Efficient photocatalytic fuel cell via simultaneous visible

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Efficient photocatalytic fuel cell via simultaneous visible-photoelectrocatalytic degradation and electricity generation on a porous coral-like WO3/W photoelectrode Donglai Pan, Shuning Xiao, Xiaofeng Chen, Ruping Li, Yingnan Cao, Dieqing Zhang, Sisi Pu, Zhangcheng Li, Guisheng Li, and Hexing Li Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b05685 • Publication Date (Web): 28 Feb 2019 Downloaded from http://pubs.acs.org on March 2, 2019

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Environmental Science & Technology

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Efficient photocatalytic fuel cell via simultaneous visible-

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photoelectrocatalytic degradation and electricity generation on a

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porous coral-like WO3/W photoelectrode

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Donglai Pana, Shuning Xiaob, Xiaofeng Chena, Ruping Lia, Yingnan Caoa, Dieqing

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Zhang a,*, Sisi Pua, Zhangcheng Lia, Guisheng Lia,*, Hexing Lia,c,*

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a. Key Laboratory of Resource Chemistry of Ministry of Education, Shanghai Key

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Laboratory of Rare Earth Functional Materials, College of Life and Environmental

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Science, Shanghai Normal University, Shanghai, 200234, China. E-mail:

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[email protected], [email protected]. [email protected]

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b. International Collaborative Laboratory of 2D Materials for Optoelectronics Science

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and Technology of Ministry of Education, College of Optoelectronic Engineering,

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Shenzhen University, Shenzhen 518060, China.

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c. Shanghai University of Electric Power, 2588 Changyang Rd., Shanghai 200090, China

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ABSTRACT: Photocatalytic fuel cells (PFCs) have proven to be effective for

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generating electricity and degrading pollutants with a goal to resolve environmental and

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energy problems. However, the degradation of persistent organic pollutants (POPs),

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such as perfluorooctanoic acid (PFOA), remains challenging. In the present work, a

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porous coral-like WO3/W (PCW) photoelectrode with a well-designed energy band

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structure was used for the photoelectrocatalytic degradation of POPs and the

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simultaneous generation of electricity. The as-constructed bionic porous coral-like

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nanostructure greatly improved the light-harvesting capacity of the PCW


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photoelectrode. A maximum photocurrent density (0.31 mA/cm2) under visible light (λ

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> 420 nm) irradiation and a high incident photon conversion efficiency (IPCE) value

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(5.72 % at 420 nm) were achieved. Owing to the unique porous coral-like structure, the

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suitable energy band position, and the strong oxidation ability, this PCW

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photoelectrode-based PFC system exhibited a strong ability for simultaneous

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photoelectrocatalytic degradation of PFOA and electricity generation under visible

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light irradiation, with a power output of 0.0013 mV/cm2 using PFOA as the fuel. This

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work provides a promising way to construct a reliable PFC using highly toxic POPs to

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generate electricity.

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Introduction

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Persistent organic pollutants (POPs) are harmful to the natural environment and

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human health.1-7 Perfluorooctanoic acid (PFOA) is recognized as the most stable and

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toxic POP in nature due to its strong C-F chemical bond with an energy of 485 kJ/mol.8

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This strong chemical bond means that the PFOA molecules are difficult to break down

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by traditional methods, such as chemical oxidation, reverse osmosis and

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nanofiltration.9-11 Recently, it was reported that PFOA could be degraded by

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photocatalysis with the assistance of the persulfate ion (S2O82-) or the ferric ion as the

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Fenton reagent.12-16 However, the applied photocatalytic process can only be driven by

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ultraviolet light and suffers from low efficiency due to the high recombination rate of

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photoelectron-hole pairs.17-21

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Compared to photocatalysis, photoelectrocatalysis (PEC) has been used as an effective

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way to remove organic pollutants and produce H2 due to the rapid separation of

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photoelectrons from holes under the bias voltage.22-35 The photogenerated electrons can

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be rapidly transferred to the cathode to form a photocurrent or drive reduction reactions



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such as H2 generation and CO2 reduction, 36-41 and the holes left on the anode are used

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for oxidative degradation of POPs.42-45 Thus, a new photocatalytic fuel cell (PFC) based

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on the entire process can be constructed for simultaneous electricity generation and

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pollutant degradation.46-49

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To construct a PFC that can decompose PFOA with a strong C-F bond, a highly

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positive valence band (VB) position for the photoanodic semiconductor is needed for a

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strong oxidizing capacity. A narrow band gap is also required for efficient utilization

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of longer wavelength light in sunlight. Considering these two aspects, tungsten trioxide

