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Photoelectrochemical Solar Cells Consisting of a Pt-Modified CdS Photoanode and an Fe(ClO4)2/Fe(ClO4)3 Redox Shuttle in a Nonaqueous Electrolyte Yosuke Kageshima, Hiromu Kumagai, Takashi Hisatomi, Tsutomu Minegishi, Jun Kubota,† and Kazunari Domen* Department of Chemical System Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan S Supporting Information *

ABSTRACT: Photoelectrochemical photovoltaic cells (PEC PVs) consisting of an n-type CdS single-crystal electrode and a Pt black counter electrode in a nonaqueous electrolyte containing an Fe(ClO4)2/Fe(ClO4)3 redox shuttle were studied as a means of obtaining photovoltages above the onset voltage for water splitting with one-step photoexcitation. To improve the photovoltaic performance, the effects of the redox concentration on the cell performance were investigated by UV−vis absorption and PEC measurements and by assessing the electrolyte using hydrodynamic voltammetry. Under visible-light irradiation (420−800 nm) from a Xe lamp, a relatively high open-circuit voltage (VOC) of approximately 1.6 V was obtained, resulting from the negative flat-band potential of the CdS and the positive redox potential of the Fe complexes. Upon optimization of the redox concentration, photocurrent for the Pt/CdS electrode was increased to approximately 30 mA cm−2, and an incident photon-to-current conversion efficiency of up to 80% was achieved at 480 nm as a result of the promotion of the anodic reaction on the Pt surface. Under simulated sunlight, the PEC PV composed of Pt/CdS in a 20 mM Fe(ClO4)2/Fe(ClO4)3 electrolyte exhibited a VOC of 1.38 V, a 3.54 mA cm−2 short-circuit current, and a 2.8% photon-to-energy conversion efficiency.

1. INTRODUCTION

Because solar radiation undergoes daily, seasonal, and meteorological fluctuations, the generation of chemical energy carriers from solar energy is vital, meaning that the conversion of solar energy to storable chemicals will be a key technology in the future.13−15 As an example, the effective integration of a PV device with a water electrolysis apparatus represents a promising technique for converting solar energy into chemical carriers such as hydrogen. This method is conventionally termed the PV + electrolyzer approach.16−18 The decomposition of water to obtain hydrogen and oxygen under standard conditions (297 K and 101.3 kPa) requires a voltage of 1.23 V based on thermodynamic principles, as this is equivalent to the ΔG° value of 247 kJ mol−1 for this reaction. Because an additional overpotential at each electrode is required for electrocatalysis, an output voltage of 1.5−1.6 V is necessary for the electrolysis of water in a practical system.19 Thus, if a single PV cell is able to generate a voltage greater than 1.5−1.6 V, it will be able to readily split water based on single-photon excitation. Unfortunately, as noted above, typical single-junction PVs are designed to generate only 0.8−0.9 V,

Because of ongoing concerns regarding energy and environmental issues, the utilization of renewable energy continues to attract significant attention. Solar energy, the most abundant renewable energy source, is one of the most important alternatives to fossil fuels. Over the past few decades, conventional single-junction solid-state photovoltaics (PVs) such as Si, Cu(In, Ga)Se2 and CdTe have been successfully developed as a means of generating electric power from solar energy.1−4 Furthermore, photoelectrochemical (PEC) cells with applications in PVs, such as dye-sensitized solar cells (DSSCs),5−7 have also been investigated for the fabrication of new PVs with reduced costs compared to conventional solidstate systems. More recently, the photoelectric conversion efficiency of DSSCs has been improved through the application of organic−inorganic perovskite compounds as photoabsorbers, and the resulting perovskite PVs have had a significant impact with regard to research into solid-state solar cells.8,9 In particular, for traditional single-junction solid-state PVs, the band gap of the semiconductor is typically in the range of 1.4− 1.5 eV so as to allow the generation of a 0.8−0.9 V open-circuit voltage (VOC). This allows the maximum level of electric power generation based on the Shockley−Queisser limit.10−12 © XXXX American Chemical Society

Received: March 7, 2016 Revised: May 10, 2016

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redox materials and to investigate more sophisticated designs for the electrodes or the cell itself. The present work examined the Fe(ClO4)2/Fe(ClO4)3 redox shuttle, which also has a positive redox potential,38,39 in combination with a CdS single-crystal photoanode, as an alternative redox shuttle. The schematic diagram of the present PEC PV cell consisting of Fe(ClO4)2/Fe(ClO4)3 as a redox shuttle is illustrated in Figure 1. At the same time, the effects of

