Boosting Photoelectrochemical Water Oxidation with Cobalt

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Boosting Photoelectrochemical Water Oxidation with Cobalt Phosphide Nanosheets on Porous BiVO4 Haili Tong, Yi Jiang, Qian Zhang, Wenchao Jiang, Kaili Wang, Xiaoxi Luo, Ze Lin, and Lixin Xia ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04405 • Publication Date (Web): 04 Dec 2018 Downloaded from http://pubs.acs.org on December 4, 2018

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ACS Sustainable Chemistry & Engineering

Boosting Photoelectrochemical Water Oxidation with Cobalt Phosphide Nanosheets on Porous BiVO4

Haili Tong, Yi Jiang,* Qian Zhang, Wenchao Jiang, Kaili Wang, Xiaoxi Luo, Ze Lin, and Lixin Xia*

College of Chemistry, Liaoning University, No. 66, Chongshan Middle Road, Huanggu District, Shenyang 110036, People’s Republic of China

*Corresponding authors. E-mail addresses: [email protected] (Y Jiang); [email protected] (L Xia).

ABSTRACT: Modification of semiconductor surface by cobalt-rich catalysts is an effective strategy to improve the photoelectrochemical (PEC) water oxidation kinetics

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and migration of photogenerated electron and hole. In this study, cobalt phosphide (CoP) nanosheet was integrated on the nanoporous bismuth vanadate (BiVO4) electrode by using the hydrothermal method. The introduction of CoP significantly improved the PEC performance of the photoanode, with a photocurrent up to 4.0 mA cm-2 at 1.23 V (versus RHE) under simulated 100 mW cm-2 irradiation, a three-fold enhancement over that obtained by the bare BiVO4. The BiVO4+CoP photoanode exhibited an impressive early onset of water oxidation, with a more than 220 mV cathodic shift of the onset potential, superior to the typical Co3O4 and Co-Pi cocatalysts modified BiVO4 photoanode. Systematic studies show that the improvement in PEC performance by CoP is mainly due to the restraint of surface charge recombination and increase in photovoltage.

KEYWORDS: bismuth vanadate (BiVO4), cobalt phosphide, photoelectrochemical, water oxidation, photoanode INTRODUCTION


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Photoelectrochemical (PEC) water splitting into hydrogen and oxygen using semiconductor materials, can effectively converse and utilize the unlimited solar energy. In a PEC system, the oxygen evolution reaction (OER) occurring on the photoanode is considered to be a challenge and a key for the overall water splitting, owing to the sluggish four-electron-based reaction kinetics and the complex intermediate species.1-5 Thus, a highly active semiconductor photoelectrode is required for efficient water oxidation. Due to its moderate band gap energy (approximately 2.4 eV) under visible light irradiation, BiVO4 is a very promising photoelectrode material. Moreover, BiVO4 has an appropriate portion of the valence band (VB) position for OER as well as a beneficial conduction band (CB) that is pretty close to the thermodynamic potential of hydrogen evolution reaction (HER).6-9 Nevertheless, the water splitting activity of unmodified BiVO4 is low because the migration of the photogenerated electrons and holes in it is difficult. Introduction of metal ions,10-11 doping or directional control of BiVO412-15 have been employed to alleviate these problems. In addition, the integration with OER catalysts is also an effective method for promoting the PEC performance.


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The OER catalysts based on precious metals (such as IrO216-18 and RuO219-21) are not suitable for large-scale commercial applications owing to their high costs and scarcity. Therefore, the development of non-noble metal co-catalysts with low cost and high performance has drawn considerable attention. Cobalt-based catalysts are extensively used both as electrocatalysts for water oxidation and as co-catalysts on semiconductor electrodes for PEC water oxidation.22-26 BiVO4 photoanodes respectively modified by Co-Pi,27-30 Co-Bi,31 Co3O4,32-33 and some molecular cobalt catalysts34-36 have also been reported to improve the charge separation and water oxidation kinetics.

