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Electrolyte effects on undoped and Mo-doped BiVO4 film for photoelectrochemical water splitting
Journal of Electroanalytical Chemistry 842 (2019) 41–49
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Electrolyte effects on undoped and Mo-doped BiVO4 film for photoelectrochemical water splitting
T
Pran Krisna Dasa, Maheswari Arunachalamb, Young Jun Seoc, Kwang-Soon Ahnd, Jun-Seok Hae, ⁎ Soon Hyung Kangc, a
Department of Advanced Chemicals and Engineering, Chonnam National University, Yongbong-ro 77, Yongbong-dong, Gwangju 500-757, Republic of Korea Department of Chemistry, Chonnam National University, Yongbong-ro 77, Yongbong-dong, Gwangju 500-757, Republic of Korea c Department of Chemistry Education and Optoelectronic Convergence Research Center, Chonnam National University, Yongbong-ro 77, Yongbong-dong, Gwangju 500757, Republic of Korea d School of Chemical Engineering, Yeungnam University, Gyeongsan 712-749, Republic of Korea e Department of Chemical Engineering, Chonnam National University, Gwangju 61186, Republic of Korea b
ARTICLE INFO
ABSTRACT
Keywords: Electrolyte BiVO4 Photoelectrochemical cell Cation Anion
As the electrolyte in a photoelectrochemical system used for solar water splitting is closely associated with the charge transfer phenomenon, the fundamental study of the electrolyte is an increasingly crucial issue. In this paper, the relation between the photoelectrochemical performance and the variation of electrolyte composition is investigated in depth. Here, the potassium phosphate (KPi) containing different anions (CH3COO−, Cl− and NO3−) or cations (NH4+, K+ and Na+) was prepared at the electrolyte with the different pH values of 5, 7, 9, and 11 in order to investigate how anions or cations affect the PEC performance. Herein, we compared the differences in the undoped and Mo-doped BiVO4 films due to the different bulk/surface states, and found different morphological and crystalline features as well as remarkably different PEC performance. At first, the pH in the electrolyte significantly influences the solar water oxidation reaction, revealing that the electrolyte with a high pH grants better PEC activity, because the higher pH solution can provide more hydroxide ions (OH−) to react with holes to form hydroxyl radicals, which are recognized as important intermediates for PEC water oxidation in the presence of O2. Further, in the electrolyte containing the different cations, the NH4+ ions exhibit the enhanced PEC performance, due to its cation size which increases the ionic dissociation and viscosity. By contrast, the anion effect can be negligible in the used electrolyte. From this fundamental research, it can be known that the optimization of electrolyte is quite a vital parameter for advancing the PEC performance.
1. Introduction Currently, an up-and-coming artificial photosynthesis strategy for a future carbon-free, storable, and sustainable energy is regarded as a renewable energy source based on solar energy [1–5]. In particular, photoelectrochemical (PEC) water splitting for solar fuel production has been considered as a desirable approach to resolve global environmental and energy problems [6–13]. The fascination toward solar water splitting in the circumstance of sufficient water resources is due to its main advantages of a small reaction potential of 1.23 eV and almost zero CO2 emission. The main mechanism of PEC water splitting is generally based on the generation of photoexcited charge carriers. That is, the sunlight absorbed by a semiconductor photoelectrode generates the photoexcited charge
⁎
carriers as well as the electron (e−) and hole (h+) pairs inside the semiconductor particles. These photogenerated carriers drive electrochemical oxidation/reduction reactions at the semiconductor/electrolyte interface, producing hydrogen and oxygen at the photocathode and photoanode, respectively. In order to realize a well-working PEC device, it must generally satisfy at least three requirements: 1) appropriate energetic band gap of the materials, 2) proper band position and 3) robust resistance to photocorrosion. Specifically, the ideal photoelectrode used for photoelectrochemical water splitting requires an appropriate band gap of more than 1.23 eV (thermodynamic equilibrium voltage) plus 0.8 eV so as to overcome kinetic barriers or loss. Furthermore, the conduction band and valence band position are important in photoelectrode for PEC water splitting. In order to make the favorable H2 and O2 evolution
Corresponding author. E-mail address: [email protected] (S.H. Kang).
https://doi.org/10.1016/j.jelechem.2019.04.030 Received 12 February 2019; Received in revised form 1 April 2019; Accepted 8 April 2019 Available online 25 April 2019 1572-6657/ © 2019 Elsevier B.V. All rights reserved.
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reaction, the photoelectrode should be positioned at the conduction band above the H+/H2 equilibrium potential for hydrogen evolution and at the valence band below the O2/H2O equilibrium potential for oxygen evolution. In addition, in consideration of the continuous measurement under solar illumination, the long-term photostability should be attained in aqueous solution [3,5,14–16]. Therefore, in the past few decades, much research has been focused on the pursuit or synthesis of novel and innovative semiconductor materials for sunlight assisted water oxidation, [8,17–20] but the ideal photoanode in terms of efficiency remains elusive. Therefore, the substantial research should continue to be concentrated on the material issue. BiVO4, considered as one of the most promising n-type semiconductors, exhibits monoclinic bismuth vanadate (BiVO4) with a narrow band gap (Eg = 2.4 eV), possesses a high light capturing yield toward the visible wavelength, and can achieve a theoretical photocurrent density of ~7 mA/cm2 [21]. However, BiVO4 photoanode also has certain limitations, such as excessive surface recombination, poor charge transport, short carrier diffusion length (~100 nm), and sluggish water oxidation kinetics. Considering that the PEC reaction occurs on the intimate surface region (~tens of nanometer), the most limitation happened on the surface-related reaction. Hence, the effective surface engineering intimately associated with the surface recombination, charge transfer event, and water oxidation kinetics may be a key factor for improving the PEC performance of BiVO4 film. It has actually been reported that Mo- or W-doped BiVO4 film shows dramatically improved PEC response due to the significantly increased electrical conductivity [22]. However, the surface state of the doped BiVO4 film is quite different than that of the intrinsic BiVO4 film. It has already been reported that the doping of BiVO4 film dramatically influences the electronic conductivity in the bulk state of the film, subsequently enhancing the PEC performance. However, as the doping can also affect the surface states in the interfacial region between the photoelectrode and electrolyte [23–25], in this research, we explored the undoped and doped BiVO4 films in order to survey the interfacial issues in the variation of the electrolyte composition. The electrode/electrolyte interfacial chemistry is crucial for photoelectrochemical (PEC) and electrocatalytic water splitting, where cations and anions in the electrolyte often play a crucial role. In this work, we fundamentally focus on the effects of cations (NH4+, K+, and Na+) and anions (CH3COO−, Cl−, and NO3−) at the different pH values of 5, 7, 9, and 11 of electrolyte using potassium phosphate so as to investigate how anions and cations affect the PEC performance of undoped and Mo-doped BiVO4 films. It was found that the PEC activities of BiVO4 photoelectrode in strong alkaline electrolytes (pH 11) show a sequence of NH4+ > K+ > Na+ with an obvious cathodic shift of onset potential, and the photocurrents of Mo-doped BiVO4 electrodes follow a sequence of NH4+ > Na+ > K+ during the reaction. This is most likely because Na+ ions can more strongly adsorb on the BiVO4 surface and serve as a more effective blocker for PEC reaction sites. The anion effect is negligible in both undoped and Mo-doped BiVO4 photoanodes. However, the Mo-doped BiVO4 photoanode shows better photocurrent in acidic electrolyte. More detailed discussion will follow in this manuscript with several results.
