Solar-powered hydrogen: exploring production, storage, and energy integration strategies

Abstract

This review explores the advancements in solar technologies, encompassing production methods, storage systems, and their integration with renewable energy solutions. It examines the primary hydrogen production approaches, including thermochemical, photochemical, and biological methods. Thermochemical methods, though highly efficient, require advanced materials and complex reactor designs, while photochemical methods offer a simpler alternative but suffer from low conversion efficiencies. Biological hydrogen production presents a low-cost option but faces limitations in scalability and production rates. The review also highlights innovative hydrogen storage technologies, such as metal hydrides, metal-organic frameworks, and liquid organic hydrogen carriers, which address the intermittency of solar energy and offer scalable storage solutions. Additionally, the potential of hybrid energy systems that integrate solar hydrogen with photovoltaics, thermal energy systems, battery storage, and smart grids is emphasized. Despite technical and economic barriers, ongoing advancements in catalyst development, material optimization, and artificial intelligence-driven energy management systems are accelerating the adoption of solar hydrogen technologies. These innovations position solar hydrogen as a pivotal solution for achieving a sustainable and low-carbon energy future.

1. Introduction

The global energy landscape is currently undergoing a significant transformation due to the increasing demand for sustainable and environmentally friendly energy solutions, which is driven by several factors [1], including the growing global population, rapid industrialization, and the urgent need to mitigate the effects of climate change. Traditional energy sources, such as fossil fuels, contribute heavily to greenhouse gas emissions and are limited in supply [2], making them unsustainable in the long term. Wind, solar, and hydropower offer promising alternatives that can significantly reduce the environmental impact of energy production, in which solar energy stands out due to its abundance and geographical flexibility, which can be captured in almost any location on Earth [3], making it a flexible and widely usable option for energy generation. However, solar energy is not always available because it depends on daylight and weather conditions, which creates a significant challenge, requiring the development of efficient energy storage systems to ensure a consistent and reliable energy supply. Solar energy can be captured and converted into various forms, including electrical energy via photovoltaics (PVs), thermal energy through solar heating systems, and chemical energy in the form of solar fuels, in which the conversion of solar energy into chemical energy represents a promising strategy for overcoming the challenges of intermittency and storage [4]. Solar fuels, such as hydrogen, store solar energy in chemical bonds that can be released on demand, providing a flexible and long-term energy storage solution. As a clean energy carrier, hydrogen can be used in fuel cells to produce electricity with water as the only byproduct, making it an attractive option for reducing carbon emissions in sectors such as transportation, industry, and power generation [5]. Furthermore, hydrogen can be stored in compressed, liquefied, or chemically bonded forms, providing a versatile means of energy storage and transport. One of the most promising avenues for producing hydrogen sustainably is through solar hydrogen production, which directly or indirectly uses solar energy to split water into hydrogen and oxygen. Unlike traditional hydrogen production methods that depend on fossil fuels and produce significant carbon emissions, solar hydrogen production is environmentally friendly and uses an unlimited energy source [6]. Solar hydrogen production can be achieved through several processes, including thermochemical water splitting, photochemical reactions, and biological processes. In addition, hydrogen can serve as both a fuel and an energy storage medium, and its ability to be stored for long periods enables it to bridge the gap between solar energy availability and demand, effectively addressing the intermittency of solar power [7], which makes it a valuable resource not only in energy production but also in stabilizing energy grids and providing backup power in off-grid applications. Despite the significant promise of solar hydrogen, there are still numerous technical and economic barriers that must be overcome before it can be deployed at scale. Challenges [8] such as low conversion efficiencies, high production costs, and the need for advanced materials in catalytic processes are currently limiting the widespread adoption of solar hydrogen technologies. However, ongoing research in areas such as catalyst development [9, 10], reactor design [11, 12], and integration with renewable energy systems [13–15] is steadily improving the viability of these technologies. This review will provide a comprehensive overview of the current state of solar hydrogen production, storage technologies, and systems integration, with a focus on the major approaches including thermochemical, photochemical, and biological methods as illustrated in Fig. 1, which presents a graphical abstract of solar-powered hydrogen technologies. By examining the underlying principles, recent advancements, and technical challenges associated with each method, the aim of this study is to highlight the potential of solar hydrogen as a key component in the future of renewable energy storage.

2. Solar hydrogen production technologies

Solar energy can be converted into hydrogen through three primary methods (as shown in Fig. 2): thermochemical, photochemical, and biological processes. Thermochemical production involves high-temperature reactions, often using metal oxides, to split water into hydrogen and oxygen, typically driven by concentrated solar power (CSP). Photochemical methods use solar energy to directly drive water-splitting reactions via photocatalysts, offering a potentially simpler and lower-cost route to hydrogen. Lastly, biological processes utilize microorganisms, such as algae or bacteria, to produce hydrogen from sunlight through their natural metabolic activities.

2.1 Thermochemical hydrogen production

Thermochemical hydrogen production is a promising method for typically utilizing CSP to drive high-temperature chemical reactions, splitting water into hydrogen and oxygen, which utilizes solar energy to heat reactants to temperatures that facilitate water decomposition, typically involving such as sulfur-iodine (S-I) [16, 17], hybrid sulfur (HyS) [18, 19], calcium-bromine [20], and metal oxide reduction-oxidation (redox) cycles [21–27]. Among these, metal oxide cycles have gained significant attention due to their high efficiency, scalability, and relative simplicity [28]. These cycles, such as ZnO/Zn, CeO₂/Ce₂O₃, and Fe₃O₄/FeO, operate through a straightforward two-step redox mechanism that effectively converts solar thermal energy into chemical energy [21]. Unlike the S-I and HyS cycles, which involve corrosive chemicals and complex multi-step reactions, metal oxide cycles rely on widely available and cost-effective materials like ZnO and CeO₂, making them practical for industrial-scale applications [24]. In comparison, methods like S-I and HyS, while capable of high yields, involve greater operational complexity and higher costs, limiting their scalability [17]. Metal oxide cycles thus strike an ideal balance between efficiency, simplicity, and scalability, positioning them as a leading candidate for sustainable and large-scale solar hydrogen production. Table 1 provides a comprehensive overview of thermochemical hydrogen production methods, summarizing their efficiency, production rates, material durability, potential advantages, and associated challenges.

2.1.1 Mechanisms of cyclic redox reactions

At the core of thermochemical hydrogen production are cyclic redox reactions involving metal oxides, which alternately undergo reduction at high temperatures and re-oxidation by water to produce hydrogen. A widely studied example is the ZnO/Zn cycle (as shown in Fig. 3), which operates through the two-step reactions [21]:

Thermal reduction: ZnO → Zn + 1/2O₂ (at T > 2000°C), during this step, solar energy is concentrated to achieve the high temperatures (>2000°C) required to reduce zinc oxide (ZnO) to metallic zinc and oxygen. Water splitting (hydrolysis): Zn + H₂O → ZnO + H₂ (at ~ 400–600°C), in this stage, the reduced zinc reacts with water vapor at lower temperatures (~400–600°C) to regenerate ZnO and produce hydrogen gas.

