From seawater to hydrogen via direct photocatalytic vapor splitting: A review on device design and system integration

Abstract

Solar-driven hydrogen production from seawater attracts great interest for its emerging role in decarbonizing global energy consumption. Given the complexity of natural seawater content, photocatalytic vapor splitting offers a low-cost and safe solution, but with a very low solar-to-hydrogen conversion efficiency. With a focus on cutting-edge photothermal–photocatalytic device design and system integration, the recent research advances on vapor splitting from seawater, as well as industrial implementations in the past decades were reviewed. In addition, the design strategies of the key processes were reviewed, including vapor temperature and pressure control during solar thermal vapor generation from seawater, capillary-fed vaporization with salt repellent, and direct photocatalytic vapor splitting for hydrogen production. Moreover, the existing laboratory-scale and industrial-scale systems, and the integration principles and remaining challenges in the future seawater-to-hydrogen technology were discussed.

Graphical abstract

Keywords

seawater / hydrogen / photocatalytic / vapor splitting / solar-driven

1 Introduction

Solar-driven water splitting for hydrogen production is an emerging solution to address the global energy crisis and industry decarbonization [1]. In comparison to pure water, seawater is widely available, even in some drought regions like the Middle East. When powered by solar energy, hydrogen generated from seawater could be an abundant renewable energy resource [2–5]. However, seawater is rich in different ions, which poses great challenges to direct seawater electrolysis such as the chlorine-related competing reactions at the anode [6]. Coupling seawater purification systems [7–10] (i.e., membrane desalination) with conventional catalysts/electrolyzers can overcome these challenges. Yet, this may increase the complexity of the system. Therefore, numerous research efforts have been dedicated to simplifying the process with novel materials and system integration, aiming to directly use seawater as the feedstock for solar-driven hydrogen production.

To date, several approaches have been proven to be feasible for producing hydrogen from seawater powered by sunlight. As shown in Fig.1, these approaches are divided into two categories: direct seawater splitting (upper dash box) and indirect ones (lower dash box). Direct seawater splitting relies on the novel photo and electrocatalyst materials that work with the existence of ions in seawater, leading to the actively investigated solar-driven direct seawater splitting systems such as photocatalysis, photovoltaic-assisted electrolysis (PV-E), and photoelectrochemical cells. Direct photocatalytic seawater splitting still lacks suitable photocatalysts, which hinders the direct photocatalytic seawater splitting performances under the complex composition of seawater. Meanwhile, other issues of direct seawater splitting also slow down its industrial implementation. For instance, the solar irradiation incident upon the photocatalyst may be reduced by the light scattering caused by the H₂/O₂ bubbles produced on the surface, particularly at higher production rates [11,12]. The bubble covered area does not contact with water any more, thus causing the reduction of the effective area of the photocatalysts [13,14]. Besides, liquid water absorbs ultraviolet (UV) light, which is the most effective wavelength range for photocatalytic reactions. The bubbles also induce perturbations when escaping from surface and may cause the peeling of materials, affecting the photocatalyst durability. In addition, chlorine evolution is unavoidable, though can be minimized, due to the small potential difference required for the oxygen evolution and chlorine evolution. A recent review summarized the research advances novel photocatalysts and underlying mechanism with the presence of ions for direct seawater splitting [15]. Meanwhile, the PV-E system for direct seawater electrolysis hydrogen production is getting popular due to the rapid price drop of PV and the easiness of PV-E system integration. Pang et al. [16] provided a recent perspective on such systems. In addition, there are also studies that combined both photocatalytic and electrocatalytic direct seawater splitting to produce hydrogen. Interested readers are referred to a recent review by Yao et al. [17]

On the other hand, the combination of the photothermal and photocatalytic processes starts to attract a lot of research attention for hydrogen production from seawater. The photothermal process aims to vaporize seawater to obtain pure water vapor. The photocatalytic process is to split the vapor for hydrogen production. Both processes are driven by the sunlight. The first step, i.e., the photothermal process, is also known as interfacial solar vapor generation (SVG), where the photothermal material absorbs the sunlight and converts the sunlight into heat which is then localized to the evaporation site of seawater. Moreover, as an emerging technique to extract pure water from seawater, the SVG technology have achieved numerous research progresses, including novel solar absorption materials, high-efficient systems design, salt repellent evaporator, and so on [18–22]. In the second step, photocatalytic reaction occurs to split water vapor for hydrogen generation. In addition, water vapor can also be condensed into liquid water, followed by conventional aqueous water splitting, e.g., alkaline water electrolysis and proton exchange membrane electrolysis. This novel integrated system allows the optimization of two sub-systems separately with more flexibility. The choice between using water vapor and liquid water as the feed for hydrogen production depends on various factors, including the energy source (i.e., concentrated or unconcentrated sunlight), the efficiency of the photocatalysis/electrolysis process, and the operational safety requirements. In this paper, this novel integrated photothermal and photocatalytic system for hydrogen production from indirect seawater splitting will be focused on.

As described above, the indirect gas phase seawater photocatalytic system is composed of integrated photothermal-photocatalytic components. The process of seawater-vapor-hydrogen transport in the system and conversion functions of different components are illustrated using a multi-layered design in Fig.2. On the bottommost layer, part of the sunlight (e.g., long wavelength light) reaches the photothermal component and is converted into heat for the vaporization of seawater. Part of the heat is also dissipated into the water and environment as a heat loss. Above that, water vapor, generated from the photothermal layer, is captured in the photocatalytic component, and simultaneously split to produce hydrogen and oxygen under the illumination of sunlight, especially with the short wavelength range.

This paper focused on the cutting-edge photothermal-photocatalytic device design and system integration from the research advances and industrial revolutions in the past decades. The role of key processing parameters such as vapor temperature, vapor pressure, and light intensity for the design of an efficient hydrogen production system were discussed. The design strategies for the key photothermal and photocatalytic components and the recent advances were reviewed. System integration, including the integration principles, advantages, and remaining challenges of the existing systems were elaborated on, and perspectives on the directions of the future seawater-to-hydrogen technology were provided.

2 Impact parameters on hydrogen production

Vapor quality highly affect its decomposition process. Thereby, in the photothermal–photocatalytic seawater splitting system, the two components need to be integrated in an optimized manner. In this section, the research findings on the influence of vapor properties (e.g., temperature and pressure), vapor generation rate, light concentration on photocatalytic reactions， and hydrogen production rate were summarized.

2.1 Vapor temperature and pressure

Faster hydrogen production rates have been observed at higher temperature under liquid phase photocatalytic reactions, as the reaction rates of the rate limiting steps increase with temperatures [23,24]. Fundamentals on temperature-dependent photocatalytic reaction has been discussed [25,26], from the aspects of reaction kinetics, adsorption of water molecular, desorption of gas products, and catalysis stability. By testing the hydrogen production via photothermal catalytic gel at various temperatures, Gao et al. [27] observed an almost doubled hydrogen production rate when the temperature increases from 20 to 60 °C. In photocatalytic vapor splitting, it is assumed that the vapor temperature is the same with the surface temperature of photocatalysis in the integrated system with well thermal insulation. In case of temperature non-uniformity, e.g., if the surface temperature of photocatalysis is lower than vapor temperature, condensation occurs. The condensate water film could become a barrier for light penetration and hydrogen diffusion, slowing down the hydrogen reaction, which is undesired. However, Shearer et al. [28] observed an opposite trend for gas phase reactions when using the same photocatalyst, i.e., the reaction rate decreasing with temperature, as shown in Fig.3(a). Herrmann [26] investigated the water vapor splitting at 25 to 55 °C, and also found that both the hydrogen and oxygen evolution rates dropped at higher temperatures when the water partial pressure was constant. Given the complexity of temperature influence, optimizing the temperature conditions for photocatalytic water splitting is essential to achieve the desired hydrogen production rates and selectivity for specific photocatalysts and conditions.

