CO2 Electrochemical Reduction as Probed through Infrared

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CO Electrochemical Reduction as Probed through Infrared Spectroscopy Shangqian Zhu, Tiehuai Li, Wen-Bin Cai, and Minhua Shao ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b02525 • Publication Date (Web): 06 Feb 2019 Downloaded from http://pubs.acs.org on February 6, 2019

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ACS Energy Letters

CO2 Electrochemical Reduction as Probed through Infrared Spectroscopy Shangqian Zhu,† Tiehuai Li,† Wen-Bin Cai,‡ and Minhua Shao†,#,*

†Department

of Chemical and Biological Engineering, The Hong Kong University of

Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China ‡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 #Energy

Institute, The Hong Kong University of Science and Technology, Clear Water

Bay, Kowloon, Hong Kong, China

AUTHOR INFORMATION

Corresponding Author *[email protected]

ACS Paragon Plus Environment

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ABSTRACT CO2 electrochemical reduction shows great promise in storing renewable energy and alleviating global warming. Its large-scale applications in the industry, however, are still in infant mainly due to the unsatisfactory performance of electrocatalysts. Rational design of advanced electrocatalysts would be the ultimate solution, which has yet to be achieved currently because of the lack of understandings of reaction mechanisms. In the past few years, in situ attenuated total reflection infrared (ATR-IR) spectroscopy has been successfully adopted to study the electrochemical interface of CO2 reduction reaction (CO2RR) and significantly advanced our understandings of this reaction. In this perspective, these advances, as well as the challenges and opportunities faced in further studying CO2RR by this technique are discussed.

TOC GRAPHICS


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Mitigating the continuous rising of CO2 atmospheric concentration and harvesting-andstoring the renewable energy are among the most urgent issues of this century. Among various technologies that can potentially tackle these challenges, electrocatalytic conversion of CO2 and H2O into various chemicals and fuels by using renewable electricity is particularly attractive.1,

2

Currently, the large-scaling adoption of CO2

electrochemical reduction has yet to be achieved, which still requires more selective and active electrocatalysts.3,

4

Combining theoretical and experimental approaches to

rationally design more advanced catalysts is desired,5 whereas the full understandings of reaction mechanisms, especially the nature of reaction intermediates and their interactions with active sites are prerequisites. In situ spectroscopic characterizations of the electrochemical interface in real time are critical components for realizing these


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goals.6 Vibrational spectroscopies (Raman, infrared (IR) and sum frequency generation) can directly provide molecular-level information and are hence frequently used. Among them, surface IR spectroscopy features the low cost, facile construction of optical pathway, and simple surface selection rule.7 In this perspective, we will focus on the in situ IR spectroscopy technique with the attenuated total reflection (ATR-IR) configuration, which is especially powerful in studying CO2 reduction reaction (CO2RR) due to its high surface sensitivity, free mass transport, and insusceptibility to gas evolution.7 In the past four years, extensive applications of ATR-IR spectroscopy were witnessed in exploring various electrochemical interfacial phenomena in CO2RR, which include the direct observation on adsorbed reaction intermediates and their interactions with electrode surfaces, speculation of reaction mechanisms, and understandings of compositional and structural dependent activities. Herein, we briefly overview these achievements and discuss challenges and opportunities of ATR-IR in studying CO2RR.

Reaction Intermediates and Mechanisms Determination


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ACS Energy Letters

Among various configurations, the ATR surface enhanced infrared spectroscopy (ATRSEIRAS) (Figure 1a) is extremely powerful for the detection of reaction intermediates since the molecules adsorbed on particle-shaped metal thin films would have tens of times higher IR absorption.8 Metal thin films are usually deposited on various ATR prisms (e.g. Si and ZnSe) by chemical or physical methods. Significant efforts have been contributed to reaction mechanism studies on Cu, which is the most promising material for direct CO2-to-hydrocarbon conversion.1 Chemisorbed CO (*CO) is the most frequently reported intermediates, which is usually detected as a broad and asymmetric band located between 2100 and 1900 cm-1 at potentials lower than -0.4 V in CO2 saturated bicarbonate-based solutions (Figure 2a).9-11 The integrated *CO band intensity would reach a maximum at about -0.8 V and then gradually decrease.9, 10 This potential is consistent with the onset of the generation of hydrocarbons,1 indicating a significant amount of surface *CO is needed to trigger the production of these higher order products. By changing the solution pH, the adsorption behavior of CO can be altered. The adsorption profile of *CO was found to be shifted by 60 mV pH-1 in the reversible hydrogen electrode (RHE) scale (Figure 2b),9 which, however, can be