This two-step cycle effectively converts solar thermal energy into chemical energy in the form of hydrogen. However, achieving and maintaining the necessary high temperatures (>2000°C) present significant engineering challenges. Advanced solar concentrator technologies and reactor designs are required to focus and retain heat with minimal losses [44]. The thermodynamics of these reactions determine the feasibility of thermochemical cycles. In other words, the energy required to break chemical bonds and the energy released during the reactions determine whether the process can efficiently produce hydrogen under given conditions, such as temperature and pressure. If the energy input needed is too high, the process may be inefficient or impractical for large-scale hydrogen production. For example, the reduction of metal oxides like ZnO requires substantial input energy, as the Gibbs free energy of the reaction is highly endothermic. The efficiency of this process depends on the ability to minimize heat loss during the high-temperature reduction phase and ensure effective heat recovery between cycles [23]. In addition to ZnO, other metal oxide pairs, such as CeO₂/Ce₂O₃ [24, 26] and Fe₃O₄/FeO [25], have been investigated due to their lower temperature requirements and favorable thermodynamic properties. However, these cycles typically exhibit lower hydrogen production efficiencies compared to the ZnO/Zn cycle, in which the exploration of alternative materials is an ongoing research focus aimed at balancing efficiency, cost, and operational temperatures.

2.1.2 Developing methods of thermochemical hydrogen production

The reaction kinetics of the redox cycle significantly influence the overall efficiency of thermochemical hydrogen production. For example, the kinetic limitations in the ZnO reduction step stem from the slow rate of oxygen release at high temperatures, which can reduce overall cycle efficiency. Researchers are investigating various strategies to enhance reaction rates, such as using catalytic coatings on metal oxides [29, 30], optimizing particle size to increase surface area [31], and developing novel reactor configurations that improve heat and mass transfer [32].

Material degradation at high temperatures is another critical factor. The extreme thermal conditions during the reduction phase can lead to material degradation, which decreases the surface area available for reactions, thus lowering efficiency over repeated cycles [45]. Furthermore, reactor components and materials must be resistant to high-temperature oxidation and thermal stress [46]. High-performance materials, such as ceramics [33] and refractory metals [34], are being explored to withstand the demanding conditions in thermochemical reactors. Coating technologies [35] that can protect metal oxides from sintering or degradation are also being developed.

Another challenge in thermochemical hydrogen production is improving the efficiency of heat transfer within the system. The need for high temperatures and the difficulty of maintaining these temperatures uniformly across the reaction environment often lead to heat losses [37]. Advanced solar concentrators capable of achieving high flux densities, such as parabolic troughs [47] and central tower receivers [36], have been employed to address this issue, but additional strategies are required to optimize heat utilization.

Heat recovery systems can significantly enhance overall cycle efficiency by capturing excess thermal energy from the high-temperature reduction phase and redirecting it to the subsequent water-splitting phase [38]. The design of recuperative heat exchangers and thermal energy storage systems [37] is a critical area of research that seeks to reduce the energy input required for each cycle, thereby improving the net solar-to-hydrogen efficiency.

Additionally, the separation of hydrogen and oxygen during the process is essential to avoid recombination [48], which can reduce the yield of hydrogen and introduce safety concerns. One promising approach is the use of membrane-based separation systems [39] that allow for the selective removal of oxygen during the reduction phase, thereby preventing recombination and ensuring high-purity hydrogen production. Recent advances in ceramic membranes [40] and perovskite oxides [41] have shown potential in this area, but their scalability and durability under high-temperature conditions require further investigation.

2.1.3 Efficiency and system limitations

Several factors limit the current efficiency of thermochemical hydrogen production systems. First, the high operational temperatures necessitate advanced materials and reactor designs, increasing the capital and operational costs. Second, heat losses due to imperfect insulation and inefficient heat transfer mechanisms reduce the overall efficiency of the system. Third, the slow kinetics of oxygen release during the reduction step hinder the rate of hydrogen production, necessitating the exploration of catalysts or optimized material structures. In response to these challenges, novel reactor designs [12] are being developed that focus on maximizing the concentration and retention of solar energy while minimizing thermal losses. For instance, rotating particle reactors [42] have been proposed to enhance the exposure of metal oxide particles to solar radiation, improving the efficiency of the reduction reaction. Similarly, fluidized bed reactors [43] offer the potential to improve heat transfer and reaction kinetics by ensuring continuous mixing and uniform heating of the reactants. Advanced material engineering also plays a critical role in addressing these limitations. The development of nanostructured metal oxides [22, 27] and composite materials [23] that can withstand extreme temperatures without degradation offers a promising pathway to improving the longevity and efficiency of thermochemical systems.

2.2 Photochemical hydrogen production

Photochemical hydrogen production utilizes solar energy to drive water-splitting reactions through photocatalysis, which has garnered significant interest due to its potential to provide a direct, low-cost pathway for converting sunlight into hydrogen [49]. However, the efficiency of current systems is limited by the properties of available photocatalysts, which typically absorb only a small portion of the solar spectrum, and by challenges related to charge separation and transfer within the catalyst material [50]. Advances in photocatalyst development and system optimization are therefore crucial to improving the overall efficiency of photochemical hydrogen production. Table 2 offers a detailed summary of photochemical hydrogen production methods, highlighting their efficiency, production rates, material durability, key advantages, and associated challenges.

2.2.1 Photocatalyst classes

Photocatalysts play a central role in photochemical hydrogen production, driving the water-splitting reaction by absorbing light, generating charge carriers (electrons and holes), and facilitating redox reactions. These catalysts can be broadly categorized into three classes:

1. Single-component photocatalysts

Single-component materials, such as titanium dioxide (TiO₂), ZnO, tungsten trioxide (WO₃), and hematite (Fe₂O₃), are widely studied due to their simplicity and well-understood properties. TiO₂, for example, is highly stable, cost-effective, and exhibits significant photocatalytic activity under ultraviolet (UV) light. However, its wide bandgap (~3.2 eV) restricts absorption to the UV spectrum, which constitutes only ~4% of solar radiation [51].

To extend absorption into the visible range, researchers have employed doping strategies, introducing elements such as Fe [52–54], Co [55], Cr [56] (transition metals) or N [57], S [58, 59], and C [60] (nonmetals). Although doping can reduce the bandgap, it often introduces recombination centers that decrease overall efficiency [51]. Nanostructuring TiO₂ into 1D (nanotubes [61, 62]) or 2D (nanosheets [63, 64]) forms improves charge separation and transport by providing shorter diffusion paths for charge carriers. Core-shell structures, where TiO₂ is coated with other materials, offer additional advantages by protecting the catalyst from recombination and maintaining high stability [53, 65].

Other single-component materials, such as ZnO [66, 67], WO₃ [68], and Fe₂O₃ [69, 70], feature narrower bandgaps for visible light absorption but suffer from poor charge transport properties and low quantum efficiencies. To address these limitations, heterojunction designs combining these materials with others have been explored [89].

