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. 2026 Sep 28;11(40):61946–61957. doi: 10.1021/acsomega.6c07951

Superhydrophobic Porous Coordination Polymer-Confined Co3O4 Nanoparticles: A Sustainable Strategy for Electrochemical Water Splitting

Ravula Nagaraju †,‡, Litun Kumar Pradhan §,∥, J N Behera §,∥, Koya Prabhakara Rao †,‡,*
PMCID: PMC13670867  PMID: 42890035

Abstract

In this work, we report three robust, cost-effective, and efficient post-synthetic nanoparticle-embedded metal–organic framework (MOF)-based electrocatalysts, Co 3 O 4 @Zn 2 M 2 (μ 2 –OH) 2 (BTMB) 2 (M = Zn (1), Co (2), and Ni (3)), for overall water splitting. The pristine frameworks, Zn 2 M 2 (μ 2 –OH) 2 (BTMB) 2 , were transformed from relatively low-conducting materials into conductive analogues through post-synthetic encapsulation of approximately 2.5–3.1 wt % Co3O4 nanoparticles via a sonochemical approach. Successful nanoparticle incorporation and preservation of framework integrity were confirmed by PXRD, SEM, TEM, XPS, EDX, ICP-OES, and TGA analyses. Notably, the Co3O4@MOF composites exhibited enhanced superhydrophobicity, displaying water contact angles of 162.41° (1), 166.42° (2), and 164.47° (3), compared to those of the corresponding pristine MOFs (155.5°, 159.3°, and 160.8°, respectively), which is one of the key steps for enhancing the electrochemical activity of this type of composite. The Co3O4@MOF composites achieved low overpotentials for the hydrogen evolution reaction (HER) of 196, 147, and 169 mV, and for the oxygen evolution reaction (OER) of 499, 425, and 486 mV, respectively, for 1–3 in 1 M KOH electrolyte. These catalysts also exhibited low Tafel slopes of 101, 94, and 96 mV dec–1 for HER and 123, 118, and 122 mV dec–1 for OER, indicating significantly improved electrocatalytic activity and reaction kinetics. Further, we also carried out an overall water-splitting study to understand their bifunctional behavior under similar electrochemical conditions, delivering a cell potential of 1.78 V at 10 mA cm–2. Compared with the parent MOFs, the nanoparticle-embedded composites demonstrated nearly fourfold enhancement in HER catalytic performance. Among the series, 2 exhibited the best overall electrocatalytic performance, combining the lowest overpotentials, superior kinetics, and remarkable operational stability, maintaining consistent activity for more than 50 h under continuous electrolysis. The integration of Co3O4 nanoparticles into a MOF matrix improves superhydrophobicity and durability, creating affordable, efficient electrocatalysts for overall water splitting and green hydrogen production.

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Introduction

In pursuit of global sustainability and net-zero emissions in energy production, demand for clean energy is increasing at an accelerated pace. The extensive combustion of conventional fossil fuels such as coal, gas, and crude oil contributes to global warming and adverse effects on the environment. , Modern research has found that “green hydrogen”, which offers high energy density and serves as an emerging and potential clean alternative to fossil fuels, facilitates a pathway toward achieving the goal of sustainable decarbonization. , The conventional hydrogen production methods, namely coal gasification, steam methane reforming, and similar processes, are unable to meet the goal of a net-zero carbon footprint due to their inherent drawbacks, including high CO2 emissions. − In this regard, researchers across the globe have shifted their focus toward the environmentally friendly process of water electrolysis. , Water electrolysis has garnered widespread attention for its environmentally compatible approach, with no harmful emissions into the atmosphere, and its efficient energy conversion method for producing green hydrogen, which involves the splitting of water into hydrogen and oxygen. , Using hydrogen as fuel produces only water vapor as a by-product without any greenhouse gas emissions like CO2 into the atmosphere. , To surmount the kinetic barriers that arise primarily and govern the sluggish multi-electron transfer kinetics of the HER and most critically the OER, the employment of highly performant electrocatalysts is indispensable. Though noble metals such as Pt, RuO2, and IrO2 exhibit predominant catalytic activity, their high cost and low abundance hinder the wide-scale real-time applications. , To mitigate these challenges, tremendous efforts have paved the way for developing more abundant, cost-effective, non-noble catalysts, including layered double hydroxides (LDHs), heterostructure materials, − transition metal sulfides, phosphides etc. To further improve the catalytic efficiency, researchers have developed porous materials, multidimensional materials, and hybrid materials and also combinations of the above-mentioned materials.

Among various materials, the metal organic frameworks (MOFs) − or porous coordination polymers (PCPs), which are a periodic array of metal ions or metal clusters coordinated to multidentate organic ligands. Indeed, MOFs have a wide range of applicability in catalysis, gas storage, sensing, drug delivery, and adsorption due to their diverse structural tenability, large surface area, and stability in chemical environments. − The catalytically active metal sites present in the MOFs promote excellent electron transfer during the HER and OER. Electrocatalytic water splitting strongly depends on electrode-electrolyte interfacial wettability. Conventionally, many researchers reported hydrophilic surface architectures are favored electrocatalytical water splitting as they facilitate electrolyte spreading over wet surface at high current densities. However, recent advances of superhydrophobic MOFs, including our own, − have challenged this paradigm by demonstrating that rationally designed superhydrophobic MOF-based materials can achieve efficient overall water splitting. , Generally, superhydrophobic materials repel water in aqueous electrolytes and minimize the electrode-electrolyte interface, hindering the accessibility of active sites and elevating the charge transfer resistance. Notably, superhydrophobic materials often exhibit intrinsically lower electrical conductivity, resulting in an insulating character. Our latest research on the embedding of RuO2 nanoparticles into superhydrophobic MOFs results in better surface conductivity and synergistically improved electrochemical water splitting by solid–gas–liquid triphase interface (Cassie-Baxter model) mechanism. Although RuO2 is relatively expensive, RuO2@MOFs show strong over potentials close to Pt/C. Our research group started looking toward low-cost electrocatalysts across potentials close to Pt/C for real-time applications for green H2 production. In this instance, we have chosen inexpensive transition metal-based Co3O4 nanoparticles because of their electrochemical HER and OER activity and relative affordability in comparison to materials based on platinum, iridium, or ruthenium. Furthermore, to the best of our knowledge, no study on post-synthetic Co3O4 nanoparticles encapsulated in MOFs has been reported in the literature.

