Reverse Steam Rising: A Novel Route to Hierarchical Nickel Organometallics for Enhanced Oxygen Evolution

Abstract

This work introduces the Reverse Steam Rising Process (RSRP), a novel dissolution method, for the preparation of highly homogeneous organo-nickel composites. This approach enables gradual material dissolution, resulting in improved material integration. We investigate two distinct synthetic pathways: a direct organic material–nickel composite and a surfactant-assisted variation. Our findings demonstrate that the inclusion of a surfactant significantly improves the properties of the resulting organo-nickel composite. The RSRP method differs from traditional synthesis methods in that it utilizes reverse steam condensation to create a highly porous, multi-level structure. This unique structure significantly boosts the material’s electrocatalytic performance, particularly for the oxygen evolution reaction (OER). The Ni-MOF-CTAB catalyst exhibits an overpotential of 397 mV at 10 mA cm⁻² and a Tafel slope of 183 mV dec⁻¹, outperforming pristine Ni-MOF. The hierarchical design promotes superior ion and gas transport, while the distinctive organometallic configuration optimizes electronic interactions critical for OER activity. This innovative process enables precise control over both the micro- and nanoscale morphology of the nickel-based catalyst, ultimately leading to superior performance metrics. This advancement offers a new pathway for developing high-performance nickel organometallic materials for diverse electrocatalytic applications.

1. Introduction

Sufficient and sustainable energy is one of the main factors contributing to the prosperity and economic growth of societies. Global energy consumption has significantly increased over the past few years. Additionally, the future projection of energy consumption is continually increasing [1,2,3]. The world relies on carbon-based fuels, such as fossil fuels, natural gas, and coal, as its principal sources of energy [4]. Although fossil fuels are cheap and reliable, and the technology used to harness the energy stored in them is well-developed, they are a non-renewable source of energy. This means that the quantity of these fuels is limited, and their reserves are not replenished naturally. Additionally, the burning of fossil fuels produces carbon dioxide in the atmosphere, which significantly contributes to several environmental issues, including climate change and global warming [5]. Therefore, the transition to clean and sustainable energy sources is urgently required to sustain continuous human development, while also limiting the adverse impact on the environment.

Energy conversion devices, such as fuel cells, produce clean electric energy by consuming H₂ fuel in the presence of oxygen in an environmentally friendly process, where the only exhaust is clean water [6,7,8,9]. Hydrogen fuel can be produced by splitting water [10,11,12].

Water splitting involves two half-reactions. First, the oxygen evolution reaction (OER) occurs at the anode, producing oxygen through the oxidation of water. Second, the hydrogen evolution reaction (HER), which occurs at the cathode, involves the reduction of water molecules into H₂ gas. The OER is a four-electron process, which is kinetically sluggish and has a very high energy barrier. Additionally, the oxygen evolution involves the formation of O–O bonds, which require a massive amount of energy and are very challenging to form [13,14].

Electrocatalysts based on precious metals, such as Pt, exhibit excellent performance towards the OER; however, the formation of an oxide layer at the Pt surface at high positive potentials leads to a decrease in catalytic activity towards the OER [15,16,17,18]. Additionally, they suffer from high costs and scarcity, which limit their practicality as catalysts for the OER [19,20]. Moreover, catalysts based on oxides of precious metals have been developed for the OER and HER [21]. Both RuO₂ and IrO₂ exhibit excellent catalytic activity toward the OER and are considered the standards for evaluating the performance of other catalysts [22]. Both materials showed outstanding efficiency with low overpotential; however, RuO₂ is unstable at high positive potential and tends to dissolve during the formation of oxygen gas [23]. IrO₂ has been reported to have better stability than RuO₂, but its efficiency is lower than that of RuO₂. The high cost and dissolution concern adversely affect the long-term stability of these catalysts, limiting their use [24]. In addition to precious metal-based catalysts, transition metals have attracted significant attention as promising candidates for the OER. Transition metal-based catalysts offer structural and compositional active sites by offering d-orbital to establish binding with oxygen species [25,26,27]. Therefore, catalysts based on transition metals are capable of facilitating the formation of intermediates at low energy during the multi-step process associated with the OER. Additionally, transition metals are more abundant than precious metals; therefore, the catalysts will be cost-effective, which maximizes the opportunity for mass production of the catalyst materials for commercial purposes. Cobalt oxides provide a good example of the outperformance of these catalysts. They have been widely used to catalyze the OER in alkaline solution because of the high stability and unique electronic configurations [28]. Although cobalt-based catalysts exhibit good performance, they still lag behind those based on precious metals. To further improve performance, cobalt oxides are doped with other metals or metal oxides. This modification could lead to synergistic effects due to electronic interactions, which enhance its physicochemical characteristics and, consequently, improve its performance [29]. Jin et al. reported a one-pot synthesis route for the fabrication of a composite catalyst of cobalt oxide and N-doped carbon. The catalyst exhibited superior catalytic activity in terms of overpotential and current density when compared with cobalt oxides [30]. Ni-based catalysts were also investigated as one of the most promising and cost-effective candidates for the OER. Ni oxides and hydroxides were extensively studied [31,32]. Ni(OH)₂ can be readily prepared by chemical and electrochemical methods. Hydrous α-Ni(OH)₂ can be prepared by immersing Ni metal in an alkaline solution [32]. The β-Ni(OH)₂ can be obtained by aging the α-Ni(OH)₂ in an alkaline solution. Both α-Ni(OH)₂ and β-Ni(OH)₂ can be oxidized electrochemically to produce γ-NiOOH and β-NiOOH, respectively. Generally, β-NiOOH has better electrocatalytic activity than γ-NiOOH [31,32]. However, despite their advantages, nickel-based materials often suffer from issues such as limited conductivity, suboptimal stability, and lower catalytic activity compared to their noble metal counterparts. Recent advancements in nanostructured nickel materials and organometallic frameworks have shown promise in overcoming some of these limitations by enhancing the surface area, electronic structure, and overall durability of the catalysts.

