A Multifunctional Nickel-Based Metal–Organic Framework (MOF) for Hydrogen Production, Supercapacitors, and Electrocatalysis

Abstract

The nickel-derived metal–organic framework (MOF), Ni-BTB, synthesized from 4,4′,4″-benzene-1,3,5-tribenzoic acid (H₃BTB), was investigated as a multifunctional platform for enhanced energy applications including production and storage. In catalytic hydrogen generation by NaBH₄ hydrolysis, Ni-BTB attained a hydrogen generation rate (HGR) of 4640 mL H₂/g•min with 1 mg of catalyst, with an activation energy of 76.44 kJ/mol. Under optimized reaction conditions (60 °C, 20 mg catalyst, and 1 g NaBH₄), the HGR increased to 9542 mL H₂/g•min, while exhibiting high recyclability throughout four successive cycles. As a supercapacitor electrode, Ni-BTB achieved a specific capacitance of 156 F/g at 1 A/g and showed remarkable cycling stability, maintaining its capacitance after 10,000 charge–discharge cycles. Furthermore, Ni-BTB exhibited exceptional electrocatalytic activity for oxygen evolution reaction (OER), requiring only 106 mV overpotential to achieve 10 mA/cm², offering a time-of-flight (TOF) of 0.0585 s⁻¹ and demonstrating significant operational longevity of at least 12 h. These findings underscore Ni-BTB as a durable, reusable, and adaptable material for hydrogen production, energy storage, and electrocatalytic applications.

1. Introduction

The high global energy consumption, coupled with intensifying environmental and climate concerns, push the society toward the development of sustainable, low-carbon energy conversion technologies [1,2,3,4]. Among the proposed solutions, clean hydrogen production stands out for its high energy density and potential as an environmentally benign energy carrier [5,6,7]. Among several methods for hydrogen production, water electrolysis represents a cornerstone technology, enabling the efficient electrochemical generation of hydrogen and oxygen [8,9]. In parallel, high-performance energy storage systems are required for mitigating the intermittency of renewable energy generation while maintaining a stable, efficient energy supply. Consequently, integrating hydrogen production, oxygen evolution, and energy storage offers a holistic strategy for realizing scalable, sustainable clean energy infrastructure [10,11].

Hydrogen has been recognized as a clean, sustainable energy vector capable of replacing conventional fossil-based energy sources and facilitating the shift toward carbon-neutral energy technologies [12,13,14,15]. Hydride-based materials play a crucial role in hydrogen storage and production by enabling safe, efficient solid-state hydrogen storage [16,17,18]. Notably, sodium borohydride (NaBH₄) is a promising material for hydrogen storage and release, attributed to its substantial hydrogen storage capacity (10.6 wt.%) and environmental compatibility. Its hydrolysis in water releases four moles of hydrogen per mole of NaBH₄. The hydrogen release requires a catalyst that exhibits high activity, durability, and structural robustness to ensure controlled and efficient hydrogen generation [19,20]. Accordingly, extensive efforts have been focused on designing advanced catalytic systems to enhance hydrogen release performance [21,22,23].

Hydrogen production via water electrolysis remains a fundamental process for converting renewable electricity into chemical energy, producing hydrogen and oxygen through two coupled half-reactions: hydrogen is generated at the cathode via the hydrogen evolution reaction (HER), while oxygen is produced at the anode through the oxygen evolution reaction (OER) [24,25,26]. Although HER generally proceeds efficiently, the OER is hindered by slower kinetics, which make it the rate-limiting step of the reaction and the primary bottleneck in overall water-splitting efficiency. While RuO₂ and IrO₂ offer excellent catalytic activity for the OER, the high expense of these metals limits large-scale implementation. Hence, the development of earth-abundant, economically viable, and durable electrocatalysts is critical to improving OER efficiency and advancing water electrolysis technologies [27,28,29].

Supercapacitors (SCs) have gained increasing attention as next-generation energy storage systems due to their high power output, fast charge–discharge rates, and prolonged cycle stability [30,31,32,33,34,35,36,37]. These properties make SCs suitable for applications in electric mobility solutions, portable electronic devices, and intelligent power grid applications [38,39]. However, traditional electrode materials like activated carbon, graphene, and oxides of transition metals often suffer from limited energy density, restricted tunability, and scalability issues [40,41]. These limitations highlight the urgent need for new electrode materials that combine high capacitance, excellent stability, and structural versatility [30,31,32,33,34,35,36,37].

