Research Background

The pursuit of economically viable and sustainable water and fuel co-production is a promising but challenging endeavor. In a recent issue of Nature Water, Pornrungroj et al. present an innovative combination of interfacial solar steam generation and photocatalysis. This approach efficiently harnesses solar energy to convert open water into pure water and hydrogen fuel.

The article has been published in Joule with the title " Unlocking solar-driven synergistic clean water harvesting and sustainable fuel production".

【Article Interpretation】

The high gravimetric energy density of hydrogen (143 MJ kg−1) and its environmentally friendly nature, with no harmful gas emissions during combustion, make it a beacon for the future of renewable energy, especially in the pursuit of carbon neutrality. 1 However, a major challenge remains: how can we produce hydrogen sustainably without imposing excessive energy demands on the process? Conventional production of hydrogen via water splitting requires an external energy source, such as electricity, because its free energy change is large and positive (ΔG0 = +237 kJ/mol). This reliance imposes limitations and highlights the need for a comprehensive assessment of its energy burden. 2 One solution lies in photocatalytic water splitting, which uses solar energy to overcome the reaction energy barrier and eliminate the need for electricity. However, the high demand for pure water in this process limits its feasibility and further exacerbates global water shortages, especially in water-scarce communities. The use of untreated water, even with only small amounts of contaminants, can lead to serious side effects, including other oxidation reactions (such as chloride oxidation) and catalyst poisoning, resulting in a sharp decrease in solar-to-hydrogen (STH) efficiency and long-term operational stability. 2 In addition, the costs associated with sanitation installation and water transportation further exacerbate these challenges. 3 In a breakthrough recently detailed by Pornrungroj et al. in Nature Water, a novel system seamlessly combines interfacial solar steam generation (SVG) and photocatalysis to provide a breakthrough approach for the simultaneous production of clean water and hydrogen. This ingenious integration involves floating a solar absorber on the ocean surface, converting solar energy into heat and activating seawater to continuously produce steam. Simultaneously, a photocatalyst (PC) is used to convert water vapor into hydrogen (Figure 1A). 4 The unreacted vapor is then condensed on an optical window for water harvesting.

The core advantages of the system are multifaceted. SVG uses low-cost materials and consumes less energy than existing desalination technologies such as reverse osmosis and multi-layer flash distillation. Carbon-based fibers and macroporous sheets were chosen for SVG due to their low density and porous structure, which enables them to float on the sea surface. The careful selection of aluminum-doped strontium titanate (Al:SrTiO3) photocatalyst ensured high reactivity, cost-effectiveness and ease of synthesis. In their design, a strategic measure was taken by introducing the co-catalyst RhCrOx to suppress the reverse reaction by preventing O2 from reaching Rh species. Subsequently, the two components were intricately embedded in the Nafion matrix, intelligently preventing direct contact with water.

The seamless integration of SVG and photocatalysis has several key implications. Pure RhCrOx Al:SrTiO3 selectively utilizes the UV region of the solar spectrum for water splitting, making the remaining spectrum, especially the near-infrared field, available for efficient SVG, thereby achieving comprehensive utilization of solar energy (Figure 1E). In terms of reactivity, the ingenious method of providing steam (gaseous water) to the photocatalyst avoids the adverse effects associated with liquid water. This not only reduces the activation energy (ΔG0(H2O,l) = +237 kJ/mol, ΔG0)(H2O,g) = +228.6 kJ/mol), but also eliminates the solar scattering caused by the liquid water film, mitigating the impact of trace contaminants on the solar absorptivity. In addition, the hydrophobic treatment of the solar absorber by silanization is emphasized to repel any contaminants dissolved in the bulk solution, which may otherwise permeate through the solar absorber to reach the photocatalyst despite the application of a physical separation layer between the feedstock and the photocatalyst. As shown in Figures 1B and 1C, the water contact angle shows an increment after the hydrophobic treatment, which is associated with higher STH efficiency. In contrast, the photocatalysts directly coated on glass exhibited the conventional trend, with a significant decrease in STH efficiency under seawater. Notably, this design improves long-term operational stability by not only isolating the photocatalyst from contaminants in seawater but also preventing the dissolution of the co-catalyst by maintaining a relatively dry environment. As shown in Figure 1D, the hydrogen evolution activity of the photocatalyst coated on glass rapidly decreased by more than 40% after 66 h, which was attributed to the loss of Cr species on the surface of RhCrOxAl:SrTiO3. In contrast, the untreated SVG photocatalyst and the treated SVG-photocatalyst remained stable in pure water for up to 154 h, retaining 89% of their initial performance. Under seawater conditions, the performance of the glass photocatalyst and the untreated SVG photocatalyst was significantly lower than their performance in pure water, producing only 0.04% of the STH efficiency in the first 22 h, with a reduced hydrogen evolution rate in subsequent cycles. In contrast, the SVG photocatalyst showed consistent performance in both seawater and pure water, maintaining an STH efficiency of 80% of its initial performance after 154 h of operation and stoichiometric total water splitting. Finally, a prototype for integrated water splitting and clean water production using Cambrian water was developed (Figures 1F and 1G). Hydrogen produced by water splitting was trapped in the gas-tight headspace of the reactor, achieving a performance of 16.1 ± 3.6 mmol m−2 h−1 at AM 1.5G, STH: 0.10 ± 0.02% (7.82 ± 1.52 mmol m²h−l under natural sunlight). Meanwhile, water vapor condensed from SVG formed droplets on the tilted windows and flowed downward into a separate partition at AM 1.5G, producing 0.94 ± 0.12 kg m−2 h−1 (0.71 ± 0.12 kg m2 h−l under natural sunlight) (Figure 1H).

In addition to optimizing the SVG-PC system, Pornrungroj et al. also envisioned the possibility of harvesting water and fuel directly from the ubiquitous atmospheric moisture. Compared with the SVG-PC system, the atmospheric water harvesting (AWH)-photocatalytic system can fundamentally solve the dependence on seawater or water raw materials. This is considered to be the next generation of water-fuel cogeneration system, which is very beneficial for inland areas with limited water access. 5,6 However, the AWH-PC system is extremely challenging and remains relatively unexplored, with several intractable bottlenecks to be addressed. 7,8 First, the developed adsorbents place high demands on satisfactory hygroscopicity and photocatalytic reactivity. Second, due to the imbalance between moisture absorption/adsorption (water supply) and desorption (water release), how to continuously harvest energy fuels and clean water is a challenging problem.

【Article Summary】

The successful establishment of the SVG-PC system undoubtedly marks a key milestone, revealing new prospects for photocatalysis in polluted water treatment, clean water production, and hydrogen release. This achievement will help inspire researchers to conduct more in-depth studies aimed at optimizing and expanding its water and fuel production capabilities. First, the interaction between the solar absorber and the photocatalyst needs to be strengthened. Ensuring the stability of the catalyst without shedding while effectively separating the pollutant water from the photocatalyst remains a key path for refinement. Second, while the current device volume is limited to less than 180 cubic centimeters, it is crucial to pay attention to large-scale manufacturing and scalability. To bridge the gap between performance and practical application standards, it is necessary to focus on upgrading the SVG-PC system. Finally, the relentless pursuit of improving the efficiency of solar-to-hydrogen remains the primary goal. Improvements in efficiency directly amplify the overall fuel production capacity, bringing the technology closer to its maximum potential. The way forward relies on interdisciplinary collaboration across physics, chemistry, materials science, and nanotechnology. In this emerging field, such symbiotic partnerships are expected to take a revolutionary step towards practical applications in the foreseeable future.

【Source】

https://doi.org/10.1016/j.joule.2024.01.019