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Article

Transition Metal Exchanged β Zeolites: CoM/β (M = Zn, Ce, and Cu) as Oxygen Electrode in Alkaline Media

by
Jadranka Milikić
1,*,
Katarina Rondović
1,
Ljiljana Damjanović-Vasilić
1,*,
Vladislav Rac
2,
Rastko Vasilić
3 and
Dalibor Stanković
4,5
1
Faculty of Physical Chemistry, University of Belgrade, Studentski trg 12-16, 11158 Belgrade, Serbia
2
Faculty of Agriculture, University of Belgrade, Nemanjina 6, 11080 Belgrade, Serbia
3
Faculty of Physics, University of Belgrade, Studentski trg 12-16, 11000 Belgrade, Serbia
4
Faculty of Chemistry, University of Belgrade, Studentski trg 12-16, 11000 Belgrade, Serbia
5
“VINČA” Institute of Nuclear Sciences-National Institute of the Republic of Serbia, University of Belgrade, Mike Petrovića Alasa 12-14, 11000 Belgrade, Serbia
*
Authors to whom correspondence should be addressed.
Processes 2025, 13(9), 2996; https://doi.org/10.3390/pr13092996
Submission received: 29 July 2025 / Revised: 12 September 2025 / Accepted: 15 September 2025 / Published: 19 September 2025
(This article belongs to the Special Issue Advances in Electrochemical Energy Storage and Conversion Technologies)
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Abstract

The zeolite structure, with its precisely distinct pores, settled cages, and adsorption sites, enables the formation and stabilization of isolated metal centers. These well-defined structures make metal-loaded zeolites promising catalysts. Three different β zeolites were synthesized by an aqueous ion-exchange procedure, firstly with cobalt (Co), and secondly with zinc (Zn), cerium (Ce), and copper (Cu), to make three bimetallic CoZn/β, CoCe/β, and CoCu/β zeolites, respectively. X-ray powder diffraction analysis, Fourier transform infrared spectroscopy, scanning electron microscopy with energy dispersive spectroscopy, and low-temperature nitrogen adsorption analysis revealed the structural, morphological, and surface properties of the studied materials, while optical properties were investigated by UV-Vis diffuse reflectance spectroscopy. The lowest onset potential of 1.67 V was obtained for both CoZn/β and CoCe/β, while the somewhat positive value of 1.70 V was observed for CoCu/β. CoZn/β exhibited the lowest value of Tafel slope of 89 mV dec−1, while slightly higher values of 109 and 113 mV dec−1 were calculated for CoCe/β and CoCu/β during ORR, respectively. CoZn/β showed four-electron pathways of ORR, CoCu/β showed a mixed ORR mechanism, while CoCe/β offered two-electron pathways of ORR. All presented results established that CoZn/β had the highest OER/ORR activity, followed by CoCu/β, while CoCe/β had the lowest activity detected.

1. Introduction

Renewable energy conversion and storage systems are the most investigated in recent decades because of huge global energy and environmental issues [1,2,3]. Namely, metal–air batteries (MABs) such as rechargeable zinc–air batteries (ZABs) [4,5,6,7,8,9,10,11,12,13,14,15] as energy storage and conversion devices have many advantages, including being environmentally friendly and having high theoretical energy density. The main reactions in ZABs during the charge–discharge processes are the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) [16,17,18,19,20]. Developing new bifunctional electrocatalysts for ZABs implies overcoming the slow kinetics of OER/ORR [21,22,23,24,25,26,27,28,29,30,31,32,33] because of their multi-step electron transfer, the high price of state-of-the-art anode or cathode electrocatalysts (such as Pt for ORR, and IrO2 and RuO2 for OER), and low abundance of these noble metal electrocatalysts [1,34]. Hence, it is crucial to make highly efficient OER/ORR catalysts [35,36,37,38] with good mechanical and chemical stability, low price, and practical synthesis [39,40,41,42,43]. Non-noble metals as bifunctional electrocatalysts could be one prospective solution for increasing the ZABs’ performances [44,45].
