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Research Article http://pubs.acs.org/journal/acscii
Electrochemical Partial Reforming of Ethanol into Ethyl Acetate Using Ultrathin Co3O4 Nanosheets as a Highly Selective Anode Catalyst Lei Dai,† Qing Qin,† Xiaojing Zhao, Chaofa Xu, Chengyi Hu, Shiguang Mo, Yu Olivia Wang, Shuichao Lin, Zichao Tang, and Nanfeng Zheng* State Key Laboratory for Physical Chemistry of Solid Surfaces, Collaborative Innovation Center of Chemistry for Energy Materials, and Engineering Research Center for Nano-Preparation Technology of Fujian Province, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, P. R. China S Supporting Information *
ABSTRACT: Electrochemical partial reforming of organics provides an alternative strategy to produce valuable organic compounds while generating H2 under mild conditions. In this work, highly selective electrochemical reforming of ethanol into ethyl acetate is successfully achieved by using ultrathin Co3O4 nanosheets with exposed (111) facets as an anode catalyst. Those nanosheets were synthesized by a one-pot, templateless hydrothermal method with the use of ammonia. NH3 was demonstrated critical to the overall formation of ultrathin Co3O4 nanosheets. With abundant active sites on Co3O4 (111), the as-synthesized ultrathin Co3O4 nanosheets exhibited enhanced electrocatalytic activities toward water and ethanol oxidations in alkaline media. More importantly, over the Co3O4 nanosheets, the electrooxidation from ethanol to ethyl acetate was so selective that no other oxidation products were yielded. With such a high selectivity, an electrolyzer cell using Co3O4 nanosheets as the anode electrocatalyst and Ni−Mo nanopowders as the cathode electrocatalyst has been successfully built for ethanol reforming. The electrolyzer cell was readily driven by a 1.5 V battery to achieve the effective production of both H2 and ethyl acetate. After the bulk electrolysis, about 95% of ethanol was electrochemically reformed into ethyl acetate. This work opens up new opportunities in designing a material system for building unique devices to generate both hydrogen and highvalue organics at room temperature by utilizing electric energy from renewable sources.
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alkaline conditions.19−28 Transition metal oxides have been thus emerging as one of the most studied systems. The activities of transition-metal-based electrocatalysts have been demonstrated to be related to several factors of the catalysts, such as the unusual electronic structures, the variable valence, and the surface oxygen binding energy.29−33 Spinel transition metal oxides, including Co3O4, are found to be the ideal electrocatalysts for renewable energy conversion.30,32−35 Together with these recent instances of progress in OER electrocatalysts, the remarkable development of hydrogen evolution reaction (HER) electrocatalysts made from earthabundant elements has been creating bright opportunities toward the green production of hydrogen.36,37 Although water electrocatalytic splitting is an ideal process to generate hydrogen, much energy of the process is used to overcome the huge energy barrier for oxygen generation at anodes. However, oxygen is not a high-value product. It would be very promising if the anodic reaction could be utilized for
INTRODUCTION Hydrogen as renewable energy provides a promising solution to overcome our dependency on fossil fuels and the energy crisis.1−4 However, currently hydrogen is mainly produced by steam reforming fossil fuels such as natural gas at high temperature. Utilizing electric energy from renewable sources for producing H2 fuel by electrochemical means has thus attracted increasing attention during the past decades.5−13 Proton-containing liquids, such as water and alcohols, are ideal source materials for the electrochemical production of hydrogen due to their nontoxicity and availability. However, one of the major technological hurdles is the development of cost-effective efficient anode electrocatalysts.9,14,15 In acid medium, state-of-the-art electrocatalysts in oxygen evolution reaction (OER), an important half-reaction in electrochemical water splitting, are mainly expensive and rare noble metals and their oxides, limiting the wide application of the electrochemical reaction process.16−18 Increasing research attention has been directed to the development of alternative efficient catalysts based on inexpensive and earth-abundant elements with promising good catalytic activity and durability for OER in © XXXX American Chemical Society
Received: June 7, 2016
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DOI: 10.1021/acscentsci.6b00164 ACS Cent. Sci. XXXX, XXX, XXX−XXX
