High-Performance Bifunctional HER/OER Electrocatalysis Enabled by Solvothermal Cobalt Growth on Screen-Printed Nickel Microparticle Interlayers

1. Introduction

The development of efficient, durable, and low-cost electrocatalysts for alkaline water splitting remains a central challenge in sustainable hydrogen production. Water electrolysis offers a direct route for converting renewable electricity into high-purity hydrogen; however, its large-scale implementation is still limited by the kinetic barriers associated with the hydrogen evolution reaction (HER) and, more critically, the oxygen evolution reaction (OER). Although noble-metal-based catalysts, such as Pt for HER and Ir/Ru oxides for OER, exhibit excellent activity, their high cost and scarcity restrict their practical applications [1,2,3]. Therefore, the design of earth-abundant bifunctional electrocatalysts capable of promoting both HER and OER with low overpotentials and long-term operational stability is of considerable importance.

Among non-noble transition metal-based systems, cobalt-containing oxides and hydroxides have attracted significant attention owing to their rich redox chemistry, tunable oxidation states, and favorable catalytic properties in alkaline media [4,5,6]. In particular, Co-based materials can participate in reversible Co²⁺/Co³⁺ redox transitions and generate hydroxylated surface species relevant to water dissociation, interfacial charge transfer, and OER-related surface reconstruction [7,8]. However, the practical performance of cobalt-based electrocatalysts depends not only on their intrinsic chemical composition but also on their structural integration with the current collector, accessibility of electroactive sites, and stability of the catalyst/support interface during gas-evolving operations [8].

Nickel foam is widely used as a three-dimensional conductive substrate for alkaline electrocatalysis because of its high electrical conductivity, open macroporous architecture, mechanical robustness, and compatibility with the growth of transition metal oxides/hydroxides [9,10]. Its interconnected porous structure facilitates electrolyte penetration and gas release, making it particularly attractive for high-current-density water-splitting electrodes [11]. Nevertheless, the direct deposition of active materials onto pristine nickel foam can lead to non-uniform catalyst coverage, limited interfacial anchoring, partial blockage of macropores, and inefficient utilization of the deposited active phase [12]. These limitations highlight the need for substrate-engineering strategies that can improve catalyst nucleation, adhesion, and electrochemical accessibility without compromising the conductivity of open-cell foams [13].

The surface engineering of nickel foam through intermediate functional layers represents an effective route to address these issues. In this context, screen printing is particularly attractive because it is scalable, low-cost, compatible with patterned deposition, and suitable for fabricating porous coatings on large electrode areas [14,15,16]. A screen-printed nickel microparticle layer can introduce additional roughness, local porosity, and metallic contact points, thereby modifying the nucleation environment for subsequent catalyst growth [14,15,16]. Unlike dense or insulating coatings, a properly designed nickel-based interlayer can preserve the electrical conductivity while increasing the number of anchoring sites available for hydrothermal deposition. However, the role of such screen-printed metallic interlayers in controlling cobalt-based bifunctional HER/OER electrodes has not been sufficiently elucidated.

In this study, we present a bifunctional alkaline water-splitting electrode developed by integrating screen printing and solvothermal cobalt growth on commercial Ni foam. In a previous study, we successfully enhanced a supercapacitor using this approach [16]. Initially, a porous nickel microparticle/polymer interlayer was deposited onto nickel foam via screen printing, resulting in a modified substrate, referred to as NF-m. This interlayer served as a scaffold for the solvothermal formation of cobalt-based oxide/hydroxide nanostructures, culminating in the fabrication of the Co@NF-m electrode. The design aims to augment the geometric roughness of nickel foam while establishing a structurally integrated conductive framework. This framework facilitates cobalt growth, improves catalyst anchoring, and enhances the electrochemical utilization of Co/Ni redox-active sites.

The fabricated electrodes were systematically evaluated using structural, morphological, surface chemical, and electrochemical characterizations. The Co@NF-m electrode exhibited superior bifunctional performance in 1 M KOH, requiring low overpotentials of 61 and 241 mV for the HER and OER, respectively, at 10 mA cm^-2. Importantly, although Co@NF exhibited a higher C_dl-derived apparent electrochemical surface area, Co@NF-m delivered stronger ECSA-normalized HER and OER responses, indicating that the improved activity cannot be attributed solely to a larger capacitive surface area. Instead, the integrated redox-charge analysis and impedance response suggest a more effective participation of electrochemically accessible Co/Ni redox-active species and a more favorable apparent interfacial electrochemical response. Long-term chronopotentiometric measurements further confirmed the operational robustness of the optimized electrode under both HER and OER conditions, while post-stability SEM analysis showed the preservation of the porous morphology without severe delamination. These findings demonstrate that screen-printed nickel microparticle interlayers can serve as effective structural platforms for engineering robust cobalt-based bifunctional electrocatalysts for alkaline water-splitting.

