Theoretical studies of transition metal surfaces as electrocatalysts for oxygen electroreduction

In the last few years the quest towards a hydrogen based economy has intensified\ninterest for effective and less expensive catalysts for fuel cell applications. Due to its\nslow kinetics, alternative catalysts for the oxygen electroreduction reaction are actively\nresearched. Platinum alloys with different transition metals (for example: Ni, Co and Fe)\nhave shown improved activity over pure Pt. The design of a Pt-free catalysts is also\nhighly desirable, and different alternatives including metalloporphyrins and Pd-based\ncatalysts are being researched. Pd-based catalysts constitute an attractive alternative to Pt\nalloys in some fuel cell applications, not only because of lower costs but also because of\nthe lower reactivity of Pt alloys towards methanol, which is important for improved\nmethanol crossover tolerance on direct methanol fuel cells.\nIn this work we apply density functional theory (DFT) to the study of four catalysts\nfor oxygen electroreduction. The electronic structure of these surfaces is characterized\ntogether with their surface reconstruction properties and their interactions with oxygen\nelectroreduction intermediates both in presence and absence of water. The energetics\nobtained for the intermediates is combined with entropy data from thermodynamic tables\nto generate free energy profiles for two representative reaction mechanisms where the\ncell potential is included as a variable. The study of the barriers in these profiles points\nto the elementary steps in the reaction mechanisms that are likely to be rate-determining.\nThe highest barrier in the series pathway is located at the first proton and charge transfer\non all four catalytic surfaces. This is in good agreement with observed rate laws for this\nreaction. The instability of hydrogen peroxide on all surfaces, especially compared with\nthe relatively higher stability of other intermediates, strongly points at this intermediate as the most likely point where the oxygen bond is broken during oxygen reduction. This\nadds to the argument that this path might be active during oxygen electroreduction.\nA better understanding behind the reaction mechanism and reactivities on these\nrepresentative surfaces will help to find systematic ways of improvement of currently\nused catalysts in the oxygen electroreduction reaction.