Pd-Modified CoP and CoFeP Catalysts as Efficient Bifunctional Catalysts for Water Splitting

Abstract

Developing highly efficient and stable electrocatalysts from inexpensive and earth-abundant elements represents a significant advancement in overall water splitting (OWS). This study focuses on the synthesis and evaluation of palladium-modified cobalt–phosphorus (PdCoP) and cobalt–iron–phosphorus (PdCoFeP) coatings for use as electrocatalysts in hydrogen evolution (HER), oxygen evolution (OER) and overall water splitting (OWS) in alkaline media. A facile electroless plating method is adopted to deposit the CoP and CoFeP coatings onto a copper surface (Cu sheet), with sodium hypophosphite (NaH₂PO₂) acting as the reducing agent. Pd crystallites were incorporated on CoP and CoFeP coatings using the galvanic displacement method. This study details morphological characterization (using SEM, EDX, and XRD), as well as electrochemical activity testing, for both HER and OER using linear sweep voltammetry (LSV) at different temperatures. The stability of the catalysts for HER was evaluated using chronoamperometry (CA) and chronopotentiometry (CP). The results show that the Pd-modified CoFeP and CoP catalysts exhibited lower overpotentials of 207 and 227 mV, respectively, for HER and 396 mV for OER at a current density of 10 mA cm⁻² compared to the unmodified CoFeP and CoP catalysts. The innovation achieved in this study lies in combining a facile, low-cost deposition method (electroless plating followed by galvanic displacement) with a novel, highly effective ternary composition (PdCoFeP) that exploits synergistic electronic and morphological effects to achieve superior bifunctional performance for alkaline OWS, achieving a low cell voltage of 1.69 V at a current density of 10 mA cm⁻². Overall, this research demonstrates that these synthesized materials are promising candidates for sustainable and economical hydrogen production.

1. Introduction

The increasing demand for clean and sustainable energy sources has been driven by two key factors: the overuse of fossil fuels and the rapid growth of environmental problems [1,2,3]. Hydrogen has been regarded as a potentially viable alternative fuel, primarily due to its clean and sustainable characteristics. Currently, the predominant method for producing hydrogen is fossil fuel reforming, which results in carbon dioxide emissions. On the other hand, water electrolysis is regarded as the most environmentally friendly technique for hydrogen production, and it can mitigate the intermittent nature of renewable electricity [4,5,6]. Electrochemical water splitting, comprising a hydrogen evolution reaction (HER) at the cathode and an oxygen evolution reaction (OER) at the anode, has been identified as a promising technology for large-scale hydrogen production. This is due to low energy consumption and high hydrogen purity, among other advantages [7,8,9]. As Van Troost Wijk and Deiman first reported in 1789, water electrolysis requires a large overpotential, and this is still true more than 200 years later due to the relatively sluggish kinetics of both cathodic HER and anodic OER. It is well established that the thermodynamic potential of water electrolysis is 1.23 V [10,11]. However, to achieve a practical rate of water dissociation, it is necessary to apply an overpotential to the water electrolyzer. It has been demonstrated that both HER and OER demand significant overpotentials to surpass the kinetic barriers that arise from the high activation energies necessary for forming reaction intermediates on the electrode surface [12,13,14]. Furthermore, these overpotentials need to drive the reaction at a specific current density, which is typically a geometric current density of 10 mA cm⁻². Using an active catalyst is greatly important in reducing the overpotential requirements of these two half-reactions, thereby increasing the reaction rate. Ideally, a catalyst should exhibit high activity (large current density at low overpotentials) and durable performance over an extended period [15]. An efficient water splitting process necessitates using electrocatalysts characterized by high efficiency, stability, and corrosion resistance. These catalysts must facilitate both HER at the cathode surface and OER at the anode surface, with both reactions occurring simultaneously and exhibiting high rates of reaction [16,17]. This will enhance efficiency, lower the cost of developing an electrolysis system, and simplify the system [10,18,19,20,21]. Various catalysts have been synthesized to enhance the reaction rate and reduce the overpotential for HER and OER. Currently, noble metals and noble metal oxides, including Pt [22,23], RuO₂ [24], and IrO₂ [25], are regarded as the most effective catalysts for both HER and OER. However, the high cost and scarcity of noble metal catalysts have raised concerns regarding economic viability. Consequently, a strong research focus has emerged on developing low-cost, high-efficiency, non-precious electrocatalysts, which serve as an alternative to precious-based materials and exhibit high activity towards OER and HER [26,27,28,29,30].

Today, earth-abundant transition metal compounds, principally Co-, Fe-, Mn-, and Ni-based materials formed either as oxides or hydroxides, are utilized for OER. These metals are often combined with other non-metallic elements, including C, N, S, P, Se, and B, to form binary or ternary compositions. These compositions are employed in both HER and OER [31,32,33,34,35,36,37,38]. Significant research has been dedicated to investigating the potential of these compounds as electrocatalysts, given to their remarkable durability, particularly cobalt-based compounds such as sulfides [39,40,41], borides [42], (oxy)hydroxides [43,44,45] and phosphides [46,47], which have been evidenced to be remarkable electrolyzed water catalysts. Researchers have highlighted transition-metal phosphides (TMPs) as a promising class of catalysts for electrolyzing water. TMPs such as Co-P [48,49], Fe-P [50,51], Ni-P [52,53], and Mo-P [54,55] have been identified as highly attractive catalysts due to the superior conversion of their oxidation states, their cost-effectiveness, and their high activity. A significant number of studies have been conducted on the catalytic role of transition metal phosphide in HER. These studies have resulted in the development of varied of catalysts that exhibit high activity in hydrogen evolution. This enhanced activity has been achieved through adjusted catalyst morphology and surface modification [56]. Given the calculation results for density functional theory (DFT), it is hypothesized that the P atom in the phosphide catalyst has electronegativity, facilitating the adsorption and capture of protons, and enabling part of the charge of Co to be transferred to the P atom. Modifying the electronic structure has been demonstrated to be an effective strategy for reducing the Gibbs hydrogen adsorption free energy, resulting in the enhanced catalytic activity of hydrogen evolution [57,58]. A considerable number of monometallic phosphides are employed in electrolytic water and have demonstrated considerable success, exhibiting high catalytic activity during the reaction. Among these, the cobalt phosphide catalyst has been shown to exhibit low overpotential, long-term stability, and other advantageous properties, and it is regarded as the most promising catalyst candidate for HER [59,60,61].

