Evergreen — Joint Journal of Novel Carbon Resource Sciences and Green Asia Strategy
Article Open Access CC BY 4.0 Vol 13 · Iss 02 · June 2026 · pp. 813–824

Zeolitic Imidazole Frameworks-8 (ZIF-8) Modified with Cu(II)/Ni(II)/Co(II) As Bifunctional ORR/OER Electrocatalytic Material

Annisa Nur Buana Wati1, Ubed Sonai Fahruddin Arrozi2, Abu Masykur1, Sri Hastuti1, Atmanto Heru Wibowo1

1 Department of Chemistry, Faculty of Mathematics and Sciences, Universitas Sebelas Maret, Jl. Ir. Sutami No. 36A. Jebres, Surakarta 57126, Central Java, Indonesia
2 Department of Chemistry, Faculty of Mathematics and Sciences, State University of Malang, Jl. Semarang 5, Malang 65145, East Java, Indonesia

Corresponding author: aheruwibowo@staff.uns.ac.id  ·  Atmanto Heru Wibowo

ReceivedFebruary 21, 2025
AcceptedSeptember 10, 2025
PublishedJune 2026
DOI10.5109/7432657

Abstract

Zeolitic Imidazole Frameworks-8 (ZIF-8) have been modified with transition metals by partially replacing the original Zn(II) metal ions with Co(II), Cu(II), or Ni(II) ions to form 50%-Co(II)/Ni(II)/Cu(II)/ZIF-8. The modification aims to create a material with a higher surface area, which is beneficial for use as an electrocatalyst. Among these MOFs, Co-modified ZIF-8 has shown the most potential electrocatalytic activity toward the oxygen reduction reaction (ORR) compared to nickel or copper modifications. Rotating ring-disk electrode (RRDE) measurements revealed kinetic parameters such as onset potential, kinetic current density, Tafel slope, and electron transfer number, offering insights into the reaction mechanism. 50%-Co(II)/ZIF demonstrated the best performance as an efficient ORR catalyst, with an onset potential of 0.8 V vs RHE and an electron transfer number reaching 3.43 at 0.7 V vs RHE, indicating a tendency towards a four-electron pathway. Chronoamperometry and 100 cycles of cyclic voltammetry measurements provided evidence of the stability of the material. Additionally, 50%-Co(II)/ZIF/C exhibited a decent overpotential of 0.509 V to achieve a current density of 10 mA cm-2 for oxygen evolution reaction (OER) catalysis.

Keywords: electrocatalyst, modified ZIF-8, oxygen evolution reaction, oxygen reduction reaction, reversible fuel cell

Outline
Abstract 1. Introduction 2. Materials and Method 2.1. Materials 2.2. Catalyst ink preparation 2.3. Electrochemical measurement 3. Results and Discussion 4. Conclusion References

1. Introduction

The increasing demand for energy has led to the development of renewable energy sources as a sustainable alternative to fossil fuels1–3). Electrochemical energy conversion is one of the prospective renewable energy systems4,5). One of the key aspects that needs to be developed in this system is a storage solution to address fluctuations in energy production and consumption6,7). The development of reversible fuel cells (RFC), which combine fuel cell and electrolyzer technologies, enables the conversion of hydrogen and oxygen into electricity as well as the electrolysis of water into the necessary gases for the conversion process within a single system8). In addition to the energy storage issue, the slow kinetics of the oxygen reduction reaction (ORR) occurring in fuel cells and the oxygen evolution reaction (OER) in electrolyzers also pose significant barriers9,10). Commercial Pt/C and RuO₂, which are commonly used as catalysts for these reactions, are expensive and lack of bifunctional properties, thus hindering the commercialization of RFC11–13). Bifunctional non-precious metal-based catalysts are being developed to address these challenges14,15).

