Electrochemical Reduction of CO2 in Proton Exchange Membrane

Aug 23, 2017 - Effects of catalyst, electrode potential, and electrolyte species on the product ... Figure 1. Flow sheet of the hydrogenation of CO2 i...
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Electrochemical Reduction of CO2 in Proton Exchange Membrane Reactor: The Function of Buffer Layer Lin Ma,† Shuai Fan,† Dongxing Zhen,† Xuemei Wu,*,† Shishui Liu,† Jingjing Lin,† Shiqi Huang,† Wei Chen,† and Gaohong He*,†,‡ †

State Key Laboratory of Fine Chemicals, Research and Development Center of Membrane Science and Technology, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, China ‡ School of Petroleum and Chemical Engineering, Panjin Campus of Dalian University of Technology, Panjin 124221, China ABSTRACT: Electroreduction of CO2 is performed in proton exchange membrane reactors (PEMRs) with a buffer layer to investigate the critical factors that determine the cell performance. The buffer layer has the function of ensuring sufficient cathode potential (above the potential threshold of Cu, Sn, and In catalysts at around −1.3 to −1.4 V) compared with the limited cathode potential in the conventional PEMR, therefore a high hydrogenation rate (i.e., 89.8 nmol cm−2s−1 at −1.8 V) is achieved. The buffer layer exhibits good ability to suppress H2 evolution, however, the current efficiency of HCOOH decreases by over 50% after the buffer solution is saturated with protons (after 10 h reaction). Improving CO2 mass transfer at the reaction interface by adding tetrahydrofuran (THF) in a buffer layer or zeolitic imidazolate framework with a leaf-like morphology (ZIFL) in the catalyst layer, the current efficiency of HCOOH can be increased by around 10−15%.

1. INTRODUCTION Nowadays, CO2 conversion has received tremendous attention because of the increasing content of CO2 in the atmosphere and excessive depletion of fossil fuels. It plays an important role in preventing global warming and offering renewable energy. Methods of CO2 conversion have been widely investigated. Many researches focus on CO2 reduction without hydrogen gas participation such as the high-temperature solid oxide electrolysis cell (SOEC)1 and the copolymerization of CO2 with alkylene.2 Most common procedures are catalytic hydrogenations such as conventional heterogeneous hydrogenation, mild electroreduction, and photocatalytic reduction.3−5 The adsorption and activation of CO2 on the catalysts surface was considered to be the control step according to the mechanism of hydrogenation of CO 26 because of the stabilization of CO2 in thermodynamics.4 For conventional heterogeneous reactors, such as fixed bed, fluidized bed, and slurry reactors, harsh environments (∼700 °C, ∼8 MPa) are indispensable because high temperature could provide sufficient activation energy for CO2 and high pressure could increase H2 partial pressure for better adsorption on catalysts surface.7 CO8 (via reverse water−gas shift (RWGS)), hydrocarbons3 (via Fischer−Tropsch synthesis (FT)) and some oxygenates9 such as methanol are the common products of CO2 gas phase hydrogenation. Electrochemical reduction of CO2 has attracted great attention due to its ambient operating conditions and unique ability to activate CO2 and H2 by electric energy. With electric power supply, CO2 is theoretically reduced at −0.61 V (vs SHE) to produce formic acid (HCOOH)10 and sufficient © 2017 American Chemical Society

adsorbed hydrogen atoms on catalyst surface could be easily provided either by protons in acidic electrolyte solution or by dissociation of H2 at theoretically 0 V (vs SHE). Reaction rate of electrochemical reduction reactors (about 14800 nmol min−1g−1)11 is significantly improved compared with the conventional heterogeneous reactors (about 40 nmol min−1 g−1).12 It is extremely attractive when electricity derives from renewable sources such as solar, wind, hydroelectric power, and so on.11−13 In general, CO2 electroreduction is mainly focused on conventional three-electrode liquid electrolytic cell configurations. Effects of catalyst, electrode potential, and electrolyte species on the product selectivity are summarized in recently published reviews.11,14 Single metal catalysts can be roughly classified into two categories by the products: CO formation metals (Cu, Au, Ag, Zn, and Pd)15 and HCOOH formation metals (Pb, Hg, In, Sn, Cd, and Tl).16 Because of the lower solubility of CO2 in aqueous electrolyte solution, high pressure operation is often needed in the three-electrode liquid electrolytic cells to reduce CO2 mass transfer resistance.17 As an alternative, organic solvents are reported as electrolyte to improve solubility of CO2, however, the proton conductivity is limited.18 Gas phase feeding reduces the mass transfer resistance of CO2 to a large extent and could be achieved in fuel cell reactors.21 With the configuration of solid electrolyte Received: Revised: Accepted: Published: 10242