Low-cost bifunctional transition metal electrocatalysts (TMEs) with nickel (Ni), cobalt (Co), iron (Fe), and zinc (Zn) are promising OER/ORR candidates [45,46,47,48]. Electrocatalysts based on Fe, Co, and nitrogen-co-doped graphene-augmented inorganic alumina nanofibers (Fe/Co-NGr) were studied for OER/ORR in alkaline media. Fe/Co-NGr shows high ORR and OER reversibility (ΔE) of 0.85–0.88 V [44]. These electrocatalysts of Co and Fe co-doping showed better catalytic activity compared to the use of only Fe, which is reflected in the highest electron transfer number (n) value of 3.4 during ORR [44]. Co/CoO hetero-nanoparticle-decorated lignin-derived N-doped porous carbon nanofibers (Co-LCFs-800) gave a half-wave potential (E1/2) of 0.834 V toward ORR and an overpotential at 10 mA cm−2 (η10) of 354 mV during OER [49]. Namely, a small potential gap ΔE of 0.750 V was observed for Co-LCFs-800, which confirmed its impressive OER/ORR activity [49]. A MnS-doped cobalt nitride catalyst (MnS/CoNx) with divergent fiber morphology is synthesized by low-temperature nitriding for the self-assembling layered double hydroxides (LDHs) [50]. MnS/CoNx with very good electrical conductivity shows E1/2 of 0.80 V in ORR and η10 of 290 mV in OER [50]. A bifunctional CoO-Mn3O4 heterostructure (CMH) cathode produced from a Mn-doped zeolitic imidazolate framework showed an n value of 3.97 at 0.6 V toward ORR [51]. Carbon nanosheets embedded with abundant graphene-encapsulated CoO and CoNx (CoO/CoNx–C) were synthesized via a simple one-step pyrolysis process and evaluated for their OER and ORR performance in alkaline media [52]. E1/2 of 0.844 V in ORR and η10 of 384 mV in OER were obtained for CoO/CoNx-C due to its morphological properties [52]. Also, impressive bifunctional OER/ORR activity was noticed in cobalt nanoparticles compressed in N-doped graphitic carbon with a core–shell structure, which was optimized at 800 °C (Co@NC-800) [53]. Co@NC-800 showed E1/2 of 0.82 V in ORR and η10 of 350 mV [53].
Zeolites are crystalline materials with a well-defined microporous structure [54,55], and their remarkable applications in adsorption processes and catalysis result from features such as a large surface area and good thermal stability. The negatively charged framework, usually built from TO4 (T = Si, Al) tetrahedral units linked by shared oxygen atoms, is neutralized by extra-framework cations (typically alkaline or earth-alkaline metals), which can be easily replaced with other cations through an ion-exchange procedure [56]. The amount of exchangeable cations mainly depends on the structure of zeolites and the Si/Al ratio [57]. Different cations, in various amounts, can be introduced into the zeolite structure at distinct sites, thus allowing the preparation of materials with desirable properties for diverse applications [57].
This study focuses on synthetic β-zeolite, which has a three-dimensional channel system with two perpendicular 12-membered ring straight channels (6.6 × 6.7 Å), and one sinusoidal 12-membered ring channel (5.6 × 5.6 Å) [58]. An aqueous ion-exchange procedure is used as a simple and economically justified procedure for the introduction of different transition metal cations in the zeolite’s structure. Thus, three bimetallic electrocatalysts (CoZn/β, CoCe/β, and CoCu/β) were prepared in this work by modification of β zeolite (Si/Al = 19) through an aqueous ion-exchange procedure, firstly with cobalt (Co), and secondly with zinc (Zn), cerium (Ce), and copper (Cu).

