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DOI: 10.1039/D4QI01587H (Research Article) Inorg. Chem. Front., 2024, 11, 6909-6918

Constructing built-in electric fields in 2D/2D Schottky heterojunctions for efficient alkaline seawater electrolysis†

Hongjun Chen a, Liming Deng a, Sheng Zhao a, Shuyi Liu a, Feng Hu *a, Linlin Li *a, Jianwei Ren b and Shengjie Peng *ac
aCollege of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China. E-mail: fenghu@nuaa.edu.cn; lilinlin@nuaa.edu.cn; pengshengjie@nuaa.edu.cn
bDepartment of Mechanical Engineering Science, University of Johannesburg, 2092, Johannesburg, South Africa
cState Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai 200050, China

Received 24th June 2024 , Accepted 23rd August 2024

First published on 30th August 2024

Abstract

Developing efficient and durable hydrogen evolution reaction (HER) electrocatalysts is critical for industrial and sustainable hydrogen production. Herein, a simple co-precipitation strategy is proposed to successfully construct catalysts with a Mott–Schottky heterojunction by coupling a transition-metal phosphate to the surface of stripped MXene thin-layer nanosheets (M3(PO4)2@MXene, M = Co, Ni, and Fe). The Co3(PO4)2@MXene with a unique tightly connected 2D/2D heterostructure and built-in electric field induces directional electron transfer at the interface, regulates the polarized structure of the active sites, and accelerates both mass and electron transport. Consequently, the optimized Co3(PO4)2@MXene demonstrates outstanding HER performance, achieving low overpotentials of 46 and 58.6 mV at 10 mA cm−2 in alkaline freshwater and seawater electrolytes, respectively. Moreover, the Co3(PO4)2@MXene heterojunction catalyst maintains stable operation at a high current density of 500 mA cm−2 for over 100 h in alkaline seawater electrolytes. More importantly, Co3(PO4)2@MXene can offer a low potential of 1.71 V at 500 mA cm−2 with stable operation for 50 h in a flow-type alkaline seawater electrolyser. This study provides a unique heterostructure in an electrocatalyst for an efficient HER and presents its potential application in seawater electrolysis.

Introduction

As a renewable energy source, hydrogen is considered an ideal energy storage material to achieve low-carbon transformation of energy systems.1–3 Electrolysis reactions are sustainable and efficient ways to produce high-purity hydrogen in alkaline media, among which the hydrogen evolution reaction (HER) is one of the critical processes to convert electric energy into hydrogen energy.4–7 Compared with the shortage of fresh-water resources, seawater is one of the richest natural resources on the Earth, accounting for 96.5% of the planet's total water resources.8–10 Therefore, electrolysis is a promising way of using abundant seawater for hydrogen evolution to achieve large-scale hydrogen production. However, numerous challenges hinder the large-scale application of seawater electrolysis in industrial situations, such as high corrosion and low conductivity caused by the relatively complex composition of seawater.11–14 Pt-based catalysts have been considered the foremost catalysts for hydrogen evolution reactions, but their scarcity, high cost, and instability hinder their large-scale application and industrial development.15–17 Thus, the top priority of current research is to explore and prepare non-precious metal HER electrocatalysts with low cost and high catalytic performance.

In recent years, transition-metal phosphates have attracted widespread attention because of their low cost and abundant resources. However, the low conductivity of transition metal phosphates hinders the full utilization of the intrinsic activity of their metal centers in the electrocatalytic process. Heterogeneous interface engineering can effectively balance the optimal conditions for the adsorption of intermediates through unique multi-component synergies, which have been proven to be an effective strategy for designing efficient HER electrocatalysts.12,17–19 Despite extensive efforts to investigate the impacts of heterogeneous interface engineering, there remains a deficiency in developing efficient electrocatalysts suitable for the alkaline HER and industrial applications.20,21 The Mott–Schottky heterojunction, a contact between a metal and a semiconductor, can effectively induce charge transfer at the interface and regulate the Fermi level until balance.20,22–25 More significantly, the phenomenon generates a built-in electric field (BEF), which not only modifies the electronic structure of the active sites but also positively impacts the electron transport efficiency and enhances the catalyst activity.26–29 It is worth noting that differences in the work function (Φ) between heterogeneous components can provide an intrinsic driving force for the transport capacity and orientation of charges at the interface.17,30–33 An effective strategy is to construct a Mott–Schottky heterojunction to improve the electrocatalytic activity of transition metal phosphates.23,34 The new carbon material, 2D MXene, as a semiconductor material, can be used as the semiconductor phase of the Mott–Schottky heterojunction because of its apparent features such as high-efficiency charge transfer capacity and excellent corrosion resistance.