and even the recent rapid progress in perovskite PVs has not resulted in any reported cases in which VOC values greater than 1.23 V have been generated by one-step photoexcitation.9 Therefore, these PVs cannot electrolyze water without the use of a series circuit of two or three PVs or a voltage converter.20,21 Furthermore, approaches to developing a PEC PV with a high output voltage through one-step photoexcitation22,23 for direct connection to a water electrolyzer have rarely been reported. PEC and photocatalytic (PC) water splitting for the direct production of hydrogen and oxygen from an aqueous electrolyte are also expected to have potential as new energy technologies.24−26 Since the Honda−Fujishima effect was reported,27 numerous photocatalyst materials and systems have been investigated with the aim of achieving effective solar hydrogen production.28−30 Although these PEC and PC water-splitting methods are attractive and their efficiency has been continually increased, many fundamental bottlenecks and disadvantages remain. First, the energy levels of the conduction band minimum (CBM) and valence band maximum (VBM) in these materials must be more negative and more positive than the hydrogen and oxygen evolution potentials, respectively, to meet the thermodynamic requirements of water splitting.26 Second, the semiconductor materials used in PEC and PC systems must, of course, function in aqueous solutions without any degradation.24 Both nitride and sulfide photoanodes are unstable in aqueous media, which is considered to be one of the primary challenges. Finally, there are kinetic disadvantages associated with water splitting, including complicated multielectron reactions, each consisting of several elementary steps.31 These multistep reactions make it quite difficult to perform multielectron hydrogen (two-electron) and oxygen (fourelectron) evolution reactions using photoexcited minority carriers in semiconductors, especially when employing visible light as the energy source. Because of these difficulties, the efficiencies of PEC and PC water-splitting systems remain limited, and only a few examples have been reported based on one-step photoexcitation by visible-light irradiation.28 In a previous report, we described a new type of PEC PV cell consisting of a CdS photoanode with a Ru(bpy)32+/3+ redox shuttle in a nonaqueous electrolyte, capable of generating voltage outputs above the value of 1.23 V required for water electrolysis.32 A PEC PV cell with CdS and Ru(bpy)32+/3+ exhibited a high VOC of 1.48 V, achieving a value greater than those previously reported for solid-state and PEC PVs.1,2,4,7,9 This work demonstrated the generation of an output voltage higher than that required for water electrolysis based on the one-step photoexcitation of the semiconductor, suggesting the possibility of applying this system to water photolysis. Although photocorrosion of CdS was greatly suppressed in this system, compared with that normally observed in aqueous solutions, the CdS still exhibited some degree of degradation even in the nonaqueous solutions,33−35 and thus, this PEC PV might still have insufficient long-term stability. In our prior work,32 we also identified an issue in that the absorption of 450-nm light by the Ru(bpy)32+ interfered with the photoexcitation of the CdS, whose absorption edge is located at 520 nm.36,37 This finding suggested that a thinner design is required for the PEC cell to enhance the efficiency. Although the previous PEC PV cell consisted of CdS and Ru(bpy)32+/3+, in principle, novel PEC PVs such as this can be composed of various semiconductor and redox materials. Therefore, we believe that our next task is to search for more stable, less expensive semiconductor or

Figure 1. Schematic diagram of the present PEC PV cell consisting of a CdS photoanode and a Pt black electrode in acetonitrile electrolyte containing Fe(ClO4)2/Fe(ClO4)3 redox shuttle.

the redox-shuttle concentration and surface modification of the CdS photoanode with Pt were also examined to determine means of enhancing the cell performance. This new type of PEC PV cell can potentially electrolyze water by simple onestep photoexcitation because of its relatively high output voltage, whereas conventional PVs require series connection of PVs or complicated electrical circuit to achieve efficient water electrolysis to increase voltage. In addition, the present PEC PV cells are expected to avoid the difficulties of PEC or PC water splitting to perform the multielectron process of hydrogen and oxygen evolutions by photoexcited minority carriers. Furthermore, in the semiconductor as a photoelectrode for the present PEC PV cells, the band positions are not required to extend over the redox potential of water, in contrast to PEC water splitting. Utilization of a nonaqueous electrolyte in this system also allows the application of photocorrosive semiconductors in the water electrolyte such as some sulfides. Therefore, we believe that the present work successfully demonstrates a new direction for solar-energy conversion by simple and inexpensive method.

2. EXPERIMENTAL SECTION Electrolyte Absorbance Measurements. UV−vis absorption spectra of the Fe(ClO4)2/Fe(ClO4)3 redox shuttle in acetonitrile at various concentrations were acquired using a spectrophotometer (JASCO, V-670) with a 10-mm quartz cell. Hydrodynamic Voltammetry. Hydrodynamic voltammetry was performed using a rotating ring-disk electrode device and a Pt rotating-disk electrode (BAS Co. Ltd.). A Pt black electrode and a Ag/AgCl reference electrode with an acetonitrile electrolyte junction were used as the counter and reference electrodes, respectively, under the same conditions as applied during the PEC assessments described below. The voltammograms were analyzed using Koutecky−Levich plots based on the equation i−1 = ik −1 + (0.62nFD2/3ω1/2ν−1/6C)−1 = ik −1 + idiff −1 (1) B