Metal–organic frameworks (MOFs), distinguished by their high porosity, extensive surface areas, and tunable chemical compositions, have emerged as highly promising materials for several applications [42,43,44]. They, including their derived material, have demonstrated excellent performance in hydrogen generation [14], supercapacitor energy storage [45,46,47,48,49,50], and OER electrocatalysis [51], owing to their accessible active sites and structural flexibility. These features suggest MOFs as ideal candidates for multifunctional energy applications, offering a unified platform for clean hydrogen production, advanced energy storage, and sustainable electrochemical processes [52].

Herein, we synthesized and tested a nickel-based MOF constructed from 4,4′,4″-benzene-1,3,5-tribenzoic acid (Ni-BTB) as a multifunctional material for advanced energy applications. Tritopic carboxylic linker, i.e., H₃BTB, was used to ensure high stability. It was characterized using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier-transform infrared (FT-IR), and transmission electron microscopy (TEM). The Ni-BTB catalyst exhibited high performance in catalyzing hydrogen release from NaBH₄, reaching a generation rate of 4640 mL H₂/g•min, which increased to 9542 mL H₂/g•min at 60 °C under optimized conditions, while maintaining excellent stability over four consecutive cycles. As a supercapacitor electrode, Ni-BTB delivered an initial capacitance of 156 F/g at 1 A/g and maintained full capacitance over 10,000 charge–discharge cycles, indicating outstanding durability and reusability. Furthermore, Ni-BTB exhibited remarkable OER activity, achieving a current density of 10 mA/cm² at a low overpotential of 106 mV. Collectively, these results highlight Ni-BTB as a versatile and multifunctional MOF for integrated hydrogen production, oxygen evolution, and high-performance energy storage, offering significant promises for next-generation energy conversion systems.

2. Results and Discussion

2.1. Materials Synthesis and Characterization

The Ni-BTB synthesis procedure is illustrated schematically in Figure 1. Ni is coordinated to BTB, forming an innovative 3D nickel-based hybrid framework with high ferromagnetism and semiconductor properties [53]. The synthesis of the Ni-BTB framework was confirmed by XRD (Figure 2a), FT-IR (Figure 2b), TEM (Figure 3), and XPS (Figure 4), providing comprehensive insights into its structural, morphological, and chemical features.

The XRD profile of Ni-BTB shows well-defined diffraction peaks, validating the establishment of a highly crystalline MOF (Figure 2a). No diffraction signals from metallic nickel or nickel oxide phases are observed, indicating that Ni species are uniformly coordinated within the BTB framework (Figure 2a). The experimental XRD pattern closely aligns with the structure of an isostructure BTB framework published under CCDC Deposition Number 2160411 for catena-[bis(μ-1,3,5-tris(4-carboxylatophenyl)benzene)-bis(DMF)-tri-iron(II)] (DMF), known as BTB-MOF-24 [54,55]. The comparatively sharp diffraction peaks indicate nanoscale crystallite sizes, which are beneficial for catalytic applications owing to the enhanced availability of accessible active sites.

The FT-IR spectroscopy further confirms the coordination between Ni²⁺ ions and the BTB ligand (Figure 2b). Distinct vibrational bands are observed in the 1600–1650 cm⁻¹ region, reflecting the asymmetric stretching of C=O bonds in the coordinated carboxylate groups. The lack of bands linked to free –COOH groups, coupled with the appearance of symmetric C–O stretching bands near 1400 cm⁻¹, validates the deprotonation and bidentate coordination of the carboxylate groups to the nickel centers. The extra vibrational bands observed in the 500–600 cm⁻¹ range are attributed to Ni–O stretching modes, providing further confirmation of the MOF’s development.

A TEM image of the Ni-BTB is presented as shown in Figure 3. The material consists of quasi-hexagonal nanoparticles. The precisely delineated nanoscale architecture indicates a high level of structural uniformity, which is advantageous for maintaining consistent surface chemistry and readily available active sites. Moreover, the quasi-hexagonal morphology and homogeneous dispersion facilitate efficient mass transport and enhance the exposure of catalytically active nickel centers, which is expected to contribute positively to the observed catalytic and electrochemical performance of Ni-BTB.

