Electrocatalysis: theory and experiment at the interface

The need to develop cleaner/greener methods for both energy production and chemical synthesis has been generating renewed interest in electrocatalysis. Experimental advances in the application of spectroscopic methods such as IR, INS, NMR, and XAS, and structural probes such as STM, AFM, high resolution TEM, and XRD are providing a wealth of data that enable structure–property relationships in electrocatalysis to be investigated. Similarly, developments in theoretical methods (MD simulations, DFT calculations, and Monte Carlo simulations combined with ab initio methodologies) are providing new insights regarding old catalysts and promise to provide direction in the search for new catalysts. The advent of high throughput catalyst preparation methods means that many more electrocatalyst formulations are being screened for an ever-wider variety of reactions. Directing this attempt will require the combined efforts of theoretical models and the development of new experimental techniques.

This Themed Issue of PCCP is timed for publication to accompany Faraday Discussion 140, “Electrocatalysis: Theory and Experiment at the Interface”, which will take place at the University of Southampton, July 7–9, 2008. The objectives of the meeting and this issue are to highlight the recent contributions of new experimental and theoretical methods in advancing both the fundamental and applied knowledge of electrocatalysis. This cross-fertilisation of experiment and theory and application of the results to an applied problem in electrochemistry follows on from other meetings that successfully built bridges between communities of researchers. For example, our current understanding of the molecular structure of the electrode/electrolyte interface and, in particular, the dependence of electrode reactions on the electrode surface structure stem from meetings held in 1979 in Snowmass, Colorado (T. E. Furtak, K. L. Kliewer and D. W. Lynch, Surf. Sci., 1980, 101, 1, and the other manuscripts in that volume), and in 1982 in Logan, Utah (W. N. Hansen, D. M. Kolb and D. W. Lynch, J. Electroanal. Chem., 1983, 150, IX, and the other manuscripts in that volume). Those two meetings brought surface scientists and electrochemists together and fostered the development of both in situ spectroscopic methods in electrochemistry and the study of single crystal electrode surfaces.

Much of our current understanding of the fundamental details of surface electrochemistry and electrocatalytic reactions has been derived from studies of single crystal electrode surfaces (see, for example, the excellent review by N. M. Markovic and P. N. Ross, Surf. Sci. Rep., 2002, 45, 121). This emphasis continues to the present day and is a recurring theme within this issue, with nearly half the papers describing the effects of particular crystal surfaces. Measurements (e.g., Cuesta and Escudero¹) and calculations (e.g., Hansen et al.²) describing reactions at the most stable surface structures (111) of Pt and other metal surfaces provide insights into the parameters that determine the rate limiting steps of electrocatalytic reactions, the reduction of NO and the oxygen reduction reaction, respectively. Practical electrocatalysts usually consist of supported nanoparticles which may be thought of as having a high degree of surface roughness or density of step and edge sites. Thus, stepped single crystal surfaces are often used as models. Inkaew and Korzeniewski report on the kinetics of CO oxidation on Pt(335) in acid solution,³ whilst Garcia and Koper describe CO oxidation at stepped surfaces in alkaline solution.⁴ The extension to nanoparticle catalysts is made by Solla-Gullón et al.⁵ and Susut et al.⁶ in studies of nanoparticles with defined shapes (cubic or octahedral/tetrahedral) so that the surface orientation of the particles was controlled.

The drive to use fuel cells in automotive applications has provided the motivation for much of the research in electrocatalysis in the last decade. Proton exchange membrane fuel cells (PEM-FC) and direct methanol fuel cells (DMFC) have been the most likely candidates. Thus the focus has been on the discovery of better anode catalysts with improved CO tolerance, which is a contaminant of the hydrogen fuel produced by reforming hydrocarbons, or methanol oxidation activity and cathode catalysts with enhanced oxygen reduction activity and/or tolerance to methanol cross-over. A variety of bimetallic, often Pt based, catalysts have been shown to provide enhanced activity for each of these reactions. The preparation of catalysts in which the surface composition is controlled offers the opportunity to explore both the physical (ensamble) and chemical (electronic) effects of the secondary element in bimetallic catalysts. In this issue Hoster et al.⁷ and Del Colle et al.⁸ both explore the effects of surface composition for the PtRu bimetallic system, the former taking the approach of modifying a Ru(0001) surface with Pt and the latter of modifying stepped Pt surfaces with Ru. Whilst PtRu appears to be the most promising catalyst for the oxidation of methanol and other small organic molecules, such as formic acid and ethanol, the exploration of a wide variety of binary and more complex mixtures continues⁹ and, in particular, there is a drive to explore less expensive alternatives, such as the PtPd and PtBi intermetllic phases as described by Wang et al.¹⁰