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cancelled in the standard hydrogen electrode (SHE) scale (Figure 2c), revealing that the adsorption of *CO is actually determined by the absolute potential. It should be mentioned that the equilibrium potentials for hydrocarbon (e.g. C2H4) production are fixed in the RHE scale (Figure 2d), hence the observed more facile generation of hydrocarbons of CO reduction in alkaline than that of typical CO2RR in neutral solutions can be partially explained by the more abundant surface *CO species at higher potentials in the former versus RHE. The detected *CO, however, were not always reactive intermediates. CO adsorbed at -1.1 V (SHE) in the CO-purged 0.1 M LiOH solution showed a major band between 2100 and 1900 cm-1 assigned to linearlybonded CO (COL) adsorbed on one Cu atom, and a minor band between 1750 and 1900 cm-1 related to bridge-bonded CO (COB) coordinated with two Cu atoms.12 By continuously purging Ar to remove the dissolved CO in the solution, the immediate drop of COL band area can be clearly observed, along with the increased of COB band. It can be speculated that the COL was converted to COB or partially desorbed. Interestingly, the COB still persists even the potential was stepped to a much more negative value (1.75 V), indicating it can hardly be further reduced. Theoretical calculations revealed


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that COB suffered significantly higher energy barriers toward *CHO intermediate as compared to COL,12 hence it is only a spectator instead of a real intermediate. The impacts of *CO adsorption configuration should be seriously considered in further theoretical and experimental analysis of CO2RR pathways. The detection of intermediates other than *CO was also reported on Cu surfaces. However, due to the much weaker surface enhancement effects than that of *CO, their band intensities were usually quite low. For instance, a very weak band near 1720 cm-1 was detected between -0.6 and -1.2 V (RHE) possibly related with the C=O double bond stretching of surface-bonded *CHO,10 which is the subsequent downstream intermediate of CO. And two bands located near 1400 cm-1 are suspected as *COOH and *COO-,10 which serve as precursors for CO and formate, respectively.13 More studies are needed to confirm the band assignments due to their relatively weak absorbance. Several vibrational features detected between 1600 and 1200 cm-1 were possibly related to *CO3, *OCH3, *OC2H5 and *HCO, according to density functional theory (DFT) calculations on the vibrational frequency.14 Such an approach combining spectroscopic and DFT calculation results might be an efficient strategy for band assignments. By using


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traditional infrared reflection absorption spectroscopy (IRRAS), two weak bands located at 1191 and 1584 cm-1 were detected and assigned to the C-O-H and C-O stretching of adsorbed OCCOH intermediates, respectively during CO reduction on single crystalline Cu electrodes in an alkaline solution.15 It is of great interest to further confirm these bands by ATR-SEIRAS with a higher surface sensitivity.

Figure 1. Schematic illustration of ATR-IR configurations for studies on a) surface enhanced metal thin films and b) nanocatalysts.


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Figure 2. a) Real-time ATR-SEIRAS spectra recorded during the cathodic scan of a Cu thin film electrode in a CO2-saturated 0.1 M KHCO3 solution.10 b) CO adsorption profiles at various pH, c) normalized CO integrated band intensities as a function of potential (SHE scale), and d) Pourbaix diagram of the equilibrium potentials for H2, CH4 and C2H4 generation.9 Reprinted from references.9, 10


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On other materials mainly generating CO and formate, ascertaining the reaction pathways toward these two-electron transferred products would provide important information on their competition and guide the design of more selective and reactive catalysts. Several post-transition metals (In, Sn, Pb, and Bi) are promising for formate production and were studied by ATR-IR.16, 17 Bands located at 1500, 1385 and 1100 cm1

were detected on Sn films which can be assigned as tin carbonate on surfaces (Figure