XPS analysis was employed to further the surface composition and electronic states of Ni-BTB (Figure 4). The survey spectrum confirms the presence of Ni, C, and O, indicating elevated sample purity. High-resolution Ni 2p spectra exhibit peaks at 855.6 eV (Ni 2p3/2) and 873.2 eV (Ni 2p1/2) along with satellite features, confirming the presence of Ni²⁺ species (Figure 4a). The O 1s spectra can be deconvoluted at 31.2 eV, 532.6 eV, and 533.8 eV, corresponding to Ni–O, C=O, and C–O bonds, respectively, indicating robust metal–ligand interactions (Figure 4b). The C 1s spectra exhibit peaks at around 284.8 eV (C–C/C=C), 286.1 eV (C–O), and 288.7 eV (O–C=O), thereby affirming the structural integrity of the BTB linker (Figure 4c).

2.2. Hydrogen Generation Using Ni-BTB

The hydrogen production efficacy of Ni-BTB was examined by the hydrolysis of NaBH₄, as depicted in Figure 5a. With a catalyst loading of 50 mg, the time required to achieve the maximum hydrogen volume was reduced to approximately 28 min across a NaBH₄ concentration range of 0.2–3.0 wt.%. The HGR attained under these conditions was 4640 mL H₂/g•min (Figure 5b). Significantly, Ni-BTB demonstrated exceptional durability and reusability, maintaining practically constant catalytic activity across at least four successive cycles, underscoring its substantial stability and resilience under operating conditions (Figure 5c,d). These findings illustrate the adaptability and efficacy of Ni-BTB for hydrogen production across a broad spectrum of NaBH₄ concentrations.

Temperature is important in the hydrolysis of NaBH₄, as illustrated in Figure 6a,b. High temperature significantly improves the hydrogen evolution rate by accelerating reaction kinetics and increasing the frequency of molecular collisions. At 60 °C, using 20 mg of Ni-BTB and 1.0 wt.% NaBH₄, the time for hydrogen evolution to reach its maximum decreased substantially to 1.31 min, yielding an HGR of 9542 mL H₂/g•min. This exceptional result highlights the significant catalytic efficiency of Ni-BTB, even under comparatively mild reaction conditions.

The kinetic and thermodynamic properties of the Ni-BTB-catalyzed NaBH₄ hydrolysis were evaluated using Arrhenius and Eyring analyses (Figure 6c,d). The calculated apparent activation energy was 76.44 kJ/mol, with a strong linear correlation coefficient (R² = 0.99), indicating reliable kinetic behavior. The activation parameters, ΔH* and ΔS*, were determined to be 73.9 kJ/mol and 0.25 kJ/mol•K, respectively, thereby substantiating the temperature-dependent enhancement of the reaction rate. NaBH₄ dissociates in aqueous solution into BH₄⁻ and Na⁺ ions, which readily adsorb onto the surface of the Ni-BTB catalyst. The BH₄⁻ species hydrolyzes at the exposed Ni active sites, generating hydrogen gas that promptly desorbs from the surface, while sodium metaborate (NaBO₂) is produced as a byproduct. The coordination environment and porous structure of Ni-BTB facilitate the effective adsorption, activation, and turnover of BH₄⁻ ions, enabling the complete release of 4 equivalents of H₂ per NaBH₄ molecule under mild circumstances.

A comparative examination of hydrogen generation performance for several nickel-based and MOF-derived catalysts documented in the literature is presented in

Table 1. Catalyst performance comparison in NaBH₄ hydrolysis.

Catalysts	Cat. (mg)	Temperature (°C)	Reaction Conditions	HGR (mL H2/g•min)	Ref.
Cu–Ni–PET MOF	100	50 °C	0.25 g NaBH4 (ethanol)	2650	[56]
Cu–Ni–C MOF	100	50 °C		2150	[56]
Ni-Co3O4	50	25 °C	10 wt% NaBH4 10 wt% NaOH	1925	[57]
Ni1Pt3/KNbO3	25	41.9 °C	0.5 mmol NaBH4	2068	[58]
Ni(1,4-NDC)	50	60 °C	50 mM NaBH4	1333	[59]
Ni-POM	2.5	25 °C	300 mM NaBH4	610.2	[60]
Ni-BTB	1	25 °C	3 wt% NaBH4	4640	This work
	20	60 °C	1 wt% NaBH4	9542

Note: NDC: Naphthalenedicarboxylate; POM: Polyoxometalate.

. The literature data indicate that both catalyst composition and operational parameters, e.g., catalyst mass, temperature, and NaBH₄ concentration, are pivotal in influencing hydrogen evolution efficiency. The Cu–Ni–PET MOF exhibits a notable HGR of 2650 mL H₂/g•min at 50 °C; however, it requires a substantial catalyst loading of 100 mg and elevated temperature [56]. The carbonized counterpart, Cu–Ni–C MOF, exhibited HGR of 2150 mL H₂/g•min, suggesting that carbonization may partially hinder active sites, albeit with enhanced conductivity [56].

