Effect of the surface roughness of copper substrate on ...

Journal of CO₂ Utilization 21 (2017) 219–223

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Journal of CO2 Utilization journal homepage: www.elsevier.com/locate/jcou

Effect of the surface roughness of copper substrate on three-dimensional tin electrode for electrochemical reduction of CO2 into HCOOH

MARK

⁎

Binhao Qin, Hongjuan Wang, Feng Peng , Hao Yu, Yonghai Cao School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, 510640, China

A R T I C L E I N F O

A B S T R A C T

Keywords: Electrochemical deposition CO2 electroreduction reaction Electropolishing Tin electrode Faradaic efficiency Formic acid

The tin electrode with three-dimensional (3D) porous structure was prepared using copper foil as the substrate by electrochemical deposition method for the electrochemical reduction of CO2 into HCOOH. The results indicate that the faradaic efficiency (FE) and the energy conversion efficiency (EE) of CO2 to HCOOH are dependent on the surface roughness of copper foil that can be controlled by the electropolishing time. The prepared 3D tin electrode on the smoothest Cu foil exhibits the highest FE (90%) and EE (50%) at −1.2 V (vs. RHE), which were 1.6 times those on the original copper foil. Meanwhile, the Sn loss ratio of this electrode in the electrochemical process is the lowest, showing good stability.

1. Introduction The ever-increasing concentration of atmospheric carbon dioxide is the main cause of greenhouse gas effect [1]. Electrochemical reduction of carbon dioxide into energy-rich fuels and useful chemicals has received considerable attention in recent years as a potential strategy for mitigating excessive emissions of carbon dioxide and effectively storing intermittent renewable energy simultaneously [2,3]. Among the products (e.g., syngas, acids, alcohols, hydrocarbon) [4] from carbon dioxide reduction reaction (CO2RR), formic acid (HCOOH) is a promising chemical as a fuel and a hydrogen storage material [5,6]. However, many metal electrodes reported (e.g., plumbum, mercury, indium, cadmium) for CO2 to HCOOH are poisonous, costly, or both [5]. Nevertheless, tin and its correlative catalysts, as nontoxic and inexpensive materials, are a kind of ideal cathode materials for CO2RR to HCOOH [5–8]. The faradaic efficiencies and final production rates of formic acid were reported from 23% to 93.6% and from 3 to 797 μmol h−1 cm−2 with different tin based cathodes, and the stability of the tin based cathode was reported more than 15 h [6]. To enhance the electroactive area and the current density, tin electrode with three dimensional (3D) porous structure has been made by electrochemical deposition with hydrogen bubbles evolution[9,10], which is a very simple and fast method. In addition, such obtained self-supported cellular structure as monolithic electrode is cheap but effective for CO2RR without a binder such as nafion solution [11] or poly-tetrafluoroethylene [7,12]. So far, the effect of experimental conditions on the performance of CO2RR to HCOOH at Sn based electrodes has been widely investigated. ⁎

Corresponding author. E-mail address: [email protected] (F. Peng).

http://dx.doi.org/10.1016/j.jcou.2017.07.012 Received 15 May 2017; Received in revised form 14 July 2017; Accepted 14 July 2017 2212-9820/ © 2017 Elsevier Ltd. All rights reserved.

It has been shown that the HCOOH formation rate depends strongly on various parameters, such as pH [13], supporting electrolyte [14], the potential and the CO2 partial pressure [15]. Recently, many researchers have devoted to study the influence of structure and morphology [5–8] of the tin electrode on the performance of CO2RR. In order to enhance the faradaic efficiency and the production rate of HCOOH, tin was deposited on different substrates. Wang et al. [16] prepared Sn/f-Cu electrode by electrodepositing Sn on a Cu foam substrate in aqueous plating solution for the electrochemical reduction of carbon dioxide in aqueous KHCO3 solution. The results demonstrate that the maximum faradaic efficiency of 83.5% can be obtained at −1.8 V vs. Ag/AgCl, the average current density and the production rate of formate with the Sn/ f-Cu electrode are twice higher than those with the Sn plate electrode. However, the effect of surface roughness of the substrate on the efficiency and the stability of the electrode for CO2RR is still unclear at present. Herein, a series of 3D tin electrodes on copper foils with different surface roughness were controllably prepared by electrochemical method and the important dependence of the energy efficiency and the stability of the electrode for CO2RR on the surface roughness of the substrate was investigated and revealed for the first time. 2. Experimental 2.1. Preparation of Cu foil supported tin electrodes Copper foil as a rectangular substrate (1 cm × 2 cm) was annealed at 500 °C for two hours in the tube furnace with argon, and then cleaned under ultrasonication with ethanol, acetone and 2 M HCl for