On the other hand, vapor pressure also plays a role in water molecular decomposition. The vapor pressure directly influences the vapor concentration available for the photocatalytic water splitting process by changing the surface coverage of chemisorbed water molecular. Therefore, a higher vapor pressure tends to result in a faster water splitting reaction. Note that relative humidity (RH) is the ratio of the vapor pressure to the saturation vapor pressure at that temperature. A linear relationship between RH and H₂ production rate were reported by Shearer et al. [28], as shown in Fig.3(a). Suguro et al. [29] also reported that the apparent quantum yield did not depend on the reaction temperature, but dropped with decreasing RH at both 24 and 50 °C. At an RH value above 0.6, their photocatalyst exhibited a higher H₂ evolution rate, thus suggesting the important role of water vapor concentration.

2.2 Vapor generation rate and feeding speed

The vapor feeding speed affects the hydrogen generation rate. Spurgeon & Lewis [30] showed that the increase of vapor feeding was favored for high H₂ evolution rate, as plotted in Fig.3(b). Therefore, the design of photothermal process is required to have a high vapor generation rate. Vapor from the photothermal process is often mixed with air, especially when the RH is low. Oxygen concentration or oxygen partial pressure in the vapor–air mixture could affect the photocatalytic water decomposition. Li et al. [31] showed that when the photocatalytic generated oxygen was captured simultaneously and transferred away effectively from photocatalyst surface, the hydrogen evolution amount was significantly enhanced in the Pt/TiO₂ photocatalytic system. The reduction of oxygen concentration, by affecting the hydrogen–oxygen recombination, was thermodynamically beneficial for the photocatalytic hydrogen production. For the same reason, Nishioka et al. [25] concluded that a degassed condition (i.e., a batch system or a flow system) was beneficial in measuring the photocatalytic water splitting reaction rate with minimal impacts from the back reaction.

2.3 Light intensity

Both photothermal and photocatalytic process rely on sunlight input. The light intensity is directly linked to the vapor generation rate and hydrogen evolution rate. As shown in Fig.3(c), Dionigi et al. [32] reported that the peak values of both hydrogen and oxygen varied linearly with light intensity. It is notable that about 43% of water–hydrogen conversion was achieved at the irradiance of 460 mW/cm² by using a UV LED with wavelength 367 nm. Guo et al. [33] studied the effect of solar intensity on hydrogen evolution based on their own photocatalytic. Based on their experimental measurements, it was found that the hydrogen evolution rate increased with the solar intensity but not linearly. The underlying reason was that the catalysts only absorbed part of the incident photons that could create electron-hole pairs to catalyze the reaction. As for the case of solar irradiance, they claimed that the high temperature caused by the increase in solar intensity on the reaction surface also contributed to the high evolution rate. Regarding the above evidence, solar concentration is another important design parameter for solar-driven hydrogen production system.

3 Design strategies

Considering the parameters summarized above, there are different design strategies to integrate the SVG and photocatalytic processes. In this section, the reported solutions for vapor temperature and pressure control, salt mitigation, and photocatalytic vapor splitting were reviewed.

3.1 Vapor temperature and pressure control

The peak solar irradiance on Earth is approximately 1 kW/m², and if all of the solar irradiance can be used to vaporize water, the theoretical limit of vapor generation rate is 1.47 kg/m² under ambient conditions. To elevate the vapor temperature above the ambient temperature is a critical technological challenge for SVG. In comparison with the solar energy density, there is a substantial energy flux mismatch to boil the water under ambient pressure, of which the heat flux demand is around 7 kW/m² [34]. Therefore, active solar concentrations are required to raise the pressure and temperature of the vapor generated, including two categories, optical concentration and thermal concentration.

Optical concentration uses optical lenses to focus the incoming solar radiation, as shown in Fig.4(a). In the SVG process, the concentrated sunlight increases the temperature and heat flux of the evaporator beyond the boiling threshold of seawater to initiate high-temperature vapor generation. Meanwhile, concentrated light is also beneficial for the photocatalytic vapor splitting process. Referring to the current concentrated solar systems, Fig.4(b) lists four typical systems that can be potentially modified for solar-driven hydrogen production. The first one is the parabolic trough, with an absorber pipe receiving the reflection of direct sunlight on parabolic mirrors [35]. The second one is a tower system, in which hundreds of mirrors are aligned in a way that all the reflections are concentrated into a single point [38]. The temperature in the tower can reach up to 1000 °C. The third one is a linear Fresnel reflector [39]. There are two parallel receivers in each row in a compact linear Fresnel reflector (CLFR) design, making it more compact than the parabolic trough collector. The last one is the parabolic dish system, which contains a set of parabolic-shape mirrors to concentrate sunlight in a focal point. This system is quite efficient as it can follow the sun in two directions. By utilizing the optical concentration, Zhao et al. [36] reported a high-pressure high-temperature SVG device. As shown in Fig.4(c), a simple non-tracking compound parabolic concentrator was incorporated with a planar absorber geometry to increase the energy density of the device. The setup is able to generate steam at 128 °C and 250 kPa, and successfully demonstrated its usage for a standard medical sterilization process. In another example, Neumann et al. [37] employed the Fresnel lens with a 26.67 cm × 26.67 cm area and 44.5 cm focal length to concentrate the sunlight in their SVG setup (see Fig.4(d)). An elevated steam temperature of up to 140 °C was observed within a ten-minute irradiation time.

On the other hand, thermal concentration can locally increase the vapor temperature and heat flux in a simpler manner. The enhancement effect can be evaluated by the ratio of the solar absorbing area to the evaporation area. The working principle is to use a large solar absorbing surface that converts sunlight to heat and then localizes the heat for evaporation in a small and localized area. In this way, the concentrated heat flux is able to provide enough energy for high-temperature vapor generation, as illustrated in Fig.5(a). The concept of thermal concentration for high-temperature solar steam generation was addressed [40], and the concept was rapidly implemented in many existing solar thermal systems. Fig.5(b) shows the laboratory-scale ambient steam generator under one sun illumination. There are three main components in the device, a spectrally selective solar absorber, a thermal insulator, and a convective cover. An evaporation slot can be observed in the picture of the absorber. To have different thermal concentrations, the evaporation slot was varied in length (1 mm width) on the absorber. In the improved design, the evaporation dots were also tested as a replacement for the evaporation slot for better vapor diffusion. As shown in Fig.5(b), by raising the thermal concentration, the vapor temperature also increased. However, the evaporation temperature cannot be beyond 100 °C because the evaporation occurs at ambient pressure. To have a better understanding of thermal concentration and heat localization, Raza et al. [44] showed the temperature distribution on the side and subsurface of the absorber with evaporation slots, which clearly demonstrated that water received heat from the absorber. The solar steam generation efficiency can be evaluated by measuring the water that is evaporated by the solar irradiation. Other than the manual opening of the evaporation slots on solar absorber, the natural porous media is another way to achieve thermal concentration. As shown in Fig.5(c), Ito et al. [41] demonstrated the use of a porous graphene solar absorber for high-efficiency steam generation by heat localization. Under the slight concentration around 9 times of the solar intensity, the vapor temperature is able to reach 100 °C in a few minutes. In Fig.5(e), some other representative choices of the porous material used as the solar absorber were also listed, including artificial porous materials, carbon black nanoparticle coated plant cellulose [42,45], carbonized lotus seedpods [43], and carbonized wood stem [46–48]. When using porous materials for thermal concentration, porosity is the key parameter affection thermal concentration and vapor temperature. The thermal concentration for a porous material can be roughly calculated as the reverse of porosity. For instance, to have a thermal concentration of 100×, the porosity is roughly around 1%.