Despite the high catalytic activity of the cobalt phosphide catalyst in hydrogen evolution reactions, a comparative analysis reveals a performance deficit for comprehensive performance when benchmarked against precious metal Pt-based catalytic materials. To enhance the activity of the catalyst, researchers have pursued a multifaceted approach involving the doping of one or more metal atoms into the single-metal phosphating process. This strategy aims to regulate the electronic structure and optimize surface properties, achieving enhanced catalytic functionality. For instance, Yang et al. synthesized Fe-doped three-dimensional Co-based phosphate Fe_xCo_3−x(PO₄)₂ on a copper wire using the electrodeposition method as an electrochemical catalyst. In an alkaline solution, a current density of 10 mA cm⁻² required only 48.9 mV of overpotential for HER, and the catalytic performance was maintained after 100 h of operation. This study, thus, demonstrates the crucial role played by Fe doping in enhancing the catalytic performance of the Fe_xCo_3−x(PO₄)₂ catalyst for HER [62]. As research has progressed, it has become evident that transition metal phosphide exhibits not only exceptional performance for HER but also noteworthy catalytic performance for OER. The catalytic effect of cobalt-based phosphide materials on OER is attributable to the adsorption of OOH species by positively charged cobalt ion centers, while negatively charged P centers facilitate surface oxygen removal. It is therefore anticipated that transition metal phosphating will function effectively as a bifunctional catalyst for both HER and OER. The development of bifunctional electrocatalysts within the same electrolyte has attracted significant attention due to their numerous advantages, including the simplified fabrication process, the reduced cost, and enhanced hydrogen productivity. The majority of catalysts composed of Co- or Ni-based materials can catalyze a single reaction (OER or HER). Consequently, it is essential to identify an efficient bifunctional catalyst that exhibits robust stability for the overall water splitting process in an alkaline environment [63]. Incorporating transition metal Fe into cobalt phosphide has been demonstrated to enhance its oxygen evolution activity, while simultaneously having an optimizing effect on hydrogen evolution electrocatalysis [64,65]. The enhanced HER/OER performance of the FeCo-P catalyst is primarily attributable to the synergistic effect of Fe, Co, and P, as well as the robust interaction between ions [66].

The majority of bimetallic phosphides have been synthesized through utilizing hydrothermal, solid-phase, and gas-phase phosphating methodologies [67]. Even though these methodologies have been employed to prepare transition bimetallic phosphides of various morphological structures, which have been shown to have superior properties, it is still necessary to further simplify the preparation of bimetallic phosphides to meet the requirements of their practical applications [68]. As indicated in the relevant literature, the conventional phosphating reaction still has many disadvantages, including high reaction temperatures, the requirement for inert gas protection, and the usage of hazardous phosphorus resources [69]. These methods demand considerable energy input and rely on complicated apparatus. Using straightforward, economical, and effective techniques for catalyst fabrication has the potential to decrease the overall cost of hydrogen production through water electrolysis [70]. Consequently, electroless deposition has emerged as a highly efficient, cost-effective, eco-friendly, and straightforward catalyst preparation method. Electroless plating, also referred to as chemical or autocatalytic plating, is a non-galvanic plating method involving multiple reactions occurring simultaneously in an aquatic solution [71], and it does not require using complex equipment or hazardous power sources [72].

This study investigates the fabrication and evaluation of palladium-modified cobalt–phosphorus (PdCoP) and cobalt–iron–phosphorus (PdCoFeP) coatings as electrocatalysts for HER and OER in alkaline media. The CoP and CoFeP coatings were deposited on the copper surface using a straightforward and highly efficient electroless metal plating technique, using sodium hypophosphite (NaH₂PO₂) as the reducing agent. Incorporating of Pd crystallites on the CoP and CoFeP coatings using the galvanic displacement method has been shown to result in significant improvements in catalytic performance. Evaluations and characterizations of PdCoP and PdCoFeP were conducted to assess their chemical state, catalytic activity, and stability during HER and OER. Moreover, the catalytic mechanisms of PdCoP and PdCoFeP catalysts for overall water splitting were investigated. The findings of this research suggest that the synthesized catalysts have the potential to be promising candidates for utilization in alkaline media for performing water splitting, which could result in a more environmentally sustainable and economical method of hydrogen production.

