Promoting the Electrochemical Reduction of Carbon Dioxide by a

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Applied Chemistry

Promoting the Electrochemical Reduction of Carbon Dioxide by a specially designed Biomimetic Electrochemical Cell Fajun Li, Zhenzhen Fan, Jian Tai, Huimin Wei, Yanqing Zhou, and Lixu Lei Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b02031 • Publication Date (Web): 03 Sep 2018 Downloaded from http://pubs.acs.org on September 3, 2018

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Industrial & Engineering Chemistry Research

Promoting the Electrochemical Reduction of Carbon Dioxide by a specially designed Biomimetic Electrochemical Cell Fajun Li, Zhenzhen Fan, Jian Tai, Huimin Wei, Yanqing Zhou, Lixu Lei* School of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, China ABSTRACT: To overcome the difficulties arisen from the slow dissolution of CO2 and the depletion of electrolyte in anodic area, so as to obtain higher space time yield for electrochemical reduction of carbon dioxide (CO2) to formic acid (ERCO2F), we have designed a biomimetic electrochemical cell (BEC). The BEC has two gas diffusion electrodes (GDEs) separated by a flowing electrolyte regulator, which forces the electrolyte to flow from one electrode to the other, and a formic acid separator in the upstream of the circulating electrolyte. Here, the cathode is a piece of Sn-electroplated Cu foam, and the anode is a piece of pure Ni foam. The results show that the space time yield is 290 µmol h−1 cm−2 at −1.8 V vs. Ag/AgCl, respectively while the Faradaic efficiency remains at 92%, which is 2 times of that in an undivided electrochemical cell (143 µmol h−1 cm−2 at −1.8 V vs. Ag/AgCl). Consequently, the BEC successfully speeds up the ERCO2F. KEYWORDS: CO2 electroreduction, Gas diffusion electrode, Biomimetic electrochemical cell, High space time yield

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INTRODUCTION Carbon dioxide (CO2) in the atmosphere has already been a serious threat to the environment because of its increasing concentration1 and contribution to global warming.2 To mitigate this global crisis,3 both physical (carbon capture4 and sequestration) and chemical methods5-10 (photochemical, electrochemical and photo-electrochemical) have been investigated. Among all the routes, the electrochemical reduction of CO2 is promising since it not only reduces CO2 concentration, but also produces valuable materials under manageable electrolytic conditions.11 Up to now, a variety of products can be obtained by means of electrochemical reduction of CO2, such as CO,12-18 formic acid,19-30 methanol,31 ethanol,32 methane.33 Among them, formic acid (HCOOH) is the most attracting because it is an important industrial chemical and a suitable fuel for the direct formic acid fuel cell;34 it is also a viable hydrogen storage material due to its high volumetric and a moderate gravimetric hydrogen storage capacity.35 In a previous work,36 we have reported that the Faradaic efficiency is above 80% at cell voltage 2.9 V for a cell with Sn cathode and IrxSnyRuzO2/Ti anode, which means that it needs 5400 kWh electric power to produce 1 ton of formic acid. In places there is cheap electricity, electrochemical reduction of carbon dioxide (CO2) to formic acid (ERCO2F) could attract commercial interest already.37 However, to commercialize ERCO2F, it is of huge eager to produce HCOOH as substantially and inexpensively as possible in given space and time. This means we need to solve any other problems against faster process. From the view point of chemical engineering,38 the total speed of a chemical process in large space-time is controlled by both the nature of the reaction and the transfers of momentum, energy and the chemical species involved in the reaction. As it is known that electrochemical half-reactions are taken place only on the surface of the electrodes, the electrochemical reactions


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are mostly, if not always, controlled by the quantity of active catalytic sites, as well as the mass transfer. Therefore, to speed up the total reaction process, we have to use electrodes with largest specific surface area and any means that speed up the transfers. The first is to find the best electrode catalyst and increase its specific surface area. To do that, we have to find the best electrode catalyst and make its surface as large as possible39 to catalyse the reaction as fast and as many as possible. For this particular ERCO2F, although there are many electrode materials (also as catalysts)40-43 being used, metallic electrodes in various forms are the most popular, including foils,44 nanofoams,45,46 nanoparticles,47 and alloys,48 which have been summarized in literature49. Among those metals, Sn is inexpensive, non-toxic and highly efficient (Faradaic efficiency up to 88.4% reported),49 and can be considered as a kind of excellent catalyst.44 The biggest shortcoming for any foil electrodes is its extremely low specific surface area,44 resulting slow reaction (low current density). Therefore, electroplating Sn on copper foam is experimented and found working properly at much higher current density.50 We believe its rough Sn surface and excellent electric conductivity of copper substrate makes the success, thus we choose Sn electroplated copper foam as the cathode. The second issue comes from the transfer of substances. The total ERCO2F reaction is a gasliquid reaction: CO2 (g) + H2O (l) = HCOOH (l) + 1/2 O2 (g)

