Construction of a CoP/MnP/Cu₃P heterojunction for efficient methanol oxidation-assisted seawater splitting

Abstract

Methanol oxidation-assisted direct seawater electrolysis has emerged as a potent technology for efficient hydrogen (H₂) production alongside high-value chemicals such as formic acid and formaldehyde. However, the large-scale application of this technology heavily relies on developing highly active and robust bifunctional electrocatalysts for methanol oxidation and hydrogen evolution reactions (MOR/HER). Herein, we report a simple hydrothermal-phosphorylation method to synthesize a heterostructured catalyst on copper foam, comprising CoP, MnP, and Cu₃P (CoP/MnP/Cu₃P@CF). The synergistic interaction among the heterogeneous components endowed CoP/MnP/Cu₃P@CF with excellent MOR, oxygen evolution reaction (OER), and HER performance in alkaline seawater electrolytes. Notably, the MOR-assisted CoP/MnP/Cu₃P@CF-based seawater electrolyzer catalyst required only 1.410 V to achieve a current density of 10 mA cm⁻², significantly lower than the 1.681 V required for an OER–HER seawater electrolyzer. Additionally, the MOR-assisted electrolyzer exhibits high faradaic efficiency and cycling stability, offering the potential for sustainable energy-efficient H₂ production.

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1. Introduction

The global surge in energy demand, compounded by climate change and reliance on fossil fuels, has spurred the quest for sustainable and clean energy solutions.^1–7 Hydrogen (H₂) is emerging as a formidable renewable energy carrier with its high energy capacity and clean combustion.^8–10 Traditional H₂ production methods, however, are both energy-intensive and emit greenhouse gases.^11–14 Utilizing renewable sources for water electrolysis offers a greener alternative. Particularly critical is developing technologies for continuous seawater electrolysis to produce H₂ on a large scale.^15,16 A primary challenge in the electrocatalytic splitting of seawater is the slow kinetics and high potential required for the oxygen evolution reaction (OER) at the anode, which leads to significant energy consumption and potential competitive chlorine evolution reactions.^17–20 Researchers have thus explored alternative anodic reactions that are thermodynamically favorable, enhancing the energy efficiency of H₂ production at the cathode. During electrolysis, a promising strategy involves oxidizing small organic molecules like alcohols, urea, glucose, 5-hydroxymethylfurfural, and hydrazine.^21–27 Methanol oxidation, in particular, offers lower overpotentials and faster kinetics than the OER, potentially enabling more efficient H₂ production. Additionally, methanol oxidation can yield value-added chemicals like formaldehyde, formic acid, and dimethyl ether, making it a viable option for low-energy, large-scale H₂ production.

Recent advances have significantly boosted the performance of non-precious metal electrocatalysts for methanol oxidation-assisted seawater electrolysis.^28,29 Innovations include crystal phase engineering, chemical doping, and the creation of heterostructures.^30–33 Constructing heterostructures promotes electron redistribution at interfaces, producing synergistic effects and enhanced electrocatalytic performance due to lattice mismatches.^34–36 Transition metal phosphides (TMPs), known for their abundant availability, high conductivity, and catalytic activity, have garnered significant attention in this context.^37–39 As a result, constructing heterostructures based on TMPs has become a promising approach to enhance the energy efficiency of methanol oxidation-assisted hydrogen production. For example, Zhang et al.⁴⁰ developed a CoP@NiFe LDH via a straightforward electrodeposition–phosphorization–electrodeposition method. This structure optimized the electronic configuration and improved the catalyst's corrosion resistance in seawater, achieving 100 mA cm⁻² at just 260 mV. Kandel et al.⁴¹ synthesized a Ni₂P-MnP@Co₂P@NF heterostructured catalyst that enhanced charge transfer and exposed more active sites, exhibiting 10 mA cm⁻² at only 255 mV during OER tests. Chang et al.⁴² designed an amorphous Ni_xCo_yFe_z–P heterostructured catalyst via mild electro-deposition. Adding 2 M methanol to the electrolysis system reduced the potential by 110 mV at 20 mA cm⁻². Nonetheless, crafting phosphide-based heterostructures that are both highly active and stable remains a formidable challenge.