Herein, we report a 2D/2D Mott–Schottky heterogeneous electrocatalyst (Co3(PO4)2@MXene) with a built-in electric field for electrolysis in an alkaline environment. Benefiting from its unique 2D/2D heterostructure, the Co3(PO4)2@MXene Mott–Schottky heterojunction electrocatalyst has a larger specific surface area, which can be conducive to the exposure of active sites. Furthermore, the built-in electric field between Co3(PO4)2 and MXene induces interfacial electron transport and accelerates the HER reaction kinetics. With the synergistic effect of accelerated mass and electron transport, Co3(PO4)2@MXene achieves a current density of 10 mA cm−2 at low HER overpotentials of 46 and 58.6 mV in 1 M KOH and alkaline seawater, respectively. Moreover, the Co3(PO4)2@MXene heterojunction catalyst operates stably at a high current density of 500 mA cm−2 for more than 100 h in alkaline seawater, effectively overcoming the problems of slow kinetics in seawater electrolysis and interference from impurities. This study provides a way to design heterojunction catalysts with high catalytic activity and stability for sustainable hydrogen production and to promote the development of seawater electrolysis.

Experimental section

Chemicals and materials

Titanium aluminium carbide (Ti3AlC2, 98%), sodium hydrogen phosphate (Na2HPO4, 98%), cobalt nitrate hexahydrate (Co(NO3)2·6H2O, 97%), nickel nitrate hexahydrate (Ni(NO3)2·6H2O, 98%), ferrous chloride tetrahydrate (FeCl2·4H2O, 98%) and graphitized carbon on platinum (Pt/C, 20%) were obtained from Aladdin (Shanghai, China).

Synthesis of MXene nanosheets

2 g of Ti3AlC2 (MAX) was slowly immersed into a mixture of 30 mL of 9 M HCl and 2 g of LiF. After being completely dissolved, the mixture was kept at 40 °C for 36 h in an oil bath. Then, the mixture was rinsed with deionized water repeatedly and the pH of the supernatant was adjusted to 7. The powder was dispersed in 500 mL of deionized water and kept in an ice water bath under argon for 10 h. The upper-layer solution was collected after centrifugation to obtain the MXene solution and this was stored in a refrigerated environment.

Synthesis of Co3(PO4)2@MXene and Co3(PO4)2

291 mg of Co(NO3)2·6H2O and 142 mg of Na2HPO4 were added into a mixed solution of 25 mL deionized water and 25 mL of the MXene-dispersed solution. The mixture was stirred in an oil bath at 50 °C for 1 h, then rinsed and centrifuged with deionized water three times. Finally, the Co3(PO4)2@MXene was obtained after freeze–drying. For comparison, the same preparation method was applied to prepare Co3(PO4)2.

Synthesis of M3(PO4)2@MXene (M = Ni, Fe) and M3(PO4)2 (M = Ni, Fe)

Ni3(PO4)2 and Fe3(PO4)2 were prepared by the same method as that used for Co3(PO4)2. Ni3(PO4)2@MXene and Fe3(PO4)2@MXene were prepared by the same method as that used for Co3(PO4)2@MXene.

Materials characterization

X-ray diffraction (XRD) data were acquired using a Bruker D8 Advance instrument (Billerica) with a guaranteed scanning rate of 10° min−1. The morphology and microstructure observations were recorded using a scanning electron microscope (SEM, Regulus 8100). Transmission electron microscopy data were obtained on an FEI Tecnai G2 F20. The Brunauer–Emmett–Teller (BET) surface area was measured using the V-Sorb 2800P (Gold APP Instruments, China. The valence state of the catalyst was measured and analyzed using an X-ray photoelectron spectrometer (XPS Escalab 250Xi). The extended X-ray absorption fine structure (EXAFS) was measured at the Beijing Photon Source (easyXAFS300, easyXAFS LLC) beamline. The work function of the catalyst was measured by ultraviolet photoemission spectroscopy (UPS), using a VG Scienta R4000 analyzer, with 5 eV bias and UV–vis diffuse reflection spectra (DRS) were recorded using a Shimadu UV2550 spectrophotometer.

