One-Compartment Hydrogen Peroxide-Photofuel Cell Using TiO₂ Photoanode and Prussian Blue Cathode

A paste containing anatase TiO₂ particles with mean size of 20 nm (PST-18NR, Nikki Syokubai Kasei) was coated on FTO electrode (sheet resistance = 10 Ω/square) by the doctor blade technique. The as-coated film was heated at 773 K for 1 h to form mesoporous TiO₂ nanocrystalline film on FTO (mp-TiO₂/FTO). The specific surface area and pore volume of mp-TiO₂ were determined to be 17.28 m² g⁻¹ and 0.72 cm³ g⁻¹, respectively, by nitrogen adsorption-desorption isotherms at 77 K with a micromeritics automatic surface area and porosimetry analyzer (TriStar 3000, Shimadzu). Pore size distribution analysis by the Barret-Joyner-Halenda method afforded a peak pore size of ∼20 nm.¹⁹

Sol-gel TiO₂ films were formed on FTO by a dip-coating method (d-TiO₂/FTO).²⁰ A solution of Ti(OiPr)₄ (5.350 g) and acethylacetone (3.775 g) was prepared. To the solution, a solution of ethanol (9.1725 g) and trifluoroacetic acid (0.4687 g) was added by stirring. After fluorine-doped tin oxide film-coated glass substrates were immersed into the mixed solution and pulled up with a speed of 2.0 mm s⁻¹, and then, they were annealed at 773 K form 1 h. The film thickness (l_d-TiO2) was changed by repeating the procedures.

Compact TiO₂ films were formed on FTO by a spray method (s-TiO₂/FTO).²¹ A coating solution was prepared by adding a solution of ethanol (9.1725 g) and trifluoroacetic acid (0.04687 g) to a solution of Ti(OiPr)₄ (5.350 g) and acethylacetone (3.775 g) by stirring. Spraying the solution on FTO heated at 773 K yielded a compact TiO₂ film.

PB films were electrodeposited on FTO from an aqueous solution of ferric-ferricyanide according to the method reported by Itaya and co-workers.²² A three-electrode photoelectrochemical cell with the structure of FTO working electrode | a mixed aqueous solution of 0.02 M FeCl₃ and 0.02 M K₃[Fe(CN)₆] (200 mL) | Ag/AgCl reference electrode (TOA-DKK) | GC counter electrode was fabricated. PB films were electrodeposited on FTO under a constant current of −40 μA cm⁻². The film thickness (l_PB) was controlled by the electrodeposition time (t_ed).

Scanning electron microscopy (SEM) observation was carried out by Hitachi S-4800 Type II at an applied voltage of 20 kV. UV/Vis diffuse reflectance spectra were recorded on a Hitachi U-4000 spectrophotometer. The reflectance (R) spectra were converted to absorption spectra by using the Kubelka–Munk function (F(R_∞) = (1 – R)²/2R). X-ray diffraction (XRD) measurements were performed by a Rigaku Mini Flex X-ray diffractometer operating at 40 kV and 100 mA. The data were collected in the range from 5° to 90° (2θ) by the use of Cu Kα radiation (λ = 1.545 Å).

A three-electrode photoelectrochemical cell with the structure of TiO₂/FTO | 0.1 M NaClO₄ without and with 0.1 M H₂O₂ (pH 3) | Ag/AgCl reference electrode (TOA-DKK) | GC was fabricated. Photocurrents were measured at the rest potential in the dark using a potentio/galvanostat (HZ-7000, Hokuto Denko) under illumination from the back side of the TiO₂ film by a UV-LED (λ  = 365 ± 25 nm, light intensity at 365 nm (I₃₆₅) = 31.2 mW cm⁻² nm⁻¹, HLV-24UV365-4WNRBT, CCS) at 298 K.

For evaluation of the performances as the H₂O₂-PFC, two-electrode photoelectrochemical cells with the structures of TiO₂/FTO photoanode (apparent area = 1.0 × 1.0 cm²) | 0.1 M NaClO₄ and 0.1 M H₂O₂ (pH 3) | GC or PB/FTO cathode (apparent surface area = 2.0 × 2.0 cm²) were fabricated. The current (J)-cell voltage (V_cell) curves were measured under irradiation of UV-light from the back side of the TiO₂ film by a UV-LED (λ  = 365 ± 25 nm, I₃₆₅ = 31.2 mW cm⁻² nm⁻¹).

