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BiOCl-based photocathode for photocatalytic fuel cell
Journal Pre-proofs Full Length Article BiOCl-based photocathode for photocatalytic fuel cell Yang Wang, Yanming Wang, Xi-ming Song, Yu Zhang, Tianyi Ma PII: DOI: Reference:
S0169-4332(19)33766-3 https://doi.org/10.1016/j.apsusc.2019.144949 APSUSC 144949
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Applied Surface Science
Received Date: Revised Date: Accepted Date:
6 August 2019 22 November 2019 3 December 2019
Please cite this article as: Y. Wang, Y. Wang, X-m. Song, Y. Zhang, T. Ma, BiOCl-based photocathode for photocatalytic fuel cell, Applied Surface Science (2019), doi: https://doi.org/10.1016/j.apsusc.2019.144949
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BiOCl-based photocathode for photocatalytic fuel cell Yang Wang a, Yanming Wang a, Xi-ming Song a, Yu Zhang a,* and Tianyi Ma b,* a
Institute of Clean Energy Chemistry, Key Laboratory for Green Synthesis and Preparative Chemistry of Advanced Materials, College of Chemistry, Liaoning University, Shenyang 110036, China. b
*
Discipline of Chemistry, The University of Newcastle, Callaghan, NSW 2308 Australia.
Corresponding authors. Email: [email protected] (Y. Zhang); [email protected] (T. Ma)
Abstract A photocatalytic fuel cell with BiOCl as the photocathode and furfural as direct fuel was developed for the first time. We synthesized a uniformly and densely distributed stable BiOCl film on F-doped Tin dioxide in-situ successfully, and X-ray photoelectron spectroscopy results showed the existence of oxygen vacancies on the surface of BiOCl. In the furfural fuel cell, the BiOCl-coated FTO was chosen as the electrode for light-induced oxygen reduction reaction. Linear sweep voltammograms test of the Pt-BiOCl furfural fuel cell showed a higher open circuit voltage than the Pt-Pt furfural fuel cell, which was consistent with the theoretical analysis of a larger electromotive force value under illumination. Gas chromatography-mass spectrometry results showed an obvious degradation effect towards furfural, 5-methyl furfural and 2-acetylfuran, which demonstrated a distinctive way of utilizing non-noble metal as the electrode in photocatalytic fuel cell.
Keywords: photocatalytic fuel cell, BiOCl, photocathode, furfural
1. Introduction Every year mountains of agricultural by-products rich in plant fibers are discarded as waste without sufficient utilization. Thus it is highly demanded to explore novel methods with regarding to transferring biomass-derived materials into usable energy that matters for developing economy and technology sustainably. Furfural (2-furaldehyde) is an aromatic aldehyde which has been widely used in furan derivative manufactures such as resins production, wetting agents and pesticides [1]. As a common reaction product of hydrolysis and dehydration processes of fiber-rich farming by-products like corncobs and bagasse, the utilization of furfural is still restricted in spite of a large output in many traditional agricultural countries every year. As reported, direct photodegradation has always been a habitual method in removing furfural easily
[2]
. Recently, some splendid TiO2-based
researches focused on the photodegradation of furfural have also been reported [3-6]. However, to some extent, direct degradation treatments make the biomass energy within furfural unutilized. As an electrochemical power generation technology which can converse biomass energy into electricity directly under mild conditions, fuel cells show a high energy conversion efficiency far over chemical or biological treatments. However, the working temperature of traditional fuel cell is always over 60 degrees Celsius when noble metal-based electrode and the catalyst are often stable for merely a short time. As an technology that can utilize organic matters and generate electricity efficiently, photocatalytic fuel cell (PFC) has been widely developed in both pollutant degradation and wastewater treatment
[7-10]
. To the best of our
knowledge, some works targeted at simple alcohol or glucose with chemical
[11-14]
or
biological
[15]
photoanode in photocatalytic fuel cells have been reported. Similarly, few
distinctive works on light-enhanced electrode reaction biofuel system have been assembled recently despite an incomplete overall cycle of the fuel cell in photo-assisted conditions [16-20]. To-date, furfural has not been explored as the direct fuel in photocatalytic fuel cells. As a class of promising photocatalyst for energy conversion and environmental remediation, bismuth oxyhalides (BiOX, X = Cl, Br and I) have been widely investigated due to their high photoactivity, moderate charge transport property and negative conduction band edge
[19, 20]
. Since BiOX have indirect-transition bandgap which can enhance the induced
dipole and reduce the photo-induced electrons and holes recombination rate, BiOX always exhibits excellent performance in the photocatalytic process
[21, 22]
. As a member of bismuth
oxyhalides, BiOCl has also received much attention in recent years such as reducing CO2 to fuels [23], H2 evolution from water and degradation of pollutants [24]. Different kinds of microor nanostructures such as BiOCl nanobelts
[25]
, nanofibers [26] or nanosheets
[27]
have been
synthesized. As a p-type semiconductor, BiOCl can accumulate more photogenerated electrons on the surface, which is beneficial to the electrocatalytic process
[28]
. The fermi
level of the p-type semiconductor is relatively low, and the downward band banding formed on the surface is conducive to the transfer of electrons to the surface of the material as well. In this work, we successfully in-situ synthesized a stable ellipsoid-liked BiOCl film on F-doped Tin dioxide (FTO) glass substrate by solvothermal reaction. Considering the instability of BiOCl under intense illumination, we composed the fuel cell by taking BiOCl as the photocathode for oxygen reduction reaction, while platinum sheet electrode (Pt) was chosen as the anode. As the oxidant of the fuel reaction, excessive oxygen introduced in the
cathode cell guaranteed the stability of BiOCl photocathode as well. 2. Experimental 2.1 In situ synthesis of BiOCl on FTO 3 mmol KCl was dissolved in 12.5 mL ethylene glycol, which was then dropped into 12.5 mL ethylene glycol containing 2.85 mmol Bi(NO3)3·5H2O. The mixture was stirred for 1 h, and then transferred into a 40 mL Teflon-lined autoclave with a FTO plate in it. The autoclave was heated at 160 °C for 12 h. After cooling down to room temperature, the FTO plate was taken out and cleaned with distilled water and ethanol for two times individually. Drying at 80 °C for 12 h to obtain the finished product, which was then treated to 1 cm × 0.9 cm reserved for the photocatalytic reactions next. 2.2 Experimental setup
Fig. 1 Setup of photocatalytic fuel cell (anode: 1% vol. furfural in 0.02 mol/L Na2SO4 electrolyte; photocathode: 0.02 mol/L Na2SO4 electrolyte).