1. Type-II heterojunction photocatalysts

Type-II heterojunctions involve interfaces between two materials with complementary band structures. These structures promote efficient charge separation by confining electrons and holes to different regions. For example, coupling ZnO with reduced graphene oxide (rGO) improves electron mobility and extends light absorption into the visible range [71–73]. These heterostructures show significant promise in improving photocatalytic performance by leveraging synergies between their components.

1. Z-Scheme photocatalysts

Inspired by natural photosynthesis, Z-scheme systems use two photocatalysts with complementary bandgaps [87], one driving the oxygen evolution reaction (OER) and the other driving the hydrogen evolution reaction (HER). An electron mediator transfers charge between the two photocatalysts, allowing for better utilization of the solar spectrum. Examples include systems incorporating plasmonic nanoparticles or 2D materials like graphene [90] and molybdenum disulfide (MoS₂) [71, 83]. Z-scheme designs are particularly promising for overcoming the limitations of single-component and type-II heterojunction systems, achieving higher overall efficiency by separating reaction sites [86].

2.2.2 Bandgap engineering and light absorption mechanisms

A fundamental aspect of photochemical hydrogen production is the bandgap of the photocatalyst (as shown in Fig. 4), which defines its ability to absorb light and generate the charge carriers necessary for water splitting. The bandgap is the energy difference between the valence band (occupied by electrons) and the conduction band (unoccupied energy levels). For a photocatalyst to efficiently drive water splitting, its bandgap must allow absorption of a significant portion of the solar spectrum and generate electrons and holes with sufficient energy for the redox reactions. The minimum theoretical bandgap required for water splitting is ~1.23 eV, corresponding to the Gibbs free energy of the reaction [49]. However, in practical systems, photocatalysts typically have bandgaps in the range of 1.5–3 eV to account for additional energy losses due to recombination, overpotential, and energy dissipation at the electrode surface [76].

The light absorption mechanism begins when photons with energy equal to or greater than the bandgap excite electrons from the valence band to the conduction band, leaving behind holes in the valence band. This process generates electron-hole pairs, or excitons, which are the primary drivers of the HER and OER. Photocatalysts with smaller bandgaps (<1.5 eV) can absorb a broader range of the solar spectrum, including visible and near-infrared light, but may lack the energy required to efficiently drive water splitting. Conversely, larger bandgaps (>3 eV) provide sufficient energy for redox reactions but are limited to absorbing UV light, which constitutes only a small fraction (~4%) of the solar spectrum [49].

Bandgap engineering techniques are essential for optimizing light absorption in photocatalysts, enabling improved performance in photochemical hydrogen production. One approach is quantum confinement [77], which involves reducing the dimensions of materials to the nanoscale, thereby altering their electronic properties, increasing the bandgap, and enhancing charge dynamics. Another strategy is defect engineering [78], where controlled defects are introduced into the material’s structure, creating localized energy states that improve light absorption and charge carrier mobility. Additionally, the incorporation of plasmonic nanoparticles, such as gold (Au) [83] or silver (Ag) [84, 85], enhances light absorption by leveraging localized surface plasmon resonance, which amplifies electromagnetic fields near the catalyst surface and boosts the generation of electron-hole pairs. These techniques collectively contribute to more efficient utilization of the solar spectrum for water splitting.

2.2.3 Optimization of photocatalytic systems

Improving the solar-to-hydrogen conversion efficiency requires not only advances in photocatalyst materials but also system-level optimization. Suspension reactors [88], where photocatalysts are dispersed in a liquid medium, enhance the interaction between light, water, and catalysts, increasing reaction rates. Layered 2D materials, such as graphene [90] and MoS₂ [71, 83], provide broad light absorption and efficient charge transport pathways. Additionally, high surface area nanostructured materials [59–64, 69, 70, 77, 85] offer more active sites for water splitting, further boosting efficiency.

Visible light utilization remains a key focus, as UV light accounts for only a small fraction of solar energy. Researchers [91, 92] are developing photocatalysts with broad-spectrum absorption, multifunctional designs, and robust frameworks to improve light harvesting and stability under real-world conditions. Metal-organic frameworks (MOFs) [93] and perovskites [74], while not photocatalysts themselves, serve as scaffolds for active components, offering tunable properties and controlled environments for hydrogen evolution [75]. However, their stability in aqueous environments [94], potential toxicity due to lead content [95], and sensitivity to moisture [96] remain major challenges. Addressing these challenges requires innovative solutions, such as developing lead-free perovskites [97], enhancing stability through surface passivation techniques [98], encapsulating the materials to protect them from environmental exposure [99], and incorporating hybrid organic-inorganic structures to improve resistance to moisture and aqueous conditions [100].

2.3 Biological hydrogen production

Biological hydrogen production is a promising approach that utilizes the natural metabolic processes of microorganisms such as algae, cyanobacteria, and photosynthetic bacteria to produce hydrogen from sunlight and water [101]. While this method has the potential to offer a sustainable and low-cost solution for hydrogen production, its efficiency and scalability remain limited by several biochemical and engineering challenges [102]. Advances in understanding the metabolic pathways involved in hydrogen production, as well as the development of efficient bioreactor systems, are critical to improving the scalability and feasibility of this technology. Table 3 presents a comprehensive overview of biological hydrogen production methods, emphasizing their efficiency, production rates, material durability, key benefits, and challenges.

2.3.1 Mechanisms of enzymes hydrogen production

Biological hydrogen production relies on specific enzymes within microorganisms that catalyze the conversion of water or organic substrates into hydrogen, in which hydrogenase and nitrogenase play crucial roles in the hydrogen evolution process.

Hydrogenase enzymes are widely distributed among microorganisms and catalyze the reversible reduction of protons to molecular hydrogen [101]. These enzymes exist in three main types [104] based on their active metal centers: [FeFe]-hydrogenases, [NiFe]-hydrogenases, and [Fe]-hydrogenases; their structures and active sites are shown in Fig. 5. The [FeFe]-hydrogenases and [NiFe]-hydrogenases are the most extensively studied, as they perform similar functions but differ in their active sites. [FeFe]-hydrogenases are typically associated with higher catalytic activity, while [NiFe]-hydrogenases are more common and often more robust under varying conditions [120]. Both types utilize electrons generated during photosynthesis or the metabolic degradation of organic compounds to reduce protons into hydrogen gas. However, a significant limitation of these enzymes is their sensitivity to oxygen, which can irreversibly inactivate the enzyme by damaging its metal centers. This sensitivity poses a major bottleneck in biological hydrogen production, particularly in photosynthetic systems where oxygen is a byproduct [121]. The third type, [Fe]-hydrogenases, contains a single iron atom at the active site without iron-sulfur clusters, distinguishing it from the other two types. These enzymes operate via a fundamentally different enzymatic mechanism, primarily catalyzing hydrogen production in methanogenic archaea [122]. Although less studied, [Fe]-hydrogenases contribute to the diversity of enzymatic pathways for hydrogen evolution and may offer unique advantages in specific microbial systems.

