Oxygen evolution reaction (OER) is a key process for green and sustainable energy storage and transfer technologies like electrocatalytic water splitting, rechargeable metal-air batteries, regenerative fuel cells, etc. Nevertheless, the high potential barrier and sluggish kinetic process limit its overall performance. Thus, designing and exploiting high-efficient, robust and noble-metal-free catalysts is one of the challenges in the field of new energy. CoFe layered double hydroxide (CoFe LDH) possesses broad prospects in OER due to the extraordinary features such as unique 2D layered structures, multiple and flexible chemical compositions, high dispersive metal cations, excellent stability and low cost. However, the poor conductivity and insufficient reactive sites hamper its industrial application. Beginning with the introduction of the structure of CoFe LDH and the elaboration of the proposed mechanisms for OER, the preparation of CoFe LDH and the current modification strategies for CoFe LDH to thoroughly boost its reactivity are summarized including intercalation and exfoliation, vacancy creation, hybridization, ions substitution and their derivatives. At last, the current challenges and future directions for LDH-based nanostructures in energy conversion and utilization are discussed.
The rapid development of science and technology and the rapid growth of population have led to the depletion of fossil fuels, environmental pollution and excessive carbon emissions, which has aroused great concern about the search for renewable energy. Therefore, exploring low-cost and high-efficiency technologies to generate green and renewable energy is one of the greatest challenges facing mankind[1⇓⇓~4]. Technologies such as solar energy, wind energy, water energy and geothermal energy are becoming more and more mature, and the power generation is increasing year by year, which can theoretically meet a large part of the global energy demand, but it is vulnerable to environmental instability factors and can not be effectively utilized, so the development of low-cost and efficient energy storage technology to make full use of green energy is one of the research hotspots in recent years[5,6]. Hydrogen, as the fuel with the highest energy density (140 MJ·kg-1) among chemical fuels and with the advantage of zero carbon emission, is the most potential medium or carrier for storage and conversion of the above renewable energy[7⇓⇓⇓~11]. However, the main technologies of hydrogen production in industry are water-gas conversion and methane reforming, which produce hydrogen with low purity, consume a large amount of fossil fuels, and emit pollutants such as CO and CO2[12⇓⇓~15]. Since the first discovery of hydrogen production from water electrolysis in the 18th century, this process has gradually developed into one of the potential technologies for hydrogen energy generation because of its green sustainability[16⇓~18]. Water electrolysis reaction mainly includes Hydrogen evolution reaction (HER) on the cathode and Oxygen evolution reaction (OER) on the anode. Compared with the two-electron conversion of HER, OER as a half-reaction of water electrolysis involves a four-electron conversion process, which requires a larger overpotential and slower reaction kinetics[19,20]. At present, the voltage of water splitting in commercial electrolyzers is generally between 1. 8 and 2. 0 V, which is much higher than the theoretical water splitting voltage of 1. 23 V. In order to reduce the energy loss of this part, scientists are committed to developing efficient, stable and inexpensive electrocatalysts[21,22].
Noble metals such as Ir and Ru have excellent OER activity in both acidic and alkaline solutions, but their reserves and prices do not have the advantages of industrialization[23]. Transition metal compounds, such as Mo, Fe, Co and Ni, exhibit excellent OER properties due to their unique d-electronic structure. Transition metal hydroxides, in particular, have attracted much attention due to their superior OER performance and good stability in alkaline solutions. Transition metal hydroxides include brucites (e.g., :Mg(OH)2, β-Co(OH)2, etc.) And hydrotalcites (e.g., CoFe LDH, CoFeMo LDH, etc.)[24].
Brucite has been studied for a long time, but its performance is low and its stability is poor. Layered double hydroxides (LDHs) can be obtained when part of the divalent metal ions are replaced by another transition metal element. There are many kinds of hydrotalcite materials, and the material components used as OER catalysts generally contain three elements: Fe, Co and Ni. Among them, CoFe LDH exhibits excellent OER catalytic activity due to the incorporation of Fe into the Co(OH)2, which makes it have both the high activity of Fe(OH)3 and the stability of Co(OH)2, and the synergistic effect of Fe and Co is also conducive to the formation of reaction intermediates[25]. However, CoFe LDH still suffers from the common disadvantages of transition metal hydroxides: few active sites and low conductivity, which hinder the further development of catalysts. Therefore, the optimal modification of CoFe LDH is still a hot issue in scientific research.
In this paper, the structural characteristics of hydrotalcite and the mechanism of OER were briefly introduced. Then the modification strategies of CoFe LDH materials in recent years are reviewed in detail, including intercalation and exfoliation, vacancy fabrication, material recombination, ion substitution, derivatives, etc., as shown in Table 1. Finally, the future challenges and opportunities of CoFe LDH-based electrocatalysts are proposed.
