Electrochemical Conversion of CO2 to 2-Bromoethanol in a

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Electrochemical Conversion of CO2 to 2Bromoethanol in a Membraneless Cell Shenghong Zhong, Zhen Cao, Xiulin Yang, Sergey M. Kozlov, KuoWei Huang, Vincent Tung, Luigi Cavallo, Lain-Jong Li, and Yu Han ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.9b00004 • Publication Date (Web): 29 Jan 2019 Downloaded from http://pubs.acs.org on January 31, 2019

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ACS Energy Letters

Electrochemical Conversion of CO2 to 2-Bromoethanol in a Membraneless Cell Shenghong Zhong, Zhen Cao, Xiulin Yang, Sergey M. Kozlov, Kuo-Wei Huang, Vincent Tung, Luigi Cavallo, Lain-Jong Li*, and Yu Han*

Physical Science and Engineering Division, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Kingdom of Saudi Arabia

AUTHOR INFORMATION Corresponding Authors Yu Han (e-mail: [email protected]) Lain-Jong Li (E-mail: [email protected])

ACS Paragon Plus Environment

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ABSTRACT: Tremendous research efforts have been made to electrochemically convert CO2 into useful chemicals, with the main products being limited to hydrocarbons and oxygenates. One-step processes that integrate CO2 reduction with a subsequent reaction to produce other types of functional chemicals are economically attractive. Here we report that direct electrochemical conversion of CO2 to 2-bromoethanol, a valuable pharmaceutical intermediate, is enabled by coupling the anodic and cathodic reactions with the presence of potassium bromide electrolyte in a membraneless electrochemical cell. The maximum Faradaic Efficiency of converting CO2 to 2bromoethanol that we achieved is 40 % at -1.01 VRHE with its partial current density of -19 mA cm-2. Our work demonstrates a new strategy for making value-added products from CO2 through a simple process.

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Electrochemical CO2 reduction reaction (CO2RR) is one promising approach to transform CO2 to useful chemicals with high energy efficiency under mild reaction conditions1. By activating chemically inert CO2 molecules2 and overcoming the kinetic energy barriers in C–C bond formation, the electrocatalytic CO2RR processes can generate products with two or more carbons (C2+) in composition. The most common products of CO2RR are hydrocarbons and oxygenates, and additional reactions are needed to further convert these products to more valuable chemicals. It is therefore highly desirable to integrate the CO2 reduction and the subsequent functionalization in a single electrochemical process. In most established CO2RR systems, “membrane” cells (two compartment cell separated by a membrane) are used for the purpose of studying the cathodic reactions3-12 or preventing unwanted side reactions caused by the convenient transport of chemical species between two electrodes. By comparison, the systems comprised of “membraneless” cells (single-chamber cells)13–15 are simpler and more suitable for large scale production, given the proper design of electrochemical reactions on cathodes and anodes, and such systems hold great potentials in directly and selectively converting CO2 to highly value-added C2+ chemicals. Bromohydrins are important building blocks in chemical and pharmaceutical synthesis, as they can be easily converted to various commodity chemicals, such as azidoalcohols, epoxides, aminoalcohols, aziridines, and hydroxy acids16. Therefore, it would be of great economic interest if bromohydrins can be one-step prepared from CO2RR. Even the simplest bromohydrin, 2bromoethanol (BrCH2CH2OH), is much more valuable ($ 21,000 per ton) than conventional products of CO2RR such as CO ($ 1,300 per ton), HCOOH ($ 1,300 per ton), and C2H4 ($ 800 per ton)17.