2. Results and Discussion

2.1. Coatings, Microstructure and Morphology Studies

The primary objective of this study was to develop an effective catalyst for HER and OER in an alkaline medium (1.0 M KOH), employing a straightforward methodology. Consequently, Co-based coatings containing Co, Fe and P were deposited on Cu substrates via a facile electroless deposition process, where sodium hypophosphite was utilized as the reducing agent. The galvanic displacement method was used to incorporate palladium crystallites into the CoP and CoFeP coatings, and this enhanced the catalytic activity of the resulting coatings. Scanning electron microscopy (SEM) and energy-dispersive X-ray analysis (EDX) were employed to investigate the surface morphology and composition of the coatings. Figure 1 illustrates the SEM images of the fabricated Co-based coatings with different compositions such as CoP (a), CoFeP (b), PdCoP (c), and PdCoFeP (d). The presence of Pd, Co, Fe, and P in the coatings is confirmed by the corresponding EDX spectra of all the samples. The results of the EDX are outlined in

Table 1. Composition of coatings deposited on Cu surface determined by EDX analysis.

Sample	Element, at%
	Pd	Co	Fe	P
CoP	-	90.22	-	9.78
CoFeP	-	83.78	9.96	6.26
PdCoP	3.54	83.72	-	12.74
PdCoFeP	8.36	64.08	12.49	15.07

, which shows that the fabricated CoP, CoFeP, PdCoP, and PdCoFeP electrocatalysts demonstrated Pd contents ranging from 3.54 to 8.36 at%, Co contents ranging from 64.08 to 90.22 at%, Fe contents ranging from 9.96 to 12.49 at%, and P contents ranging from 6.26 to 15.07 at%.

SEM analysis demonstrates distinct morphological variations induced by compositional modifications in the electrolessly deposited coatings. The CoP coating exhibited a compact and relatively homogeneous surface composition, characterized by densely packed grains [72,73], indicative of stable nucleation and uniform growth (Figure 1a). It is noteworthy that incorporating Fe into the CoFeP structure results in a significant structural transformation, creating a porous and heterogeneous morphology [74]. This substantial modification underscores the considerable impact of Fe on nucleation kinetics, surface energy, and phosphorus incorporation during electroless deposition (Figure 1b). Adding Pd to CoP leads to the surface becoming rougher and more fragmented. This is characterized by the presence of finer clusters and increased grain boundaries, indicative of an enhanced nucleation density.

The PdCoFeP coating exhibits the most irregular and nanostructured surface, characterized by textured characteristics and fine crystallites (Figure 1c,d). This observation indicates a synergistic effect of Pd and Fe in inducing high nucleation rates and complex growth pathways. The obtained results demonstrate the extreme sensitivity of electroless deposition processes to even minor compositional variations, which can dramatically alter the resulting microstructure.

Figure 2a shows the XRD pattern of the CoP catalyst. The diffraction patterns of the CoP revealed crystalline phases associated with orthorhombic CoP and metallic Co. The CoP peaks were detected at approximately 2θ of 45.1° and 48.1°. These peaks corresponded to the (021) and (121) planes, respectively, and matched with COD file No. 9,008,928 (a = 5.59900 Å, b = 5.07600 Å, c = 3.28100 Å). The 2θ peaks at approximately 41.7°, 44.9°, 47.7°, 62.9°, 76.2°, 84.9° and 93.1° corresponded to the metallic Co phase (COD No. 1512501, a—2.49680 Å, b = 2.49680 Å, c = 4.03081 Å), which were indexed to the (100), (002), (101), (102), (110), (103) and (112) crystal planes, respectively.

Following the incorporation of Fe into the CoP structure, the resulting CoFeP coating exhibits diffraction peaks at 2θ values of 44.97°, 65.48° and 82.96°, corresponding to the crystal planes of (110), (200), (211) and (220), respectively, of a CoFe-type solid solution phase (COD 1524319) (Figure 2b). Compared with pure CoP, a slight shift in peak positions is observed, confirming the successful incorporation of Fe atoms into the CoP lattice and the formation of a CoFeP alloy-like structure. Reflections corresponding to metallic Co (COD 1512501) and metallic Fe (COD 1512555) are also detected (Figure 2b). Introducing Fe induces lattice distortion and modifies the crystal structure, which could enhance electronic conductivity and catalytic active site density.

For PdCoP (Figure 2c), the additional reflections observed at 2θ = 40.24°, 46.80°, 68.34°, 82.39° and 86.93° correspond to the (111), (200), (220), (311) and (222) crystallographic planes of face-centered cubic Pd (COD 1011105), respectively, which confirms the presence of palladium on the surface. Reflections assigned to metallic Co (COD 1512501) are also detected. Incorporating Pd does not significantly alter the CoP crystal structure, as the main CoP peaks remain unchanged after Pd deposition. However, the slight broadening and reduction in intensity of the CoP peaks compared to pure CoP suggest the occurrence of surface modification and possible lattice strain due to Pd deposition. The presence of both CoP and Pd peaks indicates the formation of a bimetallic heterostructure (Pd–CoP) rather than a new intermetallic compound.

Figure 2d shows the XRD pattern of PdCoFeP, which exhibits diffraction peaks corresponding to the CoFe phase (COD 1524319), metallic Co (COD 1512501), metallic Fe (COD 1512555), and Pd (COD 1011105). These results confirm the successful deposition of Pd crystallites onto the CoFeP surface via galvanic displacement, with no alteration in the original CoFeP crystal structure.