stream. This process can guarantee crack-free deposition during the sample coating [41]. The BiVO4 and Mo-doped BiVO4 photoelectrodes were prepared by the spin-coating assisted sol-gel method. In order to prepare 20 mL of 0.2 M BiVO4 solution, 0.4679 g of ammonium metavanadate (NH4VO3) (99.0%, JUNSEI) was dissolved in 5 mL of deionized water (DI) and continuously stirred for about 10 min. To completely dissolve the NH4VO3 solution, 5 mL of 60% HNO3 solution was added to this solution, which changed the solution color from white to a transparent light yellow. Then, 1.9403 g of bismuth (III) nitrate (Bi (NO3)2·5H2O) (98% crystalline, Alfa Aesar) was added into the vanadium solution, and the mixture was thoroughly sonicated for 30 min so as to ensure the homogeneous mixing of all of the components. In order to control the viscosity and surface tension of the precursor solutions, 10 mL of absolute ethanol was incorporated with the solution under constant stirring at room temperature for 30 min to form a spinnable viscous solution. Then, 50 μL of 0.2 M BiVO4 solution was spread on the surface of the FTO substrate and subsequently spin-coated at 1500 rpm for 10 s. The samples were then dried at 70 °C for a short period (10 min) on a hot plate to remove the remnant solvents and enhance the structural stability of the film. After drying the sample for a short time, the high-temperature annealing was performed at 500 °C for 2 h under air atmosphere in order to improve the crystallinity of the sample. Similarly, 3 wt% Mo-doped BiVO4 photoanodes were synthesized by the same technique under similar conditions by adding ammonium molybdate tetrahydrate ((NH4)6Mo7O24·4H2O) (99.98% trace metals basis, SIGMA-ALDRICH) at different weight percentages ranging from 1 wt% to 5 wt% and adding bismuth (III) nitrate (Bi(NO3)2·5H2O). Herein, we explored the 3 wt% Mo-doped BiVO4 film so as to reveal the highest PEC activity. 2.2. Characterization The morphological changes and the corresponding cross-sectional views of BiVO4 and Mo-doped BiVO4 films were observed by Field Emission Scanning Electron Microscopy (FE-SEM, S4800, HITACHI Inc.) operating at 10 kV and 20 mA. The crystal structures of the films were obtained through X-ray diffraction (XRD) measurement using a (PANalytical X'Pert PRO) instrument operating at 40 kV and 30 mA. On the other hand, the absorbance of the samples were measured using an ultraviolet-visible (UV–vis) spectrophotometer (LAMBDA-900 UV/vis/ IR Spectrometer, PerkinElmer) to analyze the optical properties of BiVO4 and Mo-doped BiVO4 in the wavelength range of 350–750 nm. 2.3. Photoelectrochemical characterizations PEC measurements were conducted in a three-electrode configuration under light illumination using a potentiostat (CHI Instruments, USA). The BiVO4 and Mo-doped BiVO4 photoanodes were used as working electrodes with an active area of 0.2 cm2. The Pt sheet (area: 10 cm2) and saturated Ag/AgCl electrode with saturated KCl were used as counter and reference electrodes, respectively. The potential of homemade saturated Ag/AgCl electrode (0.11 V vs normal hydrogen electrode, NHE) was converted to be presented in this work. The potentials described in this paper refer to the reversible hydrogen electrode (RHE), which is related to the saturated Ag/AgCl electrode and the electrolyte pH by the following equation:
2. Experimental section 2.1. Preparation of BiVO4 and Mo-doped BiVO4 photoanodes
ERHE = EAg/AgCl (sat.) + E° Ag/AgCl (sat.) + 0.0591·pH
First, a fluorine-doped Tin Oxide (FTO) (Hartford Glass Corp.; sheet resistance 15 Ω/sq.) substrate with dimensions of 1.5 × 1.5 cm2 was cleaned with detergent and deionized water (DI) three to four times in order to remove impurity and dust. Next, the substrate was treated with a solution of H2SO4:H2O2:H2O (3:1:1, volume ratio) for 30 min so as to improve the surface hydrophilicity of the substrate, then rinsed with acetone and deionized water via ultrasonication, and finally dried in air
An aqueous electrolyte containing 0.5 M Na2SO4 or 0.5 M potassium phosphate (KPi) was used for the PEC measurement after nitrogen bubbling to remove the dissolved oxygen in the solution. The 0.5 M KPi solution was prepared by dissolving K2HPO4 and KH2PO4 (98.0% and 99%, SAMCHUN, respectively) in deionized (DI) water. The pH of the 0.5 M KPi solution was measured between 7.01 and 7.1. The electrolytes of pH 9 and 11 were created by adding various amounts of NaOH, 42
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KOH, and NH4OH to 0.5 M KPi solution. By contrast, the electrolyte of pH 5 was created by adding various amounts of HCl, HNO3, and CH3COOH solution to 0.5 M KPi solution. A Xe lamp was used as a light source at 150 W with a light intensity of 100 mW/cm2 (AM 1.5 filter). The current–voltage (J–V) curves and chopped light on/off curves were acquired at a scan rate of 20 mV/s during the potential sweep. Cyclic voltammetry (CV) measurements were performed at room temperature at a scan rate of 100 mV/s in 0.5 M Na2SO4 and 0.5 M KPi solution, respectively. Capacitive current scans were used to determine the extent of surface charging in the BiVO4/KPi interfacial region at room temperature. The evolved O2 gas was quantitatively determined during the photo electrochemical water oxidation in potassium phosphate by applying 1.23 V vs. RHE under illumination (100 mW/cm2) using a gas chromatograph (GC) (YL Instrument 6500GC System). A well-arranged air-tight electrochemical cell was used for the quantitative evaluation of O2. The cell was filled with 0.5 M KPi (pH = 7) electrolyte then degassed by argon gas flow for 20 min prior to measuring the amount of gas evolution. Then, it should be tightly packed so as to block the O2 gas leak into the cell. The gas was manually injected inside the gas chromatography. Uncertainties in the amount of O2 not contributing to the water splitting process were ruled out by accounting for the amount of O2 detected in GC during the O2 leakage test. However, the amount of O2 generated during blank run was very low, meaning that there was almost no leakage during the gas evolution experiments. ‘No gas leakage’ was also ensured by evaluating and comparing with the ratio of O2/N2 signals in the GC during blank run. Furthermore, in order to obtain the carrier density and flat-band potential (VFB), the typical Mott-Schottky plots (AUTOLAB/PGSTAT, 128 N) were measured at a frequency of 1 kHz using a standard potentiostat equipped with an impedance spectra analyzer (Nova) in the same electrochemical configuration and electrolyte under dark condition.
results shown in Fig. 1. The XRD pattern of the undoped BiVO4 film after calcination at 500 °C under air ambient shows a well-crystallized monoclinic structure corresponding to the (013), (004), (024), and (116) planes, which are in excellent accordance with the standard data (JCPDS No. 83-1699) [26]. No other peaks for impurity or other binary or ternary compounds were found, thereby confirming the formation of a high-quality BiVO4 film. In the case of Mo-doped BiVO4 film, the film exhibits a well-crystallized monoclinic structure, but the main peak of the monoclinic BiVO4 structure shifts to a high 2θ angle with low intensity, which clearly indicates the substitution of Mo cations into the BiVO4 lattice. In this case, the enlargement of the d-spacing of the corresponding crystal planes stems from the substitution of Mo6+ cations into V sites of BiVO4, because the ionic radius (0.41 Å) of Mo6+ cations is larger than that (0.35 Å) of V5+ cations [27]. In addition, except for the FTO substrate peaks, no impurities or other phases were detected, which confirms the high purity of the prepared film. The surface morphologies of undoped and Mo-doped BiVO4 films were compared, as shown in Fig. 2. The undoped BiVO4 film exhibits a maze-like structure containing many pores. By contrast, small pores with well-defined shapes are seen for Mo-doped BiVO4 film. The different morphologies of undoped and Mo-doped BiVO4 samples show the role of the Mo dopant as forming the BiVO4 film. It can be seen that the addition of Mo elements in the precursor solution suppresses the growth of grains in BiVO4 film, forming smaller grains with small sized pores through the surface area of BiVO4 film. It also implies that the smaller size of the nanoparticles composed of the film can provide more active sites, which can improve the PEC performance. The optical properties of the films are crucial for PEC performance. In order to survey the optical properties of the films, the UV–Vis spectra were taken, as shown in Fig. 3a. All of the films show strong absorption in the UV–Vis wavelength, and the incorporation of Mo can hardly affect the absorption edges of BiVO4. That is to say, no peak shift of UV–Vis spectrum was found in Mo-doped BiVO4 film, indicating the absence of the phase transformation from the monoclinic to tetragonal structures with Mo doping [28]. Both samples showed strong optical absorption in the 400–500 nm wavelength range and exhibited a
3. Results and discussion In order to investigate the crystalline properties of undoped and Modoped BiVO4 films, the XRD measurement was performed, with the
Fig. 1. XRD patterns of (a) undoped BiVO4 and (b) Mo-doped BiVO4 films. 43
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Fig. 2. Surface FE-SEM images of (a) undoped BiVO4 and (b) Mo-doped BiVO4 films.