2 Structure and OER Reaction Mechanism of Hydrotalcite
2.1 Structure of hydrotalcite
As shown in Fig. 1, hydrotalcite is a layered bimetallic hydroxide material mainly composed of positively charged host lamellar layers and negatively charged interlayer anions. The main lamellar structure of hydrotalcite is similar to that of brucite Mg(OH)2, which is composed of an octahedral structure composed of a transition metal ion and six hydroxyl ions in the center, and the lamellar cells are connected by edges to form a two-dimensional shape. The divalent positive ions in the main lamina of the hydrotalcite material are partially replaced by trivalent positive ions, so that the lamina is positively charged. To appear electrically neutral, the interlayer is balanced by negatively charged anions and in addition contains water molecules. When the layers are stacked on each other, the hydroxyl groups in the main layers of the hydrotalcite can form hydrogen bonds with interlayer anions and water molecules to form a stable structure[26]. The general structural formula of hydrotalcite is , where M2+ and M3+ represent divalent and trivalent metal cations, respectively, An- is the anion between the positive ion layers, and X is usually between 0.2 and 0.4. In hydrotalcite materials, metal elements are rich in composition and adjustable in proportion, including divalent ions (Co2+, Ni2+, Fe2+, etc.) and trivalent ions (Fe3+, Mn3+, etc.); The interlayer ions can regulate the composition (CO32-, NO3-, $\mathrm{SO}_{4}^{2-}$, Cl-, RCO2-, etc.) by ion exchange, so as to achieve the purpose of changing the interlayer spacing[28⇓~30]. To sum up, the composition, structure and properties of hydrotalcite are flexible and controllable, which can effectively improve its photocatalytic and photoelectrocatalytic properties[31⇓~33].
2.2 Mechanism of OER
The OER reaction is a half-reaction of technologies such as water splitting and rechargeable metal-air batteries, and its reaction equation varies with electrolyte pH.
The reaction equation in acidic and neutral environment is:[13,34]
The reaction equation in alkaline environment is:
Hydrotalcite, as a transition metal hydroxide material, is easily dissolved in acidic medium. Therefore, the OER reaction of hydrotalcite is generally carried out under alkaline conditions. At present, the OER reaction mechanism is mainly divided into two types: adsorbate evolution mechanism (AEM) and lattice oxygen oxidation mechanism (LOM) (Fig. 2)[27,35].
In alkaline environment, AEM refers to the continuous adsorption and desorption process of oxygen-containing intermediate products on the metal active site (M *), including the four-electron transfer process centered on the metal ion, as shown in equations (1 – 4)[36].
First, OH- adsorbs on the active site (*) by losing one electron to form OH* (Formula 1). The subsequent deprotonation of OH* with free OH- produces O* with the loss of an electron (Equation 2). The O* intermediate is bonded to the O atom in OH-, and one electron is lost to give OOH* (Formula 3). Finally, OOH* and OH- act to release the active site with the loss of an electron accompanied by the formation of O2 (Equation 4).
In addition to the above AEM, it was found that there may be another mechanism, LOM, in the OER reaction of perovskite-type catalysts[37⇓~39]. This mechanism is also applicable to some hydrotalcite materials (such as CoZn LDH)[40]. The LOM mechanism includes the oxidation, exchange and release of lattice oxygen ligands on the catalyst surface, as shown in equations (5 ~ 8):
First, OH* loses one electron with free OH- to form Vo and OO* on the metal active site (Equation 5). The subsequent reaction of OO* with free OH- loses one electron to give OH*, accompanied by the precipitation of O2 (Equation 6). The Vo then combines with the free OH-, losing one electron to give the (Equation 7). Finally, and free OH- react to lose an electron to form H2O and OH* (Equation 8).
To sum up, AEM contains a 4-electron transfer process involving the formation of multiple adsorption intermediates, with a theoretical minimum overpotential of ∼ 0.37 V during the conversion of OH* to OOH*[41⇓⇓~44]. However, LOM can bypass the limitation of AEM and does not involve the conversion of OH* to OOH*, but the direct coupling of OH- and lattice oxygen on the active sites of the catalyst surface to form O2 molecules, so this mechanism is beneficial to achieve a smaller overpotential[17,35,45⇓ ~47]. However, compared with AEM, the applicability of LOM in hydrotalcite materials still needs further study.
3 Synthesis of CoFe LDH
The synthesis process of CoFe LDH hydrotalcite is relatively simple. In this section, three typical synthesis methods, namely precipitation, solvothermal and electrodeposition, are mainly reviewed[48⇓~50].
