www.advenergymat.de REVIEW 1900624 (1 of 48) © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Single Atoms and Clusters Based Nanomaterials for Hydrogen Evolution, Oxygen Evolution Reactions, and Full Water Splitting Siraj Sultan, Jitendra N. Tiwari,* Aditya Narayan Singh, Shynggys Zhumagali, Miran Ha, Chang Woo Myung, Pandiarajan Thangavel, and Kwang S. Kim* DOI: 10.1002/aenm.201900624 1. Introduction The continuous growth in global energy demand, the rapid consumption of conventional coals, oils, and fossil fuels and its associated environmental issues are prompting intense research interest in developing alternative energy systems. [1,2] In particular, geopolitical anxieties and rising threats in climate variation inspire movement toward renewable, stable, ecof- riendly, and benign energy sources. The current global energy The sustainable and scalable production of hydrogen through hydrogen evo- lution reaction (HER) and oxygen through oxygen evolution reaction (OER) in water splitting demands efficient and robust electrocatalysts. Currently, state-of-the-art electrocatalysts of Pt and IrO 2 /RuO 2 exhibit the benchmark catalytic activity toward HER and OER, respectively. However, expanding their practical application is hindered by their exorbitant price and scarcity. There- fore, the development of alternative effective electrocatalysts for water split- ting is crucial. In the last few decades, substantial effort has been devoted to the development of alternative HER/OER and water splitting catalysts based on various transition metals (including Fe, Co, Ni, Mo, and atomic Pt) which show promising catalytic activities and durability. In this review, after a brief introduction and basic mechanism of HER/OER, the authors systematically discuss the recent progress in design, synthesis, and application of single atom and cluster-based HER/OER and water splitting catalysts. Moreover, the crucial factors that can tune the activity of catalysts toward HER/OER and water splitting such as morphology, crystal defects, hybridization of metals with nonmetals, heteroatom doping, alloying, and formation of metals inside graphitic layered materials are discussed. Finally, the existing challenges and future perspectives for improving the performance of electrocatalysts for water splitting are addressed. Electrocatalysts S. Sultan, Prof. J. N. Tiwari, A. N. Singh, S. Zhumagali, M. Ha, Dr. C. W. Myung, P. Thangavel, Prof. K. S. Kim Center for Superfunctional Materials Department of Chemistry Ulsan National Institute of Science and Technology (UNIST) 50 UNIST-gil, Ulsan 44919, South Korea E-mail: jitendra@unist.ac.kr; kimks@unist.ac.kr The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/aenm.201900624. demand is around 15 terawatts (TW), and the solar sun can provide higher than 50 TW energy in the light irradiance form. Since other renewable energy resource are limited and less than our present and foreseeable future social requirements, the sun represents the most plentiful source of energy that can provide enough power which is higher than the human- ity’s needs. The existing technologies and photovoltaic devices are converting light energy of sun into an electrical energy, however, in each day the sun only shines to a few hours, and due to shadowing the light from sun is distributed unevenly across the earth’s surface. Additionally, the energy from sun rays is scattered over the entire earth surface and fairly decreases their power on the basis of area. Therefore, due to the diffuse and intermittent nature of the sun’s energy, alternative methods and devices for concentrating and storing of this light energy are necessitated. Water splitting is considered to be a promising approach which can resolve the looming energy and environmental crisis because it produces clean hydrogen energy with zero CO 2 emission. [3,4] Water splitting, a combination of two half-cell reactions being hydrogen evolution reaction (HER) at a cathode and oxygen evolution reaction (OER) at an anode to produce hydrogen and oxygen gases that can be recombined and converted into electricity at the point of usage in a fuel cell, provides a promising and environmental friendly pathway for conversion and storage of these clean renewable energy. [5] But, in practice the water-splitting requires a larger thermodynamic potential due to some kinetic barriers happening at both HER and OER sides. [5] This large thermodynamic equilibrium poten- tial can be overcome by decorating the surface of the electrode with some active catalytic materials. Currently, benchmark commercial Pt/C is used for HER, and benchmark IrO 2 /RuO 2 electrocatalysts for OER. [6,7] However, their rocketing price and scarcity are major obstacles toward their practical implemen- tation. [6–8] Therefore, it is highly desirable to explore low cost, efficient, and stable catalysts especially designed from earth- abundant metals for both photo and nonphoto water splitting. In this regard, the research and development of earth-abundant materials for HER/OER and whole water splitting catalysts Adv. Energy Mater. 2019, 1900624