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Trash to treasure: Use flue gas SO2 to produce H2 via a photoelectrochemical process
Accepted Manuscript Short communication Trash to Treasure: Use Flue Gas SO2 to Produce H2 via a Photoelectrochemical Process Jin Han, Hanyun Cheng, Liwu Zhang, Hongbo Fu, Jianmin Chen PII: DOI: Reference:
S1385-8947(17)31831-4 https://doi.org/10.1016/j.cej.2017.10.116 CEJ 17900
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
22 June 2017 23 September 2017 18 October 2017
Please cite this article as: J. Han, H. Cheng, L. Zhang, H. Fu, J. Chen, Trash to Treasure: Use Flue Gas SO2 to Produce H2 via a Photoelectrochemical Process, Chemical Engineering Journal (2017), doi: https://doi.org/10.1016/ j.cej.2017.10.116
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Trash to Treasure: Use Flue Gas SO2 to Produce H2 via a Photoelectrochemical Process Jin Han, Hanyun Cheng, Liwu Zhang*, Hongbo Fu, Jianmin Chen Shanghai Key Laboratory of Atmospheric Particle Pollution and Prevention, Department of Environmental Science and Engineering, Fudan University, Shanghai 200433, China
Abstract: Sulfur dioxide (SO2), emitting from power plant and factories, has been one of the major atmospheric pollutants and causes lots of harmful environmental issues. Although some mature desulfurization technologies are available, a more economic or even profitable approach for flue gas desulfurization is greatly desirable. Herein, we report SO2 can be efficiently recycled to produce H2 via a photoelectrochemical (PEC) water splitting process based on Mo-doped BiVO4 inverse opals. Via this process, the SO2 removal rate is higher than 97%, the absorbed SO2 can act as a sacrifice reagent to produce H2 in the PEC water splitting. With the design of the highly ordered porous structure, the generation rate of H 2 is improved by tens of times. Via the proposed process, the SO2 released (0.03 kg) from coal power plant for a generation of 1 kW h electricity can be recycled to produce around 10 L H2. By combining with ammonia-based desulfurization technology, the production of H2 can greatly reduce the cost of ammonia-based desulfurization technology and make it profitable.
Keywords: Desulfurization; H2; Flue gas; PEC; BiVO4
1. Introduction Sulfur dioxide (SO2), emitting from power plant and factories, has been one of the major atmospheric pollutants. It could form sulfate aerosol in atmosphere, and lead to acid deposition. Thus, it can lead to a wide range of harmful issues from human health, agriculture, global climate to traffic, such as acid rain, haze, PM2.5 and low visibility.1-6 Accordingly, extensive researches have been studied and applied towards SO2 removal, including adsorption7-14 and reduction methods15-17. Among these strategies, wet flue gas desulfurization (WFGD) has been one of the state-of-the-art technologies for SO2 removal with high 1 / 16
efficiency and simple equipment by solution absorbing.3,
4, 12, 15-18
Though the WFGD
technology has been a mature technology, but its cost is still high and the energy of SO32- is wasted due to the ubiquitous aeration process for an oxidation of SO32- to SO42- before disposal. The cost may be reduced if the wasted energy can be efficiently recycled. As an abundant and renewable energy source, solar energy has been broadly studied for hydrogen generation using illuminated semiconductor electrodes by photoelectrolysis of water.19-26 Nevertheless, the sluggish anodic half reaction, the O2 evolution reaction (OER) greatly limits the generation rate of H2.27-29 It has been certified that the oxidation of SO32- is kinetically and thermodynamically much easier than that of water, thus it will be a realizable route to achieve solar-to-H2 energy with the SO2 removal via PEC process. Huang et al.13 reported a direct photolytic oxidation of Na2SO3 for hydrogen generation under ultraviolet (UV) illumination. But the main factor which restrains its application is Na2SO3 can only be activated by UV light, the hydrogen generation rate is thus low. Herein, we propose to combine the SO2 desulfurization process with the PEC H2 production process. Bismuth vanadate (BiVO4) with a narrow band gap of 2.4 – 2.5 eV (496 – 517 nm) is chosen as the photoanode, which possesses a broad response in visible light and has been considered as one of the most promising candidates for PEC water oxidation.20, 21, 30-32
Although in our previous work we have shown that with BiVO4 as photoanode SO2 can
be used to produce H2, however, the photocurrent is < 0.2 mA cm-2 due to the inefficient electrode structure.33 The efficiency needs to be greatly improved to make this process economically viable. Photonic crystals has been verified a great potential in manipulating light based on photonic band structure, by enhancing the interaction of light with a semiconductor through near-bandgap resonant scattering and slow photo effects. Besides, the opal structure can also reduce the amount of material required due to the highly porous structure. In this work, we report a novel method for SO2 removal with a simultaneously enhanced H2 generation via a PEC process by inverse opal Bi(Mo)VO4 (donated as IO-Bi(Mo)VO4). We show that SO2 can be efficiently recycled to produce H2 based on the Mo-doped BiVO4 inverse opals as photoanodes.
