Characterization of Electrocatalytic Water Splitting and CO2 Reduction

Oct 24, 2017 - Solar Energy Research Institute of Singapore, National University of Singapore, 7 Engineering Drive 1, Singapore 117574. ACS Catal. , 2...
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Perspective Cite This: ACS Catal. 2017, 7, 7873-7889

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Characterization of Electrocatalytic Water Splitting and CO2 Reduction Reactions Using In Situ/Operando Raman Spectroscopy Yilin Deng† and Boon Siang Yeo*,†,‡ †

Department of Chemistry, Faculty of Science, National University of Singapore, 3 Science Drive 3, Singapore 117543 Solar Energy Research Institute of Singapore, National University of Singapore, 7 Engineering Drive 1, Singapore 117574

‡

ABSTRACT: The electrocatalytic reduction of CO2 and water splitting have received significant attention because recycling CO2 into fuels and chemical feedstock is a crucial step to close the anthropogenic carbon circle, whereas splitting water produces H2 gas, which is a valuable carbon-free energy carrier. The clear identification of the catalytic-active sites and elucidation of the reaction mechanisms in these systems remain a grand challenge. It requires simultaneous characterizations of the catalysts under actual reaction conditions. Raman spectroscopy is among the handful of techniques that are suitable for the in situ/operando investigations of heterogeneous catalytic systems. This Perspective will highlight primarily recent works on the application of Raman spectroscopy in unraveling the structural changes of catalysts, their possible active sites, and the intermediates formed during water electrolysis and CO2 electroreduction. Results from complementary techniques such as X-ray absorption spectroscopy, among others, will also be presented in order to provide a more holistic discussion. The outlook for future work is discussed. KEYWORDS: operando, Raman spectroscopy, electrochemistry, oxygen evolution reaction, hydrogen evolution reaction, CO2 reduction reaction

1. INTRODUCTION Using solar-generated electricity to convert carbon dioxide (CO2) into alcohols and hydrocarbons, as well as to reduce water into hydrogen gas, is a green and sustainable way to reduce the amount of atmospheric CO2 and to produce important chemicals.1,2 Three major reactions are involved, namely, the hydrogen evolution reaction (HER), CO 2 reduction reaction (CO2RR), and oxygen evolution reaction (OER). Application of these systems in the industrial scale has been hindered by poor selectivities (for some CO2 reduction reactions), low production rates, and poor energetic efficiencies of the reactions. Some of the best catalysts for these reactions are also rare and expensive metals, for example, platinum for the HER.3 To overcome these shortcomings and pave the way toward the development of better catalysts, a proper mechanistic understanding of the reaction pathways and identification of reactive sites are indispensable. Classical electrochemical methods, for example, use of Tafel slopes, usually provide indirect and incomplete information about the reaction kinetics and mechanism of an electrochemical system.4 To obtain a clearer picture of the system of interest, operando measurements, which refers to the simultaneous characterizations of both the physical−chemical properties and activities of the catalysts under actual reaction conditions, are also required.5 A combination of these methods, alongside quantum chemical simulations, will enable a more direct establishment © XXXX American Chemical Society

between the catalytic performances and various qualities of the catalysts. Operando spectroscopy involves making real-time measurements with techniques such as nuclear magnetic resonance (NMR), electron paramagnetic resonance (EPR), Mössbauer, infrared (IR), Raman, X-ray absorption spectroscopy (XAS), and so on. Among these methods, Raman spectroscopy has been widely employed to characterize almost all types of catalysts, from bulk or supported metal particles and their oxides and sulfides, to molecular catalysts solubilized in liquids.6−10 Raman spectroscopy is especially useful for operando investigations in heterogeneous catalytic systems, because it is able to provide fingerprinting-type information about the molecular structures of the catalysts as well as reaction intermediates under reactive conditions.11,12 Raman spectroscopy is also sensitive to vibrations involving polarizations changes rather than dipole moment changes and therefore shows a low scattering cross section for water.13,14 This makes it particularly applicable toward electrochemical systems performed in aqueous media. Over the years, the combination of Raman spectroscopy and electrochemistry has Received: August 1, 2017 Revised: October 3, 2017

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ACS Catalysis led to a significant molecular level understanding of electrocatalytic systems. This work will demonstrate selected applications of in situ/ operando Raman spectroscopy in electrochemical water splitting and CO2 reduction systems. We highlight especially its potential in (1) probing the structural/compositional changes of catalyst surfaces, especially for metal oxide surfaces, (2) identification of surface-adsorbed molecules or intermediates, and (3) determination of catalytically active sites. Comparisons between operando Raman spectroscopy and other techniques, such as XAS, among others, will also be provided.

2. IN SITU/OPERANDO RAMAN SPECTROSCOPY SYSTEM Raman spectroscopy can be used to probe the vibrational, rotational, and other low-frequency modes of a material.13 The incident photons interact with the sample (for example, an organic molecule) which might cause an energy exchange between them (inelastic scattering). By measuring the energy differences between the incident and the scattered photons, information about the vibrational modes of the system, its “fingerprints”, could be obtained. Electrochemistry deals with chemical processes that involve electron transfer.15 By measuring the electric charge transfer through the system, knowledge of the electrochemical reactions could be obtained. A combination of Raman spectroscopy and electrochemistry could therefore provide real-time spectroscopic information on how an electrochemically driven reaction is evolving. An operando electrochemical/Raman setup consists of three main components: a potentiostat, an electrochemical/Raman cell, and a Raman spectrometer. A key challenge for operando Raman measurements is to increase the spectroscopic limits of detection, while not distorting the electrochemical responses.16 The type of electrochemical/Raman cells used has a big influence on whether this target can be achieved.16−19 The first generation of electrochemical/Raman cell was reported by Fleischmann et al. in 1974 (Figure 1a).20 A thick optical window and an ultrathin layer of electrolyte were used in their setup. This cell design, however, does not fit well with the new generation of confocal Raman microscopy systems. Ren et al. developed a cell made of Teflon or polychlorotrifluoroethylene, with the working electrode facing upward.17,18 In their design, a thin layer of electrolyte (ca. 0.20 mm), a quartz window (ca. 1.0 mm in thickness), and air were between the working electrode and the microscope objective (Figure 1b(i)).16,18 However, the mismatch between the refractive indices of the air, quartz, and electrolyte would significantly diminish the overall detection sensitivity of the system. In addition, the ultrathin layer of electrolyte would affect/distort the electrochemical responses. To solve these problems, Zeng et al. designed a new setup that employed a water-immersion Raman objective with a long working distance of 2.8 mm (Figure 1b(ii)).16 A drop of water placed in between the objective and the quartz window helped to reduce the attenuation of signals. A 2.0 mm thick electrolyte layer was used. This helped to avoid the serious diffusion hindrance problem: the methanol oxidation peak recorded in the cyclic voltammogram of a Pt electrode in a 2.0 mm thick 1 M methanol +0.1 M H2SO4 electrolyte was similar to that obtained in bulk electrolyte. However, we note that in the design of Ren et al., the presence of the quartz window could still affect negatively the detection limits of the setup.