similar absorption curve. In addition, the onset wavelengths of undoped and Mo-doped BiVO4 films are 503 nm and 505 nm, respectively. The exact optical band gap energies were estimated from the Tauc plots (Fig. 3(b)), existing in almost the same range of 2.44 to 2.45 eV, which is consistent with the reported value of BiVO4 [29,30]. Therefore, it was concluded that the substitution of Mo6+ cations could hardly influence the band gap of BiVO4, as it exhibited the characteristic band gap of monoclinic BiVO4 phase. In order to thoroughly examine the surface state in the photoelectrochemical testing condition, the cyclic voltammetry (CV) was measured in a 0.5 M Na2SO4 aqueous solution under dark and continuous sun light irradiation. As shown in Fig. 4, the anodic photocurrent of Mo-doped BiVO4 film was significantly increased as compared to that of the BiVO4 film, and it can be noted that the Mo-doped BiVO4 film may have a much higher specific capacitance after the discharging process. Although the separation of the photogenerated charges initially takes place, the photogenerated holes are preferentially trapped at the electrode surface, thus taking part in the charge recombination reaction. In fact, the holes trapped at the surface are one of the limiting factors of the PEC performance. For further investigation, the cyclic voltammetry under dark condition was performed at the same condition. The specific capacitance is one of the supportive phenomena to explain the specific surface areas of the samples, which is closely related to the OER activity. The specific capacitance was calculated from the CV curves according to the following equation:
Csp =
V2 V1
2 (V 2
active material, and S is the scan rate. The calculated specific capacitances of undoped and Mo-doped BiVO4 samples are 14.3572 mF/g and 57.3687 mF/g, respectively, under the dark condition. We also measured the cyclic voltammetry (CV) in a 0.5 M KPi solution under dark and continuous sun light irradiation. As shown in (Fig. S1), the anodic photocurrent of Mo-doped BiVO4 film was significantly increased as compared to that of the BiVO4 film, and it can be noted that the Modoped BiVO4 film may have a much higher specific capacitance after the discharging process. In case of the KPi solution, the calculated specific capacitances of undoped and Mo-doped BiVO4 samples are 15.2132 mF/g and 47.9955 mF/g, respectively, under the dark condition. The Mo-doped BiVO4 film is expected to exhibit better oxygen evolution reaction activity than the undoped film. Furthermore, the electrochemical capacitive current scan was performed under the dark condition so as to intimately investigate the surface states (e.g., defect or trap sites), as shown in Fig. 4(c). The pristine BiVO4 film shows a peak centered at about 0.75 VRHE, which was previously confirmed as evidence for a surface state by Bard et al. [31]. The pure BiVO4 film exhibits a steadily increased capacitance spectra as scanning to the low potential and more 0.75 VRHE, whereas the Mo-doped BiVO4 film exhibits a steepness of capacitance at near 0.75 VRHE and 1.0 VRHE, indicating the presence of surface states in the circumstance where the addition of Mo elements on the BiVO4 lattice can induce a different surface condition. Fig. 5 shows the linear sweep curves of the undoped and Mo-doped BiVO4 films obtained under the light on/off condition in electrolyte with pH 5. In order to examine the anion effects (CH3COO−, Cl−, and NO3−) in the 0.5 M potassium phosphate (KPi) electrolyte, three types of acidic solutions (CH3COOH, HCl, and HNO3; briefly expressed as AA, HA, and NA, respectively) were used to modulate the pH value of KPi
I (V ) dV V 1) Sm
where Csp is the specific capacitance, I(V)dV is the integrated area of the CV curve in the voltage window from V1 to V2, m is the mass of the
Fig. 3. (a) UV–visible absorption spectra and (b) Tauc plots of undoped BiVO4 and Mo-doped BiVO4 films. 44
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Fig. 4. Cyclic voltammetry for (a) undoped BiVO4 and (b) Mo-doped BiVO4 films, and (c) capacitances of both samples at dark state. Both dark current (D) and photocurrent (L) under AM 1.5 sunlight illumination at scan rate of 100 mV/s. Data were recorded in a three-electrode cell in 0.5 M Na2SO4 solutions (pH 6.5).
electrolyte. In the case of the undoped BiVO4 film (Fig. 5(a)), no remarkable difference was observed, indicating that the anions have no effects on solar water oxidation. Further, it was observed that in the low potential region (0–0.5 VRHE), a noticeable high dark current was formed, which can likely be ascribed to the charging effect of hydrogen ions into the BiVO4 film. By contrast, the Mo-doped BiVO4 film shows a certain amount of photoresponse in the KPi electrolyte modulated by AA and HA as compared to that of NA. In general, in the presence of AA solution, the adsorption of acetate ions on the Mo-doped BiVO4 surface was observed and the dominant photo-oxidation reaction in this solution may be the oxidation of acetate ions to carbon dioxide or ethane [32–36]. Accordingly, most of the photogenerated holes can meet with the acetate ions, enabling the formation of the carbon-based other compounds, rather than water oxidation. In a similar context, in the HA solution, the Cl2 gas is easily produced by the photo-oxidation of Cl− ions because the potential of Cl− oxidation is relatively lower than that of water oxidation, and only two photons take part in the Cl− oxidation reaction
to convert to the Cl2, as compared with the four-photon requirement for H2O oxidation reaction to convert to the O2. Therefore, it was noted that the photocurrent was mainly produced by the chloride oxidation, and reversely, the O2 evolution reaction was completely suppressed. Furthermore, in the case of NA solution, the N2 can be produced by the photo-oxidation of NO3− in the nitrate solution; however, the probability of this is extremely rare due to the high overpotential. Therefore, the absence or presence of Mo-doping on the BiVO4 film influences the PEC performance, where no difference was observed in the undoped BiVO4 film, whereas AA- and HA-mediated KPi electrolytes show slightly enhanced photocurrent density in the Modoped BiVO4 films, attributed to the increased surface acidity of BiVO4 film by the substitution of Mo6+ cation, since Mo6+ cation has a larger electronegativity than V5+ cation. The higher adsorption affinity toward electrolytes, which is due to its high surface acidity on the surface, was an important factor for the enhanced activity of Mo-doped BiVO4 against pure BiVO4. Moreover, in the minor part, the sufficiently photogenerated holes in the photoelectrode/electrolyte interface region
Fig. 5. Chopped LSV curves of (a) undoped BiVO4 film and (b) Mo-doped BiVO4 films. Data were recorded in a three-electrode PEC cell in 0.5 M potassium phosphate (KPi) solution at pH 5 containing CH3COO−, Cl−, and NO3− anions. (Here, AA = CH3COOH, HA = HCl, and NA = HNO3). 45
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Fig. 6. Chopped LSV curves of undoped BiVO4 film in the solution of (a) pH 9, (b) pH 11, and chopped LSV curves of 3 wt% Mo-doped BiVO4 film in the solution of (c) pH 9 and (d) pH 11. Data were recorded in a 3-electrode PEC cell in 0.5 M potassium phosphate (KPi) solution and the pH of its solution was modulated by the NH4+, Na+, and K+ cations. (Here, AH = NH4OH, SH = NaOH, and PH = KOH).
can react with anions, rather than the real photo-oxidation of water. Further, the Mo-doped BiVO4 film also exhibits markedly dark current in the potential range of 0–0.25 VRHE, likely due to the hydrogen intercalation process into the film. Fig. 6 shows the chopped linear sweep curves (LSV) of undoped BiVO4 and Mo-doped BiVO4 films obtained under the light illumination condition in the alkaline solution. In order to investigate the photoelectrochemical properties of both samples in the 0.5 M KPi electrolyte containing different cations (NH4+, K+, and Na+), the electrolyte with a pH of 11 was prepared to investigate the cation effect. Here, the alkaline KPi electrolyte modulated by NH4OH, KOH, and NaOH solution was prepared to make a solution of pH 11. Fig. 6(a) shows the chopped on/off photocurrent curve of undoped BiVO4 film depending on the cations, that exist in the significantly low value, listing the sequence of photocurrent; NH4+ > K+ > Na+, which follow the ionic radius trend. By contrast, the light on/off photocurrent curve of Mo-doped BiVO4 film also shows a sequence of NH4+ > Na+ > K+. This irregular ordering of PEC activity is found to be the balance effect of two points: the extent of the weakening of OeH bond on photoelectrode surface after interacting with cations in electrolytes, and the different rates of oxygen reduction reaction (ORR). Already, it proved that the water oxidation reaction in alkaline media closely involves the dissociation of OeH bonds, which are actually slow. From the preceding literature survey, the hydrated cations of M+(H2O)x (x denoting the hydration number) can interact with OH− species adsorbed on the surface of photoelectrode, forming the complexes like electrode−OH–M+(H2O)x via noncovalent interactions, including hydrogen bond and electrostatic interactions [37–40]. Such complexes can enhance the adsorption of OH− species and block the active sites for O2 and H2 adsorption, which will inhibit the water oxidation rate and induce the weakening of the OeH bond. In particular, the K+ cations not only lead to the rapid decrease of OeH bond strength, but are also most effective for retarding the back reaction, while the NH4+ cations shows the most detrimental effects on water oxidation. Our results provide important insight into the roles of cations in PEC water oxidation and indicate a new strategy for tailoring the electrode-electrolyte interface via the sagacious choice of cations in the BiVO4/KPi electrolyte interface for more effective PEC water splitting.
Moreover, the pH of the electrolyte is one of the most crucial parameters influencing the photoelectrochemical reactions, because the pH value can determine the amount of H+ or OH− ions, becoming a critical influencing factor of PEC cell working in the aqueous solution. Here, the photocurrent increases with increasing pH value. This is explained by the fact that the higher pH value can provide more hydroxide ions (OH−) to react with the photogenerated holes to form hydroxyl radicals. However, in the case of BiVO4 film, the sluggish kinetics of the photogenerated holes, the slow charge transfer at the photoelectrode/electrolyte interface, and the rapid electron-hole recombination in both bulk and surface substantially limit its application as the efficient photoanode material. Therefore, it is highly desired to enhance the separation of photogenerated charges in order to improve the photoelectrochemical efficiency of BiVO4 film. In the higher pH solution, the more hydroxide ions (OH−) can react with holes to form hydroxyl radicals (·OH). As has been reported, the ·OH is recognized to be an important intermediate for both PEC water oxidation and pollutant degradation in the presence of O2. Thus, the amount of ·OH produced during the water oxidation reaction directly affects the PEC activity. In addition, at the higher pH solution, the formation of ·OH from OH− anions can be further enhanced. Therefore, it is desirable to increase the amount of photogenerated holes on the photoelectrode surface so as to facilitate the production of hydroxyl radicals for efficient PEC process. As has been reported, the phosphate modification of photoelectrode could form the negative field on the photoelectrode surface so as to cause trapped holes, resulting in the effective separation of photogenerated charge, leading to the enhancement of PEC performance. In order to further investigate the influence of the two above mentioned cations and anions on the stability of the undoped BiVO4 and Mo-doped BiVO4 films for PEC water splitting, the photoelectrodes were tested in the electrolyte of various pH values, as shown in Fig. 7. The pH effect as well as the photostability was measured in 0.5 M KPi solution at pH 5, 9, and 11 containing different anions or cations species under AM 1.5 G continuous solar light illumination for 2000 s, using a constant potential of 1.23 VRHE. In the case of the undoped BiVO4 film, the achievable photocurrent density is somewhat low, but a significantly stable photocurrent was attained as compared to that of 46
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Fig. 7. I-t curves (a), (c), (e) of undoped BiVO4 films and (b), (d), (f) of 3 wt% Mo-doped BiVO4 electrodes at 1.23 VRHE in 0.5 M KPi at pH 5, 9, and 11 under continuous illumination for 2000 s. (a) and (b) performed in 0.5 M KPi solution at pH 5 containing CH3COO−, Cl−, and NO3− anions and (c), (d) and (e), (f) performed in 0.5 M KPi solution containing NH4+, Na+, and K+ cations at pH 9 and 11, respectively. (Here, AA = CH3COOH, HA = HCl, NA = HNO3, AH = NH4OH, SH = NaOH and PH = KOH).
