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Editorial: Surface Electrochemistry
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Editorial: Surface Electrochemistry Victor Climent Current Opinion in Electrochemistry 2017, 1:A5–A7 For a complete overview see the Issue http://dx.doi.org/10.1016/j.coelec.2017.02.003 2451-9103/© 2017 Published by Elsevier B.V.
Victor Climent University of Alicante (Spain)
V Climent completed his education (MSc) in the University of Alicante (1996), where he also did his PhD (2000). Then, he moved to the University of Oxford (UK), with a Marie Curie Fellowship (2001-2002). He returned to the University of Alicante where he obtained the position of ‘Profesor Titular’ (Associated Professor) in 2007. His main research interest is the investigation of the relationship between the structure of the electrode surface, its composition and its electrochemical reactivity.
A brief story of the close encounter between two charged phases Electrochemistry is mainly an interfacial science. Most investigations in electrochemistry involve the transfer of charged species through, at least, one interphase. In consequence, the structure of the interphase plays a significant role on almost any electrochemical process. The most common interphase includes a solid electronic conductor in contact with an electrolytic solution. In this case, the surface of the electrode is a significant part of the interphase, having great influence on the nature of electrochemical reactions. This is particularly important in the field of Electrocatalysis: in an electrocatalytic process, the intimate interaction of the electrode with the reaction intermediates is at the heart of the enhancement of reaction rates [1]. Electrosorption processes are very sensitive to the structure and composition of the electrode surface. Therefore, a deep understanding of the electrocatalytic phenomena must start with the detailed knowledge of the structure of the interphase. This includes the structure of the solid surface and its influence on electrosorption processes, as well as the arrangement of solvent and ionic species close to the surface. Recent computational efforts to understand the relation between the properties of the interphase and the electrocatalytic phenomena are reviewed in this volume in the chapter by Anderson. The investigation of the electrochemical interphase is quite old. One of the most iconic moments in this field was the publication of the well-known paper by Grahame in Chemical Reviews [2]. However, these initial studies focused on the electrolytic part of the interphase and used mainly liquid (mercury) electrodes, therefore without control of the surface structure. Studies with solid electrodes have an additional complication derived from the easy fouling of the surface by uncontrolled trace level of impurities in the solution. The strategy to overcome this problem followed by the Russian School of Frumkin et al. was to use high surface electrodes, therefore increasing significantly the surface to volume ratio. With this strategy, valuable information was obtained for electrosorption processes on platinum group metals [3]. However, these studies still lack the control of the electrode surface structure. Initial studies with solid electrodes with controlled surface structure used less reactive gold and silver electrodes [4]. Being less active means that they are more resistant to contamination and therefore easier to handle. However, for the same reason, these metals are less interesting for electrocatallytic applications. Attempts to study the electrochemistry of platinum single crystal
www.sciencedirect.com
Current Opinion in Electrochemistry 2017, 1:A5–A7
A6
electrodes were done initially in the late sixties and during the seventies [5]. These attempts combined preparation of the electrode surface in Ultra High Vacuum (UHV), where stablished protocols were available, with the transfer of the electrode to the electrochemical environment. It turned out that such transfer step is significantly difficult and we know nowadays that most of these early results are ruined by uncontrolled levels of contamination on the surface. A milestone in the development of the field of surface electrochemistry was the introduction of the flame annealing methodology for the decontamination of the electrode surface [6]. This easy treatment led to reproducibly clean surfaces with well-defined electrochemical behaviour. Most importantly, the treatment could be easily performed in any electrochemical lab what allowed many research groups to jump into this field without the requirement of sophisticated and expensive equipment. In consequence, the eighties and nineties witnessed an extraordinary proliferation of studies in this field that led to the collection of a significant amount of results. Once the protocols to study the electrochemical behaviour of single crystal electrodes were established for electrocatalytic metals, the door was open to investigate the relationship between surface structure and reactivity. As expected, most electrochemical reactions involving adsorbed intermediates exhibit significant dependence with the structure of the surface. Studies in this field paralleled the relationship between surface science and heterogeneous catalysis. In fact, the field of surface electrochemistry could benefit from the extensive knowledge already present in the field of surface science. For instance, important molecules studied in both fields are CO and NO [7– 9]. When compared with UHV, the electrochemical environment introduces two particularities. On the one hand, the influence of the solvent molecules, which cover the electrode surface and compete with the adsorption process and also solvate both solution and adsorbed species. On the other hand, the existence of an electric field gives an additional degree of freedom in the study of the electrochemical interphase. The extreme sensitivity of electrosorption processes to the structure of the surface turns the electrochemical techniques into surface analytical tools. Underpotential deposition processes (including hydrogen adsorption) can be used in combination with cyclic voltammetry to obtain detailed information about the structure of the electrode surface. Once the voltammetric profiles for the welldefined single crystal surfaces are known, this approach can be extended to characterise polycrystalline materials and true nanoparticle catalysts, allowing the deconvolution of different surface sites that conform the surface of these complex materials [10].