Figure 1. Schematic diagram of the electrochemical/Raman spectroscopy setup of (a) Fleischmann et al. (reprinted with permission from ref 20. Copyright 1974 Elsevier B.V.); and (b) Zeng et al. (i) An electrochemical/Raman setup using an air objective and (ii) a newly designed setup using a water immersion objective. CE: Counter electrode, WE: Working electrode and RE: reference electrode (reprinted with permission from ref 16. Copyright 2016 American Chemical Society).

In 2010, the group of Bell and more recently our group employed a 13 μm thick Teflon film to wrap and protect the water-immersion objective instead of using the quartz window (Figure 2).19,21 Our cell is a custom-made round Teflon dish. The working electrode, typically a disc, is sheathed in Teflon

Figure 2. Schematic diagram of an electrochemical/Raman spectroscopy setup used by our group. 7874

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ACS Catalysis fitting with its top exposed to the electrolyte. The optically transparent Teflon film, which has almost the same refractive index as water (n = 1.33), circumvented the problem of mismatch in refractive indices of the protective film and the electrolyte. An objective with a high numerical aperture (N.A. = 1.10) was used. These features allowed the laser light to be focused very tightly to a spot (with a diameter of a few hundred nanometers), and also significantly increased signal collection. The major drawback of this system, though, is the short distance between the lens of the working objective and the working electrode. Bubbles formed during a gas evolving reaction could be trapped in this gap and attenuate the Raman signals. The type of reference and counter electrodes used depends on the conditions of the electrochemical experiments being performed. For example, a Hg/HgO reference can be employed for the oxygen evolution reaction in alkaline electrolyte; either a Hg/HgSO4 or a Hg2Cl2/Hg reference can be used for the hydrogen evolution reaction under acidic conditions; a Ag/AgCl reference can be usually used for CO2 reduction which is typically performed in near-neutral pH electrolytes such as 0.1 M KHCO3 solution. The counter electrode can be a platinum wire or graphite rod. Carbon, rather than Pt counter electrodes, are preferred for HER.22 However, the former could corrode during oxygen evolution (the typical counter reaction for HER and CO2RR) to give amorphous carbon.23,24 Carbon, which typically has a very large Raman scattering cross section, could then deposit on the cathode.25 Its signals would easily overwhelm those of the analytes. In our laboratory, Raman spectra are acquired using a confocal Raman microscopy system that consists of an epiillumination microscope joined to a spectrometer with a charge coupled device (CCD) detector.21 The excitation wavelength used may depend on the material of the working electrode. For samples such as roughened Au and Ag electrodes, visible light lasers such as the HeNe 633 nm and the Ar+ 514 nm could be used to achieve the surface-enhanced Raman spectroscopy (SERS) effect.26 During the operando Raman measurements, the backscattered light collected by the water-immersion objective is filtered through an edge (or notch) filter and directed into a spectrograph/CCD for spectral readout.

intermediates, OH*, O*, and OOH*, has been proposed by Man et al:27 M + OH− → M−OH + e− (in alkali; M represents the active site)

M−OH + OH− → M−O + e− + H 2O M−O + OH− → M−OOH + e−

M−OOH + OH− → M−OO + e− + H 2O

M−OO → M + O2

Determination of the catalytic-active centers (M) remains an unresolved question on many metal oxide catalysts. It has been suggested that metal oxide centers with high valence states are responsible for the catalysis of OER.29,32 Another proposed pathway involves the formation and decomposition of a highvalence metal oxide to release O2 gas:29 MZ + OH− → MZ−OH + e− (Z represents the valence of the active site)

MZ−OH → MZ + 1−OH + e−

2MZ + 1−OH + 2OH− → 2MZ + 2H 2O + O2

In this section, we focus on examples that illustrate the determination of molecular structures of the catalysts, monitoring of the structural changes of metal oxides during OER, and identification of OER intermediates. 3.1. Iridium Oxide Catalysts. Iridium oxide has been regarded as a benchmarking catalyst for the oxygen evolution reaction,28 and has been studied using X-ray absorption spectroscopy (XAS).33 The X-ray absorption near-edge structure (XANES) of XAS provides insight into the electronic structure and symmetry of the metal sites, whereas the extended X-ray absorption fine structure (EXAFS) explores their coordination environment.34 Through in situ XANES measurements, Minguzzi et al. found that Ir3+ and Ir5+ coexisted in an electrodeposited iridium oxide film held at >1.3 V vs RHE in aqueous 0.5 M H2SO4 electrolyte.33 On this basis, the authors proposed a cycling mechanism for the OER on the catalyst, involving an Ir3+ → Ir5+ oxidation process and a consequent Ir5+ → Ir3+ reduction process that produced O2 gas. Mo et al. also examined, via XAS, the structural changes of iridium oxide in aqueous 0.5 M H2SO4 electrolyte.35 On the basis of the very similar average Ir−O distances measured on the catalyst at 0.8 V vs SCE to that measured on crystalline IrO2, the authors proposed that the catalyst was best represented as IrO2 at 0.8 V vs SCE. The authors also performed in situ SERS on iridium oxide catalyst deposited on roughened gold substrates, and identified crystalline IrO2 (based on the observation of a combination of A1g and B2g phonon modes at ∼744 cm−1) at 0.85 V vs SCE. The structure of the iridium oxide catalyst under OER conditions was not probed. Recently, Pavlovic et al. performed a systematic in situ Raman spectroscopy study to investigate the structural changes of anodically grown iridium oxide films at OER-relevant potentials in aqueous 0.5 M H2SO4.36 Density functional theory (DFT) calculations were also employed for the interpretation of the Raman results. The authors recorded and interpreted the structural changes of the iridium oxide

3. APPLICATION OF OPERANDO RAMAN SPECTROSCOPY IN THE OXYGEN EVOLUTION REACTION The electrochemical oxygen evolution reaction (4OH− → O2 + 2H2O + 4e−, in alkali), which typically requires a significant overpotential to proceed at reasonable rates, is a limiting factor for the efficient splitting of water into hydrogen and oxygen gas.27 To date, the best electrocatalysts for the OER are Ru and Ir oxides.28 However, these still require at least a few hundred millivolts to drive the OER to a current density of 10 mA/ cm2.29 Moreover, these two materials suffer from corrosion and dissolution during the OER process.30,31 The scarcity of Ru and Ir also render their large-scale application in various industries impractical. Hence, there have been numerous efforts to develop high-performance and stable OER catalysts from earthabundant materials. A fundamental understanding of the OER reaction mechanisms would be crucial to achieve this target. The four-electron transfer process for O2 evolution has been proposed to occur through different pathways. For example, a widely accepted OER mechanism which involves three surface 7875

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Figure 3. Schematic diagram depicting the transformation of anodically grown iridium oxide films over the potential range from 0 V vs RHE to the OER potential regime in aqueous 0.5 M H2SO4. The top bar shows the color changes of the catalyst as a function of the applied potential (reprinted with permission from ref 36. Copyright 2016 American Chemical Society).

Figure 4. (a(i)) In situ SER spectra of a Au electrode in 1 M HClO4 electrolyte during a linear sweep voltammetry scan from 1.0 to 1.65 V vs Ag/ AgCl (scan rate: 2 mVs−1). (a(ii)) A zoomed-in view of the spectra recorded at 1.0 and 1.4 V vs Ag/AgCl. The accompanying cartoon shows a representation of the surface-bound Au-OOH. (a(iii)) The linear sweep voltammogram of Au electrode recorded in 1 M HClO4 at 2 mV/s (reprinted with permission from ref 19. Copyright 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim). (b(i)) Linear sweep voltammogram of a polycrystalline gold electrode in 1.0 M HClO4 at 1 mV/s. The inset shows the current density below 2.0 V that was multiplied by a factor of 1000. (b(ii)) In situ SER spectra acquired on the gold electrode at various potentials. Reference electrode: RHE (adapted and reprinted with permission from ref 40. Copyright 2013 Royal Society of Chemistry).

