Introduction

Water electrolysis presents a sustainable solution for hydrogen production, leveraging the decomposition of water into oxygen and hydrogen gases through electrochemical reactions1,2. Key to this process are the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), which occur at the anode and cathode, respectively3. The effectiveness of these reactions is intrinsically linked to the properties of the electrocatalysts employed, which dictate the overall energy efficiency and viability of the electrolysis under industrial condition4,5.

Recent advances in electrocatalysis have increasingly focused on the utilization of transition metals like cobalt (Co)6 and nickel (Ni)7, which are not only abundant and economical but also exhibit remarkable electrical conductivity and chemical stability8,9,10. Among these, CoNi nanoalloys have emerged as superior candidates for OER and HER due to their enhanced electrocatalytic properties and their crucial role in facilitating the cleavage of the HO–H bond, thus serving as effective bifunctional catalysts11,12. To further augment the activity of these nanoalloys, researchers have employed various strategies such as nanoparticle size reduction, intricate morphological tailoring, doping with non-metallic atoms, and the use of conductive supports13,14,15,16. However, manipulating the metal composition often proves more straightforward than precisely controlling the size and morphology of the nanoparticles17,18. For instance, nickel oxide (NiO), known for its low cost and wide availability, has seen modifications in its structural size and surface microstructure to enhance its catalytic capabilities. A pioneering approach by Ito et al.17 involved the synthesis of porous NiO nanoparticles with adjustable pore sizes through a simplified annealing process, which, despite its simplicity, led to partial agglomeration of the nanomaterials. Addressing this, Wang et al. enhanced NiOx’s activity by integrating it onto the surface of bamboo-shaped carbon nanotubes, thus optimizing the utilization of active sites and increasing the surface area available for catalysis19.

The transformation of Ni in CoNi Prussian blue analogues (PBA) to NiO represents another avenue for improving catalytic activity. However, challenges such as the aggregation of nanoalloys can diminish the exposure of active sites and reduce the efficiency of electron and proton transfers, ultimately compromising the stability and efficacy of the catalyst20,21,22. To counter these issues, conductive supports like carbon cloth, carbon nanotubes, graphene, and activated carbon are employed to evenly disperse the nanoalloys. Among these materials, carbon nanotubes are particularly valued for their exceptional stability, electrical conductivity, and their ability to adhere strongly to catalyst particles, creating a multifunctional design that enhances electrocatalytic performance23,24.

In our study, we have developed a carbon-coated NiCoO₂/NiCo electrocatalyst through the high-temperature calcination of CoNi PBA. This novel material embeds NiCoO2/NiCo particles within a robust carbon framework and nanotubes, effectively mitigating the issue of metal agglomeration while facilitating enhanced electrical conductivity. The resulting electrocatalysts exhibit superior OER/HER performance compared to conventional commercial electrocatalysts, offering a new perspective for the design and development of high-performance bifunctional electrocatalysts for energy applications. This work not only advances the fundamental understanding of the material’s electrocatalytic mechanisms but also sets the stage for the next generation of energy conversion technologies.

Experimental

Experimental materials and reagents

All reagents were used as received without further purification. Deionized water was obtained from the Large Instrument Center at Liaoning Petrochemical University. Anhydrous ethanol (C2H5OH) and potassium hydroxide (KOH) were purchased from Liaoning Quanrui Reagent Co. Cobalt potassium cyanide (K3Co(CN)6) and pure cobalt were supplied by Alding Reagent Company. Hexahydrate nickel chloride (NiCl2·6H2O) was acquired from Shenyang Xinxi Reagent Factory. Sodium citrate (C6H5Na3O7) and ammonia solution (NH3·H2O) were sourced from Tianjin Hengxing Reagent Company.

Catalyst synthesis

Synthesis of CoNi prussian blue analogues (CoNi PBA)

Six millimoles (mmol) of NiCl₂ and twelve millimoles (mmol) of sodium citrate were dissolved in 80 mL of deionized water to create Solution A. Separately, five millimoles (mmol) of K3Co(CN)6 were dissolved in another 80 mL of deionized water to form Solution B. Solution B was then gradually added to Solution A under constant stirring. The mixture was stirred for 30 min and allowed to react for 24 h at room temperature. The resulting CoNi PBA was isolated by centrifugation, washed three times with deionized water, and freeze-dried.

