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Rational Design of Bi Nanoparticles for Efficient Electrochemical CO2 Reduction: the Elucidation of Size and Surface Condition Effects Zhiyong Zhang, Miaofang Chi, Gabriel M. Veith, Pengfei Zhang, Daniel A. Lutterman, Joel Rosenthal, Steven H. Overbury, Sheng Dai, and Huiyuan Zhu ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b01297 • Publication Date (Web): 08 Aug 2016 Downloaded from http://pubs.acs.org on August 13, 2016

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Rational Design of Bi Nanoparticles for Efficient Electrochemical CO2 Reduction: the Elucidation of Size and Surface Condition Effects Zhiyong Zhang,† Miaofang Chi,‡ Gabriel M. Veith,§ Pengfei Zhang,† Daniel A. Lutterman,† Joel Rosenthal,⊥ Steven H. Overbury,† Sheng Dai,†,∥ Huiyuan Zhu†,* †

Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, United

States ‡

Center for Nanophase Materials Science Division, Oak Ridge National Laboratory, Oak Ridge,

TN 37831, United States §

Materials Sciences and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN

37831, United States ⊥

Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware

19716, United States ∥

Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996, United States

ACS Paragon Plus Environment

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ABSTRACT We report an efficient electrochemical conversion of CO2 to CO on surfaceactivated bismuth nanoparticles (NPs) in acetonitrile (MeCN) at ambient conditions, with the assistance of 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([bmim][OTf]). Through the comparison between electrodeposited Bi films (Bi-ED) and different types of Bi NPs, we for the first time demonstrate the effects of catalyst’s size and surface condition on organic phase electrochemical CO2 reduction. Our study reveals that the surface inhibiting layer (hydrophobic surfactants and Bi3+ species) formed during the synthesis and purification process hinders the CO2 reduction, leading to a 20% drop in Faradaic efficiency for CO evolution (FECO). Bi particle size showed a significant effect on FECO when the surface of Bi was air-oxidized, but this sizedependence of FECO became negligible on surface-activated Bi NPs. After the surface activation (hydrazine treatment) that effectively removed the native inhibiting layer, activated 36 nm Bi NPs exhibited a nearly quantitative conversion of CO2 to CO (96.1% FECO), and a mass activity for CO evolution (MACO) of 15.6 mA mg-1, three-fold higher than the conventional Bi-ED, at 2.0 V (vs. Ag/AgCl). This work elucidates the importance of the surface-activation for an efficient electrochemical CO2 conversion on metal NPs and paves the way for understanding the CO2 electrochemical reduction mechanism in non-aqueous media.

KEYWORDS: bismuth nanoparticle •surface activation • electrochemical CO2 reduction • CO • ionic liquid

INTRODUCTION Electrochemical reduction of CO2 to useful fuels has been attracting research interest in recent years, since it is considered as an essential component in carbon-neutral cycle achieved by linking the storage of intermittent renewable energy (wind, solar, and tidal) with the utilization of


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anthropogenic CO2.1-4 However, electrochemical CO2 reduction reaction (CO2RR) usually suffers from sluggish kinetics and a broad product distribution, due to the involvement of multiple electron and proton transfers, and the competition with the hydrogen evolution reaction (HER). To improve the catalytic activity and selectivity, various metallic electrodes and reaction conditions have been evaluated in both aqueous and organic media.5 In buffered aqueous media, Au,6-10 Ag,11-13 and Zn14 have been identified with a good selectivity toward CO production because of their weak CO adsorption.15 CO production is further promoted when imidazoliumbased ionic liquids (IL) serve as the supporting electrolytes.16 Compared with aqueous solutions, aprotic media facilitate the CO2RR by suppressing the competing HER and enhancing the CO2 solubility.3 Especially with high pKa imidazolium-based ILs as the proton donors, aprotic media such as acetonitrile (MeCN) allow the CO2RR to CO to occur even on some earth abundant metal electrodes with lower market prices such as electrodeposited Bi,17 Sn,18 and Pb18. Recently, well-defined nanoparticle (NP) catalysts have shown improved activities for CO2RR. In contrast to the conventional electrodeposited electrodes, carefully designed NP catalysts may provide controllable facets and structures, offering an ideal platform for understanding and accurately correlating the catalytic performance with their physiochemical properties. In aqueous media, previous work on Pd,19 Au,7, 9, 10 Ag,11 and Cu,20, 21 have emphasized the importance of particle size and surface profile on CO2RR. For instance, the NP size has been considered as an important parameter that varies the adsorption energy of reactants and reaction intermediates and thus alters the competition between CO2RR and HER.7, 19, 20 Additionally, native surface oxides on the electrocatalysts have a non-trivial effect upon performance, as these oxides may be reduced or converted into more active surface structures,6,

