Pollutants degradation and power generation by photocatalytic fuel cells: A comprehensive review

Yasser Vasseghian; Alireza Khataee; Elena-Niculina Dragoi; Masoud Moradi; Samaneh Nabavifard; Gea Oliveri Conti; Amin Mousavi Khaneghah

doi:10.1016/j.arabjc.2020.07.016

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Review article

13 (

); 8458-8480

doi:

10.1016/j.arabjc.2020.07.016

Pollutants degradation and power generation by photocatalytic fuel cells: A comprehensive review

Yasser Vasseghian^{^a}, Alireza Khataee^{^a,^b}, Elena-Niculina Dragoi^{^c}, Masoud Moradi^{^d,⁎⁎}, Samaneh Nabavifard^{^e}, Gea Oliveri Conti^{^f}, Amin Mousavi Khaneghah^{^g,⁎}

a Institute of Research and Development, Duy Tan University, Da Nang 550000, Vietnam

b Research Laboratory of Advanced Water and Wastewater Treatment Processes, Department of Applied Chemistry, Faculty of Chemistry, University of Tabriz, 51666-16471 Tabriz, Iran

c Faculty of Chemical Engineering and Environmental Protection “Cristofor Simionescu”, “Gheorghe Asachi” Technical University, Iasi, Bld Mangeron No 73, 700050, Romania

d Research Center for Environmental Determinants of Health (RCEDH), Health Institute, Kermanshah University of Medical Sciences, Kermanshah, Iran

e Police Sciences and Social Studies Institute, Tehran, Iran

f Environmental and Food Hygiene Laboratories (LIAA), Department of Medical, Surgical Sciences and Advanced Technologies “G.F. Ingrassia”, Hygiene and Public Health, University of Catania, Via S. Sofia 87, 95123 Catania, Italy

g Department of Food Science, Faculty of Food Engineering, State University of Campinas (UNICAMP), 13083-862 Campinas, São Paulo, Brazil

⁎Corresponding author. mousavi@unicamp.br (Amin Mousavi Khaneghah)

⁎⁎Co-corresponding author. mahfooz60@gamil.com (Masoud Moradi),

Received: 2020-5-26, Accepted: 2020-7-27, Published: 2020-11-01

Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University.

Abstract

Schematic of pollutants degradation in PFC.

Abstract

Wastewater contains organic compounds (fatty acids, amino acids, and carbohydrates) that have a significant amount of chemical energy. In this regard, the use of wastewater for recovering energy by some appropriate energy conversion technologies can be considered as an appropriate approach to simultaneously achieve the reduction of environmental contamination and increasing supply of energy. The Photocatalytic Fuel Cell (PFC) can provide a new approach in developing technology for simultaneous organic pollutants removal from wastewaters and power generation, but it also has disadvantages, such as requires higher voltage, more cost and complexity. To present a comprehensive vision of the current state of the art, and progress the treatment efficiency and agitate new studies in these fields, this review discussed the study covering PFC aspects, with a focus on the comparison of pollutant degradation, power generation, different photoanode and photocathode materials as well as the application of the Fenton process in PFCs.

Keywords

Photocatalytic fuel cells

Pollutant degradation

Power generation

Photocatalysis

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1 Introduction

Conventional water and wastewater treatment processes are not effective in removing and/or degrading a number of pollutants, as well as, some efficient treatment techniques e.g. reverse osmosis (RO) usually require high energy consumption for purification (for example, 5–100 kwh/kgCOD) (Li et al., 2019). An efficient solution to overcome this challenge is to add an additional unit of advanced oxidation processes (AOPs) to conventional water treatment equipment (Liu et al., 2019; Liu et al., 2017). AOPs are widely used to decompose several organic substances to highly reactive chemicals (Khatri et al., 2018; Vasseghian and Dragoi, 2018; Vasseghian et al., 2020 ). Among them, semiconductor-based photocatalytic processes have been widely welcomed as an efficient option due to their eligible properties such as high efficiency, low cost, high stability, low toxicity and chemical inertness (Sarac et al., 2020; Wang et al., 2019). The low level of rich chemical energy obtained during the process of elimination from organic decomposition is one of the main disadvantages of most AOPs (Jiang et al., 2018; Pirsaheb et al., 2019; Wang et al., 2019 ). On the other hand, the organic compounds in sewerage can be used as sources of carbon and energy that can be applied. Thus, It is essential to extend sustainable methods that can degrade these organic contaminants and synchronous recover their chemical energy (Li et al., 2019; Li et al., 2017). The Photocatalytic Fuel Cell (PFC) is a successful scheme for wastewater treatment and synchronous recovery of chemical energy from the wastewater (Lee et al., 2018). The non-selective properties of the photocatalyst lead to the degradation of an extensive range of pollutants such as dyes (Nordin et al., 2019; Lee et al., 2018; Ong et al., 2019; Khalik et al., 2017; Tang et al., 2019 ), antibiotics (Li et al., 2019; Zhao et al., 2017; Deng et al., 2018; Lu et al., 2019 ), heavy metal ions (Wang et al., 2017), alcoholic compounds (Xia et al., 2016; Sfaelou et al., 2014). However, a high similarity between PFC and other fuel cells (FC) types (such as microbial FC- MFC- (Fang et al., 2016); direct methanol FC (Wu et al., 2017), proton exchange membrane FC (Qi et al., 2018); alkaline polymer electrolyte membrane FC (Lan et al., 2015), can be observed. While this technique is able to simultaneously degrade organic pollutants from the wastewater and produce power (Khalik et al., 2017).

Compared to conventional MFCs, PFCs have the following desirable features, but may not be limited to: (i) PFC can use sunlight as an energy source and lead to the fast generation and direct transfer of electrons, thus increasing the degradation efficiency of pollutants; (ii) It is possible to design high-performance electromaterials for PFC by nanotechnology; (iii) PFC can be easily fabricated and operated in finite reaction conditions by controlling the light input; and (iv) PFC system has strong oxidative ability of photogenerated holes and ^•OH by non-selective oxidation of substrates (Li et al., 2019; Kee et al., 2018).

The PFC needs a photoanode carrying photocatalyst and light radiation (LR) to generate electrons (e⁻) to decompose organic pollutants and generate power (Eq. (1)). In order to ensure that the e⁻ are excited in the conduction band (CB) and that they can leave the holes (h⁺) in the valence band (VB), the radiation energy should be higher or equal to the photocatalyst energy band. The UV irradiation of the anode detaches the photogenerated e⁻ and h⁺ (Eqs. (2) and (3)). For power generation, due to the organic compounds’ decomposition in the anode, e⁻ are provided and transmitted to the cathode by means of an external circuit (Eq. (4)). The hydrogen ions generated by the photooxidation process are transferred to the cathode by diffusion using a proton exchange membrane (PEM). Oxygen is provided at the cathode and the transferred e⁻ react with O₂, thus resulting in water (Eqs. (5)–(10)) or superoxide radicals (Eqs. (12) and (13)) (Lee et al., 2018).

Photoexcitation of photocatalyst:

(1)

P h o t o c a t a l y s t + h_{v} \to h_{VB}^{+} + e_{CB}^{-}

Generation of ^•OH

(2)

h_{VB}^{+} + {OH}^{-} (a d s) \to {OH}^{\cdot}

(3)

h_{VB}^{+} + H_{2} O \to {OH}^{\cdot} + H^{+}

Redox reactions

(4)

h_{VB}^{+} + O r g a n i c s \to {Organics}^{\cdot +} (o x i d a t i o n o f o r g a n i c s)

(5)

{OH}^{\cdot} + O r g a n i c s \to {Organics}^{\cdot} (d e g r a d a t i o n o f o r g a n i c s)

(6)

{Organics}^{\cdot} + O_{2} \to O r g a n i c s - {OO}^{\cdot} (d e g r a d a t i o n o f o r g a n i c s)

(7)

{{\cdot O}_{2}^{-} + O r g a n i c s}^{\cdot} \to O r g a n i c s - {OO}^{\cdot} (d e g r a d a t i o n o f o r g a n i c s)

(8)

e^{-} + O r g a n i c s \to {Organics}^{\cdot} (r e d u c t i o n o f o r g a n i c s)

(9)

{Organics}^{\cdot +} \to {Organics}^{2 +} + e_{CB}^{-} (i n j e c t i o n o f e l e c t r o n i n t h e C B)

(10)

{4 H}^{+} + O_{2} + 4 e^{-} \to 2 H_{2} O

(11)

{2 H_{2} O + 2 H}^{+} + 2 e^{-} \to H_{2} O_{2}

Generation of superoxides:

(12)

e_{CB}^{-} + O_{2} \to {\cdot O}_{2}^{-}

(13)

e^{-} + O_{2} \to {\cdot O}_{2}^{-} (r e a c t i o n a t t h e c a t h o d e)

Significantly, most species of PFC reactions occur mainly on the surfaces of the electrodes. Additionally, in the solution, the reaction time is limited by the partly mass transfer and thus, the processing time is delayed. Consequently, in order to raise the efficiency of reactivity of materials and improve the mass transfer in the liquid phase, the PFC is commonly used in combination with materials such as carbon nanotubes (Das et al., 2017); graphene (Xiang et al., 2012) and metal oxides (Martin et al., 2015). TiO₂, as one of the most important photocatalytic materials, is mainly employed due to high stability (physical and chemical) and unique properties (optical and electronic), easy to prepare, low cost and non-toxic (Li et al., 2014; Khataee et al., 2014). However, the fast recombination e⁻_CB and h⁺ in TiO₂ reduces the efficiency of photocatalytic degradation (Mousavi et al., 2020; Ying et al., 2016). In this regard, some new anodes have been introduced to raise the absorption capacity of radiation and e⁻- h⁺ separation on the photoelectrode surface (Table 1).

Table 1 Characteristics of some PFC that have been employed in recent years.