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ACS Central Science the oxidative production of high-value organics. The resulting electrochemical reforming methods would allow the room temperature production of high-value organic products and hydrogen at anodes and cathodes, respectively. In this regard, the selectivity of the anodic reactions is the most challenging issue. For example, the electrochemical reforming of ethanol at room temperature has been achieved using noble metal based anodes. However, no valuable products have already been obtained in pure form,5 similar to the situation met by conventional ethanol reforming that was typically carried out at high temperature and high catalyst loading.38 In this work we put forward a concept to use efficient transition metal oxide OER catalysts as selective ethanol oxidation catalysts for the development of electrochemical reforming of ethanol into ethyl acetate. An ammonia-assisted strategy was developed to prepare atomic thickness ultrathin spinel-type Co3O4 nanosheets (NSs) with (111) planes as the major exposure surface. These Co3O4 nanosheets exhibited an excellent electrocatalytic performance in OER and the electrooxidation of ethanol. Ethyl acetate was the only oxidation product of ethanol. At the potential of 1.445 V (vs RHE), the Faraday efficiency toward ethyl acetate was as high as 98%, making the Co3O4 NSs highly promising anodic catalysts for the partial electrochemical reforming of ethanol. Together with a Ni−Mo HER catalyst, Co3O4 NSs were grown on carbon cloth and used as the anode catalyst to build an overall of ethanol reforming electrolyzer cell using a 1.5 V battery for the separated production of clean hydrogen fuels and useful ethyl acetate at the cathode and anode, respectively.
Figure 1. (a, b) Representative TEM, (c) AFM, and (d, e) HRTEM images of the ultrathin Co3O4 nanosheets. (f) The corresponding FFT pattern of an individualCo3O4 nanosheet shown in panel d.
image of an individual nanosheet that lay on the TEM grid. The single-crystalline feature of the nanosheets was also verified by the corresponding fast Fourier transform (FFT) pattern (Figure 1f). All these data suggested that the (111) exposed Co3O4 NSs with atomic thickness have been successfully synthesized. Formation Mechanism of Co3O4 Nanosheets. Although CoCl2 was used as the sole Co precursor, the obtained spinel Co3O4 NSs contained both Co(III) and Co(II). We therefore considered if the presence of O2 and NH3 during the synthesis was critical to the formation of ultrathin Co3O4 NSs. Co(NH3)63+ with a formation constant Kf of 4.6 × 1033 is a much more stable complex than Co(NH3)62+ (Kf = 5.0 × 104),40,41 meaning that in the presence of NH3 the initial Co2+ in the system would be oxidized to Co3+ by O2 in air. Indeed, before transfer into the glass pressure vessel, the reaction mixture was already partially oxidized as evidenced by the UV− vis spectroscopic measurements (Figure S3a). The absorption peak at 520 nm corresponding to Co(H2O)62+ disappeared. As shown in the ESI-MS spectrum (Figure S3b), a peak with a m/z of 162.1 corresponding to Co(NH3)63+ appeared, further confirming the oxidation of Co2+ into Co3+ in the presence of NH3. When the reaction parameters were kept the same except that O2 was excluded by maintaining the reaction under the protection of N2, the reaction led to the formation of ultrathin Co(OH)2 (JCPDS #45-0031) rather than Co3O4 (Figures S4a and S5a). Interestingly, replacing NH3 by CH3NH2 also resulted in the formation of ultrathin Co(OH)2 NSs (Figures S4b and S5b). These observations suggested the importance of NH3 in the synthesis of ultrathin Co3O4 NSs. It was noteworthy that ultrathin Co3O4 NSs were also generated when Co(OAc)2 or Co(NO3)2 (Figures S4c,d and S5c,d) was used as the Co precursor, indicating not much effect of anions in the Co precursor salts on the formation of Co3O4 NSs. Such a situation is different from the previous report on the important role of anions on the formation of Co3O4 NSs.41
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RESULTS AND DISCUSSION Synthesis and Structural Characterizations of Ultrathin Co3O4 Nanosheets. In a typical synthesis of ultrathin Co3O4 NSs, 50 mg of CoCl2 was dissolved in 10 mL of pure water, yielding a clear light pink solution. After 8 mL of NH3· H2O (25% NH3) was added, the mixture was stirred in air for 10 min. The color of the solution turned into dark brown during the stirring in air (see Supporting Information for more details). The resulting mixture was then transferred into a glass pressure vessel. The vessel was heated from room temperature to 140 °C in about 30 min and kept at 140 °C for 5.0 h. After the mixture was cooled to room temperature, the black products were collected by centrifugation and washed several times with water. As