Electrochemical Partial Reforming of Ethanol into Ethyl AcetateUsing Ultrathin Co3O4 Nanosheets as a Highly Selective AnodeCatalystLei 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 ChemicalEngineering, Xiamen University, Xiamen 361005, P. R. China
*S Supporting Information
ABSTRACT: Electrochemical partial reforming of organicsprovides an alternative strategy to produce valuable organiccompounds while generating H2 under mild conditions. In thiswork, highly selective electrochemical reforming of ethanolinto ethyl acetate is successfully achieved by using ultrathinCo3O4 nanosheets with exposed (111) facets as an anodecatalyst. Those nanosheets were synthesized by a one-pot,templateless hydrothermal method with the use of ammonia.NH3 was demonstrated critical to the overall formation ofultrathin Co3O4 nanosheets. With abundant active sites onCo3O4 (111), the as-synthesized ultrathin Co3O4 nanosheetsexhibited enhanced electrocatalytic activities toward water andethanol oxidations in alkaline media. More importantly, over the Co3O4 nanosheets, the electrooxidation from ethanol to ethylacetate was so selective that no other oxidation products were yielded. With such a high selectivity, an electrolyzer cell usingCo3O4 nanosheets as the anode electrocatalyst and Ni−Mo nanopowders as the cathode electrocatalyst has been successfullybuilt for ethanol reforming. The electrolyzer cell was readily driven by a 1.5 V battery to achieve the effective production of bothH2 and ethyl acetate. After the bulk electrolysis, about 95% of ethanol was electrochemically reformed into ethyl acetate. Thiswork 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.
■ INTRODUCTION
Hydrogen as renewable energy provides a promising solution toovercome our dependency on fossil fuels and the energycrisis.1−4 However, currently hydrogen is mainly produced bysteam reforming fossil fuels such as natural gas at hightemperature. Utilizing electric energy from renewable sourcesfor producing H2 fuel by electrochemical means has thusattracted increasing attention during the past decades.5−13
Proton-containing liquids, such as water and alcohols, are idealsource materials for the electrochemical production ofhydrogen due to their nontoxicity and availability. However,one of the major technological hurdles is the development ofcost-effective efficient anode electrocatalysts.9,14,15 In acidmedium, state-of-the-art electrocatalysts in oxygen evolutionreaction (OER), an important half-reaction in electrochemicalwater splitting, are mainly expensive and rare noble metals andtheir oxides, limiting the wide application of the electrochemicalreaction process.16−18 Increasing research attention has beendirected to the development of alternative efficient catalystsbased on inexpensive and earth-abundant elements withpromising good catalytic activity and durability for OER in
alkaline conditions.19−28 Transition metal oxides have beenthus emerging as one of the most studied systems. Theactivities of transition-metal-based electrocatalysts have beendemonstrated 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 transitionmetal oxides, including Co3O4, are found to be the idealelectrocatalysts for renewable energy conversion.30,32−35
Together with these recent instances of progress in OERelectrocatalysts, the remarkable development of hydrogenevolution reaction (HER) electrocatalysts made from earth-abundant elements has been creating bright opportunitiestoward the green production of hydrogen.36,37
Although water electrocatalytic splitting is an ideal process togenerate hydrogen, much energy of the process is used toovercome the huge energy barrier for oxygen generation atanodes. However, oxygen is not a high-value product. It wouldbe very promising if the anodic reaction could be utilized for
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the oxidative production of high-value organics. The resultingelectrochemical reforming methods would allow the roomtemperature production of high-value organic products andhydrogen at anodes and cathodes, respectively. In this regard,the selectivity of the anodic reactions is the most challengingissue. For example, the electrochemical reforming of ethanol atroom temperature has been achieved using noble metal basedanodes. However, no valuable products have already beenobtained in pure form,5 similar to the situation met byconventional ethanol reforming that was typically carried out athigh temperature and high catalyst loading.38
In this work we put forward a concept to use efficienttransition metal oxide OER catalysts as selective ethanoloxidation catalysts for the development of electrochemicalreforming of ethanol into ethyl acetate. An ammonia-assistedstrategy was developed to prepare atomic thickness ultrathinspinel-type Co3O4 nanosheets (NSs) with (111) planes as themajor exposure surface. These Co3O4 nanosheets exhibited anexcellent electrocatalytic performance in OER and theelectrooxidation of ethanol. Ethyl acetate was the onlyoxidation product of ethanol. At the potential of 1.445 V (vsRHE), the Faraday efficiency toward ethyl acetate was as highas 98%, making the Co3O4 NSs highly promising anodiccatalysts for the partial electrochemical reforming of ethanol.Together with a Ni−Mo HER catalyst, Co3O4 NSs were grownon carbon cloth and used as the anode catalyst to build anoverall of ethanol reforming electrolyzer cell using a 1.5 Vbattery for the separated production of clean hydrogen fuelsand useful ethyl acetate at the cathode and anode, respectively.
