Hydrogen fuel is considered as the cleanest renewable resource and the primary alternative to fossil fuels for future energy supply. Sustainable hydrogen generation is the major prerequisite to realize future hydrogen economy. The electrocatalytic hydrogen evolution reaction (HER), as the vital step of water electrolysis to H2 production, has been the subject of extensive study over the past decades. In this comprehensive review, we first summarize the fundamentals of HER and review the recent state-of-the-art advances in the low-cost and high-performance catalysts based on noble and non-noble metals, as well as metal-free HER electrocatalysts. We systemically discuss the insights into the relationship among the catalytic activity, morphology, structure, composition, and synthetic method. Strategies for developing an effective catalyst, including increasing the intrinsic activity of active sites and/or increasing the number of active sites, are summarized and highlighted. Finally, the challenges, perspectives, and research directions of HER electrocatalysis are featured.

Recent Advances in Electrocatalytic Hydrogen Evolution Using Nanoparticles.

Hollow nanostructures offer promising potential for advanced energy storage and conversion applications. In the past decade, considerable research efforts have been devoted to the design and synthesis of hollow nanostructures with high complexity by manipulating their geometric morphology, chemical composition, and building block and interior architecture to boost their electrochemical performance, fulfilling the increasing global demand for renewable and sustainable energy sources. In this Review, we present a comprehensive overview of the synthesis and energy-related applications of complex hollow nanostructures. After a brief classification, the design and synthesis of complex hollow nanostructures are described in detail, which include hierarchical hollow spheres, hierarchical tubular structures, hollow polyhedra, and multi-shelled hollow structures, as well as their hybrids with nanocarbon materials. Thereafter, we discuss their niche applications as electrode materials for lithium-ion batteries and hybrid supercapacitors, sulfur hosts for lithium-sulfur batteries, and electrocatalysts for oxygen- and hydrogen-involving energy conversion reactions. The potential superiorities of complex hollow nanostructures for these applications are particularly highlighted. Finally, we conclude this Review with urgent challenges and further research directions of complex hollow nanostructures for energy-related applications.

Complex Hollow Nanostructures: Synthesis and Energy-Related Applications.

Seawater is one of the most abundant natural resources on our planet. Electrolysis of seawater is not only a promising approach to produce clean hydrogen energy, but also of great significance to seawater desalination. The implementation of seawater electrolysis requires robust and efficient electrocatalysts that can sustain seawater splitting without chloride corrosion, especially for the anode. Here we report a three-dimensional core-shell metal-nitride catalyst consisting of NiFeN nanoparticles uniformly decorated on NiMoN nanorods supported on Ni foam, which serves as an eminently active and durable oxygen evolution reaction catalyst for alkaline seawater electrolysis. Combined with an efficient hydrogen evolution reaction catalyst of NiMoN nanorods, we have achieved the industrially required current densities of 500 and 1000 mA cm−2 at record low voltages of 1.608 and 1.709 V, respectively, for overall alkaline seawater splitting at 60 °C. This discovery significantly advances the development of seawater electrolysis for large-scale hydrogen production. Seawater electrolysis is a promising approach to produce hydrogen fuel and is also of great significance to seawater desalination. Here, the authors prepare 3D core-shell metal-nitride catalysts from earth-abundant elements for high-performance alkaline seawater electrolysis.

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Non-noble metal-nitride based electrocatalysts for high-performance alkaline seawater electrolysis

The design of highly efficient, stable, and noble-metal-free bifunctional electrocatalysts for overall water splitting is critical but challenging. Herein, a facile and controllable synthesis strategy for nickel–cobalt bimetal phosphide nanotubes as highly efficient electrocatalysts for overall water splitting via low-temperature phosphorization from a bimetallic metal-organic framework (MOF-74) precursor is reported. By optimizing the molar ratio of Co/Ni atoms in MOF-74, a series of CoxNiyP catalysts are synthesized, and the obtained Co4Ni1P has a rare form of nanotubes that possess similar morphology to the MOF precursor and exhibit perfect dispersal of the active sites. The nanotubes show remarkable hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) catalytic performance in an alkaline electrolyte, affording a current density of 10 mA cm−2 at overpotentials of 129 mV for HER and 245 mV for OER, respectively. An electrolyzer with Co4Ni1P nanotubes as both the cathode and anode catalyst in alkaline solutions achieves a current density of 10 mA cm−2 at a voltage of 1.59 V, which is comparable to the integrated Pt/C and RuO2 counterparts and ranks among the best of the metal-phosphide electrocatalysts reported to date.