Recent efforts to enhance the OER performance have investigated metal–organic frameworks (MOFs) as scaffolds for nickel-based electrocatalysts. Ling et al. reported the preparation of Fe₂Ni-MOF grown on nickel foam as an efficient catalyst for the OER, attributed to synergistic bimetallic interactions and a high-surface-area hierarchical structure [33]. Similarly, Xie et al. prepared MOF-Ni@NiO/N–C nanowires, which exhibited stable OER activity, highlighting the benefits of controlled nanostructuring [34]. Moreover, Sondermann et al. synthesized an MOF-Ni-Co composite via a one-step solvothermal route, combining MOF-Ni with a conductive carbon additive to enhance the electrical conductivity. The obtained composite electrocatalyst showed enhanced OER activity and durability in alkaline media due to synergistic charge transfer, and increased surface area (596 m²/g) [35]. Recent work by Ye et al. demonstrated that Fe(OH)₃-embedded Ni-MOF nanorods exhibit remarkable activity at high current densities and long-term stability for the OER. The significant performance is attributed to the electrochemical reconstruction into carbonate-adsorbed NiFe oxyhydroxides during the OER. The study demonstrates the crucial role and structural flexibility of the MOF scaffold in enhancing electrocatalytic activity [36].

In this study, we report the preparation of a novel nickel organometallic framework (Ni-MOF) and Ni-MOF-CTAB using the Reverse Steam Rising Process. We investigate its structural properties through a combination of advanced characterization techniques, including FTIR spectroscopy, UV-Vis-NIR diffuse reflectance spectroscopy, thermogravimetric analysis (TGA), XPS, XRD, SEM, and EDS. Furthermore, the electrocatalytic activities of composite catalysts prepared by mixing MOF and carbon fibers are investigated. The electrochemical measurements include cyclic voltammetry, Tafel slopes, scan rates, and chronoamperometry.

2. Results and Discussion

2.1. XPS Analysis

X-ray photoelectron spectroscopy (XPS) analysis was performed to investigate the elemental composition and chemical states of the Ni-MOF and Ni-MOF-CTAB samples. The survey and high-resolution spectra are shown in Figure 1a. The survey spectra confirmed the presence of carbon (C 1s), oxygen (O 1s), nitrogen (N 1s), and nickel (Ni 2p) in both samples. The carbon content in Ni-MOF-CTAB (72.22% At.) was slightly higher than in Ni-MOF (68.48% At.), which can be attributed to the additional carbonaceous surfactant (CTAB) used during synthesis. Conversely, the oxygen atomic percentage slightly decreased from 28.73% in Ni-MOF to 23.06% in Ni-MOF-CTAB, reflecting the relatively higher organic content in the CTAB-modified sample. The N 1s signal was significantly stronger in the Ni-MOF-CTAB sample (1.66% At.) compared to Ni-MOF (0.33% At.), further supporting the incorporation of nitrogen-rich CTAB into the surface or near-surface region of the material. The high-resolution N 1s spectrum of Ni-MOF-CTAB exhibited two peaks at 399.78 eV and 400.84 eV, which can be assigned to quaternary ammonium nitrogen from CTAB and residual nitrate/solvent species, respectively. In contrast, only a single N 1s peak at 399.92 eV was observed for Ni-MOF, likely originating from traces of nitrate or DMF [37,38]. Deconvolution of the high-resolution C 1s spectrum (Figure 1b,c) revealed three major components. The peak at ~283.9 eV corresponds to sp² hybridized carbon (C=C or C–C), while the peak near 287.7 eV is attributed to carboxyl or ester groups (C=O or O–C=O) from the trimesic acid ligands. A lower-binding energy peak at ~280.6–280.9 eV was observed in both samples and may be associated with Ni–C interactions or the presence of surface Ni-carbide-like species formed under thermal treatment [39].