The structural evolution from CoP to CoFeP and subsequently to PdCoP and PdCoFeP demonstrates a clear progression in crystal structure and surface composition. The CoP coating exhibits well-defined peaks corresponding to orthorhombic CoP, indicating high crystallinity with a small amount of residual metallic Co. After introducing Fe, the diffraction peaks undergo a slight shift and broadening, confirming the successful incorporation of Fe into the CoP lattice to form a CoFeP alloy-like structure. This Fe substitution introduces lattice distortion, enhancing electronic conductivity and potentially creating more catalytically active sites. Following Pd deposition via galvanic displacement, the characteristic CoP or CoFe peaks remain largely unchanged, indicating that Pd incorporation does not disturb the bulk CoP or CoFeP structures. However, new reflections assigned to fcc Pd appear, verifying the formation of Pd nanoparticles on the CoP or CoFeP surfaces. The sequential modification in the order CoP → CoFeP → PdCoP → PdCoFeP leads to a progressive structural and compositional evolution. Fe alloying alters the bulk crystal lattice, while Pd addition tunes the surface composition and introduces additional active sites. This synergistic structural tuning is expected to improve the electronic structures, surface reactivities, and catalytic performances of the PdCoP and PdCoFeP composite catalysts.

2.2. Electrocatalytic Activity Towards HER

The electrocatalytic performances of the prepared catalysts for the hydrogen evolution reaction (HER) were investigated by recording linear sweep voltammograms (LSVs) in 1.0 M KOH solution at a potential scan rate of 5 mV·s⁻¹ at room temperature (Figure 3a). Summarized parameters for HER on the investigated catalysts in alkaline media at 25 °C are given in

Table 2. Summarized parameters for HER on the investigated catalysts in alkaline media.

Sample	Eonset *, V	η10 **, mV	Tafel Slope, mV dec−1	Ea, kJ mol−1
CoP	0.166	239	78.1	11.3
CoFeP	0.188	245	76.4	6.0
PdCoP	0.171	250	76.3	4.7
PdCoFeP	0.213	259	82.8	11.4

* Potential at −1 mA cm⁻², ** Overpotential at 10 mA cm⁻².

. The corresponding overpotential values at 10, 20, 50, and 100 mA cm⁻² are given in Figure 3b. Among the catalysts investigated, the CoP catalyst exhibited the lowest overpotential value of 239 mV for the HER compared to CoFeP (245 mV), PdCoP (250 mV), and PdCoFeP (259 mV), attaining a current density of 10 mA cm⁻² at 25 °C (Figure 3a and Table 2). However, a higher current density is delivered on PdCoFeP, indicating that more efficient hydrogen generation occurs under higher load conditions. Moreover, the overpotential of PdCoFeP at 100 mV cm⁻² is 385 mV, which is smaller than those of PdCoP (420 mV), CoFeP (428 mV), and CoP (458 mV) (Figure 3b). These improvements are attributed to the synergistic interactions between Co, Fe, P, and Pd, which enhance electron transfer and facilitate HER kinetics.

The reaction kinetics and mechanism of the catalysts can be evaluated based on the Tafel slopes. The Tafel slopes were determined using the following equation [75]:

η = b · log j/j₀

(1)

where η is the overpotential, b is the Tafel slope, j is the experimental current density, and j₀ is the exchange current density. The plot of η versus log j represents the Tafel slope. In alkaline media, the HER process involves three main steps, as shown in Equations (2)–(4) [75]:

* + H₂O + e⁻ ↔ *H_ads + OH⁻  (Volmer step, 120 mV dec⁻¹)

(2)

*H_ads + e⁻ + H₂O ↔ H₂ + OH⁻ + * (Heyrovsky step, 40 mV dec⁻¹)

(3)

2 *H_ads ↔ H₂ + *  (Tafel step, 30 mV dec⁻¹)

(4)

H_ads denotes the H₂ adsorbed to the metal sites, where * represents the metal sites. Figure 3c displays the plots of η versus log j for each catalyst. The Tafel slopes calculated were within a range of approximately 76 to 83 mV dec⁻¹ (Table 2), whereas the CoFeP and PdCoP catalysts had the lowest Tafel slope (ca. 76 mV dec⁻¹) relative to CoP (78.1 mV dec⁻¹) and PdCoFeP (82.8 mV dec⁻¹) (Figure 3c). This finding indicates that the HER may occur through the Volmer–Heyrovsky mechanism, in which water molecules or H₂ adsorbs onto an electrode to generate MH_ads species.

The activation energy (E_a) is a critical factor in evaluating the performance of the catalyst. This energy can be estimated by employing the LSV curves at varying temperatures. The obtained HER LSVs for each catalyst at temperatures from 25 to 55 °C are shown in Figure S1 and Table S1 (Supplementary Material). The results obtained outline that the HER activity of the CoP, CoFeP, PdCoP, and PdCoFeP catalysts is enhanced as the electrolyte temperature increases, whereas the overpotentials required to achieve the current density of 10 mA cm⁻² decrease with the increase in temperature for each catalyst (Table S2, Supplementary Material). Furthermore, the obtained Tafel slope values range from approximately 72 to 94 mV dec⁻¹. These results indicate that the HER kinetic process is closely related to temperature. For determining E_a, the current values at different temperatures recorded on the catalysts at −0.40 V (vs. RHE) were extracted from the LSV profiles (Figure S1, Supplementary Material). The data were plotted as log(i) against 1/T for the CoP, CoFeP, PdCoP, and PdCoFeP catalysts and are shown in Figure 3d. The E_a was calculated using the slope of the obtained straight lines for each catalyst from the Arrhenius equation [76]. It was determined that the PdCoP, CoFeP, CoP, and PdCoFeP catalysts had E_a values of 4.7, 6.0, 11.3, and 11.4 kJ mol⁻¹, respectively (Figure 3d). The obtained E_a values were significantly lower against Pt/C (20 wt%) benchmark, which were reported as follows: benchmark—16 kJ mol⁻¹ [77]; Pd100—20.86 kJ mol⁻¹ [77]; Pd₅Co₉₅—21.75 kJ mol⁻¹ [77]; Pd/C—31 kJ mol⁻¹ [77]; CoO_x—29.79 kJ mol⁻¹ [78]. The low activation energies implied that a small reaction kinetic barrier is required for the reaction to proceed efficiently. The reduction in E_a can be explained through several factors related to the catalyst’s composition, electronic structure, and morphological properties. Incorporating of Fe and subsequent modification with Pd crystallites reduces the E_a of CoP due to the synergistic interactions between Fe, Co, and Pd. These interactions are crucial because they enhance electron transfer and facilitate the kinetics of the HER.