Note that extensive research have been conducted on transition-metal phosphides recently in various fields because of their non-precious and high catalytic activity.37-42 Among them, CoP is used as a representative catalyst for HER with high catalytic activity and corrosion resistance in an alkaline environment. Previous studies have reported that various morphologies of CoP were directly used as electrocatalysts for HER by loading them on carbon cloths or Ti substrates or on carbon nanotubes.43-46 In recent years, CoP nanocrystals have also been applied as an OER catalyst for water


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oxidation.47-50 Therefore, a CoP catalyst modified BiVO4 electrode should be, in theory, a very interesting candidate photoanode for water splitting. Surprisingly, it was not until recently that CoP nanoparticles were applied to a Mo-doped BiVO4 photoanode and a Ni foam cathode as a bifunctional electrocatalyst by a drop casting method was reported.51 However, in that work, the CoP-modified BiVO4 electrode exhibited inferior PEC performance as compared to the Co-Pi-modified BiVO4 electrode and CoP nanoparticles were converted to a chemical state similar to Co-Pi in a phosphate buffer solution during the PEC water oxidation. In this study, we developed a convenient and efficient hydrothermal method to deposit CoP nanosheets on nanoporous BiVO4 photoanode. The resulting BiVO4+CoP photoanode exhibited strong PEC activity and outstanding stability for OER in a borate buffer. A high photocurrent density of 4.0 mA cm-2 at 1.23 V versus a reversible hydrogen electrode (RHE) and a low onset potential of 0.2 V were obtained. Furthermore, two well-known OER catalysts, namely Co3O4 and Co-Pi, were modified on BiVO4 electrodes for comparison (Scheme 1). The BiVO4+CoP photoanode displayed significantly higher PEC performance than the BiVO4+Co3O4 and


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BiVO4+Co-Pi photoanodes. These results show a proof-of-concept advancement by using CoP nanosheet as a co-catalyst on BiVO4 photoanode for PEC water oxidation.

Scheme 1. The schematic of CoP, Co3O4 and Co-Pi catalysts modified on BiVO4 electrodes for photocatalytic water splitting.

EXPERIMENTAL SECTION

Preparation of BiVO4 photoanode

BiVO4 photoanode was prepared by electrodeposition method.52 To be specific, 150 mL DI water was put into a clean beaker, and pH was calibrated to 1.7 by using HNO3. KI (0.4 M) was dissolved in the above solution. After stirring for 5 minutes, 2.91 g Bi(NO3)3•5H2O was added to the solution and the mixture was ultrasoniced for 10


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minutes. After dissolving 1.49 g p-benzoquinone in ethyl alcohol, mixed with the above solution and sonicated for 20 minutes. The clean FTO is used as a working electrode for electrodeposition in a three-electrode device and constant potential of -0.1 V versus Ag/AgCl for 300 s to obtain a BiOI film. Then rinse with deionized water and dry by nitrogen, 50 µL dimethyl sulfoxide (DMSO) solution containing 2.65 mg vanadyl acetylacetonate (VO(acac)2) was added to the BiOI film. After the electrode was heated for 2 hours at 450 °C in a muffle furnace. And then naturally cool to room temperature, the BiVO4 photoanode was immersed in NaOH solution (1.0 M) for about 0.5 hours, and excess V2O5 from the BiVO4 photoanode was removed. Finally, the photoanode was washed with distilled water and dried with nitrogen.

Synthesis of Co3O4 nanoparticles

The Co3O4 was synthetized by using a hydrothermal method previously reported.53 In this synthesis process, 0.64 g (Co(CH3COO)2∙4H2O) aqueous solution was firstly dissolved into 1.2 mL of deionized water and added into 24 mL C2H5OH. After the mixture solution was ultrasoniced for 10 minutes, 500 µL of NH3∙H2O were added under


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stirring at room temperature. Then, the mixture was added to a 100 mL of teflon-lined stainless autoclave and heated at 353 K for 24 hours with a heating speed of 2 °C min-1. Then, the solution was kept at 423 K for 3 hours and allowed to cool to room temperature. Finally, the product was gathered by centrifugation and rinsed with C2H5OH and dried at 333 K overnight.

Synthesis of CoP nanosheets

The CoP nanosheets were collected with a centrifugation treatment and without a centrifugation treatment, respectively. 0.2 mg of Co3O4 and 1.0 mg of NaH2PO2∙H2O were mixed together and ground into a fine powder with a mortar. After that, the powder was calcined at 575 K for 2 hours with a heating rate of 2°C min-1. The black solid obtained was rinsed with ethanol and distilled water for several times. Subsequently, the precipitate was dispersed in 50 mL ethanol ultrasonic for 1 h.