0.8 V vs RHE, the amount of Ir4+ increased and the amount of Ir3+ decreased, as revealed by the increase in the intensities of the Raman peaks (∼474 cm−1 and ∼690 cm−1 at 1.2 V vs RHE) that are linked to the Ir4+ and the attenuation of the peak (∼600 cm−1 at 1.2 V vs RHE) that is linked to Ir3+. Once all the iridium atoms reached +4 oxidation state, rutile-IrO2 began to

catalyst as a function of the applied potential (Figure 3). At potentials from 0.8 to 1.2 V vs RHE, the authors considered the catalyst as hydrated edge-sharing IrO6 polyhedra. The octahetra units were connected by μ-oxo oxygen bridges. Both Ir3+ and Ir4+ existed in the catalyst, and some of the bridge oxygen atoms were protonated. As the potential increased positively to 7876

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the μ-OO peroxide. This discovery contributes to a more comprehensive understanding of the structural changes of cobalt oxide during its oxidation. Yeo et al. examined the electrochemical behaviors of Co oxides on different metal substrates in 0.1 M KOH electrolytes.42 On a bulk Co electrode, Raman peaks at 197, 485, 620, 691, and 505 cm−1 were observed at 0 V vs Hg/HgO (prior to OER, Figure 5a). The first four aforementioned peaks were

precipitate, as indicated by the appearance of its band at ∼550 cm−1. The catalyst did not undergo significant structural changes with further increased potentials. Iridium centers with higher +6 and +8 oxidation states were also not spectroscopically observed. The authors suggested that μ-oxo connected [IrO6]n clusters with Ir4+ centers were responsible for the catalysis of the OER, while IrO2 was a side product that led to the deactivation of the catalyst at >1.2 V vs RHE. 3.2. Gold Electrodes. Yeo et al. have focused on the identification of OER intermediates on a roughened Au anode using Raman spectroscopy.19 Roughened gold is a SERS-active substrate. The Raman spectra collected on a Au electrode during a potentiodynamic scan from 1.0 to 1.65 V vs Ag/AgCl in 1 M HClO4 are presented in Figure 4a. At ≥ 1.4 V vs Ag/ AgCl (∼1.6 V vs RHE), where a rise in current was observed, a Raman peak was recorded at ∼820 cm−1. The intensity of the ∼820 cm−1 band increased as more anodic potentials were applied. This feature was assigned to the ν(O−O) stretching vibration of hydroperoxy (M-OOH) in a Au-OOH species.37−39 The Raman spectra collected using deuterated electrolytes further corroborated this assignment. On the basis of a rise in current in the voltammogram as well as the Raman spectroscopic data, it was proposed by Yeo et al. that surface-bound hydroxyperoxy (Au-OOH) species are likely reaction intermediates of oxygen evolution occurring on the Au electrode. This, in turn, supports the reaction mechanism proposed by Man et al. that involves the OOH* intermediate.27 The Raman band at ∼820 cm−1 on Au was also reported subsequently by Diaz-Morales et al. (Figure 4b).40 Isotopic labeling using 18O in lieu of 16O confirmed the assignment of the band to an O−O stretching vibration of Au-OOH. However, through the use of online electrochemical mass spectrometry, Diaz-Morales et al. reported that O2 first evolved at approximately 2.0 V vs RHE, a potential higher than that proposed by Yeo et al. The Au-OOH was thus assigned instead to a species presumably incorporated into a highly disordered surface gold oxide rather than a surface-bound OER intermediate. This was further supported by density functional theory calculations, which suggested that AuOOH formed at 1.28 V vs RHE, whereas oxygen evolution only started at 1.95 V vs RHE. An oxide decomposition or oxide disproportionation pathway for O2 evolution was instead suggested. Thus, through a combination of operando/on line techniques as well as theoretical calculations, the identification and role of Au-OOH on Au electrodes in the electrochemical oxidation of water could be determined more clearly. However, further experiments still have to be pursued to clarify the exact mechanism of the OER on Au electrodes. 3.3. Cobalt Oxide Catalysts. The oxides of cobalt and their derivatives are promising electrocatalysts for the oxygen evolution reaction. For example, electrodeposited CoOx required an overpotential of ∼400 mV to catalyze 10 mA/ cm2 of O2 evolution current in 1 M NaOH electrolyte.31 Wang et al. have studied the structural changes of the Co3O4 catalyst prior to O2 evolution in KOH electrolytes, using a combination of in situ Raman spectroscopy, X-ray absorption spectroscopy, and grazing-angle X-ray diffraction.41 They observed a coincidence between the appearance of a Raman peak at 931 cm−1 and the rise of an anodic peak at ∼1.8 V vs Zn/ Zn(OH)42− (1.36 V vs RHE) in 1 M KOH electrolyte. The Raman peak at 931 cm−1 was assigned to the vibration of μ-OO peroxide (Co-OO-Co) species. The tetrahedral Co2+ ions in the spinel Co3O4 were proposed to be vital for the formation of

Figure 5. A sequence of Raman spectra measured on (a) bulk Co, (b) Au, (c) ∼ 0.4 ML cobalt oxide deposited on Au, and (d) ∼ 87 ML cobalt oxide deposited on Au. The spectra were collected during linear sweep voltammetry scans at 2 mV/s in 0.1 M KOH electrolyte. Potentials are reported vs Hg/HgO (in 0.1 M KOH) (reprinted with permission from ref 42. Copyright 2011 American Chemical Society).

assigned to Co3O4, while the last was assigned to CoO(OH). As the potential increased to 0.7 V vs Hg/HgO, the peaks from Co3O4 attenuated while new peaks from CoO(OH) at 503 and 565 cm−1 showed up, indicating the dominant presence of a CoO(OH) phase at O2-evolving potentials. A bulk Au electrode was also studied as reference, and upon oxidation, gave only a single Au oxide band at 559−574 cm−1 (Figure 5b). Interestingly, for a 0.4 ML (ML: monolayer) cobalt oxide deposited on a Au substrate, a sharp Raman peak at 609 cm−1 was recorded at 0 V vs Hg/HgO, which gradually shifted to 579 cm−1 as the potential increased to 0.7 V vs Hg/HgO (Figure 5c). The authors assigned this peak to the CoOx species, and the decrease in its wavenumber was attributed to the progressive oxidation of CoII and CoIII centers to CoIV.43,44 Thick layers (∼87 ML) of Co oxide deposited on Au gave the same Raman spectra as that of a bulk Co electrode (Figure 5d). This suggests that the spectroscopic change observed for the 0.4 ML CoOx/Au could be related to the Co atoms being perturbed by Au. It is notable that the OER activities of the cobalt oxide deposited on top of the Au surface was significantly enhanced: the OER turnover frequency exhibited 7877