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Fig. 1. SEM images of the electropolished copper foils. Polishing time: (a) 0 s, (b) 10 s, (c) 20 s, (d) 25 s, (e) 30 s, (f) 60 s, (g) 120 s, (h) 240 s and (i) 480 s.

2.2. Electrochemical CO2RR

15 min, sequently. The foil was cleaned under ultrasonication with 2 M HCl for 15 min again, and then curdled in drying oven at 60 °C. To ensure the working area of 1 cm2 and the reserved area of about 0.2 cm2 for electrode holder, the rest and the other side of the foil were sealed with epoxy resin. In a three-electrode system, the Cu foil, Ag/ AgCl electrode (KCl saturated) and platinum plate electrode was used as the working electrode, reference electrode, and counter electrode, respectively. All the potentials in the work were converted into the reversible hydrogen electrode (RHE). Cu foil was electro-polished in 60 wt% phosphoric acid at 0.73 V (vs. RHE) with a potentiostat (CHI 630E, Shanghai). The polished Cu foils with different roughness were obtained by controlling the polishing time from 0 s to 480 s. Tin electrodes with 3D porous structure on these polished copper foils were prepared by electrochemical deposition method with the polished copper foil as cathode and tin foil as anode and the distance of 2 cm between these two electrodes. Then direct current with high current density of 2 A cm−2 was applied for 2 min in 1.2 M HCl solution containing 0.1 M SnCl2 and 0.1 M trisodium citratedihydrate (Na3C6H5O7) in ice-water bath. The as-prepared tin electrode with Cu foil substrate was cleaned with deionized water (Milli-Q®, ∼18MΩ cm) and dried in nitrogen at room temperature.

Electrochemical CO2RR was performed with a CHI 630E electrochemical workstation in an H-type cell, which has good gas and liquid tightness and even agitation (400 r min−1). The H-type cell was separated by Nafion 117 (DuPont). The prepared tin electrode as the working electrode and Ag/AgCl electrode as the reference electrode were placed in one side of H-type cell, and platinum plate electrode as the counter electrode was placed in the other side with 0.1 M KHCO3 aqueous solution used as the electrolyte (each side 40 mL). The electrolyte was bubbled with CO2 or N2 at the flow rate of 7.37 mL min−1 to construct CO2-saturated or CO2-free electrolyte at 0 °C. The electroreduction time was 1 h at preset potential (−1.0 V, −1.1 V, −1.2 V and −1.3 V vs. RHE). It is noteworthy that the electrode holder should avoid being contacted with the electrolyte in the electrochemical process (See schematic illustration Fig. S1). 2.3. Quantitative analyses and electrode characterizations CO and H2 evolved from CO2 and water as associated reactions during the electrochemical reduction of CO2 were quantified using gas chromatograph with a packed column, a flame ionization detector and a thermal conductivity detector every 30 min. The main reduced 220


