A Review of Transition Metal Oxygen-Evolving Catalysts Decorated by Cerium-Based Materials: Current Status and Future Prospects

Open AccessCCS ChemistryMINI REVIEW1 Jan 2022A Review of Transition Metal Oxygen-Evolving Catalysts Decorated by Cerium-Based Materials: Current Status and Future Prospects Yanyan Li, Xinyu Zhang and Zhiping Zheng Yanyan Li Department of Chemistry, Guangdong Provincial Key Laboratory of Energy Materials for Electric Power, Southern University of Science and Technology, Shenzhen 518055 Key Laboratory of Energy Conversion and Storage Technologies, Southern University of Science and Technology, Ministry of Education, Shenzhen 518055 , Xinyu Zhang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Department of Chemistry, Guangdong Provincial Key Laboratory of Energy Materials for Electric Power, Southern University of Science and Technology, Shenzhen 518055 Key Laboratory of Energy Conversion and Storage Technologies, Southern University of Science and Technology, Ministry of Education, Shenzhen 518055 and Zhiping Zheng *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Department of Chemistry, Guangdong Provincial Key Laboratory of Energy Materials for Electric Power, Southern University of Science and Technology, Shenzhen 518055 Key Laboratory of Energy Conversion and Storage Technologies, Southern University of Science and Technology, Ministry of Education, Shenzhen 518055 https://doi.org/10.31635/ccschem.021.202101194 SectionsAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Non-noble metal catalysts are suitable for the oxygen evolution reaction (OER) owing to their original oxidation states and oxygen coordination environments, which can regulate the adsorption of OH− at the active sites to facilitate the formation of oxygen-containing intermediates. However, the difficulties encountered in the conversion of intermediates (M–OH, M–O, and M–OOH) lead to low efficiency. Decorations of transition metal catalysts with foreign elements are regarded effective solutions, among which decoration with Ce-based materials (CeBM) is the most prominent. This review investigates the current status and future prospects of CeBM-decorated transition metal electrocatalysts. By presenting a thorough account of the latest development, we aim to set a common ground for the research community for a deeper understanding of the roles of CeBM that originate from its unique electronic structure and abundant oxygen vacancies. Moreover, we wish to provide our own perspectives as to how to further the design of Ce-based OER electrocatalysts and where such catalysts may be applied in fields beyond electrocatalysis. Download figure Download PowerPoint Introduction The increasingly serious global energy crisis and environmental pollution issues have urged people to find alternative, clean, and sustainable energies.1 Hydrogen gas is arguably the most ideal alternative as it is renewable and possesses large reserves and high gravimetric energy density.2 Electrochemical water splitting to produce H2 is considered one of the simplest hydrogen production methods.3–6 However, large-scale production of H2 by this means is greatly hindered by the sluggish kinetics of the oxygen evolution reaction (OER) happening at the anode whereby O–H bonds are broken and accompanied with the formation of the O–O bond. The commonly accepted steps involved in the OER process are as follows: M OH − ( 1 ) M − OH OH − ( 2 ) M − O OH − ( 3 ) M − OOH OH − ( 4 ) M + O 2 (1)where M denotes the active sites. The energy profile of this process is mainly determined by the energy barrier encountered in the formation and transformation of the three intermediates, namely M–OH, M–O, and M–OOH.7 The high energy barrier is the direct cause of high onset potential, high overpotential, and the resulting slow kinetics. It is believed that the overall energy scheme is largely determined by the binding energy of the M–O intermediate.7,8 Therefore, the key to solving this problem is to develop suitable OER electrocatalysts to