Innovative synthesis of cobalt-based catalysts using ionic liquids and deep eutectic solvents: A minireview on electrocatalytic water splitting

2.1 Electrodeposition technology

Electrodeposition technology is an effective method for preparing catalysts by adding substances such as metal ions into an electrolyte and obtaining the target product through the action of an electric field (Figure 2) [20,21]. By adjusting the parameters of the electrolyte, it is possible to control the structure and properties of the deposited material. The solvent in the electrolyte is usually water or organic solvents. Water is the most commonly used solvent due to its environmental friendliness, low cost, and easy availability. However, under certain circumstances, organic solvents such as acetonitrile, N,N-dimethylformamide (DMF), and N,N-dimethylacetamide are also employed to achieve better electrochemical properties or specific crystal structures. However, traditional electrolytes have a series of issues.

Figure 2

Diagram of electrodeposition technology [20,21].

Narrow electrochemical windows, which mean many electrodeposition reactions requiring high potential driving forces (e.g., deposition of highly reactive metals, synthesis of complex compounds) cannot proceed.

Volatility, leading to changes in electrolyte composition over time (increased solute concentration after solvent evaporation), which affects the repeatability and stability of the electrodeposition process.

Temperature limitations, restricting the application of electrodeposition technology in extreme environments (e.g., high-temperature metallurgy, preparation of low-temperature energy storage devices).

Strong corrosiveness, specifically causing corrosion and damage to equipment, increasing maintenance costs, posing safety hazards, and affecting the purity and performance of deposited materials [22–24].

In the preparation of transition Co-based catalysts, ILs and DESs are increasingly being used as electrolytes. This is attributed to their unique properties compared to traditional water or organic solvents, such as good conductivity, wide electrochemical windows, environmental friendliness, mild conditions, and a wider range of operating temperatures (Table 2). Murchite et al. investigated the electrical conductivity and thermal stability of N,N-dimethylethanolammonium-based ILs and found that they remained stable at high temperatures (323 K) with a conductivity >5 mS cm⁻¹. The wide electrochemical windows of ILs (e.g., [BMIM][PF₆] exhibits a window of 4.5 V) can avoid side reactions (such as water decomposition) at high potentials, thereby reducing corrosion of the stainless steel surface caused by OERs. Experiments show that at a potential of 2.5 V vs SCE, the corrosion rate of stainless steel in the ILs system is 0.08 mm/year, which is only 32% of that in an aqueous solution (0.25 mm/year) [25]. Electrochemical stability of ILs and DESs makes them well-suited for the electrodeposition process, which helps maintain the morphology and uniformity of the catalysts. Furthermore, ILs and DESs can remain stable and are less prone to reacting with other substances, which benefits the preservation of the catalysts’ chemical purity and stability. Additionally, due to their high viscosity, ILs and DESs can be used to regulate the morphology of products, leading to porous metal nanostructures. Using ILs and DESs as electrolytes can also result in more uniform particle distribution and reduced waste. The catalytic performance of cobalt-based catalysts obtained by electrodeposition method has been comprehensively summarized in Table 3.

Although ILs and DESs may have a high cost, this investment can be justified considering their benefits in improving catalyst performance and longevity. The choice of solvent should be based on specific application requirements and objectives.

