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Trimetallic oxyhydroxide modified 3D coral-like BiVO4 photoanode for efficient solar water splitting
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Trimetallic oxyhydroxide modified 3D coral-like BiVO4 photoanode for efficient solar water splitting ⁎
Wenzhang Lia,b, Libo Dua, Qiong Liua, Yang Liua, , Dongwei Lic, Jie Lia,
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a
School of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China Hunan Provincial Key Laboratory of Efficient and Clean Utilization of Manganese Resources, Central South University, Changsha 410083, China c Division of Scientific Research Management, Chongqing University of Education, Chongqing 400065, China b
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
oxyhydroxide decorated on • WCoFe coral-like BiVO photoanode. interface between WCoFe oxy• The hydroxide and BiVO accelerates the 4
4
• •
charge transfer and avoid charge recombination. The WCoFe/BiVO4 composite photoanode exhibits better PEC performance than bare BiVO4. The onset potential of BiVO4 photoanode is obviously negative shift after modification by WCoFe oxyhydroxide.
A R T I C LE I N FO
A B S T R A C T
Keywords: WCoFe oxyhydroxide BiVO4 Photoelectrochemical water splitting Photoanode Electrocatalyst
Water splitting driven by sunlight is considered to be a promising strategy for solving energy crisis and environment pollution. But the sluggish kinetics of oxygen evolution reaction (OER) is quite limited for improving the efficiency of solar conversion. In this study, a core-shell nanostructure film of WCoFe oxyhydroxide coated BiVO4 (WCoFe/BiVO4) was constructed by solvothermal method, which exhibits better photoelectrochemical (PEC) properties compared with the bare BiVO4. A high quality-interface is formed between WCoFe oxyhydroxide and BiVO4 junctions, which further facilitates the separation and transport of photogenerated carriers. Notably, WCoFe/BiVO4 photoanode shows a low onset potential of 0.28 V vs. reversible hydrogen electrode (RHE) and a significant enhancement of photocurrent density, reaching 4.35 mA cm−2 at 1.23 VRHE, which is mainly attributed to the improved OER kinetics. In addition, the charge injection efficiency of WCoFe/BiVO4 photoanode also increases to 84.75%, more than twice as much as that of bare BiVO4 photoanode. Photoelectrochemical measurements and optical studies have demonstrated that the high PEC performance can be contributed to the reduced OER barriers and charge recombination, as well as the excellent optical transparency of WCoFe oxyhydroxide.
1. Introduction Photoelectrochemical (PEC) water splitting is considered to be a highly promising strategy to produce clean energy for solving energy
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crisis and environmental issues [1,2]. Compared to the hydrogen evolution reaction (HER), the oxygen evolution reaction (OER) process is a major obstacle for overall water splitting due to the requirement of transferring four electrons in a single step, which has attracted
Corresponding authors. E-mail addresses: [email protected] (Y. Liu), [email protected] (J. Li).
https://doi.org/10.1016/j.cej.2019.123323 Received 10 August 2019; Received in revised form 10 October 2019; Accepted 29 October 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Wenzhang Li, et al., Chemical Engineering Journal, https://doi.org/10.1016/j.cej.2019.123323
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WCoFe oxyhydroxide and photoanode by an optimized loading method. In this work, we successfully coated the WCoFe oxyhydroxide on the surface of coral-like BiVO4 photoanode by a solvothermal method. The WCoFe oxyhydroxide was uniformly distributed with a thickness of ~5 nm. The well constructed composite structure with a high quality interface facilitates charge separation and transfer. Moreover, the WCoFe oxyhydroxide exhibits great OER properties with reduced OER kinetics barriers, improving the hole injection efficiency for water oxidation. As a result, WCoFe oxyhydroxide modified BiVO4 photoanode exhibits a high photocurrent density of 4.35 mA cm−2 at 1.23 VRHE and a low onset potential of 0.28 VRHE.
intensively attention to improve the PEC performance [3–6]. In particular, BiVO4 has emerged as one of the most promising PEC water splitting candidates which can harvest about 11% of the solar spectrum with the moderate bandgap (2.4 eV), as well as chemical stability and high abundance [7–10]. However, the performance of bare BiVO4 is severe limited by charge recombination and sluggish OER kinetics, which is still an urgent problem for further applications. To date, tremendous research efforts have been made to improve the PEC water splitting performance of BiVO4 by element doping [11–16], morphology engineering [17,18] and the deposition of oxygen evolution catalysts (OECs) [19]. In general, element doping is considered to be a useful strategy to accelerate the transfer of photogenerated carriers and broaden absorption range by modulating the band structure. As for BiVO4, W, Mo and In doping have been studied and used to improve the PEC water splitting performance [11,14,15]. For example, the In3+ doped BiVO4 exhibits a more positive flat band position and higher charge separation efficiency because partial sites of Bi3+ was substituted by In3+ [14]. As for Mo6+ doped BiVO4, Mo6+ replaces the partial sites of V5+, which leads to deformation of the crystal structure, and thus improving the electron mobility. Besides, it also results in a larger photovoltage and a better PEC performance [15]. Moreover, controlling the grow orientation along [0 0 1] can also improve the PEC performance because the exposed (0 0 1) facet has excellent intrinsic charge transport and OER properties [18]. Although many efforts have been made to improve the PEC properties of BiVO4, the PEC water splitting performance is still poor due to the sluggish OER kinetics, which is also responsible for the low onset potential caused by charge surface recombination. Modifying the surface of BiVO4 with an efficient OEC (such as metallic/multimetallic oxide/oxyhydroxide [20–23], LDHs [24,25] and cobalt phosphate (CoPi) [26,27]) has demonstrated to be one of the most convenient ways to reduce the dynamic barrier of OER and achieve high photocurrent density. Specially, a serious of metal oxyhydroxides have been confirmed as efficient OECs for BiVO4 like FeOOH [28–30], CoOOH [31] and NiOOH [32]. Additionally, the previous works demonstrated that multimetal oxide/oxyhydroxide were more efficient OECs [21–23]. Recent years, FeCoW oxide/oxyhydroxide has attracted numerous interests in OER field. Edward H. Sargent and co-workers [22] synthesized gelled FeCoW oxyhydroxide with a room-temperature recipe. It has a low overpotential for OER in alkaline electrolyte for OER, which is due to the modulated electronic structure and synergistic interplay among W, Co and Fe. In addition, the gelled FeCoW oxyhydroxide also show much higher catalytic activity than that of annealed control, which mainly due to the phase separation after annealing. Huang and co-workers [23] developed a wet-chemical method to directly grow FeCoW oxyhydroxide on substrates for OER, which exhibited an overpotential of 310 mV at 100 mA cm−2 with superior stability. There are also several piece of works that reported FeCoW oxide/oxyhydroxide as OEC for modifying photoanode. For example, Liang et al [21] prepared W doped BiVO4 coated with FeCoW oxide by the sol gel method with post-annealing. The FeCoW oxide existed as small particles on the surface of W:BiVO4, and the introduction of FeCoW oxide led a higher light absorption intensity and a wider photoresponse region, as well as higher photocurrent and lower flat-band potential. Xiao et al [33] examined FeCoW oxyhydroxide gel coated Fe2O3 photoanode for PEC water splitting. Different from the oxide particle, the FeCoW oxyhydroxide gel uniformly coated on the surface of Fe2O3 photoanode by spincoating method acted as a hole storage layer, which facilitated the photogenerated hole to be extracted from Fe2O3 for water oxidation. Nevertheless, the interface between the OEC and photoanode is also an important factor to accelerate the charge transport and avoid the charge recombination. Compared to dense OEC, amorphous and disordered OEC can avoid Fermi-level pinning. It also leads to an improved PEC performance when incorporated into existing photoanode systems due to the better permeability for electrolyte [34]. Thus, it requires additional consideration to construct a high quality interface between
2. Experimental section 2.1. Preparation of BiVO4 and WCoFe oxyhydroxide modified BiVO4 (WCoFe/BiVO4) photoanodes The BiVO4 photoanodes was synthesized according to the previous study [32] (Supporting Information). The WCoFe/BiVO4 composite photoanode was obtained by solvothermal method. 0.5 ml of n-Butylamine was dropwise added into 45 ml of ethanol solution containing 0.03956 g tungsten hexachloride, 0.0190 g cobalt chloride and 0.0054 g iron(III) chloride hexahydrate under stirring, and then the solution was stirred for additional 15 min. The as-prepared BiVO4 photoanode was immersed in the above solution at 160 °C for 9 h, 12 h, 15 h and 18 h, respectively. They were donated as WCoFe/BiVO4-x (x = 9 h, 12 h, 15 h and 18 h, respectively). The composite photoanodes were washed by ethanol and deionized water for several times and dried in vacuum oven at 60 °C. The synthetic route was illustrated in Scheme 1. For comparison, the samples with different ratios of W, Co and Fe were also prepared through the above method by changing the amount of tungsten hexachloride, cobalt chloride and iron(III) chloride hexahydrate.