TiO₂ films were formed on FTO using dip-coating (d-TiO₂/FTO) with the film thickness varied by the number of coating times (N). Figure 1A shows XRD patterns for the d-TiO₂(N)/FTO samples. In every pattern, strong diffraction peaks are observed at 2θ = 26.48°, 33.69°, 37.75°, and 51.52°. These peaks are assignable to the diffraction from the (110), (101), (200) and (211) crystal planes of SnO₂, respectively. As a result of d-TiO₂-film coating, a new peak indexed as the diffraction from the (101) plane of the anatase TiO₂ appears at 2θ = 25.3°. The intensity of the diffraction peak increases with an increase in N. The inset in Figure 1B shows SEM image for d-TiO₂(N = 2)/FTO. A uniform d-TiO₂ film with the thickness (l_d-TiO2) of 0.87 ± 0.05 μm is formed on FTO. Figure 1B shows plots of the thickness of the d-TiO₂/FTO bilayer (l_d-TiO2/FTO) vs. N. The l_d-TiO2/FTO value linearly increases with respect to N according to the equation of l_d-TiO2/FTO (μm) = 0.136 N + 0.609. Accordingly, the l_FTO is ∼0.61 μm, and the d-TiO₂ thickness (l_d-TiO2) can be expressed by the equation of l_d-TiO2 (μm) = 0.136 N. The crystallite size (d) determined by the Scherrer equation increased with an increase in N: d = 20.3 nm at N = 2, d = 27.1 nm at N = 3, and d = 30.0 nm at N = 4. The film thickness and crystallite size for mp-TiO₂ were 3.6 μm and 14.4 nm, respectively.

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PB films were electrodeposited on FTO (PB/FTO) with the film thickness varied by the electrodeposition time (t_ed). Figure 2A shows the XRD patterns for PB(t_ed)/FTO. The electrodeposition of PB induces a diffraction peak at 2θ = 17.58° assignable to the diffraction from its (200) plane. Figure 2B shows UV-visible absorption spectra for PB(t_ed)/FTO. While the absorption edge for pristine FTO is ∼330 nm, strong absorption due to the intervalency electron transfer appears at 500 nm < λ < 1300 nm with a peak around 600 nm.²² The absorption intensifies with an increase in t_ed. The l_PB determined by SEM observation increased in proportion to t_ed according to the following equation of l_PB(nm) = 89.5t_ed/min.

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Figure 3A shows cyclic voltammetry (CV) curves for the d-TiO₂(N)/FTO electrodes, and the pristine FTO and mp-TiO₂/FTO for comparison, in an aqueous electrolyte solution containing [Fe(CN)₆]³⁻/Fe(CN)₆⁴⁻ redox pairs. In the CV curve for FTO, a pair of redox currents is observed with a formal potential (E^') of 0.364 V vs. standard hydrogen electrode (SHE). The current hardly changes with the formation of mp-TiO₂, which indicates that the electrolyte solution freely penetrates into the pores of mp-TiO₂ to come in contact with FTO at the interface. In the CV curves for d-TiO₂(N)/FTO, the current decreases with an increase in N. Further, a compact TiO₂ underlayer was formed on FTO by a spray method (s-TiO₂),²¹ and then, the d-TiO₂ film was coated over it (d-TiO₂/s-TiO₂/FTO). Figure 3B shows a CV curve for the s-TiO₂/FTO electrodes in an aqueous electrolyte solution containing [Fe(CN)₆]³⁻/Fe(CN)₆⁴⁻ redox pairs. In this system, the current due to the reversible redox reactions disappears. Thus, the s-TiO₂ film inhibits the direct contact between FTO and the electrolyte solution.

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First, the TiO₂ photoanode effect on the cell performances of H₂O₂-PFCs with the structure of photoanode-GC cathode was studied. Table I summarizes short-circuit current (J_sc) and open-circuit voltage (V_oc) for various cells. Figure 4A compares photocurrent (J_ph)-cell voltage (V_cell) curves for the H₂O₂-PFCs using mp-TiO₂/FTO and d-TiO₂(N = 2)/s-TiO₂/FTO as the photoanode under UV-light irradiation. In each case, no current flows in the dark. Under irradiation, the d-TiO₂(N = 2)/s-TiO₂/FTO photoanode cell affords much larger J_sc (1.15 mA cm⁻²) and V_oc (684 mV) (cell 3) than the mp-TiO₂/FTO photoanode cell (J_sc = 0.328 mA cm⁻² and V_oc = 464 mV (cell 1). Figure 4B shows plots of J_ph and V_cell for the d-TiO₂(N)/s-TiO₂/FTO photoanode cell as a function of N. Both the J_sc and V_oc drastically increases with increasing N (cell 2–5), going through maxima at N = 2 or l_d-TiO2 = 0.27 μm. Figure 4C shows the photochronoamperometry curve for the d-TiO₂(N = 2)/s-TiO₂/FTO-GC cells (cell 3). The initial photocurrent reaches 1.37 mA cm⁻², steeply decaying just after irradiation. A similar sharp current spike was also observed for the mp-TiO₂/FTO-GC cell (cell 1). These results strongly suggest the presence of a large overpotential for the H₂O₂ reduction by the GC cathode.

