Efficient hydrogen evolution via neutral water electrolysis using nanocrystalline TiO₂ electrocatalyst

Abstract

Significant efforts have been focused on the development of non-noble metal electrocatalysts for efficient production of green hydrogen using renewable energy sources such as water and sunlight. Various transition metal oxides have been explored as promising candidates for hydrogen evolution reactions (HER), attracting considerable attention in the field of water splitting. In this study, we report the synthesis of anatase phase TiO₂ nanoparticles (NPs) with a crystallite size of 6.94 nm, achieved through a co-precipitation method specifically for neutral water electrochemical HER applications. The as-synthesized anatase phase of TiO₂ electrocatalyst have been characterized by X-ray diffraction, scanning electron microscope, transmission electron microscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, and ultraviolet visible spectroscopy. Transmission electron microscopy analysis revealed that the average particle size of the TiO₂ NPs was approximately 5.80 nm. The exceptional HER performance of these nanoparticles can be attributed to their optimal crystallite size, rapid charge transfer kinetics, and reduced band gap energy. The electrocatalyst demonstrated an overpotential of 470 mV to achieve a current density of 10 mA cm⁻² in a 0.2 M Na₂SO₄ electrolyte. Furthermore, the electrode maintained stable HER activity over a continuous period of 50 h, indicating its potential for practical applications. These findings highlight the critical role of crystallite size optimization in enhancing HER activity, particularly in neutral electrolytes. By advancing the understanding of non-noble metal electrocatalysts, this study contributes to the ongoing efforts to develop efficient, sustainable methods for green hydrogen production, aligning with global goals for renewable energy utilization.

Supplementary Information

The online version contains supplementary material available at 10.1038/s41598-025-93371-0.

Introduction

The significant challenges associated with energy use and environmental degradation have instigated a notable academic investigation into the rapid progress of sustainable energy storage and conversion technologies^1,2. The advancement of innovative energy technologies constitutes a paramount concern for numerous nations and regional entities, as it is imperative to address the issues instigated by environmental pollution, energy shortages, and the deleterious consequences of fossil fuel combustion³. Renewable energy sources such as solar, wind, and oceanic tidal energy present compelling alternatives; however, there remains substantial potential for enhancement regarding their efficacious utilization^4,5. The difficulties arise primarily from the intermittent characteristics of these energy sources. This issue can be alleviated through the conversion of energy during peak periods into green hydrogen derived from water^6,7. Electrocatalytic water splitting represents an imperative advancing technique for the generation of hydrogen (H₂), effectively tackling the pressing challenges associated with attaining carbon neutrality⁸. This methodology provides versatility in the deployment of renewable energy sources^9–11. While platinum-group metals are recognized as the most efficient electrocatalysts for the hydrogen evolution reaction (HER), their prohibitive cost poses a significant barrier to widespread and scalable implementation¹². Considerable efforts have been directed towards synthesizing electrocatalysts that exhibit a favorable balance of high activity and low cost. As a result, extensive resources have been allocated to the design and development of economical, high-performance catalysts intended to replace noble metal-based catalysts¹³. Recently, TiO₂ has emerged as a viable candidate for cathodic reactions, particularly for the HER^14–16. The anatase and rutile phases of TiO₂ have historically garnered considerable attention within the realm of energy conversion. These two phases are predominantly employed in photovoltaics, photocatalysis, and electrocatalysis, primarily due to their inherent stability, exceptional performance, and cost-effectiveness. Notably, the anatase phase possesses a broader band gap (Eg = 3.2 eV) in comparison to rutile (Eg = 3.0 eV)^17–19. Consequently, TiO₂ nanoparticles (NPs) have been the focus of extensive investigation by numerous researchers for their prospective applications in energy storage and conversion²⁰. TiO₂ NPs have frequently been utilized as a supporting substrate in the fabrication of electrodes exhibiting diverse morphologies^21,22. Various methodologies have been employed to diminish the band gap of TiO₂ and augment its electrochemical performance. For instance, Yamazaki et al.²³ examined the influence of altering the morphology of TiO₂ from nanorods to NPs on its photocatalytic efficacy in the oxygen evolution reaction (OER). An alternative strategy involves the doping of TiO₂ with various elements; for example, Chen et al.²⁴ illustrated that doping TiO₂ with manganese significantly enhances its electrocatalytic nitrogen reduction capabilities. Similarly, the incorporation of boron has been shown to effectively enhance electrocatalytic nitrogen fixation²⁵. The OER activity of TiO₂ NPs can be fine-tuned through the substitution of titanium within the material²⁶. Moreover, combining TiO₂ with carbon-based materials, such as reduced graphene oxide, has been investigated to enhance its photocatalytic performance for the HER²⁷. The integration of reduced graphene oxide with TiO₂ also contributes positively to the enhancement of electrocatalytic activity for OER in a 3 M KOH electrolyte²⁸. Apart from the anatase TiO₂ electrocatalyst, noble-metal-free nanocomposite electrocatalyst have been preferred for synergistically accelerating the production of green hydrogen such as MoS₂/NiO nanostructure²⁹. The synthesized MoS₂/NiO electrocatalyst has been shown excellent hydrogen production at low overpotentials with the Tafel slope for HER stands 37 mV/decade in alkaline media. However, in acidic media HER also have been produced by Mugheri et al.³⁰ by using of ZnO-MoS₂ heterojunction electrocatalyst at 0.26 V. Co₃O₄/MoS₂ nano composite electrocatalyst have been used for catalytic production of hydrogen³¹. Co₃O₄/MoS₂ nano composite electrocatalyst achieved a current density of 10 mA/cm² at overpotential of 260 mV with Tafel slope of 56 mVdec⁻¹. Subsequently, the nanocomposite such as NiO/C³², Co₃O₄/C³³, NiCo₂O₄/CuO³⁴ have been significantly used for overall water splitting.