■ RESULTS AND DISCUSSIONSynthesis and Structural Characterizations of Ultra-
thin Co3O4 Nanosheets. In a typical synthesis of ultrathinCo3O4 NSs, 50 mg of CoCl2 was dissolved in 10 mL of purewater, yielding a clear light pink solution. After 8 mL of NH3·H2O (25% NH3) was added, the mixture was stirred in air for10 min. The color of the solution turned into dark brownduring the stirring in air (see Supporting Information for moredetails). The resulting mixture was then transferred into a glasspressure vessel. The vessel was heated from room temperatureto 140 °C in about 30 min and kept at 140 °C for 5.0 h. Afterthe mixture was cooled to room temperature, the blackproducts were collected by centrifugation and washed severaltimes with water.As revealed by transmission electron microscopy (TEM), the
obtained products consist of stacked ultrathin nanosheets withuniform thickness of ∼1.6 nm (Figures 1a, 1b, and S1). Alldiffraction peaks of the obtained nanosheets are in goodagreement with the standard data (JCPDS #43-1003) of cubicspinel Co3O4 (Figure S2), suggesting that the ultrathin NSswere of spinel phase. It should be pointed out that some of theultrathin Co3O4 NSs were stacked together as a result ofenergetically favored face-to-face interactions.39 The ultrathinnature of the obtained Co3O4 NSs was also confirmed byatomic force microscopy (AFM) (Figure 1c). As measured byAFM, the thickness of the Co3O4 NSs was 1.6 nm, which wasconsistent with the number estimated from the TEM analysis.To further analyze the growth habit of the nanosheets, high-
resolution transmission electron microscopy (HRTEM) wasadopted to investigate the lattice arrangements within thenanosheets (Figures 1d and 1e). Lattice fringes with interplanarspacing of 0.47 and 0.28 nm, corresponding to (111) and (220)fringes of spinel Co3O4, were clearly observed in the HRTEM
image of an individual nanosheet that lay on the TEM grid. Thesingle-crystalline feature of the nanosheets was also verified bythe corresponding fast Fourier transform (FFT) pattern(Figure 1f). All these data suggested that the (111) exposedCo3O4 NSs with atomic thickness have been successfullysynthesized.
