A highly efficient flower-like cobalt catalyst for electroreduction of carbon dioxide

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1 Chinese Journal of Catalysis 39 (2018) 催化学报 2018 年第 39 卷第 5 期 available at journal homepage: Article A highly efficient flower-like cobalt catalyst for electroreduction of carbon dioxide Gang Yang, Zhipeng Yu, Jie Zhang *, Zhenxing Liang # Key Laboratory on Fuel Cell Technology of Guangdong Province, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou , Guangdong, China A R T I C L E I N F O A B S T R A C T Article history: Received 23 November 2017 Accepted 7 January 2018 Published 5 May 2018 Keywords: CO2 electroreduction reaction Cobalt Flower-like morphology Formate Interface Electrochemical conversion of CO2 into fuel has been regarded as a promising approach to achieve the global carbon cycle. Herein, we report an efficient cobalt catalyst with a unique flower-like morphology synthesized by a green and facile hydrothermal method, in which n-butylamine is used as the capping agent. The resultant catalyst shows superior electrocatalytic activity toward CO2 electroreduction, which is highly selective for generating formate with a Faraday efficiency of 63.4%. Electrochemical analysis reveals that the oxide on the surface is essential for the electrocatalysis of the CO2 reduction reaction. Cyclic voltammograms further suggest that this catalyst is highly active for the oxidation of reduced product, and can thus be seen as a bifunctional catalyst. 2018, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved. 1. Introduction Due to fossil fuel combustion, the concentration of CO2 in the atmosphere has recently reached 400 ppm and is expected to rise continuously, which can lead to numerous environmental problems such as global warming, rising sea levels, and more erratic weather patterns [1,2]. Therefore, CO2 reduction reaction (CORR) has become a research focus to control the atmospheric concentration of CO2. Various CO2 reduction approaches, including electrochemical [3], biochemical [4], photochemical [5,6], and thermochemical [7] methods, have been proposed and explored in the past decades. Among them, electrocatalysis is a very promising method as it can be operated under ambient conditions. During the past decades, heterogeneous electrochemical CO2 reduction has been explicitly investigated, and various catalysts have been investigated to generate products such as carbon monoxide, formic acid, methane, and methanol [8 10]. Recently, Kanan et al. [11,12] suggested that the activity of metal catalysts obtained by the reduction of metal oxides is usually higher than that in other methods. Xie et al. [13] investigated cobalt, associated with loosely bonded d electrons and high electrical conductivity, as a promising catalyst for the CORR. They synthesized two-dimensional cobalt/cobalt oxide hybrid by using a solvothermal method with high electrocatalytic activity for the CORR, and the main product was formate. It was proposed that the metal/metal oxide interface plays a key role in catalyzing the CORR. * Corresponding author. Tel: ; cejzhang@scut.edu.cn # Corresponding author. Tel: ; zliang@scut.edu.cn This work was supported by the National Natural Science Foundation of China ( , ), Science and Technology Program of Guangzhou ( ), Science and Technology Program of Guangdong (2017A ), National Key R&D Program of China (2016YFB ). DOI: /S (18) Chin. J. Catal., Vol. 39, No. 5, May 2018

2 Gang Yang et al. / Chinese Journal of Catalysis 39 (2018) In this work, we develop a green and facile synthetic strategy for unique nanostructured cobalt by using nontoxic capping and reductant agents. The electrochemistry of the CORR on the resultant Co is then explicitly investigated. The flower-like Co, which favors the exposure of the active sites and the accessibility of the species, is found to show superior electrocatalytic activity than the bulk Co. Nuclear magnetic resonance (NMR) results indicate that the main product is formate, and the yield is highly dependent on the applied potential. 