February 25, 2017 August 19, 2017 August 23, 2017 August 23, 2017 DOI: 10.1021/acs.iecr.7b00819 Ind. Eng. Chem. Res. 2017, 56, 10242−10250

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Industrial & Engineering Chemistry Research

Figure 1. Flow sheet of the hydrogenation of CO2 in a PEMR (a) and schematics of the PEMR configuration with (b) and without (c) buffer layer (a, copper plate; b, graphite with gas channel; c, diffusion layer; d, catalyst layer; e, proton exchange membrane, i.e., Nafion 115; f, buffer layer; g, Ag/ AgCl reference electrode.).

imidazolate framework-leaf (ZIF-L)) are helpful to reduce CO2 mass transfer resistance because of their better adsorption of CO2 molecule. As a result, 10−15% increase in current efficiency and 0.1 V decrease in potential threshold are achieved for CO2 electroreduction. Because liquid buffer electrolyte solution might cause the problems of electric energy consumption, catalyst flooding, leakage, etc., it is necessary to design new types of proton exchange membranes (PEMs) to replace the function of buffer layer on the base of the investigation above. With the critical functions of buffer layer investigated in the present work, research of proton exchange membranes with new structures is ongoing in our group, therefore current efficiency could be further improved in the conventional PEMR configuration.

sandwiched by two pieces of gas diffusion electrode (so-called membrane electrode assembly, MEA), an efficient three-phase reaction interface is preferable.19,20 Quite recently, proton exchange membrane reactor (PEMR) has also been developed as a CO2 hydrogenation electrolytic cell. Centi et al.21 and Lee et al.22 did pioneer works on electrocatalytic converting of CO2 into HCOOH at ambient temperature and pressure with a conventional proton exchange membrane fuel cell (PEMFC) configuration, however, the current efficiency of HCOOH was only about 5%. Wu et al. introduced a liquid electrolyte buffer layer circulation between a Nafion membrane and a cathode catalyst layer, achieving great breakthroughs on current efficiency (up to 90%) and reaction rate (about 69 nmol s−1c m−2 vs 22 nmol s−1 cm−2 of that of the conventional threeelectrode electroreduction).23−26 Two configurations of the PEMRs (with and without buffer layer) are illustrated in Figure 1b,c. The only difference between Wu and Centi and Lee is the buffer layer, which obviously plays an important role on CO2 electroreduction in the PEMR. The functions of buffer layer is reported to be suppression of H2 evolution by Wu, however, the functions are not confined to this.26 In the present work, the functions of a buffer layer are investigated to improve the performance of CO2 electroreduction in PEMRs. The PEMRs are run in two ways, i.e., with and without a buffer layer. On the basis of the contrast experiments, a buffer layer helps to provide sufficient cathode potential that is essential to CO2 activation by forming an electric double layer near the cathodic catalyst layer, however, cathode potential is always lower than the threshold values even with very high overall cell potential (∼3.0 V) in the conventional PEMR. The buffer layer exhibits good ability to suppress H2 evolution only before it is saturated with protons, and current efficiency of HCOOH decreases by over 50% after a long period of reaction time (e.g., 10 h). Even for CO2 gas phase feeding, adding functional materials either in a buffer layer (tetrahydrofuran (THF)) or in a catalyst layer (zeolitic