2. Experimental Section

2.1. Materials

All materials used in this study were employed as received, without additional purification. These materials include potassium hydroxide (KOH, technical grade, Sigma-Aldrich), ethanol (CH₃CH₂OH, 99.8%, ACROS Organics), hydrochloric acid (HCl, 37%, Sigma Aldrich), cobalt(II) nitrate hexahydrate (Co(NO₃)₂·6H₂O, 98%, Sigma Aldrich), nickel foam (99.8%, Changzhou DLX Alloy Co, 90% porosity, 90 ppi), nickel powder (Vale Type 255, 99.9%, Fisher, sub-sieve size 2.2-2.6 µm), Zirfon Perl 500 (Agfa Corporate), polyvinyl alcohol (PVA, Sigma Aldrich), polyacrylic acid (PAA, Sigma Aldrich), and polyethylene glycol (PEG, Sigma Aldrich).

2.2. Fabrication of the Modified Ni Foam Substrate

To improve the growth and adhesion of cobalt nanostructures, a nickel foam substrate was modified by incorporating a porous nickel microparticle interlayer via screen printing, as described in our previous study [16]. Briefly, the ink formulation commenced by dissolving polyvinyl alcohol and polyacrylic acid in deionized water to establish a binder-dispersant system. The solution was gently heated to approximately 70 ^οC to ensure complete polymer dissolution and subsequently cooled to room temperature. Polyethylene glycol (PEG) was added as a viscosity modifier and plasticizer to improve the printability and flexibility of the deposited layers. High-purity Ni powder was gradually incorporated into the prepared binder solution under continuous mechanical stirring. The resulting slurry was mixed under vacuum at 250 rpm for 2 h to eliminate trapped air and ensure compositional homogeneity, and was subsequently deposited onto the nickel foam through a screen-printing process to form a thin Ni-255/polymer interlayer on the nickel foam. The printed substrates were dried at room temperature for 24 h, yielding modified nickel foam substrates for subsequent cobalt deposition.

Notably, the NF-m substrate maintained sufficient structural and mechanical integrity after drying at ambient temperature, without requiring a conventional thermal sintering step. The hydrophilic PVA/PAA/PEG binder system not only provided strong adhesion between the nickel particles and nickel foam, but also promoted electrolyte wettability and accessibility of the porous network. Consequently, the modified substrate offered an enlarged accessible surface area, favoring the subsequent homogeneous growth of cobalt nanostructures.

2.3. Solvothermal Fabrication of Co@NF and Co@NF-m

The solvothermal fabrication of Co_xOy electrodes on commercial NF and NF-m was described in our previous studies [6,17]. In summary, 16 mmol of Co(NO₃)₂·6H₂O was dissolved in 40 mL pure ethanol. This solution, along with the NF or NF-M substrates, was placed in a Teflon-lined stainless-steel autoclave and subjected to heating at 120 ^oC for 10 hours. Following this, the Co@NF and Co@NF-m electrodes were repeatedly {Citation}rinsed with deionized water and ethanol, dried at 60 ^oC for 12 hours, and then calcined in air at 300 ^oC for 3 hours with a heating rate of 5 ^oC/min. The use of 16 mmol of the Co precursor was determined based on prior studies, which demonstrated that this quantity represented optimal loading [18,19]. The cobalt species loading was approximately 6 ± 0.2 mg cm^-2, calculated based on the geometric area of the electrode. This loading was determined gravimetrically by weighing the nickel foam substrate before and after solvothermal growth on an analytical balance.

Crucially, the subsequent solvothermal deposition of cobalt served a dual purpose: it drove the uniform growth of active cobalt nanostructures and simultaneously triggered in situ thermal esterification and cross-linking of the ambient-dried PVA/PAA polymer matrix [20]. The hydrothermal environment transforms the water-soluble binder into an insoluble, chemically robust, and highly hydrophilic polymeric network [21]. This in situ engineering approach successfully locks the screen-printed nickel microparticles to the nickel foam substrate, guaranteeing excellent mechanical durability during long-term stability testing while maintaining the open porosity required for high-rate hydrogen evolution.

2.4. Physicochemical Characterization

The crystalline structure, surface morphology, elemental composition, porous characteristics, and surface chemical states of the fabricated electrodes were systematically investigated using complementary physicochemical characterization techniques. The phase composition and crystallographic properties of the samples were examined by X-ray diffraction (XRD) using a Rigaku MiniFlex II diffractometer with Cu Kα radiation (λ = 1.5406 A), operated at 30 kV and 15 mA.

