Experimental and Theoretical Study of Cobalt Selenide as a Catalyst for O 2 Electroreduction Ellen Vayner, Reyimjan A. Sidik, and Alfred B. Anderson* Department of Chemistry, Case Western ReserVe UniVersity, CleVeland, Ohio 44106 Branko N. Popov Department of Chemical Engineering, UniVersity of South Carolina, Columbia, South Carolina 29208 ReceiVed: March 14, 2007; In Final Form: May 7, 2007 Cobalt sulfides have been known for more than 30 years to be active toward oxygen reduction, and cobalt selenides have shown less activity. In this paper, a theoretical analysis is made of the four-electron reduction reaction of oxygen to water over the mixed anion and cation (202) surface of the pentlandite structure Co 9 - Se 8 , one of several selenide phases. Reversible potentials for forming adsorbed reaction intermediates in acid are predicted using adsorption energies calculated with the Vienna ab initio simulation program (VASP) and the known bulk solution values together in a linear Gibbs energy relationship. Comparison with an earlier theoretical analysis of pentlandite structure Co 9 S 8 shows that the overpotential is predicted to be larger for the selenide by around 0.22 V. Cobalt selenide electrodes of unspecified stoichiometry were prepared chemically on glassy carbon discs, and polarization curves were measured using rotating discs. When heat-treated at 900 °C, the onset potential for O 2 reduction was found to be 0.5 V (normal hydrogen electrode, NHE), whereas electrodes not subject to heat-treatment were inactive. For Co 3 S 4 , onset potentials in the literature are ∼0.8 V (NHE), consistent with a ∼0.3 V higher measured overpotential for the selenide. The theoretical predictions for the pentlandite sulfide and selenide surfaces are in qualitative agreement. Introduction Cobalt sulfides were noted over 30 years ago to be active toward the four-electron electroreduction of O 2 to H 2 O in acid electrolytes. 1,2 Cobalt selenides were observed to be less active, and the cobalt tellurides were less active still. A recent theoretical study 3 explored the activities of three crystallographic surface planes of pentlandite structure Co 9 S 8 toward this reaction and produced a mechanism consistent with the activity. The Co terminated (008) surface was predicted to be inactive due to passivation by OH(ads), as were the isolated Co sites on the 1:4 Co/S (002) surface. However, the partially OH(ads) passi- vated 5:4 Co/S (202) surface presented properties allowing each of the four one-electron reduction steps to proceed at acceptable reversible potentials, accounting for good observed activity. In the predicted mechanism for this surface, O 2 dissociated exothermally over a 4-fold Co site, placing one O(ads) near its center and the other on an adjacent S. These O(ads) were subsequently reduced to OH(ads) and H 2 O, and the last OH- (ads) reduction step was predicted to have the lowest reversible potential, 0.74 V, indicating that it may be responsible for the O 2 reduction overpotential (the standard reversible potential for the reduction to water is 1.23 V). It is of interest to explain the lower activity of the cobalt selenides. This is addressed by using the same theoretical approach as for the pentlandite sulfide to study the steps in O 2 reduction over an isostructural (202) surface to see if, according to the predicted reversible potential of the electron-transfer steps, it should indeed be less active than the sulfide. This study also presents new experimental results for cobalt selenide. Developing new electrocatalysts to replace platinum for O 2 reduction remains a significant technological goal. In addition to the cobalt chalcogenides mentioned previously, many other transition metal compounds have been explored. 4-15 Alonso- Vante et al. have led the study of transition-metal chalcogenides as oxygen reduction catalysts. 15 They advanced the synthesis of Ru-based catalysts and contributed to the characterization of them using electrochemical and spectroscopic techniques. They associated the four-electron reduction of molecular oxygen to water with O 2 adsorption on adjacent cation sites, which, they believe, are required for O-O bond cleavage. 15 Pentlandite structured Co 9 S 8 and (Fe,Ni) 9 S 8 have also been subjects of fairly recent electronic structure, lattice parameter, and stability calculations. 16 Lately, cobalt sulfide compounds, such as linnaeite (Co 3 S 4 ) and pentlandite (Co 9 S 8 ), have been characterized as potential materials for the synthesis of nanoc- rystals. 17,18 The activities of nanocrystals of these compounds as oxygen reduction electrocatalysts have not been studied. The surface structures and compositions of the chalcogenide electrocatalysts have not yet been experimentally characterized in detail. This is why idealized model structures are assumed for theoretical studies. In the case of well-studied platinum and platinum alloy oxygen reduction electrocatalysts, surface struc- tures are believed to be known, and theoretical studies have been carried out for some of them. Platinum monolayer skins on platinum alloys are more active for O 2 reduction, and a series of theoretical studies has been conducted to determine properties of Pt 3 Co 19,20 and Pt 3 Cr 21-23 (111) surface Pt skins. The ∼50 mV reduction in overpotential on the skins relative to pure platinum has been explained as due to a positive shift in the OH(ads) reduction potential 19,20,22,23 and a positive shift in the * Corresponding author. E-mail: aba@case.edu; phone: (216) 368-5044; fax: (216) 368-3006. 10508 J. Phys. Chem. C 2007, 111, 10508-10513 10.1021/jp072056m CCC: $37.00 © 2007 American Chemical Society Published on Web 06/20/2007