Electrochemical Reduction of CO2 in Proton Exchange Membrane

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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 Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b00819 • Publication Date (Web): 23 Aug 2017 Downloaded from http://pubs.acs.org on August 29, 2017

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Industrial & Engineering Chemistry Research

Electrochemical Reduction of CO2 in Proton Exchange Membrane Reactor: The Function of Buffer Layer Lin Ma a, Shuai Fan a, Dongxing Zhen a, Xuemei Wu a*, Shishui Liu a, Jingjing Lin a, Shiqi Huang a, Wei Chen a, Gaohong He a,b** a

State Key Laboratory of Fine Chemicals, Research and Development Center of Membrane

Science and Technology, School of Chemical Engineering, Dalian University of Technology, Dalian, China, 116024. b

School of Petroleum and Chemical Engineering, Panjin Campus of Dalian University of

Technology, Panjin, 124221, China.

Corresponding authors: [email protected]; [email protected] ACS Paragon Plus Environment


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ABSTRACT: Electroreduction of CO2 is performed in proton exchange membrane reactors (PEMRs) with 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 high hydrogenation rate (i.e. 89.8nmol cm-2s-1 at -1.8V) 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 10h reaction). Improving CO2 mass transfer at the reaction interface by adding tetrahydrofuran (THF) in buffer layer or Zeolitic Imidazolate Framework with a Leaf like morphology (ZIF-L) in catalyst layer, the current efficiency of HCOOH can be increased by around 10-15%. 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)1and the copolymerization of CO2 with alkylene.2 Most common procedures are catalytic hydrogenations, such as conventional heterogeneous hydrogenation, mild electroreduction and photocatalytic reduction.3, 4, 5 2

ACS Paragon Plus Environment

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The adsorption and activation of CO2 on the catalysts surface was considered to be the control step according to the mechanism of hydrogenation of CO26 because of the stabilization of CO2 in thermodynamics.4 For conventional heterogeneous reactors, such as fixed bed, fluidized bed and slurry reactor, harsh environments (~700oC, ~8MPa) 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 -0.61V (vs SHE) to produce formic acid (HCOOH)

10

and sufficient adsorbed hydrogen atoms on catalyst surface could be

easily provided either by protons in acidic electrolyte solution or by dissociation of H2 at theoretically zero volt (vs SHE). Reaction rate of electrochemical reduction reactors (about 14800 nmol min-1g-1)

11

is significantly improved comparing with the

conventional heterogeneous reactors (about 40 nmol min-1g-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 3



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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 Due to 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 sandwiched by two pieces of gas diffusion electrode (so called membrane electrode assembly, MEA), efficient three-phase reaction interface is preferable.19,

20

Quite

recently, proton exchange membrane reactor (PEMR) has also been developed as CO2 hydrogenation electrolytic cell. Centi et.al

21

and Lee et.al

22

did pioneer works on

electro-catalytic converting 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 Nafion membrane and cathode catalyst layer, achieving great breakthrough on current efficiency (up to 90%) and reaction rate (about 69 nmol s-1cm-2 vs 22 nmol s-1cm-2 of that of the conventional three-electrode electroreduction).23-26 Two configurations of the PEMRs (with and without buffer layer) are illustrated in Figure 1(b) and (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 4


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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 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 buffer layer. Based on the contrast experiments, buffer layer helps to provide sufficient cathode potential that is essential to CO2 activation by forming 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 long period of reaction time (e.g. 10 h). Even for CO2 gas phase feeding, adding functional materials either in buffer layer (Tetrahydrofuran (THF)) or in catalyst layer (Zeolitic 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 0.1V decrease in potential threshold are achieved for CO2 electroreduction. Since 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 5



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conventional PEMR configuration. 2. EXPERIMENTAL 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 (i-C3H7OH 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.), and 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%)， 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 electrode (GDE) were home made by spraying catalyst inks (Nafion/catalysts weight ratio of 1: 3, isopropanol as dispersant) on commercialized GDL38BC to form catalyst layer, and then drying at 80oC. 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) on either side of Nafion115 at 140oC and 3MPa for 90s. And MEA with 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 has the same configuration as a PEMFC, with a 6