Synthesis of BiVO4+CoP electrode


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The different concentration of ultrasonic and centrifugation treatment of CoP suspension liquid (0.3, 0.5, 0.7 mg mL-1) were varied by dilution procedure, then was transferred to a 10 mL Teflon-lined stainless autoclave. The prepared BiVO4 electrodes were immersed the above suspension liquid and leaned against the wall, which were heated at 373 K for 10 hours in air. Then cool at room temperature, the electrodes were taken out and rinsed with deionized water, obtaining BiVO4+CoP electrode.

Synthesis of BiVO4+Co3O4 electrode

The different concentration of Co3O4 suspension liquid (0.2, 0.4, 0.6 mg mL-1) were varied by dilution procedure, then was transferred to a 10 mL Teflon-lined stainless autoclave. The prepared BiVO4 electrodes were immersed in it and leaned against the wall, which were heated at 373 K for 10 hours in air. After cooling to room temperature, the electrodes were taken out and rinsed with deionized water, obtaining BiVO4+Co3O4 electrode.

Synthesis of BiVO4+Co-Pi electrode


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The BiVO4+Co-Pi electrodes were prepared according to the previous paper.27 Firstly, 15 mg Co(NO3)•6H2O dissolved in 20 mL of PH = 7 PBS solution, stirring for 10 min. A standard three-electrode system was used for electrodeposition with the FTO+BiVO4 as working electrode. The deposition was carried out at 0.3 V versus Ag/AgCl for 50 s, 150 s, 300 s, the current density is 1.0~10 µA cm-2. Then rinse with distilled water and dried under N2 flow.

Materials

FTO-coated glass slides were obtained from Zhuhai Kaivo Electronic Components Co., Ltd. (Co(CH3COO)2∙4H2O) powder was obtained from Aladdin Industrial Corporation. NH3∙H2O and NaH2PO2∙H2O were obtained from Sinopharm Chemical Reagenl Co. Ltd. The organic solvents were AR grade.

Characterization

The crystalline phase of the BiVO4 and BiVO4 +Co were characterized by XRD (powder X-ray diffraction) on a Bruker Smart APEX II powder diffractometer with CuKa radiation.


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The morphologies of the electrodes were investigated by scanning electron microscopy (SEM) images and Energy-dispersive X-ray analysis spectra (EDX) using a Quanta SU8000 scanning electron microscope instrument operated at 15 kV. X-Ray photoelectron spectroscopy (XPS) datas were tested using a K-Alpha XPS spectrometer (Thermo Fisher Scientific, Waltham, MA, USA). The UV-visible spectrophotometer (Perkin Elmer Lambda 35; Norwalk, CT, USA) was employed to obtain UV-visible diffuse reflectance spectra. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) image was obtained using a JEOL 2100 operated at 200 kV. The content of elements was tested by inductively coupled plasma mass spectroscopy (ICP, PE-43008300).

Photoelectrochemical measurements

The PEC performance of cobalt-catalysts modified BiVO4 electrodes were measured by a CHI 760E electrochemical workstation (Chenhua Instrument, Shanghai, China) under the irradiation. A three-electrode device was consisted of a platinum wire as the counter electrode, the BiVO4, BiVO4+CoP, BiVO4+Co3O4 or BiVO4+Co-Pi photoanodes (active


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area = 1.0 cm2) as the working electrode and an Ag/AgCl electrode as the reference electrode. The photoanode was irradiated with a 300 W xenon lamp (Beijing, CELHXF300C) that simulates solar light (AM 1.5G), and the light intensity was 100 mW cm2.

The linear sweep voltammetry (LSV) with a scan rate of 10 mV s-1 were conducted in

a pH 9.0 0.2 M borax buffer. Electrochemical impedance spectroscopy (EIS) were carried out with an AC voltage amplitude of 5 mV at 1.23 V versus RHE, frequency range from 10 kHz to 0.01 Hz under solar light irradiation.

Intensity modulated photocurrent spectroscopy (IMPS) measurements

IMPS measurements were measured by using a Zahner IMPS electrochemical workstation (Zahner electrochemical company, Germany, IM6ex) in a three-electrode system. Modulated illumination was supplied with a light-emitting diode (LED: 420 nm, 10 W m-2) that permitted superposition of sinusoidal modulation (~10%) on a dc illumination level. The photocurrent was recorded as a function of frequency (10 kHz ~ 100 mHz) under illumination at different potentials.