Nitrogenase enzymes, found in nitrogen-fixing bacteria and cyanobacteria, also play a role in hydrogen production. Nitrogenase catalyzes the reduction of atmospheric nitrogen (N₂) into ammonia (NH₃), but hydrogen is produced as a byproduct of this reaction [103]. Unlike hydrogenases, nitrogenase can function in both aerobic and anaerobic conditions, although its hydrogen production efficiency is typically lower than that of hydrogenase. The adenosine triphosphate requirements of nitrogenase, coupled with its dual role in nitrogen fixation and hydrogen evolution, make it less efficient for large-scale hydrogen production compared to dedicated hydrogenases [104].

2.3.2 Metabolic engineering approaches

To overcome the limitations of natural microbial pathways, metabolic engineering and genetic modification approaches have been extensively explored to improve hydrogen yields. One strategy involves modifying the expression levels of hydrogenase or nitrogenase enzymes to enhance hydrogen production [123]. For example, researchers have genetically engineered algae [105] and cyanobacteria [106] to overexpress oxygen-tolerant hydrogenases, thereby improving their hydrogen production capacity under aerobic conditions. Another approach focuses on redirecting the flow of electrons within the microbial cell to favor hydrogen production. In photosynthetic microorganisms, light energy absorbed by photosystems generates high-energy electrons that are transferred to various cellular processes. By manipulating the electron transport chain, it is possible to channel more electrons towards hydrogenase or nitrogenase enzymes, increasing hydrogen output [124], which can be achieved by knocking out competing pathways, such as those involved in carbon fixation (e.g. Calvin cycle [107]) or respiration [108], which otherwise divert electrons away from hydrogen production. In addition, synthetic biology has been used to create hybrid systems that combine microbial cells with synthetic components, such as nanomaterials [109] or artificial electron donors [110], to improve the efficiency of electron transfer and hydrogen evolution. For instance, hybrid systems incorporating metal nanoparticles [111] have been developed to enhance the solar energy capture capabilities of algae and cyanobacteria, thereby increasing the overall efficiency of photosynthetic hydrogen production.

2.3.3 Hydrogenase stability and scalability

A central challenge in biological hydrogen production is improving the stability and efficiency of hydrogenase enzymes, particularly under oxygenic conditions [125]. Recent research has focused on modifying the active site of hydrogenase enzymes to increase their resistance to oxygen [126]. Another major hurdle in enzyme-based hydrogen production is scalability. While hydrogenase and nitrogenase are effective at the laboratory scale, their sensitivity to environmental conditions, particularly oxygen, poses significant challenges for industrial deployment [127]. Site-directed mutagenesis [112] has been employed to alter the metal center of the enzyme structure, reducing its affinity for oxygen while maintaining catalytic activity. Another promising approach involves engineering the electron transfer pathway of the enzyme [113] to increase the rate of hydrogen production, reducing the probability of oxygen-induced inactivation. In addition to enzyme modifications, researchers [114] are exploring cellular strategies that limit the exposure of hydrogenase to oxygen. For example, some cyanobacteria have been engineered to temporally separate photosynthesis and hydrogen production by performing these processes at different times of day [128] (e.g. photosynthesis during the day and hydrogen production at night, when oxygen levels are lower). These efforts are crucial for overcoming the oxygen sensitivity barrier that currently limits the efficiency of biological hydrogen production.

2.3.4 Bioreactor design and scaling challenges

Beyond enzymatic and metabolic improvements, the design of bioreactor systems plays a critical role in determining the overall hydrogen production rate. Bioreactors are engineered environments that support the growth and metabolic activity of microorganisms under controlled conditions [129]. Optimizing these systems for large-scale hydrogen production presents several challenges, particularly in terms of light distribution, nutrient supply, and gas exchange. Light intensity and distribution are key factors in maximizing the efficiency of photosynthetic hydrogen production. In algae and cyanobacteria-based systems, light must be evenly distributed throughout the reactor to ensure that all cells receive adequate illumination for photosynthesis [128]. Traditional flat-panel or tubular photobioreactors often face the issue of light attenuation, where cells near the reactor surface absorb most of the available light, leaving the cells deeper within the reactor with insufficient photon exposure [130]. To address this issue, optical fiber-based reactors [115, 116] and internal light sources [131] have been developed to distribute light more uniformly throughout the culture medium. Additionally, the management of CO₂ and oxygen levels is critical for maintaining high hydrogen production rates. In algal systems, CO₂ is a key substrate for photosynthesis, and its concentration must be carefully controlled to ensure efficient carbon fixation and electron generation. At the same time, oxygen produced during photosynthesis must be removed from the system to prevent the inhibition of hydrogenase activity [117]. Gas exchange systems that continuously remove oxygen and supply CO₂ are therefore essential components of any large-scale bioreactor designed for hydrogen production [130]. Recent advances in biohydrogen production have also explored the use of mixed microbial cultures that combine different species of algae, cyanobacteria, or bacteria to improve hydrogen yields. By employing species with complementary metabolic functions, such as one organism producing hydrogen and another consuming oxygen, these systems can achieve more stable and efficient hydrogen production [118]. However, maintaining the balance between different microbial species in a mixed-culture reactor is challenging, particularly as conditions within the reactor change over time.

2.3.5 Improvement of algae and cyanobacteria biohydrogen production

In controlled laboratory environments, significant progress has been made in improving the efficiency of algal and cyanobacterial hydrogen production. For instance, researchers [119] have optimized the light-to-hydrogen conversion efficiency by adjusting light intensity and wavelength to match the specific absorption characteristics of the photosynthetic machinery. Using light-emitting diode-based illumination systems, which can be tuned to specific wavelengths, scientists have been able to maximize the efficiency of photosystems involved in electron transport and hydrogen evolution. Another important factor is oxygen management, as discussed earlier. In closed photobioreactor systems, advanced membrane-based gas separators [39] are being developed to continuously remove oxygen while maintaining optimal CO₂ concentrations. These systems help maintain anaerobic or microaerobic conditions that are favorable for hydrogenase activity, thereby enhancing overall hydrogen production rates. In summary, while biological hydrogen production remains a promising technology, significant challenges must be overcome to scale up these systems for industrial use. Advances in metabolic engineering, enzyme stability, and bioreactor design will be essential for improving the efficiency and economic viability of biohydrogen production. Continued research into the optimization of light harvesting, gas management, and mixed microbial systems will likely play a crucial role in making biological hydrogen production a competitive alternative in the renewable energy landscape.

3. Solar hydrogen storage technologies

One of the key challenges in creating a sustainable hydrogen economy is the efficient and safe storage of hydrogen. The intermittent nature of solar energy necessitates reliable storage technologies to ensure that hydrogen produced via solar methods can be used when needed [132]. Hydrogen can be stored in various forms including compressed gas [133], liquefied hydrogen [134], or chemically bound to materials [135]. This section explores the physical and chemical principles governing hydrogen storage systems, including the thermodynamic and kinetic factors that influence their performance and suitability for various applications. Figures 6 and 7 present a depiction of various hydrogen storage technologies and illustrate hydrogen density and accessibility across different storage systems. Table 4 compares hydrogen storage technologies, highlighting their capacity, energy density, consumption, safety, cost, and challenges.