(1)

This reaction takes place electrochemically on the surface of 2 solid electrodes in dilute KHCO3 solution (pH ~ 6.8), therefore, it involves three states of substance: Cathode reaction CO2 (g) + H2O (l) + 2 e = HCOO− (aq) + OH− (aq)


(2)

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OH− (aq) + CO2 (g) = HCO3− (aq)

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(3)

Anode reaction H2O (l) = 2 H+ (aq) + 1/2 O2 (g) + 2 e

(4)

HCO3− (aq) + 2 H+ (aq) = CO2 (g) + H2O (l)

(5)

The cathode reaction involves gaseous CO2 as a reactant, and the anode reaction involves gaseous O2 as a product. As it is well-known, both the solubility and dissolution speed of CO2 is very low,51 which definitely delay the reaction. Gas diffusion electrodes (GDEs) can make CO2 and other species directly meet at the surface of the electrode, which make the electrode reaction much faster than the plate electrodes.52-55 Also, as the ERCO2F proceeds, the concentration of HCOOH increases. There are two problems arising: the first is that the pH of the reaction liquid decreases continuously, because HCOOH is a stronger acid than H2CO3; the second is that formic acid can be electrochemically oxidized on the anode, which makes the Faradaic efficiency decrease as the time goes.56 Although we have found that wrapping the anode with a Nafion membrane can retard the oxidation, it is better to separate the HCOOH newly-produced as soon as possible. Reaction shown in Eq. (5) could deplete the electrolyte (KHCO3) in anodic area, although the diffusion of HCO3− from cathode to anode could compensate part of the loss. Especially, when the electrochemical cell is separated with a Nafion membrane, it can result the voltage of the cell increasing rapidly because the anode potential increases.57 Thus, we need a method against the anodic depletion of the electrolyte.


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Figure 1. Structure of the biomimetic electrochemical cell.

To make all those issues solved, we have designed a device, which is called “biomimetic electrochemical cell (BEC)” (Figure 1) 58, because it could mimic the digestive system of living beings, in which there are a series of organs connected by a pipe, and all substances flow unidirectionally. Still, each organ does only a thing to make the whole process going on continuously. Basically, the BEC system consists of a BEC, a HCOOH separator and a pump (Figure 2). The BEC (Figure 1) contains an anode, a cathode and a regulator. Both the electrodes are gas diffusion electrodes (GDEs), which allow the gaseous CO2 to contact the cathode and O2 to depart the anode directly, without passing through the electrolyte solution. The anode is also covered with a Nafion membrane, which is to prevent the anodic oxidation of HCOO−.44 The regulator is actually a pipe array assembled on an insulator board, which makes the electrolyte


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solution flow unidirectionally from the inlet, then to anode, cathode, the outside separator and finally to the inlet, pushing by a pump.

Figure 2. The BEC system that shows the currents of the substances and the function of each component (organ).

When the system is working, the anode in BEC oxidises water from the inlet, exhales O2, releases 2 protons to the electrolyte and 2 electrons to the power source (not shown in the picture); the cathode inhales CO2 and compounds it with the protons and electrons into HCOOH. The HCOOH is released into the stream of the electrolyte (which can be separated by the HCOOH separator when the formic acid concentration is high enough, but not shown in this work), then the electrolyte is pumped back to the inlet and circulated. Thus, the process can go on and on. Therefore, the system digests CO2 and H2O, and produces O2 and HCOOH, with the help of electric power.