However, bare crystallite size of TiO₂ can significantly influence its electrocatalytic activity. In the current research, TiO₂ NPs were generated through the co-precipitation technique, followed by a calcination process. The morphological characteristics and structural properties of the synthesized TiO₂ NPs were assessed using a variety of analytical techniques, including powder X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), Xray photoelectron spectroscopy (XPS), Raman spectroscopy, and ultraviolet visible spectroscopy (UV-Vis). Subsequently, these synthesized TiO₂ NPs were employed in the hydrogen evolution reaction within an alkaline electrolyte medium.

Materials and methods

Chemicals

Every chemical utilized is of an analytical grade reagent. The 99% pure titanium tetrachloride (TiCl₄) acquired from Sigma Aldrich was utilized as the precursor of Ti. Absolute ethanol was used as a solvent, and 35% (w/w) hydrochloric acid (HCl) was used as a catalyst. A pH was maintained using ammonium hydroxide (NH₄OH). All solutions were prepared using our lab’s double distilled water (DDW).

Synthesis of TiO2 NPs

To synthesize titanium dioxide nanoparticles (TiO₂ NPs) utilizing the co-precipitation technique, a 10 mL aliquot of concentrated hydrochloric acid (HCl) solution was maintained at a temperature of 0℃ and subsequently combined with 5 mL of titanium tetrachloride (TiCl₄) solution, followed by stirring for a duration of one hour. Following this period, a transparent solution was achieved. Ammonium hydroxide (NH₄OH) was subsequently introduced into this solution to regulate the pH to a value of 7. The mixture was agitated for 15 min and then permitted to settle for 30 min. The resultant precipitate was subjected to centrifugation at 2000 revolutions per minute (rpm) multiple times employing deionized distilled water (DDW) and a mixture of ethanol and DDW. The synthesized TiO₂ NPs were then dried at ambient temperature prior to undergoing calcination at 600 ℃ for a duration of 2 h.

