Photochemical cells are essential components of artificial photosynthesis systems, as they directly convert solar energy into chemical energy [1]. These cells consist of a light-absorbing material, catalysts, and redox mediators that facilitate the conversion of absorbed photons into chemical reactions, such as water splitting and carbon dioxide reduction. The development of efficient and stable photochemical cells is crucial for the success of artificial photosynthesis technology and its potential applications ^[2][3][2,3].

In photochemical cells, the process of artificial photosynthesis begins with the absorption of light by a photosensitizer, a light-absorbing material that generates excited electrons upon illumination. The photosensitizer can be organic dye, inorganic dye, or a quantum dot, each with their unique light absorption characteristics ^[4][5][4,5]. The efficiency of the photosensitizer is determined by its ability to absorb a broad range of the solar spectrum and its excited state lifetime, which influences the charge separation process. Upon light absorption, the excited electrons are transferred from the photosensitizer to a suitable electron acceptor while the holes (h⁺; positive charges) are transferred to an electron donor. This charge separation process is essential for converting the absorbed light energy into chemical energy and avoiding the rapid recombination of generated charges, which would result in energy loss ^[6][7][8][6,7,8].

The separated charges drive two critical reactions in artificial photosynthesis: water oxidation and carbon dioxide reduction. In water oxidation, also known as the oxygen-evolving reaction (OER), the holes generated during the charge separation process oxidize water molecules to produce oxygen gas and protons ^[9][10][9,10]. In carbon dioxide reduction, also known as the carbon dioxide reduction reaction (CO₂RR), excited electrons reduce CO₂ to produce value-added chemicals and fuels, such as carbon monoxide, formic acid, methanol, or methane. The performance of a photochemical cell in these reactions is determined by the activity, selectivity, and stability of the catalysts used for water oxidation and CO₂ reduction [11].

The electron transfer process in photochemical cells is facilitated by redox mediators, which shuttle electrons between the photosensitizer and the catalysts. Redox mediators can be metal complexes or organic molecules, and their role is to minimize energy loss during electron transfer and prevent charge recombination. In addition, redox mediators can affect the selectivity of the CO₂RR by controlling the potential and the number of electrons transferred to the CO₂ molecule ^[12][13][14][12,13,14]. The final step in artificial photosynthesis is the formation of the desired products, which can be hydrogen gas, value-added chemicals, or fuels. The product distribution is determined by the thermodynamics and kinetics of the catalytic reactions, as well as the local concentration of reactants and products. In some cases, the photochemical cell is integrated with a membrane separator or a gas-diffusion electrode to facilitate the separation of the products and increase the overall efficiency of the system ^[15][16][15,16].

Organic dyes, such as metalloporphyrins, phthalocyanines, and ruthenium polypyridyl complexes, have been widely used as sensitizers in dye-sensitized solar cells (DSSCs) because of their strong absorption coefficients and high molar extinction coefficients. In a study conducted by Mathew and group [17], a molecularly engineered porphyrin dye, coded SM315, which features the prototypical structure of a donor–π-bridge–acceptor and both maximizes electrolyte compatibility and improves light-harvesting properties, was used in DSSCs. They found that using SM315 with the cobalt (II/III) redox shuttle resulted in dye-sensitized solar cells that exhibited a high open-circuit voltage VOC of 0.91 V, short-circuit current density J_SC of 18.1 mA cm^–2, fill factor of 0.78, and a power conversion efficiency of 13%. Even when organic dyes are relatively inexpensive and offer tunable absorption properties, their long-term stability and limited light-harvesting efficiency remain challenges ^[17][18][17,18]. Recent advances in molecular engineering have resulted in the development of new organic dyes with improved performance and stability ^[18][19][18,19].

Inorganic dyes, such as cadmium sulfide (CdS) and cadmium selenide (CdSe), have also been employed as sensitizers [20] for their higher stability and broader absorption spectra compared to organic dyes, but their toxicity and potential environmental impacts remain major concerns [21]. Perovskite materials, which have demonstrated remarkable efficiency improvements in solar cells, can also be considered inorganic dyes and have been used as promising materials [22]. Yoo and group [22] reported using a holistic approach to improve the performance of PSCs through enhanced charge carrier management. First, they developed an electron transport layer with film coverage, thickness, and composition by tuning the chemical bath deposition of tin dioxide (SnO₂). Second, the authors decoupled the passivation strategy between the bulk and the interface, leading to improved properties, while minimizing the bandgap penalty. The devices exhibited an electroluminescence external quantum efficiency of up to 17.2% and an electroluminescence energy conversion efficiency of up to 21.6%. As solar cells, they achieved a certified power conversion efficiency of 25.2%, corresponding to 80.5% of the thermodynamic limit of its bandgap.

