A high-performance electrocatalyst via graphitic carbon nitride nanosheet-decorated bimetallic phosphide for alkaline water electrolysis

Abstract

Developing renewable and clean energy systems for overall water electrolysis requires low-cost, highly efficient, and stable catalysts. With this motivation, nickel cobalt phosphorus (NiCoP) was electrodeposited onto nickel foam (NF) and then modified with graphitic carbon nitride (g-C₃N₄). The designed g-C₃N₄/NiCoP/NF electrode was used for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline water electrolysis. It exhibited a small overpotential of 80 mV@10 mA cm⁻² with a Tafel slope of 89 mV dec⁻¹ for the HER. It also exhibited an overpotential of 370 mV@10 mA cm⁻² with a Tafel slope of 64 mV dec⁻¹ for the OER. The g-C₃N₄/NiCoP catalyst exhibited satisfactory stability in an alkaline electrolyzer system, in which g-C₃N₄/NiCoP/NF was used as the anode and cathode. Meanwhile, the electrocatalyst requires only a cell voltage of 1.70 V to achieve 10 mA cm⁻² current density for overall water electrolysis.

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1. Introduction

The worldwide energy crisis has led researchers to seek alternative energy sources other than fossil fuels. Among alternative energies, hydrogen can be accepted as the energy of the future.^1,2 Some of the advantages are clean energy, high energy density and energy conversion efficiencies, different ways of storage, and energy carriers.³ In sustainable hydrogen gas (H₂) production, electrochemical and photochemical water splitting can play an important role in tackling global energy crises in an environmentally friendly way.⁴ Overall, water electrolysis occurs with two half-reactions including the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). Under standard conditions, the real potential of water electrolysis is much higher than its thermodynamic potential (1.23 V). It is well known that platinum (Pt) is the most effective HER electrocatalyst.^5,6 However, its high cost and low reserves greatly limit its commercial application.⁷ Therefore, catalyst development using non-noble metal alternatives to Pt is required for high stability, low overpotential, and high catalytic activity. Moreover, Pt oxides do not serve well for the OER as they cause the conductivity to decrease. Other precious metals such as Ir, Rh, Ru and their metal alloys are used as catalysts in the OER.⁸ Although the decomposition of water was first carried out in an acidic medium, electrolysis of water in alkaline media has been studied for industrial applications for centuries.⁹ Alkaline water electrolysis has some disadvantages, such as low current density owing to the increased ohmic loss¹⁰ and small active electrode surface area. Researchers have developed an anion-exchange membrane (AEM) technology to overcome these limitations.¹¹ It is anticipated that the AEM will provide technical and cost advantages in more advanced and large-scale hydrogen production in the future compared to traditional alkaline electrolysis technology.¹¹

Recently, many electrocatalysts such as transition metal-based alloys,¹² sulfides,¹³ phosphides,¹⁴ carbides¹⁵ and selenides¹⁶ have been developed for the HER in alkaline media. Among the electrocatalysts, transition metal phosphides (TMPs) are popular as effective electrocatalysts for water electrolysis.^17–19 Moreover, TMPs have advantages such as a tunable electronic structure, low price, and high durability over a wide pH range. For example, CoP-based materials have unique physical properties, such as superior charge transfer, owing to their P-rich components.²⁰ Acting as a proton acceptor, P can promote the formation of metal hydrides, thereby accelerating hydrogen production by electrochemical desorption. Recent studies have suggested that a second metal can be added to phosphite to improve catalytic performance.^21–23 Lian et al.²⁴ synthesized Co–Fe bimetal phosphides using a precipitation method and achieved an overpotential of 133 mV@10 mA cm⁻² in 1.0 M KOH. Liang et al. synthesized NiCoP on Ni foam using a plasma-assisted approach. The catalyst exhibited a HER overpotential of 32 mV@10 mA cm⁻² current density in alkaline media.²⁵ In another study, NiCoP nanopeapods (the morphological structures of carbon nanotubes and NiCoP's resemblance to pae and pods, respectively; therefore, they are called NiCoP nanopeapods) were synthesized by a three-step hydrothermal–carbonization–phosphorization process. They reported a decrease in the overpotential of up to 82 mV@10 mA cm⁻² with a NiCoP nanopeapod/CNT electrode.²⁶ Although good performance of HER electrodes has been achieved in previous studies, the synthesis processes are complex and time-consuming.