2. Materials and Methods 2.1 Synthesis. The photoelectrode film was synthesized following a published method.20 And 2 / 16
the details of the process are described in Supporting Information. 2.2 Characterization. The microstructure of as-prepared electrode was carried out with scanning electron microscopy (SEM, LEO GEMINI 1530VP FEG-SEM). The crystal structure of the as-prepared electrode was recorded with X-ray diffraction (XRD) patterns by a Bruker using Cu Kα radiation. UV-Vis spectrum of the as-prepared electrode was performed by a UV-Vis spectrophotometer (HP 8453 UV-Visible Spectrophotometer) with an integrating sphere attachment. 2.3 PEC tests. The PEC measurements were performed on a potentiostat (IVIUMSTAT potentiostat/galvanostat). For the PEC tests, a standard three electrode configuration was employed, with synthesized film as the working electrode, Pt as the counter electrode and Ag/AgCl as the reference electrode. The electrolyte solutions of the PEC experiments were prepared by the reaction of SO2 with NaOH solution. SO2 (500 ppm) was successively bubbled into the NaOH solutions with a flow rate of 50 ml min-1. Then the SO2 reacts with NaOH solution to form Na2SO3 (0.1 M). The concentration of SO2 is monitored by Thermo ScientificTM 43i HL. For comparison, the 0.1 M NaOH solution was also prepared. The PEC characterization was measured under illumination using a 300 W Xe lamp solar simulator with AM 1.5 G filter (100 mW cm-2) from the back side of the film (FTO side). The concentration of H2 was analyzed by gas chromatography (SHIMADZU GC-2014, TCD detector).
3. Results and Discussion The preparation of photonic nanostructured Bi(Mo)VO4 electrode is illustrated in Fig. 1a. A colloidal crystal template of polystyrene beads was first formed on top of the FTO glass by evaporation-induced self-assembly. Dip-coating was employed to infiltrate the Bi(Mo)VO4 precursor into the voids of the colloidal crystal template. After annealing in air at 500 oC for several hours, the template was removed while simultaneously crystallizing the IO-Bi(Mo)VO4 framework. A planar Bi(Mo)VO4 with similar thickness but without the inverse opal structure was also prepared for comparison under identical conditions. Typical SEM images of the IO-Bi(Mo)VO4 films produced from colloidal crystal templates of polystyrene spheres (PS) with 320 nm diameter are shown in Fig. 1b and b’. Fig. 1b is a SEM image of IO-Bi(Mo)VO4 with lower magnification, showing large area of the inverse opal 3 / 16
structure. In Fig. 1b’, the periodically ordered and full macropore interconnectivity can be clearly observed. This continuous porous structure allows unhindered electrolyte transfer throughout the entire photoanode. The XRD patterns of the electrode films (Fig. S1a) displayed the monoclinic scheelite structure (JCPDS NO. 14-0688). Besides, since there isn’t any secondary phase peaks in IO-Bi(Mo)VO4 and Bi(Mo)VO4, which suggest Mo is substitutionally incorporated in BiVO4.20 Though XRD results provide some trace that Mo have been doped in BiVO4. However, it is too rough to identify the doping sites because of the low doping concentration. To get more doping information (doping sites in the crystal lattice) of Mo, the Raman spectra were measured and the results are shown in Fig. S1b. The Raman mode located at 829 cm-1 is assigned to the symmetric stretching mode of VO43- units. The symmetric stretching mode in IO-Bi(Mo)VO4 shifts to a lower wave number because Mo6+ (95.9 g mol-1) is heavier than V5+ (50.9 g mol−1), which suggests Mo6+ substitutes V5+ in the VO43- tetrahedron.
The intense absorption in visible light range and small band gap of IO-Bi(Mo)VO4 film indicate that it could efficiently utilize the solar light (Fig. S2 and S3). As shown in Fig. S2, IO-Bi(Mo)VO4 film have intense light absorption from 220 to 800 nm. Besides, the IO-Bi(Mo)VO4 film exhibits much stronger light extinction than that of Bi(Mo)VO4 and BiVO4 film around 560 nm, which is attributed to the photonic band gap due to its special inverse opal structure. Fig. 2a displays the linear sweep voltammetry (LSV) curves of Bi(Mo)VO4 and IO-Bi(Mo)VO4 tested in different conditions under AM 1.5 G irradiation. It can be seen that all the films show photocurrent density increase with increasing applied potential under AM 1.5 G irradiation, whereas the currents are negligible in the dark. For IO-Bi(Mo)VO4, it exhibits a higher photocurrent density response with SO2 removal than that without SO2 removal when under irradiation, which indicates SO2 removal could enhance the photocurrent intensity (Fig. 2a). Chopped LSV curves of Bi(Mo)VO4 and IO-Bi(Mo)VO4 in different conditions under chopped illumination (Fig. 2b) were also studied and show consistent results with the LSV results (Fig. 2a). As compared with pristine BiVO4, Mo doping can significantly improve the photocurrent intensity due to the enhanced charge transfer (shown in Fig. S4). IO-Bi(Mo)VO4 displayed the unique inverse opal microstructure and special 4 / 16
