Paper EH-TuE7
Water as Promoter and Catalyst in Di-Oxygen Reactions at Aqueous and Organic Electrified Interfaces

Tuesday, December 9, 2014, 7:40 pm, Room Lehua

Understanding the role of water in of di-oxygen electrochemistry at atomic and molecular levels is the key to driving technological innovations that are urgently needed to deliver reliable, affordable and environmentally friendly energy [1-4]. Surprisingly, all previous studies have treated the water molecule as reactants needed to satisfy the stoichiometry of the reaction, rather than as vital hydrogen-donor molecules that can promote the rates of transformation of oxygen intermediates to final products. It is the impact of water on di-oxygen electrochemistry that constitutes the focus of our paper. First, we introduce a universal model that is capable of rationalizing, and ultimately understanding, electrocatalysis of the oxygen reaction in aqueous media, as well as in Li-O₂ electrochemistry in organic environments. The model is based on the formation of HO_ad···H₂O (alkaline) and LiO₂···H₂O (organic solvents) complexes that place water in a configurationally favorable position for proton transfer to O₂^- and HO₂^- intermediates that are formed on neighboring active sites. We propose that monometallic electrodes modified by omnipresent oxygenated spectators such as OH_ad and LiO₂ are, in fact, bifunctional catalysts capable of facilitating different parts of the overall multi-electron process: providing adsorption sites for the formation of complexes as well as bare metal sites to facilitate the electron transfer to O₂, O₂^- and HO₂^-.

Moreover, we demonstrate that water plays a dual role in Li-O₂ electrochemistry, acting simultaneously as a promoter in the production of Li₂O₂ and also as a catalyst, regenerating itself through a sequence of steps that include the recombination of H⁺ and OH^- back to water. Water acting as a catalyst has not, to the best of our knowledge, previously been reported for any electrochemical reaction.

References:

1. N. M. Markovic; “Interfacing Electrochemistry”; Nature Materials; 12(2013)101-102

2. R. Subbaraman, D. Tripkovic, K-C. Chang, D. Strmcnik, A. P. Paulikas, H.P. Hurinsit, M. Chan, J. Greeley, V. Stamenkovic and N. M. Markovic; “Trends in Activity for the Water Electrolyzer Reactions on 3d-M(Ni,Co,Fe,Mn)-Hydr(oxy)oxide Catalysts”; Nature Materials, 11 (2012) 550-557.

3. R. Subbaraman, D. Tripkovic, D. Strmcnik, K-C. Chang, M. Uchimura, A. P. Paulikas, V. Stamenkovic and N. M. Markovic; “Enhancing Hydrogen Evolution Activity in Water Splitting by Tailoring L⁺-Ni(OH)₂-Pt Interfaces”; Science, 334 (2011) 1256-1260

4. H. Gasteiger and N.M. Markovic; Fuel Cells - " Just a Dream-or Future Reality ", Science, 324 (2009) 48-49.