2. EXPERIMENTAL SECTION 2.1. Materials. The following chemicals were commercially available: potassium bicarbonate (KHCO3 99%), potassium chloride (KCl 99%), calcium chloride (CaCl2 99%), potassium sulfate (K2SO4 99%), ethanol (C2H5OH 99%), 2-propanol (iC3H7OH 99%), succinic acid (C4H6O4), anhydrous calcium chloride, D2O (99%), 5 wt % Nafion solution (DuPont Co.), Nafion 115 (DuPont Co.), the gas diffusion layer (GDL38BC, SGL Co.), the Pt gas diffusion electrode (Pt GDE, 0.5 mg cm−2 Pt, Sunrise Power Co.), nano Sn (99.9%), nano Cu (99.9%), nano In (99.9%), and tetrahydrofuran (THF 99%). ZIF-L is synthesized by Zn (NO3)2 and 2-methylimidazole (Hmim) according to our previous work.27 2.2. Preparation of MEAs. Sn, Cu, and In gas diffusion electrodes (GDE) were homemade by spraying catalyst inks (Nafion/catalysts weight ratio of 1:3, 2-propanol as dispersant) on commercialized GDL38BC to form a catalyst layer and then drying at 80 °C. Two kinds of MEAs were investigated in this work. MEA without buffer layer was prepared by hot pressing the anode (Pt GDE) and cathode GDE (Sn, Cu, or In GDE) 10243

DOI: 10.1021/acs.iecr.7b00819 Ind. Eng. Chem. Res. 2017, 56, 10242−10250

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Industrial & Engineering Chemistry Research on either side of Nafion115 at 140 °C and 3 MPa for 90s, and MEA with a buffer layer was prepared by hot pressing the anode Pt GDE on one side of Nafion115. 2.3. Hydrogenation of CO2. As shown in Figure 1, the reactor had the same configuration as a PEMFC, with a 5.29 cm2 effective reaction area and serpentine flow channels for both the anode and cathode. The electrodes were sandwiched between the flow channels with catalyst loadings of 2 mg cm−2 (Sn, Cu, In, or Pt) at the cathode and 0.5 mg cm−2 Pt at the anode. The flow rates of H2 and CO2 were 30 mL min−1 and 20 mL min−1, respectively. Then 30 mL of buffer solution was recycled between cathode and membrane at a flow rate of 15 mL min−1. The thickness of the buffer layer was 6 mm, and a Ag/AgCl reference electrode was placed in the middle of it. A thick buffer layer (12 mm) was used only for the experiment with two reference electrodes. All of the experiments were conducted at 25 °C. With potential applied (electrochemical workststion CHI1140C or GWInSTEK GPD-3303S, DC Power Supply) across the cell, H2 was reduced to protons and electrons. Protons were transferred across the proton exchange membrane to cathode to participate CO2 hydrogenation, and electrons were transferred through external circuit. 2.4. Analytic Methods. The vapor phase products were detected by gas chromatography (GC), and the gas flow rate was measured with a soap bubble flowmeter using single-point correction with external standard gas (A standard gas mixture consist of 2% CO, 2.1% H2, and 95.9% CO2 was employed).24 Liquid products were detected by the 1H NMR with succinic acid as an internal standard. The current efficiency, conversion, reaction rate, and selectivity were calculated by eqs 1 through 4. current efficiency =

conversion =

It

mol product mol reactant

reaction rate =

selectivity =

mol productnF

× 100%

× 100%

Figure 2. Partial potential and current efficiency as a function of current density (reaction conditions: cathode catalyst, Sn; buffer layer, 0.5 M KHCO3; reaction time, 0.5 h; A, anode; M, membrane; C, cathode; B, buffer layer; R, reference electrode).

Figure 3. Potential as a function of current density for different system of CO2 hydrogenation (reaction conditions: cathode catalyst, Sn; buffer layer, 0.1 M KHCO3; reaction time, 0.5 h).

(1)

(2)

mol product (3)

tA

mol CO2 converts to target product mol CO2 converts to all products

× 100% (4)

Figure 4. Schematic diagram of potential distribution in MEA without (a) and with (b) buffer layer (reaction conditions: cathode catalyst, Sn; buffer layer, 0.5 M KHCO3; reaction time, 0.5 h; A, anode; M, membrane; C, cathode; B, buffer layer).

where molreactant and molproduct are moles of CO2 and HCOOH or CO, n is moles of electrons transferred, F is the Faraday constant (96 485.34 C mol−1), I is the current observed in the experiments, t is the operational time, and A is the reaction area.

insitu on the cathode catalyst and therefore is analogous to liquid reaction.