revealed by transmission electron microscopy (TEM), the obtained products consist of stacked ultrathin nanosheets with uniform thickness of ∼1.6 nm (Figures 1a, 1b, and S1). All diffraction peaks of the obtained nanosheets are in good agreement with the standard data (JCPDS #43-1003) of cubic spinel Co3O4 (Figure S2), suggesting that the ultrathin NSs were of spinel phase. It should be pointed out that some of the ultrathin Co3O4 NSs were stacked together as a result of energetically favored face-to-face interactions.39 The ultrathin nature of the obtained Co3O4 NSs was also confirmed by atomic force microscopy (AFM) (Figure 1c). As measured by AFM, the thickness of the Co3O4 NSs was 1.6 nm, which was consistent with the number estimated from the TEM analysis. To further analyze the growth habit of the nanosheets, highresolution transmission electron microscopy (HRTEM) was adopted to investigate the lattice arrangements within the nanosheets (Figures 1d and 1e). Lattice fringes with interplanar spacing of 0.47 and 0.28 nm, corresponding to (111) and (220) fringes of spinel Co3O4, were clearly observed in the HRTEM B
DOI: 10.1021/acscentsci.6b00164 ACS Cent. Sci. XXXX, XXX, XXX−XXX
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mixtures were heated at 140 °C for 0.5, 1.0, and 3.0 h, the products readily displayed diffraction peaks of pure spinel Co3O4 (JCPDS #43-1003) (Figure S10b-d), and the crystalline feature of the products was enhanced with the heating time. As revealed by the TEM studies (Figure S11), the gradual development of the well-defined nanosheet morphology was accompanied by the increased crystalline feature of the products. While the oxidation of Co2+ by O2 in the presence of NH3 provided Co3+, the formation of Co(OH)2 nanosheets served as nice templates for the deposition of Co3+ to induce the formation of Co3O4 NSs. Figure S12 illustrates their possible formation mechanism. At the early stage of reactions, Co(OH)2 nanosheets were easily generated due to the layer structure of Co(OH)2 and also its low Ksp (1.1 × 10−15). Then NH3stabilized Co3+ species started to deposit onto the Co(OH)2 nanosheets to induce the formation of Co3O4 NSs having (111) as their main exposure facets. In this mechanism, NH3 plays two different important roles. One is to facilitate the oxidation of Co2+ into Co3+, which is a key component for Co3O4. The other is to selectively bind on Co3O4 (111) to ensure the anisotropic growth of the nanosheets. The binding of NH3 also helped to prevent the continuous deposition of Co species onto Co3O4 nanosheets and thus the growth of the nanosheets along the ⟨111⟩ direction. Electrocatalytic Performances of Co3O4 Nanosheets in Oxygen Evolution Reaction. The electrocatalytic performances of the Co3O4 NSs were first investigated in the oxygen evolution reaction (OER) in O2-saturated 1.0 M aqueous KOH (pH = 13.6) solution. To highlight the structural advantage of Co3O4 NSs in OER, Co3O4 nanocubes (NCs) were prepared and used as the control catalyst (Figure S13).42 Co3O4 NSs and Co3O4 NCs with mass 0.1 mg were deposited on carbon paper as the working electrodes. The polarization curves normalized by the electrode surface areas were first used to evaluate the OER activities of the catalysts. As shown in Figures 3a and 3b, the ultrathin Co3O4 NSs only needed an overpotential of 270 mV to achieve a current density of 10 mA/cm2. In comparison, Co3O4 NCs required overpotential of 332 mV to reach the same current density. It should be noted that the overpotential on Co3O4 NSs was also lower than that for many previously reported Co-based catalysts.19,34,35,43 Moreover, the corresponding Tafel plots (Figure 3c) indicated that the ultrathin Co3O4 NSs possessed a smaller Tafel slope (46 mV/dec) than that of Co3O4 NCs (63 mV/dec). The smaller Tafel slope makes Co3O4 NSs have large current densities at low overpotentials. Overall, the Co3O4 NSs exhibited much higher TOFs than Co3O4 NCs at the low overpotentials (Figure S14a). In addition to its high activity, high stability of an OER electrocatalyst was also critical for energy conversion systems. Impressively, as illustrated in Figures 3d and S14b, ultrathin Co3O4 NSs with (111) surface showed an excellent durability in the alkaline electrolyte. No obvious performance loss was observed within 10 h reaction or after 3000 CV cycles. In comparison, some fluctuation was revealed on Co3O4 NCs. As analyzed by TEM (Figures S15a and S15b), after the long-time electrochemical tests, the Co3O4 NSs nicely maintained their nanosheet structure, but the NCs were heavily aggregated. XRD spectra (Figure S15c) of the two samples indicated that they were still in good agreement with the standard data (JCPDS #43-1003) of cubic spinel Co3O4, and no another peaks appeared.