In this study, we have synthesized three post-synthetic Co3O4 nanoparticle encapsulated metal-organic framework (MOF) composites, Co 3 O 4 @Zn 2 M 2 (μ 2 –OH) 2 (BTMB) 2 (M = Zn (1), Co (2), and Ni (3)), via sonochemical approaches. The electrochemical water splitting of these superhydrophobic Co3O4@ MOFs composites were systematically compared with their pristine MOFs. Additionally, their overall water splitting and long-term catalytic stability was analyzed to evaluate the durability and reusability of these materials under operational conditions.

Experimental Section

Materials

Zinc nitrate hexahydrate (Zn­(NO3)2·6H2O), cobalt nitrate hexahydrate (Co­(NO3)2·6H2O), nickel nitrate hexahydrate (Ni­(NO3)2·6H2O), methanol (MeOH), N,N-dimethylformamide (DMF), and ethanol were obtained from Merck. Cobalt oxide (Co3O4) nanoparticles with 99.98% purity were sourced from SRL, while a Nafion solution was purchased from Sigma-Aldrich. All reagents were used as received, without any further purification. Deionized water was employed throughout for preparing electrolyte solutions and for rinsing electrodes.

The Post-Synthetic Incorporation of Co3O4 Nanoparticles into Zn2M2(μ2–OH)2(BTMB)2(M = Zn, Co, Ni)

The parent MOFs, Zn 2 M 2 (μ 2 –OH) 2 (BTMB) 2 (M = Zn, Co and Ni), were synthesized in accordance with previously reported procedures. , Commercially available Co 3 O 4 nanoparticles were employed as the guest species for the encapsulation process by post-synthetic method. Prior to incorporation, the three parent MOFs materials were subjected to solvent activation by dispersing them in ethanol and treating the suspension under ultrasonic irradiation (200 W) for 30 min to facilitate the removal of residual species. The resulting solids were subsequently recovered by filtration and dried under vacuum at 225 °C for 6 h to achieve complete activation of the porous network. The activated frameworks were then utilized for the encapsulation of Co 3 O 4 nanoparticles within the host matrix.

Synthesis of Co 3 O 4 @Zn 4 (μ 2 –OH) 2 (BTMB) 2 MOF composite (1)

A pre-activated, Zn 4 (μ 2 –OH) 2 (BTMB) 2 , (75 mg, 0.0633 mmol) and ethanol (18 mL) as the dispersion medium are taken into a 100 mL two-neck round-bottom flask. The heterogeneous mixture was subjected to ultrasonic irradiation (200 W) for 15 min to achieve homogeneous dispersion of the framework material. Subsequently, commercially available Co3O4 nanoparticles (15.259 mg, 0.0633 mmol) were introduced into the suspension, followed by purging with high-purity nitrogen gas for 20 min to eliminate dissolved oxygen and maintain an inert atmosphere. The resulting mixture was heated under refluxed conditions at 80 °C for 24 h under a continuous nitrogen environment to facilitate effective incorporation of Co 3 O 4 within the framework matrix. Upon completion of the reaction, the product was allowed to cool gradually to ambient temperature. The solid product was then collected via filtration and dried under vacuum conditions at 70 °C. The procedure yielded a gray solid with an isolated yield of 95%, indicating efficient integration of the components. The amount of Co3O4 nanoparticles incorporated into Zn 4 (μ 2 –OH) 2 (BTMB) 2 , based on the TGA, energy-dispersive X-ray (EDX) spectroscopy, ICP-OES and CHN elemental analysis, is found to be 2.7 wt %, which is equal to 0.56 Co3O4 nanoparticles per Zn 4 (μ 2 –OH) 2 (BTMB) 2 formula unit of MOF/PCP (Scheme ). Elemental microanalysis for 1, calculated (%): C, 59.85; H, 2.48, found (%): C, 59.69; H, 2.56. The FT-IR data is (Figure S1) (4000–400 cm–1): 3596­(br, w), 3400­(br, w), 3062­(w), 1650­(m), 1613­(m), 1609­(m), 1566(s), 1442­(w), 1418­(m), 1379(s), 1274­(m), 1252­(br, w), 1160­(w), 1111­(m), 1080­(m), 1035­(w), 1014­(w), 928­(w), 897­(w), 878­(w), 804(s), 763(s), 718­(m), 681(s), 662(s), 631­(m), 568(s), 563­(m), 489­(w),449(s).

1. Schematic Representation for the Synthesis of Co3O4@Zn2M2(μ2–OH)2(BTMB)2 (M = Zn (1), Co (2) and Ni (3) MOF Composites by Post-Synthetic Sonochemical Method.

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A comparable synthetic protocol was adopted for the preparation of the remaining two compounds, Co 3 O 4 @Zn 2 M 2 (μ 2 –OH) 2 (BTMB) 2 (M = Co (2) and Ni (3) MOF composites (Scheme ) (see Supporting information (SI)).

Synthesis of Co3O4@Zn 2 Co 2 (μ 2 –OH) 2 (BTMB) 2 (2)

The Co 3 O 4 @Zn 2 Co 2 (μ 2 –OH) 2 (BTMB) 2 (2) was obtained as a light purplish gray solid with 92% yield. The amount of Co 3 O 4 nanoparticles incorporated into Zn 2 Co 2 (μ 2 –OH) 2 (BTMB) 2 , based on the TGA, EDX, ICP-OES and CHN elemental analysis, is found to be 2.5 wt %, which is equal to 0.52 Co3O4 nanoparticles per Zn 2 Co 2 (μ 2 –OH) 2 (BTMB) 2 formula unit of MOF. Elemental microanalysis for 2 calculated (%): C, 50.72; H, 2.52, found (%): C, 50.65; H, 2.66. The FT-IR data is (Figure S1) (4000–400 cm–1): 3606­(br, w), 3410­(br, w), 3060­(w), 1650.7­(m), 1615­(m), 1609­(m), 1566(s), 1512­(w), 1443­(w), 1414­(w), 1377(s), 1272­(w), 1254­(w), 1160­(w), 1115­(m), 1080­(m), 1076­(w), 1037­(br, w), 1018­(w), 926­(w), 912­(w), 877­(m), 805­(m), 762(s), 718­(m), 681(s), 664(s), 623­(w), 568(s),547­(w), 494­(w), 453(s).