2.2. Material characterization The XRD patterns were measured on Rigaku X-ray diffractometer D/ max2250 equipped with Cu Kα (λ = 0.15406 nm) radiation. The morphologies and structures of all photoanodes were measured by Nova Nano SEM 230 scanning electron microscopy (SEM) with an energy-dispersive X-ray spectrometer (EDS) and JEM-2100F high-resolution transmission electron microscopy (HRTEM). The surface elements as well as the valence states were determined by K-Alpha 1063 X-ray photoelectron spectroscopy (XPS). And the optical properties were measured by TU-1901 double-beam UV–vis spectrophotometer.
Scheme 1. Schematic representation of procedures used to prepare WCoFe/ BiVO4 photoanode. 2
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2.3. Photoelectrochemical (PEC) measurements PEC performances of all photoanodes were conducted on Zennium electrochemical workstation with a typical three-electrode configuration. The as-prepared photoanode was used as working electrode, platinum foil was used as counter electrode and Ag/AgCl electrode (saturated KCl) was used as reference electrode. The illumination source was 500 W xenon lamp equipped with an AM 1.5 G filter to gain 1 sun illumination (100 mW cm−2). The electrolyte was 0.2 M potassium phosphate buffer solution (pH = 7) for water oxidation. As for sodium sulfite oxidation, the electrolyte was the above solution with an extra 0.2 M Na2SO3. In the experiment process, the photoanode was irradiated from the back side. Photocurrent-potential (J-V) curves were obtained by linear sweeping voltammetry (LSV) with or without illumination at a scan rate of 20 mV s−1. The AC frequency of electrochemical impedance spectroscopy (EIS) was ranging from 10 kHz to 0.1 Hz and performed at 1.23 VRHE. For each photoanode, the MottSchottky measurements were obtained at different AC frequencies, as 1 kHz, 2 kHz and 5 kHz, respectively. Incident photon to current conversion efficiency (IPCE) was conducted with a bias of 1.23 V using a xenon lamp (150 W, Oriel) equipped with a monochromator. The potentials reported in this research were adjusted to versus RHE by the Nernst equation: Fig. 2. (a–c) Transmission electron microscopy (TEM) and HRTEM images of WCoFe/BiVO4-18 h photoanode; (d-i) EDS mapping of WCoFe/BiVO4-18 h photoanode.
VRHE = VAg/AgCl + 0.059 × pH + 0.1976 The applied bias photo to current conversion efficiency (ABPE) was obtained by the equation:
explored by the XRD (Fig. 1b). The diffraction peaks are observed at 18.80°, 28.98°, 30.97°, 34.55°, 39.76°, 41.99° and 45.55°, corresponding to (1 0 1), (1 1 2), (0 0 4), (2 0 0), (1 1 4), (2 0 3) and (2 1 3) planes of monoclinic BiVO4 (JCPDS 14-0688) [30]. After coating WCoFe oxyhydroxide, three new peaks (heart-shaped in Fig. 1b) are found. The peaks at 41.94°, 53.02° and 62.50° can be assign to WCoFe oxyhydroxide, which is consistent with the crystal phases of bare WCoFe oxyhydroxide (Fig. S4). To clarify the detailed structure of WCoFe/BiVO4, TEM and HRTEM were employed. As shown in Fig. 2a and Fig. S3, a gauze-like material composed of crumpled and entangled nanosheets, which is similar as the reported WCoFe oxyhydroxide in the previous study [23], is tight supported on BiVO4 coral. HRTEM image (Fig. 2b) shows a distinct outer layer with a thickness about 5 nm. The inner material exhibits well-defined lattice fringes with a spacing of 0.226 nm, corresponding to (1 1 4) plane of monoclinic BiVO4 (JCPDS 14-0688). Meanwhile, the lattice interplanar distance of outer layer is 0.32 nm, which is well ascribed to (1 1 1) crystallographic plane of WCoFe oxyhydroxide according to previous study [23]. Energy-dispersive X-ray spectroscopy (EDS) elemental mappings (Fig. 2d–e) reveal that the W, Co and Fe elements are well distributed throughout the whole composite, which
J × (1.23 − VRHE) ABPE = × 100% P where J is the photocurrent density (mA cm−2) at the given potential (VRHE), and P is the incident light intensity (mW cm−2). 3. Results and discussion The morphologies of BiVO4 and WCoFe/BiVO4 photoanodes were examined by SEM. Fig. S1 reveals that the BiVO4 photoanode has a 3D coral-like structure with an average dendritic diameter of 200 nm [35], which is similar to that of WCoFe/BiVO4 photoanode in Fig. 1a. Compared to the bare BiVO4 photoanode (Fig. S1), the WCoFe/BiVO4 photoanode (Fig. 1a) obviously exhibits WCoFe oxyhydroxide coated on the surface of BiVO4 after solvothermal processing. The insert section, shown in Fig. 1a, demonstrates that the thickness of WCoFe/ BiVO4-18 h layer is approximately 1.2 μm, and the coated WCoFe oxyhydroxide does not change the typical geometry of BiVO4. As shown in Fig. S2, the reaction time of solvothermal process has neglected effect on the thickness of films. The crystal structure of the WCoFe/BiVO4 photoanode was further
Fig. 1. (a) Scanning electron microscopy (SEM) image of WCoFe/BiVO4-18 h photoanode, and inset shows the cross-sectional SEM image; (b) XRD patterns of all samples. 3
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Fig. 3. The XPS spectra of WCoFe/BiVO4-18 h photoanode: (a) XPS of survey spectrum; (b) XPS W 4f; (c) XPS Co 2p; (d) XPS Fe 2p; (e) XPS Bi 4f; (f) XPS V 2p.