3. RESULTS AND DISCUSSION 3.1. Cathode Potential Threshold for CO2 Hydrogenation. The rate limiting step of CO2 hydrogenation is the association of CO2 molecule with an electron (eq 5), which needs very high cathode potential to overcome the high activation energy.6 Hydrogenation of CO2 in electrolyte solution could decrease the required potential because the reaction between CO2 and electron can be directly coupled with sufficient protons in the solution.28 Reactions and electrode potentials under standard conditions (pH = 7, 25 °C, 1 atm of gases, and 1 M solution) are listed in eqs 6 and 7, respectively.6 PEMR could provide dissociated hydrogen atoms

CO2 + e− = CO2−

ΔE = − 1.90 V vs SHE

(5)

CO2 + 2H+ + 2e− = HCOOH(l) (6)

ΔE = −0.61 V vs SHE CO2 + 2H+ + 2e− = CO(g) + H 2O(1) ΔE = − 0.53 V vs SHE

(7)

With a buffer layer (between membrane and cathode with Ag/AgCl reference electrode insertion and cell structure shown in Figure 1b), partial potential and current efficiency were 10244

DOI: 10.1021/acs.iecr.7b00819 Ind. Eng. Chem. Res. 2017, 56, 10242−10250

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higher current efficiency of HCOOH is only about 5%21,22 with the conventional PEMR configuration. At first glance, with either H2 or H2O as anode feedstock, the overall potential increases with the increasing current density and reaches values above 2.0 V for H2 feedstock and 3.0 V for H2O feedstock, respectively. It seems that CO2 should be activated and involved in hydrogenation with such high potentials over the potential threshold. While taking the ohmic resistance of the cell (mainly in proton exchange membrane, also including ohmic resistance in catalyst layers and the contact interface) into account, cathode potential changes a lot. Because it is difficult to measure anode and cathode partial potentials separately without a reference electrode in the conventional two-electrode PEMR, an IR corrected curve with H2 feedstock is plotted in Figure 3 to make comparison. Just considering the ohmic resistance of proton exchange membrane, which is about 0.529 Ω in this work (Nafion 115: membrane resistance is about 0.1 Ω cm−2 measured in our previous work),29 the potential at cathode is less than 1.2 V (IR corrected curve in Figure 3), which indicates that the overall potential is consumed quite a lot by the membrane. By removing ohmic resistance, the potential on the cell is too low (less than 1.2 V) to reach the potential threshold of CO2 hydrogenation. It was noticed that current density was fairly high with H2 feedstock, which may cause serious hydrogen evolution and loss of active catalyst sites for CO2 hydrogenation. To distinguish the influence of cathode potential from hydrogen evolution competition reaction, water was used as a hydrogen source at the anode. With the electrolysis of water at anode, current density was significantly reduced to the values equivalent to that in the PEMR with a buffer layer and the overall potential increased to a high value of around 3 V. Even in that case, no product was detected. Considering the onset potential of water electrolysis (about 2.0 V in this experiment), the overall potential of the PEMR was mainly consumed by the anode reaction, and the cathode potential was still smaller than 1.0 V. According to the investigation above, the potential distribution in the PEMR with and without buffer layer could be schematically shown in Figure 4. Given a constant overall potential of 1.6 V (denoted as ΔE in Figure 4), in the conventional PEMR without buffer layer (Figure 4a), the potential consumed by Nafion membrane was calculated to be 1.23 V (ΔEM, nearly 80% of the overall potential), so the cathode potential was far below the CO2 activation potential threshold (−1.3 V). As a comparison, in the PEMR with buffer layer (Figure 4b), nearly 90% of the overall potential was mainly located near the surface of the cathode catalyst (ΔEC), which is favorable for CO2 electroreduction. Therefore, cathode potential is an essential factor for CO2 hydrogenation. In the conventional PEMR configuration without a buffer layer, cathode potential could not break through the threshold to get sufficient activation energy for