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ACS Catalysis by the ∼0.4 ML cobalt oxide on Au substrate was 40 times higher than that of bulk Co and nearly 3 times higher than that of bulk Ir. By correlating the OER performances of the catalysts to their in situ Raman spectra, the authors concluded that the enhanced activity of cobalt oxide on Au was due to a larger population of CoIV active centers as a result of enhanced oxidation of cobalt oxide mediated by the electronegative Au support. The OER catalytic behaviors of cobalt oxides have also been studied by Risch et al. using operando X-ray absorption spectroscopy (XAS).45 Alongside XAS, UV−vis-absorption and time-resolved mass spectrometry were used to track the OER reaction kinetics and products on amorphous cobalt-based oxides. Two redox transitions at potentials of 1.0 V (Co II 0.4 Co III 0.6 ↔ all-Co III ) and 1.2 V (all-Co III ↔ CoIII0.8CoIV0.2) vs NHE (normal hydrogen electrode) at pH 7 were recorded. The authors proposed that cobalt ions at the margins of Co-oxo fragments (Cox(μ-O)y(μ−OH)z) underwent Co II ↔ Co III ↔ Co IV oxidative state changes, accompanied by structural changes and deprotonation of Cooxo bridges. A proposed circle of interconversion of three structural motifs that may be involved in OER catalysis is shown in Figure 6. It is proposed that the active site for O−O bond formation is created by the encounter of two or more CoIV ions.

surfaces with different oxidation states.21 The samples studied were (1) metallic Cu, (2) Cu2O film deposited on Cu substrate (Cu2O/Cu), (3) Cu(OH)2 film on Cu (Cu(OH)2/Cu) and (4) CuO film on Cu substrate (CuO/Cu). It was found that the electrochemical behaviors of the four samples were different during cyclic voltammetry (CV) and direct chronoamperometry measurements at O2-evolving potentials in 0.1 M KOH electrolyte. During CV (anodic sweep), all the samples converted to CuO and Cu(OH)2 species, as revealed by their marker bands at 298/347 and 490 cm−1 (Figure 7a).53−56 Hence, it was expected that all the four samples showed similar OER performances. A metastable Raman peak at 603 cm−1 appeared in the spectra of all the four samples at O2-evolving potentials, ≥ 1.62 V vs RHE. The 603 cm−1 peak was identified as the marker of a CuIII oxide, since this frequency matched well with that of a NaCuIIIO2 standard. The formation of CuIII oxide was further confirmed by in situ X-ray near-edge structure spectroscopy (XANES) measurements. In contrast, if the freshly prepared samples were subjected directly to chronoamperometry at 1.7 V vs RHE, Cu and Cu2O/Cu did not oxidize to CuII species and they showed 10 times lower OER current densities compared to CuO/Cu and Cu(OH)2/Cu samples (Figure 7b). The 603 cm−1 peak of CuIII oxide was also absent on Cu and Cu2O/Cu but could be observed on Cu(OH)2/Cu and CuO/Cu during the direct chronoamperometry. By correlating the presence of the CuIII oxide peak and the OER activities of different Cu oxides, we proposed CuIII oxides as catalytically active species for OER, with CuII oxides/ hydroxides as their precursors. However, due to our limited knowledge about the structure, the coordination environment, and temporal behavior of the CuIII species, its precise role for the OER is still unclear. We suggest that DFT simulations of the OER on CuIII oxides and time-resolved techniques would be helpful. 3.5. Nickel Oxide and Ni−Fe Oxide Catalysts. Some Nibased materials have been reported to exhibit OER activities that rivaled that of Ru and Ir oxides.31 It has been proposed that the activity of NiOOH can be enhanced through either its conversion from the γ-NiOOH to the OER-active β-NiOOH phase via an aging process in base, as illustrated in the Bode scheme (Figure 8); or by addition of Fe into the NiOOH.57−60 It was not until 2014 that Trotochaud et al. showed that previously reported increased activity of β-NiOOH was actually due to the incorporation of Fe impurities from the unpurified KOH electrolytes into the Ni(OH)2/NiOOH structures.61 This result disproved the view that β-NiOOH was intrinsically more active than γ-NiOOH and had significant implications for the study of nickel−iron oxides for OER catalysis. Numerous efforts have since been devoted to the elucidation of the catalytic sites that contribute to the superior OER activities in nickel−iron (oxy)hydroxides.58,62−65 Bell and co-workers first found that incorporation of Fe into the Ni film increased the potential of the Ni(OH)2/NiOOH redox reaction and decreased the overall valence of the Ni in the catalyst at OER-relevant potentials.66 In situ Raman spectra revealed that the relative intensities between the two characteristic 474 and 554 cm−1 bands of NiOOH changed with Fe content, which the authors attribute to changes in the local environment of Ni−O. Similar changes were observed on aged Ni films, and therefore, the authors proposed that Ni sites in the Ni−Fe films catalyzed the OER. After the proposition that the enhanced OER activity of an aged Ni film was due to Fe impurities was raised, the group of Bell revisited the effects of

Figure 6. Proposed interconversion of Co-oxo species in the cobalt oxide catalyst, which leads to the evolution of oxygen. Potentials are reported vs NHE (reprinted with permission from ref 45. Copyright 2015 Royal Society of Chemistry).

Through the above examples, we show that XAS and Raman spectroscopy can provide insightful information on the structural changes of cobalt oxide during OER. We do note that some quantitative information, e.g., the ratio of metal ions with different oxidation states, could only be measured by XAS. In addition, although the structural changes and intermediates on the catalyst under reaction conditions could be detected using in situ Raman spectroscopy and XAS, their temporal behavior was not easily determined. We suggest that this information could be obtained using time-resolved techniques such as time-resolved IR spectroscopy. 3.4. Copper and Copper Oxide Surfaces. Recently, monometallic copper catalysts have been investigated and considered promising for the OER.46−52 Our group performed a systematic Raman spectroscopic study of the OER on copper 7878

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Figure 7. a(i) Cyclic voltammograms of Cu, Cu2O/Cu, Cu(OH)2/Cu,and CuO/Cu samples in 0.1 M KOH electrolyte at 1 mV/s scan rate. a(ii) Chronoamperograms of the four post-CV samples at 1.7 V. a(iii-vi) In situ Raman spectra of Cu (the inset shows a zoomed-in of the spectrum taken at 0.72 V), Cu2O/Cu, Cu(OH)2/Cu, and CuO/Cu, respectively, during the CV measurements shown in a(i). b(i) Chronoamperograms of freshly prepared Cu, Cu2O/Cu, Cu(OH)2/Cu, and CuO/Cu at 1.7 V in 0.1 M KOH. b(ii-v) In situ Raman spectra collected respectively from Cu, Cu2O/ Cu, Cu(OH)2/Cu, and CuO/Cu, before, during and after the chronoamperometry measurements shown in b(i). The signal marked with an asterisk in b(v) originates from a cosmic ray. Potentials are reported vs RHE (adapted and reprinted with permission from ref 21. Copyright 2016 American Chemical Society).

