Unveiling the Electrochemical Kinetics of an FeWO₄-NiFeOOH Anode: Electrolyte Effects on Energy Conversion

Abstract

This study aims to determine the electrochemical performance of FeWO₄ (Fe), NiFeOOH (Ni), and FeWO₄/NiFeOOH (FeNi) electrodes in 0.1 M Na₂SO₄ and 0.1 M NaOH electrolytes, highlighting the impact of SO₄²⁻ and OH⁻ ions. Nyquist analysis demonstrated that the FeWO₄/NiFeOOH electrode had the lowest charge transfer resistance, indicating superior charge transport and capacitive performance over the individual electrodes. In Na₂SO₄, SO₄²⁻ ions stabilized double-layer capacitance and enhance ionic mobility. Conversely, in NaOH, highly conductive and mobile OH⁻ ions significantly improved charge transfer and diffusion, making NaOH more effective for electrochemical applications. Tafel analysis revealed better charge transfer kinetics and greater energy efficiency in NaOH, with the composite electrode excelling in both electrolytes. Linear voltammetry showed a synergistic interaction between FeWO₄ and NiFeOOH, achieving a photocurrent density of 6.70 mA·cm⁻² in NaOH under illumination, an 830.56% increase over Na₂SO₄. Additionally, the FeWO₄/NiFeOOH composite electrode exhibited longer electron lifetimes in NaOH than in Na₂SO₄, attributed to the smaller ionic radius and higher diffusion coefficient of OH⁻ ions. Pulsed photocurrent analysis revealed notable improvements in photocurrent generation and stability in NaOH. These findings indicate that the FeWO₄/NiFeOOH composite is a highly efficient and stable material for advanced energy technologies, with NaOH providing optimal performance conditions.

1. Introduction

In recent decades, extensive research has focused on renewable energy sources [1,2]. Studies consistently show that solar energy is the most abundant and powerful alternative to fossil fuels such as coal, oil, and natural gas. Solar hydrogen production via photoelectrochemical (PEC) water splitting is a key technology in the renewable energy sector and a promising method for future energy conversion [3]. Since Honda and Fujishima first introduced the TiO₂ photoelectrode for solar-driven water splitting, various semiconductor oxides, including TiO₂, ZnO, BiVO4, Fe₂O₃, WO3, and FeOOH, have been widely studied as photoanode materials [4,5]. Each material has specific drawbacks that must be addressed: TiO₂ and ZnO have poor visible light absorption, BiVO₄ exhibits rapid charge carrier recombination, Fe₂O₃ has a very short hole diffusion length, and WO₃ demonstrates limited stability in neutral media. These limitations underscore the need for careful material selection and optimization to address these challenges and improve performance across various applications [5,6]. Recent studies show that constructing heterojunctions effectively addresses these challenges by improving charge carrier separation and transfer, enhancing visible light absorption, and significantly improving photoelectrocatalytic performance. Mao et al. [7] recently fabricated a WO₃@Fe₂O₃ core-shell nanoarray heterojunction that significantly enhanced PEC water-splitting performance. The heterojunction achieved a photocurrent density of 1.26 mAcm⁻² at 1.23 V vs. RHE and an incident photon-to-current efficiency (IPEC) of 24.4% at 350 nm, substantially higher than that of pristine WO₃, which exhibited a photocurrent density of 0.18 mAcm⁻² and an IPEC of 11.5%. Furthermore, FeWO₄ is a visible-light-responsive photoanode for PEC water splitting owing to its moderate band gap (1.8–2.0 eV), enabling effective visible light absorption, excellent water oxidation ability, chemical stability in acidic electrolytes, and suitable hole diffusion length for effective charge transport [8,9].

Similarly, iron oxyhydroxide (FeOOH) is a promising visible-light material for constructing heterojunctions to enhance PEC water-splitting efficiency owing to its abundance, chemical stability, and environmental friendliness. Moreover, FeOOH functions as an oxygen evolution reaction (OER) cocatalyst, effectively addressing sluggish oxygen evolution kinetics. Although WO₃/FeOOH composites have been extensively studied for water pollutant treatment, few studies have focused on their use as photoanodes for PEC water splitting [9]. Researchers have investigated the challenges associated with WO₃ and FeOOH, focusing on strategies such as cocatalyst doping, crystal facet tuning, and heterojunction fabrication [10,11,12]. Among various strategies, doping FeOOH/WO₃ with cocatalysts has effectively created more active sites, improved surface OER kinetics by lowering activation energy, and reduced surface recombination. Selecting an electrocatalyst (EC) as a cocatalyst requires prioritizing a high-quality interface between the EC and the oxide or hydroxide. This ensures a balance between the hydrogen evolution reaction activity of the EC and the light-harvesting efficiency of the material [13]. Consequently, developing novel and highly active cocatalysts is essential to enhance the stability and Faradaic efficiency of these materials, improving their performance in PEC applications [14]. Studies indicate that doping FeOOH with Co [15] and Ni [16] significantly improves electron transfer during electrochemical water oxidation. Ni-doped β-FeOOH (Ni:FeOOH), through an in situ electrochemical method, improved OER activity by imparting semi-metallic properties to the material [17]. This innovation has led to the incorporation of Ni:FeOOH in PEC applications owing to its superior OER activity and unique optical and electrochemical properties compared to bare FeOOH [18]. For example, Fang et al. pioneered a ternary photoanode with improved PEC performance [19]. Subsequently, Zhang et al. fabricated a ternary BiVO₄/Ni:FeOOH/CoPi photoelectrode that accelerated interfacial charge transfer [20]. These studies show that Ni:FeOOH is a versatile complementary modifier, playing different roles in various photoelectrodes for PEC applications. However, Ni:FeOOH-modified FeWO₄ remains unexplored, presenting a promising research direction. Inspired by this idea, we coupled FeWO₄ with Ni:FeOOH to enhance PEC activity. Consequently, the FeWO₄/NiFeOOH photoanode exhibits higher activity under irradiation, and its onset potential (Vonset) shifts cathodically. The Ni:FeOOH network improves charge transfer by forming a continuous conducting path. PEC response only occurs in FeWO₄/Ni:FeOOH, not in FeOOH. Therefore, Ni:FeOOH acts as a bifunctional modifier, accelerating charge transfer and transport by forming a p–n heterojunction and increasing light absorption. These synergistic effects improve both the Faradaic efficiency and stability compared to other materials. Moreover, electrolytes are critical for boosting the PEC water-splitting efficiency of photocatalytic materials. Various anions (Cl⁻, H₂PO₄⁻, ClO₄⁻, and SO₄²⁻) and cations (Li⁺, Na⁺, and K⁺) have been explored to optimize PEC performance. For example, Ding et al. reported that the PEC performance of TiO₂ electrodes with cocatalysts follows the trend Li⁺ > K⁺ > Na⁺ in strongly alkaline electrolytes containing these cations. ZnO photoanodes have demonstrated varying PEC water-splitting efficiencies with electrolytes such as NaOH, KOH, Na₂SO₄, and NaClO₄ [21,22,23,24,25,26]. FeWO₄-based catalysts, known for their excellent stability and water oxidation ability, have also been extensively studied in diverse PEC and catalytic applications [27,28]. These findings highlight the need for selecting suitable electrolytes and catalysts to optimize PEC system performance.