3.1 Physical and chemical hydrogen storage methods

Hydrogen storage materials are a crucial component of solar hydrogen systems, as they allow for the reversible storage and release of hydrogen at practical temperatures and pressures. The development of efficient storage materials involves balancing several factors: storage capacity, kinetics of hydrogen adsorption and desorption, stability under cycling conditions, cost-effectiveness, and sustainability [139]. Current research focuses on improving these properties in metal hydrides, carbon-based materials, and novel chemical storage systems such as MOFs and complex hydrides [137]. Additionally, alternative storage solutions, including liquid organic hydrogen carriers (LOHCs) and synthetic fuels, are being explored to complement material-based hydrogen storage.

Metal hydrides are one of the most well-researched classes of hydrogen storage materials. They store hydrogen by forming metal-hydrogen bonds, often leading to high hydrogen densities at relatively low pressures [136]. The general reaction for hydrogen storage in metal hydrides can be expressed as: M + H₂ → MH_2, where M represents a metal or alloy that reacts with hydrogen to form a hydride. Metal hydrides, such as MgH₂ [140], LaNi₅H₆ [141], and TiFeH₂ [142–144], offer several advantages, including high volumetric hydrogen density and safe, solid-state storage. However, many hydrides require high temperatures for hydrogen release, which limits their practical applications. For instance, magnesium hydride (MgH₂) can store up to 7.6 wt.% hydrogen, but its desorption temperature is around 300°C, necessitating substantial energy input for hydrogen release [140]. Recent advancements in metal hydride research have focused on nanostructuring [145–147] and alloying [148, 149] to reduce desorption temperatures and improve reaction kinetics. For example, nanostructured magnesium hydride [145, 146] has demonstrated faster hydrogen uptake and release due to the increased surface area and reduced diffusion path lengths for hydrogen atoms. Additionally, the incorporation of transition metals like Ti [150] or Ni [151] as catalysts has been shown to lower the activation energy for hydrogen desorption, although repeated cycling at high temperatures poses challenges for material durability and recyclability.

Carbon-based materials, including carbon nanotubes (CNTs) [152], graphene [153, 154], and activated carbons [155], offer another route for hydrogen storage, storing hydrogen primarily through physisorption, where hydrogen molecules weakly bind to the surface of the material. While carbon materials typically have lower hydrogen storage capacities compared to metal hydrides, they operate at lower temperatures and pressures, making them more energy-efficient for hydrogen release. Research into doping carbon structures with metals [156, 157] or creating porous carbon composites [158–160] aims to increase their hydrogen storage capacity. For example, Li-doped carbon nanotubes [157] have shown improved hydrogen uptake due to the increased interaction between hydrogen molecules and the doped sites. However, the resource-intensive production of advanced carbon materials and their end-of-life disposal remain sustainability concerns.

MOFs represent a novel class of hydrogen storage materials that have gained significant attention in recent years, which composed of metal ions coordinated to organic ligands, forming a highly porous crystalline structure. This porosity allows for a high surface area, making MOFs excellent candidates for hydrogen physisorption. The tunability of MOFs, where both the metal centers and organic linkers can be modified, offers the potential to optimize their hydrogen storage properties [161]. For example, MOF-5 [162] and Hong Kong University of Science and Technology-1 (HKUST-1, known as MOF-199 or Cu₃(BTC)₂ (BTC = benzene-1,3,5-tricarboxylate, Ph(COO^-)₃)) [163] have demonstrated moderate hydrogen storage capacities at cryogenic temperatures, and ongoing research is focused on designing MOFs with functionalized pores [164] that can improve hydrogen binding energies at higher temperatures. Despite their potential, challenges such as stability during cycling and the environmental impact of large-scale production remain barriers to widespread adoption. Efforts to recycle MOFs [165], such as recovering metal ions or reusing ligands, are being investigated to mitigate these issues.

Another promising material class is complex hydrides, such as borohydrides (e.g. LiBH₄ [166, 167], NaBH₄ [168]) and alanates (e.g. NaAlH₄ [169]), offering high gravimetric hydrogen capacities (e.g. LiBH₄ can store up to 18.5 wt.% hydrogen [170]), but their practical use is limited by the high desorption temperatures and slow kinetics of hydrogen release. Advances in catalysis and nanoconfinement [138] have shown potential for improving the hydrogen release properties of complex hydrides. For instance, Ti-doped NaAlH₄ [171] has been shown to reduce the desorption temperature and improve the reversibility of the hydrogenation/dehydrogenation cycles. However, the chemical reactivity and potential toxicity of some complex hydrides raise concerns about environmental sustainability and disposal.

Beyond solid-state materials such as metal hydrides, carbon-based compounds, and MOFs, alternative storage solutions, including liquid organic hydrogen carriers (LOHCs) and e-fuels (synthetic fuels), have emerged as promising candidates for scalable and transportable hydrogen storage. LOHCs are organic compounds capable of chemically binding and releasing hydrogen through catalytic processes [172]. Examples include toluene/methylcyclohexane [173], dibenzyltoluene [174], and N-ethylcarbazole systems [175]. These compounds store hydrogen via a hydrogenation reaction, such as converting toluene to methylcyclohexane, which can later release hydrogen through dehydrogenation. This process allows hydrogen to be stored in a liquid phase, operating at ambient temperatures and pressures, making it safer and more cost-effective than compressed or liquefied hydrogen. However, the hydrogenation and dehydrogenation processes require efficient catalysts and energy input, posing challenges for practical application [172]. Additionally, the stability of LOHCs over multiple cycles and the environmental impact of spills or degradation products require further research [176].

E-fuels store hydrogen chemically in the form of hydrocarbons or other energy carriers and are synthesized by combining green hydrogen with carbon dioxide or nitrogen. Common e-fuels include methane, ammonia, methanol, and hydrocarbons such as petrol, diesel, and kerosene [177]. Methane [178], produced through the Sabatier process by reacting hydrogen with CO₂, serves as a direct substitute for natural gas and can be easily stored and integrated into existing gas grids. Ammonia [179], synthesized via the Haber-Bosch process, offers carbon-free hydrogen storage with an established global transport network, though its toxicity and energy-intensive production are challenges. Methanol [180], formed by reacting hydrogen with CO₂, is a versatile liquid fuel used in engines and as a chemical precursor, and is also being developed for direct methanol fuel cells. Hydrocarbons [181] such as petrol, diesel, and kerosene, synthesized using green hydrogen and CO₂ through the Fischer–Tropsch process, mimic conventional fossil fuels, enabling their use in existing transportation and industrial systems. While these fuels offer high energy densities and compatibility with current infrastructure, their production processes are energy-intensive, requiring renewable feedstocks and efficient catalysts to minimize environmental impact.