Conversely, Ni-BTB achieves a high HGR of 4640 mL H₂/g•min at ambient temperature with only 1 mg of catalyst, underscoring its remarkable intrinsic catalytic efficacy. Oxide-based catalysts, such as Ni–Co₃O₄ [57], and noble-metal-containing systems, such as Ni₁Pt₃/KnbO₃ [58], exhibit reduced HGR values and necessitate elevated catalyst loadings, robust alkaline conditions, or expensive noble metals. Coordination polymers such as Ni(1,4-NDC) [55] and Ni-POM [56] exhibit constrained hydrogen production efficiency, primarily due to limited surface accessibility and suboptimal electronic configurations. The exceptional efficiency of Ni-BTB is attributable to the abundant, dispersed Ni active sites and robust metal–ligand coordination, which together facilitate efficient mass transfer and fast catalytic turnover. The significant rise in HGR to 9542 mL H₂/g•min at 60 °C further substantiates the system’s advantageous kinetic characteristics. Significantly, even with minimal catalyst loading and reduced NaBH₄ concentration, Ni-BTB surpasses previously reported catalysts, demonstrating its durability and scalability. These findings demonstrate that Ni-BTB serves as a remarkably effective catalyst for hydrolyzing NaBH₄ without requiring noble-metal catalysts. The combination of remarkably high hydrogen generation rates, minimal catalyst dosage, outstanding recyclability, and robust performance under mild conditions underscores the efficacy of MOF-based coordination engineering, establishing Ni-BTB as a viable candidate for hydrogen generation applications.

2.3. The Electrochemical Performance

CV analysis was conducted to assess the electrochemical performance of the Ni-BTB electrode at scan speeds of 1–200 mV/s (Figure 7). Figure 7a shows that the CV curves deviate from an ideal rectangular shape, with distinct anodic and cathodic redox peaks. It indicates that Faradaic redox reactions predominantly drive charge storage via a pseudocapacitive, i.e., battery-type, mechanism. As the scan rate increases, the current values rise correspondingly while maintaining the overall curve shape, indicating rapid charge-transfer kinetics and robust electrochemical stability of the Ni-BTB electrode (Figure 7a).

Both anodic and cathodic peak currents show an increase consistently with the scan rate, indicating the effective use of electroactive Ni sites and rapid ion/electron movement within the electrode matrix (Figure 7b). The logarithmic plots of peak current versus scan rate show remarkable linearity, with high correlation coefficients (R² = 0.98–0.99), indicating that the primary mechanism for charge storage is controlled by surface processes and exhibits a notable pseudocapacitive component (Figure 7c).

GCD curves were measured at current densities ranging from 1 to 100 A/g to further assess the energy-storage properties of the Ni-BTB electrode (Figure 8). The GCD curves display nearly symmetric charge–discharge behavior with small voltage plateaus, signifying reversible Faradaic reactions and hybrid supercapacitor functionality (Figure 8a). Although specific capacitance decreases gradually at high current densities due to kinetic constraints, the electrode retains substantial capacitance, indicating high-rate performance (Figure 8b).

The stability of the Ni-BTB electrode was examined over 10,000 charge–discharge cycles (Figure 8c). Figure 8c shows that the electrode maintains an efficiency near 100%, underscoring its superior electrochemical reversibility, structural stability, and long-term cycling stability.

A comparative overview of Ni-BTB is provided in

Table 2. Electrochemical performance comparison of Ni-BTB and other reported electrode materials.

Materials	Electron Collector	Electrolyte	Capacitance (F/g)	Recyclability (Cycles)	Ref.
CNO-ZnO	Cotton clothes	3 M KOH	125 at 1 A/g	2000	[63]
Co3O4	Glassy carbon	2MKOH	111 at 0–0.5 mV/s	1000	[64]
NiO/CuO	Carbon support	1M Na2SO4	35.63 at 0.3 A/g	1000	[62]
ZnO/3 wt.% MWCNTs	Carbon cloth	1M Na2SO4	115.72 at 5 mV/s		[67]
ZnO/CdS	fluorine-doped tin oxide	Ionic liquid	139 μF/g	5000	[65]
Ni3 (HITP)2 MOF	Nickle foam	[EMIM][BF4] ionic liquid	84 at 5 mV/s	2000	[66]
Ni3 (BHT)2	Nickel foam	1 M NEt4BF4 in acetonitrile	29 at 5 mV/s	8000	[61]
Ni-BTB	Nickel foam	6 M KOH	156 at 1 A/g	10,000	This work