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5.29 cm2 effective reaction area and serpentine flow channels for both the anode and cathode. The electrodes are sandwiched between the flow channels with catalyst loadings of 2mg cm-2 (Sn, Cu, In or Pt) at the cathode and 0.5 mg cm-2 Pt at the anode. The flow rate of H2 and CO2 is 30mLmin-1 and 20mLmin-1, respectively. 30 mL buffer solution was recycled between cathode and membrane at flow rate of 15 mL min-1. The thickness of buffer layer is 6mm and Ag/AgCl reference electrode is placed in the middle of it. Thick buffer layer (12mm) is used only for the experiment with two reference electrodes. All the experiments are conducted at 25oC. 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.

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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.)

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 internal standard. The current efficiency, conversion, reaction rate and selectivity were calculated by eqs. (1) through (4). mol product nF

Current Efficiency =

Conversion =

It

mol product mol reactant

Reaction Rate =

Selectivit y =

× 100%

× 100%

mol product

(1) (2) (3)

tA

mol CO 2 converts to target product mol CO 2 converts to all products

× 100%

(4)

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.

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 8


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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, 25oC, 1 atm of gases, and 1 M solution) are listed in eqs. (6) and (7), respectively.6 PEMR could provide dissociated hydrogen atoms insitu on the cathode catalyst, therefore is analogous to liquid reaction. -

CO2 + e- = CO2 ∆E = −1.90V vs SHE

CO2 + 2H+ + 2e− = HCOOH(l)

∆E = −0.61VvsSHE

CO2 + 2H+ + 2e− = CO(g) + H2O(l)

∆E = -0.53VvsSHE

(5) (6) (7)

With buffer layer (between membrane and cathode with Ag/AgCl reference electrode insertion and cell structure shown in Figure 1 (b)), partial potential and current efficiency are 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 -1.3V (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 50mA 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 9



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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 buffer layer, sufficient potential could be built (R2-C potential) near the surface of the catalyst and ensure high performance for CO2 electroreduction.

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

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 1(c)). Unfortunately, there is no CO2 hydrogenation products in the PEMR without buffer layer, regardless of the anode hydrogen source. It is also coincide with the data in 10


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literature, in which the higher current efficiency of HCOOH is only about 5%

21, 22

with the conventional PEMR configuration.

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

At first glance, with either H2 or H2O as anode feedstock, the overall potential increases with the increasing current density and reaches values above 2.0V for H2 feedstock and 3.0V for H2O feedstock, respectively. It seems that CO2 should be activated and involved in hydrogenation with such high potentials over the potential threshold. While taken 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. Since it is difficult to measure anode and cathode partial potentials separately without reference electrode in the conventional two-electrode PEMR, 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 0.529Ω in this work (Nafion 115: membrane resistance is about 0.1Ω cm-2 measured in our previous work),

29

the potential at cathode is less

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than 1.2V (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 1.2V) to reach the potential threshold of CO2 hydrogenation. It is noticed that current density is fairly high with H2 feedstock, which may cause serious hydrogen evolution and loss of active catalyst sites for CO2 hydrogenation. In order to distinguish the influence of cathode potential from hydrogen evolution competition reaction, water is used as hydrogen source at the anode. With the electrolysis of water at anode，current density is significantly reduced to the values equivalent to that in the PEMR with buffer layer, and the overall potential increases to high value of around 3V. Even in that case, none product is detected. Considering the onset potential of water electrolysis (about 2.0V in this experiment), the overall potential of the PEMR is mainly consumed by the anode reaction, and the cathode potential is still smaller than 1.0V. 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 1.6V (denoted as △E in Figure 4), in the conventional PEMR without buffer layer (Figure 4(a)), the potential consumed by Nafion membrane is calculated to be 1.23V (△EM, nearly 80% of the overall potential), so the cathode potential is far below the CO2 activation potential threshold (-1.3V). As comparison, in the PEMR with buffer layer (Figure 4 (b)), nearly 90% of the overall potential is mainly located near the surface of the cathode catalyst (△EC), which is favorable for 12


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CO2 electroreduction.