Pros and cons exist in both optical and thermal concentrations. Compared with the optical concentration, thermal concentration has a lower efficiency because of the additional heat loss. The thermal concentration efficiency evaluates the total amount of incoming solar energy that is converted to the concentrated thermal energy. There are two critical factors, i.e., the absorber base temperature and the heat loss coefficient, which impact the concentration efficiency. For some open systems without convective heat loss control, the energy efficiency may drop to below 20% when the vapor temperature is near 100 °C, especially under one-sun illumination. For example, recent studies on saturated steam generation at 100 °C using a thermal concertation of 1000× achieved energy efficiency of 20% only [49]. Due to this challenge, state-of-the-art steam generation at high pressures and high temperatures still relies on optical concentration [50,51], which has a higher cost and complexity than that of the thermal concentration due to the use of optical concentrator and tracking systems, especially in remote and resource-limited areas.

3.2 Capillary-fed seawater evaporation with salt-repelling

In the SVG from seawater, the saline solution is pumped to the top photothermal layer through hydrophilic wicks and then get evaporated. To have sufficient water vapor for hydrogen production, the water supply to the evaporative surface in the SVG device needs to match the water splitting rate. For the popular one-sun irradiated evaporation devices, as the vaporization enthalpy is constant as 2453 kJ/kg at 20 °C, the maximum evaporation rate is 1.47 kg/(m²·h), which is much lower than the transport-limited flow rates in typical hydrophilic porous materials at above 1000 kg/(m²·h) [52,53]. Therefore, smart designs for the water supply path are needed to lower water supply rates in order to enhance water evaporation in the SVG devices.

However, in many conventional interfacial solar evaporators, owing to the high heat localization and rapid steam generation, inevitable white salt crystals tend to precipitate on the light absorption layer, resulting in the reflection of the incident solar light that leads to a sharp decrease in the thermal efficiency [54,55]. Additionally, salt might mitigate to the photocatalytic layer and stop the water splitting reaction. A basic salt transport conservation equation for solar interfacial evaporators can be expressed as

(1)m_{\mathrm{s}\mathrm{u}\mathrm{r}\mathrm{f}\mathrm{a}\mathrm{c}\mathrm{e}}=m_{\mathrm{p}\mathrm{u}\mathrm{m}\mathrm{p}}-m_{\mathrm{d}\mathrm{i}\mathrm{f}\mathrm{f}\mathrm{u}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n}}-m_{\mathrm{r}\mathrm{e}\mathrm{m}\mathrm{o}\mathrm{v}\mathrm{e}\mathrm{d}},

where m_surface is the mass of salt that is precipitated on the evaporation surface, m_pump is the salt passively pumped saline solution to the top, m_diffusion is the mass of salt diffused back to the bulk seawater, and m_removed is the mass of salt that is physically removed from the device. To maintain a salt free evaporation surface during continuous vapor generation, it is necessary to reduce the m_pump and increase the m_diffusion. Numerous designs and techniques were proposed, which could be categorized into three main groups, back diffusion, direct salt obstruction, and contactless evaporation [56].

The first salt-mitigation strategy is to rely on back diffusion of the near-saturated brine from evaporation sites to bulk. It is driven by the high concentration gradient between the saturated salt solution on the top evaporation surface and bottom bulk water. Therefore, decreasing the difference between evaporation rate that leads to salt accumulation and back diffusion rate is a key factor to prevent salt accumulation. A simple Péclet number analysis can help to understand if the salt will tend to precipitate on the evaporation surface or not. The Péclet number (Pe) is defined as the ratio of the rate of advection of a salt concentration by the flow to the rate of diffusion driven by salt concentration gradient as [57]

(2)\mathrm{P}\mathrm{e}=\frac{uL}{D},

where u is the wicking velocity that is a function of evaporation, L is the characteristic length of the evaporator (the distance from liquid/vapor interface to the evaporation surface), and D is the diffusion coefficient of salt in water. When the Péclet number is lower than unity, the back diffusion is superior to advection and the evaporation layer is highly likely to be free from salt accumulation. Thus, having a trade-off between evaporation and back-diffusion is crucial to maintain a clean evaporation surface. Based on this concept, many solar evaporators were designed to deal with low salinity solutions (3.5 wt.%) and achieved stable and continuous evaporation performance when in operation. For instance, as shown in Fig.6, Chen and the group [58] fabricated a novel salt rejecting solar evaporator by alternating the substrate structure between synthesized wicking fabric and polystyrene foam. Their key findings indicated that modifying the ratio between evaporation surface area and water path cross-section area could rapidly reject and diffuse back excess salt crystals to preserve a salt free photothermal layer for efficient light absorption as shown in Fig.6(b). However, while using highly concentrated solutions, back-diffusion becomes weak and insufficient due to the small concentration gradient. Hence, an enhanced fluid convection for a better mass transfer is required to increase the salt exchange. In another study, Hu and the coworkers [47], as depicted in Fig.6(c), designed an interconnected and highly porous configuration from wood that includes different pore sizes to treat brine effectively. In their device, both diffusion and convection were used to reject salts through employing the large vessel channels while the interconnected tight pores were used to passively pump the water (see the left image of Fig.6(c)).

The second strategy is to obstruct salt directly. This concept mainly depends on blocking the salt ions and only transporting water molecules to reach the top photothermal layer. For instance, an ion selective membrane is used to allow water to penetrate while preventing the salt ions from passing. A well-established technique known as reverse osmosis (RO) uses this phenomenon [59–61]. In this context, Wang et al. [62] fabricated an artificial mangrove tree that contains a polymeric RO membrane as a bio-mimicked root as depicted in Fig.6(d). Their experiments showed that the synthetic leaves could produce a high negative pressure to allow the bottom membrane to reject salt ions significantly. In addition to a selective membrane, a hybrid photothermal layer with hydrophobic evaporator and a hydrophilic layer could also achieve salt repelling, such as Janus membrane. In this way, the top hydrophobic surface reject salt ions, and the hydrophilic layer beneath provide enough water supply for evaporation along with diffusing near saturated salts to prevent crystallization [62–66]. For example, Zhu and the group [64] designed a Janus evaporator that contains a top light absorption layer made from carbon black and hydrophobic poly methacrylate (PMMA). The bottom hydrophilic PAN layer showed an excellent wicking and was used as a passive water transporter as shown in Fig.6(e). Under one-sun illumination, the device remained salt free and enabled a stable evaporation efficiency of 72% with an evaporation rate of 1.3 kg/(m²·h) without salt precipitation (Fig.6(f)). To sum up, water-repelling surfaces owing to their outstanding hydrophobic nature can always maintain a clean evaporation surface, but their efficiency and stability in long-term operation should be furtherly investigated and developed [65–67].

The third strategy is contactless evaporation. The idea is to create a small gap that separates the evaporation surface from bulk seawater and totally avoid salt crystallization. In classical evaporators, usually the photothermal layer is in direct contact with the bulk water. Using this concept and replacing the thermal conduction by the contactless thermal radiation can prevent the salt precipitation problem. Recently, this unique configuration showed a great potential in maintaining a clean interface and stable vapor generation. For further illustration, selective light absorbing materials are utilized to efficiently transform the incident broad solar light to a mid-infrared (MIR) radiation which is favorable for water to absorb. As shown in Fig.6(g), by using this strategy, the abundant solar energy is efficiently restricted to have a penetration depth that is less than 100 µm in the bulk water. Confining solar heating within a small range has been proved to facilitate interfacial evaporation. For instance, Chen and coworkers [68] were the first group to demonstrate such a non-contact evaporation device as shown in Fig.6(h). Their key findings indicated that owing to the small separation, the top solar absorber layer was totally clean and free of salt even after all seawater had evaporated. Menon et al. [69], as presented in Fig.6(i), succeeded in increasing the range of applications to wastewater management by utilizing a solar absorbing umbrella. Owing to the isolated feed solution from the top photothermal layer, the device exhibited an efficient and stable evaporation. Nevertheless, even when dealing with hypersaline solutions, the device still could perform well and showed a great promise toward achieving sustainable wastewater treatment and zero brine discharge (Fig.6(j)). Although this strategy can thoroughly get rid of salt accumulation problem from the origin, a main concern that needs to be addressed is the high thermal losses driven by the elevated temperatures of the selective absorbers. In addition, the concept of downward heat transfer as well can decrease the thermal efficiency of the evaporation process [67–69].