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(WO3) is selected as an ideal candidate as an n-type semiconductor.27, 50-57

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Corals living under the sea have a unique dendrite morphology and a high sunlight

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utilization efficiency. Inspired by this ability, porous coral-like structured

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nanomaterials are desired to gain better light-harvesting properties. In this work, a

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WO3/W photoelectrode with a porous coral-like nanostructure was prepared, and it

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displayed high photoelectrochemical properties and an enhanced performance for PEC

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degradation of POPs such as PFOA. The conduction band (CB) position of WO3 is

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above -0.33 V, and the highly positive CB prevents the photogenerated electrons from

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being trapped by O2, meaning that more electrons can be used to generate electricity.

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Therefore, WO3 served as the photoanode in the PEC system to construct the PFC

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(Scheme 1).

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Scheme 1. Illustration of a photocatalytic fuel cell.


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Experiment

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Preparation of samples: Porous coral-like WO3/W photoelectrodes (PCW) were

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prepared by constant potential electrolysis in a two-electrode system, in which a

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polished tungsten foil (33 mm × 20 mm, 0.2 mm thick) and a platinum foil (20 mm ×

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20 mm) served as the anode and cathode, respectively. A 50 mL aqueous solution

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containing 0.2 mol/L NH4F and 0.1 mol/L H3PO4 was used as the electrolyte. The

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process of anodic oxidation was performed under a controlled constant voltage from

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DC 30 V to DC 80 V at 0 °C for 2.5 h. The as-obtained electrodes were then air-calcined

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at 350, 450, 550 and 650°C for 3 h. The calcined electrodes were then named PCW

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350, PCW 450, PCW 550 and PCW 650, respectively. Commercial WO3 (J&K Co. Ltd.)

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loaded on W foil was also fabricated through the doctor-blade method reported

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previously58 and then calcined at 550°C for 3 h. The resulting electrode was named

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CW. The preparation of WNT, CdS NWs and BiVO4 photoelectrodes could be found

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in the supporting information (SI Section S1-S3).

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Characterization: X-ray diffraction (XRD) patterns were collected in parallel mode

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(2θ from 20 to 40°) using a Rigaku Dmax-3C Advance X-ray diffractometer (Cu Kα

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radiation, λ= 1.5406 Å). The morphologies were studied through field emission

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scanning electron microscopy (FESEM, HITACHI S-4800) and transmission electron

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microscopy (TEM, JEOL, 2010F). Diffuse reflectance spectra over a range of 200-800

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nm were obtained through a UV-vis spectrophotometer (DRS, MC-2530) equipped

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with an integrating sphere assembly using BaSO4 as a reference. The electron

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paramagnetic resonance (EPR) measurements were conducted on a Bruker EMX-8/2.7

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Electro-Spin Resonance Spectrometer (X-band).



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Photoelectrochemical measurement: All of the photoelectrochemical measurements

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were performed on an electrochemical workstation (CHI660E). A standard three-

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electrode setup was established using porous coral-like WO3/W (PCW, 4 cm2) as the

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working electrode, a commercial platinum foil (99.99 %, 4 cm2) as the counter electrode,

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and a Ag/AgCl electrode as the reference electrode. A 300 W Xenon lamp was used as

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the visible light source with a cut off of λ < 420 nm. The photocurrent density was

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detected in an aqueous solution containing 0.5 mol/L Na2SO4 by linear sweep

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voltammetry (LSV). Electrochemical impedance spectroscopy (EIS) analyses were

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recorded in the same solution at open-circuit voltage over the frequency range from 106

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to 10-2 Hz with an AC voltage of 5 mV. The Motto-Schottky plots were obtained at a

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fixed frequency of 1 kHz to determine the flat-band potential. The electrochemical

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surface areas of different photoelectrodes were obtained by cyclic voltammetry (CV)

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measurements in different scan rates to estimate electrochemical double layer (SI

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Section S4). The incident photon conversion efficiency (IPCE) measurements were

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carried out on the electrochemical workstation (ZAHNER-elektrik) at a potential of 0.5

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V (vs. Ag/AgCl) under irradiation with λ ranging from 367 to 720 nm.