Current Opinion in Electrochemistry 2017, 1:A5–A7
In addition to the information provided by cyclic voltammetry, one classical electrochemical parameter characterising the interphase is the double layer capacitance. Electrochemical impedance spectroscopy is the best technique for its investigation since it allows the identification of dynamical behaviours by exploring its dependence with the frequency of the electrochemical perturbation. The chapter by Pajkossy et al. gives a revision on this topic. Pure electrochemical techniques can provide rich information about the interphase, in particular when analysed under the light of the electrocapillary equation [11] and even more when combined with temperature variation [12]. Another valuable, but still relatively unexplored, thermodynamic approach is that of calorimetric measurements with single crystal surfaces, as described in the chapter by Schuster. Notwithstanding the value of all these pure electrochemical and thermodynamic approaches, they usually need to be combined with other spectroscopic or microscopic approaches to obtain the detailed picture we aim for nowadays. Two approaches can be distinguished here. Ex situ techniques involve the transfer of the electrode to UHV environment to allow application of electron probe analytical tools such as LEED, Auger, XPS, TEM. Such transfer step may cast some doubts about whether the surface has suffered any alteration of its properties, especially during the evaporation of the solvent. Still, they proved valuable in the characterization of surfaces in the past. Nowadays there is a growing trend to explore similar characterization techniques to study real catalysts under true operating conditions, what has been known as in operando conditions. The chapter by Roldán-Guenya et al gives an excellent overview of several approaches under this concept. Among the different in situ techniques, vibrational spectroscopies and scanning probe microscopies are most useful. Vibrational spectroscopies allow not only the identification of compositional changes at the interphase but also provide indirect structural information through the influence of structure and chemical bonds on vibrational frequencies. The main difficulty of IR spectroscopy is the separation of the spectral contributions from bulk water. The surface enhancement obtained with rough surfaces helps in such separation but impedes its application to well defined surfaces. The review by Cai et al will help the reader to get a better idea about new trends in the application of this approach. Also Raman spectrocopies, which offers complementary information to IR spectroscopy, relies on surface enhancement to obtain measurable signals. Again, this significantly hampers its application to well defined surfaces. Two approaches have been designed to overcome this limitation. Shell Isolated Nanoparticles can be used to enhance the signal without the modification of the intrinsic properties of the surface. This is described in the review by Li et al. The second approach is the tip www.sciencedirect.com
Surface Electrochemistry Climent
enhanced spectroscopy which combines the scanning probe microscopy with Raman spectroscopy to obtain structural and compositional information of the surface [13]. However, application of this technique in electrochemical environment is still at a too early stage to be reviewed in this volume. While some uncertainties still persist in some cases, in general, the combination of the different methodologies mentioned above has provided an outstandingly detailed knowledge of the aqueous interphase. Now, this knowledge can be applied in the understanding of complex reactions on more complex electrode materials. In this regards, the challenge nowadays is to understand electrocatalytic processes such as the oxygen reduction reaction, CO2 reduction, nitrite and nitrate reduction, etc., at a level that allows the rational design of new electrocatalytic materials. These studies usually involve dispersed materials such as nanoparticle of pure or alloyed metals. Because electrocatalytic processes are sensitive to the properties of the surface, controlling the surface of the nanoparticles is a requisite for approaching this problem. Unfortunately, there is nowadays a large number of publications in this field that do not satisfy this criteria and makes comparison of results very difficult. The reader can find more about this strategy to study the electrochemistry of nanoparticle catalysts in the review by Solla-Gullon et al. Additional challenges remain in the study of nonaqueous interphases. In many realms of electrochemistry it is convenient to change the solvent, especially to increase the potential window. Also, as mentioned before, solvation has an important effect in the electrocatalytic phenomena that will be very sensitive to the nature of the solvent. Ionic liquids are considered a promising solvent for electrochemical applications. However, relatively little is known yet about the structure of the interphase in these solvents, especially when compared with the rich knowledge gathered for aqueous interphases. Presumably, this will be a subject where we expect to see significant progress in the near future. The chapter by Fu et al. presents recent advances in the characterization of these novel interphases, using scanning probe microscopies. Other chapters in this volume (those by Pajkossy, Li and Cai) also mention recent applications of different techniques to study ionic liquids, demonstrating that this is a fashionable topic today.