3.2 Thermodynamic and kinetic mechanisms of hydrogen storage

The performance of hydrogen storage materials is determined by the thermodynamics and kinetics of hydrogen adsorption and desorption processes. An ideal hydrogen storage material must strike a balance between high storage capacity, low operating temperatures, and fast kinetics for hydrogen uptake and release [137]. From a thermodynamic perspective, the enthalpy of hydrogen adsorption (ΔH) plays a critical role in determining the operating temperature of the storage system. For example, metal hydrides with strong metal-hydrogen bonds (high ΔH) typically require higher temperatures for hydrogen release, as the stored hydrogen is more tightly bound to the host material. Conversely, materials with weak hydrogen binding (low ΔH), such as physisorption-based materials, may release hydrogen at lower temperatures but suffer from lower storage capacities [182]. The trade-off between storage capacity and release temperature is one of the key challenges in hydrogen storage. For instance, light metal hydrides such as LiBH₄ [166] offer very high gravimetric hydrogen densities but require temperatures exceeding 400°C for hydrogen release. On the other hand, materials like MOFs and carbon nanotubes can release hydrogen at near-ambient conditions but typically store less hydrogen per unit mass [162]. The kinetics of hydrogen storage materials are equally important. Slow adsorption/desorption kinetics can limit the rate at which hydrogen is stored or released, making the material unsuitable for real-time applications. Metal hydrides, in particular, often exhibit sluggish kinetics due to slow hydrogen diffusion through the solid lattice [183]. To overcome this, as discussed above, researchers are exploring the use of catalysts to accelerate the hydrogenation and dehydrogenation processes, and additionally, nanostructuring metal hydrides can shorten diffusion pathways, improving reaction kinetics and reducing desorption temperatures.

3.3 Applications and operational performance

In practical applications, hydrogen storage materials must not only demonstrate high performance in controlled laboratory conditions but also perform reliably under operational conditions, including factors such as cycling stability, thermal management, and material cost [137]. Cycling stability is critical for long-term use, as hydrogen storage materials must withstand multiple charge/discharge cycles without significant degradation. For example, metal hydrides can suffer from material pulverization and sintering after repeated hydrogenation/dehydrogenation cycles, which reduces their surface area and hydrogen capacity [184]. To mitigate these issues, materials are being designed with protective coatings [185] or nanostructured materials [147, 186] that maintain the integrity of the storage medium over time. Thermal management is another important consideration, particularly for high-temperature hydrides. Effective heat exchange systems must be incorporated into hydrogen storage devices to efficiently manage the exothermic and endothermic nature of hydrogen adsorption and desorption [187]. Advances in thermal conductivity enhancers, such as incorporating graphene [154] or metal hydride [188, 189] into storage materials, have shown promise in improving heat dissipation and reducing the energy input required for hydrogen release. Lastly, material cost and scalability are crucial for real-world deployment. While materials like MOFs and nanostructured metal hydrides show excellent performance in research settings, their high production costs and complexity of synthesis currently hinder large-scale adoption [136]. For hydrogen storage technologies to be commercially feasible, materials must be not only efficient but also cost-effective and easy to produce at scale, which is especially important for industrial and transportation applications, where the cost per unit of stored energy is a significant factor [190]. Metal hydrides, for instance, while efficient, often involve expensive materials or catalysts, and their energy-intensive production processes further add to the cost [187]. On the other hand, carbon-based materials, such as activated carbon or carbon nanotubes, are relatively more cost-effective, but their lower hydrogen storage capacities present a trade-off [156]. Advances in scalable production methods, such as template synthesis for MOFs [191] or ball-milling techniques for hydrides [192], are being explored to reduce costs while maintaining high performance. In practical applications, the choice of hydrogen storage material will depend on the specific requirements of the system. For stationary applications, where weight and volume constraints are less critical, metal hydrides with high volumetric capacity and stable cycling performance may be preferred [132]. In contrast, for mobile applications such as hydrogen-powered vehicles, materials with lower weight, faster kinetics, and the ability to operate at near-ambient conditions, such as lightweight carbon materials or advanced chemical hydrides, may be more suitable [193].

4. Hybrid systems for solar hydrogen energy applications

As the transition towards cleaner energy systems, it is crucial to explore how solar hydrogen technologies can be effectively integrated with existing renewable energy sources, storage solutions, and energy distribution systems. The effective integration of solar hydrogen production with PV, thermal energy, and battery storage technologies can enhance overall system efficiency, enable better energy management, and contribute to grid stability [194]. This section discusses the scientific and technical challenges of integrating solar hydrogen with other technologies and highlights potential solutions for optimizing these hybrid systems, as illustrated in Fig. 8, which provides an overview of hybrid systems for solar hydrogen energy applications.

4.1 Advanced integration of solar hydrogen with energy systems

Integrating solar hydrogen production with PV or thermal energy systems presents several scientific and engineering challenges, in which solar hydrogen production requires efficient energy conversion from sunlight, and both PV and CSP systems can serve as primary energy sources for hydrogen production [194]. However, balancing the energy flows between electricity generation (via PV or CSP) and hydrogen production presents a significant challenge, particularly in terms of managing energy availability, storage, and conversion losses [195]. In a typical integrated system, PV cells or solar thermal collectors convert sunlight into electricity, which is then used to power electrolysis units that split water into hydrogen and oxygen. The efficiency of this process depends on several factors [196], including the capacity of the PV or CSP systems, the efficiency of the electrolysis units, and the availability of solar energy. A major challenge in this integration is managing the intermittency of solar energy, which can cause fluctuations in the power supply and affect the continuity of hydrogen production. Moreover, the design of hybrid solar systems that combine PV electricity generation with solar thermal hydrogen production (e.g. thermochemical cycles) must account for energy flow optimization. One emerging strategy [197] is the use of dual-use solar collectors that can simultaneously generate electricity and heat for thermochemical hydrogen production, requiring advanced thermal management and energy storage solutions to ensure that heat generated by the CSP system can be stored and used during periods of low solar radiation. Hybrid energy systems that combine solar hydrogen production with battery storage or fuel cells offer an additional level of flexibility. Batteries can be used to store excess electricity generated by PV systems, which can then be used to power electrolysis units during periods of low solar availability [198]. Alternatively, hydrogen produced during periods of excess solar energy can be stored and used to generate electricity through fuel cells during times of peak demand or when solar energy is unavailable. These hybrid systems enable energy shifting, where energy generated during periods of high solar availability is stored and used later, enhancing grid stability and reducing reliance on fossil fuel-based backup power systems [199].

4.1.1 Challenges in hybrid systems

One of the primary challenges in integrating solar hydrogen production with other renewable energy systems is the optimization of energy flows. In hybrid systems, there is a constant need to balance the supply and demand of energy between electricity generation, hydrogen production, and storage. Effective management of energy flow requires the integration of smart energy management systems that can dynamically distribute energy between PV systems, electrolysis units, batteries, and fuel cells, accounting for real-time variations [200] in solar irradiance, energy demand, and storage capacity to minimize energy losses and maximize efficiency. Advances in power electronics, such as bidirectional inverters [201] and intelligent controllers [202], play a crucial role in this optimization. For instance, by incorporating real-time data on solar irradiance, temperature, and grid demand, power electronics can regulate the power supply to electrolysis units or divert excess energy to battery storage, improving overall system performance [198]. Additionally, artificial intelligence (AI)-based algorithms [203] are being explored to predict energy demand and optimize the distribution of energy between hydrogen production and storage systems.