Using DESs as electrolytes, the addition of transition metals allows for the electrolytic production of transition metal compounds. By adjusting the composition of the DESs, it is possible to control the structure and performance of the catalysts. The use of ChCl/malonic acid DES with a small percentage of water as the electrolyte for electrodepositing cobalt resulted in an electrodeposited layer with a unique plate-like morphology of Co nanoparticles from cobalt salt. The structured nature of DES can influence the nucleation and growth of cobalt crystals, favoring the formation of flat planes over other shapes. The addition of a small percentage of water reduced the viscosity of the ChCl/malonic acid DES, while altering the local structure around the cobalt ions in the DES. Water content can affect reduction of cobalt salt and deposition kinetics of electrodeposited layer, which might favor the development of a plate-like morphology and the quality of the electrodeposited films. The resulting structure consisted of well-separated nanoclusters of cobalt on the surface of a dense graphite sheet. Despite the absence of stabilizing agents, the cobalt nanoparticle clusters had a size range of approximately 50–80 nm. The reason is possible to that the combination of ChCl and malonic acid in DES creates a complex hydrogen-bonding network that can stabilize specific crystal facets of cobalt during electrodeposition, promoting the growth of plate-like structures. After the catalyst is prepared, residual ILs/DESs are thoroughly removed by centrifugal washing (8,000 rpm, 10 min) and vacuum drying (80°C, 12 h). The cobalt nanoparticles displayed remarkable HER performance, achieving a current density of 10 mA cm⁻² with an overpotential of 350 mV and a Tafel slope of 76 mV dec⁻¹ in 1 M KOH solution [26].

To enhance the catalytic effects of cobalt-based catalysts, other metal salts are incorporated into ILs and DESs, thereby forming alloys, such as Ni–Co [27], Co–Cu [28], Ni–Co–Sn [29], and Zn–Co [30]. There are various methods for preparing cobalt-based alloys, including molten salt electrolysis and mechanical alloying, among others. Given that DESs have excellent solubility for a variety of metal salts, using DES as an electrolyte for electrodeposition is an effective strategy for producing cobalt-based alloys. The solubility and electrochemical behavior can be adjusted by altering the components of the DES, enabling precise control over the structure and composition of the alloy. Cobalt alloys electrodeposited in DES typically exhibit superior dispersibility, which means more active sites and thus enhanced catalytic efficiency. The unique properties of DESs can promote the growth and formation of specific crystal facets, leading to the creation of plate-like or other non-spherical structures with high activity, which are particularly advantageous for catalytic reactions. Additionally, since cobalt alloys electrodeposited in DES often possess better dispersion and higher activity, employing this strategy can enhance both the yield and quality of the alloys.

Cobalt salt, nickel salt, and tin salt are introduced into ChCl/EG DES. Ni–Co–Sn alloy were gotten by electrodeposition, which showed high corrosion current density and exchange current density potential, indicating that the Ni–Co–Sn alloy with high performance and stability was a good candidate for HER [29]. Zinc, cobalt, and zinc–cobalt coatings can also be obtained using electro-deposition methods with ChCl/EG DES. By altering the concentration of Zn²⁺ and Co²⁺ ions, crystalline deposits with precisely controlled compositions and morphologies were achieved, exhibiting different electrocatalytic HER activities in alkaline media (Figure 3). An increase in the cobalt content within the coating had a positive impact on the HER performance [30].

Figure 3

Metallic composition (a), the SEM images of Cu substrate (b), Cu/Zn (c), Cu/Co (d), Cu/Zn₉₆Co₄ (e), and Cu/Zn₃Co₉₇ (f), polarization curves carried out with different catalysts (g), the corresponding Tafel slopes (h), and electrochemical stability tests of the catalysts (i) [30].

In the context of electroplating processes, the introduction of an appropriate amount of sulfur sources enables the direct electrodeposition of cobalt sulfide. Within the aforementioned DES-metal salt electrolyte system, incorporating a non-metallic source can yield catalysts such as cobalt sulfide and cobalt phosphide. The preparation of porous S-doped Co–S films grown on nickel foam (Co–S/NF) has been successfully achieved through a one-step electrodeposition approach using DES as the electrolyte. DES had the capability to form stable Co(ii)–Cl complexes with cobalt ions, which could coordinate with Co(ii)–thiourea (TU) complexes to precisely control the generation of Co–S complexes (Figure 4). Co–S/NF exhibited features of highly exposed active sites and strong structural integrity, demonstrating excellent water splitting performance, particularly at high current densities, requiring only 1.72 V to achieve a large current density of 500 mA cm⁻² in 30 wt% KOH solution. S doping enhanced charge and mass transport while promoting catalysis kinetics by increasing the electrochemical active area and creating structural defects [31]. These defects, counterintuitively, serve to improve the catalyst’s performance by providing additional active sites and facilitating electron transfer, a phenomenon consistent with the theory of defect chemistry in materials science. The synergy between the controlled release of sulfur species from the DES medium and the unique electrochemical properties of the DES electrolyte contributes to the superior electrocatalytic activity observed in the Co–S/NF composites.

