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Ni-foam-supported and amine-functionalized TiO2 photocathode improved photoelectrocatalytic reduction of CO2 to methanol
Journal of Catalysis 349 (2017) 1–7
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Ni-foam-supported and amine-functionalized TiO2 photocathode improved photoelectrocatalytic reduction of CO2 to methanol Liwen Wang a, Yongjian Jia a, Rong Nie a, Yuqian Zhang a, Fengjuan Chen a,⇑, Zhenping Zhu b, Jianguo Wang b, Huanwang Jing a,b,⇑ a b
State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, People’s Republic of China State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 8 December 2016 Revised 17 January 2017 Accepted 21 January 2017
Keywords: Ni foam Light quantum efficiency CO2 reduction Covalent ligand Isotopic labeling experiments
a b s t r a c t We report new insights into the photoelectrochemical reduction of CO2 in a photoelectrochemical system, which is assembled from a nickel foam covered with TiO2 modified by covalent ligands as photocathode and BiVO4 as photoanode. These photoelectrocatalytic cells generate mainly methanol as a product in the liquid phase. Our results show that imine-functionalized TiO2/Ni has the highest activity. The formation rate of methanol in this excellent cell is up to 153 lM/h cm2, which is about 15 times higher than that of the TiO2/Ni electrode. If the Faradaic efficiency is considered as 100%, the light quantum efficiency of this cell reaches 1.2%, that is two times better than that of plants. Isotopic labeling experiments with 13CO2 confirm that the detected products are produced exclusively by the reduction of CO2 and water splitting. Ó 2017 Elsevier Inc. All rights reserved.
1. Introduction Global warming and the energy crisis are two serious issues that human beings face in the 21st century in view of the unsustainable usage of fossil energy and consequent emission of carbon dioxide as a greenhouse gas [1–3]. How to reduce the concentration of CO2 in atmosphere and convert it to useful carbon-based fuels is a great challenge for scientists [4–12]. Many methodologies for converting CO2 to hydrocarbons have been reported in the literature, such as electrochemical [13–18] and photochemical methods [19–24]. Hori et al. reported in the 1980s that various metals, such as Cu, Ag, Au, Ni, and Pb, can be used to catalyze the electrochemical reduction of CO2 to formate, CO, and CH4 [25]. Further investigations have mostly focused on Cu or Cu-based alloys, in light of their better activity [26–40]. Although Ni showed the worst activity in the electrochemical reduction of CO2, its hydrogen emission ability is the best among these metals and alloys at lower external voltages [41]. Semiconductors, such as TiO2, ZnO, CdS, Co3O4, and Ga2O3 [42–49], have been widely utilized as light harvesters in CO2 ⇑ Corresponding author at: State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, People’s Republic of China. Fax: +86 931 8912582. E-mail addresses: [email protected] (F. Chen), [email protected] (H. Jing). http://dx.doi.org/10.1016/j.jcat.2017.01.013 0021-9517/Ó 2017 Elsevier Inc. All rights reserved.
reduction. Although TiO2 is known as a promising semiconductor material that is nontoxic, nonpolluting, and highly stable [50], its wide energy band gap (Eg = 3.2 eV, k < 385 nm) limits its absorption for solar spectra and practical applications [51,52]. To improve the photoelectrocatalytic efficiency of TiO2 in CO2 conversion, TiO2 nanoparticles are commonly modified, hybridized, and functionalized by various techniques [53]. Here, we report a new photoelectrochemical system (Fig. 1) composed of an organic–inorganic composite photocathode, nickel-foam-supported TiO2 nanoparticles modified by amine and imine ligands, and a BiVO4 photoanode. In addition, Eosin Y disodium salt was added to the electrolyte of an aqueous KHCO3 solution to improve the absorption of simulated solar light. Therefore, our new system can generate methanol efficiently as a unique product at high yield. 2. Experimental 2.1. General All chemical reagents, unless otherwise stated, were analytical grade. Typically, Eosin Y disodium salt (Eosin Y), potassium bicarbonate (KHCO3), salicylaldehyde, sodium hydroxide, 3-aminopropyl triethoxy silane, tetrabutyl titanate, and 3-aminopropyl-triethoxysilane (APTES) were ordered from J&K Chemical.
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lowed by 2 h additional stirring at room temperature to form TiO2 sol–gel. Nickel foam (1.4 1.4 cm) was immersed in TiO2 sol–gel for 15 min to form a uniform layer. A 3D netlike TiO2 film was formed after annealing at 550 °C for 45 min in a muffle furnace. 2.2.2. Surface modification of TiO2/Ni film electrodes with organic ligands The as-prepared TiO2 films were first modified with 3-aminopropyltriethoxysilane (APTES, J&K Chemical), which was used as a coupling reagent to react with the TiO2 surface hydroxyl groups, producing an amine-modified photocathode of NH2/TiO2/ Ni. The binding amine groups could be converted to different imine groups in the presence of salicylic aldehyde, producing an aminemodified photocathode of CHO/NH2/TiO2/Ni. Thus, three different multimodified Ni-foam-supported electrodes were successfully fabricated with the net structures shown in Fig. 2. The organicligand-modified Ni-foam-supported electrodes were denominated as TiO2/Ni, NH2/TiO2/Ni, and CHO/NH2/TiO2/Ni, respectively. 2.3. Photoelectrochemical measurement
Fig. 1. Schematic for the PEC cell for CO2 reduction.
