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Recent applications of in situ ATR-IR spectroscopy in interfacial electrochemistry
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Review Article Recent applications of in situ ATR-IR spectroscopy in interfacial electrochemistry Han Wang, Ya-Wei Zhou and Wen-Bin Cai∗ In situ attenuated total reflection infrared (ATR-IR) spectroscopy is a powerful analytical tool for the molecular-level study of interfacial electrochemistry. This report presents a brief overview on selected publications over the past 3 years related to the application of in situ ATR-IR to electrochemical systems of fundamental and practical interests, and gives opinions on the technical development as well as the future applications in electrochemical systems. Address Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Collaborative Innovation Center of Chemistry for Energy Materials, Department of Chemistry, Fudan University, Shanghai 200433, China ∗
Corresponding authors: Cai, Wen-Bin ([email protected])
Current Opinion in Electrochemistry 2017, 1:73–79 This review comes from a themed issue on Surface Electrochemistry 2017 Edited by Victor Climent For a complete overview see the Issue and the Editorial Available online 25th January 2017 http://dx.doi.org/10.1016/j.coelec.2017.01.008 2451-9103/© Elsevier B.V. All rights reserved.
The electrochemical ATR-IR spectroscopy is mostly established in a so-called Kretschmann configuration [1] with the IR window/metal film/electrolyte structure, unless otherwise specified. The ATR-IR spectroscopy possesses higher surface sensitivity and specificity as compared to the IRAS, especially when molecules reside on metallic nanoparticle film electrodes, where tens of times of surface-enhanced infrared absorption (SEIRA) may occur. When significant SEIRA effect takes place and is emphasized, the term ATR-SEIRAS is widely used instead of ATR-IR [1]. Furthermore, without the thin-layer electrolyte structure, in situ ATR-IR can be applied to monitor the dynamic interfacial processes with a time resolution down to milliseconds without difficulty. The past 3 years have witnessed extensive applications of this technique to explore varieties of aspects of electrochemical interfaces, including potential-dependent interfacial structures of adsorbates, mimetic biointerface responses, reaction mechanisms relevant to energy conversion and storage processes and interfacial properties of electrocatalysts. Herein, we present a brief overview on recent applications of in situ ATR-IR in electrochemistry and highlight important achievements in questing the above aspects and significant advancements in developing this method.
Characterization of electrochemical interfacial structures Introduction The electrode/electrolyte interface is the central part of an electrochemical system, which may decide electrode behaviors and whole cell performances. To better understand the interfacial structures and processes, in situ microscopic and spectroscopic techniques are needed to combine with the traditional electrochemical techniques. In situ vibrational spectroscopies including surface Raman, IR and SFG have received intensive attention since they can directly provide molecular-level structural information about the electrochemical interfaces. Among them, surface IR spectroscopy features low-cost instrumentation, easy optical design and operation, simple surface selection rule, and wide applicability to various electrodes. Surface IR spectroscopy with either external reflection (often referred to IRAS or IRRAS) [1–3] or internal reflection (attenuated total reflection, referred to ATR-IR or ATR-FTIR) [1–4] is commonly used. www.sciencedirect.com
The electrochemical interfacial structure is largely dependent on the electrode, electrolyte and potential. The interfacial structural characterization lays the basis for understanding and tuning electrochemical processes. In situ ATR-IR has allowed the identification of the chemical nature and geometries of adsorbates of interests. Phosphate buffer solutions are widely used in electrochemistry and bioelectrochemistry. By applying ATR-SEIRAS in conjunction with isotope labeling, two types of phosphate adsorbates on Au electrode were determined with distinct pH dependences, as shown in Figure 1(a) [5• ]. Both infrared spectra and DFT calculations proved that the acid– base equilibrium of (HPO4 )ads (PO4 )ads + H + is shifted to a lower pKa value. Water and its derivatives such as hydronium and hydroxide are actively involved in such reactions as hydrogen/oxygen evolution and small organic molecule (SOM) oxidation. In situ ATR-IR enables to provide a detailed insight into interfacial behaviors of water and its derivatives. Current Opinion in Electrochemistry 2017, 1:73–79
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Figure 1
(a) Schematic illustration and spectral results of phosphate adsorption on Au electrode at different pH values. Adapted from reference [5• ]. (b) Schematic illustration of potential-induced interfacial water structure on Au electrode at the presence of adsorbed sodium dodecyl sulfate. Adapted from reference [2• ].