2. Materials and Methods

An initial material was the ammonium form of synthetic zeolite β (Si/Al = 19), obtained from Alfa Aesar (Haverhill, MA, USA). Its hydrogen form was obtained by calcination at 500 °C (5 h) in air and marked as β. In the first step, a monometalic electrocatalyst was prepared by an ion-exchange procedure using dilute aqueous solution of a divalent transition metal salt: 5 g of β zeolite was mixed at room temperature in 1 L of 0.003 M Co(NO3)2·6H2O (purity ≥ 98%, purchased from Fluka, Seelze, Germany) solution for 7 days, followed by filtration, rinsing by deionized water, and drying at 80 °C in air (2 h). The obtained Co-exchanged β zeolite exhibited a pale pink color, thus validating the presence of Co2+ (Scheme 1) [59].
In the second step, in order to prepare the bimetallic sample, 1.2 g of Co-exchanged β zeolite was suspended in 240 mL of 0.003 M Zn(NO3)2·6H2O (purity ≥ 99%, purchased from Fluka, Seelze, Germany) aqueous solution and mixed at room temperature (7 days). Afterwards, the sample was passed through a filter (ROBU® Glasfilter-Geräte GmbH, Hattert, Germany), rinsed with deionized water, and dried at 80 °C in air (2 h). The same procedure was followed for the preparation of other bimetallic samples, using 0.003 M Cu(NO3)2·3H2O (purity RPE, purchased from Carlo Erba, Cornaredo, Italy) aqueous solution and 0.003 M Ce(NO3)3·6H2O (purity 99%, purchased from Aldrich, Wyoming, IL, USA) aqueous solution. All prepared zeolites were then calcined at 500 °C in air (5 h) and designated as CoZn/β, CoCe/β, and CoCu/β (Scheme 1).
A multianalytical approach was chosen for the characterization of all materials studied in this work. For crystalline phase identification, the diffraction patterns were collected using a Rigaku Ultima IV diffractometer (Rigaku, Tokyo, Japan) in Bragg–Brentano geometry. Cu Kα radiation (λ = 1.54178 Å) was employed, scanning from 4° to 50° 2θ with 0.020° increments and an acquisition rate of 1° per minute. Additional study of the structural features of the studied zeolites was performed by analysis of the Fourier transform infrared spectra collected on a Nicolet 6700 spectrometer (Thermo Scientific, Waltham, MA, USA) using the KBr pellet technique, within the wavenumber range of 4000 cm−1 to 400 cm−1, with a resolution of 4 cm−1 and 64 acquisitions. Surface morphology and elemental composition of all analyzed materials were investigated using the Phenom ProX desktop scanning electron microscope (ThermoFisher ScientificTM, Waltham, MA, USA), which is equipped with a fully integrated Energy Dispersive Spectrometer (ThermoFisher ScientificTM, Waltham, MA, USA) and elemental identification software (verison 3.7.3.0). Before the analysis, the powders were placed on a carbon tape and covered with Cu by a CY-PSP180G-1TA plasma-coated sputtering apparatus from Zhengzhou CY Scientific. The analyses were conducted under an acceleration voltage of 15 kV. The particle size distribution was obtained using ImageJ software (version 1.53k). In order to obtain a representative composition, a minimum of five areas of 65 × 65 µm2 were analyzed on each sample. The copper content used for coating the CoCu/β sample was determined by averaging the measured values from the β, CoCe/β, and CoZn/β samples, which were coated simultaneously with CoCu/β. This average value was then subtracted from the measured copper content of the CoCu/β sample. UV-Vis diffuse reflectance spectra were recorded on Agilent Cary UV-Vis-NIR 5000 spectrophotometer (Agilent, Santa Clara, CA, USA) equipped with an integration sphere in the 200–800 nm range, 1 nm data interval, and scan rate 600 nm min−1, by polytetrafluoroethylene (PTFE) as reference material in order to study optical properties. The Microtrac Belsorp Mini X instrument (MicrotracBEL Corporation, Osaka, Japan) was used for the specific surface areas and porosity measurements. Prior to the analysis, the investigated materials were degassed in a vacuum at 350 °C for 4 h. Using the Brunauer–Emmett–Teller (BET) and t-plot methods, the textural parameters were calculated from the nitrogen adsorption–desorption isotherms.
Electrochemical investigations were completed by Ivium VO1107 Potentiostat/Galvanostat (Eindhoven, The Netherlands) and in a three-electrode electrochemical glass cell containing 1 M KOH solution (40 cm3). A graphite rod (GrafTech International Ltd., Brooklyn Heights, NY, USA) and saturated calomel electrode (SCE) (SI Analytics, Weilheim, Germany) were set as counter and reference electrodes, respectively. Here, the potentials are calculated relative to the reversible hydrogen electrode (RHE) via the following equation: ERHE = ESCE + 0.242 V + 0.059 · pH solution.
Catalytic ink was prepared with 5 mg of the found powder of CoZn/β, CoCe/β, and CoCu/β zeolites, 0.6 mg of Vulcan XC-72R, 10 µL of Nafion (5 wt%, DuPont (now part of Chemours), Wilmington, DE, USA), and 490 µL of ethanol by ultrasonic mixing for 1 h. The glassy carbon rotating disk electrode (0.19625 cm2 area) with 35 μL of pipetted catalytic ink presents the working electrode. The optimum balance between adequate active site density and effective mass or charge transfer was found by loading ranging from 0.2 to 2 mg cm−2 [60], which is in accordance with the loading presented here.
A double-layer capacitance (Cdl) investigation of three CoM/β zeolites was performed at different scan rates (5–50 mV s−1) in a nitrogen (N2)-saturated 1 M KOH solution.
The potential range from 1.2 to 2 V was set for linear sweep voltammetry (LSV) at a scan rate of 20 mV s−1, and a rotation rate of 1800 rpm was used for OER examination. Electrochemical impedance spectroscopy measurements in the frequency range of 100 kHz to 0.1 Hz, with a 5 mV amplitude, were performed at 1.9 V. Chronoamperometry investigations were performed at 1.8 V for 3600 s for CoCe/β, and CoCu/β zeolites, and at 1200 rpm for 10,000 s for CoZn/β.
ORR measurements of three β zeolites were performed by LSV methods in the potential range from 0.3 to 1 V in N2- and O2-saturated alkaline solution, respectively. The results obtained by LSV measurements were used for ORR number-exchanged electron calculations at different rotation rates (300–2400 rpm), and the CA ORR test was performed at 0.7 V for 11,000 s.