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Photoelectrocatalytic activity test: A H-cell was used as the reactor for PEC

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degradation of organic pollutants. In the anode, a 0.5 mol/L Na2SO4 aqueous solution

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containing 5 ppm methyl orange (MO), rhodamine B (RhB), methylene blue (MB), 4-

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chlorophenol (4-CP), bisphenol A (BPA) or PFOA was used as the electrolyte. An

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aqueous solution containing 0.5 mol/L Na2SO4 was used in the cathode. The H-cell was

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separated by a proton exchange membrane (Nafion 117). Before light irradiation, the

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working electrode was immersed in the reaction solution in the dark for 20 min to reach

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adsorption-desorption equilibrium. The cathode chamber electrolyte was purged with

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nitrogen (99.999%) to remove dissolved O2. During the photoelectrocatalytic process,


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a constant potential (0.5 V vs. Ag/AgCl) was applied on the working electrode with a

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constant magnetic stirring rate (600 rpm). A 300 W Xenon lamp was used as the visible

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light source with a cut off of λ < 420 nm. All reactions lasted for 2 h. For comparison,

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both the electrocatalytic (EC) degradation and the photocatalytic (PC) degradation of

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organic pollutants were performed under the same conditions without light irradiation

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or bias voltage, respectively. The concentration of MO, RhB, MB or 4-CP was

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determined by measuring the UV absorbance at the characteristic wavelengths of 465,

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564, 665 and 275 nm, respectively. The concentration of BPA was determined using an

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HPLC system (UltiMate 3000 Thermo Fisher, phenyl column at 25 °C, a 1:1 v/v

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water/methanol solution as the mobile phase, flow rate = 1.0 mL/min, detection

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wavelength = 230 nm). For the degradation of PFOA, the resulting F- concentration

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was determined using ion chromatography (DIONEX ICS-5000 Thermo Scientific),

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and the degradation yield was calculated based on the following equation:16

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PFOA defluorination ratio = CF-/(C0 × 15)

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where CF- is the concentration of fluoride ion and C0 is the initial concentration of

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PFOA. The factor of 15 is ascribed to the number of fluorine atoms in each PFOA

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molecule. Similarly, the 4-CP degradation yield could also be calculated based on the

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resulting F- concentration according to the following equation:

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4-CP dechlorination ratio = CCl-/C0

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In this equation, we used a factor of 1 instead of 15 since each 4-CP molecule contains

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only one chlorine atom.

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Photocatalytic fuel cell test: The PFC efficiency was tested in an electrochemical

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single cell with a two-electrode system using an electrochemical workstation



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(CHI660E). The as-prepared PCW electrode was used as the working electrode, and

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the Pt foil was used as the auxiliary and reference electrode. Both electrodes possessed

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the same working area of 4 cm2. The distance between those two electrodes was fixed

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at approximately 4 cm. A 300 W Xenon lamp was used as the visible light source by

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cutting off all the lights with λ < 420 nm. Various organic pollutants (MO, RhB, MB,

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4-CP, BPA, and PFOA) and methanol were used as fuel resources, respectively. An

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aqueous solution containing 0.5 mol/L Na2SO4 served as the electrolyte. The current-

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voltage (J-V) plots were recorded to characterize the PFC performance with a scan rate

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of 1 mV/s.

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Results and discussion

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The oxidation voltage played a key role in fabricating the PCW photoelectrode

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through the anodic oxidation method. Under a low oxidation voltage of DC 30 V, WO3

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growing on the W foil displayed crystal nanoparticles (Figure 1a). When the anodic

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voltage increased to DC 50 V, a porous coral-like nanostructured WO3 framework was

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obtained (Figure 1b). Further increasing the anodic oxidation voltage resulted in the

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collapse of the porous structure and the formation of large bumps with high thickness

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(Figure 1c). High-resolution transmission electron microscopy (HRTEM) of PCW 550

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showed two characteristic lattice spacings of 0.383 and 0.365 nm ascribed to the (002)

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and the (200) lattice planes, respectively, of monoclinic WO3 (Figure 1d). The XRD

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patterns (Figure S1a) also confirmed that the WO3 crystallization degree increased with

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increasing anodic voltage. The UV-vis diffuse reflectance spectroscopy (DRS) results

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(Figure S1b) demonstrated that the PCW photoelectrode prepared at 50 V DC exhibited

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the strongest light absorption intensity from 200 to 800 nm. Fixing the DC voltage at

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50 V, the porous coral-like WO3/W photoelectrode was easily obtained, and the


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thickness of the WO3 layers could be adjusted by increasing the anodic oxidation time

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from 0.5 to 2.5 h, as determined by the SEM images (Figure S2) and XRD patterns

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(Figure S3).

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Figure 1. SEM images of WO3/W photoelectrodes calcined at 550 °C for 3 h and

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treated with the anodic oxidation voltage of (a) 30 V, (b) 50 V and (c) 80 V; (d) HRTEM

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image of PCW.

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The SEM images of PCW 550 (Figure S4) revealed that the porous coral-like

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morphology was preserved after being calcined for 3 h at temperatures below 550°C.