A specialized glass device was chosen as the container for the entire photocatalytic process of the fuel cell. An unusual region on one side of the device is made of quartz glass, which is conductive to the transmission of light. Same with the salt bridge in the simple biological fuel cell which transfer ions in different parts of the fuel cell fluently and reduce the liquid junction potential as well, a proton exchange membrane is used between two electrode cells in the furfural fuel cell to facilitate the transfer of hydrogen ions from the anode region to the cathode region. Oxygen was blown into the cathode solution at a medium speed to ensure adequate reactants on the cathode. 2.3 Characterization The X-ray diffraction patterns were recorded with a Bruker (Germany) D8 Advance diffractometer with Cu Kα radiation in the range of 20° – 80° (2θ). UV-vis diffuse reflectance spectroscopy was characterized by using a Beijing General Analysis TU-1950 with BaSO4 as the reflectance standard. X-ray photoelectron spectroscopy (Kratos AXIS SUPRA, Al Kɑ radiation) measurements were carried out to analyze the surface chemical compositions and states. The morphologies of the films were observed by scanning electron microscope using a Shimadzu JSM-6700F. The high resolution transmission electron microscopy imaging was performed on a Hitachi H-7650 microscope. The elemental mapping of the material was analyzed with transmission electron microscope (TEM, jeol-2100F). Electrochemical measurements were obtained by using a electrochemical workstation produced by CH Instruments Inc. (CHI 660E). When measuring the electrochemical properties of the furfural fuel cell, cyclic voltammetry test together with linear sweep voltammograms test and Amperometric I-t curve
test were carried out. Cyclic voltammetry test of the oxygen reduction reaction activity of BiOCl photocathode was carried out at a scanning rate of 20 mV s−1 in a three-electrode system with BiOCl film as working electrode, Pt sheet as the counter electrode while Ag/AgCl electrode was chosen as the reference electrode, and the electrolyte was 0.02 mol/L Na2SO4 with continuous bubbling oxygen. Cyclic voltammetry test of furfural oxidation was performed at a scanning rate of 20 mV s−1 using a Pt-Pt three-electrode system with Ag/AgCl electrode as the reference electrode, and the electrolyte was 0.02 mol/L Na2SO4 containing 1% (vol.) furfural. Linear sweep voltammograms test of the fuel cell was carried out in a two-electrode system with BiOCl (1×0.9 cm2) in 0.02 mol/L Na2SO4 as the photocathode and Pt sheet (1×1×0.2 cm3) in 0.02 mol/L Na2SO4 with 1% furfural as the anode. Amperometric I-t curve test was carried out in the same condition with the linear sweep voltammograms test, and a 50 seconds long period was chosen as the react time with or without illumination. In this work, all photocatalytic reactions were carried out under the tense of 100 mW/cm2 (1 solar light equally) with a Xenon lamp (320 – 780 nm), and the distance from lamp to the reactor was 5 cm. The same concentration of electrolyte in the fuel cell offered an identical potential difference for the two electrode reactions. Fourier transform infrared test of BiOCl was carried out by using a Shimadzu IR-Prestige21 with potassium bromide tablet. Raman result was recorded with laser Raman microscope (inVia, Renishaw). Atomic Force Microscope results were obtained from BRUKER ICON2-SYS. Gas chromatography-mass spectrometry (Thermo, Trace GC + Polaris Q) tests were carried out to check out the results of reaction process after 20 hours (2× 10 h).
3. Results and discussion
Fig. 2 (a) Comparison of XRD patterns; (b) UV-vis DRS and bandgap estimate of BiOCl; (c-f) XPS of BiOCl.
3.1 Materials synthesis and characterization
The ellipsoidal BiOCl was synthesized on the FTO substrate in situ by solvothermal method. We performed the X-ray diffraction (XRD) analysis of BiOCl scraped down from BiOCl-coated FTO, BiOCl-coated FTO and FTO respectively. As shown in Fig. 2a, the XRD pattern of BiOCl can be well indexed to the tetragonal phase of BiOCl (JCPDS No. 06-0249), which also reveals that the prepared sample is well crystallized. The diffraction peak at 32.5 ° corresponds to the (110) plane of BiOCl with the lattice spacing of 0.275 nm. The diffraction
peaks at 12 ° and 25.9 ° are corresponded well to the (001) and (101) plane of BiOCl with the lattice spacing of 0.738 nm and 0.344 nm, which also shows a good correspondence to the high resolution transmission electron microscopy image of BiOCl as seen in Fig. 3c. Fig. 2b displays the UV-vis diffuse reflectance spectrum (UV-vis DRS) of BiOCl. It can be observed that BiOCl has a strong absorption of the light with wavelengths shorter than 360 nm. But additionally, the prepared BiOCl has a certain absorption in the visible light region which is different from reported
[24]
, indicating that the material may have an abundant
inter-band states owing to the chemisorbed molecules like H2O or O2 etc. Since BiOCl has an indirect bandgap
[29]
, we calculated the bandgap by Kubelka-Munk function [F(R∞)hν]1/2–hν
with the absorbance data from UV-vis DRS as shown in the inset of Fig. 2b. The bandgap value of BiOCl (3.34eV) is consistent with the result reported as well [30]. X-ray photoelectron spectroscopy (XPS) tests have been carried out to further confirm the valency of elements. Fig. 2c shows a full scan spectrum of BiOCl, indicating the existence of bismuth, oxygen and chlorine, while Fig. 2d-f show the test results of the elements within respectively. Two peaks at 159.4 and 164.5 eV are corresponded to Bi 4f7/2 and Bi 4f5/2 of Bi3+, and two peaks at 197.9 and 199.5 eV are found in Fig. 2f, corresponding to Cl 2p3/2 and Cl 2p1/2 of Cl-. The O 1s XPS spectrum of BiOCl has been separated into two peaks at 530.1 and 531.9 eV, which are assigned to be the oxygen in crystal lattice (O L) and oxygen vacancies (OV), while the peak at 533.2 eV is speculated to be the residual H2O
[26]
.