2D materials such as graphene oxide, graphene, graphite carbon nitride (g-C₃N₄), metal chalcogenides, and transition metal carbide, nitride or carbonitrides (MXene) have recently attracted considerable attention owing to their unique properties.^27–30 Among various 2D materials, g-C₃N₄ has a layered structure that can be considered as an N-substituted graphite framework consisting of π-conjugated graphite planes due to the presence of sp² hybridization of C and N atoms.³¹ g-C₃N₄ has high stability, an easy synthesis route, abundance, wide surface area, and tunable electronic structures.^32–34 In addition, g-C₃N₄ also has properties such as being insoluble in acidic, alkaline, and organic solvents and having high thermal stability.³⁵ Despite these important advantages, the use of g-C₃N₄ alone is insufficient because of its poor water dissociation ability.³²

In this study, we carried out the synthesis of NiCoP by electrodeposition, which is an easy and fast synthesis method. Potentiostatic electrodeposition was applied in one step to fabricate films of Ni and Co phosphides on nickel foam (NF). To increase the electrocatalytic activity of NiCoP, the electrode was modified with graphitic carbon nitride. g-C₃N₄ was synthesized by the thermal treatment of urea and deposited on NiCoP/NF by a drop-dry process. To the best of our knowledge, the electrocatalyst obtained as a result of NiCoP electrodeposition on NF and its modification with g-C₃N₄ for use in alkaline water electrolysis has not yet been reported. The electrochemical activity of the NiCoP catalyst was increased by changing the Ni to Co ion ratio. The obtained electrode (g-C₃N₄/NiCoP/NF) was used as both the anode and cathode in the overall water electrolysis (OWE) in alkaline electrolytes.

2. Experimental

2.1 Chemicals

For the synthesis of g-C₃N₄ and NiCoP, urea (ISOLAB), nickel(II) chloride hexahydrate (NiCl₂·6H₂O) (Sigma Aldrich), cobalt(II) chloride hexahydrate (CoCl₂·6H₂O) (Sigma Aldrich), sodium hypophosphite monohydrate (NaH₂PO₂) (ZAG Kimya), ammonium chloride (NH₄Cl) (Emsure) and nitric acid (Emsure) were purchased. Pt/C (5%) (Sigma Aldrich) and ruthenium(IV) oxide (RuO₂) (Sigma Aldrich) were used as reference electrocatalysts for the HER and OER, respectively. Potassium hydroxide (KOH) (Merck), HCl (Merck), ethanol (Merck) and acetone (Merck) were purchased and used without purification. Pure water (≥18 M, Millipore) was used to clean the NF and to prepare electrolytes.

2.2 Characterization

The surface characterization of the materials was carried out by scanning electron microscopy (SEM; FEI-Nova). X-ray photoelectron spectroscopy (XPS) was performed using a monochromatized Al K alpha excitation source (Thermo Scientific K-Alpha model). X-ray diffraction (XRD) analysis was performed using an X-ray device (Model: BRUKER D-8 ADVANCE). Fourier transform infrared (FT-IR) analyses were performed using a JPEGFT-IR PerkinElmer 100 instrument.