optimize the binding energy of the intermediate, reduce the energy barrier of the reaction, and improve the catalytic performance.9–11 Although significant progress has been made toward this goal, as judged by the common criteria of a good OER catalyst, including low onset/over potential, small Tafel slope, high stability, high electrical conductivity, low resistance for mass and charge transfer, high electrochemically active surface area (ECSA), and high long-term stability or reusability, efforts for further development are warranted. Noble metal catalysts (RuO2 and IrO2) show excellent OER catalytic performance,12,13 but widespread commercial applications are greatly hindered by their low durability and the high cost associated with the scarcity of these metal elements. Therefore, it has become a research focus in recent years to find high-efficiency, stable, renewable, and inexpensive non-noble metal catalysts to replace noble metal catalysts.14 Previous studies have found that as a category of non-noble metal catalysts, transition metal catalysts exhibited various oxygen-evolving properties,15 especially, transition metal oxides (TMOs), transition metal chalcogenides (TMCs), transition metal pnictides (TMPs), and transition metal borates (TMBs) have attracted the most attention from researchers.16,17 In addition to the low cost, the variable oxidation states and diverse coordination modes of transition metal ions can be utilized to regulate the adsorption of OH− at the active sites and facilitate the subsequent formation and transformation of other oxygen-containing intermediates.18–20 However, the high energy barrier associated with the strong binding of M–O remains a significant challenge in the conversion between intermediates (M–OH to M–OOH) in these transition metal catalysts. Experimentally, this is reflected by the still-high onset and overpotentials. An effective way to improve the catalytic performance is to modify the transition metal catalysts with exogenous elements or compounds to optimize the electronic structure of the active site to reduce the reaction energy barrier and lower the overpotential.11,16,21 Based on its oxygen storage capacity (OSC) properties,22–24 CeBM as a catalyst and cocatalyst has long been the focal point and received remarkable attention in various catalytic applications,25–27 particularly, in OER.28–31 Determined by powder X-ray diffraction (XRD) and high-resolution transmission electron micrographs (HRTEM), there are three different types of Ce participations: CeO2, CeOx, and Ce-doped, and the catalytic functions of these materials can be attributed to the following: (1) Promoting electron transformation. The interaction between CeBM and the parent catalyst can serve as a pathway for electron transfer (d–f coupling effect and heterointerface pathway), enhancing the conductivity.32,33 (2) Regulating active sites. This effect can be divided into two parts: (a) the decoration of CeBM can promote the formation of more active sites of new valence, which have relatively high intrinsic catalytic activity34,35; (b) the binding energy of the intermediates and active sites can be optimized by CeBM, thus facilitating the adsorption and transformation of the intermediates.36,37 (3) Introducing oxygen vacancies. Oxygen vacancies from CeBM can be used as storage sites and departing pathway for O2, thus facilitating the decomposition of M–OOH for the evolution of oxygen.38,39 (4) Enhancing structure stability. CeBM could inhibit the oxidation and depletion of metal-active sites.40 Different from previous reviews focused on CeO2 functionalization in various electrocatalytic applications,24,28–30 this review is not meant to be comprehensive, but it does offer new insights more focused on electrocatalytic OER. First, we focus our review and discussion on various transition metal-based OER catalysts: TMOs, TMCs, TMPs, and TMBs have been thoroughly surveyed. Second, this work summarizes catalysts functionalized by CeO2, CeOx, and Ce-doped ions; these functionalized