Selective Electrochemical Production of Formate from Carbon Dioxide

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Selective Electrochemical Production of Formate from Carbon Dioxide with Bismuth-Based Catalysts in an Aqueous Electrolyte Chan Woo Lee, Jung Sug Hong, Ki Dong Yang, Kyoungsuk Jin, Jun Ho Lee, Hyo-Yong Ahn, Hongmin Seo, Nark-Eon Sung, and Ki Tae Nam ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b03242 • Publication Date (Web): 19 Dec 2017 Downloaded from http://pubs.acs.org on December 19, 2017

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Selective Electrochemical Production of Formate from Carbon Dioxide with Bismuth-Based Catalysts in an Aqueous Electrolyte Chan Woo Lee,† Jung Sug Hong,† Ki Dong Yang,† Kyoungsuk Jin,† Jun Ho Lee,† Hyo-Yong Ahn,† Hongmin Seo,† Nark-Eon Sung,‡ and Ki Tae Nam*,† †

Department of Materials Science and Engineering, Seoul National University, Seoul 151-744,

Korea ‡

Pohang Accelerator Laboratory, Pohang University of Science and Technology, Pohang, 37673,

Republic of Korea *Correspondence to: [email protected]

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ABSTRACT: For the efficient electroconversion of CO2 to formate, CO and H2 evolution must be suppressed. Herein, carbon-supported BiOx nanoparticles (BiOx/C) were investigated as a potential candidate for CO2 reduction. In bicarbonate solutions, the BiOx/C catalysts exhibited a high Faradaic efficiency of 93.4% for formate from −1.37 to −1.70 V vs. Ag/AgCl with a negligible amount CO and H2. Stable partial current densities and high Faradaic efficiencies were also achieved in 0.5 M NaCl (12.5 mA cm−2 and 96.0%, respectively). The possible reaction pathways and kinetic parameters of formate formation were examined using systematic electrochemical methods, including Tafel, pH dependence, and in situ X-ray absorption near edge structure analyses. From the results of these mechanistic studies, we propose that dual mechanisms are functional on the BiOx/C catalysts. Specifically, a two-electron and one-proton transfer reaction to adsorbed CO2 or a chemical proton transfer reaction to CO2− anion are the possible RDS at low potentials, whereas a one-electron transfer reaction to CO2 is the RDS at high potentials.

KEYWORDS: bismuth, carbon dioxide, electrocatalysis, heterogeneous catalysis, reaction mechanism


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1. INTRODUCTION Global carbon dioxide levels in the atmosphere have surpassed 400 ppm for the first time in recorded history.1 Concerns about climate change due to greenhouse gases have led to an international commitment to meet the carbon emission reduction target set by the Kyoto protocol and annual UN conference.2,3 Electrochemically converting CO2 into useful chemicals and fuels represents a promising strategy to not only reduce CO2 emission, but also replace or modify current petrochemical-based processes.4–10 Among CO2-derived fuels, formic acid (HCOOH) is one of the attractive candidates as a liquid fuel for the hydrogen economy due to its high volumetric hydrogen density of 53 g H2 per liter, non-toxicity, safety and transportability.11–14 Moreover, formic acid could easily be converted into H2 under ambient conditions with the development of an efficient formic acid dehydrogenation catalyst.15 For decades, the Pd, Sn, SnO2, Pb and PbO2 electrodes have been used to selectively convert CO2 to formate (HCOO−) in an aqueous solution.16–28 Each electrode has an optimum potential window for HCOO− formation at high Faradaic efficiency (FE). However, above or below the optimum potential, the evolution of H2 or CO, which are formed in competing reactions, relatively increases. For instance, the FE for HCOO− formation of an carbon-supported SnO2 electrode is 86.2% at −1.77 V vs. Ag/AgCl, but decreases to 65.1% and 45.2% at −1.67 and −1.87 V vs. Ag/AgCl, respectively.23 For a Sn foil electrode, the maximum FE of 95.0% was achieved at −1.97 and −1.67 V vs. Ag/AgCl in 0.1 M KHCO3 and 0.1 M Na2SO4.19 Similar trends have been also shown on Sn quantum sheet and porous SnO2 electrodes.29,30 For Pd and Pb electrodes, the optimum potential is reported to be −0.79 and −1.57 V vs. Ag/AgCl.16,26 Thus, the fundamental issue for developing efficient formic acid catalysts is the suppression of competing reactions. Interestingly, CO production is reportedly suppressed on Pb and PbO2


