Oxygen electrocatalysis is of great importance for many energy storage and conversion technologies including fuel cells, metal-air batteries and water electrolysis. Replacing noble metal-based electrocatalysts with highly efficient and inexpensive non-noble metal based oxygen electrocatalysts is critical for the practical applications of these technologies. Here we report a general approach for the synthesis of hollow frameworks of nitrogen-doped carbon nanotubes derived from metal-organic frameworks, which exhibit higher electrocatalytic activity and stability for oxygen reduction and evolution than commercial Pt/C electrocatalysts. The remarkable electrochemical properties are mainly attributed to the synergistic effect from chemical compositions and the robust hollow structure composed of interconnected crystalline nitrogen-doped carbon nanotubes. The presented strategy for controlled design and synthesis of metal-organic framework-derived functional nanomaterials offers prospects in developing highly active electrocatalysts in electrochemical energy devices.

This review summarizes recent research progress and perspectives on noble-metal-free bifunctional heterogeneous electrocatalysts towards hydrogen and oxygen evolution reactions in overall water splitting.

Hollow structures with high complexity in shell architecture and composition have attracted tremendous interest because of their great importance for both fundamental studies and practical applications. Herein we report the designed synthesis of novel box-in-box nanocages (NCs) with different shell compositions, namely, Co3O4/NiCo2O4 double-shelled nanocages (DSNCs). Uniform zeolitic imidazolate framework-67/Ni-Co layered double hydroxides yolk-shelled structures are first synthesized and then transformed into Co3O4/NiCo2O4 DSNCs by thermal annealing in air. Importantly, this strategy can be easily extended to prepare other complex DSNCs. When evaluated as electrodes for pseudocapacitors, the Co3O4/NiCo2O4 DSNCs show a high specific capacitance of 972 F g(-1) at a current density of 5 A g(-1) and excellent stability with 92.5% capacitance retention after 12 000 cycles, superior to that of Co3O4 NCs with simple configuration and Co3O4/Co3O4 DSNCs. Besides, the Co3O4/NiCo2O4 DSNCs also exhibit much better electrocatalytic activity for the oxygen evolution reaction than Co3O4 NCs. The greatly improved electrochemical performance of Co3O4/NiCo2O4 DSNCs demonstrates the importance of rational design and synthesis of hollow structures with higher complexity.

Electrochemical water splitting has been considered as a promising approach to produce clean and sustainable hydrogen fuel. However, the lack of high-performance and low-cost electrocatalysts for hydrogen evolution reaction hinders the large-scale application. As a new class of porous materials with tunable structure and composition, metal-organic frameworks have been considered as promising candidates to synthesize various functional materials. Here we demonstrate a metal-organic frameworks-assisted strategy for synthesizing nanostructured transition metal carbides based on the confined carburization in metal-organic frameworks matrix. Starting from a compound consisting of copper-based metal-organic frameworks host and molybdenum-based polyoxometalates guest, mesoporous molybdenum carbide nano-octahedrons composed of ultrafine nanocrystallites are successfully prepared as a proof of concept, which exhibit remarkable electrocatalytic performance for hydrogen production from both acidic and basic solutions. The present study provides some guidelines for the design and synthesis of nanostructured electrocatalysts.

Hydrogen is expected to play a major role in the development of sustainable energy and environment. Electrocatalytic hydrogen evolution reaction (HER) is known as an efficient method for large-scale hydrogen production, and in this electrochemical process, efficient and low-cost electrocatalysts are indispensable. Recent advances have revealed that nanostructured molybdenum sulfides (MoSx) would be promising alternatives to Pt for the electrochemical generation of hydrogen from water. In this review, we focus on the recent progress on MoSx-based materials as electrocatalysts toward the HER under acidic condition. Moreover, future research scope and important challenges emerging from MoSx nanostructures are discussed toward the development of more advanced and efficient electrocatalysts for HER.