Integrating solar hydrogen into energy systems demands a comprehensive analysis of strategies to enhance system-level efficiency. In hybrid systems, energy losses can occur at several points [204], including electrolysis, hydrogen compression/storage, and conversion back to electricity via fuel cells. Reducing these losses is critical to improving the overall efficiency of the system. One approach to improve efficiency is to develop high-efficiency electrolyzers, such as proton exchange membrane [205, 206] or solid oxide electrolyzers [207] that can operate at lower energy input levels. In addition, optimizing thermal coupling between solar thermal systems and hydrogen production processes, such as using waste heat from CSP plants to preheat the water for electrolysis [208], can further reduce energy consumption.

Another scientific challenge in the integration of solar hydrogen systems with other technologies is optimizing the interaction between hydrogen production, energy storage, and grid stability in integrated systems. The intermittent nature of solar energy complicates efforts to maintain grid stability, particularly when large amounts of renewable energy are fed into the grid [209]. Hydrogen storage offers a potential solution by acting as a long-term storage medium that can absorb excess energy during periods of high solar generation and release energy during periods of low generation. However, the challenge lies in ensuring that hydrogen production and consumption are properly coordinated with grid demand. Integrating hydrogen-based systems with the grid requires the management of dispatchable power sources. Hydrogen stored during peak solar periods can be converted back into electricity through fuel cells to provide grid-balancing services. Fuel cells offer a clean and flexible means of generating electricity on demand, helping to stabilize the grid and compensate for fluctuations in solar power generation [210]. However, to achieve this level of integration, advanced grid infrastructure capable of accommodating variable hydrogen production and dispatchable hydrogen-powered electricity generation is required.

4.1.2 Smart grids and artificial intelligence-based optimization systems

The integration of solar hydrogen systems with renewable energy technologies hinges on advanced energy management solutions, with smart grids and AI-based optimization systems playing a pivotal role in enhancing efficiency, reliability, and flexibility. Smart grids, as intelligent energy distribution networks, dynamically manage the flow of energy between generation, storage, and consumption using real-time data and advanced communication systems [211]. In solar hydrogen systems, smart grids ensure surplus solar electricity is allocated to electrolysis units for hydrogen production during periods of high solar availability, while stored hydrogen can be converted back to electricity through fuel cells during low solar irradiance or high energy demand [202]. This real-time energy distribution reduces waste, enhances system efficiency, and contributes to grid stability.

AI-based optimization systems further complement smart grids by enabling predictive and adaptive energy management. By processing extensive datasets, including solar irradiance, temperature, energy demand, and grid load, AI algorithms forecast energy production and consumption patterns [212]. These insights allow for proactive adjustments, such as scheduling hydrogen production during peak solar generation or deploying stored hydrogen during periods of high demand. Additionally, AI optimizes the operation of electrolyzers by adjusting parameters like voltage and current to maximize hydrogen production efficiency, while managing thermal flows in systems combining solar thermal and hydrogen production to ensure effective use of CSP-generated heat [213].

AI-driven smart grids also play a crucial role in maintaining grid stability by monitoring grid performance, detecting potential instabilities from renewable energy fluctuations, and coordinating hydrogen storage systems and other renewable sources. Machine learning models integrated within these systems continuously enhance performance by analyzing historical data, enabling precise timing of hydrogen production to reduce costs and ensure reliability. Furthermore, AI-based demand response mechanisms allow smart grids to interact with end-users, promoting energy consumption behaviors that align with renewable energy availability [203].

The coupling of smart grids and AI in hybrid systems fosters a holistic energy management approach, enabling energy produced by solar hydrogen systems to be efficiently shared across transportation, industrial, and residential sectors. This sector coupling not only enhances system resilience but also accelerates the decarbonization of multiple energy-intensive sectors [214]. Expanding the role of smart grids and AI-based optimization systems is essential for overcoming challenges such as energy intermittency, resource allocation, and grid integration, ensuring that solar hydrogen technologies are seamlessly incorporated into a sustainable and adaptive energy infrastructure capable of meeting the demands of a low-carbon future.

4.2 Case studies and pilot projects

Case studies and pilot projects that have successfully demonstrated solar hydrogen production and its integration with other technologies provide valuable practical insights and underscore the feasibility of these systems. Table 5 presents an overview of case studies and pilot projects, highlighting their technological focus, efficiency, advantages, and associated challenges. For example, the Solar chemical reactor demonstration and Optimization for Long-term Availability of Renewable JET fuel (SOLAR-JET) project, funded by the European Union, successfully demonstrated efficiencies of up to 15%–20% in converting solar energy to hydrogen and synthetic fuels. While the approach is promising for industrial-scale hydrogen production, the primary challenges include the high temperatures (>1500°C) required for the reactions and the cost of CSP infrastructure [215, 216]. The integration of solar thermal energy with chemical processes proved viable but emphasized the need for advanced materials to withstand extreme thermal conditions and reduce operational costs.

Another notable example is the Hydrogen Pilot STorage for large Ecosystem Replication (HyPSTER) project in Europe, which focuses on integrating green hydrogen production with underground storage and renewable energy sources like wind and solar. HyPSTER combines PV-driven electrolysis with underground salt cavern storage. Electrolyzer efficiency in the project reaches ~60%–70%, with hydrogen stored at high pressures for grid stabilization during peak demand [217, 218]. The ability to scale storage capacity is a significant advantage, making it suitable for balancing intermittent renewable energy sources like wind and solar. However, the need for suitable geological formations for salt caverns and the costs of compression and transport infrastructure limit its broader applicability. The HyPSTER project highlights the practicality of coupling renewable hydrogen production with long-term storage but underscores the need for cost-effective solutions for hydrogen retrieval and distribution.

In the United States, the U.S. Department of Energy (DOE)-supported Advanced Water Splitting Materials Consortium (Hydrogen Materials Advanced Research Consortium, HyMARC) has been conducting pilot studies to integrate hydrogen production with renewable energy systems. For instance, these systems achieve efficiencies of 50%–60% in overall energy utilization by combining PV electricity generation with thermal energy recovery for thermochemical hydrogen production [219]. While this approach improves system efficiency, the complexity of managing thermal and electrical energy flows adds to operational challenges. Advanced materials for electrolyzers and thermal storage improve durability and performance, but high initial capital costs remain a barrier for widespread deployment.

Additionally, Japan’s Fukushima Hydrogen Energy Research Field (FH2R) is a large-scale demonstration project integrating solar hydrogen production with grid services. FH2R operates 20 MW PV array powering electrolysis units with an efficiency of ~60%–80% under optimal conditions [220]. The project demonstrates the feasibility of producing hydrogen at a large scale for diverse applications, including fuel cells and industrial processes. Hydrogen stored during peak solar availability is used to stabilize the grid and provide dispatchable power. Key challenges include the intermittent nature of solar energy and the high costs associated with large-scale electrolyzers. Despite these challenges, FH2R has proven to be a scalable model for integrating solar hydrogen production with grid services, achieving cost reductions through economies of scale.