Authors/Year	Type of anodes	Characteristics of the PFCs	Substrates	Light source	Maximum power density (P_max, W m⁻²)	Type of electrolyte	Effective photo-electrodes surface area (cm²)	Refs.
Antoniadou et al./2011	CdS/ZnS/ TiO₂	PFC was manufactured from two rectangular compartments, filled with aerated electrolyte and separated by a silica frit. Both compartments included the same. UVA agitation of the anode was done by Black Light fluorescent tubes. An array of parallel tubes (each of 4 W nominal power) was set in front of the cell window, generating around 4.0 mW/cm² of light on the titania film. The excitation was generated by a Xe lamp that simulated solar radiation. In all cases, glass-FTO electrode as a window passes the exciting irradiation through itself.	–	Ultraviolet A (UVA)	–	NaOH	0.5	(Antoniadou et al., 2011)
Liu et al. /2016	Ag/AgCl/GO	The PFC system had two photo-electrodes, a LED lamp (2 W) as a visible light source and an irradiation surface of 20 mm × 50 mm. The cell in a quartz tube (30 mm × 18 cm) had an aeration device, Ag/AgCl/GO-based anode and ZnIn₂S₄-based cathode placed at 2.5 cm from one-another. When the system was running, a 1 U resistance connected the anode.	Rhodamine B	Visible light	0.540	Na₂SO₄	-	(Liu et al., 2016)
Xie and Ouyang/2017	WO₃/FTO	The 2 × 2 cm² WO₃/FTO anode with an effective area of 3.0 cm² was fixed by an electrode holder and collected with the air-breathing cathode (3.0 cm diameter and 7.1 cm² effective area) through copper wire. The distance between the anode and cathode was ∼ 2 cm. The Pt/C catalyst loaded side of the air-breathing cathode was in direct contact with the electrolyte and the other side (without the catalyst) was exposed to air.	Oxytetracycline hydrochloride	Simulated solar light	0.363	Na₂SO₄	3.0 (anode), 7.1 (cathode)	(Xie and Ouyang, 2017)
Lee et al./2017	ZnO/Zn	The system had an immobilized ZnO/Zn anode and a Pt/C cathode, placed at 5 cm from each other. An ultraviolet A (UVA) lamp was place at 15 cm from the anode represented the light source. Aeration was supplied at the cathode side.	Reactive Green 19	Ultraviolet A (UVA)	0.102	Without adding any supporting electrolyte	15	(Lee et al., 2017)
Xie et al./2017	ZnFe₂O₄/ TiO₂-NTs	The system has 2 × 2 cm² ZnFe₂O₄/TiO₂-NTs anode (3.0 cm² effective area) and an air-breathing cathode (3.0 cm diameter and 7.1 cm² effective area) placed at ∼2 cm from each other. An electrode holder fixed the anode that was connected with the air-breathing cathode using copper wire. The Pt/C catalyst side of the air-breathing cathode was in direct contact with the electrolyte and the other side was exposed to the air (to receive oxygen from the surrounding environment). A 300 W Xe lamp with an AM 1.5 filter simulated the solar light.	Phenolic compounds and pharmaceutical products	Simulated solar light	0.013	Na₂SO₄	3.0 (anode), 7.1 (cathode)	(Xie et al., 2017)
Zeng et al. /2018	TiO₂/WO₃/W	The PFC was constructed using a dual-electrode configuration. The PFC was equipped an AM 1.5 illumination and 1 cm² illumination area for both the WO₃/TiO₂ anode and Pt/BJS cathode.	Atrazine	Visible light	0.563	K₂SO₄	–	(Zeng et al., 2018)
Ye et al. /2018	TiO₂ nanotube array (TNA)	The system had a TNA electrode (3 cm × 3 cm) as photoactive anode and a copper foil (Grade M2, 3 cm × 4.5 cm) as counter electrode placed at 2 cm from each other. The reaction cell (6 cm × 6 cm × 6 cm) is a rectangular quartz cube. A 450 mW UV-LED lamp (365 nm peak intensity) placed outside the reaction cell (on the side of TNA anode) generated 11.6 mW/cm² radiant power at the TNA surface.	Micropollutant 4-chloro-2-methylphenoxyacetic acid	UV	–	Na₂SO₄	9.0	(Ye et al., 2018)
Ong et al./2019	ZnO/BaTiO₃	An anode and platinum-loaded carbon (Pt/C) cathode (connected by a 1 kΩ resistivity external circuit) were placed in a 1000 ml Pyrex glass cell filled with 500 ml of RR120. Two ultraviolet A (UVA) lamps (placed at 100 mm from the anode and cathode) were used as the light source.	Reactive Red 120	Ultraviolet A (UVA)	0.005284	Without adding any supporting electrolyte	–	(Ong et al., 2019)

In this context, an overview of photocatalytic fuel cells for the degradation of pollutants and simultaneous power generation is presented. The effect of different photoanode and photocathode materials on pollutant degradation and power generation were discussed. To better understand the Fenton process on photocatalytic fuel cell systems, an analysis regarding the hybrid system of Fenton process and photocatalytic fuel cell for pollutant degradation and power generation was also performed.

2 Pollutant degradation

The water pollution crisis is one of the environmental problems generated by the development among human’ life. Discharging wastewater with organic pollutants such as dye (Iqbal et al., 2017; Mousavi et al., 2020; Xie et al., 2012; Yang et al., 2018 ) and antibiotics (Hailili et al., 2018; Osouleddini et al., 2019) are the most important segments of water pollution (Zarei et al., 2012). In this regard, recently some effective technologies were employed to reduce the toxic wastewater issue (Liu et al., 2016; Alikarami et al., 2019; Khataee et al., 2011 ). Among them, the PFC system has recently been approached as a promising technology to treat a wide range of colored wastewater (Katsumata et al., 2013).

Based on the PFC mechanism (Eqs. (1)–(13)), when the photoanode is irradiated by light, the e⁻ are excited from VB into the CB (Eq. (1)). The band gap is also known as an energy gap and it will be further referred to as Eg. Then, it could react with the hydroxide ions or water molecules to produce the ${OH}^{\cdot}$ (Eqs. (2) and (3)). Afterward, azo bonds are oxidized: i) directly, by photogenerated h⁺ (Eq. (4)); or ii) indirectly, by ${OH}^{\cdot}$ (Eq. (5)). The e⁻ move to the cathode and react with the hydrogen ions produced through photooxidation to water (Eq. (10)). In this process, the HO and O₂^– leads to photocatalytic degradation of organic contaminants (Fig. 1).

Schematic of pollutants degradation in PFC.

In the case of Methylene Blue (MB) degradation, Chen et al. utilized an aerated PFC with nanopore WO₃/W photoanode and a Pt modified commercial silicon photovoltaic cell (Pt/PVC) photocathode (Chen et al., 2014). They were able to remove 14% of MB under the photolysis condition after 90 min and 91% of MB under aerated condition after 60 min. The Fermi level discrepancy between the two electrodes can make the e⁻ to immigrate from photoanode to photocathode. Under aeration condition, the O₂ molecules use the $e^{-}$ of Pt/PVC photocathode. To produce stable electricity in the external circuit, the h⁺ of Pt/PVC can catch the e⁻ from WO₃/W. Therefore, the WO₃/W h⁺ can be generated by pollutants decomposition. According to the mentioned phenomena, in this system, the simultaneous recovery of energy and organic pollutant degradation can occur (Chen et al., 2014).

Tang et al. (Tang et al., 2014) (Tang et al., 2014) used a rotating disk PFC (RDPFC) that has as photoanode a TiO₂/Ti disk. The system degrades RhB and Reactive Brilliant Red X-3B (RBR X-3B) and is formed by a Pt black/Pt cathode, NaOH anolyte, and H₂SO₄ catholyte. The proposed system had promising results, being practical to generate electricity and hydrogen (H₂) energy under UV lighting and different types of dye wastewater. As the enhancement of dye concentration leads to a significant decrease of sample solution transmittance, the sample solution in the immersion reactor absorbs a high percentage of the ultraviolet light, thus leading to a low degradation efficiency; on the other hand, LR absorption by solution significantly decreased in the aqueous film RDPFC and retained efficiency with sensible performance and promising performance.

In another study, Wu et al. (Wu et al., 2015) (Wu et al., 2015) investigated the phenol removal efficiency using a PFC with TiO₂ NRs/FTO photoanode, and C/Cu₂O NWAs/Cu mesh photocathode. The system displays a remarkable performance for the degradation of phenol so that 84.2% TOC removal was achieved in 8 h. The reaction process in this study can be explained by Eqs. (14)–(18) (Wu et al., 2015). Moreover, the photoresponse of the photocatalyst and conductivity of the substrates can affect the PFC performance in pollutant degradation. Results indicated short circuit current (J_SC) of TiO₂ NRs/FTO PFC is far more than Jsc of TiO₂ NTs/Ti PFC. The high value of J_SC showed the PFC system has a synergistic facilitated impact on the photoresponse of the photoanode and photocathode. The Fermi level of TiO₂NRs/FTO is further negative than that of C/Cu₂O NWAs/Cu mesh, resulting in a self-driven PFC during solar LR. For this system, the residual photoanode h⁺ and the photocathode e⁻ lead to simultaneous organic compound degradation and H₂ generation (Xie et al., 2017).

(14)

Ti O_{2} + h_{v} \to h_{V B 1}^{+} + e_{C B 1}^{-}

(15)

C_{6} H_{5} O H + 5 H_{2} O + 16 h_{V B 1}^{+} \to 6 C O_{2} + {16 H}^{+}

(16)

{Cu}_{2} O + h_{v} \to h_{V B 2}^{+} + e_{C B 2}^{-}

(17)

e_{CB}^{-} + h_{V B 2}^{+} \to {h_{v}}^{'}

(18)

2 H^{+} + 2 e_{C B 2}^{-} \to H_{2}

Also, Yang et al. (Yang et al., 2015) (Yang et al., 2015) applied a dual-chamber rotating-disk PFC with TiO₂ nanotube array/Ti (TNA/Ti) rotating-disk photoanode and Cu rotating-disk cathode for degradation RhB, RBR X-3B, MB, Bisphenol A (BPA), and Glucose. They succeeded to remove 96.8%, 69.8%, 92.9%, 86.9% and 93.2% of RhB, RBR X-3B, MB, Glucose, and RhB/BPA in anode chamber and 89.8%, 50.9%, 84.7%, 54.8% and 71.3% of RhB, RBR X-3B, MB, Glucose, and RhB/BPA in cathode chamber, respectively. The results revealed that the organic degradation process can be done in both the anode and cathode chamber with different pathway. Under UV irradiation the hole could be generated in the anode which leads to direct oxidation of the pollutant, Eqs. (19)–(21).