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electrodes, with only a negligible amount of CO being produced at any given potential.25,26 In contrast, Pd and SnO2 electrodes show considerable CO evolution and FE values for CO of 12.5% and 23.2% at −1.60 and −1.57 V vs. Ag/AgCl, respectively.18,22 These differences in CO evolution as a competing reaction to formate production indicate that different reaction mechanisms exist for each electrode. Based on calculated binding energies, Pb has a greater binding affinity for a formate intermediate (*OCHO) than carboxylate (*COOH).27,28 This property is speculated to play an important role for the exclusive production of HCOO− on Pb, as *OCHO leads to only HCOOH formation, whereas both CO and HCOOH can be produced from *COOH.27,28 Electrokinetic analyses have provided substantial insight into the mechanisms underlying these reactions. For example, the electrokinetic study on PbO2 revealed that the initial formation of CO2− anion is a rate-determining step (RDS), as was shown experimentally based on the non-dependency of current density on HCO3− concentration.25 For SnO2, it was hypothesized from the Tafel slope of 67 mV dec−1 that the RDS involves a chemical proton transfer step after CO2− anion formation.20 Alternatively, it was also proposed that carbonate, rather than CO2− anion, is a key intermediate.21 To identify efficient and environmentally benign catalysts for HCOO− conversion, we focused on carbon-supported bismuth oxide (BiOx/C). Notably, Bi metal is a poor catalyst for H2 evolution due to its positive free energy of hydrogen adsorption.31,32 In addition, the Bi surface is reported to greatly favor the adsorption of *OCHO intermediate over *COOH and *H.33 However, since the activity of a bismuth metal electrode for HCOO− conversion was first reported in 1995,34 detailed electrokinetic studies in aqueous solution have not been performed.34–37 Herein, we demonstrate that the catalytic activity of BiOx/C is specific for


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HCOO− as opposed to CO and H2, and an average FE of 93.4% for HCOO− is achieved over the wide potential window from −1.37 to −1.70 V vs. Ag/AgCl. In addition, it is firstly presented that dual mechanisms of electrochemical HCOO− production operate on the BiOx/C.

2. EXPERIMENTAL METHODS 2.1. Materials. Bismuth nitrate pentahydrate (98%), sodium bicarbonate (99.7%), sodium perchlorate (98%) and sodium chloride (99%) were purchased from Sigma-Aldrich. Bismuth powder (99.999%) and beta-bismuth oxide (99.9%) were purchased from Alfa Aesar. Ethylene glycol (99.5%) and sodium bismuthate (80%) were purchased from Daejung Chemical & Metals. Bismuth carbonate basic (99.5%) was purchased from Wako Pure Chemical. Ethanol (99.9%) and Vulcan XC-72R carbon black were purchased from OCI Company and Carbot Corporation, respectively. Carbon dioxide (99.999%) and carbon-13C dioxide (99%) were purchased from PSG Corporation and Cambridge Isotope Laboratories, Inc, respectively.

2.2. Synthesis of BiOx/C catalysts. The BiOx/C was synthesized by a facile solvothermal method modified from previous reports.38 Firstly, the bismuth nitrate source was dissolved in ethylene glycol and mixed with ethanol. Then, appropriate amount of carbon black was dispersed in the solution through sonication for 30 min. The suspension was put into a Teflon-lined stainless steel autoclave and heated up to 160 °C. After 2 h, the resulting solution was cooled to room temperature and washed with deionized water. Finally, the black powder was collected by centrifugation and dried for 6 h at 80 °C. Here, the concentration of the bismuth nitrate was approximately 0.01 M. The molar ratio of the injected Bi and C (Bi/C) was 0.11.


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2.3. Electrode Preparation. The BiOx/C catalysts were dispersed in ethanol with the neutralized Nafion solution which was prepared by pH adjustment of Nafion dispersion (D-521, Alfa Aesar) with 0.1 M NaOH aqueous solution. And the catalyst dispersion (3 mg ml−1) was drop-coated on a glassy carbon plate electrode (Alfa Aesar) and dried under ambient condition. The loading amount of the catalyst was 0.3 mg. Pristine carbon black and commercial Bi2O3 electrodes was prepared by the same procedure. Bulk Bi electrode was prepared with commercial Bi particles (99.9%, US Research Nanomaterials, Inc.). The Bi particles were dispersed in ethanol with the neutralized Nafion solution. Then, the catalyst dispersion (3 mg ml−1) was dropcoated on a glassy carbon plate electrode and dried under ambient condition. The loading amount of the catalyst was 0.3 mg.