This study aims to enhance the PEC water-splitting activity of FeWO4-NiFeOOH photoanodes by optimizing their electrolytes. To our knowledge, no studies have investigated the impact of electrolytes on the PEC water-splitting activities of FeWO4-NiFeOOH photoanodes. In this study, FeWO4-NiFeOOH anodes were synthesized using simple synthetic methods, and their PEC water-splitting performance was evaluated in Na₂SO₄ and NaOH electrolytes. The FeWO₄-NiFeOOH photoanode exhibited significantly higher PEC water-splitting activity in NaOH than in Na₂SO₄.

2. Materials and Methods

FeWO₄ nanostructures were synthesized using a hydrothermal method with FeCl₃·6H₂O and Na₂WO₄·2H₂O as the main precursors. First, FeCl₃·6H₂O (3.244 g) and Na₂WO₄·2H₂O (3.96 g) were dissolved in deionized water and vigorously stirred for 2 h to ensure complete dissolution and a uniform solution. The resulting solution was transferred into Teflon-lined autoclave vessels and heated under microwave-assisted hydrothermal conditions at 180 °C for 2 h. After the reaction was complete, the autoclaves were cooled to room temperature. The synthesized product was collected, rinsed thoroughly with ethanol and deionized water to remove residual byproducts or unreacted precursors, and separated by centrifugation at 8000 rpm. Finally, the purified material was dried at 80 °C for 30 min before further analysis.

NiFeOOH nanostructures were synthesized via co-precipitation using FeCl₂·4H₂O, Ni(NO₃)₂·6H₂O, and hexamethylenetetramine (HMTA) as the primary precursors. First, FeCl₂·4H₂O (0.59 g) and HMTA (0.42 g) were dissolved in 30 mL of deionized water and stirred vigorously for 30 min to obtain Solution A. Meanwhile, Ni(NO₃)₂·6H₂O (0.87 g) and HMTA (0.42 g) were dissolved in 30 mL of deionized water and stirred for 30 min to obtain Solution B. Solution A was slowly added to Solution B in a 2:1 ratio, and the final solution was left at room temperature for 10 min. The synthesized product was collected, thoroughly rinsed with ethanol and deionized water to remove any residual byproducts or unreacted precursors, and separated by centrifugation at 8000 rpm. Finally, the purified material was dried at 80 °C for 30 min for further analysis.

The FeWO₄/NiFeOOH composite was prepared by mixing equal amounts of FeWO₄ and NiFeOOH nanostructures [29,30]. Each nanoparticle was separately dispersed in 25 mL of deionized water using probe sonication for 2 h to form a uniform and stable suspension. The NiFeOOH suspension was then slowly added to the pre-sonicated FeWO₄ solution while maintaining continuous sonication to ensure effective blending. The mixture was then sonicated for 1 h to promote strong interaction and integration between the two materials. The resulting composite was isolated by centrifugation at 10,000 rpm, washed multiple times with deionized water and ethanol to remove any residual impurities, and dried in an oven at 80 °C for 12 h to completely remove moisture, yielding the NiFeOOH composite for further use.

To fabricate the electrode, 2 mg of each synthesized material was dispersed in 3 mL of ethylene glycol and sonicated for 15 min to form a uniform suspension. This prepared dispersion was carefully drop-cast onto a pre-cleaned 1 × 1 cm² indium tin oxide (ITO) glass substrate on a hot plate maintained at 120 °C. After coating, the substrate was dried in an oven at 130 °C for 24 h to ensure complete solvent evaporation and form a stable, uniform coating on the ITO substrate.

The synthesized materials were characterized using advanced analytical techniques to evaluate their structural, morphological, chemical, and optical properties. X-ray diffraction analysis with a PANalytical X’pert PRO system (Netherlands) was used to identify the crystalline phases and confirm the structural composition of the samples. Scanning electron microscopy (SEM) with a Hitachi S-4800 instrument (Japan) was used to analyze the morphological features of FeWO₄, NiFeOOH, and the FeWO₄/NiFeOOH composite. This provided high-resolution images of their surface structure and particle distribution. X-ray photoelectron spectroscopy (XPS) was used to investigate the chemical states and valence band structures using a Thermo Fisher Scientific MultiLab 2000 system (Seoul, South Korea), providing insights into the elemental composition and oxidation states of the materials. The optical properties of the synthesized materials were evaluated through multiple spectroscopic methods. Fourier transform infrared spectroscopy was performed with a PerkinElmer Spectrum 100 system (USA) to identify functional groups and confirm chemical bonding. Additionally, UV–Vis spectroscopy was conducted using a Neogen NEO-D3117 system (South Korea) to study the light absorption characteristics and optical bandgap properties.

The PEC water-splitting performance of the synthesized materials was assessed using a three-electrode system connected to a Bio-Logic SP-200 potentiostat (Seyssinet-Pariset, France), comprising an Ag/AgCl reference electrode, a platinum counter electrode, and the synthesized materials as working electrodes, using 0.1 M Na₂SO₄ and NaOH as electrolytes. Performance was evaluated under illuminated and dark conditions, with all potentials referenced to the Ag/AgCl electrode. Electrochemical impedance spectroscopy was conducted at a 10 mV bias across a frequency range of 0.5 Hz to 100 MHz to assess charge transfer characteristics. Illumination was provided by an ABET Technologies light source (Model 10500, USA) at a power density of 100 mW/cm², with the samples placed 6 cm from the light source for consistent exposure. This setup enabled the evaluation of the photoresponse and charge transport efficiency of the materials, providing a comprehensive understanding of their physical and chemical attributes for potential applications.

