ZIF-67-Derived Carbon-Doped Cobalt Composites With Dominant Pseudocapacitance as Solid Transducing Layer for High-Sensitivity Solid-Contact Ion-Selective Electrodes

Carbon-doped cobalt composites (CDC-67s: Co@C, Co₃O₄@C, and Co₃O₄) have been synthesized via controlled pyrolysis and oxidation of ZIF-67 (a metal–organic framework) precursor. These composites exhibit both electrical double-layer capacitance and pseudocapacitance, making them promising materials for high-performance ion–electron solid transduction layers (STLs). Herein, the efficient transduction mechanism of CDC-67s has been systematically analyzed. Specifically, the Co₃O₄@C material leverages its mesoporous carbon framework to facilitate additional charge storage through reversible Co³⁺/Co²⁺ redox reactions. This mechanism results in a high specific capacitance of 103.44 F g^–1, with pseudocapacitance accounting for up to 83% of the total capacitance. Furthermore, solid-state contact ion-selective electrodes (SC-ISEs) fabricated with Co₃O₄@C as STLs show the highest sensitivity of 68.03 mV decade^–1 among the CDC-67s, significantly surpassing that of SC-ISEs based on commercial materials like graphene and multiwalled carbon nanotubes (MWCNTs). This pseudocapacitance-dominated charge storage mechanism, thus, greatly enhances SC-ISE sensitivity, expanding the design paradigm for traditional carbon-based SC-ISE materials and opening new avenues for developing high-performance SC-ISEs. The stability of the CDC-67-based SC-ISEs has been assessed by water layer tests; the Co₃O₄-based SC-ISE demonstrates superior resistance to this interfacial interference, indicating the promising application of Co₃O₄@C as ion–electron solid transduction layer for solid-contact ion-selective electrodes.