the Mo-doped BiVO4 film, owing to the stability of Bi3+ in aqueous medium. In particular, at pH 5, considerably high photostability is attainable in that the anions can release or capture the protons so as to maintain the localized pH value at the BiVO4/electrolyte interface when the solar water splitting occurs. However, at high pH solution, the excess hydroxyl ions adsorb onto the positively charged Bi surface so as to form the soluble species and increase the chemical dissolution rate. On the other hand, the Mo-doped BiVO4 photoelectrodes show lower photostability due to its high surface acidity where the V5+ cations are replaced by the Mo6+ cations, thus increasing the surface acidity. The faradaic efficiency and quantity of evolved H2 and O2 gases are presented in Fig. 8. The faradaic efficiency is calculated according to ρ = nH2 / (Q / 2F) and ρ = nO2 / (Q / 4F), where nH2 and nO2 are the amount of generated hydrogen and oxygen, respectively; Q is the total amount of charge passed through the cell; and F is the faraday constant. In this system, the undoped BiVO4 and 3 wt% Mo-doped BiVO4 photoanodes are used as the working electrode under the applied voltages of 2 V vs saturated Ag/AgCl. The faradaic efficiency and quantity of the evolved H2 and O2 gases are measured for 90 min. The initial faradaic efficiency of the undoped BiVO4 film during the initial 30 min is about 57%, and it increases with time, reaching 68.53% after 90 min for H2 and 14.66% to 36.91% for O2 under continuous illumination. In the case of the Mo-doped BiVO4 film, the initial faradaic efficiency during the initial 30 min is 67.81%, and it increases with time up to 72.88% after 90 min for H2 and 18.07% to 46.20% for O2 under continuous illumination. It also shows a steadily increased H2 and O2 evolution rate up to 90 min. This means that the faradaic efficiency and quantity of the evolved H2 and O2 gases are both high in the case of doped BiVO4 as compared to undoped BiVO4, likely resulting from the higher photocurrent density and lower onset potential. However, the amounts of evolved H2 and O2 gases are relatively lower than the extent of photocurrent enhancement in the Mo-doped BiVO4 film, indicating the sincere necessity of cocatalysts at the interfacial reaction in this system. We also measure the faradaic efficiency and amount of evolved H2 and O2 gases varying the composition of electrolytes at pH 9. At pH 9, NH4+ containing KPi electrolyte shows the initial faradaic efficiency of the undoped BiVO4 film during the initial 30 min is about 81.74%, and it increases with time, reaching 85.30% after 90 min for
H2 and 18.28% to 26.65% for O2 under continuous illumination (Fig. S2). On the other hand, in the case of Mo-doped BiVO4 film, the initial faradaic efficiency during the initial 30 min is 88.44%, and it increases with time up to 93.92% after 90 min for H2 and 23.00% to 44.32% for O2 under continuous illumination. KPi solution containing Na+ at pH 9 shows that the faradaic efficiency of the undoped BiVO4 film during the initial 30 min is about 61.23%, and it increases with time, reaching 64.50% after 90 min for H2 and 07.14% to 29.33% for O2 under continuous illumination (Fig. S3) and in the case of Mo-doped BiVO4 film, the initial faradaic efficiency during the initial 30 min is 80.43%, and it increases with time up to 84.40% after 90 min for H2 and 12.93% to 30.26% for O2 under continuous illumination. In case of KPi solution containing K+ at pH 9, the faradaic efficiency of the undoped BiVO4 film during the initial 30 min is about 77.44%, and it increases with time, reaching 81.81% after 90 min for H2 and 09.80% to 20.01% for O2 under continuous illumination (Fig. S4) and in the case of Mo-doped BiVO4 film, the initial faradaic efficiency during the initial 30 min is 68.19%, and it increases with time up to 75.18% after 90 min for H2 and 27.68% to 44.90% for O2 under continuous illumination. It shows that the faradaic efficiency is increasing with the increasing of the pH of the electrolyte, disclosing the steadily increase of H2 and O2 evolution rate up to 90 min. The faradaic efficiency and quantity of the evolved H2 and O2 gases are both high in the case of Mo-doped BiVO4 as compared to undoped BiVO4 at pH 9 and it is related to the photocurrent density trend. 4. Conclusion The prepared n-type BiVO4 and 3 wt% Mo-doped BiVO4 electrodes were used as a photoanodes of a PEC cell. The types of anions (CH3COO−, Cl− and NO3−) and cations (NH4+, K+ and Na+) contained in the electrolyte, as well as the pH conditions of the electrolyte, were systematically varied in order to investigate how these conditions affect the PEC action (i.e., photocurrent to O2 conversion efficiency). It is found that the PEC activities of the BiVO4 photoelectrode in strong alkaline electrolytes show a trend of NH4+ > K+ > Na+ with the obvious cathodic shift of onset potential, and the photocurrents of the Mo-doped BiVO4 electrodes follow a trend of NH4+ > Na+ > K+ during the PEC performance. This is most likely because Na+ ions can 47
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Fig. 8. (a and b) Faradaic efficiencies and (c and d) quantities of the detected H2 and O2 gases in a PEC system from the undoped BiVO4 and Mo-doped BiVO4 photoanodes as the working electrode under applied voltages of 2 V vs Ag/AgCl in 0.5 M KPi solution at pH 7.
more strongly adsorb on the BiVO4 surface and serve as a more effective blocker for PEC reaction sites. No significance anion effect appears in either sample. The anion effects of undoped and Mo-doped BiVO4 films act like a dissimilar motion, exhibiting no remarkable difference in the undoped film, whereas the CH3COOH and HCl solution-mediated KPi electrolytes show slightly enhanced photocurrent density in the Modoped BiVO4 films. The main reason for this is the increased surface acidity of BiVO4 by the substitution of Mo cation; since Mo6+ cations have a larger electronegativity than V5+ cations. The higher adsorption affinity toward electrolytes, due to its high surface acidity on the surface, was an important reason for the enhanced activity of Mo-doped BiVO4 film against the pure BiVO4. Moreover, in the minor part, the sufficiently photogenerated holes in the photoelectrode/electrolyte interface region can react with anions, rather than the real photo-oxidation of water. The pH value of the solution is one of the most crucial parameters influencing the photoelectrochemical reactions. It shows a photocurrent increase with increased pH value. It appears that a higher pH value can provide more hydroxide ions (OH−) to react with holes to form hydroxyl radicals. As reported, the hydroxyl radicals (·OH) are recognized to be important intermediates for PEC water oxidation in the presence of O2. Thus, the amount of ·OH produced during the PEC reaction directly affects the PEC activity. This study will provide a good foundation for formulating optimum electrolyte compositions in order to enhance the efficiency of desired photo-oxidation reactions for various photoelectrochemical cells.
Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jelechem.2019.04.030. References [1] Z.B. Chen, T.F. Jaramillo, T.G. Deutsch, A. KleimanShwarsctein, A.J. Forman, N. Gaillard, R. Garland, K. Takanabe, C. Heske, M. Sunkara, Accelerating materials development for photoelectrochemical hydrogen production: standards for methods, definitions, and reporting protocols, J. Mater. Res. 25 (1) (2010) 3–16. [2] Z. Li, W. Luo, M. Zhang, J. Feng, Z. Zou, Photoelectrochemical cells for solar hydrogen production: current state of promising photoelectrodes, methods to improve their properties and outlook, Energy Environ. Sci. 6 (2013) 347–370. [3] Y. Park, K.J. McDonald, K. Choi, Progress in bismuth vanadate photoanodes for use in solar water oxidation, Chem. Soc. Rev. 42 (2013) 2321–2337. [4] R. Abe, Recent progress on photocatalytic and photoelectrochemical water splitting under visible light irradiation, J. Photochem. Photobiol. C 11 (4) (2010) 179–209. [5] M.G. Walter, E.L. Warren, J.R. McKone, S.W. Boettcher, Q. Mi, E.A. Santori, N.S. Lewis, Solar water splitting cells, Chem. Rev. 110 (11) (2010) 6446–6473. [6] C.M. Ding, J.Y. Shi, Z.L. Wang, C. Li, Photoelectrocatalytic water splitting: significance of cocatalysts, electrolyte and interfaces, ACS Catal. 7 (2017) 675–688. [7] A.J. Nozik, Photoelectrochemistry: applications to solar energy conversion, Annu. Rev. Phys. Chem. 29 (1978) 189–222. [8] A. Fujishima, K. Honda, Electrochemical photolysis of water at a semiconductor electrode, Nature 238 (1972) 37–38. [9] N.S. Lewis, D.G. Nocera, Powering the planet: chemical challenges in solar energy utilization, Proc. Natl. Acad. Sci. U. S. A. 103 (43) (2006) 15729–15735. [10] D. Gust, T.A. Moore, A.L. Moore, Mimicking photosynthetic solar energy transduction, Acc. Chem. Res. 34 (2001) 40–48. [11] S. Chu, Y. Cui, N. Liu, The path towards sustainable energy, Nat. Mater. 16 (2016) 16–22. [12] A.J. Morris, G.J. Meyer, E. Fujita, Molecular approaches to the photocatalytic reduction of carbon dioxide for solar fuels, Acc. Chem. Res. 42 (2009) 1983–1994. [13] K. Sivula, R. van de Krol, Semiconducting materials for photoelectrochemical energy conversion, Nat. Rev. Mater. 1 (2016) 15010. [14] R. Van de Krol, M. Grätzel, Photoelectrochemical Hydrogen Production, Springer, 2012, p. 324. [15] A. Kudo, K. Ueda, H. Kato, I. Mikami, Photocatalytic O2 evolution under visible light irradiation on BiVO4 in aqueous AgNO3 solution, Catal. Lett. 53 (1998) 229–230. [16] A. Kudo, K. Omori, H. Kato, A novel aqueous process for preparation of crystal form-controlled and highly crystalline BiVO4 powder from layered vanadates at room temperature and it's photocatalytic and photophysical properties, J. Am. Chem. Soc. 121 (1999) 11459–11467.