On Au electrodes the v(OH) band for non-H-bonded water and the δ(HOH) band for adsorbed H3 O+ were observed in acid media, as well as the v(OH) band for adsorbed hydroxide in alkaline media [6]. Those bands showed peak positions shifted from the frequencies of the stretching and bending modes of bulk water bands. Interfacial water may interact strongly with surface coadsorbates. A combined application of in situ ATR-IR and IRAS provides integral molecular information at the interface and in the thin-layer solution about the interfacial structure of water–sodium dodecyl sulfate (SDS) coadsorbed Au electrode [2• ]. The potential-induced phase transition of the SDS film can be further clarified based on infrared spectral results of both reflection modes (Figure 1(b)). In ionic liquid electrolyte, the potentialinduced behavior of trace amount of water was studied on Au electrode by in situ ATR-SEIRAS [7]. The spectral results indicated that the condensation of water at the interface reduces the coulombic interaction between ions and Au surface. Interfacial structure and interaction of bio-molecules at Au electrodes could be well understood with ATR-IR. At the presence of the phospholipid bilayer on Au electrode, interfacial water was detected in the bilayer zone with potential-dependent distribution among the three Current Opinion in Electrochemistry 2017, 1:73–79
types of water structure [8]. For adenine adsorption on Au, ATR-SEIRAS allows the deconvolution of the mixed bands due to the two coadsorbed adenine forms from basic solutions, yielding the surface pKa2 value equal to the solution pKa2 , regardless of adsorption potential [9]. In situ ATR-SEIRAS and EQCM were applied to investigate the underlying mechanisms of cell adhesion on electrode surface during electroactive biofilm formation, establishing the relationship between current generation, cell biomass and relative molecular composition [10]. Interactions of cardiolipin with cytochrome c membranes on Au surface were found to be potential dependent according to the spectral results [11].
Mechanistic studies of energy conversion and storage processes The oxidation of SOMs is of great relevance to the anode processes of low-temperature fuel cells, and the mechanistic understanding is much helpful for the design of efficient catalysts. Along this line, in situ ATR-IR is an ideal tool for studying electrocatalytic mechanisms by measuring the adsorbed molecules and/or intermediates as a function of potential or time. In order to address the debating mechanisms over formic acid oxidation, the electrocatalytic oxidation of formic acid and formate to CO2 on platinum was studied over www.sciencedirect.com
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Figure 2
(a) Scheme of in situ ATR-SEIRAS cell structure. (b) ATR-SEIRAS spectra of CO formation on Cu film electrode through the reduction of CO2 at different potentials. (c) Atomic force microscopic image of the Cu film on Si prism surface. (d) Band intensity of COad vs. potential in pH 10.1 and 6.9 electrolyte, respectively. Adapted from reference [16•• ]. (e) Scheme of spectral detection of propylene carbonate decomposition at the present of TEA+ anion and O2 . (f) In situ ATR-SEIRAS spectra of Au electrode in 0.1 M TEAClO4 /propylene carbonate electrolyte at various potentials. Adapted from reference [21•• ].
the range of pH 0–12 by in situ ATR-SEIRAS coupled with cyclic voltammetry, revealing a volcano-shaped pHdependent oxidation current peaked at a pH close to the pKa of HCOOH (3.75) [12•• ]. HCOOH is suggested to be oxidized after being converted to HCOO− via the acid–base equilibrium. In acidic media, the stable bridgebonded formate on Pt may suppress HCOO− oxidation by blocking active sites, but enhance the oxidation of HCOO− at high potentials by suppressing the adsorption of OH or the surface oxidation. Also noted is the report regarding ATR-SEIRAS investigation on the oxidation of formic acid at Rh/Au(111–25 nm) electrode in acidic media, in which the authors suggest that a bridge-bonded formate is the key active intermediate [4]. In addition, in situ ATR-SEIRAS study also revealed that the presence of tens of micromolar chloride ions significantly decreases electrocatalysis of formic acid oxidation on Pd electrode. The reduced electrocatalytic activity of the Pd surface can be jointly attributed to the blocking of active surface sites by both Cl− specific adsorption and induced COad formation [13]. The ethanol oxidation reaction on Pd film electrode in alkaline media was investigated using in situ ATR-SEIRAS in conjunction with isotope kinetic effect [14•• ]. The reaction intermediates and products, such as COad , acetylad (CH3 C∗ =O) and acetate, were identified with acetate being the main product. Reaction pathways involving the CH3 C∗ =O species as the pivotal intermediate were suggested based on spectral results. Closely related is the early report in 2013 in which the reaction pathways for www.sciencedirect.com
methanol oxidation on Pd film electrode in alkaline media were investigated by combining in situ ATR-SEIRAS and IRAS measurements [3]. Surface species including CO and methoxyl (but not bridge-bonded formate) were detected by ATR-SEIRAS, while reaction products like formate, CO2 and CO3 2− /HCO3 − with different potentialdependent distributions were observed by IRAS. Notably, adsorbed hydroxyl species are believed to be reactants for the electrocatalytic oxidation of SOMs in alkaline media. Interestingly, the librational mode of OHad on ionomercoated Pt film electrode in alkaline media was identified at ca. 1130 cm−1 by ATR-FTIR at potentials higher than 0.6 V RHE, and this band was enhanced at CeO2 modified Pt film electrode, consistent with the promoted oxidation of methanol, ethanol, ammonium and CO on CeO2 -modified Pt in alkaline media [15• ]. Electrochemical CO2 reduction is considered as a viable way to carbon recycle and energy storage, triggering a rising interest of using ATR-IR to explore mechanistic aspects. COad is regarded as the key intermediate primed for further reduction to higher order fuels during the CO2 reduction on Au and Cu electrodes while the hydrogen evolution is the competing reaction. In situ ATR-SEIRAS in conjunction with electrochemical kinetic measurement provides molecular-level insight into complex CO2 reduction reaction [16•• ]. Spectroscopic data reveal that bridge CO species occupy a high population of the Au surface, consistent with the notion that catalysis proceeds at a minority fraction of surface sites. ATR-SEIRAS was also used to track the CO adsorption profile on Cu Current Opinion in Electrochemistry 2017, 1:73–79