Characterizations

Materials characterizations

The XRD pattern of TiO₂ NPs was analyzed by using an X-ray diffraction spectrometer (Bruker Germany D8 Advance) in the 2θ range of 20–80° at a scan rate of 1° min⁻¹, using a Cu Kα target (λ = 0.15418 nm, running at 40 kV and 100 mA). The surface morphology of TiO₂ NPs was observed by employing field-emission scanning electron microscope (FE-SEM; JSM-7900 F, JEOL, Japan) coupled with an energy dispersive X-ray spectroscopy analyzer (EDS; OXFORD Instruments, INCA PentaFETx3, Model 7,557). Before SEM analysis, TiO₂ NPs were coated with Pt. The UV–visible spectrometer Hitachi U-3300 was used to record optical absorption spectra. The Raman spectrometer Renishaw UK Sales UK Model Renish INVIA 0120-20 was used for the Raman spectra measurements. A transmission electron microscope (TEM) (JEM 2010 JEOL Japan) working at an acceleration voltage of 200 kV was used to evaluate fine morphology and structural analysis.

Preparation of electrode and electrochemical measurements

The 306-grade stainless steel (SS) sheet was delineated into segments measuring 6 × 1 cm² and subjected to a cleaning process utilizing sandpaper, subsequently undergoing ultrasonic washing with acetone and deionized distilled water (DDW) for a duration of 15 min each³⁵. The electrocatalyst was synthesized utilizing the drop-casting methodology. The catalyst slurry was formulated by combining TiO₂ nanoparticles, polyvinylidene fluoride (PVDF), in a proportion of 90:10. N-Methyl-2-pyrrolidine (NMP) was utilized as the solvent and was mechanically processed using a mortar and pestle until achieving a homogeneous consistency. Subsequently, the resultant mixture was applied onto a pre-cleaned stainless steel (SS) substrate measuring 1 × 1 cm² and permitted to dry under an infrared (IR) lamp for a duration of one hour, resulted the fabrication of working electrode. In this preparation of working electrode containing 0.03 mg/cm of as-synthesised TiO₂ has been used. Electrochemical analyses were conducted employing an Auto Lab potentiostat configured in a three-electrode arrangement. The electrode fabricated with TiO₂ nanoparticles affixed to the SS substrate functioned as the working electrode, a platinum wire was designated as the counter electrode, while a saturated calomel electrode served as the reference electrode. All electrochemical assessments were performed in a 0.2 M sodium sulfate (Na₂SO₄) electrolyte solution with scan rate 1 mVs⁻¹. The scan rate of 1 mVs⁻¹ has been choosed by optimising the scan rate of LSV from 1 to 50 mVs⁻¹. At 1 mVs⁻¹ of LSV shown a good electrocatalytic activity (Fig. S1). The entirety of the measurements was executed at ambient temperature. The measured potential was subsequently converted to the reversible hydrogen electrode (RHE) scale utilizing the Nernst Eqs^36,37.

1

Linear sweep voltammetry (LSV) was conducted at a scan rate of 1 mV s⁻¹, alongside the acquisition of cyclic voltammetry (CV) profiles at various scan rates. The double-layer capacitance (C_dl) was determined utilizing CV profiles obtained within a potential window ranging from (− 0.3) to 0 V (versus SCE) across scan rates extending from 5 to 100 mV s⁻¹. Additionally, electrochemical impedance spectroscopy was employed to assess the resistive characteristics of the electrochemical cell with an alternating current amplitude of 10 mV over a frequency spectrum from 100 kHz to 100 mHz. The durability of the material for hydrogen evolution reaction (HER) was appraised through chronopotentiometry experiments conducted at a current density of (− 10) mA cm⁻² for a duration of 50 h.