Figure 4

(a)–(e) FE-SEM images of the as-prepared Co–S–x/NF (x = 0, 10, 30, 50, and 70) with different TU feed concentrations. (f) The corresponding mass loading, atomic ratio of Co/S, and S doping content [31].

Moreover, ILs and DESs can also be used to prepare bimetallic sulfides and phosphides. Compared to single-metal sulfides, bimetallic sulfides exhibit higher electrical conductivity. The mixed valence states of the metal ions and the cooperative electronic interactions increase their reactivity, revealing great potential. Bimetallic sulfides and phosphides can be readily obtained through the simultaneous addition of multi-metal sources and non-metallic sources in IL and DES electrolytes.

Wang research group meticulously designed a methyltriphenylphosphonium bromide/ethylene glycol DES and utilized a potentiostatic method to electrodeposit a nickel–cobalt phosphide (NiCoP) film onto the surface of carbon cloth (CC). The formation of this film followed a diffusion-controlled three-dimensional instantaneous nucleation and growth mechanism, ensuring the quality of the film while highlighting the dominant role of the diffusion process. Initially, raw molecules diffused freely in the solution or vapor phase and then rapidly aggregated under suitable conditions to form primary crystal nuclei. Subsequently, these nuclei continued to grow through precise processes such as surface diffusion and material adsorption, ultimately shaping a structurally complete film. The synergy between the DES’s unique hydrogen-bonding network and the carefully controlled deposition potential fosters a uniform nucleation and growth process, resulting in a dense and conformal coating that optimizes the electrochemical performance. Additionally, the presence of phosphorus introduces electronic modifications to the Ni–Co lattice, enhancing the electrical conductivity and stability of the catalyst, which aligns with the principles of d-band center theory in electrocatalysis. Furthermore, the choline chloride-ethylene glycol DES’s ability to fine-tune the reduction potential of nickel and cobalt salts contributes to the precise control over the composition and structure of the NiCoP film, thereby amplifying its catalytic activity. This NiCoP film demonstrated outstanding HER activity, with an η ₁₀ value of only 93 mV and a Tafel slope as low as 48 mV dec⁻¹ in a 1 M KOH [32], reflecting its exceptional electrocatalytic efficiency. In a similar vein, Zeng et al. have successfully synthesized NiCo _x S _y by electrolyzing a mixture containing NiCl₂, CoCl₂, and TU within a ChCl/EG DES. This structure exhibited impressive water splitting performance, achieving a current density of 10 mA cm⁻² with a low overpotential of 1.57 V [33]. This achievement underscores the potential of DESs in tailoring the local electrochemical environment to facilitate the formation of desired intermetallic phases and dopant distributions, which are crucial for enhancing the catalytic performance of transition metal compounds in water splitting reactions. The combination of precise compositional control and the inherent properties of DESs offers a promising route for the rational design of advanced electrocatalysts tailored for energy conversion applications. Using either DESs or ILs as electrolytes, it is possible to incorporate metals capable of forming alloys with cobalt, such as iron, nickel, chromium, tungsten, molybdenum, copper, etc., according to specific requirements. This enables the controllable enhancement of catalyst performance, making it suitable for various industrial applications.