The use of first row transition metals such as Mn, Fe, Co, Ni, Cu, and Zn has been widely explored as electrocatalysts due to their low cost and high catalytic activity16). When incorporated into Metal Organic Frameworks (MOFs), these metals can further enhance catalytic performance. MOFs are materials composed of aromatic organic ligands and metal ions, forming a conjugated structure that can withstand oxygen radical attacks and regulate the electronic state of active sites17,18). Their versatility lies in the ability to tailor composition and structure, optimizing properties like surface area, pore size, and electronic conductivity19,20). These attributes make MOFs ideal supports for electrocatalysts, especially when paired with transition metals. By tuning the electronic environment around the metal sites, MOFs facilitate more efficient electron transfer, boosting catalytic activity. Additionally, the high surface area and well-defined porosity of MOFs accommodate numerous active sites, increasing catalytic efficiency. These pores also aid in the diffusion of electrolyte ions to active sites, ensuring high reaction rates21). The combination of transition metals' low cost and high catalytic activity with the unique structural and electronic properties of MOFs offers a new class of materials with enhanced stability, reactivity, and efficiency, which is highly beneficial for electrocatalysis22).

Several types of MOFs have been used as electrocatalysts, either in their pristine form, including lattice-strained NiFe MOF23), ZIF-67/NPC24), Co2(OH)2BDC25), or in their derivatives form, such as Fe-C catalysts derived from ZIF-826–28), MOF-derived two-dimensional N-doped carbon nanosheets29), and MOF derived Co3O4 nanoparticles embedded in N-doped mesoporous carbon layer30). Derivatives of ZIF-8 materials have been widely reported, including N, S-doped nanocarbon31), monodispersed Co in mesoporous polyhedrons32), or porous-carbon supported transition metals33). However, reports on modified ZIF-8 in its pristine form remain limited. In this study, metal–organic frameworks based on Zeolitic Imidazole Frameworks-8 (ZIF-8), which feature a 2-methylimidazolate linker, are modified by partially replacing the original Zn(II) metal ions with Cu(II), Ni(II), and Co(II) ions. This research focuses on the electrocatalytic performance of the modified ZIF-8 in its pristine form, emphasizing its simple synthesis method, which is advantageous for large-scale applications. ZIF-8 has a high nitrogen content, is well-known for its chemical stability, high surface area, and possesses good resistance in aqueous or basic conditions34). While our previous work has focused on the application of modified ZIF-8 in oxidation reactions such as benzyl alcohol oxidation35), our study extends its use toward reversible fuel cell applications by demonstrating its bifunctional electrocatalytic activity in both ORR and OER.

The purpose of this study is to evaluate a series of Cu(II)/Ni(II)/Co(II)-modified ZIF-8 materials as bifunctional catalysts for both ORR and OER, aiming to provide a low-cost, stable, and easily synthesized alternative to precious-metal-based catalysts. This work contributes to the growing field of MOF-based electrocatalysts by introducing a simple and scalable approach to tuning ZIF-8 with multiple transition metals, furthering the potential of MOFs in reversible energy conversion systems. The ease of preparation of this material is expected to provide a solution to the need for efficient, inexpensive, and stable bifunctional catalysts.

2. Materials and Method

2.1. Materials

The modified ZIF-8 with Cu(II)/Ni(II)/Co(II) was synthesized using a method reported in our previous studies35). The Pt/C (20 wt% Pt), potassium hydroxide (KOH), isopropanol (C3H8O), methanol (CH3OH), and 5 wt% Nafion were obtained from commercial suppliers.

2.2. Catalyst ink preparation

The modified ZIF-8 was combined with calcined Vulcan XC72R with a 1:1 ratio and then dispersed in a stock solution containing isopropanol:Nafion:water (20:0.4:79.6) with a concentration of 2 mg/mL. The ink was ultrasonicated for 1 hour to even out the dispersion36). The dispersed ink is dropcasted on a rotating ring disk electrode (RRDE) and rotating disk electrode (RDE), with the amount of 10 uL and 3.5 uL, respectively.