Preparation of NiCoO2/NiCo@C

Twenty-five milligrams (mg) of the dried CoNi PBA precursor were placed in a porcelain boat and introduced into a tube furnace. The system was purged with nitrogen gas (N2) for 20 min to displace any air. The furnace temperature was then increased to 400 °C at a rate of 5 °C per minute and maintained at this temperature for 2 h. After the completion of the reaction, the furnace was allowed to cool to room temperature, and the black NiCoO2/NiCo@C product was collected. The same calcination procedure was applied at different temperatures (200 °C, 300 °C, and 500 °C) to obtain samples denoted as CoNi PBAs-200, CoNi PBAs-300, and CoNi PBAs-500, respectively.

Results and discussion

Figure 1 illustrates the preparation process flowchart for NiCoO2/NiCo@C. Initially, the XRD pattern of CoNi PBA, as shown in Figure S1, perfectly matches the standard diffraction pattern for CoNi PBA (JCPDS: 89-3738)25, validating the successful synthesis of the precursor.

Fig. 1
figure 1

Flowchart of preparation process of NiCoO₂/NiCo@C.

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Post calcination at 400 °C, the XRD spectra (Fig. 2) reveal the transformation of the precursor into a multi-component composite consisting of NiCoO2 (JCPDF: 10-0188)26 and NiCo metal (JCPDF: 15-0806 and 04-0850)27. The peaks at 2θ = 36.8°, 42.8°, 61.8° are correspond to the (111), (200) and (220) crystal planes of NiCoO2 respectively, confirming the successful synthesis of NiCoO2, while other peaks at 44° and 52° belong to the (111) and (200) crystal planes of Ni and Co metal peaks. Additionally, a distinct peak at approximately 26° correlates with amorphous carbon, underscoring the successful incorporation of carbon within the composite structure.

Fig. 2
figure 2

XRD pattern of NiCoO2/NiCo@C.

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X-ray photoelectron spectroscopy (XPS) is conducted to analyze the elemental composition and valence states of the sample material. As shown in Fig. 3a, the XPS survey spectrum confirms the presence of Co, Ni, O, N, and C elements. As shown in Fig. 3b, the peaks located at 778.1 and 793.8 eV are respectively correlated with the Co 2p3/2 and Co 2p1/2 of Co0 28. Furthermore, the peaks at 781.7/798.2 eV are assigned to Co2+ and shake-up satellites29,30,31. In the deconvoluted Ni 2p region (as depicted in Fig. 3c), the 2p3/2 and 2p1/2 of Ni0 and Ni2+ ions emerge at 852.2/869.9 eV and 854.3/872.7 eV, respectively29,32,33. Meanwhile, their shake-up satellites are positioned at 859.4/878.1 eV. The O 1s spectrum (Fig. 3d) can be deconvoluted into three peaks corresponding to C–O (530.8 eV), M–O (529.1 eV) (M = Co or Ni), and O–H (527.6 eV), respectively29. The XPS results confirmed the successful formation of Co, CoO, Ni, and NiO through the heat treatment of CoNi PBA. The above results are in accordance with the outcomes of XRD analysis.

Fig. 3
figure 3

Full-scan spectrum of NiCoO₂/NiCo@C (a) and the high-resolution spectra of Co 2p (b), Ni 2p (c), O 1s (d) components.

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The morphology of the nano-catalytic materials synthesized in the experiment is characterized using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The SEM and TEM images of the NiCoO2/NiCo@C (Fig. 4a–c) clearly reveal the presence of tubular structures on the surface of NiCoO2/NiCo@C. The TEM image in Fig. 4c illustrates that some metal particles are encapsulated within carbon frameworks, while others are distributed within carbon nanotubes on the outer surface of the catalyst. The formation of carbon nanotubes enhances the electrical conductivity of the nanomaterials, thereby improving their electrochemical catalytic performance. The magnified HRTEM image (Fig. 4d) reveals that metallic metal particles are embedded within carbon frameworks (interspace of 0.36 nm)34, contributing to high structural stability and durability in an alkaline electrolyte. The lattice spacing of 0.177 nm corresponds to the (111) facet of CoNi35, while 0.213 nm corresponds to the (111) facet of NiCoO236. The elemental maps of NiCoO2/NiCo@C (Fig. 4e) indicate that Ni, Co, O, and N are uniformly distributed throughout the sample.

