Perovskite Paradigm: Unraveling Photoelectrochemical Synergies for Sustainable Transformations

Abstract

Owing to the tunable electronic properties, high carrier lifetimes, low recombination possibility, and long diffusion lengths, perovskites have gained attention for potential use in photoelectrocatalytic processes. Photoelectrochemical methods can convert sunlight into electricity or useful fuels, resulting in extensive research to develop PEC applications. This chapter embarks on a comprehensive exploration of the perovskite structure’s transformative influence on diverse photoelectrochemical cell (PEC) and monolithic Photovoltaic-Electrocatalytic (PV-EC) devices including water splitting, H₂ evolution, CO₂ reduction, N₂ reduction, degradation of pollutants, (bio) sensing, and organic synthesis. By assessing the mechanisms and kinetics involved, we aim to disclose the potential of perovskite-based photoelectrochemical systems in shaping the landscape of green energy and environmental stewardship. Furthermore, the chapter addresses the progress and challenges in enhancing the stability, selectivity, and efficiency of perovskite-based PEC and monolithic PV-EC reactions to unravel the synergistic potential for sustainable transformations in the realm of photoelectrochemistry.

1. Introduction

Photoelectrochemical (PEC) processes, also referred as artificial photosynthesis, serve as primary means to harness solar energy. These procedures transform sunlight, the most plentiful renewable energy source, into chemical fuels that can be stored, providing versatility in sequential energy transportation and storage. PEC techniques combine the advantages of both particulate photocatalytic and electrochemical systems, leading to higher energy conversion efficiencies by enhancing light absorption and improving charge separation and utilization [1]. Moreover, the PEC methods offer advantages of being cost-effective and easy to handle, enabling the production of the desired final products through thermodynamic and kinetics control [2]. By mimicking natural photosynthesis, PEC methods also provide fast reaction kinetics and selectivity for the products, all without requiring high input energy costs, which helps to minimize the rate of electron-hole recombination [3]. Unlike PEC devices, which rely solely on sunlight for power, monolithic photovoltaic-electrochemical (PV-EC) devices as a type of PEC systems, also known as integrated solar fuel generators or tandem devices, have the ability to utilize solar light during analysis and can also be powered by renewable electricity into a single device [4]. The objective of such systems is to surpass the constraints of individual solar cells and boost the overall efficiency of energy conversion. The photovoltaic cell captures sunlight and converts it into electricity. However, the electrochemical cell utilizes the generated electricity to power chemical reactions that generate fuels or other valuable products. By combining high-efficiency photovoltaic and electrochemical cells, these systems are able to capture a wide range of solar radiation and utilize surplus energy that would otherwise be wasted in individual PEC cells. PV-EC systems have the potential to enhance the durability and stability of devices. Furthermore, PV and EC components can be easily adjusted and optimized, allowing greater operational flexibility and better overall performance [5].

These techniques demonstrate significant potential in various fields, including in the case of water splitting, solar fuel production, environmental remediation, and chemical sensing [6, 7, 8].

At the core of these PEC systems lies a photoelectrode, typically crafted from nanostructured semiconductors with suitable bandgaps, enabling them to absorb light and produce electron-hole pairs for involvement in reduction/oxidation reactions [9]. Optimal semiconducting materials for photoelectrodes should exhibit favorable characteristics in light absorption, charge separation, and transmission. Yet, challenges persist due to crystal defects, unsuitable energy band structures, and electron-hole recombination, hindering the development of superior photoelectrode materials capable of efficient PEC conversions for solar energy utilization. Over the past decade, researchers have unveiled promising inherent power utilization capabilities in perovskites [10, 11, 12]. As an example, perovskite solar cells have attained a remarkable power conversion efficiency of 25.5% [13]. This remarkable improvement in photovoltaic performance has positioned perovskite solar cells (PSCs) at the forefront of developing cost-effective next-generation solar power and integrated technologies [14, 15]. Besides their ability to convert solar energy to electricity, perovskites have several other desirable characteristics including suitable band gap, high light-harvesting capability, good electron mobility, well-adjusted charge transport, redox behavior, multicolor emission, compatibility with localized surface plasmon effect, as well as cost-effective scalable fabrication processes [16, 17, 18]. These make perovskites highly valuable for various applications in the photoelectrochemical field [19].

Thanks to its compositional and structural flexibility, approximately 90% of the elements in the periodic table can be partially replaced at anionic or cationic sites in the perovskite structure [20, 21]. Furthermore, by various synthesis methods and operating factors, the multifaceted shapes, and different particle sizes of perovskite can be easily obtained. Considering the exceptional properties of perovskites, incorporating them into photoelectrode materials for PEC conversions could be deemed a prudent decision. Although they have promising applications in various fields, there are concerns about toxicity of certain perovskite materials, particularly lead-based perovskites. Also, perovskite materials are strongly criticized for their lack of stability in practical applications. For instance, halide perovskites exhibit high sensitivity to water vapor and oxygen, and they can readily dissolve or decompose in polar solvents, posing significant challenges in device operations [22]. Therefore, the development of stable perovskites through compositional engineering may be a practical effective strategy. This can be further enhanced through approaches such as crystal engineering [23, 24, 25], doping strategy [26], polaron arrangement [27], barrier layers [28], and encapsulation methods which not only can enhance the device stability but also can prevent from toxic exposure and environmental impacts.

This chapter aims to comprehensively explore the transformative influence of perovskite structures on various photoelectrochemical cell (PEC) and monolithic Photovoltaic-Electrochemical (PV-EC) devices. These devices include water splitting, H₂ evolution, CO₂ reduction, N₂ reduction, degradation of pollutants, (bio) sensing, and organic synthesis. By examining the mechanisms and kinetics involved, our goal is to uncover the potential of perovskite-based photoelectrochemical systems in promoting green energy and environmental stewardship. Additionally, the chapter addresses the progress and challenges in improving the stability, selectivity, and efficiency of perovskite-based PEC and monolithic PV-EC systems. Through these efforts, we aim to reveal the synergistic potential for sustainable transformations in the field of photoelectrochemistry and solar fuel production.

2. Perovskite structure

In 1839, Gustav Rose first discovered CaTiO₃ in the Ural Mountains. It was later named by Russian mineralogist Count Lev Alekseyevich von Perovski [29]. The term “perovskite” has come to encompass a group of organic and inorganic materials with crystal structures similar to ABX₃, now identified as the perovskite structure.

2.1 Perovskite oxide

In perovskite oxide with a typical ABO₃ structure, A-site is frequently occupied by a metal ion including alkaline, alkaline-earth, or lanthanide rare-earth metals, while B-site cations can be occupied by metal ions such as the transition metals (TMs). As shown in Figure 1(a), the ideal perovskite oxide has a cubic crystal with space group Pm-3 m, which has a high degree of symmetry. The B-sites are coordinated by six oxygens, forming the corner-sharing octahedrons of BO₆ and the cations in the A-site are surrounded by eight BO₆ octahedral arrangement [30]. Due to the lattice distortions caused by variations in ionic radii of the constituent cations, the symmetry of perovskite can be reduced to orthogonal, tetragonal, rhombohedral, monoclinic, and triclinic structures. These crystal lattice distortions impact on dipole moment and the electronic structure, leading to the emergence of new properties and applications in various fields [31].

Figure 1.

Schematic of the crystal unit cell of an ideal perovskite oxide with a cubic structure (a), Ruddlesden−Popper perovskite oxide with a layered structure (b) and halide perovskite with methylammonium as A-site cation (c).