Similarly, the O 1s spectra (Figure 1d,e) showed multiple components: a low-energy peak near ~530 eV attributed to Ni–O bonds, a dominant peak at ~532 eV corresponding to oxygen in carboxylate groups, and a higher-energy component near ~533.5 eV assigned to hydroxyl groups or adsorbed water [40,41]. The N 1s signal was significantly stronger in the Ni-MOF-CTAB sample (1.66% At.) compared to Ni-MOF (0.33% At.), as shown in Figure 1f,g, further supporting the incorporation of nitrogen-rich CTAB into the surface or near-surface region of the material. The high-resolution N 1s spectrum of Ni-MOF-CTAB exhibited two peaks at 399.78 eV and 400.84 eV, which can be assigned to quaternary ammonium nitrogen from CTAB and residual nitrate/solvent species, respectively. In contrast, only a single N 1s peak at 399.92 eV was observed for Ni-MOF, likely originating from traces of nitrate or DMF [37,38]. High-resolution Ni 2p spectra (Figure 1h,i) exhibited characteristic features of divalent and trivalent nickel species. In both samples, the peaks at ~854.1–854.2 eV (Ni 2p₃/₂) and ~871.5–871.7 eV (Ni 2p₁/₂) are consistent with Ni²⁺ in octahedral coordination, likely as part of the Ni–O bonds in the MOF. However, in addition to the expected satellite features, distinct peaks were observed at ~858.0 eV and ~874.7 eV in Ni-MOF and at ~858.8 eV and ~875.8 eV in Ni-MOF-CTAB [40,41]. These higher binding energy components are characteristic of Ni³⁺ species, indicating that a fraction of nickel exists in a +3 oxidation state, possibly due to surface oxidation, ligand interactions, or the formation of mixed-valence states (e.g., Ni²⁺/Ni³⁺). The relative intensities of these peaks suggest a greater proportion of Ni³⁺ in the CTAB-modified sample, which may be attributed to CTAB-induced structural or surface electronic effects during the synthesis process.

2.2. FTIR Analysis of Ni-MOF and Ni-MOF-CTAB

FTIR spectroscopy is essential for analyzing Ni-MOF and Ni-MOF-CTAB, as it provides valuable insights into the functional groups and the metal–organic interactions in the framework. It helps identify the presence of specific groups in the organic linkers, the interaction between metal ions (Ni²⁺) and the organic components, and any changes in the material’s structure due to the incorporation of CTAB. FTIR also reveals hydration effects and the role of surfactants in modifying the chemical stability and structural integrity of the MOF, offering a comprehensive understanding of how these materials are structured and function. Figure 2 shows the FTIR spectra of the under-studied samples. In the FTIR spectrum of the Ni-MOF sample, a broad peak at 3430 cm⁻¹ corresponds to the O–H stretching vibrations, which can be attributed to the hydroxyl groups present in the MOF structure or to adsorbed water molecules [42]. The presence of this broad peak indicates the presence of hydration or moisture in the sample. The relatively weak peaks at approximately 2934 cm⁻¹ are attributed to C–H stretching vibrations of alkyl groups in the organic linkers, indicating the presence of organic components, such as carboxylates and amines, within the framework [43]. The sharp, intense peak at 1628 cm⁻¹ corresponds to the C=O stretching vibration of the carboxylate group, a key feature of the organic linker in the Ni-MOF structure [44]. This peak is crucial in identifying the interaction between the metal centers (Ni²⁺) and the organic ligands. The moderate peaks at around 1379 cm⁻¹ and 1104 cm⁻¹ are assigned to C–H and C–O stretching vibrations and C–C bending vibrations of the organic linker [43,45]. These peaks reflect the bonding environment within the framework and provide insights into the structural stability of the MOF. The peaks at 779 cm⁻¹, 720 cm⁻¹, and 673 cm⁻¹ are related to the metal–oxygen (Ni–O) bonds and the characteristic vibrations of the metal centers in the framework, confirming the presence of nickel in the material [46].

In contrast, the FTIR spectrum of Ni-MOF-CTAB shows several differences, which can be attributed to the incorporation of CTAB into the MOF structure. The broad peak at 3381 cm⁻¹ is much wider and more intense compared to the one observed in Ni-MOF. This indicates a more substantial presence of O–H and N–H stretching vibrations, which can be attributed to the increased amount of adsorbed water or the potential influence of the CTAB surfactant in altering the hydration state of the material. The intensity of this peak suggests that the CTAB molecules may have interacted with the Ni-MOF structure, possibly leading to increased hydration or a modification of the pore structure. The peak at 2934 cm⁻¹ in Ni-MOF-CTAB is similar to that in Ni-MOF, corresponding to C–H stretching vibrations of alkyl groups. However, the overall intensity of this band is higher in the case of Ni-MOF-CTAB, reflecting the increased amount of alkyl groups from the CTAB surfactant, which is composed of long-chain alkyl groups. The C=O stretching vibration at 1661 cm⁻¹ is more intense in Ni-MOF-CTAB than in Ni-MOF, which suggests a stronger interaction between the organic linkers and the nickel centers in the presence of CTAB. This could be due to the surfactant’s role in altering the coordination environment around the Ni²⁺ ions. Another significant change is the appearance of a new peak at 1545 cm⁻¹ in Ni-MOF-CTAB, which is likely associated with N–H bending vibrations of the amine groups from the CTAB molecules [47]. This directly indicates the interaction between the CTAB and the MOF, and suggests that CTAB is interacting with the framework, possibly through ionic interactions or hydrogen bonding. The peaks at 1447 cm⁻¹ and 1381 cm⁻¹ in Ni-MOF-CTAB are significantly more intense than those in Ni-MOF. These peaks are associated with C–H bending vibrations and C–O stretching of the carboxylate group. The increased intensity indicates a more substantial contribution from the surfactant’s alkyl chains and possibly a more organized arrangement of the organic linkers in the presence of CTAB. The new peak at 1253 cm⁻¹ in Ni-MOF-CTAB can be attributed to C–N stretching vibrations, which is a clear indication of the presence of the nitrogen-containing surfactant in the framework [47]. This confirms that CTAB is not only physically adsorbed on the surface, but also incorporated into the structure, interacting with the metal centers and the organic ligands. The peak at 1110 cm⁻¹ is likely related to C–N bending or other C–O interactions in the structure, further supporting the incorporation of CTAB. Finally, the peaks at 763 cm⁻¹, 720 cm⁻¹, and 684 cm⁻¹ in Ni-MOF-CTAB are more intense than those in Ni-MOF. These peaks correspond to metal–oxygen (Ni–O) vibrations, and the increased intensity indicates that CTAB may influence the metal coordination or affect the crystallinity of the MOF. The increased intensity of these peaks suggests that CTAB may stabilize the metal centers and enhance the overall structural integrity of the MOF.