The surface morphology and microstructural features of the electrodes were analyzed by field-emission scanning electron microscopy (FE-SEM, Zeiss Crossbeam 350) at various magnifications. Elemental composition and spatial elemental distribution were determined using energy-dispersive X-ray spectroscopy (EDS) attached to the SEM system. To further investigate the surface chemical composition and oxidation states of the constituent elements, X-ray photoelectron spectroscopy (XPS) analyses were performed using a SPECS FlexMod spectrometer equipped with a monochromated Al Kα X-ray source (hν = 1486.6 eV). Survey and high-resolution core-level spectra were collected at pass energies of 100 and 20 eV, respectively. All spectra were calibrated by referencing the adventitious carbon C 1s peak to a binding energy of 284.8 eV.

2.5. Electrochemical Measurements

The electrocatalytic performance of the fabricated electrodes for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) was systematically evaluated using cyclic voltammetry (CV), linear sweep voltammetry (LSV), staircase voltammetry (SCV), electrochemical impedance spectroscopy (EIS), long-term stability measurements, and a zero-gap two-electrode setup. To assess the reproducibility of the fabrication procedure, three independently prepared electrodes of each type were tested under identical experimental conditions.

All electrochemical measurements were performed using a VersaSTAT 4 potentiostat/galvanostat in a conventional three-electrode configuration at room temperature (25 ^oC). The prepared electrodes served as the working electrodes, while a graphite rod and Ag/AgCl electrode (3.5 M KCl) were employed as the counter and reference electrodes, respectively. Three-electrode HER/OER measurements were performed in 1 M KOH, whereas the zero-gap two-electrode electrolyzer was evaluated in 6 M KOH solution. Prior to each experiment, the electrolyte was purged with high-purity nitrogen (99.999%) for at least 15 min to remove dissolved oxygen and establish stable electrochemical conditions in the cell. All measurements were repeated at least three times using independently fabricated electrodes to verify experimental reproducibility. The potentials measured versus Ag/AgCl were converted to the reversible hydrogen electrode (RHE) scale according to the Nernst equation (1):

$$E_{\mathrm{R}\mathrm{H}\mathrm{E}}=E_{\mathrm{A}\mathrm{g}/\mathrm{A}\mathrm{g}\mathrm{C}\mathrm{l}}+0.059 \mathrm{p}\mathrm{H}+E_{\mathrm{A}\mathrm{g}/\mathrm{A}\mathrm{g}\mathrm{C}\mathrm{l}}^0$$

(1)

where $E_{\mathrm{A}\mathrm{g}/\mathrm{A}\mathrm{g}\mathrm{C}\mathrm{l}}^0$is the standard potential of the Ag/AgCl reference electrode, and the pH of 1 M KOH was taken as 13.6.

For the HER, LSV measurements were performed with 95% dynamic iR compensation. The uncompensated solution resistance (R_s) was determined prior to polarization measurements by high-frequency impedance analysis. No additional post-measurement iR correction was applied. Cyclic voltammetry measurements were conducted in 1 M KOH within the potential range of 1.0 to1.6 V versus RHE. The electrochemically active surface area (ECSA) was estimated from the double-layer capacitance (C_dl) values obtained from CV measurements recorded within a non-Faradaic potential region at scan rates between 5 and 100 mV s^-1.

Staircase voltammetry (SCV) was used to obtain the OER polarization curves in 1 M KOH solution. OER measurements were performed by stepping the potential anodically from 1.0 to 1.8 V. The Tafel slopes were derived from the linear region of the log(j)-η plots selected near 1.5 V vs. RHE for each electrode, where the current response was dominated by oxygen evolution rather than surface redox transitions.

Electrochemical impedance spectroscopy measurements were carried out at a DC bias potential of 1.5 V (for OER) and -0.3 V (for HER) versus RHE using an AC perturbation amplitude of 5 and 10 mV over a frequency range of 10 mHz to 10 kHz. The impedance spectra were analyzed using ZView software and fitted with an equivalent circuit consisting of the solution resistance (R_s) in series with a parallel combination of the charge-transfer resistance (R_ct), and a constant phase element (CPE). The CPE parameters (CPE-T and CPE-P) were used to describe the deviations from the ideal capacitive behavior associated with surface heterogeneity and interfacial effects. When necessary, a Warburg diffusion element (W) was included to account for the mass transport limitations. Owing to the hierarchical porous electrode architecture and the possible overlap between the charge transfer, capacitive, and mass transport processes, the fitted R_ct values were treated as apparent interfacial descriptors rather than absolute kinetic constants. The long-term operational stability of the electrodes was evaluated using chronopotentiometric measurements at constant current densities of 100 mA cm^-2 (for HER) and 50 mA cm^-2 (for OER) for approximately 40 h in 1 M KOH solution. The electrode potential was continuously monitored throughout the test to assess the durability of the catalysts under prolonged operating conditions.

For the 2 electrode setup, a custom-made cell was used along with a Zirfon Perl 500 membrane for gas separation.