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In this study, as a proof of concept, we show that 2-bromoethanol can be directly obtained from CO2RR by using a membraneless cell in the presence of KBr in 0.1 M KHCO3 electrolyte. As illustrated in Scheme 1, such a setup allows the cathodic intermediate in situ brominated by Br2 generated from the anode to produce 2-bromoethanol. Under optimized conditions, the Faradaic Efficiency (FE) of producing 2-bromoethanol from CO2 is as high as 40% at -1.01 VRHE. Our work provides a promising strategy to tune the products of CO2RR from conventional hydrocarbons and oxygenates to highly value-added halohydrins. In our initial attempt of CO2RR in a membraneless cell without addition of KBr, we used a Cu foil with nano-cubic morphology (Figure S1) as catalyst (cathode, working electrode) for its well-known high selectivity to C2+ chemicals1,12,18,19, and 0.1 M KHCO3 solution as the electrolyte. Because the membraneless cell configuration allows the direct contact between the species generated from both electrodes, the anodic product oxygen and the cathodic intermediate can recombine to produce CO2, leading to a low CO2RR efficiency. In order to solve this problem, we added KBr into the electrolyte solution to suppress the production of O2 from the anodic reaction, as Br- can be more easily oxidized than OH- under the investigated conditions. In other words, Br2 instead of O2 was produced at the anode upon the addition of KBr in the system. Interestingly, we note that such a modification of the anodic reaction by adding KBr led to new liquid CO2RR products as revealed by 1H-NMR spectroscopy (Figure S2). Twodimensional NMR correlation spectroscopy (Figure 1) shows that the observed new peak at 3.78 ppm is coupled with the peak at 3.42 ppm, suggesting a compound having two different carbon atoms. Analysis of 1H-splitting indicates that the two peaks associated with the new product are both triplet (Figure S3), suggesting an ACH2CH2B (A ≠ B) structure. Given the reaction conditions used, it is highly possible that the new product is BrCH2CH2OH (2-bromoethanol). In


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ACS Energy Letters

order to confirm this assignment, we prepared a standard solution containing 100 ppm 2bromoethanol (v/v, in 0.1 M KHCO3) and collected its 1H-NMR spectrum (Figure S4). The result shows that both 1H-splitting and chemical shifts of 2-bromoethanol are in good agreement with those observed for the new liquid product. We also used direct-infusion Obitrap mass spectrum to examine KOH-treated (to avoid the interference of bicarbonate) electrolyte after the CO2RR process, and detected the presence of ethylene oxide, which was formed by the reaction of 2-bromoethanol and KOH (Figure S5). This result further supports that 2-bromoethanol was produced during the CO2 conversion. In addition to 2-bromoethanol, we identified a trace amount of 1,2-dibromoethane (another product of ethylene bromination) in the product, according to NMR (Figure S6 and Table S1) and head space GC-MS (Figure S7) analysis. Other identified products include hydrogen, carbon monoxide, methane, formate, ethylene, ethanol, acetate, acetaldehyde, propionaldehyde, and n-propanol. The net reaction of the production of 2-bromoethanol can be described by Eq. (1), which indicates that the production of one 2-bromoethanol molecule corresponds to the transfer of 10 electrons. 2CO2 + 11H + + 𝐵𝑟 ― + 10𝑒 ― = 𝐵𝑟𝐶𝐻2𝐶𝐻2𝑂𝐻 + 3𝐻2𝑂

𝐸𝑞. (1)

The Faradaic efficiency that represents the selectivity was calculated by Eq. (2). 𝜺 𝑭𝒂𝒓𝒂𝒅𝒂𝒊𝒄 =

𝜶𝒏𝑭 𝑸

𝐸𝑞. (2)

where α is the number of electrons transferred (α = 10 for 2-bromoethanol); n is the number of moles of the produced 2-bromoethanol, which is calculated from its concentration in the electrolyte as determined by NMR (Figure S8); F is the Faraday constant (96,485 C·mol-1); Q


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represents all the charge passed throughout the process and it is calculated by the integration of the i-t curve. We found that the production of 2-bromoethanol was directly correlated with the concentration of KBr in the electrolyte. As shown in Figure 2a, at -0.92 VRHE, when 0.2, 1, 2, 5 and 20 mM KBr were added in electrolyte, the selectivity of 2-bromoethanol was around 4.7%, 12.8%, 28.6%, 26.0% and 17.1%, respectively. The fact that more KBr in electrolyte leads to more Br2 generated by anodic oxidation can explain why the selectivity of 2-bromoethanol increased with the concentration of KBr ranging from 0.2 mM to 2 mM. The observed lower selectivity of 2-bromoethanol at 20 mM than at 2 mM of KBr suggests the formation of other products associated with Br2 at higher concentration conditions. According to the 1H-NMR spectrum, there is a new compound (single peak at 3.74 ppm) in the liquid products from the system of 20 mM of KBr (Figure S9). We surmise this is a Br-containing compound but we cannot identify it explicitly with the available data. At the optimal concentration of KBr (2 mM) in electrolyte, we further examined the CO2RR under various potentials. The results show that the on-set potential of producing 2-bromoethanol is around -0.72 VRHE, and that its selectivity increases with more negative potential applied and reaches a maximum value of 40% at -1.01 VRHE (Figure 2b). It is worth noting that ethylene was not detected until the potential was increased to -1.08 VRHE. The current density (Figure 2c) in our membraneless cell system is -47 mA cm-2 at -1.01 VRHE and -65 mA cm-2 at -1.08 VRHE, remarkably higher than the values of traditional cells12,18,19. The high current density of our system is attributed to the use of more reductive KBr additive at a much (four orders of magnitude) higher concentration compared to the oxidized species (OH-) in conventional cells9. The high current density tends to lead to a high local pH at the cathode and thus favorable kinetics for the production of C2+ chemicals 10, 20-22. Taking this advantage, the partial current of