means the well dispersed FeCoW oxyhydroxide. To further understand the surface elements and the chemical binding state, X-ray photoelectron spectroscopy (XPS) spectra of WCoFe/BiVO4 photoanode were performed and the results were shown in Figs. 3 and S5. The low-resolution XPS spectra (Fig. 3a) clearly shows W, Co, and Fe elements, indicating that the WCoFe oxyhydroxide is successfully obtained. The couple peaks of W4f located at 35.6 eV and 37.7 eV are corresponding to the W6+ oxidation states in Fig. 3b [36]. As shown in Fig. 3c, the main peaks in the Co 2p spectrum are Co 2p 3/ 2 (781.2 eV) and Co 2p 3/2 (797.3 eV) with two satellite peaks (786.3 eV and 803.0 eV), indicating the Co can be assigned as Co2+ [24]. According to Fig. 3d, the Fe 2p spectrum consists of two peaks at 711.8 and 725.4 eV, corresponding to 2p 3/2 and 2p 1/2 states, respectively, which indicates the Fe in Fe3+ oxidation state. In addition, there is a weak peak at 719.2 eV which is the satellite peak of Fe 2p 3/2 [37]. The peaks of Bi 4f are located at 159.3 and 164.7 eV, corresponding to Bi3+ in BiVO4 (Fig. 3e). The peak at 516.9 eV in Fig. 3f is indexed to V5+ of BiVO4 [38]. Fig. S3a shows two peaks of O 1 s, the former located at 530.6 eV derives from lattice oxygen, and the latter at 531.7 eV is corresponding to hydroxy species [39]. Finally, the peaks of C 1 s spectrum can be indexed to organic matter during reaction or carbon granule from instruments. The complete PEC process is affected by three factors as light harvest followed by generating electron-hole pairs, separation and transport of electron-hole pairs, and the surface reaction to produce oxygen and hydrogen. In order to investigate the optical absorption properties of BiVO4 and WCoFe/BiVO4 photoanodes, UV–vis absorption spectra were employed as shown in Fig. 4. The absorption edge of both BiVO4 and WCoFe/BiVO4 photoanodes are similar with each other, approximately 510 nm, which corresponding to band gap of ~2.4 eV. It also means that the WCoFe oxyhydroxide cannot provide additional photogenerated carriers. Besides, the UV–vis absorption characteristics of bare and the composite photoanodes have a very slight difference, indicating the WCoFe oxyhydroxide layer has no additional photoresponse. In order to understand the effect of WCoFe oxyhydroxide for reaction dynamics, the PEC behaviors of bare BiVO4 and WCoFe/BiVO4-x photoanodes for water oxidation and sulfite oxidation were conducted in corresponding electrolyte solution under AM 1.5 G illumination (100 mW cm−2) (Fig. 5a and b). As shown in Fig. 5a, the photocurrent density (solid) of WCoFe/BiVO4-18 h photoanode for water oxidation
Fig. 4. UV–vis absorption spectra of bare BiVO4 and WCoFe/BiVO4-x photoanodes.
has a significant increment compared to bare BiVO4 photoanode in the whole potential window. Specifically, the photocurrent density of bare BiVO4 photoanode is 1.60 mA cm−2 at 1.23 VRHE, but that of WCoFe/ BiVO4-18 h photoanode reaches 4.35 mA cm−2, more than 2 times as large as bare BiVO4 photoanode. The value is also compared to the BiVO4 based photoanodes reported in the literature (Table S2). Furthermore, the onset potential of WCoFe/BiVO4-18 h photoanode for water oxidation has a clearly negative shift relative to bare BiVO4 photoanode, reaching to 0.28 VRHE. The reduced OER barriers after coating by WCoFe oxyhydroxide may account for the negative shift of onset potential. The photocurrents of samples synthesized with different ratios of W, Co and Fe in precursor were also measured as shown in Fig. S8, and optimal molar ratio of W:Fe:Co is 5:1:4 in this experiment. In addition, the Mott-Schottky analysis (Fig. S6) indicates the flat band potential of WCoFe/BiVO4 with a cathode shift of approximate 0.3 V. The slopes between BiVO4 and WCoFe/BiVO4 photoanodes in same frequency have not changed, meaning the carrier concentrations are similar with each other and demonstrating none of W, Co and Fe was doped into BiVO4 [40]. These results demonstrate that the WCoFe oxyhydroxide, as oxygen evolution catalyst, significantly improves the OER kinetics by utilizing the photogenerated holes for water oxidation. The incident-photon-to-current conversion efficiencies (IPCE) of bare BiVO4 and WCoFe/BiVO4 photoanodes were shown in Fig. 5c. 4
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Fig. 5. Current-potential (J-V) curves of bare BiVO4 and WCoFe/BiVO4-x photoanodes conducted in (a) 0.2 M phosphate buffer solution (pH = 7) and (b) 0.2 M phosphate buffer solution contains 0.2 M Na2SO3 (pH = 7); (c) Incident photon to current conversion efficiency (IPCE) for BiVO4 based photoanodes; (d) Applied bias photon to current efficiency (ABPE) for bare BiVO4 and WCoFe/BiVO4-18 h photoelectrodes.
Fig. 6. The electrochemical impedance spectra (EIS) of (a) BiVO4 and WCoFe/BiVO4-x photoelectrodes; (b) The bode phase plots of the photoelectrodes; (c) Hole injection efficiency of BiVO4 based photoanodes; (d) Charge separation efficiency of BiVO4 based photoanodes.
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Fig. 7. Actual (solid line) and theoretical calculated (dashed line) evolution amount of H2 and O2 measured at 1.23 VRHE for 120 min; (b) The current-time curve for the WCoFe/BiVO4-18 h; (c) The possible mechanism of WCoFe/BiVO4 photoanode for PEC water splitting.
proposed that the WCoFe oxyhydroxide might facilitate the hole consume for water oxidation, resulting in the smaller Rct of WCoFe/BiVO4 photoanode. The calculated photogenerated electron lifetime (τd) from electrochemical impedance spectra (EIS) test was shown in Table S1. The WCoFe/BiVO4 has longer photogenerated electron lifetime (4.91 ms) than that of bare BiVO4 (2.35 ms), which can also account for the higher charge separation efficiency. To confirm that the OER at the WCoFe/BiVO4 photoanode as well as the hydrogen evolution reaction (HER) at the Pt electrode, the evolved gases were detected every 30 min by gas chromatography (GC). And the amounts of the evolved H2 and O2 gases were 140.43 and 69.85 μmol after 2 h (Fig. 7a), respectively. And the ratio of evolved H2 and O2 gases is nearly 2 with the faraday efficient of more than 90%, which means the photogenerated holes were utilized for water oxidation to produce O2 in the system indeed. Based on the results mentioned above, a possible mechanism about the solar water oxidation is proposed in Fig. 7c. When the simulated sunlight irradiated the WCoFe/BiVO4 photoanode, the photogenerated electron-hole pairs were produced inside the coral-like BiVO4. Then the holes transported to the interface between BiVO4 and WCoFe oxyhydroxide and were extracted into the WCoFe oxyhydroxide for water oxidation. Meanwhile, the electrons were transferred to the Pt electrode to produce H2. Besides, the stability of WCoFe/BiVO4 photoanode was also considered as an important factor for PEC water splitting. In this work, the as-prepared photoanode showed a high stability (Fig. S10), which can be ascribed to the highquality interface between WCoFe oxyhydroxide and BiVO4. On the one hand, it can protect the BiVO4 from electrolyte and prohibit the corrosion. On the other hand, it can reduce the density of surface states, thus accelerating the migration of hole from BiVO4 to WCoFe oxyhydroxide [42].