catalysts are collectively abbreviated as CeBM-TMs. Functionalization of a parent catalyst is not limited to its surface modification with the formation of interfaces, and the catalyst can also be functionalized with doping into its interior. Third, our particular interest and attention is given to their applications for OER, which is the more challenging and kinetically more sluggish reaction in the overall scheme of water splitting. Our discussion is centered more around the mechanistic understanding of Ce-functionalization than just the description of the commonly accepted mechanism or the introduction of various operando techniques in electrocatalysis. Finally, we offer our own perspectives as to how to achieve detailed mechanistic understanding of the catalytic OER by judicious design of Ce-decorated catalysts as well as how they may be applied to catalyze other types of reactions beyond OER (Figure 1). Figure 1 | Schematic diagram of CeBM-decorated transition metal oxygen-evolving catalysts: current status and future prospects. Download figure Download PowerPoint Synthetic Strategies CeBM-decorated transition metal oxygen-evolving catalysts provide the full extent of synergy between CeBM and the substrate or parent catalyst, promoting formation and transformation of intermediates, facilitating mass and charge transfer, and thus improving the overall reaction kinetics and efficiency. Generally, multiple strategies have been involved, such as solvothermal,41 precipitation,42 metal–organic framework (MOF) derivation,43 electrodeposition,44 self-assembly strategy,45 and so on. Below we have summarized and discussed the representative synthetic approaches based on different material types, including TMOs, TMCs, TMPs, and TMBs (Table 1). Table 1 | Summary of CeBM-Decorated Transition Metal Oxygen-Evolving Catalysts Synthesis Methods Catalyst Methods (Characteristics) Structure References TMOs LDHs FeOOH/CeO2 HLNTs Electrodeposition method (control the thickness of CeO2 layers) FeOOH/CeO2 HLNTs (length: ∼2 μm diameter: ∼330 nm) with nanotube structure 38 Ni4Ce1@CP Solvothermal method (facalitate formation of interfaces) CeO2 (∼4.5 nm, NPs) was dispersed on α-Ni(OH)2 nanosheets 36 CeO2–x–FeNi One-step method (provide abundant vacancies) CeO2–x–FeNi substrate with NiFeOOH nanosheet array 35 Ni–Fe–Ce–LDH Solvothermal method (Ce3+ ion concentration can be tuned) Ni–Fe–Ce–LDH hollow microcapsules structure (length: ∼1 μm) 46 Ce0.21@Co(OH)2 Ethanol refluxing and high-pressure microwave strategy (produce loose structure) CeO2 incorporated into the ultrathin Co(OH)2 nanosheets (length: ∼1.2 μm) 47 First-row TMOs CeO2/Co3O4 Solution-phase cation exchange method (retain precursor mophology) CeO2/Co3O4 nanotubes (length: ∼1–5 μm, diameter:150 nm) 48 Co3O4@[email protected]2 MOF-derived method (retain the polyhedron structure) Co3O4@[email protected]2 (diameter: ∼500 nm) with polyhedron structure 49 3DOM-CC-10 Template-assistant method (retain template structure) Co3O4/CeO2 on nanobranched networks with pore size of ∼200 nm 50 CeOx/CoOx Two-step electrodeposition procedure (tune the thickness and concentration of Ce ions) CeOx layer (thickness: ∼1.5 nm) on CoOx film (thickness: ∼1 μm) 51 Ce–NiO–E Sol–gel method (alter the particle sizes) CeO2 clusters/atoms doped NiO (particle size: 7.5 ± 1.4 nm) formed spherical structure 52 CeO2/LaFeO3 Chemical bath process (generate ideal interfaces) CeO2 (∼5 nm, NPs) deposited on LaFeO3 (10s to several hundred nanometers) 53 TMCs CeOx/CoS MOF-derived method (offer abundant active sites) CeOx (∼5 nm, NPs) dispered on hollow CoS (∼500 nm) with polyhedral structure 34 Co9S8/CeO2/Co–NC MOF-derived method (restrain 2D carbon layer coated structure) CeO2 (NPs) and Co9S8 on the substrate formed "senbei"-like structure 54 Co/Ce–Ni3S2/NF One-step hydrothermal method (enable in situ dopant of Ce ion) Co and Ce-codoped on Ni3S2 nanosheets on NF 55 N,Ce–CoS2 Electrodeposition method followed by sulfidation (control the length and thickness of nanosheets) Ce, N-doped-CoS2 