Photocatalytic Fuel Cells for Simultaneous Wastewater Treatment and Power Generation: Mechanisms, Challenges, and Future Prospects

Abstract

Technological advancement is accompanied by excessive consumption of fossil fuels and affluent uses of chemical substances in many sectors, including transportation and manufacturing companies, and so on. Being an exhaustible resource, the excessive use of fossil fuels and of chemical substances may lead to a serious energy crisis in the long run, and it may additionally impose environmental pollution. Attempts have been made in the solution of such serious issues from every nook and corner. Nonetheless, no method has been found to be a panacea in waste water treatment and subsequent beneficiaries. One of the attempts in the solution to such issues is the application of photocatalytic technology, which could serve as a dual function in environmental remediation and clean energy production. A photocatalytic fuel cell is a tool developed for the recovery of energy from organic wastes. A rational cell construction needs the fabrication of photoelectrodes, the design of a photoanode and a photocathode chamber, in addition to an ion-transport membrane for pollution treatment and electricity generation. In this review, comprehensive fundamental assessments and recent developments in the design of photocatalytic fuel cells, their applications, future prospects, and challenges are covered.

1. Introduction

Large scale chemical synthesis, their derivatization, and commercial applications lag with rampant development, industrialization, and urbanization, which ultimately has consequences for the generation of varieties of complicated pollutants or synthetic compounds. Cumulative and continual disposal of such hazardous chemical substances into natural water bodies can pose an immense threat to human health and the environment [1,2,3]. The benefit of rampant availability of various synthetic chemical substances for a luxury life and the present world technology has created the burden of their appropriate disposal [4]. Traditionally, such solid and liquid effluents were being disposed into natural water sources like lakes, rivers, and oceans while gaseous effluents were disposed into the open sky as per the dictum ‘dilution is the solution of the pollution’ [5]. These days, the paradigm has shifted with the dawn of subsequent reflective problems and excessive and continual waste production. The present day’s talk is about the uses of non-hazardous materials with the proper disposal of waste materials in the vein of green chemistry. Therefore, this view wakes up to develop a new technique to dispose of waste materials with simultaneous energy generation. The constituents of waste materials could be heavy metal ions, organic compounds, and many other hybrid compounds [6,7,8]. The majority of the chemical waste obtained from dyes, textile, and chemical industries are organic compounds [9,10]. Photocatalysts can be used to degrade such organic materials. Hunge et al., developed an ultrasound treated graphene oxide/titanium oxide composite as a photocatalyst for the degradation of salicylic acid under sunlight illumination [11]. However, photocatalytic degradation of organic compounds is not the end story in the field of electrochemistry. Rather, a simultaneous action of photocatalytic degradation of organic compound and energy production is possible.

Organic matter can serve as an important source of chemically stored energy. In one facet, the presence of organic compounds in the air, water, or soil, poses a great environmental problem as waste material, and in another facet it serves as an important source for the recovery of energy from wastes. In this context “filling two needs with one deed” is the exploitation of chemical energy at the cost of organic waste degradation. Microbial fuel cell (MFC) can be a green approach for the utilization of waste organic materials for bioelectricity [12,13,14]. For decades, such bio-based electrochemical technologies have been studied and operated for the simultaneous decomposition of organic waste materials and electricity generation with the aid of the microbial community. However, many such pollutants are recalcitrant and difficult to degrade biologically [15]. Besides this, stringent working conditions, complexity in bacterial cultivation, poor bio-electrochemical activities, a long time, etc., have led to the search for other alternative methods for the generation of energy from waste organic materials [16,17]. In this run, photocatalysis function has been coupled with fuel cell operation—“the marriage of photocatalysis and fuel cells”—to develop alternative technology, such as the photocatalytic fuel cell (PFC) [18,19].

A photocatalyst is a semiconductor that increases the rate of reaction. Photocatalysis is a light-induced green chemical pathway in which especially organic compounds are oxidized into other products in the presence of light [20,21,22,23]. The photocatalytic oxidation process has been widely employed in the degradation of organic pollutants in air and water [24,25,26,27,28]. Light is cost-free, perpetual, clean, and convenient energy. An electrochemical cell devised to convert chemical energy stored in organic materials into electricity under the irradiation of sunlight is commonly termed a photoelectrochemical cell (PEC). PEC comprises three types of cells, viz; a photovoltaic cell, a photoelectrosynthetic cell, and a photocatalytic cell [29]. In this review article, an overview of photoelectrodes used in PEC and PFC is given. Subsequently, electrodes and their methods of preparation with major features of PFC are discussed along with their operation mechanism. Finally, the applications, future prospects, and challenges of PFC are covered.