Formation Mechanism of Co3O4 Nanosheets. AlthoughCoCl2 was used as the sole Co precursor, the obtained spinelCo3O4 NSs contained both Co(III) and Co(II). We thereforeconsidered if the presence of O2 and NH3 during the synthesiswas critical to the formation of ultrathin Co3O4 NSs.Co(NH3)6
3+ with a formation constant Kf of 4.6 × 1033 is amuch more stable complex than Co(NH3)6
2+ (Kf = 5.0 ×104),40,41 meaning that in the presence of NH3 the initial Co
2+
in the system would be oxidized to Co3+ by O2 in air. Indeed,before transfer into the glass pressure vessel, the reactionmixture was already partially oxidized as evidenced by the UV−vis spectroscopic measurements (Figure S3a). The absorptionpeak at 520 nm corresponding to Co(H2O)6
2+ disappeared. Asshown in the ESI-MS spectrum (Figure S3b), a peak with a m/zof 162.1 corresponding to Co(NH3)6
3+ appeared, furtherconfirming the oxidation of Co2+ into Co3+ in the presenceof NH3. When the reaction parameters were kept the sameexcept that O2 was excluded by maintaining the reaction underthe protection of N2, the reaction led to the formation ofultrathin Co(OH)2 (JCPDS #45-0031) rather than Co3O4(Figures S4a and S5a). Interestingly, replacing NH3 byCH3NH2 also resulted in the formation of ultrathin Co(OH)2NSs (Figures S4b and S5b). These observations suggested theimportance of NH3 in the synthesis of ultrathin Co3O4 NSs. Itwas noteworthy that ultrathin Co3O4 NSs were also generatedwhen Co(OAc)2 or Co(NO3)2 (Figures S4c,d and S5c,d) wasused as the Co precursor, indicating not much effect of anionsin the Co precursor salts on the formation of Co3O4 NSs. Sucha situation is different from the previous report on theimportant role of anions on the formation of Co3O4 NSs.
41
Figure 1. (a, b) Representative TEM, (c) AFM, and (d, e) HRTEMimages of the ultrathin Co3O4 nanosheets. (f) The corresponding FFTpattern of an individualCo3O4 nanosheet shown in panel d.
In the synthesis of Co3O4 NSs, the amount of NH3·H2O wasalso essential. As shown in Figure S6, when the amount ofNH3·H2O was 1.0 mL, Co3O4 nanoparticles with mixedmorphologies were obtained. Few nanosheets started to appearwhen the amount of NH3·H2O was increased from 1.0 to 2.0mL. Co3O4 NSs were obtained as the major products onlywhen 4.0 mL of NH3·H2O was used. But the diameter of theobtained NSs with 4.0 mL of NH3·H2O was still relatively small(less than 100 nm), and their surfaces were not smooth. Whenthe amount of NH3·H2O was further increased to 8.0 mL,Co3O4 NSs having much larger diameter (hundreds ofnanometers) were obtained as the major products. With theincrease of NH3·H2O amount from 2.0 to 8.0 mL, the averagethickness of the nanosheets was gradually reduced from ∼3.2 to∼1.6 nm. To exclude that the effect of NH3·H2O on theformation of Co3O4 NSs was due to its role of providing analkaline condition, we used a certain concentration ofpotassium hydroxide solution instead of NH3·H2O to controlthe pH value of the reaction. However, in this case, no Co3O4NSs were obtained (Figure S7). Only the cube-like nano-particles were produced.Based on the above results, we proposed that the strong
preferential binding of NH3 on Co3O4 (111) might be the mainreason for the anisotropic 2D growth of Co3O4 NSs. In order toprove 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 theas-prepared Co3O4 NSs upon heating under vacuum. Therelease of NH3 at the temperature between 160 and 310 °C wasclearly observed (Figure 2), suggesting the rather strong
adsorption of NH3 on Co3O4 NSs. The surface adsorption ofNH3 made Co3O4 NSs have a positively charged surface, whichwas also confirmed by ζ-potential measurements (Figure S8).Moreover, the EDS-mapping also revealed that a nitrogenspecies was adsorbed on the surface of the NSs (Figure S9). Allthe evidence supported that the strong adsorption of NH3played an important role in the formation of the Co3O4 NSswith the thickness of atomic layers.To further uncover the whole formation process of Co3O4