Results and discussion

Structural, optical, surface and morphological properties of TiO₂

To elucidate the process of synthesizing TiO₂ nanoparticles via the co-precipitation methodology, has been presented. Initially, 10 mL of concentrated hydrochloric acid at 0 °C was combined with 5 mL of titanium tetrachloride and agitated for a duration of 1 h. Following the adjustment of the pH to 7 utilizing ammonium hydroxide, the resultant precipitate was subjected to centrifugation, subsequently washed with deionized distilled water and ethanol, then dried and calcined at a temperature of 600 °C for a period of 2 h. A powdered specimen was employed for X-ray diffraction (XRD) analysis to investigate the crystalline architecture of the TiO₂ sample. The resultant XRD pattern is illustrated in Fig. 1a. The pronounced peaks observed in the XRD pattern signify the crystalline characteristics of TiO₂ nanomaterials. The peaks manifested at 2θ angles of 25.50°, 37.12°, 38.06°, 38.89°, 48.34°, 54.04°, 55.28°, 62.24°, 62.97°, 68.99°, 70.44°, 75.12°, 76.25°, and 82.89°, correspondingly aligning with Bragg’s reflections from the (101), (103), (004), (112), (200), (105), (211), (213), (204), (116), (220), (215), (301), and (312) crystallographic planes of TiO₂, respectively³⁸.

Fig. 1.

a The XRD pattern, b Raman spectrum, c UV-DRS spectra, and d Tauch plot of TiO₂ NPs

The acquired X-ray diffraction (XRD) pattern exhibited a significant correspondence with the reference Powder Diffraction File (PDF) number 00-004-0477. The pronounced diffraction peak observed at 25.50° is associated with the (101) crystallographic plane, thereby corroborating the tetragonal anatase phase of TiO₂^39,40.

The lattice parameters pertinent to TiO₂ NPs are a = 3.7830 Å, b = 3.7830 Å, and c = 9.5100 Å. The determination of crystallite size was conducted utilizing Scherrer’s formula, as delineated below⁴¹.

2

where K represents a constant valued at 0.94, β denotes the line width at the half-maximum height, D signifies the crystallite size, and λ corresponds to the wavelength of the X-ray radiation (λ = 0.15406 nm)⁴². The mean crystallite size obtained for the synthesized TiO₂ nanoparticles is approximately 6.94 nm.

The Raman spectrum of synthesized TiO₂ nanoparticles is employed to investigate the vibrational modes. Various crystallographic phases of TiO₂ present unique Raman spectra, facilitating the determination of the crystal structure. Figure 1b illustrates the Raman spectrum corresponding to the synthesized TiO₂ nanoparticles. This spectrum corroborates the tetragonal anatase phase of TiO₂, characterized by localized D2d symmetry and belonging to the space group I41/amd.^42,43 According to group theory, six Raman-active modes, specifically A_1g + 2B_1g + 3E_g, are anticipated for TiO₂⁴⁴. The detected peaks include the E_g mode at a wavenumber of 142 cm⁻¹, which is notably sharp and intense, alongside additional peaks at E_g 194 cm⁻¹, B_1g 394 cm⁻¹, A_1g 514 cm⁻¹, B_1g 519 cm⁻¹, and Eg 639 cm⁻¹.

TiO₂ is a widely used compound with various applications, including as a photocatalyst in environmental and energy related fields. As TiO₂ is exposed to UV-visible light, it undergoes electronic transitions. The band gap of TiO₂ is determined by its UV-visible absorption spectra as shown in Fig. 1c. The Kubelka-Munk Eq. (3) is used to calculate the band gap⁴⁵.

3

where α, h, and v, indicate the absorption coefficient, Planck’s constant, and frequency of the light, respectively. The E_g value estimation can be done using the plot of the (αhv)² vs. photon energy (hv) curve called the Tauc plot (Fig. 1d). The band gap of TiO₂ depends on its crystal structure and the particle size. The prepared TiO₂ NPs possess an E_g value of 3.1 eV, making them highly effective.

The SEM technique examines the surface morphology of the synthesized TiO₂ NPs at varying magnifications of 20,000X and 60,000X, as illustrated in Fig. 2a, b. The morphology of the TiO₂ particles exhibits a uniform spherical configuration alongside a consistent size distribution. Nanometer-sized particles were discerned at the elevated magnification. To ascertain the chemical composition, Energy Dispersive X-ray Analysis (EDAX) was conducted, as depicted in Fig. 2c. In the synthesized TiO₂ NPs, the elemental compositions of Titanium (Ti) and Oxygen (O) are quantified at 33.11% and 66.89%, respectively, which closely aligns with the theoretical atomic ratios found in TiO₂, thereby corroborating the successful formation of TiO₂. The results obtained from the EDAX analysis were further substantiated by the findings from X-ray Diffraction (XRD), Raman spectroscopy, and UV-visible spectroscopic assessments.