3.1 Precipitation method and solvothermal method
The precipitation method is simple in process, convenient in operation, and mild and controllable in reaction conditions. Since Feitknecht et al. First used the precipitation method to synthesize hydrotalcite materials in 1942, this method is still the most widely used method to prepare nanoscale hydrotalcite materials[51]. The synthesis of CoFe LDH is generally carried out by mixing and dissolving metal salts (including Co2+ and Fe3+) and precipitants (urea, NaOH/NaCO or hexamethylenetetramine (HMT), etc.) In solvents (water, ethanol, ethylene glycol, etc.), and using solvothermal method or water bath method to obtain the material[52]3 [53][54]. For example, Tian et al. Selected Fe(NO3)3 and Co(NO3)2 as metal salts, urea as precipitant and MXene as substrate, and finally obtained the composite material of CoFe LDH/MXene at 100 ℃ for 5 H under the protection of N2 atmosphere[52]. The crystallization step of hydrotalcite can be controlled by adjusting the reaction temperature and time, and then the structure and morphology of CoFe LDH can be affected[55]. Deng et al. Selected Fe(NO3)3 and Co(NO3)2 as metal salts and urea as a precipitant to prepare CoFe LDH nanofiber structures by shortening the reaction time and reducing the crystallinity of CoFe LDH during the hydrothermal process[56].
Solvothermal methods are often used in combination with precipitation methods to improve crystallinity, control particle size, and increase product yield[48]. The precipitant is usually urea or NaOH/NaCO3, and sometimes complexing agent (ammonium fluoride, etc.) Is added to control the growth process of hydrotalcite, improve the crystallinity of hydrotalcite and increase the specific surface area. For example, Yang et al. Used urea as precipitant and ammonium fluoride as complexing agent in the hydrothermal process to prepare self-grown CoFe LDH on nickel foam, which has the advantages of high crystallinity and large specific surface area[57].
3.2 Electrodeposition
Electrodeposition is one of the efficient methods to prepare 3D hydrotalcite nanosheet arrays because of its simplicity, time-saving and environmental protection. In the electrodeposition method, the N in the electrolyte (containing metal salt) is reduced to OH- through the conductive substrate (nickel foam, carbon cloth, carbon paper, and the like) of the cathode, and with the increase of the deposition time,The OH- generated by continuous reduction will accumulate on the surface of the cathode conductive substrate, and when the surface alkalinity reaches a certain value, the metal ions near the cathode will react with the OH- to precipitate hydrotalcite and adhere to the surface of the cathode conductive substrate. The type and concentration of metal salt, deposition time and voltage have a great influence on the morphology, crystallinity and phase composition of hydrotalcite[50,58,59]. For example, Wang et al. Used in situ TEM to study the effect of different metal salt concentrations on the morphology of CoFe LDH, and found that when the metal salt concentrations were 0.015 M and 0.15 M, the structure of CoFe LDH was horizontally grown thin film and vertically grown flake, respectively[60].
In addition, CoFe LDH materials can also be prepared by laser liquid phase ablation. Gao et al. Obtained CoO @ CoFe LDH composites in NaCl solution containing CoFe alloy by laser liquid phase ablation with only 7 ns laser pulse[61]. Therefore, the mass production of CoFe LDH with specific structure by low-cost means is still one of the current research priorities.
4 Modification Strategy of CoFe LDH
CoFe LDH is one of the most potential OER catalysts because of its special layered structure and good stability, however, the disadvantages of few active sites and low conductivity hinder its further development. The factors affecting the performance of electrocatalysts can be summarized as follows: (1) the number of active sites on the catalyst surface; (2) conductivity; (3) Intrinsic activity of a single active site. In view of the above three aspects, researchers have modified CoFe LDH materials by different methods to improve their catalytic activity, including intercalation and exfoliation, vacancy fabrication, material recombination, ion substitution and derivatives.
4.1 Intercalation exfoliation
The distance between the positive ion layers of CoFe LDH is closely related to its OER performance[50,90,91]. Multilayer CoFe LDH nanosheets are thick, have small specific surface area and few active sites, thus inhibiting their electrocatalytic performance. Therefore, the above problems can be effectively improved by increasing the distance between the positive ion layers and synthesizing few-layer or single-layer nanosheets, thereby improving the OER performance of CoFe LDH.
Interlayer ion exchange is one of the common methods to increase the interlayer spacing. Formamide, as a polar molecule, can directly bind to LDH sheets and inhibit the interaction between layers, thus achieving the purpose of expanding the interlayer spacing[92,93]. Liu et al. Treated CoFe LDH with a mixed solvent of dimethylformamide and ethanol. When stirred in an argon atmosphere, the interlayer anion CO32- was hydrolyzed by the solvent to form a COOH-, and the interlayer spacing increased from 0. 8 nm to 4. 5 nm[62]. The overpotential of CoFe LDH with increased interlayer spacing at 10 mA·cm-2 is 300 mV, which is smaller than that of untreated CoFe LDH (315 mV). In addition, Zhou et al. Prepared E-CoFe LDH with increased interlayer spacing by etching CoFe LDH with HNO3 based on the principle of acid-base reaction[63]. Compared with the XRD pattern before etching, the peak of E-CoFe LDH is obviously inclined to a small angle, and the interlayer spacing is increased from 0. 75 nm to 0. 77 nm according to the Scherrer formula, which is conducive to the full contact between the electrolyte and the active site, so that it has a higher activity.