Notes: HITP, 2,3,6,7,10,11-hexaiminotriphenylene; BHT: benzenehexathiolate.

relative to previously published electrode materials. Ni-BTB exhibits the best electrochemical performance among the tested materials, a capacitance of 156 F/g at 1 A/g, and remarkable cycling durability up to 10,000 cycles. This superior behavior is attributed to its structure, which promotes rapid ion transport, and to its high concentration of active nickel sites, which enhance charge storage. Moreover, robust metal–ligand coordination ensures superior mechanical and chemical stability. While a straightforward electrochemical system using nickel foam as the current collector in a 6 M KOH electrolyte enhances practical utility, conversely, Ni₃(BHT)₂, a benzenehexathiolate compound, exhibits a much lower capacitance of 29 F/g but maintains considerable cycling stability for up to 8000 cycles [61]. The constrained electrochemical performance of Ni₃(BHT)₂ can be attributed to the scarcity of active sites and its compact crystal structure, which impede ion diffusion. Consequently, this restricts its potential for high-performance energy storage applications. Conventional oxides such as NiO/CuO exhibit suboptimal capacitance (35.63 F/g) and low stability (5000 cycles) due to their limited surface area and impeded ion transport, highlighting the intrinsic limits of nonporous, dense oxide structures [62]. CNO-ZnO attains a capacitance of 125 F/g but exhibits poor cycling stability, lasting only 2000 cycles, indicating rapid deterioration during successive charge/discharge processes [63]. Co₃O₄ exhibits modest capacitance (111 F/g); however, it suffers from poor cycling stability (1000 cycles), indicating that the interplay between low active-site density and insufficient structural integrity limits long-term performance [64]. ZnO/CdS provides satisfactory capacitance (139 μF/cm²) with enhanced stability for up to 5000 cycles; yet, the multi-component composition of this system engenders chemical heterogeneities that undermine long-term durability [65]. Ni₃(HITP)₂, where HITP refers to 2,3,6,7,10,11-hexaiminotriphenylene, is an MOF that exhibited low capacitance (84 F/g) and inadequate cycling stability (2000 cycles), attributable to limited accessibility of active sites, underscoring the essential importance of porosity and framework design in the attainment of high-performance supercapacitors [66]. Collectively, Table 2 highlights the exceptional performance of Ni-BTB, which exceeds that of all other materials in terms of capacitance, long-term stability, and high potential for practical electrochemical applications. The distinctive combination of a hybrid structure, a concentrated distribution of active sites, and robust metal–ligand interactions facilitate rapid ion transport, substantial charge storage, and remarkable cycling longevity. In contrast, most alternative materials exhibit one or more deficiencies, such as reduced charge storage capacity, compromised cycling stability, or structural and chemical limitations, underscoring the importance of framework design and active-site engineering in advancing high-performance supercapacitor materials.

2.4. Electrocatalysis: Oxygen Evolution Reaction (OER)

The OER activity of the Ni-BTB framework was systematically evaluated in an alkaline electrolyte using LSV within a standard three-electrode system (Figure 9a–c). All recorded potentials were calculated to RHE scale to ensure reliable comparison (Figure 9c). The LSV curve exhibits an anodic peak at around 0.33 V (vs. Ag/AgCl) for the Ni-BTB electrode (Figure 9a–c). This property concerns the oxidation of Ni species to electrochemically active Ni oxyhydroxide nodes in the Ni-BTB MOF, which serve as the actual catalytic sites for water oxidation. Following this activation phase, a rapid and substantial increase in current density is observed, demonstrating the superior catalytic activity and high OER efficiency of the Ni-BTB framework.

At a current density of 10 mA/cm², the Ni-BTB catalyst delivers a low overpotential (η) of 106 mV, highlighting its outstanding intrinsic OER activity (Figure 9c). This exceptional performance is primarily attributed to the unique structural features of the Ni-BTB framework, characterized by abundant, readily accessible Ni active centers and π-conjugated aromatic linkers that facilitate effective electron delocalization.

The stability of the Ni-BTB electrode was assessed via continuous electrolysis for 12 h, during which minimal decline was noted, thereby affirming its exceptional longevity and structural integrity under extended OER conditions (Figure 9d). The vigorous and continuous oxygen evolution at the electrode surface further confirms the system’s realistic catalytic efficacy. Ni-BTB demonstrates its viability as a cost-effective and scalable electrocatalyst for industrial water-splitting applications. The Tafel analysis reveals a slope of 187 mV/dec, indicating modest OER kinetics under alkaline conditions (Figure 10a). This value indicates that electron-transfer mechanisms significantly influence the overall reaction rate, consistent with findings on nickel-based OER catalysts, where the generation of high-valence Ni intermediates is often the rate-limiting step.

