High-Efficiency Water-Splitting Solar Cells with Low Diffusion

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High Efficiency Water Splitting Solar Cells with Low Diffusion Resistance in Interface between ZrO2 Corresponding to Halochromic Pigments Yi-Sheng Lai, Hsuan-Heng Lu, and Yen Hsun Su ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.7b01154 • Publication Date (Web): 05 Jul 2017 Downloaded from http://pubs.acs.org on July 11, 2017

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ACS Sustainable Chemistry & Engineering

High Efficiency Water Splitting Solar Cells with Low Diffusion Resistance in Interface between ZrO2 Corresponding to Halochromic Pigments Yi-Sheng Laia, Hsuan-Heng Lua, Yen-Hsun Su* Department of Materials Science and Engineering, National Cheng Kung University, Tainan 70101, Taiwan [email protected]

Keyword: ZrO2, water splitting, AC impedance, halochromic pigments

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract ZrO2 nanoparticles films coated with halochromic pigments are applied to the water splitting solar cells. On the basis of our results, ZrO2 nanoparticles films coated with methyl orange have remarkable water splitting properties. In positive applied voltages and AM 1.5G irradiation, the highest hydrogen gas generation rate (1.8 ml/hr∙cm2) is measured from ZrO2 nanoparticles films coated with methyl orange within 0.2 M H2SO4 water solution for electrolyte. As the electrolyte is changed from KHCO3 water solution to the H2SO4 water solution, the interface resistance between ZrO2 corresponding to halochromic pigments is reduced (from 107 ohm·m to 11.7 ohm·m) and the electron diffusion coefficient is raised (821.67%).

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Introduction For solving the risk of energy and the environmental pollutions, water splitting solar cells are the high attractive option.1-5 Generally, solar light is permanent and widely distributed on the earth. A non-toxic and storable solar energy is helpful in reducing the pollutions and rising the energy generation for our plant. Hydrogen gas is high energy density gas using in fuel cells.6-9 Water is the main constituent of Earth. Thus, water splitting for generation hydrogen gas is the best choice. ZrO2 with extraordinary wide band gap, which is necessary for preparing ultrahigh performance water splitting solar cells.8-13 As the band bap of ZrO2 is too wide to absorb the solar light for exciting the electrons from valence band to conduction band. The photosensitizers provide suitable excited energy level for acting as the step of the conduction band of ZrO2, which is necessary. Methyl violet, methylene blue, methyl orange and Congo red are the candidates which have suitable excited energy level for the conduction band of ZrO2. In the hydrogen generation from water splitting, KHCO3 and H2SO4 solutions are generally used as the electrolyte. These halochromic pigments tune the excited energy level by difference pH value.14-16 Thus, these halochromic pigments have wide application prospects in the water splitting experiments. According to the water splitting circuit, the interface resistance between 3



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electrode and electrolyte and the electron diffusion coefficient in electrolyte are involved.17-20 Thus, the electrochemical impedance spectroscopy (EIS) and the Nyquist plots are applied to investigate the determination of the interface resistance and the calculation of diffusion coefficient. The main ideal of this study are the followings: (1) The ZrO2 is used as the semiconductor material for hydrogen generation from water splitting. (2) The energy level of halochromic pigments are tuned by the pH value of electrolyte for fitting the conduction band of ZrO2. (3) EIS and the Nyquist plots are applied to investigate the electrochemical analysis and thermodynamic calculation.

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Experiment section Material. ZrO2 nanoparticles and zirconium dichloride oxide (ZrOCl2·8H2O) are purchased from Sigma-Aldrich. The following samples are analytical grade without further purification: Methyl Violet 2B (methyl violet), Methylene Blue (methyl blue) and Methyl Orange (methyl orange), which are purchased from Acros Organic. Congo red are purchased from Tokyo Chemical Industry (TCI). Potassium bicarbonate (KHCO3) and Sulfuric acid (H2SO4) are purchased from Showa Chemical Industry. High performance liquid chromatography (HPLC) grade ethanol and deionized water are used in all the experiments. Fabrication of ZrO2 nanoparticles films matrixed electrode. ZrO2 nanoparticles (0.3 g) are dispersed in 10 ml ethanol for preparing the suspension solution. The dipping volume of ZrO2 nanoparticles suspension solution on the pre-cleaned indium tin oxide (ITO) glass is 1 ml/cm2. The ZrO2 nanoparticles films are dried in air and room temperature. Then, the zirconium dichloride oxide ethanol solution (0.5g ZrOCl2 ·8H2O in 10 ml ethanol) is dipped into the ZrO2 nanoparticles films (1 ml/cm2). The ZrO2 nanoparticles films are drying in air before backed under 150℃ for 12 hr.

