Ternary Nickel-Iron-Phosphorus (NiFeP) Electrocatalysts for Alkaline Water Splitting

Abstract

In this study, ternary NiFeP coatings were fabricated on a copper substrate using a simple, fast, and cost-effective electroless deposition method. The coatings were named Ni₈₅Fe₄P₁₂, Ni₈₀Fe₈P₁₂, and Ni₇₅Fe₁₂P₁₂, indicating 4, 8, and 12 at % of Fe, respectively. The surface morphology and composition of the coatings were characterized using scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX). The activity of the prepared coatings was evaluated using the water-splitting reaction to determine the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in a 1 M KOH electrolyte solution. Electrochemical measurements were carried out in a temperature range from 25 °C to 55 °C. The HER and OER current density values increased by up to 2.58 and 2.13 times, respectively, with temperature increase compared to the result at 25 °C. All three coatings demonstrated activity in both reactions. Ni₈₅Fe₄P₁₂ exhibited the highest catalytic efficiency in the HER, with the overpotential of 340 mV at 10 mAcm⁻² and a Tafel slope of 61 mVdec⁻¹. In the OER, the efficiency of the NiFeP catalysts correlated with their Fe content. The overpotential was 412 mV for Ni₈₀Fe₈P₁₂ and 432 mV for Ni₇₅Fe₁₂P₁₂ at 10 mAcm⁻² with Tafel slopes of 96 and 91 mVdec⁻¹, respectively. This study underscores the critical influence of Fe content on the catalytic efficiency of NiFeP coatings, with reduced Fe content enhancing HER and increased Fe content benefits OER.

1. Introduction

Ever-increasing the world’s population (8.2 billion in 2025) and environmental pollution demand the development of novel energy sources to replace fossil fuels [1]. The European Commission’s initiative to curtail greenhouse gas emissions by 55% by the year 2030 served as a catalyst for the issuance of a directive that advocated for an augmentation in the consumption of renewable energy sources, with a proposed range of 38–40% [2]. One of the steps to reduce carbon dioxide emissions is wind/solar cells, metal–air batteries, different fuel cells, and geothermal energy use in the industry and households. However, nowadays, the focus is on producing green hydrogen energy as part of the global energy landscape. There is a huge amount of hydrogen used by industry for refining petroleum, treating metals, producing fertilizer and other chemicals, and processing foods, however it is still produced by steam reforming of methane, which uses methane natural gas and the resulting secondary reaction products are carbon monoxide and carbon dioxide [3]. Therefore, green hydrogen (H₂) has emerged as the most promising energy carrier for decarbonizing the industrial, building, and transportation sectors. Despite the production of low-emission hydrogen growth—almost 10% in 2023—its total output remains less than 1 Mt [4]. The attention has been focused on the water electrolysis process. However, despite the positive aspects of this technology, the production of hydrogen by the electrolysis method remains expensive due to the high cost of electricity required to carry out the electrolysis reaction and low efficiency. Considering the goals set by the European Commission, one of the areas of clean energy technology that is given priority in scientific activities is the water splitting reaction for the production of hydrogen and the catalysts used for it [2].

The electrochemical water splitting reaction (1) consist with the hydrogen evolution reaction (HER) designated as the cathodic reaction and the oxygen evolution reaction (OER) referred to as anodic reaction can be performed in an alkaline (2–3) or acidic (4–5) medium. In general, the utilization of an acidic medium has been demonstrated to expedite reaction kinetics and reduce the overpotential requirement. However, this approach necessitates the employment of costly, high-performance catalyst materials, such as noble metals. A corrosive environment limits on the available options for catalysts, thereby influencing their durability. Additionally, it necessitates the incorporation of corrosion-resistant hardware. Conversely, the employment of an alkaline medium facilitates the utilization of a more extensive array of non-noble catalysts, including but not limited to Ni, Fe, and Co. The adoption of more economical materials is possible for both catalyst and hardware components. Metals exhibit enhanced stability in their base state. However, the utilization of an alkaline medium has been demonstrated to elevate the energy barrier, a consequence of water dissociation. Consequently, the kinetics undergo a decline, particularly with regard to the Volmer step. Depending on the medium, the cathodic half-reaction proceeds through the reduction in hydronium ions (H₃O⁺) in acidic media or water molecules (H₂O) in alkaline media [5,6]. In the context of alkaline media, the OER, which entails a four-electron transfer process (2), is identified as the rate-limiting step in the water splitting process due to its sluggish kinetics. Conversely, the HER (3) entails the reduction in protons to hydrogen gas, a process that is generally considered to be faster [7,8]:

$${2H}_{2}O \to 2H_2+O_{2 } \hspace{1em}\hspace{1em}{ \hspace{1em}E}^0=-1.23 \mathrm{V}$$

(1)

Alkaline media:

$$4O{H}^{-} \to H_{2}O + O_2 + 4e^{-} E^0=-0.40 \mathrm{V}$$

(2)

$${4H}_{2}O+4e^{-}\to 2H_2+4{OH}^{-}{ \hspace{1em}\hspace{1em}E}^0=-0.83 \mathrm{V}$$

(3)

Acidic media:

$$2H_{2}O \to O_2+4H^{+}+4e^{-}{\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}E}^0=-1.23 \mathrm{V}$$

(4)

$$4H^{+}+4e^{-} \to 2H_2\hspace{1em} \hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}{E}^0=0.00 \mathrm{V}$$

(5)

The enhancement of the efficiency of both OER and HER is imperative for the advancement of overall water electrolysis performance. A promising approach to achieve this objective entails the utilization of highly active and stable electrocatalysts, which have the potential to reduce the overpotential and enhance the reaction kinetics for both reactions.