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electrode at varied pH values, potentials and CO concentrations as shown in Figure 2(a–d) [17]. Electrochemical ATR-SEIRAS in conjunction with isotope labeling revealed that hydrogen adsorption on Cu is preferred to COad during CO2 reduction at low overpotentials [18• ]. On Sn/SnOx electrode, the key electroactive intermediate during CO2 reduction was identified by electrochemical ATR-IR as carbonate species bound on SnII oxyhydroxide surface [19• ]. The study also ruled out a single one-electron charge transfer to CO2 to form CO2 •− . In acetonitrile, the electrochemical CO2 reduction on Au electrode was much boosted with the introduction of 0.5–0.7 M H2 O and the adsorbed CO2 − intermediate was suggested to interpret the double bands in ATR-SEIRAS spectra despite uncertainty [20]. Understanding the mechanistic details of solvent degradation is important toward a rational selection of the Li–O2 battery electrolytes. Propylene carbonate (PC) is widely used as the solvent for lithium–oxygen batteries and in situ ATR-SEIRAS was applied to investigate the superoxide-induced PC decomposition on Au electrode surface as shown in Figure 2(e, f) [21•• ]. The highly sensitive SEIRAS method revealed that the superoxideinduced ring-opening reaction of PC was determined by the electrolyte cation.
Spectral evaluation of electrocatalytic materials The chemical nature and population of interfacial species are closely correlated to the surface structure and reactivity of electrocatalytic materials, providing a basis for the interfacial spectral characterization or evaluation of catalytic materials. Taking advantage of the high sensitivity and frequency resolution of ATR-SEIRAS, the CO adsorption and oxidation on polycrystalline Au electrodes with controlled terraces and edges was investigated, and a detailed assignments of the COad bands was made to multiple types of CO bonding structures [22• ]. Similarly, the COad bands were also effective for characterizing shapecontrolled Pt nanocrystals [23] and Pt shells on Au and Ru substrates [24,25]. Recently, Pd-B/C was found to exhibit much higher activity and stability than Pd/C as the anode catalyst in direct formic acid fuel cells as shown in Figure 3(a–d) [26• ]. In situ ATR-SEIRAS measurement under mimetic fuel cell anode condition revealed a greatly inhibited COad formation accompanied with a promoted CO2 formation on Pd-B/C, correlating well with the enhanced formic acid oxidation as well as improved fuel cell performance. Dealloying changes the surface structure and thus reactivity of electrocatalytic alloy materials, these changes can be reflected with differing interfacial chemistry. In situ ATRSEIRAS showed enhanced COad and acetate formation on Pd–Ni–P film after dealloying, correlating well with the significantly enhanced EOR activity and stability on Current Opinion in Electrochemistry 2017, 1:73–79
the dealloyed Pd–Ni–P electrode in alkaline media [27]. To determine the EOR products, ATR-FTIR with the Otto configuration [1] was also tested by pushing a practical catalyst electrode against Ge or ZnSe ATR prism, able to identify spectrally the products in the thin-layer solution (and thus the selectivity) out of different catalysts. Unfortunately, no surface signals were detected [28–32].
Extension and development of ATR-IR technique To strengthen the power of electrochemical ATR-IR technique for broader applications, this technique per se has undergone extension and development. This may include early extensive efforts of wet-fabrication of varieties of metallic films on ATR Si prism [33] as well as the design of a well-defined flow cell capable of simultaneous measurement of ATR-FTIR and online-DEMS [34]. Interestingly, a rotating dual-electrode in situ ATRIR setup was recently developed, enabling IR spectroscopic access to both the anodic and the cathodic reactions of a direct methanol fuel cell during mimicked cell operation [35]. Extension of in situ ATR-FTIR to 2–5 monolayer graphene electrode on Si ATR element was also reported using ORR as the probe reaction [36• ]. The excellent stability and conductive nature of the graphene film promises itself a good support of nanocatalysts for desired electrocatalytic reactions. Notably, coupling ATR-IR simultaneously with onlineDEMS or even separately with IRAS [2,3,37] enables to obtain richer information about the reaction intermediates on surface and products in the solution, in recognition of that the latter two techniques favor the detection of the solution species, as compared to the ATR-IR. Recently a wall-jet electrode for a controlled mass transport in an ATR-IR cell was designed to study the oxygen reduction reaction on Pt nanocatalysts with different anions [38,39• ]. Similar wall-jet design of ATR-IR cell but with multiple internal reflections was also reported for the study of formic acid oxidation on Pt/C [40]. A breakthrough toward the development of this technique is the reported combination of spectroelectrochemistry and femtosecond 2D IR spectroscopy involving the use of ultrathin (∼nm) conductive layers of Pt and indium–tin oxide (ITO) as working electrodes on a singlereflection ATR element, which enables to detect vibrational relaxation and spectral diffusion of v(COad ) as a function of the applied potential at a time resolution of picosecond [41• ].
