Cross Sphere Electrode Reaction: the Case of Hydroxyl Desorption during the Oxygen Reduction Reaction on Pt(111) in Alkaline Media

Abstract

Hydroxide ion is a common electrolyte when electrode reactions take place in alkaline media. In the case of oxygen reduction reaction on Pt(111), we demonstrate by ab initio molecular dynamics calculations, that the desorption of hydroxyl (OH*) from the electrode surface to form a solvated OH<sup>−</sup> is a cross sphere process, with the reactant OH* in the inner sphere and the product OH<sup>−</sup> directly generated in the aqueous outer sphere. Such a mechanism is distinct from the typical inner sphere and outer sphere reactions. It is dictated by the strong hydrogen bonding interactions between a hydroxide ion and water molecules and facilitated by proton transfer through solvation layers. It should play a significant role whenever OH* desorption, or its reverse, OH<sup>−</sup> adsorption, is involved in an electrochemical reaction

OH–···Au Hydrogen Bond and Its Effect on the Oxygen Reduction Reaction on Au(100) in Alkaline Media

“…On Pt(111), OH* desorption is known to be the key step that determines the onset electrode potential of ORR in an alkaline medium. 24 On Au(100), OH* desorption is more facile and can reach a higher equilibrium electrode potential. As the OH − •••Au interaction is due to the intrinsic property of Au atoms, such hydrogen bonds should also be present on other gold surfaces or electrodes doped and/or alloyed with gold.…”

“…Only during very short durations would OH − move to the outer sphere, for q = −1.0e and −0.5e, as shown in panels a and b of Figure 3. Without such a hydrogen bond with surface metal atoms, OH − , as a strong hydrogen bond acceptor in its interactions with water molecules, would be better solvated in the outer sphere, away from the surface, as demonstrated in the case of Pt(111) 24 (also see Figure 1b).…”

“…Within this model, it is hard to desorb OH*, which could be achieved only by adding −1.0e or more negative charge to the Pt(111) slab. 24 However, after the addition of one more NaOH to afford a 2NaOH|Pt(111)|nH 2 O model, one OH − is typically observed in the solution layer while the other OH − would stay on the surface as OH* (Figure 1b). The position of the solvated OH − is often >6 Å above Pt(111), clearly in the outer sphere region, and does not come near the surface during the AIMD simulation of 10 000 steps (12.0 ps).…”

“…determines its ORR onset potential. 24 Above this potential, OH − would adsorb on the surface, and even the energetically most favorable reduction step…”

“…and turning off the reduction current. Furthermore, as the solvation interaction between OH − and H 2 O is strong, reaction 1 is achieved by an indirect process 24…”