optical property due to the photonic band gap, maximum photocurrent response during SO2 removal. For IO-Bi(Mo)VO4, with SO2 removal a photocurrent density of 3.42 mA cm-2 is achieved at 0.6 V vs. Ag/AgCl, which is almost 4 times higher than that without SO2. Compared with planar Bi(Mo)VO4, The inverse opal structure can improve the photocurrent density by a factor of 2.5. The amperometric i-t curves studied at 0.4 V vs. Ag/AgCl suggest a similar result (Fig. S5). This enhancement is attributed to the ordered porous structure of Bi(Mo)VO4. Besides, the enhanced light harvesting efficiency, the large exposed area to the electrolyte and the significantly reduced distance for photogenerated holes to reach the interface of Bi(Mo)VO4 and the electrolyte, are advantages of the inverse opal structure that will be responsible for its enhanced photoelectrochemical efficiency. Fig. 2c shows the Electrochemical Impedance Spectroscopy (EIS) Nyquist plots of IO-Bi(Mo)VO4 film under AM 1.5 G irradiation with/without SO2 removal. It is clearly observed that the diameter of the arc radius in the SO2 removal is smaller than that in the absence of SO2, indicating the existence of SO32- could efficient increase charge separation and transport at the electrode film and electrolyte solution interface. Schematic illustration of H2 generation with SO2 removal by IO-Bi(Mo)VO4 are depicted in Fig. 2d (a schematic illustration without SO2 for comparison is provided in Fig. S6). Generally, a semiconductor (IO-Bi(Mo)VO4 as an example) can be excited by light irradiation and the excited e− jumps to the conduction band (CB), which creates a corresponding hole (h+) in the valence band (VB). With the effect of external bias, the e− migrates to the cathode electrode (Pt wire) where it can reduce water to produce H2, while the h+ participates in the oxidation reaction on the anode. With the absence of SO2 (only NaOH(aq) as electrolyte solution), the half reaction on the photoanode is (Fig. S6): 4 OH−→ O2(g) + 2 H2O + 4 e−
1.23 V vs. RHE
(Eq. 1)
Whereas inletting SO2(g) in the electrolyte solution, SO32− is formed, the half reaction on the photoanode is (Fig. 2d): SO32− + 2 OH−→ SO42− + H2O + 2 e−
0.92 V vs. RHE
(Eq. 2)
In the first case, the oxidation of OH− requires 4 h+ and a higher activation energy (1.23 V vs. RHE). Since SO32− is much easier to be oxidized (only need 2 e− and lower activation energy) than OH−, thus the created h+ will preferentially participate in the oxidation of SO32− rather 5 / 16
than oxidation of OH− while SO2(g) is inletted into the electrolyte system. The essential reactions during this SO2 PEC process are thus summarized as following: SO2 flow in the electrolyte solution (NaOH(aq)): SO2(g) + 2OH−→ SO32− + H2O
(Eq. 3)
Semiconductor under AM 1.5 G irradiation: Semiconductor catalyst (BiVO4, Bi(Mo)VO4, IO-Bi(Mo)VO4)
h+ + e−
(Eq. 4)
Reaction on photoanode (electrode film): SO32− + 2 OH−→ SO42− + H2O + 2 e−
(Eq. 5)
Reaction on cathode (Pt wire): 2 H2O + 2 e− → H2(g)↑ + 2 OH−
(Eq. 6)
The total reaction: SO2(g) + 2 OH−
H2(g)↑ + SO42−
(Eq. 7)
The amounts of evolved H2 were further measured at a bias of 0.6 V (vs. Ag/AgCl) under AM 1.5 G irradiation. The SO2 removal rates, photocurrent densities, theoretical evolution rates of H2, experimental evolution rates of H2 and Faradaic efficiencies on different electrode films with/without SO2 removal are summarized in Table 1. The calculation details of theoretical evolution rate of H2 and Faradaic efficiency are described in Supporting Information. As shown in Table 1, for all the electrode films, the current density and H2 evolution rate have been distinctly improved with SO2 removal, which are consistent with the LSV results (Fig. 2a and Fig. S4). The proposed PEC process shows a high SO2 removal rate of > 97%. Furthermore, the Faradaic efficiencies for H2 production are higher than 94 % (Table 1), indicating nearly all the generated electrons in the circuit are consumed by H2 production. With SO2 removal, the H2 evolution rate is significantly enhanced by more than 4 times on IO-Bi(Mo)VO4 than that without SO2. In addition, the IO-Bi(Mo)VO4 with SO2 removal provides much higher amount of H2 generation (60.26 μmol h-1 cm-2) than that of Bi(Mo)VO4 (22.51 μmol h-1 cm-2) and BiVO4 (1.08 μmol h-1 cm-2), which mainly owning to the improved light harvesting in this special photonic crystal structure. Moreover, due to the highly
porous
structure
the
distance
from
the
photogenerated
holes
to
the
semiconductor-electrolyte interface is greatly shortened than the bulk film. The inverse opal 6 / 16
structure significantly enhanced the generation rate of H2 (0.6 V vs. Ag/AgCl) by more than 30 times compared with our previous work.33 The great leap in activity may make this approach become practically feasible and the desulfurization process be profitable. To study the influence of CO2, the electrolyte is changed from NaOH to Na2CO3 or NaHCO3, interestingly, no significant change is observed in SO2 absorption (~ 97%) and H2 production (~ 60μmol h-1 cm-2). This is attributed to that SO2 can quickly react with Na2CO3 or NaHCO3 to form Na2SO3 ( pKa1 of H2CO3 is 6.38, while pKa1 of H2SO3 is 1.90). The existence of CO2 in the flue gas can hardly influence the SO2 absorption and Na2SO3 formation. Although solar light is not controllable resource, however the produced SO32- during desulfurization is storable, which can be converted to H2 via the PEC process when the solar light is available.