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Here, we report our first batch of the experiments on the BEC (not include HCOOH separator) and compare the results with an undivided electrochemical cell to reveal its effects on the parameters, such as Faradaic efficiency, current density and long-term durability of ERCO2F. The effects of flowing rate are also explored in unit time and space (area of electrode). EXPERIMENTAL Materials SnCl2⋅2H2O, KHCO3, KCl, sodium citrate dihydrate and ethylene glycol were purchased from Hushi Reagent, Shanghai; KMnO4, K2C2O4·H2O and (NH4)2Fe(SO4)2 were purchased from Aladdin, Shanghai. All those are of analytical grade. Cu foam (99.99%) and Ni foam (99.99%) were purchased from Kunshan Jiayisheng Electronics Co., Ltd. Polytetrafluoroethylene (PTFE) membranes were purchased from Dongguan Puwei Material Co., Ltd. N2 (99.9%) and CO2 (99.9%) were purchased from Shangyuan gas plant, Nanjing. All the materials were used without further purification. All the solutions were prepared using 18.2 MΩ deionized H2O. Characterization Scanning electron microscopy (SEM) and energy dispersion spectroscopy (EDS) were performed using a Pro-X desktop scanning electron microscope at an acceleration voltage of 15 kV. X-ray diffraction pattern (XRD) was recorded on a Ultima IV using Cu Kα radiation (1.54056 Å), scan range from 10° to 80°. The N2 adsorption-desorption isotherms at 77 K and its textural properties of the prepared sorbents were determined from N2 adsorption isotherm at 77 K using an adsorption apparatus (ASAP2020). Before each measurement, the samples were degassed under a high vacuum at 393 K for 15 h in order to out-gas any remaining moisture or


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organic species. The specific surface area of the samples was calculated by the application of the Brunauer-Emmett-Teller (BET) method. Preparation of the electrodes Sodium citrate was first dissolved in deionized water to form a 0.05 mol L−1 solution, its pH is adjusted with sulphuric acid to 6. It was then mixed with 0.02 mol L−1 SnCl2 and ethylene glycol in a volume ratio of 4:2:1, which is used as the electrolyte. By using a piece of Cu foam (1 × 1 cm2) as cathode and a Sn foil as anode, thin Sn layer is electroplated on the Cu foam at 4 mA galvanostatically,50 which was used as the cathode after applying a piece of gas diffusion membrane to make the GDE. The gas diffusion membrane (GDM) is faced to the CO2 inlet in the BEC. A piece of nickel foam of 1 × 1 cm2 is used as the anode directly. It is covered with a piece of Nafion membrane on one side and a GDM on the other side that faces to the O2 outlet in the BEC. Electrochemical experiments All the electrochemical experiments were performed on a CorrTest CS350 electrochemical workstation. Cyclic voltammetry (CV) measurements were performed in an undivided cell in electrolyte after being bubbled with N2 or CO2. The potentiostatic electrolysis experiments were performed in an undivided cell or the BEC without the HCOOH separator. The distance between the electrodes is 2 cm in the undivided cell and 4 cm in BEC, respectively. The electrolyte was 0.1 mol L−1 aqueous KHCO3 solution which was saturated before and then being continuously bubbled with CO2 at 20 mL min−1 during the electrolysis. The electrolytic reaction was


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terminated when the amount of charge reached 720 C. All experiments were performed under room temperature and ambient pressure. Titration of HCOOH It has been reported that the only products observed are H2, CO, and HCOO- on the catalytic activity of Sn electrodes for CO2 reduction.49,59 So the concentrations of formic acid in the electrolyte can be determined by titrations.60 The method is as follows: firstly, formate can be oxidized by the potassium permanganate in the strong alkaline solution. Consequently, sodium hydroxide was added into the electrolyte, and then excessive but known amount (a) of potassium permanganate standard solution (previously titrated with the potassium oxalate) was added at room temperature. The reaction is as follows: HCOO− + 2 MnO4− + 3 OH− = CO32− + 2 MnO42− + 2 H2O

(6)

Secondly, the solution was acidized with sulphuric acid, which makes all the existing MnO42− disproportionate to Mn2+ and MnO4−: 5 MnO42− + 8 H+ = Mn2+ + 4 MnO4− + 4 H2O

(7)

Thirdly, an excessive but known amount (c) of standard (NH4)2Fe(SO4)2 solution was added to the solution, which makes MnO4− be reduced completely: 5 Fe2+ + MnO4− + 8 H+ = 5 Fe3+ + Mn2+ + 4 H2O

(8)

Fourthly, the excess of ferrous ions was back titrated with the standard KMnO4 solution (b): MnO4− + 5 Fe2+ + 8H+ = Mn2+ + 5 Fe3+ + 4 H2O


(9)

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The amount of formic acid (n) is calculated according to the following equation: n = [5a - (c - 5b)]/2 = (5a + 5b - c)/2 where n is the amount of the existed formate and c is the amount of (NH4)2Fe(SO4)2 in the standard solution added in step (8), a and b is the amount of KMnO4 in the standard solution added in the steps (6) and (9) respectively. RESULTS AND DISCUSSION

Figure 3. (a) Changes of cell voltage when Sn is plated on Cu foam; (b) EDS of the electroplated surface of the Cu foam at different time. Table 1. Atomic concentration at different electroplating time

Time (min)

