1 Introduction
The rapid consumption of fossil fuels has led to environmental pollution and energy crisis. With the increasing demand for materials and resources, the development and utilization of clean and sustainable energy has become particularly important
[1]. Electrochemical energy storage and conversion technologies such as fuel cells, water electrolysis, and metal-air batteries are ideal solutions
[2⇓⇓⇓~6]. Oxygen evolution reaction (OER) is one of the key electrochemical reactions, which involves the multi-step transfer of four electrons, and the reaction kinetics generally progresses slowly
[7⇓~9]. It is well known that the use of a highly active catalyst can drive the electrochemical reaction to proceed efficiently
[10⇓~12]. Precious metal-based electrocatalysts such as RuO
2 and IrO
2 are currently the most commonly used OER catalytic materials in industry. However, the scarcity and expensiveness of precious metal elements limit their further application in practical industrial sites
[13,14]. At present, researchers have discovered and developed a large number of non-noble Metal catalysts, such as hydroxides/oxyhydroxides, perovskites, sulfides, phosphides, carbides, metal-organic frameworks (MOF) materials or MOF-derived composites
[15⇓⇓~18][19⇓~21][22,23][24⇓~26][27,28][29⇓~31]. They are rich in resources, relatively low in cost, and excellent in catalytic performance, showing the application potential of replacing noble metal-based catalysts.
In the electrocatalytic reaction, there are many factors that affect the reaction efficiency, such as catalyst particle size, crystallinity, surface morphology and catalyst loading
[32⇓⇓⇓⇓⇓⇓~39]. Among them, the electrode preparation method plays a decisive role in the coverage density of electrolyte and the distribution of catalyst active sites, and has an important impact on the performance of the catalyst
[40,41]. For a high-performance electrode, in addition to the high intrinsic electrocatalytic activity of the catalyst used, it is also necessary to design the electrode structure, especially to modify and regulate the electrode surface reasonably. At present, the manufacturing strategies of electrodes can be divided into two categories: (1) Substrate-assisted electrodes, that is, catalytic materials are loaded on a pre-existing Substrate to form a composite structure to form a working electrode with catalytic activity. (2) Substrate-free electrodes, two-dimensional or three-dimensional materials with special structures are manufactured through aggregation, crosslinking or polymerization of materials, and can be used as working electrodes directly or after modification.
In this paper, the electrode preparation methods of high efficiency OER catalysts are introduced, and the current strategies and research progress in electrode preparation are summarized in detail, focusing on the advantages and disadvantages of various electrode preparation methods and loading methods. Finally, some suggestions are provided for the preparation of efficient OER electrodes, and the research direction of electrode preparation is prospected.
2 Ubstrate auxiliary electrode
Generally, in the electrochemical performance test, the electrocatalytic active material needs to be loaded on the conductive substrate by coating, deposition or in-situ growth to form the working electrode. Commonly used substrate materials are mainly the following categories: carbon-based materials, such as carbon fiber paper and carbon fiber cloth; Metal substrate materials including metal foams (e.g., nickel, copper, cobalt foams), metal meshes (e.g., titanium and stainless steel meshes), metal plates/foils (e.g., titanium plates, molybdenum and copper foils), and the like; In addition, graphene films, graphene aerogels, carbon aerogels, carbon felts and other materials with rich pore structure and high conductivity have also been widely used as electrode substrates
[42⇓~44]. At present, the manufacturing process of the conductive substrate is relatively mature. Compared with the non-substrate electrode, the application of the substrate auxiliary electrode is more extensive. Most of the existing catalytic materials can construct the working electrode through the substrate auxiliary.
Typical loading strategies include drop coating, aqueous/solvothermal, electrodeposition, vapor deposition, and layer-by-layer self-assembly. The morphology, microstructure, surface wettability and other surface properties of catalysts can be purposefully controlled by different loading methods, and these factors will have a tremendous impact on the overall performance of catalysts. Therefore, understanding the advantages and disadvantages of different loading methods and the latest research progress is of great significance for the preparation of substrate-assisted efficient OER electrodes. Table 1 summarizes the OER electrocatalytic performance of the electrodes prepared by different loading methods.
2.1 Drop coating method
The current standard technique for making electrodes is Drop casting. Generally, the electrocatalyst powder and the conductive support are dispersed in a solvent to form a catalyst "ink", which is dropped onto the surface of a conductive current collector (such as a polished glassy carbon electrode, nickel foam, or carbon cloth), and dried in an atmosphere or oven to form a working electrode. It should be noted that during the electrode manufacturing process, it is usually necessary to add an organic binder (e.g., Nafion
®) to improve the adhesion between the electrocatalyst layer and the conductive substrate
[41,45].
