Electrocatalytic Interface Engineering with Ionic Liquids

Abstract

Electrocatalyst development for reactions involving multiple charge transfer steps and more than one adsorbed intermediate is severely limited by the scaling relations that govern the free energy of adsorption of those intermediates. These scaling relations impart a minimum overpotential (thermodynamic), limiting activity independent of the identity of the catalyst. Consider the associative oxygen reduction reaction (ORR) mechanism which proceeds through *OOH, *O and *OH adsorbed intermediates1 , 2 , 3 , 4. Generally, the ORR is found to be rate limited by the conversion of *OH to H2O and it is concluded that weakening the strength of adsorption of *OH with the catalyst surface will lead to improved activity5 , 3 , 4 , 6. A shift in *OH binding strength, typically accomplished through manipulation of the electronic and geometric structure of the catalyst5 , 6, however, will also cause a shift in the energy of adsorption of other intermediates, including *OOH. This effect, termed intermediate scaling, rather than resulting in a continued increase in catalytic activity with decreasing *OH binding strength, will at some point shift the limiting reaction to the activation of O2, Sabatier/volcano relationship. Adsorbed intermediate scaling relations create a thermodynamic minimum overpotential of 0.37 V due to the fixed difference in free energy of adsorption (ΔΔGad) for *OH and *OOH of ~3.2 eV which is independent of the identity of the catalyst2 , 3 , 7 , 8. The ideal ORR catalyst, near zero overpotential, is found to have a difference in binding energy between *OH and *OOH (ΔΔGad) of 2.46 eV9. The origin of the effect of water on the progression of the ORR, as well as other elementary electrochemical reactions, is considered to be two-fold: (1) through solvation and hydrogen bonding, H2O stabilizes adsorbed reaction intermediates on the surface to differing degrees. It is found to stabilize *OH by about 0.5 eV and *OOH by 0.25 eV, contributing to the fixed ΔΔGad value9; (2) a mixed *OH-H2O hydrogen bonded structure is found to readily form on metal surfaces where charge redistribution due to the hydrogen bonding increases the binding strength and surface coverage of the *OH10 , 11 , 12 , 13. If strategies can be developed to reduce the binding energy difference between *OH and *OOH and prevent the formation of an ordered water stabilized *OH spectator structure, than the activity and selectivity of multi-intermediate reactions could be potentially maximized. Here we will present our progress in addressing intermediate scaling for the ORR through the manipulation of metal/electrolyte interface with chemically tailored ionic liquids (IL). Figure 1 demonstrates the impact of the formation of a composite electrocatalyst architecture through incorporation of a protic IL into a carbon supported PtNi alloy catalyst. The cyclic voltammogram, Fig. 1(top), shows a significant decrease in OH/O coverage at higher potentials which can be attributed to a decrease in adsorption free energy for those intermediates/spectators due to the manipulation of the interaction of water with the catalytic surface in the presence of the IL. We argue that it is this change in water interaction with the surface, potentially in combination with other properties of the IL including reactant solubility, that results in the enhanced ORR activity observed in Figure 1(bottom). Through the testing of well-defined IL/single crystal composite electrodes, we will present a systematic study of the mechanism by which ILs affect the kinetics of the ORR. The insight developed can be applied to other multi-intermediate reactions where the control of water interaction with the catalytic surface and adsorbed intermediate solvation is a viable strategy to approach the breaking of scaling relations. Figure 1: (top) CV of PtNi/C with (red) and without (blue) [MTBD][beti] IL in Ar purged 0.1 M HClO4. (bottom) ORR polarization curve for PtNi/C with (red) and without (blue) [MTBD][beti] IL in O2 saturated HClO4. 1. Gómez-Marín, A. M., et al., Catal. Sci. Technol. 4,1685 (2014). 2. Hansen, H. A., et al., J. Phys. Chem. C 118,6706–6718 (2014). 3. Nørskov, J. K. et al., J. Phys. Chem. B 108,17886–17892 (2004). 4. Qi, L., et al., J. Catal. 295,59–69 (2012). 5. Stamenkovic, V. R. et al., Science 315,493–497 (2007). 6. Stephens, I. E. L., et al., Energy Environ. Sci. 5,6744 (2012). 7. Viswanathan, V., et al., ACS Catal. 2,1654–1660 (2012). 8. Koper, M. T. M., Chem. Sci. 4,2710 (2013). 9. Viswanathan, V., et al., Top. Catal. 57,215–221 (2014). 10. Casalongue, H. S. et al., Nat Commun 4,2817 (2013). 11. Ogasawara, H., et al., Top. Catal. 59,439 (2016) 12. Pettersson, L. G. M. et al., J. Phys. Chem. C 114,10240–10248 (2010). 13. Schiros, T., et al., J. Phys. Chem. C 111, 15003–15012 (2007). Figure 1