3. Results

Figure 1 shows the diffraction peaks of FeWO₄ (Fe), NiFeOOH (Ni), and FeWO₄/NiFeOOH (FeNi). The peaks of FeWO₄ correspond to its monoclinic primitive structure (P2/c(13) space group) and unit cell parameters of a = 0.475 nm, b = 0.572 nm, c = 0.497 nm, and β = 90.17° (Figure 1), consistent with the values reported in the literature (JCPDS card No. 71-2391). The diffraction peaks at 36.23°, 38.88°, 41.37°, 54.18°, and 65.05° correspond to the (021), (202), (121), (221), and (023) planes, respectively [31]. The diffraction peaks of NiFeOOH were compared with the standard peaks of FeOOH (JCPDS No. 76-0182), which retains a rhombohedral crystal structure (R-3c space group) with the following unit cell parameters: a = 0.5031 nm, b = 0.5029 nm, c = 1.373 nm, and α = β = 90°, γ = 120° (Figure 1). The diffraction peaks at 24.07°, 33.06°, 35.58°, 40.87°, 49.38°, 54.06°, 62.46°, and 64.02° correspond to the (012), (104), (110), (113), (024), (116), (214), and (300) planes, respectively. Ni incorporation causes the diffraction peaks to closely resemble those of FeOOH, suggesting that the crystal structure of FeOOH remains largely unchanged upon Ni doping [32,33]. Figure 1 shows that incorporating NiFeOOH into FeWO4 shifts the peaks to lower angles and decreases peak intensities compared to the Fe and Ni samples. Notably, peaks at 32.71° and a small bump at 35.47° from FeOOH appeared in the FeWO₄/NiFeOOH samples. Similarly, Chen et al. reported that NiFeOOH/Fe₂O₃ had no significant effect on its crystalline structure [34]. The grain sizes of FeWO₄, NiFeOOH, and FeWO₄/NiFeOOH were approximately 59 nm, 44 nm, and 32 nm, respectively.

Figure 2a shows the SEM image of FeWO₄, revealing highly aggregated nanostructure spherical particles with an average size of 30–40 nm. Moreover, Zhou et al. recently reported that hierarchical plate-like FeWO₄ nanostructured materials exhibit excellent photocatalytic activity [31]. The NiFeOOH sample consists of nanorods and sphere-like morphologies, forming aggregates of 50–60 nm (Figure 2b). Lin et al. reported that incorporating NiFeOOH as a fluffy layer on WO₃ nanoflakes enhanced PEC photoelectrode performance [35]. The maximum photocurrent density achieved was 2.58 mAcm⁻² at 1.8 V versus RHE, which is 4.3 and 2 times higher than that of pure WO₃ and WO₃/NiFeOOH, respectively. Figure 2c shows significant agglomeration of interconnected spherical nanoparticles in the FeWO₄/NiFeOOH sample, likely caused by van der Waals forces and physicochemical attraction [36]. The observed changes in surface roughness are attributed to the NiFeOOH nanostructure decoration, which may facilitate complete electrolyte infiltration [37]. This modification improves structural stability and electron transfer within the electrode, thereby enhancing the electrochemical performance of FeWO₄. Furthermore, the results suggest that while FeWO₄/NiFeOOH has a minimal effect on surface morphology, adding hydrazine hydrate significantly influences surface roughness and nanoparticle agglomeration.

The UV–Vis absorption spectrum and bandgap analysis of Fe, Ni, and FeNi nanocomposites (Figure 3) reveal crucial insights into their optical properties, highlighting unique behaviors across the 210–800 nm range. FeWO₄, characterized by its distinct electronic structure, demonstrates strong light absorption, particularly in the visible and ultraviolet regions. This characteristic is largely attributed to electronic states induced by oxygen vacancies in the material. NiFeOOH, in contrast, displays broad visible-light absorption, making it a promising material for photocatalytic and energy conversion applications. In the FeNi nanocomposite, FeWO₄ and NiFeOOH interact synergistically, enhancing absorption capacities. This occurs through the formation of additional electronic band levels within the composite, facilitated by their structural and electronic compatibility. The interaction between FeWO₄ and NiFeOOH couples their electronic states, introducing new energy levels within the bandgap of the material. These new levels modify optical transitions, extending the photon absorption range to bridge the gap between ultraviolet and visible light absorption, effectively broadening the absorption spectrum of the composite. This expansion enhances the light-harvesting efficiency of the composite, enabling additional light absorption and increasing its potential for energy conversion. Compared to their pristine samples, Fe and Ni, the FeNi composite demonstrates enhanced light-harvesting capabilities (Figure 3a), highlighting the influence of nanoscale interactions and structural modifications in enhancing optical and electronic properties. Furthermore, the optical bandgaps of the materials—2.21 eV (FeWO₄), 1.42 eV (NiFeOOH), and 2.06 eV (FeNi composite)—highlight how the combination of these materials optimizes the light absorption spectrum, as shown in Figure 3b–d. FeWO₄, with its wide bandgap, primarily absorbs UV light, limiting its effectiveness for visible-light-driven applications despite its stability. In contrast, NiFeOOH, with a narrower bandgap, excels in visible-light absorption, making it ideal for such applications. The FeNi composite, with a bandgap of 2.06 eV, achieves an optimal balance by integrating the strengths of both materials: the visible-light activity of NiFeOOH and the UV-light absorption and stability of FeWO₄. The optimized band alignment and interface engineering of the FeNi composite enable efficient light harvesting, charge transport, and stability, establishing it as a highly promising material for energy conversion applications, such as photocatalysis and photoelectrochemical (PEC) cells. This study highlights the importance of engineered nanostructures in enhancing material performance, offering significant potential for advancing sustainable energy technologies.

Figure 4 illustrates the chemical bonds of FeWO₄, NiFeOOH, and FeWO₄-NiFeOOH compounds. Broad bands spanning from 3000 to 3600 cm⁻¹ and 1600 to 1750 cm⁻¹ correspond to metal-conjugated O-H groups from water molecules within the FeWO₄ sample. The band at 562 cm⁻¹ corresponds to Fe-O stretching vibrations in hematite particles. Furthermore, the peaks at 887–637 cm⁻¹ correspond to W–O stretching vibrations in the WO₄ tetrahedra deformation mode for FeWO₄ samples (Figure 4) [38]. The broad band at 500–1000 cm⁻¹ corresponds to Fe-O-W stretching vibrations in the Fe and FeNi samples. The FTIR spectra of the Ni sample (Figure 4) display peaks at a broad range from 3000 to 3600 cm⁻¹, attributed to water molecule stretching modes [39]. Peaks at 1054 cm⁻¹ and 829 cm⁻¹ correspond to hydroxyl bending vibrations in Fe–OH, while bands at 670 cm⁻¹, 562 cm⁻¹, and 475 cm⁻¹ represent Fe-O bending vibrations [40].

