Construction of Co/Co₂P/VN heterointerfaces enhances trifunctional hydrogen and oxygen catalytic reactions

Abstract

Designing efficient and serviceable electrocatalysts for overall water-splitting and metal–air batteries is crucial for advancing a green economy. Herein, we successfully synthesized a novel nanocomposite material composed of Co/Co₂P/VN heterointerfaces anchored on a nitrogen (N)-doped graphitic carbon matrix through a one-step pyrolysis process. The resultant Co/Co₂P/VN exhibits unparalleled electrocatalytic trifunctional properties due to abundant catalytically active sites at the heterointerfaces and the optimized electronic configuration, possessing a relatively low overpotential for the HER (η₁₀ = 111 mV) and OER (η₁₀ = 379 mV), and a high half-wave potential for the ORR (E_1/2 = 0.865 V), while maintaining the long-term stability for all three catalytic reactions in alkaline media. This work provides critical insights into the design of future efficient multicomponent materials for trifunctional electrocatalysis.

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1. Introduction

Driven by the increasing scarcity of fossil fuel resources and environmental challenges, the exploration of water-splitting devices and metal–air batteries has attracted enormous interest due to their significant potential applications.^1–3 Electrocatalytic water splitting and metal–air batteries consist of three key half-reactions, i.e., the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR).^4–6 To date, precious metal-based materials (Pt, Ru, or Ir) have been identified as benchmark catalysts for these core electrochemical reactions.⁷ Nevertheless, the prohibitive expense and scarcity severely hamper their widespread commercialization. Furthermore, precious metal-based electrocatalysts are usually unable to catalyze the HER and OER in the same electrolyte. For example, IrO₂ and RuO₂ are considered benchmarks for OER catalysts with good activity and stability, but their activity towards the HER and ORR is not satisfactory. The development of multifunctional reaction activity of precious metal catalysts is very limited.^8,9 Therefore, exploring economical multifunctional electrocatalysts with high efficiency and cost-effectiveness is highly desirable for sustainable water-splitting and metal–air battery technology.

Recently, various Earth-abundant transition metal compounds (TMCs), including transition metal nitrides,^10–12 transition metal phosphides,^13–16 transition metal chalcogenides,^17–19 and transition metal borides,^20–22 have been studied for overall water-splitting and metal–air batteries, and have made great progress in practical application. Vanadium nitride (VN), a typical representative of transition metal nitrides, has shown Pt-like properties with high electrochemical stability, good electrical conductivity, and excellent corrosion resistance under electrochemical operating conditions. While VN has demonstrated good electrochemical performance in the ORR,^23–25 it generally exhibits poor activity and stability in the HER and OER.^26,27 Furthermore, VN often forms large nanoparticles due to spontaneous agglomeration under high-temperature processes, which could significantly reduce the accessible electrochemical surface area and hence the catalytic activity. By contrast, transition metal phosphides (TMPs), especially cobalt-based phosphides (Co_xP_y), have attracted much attention owing to their high efficiency and good durability for the HER and OER.^28,29 However, Co_xP_y has shown disadvantages of poor conductivity and insufficient active sites. It is expected that the construction of heterojunction nanostructures with the combination of each active component while optimizing their electronic structures can provide multiple functional sites for synergistically enhanced electrocatalytic activity compared to single-phase structures.

Herein, a facile one-step pyrolysis method is used to construct the Co/Co₂P/VN heterojunctions supported on the N-doped carbon matrix. The as-prepared Co/Co₂P/VN electrocatalyst exhibits excellent trifunctional electrocatalytic activity for the HER, OER, and ORR with desired high stability. In addition to the heterogeneous interface, transition metal nanocrystals can enable the formation of carbon nanotubes to further improve the electrical conductivity of materials, which not only improves particle dispersibility and uniformity, but also facilitates electron transport in the electrocatalytic processes. These results demonstrate that the interfacial engineering regulation strategy for the synthesis of the Co/Co₂P/VN catalyst triggers electron redistribution and tunes the surface charge state to accelerate the reaction kinetics, resulting in excellent trifunctional properties.

2. Experimental

2.1. Chemicals and reagents

Cobalt nitrate hexahydrate (Co(NO₃)₂·6H₂O), sodium hypophosphite monohydrate (NaH₂PO₂·H₂O), vanadium trichloride (VCl₃), dicyandiamide (DCD), potassium hydroxide (KOH), isopropyl alcohol (C₃H₈O), Nafion solution (5 wt%), absolute ethanol (CH₃CH₂OH, ≥99.7%) and platinum on activated carbon (20 wt% Pt/C) were purchased from Sigma-Aldrich (USA) and used as received.

2.2. Synthesis of Co/Co₂P/VN

Typically, 160 mg of Co(NO₃)₂·6H₂O, 53 mg of VCl_3, and 600 mg of DCD were ground together to obtain a well-mixed powder. The mixed powder was then loaded into a small porcelain boat, which was placed on one side of a large porcelain boat. 800 mg sodium hypophosphite monohydrate (NaH₂PO₂·H₂O) was placed on the other side of the large porcelain boat and used as the reaction raw material. Subsequently, the large porcelain boat was placed in the center of the heating zone of a tube furnace. The NaH₂PO₂·H₂O side was on the upper tuyere, while the mixed powder was on the lower tuyere, with H₂/Ar as the reducing/protective gas at a flow rate of 60 sccm min⁻¹. The furnace was heated to 700 °C at a heating rate of 10 °C min⁻¹. The reaction was maintained for 3 h. After cooling to room temperature under a H₂/Ar atmosphere, the samples were ground and collected. Finally, a black powder sample was obtained, denoted as Co/Co₂P/VN.