nanosheets with an average size of 5 nm 56 CeO2/CoSe2 Polyol reduction method (facilitate the homogeneous generation of CeO2 NPs) CeO2 (∼3.5 nm, NPs) were grown on the CoSe2 nanobelts (width: ∼300 nm) 39 TMPs Co4N–CeO2/GP Electrodeposition followed by nitridation (introduce nitrate anion into the interlayers of GP) CeO2 (∼5 nm, NPs) are distributed around Co4N (with small rough pores) on GP 57 [email protected]2 Hydrothermal reaction, phosphorization, and electrodeposition (enable dual modulation of CoP) CeO2 (amorphous) adheres to the surface of CoP with uniform V doping 58 CoP/CeO2 Solvothermal followed by phosphidation (enable co-existence of Co(OH)3 precursor and CeO2 CeO2 (∼40 nm, NPs) coated on CoP nanosheets (width: ∼1 μm) with rough porous surface 59 TMBs 20CeO2/Co-Bi Chemical reduction method (at room temperature) CeO2 (<5 nm, NPs) coated on the surface of Co-Bi nanosheets with amorphous structure 60 TMOs Layered double hydroxides Layered double hydroxides (LDHs) are important TMOs known for their large catalytic activity area and good hydrophilicity, which are good for mass transfer and ion movement in OER applications.61,62 However, the strong interaction between intermediates and active sites often leads to sluggish kinetics and insufficient activity.63,64 CeBM-decorated LDHs have been widely studied in recent years owing to their significantly improved catalytic efficiency over pure LDHs. The main synthesis strategies focus on solvothermal65–67 and electrodeposition methods.68,69 The electrodeposition method is often used to deposit CeBM on parent LDHs. As an example, FeOOH/CeO2 heterolayered nanotubes (HLNTs) were obtained by Li and co-workers38 via electrodeposition at a specific current density and temperature (Figure 2a). ZnO nanorod arrays (NRAs) were used as a template, followed by electrodepositing CeO2 and FeOOH layers on the surfaces of ZnO. After removing ZnO, FeOOH/CeO2 HLNTs were fabricated. Zhao and co-workers70 constructed a three-dimensional (3D) self-supporting [email protected] LDH/CeOx electrode by electrodepositing CeOx nanoparticles (NPs) on NiFe LDH nanosheets with Ni foam (NF) as a substrate, in which abundant oxygen vacancies were introduced. Similarly, Du and co-workers71 prepared a self-supported 3D CeO2/Ni(OH)2 electrode through a controllable electrophoretic deposition strategy. Figure 2 | (a) The fabrication procedure of CeO2/FeOOH HLNTs-NF. Reprinted with permission from ref 38. Copyright 2016 Wiley-VCH. (b) Schematic illustration of the fabrication of hollow Ni–Fe–Ce–LDH microcapsules mediated by cerium doping in MIL-88A. Reprinted with permission from ref 46. Copyright 2020 Royal Society of Chemistry. (c) Illustration of the fabrication process of Cex@Co(OH)2. Reprinted with permission from ref 47. Copyright 2020 Elsevier. (d) Illustration of the fabrication process of the hybrid nanostructure CeO2/Co3O4. Reprinted with permission from ref 48. Copyright 2019 American Chemical Society. (e) Synthesis route for the Co3O4@[email protected]2 composites. Reprinted with permission from ref 49. Copyright 2019 Royal Society of Chemistry. (f) Diagrams of Ce–NiO–E and Ce–NiO–L. Reprinted with permission from ref 52. Copyright 2018 Wiley-VCH. Download figure Download PowerPoint Solvothermal is also an efficient approach to prepare CeBM-LDHs electrocatalysts. Huang and co-workers36 utilized carbon papers (CPs) as supports to prepare [email protected] electrocatalysts through a one-pot solvothermal method, in which abundant intimate Ni(OH)2–CeO2 interfaces were obtained. Similarly, a novel hollow 3D Ce-doped NiFe–LDH (Ni–Fe–Ce–LDH) microcapsules was prepared by Yan and co-workers via a one-step hydrothermal approach (Figure 2b). By adjusting the Ce3+/Fe3+ ratios, the morphologies of Ni–Fe–Ce–LDH microcapsules could be changed.46 Unlike the above common methods, a unique ethanol refluxing and high-pressure microwave strategy was applied by Chai and co-workers.47 Ce-doped hollow structures stacked with ultrathin Co(OH)2 nanosheets (Cex@Co(OH)2) were prepared by a