Synthesis of Co3O4@Zn 2 Ni 2 (μ 2 –OH) 2 (BTMB) 2 (3)

The Co3O4@Zn2Ni2(μ2–OH)2(BTMB)2 (3) was obtained as a gray solid with 94% yield. The amount of Co3O4 nanoparticles incorporated into Zn 2 Ni 2 (μ 2 –OH) 2 (BTMB) 2 , based on the TGA, EDX, ICP-OES and CHN elemental analysis, is found to be 3.1 wt %, which is equal to 0.64 Co3O4 nanoparticles per Zn 2 Ni 2 (μ 2 –OH) 2 (BTMB) 2 formula unit of MOF. Elemental microanalysis for 3 calculated (%): C, 49.62; H, 2.47, found (%): C, 49.48; H, 2.60. The FT-IR data is (Figure S1) (4000–400 cm–1): 3612­(br, w), 3410­(br, w), 3061­(w), 1649­(m), 1614­(m), 1610­(m), 1566­(m), 1515­(br, w), 1443­(w), 1416­(m), 1377(s), 1272­(w), 1254­(br, w), 1163­(w), 1112­(m), 1081­(w), 1000­(br, w), 928­(br, w), 912­(w), 877­(w), 805­(m), 764(s), 805­(m), 764(s), 718­(m), 661(s), 568(s), 506­(w), 457(s).

General Characterization Techniques

A Rigaku RINT diffractometer with Cu Kα radiation was used to record powder X-ray diffraction (PXRD) patterns in order to investigate the crystalline phases of the synthesized materials. Thermogravimetric analysis (TGA) was carried out on a Rigaku TG8120 instrument under a nitrogen atmosphere at a heating rate of 10 K min–1 to evaluate thermal stability. A volumetric adsorption analyzer (Belsorp Mini-X, BEL JAPAN INC., Japan) was used to obtain gas adsorption–desorption isotherms in order to evaluate surface area and porosity properties. Fourier-transform infrared (FTIR) spectra were collected at room temperature using a Cary 639 FTIR spectrometer equipped with a diamond attenuated total reflectance (ATR) accessory to identify functional groups. Surface morphology and microstructural features were examined by field-emission scanning electron microscopy (FE-SEM) using a Merlin Compact microscope fitted with a GEMINI-I electron column (Zeiss Pvt. Ltd., Germany). Inductively coupled plasma-optical emission spectrophotometer (ICP-OES) analysis carried out using iCAP 7000. High-resolution transmission electron microscopy (HR-TEM) was performed on a JEOL 2100F instrument operated at an accelerating voltage of 200 kV to obtain detailed nanoscale structural information. For HR-TEM analysis, the sample was ultrasonically dispersed in ethanol to form a homogeneous suspension. A few drops of this dispersion were deposited onto a carbon-coated copper grid (200 mesh) and allowed to dry under ambient conditions prior to imaging. Elemental composition and chemical states were analyzed using X-ray photoelectron spectroscopy (XPS) with a PHI 5000 Versa Probe III system.

Contact Angle Measurements

Contact angle analysis was performed using a HOLMARC contact angle goniometer (Model No. HO-IAD-CAM-01B, Holmarc Opto-Mechatronics Ltd., India). The instrument is equipped with an automated liquid dispensing system, a temperature regulation unit, and a rotatable sample stage to ensure precise measurements. For sample preparation, approximately 30 mg of powdered Co 3 O 4 @Zn 2 M 2 (μ 2 –OH) 2 (BTMB) 2 (M = Zn (1), Co (2) and Ni (3) MOF composites were evenly distributed and gently pressed onto an aluminum substrate to obtain a uniform and smooth surface, using a glass slide. A controlled volume of 20 μL deionized water was then dispensed onto the sample surface using an automated 150 μL syringe system. The droplet images were captured with a high-resolution CCD camera, and the contact angles were determined through drop shape analysis employing the β-spline fitting method. To evaluate thermal effects, contact angle measurements were also conducted at elevated temperatures. The substrate temperature was systematically increased from room temperature to 90 °C in increments of 10 °C using the built-in temperature controller. Each measurement was performed within a short duration (under 5 min) to minimize errors arising from rapid evaporation of the water droplet at higher temperatures.

Electrocatalyst Fabrication and Electrochemical Evaluation

Electrochemical experiments were performed using a conventional three-electrode configuration, consisting of a glassy carbon electrode (GCE) as the working electrode, a platinum wire serving as the counter electrode, and an Ag/AgCl electrode (3.5 M KCl) as the reference electrode. All experiments were conducted at room temperature using a computer-controlled electrochemical workstation (Biologic SP-300 potentiostat). To prepare the working electrode, a homogeneous catalyst ink was formulated by dispersing 6 mg of the active material in 980 μL ethanol and 20 μL of 5 wt % Nafion solution, followed by ultrasonication for at least 1 h to obtain a homogeneous suspension. Subsequently, an appropriate aliquot of the ink was drop-cast onto the polished surface of the glassy carbon electrode (3 mm diameter), yielding a catalyst loading of approximately 0.2 mg cm–2. The electrocatalytic activity toward the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) was evaluated in 1 M KOH electrolyte (pH 14). Linear sweep voltammetry (LSV) was carried out at a scan rate of 5 mV s–1. For benchmarking purposes, the electrochemical performance of the prepared catalyst was compared with that of a commercial 5 wt % Pt/C catalyst under identical experimental conditions. The measured potentials (vs. Ag/AgCl) were converted to the reversible hydrogen electrode (RHE), and all potentials were reported without IR compensation. Using the equation: ERHE = EAg/AgCl + 0.197 + (0.059 × pH) [V].

Electrochemical impedance spectroscopy (EIS) measurements were conducted under potentiostatic conditions at −0.39 V (vs. RHE) for HER and 1.76 V (vs. RHE) for OER, applying an AC perturbation amplitude of 5 mV over a frequency range spanning from 1 MHz to 0.1 Hz, in order to investigate the interfacial charge-transfer characteristics. The long-term operational stability of the catalyst was assessed through chronopotentiometry measurements at −0.5 V (vs. Ag/AgCl) over a duration of 50 h for HER. To evaluate the reaction kinetics of HER and OER, Tafel analysis was performed by plotting overpotential (η) as a function of the logarithm of current density (log J). The Tafel slope (b) was extracted from the linear region of the polarization curves according to the Tafel relationship (η = a + b log J), where a is a constant and J denotes the current density. The electrochemically active surface area (ECSA) was estimated from the double-layer capacitance (Cdl). At several scan rates (5, 10, 20, 40, 50, 80, 100, 120, 150, 180, and 200 mV s–1), non-faradaic cyclic voltammetry (CV) was carried out in the potential window of 1.10 V to 1.40 V (vs. RHE). Cdl was obtained from the linear correlation between current density and scan rate. A two-electrode cell was set up to assess the overall water-splitting behavior of 2. For the electrode preparation, 2 mg of catalyst was coated evenly onto a 1 × 1 cm2 graphite sheet. The same procedure was followed for both electrodes, which were then used as the cathode and anode with a separation of 1.5 cm.