The solid-state UV–vis absorption spectra were determined on a UV–vis spectrophotometer (UV-2600, Shimadzu). The 1H nuclear magnetic resonance (1H NMR, JNM-ECS 400 MHz, JOEL) spectrum was utilized to analyze the liquid products. The simulated sunlight source was a 300 W Xenon lamp (PLS-SXE300/300UV, Beijing Perfect Light Technology Co. Lt). The external potential was provided by an electrochemical workstation (CHI660E). The XRD data were examined by a PANalytical X Pert Pro diffractometer at 40 kV and 40 mA with Cu Ka radiation (k = 1.5405 Å). Fourier transform infrared (FT-IR) spectra were recorded on a Nicolet NEXUS 670 instrument. The morphology of CHO/NH2/TiO2/Ni was characterized by scanning electron microscopy (FESEM, Hitachi S-4800). X-ray photoelectron spectroscopy (XPS) was performed on a Kratos AXIS Nova spectrometer (Shimazu Co., Ltd.) with a monochromatic Al Ka X-ray. 2.2. Preparation of electrodes The Ni-foam-supported and amine-functionalized TiO2 photocathodes are shown in Fig. 2 and described explicitly in the following paragraphs. The fabrication process for the BiVO4 electrode is available in the Supporting Information (Fig. S2). 2.2.1. Preparation of Ni-foam-supported TiO2 electrodes TiO2 sol–gel was prepared through the following processes described in previous reports [54,55]. Tetrabutoxytitanium was first diluted with ethanol and diethanolamine with ratios tetrabu toxytitanium/ethanol/diethanolamine = 1:10:1. The resulting mixture was stirred for 1 h at room temperature, and deionized water and ethanol were added with ratio deionized/ethanol = 1:1, fol-
The linear sweep voltammetry (LSV) curves and amperometric J–t curves were measured in a three-electrode system by an electrochemical workstation (CHI660E) with a scan rate of 50 mV/s. In the new PEC cells, the Ni-foam-supported electrode was engaged as the working electrode with BiVO4 as the counter electrode and a SCE as the reference electrode. The PEC reduction of CO2 was performed in the PEC cell using a KHCO3 aqueous solution (0.1 M, pH 6.8) as electrolyte under simulated sunlight irradiation with an appropriate external potential. The electrolyte containing Eosin Y (1 mM) was bubbled with CO2 or Ar for 30 min before the measurements. The light intensity of the light source was adjusted to 200 mW/cm2, calibrated by a standard solar cell. We must emphasize that the convenient photocathodes were colored and prepared in situ after the electrodes were immerged into the electrolyte during the CO2 bubbling process. 3. Results and discussion 3.1. Characterization of photocathodes The SEM image (Fig. 3a) shows that the nickel foam skeleton was covered with the TiO2 films. On the films, the TiO2 nanoparticles are uniform and dense and have relatively the same size (13–15 nm, Fig. 3b). Three diffraction peaks at 25.2°, 48.3°, and 62.7°, corresponding to (1 0 1), (2 0 0), and (2 1 3) diffraction planes of anatase (JCPDS No. 21-1272), are observed in the XRD pattern of TiO2/Ni (Fig. S1). These results demonstrate that TiO2 particles and films are anatase phases. XPS analysis of CHO/NH2/TiO2/Ni is shown in Fig. S3. The peak at 101.8 eV was assigned to the 2p state of Si. The peaks of N 1s and Ti 2p can be detected as well, revealing that amine ligands were successfully modified onto the surface of TiO2 electrode. Their FTIR spectra (Fig. S4) can also confirm the covalent linkage with the TiO2 film. There were absorption peaks at 1628.1 cm 1 (Ar-ring breath vibration, C@C), 1457.9 cm 1 (C@N stretching vibration), 1123.6 cm 1 (SiAO stretching vibration),
Fig. 2. Schematic illustration of the process of photocathode preparation.
L. Wang et al. / Journal of Catalysis 349 (2017) 1–7
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Fig. 3. SEM images of TiO2@nickel foam.
and 2922.1 cm 1 (CAH stretching vibration), respectively. The XRD pattern of the BiVO4 photoanode is shown in Fig. S2 and is in accordance with the phase of clinobisvanite. The UV–vis absorption spectra of Ni foam, TiO2/Ni foam, NH2/ TiO2/Ni foam, and CHO/NH2/TiO2/Ni foam were obtained and are depicted in Fig. 4a. It can be seen that the absorption of CHO/ NH2/TiO2/Ni foam was stronger than that of Ni foam and TiO2/Ni foam. This result proves that the organic covalent ligand increases the absorption intensity and causes a slight red shift of the TiO2/Ni
electrode. The absorption band of the CHO/NH2/TiO2/Ni foam electrode absorption edge is enlarged to 600 nm owing to its aromatic groups. When the CHO/NH2/TiO2/Ni electrode absorbed Eosin Y dye, its absorption edge could be enlarged to the visible range of about 700 nm, indicating that the dye-sensitized photocathode has better light harvesting ability (Fig. S6). A plot of the square of absorption efficiency (a) multiplied by the photon energy ((a h m)2) versus the photoenergy shows a linear relationship (Fig. 4b), indicating that CHO/NH2/TiO2/Ni is a direct transitiontype semiconductor. The bandgap of this photocathode is estimated to be 2.56 eV by calculating the intersection of the plot line and abscissa axis. The bandgap of CHO/NH2/TiO2/Ni with dye was estimated to be 2.06 eV (Fig. S6), augmenting the outstanding efficiency of light absorption. 3.2. Photoelectrochemical properties The LSV curves of photoelectrochemical cells were obtained in the range from 0.3 to 1.1 V (vs. SCE) and are presented in Fig. 5. The experiments were conducted in KHCO3 aqueous solutions saturated with Ar or CO2 with a scan rate of 50 mV s 1 under simulated sunlight irradiation. As shown in Fig. 5, the current density of the electrochemical cell improves smoothly, along with the increase of bias voltage without light irradiation compared with the near zero current density under Ar. Under photoelectrochemical conditions, the cell gives a clear enhancement of current density from 0.5 to 1.1 V, which exhibits a successful coupling of photoelectrocatalytic water splitting and photoelectrocatalytic CO2 reduction. Therefore, the 0.6 V bias voltage is chosen as the minimum working bias voltage in the next investigations. 3.3. Photoelectrochemical reduction of CO2
Fig. 4. (a) The UV–vis spectra of different electrodes; (b) a plot analysis of the band gap of CHO/NH2/TiO2/Ni.