These projects collectively highlight the strengths and limitations of solar hydrogen technologies. While CSP systems like SOLAR-JET excel in industrial applications, their scalability is constrained by high temperatures and costs. PV-driven systems such as FH2R and HyPSTER are more adaptable to grid integration and balancing, offering practical solutions for renewable energy storage. HyMARC demonstrates the potential of hybrid approaches, combining electrical and thermal energy for higher system efficiencies. The comparative analysis underscores the importance of tailoring technology choices to specific operational and regional requirements to maximize the benefits of solar hydrogen systems.

4.3 Technological advances and future directions

Technological advances in energy storage, smart grids, and power electronics are crucial for the integration of solar hydrogen production with other energy systems. Battery systems are becoming increasingly efficient and cost-effective, providing short-term energy storage solutions that complement the long-term storage potential of hydrogen. A cost-benefit analysis must account for capital and operational costs, such as the high expense of electrolyzers and thermal management systems [221], against the long-term benefits of improved energy efficiency, reduced reliance on fossil fuels, and enhanced grid stability. Additionally, fuel cell technologies are advancing, offering higher efficiency and longer lifespans, making them suitable for a wide range of applications, from residential energy systems to large-scale power plants. One emerging trend is the development of integrated renewable energy hubs, where solar hydrogen production, battery storage, and fuel cells are co-located with other renewable energy sources such as wind and hydropower [222], allowing for energy-sharing between different technologies, improving overall system efficiency and reliability. Furthermore, advances in hydrogen transport and distribution infrastructure [223], such as hydrogen pipelines and liquid hydrogen storage, will enable the large-scale deployment of hydrogen as a key energy carrier in the renewable energy economy. Another area of innovation is the exploration of electrochemical hydrogen storage systems that combine elements of battery storage with hydrogen production. Technological advances are crucial for improving system integration and reducing costs. For instance, reversible fuel cells or flow batteries that can both store hydrogen and generate electricity are being researched as a means of simplifying the energy conversion process and improving overall system efficiency [224]. Another scientific challenge is optimizing the synergy between solar hydrogen production, energy storage, and grid stability [225], which requires the development of real-time control algorithms that can predict solar energy availability, hydrogen demand, and grid load in order to dynamically adjust the energy flow between different components of the system [226]. Additionally, the creation of grid-interactive hydrogen storage systems, where hydrogen production and fuel cell electricity generation can be used to smooth out fluctuations in renewable energy supply, will play a crucial role in ensuring grid stability [227], being seamlessly integrated into existing grid infrastructure, and ongoing research is focused on how to standardize and scale hydrogen infrastructure for use in large-scale energy systems and further enhance the economic viability of these systems.

5. Discussion and future perspectives

The development of solar hydrogen production and storage technologies presents a transformative opportunity to advance sustainable energy systems, yet their implementation faces significant technical and economic hurdles. Thermochemical, photochemical, and biological methods each offer distinct pathways for hydrogen production, but their scalability and viability remain subjects of ongoing debate, emphasizing the need for critical analysis and innovation.

Thermochemical hydrogen production, particularly through integration with CSP systems, has shown considerable promise due to its high efficiency and compatibility with advanced materials such as metal oxides in redox cycles. However, the reliance on extreme operating temperatures (>1500°C) presents challenges, including material degradation, complex reactor design, and high operational costs. While advocates highlight the potential of high-temperature materials and advanced thermal energy storage systems to mitigate these issues, concerns persist regarding the energy losses during heat transfer and the scalability of CSP infrastructure. Continued research into novel reactor designs and improved thermal coupling strategies is necessary to address these limitations effectively.

Photochemical hydrogen production provides a more direct and potentially cost-effective approach by harnessing sunlight to drive water-splitting reactions. Yet, the low solar-to-hydrogen conversion efficiencies of current photocatalysts represent a significant barrier. Advances in bandgap engineering, heterostructures, and plasmonic enhancements are promising, but questions remain about the long-term durability and stability of materials such as perovskites MOFs. While these materials show enhanced light absorption and charge separation capabilities, their environmental sensitivity, particularly to moisture, raises concerns about their practicality. Addressing these challenges will require robust testing under real-world conditions and the development of stable material configurations.

Biological hydrogen production is often lauded for its low-temperature processes and alignment with natural metabolic pathways, yet it struggles with slow production rates and the oxygen sensitivity of key enzymes such as hydrogenase. Innovations in metabolic engineering, including the development of oxygen-tolerant enzymes and synthetic biology techniques, have yielded promising results in laboratory settings. However, scaling these systems introduces additional complexities, such as nutrient management and efficient bioreactor design. Opposing perspectives on the feasibility of large-scale deployment highlight the necessity for pilot-scale projects that bridge the gap between experimental advancements and industrial applications.

Hydrogen storage technologies remain a pivotal factor in enabling the widespread adoption of solar hydrogen systems. Conventional methods like compressed gas and liquefied hydrogen are mature but constrained by low volumetric densities and energy-intensive processes. Solid-state storage materials, including metal hydrides and MOFs, offer safer and higher-density storage solutions, though they are challenged by slow hydrogen release kinetics, high material costs, and degradation during repeated cycling. Advocates for these technologies point to advancements in nanostructuring and catalytic enhancements as pathways to improve performance, while critics emphasize the need for comprehensive lifecycle analyses to ensure their cost-effectiveness and scalability.

Integrating solar hydrogen systems into renewable energy grids adds another layer of complexity. Hybrid systems that combine PV or CSP technologies with hydrogen production offer flexibility but require sophisticated energy management to balance supply and demand. While some view the energy losses in electrolyzers and fuel cells as a barrier to efficiency, advancements in smart grids and AI-driven optimization are being explored to dynamically allocate energy and enhance grid stability. However, the limited number of large-scale implementations highlights the uncertainty surrounding the scalability and reliability of such systems, underscoring the importance of demonstration projects to validate their potential.

The successful adoption of solar hydrogen technologies will depend not only on technological breakthroughs but also on supportive policy measures and economic incentives. Subsidies, tax credits, and carbon pricing mechanisms are essential to close the cost gap between renewable and fossil fuel-derived hydrogen. Furthermore, the establishment of international hydrogen certification schemes will promote transparency and distinguish sustainable hydrogen in global markets.

In conclusion, while solar hydrogen technologies hold significant potential for decarbonization and energy storage, addressing their technical, economic, and policy-related challenges will require a balanced approach that incorporates diverse perspectives. By fostering collaboration across research, industry, and government sectors, and prioritizing real-world testing, these technologies can evolve into a cornerstone of the global energy transition, supporting applications ranging from grid stabilization to industrial processes and sustainable transportation.

Author contributions

Ge Chen (Conceptualization [lead], Data curation [equal], Formal analysis [equal], Investigation [lead], Methodology [equal], Resources [equal], Software [equal], Validation [equal], Visualization [equal], Writing—original draft [lead]), Renhui Sun (Conceptualization [equal], Investigation [supporting], Resources [supporting], Validation [supporting]), and Baodong Wang (Conceptualization [equal], Formal analysis [equal], Investigation [equal], Methodology [equal], Resources [equal], Supervision [lead], Validation [equal], Writing—review & editing [lead])

Conflict of interest

None declared.

Funding

None declared.

Data availability

The data underlying this article is available in the article.

References