Determination of Faradaic efficiency


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The amounts of O2 and H2 evolution were tested by using gas chromatography (6890A) with a thermal conductivity detector. The Faradaic efficiency was calculated according to the integrated charge (Q) passed and the amount of O2 evolved.

FE (%) 

n O2 100%  Q/4     96485 

(1)

The incident photon-to-current efficiency (IPCE) of each wavelength was tested by irradiation with a 300 W xenon lamp with an AM 1.5G filter. The IPCE value was calculated by the equation:

IPCE (%) 

1240  J 100%   Pin

(2)

where: J is the photocurrent density at a specific wavelength, Pin is the measured light power density, and λ is the wavelength of incident light.

The following equation is used to calculate the applied bias photon-to-current efficiency (ABPE) from the LSV curve, where J is the photocurrent density at the potential of Vbias,


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and Pin is the simulated solar light intensity. All measured potentials were transformed into RHE (ERHE = EAg/AgCl +0.059 V × pH+0.197 V).

 J (mA / cm 2 )  (1.23  Vbias )(V )  ABPE (%)   100  Pin (mW / cm 2 )   AM 1.5G

(3)

RESULTS AND DISCUSSION The nanoporous BiVO4 photoanodes were prepared via an electrodeposition method followed by annealing reported previously,52 which led to a relatively dense and nanoporous BiVO4 nanoparticles formation. The X-ray diffraction (XRD) results (Figure S1a in Supporting Information) are in accordance with the expectation for a monoclinic phase. The BiVO4+CoP electrode was prepared via a hydrothermal method (details in Supporting Information). CoP nanosheets were prepared using a Co3O4 precursor.53 The structures of CoP and Co3O4 were examined using XRD (Figures S1b and 1c). The diffraction peaks at 19.0°, 31.3°, 36.8°, 44.8°, 59.4° and 65.2° are in accordance with the monoclinic phase of Co3O4 (JCPDS 43-1003).32 After phosphating, the diffraction peaks were consistent with major orthorhombic CoP (JCPDS 29-0497). The XRD pattern for the phosphided product shows diffraction peaks at 32.0°, 36.7°, 46.2°, 48.4°,


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52.3°and 57.2° indexes to the (011), (111), (112), (211), (103) and (301) planes of CoP, respectively.54-55 No other impurities signals such as Co3O4 except a minor peak of Co2P (112) can be detected.

56-57

Further characterizations, a typical TEM and high-

resolution TEM (HRTEM) images of the CoP nanosheets are presented in Figure S2. The CoP nanocrystals have lattice spacings of 0.28 and 0.19 nm, which are consistent with the d-spacing of the CoP (011) and (211) planes, respectively, and exhibit a sheet structure with a high specific surface area ratio.


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Figure 1. SEM images of (a) BiVO4, (b) BiVO4+CoP, (c) BiVO4+Co3O4, (d) BiVO4+Co-Pi and (e) EDX elemental mapping images of Bi, V, O, Co and P for the CoP nanosheet.

The morphologies, sizes and microstructures of the as-prepared samples (BiVO4, BiVO4+CoP, BiVO4+Co3O4 and BiVO4+Co-Pi) were characterized by SEM. As shown in Figure 1a, BiVO4 is composed of a large number of worm-like nanoparticles and had a nanoporous morphology, indicating the appearance of a highly three-dimensional (3D) network of open and porous structures. When the CoP catalyst is grown on a nanoporous BiVO4 electrode by using a simple hydrothermal method, the electrode clearly shows that the CoP nanocrystals shaped like nanosheet were embedded in the gap of BiVO4 (Figure 1b). Elemental mapping from energy-disperse X-ray spectroscopy (EDX) (Figure 1e) exhibits a uniform distribution for CoP on the surface of BiVO4, further confirming the successful modification of CoP. In addition, the shift of the V 2p and O 1s peaks in X-ray photoelectron spectroscopy (XPS) (Figure S4) suggests an effective electron interaction between BiVO4 and CoP.58 The Co3O4 and Co-Pi catalysts as the contrast catalysts were also modified on the surface of BiVO4. The dispersed Co3O4


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particles on the BiVO4 electrode are discrete and inhomogeneous, as shown in Figure 1c. Thereafter, Co-Pi nanoparticles were well-dispersed on BiVO4 electrode surface, forming a dense layer. (Figure 1d). To gain more insight, we conducted XPS and energy-dispersive spectrometry (EDS), wherein the signals of the Co and P elements are observed, respectively (Figures S3-S7).

