Monitoring the Chemical State of Catalysts for CO2 Electroreduction

Letter pubs.acs.org/acscatalysis

Monitoring the Chemical State of Catalysts for CO2 Electroreduction: An In Operando Study Abhijit Dutta,† Akiyoshi Kuzume,† Motiar Rahaman,† Soma Vesztergom,†,‡ and Peter Broekmann*,† †

Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, Bern 3012, Switzerland Department of Physical Chemistry, Eötvös Loránd University, Pázmány Péter sétány 1/A, Budapest 1117, Hungary

‡

S Supporting Information *

ABSTRACT: A major concern of electrocatalysis research is to assess the structural and chemical changes that a catalyst may itself undergo in the course of the catalyzed process. These changes can influence not only the activity of the studied catalyst but also its selectivity toward the formation of a certain product. An illustrative example is the electroreduction of carbon dioxide on tin oxide nanoparticles, where under the operating conditions of the electrolysis (that is, at cathodic potentials), the catalyst undergoes structural changes which, in an extreme case, involve its reduction to metallic tin. This results in a decreased Faradaic efficiency (FE) for the production of formate (HCOO−) that is otherwise the main product of CO2 reduction on SnOx surfaces. In this study, we utilized potential- and time-dependent in operando Raman spectroscopy in order to monitor the oxidation state changes of SnO2 that accompany CO2 reduction. Investigations were carried out at different alkaline pH levels, and a strong correlation between the oxidation state of the surface and the FE of HCOO− formation was found. At moderately cathodic potentials, SnO2 exhibits a high FE for the production of formate, while at very negative potentials the oxide is reduced to metallic Sn, and the efficiency of formate production is significantly decreased. Interestingly, the highest FE of formate production is measured at potentials where SnO2 is thermodynamically unstable; however, its reduction is kinetically hindered. KEYWORDS: carbon dioxide electroreduction, tin dioxide, formate, Raman spectroscopy, Faradaic efficiency

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Faradaic efficiencies (FEs) of formate production5 are quite sporadic and range between 10 and 90%. Without any doubt, the scatteredness of the reported FE values5 indicates that the electrocatalytic activity of Sn strongly depends on its morphology, chemical state, and the conditions of the electrolysis. This is also supported by some recent studies of the variation of product distribution as a function of pH5g or as a result of surface deactivation or etching.5f In addition, it was pointed out by Kanan et al.5h that removing the native oxide layer of tin electrodes heavily decreases their catalytic activity and that electrodeposited SnOx layers are more catalytic than bulk tin itself. A few studies dealing with CO2 reduction on nanoparticulate tin oxide have surfaced in the past year.6 Most recently, Bocarsly et al.7 applied in situ ATR−IR spectroscopy for studying the mechanism of CO2 reduction on Sn films covered by SnOx. A clear conclusion of these studies is that while using SnOx for the catalysis of CO2 reduction, the oxidation state of the catalyst cannot always be maintained at the highly cathodic

INTRODUCTION Primarily due to its profound effect on the world’s climate,1 the growing carbon dioxide content of the atmosphere raised noticeable attention in the past century. As a result, a number of technologies2 including capture and sequestration have been devised in order to reduce the levels of carbon dioxide in the air. Among these technologies, electrochemical reduction deserves particular attention because it can turn CO2an otherwise useless or even harmful materialinto fuel or other products with high added value. This advantage cannot be ignored, also from an energetic aspect, if we realize that by the reduction of CO2, we may store solar or electrical energy in the form of a reduced carbon compound. Undoubtedly, this is the reason for which the electroreduction of CO2 is in the focus of a continuously growing interest, and ever since it was first described by the pioneering work of Royer3 in 1870, the number of literature reports dealing with it is ceaselessly rising.4 It is known that on Sn (and also on Pb, Hg, Cd, and a few other metals), the electroreduction of CO2 proceeds at a relatively high overpotential; however, it selectively yields formate or formic acid,4a which are substances of high value with a very promising use, for example, in fuel cells.4b,c The electroreduction of CO2 on tin is, therefore, very widely studied; however, the reported © 2015 American Chemical Society

Received: October 16, 2015 Revised: November 17, 2015 Published: November 18, 2015 7498