At first, various Ni-foam-supported electrodes were prepared and assembled with BiVO4/FTO and SCE to build a photoelectrocatalytic cell for CO2 reduction. The products in the electrolytes were monitored and their abilities for CO2 reduction were evaluated (Fig. 6a). We can see from Fig. 6a that the simple TiO2nanoparticle-covered Ni foam photocathode (TiO2/Ni) can produce methanol as a major product at a rate of 9.5 lM/L h cm2 compared with inactive bare Ni foam electrodes at 0.6 V bias voltage. To enhance the CO2 catching ability on the surface of the photocathode in situ, organic ligands containing amine groups are bridged to TiO2 nanoparticles by APTES. The resulting electrode of NH2/TiO2/Ni is more efficient for photoelectrochemical CO2 reduction than electrode TiO2/Ni. When the amine groups are converted to imine groups by a condensation reaction with salicylaldehyde, the obtained photocathode CHO/NH2/TiO2/Ni is superior to other electrodes under the same conditions, generating methanol in a rate of 153 lM/h/cm2, 15 times that of the TiO2/Ni
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Fig. 6. The rate for methanol with different electrodes at light quantum efficiency for methanol (b).
Fig. 5. Linear sweep voltammetric curves of TiO2/Ni (a), NH2/TiO2/Ni (b), and CHO/ NH2/TiO2/Ni (c).
electrode, owing to its better light harvesting and CO2 capture ability (vide supra). As a comparison, a control experiment was carried out using an electrode of CHO/TiO2/Ni that was made by the direct adsorption of salicylaldehyde onto TiO2. The lower activity (14.5 lM/h cm2) might be attributed to the lower Lewis basicity of C@O than C@N, which can catch CO2 molecules via coordination bonds.
0.6 V (a) and the rate and
Based on the above photoelectrochemical determinations, the CHO/NH2/TiO2/Ni photocathode reduction of CO2 was then investigated at various bias potentials, such as 0.6, 0.8, and 1.0 V. The major products of electrolytes in new photoelectrochemical cells are methanol and a little ethanol; the gas phase products are hydrogen and oxygen with a small quantity of carbon monoxide. These results are listed in Table 1, in which the total amount of products at more negative potential was larger than that at less negative potential. Some formate could be also detected at 0.4 V bias potential. The highest Faradaic efficiency is up to 507.9% at 0.6 V, which reflects good synergetic interaction between CO2 reduction and water splitting. The maximum formation rate of methanol was 186 lM/L h cm2 at 1.0 V. Alternatively, hydroxyl anions could be also oxidized to oxygen when their electrons transferred directly to the HOMO of dyes rather than through an external circuit (vide infra). Therefore, the Faradaic efficiencies of PEC cells might exceed 100%. If the Faradaic efficiency is estimated to be 100%, the light quantum efficiency of the cell reaches up to 1.2%, two times better than for plants. The rate of oxygen release by the photoelectrocatalytic cell is far less than that of reduction products, which may be because of the oxidation of CO to CO2. To verify that methanol derived from CO2 reduction, photoelectrochemical experiments were performed on CHO/NH2/TiO2/Ni electrodes in Ar-saturated KHCO3 solution (without CO2) at 0.6 V. It was confirmed that no liquid was undetectable after electrolysis for 4 h, confirming that methanol was derived from CO2 reduction. The stability of the CHO/NH2/TiO2/Ni electrode for photoelectrochemical reduction of CO2 was evaluated by seven successive
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L. Wang et al. / Journal of Catalysis 349 (2017) 1–7 Table 1 The results of photoelectrocatalytic CO2 reduction using CHO/NH2/TiO2/Ni as photocathode. Photocathode
CHO/NH2/TiO2/Ni
Potential/V vs. SCE
0.4 Ratea (FE)b
0.6 Rate (FE)
0.8 Rate (FE)
1.0 Rate (FE)
MeOH
78.0 (60.8)
153.4 (452.0)
162.6 (288.5)
186.5 (27.3)
EtOH
5.2 (8.1)
7.9 (46.6)
14.5 (51.4)
17.1 (5.0)
HCOOH
6.3 (1.6)
–
–
–
H2
8.0 (2.1)
9.5 (9.3)
57.9 (34.2)
68.8 (3.4)
CO
–
–
0.26 (0.15)
0.47 (0.02)
O2
–
–
0.38
0.68
TEE (%)c
72.9 –
507.9 1.2
374.3 1.3
35.7 –
Ucell a b c
Rate = lM h 1 cm 2. Faradaic efficiency (%). Total electron efficiency (TEE) = RFE.