3.3 Photocatalytic vapor splitting

Photocatalytic water splitting in gas phase was reported as early as 1980 [70]. It shares some similarity with splitting liquid water. Fujishima and Honda [71] first reported photocatalytic water splitting and photocatalysis on TiO₂ using UV irradiation. The study involved photocatalysis to split water with a TiO₂ photo anode using an electrochemical cell connected to a platinum cathode via an external circuit. At 2011, Mao and the group [72] demonstrated a black hydrogenated TiO₂ to enhance solar absorption with substantial photocatalytic activities. Up to the present, more and more semiconductor materials such as TiO₂, MnO_x, WO₃, Fe₃O₄, and Ta₂O₅ have been studied in detail for photocatalysis [72–81], as see in Fig.7, in which, the black and blue fonts are photocatalytic water splitting in liquid and gas phase, respectively.

Semiconductors have valence bands (VBs) and conduction bands (CBs) which are separated by the band gap. Upon irradiation with light of sufficient energy, which is higher than the bang gap, the electron from the VB is shifted to the CB, which leaves positively charged holes in the VB as shown in Eq. (3),

(3)\mathrm{S}\mathrm{C}+h\cdot \nu \to {{\mathrm{h}}^{+}}_{\mathrm{V}\mathrm{B}}+{{\mathrm{e}}^{-}}_{\mathrm{C}\mathrm{B}}

where SC represents the semicondutor, h∙ν represents the energy of the absorbed photon (i.e., the incident light), h⁺_VB and e⁻_CB are the hole and electrons in the VB and CB, respectively. The electron and holes then participate in the redox reaction to complete the photocatalysis reaction [82,83]. In photocatalytic water splitting, the holes oxidize the water molecule while electrons in the CB reduce the H⁺ to hydrogen, as shown in Eqs. (4) and (5),

(4)\begin{array}{rl}\mathrm{O}\mathrm{x}\mathrm{y}\mathrm{g}\mathrm{e}\mathrm{n}& \mathrm{e}\mathrm{v}\mathrm{o}\mathrm{l}\mathrm{u}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{r}\mathrm{e}\mathrm{a}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}(\mathrm{O}\mathrm{E}\mathrm{R}):\\ & 2{\mathrm{H}}_2{\mathrm{O}}_{(\mathrm{l})}+{4{\mathrm{h}}^{+}}_{\mathrm{V}\mathrm{B}}\to {\mathrm{O}}_{2(\mathrm{g})}+4{\mathrm{H}}^{+}\end{array}

(5)\begin{array}{rl}& \mathrm{H}\mathrm{y}\mathrm{d}\mathrm{r}\mathrm{o}\mathrm{g}\mathrm{e}\mathrm{n}\mathrm{e}\mathrm{v}\mathrm{o}\mathrm{l}\mathrm{u}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{r}\mathrm{e}\mathrm{a}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}(\mathrm{H}\mathrm{E}\mathrm{R}):\\ & 4{\mathrm{H}}^{+}+{4{\mathrm{e}}^{-}}_{\mathrm{C}\mathrm{B}}\to 2{\mathrm{H}}_{2(\mathrm{g})}\end{array}

The minimum energy required to split water at room temperature is approximately 1.23 eV, and the corresponding potential for water oxidation is 0.83 V (vs. SHE (standard hydrogen electrode)) and H⁺ reduction is −0.41 V (vs. SHE) at pH 7 [84]. The Einstein-Plank relation can be used to calculate the wavelength of light required to generate electron-hole pair in a semiconductor (Eq. (6)). The energy of the incident light should be higher than the bandgap of the semiconductor [85].

(6)E=h\cdot \nu =h\cdot c/\lambda >E_{\mathrm{g}},

where h is plank’s constant (6.63 × 10⁻³⁴ J∙s); ν, the frequency of the incident light; E, the energy; c, the speed of light (3 × 10⁸ m/s); λ, the wavelength of light; and E_g, the energy of band gap.

Photocatalytic hydrogen production has a low solar-to-hydrogen (STH) efficiency, less than about 3% in most reported systems [88–90]. Vapor feeding photocatalytic systems have an even lower STH efficiency. Suguro et al. [29] performed water splitting reaction in the vapor-fed system using TiO_x coated CoOOH/Rh supported on SrTiO (also see Fig.8(a)). The simulated sunlight with a flux of 100 mW/cm² was used and the stoichiometric ratio of hydrogen and oxygen at 2:1 were generated. The STH energy conversion was 0.4% for the long-term stability test. For the photocatalytic vapor splitting processes to become feasible, STH needs to be increased [91,92]. Here, some research progresses in the past decade are listed, where some novel catalysts other than TiO₂ have also been studied, such as ZnFe₂O₄ (ZFO) nanoparticles with iron containing surfaces. The catalyst was treated with NaOH which was termed as alkali treatment, and the catalysts were treated with 0.5, 1, 3, 5 and 10 mol/L NaOH for 12 h and are denoted as 0.5MZFO, 1MZFO, 3MZFO, 5MZFO, and 10MZFO [86]. The first plot in Fig.8(b) shows the H₂ production yield after 5 h of irradiation with these five photocatalyst. The untreated ZnFe₂O₄ shows a relatively lower yield of 15.9 μmol/g compared to other catalysts which were treated with NaOH. The highest H₂ yield was 61.9 μmol/g on 3MZFO, about 3.9 times higher compared to untreated ZnFe₂O₄. In particular, the performance of water splitting for 5 h using vapor fed system for 3MZFO catalyst was recorded in the second image of Fig.8(b), where the hydrogen and oxygen yield increase over time with the rate of 12.4 and 6.5 μmol/g, respectively. The ratio of hydrogen to oxygen was also 2:1, which confirms that the reaction is water splitting. No hydrogen was observed when the test was conducted in dark. As is discussed in Section 2, the RH is one of the factors that affects hydrogen production. In Fig.8(b), it can be found that hydrogen production increases with the increase in RH to 0.5. The higher activity with RH is due to the presence of more water on the catalyst surface. At a higher RH, the higher amount of physically adsorbed water hinders the diffusion of H₂.

Noble metal free catalysts such as MoS_x have also been studied for hydrogen evolution reaction. In some studies, the MoS_x catalyst was reported to achieve similar activities compared to platinum. MoS_x has narrow band gaps which are in the range of 1.2 eV. Due to the very narrow bandgap, the complete photocatalytic water splitting is not possible [93–95]. However, the CB is well suited for hydrogen production [96]. In addition, MoS_x semiconductors have been shown to have a high conductivity which makes it good electrocatalyst for half-cell reaction for HER [97,98]. One method to use MoS_x for photoelectrodes is to use it with other semiconductors CuO, pSi, or TiO₂ [87,99]. MoS_x has been reported to have a hygroscopic nature. It was reported to bind 0.9 H₂O molecule per molecule of Mo. Due to this property, it was used along with the titania photocatalyst (P25) TiO₂ for water splitting using water vapors [87]. Fig.8(c) shows the schematic of the water splitting device. The catalyst is the mixture of MoS_x and TiO₂ which is coated on insulated substrate such as glass. Both MoS_x and TiO₂ act as photocatalysts. From the energy diagram for MoS_x and TiO₂ catalyst in Fig.8(c), it can be seen that the CB of MoS_x is well suited for hydrogen generation while the VB of TiO₂ is suited for oxygen evaluation reaction. As TiO₂ is a wide band gap material, it also provides high energy electrons for the reactions, which are proposed to transfer to MoS_x for hydrogen generation. Under the illumination of 100 mW/cm² simulated sunlight, the hydrogen production rate was 11.09 mmol/(g·h). A photocatalytic STH efficiency record was reported in early 2023, where an indium gallium nitride photocatalyst was tested in concentrated solar light to split pure water at an STH efficiency of 9.2% [100]. In addition, a scaled-up version of the system with a natural solar light capacity of 257 W also achieved a high STH efficiency of 6.2% in the same study.