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diameter. The SEM photos of the prepared tin electrode on Cu-25 are shown in Fig. 2a and b. It can be seen that a kind of self-supported three-dimensional network structure with the aperture diameter of about 65 μm is formed. Fig. S2 shows that the morphologies and the aperture statistics of the tin electrodes with different electropolishing times. It can be seen that diameters of apertures are between 61.3 μm and 67.5 μm from 0 s to 480 s. It is worth noting that the morphology of tin coating and the aperture diameter distribution of three-dimensional network structures are almost the same with different electropolishing time. At higher magnification, it can be further observed that the network is composed of needlelike dendritic structure (Fig. 2b), which is conducive to the electrochemical reduction of carbon dioxide [2]. The composition of the electrode surface is determined by EDS. The results show that the mass fractions of Sn, Cu and O are 92.69%, 5.59% and 1.72% at the electrode surface with Cu-25 as the substrate. However, Sn, Cu and O mass fractions are 86.73%, 11.45% and 1.81% with the original copper foil as the substrate, indicating that the coverage of tin on Cu-25 is much better than on the original copper foil at the same deposition conditions. The XRD patterns show that all tin electrodes with 3D porous structure contain tin and copper components (Fig. 2c). The main diffraction peaks at 30.6, 32.0, and 44.9° correspond to (200), (101), (211) crystal plane of pure metal Sn (JCPDS: 04-0673), indicating that the needlelike dendritic network is composed of Sn. The three peaks at 43.3, 50.4, and 74.1° correspond to (111), (200), (220) crystal plane of Cu (JCPDS: 70–3039), indicating that the copper foil component exists at all the samples at different electrochemical polishing time. Fig. 3a shows the linear sweep voltammetry (LSV) of 3D tin electrode in 0.1 M KHCO3 with CO2-saturated or CO2-free environment at 5 mV s−1. The current density in CO2-saturated 0.1 M KHCO3 is obviously larger than that in the CO2-free 0.1 M KHCO3 when the potential is lower than −0.6 V, which proves that tin electrode with 3D porous structure is an effective catalyst for CO2RR. The electrochemical reduction of carbon dioxide over the prepared 3D tin electrode with Cu25 as the substrate is evaluated in CO2-saturated 0.1 M KHCO3 aqueous solution at constant potential ranging from −1.0 V to −1.3 V (Fig. 3b). At −1.2 V, the FE and the EE of CO2 to HCOOH reach to the maximum values of 92% and 50%, respectively, and at the same time, the efficiencies of the side reactions of CO2 to CO and H2O to H2 are lowest, indicating that this 3D tin electrode has high selectivity for the reaction

product, HCOOH, was determined using high performance liquid chromatograph with an organic acid analysis column (HPX-87H, BioRad) and an ultraviolet detector (UVD). Before the quantitative detection of HCOOH, the electrolyte solution was acidified with sulfuric acid to pH of 2. The faradaic efficiencies (FEs) of the products [7,12] are calculated based on the Eq. (1).

FE% =

n e nF × 100% Q

(1)

where ne is the number of electrons exchanged (ne = 2 for reduction of CO2 to HCOOH and H+ to H2), n is the mole number of product, F is faraday's constant (F = 96485C mol−1), Q is the total charge passed. The energy efficiencies (EEs) are evaluated [7,12] based on the Eq. (2).

EE% =

E0 × FE% appliedcellpotential 0

(2) 0

is the equilibrium cell potential (E = 0 V where E −1.23 V = −1.23 V for hydrogen evolution reaction, E0 = −0.2 V − 1.23 V = −1.43 V for CO2RR to HCOOH). The tin electrodes with Cu foil substrate were characterized by fieldemission scanning electron microscopy (FESEM, ZEISS Merlin) at 5 kV and energy dispersive spectrometer (EDS, Oxford X-MaxN20) at 20 kV. X-Ray diffraction (XRD) patterns were obtained with an X-ray diffractometer (Bruker D8-advance). Inductively-coupled plasma spectrometer (ICP, Perkin Elmer optima 8300) was used to analyze the content of Sn dissolved in electrolyte after the CO2RR. 3. Results and discussion Fig. 1a–i show the SEM photos of copper foils electropolished with different times. It can be observed that the original copper foil is very rough (Fig. 1a). With the increase of electropolishing time, the surface of Cu foil becomes smooth gradually (Fig. 1b–d). When the time is 25 s, the copper foil is the smoothest, and it is donated as Cu-25. Further increasing the electropolishing time, the surface of the Cu foil becomes rougher and rougher conversely. In fact, some pinholes with the average diameter of 65 nm can be found on the surface of the smoothest Cu foil (Fig. 1d). With polishing time increasing further, the pinholes get bigger. The pinholes increase to about 160 nm in diameter at 60 s (Fig. 1f). And then the edges of holes are corroded gradually, and finally the pinholes turn into larger pits. At 480 s, the pits are about 1.6 μm in

Fig. 2. SEM images (a, b) of the prepared tin electrode on the Cu-25. XRD patterns (c) of the prepared tin electrodes on the electropolished Cu foils. Polishing time: a–0 s, b–10 s, c–20 s, d–25 s, e–30 s, f–60 s, g–120 s, h–240 s and i–480 s.