2.3. Synthesis of Co/Co₂P, Co/VN, and Co

The synthesis method of Co/Co₂P, Co/VN, and Co was similar to that of Co/Co₂P/VN. Specifically, to synthesize Co/Co₂P, 160 mg of Co(NO₃)₂·6H₂O and 600 mg of DCD were ground and placed on one side of a large porcelain boat with a small porcelain boat on the other side for P-treatment (800 mg of NaH₂PO₂·H₂O). H₂/Ar was used as the reducing/protective gas. The temperature was increased to 700 °C at a rate of 10 °C min⁻¹ and kept for 3 h. After that, the resulting powder was denoted as Co/Co₂P. Similarly, 160 mg of Co(NO₃)₂·6H₂O, 53 mg of VCl₃, and 600 mg of DCD were used as the precursors for synthesizing Co/VN, while 160 mg of Co(NO₃)₂·6H₂O and 600 mg of DCD were used for synthesizing Co without adding the phosphorus source.

2.4. Materials characterization

The phase composition and crystal information of the synthesized materials were characterized by using an X-ray diffractometer (XRD, Empyrean II) with CuKα radiation (λ = 0.15406 nm). The morphology and microstructure of the samples were investigated using a transmission electron microscope (TEM, FEI Tecnai G2 F20S-TWIN) coupled with energy-dispersive X-ray (EDX) spectroscopy and a field-emission scanning electron microscope (FE-SEM, Hitachi S-4800). We obtained the Raman spectra of the material using a Renishaw inVia microscopic confocal laser Raman spectrometer (excitation laser: 532 nm). An X-ray photoelectron spectrometer (XPS, Thermo ESCALAB250i, AlKα source) was used to obtain the surface species and chemical states.

2.5. Electrochemical measurements

For the HER and OER, all the experimental tests were conducted on a CHI 660E electrochemical workstation (ChenHua Instrument, Inc., Shanghai) employing a conventional three-electrode H-cell configuration at ambient temperature. A saturated calomel electrode (Hg/HgCl₂, SCE) served as the reference electrode; a graphite rod was chosen as the counter electrode; and a glassy carbon electrode (GCE, geometric area: 0.0714 cm²) served as the working electrode. 4 mg of the catalyst was dispersed in 200 μL isopropyl alcohol containing 5 μL Nafion solution (0.1 wt%) under ultrasonic treatment for 1 h to achieve uniform suspension of ink. Then, 2 μL of ink was loaded on the surface of the GCE, followed by drying naturally in air before testing. The catalyst loading density is about 0.55 mg cm⁻². The electrocatalytic HER and OER performance was stabilized by running 20 CV cycles in a 1.0 KOH solution. All the potentials were referred to the reversible hydrogen electrode (RHE) by using:

E_{vs. RHE} = E_{vs. SCE} + 0.2415 + 0.059 pH. (1)

Linear sweep voltammetry (LSV) was performed at a scan rate of 5 mV s⁻¹ in a N₂-saturated environment. The Tafel slope was obtained from the corresponding LSV curves. The frequency range for electrochemical impedance spectroscopy (EIS) testing is from 0.1 Hz to 100 kHz. The cyclic voltammogram (CV) based on the non-faradic region was obtained at different scan rates of 10–140 mV s⁻¹. Moreover, the double-layer capacitances (C_dl) obtained through CV can be used to evaluate the electrochemically active surface area (ECSA). To monitor the stability of the catalyst, the multistep chronoamperometric curve was assessed for 50 h at different potentials.

For the ORR, the measurements were performed on a rotating disc electrode (RDE) or a rotating ring disc electrode (RRDE) (RIKEN Hong Kong Ltd, AFMSRCE model). A platinum counter electrode and a SCE were used as the counter and reference electrodes, respectively. 0.1 M KOH (pH = 13) was used as the electrolyte. Before the ORR test, oxygen gas was passed through the electrolyte for more than 30 minutes to completely remove the dissolved air and obtain a saturated O₂ solution. All potentials were calibrated to RHE according to eqn (1). The current density (J) was normalized using the geometrical area of the working electrode (for the RDE: 0.1963 cm²). For the RDE, the catalyst loading density is about 1.99 mg cm⁻².

The RRDE voltammetry curves were obtained at different rotation speeds ranging from 400 to 2500 rpm at a scan rate of 5 mV s⁻¹ to obtain the selectivity in the ORR. The number of electrons transferred (n) and the percentage (%) of peroxide (HO₂⁻) were calculated according to the following eqn (2) and (3):^30,31

I_d represents the disk current, and I_r represents the ring current. The RRDE current collection efficiency N value is 0.38. The disk electrode in the RRDE is made of glassy carbon material (geometric area: 0.2376 cm²), while the ring electrode is made of Pt (geometric area: 0.2356 cm²).