22-24

and suppress the competing

HER.25 Similarly, it has been demonstrated that the catalytic activity for CO2RR is strongly


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dependent on the structure of the NP catalysts9-11, 14, 21 and the population of grain boundaries26. All these studies have been carried out in aqueous systems, leaving us immediate questions as to how the physiochemical parameters of the catalysts influence CO2RR in aprotic media, and how to develop even more efficient and practical catalysts for CO2RR. To address these issues, we have employed two types of monodispersed Bi NPs with an average size of 36 and 7 nm, respectively, and investigated their catalytic behavior for CO2RR in aprotic media. When exposed to air, the surface of Bi NPs is spontaneously oxidized. Owing to the lack of free protons in organic media, we anticipate the surface oxidized layer cannot be simply removed electrochemically as in aqueous media. Instead, the oxidized species and the bulky capping ligands on the NP surface form a surface inhibiting layer that constrains the electron transfer and mass transport during electrochemical reactions. We demonstrate that through the surface activation of NPs with hydrazine, which functions to both remove hydrophobic ligand and reduce of the oxidized layer, the inhibiting layer is substantially mitigated. Subsequently, the activated Bi NPs perform CO2RR with a Faradaic efficiency for CO evolution (FECO) of 96.1% under the optimized condition (Scheme 1). The comparison between Bi NPs with the electrodeposited Bi films (Bi-ED) allows a direct evaluation of the roles of particle size, surface oxidized layer, and organic surfactant residuals during CO2RR in aprotic media. As a result, we find that the capping ligands significantly inhibit the CO2RR, with a 20% drop in FECO. Interestingly, the size and morphology demonstrate trivial effects on FECO of activated Bi NPs, while become crucial parameters only when the surface is air-oxidized. These observations allow us to maximize the mass utilization of Bi metal, and thus cost-effectiveness, by using smaller Bi NPs. Under an optimal condition, the activated Bi NPs present a mass


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activity for CO evolution (MACO) of 15.6 mA mg-1, three-fold higher than the conventional BiED.

Scheme 1. Schematic illustration of surface-activation of Bi NPs for CO2RR. RESULTS AND DISCUSSION Bi NPs were synthesized via an organic solution chemical reduction of Bi precursors, by modifying a previously reported method.27 Typically, to prepare 36 nm Bi NPs, 1 mmol bismuth acetate (Bi(ac)3) was mixed with 1-octadecene (ODE). Then, 0.24 ml 1-dodecanethiol (DDT) was added into the reaction solution as a chelating agent to form bismuth thiolate complex, followed by the addition of 1 ml trioctylphosphine (TOP) as both the reducing agent and surfactant. A typical transmission electron microscopy (TEM) image of the as-synthesized Bi NPs is shown in Figure 1 (a), demonstrating the NPs are core-shell structured polyhedrons with an average size of 36 ± 0.5 nm. The core-shell structure is more clearly identified under the scanning transmission electron microscopy (STEM) (Figure 1 (b)) and high resolution TEM (HR-TEM) (Figure 1 (c)), which show a crystalline metallic core and an oxidized shell that resulted from the spontaneous oxidation in air. The lattice spacing imaged from the core region in Figure 1 (c) is 0.323 nm, corresponding to (110) lattice distance of rhombohedral Bi. Under the same synthesis conditions, varying the amount of TOP significantly influenced the size and dispersity of the final products. As shown in Figure S1, increasing the amount of TOP (2 ml) led to spherical NPs with a broad size distribution, while decreasing the amount of TOP (0.5 ml)


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resulted in uncontrolled, irregularly shaped large particles. Bi NPs with an average size of 7 ± 0.2 nm (Figure S2 (a)) were also synthesized by using bismuth neodecanoate as a precursor under the similar synthesis condition. The XRD patterns of both 7 and 36 nm as-synthesized Bi NPs (Figure S3) present a typical rhombohedral crystal structure, consistent with the lattice spacing observed in Figure 1 (c). Some additional diffraction peaks were observed in the XRD pattern of 36 nm as-synthesized Bi NPs, which were related to the oxidized species formed during air-exposure. Nevertheless, we did not observe any crystalline oxide phase on the 7 nm Bi NPs, indicating these oxidized species on small NPs were either amorphous or crystalline in finite regions below the detection limits.

Figure 1. (a) TEM, (b) STEM, (c) HR-TEM images of the as-synthesized 36 nm Bi NPs; and (d) TEM image of the 36 nm Bi/C.


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For comparison, we prepared a conventional Bi-ED on glassy carbon electrode (GCE) according to previous publications.17,

18, 28

The morphology of Bi-ED was characterized by

scanning electron microscopy (SEM). As shown in Figure S4, the electrodeposition produced a thick porous Bi film on the GCE, consisting of a variety of structures such as sheets, dendrites, and agglomerated particles with a broad size distribution in scales of submicrometer and micrometer. In contrast to Bi-ED, the Bi NPs clearly exhibit a more precisely controlled size and morphology, and therefore are more suitable for practical applications and structure-related mechanism investigations.