In this study, we prepared a heterostructured catalyst on copper foam composed of CoP, MnP, and Cu₃P (CoP/MnP/Cu₃P@CF) using a hydrothermal-phosphorization method. The synergistic interaction among each component optimized the electronic structures at the active centers, enhancing their MOR, OER, and HER activities. As a result, a MOR–HER seawater electrolyzer equipped with the CoP/MnP/Cu₃P@CF catalyst exhibited a current density of 10 mA cm⁻² at a low cell voltage of 1.410 V, which is 0.271 V higher than that required for an OER–HER electrolyzer. Moreover, the methanol oxidation-assisted electrolysis cell demonstrated high Faraday efficiency and stability, indicating its potential for practical applications.

2. Experimental

2.1 Chemicals and reagents

In the experiments, copper foam (CF), ethanol (C₂H₅OH), deionized water, cobalt nitrate hexahydrate (Co(NO₃)₂·6H₂O), manganese nitrate solution (50 wt%), urea, ammonium fluoride, sodium hypophosphite monohydrate, potassium hydroxide, and methanol were used. Seawater was collected from the beach in Rizhao, Shandong Province.

2.2 Material preparation

2.2.1 Synthesis of the Mn precursor. The precursor was synthesized using a simple hydrothermal method. CF was cut into small pieces of 1 × 2 cm and ultrasonically cleaned with ethanol and water for 3 minutes. A solution comprising 1.0 mL of manganese nitrate, 0.5 g of urea, and 0.6 g of ammonium fluoride was dissolved in 20 mL of deionized water. After ultrasonication for 10 minutes, a homogeneous transparent liquid was obtained. The cleaned CF pieces were immersed in the solution and placed in a Teflon-lined autoclave for heating at 120 °C for 4 hours. The resulting sample was washed with ethanol and dried at 60 °C.

2.2.2 Synthesis of the MnCo precursor. A solution of 1.0 g cobalt nitrate hexahydrate, 0.5 g urea, and 0.6 g ammonium fluoride was dissolved in 20 mL of deionized water. After 30 minutes of ultrasonication, a uniform red liquid was achieved. The Mn precursor was immersed in the solution and placed in a Teflon-lined autoclave for heating at 110 °C for 2 hours. The sample was then washed with ethanol and dried at 60 °C.

2.2.3 Synthesis of CoP/MnP/Cu₃P@CF. Sodium hypophosphite monohydrate (0.3 g) and the MnCo precursor were placed separately in upstream and downstream ceramic boats in a tubular furnace. The upstream boat was annealed at 350 °C, and the downstream boat was annealed at 300 °C for 1 hour to obtain CoP/MnP/Cu₃P@CF.

2.2.4 Synthesis of Cu₃P@NF. Sodium hypophosphite monohydrate (0.3 g) and CF were placed separately in upstream and downstream ceramic boats in a tubular furnace. The upstream boat was annealed at 350 °C and the downstream boat was annealed at 300 °C for 1 hour to obtain Cu₃P@NF.

2.2.5 Synthesis of MnP/Cu₃P@CF and CoP/Cu₃P@CF. Following a similar hydrothermal method, but without the addition of cobalt nitrate hexahydrate or manganese nitrate solution, followed by phosphidation for 1 hour using a tubular furnace, resulted in MnP/Cu₃P@CF and CoP/Cu₃P@CF, respectively.

2.3 Material characterization

X-ray diffraction (XRD) tests were performed using a Bruker D8 instrument from Germany. Scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) were conducted using a Carl Zeiss Sigma 300 and a Thermo Fisher Talos F200X, respectively. X-ray photoelectron spectroscopy (XPS) was performed using a Thermo Fisher ESCALAB 250Xi, calibrated with C 1s at 284.8 eV. Fourier-transform infrared spectroscopy (FTIR) was conducted with a DEEP-FTIR-2 by Zhonghuan Technologies Beijing.

2.4 Electrochemical analysis

Electrochemical measurements were performed on a CHI760E electrochemical workstation from Shanghai Chenhua using a three-electrode system with Cu₃P@CF, MnP/Cu₃P@CF, CoP/Cu₃P@CF, and CoP/MnP/Cu₃P@CF serving as the working electrodes. Seawater + 1 M KOH and seawater + 1 M KOH + 1 M methanol were used as electrolytes. The main ions in the seawater were Cl⁻ (19 800 ppm), K⁺ (500 ppm), Na⁺ (11 000 ppm), Ca²⁺ (300 ppm), Br⁻ (50 ppm), and Mg²⁺ (1500 ppm). In situ IR measurements were conducted using a specialized electrolyzer equipped with an MCT detector and a DEEP-FTIR-2 Fourier transform infrared spectrometer. The in situ IR data were collected using an electrochemical workstation (CHI 760E) within a potential range of 1.2 to 1.8 V vs. RHE.