Liquid-phase exfoliation is one of the common methods to prepare few-layer or single-layer nanosheets. However, the use of solvents in this process will block the active sites on the surface of nanosheets, making them unable to contact with water, thus weakening the catalytic performance of materials. The above problems can be effectively avoided by means of plasma stripping. Liu et al. Put CoFe LDH in a dielectric barrier discharge (DBD) plasma reactor for stripping for 5 min, and the water plasma in the DBD can etch the interlayer anions.The electrostatic interaction between the positive ion layer and the interlayer anion is weakened, thereby expanding the interlayer spacing (Figure 3A), and a thin layer nanosheet with a thickness of 1.5 nm is prepared, which can expose more active sites, thus showing excellent catalytic activity[64]. In addition to water plasma, they also employed the "dry exfoliation" technique of Ar plasma to synthesize CoFe LDH nanosheets with a thickness of 0.6 nm (Fig. 3 b-c)[53]. The infrared spectrum shows that the intensity of the peak corresponding to the CO32- decreases after exfoliation, which may be due to the etching of the CO32- between the layers of CoFe LDH by Ar plasma, which weakens the electrostatic interaction between the interlayer anion and cation layers. The overpotential was 237 mV when the current density reached 10 mA·cm-2, which was significantly higher than that of the unstripped CoFe LDH (321 mV) (Fig. 3D).
The preparation of few-layer or single-layer CoFe LDH by ion exchange and exfoliation technology involves two processes of synthesis and exfoliation, which takes a long time and has a low yield. Gao et al. Combined the two steps and directly added formamide solution to replace the interlayer CO32- during the preparation of CoFe LDH, and the CoFe LDH nanosheets with a thickness of 1. 3 nm were obtained after 5 min of reaction at 80 ℃ and pH ~ 10[65]. In addition, Zhao et al. Dissolved cobalt chloride, ferric chloride and precipitant HMT in deionized water, and successfully synthesized porous CoFe LDH composed of ultrathin nanosheets by oil bath method under argon atmosphere[66]. The catalyst maintains high activity in alkaline electrolyte for 20 H.
To sum up, the interlayer ion exchange method, the stripping method and the one-step method can effectively expand the distance between the positive ion layers, synthesize a few-layer or single-layer CoFe LDH, and effectively increase the active site concentration of the CoFe LDH, thereby improving the oxygen evolution reaction performance.
4.2 Vacancy fabrication
The introduction of oxygen vacancies or metal vacancies is one of the effective ways to improve the electrocatalytic performance of materials: (1) vacancies can control the electronic structure of materials and improve the conductivity of materials; (2) The introduction of vacancy will produce a large number of dangling bonds and expose more active sites[5,67,68]. Therefore, defect regulation of CoFe LDH can further improve its electrocatalytic performance.
4.2.1 Oxygen vacancy
As one of the common defects in nanomaterials, the formation of oxygen vacancies can not only ensure the internal charge balance and stability of materials, but also facilitate the adsorption of OH- on the surface of catalysts to promote the water electrolysis reaction[49,67]. The reduction method is one of the most common methods to construct oxygen vacancies. The commonly used reducing agents are SnCl2, NaBH4, etc., and the reducing atmospheres are H2S, CH4, H2, etc[49]. Karmakar et al. Used NaHSe as a reducing agent to obtain Se @ CoFe LDH rich in oxygen vacancies[67]. From the analysis of EPR spectrum (Figure 4A), it can be seen that the oxygen vacancy concentration of Se @ CoFe LDH is larger than that of CoFe LDH. At the same time, the band gap of Se @ CoFe LDH is correspondingly reduced to 1.11 eV, indicating that oxygen vacancies help to reduce the band gap of nanomaterials and thus improve the conductivity of materials (Fig. 4B). The overpotential of Se @ CoFe LDH at 50 mA·cm-2 was 251 mV, and the Tafel slope was 47 mV·dec-1. In addition, the interconversion between Ce3+ and Ce4+ in CeO2-x can optimize the electronic structure of the material, and when CeO2-x is compounded with other materials, it is beneficial to create oxygen vacancies within the composite. Hu et al. Synthesized CeO2-x@CoFe LDH/NF composite with heterostructure by one-step hydrothermal method[68]. The XPS spectrum shows that the oxygen vacancy concentration of CeO2-x@CoFe LDH/NF is 47.1%, which is higher than that of CoFe LDH (39.9%). The composite showed good catalytic activity and stable oxygen evolution performance after 35 H of continuous measurement.
4.2.2 Metal vacancy
Compared with oxygen vacancy, metal ion vacancy has special properties due to the diversity of its electron and orbital distribution, but the formation energy of metal ion vacancy is larger and the construction process is more difficult[94,95]. Zhu et al. introduced Fe vacancies into CoFe LDH by doping Rh element, and synthesized Rh-doped CoFe LDH with hollow nano-triangular structure (Fig. 4C)[69]. The XANES spectra of the composite showed that Rh atoms mainly replaced the sites of Fe atoms in CoFe LDH, resulting in Fe vacancies. The introduction of Fe vacancy can increase the chemical valence of the adjacent metal, resulting in a large number of dangling bonds, that is, the number of active sites increases, thus enhancing the electrocatalytic surface reaction.