2. Development of Single and Dual Photoelectrode PEC

Initially, hydrogen gas was produced by a water reduction process using PEC with a single photocatalyst. Electrodeposited cuprous oxide protected with nanolayers of aluminum-doped zinc oxide and titanium oxide Cu₂O/(Al/ZnO-TiO₂/Pt) was used as a promising photocathode for water reduction. In this case, the cathode was activated by the electrodeposition of platinum nanoparticles [30]. Later on, two electrodes PEC system (FTO/WO₃ nanosheet as photoanode and Cu/Cu₂O nanowire/NiO_x as photocathode) were used for hydrogen production by water reduction [31]. The different band gap of Cu₂O (2.0 eV) and WO₃ (2.6 eV) permits an efficient and complementary light absorption for sunlight-driven water splitting phenomena. However, hydrogen production via the PEC system demands an external electrical energy supply.

A bit different from the PEC, a fuel cell has been devised for the direct conversion of chemically stored energy into electrical energy. The fuel cell is an energy conversion electrochemical device, where electricity is generated at the cost of an underlying chemical reaction [32]. The incorporation of the photocatalytic oxidation process in fuel cell operation gives birth to a photocatalytic fuel cell (PFC). Therefore, at its highest performance, a photocatalytic fuel cell is noted for its trios function of simultaneous organic pollutant degradation, electricity generation, and hydrogen production. Compared to those of solar cells, PFC uses organic wastes as fuel for the generation of electricity [33].

At the same time, a solar-driven fuel cell was used for the generation of electricity from wastewater. For this purpose, a photocatalytic fuel cell with a single photoelectrode (e.g., BiOCl/Ti as photoanode and Pt as photocathode) was used for the degradation of organic pollutants and simultaneous electricity generation. Later on, dual photoelectrode (WO₃/W or TiO₂/Ti as photoanode and Cu₂O/Cu as photocathode) functioning under visible light illumination was applied for simultaneous wastewater treatment and electricity generation [34]. Based on this background, the development of a solar-driven dual photoelectrode PFC has come to the fore for simultaneous organic pollutant degradation and electricity generation. Under the irradiation of light, photogenerated holes are the majority for oxidation in n-type semiconductors and photogenerated electrons are the majority for the reduction in p-type semiconductors. In the formulation of PFC, the n-type semiconductor material is made anode which is expected to exhibit good photocatalytic properties, and the p-type semiconductor is made cathode where hydrogen production is expected. For p-type semiconductors as the photocathode, the electrode potential E_CB Vs NHE is required to be more negative than the normal hydrogen electrode potential (E_H+/H₂). A schematic diagram of a single photoelectrode with Bi-TiO₂ coupled with Pt cathode for RhB dye degradation is shown in the Figure 1 [35].

3. Electrodes Used in PFC

A typical photocatalytic fuel cell consists of two electrodes out of which one or both can be a photoelectrode. A photoanode comprises an n-type of semiconductor photocatalyst which consists of sufficient photoexcited electrons of the n-type of semiconductor [36]. It has a higher fermi energy level and functions as a strong oxidizing electrode for the degradation of organic materials. It supplies photogenerated electrons toward the cathode. The photoanode performs a dual function as a photocatalyst for photocatalysis and as an electrocatalyst for fuel cell activities. As the photoanode has a significant role in the generation of photocurrents, the nanoarchitectonics of anode materials seem to be of paramount importance. In the PFC system, the values of bandgap energy (E_g), the recombination rate of photoelectrons, and the feature of materials play a vital role in its performance. The fast induced electrons and their recombination with a hole in the photoanode can limit the performance of photoelectrochemical reactions. The bandgap energy of anode materials can be tuned by doping or by composite formation [37,38]. Commonly used photoanode in PFC includes TiO₂, ZnO, WO₃, α-Fe₂O₃, BaTiO₃, BiVO_4, etc. [39,40,41,42,43,44,45,46]. Among them, TiO₂ is the most studied photoanode material applicable for PFC [47,48,49]. An increased interest in the use of TiO₂ as a photocatalyst is preferable due to its stable physical and chemical features, low cost, facile methods of preparation, non-toxicity, profound stability, and excellent optoelectronic properties. Titanium oxide has been presented in various forms such as nanofilm, nanopowder, nanotubes, nanorods, etc. [50,51,52]. Furthermore, the use of nanoporous material avails the high surface area and increased the efficacy of photoelectrochemical performance. Due to the relatively higher bandgap of TiO₂ (3.0–3.2 eV), it can absorb approximately 4% solar radiation, necessitating UV absorption for photoexcitation of electrons. On the contrary, the photogenerated electrons are more prone to combine with the hole in TiO₂ resulting in reduced photocatalytic degradation [53]. Against these constraints, it is imperative to develop a visible light-responsive TiO₂ for its application in photoelectrochemical fuel cell. To overcome the limitations of TiO₂, various efforts have been put to facilitate visible light absorption and enhance the charge carrier separation. Some of such efforts includes surface modification [54], metal/metal oxide doping [55,56], non-metal doping [57], dye sensitization [58], etc.