2.4. Evaluation of Catalytic Properties. 2.4.1. Electrochemical measurements and product analysis. Electrolyses were conducted in a two-compartment electrochemical cell with a piece of Nafion membrane (Nafion 117) as shown in Supporting information. The BiOx/C electrodes and a piece of platinum foil were used as working and counter electrodes, respectively. All of the electrokinetic studies including pH dependence, Tafel and in situ analyses were conducted in 1.0 M NaHCO3/NaClO4 solutions. The cell was designed to have a large electrode area (2.05 cm2) and a small electrolyte volume (4 ml) in each of the compartments, with a headspace of 4.6 ml, motivated by the cell of Jaramillo’s group.6 All potentials were controlled against an Ag/AgCl reference electrode (3.0 M NaCl, BASi) utilizing a potentiostat (CHI 608C, CH Instruments) and converted to the RHE reference scale (ERHE = EAg/AgCl + 0.210 V + 0.0591 V × pH) after IR compensation. Typically, before the electrolysis, the catholyte was saturated with CO2 gas for at least 15 min to satisfy the saturation of pH. When the passed charge runs to 5 C, the electrolysis


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was stopped. Then, gas products were sampled using a syringe (0.1 ml, Hamilton) and injected to a gas chromatograph (Shimadzu) equipped with a packed Shincarbon ST 50/80 column. The gas mixtures in the column was transferred and detected in a thermal conductivity detector (TCD) and a flame ionization detector (FID). The concentration of produced HCOO− was analyzed by a 600 MHz NMR spectrometer (Avance 600, Bruker). The catholyte (700 µL) containing the liquid product was mixed with the D2O solution (35 µL) of 10 mM dimethyl sulfoxide (DMSO) and 50 mM phenol as internal standards. The 1D 1H NMR spectrum was measured with solvent suppression to cut the water peak down. 2.4.2. pH dependence. The pH was adjusted using 1.0 M solutions with different ratios of NaHCO3/NaClO4 under a constant CO2 flow (20 cc min−1). The examined concentrations of NaHCO3 were 0.05, 0.075, 0.10, 0.15, 0.2, 0.5 and 0.8 M. The corresponding pH values under CO2 saturation were 5.91, 6.06, 6.18, 6.37, 6.49, 6.91 and 7.15, respectively. The electrolyses were conducted to pass 5 C at various potentials under CO2 flow (20cc min−1 in total). After electrolysis, the initial pH values of 6.18, 6.49 and 6.91 changed into 6.22, 6.60 and 7.00. 2.4.3. PCO2 dependence. The partial pressure of CO2 was controlled by changing the ratio of CO2 and Ar flow (20cc min−1 in total) in 0.5 M NaHCO3/0.5 M NaClO4. The electrolyses were conducted to pass 5 C at the fixed potential of −1.40 V vs. Ag/AgCl. 2.4.4. Long-term bulk electrolysis. The electrolyses were conducted under the continuous CO2 flow of 20 cc min−1. After electrolyses in CO2-saturated 0.5 M NaCl at various potentials, the concentration of HCOO− was measured and the average faradaic efficiencies for HCOO− were calculated from the charge passed.


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2.5. Characterization of BiOx/C catalysts. The scanning transmission electron microscopy (STEM), high-resolution transmission electron microscopy (HRTEM) images, selected area electron diffraction (SAED) patterns of the BiOx/C catalysts were obtained using a highresolution transmission electron microscope (JEM-2100F, JEOL) at an acceleration voltage of 200 kV. The scanning transmission electron microscopy-energy dispersive spectroscopic (STEM-EDS) analysis was conducted using a transmission electron microscope (Talos F200X, FEI). For the analysis, the BiOx/C particles are dispersed in ethanol and the solution was dropped on the TEM grid and dried in ambient condition. The surface composition was evaluated by Xray photoelectron spectroscopy (XPS, Sigma Probe, Thermo VG Scientific). All the binding energies are referenced to C 1s (284.5 eV). Powder X-ray diffraction (XRD) was carried out on a D8 Advance X-ray diffractometer with Cu Kα radiation (λ=1.54056 Å). X-ray absorption near edge structure (XANES) analyses were performed at room temperature at beamline 8C at the Pohang Accelerator Laboratory (PAL), Republic of Korea. They were collected in the fluorescence mode at the bismuth L3-edge using electron energy of 2.5 GeV and a current of 250 mA. Bi L3-edge energy calibration was performed using Bi metal as a reference. During in situ XANES experiment, X-rays passed through the Kapton tape and were shined on the BiOx/C electrode immersed in the electrolyte under the applied potential. After in situ experiment, we separated the electrode from the circuit and rinsed it with distilled water. Then, the electrode was dried at ambient condition for 24 h after N2 blowing. Finally, the XANES spectra of the electrode were recorded to identify the Bi valency at the resting state.