The electrocatalytic activity of the samples was primarily evaluated using linear sweep voltammetry (LSV) at a scan rate of 5 mV s⁻¹ with a 95% iR compensation. All potentials were recalibrated to the reversible H₂ electrode potential using the following equation:

E_RHE = E_Hg/HgO + 0.098 V + 0.0592 pH (1)

where E_RHE is the reversible H₂ electrode potential and E_Hg/HgO is the mercury/mercuric oxide potential.

The polarization curve was converted to a Tafel slope plot using the following equation:

η = a + b log[j] (2)

The turnover frequency (TOF) was calculated as follows:

where j is the current density, n is the number of electrons transferred, N is the number of active sites, and F is the Faraday constant.

The measurement and calculation of N followed previously reported methods.^43,44 Cyclic voltammetry (CV) measurements were conducted at a fixed scan rate of 50 mV s⁻¹ within a potential range relative to the RHE. The integral of the charge over the entire potential range of the CV curve provided the surface charge density (Q_s), calculated as half of the charge value.

The formula for calculating N is:

The electrochemical impedance spectroscopy (EIS) measurements were typically conducted within a frequency range of 10⁻² to 10⁶ Hz, with an AC voltage amplitude disturbance of 5–10 mV.

The electrochemical surface area (ECSA) was calculated using CV measurements at different scan rates in the non-faradaic region to obtain the C_dl value, plotted as a slope that was further used to deduce the ECSA. The ECSA can be calculated using:^45,46

where C_dl is the double-layer capacitance, C_s is the specific capacitance (0.040 mF cm⁻²), and A is the working electrode area (1 cm²).

3. Results and discussion

3.1 Morphology and structure characterization

As illustrated in Fig. 1a, the CoP/MnP/Cu₃P@CF samples were synthesized through a hydrothermal-phosphating method. Specifically, CF is added to an aqueous solution containing Mn²⁺, and the Mn precursor is obtained after a hydrothermal process. Subsequently, the Mn precursor is immersed in an aqueous solution containing Co²⁺, to deliver the CoMn precursor. After a phosphating process, CoP/MnP/Cu₃P@CF was obtained.

Fig. 1 (a) Schematic diagram of the preparation route of CoP/MnP/Cu₃P@CF. (b) TEM, (c) HRTEM images, and (d) EDS mapping of CoP/MnP/Cu₃P@CF.

The morphologies of the Mn precursor, MnCo precursor, and the CoP/MnP/Cu₃P@CF were analyzed using scanning electron microscopy (SEM). As shown in Fig. S1a and b (ESI†), MnCO₃ (Mn precursor) spheres with a diameter of approximately 3 μm were grown on the copper foam skeleton. In the second step of the hydrothermal process, Co(CO₃)_0.5(OH)·0.11H₂O particles were grown on MnCO₃ spheres (Fig. S1c and d, ESI†). After the phosphatizing process, CoP/MnP/Cu₃P@CF was obtained, and the spherical morphology is basically preserved (Fig. S1e, ESI†). The transmission electron microscopy (TEM) image in Fig. 1b confirms the irregular morphology of CoP/MnP/Cu₃P@CF. A high-resolution TEM (HRTEM) image depicted lattice spacings of 0.282 nm, 0.202 nm, 0.196 nm, and 0.200 nm, corresponding to the (011) plane of CoP, (021) and (220) planes of MnP, and the (300) plane of Cu₃P, respectively (Fig. 1c). Energy dispersive spectroscopy (EDS) elemental mapping demonstrated the homogeneous Co, Mn, Cu, and P distribution within the particles (Fig. 1d).