Doping or composite formation with TiO₂ brings about a change in the band gap energy and causes a shift in the UV absorption region, resulting in enhanced photooxidative properties [59,60,61]. For instance, the E_g of TiO₂/BiVO₄ composite reported 2.5 eV, while that for pure TiO₂ and pure BiVO₄ were 3.2 eV and 2.35 eV, respectively. In this work, the composite exhibited better photooxidative properties in the degradation of stearic acid and Rhodamine B compared to that from either component [62]. Silver nanoparticles incorporated TiO₂, CdS-ZnS quantum dot sensitized TiO₂ lead to the absorption band shifting to the visible region to harvest solar radiation [63,64,65]. Furthermore, the calcination temperature of TiO₂ also affects the UV absorption properties and influences the performance of the photoelectrochemical cell. Gorska et al. used several calcination temperatures ranging from 350 °C to 750 °C to prepare TiO₂ photocatalysts and they observed the best photocatalytic activity in the TiO₂ prepared at 450 °C [66].

Commonly, platinum or platinum-based materials are used as cathode in photoelectrochemical fuel cells. A photocathode in PFC is made up of a p-type of semiconductor. Cuprous oxide (Cu₂O), hematite (α-Fe₂O₃), reduced graphene oxide (RGO), ZnO/CuO composite, CuO/Co₃O_4, etc., have been reported to be used as a photocathode in PFC [67]. It has a lower Fermi energy level. At the cathode, electrocatalyst or photocatalyst induces an oxygen reduction reaction (ORR) in the presence of oxygen (aerobic condition) and water reduction reaction (WRR) or hydrogen evolution reaction (HER) in the absence of oxygen (anaerobic condition) [68,69,70]. As a consequence, either water or hydrogen is produced at the cathode depending upon the presence or absence of oxygen, respectively. Both electrodes are in contact with electrolytes in an electrochemical assembly. The photogenerated electrons are transported to the cathode via an external circuit due to the existing potential difference between the photoanode and the cathode, resulting in the generation of electricity [71]. As a net reaction, light energy is used to convert chemical energy into electrical energy directly in the fuel cell. In some cases, a reduction reaction may not be spontaneous at the photocathode due to the development of overpotential between the photocathode and the electrolyte. Such a condition necessitates the use of an electrocatalyst such that it would create a large interface between these two phases, and it facilitates the electron exchange with the electrolyte. A commonly used electrocatalyst for PFC cathode is a nanostructured platinum-coated carbon black in which the reaction depends upon the availability of oxygen. Water is produced under aerobic conditions and a hydrogen evolution reaction occurs under anaerobic conditions. However, by all means, the use of platinum is hindered due to its aggregation and higher cost tendency. Therefore, material scientists are searching for its alternatives. One of the most popularly used photocatalysts is Cu₂O (bandgap of 2.0 eV), which is good for water reduction [67]. Besides this, cupric oxide (band gap of approximately 1.5 eV) can behave as an effective photocathode [72]. Titanium oxide and their various composite materials have been developed for the degradation of organic materials. Hunge et al., developed the titanium oxide@nanodiamond composite for the photocatalytic degradation of bisphenol A [73]. Graphene modified TiO₂ have shown a high performance for organic pollutant removal due to the several benefits offered by graphene [25,74,75,76,77,78]. According to Tan et al. [79], besides the role of graphene, the type and the number of functional groups of the organic also determines the overall photocatalytic activities.

