In Situ Electrodeposited Indium Nanocrystals for Efficient CO2

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In-Situ Electrodeposited Indium Nanocrystals for Efficient CO Reduction to CO with Low Overpotential 2

Chunmei Ding, Ailong Li, Sheng-Mei Lu, Hefeng Zhang, and Can Li ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b01795 • Publication Date (Web): 22 Aug 2016 Downloaded from http://pubs.acs.org on August 23, 2016

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In-Situ Electrodeposited Indium Nanocrystals for Efficient CO2 Reduction to CO with Low Overpotential Chunmei Dinga, Ailong Lia, b, Sheng-Mei Luc, Hefeng Zhanga, b and Can Lia, *

a

State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences; Dalian National Laboratory for Clean Energy, Dalian 116023, China. b

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University of Chinese Academy of Sciences, Beijing 100049, China

State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China.

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ABSTRACTS: The dream of artificial photosynthesis that converts CO2 to fuels or chemicals has been dimmed by the lack of efficient catalysts. Herein, indium (In) based catalyst is prepared via in-situ electrodeposition on the carbon substrate from organometallic precursor during CO2 reduction reaction. It is found to be robust for CO2 reduction to CO promoted by imidazolium ionic liquid in acetonitrile. The onset overpotential is impressively low for non-noble metal materials, rivaling that of noble metal Ag. Moreover, the CO evolution rate is stable for 15 h, with a Faradic efficiency of around 99%. In the same condition, In catalyst deposited in-situ performs much better than that prepared ex-situ and most of the catalysts previously reported. This is ascribed to the intrinsic properties of in-situ generated In nanocrystals in well contact with the porous substrate, suggesting the advantages of in-situ preparation strategy. Besides, via coupling CO2 reduction reaction with water oxidation in aqueous anolyte, CO and O2 can be produced simultaneously with high efficiency, demonstrating the good performance of non-noble In based catalyst for reducing CO2 to CO and its possible application in artificial photosynthesis from water and CO2.

KEYWORDS: CO2 reduction; indium; in-situ; non-noble; ionic liquids 1 INTRODUCTION CO2 is notorious as a kind of greenhouse gas while can be utilized and recycled as an abundant carbon feedstock through reducing CO2 to useful chemicals or fuels. CO, as a reduced product of CO2, is an important feed in chemical industries, such as producing H2 via water-gas shift reaction and generating liquid fuels by Fischer-Tropsch synthesis.1-2 Many kinds of homogeneous catalysts show high initial activity for CO2 conversion to CO but are mostly unstable for long time reaction.3-6 Electrocatalytic reduction of CO2 to CO via heterogeneous catalysts is a promising strategy which has drawn much attention.7 However, the large overpotential due to the inertness of CO2 and the low selectivity of products particularly as the competitive proton reduction reaction in aqueous make the efficiency far 2 ACS Paragon Plus Environment

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from satisfactory.8-11 To suppress the proton reduction reaction, CO2 reduction in non-aqueous system is a good tactic. What’s more, the concentration of water as a reagent can be easily regulated and the solubility of CO2 in organic solvent is higher by nearly one order of magnitude.8 Various catalysts especially noble metals Au12-13, Ag14-16 show excellent performance for CO evolution in organic and aqueous systems, but these catalysts have a common disadvantage of high cost which is undesirable from a long-term perspective. Non-noble metal materials such as and Bi17-19 and Sn17 were previously prepared via ex-situ or in-situ electrodeposition on inert glass carbon electrode and proved to be effective for CO2 reduction in acetonitrile (AN) with suitable ionic liquid (IL) promoter. Besides, Cu20-22, Zn23, MoS224-25 and MoO2/Pb26-27 are also active for CO2 reduction to CO under various conditions but the Faradic efficiencies (FE) is still unsatisfying and the overpotential is high. Indium (In) has also been studied for CO2 reduction, while it usually generates HCOOH and CO with low selectivity no matter in organic or aqueous systems.28-34 In a word, most of the catalysts reported to date still suffer from high overpotential, low activity and uncontrollable selectivity etc. And the design and fabrication of electrodes with uniform and highly dispersed catalysts and good ohmic contact between the catalyst and the underlying substrate, which is critical for the activity and stability of the electrode, remains a big challenge.18, 26 Therefore, exploring inexpensive CO2 reduction catalyst with high selectivity and efficiency, and developing novel fabrication strategies for them are imperative. Herein, we found that In nanocrystals prepared by in-situ electrodeposition (designated as In(in-situ)) from acetylacetone salt (In(acac)3) precursor on commercial carbon (C) plate substrates is impressively active for CO evolution assisted by imidazolium IL in AN. The onset overpotential is strikingly low for non-noble metal materials, rivaling that of noble metal Ag. The activity is stable during 15 h, with a FE of about 99% even after removing the In precursor. Under the same condition, the In(in-situ) nanocatalyst is more active than In prepared ex-situ (denoted as In(ex-situ)) and most of the catalysts reported before. Besides, by 3 ACS Paragon Plus Environment