Figure 4 depicts the incorporation of NiFeOOH into the FeWO₄ compound, showing the presence of Fe-O, W-O, and O-H vibrations from the FeWO₄ compound. However, the additional peaks at 1387 cm⁻¹ and 928 cm⁻¹ correspond to hydroxyl bending vibrations in Fe-OH. The vibrations corresponding to hydroxyl out-of-plane bending in Fe-OH at 1054 cm⁻¹ disappeared, probably owing to the dominance of W–O and Fe–O bands. Meanwhile, the peaks at 475 cm⁻¹, 670 cm⁻¹, and the broad band at 500–1000 cm⁻¹ remain intact, consistent with the NiFeOOH material composition. This indicates covalent bonding between the NiFeOOH and FeWO₄ materials.

XPS analysis of the FeWO₄, NiFeOOH, and FeWO₄-NiFeOOH samples provided detailed insights into their elemental composition, chemical states, and structures. The survey spectra revealed distinct peaks for W, Fe, O, and Ni species, confirming their presence in the synthesized samples (Figure 5a). High-resolution deconvoluted spectra (Figure 5b–k) further elucidated the oxidation states and bonding environments of the elements. The O 1s core level spectra provide key insights into oxygen species in the FeWO₄, NiFeOOH, and FeWO₄/NiFeOOH samples. A peak at ~529.9 eV corresponds to lattice oxygen, while a higher-energy peak at ~529.6 eV represents adsorbed oxygen (Figure 5b–d) [41].

In the NiFeOOH sample, a peak at 528.8 eV indicates oxygen related to absorbed moisture, reflecting the surface chemistry and environmental interactions of the sample. The Fe 2p spectra of FeWO₄, NiFeOOH, and FeWO₄-NiFeOOH samples displayed binding energy peaks at 710.19 eV, 709.64 eV, and 709.73 eV (Fe 2p3/2) and 723.99 eV, 723.62 eV, and 723.99 eV (Fe 2p1/2), indicative of Fe in the +2 oxidation state (Figure 5e–g) [42]. Similarly, the W 4f core-level spectra of FeWO₄ and FeWO₄-NiFeOOH display peaks at approximately 34.88 eV and 34.78 eV (W 4f₇/₂) and 37.03 eV and 36.92 eV (W 4f₅/₂), indicating that W exists predominantly in the +6 oxidation state (Figure 5h,i) [43]. Furthermore, high-resolution scans of the Ni 2p spectra for the NiFeOOH and FeWO₄/NiFeOOH samples show peaks at 854.11 eV and 854.84 eV (Ni 2p₃/₂) and 875.65 eV and 872.49 eV (Ni 2p₁/₂), indicating the presence of Ni2+ and Ni3+, respectively [44] (Figure 5j,k). However, the presence of Ni ions in the Ni 2p spectra of the FeWO₄-NiFeOOH sample optimizes the electronic structures of FeOOH, weakens other binding energies, and potentially enhances the photocatalytic activity of the FeWO₄-NiFeOOH sample.

Electrochemical evaluation of FeWO₄ (Fe), NiFeOOH (Ni), and FeWO₄/NiFeOOH (FeNi) composite electrodes was conducted to assess their potential in energy conversion applications, focusing on their performance in 0.1 M Na₂SO₄ and NaOH electrolytes, under alternating illumination (Figure 6).

Nyquist plots from impedance spectroscopy data were used to analyze charge transfer and ionic diffusion at the electrode–electrolyte interface. In these plots, the high-frequency semicircle represents the charge transfer resistance (Re), while the low-frequency slope indicates ionic diffusion resistance. Illumination significantly affected the electrochemical properties of the electrodes, particularly reducing the high-frequency semicircle radius, suggesting enhanced photocurrent generation. This phenomenon indicates that under light, the electrodes, particularly the FeNi composite, effectively utilize light energy to generate charge carriers (electrons and holes). The electrolyte plays a critical role in these electrochemical processes. In the Na₂SO₄ electrolyte, sulfate ions (SO₄²⁻) enhanced the electrochemical performance by stabilizing the double-layer capacitance (Cd), which is crucial for efficient charge storage (Figure 6a). SO₄²⁻ improves ionic mobility, reducing ionic impedance at the interface and facilitating more effective charge separation. Consequently, the FeNi composite, combining FeWO₄ and NiFeOOH, exhibited significantly lower Re than that in the individual components, indicating improved electrochemical kinetics. The results from the synergistic effects between FeWO₄ and NiFeOOH indicate enhanced light absorption and accelerated electron transfer during electrochemical reactions. Furthermore, the FeNi composite demonstrated superior capacitance, with values ranging from 72.91 × 10⁻⁶ F to 32.56 × 10⁻⁶ F, highlighting its effective charge storage in both light and dark conditions (

Table 1. The physical parameters were initially derived in 0.1 M sodium hydroxide and 0.1 M sodium sulfate solutions under alternating illumination conditions.

Electrode		Electrolyte 0.1 M	Condition	Ro (Ω)	Rb (Ω)	Re (kΩ)	Cbc (μF)	Cd (μF)
FeWO4 nanoparticles		Na2SO4	Dark	30.07	381.7	9.02	19.06	31.83
			Light	27.63	376.1	7.61	25.84	39.77
NiFeOOH nanostructures			Dark	29.30	46.53	6.86	15.71	131.61
			Light	25.61	42.86	5.98	18.96	142.89
FeWO4/NiFeOOH			Dark	26.67	24.53	3.98	32.56	65.35
			Light	22.53	26.12	3.62	36.49	72.91
FeWO4 nanoparticles		NaOH	Dark	9.32	17.94	15.94	57.02	67.99
			Light	9.11	17.84	14.56	62.10	77.53
NiFeOOH nanostructures			Dark	6.83	15.68	4.65	21.98	134.56
			Light	5.99	12.53	4.11	28.96	145.96
FeWO4/NiFeOOH			Dark	4.86	11.57	3.31	78.96	110.56
			Light	3.64	8.94	2.98	82.65	125.64

). This superior charge storage capability stems from the complementary properties of FeWO₄ and NiFeOOH, where FeWO₄ provides structural stability, and NiFeOOH enhances light absorption and charge separation. Using NaOH as the electrolyte distinctly improved electrochemical performance, primarily owing to the behavior of hydroxide ions (OH⁻) (Figure 6d). Hydroxide ions are smaller in size, with a higher diffusion coefficient than sulfate ions, enabling rapid movement through the electrolyte. This enhanced mobility of OH⁻ ions significantly reduces Re and improves charge separation and transport dynamics at the electrode–electrolyte interface. This promotes a robust Helmholtz double layer at the interface, boosting the electrochemical performance of the FeNi composite. Consequently, the FeNi composite exhibited impressive capacitance values ranging from 125.64 × 10⁻⁶ F to 78.96 × 10⁻⁶ F under various conditions, demonstrating its high charge storage capacity across different environments (Table 1).