In a typical study, organic wastewater or electrolytes such as sodium sulfate, sodium chloride, etc. are put into the reaction chamber as electrolytes. Any organic matter such as dyes (methylene blue, methylene green, Congo red, methyl orange), drugs, phenols, etc., can act as fuel in PFC [80,81]. Besides these, photoelectrical treatment of landfill leachate, oil mill, textile, pharmaceutical, and tannery wastewater with simultaneous degradation organic matters and electricity generation can be carried out [82].

Table 1. Electrodes and their methods of preparation with major features of PFC.

Photoanode Materials	Cathode/Photocathode	Methods of Preparation	Special Features	Performance	Ref.
TiO2 NRs/FTO	C/Cu2O NW As/Cu	Hydrothermal/anodization	Phenol degradation and simultaneous hydrogen production	TOC removal rate 84.2% and H2 production rate 86.8 μmol cm−2	[34]
Au/TiO2	Pt	Pulsed laser deposition (PLD)	Bisphenol A (BPA) oxidation, water splitting, hydrogen production	Photocurrent density = 58 μA/cm2	[83]
TiO2 nanotube array	Cu	Electrochemical anodization	Micropollutant removal from water	62% MCPA removal	[84]
ZnO/Zn	Pt/C	Heat attachment method	Degradation of reactive green 19	100% removal of RG19, Jsc = 0.0427 mAcm−2, and (JV)max = 0.0102 mW cm−2	[85]
BiVO4 bismuth vandate	Cu2O	-	Good stability, Electricity generation and waste water treatment.	Voc = 0.463 V and Jsc = 0.113 mA cm−2	[86]
CdS-ZnS-TiO2	Pt black on carbon black	Successive ionic layer adsorption and reaction	Organic compound degradation and electricity generation.	JVmax = 1.01 mW cm−2 at a current density of 1.4 mA cm−2. FF = 0.45	[65]
WO3/W	Fe@Fe2O3/carbon felt electro-fenton	Hydrothermal process	Waste water treatment and electricity generation.	Jsc = 0.59 mA cm−2, JVmax = 0.34 mW cm−2	[1]
WO3/TiO2	-	Spray pyrolysis method	Photoelectrolytic degradation of benzoic acid	Benzoic acid degradation rate of 46.56%, Voc = 0.485 V Isc = 0.575 mA	[87]
Ag-TiO2	Pt/C on carbon cloth	Solvothermal process	Coulombic efficiency of 9.4%	JVmax = 1.85 Wm−2 COD removal = 14.8%	[88]
MoS2/TiO2	MoS2/Ni foam	Hydrothermal process	Higher light harvesting, electron transport efficiency, Rhodamine B (20 ppm, pH =7) degradation efficiency of 69.16%.	Voc = 0.7 V Isc = 1.04 mA cm−2 JVmax = 0.114 mW cm−2.	[36]
WO3/W	Cu2O/Cu	Hydrothermal process	Waste water treatment and electricity generation	Degradation rate of phenol, Rhodamine B and Congo red were 58%, 63%, 74%, respectively.	[67]
BiOCl/Ti	Pt	Wet chemical synthesis and dip coating method	Rhodamine B and salicylic acid degradation and electricity generation	Jsc = 0.0116, Voc = 0.655 V, ff = 0.39 for Salicylic acid under UV irradiation.	[71]
Bi doped TiO2	Pt	Sol-gel method	Rhodamine B degradation and photocurrent generation	91.2% degradation of Rhodamine B	[35]
NiFe layered double oxide/TiO2	Carbon black	co-precipitation/calcination	Hydrogen peroxide production	Voc = 0.78 V Jsc = 1093 μA cm−2. Pmax = 169 μW cm−2.	[55]
BiVO4/TiO2 NTs	ZnO/CuO NWs	Hydrothermal/liquid phase deposition	Degradations of methyl orange, Congo red and methylene blue are 76%, 83%, and 90%, respectively.	JVmax = 0.116 mW cm−2.	[80]
Fe-graphene oxide-titanium phosphate	ZnIn2S4	Hummer’s modified method, wet chemical method	Degradation of Rhodamine B	90% removal capacity for Rhodamine B at pH 1.	[89]

shows some reported photoelectrodes and their methods of preparation with major features.