3. Results

3.1. Characterization of CoM/β (M = Zn, Ce, and Cu) Electrocatalyst

The XRPD analysis was employed to study degree of structural order of materials obtained by the ion-exchange procedure. The results obtained for the bimetallic catalysts are presented in Figure 1A, alongside the diffractogram of the parent zeolite for comparison. The XRD patterns of CoZn/β, CoCe/β, and CoCu/β catalysts display only the reflections characteristic for the parent β zeolite. The peaks of ion-exchanged zeolites detected at 2θ, i.e., 7.8°, 13.5°, 14.7°, 21.5°, 22.6°, 25.5°, 27.2°, 28.9°, 29.6°, 33.6°, 43.8°, correspond well with the standard diffraction angles for the crystal structure of β zeolite [61]. The observed diffraction maxima of the initial zeolite suggest that the β structure is maintained after the ion-exchange procedure, showing no noticeable changes in crystallinity. Additionally, no new phases related to Co, Cu, Zn, or Ce species were identified, indicating a low metal content and distribution of metal cations within the zeolite framework. It is also possible that small quantities of metal oxides can be present on the surface of the samples as amorphous or dispersed as nanoclusters.
Figure 1B shows the FTIR spectra of both the parent and ion-exchanged β zeolites displaying characteristic vibrations of zeolite framework: the band at 795 cm−1 originates from external symmetric T-O (T = Si, Al) stretching vibrations, the band at 1225 cm−1 originates from external T-O-T asymmetric stretching vibrations, and the band at 1095 cm−1 originates from the internal T-O-T asymmetric stretching vibrations. The bands originating from bending vibrations of T-O bonds have been detected at 465 cm−1 and 623 cm−1 [62]. Moreover, typical for β zeolite structure, the band centered at 950 cm−1, assigned to surface Si-OH stretching vibrations [63], and two bands, centered at 572 cm−1 and 525 cm−1, originating from vibrations of six-membered and five-membered rings, respectively, have been detected [62]. FTIR spectra of CoZn/β, CoCe/β, and CoCu/β zeolites show that the ion-exchange process does not alter the zeolite framework structure, which is consistent with the XRPD analysis results.
The UV-Vis DR spectra of the parent and ion-exchanged β zeolites, recorded to study the oxidation state of transition metal cations in ion-exchanged β zeolites, are shown in Figure 1C. The bands detected at 225 and 270 nm are assigned to the zeolite framework and thus are not related to the transition metals present in the electrocatalysts. The broadband, positioned at about 515 nm, detected the UV-Vis DR spectra of CoZn/β and CoCe/β zeolites, which originate from octahedrally coordinated Co2+ ions [59]. In the case of the CoCe/β zeolite, the strong absorption band at 268 nm is due to the 4f-5d transition of the Ce3+ ion [64]. No specific signal from zinc was observed in the UV-Vis DR spectrum of CoZn/β zeolite, most likely because of the overlap with the signal originating from the zeolite structure and signal from Zn2+ ions (anticipated at 250 nm) [65]. The presence of Cu2+ species in the UV-Vis DR spectrum CoCu/β zeolite is confirmed by strong absorption below 400 nm, with maxima at 230 nm [66] and a broad signal between 600 and 800 nm that could be assigned to d-d transitions of isolated, distorted octahedrally coordinated Cu2+ ions [67]. Hence, the presented UV-Vis DR spectra support the conclusion that the investigated ion-exchanged β zeolites contain Co, Zn, and Cu as divalent and Ce as trivalent ions. While UV-Vis DR spectroscopy as an optical technique provides information on the presence of active sites and their electronic structure in ion-exchanged zeolites, elemental composition and chemical state information (e.g., confirming the presence and state of surface species, such as oxides) can only be obtained by surface-sensitive techniques, such as X-Ray Photoelectron Spectroscopy (XPS). Thus, further investigation is required for thorough characterization of surface oxidation states and their roles in catalytic processes.
Low-temperature nitrogen-adsorption measurements were carried out to evaluate the surface area and pore structure of all investigated materials. The resulting isotherms are shown in Figure 1D. A summary of the textural properties of the studied zeolites is provided in Table 1.
All examined materials exhibit a type I adsorption isotherm, in accordance with the IUPAC classification, typical for microporous materials. The obtained results confirm that the porous characteristics of the parent zeolite remained intact after ion exchange. The ion-exchanged zeolites show some decrease in surface area, but the overall changes in textural properties cannot be considered significant.
SEM images shown in Figure 2 further support previous observations, indicating no morphological changes in the β zeolite after incorporation of Co, Cu, Zn, and Ce. In all three ion-exchanged β zeolites, well-defined micro crystals of different sizes were detected (Figure 2B–D). The EDS spectra of CoZn/β, CoCe/β, and CoCu/β are presented in Figures S1B, S1C, and S1D, respectively, with elemental mapping of the CoZn/β zeolite showing a uniform distribution of detected elements. The elemental composition of parent and ion-exchanged β zeolites obtained by EDS measurements is shown in Table 2.
Figure S2 shows the corresponding particle size distribution of the prepared samples. Average particle diameters of 0.60, 0.57, and 0.65 µm were found for CoZn/β, CoCe/β, and CoCu/β electrocatalysts (Figures S2A, S2B and S2C), respectively. All bimetallic electrocatalysts show diameters of particles in the 0.3 to 1.2 µm range.