tests of CO2 electrolysis at 0.6 V (each test lasted 4 h). As shown in Fig. S7, the production rates of methanol remain around 153 lM/h/cm2 for 28 h. Meanwhile, the XRD patterns (Fig. S8) of fresh and used photocathodes of CHO/NH2/TiO2/Ni are the same, which indicates no passivation occurring on the surface of the CHO/NH2/TiO2/Ni electrode. These results verify the good stability and activity of the CHO/NH2/TiO2/Ni electrode for photoelectrochemical reduction of CO2. To study the absolute energy of the electrode at the redox level, its band gap structure was detected by a Mott–Schottky (MS) relationship, which can be applied to estimate the potential of the CB edges of NH2/TiO2/Ni and CHO/NH2/TiO2/Ni. Fig. 7a shows the MS plots of CHO/NH2/TiO2/Ni and NH2/TiO2/Ni, which were measured at 0.5 kHz in the dark. The positive slopes of the linear MS plots were obtained, indicating that the electrodes behave as n-type semiconductors. Therefore, the conduction band can be determined as an x-axis point of intersection. The derived conduction band potential of NH2/TiO2/Ni in aqueous solution is 0.42 V. Meanwhile, the derived CB of CHO/NH2/TiO2/Ni in aqueous solution is 0.73 V. These results show that the imine molecules make the conduction band of the electrode move to a more positive position, so that it has enough energy to reduce CO2. Moreover, according to these data combined with the band gap values obtained from the UV–vis diffuse reflectance spectra, the valence band (VB) position of CHO/NH2/TiO2/Ni can be calculated as 1.83 V. Fig. 7b shows a band potential diagram for NH2/TiO2/Ni and CHO/NH2/TiO2/Ni together with the thermodynamic potentials for CO2 reduction to methanol. This result shows that the CB potential of n-type CHO/NH2/TiO2/Ni was high enough for CO2 reduction to methanol. Furthermore, the electronic structures of the Eosin Y and imine ligand and their geometries and energies were fully computerized using the hybrid DFT method at the level of B3LYP/6-31G(d)/ LANL2DZ [56,57]. Their molecular orbital diagrams of HOMOs and LUMOs are exhibited in Fig. S9. The conducting band of TiO2 is located between the LUMO and HOMO of dye and organic molecules (Fig. S10). These results confirm that the propulsion of electron injection from LUMOs to the conducting band of TiO2 is quite sufficient. The electrochemical impedance spectra (EIS) of photocathodes were measured by the electronic transmission performance. It can be seen from Fig. 8 that the EIS of the electrode
Fig. 7. (a) Mott–Schottky plots of NH2/TiO2/Ni (red) and CHO/NH2/TiO2/Ni (black). (b) A schematic diagram of the band gap of TiO2, NH2/TiO2/Ni, and CHO/NH2/TiO2/Ni.
CHO/NH2/TiO2/Ni was much smaller than that of the electrode TiO2/Ni. This proved that the introduction of the covalent ligand decreased the resistance of the substrate and further improved
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Fig. 8. The electrochemical impedance spectra of TiO2/Ni and CHO/NH2/TiO2/Ni.
In addition, the small amount of labeling ethanol can also be monitored by GC–MS (Fig. S17). To reveal the transportation of electrons in the photoelectrocatalytic cell clearly, a proposed mechanism is drawn in Fig. 9. The electrons in the ground states (HOMO) of dye molecules harvest photons to jump into the excited states (LUMO); then photoelectrons inject into the conduction band (CB) of anatase-TiO2, transfer to nickel, and are quickly caught by protons adsorbed onto the nickel foam, producing hydrogen atoms. There are two paths for oxidation of water: the electrons of OH transfer to the BiVO4 electrode (path a) accompanied by oxygen release, and then flow in the external circuit. After arriving to the valence band (VB) of TiO2, the external electrons fill into the HOMO of dye molecules under the action of voltage, completing the recovery of dye molecules. When the electrons of OH transfer directly to the HOMO of dyes (path b), the dye molecules are regenerated and can participate in another photocatalytic CO2 reduction process. In this process, oxygen might be formed without BiVO4 participation. With this understanding, Faradaic efficiency could be greatly increased under the conditions of electron mobility in path b a. 4. Conclusions In summary, new photoelectrocatalytic cells were designed and fabricated, in which the photocathodes were made from a Ni foam covered with a semiconductor of anatase TiO2 film that was functionalized by organic ligands. The photocathode of CHO/NH2/Ni with imine ligands shows more activity than that of NH2/TiO2/Ni owing to its better light absorption and electronic transportation. The activity of the electrode NH2/TiO2/Ni with amine ligands is higher than that of TiO2/Ni because of its better adsorption of CO2. The highest formation rate of methanol in the photoelectrocatalytic cell of CHO/NH2/TiO2/Ni can reach 1461 lM after 4 h. Its light quantum efficiency (Ucell = 1.2%) is superior to that of plants (U 0.5%). Our EIS experiments proved that the introduction of the covalent ligands could decrease the resistance of electrodes, which should further improve their electron transfer ability. Therefore, new Ni-foam-supported and amine-functionalized TiO2 photocathodes offer many advantages, such as nontoxicity, better conductivity, economy, and facile approach. Acknowledgments This study was funded by the National Natural Science Foundation of China (NSFC 21173106, 21401091), the Fundamental Research Funds for the Central Universities (lzujbky-2016-K09), and the Foundation of the State Key Laboratory of Coal Conversion (Grant No. J16-17-913).
Fig. 9. The mechanism of the photoelectrocatalytic process of CO2 reduction.
Appendix A. Supplementary material the electron transfer ability of the electrode, which could transfer more electrons for CO2 reduction.
3.4.
13
CO2 labeling experiments
To examine the carbon source of the product CH3OH, an isotopic labeling experiment of 13CO2 was carried out in the PEC cell using CHO/NH2/TiO2/Ni as the photocathode. After the whole system was purged with Ar gas, 13CO2 gas was introduced into the well-sealed PEC device for 15 min. The doublet peaks at 3.03 and 3.41 ppm are attributed to the coupling of protons with the 13C nucleus of 13 CH3OH (Fig. S11). Meanwhile, 13CO (m/z = 29) was detected by the GC–MS technique (Fig. S16). These results could verify that the carbon atoms of CH3OH and CO are indeed derived from CO2.