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However, the porous coral-like nanostructure of WO3/W collapsed after being calcined

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at 650°C, which occurred along with the serious aggregation of the WO3 pore wall. As

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shown in Figure 2a, the PCW 550 calcined at 550°C for 3 h exhibited the strongest

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visible light absorption compared with those obtained at other calcination temperatures

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from 350 to 650 °C. The cross-section SEM image (Figure S5) confirmed that the

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porous coral-like WO3 was directly grown on the metal tungsten substrate with a

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thickness of 500 nm. These results demonstrated that the WO3 layer strongly interacted

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with the tungsten substrate. This strong interaction could reduce the Schottky contact

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between the WO3 and the W foil.59, 60 As shown in Figure 2b, no significant diffraction

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peaks in the XRD patterns were found that indicated the presence of the crystal WO3

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phase on the PCW photoelectrode before calcination. However, all the PCW

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photoelectrodes calcined from 350 to 650 °C showed a stable monoclinic structure



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(PDF#43-1035) of WO361, 62 63, and the WO3 crystallization degree increased gradually

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with increasing calcination temperature.64

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Figure 2. (a) UV-vis DRS spectra and (b) XRD patterns of PCW calcined at different

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temperatures.

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Figure 3a shows the photocurrent density-potential curves of different PCW

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photoelectrodes under visible light irradiation. PCW 550 exhibited the highest

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photocurrent density of 0.31 mA/cm2 compared to PCW 350, PCW 450, PCW 650 and

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CW, with corresponding photocurrent densities of approximately 0.20, 0.18, 0.12 and

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0.08 mA/cm2 at 0.5 V (vs. Ag/AgCl). With increasing applied potential, the

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photocurrent density first increased and then gradually stabilized with densities of 0.38,

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0.57, 0.37 and 0.35 mA/cm2 for PCW 650, 550, 450 and 350 at 1.2 V (vs. Ag/AgCl),

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respectively. PCW 550 also exhibited the highest IPCE under light irradiation with

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wavelengths from 367 to 720 nm, and this result could be ascribed to the unique porous

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structure favorable for light harvesting (Figure 3b). Moreover, the EIS spectra in Figure

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3c revealed that PCW 550 displayed the smallest impedance arc diameter,

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corresponding to the lowest charge-transfer impedance.65 In other words, PCW 550


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exhibited the fastest rate of electron transfer, which could efficiently reduce the

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electron-hole recombination rate.

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Figure 3. Photocurrent density-potential curves (a) IPCE measurements (b) and EIS

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Nyquist plots (c) of all the PCW photoelectrodes calcined at different temperatures.

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The PEC activity was evaluated by the degradation of MO in an aqueous solution

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upon reaching an adsorption-desorption equilibrium in 20 min (Figure S6). As shown

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in Figure 4a, PCW 550 exhibited the highest MO degradation rate of ~68.0 % under

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visible light irradiation for 2 h, which is nearly 2 times higher than that of the CW

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electrode. Compared to the PCW electrodes calcined below 550 °C, PCW 550

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presented a higher crystallization degree of WO3, which can facilitate electron transfer

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(see Figure 3c) and thus diminish electron-hole recombination. PCW 650, with the



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highest crystallization degree of WO3, exhibited a lower PEC activity than that of PCW

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550, and this result can be ascribed to the collapse of the nanostructure and the reduced

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light absorption (see Figure 3a). In addition, the aggregation of WO3 also increased the

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impedance (see Figure 3c), which was unfavorable for electron transfer and enhanced

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electron-hole recombination. For better illustrating the advantages of porous coral-like

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structure, WO3/W photoelectrodes with other different morphologies were also

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investigated in PEC degradation process (Figure S7). The results displayed that PCW

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photoelectrode had a higher MO degradation efficiency than commercial WO3 (CW)

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and nanotube-like WO3 (WNT) photoelectrodes, which could be contributed to the

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higher adsorption capacity and surface area of PCW photoelectrode. The surface areas

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of different samples were further measured based on the electrochemical surface area

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(ESA) obtained via the electrochemical measurements and calculations. As shown in

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Figure S8 and Table S1, PCW 550 photoelectrodes displayed a higher surface area than

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both CW and WNT photoelectrodes. The decrease of electrochemical surface area from

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PCW 350 to PCW 650 was attributed to the aggregation to the WO3 crystals during the

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calcination treatment. Nevertheless, PCW 550 still possessed the highest activity owing

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to its highly crystalline degree of WO3 and higher positive valence band. Therefore,