The existence of oxygen vacancies enhances the absorbance of oxygen and water molecules on the surface of BiOCl particles, which can form the surface states, leading to the sub-bandgap transition and thus improving the photocatalytic activity of the material further
due to the additional absorbance of visible light. The analysis results of XPS spectra, taken together with the XRD patterns comparison, provide strong evidence that BiOCl has been synthesized accurately. Fourier Transform Infrared (FTIR) spectroscopy of BiOCl was obtained to analyze the structure as well. Though dried immediately before test, a sharp peak in 3450 cm-1 owing to the vibration of -OH can still be found in the FTIR spectrum of BiOCl notably, which indicates that the excess water is physically adsorbed in the oxygen vacancy of BiOCl (Fig. S2). The existence of oxygen vacancies trends to combine oxygen on the surface of photocathode, which can improve the overall efficiency by promoting the cathode oxygen reduction reaction process in the fuel cell.
Fig. 3 SEM images of BiOCl on FTO (a, b); TEM image of BiOCl (c); Elemental mapping of BiOCl (d).
The BiOCl film was further characterized by field emission scanning electron microscopy (FE-SEM) and high resolution transmission electron microscopy (HRTEM).
Although tiny cracks can be observed from Fig. 3a, FE-SEM results show that BiOCl particles are distributed uniformly and densely on the FTO substrate (Fig. 3a). When zooming in to examine the details of the surface, the acicular structure with a width of about 14 nm of BiOCl is observed, which may be capable of providing a high contact area during the reaction process. To measure the thickness of BiOCl and investigate the morphology of the coated film, atomic force microscopy (AFM) has been carried out (Fig. S4). AFM results show a height of 1.2 μm of the BiOCl particle, which is the same with the FE-SEM result, proving that the BiOCl particles on FTO are shaped like spheres, or ellipsoids, exactly. We have tested the cross-sectional SEM of the BiOCl coated FTO and the results were shown in Fig. S7. The top-right side of the image is FTO, and the bottom-left side is the BiOCl film. Clearly, a rough surface of the BiOCl film can be observed, and the thickness of the film is about 1 μm, which is consistent with the SEM and AFM results shown in Fig. 3 and Fig. S4. As shown in Fig. 3c, it is obvious that the fringe spacing of 0.74 nm and 0.17 nm can be found in the HRTEM image of BiOCl, corresponding well to the lattice space of (001) and (202), which is also consistent with the XRD pattern shown in Fig. 3c. (BiOCl, P4/nmm, more information in PDF#06-0249). The large fringe spacing of (001) which is rare in common materials also ensures the accuracy of BiOCl. TEM elemental mapping was carried out to check out the distribution of different elements in the sample (Fig. 3d). The elemental mapping test result proves the existence of element Bi, O and Cl in the sample, and the image of the selected area clearly shows the distribution of different elements in BiOCl. Fig. S3 shows the Raman spectrum of BiOCl. The strong peaks at 112.09 and 142.35 cm-1 correspond to the internal Bi-Cl stretching mode. Notice that when ramen shift increases
over 3000 cm-1, the response signal increases sharply owing to the adsorbed water in the vacancies of BiOCl , which is consistent with the FTIR spectrum. 3.2 Electrochemical characterization
Fig. 4 (a) Amperometric I-t curves without furfural (a) and with furfural (b); (b) LSV curves with light off/on; (c) CV curves of BiOCl (vs. RHE) with light off/on; (d) CV curve of furfural (vs. RHE).
We tested the performance of the as-developed furfural fuel cell. Fig. 4a shows the Amperometric I-t curve test results with light off/on. Obviously, the current value has been increased up to an order of magnitude after illuminating, indicating that the photogenerated carriers can promote the electrode reaction under photostimulation. A decay tendency in current value can be observed, which may caused by the chemisorbed furfural on the surface of Pt. Then we investigated the linear sweep voltammograms (LSV) tests as shown in Fig. 4b. A larger value of open circuit voltage (Voc) is observed (0.82 V vs. 0.65 V), showing a higher electromotive force of the fuel battery under light, which is conformed to the prediction of the electrode reaction mechanism (Fig. 5). A sharp increase on short circuit current (Isc) level (14.2 vs. 4.5) owing to the more photogenerated carriers produced by illuminating indicates
that BiOCl shows an obvious photocatalytic activity in the fuel cell reaction. The comparative LSV test of Pt-Pt system furfural fuel cell, a standard effective collocation in most of the fuel cells, was carried out in the dark with other conditions unchanged. A higher Isc than Pt-BiOCl fuel cell under illumination condition is obtained, and the Voc is measured to be 0.63 V, which is almost the same with the Pt-BiOCl fuel cell without light, but smaller than the one under illumination, confirming that the light-induced oxygen reduction reaction on the semiconductor BiOCl photocathode can actually afford an additional auxiliary effect for enhancing the performance of the furfural fuel cell. To test the electrochemical property of BiOCl, we performed the cyclic voltammetry (CV) in simple electrolyte solution (0.02 mol/L Na2SO4) which was consistent with the actual condition of the fuel cell reactions. As shown in CV test from Fig. 4c, the current of reduction peak has been increased under illumination, and the potential of reduction peak of oxygen has been moved from 0.94 V to 1.2 V, which is more closer to 1.23 V, the theoretical standard oxygen reduction potential, proving that BiOCl is an efficient material towards the reduction reaction of oxygen. The CV test of furfural with Pt sheet as both cathode and anode in 0.02 mol/L Na2SO4 electrolyte solution was also performed in the dark condition as shown in Fig. 4d. The measured potential of oxidation peaks of furfural are 0.02 V and 1 V, which indicates that furfural is easy to be oxidized in the photocatalytic fuel cell.