3. Results and Discussion

3.1. Structural and Morphological Characterization

The morphology, crystalline structure, and surface chemical composition of the fabricated electrodes were systematically examined to clarify the role of the screen-printed NF-m interlayer in the formation of the Co-based active phase. The NF-m substrate was developed by implementing a porous nickel microparticle interlayer on commercial nickel foam via screen printing. This modification was intended to enhance the surface roughness, promote cobalt growth, and improve the mechanical integration of the subsequently deposited cobalt species, as previously documented.

The SEM images in Figure 1a-d reveal clear morphological differences among the investigated electrodes. The NF-m substrate preserves the open macroporous structure of the pristine nickel foam, as shown in the inset of Figure 1a, and introduces a rough particulate surface composed of Ni microparticles. This rough architecture provides additional anchoring sites for the hydrothermal growth of cobalt-based species. After cobalt deposition, Co@NF-m exhibited a dense and relatively homogeneous coverage of interconnected cobalt-based nanostructures over the modified foam surface (Figure 1b and c). The resulting hierarchical morphology combines the macroporous conductive framework of nickel foam with a rough Ni interlayer and nanoscale Co-containing features, which is expected to improve the electrolyte penetration and increase the number of accessible electroactive sites. In comparison, Co@NF prepared directly on pristine nickel foam displays a more compact surface morphology with larger agglomerated regions (Figure 1d), suggesting that the NF-m interlayer significantly influences cobalt growth and promotes a more structurally integrated active layer structure.

The elemental composition was further supported by EDS analysis (Table 1). The NF-m substrate is mainly composed of Ni with a minor O contribution, which can be attributed to the native surface oxide/hydroxide layer formed upon exposure to air. After cobalt deposition, both Co@NF and Co@NF-m showed the presence of Co, O, and Ni, confirming the successful formation of cobalt-containing surface layers. For Co@NF-m, the detectable Ni signal indicated that the Co-based layer remained porous and did not completely block the underlying metallic framework. In contrast, the lower Ni signal observed for Co@NF suggests a more continuous cobalt-rich coverage on the pristine NF.

The crystalline structures of the electrodes were analyzed using XRD, as shown in Figure 1e. The NF-m substrate exhibited the characteristic reflections of face-centered cubic metallic Ni at 2θ ≈ 44.5^o, 51.8^o, and 76.4^o, assigned to the (111), (200), and (220) planes, respectively. The absence of additional crystalline phases indicates that the screen-printing modification preserved the metallic Ni framework. After cobalt deposition, additional diffraction peaks appeared at approximately 2θ ≈ 19.0^o, 31.3^o, 36.8^o, 44.8^o, 59.3^o, and 65.2^o, which were indexed to the (111), (220), (311), (400), (511), and (440) planes of spinel Co₃O₄, respectively. These reflections confirm the formation of crystalline cobalt oxide on both Co-deposited electrodes.

The crystallite size of Co@NF-m was estimated using the Scherrer equation from the selected XRD reflections, as summarized in Table S1. The Co₃O₄ crystallite size calculated from the intense (311) reflection at 2θ ≈ 36.8^o was approximately 21.7 nm, confirming the nanocrystalline nature of the deposited cobalt oxide. The estimated values for the Ni(111) and Co-related reflections were approximately 16.3 nm and 24.9 nm, respectively. The formation of nanoscale Co₃O₄ domains is beneficial for HER/OER electrocatalysis because it shortens the ion diffusion pathways and increases the density of electrochemically accessible active sites [22,23,24].

The surface chemical composition was evaluated using XPS survey analysis, as shown in Figure 1f. The survey spectra confirmed the presence of Ni, Co, O, and C species on the modified electrodes, verifying the successful deposition of cobalt-based species onto the Ni foam substrates. The Ni signal originates from the metallic nickel foam/interlayer and surface-oxidized nickel species, whereas the Co and O signals are associated with the cobalt oxide/hydroxide surface layer. The C 1s contribution was mainly attributed to adventitious carbon and/or residual carbon-containing surface species. The XPS measurements were calibrated using the adventitious C 1s peak at 284.8 eV, and high-resolution spectra were used to clarify the oxidation states of the surface species [16].

The high-resolution XPS spectra included in the Supporting Information section (Figs. S1-S3) provide additional insights into the surface chemistry. The Ni 2p spectra of NF-m, Co@NF-m, and Co@NF show characteristic Ni²⁺ 2p_3/2 and Ni²⁺ 2p_1/2 components, together with satellite features, indicating the presence of surface-oxidized nickel species. The Co 2p spectra of Co@NF and Co@NF-m displayed Co 2p_3/2 and Co 2p_1/2 peaks accompanied by satellite features, consistent with the mixed Co²⁺/Co³⁺ states in cobalt oxide. This mixed-valence configuration is important for alkaline electrocatalysis because it facilitates reversible surface redox transitions and charge transfer [25,26]. Moreover, the O 1s spectra were deconvoluted into metal-oxygen, surface hydroxyl, and adsorbed H₂O/oxygenated species contributions. The hydroxyl-related component is particularly relevant because surface M-OH species can participate in water dissociation, interfacial redox reactions, and OER-related surface reconstruction [27,28].