Generally speaking, most of the catalysts are in the form of powder, and the drop coating method is widely used in the electrode preparation of powder catalysts because of its simplicity and applicability
[36,46⇓⇓~49]. It should be noted that the drop coating method has almost no restrictions on the substrate, and different substrates will affect the performance of the catalyst, so extra attention should be paid when choosing the substrate. For example, Tavakkoli et al. systematically explored the regulation of the electrocatalytic activity of porous catalysts by the underlying substrate by loading the synthesized metal (manganese) nitrogen-carbon catalysts on glassy carbon electrodes, nickel sheets and gold sheets, respectively
[46]. The results show that the redox kinetic current and OER activity of the catalyst are significantly enhanced when the catalyst is deposited on the nickel substrate, which is superior to other substrates, due to the formation of the lower catalyst/nickel interface site.
At the same time, the disadvantages of the drip coating method are also very obvious. First, when "ink droplets" are applied to the substrate to prepare the working electrode, due to the high surface energy, the electrocatalyst usually aggregates, significantly reducing the effective area of the electrode and burying the active sites. Secondly, the use of non-conductive binder will make the conductivity of the electrode worse, block the active catalytic sites, and limit the best performance of the catalyst
[50,51]. Thirdly, the added binder will make the wettability of the electrode surface worse, and the electrolyte is difficult to soak into the electrode, thus reducing the interfacial concentration of reactants and further hindering the mass/electron transfer process, thus reducing the electrocatalytic performance
[52,53]. During the long-term use of the electrocatalyst, the low adhesion between the catalyst and the substrate will cause the peeling of the catalyst, resulting in a decline in stability
[54,55]. From this point of view, it is necessary to develop some new electrode preparation strategies.
2.2 Hydrothermal/solvothermal method
Hydro/Solvothermal reaction is one of the effective strategies for depositing or growing electrocatalytically active materials on substrates. The specific process is as follows: the component to be deposited is added into a closed reactor with a conductive substrate, and the nano material can be uniformly grown or deposited on the substrate after a certain time at a proper temperature
[56⇓~58]. In addition to the commonly used carbonaceous materials, metal ions can also be used for hydrothermal deposition, where they diffuse into the substrate during thermal treatment to form nanoparticles and participate in catalytic reactions. Traditional hydrothermal/solvothermal methods have many advantages, such as simple operation, environmental friendliness, low cost, mild reaction process, high crystallinity and phase purity of the product. By combining with the conductive substrate, the side effect of the binder can be avoided, the electron transfer resistance can be reduced, and the high-activity OER electrode can be prepared more easily.
Surface defects and hydroxyl groups on the conductive substrate can serve as nucleation and growth sites for active materials during hydrothermal/solvothermal processes. The controllable preparation of the nanostructures can be realized by adjusting experimental variables such as temperature, reaction time and feeding ratio of raw materials, so that the catalytic performance of the electrode can be regulated and controlled
[59,60]. For example, Gao et al. studied the effect of raw material ratio on the morphology of layered double hydroxide/nickel foam by changing the molar ratio of zinc to cobalt
[59]. The results show that the morphology of the layered double hydroxide changes from nanosheets to nanowires with the decrease of the molar ratio of zinc to cobalt.
In addition, metal-based substrates can also be used as ion sources in hydrothermal reactions and play an important role in the growth process of catalysts. For example, Kim et Al. constructed porous membranes of various geometries, MIL-53 (Al), directly on aluminum metal substrates by a simple hydrothermal method without using any additional metal source
[61]. They also fabricated HKUST-1 (Cu) thin films and ZIF-7 (Zn) thin films on corresponding metal mesh substrates, demonstrating the universality of this method. Similarly, Wang et al. reported an ultra-long quasi-two-dimensional Co-MOF nanoarray (Fig. 1) with oriented growth assisted by a metal layer, and its growth mechanism is related to the coordination of thiophene dicarboxylic acid ligand and metal layer
[31]. The Co-MOF nanoarrays grown on Ni foam show significant OER catalytic activity and excellent stability. This strategy can realize spatially localized growth of nanoarrays.
In addition to morphology control, hydrothermal method is also widely used for catalyst modification. For example, Wen et al. introduced a Cr dopant into a CoFe layered double hydroxide on nickel foam by a facile "one-step" hydrothermal method to form a substrate auxiliary electrode (Cr-CoFe LDHs/NF) (Fig. 2)
[62]. The synthesized material exhibits excellent OER electrocatalytic activity and stability with an ultralow overpotential of 238 mV at a current density of 10 mA·cm
-2. Luo et al. precisely controlled the Mo doping of Ni
3S
2 by a simple solvothermal process
[63].