After applying a specific potential, the IPCE values of WCoFe/BiVO4 photoanodes are higher than that of bare BiVO4 photoanode at same wavelength. In particular, WCoFe/BiVO4-18 h shows the IPCE value of 37.7% at the wavelength of 450 nm, while the BiVO4 photoanode shows 13.34% at the same wavelength. In addition, the applied bias photon to current efficiency (ABPE) of WCoFe/BiVO4-18 h and bare BiVO4 photoanodes were shown in Fig. 5d, and the maximum ABPE of WCoFe/ BiVO4-18 h photoanode is 1.38% at 0.71 VRHE. Meanwhile, the bare BiVO4 shows the maximum ABPE value of 0.35% at 0.84 VRHE, which is much lower than that of WCoFe/BiVO4-18 h photoanode. To clearly understand the reasons of the improved PEC performance, the hole injection efficiency and the charge separation efficiency were calculated in Fig. 6c and 6d. For sulfite oxidation, the photocurrent density of bare BiVO4 and WCoFe/BiVO4-18 h are 3.38 mA cm−2 and 5.13 mA cm−2 at 1.23 VRHE, respectively. Because the oxidation of sulfite hole scavenger is not limited by the reaction kinetics, the enhancement in Fig. 5b can be attributed to the increased charge separation efficiency, from 21.1% to 57.2% (Fig. 6d). It means that the WCoFe oxyhydroxide layer accelerates the hole extraction from BiVO4, thus reducing the charge recombination. Besides, as shown in Fig. 6c, the hole injection efficiency of WCoFe/BiVO4 (84.75%) is much higher than that of bare BiVO4 photoanode (~40%). It demonstrates that bare BiVO4 is indeed limited by the sluggish OER kinetics, and the WCoFe oxyhydroxide can effectively accelerate the hole injection into the electrolyte by reducing the dynamic barriers of OER [41]. EIS test was further measured to investigate the interfacial property of BiVO4 and WCoFe/BiVO4-x photoanodes under AM 1.5 G illumination. As shown in Fig. 6a, the EIS spectra reveal that WCoFe/BiVO4 has a smaller Rct than that of bare BiVO4, indicating the improved OER dynamics between the surface of electrode and electrolyte. It has been 6
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4. Conclusions
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In summary, an efficient OER catalyst (WCoFe oxyhydroxide) was integrated to coral-like BiVO4 photoanode for PEC water splitting. The WCoFe/BiVO4 photoanode exhibits high PEC properties, such as photocurrent density of 4.35 mA cm−2 at 1.23 VRHE and low onset potential of 0.28 VRHE. The photocurrent and EIS measurements demonstrate that the introduction of WCoFe oxyhydroxide not only reduces the dynamic barrier of OER, but also accelerates the charge separation and transport, thus leading higher charge separation and hole injection efficiency compared to the bare BiVO4 photoanode. The optical property and Mott-Schottky measurement reveal that the WCoFe oxyhydroxide layer can collect and store the holes from BiVO4 to reduce the charge recombination. All of these results indicate that solvothermal method is a promising strategy to load WCoFe oxyhydroxide as efficient electrocatalyst on the surface of photoelectrode. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This study was supported by the National Nature Science Foundation of China (No. 21471054, 51904356), the Hunan Provincial Science and Technology Plan Project, China (No. 2016TP1007), the Hunan Provincial Natural Science Foundation of China (Grant No. 2017JJ2326), the Natural Science Foundation of Chongqing, China (No. cstc2018jcyjAX0733). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2019.123323. References [1] C. Jiang, S.J.A. Moniz, A. Wang, T. Zhang, J. Tang, Photoelectrochemical devices for solar water splitting-materials and challenges, Chem. Soc. Rev. 46 (2017) 4645–4660. [2] C. Ding, J. Shi, Z. Wang, C. Li, Photoelectrocatalytic water splitting: significance of cocatalysts, electrolyte, and interfaces, ACS Catal. 7 (2016) 675–688. [3] J. Suntivich, K.J. May, H.A. Gasteiger, A perovskite oxide optimized for oxygen evolution catalysis from molecular orbital principles, Science 334 (2011) 1383–1385. [4] B. Liu, H.Q. Peng, C.N. Ho, H. Xue, S. Wu, T.W. Ng, C.S. Lee, W. Zhang, Mesoporous nanosheet networked hybrids of cobalt oxide and cobalt phosphate for efficient electrochemical and photoelectrochemical oxygen evolution, Small 13 (2017) 1701875. [5] H. Jiang, J. Gu, X. Zheng, M. Liu, X. Qiu, L. Wang, W. Li, Z. Chen, X. Ji, J. Li, Defect-rich and ultrathin N doped carbon nanosheets as advanced trifunctional metal-free electrocatalysts for the ORR, OER and HER, Energy Environ. Sci. 12 (2019) 322–333. [6] Z. Ma, H. Hou, K. Song, Z. Fang, L. Wang, F. Gao, Z. Yang, B. Tang, W. Yang, Ternary WO3/porous-BiVO4/FeOOH hierarchical architectures: towards highly efficient photoelectrochemical performance, ChemElectroChem 5 (2018) 3660–3667. [7] Z. Yang, W. Luo, Z. Li, Solar hydrogen generation from seawater with a modified BiVO4 photoanode, Energy Environ. Sci. 4 (2011) 4046–4051. [8] S. Wang, P. Chen, Y. Bai, J.H. Yun, G. Liu, L. Wang, New BiVO4 dual photoanodes with enriched oxygen vacancies for efficient solar-driven water splitting, Adv. Mater. 30 (2018) 1800486. [9] Y. Liu, B.R. Wygant, K. Kawashima, O. Mabayoje, T.E. Hong, S.-G. Lee, J. Lin, J.-H. Kim, K. Yubuta, W. Li, Facet effect on the photoelectrochemical performance of a WO3/BiVO4 heterojunction photoanode, Appl. Catal.B:Environ. 245 (2019) 227–239. [10] Z. Ma, K. Song, L. Wang, F. Gao, B. Tang, H. Hou, W. Yang, WO3/BiVO4 type-II heterojunction arrays decorated with oxygen-deficient ZnO passivation layer: a highly efficient and stable photoanode, ACS Appl. Mater. Interfaces 11 (2019) 889–897. [11] K.P.S. Parmar, H.J. Kang, A. Bist, P. Dua, J.S. Jang, J.S. Lee, Photocatalytic and photoelectrochemical water oxidation over metal-doped monoclinic BiVO4 photoanodes, ChemSusChem 5 (2012) 1926–1934. [12] W.J. Jo, J.W. Jang, K.J. Kong, H.J. Kang, J.Y. Kim, H. Jun, K. Parmar, J.S. Lee, Phosphate doping into monoclinic BiVO4 for enhanced photoelectrochemical water oxidation activity, Angew. Chem. Int. Ed. 124 (2012) 3201–3205. [13] A.J. Rettie, W.D. Chemelewski, J. Lindemuth, J.S. McCloy, L.G. Marshall, J. Zhou, D. Emin, C.B. Mullins, Anisotropic small-polaron hopping in W: BiVO4 single crystals,
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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Trimetallic oxyhydroxide modified 3D coral-like BiVO4 photoanode for efficient solar water splitting ⁎
Wenzhang Lia,b, Libo Dua, Qiong Liua, Yang Liua, , Dongwei Lic, Jie Lia,
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a
School of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China Hunan Provincial Key Laboratory of Efficient and Clean Utilization of Manganese Resources, Central South University, Changsha 410083, China c Division of Scientific Research Management, Chongqing University of Education, Chongqing 400065, China b
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
oxyhydroxide decorated on • WCoFe coral-like BiVO photoanode. interface between WCoFe oxy• The hydroxide and BiVO accelerates the 4
4
• •
charge transfer and avoid charge recombination. The WCoFe/BiVO4 composite photoanode exhibits better PEC performance than bare BiVO4. The onset potential of BiVO4 photoanode is obviously negative shift after modification by WCoFe oxyhydroxide.