3a).16 These bands can only be detected at potentials when formate with a reasonable high faradaic efficiency was measured, indicating carbonate is the intermediates toward formate generation (Figure 3b). The absence of these bands after removing the surface SnOx/Sn(OH)x by chemical etching further indicated that surface oxide species were the real active sites for tin carbonate formation and the following formate production. In situ Raman spectroscopic studies also confirmed the importance of surface SnOx, as a high formate selectivity can only be obtained when the characteristic bands of SnOx were detected (483, 623 and 762 cm-1).18 Similar reaction pathway was proposed on indium surfaces with indium carbonate detected as reaction intermediates.17 The carbonate intermediates were not detected on Pb and Bi, indicating that CO2RR proceeded


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ACS Energy Letters

directly on metal surfaces instead of oxide species. Herein, the capability of these in situ spectroscopic tools in determining the type of real active sites was well demonstrated. In the further development of post-transition metal-based nanocatalysts for selective formate generation, the surface oxidation state engineering would be indispensable.

Figure 3. a) ATR-IR spectra and b) corresponding faradaic efficiencies at various potentials during CO2RR on the Sn thin film.16 c) ATR-IR spectra and d) proposed reaction mechanisms on the Ag thin film.19 Reprinted from references.16, 19

Ag and Au are both CO-selective catalysts in CO2RR. Combined product measurements and theoretical studies indicated that the CO2-to-CO generation should


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proceed via the *COOH intermediate coordinated with catalyst surfaces via a C atom.13 Such reaction pathway was supported by in situ ATR-IR studies on Ag thin film.19 Three peaks located at 1660, 1386 and 1288 cm-1 were detected as the OH deformation, C-O stretching, and C=O stretching of *COOH, respectively (Figure 3c). More importantly, the fixed ratio between the first two bands (-1.4 to -1.55 V) ruled out the possibility of any *OCO- intermediate coordinated with the surface by two O atoms as precursors for CO production on Ag, which only has a single vibration feature near 1386 cm-1. At a higher overpotential (-1.6 V vs Ag/AgCl), the band related with *COO- coordinated with the surface by the C atom was detected, indicating the electron and proton transfer are not necessarily concerted on Ag (Figure 3d). A weak *CO band near 2034 cm-1 was also observed.19 The detection of intermediates, however, is rather challenging on Au surfaces. Even the most expected *CO intermediates were not observed in CO2RR potentials on Au thin films with a strong surface enhancement effect,10,

20, 21

which is

possibly related to the quick desorption of *CO due to its weak adsorption. It should be mentioned that proper cell design and counter electrode materials should be chosen as fake *CO bands can be detected in some cases on Au surfaces contaminated by Pt and


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ACS Energy Letters

electrolyte impurities.20,

22

And some practices on mitigating these interferences were

summarized by Dunwell et al.22

Impacts of Electrolytes

As accepted, the electrochemical interface conditions (e.g. electrolytes) are as important as the catalyst materials on electrochemical reactions. Besides detecting different reaction intermediates and mechanisms on various metal surfaces, ATR-IR also shows capability in revealing the impacts of electrolytes. Bicarbonate-based electrolytes are most frequently used media for aqueous CO2RR. For a very long period, bicarbonate anions were solely believed as the pH buffer23 and proton donor24. Recent works from two individual groups found that bicarbonate anions may be directly involved into the reaction. Although *CO cannot be directly observed on Au surfaces at CO2RR potentials, stepping the working electrode back to a high potential (e.g. 0.4 V vs RHE) would allow its re-adsorption and detection.20 Interestingly, if C atoms of the CO2 and HCO3- used were isotopically distinct, the observed *CO band is always more related to bicarbonate anions instead of the CO2 purged (Figure 4a-c). This was also supported by