NSs, we investigated the products collected from the reactionmixtures after the temperature was raised from roomtemperature to 140 °C in 30 min and kept at this temperaturefor different periods. At the temperature raising stage beforereaching 140 °C, the obtained products were a mixture ofCo(OH)2 and Co3O4 (Figure S10a). After the reaction
mixtures were heated at 140 °C for 0.5, 1.0, and 3.0 h, theproducts readily displayed diffraction peaks of pure spinelCo3O4 (JCPDS #43-1003) (Figure S10b-d), and the crystallinefeature of the products was enhanced with the heating time. Asrevealed by the TEM studies (Figure S11), the gradualdevelopment of the well-defined nanosheet morphology wasaccompanied by the increased crystalline feature of theproducts.While the oxidation of Co2+ by O2 in the presence of NH3
provided Co3+, the formation of Co(OH)2 nanosheets served asnice templates for the deposition of Co3+ to induce theformation of Co3O4 NSs. Figure S12 illustrates their possibleformation mechanism. At the early stage of reactions, Co(OH)2nanosheets were easily generated due to the layer structure ofCo(OH)2 and also its low Ksp (1.1 × 10−15). Then NH3-stabilized Co3+ species started to deposit onto the Co(OH)2nanosheets to induce the formation of Co3O4 NSs having (111)as their main exposure facets. In this mechanism, NH3 playstwo different important roles. One is to facilitate the oxidationof Co2+ into Co3+, which is a key component for Co3O4. Theother is to selectively bind on Co3O4 (111) to ensure theanisotropic growth of the nanosheets. The binding of NH3 alsohelped to prevent the continuous deposition of Co species ontoCo3O4 nanosheets and thus the growth of the nanosheets alongthe ⟨111⟩ direction.
Electrocatalytic Performances of Co3O4 Nanosheets inOxygen Evolution Reaction. The electrocatalytic perform-ances of the Co3O4 NSs were first investigated in the oxygenevolution reaction (OER) in O2-saturated 1.0 M aqueous KOH(pH = 13.6) solution. To highlight the structural advantage ofCo3O4 NSs in OER, Co3O4 nanocubes (NCs) were preparedand used as the control catalyst (Figure S13).42 Co3O4 NSs andCo3O4 NCs with mass 0.1 mg were deposited on carbon paperas the working electrodes. The polarization curves normalizedby the electrode surface areas were first used to evaluate theOER activities of the catalysts. As shown in Figures 3a and 3b,the ultrathin Co3O4 NSs only needed an overpotential of 270mV to achieve a current density of 10 mA/cm2. In comparison,Co3O4 NCs required overpotential of 332 mV to reach thesame current density. It should be noted that the overpotentialon Co3O4 NSs was also lower than that for many previouslyreported Co-based catalysts.19,34,35,43 Moreover, the corre-sponding Tafel plots (Figure 3c) indicated that the ultrathinCo3O4 NSs possessed a smaller Tafel slope (46 mV/dec) thanthat of Co3O4 NCs (63 mV/dec). The smaller Tafel slopemakes Co3O4 NSs have large current densities at lowoverpotentials. Overall, the Co3O4 NSs exhibited much higherTOFs than Co3O4 NCs at the low overpotentials (FigureS14a).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, ultrathinCo3O4 NSs with (111) surface showed an excellent durability inthe alkaline electrolyte. No obvious performance loss wasobserved within 10 h reaction or after 3000 CV cycles. Incomparison, some fluctuation was revealed on Co3O4 NCs. Asanalyzed by TEM (Figures S15a and S15b), after the long-timeelectrochemical tests, the Co3O4 NSs nicely maintained theirnanosheet structure, but the NCs were heavily aggregated.XRD spectra (Figure S15c) of the two samples indicated thatthey were still in good agreement with the standard data(JCPDS #43-1003) of cubic spinel Co3O4, and no anotherpeaks appeared.
Figure 2. TPD-MS curves of the Co3O4 nanosheets. (a) Theaccumulative ionization intensity of the decomposition products fromroom temperature to 450 °C; (b) Relative ionization intensities of themain decomposition products at different temperatures.