Concluding remarks The ATR-IR has been widely applied to study interfacial electrochemistry of broad interest, ranging from potential-induced interfacial structure change, mechanistic understanding of electrocatalytic oxidation of SOMs, reduction of CO2 and O2 and decomposition of solvent www.sciencedirect.com
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Figure 3
(a) Schematic illustration of the performance comparison between Pd/C and Pd-B/C; ATR-IR spectra of aqueous CO2 bands (b) and full range results during formic acid oxidation on Pd/C (c) and Pd-B/C (d), respectively. Adapted from reference [26• ].
in Li–O2 batteries to spectral correlation with properties of catalytic materials. Despite its obvious advantages and power, ATR-IR should be often coupled with one or more of other techniques including but not limited to electrochemical methods, DFT calculation, isotope labeling, IRAS and DEMS for comprehensive investigations. More ATR-IR applications should be directed to the reviving hot areas such as CO2 reduction and SOMs oxidation on efficient electrocatalysts, and the solvent degradation and the structural stability of solid electrolyte interface (SEI) of lithium-based batteries. Meanwhile, the ATR-IR technique should be further developed to meet the quest of ultrafast interfacial dynamics, such as transfer of redoxactive samples from bulk solution to organic monolayers and potential-dependent orientational dynamics, and to meet the operando monitoring of practical electrochemical interfaces with capabilities of tracing slow interfacial processes spanning from minutes to hours under the influences of external electricity, light and/or heat.
Acknowledgments This work is supported by the 973 Program (no. 2015CB932303) of MOST and NSFC (nos. 21473039 and 21273046).
References and recommended reading Papers of particular interest, published within the period of review, have been highlighted as: www.sciencedirect.com
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the Li–O2 battery. It signifies the use of SEIRAS technique in this direction and advanced the understanding for the selection of solvents in Li–O2 batteries. 22. Chen D-J, Allison TC, Tong YJ: Mechanistic insights into • electro-oxidation of solution CO on the polycrystalline gold surface as seen by in situ IR spectroscopy. J Phys Chem C 2016, 120:16132–16139. 23. Brimaud S, Jusys Z, Behm RJ: Shape-selected nanocrystals for in situ spectro-electrochemistry studies on structurally well defined surfaces under controlled electrolyte transport: a combined in situ ATR-FTIR/online DEMS investigation of CO electrooxidation on Pt. Beilstein J Nanotechnol 2014, 5:735–746. 24. Wang H, Jiang K, Chen Q, Xie Z, Cai W-B: Carbon monoxide mediated chemical deposition of Pt or Pd quasi-monolayer on Au surfaces with superior electrocatalysis for ethanol oxidation in alkaline media. Chem Commun 2016, 52:374–377. 25. Edited by Chen D, Atienza DO, Tong YJ: Integrated studies of Au@Pt and Ru@Pt core-shell nanoparticles by in situ electrochemical NMR, ATR-SEIRAS, and SERS: Nanomaterials for Fuel Cell Catalysis. Edited by Ozoemena IK, Chen S: Springer; 2016:225–251. 26. Jiang K, Chang J, Wang H, Brimaud S, Xing W, Behm R Jr, • Cai W-B: Small addition of boron in palladium catalyst, big improvement in fuel cell’s performance: what may interfacial spectroelectrochemistry tell? ACS Appl Mater Interfaces 2016, 8:7133–7138.
14. Yang Y-Y, Ren J, Li Q-X, Zhou Z-Y, Sun S-G, Cai W-B: •• Electrocatalysis of ethanol on a Pd electrode in alkaline media: an in situ attenuated total reflection surface-enhanced infrared absorption spectroscopy study. ACS Catal 2014, 4:798–803. This work clarified the dual pathways of ethanol oxidation on Pd in alkaline media. Acetyl intermediates were confirmed for the first time on Pd film electrode in alkaline media with ATR-SEIRAS in conjunction with isotope labeling and the acetyl species was suggested to play a pivotal role in the reaction pathways.
27. Wang Y, Jiang K, Cai W-B: Enhanced electrocatalysis of ethanol on dealloyed Pd–Ni–P film in alkaline media: an infrared spectroelectrochemical investigation. Electrochim Acta 2015, 162:100–107.
15. Katayama Y, Okanishi T, Muroyama H, Matsui T, Eguchi K: • Enhanced supply of hydroxyl species in CeO2 -modified platinum catalyst studied by in situ ATR-FTIR spectroscopy. ACS Catal 2016, 6:2026–2034.