Fe in Ni catalysts.58 The authors found that Ni films aged in unpurified KOH electrolyte could incorporate 23−26% Fe after 38 days and showed OER performances comparable to optimized Ni1−xFexOOH catalysts, while those aged in purified electrolyte exhibited much lower activities for OER. On the basis of the broader ∼476 and ∼560 cm−1 peaks, as well as the lower 476/560 cm−1 ratio of the Ni films aged in unpurified KOH compared to that of pure NiOOH, the authors suggested the formation of Ni−Fe layered double (oxy)hydroxide (LDH), which was produced by substitution of Fe into Ni(OH)2/NiOOH lattice (Figure 9a). This NiFe-LDH was further proposed to be critical for high OER activity with the Fe centers being shown, through turnover frequency calculations, to be the active sites. Trześniewski et al. performed a systematic study on Fecontaining nickel borate and nickel oxyhydroxide at different pHs.65 Roughened Au substrates were employed to achieve the SERS effect. Borate anions did not play an apparent role for OER catalysis at pH 13. SERS peaks at 900−1150 cm−1 were recorded at pH 13 on both samples but were absent at a

Figure 8. Bode scheme for Ni(OH)2/NiOOH transformations (reprinted with permission from ref 58. Copyright 2015 American Chemical Society).

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Figure 9. (a) In situ Raman spectra collected from Ni(OH)2/NiOOH films on roughened Au substrates in (i) Fe-free and (ii) unpurified 0.1 M KOH electrolytes on Day 0 and Day 6. Reference electrode: Hg/HgO. The equilibrium potential for O2 evolution is 0.365 V vs Hg/HgO in 0.1 M KOH electrolyte (reprinted with permission from ref 58. Copyright 2015 American Chemical Society). (b) In situ SER spectra of freshly prepared Ni(Fe)(OH)2 acquired at constant potentials in the potential range 1.0−1.9 V vs RHE in (i) 0.1 M KOH pH 13, (ii) 0.5 M K−Bi (potassium borate) pH 9.2, and (iii) 0.1 M KOH + 0.01 M H3BO3 (reprinted with permission from ref 65. Copyright 2015 American Chemical Society).

group of Bell, through operando X-ray absorption spectroscopy (XAS) with high energy resolution fluorescence detection (HERFD), found that Fe3+ in mixed (Ni, Fe) oxyhydroxides (Ni1−xFexOOH) occupied octahedral sites. The Fe−O bond distance was contracted compared to pure γ-FeOOH, which was imposed through edge-sharing with surrounding [NiO6] octahedra.63 The Fe cations were found remaining in the +3 state even during the OER catalysis. Density functional theory with Hubbard U (DFT + U) calculations showed that the Fe sites in Ni1−xFexOOH exhibited near-optimal adsorption of OER intermediates and hence are responsible for the low OER overpotentials. Ni sites were suggested inactive for water oxidation. The group of Strasser also employed operando XAS and differential electrochemical mass spectrometry (DEMS) to study the catalytic behaviors of mixed Ni−Fe oxides.62 These methods revealed that 75% Ni centers in Fe-free Ni-catalyst increased in their oxidation state from +2 to +3, and up to 25% Ni reached +4 oxidation state during OER. However, for the mixed NiFe catalysts with >9 atomic % of Fe, most of the Ni

moderate alkaline pH 9.2 (Figure 9b). The peaks at 900−1150 cm−1 were attributed to NiOO−, which was formed via a deprotonation process in strongly alkaline electrolytes. The authors linked the OER activities of the two Ni-based catalysts with the generation of the NiOO− sites. The OER activities of both catalysts were dependent on the pHs, which could be due to the deprotonation of the Ni-based catalysts at high pHs and hence the formation of the NiOO− sites that acted as OERcatalytic centers. Thus, in addition to the key role of the Fe impurities, the authors suggested that the pH of the electrolyte was also crucial to obtain catalytically active phases, which were proposed to be NiOO− in their work. Extensive information about the Ni sites could be obtained in the aforementioned in situ/operando Raman studies. These works, however, could not identify the local structure of the Fe sites under reactive conditions, because of the lack of characteristic Fe−O Raman signals. This information could be determined using other techniques such as XAS and Mössbauer spectroscopy, which are element-specific.62−64 The 7880

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Figure 10. Possible pathways for the electroreduction of CO2 to different products on transition metals and molecular catalysts: (a) pathways of CO2 reduced to CO, CH4 (blue arrows), methanol (black arrows), and formate (orange arrows); (b) pathways from CO2 to ethylene (gray arrows) and ethanol (green arrows); (c) pathway of CO2 insertion into a metal−H bond that yields formate (purple arrows). Species in red are reactants or products in solution, while those in black are adsorbates. RDS represents for rate-determining steps and (H++e−) indicates steps in which either separated or concerted proton−electron transfer process occurs. Potentials are reported vs RHE (reprinted with permission from ref 72. Copyright 2015 American Chemical Society).

peak to a certain Ni oxide/hydroxide species may not be totally reliable, especially when data from the appropriate standard compounds is not available. We also note that the valence state of the Fe sites measured by XAS was different from that elucidated by Mössbauer spectroscopy.63,64 Our knowledge about the structures and changes of the Ni−Fe catalyst surface during OER is thus still incomplete, and further studies are required to unravel the OER processes on Ni−Fe catalysts.

centers remained in +2 oxidation state under OER reaction conditions. The Fe cations persisted in the +3 valence state regardless of the catalyst composition and potential. On the basis of these findings, the authors proposed that Ni was the active site and there existed a kinetic competition between the (i) Ni2+ → Ni4+ oxidation process and the (ii) Ni4+ → Ni2+ reduction process that released O2. In the mixed NiFe-catalysts, step (ii) was proposed to outweigh step (i), and therefore, a low +2 oxidation state was measured for Ni centers, together with the significantly enhanced OER activity of NiFe catalysts compared to pure Ni catalysts. Another study by Chen et al. on the NiFe catalysts using Mössbauer spectroscopy recorded a different behavior of the Fe centers: up to 21% Fe3+ centers transformed to Fe4+.64 Fe4+, in contrast, was absent in pure Fe oxides during OER. Although strong evidence of the existence of Fe4+ in NiFe oxides/hydroxides during the OER was shown, this study did not indicate the catalytic role of Fe4+ on O2 evolution. Further investigations are required to elucidate the role of Fe4+ for OER. Overall, excellent progress has been achieved in the elucidation of the OER mechanism on Ni−Fe catalysts. However, we would like to highlight that for the aforementioned Raman studies, assignment of a newly discovered Raman

4. APPLICATIONS OF IN SITU/OPERANDO RAMAN SPECTROSCOPY IN CO2 ELECTROREDUCTION Depending on the catalysts and experimental conditions, the electrochemical reduction of CO2 has yielded many useful products such as CO, formate, multicarbon hydrocarbons, and alcohols.67 Zn, Ag, and Au show a good selectivity for CO production, while post-transition metals Pb, Sn, In, and Tl exhibit a high selectivity for formate (HCOO−).67 To date, Cu is the only metal catalyst that can reduce CO2 to significant amounts of hydrocarbons and alcohols.68−71 On the basis of experimental observations and density functional theory calculation results, Koper and co-workers proposed a comprehensive mechanism for CO2 reduction on Cu electrodes (Figure 10).72 CO2 could first be chemisorbed, then reduced to 7881

