Analysis of sulfur poisoning on a PEM fuel cell electrode

• Published in: Electrochimica acta Vol. 55; no. 20; pp. 5683 - 5694
• Main Authors: Sethuraman, Vijay A., Weidner, John W.
• Format: Journal Article
• Language: English
• Published: Kidlington Elsevier Ltd 01-08-2010; Elsevier
• Subjects: Adsorption; Anode poisoning; Applied sciences; Cyclic voltammetry; Deactivation; Durability; Electrodes; Energy; Energy. Thermal use of fuels; Equipments for energy generation and conversion: thermal, electrical, mechanical energy, etc; Exact sciences and technology; Exposure; Fuel cells; Hydrogen oxidation reaction; Hydrogen sulfide; Oxidation; Platinum; Polymer electrolyte membrane fuel cell; Sulfur; Sulfur poisoning; Cyclic voltammetry; Hydrogen sulfide; Polymer electrolyte membrane fuel cell; Anode poisoning; Durability; Hydrogen oxidation reaction; Sulfur poisoning; Hydrogen Sulfides; Anode; Hydrogen; Electrode poisoning; Transition metal; Adsorption; Platinum; Electrode life; Oxidation; Electrochemical reaction; Sulfur; Proton exchange membrane fuel cells
• ISSN: 0013-4686; 1873-3859
• DOI: 10.1016/j.electacta.2010.05.004

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 polymer electrolyte membrane (PEM) fuel cell. At 70
°C, sequential hydrogen sulfide (H
2S) 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 H
2S while the fuel cell is operating at a finite load, and is dependent on the local overpotential as well as the duration of exposure. Regardless of this deactivation, the H
2/O
2 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 H
2S on Pt anode is presented. H
2S dissociatively adsorbs onto Pt as two different sulfur species and at intermediate oxidation potentials, undergoes electro-oxidation to sulfur and then to sulfur dioxide. 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 H
2S. As an implication, the local potential of a PEM fuel cell anode exposed to H
2S contaminated fuel should be kept below the equilibrium potential for sulfur oxidation to prevent irreversible loss of Pt sites.