3. RESULTS AND DISCUSSION


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For the synthesis of BiOx/C, an ethylene glycol (EG)-based process was adopted.38 The size and morphology of BiOx/C were analyzed using STEM and a high-angle annular dark field (HAADF) detector. Figure 1a and the schematic representation show that white Bi oxide nanoparticles with sizes of < 10 nm could be clearly distinguished from carbon particles with sizes of several tens of nanometers. STEM-EDS mapping analysis also supported that Bi signals predominantly occurs from the white nanoparticles (Figure S1). The analyses of HRTEM (Figure 1b) and SAED patterns (inset) showed that the synthesized bismuth oxide was amorphous, which was also identified by XRD analysis (Figure S2). Furthermore, by XPS analysis, additional Bi 4f, Bi 4d and O 1s peaks were identified and were not detected with pristine carbon (Figure S3a). The XPS spectrum of the Bi 4f core levels (Figure 1c) showed that bismuth had a single oxidation state of Bi3+.39,40 In the O 1s spectrum (Figure S3b), the peaks at 530.3 eV and 532.7 eV were indexed to oxygen in the Bi oxide lattice and adsorbed water molecules, respectively.41,42 The atomic percentages of BiOx/C for Bi, O and C were 1.2%, 4.3% and 94.5%, respectively (Figure S3c). Clusters were also observed by extended X-ray absorption fine structure (EXAFS) analysis (Figure 1d). The most intense peak at 1.6 Å corresponds to the 1st coordination shell of Bi-O.43 The peak at 3.0 Å can be attributed to the interaction of Bi with farther oxygens.44 The peak at 3.6 Å can be assigned to the Bi-Bi interaction.44 In contrast to bulk Bi oxide synthesized through thermal decomposition,43 the 2nd Bi-O shell of BiOx/C was significantly suppressed using the EG-based process. This difference in structure is indicative of large structural disorder around the Bi sites, a property that is characteristic of amorphous clusters.45 Based on these characterizations, we confirmed that the synthesized BiOx/C catalyst consists of few-nanometer-sized clusters with a low atomic ratio of Bi (1.2 at% Bi/C).


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Figure 1. (a) STEM image and schematic representation of BiOx/C. (b) HRTEM image of BiOx/C. Inset shows the SAED patterns, in which no distinguishable crystalline characteristics were detected. (c) X-ray photoelectron spectroscopy spectrum of the Bi 4f core levels. (d) Fourier transform of the Bi L3-edge EXAFS spectrum.

The superior CO2 reduction capability of the BiOx/C catalyst was demonstrated by linear sweep voltammetry (LSV) in 0.5 M NaHCO3/0.5 M NaClO4 as the electrolyte. As shown in Figure 2a, the BiOx/C electrode under CO2 exhibited enhanced catalytic current compared to that under Ar. At −1.41 V vs. Ag/AgCl, the current density reached over 3.5 mA cm−2 under CO2, a value that was 4.6-fold larger than that observed under Ar. The catalytic current density always remained higher under CO2 at the examined potential range. In contrast, the pristine carbon black electrode used as a control showed a negligible current change under CO2 and Ar atmospheres (Figure S4). In addition, the BiOx/C electrode showed two times higher geometric current density compared with the electrode prepared with commercial Bi2O3 powder.


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Figure 2. (a) Cathodic linear sweep voltammetric scan at 50 mV s−1 for BiOx/C and commercial Bi2O3 in 0.5 M NaHCO3/0.5 M NaClO4 under CO2 and Ar saturation. (b) Total current density as a function of potential for BiOx/C and commercial Bi2O3 in CO2-saturated 0.5 M NaHCO3/0.5 M NaClO4. (c) Potential dependence of Faradaic efficiencies for HCOO−, H2, and CO production on BiOx/C and commercial Bi2O3. (d) Partial current densities for HCOO− production on BiOx/C and commercial Bi2O3 at the examined potential range.

To quantitatively analyze the reaction products and associated FEs of BiOx/C, bulk electrolysis was conducted at fixed potentials. Specifically, 5 C was passed in a CO2-saturated 0.5 M NaHCO3/0.5 M NaClO4 (pH 6.91) and total current density values were measured (Figure 2b). The final products and associated FEs for each reaction were analyzed by gas chromatography (GC) and proton nuclear magnetic resonance (H NMR) spectroscopy. The total current density increased from approximately −1.30 V vs. Ag/AgCl, which is well matched with the results from the LSV method. Similarly, the FE for HCOO− drastically increased from 1.8% to 95.9% over