The X-ray diffraction (XRD) patterns in Fig. 2a suggest that CoP/MnP/Cu₃P@CF comprises CoP (PDF No. 65-1474), MnP (PDF No. 51-0942), and Cu₃P (PDF No. 71-2261). X-ray photoelectron spectroscopy (XPS) was employed to characterize the elemental composition and electronic structure of the CoP/MnP/Cu₃P@CF electrocatalyst (Fig. 2b). The high-resolution XPS spectrum for Co 2p (Fig. 2c) displayed satellite peaks at 803.09 and 786.54 eV, while Co 2p_1/2 and Co 2p_3/2 satellite peaks were observed at 798.24 and 782.25 eV, respectively.⁴⁷ The Co-P satellite peaks were located at 793.81 and 778.83 eV. In the Mn 2p spectrum (Fig. 2d), satellite peaks appeared at 648.51 eV, with MnP-related peaks at 653.65 (Mn 2p_1/2) and 641.85 eV (Mn 2p_3/2). Additional peaks at 655.76 and 643.96 eV were attributed to Mn 2p_1/2 and Mn 2p_3/2 of Mn⁴⁺, respectively.⁴⁸ The Cu 2p spectrum (Fig. 2e) shows peaks at 952.82 (Cu 2p_1/2) and 933.11 eV (Cu 2p_3/2) for Cu₃P, while peaks at 955.27 and 935.92 eV were assigned to Cu 2p_1/2 and Cu 2p_3/2 of Cu²⁺, respectively.⁴⁹Fig. 2f shows the P 2p spectrum of CoP/MnP/Cu₃P@CF, in which the P–O peak was observed at 134.38 eV, and the P 2p_1/2 and P 2p_3/2 peaks were located at 130.82 eV and 129.77 eV, respectively.⁵⁰ The presence of P–O peaks is ascribed to the partial surface oxidation of the catalyst.^51,52

Fig. 2 (a) XRD pattern and (b) survey XPS spectra of CoP/MnP/Cu₃P@CF. High-resolution (c) Co 2p, (d) Mn 2p, (e) Cu 2p, (f) P 2p of CoP/MnP/Cu₃P@CF.

3.2 OER performance in alkaline seawater

The oxygen evolution reaction (OER) performance of CoP/MnP/Cu₃P@CF was investigated in a seawater electrolyte containing 1 M KOH using a three-electrode system.⁵³Fig. 3a shows that the CoP/MnP/Cu₃P@CF catalyst exhibits superior OER catalytic activity in the alkaline seawater electrolyte. Fig. 3b shows that the CoP/MnP/Cu₃P@CF requires 1.530 V, 1.590 V, and 1.621 V to achieve current densities of 10, 100, and 200 mA cm⁻², respectively, lower than 1.636 V, 1.776 V, and 1.809 V for the benchmark Cu₃P@CF, 1.586 V, 1.678 V, and 1.736 V for MnP/Cu₃P@CF, and 1.533 V, 1.617 V, and 1.658 V for CoP/Cu₃P@CF. To evaluate the reaction kinetics of the synthesized catalyst during the OER process, Tafel plots were created (Fig. 3c). The Tafel slope for CoP/MnP/Cu₃P@CF in alkaline seawater is 43 mV dec⁻¹, lower than those of Cu₃P@CF (135.6 mV dec⁻¹), MnP/Cu₃P@CF (70.8 mV dec⁻¹), and CoP/Cu₃P@CF (60.9 mV dec⁻¹), indicating enhanced reaction kinetics and suggesting the synergistic effect of each component in the electrocatalyst.^54–56 For further assessment of the intrinsic activity of the electrocatalysts in the OER, TOF values were calculated at specific potentials.⁵⁷ CV curves (Fig. S2a–d, ESI†) measured in the range of 0–0.6 V (vs. RHE) were used to compute the Q_s values for Cu₃P@CF, MnP/Cu₃P@CF, CoP/Cu₃P@CF, and CoP/MnP/Cu₃P@CF. As shown in Fig. 3d, at working potentials of 1.6 V, 1.625 V, and 1.65 V, the TOF values for CoP/MnP/Cu₃P@CF are 0.113, 0.191, and 0.305 s⁻¹, respectively, much higher than 0.013, 0.028, and 0.046 s⁻¹ of Cu₃P@CF, 0.018, 0.038, and 0.063 s⁻¹ of MnP/Cu₃P@CF, and 0.073, 0.112, and 0.172 s⁻¹ of CoP/Cu₃P@CF. These results confirm the superior OER catalytic activity of CoP/MnP/Cu₃P@CF. Electrochemical impedance spectroscopy (EIS) results confirm that CoP/MnP/Cu₃P@CF exhibits the lowest charge transfer resistance of 1.101 Ω compared to 13.940 Ω for Cu₃P@CF, 3.068 Ω for MnP/Cu₃P@CF, and 1.289 Ω for CoP/Cu₃P@CF (Fig. 3e and Fig. S3, ESI†), indicating faster electron transfer during OER electrolysis with CoP/MnP/Cu₃P@CF. Finally, constant current tests performed for 24 h at 20 mA cm⁻² during the OER, as shown in Fig. 3f, indicate stable catalytic performance with no significant degradation, demonstrating the remarkable corrosion resistance and catalytic longevity of the CoP/MnP/Cu₃P@CF electrode in the alkaline seawater electrolyte during the OER. Moreover, the OER performance of CoP/MnP/Cu3P@CF is superior to those reported in many recent reports (Table S1, ESI†).

