Mechanism of the Hydrogen/Platinum(111) Fuel Cell

Abstract

We discuss our recently proposed mechanism for the electro-oxidation/reduction on Pt(111) surfaces (J. Electroanal. Chem. 2002, 537, 7) in the presence of sulfuric acid. The bisulfate ion has a large dipole moment and is strongly adsorbed on the positive electrode. Due to the large field gradients, the oxygen atoms of the adsorbed water molecules (and the dipoles) point down and bind to the on-top positions of the platinum substrate. As the electrode becomes more negative, the field gradient changes direction, and the water dipoles gradually reverse their orientation. At a certain critical value of the orientational parameter (which depends also on the bisulfate surface concentration), a two-dimensional honeycomb array of hydrogen bonded water molecules is formed. This is a new form of solid water, a true two-dimensional “ice”. For these negative potentials, the stable structure has one of the hydrogen atoms of the water pointing down, and this means that it is adsorbed by the hollow site of the Pt lattice. To satisfy the stoichiometry of the hydrogen bonds, we need to adsorb one-third of the surface sites of H+ ions. The following reversible reaction occurs: (H5 )3 + 6e- ⇌ 6H + (H3 )3. For the (111) surface of platinum and because of the geometrical matchup (the Pt−Pt distance is 2.77 Å, and the water diameter is 2.76 Å) this reaction occurs as a first-order transition, visible in the voltammogram as a sharp peak. From the [H+] concentration dependence of this sharp spike, we get an effective charge of 1.02 ± 0.02 for the adsorbed moiety. High-accuracy quantum calculations on a five-layer platinum metal slab show that this compound is stable in the absence of bisulfate ions. The quantum calculations show also that the hydrogen atoms in the hollow positions are neutralized. Since there are two-thirds of the Pt sites in the hollow positions, our model gives a natural explanation to the well-known fact that the hydrogen yield is 2/3 on this surface. We have revised our theory to shift the turning point of the water molecules to the transition potential where the HER honeycomb phase is formed. The turning point is in general agreement with the recent laserinduced measurements of the potential of zero charge.