2.2 Solventothermal technology

Solventothermal synthesis is a chemical synthesis technique carried out under high temperature and pressure conditions within a closed system, where the use of solvents facilitates chemical reactions at temperatures lower than those traditionally required for high-temperature reactions [34]. The advantage of this method lies in its ability to achieve high-temperature reaction conditions at lower external temperatures, typically due to the natural increase in internal pressure within the sealed system rather than externally applied pressure. This approach enhances the purity and crystallinity of the products because the reactions take place in a more controlled environment, reducing the occurrence of side reactions and allowing for the formation of more complete crystal structures. By precisely controlling parameters such as temperature, pressure, and time, the reaction rate and product morphology can be controlled, allowing for precise control over the properties of the product. Although high-purity products and large-sized crystals can be synthesized in a short time, solventothermal synthesis faces challenges such as high equipment costs and complex operations [35]. Moreover, strict requirements for reaction conditions and factors such as the solubility and kinetics of reactants may affect the outcome of the reaction. Therefore, when conducting solventothermal synthesis, it is necessary to carefully select reaction conditions and solvent systems to ensure the effectiveness and reproducibility of the synthesis process [36,37]. The performance of cobalt-based catalysts prepared by solvothermal method for water splitting is shown in Table 4.

In the preparation of transition metal catalysts via the solvothermal method, commonly used solvents include water, alcohols (such as methanol, ethanol, isopropanol), organic acids (such as acetic acid, formic acid), ketones (such as acetone), esters (such as ethyl acetate), amines (such as pyridine, triethylamine), and other polar aprotic solvents (such as nitrile, DMF, dimethylsulfoxide, etc.). The choice of solvent depends on the desired reaction conditions, target product, and the solubility of reactants. In recent years, ILs and DESs have gained special attention as solvents in solvothermal reactions. Compared to traditional solvents, they offer the following advantages:

Broad liquid temperature range: ILs and DESs are liquid at room temperature or close to it, providing a wider operating temperature window.

High thermal stability: Both ILs and DESs typically possess high thermal stability, allowing their use at elevated temperatures without decomposition.

Excellent electrochemical stability: They are stable across a wide pH range, suitable for reactions related to electrochemistry.

Good solubility: Capable of dissolving various inorganic salts and organic compounds, applicable to a variety of chemical reactions.

Adjustable physical and chemical properties: By altering the composition of ILs and DESs, their polarity, dielectric constant, and other properties can be adjusted to meet different reaction requirements.

Low volatility: Their low volatility reduces evaporation losses during high-temperature reactions, facilitating continuous operation and catalyst recovery. Moreover, many ILs and DESs are non-toxic or low-toxic, renewable, and have a smaller environmental impact.

Recyclable and reusable: ILs and DESs can be recovered and reused multiple times through simple separation steps, reducing costs and waste generation. These characteristics make ILs and DESs ideal solvent choices, especially in catalytic reactions requiring mild conditions, high selectivity, and efficiency.

However, their cost and synthetic complexity are also factors to consider. When selecting solvents, one should take into account the specific needs of the reaction, cost-effectiveness, and sustainability.

Monometallic (Co, Ni, Fe) and bimetallic (FeCo, NiCo, NiFe) hydroxides were successfully synthesized by adding 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]BF₄) IL and corresponding metal salts to water and conducting a one-step rapid reaction under alkaline conditions. This IL plays a crucial role in this synthesis process; it serves not only as a reaction medium but also participates in the coordination and dispersion of metal ions, thereby influencing the morphology and structure of the hydroxides. The anion of the IL (e.g., BF 4 − ) can form coordination bonds with metal ions, which aids in the uniform dispersion of metal ions in solution, promoting the formation of hydroxide nanoparticles. Additionally, the steric and charge effects of the IL can regulate the growth and path of metal hydroxides, leading to the preferential growth and thus affecting the crystal structure and morphology of the final products. All the prepared hydroxides exhibited excellent performance in the OER. In particular, the CoFe hydroxide and CoNi hydroxide showed overpotentials of 350 and 420 mV, respectively, when the current density was at 10 mA cm⁻² [38]. These materials hold significant potential for applications in the field of electrocatalysis, particularly in areas related to energy conversion and storage, such as hydrogen production through water electrolysis.

