Facile CO2 Electro-Reduction to Formate via Oxygen Bidentate

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Facile CO2 electro-reduction to formate via oxygen bidentate intermediate stabilized by high-index planes of Bi dendrite catalyst Jai Hyun Koh, Da Hye Won, Taedaehyeong Eom, Nak-Kyoon Kim, Kwang Deog Jung, Hyungjun Kim, Yun Jeong Hwang, and Byoung Koun Min ACS Catal., Just Accepted Manuscript • Publication Date (Web): 20 Jun 2017 Downloaded from http://pubs.acs.org on June 20, 2017

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Facile CO2 electro-reduction to formate via oxygen bidentate intermediate stabilized by high-index planes of Bi dendrite catalyst Jai Hyun Koh,a‡ Da Hye Won, a‡ Taedaehyeong Eom,b‡ Nak-Kyoon Kim,c Kwang Deog Jung, a Hyungjun Kim,b* Yun Jeong Hwang, a,d,* and Byoung Koun Mina,e,* a

Clean Energy Research Center, Korea Institute of Science and Technology, 5 Hwarang-ro 14-gil,

Seongbuk-gu, Seoul 02792, Republic of Korea b

Graduate School of Energy, Environment, Water, and Sustainability (EEWS), Korea Advanced

Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea c

Advanced Analysis Center, Korea Institute of Science and Technology, 5 Hwarang-ro 14-gil,

Seongbuk-gu, Seoul 02792, Republic of Korea d

Korea University of Science and Technology, 217 Gajeong-ro, Yuseong-gu, Daejeon 34113,

Republic of Korea e

Green School, Korea University, 145 Anam-ro, Seongbuk-gu, Seoul 02841, Republic of Korea

‡These authors contributed equally. *Corresponding Authors

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ABSTRACT. Electrochemical CO2 conversion to chemical-products is a promising strategy for sustainable industrial development. However, the success of this approach requires an in-depth understanding of catalysis because it involves highly complex multi-step reactions. Herein, we suggest a rational design of a hierarchical Bi dendrite catalyst for an efficient conversion of CO2-to-formate. A high selectivity (~89% at –0.74 VRHE) and, more importantly, a stable performance during long-term operation (~12 h) were achieved with the Bi dendrite. Density functional theory (DFT) is used to investigate three possible reaction pathways in terms of surface intermediate, and the one via *OCOH surface intermediate is calculated to be the most energetically feasible. DFT calculations further elucidate the plane-dependent catalytic activity, and conclude that the high-index planes developed on the Bi dendrite are responsible for the sustainable performance of Bi dendrite. We expect that our experimental and theoretical study will provide a fundamental guideline to the CO2-to-formate conversion pathway as well as design principles for enhancing the catalytic performance.

KEYWORDS Electrocatalyst · CO2 Reduction · Bismuth · Dendrite · Formate


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INTRODUCTION Mankind has been heavily dependent on fossil fuels to meet his energy needs, and this reliance has led to various environmental problems such as global warming and the energy crisis.1 Therefore, the possibility of an artificial photosynthesis system that uses solar energy to synthesize chemical products from CO2 and water has attracted much attention as a potential breakthrough for sustainable development.2 One of the biggest challenges facing such a system is CO2 conversion because it has sluggish kinetics and is composed of highly complex multi-step reactions. To overcome this hurdle, various metal candidates (e.g., Ag3-11, Au11-20, Cu21-25, Sn26-29) have been experimentally screened for their catalytic performance on CO2 reduction and the mechanisms of the more promising ones have been extensively investigated. However, a catalyst which satisfies the requirements for practical applications, including low material cost, earthabundance, environmental friendliness and safety, is still being sorted. Bi has received attention as a catalyst of CO2 reduction due to advantages such as its low and stable cost and low-toxicity. In previous studies, almost all Bi catalysts produced CO from CO2 with high Faradaic efficiencies of 74–96% in an aprotic electrolyte (e.g., acetonitrile) containing ionic liquids.30-33 Interestingly, a few studies have recently reported that Bi-based electrodes could selectively reduce CO2 to formate in an aqueous electrolyte, which is more practical catalytic environment in terms of cost-effectiveness than an aprotic electrolyte with ionic liquid.34-36 Also, formic acid (HCOOH) and formate salts (HCOO–) are relatively highvalue commodity chemicals that are widely used in cleaning, feed preservation, and leather processing.37 A similar situation was also observed with Sn electrodes which are well-known for producing formate from CO2 in aqueous systems but in aprotic electrolytes catalyze the selective production of CO.32 These phenomena imply that the catalytic environment can tune the product


