along at least a portion of the length of said first cathode flow channel; (b) a second fuel cell comprising: (i) a second anode; (ii) a second cathode; (iii) a second proton exchange membrane electrolyte interposed between said second anode and said second cathode; (iv) a second anode flow field plate adjacent said second anode, said second anode flow field plate comprising a second anode flow channel for directing said fuel to said second anode; and (v) a second cathode flow field plate adjacent said second cathode, said second cathode flow field plate comprising a second cathode flow channel for directing said oxidant to said second cathode; and (i) a second anode; (ii) a second cathode; (iii) a second proton exchange membrane electrolyte interposed between said second anode and said second cathode; (iv) a second anode flow field plate adjacent said second anode, said second anode flow field plate comprising a second anode flow channel for directing said fuel to said second anode; and (v) a second cathode flow field plate adjacent said second cathode, said second cathode flow field plate comprising a second cathode flow channel for directing said oxidant to said second cathode; and (c) a coolant channel interposed between said first cathode flow field plate and said second anode flow field plate, for directing a coolant in a heat transfer relationship with at least one of said first cathode flow field plate and said second anode flow field plate, said coolant channel having an inlet and an outlet,