Perovskite oxides can serve as proton and oxygen conductors, redox catalysts or multi-functional catalysts, depending on the various metal ions located in the A and B-sites [32]. Besides, the composition of perovskite oxides can be customized to produce specific perovskite structures such as double-perovskite structures, layered perovskite structures, and Ruddlesden−Popper (RP) perovskite structures with some exceptional physicochemical properties. Double perovskite oxides have the general formula A₂BB′O₆, where A is a monovalent or divalent cation, B and B′ are different trivalent cations that occupy equivalent positions in the crystal lattice. Layered perovskite oxides are another class of perovskite oxides with general formula A₂B_n-1 B_nO_3n + 1, where A is a monovalent or divalent cation and, B and B′ are different trivalent cations that form distinct layers separated by layers of A cations. Similar to layered perovskite oxide, Ruddlesden−Popper perovskite oxide has a layered structure, with alternative layers of perovskite-like and rock salt-like structures (see Figure 1(b)). However, in A-site was partially replaced by a larger cation, resulting in a layered structure with an extra oxygen layer [33]. The presence of various sites provides researchers with the opportunity to substitute or integrate different elements, leading to the development of new materials exhibiting significant photoelectrocatalytic activity. Moreover, the high electronic conductivity, elevated charge carrier mobility, and robust light absorption—spanning visible and ultraviolet spectra—position perovskite oxides as promising candidates for PEC devices [34].

2.2 Halide perovskites

In 2009, Kojima and his colleagues conducted pioneering work that sparked interest in halide perovskites as promising semiconducting materials. These materials gained attention for their potential application in photovoltaic devices [35]. Halide perovskites belong to a group of materials with the formula of ABX₃, A representing monovalent organic and/or inorganic cation, B is a divalent cation especially including IVA group elements (e.g., Pb²⁺, Sn²⁺), and X is a halide or mixed halide anions [36]. Mixed halogen-based perovskites exhibit numerous intriguing properties in contrast to single halogen-based perovskites, including enhanced thermal stability and a more manageable band structure. Figure 1(c) shows a crystal structure of a halide perovskite. In this arrangement, the B cation occupies the body center of an octahedron, which comprises six halide anions [BX₆]⁴⁻ [37]. In cubic symmetry, the octahedral are interconnected by sharing their vertexes. As the temperature declines, the octahedral structure initiates to slant, leading to a sequence of lower symmetries, such as tetragonal, monoclinic, orthorhombic, and rhombohedral. When an organic cation, like methylammonium (MA⁺) or formamidinium (FA⁺), occupies the A-site in the halide perovskite structure, it is known as an organic-inorganic hybrid halide perovskite [10]. However, the presence of organic cations usually results in the instability of the structure. On the other hand, to enhance stability, the organic cation can be replaced with an inorganic cation, such as (Cs⁺), resulting in an all-inorganic halide perovskite. The B site in the halide perovskites is commonly occupied by element Pb. Nevertheless, lead (Pb) is a recognized toxic metal capable of causing harm to both the environment and human health. As a result, there has been extensive research on using tin (Sn) as a partial or complete substitute for Pb. Nevertheless, tin-based halide perovskites are more prone to degradation compared to the lead-based perovskites. Additionally, other types of halide perovskites, such as halide double perovskites, layered halide perovskites, and Ruddlesden−Popper (RP) structures, have been studied as stable and environmentally friendly alternatives to lead halide perovskite [38].

Until now, halide perovskite has been extensively studied in various photocatalytic reactions, including hydrogen evolution, CO₂ reduction, and pollutant degradation [39, 40, 41]. The investigation into halide perovskites as a photoelectrocatalyst is motivated by their fascinating properties such as adjustable bandgap, high absorption coefficients, extensive carrier diffusion lengths, and long charge carrier lifetime. The stability of halide perovskites is the main challenge in various applications. Factors such as moisture, oxygen, light, and polar solvent can lead to rapid decomposition of halide perovskites. To address this issue, several strategies have been employed based on the intrinsic stability of halide perovskites. These strategies comprise: (i) substituting the organic MA or FA with inorganic ions like Cs⁺; (ii) developing composites that expedite the swift extraction of photogenerated electrons and holes [11, 42, 43]; and (iii) The exploration of encapsulating halide perovskites has been undertaken as a method to avoid direct contact with degradable environments [44].

3. PEC processes

Sunlight is widely recognized as the most promising energy source due to its abundance, carbon-free nature, worldwide availability, non-polluting, and limitlessly renewable source of clean energy. However, solar energy is known to be intermittent and unstable, which limits direct use in practical applications. By converting solar irradiation into chemical fuels, it can be conveniently stored and transported, providing a reliable and versatile energy solution [45]. Photoelectrochemical processes serve as major means to harness solar energy and convert sunlight into storable chemical fuels. In a photochemical procedure, light energy is directly utilized to drive specific half-reactions, such as the oxygen evolution reaction (OER), and hydrogen evolution reaction (HER). When this process is combined with an additional potential bias, it is referred to as a photoelectrochemical process, also known as artificial photosynthesis [46].

Since the first PEC cells were described in 1972, extensive research has focused on PEC-based solar water splitting, which has been the most extensively studied application in this field [47]. As research on PEC cells advanced, scientists discovered that the potential of PEC extended beyond their initial applications, leading to the reporting of other applications of PECs, such as fuel production, CO₂ reduction, and pollutant degradation [3]. A schematic of a CO₂ reduction PEC cell in an acidic electrolyte has been presented in Figure 2. The PEC system comprises working and counter electrodes, with one or both of them being photoelectrodes. These photoelectrodes can be classified into various systems, including the photoanode/cathode system, anode/photocathode system, or the photoanode/photocathode system. In laboratory-scale setups, a reference electrode is typically included to monitor the precise reduction potential value of the working electrode’s half-reaction within a cell. To design an efficient PEC system, it is crucial to select a light absorber that satisfies the following criteria: (1) it should possess the capability to absorb visible light within a band gap energy range of 1.5–2.5 eV, (2) the conduction band (CB) should have minimum levels, and the valence band (VB) should have maximum levels to provide the thermodynamic driving force for the desired reaction, and (3) it must demonstrate sufficient stability in the electrolyte solution to ensure the long-term performance and durability of the PEC system [48]. Therefore, the key component for PEC systems is photoelectrode, which directly regulates the energy conversion efficiency. In addition, it is worth noting that PEC systems have the potential to operate using available and stable materials, such as TiO₂, Fe₂O₃, and Cu-based metal oxides [49]. However, as photoelectrodes in PEC systems, perovskite materials can be more attractive than simple oxides, by reason of tunable band structures, high electrocatalytic activity, enhanced light absorption, and efficient charge separation [50]. In addition, there has been significant interest in the energy conversion of perovskites through photovoltaic technology. The combination of high-performance photovoltaic cells that convert solar irradiation into electricity with electrochemical systems as a part that simultaneously stores or converts the generated electricity, has been widely studied. The integration of PV and EC in a single device, known as monolithic PV-EC systems, allows for continuous output and a high external voltage, which is essential for efficient solar reactions [51]. The mechanism of PV-EC devices consists of two parts: (1) solar energy is converted to electricity by the photovoltaics part, and (2) the electrochemical system converts the generated electricity into stored electricity or renewable fuels. Perovskite materials can be incorporated directly into the PV-EC device, permitting for a monolithic design (Figure 3). In fact, the perovskites as an absorber and the electrocatalytic cell are joined into a single device, eradicating the need for distinct components and simplifying the system design [52].