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based on these results, it indicated that such unique porous coral-like nano-structure

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could improve both the light and chemical absorption performances of WO3

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photoelectrode. Meanwhile, PFOA was electronegative with a zeta potential of -4.83

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mV (Figure S9), thus negatively-charged PFOA molecules could be easily trapped by

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the PCW photoanode via electrostatic attraction. The recombination rate of photo-

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induced charge carriers can be effectively suppressed in the PEC process under the

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external circuit and bias potential, possibly accounting for the higher pollutant

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degradation activity of PEC than that of the PC and EC processes (Figure 4b). With the


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assistance of the bias voltage, the photogenerated electron could be transferred to the

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Pt cathode for the H2 evolution reaction, leading to enhanced activity. PCW 550

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displayed the highest H2 evolution amount of 32.1 μmol within 2 h compared to that of

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PCW 350, PCW 450, PCW 650 and CW, with H2 evolution amounts of 21.0, 29.5, 31.6

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and 7.7 μmol, respectively (see Figure S10). The H2 evolution trend was also consistent

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with the trend of the PEC degradation rates of MO, as shown in Figure 4a. The influence

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of the initial MO concentration on the degradation efficiency and the H2 evolution rate

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was also investigated during the PEC process (Figure S11, S12). The MO degradation

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rate and the H2 evolution rate increased upon decreasing the concentration of MO,

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which was in accordance with the Langmuir-Hinshelwood model.66-68 The total organic

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carbon (TOC) value showed a similar trend to the trend of MO degradation yield in the

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PEC process (Table S2). Furthermore, PCW 550 exhibited a high stability without an

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obvious decrease in activity after 5 cycles (Figure S13). The long-time response of

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current density were investigated to confirm the stability of PCW 550. As shown in

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Figure S14, the current density presented no obvious decline even after 40 h, suggesting

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an excellent stability of PCW 550.



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Figure 4. (a) The PEC degradation efficiency for a 5 ppm MO solution using different

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photoelectrodes; (b) the MO degradation efficiency in PEC, PC and EC processes with

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PCW 550; (c) the active species trapping experiments for the PEC degradation process;

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and (d) the relationship of Eg and the VB value for the PCW photoelectrodes.

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Active species trapping experiments were conducted to better understand the

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degradation mechanism of the PEC process. As shown in Figure 4c, the degradation

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efficiency of MO was greatly inhibited when 2.5 % methanol (ME) was added as the

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photogenerated hole (h+) scavenger. Meanwhile, the degradation efficiency also

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decreased abruptly after adding 2.5 % isopropyl alcohol (IPA) as the hydroxyl radical

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(HO•) scavenger. We then concluded that the HO• radical acted as the major active

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species during the PEC degradation process based on these results. The photogenerated

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electrons were transferred to the counter electrode under the bias voltage, which could

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inhibit the formation of •O2- for further oxidizing organic molecules on the photoanode.


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Generally, the generation of HO• was dependent on the position of the VB of the

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semiconductor. According to the UV-vis DRS results, the band gaps of PCW 350-650

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were determined to be 2.5, 2.4, 2.4, and 2.3 eV (Figure S15 inset graph). Based on the

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Motto-Schottky plots (Figure S15), the positions of the CBs of the PCW 350-650

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samples were calculated to be approximately 0.50, 0.48, 0.40 and 0.32 V (vs. NHE).

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Thus, the VB values of PCW 350-650 could be estimated as 3.0, 2.9, 2.8 and 2.6 V (vs.

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NHE). The band gap (Eg) and the VB position were inversely related to the calcination

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temperature, as described in Figure 4d. Therefore, a suitable balance of Eg and the VB

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position could be pursued to realize both enhanced visible light harvesting and

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oxidation ability. BiVO4 and CdS, as widely used visible-light photocatalysts, were also

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used as references for treating 5 ppm MO in the PEC system as shown in Figure S7.

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Both CdS and BiVO4 exhibited a lower MO removal efficiency than PCW 550, due to

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the lower valence bandposition.69 It further illustrated the advantages of energy band

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structure for PCW photoelectrode. Among the PCW photoelectrodes, PCW 550

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exhibited an excellent visible light response, which generated more photoelectrons and

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holes. The photoelectrons transfer to the counter electrode for H2 production via water

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splitting (Figure S16). Due to the high oxidation potential, the photogenerated holes

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left on the photoelectrodes could directly oxidize organic pollutants into CO2 and H2O.