Acknowledgements This research was supported by Basic Science Research Program through the National ResearchFoundation of Korea (NRF) (grant number: NRF-2017R1D1A3B03031602). Also, this work was supported by Priority Research Centers Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2018R1A6A1A03024334).
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Electrolyte effects on undoped and Mo-doped BiVO4 film for photoelectrochemical water splitting
T
Pran Krisna Dasa, Maheswari Arunachalamb, Young Jun Seoc, Kwang-Soon Ahnd, Jun-Seok Hae, ⁎ Soon Hyung Kangc, a
Department of Advanced Chemicals and Engineering, Chonnam National University, Yongbong-ro 77, Yongbong-dong, Gwangju 500-757, Republic of Korea Department of Chemistry, Chonnam National University, Yongbong-ro 77, Yongbong-dong, Gwangju 500-757, Republic of Korea c Department of Chemistry Education and Optoelectronic Convergence Research Center, Chonnam National University, Yongbong-ro 77, Yongbong-dong, Gwangju 500757, Republic of Korea d School of Chemical Engineering, Yeungnam University, Gyeongsan 712-749, Republic of Korea e Department of Chemical Engineering, Chonnam National University, Gwangju 61186, Republic of Korea b
ARTICLE INFO
ABSTRACT
Keywords: Electrolyte BiVO4 Photoelectrochemical cell Cation Anion
As the electrolyte in a photoelectrochemical system used for solar water splitting is closely associated with the charge transfer phenomenon, the fundamental study of the electrolyte is an increasingly crucial issue. In this paper, the relation between the photoelectrochemical performance and the variation of electrolyte composition is investigated in depth. Here, the potassium phosphate (KPi) containing different anions (CH3COO−, Cl− and NO3−) or cations (NH4+, K+ and Na+) was prepared at the electrolyte with the different pH values of 5, 7, 9, and 11 in order to investigate how anions or cations affect the PEC performance. Herein, we compared the differences in the undoped and Mo-doped BiVO4 films due to the different bulk/surface states, and found different morphological and crystalline features as well as remarkably different PEC performance. At first, the pH in the electrolyte significantly influences the solar water oxidation reaction, revealing that the electrolyte with a high pH grants better PEC activity, because the higher pH solution can provide more hydroxide ions (OH−) to react with holes to form hydroxyl radicals, which are recognized as important intermediates for PEC water oxidation in the presence of O2. Further, in the electrolyte containing the different cations, the NH4+ ions exhibit the enhanced PEC performance, due to its cation size which increases the ionic dissociation and viscosity. By contrast, the anion effect can be negligible in the used electrolyte. From this fundamental research, it can be known that the optimization of electrolyte is quite a vital parameter for advancing the PEC performance.
1. Introduction Currently, an up-and-coming artificial photosynthesis strategy for a future carbon-free, storable, and sustainable energy is regarded as a renewable energy source based on solar energy [1–5]. In particular, photoelectrochemical (PEC) water splitting for solar fuel production has been considered as a desirable approach to resolve global environmental and energy problems [6–13]. The fascination toward solar water splitting in the circumstance of sufficient water resources is due to its main advantages of a small reaction potential of 1.23 eV and almost zero CO2 emission. The main mechanism of PEC water splitting is generally based on the generation of photoexcited charge carriers. That is, the sunlight absorbed by a semiconductor photoelectrode generates the photoexcited charge
⁎
carriers as well as the electron (e−) and hole (h+) pairs inside the semiconductor particles. These photogenerated carriers drive electrochemical oxidation/reduction reactions at the semiconductor/electrolyte interface, producing hydrogen and oxygen at the photocathode and photoanode, respectively. In order to realize a well-working PEC device, it must generally satisfy at least three requirements: 1) appropriate energetic band gap of the materials, 2) proper band position and 3) robust resistance to photocorrosion. Specifically, the ideal photoelectrode used for photoelectrochemical water splitting requires an appropriate band gap of more than 1.23 eV (thermodynamic equilibrium voltage) plus 0.8 eV so as to overcome kinetic barriers or loss. Furthermore, the conduction band and valence band position are important in photoelectrode for PEC water splitting. In order to make the favorable H2 and O2 evolution
Corresponding author. E-mail address: [email protected] (S.H. Kang).
https://doi.org/10.1016/j.jelechem.2019.04.030 Received 12 February 2019; Received in revised form 1 April 2019; Accepted 8 April 2019 Available online 25 April 2019 1572-6657/ © 2019 Elsevier B.V. All rights reserved.
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reaction, the photoelectrode should be positioned at the conduction band above the H+/H2 equilibrium potential for hydrogen evolution and at the valence band below the O2/H2O equilibrium potential for oxygen evolution. In addition, in consideration of the continuous measurement under solar illumination, the long-term photostability should be attained in aqueous solution [3,5,14–16]. Therefore, in the past few decades, much research has been focused on the pursuit or synthesis of novel and innovative semiconductor materials for sunlight assisted water oxidation, [8,17–20] but the ideal photoanode in terms of efficiency remains elusive. Therefore, the substantial research should continue to be concentrated on the material issue. BiVO4, considered as one of the most promising n-type semiconductors, exhibits monoclinic bismuth vanadate (BiVO4) with a narrow band gap (Eg = 2.4 eV), possesses a high light capturing yield toward the visible wavelength, and can achieve a theoretical photocurrent density of ~7 mA/cm2 [21]. However, BiVO4 photoanode also has certain limitations, such as excessive surface recombination, poor charge transport, short carrier diffusion length (~100 nm), and sluggish water oxidation kinetics. Considering that the PEC reaction occurs on the intimate surface region (~tens of nanometer), the most limitation happened on the surface-related reaction. Hence, the effective surface engineering intimately associated with the surface recombination, charge transfer event, and water oxidation kinetics may be a key factor for improving the PEC performance of BiVO4 film. It has actually been reported that Mo- or W-doped BiVO4 film shows dramatically improved PEC response due to the significantly increased electrical conductivity [22]. However, the surface state of the doped BiVO4 film is quite different than that of the intrinsic BiVO4 film. It has already been reported that the doping of BiVO4 film dramatically influences the electronic conductivity in the bulk state of the film, subsequently enhancing the PEC performance. However, as the doping can also affect the surface states in the interfacial region between the photoelectrode and electrolyte [23–25], in this research, we explored the undoped and doped BiVO4 films in order to survey the interfacial issues in the variation of the electrolyte composition. The electrode/electrolyte interfacial chemistry is crucial for photoelectrochemical (PEC) and electrocatalytic water splitting, where cations and anions in the electrolyte often play a crucial role. In this work, we fundamentally focus on the effects of cations (NH4+, K+, and Na+) and anions (CH3COO−, Cl−, and NO3−) at the different pH values of 5, 7, 9, and 11 of electrolyte using potassium phosphate so as to investigate how anions and cations affect the PEC performance of undoped and Mo-doped BiVO4 films. It was found that the PEC activities of BiVO4 photoelectrode in strong alkaline electrolytes (pH 11) show a sequence of NH4+ > K+ > Na+ with an obvious cathodic shift of onset potential, and the photocurrents of Mo-doped BiVO4 electrodes follow a sequence of NH4+ > Na+ > K+ during the reaction. This is most likely because Na+ ions can more strongly adsorb on the BiVO4 surface and serve as a more effective blocker for PEC reaction sites. The anion effect is negligible in both undoped and Mo-doped BiVO4 photoanodes. However, the Mo-doped BiVO4 photoanode shows better photocurrent in acidic electrolyte. More detailed discussion will follow in this manuscript with several results.
