New Method to Determine Eley-Rideal Barriers for 2e- and 4e- Oxygen Reduction Reactions in Fuel Cells

We use atomic level simulations with density functional theory (DFT) to calculate barriers on Pt and Pd catalysts for Eley-Rideal (ER) reactions during the oxygen reduction reaction (ORR) in fuel cells.  In our new method, we simulate the hydrogen coming into the reaction from H⁺ in the solution.  Our method of calculating potential dependent ER barriers in fuel cells does not use external potential or nonzero charged calculations.  The barriers are determined using a single nudged elastic band (NEB) calculation.  A new scheme determines simultaneous zero-capacitance anode/cathode barriers.  An accurate solvation method is used to include the effect of water solvation¹

The following new ER calculated barriers supplement our previous calculations of Langmuir-Hershelwood (LH) barriers².

OOH_ad + H⁺ + e^- -> HOOH_ad

OH_ad + H⁺ + e^-  -> HOH_ad

OO_ad + H⁺ + e^- -> OOH_ad

OOH_ad + H⁺ + e^- -> HOOH_ad

OOH_ad + H⁺ + e^- -> 2OH_ad

Using the combined ER and LH barriers, we determine the rate determining step for ORR for Pt and Pd at different potentials.  The ER barriers calculated agree well with previous methods³.  They also can predict whether a catalyst favors the 2e^- or 4e^-ORR. 

Figure 1:  The barriers differ significantly for 2e^- and 4e^- ORR on Pt and Pd.  For Pd, HOOH formation is favored by 0.06 eV at 0.8 V.  On the other hand for Pt, water formation is favored by 0.33 eV at 0.8 V.  The Pt water formation barrier also agrees with previous methods³.

References:

1.            R. Sundararaman, K. Letchworth-Weaver and T. A. Arias, J Chem Phys 137(4), 044107 (2012).

2.            T. H. Yu, T. Hofmann, Y. Sha, B. Merinov, D. J. Myers, C. Heske and W. A. Goddard, J Phys Chem C (117), 26598-26607 (2013).

3.            V. Tripkovic, E. Skulason, S. Siahrostami, J. K. Norskov and J. Rossmeisl, Electrochim Acta 55, 7975-7981 (2010).