In the 1970s, Miles and Thomason published a series of studies that evaluated the catalytic activity of various metals and their compounds for water splitting in both acidic and alkaline environments. These publications laid the foundation for future research in the field [9,10]. Noble metal catalysts, including platinum (Pt), iridium (Ir), and ruthenium (Ru), have long been regarded as the gold standard for electrocatalysis due to their exceptional activity and stability. Pt is widely recognized as the most effective HER catalyst due to its optimal hydrogen adsorption free energy, which facilitates rapid proton reduction and desorption [11]. Similarly, Ir- and Ru-based catalysts exhibit outstanding performance for water splitting, not only in their oxide forms (IrO₂, RuO₂) [12] but also in homogeneous and heterogeneous configurations. Homogeneous molecular catalysts incorporating Ir and Ru complexes offer tunable electronic structures and well-defined active sites, while heterogeneous systems combining these noble metals with transition metals, chalcogenides, or phosphides exhibit synergistic effects that enhance catalytic performance [11,13,14]. Despite their effectiveness, the scarcity and high cost of noble metals significantly limit their large-scale application, highlighting the urgent need for earth-abundant and cost-effective alternatives [11,14,15,16]. The focus of research was shifted to the transition metals and their compounds as oxides, sulfides, phosphides as well as bifunctional catalysts capable of catalyzing both HER and OER, which could simplify practical application and benefit the industrialization of energy sources. A significant body of research has been dedicated to the investigation of transition metals, including Co, Ni, Fe, Cu, Mo, W, and V, along with their compounds. The primary objective of these studies is to enhance the activity of water-splitting reactions [17,18,19,20,21].

Transition metal-based catalysts, particularly nickel-iron (NiFe) materials, have emerged as some of the most promising candidates for alkaline water electrolysis. Nickel-based compounds have been shown to exhibit high catalytic activity due to their favorable electronic structure and redox flexibility [22]. Furthermore, iron incorporation has been demonstrated to enhance their efficiency through synergistic effects [16,23]. Scientists have demonstrated a strong interest in further enhancing the electronic properties, active site accessibility, and catalytic activity of NiFe catalysts. This pursuit has led to the incorporation of reactive nonmetals, including sulfur (S) [24,25], phosphorus (P) [25], and carbon (C) [26].

In the realm of scientific research, transition metal phosphides (TMPs) have emerged as a subject of significant interest in recent decades. This interest can be attributed, at least in part, to the inherent properties of these compounds, which include their abundance in the earth’s crust and their versatility in applications across diverse catalytic domains, such as thermocatalysis [27,28,29], photocatalysis [30,31,32], and electrocatalysis [33,34,35,36,37]. TMPs exhibit a wide range of compositions and structures, with the most prevalent types being binary, ternary, and more complex phases. TMPs derived from transition metals, including cobalt (Co), nickel (Ni), and iron (Fe), have demonstrated remarkable efficacy as catalysts for both the hydrogen evolution reaction and the oxygen evolution reaction [38,39]. The P. Jiang group studied FeP catalyst deposited on Ti nanowires. The presented results showed exceptionally high catalytic activity, reaching overpotential of 55 mV for 10 mAcm⁻² and Tafel slope of 38 mVdec⁻¹ at scan rate of 2 mVs⁻¹ in a 0.5 M H₂SO₄ solution for HER [40]. Ni₂P nanoparticles obtained on glassy carbon prepared by thermal reaction have been studied by the L. Feng group [41]. The results demonstrate that a maximum potential of approximately 225 mV for 10 mAcm⁻² and Tafel slope of 102 mVdec⁻¹ was observed at scan rate of 5 mVs⁻¹ in a 1 M KOH solution for HER. Moreover, NiFe phosphides that exhibited high efficiency for water splitting were presented. The H. Chen group investigated NiFeP coatings that were electrolessly deposited on palladium-coated filter paper. They calculated an overpotential of 158 mV for HER and 282 mV for OER in an alkaline environment to deliver a current density of 10 mAcm⁻² [35]. Furthermore, the G. Dong group presented NiFeP/NF catalysts deposited on a Ni foam substrate with an overpotential of 102 mV for HER and a Tafel slope of 101 mVdec⁻¹. For OER, the authors refer to an overpotential of 216 mV and a Tafel slope of 28.9 mVdec⁻¹ to deliver a current density of 10 mAcm⁻² in a 1 M NaOH solution [36]. The versatility, tenability, and tunability of these catalysts make them promising candidates for improving the overall efficiency of water splitting and other electrochemical processes.

