Metal-organic framework (MOF) nanoparticles, also called porous coordination polymers, are a major part of nanomaterials science, and their role in catalysis is becoming central. The extraordinary variability and richness of their structures afford engineering synergies between the metal nodes, functional linkers, encapsulated substrates, or nanoparticles for multiple and selective heterogeneous interactions and activations in these MOF-based nanocatalysts. Pyrolysis of MOF-nanoparticle composites forms highly porous N- or P-doped graphitized MOF-derived nanomaterials that are increasingly used as efficient catalysts especially in electro- and photocatalysis. This review first briefly summarizes this background of MOF nanoparticle catalysis and then comprehensively reviews the fast-growing literature reported during the last years. The major parts are catalysis of organic and molecular reactions, electrocatalysis, photocatalysis, and views of prospects. Major challenges of our society are addressed using these well-defined heterogeneous catalysts in the fields of synthesis, energy, and environment. In spite of the many achievements, enormous progress is still necessary to improve our understanding of the processes involved beyond the proof-of-concept, particularly for selective methane oxidation, hydrogen production, water splitting, CO2 reduction to methanol, nitrogen fixation, and water depollution.

State of the Art and Prospects in Metal-Organic Framework (MOF)-Based and MOF-Derived Nanocatalysis.

The number of research reports published in recent years on electrochemical water splitting for hydrogen generation is higher than for many other fields of energy research. In fact, electrochemical water splitting, which is conventionally known as water electrolysis, has the potential to meet primary energy requirements in the near future when coal and hydrocarbons are completely consumed. Due to the sudden and exponentially increasing attention on this field, many researchers across the world, including our group, have been exerting immense efforts to improve the electrocatalytic properties of the materials that catalyze the oxygen evolution reaction (OER) at the anode and the hydrogen evolution reaction (HER) at the cathode, aided by the recent revolutionary discovery of nanomaterials. However, the pressure on the researchers to publish their findings rapidly has caused them to make many unnoticed and unintentional errors, which is mainly due to lack of clear insight on the activity parameters. In this perspective, we have discussed the use and validity of ten important parameters, namely overpotential at a defined current density, iR-corrected overpotential at a defined current density, Tafel slope, exchange current density (j0), mass activity, specific activity, faradaic efficiency (FE), turnover frequency (TOF), electrochemically active surface area (ECSA) and measurement of double layer capacitance (Cdl) for different electrocatalytic materials that are frequently employed in both OER and HER. Experimental results have also been provided in support of our discussions wherever required. Using our critical assessments of the activity parameters of water splitting electrocatalysis, researchers can ensure precision and correctness when presenting their data regarding the activity of an electrocatalyst.

Precision and correctness in the evaluation of electrocatalytic water splitting: revisiting activity parameters with a critical assessment

A decade after the first report, the family of two-dimensional (2D) carbides and nitrides (MXenes) includes structures with three, five, seven, or nine layers of atoms in an ordered or solid solution form. Dozens of MXene compositions have been produced, resulting in MXenes with mixed surface terminations. MXenes have shown useful and tunable electronic, optical, mechanical, and electrochemical properties, leading to applications ranging from optoelectronics, electromagnetic interference shielding, and wireless antennas to energy storage, catalysis, sensing, and medicine. Here we present a forward-looking review of the field of MXenes. We discuss the challenges to be addressed and outline research directions that will deepen the fundamental understanding of the properties of MXenes and enable their hybridization with other 2D materials in various emerging technologies.

The world of two-dimensional carbides and nitrides (MXenes)

The emergence of metal-organic frameworks (MOFs) as a new class of crystalline porous materials is attracting considerable attention in many fields such as catalysis, energy storage and conversion, sensors, and environmental remediation due to their controllable composition, structure and pore size. MOFs are versatile precursors for the preparation of various forms of nanomaterials as well as new multifunctional nanocomposites/hybrids, which exhibit superior functional properties compared to the individual components assembling the composites. This review provides an overview of recent developments achieved in the fabrication of porous MOF-derived nanostructures including carbons, metal oxides, metal chalcogenides (metal sulfides and selenides), metal carbides, metal phosphides and their composites. Finally, the challenges and future trends and prospects associated with the development of MOF-derived nanomaterials are also examined.

Nanoarchitectured Design of Porous Materials and Nanocomposites from Metal-Organic Frameworks.