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Journal of Catalysis journal homepage: www.elsevier.com/locate/jcat
Ni-foam-supported and amine-functionalized TiO2 photocathode improved photoelectrocatalytic reduction of CO2 to methanol Liwen Wang a, Yongjian Jia a, Rong Nie a, Yuqian Zhang a, Fengjuan Chen a,⇑, Zhenping Zhu b, Jianguo Wang b, Huanwang Jing a,b,⇑ a b
State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, People’s Republic of China State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 8 December 2016 Revised 17 January 2017 Accepted 21 January 2017
Keywords: Ni foam Light quantum efficiency CO2 reduction Covalent ligand Isotopic labeling experiments
a b s t r a c t We report new insights into the photoelectrochemical reduction of CO2 in a photoelectrochemical system, which is assembled from a nickel foam covered with TiO2 modified by covalent ligands as photocathode and BiVO4 as photoanode. These photoelectrocatalytic cells generate mainly methanol as a product in the liquid phase. Our results show that imine-functionalized TiO2/Ni has the highest activity. The formation rate of methanol in this excellent cell is up to 153 lM/h cm2, which is about 15 times higher than that of the TiO2/Ni electrode. If the Faradaic efficiency is considered as 100%, the light quantum efficiency of this cell reaches 1.2%, that is two times better than that of plants. Isotopic labeling experiments with 13CO2 confirm that the detected products are produced exclusively by the reduction of CO2 and water splitting. Ó 2017 Elsevier Inc. All rights reserved.
1. Introduction Global warming and the energy crisis are two serious issues that human beings face in the 21st century in view of the unsustainable usage of fossil energy and consequent emission of carbon dioxide as a greenhouse gas [1–3]. How to reduce the concentration of CO2 in atmosphere and convert it to useful carbon-based fuels is a great challenge for scientists [4–12]. Many methodologies for converting CO2 to hydrocarbons have been reported in the literature, such as electrochemical [13–18] and photochemical methods [19–24]. Hori et al. reported in the 1980s that various metals, such as Cu, Ag, Au, Ni, and Pb, can be used to catalyze the electrochemical reduction of CO2 to formate, CO, and CH4 [25]. Further investigations have mostly focused on Cu or Cu-based alloys, in light of their better activity [26–40]. Although Ni showed the worst activity in the electrochemical reduction of CO2, its hydrogen emission ability is the best among these metals and alloys at lower external voltages [41]. Semiconductors, such as TiO2, ZnO, CdS, Co3O4, and Ga2O3 [42–49], have been widely utilized as light harvesters in CO2 ⇑ Corresponding author at: State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, People’s Republic of China. Fax: +86 931 8912582. E-mail addresses: [email protected] (F. Chen), [email protected] (H. Jing). http://dx.doi.org/10.1016/j.jcat.2017.01.013 0021-9517/Ó 2017 Elsevier Inc. All rights reserved.
reduction. Although TiO2 is known as a promising semiconductor material that is nontoxic, nonpolluting, and highly stable [50], its wide energy band gap (Eg = 3.2 eV, k < 385 nm) limits its absorption for solar spectra and practical applications [51,52]. To improve the photoelectrocatalytic efficiency of TiO2 in CO2 conversion, TiO2 nanoparticles are commonly modified, hybridized, and functionalized by various techniques [53]. Here, we report a new photoelectrochemical system (Fig. 1) composed of an organic–inorganic composite photocathode, nickel-foam-supported TiO2 nanoparticles modified by amine and imine ligands, and a BiVO4 photoanode. In addition, Eosin Y disodium salt was added to the electrolyte of an aqueous KHCO3 solution to improve the absorption of simulated solar light. Therefore, our new system can generate methanol efficiently as a unique product at high yield. 2. Experimental 2.1. General All chemical reagents, unless otherwise stated, were analytical grade. Typically, Eosin Y disodium salt (Eosin Y), potassium bicarbonate (KHCO3), salicylaldehyde, sodium hydroxide, 3-aminopropyl triethoxy silane, tetrabutyl titanate, and 3-aminopropyl-triethoxysilane (APTES) were ordered from J&K Chemical.
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lowed by 2 h additional stirring at room temperature to form TiO2 sol–gel. Nickel foam (1.4 1.4 cm) was immersed in TiO2 sol–gel for 15 min to form a uniform layer. A 3D netlike TiO2 film was formed after annealing at 550 °C for 45 min in a muffle furnace. 2.2.2. Surface modification of TiO2/Ni film electrodes with organic ligands The as-prepared TiO2 films were first modified with 3-aminopropyltriethoxysilane (APTES, J&K Chemical), which was used as a coupling reagent to react with the TiO2 surface hydroxyl groups, producing an amine-modified photocathode of NH2/TiO2/ Ni. The binding amine groups could be converted to different imine groups in the presence of salicylic aldehyde, producing an aminemodified photocathode of CHO/NH2/TiO2/Ni. Thus, three different multimodified Ni-foam-supported electrodes were successfully fabricated with the net structures shown in Fig. 2. The organicligand-modified Ni-foam-supported electrodes were denominated as TiO2/Ni, NH2/TiO2/Ni, and CHO/NH2/TiO2/Ni, respectively. 2.3. Photoelectrochemical measurement
Fig. 1. Schematic for the PEC cell for CO2 reduction.