With the introduction of Mo element, the original Ni3S2 nanosheets aggregate to form Mo-Ni3S2 flower-like nanosheet clusters. In this work, high-purity nickel foam was selected as the Ni source to participate in the reaction and as the conductive substrate for the construction of the electrode due to its interconnected porous network and multi-dimensional electron transport pathways. After phosphating treatment, the formed Mo-Ni3S2/NixPy/NF electrode has an ultralow overpotential of 238 mV at a current density of 50 mA·cm-2, and can work stably for more than 72 H at a current density of 10 mA·cm-2.
Recently, some researchers have prepared new efficient OER electrodes by hydrothermal method. For example, Yao et al. prepared a flexible Co-Mo-N/Au electrode on a novel nanoporous gold conductive substrate (Fig. 3)
[64]. Bimetallic cobalt – molybdenum nitride nanosheets were vertically grown on a 3D porous gold framework via a hydrothermal method with a large-area, accessible CoMoO
x active surface. Moreover, the nanoporous gold framework helps to promote the electron transfer and mass transfer during the reaction. Therefore, the Co-Mo-N/Au electrode exhibits excellent OER performance in 1 mol·L
-1KOH solution with ultralow onset overpotential (166 mV) and Tafel slope (46 mV·dec
-1, requiring only an overpotential of 370 mV to reach an ultrahigh current density of 1156 mA·cm
-2.
Finally, it should be mentioned that although hydrothermal reaction can obtain efficient and stable electrodes, there are still some problems, such as long reaction time and large-scale application. In addition, how to control specific factors (temperature, time, pH, solvents, surfactants and additives, etc.) To optimize the morphology and structure of nanomaterials and prepare electrodes with excellent catalytic performance still has a lot of room for exploration.
2.3 Electrodeposition
Electrodeposition is a solution-based preparation process, usually carried out in a three-electrode cell. Generally, by using a selected conductive substrate as a working electrode, the electroactive material can be uniformly and orderly grown in a short time under the action of an applied electric field, which has the possibility of mass production
[65⇓~67]. Moreover, the deposition process can be implemented on any conductive substrate and is not limited to surface flatness and size issues. At present, the electrodeposition method has become a general technology for the preparation of substrate auxiliary electrode, which is widely used in the preparation of alloys, metal-based compounds and other materials. Moreover, electrodeposition technology is relatively mild and can be better coordinated with other technologies, which greatly expands the application scope of electrodeposition technology.
By adjusting the deposition time, working temperature, applied voltage or current density, as well as the type and concentration of metal salts and additives, the nucleation and growth processes of substances can be precisely regulated and controlled, thereby controlling the phase composition, morphology, crystallinity, loading capacity and other parameters of the catalyst. Therefore, electrodeposition technology can better control the structure and composition of the electrode, and control the conductivity and packing density of each deposition layer, which plays an important role in the efficiency of the electrocatalyst. For example, Pei et al. studied the effect of electrodeposition time on CoFe LDH nanosheets
[68]. The experimental results show that the increase of electrodeposition time will affect the formation of porous structure of the material, which will further increase the loading of CoFe LDH.
Chen et al. developed a fast and scalable electrodeposition method to synthesize amorphous Co-Fe-W trimetallic oxyhydroxides rich in $\mathrm{CO}_{\mathrm{Oh}}^{3+}$ (Fig. 4)
[69]. The peroxide ligand released from the WO
4(O
2)
4- precursor oxidizes the Co
2+ to Co
3+,And then electrodeposit with Fe
3+,
, and OH
- to form a Co-Fe-W oxyhydroxide. The catalyst-deposited nickel foam can be directly used as a stable OER electrode in a water electrolyser, exhibiting excellent catalytic performance and excellent durability in alkaline electrolytes. At the current density of 100 mA·cm
-2, the overpotential is 234 mV, and it can work stably for more than 120 H, showing the potential application in practical water splitting.
In addition to the preparation of electrodes by "one-step" electrodeposition, the combination of electrodeposition technology with other synthesis methods can greatly enrich the preparation strategies of catalysts and promote the birth of high-performance OER electrodes. Dong et al. synthesized Co, Ni Co-doped Fe
3N nanotube material (CoNi-Fe
3N NTs) in situ on iron foil by the combined process of anodic oxidation/electrodeposition/nitridation (Fig. 5)
[70]. The optimized catalyst can reach a current density of 10 mA·cm
-2 with an overpotential of only 285 mV, and its Tafel slope is 34 mV·dec
-1, which is superior to the commercial RuO
2 catalyst. Density function theory (DFT) further shows that the Ni site in CoNi-FeOOH can regulate the adsorption of OER intermediates and provide lower overpotential than Fe and Co sites, which is the best active site to obtain excellent OER performance.
