G-C₃N₄ with structural defects and low polymerization synthesized by urea as the precursor for photocatalytic H₂ production under visible light.

Solar-assisted water splitting using photoelectrochemical cells (PECs) is one of the promising pathways for the production of hydrogen for renewable energy storage. The nature of the semiconductor material is the primary factor that controls the overall energy conversion efficiency. Finding semiconductor materials with appropriate semiconducting properties (stability, efficient charge separation and transport, abundant, visible light absorption) is still a challenge for developing materials for solar water splitting. Owing to the suitable bandgap for visible light harvesting and the abundance of iron-based oxide semiconductors, they are promising candidates for PECs and have received much research attention. Spinel ferrites are subclasses of iron oxides derived from the classical magnetite ($FeIIFe2IIIO4$) in which the $FeII$ is replaced by one (some cases two) additional divalent metals. They are generally denoted as $MxFe3−xO4$ ($M=Ca$, Mg, Zn, Co, Ni, Mn, and so on) and mostly crystallize in spinel or inverse spinel structures. In this mini review, we present the current state of research in spinel ferrites as photoelectrode materials for PECs application. Strategies to improve energy conversion efficiency (nanostructuring, surface modification, and heterostructuring) will be presented. Furthermore, theoretical findings related to the electronic structure, bandgap, and magnetic properties will be presented and compared with experimental results.

Electrolysis is one of the clean, environmentally friendly, and sustainable pathways to produce hydrogen for renewable energy storage. However, to make electrolysis a competitive technology for hydrogen production, developing nonprecious metal‐based catalysts for oxygen evolution reaction (OER) is mandatory. Several new classes of electrocatalysts are developed with outstanding OER catalytic activity, stability, and commercial viability. Owing to the structural diversity, porosity, and accessibility of catalytically active metal centers, nickel‐based metal–organic frameworks (MOFs) are intensively explored as OER catalysts. In particular, bi‐ and trimetallic Ni MOFs with Fe and Co as additional metal nodes show excellent OER activity which can be tailored through the fine tuning of the metal compositions. Herein, the current state of research in Ni‐based MOFs as OER catalyst materials for alkaline electrolysis is presented. Strategies to improve the catalytic performance like compositional variations, choice of synthetic routes, and support materials are presented. Furthermore, OER activities are compared and presented based on the performance metrics (current density, overpotential, and Tafel slopes). Finally, concluding remarks featuring the key findings in Ni‐based MOFs and the possible rooms for future developments are summarized.

Hexagonal perovskite YFeO₃ was synthesized by a complex-assisted sol-gel technique allowing crystallization at calcination temperatures below 700 °C. As determined by diffuse reflectance spectroscopy (DRS) and Tauc plots, the hexagonal YFeO₃ exhibits a lower optical band gap (1.81 eV) than the orthorhombic structure (about 2.1 eV or even higher) being typically obtained at elevated temperatures (>700 °C), and thus enables higher visible light photocatalysis activity. Structure and morphology of the synthesized YFeO₃ perovskites were analyzed by powder X-ray diffraction (XRD) and nitrogen adsorption, proving that significantly smaller crystallite sizes and higher surface areas are obtained for YFeO₃ with a hexagonal phase. The photocatalytic activity of the different YFeO₃ phases was deduced via the degradation of the model pollutants methyl orange and 4-chlorophenol. Experiments under illumination with light of different wavelengths, in the presence of different trapping elements, as well as photoelectrochemical tests allow conclusions regarding band positions of YFeO₃ and the photocatalytic degradation mechanism. X-ray photoelectron spectroscopy indicates that a very thin layer of Y₂O₃ might support the photocatalysis by improving the separation of photogenerated charge carriers.

The photoelectrochemical activity of hematite films prepared by electrochemical deposition (ED) in the presence of organic additives is discussed. The studies focus on the role of small organic additive molecules in the tuning of the morphology of the films and their influence on the photoelectrochemical oxidation of water. The organic additives, namely, coumarin 343 (C343), γ-glucuronic acid (GA) and sodium dodecyl sulfonate (Sds), possess functional moieties to interact with iron ions in the ED bath electrostatically or through metal–ligand complexation reactions. XPS measurements prove that the organic additives are incorporated, and the oxidation state of Fe³⁺ rules out the presence of mixed valences in the films. SEM and XRD measurements present morphological and structural evidence, respectively. The photoelectrochemical study shows that organically modified hematite films exhibit enhanced photoactivity; the photocurrent density at 1.4 V vs. RHE on a GA-modified electrode is up to 5–6 times higher than on the unmodified electrode. Electrochemical impedance results reveal the role of the organic additives in reducing the charge transfer resistance from the hematite surface to the solution. In addition, a simple Ti post treatment greatly enhances the photoactivity of all electrodes under investigation.