4. Mechanism of Photocatalytic Fuel Cell

The working of the photocatalytic fuel cell can well be described in terms of the band theory of solids. As an isolated atom is characterized by filled and vacant atomic orbitals, their assembly (lattice containing many atoms) constitutes the form of closely spaced filled and vacant orbitals, essentially forming a continuous band. The filled bonding orbitals (Highest occupied molecular orbital, HOMO) constitute the valence band (VB) and the vacant antibonding orbitals (lowest unoccupied molecular orbital, LUMO) forms the conduction band (CB) [90]. The energy difference (in electron volt) between the top of the valence band and the bottom of the conduction band is termed the band gap energy (E_g). It is the band gap energy that strongly influences the optical and the electrical properties of the electrode materials in an electrochemical setup [91,92]. When the band gap is very small, the conduction band and the valence band overlap such that electrons can move from the valence band to the conduction band easily, as that happened in a metallic conductor.

For larger values of E_g, the valence band is almost filled and the conduction band is almost vacant. Thermal excitation of electrons from the valence band to the conduction band makes a possibility of conduction in the materials. This constitutes a hole (electron deficit center) at the valence band where rearrangement of electrons is possible resulting in the mobility of holes. In case the band gap is greater than 1.5 eV, only a few charge carriers are available, which are ineffective for electrical conductivity. The different band gap regions are associated with the strong absorption edges of the electromagnetic spectrum. For instance, a band gap of approximately 1.2 eV corresponds to the near infrared region and a band gap of approximately 2.0 eV corresponds to the mid-visible region. A band energy of approximately 3.0 eV corresponds from the blue region to the ultraviolet region. A composite formation results in the band gap shifting. [93]. The addition of dopants (n-type or p-type) can introduce holes in the valence band and electrons in the conduction band. An n-type of dopant avails electrons, while a p-type of dopant pulls the resulting electrons into the formation of holes [94].

Tuning of electrochemical response in semiconductor electrodes can be made by altering the impurity levels, surface treatment, or surface adsorption [95]. The semiconductor–solution interface is very crucial in the conversion of light energy into electrical energy by photo-assisting the electron transfer between the semiconductor and the chemical species in a solution [96,97]. Besides this, the potential of donor/acceptor redox couples has also a crucial role in the photocatalysis process. The Fermi level of photoexcited electrons in the CB should be higher than the energy level of the acceptor otherwise, the reducible chemical species is unable to receive electrons at the cathode. In addition, the level of the holes (h⁺) in the VB of the semiconductor electrode should be at a lower position than that of the donor [83]. In the PFC couple designed by Chen et al., the WO₃/W acts as a photoanode, and Cu₂O/Cu acts as a photocathode where organic compounds can be decomposed (oxidized) by the holes of WO₃/W photoanode only due to its higher oxidation power (approximately +3.1 V_NHE) [83]. The WO₃/W photoanode is an n-type semiconductor where the Fermi energy level is near the E_VB, and the Cu₂O/Cu acts as a p-type semiconductor where fermi energy lies near the E_CB. The schematic diagram is shown in Figure 2.

Both the photoelectrodes can generate electron/hole pairs. The Fermi energy level of WO₃/W photoanode is lower (more negative) than that of Cu₂O/Cu as a cathode. Therefore, an interior bias is produced to cause the migration of photoelectrons of WO₃/W photoanode to combine with the hole of Cu₂O/Cu photocathode via an external circuit. The holes of WO₃/W photoanode as well as electrons of Cu₂O/Cu photocathode can oxidize the organic matter (R) with simultaneous water cleaning and energy recovery.

Reaction of hole of photoanode: nh⁺ + R → R’ + nH⁺

Reaction at photocathode: 2H⁺ + 2e⁻ → H₂

When the energy of irradiated light exceeds the bandgap energy, the photoexcited electrons can be promoted from VB to CB, which facilitates the oxidation reaction at the anode and the reduction reaction at the cathode.

Photoanode + hυ → h⁺ + e⁻

During this process, the photoexcited electron (e⁻) is transferred to the CB of the semiconductor electrode, while the photoexcited hole (h⁺) remains with the VB. The photoexcited charges (electrons and holes) migrate on the surface of the semiconductor electrode, and they accomplish the underlying redox reaction.

4.1. Photoreduction Reaction

The photoexcited electrons migrate to the cathode and may react with dissolved oxygen to produce superoxide radicals (^•O⁻₂) or hydrogen peroxide (H₂O₂) or water (H₂O) or hydroxyl radical (OH⁻).