3. Results and discussion

3.1. Structural and morphological characterization

Fig. 1a shows a schematic representation of the synthesis process for the Co/Co₂P/VN nanocomposite. Heterogeneous Co/Co₂P/VN nanoparticles uniformly embedded in the N-doped carbon matrix were successfully prepared via one-step pyrolysis at 700 °C. It is worth noting that under low-temperature conditions (300 °C), NaH₂PO₂ can undergo pyrolysis to generate PH₃ and Na₂HPO₄. When placed in a porcelain boat separately for calcination, PH₃ gas flowed into the small ceramic boat for the phosphating reaction to generate Co₂P, while avoiding other impurities that might affect the purity of the final product.^14,32 When the temperature increased to 700 °C, VCl₃ and DCD began to react to generate VN. Meanwhile, metallic Co nanoparticles were obtained under the reducing Ar/H₂ gas at high temperatures, which promoted the formation of carbon nanotubes.^33,34 The resultant nanoparticles were coupled with each other to generate Co/Co₂P/VN materials with abundant heterostructures.

Fig. 1 (a) Schematic illustration of the synthesis of Co/Co₂P/VN. (b) XRD patterns and (c) Raman spectra of Co, Co/Co₂P, Co/VN, and Co/Co₂P/VN.

The crystalline structures of as-synthesized samples were probed by XRD. As shown in Fig. 1b, the diffraction peaks can be assigned to Co (PDF #15-0306), Co₂P (PDF #32-0306), and VN (PDF #35-0768). Fig. S1† shows the enlarged view of XRD spectra in the 2θ range of 20° to 40°, where the peaks at about 26° and 35° could be assigned to graphitic carbon and the (111) crystal plane of VN, respectively. The effect of pyrolysis temperature on the crystallinity of materials was also evaluated. As shown in Fig. S2,† the phases of materials synthesized at different pyrolysis temperatures did not change significantly, with Co/Co₂P/VN synthesized at 700 °C possessing the highest relative Co₂P content. Meanwhile, the Co peak and VN peak of Co/Co₂P/VN-800 increased significantly, suggesting that as the temperature increased, Co and VN nanoparticles began to aggregate more, and the graphite carbon peak also showed an upward trend. These XRD results showed that Co, Co₂P, VN, and graphitic carbon species co-existed in the sample, leading to abundant heterojunctions within the nanocomposite.

The Raman spectra are shown in Fig. 1c. The spectrum presents two kinds of peaks at around 1350 and 1600 cm⁻¹, which can be ascribed to the defect-induced D-bands and in-plane vibrational graphitic G-bands.^35,36 According to the XRD (Fig. S1†) pattern, it can be seen that almost no graphite carbon (about 26°) peak was found in Co and Co/Co₂P samples, while weak graphite carbon was present in Co/VN and Co/Co₂P/VN samples. Therefore, similar conclusions were also shown in the Raman spectra (Fig. 1c) of these four samples, with clear D and G peaks observed in Co/VN and Co/Co₂P/VN samples. The I_d/I_g ratio is an important parameter for measuring the defect density in graphite carbon materials.³⁷ Among them, the I_d/I_g value of the Co/Co₂P/VN sample is 0.8407, higher than that of Co/VN (0.8386), further verifying that the graphite carbon contained in the sample has abundant defects and a disordered structure. According to reports,^1,25 the increase in defects in carbon-based catalyst materials means an increase in catalytically active sites, which is conducive to rapid charge transfer and promotes catalytic reaction kinetics.

Graphitization refers to the ordered transformation of carbon atoms from a disordered layer structure into a graphite crystal structure. For carbon materials, the higher the degree of graphitization, the better the conductivity. The growth and ordering of graphite grains (i.e. graphitization) are often related to the temperature of carbonization pyrolysis.³⁶ Fig. S2† shows the XRD patterns of samples synthesized at different temperatures. The samples at 700 and 800 °C exhibit distinct graphite carbon peaks at around 26°, indicating that as the temperature increases, graphite carbon is generated, the degree of graphitization increases, and the conductivity of the material is enhanced. However, due to the competition between the generation of defects and graphitization in carbon materials,³⁸ we speculate that although the increase in temperature promotes graphitization of the material, it may also lead to a reduction in defects. Hence, the Co/Co₂P/VN samples prepared at a reasonable pyrolysis temperature (700 °C) in this work exhibit abundant defects and an appropriate graphitization degree (conductivity), thus demonstrating optimal catalytic activity.