Fig. 5 Schematic of the electrode reaction mechanism in photocatalytic fuel cell.
In theory, the difference of the oxidation potential of furfural and reduction potential of oxygen is the electromotive force of the fuel cell. The measured electromotive force value of the furfural fuel cell under illumination (V’, Fig. 5) is 1.18 V, while the electromotive force value without light (V) is 0.92 V (calculated from Fig. 4c and 4d). Under light conditions, a holes’ quasi Fermi level (EFP) of BiOCl which acts as the actual potential of cathode is formed. Since EFP is lower than the standard oxygen reduction potential, a higher electromotive force should be obtained, indicating that solar energy can promote the catalytic process of furfural in the fuel cell. Obviously, the obtained higher electromotive force value with photostimulation confirms our inference, indicating that solar energy can be successfully utilized in the fuel cell together with biomass energy. Gas chromatography-mass spectrometry (GC-MS) tests were carried out to verify the existence of the photodegradation process of furfural (Fig. S6). By using dichloromethane as the extractant, the organic parts in the solution were successfully extracted and gathered. It is quite obvious that the amount of furfural has been decreased to 60.2% (Fig. S6a and Fig. S6b), proving the existence of the photocatalytic energy conversion from chemical energy
into electricity. More than that, as a kind of chemically active aromatic aldehyde, furfural is unstable and is prone to be oxidized, especially when exposed to light and air. Two non-ignorable peaks in the gas chromatograms have also been identified as 5-methyl furfural and 2-acetylfuran. Comparing with the original furfural ingredient, the amount of 5-methyl furfural and 2-acetylfuran after photocatalytic process have been decreased to 43.5% (Fig. S6c and Fig. S6d) and 43.1% respectively (Fig. S6e and Fig. S6f), indicating that these organic compounds can be photodegraded together with furfural in the photocatalytic process of the as-developed fuel cell at the same time. We also performed XRD of the BiOCl photocathode after the photocatalytic reaction of the furfural fuel cell (Fig. S1a). Two comparative experiments have been carried out to check out the stability of BiOCl. As shown in Fig. S1c, the BiOCl-coated FTO turns into dark after 5 seconds long illumination (100mW/cm2) owing to the Bi0 reduced from Bi3+. In addition, a BiOCl-coated FTO turns into dark in 0.02 mol/L Na2SO4 aqueous solution after 3 minutes under illumination. By comparing the XRD patterns of a fresh BiOCl-coated FTO and the BiOCl photocathode after the photocatalytic reaction of the furfural fuel cell, it can be seen that BiOCl photocathode is still stable after the photocatalytic reaction process. 4. Conclusion We demonstrated the synthesis of an evenly distributed BiOCl film on FTO in situ as the novel photocathode for the furfural fuel cell. Under illumination, the p-type semiconductor BiOCl accumulates more photogenerated electrons on the surface, which can enhance the electrocatalytic process further. By choosing semiconductor as the photocathode material, we assembled a fuel cell which can take advantage of solar energy at the same time without
using the noble metal cathode. Not only that, the fuel cell with light responsive material as the photoanode and photocathode may improve the performance as well. Note that the efficiency and power density of this proof-of-concept photocatalytic fuel cell is limited which has substantial room for further improvement. An in-depth research focused on the synthesis and decoration of BiOCl is deserved to be carried out next. There is no exaggeration that the successful design of the photocatalytic furfural fuel cell paves the way for dealing with more kinds of biological wastes in a distinctive way. Nevertheless, this study shows a peculiar way of developing photocathode for photocatalytic fuel cell, which opens up an approach to dealing with agricultural product metabolites and enhancing photoelectric reaction process in future.
Acknowledgments This work was financially supported by Liaoning Revitalization Talents Program - Pan Deng Scholars (XLYC1802005), Liaoning BaiQianWan Talents Program, the National Science Fund of Liaoning Province for Excellent Young Scholars, Science and Technology Innovative Talents Support Program of Shenyang (RC180166), Australian Research Council (ARC) through Discovery Early Career Researcher Award (DE150101306) and Linkage Project (LP160100927), Faculty of Science Strategic Investment Funding 2019 of University of Newcastle, and CSIRO Energy.
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The highlights of this work are: 1. Utilization of biological waste A furfural fuel cell was designed, showing a novel way utilizing biological wastes. 2. Synthesis of a densely distributed BiOCl film on FTO in situ A densely distributed ellipsoid-liked BiOCl film was in-situ synthesized on FTO. 3. BiOCl semiconductor photocathode in the photocatalytic fuel cell The BiOCl photocathode showed a higher electromotive force under illumination. 4. A higher open circuit voltage than the Pt-Pt system Under illumination, I-V test showed a higher Voc than Pt-Pt fuel cell.
Declaration of Interest Statement
BiOCl-based photocathode for photocatalytic fuel cell Yang Wang a, Yanming Wang a, Xi-ming Song a, Yu Zhang a,* and Tianyi Ma b,*
Author Contribution Statement
BiOCl-based photocathode for photocatalytic fuel cell Yang Wang a, Yanming Wang a, Xi-ming Song a, Yu Zhang a,* and Tianyi Ma b,*
Yang Wang: Formal analysis; Investigation; Methodology; Roles/Writing - original draft. Yanming Wang: Supervision. Xi-ming Song: Resources; Software. Yu Zhang: Formal analysis; Investigation; Methodology; Writing - review & editing; Conceptualization; Data curation. Tianyi Ma: Formal analysis; Investigation; Methodology; Writing - review & editing; Funding acquisition.