stream. This process can guarantee crack-free deposition during the sample coating [41]. The BiVO4 and Mo-doped BiVO4 photoelectrodes were prepared by the spin-coating assisted sol-gel method. In order to prepare 20 mL of 0.2 M BiVO4 solution, 0.4679 g of ammonium metavanadate (NH4VO3) (99.0%, JUNSEI) was dissolved in 5 mL of deionized water (DI) and continuously stirred for about 10 min. To completely dissolve the NH4VO3 solution, 5 mL of 60% HNO3 solution was added to this solution, which changed the solution color from white to a transparent light yellow. Then, 1.9403 g of bismuth (III) nitrate (Bi (NO3)2·5H2O) (98% crystalline, Alfa Aesar) was added into the vanadium solution, and the mixture was thoroughly sonicated for 30 min so as to ensure the homogeneous mixing of all of the components. In order to control the viscosity and surface tension of the precursor solutions, 10 mL of absolute ethanol was incorporated with the solution under constant stirring at room temperature for 30 min to form a spinnable viscous solution. Then, 50 μL of 0.2 M BiVO4 solution was spread on the surface of the FTO substrate and subsequently spin-coated at 1500 rpm for 10 s. The samples were then dried at 70 °C for a short period (10 min) on a hot plate to remove the remnant solvents and enhance the structural stability of the film. After drying the sample for a short time, the high-temperature annealing was performed at 500 °C for 2 h under air atmosphere in order to improve the crystallinity of the sample. Similarly, 3 wt% Mo-doped BiVO4 photoanodes were synthesized by the same technique under similar conditions by adding ammonium molybdate tetrahydrate ((NH4)6Mo7O24·4H2O) (99.98% trace metals basis, SIGMA-ALDRICH) at different weight percentages ranging from 1 wt% to 5 wt% and adding bismuth (III) nitrate (Bi(NO3)2·5H2O). Herein, we explored the 3 wt% Mo-doped BiVO4 film so as to reveal the highest PEC activity. 2.2. Characterization The morphological changes and the corresponding cross-sectional views of BiVO4 and Mo-doped BiVO4 films were observed by Field Emission Scanning Electron Microscopy (FE-SEM, S4800, HITACHI Inc.) operating at 10 kV and 20 mA. The crystal structures of the films were obtained through X-ray diffraction (XRD) measurement using a (PANalytical X'Pert PRO) instrument operating at 40 kV and 30 mA. On the other hand, the absorbance of the samples were measured using an ultraviolet-visible (UV–vis) spectrophotometer (LAMBDA-900 UV/vis/ IR Spectrometer, PerkinElmer) to analyze the optical properties of BiVO4 and Mo-doped BiVO4 in the wavelength range of 350–750 nm. 2.3. Photoelectrochemical characterizations PEC measurements were conducted in a three-electrode configuration under light illumination using a potentiostat (CHI Instruments, USA). The BiVO4 and Mo-doped BiVO4 photoanodes were used as working electrodes with an active area of 0.2 cm2. The Pt sheet (area: 10 cm2) and saturated Ag/AgCl electrode with saturated KCl were used as counter and reference electrodes, respectively. The potential of homemade saturated Ag/AgCl electrode (0.11 V vs normal hydrogen electrode, NHE) was converted to be presented in this work. The potentials described in this paper refer to the reversible hydrogen electrode (RHE), which is related to the saturated Ag/AgCl electrode and the electrolyte pH by the following equation:
2. Experimental section 2.1. Preparation of BiVO4 and Mo-doped BiVO4 photoanodes
ERHE = EAg/AgCl (sat.) + E° Ag/AgCl (sat.) + 0.0591·pH
First, a fluorine-doped Tin Oxide (FTO) (Hartford Glass Corp.; sheet resistance 15 Ω/sq.) substrate with dimensions of 1.5 × 1.5 cm2 was cleaned with detergent and deionized water (DI) three to four times in order to remove impurity and dust. Next, the substrate was treated with a solution of H2SO4:H2O2:H2O (3:1:1, volume ratio) for 30 min so as to improve the surface hydrophilicity of the substrate, then rinsed with acetone and deionized water via ultrasonication, and finally dried in air
An aqueous electrolyte containing 0.5 M Na2SO4 or 0.5 M potassium phosphate (KPi) was used for the PEC measurement after nitrogen bubbling to remove the dissolved oxygen in the solution. The 0.5 M KPi solution was prepared by dissolving K2HPO4 and KH2PO4 (98.0% and 99%, SAMCHUN, respectively) in deionized (DI) water. The pH of the 0.5 M KPi solution was measured between 7.01 and 7.1. The electrolytes of pH 9 and 11 were created by adding various amounts of NaOH, 42
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KOH, and NH4OH to 0.5 M KPi solution. By contrast, the electrolyte of pH 5 was created by adding various amounts of HCl, HNO3, and CH3COOH solution to 0.5 M KPi solution. A Xe lamp was used as a light source at 150 W with a light intensity of 100 mW/cm2 (AM 1.5 filter). The current–voltage (J–V) curves and chopped light on/off curves were acquired at a scan rate of 20 mV/s during the potential sweep. Cyclic voltammetry (CV) measurements were performed at room temperature at a scan rate of 100 mV/s in 0.5 M Na2SO4 and 0.5 M KPi solution, respectively. Capacitive current scans were used to determine the extent of surface charging in the BiVO4/KPi interfacial region at room temperature. The evolved O2 gas was quantitatively determined during the photo electrochemical water oxidation in potassium phosphate by applying 1.23 V vs. RHE under illumination (100 mW/cm2) using a gas chromatograph (GC) (YL Instrument 6500GC System). A well-arranged air-tight electrochemical cell was used for the quantitative evaluation of O2. The cell was filled with 0.5 M KPi (pH = 7) electrolyte then degassed by argon gas flow for 20 min prior to measuring the amount of gas evolution. Then, it should be tightly packed so as to block the O2 gas leak into the cell. The gas was manually injected inside the gas chromatography. Uncertainties in the amount of O2 not contributing to the water splitting process were ruled out by accounting for the amount of O2 detected in GC during the O2 leakage test. However, the amount of O2 generated during blank run was very low, meaning that there was almost no leakage during the gas evolution experiments. ‘No gas leakage’ was also ensured by evaluating and comparing with the ratio of O2/N2 signals in the GC during blank run. Furthermore, in order to obtain the carrier density and flat-band potential (VFB), the typical Mott-Schottky plots (AUTOLAB/PGSTAT, 128 N) were measured at a frequency of 1 kHz using a standard potentiostat equipped with an impedance spectra analyzer (Nova) in the same electrochemical configuration and electrolyte under dark condition.
results shown in Fig. 1. The XRD pattern of the undoped BiVO4 film after calcination at 500 °C under air ambient shows a well-crystallized monoclinic structure corresponding to the (013), (004), (024), and (116) planes, which are in excellent accordance with the standard data (JCPDS No. 83-1699) [26]. No other peaks for impurity or other binary or ternary compounds were found, thereby confirming the formation of a high-quality BiVO4 film. In the case of Mo-doped BiVO4 film, the film exhibits a well-crystallized monoclinic structure, but the main peak of the monoclinic BiVO4 structure shifts to a high 2θ angle with low intensity, which clearly indicates the substitution of Mo cations into the BiVO4 lattice. In this case, the enlargement of the d-spacing of the corresponding crystal planes stems from the substitution of Mo6+ cations into V sites of BiVO4, because the ionic radius (0.41 Å) of Mo6+ cations is larger than that (0.35 Å) of V5+ cations [27]. In addition, except for the FTO substrate peaks, no impurities or other phases were detected, which confirms the high purity of the prepared film. The surface morphologies of undoped and Mo-doped BiVO4 films were compared, as shown in Fig. 2. The undoped BiVO4 film exhibits a maze-like structure containing many pores. By contrast, small pores with well-defined shapes are seen for Mo-doped BiVO4 film. The different morphologies of undoped and Mo-doped BiVO4 samples show the role of the Mo dopant as forming the BiVO4 film. It can be seen that the addition of Mo elements in the precursor solution suppresses the growth of grains in BiVO4 film, forming smaller grains with small sized pores through the surface area of BiVO4 film. It also implies that the smaller size of the nanoparticles composed of the film can provide more active sites, which can improve the PEC performance. The optical properties of the films are crucial for PEC performance. In order to survey the optical properties of the films, the UV–Vis spectra were taken, as shown in Fig. 3a. All of the films show strong absorption in the UV–Vis wavelength, and the incorporation of Mo can hardly affect the absorption edges of BiVO4. That is to say, no peak shift of UV–Vis spectrum was found in Mo-doped BiVO4 film, indicating the absence of the phase transformation from the monoclinic to tetragonal structures with Mo doping [28]. Both samples showed strong optical absorption in the 400–500 nm wavelength range and exhibited a
3. Results and discussion In order to investigate the crystalline properties of undoped and Modoped BiVO4 films, the XRD measurement was performed, with the
Fig. 1. XRD patterns of (a) undoped BiVO4 and (b) Mo-doped BiVO4 films. 43
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Fig. 2. Surface FE-SEM images of (a) undoped BiVO4 and (b) Mo-doped BiVO4 films.