EIS spectra were obtained on the PdCoFeP and CoFeP samples to investigate the influence of modification with Pd on HER activity. The Nyquist plots shown in Figure 4 are characterized by a broad semicircle, while at low frequencies (1.5–0.5 Hz), a small but pronounced inductive response emerges as Z_im values become positive.

This phenomenon typically signals the slow relaxation of adsorbates in multistep reactions such as corrosion or electrocatalysis [79], suggesting that the electrode surface may undergo some restructuring or phase transformation during operation. Therefore, the impedance spectra were fitted to an equivalent circuit model that included an inductive element to account for the low-frequency inductive response [80]. The constant phase element (CPE_dl) values were recalculated into effective capacitance C_eff vias Brug et al.’s formula [81], and the obtained values for CoFeP and PdCoFeP were 3.5 mF cm⁻² and 7.4 mF cm⁻², respectively. Additionally, the associated charge transfer resistance (R_ct) of CoFeP (5.5 Ω cm²) was two times larger than that of the more electrochemically active PdCoFeP (3.7 Ω cm²). These observations correspond well to the HER activity results presented in Figure 3. The palladium-decorated film exhibits better HER activity because it has a significantly larger electrochemically active surface area and faster HER kinetics, as demonstrated by the larger C_eff and lower R_ct values.

2.3. Electrocatalytic Activity Towards OER

The activity of the Co-based catalysts in the OER was also evaluated. The obtained data are given in Figure 5 and

Table 3. Electrochemical performance of the investigated catalysts for OER in alkaline media at 25 °C.

Sample	Eonset *, V	ηonset, mV	E **, V	η10 **, mV	Tafel Slope, mV dec−1	Ea, kJ mol−1
CoP	1.5205	290	1.6607	431	120.3	20.7
CoFeP	1.5846	355	1.6650	435	74.5	21.2
PdCoP	1.5426	313	1.6255	396	77.8	12.2
PdCoFeP	1.5577	328	1.6256	396	70.5	11.8
Cu	1.7638	534	1.8750	645	-	-

* Values at 1 mA cm⁻², ** Values at 10 mA cm⁻².

. In addition, the LSV was recorded on the Cu substrate for comparison. Higher OER current densities were obtained on the CoP, CoFeP, and Pd-modified CoP and CoFeP catalysts compared to the Cu substrate (Figure 5a), indicating that they demonstrated high activity during the OER. As shown by the LSVs recorded for the aforementioned catalysts in a 1 M KOH solution, the lowest OER overpotential of 396 mV was observed for the Pd-modified CoP and CoFeP, achieving a current density of 10 mA cm⁻², compared to the unmodified CoP (431 mV) and CoFeP (435 mV), and Cu (645 mV) (Table 3). Lower Tafel slope values (in the range of 70.5 to 77.8 mV dec⁻¹) were obtained for PdCoFeP, CoFeP, and PdCoP catalysts compared to the CoP catalyst (120.3 mV dec⁻¹). These results indicated superior OER kinetics (Table 3). Higher current density values for OER were obtained on the Pd-modified CoP and CoFeP catalysts compared to the unmodified CoP and CoFeP catalysts (Figure 5b). The overpotential of PdCoFeP at 100 mA cm⁻² is 549 mV, which is smaller than those of PdCoP (595 mV), CoFeP (747 mV), and CoP (778 mV). The obtained OER LSVs for each catalyst at temperatures ranging from 25 to 55 °C are shown in Figure S2 and Table S2 (Supplementary Material). The OER activity of the CoP, CoFeP, PdCoP, and PdCoFeP catalysts also increases as the temperature of electrolyte increases. An increase in temperature for each catalyst also results in a decrease in the overpotential values, which need to attain 10 mA cm⁻² (Table S2, Supplementary Material). The obtained Tafel slope values, which also decrease with the increase in temperature for each catalyst, range from approximately 120 to 95 mV dec⁻¹ for CoP, from 74 to 60 mV dec⁻¹ for CoFeP, from 78 to 77 mV dec⁻¹ for PdCoP, and from 70 to 65 mV dec⁻¹ for PdCoFeP at a temperature from 25 and 55 °C, respectively (Table S2, Supplementary Material). To determine E_a for the OER, the current value and temperature value of the catalysts at 1.8 V (vs. RHE) were extracted from the LSV profiles (Figure S2, Supplementary Material). The PdCoFeP, PdCoP, CoP, and CoFeP catalysts have E_a values of 11.8, 12.2, 20.7 and 21.2 kJ mol⁻¹, respectively (Figure 5d). In addition, the PdCoFeP has the lowest activation energy, i.e., 11.8 kJ mol⁻¹, implying that it has the smallest reaction kinetic barrier.