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The halochromic pigments are dissolved into the ethanol for preparing the halochromic pigments solution with standard concentration (2 × 10-5 M). ZrO2 nanoparticles films are soak in these halochromic ethanol solutions for 12 hours. The halochromic pigments are coated on ZrO2 nanoparticles films. Fabrication of Pt films. Pt are depositing on the pre-cleaned ITO glass by ionic reaction deposition sputter method. The sputter time is 1000 seconds and the sputter current is 20 mA. The thickness of Pt film on ITO is about 20 nm. The conductivity is about 2.79 (S/m). Characterization. The X-ray diffraction (XRD) patterns of ZrO2 nanoparticles are measured and obtained by Bruker AXS (D8) X-ray diffractometer with Cu Kα radiation. The images of high resolution field emission scanning electron microscopy is performed on a Hitachi 6700F microscope. The UV-vis absorbance of the halochromic pigments solution are measured by CT-2200 spectrophotometer. The solar light is simulated by AM 1.5G (1000 W/m2). The electrochemical analyzer (CH Instruments 6273E) is combined with AM1.5G for carrying out the experiments of hydrogen generation and obtaining the electro impedance spectrums (EIS). Hydrogen generation from water splitting. The water splitting efficiency for hydrogen generation is carried by ZrO2 6


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nanoparticles films coated with halochromic pigments under AM 1.5G irradiation within electrolyte. KHCO3 water solution (0.4 M) and H2SO4 water solution (0.2 M) are served as the electrolyte for water splitting, respectively. The pH value of 0.4 M KHCO3 water solution is 9.0. The pH value of 0.2 M H2SO4 water solution is 1.5. The temperature is effect the water splitting experiments. Thus, the temperature is setting at room temperature (25℃). Electrochemical impedance spectroscopy test. AM 1.5G is used as a simulated solar source in the ESI experiment. The measurements are performed at room temperature at 0.7 V. The EIS spectra are fitted to an appropriate equivalent circuit by CH Instruments. The thermodynamic diffusion coefficient of the electrons in the electrolyte is discussed. The experiment setting is following: frequency from 100000 to 1 Hz with amplitude 0.7 V.

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Results and discussion UV-vis Absorbance Spectrum. The absorption spectrum of halochromic pigments within different electrolyte are shown in Figure 1. From Figure 1(a), in the ethanol and 0.4 M KHCO3 water solution, the methyl violet has a strong absorption at 587 nm. In the 0.2 M H2SO4 water solution, methyl violet has two strong absorptions at 622 nm and at 423 nm. From Figure 1(b), the methylene blue which is dissolved in ethanol solution demonstrates two strong absorption peaks at 657 nm and at 293 nm. While the methylene blue is dissolved in 0.4 M KHCO3 water solution and 0.2 M H2SO4 water solution, the strong absorption peaks are shown at 667 nm and at 293 nm. From Figure 1(c), the methyl orange has the strongest absorption peak at 425 nm within ethanol solution. When methyl orange is dissolved in 0.4 M KHCO3 water solution, the strongest absorption peak shifts to 472 nm. As methyl orange is dissolved in 0.2 M H2SO4 water solution, the strongest absorption peak is at 510 nm. From Figure 1(d), the Congo red shows a strong absorption peak at 511nm within ethanol solution. In the 0.4 M KHCO3 water solution, Congo red has a strong absorption peak at 486 nm. In the 0.2 M H2SO4 water solution Congo red demonstrates a wide absorption range between 200 nm and 800 nm. According to Figure 1, the halochromic pigments demonstrate different absorption peaks within different electrolyte (under the same 8


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concentration). Thus, the energy level of the halochromic pigments are tuned by different electrolyte. Photocurrent density and the efficiency of the water splitting solar cells. The efficiency of water splitting follows the equation (1).