Figure 2. Photocurrent-potential characteristics of BiVO4 electrode modified by with (using drop-casting and hydrothermal method, respectively) and without the centrifugation treatment of CoP under illumination (scan rate: 10 mV s-1).

To explore the PEC performance of the photoanodes, a conventional three-electrode device was conducted with the sample electrodes as the working electrodes. All of the (photo)electrochemical measurements were conducted at pH 9.0 in a 0.2 M borate


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buffer. Since CoP was extensively investigated as an electrocatalyst, we first tested the activity of CoP in an electrochemical water oxidation system. The linear sweep voltammetry (LSV) curves of the sample electrodes are shown in Figure S8. The CoP and Co3O4 electrodes exhibit similar OER activity, which is inferior to that of Co-Pi. The potentials of CoP and Co-Pi at 1.0 mA cm-2 are 2.0 V and 1.87 V, respectively. It is concluded that CoP is not a robust OER electrocatalyst on FTO. Based on the above results, we compared the PEC performance of the as-prepared BiVO4+CoP, BiVO4+Co3O4 and BiVO4+Co-Pi photoanodes. The loading amount of the catalyst is also a critical influence on the PEC performance. Therefore, the loading amount of different cobalt-based catalysts on the electrode was discussed. The mass of CoP on the BiVO4 electrode was adjusted by varying the concentration of the CoP suspension liquid (details in Supporting Information). The loading amount of CoP was quantified to be 10.7, 16.7 and 37.3 nmol cm-2, respectively, according to the inductively coupled plasma mass spectroscopy (ICP) measurement. Similarly, the BiVO4+Co3O4 photoanodes were also prepared by a hydrothermal method, and the amount of Co3O4 on the BiVO4 electrode was 7.1, 14.3 and 18.7 nmol cm-2, respectively. Co-Pi was


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applied onto the BiVO4 photoanode using an electrodeposition method in a phosphate buffer solution containing a certain concentration of Co(NO3)2·6H2O at 0.3 V (vs. Ag/AgCl) for different time periods, as described previously.27 The PEC performances of all the electrodes evaluated by LSV are shown in Figures S9-S11, the presented curves clearly reflect the importance of the loading amount of the catalysts on the photoelectrode.

In addition, the loading process of the photoanodes has a considerable impact on the PEC performance. A CoP modified BiVO4 electrode by using the drop casting method as reported was prepared for comparison, and the PEC performance of the two BiVO4+CoP photoanodes are shown in Figure 2. The BiVO4+CoP photoanode prepared using the drop coating method exhibits an unremarkable photocurrent, with a value of 3.0 mA cm-2 at 1.23 V versus RHE, which is the same as that of the BiVO4+Co3O4 and BiVO4+Co-Pi photoanodes and is considerably lower than that of the photoanode prepared using the hydrothermal method in this work. Furthermore, the sizes and the dispersity of the CoP nanosheets are crucial. The ultrasonic and centrifugation


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treatments of the CoP suspension liquid were necessary to remove the large sheet or bulk layers, therefore, CoP nanosheets were easier to implant in the pores of BiVO4. The photocurrents of the two BiVO4+CoP photoanodes with and without the centrifugation treatment of CoP are shown in Figure 2. The BiVO4 photoanode modified with untreated CoP shows considerably poorer PEC performance, with less than half the photocurrent density of the treated photoanode.

Figure 3. (a) LSV curves and (b) photo conversion efficiencies of the BiVO4, BiVO4+CoP, BiVO4+Co3O4 and BiVO4+Co-Pi photoanodes measured with and without illumination (scan rate: 10 mV s-1).