Complex nano-architectures with ordered two-dimensional (2D) building blocks are a class of promising electrocatalysts for different electrochemical technologies. In this work, a novel template-engaged strategy followed by sequential etching and phosphorization treatments is demonstrated to fabricate open and hierarchical Ni–Co–P hollow nanobricks (HNBs) via the assembly of oriented 2D nanosheets. Benefiting from the unique nano-architectures with large electrolyte-accessible surface and abundant mass diffusion pathways, the as-prepared Ni–Co–P HNBs exhibit high electrocatalytic activity, which affords the current density of 10 mA cm−2 at low overpotentials of 270 mV and 107 mV for oxygen and hydrogen evolution reactions respectively, and excellent stability in an alkaline medium. Remarkably, when used as both the anode and cathode, a low cell voltage of 1.62 V is required to reach the current density of 10 mA cm−2, making the Ni–Co–P HNBs an efficient bifunctional electrocatalyst for overall water splitting.

Construction of hierarchical Ni–Co–P hollow nanobricks with oriented nanosheets for efficient overall water splitting

It is of prime importance to develop dual-functional electrocatalysts with good activity for overall water splitting, which remains a great challenge. Herein, we report the synthesis of a Co-doped nickel selenide (a mixture of NiSe2 and Ni3Se4)/C hybrid nanostructure supported on Ni foam using a metal–organic framework as the precursor. The resulting catalyst exhibits excellent catalytic activity toward the oxygen evolution reaction (OER), which only requires an overpotential of 275 mV to drive a current density of 30 mA cm−2. This overpotential is much lower than those reported for precious metal free OER catalysts. The hybrid is also capable of catalyzing the hydrogen evolution reaction (HER) efficiently. A current density of −10 mA cm−2 can be achieved at 90 mV. In addition, such a hybrid nanostructure can achieve 10 and 30 mA cm−2 at potentials of 1.6 and 1.71 V, respectively, along with good durability when functioning as both the cathode and the anode for overall water splitting in basic media.

MOF-derived Co-doped nickel selenide/C electrocatalysts supported on Ni foam for overall water splitting

Herein we report the control synthesis of lepidocrocite VOOH hollow nanospheres and further their applications in electrocatalytic water splitting for the first time. By tuning the surface area of the nanospheres, the optimal performance can be achieved with low overpotentials of 270 mV for the oxygen evolution reaction (OER) and 164 mV for the hydrogen evolution reaction (HER) at 10 mA cm−2 in 1 m KOH, respectively. Furthermore, when used as both the anode and cathode for overall water splitting, a low cell voltage of 1.62 V is required to reach the current density of 10 mA cm−2, making the VOOH hollow nanospheres an efficient alternative to water splitting.

Efficient Overall Water-Splitting Electrocatalysis Using Lepidocrocite VOOH Hollow Nanospheres.

Solution Growth of Vertical VS2 Nanoplate Arrays for Electrocatalytic Hydrogen Evolution

MXene (e.g., Ti3 C2 ) represents an important class of two-dimensional (2D) materials owing to its unique metallic conductivity and tunable surface chemistry. However, the mainstream synthetic methods rely on the chemical etching of MAX powders (e.g., Ti3 AlC2 ) using hazardous HF or alike, leading to MXene sheets with fluorine termination and poor ambient stability in colloidal dispersions. Here, we demonstrate a fluoride-free, iodine (I2 ) assisted etching route for preparing 2D MXene (Ti3 C2 Tx , T=O, OH) with oxygen-rich terminal groups and intact lattice structure. More than 71 % of sheets are thinner than 5 nm with an average size of 1.8 μm. They present excellent thin-film conductivity of 1250 S cm-1 and great ambient stability in water for at least 2 weeks. 2D MXene sheets with abundant oxygen surface groups are excellent electrode materials for supercapacitors, delivering a high gravimetric capacitance of 293 F g-1 at a scan rate of 1 mV s-1 , superior to those made from fluoride-based etchants (<290 F g-1 at 1 mV s-1 ). Our strategy provides a promising pathway for the facile and sustainable production of highly stable MXene materials.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/anie.202015627

Ambient-Stable Two-Dimensional Titanium Carbide (MXene) Enabled by Iodine Etching.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/adfm.202002664

Huanhuan Shi

Papers

MOF-derived Co-doped nickel selenide/C electrocatalysts supported on Ni foam for overall water splitting

Efficient Overall Water-Splitting Electrocatalysis Using Lepidocrocite VOOH Hollow Nanospheres.

Solution Growth of Vertical VS2 Nanoplate Arrays for Electrocatalytic Hydrogen Evolution

Ambient-Stable Two-Dimensional Titanium Carbide (MXene) Enabled by Iodine Etching.

Phthalocyanine‐Based 2D Conjugated Metal‐Organic Framework Nanosheets for High‐Performance Micro‐Supercapacitors