A R T I C LE I N FO
A B S T R A C T
Keywords: WCoFe oxyhydroxide BiVO4 Photoelectrochemical water splitting Photoanode Electrocatalyst
Water splitting driven by sunlight is considered to be a promising strategy for solving energy crisis and environment pollution. But the sluggish kinetics of oxygen evolution reaction (OER) is quite limited for improving the efficiency of solar conversion. In this study, a core-shell nanostructure film of WCoFe oxyhydroxide coated BiVO4 (WCoFe/BiVO4) was constructed by solvothermal method, which exhibits better photoelectrochemical (PEC) properties compared with the bare BiVO4. A high quality-interface is formed between WCoFe oxyhydroxide and BiVO4 junctions, which further facilitates the separation and transport of photogenerated carriers. Notably, WCoFe/BiVO4 photoanode shows a low onset potential of 0.28 V vs. reversible hydrogen electrode (RHE) and a significant enhancement of photocurrent density, reaching 4.35 mA cm−2 at 1.23 VRHE, which is mainly attributed to the improved OER kinetics. In addition, the charge injection efficiency of WCoFe/BiVO4 photoanode also increases to 84.75%, more than twice as much as that of bare BiVO4 photoanode. Photoelectrochemical measurements and optical studies have demonstrated that the high PEC performance can be contributed to the reduced OER barriers and charge recombination, as well as the excellent optical transparency of WCoFe oxyhydroxide.
1. Introduction Photoelectrochemical (PEC) water splitting is considered to be a highly promising strategy to produce clean energy for solving energy
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crisis and environmental issues [1,2]. Compared to the hydrogen evolution reaction (HER), the oxygen evolution reaction (OER) process is a major obstacle for overall water splitting due to the requirement of transferring four electrons in a single step, which has attracted
Corresponding authors. E-mail addresses: [email protected] (Y. Liu), [email protected] (J. Li).
https://doi.org/10.1016/j.cej.2019.123323 Received 10 August 2019; Received in revised form 10 October 2019; Accepted 29 October 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Wenzhang Li, et al., Chemical Engineering Journal, https://doi.org/10.1016/j.cej.2019.123323
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WCoFe oxyhydroxide and photoanode by an optimized loading method. In this work, we successfully coated the WCoFe oxyhydroxide on the surface of coral-like BiVO4 photoanode by a solvothermal method. The WCoFe oxyhydroxide was uniformly distributed with a thickness of ~5 nm. The well constructed composite structure with a high quality interface facilitates charge separation and transfer. Moreover, the WCoFe oxyhydroxide exhibits great OER properties with reduced OER kinetics barriers, improving the hole injection efficiency for water oxidation. As a result, WCoFe oxyhydroxide modified BiVO4 photoanode exhibits a high photocurrent density of 4.35 mA cm−2 at 1.23 VRHE and a low onset potential of 0.28 VRHE.
intensively attention to improve the PEC performance [3–6]. In particular, BiVO4 has emerged as one of the most promising PEC water splitting candidates which can harvest about 11% of the solar spectrum with the moderate bandgap (2.4 eV), as well as chemical stability and high abundance [7–10]. However, the performance of bare BiVO4 is severe limited by charge recombination and sluggish OER kinetics, which is still an urgent problem for further applications. To date, tremendous research efforts have been made to improve the PEC water splitting performance of BiVO4 by element doping [11–16], morphology engineering [17,18] and the deposition of oxygen evolution catalysts (OECs) [19]. In general, element doping is considered to be a useful strategy to accelerate the transfer of photogenerated carriers and broaden absorption range by modulating the band structure. As for BiVO4, W, Mo and In doping have been studied and used to improve the PEC water splitting performance [11,14,15]. For example, the In3+ doped BiVO4 exhibits a more positive flat band position and higher charge separation efficiency because partial sites of Bi3+ was substituted by In3+ [14]. As for Mo6+ doped BiVO4, Mo6+ replaces the partial sites of V5+, which leads to deformation of the crystal structure, and thus improving the electron mobility. Besides, it also results in a larger photovoltage and a better PEC performance [15]. Moreover, controlling the grow orientation along [0 0 1] can also improve the PEC performance because the exposed (0 0 1) facet has excellent intrinsic charge transport and OER properties [18]. Although many efforts have been made to improve the PEC properties of BiVO4, the PEC water splitting performance is still poor due to the sluggish OER kinetics, which is also responsible for the low onset potential caused by charge surface recombination. Modifying the surface of BiVO4 with an efficient OEC (such as metallic/multimetallic oxide/oxyhydroxide [20–23], LDHs [24,25] and cobalt phosphate (CoPi) [26,27]) has demonstrated to be one of the most convenient ways to reduce the dynamic barrier of OER and achieve high photocurrent density. Specially, a serious of metal oxyhydroxides have been confirmed as efficient OECs for BiVO4 like FeOOH [28–30], CoOOH [31] and NiOOH [32]. Additionally, the previous works demonstrated that multimetal oxide/oxyhydroxide were more efficient OECs [21–23]. Recent years, FeCoW oxide/oxyhydroxide has attracted numerous interests in OER field. Edward H. Sargent and co-workers [22] synthesized gelled FeCoW oxyhydroxide with a room-temperature recipe. It has a low overpotential for OER in alkaline electrolyte for OER, which is due to the modulated electronic structure and synergistic interplay among W, Co and Fe. In addition, the gelled FeCoW oxyhydroxide also show much higher catalytic activity than that of annealed control, which mainly due to the phase separation after annealing. Huang and co-workers [23] developed a wet-chemical method to directly grow FeCoW oxyhydroxide on substrates for OER, which exhibited an overpotential of 310 mV at 100 mA cm−2 with superior stability. There are also several piece of works that reported FeCoW oxide/oxyhydroxide as OEC for modifying photoanode. For example, Liang et al [21] prepared W doped BiVO4 coated with FeCoW oxide by the sol gel method with post-annealing. The FeCoW oxide existed as small particles on the surface of W:BiVO4, and the introduction of FeCoW oxide led a higher light absorption intensity and a wider photoresponse region, as well as higher photocurrent and lower flat-band potential. Xiao et al [33] examined FeCoW oxyhydroxide gel coated Fe2O3 photoanode for PEC water splitting. Different from the oxide particle, the FeCoW oxyhydroxide gel uniformly coated on the surface of Fe2O3 photoanode by spincoating method acted as a hole storage layer, which facilitated the photogenerated hole to be extracted from Fe2O3 for water oxidation. Nevertheless, the interface between the OEC and photoanode is also an important factor to accelerate the charge transport and avoid the charge recombination. Compared to dense OEC, amorphous and disordered OEC can avoid Fermi-level pinning. It also leads to an improved PEC performance when incorporated into existing photoanode systems due to the better permeability for electrolyte [34]. Thus, it requires additional consideration to construct a high quality interface between
2. Experimental section 2.1. Preparation of BiVO4 and WCoFe oxyhydroxide modified BiVO4 (WCoFe/BiVO4) photoanodes The BiVO4 photoanodes was synthesized according to the previous study [32] (Supporting Information). The WCoFe/BiVO4 composite photoanode was obtained by solvothermal method. 0.5 ml of n-Butylamine was dropwise added into 45 ml of ethanol solution containing 0.03956 g tungsten hexachloride, 0.0190 g cobalt chloride and 0.0054 g iron(III) chloride hexahydrate under stirring, and then the solution was stirred for additional 15 min. The as-prepared BiVO4 photoanode was immersed in the above solution at 160 °C for 9 h, 12 h, 15 h and 18 h, respectively. They were donated as WCoFe/BiVO4-x (x = 9 h, 12 h, 15 h and 18 h, respectively). The composite photoanodes were washed by ethanol and deionized water for several times and dried in vacuum oven at 60 °C. The synthetic route was illustrated in Scheme 1. For comparison, the samples with different ratios of W, Co and Fe were also prepared through the above method by changing the amount of tungsten hexachloride, cobalt chloride and iron(III) chloride hexahydrate.