In this work, as the energy of SO2 is used to produce high-value-added product H2, which can greatly reduce the cost of desulfurization process. Furthermore, the energy input in this PEC process is mainly from solar light. The small external bias can be provided from a solar cell or by replacing the Pt cathode with a p-type semiconductor as a photocathode. In practical application, the sodium hydroxide is expensive and the product of sodium sulfate is of low value. Instead, the proposed approach can readily be combined with the existing ammonia-based desulfurization technology. The (NH4)2SO3 produced in this technology can be used to produce H2, replacing the current aeration treatment, which leads to a waste of the energy of SO32-. Via the proposed process, the SO2 released (0.03 kg) from coal power plant for a generation of 1 kW h electricity can be recycled to produce around 10 L H2 based on the total reaction (Eq. 7). Via the existing ammonia-based desulfurization technology, the cost of dealing with SO2 in flue gas for producing 1 MWh electricity is around 5 USD. If the SO32produced in this desulfurization technology are recycled to produce H2, then 104 H2 can be produced, which has a value of 6.5 USD. It is recently reported that Mo-doped BiVO4 can stably operate for at least 1100 h,34 the raw material cost of this PEC process is thus calculated to be around 0.3 USD/MWh (see Table S1 in Supporting Information for calculation details), so the total cost for dealing with flue gas from 1 MWh electricity is: 5 – 6.5 + 0.3 = -1.2 USD. That means ammonia-based desulfurization technology can become profitable when it is combined with the proposed PEC process, a profit of 1.2 USD can be made by dealing with 1 MWh electricity. In China, 6 million MWh electricity were produced 7 / 16
from coal burning power plant in 2016, potentially a profit of 7.2 billion USD/year can be made through this PEC process. The instable solar energy may lead to additional cost, however, the stability of BiVO4 and efficiency of the PEC process can be further improved in the future research, a greatly reduced operation cost of PEC technology and higher benefit can thus be expected.
Associated content Support Information The supporting information is available free of charge. Cost calculation details. The synthesis process and related calculation details of theoretical evolution rate of H2 and Faradaic efficiency; XRD patterns, UV-Vis absorption spectra and Tauc plots of electrode films; chopped J-V curves of Bi(Mo)VO4 and BiVO4 with/without SO2 removal; amperometric i-t curves under chopped illumination at applied potentials; mechanism of H2 generation by IO-Bi(Mo)VO4 during PEC process without removal of SO2.
Author Information Corresponding Author E-mail: [email protected] Phone/Fax: +86-21-6564-2781
Notes The authors declare no competing financial interest.
Acknowledgements The authors gratefully acknowledge financial support from National Natural Science Foundation of China (No. 21507011 and No. 21677037), and Ministry of Science and Technology of the People’s Republic of China (2016YFE0112200 and 2016YFC0203700).
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Fig. 1. (a) Illustration of synthetic strategy of IO-Bi(Mo)VO4, low-magnification (b) and highmagnification (b’) SEM images of IO-Bi(Mo)VO4.
13 / 16
Fig. 2. (a) Linear Sweep Voltammetry (LSV) curves of Bi(Mo)VO4 and IO-Bi(Mo)VO4 tested in different conditions with/without AM 1.5 G irradiation (100 mW/cm-2); (b) Chopped LSV curves of Bi(Mo)VO4 and IO-Bi(Mo)VO4 in different conditions under chopped illumination; (c) Electrochemical Impedance Spectroscopy (EIS) Nyquist plots of IO-Bi(Mo)VO4 with/without SO2 removal under illumination at 0.7 V (vs. Ag/AgCl); (d) Mechanism of H2 generation by IO-Bi(Mo)VO4 during PEC process with removal of SO2.
14 / 16
Table 1. The current density, theoretical evolution rate and experimental evolution rate of H2, Faradaic efficiency in different electrolyte solutions with different catalysts at 0.6 V (vs. Ag/AgCl) and SO2 absorbing efficiency. BiVO4
Current Density/mA cm-2
Bi(Mo)VO4
IO-Bi(Mo)VO4
NaOH(aq)
NaOH(aq)+SO2(g)
NaOH(aq)
NaOH(aq)+SO2(g)
NaOH(aq)
NaOH(aq)+SO2(g)
0.007
0.06
0.32
1.26
0.83
3.42
0.13
1.1
6.02
23.7
15.6
64.11
0.126
1.08
5.84
22.51
14.82
60.26
97
98
97
95
95
94
/
~97
/
~98
/
~97
Theoretical evolution rate of H2/μmol h-1 cm-2 Experimental evolution rate of H2/μmol h-1 cm-2 Faradaic efficiency/% SO2 absorbing efficiency/%
Mo-doped BiVO4 inverse opal photoanode was fabricated by self-assembly method. Flue gas SO2 can greatly improve the H2 production via PEC water splitting The inverse opal structure shows higher photoactivity due to its special structure SO2 removal with a simultaneous H2 production is realized
15 / 16
H2 Production with Simultaneous SO2 Removal on Photonic Crystals.