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The Journal of Physical Chemistry C where ik, idiff, n, F, D, ω, ν, and C are the kinetically controlled current density (mA cm−2), the limiting current density (mA cm−2) for diffusion in the bulk electrolyte, the number of electrons in the reaction, Faraday’s constant (C mol−1), the diffusion coefficient of the reactant (cm2 s−1), the rotation rate of the disk electrode (rad s−1), the kinematic viscosity of the electrolyte (cm2 s−1), and the bulk concentration of the reactant (mol cm−3), respectively.40,41 The kinetically controlled current density, ik, represents the kinetic current of the reaction at a relatively large overpotential and can be employed to evaluate the electrocatalytic activity without the effects of mass transport in the bulk electrolyte.40,41 The value of ik at each electrode potential, E, can be obtained from the intercept of the Koutecky−Levich plot as a function of ω−1/2. Tafel plots of the kinetically controlled current density, ik (electrode potential E vs log |ik|), were generated, and the Tafel slopes, banode and bcathode (mV decade−1), respectively, were estimated from the linear portions of these plots. Preparation of the Photoelectrodes and Electrolytes. A commercially available CdS single crystal (10 × 10 mm, 1 mm thick; Crystal Base Co., Ltd.) was cut into pieces approximately 5 mm × 5 mm in size and were used as photoanodes. Wiring was attached on the back side of the CdS using indium soldering.42 The back and sides of the crystal were subsequently covered with epoxy resin (Nichiban Co., Ltd.). The surface of the CdS was etched and cleaned by immersion in concentrated HCl. In the case of surface modification, a vacuum-evaporated Pt layer (1 nm) was deposited on the CdS surface after HCl etching, with the thickness of the Pt layer estimated using a quartz crystal oscillator sensor. Iron perchlorate [Fe(ClO4)2·6H2O and Fe(ClO4)3·nH2O, Wako Chemicals] was employed as the redox shuttle. Iron perchlorate redox-shuttle electrolyte solutions ranging from 2 to 1000 mM were prepared at Fe(ClO4)2/Fe(ClO4)3 molar ratios of unity. Anhydrous acetonitrile (Wako Chemicals) and 100 mM tetrabutylammonium perchlorate (TBAP, Wako Chemicals) were used as the solvent and supporting electrolyte, respectively, for all electrochemical and PEC measurements. Because the water of hydration in the iron perchlorate reagents was believed to have the potential to initiate photocorrosion of the CdS, the electrolyte was dehydrated with anhydrous magnesium sulfate, prepared by heating MgSO4·7H2O (Wako Chemicals), before each electrochemical or PEC measurement. Electrochemical and PEC Measurements. All electrochemical and PEC measurements were conducted using a cell with a commercial water jacket (BAS Co., Ltd., used as received). To minimize any interference of the visible-light absorption of the electrolyte with the band gap photoexcitation of the CdS, the surface of the photoelectrode was positioned as close to the irradiated section of the cell as possible, such that the optical path length was less than 1 mm. Pt black deposited on a Pt wire (0.5 mm diameter, approximately 90 mm length) was used as the counter electrode in both the three-electrode and two-electrode setups. In the three-electrode system, the potential was measured using a Ag/AgCl reference electrode made in-house, in conjunction with an acetonitrile electrolyte junction with double bridges to avoid leakage of Cl− and aqueous solution to the main vessel. The Fe(ClO4)2/Fe(ClO4)3 redox potential was determined with a Pt disk electrode (BAS Co., Ltd.) as the working electrode. A CdS photoanode prepared as described above was used as the working electrode during each PEC measurement. During each PEC measurement, the electrolyte was stirred, and the cell was purged with

Ar to remove air prior to the trial. Either a 300-W Xe lamp emitting at 420−800 nm in conjunction with a cutoff filter and a cold mirror or a solar simulator (XES-40S2-CE, SAN-EI Electric Co., Ltd.) generating AM1.5G irradiation at 100 mW cm−2 was utilized. During the PEC measurements, cooling water was circulated around the cell to prevent a temperature rise due to irradiation by the light source, which would otherwise result in evaporation of the electrolyte.

3. RESULTS AND DISCUSSION Effects of the Light Absorption of Each Electrolyte. UV−vis transmission spectra of Fe(ClO4)2/Fe(ClO4)3 acetonitrile solutions of varying concentrations (0.2−200 mM) were acquired and are shown in Figure 2. The shaded area in this

Figure 2. Transmittance spectra of 0.2−200 mM Fe(ClO4)2/ Fe(ClO4)3 [Fe(ClO4)2/Fe(ClO4)3 = 1] and 100 mM TBAP in acetonitrile. The shaded area (below 520 nm) indicates the wavelength region over which the CdS photoanode can absorb the light of a 300W Xe lamp equipped with visible filters.

figure (420−520 nm) indicates the wavelength region of interest for the present PEC PV cell based on a light source (a Xe lamp with a cutoff filter and a cold mirror) emitting at 420− 800 nm and the absorption edge of CdS at approximately 520 nm. The Fe(ClO4)2/Fe(ClO4)3 redox shuttle produced strong absorption peaks at about 350 nm along with relatively weak absorption at approximately 870 nm, due to the photoexcitation of Fe3+ and Fe2+, respectively.43,44 The molar absorption coefficients for the 350- and 870-nm peaks were estimated to be 4.01 × 103 and 5.93 dm3 mol−1 cm−1, respectively, based on the data in Figure S1. Although the main peak at 350 nm is not itself within the visible region, it has a broad shoulder that extends into the visible region, from 400 to 600 nm. The molar absorption coefficient at 420 nm, serving as a benchmark, was estimated to be approximately 1.83 × 102 dm3 mol−1 cm−1 and, thus, was sufficiently high to interfere with the photoexcitation of the CdS to some extent. The light path length in the present PEC PV cells was fixed at less than 1 mm, and the transmittances of the 2 mM Fe(ClO4)2/Fe(ClO4)3 electrolyte (Fe2+/Fe3+ = 1) over a 1-mm path length at 350 and 420 nm were estimated to be 39.7% and 95.9%, respectively, from the above molar absorption coefficients. Thus, although UV light could be absorbed by the electrolyte, the majority of the visible light was believed to reach the photoelectrode because the absorption coefficient of the electrolyte in the visible-light region was relatively small compared with that in the UV region. This result indicates that the present PEC PV cells with an Fe(ClO4)2/Fe(ClO4)3 electrolyte have the potential to utilize visible light. NeverC