DOI: 10.1021/acscatal.5b02322 ACS Catal. 2015, 5, 7498−7502

Letter

ACS Catalysis operating conditions, and the reduction of SnOx to Sn often results in a decreased FE for formate production. In this paper, in operando Raman spectroscopy is applied in order to investigate this problem and to establish a correlation between the stability of SnO2 and its activity toward formate formation. For the purposes of this study, tin oxide nanoparticles (SnO2 NPs) were synthesized both with and without a support of reduced graphene oxide (rGO) layers. The catalyst was applied for the electrolysis of CO2 in aqueous solutions of different pH values. Previous studies in the literature6d,8 revealed that the stability of tin oxides depends heavily both on the pH8c and the applied electrode potential,8d and thus, we refrained from pH < 7 values where SnO2 is more expected to undergo electroreduction, even at moderate cathodic potentials. Instead, measurements were carried out with slightly or heavily alkaline solutions of pH = 8.5, 9.7, and 12, which we prepared by bubbling CO2 through a 0.5 mol dm−3 (pH = 13.5) NaOH solution, until the desired pH was reached. At pH = 8.5, the prevalent carbonate species in the solution is HCO−3 , at pH = 12 it is CO2− 3 , whereas at pH = 9.7, both carbonate and hydrocarbonate ions are present in the system at approximately equal concentrations.6d By the analysis of Raman spectra acquired in operando (that is, during the electrolysis of CO2), and by determining the FE of formate production with ion chromatography, we found a strong correlation between the oxidation state of the SnO2 NPs and their FE for the formation of HCOO− ions.

Figure 1. SnO2NPs@rGO catalyst. (a) TEM image with a size distribution histogram shown in the inset. (b) High-resolution TEM image of a catalyst NP showing a fringe-like pattern; the d-spacing is established based on the FFT image in the inset. (c) High-resolution XPS spectrum of the Sn 3d region. (d) Normal Raman spectrum of the catalyst, as-prepared.

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RESULTS AND DISCUSSION The tin oxide NPs were synthesized both with and without a support of reduced graphene oxide (rGO) via a nonhydrolytic solvothermal reaction with hexanoate complexes (see the Supporting Information for details of the synthesis). Transmission electron microscopy (TEM, Figure 1a) showed that the SnO2 NPs with an average particle size of (4.4 ± 0.9) nm are uniformly dispersed on the rGO surface without aggregation. For SnO2 NPs without an rGO support, a very similar size distribution is observed by TEM (see Figure S1a in the Supporting Information). The high-resolution (HR) TEM image of a single rGO-supported NP (Figure 1b) revealed a fringe-like pattern: aided by the corresponding FFT (inset of Figure 1b), an interplanar d-spacing of ∼0.334 nm was determined, which can be attributed to the (110) plane of SnO2. In the selected area electron diffraction pattern (SAED, see Figure S1b in the Supporting Information), diffraction rings were indexed by the crystal planes (110), (101), (211), (112), and (301) of SnO2, which clearly confirms the presence of SnIV in the nanocomposite material. In addition, the X-ray diffraction (XRD) pattern of the SnO2 NPs (Figure S2) showed X-ray diffraction lines which can be assigned9 to cassiterite, a SnO2 phase of tetragonal rutile structure. By means of X-ray photoelectron spectroscopy (XPS), the chemical composition of SnO2NPs@rGO was analyzed as well. Only Sn, O, and C were observed in the survey spectrum (see Figure S3a in the Supporting Information), and no other peaks of metallic impurities were detected. Figure 1c shows the highresolution XPS spectrum of Sn 3d in SnO2NPs@rGO. The binding energies (BEs) of Sn 3d5/2 and 3d3/2 were determined as 486.6 and 495.0 eV, respectively, suggesting10 that the oxidation state of Sn in the nanoparticles is SnIV and that no other oxidation states prevail (as shown also by the fitted curves in Figure 1c). The deconvolution of the C 1s region from rGO gave four peaks assigned to C−C/CC, C−O, CO, and

OC−O at 284.4, 286.0, 287.3, and 289.0 eV, respectively (see Figure S3b in the Supporting Information). The peak associated with C−C/CC is predominant, confirming the nearly complete reduction of GO. The high-resolution O 1s spectrum of SnO2NPs@rGO (Figure S3c in Supporting Information) shows asymmetric peaks that can be deconvoluted to two peaks at 531.7 and 533.6 eV; the former peak can be assigned to the coordination of oxygen bounded to tin atoms, whereas the latter is due to the loss of oxygen.10c A normal Raman spectrum of as-prepared SnO2NPs@rGO is shown in Figure 1d. Here the two peaks appearing at 1355 and 1593 cm−1 correspond to the D and G bands of carbon materials, respectively. Usually the D band is correlated with the breathing mode of k-point phonons of A1g symmetry, while the G band is related to the E2g phonons of sp2 carbon atoms.11 In addition, Figure 1d also displays three other peaks at 482, 623, and 762 cm−1, which can be assigned to the Eg, A1g, and B2g modes of SnO2 crystallites, respectively.12 Figure 2 shows linear sweep voltammograms (LSVs) recorded on SnO2NPs@rGO (cast-and-dried on glassy carbon) in four different solutions. One of these is 0.5 mol dm−3 NaOH (pH = 13.5) and contains no CO2; the rest of the solutions were prepared by dissolving CO2 in this solution until the given pH values (12, 9.7 and 8.5) were reached. For all CO2-containing solutions, the LSVs of Figure 2 exhibit a moderate cathodic current at low cathodic potentials that can be attributed to the reduction of CO2.6d By a closer inspection of the curves, it is apparent that the highest current related to CO2 reduction can be measured in the pH = 9.7 solution. As the potential reaches more cathodic values, the current increases; however, some other processes−hydrogen evolution13 and the reduction14 of SnO2 to Sn−are also triggered. At E = −1.6 V versus Ag | AgCl, the total current is already dominated by hydrogen evolution as it is suggested by 7499