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selectivity of the reaction by influencing the mechanism. Therefore, to design a catalyst rationally, it is important not only to modify the intrinsic properties of a catalyst experimentally, but also to understand and consider the specific mechanisms of catalysis under different operating conditions. Empirical and theoretical studies can be usefully integrated with the assistance of the density functional theory (DFT) method which can either suggest the origin of a catalytic activity based on experimental results or predict an appropriate structure for a catalyst. For instance, Zhu et al. experimentally found the optimum size of monodispersed Au nanoparticles, and their DFT study suggested that the origin of the superior CO2 electro-reduction activity was the numerous edge sites, which stabilize the reaction intermediates.13 On the contrary to this, an Au nanowire structure with a high edge-to-corner ratio was suggested based on the DFT calculations and the following experiments verified that those predicted structure did indeed exhibit the outstanding performance for electro-reduction of CO2.14 Collaborative studies integrating experimental and theoretical approaches have been conducted for the CO2 reduction reaction, and consequently, a rational design of a CO2-reducing catalyst has been proposed: the coordinately unsaturated site can effectively stabilize the reaction intermediate by lowering the energy barrier for its binding to the site.8,18,22,38-40 Based on the proposed strategy, we developed a nanostructured Bi electrode with a high density of low-coordinated sites for CO2 reduction in an aqueous solution. To enhance the product selectivity in a controlled manner, dendritic nanostructures were carefully incorporated to the surface of the Bi electrode. A dendritic morphology is promising for electrocatalysts due to their large surface area, and high degree of connectivity accompanied by a large number of active local sites.8,29 For example, these structural advantages of dendrites have been


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demonstrated by Sn29 and Zn8 with remarkable catalytic activities. The hierarchical Bi dendrite demonstrated a promising potential as a catalyst for CO2 reduction, showing the highest formate production rate among the Bi catalysts reported in an aqueous solution. The accompanying DFT study explored three possible reaction pathways and calculated that the most energetically feasible path was via *OCOH surface intermediate formation on the prepared Bi electrode. Furthermore, the plane dependence of the Bi catalyst proposed by the DFT calculation suggests fundamental guidelines and design principles for the development of a high performing CO2 electro-reduction catalyst.

EXPERIMENTAL SECTION Preparation of Bi dendrite electrode. Bi dendrite electrodes were deposited by a modified literature procedure.41 An electrodeposition of Bi3+ precursor was conducted in one-compartment cell using a potentiostat (PGSTAT128N, Metrohm Autolab). A three electrodes setup was employed with an Ag/AgCl (3 M NaCl, CH Instruments) and a platinum gauze (100 mesh, 99.9% trace metals basis, Sigma-Aldrich) as a reference electrode and a counter electrode, respectively. Bi thin film of 100 nm prepared by e-beam evaporation on a Cu substrate (99.98%, trace metals basis, Sigma-Aldrich) was used as a working electrode in order to minimize an amount of Bi used for the electrode. An electrolyte was 20 mM of Bi(NO3)3·5H2O (98%, Sigma-Aldrich) in ethylene glycol (anhydrous, 99.8%, Sigma-Aldrich) solution. Bi dendrite was deposited on the working electrode by repeated cycles of an electrodeposition and a resting step. A single cycle of deposition consists of a constant-potential electrodeposition at ‒1.8 V vs. Ag/AgCl for 60 s and a subsequent resting step for 2 s (Figure S1). This cycle was repeated for 10, 20, and 30 times to prepare three different Bi dendrite electrodes for electrochemical CO2 reduction. After the