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Figure 11. Operando Raman spectroscopy of SnO2 catalysts at varied potentials and pHs. (a) Operando Raman spectra of SnO2 catalysts as a function of the applied potential for each studied pH. (b) The relative intensities of the SnIV-related A1g Raman peaks (○, solid line) and the Faradaic efficiencies for formate production (×, dashed line) recorded from the SnO2 samples as a function of the applied potential. The three distinct potential regions represented by the shaded background indicated three forms of the catalyst: (I) fully oxidized SnO2, (II) a partially reduced compound of mixed oxidation state, and (III) completely reduced metallic Sn, as illustrated by the scheme shown in (c) (reprinted with permission from ref 76. Copyright 2015 American Chemical Society).

adsorbed formate intermediates (HCOO*), which finally desorb as formate ions. On the other hand, the *COOH intermediate, which is adsorbed via its C atom, would be converted to *CO intermediate. *CO is the key intermediate in the formation of hydrocarbons and alcohols.73,74 Some of the major unanswered questions pertaining to the electroreduction of CO2 are (1) the chemical and structural changes of the catalysts during the reaction; and (2) identification of the intermediates during CO 2 reduction.6,9,68,75−78 The chemical and structural changes of the catalysts may affect their stabilities and selectivities toward certain product(s), while detection of the intermediates provides insightful information about the mechanisms of CO2 reduction processes. The examples selected in this section will address these topics through the application of in situ/ operando Raman spectroscopy. 4.1. Tin Oxides. Tin oxide (SnO2) nanoparticles have been reported to give a high selectivity toward formate production.79−82 In order to probe the oxidation state changes of SnO2 nanoparticles during CO2 reduction, Dutta et al. performed potential- and time-dependent operando Raman spectroscopy on the catalysts in electrolytes with different alkaline pHs.76

The operando Raman spectra during CO2 reduction along with the corresponding performances of the catalysts are presented in Figure 11. Three bands at 482, 623, and 762 cm−1 were observed on the as-prepared catalysts and ascribed, respectively, to the Eg, A1g, and B2g modes of SnO2 crystallites.83 The peak at 623 cm−1 was characteristic of SnO2. Taking the results in pH 9.7 electrolyte as an example: at a moderately negative potential of −1 V vs Ag/AgCl, the intensity of the marker band of SnO2 at 623 cm−1 only slightly decreased, indicating the catalyst was only partially reduced. This is different from the prediction based on the Pourbaix diagram that the SnO2 should have been reduced to Sn metal.76,84 A high Faradaic efficiency (FE) of ∼80% for formate was obtained at this potential. When a more negative potential of ∼ −1.5 V vs Ag/AgCl was applied, SnO2 was then fully reduced to metallic Sn, accompanied by a significant decrease in FE for formate. Similar results were also obtained at pH 8.5 and 12. The authors concluded that (1) the practical kinetic stability region of SnO2 exceeded the thermodynamic stability window (from the Pourbaix diagram); and (2) there are strong correlations between the oxidation state of Sn-catalysts and their selectivities for formate: when 7882

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Figure 12. (a) Raman spectra of freshly prepared Cu, Zn, and CuxZn catalysts in air. (b−d) Operando Raman spectra of Cu, Zn, and Cu4Zn catalysts, respectively, as a function of the reaction time during the electrochemical reduction of CO2 at −0.85 V vs RHE in 0.1 M KHCO3 (the insets show the simultaneously acquired chronoamperograms) (reprinted with permission from ref 9. Copyright 2016 American Chemical Society).

surface.86−89 Hence, DAT ligands led to a weaker adsorption of the CO intermediates on the surface. This effect was proposed to be the reason for the high performance of AgDAT catalysts: once formed, the CO molecules would desorb rather than stay on the surface. Therefore, the overall turnover frequency of CO production was enhanced. Although it detected surface-adsorbed CO, the above in situ SERS study did not record the precursors for CO, which could be COO−* and/or COOH* intermediates.72 Through the use of in situ attenuated total reflection Fourier transform infrared spectroscopy, Firet et al. managed to record the reaction intermediates formed during the electroreduction of CO2 to CO on Ag thin films.77 At potentials from −1.40 V to −1.55 V vs Ag/AgCl in CO2-saturated 0.1 M KCl electrolyte, signals belonging to COOH* moieties were recorded, which therefore led to the proposal of a proton coupled electron transfer reaction mechanism, CO2(g) + * + H+(aq) + e− ↔ COOH*.72 However, at more negative potentials than −1.55 V vs Ag/ AgCl, signals from both COO−* and COOH* intermediates were recorded, indicating individual proton and electron transfer steps have occurred (CO2(g) + * + e− ↔ COO−* and then COO−* + H+ ↔ COOH*). This study indicates that the reaction pathways for CO2 reduction can depend on the applied potentials and did not always involve a concerted proton coupled electron transfer. 4.3. Copper-Based Catalysts. Cu-based materials are the most promising catalysts for the electroreduction of CO2 to hydrocarbons and alcohols.68−71 As the key intermediate to hydrocarbons and alcohols, the formation and behavior of CO intermediates on Cu-based electrodes have received significant attention.9,71,75,78 The measurements of CO intermediates on Cu electrodes by Raman spectroscopy have been extensively reported.75,78 On a polycrystalline Cu electrode in 3.5 M KCl electrolyte at −1.4 V vs SHE during CO2 reduction, Oda et al. recorded the C−O stretching and Cu−CO stretching modes of

SnO2 was partially reduced, a high FE (∼80%) for formate could be obtained. Through in situ attenuated total reflectance infrared spectroscopy (ATR-IR), Baruch et al. successfully measured the peaks of a surface-bound monodentate tin carbonate species at 1500, 1385, and 1100 cm−1 on tin oxide catalysts when CO2 reduction occurred.82 On this basis, the authors proposed a mechanism for formate formation that involved tin carbonate as the key intermediate: native SnO2 was first reduced to a SnII oxyhydroxide, a catalytic resting state. This SnII species could then react with CO2 to form the observed tin carbonate intermediates. The surface-bound carbonate then undergoes two electrons and one proton transfers to give formate. The adsorbed formate quickly desorbed and the adsorption site then reverts to SnII oxyhydroxide. 4.2. Silver-Based Catalysts. Ag is an ideal catalytic surface for studying the reduction of CO2 to CO, since it produces mainly CO gas with small amount of H2, hydrocarbons, alcohols, and formic acid/formate.85 Efforts have been made to investigate the reaction processes during CO production on Ag and Ag-based catalysts.6,77 To investigate the beneficial influence of 3,5-diamino-1,2,4triazole (DAT) on the electrochemical reduction of CO2 to CO on a silver electrode, Schmitt and Gewirth performed in situ SERS measurements, with and without the presence of DAT in the electrolyte.6 On a silver electrode in the absence of DAT, Raman bands of adsorbed CO were recorded at ∼1880 and 1945 cm−1 at −0.05 V vs Ag/AgCl. These were, respectively, assigned to CO adsorbed on a 3-fold hollow site and a bridge site on the Ag surface.86−88 In the presence of DAT, the 1880 cm−1 band was absent during electrolysis. The band associated with bridge-bound CO was still observed, but at a lower wavenumber of 1924 cm−1. Two new bands appeared at 2049 and 2099 cm−1, respectively attributed to CO top-bound and CO physisorbed/weakly coordinated with the electrode 7883