Figure 5. Current efficiency, conversion, and reaction rate as a function of cathode potential of CO2 hydrogenation with buffer layer (reaction conditions: cathode catalyst, Sn; buffer layer, 0.5 M KHCO3; reaction time, 0.5 h).

investigated as a function of current density in the PEMR using H2 and CO2 as anode and cathode feedstock, respectively. As shown in Figure 2, the potential threshold of CO2 hydrogenation is −1.3 V (vs Ag/AgCl). The reduction products of CO2 mainly include HCOOH and CO. Current efficiency of both HCOOH and CO increases with the increase of current density above −1.3 V and gradually reaches stable values of around 50% and 10%, respectively. Current density keeps small values (within 50 mA cm−2) even at high overall potential (anode to cathode, denoted as A−C potential), which is helpful to suppress H2 evolution. Partial potentials at anode, cathode, and buffer solution are measured in galvanostatic mode in which the potential distribution is investigated by inserting two reference electrodes in different location in a thick buffer layer (one is close to Nafion membrane (R1), and the other is close to catalyst layer (R2), as schematically shown in Figure 2). It is seen that around 80% of the A−C, overall potential is consumed by the cathode compartment (R1-C potential), and most of the cathode potential (around 80%) is located near the cathodic catalyst layer (R2-C potential) that is certainly higher than the potential threshold of CO2 hydrogenation. The difference between R1-C potential and R2-C potential can be roughly considered as the potential consumption in the buffer layer. The results in Figure 2 infer that with a buffer layer, sufficient potential could be built (R2-C potential) near the surface of the catalyst and ensure high performance for CO2 electroreduction. Figure 3 shows the overall potential as a function of current density with either H2 or H2O as anode hydrogen source in the conventional PEMR (a two-electrode system with anode− membrane−cathode MEA structure as shown in Figure 1c). Unfortunately, there are no CO2 hydrogenation products in the PEMR without a buffer layer regardless of the anode hydrogen source. It also coincides with the data in literature, in which the

Table 1. The Properties of Buffer Solution before and after CO2 Hydrogenation

buffer solution 0.1 M KHCO3 0.1 M KCl 0.05 M K2SO4 0.05 M CaCl2

before reaction 8.95 6.46 6.16 8.51

conductivity ms cm−1

concentration of H+

pH after reaction 7.90 2.36 2.91 2.10

before reaction

after reaction

−10

−8

5.248 3.467 6.918 3.090

× × × ×

10 10−7 10−7 10−9

10245

3.162 4.365 1.230 7.943

× × × ×

10 10−3 10−3 10−3

increase multiple 6.025 1.259 1.778 2.571

× × × ×

10 104 103 106

before reaction

after reaction

8.93 12.0 9.55 8.97

9.16 12.81 9.95 12.12

DOI: 10.1021/acs.iecr.7b00819 Ind. Eng. Chem. Res. 2017, 56, 10242−10250

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Figure 6. Effects of type and concentration of buffer solution on current density and current efficiency. (a,b) different types of buffer solution; (c,d) KHCO3 with different concentrations (reaction conditions: cathode catalyst, Sn; reaction time, 0.5 h).

to the low tendency of HCOOH to bind the CO 2 − intermediate,30 and the selectivity of HCOOH is about 80% on Sn catalyst. The reaction rate and conversion of CO2 increase almost linearly with the increase of cathode potential because of the higher energy input. The reaction rate exhibits values greater than 89.8 nmol s−1 cm−2 at −1.8 V, which is more than that in the conventional three-phase liquid reactors (about 22 nmol s−1 cm−2).31 3.2. Suppression of H2 Evolution by Buffer Layer. As investigated in the conventional PEMR without buffer layer (Figure 3), current density is fairly high, indicating very serious hydrogen evolution competition reaction. For the PEMR with buffer layer, pH, concentration of H+, and conductivity of different types of buffer solutions were tested to understand the suppression effect of a buffer layer on H2 evolution. As listed in Table 1, concentration of H+ in the buffer solution significantly increased after a 0.5 h reaction period. It means that a large number of protons oxidized by the anode catalyst could be retained in the buffer solution and suppress hydrogen evolution to some extent. Although pH values before reaction differ in different buffer solutions, they all decreased after a 0.5 h reaction period. KHCO3 exhibited the smallest variation of pH value, which showed the best buffer ability because of the ion equilibrium of the weak electrolyte HCO3− (HCO3− + H+ ↔ H 2O + CO2 ). The effects of species and concentration of a buffer solution on CO2 hydrogenation were further investigated. As shown in Figure 6a,b, HCOOH current efficiency exhibited highest values with KHCO3 solution regardless of the higher current density with KCl solution. In combining the data in Table 1, it was found that HCOOH current efficiency increased with the