Of course, as a new preparation technology of catalyst electrode, electrodeposition method still needs to be further improved and upgraded in the future development. Composite materials have multiple highly sensitive factors in the process of deposition, which requires more accurate exploration of the specific impact of parameter changes on electrode preparation. In addition, due to the complexity of the composition of composite deposited films, there is still a lack of research on the catalytic behavior mechanism of electrodeposited materials, which will be the focus of researchers in the future. It is worth mentioning that more diverse electrocatalytic electrodes can be prepared by combining different substrate materials with electrodeposition technology. The exploration of advanced supporting substrate manufacturing technology is helpful to promote the rapid development of electrodeposition process.
2.4 Chemical Vapor Deposition
Chemical vapor deposition (CVD) is one of the most powerful methods for the synthesis of two-dimensional materials. At a certain temperature, gas-phase atoms and molecules react in the gas atmosphere or between gas-solid interfaces to form solid products with high quality, uniform thickness and less structural defects
[71,72]. The morphology, layer number, lateral size, orientation, defect degree and the like of the two-dimensional nanomaterial can be regulated and controlled by changing a reactant substrate or regulating kinetic growth parameters (temperature, pressure, gas flow rate, reaction time and the like). Therefore, many researchers use CVD methods to modify and control the surface/interface of materials, such as geometric size control, defect engineering modification, doping modification and heterostructure construction. Obviously, the use of chemical vapor deposition to construct electrodes allows us to fundamentally understand the relationship between the structure and performance of electrocatalytic electrodes. As a typical "bottom-up" synthesis method, CVD method takes into account high quality, high efficiency, controllability and scalability to a certain extent, which has attracted special attention of researchers.
Chemical vapor deposition methods also have many variations, such as aerosol-assisted vapor deposition and plasma-enhanced vapor deposition. Sagu et al. Used Aerosol-assisted chemical vapor deposition (AACVD) to grow assisted chemical vapor deposition thin films on Fluorine-doped tin oxide (FTO) glass, which can be directly used as OER working electrodes
[73]. Stienen et al. grew Fe
2O
3 directly on nickel foam by metal organic chemical vapor deposition (MOCVD) using Metalorganic chemical vapor deposition as a precursor, and successfully fabricated a three-dimensional electrode for OER reaction, showing excellent catalytic performance
[74]. Zheng et al. improved the method of preparing carbon nanotubes by direct pyrolysis, developed a novel and simple "self-spraying" vapor phase growth method, and prepared uniformly dispersed nitrogen-doped carbon nanotubes (Fig. 6)
[75]. Recently, Bigiani et al. prepared functionalized chemical vapor deposition and Mn
2O
3 nanocomposites by Plasma enhanced-chemical vapor deposition (PE-CVD), which were used as anodes for seawater cracking reaction
[76]. It can decompose water while avoiding electrode corrosion caused by chloride. The results show that the MnO
2 modified by Co
3O
4 performs best in the application, with a Tafel slope of 40 mV·dec
-1 and an overpotential of 450 mV, which is superior to most of the oxide catalysts currently used for seawater decomposition.
Overall, chemical vapor deposition is a reliable electrode preparation strategy, but there are still some process and technical challenges. The equipment and related operations involved in the CVD synthesis process are more professional, the synthesis route is relatively difficult, the yield is low, and it is difficult to achieve large-scale industrial production. Moreover, it is still difficult to synthesize "perfect" two-dimensional nanomaterials by CVD method. How to obtain atomically defect-free low-dimensional materials by controlled vapor deposition is an attractive and challenging task.
2.5 Layer-by-layer self-assembly
Layer-by-Layer assembly (LbL) is a general method for preparing multilayer thin-film electrodes (fig. 7)
[77⇓~79]. In the preparation process, the molecular material can be alternately adsorbed on the substrate by virtue of the weak interaction between molecules and deposited layer by layer to form a nano-film with certain functions. Nanomaterials with different structures and compositions can be tightly and orderly combined in the LbL assembly process. The number of the deposition layers can be accurately controlled by changing the concentration of the solution, the adsorption time, the deposition times and the like, and the ideal layer thickness can be obtained.
The traditional LbL assembly method is based on electrostatic interaction. With the deepening of research, other interactions can also be used in LbL assembly, such as hydrogen bonding interaction, charge transfer interaction, biospecific interaction, covalent bond interaction, coordination chemistry interaction and intermolecular interaction. Moreover, more and more LbL assembly technologies have been reported, such as immersion, rotation, spray, electromagnetic and jet, which provide different options for researchers. Therefore, LbL technology is widely used in many fields, such as field effect transistors, lithium ion batteries, supercapacitors, solar cells, sensors, fuel cells and photo/electrochemical catalysis
[80,81].