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coupling the CO2 reduction half reaction with water oxidation in aqueous anolyte, CO and O2 can be evolved simultaneously with high efficiency. 2 RESULTS AND DISCUSSION The CO2 reduction activity was firstly tested by linear sweep voltammetric (LSV) scans and calculating the current density (J) difference in Ar and CO2 atmosphere (Figure S1a,b). As shown in Figure 1a, the CO2 reduction activity is nearly zero with the blank C electrode. In contrast, the C electrode with In(ex-situ) catalyst shows significant CO2 reduction current, suggesting that In is rather effective for CO2 reduction. And the activity in the presence of 1-butyl-3-methylimidazolium hexafluorophosphate (BMIMPF6) IL is much higher than that with tetrabutylammonium hexafluorophosphate (TBAPF6), with the onset potential positively shifted by 200 mV (from -1.89 V to -1.69 V), possibly as the IL mediator plays an important role in activating CO2 and stabilizing the intermediates through the formation of BMIM--CO2 complex16, 19, 35, as described in Scheme S1. Further, we tried to deposit In catalyst via in-situ method from molecular precursor In(acac)3 during CO2 reduction. From cyclic voltammetry (CV) measurements, the reduction of In(acac)3 starts at around -1.5 V (Figure S1c). With In(in-situ) catalyst, the onset potential of CO2 reduction further moves to -1.65 V and the J is also increased. From Figure 1b, the J of In(in-situ) catalyst at -1.9 V is about threefold that of Ag and sevenfold that of Bi catalyst which is deposited in-situ from Bi(OTf)3 precursor and has been one of the most effective non-noble metal catalysts for CO evolution in AN18. An enlarged view of the LSV curves is given in Figure S1d for more accurate comparison. The onset potential of In(in-situ) is the same with the most popular noble metal catalyst Ag. Compared with the onset potential of Bi (-1.8 V, consistent with the reported value18), it is more positive by 150 mV, and the potential at 10 mA cm-2 is positively shifted by about 200 mV. The standard E°(CO2/CO) value and the overpotential of CO2 reduction are difficult to be evaluated in organic systems and can only be estimated from the pKa,17-19, 36 as they


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depend on the proton donating ability of the electrolyte. For In(in-situ) catalysts, the estimated onset overpotential is -130 mV which is close to the value previously reported for Ag.18 To be more confident about the onset potential, we measured the CO evolution at -1.65 V using a C electrode in In(acac)3 solution or that predeposited with In(in-situ) catalysts, and it is confirmed that CO can be produced consistently during 5 h with an activity comparable with that of Ag (Figure 2a). What’s more, the activity is much higher in the presence of BMIMPF6 than the cases of 1-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF4) and TBAPF6 under the same condition (Figure 2b). Obvious CO evolution is observed at -1.7 V in the presence of IL while there is no activity with TBAPF6. Besides, the activity is optimized with the concentration of In(acac)3 and the species of solvents, substrates and In precursors (Figure S2). Figure 3a shows that the optimized activity of In(in-situ) from In(acac)3 is about fourfold that of In foil and twofold that of In(ex-situ) (its activity is also optimized, Figure S3) under the same potential -1.9 V, suggesting the superiority of In(in-situ) catalyst. And the CO evolution rates are consistent with the LSV results above. Most importantly, the results of control experiments show that there is no CO production without CO2, and the catalytic activities of blank C and Ni foil are negligible even at very negative potential -2.2 V (Figure 3b). This certifies that CO is indeed generated via the CO2 reduction reaction catalyzed by In catalyst instead of other side reactions of the electrolyte or the electrode itself. Besides, various other catalysts can also be deposited on C electrodes by similar methods (Figure S4), and there are obvious CO evolution with Bi, Cu etc. which are all inferior to In(in-situ) from In(acac)3 (Figure S5). This suggests that efficient CO production is possible only with appropriate materials and catalyst precursors, and the interactions between the IL, CO2, and the cathode surface may be critical. To understand the results above, we further characterized the catalysts in detail. The scanning electron microscopy (SEM) images (Figure 4a) shows that the smooth surface of C plate is covered by a layer of In particles with about 5 µm in size after depositing In(ex-situ) 5 ACS Paragon Plus Environment