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the potential range of −1.20 to −1.51 V vs. Ag/AgCl (Figure 2c). As the thermodynamic potential for the CO2/HCOO− couple is −0.64 V vs. Ag/AgCl at pH 6.91 (−0.02 V vs. RHE),23 the overpotential of BiOx/C at which the partial current density for HCOO− (jHCOO-) was 1 mA cm−2 was determined to be 0.72 V (Figure 2d). As a comparison, the overpotentials of reported Pd/C, PbO2, SnO2/C and commercial Bi2O3 are 0.04, 0.88, 0.92 and 0.77 V, respectively (Figure S5a and 2d).16,23,25 Besides, the performances of the current state of the art catalysts for HCOO− production were plotted in Figure S5b.29,30,46–48 In constrast to BiOx/C, commercial Bi2O3 exhibited an abrupt current drop at the high potential which was due to detachment of agglomerated bulk particles from glassy carbon electrode during electrolysis. The FE for HCOO− reached 92.1% at −1.37 V vs. Ag/AgCl and was stably maintained at an average value of 93.4% until the potential reached −1.70 V vs. Ag/AgCl (Figure 2c). At −1.75 V vs. Ag/AgCl, the FE for HCOO− declined, and the FE for H2 gradually increased from 4.1% to 27.2%. Conversely, near the onset potential for HCOO− formation, the FE for H2 decreasd from 16.6% to 1.6%. In contrast to the competing trend between H2 and HCOO−, the CO evolution by BiOx/C was largely suppressed over the whole potential range. As displayed in Figure S6, the FE for CO increased only 1.4% from −1.20 to −1.30 V vs. Ag/AgCl. At higher potentials, the FE was reduced and remained at 0.3% on average. In the case of Pd, SnO2 and PbO2 electrodes,18,22,25 the FE for CO was reported to be 12.5%, 23.2% and 0.0% at −1.60, −1.57 and −1.63 V vs. Ag/AgCl, respectively. Thus, BiOx/C catalyst has a similar behavior to PbO2 with respect to the repression of CO production. Due to the wide potential range at which the FE for HCOO− remains high, a high jHCOO- was also obtained (Figure 2c). Over the potential range from −1.34 to −1.83 V vs. Ag/AgCl, the FE


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for HCOO− was maintained at more than 83.0%, and jHCOO- increased with increasing potential (Figure 2d). Under the present experimental conditions, the jHCOO- value reached 16.1 mA cm−2 at −1.75 V vs. Ag/AgCl, which corresponds to 300 µmol cm−2 h−1. We speculate that the wide potential window of BiOx/C is attributable to the intrinsic properties of bismuth. Even bulk-sized Bi metal electrodes prepared using commercial Bi particles exhibited a similar trend of FE distribution as BiOx/C (Figure S7). Likewise, PbO2 exhibited the FE more than 85% for HCOO− over a wide potential window from -1.34 to -1.64 V vs. Ag/AgCl (Figure S8).25 In contrast, SnO2 and Sn represent a peak shape of FE distribution for HCOO− (Figure S8).21,23,49 We also confirmed that carbon in the produced HCOO− originated from the supplied CO2. Figure S9a shows 1H NMR spectra of the resultant solutions after bulk electrolysis with BiOx/C electrodes under CO2, Ar and

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CO2 flow. Under 13CO2, the spectrum of the solution showed a

strong doublet (JCH = 194 Hz) at 8.35 ppm, which was attributed to the proton coupled to 13C of H13COO−.50 In Ar-saturated condition, a weak singlet of HCOO− was detected at 8.35 ppm and the corresponding FE was only 4.5%. The tiny amount of HCOO- was produced through the reduction of CO2 supplied from the dissociation reaction of HCO3−.25 Moreover, the H13COO− was also observed at 171.0 ppm in the

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C NMR spectrum of the resultant solution after

electrolysis under 13CO2 flow (Figure S9b).6 To determine the underlying mechanism of HCOO− formation by BiOx/C, the pH dependence of the reaction was investigated at various potentials. The pH was adjusted using 1.0 M solutions of NaHCO3/NaClO4 at different ratios under a constant CO2 flow in an open system. For the control of bulk pH, an open system was required because a closed system may result in the buildup of pressure, which can irregularly increase the concentrations of HCO3− and H+ from the equilibrium of CO2 + H2O ↔ HCO3− + H+. In addition, the use of continuous CO2 flow helped to