A comparison of the OER performances of the investigated catalysts with those of previously reported Co-P-based and most advanced noble-metal catalysts is shown in

Table 4. Comparison of OER performance of the investigated catalysts with some previously reported Co-P-based and most advanced noble–metal catalysts.

Catalyst	η10 *, mV	Tafel Slope, mV dec−1	Electrolyte	Ref.
PdCoFeP	396	70.5	1 M KOH	This study
PdCoP	396	77.8	1 M KOH	This study
CoOx	423	42	1 M KOH	[82]
Co2P2O7	490	86	1 M KOH	[83]
Co(PO3)2 nanosheets	574	106	1 M KOH	[83]
CoP hollow polyhedron	400	57	1 M KOH	[84]
Reduced mesoporous Co3O4 nanowires	400	72	1 M KOH	[85]
CoP-MNA/Ni foam	390	65	1 M KOH	[86]
CoP	350	47	1 M KOH	[18]
CoSi-P	309	121	1 M KOH	[87]
CoP	300	65	1 M KOH	[88]
Mn-CoP	288	77.2	1 M KOH	[89]
RuO2/CF	360	164	1 M KOH	[90]
RuO2 on NF	290	81	1 M KOH	[91]
IrO2 commercial	339	94.5	1 M KOH	[92]
Ir/C	254	71.9	1 M KOH	[93]
Ni0.3Co2Fe-P	290	-	1 M KOH	[94]
Co2Fe-P	367	-	1 M KOH	[94]
CuO@FeCoP/CF	240	-	1 M KOH	[95]
Mo-CoP@C	280	53.1	1 M KOH	[96]
CoFe@Co2C@Co-0.2/NF	246	-	1 M KOH	[97]

* Values at 10 mA cm⁻².

. The obtained overpotential values of 396 mV for the PdCoP/Cu and PdCoFeP/Cu catalysts are lower compared to CoO_x (423 mV) [82], Co₂P₂O₇ (490 mV) [83], CoP hollow polyhedron (400 mV) [84], and reduced mesoporous Co₃O₄ nanowires (400 mV) [85] but higher compared to the overpotential values for CoP-MNA/Ni foam (390 mV) [86], CoP film (350 mV) [18], CoSi-P (309 mV) [87], CoP (300 mV) [88], Mn-CoP (288 mV) [89], RuO₂/CF catalyst (360 mV) [90], RuO₂ on NF (290 mV) [91], IrO₂ commercial (339 mV) [92], Ir/C (254 mV) [93], Ni_0.3Co₂Fe-P (290 mV) [94], Co₂Fe-P (367 mV) [94], CuO@FeCoP/CF (240 mV) [95], Mo-CoP@C (280 mV) [96], and CoFe@Co₂C@Co-0.2/NF (246 mV) [97].

2.4. Determination of Electrochemically Active Surface Areas

The electrochemically active surface areas (ECSAs) of the unmodified and Pd-modified CoP and CoFeP catalysts were determined from the measurements of the electrochemical double-layer capacitance (C_dl). The CV curves were recorded at different scan rates in the non-Faradaic region (Figure 6a–d), which was followed by calculating the slope of the curve obtained by plotting the differences in the anodic and cathodic current versus the scan rate (Figure 6e). The C_dl was 2458.5, 11,652.2, 10,858.9 and 56,291.2 µF for CoP, CoFeP, PdCoP and PdCoFeP (Figure 6e), whereas the calculated ECSA values were 61.5, 291.3, 271.5 and 1407.3 cm² for CoP, CoFeP, PdCoP and PdCoFeP, respectively.

The high ECSA of PdCoFeP was due to the higher number of active sites, which contributed to the higher OER electrocatalytic activity. The incorporation of Fe into the CoP coating, followed by its modification with Pd crystallites, allowed it achieve the lowest OER overpotential, Tafel slope, and E_a, which suggests that factors beyond active site availability influence OER performance. Other factors, such as electronic structure and conductivity, as well as a high ECSA, also significantly influence catalyst performance. The catalyst with 8.36 at% Pd had the best activity during the OER.

2.5. Investigation of Stability of Catalysts

Additionally, the stabilities of the most promising Pd-modified CoP and CoFeP catalysts were investigated using chronoamperometry by recording the chronoamperometric curves at a constant potential of −0.26 V for 10 h on the PdCoP and PdCoFeP catalysts (Figure 7). The PdCoFeP/Cu catalyst possessed excellent stability, with current retention of 96.02% over 10 h compared to the PdCoP/Cu catalyst.

Figure 8 shows the chronopotentiometric curves recorded on the PdCoP (a) and PdCoFeP (b) catalysts in a 1 M KOH solution at a constant current density of −10 mA cm⁻² and 25 °C. It is evident that the potential required to attain −10 mA cm⁻² slightly increased from −0.1945 V to −0.2131 V for PdCoP (Figure 8a) and from −0.1551 V to −0.1740 V for PdFeCoP (Figure 8b) over 10 h, demonstrating the good stability of both catalysts.

2.6. Investigation of Overall Water Splitting

To confirm the bifunctional activity of the unmodified and Pd-modified CoP and CoFeP catalysts, a two-electrode water electrolysis cell was constructed using two of the same investigated catalysts as the anode and the cathode. Figure 9 shows the overall catalytic performance of a two-electrode alkaline electrolyzer cell configuration using the same catalysts for both the anode and the cathode: CoP‖CoP, CoFeP‖CoFeP, PdCoP‖PdCoP, and PdCoFeP‖PdFeCoP.