4.2.3 Coexistence of oxygen vacancy and metal vacancy
In addition to oxygen vacancies and metal vacancies alone, CoFe LDH with both oxygen vacancies and metal vacancies was constructed by unique surface control methods (such as plasma stripping, etching and organic solvent reduction), which is conducive to further optimizing the electrocatalytic performance of CoFe LDH. Using plasma (Ar plasma, H2O plasma, and N2 plasma) exfoliation technique, Wang et al. Formed various vacancies (O, Co, and Fe vacancies) on the surface of CoFe LDH nanosheets (Figures 3A-B, 5A-B)[53][64][70]. The XPS spectrum analysis results show that the treated CoFe LDH material has a higher concentration of O vacancies (Fig. 5c-d). These vacancies can not only adjust the surface electronic structure of CoFe LDH nanosheets, but also effectively reduce the coordination number around the active sites, thus promoting the adsorption of OH- and OER reaction intermediates, and significantly improving its OER performance. In addition, strong acid etching is also one of the effective methods to create defects on the surface of materials. Zhou et al. Obtained a multi-vacancy CoFe LDH by 1 M HNO3 etching, and the overpotential of the treated CoFe LDH was reduced from 346 mV to 302 mV at 10 mA·cm-2, and it had high stability[63]. However, strong acid etching will cause some damage to the structure of the material, and the mild organic solvent reduction reaction technology can effectively improve this disadvantage. For example, Zhou et al. Prepared CoFe LDH with various vacancies by reducing CoFe LDH with ethylene glycol at 140 ℃ for 6 H[71]. Its overpotential at 10 mA·cm-2 was 67 mV lower than that of the untreated one.
To sum up, the formation of vacancies can cause changes in the atomic structure and electronic structure of the material, thereby changing the conductivity of the material or increasing the number of surface active sites to achieve the purpose of optimizing the performance of CoFe LDH.
4.3 Material compounding
Composite materials improve catalyst activity mainly by constructing special structures[96⇓~98]. CoFe LDH can be complexed with transition metal oxides, transition metals, transition metal sulfides, transition metal phosphides and carbon materials to improve the conductivity of materials, increase the number of active sites of materials, reduce the adsorption energy of intermediate products, and achieve the purpose of improving the electrochemical performance[61,73,74][75][77][76][78,89,99].
4.3.1 Compositing with transition metal based materials
Transition metal oxides can be widely used in OER reaction because of their high electrocatalytic activity, simple preparation process and good stability. Gao et al. Synthesized CoO/CoFe LDH by depositing CoO nanoclusters on CoFe LDH nanosheets using a single-step pulsed laser liquid ablation method (Fig. 6a)[61]. Compared with the Co2+ in CoO and CoFe LDH materials alone, some Co3+ ions with lower coordination number and smaller adsorption energy of H2O molecules appear in the composite material, which promotes the electron transfer from Co to Fe through the Co-O-Fe chemical bond (Fig. 6B). It can be seen from Fig. 6 C that the overpotential of CoO/CoFe LDH is 254 mV at a current density of 10 mA·cm-2. Because high valence Co or Fe ions can significantly affect the OER performance of materials, Ma et al. Directly compounded Co3O4 containing Co3+ with CoFe LDH to obtain Co3O4/CoFe LDH composite[72]. The overpotential at a current density of 10 mA·cm-2 is 290 mV, and continuous operation for 10 H can also ensure the voltage stability. The main reason for the improvement of the properties of the above composites is the large area of the two-phase interface produced when the two substances are compounded. The interface structure is very important for the performance of materials, which is mainly reflected in the following aspects: (1) The metal atoms near the interface are usually in a coordinatively unsaturated state and have a large adsorption energy, which can be used as active sites for reactions to improve the inherent activity of materials; (2) The electronic structure near the interface will change, which will lead to the acceleration of electron transport rate and the improvement of material conductivity. The construction of 3D network structure can further improve the OER performance of materials. On the one hand, the 3D structure is conducive to exposing more active sites and accelerating surface reactions; On the other hand, the network structure can promote the penetration of electrolyte ions, especially when the conductive material is combined with CoFe LDH with poor conductivity, which is conducive to the creation of electron/ion transport paths. Zhang et al. Used NiCo2O4 nanofibers as a substrate to grow CoFe LDH on it by co-precipitation method, and successfully prepared NiCo2O4@CoFe LDH/NF 3D structural materials[73]. When the current density is 20 mA·cm-2, the overpotential of NiCo2O4@CoFe LDH/NF is 273 mV. In addition to Co, Cu is widely used in the field of electrocatalysis because of its high stability and abundant reserves. Wang et al. Successfully prepared CuO nanofibers coated with CoFe LDH nanosheets (CuO @ CoFe LDH/CF) by electrodeposition (Fig. 7A, B)[74]. Benefiting from the 3D composite structure, CuO @ CoFe LDH/CF has a large number of active sites, and its double layer capacitance (Cdl) is 14 times higher than that of CoFe LDH/CF, and its performance is obviously improved (Fig. 7 C, d).