Metal-Organic Frameworks Derived Nanotube of Nickel–Cobalt Bimetal Phosphides as Highly Efficient Electrocatalysts for Overall Water Splitting

Carbon materials are generally preferred as anodes in supercapacitors; however, their low capacitance limits the attained energy density of supercapacitor devices with aqueous electrolytes. Here, we report a low-crystalline iron oxide hydroxide nanoparticle anode with comprehensive electrochemical performance at a wide potential window. The iron oxide hydroxide nanoparticles present capacitances of 1,066 and 716 F g−1 at mass loadings of 1.6 and 9.1 mg cm−2, respectively, a rate capability with 74.6% of capacitance retention at 30 A g−1, and cycling stability retaining 91% of capacitance after 10,000 cycles. The performance is attributed to a dominant capacitive charge-storage mechanism. An aqueous hybrid supercapacitor based on the iron oxide hydroxide anode shows stability during float voltage test for 450 h and an energy density of 104 Wh kg−1 at a power density of 1.27 kW kg−1. A packaged device delivers gravimetric and volumetric energy densities of 33.14 Wh kg−1 and 17.24 Wh l−1, respectively. Carbons dominate anode materials for supercapacitors, however the attained energy density remains low. Here the authors fabricate low-crystalline iron oxide-hydroxide nanoparticle anodes with good electrochemical characteristics, exhibiting high stability and energy/power densities in a hybrid supercapacitor.

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Low-crystalline iron oxide hydroxide nanoparticle anode for high-performance supercapacitors.

Fe-Ni-Mo Nitride Porous Nanotubes for Full Water Splitting and Zn-Air Batteries

Designing low-cost, highly active and stable electrocatalysts is very important to various renewable energy storage and conversion devices Herein we develop a facile method to fabricate bimetallic Ni–Mo nitride nanotubes, which can serve as highly active and stable bifunctional electrocatalysts for full water splitting To drive a current density of 10 mA cm−2, the bimetallic Ni–Mo nitride nanotubes require an overpotential of 295 mV for the OER and 89 mV for the HER The alkaline water electrolyzer with the nanotubes as cathode and anode catalysts requires a cell voltage of ca 1596 V to achieve a current density of 10 mA cm−2 Furthermore, the nanotubes for full water splitting show excellent stability even at a high current density of 370 mA cm−2, superior to the integrated performance of commercial Pt and IrO2 Our experimental results show that the NiOOH and NH groups formed at the catalyst surface during the OER process are active species for the OER, while the Ni(OH)2, NH and Mo species at the catalyst surface play a key role in the HER The present strategy may open an avenue for fabrication of low-cost, highly active and stable electrocatalysts for large-scale water splitting

Bimetallic Ni–Mo nitride nanotubes as highly active and stable bifunctional electrocatalysts for full water splitting

Hierarchical nanosheet-based NiMoO4 nanotubes were synthesized by a hydrothermal treatment and a subsequent in situ diffusion reaction method. The nanotubes with a high surface area of 128.5 m2 g−1 were composed of highly ordered ultrathin nanosheets with a thickness of less than 10 nm. When used as electrodes in electrochemical capacitors, the hierarchical nanotubes exhibited excellent electrochemical performances. The specific capacitance of the hierarchical nanotubes was up to 864 F g−1 at a current density of 1 A g−1. The results indicate that the hierarchical nanosheet-based NiMoO4 nanotubes are very promising for applications in energy storage and other electrochemical devices. Furthermore, the strategy presented here is facile, and may be expended as a typical method to synthesize other kinds of metal molybdate nanotubes.