2. Experimental 2.1. Synthesis of flower-like Co, bulk Co, and Co(OH)2 Flower-like Co: First, CoCl2 6H2O powder was dissolved in a liquid mixture of 21 ml deionized water and 9.0 ml n-butylamine to yield a concentration of 50 mmol/l. Second, NaH2PO2 H2O was added as a reductant with a concentration of 0.40 mol/l. After vigorous stirring, the mixture was transferred to a Teflon cup in a stainless steel-lined autoclave for conditioning at 140 C for 24 h. Then, the system was allowed to naturally cool down to room temperature, and the black fluffy solid product was collected by centrifuging the mixture. The product was rinsed with deionized water and ethanol in sequence, and then dried in a vacuum oven at 60 C for 4 h. Bulk Co and Co(OH)2: The preparation of bulk Co and Co(OH)2 followed the abovementioned procedure, except that n-butylamine was not used for bulk Co and the hydrothermal treatment time was shortened to 3 h for Co(OH) Physicochemical characterization X-ray diffraction (XRD) measurements were carried out on a TD-3500 X (Tongda Technology) diffractometer with a Cu Kα radiation source operated at 40 kev. The patterns were collected at a scan rate of 0.05 /s in the 2θ range 10 to 80. Scanning electron microscopy (SEM) observations were carried out on a Nova NanoSEM 430 scanning electron microscope Electrochemical characterization The electrochemical behavior was characterized by cyclic voltammetry and linear sweep voltammetry using a three-electrode cell on an electrochemical workstation (CHI660E), at room temperature (25 C). A gold wire and a saturated calomel electrode (SCE) were used as the counter and reference electrodes, respectively. The working electrode was a glassy carbon disk (5.0 mm in diameter, PINE) covered with a thin layer of Nafion-impregnated catalyst. Typically, the thin-film electrode was prepared as follows: 10 mg of the catalyst was dispersed in 1.0 ml Nafion/ethanol (0.84 wt% Nafion) by sonication for 120 min. Then, 10 µl of the dispersion was transferred onto the glassy carbon disk by using a pipette, yielding a loading of 0.50 mg/cm 2. The electrolyte solution, 0.10 mol/l of KHCO3, was bubbled with either Ar (99.999%) or CO2 (99.999%) for 30 min before the test. The cyclic voltammograms (CVs) were collected at 20 mv/s in the potential range between 1.25 and 0.4 V (vs. SCE) in Ar- and CO2-saturated electrolyte solution. The linear sweep voltammograms (LSV) were collected by scanning the potential from 0.6 down to 1.2 V in Ar- or CO2-saturated electrolyte solution. All the potentials are referenced to SCE in this study. For controlled-potential electrolysis, 50 µl of the dispersion was loaded onto a gas-diffusion carbon paper with an area of 1 cm 2 to form a working electrode. The counter and reference electrodes were gold wire and SCE, respectively. The electrolysis was performed in an H-type cell with a piece of Nafion 117 cation-exchange membrane (H + form) as a separator. 20 ml of KHCO3 solution (0.10 mol/l) was introduced into the cathode chamber, and CO2 was continuously bubbled throughout the experiment to ensure saturation. The electroreduction of CO2 was performed at a constant potential of 0.75, 0.85, 0.95, 1.05, and 1.15 V. The electrolysis was prolonged for 10 h to enrich the reduction product. The liquid products were quantified by 1 H NMR (Bruker AVANCE III 400) spectroscopy, for which 0.25 ml electrolyte was mixed with 0.50 ml dimethyl sulfoxide-d6 and 5.0 mg methylbenzene was added as an internal standard. The faradaic efficiency of formate (ηformate) was calculated from the total amount of charge Q (C) passed through the material and the total amount of formate produced nformate (mol). ηformate = 2Fnformate/Q = 2Fnformate/ Idt where I (A) is the reduction current at a specific applied potential and t (s) is the time. 3. Results and discussion 3.1. Physical characterization Fig. 1(a) reveals that cobalt hydroxide has a hexagonal sheet structure. After reduction, serious aggregation of Co, namely bulk Co, is observed in the absence of n-butylamine (see Fig. 1(b)); in comparison, a flower-like Co covered with curved nanosheets, which inherits the morphology features of Co(OH)2, is synthesized by adding n-butylamine (see Fig. 1(c)). The difference in the morphology can be attributed to the chemical adsorption of n-butylamine on the facets of Co(OH)2, thus preventing the aggregation of Co. The unique flower-like morphology of Co favors both the exposure of the active sites and the accessibility of the active species, thus facilitating the electrocatalysis (vide infra). The corresponding XRD patterns are shown in Fig. 2. As seen in Fig. 2(1), Co(OH)2 shows the characteristic peaks of β-co(oh)2 (No ). Both bulk Co and flower-like Co basically show the same patterns with the diffraction peaks at 41.68, 44.76, 47.57, 62.73, and (see Fig. 2(2, 3)), indicating that they are bulk metal of the hexagonal close-packed (hcp) phase (No ) Electrochemical measurements The CO2 reduction activity of flower-like Co, bulk Co, and Co(OH)2 is evaluated by collecting the LSVs. As seen in Fig. 3(a), for the three catalysts, a broad cathodic reduction wave is observed between 1.2 and 0.6 V, which is then followed by a

