Sustainable hydrogen production is a key target for the development of alternative, future energy systems that will provide a clean and affordable energy supply. The Sun is a source of silent and precious energy that is distributed fairly all over the Earth daily. However, its tremendous potential as a clean, safe, and economical energy source cannot be exploited unless the energy is accumulated or converted into more useful forms. The conversion of solar energy into hydrogen via the water-splitting process, assisted by photo-semiconductor catalysts, is one of the most promising technologies for the future because large quantities of hydrogen can potentially be generated in a clean and sustainable manner. This Minireview provides an overview of the principles, approaches, and research progress on solar hydrogen production via the water-splitting reaction on photo-semiconductor catalysts. It presents a survey of the advances made over the last decades in the development of catalysts for photochemical water splitting under visible-light irradiation. The Minireview also analyzes the energy requirements and main factors that determine the activity of photocatalysts in the conversion of water into hydrogen and oxygen using sunlight. Remarkable progress has been made since the pioneering work by Fujishima and Honda in 1972, but he development of photocatalysts with improved efficiencies for hydrogen production from water using solar energy still faces major challenges. Research strategies and approaches adopted in the search for active and efficient photocatalysts, for example through new materials and synthesis methods, are presented and analyzed.

This review article reports the most significant advances made in H 2 production via water-splitting and the challenges that need to be addressed over the coming years to verify the feasibility of H 2 production by both inorganic semiconductors and living microorganisms as a competitive process in the hydrogen economy.

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Nearly 2 % of the world's primary energy is stored in the 65 million tonnes of hydrogen generated each year, almost all of which is for captive use in the chemical and refinery industries. [1] Currently, the main processes for producing industrial hydrogen are catalytic steam reforming of natural gas (48 %) and oil-derived naphtha (30 %), coal gasification (18 %), and the electrolysis of water (4 %). [2] Apart from its traditional uses, hydrogen is considered an ideal energy carrier in the future energy systems that need to be economically and environmentally sustainable. [3] The possibility of using hydrogen as an alternative energy carrier has intensified the exploration of hydrogen production processes from a wide range of primary sources such as natural gas, fuels, methanol, biomass, coal, solar, and nuclear power. [4][5][6][7][8] Although hydrogen production, storage, and distribution are commercially viable in the chemical and refining industries, the cost and efficiency of the infrastructures for its storage and distribution for energy use is currently unacceptable compared to existing petroleum distillate facilities. [9] Additionally, current commercial options for H 2 storage (high pressure or liquefaction) do not fully meet requirements for compactness, drive range, and cost in transport applications (2 kWh kg À1 and 4 $ kWh À1 ). To achieve effective hydrogen storage, research activities have focused on the development of in situ production processes based on the reforming of high-density liquids that contain hydrogen, such as methanol, ethanol, or fuels. Several studies [10][11][12][13][14] have analyzed [a] Dr.