Quantum dots, or semiconductor nanocrystals, have also emerged as promising sensitizers for artificial photosynthesis systems as a result of their unique optical properties, such as a size-tunable bandgap and multiple exciton generation ^[23][24][25][23,24,25]. They have shown improved efficiencies compared to organic dyes, but their toxicity and potential environmental impacts remain major concerns. Recent studies have focused on developing alternative, less toxic quantum dot materials, such as copper indium sulfide (CIS) and silver indium sulfide (AgInS₂) [23]. For example, researchers [23] developed CuInS₂ (CIS)-based solar cell devices by sensitizing TiO₂ photoanodes with CIS quantum dots (CISQDs). The research group reported a maximum efficiency of 3.8% (with JSC ≈ 6.2 mA, VOC ≈ 926 mV and FF ≈ 66 for cell area ≈ 0.25 cm² and thickness ≈ 20 µm) when 4.6 nm CISQDs that were sensitized on composite photoanode were used. The group explains that the high VOC observed was possible because of the combined effect of the P25 composite photoanode’s properties (such as fewer defects, good connectivity between particles, effective light scattering, and minimum recombination) with an effective electron transport and the size of the optimized CuInS₂QDs.

Silicon-based mesoporous materials also play an important role in the design and implementation of photoelectrochemical cells for artificial photosynthesis because of their inherent semiconductor properties, high surface area, and controllable pore size ^[2][4][5][6][2,4,5,6]. Silicon, particularly in its nanostructured form, possesses direct band gaps that facilitate efficient charge transfers [2]. Furthermore, silicon’s natural abundance and non-toxicity contribute to the sustainability and potential large-scale applications of these systems ^[7][8][7,8].

Recent advances in mesoporous silicon fabrication technologies, such as electrochemical etching and magnesiothermic reduction, have allowed for the creation of highly ordered, crystalline structures that are beneficial for photon absorption and charge transportation ^[9][14][9,14]. Additionally, strategies for the modification of mesoporous silicon, such as doping with other elements or coupling with suitable co-catalysts, have been explored for improving its photoelectrochemical performance [6]. These efforts have demonstrated promising results in enhancing the stability and efficiency of silicon-based photoelectrochemical cells, paving the way for practical applications of artificial photosynthesis [9].

Another approach that has been employed is the use of molecular catalysts as a result of their ability to facilitate a redox reaction that converts solar energy into chemical energy [26]. Examples of molecular catalysts for artificial photosynthesis include transition metal complexes, such as cobalt (Co) [27], manganese (Mn) [28], and iron-based complexes [29]. These catalysts offer the advantage of cost-effectiveness and sustainability compared to noble metal catalysts, but their catalytic activity and stability often lag behind ^{[27][28][29][30]}[27,28,29,30]. Recent research efforts have focused on developing more robust molecular catalysts with improved performance and stability ^[29][30][31][29,30,31]. A study conducted by Wolff and group [31] reported simultaneous H₂ and O₂ evolution by CdS nanorods decorated with nanoparticulate reduction and molecular oxidation co-catalysts. The authors explained that the process proceeded entirely without sacrificial agents and relied on the nanorod morphology of CdS to spatially separate the reduction and oxidation sites. They further explained that hydrogen was generated on Pt nanoparticles grown at the nanorod tips, whereas Ru(tpy)(bpy)Cl₂-based oxidation catalysts were anchored through dithiocarbamate bonds onto the sides of the nanorod. In the case of O₂ generation from water, the research group explained that the process was verified using ¹⁸O isotope-labeling experiments, and time-resolved spectroscopic results confirmed efficient charge separation and ultrafast electron and hole transfer to the reaction sites. The authors ended by arguing that the system demonstrated that combining nanoparticulate and molecular catalysts on anisotropic nanocrystals can provide an effective pathway for visible-light-driven photocatalytic water splitting.