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mass spectroscopic measurements on the isotopic composition of CO gases generated. Kinetic analysis based on effects of CO2 partial pressure and HCO3- concentration indicated that CO2 in equilibrium with bicarbonates is the real source of CO2RR. Hence the CO2 mass transport can be facilitated by bicarbonate anions. The similar issue was also examined on Cu surfaces.10 Since *CO can be observed in CO2RR potentials, the direct and dynamic tracking on the role of bicarbonate anions can be achieved. It should be mentioned that *CO bands can still be detected in the Ar-saturated KHCO3 solution, indicating CO2 in equilibrium with bicarbonate anions is sufficient to drive the reaction.10 In another experiment, the potential was switched from 0.2 to -0.6 for the start of CO2RR in the

12CO

2-saturated

KH13CO3 solution, and real-time spectra were

continuously recorded. Similar to the findings on Au,20 the initial *CO band detected was always related to the bicarbonate anion used. *CO bands associated with the CO2 gas purged can only be observed after a certain period (~18 s) due to the isotopic composition changes of bicarbonate anions from 13C-labelled to a mixed pattern (Figure 4d). As a consequence, the bicarbonate anion-mediated CO2 mass transport was also validated on Cu surfaces. The detection of predominant bicarbonate-related products in


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ACS Energy Letters

both gas (CO) and liquid (formate) phases on N-doped carbon25 and Au-Sn26 nanoparticles, respectively by mass spectrometry further supported the universality of this mechanism and the unique role of bicarbonte anions in CO2RR (Figure 4e). The widely believed pH buffering effects of bicarbonate anions can also be confirmed by ATR-IR as well since the elevated local pH upon the significant consumption of H+ would alter the equilibrium between HCO3- and CO32- anions, and a strong solution CO32- band can be clearly observed near 1390 cm-1.10 The quantitative determination on local pH changes in bicarbonate solutions with different concentrations during CO2RR can be achieved by measuring the CO32-/HCO3- band area ratio.27 The significant changes in local pH would lead to the underestimation of real overpotential to even 150 mV,27 which should be considered in traditional electrochemical analyzing techniques (e.g. Tafel slopes). It is still unclear whether the H of HCO3- anions can serve as the proton source by spectroscopic techniques.


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Figure 4. a-c) ATR-SEIRAS study of CO produced in the 13CO2 purged 0.5 M NaH12CO3 electrolyte in real time.20 d) *CO composition and band intensity changes in

12CO

2-

saturated 0.1 M KH12CO3 solution at -0.6 V and e) proposed bicarbonate-mediated CO2 mass transport phenomenon.10 Reprinted from references.10, 20

The pH buffering effect was also detected on cations by ATR-IR using the same strategy. The increase of local pH would be alleviated if larger cation-contained electrolytes were used, which is possibly associated with the cation hydrolysis.28


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ACS Energy Letters

Besides, cation was found to alter the stability of reaction intermediates. A lower *CO stretching wavenumber can be detected if cations with a larger size are used by ATRSEIRAS on Cu thin films, which indicated an enhanced *CO binding strength.29 In addition, the further reduction of *CO into other higher order species was facilitated by larger cations via tracking the *CO band intensity evolution.29 The spectroscopic evidence supports the theoretical prediction that larger cations can enhance the local electric field and promote hydrocarbon production.30 The effects of large organic-based cations were also explored by several groups. The presence of protonated pyridine (PyH+) was found to accelerate the formate production rate on Pt surfaces by facilitate the further reduction of surface *COOHL (near 1800 cm-1) toward formate instead of CO.31 This study excluded the previous proposal that formate was produced from solution Py-CO2 complex with surface *H.32 Papasizza et al. studied CO2RR on Au thin films in a mixture solution consisting of ionic liquid ([EMIM]BF4) and water,33 which was believed to significantly lower the CO2RR energy barrier (initial CO2-to-CO2- conversion) due to the complexation between CO2- radical and EMIM+ cation.34 *CO bands can be detected in this solution,33 indicating the altered Au-*CO binding strength as compared


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to that in bicarbonate-based solutions.10, 20 The EMIM+-CO2 related species, however, were not detected by ATR-SEIRAS.33 Besides, adsorbed water molecules was found to have two configurations in the water- (3415 cm-1) and ionic-liquid-rich (3610 cm-1) region, respectively, and the preferential consumption of the later indicating the facilitated intermediate protonation processes by EMIM+ cations.33