Electrochemical measurements

The catalyst, conductive agent (carbon black), and binder (polyvinylidene fluoride, PVDF) were mixed in a mass ratio of 7[thin space (1/6-em)]:[thin space (1/6-em)]2[thin space (1/6-em)]:[thin space (1/6-em)]1 to obtain the catalyst paste. The working electrode was prepared by coating the slurry on 1 × 1 cm2 nickel foam. A standard three-electrode system was used to test the electrocatalytic activity of the catalyst at an electrochemical workstation (Autolab Instruments). The prepared catalyst was used as the working electrode, a graphite rod was used as the counter electrode (CE), and a KCl-saturated Ag/AgCl electrode was used as the reference electrode (RE) for the HER tests. All of the potentials reported were converted relative to the reversible hydrogen electrode (RHE) according to the Nernst equation (ERHE = EAg/AgCl + 0.197 + 0.0591 × pH). The electrochemically active surface area (ECSA) was obtained by measuring the capacitance at scan rates of 40–120 mV s−1. Electrochemical impedance spectroscopy (EIS) was conducted over a frequency range of 0.01–105 kHz with an amplitude of 10 mV.

Results and discussion

As illustrated in Fig. 1a, a simple co-precipitation method is employed to synthesize Co3(PO4)2@MXene with a 2D/2D Mott–Schottky heterostructure, where Co3(PO4)2 is in situ coupled on the surface of MXene thin-layer nanosheets. The morphological evolution of the monolayer MXene is observed by scanning electron microscopy (SEM). The MXene nanosheets formed after acid etching present a thin-layer accordion shape, which can be used as a suitable carrier for binding active catalysts (Fig. 1b; Fig. S1†). Co3(PO4)2 prepared separately shows an apparent lamellar structure, and the diameter of the nanosheets is about 200–500 nm (Fig. S2†). After co-precipitation, nano-flake Co3(PO4)2 grows uniformly on the MXene nanosheets, indicating the successful synthesis of the 2D/2D heterostructure (Fig. 1c and d). Notably, the uniform dispersion of Co3(PO4)2 nanosheets on MXene, forms a unique 2D/2D heterostructure, which not only has a large specific surface area exposing more active sites but also facilitates the rapid diffusion of electrons/ions at the interface, causing rapid electrochemical reaction kinetics. The transmission electron microscopy (TEM) images reveal that well-defined and high-contrast Co3(PO4)2 nanosheets can be observed on MXene nanosheets. As shown in Fig. 1e, high-resolution TEM (HRTEM) images show light and dark interfaces corresponding to Co3(PO4)2 and MXene, forming a heterogeneous structure with prominent interface contacts. The lattice spacing of MXene is 0.48 nm, corresponding to the (002) plane, while the lattice spacing of 0.27 nm corresponds to the (111) plane of Co3(PO4)2 (Fig. 1f). The HRTEM image indicates that the MXene's layer spacing ≈ 0.98 nm, indicating that MXene still maintains the thin layer nanosheet structure after forming the composite material (Fig. 1g). The elemental mappings indicate that Ti, Co, P, and O elements are evenly distributed, which further confirms the formation of the Co3(PO4)2@MXene heterostructure (Fig. 1h).
image file: d4qi01587h-f1.tif
Fig. 1 Preparation and morphology characterization of Co3(PO4)2@MXene. (a) Schematic diagram of the preparation of Co3(PO4)2@MXene. (b) SEM image of MXene. (c and d) SEM, (e) TEM and (f) HRTEM images of Co3(PO4)2@MXene. (g) HRTEM image of layer spacing of MXene in Co3(PO4)2@MXene. (h) Elemental mappings of Ti, Co, P, and O in Co3(PO4)2@MXene.

