Not trying to be a dick but this idea has been thought of many of times, its just too ineffective at this point in time. One day in future if one could potentially harness only the c02 we exhale and separate it from whatever else pollution etc. compress that into a tank with noozle they could have unlimited c02 but they would need many hours, at that point live human slaves could be used as c02 batteries technically! hah sci fie of course.