Figure 1 illustrates that the NF-m interlayer functions beyond merely serving as a passively conductive support. This alters the surface architecture and morphology, suggesting enhanced growth and anchoring. Additionally, it facilitates the formation of a porous, nanocrystalline, mixed-valence cobalt oxide/hydroxide layer in close contact with the metallic Ni framework. The combination of open-cell foam porosity, screen-printed Ni microparticle roughness, nanocrystalline Co₃O₄ domains, and hydroxyl-rich surface chemistry provides a favorable structural and chemical platform for enhanced bifunctional HER/OER electrocatalytic activity.

Figure 1. SEM images of the fabricated electrodes: NF-m and pristine NF (inset) (a), Co@NF-m (b, c), and Co@NF (d). The XRD patterns (e) and XPS survey spectra (f) verify the presence of Ni, Co, O, and C species on the modified electrodes.

Figure 1. SEM images of the fabricated electrodes: NF-m and pristine NF (inset) (a), Co@NF-m (b, c), and Co@NF (d). The XRD patterns (e) and XPS survey spectra (f) verify the presence of Ni, Co, O, and C species on the modified electrodes.

3.2. Electrochemical Measurements

The electrochemical behavior of the NF-m, Co@NF, and Co@NF-m electrodes was first examined by cyclic voltammetry in 1 M KOH. As shown in Figure 2a, Co@NF-m exhibited a larger CV envelope than both Co@NF and NF-m within the investigated potential window, indicating a stronger overall electrochemical response. Since this potential region includes Co-based surface redox/faradaic processes, the enlarged CV area of Co@NF-m suggests enhanced participation of electrochemically accessible cobalt redox sites rather than a purely capacitive contribution. The quantitative integration of the CV profiles recorded at 10 mV s^-1 further supports this interpretation, showing that Co@NF-m possesses an approximately 3.5 times higher integrated redox charge than Co@NF, indicating a larger apparent contribution from the electrochemically accessible Co/Ni redox-active species (Table 2 and Table S1). This result should be interpreted as a comparative indication of enhanced redox participation rather than an absolute quantification of the number of active sites.

To separate the capacitive contribution from the faradaic processes, the double-layer capacitance was estimated from the CV measurements recorded in a non-faradaic potential region at different scan rates (Figure S4). The average capacitive current density was plotted as a function of scan rate, as shown in Figure 2b. The calculated C_dl values were 10.9 ± 0.4, 23.7 ± 1.3, and 7.2 ± 0.7 mF cm^-2 for NF-m, Co@NF, and Co@NF-m, respectively. The corresponding ECSA values, summarized in Table 1, were estimated according to eq. 2:

ECSA = C_dl/C_s,

(2)

where C_s is the specific capacitance of an ideally smooth electrode surface, which is commonly taken as 40 μF cm^-2 [29]. Based on this approximation, Co@NF exhibited the highest C_dl-derived ECSA, followed by NF-m and Co@NF-m.

Table 2. C_DL and ECSA values of the NF-m, Co@NF, and Co@NF-m electrodes.

Electrode	Cdl (mF cm-2)	Apparent ECSA factor	Relative active charge
NF-m	10.9 ± 0.4	272.5	0.10
Co@NF	23.7 ± 1.3	592.5	1.00
Co@NF-m	7.2 ± 0.7	180	3.51

Table 2. C_DL and ECSA values of the NF-m, Co@NF, and Co@NF-m electrodes.

Electrode	Cdl (mF cm-2)	Apparent ECSA factor	Relative active charge
NF-m	10.9 ± 0.4	272.5	0.10
Co@NF	23.7 ± 1.3	592.5	1.00
Co@NF-m	7.2 ± 0.7	180	3.51

Figure 2. CV at 10 mV s-1 in 1 M KOH (a), and linear relationship between the average capacitive current density and scan rate, obtained from non-faradaic CV measurements, for C_dl estimation of the NF-m, Co@NF, and Co@NF-m electrodes.

Figure 2. CV at 10 mV s-1 in 1 M KOH (a), and linear relationship between the average capacitive current density and scan rate, obtained from non-faradaic CV measurements, for C_dl estimation of the NF-m, Co@NF, and Co@NF-m electrodes.

