Chemistry Department Colloquium

Given a reduction in aqueous electrolyte with standard reversible potential Uo(aq):
R(aq) + H+(aq) + e- ⇄ P(aq), (1)
the corresponding description when reactant and product are adsorbed on the electrode is
R(ads) + H+(aq) + e- ⇄ P(ads) (2)
where R and P stand for reactant and product. We have found that when these species are adsorbed, the reversible potential Uo(ads) is given to good approximation by
Uo(ads) = Uo(aq) + Do(P) – Do(R) (3)
where Do are dissociation energies of the bonds to the surface. Using eq 3 for Uo(ads)
has the advantage that it does not require calculating solvation energies of charged ions such as hydronium and hydroxyl because they are contained in Uo, which is available from measurement. For neutral reactants and products, solvation energies are small, allowing eq 3 to rapidly yield useful predictions based on energy calculations using available commercial codes.
The full Gibbs energy change for a reduction reaction, including potential dependence, is
given by
ΔG(U) = {GRed(U) – GOx(U)} + n(φ + FU) (4)
where -(φ + FU) is the energy of an electron on the vacuum scale, φ being the
thermodynamic work function of the standard hydrogen electrode. We have developed
a theory that fully implements eq (4) in a code called Interface 1.0. It uses two-dimensional density functional band theory for periodic systems with linear combinations of pseudo-atomic orbitals, norm-conserving pseudopotentials, and projector expansions. In it the surface potential is adjusted by adding surface charge and a counter charge distribution in the double layer is determined self-consistently using a modified Poisson-Boltzmann theory within a dielec