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To estimate the kinetic aspects of the Fe(ClO4)2/Fe(ClO4)3 redox reaction, hydrodynamic voltammetry using a rotatingdisk electrode (RDE) was performed, and the results are provided in Figure 4. In the present study, a Pt disk electrode was applied as the working electrode. In the CVs obtained with a Pt RDE at different rotating rates in a 2 mM Fe(ClO4)2/ Fe(ClO4)3 redox solution (Figure 4a), the anodic and cathodic reactions reached a diffusion-based limit in the bulk electrolyte at values in the ranges of 1.7−1.8 and 0.6−0.7 VAg/AgCl, respectively, and hysteresis was observed between the anodic and cathodic scans. Koutecky−Levich plots40,41 for the anodic reaction on the Pt surface were generated by plotting the reciprocal of the current density, 1/i, at a given electrode potential, E, as a function of ω−1/2, according to eq 1. These are provided in Figure 4b, and it is evident that different straight lines were obtained at each electrode potential. These Koutecky−Levich plots all have similar slopes but different intercepts for each electrode potential, indicating that the total number of exchanged electrons, n, is independent of the electrode potential, whereas the kinetically controlled current tends to vary. Assuming that the kinematic viscosity of the electrolyte equals that of acetonitrile (ν = 4.45 × 10−3 cm2 s−1) and that this reaction is a one-electron process, the diffusion coefficient for the reductant (Fe2+) was calculated to be approximately 1.11 × 10−5 cm2 s−1. This value is almost equal to the diffusion coefficient for common redox reactions, such as that of ferrocene (2.37 × 10−5 cm2 s−1),46 indicating that the diffusion of these iron species is comparable to that of ferrocene. Assuming that the diffusion of the reactants is extremely slow, the local distributions of the redox concentrations around both the anode and cathode can be readily derived from the applied ratio [Fe(ClO4)2/Fe(ClO4)3 = 1], and the photocurrent obtained from the PEC PV cells should be limited by the diffusion process. Thus, if the diffusion coefficients for the present redox species are sufficient, this will minimize the diffusion limitation of the photovoltaic performance. Finally, these kinetically controlled current density values were converted into the Tafel plots shown in Figure 4c, and the slopes were used to evaluate the redox reaction at the Pt surface. The slopes for the anodic and cathodic reactions on the Pt surface, banode and bcathode, were estimated to be 98 and 86 mV decade−1, respectively, when using 2 mM Fe(ClO4)2/Fe(ClO4)3. Assuming a transfer coefficient of α = 0.5, the Tafel slope for a one-electron reaction should be 120 mV decade−1. The Tafel slopes for the Fe(ClO4)2/Fe(ClO4)3 redox reaction were somewhat less than this value, indicating that the redox reaction was not fully a oneelectron process. Nevertheless, the number of electrons in the redox reaction on the Pt surface was certainly much less than that for the standard water-oxidation multielectron reaction. Therefore, the present PEC PV cells could avoid the difficulties associated with conventional direct water splitting on photocatalysts. Based on the calculated Tafel slopes, the Pt catalyst evidently generates effective electrocatalytic activity in conjunction with the Fe(ClO4)2/Fe(ClO4)3 redox reaction. In the following section, the modification of the CdS surfaces with Pt to produce effective active sites that promote the anodic reaction and enhancement of the photocurrent is examined. Effect of the Concentration of the Fe(ClO4)2/Fe(ClO4)3 Redox System on the PEC Performance. Figure 5a shows the potential−current curves for an unmodified CdS photoelectrode, acquired in 2−1000 mM Fe(ClO4)2/Fe(ClO4)3 and 100 mM TBAP in acetonitrile under irradiation with a 300-W

theless, because of some light absorption by the electrolyte, we predicted that the enhancement of the CdS photoanode photocurrent and the interference of the visible-light absorption by the electrolyte as its concentration was increased would compete with one another, which agrees with the observed trends in the potential−current and voltage−current properties (see Figures 4 and 5 below). Thus, further suppression of interference with visible-light absorption in semiconductors by the photoexcitation of metal-ion complexes is an important future task associated with the improvement of the photocurrent and efficiency of the present PEC PV cells. Kinetic Analyses of the Fe(ClO4)2/Fe(ClO4)3 Redox Solution by Hydrodynamic Voltammetry with a Pt RDE. Cyclic voltammograms (CVs) were obtained using a Pt disk electrode in 2−1000 mM Fe(ClO4)2/Fe(ClO4)3 [Fe(ClO4)2/ Fe(ClO4)3 = 1] and 100 mM TBAP in acetonitrile at a scan rate of ±50 mV s−1, as shown in Figure 3. The oxidation