4 Integrated systems: laboratory-scale prototyping and industrial-level implementation

The laboratory-scale systems of hydrogen generation from seawater via an integrated photothermal-photocatalysis process have been successfully demonstrated. Schrauzer & Guth [101] and Domen et al. [102] were among the first who demonstrated photocatalytic water vapor splitting. Later, Lee et al. [103] designed a floatable photocatalytic platform constructed from porous elastomer-hydrogel nanocomposites, as shown in Fig.9(a). The nanocomposites serve several critical functions to deliver light, supply water, and separate the gas products. A hydrogen evolution rate as high as 163 mmol/(h·m²) was demonstrated on Pt/TiO₂ cryoaerogel. They also scaled up the platform to an area of 1 m² using single-atom Cu/TiO₂ photocatalysts, and the daily hydrogen production rate was as high as 79.2 mL under natural sunlight with stable long-term operation.

As discussed in Section 3.1, some natural porous materials such as wood stems are employed as solar steam generators and vapor splitters. For example, Guo et al. [33] used charred wood substrates in their efficient and integrated photothermal-photocatalytic system. As shown in Fig.9(b), natural wood was carbonized and CoO catalysts were incorporated in their system to evaporate water and then split it. The hydrogen production rate was as high as 220.74 μmol/(h·cm²) under light illumination. It was claimed that because the integrated photothermal–photocatalytic system exhibits interface of steam/photocatalyst/hydrogen only and no liquid phase is involved, the interface barrier is reduced and the transport resistance of the hydrogen gas is lowered by nearly two orders of magnitude in comparison with liquid water splitting. This serves as a good demonstration of a cost-effective photothermal–photocatalytic vapor splitting system to produce hydrogen from seawater. Similarly, a carbonized wood with TiO₂ as the photothermal–photocatalytic system was utilized by Han et al. [104] for water vapor splitting. Carbonized wood surface was found to have two critical functions, to enhance solar spectrum absorption, and to transfer photogenerated electrons quickly from TiO₂. Their system could produce hydrogen from seawater at a rate of 248.1 μmol/(m²·h) under the simulated one-sun irradiation with a long-term stability proven by their cycling experiments.

Industrial implementations for hydrogen production at large scale are limited by many factors, such as scalability of photothermal/photocatalysis fabrication method, and device dimension limits. The large-scale pilot projects for vapor generation appears much earlier than that for hydrogen production. Potentially, the photothermal devices can be upgraded for hydrogen production after integration with photocatalytic vapor splitting process. One of the most common designs is the floating passive SVG device [107], which is suitable for small-scale distributed hydrogen production. Fig.10 provides some other designs of the existing and future pilot projects at different scales. Fig.10(a) shows a tube-type prototype presented by Gao et al. [105], which achieved a hydrogen production rate of 13.3 mmol/(g·h), along with water production after condensing the excess vapor. Another design is the “hydro panel” from the pilot project by Desolenator at the Netherlands for photothermal desalination [99]. Similar concept of the solar panel, seawater flows through the hydro panel and then evaporates. The panel design has advantages such as being modulable and scalable. The modulable panel design is also an option as demonstrated by Nishiyama et al. [92], who scaled up an earlier demonstration of a 1 m² panel reactor system to a 100 m² array of panel reactors (see Fig.10(b)). The reactors used a modified aluminum-doped strontium titanate particulate photocatalyst and a commercial polyimide membrane for vapor generation. The scaled-up system operated over several months on its own with a maximum STH of 0.76%. To the best of the authors’ knowledge, this is the largest scale of photocatalytic solar hydrogen production setup reported by 2021. Another example is an on-going project “solar dome” by NEOM [106,108], which is planned to cover an area of 26500 km² for vapor generation from the photothermal process. The schematic illustration of the project is shown in Fig.10(c). In the NEOM project plan, the vapor is condensed as fresh water. But the vapor from the solar dome can also be potentially used as the feed to a photocatalytic hydrogen production.

5 Summary

The integrated photothermal-photocatalytic system is shown to be a feasible way for hydrogen production from indirect seawater splitting. Currently at low STH efficiencies, the photocatalytic water splitting reactions in gas phase have several advantages in terms of the simplicity, safety, and costs in system design and operation [109,110]. The stoichiometric hydrogen and oxygen product mixture can be handled safely by providing the moist, and thus it is much simple, cost-effective, and flexible to scale-up the vapor splitting system design.

Essential next steps are required to improve the system performance, such as new photothermal/photocatalytic material development, optimized structural design for mass transport, functional device integration, and process optimization. The goal is to maximize the use of harvested solar energy and use it smartly, either for vapor generation or vapor splitting process, to improve the STH efficiency.

The bottleneck of photocatalytic hydrogen generation from seawater is the slow photocatalytic reaction. Often, the photocatalytic process is limited by the low solar light absorption and charge transfer. Therefore, new photocatalytic materials are required to increase the STH efficiency. Most materials predominantly respond to ultraviolet light, which constitutes only a small fraction (less than 3%) of the solar spectrum. Limited photocatalysts were reported to use some visible light spectrum, e.g., 400–485 nm, while the entire visible spectrum (400–700 nm) accounts for nearly 40% of solar light energy. This narrow response range restricts the efficiency of photocatalytic hydrogen production. To addressing these challenges, continuous research efforts are required to broaden the effective spectrum for photocatalytic process with novel materials.

Enhancing the vapor splitting process also requires effective vapor and gas transport. Thus, the optimized structural design is necessary for photothermal and photocatalytic material morphologies. The majority of such materials are porous structures with random pores. The pore shapes, sizes, and connectivity cannot be well controlled through the conventional material synthesis processes. The advanced micro/nano fabrication technologies (i.e., high resolution 3D printing) bring new opportunities to fabricate the on-demand porous catalytic materials with controllable topology. Moreover, coupling with multi-physics simulation and topology optimization, the ideal vapor and gas pathways can be designed in the porous structure for photocatalytic reaction.

Last but not the least, a high-performance vapor splitting system is resulted from the optimized cooperation between photothermal and photocatalytic process. Photothermal and photocatalytic materials response to different wavelength ranges from the solar spectra [111–116], as illustrated in Fig.11. By using long-range wavelength light for photothermal processes, combined with the photocatalytic process using short-range wavelength light, the sunlight utilization can be improved. It is worth mentioning that the photomolecular effect is able to lead to an increased evaporation rate above the thermal evaporation limit. The photomolecular effect happens only for photons with a wavelength shorter than 650 nm and peaks at 520 nm [117,118]. Therefore, carefully utilizing the targeted wavelength with proper materials for the photothermal and photocatalytic processes is essential to maximize the use of solar spectra for STH enhancement, and then substantially reduce costs, photocatalyst stability and gas separation efficiency.