O₂ + e⁻ → ^•O⁻₂ (strong oxidant)

The transportation of photoexcited electrons to the counter electrode via an external circuit generates electricity. The transport of electrons from electrodes to the electrolyte solution is governed by the Butler–Volmer equation [98].

$$\frac{i}{i0}=\exp \left(\frac{{\alpha }_{\mathrm{A}}\mathrm{n}\mathrm{F}}{\mathrm{R}\mathrm{T}}\mathsf{\eta }\right) – \exp \left(\frac{{\mathsf{\alpha }}_{\mathrm{C}}\mathrm{n}\mathrm{F}}{\mathrm{R}\mathrm{T}}\mathsf{\eta }\right)$$

where i and i₀ are instantaneous current density and exchange current density (in Am⁻² unit), α_A and α_C are transfer coefficient (unitless) for anodic and cathodic current, respectively, n is the number of electrons transferred, F is Faraday’s constant, R is the universal gas constant, T is the absolute temperature and η is the over potential (deviation from equilibrium potential due to electrode polarization), respectively.

Furthermore, hydrogen ions are consumed in the presence of oxygen to give the hydrogen peroxide or water in an acidic or neutral medium. These are the cases of oxygen reduction reaction (ORR).

O₂ + 2 H⁺ + 2 e⁻ → H₂O₂        E° = + 0.68 V (Oxygen reduction reaction)

O₂ + 4 H⁺ + 4 e⁻ → 2 H₂O        E° = + 1.23 V (Oxygen reduction reaction)

However, water reduction reaction (WRR) occurs at a basic medium.

$$\frac{1}{2}{\mathrm{O}}_2 +{\mathrm{H}}_2\mathrm{O}+2 {\mathrm{e}}^{-} \to 2 {\mathrm{OH}}^{-} (\mathrm{Water} \mathrm{reduction} \mathrm{reaction})$$

In the absence of oxygen at the cathode, the reduction of hydrogen ions takes place to produce hydrogen gas. This is often referred to as a hydrogen evolution reaction (HER) [70].

2 H⁺ + 2 e⁻ → H₂ (proton reduction, at acidic pH, in absence of oxygen)

4.2. Photooxidation Reaction

The photoexcited hole (h⁺) in the VB may react with water molecules or hydroxyl ions to produce free hydroxyl radicals (^•OH). The H⁺ ions produced in the photooxidation process are brought to the cathode by a diffusion process in the electrolyte.

H₂O + h⁺ → ^•OH + H⁺

OH⁻ + h⁺ → ^•OH (strong oxidant)

4.3. Decomposition of Organic Compound

Such photogenerated holes can act as an oxidant to decompose organic matter in wastewater. As a consequence, reactive chemical species such as superoxide oxygen radicals, hydroxyl radicals, etc., decompose the organic compound due to their strong oxidizing action. Besides these, some of the photoexcited charges may undergo recombination with the production of heat [99].

Organic compound + ^•OH → CO₂ + H₂O + oxidized organic compound

$$\mathrm{Organic} \mathrm{compound}+{}^{•}{\mathrm{O}}_2^{-}\to {\mathrm{CO}}_2+{\mathrm{H}}_2\mathrm{O}+\mathrm{oxidized} \mathrm{organic} \mathrm{compound}$$

In the photocatalytic fuel cells, organic pollutants act as fuel, which are brought to decompose under the irradiation of light at photoanode whereby electrons are photoexcited, which are driven by the potential difference between photoanode and photocathode. The efficacy of photocatalytic fuel cells is governed by many factors such as choice of visible-light photoelectrodes, cell design, duel photoelectrode setup, and inter-relation of these factors. Therefore, optimization in the formulation of effective semiconductors and rational design of photocatalytic fuel cells for organic waste degradation and electricity generation is a crucial aspect in the design of photocatalytic fuel cells.

The power density in PFC can be calculated by the relation,

Power density (P) = current density (J) × voltage (V)

Power density (P) can be expressed in mW cm⁻², current density (J) can be expressed in mA cm⁻², and voltage (V) can be expressed in mV [100].

5. Recent Development in the Design of PFC

In the early days, one of the most frequently used photoanodes was titanium oxide and the cathode was platinum. Later on, modification in electrode materials was carried out for a more efficient and versatile photoelectrode. These days, findings show that a dual photoelectrode could be more effective in the performance of PFC. In the development of a photoelectrode, binary or ternary materials are synthesized in search of better performance [101,102]. Besides these, many techniques are incorporated in the designing and the functioning of PFC. Great attention has been paid to the selection of electrode materials and their design in the formulation of PFC that is capable of performing multiple functions, including wastewater treatment, photoelectricity generation, hydrogen evolution, and carbon dioxide reduction [103]. Some PFCs are also capable of removing heavy metal ions such as Cr(VI) [104].