Nanostructured catalysts, such as metal oxides, metal sulfides, and metal-organic frameworks (MOFs), have also been explored for artificial photosynthesis applications. These materials offer a high surface area and tunable electronic properties, making them attractive candidates for catalytic applications ^{[32][33][34][35][36][37]}[32,33,34,35,36,37]. Examples of nanostructured catalysts for artificial photosynthesis include cobalt oxide (Co₃O₄) [35], nickel oxide (NiO) [36], and iron sulfide (FeS₂) [37]. Alam and group [37] reported Pyrite (FeS₂)-decorated 1D TiO₂ nanotubes in a bilayer as a sustainable photoanode for photoelectrochemical water splitting activity. The results of the catalyst (15-FeS₂@TiO₂) showed a higher photocurrent density of 1.59 mA/cm² at 0.3 V versus a reference electrode of Ag/AgCl (or at 1.23 V versus a reversible hydrogen electrode) using a 100 mW/cm² intensive light source and a donor density (N_D) of 3.68 × 10⁻¹³ cm⁻³ as compared to that of pure TiO₂NTs (0.09 mA/cm²), 05-FeS₂@TiO₂NTs (0.19 mA/cm²), 10-FeS₂@TiO₂NTs (0.53 mA/cm²), and 20-FeS₂@TiO₂NTs (0.61 mA/cm²). The authors explained that the photoelectrochemical activity results were attributed to the homogenous integration of FeS₂ that not only increased the charge separation but also intensively interacted with the substrate (TiO₂ nanotubes), which resulted in excellent photoelectrochemical activity. Even when these materials offer the advantages of cost-effectiveness and sustainability, their catalytic activity and stability are often less than those of noble metal catalysts [32].

Cobalt-based redox mediators, such as cobalt bipyridine and cobalt phenanthroline complexes, have been widely used in artificial photosynthesis systems as a result of their favorable redox properties and stability ^[38][39][38,39]. These mediators can efficiently shuttle electrons between the photoanode and the counter electrode, reducing the overall overpotential of the system and improving its efficiency. However, cobalt-based mediators can suffer from high recombination rates and limited diffusion coefficients, which can adversely impact their overall performance [40]. Copper-based redox mediators, such as copper phenanthroline and copper bipyridine complexes, have also been explored as alternatives to cobalt-based mediators in artificial photosynthesis systems [41]. Copper-based mediators offer several advantages, such as a lower cost and abundant availability compared to cobalt mediators. They also demonstrate good electron transfer properties and stability. However, their catalytic activity and stability may not be as high as those of cobalt-based mediators, and their applications in artificial photosynthesis systems require further optimization ^[42][43][42,43]. Organic redox mediators, such as organic molecules containing viologen, TEMPO, and ferrocene moieties, have also been investigated for use in artificial photosynthesis systems [44]. These mediators offer several advantages, including a low cost, good solubility, and tunable redox properties, but their long-term stability and compatibility with other materials in the system remain challenges ^[45][46][45,46]. Recent research efforts have focused on developing new organic mediators with improved stability and performance for artificial photosynthesis applications [47].

One approach to enhance the performance of photochemical cells is broadening their absorption spectra, which allows them to capture more sunlight and convert it into useful energy. This can be achieved by designing novel photosensitizers with extended absorption profiles, employing multiple photosensitizers with complementary absorption spectra, or introducing additional light-harvesting materials into the system ^{[48][49][50][51]}[48,49,50,51]. For example, Cheema et al. [52] synthesized and characterized seven organic sensitizers, employing thienopyrazine (TPz) as a π-bridge in a double donor, double acceptor organic dye design. The author reported that the thienopyrazine (TPz) building block allows for NIR photon absorption in dye-sensitized solar cells (DSCs) when used as a π-bridge and that the dye design was found to be remarkably tunable with solution absorption onsets ranging from 750 to nearly 1000 nm. Furthermore, the incorporation of quantum dots with tunable absorption properties has been shown to improve the light-harvesting capabilities of photochemical cells [53].