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S1385-8947(17)31831-4 https://doi.org/10.1016/j.cej.2017.10.116 CEJ 17900
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
22 June 2017 23 September 2017 18 October 2017
Please cite this article as: J. Han, H. Cheng, L. Zhang, H. Fu, J. Chen, Trash to Treasure: Use Flue Gas SO2 to Produce H2 via a Photoelectrochemical Process, Chemical Engineering Journal (2017), doi: https://doi.org/10.1016/ j.cej.2017.10.116
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Trash to Treasure: Use Flue Gas SO2 to Produce H2 via a Photoelectrochemical Process Jin Han, Hanyun Cheng, Liwu Zhang*, Hongbo Fu, Jianmin Chen Shanghai Key Laboratory of Atmospheric Particle Pollution and Prevention, Department of Environmental Science and Engineering, Fudan University, Shanghai 200433, China
Abstract: Sulfur dioxide (SO2), emitting from power plant and factories, has been one of the major atmospheric pollutants and causes lots of harmful environmental issues. Although some mature desulfurization technologies are available, a more economic or even profitable approach for flue gas desulfurization is greatly desirable. Herein, we report SO2 can be efficiently recycled to produce H2 via a photoelectrochemical (PEC) water splitting process based on Mo-doped BiVO4 inverse opals. Via this process, the SO2 removal rate is higher than 97%, the absorbed SO2 can act as a sacrifice reagent to produce H2 in the PEC water splitting. With the design of the highly ordered porous structure, the generation rate of H 2 is improved by tens of times. Via the proposed process, the SO2 released (0.03 kg) from coal power plant for a generation of 1 kW h electricity can be recycled to produce around 10 L H2. By combining with ammonia-based desulfurization technology, the production of H2 can greatly reduce the cost of ammonia-based desulfurization technology and make it profitable.
Keywords: Desulfurization; H2; Flue gas; PEC; BiVO4
1. Introduction Sulfur dioxide (SO2), emitting from power plant and factories, has been one of the major atmospheric pollutants. It could form sulfate aerosol in atmosphere, and lead to acid deposition. Thus, it can lead to a wide range of harmful issues from human health, agriculture, global climate to traffic, such as acid rain, haze, PM2.5 and low visibility.1-6 Accordingly, extensive researches have been studied and applied towards SO2 removal, including adsorption7-14 and reduction methods15-17. Among these strategies, wet flue gas desulfurization (WFGD) has been one of the state-of-the-art technologies for SO2 removal with high 1 / 16
efficiency and simple equipment by solution absorbing.3,
4, 12, 15-18
Though the WFGD
technology has been a mature technology, but its cost is still high and the energy of SO32- is wasted due to the ubiquitous aeration process for an oxidation of SO32- to SO42- before disposal. The cost may be reduced if the wasted energy can be efficiently recycled. As an abundant and renewable energy source, solar energy has been broadly studied for hydrogen generation using illuminated semiconductor electrodes by photoelectrolysis of water.19-26 Nevertheless, the sluggish anodic half reaction, the O2 evolution reaction (OER) greatly limits the generation rate of H2.27-29 It has been certified that the oxidation of SO32- is kinetically and thermodynamically much easier than that of water, thus it will be a realizable route to achieve solar-to-H2 energy with the SO2 removal via PEC process. Huang et al.13 reported a direct photolytic oxidation of Na2SO3 for hydrogen generation under ultraviolet (UV) illumination. But the main factor which restrains its application is Na2SO3 can only be activated by UV light, the hydrogen generation rate is thus low. Herein, we propose to combine the SO2 desulfurization process with the PEC H2 production process. Bismuth vanadate (BiVO4) with a narrow band gap of 2.4 – 2.5 eV (496 – 517 nm) is chosen as the photoanode, which possesses a broad response in visible light and has been considered as one of the most promising candidates for PEC water oxidation.20, 21, 30-32
Although in our previous work we have shown that with BiVO4 as photoanode SO2 can
be used to produce H2, however, the photocurrent is < 0.2 mA cm-2 due to the inefficient electrode structure.33 The efficiency needs to be greatly improved to make this process economically viable. Photonic crystals has been verified a great potential in manipulating light based on photonic band structure, by enhancing the interaction of light with a semiconductor through near-bandgap resonant scattering and slow photo effects. Besides, the opal structure can also reduce the amount of material required due to the highly porous structure. In this work, we report a novel method for SO2 removal with a simultaneously enhanced H2 generation via a PEC process by inverse opal Bi(Mo)VO4 (donated as IO-Bi(Mo)VO4). We show that SO2 can be efficiently recycled to produce H2 based on the Mo-doped BiVO4 inverse opals as photoanodes.