Electrocatalytic Performances of Co3O4 Nanosheets inEthanol Oxidation. The excellent OER performance ofCo3O4 nanosheets suggested the unique properties of Co3O4(111) in electrocatalytic oxidation reactions. Considering thatOER itself does not produce valuable products, we haveattempted to apply the Co3O4 NSs in the electrocatalyticoxidation of alcohols which can lead to the production of morevaluable products such as carboxylic acids or esters. Electro-oxidation of ethanol was then performed in an aqueous KOH(1.0 M) solution of ethanol (1.0 M) by using Co3O4 NSs as theanodic catalyst. As shown in Figures 4a and S16, anamperometric response was readily observed. On Co3O4 NSs,the onset potential (∼1.32 V vs RHE) required for theelectrooxidation of ethanol was much lower than that for OER(∼1.45 V vs RHE), suggesting the easier oxidation of ethanolthan H2O on Co3O4 NSs at the low oxidation potentials. Itshould be pointed out that the carbon paper only showed littleelectrocatalytic activity in both OER and ethanol oxidationreactions 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 10mA/cm−2, the required oxidation potentials were 1.445 and1.550 V (vs RHE) on Co3O4 NSs and Co3O4 NCs, respectively.On Co3O4 NSs, the oxidation current densities were increasedfrom 10 to 50 mA/cm2 with the potential raised from 1.445 to1.545 V. With such a potential increment, the current densitieson Co3O4 NCs were increased only from 3.06 to 10.6 mA/cm2.Moreover, the corresponding Tafel plots (Figure 4b) indicatedthat a smaller Tafel slope was achieved on the ultrathin Co3O4NSs (138 mV/dec) than that on Co3O4 NCs (192 mV/dec).The results suggested that the Co3O4 (111) facet should alsohave better electrocatalytic performance than its (100)counterpart in the electrooxidation of ethanol.
The ultrathin Co3O4 NSs also exhibited excellent durabilityin the electrochemical oxidation of ethanol (Figures 4c andS18). During the chronoamperometric experiment, there wasno obvious gas bubble formation on the Co3O4 modifiedworking electrodes, suggesting that no O2 was evolved and aliquid product might be obtained. The obtained products afterchronoamperometric tests with different potentials applied onthe Co3O4 NSs modified working electrodes were analyzed byGC−MS spectrometry and 1H NMR spectroscopy. The GC−MS measurement suggested the presence of ethyl acetate as themain product (Figure S19). Besides a triplet at 0.99 ppm and aquartet at 3.45 ppm corresponding to the protons in ethylgroup, a new singlet at chemical shift of 1.72 ppm was clearlyrevealed in the NMR spectra of the product. This peak can beattributed to the methyl group adjacent to carboxylate in ethylacetate (Figure S20). Although the peak intensities increasedwith the applied potential, there was no appearance of newpeaks upon the change of potential (Figure 4d). These dataindicated the high selectivity of the electrochemical oxidation ofethanol toward ethyl acetate. The Faraday efficiency wascalculated 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 electrochemicaloxidation of ethanol into ethyl acetate. One involves theoxidation of ethanol into acetate and its coupling with ethanol.However, under the alkaline condition in this work, theesterification between acetate and ethanol did not proceedmuch to achieve the high-yield production of ethyl acetate.Therefore, the formation of ethyl acetate might undergo theother pathway via the acetaldehyde formation under theelectrochemical conditions.
Figure 3. OER performances of Co3O4 multilayer nanosheets andnanocubes. (a) The normalized polarization curves of nanosheets andnanocubes by the electrode surface area of electrocatalysts. (b) Theoverpotential 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 aconstant current density of 10 mA/cm2.
