Observation of O/O isotope effects at the cathode of a polymer electrolyte membrane fuel cell.

O/O isotope effects were observed at the cathode of a polymer electrolyte membrane fuel cell at 25 and 35°C. Results of experiments in which the O/O isotope ratios of the oxygen gases supplied to and exhausted from the cell were measured revealed that the lighter isotope O reacted more preferentially to form water molecules at the cathode than the heavier one, O. The value of the oxygen isotope separation factor, S, defined as the ratio of the O/O isotope ratios of the oxygen gases supplied to and exhausted from the cell, ranged from 1.0030 to 1.0139, and tended to decrease with decreasing rate of oxygen utilisation (θ) and with increasing flow rate of the feed oxygen gas (D). The value of another separation factor, S, defined as the ratio of the O/O isotope ratios of the exhausted oxygen gas and oxygen having reacted to form water molecules at the cathode, ranged from 1.0049 to 1.0304. The S value was much less affected by the change in θ and D than the S value with the majority of the S value being in the range of 1.0240-1.0304.