Figure 2. XPS spectra of (a) as-synthesized and (b) hydrazine treated 36 nm Bi/C; (c) assynthesized and (d) hydrazine treated 7 nm Bi/C


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The chemical nature of the as-synthesized Bi NPs was analysed by X-ray photoelectron spectroscopy (XPS), as shown in Figure 2 (a, c). The peaks at 157.0 and 159.0 eV in the highresolution XPS spectra were attributed to Bi0 and Bi3+ 4f7/2 respectively, clearly demonstrating the oxidation of these Bi NPs after exposure to air. The charge of Bi3+ was mainly balanced by oxygen, as detected by XPS (Figure S5). The integration shows a Bi0/Bi3+ ratio of 1:4 for the 36 nm as-synthesized Bi NPs (Figure 2 (a)). The Bi0/Bi3+ ratio for the 7 nm Bi NPs is only 1:13, indicating the smaller Bi NPs are more vulnerable to air-oxidation. In addition to being spontaneously surface oxidized, the as-synthesized Bi NPs are surrounded by the long carbon-chain TOP ligands, forming an inhibiting layer. The inhibiting layer may block the active site, prevent efficient mass and electron transfer, and thus impede efficient electrochemical reactions.29,

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To remove this layer, the as-synthesized Bi NPs were first

deposited on acetylene black (AB) with a metal loading of ~ 60 wt% (denoted as as-synthesized Bi/C and shown in Figure 1 (d) and S2 (b)). The Bi/Cs were then soaked in hydrazine monohydrate and washed with copious EtOH (Figure 3 (a)). During this process, hydrazine served as a Lewis base and gradually replaced the bulky long chain phosphine surfactants. The excess hydrazine was then washed away by EtOH. The existence of TOP on the as-synthesized NPs and its removal by hydrazine treatment were verified by Fourier-Transform infrared (FT-IR) spectra of unsupported Bi NPs (Figure 3 (b)). In FT-IR spectra, three strong adsorption peaks were observed on the as-synthesized Bi NPs at 2960, 2919, and 2850 cm-1, corresponding to the C-H stretching in TOP residuals.31 The intensities of these peaks significantly decreased after the hydrazine treatment, suggesting that most TOP was successfully removed, consistent with previous reports.32, 33 More importantly, the hydrazine treatment effectively reduced the surface oxidized layer. As evident in the XPS spectra, the Bi0/Bi3+ ratios for both 36 and 7 nm Bi NPs


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increased to ~ 1:2, after the hydrazine treatment (Figure 2 (b, d)). The remaining Bi3+ was probably due to the re-exposure of Bi/C to the air during the sample processing for XPS analyses. The reduction of Bi3+ was further confirmed in the XRD pattern (Figure S3), since most of the diffraction peaks associated with oxidized species disappeared after the hydrazine treatment.

Figure 3. (a) Scheme of the surface activation by hydrazine treatment. (b) FT-IR spectra of trioctylphosphine, as-synthesized Bi NPs, and hydrazine treated Bi NPs. We first evaluated the activity of 36 nm Bi/C for CO2RR in the MeCN solution containing 0.1 M 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([bmim][OTf]). To eliminate any possible surface oxidation during storage and electrode preparation, the as-prepared Bi/Cmodified GCE was immersed in hydrazine monohydrate overnight before the electrochemical measurements (denoted as freshly reduced Bi/C). As shown by solid lines in Figure 4 (a), the 36 nm freshly reduced Bi/C presents only a typical double-layer capacitive CV profile in inert N2 atmosphere, primarily arising from the carbon supports. With the supply of CO2, a cathodic current is observed that quickly increased at potentials more negative than -1.6 V, indicating CO2 is electrochemically reduced on the 36 nm freshly reduced Bi/C. In comparison with the CV trace on Bi-ED (dashed line in Figure 4 (a)), the freshly reduced Bi/C demonstrates an onset


