Electrocatalytic Pathways for Oxygen Reduction and Evolution in Aqueous and Non Aqueous Electrolytes
Oxygen reduction (ORR) and evolution reaction (OER) at an electrochemical interface has important connotations with several key technologies ranging from the use of oxygen depolarized cathodes in chlorine generation to fuel cells, electrolyzers, water treatment, hybrid batteries (including Li/Air) and even life support. In all the decades of research conducted in a wide range of pH (aqueous) and non aqueous electrolytes some very interesting threads have emerged pointing to revolutionary materials development. For example, recent advances in our understanding of alternative active sites for oxygen reduction has provided for the basis of its molecular design. This presentation will put these developments in the context of more than two decades of effort devoted to engendering such non noble metal electrocatalysts. In this presentation we will present our latest data on the most active analogs which comprise of a FeNx coordinated active site in close concert with Fe nano-particles either present in some polymer composite or more ideally as edge defects close to the Fe-Nx coordinated structure. We will show how these active sites evolve on carbon supports as graphene defect structures. Such active site determinations are made with the use of a special in situ synchrotron x-ray absorption method using the near edge spectra referred to as x-ray absorption near edge structure (XANES), in a subtractive mode wherein the signal contribution from the bulk is successfully subtracted from the effect of the surface adsorbed species. When combined with our ability to simulate the same signatures using models with specifically adsorbed moieties, a powerful tool emerges to study electrochemical interfaces under actual in situ and operando conditions. This technique, commonly called the ‘Delta Mu (Dμ) Technique’ has been applied to a wide variety of transition metal surfaces and alloys[i],[ii] including non-Pt based metal electrocatalysts with element specificity. EXAFS data taken concurrently provide information on the changes in short range atomic order around the absorber atom thereby providing structural information such as bond distances and coordination numbers (thereby information on average cluster size, homogeneity and surface segregation etc.).
In this presentation among other things we will provide a picture of the electrocatalytic pathways in aqueous (at both the extreme edges of the pH scale) and non-aqueous environments. In the latter, our prior efforts to understand the effect of electrolyte both in terms of solvent[iii] and choice of salt will be presented in terms of engendering inner our outer sphere charge transfer[iv] during ORR and OER reactions. The technological consequence of such materials in power generation, electrolysis and energy storage will be described.
The authors deeply appreciate the financial assistance from the Army Research Office (Single Investigator award, W911NF-09-1) and the Department of Energy (EERE, DE-EE-000459). Authors also express their thanks to the Department of Energy, Materials Science Division for building and maintaining the National Synchrotron Light Source, in Brookhaven National Laboratory, Upton, NY.
[i] Contrast to Metal Ligand Effects on PtnM Electrocatalysts with M equal to Ru vs. Mo and Sn as Exhibited by in situ XANES and EXAFS Measurements in Methanol’, F. C. Scott, S. Mukerjee, and D. E. Ramaker, J. Phys. Chem. C., 114, 442 (2010).
[ii] ‘Electrochemical Kinetics and X-ray Absorption Spectroscopic Investigation of Oxygen Reduction on Chalcogen-Modified Ru Catalysts in Alkaline Medium’, N. Ramaswamy, R. J. Allen and S. Mukerjee, J. Phys. Chem. C., 115, 12650 (2011).
[iii] ‘Oxygen Reduction Reactions in Ionic Liquids and the Formulation of General ORR Mechanism for Li-Air Batteries,’ C. J. Allen, H. Hwang, R. Kautz, S. Mukerjee, E. J. Plichta, M. A. Hendrickson and K. M. Abraham, J. Phys. Chem. C., 116, 20755 (2012).
[iv] Influence of Inner and Outer Sphere Electron Transfer Mechanisms during Electrocatalysis of Oxygen in Alkaline Medium’, N. Ramaswamy and S. Mukerjee, J. Phys. Chem. C., 115, 18015 (2011).