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

Therefore, cathode potential is an essential factor for CO2 hydrogenation. In the conventional PEMR configuration without buffer layer, cathode potential could not breakthrough the threshold to get sufficient activation energy for CO2 hydrogenation, resulting in few products. When the catalyst layer is in contact with electrolyte solution in buffer layer (Figure 4(b)), 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 case of the conventional PEMR without buffer layer (Figure 4(a)), the cathode catalyst layer directly contacts 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 13



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the surface of Sn catalyst because of the fast hydrogen evolution reaction. As a result, higher current density and lower cathode potential are 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 buffer layer, pure H2 and CO2 as anode and cathode feedstock, respectively, high cathode potential is observed in the PEMR and the hydrogenation performance has been greatly improved. As shown in Figure 5, the potential threshold at around -1.3V is observed obviously to trigger the hydrogenation of CO2. After then, current efficiency of HCOOH increases fast with the increase of cathode potential and then tends to be stable at around 62.1% as the result of competition balance between hydrogenation and hydrogen evolution, while the current efficiency of CO is stabilized at around only 10%. HCOOH is the most favorable product due to the low tendency of HCOOH to bind the CO2- 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-1cm-2 at -1.8V, which is more than that in the conventional three-phase liquid reactors (about 22 nmol s-1cm-2)31.

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

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 are tested to understand the suppression effect of buffer layer on H2 evolution. As listed in Table 1, concentration of H+ in the buffer solution significantly increases after 0.5h 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 decrease after 0.5h reaction period. KHCO3 exhibits smallest variation of pH value, which shows the best buffer ability because

of

the

ion

equilibrium

of

the

weak

electrolyte

HCO3-.

(HCO3 - + H+ ↔H2 O+ CO2). Table 1. The properties of buffer solution before and after CO2 hydrogenation. 15



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Buffer

Concentration of H+

pH

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Conductivity ms cm-1

Before

After

Before

After

Increase

Before

After

reaction

reaction

reaction

reaction

multiple

reaction

reaction

0.1MKHCO3

8.95

7.90

5.248×10-10

3.162×10-8

6.025×10

8.93

9.16

0.1MKCl

6.46

2.36

3.467×10-7

4.365×10-3

1.259×104

12.0

12.81

0.05MK2SO4

6.16

2.91

6.918×10-7

1.230×10-3

1.778×103

9.55

9.95

0.05MCaCl2

8.51

2.10

3.090×10-9

7.943×10-3

2.571×106

8.97

12.12

solution

The effects of species and concentration of buffer solution on CO2 hydrogenation are further investigated. As shown in Figure 6 (a) and (b), HCOOH current efficiency exhibits highest values with KHCO3 solution, regardless of the higher current density with KCl solution. Combined the data in Table 1, it is found that HCOOH current efficiency increases with the decrease of the H+ concentration variation, becauseKHCO3 solution keeps basic during the experimental period, while KCl and CaCl2 solutions gradually turn 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 participate in the hydrogenation reaction in the form of HCO3- and act as an intermediate product.32 As a comparison, in the conventional PEMR system, the solid electrolyte Nafion membrane has strong acidity and the conductivity is around 5-8 times of that of the buffer solution, so the ability to suppress H2 evolution is poor. As shown in Figure 6(c) and (d), with KHCO3 buffer solution, current efficiency slightly decreases with the increasing concentration, although current density increases significantly due to the increase of conductivity. 16


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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 3M.