0

20

40

60

70

80

Atomic concn. of Sn

0

18

49

78

98

100


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Figure 3a shows the variations of the cell voltage with time during electroplating Sn on the copper foam under N2 atmosphere. It can be seen that the cell voltage slightly declines in the first 75 minutes, but drops thereafter suddenly to 0.38 V and then stable. According to the EDS of the electroplated surface of the copper foam at different time in Figure 3b, and the atomic concentration of Sn revealed by them in Table 1, the content of Sn increases gradually until 70 minutes, then reaches 100% in 80 minutes. Therefore, electroplating of Sn causes more and more Sn covering on Cu surface, and Sn fully covers the Cu surface at about 75 minutes; also, it can be concluded that Sn-electroplating on Cu surface requires higher over-potential than that on Sn surface.

Figure 4a shows the XRD patterns of the Cu foam and the Sn-plated Cu foam together with PDF card Cu (03-1005) and Sn (01-0926). It can be seen that Cu can be identified because the Sn shell is very thin which makes X-ray able to penetrate through. Figure 4b shows that the specific surface area of Sn-plated Cu foam (11.5357m2/g) can be larger than that of the plate Sn (0.0215m2/g). This will provide more catalytically active sites for ERCO2F.


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Figure 4. (a) XRD patterns of Cu foam and Sn-plated Cu foam; (b) Adsorption/desorption isotherms of N2 of different electrodes; (c) SEM images of Cu foam and (d) Sn-plated Cu foam.

Figures 4c and 4d present the morphologies of the Cu foam and the Sn-plated Cu foam electrodes, which show that the surface of Cu foam is very smooth, and there are micro pores; the foam turns very rough after being covered by the irregular Sn crystals of 200 nm ~ 400 nm after electroplating, but the pores are still there with little change, which can make the fluid materials permeable. Such a surface makes higher specific surface area, and should help the ERCO2F.


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Figure 5. CV curves vs. (a) Ag/AgCl and (b) RHE obtained on the electrode Sn-plated Cu foam in 0.1 mol L−1 KHCO3 solution saturated with N2 and CO2 at a scan rate of 0.01 V s−1; (c) Variations of the electrolysis current density on the Sn-plated Cu foam electrode in the BEC at different flow rate at −1.8 V; (d) Long-term durability of electrolysis in the BEC.

CV curves in Figure 5a were obtained with the Sn-plated Cu foam electrode. The anodic peak at −0.75 V and the cathodic peak at −1.11 V vs. Ag/AgCl under N2 can be attributed to the formation and reduction of SnO, respectively. This is because that the peaks appear at the same potential when the same data are drawn with the potentials relative to RHE (Reversible Hydrogen Electrode) (Figure 5b).56 It is known that pH of the KHCO3 solution lowers if the


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solution is saturated with CO2, and it increases continuously if it is bubbled with N2 because of the dissipation of CO2.44 Below −0.58 V (vs. RHE), it can be seen that higher current is obtained when CO2 is bubbled in, which means ERCO2F takes place at lower potentials than HER.45 Figure 5c depicts the current density at different flow rate of the electrolyte, which is carried out at −1.8 V in the BEC. It can be observed that the current density increases as the electrolyte flows faster, and when it reaches 400 mL min−1, the current is ~3.4 times of that in still. Therefore, the flowing electrolyte in the BEC can enhance the transfer of HCOO− and H+, and stop the depletion of HCO3− in the nearby of the anode, thus, break the limitation of mass transfer, and accelerate the electrode reaction, until the limitation from the electrode reaction reaches. Consequently, the following experiments in the BEC were all performed at the flow rate of 400 mL min−1. Figure 5d shows the long-term durability of electrolysis in the BEC. The variation of current density with time has been observed at potentials of −1.1 V, −1.2 V, −1.4 V, −1.6 V and −1.8 V in the BEC (inset of Figure 5d). It can be seen that the current remains stable in 2 hours. The fluctuation in current density can be observed, especially at more negative potentials, which must be caused by the H2 bubbles on cathode and O2 bubbles formed in the electrolyte side, which can be seen during the experiments. Those bubbles cover the electrode, which makes the electrode difficult to contact the reactants, thus increases the polarization of electrode. Figure 5d also shows the variations of the Faradaic efficiency with time in the BEC at −1.8 V vs. Ag/AgCl. It can be found that the Faraday efficiency is almost unchanged in the electrolysis experiment for up to 5 hours.


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Figure 6. (a) Partial current density and (b) Faradaic efficiency, space time yield in different experiment conditions.