Additional insight into the inherent catalytic activity was obtained by assessing turnover frequency (TOF) at overpotential, as shown in Figure 10b. The TOF increases significantly with increasing overpotential, indicating enhanced oxygen evolution kinetics under greater driving forces. Significantly, Ni-BTB exhibits a TOF of 0.0585 s⁻¹.

Table 3. Summary for electrocatalysts used for OER.

Materials	Compositions	Synthesis Method	Condition	Electrolyte	Overpotential (mV) at 10 mA/cm2	Tafel Slope (mV·dec−1)	TOF (s−1)	Ref.
NiO/CuO@C	Solvothermal Carbonization	Solvothermal Carbonization	100 °C for 5 h 800 °C for 3h	6 M KOH	300	60	0.21	[35]
Ni/NiO/Ni3C_graphitic shell	Ni NiO, Ni3C Graphitic shell	Heating	heating at 4 °C/min under nitrogen flow to 600 °C	1.0 M KOH	170	49	52.8	[68]
NiFeCoOx	Ni Fe Co	Precipitation Calcination	Room temperature, alkaline, 2h 300 °C for 5 h	1 M KOH	228	511		[69]
Ni10Co-BTC	Ni Co BTC	Solvothermal	170 °C for 48 h	1 M KOH	346	47		[70]
[Ni3(BPE)4(BTB)2(H2O)2]·2DMF·2H2O	BTB BPE, 1,2-bis(4-pyridyl) ethane]	Solvothermal	100 °C for 3 days	0.1 M KOH	700	198	0.84	[71]
NiBTB	Ni BTB	Solvothermal	Heating at 120 °C for 24 h	6 M KOH	106	187	0.0585	This study

Notes: BTC, 1,3,5-benzenetricarboxylate.

summarizes electrocatalysts used for the OER process compared to the Ni-BTB system, highlighting the superior catalytic efficiency of the latter. For instance, Ni/C composites comprising metallic Ni, NiO, and Ni₃C encased in a graphitic shell necessitate approximately 170 mV to achieve 10 mA/cm² [68], whereas ternary NiFeCoO_x oxides require around 228 mV at the identical current density [69]. MOF-derived catalysts, such as Ni₁₀Fe-BTC and Ni₁₀Co-BTC, exhibit high overpotentials of 337–378 mV at 10 mA/m² [70]. Likewise, carbonized NiCuBDC at 800 °C displays an overpotential near 300 mV at 10 mA/cm², underscoring the constraints of MOF-derived composites in preserving accessible nickel active sites [35]. In contrast, Ni-BTB demonstrates an overpotential of only 106 mV at 10 mA/cm², surpassing nickel-based electrocatalysts by more than double (Table 3). This exceptional activity stems from the inherent structural and electrical benefits of the Ni-BTB architecture. The nickel MOF was also used without carbonization at 800 °C (Table 3). The inflexible, π-conjugated BTB linker establishes a robust structure that optimizes the accessibility of Ni active sites and promotes swift electron transport via prolonged conjugation pathways. Collectively, these attributes yield enhanced OER kinetics, high catalytic turnover, and remarkable operational stability, establishing Ni-BTB as a reference electrocatalyst for alkaline water oxidation. In comparison, nickel-based hybrid MOF, [Ni₃(BPE)₄(BTB)₂(H₂O)₂]·2DMF·2H₂O (Ni-BTB-BPE; BPE = 1,2-bis(4-pyridyl)ethane), was reported as a bifunctional electrocatalyst for the oxygen reduction reaction (ORR) and OER (Table 3) [71]. BTB ligands form two-dimensional Kagome-type layers with Ni centers, which are further connected by BPE linkers to form a three-dimensional framework. The surface concentration of electrochemically active Ni²⁺/Ni³⁺ was determined to be 5.49 nM/cm². When supported on carbon, Ni-BTB-BPE/C demonstrated a significant ORR mass activity of 108.95 A/g_Ni at 0.75 V vs. RHE. The catalyst promoted a four-electron ORR pathway, with a turnover frequency of 2.24 electrons [Ni]⁻¹ s⁻¹ at 0.70 V versus RHE, 2.67 times superior to the two-electron pathway, and exhibited proficient bifunctional ORR/OER performance with a bifunctionality index of 0.9 at 1 mA/cm² [71].