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maintain the CO2 partial pressure (PCO2) and to offset bulk pH changes that can occur due to proton consumption during electrolysis.51 After electrolysis, the bulk pH change was identified to be negligible as described in experimental section. In detail, the proton transfer from HCO3− produces CO32− ions which react with water to form HCO3− and OH− ions. At this stage, the continuous CO2 flow countervails OH− ions through the formation of HCO3−.52 In this system, an increase in bulk pH was accompanied by an increase in a bulk HCO3− concentration, with the bulk CO2 concentration being fixed at 0.033 M.51 Also, through bulk electrolysis, rather than a voltammetric method, the jHCOO- values were obtained. Using the open system, HCOO− formation by BiOx/C was found to be pH dependent, with differences being observed at low and high potentials (Figure 3a). As the pH increased from 6.18 to 6.91, the jHCOO- increased at −1.39 V vs. Ag/AgCl but decreased at −1.51 V vs. Ag/AgCl. An identical result was observed when the total current densities were plotted in place of jHCOO-. These results represent that the reaction mechanism of the BiOx/C changes depending on the potential. In comparison, SnO2 and PbO2 electrode showed increased and decreased current density, respectively, at −1.58 V vs. Ag/AgCl with increasing pH.23,25 When compared at high potential, the pH-dependent behavior of the BiOx/C catalyst was similar to that of PbO2. Notably, however, pH dependence at low potential was not presented in previous reports. To our knowledge, the present findings for BiOx/C are the first to demonstrate a change in pH dependence based on potential. The present experimental design allowed for the detection of this novel property and gave reproducible results.


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Figure 3. (a) Change of the partial current density for HCOO− as a function of pH at various potentials. (b) Plot of the partial current density for HCOO− as a function of pH at a fixed potential of −1.39 V vs. Ag/AgCl. (c) Tafel plots for HCOO− production with a linear fit. The Tafel slopes were estimated in several pH conditions. (d) Normalized Bi L3-edge XANES spectra of Bi metal and BiOx/C at the initial state, −1.51 V vs. Ag/AgCl and the resting state.

To more precisely determine the pH dependence in the low potential range, the examined pH values were delicately controlled at −1.39 V vs. Ag/AgCl and the corresponding jHCOO- values were plotted against pH (Figure 3b). In the pH range from 5.91 to 6.49, the pH and Log(jHCOO-) values were linearly correlated with a slope of one. This result clearly indicates the first-order dependence of jHCOO- on HCO3−, because the [HCO3−] is proportional to the reciprocal of [H+] at a fixed CO2 concentration.51 Taken together, these results indicate that HCO3- is involved in the RDS at low potential. During HCOO− formation, it is well known that HCO3− functions as a proton donor and has higher proton-donating ability compared with H2O.23 Here, we also


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identified that an increase in PCO2 results in higher jHCOO- (Figure S10). Therefore, both CO2 and HCO3− were determined to be the reactants involved in RDS at low potential. To determine the number of electrons involved in the RDS at low potential, Tafel analysis was conducted with BiOx/C in CO2-saturated 1.0 M NaHCO3/NaClO4 solutions as the electrolyte. As shown in Figure 3c, the Tafel slopes were estimated to be 66, 60 and 68 mV dec−1 at pH 6.18, 6.49 and 6.91, respectively. According to previous studies,20,21,23 Tafel slopes in the vicinity of 59 mV dec−1 indicate either: (1) chemical RDS with one-electron pre-equilibrium or (2) twoelectron RDS with chemical pre-equilibrium. Additionally, considering that both CO2 and HCO3− were involved in the RDS, the reaction pathway may involve a one-electron transfer to CO2, followed by a chemical proton transfer as the RDS,20,23 or the chemical adsorption of CO2 may be followed by two-electron and one-proton transfer as the RDS.21 In contrast to the findings at low potential, at the high potential, it was observed that jHCOOdecreases as the pH and HCO3− concentration increased (Figure 3a), demonstrating that HCO3− was not involved in the RDS. Rather, from the observed current response under CO2, it was determined that only CO2 was involved in the RDS (Figure 2a). Therefore, the formation of CO2− anion is proposed to be the RDS at high potential. It should be also considered that the HCOO− formation with high jHCOO- may increase the local concentration of OH−, which causes local depletion of HCO3− concentrations near the catalyst surface by the reaction, HCO3− + OH− ↔ CO32− + H2O.53,54 The higher buffer strength of HCO3− leads to higher local concentrations of HCO3−, a proton donor.54 When the buffer strengths of HCO3− are 0.1, 0.2 and 0.5 M, the FEs for HCOO− formation by BiOx/C were measured to be 92.0%, 90.6% and 95.9% around -1.51 V vs. Ag/AgCl, respectively. These results represent that the BiOx/C catalyst is highly resistive to H2 evolution. In contrast, Sn nanofoam electrodes showed a significant decrease in FE for HCOO−


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from 90.0% to 63.0% as the buffer strength increases from 0.1 to 0.5 M.55 It is also notable that CO evolution was much more suppressed at higher potentials than at lower potentials. As shown in Figure S6, the FE for CO reached 1.4% at −1.30 V vs. Ag/AgCl, but remained at an average of 0.3% above −1.45 V vs. Ag/AgCl. As the observed pH dependence and reaction mechanism were also altered with changing potential, the increase and suppression of CO evolution by BiOx/C are also considered to be affected by mechanistic variations. We speculate that the potential-dependent changes originated from a valency change of Bi at the electrode surface.

