2017-12-4

Oringinal title：Multifunctional Mo-N/C@MoS2electrocatalyst for HER, OER, ORR and Zn-air batteries

Fuel cells and high-density rechargeable batteries have the potentials to revolutionize the future of electric vehicles, consumer electronics, and even grid power supply. On the one hand, a successful implementation of fuel cells is critically dependent on unlimited supply of hydrogen fuel, which can be produced via water splitting. Unfortunately, owing to the high stability of water, a highly active and stable electrocatalyst is required to drive not only the hydrogen evolution reaction (HER) at the cathode but also the oxygen evolution reaction (OER) at the anode side of the electrolytic cell. Meanwhile, a catalyst with boosted activity and stability to overcome the sluggish oxygen reduction reaction (ORR) is required to effectively operate such fuel cells. Rechargeable solid Zn-air batteries (ZABs) have a high theoretical energy density as high as ～1086 Wh kgZn–1 and are less complex in design, which make them very attractive for future flexible and wearable electronic devices. Nonetheless, their practical application is hindered by the sluggish ORR/OER kinetics on the air-electrode. Although noble metal (e.g. Pt, Ru, Ir, etc.) based electrocatalysts are highly active toward HER, ORR and OER, their implementations are hugely hampered by their exorbitant cost, inadequate supply, poor immunity to oxidation, and unsatisfactory electrochemical durability.

Therefore, strategic and well-balanced synthesis techniques that can effectively address these requirements are urgently needed. Various cheap materials with different morphologies and structures, including 3D metal-organic composites and 2D polymeric carbon frameworks such as graphene or tranisition-metal dichalcogenides such as MoS2, are being rigorously researched for various electrochemical energy conversion and storage application. Building a composite structure of 3D and 2D materials are particularly attractive as they stand to take advantage of their unique individual material properties and opens the path to the design of functionally-oriented composite materials. Nonetheless, such composite materials are rarely exploited.

In a recent article, the group of Prof. Shichun Mu at Wuhan University of Technology, China, developed an interface engineered heterostructure of 3D@2D framework. In this work, a metal-organic framework (MOF) derived 3D N-dope carbon framework (N/C) with graphene analogue particles is encapsulated by vertically grown hierarchical porous MoS2 nanosheets, leading to the creation of a Mo–N coupling interface. Such interface is activated by the Mo-metal catalyzed reaction with the graphene analogue particles on the surfaces of the N/C framework. The creation of the Mo–N interface contributes to the formation of three distinctive structural phases while maintaining the hierarchical framework of 3D@2D “core-shell” structure. The obtained Mo-N/C@MoS2 catalyst exhibits a significantly high multifunctional activity and stability toward HER (η _j=10 ～117 mV), OER (η _j=10 ～0.39 V) and ORR (E1/2 ～0.81 V). Interestingly, the high ORR/OER performance also facilitates the design of high capacity electrodes for primary Zn-air batteries (～63.9 % efficient at 5 mA cm^–2) and all-solid-state (ASS) miniature rechargeable Zn-air batteries (open circuit voltage ～1.34 V). Such batteries deliver an energy density as high as ca.≈846.07 Wh kgZn^–1 which is about ≈77.9% of the theoretical energy density. The outstanding multifunctional performance is attributed to the synergistic  contribution of all three unique phases; constituting the active sites on the exposed edges of the MoS2 nanosheets, the active interfacial Mo-N coupling centers, and the N-induced active sites on neighboring carbon atoms of the N/C framework. Both experimental analysis and DFT calculations revealed that the Mo-N coupling at the interface is the most active site and plays the dominant trifunctional role toward enhancing the multifunctional activity and stability of Mo-N/C@MoS2. The hierarchical pore-enriched geometry and high surface area also  significantly boost the diffusion and mass transport efficiency for further enhancing the catalyst performances. Their strategy shows that the activity of otherwise inactive materials for multifunctional electrochemical applications can be significantly boosted via interface engineering. Therefore, this work contributes to the emerging research on multifunctional electrocatalysis for the fast-growing technology of energy conversion and storage devices, and could open new avenues for rethinking material design.

To learn more about this and other interesting research works, visit the Advanced Functional Materials homepage http://onlinelibrary.wiley.com/doi/10.1002/adfm.201702300/full) or MaterialsViewsChina.com.