Results and Discussion

Synthesis of Co3O4@Zn2M2(μ2–OH)2(BTMB)2 (M = Zn (1), Co (2), and Ni (3)) MOF Composites

Initially, the pristine MOFs, Zn 2 M 2 (μ 2 –OH) 2 (BTMB) 2 (M = Zn, Co and Ni) were synthesized in bulk following our previously reported procedures. , Similarly, commercially available Co 3 O 4 nanoparticles were employed as the guest species for the encapsulation process by post-synthetic method. Prior to encapsulation, the three pristine MOFs were pre-treated with ethanol under ultrasonication, followed by filtration and vacuum drying to remove residual solvents and impurities. Encapsulation of Co 3 O 4 nanoparticles into the MOF matrices was subsequently carried out via ultrasonication in various solvents, including ethanol, propanol, water, tetrahydrofuran (THF), and n-hexane, followed by a reflux process. Among these, ethanol was the most effective medium for achieving uniform incorporation of Co3O4 nanoparticles into the MOF frameworks. The resulting gray, light purplish gray, and gray homogeneous solids (1–3) are isolated by filtration, dried in a hot-air oven, and obtained in good yields (92%–95%). These materials were subjected to comprehensive characterization using PXRD, SEM, TEM, XPS, EDX, ICP-OES, TGA, contact-angle measurements, and adsorption analyses, followed by evaluation of their electrochemical water-splitting performance.

Structural Investigation and Characterization of Co3O4@Zn2M2(μ2–OH)2(BTMB)2 (M = Zn (1), Co (2), and Ni (3)) MOF Composites

The effective incorporation of Co3O4 nanoparticles into the Co 3 O 4 @Zn 2 M 2 (μ 2 –OH) 2 (BTMB) 2 (M = Zn, Co and Ni) MOFs was substantiated through elemental and surface analyses. EDX and ICP-OES, confirmed the presence of cobalt species in conjunction with the intrinsic metal nodes of the MOFs (Zn in compound 1, Zn/Co in compound 2, and Zn/Ni in compound 3), as evidenced in Figure S2 (Table S1). Comprehensive structural characterization was further performed using PXRD, FE-SEM, TGA, and TEM. The PXRD profiles of Co 3 O 4 @Zn 2 M 2 (μ 2 –OH) 2 (BTMB) 2 (M = Zn (1), Co (2), and Ni (3)) closely resemble those of the corresponding pristine MOFs indicating that the long-range crystalline order of the host MOFs, is well preserved following nanoparticle encapsulation. Moreover, additional diffraction reflections characteristic of crystalline Co3O4 were clearly identified at 2θ values of 19.1°, 31.3°, 36.8°, 38.5°, 44.8°, 55.6°, 59.3°, and 65.2°, which can be indexed to the (111), (220), (311), (222), (400), (422), (511), and (440) lattice planes, respectively. These peaks coexist with the dominant reflections of the parent MOFs, confirming the successful incorporation of Co3O4 within the MOFs matrix (Figures S3–S5). Importantly, the intrinsic two-dimensional layered architecture of the pristine MOFs remains largely unaltered upon encapsulation. However, we could observe minor changes of d-spacing values corresponding to [100] peaks in PXRDs of three compounds, 1–3 (Figures S3, S4, and S5). For example, in the case of 2, after Co3O4 nanoparticle encapsulation, the 2θ values of the [100] peak decreased from 7.80 to 6.87 degrees; correspondingly, their d-spacing values increased from 11.33 to 12.86 nm (Figure S4), as confirmed by Co3O4 nanoparticles encapsulated into the MOF frameworks. The TGA of the three compounds (1–3) depicts a single weight loss step of 6–9% below 250 °C, attributable to the desorption of physisorbed solvent molecules, predominantly ethanol, substantiating that no residual DMF is retained within the Co3O4-encapsulated frameworks. The Co3O4@MOF composites exhibited thermal stability, maintaining structural integrity up to ∼420 °C; beyond this temperature progressive decomposition of the BTMB ligand is observed. A slight deviation in the total weight loss of the Co3O4@MOFs (by 2.0–3.5%) relative to their pristine MOFs (Figure S6) indicates the successful incorporation of Co3O4 nanoparticles. The estimation of Co3O4 loadings determined to be 2.7, 2.5, and 3.1 wt % for compounds 1–3, respectively. The variation in Co3O4 loadings in compounds 1–3 could be due to the Lewis acidic open metal interaction strength of the respective MOF metal with Co3O4 nanoparticles.

Field-Emission Scanning Electron Microscope (FE-SEM) and Transmission Electron Microscopy (TEM)

The FE-SEM images of compounds 1–3 are presented in Figure a–c. As shown in the micrographs, the resulting materials are homogeneous, and their morphologies make it evident that they are different from the pristine MOFs. Due to the incorporation of Co3O4 nanoparticles, the particle morphology transformed from flake-like structures (observed in the pristine MOFs) , to almost spherical particles with diameters ranging from 30 to 150 nm. The consistent distribution of Co3O4 nanoparticles across the MOF matrices is further confirmed by the matching TEM images (Figure d). The darker areas indicated by yellow circles correspond to Co3O4 nanoparticles immersed in the MOF framework, confirming that their encapsulation was successful. Compound 2, selected-area electron diffraction (SAED) pattern (Figure f) shows discrete diffraction spots that are suggestive of good crystallinity. Furthermore, the presence of Co3O4 nanoparticles in 2 is confirmed by the HRTEM analysis (Figure e), the well-distinguished fringes (Figure e) at 0.2456 and 0.1514 nm, corresponding to the (311) and (511) crystal planes of Co3O4, which in turn coincide with the SAED (Figure f) as well as the PXRD analysis. Among these, the (311) plane showed the highest intensity in the PXRD pattern, indicating its dominant crystallographic orientation.