The solid-state UV–vis absorption spectra were determined on a UV–vis spectrophotometer (UV-2600, Shimadzu). The 1H nuclear magnetic resonance (1H NMR, JNM-ECS 400 MHz, JOEL) spectrum was utilized to analyze the liquid products. The simulated sunlight source was a 300 W Xenon lamp (PLS-SXE300/300UV, Beijing Perfect Light Technology Co. Lt). The external potential was provided by an electrochemical workstation (CHI660E). The XRD data were examined by a PANalytical X Pert Pro diffractometer at 40 kV and 40 mA with Cu Ka radiation (k = 1.5405 Å). Fourier transform infrared (FT-IR) spectra were recorded on a Nicolet NEXUS 670 instrument. The morphology of CHO/NH2/TiO2/Ni was characterized by scanning electron microscopy (FESEM, Hitachi S-4800). X-ray photoelectron spectroscopy (XPS) was performed on a Kratos AXIS Nova spectrometer (Shimazu Co., Ltd.) with a monochromatic Al Ka X-ray. 2.2. Preparation of electrodes The Ni-foam-supported and amine-functionalized TiO2 photocathodes are shown in Fig. 2 and described explicitly in the following paragraphs. The fabrication process for the BiVO4 electrode is available in the Supporting Information (Fig. S2). 2.2.1. Preparation of Ni-foam-supported TiO2 electrodes TiO2 sol–gel was prepared through the following processes described in previous reports [54,55]. Tetrabutoxytitanium was first diluted with ethanol and diethanolamine with ratios tetrabu toxytitanium/ethanol/diethanolamine = 1:10:1. The resulting mixture was stirred for 1 h at room temperature, and deionized water and ethanol were added with ratio deionized/ethanol = 1:1, fol-
The linear sweep voltammetry (LSV) curves and amperometric J–t curves were measured in a three-electrode system by an electrochemical workstation (CHI660E) with a scan rate of 50 mV/s. In the new PEC cells, the Ni-foam-supported electrode was engaged as the working electrode with BiVO4 as the counter electrode and a SCE as the reference electrode. The PEC reduction of CO2 was performed in the PEC cell using a KHCO3 aqueous solution (0.1 M, pH 6.8) as electrolyte under simulated sunlight irradiation with an appropriate external potential. The electrolyte containing Eosin Y (1 mM) was bubbled with CO2 or Ar for 30 min before the measurements. The light intensity of the light source was adjusted to 200 mW/cm2, calibrated by a standard solar cell. We must emphasize that the convenient photocathodes were colored and prepared in situ after the electrodes were immerged into the electrolyte during the CO2 bubbling process. 3. Results and discussion 3.1. Characterization of photocathodes The SEM image (Fig. 3a) shows that the nickel foam skeleton was covered with the TiO2 films. On the films, the TiO2 nanoparticles are uniform and dense and have relatively the same size (13–15 nm, Fig. 3b). Three diffraction peaks at 25.2°, 48.3°, and 62.7°, corresponding to (1 0 1), (2 0 0), and (2 1 3) diffraction planes of anatase (JCPDS No. 21-1272), are observed in the XRD pattern of TiO2/Ni (Fig. S1). These results demonstrate that TiO2 particles and films are anatase phases. XPS analysis of CHO/NH2/TiO2/Ni is shown in Fig. S3. The peak at 101.8 eV was assigned to the 2p state of Si. The peaks of N 1s and Ti 2p can be detected as well, revealing that amine ligands were successfully modified onto the surface of TiO2 electrode. Their FTIR spectra (Fig. S4) can also confirm the covalent linkage with the TiO2 film. There were absorption peaks at 1628.1 cm 1 (Ar-ring breath vibration, C@C), 1457.9 cm 1 (C@N stretching vibration), 1123.6 cm 1 (SiAO stretching vibration),
Fig. 2. Schematic illustration of the process of photocathode preparation.
L. Wang et al. / Journal of Catalysis 349 (2017) 1–7
3
Fig. 3. SEM images of TiO2@nickel foam.
and 2922.1 cm 1 (CAH stretching vibration), respectively. The XRD pattern of the BiVO4 photoanode is shown in Fig. S2 and is in accordance with the phase of clinobisvanite. The UV–vis absorption spectra of Ni foam, TiO2/Ni foam, NH2/ TiO2/Ni foam, and CHO/NH2/TiO2/Ni foam were obtained and are depicted in Fig. 4a. It can be seen that the absorption of CHO/ NH2/TiO2/Ni foam was stronger than that of Ni foam and TiO2/Ni foam. This result proves that the organic covalent ligand increases the absorption intensity and causes a slight red shift of the TiO2/Ni
electrode. The absorption band of the CHO/NH2/TiO2/Ni foam electrode absorption edge is enlarged to 600 nm owing to its aromatic groups. When the CHO/NH2/TiO2/Ni electrode absorbed Eosin Y dye, its absorption edge could be enlarged to the visible range of about 700 nm, indicating that the dye-sensitized photocathode has better light harvesting ability (Fig. S6). A plot of the square of absorption efficiency (a) multiplied by the photon energy ((a h m)2) versus the photoenergy shows a linear relationship (Fig. 4b), indicating that CHO/NH2/TiO2/Ni is a direct transitiontype semiconductor. The bandgap of this photocathode is estimated to be 2.56 eV by calculating the intersection of the plot line and abscissa axis. The bandgap of CHO/NH2/TiO2/Ni with dye was estimated to be 2.06 eV (Fig. S6), augmenting the outstanding efficiency of light absorption. 3.2. Photoelectrochemical properties The LSV curves of photoelectrochemical cells were obtained in the range from 0.3 to 1.1 V (vs. SCE) and are presented in Fig. 5. The experiments were conducted in KHCO3 aqueous solutions saturated with Ar or CO2 with a scan rate of 50 mV s 1 under simulated sunlight irradiation. As shown in Fig. 5, the current density of the electrochemical cell improves smoothly, along with the increase of bias voltage without light irradiation compared with the near zero current density under Ar. Under photoelectrochemical conditions, the cell gives a clear enhancement of current density from 0.5 to 1.1 V, which exhibits a successful coupling of photoelectrocatalytic water splitting and photoelectrocatalytic CO2 reduction. Therefore, the 0.6 V bias voltage is chosen as the minimum working bias voltage in the next investigations. 3.3. Photoelectrochemical reduction of CO2
Fig. 4. (a) The UV–vis spectra of different electrodes; (b) a plot analysis of the band gap of CHO/NH2/TiO2/Ni.