www.sciencedirect.com
A7
We hope the reader will find in the pages that follows a brief but rich overview of the recent trends and future directions in the topic of surface electrochemistry and electrocatalysis. References 1.
Herrero E, Feliu JM, Aldaz A: Electrocatalysis, vol 2. In Encyclopedia of Electrochemistry. Edited by Bard AJ, Stratmann M. Weinheim: Wiley-VCH Verlag; 2003:443.
2.
Grahame DC: The electrical double layer and the theory of electrocapillarity. Chem Rev 1947, 41:441–501.
3.
Frumkin AN, Petrii OA: Potentials of zero total charge and zero free charge of platinum group metals. Electrochim Acta 1975, 20:347–359.
4.
Hamelin A: Double layer properties at sp and sd metal single-crystal electrodes. New York: Plenum; 1985:1–101.
5.
Climent V, Feliu JM: Thirty years of platinum single crystal electrochemistry. J Solid State Electrochem 2011, 15:1297–1315.
6.
Clavilier J, Faure R, Guinet G, Durand R: Preparation of monocrystalline Pt microelectrodes and electrochemical study of the plane surfaces cut in the direction of the {111} and {110} planes. J Electroanal Chem 1980, 107:205–209.
7.
Chang SC, Weaver MJ: In-situ infrared spectroscopy at single-crystal electrodes: an emerging link between electrochemical and UHV surface science. J Phys Chem 1991, 95:5391.
8.
Villegas I, Gómez R, Weaver MJ: Nitric oxide as a probe adsorbate for linking Pt(111) electrochemical and model UHV interfaces using IR spectroscopy. J Phys Chem 1995, 99:14832.
9.
Rodes A, Gómez R, Perez JM, Feliu JM, Aldaz A: On the voltammetric and spectroscopic characterization of nitric oxide adlayers formed from nitrous acid on Pt(h,k,1) and Rh(h,k,1) electrodes. Electrochim Acta 1996, 41:729–745.
10. Solla-Gullón J, Rodríguez P, Herrero E, Aldaz A, Feliu JM: Surface characterization of platinum electrodes. Phys Chem Chem Phys 2008, 10:1359–1373. 11. Garcia-Araez N, Climent V, Herrero E, Feliu J, Lipkowski J: Thermodynamic studies of bromide adsorption at the Pt(111) electrode surface perchloric acid solutions: Comparison with other anions. J Electroanal Chem 2006, 591:149–158. 12. Garcia-Araez N, Climent V, Feliu JM: Temperature effects on platinum single crystal/aqueous solution interphases Combining Gibbs thermodynamics with laser pulsed experiments: Modern Aspects of Electrochemistry. Edited by Vayenas CG: Springer; 2011:1–105. 13. Pettinger B, Ren B, Picardi G, Schuster R, Ertl G: Nanoscale probing of adsorbed species by tip-enhanced Raman spectroscopy. Phys Rev Lett 2004:92.