Bode plots and phase angle analyses were performed to validate these observations (Figure 6b,c,e,f). These analyses showed that the FeNi composite in NaOH exhibited minimal impedance across a wide frequency range, indicating excellent ionic conductivity and rapid charge transfer kinetics. At low frequencies, NaOH electrolytes significantly enhanced ionic diffusion, promoting greater charge accommodation within the Helmholtz layer, which is crucial for efficient charge storage. At high frequencies, the electrolyte maintained rapid ionic mobility, which is crucial for high-frequency electrochemical applications. In contrast, Na₂SO₄ exhibited higher impedance across the frequency spectrum, indicating slower ionic diffusion and reduced charge transport efficiency. The larger ionic radius and lower diffusion coefficient of sulfate ions hindered rapid ion migration, slowing charge transfer and decreasing overall electrochemical performance. Additionally, the electron lifetime (τ_e), indicative of charge carrier stability, was estimated using impedance spectroscopy and phase analysis. The electron lifetime was longer in NaOH (4.72 × 10⁻⁶ s) than in Na₂SO₄ (4.62 × 10⁻⁶ s). This longer electron lifetime in NaOH suggests that the smaller ionic radius and faster diffusion coefficient of OH⁻ ions enhance charge transport and reduce recombination rates, improving overall electrochemical performance. In contrast, the shorter electron lifetime in Na₂SO₄ indicates increased recombination, as larger sulfate ions hinder ionic mobility and slow charge transfer. The FeWO₄/NiFeOOH composite combined with the optimal NaOH electrolyte demonstrates superior electrochemical performance. The enhanced ionic mobility, efficient charge separation, and long electron lifetime in NaOH highlight its suitability for energy conversion and storage applications. The synergistic effects of the FeWO₄/NiFeOOH composite and the advantageous properties of NaOH position the FeNi composite as a highly promising material for energy storage systems, where efficient charge transfer, rapid ionic diffusion, and long-term stability are crucial. These findings highlight the importance of electrolyte composition and its unique material properties in optimizing electrochemical device performance.

Tafel analysis of the Fe, Ni, and FeNi anodes revealed distinct electrochemical behavior when tested in Na₂SO₄ and NaOH electrolytes under both illuminated and dark conditions (Figure 7).

Across all experiments, NaOH consistently outperformed Na₂SO₄ (Figure 7a,b). Illumination caused an anodic shift for all anodes in both electrolytes, indicating enhanced charge carrier generation. This shift refers to a move of the electrode potential towards more positive values, typically induced by illumination or conditions that increase charge carrier activity. The anodic shift for the FeNi anode in Na₂SO₄ between dark and illuminated states was 80 mV, slightly higher than the 70 mV shift in NaOH. This indicates that while Na₂SO₄ enhances charge carrier generation and oxidation under illumination, it also causes higher overpotentials and slower charge transfer kinetics. This is attributed to the lower ionic conductivity of Na₂SO₄ and weaker electrode interactions, restricting its electrochemical efficiency. Conversely, NaOH promotes more efficient charge transfer owing to its superior ionic conductivity, reduced overpotentials, and enhanced stabilization of reaction intermediates despite a smaller anodic shift. The reduced shift in NaOH is attributed to its favorable electrochemical environment, where rapid OH⁻ ion mobility improves charge transport and lowers energy barriers. Under illumination, the Tafel slope of the FeNi anode was 53.11 mV·dec⁻¹ in Na₂SO₄, significantly higher than the 28.91 mV·dec⁻¹ in NaOH (Figure 7a). This difference highlights the superior charge transfer efficiency in the alkaline medium, reducing energy losses and enhancing reaction kinetics (

Table 2. Tafel fitting results for the electrodes measured in 0.1 M NaOH and 0.1 M Na₂SO₄ electrolytes under dark and light conditions.

Electrolyte 0.1 M	Photoelectrode	Tafel Slopes		J1		J2
		Dark mVdec−1	Light mVdec−1	Dark mAcm−2	Light mAcm−2	Dark mAcm−2	Light mAcm−2
Na2SO4	FeWO4	89.85	82.65	−0.24	0.01	−2.85	−2.67
	NiFeOOH	78.31	67.29	−0.86	−0.21	−3.74	−3.71
	FeWO4/NiFeOOH	61.26	53.11	−0.08	0.06	−2.41	−1.98
NaOH	FeWO4	55.19	46.37	−0.08	−0.04	−2.66	−2.47
	NiFeOOH	68.35	57.56	−0.31	−0.21	−2.88	−2.62
	FeWO4/NiFeOOH	32.61	28.91	−0.08	0.08	−1.96	−1.32

). Photocurrent density values for a diffusion-limited current (J1) and charge exchange current (J2) were lower in NaOH than in Na₂SO₄. This indicates that the high reactivity and ionic mobility of NaOH improved charge transport dynamics and interfacial interactions. Hydroxyl ions in NaOH enhance adsorption at anode active sites, stabilize catalytic intermediates, and reduce recombination losses, enhancing charge generation and transfer (Table 2). In contrast, Na₂SO₄ is hindered by its lower ionic conductivity and weaker electrode interactions, resulting in slower charge dynamics and higher energy losses. Under both illuminated and dark conditions, the FeNi anode demonstrated optimal performance with NaOH (Figure 7b). This is evident in its lower Tafel slopes and higher photocurrent values (J1 and J2). The synergy between the FeNi anode and the alkaline medium facilitated reduced overpotentials, accelerated charge transfer, and sustained electrochemical activity. High hydroxyl ion mobility and strong electrode–electrolyte interactions in NaOH contributed to efficient ionic transport, intermediate stabilization, and minimizing resistive losses. The comparison between Na₂SO₄ and NaOH highlights the superiority of NaOH as an electrolyte for the FeNi anode. Its ability to optimize electrochemical processes through enhanced charge kinetics, reduced energy barriers, and improved ionic conductivity establishes it as the preferred medium for high-efficiency and stable applications.

Linear voltammetry analysis was performed on the synthesized samples in different electrolytes under illuminated and non-illuminated conditions (Figure 8).