Interestingly, the C_dl-derived ECSA trend did not follow the trend observed in the wider faradaic CV window. Although Co@NF exhibited the largest apparent capacitive surface area, Co@NF-m displayed the strongest redox response and the highest integrated redox charge. This distinction indicates that the C_dl-derived ECSA should be considered an apparent electrochemical surface descriptor rather than a direct measure of the number or catalytic efficiency of the Co active sites [30]. The superior activity of Co@NF-m is likely related to the more effective integration of the Co-based active phase with the screen-printed NF-m interlayer, which promotes a higher population of electrochemically accessible redox-active sites. Furthermore, as discussed below, Co@NF-m maintained the highest HER and OER responses even after ECSA normalization. This indicates that the improved bifunctional performance was not governed solely by the apparent capacitive surface area.

The bifunctional electrocatalytic activity of the NF-m, Co@NF, and Co@NF-m electrodes was further evaluated for HER and OER in 1 M KOH. The HER polarization curves are shown in Figure 3a, while the corresponding Tafel plots are presented in Figure 3b. Among the examined electrodes, Co@NF-m exhibited the most favorable HER response, requiring an overpotential of only 61 mV to reach 10 mA cm^-2, which is substantially lower than those of NF-m (203 mV) and Co@NF (220 mV). The improved HER activity of Co@NF-m is also reflected in its lower Tafel slope of 76.6 mV dec^-1 compared to those of the NF-m and Co@NF electrodes (Table 3), indicating faster HER kinetics.

A similar trend was observed for the OER activity, as shown in Figure 3c. Co@NF-m again demonstrated the best catalytic performance, reaching 10 mA cm^-2 at an overpotential of 241 mV, whereas Co@NF and NF-m required 304 mV and 420 mV, respectively. The corresponding OER Tafel plots in Figure 3d show Tafel slopes of 92.5, 91.4, and 100.9 mV dec ^-1 for Co@NF-m, Co@NF, and NF-m, respectively. Although the OER Tafel slopes of Co@NF-m and Co@NF are relatively close, the markedly lower OER overpotential of Co@NF-m confirms its superior overall OER activity.

To further distinguish the contribution of the apparent electrochemical surface area from the intrinsic catalytic response, the HER and OER polarization curves were normalized using the corresponding C_dl-derived ECSA values. The ECSA-normalized HER curves are presented in Figure 4a. Despite exhibiting the lowest C_dl-derived ECSA among the Co-containing electrodes, Co@NF-m maintained the highest normalized HER current density over the investigated potential range. This behavior confirms that the superior HER performance of Co@NF-m is not caused by a larger capacitive surface area but rather by a more efficient electrochemical utilization of the active Co/Ni sites.

The same conclusion is supported by the ECSA-normalized OER polarization curves shown in Figure 4b. Co@NF-m again displayed the strongest normalized OER response, clearly outperforming both Co@NF and NF-m. This result is particularly important because Co@NF exhibits the highest C_dl-derived ECSA, whereas Co@NF-m shows the best ECSA-normalized activity. Therefore, the activity trend follows the integrated redox-charge trend more closely than the C_dl-derived ECSA trend. This indicates that the higher bifunctional activity of Co@NF-m is associated with the increased participation of electrochemically accessible redox-active sites, as supported by the approximately 3.5-fold higher integrated redox charge discussed above.

Electrochemical impedance spectroscopy was employed to further evaluate the interfacial electrochemical responses of the NF-m, Co@NF, and Co@NF-m electrodes under HER- and OER-relevant conditions. The corresponding Nyquist plots are shown in Figure 4c and d, and the fitted parameters are listed in Table 4.

Under HER conditions, NF-m exhibited a substantially larger apparent R_ct, indicating sluggish interfacial charge transfer in the absence of the Co-based active phase. After cobalt deposition, both Co-containing electrodes showed a pronounced decrease in the apparent R_ct, with Co@NF-m displaying the lowest value among the HER-tested electrodes. This trend is consistent with the superior HER polarization response and lower Tafel slope of Co@NF-m. However, the relatively low CPE exponent of Co@NF-m indicates a more distributed and non-ideal capacitive response, which is expected for a hierarchical porous electrode architecture.

Under OER-relevant conditions, Co@NF-m also exhibited the most favorable impedance response, with a markedly lower apparent R_ct than both NF-m and Co@NF. This behavior agrees well with the lower OER overpotential and enhanced ECSA-normalized OER activity. The inclusion of Warburg-type contributions in most fits further suggests that ion transport and diffusion-related processes contribute to the overall impedance response of these porous electrodes.

Table 4. EIS fitting parameters of NF-m, Co@NF, and Co@NF-m measured at 1.5 V vs RHE for OER and -0.3 V vs RHE for HER. The fitted R_ct values were treated as apparent charge-transfer resistances owing to the porous electrode architecture and distributed capacitive/transport contributions.

