Carbon-Dot-Enhanced Electrocatalytic Hydrogen Evolution

ConspectusHydrogen evolution by the electrolysis of water is an important energy conversion process, especially for some intermittent energy systems. However, its practical applicability is limited by its low energy-conversion efficiency and the need for expensive electrocatalysts. In response, carbon-based nanocomposites have gained substantial interest as promising alternatives to the currently used precious metal catalysts. Among them, carbon dots (CDs) have exhibited many outstanding features, including flexible composition, high conductivity, good dispersion, and strong metal coordination. The past decade has seen remarkable advances in CD-based electrocatalysts for hydrogen evolution, where CDs and their derived hybrids have shown impressive performance and indispensable prospects for future hydrogen utilization. However, the widespread use of CD-based electrocatalysts still requires the development of high-quality CDs and advanced synthesis strategies. New assessment techniques are also required to elucidate the unique functions of CDs in composition regulation, structure fabrication, surface modification, and host–guest interactions in electrocatalysts and ultimately to establish the relationships among structure, composition, and activity.This Account is based on previous studies by our group and focuses on the synthesis of CDs and the types of CDs that make excellent electrocatalysts for hydrogen evolution. It draws on evidence from a range of advanced characterization techniques such as aberration-corrected scanning transmission electron microscopy, X-ray absorption spectroscopy, X-ray photoelectron spectroscopy, in situ Fourier transform infrared spectroscopy, and atomic force microscopy. As a result, a general route to fabricating efficient CD-based electrocatalysts for hydrogen evolution is reported, and the structure–activity correlations of CD-based electrocatalysts are demonstrated. Heteroatomic modification of electrocatalysts via composition regulation of CDs is also investigated. For example, first-principles density functional theory is used to investigate the unique properties of molybdenum phosphide modified with nitrogen-doped CDs: the introduction of nitrogen atoms is shown to generate carbon vacancies, thus reducing the coordination number of reaction sites and enhancing their electrocatalytic activity. The effect of the CDs' spatial confinement due to self-assembly is also elucidated, where the splicing of CDs and novel CD–metal interfacial effects also greatly stabilize the metal active components and improve their catalytic efficiency. Finally, we offer some of our knowledge and insights on the current challenges and future research directions in this field from the perspectives of CD growth mechanisms, electrocatalyst synthesis, performance optimization, and expanding applicability.