Fig. 3 (a) OER LSV curves, (b) potential plots, (c) Tafel plots, (d) TOF values, and (e) Nyquist plots of Cu₃P@CF, MnP/Cu₃P@CF, CoP/Cu₃P@CF, and CoP/MnP/Cu₃P@CF in seawater + 1 M KOH electrolyte. (f) OER stability test of CoP/MnP/Cu₃P@CF at 20 mA cm⁻².

3.3 Methanol oxidation reaction performance in alkaline seawater

Similarly, we explored the methanol oxidation reaction (MOR) performance of CoP/MnP/Cu₃P@CF in a seawater electrolyte containing 1 M KOH and 1 M methanol, employing a three-electrode setup. Fig. 4a reveals the superior MOR catalytic activity of the CoP/MnP/Cu₃P@CF catalyst compared to that of the comparison samples. The introduction of methanol significantly lowers the onset potentials for oxidation reactions compared to the OER process. As depicted in Fig. 4b, the specific potentials required to achieve current densities of 10, 100, and 200 mA cm⁻² are 1.356 V, 1.443 V, and 1.489 V, respectively. In contrast, the potential values for Cu₃P@CF stand at 1.447 V, 1.556 V, and 1.612 V; for MnP/Cu₃P@CF at 1.375 V, 1.543 V, and 1.603 V; and for CoP/Cu₃P@CF at 1.380 V, 1.474 V, and 1.525 V. These results highlight the pronounced reactivity of CoP/MnP/Cu₃P@CF during the MOR process. Fig. 4c presents the Tafel plots, where the CoP/MnP/Cu₃P@CF catalyst exhibits a lower slope of 76.2 mV dec⁻¹ in alkaline seawater compared to 165.4 mV dec⁻¹ for Cu₃P@CF, 150.3 mV dec⁻¹ for MnP/Cu₃P@CF, and 91.5 mV dec⁻¹ for CoP/Cu₃P@CF. CV curves were plotted in the range of 0–0.6 V (vs. RHE) (Fig. S4a–d, ESI†) to calculate the Q_s values for Cu₃P@CF, MnP/Cu₃P@CF, CoP/Cu₃P@CF, and CoP/MnP/Cu₃P@CF, and the calculated values are 0.0407, 0.0574, 0.190, and 0.142, respectively. Fig. 4d shows the calculated TOF for the MOR at working potentials of 1.45, 1.475, and 1.5 V. The TOF values for CoP/MnP/Cu₃P@CF were calculated to be 0.132, 0.195, and 0.268 s⁻¹, respectively, higher than 0.044, 0.074, and 0.127 s⁻¹ for Cu₃P@CF, 0.071, 0.103, and 0.153 s⁻¹ for MnP/Cu₃P@CF, and 0.057, 0.089, and 0.128 s⁻¹ for CoP/Cu₃P@CF. As depicted in Fig. 4e, EIS tests were carried out in methanol-containing alkaline seawater at an applied potential of 1.6 V. The CoP/MnP/Cu₃P@CF catalyst demonstrated an R_ct value of 0.378 Ω, approximately 2.9 times lower than the resistance observed during the OER at the same potential, indicating that the introduction of methanol accelerates charge transfer (Fig. 4e and Fig. S3, ESI†). To assess the corrosion resistance of the CoP/MnP/Cu₃P@CF catalyst in a methanol-containing seawater electrolyte, a constant current test was performed at 20 mA cm⁻² (Fig. 4f). The electrolyte was replaced after 12 hours due to methanol consumption, and no discernible deterioration in catalyst performance was observed, demonstrating excellent stability throughout the MOR process. It is worth mentioning that after 24 h of cycling, there is no significant change in R_ct, further suggesting the satisfactory stability of the catalyst (Fig. S5, ESI†). Moreover, the MOR performance of CoP/MnP/Cu₃P@CF is superior to those reported in many recent reports (Table S2, ESI†), revealing excellent catalytic performance.