2.2. Material characterization The XRD patterns were measured on Rigaku X-ray diffractometer D/ max2250 equipped with Cu Kα (λ = 0.15406 nm) radiation. The morphologies and structures of all photoanodes were measured by Nova Nano SEM 230 scanning electron microscopy (SEM) with an energy-dispersive X-ray spectrometer (EDS) and JEM-2100F high-resolution transmission electron microscopy (HRTEM). The surface elements as well as the valence states were determined by K-Alpha 1063 X-ray photoelectron spectroscopy (XPS). And the optical properties were measured by TU-1901 double-beam UV–vis spectrophotometer.
Scheme 1. Schematic representation of procedures used to prepare WCoFe/ BiVO4 photoanode. 2
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2.3. Photoelectrochemical (PEC) measurements PEC performances of all photoanodes were conducted on Zennium electrochemical workstation with a typical three-electrode configuration. The as-prepared photoanode was used as working electrode, platinum foil was used as counter electrode and Ag/AgCl electrode (saturated KCl) was used as reference electrode. The illumination source was 500 W xenon lamp equipped with an AM 1.5 G filter to gain 1 sun illumination (100 mW cm−2). The electrolyte was 0.2 M potassium phosphate buffer solution (pH = 7) for water oxidation. As for sodium sulfite oxidation, the electrolyte was the above solution with an extra 0.2 M Na2SO3. In the experiment process, the photoanode was irradiated from the back side. Photocurrent-potential (J-V) curves were obtained by linear sweeping voltammetry (LSV) with or without illumination at a scan rate of 20 mV s−1. The AC frequency of electrochemical impedance spectroscopy (EIS) was ranging from 10 kHz to 0.1 Hz and performed at 1.23 VRHE. For each photoanode, the MottSchottky measurements were obtained at different AC frequencies, as 1 kHz, 2 kHz and 5 kHz, respectively. Incident photon to current conversion efficiency (IPCE) was conducted with a bias of 1.23 V using a xenon lamp (150 W, Oriel) equipped with a monochromator. The potentials reported in this research were adjusted to versus RHE by the Nernst equation: Fig. 2. (a–c) Transmission electron microscopy (TEM) and HRTEM images of WCoFe/BiVO4-18 h photoanode; (d-i) EDS mapping of WCoFe/BiVO4-18 h photoanode.
VRHE = VAg/AgCl + 0.059 × pH + 0.1976 The applied bias photo to current conversion efficiency (ABPE) was obtained by the equation:
explored by the XRD (Fig. 1b). The diffraction peaks are observed at 18.80°, 28.98°, 30.97°, 34.55°, 39.76°, 41.99° and 45.55°, corresponding to (1 0 1), (1 1 2), (0 0 4), (2 0 0), (1 1 4), (2 0 3) and (2 1 3) planes of monoclinic BiVO4 (JCPDS 14-0688) [30]. After coating WCoFe oxyhydroxide, three new peaks (heart-shaped in Fig. 1b) are found. The peaks at 41.94°, 53.02° and 62.50° can be assign to WCoFe oxyhydroxide, which is consistent with the crystal phases of bare WCoFe oxyhydroxide (Fig. S4). To clarify the detailed structure of WCoFe/BiVO4, TEM and HRTEM were employed. As shown in Fig. 2a and Fig. S3, a gauze-like material composed of crumpled and entangled nanosheets, which is similar as the reported WCoFe oxyhydroxide in the previous study [23], is tight supported on BiVO4 coral. HRTEM image (Fig. 2b) shows a distinct outer layer with a thickness about 5 nm. The inner material exhibits well-defined lattice fringes with a spacing of 0.226 nm, corresponding to (1 1 4) plane of monoclinic BiVO4 (JCPDS 14-0688). Meanwhile, the lattice interplanar distance of outer layer is 0.32 nm, which is well ascribed to (1 1 1) crystallographic plane of WCoFe oxyhydroxide according to previous study [23]. Energy-dispersive X-ray spectroscopy (EDS) elemental mappings (Fig. 2d–e) reveal that the W, Co and Fe elements are well distributed throughout the whole composite, which
J × (1.23 − VRHE) ABPE = × 100% P where J is the photocurrent density (mA cm−2) at the given potential (VRHE), and P is the incident light intensity (mW cm−2). 3. Results and discussion The morphologies of BiVO4 and WCoFe/BiVO4 photoanodes were examined by SEM. Fig. S1 reveals that the BiVO4 photoanode has a 3D coral-like structure with an average dendritic diameter of 200 nm [35], which is similar to that of WCoFe/BiVO4 photoanode in Fig. 1a. Compared to the bare BiVO4 photoanode (Fig. S1), the WCoFe/BiVO4 photoanode (Fig. 1a) obviously exhibits WCoFe oxyhydroxide coated on the surface of BiVO4 after solvothermal processing. The insert section, shown in Fig. 1a, demonstrates that the thickness of WCoFe/ BiVO4-18 h layer is approximately 1.2 μm, and the coated WCoFe oxyhydroxide does not change the typical geometry of BiVO4. As shown in Fig. S2, the reaction time of solvothermal process has neglected effect on the thickness of films. The crystal structure of the WCoFe/BiVO4 photoanode was further
Fig. 1. (a) Scanning electron microscopy (SEM) image of WCoFe/BiVO4-18 h photoanode, and inset shows the cross-sectional SEM image; (b) XRD patterns of all samples. 3
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Fig. 3. The XPS spectra of WCoFe/BiVO4-18 h photoanode: (a) XPS of survey spectrum; (b) XPS W 4f; (c) XPS Co 2p; (d) XPS Fe 2p; (e) XPS Bi 4f; (f) XPS V 2p.
