This review describes an in-depth overview and knowledge on the variety of synthetic strategies for forming metal sulfides and their potential use to achieve effective hydrogen generation and beyond.

Over the past few decades, owing to their unique functional properties such as physical, chemical, optical and electronic properties, spinel materials have attracted significant scientific attention in heterogeneous photocatalyst research. Here, we review the main fundamental understanding of the correlations between the performance of spinel structures and their particle shape, size, chemical composition, and photo-Fenton reactions for photocatalytic applications; these include photocatalytic dye degradation for environmental remediation, photocatalytic hydrogen generation, CO₂ reduction and photoelectrochemical water splitting. In addition, the key factors and essential strategies to improve their performance and functionality are discussed in detail. Future research pathways and perspectives on the progress of these high performance and cost effective renewable energy materials are provided, along with the improvements in material properties that are necessary to replace current commercial energy materials. It is envisioned that further investigations should focus on surface modification, integrating conductive matrixes and regulating the spinel composition, which will make spinels promising photocatalysts.

Rechargeable zinc-air batteries (ZABs) are a promising energy conversion device, which rely critically on electrocatalysts to accelerate their rate-determining reactions such as oxygen reduction (ORR) and oxygen evolution reactions (OER). Herein, we fabricate a range of bifunctional M–N–C (metal-nitrogen-carbon) catalysts containing M–N_x coordination sites and M/M_xC nanoparticles (M = Co, Fe, and Cu) using a new class of γ-cyclodextrin (CD) based metal–organic framework as the precursor. With the two types of active sites interacting with each other in the catalysts, the obtained Fe@C-FeNC and Co@C-CoNC display superior alkaline ORR activity in terms of low half-wave (E_1/2) potential (~ 0.917 and 0.906 V, respectively), which are higher than Cu@C-CuNC (~ 0.829 V) and the commercial Pt/C (~ 0.861 V). As a bifunctional electrocatalyst, the Co@C-CoNC exhibits the best performance, showing a bifunctional ORR/OER overpotential (ΔE) of ~ 0.732 V, which is much lower than that of Fe@C-FeNC (~ 0.831 V) and Cu@C-CuNC (~ 1.411 V), as well as most of the robust bifunctional electrocatalysts reported to date. Synchrotron X-ray absorption spectroscopy and density functional theory simulations reveal that the strong electronic correlation between metallic Co nanoparticles and the atomic Co-N₄ sites in the Co@C-CoNC catalyst can increase the d-electron density near the Fermi level and thus effectively optimize the adsorption/desorption of intermediates in ORR/OER, resulting in an enhanced bifunctional electrocatalytic performance. The Co@C-CoNC-based rechargeable ZAB exhibited a maximum power density of 162.80 mW cm⁻² at 270.30 mA cm⁻², higher than the combination of commercial Pt/C + RuO₂ (~ 158.90 mW cm⁻² at 265.80 mA cm⁻²) catalysts. During the galvanostatic discharge at 10 mA cm⁻², the ZAB delivered an almost stable discharge voltage of 1.2 V for ~ 140 h, signifying the virtue of excellent bifunctional ORR/OER electrocatalytic activity.