Electrochemical instability of metal−organic frameworks : in situ spectroelectrochemical investigation of the real active sites

Abstract

Despite recent attempts using metal-organic frameworks (MOFs) directly as electrocatalysts, the electrochemical stability of MOFs and the role of in situ-formed species during electrochemistry are elusive. Using in situ spectroelectrochemistry, we present herein a comprehensive discussion on the structural and morphological evolution of MOFs (zeolitic imidazolate framework-67, ZIF-67) during both cyclic voltammetry and amperometry. Dramatic morphological changes exposing electron-accessible Co sites are evident. The intense conversion from tetrahedral Co sites in ZIF-67 to tetrahedral α-Co(OH)2 and octahedral β-Co(OH)2, and the formation of their corresponding oxidized forms (CoOOH), is observed during both the electrochemical treatments. Subsequent oxygen evolution reaction suggests the CoOOH produced from α/β-Co(OH)2 as the dominating active sites, not the metal nodes of ZIF-67. Specifically, the CoOOH from α-Co(OH)2 is most active (turnover frequency = 0.59 s-1) compared to that from β-Co(OH)2 (0.06 s-1). Our study demonstrates the importance of examining the electrochemical stability of MOFs for electrocatalyst design.