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In addition, partial holes could also oxidize H2O to ·OH radical (E0(H2O/·OH) = 2.4 V

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vs. NHE), which could further accelerate the oxidative degradation of organic

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pollutants.70, 71 The overall PEC reaction process is described as follows:

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1) WO3 + hv → h+ + e-

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2) H2O → H+ + OH-

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3) h+ + OH- → HO•



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4) h+ + H2O → HO• + H+

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5) h+ + organic pollutant → CO2 + H2O

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6) HO• + organic pollutant → CO2 + H2O

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The degradation efficiency of organic pollutants on PCW 550 was also evaluated using

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5 ppm of RhB, MB, 4-CP, BPA and PFOA in the PEC, PC and EC processes (Figure

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5). Meanwhile, the changes in the TOC value and H2 evolution amount within the 2 h

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reaction were also detected (Table S3). For MB and RhB, the PEC process showed a

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greatly improved performance in both the degradation efficiency and the H2 evolution

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rate compared to the PC and EC processes (Figure 5a and b), and this result could be

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attributed to the rapid separation of photoelectrons from holes in the presence of a bias

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voltage. For POPs (4-CP, BPA and PFOA), the degradation performances were similar

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to the dye-degradation process, as shown in Figure 5c-f and Table S3. The PEC

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degradation rates of 4-CP and BPA can reach 19.2 % and 57.1 %, respectively. In

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addition, the dechlorination ratio of 4-CP and the defluorination ratio of PFOA were

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44.2 % and 3.4 %, respectively, after 2 h of reaction. It is suggested that the degradation

308

of PFOA in a PEC system using a PCW photoelectrode could be achieved under visible

309

light irradiation.72-74 The kinetic study of the pollutants-degradation process on PCW

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550 (Figure S17) indicated that the degradation process was a first-order reaction. More

311

experiments including active species trapping, recyclability for treating PFOA solution

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with various concentrations were studied to investigate the PEC degradation process of

313

PCW 550 photoelectrode. As shown in Figure S18a, the defluorination ratio declined

314

by using ME as the holes scavenger, indicating that the photogenerated holes were the

315

main activity species during the PFOA degradation process. Such slight decrease of

316

PFOA degradation rate in the presence of ME could be ascribed to strong electrostatic


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adsorption effect of PFOA molecules on the WO3 electrodes, thus, PFOA was still

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easily trapped by the electrodes for further degradation. The EPR analysis also

319

presented four feature signals (hfsc αN =αH = 14.95 G) of DMPO-OH adducts in the

320

Na2SO4 electrolyte (Figure S18b), providing a direct evidence for the generation of ·OH

321

radical in the PEC degradation process. PFOA was considered one of the most difficult

322

pollutants to degrade, the high oxidation ability of photogenerated holes for PCW 550

323

could still effectively break the stable C-F bond on the surface of WO3 during the PEC

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process. The repeated tests also verified that the as-prepared PCW 550 photoelectrode

325

exhibited an excellent stability (Figure S18c). Besides, the PEC performance for

326

degrading PFOA with various concentrations (Figure S18d) suggested that the as-

327

formed PEC system was effective to degrade PFOA with a Langmuir-Hinshelwood

328

model. The above degradation results indicated that the PCW photoelectrode exhibited

329

excellent universality for pollutant degradation and H2 generation in the PEC process

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under visible light irradiation.

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Figure 5. The degradation rates for various organic pollutants in the PEC, PC and EC

333

processes: (a) RhB, (b) MB, (c) BPA and (d) 4-CP; dechlorination ratio of 4-CP (e) and

334

defluorination ratio of PFOA (f).

335

During the H2 generation process, the photogenerated electron could be transferred to

336

the cathode through an external circuit to construct a PFC system.75 The current-voltage

337

(J-V) plots and the current-power (J-JV) plots from the PCW electrodes are shown in

338

Figure 6. The open-circuit voltage (VOC), short-circuit current density (JSC) and fill

339

factor (ff) of the cell are listed in Table 1. The ff was calculated by the following

340

equation, which could directly reflect the performance of the PFC system:76

341

ff = J*Vmax/(Jsc*Voc)

342

where J*Vmax is the maximum power density of the cell obtained from the J-JV plots.

343

As shown in Figures 6a and 6b, the JSC was 0.0279 mA/cm2 in the presence of MO

344

using PCW 550 as the photoelectrode under visible light irradiation (λ > 420 nm).