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which shows obvious metallic In signals in X-ray diffraction (XRD) patterns (Figure S6a). While after in-situ deposition of In, the electrode surface becomes porous possibly stems from the evolution of large amounts of CO gas bubbles and the immersion in organic solution for a long time. And there are many graphite nanosheets covered uniformly with In nanocrystals of around 30 nm (Figure 4b,c). Clear lattice fringes of In(101), (002) and (112) facets can be observed in the high resolution transmission electron microscopy (HRTEM) images (Figure 4d and Figure S6b,c). Furthermore, the activities of various materials normalized with the electrochemical surface area (ECA) and the catalyst amount were compared (Figure S7, S8). The relative ECA-normalized activities of In(in-situ), In(ex-situ) and Bi under -1.9 V follow a ratio of 10 : 7 : 1, close to the results based on the geometric area in Figure 3a. Besides, the activity normalized with the catalyst amount of In(in-situ) is also much higher than those of other materials. Therefore, it is concluded that the activity difference is mainly caused by the intrinsic properties of in-situ formed In nanocrystals on C substrates. And the in-situ electrodeposition strategy with organic precursors is critical for the formation of these highly active nanoparticles. The local micro environment and the adsorption of the molecular precursor on the substrate during the deposition may be favorable for the uniform deposition of In nanoparticles in intimate contact with the substrate, which needs further research. Besides, In nanoparticles on the C substrate may generate a metastable interface between IL cations, which may be especially critical for the adsorption and activation of CO2 on the active site, as it has been demonstrated that the interplay between adsorbed imidazolium cations and CO2 at bulk Pt electrodes can significantly enhance CO2 reduction reaction.16, 37-38 Figure 5 shows the I-t curve, the rate and FE of CO evolution of a C electrode with In(in-situ) catalysts during long time reaction under -1.9 V. During the initial 4.5 h, the electrolyte contained 1 mM In(acac)3 and it was then changed to fresh solution without In(acac)3. The activity is relatively stable (about 15 mA cm-2) during 15 h even after removing 6 ACS Paragon Plus Environment

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In(acac)3. The initial CO evolution rate is relatively low, and then it increases as the deposition of In catalyst, and reaches an optimal and relatively stable value after about 4 h with a steady state FE of above 97%. After In(acac)3 is removed from the system, the activity can recover to the value before and then operates with an average FE of about 99% (Movie S1). Besides, In(acac)3 itself shows no activity for CO2 reduction, analyzed from the LSV results (Figure S9). And the activity of In is decreased remarkably after being oxidized in air although it can be partially recovered after several cathodic scans (Figure S10). In other words, the in-situ preparation method has an advantage of avoiding the oxidation deactivation of catalysts. What’s more, these results make it believable that the in-situ generated metallic In is the active species for CO2 reduction. The turnover-frequency is about 0.03 s-1 calculated from the deposited In amount and the CO evolution rate at -1.9 V (details in the supporting information). Possible products in the liquid phase were also confirmed, and only a small amount of HCOOH (3.7 µmol h-1 cm-2, FE 0.2% in average) was detected by Ion Chromatography. The activities above are all about CO2 reduction half reaction. The proton donors are supposed to be [BMIM]+17-19, 36 (details in the supporting information). To further couple CO2 reduction with water oxidation reaction acting as the proton source, we used aqueous anolyte separated from the catholyte by a nafion membrane, and the activity with sodium phosphate is the highest among various electrolytes (Figure S11). More importantly, Figure 6 shows that CO and O2 can be simultaneously evolved from the system for a long time. The FE of O2 evolution is constantly 100%, while it is around 85% for CO evolution. That’s because water permeates slowly to the cathodic side through the membrane resulting in the production of liquid products (HCOOH is detected by Ion Chromatography). Nevertheless, the steady partial current of CO evolution (JCO) after 5 h at -1.9 V (about 10 mA cm-2) is still much higher than that previously reported (< 0.1 mA cm-2 at -1.95 V19). This result displays the potential application of the In(in-situ) catalyst for artificial photosynthesis with water and 7 ACS Paragon Plus Environment

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CO2. This kind of catalysts on porous substrates may be directly used as dark cathodes in a photoelectrochemical or photovoltaic-electrolysis cell. While it is difficult to directly apply them on photoelectrodes as they scatter light that might otherwise be absorbed by the photoabsorber. Other deposition methods of these catalysts on photoelectrodes should be further developed. 3 CONCLUSION In nanocrystals in-situ deposited on C substrates via electrodeposition from In(acac)3 precursor are found to be robust for CO evolution promoted by imidazolium IL in AN. The activity is stable during 15 h, with a FE of about 99% even after removing In precursor. Under the same condition, In(in-situ) catalyst is much more active than In(ex-situ) and most of the catalysts reported before including Ag and Bi. The onset overpotential is one of the lowest values ever reported for non-noble materials. The superior activity of In(in-situ) is mainly ascribed to the intrinsic properties of In nanocrystals in well contact with the C substrate, indicating the advantages of in-situ catalyst preparation strategy. Via coupling the CO2 half reaction with water oxidation in aqueous anolyte, CO and O2 can be simultaneously produced with high efficiency. This demonstrates the excellent performance of inexpensive In based catalyst for CO2 reduction, and predicts its application potential in solar fuel production from water and CO2. 4 EXPERIMENTAL SECTION Electrode substrates were ultrasonically cleaned with acetone, ethanol and water prior to use. Graphite C plates were precalcined in air at 500 oC for 3 h. All other materials were used as purchased without further purification (Table S1). Electrochemical measurements were conducted in a three-electrode setup with Pt counter electrode and SCE reference electrode. All potentials were versus SCE without iR compensation, and were controlled by a potentiostat (CHI 760d). Although SCE is more suitable for aqueous systems, it is found that