2.3 Electrochemical characterization

Linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS) and chronoamperometry (CA) were run in a three-electrode system using a Gamry 1010B potentiostat. The three-electrode configuration was created with an NF (working, 0.5 × 0.5 cm², thickness: 1.6 mm), a platinum plate (counter), and mercury/mercury oxide (Hg/HgO) (reference) in the luggin capillary. The data recorded according to the Hg/HgO were adjusted to a reversible hydrogen electrode (RHE) using the equation E_RHE = E_Hg/HgO + 0.098 + 0.0591 × pH. All the electrochemical analyses were conducted in 8 mL of 1.0 M KOH. NF electrodes were cleaned in 6.0 M HCl, ethanol, acetone, and ultra-pure water by ultrasonication for 5 min. LSV studies were conducted at a 2 mV s⁻¹ scan rate at a potential between 0.3 V and −0.3 V for the HER and 1.0 V and 1.65 V for the OER versus RHE. The logarithmic values of the current density (from the LSV data) versus the overpotential values were plotted in the Tafel graph. Subsequently, a linear fit was applied to the Tafel region, and the Tafel slopes were calculated. EIS measurements were conducted in the frequency range of 0.1 Hz to 10 kHz with an AC amplitude of 10 mV. EIS data were fitted by using the Gamry Echem Analyst program. For the stability test, repetitive LSVs were performed at 100 mV s⁻¹ over 1000 cycles.

2.4 Synthesis of g-C₃N₄

The synthesis procedure of Liu et al.³⁶ was used for g-C₃N₄. Briefly, 10 g of urea was placed in a covered crucible in a muffle furnace (Protherm). Urea was heated to 550 °C for 3 hours under an air atmosphere. Then, the product (yellow color) was cleaned with 0.1 M nitric acid and then with distilled water three times. The obtained bulk material was dried at 80 °C for 24 h in a furnace.

Subsequently, g-C₃N₄ (5 mg) in 5 μL of Nafion and DMF (0.5 mL) was dispersed by ultrasonication for 30 minutes. 50 μL of the solution was deposited on the NF. The electrode was then dried on a hotplate at 80 °C. The drop-drying process was repeated 1, 2, and 3 times to determine the effect of the amount of g-C₃N₄ catalyst on the overpotential. A similar process was used to prepare RuO₂/NF and Pt/C/NF electrodes.

2.5 Synthesis of NiCoP

NiCoP was synthesized by the one-step electrodeposition on the NF.³⁷ First, 2 mM NiCl₂·6H₂O, 2 mM CoCl₂·6H₂O, 5 mM NH₄Cl, and 5 mM NaH₂PO₂ were mixed with 50 mL of distilled water. The solution was transferred to a three-electrode cell. Electrodeposition was carried out at a deposition potential of −1.0 V vs. Hg/HgO and electrodeposition times of 50, 100, 200, 300 and 400 s using CA. For comparison, NiCoP catalysts with different Ni : Co weight ratios were prepared and are represented as NiCoP(0 : 1), NiCoP(1 : 0), NiCoP(1 : 1), NiCoP(3 : 1) and NiCoP(1 : 3).

2.6 Preparation of the g-C₃N₄/NiCoP/NF electrode

An optimized amount of g-C₃N₄ catalyst was dropped on the NiCoP/NF electrode and dried under vacuum at room temperature. The prepared g-C₃N₄/NiCoP/NF electrode was used as the working electrode in a three-electrode configuration in an alkaline electrolyte. The g-C₃N₄/NiCoP/NF electrode also served as both the anode and cathode for overall alkaline water electrolysis in an H-type electrolyzer.

2.7 Measurement of overall water electrolysis

The prepared g-C₃N₄/NiCoP/NF (0.5 × 0.5 cm²) served as an anode and cathode in an H-type electrolyzer. An N-117 proton exchange membrane was used in the H-cell. Electrolysis was carried out in 1.0 M KOH.