345

Under the same conditions, the JSC values were approximately 0.0242, 0.0279, 0.0157,

346

and 0.0143 mA/cm2 for PCW 350, PCW 450, PCW 650 and CW photoelectrodes,

347

respectively. The ff value reached 0.213 for PCW 550 (Table 1), which was

348

significantly larger than those of the other photoelectrodes. PCW 550 also exhibited the

349

highest JVmax (0.003 mW/cm2) at JSC and VOC values of 0.0279 mA/cm2 and 0.505 V,

350

respectively. These excellent performances for PCW550 were ascribed to the suitable

351

Eg and VB position of PCW 550, which can enhance visible light absorption and

352

oxidation ability (see Figure 4d). In addition, the bionic porous structure of the PCW

353

photoelectrodes also improved the light-harvesting capacity over that of CW, thus

354

improving the PFC performance. However, the power output and the JSC of all the PCW

355

photoelectrodes decreased dramatically in the dark (Figure S19 and Table 2), which


Page 18 of 32

Page 19 of 32


356

clearly confirmed the characteristics of the PFC. A series of organic molecules were

357

chosen as the fuel to obtain the J-V and J-JV plots to further investigate the PFC

358

performance of the PCW 550-Pt foil system. As shown in Figure 6c, surprisingly, the

359

Voc values in the presence of a POP (4-CP, BPA or PFOA) are higher than that in

360

methanol (0.180 V), which is commercially used in fuel cells. These higher Voc values

361

suggested that the as-formed PCW 550-Pt PFC system may possess a high power output

362

(JVmax) by using POPs as the fuel (Figure 6d). Methanol is much easier to oxidize

363

than POPs, and the Jsc value in a methanol solution should be much higher than those

364

obtained in a POP solution.75 Nevertheless, comparative Jsc values could still be

365

produced by choosing POPs as the fuel in the PCW 550-Pt PFC system (Table 1). Even

366

in a PFOA solution, a JSC of 0.0177 mA/cm2 was maintained, although the strong C-F

367

bond was very difficult to oxidize. These results suggested that the high oxidation

368

ability of PCW 550 could be used to directly oxidize the PFOA molecule to generate

369

electricity. The positive CB position of PCW 550 also prevented the reaction between

370

the photogenerated electrons and O2 to form •O2- (Fig S16); thus, more electrons could

371

be transferred to the external circuit to maintain the Jsc values. Nevertheless, the ff

372

values in the presence of POPs were slightly lower than those in the presence of

373

methanol. These results could be mainly ascribed to the slow oxidization process of

374

POPs under visible light irradiation. The decreased TOC values of different organic

375

pollutant solutions in the PFC process by using PCW 550 (Table S4), indicating that

376

organic pollutants could be used as the fuels to generate electricity during the PFC

377

process. These results confirmed that the as-formed PFC system could allow POPs to

378

be oxidized on the porous coral-like WO3/W photoanode; thus, more electrons could

379

be transferred to the Pt cathode to generate a higher photocurrent under visible light

380

irradiation. Moreover, the current-time plots under different output voltages in 5 ppm



381

PFOA solution illustrated that the power output increased with the increasing of voltage

382

window in the PFC system (Figure S20a). And the stability of PCW photoelectrode

383

under acidic condition also could be verified by the long-time output tests (Figure S20b),

384

which showed that the PCW photoelectrode could maintain a stabilized power output

385

even in the acidic condition.

386 387

Figure 6. J-V (a) and J-JV (b) plots using different photoelectrodes with 5 ppm MO in

388

the PFC system. J-V (c) and J-JV (d) plots on PCW 550 photoelectrodes with 5 ppm of

389

different organic pollutants or 2.5 % of methanol. All tests were performed under

390

visible light irradiation in an aqueous solution containing 0.5 mol/L Na2SO4.

391

Table 1. Current-voltage characteristics from different photoelectrodes with 5 ppm of

392

different organic pollutants or 2.5 % methanol in a PFC system under visible light

393

irradiation or in the dark.


Page 20 of 32

Page 21 of 32


Photoelectrode Jsc/mA*cm-2

Light source

Fuel

Voc/V

ff

Visible light

No fuel

PCW 550

0.0154

0.375

0.196

Visible light

4-CP

PCW 550

0.0198

0.451

0.156

Visible light

BPA

PCW 550

0.0187

0.441

0.154

Visible light

MB

PCW 550

0.0179

0.454

0.196

Visible light

Methanol

PCW 550

0.0186

0.180

0.252

Visible light

PFOA

PCW 550

0.0177

0.507

0.146

Visible light

MO

CW

0.0143

0.285

0.203

Visible light

MO

PCW 350

0.0242

0.518

0.183

Visible light

MO

PCW 450

0.0279

0.493

0.152

Visible light

MO

PCW 550

0.0279

0.505

0.213

Visible light

MO

PCW 650

0.0157

0.471

0.162

Dark

MO

CW

0.0124

0.282

0.208

Dark

MO

PCW 350

0.0184

0.496

0.192

Dark

MO

PCW 450

0.0252

0.491

0.162



394

Page 22 of 32

Dark

MO

PCW 550

0.0184

0.504

0.237

Dark

MO

PCW 650

0.0074

0.432

0.165

All tests were performed in an aqueous solution containing 0.5 mol/L Na2SO4.