Beyond Model Catalyst Surfaces

The surface enhancement effect is indispensable in detecting very weak vibrational features. However, it is well identified that the catalytic properties of these single-metal bulk materials are distinct as compared to those of counterpart nanocatalysts. Besides, engineering various nanostructures (e.g. core-shell and shape-controlled) and compositions (e.g. alloy) on ATR thin films are challenging. These drawbacks would limit the applications of ATR-SEIRAS in analyzing reaction mechanisms on practical catalysts. In the past two years, a modified ATR configuration was successfully adopted to study CO2RR on synthesized catalysts, which consists of a conductive metal thin film


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ACS Energy Letters

substrate (usually Au due to the absence of any intermediate vibrational features in CO2RR),

and an overlayer film prepared by simply casting catalyst suspension (Figure 1b).

It should be mentioned that the high surface area of nanocatalysts may partially compensate for the loss of surface enhancement effect, and allow the detection of various intermediates.

Surface-bonded carboxyl intermediate *COO- located between 1544 and 1520 cm-1 was detected in CO2RR on Li tuned ZnO nanoparticles.35 The evolution of the band intensity as a function of applied potentials shows a similar trend to that of CO faradaic efficiency, hence confirming *COO- as the intermediate for CO generation.35 On carbon supported Pd nanoparticles, *CO, *COOH, and *OCHO were detected,36 which helped draw relatively complete reaction pathways toward CO and formate generation. Oxidederived Cu (OD-Cu) is well-known for its superior high order hydrocarbon production capability.37 It was found that the CO coverage was similar on (100), (111) and (110) facets dominated Cu2O nanocrystals possibility due to the surface reconstruction under reaction conditions.38 Instead, the cation seemed to play a more important role on the


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adsorption/reduction of *CO,38 and was consistent with previous study on surface enhanced Cu thin film.29 The Xu group detected an *CO band centered at 2058 cm-1 on OD-Cu nanoparticles in alkaline solutions. Its lowered wavenumber as compared to those on Cu (2131, 2089 and 2073 cm-1) indicated the stronger Cu-CO interaction.39 The redshift of *CO band center was also observed on Cu2O thin film with surface enhancement effects prepared by electrodeposition in lactic acid-contained solutions.40 Findings from these two studies support the theoretical prediction that some strong CO binding sites are mainly responsible for the improved hydrocarbon production on ODCu.41 It should be mentioned that in situ Raman spectroscopic studies by several groups found that the CO2RR related intermediates can only be observed on metallic Cu instead of CuOX (several bands between 100 and 700 cm-1) surfaces.42,

43

Nevertheless, the possibility that the altered Cu-CO interaction on OD-Cu was caused by trapped O atoms in sub-surfaces cannot be fully excluded, which is still under debate.44 Cysteamine-functionalized Ag nanoparticles showed a high selectivity for CO at low overpotentials.45 The ATR-IR study found CO2 could form chemical bonds with – NH2 groups as indicated by a band (~839 cm-1) that can be only observed under CO2


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purging situation, which possibly lowered the CO2 activation energy barrier.45 This ATRIR configuration was also capable of analyzing more complicated structural and compositional effects of nanocatalysts on CO2RR results. Abundant grain boundaries in twisted Pd nanowires significantly improved the CO faradaic efficiency as compared to Pd nanoparticles.46 The reaction rate, however, was sacrificed. By alloying Pd with Au forming a unique core-shell structure consisting of a pure Pd shell, the CO faradaic efficiency and mass activity could be simultaneously increased.46 The ATR-IR study revealed that the formation of *CO could be facilitated on Pd and Pd-Au nanowires by reducing the overpotential by more than 100 mV (Figure 5a-e), indicating an improved CO2-to-*COOH conversion due to the presence of defects. More importantly, the incorporation of Au would promote the generation of more weakly adsorbed COL than strongly adsorbed COB (Figure 5f), hence accelerating the overall reaction rate by easier *CO desorption.46