(1) The photocurrent density at Eop is Jop, Eop is the applied voltage under optimal operating conditions. The intensity of incident solar light (AM 1.5 G) is marked as Int. The theoretical voltage for electrolyzing water into O2 and H2 is 1.23 V. To investigate the water splitting efficiency of ZrO2 nanoparticles films coated with halochromic pigments, the experiment is set under AM 1.5G irradiation within 0.4 M KHCO3 water solution or 0.2 M H2SO4 water solution. In positive applied voltages, the halochromic pigments which is coated on ZrO2 nanoparticles films generate the excited electrons by absorbing the solar energy. The excited electrons transport into the conduction band of ZrO2. Then, the electrons inject into the electrolyte for hydrogen gas generation by Pt films cathode. In negative applied voltages conditions, the excited electrons generated from halochromic pigments inject into the electrolyte, directly. The excited electrons reduce the H+ ions into hydrogen gas. Under AM 1.5 G irradiation and 0.4 M KHCO3 water solution for electrolyte, the 9



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photocurrent density and the water splitting efficiency of ZrO2 nanoparticles films coated with halochromic pigments are measured and studied. The related results are shown in Figure 2 and Table 1. From Figure 2(a), the photoelectric current density of ZrO2 nanoparticles films coated with methyl orange is the highest. The current density at Jop is 1.81 A/m2 and the water splitting efficiency is 0.077%. The results of the hydrogen generation of ZrO2 nanoparticles films coated with halochromic pigments are plotted in Figure 2(b). From the Figure 2(b), the hydrogen generation rate of ZrO2 nanoparticles films coated with methyl orange is faster than that of the others ZrO2 nanoparticles films coated with halochromic pigments. Hydrogen gas 0.43 ml is generated in 1800 s from ZrO2 nanoparticles films coated with methyl orange (the sample size is 1 cm2). The hydrogen generation rate of ZrO2 nanoparticles films coated with methyl orange is 0.83 ml/hr·cm2. Then, 0.2 M H2SO4 water solution is replaced by 0.4 M KHCO3 water solution for electrolyte. The related results of water splitting efficiency of ZrO2 nanoparticles films coated with halochromic pigments was studied under AM 1.5G irradiation, positive applied voltages and 0.2 M H2SO4 water solution for electrolyte, as shown in Figure 3 and Table 2. From the Figure 3(a), the photoelectric current density of ZrO2 nanoparticles films coated with methyl orange is the highest. The current density at Jop is 3.83 A/m2 and the water splitting efficiency is 0.236%. From 10


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the Figure 3(b), hydrogen gases 0.9 ml is generated by ZrO2 nanoparticles films coated with methyl orange in 1800 s (the sample size is 1 cm2). The hydrogen generation rate of ZrO2 nanoparticles films coated with methyl orange is 1.76 ml/hr·cm2. The experimental results from the different electrolytes demonstrate that the ZrO2 nanoparticles films coated with methyl orang have the best water splitting efficiency and hydrogen generation rate. From Figure 1, the excited state energy level of methyl orange within 0.2 M H2SO4 water solution is calculated (-2.18 eV), which is much higher than the conduction band of ZrO2 (-2.5 eV). While methyl orange is generated the excited electrons by absorbing the light from AM 1.5G irradiation, the excited electrons transfer to the conduction band of ZrO2. The excited electrons migrate to the Pt electrode and injected into electrolyte for hydrogen gas generation. Electrochemical Impedance Spectroscopy (EIS) analysis. The EIS is modeled by the equivalent electrical analogs, as shown in Figure 4. The model consists of the interface resistance (RS), cell resistance (RI), effective capacitive (CP). The operating setting for EIS measurements of ZrO2 nanoparticles films coated with halochromic pigments is under AM 1.5G irradiation with a potential bias of 0.7 V and 100000 to 1 Hz for testing range. The related EIS results and Nyquist plots of ZrO2 nanoparticles films coated with 11



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halochromic pigments within 0.4 M KHCO3 water solution are shown in Figure 5. From the Figure 4 (a), ZrO2 nanoparticles films coated with methyl orange shows the smallest Nyquist plots, which means the smallest charge transfer resistances between semiconductor and pigment. As seen the Bode plot and the phase angle plot (log |Z| vs log f, and vs log f) in Figure 4 (b), the ideal capacitor21 is related to the slope value (

| |

。

approach -1) and the phase angle values (approach 90 ). Thus, the ZrO2

nanoparticles films coated with Congo red is referred to the ideal capacitor. The EIS results of ZrO2 nanoparticles films coated with halochromic pigments within 0.2 M H2SO4 water solution are shown in Figure 5. From Figure 5(a), the semicircle of ZrO2 nanoparticles films coated with methyl orange shows the smallest Nyquist plots. From Figure 5(b), the slope value and the phase angle demonstrate that the water splitting cell of ZrO2 nanoparticles films coated with methyl orange under AM 1.5G irradiation within 0.2 M H2SO4 water solution electrolyte is far away from the ideal capacitor. The electrons transporting phenomena is determined by the result of thermodynamic calculation17-20. The electron mobility, electrons diffusion life time, diffusion coefficient and average distance are include in the results of thermodynamic calculation, which is shown in Table 3. The electron mobility (µ) is determined by using Equation 2 12


