Theoretical studies of transition metal surfaces as electrocatalysts for oxygen electroreduction

摘要

In the last few years the quest towards a hydrogen based economy has intensifiedinterest for effective and less expensive catalysts for fuel cell applications. Due to itsslow kinetics, alternative catalysts for the oxygen electroreduction reaction are activelyresearched. Platinum alloys with different transition metals (for example: Ni, Co and Fe)have shown improved activity over pure Pt. The design of a Pt-free catalysts is alsohighly desirable, and different alternatives including metalloporphyrins and Pd-basedcatalysts are being researched. Pd-based catalysts constitute an attractive alternative to Ptalloys in some fuel cell applications, not only because of lower costs but also because ofthe lower reactivity of Pt alloys towards methanol, which is important for improvedmethanol crossover tolerance on direct methanol fuel cells.In this work we apply density functional theory (DFT) to the study of four catalystsfor oxygen electroreduction. The electronic structure of these surfaces is characterizedtogether with their surface reconstruction properties and their interactions with oxygenelectroreduction intermediates both in presence and absence of water. The energeticsobtained for the intermediates is combined with entropy data from thermodynamic tablesto generate free energy profiles for two representative reaction mechanisms where thecell potential is included as a variable. The study of the barriers in these profiles pointsto the elementary steps in the reaction mechanisms that are likely to be rate-determining.The highest barrier in the series pathway is located at the first proton and charge transferon all four catalytic surfaces. This is in good agreement with observed rate laws for thisreaction. The instability of hydrogen peroxide on all surfaces, especially compared withthe 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. Thisadds to the argument that this path might be active during oxygen electroreduction.A better understanding behind the reaction mechanism and reactivities on theserepresentative surfaces will help to find systematic ways of improvement of currentlyused catalysts in the oxygen electroreduction reaction.