2.3. UV-Vis Spectroscopy and Optical Band Gap Analysis

The optical properties of the synthesized Ni-MOF and Ni-MOF-CTAB samples were studied using UV-Vis-NIR diffuse reflectance spectroscopy. The corresponding spectra, presented in Figure 3a, reveal several distinct absorption features indicative of electronic transitions associated with the organic linker, nickel centers, and d–d transitions. Furthermore, broad absorption bands in the near-infrared (NIR) region suggest the presence of localized surface plasmon resonance (LSPR) or defect-related transitions.

For both Ni-MOF and Ni-MOF-CTAB, five characteristic absorption peaks were observed: (i) a sharp peak around 298 nm, which can be attributed to π → π* transitions of the aromatic rings in the trimesic acid linker [48,49]; (ii) a moderate-intensity peak near 408 nm, likely corresponding to ligand-to-metal charge transfer (LMCT) transitions between the carboxylate groups and Ni²⁺ centers [50]; (iii) a broader peak at 672 nm, typically associated with d–d transitions of octahedrally coordinated Ni²⁺ ions (³A₂g → ³T₁g transitions) [51]; and (iv) a broad absorption band in the range of 1000–1500 nm, and (v) at 1800–2000 nm, which are more pronounced in Ni-MOF-CTAB. These bands may arise from plasmonic behavior due to Ni³⁺ species, mid-gap defect states, or vibronic overtones related to adsorbed CTAB molecules [52,53].

A comparison of the spectra reveals that the intensities of the first three peaks (298, 408, and 672 nm) are higher in Ni-MOF, suggesting stronger π → π*, LMCT, and d–d transitions in the absence of CTAB. In contrast, the two broad bands in the NIR region are significantly more intense in Ni-MOF-CTAB, indicating enhanced sub-bandgap absorption, possibly due to the introduction of Ni³⁺ species and surface/interface states facilitated by the CTAB surfactant. To determine the optical band gap energies, the Tauc method was applied using the following relationship [54]:

(αhν)ⁿ = A(hν − Eg)

where α is the absorption coefficient, hν is the photon energy, Eg is the optical band gap, A is a constant, and n depends on the nature of the electronic transition. For direct allowed transitions, n = 2. The plots of (αhν)² versus hν (Figure 3b) revealed three distinct linear regions for each sample, suggesting the presence of multiple optical band gaps due to different transition mechanisms or energy states.

By extrapolating the linear segments of the Tauc plots to the energy axis (Figure 3), the following band gap values were determined:

Ni-MOF: 1.61 eV, 2.32 eV, and 3.41 eV

Ni-MOF-CTAB: 1.27 eV, 2.44 eV, and 3.85 eV

The presence of three optical band gaps in each material indicates a complex electronic structure, likely influenced by the coexistence of Ni²⁺/Ni³⁺ species, organic–inorganic charge transfer states, and defect levels. The lower-energy band gaps (1.27–1.61 eV) are attributed to sub-bandgap transitions involving defects or mid-gap states. The intermediate and higher band gaps (2.32–2.44 eV and 3.41–3.85 eV) likely correspond to LMCT and π → π* transitions, respectively.

Interestingly, the lowest band gap in Ni-MOF-CTAB (1.27 eV) is significantly lower than that of Ni-MOF (1.61 eV), suggesting enhanced light absorption in the visible and near-infrared regions due to the electronic modifications induced by CTAB. The higher band gap in the UV region (3.85 eV for Ni-MOF-CTAB vs. 3.41 eV for Ni-MOF) may be attributed to structural alterations in the organic framework or stronger electron delocalization facilitated by CTAB interaction.