3.2. OER Study of CoM/β Zeolites

Cyclic voltammograms recorded in an N2-saturated solution for the prepared zeolites were found within the non-Faradaic potential region at various scan rates (Figure S3). The double-layer capacitance (Cdl) was calculated from the CV data and is directly proportional to the electrochemically active surface area (ECSA) [68,69,70]. Cdl is a kind of supercapacitor that stores energy by collecting an electrical charge transferred between the electrodes and the electrolyte [71]. Cdl of 0.7, 0.6, and 0.6 mF cm−2 were observed for CoZn/β, CoCe/β, and CoCu/β, respectively.
Figure 3A shows OER polarization curves of CoZn/β, CoCe/β, and CoCu/β electrocatalysts. One oxidation peak of all three cobalt zeolites was noticed in the potential range from 1.46 to 1.51 V, which is associated with the oxidation of Co(III) to Co(IV) before OER starts, which is typical for alkaline media, and it follows the oxidation of Co(II) to Co(III) [72]. The lowest onset potential (Eonset) and corresponding overpotentials (ηonset) of 1.67 V and 470 mV were obtained for both CoZn/β and CoCe/β, while the somewhat positive values of 1.70 V and 500 mV were observed for CoCu/β. Again, CoZn/β and CoCe/β showed similar Tafel slopes, while CoCu/β showed a somewhat higher value. OER current density of 15.8 mA cm−2 was noticed for CoZn/β, followed by 11.1 and 5.4 mA cm−2 at 2 V for CoCe/β and CoCu/β, respectively. Four different cerium-exchanged zeolites, synthetic 13X, and natural clinoptilolite (Ce-13X cal, Ce-13X, Ce-Cli cal, and Ce-Cli) showed Eonset from 1.60 to 1.73 V, which are almost identical to the values obtained in this work [73]. Another type of zeolite, zeolite ZSM-5 and zeolite β, also altered with cerium and then calcined to prepare Ce-ZSM-5, Ce-ZSM-5 cal, Ce-β, and Ce-β cal electrodes, gave Eonset from 1.61 to 1.72 V, like values obtained here [74]. To some extent, lower values of Eonset of 1.61 and 1.64 V are noticed for CoNi-ZSM5 and CoZn-ZSM5 zeolites, which are prepared by the ion-exchanged procedure of synthetic zeolite ZSM-5 [70].
The Tafel slopes of 129, 133, and 168 mV dec−1 for CoZn/β, CoCe/β, and CoCu/β (Table 3) are calculated and presented in Figure 3B. These Tafel values are comparable with values obtained for similar zeolite OER electrocatalysts [70,73,74]. Graphene-wrapped CoO and CoNx (CoO/CoNx-C and P-CoO/CoNx-C) electrocatalysts showed 180 and 198 mV dec−1 during ORR, which is higher than all three Tafel values obtained here [52], while the cobalt nitride catalyst (MnS/CoNx) [50] with 116.9 mV dec−1 gave comparable values with the value obtained for CoZn/β (Table 3). Defect-rich N-doped carbon nanosheets supported Co3O4 nanoparticles named Co-NC and Co3O4@NCNs and gave 220 and 107 mV dec−1 toward OER, respectively [75]. Both values are similar to those obtained here. Electrocatalysts containing Pt and Ni nanoparticles anchored on nitrogen-doped mixed metal oxides (Pt/Mn2O3-NiO-N (1:1) and Pt/Mn2O3-NiO- N (1:2)) gave significantly higher Tafel slopes of 249 and 245 mV dec−1 than those observed here [76].
Electrochemical impedance spectroscopy parameters of three CoM/β zeolites are obtained by fitting EIS data through an appropriate equivalent circuit (Figure 3C inset). Similar values for charge-transfer resistance (Rct) of 54.6 and 60.4 Ω were observed for CoCe/β and CoZn/β, while a significantly higher value of 268.7 Ω was found for CoCu/β (Table 4). These EIS results are in agreement with the results attained by LSV analysis during OER, where CoZn/β and CoCe/β showed similar current densities, while CoCu/β gave even three times lower OER current density than CoZn/β and almost two times lower value than CoCe/β. Slightly higher values of Rct of 69.3 and 79.8 Ω were obtained for the same type of zeolite Ce-β and Ce-β cal [74] than the values obtained here for CoZn/β and CoCe/β, respectively.
Figure 3D displays the chronoamperometry curves of three bimetal β zeolites, where the same trend in values of OER current density was observed, as from LSV and EIS results. Namely, CoCe/β gave current densities of 2.1 mA cm−2, while a two times lower value of 1.0 mA cm−2 was noticed for CoCu/β. Long-term test stability of CoZn/β was run at 1200 rpm for 10000 s (Figure 3E) to remove lots of bubbles on the surface of the electrode during OER. Namely, OER current densities noted at 180th and 10000th s were decreased by 20%. The stability of the CoZn/β and other zeolites could be additionally improved with an increasing rotation rate during OER. The obtained stability (Figure 3D,E) of these zeolites is not so good and could be a result of the existence of lots of bubbles on the electrode surface during OER. Namely, nucleation, growth, coalescence, and detachment present the four main steps in the life cycle of the gas bubble on the surface of the electrode. The first step begins when a small gas cluster forms in a supersaturated solution (nucleation) [79]. The bubble then grows as it absorbs more gas, and this action presents the second step (growth). As the bubble becomes larger, the buoyant force increases until it overcomes the adhesion to the electrode, causing the bubble to detach. If two bubbles come into contact, they may merge to reduce surface energy (coalescence) [79]. All of these actions have a huge impact on mass transfer, interfacial supersaturation, and ohmic resistance during OER. Additionally, covering the surface of the electrode leads to a decrease in currents during OER and decreases catalyst stability [79].
The highest OER activity of the CoZn/β electrode could be a consequence of the incorporation of Zn2+ in the Co/β structure [80,81]. Namely, increasing the distance between the Co species and the O bonds due to Zn2+ incorporation could enhance the adsorption of OH− on the Co3+ to form Co4+ by the next equation, CoOOH + OH− → CoO2 + H2O + e−, where Co4+ presents an active site for OER [80].

