The bright future of solar-driven hydrogen production

Abstract

Sunlight is the most abundant sustainable energy source available to our planet. Total power needed by mankind is less than 0.1% of what is available from the sun. By comparison, the potential of other energy sources, including wind, geothermal, tidal, renewable biomass and hydropower is hundreds of times smaller. Mankind currently consumes most of its energy via historical sunlight accumulated and stored in fossil carbon sources with severe environmental repercussions. Theoretically, solar energy could easily replace fossil fuels as our main energy source, however, the technical challenges to extract and convert solar radiation into usable energy sources are formidable.The main challenge with solar energy lies in its intermittency. Apart from the day and night and seasonal cycles, solar radiation varies widely across different regions (i.e., more solar radiation reaches the equator than the poles. Another challenge is the low solar flux density, which means the energy from sunlight is spread out over a large area. Consequently, collecting substantial amount of solar energy results requires significant space. Sunlight itself cannot be transported and needs to be converted into an energy vector which makes it easier to move and stock. Electricity provides such an option, but its transportation via intercontinental cables is expensive. Chemicals in gaseous, liquid and solid forms are more convenient to transport, as is currently done with natural gas, petroleum and coal. However, if we aim to avoid the involvement of carbon atoms, hydrogen is the ideal molecule to be produced from sunlight. While transport of hydrogen gas needs strong compression and very low temperatures for liquefaction, converting it to ammonia enables transport and storage of large volumes of hydrogen over long distances. The ammonia can subsequently be cracked (decomposed) again to recover hydrogen, the desired fuel, as needed.Solar-driven hydrogen production through water splitting has emerged as a feasible pathway for green energy generation. In their Frontiers in Science lead article, Hisatomi et al. (2024) provided an in-depth discussion of the recent developments in green hydrogen production through photocatalytic water splitting. Scaling up hydrogen production via overall water splitting using photocatalysts has been the focus of development in this area.Before the emergence of photocatalysts for generating hydrogen, other methods were used to produce hydrogen from water. One of the oldest is electrolysis, which was eventually abandoned in favor of steam methane reforming, which has now resurfaced as a key method for generating green hydrogen.The splitting of water molecules yields hydrogen gas being an industrially important chemical and energy vector, and oxygen gas also needed in massive amount in all kinds of industrial processes. Hydrogen production through electrolysis is an old process. It has historically been abandoned when steam methane reforming took over, but it is back as one of the mature approaches of green hydrogen production (Table 1). Current electrolyzers have been growing increasingly powerful, ranging from megawatt (MW) to even gigawatt (GW) scale (International Energy Agency, 2023). Zero-carbon hydrogen can be produced if the electrolyzer is fueled via solar, wind, or nuclear energy. However, producing electricity solely through a photovoltaic park is economically less attractive because of the handicap of the diurnal cycle providing no power to electrolyzers at night. Instead, solar thermal systems generating high temperatures for catalytic or even spontaneous splitting of water molecules is an alternative emerging technology being demonstrated in regions with intense solar radiation. It is an example of a solar hydrogen generator not needing electricity as intermediate energy vector.Other types of devices directly using sunlight are photoelectrochemical (PEC) and photocatalytic (PC) cells. The distinction between these technologies lies in the separation distance of electric charge generation within photovoltaic semiconductors and the catalytic sites for water splitting. In PEC cells, this distance can be several centimeters, while in photocatalytic systems, it is limited to the nanoscale. Both are emerging technologies gaining technological readiness (Table 1). Both systems are also intrinsically modular and autonomous.Photocatalytic systems and PECs offer the advantage of being decentralized, with less reliance on specific locations due to their disconnection from the electricity grid and lower water consumption. This flexibility makes the economic optimal location less critical compared to large-scale electrolyzer systems. In contrast, electrolyzers require large volumes of water, which must be sourced either from seawater via desalination or from local freshwater supplies, often competing with other uses. This dependence on water availability, along with the need for access to harbors or pipelines for exporting the produced hydrogen, somehow limits the location of electrolyzer-based hydrogen production plants.Fresh water production by atmospheric water vapor capturing is another research field gaining momentum. An attractive option is to integrate an atmospheric water vapor collection function in these systems making them fully independent of local water resources, thereby providing a local solution to the water-energy nexus. These direct solar hydrogen production technologies can in principle be implemented anywhere, having access to sunlight as the only requirement. They are modular and useful at any scale. Solar-to-hydrogen efficiencies of photoelectrochemical hydrogen production systems with illuminated photoelectrodes can be very high. Owing to the less efficient charge separation in state-of-the-art photocatalysis systems, the efficiencies are still lower, but the perspective is to reach ultimately similar performances. High efficiency justifies working "at the rhythm of the sun" while producing hydrogen at a competitive prize. In addition, durability of materials is likely to be less of an issue as compared to electrodes and membranes of electrolyzers working at very high electric current densities. In situ photocatalytic installations at small and medium size for local consumption at remote locations can be made safe by limiting the pressure and avoiding the need of transporting. This has been demonstrated by Histaomi et al. (2024) in a 100m 2 photocatalyst array system for hydrogen production, utilizing photocatalyst sheets on panel reactor.Hydrogen producing devices based on photocatalysis offer a solution to the biggest technological challenges. Clean cooking is one of these challenges recognized by the World Health Organization (WHO) as the number one problem to be solved (World Health Organization, n.d.). About 4 billion people have access only to wood and charcoal as their household energy source. As a result, daily exposure to smoke from wood fires is responsible for serious health problems (World Health Organization, 2014). This use of wood due to a lack of alternate sustainable energy sources also contributes significantly to deforestation across the Global South as well as global climate change with roughly 2-7% of global greenhouse gas (GHG) emissions resulting from use of wood and charcoal as household energy source (Food and Agriculture Organization, 2017). There is no obvious solution for this problem but improving the efficiency of cookstoves and reducing air polluting emissions should be considered as an option, as well as the use of liquid petroleum gas. Hower, hydrogen cookstoves fed with safe hydrogen gas produced locally by photoelectrochemical and photocatalytic water splitting present an imaginative disruptive clean technology, preserving nature, limiting carbon dioxide (CO2) emissions, and producing water vapor instead of polluting carbonaceous particulate matter.Another exciting and promising development in solar-driven technologies is expanding their use to convert molecules other than water, such as CO2. Artificial photosynthesis captures atmospheric CO2 and transforms it into fuels or chemicals, creating a sustainable cycle that reduces emissions and provides renewable energy. This is considered the "holy grail" of renewable energy since it could address both climate change and energy needs simultaneously. However, developing photocatalytic systems for CO2 may take decades to realize. Initial successes have been demonstrated by Hisatomi et al. (2024) using photocatalytic sheets and exploring biomolecular catalysts.In conclusion, the push for renewable energy, particularly solar-driven technologies like photocatalytic hydrogen water splitting, is crucial for transitioning from fossil fuels and addressing climate change. Photocatalytic hydrogen production is key to energy sustainability because of the direct use of solar energy and its suitability for decentralized applications in regions where the majority of the human population is currently living without access to clean energy sources. There are no fundamental limitations to the solar-to-hydrogen efficiency but further advancement in efficiency, material durability, and scalability are still needed, and supportive infrastructure and regulatory frameworks are essential. Nevertheless, the future for photocatalytic hydrogen production is bright and with continuous developments, it can be expected to become an essential asset for future generations.De Rose, A., Buna, M., Strazza, C., Olivieri, N., Stevens, T., Peeters, L. and Tawil-Jamault, D., [online]