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FE-SEM images (a, b, and c) Co3O4@Zn2M2(μ2–OH)2(BTMB)2 (M = Zn (1), Co (2) and Ni (3) MOFs and (d) TEM, (e) HRTEM, and (f) SAED images of 2.

Gas Adsorption Studies

The surface area properties of compounds 1–3 were evaluated through N2 adsorption – desorption analysis at 77 K. As documented in earlier studies, the pristine Zn 2 M 2 (μ 2 –OH) 2 (BTMB) 2 (M = Zn, Co, and Ni) MOFs show minimal nitrogen uptake, mainly attributed to pore blockage within their microporous structure at low temperature. , However, the Co3O4@MOFs demonstrated a noticeable improvement in nitrogen adsorption, indicating an increase in accessible surface area after the incorporation of Co3O4 nanoparticles. Among these three compounds, Co 3 O 4 @Zn 2 Co 2 (μ 2 –OH) 2 (BTMB) 2 exhibited the highest surface area (27.75 M2 g–1), followed by compounds 3 (14.80 M2 g–1) and 1 (7.7 M2 g–1), respectively, (Figure S7) aligning with the trend observed for their corresponding pristine MOFs. ,

X-ray Photoelectron Spectroscopic (XPS) Analysis

The surface elemental composition, oxidation states, and the elaborate electronic interactions of the as-obtained products were analyzed by high-performance scanning XPS analysis. The full XPS survey spectra of this compound are shown in Figure S8; the wide survey spectrum of O 1s, C 1s, Zn 2p, and Co 2p of 2, confirms the presence of O, C, Zn, and Co respectively (Figure a to d). The Co 2p spectrum of the temperature-optimized sample exhibited two doublets (Figure d). The presence of Co2+ ions is confirmed by the measured peak locations at 781.3 and 798.3 eV, which indicate the existence of Co 2p3/2 and Co 2p1/2 of Co2+, respectively. Similarly, the presence of the Co3+ ion was indicated by the distinctive peak locations of Co 2p3/2 and Co 2p1/2 at 778.2 and 796.3 eV, respectively. , In addition, satellite peaks were observed at the binding energy of 805.3 and 798.4 eV, indicating the presence of both Co2+ and Co3+ species (in the ratio of Co2+ and Co3+ is 2.5:1) in 2 (see S.I., other elements analysis of Zn, C and O).

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High-performance scanning X-ray photoelectron spectroscopy: (a) C 1s, (b) O 1s, (c) Zn 2p, and (d) Co 2p of Co3O4@Zn2Co2(μ2–OH)2(BTMB)2(2).

Super Hydrophobicity Studies

The three parent MOFs, Zn 2 M 2 (μ 2 –OH) 2 (BTMB)2(M = Zn, Co, and Ni), , along with their nanosheet phases, are well known for exhibiting outstanding superhydrophobic behavior, typically showing water contact angles of ≥150°. In this study, it was important to assess whether the incorporation of Co 3 O 4 nanoparticles into these frameworks (1–3) would preserve or modify their wettability, as surface hydrophobicity plays a crucial role in electrochemical water-splitting of our recently reported MOF-based catalysts. , Contact angle measurements conducted on powdered Co 3 O 4 @MOF samples (1–3) exhibited significantly elevated values of 162.41°, 166.42°, and 164.47° for Co 3 O 4 @Zn 2 M 2 (μ 2 –OH) 2 (BTMB)2(M = Zn (1), Co (2), and Ni (3)), respectively (contact angles in Figure a–c and the respective photographs of water droplets on the powder samples are shown in Figure d–f). These values are higher than those of the corresponding pristine frameworks (155.5°, 159.3°, and 160.8°) (contact angles in Figure g–i and the respective photographs of water droplets on the powder samples shown in Figure j–l, clearly demonstrating an enhancement in surface hydrophobicity. Such improvement can be attributed to increased surface roughness arising from Co 3 O 4 nanoparticle incorporation, which is consistent with the observed increase in accessible surface area. To further examine the thermal robustness of this behavior, contact angle measurements were performed over a temperature range from room temperature to 90 °C. All three materials maintained their hydrophobic nature across this range, exhibiting only a slight reduction in contact angle with increasing temperature (Figure S9). Overall, these results demonstrate that the Co 3 O 4 @MOFs (1–3) possess stable and highly superhydrophobic surfaces, along with excellent water resistance, highlighting their suitability for long-term electrochemical applications.

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Contact angle images measured at room temperature (a, b, and c) and the pictures of water droplets (d, e, and f) on the powder samples of Co3O4@Zn2M2(μ2–OH)2(BTMB)2(M = Zn (1), Co (2) and Ni (3) MOF composites. Similarly, the contact angles measured at room temperature (g, h, and i) and the pictures of water droplets (j, k, and l) on the powder samples of their pristine MOFs.

Electrochemical Water Splitting

Electro Catalytic Performance for HER in Alkaline Media

Our primary goal is to provide affordable, highly efficient electrochemical water splitting catalysts for the production of green hydrogen, a sustainable energy source that addresses environmental challenges globally. The hydrogen evolution reaction (HER) constitutes a fundamental cathodic process in overall water splitting. The electrocatalytic performance of the Co 3 O 4 @MOF composite catalysts (1–3) was systematically scrutinized using a Bio-Logic SP-300 electrochemical workstation within a well-established three-electrode system under alkaline conditions (1.0 M KOH, pH 14) as the electrolyte medium. , Linear sweep voltammetry (LSV) measurements were conducted at a scan rate of 5 mV s–1 to probe and compare HER activities against their corresponding pristine frameworks, namely Zn 2 M 2 (μ 2 –OH) 2 (BTMB)2(M = Zn, Co, and Ni). As demonstrated in Figure a, the polarization profiles indicate that Co 3 O 4 @Zn 2 Co 2 (μ 2 –OH) 2 (BTMB) 2 delivers enhanced catalytic performance among the investigated materials, achieving a current density of 10 mA cm–2 at a relatively low overpotential (η10) of 147 mV. In contrast, the state-of-the-art 5 wt % Pt/C catalysts exhibits the lowest overpotential of 84 mV under identical conditions. The remaining Co 3 O 4 @MOFs hybrids demonstrate moderate HER activity, with overpotentials of 196 mV and 169 mV for compounds 1 and 3, respectively. Conversely, the pristine MOFs shows markedly inferior performance, requiring substantially higher overpotentials of 796, 438, and 603 mV. The pronounced enhancement in catalytic efficiency upon incorporation of Co3O4 nanoparticles can be ascribed to improved electrical conductivity and accelerated interfacial charge transfer within the hybrid architecture. This synergistic integration significantly outperforms both the parent MOFs and standalone commercial Co3O4 nanoparticles (100%) (η10 = 270 mV). Notably, the good activity of compound 2, is likely governed by its optimized electronic configuration, facilitated charge transport pathways, and robust catalyst electrolyte interfacial interactions, collectively promoting efficient proton reduction kinetics.