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deposition, the Bi dendrite electrodes were gently dipped in an ethanol to wash out the residual electrolytes on the surface and naturally dried at room temperature. Physical Characterization. The physical characteristics of the prepared Bi electrodes (i.e. Bi foil, e-beam Bi and Bi dendrite) were analyzed with a scanning electron microscopy (SEM), an X-ray diffraction (XRD), a high-resolution transmission electron microscopy (HR-TEM). The surface morphology of prepared Bi electrodes were confirmed by a SEM (Inspect F, FEI) at an acceleration voltage of 15.0 kV and an average working distance of 10.0 mm. The crystal structure of the prepared Bi electrodes was investigated by XRD (XRD-6000, Shimadzu) using a radiation from a Cu Kα source at 40 kV and 30 mA. Figure S2 shows the XRD patterns of Bi dendrite, e-beam Bi and polycrystalline Cu, which was used as the substrate. The morphology and crystal structure were also observed by HR-TEM (Titan 80-300TM, FEI) images and selected area diffraction (SAED) patterns. Electrochemical CO2 reduction. All electrochemical measurements were conducted in a gastight two-compartment cell of poly(ether ether ketone) (PEEK), and a proton exchange membrane (Nafion® 117) was placed in the middle of the electrochemical cell to separate catholyte and anolyte. Electrochemical CO2 reduction experiments were carried out in a CO2saturated 0.5 M KHCO3 (≥99.95%, Sigma-Aldrich) with pH of 7.3 by using a potentiostat (PGSTAT128N, Metrohm Autolab). A platinum gauze (100 mesh, 99.9% trace metals basis, Sigma-Aldrich) and the prepared Bi dendrite electrodes were used as a counter and working electrodes, respectively. A typical constant-potential electrolysis (chronoamperometry) of CO2 was conducted for an hour and steady-state current density was plotted against applied potential. All potentials were measured with a Ag/AgCl reference electrode (3 M NaCl, CH Instruments) and converted to reversible hydrogen electrode (RHE) scale using E ( . RHE) = E ( . Ag/


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AgCl) + 0.21 V + 0.059 V × pH . Prior to each electrochemical CO2 reduction experiment, solution resistance (Rs) was measured by electrochemical impedance spectroscopy taken at various potentials (Figure S3). The potential converted to RHE scale was further corrected for ohmic losses (iRs). Quantification of CO2 reduction products. The gas and liquid products from CO2 reduction were analyzed by using a gas chromatography (GC) and a nuclear magnetic resonance (NMR) spectrometer, respectively. Gas products as H2 and CO in the headspace of the electrochemical cell were quantitatively analyzed by GC (Younglin 6500 GC system) equipped with a 80/100 Carbosieve SII column (Supleco Analytical), a thermal conductivity detector (TCD), and a flame ionization detector (FID). The quantification of gas products were performed at an interval of ten minutes during the electrolysis for an hour. Ultra-high purity Ar (99.9999%) was used as a carrier gas. GC was directly connected to the cathodic side of the electrochemical cell and the gas products were injected through a six-port valve. Each compartment of the electrochemical cell contained 38 mL of electrolyte and 34 mL of headspace, and CO2 gas was purged through the electrolyte at 20 sccm. The Faradaic efficiency of gas products were evaluated from the area of each peak in the gas chromatogram by using the following equation: Faradaic efficiency =  ×   ×

2  

where Vi is the volume concentration of H2 or CO based on a calibration of the GC, F = 96,485 C mol ,  = 1.013 bar , R = 8.314 J mol K  , T = 298.15 K , and itotal is total steady-state current measured during a constant-potential electrolysis. The quantification of liquid products were performed after the electrolysis for an hour on an Agilent 600 MHz NMR spectroscopy. 450 μL of electrolyte after CO2 reduction electrolysis was mixed with an internal standard, 50 μL of 1% 3-(trimethylsilyl)-1-propanesulfonic acid