The morphological and microstructural characteristics of the synthesized samples were studied using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As depicted in Fig. 2a, carbon nanotubes with a diameter of about 20 to 40 nm were seen as the main body of the matrix. Compared with Co/Co₂P/VN synthesized at 700 °C, the sample synthesized at 600 °C only showed an agglomerated carbon structure without the formation of nanotubes (Fig. S3a and d†), while at 800 °C, the overgrowth of carbon nanotubes resulted in large diameters with thick walls (Fig. S3c and f†). This is because as the temperature increases, the generation of Co nanoparticles gradually increases. At 600 °C, there are insufficient Co nanoparticles and carbon nanotubes cannot grow, resulting in fewer active sites. At 700 °C, uniformly dispersed Co nanoparticles were generated, which is highly conducive to the uniform growth of carbon nanotubes, resulting in sufficient exposure of active sites. At the same time, only Co, VN, and Co₂P generated at 700 °C can effectively form heterogeneous interfaces. However, at 800 °C, a large number of Co particles severely agglomerate into bulk Co, which cannot assist in the generation of carbon nanotubes, resulting in oversized or even broken carbon nanotubes, which seriously hinders the exposure of active sites. Interestingly, the TEM image of Co/Co₂P/VN synthesized at 700 °C suggested that numerous nanoparticles were uniformly embedded in the carbon nanotube matrix (Fig. 2b). The crystalline planes of Co (111), Co (200), Co₂P (121), and VN (200), as well as the interfaces between these planes, can be clearly distinguished from the high-resolution TEM (HRTEM) images (Fig. 2c and d), confirming the existence of heterointerfaces in Co–Co₂P–VN. In addition, weak CoO (200) was found near the Co particles. However, we did not observe any obvious characteristic peaks of CoO in XRD patterns (Fig. 1b) and Raman spectra (Fig. 1c), indicating that CoO may originate from slight oxidation on the catalyst surface. The corresponding EDX mapping (Fig. 2e–k) indicated that the C, N, O, Co, V, and P elements were dispersed in the hetero-nanoparticles surrounded by the carbon element, which agreed well with the proposed heterogeneous structure of Co/Co₂P/VN nanoparticles embedded in carbon nanotubes. It was noted that carbon nanotubes could not only improve the conductivity of Co/Co₂P/VN nanoparticles, but also provide extra anchoring sites to the heterostructures for improved accessibility and catalytic activity. Meanwhile, in addition to the O element scattered on the carbon substrate, it can also be observed that the O element is more concentrated around the Co particles (Fig. 2h and i). We also carefully observed the presence of weak cobalt oxide species (e.g. CoO and Co₃O₄) in the HRTEM images of the other three comparative samples Co, Co/Co₂P, and Co/VN (Fig. S4†). The above analysis proves that the presence of trace cobalt oxide species on the surface of the materials is due to the oxidation of Co, which is commonly observed in nano-sized materials exposed to the atmosphere. Besides, compared to Co/Co₂P/VN, the TEM images of Co, Co/Co₂P, and Co/VN comparison samples (Fig. S4a, c and f†) show poor microstructures and severely aggregated large particle morphology, which hinders the rapid transfer of electrons/charges in the electrochemical process and is not conducive to the improvement of catalytic activity.

Fig. 2 Morphologic and microstructural characterization of Co/Co₂P/VN. (a) SEM image; (b) TEM image; (c and d) HRTEM image; (e–k) EDX element mapping images of Co/Co₂P/VN.

Owing to the lattice mismatch between the Co, Co₂P, and VN phases, interfacial electron interactions among them were unavoidably triggered. To clarify this point, X-ray photoelectron spectroscopy (XPS) measurements were conducted. The XPS survey spectra of the Co/Co₂P/VN sample identified the existence of C, N, O, Co, V, and P (Fig. 3a). The high-resolution C 1s spectra of Co/Co₂P/VN samples can be resolved into three obvious peaks of C–C/C=C (284.6 eV), C–N/C–P (285.21 eV) and O–C=O (287.36 eV) bonds (Fig. 3b), with the adventitious carbon peak calibrated to 284.6 eV.^39,40 In the high-resolution Co 2p spectra of Co/Co₂P/VN, peaks at 778.45 eV and 793.53 eV could be attributed to metallic cobalt, in good agreement with the XRD and TEM characterization studies.⁴¹ Peaks at 780.58 eV and 796.79 eV were assigned to the Co–O bonds generated by the inevitable oxidation of the material surface, while peaks at 782.37 and 803.02 eV were ascribed to the shakeup satellite peaks (Fig. 3c).⁴² It is worth noting that compared with the single Co sample, the peak position of Co⁰ in Co/VN showed similar positions with almost no peak shift, indicating that there is no direct strong electron interaction between VN and Co, and the introduction of VN has a nearly negligible effect on Co. Compared with Co/VN, the Co⁰ peak of the Co/Co₂P/VN sample showed a slight positive shift (electron loss), indicating that the introduction of Co₂P leads to a trend of electron loss around the original Co cluster. Besides, the Co⁰ peak of the Co/Co₂P/VN sample showed a negative shift (electron gain) compared to that of the Co/Co₂P sample, indicating that introducing VN can enable Co atoms in Co₂P or Co clusters to acquire d electrons. Because the above analysis shows no direct interaction between Co and VN, hence, the introduction of VN mainly promotes the electron acquisition ability of Co atoms in Co₂P.

Fig. 3 (a) XPS survey spectrum of Co, Co/Co₂P, Co/VN, and Co/Co₂P/VN samples. The corresponding high-resolution XPS spectra: (b) C 1s; (c) Co 2p; (d) N 1s; (f) V 2p. (e) Relative content of different nitrogen species in Co/VN and Co/Co₂P/VN. (g) Schematic diagram of electronic coupling between Co, P, N, and V in Co/Co₂P/VN.