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Linear sweep voltammetry (LSV) graphs (a) and the tafel slopes (b) of (1) (black), (2) (red), (3) (blue), and Zn2M2(μ2–OH)2(BTMB)2 (M = Zn (purple), Co (orange) and Ni (pink)) MOFs, Co3O4 (olive), Pt/C (dark grey) at current density 10 mA cm–2 for HER.

The intrinsic reaction kinetics were additionally elucidated through Tafel plots obtained from the LSV profiles, following the classical Tafel relationship (η = a + b log J), where η represents the overpotential, J denotes the current density, and b corresponds to the Tafel slope. , The calculated Tafel slopes for the Co 3 O 4 @MOFs (1–3) their pristine MOFs, and pure Co3O4 were determined to be 101, 94, 96, 182, 110, 135, and 120 mV dec–1, respectively (Figure b). Notably, compound (2) exhibits a comparatively lower Tafel slope, indicative of more rapid reaction kinetics and facilitated charge-transfer dynamics during the HER. This behavior suggests that the HER pathway predominantly follows the Heyrovsky–Volmer mechanism, with the Volmer step associated with electrochemical hydrogen adsorption acting as the rate-limiting process. Collectively, these observations substantiate that compound (2) demonstrates the most efficient interfacial charge-transfer characteristic among the prepared electrocatalysts, thereby highlighting its superior kinetic competence toward HER.

Nyquist plots or electrochemical impedance spectroscopy (EIS) measurements were acquired to evaluate the interfacial charge-transfer dynamics of compounds 1–3 along with their respective pristine MOFs and pure Co 3 O 4 nanoparticles, based on the Randles circuit model. The analyses were conducted using an AC amplitude of 5 mV across a frequency window ranging from 0.1 Hz to 1 MHz at an applied potential of −0.39 V vs RHE. The recorded Nyquist plots reveal well-defined semicircular features in the high- to mid-frequency region, where the diameter of each semicircle reflects the charge-transfer resistance (Rct). A reduced semicircle diameter indicates more efficient interfacial electron transfer and improved conductivity at the electrode-electrolyte boundary. All investigated samples exhibit nearly identical solution resistance values (Rs ≈ 12 Ω) in the high-frequency region, arising from contributions of the electrolyte, intrinsic electrode resistance, and contact interfaces. The Rct values for compounds 1–3, and pure Co 3 O 4 were found to be 60.2, 24.9, 34.5, and 97.8 Ω, respectively (Figure ). Significantly, compound 2 shows the lowest Rct value, highlighting its superior charge-transfer kinetics. In contrast, the parent Zn 2 Co 2 (μ 2 –OH) 2 (BTMB) 2 MOF shows an Rct value of 79 Ω, while the remaining parent MOFs display even higher Rct values (Figure S10), suggesting inferior charge-transfer characteristics. This facilitates more efficient hydrogen evolution reaction (HER) activity in alkaline conditions (1 M KOH). The enhanced interfacial electron transport between the catalyst surface and electrolyte is consistent with the improved electrocatalytic performance observed from linear sweep voltammetry (LSV) results. The related equivalent circuit diagram is shown in the inset of figure.

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EIS data for the smaller charge transfer resistance (Rct) for Co3O4 @MOFs (1) (black), (2) (red), (3) (blue) and Co3O4 (olive) MOFs for HER, respectively.

To further probe the electrochemically active surface area (ECSA), cyclic voltammetry (CV) measurements were carried out at varying scan rates ranging from 5 to 200 mVs–1 within a non-faradaic potential window of 0.63–0.94 V vs. RHE (Figure S11). , The double-layer capacitance (Cdl) was estimated from the linear slope obtained by plotting ΔJ/2, defined as the difference between anodic and cathodic current densities at 0.79 V vs. RHE derived from the CV plot in Figure a against the scan rate. A linear fit of this relationship was used to extract the Cdl values for all samples. The calculated Cdl values for compounds 1–3, Co3O4 and the corresponding pristine MOFs were calculated to be 1.64, 3.13, 2.78, 0.21, 0.37, 0.25, and 0.41 mF cm–2, respectively. The electrochemically active surface areas (ECSAs) of 1–3, Co 3 O 4 , and their pristine MOFs were measured to be 41, 78.25, 69.5, 5.25, 9.25, 6.25, and 10.25 cm2, respectively. The higher Cdl value (3.13 mF cm–2) and larger ECSA (78.25 cm2) of Co 3 O 4 @Zn 2 Co 2 (μ 2 –OH) 2 (BTMB) 2 indicate the presence of a greater electrochemically active surface area and a larger number of accessible catalytic sites, consistent with its superior HER performance. The durability of an electrocatalyst is a crucial parameter for assessing its suitability for practical implementation. The long-term performance of 2, electrocatalyst toward the HER was examined through the chronopotentiometry measurements under at constant current density. Sustained catalytic activity of 2, was observed in 1 M KOH over an extended electrolysis period of 50 h at a steady current density of 10 mA cm–2 (Figure b). Only a minimal decrease in overpotential was recorded during the continuous electrolysis, indicating negligible performance degradation. Moreover, persistent hydrogen bubble generation at the electrode interface confirms the sustained catalytic activity throughout the testing duration. Post-electrolysis transmission electron microscopy (TEM) characterization of compound, 2 confirmed that the morphology and structure remained largely unchanged compared to the fresh sample (Figure S12). Additionally, FESEM, PXRD, and ICP-OES also confirm morphology and structure remained largely unchanged after the long-term durability test (Figure S13 and Table S1). These findings demonstrate the excellent electrochemical stability and structural resilience of the catalyst under alkaline conditions.