The change of average Bi valency was monitored by in situ XANES analysis conducted at a potential of −1.51 V vs. Ag/AgCl. Figure 3d shows the normalized Bi L3-edge XANES spectra of BiOx/C at the initial state, −1.51 V vs. Ag/AgCl and resting state. The white line for BiOx/C at the initial state overlapped with those of the reference β-Bi2O3 and Bi2O2CO3, which contains Bi3+, showing that BiOx/C has an oxidation state of 3+ (Figure S11). At −1.51 V vs. Ag/AgCl and resting state, the white line of BiOx/C shifted toward a lower energy. By comparing the positions of the white lines, the average Bi valency was calculated (Figure S12). At −1.51 V vs. Ag/AgCl, the average valency was +0.5, which indicates that only 16.6% of Bi3+ are present and that the remaining molecules are Bi0. Therefore, at high potential, BiOx/C has a Bi0-rich surface. The XRD pattern measured after electrolysis at −1.51 V vs. Ag/AgCl also showed that Bi metal was formed (Figure S13). It is notable that the FE for HCOO− formation by BiOx/C can be maintained as high as 95.9% at −1.51 V even though a Bi0-rich surface is formed at the high potential. This mechanism is different from that of SnO2 where CO2 activation is mediated by hydroxyl species (*OH) of oxide layer.21,24,36 As a starting point to understand the effects of oxide layers and their


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interaction with intermediates, we compared the valency change of BiOx, SnO2 and PbO2 catalysts during electrolysis using in situ XANES analysis. The carbon-supported SnO2 (SnO2/C) and PbO2 film were synthesized using previously known methods,56,57 and their phases were confirmed to be tetragonal SnO2 and the mixture phase of α-PbO2 and β-PbO2 by XRD analysis (Figure S14). In situ XANES analysis was conducted at the potential that achieve a maximum FE for HCOO- in CO2-saturated 0.1 M NaHCO3/0.9 M NaClO4 (Figure S15). During electrolysis, the valency of PbO2 film drastically lowered from +4.0 to +1.3 while SnO2/C was slightly reduced from +4.0 to +3.6 (Figure S16). In addition, the XANES spectrum of PbO2 film obtained during electrolysis showed the characteristic peaks of metallic Pb at 13041.6 and 13055.7 eV. These results indicate that both BiOx/C and PbO2 film form metallic surfaces during electrolysis different from SnO2/C. According to recent calculation studies,27,58,59 reaction mechanism is affected by valency state of catalysts. For example, on SnOx surface, *OH species reacts with CO2 to stabilize a bicarbonate intermediate (*HCO3).58 Then, the *HCO3 can be reduced to *OCHO or *COOH. In contrast, on metal surfaces such as Pb and Sn, the *OCHO is directly stabilized by protoncoupled electron transfer or by reaction of CO2 with *H.27,59 In this regard, it can be thought that the slightly reduced SnO2/C operates via the *HCO3 pathway while zero-valent surfaces of BiOx/C and PbO2 film directly stabilize *OCHO. Therefore, we think that the reaction mechanism of the BiOx/C is similar with that of PbO2 film but different from that of SnO2/C. Furthermore, the difference in reaction mechanism can contribute to CO selectivity. This is because *HCO3 pathway can produce CO through *COOH whereas direct stabilization of *OCHO leads to only HCOOH formation.27,58,60 Actually, at an optimum potential for HCOO-, the CO FEs of SnO2/C, BiOx/C, and PbO2 film were 4.0%, 0.2% and 0.1% (Figure S6 and S15).


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Inspired from the calculation studies,27,58 we speculate that oxygen-bound CO2− anion stabilized directly on Bi0-rich surface enable complete CO suppression at high potentials. As was also previously reported,61 oxygen-bound anion ease C-H binding, which leads to HCOO− without CO production. However, in the present system, we are unable to explain how CO2− anion can be stabilized by specific adsorption. Further investigations, such as in situ surface analyses, to determine the origin of this stabilization are therefore needed. Bi-based catalyst can also be used for efficient CO2 reduction in a NaCl solution that mimics seawater (Figure 4a). In the cathode compartment, CO2 was reduced to HCOO−, whereas Cl− was oxidized to chlorine in the anode compartment. The subsequent hydrolysis of chlorine leads to the generation of protons, which are transferred across the Nafion membrane. Figure 4b shows the results of bulk electrolysis with BiOx/C in 0.5 M NaCl at several potentials under continuous CO2 flow. Average current densities of approximately 8.1, 10.6 and 13.0 mA cm−2 were achieved at −1.64, −1.69 and −1.75 V vs. Ag/AgCl, respectively. Moreover, the resultant FEs for HCOO− exceeded 96.0% at the examined potentials. The high FE for HCOO− was stably maintained even after prolonged electrolysis (Figure S17). The outstanding performance of BiOx/C in CO2saturated NaCl shows that this material is a promising candidate for HCOO− production in seawater-based electrolysis systems.