In the synthesis of Co3O4 NSs, the amount of NH3·H2O was also essential. As shown in Figure S6, when the amount of NH3·H2O was 1.0 mL, Co3O4 nanoparticles with mixed morphologies were obtained. Few nanosheets started to appear when the amount of NH3·H2O was increased from 1.0 to 2.0 mL. Co3O4 NSs were obtained as the major products only when 4.0 mL of NH3·H2O was used. But the diameter of the obtained NSs with 4.0 mL of NH3·H2O was still relatively small (less than 100 nm), and their surfaces were not smooth. When the amount of NH3·H2O was further increased to 8.0 mL, Co3O4 NSs having much larger diameter (hundreds of nanometers) were obtained as the major products. With the increase of NH3·H2O amount from 2.0 to 8.0 mL, the average thickness of the nanosheets was gradually reduced from ∼3.2 to ∼1.6 nm. To exclude that the effect of NH3·H2O on the formation of Co3O4 NSs was due to its role of providing an alkaline condition, we used a certain concentration of potassium hydroxide solution instead of NH3·H2O to control the pH value of the reaction. However, in this case, no Co3O4 NSs were obtained (Figure S7). Only the cube-like nanoparticles were produced. Based on the above results, we proposed that the strong preferential binding of NH3 on Co3O4 (111) might be the main reason for the anisotropic 2D growth of Co3O4 NSs. In order to prove the binding of NH3 on the surface of Co3O4 NSs, temperature-programmed decomposition/mass spectrometry (TPD-MS) was used to detect the released species from the as-prepared Co3O4 NSs upon heating under vacuum. The release of NH3 at the temperature between 160 and 310 °C was clearly observed (Figure 2), suggesting the rather strong
Figure 2. TPD-MS curves of the Co3O4 nanosheets. (a) The accumulative ionization intensity of the decomposition products from room temperature to 450 °C; (b) Relative ionization intensities of the main decomposition products at different temperatures.
adsorption of NH3 on Co3O4 NSs. The surface adsorption of NH3 made Co3O4 NSs have a positively charged surface, which was also confirmed by ζ-potential measurements (Figure S8). Moreover, the EDS-mapping also revealed that a nitrogen species was adsorbed on the surface of the NSs (Figure S9). All the evidence supported that the strong adsorption of NH3 played an important role in the formation of the Co3O4 NSs with the thickness of atomic layers. To further uncover the whole formation process of Co3O4 NSs, we investigated the products collected from the reaction mixtures after the temperature was raised from room temperature to 140 °C in 30 min and kept at this temperature for different periods. At the temperature raising stage before reaching 140 °C, the obtained products were a mixture of Co(OH)2 and Co3O4 (Figure S10a). After the reaction C
DOI: 10.1021/acscentsci.6b00164 ACS Cent. Sci. XXXX, XXX, XXX−XXX
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Figure 4. Electrocatalytic performances of Co3O4 nanosheets and nanocubes in ethanol oxidation. (a) The normalized polarization curves of nanosheets and nanocubes by the electrode surface area of electrocatalysts. The scan rate was 1.0 mV/s. (b) Corresponding Tafel plots of nanosheets and nanocubes. (c) Chronoamperometric curves of ultrathin Co3O4 nanosheets at different potentials for 1 h. (d) 1H NMR spectra of products before and after bulk electrolysis at different potentials for 1 h on Co3O4 nanosheet modified carbon paper electrode.