Fig. 2.

a, b SEM images at various magnifications of ×20,000 and ×60,000, and c EDAX of TiO₂ NPs

As illustrated in Fig. 3a, b, TEM images were used to observe the morphology, and particle size distribution of TiO₂ NPs. TiO₂ NPs calcinated at 550 ℃ showed spherical morphology with an average particle size is about 5.80 nm calculated from the TEM image (Fig. 3e). The HR-TEM pictures displayed in Fig. 3d, f, show a lattice distance of 0.352 nm, which is associated with TiO₂ anatase phase³⁸. The SEAD ring pattern shown in Fig. 3c is in good agreement with the crystal planes observed in the XRD pattern. The rings associated with (116), (211), (200), (112), (103), and (101) crystal planes of anatase TiO₂ were observed in the SEAD pattern. Furthermore, the EDAX and elemental mapping of TiO₂ NPs obtained from TEM is shown in Fig. 4 (red for Ti and yellow for O) confirming the uniform distribution of Ti and O. The TEM images were utilized to assess the average size and distribution of particle sizes of prepared TiO₂ NPs.

Fig. 3.

a, b TEM images, d–f HR-TEM images, c the SAED pattern of TiO₂ NPs, and e particle size distribution of TiO₂ NPs

Fig. 4.

a EDAX and b–d Element mapping of TiO₂ particle obtained from TEM

The chemical composition and oxidation states of the TiO₂ electrocatalyst has been investigated using X-ray photoelectron spectroscopy (XPS) after electrocatalytic performance as illustrated in Fig. 5. The survey spectrum (Fig. 5a) confirms the presence of Ti, O, and C elements. As shown in Fig. 5b, the XPS measurements of TiO₂ at the Ti 2p core levels reveal binding energy peaks at 458.7 eV for Ti 2p_3/2 and 464.4 eV for Ti 2p_1/2, corresponding to the Ti⁴⁺ oxidation state^46,47. The Ti 2p_3/2 core level was analysed, revealing a dominant peak cantered at 458.7 eV, which corresponds to Ti ions with a formal valence of Ti⁴⁺⁴⁸. Additionally, a peak at the lower binding energy of 464.4 eV is attributed to Ti ions in a reduced charge state (Ti³⁺). In the O 1s region (Fig. 5c), the peak centered at 530 eV is attributed to oxygen atoms within the TiO₂ lattice⁴⁹. and these O 1s peak is associated with surface hydroxide species, which are considered beneficial for enhancing hydrogen generation. The carbon peak is attributed to adventitious hydrocarbons introduced from the XPS instrument.

Fig. 5.

a Survey spectrum of TiO₂ nanoparticles, high resolution spectra of b Ti 2p and c O1s

Electrochemical measurement

The LSV curves of TiO₂, measured for HER activity evolution are shown in Fig. 6a. The TiO₂ NP-based electrocatalyst pt/c and stainless steel (SS) achieved a current density of 10 mA cm⁻² at the expense of an overpotential of 470 mV, 180 mV and 690 mV for HER respectively. To further investigate the rate of reaction for HER, the Tafel plots were plotted as illustrated in Fig. 6b. The slope of Tafel plots of TiO₂, Pt/c and SS are 165 mV dec⁻¹, 156 mV dec⁻¹ and 167 mV dec⁻¹, respectively. Among them TiO₂ demonstrating good electrocatalytic activity for HER. The higher value of overpotential suggest that the applied potential is utilized for polarization and acts as concentration overpotential. The Tafel slope of ~ 30 mV dec⁻¹ can be attributed to the Tafel step, whereas a value of ~ 120 mV dec⁻¹ attributed to Heyrovsky step^50–52. In the current study the higher value of the Tafel slope can be interpreted as Volmer-limited reactions are rate determining steps. Therefore, we propose that the HER on the TiO₂ electrode follows the Volmer-Heyrovsky-Tafel mechanism, that can be written as follows:^53,54

4

5

Fig. 6.

a LSV, b Tafel slope, c CV measured at scan rates between 20 to 100 mV s⁻¹. d Durability test of TiO₂ electrode

Here the metal active site (M) adsorbs hydrogen atom forming intermediate (M-H).