Fig. 4
figure 4

SEM (a, b) and TEM (c) and HRTEM (d) and elemental maps of NiCoO₂/NiCo@C.

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The OER performance of carbon-coated NiCoO₂/NiCo@C nanocatalysts is evaluated in an alkaline environment and compared to that of CoNi PBA, NiO, pure Co, Ni, and IrO₂. As shown in Fig. 5a, the polarization curve indicates that NiCoO₂/NiCo@C exhibits the highest catalytic activity for OER, achieving an overpotential of 329 mV at 10 mA cm−2. This value is significantly lower than those of CoNi PBA (390 mV), NiO (351 mV), IrO₂ (362 mV), Ni (393 mV), and Co (400 mV) in Fig. 5b as well as previously reported Co-based or Ni-based catalysts (Table S1). In addition, the overpotential of NiCoO₂/NiCo@C is only 392 mV at 100 mA cm−2 which is lower than CoNi PBA, NiO, IrO₂, Ni, and Co. The Tafel slopes (Fig. 5c) of NiCoO₂/NiCo@C, CoNi PBA, NiO, IrO₂, Ni, and Co are 61.88 mV dec−1, 69.64 mV dec−1, 65.37 mV dec−1, 67.9 mV dec−1, 80.2 mV dec−1 and 78.6 mV dec−1, respectively. Among all samples, the composite nano-catalyst NiCoO2/NiCo@C has the lowest Tafel slope and the highest OER catalytic kinetic properties. Electrochemical impedance spectroscopy (EIS) is performed and the fitted circuit of Nyquist plot in Figure S2. Figure 5d shows that NiCoO2/NiCo@C exhibits a smaller charge transfer resistance (Rct) of 3.5 Ω compared to NiO, Ni, and Co, thereby ensuring faster electron transfer. As shown in Fig. 5e and Figure S3, the Cdl value of NiCoO2/NiCo@C is 41.1 mF cm−2, which is higher than that of NiO (24.57 mF cm−2), CoNi PBA (15.77 mF cm−2), Ni (13.02 mF cm−2), and Co (14.77 mF cm−2). This indicates that NiCoO2/NiCo@C exposes more active sites during the catalytic reaction, thereby increasing the number of effective OER reactions. As shown in Fig. 5f, after 15 h of testing, NiCoO2/NiCo@C maintains a stable linear response under a current density of 13 mA cm−2, confirming its strong stability. Additionally, the LSV curve measured before and after 5000 CV cycles shows no significant increase in overpotential at 10 mA cm−2 compared to the first cycle, further demonstrating the catalyst’s durability. The excellent stability and durability arise from the protection provided by the 3D carbon network. The above results show the NiCoO2 and NiCo nanoparticles are incorporated into the heterojunction that boasts a large interface area and abundant exposed active sites and the NiCoO₂/NiCo heterojunction is further encapsulated by a stable and conductive three-dimensional (3D) graphitic carbon network37. Owing to the favorable interactions between NiCoO₂ and NiCo, modulated outstanding charge/mass transfer ability of the 3D carbon network22, NiCoO2/NiCo@C heterojunction shows high catalytic activity.

Fig. 5
figure 5

(a) LSV curves, (b) Overpotential required for j = 10 and 100 mA cm−2, (c) Tafel slope plots, (d) electrochemical impedance diagram, (e) Cdl to characterize the electrochemically active area for the four catalysts, (f) Electrochemical stability test for 5000 cycles of CV and i−t stability test for up to 15 h at a current density of 13 mA cm−2.