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(a) Double layer capacitance of Co3O4 @MOFs (1) (black), (2) (red), (3) (blue), Zn2M2(μ2–OH)2(BTMB)2 (M = Zn (purple), Co (orange) and Ni (pink)) MOFs, Co3O4 (olive) and (b) long-term durability of catalytical activities performed at current density of 10 mA cm–2 of (2) MOFs for HER.

The integrated electrochemical evaluations, including EIS, Cdl, and prolonged stability testing, collectively reveal the enhanced HER activity of compound 2. The observed decrease in charge-transfer resistance (Rct) together with the increased Cdl value, suggests enhanced interfacial electron transport and a greater number of accessible catalytic sites, both of which are vital for improving HER kinetics. Moreover, the strong interfacial interaction between the embedded Co3O4 nanoparticles and Zn 2 Co 2 (μ 2 –OH) 2 (BTMB) 2 , facilitates rapid electron migration and reduces interfacial resistance, thereby accelerating charge-transfer processes during catalysis. Furthermore, the cooperative electronic interactions between the Co 3 O 4 nanoparticles and Co2+ active centers enhance the adsorption of water molecules and promote efficient H–H bond formation, ultimately lowering the activation energy required for hydrogen generation. In addition to activity, 2 exhibits excellent long-term stability, as demonstrated by sustained performance over a 50 h continuous operation; this further confirms the structural resilience of the Co3O4@MOF composite architecture. The durability of the catalyst is likely associated with the strong coordination between metal ions and organic linkers, a hydrophobic surface that preserves the framework integrity, along with surface characteristics that resist degradation under alkaline conditions. Taken together, these results highlight that compound, 2, achieves a well-balanced combination of electrical conductivity, abundant active-site accessibility, structural stability, and fast charge-transfer kinetics. Such synergistic features contribute significantly to its high efficiency and durability for HER in alkaline electrolytes. Consequently, this work highlights hydrophobic surface and Co 3 O 4 nanoparticle encapsulation as an effective strategy for designing advanced MOF-based electrocatalysts with high activity and long-term durability for hydrogen evolution.

OER Performance in Alkaline Media

To further evaluate the electrocatalytic bifunctionality of the newly synthesized composite materials, OER measurements were conducted for 1–3 under conditions identical to those used for HER. At an anodic current density of 10 mA cm–2, the overpotentials for 1–3 were determined to be 499, 425, and 486 mV, respectively, which are notably lower than those of their pristine MOFs (560, 450, and 530 mV, respectively) (Figure a). These results demonstrate a substantial enhancement in OER performance upon Co 3 O 4 nanoparticle encapsulation (∼2.5–3.1 wt %), with the overpotential values better than that of commercial Co 3 O 4 (100%) (521 mV). Among them, compound 2, exhibited the best OER activity, likely due to its faster electron-transfer kinetics and the availability of a greater number of electrochemically active sites. The reaction kinetics were further examined using the Tafel plots derived from LSV data (Figure b). The computed Tafel slopes for 1–3, their pristine MOFs, and pure Co 3 O 4 nanoparticles were 123, 118, 122, 149, 120, 124, and 126 mV dec–1, respectively. The smaller Tafel slope for compound 2 indicates its superior catalytic efficiency and faster reaction kinetics during the OER process.

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LSV curves for overpotential (a) and tafel plots (b) of (1) (black), (2) (red), (3) (blue), Zn2M2(μ2–OH)2(BTMB)2 (M = Zn (purple), Co (orange) and Ni (pink)) MOFs and Co3O4 (olive) at current density 10 mA cm–2 1.0 M KOH for OER.

To probe the charge transfer behavior, EIS measurements were performed in the frequency range of 0.1 Hz–1 MHz at an applied potential of 1.76 V vs RHE (Figure a). All of the samples displayed comparatively identical solution resistance (Rs ≈ 12.5 Ω) in the high-frequency range, while the charge transfer resistance (Rct) values for 1–3, Co 3 O 4 and their pristine MOFs were found to be 41.7, 21.5, 38.3, 48.4 Ω and 83.4, 28.1, 56.5 Ω, respectively. The lowest Rct of 2 confirms more efficient interfacial electron transfer between the electrode and electrolyte. Additionally, the electrochemically active surface area (ECSA) was estimated from cyclic voltammetry measurements (SI, Figure S14) in the non-faradaic potential range (1.1–1.4 V vs RHE), and the corresponding double-layer capacitance (Cdl) values were obtained (Figure b). The Cdl for compound 2 was determined to be 11.2 mF significantly higher than those of 1 (5.1 mF cm–2), 3 (5.5 mF cm–2), Co 3 O 4 (0.64 mF cm–2), and the pristine MOFs (0.26, 0.45, 0.58 mF cm–2). The high Cdl value of 2 further supports the presence of a larger number of active sites and better catalytic accessibility. Based on these results and previous studies on nanosheet MOFs and RuO2 composites, the most plausible HER and OER mechanisms for these systems involve strong Lewis acidic open-metal sites that promote rapid adsorption–desorption of water molecules and reaction intermediates (Figure S15). , In fact, uncompensated solution resistance can influence the measured potential and consequently affect the calculated overpotential and Tafel slope. We calculated influence resistance (iR) for HER and OER data for all three compounds, 1–3. After iR compensation, the HER overpotentials were 185, 134, and 161 mV, while the corresponding Tafel slopes were 99, 91, and 94 mV dec–1, respectively, for 1–3 (Figure S16). Similarly, for OER iR compensation overpotentials were also carried out (SI, Figure S17), indicating that true catalyst performance is not much affected by instrument artifacts.

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Nyquist plots (a), and double layer capacitance (b) of these catalysts of (1) (black), (2) (red), (3) (blue), Zn2M2(μ2–OH)2(BTMB)2 (M = Zn (purple), Co (orange) and Ni (pink)) MOFs, and Co3O4 (olive) for OER.

