Modified g-C₃N₄/NiO nanocomposites for enhanced electrocatalytic efficiency in hydrogen evolution under alkaline conditions

1. Introduction

Growing global energy demands and mounting environmental challenges have intensified the pursuit of clean and sustainable energy alternatives. Among these, hydrogen (H₂) stands out as a highly promising energy source due to its high energy density, carbon-free nature, and adaptability for use in fuel cells and various chemical applications (Zhu et al., 2014). Electrochemical water splitting reactions offer an efficient route for green H₂ production. Though, the large overpotentials and slow kinetics of HER in alkaline media limit the efficiency of this process (Mondal et al., 2018, QayoomMugheri et al., 2019, Manjunath et al., 2022). Transition-metal-based catalysts, especially nickel oxide (NiO), have attracted attention for HER because of their relatively low cost, abundance, and promising catalytic activity (Mondal et al., 2018, Manjunath et al., 2022). NiO nanoparticles, with their tunable morphology and electronic structure, can offer a moderate catalytic surface for hydrogen evolution. Nevertheless, pristine NiO often suffers from poor electronic conductivity and limited active sites, which hinder its electrochemical performance (QayoomMugheri et al., 2019). The g-C₃N₄ consists of a 2D (two-dimensional) polymeric network where tri-s-triazine rings were bonded together by nitrogen-linkages. It exhibits exceptional chemical stability, high surface area, and intrinsic semiconductor properties, which make it a promising material for energy conversion (Zhu et al., 2014, Ismael et al., 2024). However, its catalytic efficiency is restricted by low electrical conductivity and rapid charge recombination. Recently, zinc stannate and magnesium gallate were reported as efficient electrocatalysts for overall water-splitting reactions to HER (Parayil et al., 2024, Parayil et al., 2025).

g-C₃N₄/NiO Nanocomposites exploit the electro-catalytic properties of both materials to enhance the HER activities. g-C₃N₄ provides a conductive matrix with abundant active sites, while NiO contributes additional catalytic centers and facilitates electron transfer (Ismael and Wark 2022, Ismael 2024). Such interface-engineered nanocomposites not only improve electron transport but also enhance structural stability, which is crucial for long-term HER operation. Although commonly used noble metals such as Ir, Ru, Pt etc are still considered as standard electro-catalysts for water-splitting because of their minimal overpotential and excellent reaction kinetics (Fang and Liu 2010, Ahmed and Mao 2016, Audichon et al., 2016), but their high price and limited availability hinder large-scale application. Consequently, designing cost-effective, transition-metal-based catalysts with comparable activity is a key challenge in electrochemical HER (Ahmed et al., 2017, Li et al., 2021, Ahmed et al., 2022). Recent studies have demonstrated that g-C₃N₄-based composites, when combined with metal oxides, exhibit remarkable photocatalytic and electrocatalytic performance (Ismael and Wark 2022, Ismael 2024, Ismael et al., 2024). Based on the previous findings, this work aims to synthesize g-C₃N₄/NiO nanocomposites through a molten salt-assisted approach, followed by ultrasonic treatment, to achieve homogeneous dispersion and optimized interfacial interactions for HER activities. The g-C₃N₄/NiO nanocomposites were employed as efficient electrocatalysts for generating hydrogen through ethanol electrolysis (Chebanenko et al., 2020) and also demonstrated excellent photocatalytic performance in environmental remediation applications (Yang et al., 2024). The g-C₃N₄/α-NiS nanocomposites were also employed as photocatalysts for hydrogen production through photocatalytic processes (Qi et al., 2023). The synthesized nanocomposites are systematically characterized and evaluated for HER in alkaline medium. Our results show that g-C₃N₄/NiO nanocomposites exhibit excellent HER activity over the NiO nanoparticles, offering a low-cost and robust electro-catalyst for alkaline water splitting.

2. Materials and Methods

3. Results and Discussion

The crystalline structure of NiO, and the g-C₃N₄/NiO nanocomposites was confirmed by XRD. Black coloured XRD pattern represents NiO nanoparticles, while red coloured XRD pattern represents g-C₃N₄/NiO nanocomposite (Fig. 1). A prominent diffraction peak appears at 27.4°, corresponding to the (002) plane of g-C₃N₄, indicating the in-plane structural organization and interlayer stacking of its conjugated aromatic framework (Ismael 2024, Ismael et al., 2024). NiO nanoparticles display a cubic phase with prominent reflections at 37.2°, 43.3°, and 62.9°, indexed to the (111), (200), and (220) planes (Mondal et al., 2018, Manjunath et al., 2022). XRD pattern of NiO nanoparticles is matched with JCPDS card # 78-0643 and can be indexed in cubic unit cell structure. The characteristic peaks of both g-C₃N₄ and NiO are clearly present in the g-C₃N₄/NiO nanocomposites, with no extra peaks detected, indicating the successful formation of a physically and chemically combined heterostructure (Ismael and Wark 2022, Ismael 2024). Notably, slight peak broadening and intensity variation in the composite indicate strong interfacial interactions, which can facilitate electron transfer during electrocatalysis (Ismael 2024).

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Fig. 1.

XRD of NiO nanoparticles and g-C₃N₄/NiO nanocomposite.

