Recent advances in two-dimensional nanomaterials as bifunctional electrocatalysts for full water splitting

Two-dimensional (2D) nanomaterials are some of the most promising bifunctional electrocatalysts that can simultaneously catalyze the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). They have attracted extensive attention owing to their decent electrocatalytic performance and cost advantages. In this review, we first provide an overview of 2D bifunctional electrocatalyst categories including layered double hydroxides, graphitic carbon nitride, metal-organic frameworks, transition metal dichalcogenides, and transition metal carbon/nitrogen compounds, and discuss their structures and synthesis methods, presenting detailed discussion on how the special 2D systems are synthesized and characterized experimentally, which complement a comprehensive understanding of the characteristics of 2D bifunctional electrocatalysts. Moreover, we focus on the recent development of 2D nano-catalysts with both HER and OER activity in the field of overall water splitting and summarize some typical strategies for enhancing the bifunctional activities, including doping, nanocomposite construction, interface formation, defect engineering and morphology modulation. The structure-activity relationships are also discussed with the aim of providing guidance in designing novel 2D bifunctional electrocatalysts. Finally, we explore some of the challenges and perspectives that remain in this rapidly growing field. We believe that the systematic and comprehensive contents of this review will stimulate wider research and attention.