Using 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM]PF₆) as an auxiliary agent, nitrogen and phosphorus-doped carbon/nickel cobalt phosphide (N,P-C/NiCoP) composites were successfully prepared. Density functional theory analysis indicated that the doping of nitrogen and phosphorus in the carbon-based materials facilitated electron transfer and increased active sites. The templating effect of this IL endowed the material with a unique wrinkled surface structure, significantly enhancing its specific surface area to five times that of the undoped sample. N,P-C/NiCoP exhibited excellent HER performance, with ƞ ₁₀ values of 108, 128, and 108 mV under acidic, alkaline, and neutral conditions, respectively [39].

Mu and other researchers developed an innovative DES composed of polyethylene glycol (PEG)200 and TU, and successfully synthesized various sulfides, including CoS₂, NiS₂, and Ni₂CoS₄, through a simple solvothermal technique. In this method, DES had significant advantages. First, the use of DES as a structure-directing agent effectively controlled the micro-morphology of the products. For example, NiCo₂S₄ exhibited a unique urchin-like micro/nanostructure with uniformly connected nanorods forming a network. The components in DES influence the aggregation state and solubility of the precursors through intermolecular forces like hydrogen bonding and van der Waals interactions, guiding the morphological development of sulfides. This structural directing effect helps to form specific microstructures, such as the sea urchin-like micro/nanostructures of NiCo₂S₄, which may provide more active sites and shorter ion transport pathways, thereby enhancing the electrochemical performance. Second, the DES also acted as a sulfur source, simplifying the reaction process, reducing the likelihood of side reactions, and lowering the overall preparation cost. Under solvothermal conditions, the heat stability and solvent effects provided by DES contribute to optimizing the thermodynamic and kinetic conditions of the reaction, enabling the formation of sulfides under relatively milder conditions, reducing unnecessary side reactions, and improving the purity and performance of the products. The synthesized NiCo₂S₄ demonstrated good oxygen evolution activity, with an overpotential of 337 mV and a Tafel slope of 64 mV dec⁻¹ [40].

Wang et al. utilized NiCl₂·6H₂O/CoCl₂·6H₂O/TU DES to grow CoNi₂S₄ nanosheets on CC substrates through solvothermal synthesis (Figure 5a). This method allowed for an even coverage of the CC with the DES due to its strong adhesive properties and liquid state under ambient temperature conditions. The coupling of the CoNi₂S₄ nanosheets with the CC substrate enhanced electron transport efficiency while preventing the catalyst from aggregating and corroding during catalysis processes. The self-supported CoNi₂S₄/CC exhibited excellent HER activity (with an overpotential of 155 mV at 50 mA cm⁻² and 30 mV at 10 mA cm⁻²) and OER activity (with an overpotential of 372 mV at 100 mA cm⁻² and 153 mV at 10 mA cm⁻²). Additionally, this material demonstrated good performance and stability during the electrolysis of seawater (Figure 5b) [41].

Figure 5

Schematic illustration of the synthesis procedures of CoNi₂S₄/CC (a), LSV curves for the CoNi₂S₄/CC@CoNi₂S₄/CC coupled electrolyzer in different electrolytes (b) [41]. Synthetic process for fabricating NBF-CoSe/Mo₂CT _x (c) [42].