means the well dispersed FeCoW oxyhydroxide. To further understand the surface elements and the chemical binding state, X-ray photoelectron spectroscopy (XPS) spectra of WCoFe/BiVO4 photoanode were performed and the results were shown in Figs. 3 and S5. The low-resolution XPS spectra (Fig. 3a) clearly shows W, Co, and Fe elements, indicating that the WCoFe oxyhydroxide is successfully obtained. The couple peaks of W4f located at 35.6 eV and 37.7 eV are corresponding to the W6+ oxidation states in Fig. 3b [36]. As shown in Fig. 3c, the main peaks in the Co 2p spectrum are Co 2p 3/ 2 (781.2 eV) and Co 2p 3/2 (797.3 eV) with two satellite peaks (786.3 eV and 803.0 eV), indicating the Co can be assigned as Co2+ [24]. According to Fig. 3d, the Fe 2p spectrum consists of two peaks at 711.8 and 725.4 eV, corresponding to 2p 3/2 and 2p 1/2 states, respectively, which indicates the Fe in Fe3+ oxidation state. In addition, there is a weak peak at 719.2 eV which is the satellite peak of Fe 2p 3/2 [37]. The peaks of Bi 4f are located at 159.3 and 164.7 eV, corresponding to Bi3+ in BiVO4 (Fig. 3e). The peak at 516.9 eV in Fig. 3f is indexed to V5+ of BiVO4 [38]. Fig. S3a shows two peaks of O 1 s, the former located at 530.6 eV derives from lattice oxygen, and the latter at 531.7 eV is corresponding to hydroxy species [39]. Finally, the peaks of C 1 s spectrum can be indexed to organic matter during reaction or carbon granule from instruments. The complete PEC process is affected by three factors as light harvest followed by generating electron-hole pairs, separation and transport of electron-hole pairs, and the surface reaction to produce oxygen and hydrogen. In order to investigate the optical absorption properties of BiVO4 and WCoFe/BiVO4 photoanodes, UV–vis absorption spectra were employed as shown in Fig. 4. The absorption edge of both BiVO4 and WCoFe/BiVO4 photoanodes are similar with each other, approximately 510 nm, which corresponding to band gap of ~2.4 eV. It also means that the WCoFe oxyhydroxide cannot provide additional photogenerated carriers. Besides, the UV–vis absorption characteristics of bare and the composite photoanodes have a very slight difference, indicating the WCoFe oxyhydroxide layer has no additional photoresponse. In order to understand the effect of WCoFe oxyhydroxide for reaction dynamics, the PEC behaviors of bare BiVO4 and WCoFe/BiVO4-x photoanodes for water oxidation and sulfite oxidation were conducted in corresponding electrolyte solution under AM 1.5 G illumination (100 mW cm−2) (Fig. 5a and b). As shown in Fig. 5a, the photocurrent density (solid) of WCoFe/BiVO4-18 h photoanode for water oxidation
Fig. 4. UV–vis absorption spectra of bare BiVO4 and WCoFe/BiVO4-x photoanodes.
has a significant increment compared to bare BiVO4 photoanode in the whole potential window. Specifically, the photocurrent density of bare BiVO4 photoanode is 1.60 mA cm−2 at 1.23 VRHE, but that of WCoFe/ BiVO4-18 h photoanode reaches 4.35 mA cm−2, more than 2 times as large as bare BiVO4 photoanode. The value is also compared to the BiVO4 based photoanodes reported in the literature (Table S2). Furthermore, the onset potential of WCoFe/BiVO4-18 h photoanode for water oxidation has a clearly negative shift relative to bare BiVO4 photoanode, reaching to 0.28 VRHE. The reduced OER barriers after coating by WCoFe oxyhydroxide may account for the negative shift of onset potential. The photocurrents of samples synthesized with different ratios of W, Co and Fe in precursor were also measured as shown in Fig. S8, and optimal molar ratio of W:Fe:Co is 5:1:4 in this experiment. In addition, the Mott-Schottky analysis (Fig. S6) indicates the flat band potential of WCoFe/BiVO4 with a cathode shift of approximate 0.3 V. The slopes between BiVO4 and WCoFe/BiVO4 photoanodes in same frequency have not changed, meaning the carrier concentrations are similar with each other and demonstrating none of W, Co and Fe was doped into BiVO4 [40]. These results demonstrate that the WCoFe oxyhydroxide, as oxygen evolution catalyst, significantly improves the OER kinetics by utilizing the photogenerated holes for water oxidation. The incident-photon-to-current conversion efficiencies (IPCE) of bare BiVO4 and WCoFe/BiVO4 photoanodes were shown in Fig. 5c. 4
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Fig. 5. Current-potential (J-V) curves of bare BiVO4 and WCoFe/BiVO4-x photoanodes conducted in (a) 0.2 M phosphate buffer solution (pH = 7) and (b) 0.2 M phosphate buffer solution contains 0.2 M Na2SO3 (pH = 7); (c) Incident photon to current conversion efficiency (IPCE) for BiVO4 based photoanodes; (d) Applied bias photon to current efficiency (ABPE) for bare BiVO4 and WCoFe/BiVO4-18 h photoelectrodes.
Fig. 6. The electrochemical impedance spectra (EIS) of (a) BiVO4 and WCoFe/BiVO4-x photoelectrodes; (b) The bode phase plots of the photoelectrodes; (c) Hole injection efficiency of BiVO4 based photoanodes; (d) Charge separation efficiency of BiVO4 based photoanodes.
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Fig. 7. Actual (solid line) and theoretical calculated (dashed line) evolution amount of H2 and O2 measured at 1.23 VRHE for 120 min; (b) The current-time curve for the WCoFe/BiVO4-18 h; (c) The possible mechanism of WCoFe/BiVO4 photoanode for PEC water splitting.
proposed that the WCoFe oxyhydroxide might facilitate the hole consume for water oxidation, resulting in the smaller Rct of WCoFe/BiVO4 photoanode. The calculated photogenerated electron lifetime (τd) from electrochemical impedance spectra (EIS) test was shown in Table S1. The WCoFe/BiVO4 has longer photogenerated electron lifetime (4.91 ms) than that of bare BiVO4 (2.35 ms), which can also account for the higher charge separation efficiency. To confirm that the OER at the WCoFe/BiVO4 photoanode as well as the hydrogen evolution reaction (HER) at the Pt electrode, the evolved gases were detected every 30 min by gas chromatography (GC). And the amounts of the evolved H2 and O2 gases were 140.43 and 69.85 μmol after 2 h (Fig. 7a), respectively. And the ratio of evolved H2 and O2 gases is nearly 2 with the faraday efficient of more than 90%, which means the photogenerated holes were utilized for water oxidation to produce O2 in the system indeed. Based on the results mentioned above, a possible mechanism about the solar water oxidation is proposed in Fig. 7c. When the simulated sunlight irradiated the WCoFe/BiVO4 photoanode, the photogenerated electron-hole pairs were produced inside the coral-like BiVO4. Then the holes transported to the interface between BiVO4 and WCoFe oxyhydroxide and were extracted into the WCoFe oxyhydroxide for water oxidation. Meanwhile, the electrons were transferred to the Pt electrode to produce H2. Besides, the stability of WCoFe/BiVO4 photoanode was also considered as an important factor for PEC water splitting. In this work, the as-prepared photoanode showed a high stability (Fig. S10), which can be ascribed to the highquality interface between WCoFe oxyhydroxide and BiVO4. On the one hand, it can protect the BiVO4 from electrolyte and prohibit the corrosion. On the other hand, it can reduce the density of surface states, thus accelerating the migration of hole from BiVO4 to WCoFe oxyhydroxide [42].