3.3. ORR Study of CoM/β Zeolites

CoZn/β, CoCe/β, and CoCu/β were examined during ORR by CV in N2- and O2-saturated electrolyte solutions (Figure S4). All three CoM/β zeolites showed no reduction peak in the N2-saturated solution, while a clear ORR peak was observed in the O2-saturated solution, which confirms that bimetal β zeolites are active for the reduction in oxygen in alkaline media. The highest peak current density of -0.53 mA cm−2 at 0.65 V was noticed for CoZn/β, followed by −0.40 and −0.36 mA cm−2 for CoCe/β, and CoCu/β at 0.67 and 0.62 V, respectively. These outcomes correspond well with similar studies in the literature [45]. In our previous work, Ce-β cal gave an ORR peak at 0.61 V [74], as examined here for CoCu/β, while Ce-β showed an ORR peak at 0.71 V [74], which is near the obtained ORR potential values for CoCe/β.
LSV RDE curves (Figure 4A) show a similar ORR current density trend of the investigated bimetal β zeolites where CoZn/β gave the highest value. The same value of ORR onset potential (Eonset) of 0.76 V and half-wave potential (E1/2) of 0.66 V was noticed for all three β zeolites (Table 5). The obtained Eonset is equal to the Eonset noticed for NiCo/β and NiCu/β during ORR [67] and is lower than values obtained for NiZn/β [67], Ce-ZSM-5 [74], Ce-ZSM-5 cal [74], Ce-β cal [74], Ce-Cli [73], Ce-Cli cal [73], and Ce-13X [73] in our previous works. The same comparing trend of similar zeolite ORR electrocatalysts was noticed for the obtained E1/2 values. An E1/2 value of 0.68 V was observed for Co/Fe-NGr-1 [44], which is almost the same as the value obtained for the zeolites examined here, while MnO/CoNx and CoNx, with values of 0.748 and 0.752 V [50], showed higher E1/2 values than those observed here.
Figure 4B shows Tafel plots and slope values (Table 5) obtained from LSV RDE curves at 1600 rpm, where CoZn/β showed the lowest value of 89 mV dec−1, while higher values of 109 and 113 mV dec−1 were obtained for CoCe/β and CoCu/β, respectively. Approximate Tafel values are observed for comparable zeolite electrocatalysts, ranging from 72 to 102 mV dec−1 [73,74], while some zeolite electrocatalysts showed significantly higher Tafel slopes, from 145 to 399 mV dec−1 [73,74]. Other materials doped with Co exhibited Tafel slopes from 53 to 115 mV dec−1, which are comparable with the reported values [33,44,50,52].
CA curve of CoZn/β during ORR is presented in Figure 4C. Approximate value of the current densities of −1.03 mA cm−2 and −0.98 mA cm−2 were found at the 198th and 11000th s, respectively. Decreasing ORR current densities of only 5% were noticed for CoZn/β during ORR, which presents its excellent stability.
Figure 5 shows ORR LSV RDE polarization curves of CoZn/β, CoCe/β, and CoCu/β. The number of electrons exchanged (n) during ORR was calculated by the Koutecky–Levich equation based on the LSV results obtained with the RDE [73]. Namely, CoZn/β offered n values from 3.4 to 3.6, followed by CoCu/β with n values from 2.6 to 2.8, and CoCe/β from 2.0 to 2.6 (Table 3, Figure 5D). As it is presented, CoZn/β showed four-electron pathways of ORR, CoCu/β showed a mixed ORR mechanism, while CoCe/β offered two-electron pathways of ORR. In our previous paper, NiZn/β [77] gave n values up to 3.3, which is almost the same as that obtained here for CoZn/β. Four examined cerium zeolites [73] showed n values from 2.2 to 2.7 during ORR, which are comparable n values to those calculated here for CoCu/β and CoCe/β.
Density functional theory (DFT) and X-ray adsorption spectroscopy (XAS) showed that the enhanced ORR activity of CoZn/β could be attributed to synergistic effects between cobalt (Co2+) and zinc (Zn2+), which lead to changes in the electronic structure and facilitate improved charge transfer pathways [82]. Namely, zinc in the bimetallic catalyst modulates the electronic properties of cobalt sites, optimizing the adsorption and activation of reaction intermediates [82,83]. This synergy enhances the overall catalytic ORR efficiency [83].

4. Conclusions

CoZn/β, CoCe/β, and CoCu/β were successfully synthesized, thoroughly characterized, and observed for OER/ORR in alkaline media. All three zeolites exhibited bifunctional activity, where CoZn/β presented the best catalytic activity, followed by CoCu/β, while CoCe/β presented the lowest OER/ORR activity. The Tafel slopes of 129, 133, and 168 mV dec−1 for CoZn/β, CoCe/β, and CoCu/β are considered during OER. Charge-transfer resistances of 54.6 and 60.4 Ω were observed for CoCe/β and CoZn/β, while a significantly higher value of 268.7 Ω was found for CoCu/β toward OER. The same value of ORR onset potential of 0.76 V and half-wave potential of 0.66 V was noticed for all three β zeolites. CoZn/β gave n values from 3.4 to 3.6, followed by CoCu/β with n values from 2.6 to 2.8, and CoCe/β from 2.0 to 2.6. Overall, all materials synthesized here showed promising bifunctional activity, demonstrating that these inexpensive and widely available materials could be a favorable alternative for applications in these fields.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/pr13092996/s1, Figure S1. The corresponding EDS spectra of β (A), CoZn/β (B), CoCe/β (C), and CoCu/β (D), and the elemental mapping of CoZn/β. Figure S2. Particle size distribution of CoZn/β (A), CoCe/β (B), and CoCu/β (C); Figure S3. CVs of CoZn/β (A), CoCe/β (B), and CoCu/β (C) at different scan rates ranging from 5 to 50 mV s−1 with the corresponding double-layer capacitance plots (D) in an N2-saturated 1 M KOH solution; Figure S4. CVs of CoZn/β (A), CoCe/β (B), and CoCu/β (C) at a scan rate of 20 mV s−1 in N2- and O2-saturated 1 M KOH solution.

Author Contributions

Conceptualization, J.M. and L.D.-V.; formal analysis, J.M., K.R., V.R., R.V., and L.D.-V.; investigation, J.M., L.D.-V., K.R., V.R., and R.V.; supervision, J.M. and L.D.-V.; validation, J.M. and L.D.-V.; visualization, J.M., K.R., and L.D.-V.; writing—original draft, J.M., K.R., and L.D.-V.; writing—review and editing, J.M., L.D.-V., and D.S. All authors have read and agreed to the published version of the manuscript.