The X-ray diffraction (XRD) is performed to investigate the phase composition of the synthesized electrocatalysts. MXene nanosheets show a unique diffraction peak at 6.07°, which corresponds to the (002) crystal face. The broad diffraction peak at 21.29° is due to the ultra-thin lamellar structure formed by the stripping of MXene. The XRD pattern of Co3(PO4)2@MXene shows characteristic peaks located at 11.05°, 13.20°, 18.20°, 30.27° and 33.28°, which correspond to Co3(PO4)2 (JCPDS#1-121) (Fig. 2a). In comparison, the peak at 6.07° is correlated to the (002) crystal planes of MXene, suggesting the formation of the Co3(PO4)2@MXene heterostructure interface. The Brunauer–Emmett–Teller (BET) specific surface area of Co3(PO4)2@MXene (194.83 m2 g−1) is much larger than that of Co3(PO4)2 (98.43 m2 g−1) and MXene (23.61 m2 g−1) (Fig. 2b). Moreover, X-ray photoelectron spectroscopy (XPS) is conducted to analyze the valence state and electronic structure state of Co3(PO4)2, MXene and Co3(PO4)2@MXene. The high-resolution Co 2p spectrum of Co3(PO4)2@MXene shows that the peaks at 781.52 eV and 797.45 eV point to Co 2p3/2 and Co 2p1/2, respectively (Fig. 2c). Compared with Co3(PO4)2, the binding energy of the Co 2p peak shifts towards higher binding energy, indicating that Co in Co3(PO4)2@MXene has a higher valence state, which is conducive to the adsorption/dissociation of water in the HER process. Conversely, the Ti 2p binding energy shows a negative shift relative to MXene, indicating a higher electron density (Fig. 2d), confirming the electron transfer from Co to Ti across the heterojunction interface.35,36 The above results demonstrate that the obvious electron transfer between the interfaces can improve the interaction between Co3(PO4)2 and MXene.37–39


image file: d4qi01587h-f2.tif
Fig. 2 Phase characterization of Co3(PO4)2@MXene. (a) XRD patterns of Co3(PO4)2, MXene and Co3(PO4)2MXene. (b) N2 adsorption and desorption isotherms for Co3(PO4)2, MXene, and Co3(PO4)2@MXene. (c) XPS spectra of the Co 2p region for Co3(PO4)2 and Co3(PO4)2@MXene, respectively. (d) XPS spectra of the Ti 2p region for MXene and Co3(PO4)2@MXene, respectively. (e) UPS spectra of Co3(PO4)2 and MXene. (f) Energy band diagram before and after Mott–Schottky contact.

According to the theory of semiconductor physics, the minimum energy required for electrons to move from the Fermi level to the vacuum level is the work function (Φ), which can control the direction of charge transport at the interface.37,40 To analyze the synergistic effect between metallic phase Co and semiconductor phase MXene in the Co3(PO4)2@MXene Mott–Schottky heterojunction electrocatalyst, ultraviolet photoelectron spectroscopy (UPS) and UV–vis diffuse reflection spectra are utilized to calculate the work function to further verify the influence of Φ on the BEF (Fig. 2e; Fig. S3†).41–43 It can be calculated according to the equation Φ = 21.22 eV − |Ef − Ecutoff| (where 21.22 eV, Ef and Ecutoff represent the photon energy, Fermi level energy and cutoff energy, respectively). As shown in Fig. 2e, the Φ values of Co3(PO4)2 and MXene are 2.42 eV and 3.85 eV, respectively. The difference of the Φ value of Co3(PO4)2 and MXene satisfies the condition for constructing the Mott–Schottky heterojunction. When the heterointerface is formed, the MXene band is induced to bend downward. The electrons are spontaneously transferred from Co3(PO4)2 to MXene, until the Fermi energy level reaches equilibrium (Fig. 2f). Meanwhile, electrons are concentrated on the nucleophilic MXene side while holes are focused on the side of the electrophilic Co3(PO4)2, and a built-in electric field is formed spontaneously.44 The formation of the built-in electric field can effectively facilitate the polarization of local charges, improve the electrochemical activity of the interface active sites, and reduce the reaction energy barrier, which is conducive to enhancing the kinetics rate of the HER reaction.