The photocatalytic activities of the PFC device can be increased by developing optofluidic technology, which is derived from microfluidic technology and optic fluidic technology. He et al., developed an optofluidic photocatalytic fuel cell composed of bismuth vanadate (BiVO₄) as photoanode and cupric oxide (Cu₂O) as photocathode for the generation of electricity from wastewater treatment [86]. A solar responsive photocatalytic fuel cell with CdS-ZnS quantum dot sensitized TiO₂ as anode and Pt coated carbon as cathode was developed by He et al. and applied for simultaneous wastewater treatment and electricity generation [65]. Lui et al. developed a flow-photocatalytic fuel cell using Ag-TiO₂ as a photoanode and Pt/C as a photocathode. When fed with real brewery wastewater, a continuous power generation of 1.85 W m⁻² under simulated light was observed [88].

Chen et al. developed WO₃/W photoanode and Cu₂O/Cu photocathode for the simultaneous wastewater treatment and generation of electricity [67].

To enhance the efficacy of PFC, Li et al. have designed a dye self-photosensitization photo fuel cell (DSPFC), using bismuthoxychloride/titanium as photoanode. In this case, BiOCl is not activated directly in the visible region due to its wide band gap. Adsorption of some dyes such as rhodamine B and methylene green onto the surface of BiOCl can cause self-photosensitization. Dye adsorbed onto the BiOCl surface is excited by the visible light whereby electrons are injected from excited dye to the CB of BiOCl. Such excited electrons are scavenged by molecular oxygen to give oxygen ion radicals (O₂^{• −}). Herein, the conduction band of BiOCl (−1.1 eV) is more negative than that of O₂/ O₂^{• −} (–0.046 eV) and hence the oxidation activity of O₂^{• −} is high which is sufficient to cause decomposition of the dye [71]. The selection and the formulation of the photoanode and the cathode have been done in such a way that the output of PFC in terms of open-circuit voltage (V_oc), short circuit current (J_sc), maximum power density (P_max), and photocurrent density have been greatly considered. Some of the PFCs with the corresponding photoanode, cathode/photocathode, the V_oc, J_sc, P_max, and PCD values are given in

Table 2. Some electrochemical parameters of MFC.

SN	Photoanode	Cathode/ Photocathode	OCV (Voc)	SCC (Jsc) mA cm−2	MPD (Pmax) μW cm−2	PCD mA cm−2	Degradation Profile	Ref
1	g-C3N4/WO3/TiO2/Ti	Pt	-	0.20	19.35	2.27 at 1.1 V vs. SCE	RhB degradation by 87.7%	[105]
2	NiFe-layered double hydroxide/TiO2	Cu2O	0.690	0.622	206	1.85 at 1.6 V vs RHE	MB degradation (10 ppm) 94.6%	[106]
3	Bi2WO6/ZnO nanorods	Pt	563 mv	37.10	3.30			[107]
4	ZnO/C	Pt/C	1000 mV	0.0069	2.34	-	Degradation of Azo dye reactive black 5 (13.6% at anode and 8.7% at cathode)	[70]
5	BiVO4/TiO2 NTs	ZnO/CuO NWs	0.53 V	0.43	0.116 mW cm−2	-	Degradation of Methyl orange (76%), Congo red (90%), and Methylene blue (83%)	[80]
6	TiO2NRs/FTO	C/Cu2O/Cu	0.41 V	0.50	-	-	Phenol removal (84.2%)	[34]
7	ZnO/Zn	Pt/C	1050 mV	0.0327	0.0076 mW cm−2	-	Degradation of reactive green 19	[85]
8	TiO2/FTO	TiO2	0.98 V	0.175	0.110 mW cm−2	-	CO2 utilization	[108]

OCV = Open circuit voltage (V_oc), SCC = short circuit current (J_sc), MPD = maximum power density (P_max), PCD = Photocurrent density.

.

6. Applications of PFC

PFC has been exploited for many purposes. Some of the notable applications are as follows.

a. 

Wastewater treatment

Organic materials present in wastewater owe a significant amount of chemical energy, which can be transformed into electrical energy. Primarily, microbial fuel cells are being used to retrieve the electrical energy from such chemical storages. However, due to the biorefractory nature, MFC is not an appropriate device to treat persistent organic compounds such as azo dyes. In these contexts, PFC systems are getting more preference over MFC due to simple operational methods, efficacy of organic matter degradation, and direct electron transfer. The major constituents of textiles discharges consist of methyl oranges, methylene blue, and Congo red. The PFC can treat azo dye effectively.

b. 

