crisis, [1] which was proposed by Fujishima for the first time by employing TiO 2 as a photoanode to oxidize water into oxygen and hydrogen under sunlight. [2] Despite the great progress in the development of photoanodes, the solar-to-chemical energy conversion efficiency still suffers from the low number of electron-hole pairs and ineffective charge transfer ability. An effective solution to PEC research is developing novel, highly active, and efficient photoanodes materials. Among various 2D functional materials, carbon nitride (g-C 3 N 4 ) is a great metal-free graphitic alternative as photoanode material for PEC water oxidation devices due to its proper band structure, stability, simple preparation, and low cost. [3][4][5] However, owing to its narrow spectral absorption (the bandgap is ≈2.7 eV), high recombination of photocarriers (due to its slow holes transfer rate) and sluggish interfacial kinetics for O 2 production, the application of pure g-C 3 N 4 as a photoanode for water oxidation has still remained a challenge. [4,6,7] Recently, construction heterojunction [8][9][10] with a secondary semiconductor such as TiO 2 , [11] SnO 2 , [12] CdS, [13] and BiVO 4 [14,15] is an effective strategy to improve the performance of g-C 3 N 4 based photoanodes. Although the success has been achieved from the above heterojunctions, their stabilities were still not well controllable. 2D/2D heterojunctions which can strongly couple with g-C 3 N 4 by faceto-face interface provide a more effective way to improve the PEC water splitting performance. [16] Therefore, it is imperative to find appropriate materials, which can strongly couple with g-C 3 N 4 in order to improve the photogenerated holes transfer and accelerate the oxygen evolution reaction (OER) kinetics.Graphdiyne (GDY) is a novel carbon allotrope due to its sp and sp 2 -hybridized π bonds carbon network and it has a large cavity structure [17] endowing the bandgap and anticipated high charge carrier mobility [18] with excellent catalytic performances. [19][20][21][22][23] Previously, GDY has been incorporated into BiVO 4[24] and g-C 3 N 4 materials and employed as photocathodes, [7] aimed to improve the hole mobility and thus catalyzing hydrogen evolution. However, the hole extraction property of GDY for improving the performance of OER devices has not been studied yet. Recently, our group synthesized GDY, which possesses suitable bandgap (≈2.1 eV) band alignment, and analogous π-conjugated plane to g-C 3 N 4 . [25,26] Recently discovered graphdiyne (GDY) is a unique two-dimensional (2D) planar structure with a high-degree π-conjunction network composed of sp and sp 2 hybridized carbon bonds. GDY has high carrier mobility, rich chemical bond properties, and having a bandgap of ≈2.1 eV. In this study, for the first time, g-C 3 N 4 /NiFe-layered double hydroxide (LDH) is decorated by GDY to obtain a new strongly coupled ternary nanocomposite g-C 3 N 4 /GDY/NiFe-LDH, which has hierarchical mesoporous layered structure, large surface area, and broad visible spectrum...