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the absolute potential of the SCE electrode changes little (< 5 mV) before and after the reaction for about 20 h, so it is safe to be used as reference electrode here in the AN system. In(ex-situ) was electrodeposited (-0.85 V, 40 min) on C plate in an aqueous solution containing InCl3 (10 mM), HCl (1 M) and KBr (0.5 M). CO2 reduction electrolysis and in-situ preparation of catalysts were conducted in a gas-tight two-compartment cell with nafion membrane (117, Dupont) separating the working and counter electrode compartments (Figure S12). Both sides contained AN solution with BMIMPF6 (300 mM), BMIMBF4 (300 mM) or TBAPF6 (100 mM) as supporting electrolyte. The concentration of catalyst precursors is 1 mM, and the electrolytes were presaturated and bubbled with CO2 during the measurements. The gas evolution rate was determined by an online gas chromatograph (Agilent GC7890, Ar carrier and TDX-01 column) equipped with TCD and FID detector with a methanizer. Liquid products were detected by Ion Chromatography (SHINEHA CIC-100i, Shodex IC SI-52 4E column). For the LSV and physical characterizations, the in-situ prepared catalysts were predeposited at -1.9 V for 5 h. And for the optimization and comparisons of CO2 reduction activities, the activities were taken as the relatively stable values during step-potential electrolysis at -1.7 V, -1.9 V and -2.1 V for 1–2 h respectively, otherwise mentioned. To determine the overpotential of the reaction (CO2 + 2 H+ → CO + H2O), the strongest acid in the reaction system must be considered. Herein, the most possible H+ source in the system is the de-protonation of the C-H bonds in [BMIM]+ (pKa ~32 in AN) forming a imidazolylidene N-heterocyclic carbene.39 Therefore, the standard E°(CO2/CO) is estimated to be -1.78 V vs. SCE according to the equation E°(CO2/CO) = 0.105 - RT·(ln10)·pKa/F (vs. SCE).17-19, 36 So the onset overpotential of CO2 reduction to CO on Ag foil and In(in-situ) is -130 mV, comparable with that previously reported for Ag.18 The ECA is determined by the double-layer capacitances calculated from the plot of discharging current densities vs. the scan rate[7, 8] (Figure S7). The amounts of catalysts on C electrodes are determined by ICP measurements. And for the cases of metal foils, it is 9 ACS Paragon Plus Environment

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calculated from the mass of metal foil (1 cm2) supposing the active thickness is the upper 3 µm (consistent with the thickness of the In(ex-situ) electrode). The results of catalyst amounts are summarized in Table S2.


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Figure 3. (a) The optimized CO evolution rates of C electrodes with In(in-situ), In(ex-situ), In foil, Bi (in situ formed from Bi(OTf)3) and Ag foil in AN-BMIMPF6 solution at -1.7 V and -1.9 V; (b) The CO evolution rates with Ni foil and C electrodes with In(in-situ) catalyst under Ar atmosphere and bare Ni and C electrode without In catalyst in the presence of CO2 at potentials of -1.7 V, -1.9 V, -2.1 V and -2.2 V.


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Figure 4. SEM images of C electrodes with (a) In(ex-situ) and (b) In(in-situ) catalysts; The corresponding (c) TEM and size distribution of In particles and (d) HRTEM images of In(in-situ) nanocrystals scrapped off from the C electrode.


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Figure 6. The time courses of O2 and CO evolution rates and FECO of the C electrode in AN-BMIMPF6 catholyte containing In(acac)3 (1 mM) at -1.9 V. Anolyte: sodium phosphate (0.5 M, pH 7). ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website. Details about chemicals and materials, electrochemical measurement, XRD, SEM, (HR)TEM, XPS, ICP, TOF calculation, products analysis and activity normalization. 14 ACS Paragon Plus Environment

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ACKNOWLEDGMENT This work was financially supported by 973 National Basic Research Program of the Ministry of Science and Technology of China (No. 2014CB239400), the National Natural Science Foundation of China (No. 21603225, 21573230). AUTHOR INFORMATION * Corresponding authors: Email: [email protected]; NOTES The authors declare no competing financial interest. REFERENCES (1)

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