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potential for CO2RR ~ 0.1 V more positive and over three-fold higher mass current density (at 2.0 V), clearly demonstrating the freshly reduced Bi/C exhibits a more efficient CO2RR. To probe the product formation, controlled potential electrolyses (CPE) were performed in the range of -1.8 to -2.2 V for two hours at each investigated potential. Figure 4 (b) presents the corresponding current transients, indicating that the continuous CO2RR occurs at potentials more positive than -2.0 V with almost constant cathodic currents. Further lowering the electrode potential, however, resulted in a higher initial current followed by a quick current decay, suggesting the system became unstable at such negative potentials. Especially at -2.2V, we noticed the decomposition of IL, with a light yellow color generated in the catholyte after CPE (Figure S6). The decomposition of IL was confirmed by Nuclear Magnetic Resonance (NMR) spectra, with the observation of multiple new peaks after CPE (Figure S7). Analyses of the cathode headspace during CPE with an online gas chromatograph suggest CO as the sole gaseous product under the investigated potentials (-1.8 to -2.2 V). The FECO on the 36 nm freshly reduced Bi/C is summarized in Figure 4 (c). A maximum FECO of 96.1 ± 0.7% is obtained at the potential of –2.0 V, showing that the electrochemical conversion of CO2 to CO on freshly reduced Bi/C occurs nearly quantitatively in the present IL/MeCN system. The FECO we observed on 36 nm freshly reduced Bi/C shows a significant enhancement compared with other nonprecious metal electrocatalysts in typical aqueous systems, such as Zn (79%),5, 14 Cu (20 – 60%),20, 34-37 and Sn (60%).23 It is also on par with precious metal electrocatalysts in aqueous conditions, for example, Ag (84 – 93%),11-13 Pd (91%),19 and Au (45 – 98%).6-8, 26, 38, 39 In aqueous media, the standard potential for electrochemical reduction of CO2 to CO is -0.52 V vs. SHE (-0.719 V vs. Ag/AgCl), at pH = 7.

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Given the low proton availability of the present IL/MeCN system, this

standard potential has been estimated to be -1.735V vs. Ag/AgCl on the basis of a pKa of 32 for


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the deprotonation of [bmim]+.17 This indicates the 36 nm freshly reduced Bi/C requires only 265 mV overpotential to achieve over 90% FECO. The FECO decreased at both higher and lower potentials. While the lower FECO at -2.2 V was mainly due to the electrochemical decomposition of ILs, the drop in FECO at -1.8 V should be attributed to the insufficient overpotential for CO2 to CO conversion. However, no H2 or other gaseous product was detected. Liquid phase analyses of the catholyte after CPE experiments by NMR spectroscopy suggested no detectable amount of other possible products, such as methanol, formate, or oxalate that have been reported in aqueous and organic phases CO2RR.3, 37 Considering the small current density at -1.8 V (4.5 mA mg-1), it is reasonable to assign the undefined FE to the CO2RR species that are below the NMR detection limits.

Figure 4. (a) Cyclic voltammograms of 36 nm freshly reduced Bi/C and Bi-ED in MeCN containing 100 mM [bmim][OTf]; (b) Current density transients and (c) FECO under constant potential electrolysis during CO2 reduction at different applied potentials; (d) FECO and MACO on Bi-ED, 36 nm, and 7 nm freshly reduced Bi/C.


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To exclude the possibility of CO generation on carbon supports, we employed pure AB as the working electrode for CO2RR in the present system. Reasonable cathodic currents were detected at potentials more negative than -2.0 V. Nevertheless, the FECO is extremely low (< 2%, Figure S8). These results confirm that the observed CO2 to CO conversion occurs via a heterogeneous electrocatalytic process,17 in which the carbon support has minimal contribution to the CO evolution. Similar to Bi/C, the AB demonstrated a current decay at -2.2 V in the first 20 min, providing further evidence that the [bmim][OTf] was electrochemically unstable in the present conditions. To elucidate the size and morphology effects of Bi catalysts on FECO and demonstrate the benefit of NPs for CO2RR in the present platform, we further conducted a comprehensive comparison between the 36 nm freshly reduced Bi/C with the Bi-ED. Besides a more positive (~0.1 V positive) onset potential and a higher mass current density demonstrated in the CV analyses (Figure 4 (a)), 36 nm freshly reduced Bi/C also shows a slightly higher FECO than BiED. According to Figure 4 (d), the FECO on Bi-ED at -2.0 V (92.0 ± 2.7%) resembles previously reported results (78 – 87%) under similar conditions (1.95 V vs. SCE, in 100 or 300 mM [bmim][OTf]),18, 28 and is similar to the FECO on commercial high purity Bi foil (88.7%, Figure S9), while that on freshly reduced 36 nm Bi/C is 96.1 ± 0.7%. Nevertheless, considering the distinctive size and morphology of 36 nm Bi/C and Bi-ED (Figure 1 (a) and Figure S4), the comparable FECO (96.1% vs. 92.0%) clearly indicates that the size and morphology of the Bi are not major controls over FECO in the CO2RR in IL/MeCN. Previous studies in aqueous conditions have revealed a size-dependent FE over metallic catalysts, such as Au,7 Pd,19 Ag,12 and Cu20. This size-dependence is believed to stem from the population of edges and corners containing more atoms with low coordination numbers, which increases with the decreasing of particle