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To investigate structural changes in CoNi PBAs calcined at different temperatures, TEM was used to examine the morphology of CoNi PBAs samples calcined at 200 °C, 300 °C, 400 °C, and 500 °C. The TEM images reveal that at a calcination temperature of 200 °C, the sample retains its cubic external structure with no significant changes (Figure S4a). When the temperature is increased to 300 °C, the cubic structure is still preserved; however, numerous internal pores become evident (Figure S4b). The presence of pores facilitates electrolyte penetration and gas expulsion. At 400 °C, the cubic structure of NiCoO2/NiCo@C disappears entirely, and nanotubular structures are observed on the surface (Figure S4c). The metal particles are embedded within the carbon framework, preventing agglomeration22. The formation of carbon nanotubes improves the electrical conductivity of the nanomaterials, thereby enhancing their electrochemical catalytic performance. In contrast, at 500 °C (Figure S4d), few tubular structures are formed, and significant agglomeration of metal particles occurs. In addition, the electrochemical OER performance of CoNi PBAs calcined at different temperatures is evaluated. The polarization curves of the electrocatalysts (Figure S5) indicate that the electrochemical activity is highest when the calcination temperature is 400 °C for OER (Figure S5a) and HER (Figure S5b), compared to catalysts calcined at other temperatures.

Similarly, the electrocatalytic hydrogen evolution properties of NiCoO2/NiCo@C, CoNi PBA, Pt/C, NiO, Ni, and pure Co are investigated under alkaline conditions. The polarization curve (Fig. 6a) shows that NiCoO2/NiCo@C has excellent catalytic activity of HER. When the current density is 10 mA cm−2, in Fig. 6b, the overpotential is 61.9 mV, which is better than CoNi PBA (140.9 mV), NiO (182.25 mV), Pt/C (94.51), Ni (281.3) and pure Co (260.0 mV). Tafel slope, as an important index to evaluate the performance of electrocatalysts, can reflect the rate control steps of electrocatalytic reactions and reveal the intrinsic activity mechanism of catalysts. As shown in Fig. 6c, the NiCoO2/NiCo@C has a lower Tafel slope of 42.6 mV dec−2 than that of CoNi PBA, Pt/C, NiO, Ni and pure Co, respectively. Figure 6d presents the EIS curves of each catalyst, illustrating the variation in charge transfer resistance during the HER process. Figure 6d shows that NiCoO2/NiCo@C exhibits a smaller charge transfer resistance (Rct) of 41.05 Ω compared to NiO, Ni, and Co, thereby ensuring faster electron transfer. Figure 6e shows the electrochemical Cdl values of NiCoO2/NiCo@C, CoNi PBA, NiO, Ni, and pure Co, which are 16.28 mF cm−2, 8.67 mF cm−2, 7.09 mF cm−2, 3.81 mF cm−2, and 5.48 mF cm−2, respectively. These results indicate that NiCoO2/NiCo@C possesses more active sites.

Fig. 6
figure 6

(a) LSV curves, (b) Overpotential required for j = 10 mA cm−2, (c) Tafel slope plots (d) EIS spectra for NiCoO2/NiCo@C, CoNi PBA, NiO@C and pure Co, (e) Cdl to characterize the electrochemically active area for the four catalysts.

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Conclusions

In this study, CoNi PBA is used as a precursor to prepare carbon-coated NiCoO₂/NiCo composite nanomaterials by sintering at 400 °C under N2. NiCoO₂ and NiCo nanoparticles are integrated into a heterojunction with a large interface area and numerous exposed active sites. Furthermore, the NiCoO₂/NiCo heterojunction is encapsulated by a stable and conductive three-dimensional (3D) graphitic carbon network. The morphology of the precursor CoNi PBA is affected by high temperature sintering. The catalyst exhibits excellent OER performance, with an overpotential of 329 mV and a Tafel slope of 61.88 mV dec−1 at a current density of 10 mA cm−2. Additionally, the material demonstrates good HER performance, achieving an overpotential of 61.9 mV at the same current density. The concepts, synthesis methods, and material properties described here offer valuable insights for the future development of high-efficiency OER and HER catalysts for commercial water splitting.