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Fig. 3. (a) CVs of the tin electrode in 0.1 M KHCO3 under CO2-saturated and CO2-free environment. (b) Faradaic efficiencies and energy efficiencies of HCOOH and H2 at −1.0 to −1.3 V over the prepared 3D tin electrode with Cu-25 as substrate.

tin electrodes were tested after the electrochemical reduction, as also shown in Fig. 5a. The change rule of Sn loss is contrary to that of the efficiencies from CO2 to HCOOH. On the smoothest Cu substrate (Cu25), the Sn loss ratio is the lowest and the efficiency to HCOOH is the highest, suggesting that the smooth surface of the Cu substrate can afford the high stability of the prepared tin electrode, resulting in the high activity of the tin electrode in current study. The long-term stability of the tin electrode on Cu-25 is further evaluated with the electroreduction of CO2 to HCOOH at −1.2 V for 16 h, as shown in Fig. 5b. The current density of electrochemical reduction remains relative stable and the FEs of gaseous products fluctuates between 6% and 11% and the total FEs of HCOOH are about 86.8% after 10-h electrolysis and 86.5% with fresh electrolyte after 6-h electrolysis, and the final production rates of HCOOH are 110.2 μmol h−1 cm−2 and 115.2 μmol h−1 cm−2, respectively, showing a good long-term durability of the tin electrode. In summary, the important dependence of energy efficiency and stability for CO2RR on the surface roughness of copper foil substrate has been revealed for the first time. The tin electrode with 3D porous structure on the smoothest copper foil exhibits the highest FE (∼90%) and EE (50%) of CO2 to HCOOH at −1.2 V (vs. RHE).

of CO2 to HCOOH, instead of side reactions of CO2 to CO and H2O to H2. At this potential, the average current density and HCOOH production rate are 6.56 mA cm−2 and 113.3 μmol h−1 cm−2, respectively (Fig. S3). Under different concentrations of KHCO3 and temperatures, the faradaic efficiencies, average current densities and the production rates for CO2RR to HCOOH are tested and the results are shown in Fig. 4. With the concentration of KHCO3 and temperature increasing, the faradaic efficiency of HCOOH decreases gradually. The average current density and the production rate of HCOOH increase with the addition of KHCO3 concentration. However, low temperature is favorable for 3D tin electrode to obtain high faradaic efficiency and production rate of HCOOH although the current density is relative low. Under the KHCO3 concentration of 0.3 M and the temperature of 0 °C, the average current density reaches to 17.2 mA cm−2 and HCOOH production rate is 280 μmol h−1 cm−2 with 86.4% FE. Fig. 5a shows the FE and the EE of CO2 to HCOOH at −1.2 V for 3D tin electrodes with Cu foil as the substrate prepared with different electropolishing times with 0.1 M KHCO3 at 0 °C. When the original copper foil is used as the substrate, the tin electrode exhibits the lowest FE (∼55%) and EE (∼30%) of CO2 to HCOOH. With the increase of electropolishing time, the FE and the EE increase gradually. When the electropolishing time was 25 s, the highest FE and EE of CO2 to HCOOH were obtained, which are 1.6-times those on the tin electrode prepared with original copper foil as the substrate. However, with the further increase of the polishing time, both the FE and the EE decrease gradually. The results indicate that the efficiency of CO2 to HCOOH is dependent on the roughness of copper foils, which can be controlled by electropolishing time. The average current densities and production rates of HCOOH with different polishing time are also shown in Fig. S4. In order to investigate the stability of tin electrode, the Sn loss ratios of

Acknowledgements This work was supported by National Natural Science Foundation of China (No. 21373091), the Provincial Science and Technology Project of Guangdong (No. 2014A030312007) and the Fundamental Research Funds for the Central Universities (No. 2015ZP021).

Fig. 4. Faradaic efficiencies, average current densities and production rates for CO2RR to HCOOH on 3D tin electrode with Cu-25 as substrate under different concentrations of KHCO3 at 0 °C (a) and under different temperatures with 0.1 M KHCO3 (b).

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Fig. 5. (a) Faradaic efficiency and energy efficiency of HCOOH at −1.2 V, and the Sn loss ratio of the tin electrodes for different electropolishing time. (b) Long-term stability of the tin electrode on the Cu-25.

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