Figure 3. Cyclic voltammograms obtained from a Pt electrode in (a) 2, (b) 20, (c) 200, and (d) 1000 mM Fe(ClO4)2/Fe(ClO4)3 [Fe(ClO4)2/Fe(ClO4)3 = 1] and 100 mM TBAP in acetonitrile. The potential of the Pt electrode was swept cyclically at 50 mV s−1.

current was observed to range from 1.2 to 1.5 V vs Ag/AgCl (VAg/AgCl), whereas the reduction current spanned the range from 0.9 to 1.2 VAg/AgCl. The current density increased with increasing reactant concentration. The redox equilibrium potential (Eeq) for the Fe(ClO4)2/Fe(ClO4)3 system was estimated to be in the vicinity of 0.9−1.5 VAg/AgCl based on the midpoints of the reduction and oxidation waves.38,39 However, this redox reaction was found to have a large overpotential of 0.3−0.5 V between the reduction and oxidation onset potentials, as clearly seen in Figure 3. Although the Eeq value of a reversible redox reaction typically does not change with concentration when the ratio of oxidant to reductant is held constant,45 it is difficult to define the exact value of Eeq in such a slow reaction. Assuming that the midpoint between the oxidation and reduction peaks (Em) represents Eeq, the potentials of the oxidation and reduction peaks (Eox and Ered) and Em for each electrolyte concentration are summarized in Table S1 as benchmarks. D

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Figure 4. (a) Cyclic voltammograms obtained from a Pt rotating-disk electrode (RDE) at 0−3600 rpm in 2 mM Fe(ClO4)2/Fe(ClO4)3 [Fe(ClO4)2/ Fe(ClO4)3 = 1] and 100 mM TBAP in acetonitrile. The potential of the Pt RDE was swept cyclically at 50 mV s−1. (b) Koutecky−Levich plots of the anodic reaction in panel a at each electrode potential. (c) Tafel plots of the kinetically controlled current density values, ik, of the anodic and cathodic reactions in panel a.

Xe lamp during a 10 mV s−1 anodic scan using a three-electrode setup. Here, each differently colored line corresponds to the results obtained at a specific concentration. The anodic photocurrent was observed to range from −0.25 to 0 VAg/AgCl and gradually increased as the anodic potential shifted. These potential−current curves, however, do not exhibit any obvious photocurrent plateau regions, whereas the photocurrent for a CdS photoanode in an acetonitrile electrolyte with a Ru complex redox shuttle has been reported to exhibit a plateau.32 As discussed above, the Fe(ClO4)2/Fe(ClO4)3 system has a significantly large overpotential, which could possibly result in a slow reaction on the CdS surface and thus lead to the observed lack of a plateau region. The CdS photoanode in a 20 mM redox-shuttle solution exhibited a slightly enhanced current density compared to the results for a 2 mM solution, although the more concentrated redox solution produced a decline in the current density. This might have resulted from light absorbance in the concentrated redox solution itself, as noted earlier. Figure 5a shows that a photocurrent of 10−14 mA cm−2 could be obtained from the CdS photoanode at approximately 1.0 VAg/AgCl and that this current was almost independent of the concentration. In contrast, the onset potential (Eonset) for the CdS photoanodic current (from −0.25 to 0 VAg/AgCl) shifted anodically as the concentration was increased. This is believed to have resulted from the large overpotential of the redox reaction and promotion of the back-reaction with increasing concentration. In the potential−current curves for the CdS photoelectrode in Figure 5b, in the vicinity of the onset potential under dark conditions, it can be seen that the cathodic dark current obtained from the CdS photoanode in the

Fe(ClO4)2/Fe(ClO4)3 electrolyte was gradually enhanced with increasing concentration. Based on these results, in the case of a low redox-shuttle concentration, the dark current generated by a reductive reverse reaction at the CdS surface might be suppressed, especially in the region near Eonset. This results in a small photocurrent and cathodic enhancement of Eonset. Eonset for CdS in 2 mM Fe(ClO4)2/Fe(ClO4)3 (−0.25 VAg/AgCl) is considered to be the value closest to the flat band potential of the CdS photoanode.47,48 Because the potential of the Pt black counter electrode in the two-electrode setup is assumed to be approximately equal to the onset potential for the reduction current in the CVs (from Figure 3, approximately 0.9−1.2 VAg/AgCl), the difference between the potential of the Pt black electrode and Eonset of the CdS anode (from −0.25 to 0 VAg/AgCl) at each concentration can be taken as equal to the VOC value of the PEC PV.32 Table 1 summarizes the photovoltaic performance from the voltage−current curves (Figure 5c) obtained from the PEC PV cells in a two-electrode setup using an unmodified CdS photoanode and a Pt black counter electrode in 2−1000 mM Fe(ClO4)2/Fe(ClO4)3 and 100 mM TBAP in acetonitrile under irradiation by a 300-W Xe lamp. The short-circuit current (ISC) for each PEC PV cell at different redox concentrations shows almost the same trend as the potential−current curves (Figure 5a) generated in the three-electrode setup and discussed in the previous paragraph. Optimization of the redox-shuttle concentrations is seen to have improved the current density for the cell to some extent. However, even though the surface of the CdS was fixed near the optical window of the cell, the deleterious effect of light absorption by the redox solution remained an E

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Figure 5. Potential−current curves obtained from a CdS electrode (a) irradiated by a Xe lamp and (b) under dark conditions in 2−1000 mM Fe(ClO4)2/Fe(ClO4)3 and 100 mM TBAP in acetonitrile. (c) Voltage−current curves for a PEC PV cell with a CdS photoanode and a Pt-black electrode in 2−1000 mM Fe(ClO4)2/Fe(ClO4)3 and 100 mM TBAP in acetonitrile. The potential or voltage of the CdS photoanode was swept at 10 mV s−1 anodically. A 300-W Xe lamp (420−800 nm) was used as the light source.