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Figure 13. (a) In situ Raman spectra of (i) hydrogen adsorption in 0.5 M H2SO4 + H2O and (ii) deuterium adsorption in 0.5 M H2SO4 + D2O as a function of the applied potential at roughened Pt surfaces. Potentials are quoted vs SCE (reprinted with permission from ref 100. Copyright 1996 Elsevier B.V.). (b) Potential-dependent infrared reflection absorption spectra collected from Pt(111), Pt(100), and Pt(110) surfaces in 0.5 M H2SO4. Potentials are reported vs RHE (reprinted with permission from ref 103. Copyright 2011 Elsevier B.V.).

surface-bound CO intermediates at ∼2000 and ∼360 cm−1, respectively.78,90 Smith and co-workers also reported the formation of CO intermediates (2090/358/280 cm−1) on Cu electrodes, together with other probable surface-adsorbed species such as formate and CHx adsorbates (broad 2900 cm−1 and weak 1450 cm−1 bands) at −1.06 V vs NHE in CO2saturated 0.1 M NaHCO3 electrolyte.75,90 Ren et al., through the measurement of CO intermediates by Raman spectroscopy, combined with the identification and quantification of the products by online gas chromatography (GC) and 1H NMR, gained mechanistic insights into the active site and reaction pathway toward ethanol production on a oxide-derived Cu−Zn catalyst during CO2 reduction reaction.9 It was discovered that the incorporation of different amounts of Zn into Cu oxides could tune the selectivities of ethanol versus ethylene production. Ethanol formation was maximized for a catalyst with a Cu: Zn ratio of 4:1, with a Faradaic efficiency of 29.1% and partial current density of −8.2 mA/cm2 at −1.05 V vs RHE. The Raman spectra collected on the Cu4Zn oxide as well as Cu oxide and ZnO electrodes are shown in Figure 12. The as-deposited Cu and Zn electrodes showed Raman bands that were ascribed respectively to Cu2O (142, 216, 525, 630 cm−1) and ZnO (430, 560 cm−1) phases, and the Cu4Zn showed both Cu2O and ZnO characteristic peaks.91,92 At −0.85 V in CO2-saturated 0.1 M KHCO3 electrolyte, Cu2O was reduced to Cu metal as shown by the disappearance of Cu2O bands. Three new bands at 280, 365, and 2060 cm−1, arising from adsorbed CO intermediates, were observed.75,90 The ZnO electrode was fully reduced to Zn metal at −0.85 V. However, bands of CO intermediates were not observed on this electrode, even though Zn itself was excellent for reducing CO2 to CO. The authors attributed this to the weak adsorption of CO on Zn surfaces.93 On the Cu4Zn catalyst, signals of both Cu2O and ZnO phases disappeared within 300 s. Peaks associated with CO were recorded at 287, 365, and 2010 cm−1 on the catalyst. It is notable that the CO-related vibrations were not observed until all the signals from the oxides had disappeared. This indicated that CO2 reduction to CO probably occurred on metallic sites rather than on oxides. On the basis of

the Raman results as well as other data obtained from CO2 reduction measurements using GC and 1H NMR, a two-site mechanism was proposed by the authors for the formation of ethanol on CuxZn catalysts. It was suggested that CO formed on Zn sites could diffuse and spill over onto Cu sites, which then combined with *CH2 intermediate (on Cu) to form *COCH2 and be further reduced to acetaldehyde and finally to ethanol.73,74,94,95 Through the above examples, we show that a combination of operando Raman spectroscopy that measures the reaction intermediates and other techniques, i.e., GC and NMR, provides a way to understand how CO2 is reduced to hydrocarbons and alcohols.

5. APPLICATIONS OF IN SITU/OPERANDO RAMAN SPECTROSCOPY IN THE HYDROGEN EVOLUTION REACTION The electrochemical hydrogen evolution reaction is the cathodic half-reaction of water splitting. The half-reaction equations are in acidic electrolytes: 2H+ + 2e− → H 2 in alkaline electrolytes: 2H 2O + 2e− → H 2 + 2OH−

The standard reduction potential of HER is defined as 0 V versus a standard hydrogen electrode. The best known material to catalyze H2 evolution in acid is platinum, which requires an overpotential of less than 50 mV to drive it to a current density of −10 mA/cm2.3 Metal sulfides, especially molybdenum sulfides, have received a lot of attention in recent years due to their efficacious HER activities. For example, at pH 0, Mo− S-based materials showed an overpotential of only 0.1−0.2 V to catalyze H2 production at −10 mA/cm2.96−99 The examples in this section will show insights that have been gained about the structural changes, active sites, and intermediates on the abovementioned HER catalysts. 5.1. Platinum Electrodes. The group of Tian recorded the Raman spectra of Pt electrodes under HER conditions.100 To achieve stable and reproducible Raman measurements as well as 7884

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also unclear which sites (Mo or S, or both) are catalytically active for HER catalysis. Using operando Raman and electron paramagnetic resonance spectroscopy, Tran et al. investigated the structural changes and active sites of anodically deposited amorphous MoSx during HER in a pH 7 phosphate buffer electrolyte (Figure 15a).8 The terminal S22− atoms of MoSx were found to be removed during HER, as revealed by a decrease in the intensity of the ν(S− S)terminal signals. At the same time, ν(MoO) signals increased in intensity. The authors proposed that the removal of the terminal sulfur could have led to the formation of unsaturated Mo sites. These Mo sites were further suggested to coexist with MoO species. The authors regarded ν(MoO) as an indicator of Mo active sites. However, we note that the observation of MoO did not demonstrate that H2 had evolved on the Mo sites; evidence such as ν(Mo−Hads) signals were not provided in the study. Our group also performed operando Raman spectroscopy on electrodeposited MoSx catalysts during HER in 1 M HClO4 electrolyte (Figure 15b).107 The anodically deposited MoSx(x≈3) films (MoSx-AE) showed four broad Raman peaks at 320, 445, 520, and 550 cm−1, assigned respectively to ν(Mo−S)coupled, ν(S apical -Mo), ν(S−S) terminal , and ν(S−S) bridging vibrations.106,110−112 The cathodically deposited MoSx(x≈2) (MoSxCE) exhibited two broad Raman bands at 320 and 415 cm−1, respectively ascribed to their ν(Mo−S)coupled and ν(Mo−S− Mo) vibrations.106,111,113 The Raman spectra of MoSx-AE were collected simultaneously while its cyclic voltammetry (CV) was measured in 1 M HClO4 electrolyte. At potentials negative to −0.07 V vs RHE, the bands at 445, 520, and 550 cm−1 attenuated, while a new band at 415 cm−1 ascribed to ν(Mo−S−Mo) of MoSx-CE (cathodically deposited) appeared. The overall spectral evolution indicated that MoSx-AE transformed to a MoSx-CE-like structure. Interestingly, a new peak at 2530 cm−1 appeared at −0.07 V vs RHE and disappeared before +0.28 V vs RHE during the anodic scan. The peak was assigned to S−H stretching vibration (ν(S−H)) since its frequency matched well with the ν(S−H) vibrations of SH adsorbates on Mo surfaces and SH ligands coordinated to Mo centers (in molecular complexes).114−118 The assignment of the 2530 cm−1 to ν(S−H) was further corroborated by H/D isotopic exchange experiments. The same 2530 cm−1 peak of ν(S−H) was recorded on MoSx-CE films as well. DFT calculations on ν(S−H) vibrational frequencies supported the assignments of the observed peaks and further revealed that only nonapical S atoms were involved in the HER (the ν(Sapical−H) band was simulated by DFT to be at 2289 cm−1, which is ∼200 cm−1 lower than that observed from experiment). Hence, we proposed that S atoms in MoSx, particularly the nonapical S atoms, were the catalytically active sites for evolving H2. Amorphous MoSx catalyst had been investigated using operando XAS and near ambient pressure XPS previously.119,120 Since hydrogen cannot be detected by these Xray-based techniques, none of these studies had successfully measured the interactions between the active sites and Hads. Our group, using operando Raman spectroscopy, recorded the presence of S−Hads bonds. However, we acknowledge that the temporal behavior of the S−Hads intermediate still needs to be elucidated, which could be by time-resolved techniques such as time-resolved IR spectroscopy.

