Skeletal Nitrogen Functionalization of Isostructural 2D Conjugated MOFs for Enhancement of the Dual-Ion Storage Capacity

Abstract

Two-dimensional conjugated metal–organic frameworks (2D c-MOFs) are emerging as promising electrode materials for electrochemical energy storage devices. However, a viable path to realize superior dual-ion storage in 2D c-MOFs has remained elusive. Here, we report the synthesis of Cu2(Nx−OHPTP) 2D c-MOFs (x=0,1,2; OHPTP=octahydroxyphenanthrotriphenylene) with precise aromatic carbon-nitrogen arrangements, based on the π-conjugated OHPTP ligand incorporated with one or two nitrogen atoms. The skeletal nitrogen modification in Cu2(Nx−OHPTP) allows the synergistic introduction of additional redox sites, and thus substantially favors the unique dual-ion adsorption capacity. Consequently, the Cu2(N2−OHPTP) cathode exhibits a largely enhanced electrochemical performance for dual-ion storage (i.e., Li+ and Cl-) with a high specific capacity of 53.8 mAh g−1, which is twice that of Cu2(N0−OHPTP) and 1.3 times that of Cu2(N1−OHPTP). Furthermore, the Cu2(N2−OHPTP) electrode displays a favorable rate performance of 52 % and good cycling stability of 96 % after 1000 cycles. We identify N-centered redox sites as additional Li+ adsorption sites by combining ex situ and in situ spectroscopy measurements and theoretical calculations. In addition, calculations underline the synergistic enhancement of the Cl− adsorption energy by about 1.0 eV at the more electron-poor CuO4 linkages after N-incorporation. This work paves the way for the precise design of 2D c-MOFs with superior electrochemical properties, advancing their application in dual-ion storage applications.