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Relationships between atomic level surface structure and stability/ activity of platinum surface atoms in aqueous environments Pietro Papa Lopes, Dusan Strmcnik, Dusan Tripkovic, Justin G. Connell, Vojislav R. Stamenkovic, and Nenad M. Markovic ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b02920 • Publication Date (Web): 07 Mar 2016 Downloaded from http://pubs.acs.org on March 8, 2016
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ACS Catalysis
Relationships between atomic level surface structure and stability/activity of platinum surface atoms in aqueous environments Pietro P. Lopes, Dusan Strmcnik, Dusan Tripkovic, Justin G. Connell, Vojislav Stamenkovic and Nenad M. Markovic* Materials Science Division, Argonne National Laboratory, Lemont, IL, USA
Abstract The development of alternative energy systems for the clean production, storage and conversion of energy is strongly dependent on our ability to understand, at atomic-molecularlevels, the functional links between activity and stability of electrochemical interfaces. Whereas structure-activity relationships are rapidly evolving, the corresponding structure-stability relationships are still missing. This is primarily because there is no adequate experimental approach capable of monitoring the stability of well-defined single crystals in situ. Here, by utilizing the power of Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) connected to a stationary probe and coupling this technique to the rotating disk electrode method, it was possible to simultaneously measure the dissolution rates of surface atoms (as low as 0.4 pg cm-2s-1) and correlate them with the kinetic rates of electrochemical reactions in real time. Making use of this unique probe, it was possible to establish almost “atom-by-atom” structure-stability-activity relationships for platinum single crystals in both acidic and alkaline environments. We found that the degree of stability is strongly dependent on the coordination of surface atoms (less coordinated yields less stable), the nature of covalent and non-covalent interactions (i.e. adsorption of hydroxyl groups, oxygen atoms and halide species vs. interactions between hydrated Li cations and surface oxide), the thermodynamic driving force for Pt complexation (Pt ion speciation in solution) and the nature of the electrochemical reaction (the oxygen reduction/evolution and CO oxidation reactions). These findings open new opportunities for elucidating key fundamental descriptors that govern both activity and stability trends, and will ultimately assist in the development of real energy conversion and storage systems.
Keywords: Electrocatalysis; Structure-Stability relationships; Corrosion; Oxide formation; Double layer effects; Oxygen reduction reaction; Oxygen evolution reaction; CO oxidation.
The development of clean, reliable and cost-effective energy must be guided by two equally important fundamental principles: improving the catalytic activity of electrode materials1–3 and increasing their long-term stability in hostile electrochemical environments4–7. For decades, structure-activity relationships have formed the basis for any predictive ability in 1 ACS Paragon Plus Environment
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tailor-making nanomaterials with desirable reactive properties8–11 (e.g., the materials-by-design strategy). Very recently, however, it has been shown that the activity of electrochemical interfaces can also be improved by the precise organization of electrolyte components in the double layer12– 14
(e.g., the double-layer-by-design strategy). Today, the material-/electrolyte-by-design strategies
offer the ability to correlate the surface reactivity of well-characterized single crystal surfaces to (i) the adsorption of covalently bonded spectator species and reaction intermediates; (ii) non-covalent interactions between adsorbates and hydrated ions in the compact part of double layer; and (iii) catalytic processes that involve making and breaking H-H, H-O and O-O bonds in fuel cells15–17, electrolyzers18–22 and Li-air batteries23–25. In many respects, understanding correlations between the stability of atoms with atomic-level surface structure (structure-stability relationships) is much less advanced than the corresponding understanding of structure-activity relationships. In earlier reports, the stability of surface atoms has been examined by monitoring the potential-dependent change in surface morphology, either by utilizing cycling voltammetry (CV) alone microscopy (STM)
27–30
26
or by a combination of CVs and scanning tunneling
or surface X-ray scattering (SXS)
31–34
. Although these approaches were