Fig. 6 reports the comparison of the electrochemical property in different designs of the cell, in which the Sn plate cathode and the Sn-plated Cu foam cathode were immersed in the electrolyte in an undivided cell, and the electrolyte solution stands still; also, the Sn-plated Cu foam cathode with a gas diffusion layers (GDLs) was assembled in the BEC. In all the cases, the anode was just a piece of nickel foam. In particular, Figure 6a shows the partial cathode current density (ja) for the ERCO2F, and the error bars in it were obtained from several parallel experiments, and Figure 6b shows their Faradaic efficiencies and space time yields at different potential. In the undivided cell, it can be seen that ja on the Sn-plated Cu foam cathode (green) is always higher than that on the plate Sn cathode (red line), which must be related to its larger specific surface area. The ja of Sn-plated Cu foam immersed in the undivided cell are higher than that of the GDE in the BEC from −1.1 V to −1.4 V, but it is lower when the potential is lower than −1.5 V vs. Ag/AgCl. One reason is that the distance between the electrodes is shorter (2 cm) in the undivided cell than that in BEC (4 cm), which definitely makes the cell reaction faster. Another reason is that when the electrode is immersed in the electrolyte, both side of the electrode contact


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the reactants, however, in the BEC, only one side of the GDE contacts the reactants in the electrolyte, which results only part of the electrocatalyst sites are in action. When the reaction speed is lower than CO2 feeding speed, more catalyst sites result faster speed. However, when the reaction rate is faster than the diffusion speed in electrolyte, the GDE will be much better CO2 supplier, and the flowing electrolyte make the mass transfer between the electrodes faster, which certainly makes the reaction proceed faster. It is known that the dissolution speed of CO2 in the electrolyte is very low, while GDE allows CO2 to reach at the three-phase interfaces directly, which makes CO2 has not to dissolve in the electrolyte to participate the reaction, thus GDE allows higher reaction speed. Figure 6b shows both the Faradaic efficiency (FE) and the space time yield (STY) versus electrode potential. It reveals that the maximum FEs for ERCO2F are 79%, 82% and 92% at −1.7 V, −1.6 V and −1.6 V for the plate Sn electrode, the Sn-plated Cu foam electrode in the undivided cell and the Sn-plated Cu foam electrode with a GDL in the BEC, respectively. Compared with the value in a literature that also using a GDE but the electrolyte is motionless, where the maximum FE at −1.8 V 61 was 86.75 ± 2.89%, the value is a bit higher (92%), even at lower potential (−1.6 V). Therefore, with the Sn-plated Cu foam electrode in the BEC, a higher FE at a relatively lower over-potential can be achieved, which is beneficial for reducing energy consumption. Although the FE reaches its maxima between −1.6 V and −1.7 V, the space time yield (STY) of formic acid still rises because of the increase of ja. The STY is 290 µmol h−1 cm−2 at −1.8 V for the Sn-plated Cu foam electrode in BEC, which is about 2 times of the same electrode immersed in the electrolyte in the undivided cell (143 µmol h−1 cm−2), which is similar to a literature value, 118 µmol h−1 cm−2 for a similar electrode in an undivided cell.50


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CONCLUSIONS In summary, the specially designed biomimetic electrochemical cell (BEC) is highly effective in enhancing the transfers of CO2, O2 (through GDL) and species in the electrolyte solution by forcing the electrolyte to flow from one electrode to the other at high speeds, and makes the current density 3.4 times of that when the electrolyte is stationary. At cathode potential of −1.8 V vs. Ag/AgCl, the space time yield of formic acid can be raised to 290 µmol h−1 cm−2, in sharp contrast of 143 µmol h−1 cm−2 in an undivided cell with stationary electrolyte. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] ORCID Lixu Lei: 0000-0002-7787-0704 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work is supported by the Fundamental Research Funds for the Central Universities (3207047452) and the Jiangsu Key Laboratory for Advanced Metallic Materials (BM2007204). REFERENCES (1) Friedlingstein, P.; Andrew, R. M.; Rogelj, J.; Peters, G. P.; Canadell, J. G.; Knutti, R.; Luderer, G.; Raupach, M. R.; Schaeffer, M.; Van Vuuren, D. P.; Le Quéré, C. Persistent growth of CO2 emissions and implications for reaching climate targets. Nat. Geosci. 2014, 7, 709-715.


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For Table of Contents Only

The BEC is highly effective in enhancing the transfers of CO2, O2 (through GDL) and species in the electrolyte solution by forcing the electrolyte to flow from one electrode to the other at high speeds, and makes the current density 3.4 times of that when the electrolyte is stationary.


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Promoting the Electrochemical Reduction of Carbon Dioxide by a

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