29. Fontes E, da Silva SG, Spinace E, Neto A, de Souza R: In situ ATR-FTIR studies of ethanol electro-oxidation in alkaline medium on PtRh/C electrocatalyst prepared by an alcohol reduction process. Electrocatalysis 2016, 7:1–8.
16. Wuttig A, Yaguchi M, Motobayashi K, Osawa M, Surendranath Y: •• Inhibited proton transfer enhances Au-catalyzed CO2 -to-fuels selectivity. Proc Natl Acad Sci 2016:4585–4593. This work applied in situ ATR-SEIRAS for the kinetic investigation of electrochemical CO2 reduction on Au electrode. Spectroscopic data reveal that bridge bonded CO species occupy a high population of the Au surface, consistent with the notion that catalysis proceeds at a minority fraction of surface sites. The results established the value of combining electrochemical kinetic and in situ IR studies to gain a molecular-level understanding of electrochemical processes.
30. Fontes E, Piasentin R, Ayoub J, da Silva J, Assumpção M, Spinacé E, Neto A, De Souza R: Electrochemical and in situ ATR-FTIR studies of ethanol electro-oxidation in alkaline medium using PtRh/C electrocatalysts. Mater Renewable Sustainable Energy 2015, 4:1–10.
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32. Geraldes AN, Da Silva DF, Pino ES, Da Silva JCM, De Souza RFB, Hammer P, Spinacé EV, Neto AO, Linardi M, Dos Santos MC: Ethanol electro-oxidation in an alkaline medium using Pd/C, Au/C and PdAu/C electrocatalysts prepared by electron beam irradiation. Electrochim Acta 2013, 111:455–465.
18. Heyes J, Dunwell M, Xu B: CO2 reduction on Cu at low • overpotentials with surface-enhanced in situ spectroscopy. J Phys Chem C 2016, 120:17334–17341. 19. Baruch MF, Pander III JE, White JL, Bocarsly AB: Mechanistic • insights into the reduction of CO2 on tin electrodes using in situ ATR-IR spectroscopy. ACS Catal 2015, 5:3148–3156. 20. Rudnev AV, Zhumaev UE, Kuzume A, Vesztergom S, Furrer J, Broekmann P, Wandlowski T: The promoting effect of water on the electroreduction of CO2 in acetonitrile. Electrochim Acta 2016, 189:38–44. 21. Vivek JP, Berry N, Papageorgiou G, Nichols RJ, Hardwick LJ: •• Mechanistic insight into the superoxide induced ring opening in propylene carbonate based electrolytes using in situ surface-enhanced infrared spectroscopy. J Am Chem Soc 2016, 138:3745–3751. This work applied in situ ATR-SEIRAS to investigate the cation effect on superoxide induced decomposition of propylene carbonate solvent in
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Review Article Recent applications of in situ ATR-IR spectroscopy in interfacial electrochemistry Han Wang, Ya-Wei Zhou and Wen-Bin Cai∗ In situ attenuated total reflection infrared (ATR-IR) spectroscopy is a powerful analytical tool for the molecular-level study of interfacial electrochemistry. This report presents a brief overview on selected publications over the past 3 years related to the application of in situ ATR-IR to electrochemical systems of fundamental and practical interests, and gives opinions on the technical development as well as the future applications in electrochemical systems. Address Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Collaborative Innovation Center of Chemistry for Energy Materials, Department of Chemistry, Fudan University, Shanghai 200433, China ∗
Corresponding authors: Cai, Wen-Bin ([email protected])
Current Opinion in Electrochemistry 2017, 1:73–79 This review comes from a themed issue on Surface Electrochemistry 2017 Edited by Victor Climent For a complete overview see the Issue and the Editorial Available online 25th January 2017 http://dx.doi.org/10.1016/j.coelec.2017.01.008 2451-9103/© Elsevier B.V. All rights reserved.
The electrochemical ATR-IR spectroscopy is mostly established in a so-called Kretschmann configuration [1] with the IR window/metal film/electrolyte structure, unless otherwise specified. The ATR-IR spectroscopy possesses higher surface sensitivity and specificity as compared to the IRAS, especially when molecules reside on metallic nanoparticle film electrodes, where tens of times of surface-enhanced infrared absorption (SEIRA) may occur. When significant SEIRA effect takes place and is emphasized, the term ATR-SEIRAS is widely used instead of ATR-IR [1]. Furthermore, without the thin-layer electrolyte structure, in situ ATR-IR can be applied to monitor the dynamic interfacial processes with a time resolution down to milliseconds without difficulty. The past 3 years have witnessed extensive applications of this technique to explore varieties of aspects of electrochemical interfaces, including potential-dependent interfacial structures of adsorbates, mimetic biointerface responses, reaction mechanisms relevant to energy conversion and storage processes and interfacial properties of electrocatalysts. Herein, we present a brief overview on recent applications of in situ ATR-IR in electrochemistry and highlight important achievements in questing the above aspects and significant advancements in developing this method.