The double-layer capacitance (C_dl), which is proportional to the electrochemically active surface area (ECSA) of a catalyst is utilized for assessment of its effectiveness for HER. The CV curves, used for ECSA determination at various scan rates from 20 to100 mVs⁻¹ are displayed in Fig. 6c. The durability assessment of an electrocatalyst is a crucial aspect of its performance. An electrocatalyst based on TiO₂-NPs exhibited stability over 50 h at the current density of − 200 mA cm⁻² with a potential of 695 mV, as seen in Fig. 6d. Its excellent stability, with no discernible change for more than 50 h, is shown by the galvanostatic curves in 0.2 M Na₂SO₄ overpotential 695 mV at 200 mA cm⁻². Subsequently, we have studied the HER performance in acidic medium (0.2 M H₂SO₄), and basic medium (0.2 M KOH). The results are mentioned in Fig S2.

Based on the CV curves used in Fig. 6c were used to plot the graph of current density at 0.2 V vs. RHE against scan rate as shown in Fig. 7b. The slope of about 2.86 mF cm⁻² indicated that the nanostructured TiO₂ particles provided higher active sites. Furthermore, the EIS measurements were applied to explore the electrode-electrolyte interface and the processes occurring at the electrode surface Fig. 7a. The electrical equivalent circuit that was utilized to fit the experimental data made up of the solution resistance (R_s), charge transfer resistance (R_ct), polarization resistance (R_p), and constant phase element (Q), is provided in the inset. The Nyquist plot consists of a single, depressed semicircle, indicating a facile charge transfer mechanism. The R_ct for TiO₂ particle is 59.89 Ω cm⁻². The values of other parameters are R_s = 1.01 Ω cm⁻², and W = 0.09 Ω.

Fig. 7.

a Nyquist plot, b plot of current density at 0.2 V vs. RHE against scan rate for double layer capacitance calculation

Conclusions

In this study, TiO₂ NPs with an anatase phase and a crystallite size of 6.94 nm were synthesized using the co-precipitation method for electrochemical hydrogen evolution reaction (HER). HR-TEM confirmed an average particle size of 5.80 nm. The synthesized TiO₂ NPs demonstrated outstanding HER performance, attributed to their optimal crystallite size, fast charge transfer kinetics, and small band gap energy. The electrocatalyst required an overpotential of 470 mV to reach a current density of 10 mA cm⁻² in a neutral 0.2 M Na₂SO₄ electrolyte. The electrode also maintained stability, showing sustained HER activity over a period of 50 h. This research highlights the significance of optimizing the crystallite size to enhance HER performance, especially in neutral electrolytes, where the activity is typically lower than in acidic or alkaline media. The study opens pathways for further exploration of TiO₂-based catalysts, focusing on tuning particle size, improving conductivity, and modifying surface properties to enhance catalytic efficiency. Future research should investigate the role of dopants and surface modifications on the catalytic properties of TiO₂ NPs to further reduce the overpotential and enhance stability over extended periods. Additionally, exploring hybrid systems or composite materials incorporating TiO₂ NPs with other conductive supports could lead to further advancements in HER activity. This would contribute to developing cost-effective and efficient electrocatalysts for large-scale hydrogen production in neutral pH environments.

Electronic supplementary material

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Author contributions

PDS: Methodology, Writing – original draft. RKC: Resources. DBM: Validation, Resources. JHK: Validation, Resources. SM: Data curation. G-PC-C: Visualization & Investigation. Y-CL: Validation, Resources. SSK: Supervision. GSK: Methodology, Supervision, Project administration, Validation, Investigation, Conceptualization.

Data availability

The authors declare that the data supporting the findings of this study are available within the paper. Should any raw data files be needed in another format they are available from the corresponding author upon reasonable request.

Declarations

Competing interests

The authors declare no competing interests.