At first, various Ni-foam-supported electrodes were prepared and assembled with BiVO4/FTO and SCE to build a photoelectrocatalytic cell for CO2 reduction. The products in the electrolytes were monitored and their abilities for CO2 reduction were evaluated (Fig. 6a). We can see from Fig. 6a that the simple TiO2nanoparticle-covered Ni foam photocathode (TiO2/Ni) can produce methanol as a major product at a rate of 9.5 lM/L h cm2 compared with inactive bare Ni foam electrodes at 0.6 V bias voltage. To enhance the CO2 catching ability on the surface of the photocathode in situ, organic ligands containing amine groups are bridged to TiO2 nanoparticles by APTES. The resulting electrode of NH2/TiO2/Ni is more efficient for photoelectrochemical CO2 reduction than electrode TiO2/Ni. When the amine groups are converted to imine groups by a condensation reaction with salicylaldehyde, the obtained photocathode CHO/NH2/TiO2/Ni is superior to other electrodes under the same conditions, generating methanol in a rate of 153 lM/h/cm2, 15 times that of the TiO2/Ni
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Fig. 6. The rate for methanol with different electrodes at light quantum efficiency for methanol (b).
Fig. 5. Linear sweep voltammetric curves of TiO2/Ni (a), NH2/TiO2/Ni (b), and CHO/ NH2/TiO2/Ni (c).
electrode, owing to its better light harvesting and CO2 capture ability (vide supra). As a comparison, a control experiment was carried out using an electrode of CHO/TiO2/Ni that was made by the direct adsorption of salicylaldehyde onto TiO2. The lower activity (14.5 lM/h cm2) might be attributed to the lower Lewis basicity of C@O than C@N, which can catch CO2 molecules via coordination bonds.
0.6 V (a) and the rate and
Based on the above photoelectrochemical determinations, the CHO/NH2/TiO2/Ni photocathode reduction of CO2 was then investigated at various bias potentials, such as 0.6, 0.8, and 1.0 V. The major products of electrolytes in new photoelectrochemical cells are methanol and a little ethanol; the gas phase products are hydrogen and oxygen with a small quantity of carbon monoxide. These results are listed in Table 1, in which the total amount of products at more negative potential was larger than that at less negative potential. Some formate could be also detected at 0.4 V bias potential. The highest Faradaic efficiency is up to 507.9% at 0.6 V, which reflects good synergetic interaction between CO2 reduction and water splitting. The maximum formation rate of methanol was 186 lM/L h cm2 at 1.0 V. Alternatively, hydroxyl anions could be also oxidized to oxygen when their electrons transferred directly to the HOMO of dyes rather than through an external circuit (vide infra). Therefore, the Faradaic efficiencies of PEC cells might exceed 100%. If the Faradaic efficiency is estimated to be 100%, the light quantum efficiency of the cell reaches up to 1.2%, two times better than for plants. The rate of oxygen release by the photoelectrocatalytic cell is far less than that of reduction products, which may be because of the oxidation of CO to CO2. To verify that methanol derived from CO2 reduction, photoelectrochemical experiments were performed on CHO/NH2/TiO2/Ni electrodes in Ar-saturated KHCO3 solution (without CO2) at 0.6 V. It was confirmed that no liquid was undetectable after electrolysis for 4 h, confirming that methanol was derived from CO2 reduction. The stability of the CHO/NH2/TiO2/Ni electrode for photoelectrochemical reduction of CO2 was evaluated by seven successive
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L. Wang et al. / Journal of Catalysis 349 (2017) 1–7 Table 1 The results of photoelectrocatalytic CO2 reduction using CHO/NH2/TiO2/Ni as photocathode. Photocathode
CHO/NH2/TiO2/Ni
Potential/V vs. SCE
0.4 Ratea (FE)b
0.6 Rate (FE)
0.8 Rate (FE)
1.0 Rate (FE)
MeOH
78.0 (60.8)
153.4 (452.0)
162.6 (288.5)
186.5 (27.3)
EtOH
5.2 (8.1)
7.9 (46.6)
14.5 (51.4)
17.1 (5.0)
HCOOH
6.3 (1.6)
–
–
–
H2
8.0 (2.1)
9.5 (9.3)
57.9 (34.2)
68.8 (3.4)
CO
–
–
0.26 (0.15)
0.47 (0.02)
O2
–
–
0.38
0.68
TEE (%)c
72.9 –
507.9 1.2
374.3 1.3
35.7 –
Ucell a b c
Rate = lM h 1 cm 2. Faradaic efficiency (%). Total electron efficiency (TEE) = RFE.