Compared with transition metal oxides, transition metals, transition metal phosphides and transition metal sulfides have the advantages of high electronic conductivity, good catalytic activity and stability, and can be used as substrate materials for electrodes[100]. Yu et al. Prepared Cu nanowire-coated copper foam by chemical oxidation, annealing and electrochemical reduction, and then electrodeposited CoFe LDH on its surface to obtain Cu @ CoFe LDH/CF composite[75]. Compared with CuO nanowires, Cu nanowires have higher conductivity, which can reduce the charge transfer resistance and make the electrons migrate quickly during the reaction. The Cu @ CoFe LDH/CF can also be used as a bifunctional catalyst (both as the anode and cathode of the cell), and its overpotential is 60 mV lower than that of the commercial cell IrO2(+)/Pt(-) at a current density of 10 mA·cm-2, while it also shows high stability at a current density of 10 mA·cm-2.
Feng et al. Prepared CoP @ CoFe LDH/NF by electrodeposition using CoP/NF with good conductivity as substrate material[76]. CoP/NF as a substrate not only increases the conductivity of the material, but also its three-dimensional structure is conducive to the contact between the active site and the electrolyte. The overpotential was 278 mV at a current density of 100 mA·cm-2 and was stable at a voltage of 1.47 V for 20 H. Huang et al. Prepared the composite structure (FeCo2S4@CoFe LDH/NF) of FeCo2S4 nanofibers and CoFe LDH materials by the method of hydrothermal-sulfurization-electrodeposition[77]. The overpotential at a current density of 100 mA·cm-2 was 259 mV. When the current density is kept at 10 mA·cm-2, it can work continuously for 24 H and remain stable.
4.3.2 Composite with carbon material
Carbon materials have the advantages of low cost, large reserves and good conductivity. Carbon materials with high specific surface area are used as substrates, which can not only promote electron transfer, but also facilitate the contact between electrolyte and active sites. Yu et al. Dispersedly grew CoFe LDH in the framework of thermally exfoliated graphite (TEG) and successfully synthesized CoFe LDH/TEG composite[89]. Compared with the bulk CoFe LDH synthesized alone, the composite has a higher density of active sites exposed at the edge. Secondly, the lamellar structure of TEG itself is conducive to the transport of ions and gases, and the good conductivity can also accelerate the electron transfer during the OER reaction. Similarly, g-CNx is also a substrate material with good conductivity, and the doping of N element will cause the polarization of C atoms, which is beneficial to the adsorption of OH- on the active sites. Bhowmik et al. Used a one-step precipitation method to self-grow CoFe LDH,CoFe LDH/g-CNx on g-CNx with an overpotential of 280 mV at a current density of 10 mA·cm-2, which is better than 420 mV and 460 mV for Co(OH)2/g-CNx and FeO(OH)/g-CNx[78].
In a word, the main mechanisms of improving the catalytic performance of CoFe LDH by the composite method include: (1) the highly conductive substrate material promotes the electron transfer; (2) The 3D network structure exposes a high density of active sites to accelerate surface reactions; (3) The close interaction between CoFe LDH and its composite materials leads to the change of electronic structure and reduces the adsorption energy of intermediate products.
4.4 Ionic substitution
On the one hand, the partial substitution of cations (such as V, Mo, Cr ions replacing Co or Fe ions) can control the electronic structure of materials, reduce the band gap or form amorphous phase, and improve the conductivity[81]; On the other hand, it can change the atomic structure and construct active sites[79]. In addition, it has the advantage of reducing the adsorption energy of reactants and intermediate products, thus optimizing the electrocatalytic performance[82,83]. Therefore, the OER performance of CoFe LDH can be effectively improved by element doping.