In this study, we present the synthesis of ternary TMPs- NiFeP catalysts with elemental compositions of Fe atomic percentage 4%, 8%, and 12% and constant percentage of P—12%, followed by a comparative analysis of their properties and application for water splitting. The NiFeP coatings (Ni₈₅Fe₄P₁₂, Ni₈₀Fe₈P₁₂, and Ni₇₅Fe₁₂P₁₂) were prepared using the electroless deposition method. A thorough examination of the morphology and composition of the resulting coatings was conducted using scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDX) analysis. Furthermore, the catalytic activity of the combination of transition metals (Ni, Fe) and the non-metal (P) for water splitting reactions, specifically the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER), was assessed in an alkaline condition (1 M KOH) using linear sweep voltammetry (LSV).

2. Materials and Methods

2.1. Chemicals

The following materials were used for the experiments: copper sheet (Cu, 99.8% purity), glycine (NH₂CH₂COOH, 99%), nickel (II) sulfate (NiSO₄·6H₂O, 99%), disodium malonate (CH₂(COONa)₂, 97.0%), sodium hypophosphite (NaH₂PO₂·H₂O, ≥99%), iron (II) sulfate (FeSO₄·7H₂O, 99%), ethylenediaminetetraacetic acid (C₁₀H₁₆N₂O₈, ≥98%), sodium citrate (Na₃C₆H₅O₇, ≥99%) supplied by Sigma–Aldrich (Taufkirchen, Germany). Palladium chloride (PdCl₂, 99.95%) and potassium hydroxide (KOH, 98.8%) were purchased from Chempur Company (Karlsruhe, Germany).

2.2. Preparation of Coatings

NiFeP coatings were deposited onto copper (Cu) substrates using an electroless plating method. Prior to the deposition process, the 1 cm × 1 cm Cu sheets underwent a mechanical surface cleaning procedure. This process involved treatment with 50–100% calcium magnesium oxide, commonly referred to as “Vienna Lime” (Kremer Pigmente GmbH and Co. KG, Aichstetten, Germany). This was followed by rinsing with deionized water to ensure the removal of any residual impurities. Subsequently, the cleaned Cu surfaces were subjected to activation by means of it immersion in a 0.5 g·L⁻¹ PdCl₂ solution for a duration of 10 s, thereby introducing Pd(II) ions. Thereafter, the surfaces were rinsed once more with deionized water. Subsequent to activation, the Cu sheets were immersed in electroless plating baths at a temperature of 85 °C for a duration of 15 min, as depicted in Figure 1. The detailed electroless deposition solutions are presented in

Table 1. Electroless deposition conditions for prepared coatings.

Coatings	Glycine, M	Disodium Malonate, M	Sodium Hypophosphite, M	EDTA, M	Sodium Citrate, M	Ni2+, M	Fe2+, mM	pH	T, °C	t, min
Ni85Fe4P12	0.4	0.098	0.301	0.07	0.15	0.107	54	9	85	15
Ni80Fe8P12	0.4	0.098	0.301	0.07	0.15	0.107	22	10	85	15
Ni75Fe12P12	0.4	0.098	0.301	0.07	0.15	0.107	54	10	85	15

. It is seen that the formation of the coatings depends on the Fe²⁺ concentration and the pH of the solution in the plating bath. The evaluated optimal pH value of plating solution was achieved and measured during the preparation of solution and further not controlled. As a consequence, catalysts with different Fe contents of 4−12 at.% were prepared comprising the following compositions Ni₈₅Fe₄P₁₂, Ni₈₀Fe₈P₁₂, and Ni₇₅Fe₁₂P₁₂. It is evident that an increase in iron content results in a concomitant decrease in nickel content within the coating. The thickness of NiFeP coatings was determined gravimetrically and found to be 0.95, 1.29, and 1.02 µm for the Ni₈₅Fe₄P₁₂, Ni₈₀Fe₈P₁₂, and Ni₇₅Fe₁₂P₁₂, respectively.

2.3. Characterization of Coatings

A tabletop scanning electron microscope (SEM) (TM4000Plus, HITACHI, Chiyoda City, Tokio, Japan), equipped with an energy-dispersive X-ray (EDX) spectrometer (Oxford Instruments, Abingdon, UK), was used to examine the morphology and composition of the prepared coatings.