Electrode	Condition	Rs Ω cm2	Rct Ω cm2	CPE-T Fsn-1cm-2	CPE-P, n	W Ω s-1/2
NF-m	HER	3.96	1113	0.01075	0.946	11.04
Co@NF	HER	3.25	46.24	0.283	0.98	3.66
Co@NF-m	HER	4.04	41.25	0.018	0.51	2.62
NF-m	OER	0.74	79.6	0.168	0.98	2.05
Co@NF	OER	1.65	93.25	0.004	0.88	-
Co@NF-m	OER	0.69	1.67	0.0002	0.865	2.59

Table 4. EIS fitting parameters of NF-m, Co@NF, and Co@NF-m measured at 1.5 V vs RHE for OER and -0.3 V vs RHE for HER. The fitted R_ct values were treated as apparent charge-transfer resistances owing to the porous electrode architecture and distributed capacitive/transport contributions.

Electrode	Condition	Rs Ω cm2	Rct Ω cm2	CPE-T Fsn-1cm-2	CPE-P, n	W Ω s-1/2
NF-m	HER	3.96	1113	0.01075	0.946	11.04
Co@NF	HER	3.25	46.24	0.283	0.98	3.66
Co@NF-m	HER	4.04	41.25	0.018	0.51	2.62
NF-m	OER	0.74	79.6	0.168	0.98	2.05
Co@NF	OER	1.65	93.25	0.004	0.88	-
Co@NF-m	OER	0.69	1.67	0.0002	0.865	2.59

Overall, the ECSA-normalized polarization curves and EIS analysis confirm that the enhanced bifunctional activity of Co@NF-m cannot be explained by its apparent capacitive surface area. Instead, the improved HER/OER performance is mainly associated with higher redox-active site participation, favorable interfacial electrochemical behavior, and effective integration of the Co-based active layer within the screen-printed NF-m architecture.

Figure 4. ECSA-normalized HER polarization curves (a) and ECSA-normalized OER polarization curves (b) for the NF-m, Co@NF, and Co@NF-m electrodes in 1 M KOH. Nyquist plots obtained under HER-relevant conditions (c) and OER-relevant conditions (d) are presented, with the inset illustrating the enlarged low-frequency/high-impedance region and the equivalent circuit employed for EIS fitting.

Figure 4. ECSA-normalized HER polarization curves (a) and ECSA-normalized OER polarization curves (b) for the NF-m, Co@NF, and Co@NF-m electrodes in 1 M KOH. Nyquist plots obtained under HER-relevant conditions (c) and OER-relevant conditions (d) are presented, with the inset illustrating the enlarged low-frequency/high-impedance region and the equivalent circuit employed for EIS fitting.

The long-term operational stability of the electrodes was further evaluated under constant current conditions in 1 M KOH. As shown in Figure 5a, during the HER operation at -100 mA cm^-2, Co@NF-m maintained the lowest overpotential throughout the test, indicating a more favorable hydrogen evolution performance under prolonged cathodic polarization. In comparison, Co@NF required a slightly more negative potential, whereas NF-m exhibited the largest cathodic potential, consistent with its inferior HER activity observed in the polarization and Tafel analyses. The nearly stable potential profile of Co@NF-m over the entire testing period demonstrates its good durability under the demanding HER conditions.

A similar stability trend was observed during the OER operation at 50 mA cm^-2, as shown in Figure 5b. Co@NF-m displayed the lowest and most stable anodic potential among the tested electrodes, whereas NF-m required a considerably higher potential and exhibited more pronounced potential fluctuations during anodic polarization. This behavior confirms the limited OER activity of the modified substrate in the absence of the Co-based active phase during the OER. In addition, the less stable response of NF-m may be partly associated with the fact that this substrate consists of an ambient-dried screen-printed Ni/polymer interlayer without the subsequent solvothermal deposition and calcination treatment applied during the fabrication of Co@NF-m. In contrast, the solvothermal growth of the Co-based phase, followed by thermal treatment, is expected to promote stronger structural integration of the Ni microparticle interlayer and the active cobalt oxide/hydroxide phase, thereby contributing to the more stable OER response of Co@NF-m.

To further assess the durability of Co@NF-m after prolonged operation, polarization curves were recorded before and after stability tests. For HER (Figure 5c), the post-stability polarization curve showed only a moderate shift relative to the initial response, indicating that the electrode largely preserved its cathodic activity after operation at -100 mA cm^-2. Similarly, the OER polarization curves before and after stability testing (Figure 5d) remained comparable, with only a slight decrease in the current density after prolonged anodic polarization. This minor activity loss may be associated with surface reconstruction, partial blocking of active sites, or gas bubble-related effects during long-term operation.

Figure 5. Chronopotentiometric stability tests of the NF-m, Co@NF, and Co@NF-m electrodes under HER operation at -100 mA cm^-2 (a) and OER operation at 50 mA cm^-2 (b) in 1 M KOH solution. HER polarization curves of Co@NF-m before and after the stability test (c). OER polarization curves of Co@NF-m before and after the stability test (d). The current densities were normalized to the geometric electrode area.

