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Letter
Activity-Stability Trends for the Oxygen Evolution Reaction on Monometallic Oxides in Acidic Environments Nemanja Danilovic, Ram Subbaraman, Kee-Chul Chang, Seo Hyoung Chang, Yijin Kang, Joshua David Snyder, Arvydas P. Paulikas, Dusan Strmcnik, YongTae Kim, Deborah J. Myers, Vojislav R. Stamenkovic, and Nenad M. Markovic J. Phys. Chem. Lett., Just Accepted Manuscript • Publication Date (Web): 24 Jun 2014 Downloaded from http://pubs.acs.org on June 28, 2014
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The Journal of Physical Chemistry Letters
Activity-Stability Trends for the Oxygen Evolution Reaction on Monometallic Oxides in Acidic Environments Nemanja Danilovic1 and Ramachandran Subbaraman1, Kee-Chul. Chang1, Seo Hyoung Chang1, Yijin J. Kang1, Joshua Snyder1, Arvydas P. Paulikas1, Dusan Strmcnik1, Yong-Tae. Kim2, Deborah Myers3, Vojislav R. Stamenkovic1 and Nenad M. Markovic1* 1
Materials Science Division, Argonne National Laboratory, 9700 Cass Ave, Argonne, Illinois 60439 2
3
Pusan National University, 30 Jangjeon-dong, Geumjeong-gu, Busan 609-735, Korea
Chemical Science and Engineering Division, Argonne National Laboratory, 9700 Cass Ave, Argonne, Illinois 60439
Abstract In the present study, we used a surface-science approach to establish a functional link between activity and stability of monometallic oxides during the OER in acidic media. We found that the most active oxides (AuOs) materials. We suggest that the relationships between stability and activity are controlled by both the nobility of oxides as well as by the density of surface defects. This functionality is governed by the nature of metal cations and the potential transformation of stable metal cation with a valance state of n= +4 to unstable metal cation with n > +4. A practical consequence of such a close relationship between activity and stability is that the best materials for the OER should balance stability and activity in such a way that the dissolution rate is neither too fast nor too slow. TOC GRAPHIC
KEYWORDS Electrochemistry, Oxygen evolution reaction, monometallic oxides, The development of materials that can significantly influence the efficacy of the oxygen evolution reaction (OER: 2H2O↔O2 + 4H+ + 4e-) in electrolyzers must be guided by two equally important fundamental principles: (i) improving the catalytic activity for the desired reaction and (ii) long-term stability in hostile electrochemical environments. Traditionally, activity of the OER 1 ACS Paragon Plus Environment
The Journal of Physical Chemistry Letters
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has, for the most part, been correlated primarily in terms of energetic factors whereby the binding energy between the elusive “active sites” and the oxygenated species is assumed to control the kinetics of the OER1. Indeed, such considerations have formed the backbone of the well-known volcano plot that, ever since its establishment by Trasatti almost four decades ago2–4, is generally used to express the kinetics of the OER as a function of more fundamental properties of the oxide materials (e.g. oxygen binding energy, enthalpy of oxide formation, etc…, known as descriptors)5– 8
. It is now generally accepted that it is possible to identify materials with unique electronic
properties that bind one intermediate not too weakly and another intermediate not too strongly, for example RuO22,9. Although these studies have offered important insights into possible relationships between activity of the OER and the oxygen binding energy, few attempts have been made toward an even more important aspect of the electrocatalysts: the fundamental link between activity and stability under electrolyzer’s operating conditions10. Activity and stability trends Figure 1 shows the apparent activity for the OER (expressed as measured overpotentials at a constant current density of 5mAcm-2) in acidic environments on five monometallic oxides, ranging from noble Au, Pt and Ir to less-noble Ru and Os. Together with the data for the stability of these oxides during the OER, expressed as the quantifiable dissolution of metal cations during the very first potentiodynamic OER sweep from 1.23V up to potential at 5 mAcm-2. These findings enabled us to explore periodic trends in activity and stability, as shown in Fig. 1a. Two types of oxides with strikingly different morphologies are studied: crystalline “thermal chemical” oxides (TC-oxide) grown by thermal O2 exposure of the metal samples, and highly defective amorphous electrochemical oxides (EC-oxide) involving hydroxyl and related species in “hydrous oxides” formed by water electrooxidation at high anodic potentials (see schematics of TC- and EC-oxides in Figure 1b, and Supplementary Fig. S1 and S2). Presented below are the salient findings on how the relationships between the nature (noble vs. non-noble), morphology (crystalline vs. hydrous/amorphous), and structure (single crystal vs. polycrystalline) of oxides affect the stability and reactivity of these surfaces. In Figure 1a, three key features are noteworthy. First, the degree of OER activity increases with the order of oxophilicity (tendency of the metal to bind to oxygen7,11) of the respective element (Au