Fig. 3d shows the high-resolution N 1s spectrum of Co/Co₂P/VN, which could be deconvoluted into five peaks at 397.6, 398.41, 399.26, 401.07, and 404.93 eV, corresponding to M–N (where M is V or Co), pyridinic N, pyrrolic N, graphitic N and oxidized N, respectively.⁴³ As compared to the spectrum of Co/VN (Fig. 3d), the peak position of pyrrolic N in Co/Co₂P/VN was shifted to lower binding energies. This indicated that the N atom of VN can obtain d electrons from Co₂P. Due to the stronger electronegativity of N compared to Co, the N-site can obtain d electrons from Co in Co₂P (Co → N), suggesting that the N dopants could be effective catalytically active sites for promoting the adsorption/dissociation kinetics of water.¹⁰ The relative contents of different nitrogen bonds are shown in Table S1† and Fig. 3e. It is found that the atomic concentration of graphitic N in Co/Co₂P/VN (39.22%) was much higher than that in Co/VN (17.69%), suggesting the increased degree of graphitization which could contribute to fast electron transfer and kinetics during electrochemical processes. The high-resolution V 2p spectrum (Fig. 3f) showed that V species also exhibited a similar behavior. The V 2p peaks of Co/Co₂P/VN consisted of four peaks, among which two at 514.16 and 521.17 eV correspond to the V–N bond, which agreed well with the N 1s spectrum (Fig. 3d).⁴⁴ Moreover, the peaks located at 517.05 and 524.47 eV were attributed to the V–O species, arising from surface oxidization of VN when exposed to air.⁴⁵ Compared with the Co/VN sample, the peak positions of V–N in Co/Co₂P/VN showed a slight negative shift, indicating that V in VN can also obtain d electrons from Co₂P. Thereby, the V sites can also be potential active sites that promote the kinetics of electrocatalytic reactions. According to these XPS analyses, it can be concluded that there were strong electronic interactions among the heterointerfaces of Co–Co₂P and Co₂P–VN within the Co/Co₂P/VN composite. For the XPS spectrum of P 2p (Fig. S5a†), two deconvolved peaks located at 129.75 and 130.70 eV are related to P 2p_3/2 and P 2p_1/2 orbitals of P^δ−, while another peak at 133.20 eV is attributed to phosphate species resulting from the surface oxidation of phosphides in air.^6,16,46 The P 2p peak of Co/Co₂P/VN showed a positive shift (electron loss) compared to Co/Co₂P, suggesting that P tends to lose electrons to adjacent atoms or compounds. Besides, the peaks at 531.65 and 530.27 eV correspond to typical peaks of mild carbon oxidation and surface metal adsorption of OH⁻.⁴⁷ Based on the above analysis, the successful synthesis of the Co/Co₂P/VN material heterojunction in terms of morphology and structure has induced the occurrence of electronic synergistic effects between the active components of Co, Co₂P, and VN. This strong interface electronic interaction effectively regulates the surface charge distribution and d-band electron energy level structure of the material, which has great potential for enhancing the catalytic activity of electrode materials.

3.2. Electrocatalytic performance of Co/Co₂P/VN

3.2.1. HER performance. The HER performance was evaluated in an N₂-saturated 1.0 M KOH solution in a conventional three-electrode H-cell configuration. Before testing, we usually stabilize and activate the catalyst by running 20 CV cycles to remove impurities such as oxides (e.g. CoO and Co₃O₄) on the material surface and eliminate the influence of these impurities during the electrochemical testing process. Fig. 4a reveals the polarization curves of different electrocatalysts, including Co, Co/Co₂P, Co/VN, Co/Co₂P/VN, and commercial 20% Pt/C. It is seen that Co, Co/Co₂P, and Co/VN catalysts possessed large overpotentials (with iR-compensation) of 286 mV, 214 mV, and 187 mV, respectively, to deliver a current density of 10 mA cm⁻² (Fig. 4b), suggesting a relatively poor HER activity in the 1.0 M KOH alkaline solution. The Co/Co₂P/VN electrode exhibits significantly enhanced electrocatalytic HER activity, showing a low overpotential of 111 mV at a current density of 10 mA cm⁻². Fig. S6† confirms that Co/Co₂P/VN synthesized at 700 °C had the lowest overpotential. This further indicated that the presence of Co–Co₂P–VN heterointerfaces could improve the HER performance. And the presence of more carbon nanotubes at 700 °C is also beneficial for increasing the electrochemical active area and active sites, enhancing conductivity, facilitating rapid electron transfer, and improving catalytic performance.

Fig. 4 Electrochemical HER tests of Co, Co/Co₂P, Co/VN, Co/Co₂P/VN, and 20% Pt/C in 1.0 M KOH electrolyte: (a) LSV curves; (b) comparison of HER overpotentials; (c) Tafel slopes; (d) Nyquist plots; (e) double-layer capacitance (C_dl); (f) polarization curves of Co/Co₂P/VN before and after 10 000 CV cycles (inset illustrates the chronopotentiometry curve at 10 mA cm⁻²).

