  The extent of irreversible deactivation of Pt towards hydrogen oxidation
reaction (HOR) due to sulfur adsorption and subsequent electrochemical
oxidation is quantified in a functional PEM fuel cell. At 70 {\deg}C,
sequential hydrogen sulfide (H2S) exposure and electrochemical oxidation
experiments indicate that as much as 6% of total Pt sites are deactivated per
monolayer sulfur adsorption at open circuit potential of a PEM fuel cell
followed by its removal. The extent of such deactivation is much higher when
the electrode is exposed to H2S when the fuel cell is operating at a finite
load, and is dependent on the local overpotential and the duration of exposure.
Regardless of this deactivation, the H2/O2 polarization curves obtained on
post-recovery electrodes do not show performance losses suggesting that such
performance curves alone cannot be used to assess the extent of recovery due to
sulfur poisoning. A concise mechanism for the adsorption and electro-oxidation
of H2S on Pt anode is presented. H2S dissociatively adsorbs onto Pt as two
different sulfur species and at intermediate oxidation potentials, undergoes
electro-oxidation to sulfur and then to sulfur dioxide (SO2). This mechanism is
validated by charge balances between hydrogen desorption and sulfur
electro-oxidation on Pt. The ignition potential for sulfur oxidation decreases
with increase in temperature, which coupled with faster electro-oxidation
kinetics result in the easier removal of adsorbed sulfur at higher
temperatures. Furthermore, the adsorption potential is found to influence
sulfur coverage of an electrode exposed to H2S. As an implication, the local
potential of a PEM fuel cell anode exposed to H2S contaminated fuel should be
kept below the equilibrium potential for sulfur oxidation to prevent
irreversible loss of Pt sites.