S0169-4332(19)33766-3 https://doi.org/10.1016/j.apsusc.2019.144949 APSUSC 144949
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
6 August 2019 22 November 2019 3 December 2019
Please cite this article as: Y. Wang, Y. Wang, X-m. Song, Y. Zhang, T. Ma, BiOCl-based photocathode for photocatalytic fuel cell, Applied Surface Science (2019), doi: https://doi.org/10.1016/j.apsusc.2019.144949
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© 2019 Published by Elsevier B.V.
BiOCl-based photocathode for photocatalytic fuel cell Yang Wang a, Yanming Wang a, Xi-ming Song a, Yu Zhang a,* and Tianyi Ma b,* a
Institute of Clean Energy Chemistry, Key Laboratory for Green Synthesis and Preparative Chemistry of Advanced Materials, College of Chemistry, Liaoning University, Shenyang 110036, China. b
*
Discipline of Chemistry, The University of Newcastle, Callaghan, NSW 2308 Australia.
Corresponding authors. Email: [email protected] (Y. Zhang); [email protected] (T. Ma)
Abstract A photocatalytic fuel cell with BiOCl as the photocathode and furfural as direct fuel was developed for the first time. We synthesized a uniformly and densely distributed stable BiOCl film on F-doped Tin dioxide in-situ successfully, and X-ray photoelectron spectroscopy results showed the existence of oxygen vacancies on the surface of BiOCl. In the furfural fuel cell, the BiOCl-coated FTO was chosen as the electrode for light-induced oxygen reduction reaction. Linear sweep voltammograms test of the Pt-BiOCl furfural fuel cell showed a higher open circuit voltage than the Pt-Pt furfural fuel cell, which was consistent with the theoretical analysis of a larger electromotive force value under illumination. Gas chromatography-mass spectrometry results showed an obvious degradation effect towards furfural, 5-methyl furfural and 2-acetylfuran, which demonstrated a distinctive way of utilizing non-noble metal as the electrode in photocatalytic fuel cell.
Keywords: photocatalytic fuel cell, BiOCl, photocathode, furfural
1. Introduction Every year mountains of agricultural by-products rich in plant fibers are discarded as waste without sufficient utilization. Thus it is highly demanded to explore novel methods with regarding to transferring biomass-derived materials into usable energy that matters for developing economy and technology sustainably. Furfural (2-furaldehyde) is an aromatic aldehyde which has been widely used in furan derivative manufactures such as resins production, wetting agents and pesticides [1]. As a common reaction product of hydrolysis and dehydration processes of fiber-rich farming by-products like corncobs and bagasse, the utilization of furfural is still restricted in spite of a large output in many traditional agricultural countries every year. As reported, direct photodegradation has always been a habitual method in removing furfural easily
[2]
. Recently, some splendid TiO2-based
researches focused on the photodegradation of furfural have also been reported [3-6]. However, to some extent, direct degradation treatments make the biomass energy within furfural unutilized. As an electrochemical power generation technology which can converse biomass energy into electricity directly under mild conditions, fuel cells show a high energy conversion efficiency far over chemical or biological treatments. However, the working temperature of traditional fuel cell is always over 60 degrees Celsius when noble metal-based electrode and the catalyst are often stable for merely a short time. As an technology that can utilize organic matters and generate electricity efficiently, photocatalytic fuel cell (PFC) has been widely developed in both pollutant degradation and wastewater treatment
[7-10]
. To the best of our
knowledge, some works targeted at simple alcohol or glucose with chemical
[11-14]
or
biological
[15]
photoanode in photocatalytic fuel cells have been reported. Similarly, few
distinctive works on light-enhanced electrode reaction biofuel system have been assembled recently despite an incomplete overall cycle of the fuel cell in photo-assisted conditions [16-20]. To-date, furfural has not been explored as the direct fuel in photocatalytic fuel cells. As a class of promising photocatalyst for energy conversion and environmental remediation, bismuth oxyhalides (BiOX, X = Cl, Br and I) have been widely investigated due to their high photoactivity, moderate charge transport property and negative conduction band edge
[19, 20]
. Since BiOX have indirect-transition bandgap which can enhance the induced
dipole and reduce the photo-induced electrons and holes recombination rate, BiOX always exhibits excellent performance in the photocatalytic process
[21, 22]
. As a member of bismuth
oxyhalides, BiOCl has also received much attention in recent years such as reducing CO2 to fuels [23], H2 evolution from water and degradation of pollutants [24]. Different kinds of microor nanostructures such as BiOCl nanobelts
[25]
, nanofibers [26] or nanosheets
[27]
have been
synthesized. As a p-type semiconductor, BiOCl can accumulate more photogenerated electrons on the surface, which is beneficial to the electrocatalytic process
[28]
. The fermi
level of the p-type semiconductor is relatively low, and the downward band banding formed on the surface is conducive to the transfer of electrons to the surface of the material as well. In this work, we successfully in-situ synthesized a stable ellipsoid-liked BiOCl film on F-doped Tin dioxide (FTO) glass substrate by solvothermal reaction. Considering the instability of BiOCl under intense illumination, we composed the fuel cell by taking BiOCl as the photocathode for oxygen reduction reaction, while platinum sheet electrode (Pt) was chosen as the anode. As the oxidant of the fuel reaction, excessive oxygen introduced in the
cathode cell guaranteed the stability of BiOCl photocathode as well. 2. Experimental 2.1 In situ synthesis of BiOCl on FTO 3 mmol KCl was dissolved in 12.5 mL ethylene glycol, which was then dropped into 12.5 mL ethylene glycol containing 2.85 mmol Bi(NO3)3·5H2O. The mixture was stirred for 1 h, and then transferred into a 40 mL Teflon-lined autoclave with a FTO plate in it. The autoclave was heated at 160 °C for 12 h. After cooling down to room temperature, the FTO plate was taken out and cleaned with distilled water and ethanol for two times individually. Drying at 80 °C for 12 h to obtain the finished product, which was then treated to 1 cm × 0.9 cm reserved for the photocatalytic reactions next. 2.2 Experimental setup
Fig. 1 Setup of photocatalytic fuel cell (anode: 1% vol. furfural in 0.02 mol/L Na2SO4 electrolyte; photocathode: 0.02 mol/L Na2SO4 electrolyte).