Generally speaking, the HER process typically involves three steps in an alkaline medium (eqn (4)–(6)). In this work, under alkaline conditions, the water molecules in the solution participate in the reaction, generating intermediate adsorbed hydrogen atoms (H_ads), which is the Volmer reaction step. Subsequently, there are two pathways involved in the hydrogen desorption process. One is the Heyrovsky reaction step (electrochemical desorption behavior), where H_ads undergoes a secondary electron transfer reaction with H⁺ in the solution to generate H₂. Another way is for two H_ads in the solution to combine and generate H₂, which is called the Tafel step.³³

H₂O + e⁻ + M → M–H_ads + OH⁻ (Volmer) (4)

M–H_ads + H₂O + e⁻ → H₂ + OH⁻ + M (Heyrovsky) (5)

And/or

M–H_ads + M–H_ads → H₂ + M (Tafel) (6)

The theoretical Tafel slopes of the Volmer, Heyrovsky, and Tafel reactions, being the HER rate-limiting steps, are 120, 40, and 30 mV dec⁻¹, respectively.¹⁴ The reaction mechanism of the HER can be explored using the Tafel slope. As revealed in Fig. 4c, the Tafel slope of the Co/Co₂P/VN catalyst was 154.4 mV dec⁻¹, smaller than those of Co (209.1 mV dec⁻¹), Co/Co₂P (162.6 mV dec⁻¹) and Co/VN (200.6 mV dec⁻¹), indicating that the four catalysts follow the Volmer mechanism in the HER process. This implies faster electrochemical kinetics on the Co/Co₂P/VN surface. We can speculate that VN with insufficient d-band electron density can show complementary advantages with Co rich in d-electrons. Meanwhile, Co₂P, as a bridge for electron transfer, can receive electrons pumped by Co and transfer them to VN, thereby obtaining an optimized electronic structure and surface charge distribution, reducing the energy barriers for water molecule adsorption and decomposition in water splitting reactions.

To understand the charge transfer mechanism, EIS measurements were performed. As shown in the Nyquist plots (Fig. 4d), the charge transfer resistance (R_ct) of Co/Co₂P/VN (23.55 Ω) was much lower than that of Co (80.96 Ω), Co/Co₂P (47.02 Ω), and Co/VN (39.5 Ω), indicating faster charge transfer between the Co/Co₂P/VN electrode and the electrolyte. The illustrations in Fig. 4d and Table S2† show the equivalent circuit diagram and corresponding R_ct values. The C_dl of catalysts was calculated based on the CV (Fig. S7†) to estimate the ECSA.^48,49 As illustrated in Fig. 4e, the double-layer capacitance value (C_dl) is half the slope of the linear curve. The C_dl value of Co/Co₂P/VN was 88.95 mF cm⁻², which was larger than those of Co (0.235 mF cm⁻²), Co/Co₂P (12.85 mF cm⁻²) and Co/VN (69.35 mF cm⁻²), suggesting the increased number of active sites. In addition, long-term electrochemical stability is also a key factor in evaluating the HER performance of electrocatalysts. For Co/Co₂P/VN, the overpotential increased by only 13 mV after 1000 CV cycles in 1.0 M KOH (Fig. 4f). The chronoamperometry test further demonstrated that the Co/Co₂P/VN catalyst can keep working efficiently for 50 h at a constant current density of 10 mA cm⁻² with negligible fluctuation, demonstrating better electrochemical stability than most HER catalysts reported in the recent literature (Table S3†). Besides, the microstructure, composition, and morphology of Co/Co₂P/VN remained almost intact after 50 h of long-term stability testing, projecting the excellent structural stability of the material even after the HER (Fig. S8†).

3.2.2. OER performance. The electrocatalytic oxygen evolution reaction (OER) often involves a four-electron transfer process, which is more complex than the HER mechanism. In the electrocatalytic OER process, water molecules in an alkaline solution undergo four steps on the catalyst surface, namely adsorption of hydroxide ions, breaking of hydrogen–oxygen bonds, generation of oxygen bonds, and desorption of oxygen, as shown in eqn (7)–(10).⁴⁶

* + OH⁻ ⇌ OH* + e⁻ (7)

OH* + OH⁻ ⇌ O* + H₂O + e⁻ (8)

O* + OH⁻ ⇌ OOH* + e⁻ (9)

OOH* + OH⁻ ⇌ * + O₂ + H₂O + e⁻ (10)

In the formula, * represents the active site of the catalyst, and OH*, O*, and OOH* represent the adsorbed intermediate species. According to the above mechanism, adjusting the adsorption energy of OOH* or O* will directly affect the performance of the OER.