Plasmonic enhancement is another strategy for improving the light-harvesting efficiency of photochemical cells. Plasmonic nanoparticles, such as gold and silver, can concentrate and scatter light, leading to enhanced absorption by photosensitizers [54]. Several studies have demonstrated the benefits of incorporating plasmonic nanoparticles into photochemical cells, resulting in increased power conversion efficiencies ^[55][56][55,56]. Liu and group [57] reported on the integration of gold nanoparticles (Au NPs) into the mesoporous TiO₂ layer of dye-sensitized solar cells, obtaining a power conversion efficiency of 6.4%, which was significantly higher than a TiO₂ DSSC. The short circuit current density was increased by 23% and the conversion efficiency was improved by 28% with the addition of Au NPs. This improvement is attributed to the increase in light harvesting efficiency and lower charge carrier recombination rate of the TiO₂-Au DSSC.

The performance of photochemical cells can also be improved by optimizing the interfaces between various materials in the system. Proper interface engineering can enhance charge separation and transport rates, reduce recombination losses, and, ultimately, increase overall efficiency [58]. This can be achieved by introducing additional layers, such as holes or electron transport layers, or by modifying the interface with functional groups or molecules ^[59][60][59,60]. For instance, Yang et al. [61] reported a simple and effective interface engineering method for achieving highly efficient planar perovskite solar cells (PSCs), employing SnO₂ electron selective layers (ESLs). A 3-aminopropyltriethoxysilane (APTES) self-assembled monolayer (SAM) was used to modify the SnO₂ ESL–perovskite layer interface. The APTES SAM demonstrated multiple functions: (1) It increased the surface energy and enhanced the affinity of the SnO₂ ESL, which induced the formation of high-quality perovskite films with a better morphology and enhanced crystallinity. (2) The terminal functional groups formed dipoles on the SnO₂ surface, leading to a decreased work function of SnO₂ and an enlarged built-in potential of SnO₂/perovskite heterojunctions. (3) The terminal groups passivated the trap states at the perovskite surface via hydrogen bonding. (4) The thin insulating layer at the interface hindered electron back transfer and reduced the recombination process at the interface effectively. These results suggest that using an ESL–perovskite interface engineered with APTES SAM is a promising method for fabricating efficient and hysteresis-less PSCs.

The use of nanostructured materials can also enhance rates of charge transport and separation in photochemical cells. These materials have a high surface area and can provide short pathways for charge transport, leading to reduced recombination losses [62]. Examples of nanostructured materials employed in photochemical cells include mesoporous metal oxides, such as TiO₂ and ZnO, and graphene-based materials ^[63][64][63,64]. In particular, the incorporation of graphene into dye-sensitized solar cells has been shown to improve electron transport and reduce recombination, resulting in enhanced cell performance [65].

Bimetallic catalysts have gained significant interest in recent years as a result of their potential for improved catalytic activity and stability compared to their monometallic counterparts [66]. These catalysts often exhibit synergistic effects, where the combination of two metals results in enhanced performance compared to the individual metals alone. Bimetallic catalysts have been applied to various photochemical cell systems, including dye-sensitized solar cells and water-splitting devices ^[67][68][69][67,68,69]. For example, Lim et al. [70] reported the use of a bimetallic NiFe-based alloy for oxygen evolution in a photochemical water-splitting system, which demonstrated improved catalytic activity and stability compared to the monometallic Ni and Fe catalysts. Moreover, the alloy catalyst exhibited substantial long-term durability after 1000 cyclic voltammetry tests. This electrochemical performance mainly originated from the synergistic effects of Fe incorporation into Ni species, leading to the improved charge transfer kinetics and intrinsic activity of the catalyst.

Another approach for enhancing the catalytic activity and stability of photochemical cells is the use of co-catalysts. Co-catalysts can work in synergy with a primary catalyst, promoting the desired reaction and improving the overall performance of the system. For instance, the introduction of co-catalysts such as Pt, Au, or Pd in semiconductor photocatalysts has been shown to improve the efficiency of photocatalytic water splitting by enhancing hydrogen evolution and reducing charge recombination ^[71][72][73][71,72,73]. In dye-sensitized solar cells, the use of co-catalysts, such as NiO or CuCrO₂, can improve the performance of the system by facilitating hole transport and reducing recombination loss ^[74][75][74,75].