The results show significant variations in current density and electrochemical behavior, influenced by the electrolyte type and lighting conditions. In the Na₂SO₄ electrolyte (Figure 8a), all anodes displayed increased current density with rising voltage, indicating effective catalytic activity and charge transfer. Illumination further improved performance, highlighting the photo-responsive nature of the anodes. Among the materials tested, the FeNi anode exhibited the highest current density, attributed to the complementary properties of its components. The superior electron transport from Fe, coupled with the photoactivity of Ni, facilitated efficient electron–hole pair generation and minimized recombination. This synergy increased active site availability for electrochemical reactions. The FeNi anode achieved a peak current density of approximately 6.70 mA·cm⁻², 17.75% higher than that of the Fe anode alone, demonstrating significantly improved catalytic performance under illumination. The onset voltages in Na₂SO₄ for Fe (0.82 V), Ni (0.68 V), and FeNi (0.69 V) showed varying electrochemical efficiencies. The Fe anode exhibited the highest onset voltage, requiring higher energy to initiate reactions and lower charge transfer efficiency. The reduced onset voltage of the Ni anode enhanced charge separation and suppressed electron–hole recombination. The FeNi anode showed a 0.13 V lower onset potential than that in Fe, demonstrating more efficient charge transfer and enhanced reaction kinetics. It exhibited the lowest onset voltage, indicating synergy between Fe and Ni, which optimized active site availability, improved ionic conductivity, and facilitated efficient charge transfer. The FeNi anode demonstrated superior catalytic activity in the neutral Na₂SO₄ electrolyte, making it ideal for applications with moderate electrochemical conditions. In the NaOH electrolyte (Figure 8b), its performance improved significantly, achieving a photocurrent density of approximately 6.70 mA·cm⁻² under illumination—nearly 830.56% higher than that in Na₂SO₄. This improvement is attributed to the alkaline NaOH environment, which enhances ionic conductivity, reduces resistive losses, and accelerates electron transport. The conductive properties of Fe and the photoactivity of Ni synergistically optimize charge separation. OH⁻ ions present in NaOH are crucial in enhancing electrochemical reactivity at the anode–electrolyte interface, further improving reaction kinetics and overall performance. The onset voltages in NaOH for Fe (0.68 V), Ni (0.58 V), and FeNi (0.55 V) confirmed significant improvements in electrochemical performance owing to the favorable properties of the alkaline medium. Compared with Fe, the FeNi anode showed a 0.13 V lower onset potential, indicating that FeNi can achieve advanced charge transfer efficiency and improved reaction kinetics. This superior performance is attributed to the synergistic interaction between Fe and Ni, which maximized active site utilization, improved electron transport, and reduced energy barriers for reactions. The use of NaOH amplified these benefits. Its high ionic conductivity enhanced OH⁻ mobility, minimized resistive losses, and stabilized charge carriers. Additionally, the alkaline environment accelerated reaction kinetics, facilitating faster electron transfer and higher catalytic efficiency. The significant increase in photocurrent density observed in NaOH compared to that in Na₂SO₄ underscores the advantages of an alkaline medium. NaOH promotes faster charge transport, enhances charge separation, and mitigates recombination losses, creating an ideal environment for electrochemical reactions. These effects stem from enhanced electron mobility, stabilized charge carriers, and increased electrochemical reactivity, all of which contribute to the higher efficiency of the system. The FeNi anode demonstrated outstanding photoelectrochemical performance, particularly in the NaOH electrolyte, establishing it as a promising candidate for renewable energy applications, such as photoelectrochemical water splitting. Its superior efficiency and operational stability highlight its potential for driving sustainable energy solutions.

The photocatalytic performance of the synthesized photoanodes was systematically evaluated under diverse applied bias potentials in two different electrolytes: 0.1 M Na₂SO₄ and 0.1 M NaOH (Figure 9a–f). In the Na₂SO₄ electrolyte, the photocurrent density increased steadily as the applied bias potential rose, peaking at +0.6 V. This increase, observed from 0.4 to 0.6 V, can be attributed to the enhanced electric field, which facilitated the efficient separation of photogenerated electron–hole pairs. The stable Na₂SO₄ environment supported moderate charge transport, minimizing surface instability and enabling consistent photocurrent generation within this potential range. At lower bias potentials, the electric field was insufficient to drive effective charge carrier movement, leading to higher recombination rates. As the bias potential increased, the stronger electric field directed electrons towards the external circuit and holes to the photoanode surface, lowering recombination and promoting oxidation reactions. At higher potentials, the activation of favorable surface states further enhanced photocurrent generation. However, beyond 0.5 V, the photocurrent density began to decline, possibly due to the inability of the electrolyte to effectively suppress recombination processes, resulting in a reduction in the driving force for charge separation. In contrast, in the NaOH electrolyte, the photocurrent density increased up to 0.5 V, driven by the enhanced electric field at moderate bias potentials, which facilitated efficient separation and transport of photogenerated charge carriers (Figure 9e).

The alkaline nature of NaOH played a critical role in suppressing recombination, stabilizing surface states, and minimizing charge trapping at defect sites. However, beyond 0.5 V, the photocurrent density decreased owing to increased electron–hole recombination and surface instability. The stronger electric field at higher potentials exacerbated recombination by inducing charge trapping at grain boundaries or triggering undesirable surface reactions, such as excessive oxidation of the photoanode material. These findings indicate the need to optimize the bias potential to balance charge separation efficiency and photoanode stability. The FeNi composite exhibited a higher photocatalytic performance than that of the pristine Fe and Ni samples in both the Na₂SO₄ and NaOH electrolytes across all applied voltages. This enhanced performance is attributed to the synergistic effects of the composite structure, which integrates the advantageous properties of both components. The FeNi composite exhibited enhanced light absorption, efficient charge carrier generation, and improved separation and transport dynamics, superior to those of the individual components. Furthermore, the formation of heterojunctions between the Fe and Ni phases reduced recombination rates by directing electrons and holes to distinct regions within the composite. Additionally, the FeNi structure, characterized by reduced grain boundary resistance and increased surface area, significantly enhances photocatalytic activity. In the NaOH electrolyte, the alkaline environment further amplifies these advantages by stabilizing surface states and minimizing charge trapping. At an applied voltage of 0.5 V, the electric field effectively separates charge carriers without inducing significant recombination or surface instability, leading to the highest observed photocurrent density. The superior ionic conductivity of NaOH also facilitates oxidation reactions at the photoanode surface, thus improving the overall PEC performance of the FeNi composite. The FeNi photoanode demonstrated outstanding stability during extended testing, maintaining a consistent photocurrent output for over 300 s under applied bias conditions. In the NaOH electrolyte, the photoanode exhibited stable photocurrent without significant degradation, indicating strong interaction with the electrolyte and effective suppression of recombination. The absence of photocurrent overshoots upon switching to the light ON state highlights the rapid and efficient utilization of photogenerated charge carriers. This responsiveness is essential for practical applications requiring stable and reproducible performance under dynamic light conditions. A comparative analysis of the two electrolytes revealed that NaOH offers a more favorable environment for PEC activity than Na₂SO₄. The alkaline medium enhances photocurrent generation, stability, and charge transport efficiency, establishing it as the preferred choice for solar-driven water-splitting applications. The significant performance of the FeNi composite in NaOH underscores its potential as a high-efficiency, durable photoanode material for renewable energy applications.