References

[1] InternationalEnergy Agency. Global hydrogen review 2021. 2023–10–6, available at website of IEA

[2] HausmannJ N, Schlögl R, MenezesP W, et al. Is direct seawater splitting economically meaningful? Energy & Environmental Science, 2021, 14(7): 3679–3685 10.1039/D0EE03659E

[3] Guo J, Zheng Y, Hu Z. . Direct seawater electrolysis by adjusting the local reaction environment of a catalyst. Nature Energy, 2023, 8: 264–272

[4] KhanM A, Al-Attas T, RoyS, et al. Seawater electrolysis for hydrogen production: a solution looking for a problem? Energy & Environmental Science, 2021, 14(9): 4831–4839 10.1039/D1EE00870F

[5] Kronawitter C X, Vayssieres L, Shen S. . A perspective on solar-driven water splitting with all-oxide hetero-nanostructures. Energy & Environmental Science, 2011, 4(10): 3889–3899

[6] XuSYuB. Current development and prospect of hydrogen energy technology in China. Journal of Beijing Institute of Technology (Social Sciences Edition), 2021, 23(6): 1-12 (in Chinese)

[7] Davenport D M, Deshmukh A, Werber J R. . High-pressure reverse osmosis for energy-efficient hypersaline brine desalination: Current status, design considerations, and research needs. Environmental Science & Technology Letters, 2018, 5(8): 467–475

[8] Fujiwara M, Kikuchi M. Solar desalination of seawater using double-dye-modified PTFE membrane. Water Research, 2017, 127: 96–103

[9] Shaheen A, AlBadi S, Zhuman B. . Photothermal air gap membrane distillation for the removal of heavy metal ions from wastewater. Chemical Engineering Journal, 2022, 431(1): 133909

[10] Lee A, Elam J W, Darling S B. Membrane materials for water purification: Design, development, and application. Environmental Science: Water Research & Technology, 2016, 2(1): 17–42

[11] Stoll T, Zafeiropoulos G, Tsampas M N. Solar fuel production in a novel polymeric electrolyte membrane photoelectrochemical (PEM-PEC) cell with a web of titania nanotube arrays as photoanode and gaseous reactants. International Journal of Hydrogen Energy, 2016, 41(40): 17807–17817

[12] Döscher H, Geisz J F, Deutsch T G. . Sunlight absorption in water-efficiency and design implications for photoelectrochemical devices. Energy & Environmental Science, 2014, 7(9): 2951–2956

[13] Gopinath C S, Nalajala N. A scalable and thin film approach for solar hydrogen generation: A review on enhanced photocatalytic water splitting. Journal of Materials Chemistry, A.Materials for Energy and Sustainability, 2021, 9(3): 1353–1371

[14] Guo L, Chen Y, Su J. . Obstacles of solar-powered photocatalytic water splitting for hydrogen production: A perspective from energy flow and mass flow. Energy, 2019, 172: 1079–1086

[15] Zhang J, Hu W, Cao S. . Recent progress for hydrogen production by photocatalytic natural or simulated seawater splitting. Nano Research, 2020, 13(9): 2313–2322

[16] Pang X, Das S, Davis J T. . Membraneless electrolyzers for low-cost hydrogen production. ECS Meeting Abstracts, 2020, MA2020(1): 1587

[17] Yao Y, Gao X, Meng X. Recent advances on electrocatalytic and photocatalytic seawater splitting for hydrogen evolution. International Journal of Hydrogen Energy, 2021, 46(13): 9087–9100

[18] Alketbi A S, Raza A, Sajjad M. . Direct solar vapor generation with micro-3D printed hydrogel device. EcoMat, 2022, 4(1): 12157

[19] Tao F, Green M, Garcia A V. . Recent progress of nanostructured interfacial solar vapor generators. Applied Materials Today, 2019, 17: 45–84

[20] Zhou L, Li X, Ni G W. . The revival of thermal utilization from the Sun: Interfacial solar vapor generation. National Science Review, 2019, 6(3): 562–578

[21] Zhu L, Gao M, Peh C K N. . Recent progress in solar-driven interfacial water evaporation: Advanced designs and applications. Nano Energy, 2019, 57: 507–518

[22] He H, Song Z, Lan Y. . Photocorrosion-based BiOCl photothermal materials for synergistic solar-driven desalination and photoelectrochemistry energy storage and release. ACS Applied Materials & Interfaces, 2023, 15(14): 17947–17956

[23] Goto Y, Hisatomi T, Wang Q. . A particulate photocatalyst water-splitting panel for large-scale solar hydrogen generation. Joule, 2018, 2(3): 509–520

[24] Hisatomi T, Maeda K, Takanabe K. . Aspects of the water splitting mechanism on (Ga_1–xZn_x)(N_1–xO_x) photocatalyst modified with Rh_2–yCr_yO₃ cocatalyst. Journal of Physical Chemistry C, 2009, 113(51): 21458–21466

[25] Nishioka S, Osterloh F E, Wang X. . Photocatalytic water splitting. Nature Reviews Methods Primers, 2023, 31(3): 1–15

[26] Herrmann J M. Heterogeneous photocatalysis: fundamentals and applications to the removal of various types of aqueous pollutants. Catalysis Today, 1999, 53(1): 115–129

[27] Gao M, Peh C K, Zhu L. . Photothermal catalytic gel featuring spectral and thermal management for parallel freshwater and hydrogen production. Advanced Energy Materials, 2020, 10(23): 2000925

[28] Shearer C J, Hisatomi T, Domen K. . Gas phase photocatalytic water splitting of moisture in ambient air: Toward reagent-free hydrogen production. Journal of Photochemistry and Photobiology A Chemistry, 2020, 401(1): 112757

[29] Suguro T, Kishimoto F, Kariya N. . A hygroscopic nano-membrane coating achieves efficient vapor-fed photocatalytic water splitting. Nature Communications, 2022, 13(1): 5698

[30] Spurgeon J M, Lewis N S. Proton exchange membrane electrolysis sustained by water vapor. Energy & Environmental Science, 2011, 4(8): 2993

[31] Li Z, Tian B, Zhen W. . Inhibition of hydrogen and oxygen recombination using oxygen transfer reagent hemin chloride in Pt/TiO₂ dispersion for photocatalytic hydrogen generation. Applied Catalysis B: Environmental, 2017, 203: 408–415

[32] Dionigi F, Vesborg P C K, Pedersen T. . Gas phase photocatalytic water splitting with Rh₂–_yCr_yO₃/GaN:ZnO in μ-reactors. Energy & Environmental Science, 2011, 4(8): 2937–2942

[33] Guo S, Li X, Li J. . Boosting photocatalytic hydrogen production from water by photothermally induced biphase systems. Nature Communications, 2021, 12(1): 1343

[34] Cheng P, Quan X, Gong S. . Recent analytical and numerical studies on phase-change heat transfer. Advances in Heat Transfer, 2014, 46: 187–248

[35] Zhang H L, Baeyens J, Degrève J. . Concentrated solar power plants: Review and design methodology. Renewable & Sustainable Energy Reviews, 2013, 22: 466–481

[36] Zhao L, Bhatia B, Zhang L. . A passive high-temperature high-pressure solar steam generator for medical sterilization. Joule, 2020, 4(12): 2733–2745

[37] Neumann O, Urban A, Day J. . Solar vapor generation enabled by nanoparticles. ACS Nano, 2013, 7, (1): 42–49

[38] Zavoico A B. Solar Power Tower Design Basis Document, Revision 0. Sandia National Laboratory Technical Report SAND2001-2100. 2001

[39] Abbas R, Montes M J, Piera M. . Solar radiation concentration features in Linear Fresnel Reflector arrays. Energy Conversion and Management, 2012, 54(1): 133–144

[40] Ni G, Li G, Boriskina S V. . Steam generation under one sun enabled by a floating structure with thermal concentration. Nature Energy, 2016, 1(9): 16126

[41] Ito Y, Tanabe Y, Han J. . Multifunctional porous graphene for high-efficiency steam generation by heat localization. Advanced Materials, 2015, 27(29): 4302–4307