X-ray photoelectron spectroscopy (XPS) was used to analyze a series of NiFeP catalysts, with measurements carried out on a Kratos AXIS Supra+ (with device control program ESCApe 1.6.1.1234; Kratos Analytical, Manchester, UK) spectrometer equipped with a monochromatic Al Kα X-ray source (1486.6 eV) operating at 225 W. The pressure within the analysis chamber was maintained below 1 × 109 mbar, and a low-energy electron flood gun served for charge compensation. Survey scans were acquired at a pass energy of 160 eV with 1 eV energy steps, while high-resolution scans were collected at a 20 eV pass energy with 0.1 eV steps focused on specific elemental peaks. The binding energy scale was referenced to the C 1s peak of adventitious carbon at 284.8 eV. Collected XPS data were converted into VAMAS format and analyzed using CasaXPS 2.3.26PR1.0 software (Casa software Ltd., Teignmouth, Devon, UK).

2.4. Electrochemical Measurements

Electrochemical measurements were performed using an a PGSTAT100 potentiostat/galvanostat (MetrohmAutolab B. V., Utrecht, The Netherlands) in a three-electrode system comprising an Ag/AgCl (3 M KCl) reference electrode, a platinum (Pt) counter electrode, and prepared NiFeP coatings as the working electrodes, with 1 M potassium hydroxide (KOH) serving as the electrolyte.

HER and OER polarization curves were recorded at set potentials from −1.5 to 0.0 V (vs. Ag/AgCl) and 0.0 to 1.0 V (vs. Ag/AgCl), respectively, at a scan rate of 2 mVs⁻¹. Polarization curves for each coating were measured at 4 temperatures ranging from 25 to 55 °C (with a temperature increase of 10 °C each time) in a double-walled jacketed cell connected to a Lauda Alpha RA 8 thermostat. The kinetics of the electrocatalysts was investigated through Tafel plots. The Tafel equation comes from the Arrhenius and Nernst Equation (6), providing information about the relationship between current and overpotential during oxidation or reduction processes on the electrode surface:

$$\eta =a+blogj$$

(6)

where $\eta$—applied overpotential (V), $a$—curve intercept (V), $b$—Tafel slope (V dec⁻¹), and $j$ (A cm⁻²)—current density [42].

Chronoamperometry (CA) analysis has been used to evaluate the long-term stability of the fabricated catalysts. The chronoamperometry curves were recorded in 1 M KOH solution for 10 h at a static potential of −0.44 V (vs. RHE) for HER and of 1.93 V (vs. RHE) for OER. A multi-step chronopotentiometry test was carried out with current ranging from −10 to −100 mAcm⁻² for HER and from 10 to 100 mAcm⁻² for OER.

All potentials in this work were converted to the reversible hydrogen electrode (RHE) scale using the following equation E_RHE = E_Ag/AgCl + 0.1976 V + 0.059 × pH_electrolyte. Current density was calculated using the working electrode geometric area of 2 cm².

In this work, IR compensation correction was implemented for all graphs.

3. Results and Discussion

3.1. Coatings, Microstructure and Morphology Studies

The present work delineates three-component NiFeP coatings with varying amounts of Fe deposited on Cu sheets via a straightforward electroless deposition process. This process employs sodium hypophosphite as the reducing agent in a citrate-based solution. Three distinct coatings were deposited on Cu sheets serving as working electrodes. The elemental composition of the coatings was determined via EDX analysis, with measurements obtained from three zones of each coating. The resulting standard deviation values are presented in

Table 2. EDX analysis results of Ni₈₅Fe₄P₁₂, Ni₈₀Fe₈P₁₂ and Ni₇₅Fe₁₂P₁₂.

Coating	Elemental Composition, at.%
	Ni	Fe	P
Ni85Fe4P12	84.56 ± 0.66	3.85 ± 0.14	11.59 ± 0.17
Ni80Fe8P12	80.26 ± 0.41	7.85 ± 0.24	11.89 ± 0.25
Ni75Fe12P12	75.48 ± 0.70	12.42 ± 0.14	12.10 ± 0.22

. The composition of each coating is as follows: 4 at% Fe in the Ni₈₅Fe₄P₁₂, 8 at% Fe in the Ni₈₀Fe₈P₁₂, and 12 at% Fe in the Ni₇₅Fe₁₂P₁₂. It is evident that an increase in iron content results in a concomitant decrease in nickel content within the coating. The thickness of the NiFeP coatings was determined gravimetrically and found to be 0.95 µm, 1.29 µm, and 1.02 µm for the Ni₈₅Fe₄P₁₂, Ni₈₀Fe₈P₁₂, and Ni₇₅Fe₁₂P₁₂ coatings, respectively. A thorough examination of the fabricated Ni₈₅Fe₄P₁₂, Ni₈₀Fe₈P₁₂, and Ni₇₅Fe₁₂P₁₂ coatings through scanning electron microscopy (SEM) revealed that the morphology of the prepared coatings is somewhat different (Figure 2). Ni₈₅Fe₄P₁₂ (Figure 2a) and Ni₇₅Fe₁₂P₁₂ (Figure 2c) coatings are characterized by oval-shaped agglomerates and a rough and evenly distributed surface. Ni₈₀Fe₈P₁₂ (Figure 2b) coating is characterized by an evenly distributed surface without significant agglomeration.