2.4. Thermal Stability and Decomposition Behavior

The thermal stability and decomposition profiles of Ni-MOF and Ni-MOF-CTAB were analyzed using thermogravimetric analysis (TGA) under a helium atmosphere, from room temperature up to 800 °C, and the obtained thermograms are shown in Figure 4. The thermal degradation patterns of the two samples reveal multi-step decomposition processes, each associated with specific structural or compositional changes.

For the Ni-MOF sample, four distinct weight loss steps were observed. (i) The first weight loss of 3.27% occurred between 35 °C and 165 °C, which is attributed to the desorption of physically adsorbed water molecules and residual solvent trapped in the pores of the MOF structure [55,56]. (ii) The second step, corresponding to a weight loss of 12.20% from 165 °C to 240 °C, is likely associated with the removal of more strongly bound guest molecules or the decomposition of volatile organic components such as residual DMF or partially decomposed linker fragments [57,58]. (iii) A significant weight loss of 25.06% was observed in the range of 240 °C to 360 °C, which can be assigned to the initial thermal decomposition of the organic ligand (trimesic acid), indicating the onset of framework breakdown [59]. (iv) The major decomposition stage occurred between 360 °C and 450 °C, with a weight loss of 34.16%, corresponding to the complete collapse of the MOF and oxidative degradation of the organic backbone [60]. Beyond 450 °C and up to 800 °C, no substantial weight loss was observed, suggesting that the decomposition was complete and a stable inorganic residue, likely composed of nickel oxide or related nickel-containing species, remained. At 800 °C, the remaining mass was 24.97%, representing the final residue.

In comparison, the Ni-MOF-CTAB sample exhibited a somewhat different decomposition behavior, reflecting the influence of the surfactant on the structure and composition of the MOF. (i) The initial weight loss of 7.53% from 35 °C to 160 °C is larger than that of Ni-MOF and can be attributed to both surface-adsorbed water and the removal of intercalated CTAB molecules, which are more volatile or less strongly held [61]. (ii) A subsequent weight loss of 12.78% occurred between 160 °C and 260 °C, likely due to the decomposition of partially embedded CTAB, residual solvent, or weakly coordinated organic moieties. (iii) The third stage, from 260 °C to 380 °C, resulted in a weight loss of 14.66%, corresponding to the partial degradation of the MOF and ligand backbone [60]. (iv) A substantial weight loss of 46.56% was observed in the range of 380 °C to 460 °C, indicating a more abrupt and complete collapse of the framework and decomposition of both the trimesic acid and CTAB components. The presence of CTAB appears to lower the thermal stability of the composite structure in this decomposition stage. (v) Above 460 °C, the decomposition plateaued with no further significant weight loss, and 18.20% of the total mass remained at 800 °C, indicating the formation of a nickel-containing oxide residue.

Overall, the Ni-MOF-CTAB sample shows slightly lower thermal stability compared to the pristine Ni-MOF, particularly in the final stages of decomposition. This behavior may result from the incorporation of CTAB, which introduces additional organic components that decompose at lower temperatures and alter the microstructure of the MOF. The higher total mass loss and lower residual weight in Ni-MOF-CTAB also suggest a higher organic content due to the presence of the surfactant.

As shown in Supplementary Figure S1, the Differential Thermogravimetric (DTG) curves (blue lines) provide a clearer view of the thermal degradation steps by showing the rate of mass loss as a function of temperature. The DTG profiles for both the Ni-MOF and Ni-MOF-CTAB samples exhibit multiple peaks, with each peak corresponding to a specific decomposition event. The most significant difference between the two DTG curves is the presence of an additional decomposition peak in the Ni-MOF-CTAB sample, centered at approximately 270 °C. This distinct peak is direct evidence of the degradation of the incorporated CTAB surfactant, a feature that is absent in the pristine Ni-MOF. While the final decomposition peaks for both materials are comparable, they are slightly shifted in temperature and altered in shape. This strongly suggests that the presence of CTAB influences the overall thermal degradation mechanism of the MOF. This observation is consistent with the broader findings of the study, which demonstrate how the surfactant’s incorporation affects the MOF’s thermal stability.

2.5. XRD and EDX Analyses

The X-ray diffraction (XRD) patterns of the synthesized samples, Ni-MOF and Ni-MOF-CTAB, are illustrated in Figure 5. The XRD patterns of both samples exhibit sharp and well-defined peaks, indicating high crystallinity. The diffraction peaks of the Ni-MOF sample were found to be in close agreement with the standard reference pattern of a material with the chemical formula NiC₂₀H₁₈N₂O₆ (JCPDS No. 96-411-2555). This compound crystallizes in a cubic structure with a space group of I 41 3 2. In contrast, the diffraction pattern of the Ni-MOF-CTAB sample matched well with a reference pattern corresponding to NiC₁₅H₁₇N₂O₈ (JCPDS No. 96-720-2893), which possesses a hexagonal crystal structure. These identifications confirm that the introduction of CTAB during synthesis induces a significant change in the crystal structure, likely due to its surfactant-mediated modulation of the coordination environment and crystal growth direction. The increase in the crystallite size upon the addition of CTAB suggests that the presence of the surfactant facilitated the growth of larger crystals, possibly by reducing surface energy or by controlling nucleation and growth kinetics.

