ARTICLE pubs.acs.org/JPCC
Effective Reversible Potential, Energy Loss, and Overpotential on Platinum Fuel Cell Cathodes Feng Tian and Alfred B. Anderson* Department of Chemistry, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, Ohio 44106-7078, United States ABSTRACT: It is shown using quantum theory that the ∼0.9 V experimentally observed onset potential for O2 reduction to water over platinum electrodes, which is much less than the standard reversible potential, 1.229 V, is caused by the ∼1.2 eV exergonicity of the OOH(ads) dissociation step. The reduction in Gibbs energy available for electrical work leads to an effective reversible potential of about 0.93 V. These findings come from a fully self-consistent density functional treatment that includes surface charging for changing the electrode potential, and double-layer relaxation in response. It is shown that the adsorption Gibbs energies for reaction intermediates on an ideal catalyst can be predicted from solution phase reversible potentials. For O2 reduction, the ideal catalyst will have OOH(aq), O(aq), and OH(aq) adsorption energies of, respectively, 1.35, 2.41, and 1.49 eV at 1.229 V electrode potential.
’ INTRODUCTION The high overpotential for the four-electron reduction of oxygen is a continuing bottleneck to improving efficiencies on fuel cells. On platinum electrodes, it is in excess of 0.4 V at a current density 1.0 A 3 cm-2, meaning that, for this current density, it occurs at about 0.8 V relative to the standard reduction potential of 1.229 V on the standard hydrogen electrode (SHE) scale.1 The overpotential has been attributed to a combination of the blocking of O2 adsorption sites and slow electron transfer kinetics, and the OH(ads) intermediate, which is removed by reduction to H2O only at high overpotential, is believed to be a site blocker.1-7 The slow electron transfer kinetics could be dominated by one or more of the four one-electron transfer steps.2 In this paper, it is proposed that any exergonic reaction that does not include the transfer of an electron during the course of the overall reaction will cause there to be an overpotential, even if all of the electron transfer steps are activationless. To the authors’ knowledge, the effect of this simple fact in causing overpotentials in electrocatalysis has gone unrecognized. This means that it will be important to search for a catalyst that minimizes the adsorption free energy of O2 on its surface and minimizes the reaction free energy for all nonelectron transfer steps that might occur following its adsorption. Steps to consider include O2(ads) dissociation to 2O(ads) and OOH(ads) dissociation to O(ads) þ OH(ads). This paper will show that such an energy loss leads to an “effective reversible potential” that is about 0.4 V less than 1.229 V. In consequence, the reaction rate will drop to zero at this potential unless a different mechanism with less Gibbs energy loss can come into play as the effective reversible potential is approached, and obviate the effect. It will be concluded that a r 2011 American Chemical Society
new focus of catalysis development should be on eliminating exergonic steps in which no electron transfers occur. It will also be shown that the adsorption Gibbs energies for the intermediates formed during the electron transfer steps can be predicted for the ideal catalyst, that is, a catalyst for which the reversible potential for each step equals the reversible potential for the overall multielectron reduction. The adsorption energies are simply functions of the solution phase reversible potentials. It is generally believed that the reduction of O2(ads) to OOH(ads) is rate-limiting over platinum in acid electrolytes,2-4 though O2(ads) dissociation to 2O(ads) has been considered for phosphoric acid electrolytes.2,5 Measured Tafel slopes in the potential regions of low and high current densities have been tentatively associated with Temkin and Langmuir conditions, respectively.6,7 The model for Temkin kinetics includes weakening of the catalytic ability of active sites due to electronic effects, which are induced by adsorbed species, and Langmuir kinetics are based on site blocking by adsorbates, which reduces the density of active sites. Recent kinetic modeling studies assumed that two initial steps participated: 1 O2 f OðadsÞ 2
ð1Þ
1 O2 þ Hþ ðaqÞ þ e- f OHðadsÞ 2
ð2Þ
and
Received: October 19, 2010 Revised: January 24, 2011 Published: February 23, 2011 4076
dx.doi.org/10.1021/jp1100126 | J. Phys. Chem. C 2011, 115, 4076–4088
The Journal of Physical Chemistry C
ARTICLE
The model used potential-dependent activation energies obtained by linear Butler-Volmer extrapolations from values used for the 1.229 V standard reversible potential.8,9 The reduction steps in eq 2 and in the following two equations for O(ads) and OH(ads) reduction were used:
For the electron transfer steps, such as eqs 3-5, there is a linear dependence of the energy of the electron on the electrode’s potential
OðadsÞ þ Hþ ðaqÞ þ e- f OHðadsÞ
ð3Þ
OHðadsÞ þ Hþ ðaqÞ þ e- f H2 OðadsÞ
ð4Þ
where ΔG(e) is the change in the electron’s Gibbs energy, ΔU is the change in the potential of the electrode, and F is the Faraday constant. The energy of an electron participating in a reduction reaction changes rapidly with potential at the rate of 1 eV/V. The energy of a reducing electron coming from an electrode surface was used in a resonant electron transfer (RET) theory, which implements the pioneering resonant electron transfer ideas of Gurney18 and Butler.19 A local reaction center (LRC) model was employed to calculate the electrode potential dependencies of activation energies for the reactions in eqs 3-5.20-22 Recently, the method was applied to calculating activation barriers for O2 reduction in base, where the first reduction product was predicted to be superoxide, O2-(ads).23 In all of the LRC studies, including O(ads) reduction to OH(ads) and OH(ads) reduction to H2O(ads), the calculated activation energies at the reversible potentials were