Figure 2.

Schematic of a CO₂ photoelectrochemical reduction cell (acidic electrolyte), resembling natural photosynthesize organisms for production of valuable chemicals and oxygen from direct air capture (DAC), water and sunlight.

Figure 3.

Schematic of a PV-EC system containing perovskite solar cells and CO₂ electroreduction units in polylithic (a) and monolithic (b) device configuration.

3.1 Water splitting

Solar-driven water-splitting is considered as the most substantial and rapidly growing method for producing renewable H₂ to address the urgent energy crisis [53]. Hydrogen is widely recognized as a crucial clean fuel for the future. However, finding a cost-effective and sustainable approach to produce H₂ that can replace fossil fuels is a significant challenge. Inspired by the initial findings in 1967 by Honda and Fujishima, it was demonstrated that a photoelectrochemical cell comprising a single-crystalline TiO₂ (rutile) anode and a Pt cathode, combined with ultraviolet (UV) irradiation and an external bias, can effectively achieve overall water splitting (see Figure 4) [54].

Figure 4.

PEC cell fabricated by Honda and Fujishima. Reproduced with permission from ref. [54].

Until now, many metal oxides as a photocatalyst for water splitting have been discovered [55]. However, most of them have large band gaps and can only be activated by ultraviolet (UV) light irradiation (l < 400 nm). It is worth noting that a significant portion of solar energy on Earth’s surface falls within the visible light region (400 nm < l < 800 nm), and near-infrared (NIR) region (800–2500 nm) [56]. So, the efficient operation of visible light is crucial for the photocatalytic water splitting. In the PEC systems (acidic medium), the water splitting procedure consists of two half-reactions [57]. The oxidation reaction, known as, OER, occurs on the photoanode: H₂O + 2 h⁺⟶2H⁺ + 1/2 O₂ and the reduction reaction, known as HER, takes place on the photocathode: 2H⁺+ 2e^_ ⟶ H₂.

In PEC cells, three processes are vital: (I) formation of photogenerated electron–hole pairs when the semiconductor materials are excited by light; (II) the separation and transfer of electron–hole pairs; (III) the participation of electrons or holes in the reduction or oxidation reactions. Due to the positive Gibbs free energy changes, water splitting is not a reaction that occurs spontaneously at atmospheric pressure and room temperature. According to theoretical calculation, a potential of 1.23 eV is needed to drive this reaction [58]. However, several factors lead to an increase in the theoretical energy quantities, including the recombination of photogenerated electron-hole pairs, the resistances of the electrodes and components, and voltage losses. To achieve efficient practical PEC- based water splitting, a semiconductor should have the following features: appropriate conduction and valence band positions, a band gap within the range of 1.0–2.2 eV in order to efficiently convert visible and NIR light, proper electron-hole pair separation, low cost, and high stability. Perovskite materials have recently emerged as promising candidates for photoelectrocatalytic performance in water splitting even under visible light excitation [59]. Besides, they can be integrated with other photovoltaic materials to create multi-junction devices.. The PV units offer a photovoltage for water splitting without needing additional bias.

A simple and efficient technique for the creation of porous BiFeO₃ as photoanodes, was investigated by Liu et al. [60]. Additionally, successive engineering procedures were applied to improve the photoelectrochemical performance in water splitting. A TiO₂ overlayer and a cobalt oxide/oxyhydroxide co-catalyst were prepared by atomic layer deposition and photo-assisted electrodeposition, respectively. Under 1 sun radiation (100 mW cm⁻², AM 1.5G spectrum), the engineered photoanode showed the photocurrent density of 0.16 mA cm⁻² for oxygen evolution reaction (1 M NaOH). The Nyquist plots revealed that the charge-transfer resistance at the photoanode|electrolyte interface was reduced by the modification of BiFeO₃ by TiO₂ overlayer and CoO_x deposition. Furthermore, the deposition of CoOx on the photoanode surface amplified the capacitance approximately 15 times. These results indicated a higher charge density on the BiFeO₃ surface, which enhances charge transfer for the oxygen evolution reaction.

The mixed-cation perovskite (HOOC(CH₂)₄NH₃)_x(CH₃NH₃)_1 − xPbI₃ ((5-AVA)_x(MA)_1 − xPbI₃) was suggested for highly efficient water splitting [61]. The presence of 5-AVA cations can enhance the creation of perovskite crystals within the mesoporous oxide host, promoting their growth in the preferred direction. Compared to the MAPbI₃, the (5-AVA)_x(MA)_1 − xPbI₃ exhibited lower defects and improved surface interaction with the TiO₂ surface. The obtained photoanode encapsulated with inexpensive and stable conductive carbon paste and waterproof silver conductive paint for PEC water splitting. The structure acts as a protective barrier, preventing the electrolyte from invading the perovskite. Besides, transportation of the generated holes from the perovskite layer to the electrolyte is facilitated by the tight interface between the conductive carbon paste layer, silver conductive paint layers, and the perovskite layer. This interface not only reduces resistance but also allows for efficient movement of the holes. Under AM 1.5G illumination in KOH electrolyte, the perovskite photoanode exhibited a noteworthy photocurrent density of 12.4 mA/cm² at 1.23 V versus the reversible hydrogen electrode (RHE) and maintained stability for 48 hours in a highly oxidized environment.

A conductive adhesive-barrier (CAB) that allows for an all-in-one, integrated assimilation and transformation of any photovoltaic system to a PEC cell was studied by Fehr et al. [62]. The discrete PEC cells constructed by p-i-n perovskite solar cell (PSC)|CAB|Pt and n-i-p PSC|CAB|IrO_x as photocathode and photoanode, respectively. The prepared system showed more than 99% conversion of the perovskite photovoltaic power into a chemical reaction. The unassisted water-splitting analyses on the photovoltaic cells were carried out with two different strategies. The first strategy involves connecting a photocathode and photoanode protected by CAB electrically in series, while illuminating them optically in parallel geometry, leading to a Solar-to-Hydrogen (STH) efficiency of 13.4%. Other system was fabricated by halide perovskite (HaP)/Si tandems presenting a hopeful low-priced photovoltaic system that showed a photovoltaic efficiency of about 30%. Water-splitting examines were conducted using a photoanode fabricated by monolithic HaP/Si tandem an IrO_x-coated CAB and Pt foil cathode, resulting in STH efficiency of 20.8% with 102 h of operation.

A metal-encapsulated photoanode based on black α-phase formamidinium lead triiodide (FAPbI₃) as an organic–inorganic metal halide perovskite (PSK) was suggested by Hansora et al. for efficient PEC water splitting (see Figure 5) [63]. To construct a photoanode with high efficiency and good durability for water splitting in an alkaline electrolyte, Ni foil as the passivation layer and nickel–iron oxyhydroxide (NiFeOOH) as an oxygen-evolving catalyst (OEC) were used to encapsulate FAPbI₃ and form NiFeOOH/Ni/FAPbI₃ photoanode. Actually, Ni metal foil was applied to enhance the activity and stability of the FAPbI₃ photoanode when immersed in water, and entirely block the infusion of the electrolyte. Without this protective layer, the photoanode becomes susceptible to degradation and loss of performance due to moisture, contaminants, and oxidation, which can negatively impact its functionality. To deposit NiFeOOH as an oxygen evolution reaction (OER) co-catalyst, the employed method was drop-casting of Ni and Fe precursor solutions on the Ni foil. Following the deposition of NiFeOOH on the photoanode, the most efficient charge transfer occurs during the mechanism of the OER. The synergistic effect of Ni and Fe within NiFeOOH has been thoroughly validated to enhance the OER kinetics in the NiFeOOH/Ni/FAPbI₃ photoanode. The Ag paste serves a critical role as a close ohmic connection metal between the Ni foil and the FAPbI₃ layer, crucial for maintaining optimal performance. In the absence of the Ag paste, both the Ni foil and the FAPbI₃ layer experience a significant drop in performance. The PEC water splitting performance of the NiFeOOH/Ni/FAPbI₃, photoanode was assessed in a setup employing a three-electrode configuration, with the photoanode as the working electrode, Hg/HgO (1 M NaOH) as the reference electrode, and a Pt wire as the counter electrode. The measurements were conducted under 1-sun illumination in 1 M KOH. At 1.23 V_RHE (the reversible hydrogen electrode), this photoanode demonstrated a photocurrent density of 22.8 mA cm⁻² and exhibited superb stability for a duration of 3 days.