When these structural findings are compared with the surface composition results obtained from X-ray photoelectron spectroscopy (XPS), a consistent trend is observed. The Ni-MOF sample, with the assigned formula NiC₂₀H₁₈N₂O₆, shows a lower surface nitrogen content and a higher carbon content, as expected from its chemical composition. Conversely, the Ni-MOF-CTAB sample, with the formula NiC₁₅H₁₇N₂O₈, shows a significantly higher nitrogen content and a slightly increased nickel concentration, which is consistent with the incorporation of CTAB, a nitrogen-containing surfactant, during synthesis.

Figure 6 shows the EDX analysis results for NiC₂₀H₁₈N₂O₆. The atomic percentages of Ni, C, N, and O are approximately 2.13%, 42.55%, 4.26%, and 12.77%, respectively. For NiC₁₅H₁₇N₂O₈, these values are approximately 2.33%, 34.88%, 4.65%, and 18.60%, respectively. Although surface-sensitive XPS values differ quantitatively due to detection limits, surface orientation, and possible preferential adsorption or degradation, the general trend of increased nitrogen and nickel in the CTAB-modified sample is in qualitative agreement with the proposed structures.

2.6. SEM and TEM Analysis

Figure 7 depicts the SEM analysis of the Ni-MOF structures produced. Figure 7(1a) illustrates the shape of the Ni-MOF particles, which resemble a crystalline, rod-like, cubic structure. Figure 7(2a) shows the same structure, but uses color mapping to highlight surface texture and topography. Figure 7(1b) indicates that preparing Ni-MOF-CTAB with the surfactant CTAB results in the formation of hexagonal crystals. This suggests that CTAB has a significant influence on crystal growth and shape. Figure 7(2b) is a colorized image of Figure 7(1b), emphasizing smoother and more defined edges. The color mapping in Figure 7(2a,2b) reveals differences in particle uniformity and surface roughness. Ni-MOF displays an uneven texture without CTAB, but with CTAB, it becomes more symmetrical and compact. These findings imply that CTAB aids in controlling particle shape during synthesis.

Figure 8 presents high-resolution transmission electron microscopy images of Figure 8a Ni-MOF and Figure 8b Ni-MOF-CTAB. The distinctly defined lattice fringes in image Figure 8a illustrate the elevated crystallinity of Ni-MOF. The distinct spots in the small SAED (selected area electron diffraction) pattern (inset) further substantiate the crystalline structure of Ni-MOF. Conversely, Figure 8b illustrates that Ni-MOF-CTAB possesses a less organized structure. The indistinct rings in the selected area electron diffraction (SAED) pattern of Ni-MOF-CTAB indicate the presence of numerous small crystals. This discovery indicates that the systematic configuration of the Ni-MOF may be disrupted by the incorporation of CTAB, leading to a less organized substance. The structural distinctions between Ni-MOF and Ni-MOF-CTAB may substantially influence their characteristics and prospective applications across various industries. The presence of CTAB during synthesis likely disrupts normal crystal growth. This modification affects both particle formation and the configuration of local atomic structures. These disparities illustrate the influence of the surfactant on the modulation of structural order at the nanoscale.

2.7. Electrochemical Characterization of Catalysts

Cyclic voltammetry was used to investigate the performance of Ni-MOF and Ni-MOF-CTAB composite electrocatalysts in an aqueous solution containing 1 M KOH. Figure 9 illustrates comparative CVs for the GCE, GCE modified with Ni-MOF, and Ni-MOF-CTAB composites, respectively. The CVs were recorded at a scan rate of 5 mVs⁻¹. Ni-MOF showed two small current peaks at 1.48 V and 1.35 V, which could be attributed to the oxidation of Ni²⁺ to Ni³⁺ and the reduction of Ni³⁺ to Ni²⁺, respectively. By contrast, Ni-MOF-CTAB showed two distinctive redox peaks with very high currents and low voltage. The Ni-MOF-CTAB exhibited the best catalytic activity, producing the highest current density at a given potential in the OER region, as highlighted in Figure 10.

Electrocatalytic kinetic parameters, e.g., overpotential ($\eta )$ and the Tafel slope are usually used to evaluate the performance of the electrocatalyst. The overpotential ($\eta$) calculates the difference between the applied potential (E) and the potential under equilibrium conditions (E_eq), using Equation (1):

η = E − E_eq

(1)

where E is the applied potential, which is required to perform OER, and E_eq is the potential of the OER at equilibrium conditions. The difference between the two values provides ($\eta$), which is defined as the energy required to exceed the electrode kinetic energy barrier of the OER. Both the current density and the overpotential $\eta$ are related to each other. Therefore, the $\eta$ is commonly measured at a current density of 10 mA/cm² when comparing the performance of various electrocatalysts. A low $\eta$ value is an indicator of enhanced catalytic performance [18]. The rate of oxygen evolution reaction was calculated using Tafel Equation (2), correlating the rate of the electrochemical reaction and the overpotential:

$$\mathit{log}\left(I\right)=\mathit{log}\left(I_o\right)+ {\displaystyle \frac{\eta }{b}}$$