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size.7 This variation in size alters the adsorption energy of protons and CO2RR intermediates and then changes the product distribution.7, 19, 20 Except Pd,19 a smaller particle size of Au,7 Ag,12 and Cu20 favors the hydrogen evolution reaction, resulting in a low FE for CO2RR. Other studies show that the adsorption of protons and reaction intermediates varies on different facets and edges/corners, resulting in structure/morphology sensitive FE.11,

21, 38

Interestingly, the

comparison between multi-structured Bi-ED and monodispersed 36 nm freshly reduced Bi/C here shows a negligible dependence of FECO on the size and morphology, which suggests a different reaction mechanism for CO2RR in the IL/MeCN system. Because of the lack of free protons in IL/MeCN system, the CO2RR should occur via a direct proton transfer between the adsorbed CO2 and ILs rather than pre-adsorbed protons on the catalyst surface; the latter usually takes place in aqueous conditions. The similar FECO on Bi-ED and Bi/C we observed here indicates this proton transfer step is not affected by the Bi morphology. Previous investigations by other groups suggest the formation of CO in the IL/MeCN system may be related with the interaction between CO2 and imidazolium. Rosen et al. performed an in situ sum-frequency generation (SFG) spectroscopic investigation of the CO2RR in 1-ethyl-3-methylimidazolium tetrafluoroborate ([emim][BF4]) on a Pt electrode, and proposed that the IL adsorbed on the catalyst surface prior to the CO2 adsorption.40 Using the same approach and a Ag electrode, García Rey et al further pointed out that the key step that controls the CO2RR is actually a structural transition of the adsorbed IL, occurring at -1.33 V (vs. Ag/AgCl).41 Based on the study on a Pb electrode, Sun et al suggested that the CO evolution is promoted through the stabilization of CO2•– by imidazolium cations, without which oxalate is formed instead.42 Although the detailed CO2RR mechanism in IL/MeCN is still unknown and needs further investigation, it is generally accepted that the CO evolution is correlated to the interaction between IL and CO2.


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Our observation of the independence of FECO on size and morphology of these Bi catalysts further confirms this concept and allows the use of smaller Bi NPs to increase the catalyst’s mass utilization. As shown in Figure 4 (d), the mass current activity for CO evolution (MACO) on 36 nm freshly reduced Bi/C is 15.6 mA mg-1, almost three-fold higher than that on Bi-ED. The higher MACO and facile synthetic approach present a promising application of these Bi NPs in the future industrial CO2 reduction process.

Figure 5. Electrochemical reduction of CO2 on 7 nm freshly reduced Bi/C. (a) Current transients during two hours’ CPE at different potentials; (b) FECO averaged in two hours at different applied potential.

Particle size insensitivity of FECO on Bi catalyst is further supported by the investigation of CO2RR on 7 nm freshly reduced Bi/C. As shown in Figure 5 and S10, the 7 nm freshly reduced Bi/C behaves similarly to the 36 nm Bi/C (Figure 4), with the CO2RR taking place at potentials negative than -1.6 V (Figure S10). At -2.0 V, the maximum FECO of 90.7 ± 4.5% is achieved (Figure 5 (b)), which is in good agreement with that on Bi-ED and 36 nm freshly reduced Bi/C (Figure 4 (d)). A MACO of 15.7 mA mg-1 is obtained at -2.0 V (Figure 5 (a)). However, we have noticed the instability of the 7 nm NP catalyst. After sequential CPEs from -1.8 to -2.2 V with an interval of -0.1 V for 2 hours at each potential, the 7 nm Bi NPs agglomerated significantly on


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supports (Figure S11 (a)). Even under the protection of surfactants, the instability of 7 nm Bi NPs is not alleviated, with the evidence of agglomeration on as-synthesized Bi/Cs after the same sequential experiments (Figure S12). The agglomeration of NPs is usually attributed to the Ostwald ripening or migration and coalescence, which are widely observed for NP catalysts43 and electrocatalysts44. By contrast, most 36 nm Bi NPs well maintained their original morphology after the CO2RR (Figure S11 (b)).

Figure 6. (a) FECO and (b) mass current densities on different 36 nm Bi/Cs; (c) FECO and (d) mass current densities on different 7 nm Bi/Cs at -2.0 V for 2 hours. As discussed above, we have probed the effects of surfactants and the surface oxidized layer on CO2RR on as-synthesized Bi/Cs. To distinguish the roles of surfactant vs. the surface oxidized layer, the post oxidized Bi/C, produced by intentional air-exposure for 12 hours after hydrazine treatment, was also investigated. TEM images of the post oxidized Bi/Cs (Figure S13) confirm that there is no dispersion or morphology change after the removal of surfactant