[42] H . Ghasemi H, Ni G, Marconnet A M, et al. Solar steam generation by heat localization. Nature Communications, 2014, 5: 449

[43] Fang J, Liu J, Gu J. . Hierarchical porous carbonized lotus seedpods for highly efficient solar steam generation. Chemistry of Materials, 2018, 30(18): 6217–6221

[44] Raza A, Lu J, Alzaim S. . Novel receiver-enhanced solar vapor generation: Review and perspectives. Energies, 2018, 11(1): 253

[45] Li X, Li J, Lu J. . Enhancement of interfacial solar vapor generation by environmental energy. Joule, 2018, 2(7): 1331–1338

[46] Jia C, Li Y, Yang Z. . Rich mesostructures derived from natural woods for solar steam generation. Joule, 2017, 1(3): 588–599

[47] He S, Chen C, Kuang Y. . Nature-inspired salt resistant bimodal porous solar evaporator for efficient and stable water desalination. Energy & Environmental Science, 2019, 12(5): 1558–1567

[48] Li W, Li F, Zhang D. . Porous wood-carbonized solar steam evaporator. Wood Science and Technology, 2021, 55(3): 625–637

[49] Liang J, Ji X, Han J. . Modeling and experimental investigation on a direct steam generation solar collector with flat plate thermal concentration. Energy Exploration & Exploitation, 2020, 38(5): 1879–1892

[50] Neumann O, Feronti C, Neumann A D. . Compact solar autoclave based on steam generation using broadband light-harvesting nanoparticles. Proceedings of the National Academy of Sciences of the United States of America, 2013, 110(29): 11677–11681

[51] Li J, Du M, Lv G. . Interfacial solar steam generation enables fast-responsive, energy-efficient, and low-cost off-grid sterilization. Advanced Materials, 2018, 30(49): 1805159

[52] Alhosani M H, Li H, Alketbi A S. . Enhanced liquid propagation and wicking along nanostructured porous surfaces. Advanced Engineering Materials, 2021, 23(7): 2100118

[53] Vaartstra G, Zhang L, Lu Z. . Capillary-fed, thin film evaporation devices. Journal of Applied Physics, 2020, 128(13): 130901

[54] Abdelsalam M A. Bio-inspired solar thermal brine treatment with direct vapor generation. Thesis for the Master’s Degree. Abu Dhabi: Khalifa University, 2023

[55] Xu K, Wang C, Li Z. . Salt mitigation strategies of solar-driven interfacial desalination. Advanced Functional Materials, 2020, 31(8): 2007855

[56] Xia Y, Kang Y, Wang Z. . Rational designs of interfacial-heating solar-thermal desalination devices: Recent progress and remaining challenges. Journal of Materials Chemistry, A. Materials for Energy and Sustainability, 2021, 9(11): 6612–6633

[57] Zhang L, Li X, Zhong Y. . Highly efficient and salt rejecting solar evaporation via a wick-free confined water layer. Nature Communications, 2022, 13(1): 849

[58] Ni G, Zandavi S H, Javid S M. . A salt-rejecting floating solar still for low-cost desalination. Energy & Environmental Science, 2018, 11(6): 1510–1519

[59] Shalaby S M, Sharshir S W, Kabeel A E. . Reverse osmosis desalination systems powered by solar energy: Preheating techniques and brine disposal challenges—A detailed review. Energy Conversion and Management, 2022, 251: 114971

[60] Gebreslase G A. Review on membranes for the filtration of aqueous based solution: Oil in water emulsion. Journal of Membrane Science & Technology, 2018, 8(2): 1000188

[61] Deshmukh A, Boo C, Karanikola V. . Membrane distillation at the water-energy nexus: Limits, opportunities, and challenges. Energy & Environmental Science, 2018, 11(5): 1177–1196

[62] Wang Y, Lee J, Werber J R. . Capillary-driven desalination in a synthetic mangrove. Science Advances, 2020, 6(8): eaax5253

[63] Yang Y, Zhao H, Yin Z. . A general salt-resistant hydrophilic/hydrophobic nanoporous double layer design for efficient and stable solar water evaporation distillation. Materials Horizons, 2018, 5(6): 1143–1150

[64] Xu W, Hu X, Zhuang S. . Flexible and salt resistant Janus absorbers by electrospinning for stable and efficient solar desalination. Advanced Energy Materials, 2018, 8(14): 1702884

[65] Hu R, Zhang J, Kuang Y. . A Janus evaporator with low tortuosity for long-term solar desalination. Journal of Materials Chemistry, A. Materials for Energy and Sustainability, 2019, 7(25): 15333–15340

[66] Gao S, Dong X, Huang J. . Bioinspired soot-deposited Janus fabrics for sustainable solar steam generation with salt-rejection. Global Challenges, 2019, 3(8): 1800117

[67] Liu G, Chen T, Xu J. . Salt-rejecting solar interfacial evaporation. Cell Reports. Physical Science, 2021, 2(1): 100310

[68] Cooper T A, Zandavi S H, Ni G W. . Contactless steam generation and superheating under one sun illumination. Nature Communications, 2018, 9(1): 5086

[69] Menon A K, Haechler I, Kaur S. . Enhanced solar evaporation using a photo-thermal umbrella for wastewater management. Nature Sustainability, 2020, 3(2): 144–151

[70] Domen K, Naito S, Soma M. . Photocatalytic decomposition of water vapour on an NiO–SrTiO₃ catalyst. Journal of the Chemical Society. Chemical Communications, 1980, (12): 543–544

[71] Fujishima A, Honda K. Electrochemical photolysis of water at a semiconductor electrode. Nature, 1972, 238(5358): 37–38

[72] Chen X, Shen S, Guo L. . Semiconductor-based photocatalytic hydrogen generation. Chemical Reviews, 2010, 110(11): 6503–6570

[73] WahabA K, Idriss H. Study of the photocatalytic reforming and oxidation of Glycerol over Ag–Pd/TiO₂. International Journal of Hydrogen Energy, 2024, 52(Part B), 159−171.DOI: 10.1016/j.ijhydene.2023.05.344

[74] Wahab A K, Nadeem M A, Idriss H. Hydrogen production during ethylene glycol photoreactions over Ag-Pd/TiO₂ at different partial pressures of oxygen. Frontiers in Chemistry, 2019, 7: 476835

[75] Zhang W, Banerjee-Ghosh K, Tassinari F. . Enhanced electrochemical water splitting with chiral molecule-coated Fe₃O₄ nanoparticles. ACS Energy Letters, 2018, 3(10): 2308–2313

[76] Leduc J, Goenuellue Y, Ghamgosar P. . Electronically-coupled phase boundaries in α-Fe₂O₃/Fe₃O₄ nanocomposite photoanodes for enhanced water oxidation. ACS Applied Nano Materials, 2019, 2(1): 334–342

[77] Amano F, Ishinaga E, Yamakata A. Effect of particle size on the photocatalytic activity of WO₃ particles for water oxidation. Journal of Physical Chemistry C, 2013, 117(44): 22584–22590

[78] Wang F, DiValentin C, Pacchioni G. Rational band gap engineering of WO₃ photocatalyst for visible light water splitting. ChemCatChem, 2012, 4(4): 476–478

[79] Hamid S B A, Teh S J, Lai C W. Photocatalytic water oxidation on ZnO: A review. Catalysts, 2017, 7(3): 93

[80] Ni M, Leung M K H, Leung D Y C. . A review and recent developments in photocatalytic water-splitting using TiO₂ for hydrogen production. Renewable & Sustainable Energy Reviews, 2007, 11(3): 401–425