Figure 5.

Encapsulated FAPbI₃ photoanode for PEC water splitting.

A PV-EC water splitting cell was made by incorporating the Pt/C/CFP||Ni-TFBDC/CFP (CFP: carbon fiber paper, TFBDC: tetrafluoroterephthalate) electrochemical cell with a tandem perovskite solar cell by Li et al. [64]. The OER part in the EC cell is joined to the cathode of the solar cell and the HER is coupled with the anode of the solar cell via a copper wire (see Figure 6). The PV cell was crafted by blending precursor solutions of FABr, FAI, CsBr, CsI, PbBr₂, and PbI₂ in DMF/DMSO with a ratio of 4:1 at a 1 M concentration. The Ni-TFBDC MOF was synthesized by substituting hydrogen atoms in terephthalate (BDC) with fluorine atoms, thereby altering the electronic structure of the MOF and enhancing its electrocatalytic activity. This system showed solar-to-hydrogen efficiency (η_STH) of 10.17% at 206.7 mA.

Figure 6.

Schematic of a PV-EC water splitting cell containing Pt/C cathode and Ni-TFBDC anode which is incorporated with a tandem perovskite solar cell.

3.2 Hydrogen production

The inimitable characteristics of hydrogen, including its great thermal properties, high energy density per unit mass (143 MJ kg⁻¹), and economic efficiency as a fuel, make H₂ a sustainable and crucial clean energy source [45]. Due to its lightweight nature (0.08988 g/L), hydrogen has the energy density much higher than that of other fuels like gasoline. One gram of hydrogen can release140 kJ of energy, almost four times the energy of methane (33 kJ/g) [65]. Currently, the promising usage of hydrogen still faces many challenges such as storage and handling, and an economically and environmentally clean production way. Photoelectrochemical hydrogen evolution offers a promising pathway toward a sustainable and low-carbon energy future by leveraging solar energy to produce clean and renewable hydrogen fuel. By developing suitable photocathode materials for PEC cells, researchers can enhance the efficiency, stability, and scalability of H₂ evolution processes driven by solar energy conversion [48]. Among different photocathodes, perovskites offer tailorable surface chemistry, versatility in operating conditions, and low cost for H₂ evolution reactions.

Water splitting for H₂ production is inherently slow and challenging, primarily due to the transfer of four electrons, necessitating a high overpotential. Additionally, the production of oxygen (O₂) through water oxidation raises safety concerns, requiring an additional separation process and resulting in increased costs [66]. In the past few years, H₂ evolution through PEC hydrohalic acids (HX, X = Cl, Br, I) splitting has attracted significant interest. PEC splitting of hydrohalic acids involves a process similar to water splitting, reduction of H⁺ to H₂ and oxidation of X⁻ to X₂/X₃⁻. Because of the solubility of X₂/X₃⁻ species in hydrohalic acid, the H₂ gas can be collected separately [38]. Lou et al. proposed an effective PEC system utilizing the MAPbI₃-TiO₂ nanorod array (TNA) for stable and efficient H₂ production in an aqueous HI solution [67]. Figure 7 shows a schematic of the fabrication of the MAPbI₃-TNA photoelectrode for hydrogen evolution from HI splitting. During the PEC process, MAPbI₃ is capable of absorbing visible light with wavelengths up to 800 nm to create electron-hole pairs. The photogenerated electrons are efficiently injected into the TiO₂ nanorod array, where they are subsequently transported to the Pt counter electrode. At the counter electrode, the electrons participate in the reduction of protons, ultimately resulting in the evolution of hydrogen. In fact, TiO₂ nanorod array has a crucial role in blocking the transfer of holes from MAPbI₃ to Fluorine-doped Tin Oxide (FTO). In fact, the holes in MAPbI₃ take part in an oxidation process that converts $I^{-}$ to $I_3^{-}$, which contributes to the stabilization of the CH₃NH₃PbI₃ crystals. Under AM 1.5 G illumination at 0.14 V (vs. Ag/AgCl), the optimized CH₃NH₃PbI₃-TNA PEC cell achieves a photocurrent density of 1.75 mA cm⁻², and remarkable stability in producing H₂ at a rate of 33.3 mmol cm⁻² h⁻¹ for over 8 h.

Figure 7.

MAPbI₃-TNA photoelectrode for PEC H₂ evolution from HI splitting.

Choi et al. reported a PEC cell based on Cs_0.05(FA_0.83MA_0.17)_0.95(PbI_0.83Br_0.17)₃ (CsFAMA) perovskite as a photocathode and a CNT paper anode [68]. The photocathode was submerged in an acidic solution (0.5 M H₂SO₄ in water), while the anode was situated in a distinct 0.5 M H₂SO₄ solution containing reduced phosphomolybdic acid (PMA) as an electron mediator. PMA was chosen as a soluble catalyst for the extraction of electrons during biomass depolymerization because of its distinctive color transformation from yellow (PMA³⁻) to dark green (PMA⁵⁻) when reduced, especially with absorbance at 700 nm. Additionally, at a relatively low potential, PMA exhibits high solubility and reversible redox performance. Pre-reduction of PMA was carried out over biomass oxidation including lignin, hemicellulose, cellulose, or oak biomass. Furthermore, by selectively depolymerizing lignin in lignocellulosic biomass, they can produce value-added chemicals such as vanillin and acetovanillone (see Figure 8). This PEC system showed the current density of 19.8 mAcm⁻² under simulated AM1.5G 1-sun radiation with stability of more than 20 h without significant performance deprivation.

Figure 8.

A perovskite photocathode integrated with a Pt catalyst and an electron extraction system from biomass is utilized to achieve bias-free hydrogen evolution. Reproduced with permission from ref. [68].