Figure 3. OER performances of Co3O4 multilayer nanosheets and nanocubes. (a) The normalized polarization curves of nanosheets and nanocubes by the electrode surface area of electrocatalysts. (b) The overpotential required for a current density of 10 mA/cm2. (c) Corresponding Tafel plots of nanosheets and nanocubes. (d) Chronopotentiometric curves of nanosheets and nanocubes at a constant current density of 10 mA/cm2.
Electrocatalytic Performances of Co3O4 Nanosheets in Ethanol Oxidation. The excellent OER performance of Co3O4 nanosheets suggested the unique properties of Co3O4 (111) in electrocatalytic oxidation reactions. Considering that OER itself does not produce valuable products, we have attempted to apply the Co3O4 NSs in the electrocatalytic oxidation of alcohols which can lead to the production of more valuable products such as carboxylic acids or esters. Electrooxidation of ethanol was then performed in an aqueous KOH (1.0 M) solution of ethanol (1.0 M) by using Co3O4 NSs as the anodic catalyst. As shown in Figures 4a and S16, an amperometric response was readily observed. On Co3O4 NSs, the onset potential (∼1.32 V vs RHE) required for the electrooxidation of ethanol was much lower than that for OER (∼1.45 V vs RHE), suggesting the easier oxidation of ethanol than H2O on Co3O4 NSs at the low oxidation potentials. It should be pointed out that the carbon paper only showed little electrocatalytic activity in both OER and ethanol oxidation reactions even at a high potential up to 1.7 V (Figure S17). Moreover, a much better performance of Co3O4 NSs than Co3O4 NCs was observed. To achieve a current density of 10 mA/cm−2, the required oxidation potentials were 1.445 and 1.550 V (vs RHE) on Co3O4 NSs and Co3O4 NCs, respectively. On Co3O4 NSs, the oxidation current densities were increased from 10 to 50 mA/cm2 with the potential raised from 1.445 to 1.545 V. With such a potential increment, the current densities on Co3O4 NCs were increased only from 3.06 to 10.6 mA/cm2. Moreover, the corresponding Tafel plots (Figure 4b) indicated that a smaller Tafel slope was achieved on the ultrathin Co3O4 NSs (138 mV/dec) than that on Co3O4 NCs (192 mV/dec). The results suggested that the Co3O4 (111) facet should also have better electrocatalytic performance than its (100) counterpart in the electrooxidation of ethanol.
The ultrathin Co3O4 NSs also exhibited excellent durability in the electrochemical oxidation of ethanol (Figures 4c and S18). During the chronoamperometric experiment, there was no obvious gas bubble formation on the Co3O4 modified working electrodes, suggesting that no O2 was evolved and a liquid product might be obtained. The obtained products after chronoamperometric tests with different potentials applied on the Co3O4 NSs modified working electrodes were analyzed by GC−MS spectrometry and 1H NMR spectroscopy. The GC− MS measurement suggested the presence of ethyl acetate as the main product (Figure S19). Besides a triplet at 0.99 ppm and a quartet at 3.45 ppm corresponding to the protons in ethyl group, a new singlet at chemical shift of 1.72 ppm was clearly revealed in the NMR spectra of the product. This peak can be attributed to the methyl group adjacent to carboxylate in ethyl acetate (Figure S20). Although the peak intensities increased with the applied potential, there was no appearance of new peaks upon the change of potential (Figure 4d). These data indicated the high selectivity of the electrochemical oxidation of ethanol toward ethyl acetate. The Faraday efficiency was calculated as high as 98% at the potential of 1.445 V (vs RHE). Ethyl acetate is a product of four-electron oxidation of ethanol. There are two possible pathways for the electrochemical oxidation of ethanol into ethyl acetate. One involves the oxidation of ethanol into acetate and its coupling with ethanol. However, under the alkaline condition in this work, the esterification between acetate and ethanol did not proceed much to achieve the high-yield production of ethyl acetate. Therefore, the formation of ethyl acetate might undergo the other pathway via the acetaldehyde formation under the electrochemical conditions. D