In a conventional three-electrode H-cell configuration, the OER performance of Co/Co₂P/VN was assessed in 1.0 M KOH alkaline electrolyte. As shown in Fig. 5a, Co/Co₂P/VN displayed outstanding activity with an overpotential of about 379 mV (with iR-compensation) at a current density of 10 mA cm⁻², which was smaller than those of Co (465 mV), Co/Co₂P (449 mV), and Co/VN (412 mV), as well as commercial RuO₂ (410 mV). Meanwhile, Co/Co₂P/VN synthesized at 700 °C exhibited lower overpotentials than those synthesized at 600 °C and 800 °C (Fig. S9†). The Co/Co₂P/VN heterojunction also showed the smallest Tafel slope (181.4 mV dec⁻¹) among all electrocatalysts (Fig. 5c), showing accelerated reaction kinetics and good intrinsic activity. As shown in Fig. 5d and Table S4,† Co/Co₂P/VN gave an R_ct value of 55.71 Ω, lower than those of Co (118.6 Ω), Co/Co₂P (92.36 Ω), and Co/VN (86.17 Ω), suggesting more efficient and faster electron transfer during the OER process. Moreover, the C_dl value of Co/Co₂P/VN, half the slope of the linear curve (Fig. 5e), acquired from CV at different scanning rates was 95.95 mF cm⁻², the largest among those of Co (0.8 mF cm⁻²), Co/Co₂P (1.25 mF cm⁻²), and Co/VN (30.5 mF cm⁻²) (Fig. 5e and S10†). Since the C_dl of electrocatalysts is often positively related to their water oxidation activities, Co/Co₂P/VN is thus expected to have the most favorable OER performance.^50,51 Additionally, the results of stability tests are shown in Fig. 5f. The LSV polarization curves before and after 10 000 cycles overlayed well and the potential response displayed no noticeable attenuation after 50 h chronopotential measurement at 10 mA cm⁻² (see the inset of Fig. 5f), confirming that the synergistic effect of abundant interface heterojunctions leads to excellent electrocatalytic OER performance (Table S5†) with outstanding stability and structural stability (Fig. S11†). In this work, due to the presence of the Co component, a large amount of oxygen generated during the OER process will oxidize Co into active OER species such as Co₃O₄ and CoOOH.^9,14,20 The generation of oxidizing substances such as CoOOH increases the adsorption of OOH* and O* in the electrolyte, which is beneficial for promoting the forward progress of the OER.

Fig. 5 Electrochemical OER tests of Co, Co/Co₂P, Co/VN, Co/Co₂P/VN, and 20% Pt/C in 1.0 M KOH solution: (a) LSV curves; (b) comparison of OER overpotentials; (c) Tafel slopes; (d) Nyquist plots; (e) double-layer capacitance (C_dl); (f) LSV polarization curves of Co/Co₂P/VN before and after 10 000 CV cycles (inset illustrates the chronopotentiometry curve at 10 mA cm⁻²).

3.2.3. ORR performance. Finally, the ORR catalytic activities of as-prepared samples and commercial Pt/C were measured by LSV in O₂-saturated 0.1 M KOH at a scan rate of 5 mV s⁻¹ using an RDE (Fig. 6a). The Co/Co₂P/VN electrode demonstrated higher activity than other samples in terms of onset potential (E_onset = 1.070 V) and half-wave potential (E_1/2 = 0.865 V), as shown in Fig. 6a and b. Fig. S12† also shows that Co/Co₂P/VN synthesized at 700 °C has better performance compared to those synthesized at 600 °C and 800 °C. Tafel plots were further analyzed to get insights into ORR performance. As shown in Fig. 6c, Co/Co₂P/VN demonstrated a Tafel slope of 158.0 mV dec⁻¹, significantly smaller than those of Co (224.6 mV dec⁻¹), Co/Co₂P (199.2 mV dec⁻¹), Co/VN (210.2 mV dec⁻¹) and commercial 20% Pt/C (199.3 mV dec⁻¹). The smaller Tafel slope indicated accelerated ORR kinetics on Co/Co₂P/VN.⁵² Such ORR performance can be attributed to the optimized electronic structure of the double heterostructure.

Fig. 6 (a) RDE voltammetry curves of Co, Co/Co₂P, Co/VN, Co/Co₂P/VN, and 20% Pt/C at a scan rate of 5 mV s⁻¹ and a rotation speed of 1600 rpm in 0.1 M KOH solution. (b) Onset potential (E_oneset) and half-wave potential (E_1/2) curves of Co, Co/Co₂P, Co/VN, Co/Co₂P/VN, and 20% Pt/C. (c) The corresponding Tafel plots of the ORR at 1600 rpm derived from (a). (d) RRDE voltammograms of the Co/Co₂P/VN catalyst at a scan rate of 5 mV s⁻¹ and different rotation speeds ranging from 400 to 2500 rpm, and the Koutecky–Levich (K–L) plots (insets). (e) The electron transfer number (n) and the percentage (%) of peroxide (H₂O₂) yield of Co/Co₂P/VN and commercial Pt/C. (f) The current versus time (i–t) chronoamperometric response of Co/Co₂P/VN and Pt/C at 0.7 V in O₂-saturated 0.1 M KOH at 200 rpm.