(19)

RhB + h_{VB}^{+} \to {RhB}^{\cdot +}^{'}

(20)

h_{VB}^{+} + H_{2} O \to {HO}^{\cdot}

(21)

RhB + {HO}^{\cdot} \to o x i d a t i o n p r o d u c t s

While both direct and indirect processes are involved in the pollutant degradation by cathode chamber as depicted in the following equations:

Direct pathway:

(22)

RhB + e^{-} \to {RhB}^{\cdot -}^{'}

Indirect pathway:

(23)

O_{2} + {2 H}^{+} + e^{-} \to H_{2} O_{2}

(24)

RhB + H_{2} O_{2} \to o x i a t i o n p r o d u c t s

(25)

H_{2} O_{2} + e^{-} \to {HO}^{\cdot} + {OH}^{-}^{'}

(26)

RhB + {HO}^{\cdot} \to o x i d a t i o n p r o d u c t s

To decompose the existing organic pollutants from water samples, the organic compounds represented the source of energy. It has a series of advantages that include: i) increased efficiency of LR utilization attained by a thin-film photocatalytic reaction and TNA photoanode; ii) more suitable h⁺–e⁻ separation through the TNA microstructure; iii) enhanced mass transfer via electrodes rotation; and iv) in situ production of H₂O₂ and ^•OH (that simplifies the decomposition of organic compounds). The appropriate efficiency was ascribed to various decomposition pathways such as direct e⁻ reduction, direct h⁺ oxidation, indirect ^•OH oxidation, and indirect H₂O₂ oxidation. The e⁻ migration could produce usable electricity, a fact that represents an advantage over conventional PFCs (Zeng et al., 2018).

Bai et al. (Bai et al., 2016) (Ye et al., 2018) investigated Methyl Orange (MO), CR and MB removal efficiency by a PFC with BiVO₄/TiO₂nano-tubes/FTO photoanode and ZnO/CuO nanowires/FTO photocathode. They achieved to remove 76%, 83%, and 90% of MO, CR, and MB, respectively. The mechanism of pollutants degradation is due to this fact that the Fermi level of BiVO₄/TiO₂ is far more negative than ZnO/CuO which leads to migration of the he BiVO₄/TiO₂ e⁻ and the ZnO/CuO h⁺ can be moved by an internal bias and then discharged to simultaneously recover energy and decompose organic compounds. On the other hand however, the photocathode electrons and photoanode holes would be released and contribute to the organic pollutants (Eqs. (27)–(29)). Moreover, the type of substrate affects the PFC performance so that PFC performed better in the presence of phenol and glucose. This can correspond to differences in the molecular structure of organic matter (Bai et al., 2016).

(27)

{RH}_{n} \to R^{,} + {nH}^{+} + {ne}^{-} (p h o t o a n o d e)

(28)

{nH}^{+} + n / {4 O}_{2} + {ne}^{-} \to n / {nH}_{2} O (p h o t o c a t h o d e)

(29)

{RH}_{n} + n / {4 O}_{2} \to R^{,} + n / {nH}_{2} O (P F C)

Furthermore, for the Reactive Green 19 (RG19) degradation, Lee et al. employed a PFC having a ZnO/Zn photoanode and platinum loaded carbon cathode. In this study, the effect of several inert electrolytes such as NaCl, Na₂SO₄, and MgSO₄, aeration, and initial concentration of dye on performance was investigated. It was shown that NaCl inert electrolyte has the highest RG19 degradation efficiency (100% after 8 h) due to the direct reaction of NaCl with ^•OH generated on the surface of the ZnO Photoanode. On the other hand, hypochlorite ions could be produced from NaCl solution (Eqs. (30)–(32)) which leads to indirect oxidation of RG19 (Lee et al., 2016).

(30)

2 {Cl}^{-} \to {Cl}_{2} + {2 e}^{-}

(31)

{Cl}_{2} + H_{2} O \to H O C l + H^{+} + {Cl}^{-} +

(32)

HOCl \to H^{+} + {Cl}^{-}

The addition of Na₂SO₄, NaCl, and MgSO₄ as inert electrolytes lead to a high Jsc and Pmax. This can be explained by the fact that the conductivity of the media enhanced with the inert electrolyte leads to a decreased PFC internal resistance. Furthermore, the existence of chloride ions contributes to a direct, strong oxidation reaction at the anode surface that, as a result, enhances degradation. For MgSO₄ and Na₂SO_4, due to the absence of chloride, the degradation rates were not as good as that of NaCl. Totally, the presence of supporting electrolyte significantly enhances the removal of color. Because the electrolyte causes increased the potential difference on the electrodes. This phenomenon facilitates the spontaneous flow of photogenerated electrons to the cathode and subsequent generation of oxidizing photogenerated holes in the valence band of the anode (Shen et al., 2006).

Initial concentration of the pollutant is the other factor that affects the PFC performance. The removal efficiency of RG19 was decreased with increasing of RG19 up to 30 mg/L. This due to prevent of UV transmission from the solution to the photoanode which leads to restricted availability of photon for ^•OH generation. On the other hand, the photoexcitation of photoanode can be decreased with increasing of initial concentration of RG19 (Saquib and Muneer, 2003; Saquib and Muneer, 2003).

RhB removal was also studied by Sui et al. (Sui et al., 2017), as a PFC equipped with a biocathode and TiO₂ and Ag/TiO₂ photoanodes was used. The anode was designed by doping Ag into the three-dimensional TiO₂ nanoarrays formed on Ti mesh. A 99.5% removal rate was obtained with Ag/TiO₂ photoanode and 97.3% with TiO₂ photoanode. For both types of photoanode, the RhB removal followed pseudo-first-order kinetics. The photoanode made of Ag/TiO₂ is significantly better (in terms of RhB decomposition and electricity production performance) than that made of TiO₂.

After 90 min operation, Zhao et al. (Zhao et al., 2017) reported a removal rate of 91.98%, 98.57%, 92.36% and 68.09% for MO, MB, CR and Tetracycline (TC), respectively using a hybrid Fenton-PFC system with TiO₂ TNA photoanode and the Pt/SiPVC photocathode. The degradation efficiency for these compounds in traditional PFC (without ferrous ions) was 53.61%, 45.38%, 51.09%, and 30.65%, respectively. The main reason for this increase can be correlated with the introduction of Fe²⁺ to the system which leads to an intensification of radical reactions and thus, the degradation efficiency increases (more details about this issue are provided in section 7).

Also, Zhao et al. (2017) (Zhao et al., 2017) proposed a system that increases organic decomposition and electricity generation concurrently by using Fe²⁺ ions to intensify the reaction of radicals. The system had TiO₂ TNA photoanode and Pt-black/Pt photocathode and was used for degradation of MO, MB, Congo Red (CR), and TC. The degradation efficiency for MO, MB, CR, and TC in traditional PFC was 34.99%, 43.75%, 40.58%, and 34.40%, respectively, but after adding the ferrous ions to the system, the degradation efficiency for these compounds increased to 97.34%, 95.36%, 93.23%, and 73.80%, respectively. In addition, the process parameters (pH, electrolyte, Fe²⁺concentration) were evaluated.

The mechanism of pollutant degradation is assigned to hydroxyl radical via electron-hole pairs are generation during UV irradiation as presented in the Eqs. (33) and (34).

(33)

{TiO}_{2} + h ϑ \to h^{+} + e^{-}

(34)

H_{2} O + h^{+} \to {HO}^{\cdot} + H^{+}

On the other hand, continuous current in the cathode leads to electrons reducing O₂ which causes the generation of different reactive radicals Eqs. (35)–(38).

(35)

O_{2} + e^{-} \to O_{2}^{\cdot -}

(36)

O_{2} + 2 H^{+} + 2 e^{-} \to H_{2} O_{2}

(37)

O_{2}^{\cdot -} + H^{+} \to {HO}_{2}^{\cdot}

(38)

{HO}^{\cdot} + H_{2} O_{2} \to {HO}_{2}^{\cdot} + H_{2} O

Also, the electrolyte concentration influence the PFC performance, so that increasing of sodium sulfate concentration from 0.00 mol/L to 0.10 mol/L cause the enhancement of color removal from 58.47% to 97.34%. This due to increasing of solution conductivity which leads to transmitted of photoinduced e⁻ through the external circuit of the PFC (Zhao et al., 2017).

To remove RG19, Lee et al. (2018) (Lee et al., 2018) applied a membrane-less PFC with a photoanode made of ZnO loaded carbon and a cathode made of Pt loaded carbon. The effect of dissolved oxygen (DO) was investigated on the degradation efficiency and it was shown that because the DO maintains the e⁻- h⁺ pair separation of ZnO, the DO concentration increase leads to a rise of RG19 removal rate. It was also shown that the decolorization performance is 37% at a slow rate (k = 0.033 h⁻¹), low DO concentration (roughly 0.2 mg L⁻¹), and N₂ gas for 8 h. Moreover, the e⁻, h⁺, ^•OH, O₂^•−, OO^• and H₂O₂ are the main radicals which contribute in the dye degradation process (Eqs. (1)–(13)).

In another study, Li et al. (Li et al., 2018) applied a PFC coupling with persulfate (PS) activation TiO₂ nanotube arrays on a titanium sheet as the photoanode to remove MO. The proposed system could enhance the reaction of photocatalytic MO removal using the formation of active species (SO4^•− and ^•OH) from the solid electrodes surface to the total liquid phase. For this reason, PFC/PS represented significant cell efficiency, raising the MO degradation by 40.36% (compared to PFC without PS), reaching 90.19%. Furthermore, the MO degradation process conforms the pseudo-first-order kinetics model, the removal mechanism being described by Eqs. (39)–(47). In this connecting system, electricity production was higher by 44.97% than the bare PFC (Li et al., 2018).

(39)

Ti O_{2} + h_{v} \to h^{+} + e^{-}

(40)

H_{2} O_{2} + h^{+} \to {OH}^{\cdot} + H^{+}

(41)

O_{2} + {4 H}^{+} + 4 e^{-} \to {2 H}_{2} O (φ^{θ} = 1.23 V, p H = 0)

(42)

{O_{2} + e}^{-} \to {\cdot O}_{2}^{-}

(43)

{O_{2} + 2 H}^{+} + 2 e^{-} \to H_{2} O_{2}

(44)

{\cdot O}_{2}^{-} + H^{+} \to H_{2} O^{\cdot}

(45)

H O^{\cdot} + H_{2} O_{2} \to H {O_{2}}^{\cdot} + H_{2} O

(46)

{S_{2} O_{8}}^{2 -} + h_{v} \to {2 S O_{4}}^{\cdot -}

(47)

{S_{2} O_{8}}^{2 -} + e^{-} \to {S O_{4}}^{\cdot -} + {S O_{4}}^{2 -}

In another study, Lu et al. (Lu et al., 2018) (Lu et al., 2018) applied a PFC with BiOBr/TiO₂ TNAs with 5, 10, 15, and 20 consecutive ionic layer adsorption and reaction cycles photoanode and Pt cathode for RhB removal. Using this system, 99.7% of RhB was removed; the working mechanism is described by Eqs. (48)–(49) at the photoanode and Eqs. (50)–(51) at the photocathode.