Figure 3a exhibits the LSV curves of the bare BiVO4 photoanode and the BiVO4 photoanodes with the optimized loading of CoP, Co3O4 and Co-Pi. All the sample


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photoanodes exhibit negligible current in the dark. Under illumination with simulated sunlight (AM 1.5G, 100 mW cm-2), the bare BiVO4 exhibits a typical response with a photocurrent of 1.32 mA cm-2 at 1.23 V versus RHE. Evidently, the photocurrents are considerably enhanced upon the loading of CoP, Co3O4 and Co-Pi. However, the BiVO4+CoP photoanode exhibits considerably higher activity than the BiVO4+Co3O4 and BiVO4+Co-Pi photoanodes. An AM 1.5G photocurrent of 4.0 mA cm-2 is achieved at 1.23 V, which is three times higher than that of the bare BiVO4, whereas the photocurrents at 1.23 V versus RHE of the BiVO4+Co3O4 and BiVO4+Co-Pi photoanodes are 2.9 and 2.4 mA cm-2, respectively. Additionally, the BiVO4+CoP photoanode exhibits an extremely low onset potential (0.2 V) toward water oxidation, which is cathodically shifted by 220 mV compared with that of BiVO4 and obviously lower than that of the BiVO4+Co3O4 and BiVO4+Co-Pi photoanodes. Moreover, the potential at a photocurrent density of 1.0 mA cm-2 of the BiVO4+CoP, BiVO4+Co3O4, BiVO4+Co-Pi and the bare BiVO4 photoanodes are 0.43 V, 0.61 V, 0.6 V and 1.07 V versus RHE, respectively, further confirming the highest activity of CoP on the electrode. The remarkable increased photocurrent and decreased onset potential


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demonstrate that CoP is a highly efficient OER catalyst on the photoanode and can effectively improve the PEC performance of the photoanode.

Figure 4. Open-circuit measurements for BiVO4, BiVO4+CoP, BiVO4+Co3O4 and BiVO4+Co-Pi photoanodes under darkness and illumination in 0.2 M borate buffer.

The open-circuit potential (VOC) of the sample photoanodes was measured under darkness and illumination to understand the onset potential shift of CoP.58-60 (Figure 4) The bare BiVO4 electrode exhibits an equilibrium potential of 0.44 V under darkness and 0.3 V under illumination, corresponding to a photovoltage of 0.14 V. Similarly, the BiVO4+CoP, BiVO4+Co3O4 and BiVO4+Co-Pi photoanodes exhibit photovoltages of 0.3 V, 0.22 V and 0.2 V, respectively. This difference in the photovoltage is consistent with the cathodic shift of the onset potential toward water oxidation.61 These results together


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with the electrochemical activities in darkness demonstrate that cathodic onset shift for BiVO4+CoP is mainly owing to the thermodynamic contribution (increase in photovoltage) rather the kinetic contribution, indicating that CoP could suppress the surface electron-hole recombination and provide a greater driving force for water oxidation than the other co-catalysts.

On the basis of the LSV results, the maximum applied bias photon-to-current efficiency (ABPE) for the BiVO4+CoP photoanode is calculated to be 1.24% at 0.7 V versus RHE, which is approximately 1.6 times that of the BiVO4+Co3O4 photoanode (0.78% at 0.76 V versus RHE), 1.8 times that of the BiVO4+Co-Pi photoanode (0.71% at 0.74 V versus RHE), and 6.2 times that of the bare BiVO4 photoanode (0.20 % at 0.93 V versus RHE) (Figure 3b).

In order to further evaluate the surface recombination of CoP, sodium sulfate (Na2SO3) was introduced as a hole scavenger to eliminate the hole injection barrier. The PEC Na2SO3 oxidation was performed in the 0.2 M borate buffer mixed with 1 M Na2SO3 (Figure S12). In the presence of Na2SO3, an obvious increase in the


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photocurrent density and a reduction of the onset potential are observed for all the sample photoanodes, showing that the surface recombination was eliminated by Na2SO3. The BiVO4+CoP, BiVO4+Co3O4 and BiVO4+Co-Pi photoanodes show almost the same photocurrent as the bare BiVO4 electrode in the presence of Na2SO3, suggesting that the modification of the cobalt catalyst did not change the separation efficiency. Thus, the hole injection efficiencies of the BiVO4, BiVO4+CoP, BiVO4+Co3O4 and BiVO4+Co-Pi photoanodes could be calculated using the following formula:28,52

 ch arg e injection  J H O / J Na SO 2

2

3

(4)

where JH2O and JNa2SO3 represent the oxidation photocurrent densities H2O and Na2SO3, respectively. Further, ηinj is the charge injection yield to the electrolyte (more details are provided in supporting information). As shown in Figure 5a, the ηinj value of the BiVO4 photoanode is less than 35% in the overall voltage range, suggesting that most of the charge recombination occurred on the surface of the electrode without participation in water oxidation reaction. The modification with Co catalysts considerably enhanced the ηin value. BiVO4+CoP shows the highest ηinj, reaching 90.3% at 1.23 V, considerably


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higher than that of the BiVO4+Co3O4 photoanode (61.3%) and the BiVO4+Co-Pi photoanode (52.0%). These results demonstrate that CoP could effectively inhibit the surface recombination and promote the hole injection into the electrolyte.