tests of CO2 electrolysis at 0.6 V (each test lasted 4 h). As shown in Fig. S7, the production rates of methanol remain around 153 lM/h/cm2 for 28 h. Meanwhile, the XRD patterns (Fig. S8) of fresh and used photocathodes of CHO/NH2/TiO2/Ni are the same, which indicates no passivation occurring on the surface of the CHO/NH2/TiO2/Ni electrode. These results verify the good stability and activity of the CHO/NH2/TiO2/Ni electrode for photoelectrochemical reduction of CO2. To study the absolute energy of the electrode at the redox level, its band gap structure was detected by a Mott–Schottky (MS) relationship, which can be applied to estimate the potential of the CB edges of NH2/TiO2/Ni and CHO/NH2/TiO2/Ni. Fig. 7a shows the MS plots of CHO/NH2/TiO2/Ni and NH2/TiO2/Ni, which were measured at 0.5 kHz in the dark. The positive slopes of the linear MS plots were obtained, indicating that the electrodes behave as n-type semiconductors. Therefore, the conduction band can be determined as an x-axis point of intersection. The derived conduction band potential of NH2/TiO2/Ni in aqueous solution is 0.42 V. Meanwhile, the derived CB of CHO/NH2/TiO2/Ni in aqueous solution is 0.73 V. These results show that the imine molecules make the conduction band of the electrode move to a more positive position, so that it has enough energy to reduce CO2. Moreover, according to these data combined with the band gap values obtained from the UV–vis diffuse reflectance spectra, the valence band (VB) position of CHO/NH2/TiO2/Ni can be calculated as 1.83 V. Fig. 7b shows a band potential diagram for NH2/TiO2/Ni and CHO/NH2/TiO2/Ni together with the thermodynamic potentials for CO2 reduction to methanol. This result shows that the CB potential of n-type CHO/NH2/TiO2/Ni was high enough for CO2 reduction to methanol. Furthermore, the electronic structures of the Eosin Y and imine ligand and their geometries and energies were fully computerized using the hybrid DFT method at the level of B3LYP/6-31G(d)/ LANL2DZ [56,57]. Their molecular orbital diagrams of HOMOs and LUMOs are exhibited in Fig. S9. The conducting band of TiO2 is located between the LUMO and HOMO of dye and organic molecules (Fig. S10). These results confirm that the propulsion of electron injection from LUMOs to the conducting band of TiO2 is quite sufficient. The electrochemical impedance spectra (EIS) of photocathodes were measured by the electronic transmission performance. It can be seen from Fig. 8 that the EIS of the electrode
Fig. 7. (a) Mott–Schottky plots of NH2/TiO2/Ni (red) and CHO/NH2/TiO2/Ni (black). (b) A schematic diagram of the band gap of TiO2, NH2/TiO2/Ni, and CHO/NH2/TiO2/Ni.
CHO/NH2/TiO2/Ni was much smaller than that of the electrode TiO2/Ni. This proved that the introduction of the covalent ligand decreased the resistance of the substrate and further improved
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Fig. 8. The electrochemical impedance spectra of TiO2/Ni and CHO/NH2/TiO2/Ni.
In addition, the small amount of labeling ethanol can also be monitored by GC–MS (Fig. S17). To reveal the transportation of electrons in the photoelectrocatalytic cell clearly, a proposed mechanism is drawn in Fig. 9. The electrons in the ground states (HOMO) of dye molecules harvest photons to jump into the excited states (LUMO); then photoelectrons inject into the conduction band (CB) of anatase-TiO2, transfer to nickel, and are quickly caught by protons adsorbed onto the nickel foam, producing hydrogen atoms. There are two paths for oxidation of water: the electrons of OH transfer to the BiVO4 electrode (path a) accompanied by oxygen release, and then flow in the external circuit. After arriving to the valence band (VB) of TiO2, the external electrons fill into the HOMO of dye molecules under the action of voltage, completing the recovery of dye molecules. When the electrons of OH transfer directly to the HOMO of dyes (path b), the dye molecules are regenerated and can participate in another photocatalytic CO2 reduction process. In this process, oxygen might be formed without BiVO4 participation. With this understanding, Faradaic efficiency could be greatly increased under the conditions of electron mobility in path b a. 4. Conclusions In summary, new photoelectrocatalytic cells were designed and fabricated, in which the photocathodes were made from a Ni foam covered with a semiconductor of anatase TiO2 film that was functionalized by organic ligands. The photocathode of CHO/NH2/Ni with imine ligands shows more activity than that of NH2/TiO2/Ni owing to its better light absorption and electronic transportation. The activity of the electrode NH2/TiO2/Ni with amine ligands is higher than that of TiO2/Ni because of its better adsorption of CO2. The highest formation rate of methanol in the photoelectrocatalytic cell of CHO/NH2/TiO2/Ni can reach 1461 lM after 4 h. Its light quantum efficiency (Ucell = 1.2%) is superior to that of plants (U 0.5%). Our EIS experiments proved that the introduction of the covalent ligands could decrease the resistance of electrodes, which should further improve their electron transfer ability. Therefore, new Ni-foam-supported and amine-functionalized TiO2 photocathodes offer many advantages, such as nontoxicity, better conductivity, economy, and facile approach. Acknowledgments This study was funded by the National Natural Science Foundation of China (NSFC 21173106, 21401091), the Fundamental Research Funds for the Central Universities (lzujbky-2016-K09), and the Foundation of the State Key Laboratory of Coal Conversion (Grant No. J16-17-913).
Fig. 9. The mechanism of the photoelectrocatalytic process of CO2 reduction.
Appendix A. Supplementary material the electron transfer ability of the electrode, which could transfer more electrons for CO2 reduction.
3.4.
13
CO2 labeling experiments
To examine the carbon source of the product CH3OH, an isotopic labeling experiment of 13CO2 was carried out in the PEC cell using CHO/NH2/TiO2/Ni as the photocathode. After the whole system was purged with Ar gas, 13CO2 gas was introduced into the well-sealed PEC device for 15 min. The doublet peaks at 3.03 and 3.41 ppm are attributed to the coupling of protons with the 13C nucleus of 13 CH3OH (Fig. S11). Meanwhile, 13CO (m/z = 29) was detected by the GC–MS technique (Fig. S16). These results could verify that the carbon atoms of CH3OH and CO are indeed derived from CO2.
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