Characterization of electrochemical interfacial structures Introduction The electrode/electrolyte interface is the central part of an electrochemical system, which may decide electrode behaviors and whole cell performances. To better understand the interfacial structures and processes, in situ microscopic and spectroscopic techniques are needed to combine with the traditional electrochemical techniques. In situ vibrational spectroscopies including surface Raman, IR and SFG have received intensive attention since they can directly provide molecular-level structural information about the electrochemical interfaces. Among them, surface IR spectroscopy features low-cost instrumentation, easy optical design and operation, simple surface selection rule, and wide applicability to various electrodes. Surface IR spectroscopy with either external reflection (often referred to IRAS or IRRAS) [1–3] or internal reflection (attenuated total reflection, referred to ATR-IR or ATR-FTIR) [1–4] is commonly used. www.sciencedirect.com
The electrochemical interfacial structure is largely dependent on the electrode, electrolyte and potential. The interfacial structural characterization lays the basis for understanding and tuning electrochemical processes. In situ ATR-IR has allowed the identification of the chemical nature and geometries of adsorbates of interests. Phosphate buffer solutions are widely used in electrochemistry and bioelectrochemistry. By applying ATR-SEIRAS in conjunction with isotope labeling, two types of phosphate adsorbates on Au electrode were determined with distinct pH dependences, as shown in Figure 1(a) [5• ]. Both infrared spectra and DFT calculations proved that the acid– base equilibrium of (HPO4 )ads (PO4 )ads + H + is shifted to a lower pKa value. Water and its derivatives such as hydronium and hydroxide are actively involved in such reactions as hydrogen/oxygen evolution and small organic molecule (SOM) oxidation. In situ ATR-IR enables to provide a detailed insight into interfacial behaviors of water and its derivatives. Current Opinion in Electrochemistry 2017, 1:73–79
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Figure 1
(a) Schematic illustration and spectral results of phosphate adsorption on Au electrode at different pH values. Adapted from reference [5• ]. (b) Schematic illustration of potential-induced interfacial water structure on Au electrode at the presence of adsorbed sodium dodecyl sulfate. Adapted from reference [2• ].
On Au electrodes the v(OH) band for non-H-bonded water and the δ(HOH) band for adsorbed H3 O+ were observed in acid media, as well as the v(OH) band for adsorbed hydroxide in alkaline media [6]. Those bands showed peak positions shifted from the frequencies of the stretching and bending modes of bulk water bands. Interfacial water may interact strongly with surface coadsorbates. A combined application of in situ ATR-IR and IRAS provides integral molecular information at the interface and in the thin-layer solution about the interfacial structure of water–sodium dodecyl sulfate (SDS) coadsorbed Au electrode [2• ]. The potential-induced phase transition of the SDS film can be further clarified based on infrared spectral results of both reflection modes (Figure 1(b)). In ionic liquid electrolyte, the potentialinduced behavior of trace amount of water was studied on Au electrode by in situ ATR-SEIRAS [7]. The spectral results indicated that the condensation of water at the interface reduces the coulombic interaction between ions and Au surface. Interfacial structure and interaction of bio-molecules at Au electrodes could be well understood with ATR-IR. At the presence of the phospholipid bilayer on Au electrode, interfacial water was detected in the bilayer zone with potential-dependent distribution among the three Current Opinion in Electrochemistry 2017, 1:73–79
types of water structure [8]. For adenine adsorption on Au, ATR-SEIRAS allows the deconvolution of the mixed bands due to the two coadsorbed adenine forms from basic solutions, yielding the surface pKa2 value equal to the solution pKa2 , regardless of adsorption potential [9]. In situ ATR-SEIRAS and EQCM were applied to investigate the underlying mechanisms of cell adhesion on electrode surface during electroactive biofilm formation, establishing the relationship between current generation, cell biomass and relative molecular composition [10]. Interactions of cardiolipin with cytochrome c membranes on Au surface were found to be potential dependent according to the spectral results [11].
Mechanistic studies of energy conversion and storage processes The oxidation of SOMs is of great relevance to the anode processes of low-temperature fuel cells, and the mechanistic understanding is much helpful for the design of efficient catalysts. Along this line, in situ ATR-IR is an ideal tool for studying electrocatalytic mechanisms by measuring the adsorbed molecules and/or intermediates as a function of potential or time. In order to address the debating mechanisms over formic acid oxidation, the electrocatalytic oxidation of formic acid and formate to CO2 on platinum was studied over www.sciencedirect.com
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Figure 2
(a) Scheme of in situ ATR-SEIRAS cell structure. (b) ATR-SEIRAS spectra of CO formation on Cu film electrode through the reduction of CO2 at different potentials. (c) Atomic force microscopic image of the Cu film on Si prism surface. (d) Band intensity of COad vs. potential in pH 10.1 and 6.9 electrolyte, respectively. Adapted from reference [16•• ]. (e) Scheme of spectral detection of propylene carbonate decomposition at the present of TEA+ anion and O2 . (f) In situ ATR-SEIRAS spectra of Au electrode in 0.1 M TEAClO4 /propylene carbonate electrolyte at various potentials. Adapted from reference [21•• ].