A method was proposed for high-efficiency hydrogen production via the oxidation of glucose using a photoelectrochemical approach. This method involves utilizing an ultrathin carbon-coated perovskite-modified TiO₂ nanotube photonic crystal (C@Cr-SrTiO₃/TiO₂ NTPC) as the photoanode [66]. Compared with water oxidation, the oxidation of biomass requires less external force. The oxidation of biomass derivatives through PEC process is a more favorable method for hydrogen generation, as it can be powered by solar energy. Glucose, a significant biomass component, is a key product of photosynthesis and is abundant in nature. Moreover, it serves as one of the primary sources of waste in agricultural and food industries. If not properly managed, this waste can cause significant environmental pollution. The PEC H₂ evolution from biomass derivatives oxidation process consists of two half-reactions:

Oxidation reaction:

$$\begin{array}{l}{\mathrm{C}}_{\mathrm{x}}{\mathrm{H}}_{\mathrm{y}}{\mathrm{O}}_{\mathrm{z}}+\left(\text{2x}–\mathrm{z}\right){\mathrm{H}}_2\mathrm{O}\to {\mathrm{xCO}}_2+\left(\text{4x}–\text{2z}+\mathrm{y}\right)\left({\mathrm{H}}^{+}+{\mathrm{e}}^{-}\right),\\ {\mathrm{E}}_{\text{anode}}={\mathrm{E}}^{\mathrm{\theta }}–0.059\left(\mathrm{pH}\right)\left(\mathrm{vs}.\mathrm{NHE}\right).\end{array}$$E1

Reduction reaction:

$$\begin{array}{l}\left(\text{4x}–\text{2z}+\mathrm{y}\right){\mathrm{H}}^{+}+\left(\text{4x}–\text{2z}+\mathrm{y}\right){\mathrm{e}}^{-}\to \left(\text{2x}–\mathrm{y}+\mathrm{y}/2\right){\mathrm{H}}_2,\\ {\mathrm{E}}_{\text{cathodic}}=0\mathrm{V}–0.059\left(\mathrm{pH}\right)\mathrm{V}\left(\mathrm{vs}.\mathrm{NHE}\right).\end{array}$$E2

Total:

$${\mathrm{C}}_{\mathrm{x}}{\mathrm{H}}_{\mathrm{y}}{\mathrm{O}}_{\mathrm{z}}+\left(\text{2x}-\mathrm{z}\right){\mathrm{H}}_2\mathrm{O}\to {\mathrm{xCO}}_2+\left(\text{2x}–\mathrm{z}+\mathrm{y}/2\right){\mathrm{H}}_2.$$E3

In glucose electrolyte, the C@Cr-SrTiO₃/TiO₂ NTPC photoanode revealed the photocurrent density of 0.43 mA cm⁻² at the potential of 0.6 V (vs. SCE).

3.3 CO₂ reduction

Due to the fast population growth and the rapid progress of the global economy, the global demand for fossil fuels is continuously growing. This leads to higher levels of greenhouse gases especially large amounts of CO₂ emissions and numerous environmental problems such as ozone depletion, climatic change, melting ice caps, acid rain, and rising sea levels. According to data from the National Oceanic and Atmospheric Administration (NOAA) research, in July 2017 the atmospheric CO₂ concentration reached approximately ∼403.95 ppm, and it is anticipated to reach about 600 ppm by 2100 [69]. Converting CO₂ into valuable chemical feedstocks is a promising approach to address the problem of controlling CO₂ levels and growing the new energy storage. Since the late 1970s, there has been significant research on photoelectrochemical CO₂ reduction as “artificial photosynthesis”. This process has garnered considerable attention for its potential for CO₂ reduction. CO₂ reduction reactions offer energy-saving capabilities and generate extensive products such as carbon monoxide (CO), formic acid (HCOOH), formaldehyde (HCHO), methane (CH₄), methanol (CH₃OH), olefins, and other hydrocarbons. Table 1 shows the obtained products and the number of electrons and protons involved in the CO₂ reduction reaction (CO₂RR) potentials [70].

Electrochemical thermodynamic half-reactions	Standard potentials (V vs. SHE)
CO2(g) + 4H^+ + 4e^− = C(s) + 2 H2O(l)	0.210
CO2(g) + 2H^+ + 2e^− = HCOOH (l)	−0.250
CO2(g) + 2H^+ + 2e^− = CO(g) + H2O(l)	−0.106
CO2(g) + 4H^+ + 4e^− = CH2O(l) + H2O(l)	−0.070
CO2(g) + 6H^+ + 6e^− = CH3OH(l) + H2O(l)	0.016
CO2(g) + 8H^+ + 8e^− = CH4(g) + 2H2O(l)	0.169
2CO2(g) + 2H^+ + 2e^− = H2C2O4(aq)	−0.500
2CO2(g) + 12H^+ + 12e^− = CH2CH2(g) + 4H2O(l)	0.064

Table 1.

Different products under standard potentials of CO₂ reduction (versus standard hydrogen electrode, SHE).

It should be considered that the nature of the electrocatalysts and the applied potential significantly impact the creation of final products. Inherently, the CO₂ molecule is highly stable because of its linear structure, which consists of two symmetric C〓O bonds, and the C-O bond length in CO₂ is approximately 116.3 pm, requiring a significant energy of ∼750 kJ/mol to break the initial bond in CO₂. Also, the reduction of protons can occur at the potential demand for CO₂ reduction reactions, leading to the low efficiency and low selectivity of a convenient reaction [71]. Moreover, most of the existing electrocatalysts still face several challenges such as poor stability, low conversion efficiency, low selectivity, and inability to conquer the hydrogen evolution as a competitive reaction.

In recent years, perovskite materials have garnered considerable interest in the field of photoelectrochemical CO₂ reduction. Crucially, the band positions of the majority of perovskites satisfy the essential thermodynamic criteria for CO₂ reduction [72]. As a result of these features, these materials have been effectively employed in PEC CO₂ reduction. Fundamentally, the PEC CO₂ cells contained the cathodic and anodic parts (Figure 9). They are separated by an ion-selective membrane. Considering the PEC CO₂ reduction in acidic solutions, in the cathodic compartment, the CO₂ reduction reaction occurs, while in the anodic compartment, the water oxidation reaction takes place. During water oxidation (2H₂O + 4 h⁺ → O₂ + 4H⁺) in the anodic section, protons are generated and subsequently transported to the cathodic part through the ion-selective membrane, propelled by a concentration gradient [73].

Figure 9.

Schematic representations of possible two-compartment PEC cells, designed for CO₂ reduction as: semiconductors employed as photocathodes (a), semiconductors utilized as photoanodes (b) and semiconductors serving as both photocathodes and photoanodes (c). Reproduced with permission from ref. [73].

Zhang et al. investigated a PEC cell for the conversion of CO₂ to CO, employing a perovskite-based CsPbBr₃ (CPB) thin film as the photocathode, a Pt foil as the counter electrode, and an Ag/AgCl (3 M KCl) reference electrode [74]. Figure 10 displays the schematic of this designed system. A two-step procedure was utilized to obtain perovskite photocathode. First, by a spin-coating process, a PbBr₂ layer film was made. Then, the solutions of the CsBr-based materials were spin coated onto the surface. Next, to prepare CsPbBr₃-F-N-Au thin films, fluorine, Nafion, and Au were added to the above solution. When F-ions are added in perovskite structure, they are able to promote the chemical interactions. Additionally, by Nafion addition, an adjustment in the band gap of CsPbBr₃ and a decreased electron-hole recombination rate were obtained. Besides, Au can accelerate separation of charge carriers in the photocathode. The photocathode exposed the best PEC performance at a potential of −0.5 V (vs Ag/AgCl), with the maximum photocurrent of −0.23 mA cm⁻². This investigation proved that a halide perovskite can be directly utilized to assist CO₂ reduction.

Figure 10.

A PEC cell for CO₂RR based on CsPbBr₃ (CPB) thin-film photocathode.

Moreover, PV-EC systems offer significant benefits in terms of scalability, durability and optimization compared to the PEC approach for CO₂ reduction. By construction of a monolithic PV-CE system, Schreier and colleagues demonstrated an efficient CO₂ to CO conversion with 90% faradaic efficiency [75]. Figure 11 displays the designed PV-EC cell including an electrochemical unit with oxidized gold and IrO₂ as cathode and anode, respectively coupled to CH₃NH₃PbI₃-based photovoltaics. This study demonstrated the prolonged and consistent operation of a PV-EC cell utilizing perovskite photovoltaics. The current density remained stable at approximately 5.8 mA cm⁻², with only minor fluctuations observed for over 18 hours.