At the photoanode,

(48)

BiOBr + h_{v} \to h^{+} + e^{-}

(49)

RhB + h^{+} \to d e g r a d a t i o n p r o d u c t

At the cathode,

(50)

O_{2} {+ 4 H}^{+} + 4 e^{-} \to 2 H_{2} O

(51)

O_{2} {+ 2 H}^{+} + 2 e^{-} \to H_{2} O_{2}

Besides, for the removal of Reactive Red 120 (RR120), Ong et al. (Ong et al., 2019) employed a PFC having as photoanode a ZnO/BaTiO₃ loaded carbon plate heterojunction. This study confirmed that Z1B1/C/Z1B1 system has the highest dye removal efficiency (93.67%) due to sufficient ZnO loading into the Z1B3 system and excellent absorption of UV light. The Langmuir-Hinshelwood kinetics model illustrated a good agreement (R² ≈ 1) between the kinetic model and the experimental data. The Z1B1/C/Z1B1system has the highest rate constant (0.3554 h⁻¹) which shows the higher removal efficiency rate of the RR120 (Ong et al., 2019). However, the direct photolysis is dramatically slower than the other processes and has the lowest rate constant of aerated PFC system. The possible reactions for RR120 degradation are described in the following equations (Ong et al., 2019):

Photoanode

(52)

ZnO / B a T i O_{3} + h_{v} \to Z n O / B a T i O_{3} (e_{CB}^{-} + h_{VB}^{+})

(53)

ZnO / B a T i O_{3} (h_{VB}^{+}) + H_{2} O \to Z n O / B a T i O_{3} + {OH}^{\cdot} + H^{+}

(54)

ZnO / B a T i O_{3} (h_{VB}^{+}) + + {OH}^{-} (a d s) \to {ZnO / B a T i O_{3} + O H}^{\cdot}

(55)

{Dye + O H}^{\cdot} \to {Dye}^{*} (d e g r a d a t i o n o f d y e)

(56)

ZnO / B a T i O_{3} (h_{VB}^{+}) + D y e \to Z n O / B a T i O_{3} + {Dye}^{* -} (o x i d a t i o n o f d y e)

(57)

Dye + h_{v} \to {Dye}^{*} (d i r e c t p h o t o l y s i s)

Cathode (in the presence of oxygen)

(58)

{(e_{CB}^{-})}_{Pt / C} + O_{2} \to {\cdot O}_{2}^{-}

(59)

Dye + {(e_{CB}^{-})}_{Pt / C} \to {Dye}^{*} (r e d u c t i o n o f d y e)

(60)

n {(e_{CB}^{-})}_{Pt / C} {+ n H}^{+} + n / 2 O_{2} \to H_{2} O_{2}

(61)

n {(e_{CB}^{-})}_{Pt / C} {+ n H}^{+} + n / 4 O_{2} \to {n / 2 H}_{2} O

(62)

n {(e_{CB}^{-})}_{Pt / C} {+ n / 2 H}_{2} O + n / 4 O_{2} \to n O H^{-}

Li et al. (2019) (Li et al., 2019) investigated the effects of adding peroxymonosulfate (PMS) into a PFC with TiO₂/Ti photoanode and Pt cathode on TC removal from aqueous solution. The results demonstrated that the addition of PMS could significantly enhance the TC decomposing and accelerate the reaction rate because it has a strong oxidative capacity and can inactivate part of TC. When PMS is activated in presence of UV irradiation (Eqs. (63) and (64)), the produced sulfate radical, ^•OH, singlet oxygen (¹O₂) and PMS direct oxidation can improve the TC degradation (Sharma et al., 2015; Chen et al., 2018; Yang et al., 2018 ). Furthermore, adding PMS to the PFC can play the role of sacrificial agent that reacts with e⁻ (Eqs. (65) and (66)) (Sayed et al., 2017) and decreases the recombination of photogenerated e⁻ and h⁺ on the photoanode, thus increasing the production of reactive radicals.

(63)

HS {O_{5}}^{-} + h_{v} \to {2 S O_{4}}^{\cdot -} + {OH}^{\cdot}

(64)

HS {O_{5}}^{-} + {S O_{5}}^{2 -} \to H S {O_{4}}^{-} + {S O_{4}}^{2 -} + 1 O_{2}

(65)

HS {O_{5}}^{-} {+ e}^{-} \to {OH}^{\cdot} + {S O_{4}}^{2 -}

(66)

HS {O_{5}}^{-} {+ e}^{-} \to {S O_{4}}^{\cdot -} + {OH}^{-}

In summary, Table 2 illustrates a summary of the different works tackling the issue of pollutant degradation with various PFCs systems.

Table 2 Degradation of different pollutants in PFCs.

Authors/Year	Photoanodes	Photocathodes	Substrates	Degradation Efficiency (%)		Process Time (min)	Maximum power density (P_max, W m⁻²)	Open circuit voltage (V_oc, V)	Refs.
Chen et al./2012	WO₃/W	Cu₂O/Cu	Rhodamine B	63		300	0.677	0.33	(Chen et al., 2012)
			Congo Red	74
			Phenol	58
Chen et al./2014	nanopore WO₃/W	Pt/PVC	Methylene Blue	91		90	1.33	0.38	(Chen et al., 2014)
Tang et al./2014	TiO₂/Ti disk	Pt black/Pt electrode	Rhodamine B	≈ 100		240	5.049	0.953	(Tang et al., 2014)
Tang et al./2014	TiO₂/Ti disk	Pt black/Pt electrode	Reactive Brilliant Red X-3B	≈ 100		240	6.788	0.978	(Tang et al., 2014)
Li et al./2014	TiO₂/FTO	carbon paper deposited with carbon black and Pt black	Methylene Blue	83.9		360	5.80	1.06	(Li et al., 2014)
Wu et al./2015	TiO₂ NRs/FTO	C/Cu₂O NWAs/Cu mesh	Phenol	84.2 of TOC		480	7.3	0.41	(Wu et al., 2015)
Yang et al./2015	TiO₂ nanotube array/Ti rotating-disk	Cu rotating-disk		Anode Chamber	Cathode Chamber	120	-	(Yang et al., 2015)	(Wu et al., 2015)
			Rhodamine B	96.8	89.8
			Reactive Brilliant Red X-3B	69.8	50.9
			Methylene Blue	92.9	84.7
			Glucose	86.9	54.8
			Rhodamine B/ Bisphenol A	93.2	71.3
Bai et al./2016	BiVO₄/TiO₂nano-tubes/FTO	ZnO/CuO nanowires/FTO	Methyl Orange	76		80	1.16	0.53	(Bai et al., 2016)
			Congo Red	83
			Methylene Blue	90
Khalik et al./2016	ZnO onto carbon felt	Pt-coated carbon paper	Reactive Black 5	≈ 100		1440	3.35 (Na₂SO₄ electrolyte), 2.56 (MgSO₄ electrolyte) and 2.45 (NaCl electrolyte)	1.129 (Na₂SO₄ electrolyte), 1.070 (MgSO₄ electrolyte) and 1.100 (NaCl electrolyte)	(Khalik et al., 2016)
Lee et al./2016	ZnO/Zn	platinum loaded carbon	Reactive Green 19	100		480	12.696 (Na₂SO₄ electrolyte), 1.077 (MgSO₄ electrolyte) and 2.817 (NaCl electrolyte)	1.128 (Na₂SO₄ electrolyte), 0.990 (MgSO₄ electrolyte) and 1.049 (NaCl electrolyte)	(Lee et al., 2016)
Khalik et al./2017	ZnO/carbon felt	Pt-loaded carbon	Reactive Black 5	80		360	6.571 (UV) and 16.651 (solar)	0.75 (UV) and 0.82 (solar)	(Khalik et al., 2017)
Sui et al./2017	TiO₂/Ti mesh	activated carbon/ poly tetra fluoro ethylene	Rhodamine B	98.5		150	0.222	-	(Sui et al., 2017)
Sui et al./2017	TiO₂	biocathode with carbon fiber brushes	Rhodamine B	97.3		120	0.211	0.350	(Sui et al., 2017)
Sui et al./2017	Ag/ TiO₂	biocathode with carbon fiber brushes	Rhodamine B	99.5		120	0.318	0.458	(Sui et al., 2017)
Zhao et al./2017	TiO₂ TNA	Pt/SiPVC	Methyl Orange	91.98		90	0.84 (PFC) and 1.57 (PFC(Fe²⁺)	0.75 (PFC) and 0.76 (PFC(Fe²⁺)	(Zhao et al., 2017)
			Methylene Blue	98.57			0.65 (PFC) and 1.01 (PFC(Fe²⁺)	0.82 (PFC) and 0.81 (PFC(Fe²⁺)
			Congo Red	92.36			0.52 (PFC) and 1.24 (PFC(Fe²⁺)	0.84 (PFC) and 0.85 (PFC(Fe²⁺)
			Tetracycline	68.09			0.48 (PFC) and 0.97 (PFC(Fe²⁺)	0.77 (PFC) and 0.81 (PFC(Fe²⁺)
Zhao et al./2017	TiO₂ TNA	Pt-black/Pt	Methyl Orange	97.34		60	3.30 (PFC) and 5.70 (Fenton-PFC)	1.26 (PFC) and 1.23 (Fenton-PFC)	(Zhao et al., 2017)
			Methylene Blue	95.36			2.70 (PFC) and 3.70 (Fenton-PFC)	1.26 (PFC) and 1.26 (Fenton-PFC)
			Congo Red	93.23			3.00 (PFC) and 4.30 (Fenton-PFC)	1.25 (PFC) and 1.25 (Fenton-PFC)
			Tetracycline	73.80			2.50 (PFC) and 3.40 (Fenton-PFC)	1.20 (PFC) and 1.21 (Fenton-PFC)
Lee et al./2018	ZnO loaded carbon	platinum loaded carbon	Reactive Green 19	37		480	0.0028	-	(Lee et al., 2018)
Lee et al./2018	immobilizing ZnO onto zinc	platinum loaded carbon	Reactive Green 19	93-sunlight 86-ultraviolet		480	0.026 (sunlight) and 0.021 (ultraviolet)	0.704 (sunlight) and 0.814 (ultraviolet)	(Lee et al., 2018)
			Acid Orange 7	62-sunlight 74-ultraviolet			0.010 (sunlight) and 0.012 (ultraviolet)	0.669 (sunlight) and 0.683 (ultraviolet)
			Methylene Blue	87-sunlight 36-ultraviolet			0.032 (sunlight) and 0.017 (ultraviolet)	0.924 (sunlight) and 0.855 (ultraviolet)
Nahyoon et al./2018	Fe/GTiP	ZnIn₂S₄	Rhodamine B	79.5		100	–	0.4	(Nahyoon et al., 2018)
Li et al./2018	TiO₂ TNA on the titanium sheet	platinum plate	Methyl Orange	92		240	0.110 (PFC) and 0.1925 (PFC/PS)	0.682 (PFC) and 0.856 (PFC/PS)	(Li et al., 2018)
Kee et al./2018	ZnO/Zn	CuO/Cu	Methyl Green	92		240	0.164 (Na₂SO₄ electrolyte), 0.142 (MgSO₄ electrolyte), 0.159 (NaCl electrolyte) and 0.054 (No electrolyte)	0.919 (Na₂SO₄ electrolyte), 0.798 (MgSO₄ electrolyte), 0.898 (NaCl electrolyte) and 0.702 (No electrolyte)	(Kee et al., 2018)
Lu et al./2018	BiOBr/TiO₂ TNAs with 15 SILAR	Pt	Rhodamine B	99.7		240	0.022	0.498	(Lu et al., 2018)
Zeng et al./2018	TiO₂/WO₃/W	Pt/BJS	Rhodamine B	99.3		180	0.694	0.792	(Zeng et al., 2018)
			Methylene Blue	98.6			0.588	0.768
			Phenol	91.7			0.498	0.753
			Bisphenol A	95.2			0.526	0.766
			Dichlorophen	96.5			0.563	0.757
			Tetracycline	92.3			0.554	0.772
Deng et al./2018	Ag-TiO₂	Pt	4-chlorophenol	32.6		360	0.03398	0.492	(Deng et al., 2018)
			Phenol	37.9			0.08995	0.499
			Tetracycline	96.4			0.10212	0.499
Ong et al./2019	ZnO/BaTiO₃loaded carbon plate heterojunction	Z1B1/C/Z1B1	Reactive Red 120	93.67		480	0.005284	0.35	(Ong et al., 2019)
Rabé et al./2019	Z-scheme g-C₃N₄/Fe⁰/TiO₂	WO₃	berberine chloride	91		90	16.4	0.8	(Rabé et al., 2019)
Li et al./2019	TiO₂/Ti	platinum	Tetracycline	90.14		60	0.015 (PFC) and 0.034 (PFC/PMS)	0.587 (PFC) and 0.749 (PFC/PMS)	(Li et al., 2019)
Wang et al./2020	TiO₂ nanorods	Cu₂O	Formaldehyde	–		3.3	–	0.58	(Wang et al., 2020)
Yu et al./2020	Ag₃PO₄	Cu₂O	Tetracycline	30		240	0.0416	0.095	(Yu et al., 2020)
	g-C₃N₄			20.82			0.00067	0.080
	Ag₃PO₄@g-C₃N₄			76			0.0606	0.18
Lam et al./2020	TiO₂/ZnO/Zn	CuO/Cu	Escherichia coli	99		60	0.172	0.868	(Lam et al., 2020)
Chen et al./2020	TiO₂ nanotube arrays	Pt	Ofloxacin	96.8		60	0.247	1.35	(Chen et al., 2020)
Lam et al./2020	ZnO/Zn	Pt	Sunset yellow	–		240	0.000565	0.411	(Lam et al., 2020)
	ZnO/Zn		Phenol	–			0.00031	0.398
	WO₃/ZnO/Zn		Sunset yellow	84.6			0.001052	0.823
	WO₃/ZnO/Zn		Phenol	67.5			0.000574	0.759
Kee et al./2020	TiO₂/ZnO/Zn	TiO₂/CuO/Cu	Palm oil mill effluent treatment	90		240	0.734	1.173	(Kee et al., 2020)