Additionally, the photocurrents were monitored over 300 s in the Na₂SO₄ and NaOH electrolytes, with no switching behavior observed (Figure 10a–f). The results showed that the induced photocurrents were consistently generated under steady-state conditions without significant fluctuations or overshoots, emphasizing the photoanode material stability during prolonged illumination in both electrolytes. An increase in the applied bias potential resulted in higher photocurrent density, indicating that elevated potentials facilitate more efficient separation and transport of the photogenerated charge carriers in both electrolytes. This behavior aligns with the expected photoanode response to the applied electric field, where higher potentials drive electrons and holes to the external circuit and photoanode surface, respectively, enhancing overall photocatalytic performance. Photocurrent generation is significantly influenced by the applied bias potential, demonstrating that photocatalytic efficiency depends on how well the applied electric field can promote charge separation and transport in the photoanode material. However, the photoanodes in NaOH exhibited more stable and higher photocurrent generation than those in Na₂SO₄ (Figure 10a–f). As mentioned earlier, this may be attributed to the enhanced ionic conductivity, lower resistance, and superior capacitance of the photoanodes in NaOH. These findings underscore the importance of optimizing the applied potential and the electrolyte composition to maximize photocurrent generation and ensure stable, high-performance operation of the photoanode materials in practical photoelectrochemical applications.

A comprehensive post-analysis of the electrochemical behavior of the anodes was performed in the Na₂SO₄ and NaOH electrolytes under similar experimental conditions to ensure a valid comparison. Nyquist plots were recorded for both dark and illuminated states (Figure 11a–f), revealing two key features: a semicircular arc in the high-frequency region, representing charge transfer resistance, and a sloped line in the low-frequency region, indicating ionic diffusion at the photoelectrode–electrolyte interface.

Under illumination, the anodes consistently exhibited a smaller semicircle radius than that under the dark state, confirming that light exposure enhances charge generation. However, the arc radius in Na₂SO₄ was significantly larger than in NaOH, reflecting the distinct properties of the electrolytes. Sulfate-based electrolytes typically exhibit lower ionic conductivity than that of alkaline electrolytes, resulting in increased charge transfer resistance. Additionally, the SO₄²⁻ ions in Na₂SO₄ interact more strongly with the electrode surface than the OH⁻ ions in NaOH, potentially causing sulfate ion adsorption or structural modifications. These interactions hinder charge carrier mobility and impede electrochemical reactions, contributing to the higher resistance observed in Na₂SO₄, as reflected in the larger arc radius in the Nyquist plot. To gain deeper insight, the Nyquist plots were analyzed using an equivalent circuit model (inset of Figure 11a,d), and

Table 3. The anodes’ Nyquist-fitted parameters were recorded after analysis in 0.1 M NaOH and 0.1 M Na₂SO₄ electrolytes under ON/OFF states.

Electrode	Electrolyte 0.1 M	Condition	RAo (Ω)	RAb (Ω)	RAe (kΩ)	CAbc (μF)	CAd (μF)
FeWO4 nanoparticles	Na2SO4	Dark	31.62	383.20	9.12	18.25	28.63
		Light	28.95	379.56	8.12	23.56	36.54
NiFeOOH nanostructures		Dark	29.65	48.65	7.59	14.65	128.65
		Light	26.54	43.72	6.12	17.98	140.21
FeWO4/NiFeOOH		Dark	26.89	27.76	4.10	31.27	62.37
		Light	23.51	25.11	4.01	35.78	69.14
FeWO4 nanoparticles	NaOH	Dark	10.41	19.36	20.21	55.56	65.52
		Light	9.89	18.01	16.15	61.17	72.21
NiFeOOH nanostructures		Dark	7.58	16.74	5.68	20.11	132.52
		Light	6.19	13.28	5.08	29.26	140.89
FeWO4/NiFeOOH		Dark	5.23	12.09	3.99	76.16	109.33
		Light	4.77	9.14	3.18	79.78	119.78