3 916 Gang Yang et al. / Chinese Journal of Catalysis 39 (2018) Fig. 1. SEM images of Co(OH)2 (a), bulk Co (b), and flower-like Co (c). (3) (2) (1) o 2 /( ) Co No Fig. 2. XRD patterns of Co(OH)2 (1), bulk Co (2), and flower-like Co (3). more significant reduction wave below 1 V. The first cathodic wave is not seen in the Ar-saturated solution, intuitively indicating that this peak originates from the direct reduction of dissolved CO2 molecules. To further investigate the origin of the wave, the peak current is plotted with respect to the scan rate, as seen in Fig. 3(b). The logarithm of the cathodic peak current shows a linear relationship with the scan rate, and the linear (b) (a) Co(OH)2-Ar Co(OH)2-CO2 Flower-like Co-Ar Flower-like Co-CO2 Bulk Co-Ar Bulk Co-CO mv/s 5 mv/s 20 mv/s mv/s 1 V/s Current density (ma/cm) Current density (ma/cm2) 0.0 Current density (ma/cm ) Co(OH)2 No slope is close to 0.5, indicating that the cathodic peak is controlled by the CO2 diffusion. Among the three catalysts, the flower-like Co exhibits the most positive onset potential at 0.7 V and the highest reduction current than do the other two catalysts. This superior electrocatalytic activity should be attributed to the unique morphological features of the flower-like Co, as mentioned above. The current of the second reduction wave (< 1 V) increases with decreasing potential, which is much larger than that of the first reduction wave. This wave has been claimed to originate from the hydrogen evolution reaction (HER)[13]. However, this does not seem convincing because the background current is much smaller in Ar-saturated solution. Here, we propose that besides the hydrogen evolution reaction, this wave should also originate from both the direct reduction of HCO3 (vide infra) and reduction of CO2 to different products. Fig. 4 shows the CVs of flower-like cobalt in KHCO3 (0.10 mol/l) in different potential windows. Fig. 4(a) shows that a negligible faradaic current response is seen in the range from 0.7 to 0.4 V (see black trace) under Ar atmosphere. The Pourbaix diagram reveals that Co remains in the divalent state and no charge transfer occurs in this potential range. Two reduction waves are seen in the negative-going scan when the lower limit potential is scanned to 1.25 V (green trace). The first wave, in the potential range from 1 to 0.85 V, can be attributed to the reduction of the divalent Co. Then, a dramatic Scan rate (mv/s) Fig. 3. (a) Linear sweep voltammograms of flower-like Co, bulk Co, and Co(OH)2 in either Ar- or CO2-saturated KHCO3 (0.10 mol/l) electrolytes; (b) Linear sweep voltammograms of flower-like Co at different scan rates in CO2-saturated KHCO3 (0.10 mol/l) electrolyte. (Inset: relationship of cathodic peak current densities vs. scan rate).

4 Gang Yang et al. / Chinese Journal of Catalysis 39 (2018) (a) Co Co (b) Fig. 4. Cyclic voltammograms of flower-like Co in (a) Ar- and (b) CO2-saturated KHCO3 (0.10 mol/l) electrolytes. increase in current is seen as the potential is further negatively scanned, and originates from both the HER and the direct reduction of HCO3. In the positive-going scan, a remarkable anodic peak is observed at potentials greater than 1.1 V, which can be attributed to the electrochemical oxidation of metal Co. It is, thus, concluded that Co is in the metal state at potentials more negative than 0.85 V and is converted into oxide at more positive potentials, and this is understandable from the thermodynamics point of view. To make the transition clearer, the CV is collected in the potential range from 1 to 0.7 V (see red trace). One pair of cathodic/anodic waves corresponding to the electrochemical transition between the divalent state and the metal state can be clearly recognized. Finally, the CV in the potential range from 1.25 to 1 V (see blue trace) reveals that metal Co is active for the HER and the direct reduction of HCO3. Fig. 4(b) shows the CVs of CO2-saturated KHCO3 (0.10 mol/l) electrolytes. The black trace reveals negligible current in the potential range from 0.7 to 0.4 V. As the lower limit potential is scanned down to 1.25 V (see green trace), two reduction waves are seen in negative-going scan with the onset potential at 1 and 0.75 V, respectively. The two waves have been recognized in the LSVs (see Fig. 3), as discussed above. It is interesting to note that in the positive-going scan, the first wave is not seen, while the second wave remains unchanged. This