Figure 11.

(a) Schematic of the apparatus integrating photovoltaics with an electrochemical cell. (b) Comprehensive energy diagram depicting the conversion of CO₂ into CO utilizing three perovskite solar cells. The PSCs connected in series generate a voltage adequate to surpass the combined energy of the reaction free energy (ΔE) and the reaction overpotentials (η) at the electrodes. Reproduced with permission from ref. [75].

A CsPbBr₃-based PEC system was fabricated for CO₂ reduction into oxalic acid [76]. To improve the stability of CsPbBr₃ in water, it was coated with platinum and graphite (C). Graphite was selected for its stability under standard temperature and pressure conditions, as well as its affordability. Platinum was chosen due to its widely acknowledged high electrocatalytic activity. Chronoamperometric tests were conducted to study photoelectrochemical properties of CsPbBr₃/Pt and CsPbBr₃/C for CO₂ reduction at low external potentials applied (− 0.5 V and - 0.8 V vs. Ag/AgCl) under illumination. During CO₂ conversion, it was observed that the CsPbBr₃/Pt samples exhibited low charge transfer resistance to the electrolyte. As a result, these samples ultimately produced the highest photo-current (12.6 mA cm⁻²). The CsPbBr₃/C showed the highest Faradic Efficiencies of 22% (at −0.5 V) and 44% (at −0.8 V) vs. Ag/AgCl, respectively. This behavior is attributed to the properties of the carbon surface, which catalyze the formation of oxalic acid more effectively than platinum. In contrast, platinum promotes the competing reaction of hydrogen formation.

Abarca and colleagues introduced a novel photoanode comprising a combination of perovskite-based calcium titanate (CaTiO₃) as an electron collector and BiVO₄ as a catalyst for visible-light-driven water oxidation, with the goal of enhancing PEC CO₂ conversion [77]. The designed photoanode is incorporated into an electrolyzer to continuously convert CO₂ into formate using visible light. The attained concentration was 63.8 g L⁻¹, with a Faradaic Efficiency of 79.1% and a solar-to-fuel (STF) conversion efficiency of 7.6%. The combination of BiVO₄ and CaTiO₃ creates a p-n junction, which enhances the separation of charges. This junction generates an interfacial electric field at the interface of both catalysts. Consequently, electrons can migrate from CaTiO₃ to the conduction bands (CB) of BiVO₄, while holes can move from BiVO₄ to the valence band (VB) of CaTiO₃. Moreover, the negative CB and VB levels in the electronic structure of CaTiO₃ lead to a spontaneous charge transfer from a thermodynamic perspective.

3.4 N₂ evolution

Nitrogen (N₂) making up about 78% of Earth^’s atmosphere, has the potential to be converted into valuable industrial products, profiting the environment and humanity. Conversion of N₂ to ammonia (NH₃) is crucial for industrial processes, and the production of pharmaceuticals, dyes, and fertilizers. Besides, NH₃ is highly valuable as a hydrogen carrier and is recognized for its ability to generate CO₂-free energy [78]. Considering the kinetic and energy aspects, N₂ Reduction Reaction (N₂RR) presents a challenging chemical process. This is primarily owing to the enthalpy of triple bonds between nitrogen atoms as 940.95 kJ mol⁻¹. At present, the industrial production of NH₃ continues to depend on the conventional Haber-Bosch method, which operates under high temperatures and pressures. This approach gives rise to considerable challenges, including excessive CO₂ emissions and high energy consumption [79]. Hence, it is crucial and urgent to devise an economically viable approach for N₂ reduction. Photoelectrochemical methods provide a more energy-efficient and environmentally friendly alternative to the conventional Haber-Bosch process. Photoelectrochemical cells potentially offer the advantages of both photochemical and electrochemical systems by employing solar energy, separating NH₃ from other byproducts and by applied electrical bias, these systems develop the reduction capability of photogenerated electrons. In a PEC N₂RR system, at the cathodic surface the N₂ reduction reaction occurs (N₂ + 6H⁺ + 6e − → 2NH₃), while on the anodic part oxidation of H₂O takes place [80].

In 1977, Schrauzer and Guth converted N₂ to NH₃ using TiO₂ in UV light consequently, a number of photoelectrocatalytic systems including TiO₂, MoS₂, LDH, V₂O₃·nH₂O, and g-C₃N₄ were studied for N₂ PEC reduction [81]. However, many of these materials still encounter numerous challenges, such as low efficiency, limited light absorption range, and instability, significantly restricting their applicability for industrial N₂ reduction. Consequently, there is a pressing need for the development of a suitable photoelectrode for N₂ conversion. In this regard, perovskite materials-driven photocathodes have been identified as a suitable and promising option.

For photocatalytic and photoelectrocatalytic N₂ reduction, a photocatalyst system composed of CoTiO₃/N-rGO p-n junction, was suggested by Paramanik et al. [82]. CoTiO₃ as a p-type semiconductor with band gap energy of about 2.25 eV, great charge carrier mobility, large absorption coefficient, and active in visible-light regions, reveals potential for well-organized PEC applications. In addition, the satisfactory N₂-adsorption sites can be generated by N-dopant in reduced graphene oxide that creates a positive charge on the graphene surface, resulting in more N₂ activation. Under light irradiation, the produced electrons on the conduction band of CoTiO₃ transfer to CB of N-rGO, and the generated holes on the valence band of N-rGO flow to CoTiO₃ (Figure 12). In order to achieve the hydrogenation procedure of N₂ and conversion to NH₃, the activated N₂ molecules are attacked by photogenerated electrons and active H atoms from water to produce the final product. This system showed great performance with a faradaic efficiency of 18.05%.

Figure 12.

Schematic of PEC N₂RR cell based on the CoTiO₃/N-rGO.

3.5 Degradation of pollutants

The release of organic pollutants from industries such as textiles, dyeing, cosmetics, and food poses significant environmental challenges related to dangerous wastes, and polluted groundwater [83]. As a result, it is vital and desirable to adopt sustainable approaches to minimize the discharge of pollutants, protecting both the environment and human well-being. Overall, the PEC technique, with its integration of light response and electrochemical analysis, has opened up exciting possibilities for the degradation of pollutants [84]. The photoelectrocatalytic procedures are greater than traditional approaches owing to their capability to remove organic waste products through operation at room temperature and pressure, cost-effectiveness, and the avoidance of polycyclic compound generation. In a PEC system, the photogenerated electrons in conduction band (e_CB⁻) and generated holes in the valence band (h_VB⁺) can engage in several reactions, resulting in the production of highly Reactive Oxidation Species (ROSs) such as hydroxyl radical (^•OH) and superoxide radical (O^2•−). These species have the capability to react with organic contaminants in a non-selective manner. Ultimately, this process leads to the mineralization of organic contaminants, converting them into H₂O and CO₂ [85]. Semiconductors like TiO₂, CdS, ZnO, and MoS₂ have been widely used as photoelectrocatalyst in degradation via PEC systems. Nonetheless, their photoelectrocatalytic efficiencies are impeded by their wide band gaps, limiting their capacity to efficiently absorb visible light [86]. So, research has focused on the visible light-active electrocatalysts that possess narrow band gaps and exhibit minimal electron-hole recombination. Perovskite materials have demonstrated exceptional potential in the degradation of pollutants due to their remarkable catalytic properties. These materials exhibit high surface area and reactivity, enabling efficient conversion of harmful pollutants into less toxic byproducts.