3. Results and discussion

3.1 Optimization of conditions

The standard reduction potentials of and at 298 K are −0.28 V and −0.23 V vs. SHE. The closeness of standard reduction potentials of Ni and Co allows co-deposition. It is known theoretically that P cannot be deposited alone in aqueous solutions but can be deposited in the presence of iron group elements (Fe, Ni Co).³⁸ Anuratha et al. reported that the cathodic reduction peak of metal phosphide (Co–P, Ni–P or Ni–Co–P) was around −0.7 V vs. Ag/AgCl and that there were small differences in the peak current densities.³⁸ Different electrodeposition potential values (in the range from 0.7 V to −1.2 V) were tried at CA. It was optimized at −1.0 V. To determine the optimal conditions of the catalysts, the effects of experimental parameters such as the thickness of the catalyst, the ratio of Ni : Co, the electrodeposition time of NiCoP, and the amount of g-C₃N₄ were tested. These parameters were determined by recording the LSV in 1.0 M KOH. First, NiCoP was electrodeposited on the NF electrode using CA at a constant voltage of −1.0 V for 200 s to optimize the Ni : Co ratio. Then, LSV for the OER was applied in the potential range of 1.0 V and 2.1 V (vs. RHE), as shown in Fig. S1a (ESI†). Among the NiCoP(0 : 1), NiCoP(1 : 0), NiCoP(1 : 1), NiCoP(3 : 1), and NiCoP(1 : 3) catalysts, NiCoP(1 : 1) exhibited the highest current density and lowest onset potential. Moreover, the bar graph derived from LSV clearly shows that the Ni : Co ratio of 1 : 1 indicates the lowest overpotential compared to others for the OER (Fig. S1b, ESI†). The thickness of NiCoP was optimized by changing the electrodeposition times to 50, 100, 200, 300, and 400 s, and then, LSV was applied to the prepared electrode, and bar graphs were obtained (Fig. S1c, ESI†). The NiCoP/NF film reached a minimum overpotential value at an electrodeposition time of 100 s and then increased with further electrodeposition time. Fig. S1d (ESI†) shows a bar graph of overpotential at 50 mA cm⁻²versus the thickness of g-C₃N₄ on the NF. g-C₃N₄ on the NF was deposited according to the drop-dry process, and the lowest overpotential value was recorded for the prepared electrode with 100 μL of g-C₃N₄. Finally, g-C₃N₄ (100 μL)/NiCoP (Ni : Co, 1 : 1) (electrodeposition time: 100 s)/NF electrode was used for the HER, OER, and OWE in alkaline electrolytes.

3.2 Evaluation of structural and morphological analyses of the catalysts

FT-IR and XRD analysis were employed to confirm the successful synthesis of g-C₃N₄ and g-C₃N₄/NiCoP catalysts. The FT-IR spectrum of g-C₃N₄ (Fig. 1a) exhibited vibration peaks at 806.8. 1625.8, 1546.0, 1460.3, 1399.2 cm⁻¹, which can be attributed to the bending vibration of the tri-s-triazine ring and stretching vibration of C–N and C=N, respectively.³⁹ The bands at 1313.0 and 1230.5 cm⁻¹ corresponded to the stretching vibration of the C–N(–C)–C or C–NH–C connected units.³⁶ The wide band at 3178.8 cm⁻¹ is attributed to the stretching modes of NH.⁴⁰ In addition, there were no significant differences between the g-C₃N₄ and g-C₃N₄/NiCoP spectra, suggesting that the g-C₃N₄/NiCoP catalyst maintained its chemical structure, similar to that of g-C₃N₄. The FT-IR spectra were compatible with those reported in the literature.⁴¹

Fig. 1 (a) FT-IR and (b) XRD analyses of g-C₃N₄/NF and g-C₃N₄/NiCoP/NF electrodes.

Crystallographic analyses were performed to determine the crystalline phases of the g-C₃N₄/NiCoP catalysts. The XRD patterns of g-C₃N₄ and g-C₃N₄/NiCoP/NF are shown in Fig. 1b. Ni foam shows three diffraction peaks at 2θ = 44.5°, 52.0,° and 76.5°, corresponding to the (1 1 1), (2 0 0) and (2 2 0) lattice planes, respectively.^42,43 The XRD peaks of g-C₃N₄ centered at 27.7° and 13.1° are related to the interlayer stacking of aromatic ring segments and repeating units of tri-s-triazine, respectively.^41,44 The diffraction peaks of g-C₃N₄/NiCoP/NF were located at 2θ = 11.8°, 44.6°, 51.9° and 76.3°. Moreover, no additional diffraction peaks were observed, except for the peak originating from g-C₃N₄, which suggests the presence of NiCoP.