the range of pH 0–12 by in situ ATR-SEIRAS coupled with cyclic voltammetry, revealing a volcano-shaped pHdependent oxidation current peaked at a pH close to the pKa of HCOOH (3.75) [12•• ]. HCOOH is suggested to be oxidized after being converted to HCOO− via the acid–base equilibrium. In acidic media, the stable bridgebonded formate on Pt may suppress HCOO− oxidation by blocking active sites, but enhance the oxidation of HCOO− at high potentials by suppressing the adsorption of OH or the surface oxidation. Also noted is the report regarding ATR-SEIRAS investigation on the oxidation of formic acid at Rh/Au(111–25 nm) electrode in acidic media, in which the authors suggest that a bridge-bonded formate is the key active intermediate [4]. In addition, in situ ATR-SEIRAS study also revealed that the presence of tens of micromolar chloride ions significantly decreases electrocatalysis of formic acid oxidation on Pd electrode. The reduced electrocatalytic activity of the Pd surface can be jointly attributed to the blocking of active surface sites by both Cl− specific adsorption and induced COad formation [13]. The ethanol oxidation reaction on Pd film electrode in alkaline media was investigated using in situ ATR-SEIRAS in conjunction with isotope kinetic effect [14•• ]. The reaction intermediates and products, such as COad , acetylad (CH3 C∗ =O) and acetate, were identified with acetate being the main product. Reaction pathways involving the CH3 C∗ =O species as the pivotal intermediate were suggested based on spectral results. Closely related is the early report in 2013 in which the reaction pathways for www.sciencedirect.com
methanol oxidation on Pd film electrode in alkaline media were investigated by combining in situ ATR-SEIRAS and IRAS measurements [3]. Surface species including CO and methoxyl (but not bridge-bonded formate) were detected by ATR-SEIRAS, while reaction products like formate, CO2 and CO3 2− /HCO3 − with different potentialdependent distributions were observed by IRAS. Notably, adsorbed hydroxyl species are believed to be reactants for the electrocatalytic oxidation of SOMs in alkaline media. Interestingly, the librational mode of OHad on ionomercoated Pt film electrode in alkaline media was identified at ca. 1130 cm−1 by ATR-FTIR at potentials higher than 0.6 V RHE, and this band was enhanced at CeO2 modified Pt film electrode, consistent with the promoted oxidation of methanol, ethanol, ammonium and CO on CeO2 -modified Pt in alkaline media [15• ]. Electrochemical CO2 reduction is considered as a viable way to carbon recycle and energy storage, triggering a rising interest of using ATR-IR to explore mechanistic aspects. COad is regarded as the key intermediate primed for further reduction to higher order fuels during the CO2 reduction on Au and Cu electrodes while the hydrogen evolution is the competing reaction. In situ ATR-SEIRAS in conjunction with electrochemical kinetic measurement provides molecular-level insight into complex CO2 reduction reaction [16•• ]. Spectroscopic data reveal that bridge CO species occupy a high population of the Au surface, consistent with the notion that catalysis proceeds at a minority fraction of surface sites. ATR-SEIRAS was also used to track the CO adsorption profile on Cu Current Opinion in Electrochemistry 2017, 1:73–79
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electrode at varied pH values, potentials and CO concentrations as shown in Figure 2(a–d) [17]. Electrochemical ATR-SEIRAS in conjunction with isotope labeling revealed that hydrogen adsorption on Cu is preferred to COad during CO2 reduction at low overpotentials [18• ]. On Sn/SnOx electrode, the key electroactive intermediate during CO2 reduction was identified by electrochemical ATR-IR as carbonate species bound on SnII oxyhydroxide surface [19• ]. The study also ruled out a single one-electron charge transfer to CO2 to form CO2 •− . In acetonitrile, the electrochemical CO2 reduction on Au electrode was much boosted with the introduction of 0.5–0.7 M H2 O and the adsorbed CO2 − intermediate was suggested to interpret the double bands in ATR-SEIRAS spectra despite uncertainty [20]. Understanding the mechanistic details of solvent degradation is important toward a rational selection of the Li–O2 battery electrolytes. Propylene carbonate (PC) is widely used as the solvent for lithium–oxygen batteries and in situ ATR-SEIRAS was applied to investigate the superoxide-induced PC decomposition on Au electrode surface as shown in Figure 2(e, f) [21•• ]. The highly sensitive SEIRAS method revealed that the superoxideinduced ring-opening reaction of PC was determined by the electrolyte cation.