Current Opinion in Electrochemistry 2017, 1:A5–A7
Editorial: Surface Electrochemistry Victor Climent Current Opinion in Electrochemistry 2017, 1:A5–A7 For a complete overview see the Issue http://dx.doi.org/10.1016/j.coelec.2017.02.003 2451-9103/© 2017 Published by Elsevier B.V.
Victor Climent University of Alicante (Spain)
V Climent completed his education (MSc) in the University of Alicante (1996), where he also did his PhD (2000). Then, he moved to the University of Oxford (UK), with a Marie Curie Fellowship (2001-2002). He returned to the University of Alicante where he obtained the position of ‘Profesor Titular’ (Associated Professor) in 2007. His main research interest is the investigation of the relationship between the structure of the electrode surface, its composition and its electrochemical reactivity.
A brief story of the close encounter between two charged phases Electrochemistry is mainly an interfacial science. Most investigations in electrochemistry involve the transfer of charged species through, at least, one interphase. In consequence, the structure of the interphase plays a significant role on almost any electrochemical process. The most common interphase includes a solid electronic conductor in contact with an electrolytic solution. In this case, the surface of the electrode is a significant part of the interphase, having great influence on the nature of electrochemical reactions. This is particularly important in the field of Electrocatalysis: in an electrocatalytic process, the intimate interaction of the electrode with the reaction intermediates is at the heart of the enhancement of reaction rates [1]. Electrosorption processes are very sensitive to the structure and composition of the electrode surface. Therefore, a deep understanding of the electrocatalytic phenomena must start with the detailed knowledge of the structure of the interphase. This includes the structure of the solid surface and its influence on electrosorption processes, as well as the arrangement of solvent and ionic species close to the surface. Recent computational efforts to understand the relation between the properties of the interphase and the electrocatalytic phenomena are reviewed in this volume in the chapter by Anderson. The investigation of the electrochemical interphase is quite old. One of the most iconic moments in this field was the publication of the well-known paper by Grahame in Chemical Reviews [2]. However, these initial studies focused on the electrolytic part of the interphase and used mainly liquid (mercury) electrodes, therefore without control of the surface structure. Studies with solid electrodes have an additional complication derived from the easy fouling of the surface by uncontrolled trace level of impurities in the solution. The strategy to overcome this problem followed by the Russian School of Frumkin et al. was to use high surface electrodes, therefore increasing significantly the surface to volume ratio. With this strategy, valuable information was obtained for electrosorption processes on platinum group metals [3]. However, these studies still lack the control of the electrode surface structure. Initial studies with solid electrodes with controlled surface structure used less reactive gold and silver electrodes [4]. Being less active means that they are more resistant to contamination and therefore easier to handle. However, for the same reason, these metals are less interesting for electrocatallytic applications. Attempts to study the electrochemistry of platinum single crystal
www.sciencedirect.com
Current Opinion in Electrochemistry 2017, 1:A5–A7
A6
electrodes were done initially in the late sixties and during the seventies [5]. These attempts combined preparation of the electrode surface in Ultra High Vacuum (UHV), where stablished protocols were available, with the transfer of the electrode to the electrochemical environment. It turned out that such transfer step is significantly difficult and we know nowadays that most of these early results are ruined by uncontrolled levels of contamination on the surface. A milestone in the development of the field of surface electrochemistry was the introduction of the flame annealing methodology for the decontamination of the electrode surface [6]. This easy treatment led to reproducibly clean surfaces with well-defined electrochemical behaviour. Most importantly, the treatment could be easily performed in any electrochemical lab what allowed many research groups to jump into this field without the requirement of sophisticated and expensive equipment. In consequence, the eighties and nineties witnessed an extraordinary proliferation of studies in this field that led to the collection of a significant amount of results. Once the protocols to study the electrochemical behaviour of single crystal electrodes were established for electrocatalytic metals, the door was open to investigate the relationship between surface structure and reactivity. As expected, most electrochemical reactions involving adsorbed intermediates exhibit significant dependence with the structure of the surface. Studies in this field paralleled the relationship between surface science and heterogeneous catalysis. In fact, the field of surface electrochemistry could benefit from the extensive knowledge already present in the field of surface science. For instance, important molecules studied in both fields are CO and NO [7– 9]. When compared with UHV, the electrochemical environment introduces two particularities. On the one hand, the influence of the solvent molecules, which cover the electrode surface and compete with the adsorption process and also solvate both solution and adsorbed species. On the other hand, the existence of an electric field gives an additional degree of freedom in the study of the electrochemical interphase. The extreme sensitivity of electrosorption processes to the structure of the surface turns the electrochemical techniques into surface analytical tools. Underpotential deposition processes (including hydrogen adsorption) can be used in combination with cyclic voltammetry to obtain detailed information about the structure of the electrode surface. Once the voltammetric profiles for the welldefined single crystal surfaces are known, this approach can be extended to characterise polycrystalline materials and true nanoparticle catalysts, allowing the deconvolution of different surface sites that conform the surface of these complex materials [10].
