# Two-Dimensional Metal-Organic Frameworks and Their Derivative Electrocatalysts for Water Splitting
## 1 Introduction  
The exacerbating issue of global pollution, coupled with the compelling necessity for an energy paradigm shift, has amplified the demand for renewable energy to decrease our dependence on fossil fuels. Among various alternatives, hydrogen energy [1; 2; 3] has been lauded as a potential panacea due to its superior energy density and eco-friendly nature. Electrocatalytic water splitting [4; 5; 6], which yields hydrogen from renewable energy sources like wind and sunlight, is an area of intense scientific investigation [7; 8; 9; 10; 11]. This process engages two fundamental reactions: hydrogen evolution reactions (HERs) [12; 13; 14] and oxygen evolution reactions (OERs) [15; 16; 17]. Despite the efficacy of these reactions, their inherent kinetics [18; 19] are relatively slow, necessitating catalysts to expedite the processes. Researchers have been meticulously exploring various high-performance catalysts to surmount this obstacle [20; 21; 22; 23; 24; 25; 26]. State-of-the-art catalysts such as Pt [27; 28] stand out due to their commendable electrical conductivity, high electrocatalytic activity, and near-zero Gibbs free energy, signifying spontaneous reaction. However, the prohibitively high cost and relative scarcity of these noble metals impede their broad-scale application in water electrolysis. Consequently, there is an urgent need to develop highly efficient, non-noble metal electrocatalysts [29; 30; 31] that can simultaneously offer high activity and durable longevity for sustained electrocatalysis. Achieving this aim could herald a new era of large-scale hydrogen production from renewable sources, catalyzing the transition towards a more sustainable energy ecosystem. Introduced in the last two decades, metal-organic frameworks (MOFs) are coordination polymers with intricate three-dimensional (3D) pore structures [32; 33; 34; 35; 36]. They aretypically composed of metal ions acting as binding nodes and organic ligands serving as linkers, which collectively form a crystalline, porous network. MOFs exhibit a suite of advantageous properties, including a large specific surface area, low density, high porosity, topological diversity, tunable pore size, regular pore channels, and tailorable functionality [37, 38]. Owing to their superior structural controllability and expansive surface area, MOFs are capable of effectively immobilizing enzymes. The tunable pore size and surface properties offered by these frameworks facilitate high efficiency and selectivity in bioseparation and purification processes. Their stable structure, high efficiency in carrier separation and transfer, along with abundant light absorption and catalytic sites render MOFs exceptional photocatalysts. Furthermore, the extensive specific surface area of MOF materials and the interaction between metal ions, ligands, and gases make them particularly suitable for gas adsorption. The versatile MOFs have shown immense potential across a range of applications, including capacitive energy storage [39, 40, 41], chemical sensing [42, 43, 44], biomedicine [45, 46, 47], gas separation [48, 49, 50], and storage [51, 52, 53]. Yet, the presence of metal nodes enveloped by organic ligands in MOFs often results in poor electrical conductivity, low metal utilization, and hindered ion diffusivity. These drawbacks have fueled research into augmenting the catalytic properties of MOFs and MOF composites through modification of their size, morphology, and composition. In the wake of the [PERSON] group's groundbreaking isolation of graphene, a single atomic layer of graphite, in 2004, two-dimensional (2D) materials [54, 55, 56, 57, 58, 59] have become a focal point of scientific attention. These materials boast extraordinary charge-carrier mobility, efficient thermal diffusion, and unique structural and electronic properties. In contrast to their one-dimensional or 3D counterparts, 2D nanomaterials [60, 61, 62, 63, 64, 65] offer expanded specific surface areas and increased exposed interfaces.