Recovery of chemical energy from organic matter

Photoexcitation onto the photocatalyst is accompanied by the formation of electrons and holes. The holes are consumed in the oxidation of organic matter at the photoanode and the photoexcited electrons are transported to the photocathode. The quantitative generation of electricity depends upon various factors such as the nature of the electrolyte, concentration of electrolyte, efficacy of photoanode and photocathode, light intensity, pH, etc. The short-circuit current, open-circuit voltage, and power density change with the change in different parameters like concentration of electrolyte, pH, nature of organic compounds, etc. [99]. Li Kan et al., showed the uses of real textile wastewaters as a fuel for the generation of electricity and employed as a bias voltage (0.6–0.75 V) in a PFC reactor [109].

c. 

Hydrogen production

Wu et al., developed a solar-driven PFC with dual photoelectrodes using TiO₂NRs/FTO as photoanode and C/Cu₂O/Cu as photoanode for simultaneous wastewater treatment and hydrogen production. The operating mechanism of dual photoelectrode PFC is given in Figure 3. The PFC showed superior performance for the degradation of phenol and hydrogen production. In this case, the total hydrogen production rate was 86.8 μmol cm⁻² [34].

d. 

Carbon dioxide reduction

These days, PFC has found its application in the reduction of carbon dioxide gas. Xie et al. have developed a coupled system of photocatalytic reduction and a photocatalytic fuel cell. In this system, carbon dioxide is first reduced to hydrocarbon by photocatalytic reduction, and the as-obtained compound is used as a fuel in PFC to generate electricity directly [108]. In the work of Yu et al., the photocatalytic fuel cell has been fueled with carbon dioxide saturated methanol-sulphuric acid solution. The design could hinder the recombination of electron-hole pairs and enhanced fuel cell performance in this engineering has been found [110]. As in the Figure 4, the CH₃OH and H₂SO₄ mixed solution saturated with CO₂ was supplied to the photoanode and the H₂SO₄ solution was supplied to the cathode.

7. Future Prospect and Challenges

Effective cell design and the architecture of photoanode has a crucial impact on the efficacy of photocatalytic fuel cell as it affects the generation of current by the transport of electron from photoanode to cathode. Furthermore, the conductive substrate-coated photocatalytic semiconductor can play a substantial role in the development of photocatalytic fuel cells with profound light-harvesting capacity. Conductive polymers such as polyaniline, polypyrrole, polythiophene, [111,112] etc., can be used in the development of composite materials either using with metallic nanoparticles or with nanofibers [113]. Surface functionalization can be performed to introduce some desired properties [114]. To enhance the surface area, nanostructured semiconductor conductive materials coated can be used with designed nanoarchitectonics. Such designs are required to be environmentally friendly and available at a low price. The generation of non-toxic byproducts in a PFC system could be supportive from the view of green chemistry. Some of the challenges in the synthesis of efficient PFC are their stability, purity, ease of synthesis, and cost factor. In some cases, the performance of PFC has been found to be reduced due to poor stability and photocorrosion, poor photoexcited charge separation, etc. For efficient performance, these methods and materials are required to exhibit high efficiency and good recycling stability. The development of PFC responding to both UV and visible light is a critical benefit in the successful formulation of PFC. Besides this, cost factor (using Pt as an electrode), toxicity concern (salt of cadmium), stability, and reusability are major issues to be considered in the development of PFC. Besides this, agglomeration is a major challenge in photocatalysis.

8. Conclusions and Recommendations

In real-world practice, the stability and the reusability of photoelectrodes is crucial for their sustainability and economic practicality. For the use of crystalline materials, XRD analysis of electrode materials before and after a certain duration of uses reveals the state of stability and reusability of electrodes. If the crystalline phase of the electrode materials remains almost constant even after several consecutive periods, the electrode is said to be stable, and it can be used successively. Further research work is necessary to increase the longevity of electrode materials. Uses of nanomaterial-based electrodes can avail a large surface area, inducing a higher rate of surface reaction. State-of-the-art nanotechnology and surface science can be applied to increase cell efficacy. Nanoarchitechtonics involves nano-creation and nano-organization with self-assembly, molecular manipulation, and chemical manipulation. Sustainable use of electrode material is necessary from the economic point of view.