Table 1. VOC, ISC, and FF Values for PEC PV Cells with a CdS Photoanode and a Pt Black Electrode in 2−1000 mM Fe(ClO4)2/Fe(ClO4)3 and 100 mM TBAP in Acetonitrile under Irradiance by a 300-W Xe Lamp concentration (mM)

Voc (V)

Isc (mA cm−2)

FF

2 20 200 1000

−1.64 −1.51 −1.23 −1.10

2.81 13.5 7.55 2.07

0.22 0.20 0.33 0.78

reduced both the FFs and the somewhat elevated VOC, especially in the low-concentration solution. This might indicate suppression of the dark current as a result of the reductive reverse reaction at the CdS surface in the region near Eonset, as noted above. In the case of a slow redox reaction, efficient utilization of the minute photocurrent around Eonset through suppression of the reductive reverse reaction at the CdS surface through surface modification or multilayered structuring of the photoelectrode will be important and could lead to dramatic improvements in the photocurrent, VOC, and FF.49−52 Enhancement of Photovoltaic Performance by Pt Modification of the CdS Surface. As discussed in the previous section, the photocurrent density on the bare CdS surface gradually increased with an anodic shift in potential and did not reach an obvious plateau, implying that the redox reaction on the CdS surface was quite slow because of the large overpotential. Therefore, Pt deposition on the CdS surface, acting as a model catalyst, served to introduce effective active sites, promoting the forward anodic reaction and enhancing the photocurrent for the PEC PV cells. At the same time, banode (98 mV decade−1) and bcathode (86 mV decade−1) were almost equal on the Pt surface, meaning that the cathodic reaction must proceed as quickly as the anodic reaction. Therefore, there is a concern that the deposition of a Pt catalyst on the CdS surface might possibly enhance the dark current as a result of the promotion of the backward cathodic reaction on the Pt surface, resulting in a reduction in the VOC values of the PEC PV cells. The potential−current properties for a 1-nm-Pt-deposited CdS photoanode (Pt/CdS) in 2−1000 mM Fe(ClO4)2/ Fe(ClO4)3 and 100 mM TBAP in acetonitrile obtained with

issue, indicating the need for an improved cell design. The VOC value for each PEC PV also decreased with increasing redox concentration, just as was previously observed for the CdS Eonset values (Figure 5a). Judging from the difference between the Eeq (∼0.9−1.5 VAg/AgCl) and Eonset (from −0.25 to 0 VAg/AgCl) values resulting from the three-electrode measurements, these PEC PVs evidently generated photovoltages reasonably close to the predicted values. The declines in VOC following increases in the redox concentration are considered to be primarily caused by an anodic shift in the Eonset value of the CdS, based on the increasing redox concentrations in the three-electrode setup. Although these VOC values were definitely greater than the onset voltage for water electrolysis (1.23 V), the fill factors (FFs) for the PEC PVs incorporating an iron perchlorate redox system were much less than those previously reported for PEC PVs with Ru complexes.32 This might have been caused by the large reaction overpotential between the reduction and oxidation of the iron perchlorate. However, the relatively high overpotential for the Fe(ClO4)2/Fe(ClO4)3 redox reaction F

DOI: 10.1021/acs.jpcc.6b02406 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C a 10 mV s−1 anodic scan using a three-electrode setup are shown in Figure 6a. Here, almost the same trends are observed

of the bare CdS, the ISC and VOC values of the Pt/CdS cells produced reasonable values for these parameters, judging from the potential−current curves (Figure 6a) obtained with the three-electrode setup. It is also notable that the Pt/CdS cells produced photocurrents very similar to those seen in the potential−current curves (Figure 6a) up to approximately 30 mA cm−2 under the Xe lamp. The Pt/CdS cells in 2 and 20 mM Fe(ClO4)2/Fe(ClO4)3 generated photovoltages (1.46 and 1.41 V VOC values for 2 and 20 mM redox solutions, respectively) much higher than the thermodynamically required voltage for water electrolysis (1.23 V). However, the VOC values for these cells were actually slightly lower than those obtained with bare CdS (Table 1). This slight reduction in VOC following Pt modification is believed to result from the promotion of the backward cathodic reaction on the Pt surface, as addressed in the previous discussion concerning the kinetic analyses. Therefore, Pt modification of the CdS surface definitely improves the photocurrent by promoting the forward anodic reaction, but the cathodic reaction is also accelerated on the Pt surface, resulting in a slight decline in VOC. Hence, further investigations aimed at finding a more suitable catalyst that promotes only the forward anodic reaction will be required to enhance the photovoltaic performances of these PEC PV cells by increasing both the photocurrent and the photovoltage. The incident photon-to-current conversion efficiency (IPCE) of the PEC PV cell with the best photovoltaic performance, consisting of 1-nm-Pt-modified CdS and a Pt black counter electrode in a 20 mM Fe(ClO4)2/Fe(ClO4)3 redox system, was measured under short-circuit conditions in the two-electrode setup. The data were compared to the results obtained with bare CdS (Figure 7). These IPCE spectra are in good