395 396

In summary, a novel WO3/W photoelectrode was developed to construct a PFC by

397

using various organic pollutants as the fuel under visible light irradiation. The prepared

398

photoelectrode displayed excellent performance for degrading POPs via visible light-

399

driven photoelectrocatalysis and showed a powerful ability to generate electricity due

400

to its unique porous coral-like nano-structure and well-designed energy band structure.

401

This work provides a new approach to fabricate functional photoelectrodes and may

402

offer more opportunities for designing PFC systems.

403

ASSOCIATED CONTENT

404

Supporting Information

405

The Supporting Information is available free of charge on the ACS Publication website at DOI:

406

10.1021/acs.est.

407

The preparation of nanotube-like WO3/W photoelectrode (WNT); The preparation of CdS

408

nanowires (CdS NWs) photoelectrode; The preparation of BiVO4 photoelectrode; The

409

measurement of electrochemical surface area (ESA); The standard potential conversion; Figure

410

with XRD patterns and UV-vis DRS spectra; SEM images of the as-obtained PCW

411

photoelectrodes; XRD patterns of PCW under different anodic oxidation times; SEM images

412

of the as-prepared PCW calcined at different temperatures; The cross-section SEM image of

413

PCW 550; The adsorption-desorption equilibrium for different photoelectrodes in 5 ppm


Page 23 of 32


414

MO solution; The PEC degradation tests for different photoelectrodes at 0.5 V (vs.

415

Ag/AgCl) under the visible-light irradiation (300 W Xe lamp with a 420 nm cutoff

416

filter), the insect graphs of the morphologies for different photoelectrodes;

417

Electrochemical surface area measurements for different photoelectrodes; Zeta

418

potential of PFOA solution (5 ppm); The amount of H2 evolution during the process of

419

the PEC degradation of 5 ppm MO using different photoelectrodes; PEC degradation

420

efficiency of PCW 550 with different initial concentrations; The amount of H2

421

evolution in the PEC degradation process with different concentrations of MO by using

422

PCW 550; The cyclic measurements for the PCW 550 photoelectrode; The

423

photocurrent stability tests for PCW 550 in a 0.5 M Na2SO4 solution in the absence of

424

organic pollutant; Motto-Schottky plots and UV-vis DRS spectrum for the as-prepared

425

PCW photoelectrode; The proposed mechanism for the PEC pollutant degradation

426

process; The kinetics curves and constants for the PEC degradation process in different

427

pollutant solution using PCW 550 as the photoelectrode; PEC degradation of PFOA

428

solution: (a) The active species trapping experiments using different scavengers; (b)

429

EPR measurements for the DMPO-OH adducts; (c) The recyclability for the PCW 550

430

photoelectrode; (d) PEC degradation for different PFOA concentrations; J-V plots and

431

J-JV plots for both CW and PCW 550; The current-time plots at different output voltage

432

in a 5 ppm PFOA solution (a), The stability of PCW photoelectrode in acidic condition

433

(b); Table with Electrochemical surface areas of different photoelectrodes; The changes

434

of total organic carbon (TOC) for the MO degradation process after 2 h reaction with

435

the different photoelectrodes; The changes of total organic carbon (TOC) and the

436

amounts of H2 evolution for different organic degradation processes after 2 h reaction

437

time with the same photoelectrode; The TOC changes of different pollutants during the

438

PFC process by using PCW 550 as the photoelectrode for 4 h at 0.1 V.



439

AUTHOR INFORMATION

440

Corresponding Author

441

* [email protected], [email protected], [email protected]

442

Notes

443

The authors declare no competing financial interest.

444

ACKNOWLEDGMENTS

445

This work was supported by the National Natural Science Foundation of China

446

(21207090, 21477079, 21876113, 21261140333, 21876112, 21803038), PCSIRT

447

(IRT1269), the China Postdoctoral Science Foundation (2018M630981) and a scheme

448

administrated by Shanghai Normal University (DXL122, and S30406).

449

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Fe2O3–MoS2–Cu2O

ternary

nanofilm

with

689 690 691 692 693 694 695 696 697 698


significantly

enhanced


699

TOC Art

700


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Efficient photocatalytic fuel cell via simultaneous visible

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