Figure 5. Chronopotentiometric stability tests of the NF-m, Co@NF, and Co@NF-m electrodes under HER operation at -100 mA cm^-2 (a) and OER operation at 50 mA cm^-2 (b) in 1 M KOH solution. HER polarization curves of Co@NF-m before and after the stability test (c). OER polarization curves of Co@NF-m before and after the stability test (d). The current densities were normalized to the geometric electrode area.

Post-stability SEM analysis of Co@NF-m (Figure S5), further supporting the electrochemical durability results. The electrode retained a rough and porous surface morphology after the stability test, without evidence of severe structural collapse or large-scale delamination of the Co-based active layer. Although local surface rearrangement after prolonged HER/OER operation cannot be excluded, the preservation of the hierarchical texture indicates good mechanical integration between the solvothermal-grown Co-based phase and the screen-printed NF-m interlayer. This morphological robustness is consistent with the stable chronopotentiometric profiles and retained polarization response after durability testing.

In addition, the zero-gap bifunctional two-electrode water-splitting performance was evaluated using the optimized electrode configuration separated by a Zirfon Perl 500 diaphragm in 6 M KOH solution at room temperature. In this configuration, the anode (Co@NF-m) and cathode (Co@NF-m) were pressed against the separator to minimize the interelectrode distance and reduce the ohmic losses while maintaining physical separation between the evolved gases. Chronopotentiometric measurements were performed under galvanostatic conditions, and the recorded values were reported as absolute cell voltages. The assembled zero-gap electrolyzer rapidly reached a stable operating voltage after the initial activation period, indicating efficient ionic transport through the Zirfon separator and good electrical contact in the cell. At a current density of 200 mA cm^-2, the cell maintained a low and nearly steady voltage, demonstrating the suitability of the Co-based NF electrodes for practical alkaline water-splitting operations under intensified conditions. The zero-gap electrolyzer required approximately 1.87 V at 200 mA cm^-2, suggesting a promising high-current two-electrode alkaline water-splitting performance under intensified testing conditions.

Figure 6. Chronopotentiometric response of the zero-gap two-electrode alkaline electrolyzer using a Zirfon Perl 500 separator at 200 mA cm^-2 in 6 M KOH solution.

Figure 6. Chronopotentiometric response of the zero-gap two-electrode alkaline electrolyzer using a Zirfon Perl 500 separator at 200 mA cm^-2 in 6 M KOH solution.

Overall, the combined electrochemical evidence suggests that the superior bifunctional response of Co@NF-m is not controlled by a single performance descriptor, such as the apparent capacitive surface area. Rather, the activity enhancement results from the interplay between electrochemically accessible Co/Ni redox-active sites, efficient integration of the cobalt-based phase with the screen-printed Ni microparticle interlayer, and favorable interfacial electrochemical behavior within the hierarchical porous structure. This interpretation is further supported by the ECSA-normalized polarization response, which decouples the activity trend from the C_dl-derived ECSA, and by the impedance analysis, which indicates improved apparent charge transfer characteristics while retaining distributed porous electrode behavior. The stable chronopotentiometric response and preserved post-operative morphology further confirm that the engineered Co@NF-m architecture combines high activity with structural robustness. Thus, the role of the NF-m interlayer extends beyond simple roughness enhancement, acting as an integrated conductive and mechanically stable platform that improves the utilization and durability of the Co-based active phase.

4. Conclusion

In the present work a novel approach was followed to fabricate a bifunctional HER/OER electrode for effective alkaline water electrolysis. In specific, a porous nickel interlayer was initially applied to a commercial nickel foam via screen printing (NF-m), followed by the solvothermal growth of cobalt-based oxide/hydroxide nanostructures to obtain a multifunctional Co@NF-m electrode. The results clearly demonstrated its superior performance compare to both NF and Co@NF electrodes. Notably, this enhanced performance cannot be explained by a simple increase in the C_dl-derived apparent ECSA, since Co@NF exhibited the highest capacitive surface area. Instead, Co@NF-m exhibited the lowest HER and OER overpotentials, the highest ECSA-normalized activity, and an approximately 3.5-fold higher integrated redox charge than Co@NF. These findings indicate a more efficient utilization of electrochemically accessible Co/Ni redox-active sites. EIS analysis further supported the improved apparent interfacial response of Co@NF-m, while long-term chronopotentiometry and post-stability SEM confirmed stable operation and preservation of the porous morphology of the Co@NF-m electrode. Therefore, the screen-printed nickel microparticle interlayer acts as an effective structural and electrochemical platform for enhancing cobalt-based bifunctional HER/OER electrocatalysis performance.