A specialized glass device was chosen as the container for the entire photocatalytic process of the fuel cell. An unusual region on one side of the device is made of quartz glass, which is conductive to the transmission of light. Same with the salt bridge in the simple biological fuel cell which transfer ions in different parts of the fuel cell fluently and reduce the liquid junction potential as well, a proton exchange membrane is used between two electrode cells in the furfural fuel cell to facilitate the transfer of hydrogen ions from the anode region to the cathode region. Oxygen was blown into the cathode solution at a medium speed to ensure adequate reactants on the cathode. 2.3 Characterization The X-ray diffraction patterns were recorded with a Bruker (Germany) D8 Advance diffractometer with Cu Kα radiation in the range of 20° – 80° (2θ). UV-vis diffuse reflectance spectroscopy was characterized by using a Beijing General Analysis TU-1950 with BaSO4 as the reflectance standard. X-ray photoelectron spectroscopy (Kratos AXIS SUPRA, Al Kɑ radiation) measurements were carried out to analyze the surface chemical compositions and states. The morphologies of the films were observed by scanning electron microscope using a Shimadzu JSM-6700F. The high resolution transmission electron microscopy imaging was performed on a Hitachi H-7650 microscope. The elemental mapping of the material was analyzed with transmission electron microscope (TEM, jeol-2100F). Electrochemical measurements were obtained by using a electrochemical workstation produced by CH Instruments Inc. (CHI 660E). When measuring the electrochemical properties of the furfural fuel cell, cyclic voltammetry test together with linear sweep voltammograms test and Amperometric I-t curve
test were carried out. Cyclic voltammetry test of the oxygen reduction reaction activity of BiOCl photocathode was carried out at a scanning rate of 20 mV s−1 in a three-electrode system with BiOCl film as working electrode, Pt sheet as the counter electrode while Ag/AgCl electrode was chosen as the reference electrode, and the electrolyte was 0.02 mol/L Na2SO4 with continuous bubbling oxygen. Cyclic voltammetry test of furfural oxidation was performed at a scanning rate of 20 mV s−1 using a Pt-Pt three-electrode system with Ag/AgCl electrode as the reference electrode, and the electrolyte was 0.02 mol/L Na2SO4 containing 1% (vol.) furfural. Linear sweep voltammograms test of the fuel cell was carried out in a two-electrode system with BiOCl (1×0.9 cm2) in 0.02 mol/L Na2SO4 as the photocathode and Pt sheet (1×1×0.2 cm3) in 0.02 mol/L Na2SO4 with 1% furfural as the anode. Amperometric I-t curve test was carried out in the same condition with the linear sweep voltammograms test, and a 50 seconds long period was chosen as the react time with or without illumination. In this work, all photocatalytic reactions were carried out under the tense of 100 mW/cm2 (1 solar light equally) with a Xenon lamp (320 – 780 nm), and the distance from lamp to the reactor was 5 cm. The same concentration of electrolyte in the fuel cell offered an identical potential difference for the two electrode reactions. Fourier transform infrared test of BiOCl was carried out by using a Shimadzu IR-Prestige21 with potassium bromide tablet. Raman result was recorded with laser Raman microscope (inVia, Renishaw). Atomic Force Microscope results were obtained from BRUKER ICON2-SYS. Gas chromatography-mass spectrometry (Thermo, Trace GC + Polaris Q) tests were carried out to check out the results of reaction process after 20 hours (2× 10 h).
3. Results and discussion
Fig. 2 (a) Comparison of XRD patterns; (b) UV-vis DRS and bandgap estimate of BiOCl; (c-f) XPS of BiOCl.
3.1 Materials synthesis and characterization
The ellipsoidal BiOCl was synthesized on the FTO substrate in situ by solvothermal method. We performed the X-ray diffraction (XRD) analysis of BiOCl scraped down from BiOCl-coated FTO, BiOCl-coated FTO and FTO respectively. As shown in Fig. 2a, the XRD pattern of BiOCl can be well indexed to the tetragonal phase of BiOCl (JCPDS No. 06-0249), which also reveals that the prepared sample is well crystallized. The diffraction peak at 32.5 ° corresponds to the (110) plane of BiOCl with the lattice spacing of 0.275 nm. The diffraction
peaks at 12 ° and 25.9 ° are corresponded well to the (001) and (101) plane of BiOCl with the lattice spacing of 0.738 nm and 0.344 nm, which also shows a good correspondence to the high resolution transmission electron microscopy image of BiOCl as seen in Fig. 3c. Fig. 2b displays the UV-vis diffuse reflectance spectrum (UV-vis DRS) of BiOCl. It can be observed that BiOCl has a strong absorption of the light with wavelengths shorter than 360 nm. But additionally, the prepared BiOCl has a certain absorption in the visible light region which is different from reported
[24]
, indicating that the material may have an abundant
inter-band states owing to the chemisorbed molecules like H2O or O2 etc. Since BiOCl has an indirect bandgap
[29]
, we calculated the bandgap by Kubelka-Munk function [F(R∞)hν]1/2–hν
with the absorbance data from UV-vis DRS as shown in the inset of Fig. 2b. The bandgap value of BiOCl (3.34eV) is consistent with the result reported as well [30]. X-ray photoelectron spectroscopy (XPS) tests have been carried out to further confirm the valency of elements. Fig. 2c shows a full scan spectrum of BiOCl, indicating the existence of bismuth, oxygen and chlorine, while Fig. 2d-f show the test results of the elements within respectively. Two peaks at 159.4 and 164.5 eV are corresponded to Bi 4f7/2 and Bi 4f5/2 of Bi3+, and two peaks at 197.9 and 199.5 eV are found in Fig. 2f, corresponding to Cl 2p3/2 and Cl 2p1/2 of Cl-. The O 1s XPS spectrum of BiOCl has been separated into two peaks at 530.1 and 531.9 eV, which are assigned to be the oxygen in crystal lattice (O L) and oxygen vacancies (OV), while the peak at 533.2 eV is speculated to be the residual H2O
[26]
.