(2)

where I is the current density, I_o is the exchange current density (i.e., the current at zero potential), and $b$ is the slope of the Tafel plot. Tafel slopes were calculated by plotting log(I) versus η, producing a straight line with a slope of b. A small Tafel slope value indicates that the current density will increase significantly with a slight change in overpotential, thus indicating a faster reaction rate for the OER. Hence, small Tafel slopes were used as indicators of better-performing electrocatalysts. Tafel slopes were calculated according to Equation (3): Equations (2) and (3) are explicitly linked to show how overpotential relates to the Tafel slope.

$$b={\displaystyle \frac{\mathit{\partial }\eta }{\mathit{\partial }\mathit{log}i}}$$

(3)

Figure 10 shows the Tafel plots for the OER current for the curves shown in Figure 9.

Table 1. Tafel slopes (b) and exchange current density (i_o) for the investigated electrodes.

	η at 5 mA cm−2	η at 10 mA cm−2	Tafel Slope (b)	Exchange Current Density (io)
GCE	-	-	302 mV · dec−1	0.00220 mA cm−2
Ni-MOF	560 mV	-	278 mV · dec−1	0.055 mA cm−2
Ni-MOF-CTAB	343 mV	397 mV	183 mV · dec−1	0.07030 mA cm−2

reports the overpotential and Tafel plots for the three investigated electrodes. The GCE modified with Ni-MOF composite catalyst yielded a maximum current density of 5.94 mA cm⁻² with a corresponding overpotential of 593 mV vs. RHE. Nevertheless, the GCE modified with Ni-MOF-CTAB composite catalyst exhibited notable performance toward OER, achieving a current density of 10 mA cm⁻² with an overpotential of 397 mV vs. RHE. These outcomes indicate a high performance of Ni-MOF-CTAB toward the OER. Therefore, the upcoming sections will focus on Ni-MOF-CTAB.

The concentration of Ni2+/Ni3+ on the Ni-MOF-CTAB composite catalyst surface was quantified by surface coverage (Г), using Equation (4).

$$\mathrm{Г}={\displaystyle \frac{Q}{nF}}$$

(4)

where $\mathrm{Г}$ is the surface coverage, Q is the charge in coulombs (C), F is the Faraday constant (96485 C mol⁻¹), and n is the number of electrons (n = 1). The Q was calculated by the integration of the area under the oxidation peak obtained at a scan rate of 5 mV s⁻¹ and is shown in Figure 11. The calculated $\mathrm{Г}$ value for the Ni-MOF-CTABwas found to be 3.91 × 10¹⁷ Ni atoms/cm².

Furthermore, the effect of various scan rates of Ni-MOF-CTAB on the OER was investigated. Figure 12 shows the CVs recorded for the Ni-MOF-CTAB composite catalyst in 1 M KOH solution. The anodic and cathodic current densities of Ni-MOF-CTAB were found to increase linearly with the square root of the scan rates (inset of Figure 12), indicating that the process taking place at the Ni-MOF-CTAB surface is controlled by the diffusion of hydroxide ions from the bulk solution to the composite electrode surfaces. The slight shift in the anodic and cathodic peaks may demonstrate a quasi-reversible process.

The stability of the Ni-MOF-CTAB was investigated using chronoamperometry and cyclic voltammetry techniques in a 1 M KOH solution. The GC electrode modified with Ni-MOF-CTAB composite catalyst was stepped from a potential of 0 V to a final potential of 1.62 V vs. RHE. The current density was recorded as a function of time, as illustrated in Figure 13. The composite electrode maintained its catalytic activity for up to 30 min. The decrease in current density with time could be attributed to the increase in diffusion layer thickness resulting from catalysis. In addition, to further investigate the stability of the Ni-MOF-CTAB, consecutive CVs were recorded under the same conditions. The working electrode was scanned approximately 100 times until constant anodic and cathodic peaks were achieved, as shown in Figure 14.

3. Experimental Section

3.1. Materials and Chemicals

The nickel nitrate hexahydrate (Ni(NO₃)₂.6H₂O), trimesic acid (H₃BTC), and hexadecyltrimethylammonium bromide (CTAB, 98%) were obtained from Sigma-Aldrich (Burlington, MA, USA). N,N-Dimethylformamide was purchased from PanReac AppliChem (Castellar del Vallès, Spain). Methanol and ethanol were obtained from Prolabo, (Žalec, Slovenia). All of the reagents were used as received, without further purification.