successful in measuring oxide-induced changes in both the local structure of surface atoms (STM) and long range roughening of well-defined single crystals (SXS), these methods were unable to provide any information about the corrosion of metal surface atoms – a subject that is of great fundamental and technological importance. Very recently, however, the development of a scanning flow cell (SFC) coupled to an Inductively Coupled Plasma-Mass Spectrometer (ICPMS) has enabled in situ measurements of the dissolution of polycrystalline metal electrodes 35–45. By utilizing this method it was possible to establish relationships between potential-dependent oxide formation in various environments38,39,42,46 (pH 1 to 13, presence of Cl- anions and reactive atmosphere such as H2, O2, and CO) and the dissolution of the corresponding cations with high sensitivity6 (ca. 3 pg cm-2 s-1). Despite the breadth of these experiments, knowledge of potentialinduced surface stability at atomic-/molecular-levels still remains incomplete, even for platinumbased materials. Two key fundamental and technical barriers for this understanding are that: (i) current in-situ ICP-MS methodologies are not sensitive enough to probe the stability of various defects such as ad-islands and step edges that are inherently present on single crystal surfaces47,48 and (ii) there is no experimental strategy capable of simultaneously monitoring stabilityreactivity relationships at well-defined surfaces and at well-established diffusion/kinetic 2 ACS Paragon Plus Environment
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conditions. The development of such a methodology would offer the ability to embrace a science-based strategy capable of exploring, at atomic-/molecular-levels, the role of covalent and non-covalent interactions in metal dissolution/activity rates. It is reasonable to anticipate that establishing such activity-stability trends on well-defined single crystals in various electrochemical environments would open new opportunities for elucidating key fundamental descriptors that govern these processes and, ultimately, make use of these descriptors to develop real energy conversion and storage systems. In this paper, we develop a unique method that utilizes a stationary probe (SP) coupled to ICP-MS, combined with a rotating Pt(hkl)-disk electrode (RDE), in order to study the role of surface geometry on the stability of surface atoms and enable “atom-by-atom” detection of the adsorbate-induced dissolution of Pt atoms in acidic and alkaline environments. We found that the degree of stability of Pt(hkl) surfaces [Pt(110) 10-4 M). Taken altogether, the observed enhancement of Pt dissolution in the presence of Cl- is triggered by the combined action of irreversible oxide formation and Pt complexation with dissolved chloride anions. Having thus far focused on the role of covalent interactions in oxide formation resulting in Pt dissolution, we will now focus on understanding whether the rather weak, non-covalent interactions that take place in the double layer may also play a role in determining the stability of platinum surface atoms. These types of interactions strongly depend on the nature of hydrated cations present in alkaline solutions12,19. Figure 2b summarizes CVs and the corresponding ICPMS Pt dissolution profiles of Pt(111) in 0.1 M KOH with and without the addition of 5mM Li+. In line with our previous results12, the formation of oxide is significantly shifted towards more positive potentials, with the formation of OHad enhanced in the presence of Li+ in the same electrolyte. This is reflected in the overall Pt dissolution profile (Figure 2b and Table 2), which becomes inhibited due to the non-covalent, Li-mediated stabilization of OHad and in turn results in a lower surface coverage of Pt-Oad. As expected, the Pt(111) surface is very stable in the butterfly potential region regardless of the nature of the alkali cation and/or pH of the electrolyte, confirming that the reversible adsorption of OHad does not trigger Pt dissolution. In contrast, after the formation of Pt-Oad on the surface, the presence of OH- in alkaline electrolytes also increases the amount of Pt corrosion simply due to bulk complexation effects (Table 2), which is in line with previous reports46. Therefore, although alkaline solutions exhibit overall higher Pt dissolution in the early stages of oxide formation as compared with acid solutions, the dissolution rate can be controlled via non-covalent (double layer) effects.
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Functional links between activity and stability Having established the role of covalent and non-covalent interactions in structurestability relationships, we shall explore the functional links between the stability of Pt surface atoms and their activity for the ORR (Figure 3a), the CO oxidation reaction (Figure 3b) and the OER (Figure 3c) in acidic solutions. We note that the results summarized in Figure 3 are obtained simultaneously with the reaction, thus providing a unique opportunity to follow the kinetics of the reaction and correlate them with surface stability; e.g., to establish in situ stabilityreactivity relationships. One particular advantage of using the rotating disk configuration is that the kinetic limitations can be separated from mass transport limitations while in the SPRDE one can also simultaneously measure dissolution of surface atoms from well-defined Pt single crystals. The second advantage of a rotating disk configuration is that the kinetics of the OER are not affected by bubble formation, which can disturb the kinetics of the reaction in the case of a stagnant electrode and obstruct the flow path of ions to the ICP-MS in a flow cell configuration. We begin by summarizing activity-stability relationships for the ORR on Pt(hkl) surfaces in 0.1 M HClO4. As shown in Figure 3a, the ORR is a structure-sensitive process for which the activity is entirely controlled by the structure-dependent adsorption of spectator OHad (E>0.6V) and Had (E