After applying a specific potential, the IPCE values of WCoFe/BiVO4 photoanodes are higher than that of bare BiVO4 photoanode at same wavelength. In particular, WCoFe/BiVO4-18 h shows the IPCE value of 37.7% at the wavelength of 450 nm, while the BiVO4 photoanode shows 13.34% at the same wavelength. In addition, the applied bias photon to current efficiency (ABPE) of WCoFe/BiVO4-18 h and bare BiVO4 photoanodes were shown in Fig. 5d, and the maximum ABPE of WCoFe/ BiVO4-18 h photoanode is 1.38% at 0.71 VRHE. Meanwhile, the bare BiVO4 shows the maximum ABPE value of 0.35% at 0.84 VRHE, which is much lower than that of WCoFe/BiVO4-18 h photoanode. To clearly understand the reasons of the improved PEC performance, the hole injection efficiency and the charge separation efficiency were calculated in Fig. 6c and 6d. For sulfite oxidation, the photocurrent density of bare BiVO4 and WCoFe/BiVO4-18 h are 3.38 mA cm−2 and 5.13 mA cm−2 at 1.23 VRHE, respectively. Because the oxidation of sulfite hole scavenger is not limited by the reaction kinetics, the enhancement in Fig. 5b can be attributed to the increased charge separation efficiency, from 21.1% to 57.2% (Fig. 6d). It means that the WCoFe oxyhydroxide layer accelerates the hole extraction from BiVO4, thus reducing the charge recombination. Besides, as shown in Fig. 6c, the hole injection efficiency of WCoFe/BiVO4 (84.75%) is much higher than that of bare BiVO4 photoanode (~40%). It demonstrates that bare BiVO4 is indeed limited by the sluggish OER kinetics, and the WCoFe oxyhydroxide can effectively accelerate the hole injection into the electrolyte by reducing the dynamic barriers of OER [41]. EIS test was further measured to investigate the interfacial property of BiVO4 and WCoFe/BiVO4-x photoanodes under AM 1.5 G illumination. As shown in Fig. 6a, the EIS spectra reveal that WCoFe/BiVO4 has a smaller Rct than that of bare BiVO4, indicating the improved OER dynamics between the surface of electrode and electrolyte. It has been 6
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4. Conclusions
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In summary, an efficient OER catalyst (WCoFe oxyhydroxide) was integrated to coral-like BiVO4 photoanode for PEC water splitting. The WCoFe/BiVO4 photoanode exhibits high PEC properties, such as photocurrent density of 4.35 mA cm−2 at 1.23 VRHE and low onset potential of 0.28 VRHE. The photocurrent and EIS measurements demonstrate that the introduction of WCoFe oxyhydroxide not only reduces the dynamic barrier of OER, but also accelerates the charge separation and transport, thus leading higher charge separation and hole injection efficiency compared to the bare BiVO4 photoanode. The optical property and Mott-Schottky measurement reveal that the WCoFe oxyhydroxide layer can collect and store the holes from BiVO4 to reduce the charge recombination. All of these results indicate that solvothermal method is a promising strategy to load WCoFe oxyhydroxide as efficient electrocatalyst on the surface of photoelectrode. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This study was supported by the National Nature Science Foundation of China (No. 21471054, 51904356), the Hunan Provincial Science and Technology Plan Project, China (No. 2016TP1007), the Hunan Provincial Natural Science Foundation of China (Grant No. 2017JJ2326), the Natural Science Foundation of Chongqing, China (No. cstc2018jcyjAX0733). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2019.123323. References [1] C. Jiang, S.J.A. Moniz, A. Wang, T. Zhang, J. Tang, Photoelectrochemical devices for solar water splitting-materials and challenges, Chem. Soc. Rev. 46 (2017) 4645–4660. [2] C. Ding, J. Shi, Z. Wang, C. Li, Photoelectrocatalytic water splitting: significance of cocatalysts, electrolyte, and interfaces, ACS Catal. 7 (2016) 675–688. [3] J. Suntivich, K.J. May, H.A. Gasteiger, A perovskite oxide optimized for oxygen evolution catalysis from molecular orbital principles, Science 334 (2011) 1383–1385. [4] B. Liu, H.Q. Peng, C.N. Ho, H. Xue, S. Wu, T.W. Ng, C.S. Lee, W. Zhang, Mesoporous nanosheet networked hybrids of cobalt oxide and cobalt phosphate for efficient electrochemical and photoelectrochemical oxygen evolution, Small 13 (2017) 1701875. [5] H. Jiang, J. Gu, X. Zheng, M. Liu, X. Qiu, L. Wang, W. Li, Z. Chen, X. Ji, J. Li, Defect-rich and ultrathin N doped carbon nanosheets as advanced trifunctional metal-free electrocatalysts for the ORR, OER and HER, Energy Environ. Sci. 12 (2019) 322–333. [6] Z. Ma, H. Hou, K. Song, Z. Fang, L. Wang, F. Gao, Z. Yang, B. Tang, W. Yang, Ternary WO3/porous-BiVO4/FeOOH hierarchical architectures: towards highly efficient photoelectrochemical performance, ChemElectroChem 5 (2018) 3660–3667. [7] Z. Yang, W. Luo, Z. Li, Solar hydrogen generation from seawater with a modified BiVO4 photoanode, Energy Environ. Sci. 4 (2011) 4046–4051. [8] S. Wang, P. Chen, Y. Bai, J.H. Yun, G. Liu, L. Wang, New BiVO4 dual photoanodes with enriched oxygen vacancies for efficient solar-driven water splitting, Adv. Mater. 30 (2018) 1800486. [9] Y. Liu, B.R. Wygant, K. Kawashima, O. Mabayoje, T.E. Hong, S.-G. Lee, J. Lin, J.-H. Kim, K. Yubuta, W. Li, Facet effect on the photoelectrochemical performance of a WO3/BiVO4 heterojunction photoanode, Appl. Catal.B:Environ. 245 (2019) 227–239. [10] Z. Ma, K. Song, L. Wang, F. Gao, B. Tang, H. Hou, W. Yang, WO3/BiVO4 type-II heterojunction arrays decorated with oxygen-deficient ZnO passivation layer: a highly efficient and stable photoanode, ACS Appl. Mater. Interfaces 11 (2019) 889–897. [11] K.P.S. Parmar, H.J. Kang, A. Bist, P. Dua, J.S. Jang, J.S. Lee, Photocatalytic and photoelectrochemical water oxidation over metal-doped monoclinic BiVO4 photoanodes, ChemSusChem 5 (2012) 1926–1934. [12] W.J. Jo, J.W. Jang, K.J. Kong, H.J. Kang, J.Y. Kim, H. Jun, K. Parmar, J.S. Lee, Phosphate doping into monoclinic BiVO4 for enhanced photoelectrochemical water oxidation activity, Angew. Chem. Int. Ed. 124 (2012) 3201–3205. [13] A.J. Rettie, W.D. Chemelewski, J. Lindemuth, J.S. McCloy, L.G. Marshall, J. Zhou, D. Emin, C.B. Mullins, Anisotropic small-polaron hopping in W: BiVO4 single crystals,
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