The perovskite BaSnO₃ was synthesized using a chemical method with a cubic crystalline structure (average size of 63 ± 2 nm) by Sahmi et al. [87]. BaSnO₃ is both safe and cost-effective and exhibits chemical stability across a wide pH range. The active surface area is increased when it is synthesized through a chemical route, which leads to improved photo activity. The waste containing ibuprofen (C₁₃H₁₈O₂: propionic acid 2-(4-isobutylphenyl)) which classify as a hazardous content and the BaSnO₃ was successfully applied for ibuprofen degradation by its mineralization. Through electrocatalysis on BaSnO₃, ibuprofen has been observed to undergo elimination at a conversion rate of 55% when a current of 150 mA is applied. Furthermore, when subjected to UV illumination, the conversion rate has been increased to 68% through a PEC process. The electrons in conduction band of BaSnO₃ produce O₂^•− and H₂O₂. The H₂O₂ serves as an electron acceptor and reacts with the O₂^•− to form ^•OH radicals. These ^•OH radicals are used to oxidize ibuprofen. Additionally, the high potential of the ^•OH/H₂O couple (2.27 V) allows for the production of ^•OH radicals, which are essential for the mineralization of ibuprofen. These ^•OH radicals are covalently bonded to BaSnO₃. Morcoso et al. investigated the photocatalytic and photoelectrochemical oxidation of 2-mercaptobenzothiazole (MBT) using CsPbBr₃ quantum dots [88].

The remarkable photoluminescence quantum yield displayed by perovskite quantum dots (QDs) provides compelling evidence of the notable reduction in nonradiative recombination procedures. As a result, when these QDs are photoexcited, a surplus of energized carriers is generated, presenting exciting possibilities for their utilization. They confirmed that complete degradation of MBT occurs as a result of hole injection from CsPbBr₃ to MBT. MBT is classified as a member of the heterocyclic aromatic compound group known as benzothiazole. It finds widespread application in various industrial sectors, serving as a rubber accelerator, corrosion inhibitor, fungicide, and antialgal agent. However, the poor biodegradability, aquatic toxicity, tumor-inducing properties, and role as a frequent allergen and human carcinogen make MBT a substance with significant concern. CsPbBr₃ QDs can be utilized for photoelectrocatalytic degradation of MBT, primarily due to their advantage of not overlapping with the characteristic band of MBT at 320 nm, which is monitored in PEC degradation. The presence of CsPbBr₃ QDs plays a significant role in the degradation of MBT under different lighting conditions. Without the existence of CsPbBr₃, no degradation occurs under visible illumination or in the dark and only UV light is effective in the MBT degradation. The addition of CsPbBr₃ QDs in the MBT solution has a remarkable impact on the degradation rate under different lighting conditions. Specifically, when CsPbBr₃ QDs are added, under UV-vis irradiation, the degradation rate doubles, while under visible light, it multiplies.

In another activity, Kuo et al. reported a photoelectrode utilizing YFeO₃/CeO₂ synthesized via a sol-gel method to tackle the challenge of recombination of photogenerated electron-hole pairs, thereby enhancing the degradation efficiency of the catalyst [89]. Due to its narrower bandgap relative to other perovskites, YFeO₃ has garnered considerable interest as a promising material for photoelectrocatalysis under visible light. Additionally, the ferromagnetic properties of YFeO₃ facilitate its magnetic retrieval post-utilization, rendering its reuse feasible. Hexagonal YFeO₃ exhibits superior photoelectrocatalytic activity compared to orthorhombic YFeO₃. However, metastable nature and thermodynamically unstable crystal structure of hexagonal YFeO₃ present significant challenges for its formation. Therefore, employing a heterojunction electrocatalyst by combining YFeO₃ with another material presents an effective alternative strategy to enhance photoelectrochemical processes under visible light. The designed photoelectrode demonstrates a highly promising PEC efficiency of 75.2% for Reactive Black 5 dye (RB5) degradation under visible light irradiation, which is significantly higher compared to YFeO₃ and CeO₂ photoelectrodes with efficiency of 31.9 and 46.9%, respectively. As is observed in Figure 13, under visible illumination, electrons and holes were produced on the conduction band (CB) and valence band (VB) of YFeO₃/CeO₂ photoelectrode. Therefore, by reason of the reactions between the cumulative holes and OH ions, the creation of hydroxyl radical was elevated, which simplified degradation. The generated hydroxyl radical is capable of reacting with RB5 and degrading the pollutant.

Figure 13.

Schematic of PEC mechanism of YFeO₃/CeO₂ for RB5 degradation.

3.6 Sensor devices

Photoelectrochemical systems have emerged as a highly valuable and efficient detection procedure in analytical chemistry owing to their superior recognition properties, stability, precision, portable apparatus and low cost. Because of the complete segregation between the excitation light source and the display current signal, they have emerged as a valuable tool with minimal background signal in trace analysis [90]. Photoactive materials play a critical role in influencing the sensitivity of a PEC device. These materials should possess specific characteristics including improved and steady photocurrent production, effective light harvesting, and high sensitivity. Recently, perovskite materials have been demonstrated outstanding analytical features in PEC sensors including distinctive carrier separation, fast response times, excellent robustness, and highly accurate detection capabilities.

A PEC aptasensor based on the “on-off-on” approach was investigated for ultrasensitive analysis of Prostate-Specific Antigen (PSA) as a target [91]. As it observed in Figure 14, principally, in order to development of the PEC system stability, Cd: Sb₂S₃ as a photosensitizer loaded on La₂Ti₂O₇ surface to produce the heterojunction related to the first “signal-on” state, and obtained a greater photocurrent because of the matched levels of band energy. Then, to realize the “signal-off” state, the V₂O₅ nanosphere as a H₂O₂ catalyst (electron donor in electrolyte solution) was utilized to mark aptamer DNA. The PSA was accurately detected by a DNA, leading to the release of V₂O₅ from the electrode, thus reinstating the PEC “signal-on” state. The suggested PEC sensor for PSA detection exposed a wide linear range from 1.000 × 10 ⁵ to 500.0 ng/mL, as well as a low detection limit of 4.300 fg/mL. This sophisticated strategy not only progresses the sensor sensitivity but also expands the application of perovskites in the field of PEC sensing.

Figure 14.

Mechanism of Electron transfer (a) signal-on state and (b) signal-off state of a photoelectrochemical aptasensor based on La₂Ti₂O₇/Sb₂S₃ and V₂O₅ for effectively signal change strategy for cancer marker detection.

A novel highly stable, sensitive and selective PEC sensor for the recognition of dopamine (DA) was fabricated using CsPbBr_1.5I_1.5 QDs and immobilized them on the TiO₂ with porous structure [92]. The TiO₂ and CsPbBr_1.5I_1.5 QDs possess band gaps of approximately 3.2 and 2.1 eV, respectively. Their conduction band edges are at approximately −3.9 and − 3.5 eV, respectively, while their valence band edges are around −7.6 and − 5.6 eV, respectively. As depicted in Figure 15, under visible light excitation, the close proximity of the conduction band edge of CsPbBr_1.5I_1.5 QDs to that of TiO₂ suggests the potential for efficient transportation of photogenerated electrons from the conduction band of CsPbBr_1.5I_1.5 QDs to the FTO through the TiO₂ layer. This procedure efficiently conceals the direct electron-hole pair recombination that occurs in perovskite, resulting in an enhancement in the sensor sensitivity. In fact, it is easier for the photogenerated holes to transport from the valence band of perovskite to its surface. Additionally, the energy level of DA (−4.91 eV) is higher than the valence band (−5.6 eV) of perovskite, providing a likelihood for the detection of DA.