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σ=µnq

(2)

where σ is the electric conductivity (S·m−1), n is the electron concentration, q is the electricity(1.6×10-19C). The diffusion coefficient D is determined by Equation 3,

kB is the Boltzmann constant (1.38×10-23JK-1) and T is the Kelvin temperature. D=µkBT/e

(3)

The electrons diffusion life time (τ) is calculated by the following Equation 4,

τ=µmH+/e where mH

+

(4)

is the mass of the hydrogen ion. The average distance is

determined by Equation 5.

=√Dτ

(5)

The values of electric conductivity are calculated from Equation 6

l A

R=

(6)

where R is interface resistance, l is the length of the resistance and A is the area of the resistance. Equation 1 suggests that electron mobility is related to electric conductivity. From Table 3, the ZrO2 nanoparticles films coated with methyl orange have the smallest interface resistance and capacitive in 0.4 M KHCO3 and 0.2 M H2SO4 water solutions. According to the interface resistance between ZrO2 and halochromic pigments, the low interface resistance induces the high electron mobility and the high diffusion 13



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coefficient of the electrons in the interface. As the electron mobility is carried into the Equation 4, the electrons diffusion life time is calculated. The average diffusion distance is calculated by Equation 5 and the diffusion life time which is from Equation 4. In the water splitting experiments, the ZrO2 nanoparticles films coated with methyl orange have the highest water splitting efficiency. The EIS results show that the charge transfer resistances between the ZrO2 nanoparticles films and methyl orange is the smallest. The energy level of the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) for the ZrO2 and halochromic pigments are shown in Figure 6. The HOMO state of the halochromic pigments is measured by the cyclic voltammetry methods and the LUMO states is calculated from the onset absorption peak of the UV-vis spectrum. From the Figure 6, the excited energy level of the halochromic pigments is higher than the conduction band of ZrO2. The valence band of the ZrO2 is lower than all ground state energy level of halochromic pigments. According to the characterization results, the mechanism of the hydrogen generation from water splitting is proposed in Figure 8. The electrons are excited to the LUMO state by the halochromic pigments absorbing the incident solar energy (AM 1.5G irradiation). The excited electrons migrate to the conduction band of ZrO2, follow the circuit, 14


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and then inject into the electrolyte for reducing H+ ions into hydrogen gas. The electron holes are migrated into the water for oxidizing the OH- into oxygen gas. The energy difference between the LUMO state energy level of photosensitizer and the conduction energy level of ZrO2 is 0.32 eV.22-25 The methyl orange is able to be tuned the LUMO state energy level by 0.2 M H2SO4 water solution. Thus, the efficiency of the hydrogen generation enhances.

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Conclusions ZrO2 nanoparticles films coated with methyl orange demonstrate the highest efficiency of hydrogen generation from water splitting within 0.2 M H2SO4 water solution in positive applied voltage. The photocurrent density is 3.83 A/m2 and the hydrogen generation rate is 1.76 ml/hr·cm2.Simultaneously, the energy level of LUMO states of halochromic pigments are tuned by 0.4 M KHCO3 and 0.2 M H2SO4 water solution due to the pH value modulation. When the electrolyte is changed from KHCO3 water solution into the H2SO4 water solution, the interface resistance is reduced from 107 ohm·m to 11.7 ohm·m.

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Figure Legends.

Figure 1. Absorption spectra of methyl violet (a), methylene blue (b), methyl orange (c) and Congo red (d) solution (black line: ethanol solution, blue line: 0.4 M KHCO3 solution and red line: 0.2 M H2SO4 solution).