Funding

The authors acknowledge the financial support from the Ministry of Science, Technological Development and Innovation of the Republic of Serbia (contract no. 451-03-136/2025-03/200146, 451-03-137/2025-03/200146, 451-03-136/2025-03/200162, 451-03-137/2025-03/200116 and 451-03-136/2025-03/200168).

Data Availability Statement

Data are available upon request from the corresponding authors.

Conflicts of Interest

The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

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Processes 13 02996 sch001
Scheme 1. Procedure of the synthesis of CoZn/β, CoCe/β, and CoCu/β zeolites.
Scheme 1. Procedure of the synthesis of CoZn/β, CoCe/β, and CoCu/β zeolites.
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Processes 13 02996 g001
Figure 1. Diffraction patterns (A), FTIR spectra (B), UV-Vis DR spectra (C), and N2 adsorption–desorption isotherms (D) of parent and ion-exchanged β zeolites.
Figure 1. Diffraction patterns (A), FTIR spectra (B), UV-Vis DR spectra (C), and N2 adsorption–desorption isotherms (D) of parent and ion-exchanged β zeolites.
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Figure 2. SEM images of β (A), CoZn/β (B), CoCe/β (C), and CoCu/β (D).
Figure 2. SEM images of β (A), CoZn/β (B), CoCe/β (C), and CoCu/β (D).
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Figure 3. OER polarization curves recorded at a rotation rate of 1800 rpm (A), with the corresponding Tafel plots (B), Nyquist plots measured at 1.9 V with the equivalent circuit shown in the inset (C), chronoamperometric curves at 1.8 V for CoCe/β and CoCu/β (D), and the chronoamperometric curve of CoZn/β recorded at 1200 rpm (E) in 1 M KOH.
Figure 3. OER polarization curves recorded at a rotation rate of 1800 rpm (A), with the corresponding Tafel plots (B), Nyquist plots measured at 1.9 V with the equivalent circuit shown in the inset (C), chronoamperometric curves at 1.8 V for CoCe/β and CoCu/β (D), and the chronoamperometric curve of CoZn/β recorded at 1200 rpm (E) in 1 M KOH.
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Figure 4. ORR polarization curves obtained at 1800 rpm (A), with corresponding Tafel plots (B), and chronoamperometric (CA) curve at 0.7 V (C) of CoZn/β in O2-saturated 1 M KOH.
Figure 4. ORR polarization curves obtained at 1800 rpm (A), with corresponding Tafel plots (B), and chronoamperometric (CA) curve at 0.7 V (C) of CoZn/β in O2-saturated 1 M KOH.
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Figure 5. ORR RDE polarization curves at 20 mV s−1 of CoZn/β (A), CoCe/β (B), and CoCu/β (C) from 300 to 2400 rpm, in O2-saturated 1 M KOH with the corresponding number of exchanged electrons of all zeolites (D).
Figure 5. ORR RDE polarization curves at 20 mV s−1 of CoZn/β (A), CoCe/β (B), and CoCu/β (C) from 300 to 2400 rpm, in O2-saturated 1 M KOH with the corresponding number of exchanged electrons of all zeolites (D).
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Table 1. Textural properties of parent and ion-exchanged β zeolites obtained by BET analysis and t-plot methods.
Table 1. Textural properties of parent and ion-exchanged β zeolites obtained by BET analysis and t-plot methods.
ZeolitesSBET a,
m2/g		Sexternal b,
m2/g		Vmicro c,
cm3/g		Vtot d,
cm3/g
β6381320.2000.361
CoZn/β6371390.1980.436
CoCe/β6221420.1910.403
CoCu/β6281380.1940.437
a SBET—specific surface area calculated using Brunauer–Emmett–Teller (BET) method from N2 isotherms; Rouquerol correction for microporous materials, ISO 9277 p/p0 ≈ 0–0.1. b Sexternal—calculated by t-plot, t = 0.40–0.6. c Vmicro—micropore volume, calculated by t-plot, t = 0.40–0.6. d Vtot—total pore volume calculated from the desorption isotherm at p/p0 = 0.99.
Table 2. Surface elemental compositions of parent and ion-exchanged β zeolites obtained by EDS measurements. Standard deviations are given in the parentheses.
Table 2. Surface elemental compositions of parent and ion-exchanged β zeolites obtained by EDS measurements. Standard deviations are given in the parentheses.
ElementOSiAlCoZnCeCu
wt %