Table 2. Degradation of different pollutants in PFCs (Chen et al., 2014; Wu et al., 2015; Tang et al., 2014; Yang et al., 2015; Bai et al., 2016; Chen et al., 2012; Li et al., 2014 ).

2.1

2.1 Bacteria inactivation

Real wastewater contains countless microorganisms and bacteria types that can threaten human health and environmental stability (Zhang et al., 2018). Because conventional disinfection processes e.g. chlorination have led to the generation of toxic byproducts (Ren et al., 2009), there has been a great interest in recent years in the promotion of photocatalysis for disinfection process. The first example of semiconductor photocatalysis used as a disinfection process was reported by Matsunaga et al. (Matsunaga et al., 1985). Since then, due to its safety, low cost and efficiency, the photocatalytic disinfectant TiO₂ has been studied. However, its low quantum efficiency and absorption only near the UV region (400 nm) limit its practical application. So far, many efforts to increase the photocatalytic performance and use of visible light from TiO₂ with rectification methods such as impurity doping, metallization, sensitivity and coating, to extend the edge of TiO₂ adsorption to the optical region can be seen (Yu et al., 2005; Elahifard et al., 2007; Wang et al., 2019; Shanmugam et al., 2020 ). However, the correction processes are complex and the prepared samples usually suffer from disadvantages e.g. low stability. In recent years, the development of PFCs to inactivate bacteria has become such an important issue that the technology can use a combination of different nanomaterials for use in photocathodes and photododes to significant efficiencies to inactivate bacteria.

Lam et al. (2020) applied a PFC with TiO₂/ZnO/Zn as anode and CuO/Cu as cathode for greywater and bacteria removal and simultaneous energy generation. They found that E. coli cells decreased significantly after 60 min process in PFC. Such efficient treatment efficiency of PFC was due to action of ROS, containing ^•O₂^–, ^•OH, H₂O₂, and h⁺ by anodic TiO₂/ZnO/Zn, which could directly inactivate the bacterial cells.

3 Power generation

3.1

3.1 Electricity generation

Under LR, the e⁻ and h⁺ of a semiconductor is separated and a photovoltage is created. The photo-generated h⁺ and e⁻ are the major cause of oxidation and reduction in the n-type and p-type semiconductors, respectively (Zarei et al., 2012; Alikarami et al., 2019). The fuel used to feed a PFC can be either organic or inorganic and it is dissolved in the electrolyte of anode compartment (Ong et al., 2019; Lianos, 2017). The compounds with the CxHyOz structure (alcohols, diols, organic acids, and carbohydrates), usual products of biomass, are the most efficient organic fuels (Sfaelou and Lianos, 2016). In the system, as a sustainable green technology, it is capable of decomposing organic pollutants and producing electricity simultaneously (Sui et al., 2017) as LR of photocatalytic sites of photoanode in a certain range of wavelengths led to the production of e⁻-h⁺ pairs (Zhao et al., 2017; Zeng et al., 2018; Xia et al., 2016 ) (Fig. 2). However, in order to ensure that the photogenerated e⁻ would be excited from the photocatalyst VB to the CB, light energy ( $\geq$ than the photocatalyst Eg) is required. The h⁺, which have strong oxidability to decompose different organics, react with the organic pollutants to oxidize and degrade them to CO₂, e⁻ and protons (C_xH_yO_z + ^•OH → CO₂ + H⁺ + e⁻) (Lee et al., 2018; Khalik et al., 2017). At this time, , the e⁻generated from photocatalysis and oxidation of organics at the photoanode side are transferred to the cathode side, generating electricity (Xie and Ouyang, 2017; Xia et al., 2016).

Electricity generation and hydrogen production in PFC.

It is noteworthy that organic fuel oxidation, or an inorganic sacrificial agent (such as S⁻², SO₃⁻²,…) or H₂O itself leads to the electricity generation and H₂ production, while electricity alone is generated in the presence of O₂ (Lianos, 2017). Indeed, a PFC can operate in two main operation manners: electricity or H₂. production In the absence of O₂, PFC produces molecular H₂ either by protons reduction (at acidic pH) or by water reduction (at alkaline pH) (Sfaelou and Lianos, 2016). The potential discrepancy between the Fermi levels of two electrodes creates the driving force that causes e⁻ transfer from photoanode to the cathode. For practical aims, the function of the cell can be approximated using the following strategies. The photoanode potential for n-type semiconductor photocatalysts is close to the CB potential (Lianos, 2017).

For the cell efficiency evolution, the first aspect considered is the estimation of produced e⁻ number as a percentage of absorbed photons. This is indicated by the incident photon-to-e⁻ conversion efficiency (IPCE) value. It can be calculated by the Eq. (67) (Lianos, 2011; Varghese and Grimes, 2008):

(67)

IPCE (%) = [\frac{1240 \times J_{SC}}{λ \times P}] \times 100

where Jsc is the short-circuit current density (mA cm⁻²), P the incident irradiance (mW cm⁻²), and λ is the incident light wavelength. The number 1240 used in Eq (67) carries the matching units. IPCE% is expected to change between 0 and 100. Sometimes, due to current doubling phenomena, IPCE can have larger values than 100% (Lianos, 2011; Antoniadou et al., 2012).

Another parameter used to express the PFC efficiency is the Fill factor (FF). It is computed using Eq. (68) (O'regan and Grätzel, 1991):

(68)

FF = P_{\max} / J_{SC} V_{SC}

where P_max represents the real maximum power density (as obtained from the JV vs. V plot) and J_scV_SC is the theoretical maximum power density. The FF also shows the amount of diversion between the actual maximum power density produced and the product of J_scV_SC. It is mentioned that pure electrolyte with no organic additive shows the smallest J_sc, V_SC, and P_max.

The next parameter employed for all types of fuel cells is power density. The generated power per unit anode area is calculated by Eq. (69) (Lee et al., 2016).

(69)

P = J \times V

where P is the power density (mW cm⁻²), J is the current density (mA cm⁻²), and V is the voltage_(mV).

In the case of PFC performance for the degradation of pollutants, a parameter called degradation efficiency or removal efficiency is applied. It is estimated by the Eq. (70) (Li et al., 2019):

(70)

Degradation (%) = \frac{C_{0} - C_{t}}{C_{0}} \times 100

where C₀ illustrates the initial concentration of pollutant solution and C_t expresses the pollutant concentration at time t.