Subsequently, XPS spectra analysis was conducted to ascertain the chemical composition of the prepared NiFeP coatings and to evaluate the interaction between atoms in the coatings. Figure 3 reports the X-ray photoelectron spectra of the Ni 2p (Figure 3a–c), O 1s (Figure 3d–f), and P 2p (Figure 3g–i) electron levels in the Ni₈₅Fe₄P₁₂, Ni₈₀Fe₈P₁₂, and Ni₇₅Fe₁₂P₁₂ coatings. As illustrated in Figure 3a–c, two prominent peaks are evident in the Ni 2p spectra, with binding energies of 852.5 ± 0.5 eV and 869.7 ± 0.5 eV. The calculated split spin–orbit component of Δ_metal = 17.2 eV was assigned to Ni⁰ 2p_3/2 and Ni⁰ 2p_1/2, respectively [43,44]. The Ni 2p₃/₂ peaks with lower intensity, observed at binding energies between 853.2 and 855.5 eV, were attributed to Ni²⁺, consistent with previous reports [43] (Figure 3a–c). As demonstrated in Figure 3d–f, two peaks emerge at binding energies of 129.4 ± 0.5 eV and 130.4 ± 0.4 eV, accompanied by a split spin–orbit component of Δ_element = 0.86 ± 0.5 eV, as indicated by the P 2p spectra. The peaks of the P 2p spectra originating at binding energies of 128.7–130.4 ± 0.5 eV can be attributed to the formation of metal-phosphides in the coating [45,46]. Moreover, a broad peak in the P 2p spectra at the binding energies ranging from 132 to 135 ± 0.5 eV, as observed for the catalysts Ni₈5Fe₄P₁₂ and Ni₈₀Fe₈P₁₂ may be attributable to the presence of metal phosphates with a binding energy of 133.9 eV related to the P–O bond in PO₄³⁺caused by surface oxidization [45,46]. It has been observed that the Ni₇₅Fe₁₂P₁₂ coating exhibits an absence of this particular peak, which could be indicative of its elevated crystallinity. As shown in Figure 3g–i, the O 1s XPS spectra display spin–orbit splitting peaks at binding energies ranging from 530.7 to 531.7 eV, which are attributed to the metal oxides and hydroxides. The dominant peak at 532.4 ± 0.5 eV is likely associated with oxygen species chemisorbed by the surface [43,44,47]. However, despite the presence of a small amount of Fe in the coatings, the Fe 2p signal was not detected. Theoretically, this may be due to the overlap or interference from the Ni LMM Auger peaks, which can potentially obscure the Fe 2p spectra [48,49]. Auger peaks, which arise from secondary electron transitions and can overlap with core-level signals of other elements. The Ni LMM Auger peak is typically located in the energy range of ~710–720 eV, depending on the oxidation state and environment. This range directly overlaps with the Fe 2p region, especially the Fe 2p₃/₂ and Fe 2p₁/₂ doublets [50].

3.2. Hydrogen Evolution Reaction Study

The hydrogen evolution reaction is undoubtedly related to H₂ production. Fundamentally HER is divided in to three steps occur on the electrode surface, i.e., primary hydrogen atoms adsorption (7), electrochemical desorption of H₂ (8) and chemical desorption of H₂ (9) from the cathode [51]:

$$\mathrm{M} + {\mathrm{H}}_2\mathrm{O} + {\mathrm{e}}^{-}\to \mathrm{M}\mathrm{H}* + \mathrm{O}{\mathrm{H}}^{-}\hspace{1em}\hspace{1em}(Volmer mechanism, b ~ 120 {\mathrm{mVdec}}^{-1})\hspace{1em}$$

(7)

$$MH*+{\mathrm{H}}_2\mathrm{O}+{\mathrm{e}}^{-}\to {\mathrm{H}}_2 + M + \mathrm{O}{\mathrm{H}}^{-} (Heyrovsky mechanism, b ~ 40 {\mathrm{mVdec}}^{-1})$$

(8)

$$\mathrm{MH}*+ \mathrm{MH}*\to {\mathrm{H}}_2 + 2M\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}(Tafel mechanism, b ~ 30 {\mathrm{mVdec}}^{-1})$$

(9)

where M—electrocatalyst, MH*—adsorbed hydrogen on electrocatalysts surface.

Each step is (7–9) is described by the theoretical Tafel slope, which reflects the rate of reaction—the lower the Tafel slope value, the faster the reaction kinetics.