The RRDE voltammetry curves can reflect the ORR kinetics of the Co/Co₂P/VN catalyst, as shown in Fig. 6d. As the rotation speed increases (from 400 to 2500 rpm), the convective mass transfer of oxygen molecules from the electrode bulk to the electrode surface is enhanced, resulting in a corresponding increase in current density. Above all, the K–L plots of Co/Co₂P/VN at different rotation speeds exhibited perfect linearity and consistent slopes (see the inset of Fig. 6d). This illustrates the first-order kinetics in the ORR process. In general, the ORR process is considered in the premise of the OOH dissociation mechanism as follows (eqn (11)–(14)):²

* + O₂ + H₂O + e⁻ → OOH* + OH⁻ (OOH formation) (11)

OOH* + e⁻ → O* + OH⁻ (OOH dissociation) (12)

O* + H₂O + e⁻ → OH* + OH⁻ (O hydration) (13)

OH* + e⁻ → * + OH⁻ (OH desorption) (14)

Note that * denotes the reaction site of N in VN. Typically, the adsorption of O₂ on the catalyst surface is the prerequisite step for an effective ORR. As mentioned above, the four comparative samples have different morphologies and structures. The Co, Co/Co₂P, and Co/VN catalysts have lower ORR catalytic activity, which may be due to their poor nanostructure (Fig. S4†), fewer defects, and low degree of graphitization (Fig. 1c, indistinct D and G bands), resulting in hindered electron transport and poor conductivity. The excellent ORR performance of the Co/Co₂P/VN catalyst is mainly attributed to its well-structured multi-level carbon encapsulated dispersed particles, crystal structure rich in defects with a good graphitization degree, and active components with strong electron synergistic effects. These advantages make the material surface rich in active sites, enhance O₂ adsorption, and accelerate surface electron transfer rate, greatly promoting the ORR process.

The RRDE measurements confirmed the high selectivity of Co/Co₂P/VN for the 4e⁻ oxygen reduction, with the electron transfer number (n) of ∼3.9 derived from the slopes of the K–L plots (Fig. 6e). The percentage (%) of peroxide (H₂O₂) yield is less than 6% in the potential range of 0.2–0.8 V (using eqn (3)), also indicating a direct 4e⁻ transfer process of Co/Co₂P/VN towards H₂O.^2,53 In addition, the Co/Co₂P/VN sample had excellent long-term stability. The reduction current density remained at 96.8% of its original value after 36 000 s of continuous testing, which was much better than the figure obtained on the Pt/C electrode (77.4%) (Fig. 6f). The microstructure and components of the Co/Co₂P/VN electrocatalyst remained at an intact state after the stability test, proving the excellent structural stability throughout the electrochemical tests (Fig. S13†). Moreover, Co/Co₂P/VN exhibited high tolerance toward 3.0 M methanol (see Fig. S14†), whereas the ORR current of the Pt/C catalyst exhibited a sharp drop after 10 mL of methanol was injected into the electrolyte at 300 s. Overall, the Co/Co₂P/VN heterointerfaces were highly promising as an outstanding ORR catalyst with high catalytic activity and excellent electrochemical stability.

3.2.4. Electrochemical enhancement mechanism. To further analyze the catalytic performance enhancement mechanism of Co/Co₂P/VN, as shown in Fig. 3g, we constructed a Co–Co₂P–VN model to explain the reasons for the electron transfer behavior of each component in Co/Co₂P/VN. Based on the XPS analysis mentioned earlier, it can be concluded that in Co/Co₂P/VN heterogeneous electrocatalysts, there exists a relatively complete electron transfer pathway during the oxidation–reduction reaction of water splitting: Co → Co₂P → VN. The electron arrangement of the Co atom is 3d⁷4s²;¹⁴ it has a high d-band density and can transfer electrons to “bridge” Co₂P (strong electron transfer). Due to the stronger electronegativity of P compared to Co,⁵⁴ the electrons transferred to the surface of Co₂P may be more concentrated on P atoms, thereby optimizing the Gibbs binding energy (ΔG_H*) of the P site in Co₂P. However, with the enrichment of P-site electrons, a portion of electrons begin to overflow onto the Co site of Co₂P, and Co, which is not deficient in electrons, can only transfer the surface surplus electrons to the adjacent VN of the heterojunction. At this time, since the electron arrangements of V atoms are 3d³4s²,¹⁰ the VN with insufficient d-band electron density can receive the surplus electrons on the P site in Co₂P.²⁹ Within VN, the electronegativity of N is stronger than that of V,⁵⁴ and more electrons will eventually be accepted by the N site. Therefore, the N site in VN is the best catalytically active site for the overall material. Therefore, in the Co/Co₂P/VN catalyst, it is plausible that Co₂P acted as a bridge for electron transport, receiving electrons pumped by Co and transferring them to VN, thereby obtaining an optimized electronic structure and surface charge distribution which affected the adsorption-bonding behavior of the material to reactive species.

4. Conclusions

In summary, a novel Co/Co₂P/VN catalyst with abundant heterointerfaces was successfully constructed. Benefiting from the respective roles and advantages of the components in the composite, the material exhibited excellent HER (111 mV at 10 mA cm⁻²), OER (379 mV at 10 mA cm⁻²), and ORR performance (E_1/2 = 0.865 V). The formation of the heterointerface between multiple components leads to the charge transfer of Co to VN through Co₂P, and the rapid electron transport efficiency enhances the intrinsic adsorption capacity of the reaction intermediates, effectively improving the trifunctional catalytic activity of Co/Co₂P/VN. This work confirms the beneficial effects of introducing heterointerfaces on the electronic structure and properties of materials, laying the foundation for the synthesis and development of multi-component electrocatalysts.

Data availability

All data that support the findings of this study are included within the article.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

Z. H. acknowledges the Future Fellowship (FT220100209) from the Australian Research Council.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ta06778a

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