Figure 5. (a) Charge injection efficiencies and (b) EIS spectra of BiVO4, BiVO4+CoP, BiVO4+Co3O4 and BiVO4+Co-Pi photoanodes measured under AM 1.5G illumination.

To gain a better sight of the interface charge transfer capability of CoP on the electrode, electrochemical impedance spectroscopy (EIS) was carried out. As shown in Figure 5b, the BiVO4 photoanode exhibits a large semicircle under irradiation, which displays a high interface charge transfer barrier. However, the co-catalysts decrease the resistance of the BiVO4 films for PEC water splitting. The BiVO4+CoP photoanode exhibits a smaller semicircle than that of BiVO4+Co3O4 and BiVO4+Co-Pi photoanodes,


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further illustrating the promotion of the charge transfer by CoP across the photoanode/electrolyte interface.

Figure 6. IPCE spectra of BiVO4 and BiVO4+CoP photoanodes measured at 0.7 V versus RHE under illumination.

The wavelength dependence of the incident photon-to-current efficiencies (IPCEs) for the BiVO4+CoP photoanode and the bare BiVO4 photoanode were examined at 0.7 V versus RHE (Figure 6). The BiVO4+CoP reveals the similar photocurrent response spectrum as the bare BiVO4, indicating that the primary photoresponse is from the nanoporous BiVO4 electrode rather than CoP, which is in accordance with their uniform response of the absorption spectra (Figure S13). At 450 nm, the IPCE value of the BiVO4+CoP photoanode reaches 47%, while that of the BiVO4 electrode is only 20%,


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suggesting that the CoP catalyst can improve the electron injection efficiency and the light harvesting efficiency.

Figure 7. (a) IMPS responses of BiVO4+CoP photoanode in 0.2 M borate buffer with different applied potentials. (b) ktrans of the BiVO4 and BiVO4+CoP photoanodes, (c) krec of the BiVO4 and BiVO4+CoP photoanodes, (d) The electron transfer time of each photoanode was estimated from IMPS analysis.


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IMPS was performed to further explore the charge transfer and surface recombination kinetics. Figure 7a exhibits typical IMPS responses of BiVO4+CoP photoanode at different applied potentials. In addition, IMPS responses of bare BiVO4 at 0.9 V versus RHE was shown in Figure S14. The frequency of the maximum imaginary number equals the charge transfer rate constant (ktrans) plus the charge recombination rate constant (krec) (2πfmax = ktrans + krec). Moreover, the charge transfer efficiency (ktrans/(ktrans + krec)) could be obtained by normalization of low frequency intercept (Im(jphoto/jh) = 0). Therefore, the key parameters ktrans and krec are easily obtained.62-63 As shown in Figure 7b, BiVO4+CoP exhibits higher ktrans values than the unmodified BiVO4 within the whole voltage range, showing that CoP on the electrode surface serves as an OER catalyst. Moreover, the krec values for BiVO4+CoP electrode is remarkably diminished as presented in Figure 7c. The electron transfer time (τd) of each sample photoanode can be reckoned by the frequency of the minimum imaginary part (τd =1/2πfmin),64 which is shown in Figure 7d. BiVO4+CoP shows shorter τd than BiVO4, and for both BiVO4 and BiVO4+CoP, the value is decreased with the increase of the bias potential. These results


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demonstrate CoP serves as an efficient OER catalyst, is beneficial for the charge transfer, again supporting the higher photocurrent and lower onset potential.

The stability of the photoanode is also an important issue. The long-term stability test of the four sample photoanodes (BiVO4, BiVO4+CoP, BiVO4+Co3O4 and BiVO4+Co-Pi) was conducted in a borate buffer (pH 9.0) under illumination at a constant potential of 0.7 V versus RHE (Figure 8a). The BiVO4 electrode exhibits a rapid decrease in the photocurrent within 10 min and then keeps stable with approximately 50% of the initial value. A severe photocurrent loss of the BiVO4+Co3O4 and BiVO4+Co-Pi photoanodes are observed, with the photocurrent density retention of 0.7 mA cm-2 and 0.55 mA cm-2 within 4 h, respectively. The introduction of CoP led to improved stability. The photocurrent density of BiVO4+CoP is consistently much higher than that of the other sample photoanodes, maintains at 1.6 mA cm-2 after the 4 h measurement with a slight decay (22% of the initial photocurrent density).