The existence of oxygen vacancies enhances the absorbance of oxygen and water molecules on the surface of BiOCl particles, which can form the surface states, leading to the sub-bandgap transition and thus improving the photocatalytic activity of the material further
due to the additional absorbance of visible light. The analysis results of XPS spectra, taken together with the XRD patterns comparison, provide strong evidence that BiOCl has been synthesized accurately. Fourier Transform Infrared (FTIR) spectroscopy of BiOCl was obtained to analyze the structure as well. Though dried immediately before test, a sharp peak in 3450 cm-1 owing to the vibration of -OH can still be found in the FTIR spectrum of BiOCl notably, which indicates that the excess water is physically adsorbed in the oxygen vacancy of BiOCl (Fig. S2). The existence of oxygen vacancies trends to combine oxygen on the surface of photocathode, which can improve the overall efficiency by promoting the cathode oxygen reduction reaction process in the fuel cell.
Fig. 3 SEM images of BiOCl on FTO (a, b); TEM image of BiOCl (c); Elemental mapping of BiOCl (d).
The BiOCl film was further characterized by field emission scanning electron microscopy (FE-SEM) and high resolution transmission electron microscopy (HRTEM).
Although tiny cracks can be observed from Fig. 3a, FE-SEM results show that BiOCl particles are distributed uniformly and densely on the FTO substrate (Fig. 3a). When zooming in to examine the details of the surface, the acicular structure with a width of about 14 nm of BiOCl is observed, which may be capable of providing a high contact area during the reaction process. To measure the thickness of BiOCl and investigate the morphology of the coated film, atomic force microscopy (AFM) has been carried out (Fig. S4). AFM results show a height of 1.2 μm of the BiOCl particle, which is the same with the FE-SEM result, proving that the BiOCl particles on FTO are shaped like spheres, or ellipsoids, exactly. We have tested the cross-sectional SEM of the BiOCl coated FTO and the results were shown in Fig. S7. The top-right side of the image is FTO, and the bottom-left side is the BiOCl film. Clearly, a rough surface of the BiOCl film can be observed, and the thickness of the film is about 1 μm, which is consistent with the SEM and AFM results shown in Fig. 3 and Fig. S4. As shown in Fig. 3c, it is obvious that the fringe spacing of 0.74 nm and 0.17 nm can be found in the HRTEM image of BiOCl, corresponding well to the lattice space of (001) and (202), which is also consistent with the XRD pattern shown in Fig. 3c. (BiOCl, P4/nmm, more information in PDF#06-0249). The large fringe spacing of (001) which is rare in common materials also ensures the accuracy of BiOCl. TEM elemental mapping was carried out to check out the distribution of different elements in the sample (Fig. 3d). The elemental mapping test result proves the existence of element Bi, O and Cl in the sample, and the image of the selected area clearly shows the distribution of different elements in BiOCl. Fig. S3 shows the Raman spectrum of BiOCl. The strong peaks at 112.09 and 142.35 cm-1 correspond to the internal Bi-Cl stretching mode. Notice that when ramen shift increases
over 3000 cm-1, the response signal increases sharply owing to the adsorbed water in the vacancies of BiOCl , which is consistent with the FTIR spectrum. 3.2 Electrochemical characterization
Fig. 4 (a) Amperometric I-t curves without furfural (a) and with furfural (b); (b) LSV curves with light off/on; (c) CV curves of BiOCl (vs. RHE) with light off/on; (d) CV curve of furfural (vs. RHE).
We tested the performance of the as-developed furfural fuel cell. Fig. 4a shows the Amperometric I-t curve test results with light off/on. Obviously, the current value has been increased up to an order of magnitude after illuminating, indicating that the photogenerated carriers can promote the electrode reaction under photostimulation. A decay tendency in current value can be observed, which may caused by the chemisorbed furfural on the surface of Pt. Then we investigated the linear sweep voltammograms (LSV) tests as shown in Fig. 4b. A larger value of open circuit voltage (Voc) is observed (0.82 V vs. 0.65 V), showing a higher electromotive force of the fuel battery under light, which is conformed to the prediction of the electrode reaction mechanism (Fig. 5). A sharp increase on short circuit current (Isc) level (14.2 vs. 4.5) owing to the more photogenerated carriers produced by illuminating indicates
that BiOCl shows an obvious photocatalytic activity in the fuel cell reaction. The comparative LSV test of Pt-Pt system furfural fuel cell, a standard effective collocation in most of the fuel cells, was carried out in the dark with other conditions unchanged. A higher Isc than Pt-BiOCl fuel cell under illumination condition is obtained, and the Voc is measured to be 0.63 V, which is almost the same with the Pt-BiOCl fuel cell without light, but smaller than the one under illumination, confirming that the light-induced oxygen reduction reaction on the semiconductor BiOCl photocathode can actually afford an additional auxiliary effect for enhancing the performance of the furfural fuel cell. To test the electrochemical property of BiOCl, we performed the cyclic voltammetry (CV) in simple electrolyte solution (0.02 mol/L Na2SO4) which was consistent with the actual condition of the fuel cell reactions. As shown in CV test from Fig. 4c, the current of reduction peak has been increased under illumination, and the potential of reduction peak of oxygen has been moved from 0.94 V to 1.2 V, which is more closer to 1.23 V, the theoretical standard oxygen reduction potential, proving that BiOCl is an efficient material towards the reduction reaction of oxygen. The CV test of furfural with Pt sheet as both cathode and anode in 0.02 mol/L Na2SO4 electrolyte solution was also performed in the dark condition as shown in Fig. 4d. The measured potential of oxidation peaks of furfural are 0.02 V and 1 V, which indicates that furfural is easy to be oxidized in the photocatalytic fuel cell.