Figure 4. Electrocatalytic performances of Co3O4 nanosheets andnanocubes in ethanol oxidation. (a) The normalized polarizationcurves of nanosheets and nanocubes by the electrode surface area ofelectrocatalysts. The scan rate was 1.0 mV/s. (b) Corresponding Tafelplots of nanosheets and nanocubes. (c) Chronoamperometric curvesof ultrathin Co3O4 nanosheets at different potentials for 1 h. (d) 1HNMR spectra of products before and after bulk electrolysis at differentpotentials for 1 h on Co3O4 nanosheet modified carbon paperelectrode.
To exclude the contribution of the surface areas of Co3O4 onthe electrochemical activity, the surface areas of Co3O4 NSs andNCs were measured using the BET method from nitrogen gasadsorption−desorption isotherms at 77 K (Figure S21a). Thesurface areas of the Co3O4 NSs and NCs were calculated to be43.4 and 10.7 m2/g, respectively. The currents in OER andethanol oxidation were then normalized by their surface areasusing the same mass loading of Co3O4 (0.1 mg/cm2). As shownin Figures S21b and S21c, at a certain oxidation potential, thecurrent densities on Co3O4 NSs were both higher than those byCo3O4 NCs in OER and ethanol oxidation.The greatly improved electrocatalytic properties of the
ultrathin Co3O4 NSs with (111) facets can be ascribed to thesynergistic effect between their macroscopic morphologicalfeature and their microscopic atomic/electronic structure.44,45
Previous work has demonstrated that rich Co3+ ions are presenton the (111) surface than on the (100) surface (FigureS22).46,47 Co3+ has been regarded as the catalytically activesites43,48−53 for transferring multiple electrons in electro-chemical processes, such as OER. Co3O4 NSs with (111) astheir major exposure facets have more Co3+ in their surface.The enrichment of surface Co3+ on Co3O4 NSs wasinvestigated 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 largerthan that at 781.5 eV (Co2+). As estimated from the peak areas,the molar ratio of Co3+/Co2+ on Co3O4 NSs was higher thanthat on Co3O4 NCs (Table S1), suggesting the presence ofmore surface Co3+ on Co3O4 NSs. This result is consistent withthe 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 onlyone major oxidation peak with much intense current density at1.44 V corresponding to the oxidation of Co3+ to Co4+. Theresult confirmed the higher density of surface Co3+ on Co3O4NSs than NCs. The rich surface Co3+ could be one of theimportant factors for the enhanced catalytic performance ofCo3O4 NSs over NCs. Moreover, better superior electricalconductivity and ion transport kinetics of Co3O4 NSs alsocontributed to their excellent catalytic performance. Asindicated by electrical impedance spectroscopy (Figure S25),the ultrathin Co3O4 NSs exhibited the lowest charge-transferresistance of 3.5 ohm, much smaller than that of Co3O4 NCs(∼15.8 ohm) at the potential of 1.50 V (vs RHE). This resultsuggested the enriched active surface sites, thus acceleratedcharge transport, and shortened ion diffusion paths on Co3O4NSs. As revealed in their TEM images, the large diameter ofCo3O4 NSs made the main excellent self-supporting electro-catalyst. During catalysis, such a self-supporting feature wouldfacilitate the mass transfer and also effectively prevent theoccurrence of agglomeration, thus helping to improve bothelectrochemical activity and stability.
