Guoliang Xia

Non-precious alloy encapsulated in nitrogen-doped graphene layers derived from MOFs as an active and durable hydrogen evolution reaction catalyst

Non-precious metal based catalysts are emerging as the most promising alternatives to Pt-based ones for hydrogen evolution reaction (HER) due to their low cost and rich reserves. However, their low efficiency and stability due to inherent corrosion and oxidation in acid media are the main barriers blocking sustainable hydrogen production. Metal–organic frameworks, with both designable metal ion centers and organic ligands, are promising precursors for the one-step synthesis of metal/alloy@carbon composites for HER. Herein, we synthesized FeCo alloy nanoparticles encapsulated in highly nitrogen-doped (8.2 atom%) graphene layers by direct annealing of MOF nanoparticles at 600 °C in N2. The catalyst shows a low onset overpotential (88 mV) and an overpotential of only 262 mV at 10 mA cm−2. Besides, it exhibits an excellent long-term durability performance even after 10 000 cycles due to the protection of the graphene layers. Our density functional theory calculations reveal that the nitrogen dopants can provide adsorption sites for H* and the appropriate increase of nitrogen will decrease ΔGH* for HER. Besides, the unique structure of the metal and graphene composites derived from MOFs can also decrease ΔGH* thereby promoting the catalytic activity.

Ruthenium-cobalt nanoalloys encapsulated in nitrogen-doped graphene as active electrocatalysts for producing hydrogen in alkaline media

The scalable production of hydrogen could conveniently be realized by alkaline water electrolysis. Currently, the major challenge confronting hydrogen evolution reaction (HER) is lacking inexpensive alternatives to platinum-based electrocatalysts. Here we report a high-efficient and stable electrocatalyst composed of ruthenium and cobalt bimetallic nanoalloy encapsulated in nitrogen-doped graphene layers. The catalysts display remarkable performance with low overpotentials of only 28 and 218 mV at 10 and 100 mA cm−2, respectively, and excellent stability of 10,000 cycles. Ruthenium is the cheapest platinum-group metal and its amount in the catalyst is only 3.58 wt.%, showing the catalyst high activity at a very competitive price. Density functional theory calculations reveal that the introduction of ruthenium atoms into cobalt core can improve the efficiency of electron transfer from alloy core to graphene shell, beneficial for enhancing carbon–hydrogen bond, thereby lowing ΔGH* of HER.

Co3ZnC/Co nano heterojunctions encapsulated in N-doped graphene layers derived from PBAs as highly efficient bi-functional OER and ORR electrocatalysts

The oxygen evolution reaction (OER) is a key half reaction involved in electrochemical water splitting. However, it is kinetically not favored and hence requires highly active electrocatalysts. Non-precious metal based catalysts have emerged as the most promising materials for the replacement of state-of-the-art noble metal catalysts. However, more progress is still needed to improve the activity and stability of these catalysts. Herein, we synthesized Co3ZnC/Co nanojunctions encapsulated in highly nitrogen doped graphene layers by one-step annealing of Prussian blue analogues (PBA). The catalyst shows a low overpotential of only 366 mV at 10 mA cm−2, which is superior to state-of-the-art commercial RuO2 catalysts. Besides, it exhibits a better performance compared with similar metallic carbide-based catalysts towards ORR with an onset potential and cathodic peak potential at 0.912 V and 0.814 V, respectively.

Preparation of porous MoO2@C nano-octahedrons from a polyoxometalate-based metal–organic framework for highly reversible lithium storage

Molybdenum oxide has been investigated as a host material for lithium-ion batteries due to its high theoretical capacity (838 mA h g−1), low electrical resistivity, high stability and high electrochemical activity toward lithium. However, a dramatic volume expansion occurred during the charge/discharge process, which results in anode pulverization and thereafter a rapid capacity fading and limits its application as an anode material. Herein, we report the preparation of MoO2 nanoparticles embedded in a porous octahedral carbon matrix (denoted as MoO2@C) by using a polyoxometalate-based metal–organic framework (PMOF) as the precursor. This facile strategy ensures the in situ formation of a porous carbon matrix which could increase active sites to store redox ions and enhance the ionic diffusivity of the encapsulated MoO2 nanoparticles. Benefiting from such unique structures, the MoO2@C composite is capable of delivering a high reversible specific capacity of 1442 mA h g−1 after 50 cycles at a current density of 100 mA g−1 and 443.8 mA h g−1 after 850 cycles at a current density of 1000 mA g−1 as an anode material.

Active and Durable Hydrogen Evolution Reaction Catalyst Derived from Pd-Doped Metal–Organic Frameworks

The water electrolysis is of critical importance for sustainable hydrogen production. In this work, a highly efficient and stable PdCo alloy catalyst (PdCo@CN) was synthesized by direct annealing of Pd-doped metal-organic frameworks (MOFs) under N2 atmosphere. In 0.5 M H2SO4 solution, PdCo@CN displays remarkable electrocatalytic performance with overpotential of 80 mV, a Tafel slope of 31 mV dec(-1), and excellent stability of 10 000 cycles. Our studies reveal that noble metal doped MOFs are ideal precursors for preparing highly active alloy electrocatalysts with low content of noble metal.