For electrocatalysis, biphasic or multiphasic materials can be alternatively assembled by LbL to construct efficient OER catalysts at the molecular scale. The prepared electrode has a unique structure, which can promote the exposure of active sites, accelerate mass transport and oxygen bubble diffusion, and thus enhance the electrocatalytic activity. Zhang et al. used layered double hydroxide nanosheets (LDH NSs) and iron porphyrin (Fe-PP) as materials to prepare ordered Ultrathin films (UTF) by layer-by-layer assembly of charge attraction (Fig. 8)
[82]. Compared with the drop-coated sample, the electrochemical active area of the ultrathin film electrode was significantly increased, and the synthesized CoNi-LDH NSs/Fe-PP UTF
30 exhibited the best OER activity: an overpotential of 264 mV and a Tafel slope of 37.6 mV·dec
-1 at a current density of 10 mA·cm
-2, which was far superior to that of the IrO
2 catalyst. In addition, the method shows broad applicability to the fabrication of other LDHs and Fe-PP based ultrathin film electrodes, both of which exhibit significantly improved OER performance relative to pristine LDH nanosheets.
In addition, the alternate deposition between molecules can make the catalyst layer closely contact with the conductive support, accelerate charge transfer and prevent the catalyst from falling off, and enhance the stability of the system. Urgunde et al. Prepared a catalyst electrode with optimal loading on conductive carbon cloth by coating catalyst ink layer by layer
[83]. It was found that the Co
3O
4-16/CC with 16 dip-coating cycles exhibited significant stability within 24 H, which was better than that with only one dip-coating cycle.
Remarkably, the layer-by-layer assembly technique can also control the growth distribution of the nanocomponents at the molecular level, opening up a new route for the fabrication of high-performance OER electrocatalytic electrodes. For example, Li et al. Grown a series of Surface-mounted metal-organic framework (SURMOF) films with precise thickness on electrode substrates by a highly controllable "quasi-epitaxial" and "self-terminated" layer-by-layer deposition method (fig. 9)
[84]. The excellent morphology of the ultrathin electrocatalyst coating maximizes the density of active sites, resulting in excellent electrocatalytic performance in alkaline solution: a mass catalytic activity of up to 2.5 mA·μg
-1 is obtained at an overpotential of 300 mV, which is one order of magnitude higher than that of the commercial IrO
2 electrocatalyst; When the current density is 500 mA·cm
-2, it can work stably for at least 100 H, showing excellent durability.
Obviously, the assembly process using LbL technology is flexible and has strong versatility. But it is undeniable that it also has some problems. After the assembly process is completed, the remaining colloidal solution is generally unable to continue the new assembly work, resulting in a lot of waste. In addition, in the assembly process, polymer electrolytes (PSS, PDDA, etc.) Are generally used to enhance the adsorption of materials. Although the dosage is very small, the introduction of such substances will have a certain impact on the electron transfer of the system. We found that although LbL technology has been used in many research works, its related applications in the field of electrocatalysis are relatively rare. How to give full play to its advantages as an electrode preparation technology and expand its application results is still a subject that needs to be constantly considered and explored.
2.6 Integrated Strategy
As mentioned above, the substrate-assisted method is widely used in the field of electrode preparation, and various loading methods have their own advantages and disadvantages (Table 2), providing a variety of options for electrode preparation. In order to make up for the shortcomings of a single strategy, multiple load modes have also been reported to work together, showing unique advantages. For example, Li et al. constructed a substrate-assisted electrode with a layered structure through a synthesis process of electrodeposition and hydrothermal treatment
[85]. They used electrodeposited Co(OH)
2 as template and precursor to grow two-dimensional bimetallic cobalt-nickel composite coatings on carbon fiber paper. Then Ni ions and Hexaaminobenzene (HAB) were added in the hydrothermal process to react with Co(OH)
2 and form flower-like conjugated coordination polymers. The for substrate auxiliary electrode (CoNi-HAB/Co(OH)
2/CFP) having a hierarchical structure exhibit excellent oxygen evolution performance. Meanwhile, the conjugated coordination polymer is tightly coupled with the Co(OH)
2/CFP template, so that the defect of using a polymer adhesive is avoided, and the electrode has better cycling stability.