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Next, the effect of the cathode on the cell performances was examined. As shown in Table I, the replacement of the GC cathode by PB/FTO greatly improves the short-circuit current (J_sc) and open-circuit voltage (V_oc) for both the mp-TiO₂/FTO and d-TiO₂/s-TiO₂/FTO photoanode cells.

Figure 5A compares the performances of mp-TiO₂/FTO-PB(t_ed = 10 min)/FTO cell (cell 6) and d-TiO₂(N = 2)/s-TiO₂/FTO-PB(t_ed = 10 min)/FTO cell (cell 15). The cell performance for the latter is much superior to the former. Strikingly, the J_sc of 1.57 mA cm⁻² and the V_oc of 787 mV are achieved for the d-TiO₂(N = 2)/s-TiO₂/FTO-PB(t_ed = 10 min)/FTO cell, whereas the dark current is negligibly small in this potential range. Thus, the excellent cell performance is ascribable to the concerted effect of the dense TiO₂ bilayer photoanode and the PB cathode. Figure 5B shows the photochronoamperometry curve for the d-TiO₂(N = 2)/s-TiO₂/FTO-PB(t_ed = 10 min)/FTO cell (cell 15). The sharp current spike observed for the mp-TiO₂/FTO-GC and d-TiO₂/s-TiO₂/FTO-GC cells disappears although the photocurrent gradually decreases with irradiation. Evidently, the remarkable enhancement of the cell performances stems from the excellent electrocatalytic activity of PB. PB possesses a cubic unit cell dimension of 1.02 nm to form unique nanopores.²³ Table I also shows that the J_sc drastically increases with increasing t_ed to reach an almost constant at t_ed ≈ 2 min or l_PB ≈ 180 nm (cell 7–15). Importantly, this finding indicates that the PB-catalyzed H₂O₂ reduction occurs at not only the external surfaces but also the internal surfaces.

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The action mechanism of the present H₂O₂-PFC can be explained by Scheme 1. UV-light irradiation of the TiO₂ anode promotes the electrons in the valence band (VB) to the conduction band (CB). The VB-holes with the potential (E) of +2.89 V (vs. SHE at pH 3) easily oxidize H₂O₂ to O₂ (E = 0.518 V vs. SHE at pH 3), of which evolution was confirmed by gas chromatography.¹⁰ H₂O₂ is known to be adsorbed on TiO₂ to form a surface Ti_s-OOH complex, where the subscript s denotes the surface atom.²⁴ We verified the formation of the H₂O₂ complex on the mp-TiO₂ surface by UV-visible absorption spectroscopy.¹⁰ During the transport of the CB-electrons to the electron-collecting FTO electrode, part of them can be lost by the reduction of H₂O₂ adsorbed on TiO₂ (loss 1 expressed by Eq. 3). This loss would become larger as the surface area of the TiO₂ electrode increases. In addition, in the mp-TiO₂/FTO anodes, the electrons can also be consumed on FTO at the TiO₂/FTO interface (loss 2 expressed by Eq. 4). The compact structure of the d-TiO₂/s-TiO₂ bilayer effectively suppresses the penetration of the electrolyte solution into the film to minimize the losses. The increase in l_d-TiO2 increases the light absorption amount of TiO₂, but the probability for the CB-electrons to reach FTO decreases due to the recombination and/or electron trapping. The balance between them would explain the presence of the optimum l_d-TiO2. Further, the electrons move to the PB cathode through the external circuit to efficiently reduce H₂O₂ to H₂O due to the excellent electrocatalytic activity.²²

Scheme 1. The action mechanism of the H₂O₂-PFC using a compact TiO₂/FTO as the photoanode and PB/FTO as the cathode.

A one-compartment PFC cell uing d-TiO₂/s-TiO₂/FTO and PB/FTO as the photoanode and cathode, respectively, was constructed. Under optimum conditions, this PFC provides J_sc = 1.57 mA cm⁻² and V_oc = 787 mV, which are much greater than the original mp-TiO₂/FTO-GC cell (J_sc = 0.328 mA cm⁻², V_oc = 464 mV). The remarkable improvement in the cell performances can be attributed to the decrease in the losses at the compact TiO₂ photoanode and the excellent electrocatalytic activity of the PB cathode for H₂O₂ reduction.

This work was supported by a Grant-in-Aid for Scientific Research (C) No. 15K05654, and MEXT-Supported Program for the Strategic Research Foundation at Private Universities.

Hiroaki Tada 0000-0001-8638-0697