Anode exhaust gas dilution method and apparatus in PEM fuel cell powered system

A method of operating a fuel cell system comprising:

• (a) selectively supplying hydrogen-containing anode effluent from an anode side of a fuel cell stack to an accumulator;
• (b) storing said anode effluent in said accumulator;
• (c) routing said stored anode effluent from said accumulator to a catalytic combustion device;
• (d) maintaining a pressure difference between said accumulator and said anode side of said fuel cell stack below a predetermined value;
• (e) selectively mixing said hydrogen-containing anode effluent from said accumulator with an oxidant-containing cathode effluent from a cathode side of said fuel cell stack to form a fluid stream;
• (f) reducing a hydrogen content in said fluid stream by catalytically combusting a portion of said hydrogen in said fluid stream in said catalytic combustion device;
• (g) selectively supplying said fluid stream from said catalytic combustion device to said cathode side of said fuel cell stack;
• (h) supplying an oxidant stream to said cathode side of said fuel cell stack;
• and
• (i) further reducing said hydrogen content in said fluid stream in said cathode side of said fuel cell stack by catalytically reacting said fluid stream in said cathode side of said fuel cell stack,
• wherein steps (e) and (f) are conducted essentially simultaneously by mixing said anode effluent with said cathode effluent in a mixing chamber containing a catalyst within said catalytic combustion device and maintaining said anode effluent and said cathode effluent separate from one another until in the presence of said catalyst.