In addition, chemical vapor deposition in combination with hydrothermal or electrodeposition techniques has been widely used to synthesize substrate-assisted transition metal-based electrocatalysts. For example, Lee et al. prepared a bifunctional electrode (Fig. 10) with excellent oxygen and Hydrogen evolution (Hydrogen evolution reaction, HER) performance through a three-step synthesis strategy, which can be used for overall water splitting
[86]. First, reduced graphene oxide was deposited on nickel foam by hydrothermal method. Then, the N-rGO/NF electrode was prepared by introducing nitrogen atoms from urea precursor by chemical vapor deposition. Finally, vertically oriented NiCo
2S
4 nanosheets were hydrothermally grown on Ni foam again. The OER performance of the modified NCS @ N-rGO/NF electrode was significantly enhanced, and the current density could reach vs. RHE at the potential of 1. 46 V (vs. RHE).
3 Substrate-free electrode
Various conductive materials may be selected as the supporting substrate for the electrocatalyst when the electrode is manufactured using the substrate-assisted method. However, the low catalytic activity of the substrate itself and its inherent structure limit the intrinsic activity of the electrocatalytic materials to some extent. In addition, limited by the high temperature furnace technology and the brittleness of some carbon fiber materials, the production capacity of most traditional substrates is difficult to improve, resulting in high prices, which limits the development of such electrodes. Therefore, it is urgent to develop electrode materials with good structure, low cost and durability. A large number of reports have shown that efficient catalytic electrodes can be achieved by synthesizing self-supporting structures with a certain porosity (such as films, foams, fibrous webs, etc.), which usually have certain catalytic activity and can be used as working electrodes or electrode substrates. Compared with traditional substrate materials, the electrode usually has the characteristics of large specific surface area and high mechanical strength, which can promote gas transmission and improve durability
[87,88]. Benefiting from the adjustability of the structure of the synthesized materials, the expansion of the performance of such synthesized electrodes has a greater possibility, which enriches the choice of electrode preparation. The synthesized material can also be modified by appropriate technology to improve the performance and stability of the electrode and produce the electrode which is more in line with industrial requirements.
At present, the commonly used manufacturing strategies are electrospinning, 3D printing technology and so on. Table 3 summarizes the OER electrocatalytic performance of the electrodes obtained by different preparation methods. It should be noted that substrate-free catalytic electrodes are mostly made of carbon nanomaterials (carbon nanotubes, graphene, etc.), and other materials are rarely used, which limits the development of such electrodes to a certain extent and needs further expansion.
3.1 Electrospinning
Electrospinning is a common method to prepare nanoscale fiber structures with controllable morphology
[42,89,90]. Its basic principle is to use electrostatic force to stretch charged polymer solution or polymer melt into ultrafine fibers with an average diameter of hundreds of nanometers. By pre-oxidation or changing the ratio of spinning solution, process parameters and annealing conditions, the precise control of fiber morphology, microstructure and porosity can be achieved. So far, more than 100 polymer materials have been used in electrospinning technology, including polyvinylpyrrolidone, polyvinyl alcohol, polyacrylonitrile, polyimide and polystyrene. It can be seen that electrospinning technology is helpful to synthesize conductive electrodes with controllable structure and composition, flexible and easy-to-use, and high mechanical strength, which has broad prospects in the field of energy conversion and storage.
Electrospinning technology is a new means of material preparation. On the one hand, the nanofibers synthesized by electrospinning technology can be deposited on conductive substrates as working electrodes by dripping or other means.On the other hand, self-supporting films or three-dimensional substrates with hierarchical structure and ordered arrangement can be assembled by stacking and folding to change the arrangement mode of fibers. Moreover, high-temperature carbonization can convert the polymer nanofiber network into a carbon fiber conductive network with a certain diameter and pore structure, which is one of the most widely used oxygen evolution electrodes. For example, Liu et al. prepared Polyimide (PI) nanofibers using a classical electrospinning process and converted them into carbon nanofibers in an argon atmosphere (Figure 11A)
[91]. The scanning electron microscope image (Fig. 11e) shows that the interior of the Nanoporous carbon nanofiber films (NCNF-1000) pyrolyzed at 1000 ℃ is a typical randomly wound three-dimensional fiber network structure with a thickness of about 60 μm. The specific surface area of the carbon nanofiber network is 1249 m
2·g
-1,OER, and the onset potential reaches the level of a commercial catalyst.
Li et al. fabricated a graphitized carbon nanofiber network for electrocatalysis through an electrospinning process (Fig. 12). The material has a large specific surface area, good conductivity and excellent durability, showing great application potential as a supporting material
[92]. In order to enhance the catalytic performance of the electrode, the researchers used an electrodeposition process to uniformly grow vertical Fe/Ni hydroxide nanosheet arrays on the surface of nanofibers, and then used NaH
2PO
2 to phosphorize the hybrid structure, thus preparing Fe/Ni phosphate nanosheet arrays supported on graphitized electrospun carbon nanofiber membranes. The freestanding 3D nanomaterials can be directly used as the working electrode for OER reaction, showing an overpotential as low as 215 mV when the current density of 10 mA·cm
-2 is reached in 1 mol·L
-1KOH solution.
