Interplanar Growth of 2D Non‐Van der Waals Co₂N‐Based Heterostructures for Efficient Overall Water Splitting

Abstract

The highly efficient water splitting is characterized by low overpotential, low Tafel slope, and long cyclability for both OER and HER, where OER is the bottleneck. In order to lower the energy barrier of four-electron transfer OER process (4OH-↔ 2H 2 O + O 2 + 4e-), the major challenge is to fabricate the catalyst that could form the optimized oxygen binding with intermediate adsorbates, which should be neither too strong nor too week to balance the energetics of the processes between the deprotonation of OH* and the formation of OOH*. [4] Interestingly, heterostructured materials can adjust the formation energy of this oxygen binding, and as such, improve the water splitting performance. [1b,5] Among these materials, 2D heterostructures have attracted great attention recently, [6] e.g., 2D-MoS 2 / Co(OH) 2 , [7] MoS 2 /WS 2 , [8] and MoS 2 /CoNC, [9] because of the extensive exposure of the heterostructured and active regions. Furthermore, fast electron transfer is another key factor that can reduce the overpotential and Tafel slope. [10] In this sense, the homogenous distribution of active sites on 2D conductive nanosheets is also a promising design, e.g., 2D NiS/graphene. [11] However, prior studies only dealt with 2D heterostructures comprising van der Waals (vdW) nanosheets (e.g., MoS 2 , graphene, etc.) as the support for the second materials in water splitting. [5] vdW solids are normally featured with layered structures bonded through relatively weak vdW interactions, and hence their exfoliation to be nanosheets is readily achievable. [5] One drawback of using heterostructures comprising vdW nanosheets in water splitting is that the electron transport can be suppressed across the vdW gap. [12] Another drawback is that their attachment to the current collector is neither conductive nor stable enough for the sustainable improvement of electrocatalytic performance. These obstacles urge us to search for the solutions from 2D heterostructures consisting of non-vdW nanosheets that may bridge active materials and current collectors strongly and conductively. To achieve this construction, a typical non-vdW solid, Co 2 N, attracts our attention, due to its unique metal characteristics with zero band gap that significantly facilitate The design and synthesis of 2D heterostructured materials for water splitting are normally based on van der Waals (vdW) nanosheets, but this approach is gradually approaching a performance ceiling in terms of activity and stability. Herein, a novel heterostructured system is explored based on 2D non-vdW and conductive nanosheets. Notably, the interplanar growth of 2D non-vdW Co 2 N nanosheets is realized between graphene layers in the current collector of carbon fiber papers (CPs), generating an interlocking structure within CPs. The exposed surface of Co 2 N nanosheets possesses a high surface energy that anchors highly active CoNC, forming 2D CoNC@Co 2 N heterostructures outside CPs. This integrated electrocatalytic system bridged by non-vdW Co 2 N nanosheets presents a low overpote...

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