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residuals and the regeneration of surface oxidized layer. The XPS analyses (Figure S14), however, show that the surfaces of both 7 and 36 nm post oxidized Bi/Cs are fully oxidized, indicating the Bi NPs are more vulnerable to air-oxidation without the protection of surfactants. The CVs on these Bi/Cs are shown in Figure S15, clearly demonstrating that the surfactant residuals inhibit the CO2RR, with the negative shifts for onset potential about 100 mV (Figure S15 (a) and (c) vs. (b) and (d)). As shown in Figure 6 (a), for 36 nm Bi/C, the FECO significantly drops from 96.1 ± 0.7% (on the freshly reduced Bi/C) to 76.2 ± 5.7% (on as-synthesized one). This low FECO is due to the capping ligands (TOP) binding on the Bi surface, which prohibits the adsorption of CO2 and the approach of the IL. The depletion of IL at the surface of electrode not only decreases the CO generation, but also slows down the reduction of oxidized layer. This assumption is verified by the recovery of FECO on the 36 nm post oxidized Bi/C (90.3 ± 1.7%), on which most TOP surfactants were removed. It is noted that this FECO is close to that on the corresponding freshly reduced catalyst. One possible explanation is that the Bi3+ species on 36 nm Bi/Cs can be reduced quickly to Bi0 and therefore will not consume the current passed through the electrode in the following reaction. This assumption is also indicated by the comparison of average mass current densities during CPE (Figure 6 (b)). As the oxidized layer on 36 nm post oxidized Bi/C is quickly removed, the passed current during CPE is mainly due to the CO2RR, which allows the post oxidized Bi/C to work similarly to the freshly reduced one. Therefore, the average mass current density collected on the 36 nm post oxidized Bi/C electrode is close to that on the freshly reduced one (17.2 ± 1.7 vs. 16.2 ± 1.9 mA mg-1). For 7 nm Bi/C, the freshly reduced catalyst presents similar FECO (90.7 ± 4.6%, Figure 6 (c)) and mass current density (17.3 ± 2.3 mA mg-1), compared with the corresponding 36 nm one. However, the 7 nm post oxidized Bi/C shows an unexpected drop in FECO (70.6 ± 5.4% on 7 nm post oxidized Bi/C


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vs. 90.3 ± 1.7% on the corresponding 36 nm one). And the mass current density on 7 nm post oxidized Bi/C also increases to 26.3 ± 3.0 mA mg-1 (Figure 6 (d)). As the surface of both 7 and 36 nm post oxidized Bi/Cs are fully oxidized (Figure S14), the difference in FECO suggests that for the post oxidized Bi/Cs the particle size plays an important role on the catalytic activity for CO2RR. Generally, the decrease in particle sizes leads to a higher surface area and boosts the surface oxidation, which has been demonstrated on multiple metal NPs, such as Ag45 and Al46. Correspondingly, the reduction of oxidized species on a smaller NP to metallic states may require a larger overpotential or longer time under electrochemical conditions than that on a larger NP. We envision that this effect would be even more prominent in the IL/MeCN system than in aqueous media, as the deficiency of free protons may cause the reduction of metal oxides to be more size-dependent. As a result, the oxidized species on 36 nm post oxidized Bi/C are quickly reduced to the metallic state and catalyze CO2RR similarly to the freshly reduced Bi/C, while the reduction of 7 nm post oxidized Bi/C is more thermodynamically unfavorable, and may consume a significant portion of cathodic current, decreasing FECO during CO2RR. Due to the high reactivity of the electrochemically reduced Bi NPs, the surface of 36 nm freshly reduced Bi NPs is quickly oxidized right after the CO2RR when losing the protection of applied potential, and the Open-circuit potential (OCP) drifted from -2.0 V to -0.2 V within 5 min even under the CO2 atmosphere (Figure S16). After the relaxation in CO2 atmosphere, the 36 nm freshly reduced Bi/C was re-examined using XPS spectrum, showing a Bi0/Bi-F/Bi-O ratio of 1:2.5:1.9 (Figure S17). This result indicates that the surface oxidation may be resulted from the reaction with Nafion binder and the trace amount of H2O generated from the CO2RR. However, as described previously, the catalytic activity of 36 nm Bi NPs for CO2RR is not sensitive to the surface oxidized layer (comparison between 36 nm freshly reduced and post


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oxidized Bi/Cs in Figure 6 (a) and (b)), and the surface oxidation during the relaxation does not affect the reusability of the 36 nm Bi/C catalysts. As shown in Figure S18, after fully relaxed in CO2 and exposed to air for ~ 10 min, the 36 nm freshly reduced Bi/C still performs efficient CO2RR, with only 4% drops in FECO.