2.3. Electrochemical measurement

The electrocatalytic activity of the catalyst was studied using a three-electrode cell with RDE or RRDE, Ag/AgCl (sat. KCl), and GC plate (connected with gold wire) as working, reference and counter electrodes, respectively. The potential data in this study were converted to potential versus the reversible hydrogen electrode (RHE) using the Nernst equation. RRDE was exclusively used for RRDE measurement. The measurement were conducted in O2- and N2-saturated 0.1 M KOH solution for ORR and OER measurement, correspondingly.

RRDE measurements were conducted to study the performance of the material as a catalyst for the oxygen reduction reaction. The tests were performed with a measurement range of 1 to 0.05 V vs RHE (Ering = 1.2 V vs RHE) at rotation rates of 400, 900, 1600, and 2500 rpm and a scan rate of 10 mV s−1. The catalyst stability was measured using chronoamperometry at 0.7 V vs RHE and cyclic voltammetry method (CV) with a measurement range of 0.05 to 1 V vs RHE at a scan rate of 50 mV s−1 for 100 cycles37). Methanol crossover tests were also carried out by measuring CV using a 3 M methanol solution in 0.1 M KOH as the electrolyte38). Oxygen evolution reaction catalysis is investigated using linear sweep voltammetry measurements (LSV) with a measurement range of 1.2 to 2.0 V vs RHE at a scan rate of 10 mV s-1 39).

3. Results and Discussion

The rotating ring-disk electrode (RRDE) technique was employed to obtain polarization curves in oxygen-saturated 0.1 M KOH. These data were then analyzed to assess the electrocatalytic activities and kinetic behaviors of the catalysts. Figures 1a-c showed the polarization curves at the disk electrode for each modified ZIF-8 at different rotation speeds, while Figure 1d depicted the polarization curves at 1600 rpm for both the disk and ring electrodes to compare the performance of the catalysts. Analysis of the polarization curve in the activation region at a rotation speed of 1600 rpm allows the determination of the onset potential (Eonset). The higher the potential, the more active the catalyst for ORR. As illustrated in the diagram, Co(II)-modified ZIF-8 exhibited the best ORR performance. The onset potential for Co(II)/ZIF-8/C was more positive than that for Cu(II)/ZIF-8/C or Ni(II)/ZIF-8/C, suggesting that Co active sites have a superior ability to interact with oxygen and intermediates during oxygen reduction. The presence of transition metal redox couples facilitates electron transfer during the oxygen reduction reaction (ORR). Notably, the redox potential of the Co(II)/Co(III) couple is close to the theoretical thermodynamic potential of ORR, making it more energetically favorable40,41). Moreover, modification with Co demonstrated the highest success in Zn replacement, with the percentage of Co in the material reaching 3.2% (refer to Figure S1). The Eonset of the modified ZIF-8 were 0.77; 0.77; 0.80 V vs RHE for Cu(II), Ni(II), and Co(II), respectively. Analysis in the kinetic diffusion region provides information regarding the half-reaction potential (E1/2), where each modified ZIF-8 showed an E1/2 of 0.62 V vs RHE. Additionally, it was observed that as the rotation rate increased, the current density also rose significantly. This is due to the enhanced mass transport to and from the electrode surface, as a thinner diffusion layer facilitates faster exchange of reactants and products. Furthermore, higher rotation rate improves oxygen supply, which is the limiting factor, thereby enhancing the ORR process42).

Figure 1(a)
(a)
Figure 1(b)
(b)
Figure 1(c)
(c)
Figure 1(d)
(d)
Fig. 1: ORR Polarization curves of the modified ZIF-8 in 0.1 M KOH under various rotation rates at a scan rate of 10 mV s-1

Using the Koutecky-Levich equation (Eq. 1), the kinetic parameters of the catalyst were analyzed. The resulting Koutecky-Levich plot (Figure 2) exhibited a straight-line relationship, with the slope defined as αK-L = (nexB)-1. The intercept, βK-L, was associated with jk-1. The linearity of the plot indicated that the reaction followed first-order kinetics with respect to the oxygen concentration (refer to Figure S2 for K-L plots for each catalyst at different potentials)43). This suggests that the reduction of oxygen in the presence of the modified ZIF-8 is a first-order reaction. Limiting current density (jL) was determined by extrapolating the plot from Eq. 2. The jL value obtained through calculation was greater than j0, suggesting that the electron transfer step is the rate determining step in this system44). Figure 3 illustrates the relative Tafel plot for the modified ZIF-8, providing insights into the Tafel slope (b) and exchange current density (j0), which were calculated using Eq. 3 and Eq. 4. The value of jk, jL, and j0 were listed in Table 1.