Many studies focused on improving different aspects of a PFC. For example, a hybrid system of PFC-peroxi-coagulation (PFC-PC) was introduced to decompose organic pollutants and to simultaneously produce electricity (Nordin et al., 2019). In this hybrid system, three forms of photocatalysts (TiO₂, ZnO, and α-Fe₂O₃ immobilized on carbon_cloth –CC-) were employed asphotoanodes and degradation of Amaranth dye (93.8%) were obtained (Nordin et al., 2019). Most of the research (Chen et al., 2012; Xia et al., 2016; Liu et al., 2011; Li et al., 2013; Li et al., 2015; Yang et al., 2014 ) was focused on the operating parameters and optimal performance. The Ho research group reported the effect of different DO concentrations on the mechanism of electricity generation and dye degradation (diazo dye RG19) in the single-chambered PFC based on ZnO/C photoanode (Lee et al., 2018). RG19 has a high resistance to microbial and chemical decomposition (Zuorro and Roberto, 2014). However, the RG19 degradation rate and the electricity generation was raised under high DO concentration (Wang et al., 2019). In all the previous studies, the used systems are single photoelectrode PFC consisting of one photoelectrode (photoanode). It should be noted that systems with dual photoelectrodes of photoanode and photocathode were also proposed (Liu et al., 2016; Bai et al., 2016; Chen et al., 2012; Pan et al., 2018 ). A dual photoelectrode PFC is formed from an n-type semiconductor photoanode and p-type photocathode. Under light/UV radiation, both photoelectrodes can produce e⁻- h⁺ pairs. The more negative Fermi level of the photoanode compared to the photocathode forms a bias that led to the e⁻ migration from the photoanode via the external circuit incorporated into the photocathode h⁺ (Wu et al., 2015; Chen et al., 2012). The accumulation of h⁺ occurs on the photoanode and of e⁻ on the photocathode; therefore, it is a Z-scheme (Tada et al., 2006; Maeda, 2013; Zhou et al., 2014 ).

3.2

3.2 Hydrogen generation

As we know, solar energy can be turned into useful forms of energy (electricity or chemical energy) using photoelectrochemical cells (PECs). Since the (Fujishima and Kenichi, 1972) paper, the H₂ generation through photocatalysis has been the topic of exhaustive studies. In fact, one way to store energy is to convert solar into chemical energy, particularly H₂. The photoelectrochemical conversion of solar energy provides the most promising routes for this purpose (Sfaelou and Lianos, 2016; Darbandi et al., 2019). Because of its zero carbon emission, high heat combustion, high energy capacity per unit volume, simple generation and on-site storage, hydrogen is considered a key clean energy option (Veziroǧlu, 2008; Ruiz-Gómez, 2013; Kelly and Thomas, 2008 ). As any standard PEC, oxidation_half-reaction on the photoanode of PFC is balanced by reduction_half-reaction on the cathode. The reduction_half-reaction does not originally depend on the organic fuel type or inorganic sacrificial agent but it is a standard reaction affected merely by pH and O₂ (Sfaelou and Lianos, 2016; Michal et al., 2014; Papagiannis et al., 2020 ). Therefore, oxidation half-reaction (71) is balanced by either half-reaction (72) or (73) (depending on if there is an inert environment or O₂ is present) (Lianos, 2017) (Fig. 2).

(71)

C_{x} H_{y} O_{z} + (4 x + y - 2 z) \cdot O H \to x C O_{2} + (2 x + y - z) H_{2} O

(72)

2 H_{2} O + 2 e^{-} \to H_{2} + 2 {OH}^{-} A t a l k a l i n e p H (- 0.83 V v s . N H E a t p H = 14)

(73)

H_{2} O + \frac{1}{2} O_{2} + 2 e^{-} \to 2 {OH}^{-} A t a l k a l i n e p H (0.4 V v s . N H E a t p H = 14)

The overall reaction becomes Eq. (74) (Lianos, 2017); which is consistent with photo electrocatalytic reforming for hydrogen production (Chiarello et al., 2010; Nomikos et al., 2014).

(74)

C_{x} H_{y} O_{z} + (2 x - z) H_{2} O \to x C O_{2} + (2 x + \frac{y}{2} - z) H_{2}

For hydrogen generation, the efficiency of solar to H₂ (η_STH) is determined as (Li et al., 2013; Kim et al., 2016):

(75)

η_{SHT} = \frac{J (m A . {cm}^{- 2}) \times (1.23 - V_{app}) (V)}{P (m W . {cm}^{- 2})} \times 100

where V_app = V_bias -V_oc. V_oc is the open circuit potential assessed under the same conditions as the bias potential (Khan et al., 2002; Lin et al., 2015).

In reality, η_STH represents the device efficiency and indicates its current generation capacity. However, the H₂ generation is not guaranteed by the flow of current and the η_STH determination should be accompanied by faradaic efficiency calculation (the percentage of current converted into H₂) (Lianos, 2017):

(76)

1 \frac{μ m o l}{m i n} \to \frac{10^{- 6} \times 6.022 \times 10^{23} \times 2 \times 1.6 \times 10^{- 19} C}{60 S} = 3.21 m A

Furthermore, as discussed in Section 3.1, a new application of PFC was introduced for H₂ generation by a solar-driven dual-photoelectrode PFC (Wu et al., 2015). To design such a system, with the aim of simultaneously generating electricity and hydrogen, the photoanode should be an n-type semiconductor and have a good photocatalytic activity (Wu et al., 2015). The photocathode should be a p-type semiconductor whose E_cb (vs. NHE) should be more negative than 0 V ( ${E^{°}}_{H^{+} / H_{2}}$ ) for the purpose of hydrogen generation, such as Cu₂O (Wang et al., 2013; Huang et al., 2011).

4 Photoanodes

4.1

4.1 The effects of different photoanode materials

Considering that the photoanode plays a significant role, the researchers try to reduce the photoanode limitations by improving the photocurrent and VLR response and decreasing the cost of construction and practical applications (Venkatkarthick et al., 2017; Venkatkarthick et al., 2016). Moreover, the fast induced e⁻ and recombination of e⁻-h⁺ pairs in the_photoanode would limit the photoelectrochemical_reaction (Khataee et al., 2011), the Eg and the recombination of photogenerated e⁻and h⁺ are two main parameters that affect the photocatalytic activity to produce ^•OH and electricity (Ong et al., 2018) (Fig. 3).

Reduction/oxidation mechanism in photocatalytic reactions and gap band of some semiconductors which used in photoanode and photocathode of PFC.

Accordingly, the photoanode_material with different Eg, recombination rate and features of e⁻ mobility play a fundamental role in the PFC systems, which can affect their performance. For these, numerous photoanode such as TiO₂, ZnO, WO₃, α-Fe₂O₃, BiVO₄, BaTiO₃ have been studied so far (Ong et al., 2019; Strataki et al., 2010; Monfort et al., 2016; Gan et al., 2014; Horiuchi et al., 2013; Bhatt and Lee, 2015; Pop et al., 2015; Gao et al., 2014; Alexander et al., 2008 ). Based on a recent review (Lianos, 2017), most studies on the_PFC to date are focused on TiO₂_photoanode. Usually, the Degussa P25 TiO₂ powder (UV activated) is the commercially accessible photocatalyst used for the PFC photoanode (Kaneko et al., 2006; Seger et al., 2012; Antoniadou and Lianos, 2010 ). TiO₂ has raised an excessive interest as photocatalyst among semiconductors and, since it possesses stable physical and chemical features such as low cost, easy to prepare, nontoxicity, outstanding stability, and excellent electronic and optical properties (Li et al., 2014; Liu et al., 2011). For instance, Kaneko et al. (Kaneko et al., 2006) used a nanoporous titanium dioxide film_photoanode and an O₂-reducing cathode to construct a PFC. Then, Lianos et al. (Lianos, 2011), Liu et al. (Liu et al., 2012; Liu et al., 2011) and Li et al. (Li et al., 2013) obtained efficient electricity production and simultaneous refractory organic compounds degradation using different types of TiO₂ photoanodes such as TiO₂ nanoparticles and TiO₂ nanotube arrays/Ti. Due to its higher Eg (3.0–3.2 eV), TiO₂ can only absorb 4% solar LR (Wang et al., 2014). Hence, the use of TiO₂ is more suitable under UV compared to the solar counterpart (Hashimoto et al., 2005) and, under UV irradiation, the photogenerated e⁻ can only migrate from TiO₂ to the cathode (Paulose et al., 2007). This limits the potential of PFC in industrial applications with solar energy. The photogenerated e⁻ and h⁺ are easy to recombine in TiO₂ which reduces photocatalytic_degradation (Li et al., 2019).

The literature presents many viewpoints concerning the efficiency raise of organic wastewater treatment by decreasing the adverse effects of e⁻-h⁺ recombination and improving the VLR absorption ability. This includes metal doping (Kim et al., 2015; Li et al., 2013), coupling other semiconductors with TiO₂ (Jiao et al., 2014) and changing the fabrication methods to optimize the micro/nanostructure of TiO₂ (Pang et al., 2014). Because the functional characterization of materials greatly depends on their micro/nanostructure, TiO₂ has been deposited as different types of nanostructures including nanoparticles, nanorods, nanotubes, nanowires, etc., (Sfaelou and Lianos, 2016) and different deposition methods were used for photoanode fabrication using powdered TiO₂ (spin coating (Kaneko et al., 2005), screen printing (Panagiotopoulou et al., 2010), and doctor blade method (Antoniadou et al., 2010).

Other than TiO₂, the most popular metal oxide semiconductors which have been used by researchers as photocatalyst in the photoanode so far are ZnO, WO₃, BiVO₄, α-Fe₂O₃ and recently BaTiO₃ (Ong et al., 2019; Sfaelou and Lianos, 2016). Based on the recent review by Sfaelou and Lianos (Sfaelou and Lianos, 2016), they are not better than TiO₂, if all parameters are considered together. The only reason for selecting some other metal oxide semiconductors is to use a material that absorbs VLR and does not necessitate the presence of a sensitizer, especially, the toxic metal sulfides (CdS) and selenides (ZnSe).

Hematite (α-Fe₂O₃) is one of the semiconductors employed as a photocatalyst in the PFC photoanode. α-Fe₂O₃ is a low-cost n-type semiconductor with relatively small Eg (2.0–2.2 eV) able to absorb the close -infrared spectrum (up to 600 nm) and roughly 40% of the solar energy. Nonetheless, the photocatalytic activity of α-Fe₂O₃ is affected by some parameters include short diffusion lengths of h⁺ (2.0–4.0 nm), high carrier recombination, confined charge transport feature and electrical conductivity (Nordin et al., 2019).