Figure 6. (a) Potential−current curves obtained from a CdS electrode modified with 1 nm of Pt in 2−1000 mM Fe(ClO4)2/Fe(ClO4)3 and 100 mM TBAP in acetonitrile. (b) Voltage−current curves for a PEC PV cell with a CdS photoanode modified with 1 nm of Pt and a Pt black electrode in 2−1000 mM Fe(ClO4)2/Fe(ClO4)3 and 100 mM TBAP in acetonitrile. The potential or voltage of the CdS electrode was swept at 10 mV s−1 anodically. The Pt-modified CdS was irradiated with a 300-W Xe lamp (420−800 nm).

as were reported for the bare CdS anode in the previous section. That is, Eonset for the photoanodic current of the Pt/ CdS was shifted anodically by approximately −0.3 to −0.1 VAg/AgCl following an increase in the concentration of the redox shuttle. Second, the highest photocurrent was achieved with 20 mM Fe(ClO4)2/Fe(ClO4)3, and the photocurrent declined when more concentrated electrolyte solutions were used, presumably because of light absorption by the electrolyte. In contrast to the results for bare CdS, the anodic photocurrent generated by Pt/CdS showed an obvious plateau at a positive potential, with the exception of Pt/CdS in 20 mM Fe(ClO4)2/ Fe(ClO4)3, which had a peak at around 0.47 VAg/AgCl. These data indicate that loading with Pt works to produce effective active sites and to promote the anodic reaction. The Pt/CdS system generated a maximum photocurrent of almost 30 mA cm−2 under irradiation by the Xe lamp, whereas the highest photocurrent obtained from the bare CdS (Figure 5) in 20 mM Fe(ClO4)2/Fe(ClO4)3 was only 14 mA cm−2. This significant enhancement of the photocurrent is attributed to an increase in the active sites on the CdS photoanode following Pt deposition, compared with the relatively flat surface of a single-crystalline electrode. In addition, the high electrocatalytic activity of Pt itself is also believed to make a contribution. The photovoltaic performance parameters (VOC, ISC, and FF) indicating the voltage−current properties of the PEC PV cells with Pt/CdS and Pt black in a two-electrode setup in 2−1000 mM Fe(ClO4)2/Fe(ClO4)3 and 100 mM TBAP in acetonitrile (Figure 6b) are summarized in Table 2. As was seen in the case

Figure 7. Incident photon-to-current conversion efficiency (IPCE) spectra of a PEC PV cell with a 1-nm-Pt-modified CdS photoanode (○) or bare CdS (□) in a 20 mM Fe(ClO4)2/Fe(ClO4)3 electrolyte under short-circuit conditions in a two-electrode setup (right). The diffusion reflection spectrum (DRS) of CdS powder is also shown as a Kubelka−Munk function (left). A 300-W Xe lamp with a band-pass filter was used as the light source.

Table 2. VOC, ISC, and FF Values for PEC PV Cells with a CdS Photoanode Modified with 1 nm of Pt and a Pt Black Electrode in 2−1000 mM Fe(ClO4)2/Fe(ClO4)3 and 100 mM TBAP in Acetonitrile under Irradiation froma 300-W Xe Lamp concentration (mM)

VOC (V)

ISC (mA cm−2)

FF

2 20 200 1000

−1.46 −1.41 −1.19 −1.07

7.46 22.3 15.9 2.10

0.20 0.66 0.39 0.62

agreement with the CdS UV−vis spectrum, which has an absorption edge at approximately 520 nm. Judging from the IPCE spectra, Pt modification significantly enhances the apparent quantum efficiency of the CdS photoanode, to approximately 80%, which is consistent with the results of photoelectrochemical measurements. It is also notable that the apparent quantum efficiency at 400 nm was slightly lower than that at other wavelengths. As discussed, this Fe(ClO4)2/ Fe(ClO4)3 redox shuttle exhibits strong absorbance in the visible-light region, which will affect the visible-light absorption of the semiconductor, especially with a concentrated electrolyte G