DOI: 10.1021/acscatal.5b02322 ACS Catal. 2015, 5, 7498−7502

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ACS Catalysis

Figure 2. Electrochemical investigations. (a) Linear sweep voltammograms (sweep rate: 10 mV s−1) showing the pH dependence of the cathodic current while CO2 is reduced on SnO2NPs@rGO. (b) Cyclic voltammograms (sweep rate: 50 mV s−1) indicating the reduction of SnO2 to Sn after a certain cathodic potential is reached (compare the solid and the dashed curves with different lower vertices: the small peak marked by an * in the anodic segment appears only on the solid curves).

the dependence of current versus pH. In general, it seems reasonable to interpret the total voltammetric response shown in Figure 2a as a result of three parallel processes: (i.) the reduction of CO2 or another related carbonate species yielding formate as a main product; (ii.) the hydrogen evolution reaction; and (iii.) the reduction of the SnO2 catalyst which yields tin species of lower oxidation number (that is, metallic Sn and probably SnII species as well). By means of cyclic voltammetry (CV, Figure 2b), it is possible to estimate the electrode potential limit below which the reduction of the catalyst may be expected (for additional data, see Figure S4 in the Supporting Information). The small positive peak in the anodic scans of the CVs (marked by * in Figure 2b) appears after the lower vertex is extended to a sufficiently negative value. This peak is attributed to the reoxidation of tin that was formed in the preceding cathodic sweep.14 (The formation of Sn at these negative potentials can also be verified by means of XRD measurements, see Figure S2 in the Supporting Information.) The potential limit triggering the reduction is slightly pH-dependent, and it is determined as −1.25, −1.40, and −1.40 V versus Ag | AgCl in the pH = 8.5, 9.7, and 12 solutions, respectively. This, as we will see below, is in agreement with the observations made by Raman spectroscopy. Results of demonstrative in operando Raman experiments are shown in Figure 3. For these studies, the synthesized SnO2 NPs (without an rGO support) were drop-cast onto glassy carbon. The potential dependence of the steady-state Raman spectra, acquired a few minutes after the application of the potential set-points, are presented in Figure 3a for the pH = 8.5, 9.7, and 12 solutions. The relative intensities of the A1g peaks in the spectra, attributed to crystalline SnO2,15 are plotted in Figure 3b as a function of the applied electrode potential. Three potential regions labeled from I to III are to be distinguished here: in region I, the catalyst is present in its native SnO2 form; in the intermediate region II, the catalyst is partially reduced; in region III, the reduction (to Sn) is already complete. It is well demonstrated by the Pourbaix diagram of Figure 4 that under the operating conditions of CO2 electrolysis the

Figure 3. In operando Raman studies at varied potential and pH. (a) The potential dependence of the Raman spectra for each studied pH. (b) The relative intensities of the SnIV-related A1g Raman peaks (○, solid line) and the Faradaic efficiencies of formate production (× , dashed line) as a function of electrode potential. In the three distinct potential regions represented by the shaded background, the catalyst is in the form of fully oxidized SnO2 (I), a partially reduced compound of mixed oxidation state (II) and completely reduced metallic Sn (III), as illustrated by the scheme of (c).

Figure 4. Pourbaix diagram of Sn and its hydrous oxides, adapted from ref 8b. The colored background matches that of Figure 3 and indicates that under the operating conditions of CO2 electrolysis, the actual formation of metallic Sn takes place only at potentials more negative than the thermodynamically predicted ones. The intensities of the Raman peaks related to the A1g mode of SnO2, however, already start to decrease mildly as the potential exceeds the stability region of hydrous SnIV oxide. The maximal FE values (for the production of formate) can be measured in this potential region.

actual transition of the SnO2 NPs to metallic Sn requires more negative potentials than what could be predicted based on 7500

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stabilized and showed no further changes with time, even for a long-lasting electrolysis. This is well-demonstrated by Figure 5 for the pH = 9.7 solution: here electrolyses were carried out at different potentials (E = −0.25, −1.1, and −1.5 V versus Ag | AgCl), while the currents shown in Figure 5a were measured. The simultaneously collected Raman spectra (Figure 5c), although they had different peak intensities at each potential (Figure 5b), did not show any significant temporal change as the electrolysis continued for 1 h. More details of the in operando Raman studies are available in the Supporting Information, see Figure S6.