Fig. 4 (a) MOR LSV curves, (b) potential plots, (c) Tafel plots, (d) TOF values, and (e) Nyquist plots of Cu₃P@CF, MnP/Cu₃P@CF, CoP/Cu₃P@CF, and CoP/MnP/Cu₃P@CF in seawater + 1 M KOH + 1 M methanol electrolyte. (f) MOR stability test of CoP/MnP/Cu₃P@CF at 20 mA cm⁻².

3.4 HER performance in alkaline seawater

We assessed the H₂ evolution reaction (HER) performance of CoP/MnP/Cu₃P@CF using a three-electrode system in a seawater electrolyte containing 1 M KOH.⁵⁸ As shown in Fig. 5a, the polarization curves reveal that CoP/MnP/Cu₃P@CF exhibits superior HER performance compared to Cu₃P@CF, MnP/Cu₃P@CF, and CoP/Cu₃P@CF. Notably, Fig. 5b illustrates that at current densities of 10, 100, and 200 mA cm⁻², the overpotentials of CoP/MnP/Cu₃P@CF are 0.146, 0.252, and 0.293 V, respectively, which are significantly lower than those of Cu₃P@CF (0.204, 0.319, 0.364 V), MnP/Cu₃P@CF (0.187, 0.289, 0.332 V), and CoP/Cu₃P@CF (0.154, 0.263, 0.308 V). Further analysis through Tafel results, presented in Fig. 5c, reveals a notable reduction in the slope for CoP/MnP/Cu₃P@CF (85 mV dec⁻¹) compared to Cu₃P@CF (101.2 mV dec⁻¹), MnP/Cu₃P@CF (96.2 mV dec⁻¹), and CoP/Cu₃P@CF (94.3 mV dec⁻¹). Additionally, cyclic voltammetry (CV) curves measured within the range of 0 to 0.6 V vs. RHE (Fig. S6, ESI†) yielded Q_s values of 6.49 × 10⁻², 8.79 × 10⁻², 8.28 × 10⁻², and 7.78 × 10⁻² for Cu₃P@CF, MnP/Cu₃P@CF, CoP/Cu₃P@CF, and CoP/MnP/Cu₃P@CF, respectively, with corresponding N values of 6.72 × 10⁻⁷, 9.11 × 10⁻⁷, 8.58 × 10⁻⁷, and 8.06 × 10⁻⁷.

Fig. 5 (a) HER LSV curves, (b) potential plots, (c) Tafel plots, (d) TOF values, and (e) Nyquist plots of Cu₃P@CF, MnP/Cu₃P@CF, CoP/Cu₃P@CF, and CoP/MnP/Cu₃P@CF in seawater + 1 M KOH electrolyte. (f) HER stability test of CoP/MnP/Cu₃P@CF at 20 mA cm⁻².

As depicted in Fig. 5d, the turnover frequency (TOF) values for the CoP/MnP/Cu₃P@CF catalyst at potentials of 0.25, 0.275, and 0.3 V are 0.627, 0.962, and 1.436 s⁻¹, respectively, higher than 0.219, 0.354, and 0.559 s⁻¹ for Cu₃P@CF, 0.269, 0.443, and 0.693 s⁻¹ for MnP/Cu₃P@CF, and 0.485, 0.746, and 1.093 s⁻¹ for CoP/Cu₃P@CF. Furthermore, the electrochemical surface area (ECSA) was calculated via C_dl to determine the number of exposed active sites.⁵⁹ As shown in Fig. S7 (ESI†), the C_dl value for the CoP/MnP/Cu₃P@CF heterostructure catalyst is approximately 1.74, 1.55, and 1.49 times higher than those of Cu₃P@CF, MnP/Cu₃P@CF, and CoP/Cu₃P@CF, respectively, suggesting that more active sites are exposed in CoP/MnP/Cu₃P@CF compared to other samples. Moreover, EIS tests were performed for Cu₃P@CF, MnP/Cu₃P@CF, CoP/Cu₃P@CF, and CoP/MnP/Cu₃P@CF during the HER in alkaline seawater electrolysis (Fig. 5e). The results indicate that the CoP/MnP/Cu₃P@CF catalyst exhibits a lower charge transfer resistance (1.256 Ω) compared to Cu₃P@CF (3.058 Ω), MnP/Cu₃P@CF (1.818 Ω), and CoP/Cu₃P@CF (1.348 Ω), suggesting that the CoP/MnP/Cu₃P heterostructures are more favorable for electron transfer (Fig. S3, ESI†).⁶⁰ Additionally, a stability test was conducted at 20 mA cm⁻² for 24 hours during the HER (Fig. 5f). Remarkably, the overpotential exhibits no obvious changes, indicating its potential applicability in seawater electrolysis. Importantly, the HER performance of CoP/MnP/Cu₃P@CF is superior to those reported in many recent reports (Table S3, ESI†).