The surface modification of catalysts is another strategy for enhancing their activity and stability in photochemical cells. This can be achieved by introducing functional groups or molecules onto the catalyst surface, which can alter its electronic properties and promote the desired reactions [76]. Surface modification can also improve the stability of catalysts by providing a protective layer against degradation [77]. For example, Zhang et al. [78] developed a room-temperature synthesis to produce gelled oxyhydroxide materials with an atomically homogeneous metal distribution. These gelled FeCoW oxyhydroxides exhibited the lowest overpotential (191 millivolts) and were reported at 10 milliamperes per square centimeter in an alkaline electrolyte. The catalyst showed no evidence of degradation after more than 500 h of operation. X-ray absorption and computational studies revealed a synergistic interplay between tungsten, iron, and cobalt in producing a favorable local coordination environment and electronic structure that enhance the energetics for an OER. Similarly, the surface modification of TiO₂ with organic molecules has been shown to improve the performance of dye-sensitized solar cells by enhancing electron transfers between the dye and the semiconductor [79].

One of the primary challenges in the development and implementation of photochemical cells for artificial photosynthesis is scalability. Although many laboratory-scale systems have demonstrated promising results, transitioning these technologies to a large scale remains a significant hurdle [80]. The scalability challenge is multifaceted, involving the need for efficient and cost-effective production methods, the integration of photochemical cells into existing infrastructure, and the development of large-scale, stable systems that can maintain high performance over extended periods of time [81]. Overcoming these challenges is crucial for the widespread adoption and commercialization of artificial photosynthesis technologies.

Another critical challenge is the durability and stability of photochemical cells. Many of the materials and components currently used in these systems, such as organic dyes, molecular catalysts, and redox mediators, can suffer from degradation and loss of performance over time as a result of various factors, such as photobleaching, chemical instability, and mechanical stress ^[82][83][82,83]. Developing materials and systems that can withstand the harsh operating conditions associated with artificial photosynthesis, including high light intensities, elevated temperatures, and corrosive electrolytes, is essential for the long-term success of these technologies [84].

The cost and resource efficiency of artificial photosynthesis technologies is another significant challenge that must be addressed for widespread implementation. Many of the materials and processes currently used in photochemical cells, such as noble metal catalysts and complex fabrication techniques, can be expensive and resource intensive [85]. To make these technologies economically viable and reduce their environmental impact, it is crucial to develop more cost-effective and sustainable materials and production methods [86]. This may involve the exploration of earth-abundant alternatives to scarce and expensive materials, as well as the development of more efficient and scalable fabrication techniques [87].

In addition to the technological challenges, the environmental and social implications of artificial photosynthesis must also be considered. Although these technologies have the potential to reduce greenhouse gas emissions and contribute to a more sustainable energy future, their large-scale deployment could have unintended consequences. For example, the production of photochemical cells and their associated infrastructure may consume significant amounts of energy, water, and other resources, leading to potential trade-offs between the benefits and the environmental costs [88]. Moreover, the social implications of artificial photosynthesis, such as potential job displacement in traditional energy sectors and the equitable distribution of benefits, must be carefully considered and addressed [89].

Despite the numerous challenges associated with artificial photosynthesis, there are many exciting research directions and opportunities to explore. One promising area of research is the development of novel materials and architectures that can significantly improve the performance and stability of photochemical cells. For instance, research into perovskite materials, two-dimensional materials, and metal–organic frameworks has shown great potential for enhancing light absorption, charge transport, and catalytic activity in these systems ^[90][91][92][90,91,92]. Another important research direction is the integration of artificial photosynthesis technologies with other renewable energy systems, such as solar cells, batteries, and fuel cells, to create more efficient and sustainable energy systems [93]. Furthermore, advances in computational modeling and materials informatics can help accelerate the discovery and optimization of new materials and systems for artificial photosynthesis [94]. These approaches can provide valuable insights into the fundamental mechanisms underlying the performance of photochemical cells and guide the design of more effective materials and architectures [95]. Finally, interdisciplinary collaborations between researchers in chemistry, materials science, engineering, and other fields can foster the development of innovative solutions to the many challenges facing artificial photosynthesis and contribute to the realization of its full potential as a sustainable energy technology [96].

Figure 1 presents a schematic of the challenges, strategies, and opportunities of photoelectrochemical cell performance in artificial photosynthesis. Overall, the strategies, challenges, and opportunities associated with photochemical cells in artificial photosynthesis require interdisciplinary research efforts, combining materials science, catalysis, engineering, and energy policy to overcome the technical barriers and unlock their full potential for a sustainable energy future.