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Figure 5. Real-time ATR-IR spectra recorded during stepping the potential from 0 V to 0.8 V on a) Pd nanoparticle, b) Pd nanowire and c) Pd-Au nanowire in the CO2saturated 0.5 M KHCO3 solution. d) Schematic illustration of different CO adsorption configurations and the corresponding wavenumber range. e) Normalized total *CO band intensity and f) COL/COB ratio as a function of applied potentials. Reproduction with permission from John Wiley and Sons.46

Table 1. Summary of detected intermediates and corresponding band positions in CO2RR Surface

Electrolyte

Band center (cm-1)

Assignment

Cu

CO2-sat. 0.1 M NaHCO3

~20759

CO

CO2-sat. 0.1 M KHCO3

2083-205210

CO


22

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CO-sat. 0.1 M KHCO3

~208011

CO

~205011

CO on undercoordinated sites

CO-sat. 0.05 M KOH

205839

CO on reconstructed Cu (100)

CO-sat. 0.1 M LiOH

~2080-205012

COL

~1850-181012

COB

172010

CHO, C coordinated

140010

COO-, two O coordinated

137010

COOH, C coordinated

147714

HCO, C coordinated

139014

OCH3, O coordinated

134014

OC2H5, O coordinated

124014

HCOO, two O coordinated

CO-sat. 0.1 M LiOH

1584, 119115

OCCOH, two C coordinated

Sn, In

CO2-sat. 0.1 M K2SO4

1500, 1385, 110016, 17

tin and indium carbonate

Ag

CO2-sat. 0.1 M KCl

203419

CO

1660, 1386, 128819

COOH, C coordinated

1559, 139919

COO-, C coordinated

~210020

CO


CO2-sat.1 M KHCO3

Au

CO-sat. 0.5 M NaHCO3 CO2-sat. [EMIM]BF4/H2O

1929,

Pt

0.5 M KCl + 0.1 M Py

~180031

COOHL, C coordinated

Zn


1544-152035

COO-, C coordinated

Pd


2000-196847

COL

1890-183447

COB

1796-178347

COM

~197736

COL

~187236

COB

~178436

COM

133836

HCOO, two O coordinated

128836

COOH, C coordinated

2004-196147

COL

1874-183247

COB

179147

COM

CO2-sat. 1 M KHCO3

Pd/AuP d


180033

CO


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Summary and Future Outlook

The development of more advanced electrocatalysts for CO2RR requires the gradual switch from traditional try-and-error methodology to a rational design approach, where deep fundamental understandings of the reaction itself, as well as impacts of catalyst structure and composition on the catalytic activities are pre-requisites. In this perspective, we focused on in situ ATR-IR spectroscopic technique and briefly summarized its recent applications in studying CO2RR. This technique allows the direct observation of many important intermediates (Table 1), and assessment of reaction conditions on catalytic consequences. More importantly, the successful adoption of novel ATR-IR configurations extends its capability from solely analyzing model metal thin films to real nanocatalysts. The direct spectroscopic evidence obtained via this technique has complemented/rejected various proposals from theoretical and traditional electrochemical studies, and also added significant new insights into the understandings of this specific reaction. Its role in further studies of CO2RR is indispensable. Herein, we propose some directions of great research interest.


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1) Cu is the most promising material for direct CO2-to-hydrocarbon electrocatalytic conversion. However, the detection of various reaction intermediates in the downstream of *CO has not been well addressed even on well-prepared Cu thin film with a strong surface enhancement effect. The short residual time and low coverage of these intermediates on the surfaces, along with the limited IR transmission at low wavenumbers on Si prisms (below 1200 cm-1) are possibly the main hurdles. In order to get key information of the C-C coupling process, advances in IR spectroscopy and ATR configurations should be highlighted. Ultrafast two-dimensional IR spectroscopy48 combined with surface enhancement effect may capture the transient high order reaction intermediates and deserve further study. The application of more powerful and purer IR source is also promising to enhance the detection capability toward very weak vibrational features. In addition, the detection of reaction intermediates at low wavenumbers needs the modification of ATR prism structures. Developing methods to prepare robust Cu thin films with strong surface enhancement effects on prisms with better IR penetration (e.g. ZnSe, Ge) to substitute commonly used Si may achieve this goal. However, the low electrochemical stability of ZnSe during potential cycling