similar absorption curve. In addition, the onset wavelengths of undoped and Mo-doped BiVO4 films are 503 nm and 505 nm, respectively. The exact optical band gap energies were estimated from the Tauc plots (Fig. 3(b)), existing in almost the same range of 2.44 to 2.45 eV, which is consistent with the reported value of BiVO4 [29,30]. Therefore, it was concluded that the substitution of Mo6+ cations could hardly influence the band gap of BiVO4, as it exhibited the characteristic band gap of monoclinic BiVO4 phase. In order to thoroughly examine the surface state in the photoelectrochemical testing condition, the cyclic voltammetry (CV) was measured in a 0.5 M Na2SO4 aqueous solution under dark and continuous sun light irradiation. As shown in Fig. 4, the anodic photocurrent of Mo-doped BiVO4 film was significantly increased as compared to that of the BiVO4 film, and it can be noted that the Mo-doped BiVO4 film may have a much higher specific capacitance after the discharging process. Although the separation of the photogenerated charges initially takes place, the photogenerated holes are preferentially trapped at the electrode surface, thus taking part in the charge recombination reaction. In fact, the holes trapped at the surface are one of the limiting factors of the PEC performance. For further investigation, the cyclic voltammetry under dark condition was performed at the same condition. The specific capacitance is one of the supportive phenomena to explain the specific surface areas of the samples, which is closely related to the OER activity. The specific capacitance was calculated from the CV curves according to the following equation:
Csp =
V2 V1
2 (V 2
active material, and S is the scan rate. The calculated specific capacitances of undoped and Mo-doped BiVO4 samples are 14.3572 mF/g and 57.3687 mF/g, respectively, under the dark condition. We also measured the cyclic voltammetry (CV) in a 0.5 M KPi solution under dark and continuous sun light irradiation. As shown in (Fig. S1), the anodic photocurrent of Mo-doped BiVO4 film was significantly increased as compared to that of the BiVO4 film, and it can be noted that the Modoped BiVO4 film may have a much higher specific capacitance after the discharging process. In case of the KPi solution, the calculated specific capacitances of undoped and Mo-doped BiVO4 samples are 15.2132 mF/g and 47.9955 mF/g, respectively, under the dark condition. The Mo-doped BiVO4 film is expected to exhibit better oxygen evolution reaction activity than the undoped film. Furthermore, the electrochemical capacitive current scan was performed under the dark condition so as to intimately investigate the surface states (e.g., defect or trap sites), as shown in Fig. 4(c). The pristine BiVO4 film shows a peak centered at about 0.75 VRHE, which was previously confirmed as evidence for a surface state by Bard et al. [31]. The pure BiVO4 film exhibits a steadily increased capacitance spectra as scanning to the low potential and more 0.75 VRHE, whereas the Mo-doped BiVO4 film exhibits a steepness of capacitance at near 0.75 VRHE and 1.0 VRHE, indicating the presence of surface states in the circumstance where the addition of Mo elements on the BiVO4 lattice can induce a different surface condition. Fig. 5 shows the linear sweep curves of the undoped and Mo-doped BiVO4 films obtained under the light on/off condition in electrolyte with pH 5. In order to examine the anion effects (CH3COO−, Cl−, and NO3−) in the 0.5 M potassium phosphate (KPi) electrolyte, three types of acidic solutions (CH3COOH, HCl, and HNO3; briefly expressed as AA, HA, and NA, respectively) were used to modulate the pH value of KPi
I (V ) dV V 1) Sm
where Csp is the specific capacitance, I(V)dV is the integrated area of the CV curve in the voltage window from V1 to V2, m is the mass of the
Fig. 3. (a) UV–visible absorption spectra and (b) Tauc plots of undoped BiVO4 and Mo-doped BiVO4 films. 44
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Fig. 4. Cyclic voltammetry for (a) undoped BiVO4 and (b) Mo-doped BiVO4 films, and (c) capacitances of both samples at dark state. Both dark current (D) and photocurrent (L) under AM 1.5 sunlight illumination at scan rate of 100 mV/s. Data were recorded in a three-electrode cell in 0.5 M Na2SO4 solutions (pH 6.5).
electrolyte. In the case of the undoped BiVO4 film (Fig. 5(a)), no remarkable difference was observed, indicating that the anions have no effects on solar water oxidation. Further, it was observed that in the low potential region (0–0.5 VRHE), a noticeable high dark current was formed, which can likely be ascribed to the charging effect of hydrogen ions into the BiVO4 film. By contrast, the Mo-doped BiVO4 film shows a certain amount of photoresponse in the KPi electrolyte modulated by AA and HA as compared to that of NA. In general, in the presence of AA solution, the adsorption of acetate ions on the Mo-doped BiVO4 surface was observed and the dominant photo-oxidation reaction in this solution may be the oxidation of acetate ions to carbon dioxide or ethane [32–36]. Accordingly, most of the photogenerated holes can meet with the acetate ions, enabling the formation of the carbon-based other compounds, rather than water oxidation. In a similar context, in the HA solution, the Cl2 gas is easily produced by the photo-oxidation of Cl− ions because the potential of Cl− oxidation is relatively lower than that of water oxidation, and only two photons take part in the Cl− oxidation reaction
to convert to the Cl2, as compared with the four-photon requirement for H2O oxidation reaction to convert to the O2. Therefore, it was noted that the photocurrent was mainly produced by the chloride oxidation, and reversely, the O2 evolution reaction was completely suppressed. Furthermore, in the case of NA solution, the N2 can be produced by the photo-oxidation of NO3− in the nitrate solution; however, the probability of this is extremely rare due to the high overpotential. Therefore, the absence or presence of Mo-doping on the BiVO4 film influences the PEC performance, where no difference was observed in the undoped BiVO4 film, whereas AA- and HA-mediated KPi electrolytes show slightly enhanced photocurrent density in the Modoped BiVO4 films, attributed to the increased surface acidity of BiVO4 film by the substitution of Mo6+ cation, since Mo6+ cation has a larger electronegativity than V5+ cation. The higher adsorption affinity toward electrolytes, which is due to its high surface acidity on the surface, was an important factor for the enhanced activity of Mo-doped BiVO4 against pure BiVO4. Moreover, in the minor part, the sufficiently photogenerated holes in the photoelectrode/electrolyte interface region
Fig. 5. Chopped LSV curves of (a) undoped BiVO4 film and (b) Mo-doped BiVO4 films. Data were recorded in a three-electrode PEC cell in 0.5 M potassium phosphate (KPi) solution at pH 5 containing CH3COO−, Cl−, and NO3− anions. (Here, AA = CH3COOH, HA = HCl, and NA = HNO3). 45
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Fig. 6. Chopped LSV curves of undoped BiVO4 film in the solution of (a) pH 9, (b) pH 11, and chopped LSV curves of 3 wt% Mo-doped BiVO4 film in the solution of (c) pH 9 and (d) pH 11. Data were recorded in a 3-electrode PEC cell in 0.5 M potassium phosphate (KPi) solution and the pH of its solution was modulated by the NH4+, Na+, and K+ cations. (Here, AH = NH4OH, SH = NaOH, and PH = KOH).
can react with anions, rather than the real photo-oxidation of water. Further, the Mo-doped BiVO4 film also exhibits markedly dark current in the potential range of 0–0.25 VRHE, likely due to the hydrogen intercalation process into the film. Fig. 6 shows the chopped linear sweep curves (LSV) of undoped BiVO4 and Mo-doped BiVO4 films obtained under the light illumination condition in the alkaline solution. In order to investigate the photoelectrochemical properties of both samples in the 0.5 M KPi electrolyte containing different cations (NH4+, K+, and Na+), the electrolyte with a pH of 11 was prepared to investigate the cation effect. Here, the alkaline KPi electrolyte modulated by NH4OH, KOH, and NaOH solution was prepared to make a solution of pH 11. Fig. 6(a) shows the chopped on/off photocurrent curve of undoped BiVO4 film depending on the cations, that exist in the significantly low value, listing the sequence of photocurrent; NH4+ > K+ > Na+, which follow the ionic radius trend. By contrast, the light on/off photocurrent curve of Mo-doped BiVO4 film also shows a sequence of NH4+ > Na+ > K+. This irregular ordering of PEC activity is found to be the balance effect of two points: the extent of the weakening of OeH bond on photoelectrode surface after interacting with cations in electrolytes, and the different rates of oxygen reduction reaction (ORR). Already, it proved that the water oxidation reaction in alkaline media closely involves the dissociation of OeH bonds, which are actually slow. From the preceding literature survey, the hydrated cations of M+(H2O)x (x denoting the hydration number) can interact with OH− species adsorbed on the surface of photoelectrode, forming the complexes like electrode−OH–M+(H2O)x via noncovalent interactions, including hydrogen bond and electrostatic interactions [37–40]. Such complexes can enhance the adsorption of OH− species and block the active sites for O2 and H2 adsorption, which will inhibit the water oxidation rate and induce the weakening of the OeH bond. In particular, the K+ cations not only lead to the rapid decrease of OeH bond strength, but are also most effective for retarding the back reaction, while the NH4+ cations shows the most detrimental effects on water oxidation. Our results provide important insight into the roles of cations in PEC water oxidation and indicate a new strategy for tailoring the electrode-electrolyte interface via the sagacious choice of cations in the BiVO4/KPi electrolyte interface for more effective PEC water splitting.