The slow kinetics of the oxygen reduction reaction remains one of the great challenges in terms of the commercial application of low temperature fuel cells, as without a much more active catalyst, the cost of the cathode remains too high. A library approach to the discovery of new ternary formulations based on PtTi binary catalysts is reported here by He and Kreidler.¹¹ Another approach to enhance oxygen reduction activity has been the modification of Pt based binary catalysts by dealloying, such as the Pt–Cu system described by Strasser et al.¹² This latter paper takes up the theme of the issue in the fullest sense, combining both experimental work and theoretical, DFT, calculations.

The contribution of theoretical calculations and models is still required in describing the fundamental aspects of the electrode/electrolyte interface and electrochemical reactions important in electrocatalysis. In this issue particular attention is drawn to the role of water at the interface by Otani et al.¹³ and the role of water and co-adsorbed ions by Wasileski and Janik¹⁴ using ab initio molecular dynamics simulations, the former describing the interfacial structure and the latter exploring the influence of co-adsorbed sodium ions on the dissociation of O₂ at Pt surfaces.

Whilst I have not introduced each of the papers in this Themed Issue, I hope that this brief introduction gives the reader a flavour of the excitement of the field. The continued advancement in experimental methods, especially in situ spectroscopic techniques (not detailed in this introduction) and the application of theoretical methodologies should enable the design of the next generation of electrocatalysts. The field is moving beyond the hydrogen and methanol oxidation and oxygen reduction reactions of low temperature fuel cells that have provided the motivation in recent years and those working in electrocatalysis will have considerable contributions to make in areas as diverse as synthetic chemistry and environmental clean up. Clearly electrocatalysis has an important role to play in the future.

Finally, I would like to thank all the authors who have submitted material to this Themed Issue of PCCP and all the staff of the journal at the RSC who have made the issue such a success.

Andrea E. Russell

University of Southampton, UK

Papers in this issue
1	A. Cuesta and M. Escudero, DOI: 10.1039/b717396b
2	J. Rossmeisl et al., DOI: 10.1039/b803956a
3	P. Inkaew and C. Korzeniewski, DOI: 10.1039/b804507k
4	G. Garcia and M. T. M. Koper, DOI: 10.1039b803503m
5	J. M. Feliu et al., DOI: 10.1039/b802703j
6	Y. Y. Tong et al., DOI: 10.1039/b802708k
7	H. E. Hoster et al., DOI: 10.1039/b802169d
8	E. Herrero et al., DOI: 10.1039/b802683a
9	A. Czerwinski et al., DOI: 10.1039/b718286b
10	H. Wang et al., DOI: 10.1039/b801473f
11	T. He and E. Kreidler, DOI: 10.1039/b802818b
12	P. Strasser and S. Koh, DOI: 10.1039/b803717e
13	M. Otani et al., DOI: 10.1039/b803541e
14	S. A. Wasileski and M. J. Janik, DOI: 10.1039/b803157f
15	D. G. Wittstock et al., DOI: 10.1039/b802688b
16	S.-G. Sun et al., DOI: 10.1039/b802047g
17	M. Osawa et al., DOI: 10.1039/b805955a
18	L. Kibler et al., DOI: 10.1039/b802915f
19	A. Kucernak and G. J. Offer, DOI: 10.1039/b802816h
20	W. Lin and P. A. Christensen, DOI: 10.1039/b802701c
21	J. Fuhrmann et al., DOI: 10.1039/b802812p
22	M. Michel et al., DOI: 10.1039/b802813n

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