In the photocatalysis field, ferroelectric materials are a novel alternative to classic materials. Due to its robust ferroelectric feature, BaTiO₃ with perovskite structure has attained much interest (Cao et al., 2014). BaTiO₃ with a wide Eg (3.18 eV) has been considered to remove different types of dyes such as methyl orange (Liu et al., 2012) and Rhodamine B (Cui et al., 2013).

4.2

4.2 Photoanode materials characterization

The characterization of photocatalyst used in the photoanode compartment is a key step in the construction and evaluation of PFC systems. This involves structural, optical, and photoelectrochemical characterizations. The photoanodes morphology is identified by using the scanning electron microscopy (SEM) technique. In fact, this technique can provide sufficient information about the electrode morphology by providing low/high magnification and cross-sectional images. The result of SEM images showed the well-aligned tube-like morphology of the TiO₂ electrode fabricated by electrochemical anodization method with an average diameter of 0.5 µm and length of 5 µm. SEM images indicate the naturally created Schottky-type contact between the TiO₂ nanotubes and Ti substrate, which can prepare a unidirectional electric channel for the transfer of photogenerated e⁻. Due to their geometric characteristics, these nanotubes can absorb reflected/refracted light, thus minimizing the photon's loss resulting from light scattering in the liquid. In other words, the multi-tube structure of TiO₂ nanotube arrays can increase the semiconductor surface area, improving the absorption properties and LR utilization, and also adsorption of organic pollutants at the electrode surface (Li et al., 2018; Sui et al., 2015).

Detailed microstructural features of the photoanodes are studied by transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HR-TEM). For example, the presence of Ag nanoparticles (Sui et al., 2017) and CdS-ZnS sensitizers (He et al., 2018) on the TiO₂ nanotubes was further confirmed by TEM and HRTEM, which revealed the well-defined lattice fringes of the Ag (Zhao et al., 2017) plane in Ag/TiO₂ (Sui et al., 2017).

The crystalline structures of the photocatalysts are analyzed by X-ray diffraction (XRD) (Sfaelou and Lianos, 2016). Hence, during the fabrication of TiO₂-based photocatalysts used in PFCs as photoanode, the XRD can be applied as a powerful technique to confirm the successful formation of the photo-catalytically active type (He et al., 2018). The identifying peaks in the XRD patterns represent that TiO₂ was easily crystallized in anatase and rutile phases. Stronger diffraction peaks are observed for the anatase phase compared to the rutile counterpart, suggesting the content of the anatase phase is higher. It is evident that the peaks intensity of TiO₂ is much stronger after calcination treatment.

As noted, the photoelectrochemical efficiency of photoanode can be characterized by the cyclic voltammetry (CV), linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS) measurements (He et al., 2018). There is a clear difference between them when the applied voltage vs. the Ag/AgCl reference electrode is more than −0.5 V; the current density can reach 2.42 mA cm⁻² (under LR) and 0 (without LR). This result illustrated that the developed photoanode has an excellent activity under VLR.

The Ag/TiO₂ and TiO₂ LSV curves show that, in the absence of LR, the photocurrent value swiftly decreases to zero. At 0.4 V electrode potential, the Ag/TiO₂ photocurrent (3.9 mA) is much higher than that of TiO₂ (1.9 mA). This indicates the lower recombination rate of photo-induced e⁻- h⁺ pairs, as well as the higher LR absorbance for the electrode, which can improve the bio-PFC performance.

UV–Vis diffuse reflectance spectroscopy (UV– Vis DRS) as an optical characterization method commonly used in order to investigate the ability of prepared photoanodes to respond to the VLR. Eg is an important feature of the semiconductors which can be computed with the following formula (Bai et al., 2016):

(77)

α h v = A {(h v - E_{g})}^{n \ 2}

where α is the absorption coefficient, hυ is the photon energy, A is a constant and n indicates the type of optical transition of semiconductor (n = 1 for a direct transition and n = 4 for an indirect transition) (Jun et al., 2012). As previously reported, for photocatalysts such as TiO₂, ZnO, BiVO₄, which are indirect Eg semiconductors (Bai et al., 2016), n is equal to 4.

In the UV region, pure TiO₂NTs has 361 nm absorption edge. In the visible region, BiVO₄/TiO₂NTs (as a composite photocatalyst) have a 530 nm absorption edge and is about 170 nm red-shifted (compared to pure TiO₂NTs). This indicates that TiO₂NTs can extend the BiVO₄ absorption spectrum into the visible region. The Eg of pure TiO₂NTs and BiVO₄/TiO₂NTs was estimated to be 3.32 and 2.34 eV. This result reveals that the incorporation of BiVO₄ into TiO₂NTs would lead to a lower Eg.

In other studies, the UV–Vis showed that the combination TiO₂ semiconductor with nanoparticles such as Ag (Sui et al., 2017) and quantum dots such as CdS and CdS-ZnS (He et al., 2018; Wang et al., 2014) leads to absorption band shifting to the visible region. Consequently, the response of prepared photoanodes to VLR is an improved, and effective way to harvest solar radiation being achieved.

The photoluminescence (PL)_spectrum analysis is also employed to evaluate the separation and recombination efficiency of photoinduced e⁻ and h⁺. For example, the Ag/TiO₂ and TiO₂ electrodes illustrated analogous broad PL emission bands (Sui et al., 2017). The PL spectrum of Ag/TiO₂ shows a weak peak compared to TiO₂, implying that the Ag doping on TiO₂ nanotubes increases the probability of e⁻-h⁺ recombination that leads to PL_intensity reduction.

5 Photocathodes

5.1

5.1 Photocathode materials for electricity generation and dye degradation

Due to the presence of over potential developed between the cathode and electrolyte, reduction reactions do not occur automatically. To overcome this overpotential, an electrocatalyst is necessary. Since the role of the electrocatalyst is to create a large interface between two phases, facilitate the e⁻ exchange with electrolyte, and eliminate the over-potential, the counter electrode operation potential is similar to the electrochemical potential of the corresponding reduction reactions (Sfaelou and Lianos, 2016) (Fig. 3). A standard and common electrocatalyst used for PFC cathodes fabrication is a mixture of carbon black with nanoparticulate Pt (Pt/C) (Antoniadou and Lianos, 2010). An example of using Pt-coated carbon for O₂ reduction reaction at the surface of the cathode is (Kaneko et al., 2009). The reaction at Pt-coated carbon is dependent on O₂ accessibility. In the presence of O₂, the reaction produces water, while the hydrogen evolution reduction occurs in the absence of oxygen (Wang et al., 2014). The use of an O₂-reducing cathode involves a continuous entry of O₂ into the electrolyte. Several constraints such as high cost and low availability of Pt have become major concerns for large scale application of Pt. Furthermore, Pt has the tendency to aggregate so that its efficiency is progressively diminished. Therefore, low-cost materials with comparable activity (that can efficiently substitute Pt) are being developed for PFC applications (Sfaelou and Lianos, 2016). To overcome the aforementioned limitations, the use of different Fermi levels photoanodes and photocathodes is encouraged.

One of the most popular p-type photocatalysts is Cu₂O (2.0 eV Eg) (Paracchino et al., 2011). Its CB is well suited for water reduction, but for water oxidation, a proper bias is needed. Due to its optimal Eg, Cu₂O can be also activated under VLR, and it was shown that it can be an effective photocathode (Chen et al., 2012). Therefore, in order to construct low-cost PFC systems, Cu₂O can be used to replace Pt. On the other hand, CuO (1.4 eV direct Eg) is another p-type semiconductor abundant in nature that has been proven as stable and effective photocathode (Masudy-Panah et al., 2017; Masudy-Panah et al., 2016).

5.2

5.2 Photocathode materials characterization

Various structures and compositions can possess considerably different properties and can, therefore, through complex structure–property relationships, they can affect the overall photoelectrode performance (Esposito et al., 2015). In this respect, the characterization of photocathode materials in the PFC systems is as important as the photoanode counterparts. In general, the techniques used to characterize the photocathodes are the same as those used for the photoanodes characterization (and mentioned in Section 5.1). For example, in order to study the crystalline phase of heterojunction ZnO/CuO NWs photocathode, Bai et al. (Bai et al., 2016) employed XRD analysis. Except for the SnO₂ background which belongs to the FTO_glass, the chief components of photocathode were identified as CuO and ZnO according to the_peak_intensity. Meantime, the CuO reduction leads to small quantities of Cu₂O. It is worth noting that no peaks_of_Cu are observed, suggesting that Cu film was completely transformed into copper oxides (CuO and Cu₂O).

6 Anodic nanocomposites

As previously mentioned, the wastewater organic compounds are an important source of energy. According to the statistics, due to biowaste discharge, about one-third_of_the the global_yearly_energy demand (130 EJ per year) resulting from organic matter is lost. Therefore, attempts to reach a sustainable methodology that recovers the energy_from this energy-rich_organic_waste and decompose them to non-dangerous products are strongly advisable (Liu et al., 2011). For this purpose, most PFC studies focus_on_developing_TiO₂ photoanode (which can merely respond to UV). To improve the light utilization efficiency and the possibility to use in industrial applications, the development of VLR responsive photoanodes is crucial (Lu et al., 2018). Many PFC works attempted to design the photoanodes by coupling_it_with_narrow Eg semiconductors (that can simultaneously enhance effectiveness and electricity generation). For this reason, many photoanodes (such as CdS-ZnS/TiO₂, BiVO₄/TiO₂-NTs, BiVO₄/WO₃/W, α-Fe₂O₃/TiO₂/Ti, ZnFe₂O₄/TiO₂-NTs, Ag/AgCl/TiO₂, CdS/TiO₂ and BiOBr/Ti) have been investigated (Ong et al., 2019; Li et al., 2019; Zhao et al., 2017 ). In addition, in expanding the LR absorption from UV_to_visible_region, different doping strategies using metal ions, non-metal ions and different semiconductors with GO have been tested (Mecha et al., 2017). Modifying the_photocatalytic_activity of pure_semiconductor catalyst using metal-ion doping leads the absorption of visible range and causes an increase in the photoreactions. It was shown that GO-based nanocomposites such as Gr–TiO₂ (Liu et al., 2010), Gr–ZnO (Neelgund et al., 2014), Gr–CdS (Thakur et al., 2017), and Gr–SnO₂ (Chen et al., 2017) increase the photocatalytic degradation efficiency (Fig. 4).

Effect of different anodic nanocomposite on electricity generation and dye degradation in PFC.