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(DSS, Sigma-Aldrich) in D2O, for NMR analysis. The H2O peak was suppressed by presaturation method. The ratio of the area of each liquid product peak to that of DSS was acquired to quantify the amount of the liquid product of CO2 reduction. The amount of charge passed to produce each liquid product was divided by the amount of total charge passed during the electrolysis to evaluate each Faradaic efficiency. The sum of Faradaic efficiencies of all gaseous and liquid products was normalized to 100%. Computational details & reduction potential calculation for CO2 reduction. We performed spin-polarized DFT calculations using Vienna Ab initio simulation package (VASP)42 with Perdew-Burke-Ernzerhof (PBE) exchange correlational functional43 and plane-wave basis sets. We first optimized the bulk rhombohedral Bi structure, yielding the lattice parameters of a = 4.809 Å and a = 57.03°, which are comparable to the experimental lattice parameters of a = 4.746 Å and a = 57.24° (Table S1). We then built slap models by cleaving the optimized bulk structure of Bi with the surface orientation of (003), (012), (110), and (104) (Figure S4), where the bottom layer is fixed and ~15 Å of vacuum layer is additionally included to avoid the artificial inter-slap interaction. For the reciprocal space sampling, we used 9×9×9 mesh for the optimization of primitive Bi bulk structure, 2×2×1 mesh for Bi (003) and (012) slab models, 2×3×1 mesh for (110) slab model, and 3×2×1 mesh for (104) slab model. We set the kinetic energy cutoff as 600 eV, and used the projector augmented wave (PAW) method44 to describe the core electrons. We computed the binding free energies (ΔGB) of possible intermediate states of H, COOH, OCOH, and HCOOH by diagonalizing the partial Hessian matrix to include the vibrational entropy and enthalpy to the DFT self-consistent-field (SCF) energy. We note that our calculation yields the thermodynamic potential of CO2 reduction to HCOOH as –0.22 V vs.


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standard hydrogen electrode (SHE), which is comparable to the experimental value of –0.20 V vs. SHE.45


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RESULTS AND DISCUSSION

Scheme 1. Schematic illustration for the preparation of Bi dendrite electrode. Bi dendrite was prepared by e-beam evaporation of Bi source on a Cu foil, followed by electrodeposition of Bi3+ precursor in ethylene glycol solution. The Bi dendrite electrode was prepared by multi-step electrodeposition of Bi3+ precursor in ethylene glycol solution onto a 100 nm-thick Bi thin film substrate using different step cycles (Scheme 1). A Bi thin film substrate was prepared by e-beam deposition on a Cu substrate and had a rough surface comprised of hundreds-of-nanometer-sized particles (Figure 1b), which could facilitate deposition by improving the adhesion of deposits. According to the SEM images, the Bi dendrites deposited over 30 cycles showed a well-defined hierarchical structure in which smaller branches grew on bigger branches (Figure 1c and S5). The CO2 reduction activity of Bi dendrite electrodes increased with the number of deposition cycles used to create them and leveled at 30 cycles (Figure S6). Therefore, 30 cycles was determined to be the optimum number of cycles for Bi dendrite deposition and the subsequent CO2 reduction experiments were conducted on the corresponding electrode. The crystalline structure of the prepared Bi dendrite was examined by XRD. As shown in Figure 1d, the XRD patterns of the Bi dendrite wellmatched rhombohedral polycrystalline Bi (JCPDS No. 85-1330). Compared to a pristine Bi foil, relatively higher index planes such as (012), (107), (116), (214) and (009) were developed on the Bi dendrite surface, suggesting a well-formed dendrite structure (Figure S7).