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FTIR spectra (Fig. 2) provide insights into chemical bonding and surface functional groups. g-C₃N₄ exhibits typical stretching vibrations of C–N heterocycles in the 1200–1650 cm⁻¹ range and a sharp peak at 810 cm⁻¹ corresponding to triazine ring breathing modes (Ismael 2024, Ismael et al., 2024). NiO shows a broad Ni–O stretching vibration around 430 cm⁻¹ (Mondal et al., 2018, Manjunath et al., 2022). In the g-C₃N₄/NiO nanocomposite, the characteristic g-C₃N₄ bands remain, while Ni–O peaks are retained with minor shifts. This suggests strong interfacial contact between g-C₃N₄ sheets and NiO nanoparticles, which enhances charge carrier mobility and provides active sites for hydrogen evolution (Ismael and Wark 2022, Ismael 2024).

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Fig. 2.

FTIR spectra of NiO nanoparticles and g-C₃N₄/NiO nanocomposite.

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Fig. 3 shows the FESEM micrographs of NiO nanoparticles and g-C₃N₄/NiO nanocomposite. FESEM studies clearly reveal that NiO nanoparticles exhibit uniform, quasi-spherical morphology (Fig. 3a, b). In the g-C₃N₄/NiO nanocomposites, NiO nanoparticles are homogeneously distributed on g-C₃N₄ sheets without significant agglomeration, confirming successful interface engineering (Fig. 3c, d). This morphology ensures maximum exposure of active sites, improving electrocatalytic activity. The strong interface is expected to facilitate fast electron transfer from NiO to g-C₃N₄ during the hydrogen evolution reaction (HER).

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Fig. 3.

(a, b) FESEM micrographs of NiO nanoparticles and (c, d) g-C₃N₄/NiO nanocomposite.

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The electrochemical studies of the HER activity of NiO nanoparticles and g-C₃N₄/NiO nanocomposites were evaluated in 1.0 M KOH alkaline electrolyte solution vs Ag/AgCl. CV and LSV experiments were conducted in the potential window from 0.0 V to – 1.70 V at 25 mV/s for HER (Figs. 4a, b). CV and LSV experiments reveal that g-C₃N₄/NiO nanocomposites show superior HER activities over NiO nanoparticles. These studies show that the g-C₃N₄/NiO nanocomposites generate three times higher current density than pure NiO nanoparticles. Note that the induced current density of any material is directly proportional to the evolved hydrogen gas during electrochemical water splitting reactions. Furthermore, the onset potential of g-C₃N₄/NiO nanocomposites (-1.30 V) was found to be lesser than the onset potential of NiO nanoparticles (-1.50 V). Low onset potential confirmed the low energy loss during the HER reaction. The reaction mechanism for HER in an alkaline medium can be followed as 2H₂O_(l)+2e- <ZAZAE_UE>&ZAZAE_ue_wingdings_F0E0;</ZAZAE_UE>H₂(g)+2OH^-(aq). The nanocomposites typically possess a high specific surface area, providing plentiful active sites for HER. The increased surface area allows for more efficient electrochemical reactions and improves the overall catalytic performance.

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Fig. 4.

(a) CV, (b) LSV, (c) Tafel, and (d) CA studies of NiO nanoparticles and g-C₃N₄/NiO nanocomposites.

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Fig. 4(c) shows Tafel plots of NiO nanoparticles and g-C₃N₄/NiO nanocomposites. Tafel value of the g-C₃N₄/NiO nanocomposites was found to be ∼40 mV/dec, nearly three times lower than the pure NiO nanoparticles (i.e. ∼112 mV/dec) for HER. The prepared g-C₃N₄/NiO nanocomposites also show better HER performances than the reported NiO and NiO-based composites. Table 1 shows Tafel values of present and previous reports on HER. Fig. 4(d) shows the chronoamperometry (CA) plots of the NiO nanoparticles and the g-C₃N₄/NiO nanocomposites for the stability check of the electrode materials. The combination of g-C₃N₄ and NiO can improve the stability of the catalyst under harsh reaction conditions. The g-C₃N₄ matrix provides protection to the NiO nanoparticles, preventing aggregation and maintaining their catalytic activity over prolonged cycling. The enhanced electrochemical HER performance of the synthesized g-C₃N₄/NiO nanocomposites can be attributed to their unique structure, interface engineering, increased surface area, and improved stability. These properties make them favourable candidates for the renewable energy technologies. To further enhance the electrocatalytic activity, g-C₃N₄ can be doped with different heteroatoms, such as sulfur, phosphorus, or boron by modifying the electronic structure and introduces additional active sites for enhancing the overall HER activity. Remarkably, g-C₃N₄/NiO nanocomposites display significantly reduced energy losses, highlighting the synergistic effect of interface engineering. This enhancement could be ascribed to the efficient electron transfer across the g-C₃N₄/NiO interface, increased electrochemically active surface area due to uniform NiO dispersion, and improved adsorption/desorption of H intermediates facilitated by NiO active sites.

4. Conclusions

Here, interface-engineered g-C₃N₄/NiO nanocomposites were successfully prepared and systematically evaluated as HER electrocatalysts in alkaline media. The morphological characterization confirms the uniform dispersion of NiO nanoparticles on g-C₃N₄ sheets, establishing strong interfacial contact that facilitates efficient electron transfer. Electrochemical measurements demonstrated that the g-C₃N₄/NiO nanocomposite exhibits a significantly reduced the energy losses during the electrochemical reactions, and outstanding long-term stability, outperforming the individual components. The present studies do not only offer the valuable mechanistic insight in the role of g-C₃N₄/NiO interfaces but suggests a favourable route for developing high-performance, earth-abundant HER catalysts for sustainable hydrogen production.