The developed CoP, CoFeP, PdCoP and PdFeCoP electrodes exhibited cell potentials of 1.86 V, 1.81 V, 1.78 V and 1.69 V, respectively, at 10 mA cm⁻² (Figure 9), comparable to the cell potential values recorded for Pt/C‖IrO₂ (1.71 V), Pt/C‖Pt/C (1.83 V) [98], CoP/rGO-400‖CoP/rGO-400 (1.70 V) [98], Co(OH)₂@NCNTs@NF‖Co(OH)₂@NCNTs@NF (1.72 V) [99], Co-P/NC/CC‖Co-P/NC/CC (1.77 V) [100], and Co-P/NC-CC‖Co-P/NC-CC (1.95 V) [100] as well as for other studies reported in the literature (

Table 5. Comparison with various electrocatalysts for overall water splitting.

Anode II Cathode	Cell Voltage, V	Electrolyte	Ref.
PdCoFeP‖PdCoFeP	1.69	1 M KOH	This study
PdCoP‖PdCoP	1.78	1 M KOH	This study
Pt/C‖IrO2	1.71	1 M KOH	[98]
Pt/C‖Pt/C	1.83	1 M KOH	[98]
CoP/rGO-400‖CoP/rGO-400	1.70	1 M KOH	[98]
Co(OH)2@NCNTs@NF‖ Co(OH)2@NCNTs@NF	1.72	1 M KOH	[99]
Co-P/NC/CC‖Co-P/NC/CC	1.77	1 M KOH	[100]
Co-P/NC-CC‖Co-P/NC-CC	1.95	1 M KOH	[100]

).

The energy efficiencies of the cells were calculated. The energy efficiencies of the CoP, CoFeP, PdCoP and PdCoFeP cells were 66.13%, 67.96%, 69.10%, and 72.78%, respectively. These catalysts appear to be promising candidates for practical overall water splitting applications.

3. Materials and Methods

3.1. Chemical Reagents

Cobalt(II) sulfate CoSO₄·7H₂O (99.5%, Chempur, Piekary Śląskie, Poland), sodium hypophosphite (NaH₂PO₂, 97%, Alfa Aesar, Kandel, Germany), glycine (NH₂CH₂COOH, 99%, Chempur, Piekary Śląskie, Poland), palladium(II) chloride PdCl₂ (59.5%, Alfa Aesar, Ward Hill, MA, USA), hydrochloric acid (HCl, 35–38%, Chempur, Piekary Śląskie, Poland), potassium hydroxide KOH (98.8%, Chempur, Piekary Śląskie, Poland), Cu sheets (99%, Goodfellow Cambridge Limited, Huntingdon, UK), and calcium magnesium oxide, known as “Vienna Lime” (50–100%, Kremer Pigments GmbH & Co. KG, Aichstetten, Germany) were used. All the chemicals were analytical grade and used without any further purification. The electrolytes were prepared using deionized water from a Millipore Milli-Q Ultra system with a resistivity of 18.2 MW-cm or higher.

3.2. Preparation of Catalysts

CoP and CoFeP catalysts were deposited onto Cu substrates (1 × 1 cm) via an electroless deposition method. The process consisted of the following steps:

• Pretreatment: The Cu substrate was pretreated with calcium magnesium oxide and subsequently rinsed with deionized water (DI).
• Decapitation: The substrate was immersed in a HCl:H₂O (1:1, v/v) solution for 1 min at 25 °C, followed by rinsing with DI.
• Activation: The cleaned Cu substrate was activated via immersion in a 0.5 g L⁻¹ PdCl₂ solution for 1 min at 25 °C, and then rinsed with DI.
• Electroless deposition:

  (i)

  For CoP deposition, the Pd-activated Cu substrate was immersed in a plating bath containing 0.1 M CoSO₄, 0.6 M glycine, and 0.75 M NaH₂PO₂ (pH = 11) for 30 min at 60 °C.

  (ii)

  For CoFeP deposition, the same plating bath was used with the addition of 0.01 M FeSO₄.

Finally, Pd crystallites were deposited on the CoP and CoFeP coatings via galvanic displacement, achieved by immersing the samples in a 1 mmol L⁻¹ PdCl₂ solution for 1 min at 25 °C, followed by rinsing with DI.

3.3. Characterization of Catalysts

An FEI Helios Nanolab 650 (Hillsboro, OR, USA) dual beam system with an energy dispersive X-ray (EDX) spectrometer INCA Energy 350 (Oxford Instruments, Abingdon, Oxfordshire, UK) and an X-Max 20 mm² (Oxford Instruments, Abingdon, Oxfordshire, UK) detector was used to observe the surface morphology via scanning electron microscopy.

X-ray diffraction patterns were recorded using an X-ray diffractometer D2 Phaser using CuK_α radiation (Bruker, Karlsruhe, Germany). A step-scan mode was used in a 2θ range from 10° to 100° with a step length of 0.04° and a counting time of 1 s per step. The samples were crushed prior to measurement. For XRD measurements, the CoP, CoFeP, PdCoP and PdCoFeP catalysts were deposited on the glass surface.