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Figure 1. Characterization of Bi dendrite electrode. Top-down SEM images of (a) pristine Bi foil, (b) e-beam deposited Bi, and (c) Bi dendrite. (d) XRD patterns of as-prepared Bi dendrite and the pristine Bi foil. To investigate the catalytic activity and the selectivity on the prepared Bi dendrite in the aqueous solution, the electrochemical CO2 reduction was conducted at various constant potentials from ‒0.56 to ‒1.19 V [vs. RHE; all potentials are given with respect to RHE] for an hour in a CO2-saturated 0.5 M KHCO3 aqueous electrolyte (Figure S8). A commercial pristine Bi foil was employed as a control group in terms of CO2 reduction activity. The liquid and gas products of the CO2 reduction were analyzed by a NMR spectrometer and a GC, respectively, and the resulting data were used to calculate Faradaic efficiencies and partial current densities (Figure S9). The primary product of CO2 reduction on the prepared Bi electrodes was formate (HCOO–), and a trace of CO was also detected. H2 was produced as a byproduct by the hydrogen evolution reaction (HER), which is a competitive reaction with CO2 reduction. During CO2 electrolysis, the Bi dendrite showed a much higher current density with an improved Faradaic efficiency for formate compared to the pristine Bi foil (Figure 2a‒b). As shown in Figure 2b, the


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Bi dendrite exhibited a stable current density of ‒2.7 mA cm‒2 with a Faradaic efficiency of ~89% for formate at ‒0.74 V whereas the pristine Bi foil exhibited a current density of only ‒0.4 mA cm‒2 with a Faradaic efficiency of ~56% at the same potential. Compared to the pristine Bi foil, the Bi dendrite electrode had a positively shifted onset potential, i.e., the potential at the start of the CO2 reduction reaction (Figure 2c‒d). Similarly, the overpotential of the dendritic electrode at the maximum formate production Faradaic efficiency was reduced by 180 mV. While the pristine Bi foil required an overpotential of 890 mV to reach its maximum formate Faradaic efficiency of ~79%, the Bi dendrite showed an anodically shifted overpotential of 710 mV to achieve its maximum Faradaic efficiency of ~89%. Furthermore, due to outstanding current densities and Faradaic efficiencies, the production rates of formate on the Bi dendrite surpassed not only those on the Bi foil, but also those on state-of-the-art Bi-based catalysts and on other formate-producing catalysts, such as Sn and Pb (Figure 2e and Table S3). Although the HER rate on the Bi dendrite exceeded the CO2 reduction rate at a high overpotential region over ‒1.04 V (Figure S10), this could be attributed to insufficient mass transport of CO2 into the layers of hierarchical dendritic structures.8,25


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Figure 2. Catalytic activities of pristine Bi foil and Bi dendrite. (a) Current densities during electrochemical CO2 reduction at various constant potentials in a CO2-saturated 0.5 M KHCO3. (b) Comparison of performance in terms of total current density (line; left axis) and Faradaic efficiency of formate (dot; right axis) at ‒0.74 V during 1 h. Faradaic efficiencies of (c) Bi foil and (d) Bi dendrite electrodes calculated after operation for 1 h of CO2 reduction at various applied potentials. (e) Formate production rate at various applied potentials on Bi foil and Bi dendrite electrodes. Since a durability of a catalyst is crucial for practical applications, the performance profile of the Bi dendrite during a long-term operation was examined by collecting the current density and Faradaic efficiency at ‒0.73 V (Figure 3). During constant-potential electrolysis, the Bi dendrite showed an outstanding stability over 12 h without any significant performance degradation. Faradaic efficiency, as well as current density, of formate production were also


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maintained above 90% of their initial performances. In addition, negligible change in morphology (Figure 3b–c) and XRD patterns (Figure S11) implies that the Bi dendrite morphology is sufficiently stable during the long-term electrolysis. This result is particularly promising because Bi-based electrodes including their durability issues have not yet been extensively investigated in previous studies. Thus, our Bi dendrite electrode demonstrated significant potential as an electrocatalyst for CO2 reduction with high catalytic activity, selectivity, and stability.