2. Materials and Methods 2.1 Synthesis. The photoelectrode film was synthesized following a published method.20 And 2 / 16
the details of the process are described in Supporting Information. 2.2 Characterization. The microstructure of as-prepared electrode was carried out with scanning electron microscopy (SEM, LEO GEMINI 1530VP FEG-SEM). The crystal structure of the as-prepared electrode was recorded with X-ray diffraction (XRD) patterns by a Bruker using Cu Kα radiation. UV-Vis spectrum of the as-prepared electrode was performed by a UV-Vis spectrophotometer (HP 8453 UV-Visible Spectrophotometer) with an integrating sphere attachment. 2.3 PEC tests. The PEC measurements were performed on a potentiostat (IVIUMSTAT potentiostat/galvanostat). For the PEC tests, a standard three electrode configuration was employed, with synthesized film as the working electrode, Pt as the counter electrode and Ag/AgCl as the reference electrode. The electrolyte solutions of the PEC experiments were prepared by the reaction of SO2 with NaOH solution. SO2 (500 ppm) was successively bubbled into the NaOH solutions with a flow rate of 50 ml min-1. Then the SO2 reacts with NaOH solution to form Na2SO3 (0.1 M). The concentration of SO2 is monitored by Thermo ScientificTM 43i HL. For comparison, the 0.1 M NaOH solution was also prepared. The PEC characterization was measured under illumination using a 300 W Xe lamp solar simulator with AM 1.5 G filter (100 mW cm-2) from the back side of the film (FTO side). The concentration of H2 was analyzed by gas chromatography (SHIMADZU GC-2014, TCD detector).
3. Results and Discussion The preparation of photonic nanostructured Bi(Mo)VO4 electrode is illustrated in Fig. 1a. A colloidal crystal template of polystyrene beads was first formed on top of the FTO glass by evaporation-induced self-assembly. Dip-coating was employed to infiltrate the Bi(Mo)VO4 precursor into the voids of the colloidal crystal template. After annealing in air at 500 oC for several hours, the template was removed while simultaneously crystallizing the IO-Bi(Mo)VO4 framework. A planar Bi(Mo)VO4 with similar thickness but without the inverse opal structure was also prepared for comparison under identical conditions. Typical SEM images of the IO-Bi(Mo)VO4 films produced from colloidal crystal templates of polystyrene spheres (PS) with 320 nm diameter are shown in Fig. 1b and b’. Fig. 1b is a SEM image of IO-Bi(Mo)VO4 with lower magnification, showing large area of the inverse opal 3 / 16
structure. In Fig. 1b’, the periodically ordered and full macropore interconnectivity can be clearly observed. This continuous porous structure allows unhindered electrolyte transfer throughout the entire photoanode. The XRD patterns of the electrode films (Fig. S1a) displayed the monoclinic scheelite structure (JCPDS NO. 14-0688). Besides, since there isn’t any secondary phase peaks in IO-Bi(Mo)VO4 and Bi(Mo)VO4, which suggest Mo is substitutionally incorporated in BiVO4.20 Though XRD results provide some trace that Mo have been doped in BiVO4. However, it is too rough to identify the doping sites because of the low doping concentration. To get more doping information (doping sites in the crystal lattice) of Mo, the Raman spectra were measured and the results are shown in Fig. S1b. The Raman mode located at 829 cm-1 is assigned to the symmetric stretching mode of VO43- units. The symmetric stretching mode in IO-Bi(Mo)VO4 shifts to a lower wave number because Mo6+ (95.9 g mol-1) is heavier than V5+ (50.9 g mol−1), which suggests Mo6+ substitutes V5+ in the VO43- tetrahedron.
The intense absorption in visible light range and small band gap of IO-Bi(Mo)VO4 film indicate that it could efficiently utilize the solar light (Fig. S2 and S3). As shown in Fig. S2, IO-Bi(Mo)VO4 film have intense light absorption from 220 to 800 nm. Besides, the IO-Bi(Mo)VO4 film exhibits much stronger light extinction than that of Bi(Mo)VO4 and BiVO4 film around 560 nm, which is attributed to the photonic band gap due to its special inverse opal structure. Fig. 2a displays the linear sweep voltammetry (LSV) curves of Bi(Mo)VO4 and IO-Bi(Mo)VO4 tested in different conditions under AM 1.5 G irradiation. It can be seen that all the films show photocurrent density increase with increasing applied potential under AM 1.5 G irradiation, whereas the currents are negligible in the dark. For IO-Bi(Mo)VO4, it exhibits a higher photocurrent density response with SO2 removal than that without SO2 removal when under irradiation, which indicates SO2 removal could enhance the photocurrent intensity (Fig. 2a). Chopped LSV curves of Bi(Mo)VO4 and IO-Bi(Mo)VO4 in different conditions under chopped illumination (Fig. 2b) were also studied and show consistent results with the LSV results (Fig. 2a). As compared with pristine BiVO4, Mo doping can significantly improve the photocurrent intensity due to the enhanced charge transfer (shown in Fig. S4). IO-Bi(Mo)VO4 displayed the unique inverse opal microstructure and special 4 / 16