simple ethanol refluxing and high-pressure microwave treatment. It is worth noting that the pretreatment of reflux is necessary to form loose structures for the attachment of Ce species. Then after short-time microwave heating, Ce species are incorporated simultaneously into the ultrathin nanosheets with full exposure of active sites and electron transfer (Figure 2c). In another work, Yang and co-workers synthesized high-valence Ni-doped CeO2–x covered with FeOOH nanosheets through a one-step synthesis. In detail, oxygen-vacancy-rich CeO2–x coated on carbon cloth (CC) served as the substrate, and in the presence of Ni2+/Fe3+, Ni was oxidized to higher valence states, initiated by H+ from the hydrolysis of Fe3+.35 Additionally, through a facile in situ self-assembly strategy, Tang and co-workers72 constructed Ce-doped NiFe–LDH nanosheets with reinforced electrochemical surface areas. First-row TMOs First-row TMOs such as iron, cobalt, and nickel oxides have been recently investigated as low-cost alternatives to RuO2 owing to their good conductivity and charge effect.73 The decoration of CeBM can further enhance their OER activity to meet the requirements of replacing precious metal-based electrocatalysts. These synthetic strategies involve template-assisted, solvothermal, electrodeposition, and postannealing treatment methods. As for the synthesis of Ce-based cobalt oxides, template-assisted method is the most widely used. For instance, novel CeO2/Co3O4 interface nanotubes were synthesized by Chai and co-workers via Cu2O nanowires as templates. As shown in Figure 2d, using a solution-phase cation exchange method, where CoCl2, Ce(NO3)2, and Na2S2O3 were used as precursors and etching agent, Cu2O nanowires fabricated by Fehling's reaction were converted into Ce(OH)x/Co(OH)2 nanotubes. Then by further thermal treatment, Ce(OH)x/Co(OH)2 was converted into CeO2/Co3O4 nanotubes.48 Similarly, Cu foam can also be used as a template to prepared CeO2 NPs decorated on Co3O4 nanoneedle arrays as an efficient electrocatalyst.74 In addition, MOFs as self-sacrificing templates have received tremendous attention.75–77 Zeolitic imidazolate framework (ZIF)-67 polyhedrons were carbonized at different temperatures to generate N-doped Co3O4@carbon (Co3O4@Z67-NT), and then through a facile hydrothermal process, CeO2 NPs were uniformly coated onto the surface of Co3O4@Z67-NT matrix to form porous Co3O4@[email protected]2 with an original polyhedron morphology (Figure 2e).49 In other research, using highly ordered 3D-poly(methyl methacrylate) (PMMA) structure as a template, "precursor [email protected]" monoliths were formed with Co and Ce nitrates and ascorbic acid in precursor solution. Then a heating treatment was applied to obtain the final 3D Co3O4/CeO2 interface electrocatalysts with abundantly-ordered multistage interconnected mesoporous channels.50 Different from the template-assisted method, Dai and co-workers32 prepared advanced Co3O4/CeO2 nanohybrids with CeO2 nanocubes anchored on Co3O4 nanosheets through a two-step solvothermal method. In the first step, CeO2 nanocubes were synthesized with Ce(NH4)2(NO3)6 as a precursor at 180 °C. Then Co(acac)3 was added to the autoclave and heated at 140 °C to obtain Co3O4/CeO2 nanohybrids with nanocubes-curly nanosheets morphology. By combining hydrothermal and annealing processes, Pan and co-workers78 constructed an urchin-like CeO2–CuCoO/NF electrocatalyst. In addition, an electrodeposition method was used to obtain a Ce-doped Co3O4 (Co3–xCexO4) electrode, where NF was used as a substrate for electrode position Co and Ce species.79 Similarly, by a scalable electrostatic spray deposition method, Zhang and co-workers80 prepared a thin film of Ce-doped CoOx (CoOxCe) on the surface of a carbon fiber paper (CFP), in which the doped Ce could induce CoOx to an amorphous structure. Moreover, some additional strategies such as reflux and thermal treatment,81,82 surfactant-assisted chemical route,83 and photochemical metal–organic deposition (PMOD)84 have also been applied