Hierarchical nanosheet-based NiMoO4 nanotubes: synthesis and high supercapacitor performance

To improve the efficiency of overall water splitting, highly active and stable bifunctional electrocatalysts are highly desirable. Herein, we fabricated mixed Ni–Co phosphides (Ni1−xCox–P) as bifunctional catalysts for overall water splitting. Structural characterizations indicated that the Ni1−xCox–P catalysts (0 > x > 1) exhibited hierarchical yolk-shelled morphologies, with a total diameter of 1–2 μm. Interestingly, the shell was assembled by numerous nanosheets with a thickness less than 10 nm. The electrochemical measurements indicated that the Ni1−xCox–P catalysts (0 < x < 1) showed significantly enhanced OER and HER activities in comparison to the pure Ni–P and Co–P catalysts, and the highest OER and HER activities were achieved as x = 0.31. To drive a current density of 10 mA cm−2, the Ni0.69Co0.31–P catalyst required an overpotential of 266 mV for OER and 96 mV for HER, respectively. The alkaline water electrolyzer with the Ni0.69Co0.31–P catalysts as the cathode and anode catalysts required a cell voltage of ca. 1.59 V to achieve a current density of 10 mA cm−2, which was comparable to the integrated performance of commercial Pt/C and IrO2. Furthermore, the electrolyzer assembled by the bufictional electrocatalysts showed a more stable performance than one assembled by commercial Pt/C and IrO2 operated at the simialr current density. The superior activity and long-term stability demonstrate that the hierarchical mixed Ni–Co phosphides have promising potential for application in large-scale water splitting.

Hierarchical nickel-cobalt phosphide yolk-shell spheres as highly active and stable bifunctional electrocatalysts for overall water splitting.

Hollow structures with hierarchical architecture and multi-composition have attracted extensive interest because of their fascinating physicochemical properties as well as wide applications. Herein we report the designed synthesis of hierarchical nanosheet-based CoMoO4–NiMoO4 nanotubes by a hydrothermal treatment and a subsequent calcination method. The walls of the hierarchical nanotubes are composed of interconnected NiMoO4 nanosheets and CoMoO4 nanoparticles anchored on the surface of the nanosheets. The diameter of the hollow interior, the thickness of NiMoO4 nanosheets and the diameter of CoMoO4 nanoparticles are around 180 nm, less than 6 nm and 3 nm, respectively, providing the hierarchical nanotubes with a high surface area and a large pore volume. When evaluated as electrodes for pseudocapacitors, the hierarchical nanotubes with an appropriate amount of CoMoO4 show a high specific capacitance of 1079 F g−1 at a current density of 5 A g−1 and excellent stability with 98.4% capacitance retention after 1000 cycles. Furthermore, an asymmetric capacitor, consisting of active carbon and hierarchical nanosheet-based CoMoO4–NiMoO4 nanotubes as negative and positive electrodes, respectively, delivers an energy density of 33 W h kg−1 at a power density of 375 W kg−1, and 16.3 W h kg−1 even at a high power density of 6000 W kg−1. The supercapacitive properties are much higher than those of single-phase NiMoO4 nanotubes, and most of the other metal molybdates and metal oxides reported previously. Besides, the hierarchical nanotubes also exhibit much better electrocatalytic activity for the oxygen evolution reaction than single-phase NiMoO4 nanotubes, and most of the other metal molybdates and metal oxides. Our results demonstrate the importance of rational design of complex hollow structures with enhanced properties for a wide range of practical applications.

Zhuoxun Yin

Papers

Fe-Ni-Mo Nitride Porous Nanotubes for Full Water Splitting and Zn-Air Batteries

Bimetallic Ni–Mo nitride nanotubes as highly active and stable bifunctional electrocatalysts for full water splitting

Hierarchical nanosheet-based NiMoO4 nanotubes: synthesis and high supercapacitor performance

Hierarchical nickel-cobalt phosphide yolk-shell spheres as highly active and stable bifunctional electrocatalysts for overall water splitting.

Hierarchical nanosheet-based CoMoO4–NiMoO4 nanotubes for applications in asymmetric supercapacitors and the oxygen evolution reaction