X-ray absorption spectroscopy (XAS) is employed to analyze the coordination environment of Co in Co3(PO4)2@MXene and its electronic structure. The Co K-edge position of the Co3(PO4)2@MXene heterojunction catalyst shifts to a higher energy compared to Co foil and CoO, indicating a decrease in electronic density around the Co sites and an increase in the valence state after coupling with MXene (Fig. 3a), which is consistent with the results of XPS.45 By employing the first derivative curves and the fitting average oxidation state of the Co K-edge XANES, the oxidation state of Co in Co3(PO4)2@MXene is further quantitatively estimated (the inset of Fig. 3a). The average oxidation state of Co in Co3(PO4)2@MXene is 2.13, which is higher than that in Co3(PO4)2 (2.07) (Fig. 3b). The increase in the valence state of Co is due to the formation of the BEF, which induces electron redistribution at the Co3(PO4)2@MXene interface and regulates the electronic structure. Furthermore, the significant amplitude change in oscillation observed in the Co K-edge EXAFS indicates disparate local atomic arrangements between Co3(PO4)2@MXene and Co3(PO4)2 (Fig. S4†). The Co K-edge extended X-ray absorption fine structure (EXAFS) spectra in Fig. 3c reveal a slight change in the coordination environment of Co sites in Co3(PO4)2@MXene. The Co–O bond of the Co3(PO4)2@MXene has been shortened compared with that in Co3(PO4)2, indicating that the covalency is enhanced and the formation of the Mott–Schottky interface contributes to improving the stability of the catalyst.36,37 The wavelet transform (WT) plots for Co3(PO4)2@MXene, Co3(PO4)2, CoO and Co foil shown in Fig. 3d suggest that the coordination environment of Co in Co3(PO4)2@MXene is different from that of Co in Co3(PO4)2. The maximum-intensity Co K-edge value for Co3(PO4)2@MXene is observed at k ≈ 1.45 Å−1, attributed to the Co–O scattering path. Additionally, the Co atoms in Co3(PO4)2@MXene are not only arranged through the Co–O coordination environment but also the Co–O–Co/Ti coordination environment, implying that Co3(PO4)2@MXene has a significantly strong interaction at the heterostructure interface.46


image file: d4qi01587h-f3.tif
Fig. 3 Structural characterizations of the Co3(PO4)2@MXene. (a) The normalized XANES spectra. (b) Relation between the Co K-edge absorption energy (E0) and valence states for Co foil, CoO, Co3(PO4)2 and Co3(PO4)2@MXene. (c) FT-EXAFS spectrum. (d) WT-EXAFS of Co foil, CoO, Co3(PO4)2 and Co3(PO4)2@MXene.

To further investigate the effects of the built-in electric field on the electrochemical performance of Co3(PO4)2@MXene, the alkaline HER performance is tested by using a standard three-electrode system in 1 M KOH. As shown in Fig. 4a and b, the Co3(PO4)2@MXene heterojunction catalyst only requires a low overpotential of 48 mV to achieve 10 mA cm−2 for the HER, which is comparable to Pt/C (31 mV) and significantly lower than that of Co3(PO4)2 (η10 = 151.8 mV) and MXene (η10 = 263.1 mV), implying that the HER activity can be effectively improved by constructing the built-in electric field. The Tafel slope is calculated to verify the HER kinetics rate of Co3(PO4)2@MXene. Co3(PO4)2@MXene exhibits a much smaller Tafel slope (57.9 mV dec−1), which fully confirms that the presence of the BEF causes fast HER kinetics (Fig. 4c).47,48 To further reveal the inherent electrochemical activity of the heterojunction catalyst, cyclic voltammetry is utilized to measure and calculate the double-layer capacitance (Cdl), which is directly proportional to the electrochemically active surface area (ECSA) (Fig. S5†).49,50 The Cdl value of Co3(PO4)2@MXene is distinctly larger than that of Co3(PO4)2 (4.35 mF cm−2), MXene (7.44 mF cm−2), and Pt/C (33.69 mF cm−2), possessing the largest Cdl value of 54.84 mF cm−2, implying the larger electrochemically active surface area and higher intrinsic activity of Co3(PO4)2@MXene (Fig. 4d). The Cdl value of Co3(PO4)2@MXene is 12.6 and 7.4 times that of Co3(PO4)2 and MXene, respectively, indicating that the unique 2D/2D heterostructure of Co3(PO4)2@MXene exhibits a larger specific surface area and favorably exposes more electrochemically active sites. The presence of the BEF can enhance the electron transport capacity at the interface and contribute to forming new highly active sites at the interface. Additionally, electrochemical impedance spectroscopy (EIS) for the HER in 1 M KOH electrolyte is used to further investigate the reaction kinetics. Co3(PO4)2@MXene has a lower HER charge transfer resistance (Rct) value of only 0.83 Ω (Fig. 4e). The above results validate that the BEF can effectively promote electron transfer between semiconductor and metal phases, enhancing charge transfer capability.51 The stability of Co3(PO4)2@MXene is tested at a constant current of 10 mA cm−2, showing that the potential change is negligible after 100 h of testing (Fig. 4g). The low overpotential and excellent durability of the Co3(PO4)2@MXene heterojunction catalyst are superior to those of most HER catalysts previously reported in alkaline electrolytes (Fig. 4h; Table S1†).52