[81] Hong Y, Fang Z, Yin B. . A visible-light-driven heterojunction for enhanced photocatalytic water splitting over Ta₂O₅ modified g-C₃N₄ photocatalyst. International Journal of Hydrogen Energy, 2017, 42(10): 6738–6745

[82] Marschall R. Semiconductor composites: Strategies for enhancing charge carrier separation to improve photocatalytic activity. Advanced Functional Materials, 2014, 24(17): 2421–2440

[83] Qian R, Zong H, Schneider J. . Charge carrier trapping, recombination and transfer during TiO₂ photocatalysis: An overview. Catalysis Today, 2019, 335: 78–90

[84] Fu C F, Wu X, Yang J. Material design for photocatalytic water splitting from a theoretical perspective. Advanced Materials, 2018, 30(48): 1802106

[85] Dingenen F, Verbruggen S W. Tapping hydrogen fuel from the ocean: A review on photocatalytic, photoelectrochemical and electrolytic splitting of seawater. Renewable and Sustainable Energy Reviews, 2021, 142: 110866

[86] Zhu C C, Jiang T, Yang H C. . ZnFe₂O₄ nanoparticles with iron-rich surfaces for enhanced photocatalytic water vapor splitting. Applied Surface Science, 2023, 636: 157842

[87] Daeneke T, Dahr N, Atkin P. . Surface water dependent properties of sulfur-rich molybdenum sulfides: Electrolyteless gas phase water splitting. ACS Nano, 2017, 11(7): 6782–6794

[88] Chowdhury F A, Trudeau M L, Guo H. . A photochemical diode artificial photosynthesis system for unassisted high efficiency overall pure water splitting. Nature Communication, 2018, 9: 1707

[89] Wang Q, Hisatomi T, Jia Q. . Scalable water splitting on particulate photocatalyst sheets with a solar-to-hydrogen energy conversion efficiency exceeding 1%. Nature Materials, 2016, 15: 611–615

[90] Maeda K, Teramura K, Lu D. . Noble-metal/Cr₂O₃ core/shell nanoparticles as a cocatalyst for photocatalytic overall water splitting. Angewandte Chemie International Edition, 2006, 45(46): 7806–7809

[91] Ng K H, Lai S Y, Cheng C K. . Photocatalytic water splitting for solving energy crisis: Myth, fact or busted?. Chemical Engineering Journal, 2021, 417: 128847

[92] Nishiyama H, Yamada T, Nakabayashi M. . Photocatalytic solar hydrogen production from water on a 100-m² scale. Nature, 2021, 598(7880): 304–307

[93] Jin Z, Yan X, Hao X. Rational design of a novel p-n heterojunction based on 3D layered nanoflower MoS_x supported CoWO₄ nanoparticles for superior photocatalytic hydrogen generation. Journal of Colloid and Interface Science, 2020, 569: 34–49

[94] Bhattacharya R N, Lee C Y, Pollak F H. . Optical study of amorphous MoS₃: Determination of the fundamental energy gap. Journal of Non-Crystalline Solids, 1987, 91(2): 235–242

[95] Tang M L, Grauer D C, Lassalle-Kaiser B. . Structural and electronic study of an amorphous MoS₃ hydrogen-generation catalyst on a quantum-controlled photosensitizer. Angewandte Chemie International Edition, 2011, 50(43): 10203–10207

[96] Zhang S, Zhao H, Li X. . A hierarchical Si_PN/CN/MoS_x photocathode with low internal resistance and strong light-absorption for solar hydrogen production. Applied Catalysis B: Environmental, 2022, 300: 120758

[97] Shin S, Jin Z, Kwon D H. . High turnover frequency of hydrogen evolution reaction on amorphous MoS₂ thin film directly grown by atomic layer deposition. Langmuir, 2015, 31(3): 1196–1202

[98] Benck J D, Chen Z, Kuritzky L Y. . Amorphous molybdenum sulfide catalysts for electrochemical hydrogen production: Insights into the origin of their catalytic activity. ACS Catalysis, 2012, 2(9): 1916–1923

[99] Bourgeteau T, Tondelier D, Geffroy B. . A H₂-evolving photocathode based on direct sensitization of MoS₃ with an organic photovoltaic cell. Energy & Environmental Science, 2013, 6(9): 2706

[100] Zhou P, Navid I A, Ma Y. . Solar-to-hydrogen efficiency of more than 9% in photocatalytic water splitting. Nature, 2023, 613(7942): 66–70

[101] Schrauzer G N, Guth T D. Photolysis of water and photoreduction of nitrogen on titanium dioxide. Journal of the American Chemical Society, 1977, 99(22): 7189–7193

[102] Domen K, Naito S, Onishi T. . Study of the photocatalytic decomposition of water vapor over a nickel(II) oxide-strontium titanate (SrTiO₃) catalyst. ChemInform, 1982, 86(18): 3657–3661

[103] Lee W H, Lee C W, Cha G D. . Floatable photocatalytic hydrogel nanocomposites for large-scale solar hydrogen production. Nature Nanotechnology, 2023, 18(7): 754–762

[104] Han H, Huang K, Yao Y. . Enhanced photocatalytic splitting of photothermally induced water vapor to evolve hydrogen. Chemical Engineering Journal, 2022, 450: 138419

[105] Gao M, Connor P K N, Ho G W. Plasmonic photothermic directed broadband sunlight harnessing for seawater catalysis and desalination. Energy & Environmental Science, 2016, 9(10): 3151–3160

[106] Sansom C, Patchigolla K, Jonnalagadda K. . Design of a novel CSP/MED desalination system. In: Proceedings of the 26th International Conference on Concentrating Solar Power and Chemical Energy Systems, Freiburg. New York: AIP Publishing, 2022, 2445(1): 140012

[107] Chen S, Zhao P, Xie G. . A floating solar still inspired by continuous root water intake. Desalination, 2021, 512: 115133

[108] NEOM. NEOM adopts pioneering solar dome technology for sustainable desalination project 2020. 2023-10-6, available at website of NEOM

[109] Zheng Y, Ma M, Shao H. Recent advances in efficient and scalable solar hydrogen production through water splitting. Carbon Neutrality, 2023, 2(1): 23

[110] Schreck M, Niederberger M. Photocatalytic gas phase reactions. Chemistry of Materials, 2019, 31(3): 597–618

[111] Jenny S, Matsuoka M, Takeuchi M. . Understanding TiO₂ photocatalysis: Mechanisms and materials. Chemical Reviews, 2014, 114(19): 9919–9986

[112] Patial S, Hasija V, Raizada P. . Tunable photocatalytic activity of SrTiO₃ for water splitting: Strategies and future scenario. Journal of Environmental Chemical Engineering, 2020, 8(3): 103791

[113] Acar C, Dincer I, Naterer G F. Review of photocatalytic water-splitting methods for sustainable hydrogen production. International Journal of Energy Research, 2016, 40(11): 1449–1473

[114] Mikaeili F, Gilmore T, Gouma P I. Photochemical water splitting via transition metal oxides. Catalysts, 2022, 12(11): 1303

[115] Yang M Q, Gao M, Hong M. . Visible-to-NIR photon harvesting: Progressive engineering of catalysts for solar-powered environmental purification and fuel production. Advanced Materials, 2018, 30(47): 1802894

[116] Zhu L, Gao M, Peh C K N. . Solar-driven photothermal nanostructured materials designs and prerequisites for evaporation and catalysis applications. Materials Horizons, 2018, 5(3): 323–343

[117] Tu Y, Zhou J, Lin S, et al. Photomolecular effect leading to water evaporation exceeding thermal limit. 2022, arXiv: 2201. 10385 2022

[118] Tu Y, Chen G. Photomolecular effect: Visible light absorption at water-vapor interface. 2022, arXiv:2202.10646 10.48550/arXiv.2202.10646

RIGHTS & PERMISSIONS

Higher Education Press