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prevents it from directly using as a substrate for working electrodes. Instead, adopting a modified structure with a metal film deposited thin Si wafer pressed against a ZnSe prism is more promising.49, 50

2) In situ ATR-IR is capable of distinguishing the adsorption configuration of certain reaction intermediates, which can hardly be considered in traditional electrochemical techniques. For instance, *CO is well known to have various adsorption configurations (Figure 5d) on metal surfaces (COL, COB, and COM) with different vibrational frequencies. Their various binding strengths may greatly affect the reaction pathways. ATR-IR characterization on the relative ratio of different *CO adsorption configurations and possibly other intermediates can help construct more suitable models in theoretical approaches to understand the site-specific reaction pathways. Besides, the examination on the reactivity of detected species is needed to exclude the possibility that the observed IR bands are only related to spectators instead of real intermediates. The removal of reactants from the solution (e.g. depleting CO and CO2 by purging inert


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gases) after the detection of certain IR bands, followed by the real-time tracking on band intensity evolution is a possible strategy to achieve this goal.

3) The spectroscopic studies on CO2RR reaction mechanisms on real nanocatalysts are still at the beginning and deserve more efforts. Since the structure and composition of nanocatalysts can be relatively well and systematically controlled in the synthesis than those of prepared thin films, the impacts of these factors in CO2RR can be better understood.

AUTHOR BIOGRAPHIES

Shangqian Zhu is a PhD candidate in chemical and biomolecular engineering at the Hong Kong University of Science and Technology under the supervision of Prof. Minhua Shao. His research is focused on electrochemical and in situ spectroscopic studies on CO2 reduction reaction. Tiehuai Li is an MPhil candidate in chemical and biomolecular engineering at the Hong Kong University of Science and Technology under the supervision of Prof. Minhua Shao. His research is focused on advanced electrocatalysts for CO2 reduction reaction.


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Wen-Bin Cai is a professor at the Department of Chemistry at Fudan University. His research

interest

includes

spectroelectrochemistry

and

electrocatalysis.

http://www.echem.fudan.edu.cn/Default.aspx Minhua Shao is an associate professor of the Department of Chemical and Biological Engineering and the associate director of Energy Institute at the Hong Kong University of Science and Technology. His main research interest includes electrocatalysis and advanced

batteries.

Electrochemical

Energy

Laboratory

HKUST:

http://minhuashaogroup.wixsite.com/7102

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT

S. Zhu, T. Li and M. Shao acknowledge the support from Research Grant Council (26206115 and 16309418). S. Zhu thanks HKJEBN Group for providing the PhD scholarship. W. B. Cai acknowledges the financial supports from the 973 Program of the


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ACS Energy Letters

Chinese Ministry of Science and Technology (2015CB932303), and Natural Science Foundation of China (21473039 and 21733004).

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High-Quality Frequency-Extended Detection of Surface Species at Electrodes. Anal.

Chem. 2008, 80, 166-171. (50) Shao, M.; Liu, P.; Adzic, R. R. Superoxide Anion Is the Intermediate in the Oxygen Reduction Reaction on Platinum Electrodes. J. Am. Chem. Soc. 2006, 128, 7408-7409.

Quotes to highlight in paper In the past four years, extensive applications of ATR-IR spectroscopy were witnessed in exploring various electrochemical interfacial phenomena in CO2RR, which include the direct observation on adsorbed reaction intermediates and their interactions with electrode surfaces, speculation of reaction mechanisms, and understandings of compositional and structural dependent activities.

The development of more advanced electrocatalysts for CO2RR requires the gradual switch from traditional try-and-error methodology to a rational design approach, where deep fundamental understandings of the reaction itself, as well as impacts of catalyst structure and composition on the catalytic activities are pre-requisites.


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CO2 Electrochemical Reduction as Probed through Infrared

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