CO2 hydrogenation, resulting in few products. When the catalyst layer was in contact with electrolyte solution in buffer layer (Figure 4b), an electric double layer, induced by the electrostatic attraction between cations (such as H+ and K+) in the solution and the negatively charged cathode, would be formed near the surface of Sn catalyst and produce significant cathode potential for CO2 hydrogenation. While in the case of the conventional PEMR without a buffer layer (Figure 4a), the cathode catalyst layer directly contacted with the proton exchange membrane, in which the fixed ions (such as negatively charged −SO3−) could not participate effectively in the formation of the electric double layer due to the repulsion between the same kinds of charges, and the only flexible cation species H+ could also not build up sufficient high potential around the surface of Sn catalyst because of the fast hydrogen evolution reaction. As a result, higher current density and lower cathode potential were observed as mentioned above. It is suggested that proton exchange membrane has to be redesigned to fit for offering high potential on the interface with catalyst layer. With a buffer layer, pure H2 and CO2 as anode and cathode feedstock, respectively, high cathode potential was observed in the PEMR and the hydrogenation performance had been greatly improved. As shown in Figure 5, the potential threshold at around −1.3 V is observed obviously to trigger the hydrogenation of CO2. After that, current efficiency of HCOOH increased fast with the increase of cathode potential and then tended to be stable at around 62.1% as the result of competition balance between hydrogenation and hydrogen evolution, while the current efficiency of CO was stabilized at around only 10%. HCOOH is the most favorable product due 10246

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Figure 9. Current efficiency and reaction rate with and without THF as a function of cathode potential (reaction conditions: cathode catalyst, Sn; buffer solution, 0.5 M KHCO3; mass fraction of THF, 5 wt %; reaction time, 0.5 h).

evolution is poor. As shown in Figure 6c,d, with KHCO3 buffer solution, current efficiency slightly decreased with the increasing concentration, although current density increased significantly due to the increase of conductivity. The possible reason lies in the trade-off effect of concentration. With the increase of concentration, ohmic resistance of the buffer solution decreases, while the hydrogen evolution increases and the electrolyte is more likely to precipitate from the solution when concentration is as high as 3 M. However, the effect of suppressing H2 evolution by a buffer solution is confined to certain extent. As shown in Figure 7a, pH value of the buffer solution decreased to less than 4.0 after a 5 h period of reaction and then kept stable. As a result, current efficiency of H2 evolution significantly goes up to around 50% at a long period of reaction time, and both the reaction rate and the current efficiency of HCOOH appear to decrease dramatically. Besides the intensified H2 evolution competition at a long period of reaction time, there is another possible reason for the declined efficiency of CO2 electroreduction, i.e., product inhibition. It is often observed in hydrogenation investigations with PEMR configuration.33−35 As comparison, KOH was added occasionally to maintain the constant pH of the buffer solution, and the results are shown in Figure 7a,b as dash line. With a constant pH value, H2 evolution was suppressed obviously to about 5%. Current efficiency, conversion, and reaction rate of HCOOH exhibited similar

Figure 7. Current efficiency of H2 and pH of buffer layer (a) conversion and reaction rate of HCOOH (b) as a function of reaction time (reaction conditions: cathode catalyst, Sn; buffer layer, 0.5 M KHCO3; operation potential, −2 V).