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CONCLUSION The results presented in this paper point out the necessity of the in operando characterization of electrocatalysts. By a Raman spectroscopic survey of nanoparticulate SnO2, a strong correlation has been established between the oxidation state of the NPs and their FE for the production of formate. In agreement with previous literature studies,5−7 we found that formate is produced with a high FE on SnO2 catalysts; however, the FE drops when the applied potential is negative enough for reducing the catalyst to metallic Sn. In addition to this, we have also shown by in operando Raman monitoring that the practical (kinetic) stability region of SnO2 well exceeds the thermodynamic stability window (as determined based on the Pourbaix diagram), and furthermore, we have shown that in fact the highest selectivity for the production of formate (with FE ≈ 80%) in alkaline CO2 solutions can be measured in a potential range, where the SnO2 phase is metastable but still present in the NPs. At these potentials, the SnO2-related Raman signals are only mildly decreased, hinting a partial reduction of the NPs. The total reduction to metallic Sn takes place at potentials more negative than what the Pourbaix diagram of the system predicts; at this potential range, both the intensity of the SnO2related A1g modes and the FE of formate production are dramatically decreased. Among the studied pH levels, the highest overall formate production rate (as established by ion chromatography) was measured at pH = 9.7. The FE of formate production also shows a maximum at this pH level, approximately at −1.1 V versus Ag | AgCl.

Figure 5. Potentiostatic electrolyses at pH = 9.7. (a) Electrolyzing currents at −0.25, − 1.1, and −1.5 V, each measured for 1 h. (b) The relative intensities of the A1g Raman mode remain constant during the time of the electrolysis. (c) The overall shape of the Raman spectra remains practically unchanged as the electrolysis proceeds for 1 h.

thermodynamic data.8b The significant overpotential of the reduction can be explained by the band-edge pinning that occurs in semiconductor tin oxides8d and that also results in a broadened potential range of SnO2 reduction. As indicated by the results of cyclic voltammetry, Raman, and XRD surveys, the full reduction of the SnO2 NPs (to metallic Sn) occurs at very negative potentials; however, at less negative potentials, the formation of SnII species can already be expected and can involve the partial dissolution as well as some structural changes of the catalyst NPs. In order to correlate the results of in operando Raman experiments with the selectivity of the catalyst for the production of formate, ion-chromatographic analysis (see Figure S5 in the Supporting Information) has been utilized. The concentration of formate was measured in a cell of known volume after electrolyses consuming known amounts of charges were carried out at SnO2 NPs supported by rGO; the thus obtained FE values were also plotted in Figure 3b. By comparing the measured FEs with the results of Raman spectroscopy, it is apparent that the FE of formate production strongly depends on the oxidation state of the surface. At moderately cathodic potentials, the FE is increasing with decreasing E; however, as E tends to be more negative, this tendency breaks and the FE curves go over a maximum. At very negative potentials, where the catalyst is completely reduced to metallic Sn, the FE for the production of formate and the intensity of the SnO2-related A1g peaks are both heavily decreased. Interestingly, the maxima of the FE curves are at such potentials where the thermodynamically stable phase should be metallic Sn; however, the reduction of the SnO2 NPs is kinetically hindered (see also Figure 4). Consequently, the Raman peak related to the SnO2-specific A1g mode is still of considerable intensity, indicating that the catalyst is only partially reduced (either to Sn0 or SnII) and the SnO2 phase is still prevalent. With respect to Figure 3, it is important to emphasize that the presented Raman spectra (acquired at different values of pH and electrode potential) were stationary in the sense that after the application of a certain potential, they were quickly

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.5b02322. SAED, XRD, XPS, and Raman characterization of the SnO2 nanoparticles, details concerning their synthesis, and results of further experiments on the reduction of CO2 (data of electrochemical, Raman, and ion-chromatographic measurements) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the EraNet program (ECOON, project no. 156) and the University of Bern. The support by the 7501

DOI: 10.1021/acscatal.5b02322 ACS Catal. 2015, 5, 7498−7502

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ACS Catalysis CTI Swiss Competence Center for Energy Research (SCCER Heat and Electricity Storage) is gratefully acknowledged.

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DOI: 10.1021/acscatal.5b02322 ACS Catal. 2015, 5, 7498−7502