Hu et al. Prepared CoFeV LDH materials by introducing V element into CoFe LDH by hydrothermal method. The high valence of V element causes the valence of adjacent metal elements to rise, resulting in high valence of Co or Fe, which is conducive to OER reaction[79]. The overpotential at a current density of 10 mA·cm-2 was 242 mV and exhibited a Tafel slope of 57 mV·dec-1. Through DFT + U theoretical simulation, Yang et al. Further explored and found that high-valence V element doping would enhance the covalent bond between Co or Fe and oxygen in the adsorbed hydroxide, thereby promoting the transfer of electrons from oxygen atoms to metal atoms and reducing the energy required for charge transfer in the OER process[80]. Ding et al. Successfully prepared Mo-doped CoFe LDH nanorods by hydrothermal method[81]. The adsorption energy of CoFeMo LDH for water (− 0.86 eV) is much lower than that of CoFe LDH for water (− 2.58 eV) due to the characteristic that Mo element can adjust the electronic structure of the material. At the same time, the theoretical simulation shows that the density of States of CoFeMo LDH increases near the Fermi level, which indicates that the introduction of Mo element is conducive to the formation of amorphous phase, thus accelerating the electron transfer, which improves the electrocatalytic performance of CoFeMo LDH. In addition, Wen et al. Synthesized Cr-doped CoFe LDH by hydrothermal method (Fig. 8 a)[82]. Because the electronegativity of Cr element is greater than that of Co element, DFT simulation calculation shows that the adsorption energy of CoFeCr LDH for the reaction intermediate product -OOH is lower than that of CoFe LDH (Fig. 8B), which is beneficial to the improvement of OER performance. The overpotential at 10 mA·cm-2 was 238 mV and exhibited a Tafel slope of 107 mV·dec-1 (Fig. 8 C). The introduction of high-valence transition metal elements is not only beneficial to the adsorption of reaction intermediates, but also can maintain the stability of active sites. Wang et al. Believed that the key to improve the performance of CoFe LDH materials by Cr doping was that the outermost electron arrangement of Cr3+ was , and the empty eg orbital showed strong electron capture ability[83]. But also has stronger electronegativity when the Cr3+ is converted into the Cr6+. The introduction of Cr6+ keeps Co ions in a high valence state, and the high valence Co ions are usually in a coordinatively unsaturated state, which is beneficial to the adsorption of intermediates and the improvement of OER performance.
To sum up, due to the special layered structure of CoFe LDH, which leads to the substitutability of cations, CoFe LDH can be converted into trimetallic hydrotalcite by element doping. The main reasons for the outstanding OER performance of trimetallic hydrotalcite are as follows: (1) Compared with the original CoFe LDH, the doping atom itself can be used as the active center of the reaction, which increases the surface active sites without destroying the original morphology; (2) The electron-withdrawing characteristics of doping elements (especially high-valence cations) can significantly increase the concentration of high-valence Co or Fe ions in CoFe LDH, and reduce the adsorption energy of water molecules or intermediate products; (3) The appropriate introduction of the third element will adjust the electronic structure of CoFe LDH, resulting in an electronic structure similar to that of noble metal catalysts and reducing the adsorption energy of intermediate products.
4.5 Derivative
In addition to the above modification measures, CoFe LDH can also be used as a precursor to be converted into high-performance transition metal compounds by selenization, phosphating, nitriding and other treatments, which can be used in the field of electrocatalysis[84][85][86]. In addition, the electrochemical oxidation of CoFe LDH can also change the surface structure of the material, thereby improving the oxygen evolution activity[87,101].
The transition metal compounds obtained by selenization, nitridation and phosphating have better OER performance, on the one hand, because the non-metal atoms can change the electronic state of metal atoms and reduce the adsorption energy of OH- on the material; On the other hand, the transition metal compound obtained after the treatment is usually a porous sheet structure, which is beneficial to the full contact between the electrolyte and the active site. Zhang et al. Converted CoFe LDH into Co1-xFexSe2 nanosheets composed of NiSe and CoSe2 phases by hydrothermal treatment of CoFe LDH, dimethylformamide, hydrazine and selenium powder at 200 ° C for 6 H[84]. Co0.4Fe0.6Se2 nanosheets with a thickness of 27.3 nm were obtained by selenization at a Co/Fe ratio of 0.6. Compared with the untreated CoFe LDH nanosheets (thickness of 63.9 nm), this thinner and porous structure can expose more active sites in the electrolyte and provide a fast transfer path for electrons/ions. It has an overpotential of 217 mV at a current density of 10 mA·cm-2, a Tafel slope of 41 mV·dec-1, and can operate stably for 72 H. Zhang et al. Synthesized CoFeP nanosheets by NaH2PO2 treatment of CoFe LDH[85]. The XPS results show that the phosphating treatment of CoFe LDH not only makes the surface contain oxygen vacancies, but also produces high valence Co. Transition metal nitrides are one of the ideal materials to replace noble metals in the field of electrocatalysis because of their good electrical conductivity, excellent corrosion resistance (especially in alkaline solution) and hardness. Li et al. Successfully constructed a CoFeNx porous nanosheet structure by heat-treating CoFe LDH at high temperature in an atmosphere of NH3/Ar (Fig. 9 a – C)[86]. The overpotential at a current density of 50 mA·cm-2 is 259 mV, which is better than 361 mV of CoFe LDH. The intrinsic enhancement mechanism is that the :(1)CoFeNx is composed of Fe4N and Co2N, and the tight interfacial interaction makes it easy for electrons to transfer from Fe4N to Co2N, which optimizes the adsorption energy of reactants and intermediates, thus improving the catalytic activity. The porous structure of (2)CoFeNx enlarges the specific surface area of the material and provides high-density active sites for OER reaction. It can be seen from the above that the process of selenization, phosphination and nitridation is to introduce non-metallic elements into transition metal compounds, increase the density of States near the Fermi level of the material, optimize the electronic structure of the material, and make it have an electronic structure similar to that of precious metals such as Ir and Ru, which is conducive to the adsorption of reaction intermediates, thereby improving the reactivity of OER.