Figure 2. The photoelectric current density (a) and hydrogen generation rate (b) of ZrO2 nanoparticles films coated with halochromic pigments on the ITO glass substrate under AM 1.5 G irradiation and applied positive voltages within 0.4 M KHCO3 water solution as the electrolyte. (Methyl violet: ZrO2 nanoparticles films coated with methyl violet, Methylene blue: ZrO2 nanoparticles films coated with methylene blue, Methyl orange: ZrO2 nanoparticles films coated with methyl orange, and Congo red: ZrO2 nanoparticles films coated with Congo red).

Figure 3. The photoelectric current density (a) and hydrogen generation rate (b) of ZrO2 nanoparticles films coated with halochromic pigments on the ITO glass substrate under AM 1.5 G irradiation and applied positive voltages within 0.2 M H2SO4 water solution as the electrolyte for water splitting. (Methyl violet: ZrO2 nanoparticles films coated with methyl violet, Methylene blue: ZrO2 nanoparticles films coated with methylene blue, Methyl orange: ZrO2 nanoparticles films coated 17



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with methyl orange, and Congo red: ZrO2 nanoparticles films coated with Congo red).

Figure 4. The Nyquist diagram (a) and Bode plots (b) of ZrO2 nanoparticles films coated with halochromic pigments under AM 1.5G irradiation, at 0.7 V in a 2-electrode configuration within 0.4 M KHCO3 electrolyte (Methyl violet: ZrO2 nanoparticles films coated with methyl violet, Methylene blue: ZrO2 nanoparticles films coated with methylene blue, Methyl orange: ZrO2 nanoparticles films coated with methyl orange, and Congo red: ZrO2 nanoparticles films coated with Congo red). Equivalent electrical analogs for modeling the electrochemical cell of ZrO2 nanoparticles films coated with halochromic pigments in a 2-electrode configuration at a potential bias of 0.7 V (RS: interface resistance between electrode and electrolyte, RI: cell resistance, CP: effective capacitive).

Figure 5. The Nyquist diagram (a) and Bode plots (b) of ZrO2 nanoparticles films coated with halochromic pigments under AM 1.5G irradiation, at 0.7 V in a 2-electrode configuration within 0.2 M H2SO4 electrolyte (Methyl violet: ZrO2 nanoparticles films coated with methyl violet, Methylene blue: ZrO2 nanoparticles films coated with methylene blue, Methyl orange: ZrO2 nanoparticles films coated with methyl orange, and Congo red: ZrO2 nanoparticles films coated with Congo red). 18


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Figure 6. The energy levels of the halochromic pigments within the KHCO3 and H2SO4 water solution.

Figure 7. Scheme of sandwich structure for ZrO2 nanoparticles films coated with halochromic pigments on the ITO glass substrate under AM 1.5 G irradiation for water splitting.

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Table Legends.

Table 1. Water splitting properties of ZrO2 nanoparticles films coated with halochromic pigments on the ITO glass substrate within 0.4 M KHCO3 as electrolyte under AM 1.5 G irradiation. (Methyl violet: ZrO2 nanoparticles films coated with methyl violet, Methylene blue: ZrO2 nanoparticles films coated with methylene blue, Methyl orange: ZrO2 nanoparticles films coated with methyl orange, and Congo red: ZrO2 nanoparticles films coated with Congo red).

Table 2. Water splitting properties of ZrO2 nanoparticles films coated with halochromic pigments on the ITO glass substrate within 0.2 M H2SO4 as electrolyte under AM 1.5 G irradiation. (Methyl violet: ZrO2 nanoparticles films coated with methyl violet, Methylene blue: ZrO2 nanoparticles films coated with methylene blue, Methyl orange: ZrO2 nanoparticles films coated with methyl orange, and Congo red: ZrO2 nanoparticles films coated with Congo red)

Table 3. The electron diffusion activation of thermodynamic calculation.

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(a)

(b) 3.0

3.0 Ethanol 0.4M KHCO3

Ethanol 0.4M KHCO3

2.5

0.2M H2SO4

0.2M H2SO4

Adsorption(a.u.)

Adsorption(a.u.)

2.5 2.0 1.5 1.0 0.5 0.0 200

2.0 1.5 1.0 0.5

300

400

500

600

700

0.0 200

800

300

400

Wavelength(n.m.)

700

800

700

800

3.0 Ethanol 0.4M KHCO3

2.5

0.2M H2SO4

Ethanol 0.4M KHCO3

Adsorption(a.u.)

0.2M H2SO4

2.0 1.5 1.0 0.5 0.0 200

600

(d)

3.0 2.5

500

Wavelength(n.m.)