Recently, Patil et al. fabricated nitrogen-doped PIM-1-based carbon nanofibers as flexible electrodes
[93]. Atomic layer deposition (ALD) method was used to precisely control the deposition of Atomic layer deposition catalyst layer on flexible free-standing carbon nanofibers. The synergistic effect between carbon, nitrogen doping and nickel compounds makes the electrode have better OER activity and long-term stability in alkaline medium. The results show that the nanofibers with higher specific surface area and microporous structure can be used as an effective support for catalyst deposition. The electrode does not need to use adhesive and is easy to be flexible, so it has good applicability in flexible energy storage devices and is expected to be more widely used in electrode preparation strategies.
Electrospinning technology has the advantages of large-scale preparation, mild production conditions and easy recovery. However, there are still some technical bottlenecks for large-scale manufacturing and practical applications. The spinneret used in electrospinning is almost made by hand, and its poor stability will have a huge impact on the nanostructure. Moreover, the mechanical strength of the multilayer spun material is insufficient, and it is difficult to maintain a relatively stable support structure. These problems can be effectively solved by improving the original electrospinning technology or combining with the new post-processing technology. It can be seen that the related technology of electrospinning nanofibers will play a further role in the fields of catalysis and energy.
3.2 3D printing
3D printing is a technology that can quickly prototype simple or complex three-dimensional structures. Researchers can control the layer-by-layer deposition of materials and create three-dimensional structures through three-dimensional scanning, photogrammetry, combined with computer-aided design software. Nowadays, 3D printing has become a diversified manufacturing technology, which can use various precursor materials, such as polymers, graphene oxide and stainless steel, to create products with different physical and chemical properties
[94,95]. Moreover, in the process of printing different scenes, there are many 3D technologies to choose from, such as Fused deposition modeling (FDM), Selective laser sintering (SLS), Selective laser melting (SLM), Direct ink writing (DIW), Binder jetting (BJ) and Digital light processing (DLP)
[96,97]. Because of the diversity of 3D printing technology and its precursor materials, researchers can arbitrarily control the size, geometry, rigidity and porosity of materials in additive manufacturing, so as to achieve a reasonable electrode design. At present, the size of 3D printed additive elements ranges from millimeter to meter, which can be directly used to manufacture conductive electrodes and support substrates.
Generally speaking, in order to fabricate efficient OER electrodes by 3D printing technology, additional steps (such as electroplating, spraying, sputtering or hydrothermal growth) are needed to modify or modify the electrode surface to facilitate the deposition of catalysts and improve the overall activity. Su et al. Plated high-quality NiP on a polymer surface with a complex three-dimensional printing structure (fig. 13)
[98]. The success of polymer metallization brings high conductivity of 4.7×10
4S·m
-1 to the prepared NiP @ Truss electrode. It is worth noting that the metal coating on the flexible structure still has excellent conductivity after multiple times of severe extrusion. Nickel-iron- (oxo) hydroxides nano sheets (NFNS) were further electrodeposited on the NiP layer to form a stable NFNS @ NiP @ Truss electrode, which showed excellent water splitting ability in hydroxides nano sheets. The overpotential of the OER reaction was 197 mV at a current density of 10 mA·cm
-2.
Peng et al. used 3D printing technology and partially reduced graphene oxide/carbon nanotube mixed ink to prepare 3D printed graphene biosensor electrodes (3D printed graphene/CNT, 3DP GC) with mechanical robustness and high inherent bending strength
[99]. As shown in fig. 14, the porous hierarchical structure provides a larger surface area and interconnected channels, which can accelerate mass transfer and improve the rate of charge and ion transmission, making it an ideal catalyst support and also has the possibility of large-scale application. After further in-situ growth of NiFeP nanoarrays on the 3DP GC, the formed 3DP GC/NiFeP-24L electrode showed an OER overpotential of 214 mV and an HER overpotential of 133 mV at a current density of 30 mA·cm
-2, showing good cycling stability in the water splitting reaction when used as cathode and anode, respectively, outperforming the reported Ni-based, Co-based, Fe-based catalytic materials.
