Strategic electrode engineering of bimetallic ZIF-8 frameworks: Tuning Mn, Ce, Al, and Fe with Zn for asymmetric supercapacitor

Zeolitic imidazolate framework-8 (ZIF-8) has attracted attention as a potential supercapacitor (SC) electrode material, but its single-metal configurations often suffer as positive electrodes due to limited redox activity and poor electronic conductivity, which restricts their overall electrochemical charge storage. To overcome these limitations of monometallic ZIF-8 materials for asymmetric supercapacitors (ASCs), this paper strategically explores Mn, Ce, Al, and Fe metal cations to synergistically adjust the electronic density and lattice environment of ZIF-8, thereby improving the material's intrinsic electrochemical energy storage performance. This controlled incorporation yields compositionally uniform and structurally coherent bimetallic ZIF-8 frameworks, such as Zn-Mn-ZIF-8, Zn-Ce-ZIF-8, Zn-Al-ZIF-8, and Zn-Fe-ZIF-8, which are further used to evaluate energy storage performance. Among all the electrodes, the Zn-Mn-ZIF-8 exhibited a high specific capacitance of 520.88 F g −1 at a current density of 1 A/g, surpassing those of Zn-Ce-ZIF-8 (421 F g −1 ), Zn-Al-ZIF-8 (404 F g −1 ), and Zn-Fe-ZIF-8 (252.52 F g −1 ). Afterward, an asymmetric supercapacitor based on Zn–Mn–ZIF-8 as the positive electrode and activated carbon as the negative electrode achieved an energy density of 40.625 Wh kg −1 at a power density of 1250 W kg −1 and retained 85% of its initial capacitance after 5000 GCD cycles. Characterization using HR-TEM, AFM, and XPS shows that hierarchical rod-particle structures and mixed-valence metal states enhance charge-storage kinetics, while contact angle and zeta potential measurements indicate that Zn-Mn-ZIF-8's hydrophilicity improves electrode-electrolyte interactions and performance. These results highlight the role of metal-ion incorporation in bimetallic ZIF-8 and provide a strategic approach to enhance the electrochemical energy storage performance of ZIF-8-based materials.