Enhancing electrochemical hydrogen storage in nickel-based metal-organic frameworks (MOFs) through zinc and cobalt doping as bimetallic MOFs

Abstract

Pure nickel-based metal-organic frameworks (Ni-MOFs) represent highly promising materials for electrochemical applications, attributed to their cost-effectiveness, natural abundance, capacity to form Ni2⁺/Ni³⁺ redox couples, and exceptional catalytic activity. Nonetheless, their practical utility is limited by inherent challenges, including poor electrical conductivity, a propensity for stacking, and instability in aqueous environments, particularly under the demanding conditions of water-splitting reactions for hydrogen production. In this study, we aimed to address these limitations by designing and synthesizing enhanced porous Ni-terephthalic acid [Ni(TPA)] MOFs using a metal doping strategy. Zinc (Zn) and cobalt (Co) were selected as dopants due to their unique properties. Zn–Ni(TPA) and Co–Ni(TPA) MOFs were synthesized via a facile solvothermal method and subsequently compared with pure Ni(TPA). FE-SEM, XRD, EDS, FT-IR, and BET analyses were conducted to characterize the synthesized samples, confirming the formation of layered morphologies in Zn–Ni(TPA) and Co–Ni(TPA) with no detectable impurities. The electrical conductivities of the synthesized MOFs were evaluated using electrochemical impedance spectroscopy (EIS), and the corresponding Nyquist plots are presented. Moreover, the corrosion potential (Ecorr) analysis was conducted, revealing the superior anti-corrosion properties of the Co-Ni (TPA) MOF. Electrochemical performance evaluations through cyclic voltammetry (CV) and chronopotentiometry (CP) revealed that the twentieth discharge capacity of Co–Ni(TPA) (4000 mAhg⁻1) significantly outperformed pure Ni (TPA) (1850 mAhg⁻1). Furthermore, the hydrogen storage capacities of pure Ni(TPA), Zn-doped Ni(TPA), and Codoped Ni(TPA) were comparatively investigated to assess the impact of Zn and Co doping. The results revealed that Co–Ni(TPA) demonstrated a superior hydrogen storage capacity compared to Zn–Ni(TPA), likely due to the partial substitution of Ni2⁺ with Co2⁺, which increases the availability of free holes for gas adsorption.