Zhang et al. have developed a novel composite material called NBF–CoSe/Mo₂CT _x , which includes B, N, and F-doped CoSe and Mo₂CT _x MXene. This material was synthesized using a hydrothermal process with [EMIM][BF₄], urea, and Co(NO₃)₂·6H₂O (Figure 5). During this process, the elements B, N, and F from the IL were introduced into the layered double hydroxide (LDH) nanosheets, which were then calcined to form the final NBF–CoSe/Mo₂CT _x composite. This composite material exhibited enhanced HER activity across a range of pH levels, requiring overpotentials of just 70 and 81 mV to achieve a current density of 10 mA cm⁻² in 0.5 M H₂SO₄ and 1 M KOH, respectively. Theoretical studies indicated that B, N, and F doping creates additional active sites and improves electronegativity, leading to enhanced proton adsorption capabilities. The combination of Se vacancies, heteroatom doping, and the Mo₂CT _x structure contributed to the excellent HER activity observed in this material [42].

2.3 Specific technology

2.3.1 Hot injection technology

The hot injection technology is a common technique for preparing nanomaterials and quantum dots, involving steps such as precursor solution preparation, heating, rapid injection into a hot solvent, and product formation [43,44]. This technology offers advantages such as fast reaction, controllable size, high crystallinity, and versatility, allowing for the precise synthesis and customization of nanomaterials. In brief, the hot injection technology is a rapid, controllable, and versatile approach to nanomaterial fabrication, providing an important tool for the science and application fields of nanomaterials [45].

Researchers utilized the hot injection technology to prepare CoFe LDHs with large interlayer spacing. They added measured quantities of CoCl₂·6H₂O and FeCl₃·6H₂O to a ChCl/urea DES and then heated the mixture. Water was then rapidly injected in increments. The urea decomposed to produce hydroxide ions (OH⁻), which reacted with the metal ions to form the LDH structure. Controlling the speed of nucleation was key, and the swift addition of water helped in the uniform growth of the crystals. X-ray diffraction (XRD) analysis revealed that the maximum interlayer spacing of the LDH reached 11.3 Å. Fourier-transform infrared spectroscopy results showed that the molecules intercalated within the layers were decomposition products of the DES, indicating that the DES played a role not only as a solvent but also in the formation of the interlayer composition of the LDH, beneficial for OER (Table 5) [46]. Taubert and colleagues heated a type of IL containing cobalt and copper ions, where the anions and N-butylpyridinium ions served as the anions and cations, respectively. After heating, they rapidly injected hexamethyldisilathiane (TMS)₂S to obtain nanoflower-like CuCo₂S₄ with a lower onset potential and improved durability during the OER (Figure 6 and Table 5) [47].

Table 5

Water splitting performance of cobalt-based catalysts prepared by hot injection technology, inkjet printing technology, and microwave-assisted technology, respectively

Catalyst	Applied IL/DES	Catalytic performance								Refs.
		HER			OER			Overall water splitting
		Electrolyte	ƞ (mV)@Current density (mA cm−2)	Tafel slope (mV dec−1)	Electrolyte	ƞ (mV)@Current density (mA cm−2)	Tafel slope (mV dec−1)	Electrolyte	Potential (V)@Current density (mA cm−2)
CoFe-LDH	ChCl/urea	—	—	—	0.5 M KOH	—	—	—	—	[46]
CuCo2S4	Cobalt(ii) bis(n-butylpyridine) tetrachlorocuprate(ii)	—	—	—	1 M KOH	230@10	211	—	—	[47]
CoP/carbon fiber	[BMIM]PF6	0.5 M H2SO4	97@10	50.2	—	—	—	—	—	[50]
NiP/carbon fiber	[BMIM]PF6	0.5 M H2SO4	102@10	53.3	—	—	—	—	—	[50]
FeP/carbon fiber	[BMIM]PF6	0.5 M H2SO4	111@10	63.4	—	—	—	—	—	[50]
MoP/carbon fiber	[BMIM]PF6	0.5 M H2SO4	87@10	49.1	—	—	—	—	—	[50]
Co2P/CNT	[P66614]2CoCl4	0.5 M H2SO4	135@10	58	—	—	—	—	—	[54]

Figure 6

SEM images of the synthesized CuS particles at 160°C at different reaction times: 1 (a), 4 (b), 6 (c), and 8 h (d), their respective XRD patterns (e) [47].