image file: d4qi01587h-f4.tif
Fig. 4 Electrochemical characterization of Co3(PO4)2@MXene. (a) LSV polarization curves, (b) plotted overpotential comparison diagram in 1 M KOH electrolyte of Co3(PO4)2, MXene, Co3(PO4)2@MXene and Pt/C. (c) Tafel plots, (d) Cdl values, and (e) EIS curves of Co3(PO4)2, MXene, Pt/C and Co3(PO4)2@MXene. (f) Polarization curves of LSV of Co3(PO4)2, MXene, Pt/C and Co3(PO4)2@MXene in alkaline seawater. (g) Co3(PO4)2@MXene durability test in 1 M KOH and alkaline seawater electrolytes at constant current densities of 10 and 500 mA cm−2. (h) Comparison with reported overpotential and stability of catalysts in 1 M KOH.

To explore the application of the electrocatalyst in actual environments, considering the excellent alkaline HER performance of Co3(PO4)2@MXene heterojunction catalyst in 1 M KOH, the HER electrochemical performance in seawater/alkaline seawater is tested.53 Co3(PO4)2@MXene shows the lowest overpotential (112 mV at 10 mA cm−2) and Tafel slope (90.03 mV dec−1) in the seawater, suggesting fast reaction kinetics (Fig. S6†). As shown in Fig. 4f, the Co3(PO4)2@MXene heterojunction catalyst only needs 58.6 mV and 182.9 mV overpotential in alkaline seawater to reach the current densities of 10 mA cm−2 and 100 mA cm−2, respectively. Significantly, only a lower overpotential of 240.9 mV is required at the higher current density of 200 mA cm−2. Remarkably, Co3(PO4)2@MXene demonstrates a higher overpotential at low current densities compared to Pt/C. However, Co3(PO4)2@MXene exhibits a lower overpotential when the current density surpasses 400 mA cm−2, indicating significant application potential in alkaline seawater at high current densities. The lower Tafel slope and Rct indicate that Co3(PO4)2@MXene has a fast kinetics rate for the HER in alkaline seawater (Fig. S7†). Another crucial factor to evaluate the industrial application of electrocatalysts is stability. The Co3(PO4)2@MXene exhibits superior electrocatalytic stability for 100 h at the constant level of 500 mA cm−2 and only loses 110 mV (Fig. 4g). These data demonstrate that the prepared Co3(PO4)2@MXene has good Cl− corrosion resistance in alkaline seawater.54 The distinctive 2D/2D structure and large specific surface area of the Co3(PO4)2@MXene heterojunction catalyst provide richer active sites during water electrolysis, while the hydrophilic surface facilitates the rapid release of gas. At the same time, the BEF effectively reduces the reaction energy barrier and accelerates the transfer of electrons. Therefore, the Co3(PO4)2@MXene heterojunction catalyst is efficient for alkaline fresh-water and seawater electrolysis.

We analyzed the microstructure and electronic structure of Co3(PO4)2@MXene after the HER stability test in 1 M KOH to explore the true active species during the HER process. After the HER durability test, the sample is characterized by SEM and XRD (Fig. 5a and b), showing that Co3(PO4)2@MXene well retains its 2D/2D structure and original sheet structure, which confirms the outstanding structural stability and corrosion resistance of the heterojunction catalyst. Moreover, XPS analysis is performed to investigate the changes in the chemical valence states of Co3(PO4)2@MXene after the stability test. As shown in Fig. 5c and d, Co 2p, and Ti 2p spectra shift towards lower binding energies, resulting in a decrease in valence states, which is conducive to the complementary adsorption of intermediates at heterogeneous interfaces or active sites. All of the XPS results confirm that the chemical state of Co3(PO4)2@MXene is retained throughout the HER process. The Bode and Nyquist plots of Co3(PO4)2 and Co3(PO4)2@MXene are recorded to pinpoint dynamic impedance under the HER (Fig. 5e and f). The smaller phase angle of Co3(PO4)2@MXene in the low-frequency region demonstrates that the interface interaction between the electrolyte and catalyst facilitates electron transfer at the interface, enhancing HER kinetics. Meanwhile, the phase angle shift of Co3(PO4)2@MXene is more significant than that of Co3(PO4)2 when the overpotential is increased gradually, indicating that Co3(PO4)2@MXene is more sensitive to potential response and has a faster Heyrovsky step.55