DOI: 10.1021/acscentsci.6b00164 ACS Cent. Sci. XXXX, XXX, XXX−XXX
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Electrochemical Reforming of Ethanol into Ethyl Acetate and H2. Equation 1 gives an overall anodic oxidation process of ethanol into ethyl acetate. As for electrochemical synthesis, the selectivity is always a big issue. Over Co3O4 NSs, it is surprising that the electrooxidation from ethanol to ethyl acetate was so selective that no other oxidation products were detected. With such a high selectivity, we have attempted to utilize Co3O4 NSs to build up an effective electrolyzer cell for ethanol reforming. It would be ideal if the cathodic reaction (eq 2) could be coupled together for H2 production. In that case, as shown in eq 3, the overall electrochemical process would lead to the formation of ethyl acetate in the anode and H2 in the cathode. To experimentally prove the concept, an ethanol electrochemical reforming cell was built (Figure 5). Co3O4 NSs were
To exclude the contribution of the surface areas of Co3O4 on the electrochemical activity, the surface areas of Co3O4 NSs and NCs were measured using the BET method from nitrogen gas adsorption−desorption isotherms at 77 K (Figure S21a). The surface areas of the Co3O4 NSs and NCs were calculated to be 43.4 and 10.7 m2/g, respectively. The currents in OER and ethanol oxidation were then normalized by their surface areas using the same mass loading of Co3O4 (0.1 mg/cm2). As shown in Figures S21b and S21c, at a certain oxidation potential, the current densities on Co3O4 NSs were both higher than those by Co3O4 NCs in OER and ethanol oxidation. The greatly improved electrocatalytic properties of the ultrathin Co3O4 NSs with (111) facets can be ascribed to the synergistic effect between their macroscopic morphological feature and their microscopic atomic/electronic structure.44,45 Previous work has demonstrated that rich Co3+ ions are present on the (111) surface than on the (100) surface (Figure S22).46,47 Co3+ has been regarded as the catalytically active sites43,48−53 for transferring multiple electrons in electrochemical processes, such as OER. Co3O4 NSs with (111) as their major exposure facets have more Co3+ in their surface. The enrichment of surface Co3+ on Co3O4 NSs was investigated by using X-ray photoelectron spectroscopy (XPS). As illustrated in the XPS spectrum of Co3O4 NSs (Figure S23), the peak area at 799.6 eV (Co3+) was much larger than that at 781.5 eV (Co2+). As estimated from the peak areas, the molar ratio of Co3+/Co2+ on Co3O4 NSs was higher than that on Co3O4 NCs (Table S1), suggesting the presence of more surface Co3+ on Co3O4 NSs. This result is consistent with the voltammetric curves of Co3O4 NSs and NCs (Figure S24). Co3O4 NCs exhibited two oxidation peaks at 1.15 and 1.48 V (vs RHE), corresponding to the oxidation of Co2+ and Co3+. Under the same conditions, Co3O4 nanosheets displayed only one major oxidation peak with much intense current density at 1.44 V corresponding to the oxidation of Co3+ to Co4+. The result confirmed the higher density of surface Co3+ on Co3O4 NSs than NCs. The rich surface Co3+ could be one of the important factors for the enhanced catalytic performance of Co3O4 NSs over NCs. Moreover, better superior electrical conductivity and ion transport kinetics of Co3O4 NSs also contributed to their excellent catalytic performance. As indicated by electrical impedance spectroscopy (Figure S25), the ultrathin Co3O4 NSs exhibited the lowest charge-transfer resistance of 3.5 ohm, much smaller than that of Co3O4 NCs (∼15.8 ohm) at the potential of 1.50 V (vs RHE). This result suggested the enriched active surface sites, thus accelerated charge transport, and shortened ion diffusion paths on Co3O4 NSs. As revealed in their TEM images, the large diameter of Co3O4 NSs made the main excellent self-supporting electrocatalyst. During catalysis, such a self-supporting feature would facilitate the mass transfer and also effectively prevent the occurrence of agglomeration, thus helping to improve both electrochemical activity and stability.