We initially investigated the catalytic activity of our prepared Ni₈₅Fe₄P₁₂, Ni₈₀Fe₈P₁₂, and Ni₇₅Fe₁₂P₁₂ coatings towards the hydrogen evolution reaction using the LSV method in 1 M KOH at temperatures ranging from 25 to 55 °C with a 10 °C increase. An increase in the temperature of the hydrogen evolution reaction resulted in a higher current density, likely due to improved ion mobility or reduced activation energy. As a result, the current density increased by a factor of 2.11 to 2.58 compared to its initial value (Figure 4a–c). Furthermore, the LSV analysis of the pure Pt electrode was performed under identical conditions at 25 °C for the purpose of comparison. The onset potential of HER taken at −1 mAcm⁻² current density value has been obtained at −0.254; −0.308, −0.277, and −0.019 V for Ni₇₅Fe₁₂P₁₂, Ni₈₀Fe₈P₁₂, Ni₈₅Fe₄P₁₂, and Pt, respectively. The findings indicated that the Ni₈₅Fe₄P₁₂ coating with the minimal amount of Fe exhibited the highest current density for HER, in comparison to the Ni₈₀Fe₈P₁₂, and Ni₇₅Fe₁₂P₁₂ coatings. At a temperature of 25 °C, the current densities attained at the end of experiment were −66.8, −45.2, and −38.2 mAcm⁻², respectively (Figure 4d). In addition, the pure Pt electrode exhibited a current density of −102.1 mAcm⁻² at the same conditions. Furthermore, all three Ni₈5Fe₄P₁₂, Ni₈₀Fe₈P₁₂, and Ni₇₅Fe₁₂P₁₂ investigated coatings exhibited an overpotential of 340 mV, 381 mV and 361 mV, respectively, enabling the achievement of a 10 mAcm⁻² current density (Figure 4f). Additionally, the Tafel slope values were determined to be 61, 70, and 106 mVdec⁻¹ for the prepared Ni₈₅Fe₄P₁₂, Ni₈₀Fe₈P₁₂, and Ni₇₅Fe₁₂P₁₂ coatings, indicating the Volmer–Heyrovsky mechanism for the reactions (Figure 4e). The Tafel slope value for Pt was determined to be 60 mVdec⁻¹ with the overpotential of 78 mV at a current density of 10 mAcm⁻². Undoubtedly, it is difficult to outperform the activity of Pt-supported catalysts. However, our prepared Ni₈₅Fe₄P₁₂ catalyst demonstrated Tafel slopes very close to those of pure Pt. Therefore, the long-term stability of the Ni₈₅Fe₄P₁₂ catalyst was evaluated using chronoamperometry. Figure 5a shows that approximately 88% of the initial current density was retained throughout the investigation at a static potential of −0.44 V (vs. RHE) for 10 h. Furthermore, multi-step chronopotentiometry results, which were obtained by applying variable current densities ranging from −10 mAcm⁻² to −100 mAcm⁻², confirm the long-term stability of the Ni₈₅Fe₄P₁₂ catalyst (Figure 5b). The efficiency of catalytic activity exhibited by the provided catalyst may be attributable to the synergistic effects of the Ni and Fe intermetallic phases. It is hypothesized that the negatively charged phosphorus atoms and positively charged metal atoms function as hydrogen acceptor centers, thereby promoting hydrogen evolution reaction [38]. It is noteworthy that our prepared catalysts are not of noble origin and offer a rapid and straightforward method of preparation.

3.3. Oxygen Evolution Reaction Study

The oxygen evolution reaction (OER) is another one occurs on the anode side during the water splitting process. It is known that OER complicates the water splitting reaction. The oxygen evolution reaction necessitates the transfer of four electrons, the cleavage of an O–H bond, the formation of an O–O bond, and a high potential to overcome the kinetic energy barrier. In this reaction, the generation of O₂ can be achieved through two distinct mechanisms: the direct combination of O* to yield O₂ and the release of a free active site (*) (12), and the nucleophilic attack on O* by OH⁻ to generate the OOH* intermediate (13). Subsequent proton-coupled electron transfer of OOH* results in the formation of O₂ and the realization of the active site (*) (14). Of course, firstly the reactions 10, 11 occur on the electrode surface to obtain O* [52].

OH⁻ + * → OH* + e⁻

(10)

OH* + OH⁻ → O* + H2O + e⁻

(11)

2O* →2* + O₂

(12)

O* + OH⁻→ OOH* + e⁻

(13)

OOH* + OH⁻→ * + O₂ + H₂O + e⁻

(14)