The valence states and elemental compositions of NiCoP/NF and g-C₃N₄/NiCoP/NF electrodes were determined by XPS. Fig. 2a shows the XPS survey spectrum, which confirms the presence of Ni, Co, and P. In addition, N originating from g-C₃N₄ was seen at 409.6 eV. Fig. 2b presents the high-resolution C 1s spectrum of the g-C₃N₄/NiCoP/NF electrode. As depicted in Fig. 2b, the C1s peak is observed at 284.91 and 287.39 eV, which can be attributed to the typical C–C bond and the N–C=N bonds in the tri-s-triazine ring of g-C₃N₄.⁴⁵Fig. 2c depicts the high-resolution O 1s spectra, where the NiCoP/NF and g-C₃N₄/NiCoP/NF electrodes show one peak at 531.44 and 531.77 eV, respectively, related to the P–O bond due to adsorption of NaH₂PO₂.⁴³Fig. 2d exhibits the presence of two peaks at around 874.20 and 856.40 eV, assigned to Ni 2p_1/2 and Ni 2p_3/2, respectively, with two satellite (sat.) peaks stemming from the presence of Ni(II).⁴⁶ The high-resolution Co 2p shows two peaks at 797.55 eV belonging to Co 2p_1/2 and 781.65 eV belonging to Co 2p_3/2, which can be related to the Co–P bond stemming from transition metal phosphides, and its satellite can be attributed to oxidized Co(II) (Fig. 2e).⁴⁷ The high-resolution P 2p indicates one peak at 133.15 eV, which can be associated with the phosphate species (P–O) (Fig. 2f).⁴⁸ According to the above results, the comparison of g-C₃N₄/NiCoP and NiCoP showed small positive and negative shifts in the bond energies of C 1s, O 1s, Ni 2p, Co 2p, and P 2p. This suggests that the addition of g-C₃N₄ increases the electron interactions between the elements. The detailed XPS fitting results of g-C₃N₄/NiCoP and NiCoP are listed in Table S2 (ESI†). The peak intensities of C 1s, O 1s, Ni 2p and Co 2p for g-C₃N₄/NiCoP/NF were higher than those of NiCoP/NF, which demonstrates strong electron interactions after g-C₃N₄. Interestingly, the peak intensity decreases for P 2p, which may be responsible for the charge transfer of g-C₃N₄ from Ni and Co to P, as P may have a lower partial negative charge.⁴⁹

Fig. 2 XPS analyses of NiCoP/NF and g-C₃N₄/NiCoP/NF electrodes: (a) survey spectrum and the high-resolution (b) C 1s, (c) O 1s, (d) Ni 2p, (e) Co 2p, and (f) P 2p.

The surface morphologies of the g-C₃N₄/NF and g-C₃N₄/NiCoP/NF electrodes were investigated by SEM at different magnifications (250× and 5000×), as shown in Fig. 3. The SEM images of g-C₃N₄ show a nanosheet-type morphology (Fig. 3a and b). The high-magnification SEM images of g-C₃N₄/NiCoP indicate uniformly dispersed regular spherical NiCoP nanoparticles with an average size of approximately 45 nm, which can be attributed to the controlled electrodeposition and g-C₃N₄ nanosheets (Fig. 3c and d). These results are consistent with the literature.⁴¹ The EDS analysis is presented in Fig. 3e and f, confirming the simultaneous presence of C, N, Ni, Co, and P.

Fig. 3 Low- and high-resolution SEM images of (a) and (b) g-C₃N₄/NF, (c) and (d) g-C₃N₄/NiCoP/NF. EDS analysis of (e) g-C₃N₄/NF and (f) g-C₃N₄/NiCoP/NF.

