Single Atom Catalysts Based on Earth-Abundant Metals for Energy-Related Applications.

Anthropogenic activities related to population growth, economic development, technological advances, and changes in lifestyle and climate patterns result in a continuous increase in energy consumption. At the same time, the rare metal elements frequently deployed as catalysts in energy related processes are not only costly in view of their low natural abundance, but their availability is often further limited due to geopolitical reasons. Thus, electrochemical energy storage and conversion with earth-abundant metals, mainly in the form of single-atom catalysts (SACs), are highly relevant and timely technologies. In this review the application of earth-abundant SACs in electrochemical energy storage and electrocatalytic conversion of chemicals to fuels or products with high energy content is discussed. The oxygen reduction reaction is also appraised, which is primarily harnessed in fuel cell technologies and metal-air batteries. The coordination, active sites, and mechanistic aspects of transition metal SACs are analyzed for two-electron and four-electron reaction pathways. Further, the electrochemical water splitting with SACs toward green hydrogen fuel is discussed in terms of not only hydrogen evolution reaction but also oxygen evolution reaction. Similarly, the production of ammonia as a clean fuel via electrocatalytic nitrogen reduction reaction is portrayed, highlighting the potential of earth-abundant single metal species.

Ultrasound-Driven Defect Engineering in TiO2-x Nanotubes─Toward Highly Efficient Platinum Single Atom-Enhanced Photocatalytic Water Splitting.

Single-atom catalysts (SACs) have demonstrated superior catalytic activity and selectivity compared to nanoparticle catalysts due to their high reactivity and atom efficiency. However, stabilizing SACs within hosting substrates and their controllable loading preventing single atom clustering remain the key challenges in this field. Moreover, the direct comparison of (co-) catalytic effect of single atoms vs nanoparticles is still highly challenging. Here, we present a novel ultrasound-driven strategy for stabilizing Pt single-atomic sites over highly ordered TiO2 nanotubes. This controllable low-temperature defect engineering enables entrapment of platinum single atoms and controlling their content through the reaction time of consequent chemical impregnation. The novel methodology enables achieving nearly 50 times higher normalized hydrogen evolution compared to pristine titania nanotubes. Moreover, the developed procedure allows the decoration of titania also with ultrasmall nanoparticles through a longer impregnation time of the substrate in a very dilute hexachloroplatinic acid solution. The comparison shows a 10 times higher normalized hydrogen production of platinum single atoms compared to nanoparticles. The mechanistic study shows that the novel approach creates homogeneously distributed defects, such as oxygen vacancies and Ti3+ species, which effectively trap and stabilize Pt2+ and Pt4+ single atoms. The optimized platinum single-atom photocatalyst shows excellent performance of photocatalytic water splitting and hydrogen evolution under one sun solar-simulated light, with TOF values being one order of magnitude higher compared to those of traditional thermal reduction-based methods. The single-atom engineering based on the creation of ultrasound-triggered chemical traps provides a pathway for controllable assembling stable and highly active single-atomic site catalysts on metal oxide support layers.

Magnetic Polaron States in Photoluminescent Carbon Dots Enable Hydrogen Peroxide Photoproduction.

Photoactivation of aspartic acid-based carbon dots (Asp-CDs) induces the generation of spin-separated species, including electron/hole (e- /h+ ) polarons and spin-coupled triplet states, as uniquely confirmed by the light-induced electron paramagnetic resonance spectroscopy. The relative population of the e- /h+ pairs and triplet species depends on the solvent polarity, featuring a substantial stabilization of the triplet state in a non-polar environment (benzene). The electronic properties of the photoexcited Asp-CDs emerge from their spatial organization being interpreted as multi-layer assemblies containing a hydrophobic carbonaceous core and a hydrophilic oxygen and nitrogen functionalized surface. The system properties are dissected theoretically by density functional theory in combination with molecular dynamics simulations on quasi-spherical assemblies of size-variant flakelike model systems, revealing the importance of size dependence and interlayer effects. The formation of the spin-separated states in Asp-CDs enables the photoproduction of hydrogen peroxide (H2 O2 ) from water and water/2-propanol mixture via a water oxidation reaction.