2D Metal–Organic Frameworks as Competent Electrocatalysts for Water Splitting

Abstract

water splitting (2H 2 O → 2H 2 + O 2 ) comprises two half-reactions: hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Theoretically, the decomposition voltage of water splitting is 1.23 V. [4] Nevertheless, the actual voltage is greater than the theoretical voltage of water dissociation due to the large activation energy. [5] To accelerate the sluggish HER and OER kinetics, efficient electrocatalysts are required to reduce kinetic energy barriers. Currently, the most efficient electrocatalysts toward water splitting are precious metal-based catalysts such as Pt/C for HER and RuO 2 or IrO 2 for OER. [6] Unfortunately, the large-scale application is hindered by the high price and scarcity of noble metals. Therefore, it is urgent to seek highly active and inexpensive electrocatalysts.Metal-organic frameworks (MOFs) are an emerging class of porous crystalline materials constructed by metal ions or clusters and organic ligands. [7][8][9] Due to their unique merits of large porosities, diversified structures, and designable compositions, MOFs have gained extensive attention in various applications, such as chemical sensors, [10] gas absorption, [11] and energy conversion/storage. [12][13][14][15][16] From an electrochemical perspective, MOFs can be perceived as a spatial architecture, where discrete functional units can be designed to lie in close proximity to facilitate bond breaking/forming reactions. [17] Generally, active sites and conductivity are considered as the key factors affecting the electrochemical performance of HER and OER catalysts. Despite abundant intrinsic molecular metal sites, large size and poor conductivity (10 −10 S cm −1 ) of bulk MOFs restrict the full use of their distinct advantages and lead to poor electrochemical performance. [18] To improve electrocatalytic activity of MOFs, they can be used as sacrificial precursors or templates to fabricate various conductive carbides, [19] oxides/hydroxides, [20] phosphides, [21] chalcogenides, [22] single-atom catalysts [23] or pure carbon materials with rich morphologies and sizes. [24,25] However, the high temperatures calcination inevitably leads to the loss of intrinsic active sites and long-range orders in MOFs. Therefore, improving the catalytic properties of pristine MOFs for highly efficient electrocatalysts is quite appealing.Compared with bulk MOFs, 2D MOFs emerge as a category of promising electrocatalysts toward HER or OER due to their unique characteristics, including enlarged surface areas, rapid mass transport, and enhanced conductivities (Figure 1). [26,27] Hydrogen, a clean and flexible energy carrier, can be efficiently produced by electrocatalytic water splitting. To accelerate the sluggish hydrogen evolution reaction and oxygen evolution reaction kinetics in the splitting process, highly active electrocatalysts are essential for lowering the energy barriers, thereby improving the efficiency of overall water splitting. Combining the distinctive advantages of metal-organic frameworks (MOFs) with the physicochemi...

Ultrafine Pt Nanoparticles Anchored on 2D Metal−Organic Frameworks as Multifunctional Electrocatalysts for Water Electrolysis and Zinc–Air Batteries

“…There is no obvious MOF characteristic peaks in the XRD pattern of the post‐OER Pt@Fe‐MOF (Figure S17, Supporting Information), corroborating a phase transition from crystalline materials to amorphous structure or oxyhydroxides, which could function as the highly active phase to catalyze OER. [ 40 ]…”

Electrocatalysts for the oxygen evolution reaction: mechanism, innovative strategies, and beyond

“…94,95 The unique phase interface structure fosters bond cooperation, which has several benefits, including accelerating electron transmission, enhancing catalyst conductivity, boosting hydrophilicity, and augmenting the active electrochemical area. [96][97][98] The varied band structures present at the phase interface induce charge transfer between different phases, regulating interface electrons and reducing the adsorption energy of catalytic reactions. Furthermore, the strain and synergistic effects arising from the crystal structure of the phase interface influence the catalyst's adsorption energy to the intermediate, further enhancing the electrocatalytic activity of the OER.…”

Rational design of efficient electrocatalysts for hydrogen production by water electrolysis at high current density

“…It is worth noting that metal–organic frameworks (MOFs) and derived materials show potential for achieving the simultaneous optimization of a multilevel structure. 129–131…”