decrease of the H+ concentration variation, because KHCO3 solution stayed basic during the experimental period, while KCl and CaCl2 solutions gradually turned to be acidic and favorable to H2 evolution. The improvement of current efficiency with KHCO3 solution might also be owing to the dissolved CO2 molecules in KHCO3 solution, which participated in the hydrogenation reaction in the form of HCO3− and acted as an intermediate product.32 As a comparison, in the conventional PEMR system, the solid electrolyte Nafion membrane had strong acidity and the conductivity was around 5−8 times of that of the buffer solution, so the ability to suppress H2

Figure 8. Current efficiency (a) and reaction rate (b) as a function of cathode potential for different types of catalyst (reaction conditions: buffer layer, 0.1 M KHCO3; reaction time, 0.5 h). 10247

DOI: 10.1021/acs.iecr.7b00819 Ind. Eng. Chem. Res. 2017, 56, 10242−10250

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Figure 10. SEM images of the Sn/ZIF-L catalyst before (a) and after (b) reaction of 10 h.

Figure 11. Current efficiency (a) and conversion and reaction rate (b) as a function of cathode potential on Sn-based catalyst with and without ZIFL (reaction conditions: cathode catalyst, Sn or Sn + 5 mg of ZIF-L; buffer layer, 0.5 M KHCO3; reaction time, 0.5 h).

trends in the first 4 h, in which pH was kept acidic. After that, all of the three parameters appeared to decrease dramatically but showed better performances with constant pH. It indicates that the possible reasons for the declined performance are the combination of intensified H2 evolution and product inhibition of HCOOH. It is necessary to change or adjust pH of the buffer solution at regular intervals to avoid saturation of the buffer solution with protons. The effect of suppressing H2 evolution by a buffer solution is also limited by the type of catalysts. As shown in Figure 8a, there is a potential threshold roughly at around −1.3 V for CO2 hydrogenation on some metal catalysts with high hydrogen evolution potential (1.0−1.5 V), such as Cu, Sn and In, which indicates the activation barrier. Sn and In have higher reaction rates and selectivities of HCOOH, while Cu shows a relatively higher current efficiency of CO for nearly 30%, which coincides with the reported values in the literature,12 while with catalysts of much lower hydrogen evolution potential, such as Pt, hydrogen evolution competition reaction dominates from a very low potential (0.1−0.3 V) so that HCOOH and CO could hardly be detected. For Pt catalyst, the hydrogen evolution suppression effect of the buffer solution could not take effect. Figure 8b shows the reaction rate for different catalysts with 0.1 M KHCO3 buffer solution (because it shows the best selectivity of HCOOH). The reaction rate of HCOOH increases almost linearly with the increasing cathode potential and exhibits the highest value on Sn catalyst. 3.3. Reduction of CO2 Mass Transfer Resistance. Mass transfer has important influence on reaction performance. Fuel

cell reactors have efficient gas diffusion electrodes to eliminate the mass transfer resistant of CO2 through electrolyte solutions compared as the conventional three-electrode electrolytic cells. However, it is still important to improve mass transfer of CO2 on the reaction interface with buffer layer. Because the catalyst layer directly contacts with both CO2 gas and the buffer solution, two strategies are proposed, i.e., increase of the CO2 solubility and adsorption. One strategy is to increase the solubility of CO2 in the buffer solution by adding organics on the basis of the fact that CO2 has large solubility in some organic solvents. In this work, a small amount of THF (5 wt %) was added into KHCO3 aqueous solution because THF is known to be favor of the formation of CO2 containing clathrate hydrates at much lower pressure and increaseed CO 2 solubility.36 As shown in Figure 9, THF improved the hydrogenation performance. With the increase of CO 2 concentration in the buffer solution, more CO2 molecules could access the surface of the catalyst and decrease the reaction over potential. Current efficiency of HCOOH and reaction rate increased by around 8−10%. Potential threshold of CO2 hydrogenation decreased to about −1.2 V. Another strategy is to improve the adsorption of CO2 molecules onto the catalyst surface. Increase of CO2 adsorption is favorable to the formation of CO2− that is reported to be the rate limiting step of the CO2 catalytic hydrogenation.6 ZIF-L is a kind of metal−organic framework that has porous structure, high surface area, and N atom doping. It has a two-dimensional structure with a cushion-shaped cavity between layers, which is unique and well suited to accommodate CO2 molecules. It is 10248