CONCLUSION In summary, two types of monodispersed Bi NPs (36 and 7 nm) were synthesized via an organic solution phase chemical method and were studied for CO2RR in [bmim][OTf]/MeCN. After being deposited on carbon supports and activated by hydrazine treatment, the Bi/Cs exhibited exceptionally high catalytic activity. Especially, the freshly reduced 36 nm Bi/C displayed a FECO of 96.1%, a three-time higher MACO and a ~0.1 V more positive onset potential than the conventional Bi-ED electrodes. By the comparison between Bi-ED, 36 nm Bi NPs and 7 nm Bi NPs, we demonstrate that the FECO decreases on small Bi NPs when being oxidized in air, but this size-dependence is lessened by the surface activation in hydrazine. This study provides clear evidence that the post-chemical reduction and ligand removal treatment are crucial to efficient CO2 reduction on metal NPs. The surface activation strategy employed in this work is applicable to a broad range of metallic catalysts synthesized through the colloidal methods, which not only allows the optimization of catalyst performance, but also provides the cleaner and simpler interfacial environments to enable the mechanistic investigation of catalytic reactions over NP electrocatalysts.

EXPERIMENTAL METHODS


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Materials. 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([bmim][OTf], > 98%), KBr (> 99%), HCl (37%), bismuth acetate (Bi(ac)3, > 99.99%), bismuth neodecanoate, 1-octadecene (ODE, > 90%), 1-dodecanethiol (DDT, > 98%), and trioctylphosphine (TOP, > 97%) were purchased from Sigma Aldrich. Acetonitrile (ACS reagent), acetone (ACS reagent), Chloroformd (99.8 atom%), and tetrahydrofuran (THF, ACS reagent) were purchased from Fisher Scientific. Carbon black, acetylene (100% compressed) was purchased from Alfa Aesar. CO2 (99.99%) was purchased from Air Liquide. All the chemicals were used as-received without any further purification.

Characterizations. The structure, morphology, composition, and surface functionality of the Bi NPs and Bi/Cs were analyzed by X-ray diffraction (XRD), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HR-TEM), scanning transmission electron microscopy (STEM), X-ray photoelectron spectroscopy (XPS), inductively coupled plasma optical emission spectrometry (ICP-OES), and Fourier-Transform infrared spectroscopy (FT-IR). XRD patterns were collected by a Panalytical Emyrean diffractometer equipped with Cu Kα radiation (λ = 1.5406 Å). TEM images were obtained on a JEOL 1400 (120 kV). HRTEM and STEM imaging was performed on an aberration-corrected FEI Titan S 80-300 TEM/STEM. HAADF-STEM imaging was carried out with a probe convergence angle of 30 mrad and a large inner collection angle of 65 mrad. The XPS spectra were collected on PHI 3056 spectrometer at 15 kV and an applied power of 350 W. Prior to XPS analyses, samples were load locked in from an argon filled glove box to prevent additional surface oxidation. To investigate the surface composition of Bi/Cs after the CO2RR, the electrochemical cell was dissembled in an argon filled glove box to minimize the air-exposure. ICP-OES was performed for the quantitative


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elemental analysis on a Perkin-Elmer Optima 2000 DV ICP spectrometer. The FT-IR spectra were recorded by an attenuated total reflection FTIR spectrophotometer (PerkinElmer Frontier FTIR spectrometer). Scanning electron microscopy image of the electrodeposited Bi was acquired with a Hitachi S-4700 Scanning Electron Microscope (SEM).

Synthesis of Bi NPs. To prepare 36 nm Bi NPs, 1 mmol of Bi(ac)3 was mixed with 5 ml of ODE and heated to 120 °C for two hours under a gentle nitrogen (N2) flow to remove dissolved moisture and oxygen. After that, the system was air-cooled down to 80 °C while 0.24 ml of DDT was injected into the solution to form Bi intermediate complex. After 5 mins, 1.0 ml of TOP was added into the solution and the system was further cooled down to 60 °C and kept at this temperature for 30 mins. After being cooled down to room temperature, the 36 nm Bi NPs were collected and washed by the addition of acetone (25 ml) and subsequent centrifugation at 9000 rpm for 8 min. The product was further purified twice by addition of acetone (25 ml) and centrifugation to remove all residual impurities. Final NPs were dispersed in THF for further use. Under the exact same reaction condition, by changing 1 mmol Bi(ac)3 into 1 mmol bismuth neodecanoate, 7 nm Bi NPs were synthesized.

Preparation of Bi/Cs and surfactant removal. Acetylene black (AB) is of high electrical conductivity, low specific surface area, and negligible surface functional groups. These properties make AB an excellent electrocatalyst support with a smaller double-layer capacitance and minimal electrochemical reactivity compared with many other carbon supports. The assynthesized Bi NPs were sonicated with appropriate amount of AB in ethanol, followed by centrifugation at 8500 rpm for 8 min to achieve a uniform dispersion of Bi NPs on the carbon


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supports. This carbon supported Bi is then named as as-synthesized Bi/C. The metal loadings of the as-synthesized Bi/C were determined by ICP-OES, which were 55 and 68% for 7 and 36 nm Bi/Cs, respectively. The removal of surfactant was achieved by soaking the as-synthesized Bi/C in hydrazine monohydrate for overnight and washing with copious of EtOH. The final products were dried under vacuum before use.