3.3 Evaluation of the HER results

Electrocatalytic activity tests of g-C₃N₄ and g-C₃N₄/NiCoP were performed in a three-electrode system in a 1.0 M KOH electrolyte. The electrocatalytic HER performances of the g-C₃N₄, NiCoP, g-C₃N₄/NiCoP, and Pt/C catalysts are recorded at a scanning speed of 2 mV s⁻¹ by LSV, as shown in Fig. 4. It is known that a low overpotential is a very important parameter in determining the activity of a catalyst.⁵⁰ The commercial Pt/C catalyst exhibited an overpotential of 80 mV at a current density of 10 mA cm⁻² with iR correction (50%) (Fig. 4a). The iR correction was developed to correct the voltage loss caused by the electrolyte solution between the working and the reference electrodes, where R is electrolyte solution resistance.⁵¹ The g-C₃N₄/NiCoP catalyst exhibited an overpotential of 80 mV@10 mA cm⁻², similar to that of the Pt/C catalyst. The g-C₃N₄/NiCoP catalyst exhibited a higher electrocatalytic activity than NiCoP and g-C₃N₄. When compared with the studies in the literature, the HER overpotential competes with those in the literature, as shown in Table S1 (ESI†). The Tafel slope can be found through the reaction mechanism and is often used to determine the HER and OER performance of the catalyst.⁵² Tafel plots were derived from the LSV curves to investigate the HER mechanism of the electrocatalysts (Fig. 4b). The Tafel slopes of g-C₃N₄, NiCoP, g-C₃N₄/NiCoP, and Pt/C were estimated as 121 mV dec⁻¹, 112 mV dec⁻¹, 96 mV dec⁻¹ and 32 mV dec⁻¹, respectively. The Tafel slope of g-C₃N₄/NiCoP indicates that the reaction occurs via the Volmer–Heyrovsky mechanism. Electrochemical desorption of H₂ is the rate-determining step in the kinetic process.⁵³ It has been reported that the Tafel step is the rate-determining step if the Tafel slope is below 30 mV, the Volmer step if the Tafel slope is above 120 mV dec⁻¹, and the Heyrovsky step if the slope is between 40–120 mV dec⁻¹.⁵⁴ The Tafel slope of g-C₃N₄/NiCoP is in the range of 40–120 mV dec⁻¹, and the Heyrovsky step is the rate-determining step. The Tafel slope of g-C₃N₄/NiCoP is lower than those of the other electrocatalysts, indicating superior HER kinetics. The typical HER mechanism for the alkaline medium is as follows:⁵⁵

H₂O + e⁻ + M → MH_ads + OH⁻, Volmer reaction

MH_ads + H₂O + e⁻ → H₂(g) + OH⁻ + M, Heyrovsky reaction

MH_ads + MH_ads → H₂(g) + 2M, Tafel reaction

where M represents the metal active site and H_ads is the adsorbed H species. First, the adsorption of water onto the surface leads to the formation of a hydrogen atom adsorbed surface (MH_ads) and OH⁻ intermediate (Volmer reaction). Then, H₂ desorption from the surface occurs during the Heyrovsky or Tafel reactions.

Fig. 4 LSVs of the catalysts of g-C₃N₄, NiCoP, g-C₃N₄/NiCoP, and Pt/C (a) (50% of iR-correction), (b) Tafel curves, and (c) EIS spectra. (d) LSVs of the g-C₃N₄/NiCoP for 1st, 500th, and 1000th cycles (inset: Nyquist plot of g-C₃N₄/NiCoP in initial and post-1000 cycles.)

The effect of g-C₃N₄ on HER activity was investigated, and the charge-transfer resistance (R_ct) of the catalysts was compared. The Nyquist plots of the catalysts are shown in Fig. 4c. The data fitted by the constant phase element (CPE) equivalent (inset of Fig. 4c) and estimated values are tabulated in Table S3 (ESI†). The g-C₃N₄/NiCoP catalyst has a low R_ct value of 4.84 kΩ, while the g-C₃N₄ and NiCoP have a large R_ct of 7.29 and 5.16 kΩ, respectively. The small R_ct value indicates a faster electron transfer rate of the g-C₃N₄/NiCoP catalyst than g-C₃N₄ and NiCoP, which contributes to enhancing the HER activity.^56,57 The stability of the g-C₃N₄/NiCoP catalyst was investigated for the HER in alkaline media by repeated 1000 LSV at 100 mV s⁻¹ (Fig. 4d). The relative standard deviation (RSD) was estimated to be 27% at 10 mA cm⁻². Moreover, an amperometric current–time plot at a constant overpotential value (80 mV vs. RHE) was recorded for 15 h, as shown in Fig. S2a (ESI†). The catalyst showed an excellent HER stability. The SEM image and corresponding XRD pattern after the HER stability test are shown in Fig. S3 and S4 (ESI†), respectively. When the SEM images of the g-C₃N₄/NiCoP catalyst before and after HER are compared, no significant difference is observed (Fig. S3a–c, ESI†). The XRD structural characterization also supports these conclusions (Fig. S4, ESI†).