Overall Water Splitting

Based on the HER and OER results of compounds 1–3, they may be beneficial as bifunctional catalysts for overall water splitting. We performed total (full-cell) water-splitting on the best-performing catalyst, 2, using a comparable electrolytic of 1 M KOH under two-electrode circumstances. The LSV curve showed cell delivering 1.78 V at 10 mA cm–2 (Figure a). In comparison, the RuO2(+)∥Pt/C­(−) system needed a cell voltage of 1.72 V to reach a current density of 10 mA cm–2. Furthermore, the catalyst’s long-term stability has been evaluated using a 21-hour chronopotentiometric test, during which it maintained a practically constant operating potential, exhibiting remarkable durability and sustained electrochemical activity (Figure b). The cell exhibited a consistent electrochemical response over long periods of operation, indicating the good stability of 2, under measured conditions. The results were also compared to previously published electrocatalysts, as shown in Table S3 (SI). These dual-purpose electrochemical catalysts are critical for niche, small-scale systems that require minimal mechanical complexity while optimizing chemical efficiency.

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(a) LSV curve and (b) chronoamperometry test of 2 for overall water splitting.

To examine the effect of electrolyte concentration on overpotentials, the catalyst was additionally tested for the best-performing catalysts, 2, in two other different electrolytic concentrations, such as 3 M KOH and 5 M KOH conditions. The HER and OER overpotentials obtained for 2, were 240 and 266 mV and 340 and 321 mV, respectively (Figure S18 a and b). From the above measurements, high HER overpotentials and improved OER overpotentials were observed with increasing electrolytic concentrations. Moreover, the catalysts were also evaluated in seawater electrolytic conditions; the HER and OER overpotentials obtained were 419 and 1105 mV, respectively, at 10 mA cm–2 (Figure S18 a and b). The lower activity in seawater can be attributed to its complex ionic composition, lower effective OH– availability, and altered interfacial and mass-transport conditions. Additionally, we intentionally carried out LSV on the samples of physical mixtures of the Co3O4 nanoparticles and corresponding shows no HER activity and high OER, 542 mV overpotential at 10 mA cm–2 (Figure S18c), were observed compared with LSV data of the Co3O4@MOF (2) composite (HER, 147 mV; and OER, 425 mV at 10 mA cm–2). From the above, we can clearly confirm that Co3O4 nanoparticles are not surface-loaded; rather, they are encapsulated into MOF structures.

In fact, this is the unique electrochemical water splitting reported by us earlier, the surface is superhydrophobic in nature but adsorbs gaseous water vapor (Cassie–Baxter self-cleaning model), similar to plants exhibiting photosystem II, where water splitting occurs in the gaseous form of water (molecular level). Superhydrophobic catalysts create a stable solid–gas–liquid triphase interface by trapping a microscopic air layer that prevents full liquid wetting. This structure enables a localized, gaseous-phase water-splitting reaction, rather than the standard solid–liquid biphase reaction. Further, we believe that in the case of superhydrophobic surface catalysts surrounded by water in a gaseous state, which promote the rapid desorption of gas bubbles (H2 and O2) they reduce the shielding of active sites by bubbles. This phenomenon could lower the overpotentials of catalysts used for this study compared with the conventional catalysts surrounded by liquid water, which can have more shielding of active sites. In fact, we reported recently the first metal oxide nanoparticle, RuO2-embedded MOFs post-synthetic method exhibiting good HER and OER overpotentials using this phenomenon of superhydrophobic surface. However, RuO2 is relatively expensive for real-time applications, we chose relatively, inexpensive nanoparticles Co 3 O 4 , which exhibit a similar phenomenon of water splitting after embedding into Zn 2 M 2 (μ 2 –OH) 2 (BTMB)2(M = Zn, Co, and Ni). Nevertheless, this is the 2nd example of metal oxide nanoparticles embedded into MOFs by post-synthetic methods for water splitting for green hydrogen production. We provide the data on metals and metal oxides embedded into MOFs for the purpose of comparison (S.I., Table S2).

Conclusion

In summary, three effective and economical electrocatalysts, Co 3 O 4 @Zn 2 M 2 (μ 2 –OH) 2 (BTMB) 2 (M = Zn (1), Co (2), and Ni (3)), were successfully synthesized using a post-synthetic reflux-assisted method and completely examined by PXRD, FE-SEM, TEM, ICP-OES and EDX studies. In comparison to their pristine counterparts, the addition of Co 3 O 4 nanoparticles to the MOF framework resulted in improved N2 adsorption behavior, indicating an increase in surface area upon Co 3 O 4 embedding. Additionally, all three Co 3 O 4 @MOF composites showed strong superhydrophobicity, as seen by their greater contact angles in comparison to the pristine materials, suggesting enhanced interfacial stability. Compounds 1–3 showed exceptional bifunctional activity toward total water splitting electrocatalytically, with much lower HER and OER overpotentials and smaller Tafel slopes than their parent MOFs, a 4-fold improvement in HER performance. Among them, Co 3 O 4 @Zn 2 Co 2 (μ 2 –OH) 2 (BTMB) 2 (2) depicted the lowest overpotential values for both HER and OER, coupled with rapid electron transfer kinetics, a higher electrochemically active surface area, and excellent long-term stability, maintaining catalytic integrity for over 50 h of continuous electrolysis. Remarkably, the HER and OER activities of compounds 1–3 (∼2.5–3.1 wt % Co3O4) approach those of benchmark Pt/C (5%) and pure Co 3 O 4 catalysts, respectively. The catalyst demonstrated good stability and efficient performance in a two-electrode overall water-splitting cell, requiring 1.78 V to achieve 10 mA cm–2 compared with RuO2(+)∥Pt/C­(−) (1.72 V). Overall, this study highlights the synergistic effect between Co 3 O 4 nanoparticles and the superhydrophobic MOF matrix (Cassie-Baxter state) in enhancing charge transport, surface accessibility, and active-site exposure. The findings provide valuable insight into the rational design of Co 3 O 4 @MOF composites as robust, recyclable, and cost-effective bifunctional electrocatalysts for efficient water splitting, advancing progress toward sustainable clean-energy technologies.

Supplementary Material

ao6c07951_si_001.pdf (2.7MB, pdf)

Acknowledgments

This work was financially supported by DST-SERB under Grant No. CRG/2019/002217. The authors acknowledge CoExAMMPC, VFSTR, for some of the sample’s characterizations. LKP and JNB acknowledge the Department of Atomic Energy (DAE) for providing a research grant to NISER.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.6c07951.

  • Additional information about syntheses, images of FTIR, EDX, PXRD, TGA, adsorption study graphs, contact angle images, electrochemical graphs (PDF)

R.N and K.P.R directed and performed all the experiments. All the authors were involved in the analysis of the samples and contributed to writing the manuscript.

The authors declare no competing financial interest.

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