3.4. Evaluation of Catalysts Activity for HER and OER

The electrocatalytic activities of the prepared catalysts towards the HER and OER were evaluated via linear sweep voltammetry (LSV). This was conducted using a PGSTAT100 potentiostat/galvanostat (Metrohm Autolab B. V., Utrecht, The Netherlands) with a standard three-electrode electrochemical cell setup. The fabricated catalysts with geometric areas of 2 cm² were employed as working electrodes. A Pt sheet was used as a counter electrode, and an Ag/AgCl (3 M KCl) electrode was used as a reference. All the reported potential values reported in this study were converted to the reversible hydrogen electrode (RHE) scale using the following equation:

E_RHE = E_measured + 0.059·pH + E_{Ag/AgCl (3 M KCl)}

(5)

where E_{Ag/AgCl (3 M KCl)} = 0.210 V.

Linear sweep voltammograms (LSVs) were recorded in an N₂-saturated 1 M KOH solution at different temperatures from the open circuit potential (OCP) to ca. −0.5 V (vs. RHE) for the HER and from the OCP to ca. 2.1 V (vs. RHE) for the OER at a potential scan rate of 5 mV s⁻¹. Additionally, to evaluate the long-term stability of the fabricated catalysts, chronoamperometric curves were recorded at a constant potential of −0.26 V in a 1.0 M KOH solution at 25 °C for 10 h, while chronopotentiometric curves were recorded at a constant current density of −10 mA cm⁻² at 25 °C for 10 h.

To determine E_a, the current values from the LSVs at different temperatures at potentials of −0.40 V (vs. RHE) for the HER and 1.8 V (vs. RHE) for the OER were extracted from the LSV profiles presented in Supplementary Material (Figures S1 and S2). The data were plotted as log(i) against 1/T, resulting in a straight line with a gradient of −E_a/2.3R. E_a was calculated using the slope of this line according to Equation (6):

∂logi/∂(1/T) = −E_a/2.3R

(6)

where i is the current (mA), T is the temperature (K), and R is the gas constant (8.314 J mol⁻¹ K⁻¹) [76].

The HER and OER current densities presented in this study have been scaled to the geometric area of the catalysts.

To evaluate the ECSA of the catalysts, the double-layer capacitance (C_dl) was determined by recording the CVs at various scan rates under the non-faradaic region followed by the calculation of the slope of the curve obtained by plotting the difference in the anodic and cathodic current against the scan rate [101,102,103]. From the CVs, the charging current, I_c, of the electrodes at each scan rate was determined via Equation (7):

I_c [A] = (I_anodic − I_cathodic)_OCP

(7)

The C_dl values were evaluated by plotting a graph of the charging current vs. the scan rate and calculating the slope, as shown by Equation (8):

Slope = C_dl [F] = ΔI_C [A]/Δν [V s⁻¹]

(8)

Then, the ECSA values were calculated using the specific capacitance (C_s) of 40 μF cm⁻² [101,102,103] and Equation (9):

ECSA [cm²] = C_dl [μF]/C_s [μF cm⁻²]

(9)

A two-electrode water electrolysis cell was constructed using two of the same Co-based catalysts as the anode and the cathode. The energy efficiency of the cell was calculated using the following Equation (10):

η_electrolyzer = Eth/V_{e at j}

(10)

where E_th = 1.23 V; V_{e at j} is the input voltage required to drive the electrolysis at the current density of interest. The energy efficiency calculated in this study was obtained at j = 10 mA cm⁻².

Electrochemical impedance spectroscopy (EIS) spectra were obtained on a Zahner Zennium potentiostat in the 10 kHz–50 mHz frequency range, with a 10 mV perturbation amplitude. Measurements were carried out potentiostatically at −0.2 V (vs. RHE) in 1 M KOH using the same three-electrode configuration. Equivalent circuit fitting was used to calculate double-layer capacitance and charge transfer resistance values.

4. Conclusions

In summary, Pd-decorated CoP and CoFeP coatings were successfully fabricated on Cu substrate via a two-step process involving electroless deposition followed by galvanic displacement. Structural and morphological characterization, performed using techniques such as scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS), confirmed the successful formation of uniform surface coverage. The synthesized catalysts showed significant activity for both the HER and the OER, which are essential half-reactions in OWS. Specifically, for the HER, the PdCoFeP catalyst exhibited the lowest overpotential value of 207 mV compared to PdCoP, which required 227 mV to attain a current density of 10 mA cm⁻² at 25 °C. However, it delivered a higher current density, indicating more efficient hydrogen generation under higher load conditions. In addition, the OER performance of the PdCoFeP and PdCoP catalysts demonstrated a marginally increased overpotential value of 396 mV at a current density of 10 mA cm⁻² and 25 °C compared to the unmodified CoP and CoFeP catalysts, which exhibited overpotentials of 431 and 435 mV, respectively. These improvements are attributed to the synergistic interactions between Co, Fe, P, and Pd, which enhance electron transfer and facilitate HER/OER kinetics. Meanwhile, the assembled electrolyzer that used PdCoFeP as the cathode and anode required a cell voltage of 1.69 V to achieve 10 mA cm⁻². This study demonstrates that all the synthesized variants of CoP, CoFeP, PdCoP, and PdCoFeP are effective and stable electrocatalysts for overall alkaline water splitting. Among these catalysts, PdCoFeP exhibits particular promise for applications requiring high current densities. EIS measurements showed that the PdCoFeP films exhibited a larger double-layer capacitance and lower charge transfer resistance, confirming the beneficial effects of modification with Pd for HER activity. Overall, these materials have significant potential for use in large-scale hydrogen production, renewable energy storage, and clean fuel cell technologies to support the transition to a sustainable energy future.