Figure 15.

Schematic of an all-inorganic perovskite quantum dot/TiO₂ inverse opal electrode platform for photoelectrochemical sensing of dopamine under visible light. Reproduced with permission from ref. [92].

Leng et al. introduced a self-powered PEC-based biosensor composed of a BiOI as photocathode and a WO₃/SnS₂/ZnS as photoanode [93]. Moreover, CsPbBr₃@COF-V was employed as a signal quenching agent for the quantitative detection of heart fatty acid binding protein (H-FABP) on the photocathode. In addition, the secondary antibody marker (Ab₂) was loaded to CsPbBr₃@COF–V to form CsPbBr₃@COF–V-Ab₂ for connection to FABP. This biosensor not only offers a continuous and powerful photocurrent response for FABP recognition, but also efficiently eradicates the interference of constituents. The photocurrent response of CsPbBr₃@COF–V is significantly influenced by the quenching source, which includes weak conductivity, steric interruption, and struggle against the substrate for dissolved oxygen as well as competition for the excitation light. Achieving multiple signal intensifications is possible with the intervention of this critical factor, unveiling an inventive approach to self-operating PEC biosensing devices. Actually, once FABP molecules were joined to the CsPbBr₃@COF–V-Ab₂ electrode, the response signal reduced because of steric hindrance and quenching process of Ab-2. This biosensor showed a wide-ranging from 0.0005 to 150 ng/mL with detection limit of 0.19 pg./mL for FABP detection.

In another research, Dong et al. reported a PEC sensor for trenbolone (TB) detection using Cs₃Bi₂Br₉/BiOBr/Bi₂S₃ as a working electrode with Ag-Au/CdS as a signal enhancer, an Hg/Hg₂Cl₂ reference electrode and a Pt wire as the counter electrode [94]. Cs₃Bi₂Br₉/BiOBr/Bi₂S₃ can efficiently avoid the electron-hole pairs recombination. Moreover, the working electrode was incubated with TB antibodies (Ab) of the same concentration. Subsequently, the designed sensor was applied for detection of various concentrations of TB. Trenbolone is a synthetic anabolic steroid that is used in veterinary medicine to promote muscle growth and appetite in livestock. It is known for its powerful effects on increasing muscle mass and strength, making it popular among bodybuilders and athletes looking to enhance their performance. Trenbolone is classified as a controlled substrate in many countries due to its potential for misuse and abuse, so, its detection is important. The concentration of TB can be assessed by monitoring the alteration in output current caused by the competitive binding of Ab with TB or Ag-Au/CdS. A decrease in TB concentration leads to an increase in connected Ag-Au/CdS on Ab. Therefore, the PEC sensor has the capability to reliably detect TB with a detection limit of only 29.7 fg mL⁻¹.

3.7 Synthesis of small organic compounds

PEC devices can be utilized not only for fuel production but also for the production of high-value simple organic molecules. PEC methods represent a cutting-edge approach to organic molecule synthesis that offers green and sustainable solutions, precise control over reactions, mild reaction conditions, versatility, rapid reaction kinetics, and compatibility with complex substrates [95]. 2,5-Dimethoxy-2,5-dihydrofuran (DMDF) is an important compound in organic chemistry due to potential applications in various synthetic processes as a vital intermediate for creating organic compounds such as benzenoid compounds, pyridines, pyridazines, and pyrroles. Using the conventional chemical synthetic method for DMDF, a significant quantity of bromine is utilized as an oxidizing agent, which poses concerns regarding environmental pollution [96]. PEC processes as an alternative method offer a sustainable and environmental approach to producing DMDF without additional catalyst consumption by harnessing renewable energy for driving chemical transformations.

Aimed at PEC synthesis of 2,5-Dimethoxy-2,5-dihydrofuran (DMDF) from furan, a thin film of methylammonium lead bromide (MAPbBr₃) single crystal (SCTF) as a photoanode was deposited on a FTO glass [97]. Then, an ultrathin Al₂O₃ layer was grown on the perovskite surface and a Ti layer was deposited on the Al₂O₃ layer. It was found that ultrathin Al₂O₃ layer efficiently inhibits surface defects of perovskite and Ti layer as a protective catalytic layer further improving PEC performance. Indeed, these layers significantly enhance the stability and photocatalytic activity of perovskite crystals. Figure 16 displays the schematic of the PEC cell fabricated by MAPbBr₃ perovskite on FTO/TiO₂ substrate, and Pt mesh as the photoanode, and counter electrode, respectively, in an electrolyte including a Br⁻/Br⁺ in an acetonitrile/methanol mixed solution. Under light radiation, by photogenerated holes, the Br⁻ ions oxidize into Br⁺. Generated Br⁺ species act as oxidizing reagents to produce DMDF from furan. The PEC performance and stability of the designed photoanode were considerably enhanced resulting in 6 h constant DMDF evolution examination with a high Faraday efficiency of 93%.

Figure 16.

(a) Schematic depicting the operational concept of MAPbBr₃ SCTF-based (or PCTF-based) PEC cells. Linear sweep voltammetry (LSV) (b) and Chronoamperometric trace (c) of MAPbBr₃ SCTF-based and PCTF-based photoelectrodes. Reproduced with permission from ref. [97].

4. Conclusions and prospective

The advantages of perovskites in photoelectrochemical applications profit from their exceptional properties, including appropriate band gap, high optical absorption coefficient, good electron mobility, high light-harvesting capability, simple manufacturing procedure, low cost, adaptable structure, and morphology. PEC application of perovskites holds great promise for advancing renewable energy technologies. While numerous chapters focus on the use of perovskite materials for solar cells, at present the performances of perovskites in PEC devices have not been completely investigated. This chapter thoroughly examines the significant impact of the perovskite structure on various photoelectrochemical (PEC) and monolithic Photovoltaic-Electrocatalytic (PV-EC) devices. Despite these advancements, the practical application of perovskites in PEC devices is still in its infancy stages. There are several challenges that need to be addressed in order to facilitate their practical use. One of the main challenges is the stability of perovskites under harsh operating conditions, which can limit their long-term performance. Crystal engineering or the encapsulation with appropriate semiconductor materials is an effective approach for developing stable perovskites for the practical applications. In addition, the toxicity of lead is a major concern when it comes to environmental issues which can be addressed by lead-free alternatives such as Ti, Bi, and encapsulating or coating of the lead-based perovskites to minimize the risk of lead exposure during the use of perovskite-based devices. Besides, the efficiency of perovskite-based PEC devices still needs to be improved to compete with other established technologies. Despite the obstacles, scientists are achieving rapid advancements, and there is no doubt that the current challenges will be overcome. The future of perovskite-based PEC devices is undeniably promising. To drive further progression in this realm, future research could be focused on designing perovskite architectures with various dimensions, integrating desired properties, and exploring the potential applications of perovskites in PEC processes.

Acknowledgments

M.Z. gratefully acknowledges the NEXTCCUS project (ID: 327327), which is funded through the ACT program (Accelerating CCS Technologies, Horizon2020; Project No. 691712), and financial contributions made by the Iritaly Trading Company Srl and the Italian Ministry of Education, University and Research (MIUR). N.Y. gratefully acknowledges the P4SPACE project (ID: 101067838), funded by the European Union under HORIZON-MSCA-2021-PF-01.