image file: d4qi01587h-f5.tif
Fig. 5 Reaction mechanism of Co3(PO4)2@MXene in alkaline electrolyte. (a) SEM image, (b) XRD patterns of Co3(PO4)2@MXene after the HER. High-resolution XPS spectra of (c) Co 2p and (d) Ti 2p. Corresponding Bode-phase plots for (e) Co3(PO4)2@MXene and (f) Co3(PO4)2 collected under different polarization potentials. (g) Overpotential comparison diagram in 1 M KOH electrolyte of M3(PO4)2 (M = Ni, Co, Fe) and M3(PO4)2@MXene (M = Ni, Co, Fe). (h) The schematic illustration of the HER pathway of Co3(PO4)2@MXene in 1 M KOH.

As a supplement and contrast to these data, the same method of co-precipitation is used to synthesize Ni3(PO4)2@MXene and Fe3(PO4)2@MXene. The electrochemical properties of heterojunction catalysts formed by the combination of iron group element (Ni, Co, Fe) phosphates and MXene are investigated. The successful synthesis of the two heterojunction catalysts is demonstrated by XRD and SEM (Fig. S8–S11†), and the HER activity of M3(PO4)2@MXene (M = Ni, Co, Fe) is tested and compared in 1 M KOH electrolyte. The results show that the HER electrochemical performance of M3(PO4)2@MXene (M = Ni, Co, Fe) has been greatly improved compared with that of M3(PO4)2 (M = Ni, Co, Fe) (Fig. 5g; Fig. S12†). Compared with Ni3(PO4)2@MXene (117.9 mV) and Fe3(PO4)2@MXene (182.5 mV), Co3(PO4)2@MXene possesses the lowest overpotential at 10 mA cm−2, exhibiting better alkaline HER performance. Meanwhile, the Tafel slopes of Ni3(PO4)2@MXene and Fe3(PO4)2@MXene are determined to be 73.1 mV dec−1 and 132.9 mV dec−1 for the HER, respectively. The Co3(PO4)2@MXene distinguishes itself by exhibiting the most advantageous Tafel slope and the lowest charge transfer resistance among all tested samples (Fig. S13†). It is proved that the electrochemical kinetics rate of the Co3(PO4)2@MXene heterojunction catalyst is faster than that of Ni3(PO4)2@MXene and Fe3(PO4)2@MXene. This is attributed to the rapid and efficient electron transport ability at the interface and the lower reaction energy barrier during the electrochemical reaction process. To further elucidate the HER mechanism of M3(PO4)2@MXene (M = Ni, Co, Fe) catalysts in alkaline electrolyte, we construct a schematic diagram of the alkaline HER catalytic mechanism (Fig. 5h). Under the coupling of strong interfacial electrons and the Fermi level, some interface electrons transfer from M3(PO4)2 to MXene, weakening the hydrogen adsorption strength. The redistribution of electrons can enhance the absorption of reaction intermediates and increase the catalytic activity. In the electrolysis process, H2O molecules are first adsorbed on the surface of the catalyst and dissociated into H* and OH. Then, the H* preferentially adsorbs on the negatively charged MXene side, combining with another H* and two electrons to form H2 molecules. It can be concluded that the presence of the built-in electric field promotes the transfer of interfacial electrons from Co3(PO4)2 to MXene, effectively promoting the water dissociation ability and optimizing the hydrogen adsorption/desorption, further facilitating HER kinetics. Overall, the Gibbs free energy of the HER process and the electronic structure can be effectively optimized by the built-in electric field between Co3(PO4)2 and MXene, ultimately promoting the HER dynamic process.