Spectral evaluation of electrocatalytic materials The chemical nature and population of interfacial species are closely correlated to the surface structure and reactivity of electrocatalytic materials, providing a basis for the interfacial spectral characterization or evaluation of catalytic materials. Taking advantage of the high sensitivity and frequency resolution of ATR-SEIRAS, the CO adsorption and oxidation on polycrystalline Au electrodes with controlled terraces and edges was investigated, and a detailed assignments of the COad bands was made to multiple types of CO bonding structures [22• ]. Similarly, the COad bands were also effective for characterizing shapecontrolled Pt nanocrystals [23] and Pt shells on Au and Ru substrates [24,25]. Recently, Pd-B/C was found to exhibit much higher activity and stability than Pd/C as the anode catalyst in direct formic acid fuel cells as shown in Figure 3(a–d) [26• ]. In situ ATR-SEIRAS measurement under mimetic fuel cell anode condition revealed a greatly inhibited COad formation accompanied with a promoted CO2 formation on Pd-B/C, correlating well with the enhanced formic acid oxidation as well as improved fuel cell performance. Dealloying changes the surface structure and thus reactivity of electrocatalytic alloy materials, these changes can be reflected with differing interfacial chemistry. In situ ATRSEIRAS showed enhanced COad and acetate formation on Pd–Ni–P film after dealloying, correlating well with the significantly enhanced EOR activity and stability on Current Opinion in Electrochemistry 2017, 1:73–79
the dealloyed Pd–Ni–P electrode in alkaline media [27]. To determine the EOR products, ATR-FTIR with the Otto configuration [1] was also tested by pushing a practical catalyst electrode against Ge or ZnSe ATR prism, able to identify spectrally the products in the thin-layer solution (and thus the selectivity) out of different catalysts. Unfortunately, no surface signals were detected [28–32].
Extension and development of ATR-IR technique To strengthen the power of electrochemical ATR-IR technique for broader applications, this technique per se has undergone extension and development. This may include early extensive efforts of wet-fabrication of varieties of metallic films on ATR Si prism [33] as well as the design of a well-defined flow cell capable of simultaneous measurement of ATR-FTIR and online-DEMS [34]. Interestingly, a rotating dual-electrode in situ ATRIR setup was recently developed, enabling IR spectroscopic access to both the anodic and the cathodic reactions of a direct methanol fuel cell during mimicked cell operation [35]. Extension of in situ ATR-FTIR to 2–5 monolayer graphene electrode on Si ATR element was also reported using ORR as the probe reaction [36• ]. The excellent stability and conductive nature of the graphene film promises itself a good support of nanocatalysts for desired electrocatalytic reactions. Notably, coupling ATR-IR simultaneously with onlineDEMS or even separately with IRAS [2,3,37] enables to obtain richer information about the reaction intermediates on surface and products in the solution, in recognition of that the latter two techniques favor the detection of the solution species, as compared to the ATR-IR. Recently a wall-jet electrode for a controlled mass transport in an ATR-IR cell was designed to study the oxygen reduction reaction on Pt nanocatalysts with different anions [38,39• ]. Similar wall-jet design of ATR-IR cell but with multiple internal reflections was also reported for the study of formic acid oxidation on Pt/C [40]. A breakthrough toward the development of this technique is the reported combination of spectroelectrochemistry and femtosecond 2D IR spectroscopy involving the use of ultrathin (∼nm) conductive layers of Pt and indium–tin oxide (ITO) as working electrodes on a singlereflection ATR element, which enables to detect vibrational relaxation and spectral diffusion of v(COad ) as a function of the applied potential at a time resolution of picosecond [41• ].
Concluding remarks The ATR-IR has been widely applied to study interfacial electrochemistry of broad interest, ranging from potential-induced interfacial structure change, mechanistic understanding of electrocatalytic oxidation of SOMs, reduction of CO2 and O2 and decomposition of solvent www.sciencedirect.com
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Figure 3
(a) Schematic illustration of the performance comparison between Pd/C and Pd-B/C; ATR-IR spectra of aqueous CO2 bands (b) and full range results during formic acid oxidation on Pd/C (c) and Pd-B/C (d), respectively. Adapted from reference [26• ].
in Li–O2 batteries to spectral correlation with properties of catalytic materials. Despite its obvious advantages and power, ATR-IR should be often coupled with one or more of other techniques including but not limited to electrochemical methods, DFT calculation, isotope labeling, IRAS and DEMS for comprehensive investigations. More ATR-IR applications should be directed to the reviving hot areas such as CO2 reduction and SOMs oxidation on efficient electrocatalysts, and the solvent degradation and the structural stability of solid electrolyte interface (SEI) of lithium-based batteries. Meanwhile, the ATR-IR technique should be further developed to meet the quest of ultrafast interfacial dynamics, such as transfer of redoxactive samples from bulk solution to organic monolayers and potential-dependent orientational dynamics, and to meet the operando monitoring of practical electrochemical interfaces with capabilities of tracing slow interfacial processes spanning from minutes to hours under the influences of external electricity, light and/or heat.
Acknowledgments This work is supported by the 973 Program (no. 2015CB932303) of MOST and NSFC (nos. 21473039 and 21273046).
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