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Among renewable energies, wind and solar are inherently intermittent and therefore both require efficient energy storage systems to facilitate a round-the-clock electricity production at global scale. In this context, concentrated solar power (CSP) stands out among other sustainable technologies because it offers the interesting possibility of storing energy collected from the sun as heat, by sensible, latent or thermochemical means. Accordingly, continuous electricity generation in the power block is possible even during off-sun periods, providing CSP plants with a remarkable dispatchability. Sensible heat storage has been already incorporated to commercial CSP plants. However, due to its potentially higher energy storage density, thermochemical heat storage (TCS) systems emerge as an attractive alternative for the design of next generation power plants, which are expected to operate at higher temperatures. Through these systems, thermal energy is used to drive endothermic chemical reactions, which can subsequently release the stored energy when needed through a reversible exothermic step. This review analyzes the status 2 of this prominent energy storage technology, its major challenges and future perspectives, covering in detail the numerous strategies proposed for the improvement of materials and thermochemical reactors. Thermodynamic calculations allow selecting high energy density systems, but experimental findings indicate that sufficiently rapid kinetics and long-term stability trough continuous cycles of chemical transformation are also necessary for practical implementation. In addition, selecting easy to handle materials with reduced cost and limited toxicity is crucial for large-scale deployment of this technology. In this work, the possible utilization of materials as diverse as metal hydrides, hydroxides or carbonates for thermochemical storage is discussed. Furthermore, especial attention is paid to the development of redox metal oxides, such as Co3O4/CoO, Mn2O3/Mn3O4 and perovskites of different compositions, as an auspicious new class of TCS materials due to the advantage of working with atmospheric air as reactant, avoiding the need of gas storage tanks. Current knowledge about the structural, morphological and chemical modifications of these solids, either caused during redox transformations, or induced wittingly as a way to improve their properties, is revised in detail. In addition, the design of new reactors concepts proposed for the most efficient use of TCS in concentrated solar facilities is also critically considered. Finally, strategies for the harmonic integration of these units in functioning solar power plants, as well as the economic aspects are also briefly assessed.

Research on photocatalytic materials has been a field in continuous expansion in the recent decades, as it is evidenced by the large number of articles published every year. So far, more than 190 different semiconductors have been assayed as suitable photocatalysts. To this figure, it is necessary to add the combinations with other functional materials or between different semiconductors, as well as their morphological modifications. Summing up the outcome of these different preparation strategies eventually leads to the enormous number of photocatalytic systems that have been reported in the scientific literature. Dealing with such an amount of information requires updated and educated guidance to select the most significant realizations, and it also calls for critical assessments on how the expectations are being fulfilled. This perspective article intends to assess the state of the art of photocatalysis with regard to materials and systems, considering the well-established results, but also the emerging aspects, and the envisaged new directions of this technology in the near future. In the first part, the most relevant achievements in this area, some of them already in the market while others still in development, will be reviewed according to the current understanding. The second part of the article is devoted to the most innovative and promising photocatalysts and related systems described in the open literature.

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Nanostructured TiO2 samples with primary particle size in the 4−20 nm range were prepared by either hydrothermal (H) or thermal (T) treatment of an amorphous precursor, and their behavior under UV illumination at 77 K was studied by means of EPR spectroscopy. The samples of the H series present the smallest crystallite size and after irradiation in a vacuum show some Ti3+ centers. In contrast, under these conditions only weak signals associated with oxygenated radicals are observed for the T samples. However, when oxygen is preadsorbed, several oxygenated complexes (O-, O2 -, O2H•, and O3 -) are photogenerated in proportions that depend on the characteristics of the material. Subsurface O- species are exclusively detected in the case of the samples of the H series, whereas ozonide radicals and surface O- are stabilized on materials with larger crystalline domains. These oxygenated complexes are thermally unstable, and they disappeared after warming to room temperature in the case of the T samples, but they are transformed to O2 - on the surface of the hydrothermally treated TiO2. Since adsorbed water and different types of free hydroxyls are present on these materials, as revealed by FTIR, a number of surface reactions have to be considered in order to account for the formation and stability of such photogenerated species.