β53.79 (1.57)43.75 (1.23)2.46 (0.05)
CoZn/β51.16 (0.96)44.93 (0.71)2.43 (0.05)0.15 (0.10)1.34 (0.97)
CoCe/β49.35 (0.83)45.95 (0.20)2.43 (0.02)0.23 (0.29) 2.04 (0.26)
CoCu/β52.15 (0.53)44.54 (0.44)2.43 (0.05)0.22 (0.28) 0.66 (0.27)
Table 3. Comparing the OER results of CoZn/β, CoCe/β, and CoCu/β electrodes with similar zeolite materials from the literature reports.
Table 3. Comparing the OER results of CoZn/β, CoCe/β, and CoCu/β electrodes with similar zeolite materials from the literature reports.
OER
Electrocatalysts		ElectrolyteMass Loading/mg cm−2Eonset/VȠonset/mVb/mV dec−1j at 2 V/mA cm−2References
CoZn/β1 M KOH21.6747012915.8This work.
CoCe/β1 M KOH21.6747013311.1This work.
CoCu/β1 M KOH21.705001685.4This work.
NiCo/β1 M KOH21.6646011934.2[77]
NiCu/β1 M KOH21.816102003.0[77]
NiZn/β1 M KOH21.6343017119.4[77]
Co-ZSM51 M KOH31.614102696.8[70]
CoZn-ZSM51 M KOH31.644402342.3[70]
CoNi-ZSM51 M KOH31.614101349.5[70]
Ce-ZSM-51 M KOH31.684802071.6[74]
Ce-ZSM-5 cal1 M KOH31.664602021.7[74]
Ce-β cal1 M KOH31.614101147.3[74]
Ce-Cli 1 M KOH31.735302201.2[73]
Ce-Cli cal1 M KOH31.694902781.54[73]
Ce-13X 1 M KOH31.674702802.14[73]
Ce-13X cal1 M KOH31.604002964.6[73]
NiA1 M KOH1//463~13[78]
NiX1 M KOH1//842~3[78]
CoO/CoNx-C0.1 M KOH0.5//180/[52]
P-CoO/CoNx-C0.1 M KOH0.5//198/[52]
MnS/CoNx0.1 M KOH0.26//116.9/[50]
Co-NC0.1 M KOH///220/[75]
Co3O4@NCNs0.1 M KOH///107/[75]
Pt/Mn2O3-NiO-N (1:1)0.1 M KOH0.2//249∼25[76]
Pt/Mn2O3-NiO- N (1:2)0.1 M KOH0.2//245∼27[76]
Table 4. EIS results of CoZn/β, CoCe/β, and CoCu/β in 1 M KOH at 1.9 V.
Table 4. EIS results of CoZn/β, CoCe/β, and CoCu/β in 1 M KOH at 1.9 V.
ElectrocatalystRs (Ω)Rct (Ω)Qe (mF)
CoZn/β34.960.41.9 × 10−4
CoCe/β39.854.61.1 × 10−4
CoCu/β34.7268.71.1 × 10−4
Rs—electrolyte resistance, Rct—resistances of the charge transfer reaction, Qe—quasi-double-layer capacitor.
Table 5. Comparison of ORR results of CoZn/β, CoCe/β, and CoCu/β electrocatalysts from the literature reports.
Table 5. Comparison of ORR results of CoZn/β, CoCe/β, and CoCu/β electrocatalysts from the literature reports.
ORR ElectrocatalystsElectrolyteMass Loading/mg cm−2Eonset/VE1/2/Vb/mV dec−1nReferences
CoZn/β1 M KOH20.760.66893.4–3.6This work.
CoCe/β1 M KOH20.760.661092.6–2.8This work.
CoCu/β1 M KOH20.760.661132.0–2.6This work.
NiCo/β1 M KOH20.740.651022.2–2.4[77]
NiCu/β1 M KOH20.740.66912.3–2.4[77]
NiZn/β1 M KOH20.760.68813.0–3.3[77]
Ce-ZSM-51 M KOH30.820.77792.3–2.5[74]
Ce-ZSM-5 cal1 M KOH30.810.73702.3–2.4[74]
Ce-β cal1 M KOH30.790.711781.6–1.9[74]
Ce-Cli 1 M KOH30.820.713992.4–2.7[73]
Ce-Cli cal1 M KOH30.820.702282.2–2.6[73]
Ce-13X 1 M KOH30.820.711452.3–2.7 [73]
Ce-13X cal1 M KOH30.840.75802.2–2.4[73]
CoO/CoNx-C0.1 M KOH0.5/0.844100/[52]
P-CoO/CoNx-C0.1 M KOH0.5/0.788103/[52]
MnO/CoNx0.1 M KOH0.26/0.748100.9/[50]
CoNx0.1 M KOH0.26/0.75293.2/[50]
Fe/Co-NGr-10.1 M KOH0.2/0.6853/[44]
Co3O4@NGC@CuO-0.60.1 M KOH/0.960.865115 [33]
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Milikić, J.; Rondović, K.; Damjanović-Vasilić, L.; Rac, V.; Vasilić, R.; Stanković, D. Transition Metal Exchanged β Zeolites: CoM/β (M = Zn, Ce, and Cu) as Oxygen Electrode in Alkaline Media. Processes 2025, 13, 2996. https://doi.org/10.3390/pr13092996

AMA Style

Milikić J, Rondović K, Damjanović-Vasilić L, Rac V, Vasilić R, Stanković D. Transition Metal Exchanged β Zeolites: CoM/β (M = Zn, Ce, and Cu) as Oxygen Electrode in Alkaline Media. Processes. 2025; 13(9):2996. https://doi.org/10.3390/pr13092996

Chicago/Turabian Style

Milikić, Jadranka, Katarina Rondović, Ljiljana Damjanović-Vasilić, Vladislav Rac, Rastko Vasilić, and Dalibor Stanković. 2025. "Transition Metal Exchanged β Zeolites: CoM/β (M = Zn, Ce, and Cu) as Oxygen Electrode in Alkaline Media" Processes 13, no. 9: 2996. https://doi.org/10.3390/pr13092996

APA Style

Milikić, J., Rondović, K., Damjanović-Vasilić, L., Rac, V., Vasilić, R., & Stanković, D. (2025). Transition Metal Exchanged β Zeolites: CoM/β (M = Zn, Ce, and Cu) as Oxygen Electrode in Alkaline Media. Processes, 13(9), 2996. https://doi.org/10.3390/pr13092996

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