Figure 3. (a) Current density (line; left axis) and Faradaic efficiency of formate (dot; right axis) on the Bi dendrite electrode during long-term operation of 12 h at ‒0.73 V. Morphology of the Bi dendrite electrode (b) before and (c) after CO2 reduction reaction for 12 h. It should be noted that the Bi dendrite exhibited 11 times higher CO2 reduction activity than that of the pristine Bi foil in terms of formate production rate at ‒0.74 V (Figure 2e and S10e). This 11 times higher CO2 reduction activity, despite the fact that Bi dendrite having only


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a 3.1-fold higher surface area than Bi foil according to the electrochemical surface area measurements (Figure S12), implies that the intrinsic catalytic activity of Bi dendrite was remarkably enhanced, perhaps mechanistically. To understand the mechanism of CO2 reduction on the electrode surface, Tafel plots were acquired at regions of low current density, where the reaction is mainly limited by electrokinetics (Figure S13). The dendritic Bi electrode had a Tafel slope of 92 mV dec‒1, smaller than that of pristine Bi foil of 150 mV dec‒1, and both values differed from 118 mV dec‒1. It is generally accepted that a reaction having a Tafel slope near 118 mV dec‒1 involves an initial one electron transfer step from catalyst to CO2 as the ratedetermining step (RDS), but neither electrode showed this value.12 Therefore, we can conclude that the RDSs on the Bi surfaces differ from the typical one electron transfer step, and that dendritic surfaces can lead to different mechanistic pathways from the pristine Bi foil as high index surfaces are exposed. In general, it is a well-known strategy to develop nanostructures on a catalyst surface to achieve enhanced catalytic performance.8,11,17,25,46 To gain a deeper understanding on the origin of the improved catalytic performance other than the effect of nanostructuring, DFT calculations were conducted considering possible reaction paths under the operating conditions (Figure 4). Three different possible reaction paths were suggested, as follows (an asterisk * indicates the catalytic surface): Path 1 via *COOH * + CO2 (g) + [H+ + e‒] ® *COOH (aq)

(R1-1; slow)

*COOH (aq) + [H+ + e‒] ® *HCOOH (aq)

(R1-2; fast)

*HCOOH (aq) ® * + HCOOH (aq)

(R1-3)


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Path 2 via *OCOH * + CO2 (g) + [H+ + e‒] ® *OCOH (aq)

(R2-1; slow)

*OCOH (aq) + [H+ + e‒] ® *HCOOH (aq)

(R2-2; fast)


(R2-3)

Path 3 via *H * + [H+ + e‒] ® *H (aq)

(R3-1; slow)

*H (aq) + CO2 (g) + [H+ + e‒] ® *HCOOH (aq)

(R3-2; fast)


(R3-3)

They are schematically shown in Figure 4a. The difference between path 1 and path 2 is merely whether the COOH intermediate is adsorbed via a Bi-C interaction (i.e. *COOH) or a Bi-O interaction (i.e. *OCOH), respectively. In path 3 dissolved CO2 is added to surface hydrogen after a reversible hydrogen adsorption/desorption on the electrode surface, a mechanistic path which, as reported in previous research, could be possible in the aqueous electrolyte.47


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Figure 4. DFT calculation results on the CO2-to-formate reduction reaction pathways for various possible Bi planes of (003), (012), (110), and (104). (a) Schematic diagram of three possible reaction pathways; path 1 via the formation of *COOH intermediate, path 2 via the formation of *OCOH intermediate, and path 3 via the formation of *H. Reaction free energy diagrams for the reaction (b) path 1, (c) path 2, and (d) path 3, when zero overpotential is applied (bias potential U = ‒0.21 VSHE). Noting that (012), (110), and (104) planes are preferentially developed for Bi dendrite compared with the close-packed surface of (003) that is dominant in Bi foil. Using DFT, we calculated the reaction free energies of the above three reaction paths by diagonalizing the partial Hessian to evaluate zero point energies and vibrational free energies.