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

However, the effect of suppressing H2 evolution by buffer solution is confined to certain extent. As shown in Figure 7(a), pH value of the buffer solution decreases to less than 4.0 after 5h period of reaction and then keeps stable. As a result, current efficiency of H2 evolution significantly goes up to around 50% at a long period of 17



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reaction time. And both the reaction rate and the current efficiency of HCOOH appear to decrease dramatically. Besides the intensified H2 evolution competition at 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 is added occasionally to maintain the constant pH of the buffer solution and the results are shown in Figure 7(a) and (b) as dash line. With constant pH value, H2 evolution is suppressed obviously to about 5%. Current efficiency, conversion and reaction rate of HCOOH exhibit similar trends in the first 4h, in which pH keeps acidic. After then, all the three parameters appear to decrease dramatically but shows 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.

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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 condition: cathode catalyst: Sn，buffer layer: 0.5M KHCO3，operation potential：-2V )

And the effect of suppressing H2 evolution by buffer solution is also limited by the type of catalysts. As shown in Figure 8(a), there is a potential threshold roughly at around -1.3V for CO2 hydrogenation on some metal catalysts with high hydrogen evolution potential (1.0~1.5V), such as Cu, Sn and In, which indicates the activation barrier. Sn and In have higher reaction rate and selectivity 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 low hydrogen evolution potential, such as Pt, hydrogen evolution competition reaction dominates from very lower potential (0.1-0.3V), 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 8(b) shows the reaction rate for different catalysts with 0.1M KHCO3 buffer solution (because it shows the best selectivity of

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HCOOH). The reaction rate of HCOOH increases almost linearly with the increasing cathode potential and exhibits the highest value on Sn catalyst.

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

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 comparing 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. Since 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 (5wt.%) is 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 increases CO2 solubility.36

As shown in Figure 9, THF improves the hydrogenation 20


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performance. With the increase of CO2 concentration in the buffer solution, more CO2 molecules could access to the surface of the catalyst and decrease the reaction over potential. Current efficiency of HCOOH and reaction rate increase by around 8-10%. Potential threshold of CO2 hydrogenation decreases to about -1.2V.

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

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 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 reported to have good adsorption and separation ability to CO2.37 In this work, a small amount of homemade ZIF-L (25wt. %) is hybrid with Sn catalyst to improve the adsorption of CO2. Morphology of the ZIF/Sn catalyst is investigated by SEM, as shown in Figure 10, ZIF-L presents the leaf like lamellar structure of around 4~5µm in length. It is 21



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obvious that the Sn catalyst particles (50-300nm) 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 (Figure 10(a) and (b), respectively), the Sn/ZIF-L catalyst keeps good morphology after 10h reaction.

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

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 11(a). The improvement of current efficiency is more obvious when the cathode potential is higher than -1.8V, which indicates the influence of local CO2 concentration around 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. Since 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 -1.8V. It is noticed from Figure 11 (b) that the reaction rate and conversion slightly decline with the addition 22


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of ZIF-L. It directly relates to the slightly 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.

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

4. CONCLUSION The PEMRs are run in two ways, i.e. with and without buffer layer to give insight into the critical factors that determine the performance of CO2 hydrogenation in PEMRs. Based on the contrast experiments, buffer layer helps to ensure sufficient cathode potential (above -1.3V~ -1.4V), which is essential to CO2 activation. Reaction rate with buffer layer reaches much higher values (for instance 89.8nmol s-1cm-2 at -1.8V). However, the cathode potential could not break through the threshold in the conventional PEMR without buffer layer due to a large portion of potential consumption on PEM, which leads to few product. Buffer solution can effectively reduce the current efficiency of H2 evolution competition to about 5% only before it is saturated with proton and shows acidic pH value. Meanwhile, it shows 23



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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. ACKNOWLEDGMENTS The authors thank the Joint Funds of the National Natural Science Foundation of China (U1663223), National Science Foundation of China (21476034 and 21776044), 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. The authors also thank Sunrise Power Co. for kindly supply of Pt gas diffusion electrode material.

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Electrochemical Reduction of CO2 in Proton Exchange Membrane

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