finding indicates that in the potential range from 1 to 0.75 V, pure metal Co may not be inactive to the CORR, and divalent oxide plays a key role as the active sites. This analysis is further validated by the red trace. The first cathodic reduction wave is remarkably decreased when the upper limited potential is restricted to 0.7 V, at which much less divalent oxide is generated on the surface. Furthermore, the comparison of blue trace and green trace indicates that the upper limit potential has a negligible effect on the second cathodic reduction wave (< 1 V). This result is unsurprising as Co always remains in the metal state in this potential range. In the positive-going scan, a remarkable anodic wave above 0.7 V is seen in green trace, and the current is much larger than that in Fig. 4(a), suggesting that another electrochemical oxidation process occurs. It is proposed that the reduced product, generated in the negative-going scan, can be electro-oxidized at higher potentials in the positive-going scan on the Co catalyst. As such, this catalyst can be seen as a bifunctional one for the CORR. To recognize the origin of the second reduction wave, the effect of KHCO3 concentration on the LSVs was investigated, as shown in Fig. 5. It is seen that the cathodic current linearly increases with the concentration of the HCO3 ions. This result intuitively suggests that the reduction of HCO3 ions contributes to the second cathodic wave. Notably, the first wave gradually decreases as the concentration of KHCO3 increases. This may be due to the fact that the solution viscosity increases in this course, and decreases the diffusion coefficient of CO2 [14]. Controlled-potential electrolysis is conducted to recognize the product and evaluate the faradaic efficiency by using the flower-like Co catalyst. All experiments are performed in CO2-saturated KHCO3 (0.10 mol/l), and NMR is used to quantify the solution-phase products (see Fig. 6(a)). Formate is found to be the main product in the investigated potential range. Fig. 6(b) summarizes the yield of formate and the faradaic efficien KHCO 3 (mol/l) KHCO 3 concentration (mol/l) Fig. 5. Linear sweep voltammograms of flower-like Co in CO2-saturated KHCO3 solutions of various concentrations. Inset: cathodic current 1.2 V vs. KHCO3 concentration

5 918 Gang Yang et al. / Chinese Journal of Catalysis 39 (2018) (a) HCOO - H 2 O DMSO Chemical shift (ppm) Faradaic efficiency of formate (%) Fig. 6. (a) Representative NMR spectrum of the electrolyte solution after the electrolysis at 0.85 V; (b) Formate yield and faradaic efficiency vs. electrode potentials (b) Formate production (mg/l) cy. Both the yield and the faradaic efficiency are strongly dependent on the given potential. The highest faradaic efficiency (63.4%) is achieved at 0.85 V, and the formate yield reaches at 136 mg/l. The formate yield decreases at more negative potentials, and this can be attributed to the serious concomitant reactions, including the reduction of H2O and the reduction of CO2 to generate other products. 4. Conclusions In this work, a novel flower-like cobalt was synthesized through a facile hydrothermal method with the aid of n-butylamine. The morphological features facilitate both the exposure of the active sites and the mass transfer of the active species, favoring the electrocatalysis of the carbon dioxide reduction reaction. The electrocatalytic activity of flower-like cobalt is found to be 3.5 and 4 times higher than that of the Co(OH)2 and bulk Co metal, respectively. As a result, high faradaic efficiency up to 63.4% is achieved and the yield of the product formate reaches 136 mg/l. Electrochemical analysis reveals that the divalent oxide is essential to constitute the active sites for the CO2 reduction reaction. In addition, this catalyst is highly active for the oxidation of reduced product, thus acting as a bifunctional catalyst. References [1] W. H. Wang, Y. Himeda, J. T. Muckerman, G. F. Manbeck, E. Fujita, Chem. Rev., 2015, 115, [2] S. Chu, A. Majumdar, Nature, 2012, 488, Chin. J. Catal., 2018, 39: Graphical Abstract doi: /S (18) A highly efficient flower-like cobalt catalyst for electroreduction of carbon dioxide Gang Yang, Zhipeng Yu, Jie Zhang *, Zhenxing Liang * South China University of Technology An efficient cobalt catalyst with unique flower-like morphology was synthesized by a facile hydrothermal method, which yielded a superior electrocatalytic activity towards CO2 electroreduction with a high Faraday efficiency of formate (63.4%).

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