stronger peak intensities, the surfaces of the Pt electrodes were pretreated by an electrochemical roughening procedure, so as to obtain SERS-active substrates. The SER spectra of a roughened Pt electrode at different potentials are shown in Figure 13a.100 The band at 1644 cm−1 was attributed to a HOH bending vibration from bulk water. A new band at 2088 cm−1, assigned to the adsorption of a hydrogen atom coordinated to a surface Pt atom, appeared on the electrode surface at a H2evolving potential of −0.3 V vs SCE.101 The assignment of 2088 cm−1 to Pt−H vibration was corroborated by isotopic Raman measurements performed in deuterated electrolyte. The 2088 cm−1 shifted by a factor of 1.39 to 1496 cm−1, consistent with that predicted using the harmonic oscillator model. Hence, the authors successfully recorded the Hads intermediates on the roughened Pt electrode. Due to the large IR cross section of metal−hydrogen vibrational bonds, the HER process on Pt could be advantageously studied by IR spectroscopy.101−103 Nakamura et al. discovered that hydrogen was adsorbed at the atop site on Pt(110) surface during HER, as revealed by the ν(Pt−Hads) band at 2081 cm−1; whereas on Pt(100), hydrogen was adsorbed at the asymmetric bridge site, revealed by a tail that appeared in the higher-frequency region of δHOH (HOH bending vibration) peak at ∼1630 cm−1 (Figure 13b).103 No adsorbed H was observed on Pt(111). Therefore, through in situ IR study, the authors revealed the structural-dependent adsorption of reaction intermediates formed during HER on different Pt single crystal surfaces. 5.2. Amorphous Molybdenum Sulfide (MoSx). Amorphous MoSx have attracted great attention due to their ease of preparation and excellent activities toward HER. These materials can be easily synthesized either through thermal decomposition or electrochemical deposition from (NH4)2MoS4 precursors.104,105 The structures of MoSx are generally agreed to be polymeric aggregations of MoIV3 clusters, with bridging S22−, terminal S22−, unsaturated S2−, and apical S2− ligands (Figure 14).106 Ex situ characterizations of the anodically deposited MoSx showed that the material underwent a compositional change, from MoS3-like to MoS2-like, when it was used for HER catalysis.107−109 However, it is not clear how this compositional change takes place and whether it is associated with the HER activity of the catalyst. In addition, it is

Figure 14. Amorphous MoSx polymer chain constructed from Mo3 cluster units. The different types of S ligands present are indicated by different colors. Green: bridging S22−; blue: apical S2−; yellow: terminal S22− and red: unsaturated S2− (reprinted with permission from ref 107. Copyright 2016 American Chemical Society). 7885

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Figure 15. (a) Raman spectra (532 nm green laser excitation with low power 0.1 mW) of freshly electrodeposited amorphous MoSx thin film (red trace; the inset shows zoomed-in of the MoO vibration bands multiplying the intensity by a factor of 5), and the same material after equilibration in pH 7 phosphate solution at constant potential of −0.45 V (green trace), −0.55 V (blue trace), and −0.71 V vs NHE (black trace) (reprinted with permission from ref 8. Copyright 2016 Nature Publishing Group). (b) Sequence of Raman spectra recorded from (i-ii) anodically deposited MoSx(x≈3) (MoSx-AE) and (iii-iv) cathodically deposited MoSx(x≈2) (MoSx-CE) during the (i, (iii) cathodic half sweep and (ii, (iv) anodic half sweep of the cyclic voltammetry in 1 M HClO4 at 0.5 mV/s. The signals marked with an asterisk originated from a cosmic ray. Reference electrode: RHE (adapted and reprinted with permission from ref 107. Copyright 2016 American Chemical Society).

6. CONCLUSIONS AND OUTLOOK

ascribed as the active species. For HER and CO2 reduction reactions, the sequential recording of Raman spectra allows monitoring of the surface structural changes and adsorbed intermediates on the catalyst surfaces, as shown in cases of the amorphous MoSx films during HER, the Cu4Zn catalyst, and SnO2 nanoparticles during CO2 reduction. Partially reduced SnO2 was determined to be responsible for reducing CO2 to formate, as when it was fully reduced to Sn metal, a significant decrease in formate production was found. By correlating the changes of these catalysts to their catalytic performances, a thorough understanding of how these materials catalyze the reactions and why catalysts deactivate could be obtained. We expect that this knowledge will guide in the rational design of a

Electrochemical water splitting and CO2 reduction processes have emerged as possible strategies to produce valuable transportation fuels and chemicals. In this Perspective, through recent studies, we have shown how in situ/operando Raman spectroscopy can be used to characterize and understand these electrocatalytic systems. Raman spectroscopy has the capability to provide unique fingerprint-type information about the catalysts’ structures and their adsorbed intermediates during reactions. For example, it was used to identify metastable oxides, such as CoIV oxide on Co3O4 and CuIII oxide on Cu electrodes during OER. Their presence was correlated to the OER performances of the respective catalysts and therefore 7886

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new generation of more efficient catalysts and the judicious selection of appropriate reaction conditions. We highlight that the high-speed acquisition of data (in the order of seconds) by in situ Raman spectroscopy cannot be overlooked. This capability allows the real-time analysis of the activity of a catalyst, which could change/deteriorate rapidly over minutes. This information cannot be obtained with the sole use of in situ XAS, which requires tens of minutes to collect a set of XANES and EXAFS spectra. This XAS data, when acquired over an extended period of time and hence provide only an average picture of the catalyst, could be wrongly presented as being reflective of the catalyst in a particular working state. Another advantage of Raman spectroscopy is that all the chemical information on the system under study is seen in one spectrum (200−4000 cm−1), while XAS could only focus on one specific element per measurement. The relative low cost and availability of Raman microscopes also mean that they are more accessible, as compared to the use of synchrotron-based XAS techniques. We would like to highlight that no single technique provides every detail of a complex catalytic system, and Raman spectroscopy is of no exception. The conventional Raman techniques hitherto mentioned cannot reveal the type of electrochemical processes occurring at nanometer-sized catalytically active sites. In the case of Raman spectroscopy using a conventional microscope, the spatial resolving power of such an optical system is approximately λ/2, where λ is the wavelength of the incident laser light (Rayleigh criterion). Using visible light at 633 nm, the best possible spatial resolution with these optics is ∼300 nm. The most straightforward way to surpass this limit is to create a point light source whose size is smaller than λ/2. We suggest that tip-enhanced Raman spectroscopy (TERS), with its ability to probe the chemical identities of materials with spatial resolutions of