3. Materials and Methods

H₃BTB, Ni(NO₃)₂•6H₂O, N, N-dimethylformamide (DMF), poly(vinylidene fluoride) (PVDF), and NaBH₄ were purchased from Sigma-Aldrich (Hamburg, Germany).

3.1. Synthesis of Ni-BTB

A mixture of Ni(NO₃)₂·6H₂O (0.025 mmol) and H₃BTB (0.0125 mmol) was dissolved in a solvent combination of DMF (10 mL) and distilled water (2 mL) in a vial. Subsequently, two drops of 0.2 N diluted hydrochloric acid were added. The vial was thereafter placed within a Teflon-lined stainless-steel container and heated to 120 °C at a rate of 10 °C/min for 24 h.

3.2. Characterizations

The XRD pattern of Ni-BTB was obtained utilizing a Phillips (Amsterdam, The Netherlands) 1700 X’Pert diffractometer employing Cu Kα radiation. XPS measurements were conducted, using the C 1s peak at 284.4 eV as a calibration reference. A TEM image was captured with a JEOL TEM-2100 instrument from Tokyo, Japan. FT-IR spectrum of NiBTB was collected on a Nicolet (Mountain, WI, USA) spectrophotometer (Model 6700).

3.3. The Production of Hydrogen from NaBH₄

Hydrogen production from NaBH₄ was assessed utilizing a water displacement technique. Hydrogen displaced water in a 50 mL burette, allowing accurate determination of gas volume over time. The effect of NaBH₄ concentration was investigated using the same catalyst without separation between runs. Experiments were conducted using various catalyst loadings (1, 5, 10, 20, and 50 mg) and NaBH₄ concentrations (0.2, 0.5, 1.0, 2.0, and 3.0 wt.%). The influence of reaction temperature on hydrogen evolution was examined in a 250 mL conical flask containing 20 mg of Ni-BTB and 1 g of NaBH₄, with temperatures varied between 30 and 60 °C. The hydrogen generation rate (HGR, mL H₂/g•min) was determined using Equation (1):

$$\mathrm{HGR} = {\displaystyle \frac{{\mathrm{V}}_{H_2}}{\mathrm{t}\times {\mathrm{m}}_{\mathrm{c}\mathrm{a}\mathrm{t}}}}$$

(1)

where ${\mathrm{V}}_{H_2}$ represents the generated hydrogen volume (mL), t corresponds to the reaction duration (min), and m_cat denotes the catalyst weight (g).

Arrhenius and Eyring analyses were employed to determine the thermodynamic parameters of the Ni-BTB-catalyzed hydrolysis reaction (Equation (2)) [72]:

$$\mathrm{L}\mathrm{n}\mathrm{k}=-{\displaystyle \frac{{\mathrm{E}}_{\mathrm{a}}}{\mathrm{R}}}\left({\displaystyle \frac{1}{\mathrm{T}}}\right)+\mathrm{L}\mathrm{n}\mathrm{A}$$

(2)

Here, k is the kinetic rate constant, A is the frequency factor, Ea signifies the activation energy, R stands for the gas constant, and T is the absolute temperature (Equation (3)):

$$\mathrm{L}\mathrm{n}{\displaystyle \frac{\mathrm{K}}{\mathrm{T}}}=\mathrm{L}\mathrm{n}\left({\displaystyle \frac{{\mathrm{K}}_{\mathrm{B}}}{\mathrm{h}}}\right)+{\displaystyle \frac{-∆{\mathrm{H}}^{\ast }}{\mathrm{R}\mathrm{T}}}+{\displaystyle \frac{∆{\mathrm{S}}^{\ast }}{\mathrm{R}}}$$

(3)

Here, K_B is a constant, h corresponds to Planck’s constant, and the quantities ΔH^∗ and ΔS^∗ describe the activation enthalpy and activation entropy.

Calculations were performed to obtain the Gibbs free energy (ΔG^∗) using Equation (4):

$$∆{\mathrm{G}}^{\ast }=∆{\mathrm{H}}^{\ast }-{∆\mathrm{S}}^{\ast }$$

(4)

Catalyst recyclability was assessed by performing four consecutive hydrogen generation reactions using 5 mg of Ni-BTB, with 3 g of NaBH₄ added fresh in each run.