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The computational hydrogen electrode method was used to determine the chemical potential of the proton and electron pair (H+ + e‒)48. We considered different planes of Bi surfaces, namely the close-packed surface of (003) and high-index surfaces of (012), (110), and (104) planes. For all three possible reaction paths on the close-packed surface of (003) plane, DFT results show that the first electron transfer step (R1-1, R2-1, or R3-1) is the most energetically difficult step. We also find that path 2, which is via the formation of *OCOH surface intermediate, provides the energetically most favorable path regardless of the choice of surface plane. It is of noted that the water participation in the reaction (which is not considered in our calculations) might kinetically prefer the O-H bond formation (i.e., *COOH path) over the C-H bond formation (i.e., *OCOH path)49-52. However, due to the strong thermodynamic preference toward *OCOH path, we still expect the dominant contribution of *OCOH path. Given that the dominant reaction pathway is path 2, we find that the *OCOH surface intermediate can be substantially more stabilized on all the explored high-index planes, (012), (110), and (104) than on the (003) plane. For the other possible pathways, namely path 1 and path 3, which could have a non-negligible contribution particularly under a high overpotential regime, we find that the (110) plane can serve as a better catalytic active site than the (003) plane. As our discussion being solely based on thermodynamic barriers, we estimated the potential dependent activation barriers (Eact)52-53 for the 1st and 2nd elementary steps of path 2 on the (012) surface as a representative example (Figures S14 and S15, and Table S2), leading to no barrier for the 1st step and only Eact ~ 0.2 eV for the 2nd step under the bias potential of –0.75 V (experimental condition). This shows that the barriers become surmountable at the applied potential, and thus confirms the validity of our discussion based on thermodynamic barriers.


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Indeed, as shown in the XRD pattern of Figure 1d, the (012), (110) and (104) planes were preferentially developed on the Bi dendrite, while the (003) plane was preferentially developed on the pristine Bi foil. In conjunction with DFT results, this suggests that the morphological deviation of the Bi dendrite (exposing high-index planes) contributes to the superior CO2 reduction to formate. Although we were not able to directly perform further DFT calculations due to limited computational resources, we further conceive that additional high index surfaces only present on the Bi dendrite electrode, such as (107), (116), (214) and (009) amongst others, could also positively affect CO2 reduction by providing low-coordinated sites with appropriate binding energy.

CONCLUSIONS In summary, a hierarchical Bi dendrite electrode was developed for the selective conversion of CO2 to formate in an aqueous electrolyte. The Bi dendrite exhibits high catalytic activity and selectivity with a Faradaic efficiency of ~89% at an overpotential of 710 mV as well as stable performance during 12 h of operation. Using DFT calculation, we investigated three possible CO2 reduction pathways, among which the path via the formation of *OCOH surface intermediate is the most favorable in terms of reaction free energetics. DFT results further elucidated that the high-index planes can efficiently stabilize the *OCOH intermediate to enhance CO2 reduction activity, which successfully explains the origin of the superior performance of the Bi dendrite compared to that of the Bi foil. Based on our structure-DFT corroborated study, we provide a greater understanding of the governing factors of CO2-toformate reduction catalysis, which can be utilized in the further development of various advanced metal electrodes for CO2 reduction with high activity, selectivity, and stability.


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AUTHOR INFORMATION Corresponding Author *Email: [email protected], Phone: +82-42-350-1725 *Email: [email protected], Phone: +82-2-958-5227, Fax: +82-2-958-5809 *Email: [email protected], Phone: +82-2-958-5853, Fax: +82-2-958-5809 Author Contributions ‡These authors contributed equally. Notes The authors declare no competing financial interests. ASSOCIATED CONTENT Supporting Information. Additional information about electrode preparation and DFT calculations, physical characterizations of the electrode, detail electrochemical results of CO2 reduction, and a supplementary table for performance comparison. ACKNOWLEDGMENT This work was supported by the program of the Korea Institute of Science and Technology (KIST) and partly by the KU-KIST program by the Ministry of Science, ICT and Future Planning.


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TOC GRAPHICS


Facile CO2 Electro-Reduction to Formate via Oxygen Bidentate

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