Two-dimensional metal-organic frameworks represent a family of materials with attractive chemical and structural properties, which are usually prepared in the form of bulk powders. Here we show a generic approach to fabricate ultrathin nanosheet array of metal-organic frameworks on different substrates through a dissolution–crystallization mechanism. These materials exhibit intriguing properties for electrocatalysis including highly exposed active molecular metal sites owning to ultra-small thickness of nanosheets, improved electrical conductivity and a combination of hierarchical porosity. We fabricate a nickel-iron-based metal-organic framework array, which demonstrates superior electrocatalytic performance towards oxygen evolution reaction with a small overpotential of 240 mV at 10 mA cm−2, and robust operation for 20,000 s with no detectable activity decay. Remarkably, the turnover frequency of the electrode is 3.8 s−1 at an overpotential of 400 mV. We further demonstrate the promise of these electrodes for other important catalytic reactions including hydrogen evolution reaction and overall water splitting.

To address the aggravating energy and environment issues, the cheap, highly active, and durable electrocatalysts as noble metal substitutes both at the anode and cathode are actively pursued. Among them, heteroatom-doped graphene-based materials show extraordinary electrocatalytic performance, some even close to or outperforming the state-of-the-art noble metals such as Pt and IrO 2 -based materials. This review provides a concise appraisal on graphene doping methods, the possible doping configurations and their unique electrochemical properties, including single-and double-doping with N, B, S, and P. In addition, heteroatom-doped graphene-based materials are reviewed as electrocatalysts for oxygen reduction (ORR), hydrogen evolution (HER), and oxygen evolution reactions (OER) in terms of their electrocatalytic mechanisms and performance. Significantly, three-dimensional (3D) heteroatom-doped graphene structures have been discussed, especially those that can be directly utilized as catalyst electrodes without extra binders and supports.

Pt-free electrocatalysts for hydrogen evolution reaction (HER) with high activity and low price are desirable for many state-of-the-art renewable energy devices, such as water electrolysis and photoelectrochemical water splitting cells. However, the design and fabrication of such materials remain a significant challenge. This work reports the preparation of a flexible three-dimensional (3D) film by integrating porous C3N4 nanolayers with nitrogen-doped graphene sheets, which can be directly utilized as HER catalyst electrodes without substrates. This nonmetal electrocatalyst has displayed an unbeatable HER performance with a very positive onset-potential close to that of commercial Pt (8 mV vs 0 mV of Pt/C, vs RHE @ 0.5 mA cm(-2)), high exchange current density of 0.43 mA cm(-2), and remarkable durability (seldom activity loss >5000 cycles). The extraordinary HER performance stems from strong synergistic effect originating from (i) highly exposed active sites generated by introduction of in-plane pores into C3N4 and exfoliation of C3N4 into nanosheets, (ii) hierarchical porous structure of the hybrid film, and (iii) 3D conductive graphene network.

A three-dimensional (3D) electrode composed of nitrogen, oxygen dualdoped graphene-carbon nanotube hydrogel film is fabricated, which greatly favors the transport and access of gas and reaction intermediates, and shows a remarkable oxygen-evolution catalytic performance in both alkaline and acidic solutions.

A highly hydrated structure was fabricated for catalyzing the oxygen evolution reaction (OER), which demonstrated significantly enhanced catalytic activity, favorable kinetics, and strong durability. The enhanced performance is correlated with the dual-active-site mechanism, and high hydrophilicity of the electrode can dramatically expedite the process of water oxidation into molecular oxygen.

Low efficiency and poor stability are two major challenges we encounter in the exploration of non-noble metal electrocatalysts for the hydrogen evolution reaction (HER) in both acidic and alkaline environment. Herein, the hybrid of cobalt encapsulated by N, B codoped ultrathin carbon cages (Co@BCN) is first introduced as a highly active and durable nonprecious metal electrocatalysts for HER, which is constructed by a bottom-up approach using metal organic frameworks (MOFs) as precursor and self-sacrificing template. The optimized catalyst exhibited remarkable electrocatalytic performance for hydrogen production from both both acidic and alkaline media. Stability investigation reveals the overcoating of carbon cages can effectively avoid the corrosion and oxidation of the catalyst under extreme acidic and alkaline environment. Electrochemical active surface area (EASA) evaluation and density functional theory (DFT) calculations revealed that the synergetic effect between the encapsulated cobalt nanoparticle and the N, B codoped carbon shell played the fundamental role in the superior HER catalytic performance.

A three-dimensional (3D) catalyst was fabricated by using N-doped graphene films as scaffolds and nickel nanoparticles as building blocks via a heterogeneous reaction process. This unique structure enables high catalyst loadings and optimal electrode contact, leading to a surprisingly high catalytic activity towards OER, which almost approaches that of the state-of-the-art precious OER electrocatalysts (IrO 2 ). Moreover, the catalytic process features favourable electrode kinetics and strong durability during long-term cycling. The dual-active-site mechanism was proposed for this 3D catalyst, i.e., Ni/NiOOH and Ni-N(O)-C are both active sites. The enhanced performance is attributed to synergistic effects of N-doped graphene and Ni, which enhance the activities of both components for OER. Broader contextOxygen evolution reaction (OER) is coupled with a number of key renewable energy systems like solar cells, metal-air batteries and water splitting. OER proceeds through a multistep electron transfer process, and is kinetically sluggish. Recently, three-dimensional (3D) macroscopic catalysts have attracted considerable interest due to their multiple advantages such as high catalyst loadings and excellent electrode contact. However, challenges with current 3D catalysts are their low efficiency and inferior durability. This work presents a new class of 3D electrodes for catalyzing OER. It is shown that the incorporation of nickel nanoparticles into N-doped graphene lms results in 3D catalysts that exhibit remarkable catalytic efficiency as well as high stability.

As substitutes for precious cathodic Pt/C and anodic IrO2 in electrolytic water splitting cells, a bifunctional catalyst electrode (Fe- and O-doped Co2P grown on nickel foam) has been fabricated by manipulating the cations and anions of metal compounds. The modified catalyst electrode exhibits both superior HER and OER performances with high activity, favorable kinetics, and outstanding durability. The overall ability toward water splitting is especially extraordinary, requiring a small overpotential of 333.5 mV to gain a 10 mA cm(-2) current density. A study on the electrocatalytic mechanism reveals that the atomic modulation between cation and anion plays an important role in optimizing the electrocatalytic activity, which greatly expands the active sites in the electrocatalyst. Further, the three-dimensional conductive porous network is highly advantageous for the exposure of active species, the transport of bubble products, and the transfer of electrons and charges, which substantially boosts reaction kinetics and structure stability.