Pt-like electrocatalytic behavior of Ru–MoO2 nanocomposites for the hydrogen evolution reaction

The design and development of inexpensive highly efficient electrocatalysts are critically important for their practical applications in the hydrogen evolution reaction (HER). Plain Ru and MoO2 are not very active for the HER under alkaline conditions. In this work, we report the preparation of Ru–MoO2 nanocomposites via a facile in situ carburization of a Ru modified Mo-based metal–organic framework. The nanocomposites exhibited very low overpotential and superior stability to achieve 10 mA cm−2 under both acidic and alkaline conditions (55 mV in 0.5 M H2SO4 and 29 mV in 1 M KOH). Their excellent performance under alkaline conditions was even better than that of 20% Pt/C. Our experimental and computational (DFT) results reveal that the remarkable activity stems from the synergistic interplay produced by strong electronic interactions between MoO2 and Ru nanoparticles. This modulated electronic structure accompanying enhanced electrical conductivity would significantly improve the catalytic activity. This strategy provides an insight into the design and synthesis of a low-cost and high performance alternative to Pt-based catalysts for the HER.

MOF-derived RuO2/Co3O4 heterojunctions as highly efficient bifunctional electrocatalysts for HER and OER in alkaline solutions

The oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) are key half reactions involved in electrochemical water splitting. The design of active and robust Co3O4-based electrocatalysts for overall water splitting in basic media is highly desirable but still remains a great challenge. Herein, a catalyst of a combined metal oxide heterojunction (RuO2/Co3O4) was synthesized by directly annealing a MOF-derived Co–Ru complex under an air atmosphere. The catalyst shows a low OER and HER overpotential of only 305 mV and 89 mV at 10 mA cm−2 in 1 M KOH solution, respectively. It contains only a small amount of precious metal oxides, however, and demonstrates a better performance than most of the other Co3O4-based electrocatalysts reported at the present stage.

Enhanced Activity for Hydrogen Evolution Reaction over CoFe Catalysts by Alloying with Small Amount of Pt

The hydrogen evolution reaction highly relied on Pt electrocatalysts, with high activity and stability. In the past few years, a host of efforts have been made in the development of novel platinum nanostructures with a low amount of Pt because the scarcity and high price of Pt hinder its practical applications. Here, we report the preparation of PtCoFe@CN electrocatalysts with a remarkably reduced Pt loading amount of 4.60% by annealing Pt-doped metal-organic frameworks (MOFs). The electrocatalyst demonstrated an outstanding performance with only 45 mV overpotential to achieve the 10 mA cm-2 current density, which is quite close to that of the commercial 20% Pt/C catalyst. The enhanced catalytic capability is originated from the modification of the electronic structures of CoFe by alloying with Pt. The results indicate that robust and superstable alloy electrocatalysts which contain a very small amount of noble metal could be prepared by annealing noble metal-doped MOFs.

Experimental and theoretical investigations of nitro-group doped porous carbon as a high performance lithium-ion battery anode

Doping is an effective solution to improve the capacity of carbon based anode materials such as introducing nitrogen, boron, sulfur, and phosphorus heteroatoms into the graphite lattice. However, most of the previous doping methods are confined to the crystal lattice and edge doping is rarely studied. Here, using first-principles quantum chemical calculations, we studied the lithium adsorption ability of various functional groups (NH2, NO2, SO3H, Cl, Br, I, OH, and P) which were doped at the edge of graphene sheets. Among all the groups, the nitro-group shows the best lithium adsorption properties. On the basis of theoretical predictions, we successfully synthesized nitro group edge modified porous carbon through the pyrolysis of Cu-based metal–organic frameworks (MOFs) at 600 °C under a nitrogen atmosphere and post-acid treatment. As an anode material for lithium ion batteries, it retains a capacity of 588 mA g−1 after 1500 cycles at a high current density of 1 A g−1. The lithium anodic performance of nitro-group doped carbon is superior to other edge doped carbon based materials reported in the literature such as halogen, sulfur and phosphorus. The excellent cycling performance at high current densities is ascribed to the improved lithium adsorption ability of the nitro-group doped at the edge of carbon.

A MOF-derived self-template strategy toward cobalt phosphide electrodes with ultralong cycle life and high capacity

Phosphides have high theoretical capacity and low redox voltage, and thus could be favorable for lithium storage. Still, huge volume changes and low electroconductivity hinder their application as the anode materials in lithium-ion batteries. Here, cobalt phosphide nanoparticles encapsulated in a nitrogen-doped carbon matrix by using metal–organic frameworks (ZIF-67) as a self-template have been successfully synthesized and showed excellent electrochemical performance as an anode material for lithium-ion batteries. Cobalt-phosphide-based nanohybrids with different phases can be tailored by accurately controlling the pyrolysis temperature. Electrochemical measurements reveal that the electrochemical performance is closely related to the material phase, and CoxP-NC-800 nanohybrids with two phases exhibit an ultralong cycle life of 1800 cycles at a current density of 1 A g−1. And a high reversible specific capacity of 1224 mA h g−1 could be delivered after 100 cycles at a current density of 0.1 A g−1.