1j=1nexBΩ12+1jlfilm+1jlads+1j0θθeqeηb⇒j-1=(nexB)-1Ω-12+jk-1 (1)
1jk=1jL+1j0θθeqeηb'=1jL+1j0θθeqe-E-Eeqb'⇒𝑙𝑖𝑚η→∞⁡(1jk)=1jL (2)
αTafel=∂η∂(𝑙𝑜𝑔⁡(jkjL-jk))=-b(3)
βTafel=-blog(jLj0)⇒j0=jL×10-βTafelb(4)
Figure 2
Fig. 2: Koutecky-Levich plots determined from Fig. 1 of the modified ZIF-8 at 0.62 V vs RHE
Figure 3
Fig. 3: The ORR relative Tafel plots of the modified ZIF-8

Table 1: The kinetic parameters of the modified ZIF-8

Catalystjk
(mA cm-2 mg-1)		jL
(mA cm-2)		j0
(mA cm-2)		n
at 0.7 V vs RHE
50%-Cu(II)/ZIF-8/C0.9219.896.40x10-92.68
50%-Ni(II)/ZIF-8/C0.7225.567.51x10-92.65
50%-Co(II)/ZIF-8/C1.6018.043.50x10-73.43

RRDE measurement can determine the number of exchange electrons (n) and the amount of intermediate generated (Figure 4), which can be done by comparing the current between the ring and disk according to Eq. 5 and 6, respectively. The number of exchanged electrons were listed in Table 1. All of the transition metal-modified ZIF-8 exhibited higher n than the unmodified ZIF-8, which showed an n value of 2.4, indicating that the incorporation of transition metals enhanced their electrocatalytic activity31). In the case of Co-modified ZIF-8 the value of n close to 4 indicated that ORR mechanism of Co(II)/ZIF-8/C mainly occurred via 4-electron pathway, converting oxygen directly to water. Meanwhile, Cu(II) /ZIF-8/C and Ni(II) /ZIF-8/C followed a two-step pathway where O2 was firstly reduced to HO2-, then HO2- was reduced to OH-. It can be seen that metal modification in 50%-Co(II)/ZIF-8 has the most significant increase in performance as an oxygen reduction reaction catalyst. In addition to the presence of Co(II) as the active site, the superiority of Co(II)/ZIF-8 is also attributed to its surface area. The surface area of the modified ZIF-8, measured via nitrogen physisorption, was 1101, 943, and 1137 m2 g-1 for Cu(II), Ni(II), and Co(II), respectively (refer to Figure S3).

n=4N|ID|N|ID|+IR=41+IRN|ID|≤4(5)
χH2O2=2IR/NID+IR/N(6)

The stability of the material for ORR is also studied. The reduction of relative current observed after 2000 s using current–time chronoamperometric on the modified ZIF-8. 50%-Ni(II)/ZIF-8/C and 50%-Co(II)/ZIF-8/C have a relatively stable current, with a relative decrease in current not exceeding 20% (Figure 5). To further investigate the stability of 50%-Co(II)/ZIF-8/C, 100 cycles of CV was performed (Figure 6). The voltammogram displayed a slight decrease in the ORR peak current density, while the potential of the ORR peak remained consistent. A similar result was noted upon comparing the polarization curves before and after 100 cycles of CV, where a slight reduction

Figure 4
Fig. 4: Number of exchange electrons and intermediate generated from the modified ZIF-8
Figure 5
Fig. 5: Current-time chronoamperometric responses of the modified ZIF-8 at 0.7 V vs RHE in the O2-saturated 0.1M KOH

in current density observed, while the onset potential remained unchanged. (Figure 7).