optical property due to the photonic band gap, maximum photocurrent response during SO2 removal. For IO-Bi(Mo)VO4, with SO2 removal a photocurrent density of 3.42 mA cm-2 is achieved at 0.6 V vs. Ag/AgCl, which is almost 4 times higher than that without SO2. Compared with planar Bi(Mo)VO4, The inverse opal structure can improve the photocurrent density by a factor of 2.5. The amperometric i-t curves studied at 0.4 V vs. Ag/AgCl suggest a similar result (Fig. S5). This enhancement is attributed to the ordered porous structure of Bi(Mo)VO4. Besides, the enhanced light harvesting efficiency, the large exposed area to the electrolyte and the significantly reduced distance for photogenerated holes to reach the interface of Bi(Mo)VO4 and the electrolyte, are advantages of the inverse opal structure that will be responsible for its enhanced photoelectrochemical efficiency. Fig. 2c shows the Electrochemical Impedance Spectroscopy (EIS) Nyquist plots of IO-Bi(Mo)VO4 film under AM 1.5 G irradiation with/without SO2 removal. It is clearly observed that the diameter of the arc radius in the SO2 removal is smaller than that in the absence of SO2, indicating the existence of SO32- could efficient increase charge separation and transport at the electrode film and electrolyte solution interface. Schematic illustration of H2 generation with SO2 removal by IO-Bi(Mo)VO4 are depicted in Fig. 2d (a schematic illustration without SO2 for comparison is provided in Fig. S6). Generally, a semiconductor (IO-Bi(Mo)VO4 as an example) can be excited by light irradiation and the excited e− jumps to the conduction band (CB), which creates a corresponding hole (h+) in the valence band (VB). With the effect of external bias, the e− migrates to the cathode electrode (Pt wire) where it can reduce water to produce H2, while the h+ participates in the oxidation reaction on the anode. With the absence of SO2 (only NaOH(aq) as electrolyte solution), the half reaction on the photoanode is (Fig. S6): 4 OH−→ O2(g) + 2 H2O + 4 e−
1.23 V vs. RHE
(Eq. 1)
Whereas inletting SO2(g) in the electrolyte solution, SO32− is formed, the half reaction on the photoanode is (Fig. 2d): SO32− + 2 OH−→ SO42− + H2O + 2 e−
0.92 V vs. RHE
(Eq. 2)
In the first case, the oxidation of OH− requires 4 h+ and a higher activation energy (1.23 V vs. RHE). Since SO32− is much easier to be oxidized (only need 2 e− and lower activation energy) than OH−, thus the created h+ will preferentially participate in the oxidation of SO32− rather 5 / 16
than oxidation of OH− while SO2(g) is inletted into the electrolyte system. The essential reactions during this SO2 PEC process are thus summarized as following: SO2 flow in the electrolyte solution (NaOH(aq)): SO2(g) + 2OH−→ SO32− + H2O
(Eq. 3)
Semiconductor under AM 1.5 G irradiation: Semiconductor catalyst (BiVO4, Bi(Mo)VO4, IO-Bi(Mo)VO4)
h+ + e−
(Eq. 4)
Reaction on photoanode (electrode film): SO32− + 2 OH−→ SO42− + H2O + 2 e−
(Eq. 5)
Reaction on cathode (Pt wire): 2 H2O + 2 e− → H2(g)↑ + 2 OH−
(Eq. 6)
The total reaction: SO2(g) + 2 OH−
H2(g)↑ + SO42−
(Eq. 7)
The amounts of evolved H2 were further measured at a bias of 0.6 V (vs. Ag/AgCl) under AM 1.5 G irradiation. The SO2 removal rates, photocurrent densities, theoretical evolution rates of H2, experimental evolution rates of H2 and Faradaic efficiencies on different electrode films with/without SO2 removal are summarized in Table 1. The calculation details of theoretical evolution rate of H2 and Faradaic efficiency are described in Supporting Information. As shown in Table 1, for all the electrode films, the current density and H2 evolution rate have been distinctly improved with SO2 removal, which are consistent with the LSV results (Fig. 2a and Fig. S4). The proposed PEC process shows a high SO2 removal rate of > 97%. Furthermore, the Faradaic efficiencies for H2 production are higher than 94 % (Table 1), indicating nearly all the generated electrons in the circuit are consumed by H2 production. With SO2 removal, the H2 evolution rate is significantly enhanced by more than 4 times on IO-Bi(Mo)VO4 than that without SO2. In addition, the IO-Bi(Mo)VO4 with SO2 removal provides much higher amount of H2 generation (60.26 μmol h-1 cm-2) than that of Bi(Mo)VO4 (22.51 μmol h-1 cm-2) and BiVO4 (1.08 μmol h-1 cm-2), which mainly owning to the improved light harvesting in this special photonic crystal structure. Moreover, due to the highly
porous
structure
the
distance
from
the
photogenerated
holes
to
the
semiconductor-electrolyte interface is greatly shortened than the bulk film. The inverse opal 6 / 16
structure significantly enhanced the generation rate of H2 (0.6 V vs. Ag/AgCl) by more than 30 times compared with our previous work.33 The great leap in activity may make this approach become practically feasible and the desulfurization process be profitable. To study the influence of CO2, the electrolyte is changed from NaOH to Na2CO3 or NaHCO3, interestingly, no significant change is observed in SO2 absorption (~ 97%) and H2 production (~ 60μmol h-1 cm-2). This is attributed to that SO2 can quickly react with Na2CO3 or NaHCO3 to form Na2SO3 ( pKa1 of H2CO3 is 6.38, while pKa1 of H2SO3 is 1.90). The existence of CO2 in the flue gas can hardly influence the SO2 absorption and Na2SO3 formation. Although solar light is not controllable resource, however the produced SO32- during desulfurization is storable, which can be converted to H2 via the PEC process when the solar light is available.