In addition, electrochemical oxidation is also one of the methods to improve the composition and structure of materials. Zhong et al. Prepared partially oxidized CoFe LDH (PO-CoFe-OH) by electrochemical oxidation of CoFe LDH[87]. The obtained material has an overpotential of 365 mV when the electrolyte is 0.1 M KHCO3 and the current density reaches 10 mA·cm-2. The reaction mechanism of PO-CoFe-OH was analyzed by in situ Raman spectroscopy, and it was found that the OER reaction mechanism of PO-CoFe-OH was not the traditional AEM, but LOM. The LOM mechanism requires direct O-O coupling, so the energy barrier in the OER reaction process is low, so the catalytic activity can be effectively improved. However, the direct electrochemical oxidation of CoFe LDH can only act on the near-surface region of the material. Han et al. Used the method shown in Fig. 10a to electrochemically oxidize CoFeP @ C synthesized through a series of steps as a precursor to obtain highly active amorphous CoFeOOH @ C nanosheets[88]. The amorphous structure of CoFeP and the Co3+, Fe3+, and oxygen vacancies generated by electrochemical oxidation increase the active sites of the material and improve the OER activity. The overpotential at a current density of flow 10 mA·cm-2 is 254 mV, the Tafel slope is 33 mV·dec-1, and the corresponding double layer capacitance Cdl is 99.4 mF·cm-2 (Fig. 10 B, C). On the one hand, the electrochemical oxidation oxidizes Co or Fe in CoFe LDH to a high valence state, thereby increasing the number of active sites of the material; On the other hand, a large number of defects can be generated on the surface of CoFe LDH, which can serve as additional active sites to further promote the OER reaction.
To sum up, strategies such as intercalation exfoliation, vacancy fabrication, material recombination, ion substitution and derivatives can effectively improve the electrocatalytic performance of CoFe LDH, and their corresponding advantages and disadvantages are shown in Table 2. Therefore, it is still a hot research topic to synthesize CoFe LDH-based electrocatalysts with superior performance and good stability through a simple, low-cost and efficient method by combining various modification strategies.
5 Conclusion and prospect
CoFe LDH has many advantages, such as unique two-dimensional sheet structure, rich and variable chemical components, highly dispersed metal cations, excellent stability, and low cost, which make it promising for electrocatalytic OER reaction. However, the limited concentration of active sites and low conductivity limit its large-scale application. In this paper, the structural characteristics of CoFe LDH were introduced and the corresponding OER reaction mechanism was briefly described. The modification strategies of CoFe LDH in recent years are summarized in detail, including intercalation and exfoliation, vacancy fabrication, material recombination, ion substitution and derivatives. On the one hand, the active sites of the material are increased and the adsorption energies of reactants and intermediates are reduced. On the other hand, the conductivity is improved, the electron transmission rate is increased, and the surface reaction is accelerated; The purpose of improving the electrocatalytic performance of the CoFe LDH is achieved. Although a series of progress has been made in improving the catalytic performance of CoFe LDH, there are still some problems restricting the industrial use of the catalyst.
(1) Generally, the electrolyte used in the OER test of CoFe LDH is alkaline, but the ideal electrocatalyst should still have high catalytic activity and stability in any pH electrolyte. The stability of CoFe LDH can be effectively improved by coating the surface of CoFe LDH with other materials such as carbon materials, metal oxides or organic materials to form a multi-level structure and avoid direct contact with the electrolyte.
(2) At present, the OER reaction mechanism of CoFe LDH is still not clear, for example, the mechanism in the reaction process is AEM or LOM, and the active material is Fe or Co, which are still controversial. Advanced characterization instruments such as spherical aberration scanning transmission electron microscopy and synchrotron radiation, as well as in situ characterization instruments such as in situ XPS and environmental transmission electron microscopy, are used to monitor the changes of phase structure, surface and interface atoms and electronic structure of materials during the reaction process, and to observe the real active sites, so as to provide new knowledge and understanding for the catalytic reaction mechanism.
(3) Theoretical calculation is helpful to design new catalysts with optimal structure, and special chemical components and specific chemical structure are helpful to reduce the adsorption energy of reaction intermediates. However, there may be a deviation between the theoretical assumption of catalyst structure and activity and the actual kinetic relationship. Therefore, the development time of high-performance CoFe LDH-based electrocatalysts can be shortened by combining theoretical calculations with experimental methods to construct a perfect relationship between structure and activity. It is also beneficial to further explore and analyze the electron transport and surface reaction of OER reaction.
(4) At present, the main index to evaluate the catalytic performance of CoFe LDH-based catalyst is its overpotential at a fixed current density, but it will be affected by electrode materials (such as NF compared with GC), catalyst loading and electrode preparation process (such as self-growth and the use of adhesives), so it is still an important issue to reasonably regulate the evaluation criteria of material performance.
(5) It is very important to explore a simple, inexpensive and high-yield technology to prepare high-performance CoFe LDH-based electrocatalysts for their industrial application.
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