Figure 5. An electrochemical reforming cell run by a 1.5 V battery to produce ethyl acetate and hydrogen from ethanol.
in situ grown on carbon cloth (Figure S26a) and used as the anode catalyst. The loading of Co3O4 NSs was ∼0.5 mg/cm2. Ni−Mo nanopowders were prepared according to a reported method and used as the cathode catalyst for hydrogen evolution (Figure S26b).54 In the reforming cell, the Ni−Mo nanopowders were loaded on Ni foam at ∼0.1 mg/cm2. The anode and cathode compartments were separated by a Nafion membrane, and the volume of a single compartment was 25 mL. The electrochemical reforming of ethanol was performed in an aqueous mixture of 1.0 M KOH and 1.0 M CH3CH2OH. The electrochemical reformer was readily driven by a 1.5 V battery, and evolution of H2 bubbles was clearly observed on the cathode. The stability of the electrolyzer cell was tested on the electrochemical workstation using a two-electrode system at the potential 1.5 V, and the current of the electrolyzer cell was about 22 mA. After bulk electrolysis, the concentration of ethyl acetate was quantified by quantitative 1H NMR and GC−MS (Figure S27). About 95% of ethanol was electrochemically reformed into ethyl acetate within 48 h. The Faraday efficiency was as high as 96%, consistent with the number obtained from the three-electrode measurements at the potential of 1.445 V (vs RHE).
anode: 2CH3CH 2OH + 4OH− → CH3COOCH 2CH3 + 4H 2O + 4e−
cathode: overall:
(1)
2H 2O + 2e− → H 2 + 2OH−
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CONCLUSION In this study, a highly efficient electrochemical reforming system based on earth-abundant elements has been built up to allow the room temperature partial reforming of ethanol into
(2)
2CH3CH 2OH → CH3COOCH 2CH3 + 2H 2 (3) E
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ethyl acetate while also generating hydrogen. The synthesis and use of ultrathin Co3O4 NSs as the selective ethanol oxidation anodic electrocatalyst was the key for the successful construction of the electrochemical reforming system. The Co3O4 NSs with (111) facets as their exposure surface were synthesized by using NH3 as a morphology controller. Three important roles of NH3 in the formation of Co3O4 NSs were revealed: (1) The presence of NH3 promoted the oxidation of Co2+ into Co3+ in air. (2) The formation of Co(NH3)63+ stabilizes Co3+ from being easily precipitated and thus allows the first formation of ultrathin Co(OH)2 nanosheets for the subsequent deposition of Co3+ to form Co3O4 NSs. (3) The strong binding of NH3 on Co3+ on Co3O4 prevents the continuous growth of nanosheets into thicker structures. Thanks to the abundant exposed active sites, the Co3O4 NSs exhibited excellent electrocatalytic performances in water oxidation and ethanol oxidation. Small onset potential, large anodic currents at low overpotential, and enhanced durability over an extended period were achieved. More importantly, the only electrooxidation product of ethanol on Co3O4 NSs was ethyl acetate. By using Ni−Mo nanopowders as the HER catalyst and Co3O4 NSs as the anodic catalyst, an overall ethanol reforming electrolyzer cell based on earth-abundant elements was built. The electrolyzer was readily driven by a 1.5 V battery for the simultaneous production of clean hydrogen fuels and useful ethyl acetate at the cathode and anode, respectively. The reaction temperature was mild and similar to that of fermentation. This work opens up new opportunities in designing a material system for building unique devices to generate both hydrogen and high-value organics at room temperature by utilizing electric energy from renewable sources.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscentsci.6b00164. TEM images, XRD, XPS, UV absorption spectra, and detailed electrochemical characterization (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions †
L.D. and Q.Q. contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the MOST of China (2015CB932303) and the NSFC of China (21420102001, 21131005, 21390390, 21227001, 21333008) for financial support.
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REFERENCES
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DOI: 10.1021/acscentsci.6b00164 ACS Cent. Sci. XXXX, XXX, XXX−XXX