(c)

Adsorption(a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.0 1.5 1.0 0.5

300

400

500

600

700

0.0 200

800

300

400

Wavelength(n.m.)

500

600

Wavelength(n.m.)

Figure 1. Absorption spectra of methyl violet (a), methylene blue (b), methyl orange (c) and Congo red (d) solution (black line: ethanol solution, blue line: 0.4 M KHCO3 solution and red line: 0.2 M H2SO4 solution).

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(a)

2

Current density (A/m )

2.5 Methyl Violet Methylene Blue Methyl Orange Congo Red

2.0

1.5

1.0

0.5

0.0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

Potential(V)

(b)

H2 generation (ml/cm2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.5 Methyl violet Methylene blue Methyl orange Congo red

0.4

0.3

0.2

0.1

0.0 0

200

400

600

800 1000 1200 1400 1600 1800

Time (seconds)

Figure 2. The photoelectric current density (a) and hydrogen generation rate (b) of ZrO2 nanoparticles films coated with halochromic pigments on the ITO glass substrate under AM 1.5 G irradiation and applied positive voltages within 0.4 M KHCO3 water solution as the electrolyte. (Methyl violet: ZrO2 nanoparticles films coated with methyl violet, Methylene blue: ZrO2 nanoparticles films coated with methylene blue, Methyl orange: ZrO2 nanoparticles films coated with methyl orange, and Congo red: ZrO2 nanoparticles films coated with Congo red).

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(a)

Current density (A/m2)

6 Methyl Violet Methylene Blue Methyl Orange Congo Red

5 4 3 2 1 0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

Potential(V)

(b) 1.0

H2 generation (ml/cm2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Methyl violet Methylene blue Methyl orange Congo red

0.8

0.6

0.4

0.2

0.0 0

200

400

600

800 1000 1200 1400 1600 1800

Time (seconds)

Figure 3. The photoelectric current density (a) and hydrogen generation rate (b) of ZrO2 nanoparticles films coated with halochromic pigments on the ITO glass substrate under AM 1.5 G irradiation and applied positive voltages within 0.2 M H2SO4 water solution as the electrolyte for water splitting. (Methyl violet: ZrO2 nanoparticles films coated with methyl violet, Methylene blue: ZrO2 nanoparticles films coated with methylene blue, Methyl orange: ZrO2 nanoparticles films coated with methyl orange, and Congo red: ZrO2 nanoparticles films coated with Congo red).

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-10000

(a)


-8000

Z'' (ohm)

-6000

-4000

-2000

0 0

500

1000

1500

2000

2500

3000

5

6

Z' (ohm)

(b)

6

2

log |Z|(ohm.cm )

5 4 Methyl violet Methylene blue Methyl orange Congo red

3 2 1 0 0

1

2

3

4

log f (Hz)

80

Phase angle (theta)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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60

40

20

0 0

1

2

3

4

5

6

log f (Hz)

Figure 4. The Nyquist diagram (a) and Bode plots (b) of ZrO2 nanoparticles films coated with halochromic pigments under AM 1.5G irradiation, at 0.7 V in a 2-electrode configuration within 0.4 M KHCO3 electrolyte (Methyl violet: ZrO2 nanoparticles films coated with methyl violet, Methylene blue: ZrO2 nanoparticles films coated with methylene blue, Methyl orange: ZrO2 nanoparticles films coated with methyl orange, and Congo red: ZrO2 nanoparticles films coated with Congo red). 24


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Equivalent electrical analogs for modeling the electrochemical cell of ZrO2 nanoparticles films coated with halochromic pigments in a 2-electrode configuration at a potential bias of 0.7 V (RS: interface resistance between electrode and electrolyte, RI: cell resistance, CP: effective capacitive).

25



-10000

(a) Methyl violet Methylene blue Methyl orange Congo red

-8000

Z'' (ohm)

-6000

-4000

-2000

0 0

500

1000

1500

2000

2500

Z' (ohm)

6

(b) log |Z| (ohm.cm2)

5 4 Methyl violet Methylene blue Methyl orange Congo red

3 2 1 0 0

1

2

3

4

5

6

log f (Hz)

100

Phase angle (theta)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 35


80

60

40

20

0 0

1

2

3

4

5

6

log f (Hz)

Figure 5. The Nyquist diagram (a) and Bode plots (b) of ZrO2 nanoparticles films coated with halochromic pigments under AM 1.5G irradiation, at 0.7 V in a 2-electrode configuration within 0.2 M H2SO4 electrolyte (Methyl violet: ZrO2 nanoparticles films coated with methyl violet, Methylene blue: ZrO2 nanoparticles films coated with methylene blue, Methyl orange: ZrO2 nanoparticles films coated with methyl orange, and Congo red: ZrO2 nanoparticles films coated with Congo red). 26


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6. The energy levels positions of the halochromic pigments within the KHCO3 and H2SO4 water solution.

