Single Atoms for Energy Applications

Abstract

The atom is the smallest particle to maintain chemical properties of an element. Therefore, single-atom methods are practical way to maximize the chemical properties and reduce the usage of noble/expensive elements. To date, single-atom catalysts (SACs) are of great interest and significance for sustainable energy applications. In this special issue of Small Methods, we focus on the single-atom methods for energy applications. This special issue could be called the “smallest method.” In this issue, we mix reviews with articles in every aspect of single-atom methods and their applications in water splitting, batteries, nitrogen reduction, carbon dioxide reduction, and so on. SACs have to be anchored on supports to be isolated from other metal atoms. Single-atom catalysis is considered to be a hot spot for material science and practical applications. Since the stability of single atoms on supports is the foundation to achieve long-term stability and high performance, our group summarizes (1800497) advanced preparation methods for single metal atoms, with a particular focus on how to stabilize these single metal atoms against migration and aggregation. Understanding the interactions between the single-atom metal and the support is extremely important for the design and fabrication of highly stable SACs for improved electrocatalytic activity. The defect sites on supports help to stabilize single atoms since the defects can create more reaction sites. Wang's group summarizes (1800440) the effects of defects on supporting materials for designing and fabricating highly powerful single atomic electrocatalysts for practical applications. Metal organic framework (MOF)-derived carbon materials can be served as ideal supports to anchor atomically dispersed metal atoms, due to their tunable particle size and shape features, by providing high surface area, porosity, and thermal and chemical stability. Sun's group offers (1800471) a valuable insight into the current challenges and future opportunities for MOF-derived carbon-supported single-atom catalysts. Du's group (1800376) use the first-principle method to calculate 11 transition metal atoms supported on a graphdiyne (GDY) monolayer ([email protected], where TM represents a transition metal from Sc to Zn and Pt) as electrocatalysts. 2D materials recently have become a hot topic too. The suitable synthetic strategies and applications of single atoms on different 2D material supports are summarized by Fan (1800438) and Jang (1800492) respectively. Niu's group links (1800443) the fabrication strategies of single atoms on graphene and their electrochemical applications. Zhang et al. report (1800481) the recent progress of SACs on supports and synthetic strategies and the electrochemical application toward water splitting. For oxygen evolution reaction (OER), Lee's group describes (1800293) a simple leaching-derived synthesis of atomically dispersed gold on mesoporous cobalt oxides as highly efficient catalysts for OER. The gold contents can be reduced to lower than 1 wt% with twice as much current density than mesoporous cobalt oxide at a 400 mV overpotential. For oxygen reduction reaction (ORR), Jia's group report (1800450) non-noble metal electrocatalysts using a stable Co single atom with a content of about 1.52 wt% on mesoporous carbon materials exhibiting a high electrochemical performance with long-term stability for Zn–air batteries. This work also provides a new insight on simultaneous regulation of electronic structure and hierarchical morphology to boost ORR reactivity. Yao's group reveal (1800439) that the single Co species coordinated with the nitrogen in the carbon layers of the core–shell catalysts are the actual ORR active sites. Wang's group (1800315) proposes an atom vacancy interface (AVI) model based on a case study of atomically dispersed Co atoms distributed on 2H-MoS2 surfaces as promising catalysts for hydrodeoxygenation (HDO) reactions. The results show that the reactive single Co atom promotes the H2 activation and leads to largely increased sulfur vacancies adjacent to the metals and the formation of metal vacancy interfaces on the MoS2 surface. For battery applications, Zhang's group present (1800354) how atomically dispersed CoNx-doped graphene is exploited as a host to accommodate dendrite-free lithium deposits. The coordination between Co and N in the conductive matrix can effectively regulate the local electronic structure, and thereby enhance the adsorption of lithium ions and promote the following nucleation process. Meanwhile, the doped N facilitates high dispersion of Co atoms into graphene through the CoN coordination bond. The strong lithiophilicity provided by N coordinated with Co promises a uniform lithium nucleation behavior, further contributing to the smooth lithium deposition morphology. Ling et al. use (1800376) a high-throughput screening of catalysts for N2 reduction among (nitrogen-doped) graphene-supported single-atom catalysts to find that single W atoms embedded in graphene with three C atom coordination (WC3) exhibit the best performance with an extremely low onset potential of 0.25 V. Yan systematically presents the readers with the latest advances in this field, and more importantly, sheds light on the future development of nitrogen reduction reaction catalysis with the single atomic feature. Cheng et al. updates (1800440) the development of SACs for CO2RR, and their opportunities and challenges are discussed. Shen's group discusses the unique characteristics, including geometric and electronic properties, of single metal atom catalysts and reviews their most recent development in photocatalysis. In summary, the synthesis of SACs with excellent durability is highly desirable but challenging. It requires the combination of experiment, simulation, and practical applications. The lower usage of noble/expensive metal could help to pave a sustainable way for the future renewable and sustainable society. We are expecting to see the boost of the research on single atoms. Shu-Lei Chou is a Principal Research Fellow at ISEM in the University of Wollongong (UOW). He obtained his Bachelor's (1999) and Master's degree (2004) from Nankai University, China. He received his Ph.D. degreefrom UOW. His research has been focused on energy storage materials for battery applications, especially on novel composite materials, new binders, and new electrolytes for Li/Na-ion and metal–air batteries. Yuen Wu received his B.Sc. and Ph.D. degrees from the Department of Chemistry, Tsinghua University in 2009 and 2014 at Beijing (P. R. China), respectively. He is currently a professor in the Department of Chemistry, University of Science and Technology of China. His research interests are focused on the synthesis, assembly, characterization, and application exploration of functional nanomaterials. Qiang Zhang received his B.Sc. and Ph.D. degrees from Tsinghua University in 2004 and 2009 and then he stayed at the Case Western Reserve University, USA, and the Fritz Haber Institute of the Max Planck Society, Germany. He was appointed as a faculty member at Tsinghua University in 2011. His research focuses on energy materials such as Li–S batteries, Li metal anodes, 3D graphene, and electrocatalysts. Yong-Mook Kang completed his B.S. (1999), M.S. (2001), and Ph.D. (2004) in Korea Advanced Institute of Science and Technology. He has been a senior researcher in Samsung SDI Co., Ltd. He is now a professor at Department of Materials Science and Engineering in Korea University. His research area covers electrode or catalyst materials for Li rechargeable batteries and various post Li batteries, such as Li–air batteries, Na rechargeable batteries, etc.