summarizes the extracted fitted values. In this model, RAo, RAb, RAe, film capacitance (CAbc), and double-layer capacitance (Cad) represent the solution resistance, internal electrode resistance, charge transfer resistance at the electrode–electrolyte interface, film capacitance, and double-layer capacitance, respectively. The RAo values for the anodes in dark and illuminated states in Na₂SO₄ and NaOH were consistently higher than the corresponding Ro values, likely owing to variations in the electrode surface state. Among the tested anodes, the FeNi anode exhibited the lowest resistance values in both electrolytes, outperforming the Fe and Ni anodes. Moreover, the FeNi anode exhibited lower resistance in NaOH than in Na₂SO₄. The RAo values for the FeNi anode under dark and illuminated conditions were 26.89, 5.23, 23.51, and 4.77 Ω (Table 3). These values were higher than the corresponding Ro values (26.67, 4.86, 22.53, and 3.64 Ω, respectively, Table 1), indicating a reduction in the energetic states of the electrode surface in both electrolytes. The RAb values, representing the path resistance within the electrode, were consistently higher than the corresponding Rb values for all anodes under both dark and illuminated states. This increase may be attributed to the adsorption and desorption of ions or molecules that influence the conductive pathways within the anodes. Ion accumulation or depletion of ions during repeated electrochemical reactions can alter these pathways, leading to variations in internal resistance. Among all the anodes, FeNi exhibited the lowest path resistance in both electrolytes, particularly in NaOH, highlighting its superior ionic transport. The RAb values for FNi were 27.76, 12.09, 25.11, and 9.14 Ω under dark and illuminated conditions (Table 3), slightly exceeding the corresponding Ro values of 24.53, 11.57, 26.12, and 8.94 Ω (Table 1). The RAe values, corresponding to the charge transfer resistance at the electrode–electrolyte interface, were higher than the Re values in both dark and illuminated states. This finding suggests that the accumulation of reaction byproducts or oxide layer formation during electrochemical cycling may impede electron transfer. Additionally, the depletion or redistribution of reactive species near the electrode surface could limit ion availability, thereby lowering the charge transfer efficiency. FeNi demonstrated the lowest charge transfer resistance in the NaOH and Na₂SO₄ electrolytes, with lower resistance in NaOH. This improved performance is likely attributed to the enhanced ionic conductivity of OH⁻ in NaOH, which facilitates more efficient ionic transport at the electrode–electrolyte interface. Under dark and illuminated conditions, the RAe values for FeNi were 4.10, 3.99, 4.01, and 3.18 kΩ (Table 3), while the corresponding Re values were 3.98, 3.31, 3.62, and 2.98 kΩ (Table 1). The slight increase in resistance during cycling may result from surface modifications or localized ion depletion near the electrode. The CAbc and Cad values were generally lower in both electrolytes than the corresponding Cbc and Cd values. This reduction in capacitance may arise from surface modifications during electrochemical cycling, which alters the ion adsorption capacity of the electrode. Furthermore, changes in electrolyte composition near the electrode–electrolyte interface—such as ion depletion or shifts in ionic species—could disrupt the stability of the electrical double layer, leading to diminished capacitance. Accumulation of reaction products may further impede ion diffusion, and interface polarization, which becomes more pronounced during cycling, could lower the effective charge storage capacity of the electrode. FeNi exhibited increased CAbc and CAd values compared to those of Fe and Ni, likely due to its optimized material composition, which enhances electron transfer and reduces internal resistance. This results in a more stable electrochemical network. Moreover, FeNi showed higher CAbc and CAd values in NaOH than those in Na₂SO₄, likely owing to the higher ionic conductivity of OH⁻ in NaOH, which promotes improved ion interactions and electron flow. The CAbc and CAd values for the FeNi anode ranged from 31.27 to 79.78 μF and 62.37 to 1119.78 µF under dark and illuminated conditions, respectively, which were lower than the corresponding Cbc and Cd values of 32.56–82.65 μF and 65.35–125.64 µF (Table 1 and Table 3). The differences in electrochemical performance between NaOH and Na₂SO₄ can be attributed to the specific ionic properties of their electrolytes—OH⁻ in NaOH and SO₄²⁻ in Na₂SO₄. The OH⁻ ions in NaOH have smaller ionic radii and higher ionic mobility than those of the SO₄²⁻ ions in Na₂SO₄. This leads to more efficient ionic diffusion and enhanced ionic conductivity in NaOH, facilitating faster charge transfer at the electrode–electrolyte interface. This is evidenced by the sharper impedance drop with increasing frequency in NaOH, suggesting more rapid charge transfer kinetics. Additionally, OH⁻ ions exhibit a lower tendency to form strong bonds with the electrode surface, reducing surface passivation and preserving active sites for charge transfer. Consequently, NaOH shows lower impedance values and smaller phase angles across the frequency spectrum, further confirming its superior electrochemical performance. In contrast, SO₄²⁻ ions in Na₂SO₄, with their larger ionic radii and lower mobility, hinder ionic diffusion and increase resistance to charge transfer. This is reflected in the higher impedance and phase angle values observed across the frequency spectrum in Na₂SO₄. The stronger interactions of SO₄²⁻ ions with the electrode surface may lead to adsorption or the formation of surface layers, increasing charge transfer resistance and impeding ionic movement. These factors contribute to the higher impedance and phase angles observed in Na₂SO₄, indicating less efficient charge transfer and slower electrochemical dynamics. The τ_e values in NaOH and Na₂SO₄ are slightly lower in the updated analysis than in the initial values, likely due to electrochemical stability factors and ionic interactions. In NaOH, the electron lifetime decreased from 4.72 × 10⁻⁶ s to approximately 4.55 × 10⁻⁶ s. This reduction in electron lifetime may result from prolonged cycling or the gradual accumulation of surface passivation layers, which impede electron transport over time. Although NaOH maintains relatively stable charge dynamics, this decrease in electron lifetime suggests minor performance degradation as cycling progresses. In contrast, Na₂SO₄ shows a slightly higher electron lifetime of 4.57 × 10⁻⁶ s than that of NaOH, but this is still lower than its initial value of 4.62 × 10⁻⁶ s. This reduction reflects the combined effects of slower ionic diffusion and increased surface interactions involving the larger SO₄²⁻ ions, which accelerate electron recombination and reduce charge carrier stability over time. Initially, the higher ionic mobility of OH⁻ in NaOH supported a more stable electron lifetime. However, as cycling-induced degradation affects both electrolytes, the reductions in electron lifetime underscore the effects of the ionic environment and surface interactions on the long-term electrochemical performance. In overextended cycling, the performance of both electrolytes deteriorates. However, NaOH outperforms Na₂SO₄ owing to its higher ionic conductivity, lower impedance, and reduced surface passivation effects. The superior mobility of OH⁻ ions in NaOH facilitates efficient charge transfer, minimizes recombination losses, and prevents significant impedance buildup. In contrast, Na₂SO₄ experiences a more pronounced increase in impedance and phase angles, suggesting a greater electrochemical performance decline over time. Thus, NaOH emerges as the more favorable electrolyte for high-performance electrochemical applications due to its ability to support faster ionic movement, mitigate passivation effects, and maintain lower impedance and phase angles.

4. Conclusions

This study examines the electrochemical behavior of Fe, Ni, and their composite FeNi electrodes, focusing on the effects of SO₄²⁻ and OH⁻ ions in Na₂SO₄ and NaOH electrolytes, respectively. The findings indicate the critical influence of electrolyte ionic properties on charge transfer dynamics and overall system efficiency. In NaOH, the smaller ionic radius and higher mobility of OH⁻ ions enhanced ionic conductivity, improved charge transfer kinetics, and minimized recombination losses. These properties led to superior photocurrent generation and capacitive behavior in the FeNi composite, reflecting efficient charge separation and transport mechanisms. In comparison, Na₂SO₄ exhibited slower ionic diffusion and increased charge transfer resistance, primarily due to the larger size of SO₄²⁻ ions and their stronger interactions with the electrode surface. The FeNi composite showed markedly improved functionality in NaOH, with significantly lower charge transfer resistance and higher photocurrent density, emphasizing the essential role of OH⁻ ions in enabling efficient energy conversion processes. The synergistic interaction between Fe and Ni within the composite further enhanced catalytic activity, particularly in OH⁻-rich environments, demonstrating the potential of the FeNi composite for advanced energy conversion applications. Moreover, the electron lifetime (τ_e) in NaOH decreased slightly to 4.55 × 10⁻⁶ s owing to cycling effects but remained stable, supporting efficient and sustained charge transport. Conversely, Na₂SO₄ exhibited a reduction in τ_e to 4.57 × 10⁻⁶ s, indicating slower ionic mobility and higher recombination rates. These findings highlight the superiority of OH⁻ ions in promoting long-term electrochemical stability and overall functionality, positioning the FeNi composite as a promising candidate for energy conversion technologies.