3.5 Methanol oxidation assisted seawater electrolysis performance

Based on the excellent catalytic performance of the CoP/MnP/Cu₃P@CF catalyst in the MOR, HER, and OER, we developed a two-electrode seawater electrolyzer using seawater + 1 M KOH and seawater + 1 M KOH + 1 M methanol as electrolytes (Fig. 6a), respectively. Polarization curves in Fig. 6b show that the MOR-assisted seawater electrolyzer requires cell voltages of 1.410 V and 1.697 V to achieve current densities of 10 and 100 mA cm⁻², much lower than 1.681 V and 1.873 V for a seawater electrolyzer. Fig. 6c shows a linear correlation between H₂ production and operational time. The H₂ production rate at 20 mA cm⁻² is calculated to be 10 mL h⁻¹. Importantly, the CoP/MnP/Cu₃P@CF-based MOR-assisted seawater electrolyzer exhibits high Faraday efficiency during the test period. To gain insight into the MOR mechanism, electrochemical in situ Fourier transform infrared spectroscopy (FTIR) was applied to investigate intermediate products on the CoP/MnP/Cu₃P@CF catalyst. As shown in Fig. 6d, in the alkaline seawater electrolyte, a band centered around 1190 cm⁻¹ corresponding to OOH* was observed. Notably, in the electrolyte containing methanol, a characteristic peak of COOH* appears at 1484 cm⁻¹, indicating the formation of the corresponding species.^61,62 The constant current test results show that CoP/MnP/Cu₃P@CF-based electrolyzers with and without methanol exhibit negligible performance degradation after 24 hours of continuous operation at 20 mA cm⁻², suggesting satisfactory stability (Fig. 6e). Moreover, the performance of the MOR-assisted seawater electrolyzer using CoP/MnP/Cu₃P@CF both as the anode and cathode is superior to those reported in many recent reports (Table S4, ESI†).

Fig. 6 (a) Schematic diagram of methanol-assisted alkaline seawater electrolysis. (b) Polarization curves of the CoP/MnP/Cu₃P@CF||CoP/MnP/Cu₃P@CF seawater electrolyzer with and without methanol; (c) H₂ production and faradaic efficiency in a methanol-assisted seawater electrolyzer at 20 mA cm⁻²; (d) in situ IR of CoP/MnP/Cu₃P@CF in alkaline seawater electrolytes with and without methanol; (e) stability test of CoP/MnP/Cu₃P@CF in a MOR-assisted seawater electrolyzer at 20 mA cm⁻².

4. Conclusion

In summary, a unique heterostructured electrocatalyst comprising CoP, MnP, and Cu₃P on copper foam (CoP/MnP/Cu₃P@CF) is designed and successfully synthesized via a hydrothermal-phosphating process. The synergistic interaction among heterogeneous components endows CoP/MnP/Cu₃P@CF with impressive OER, MOR, and HER performance. Consequently, the CoP/MnP/Cu₃P@CF-based MOR-assisted seawater electrolyzer needs only 1.410 V to achieve a current density of 10 mA cm⁻², much lower than the 1.681 V of the seawater electrolyzer without methanol. Moreover, the MOR-assisted seawater electrolyzer shows high faradaic efficiency and stability, providing a feasible way to produce H₂ by energy-saving seawater electrolysis.

Data availability

Data will be made available on request.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was financially supported by the Guangxi Natural Science Fund for Distinguished Young Scholars (2024GXNSFFA010008), the National Natural Science Foundation of China (22469002 and 22275166), and the Fundamental Research Funds for the Provincial Universities of Zhejiang (RF-B2023002).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4qm01067a

‡ These authors contributed equally to this work.

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