27



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 7. Scheme of sandwich structure for ZrO2 nanoparticles films coated with halochromic pigments on the ITO glass substrate under AM 1.5 G irradiation for water splitting.

28


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1. Water splitting properties of ZrO2 nanoparticles films coated with halochromic pigments on the ITO glass substrate within 0.4 M KHCO3 as electrolyte under AM 1.5 G irradiation. (Methyl violet: ZrO2 nanoparticles films coated with methyl violet, Methylene blue: ZrO2 nanoparticles films coated with methylene blue, Methyl orange: ZrO2 nanoparticles films coated with methyl orange, and Congo red: ZrO2 nanoparticles films coated with Congo red). J at 1.23V 2

(A/m )

Jop (A/m2)

1.23 - Eop (V)

η(%)

H2:ml/hr·cm2

Methyl Violet

2.18

1.09

0.157

0.017%±0.005%

0.51

Methyl Blue

0.91

0.47

0.257

0.012%±0.004%

0.21

Methyl Orange

2.49

1.81

0.425

0.077%±0.005%

0.83

Congo Red

0.12

0.02

0.355

0.001%±0.002%

0.01

Scan range: 0~1.23V Scan rate: 50mV/s

29



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 35

Table 2. Water splitting properties of ZrO2 nanoparticles films coated with halochromic pigments on the ITO glass substrate within 0.2 M H2SO4 as electrolyte under AM 1.5 G irradiation. (Methyl violet: ZrO2 nanoparticles films coated with methyl violet, Methylene blue: ZrO2 nanoparticles films coated with methylene blue, Methyl orange: ZrO2 nanoparticles films coated with methyl orange, and Congo red: ZrO2 nanoparticles films coated with Congo red) J at 1.23V 2

(A/m )

Jop (A/m2)

1.23 - Eop (V)

η(%)

H2:ml/hr·cm2

Methyl Violet

20.83

2.99

0.725

0.217%±0.09%

1.37

Methyl Blue

3.62

1.35

0.535

0.072%±0.002%

0.62

Methyl Orange

4.95

3.83

0.615

0.236%±0.08%

1.76

Congo Red

0.43

0.16

0.297

0.005%±0.002%

0.07

Scan range: 0~1.23V Scan rate: 50mV/s

30


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48


Table 3. The electron diffusion activation of thermodynamic calculation. pigments

R (ohm·m)

Capacitance(F) -5

(S·m−1)

(m2/(V·s)) 19

Methyl violet

223.8

1.76×10

4.47

6.98×10

KHCO3

Methylene blue

1606.0

3.22×10-5

0.62

9.73×10

0.4M

Methyl orange

107.0

1.71×10-5

9.35

1.46×10

Congo red

14150.0

6.25×10-5

1.10×10

Methyl violet

26.7

1.56×10-5

7.07×10 37.41

H2SO4

Methylene blue

423.2

2.34×10-5

2.36

3.69×10

0.2M

Methyl orange

11.7

1.46×10-5

85.54

1.34×10

Congo red

12400.0

5.25×10-5

8.06×10

18 20

-2

18 20

5.85×10

19 21

-2

31


18

1.26×10

D (m2/s)

(second) 15

0.29

1.45×10

0.04

2.02×10

0.60

3.03×10

0.0046

2.29×10

2.42

1.21×10

0.15

7.66×10

5.53

2.77×10

0.01

2.62×10

14 15 13 16 14 16 13

(m) 7

2.05×10

6

2.85×10

7

4.28×10

5

3.24×10

8

1.71×10

7

1.08×10

8

3.92×10

5

3.69×10


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Synopsis Halochromic pigments as photosensitizers controlled by pH value electrolytes to tune energy levels for ZrO2 based water splitting solar cell.


High-Efficiency Water-Splitting Solar Cells with Low Diffusion

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