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2-bromoethanol is as high as -19 mA cm-2 at -1.01 VRHE (Figure 2d). We measured the electrochemical surface area (ECSA) of the electrode using the cyclic voltammetry method23 to calculate the ECSA-normalized current densities (Figure 3). Specifically, the electrodes were cycled in the non-Faradaic range at different scan rates (Figure 3a, 3b); the electrical doublelayer capacitances (Cdl) of the electrodes were derived from the slope of the plots (Fig. 3c); the roughness factor (RF) of the nanocubic Cu catalyst was then determined using a pristine flat Cu foil as the reference surface (RF= Cdl/Cdlref) to be 24.75 (1.98 mF cm-2 / 0.08 mF cm-2, Figure 3c), according to which the ECSA-normalized current density was -2.0 mA cm-2 at -1.01 VRHE and -2.5 mA cm-2 at -1.08 VRHE, and the highest ECSA-normalized partial current density of 2bromoethanol was -0.75 mA cm-2 at -1.01 VRHE (Figure 3d). We characterized the Cu electrodes before and after CO2RR at – 0.92 VRHE in 0.1 M KHCO3 and 2mM KBr electrolyte using SEM, cyclic voltammetry, X-ray photoelectron spectroscopy, and Auger spectroscopy (see Figure S1). The results show that the electrode surface became rougher with the original nanocubes evolving into ill-defined morphologies (Figure S1a), while their initial chemical state (Cu(0)) was retained during electrolysis. We confirmed with control experiments that a membraneless cell configuration and the addition of KBr are both the pre-requisites for producing 2-bromoethanol from CO2RR (Figure 4). Without adding KBr in the electrolyte, there is no source of bromine to generate 2bromoethanol; on the other hand, bromination cannot take place in a membrane cell even using KBr as an additive, due to the lack of contact between the cathode-generated intermediate and anode-generated Br2. In fact, when KBr is used in the configuration of a membrane cell, the cathode compartment contains both intermediate of CO2RR and Br-, but 2-bromoethanol is not produced, suggesting that it is Br2 instead of Br- anion that accounts for the bromination. This


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also explains why no halohydrins have been detected in previous studies using KBr or KCl as additives in the electrolyte9,24–26. We performed more experiments to locate where 2-bromoethanol is formed in this membraneless cell system. There are three possible scenarios: (1) the cathodic product ethylene and anodic product Br2 react in the bulk electrolyte; (2) the cathodic product ethylene diffuse to the anode surface to be brominated; (3) the anodic product Br2 diffuse to cathode surface to bromide the cathode intermediates. We performed three control experiments to mimic the three scenarios, respectively (Figure S10). In the first experiment, according to the concentration of 2bromoethanol produced in our membraneless cell, we bubbled a comparable amount of ethylene into a Br2 solution during one hour, without catalysts or applying any potentials, and we found that only 24.7% ethylene was converted into 2-bromothanol, as determined by GC and NMR (Figure S10a). This conversion is far less than that observed in the CO2RR process using a membraneless cell, indicating that 2-bromoethanol is unlikely formed in the bulk solution from ethylene and Br2. In the secondary experiment, we used a membrane cell configuration and continuously bubbled ethylene into the anodic half-cell (Pt as the anodic counter electrode) with 0.1 M KHCO3 and 2 mM KBr as the electrolyte. We found that after 1 hour of electrolysis at – 0.96 VRHE (Cu cathode working electrode vs. RHE), only 4.0% ethylene was converted into 2bromothanol (Figure S10b), suggesting that the generation of 2-bromoethanol is not in the way that ethylene diffuse to the anode surface to be brominated. The results of these two experiments rule out the first two scenarios discussed above. In the third experiment, we reduced CO2 in a membrane cell with Br2 (ex-situ produced) in 0.1 M KHCO3 electrolyte at – 0.96 VRHE for 1 hour. As shown in Figure S10c, no ethylene was detected by GC, while other CO2RR products were founded, such as CO (GC peak retention time at ~6.63 min, peak area ~7.4), CH4 (GC peak