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Figure 8. (a) Amperometric i-t curves of BiVO4, BiVO4+CoP, BiVO4+Co3O4 and BiVO4+Co-Pi photoanodes at a bias of 0.7 V versus RHE. (b) O2 production from water splitting reaction in a PEC cell of BiVO4 and BiVO4+CoP photoanodes measured under AM 1.5G illumination.

To confirm that the photocurrent is caused by the generation of O2, the evolved O2 were detected quantitatively by gas chromatography. As shown in Figure 8b, the O2 evolved at speeds of 13.14 and 1.40 μmol h-1 respectively on the BiVO4+CoP and BiVO4 photoanodes within a 3.5 h irradiation at 0.7 V. For BiVO4+CoP, a high faradaic efficiency of 95% was obtained for oxygen production, indication of an approximate theoretical maximum value based on charge transfer. In contrast, the faradaic efficiency of the bare BiVO4 is only 68%. The CoP significantly improved the faradaic efficiency.


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These results indicate that CoP serving as a water oxidation catalyst adsorbed on the electrode surface can not only enhance the photocurrent, but also selectively increase the water oxidation kinetics via a four-electron transfer process.

CONCLUSIONS

In summary, we present the CoP nanosheet as a highly active and stable OER catalyst integrated on the BiVO4 electrode. The introduction of CoP led to a significant improvement in PEC performance due to the restraint of surface charge recombination and increase in photovoltage. The BiVO4+CoP photoanode exhibited a cathodic shift of the onset potential for water oxidation by more than 220 mV and an impressive photocurrent density of 4.0 mA cm-2 at 1.23 V versus RHE, which is three times that of bare BiVO4 and considerably higher than that of Co3O4 or Co-Pi modified BiVO4. Moreover, the BiVO4+CoP photoanode was stable for long-term operation and could drive the OER with a faradaic efficiency of almost 95%. The CoP considered in this work can serve as a competitive co-catalyst for PEC water oxidation, which can provide insights into the design of an efficient OER catalyst/semiconductor system.


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ASSOCIATED CONTENT

Supporting Information Additional information related to the XRD spectra (Figure S1), TEM and HRTEM images (Figure S2), XPS spectra (Figure. S3-S6), EDS spectra (Figure S7), additional J-V curves (Figure S8-S12), UV-Vis absorption spectra (Figure S13) and IMPS responses (Figure S14).

ACKNOWLEDGEMENTS

This work was supported by the National Natural Science Foundation of China (21671089, 21401092), the Natural Science Foundation of Liaoning Province (LT2017010, 20170540409), the Shenyang Natural Science Foundation (F16-103-400).

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(63) Liu, G.; Ye, S.; Yan, P.; Xiong, F.; Fu, P.; Wang, Z.; Chen, Z.; Shi, J.; Li, C. Enabling an integrated tantalum nitride photoanode to approach the theoretical photocurrent limit for solar water splitting. Energy Environ.Sci. 2016, 9 (4), 1327-1334, DOI 10.1039/c5ee03802b. (64) Kruger, J.; Plass, R.; Gratzel, M.; Cameron, P. J.; Peter, L. M. Charge Transport and Back Reaction in Solid-State Dye-Sensitized Solar Cells: A Study Using IntensityModulated Photovoltage and Photocurrent Spectroscopy. J. Phys. Chem. B, 2003, 107 (31), 7536-7539, DOI 10.1021/jp0348777. Table of Contents (TOC) Graphic

A cobalt phosphide (CoP) catalyst was loaded on porous bismuth vanadate (BiVO4) and the BiVO4+CoP photoanode exhibits remarkably high photoelectrochemical performance.


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TOC/Abstract Graphic

A cobalt phosphide (CoP) catalyst was loaded on porous bismuth vanadate (BiVO4) and the BiVO4+CoP photoanode exhibits remarkably high photoelectrochemical performance.


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Boosting Photoelectrochemical Water Oxidation with Cobalt

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