The methanol crossover effect was observed by comparing the cyclic voltammograms, with and without methanol in the electrolyte (Figure 8). The presence of methanol in the system reduced the ORR peak current and shifted its potential to more negative values. This shift in potential is not entirely attributed to a decrease in performance but rather because the potential vs RHE is relative to the electrolyte's pH. Additionally, Co(II)/ZIF-8/C showed resistance to methanol oxidation, as indicated by the absence of a new oxidation peak when methanol was added to the system.

The potential of the Co(II)/ZIF-8 to be used as a catalyst for OER is studied using LSV method. The polarization curve (Figure 9) recorded an overpotential (η) of 0.509 V at a current density of 10 mA cm-2, lower than the overpotential of unmodified ZIF-8 (0.579 V)45).

Figure 6
Fig. 6: Cyclic voltammograms of %50-Co(II)/ZIF-8/C at scan rate of 50 mV/s in the O2-saturated 0.1 M KOH for 100 cycles
Figure 7
Fig. 7: ORR Polarization curves of %50-Co(II)/ZIF-8/C before and after 100 cycles of CV
Figure 8
Fig. 8: Cyclic voltammograms of %50-Co(II)/ZIF-8/C at scan rate of 50 mV/s in the O2-saturated 3 M MeOH in 0.1 M KOH
Figure 9
Fig. 9: OER polarization curve of %50-Co(II)/ZIF-8/C in 0.1 M KOH

Tafel plot (Figure 10) was generated from the polarization curve to investigate the reaction kinetics and mechanisms, providing insights into how effectively an electrode generates current in response to the applied potential. The Co-modified ZIF-8 has a Tafel slope of 125 mV/dec, which is relatively high. Generally, there are two mechanisms for OER in alkaline media: the adsorbate evolution mechanism (AEM) and the lattice oxygen evolution mechanism (LOM). A higher Tafel slope suggests that the OER proceeds through the AEM pathway, where the reaction occurs at a single active site, and its activity is significantly affected by the adsorption energies of the oxygen intermediates46).

The turnover frequency (TOF) value (Figure 11) was determined using the estimated number of active sites. The total oxygen turnovers were calculated from the current density obtained in the OER polarization measurements39). It was assumed that all Co atoms in the catalysts contribute to the OER activity.

Figure 10
Fig. 10: The ORR relative Tafel plots of 50%-Co(II)/ZIF-8/C
Figure 11
Fig. 11: Turnover frequency (TOF) versus η during the OER evaluation

4. Conclusion

Zeolitic Imidazole Frameworks-8 (ZIF-8) modified with transition metals Co(II), Cu(II), or Ni(II) from the original Zn(II) have shown their electrocatalytic activities for ORR and OER. Among the modified ZIF-8s, 50%-Co(II)/ZIF-8/C demonstrated a prominent potential as a catalyst and exhibited the best performance and good stability for oxygen reduction. The electron exchange value of the material reached 3.43 at 0.7 V vs. RHE, indicating a near four-electron pathway, which supports its high catalytic efficiency. As a bifunctional catalyst, 50%-Co(II)/ZIF/C also exhibited a decent overpotential of 0.509 V for OER.

Acknowledgements

This work was developed within the scope of the projects given by Universitas Sebelas Maret through 'HGR-UNS A' (194.2/UN27.22/PT.01.03/2024).

Nomenclature

jthe measured current density (mA/cm2)
jkkinetic current density (mA/cm2)
jlfilmdiffusion-limiting current density in catalyst film (mA/cm2)
jladsdiffusion-limiting current density associated with O2 adsorption in the active site
j0exchange current density (mA/cm2)
nexchange number of electrons
bTafel slope (mV dec-1)
Ncollection efficiency
IDdisk current
IRring current

Greek symbols

Ωrotation rate (rpm)
θdegree of coverage of the catalyst surface (active sites) by oxygen at potential E
θeqdegree of coverage of the catalyst surface (active sites) by oxygen at the equilibrium potential Eeq
ηOverpotential (V)

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