Further an investigation was conducted into the performance of our prepared NiFeP coatings for the OER. The investigation was carried out in 1 M KOH at temperatures ranging from 25 to 55 °C with a 10 °C increase by means of the LSV method. The increase in temperature has been shown to enhance the current density of the oxygen evolution reaction. This enhancement has been observed to increase from 1.67 to 2.13 times its initial value (Figure 6a–c). The LSV analysis of the pure Pt electrode was measured at 25 °C as well. The current density reached at the end of the OER measurement was obtained in the following line 129.4, 100.5, 51.6, and 13.9 mAcm⁻² for Ni₇₅Fe₁₂P₁₂, Ni₈₀Fe₈P₁₂, Ni₈₅Fe₄P₁₂ and Pt, respectively. The onset potential of reaction taken at 1 mAcm⁻² current density value has been obtained at 1.57, 1.55, 1.61 and 1.71 V for Ni₇₅Fe₁₂P₁₂, Ni₈₀Fe₈P₁₂, Ni₈₅Fe₄P₁₂, and Pt, respectively (Figure 6d). The overpotential of OER at a current density of 10 mAcm⁻² was calculated using the theoretical thermodynamic voltage for OER, which is reported in references [52,53] and equal to 1.23 V. The obtained overpotential values decreased in order Pt (733 mV) > Ni₈₅Fe₄P₁₂ (548 mV) > Ni₇₅Fe₁₂P₁₂ (432 mV) > Ni₈₀Fe₈P₁₂ (412 mV) (Figure 6f). The calculated Tafel slope is directly related to the efficiency of the electrocatalyst in both the anodic and cathodic side reactions. A smaller Tafel slope value indicates better catalytic activity because it requires less overpotential to achieve a higher current density. The calculated Tafel slope values of our prepared catalysts decreased in order Pt (206 mVdec⁻¹) > Ni₈₅Fe₄P₁₂ (159 mVdec⁻¹) >Ni₈₀Fe₈P₁₂ (96 mVdec⁻¹) > Ni₇₅Fe₁₂P₁₂ (91 mVdec⁻¹) indicating efficient catalytic activity of non-precious NiFeP catalysts for OER. Also, OER chronoamperometry results using the Ni₇₅Fe₁₂P₁₂ catalyst in 1 M KOH at a static potential of 1.93 V (vs. RHE) showed approximately 79% current density retention after 10 h (Figure 7a). Multi-step chronopotentiometry confirmed the long-term stability of the Ni₈₅Fe₁₂P₁₂ catalyst during the test when variable current densities ranging from 10 mAcm⁻² to 100 mAcm⁻² were applied (Figure 7b).

It has been established that the incorporation of iron (Fe) exerts a significant influence on the electronic structure of Ni-based catalysts. This incorporation leads to the formation of high-valence active species, thereby enhancing both conductivity and oxygen adsorption. It has been observed that the amount of iron doped influences the activity of the catalysts and boosts OER electrocatalytic activity by enhancing site synergy and electron transfer [33,35,36]. These results indicate that non-noble catalysts may be more effective for OER then precious.

Herein, we also present a comparison of the HER and OER activities of the NiFeP coatings investigated in this study with those of some Ni-, Fe-, and P-based coatings recently reported in the literature in an alkaline electrolyte (

Table 3. The comparison of the electrochemical performance of the coatings tested herein towards HER in alkaline media with that of transition metal-based electrodes reported in the literature.

Catalysts	Tafel Slope (mV dec−1)	η10 (mV)	Electrolyte	Reference
Ni85Fe4P12	61	340	1 M KOH	This work
Ni80Fe8P12	71	381	1 M KOH	This work
Ni75Fe12P12	106	361	1 M KOH	This work
Pt	60	78	1 M KOH	This work
NiSe2	139	540	1 M KOH	[55]
Ni foam	151	273	1 M KOH	[57]
NiFeP	83	277	1 M KOH	[17]
Ni2P	103	200	1 M KOH	[17]
Pt	125	60	1 M KOH	[17]
Ni90Fe10	117.3	211.9	1 M KOH	[56]
Ni80Fe20	76.9	202.7	1 M KOH	[56]
NiO NP	158	576	1 M KOH	[55]
Fe2O3 NP	236	424	1 M KOH	[55]
np-NiP	71.3	250	1 M KOH	[58]

and

Table 4. The comparison of the electrochemical performance of the coatings tested herein towards OER in alkaline media with that of transition metal-based electrodes reported in the literature.

Catalysts	Tafel Slope (mV dec−1)	η10 (mV)	Electrolyte	Reference
Ni85Fe4P12	159	548	1 M KOH	This work
Ni80Fe8P12	96	412	1 M KOH	This work
Ni75Fe12P12	91	432	1 M KOH	This work
Pt	206	733	1 M KOH	This work
NiSe2	38	250	1 M KOH	[55]
Ni foam	149	590	1 M KOH	[57]
NiFeP	-	280	1 M KOH	[17]
Ni2P	-	339	1 M KOH	[17]
Ni90Fe10	69.6	454.2	1 M KOH	[56]
Ni80Fe20	67.2	450.2	1 M KOH	[56]
NiO NP	238	481	1 M KOH	[55]
Fe2O3 NP	255	691	1 M KOH	[55]
np-NiP	185.8	415	1 M KOH	[58]