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ACS Energy Letters

retention time at ~1.92 min, peak area ~1.8) and 2-bromoethanol (by NMR). The observed high selectivity of 2-bromoethanol relative to ethylene was consistent with the result obtained in our membraneless configuration, confirming that 2-bromoethanol was formed by Br2 reacting with CO2RR intermediates at the cathode surface. Figure 5a shows the free energy diagram of possible pathways for CO2 reduction to ethylene and 2-bromoethanol on the (100) facet of Cu starting from the adsorbed CO intermediate (*CO), based on density functional theory (DFT) calculations. The process of multi-electron reduction of *CO to 2-bromoethanol (and ethylene) was schematically illustrated in Figure 5b. In general, the first reaction step is energetically uphill, while all the rest steps are spontaneous. The last steps in the diagram explain the preferred production of 2-bromoethanol over ethylene in this system: the conversion of the *CHCH2 intermediate to ethylene and to 2-bromoethanol corresponds to an energy decrease of 0.56 eV and 3.31 eV, respectively, indicating that the latter is energetically more favorable. Even though the model simplified the realistic condition to certain extent, the result is consistent with the experimental observation that 2-bromoethanol (other than ethylene) is dominant in the system. In summary, we report a simple membraneless setup that enables the direct conversion of CO2 to 2-bromoethanol with the assistance of KBr. This is the first report to couple the cathodic and anodic reactions in CO2RR for producing compounds other than hydrocarbons and oxygenates. Our system gives rise to a marked selectivity (40%) and a high partial current density (-19 mA cm-2) of 2-bromoethanol under optimized conditions (2mM KBr; at -1.01 VRHE). Our study demonstrates the possibility of broadening the CO2RR product category from conventional fuels to more valuable chemical building blocks and pharmaceutical intermediates.


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ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: XXX. This file includes: Materials and Methods, Figures S1 to S10, Table S1, and References. AUTHOR INFORMATION Yu Han, [email protected] Lain-Jong Li, [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors thank the support from King Abdullah University of Science and Technology.

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Scheme 1. Schematic illustration of electrochemical conversion of CO2 to 2-bromoethanol in a membraneless cell, with a proposed reaction pathway that anodic product Br2 diffuses to cathode and reacts with cathodic intermediates to form 2-bromoethanol. The white bubbles represent CO2.


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Figure 1. Two-dimensional NMR correlation spectroscopy of liquid products, showing the presence of ethanol (black dot rectangular), 1-propanol (pink dot rectangular), and 2bromoethanol (red dot rectangular) that has two correlated peaks at 3.78 ppm and 3.42 ppm.


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Figure 2. (a) The FE of producing 2-bromoethanol at different concentrations of KBr (0.2, 1, 2, 5, and 20 mM) and -0.92 VRHE. (b, c, d) CO2RR performance at different potentials with 2 mM of KBr in electrolyte: (b) FE, (c) total current density, (d) 2-bromoethanol partial current density.


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Figure 3. (a-b) Cyclic voltammetry at the range of -0.15~-0.05 V (vs. Ag/AgCl) under different scanning rates: (a) pristine Cu foil and (b) nanocubic Cu electrodes. (c) Plot of capacitive current densities of pristine Cu foil (black) and nanocubic Cu (red) vs. scanning rate. The slope represents the double layer capacitance. (d-e) ECSA-normalized total current density (d) and partial current density of 2-bromoethanol (e).


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Figure 4. CO2RR performance in different conditions (membrane or membraneless cell; with or without KBr in electrolyte) at -0.96 VRHE. The conditions are labelled at the top right corner of each panel. The results show that 2-bromoethanol is only produced in the configuration of a membraneless cell with KBr in electrolyte.


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Figure 5. Proposed pathway for the formation of 2-bromoethanol based on DFT calculations. (a) Free energy diagrams for CO2 reduction to ethylene and 2-bromoethanol on (100) facet of Cu, with the intermediate of each elementary step labelled; (b) the corresponding energetically favorable structures of each step (orange = Cu, black = C, pink = Br, red = O, gray = H; Cu atoms directly interacting with intermediates are highlighted in green).


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Electrochemical Conversion of CO2 to 2-Bromoethanol in a

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