3.4 Evaluation of the OER results

The most probable mechanism for the OER in the alkaline electrolyte is as follows:⁵⁸

M + OH⁻ → MOH + e⁻

MOH + OH⁻ → MO + H₂O(l) + e⁻

MO + OH⁻ → MOOH + e⁻

MOOH + OH⁻ → MOO + H₂O(l) + e⁻

MOO → M + O₂(g)

This mechanism is believed to occur in the presence of transition metal sites and phosphate catalysts, which are generally considered to be catalytically active centers.^59,60 In the above equations, M represents a metal element with an active surface site. During this process, M reacts with the hydroxyl anions to form MOH. Then, MOH splits into a proton to form a water molecule and MO. MO recombines with hydroxyl anions to form MOOH. This process continues with MOO in the presence of hydroxyl anions, followed by the evolution of O₂.

The electrocatalytic activity of the g-C₃N₄/NiCoP catalyst in the OER was evaluated at a scan rate of 2 mV s⁻¹ in 1.0 M KOH using the LSV technique. As shown in Fig. 5a, the g-C₃N₄/NiCoP catalyst exhibits excellent OER performance, not only as a commercial RuO₂ catalyst, which is known to be the best electrocatalyst for the OER with a low overpotential value at a current density of 10 mA cm⁻², but is also superior to NiCoP and g-C₃N₄. The g-C₃N₄/NiCoP catalyst only required an overpotential of 370 mV@10 mA cm⁻² with iR correction (50%). The catalytic current density of g-C₃N₄/NiCoP is 1.7 times higher than that of g-C₃N₄. The possibility of the formation of double Co–N and Ni–N bonds between g-C₃N₄ and NiCoP can facilitate charge transfer between Ni, Co, and P in the g-C₃N₄/NiCoP catalyst, triggering more active reaction sites and enhancing the electrocatalytic activity. The OER overpotential is moderate compared to that reported in the literature (Table S1, ESI†). The Tafel slopes of the catalysts are shown in Fig. 5b. The g-C₃N₄/NiCoP catalyst shows a Tafel slope of 64 mV dec⁻¹, whereas the g-C₃N₄ and NiCoP catalysts exhibit higher values of 65 and 70 mV dec⁻¹, respectively. This proves that g-C₃N₄ can lead to superior activity due to an increase in the reaction rate and kinetics. The Nyquist plots are shown in Fig. 5c. The g-C₃N₄/NiCoP catalyst exhibited a low R_ct according to g-C₃N₄ and NiCoP catalysts. Consecutive 1000 LSV measurements were performed at a scan rate of 100 mV s⁻¹ to determine the stability of the g-C₃N₄/NiCoP catalyst, as shown in Fig. 5d. Compared to the initial and post-1000 LSV cycles, there was no significant change in the current densities and onset potentials. This was also supported by the current–time plot at 370 mV of overpotential for 15 h (Fig. S2b, ESI†). The post-OER SEM images are similar to those of the pre-HER of the g-C₃N₄/NiCoP catalyst (Fig. S3a, b and d, ESI†). Post-OER XRD measurements support the stability of the catalyst (Fig. S4, ESI†). These results were consistent with the EIS results (inset of Fig. 5d). Stability testing showed that this electrode was stable for a long time without any significant deviation.

Fig. 5 The LSVs of catalysts of g-C₃N₄, NiCoP, g-C₃N₄/NiCoP, and RuO₂ (a) (50% of iR-correction), (b) Tafel curves, (c) EIS spectra. (d) LSVs of the g-C₃N₄/NiCoP for 1st, 500th, and 1000th cycles (inset: Nyquist plot of g-C₃N₄/NiCoP in initial and post-1000 cycles).