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Figure 4. (a) Schematic illustration of a seawater electrolysis cell. (b) Total current density vs. time and Faradaic efficiencies for HCOO− at several potentials in 0.5 M NaCl.

4. CONCLUSIONS In summary, we demonstrated that BiOx/C is a highly efficient catalyst for the electrochemical reduction of CO2 to HCOO−. At potentials from −1.37 to −1.70 V vs. Ag/AgCl, an average FE of 93.4% was achieved for HCOO−. In addition, CO production was suppressed to less than 1.5% over the examined potential range. Due to the wide operational potential window and high selectivity, the BiOx/C electrodes exhibited a high partial current density of 16.1 mA cm−2 at −1.75 V vs. Ag/AgCl, corresponding 300 µmol cm−2 h−1. The results of mechanistic studies, including Tafel and pH dependent activity analyses, revealed that at potentials near the onset of activity, a two-electron and one-proton transfer reaction to adsorbed CO2 or a chemical proton transfer reaction to CO2− anion are the possible RDS for HCOO− formation. In contrast, the observed pH dependence for HCOO− production at higher potentials suggests that the formation of CO2− anion is the RDS because BiOx/C is reduced to form a Bi0-rich surface.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publication website at DOI: 10.1021/acscatal.xxxxxxx. STEM-EDS elemental mapping, XRD patterns and XPS spectra of BiOx/C, LSV curves of carbon black, comparison with reported catalysts, potential dependence of FE, characterization


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of bulk Bi particles, 1H NMR and 13C NMR spectra after electrolysis, PCO2 dependence, XANES spectra of reference materials, Bi valencies of BiOx/C, XRD pattern of BiOx/C after electrolysis, XRD patterns of SnO2/C and PbO2 film, FE results of SnO2/C and PbO2 film, in situ XANES spectra of SnO2/C and PbO2 film, time dependence of FE in NaCl, picture of CO2 reduction cell (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the Global Frontier R&D Program on Center for Multiscale Energy System (2011-0031571), and the KIST Institutional Program (2E00000) through the National Research Foundation of Korea funded by the Ministry of Science, ICT & Future. This research was also supported by the AOARD (FA2386-15-1-4019) and Ministry of Science, ICT & Future, through the Research Institute of Advanced Materials (RIAM) to K.T.N.

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Table of Contents Graphic


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Figure 1. (a) STEM image and schematic representation of BiOx/C. (b) HRTEM image of BiOx/C. Inset shows the SAED patterns, in which no distinguishable crystalline characteristics were detected. (c) X-ray photoelectron spectroscopy spectrum of the Bi 4f core levels. (d) Fourier transform of the Bi L3-edge EXAFS spectrum. 85x83mm (300 x 300 DPI)


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Figure 2. (a) Cathodic linear sweep voltammetric scan at 50 mV s-1 for BiOx/C and commercial Bi2O3 in 0.5 M NaHCO3/0.5 M NaClO4 under CO2 and Ar saturation. (b) Total current density as a function of potential for BiOx/C and commercial Bi2O3 in CO2-saturated 0.5 M NaHCO3/0.5 M NaClO4. (c) Potential dependence of Faradaic efficiencies for HCOO-, H2, and CO production on BiOx/C and commercial Bi2O3. (d) Partial current densities for HCOO− production on BiOx/C and commercial Bi2O3 at the examined potential range. 85x74mm (300 x 300 DPI)


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Figure 3. (a) Change of the partial current density for HCOO- as a function of pH at various potentials. (b) Plot of the partial current density for HCOO- as a function of pH at a fixed potential of -1.39 V vs. Ag/AgCl. (c) Tafel plots for HCOO- production with a linear fit. The Tafel slopes were estimated in several pH conditions. (d) Normalized Bi L3-edge XANES spectra of Bi metal and BiOx/C at the initial state, -1.51 V vs. Ag/AgCl and the resting state. 85x77mm (300 x 300 DPI)


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Figure 4. (a) Schematic illustration of a seawater electrolysis cell. (b) Total current density vs. time and Faradaic efficiencies for HCOO- at several potentials in 0.5 M NaCl. 85x35mm (300 x 300 DPI)


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