Electrodeposition of Bi on glassy carbon electrodes. The electrodeposition method was modified from previous publications,17 and was performed using a biologic SP-200 poentiostat/galvanostat, with a glassy carbon electrode (GCE, 5 mm in diameter), an Ag/AgCl (sat’d KCl, -0.044 V vs. SCE) reference electrode, and a Pt gauze counter electrode. The Bi catalyst was deposited on the freshly polished glassy carbon electrodes in a single compartment cell containing 50 ml of 20 mM bismuth acetate, 0.5 M KBr, and 1.0 M HCl electrolyte. The electrode was preconditioned by cycling the potential from -0.1 to -0.6 V vs. Ag/AgCl at a scan rate of 100 mV s-1 for 10 cycles, and then was agitated in the electrolyte to remove any exfoliated materials for the electrode. The controlled potential deposition was then carried out in the same electrolyte at -0.15 V vs. Ag/AgCl, with a total passed charge of 3.0 C cm-2. The asprepared electrode was then sequentially rinsed with 1.0 M HCl, de-ionized water, and MeCN to remove the trapped ions and water. The amount of electrodeposited Bi on GCE was estimated by the passed charge (3.0 C cm-2) with a final loading of 425 µg.

Electrochemical Measurements and Products analysis. All electrochemical measurements were conducted in acetonitrile (MeCN) solution containing 0.1 M [bmim][OTf] supporting electrolyte. To prepare the Bi/C working electrode, a catalyst ink containing 90% of Bi/C and


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10% of Nafion was first dispersed uniformly in EtOH, followed by drop-casting on a freshly polished GCE to achieve a Bi metal loading of ~ 100 µg. Cyclic voltammograms were recorded in a gas sealed single compartment cell at 100 mV s-1 with iR compensation. The controlled potential electrolysis (CPE) was performed for two hours in a self-designed gas-tight H-type cell that was separated by a Nafion (212) membrane and charged with 20 ml of [bmim][OTf]/MeCN in each compartment. CO2 was purged through the catholyte for 30 min to achieve a steady state prior to electrolysis. During the electrolysis, a steady supply of CO2 at a rate of 5 sccm was introduced to the cathode compartment, while the gas phase effluent in the headspace of the cathode was continuously introduced to the sampling loop of a Buck 910 gas chromatograph (GC). The sampled gas effluent was injected automatically every 25 min into parallel columns of MS 5A and Haysep D, and analyzed by a thermal conductivity detector (TCD) and a flame ionization detector (FID) equipped with a methanizer, using Argon as the carrier gas. The concentration of CO was derived by comparing the corresponding peak area of FID with that of standards, which was further used to calculate the Faradaic efficiency (FECO). Mass current activity for CO evolution was calculated by normalizing the cathodic current corresponding to CO generation (total current × FECO) to the mass of Bi metal on the GCE. All the FECO, current density, and mass activity were reported as averaged values during two hours’ CPE based on at least four individual measurements unless mentioned otherwise. In order to check for the formation of any nonvolatile CO2 reduction products, a 0.9 ml aliquot of the catholyte at the completion of each CPE was mixed with 0.1 ml CDCl3, and was analyzed on a Bruker 400 MHz Nuclear Magnetic Resonance Spectrometer. To confirm the decomposition of [bmim][OTf], the MeCN in the catholyte after CPE was first evaporated, and the residual was redissolved into 1.0 ml CDCl3 for NMR analysis. To understand the effects of re-oxidation at Open-circuit potentials,


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the electrode was first fully relaxed in the CO2 saturated system after a 2 hours’ reaction at -2.0 V, and then exposed in air for ~ 10 min, followed by the re-examination for CO2RR in a newly prepared [bmim][OTf]/MeCN solution under the same conditions.

AUTHOR INFORMATION Corresponding Author [email protected] ASSOCIATED CONTENT Supporting Information. TEM images of Bi NPs synthesized with different amounts of surfactants; XRD patterns; SEM image of Bi-ED; additional XPS spectrum; pictures and NMR spectra of catholytes before and after CPE; CO2RR on AB; TEM images of Bi/C after CPEs; This material is available free of charge via the Internet at http://pubs.acs.org. ACKNOWLEDGMENT This work was supported as part of the FIRST Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences. XPS data (G. M. V.) was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Materials Science and Engineering Division. H. Z. was supported by Liane B. Russell Fellowship sponsored by the Laboratory Directed Research and Development Program at the Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the US Department of Energy.


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