In this work, as the energy of SO2 is used to produce high-value-added product H2, which can greatly reduce the cost of desulfurization process. Furthermore, the energy input in this PEC process is mainly from solar light. The small external bias can be provided from a solar cell or by replacing the Pt cathode with a p-type semiconductor as a photocathode. In practical application, the sodium hydroxide is expensive and the product of sodium sulfate is of low value. Instead, the proposed approach can readily be combined with the existing ammonia-based desulfurization technology. The (NH4)2SO3 produced in this technology can be used to produce H2, replacing the current aeration treatment, which leads to a waste of the energy of SO32-. Via the proposed process, the SO2 released (0.03 kg) from coal power plant for a generation of 1 kW h electricity can be recycled to produce around 10 L H2 based on the total reaction (Eq. 7). Via the existing ammonia-based desulfurization technology, the cost of dealing with SO2 in flue gas for producing 1 MWh electricity is around 5 USD. If the SO32produced in this desulfurization technology are recycled to produce H2, then 104 H2 can be produced, which has a value of 6.5 USD. It is recently reported that Mo-doped BiVO4 can stably operate for at least 1100 h,34 the raw material cost of this PEC process is thus calculated to be around 0.3 USD/MWh (see Table S1 in Supporting Information for calculation details), so the total cost for dealing with flue gas from 1 MWh electricity is: 5 – 6.5 + 0.3 = -1.2 USD. That means ammonia-based desulfurization technology can become profitable when it is combined with the proposed PEC process, a profit of 1.2 USD can be made by dealing with 1 MWh electricity. In China, 6 million MWh electricity were produced 7 / 16
from coal burning power plant in 2016, potentially a profit of 7.2 billion USD/year can be made through this PEC process. The instable solar energy may lead to additional cost, however, the stability of BiVO4 and efficiency of the PEC process can be further improved in the future research, a greatly reduced operation cost of PEC technology and higher benefit can thus be expected.
Associated content Support Information The supporting information is available free of charge. Cost calculation details. The synthesis process and related calculation details of theoretical evolution rate of H2 and Faradaic efficiency; XRD patterns, UV-Vis absorption spectra and Tauc plots of electrode films; chopped J-V curves of Bi(Mo)VO4 and BiVO4 with/without SO2 removal; amperometric i-t curves under chopped illumination at applied potentials; mechanism of H2 generation by IO-Bi(Mo)VO4 during PEC process without removal of SO2.
Author Information Corresponding Author E-mail: [email protected] Phone/Fax: +86-21-6564-2781
Notes The authors declare no competing financial interest.
Acknowledgements The authors gratefully acknowledge financial support from National Natural Science Foundation of China (No. 21507011 and No. 21677037), and Ministry of Science and Technology of the People’s Republic of China (2016YFE0112200 and 2016YFC0203700).
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Fig. 1. (a) Illustration of synthetic strategy of IO-Bi(Mo)VO4, low-magnification (b) and highmagnification (b’) SEM images of IO-Bi(Mo)VO4.
13 / 16
Fig. 2. (a) Linear Sweep Voltammetry (LSV) curves of Bi(Mo)VO4 and IO-Bi(Mo)VO4 tested in different conditions with/without AM 1.5 G irradiation (100 mW/cm-2); (b) Chopped LSV curves of Bi(Mo)VO4 and IO-Bi(Mo)VO4 in different conditions under chopped illumination; (c) Electrochemical Impedance Spectroscopy (EIS) Nyquist plots of IO-Bi(Mo)VO4 with/without SO2 removal under illumination at 0.7 V (vs. Ag/AgCl); (d) Mechanism of H2 generation by IO-Bi(Mo)VO4 during PEC process with removal of SO2.
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Table 1. The current density, theoretical evolution rate and experimental evolution rate of H2, Faradaic efficiency in different electrolyte solutions with different catalysts at 0.6 V (vs. Ag/AgCl) and SO2 absorbing efficiency. BiVO4
Current Density/mA cm-2
Bi(Mo)VO4
IO-Bi(Mo)VO4
NaOH(aq)
NaOH(aq)+SO2(g)
NaOH(aq)
NaOH(aq)+SO2(g)
NaOH(aq)
NaOH(aq)+SO2(g)
0.007
0.06
0.32
1.26
0.83
3.42
0.13
1.1
6.02
23.7
15.6
64.11
0.126
1.08
5.84
22.51
14.82
60.26
97
98
97
95
95
94
/
~97
/
~98
/
~97
Theoretical evolution rate of H2/μmol h-1 cm-2 Experimental evolution rate of H2/μmol h-1 cm-2 Faradaic efficiency/% SO2 absorbing efficiency/%
Mo-doped BiVO4 inverse opal photoanode was fabricated by self-assembly method. Flue gas SO2 can greatly improve the H2 production via PEC water splitting The inverse opal structure shows higher photoactivity due to its special structure SO2 removal with a simultaneous H2 production is realized
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H2 Production with Simultaneous SO2 Removal on Photonic Crystals.
16 / 16