Fig. 5 Schematic of the electrode reaction mechanism in photocatalytic fuel cell.
In theory, the difference of the oxidation potential of furfural and reduction potential of oxygen is the electromotive force of the fuel cell. The measured electromotive force value of the furfural fuel cell under illumination (V’, Fig. 5) is 1.18 V, while the electromotive force value without light (V) is 0.92 V (calculated from Fig. 4c and 4d). Under light conditions, a holes’ quasi Fermi level (EFP) of BiOCl which acts as the actual potential of cathode is formed. Since EFP is lower than the standard oxygen reduction potential, a higher electromotive force should be obtained, indicating that solar energy can promote the catalytic process of furfural in the fuel cell. Obviously, the obtained higher electromotive force value with photostimulation confirms our inference, indicating that solar energy can be successfully utilized in the fuel cell together with biomass energy. Gas chromatography-mass spectrometry (GC-MS) tests were carried out to verify the existence of the photodegradation process of furfural (Fig. S6). By using dichloromethane as the extractant, the organic parts in the solution were successfully extracted and gathered. It is quite obvious that the amount of furfural has been decreased to 60.2% (Fig. S6a and Fig. S6b), proving the existence of the photocatalytic energy conversion from chemical energy
into electricity. More than that, as a kind of chemically active aromatic aldehyde, furfural is unstable and is prone to be oxidized, especially when exposed to light and air. Two non-ignorable peaks in the gas chromatograms have also been identified as 5-methyl furfural and 2-acetylfuran. Comparing with the original furfural ingredient, the amount of 5-methyl furfural and 2-acetylfuran after photocatalytic process have been decreased to 43.5% (Fig. S6c and Fig. S6d) and 43.1% respectively (Fig. S6e and Fig. S6f), indicating that these organic compounds can be photodegraded together with furfural in the photocatalytic process of the as-developed fuel cell at the same time. We also performed XRD of the BiOCl photocathode after the photocatalytic reaction of the furfural fuel cell (Fig. S1a). Two comparative experiments have been carried out to check out the stability of BiOCl. As shown in Fig. S1c, the BiOCl-coated FTO turns into dark after 5 seconds long illumination (100mW/cm2) owing to the Bi0 reduced from Bi3+. In addition, a BiOCl-coated FTO turns into dark in 0.02 mol/L Na2SO4 aqueous solution after 3 minutes under illumination. By comparing the XRD patterns of a fresh BiOCl-coated FTO and the BiOCl photocathode after the photocatalytic reaction of the furfural fuel cell, it can be seen that BiOCl photocathode is still stable after the photocatalytic reaction process. 4. Conclusion We demonstrated the synthesis of an evenly distributed BiOCl film on FTO in situ as the novel photocathode for the furfural fuel cell. Under illumination, the p-type semiconductor BiOCl accumulates more photogenerated electrons on the surface, which can enhance the electrocatalytic process further. By choosing semiconductor as the photocathode material, we assembled a fuel cell which can take advantage of solar energy at the same time without
using the noble metal cathode. Not only that, the fuel cell with light responsive material as the photoanode and photocathode may improve the performance as well. Note that the efficiency and power density of this proof-of-concept photocatalytic fuel cell is limited which has substantial room for further improvement. An in-depth research focused on the synthesis and decoration of BiOCl is deserved to be carried out next. There is no exaggeration that the successful design of the photocatalytic furfural fuel cell paves the way for dealing with more kinds of biological wastes in a distinctive way. Nevertheless, this study shows a peculiar way of developing photocathode for photocatalytic fuel cell, which opens up an approach to dealing with agricultural product metabolites and enhancing photoelectric reaction process in future.
Acknowledgments This work was financially supported by Liaoning Revitalization Talents Program - Pan Deng Scholars (XLYC1802005), Liaoning BaiQianWan Talents Program, the National Science Fund of Liaoning Province for Excellent Young Scholars, Science and Technology Innovative Talents Support Program of Shenyang (RC180166), Australian Research Council (ARC) through Discovery Early Career Researcher Award (DE150101306) and Linkage Project (LP160100927), Faculty of Science Strategic Investment Funding 2019 of University of Newcastle, and CSIRO Energy.
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The highlights of this work are: 1. Utilization of biological waste A furfural fuel cell was designed, showing a novel way utilizing biological wastes. 2. Synthesis of a densely distributed BiOCl film on FTO in situ A densely distributed ellipsoid-liked BiOCl film was in-situ synthesized on FTO. 3. BiOCl semiconductor photocathode in the photocatalytic fuel cell The BiOCl photocathode showed a higher electromotive force under illumination. 4. A higher open circuit voltage than the Pt-Pt system Under illumination, I-V test showed a higher Voc than Pt-Pt fuel cell.
Declaration of Interest Statement
BiOCl-based photocathode for photocatalytic fuel cell Yang Wang a, Yanming Wang a, Xi-ming Song a, Yu Zhang a,* and Tianyi Ma b,*
Author Contribution Statement
BiOCl-based photocathode for photocatalytic fuel cell Yang Wang a, Yanming Wang a, Xi-ming Song a, Yu Zhang a,* and Tianyi Ma b,*
Yang Wang: Formal analysis; Investigation; Methodology; Roles/Writing - original draft. Yanming Wang: Supervision. Xi-ming Song: Resources; Software. Yu Zhang: Formal analysis; Investigation; Methodology; Writing - review & editing; Conceptualization; Data curation. Tianyi Ma: Formal analysis; Investigation; Methodology; Writing - review & editing; Funding acquisition.