Ni-based metal organic frameworks (Ni-MOFs) with unique hierarchical hollow ball-in-ball nanostructure were synthesized by solvothermal reactions. After successive carbonization and oxidation treatments, hierarchical NiO/Ni nanocrystals covered with a graphene shell were obtained with the hollow ball-in-ball nanostructure intact. The resulting materials exhibited superior performance as the anode in lithium ion batteries (LIBs): they provide high reversible specific capacity (1144 mAh/g), excellent cyclability (nearly no capacity loss after 1000 cycles) and rate performance (805 mAh/g at 15 A/g). In addition, the hierarchical NiO/Ni/Graphene composites demonstrated promising performance as anode materials for sodium-ion batteries (SIBs). Such a superior lithium and sodium storage performance is derived from the well-designed hierarchical hollow ball-in-ball structure of NiO/Ni/Graphene composites, which not only mitigates the volume expansion of NiO during the cycles but also provides a continuous highly conductive graphene matrix to facilitate the fast charge transfer and form a stable SEI layer.

Metal Organic Frameworks Derived Hierarchical Hollow NiO/Ni/Graphene Composites for Lithium and Sodium Storage

An effective electrocatalyst being highly active in all pH range for oxygen reduction reaction (ORR) is crucial for energy conversion and storage devices. However, most of the high-efficiency ORR catalysis was reported in alkaline conditions. Herein, we demonstrated the preparation of atomically dispersed Fe-Zn pairs anchored on porous N-doped carbon frameworks (Fe-Zn-SA/NC), which works efficiently as ORR catalyst in the whole pH range. It achieves high half-wave potentials of 0.78, 0.85 and 0.72 V in 0.1 M HClO4, 0.1 M KOH and 0.1 M phosphate buffer saline (PBS) solutions, respectively, as well as respectable stability. The performances are even comparable to Pt/C. Furthermore, when assembled into a Zn-air battery, the high power density of 167.2 mWcm−2 and 120 h durability reveal the feasibility of Fe-Zn-SA/NC in real energy-related devices. Theoretical calculations demonstrate that the superior catalytic activity of Fe-Zn-SA/NC can be contributed to the lower energy barriers of ORR at the Fe-Zn-N6 centers. This work demonstrates the potential of Fe-Zn pairs as alternatives to the Pt catalysts for efficient catalytic ORR and provides new insights of dual-atom catalysts for other energy conversion related catalytic reactions.

Atomic Fe-Zn dual-metal sites for high-efficiency pH-universal oxygen reduction catalysis

With the development of renewable energy systems, clean hydrogen is burgeoning as an optimal alternative to fossil fuels, in which its application is promising to retarding the global energy and environmental crisis. The hydrogen evolution reaction (HER), capable of producing high-purity hydrogen rapidly in electrocatalytic water splitting, has received much attention. Abundant research about HER has been done, focusing on advanced electrocatalyst design with high efficiency and robust stability. As potential HER catalysts, metal nanoclusters (MNCs) have been studied extensively. They are composed of several to a hundred metal atoms, with sizes being comparable to the Fermi wavelength of electrons, that is, < 2.0 nm. Different from metal atoms/nanoparticles, they exhibit unique catalytic properties due to their quantum size effect and low-coordination environment. In this review, the activity-enhancing approaches of MNCs applied in HER electrocatalysis are mainly summarized. Furthermore, recent progress in MNCs classified with different stabilization strategies, that is, the freestanding MNCs, MNCs with organic, metal and carbon supports, are introduced. Finally, the current challenges and deficiencies of these MNCs for HER are prospected.

Advances in the Electrocatalytic Hydrogen Evolution Reaction by Metal Nanoclusters-based Materials.