Current Opinion in Electrochemistry 2017, 1:A5–A7
In addition to the information provided by cyclic voltammetry, one classical electrochemical parameter characterising the interphase is the double layer capacitance. Electrochemical impedance spectroscopy is the best technique for its investigation since it allows the identification of dynamical behaviours by exploring its dependence with the frequency of the electrochemical perturbation. The chapter by Pajkossy et al. gives a revision on this topic. Pure electrochemical techniques can provide rich information about the interphase, in particular when analysed under the light of the electrocapillary equation [11] and even more when combined with temperature variation [12]. Another valuable, but still relatively unexplored, thermodynamic approach is that of calorimetric measurements with single crystal surfaces, as described in the chapter by Schuster. Notwithstanding the value of all these pure electrochemical and thermodynamic approaches, they usually need to be combined with other spectroscopic or microscopic approaches to obtain the detailed picture we aim for nowadays. Two approaches can be distinguished here. Ex situ techniques involve the transfer of the electrode to UHV environment to allow application of electron probe analytical tools such as LEED, Auger, XPS, TEM. Such transfer step may cast some doubts about whether the surface has suffered any alteration of its properties, especially during the evaporation of the solvent. Still, they proved valuable in the characterization of surfaces in the past. Nowadays there is a growing trend to explore similar characterization techniques to study real catalysts under true operating conditions, what has been known as in operando conditions. The chapter by Roldán-Guenya et al gives an excellent overview of several approaches under this concept. Among the different in situ techniques, vibrational spectroscopies and scanning probe microscopies are most useful. Vibrational spectroscopies allow not only the identification of compositional changes at the interphase but also provide indirect structural information through the influence of structure and chemical bonds on vibrational frequencies. The main difficulty of IR spectroscopy is the separation of the spectral contributions from bulk water. The surface enhancement obtained with rough surfaces helps in such separation but impedes its application to well defined surfaces. The review by Cai et al will help the reader to get a better idea about new trends in the application of this approach. Also Raman spectrocopies, which offers complementary information to IR spectroscopy, relies on surface enhancement to obtain measurable signals. Again, this significantly hampers its application to well defined surfaces. Two approaches have been designed to overcome this limitation. Shell Isolated Nanoparticles can be used to enhance the signal without the modification of the intrinsic properties of the surface. This is described in the review by Li et al. The second approach is the tip www.sciencedirect.com
Surface Electrochemistry Climent
enhanced spectroscopy which combines the scanning probe microscopy with Raman spectroscopy to obtain structural and compositional information of the surface [13]. However, application of this technique in electrochemical environment is still at a too early stage to be reviewed in this volume. While some uncertainties still persist in some cases, in general, the combination of the different methodologies mentioned above has provided an outstandingly detailed knowledge of the aqueous interphase. Now, this knowledge can be applied in the understanding of complex reactions on more complex electrode materials. In this regards, the challenge nowadays is to understand electrocatalytic processes such as the oxygen reduction reaction, CO2 reduction, nitrite and nitrate reduction, etc., at a level that allows the rational design of new electrocatalytic materials. These studies usually involve dispersed materials such as nanoparticle of pure or alloyed metals. Because electrocatalytic processes are sensitive to the properties of the surface, controlling the surface of the nanoparticles is a requisite for approaching this problem. Unfortunately, there is nowadays a large number of publications in this field that do not satisfy this criteria and makes comparison of results very difficult. The reader can find more about this strategy to study the electrochemistry of nanoparticle catalysts in the review by Solla-Gullon et al. Additional challenges remain in the study of nonaqueous interphases. In many realms of electrochemistry it is convenient to change the solvent, especially to increase the potential window. Also, as mentioned before, solvation has an important effect in the electrocatalytic phenomena that will be very sensitive to the nature of the solvent. Ionic liquids are considered a promising solvent for electrochemical applications. However, relatively little is known yet about the structure of the interphase in these solvents, especially when compared with the rich knowledge gathered for aqueous interphases. Presumably, this will be a subject where we expect to see significant progress in the near future. The chapter by Fu et al. presents recent advances in the characterization of these novel interphases, using scanning probe microscopies. Other chapters in this volume (those by Pajkossy, Li and Cai) also mention recent applications of different techniques to study ionic liquids, demonstrating that this is a fashionable topic today.