A two-electrode electrolyzer is constructed to assess the potential for overall seawater electrolysis (represented by Co3(PO4)2@MXene||NiFe LDH) (Fig. 6a) to explore the feasibility of the Co3(PO4)2@MXene electrode in practical industrial seawater electrolysis applications and realize large-scale hydrogen production.10,56 The electrolytic cell shows excellent overall seawater splitting activity in alkaline seawater, and the two-electrode electrolyzer assembled can drive a current density of 100 mA cm−2 at a voltage of 1.81 V (Fig. 6b). As shown in Fig. 6c, the Co3(PO4)2@MXene||NiFe LDH electrolyzer can provide a current density of 500 mA cm−2 at a low voltage of 2.01 V, which remains stable after 165 h of operation with negligible voltage loss. All the results indicate that the Co3(PO4)2@MXene heterojunction catalyst can effectively resist the interference of seawater impurities in alkaline seawater and maintain excellent durability at high current densities, which has broad industrial application prospects. The flow-type anion exchange membrane (AEM) electrolyzer is also assembled to evaluate the feasibility and potential of the prepared HER catalyst in the electrolytic system (Fig. 6d; Fig. S14†). Intriguingly, the Co3(PO4)2@MXene||NiFe LDH electrolyzer exhibits exceptional performance, which can drive the overall seawater splitting with low voltages of 1.71 V and 2.0 V (1 M KOH) at 500 and 1000 mA cm−2 (Fig. 6e). At a high constant current density of 500 mA cm−2, the electrolyzer can operate continuously for 50 h in alkaline seawater with only slight fluctuations (Fig. 6f). After carefully evaluating previously reported catalysts, Co3(PO4)2@MXene can be considered one of the most efficient catalysts for the HER in alkaline electrolytes (Tables S2 and S3†). All the results collectively illustrate the distinguished overall water-splitting activity and stability of the Co3(PO4)2@MXene catalyst, showing its great potential for industrial electrolysis to produce renewable hydrogen.


image file: d4qi01587h-f6.tif
Fig. 6 The performance of overall water splitting. (a) Conceptual model of the water splitting in alkaline seawater. (b) LSV polarization curves of Co3(PO4)2@MXene||NiFe LDH for alkaline seawater splitting. (c) The durability of the electrolytic cell is tested in alkaline seawater at a constant current density of 500 mA cm−2. (d) Schematic of the AEM electrolyzer using Co3(PO4)2@MXene and NiFe LDH as the anode and cathode, respectively. (e) Overall water splitting performance of Co3(PO4)2@MXene||NiFe LDH. (f) Chronopotentiometry curve of Co3(PO4)2@MXene||NiFe LDH at 500 mA cm−2 in 1 M KOH.

Conclusion

In summary, the Mott–Schottky heterojunction catalyst (Co3(PO4)2@MXene) with a unique 2D/2D structure is designed and synthesized by a simple co-precipitation method, which can effectively accelerate the reaction kinetics of the alkaline HER. The 2D/2D heterostructure enables Co3(PO4)2@MXene to have a larger specific surface area, exposing more active sites and improving electrocatalytic activity. In particular, a strong built-in electric field constructed at the metal–semiconductor interface can effectively modify the electronic structure of the active sites and induce electron transfer at the interface, improving the intrinsic activity of the HER. Due to the unique morphology and the built-in electric field, the catalyst exhibits excellent electrocatalytic activity and stability. The Co3(PO4)2@MXene heterojunction catalyst only requires low overpotentials of 46 and 58.6 mV in 1 M KOH and alkaline seawater to achieve a current density of 10 mA cm−2, respectively. Moreover, the Co3(PO4)2@MXene||NiFe LDH pair demonstrates remarkable performances in overall water/alkaline seawater electrolysis, exhibiting promising prospects for large-scale hydrogen production. This work provides new opportunities for rationally designing efficient heterojunction electrocatalysts based on built-in electric fields for clean energy conversion and storage.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (22075141 and 52371226), the Scientific and Technological Innovation Special Fund for Carbon Peak and Carbon Neutrality of Jiangsu Province (BK20220039), the Opening Project of the State Key Laboratory of High Performance Ceramics and Superfine Microstructure (SKL202208SIC), the Natural Science Foundation of Jiangsu Province (BK20210311 and BK20221482), the Fundamental Research Funds for the Central Universities (NS2023032), State Key Laboratory for Modification of Chemical Fibers and Polymer Materials (KF2312), the Postgraduate Research & Practice Innovation Program of NUAA (xcxjh20230603), the Key Research and Development Program (Industrial) of Yancheng City (BE2023001), and the Open Foundation of Shanghai Jiao Tong University Shaoxing Research Institute of Renewable Energy and Molecular Engineering (Grant No. JDSX2023019). The authors acknowledge EV Energies Jiangsu Co., Ltd. for its technical support.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4qi01587h
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