An example of using a composite photoanode in the PFC systems is the work of Liu et al (Liu et al., 2011). A short TiO₂_nanotube_array (STNA)-based PFC system was constructed for electrical power generation from different refractory organic compounds and simultaneous wastewater treatment. The current-voltage_characteristics_of PFC systems using different photoanode materials (pure and composite) were compared under solar LR. It was shown that decorating the pure_STNA_electrode_material with narrow Eg semiconductors (e.g. Cu₂O and CdS), under simulated solar LR, significantly increases J_sc and JV_max values. For the CdS/STNA photoanode, J_sc and JV_max were 1.66–1.75 and_1.38–2.41_times_higher than those of the respective pure STNA_photoanode for three different organic compounds solution. That means the PFC system can work outdoors_under_solar_irradiation and light a LED indicative lamp by using this composite photoanode material.

7 Photocatalytic fuel cell and Fenton process

As already mentioned, due to its significant advantages, PFC is an important technology for wastewater treatment and simultaneous power generation. Recently, much attention has been paid to the_development_of photoanode and photocathode to improve performance (Li et al., 2014; Bai et al., 2016; Chen et al., 2012; Zang et al., 2014; Wang et al., 2015 ). During this time, novel photoelectrodes were designed to increase LT absorption and the separation of the e⁻ - h⁺ or surface area, which partially enhances the performance in the degradation of organic pollutants and simultaneous power generation. However, the efficiency is now restricted, as there are many challenges from the aspect of intrinsic semiconductor constraints. The most important constraints of PFCs are the radical reactions taking place on the photoanode and photocathode surfaces. However, the surface area cannot be significantly enlarged, which is a serious constraint on PFCs in the degradation of organic pollutants (Zhao et al., 2017). So how to increase the efficiency of radical reactions that only affects the surface of electrodes is the key to solving this problem. Lately, the Bio electro-Fenton system, which includes MFCs and the Fenton process, has_been_studied as a promising method for organic pollutant treatment (Zhang et al., 2015; Zhuang et al., 2010; Fu et al., 2010; Thor et al., 2020 ). This integration is an attempt to extend H₂O₂ production technologies that are cost-effective, efficient_and_sustainable for pollutant treatment (Zhang et al., 2015). The e⁻ produced by a microbial reaction are applied by electro-Fenton reactions to treat pollutants in the MFC cathode. The reaction of ferrous ions with H₂O₂ produced by the process of oxygen reduction leads to ^•OH production (Feng et al., 2010). This process is identified as the_electro-Fenton_process (Brillas et al., 2009).

The PFC_system can generate its external voltage through the self-bias between photoanode_and_cathode, the process which is accompanied by degradation of organic pollutants. If ferrous ions are added, a self-bias Fenton-PFC system will be created to decompose organic pollutants and simultaneously produce power. Since PFC is more useful than other fuel cells for the oxidation process of organic_and_inorganic materials, it could be a more appropriate partner for the Fenton process, given the considerable challenges in developing and exploiting these systems. The system can minimize maintenance costs and since it uses a PEC system that employs photoexcitation to generate e⁻ and h⁺ using n-type semiconductor as a photocatalyst, it does not require monitoring (Yang et al., 2014). The hybrid Fenton-PFC_system_has the following characteristics (Zhao et al., 2017):

It develops radical reactions from the_electrode_surface to the overall solution with the entry of the Fenton_reaction_in_the_system, which significantly increases the efficiency of radical reactions in the organic matter degradation and the electrical power generation relative to the traditional PFC.
It does not require external H₂O₂ as an oxidizing agent because HO $\cdot$ , HO₂ $\cdot$ , and H₂O₂ can be generated on the electrodes under LR, which decreases operating costs.
It does not need external voltages and additional ferrous_ions_because the self-bias voltage yielded can transmit $e^{-}$ to the cathode and achieve conversion to Fe²⁺/Fe³⁺.
It extremely produces sludge because of the radical reaction by using low levels of ferrous ions.

The mechanism of pollutant removal in the hybrid Fenton-PFC_system is as follows (Nordin et al., 2017):

At PFC anode

(78)

anode \to a n o d e (h^{+} + e^{-})

(79)

h^{+} + R e a c t a n t \to O x i d i z e d p r o d u c t

At Fenton cathode

(80)

O_{2} + 2 H^{+} + 2 e^{-} \to H_{2} O_{2} (E^{0} = 0.695 V / S H E)

At Fenton anode

(81)

Fe \to {Fe}^{2 +} + {2 e}^{-}

At PFC cathode

(82)

O_{2} + 4 H^{+} + 4 e^{-} \to {2 H}_{2} O_{2} (E^{0} = 1.23 V / S H E)

Overall Fenton process

(83)

{Fe}^{2 +} + H_{2} O_{2} \to {{OH}^{\cdot} + {OH}^{-} + F e}^{3 +}

It can be concluded that the hybrid Fenton-PFC system is a promising technology for organic pollutants removal and generating electricity simultaneously. In this hybrid system, PFC is employed to produce $e^{-}$ to produce H₂O₂ electrons as Fenton's reagent. Fig. 5 illustrates the schematic of the hybrid Fenton-PFC system for pollutant removal. Generally, the unique features of the hybrid Fenton-PFC system result in a fast and stable catalytic system with excellent efficiency (Nordin et al., 2017).

An example of the hybrid Fenton-PFC system.

8 Conclusions and recommendations

In this work, a review of the most representative research specific to PFCs for wastewater treatment and synchronous power generation has been performed. Since the performance of PFC is usually specified by the electrodes, most researches focus on the production of electrode materials in recent studies. Given that the features of functional electrode materials are significantly dependent on their structure, recent advances in nanotechnology produces efficient electrodes. In this regard, various nanomaterials were tested such as CdS–TiO₂, CdS/ZnS/TiO₂, Ag/AgCl/GO, Ag-TiO₂, WO₃/FTO, ZnO/BaTiO₃, ZnO/Zn. These electrodes are based on nanomaterials can led to enhance efficiency of the degradation of organic contaminants, energy performance, or both. But among these nanomaterials, TiO₂ is employed as the most efficient photodanode catalyst, due to its high oxidizing power, excellent stability low cost, low recombination rate (that can enhance efficiency in the utilization of the photogenerated e⁻ to produce the Fenton’s reagent) and also because it is an n-type semiconductor. But, this application is still limited because TiO₂ only responds to UV light, which makes up <5% of solar energy, and the recombinant high electron / hole pair reduces a large amount of energy lost due to heat, decreasing quantum efficiency and thus treatment performance. Thus, the use of effective methods to overcome TiO₂ deficiency is essential to enhance the catalytic activity of the anode. Among the researches, TiO₂ modification with a narrow band gap semiconductor shows excellent potential. Because such modifications can effectively enlarge the catalyst light response area due to the semiconductor excitation of the narrow band gap, especially in the visible light region. On the other hand, the heterogeneous performance between the two semiconductors separates the generated carriers from the photogenerated and increases the lifetime and improves the quantum performance of the composite catalyst.

However, a more in-depth analysis of the photoanode materials should be more performed. Due to insignificant conductivity characterizations and its photo absorbency in the range of the VLR up to approximately infrared, the photoanode materials with low Eg have led the negligible change of power density, photocurrent and voltage output in the attendance and inexistence of the LR source and pollutant degradation. Moreover, the degradation trend of pollutants is affected by the e⁻ activity recombination rate and Eg of photoanode materials. In the PFCs, as it maintains the e⁻ - h⁺ pair separation of photoanode, the presence of dissolved oxygen can improve the degradation of pollutants and power generation.

Durability and reusability of photonode is essential for its application in the real world and economic practicality. One method to determine the stability and reusability of electrodes is XRD test. In this regard, XRD test can be applied for fresh and used photoanodes and if the crystalline phase of the photoanode remains constant after several consecutive periods, it can be seen that the used photoanode can be reused and therefore can be used in the wastewater treatment.

Regarding dye removal by PFCs, it can be concluded that the molecular structure of the dyes and LR source significantly influence on dye degradation and power generation. Due to the formation of HO, higher aeration rate, electrolyte pH and degradability of pollutants, as well as a lower concentration of a pollutant in the PFCs, led to an increase of pollutant degradation. However, an increase in resistance leads to reduced pollutant degradation. Furthermore, as raising the LR intensity generates more photo-excited e⁻- h⁺ pairs, the efficiency is enhanced. The results of the analyzed studies show that PFCs with the membrane electrode assembly structure has a higher pollutant decomposition and simultaneous power generation. Meanwhile, the increase of organic pollutants in the wastewater led to enhance electricity generation.

In the hybrid PFC-Fenton system, as a result of the introduction of ferrous ions into the PFC to strengthen the radicals' reaction and promote ^•OH production, the performance of the system was improved for both degradations of pollutant and power production. The use of photoanode and photocathode materials in different forms can lead to an outstanding improvement of both pollutant decomposition and power generation.

Nowadays, the utilization of PFC systems was mostly concentrated on the expansion of high-efficiency photoanode substances, binary photoelectrodes PFCs, and multi-purpose PFCs. Further research work is necessary to improve the performance and economic viability of these systems, the main aspects for each answer are still pending focusing on: i) what is the relevance between the physicochemical characteristics of electrode materials and the performance of related cell?; ii) how can state-of-the-art nanotechnology be combined to further increase the cell yield?; iii) can a PFC device be applied for practical usage?; and iv) how to efficiently employ the generated electricity or H₂? Therefore, these topics are of critical significance for future expansions of any PFC device. Merged approaches that combine advances in theoretical computation and experimental data can supply the needed insights towards the essential related between electrode materials and cell efficiency. This can sufficiently progress the basic knowledge of electrode structures and result in a new strategy of energy generation with practical and efficient real-life use.

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.

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Pollutants degradation and power generation by photocatalytic fuel cells: A comprehensive review

Abstract

Abstract

Keywords

Photocatalytic fuel cells

Pollutant degradation

Power generation

Photocatalysis

1 Introduction

2 Pollutant degradation

2.1 Bacteria inactivation

3 Power generation

3.1 Electricity generation

3.2 Hydrogen generation

4 Photoanodes

4.1 The effects of different photoanode materials

4.2 Photoanode materials characterization

5 Photocathodes

5.1 Photocathode materials for electricity generation and dye degradation

5.2 Photocathode materials characterization

6 Anodic nanocomposites

7 Photocatalytic fuel cell and Fenton process

8 Conclusions and recommendations

Declaration of Competing Interest

References

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