(19a) A First Principles Study of the Effects of Sulfur Adsorption on the Activity of Pt, Ni and Pt3Ni

Low temperature fuel cells are attracting more attention as a means of producing electricity by harnessing the electrochemical energy produced by conversion of hydrogen and oxygen into water. However, the cost and efficiency of these systems need to be improved dramatically in order for fuel cells to be utilized on an everyday basis. A critical issue in proton exchange membrane fuel cells (PEMFCs) is the slow rate of oxygen reduction reaction (ORR) on the Pt cathode electrocatalyst. Recent results have shown that alloyed systems such as Pt₃Ni have a higher activity than Pt metals towards the ORR which improves the overall efficiency of the fuel cell.

Alloying Pt metals results in a dramatic change in the electronic properties of the catalyst and consequently alters their chemical reactivity. Therefore tailoring the electronic structure of metal catalysts could result in great technological relevance. We have analyzed the electronic and geometric properties of Pt, Ni and Pt₃Ni (111) surfaces using first principles calculations. A comparison of the d-band center of these metal systems correlates well to the adsorption energy of oxygen and hydrogen gaseous species relevant in fuel cell applications. Here we have also analyzed bonding and diffusion mechanisms of these species on the (111) surfaces. These results agree well with current experimental observation and previous first principles studies.

In addition, we have considered the effect of sulfur contaminants on the chemical reactivity of these systems. Preliminary calculations have shown that S atoms can reduce the adsorption energy between oxygen and hydrogen adsorbates and the (111) metal surfaces by 10% and in certain cases block adsorbates, which could affect the fuel cell efficiency. Our density functional theory calculation results indicate that a single S atom can affect 13 neighboring Pt surface adsorption sites which is in good agreement with experimental data. This effect is due to electrostatic interactions as well as a change in the electronic properties of these surfaces upon S adsorption. The 5d band-center of Ni(111) surface atoms changes significantly when sulfur is adsorbed on the surface. Similar changes are calculated for Pt(111) and Pt₃Ni(111) metal surfaces. We will discuss (i) electronic and geometric properties of the metal surfaces, (ii) the adsorption geometry and bonding mechanism of oxygen and hydrogen gaseous species and (iii) the affect of S contaminants on further oxygen and hydrogen adsorption.