The water electrolysis turnover frequency (TOF) of the electrocatalyst is an important activity parameter that reflects the HER or OER kinetics of the catalytic material.⁶¹ The TOF is calculated as follows^61,62:

where j is the current density (A cm⁻²), N_A is the Avagodro number, n is the number of electrons (2 for H₂, 4 for O₂), F is the Faraday constant (96 485 C) and Γ is the surface concentration of active sites. Γ is calculated from the recording of chronoamperomograms at the same overpotential and the current–charge relationship. The TOF parameters of the catalysts are presented in detail in Table S4 (ESI†). The TOF values for the HER were calculated for g-C₃N₄/NiCoP/NF (0.029 s⁻¹), NiCoP/NF (0.027 s⁻¹) and g-C₃N₄/NF (0.027 s⁻¹) at the same overpotential. The TOF values of g-C₃N₄/NiCoP/NF, NiCoP/NF and g-C₃N₄/NF for the OER were calculated as 0.047 s⁻¹, 0.021 s⁻¹ and 0.022 s⁻¹, respectively. Remarkably, the TOF value of the g-C₃N₄/NiCoP/NF electrode was much higher than those of NiCoP/NF and g-C₃N₄/NF. These results show the superior intrinsic activity for the HER and OER of the g-C₃N₄/NiCoP electrocatalyst.

3.5 Evaluation of the overall water electrolysis results

Based on the HER and OER performance of the g-C₃N₄/NiCoP catalyst, the g-C₃N₄/NiCoP/NF electrode was used as both the anode and cathode material to determine the catalytic properties of this electrode in the OWE system (Fig. 6). The cell voltage of the NF/NiCoP/g-C₃N₄‖g-C₃N₄/NiCoP/NF pair needs to be 1.70 V@10 mA cm⁻². This value is very close to the cell voltage (1.69 V@10 mA cm⁻²) of the NF/Pt/C‖RuO₂/NF pair, which is used as a reference. The performance of the g-C₃N₄/NiCoP/NF electrolyzer was compared with that of the electrocatalysts containing NiCoP in the literature and it was found to be competitive, as shown in Table S1 (ESI†). Moreover, the electrochemical stability of the NF/NiCoP/g-C₃N₄‖g-C₃N₄/NiCoP/NF electrolyzer was examined by LSV measurements for the initial and post-1000 cycles at 100 mV s⁻¹ (Fig. 6b). The photograph shows the used water electrolysis system, as shown in the inset of Fig. 6b. As a result, the NF/NiCoP/g-C₃N₄‖g-C₃N₄/NiCoP/NF electrolyzer exhibited a low cell voltage and high stability for the OWE in alkaline media.

Fig. 6 (a) LSV curves of catalysts for overall water electrolysis, (b) electrochemical stability test on g-C₃N₄/NiCoP electrocatalysts, and (c) photograph of the alkaline water electrolysis system.

4. Conclusions

In summary, NiCoP is successfully synthesized by electrodeposition on the nickel foam. Then, the NiCoP/NF electrode is modified with g-C₃N₄ using a drop-dry process. g-C₃N₄ accelerates the charge-transfer process and enhances the electrocatalytic activity of the NiCoP catalyst. The g-C₃N₄/NiCoP/NF electrode exhibits excellent catalytic activity and stability toward both the HER and OER. The electrode requires a low overpotential of 80 mV and 370 mV at 10 mA cm⁻² for the HER and OER, respectively. The g-C₃N₄/NiCoP/NF electrode is employed as the cathode and anode in an alkaline water electrolyzer. It requires a cell voltage of 1.70 V to achieve a current density of 10 mA cm⁻². Moreover, the water electrolyzer exhibits a good long-term electrochemical stability. The g-C₃N₄/NiCoP/NF electrode can be used as an efficient electrocatalyst for overall water electrolysis because of its both low overpotential and high current density.

Conflicts of interest

There are no conflicts to declare.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4cp00020j

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