). According to the Tafel slopes, which varied from 67 to 137 mVdec⁻¹, Table 3 indicates that the Volmer–Heyrovsky mechanism takes place on the electrode during the HER for all reviewed catalysts. A comparison of the overpotentials obtained at 10 mAcm⁻² reveals that our prepared Ni₈₅Fe₄P₁₂ (340 mV) catalyst demonstrates superiority over other catalysts, including NiSe₂ (540 mV) [54], NiO NP (576 mV) [55], and Fe₂O₃ NP (424 mV) [55]. Nevertheless, D. Shyshkin et al. [56] reported Ni₉₀Fe₁₀ (211.9 mV) and Ni₈₀Fe₂₀ (202.7 mV) coatings (where morpholine borane was used as a reducing agent) which are more efficient in HER (Table 3). However, the same Ni₉₀Fe₁₀ (454.2 mV) and Ni₈₀Fe₂₀ (450.2 mV) catalysts exhibit more sluggish kinetics regarding OER than the catalysts synthesized in this work, i.e., Ni₈₀Fe₈P₁₂ (412 mV) and Ni₇₅Fe₁₂P₁₂ (432 mV) (Table 4). In addition, our prepared Ni₈₀Fe₈P₁₂ (412 mV) catalyst demonstrated lower overpotential required for the OER when compared to Ni Foam (590 mV) [57], NiO NP (481 mV) [55], and Fe₂O₃ NP (691) [55]. Furthermore, its performance was analogous to that of np-NiP (415 mV) electrocatalysts (Table 4). These findings underscore the ongoing significance of the search for efficient, stable, and advanced catalytic materials.

In addition, the polarization curves were replotted to investigate cathodic (HER) and anodic (OER) activity of the NiFeP catalysts for overall water splitting (Figure 8). The potential difference (Δη₁₀) between HER and OER current density of ±10 mAcm⁻² (η₁₀^OER –η₁₀^HER) for Ni₈₅Fe₄P₁₂, Ni₈₀Fe₈P₁₂, Ni₇₅Fe₁₂P₁₂, and Pt catalysts represent an expected full-cell potential window. The full-cell potential values of 2.05 V for Ni₈₅Fe₄P₁₂ and Pt and 2.03 V for Ni₈₀Fe₈P₁₂ and Ni₇₅Fe₁₂P₁₂ have been calculated from the corresponding HER and OER polarization curves.

The resulting values indicate that these catalysts hold promise for application in practical overall water splitting (OWS) systems operating in alkaline media, utilizing identical electrode materials for both the anode and cathode. The catalytic performance in a two-electrode alkaline electrolyzer configuration was evaluated using Ni₈₅Fe₄P₁₂ as the anode and Ni₇₅Fe₁₂P₁₂ as the cathode (Figure 9). The assembled Ni₈₅Fe₄P₁₂(−)||Ni₇₅Fe₁₂P₁₂(+) electrolyzer demonstrated cell potential of 1.945 V at 10 mAcm⁻². In conclusion, a comparison of our prepared NiFeP coatings with analogous non-precious catalysts demonstrates the comparable or enhanced activity of hydrogen and oxygen evolution reactions and cell potential, as well as the merits of our method, which is characterized by its simplicity, rapidity, and cost-effectiveness.

4. Conclusions

In summary, the simple and cost-effective electroless metal deposition method was used for the preparation of efficient Ni₈₅Fe₄P₁₂, Ni₈₀Fe₈P₁₂, and Ni₇₅Fe₁₂P₁₂ catalysts for hydrogen and oxygen evolution. The preparation of NiFeP coatings with varying Fe concentrations (4, 8, and 12 at%) was achieved by employing sodium hypophosphite as a reducing agent. The experimental parameters, including the Fe²⁺ concentration and the pH of the plating bath solution, were meticulously controlled to ensure the optimal synthesis of the coatings. SEM analysis confirmed a crack-free morphology of the deposited NiFeP coatings. The electrocatalytic activity of the prepared NiFeP coatings was assessed through linear sweep voltammetry (LSV) in an alkaline medium across a temperature range from 25 to 55 °C, using a potential scan rate of 2 mVs⁻¹. It has been demonstrated that the HER and OER current density values increased from 2.11- to 2.58-fold and from 1.67- to 2.13-fold, respectively, with the increase in temperature from 25 °C to 55 °C. The catalyst Ni₈₅Fe₄P₁₂ with the lowest amount of Fe (4 at%) exhibited the highest catalytic efficiency for the hydrogen evolution reaction with the overpotential of 340 mV obtained at 10 mAcm⁻² and a Tafel slope of 61 mVdec⁻¹. Conversely, the NiFeP coatings with a larger quantity of Fe demonstrated significant efficiency for the oxygen evolution reaction with overpotential of 412 mV obtained at 10 mAcm⁻² for Ni₈₀Fe₈P₁₂ and Tafel slope of 91 mVdec⁻¹ for Ni₇₅Fe₁₂P₁₂. This study underscores the critical influence of Fe content on the catalytic efficacy of NiFeP coatings, with reduced Fe content enhancing HER and increased Fe content benefits OER. The long-term stability test demonstrated that approximately 88–79% of the current density was retained after 10 h on Ni₈₅Fe₄P₁₂ and Ni₇₅Fe₁₂P₁₂ coatings. Moreover, cell potential of 1.945 V at 10 mAcm⁻² was achieved by employing the most efficient catalyst of both reactions in a two-electrode alkaline Ni₈₅Fe₄P₁₂ (−)||Ni₇₅Fe₁₂P₁₂ (+) electrolyzer.

These findings contribute valuable insights into the design of cost-effective electrocatalysts for clean hydrogen production.