3.4. Electrochemical Analyses

For the electrochemical experiments, the working electrode was prepared by mixing the active material (Ni-BTB, 1 mg) with conductive carbon black, and PVDF was mixed with the other components in an 80:10:10 weight ratio using a mortar and pestle. The resulting powder was dispersed in DMF to form a uniform slurry, which was subsequently applied onto a nickel foam substrate (1 × 1.5 cm²). The coated electrodes were then dried in an oven at 80 °C overnight to ensure complete removal of any remaining solvent.

Electrochemical studies were conducted in a three-electrode configuration, with Ni-BTB/Ni foam as the working electrode, Ag/AgCl as the reference electrode, and a platinum plate as the counter electrode. Cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) measurements were performed with a Corrtest^® CS150M electrochemical workstation (Wuhan, China). The CV and GCD experiments were conducted over a potential window of 0–0.5 V (vs. Ag/AgCl) at varying scan rates and current densities. The specific capacitance was calculated from the GCD data according to Equation (5):

$$\mathrm{Capacitance} = {\displaystyle \frac{\mathrm{I}\times ∆\mathrm{t}}{\mathrm{m}\times ∆\mathrm{V}}}$$

(5)

where I denotes the discharge current, m represents the total mass of active material loaded on the electrode, V denotes the potential window, and t denotes the discharge time.

The electrode’s long-term stability was tested through 10,000 continuous charge–discharge cycles at a current density of 10 A/g utilizing a Corrtest^® CS150M workstation (China).

The linear sweep voltammetry (LSV) measurements were carried out at a scan rate of 5 mV/s, and the obtained potentials were converted to the reversible hydrogen electrode (RHE) scale using Equation (6):

E_RHE = E_measured + E_ref + 0.059 × pH

(6)

The η_OER for the oxygen evolution reaction was determined according to Equation (7):

η_OER = E_RHE − 1.23 V

(7)

The turnover frequency (TOF) was determined by estimating the density of active catalytic sites from CV experiments performed in 6 M KOH over a 0–0.5 V potential range (vs. Ag/AgCl) at scan rates ranging from 1 to 200 mV/s. The peak current for the anodic redox reaction showed a linear correlation with the scan rate, confirming a surface-governed electrochemical process. The slope obtained from the linear regression was subsequently evaluated using the following Equation (8):

$$\mathrm{Slope} = {\displaystyle \frac{{\mathrm{n}}^2{\mathrm{F}}^2\mathrm{A}{\Gamma }^0}{4\mathrm{R}\mathrm{T}}}$$

(8)

Here, n denotes the number of electrons involved in oxidation, which is 1 for the Ni²⁺/Ni³⁺ redox pair in Ni-BTB. F stands for the Faraday constant (96,485 C/mol), A indicates the electrode’s surface area, and Γ⁰ defines the coverage of active sites on the electrode surface (mol/cm²). R denotes the ideal gas constant, while T corresponds to the absolute temperature.

Subsequently, TOF values were evaluated using the formula presented in Equation (9):

$$\mathrm{TOF} = {\displaystyle \frac{\mathrm{J}\mathrm{A}}{4\mathrm{F}\mathrm{m}}}$$

(9)

Here, J represents the current density, and A corresponds to the geometric area of the working electrode; the factor 4 accounts for the four-electron transfer required for the evolution of one mole of O₂, while m represents the total amount of electrochemically active sites in moles.

To assess the catalyst’s durability, chronopotentiometric measurements were performed at a steady current density of 10 mA/cm² for 12 h of continuous operation.

4. Conclusions

In this study, Ni-BTB was successfully prepared and demonstrated multifunctionality for advanced energy applications, exhibiting excellent catalytic activity for hydrogen production via NaBH₄ hydrolysis, achieving a peak rate of 9542 mL H₂/g•min under optimal conditions, and showing remarkable stability and reusability across multiple cycles. In addition to hydrogen production, Ni-BTB functioned as an effective supercapacitor electrode, achieving a specific capacitance of 156 F/g at a current density of 1 A/g, while retaining nearly all its initial capacitance after 10,000 cycles, indicating excellent cycling stability, exceptional electrochemical stability, and longevity. Furthermore, Ni-BTB exhibited enhanced electrocatalytic performance for OER, achieving an overpotential of 106 mV at 10 mA/cm² while maintaining stable operation for 12 h. These results demonstrate that Ni-BTB is a resilient, efficient, and adaptable platform that can integrate hydrogen release from hydride hydrolysis, oxygen evolution, and electrochemical energy storage within a single material. Its multifunctional capabilities underscore its significant promise for next-generation integrated renewable energy conversion and storage systems.