The sluggish kinetics of oxygen evolution reaction (OER) hampers the conversion of renewable energies into hydrogen through an electrocatalytic process. Then, the hydrazine oxidation reaction (HzOR) is a promising anodic reaction to substitute OER, which can decrease the water-splitting voltage. Herein, self-supported ruthenium-doped cuprous oxide nanochains were synthesized on commercial copper foam (Ru-Cu2O/CF). The Ru-Cu2O/CF provides superior hydrogen evolution reaction (HER) and HzOR performances owing to its specific morphology, superhydrophilic surface, porous support and Ru doping. When used in alkaline or neutral electrolytes, the Ru-Cu2O/CF is capable of driving 10 mA cm−2 only at 31 and 51 mV, respectively. In HzOR, it is possible to achieve a superlow potential of − 41 mV at 10 mA cm−2. In an electrolytic cell with a Ru-Cu2O/CF both as cathode and anode, a voltage of 17.4 mV is needed to reach 10 mA cm−2, which is powered by renewable energies. 

Ruthenium-doped 3D Cu2O Nanochains as Efficient Electrocatalyst towards Hydrogen Evolution and Hydrazine Oxidation

Improving the carbon dioxide and nitrogen reduction reactions (CO2RR and NRR) can reduce anthropogenic greenhouse gas emissions while selectively producing chemicals needed for the fuel, plastic, and chemical industries. Efficient CO2RR can be used to replace fossil fuels as well as repurpose captured CO2, while new NRR pathways can be used to supplement or replace the energy intensive Haber–Bosch process for NH3 generation with no CO2 emissions. Therefore, this review article focuses on (photo)electrocatalytic and photocatalytic conversion of CO2 and N2 molecules into useful products, such as carbon monoxide, methanol, formic acid, and ammonia, using 2D transition metal dichalcogenides (TMDCs) and metal carbides/nitrides (MXenes). These highly tunable 2D catalysts will be evaluated for their ability to selectively and efficiently undergo CO2RR and NRR by controlling defects, phases, edge sites, interfaces, and functional groups. We first address the CO2RR and NRR challenges, with a particular focus on theoretical mechanisms and minimum energy pathways. We follow this discussion with a detailed review of state-of-the-art 2D TMDC and MXene experimental catalysts for CO2RR and NRR (photo)electrocatalytic and photocatalytic reactions, and then address areas of opportunity for these catalytic reactions.

Carbon dioxide and nitrogen reduction reactions using 2D transition metal dichalcogenide (TMDC) and carbide/nitride (MXene) catalysts

Cu2O-Derived PtCu Nanoalloy toward Energy-Efficient Hydrogen Production via Hydrazine Electrolysis under Large Current Density

Semimetal 1H-SnS2 Enables High-Efficiency Electroreduction of CO2 to CO

Transition-metal oxides with low valence states are promising candidates as anodes for advanced rechargeable Li-ion batteries. Surprisingly, the capacities of such anode materials initially decrease and then increase after long-term cycling. Herein, MnO is selected as a representative material to study the structure-function relationship and elucidate the above-mentioned phenomena during long-term cycling. To this end, the surface reconstruction to bulk transformation of MnO anode materials during the cycling procedures has been revealed. The atomic scanning transmission electron microscopy images and theoretical modeling results illustrate the formation of stable surface-phase Mn3O4 and Li2MnO4, which promote the migration of Li ions. The complete bulk-phase transformation of MnO is then revealed, during which Mn2+ was found to be initially oxidized to Mn4+ and subsequently reduced to a mixed valence of Mn2+ and Mn3+, correlating with the tendency of their discharge capacity variation upon cycling. These direct atomic-scale observations of the migration behavior of Li ions in the MnO anode offer an essential step toward understanding the electrochemical performance evolution of transition-metal oxide anodes and guide the anode preparation for Li-ion batteries.

The phase-change evolution from surface to bulk of MnO anodes upon cycling.

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Cu2O-Derived PtCu Nanoalloy toward Energy-Efficient Hydrogen Production via Hydrazine Electrolysis under Large Current Density

Semimetal 1H-SnS2 Enables High-Efficiency Electroreduction of CO2 to CO

The phase-change evolution from surface to bulk of MnO anodes upon cycling.