It should be noted that most of the carbon-based composites used in 3D printing need to add "binder" materials to make them easy to print and print, at the same time, this also makes the conductivity of the materials worse, and various modification technologies increase the complexity and cost of the whole manufacturing process. Like other printing technologies, the technical system of 3D printing is not yet mature, and there is still a long way to optimize and improve in many aspects, such as technology and materials. In any case, 3D printing technology provides a new way for electrode preparation and modification, which will have a huge impact on the field of electrocatalytic applications.
3.3 Other technologies
Templating. At present, a variety of template materials are widely used to assist the synthesis of nanostructures with certain morphologies (such as hollow and mesoporous structures). After the template is removed, the synthesized material retains the original spatial structure
[100]. It is reported that some metal foams (such as Ni foam) exhibit well-ordered macroporous structures, which can be used as template materials for the construction of free-standing electrodes
[101]. The template method can precisely control the morphology of the catalyst, form the desired layered porous structure, facilitate oxygen diffusion, and obtain more catalytic sites
[102]. Christodouleas et al. Made fibrous materials composed of precious metals (such as gold, silver, platinum, etc.) At high temperatures using paper sheets or cotton fabrics as templates
[103]. The material is permeable to gases and liquids, and is electrically conductive and electrochemically active. However, in some cases, how to effectively remove the template is still a key issue for further study.
Film casting. A method of manufacturing a flexible Film by evaporating or vacuum filtering a solution or suspension is called a "film casting method"
[104⇓~106]. It is widely used in carbon nanotubes, carbon fibers and graphene, which can enhance the mechanical strength and conductivity of films. The operation of the fluid film forming is simple, and the fluid film forming method is suitable for casting the film with the thickness of micron level, and the thickness of the film can be regulated and controlled by changing the reaction time and the solution concentration. This method often requires the use of a semipermeable membrane (e.g., filter paper) to effect film deposition of the catalyst. However, it is challenging to transfer the prepared ultrathin film from the substrate.
4 Conclusions and outlook
With the development and exploration of high-performance OER catalysts, more and more research efforts have been devoted to the design and fabrication of novel OER electrodes. In this review, we summarize the current preparation strategies of OER electrodes and the research status in this field. Among them, the substrate-assisted method is the most widely used electrode preparation method at present. The commonly used loading methods include drop coating, water/solvothermal, electrodeposition, vapor deposition, layer-by-layer self-assembly and so on. In addition, some emerging substrate-free electrodes are also developing, and technologies such as electrospinning and 3D printing have brought us new ideas for electrode preparation strategies. In this paper, the advantages and disadvantages of various electrode preparation methods are introduced in detail from the perspective of methodology, and the latest research progress is shown.
With the deepening of future research, researchers will develop and explore more and more electrode preparation technologies and loading methods. For the current preparation method of catalyst electrode, its development direction can be considered from the following four aspects.
(1) Prepare an independent electrode without binder. Although the drop coating method is the most widely used loading method in the preparation of substrate-assisted electrodes, the presence of binders brings inevitable negative effects to the reaction system. Binderless independent electrode has the advantages of high conductivity, high active area and high stability, which is an important development direction in the field of electrode design and preparation.
(2) Explore new ways of substrate materials. Current commercial substrates, such as carbon paper and metal foam, generally have very low catalytic performance. Modifying the substrate or exploring new substrate materials will be beneficial to the construction of efficient OER electrodes. The development of new technologies such as electrospinning and 3D printing provides us with more choices, and it will have broad exploration prospects to improve the catalytic performance of the substrate itself.
(3) Composite use of multiple load modes. With the continuous development of preparation technology, a single electrode preparation method has gradually been unable to meet our requirements for efficient electrocatalytic electrodes. By taking multi-step treatment and combining the advantages of various methods, it is easier to prepare electrodes with high OER catalytic activity.
(4) Develop the surface/interface modification of the electrode. Since OER is an electrocatalytic reaction involving gas, the surface/interface regulation of electrocatalysts plays an important role in optimizing their catalytic activity and stability. In addition, adjusting the wettability of the electrocatalyst by certain post-treatment means, such as plasma treatment, can improve the mass transfer ability of the electrocatalyst, promote the release of bubbles and improve the catalytic efficiency.
In general, freestanding electrodes for oxygen evolution reactions are expected to play an important role in the development of energy devices. We hope that this review can inspire researchers in the design and preparation of efficient OER electrodes, and stimulate more researchers to improve the electrocatalytic performance of independent electrodes, so that they can better meet the needs of energy conversion devices in actual production.
National Natural Science Foundation of China(51873088)
National Natural Science Foundation of China(12004195)
Natural Science Foundation of Tianjin(18JCZDJC38400)
Natural Science Foundation of Tianjin(20JCQNJC01820)
“111” Project of China's Higher Education(B18030)
Fundamental Research Funds for the Central Universities from Nankai University(63201061)
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