Electrochemical Reforming of Ethanol into EthylAcetate and H2. Equation 1 gives an overall anodic oxidationprocess of ethanol into ethyl acetate. As for electrochemicalsynthesis, the selectivity is always a big issue. Over Co3O4 NSs,it is surprising that the electrooxidation from ethanol to ethylacetate was so selective that no other oxidation products weredetected. With such a high selectivity, we have attempted toutilize Co3O4 NSs to build up an effective electrolyzer cell forethanol reforming. It would be ideal if the cathodic reaction (eq2) could be coupled together for H2 production. In that case, asshown in eq 3, the overall electrochemical process would leadto the formation of ethyl acetate in the anode and H2 in thecathode.To experimentally prove the concept, an ethanol electro-
chemical reforming cell was built (Figure 5). Co3O4 NSs were
in situ grown on carbon cloth (Figure S26a) and used as theanode catalyst. The loading of Co3O4 NSs was ∼0.5 mg/cm2.Ni−Mo nanopowders were prepared according to a reportedmethod and used as the cathode catalyst for hydrogenevolution (Figure S26b).54 In the reforming cell, the Ni−Monanopowders were loaded on Ni foam at ∼0.1 mg/cm2. Theanode and cathode compartments were separated by a Nafionmembrane, and the volume of a single compartment was 25mL. The electrochemical reforming of ethanol was performedin an aqueous mixture of 1.0 M KOH and 1.0 M CH3CH2OH.The electrochemical reformer was readily driven by a 1.5 Vbattery, and evolution of H2 bubbles was clearly observed onthe cathode. The stability of the electrolyzer cell was tested onthe electrochemical workstation using a two-electrode system atthe potential 1.5 V, and the current of the electrolyzer cell wasabout 22 mA. After bulk electrolysis, the concentration of ethylacetate was quantified by quantitative 1H NMR and GC−MS(Figure S27). About 95% of ethanol was electrochemicallyreformed into ethyl acetate within 48 h. The Faraday efficiencywas as high as 96%, consistent with the number obtained fromthe three-electrode measurements at the potential of 1.445 V(vs RHE).
■ CONCLUSIONIn this study, a highly efficient electrochemical reformingsystem based on earth-abundant elements has been built up toallow the room temperature partial reforming of ethanol into
Figure 5. An electrochemical reforming cell run by a 1.5 V battery toproduce ethyl acetate and hydrogen from ethanol.
ethyl acetate while also generating hydrogen. The synthesis anduse of ultrathin Co3O4 NSs as the selective ethanol oxidationanodic electrocatalyst was the key for the successfulconstruction of the electrochemical reforming system. TheCo3O4 NSs with (111) facets as their exposure surface weresynthesized by using NH3 as a morphology controller. Threeimportant roles of NH3 in the formation of Co3O4 NSs wererevealed: (1) The presence of NH3 promoted the oxidation ofCo2+ into Co3+ in air. (2) The formation of Co(NH3)6
3+
stabilizes Co3+ from being easily precipitated and thus allowsthe first formation of ultrathin Co(OH)2 nanosheets for thesubsequent deposition of Co3+ to form Co3O4 NSs. (3) Thestrong binding of NH3 on Co3+ on Co3O4 prevents thecontinuous growth of nanosheets into thicker structures.Thanks to the abundant exposed active sites, the Co3O4 NSs
exhibited excellent electrocatalytic performances in wateroxidation and ethanol oxidation. Small onset potential, largeanodic currents at low overpotential, and enhanced durabilityover an extended period were achieved. More importantly, theonly electrooxidation product of ethanol on Co3O4 NSs wasethyl acetate. By using Ni−Mo nanopowders as the HERcatalyst and Co3O4 NSs as the anodic catalyst, an overallethanol reforming electrolyzer cell based on earth-abundantelements was built. The electrolyzer was readily driven by a 1.5V battery for the simultaneous production of clean hydrogenfuels and useful ethyl acetate at the cathode and anode,respectively. The reaction temperature was mild and similar tothat of fermentation. This work opens up new opportunities indesigning a material system for building unique devices togenerate both hydrogen and high-value organics at roomtemperature by utilizing electric energy from renewable sources.
■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acscentsci.6b00164.
■ AUTHOR INFORMATIONCorresponding Author*E-mail: [email protected] Contributions†L.D. and Q.Q. contributed equally.NotesThe authors declare no competing financial interest.
■ ACKNOWLEDGMENTSWe thank the MOST of China (2015CB932303) and theNSFC of China (21420102001, 21131005, 21390390,21227001, 21333008) for financial support.
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