3.2. Preparation of Nickel Organometallic Precursors via Rising Vapor Process

The synthesis of Ni-MOF involves the precise combination of nickel (II) nitrate hexahydrate Ni(NO₃)₂⋅6H₂O, (0.7744 g, 2.66 mmol) and trimesic acid (0.56035 g, 2.66 mmol). These precursors are ground until thoroughly combined and reduced to a fine powder, and are then carefully spread out on Whatman filter paper, which is folded into a star shape. This folded paper is used as a controlled precursor reservoir and is placed on the top of a Pyrex tube that contains 20 mL of N,N-dimethylformamide (DMF). After that, the tube is sealed and autoclaved for 24 h at 120 °C. The tube is then left to cool down and then centrifuged. The supernatant is decanted and the greenish precipitated product is washed twice with 10 mL of N,N-dimethylformamide, followed by washing with 10 mL of ethanol and, finally, twice with 10 mL of methanol. The product is dried under vacuum at 70 °C for 24 h. Figure 15 shows a schematic diagram of the Reverse Steam Rising Process in which the ‘rising vapor’ step occurs during the condensation-driven dissolution of the precursors.

The preparation and purification of Ni-MOF-CTAB were performed in the same process as batch Ni-MOF, except that 0.37 mmol of CTAB was added to the Ni(NO₃)₂⋅6H₂O and trimesic acid mixture.

3.3. Preparation of the Ni-MOF and Ni-MOF-CTAB Inks

The composite catalyst ink was prepared by dispersing 5 mg of carbon fibers (CFs) in 1 mL of isopropanol. Then, 10 mg of the Ni-MOF or Ni-MOF-CTAB was added to the dispersion, followed by sonication for 20 min. Finally, 30 μL of Nafion oil was added to increase the mechanical integrity of the composite catalysts.

3.4. Fabrication of the Composite Electrode

A glassy carbon electrode (0.071 cm²) was used as a working electrode. The electrode was first cleaned several times with deionized water and then polished with alumina slurry (0.05 µm purchased from Metrohm). Subsequently, the electrode was polished electrochemically by cycling the electrode in 0.5 M H₂SO₄ for 30 cycles at a scan rate of 50 mV. s⁻¹. To prepare the working electrodes, the Ni-MOF/CFs or Ni-MOF-CTAB/CFs were first dispersed in a small volume of isopropanol to form a homogeneous suspension. The glassy carbon electrode was then modified by drop-casting 20 μL of this suspension onto its surface (the loading amount of catalyst was 0.2 mg). Following this, the modified electrodes were transferred to a drying oven and baked at 50 °C for 30 min to ensure solvent evaporation and stable film formation.

3.5. Characterization

The samples were characterized by mid-infrared absorption spectra obtained under vacuum optics utilizing an FT-IR spectrometer (Vertex 70, Bruker, Bremen, Germany) with a resolution of 4 cm⁻¹. Here, 64 scans were applied, and a multiple-crystal diamond was utilized for the analysis. The spectra of the catalysts were obtained using UV–Vis diffuse reflectance spectra by a PerkinElmer Lambda 950 spectrophotometer equipped with an integrating sphere. X-ray diffractometry and phase characterization were obtained using a D8 Advance (Bruker, Germany) with a Cu-Kα X-ray 2.2 kW for the X-ray source, and the applied parameters were 40 Kv and 40 mA. The sample weight loss was determined with a TGA thermal analyzer (STD-Q 600, New Castle, DE, USA) at room temperature and up to 800 °C at 10 °C/min in a helium environment. The binding energy shift was corrected based on the 1s peak binding energy of the carbon, which was centered at 285 eV. Before analysis, the samples were placed under an evacuated vacuum (12 h at 1.0 × 10⁻⁶ Torr). A microstructure analysis of the prepared catalysts was carried out using a scanning electron microscope (JSM-7800F, JEOL, Akishima, Japan), and EDS (electrodispersive X-ray spectroscopy) was performed using an X-Max^N (OXFORD Instruments, UK). Transmission electron microscopy (TEM; JEOL-JEM-2100F, Akishima, Japan) was conducted at 200 kV to examine the nanostructure of the catalysts.

4. Conclusions

Ni-MOF and Ni-MOF-CTAB were successfully synthesized using the Reverse Steam Rising Process. The reported method is simple, low-temperature, cost-effective, and rapid. The XPS analysis confirmed that incorporating CTAB into the Ni-MOF structure not only enhanced the nitrogen content, but also increased the fraction of Ni³⁺ species, suggesting improved electrocatalytic activity. Additionally, the FTIR results confirmed the incorporation of CTAB into the structure, which altered the vibrational modes of the MOF, indicating strong interactions between CTAB and the framework and enhancing structural flexibility. The XRD analyses demonstrated that the incorporation of CTAB during the synthesis process significantly influenced the crystalline phase of Ni-MOF, resulting in larger crystallite sizes and a hexagonal phase for Ni-MOF-CTAB, thus confirming the structural evolution of the material. The full electrochemical characterizations showed that the incorporation of CTAB improved the electrocatalytic activity and stability of the Ni-MOF-CTAB composite catalyst. The Ni-MOF-CTAB exhibited promising stability (30 min chronoamperometry and 100 CV cycles). Additionally, the Ni-MOF-CTAB exhibited an overpotential of 397 mV at 10 mA cm⁻² and a Tafel slope of 183 mV dec⁻¹, outperforming both the Ni-MOF composite catalyst and the bare glassy carbon electrodes. These findings highlight the key role played by the surfactant in designing and tailoring MOF-based electrocatalysts, offering a promising path for developing high-performance, cost-effective water-splitting systems.