Moreover, the pH of the electrolyte is one of the most crucial parameters influencing the photoelectrochemical reactions, because the pH value can determine the amount of H+ or OH− ions, becoming a critical influencing factor of PEC cell working in the aqueous solution. Here, the photocurrent increases with increasing pH value. This is explained by the fact that the higher pH value can provide more hydroxide ions (OH−) to react with the photogenerated holes to form hydroxyl radicals. However, in the case of BiVO4 film, the sluggish kinetics of the photogenerated holes, the slow charge transfer at the photoelectrode/electrolyte interface, and the rapid electron-hole recombination in both bulk and surface substantially limit its application as the efficient photoanode material. Therefore, it is highly desired to enhance the separation of photogenerated charges in order to improve the photoelectrochemical efficiency of BiVO4 film. In the higher pH solution, the more hydroxide ions (OH−) can react with holes to form hydroxyl radicals (·OH). As has been reported, the ·OH is recognized to be an important intermediate for both PEC water oxidation and pollutant degradation in the presence of O2. Thus, the amount of ·OH produced during the water oxidation reaction directly affects the PEC activity. In addition, at the higher pH solution, the formation of ·OH from OH− anions can be further enhanced. Therefore, it is desirable to increase the amount of photogenerated holes on the photoelectrode surface so as to facilitate the production of hydroxyl radicals for efficient PEC process. As has been reported, the phosphate modification of photoelectrode could form the negative field on the photoelectrode surface so as to cause trapped holes, resulting in the effective separation of photogenerated charge, leading to the enhancement of PEC performance. In order to further investigate the influence of the two above mentioned cations and anions on the stability of the undoped BiVO4 and Mo-doped BiVO4 films for PEC water splitting, the photoelectrodes were tested in the electrolyte of various pH values, as shown in Fig. 7. The pH effect as well as the photostability was measured in 0.5 M KPi solution at pH 5, 9, and 11 containing different anions or cations species under AM 1.5 G continuous solar light illumination for 2000 s, using a constant potential of 1.23 VRHE. In the case of the undoped BiVO4 film, the achievable photocurrent density is somewhat low, but a significantly stable photocurrent was attained as compared to that of 46
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Fig. 7. I-t curves (a), (c), (e) of undoped BiVO4 films and (b), (d), (f) of 3 wt% Mo-doped BiVO4 electrodes at 1.23 VRHE in 0.5 M KPi at pH 5, 9, and 11 under continuous illumination for 2000 s. (a) and (b) performed in 0.5 M KPi solution at pH 5 containing CH3COO−, Cl−, and NO3− anions and (c), (d) and (e), (f) performed in 0.5 M KPi solution containing NH4+, Na+, and K+ cations at pH 9 and 11, respectively. (Here, AA = CH3COOH, HA = HCl, NA = HNO3, AH = NH4OH, SH = NaOH and PH = KOH).
the Mo-doped BiVO4 film, owing to the stability of Bi3+ in aqueous medium. In particular, at pH 5, considerably high photostability is attainable in that the anions can release or capture the protons so as to maintain the localized pH value at the BiVO4/electrolyte interface when the solar water splitting occurs. However, at high pH solution, the excess hydroxyl ions adsorb onto the positively charged Bi surface so as to form the soluble species and increase the chemical dissolution rate. On the other hand, the Mo-doped BiVO4 photoelectrodes show lower photostability due to its high surface acidity where the V5+ cations are replaced by the Mo6+ cations, thus increasing the surface acidity. The faradaic efficiency and quantity of evolved H2 and O2 gases are presented in Fig. 8. The faradaic efficiency is calculated according to ρ = nH2 / (Q / 2F) and ρ = nO2 / (Q / 4F), where nH2 and nO2 are the amount of generated hydrogen and oxygen, respectively; Q is the total amount of charge passed through the cell; and F is the faraday constant. In this system, the undoped BiVO4 and 3 wt% Mo-doped BiVO4 photoanodes are used as the working electrode under the applied voltages of 2 V vs saturated Ag/AgCl. The faradaic efficiency and quantity of the evolved H2 and O2 gases are measured for 90 min. The initial faradaic efficiency of the undoped BiVO4 film during the initial 30 min is about 57%, and it increases with time, reaching 68.53% after 90 min for H2 and 14.66% to 36.91% for O2 under continuous illumination. In the case of the Mo-doped BiVO4 film, the initial faradaic efficiency during the initial 30 min is 67.81%, and it increases with time up to 72.88% after 90 min for H2 and 18.07% to 46.20% for O2 under continuous illumination. It also shows a steadily increased H2 and O2 evolution rate up to 90 min. This means that the faradaic efficiency and quantity of the evolved H2 and O2 gases are both high in the case of doped BiVO4 as compared to undoped BiVO4, likely resulting from the higher photocurrent density and lower onset potential. However, the amounts of evolved H2 and O2 gases are relatively lower than the extent of photocurrent enhancement in the Mo-doped BiVO4 film, indicating the sincere necessity of cocatalysts at the interfacial reaction in this system. We also measure the faradaic efficiency and amount of evolved H2 and O2 gases varying the composition of electrolytes at pH 9. At pH 9, NH4+ containing KPi electrolyte shows the initial faradaic efficiency of the undoped BiVO4 film during the initial 30 min is about 81.74%, and it increases with time, reaching 85.30% after 90 min for
H2 and 18.28% to 26.65% for O2 under continuous illumination (Fig. S2). On the other hand, in the case of Mo-doped BiVO4 film, the initial faradaic efficiency during the initial 30 min is 88.44%, and it increases with time up to 93.92% after 90 min for H2 and 23.00% to 44.32% for O2 under continuous illumination. KPi solution containing Na+ at pH 9 shows that the faradaic efficiency of the undoped BiVO4 film during the initial 30 min is about 61.23%, and it increases with time, reaching 64.50% after 90 min for H2 and 07.14% to 29.33% for O2 under continuous illumination (Fig. S3) and in the case of Mo-doped BiVO4 film, the initial faradaic efficiency during the initial 30 min is 80.43%, and it increases with time up to 84.40% after 90 min for H2 and 12.93% to 30.26% for O2 under continuous illumination. In case of KPi solution containing K+ at pH 9, the faradaic efficiency of the undoped BiVO4 film during the initial 30 min is about 77.44%, and it increases with time, reaching 81.81% after 90 min for H2 and 09.80% to 20.01% for O2 under continuous illumination (Fig. S4) and in the case of Mo-doped BiVO4 film, the initial faradaic efficiency during the initial 30 min is 68.19%, and it increases with time up to 75.18% after 90 min for H2 and 27.68% to 44.90% for O2 under continuous illumination. It shows that the faradaic efficiency is increasing with the increasing of the pH of the electrolyte, disclosing the steadily increase of H2 and O2 evolution rate up to 90 min. The faradaic efficiency and quantity of the evolved H2 and O2 gases are both high in the case of Mo-doped BiVO4 as compared to undoped BiVO4 at pH 9 and it is related to the photocurrent density trend. 4. Conclusion The prepared n-type BiVO4 and 3 wt% Mo-doped BiVO4 electrodes were used as a photoanodes of a PEC cell. The types of anions (CH3COO−, Cl− and NO3−) and cations (NH4+, K+ and Na+) contained in the electrolyte, as well as the pH conditions of the electrolyte, were systematically varied in order to investigate how these conditions affect the PEC action (i.e., photocurrent to O2 conversion efficiency). It is found that the PEC activities of the BiVO4 photoelectrode in strong alkaline electrolytes show a trend of NH4+ > K+ > Na+ with the obvious cathodic shift of onset potential, and the photocurrents of the Mo-doped BiVO4 electrodes follow a trend of NH4+ > Na+ > K+ during the PEC performance. This is most likely because Na+ ions can 47
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Fig. 8. (a and b) Faradaic efficiencies and (c and d) quantities of the detected H2 and O2 gases in a PEC system from the undoped BiVO4 and Mo-doped BiVO4 photoanodes as the working electrode under applied voltages of 2 V vs Ag/AgCl in 0.5 M KPi solution at pH 7.
more strongly adsorb on the BiVO4 surface and serve as a more effective blocker for PEC reaction sites. No significance anion effect appears in either sample. The anion effects of undoped and Mo-doped BiVO4 films act like a dissimilar motion, exhibiting no remarkable difference in the undoped film, whereas the CH3COOH and HCl solution-mediated KPi electrolytes show slightly enhanced photocurrent density in the Modoped BiVO4 films. The main reason for this is the increased surface acidity of BiVO4 by the substitution of Mo cation; since Mo6+ cations have a larger electronegativity than V5+ cations. The higher adsorption affinity toward electrolytes, due to its high surface acidity on the surface, was an important reason for the enhanced activity of Mo-doped BiVO4 film against the pure BiVO4. Moreover, in the minor part, the sufficiently photogenerated holes in the photoelectrode/electrolyte interface region can react with anions, rather than the real photo-oxidation of water. The pH value of the solution is one of the most crucial parameters influencing the photoelectrochemical reactions. It shows a photocurrent increase with increased pH value. It appears that a higher pH value can provide more hydroxide ions (OH−) to react with holes to form hydroxyl radicals. As reported, the hydroxyl radicals (·OH) are recognized to be important intermediates for PEC water oxidation in the presence of O2. Thus, the amount of ·OH produced during the PEC reaction directly affects the PEC activity. This study will provide a good foundation for formulating optimum electrolyte compositions in order to enhance the efficiency of desired photo-oxidation reactions for various photoelectrochemical cells.
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Acknowledgements This research was supported by Basic Science Research Program through the National ResearchFoundation of Korea (NRF) (grant number: NRF-2017R1D1A3B03031602). Also, this work was supported by Priority Research Centers Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2018R1A6A1A03024334).
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