DOI: 10.1021/acs.iecr.7b00819 Ind. Eng. Chem. Res. 2017, 56, 10242−10250

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Industrial & Engineering Chemistry Research Notes

reported to have good adsorption and separation ability to CO2.37 In this work, a small amount of homemade ZIF-L (25 wt %) was hybrid with a Sn catalyst to improve the adsorption of CO2. Morphology of the ZIF/Sn catalyst was investigated by SEM, as shown in Figure 10, ZIF-L presented the leaf-like lamellar structure of around 4−5 μm in length. It is obvious that the Sn catalyst particles (50−300 nm) cluster on the edge of ZIF-L because the edge has a higher free energy than the smooth surface and is easier to combine with other solid or gas molecules. Comparing the SEM images before and after hydrogenation (parts a and b of Figure 10, respectively), the Sn/ZIF-L catalyst keeps good morphology after a 10 h reaction. As shown in Figure 11, CO2 hydrogenation performance shows similar trends with and without the addition of ZIF-L. Current efficiency of HCOOH increases by around 10−15% with Sn/ZIF-L catalyst compared with pure Sn, as shown in Figure 11a. The improvement of current efficiency is more obvious when the cathode potential is higher than −1.8 V, which indicates the influence of local CO2 concentration around a Sn catalyst is more prominent at higher potential. Better CO2 adsorption ability by hybrid of ZIF-L helps to reduce mass transfer resistance of CO2. Because ZIF-L has little contribution to the formation of CO, selectivity of HCOOH shows an upward trend, increasing from 81.2% to 92.4% at −1.8 V. It is noticed from Figure 11 that the reaction rate and conversion slightly decline with the addition of ZIF-L. It directly relates to the slight decrease (about 10%) of the current density caused by the lower electronic conductivity of ZIF-L. In the subsequent work, it could be improved by the carbonation of ZIF-L.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Joint Funds of the National Natural Science Foundation of China (U1663223, 21476034 and 21776044), MOST innovation team in key area (2016RA4053), Education Department of the Liaoning Province of China (LT2015007), Fundamental Research Funds for the Central Universities (DUT16TD19), and the Changjiang Scholars Program (T2012049) for financial support of this work. We also thank Sunrise Power Co. for kindly supply of Pt gas diffusion electrode material.

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4. CONCLUSION The PEMRs were run in two ways, i.e., with and without a buffer layer, to give insight into the critical factors that determine the performance of CO2 hydrogenation in PEMRs. On the basis of the contrast experiments, a buffer layer helps to ensure sufficient cathode potential (above −1.3 V ∼ −1.4 V), which is essential to CO2 activation. Reaction rate with a buffer layer reaches much higher values (for instance 89.8 nmol s−1 cm−2 at −1.8 V). However, the cathode potential could not break through the threshold in the conventional PEMR without a buffer layer due to a large portion of potential consumption on PEM, which leads to few products. Buffer solution can effectively reduce the current efficiency of H2 evolution competition to about 5% only before it is saturated with protons and shows acidic pH value. Meanwhile, it shows superior performance after adding some functional materials that improve adsorption of CO2 on the catalyst surface, such as organic solvent (THF) in buffer solution and MOFs (ZIF-L) in catalyst, reduce the mass transfer resistance of CO2 and thus current efficiency increases by about 15%. These investigations are meaningful to the design of key components of PEMR for CO2 electroreduction such as proton exchange membrane and carbonized MOF catalysts.

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AUTHOR INFORMATION

Corresponding Authors

*For X.W.: E-mail, [email protected]. *For G.H.: E-mail, [email protected]. ORCID

Xuemei Wu: 0000-0002-0930-7602 10249

DOI: 10.1021/acs.iecr.7b00819 Ind. Eng. Chem. Res. 2017, 56, 10242−10250

Article

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DOI: 10.1021/acs.iecr.7b00819 Ind. Eng. Chem. Res. 2017, 56, 10242−10250