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absorption by the electrolyte was also assessed, and kinetic analyses with hydrodynamic voltammetry were performed. After the concentration of the reactants had been adjusted, PEC PV cells with CdS and an Fe(ClO4)2/Fe(ClO4)3 redox shuttle generated maximum VOC and ISC values of 1.64 V and 13.5 mA cm−2, respectively, although VOC and ISC decreased with increasing electrolyte concentration, possibly because of the strong visible-light absorption of the redox solution and the dark current resulting from the backward cathodic reaction, especially in the vicinity of Eonset. These high VOC values reflect the differences in the negative Fermi levels of the CdS photoanode and the positive redox equilibrium potential of the Fe(ClO4)2/Fe(ClO4)3 system. These values exceed the thermodynamically required voltage for water electrolysis (1.23 V) with one-step photoexcitation. Kinetic analyses by hydrodynamic voltammetry demonstrated that Pt catalyzes the Fe(ClO4)2/Fe(ClO4)3 redox reaction, so Pt deposition on the CdS surface was used to enhance the photovoltaic performances of the PEC PV cells. The cells incorporating a Pt/CdS photoanode exhibited improved photocurrents up to approximately 30 mA cm−2 under irradiance by a Xe lamp. The VOC values for PEC PVs with Pt/CdS were slightly deteriorated compared to those obtained with bare CdS because of the promotion of both the anodic and cathodic reactions on the Pt surface. Therefore, more suitable catalysts that accelerate only the desired anodic reaction should be examined in the future. Finally, the PEC PV cell showing the highest photovoltaic performance, composed of a 1-nm-Pt-modified CdS and a Pt black counter electrode in a 20 mM Fe(ClO4)2/Fe(ClO4)3 redox solution, exhibited a significantly improved apparent quantum efficiency of up to 80% and a considerable power conversion efficiency of 2.8% under irradiance with simulated sunlight. In summary, we designed a novel PEC PV system consisting of a CdS photoanode in conjunction with an inexpensive and slightly slower Fe(ClO4)2/Fe(ClO4)3 redox shuttle and demonstrated a potential means of increasing the photocurrent by adjusting reactant concentrations and modifying the surface of the photoelectrode with a noblemetal catalyst.

solution. Nevertheless, because the main absorption peak for the Fe(ClO4)2/Fe(ClO4)3 system is situated in the nearultraviolet region (at 350 nm), the negative effects of light absorption by the redox solution should be minimized. Finally, a PEC PV cell with 1-nm-Pt-modified CdS and a Pt black counter electrode in a 20 mM Fe(ClO4)2/Fe(ClO4)3 redox solution was also assessed under illumination with simulated sunlight, as shown in Figure 8. Here, a VOC value of

Figure 8. Voltage−current curves obtained from a PEC PV cell with a CdS photoanode modified with 1 nm of Pt and a Pt black electrode in 20 mM Fe(ClO4)2/Fe(ClO4)3 and 100 mM TBAP in acetonitrile. The voltage of the Pt/CdS photoanode was swept from negative to positive at 10 mV s−1. Simulated sunlight (AM1.5G) was used as the light source.

1.38 V, an ISC value of 3.54 mA cm−2, and an FF value of 0.56 were obtained even under simulated sunlight, with a power conversion efficiency estimated at 2.8%. It should be noted that the photocurrent obtained from the Pt/CdS system under these conditions plateaued at an output voltage of approximately 0.9 V, whereas the photocurrent for Pt/CdS in a 20 mM Fe(ClO4)2/Fe(ClO4)3 electrolyte under irradiation by the Xe lamp peaked at 0.47 VAg/AgCl (see Figure 6a) or a 0.71 V output voltage (Figure 6b). The photon flux values generated below 520 nm by the solar simulator and the Xe lamp (shaded area in Figure S4) were equivalent to 7.45 and 62.5 mA cm−2, respectively, as calculated per ampere. The photon flux calculated for the solar simulator is considered to be reasonable when compared to the experimental photocurrent (3.54 mA cm−2 ISC in Figure 8) and the IPCE spectrum (70−80% in Figure 7), although the value calculated for the Xe lamp seems too high. Even though Pt deposition is expected to increase the number of active sites on the photoanode and the PEC measurements were performed with vigorous stirring, the quantity of incident photons from the 300-W Xe lamp was still so large that the photocurrent would be limited by the diffusion rates of the reactants, resulting in the peak seen in Figure 6. This could also possibly lead to photocorrosion to some extent. At this stage of our work, we have successfully demonstrated the principles of enhancing the photocurrent by adjusting the concentrations of reactants and modifying the photoelectrode surface with a metal catalyst. In the future, we intend to perform further investigations with the aim of determining the optimal metal catalyst.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b02406. Molar absorption coefficients of the electrolyte (Figure S1), summary of the redox potential for the Fe(ClO4)2/ Fe(ClO4)3 electrolyte (Table S1), electric power values for the present PEC PV cells (Figures S2 and S3), output spectra of the Xe lamp and solar simulator utilized in this study (Figure S4), and electrochemical background measurement for the present supporting electrolyte (Figure S5) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address

4. CONCLUSIONS Novel PEC PV cells with CdS photoanodes and various concentrations of an Fe(ClO4)2/Fe(ClO4)3 redox shuttle were investigated by comparison between electrochemical measurements in three-electrode and two-electrode setups. Light

†

Department of Chemical Engineering, Fukuoka University, 819-1 Nanakuma, Jonan-ku,Fukuoka, 810-0180, Japan Notes

The authors declare no competing financial interest. H

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ACKNOWLEDGMENTS This work was financially supported by a Grant-in-Aid for Specially Promoted Research (No. 23000009) from the Japan Society for the Promotion of Science (JSPS). This work contributes to the International Exchange Program of the A3 Foresight Program of JSPS.

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ABBREVIATIONS

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REFERENCES

PEC PVs, photoelectrochemical photovoltaic cells; IPCE, incident photon-to-current conversion efficiency

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