www.sciencedirect.com
A7
We hope the reader will find in the pages that follows a brief but rich overview of the recent trends and future directions in the topic of surface electrochemistry and electrocatalysis. References 1.
Herrero E, Feliu JM, Aldaz A: Electrocatalysis, vol 2. In Encyclopedia of Electrochemistry. Edited by Bard AJ, Stratmann M. Weinheim: Wiley-VCH Verlag; 2003:443.
2.
Grahame DC: The electrical double layer and the theory of electrocapillarity. Chem Rev 1947, 41:441–501.
3.
Frumkin AN, Petrii OA: Potentials of zero total charge and zero free charge of platinum group metals. Electrochim Acta 1975, 20:347–359.
4.
Hamelin A: Double layer properties at sp and sd metal single-crystal electrodes. New York: Plenum; 1985:1–101.
5.
Climent V, Feliu JM: Thirty years of platinum single crystal electrochemistry. J Solid State Electrochem 2011, 15:1297–1315.
6.
Clavilier J, Faure R, Guinet G, Durand R: Preparation of monocrystalline Pt microelectrodes and electrochemical study of the plane surfaces cut in the direction of the {111} and {110} planes. J Electroanal Chem 1980, 107:205–209.
7.
Chang SC, Weaver MJ: In-situ infrared spectroscopy at single-crystal electrodes: an emerging link between electrochemical and UHV surface science. J Phys Chem 1991, 95:5391.
8.
Villegas I, Gómez R, Weaver MJ: Nitric oxide as a probe adsorbate for linking Pt(111) electrochemical and model UHV interfaces using IR spectroscopy. J Phys Chem 1995, 99:14832.
9.
Rodes A, Gómez R, Perez JM, Feliu JM, Aldaz A: On the voltammetric and spectroscopic characterization of nitric oxide adlayers formed from nitrous acid on Pt(h,k,1) and Rh(h,k,1) electrodes. Electrochim Acta 1996, 41:729–745.
10. Solla-Gullón J, Rodríguez P, Herrero E, Aldaz A, Feliu JM: Surface characterization of platinum electrodes. Phys Chem Chem Phys 2008, 10:1359–1373. 11. Garcia-Araez N, Climent V, Herrero E, Feliu J, Lipkowski J: Thermodynamic studies of bromide adsorption at the Pt(111) electrode surface perchloric acid solutions: Comparison with other anions. J Electroanal Chem 2006, 591:149–158. 12. Garcia-Araez N, Climent V, Feliu JM: Temperature effects on platinum single crystal/aqueous solution interphases Combining Gibbs thermodynamics with laser pulsed experiments: Modern Aspects of Electrochemistry. Edited by Vayenas CG: Springer; 2011:1–105. 13. Pettinger B, Ren B, Picardi G, Schuster R, Ertl G: Nanoscale probing of adsorbed species by tip-enhanced Raman spectroscopy. Phys Rev Lett 2004:92.
Current Opinion in Electrochemistry 2017, 1:A5–A7