to synthesize Ce-based cobalt oxides. Ce-based nickel and other oxides are also a focus, and postannealing treatment is often applied to the synthesis of these materials. As a typical example, a sol–gel method followed by high-temperature annealing was used to prepare CeO2-decorated NiO catalysts by Qu and co-workers (Figure 2f).52 It is worth noting that by changing the addition sequence of cerium precursor, two forms of CeO2–NiO catalysts can be obtained, one with CeO2 clusters embedded in NiO (Ce–NiO–E) matrix and the other featuring surface-loaded CeO2 NPs (Ce–NiO–L). Delaunay and co-workers reported a simple two-step dip-coating/annealing process to prepare defect-rich NiCeOx on NF. During the annealing step, the Ni from the Ni substrate diffused into the deposited CeOx film, which can introduce abundant oxygen vacancy defects into NiCeOx.85 Similarly, by a facile postannealing treatment, NiMoO4 with absorbed Ce3+ ions was converted into CeO2 NPs-decorated NiMoO4 nanosheets, with extensive defects as catalytic sites.86 Li and co-workers87 prepared a novel self-assembled two-dimensional (2D) NiO/CeO2 heterostructure with abundant oxygen vacancies via a facile two-step (solvothermal followed by calcination) process. Through a similar process, Bharali and co-workers88 synthesized a nanostructured CuOx–CeO2/C electrocatalyst with abundant surface oxygen vacancies and distinguished oxide–oxide/carbon interfaces. It is noteworthy that Ni and co-workers developed a novel CeO2/LaFeO3 hybrid via a facile chemical bath method. Interestingly, due to the strong interaction between the two phases, selective interfacial La diffusion from LaFeO3 to CeO2 was observed, which can promote catalytic performance.53 TMCs TMCs mainly include transition metal sulfides and transition metal selenides. These materials are of interest because of their excellent electrical conductivity and chemical resistance against acid/alkaline electrolyte.89 However, the bonding of the intermediates (M–OH and M–OOH) in OER is very strong, leading to low activity. Construction of CeBM–TMCs is of great significance to improve their intrinsic catalytic performance. MOF-derived and electrodeposition methods are primary and common synthetic strategies. MOF-derived method is universal for the synthesis of cobalt sulfides. Specifically, using ZIF-67 derived CoS as a carrier for the in situ generation of CeOx NPs, Tang and co-workers34 obtained a hollow CeOx/CoS hybrid nanostructure, as shown in Figure 3a. During the sulfidation, the as-formed ZIF-67 particles were dispersed in ethanol solution of thioacetamide (TAA), followed by refluxing at elevated temperature to form amorphous CoS hollow nanocages. Then, CeOx NPs were generated in situ and anchored on the surface of CoS hollow nanocages via an in situ surface coating process, in which the mixture containing CoS nanocages, Ce(NO3)3, and hexamethylenetetramine (HMT) was heated at 180 °C. Furthermore, they prepared a CeOx/[email protected]–CeO2NRs with a structure similar to Chinese tanghulu through a similar process, except that homologous long CeO2 nanorods (L–CeO2NRs) were chosen as hard templates to induce the nucleation and growth of ZIF-67.90 In addition, Ren and co-workers54 reported a novel 2D "senbei"-like Co9S8/CeO2/Co heterostructural nitrogen-doped carbon nanosheets (Co9S8/CeO2/Co–NC) with a 2D Co/Ce bimetallic ZIF as precursor (Figure 3b). After heating the Co/Ce-ZIF at high temperature, CeO2/Co–NC precursors were obtained. Then CeO2/Co–NC precursors were sulfided in ethanol with the addition of TAA in an oil bath (60 °C) to form "senbei"-like Co9S8/CeO2/Co–NC, which can provide a large specific surface area and shorten the electron transmission path. By utilizing a CoMo-bimetallic hybrid ZIF (HZIF) as a precursor, Huang and co-workers91 produced a mesoporous CoS/MoS2 polyhedron, which had abundant Co/Mo active sites and oxygen vacancies/defects. Moreover, for the synthesis of Ce-based cobalt selenides, a polyol reduction method was applied by Yu and co-workers to obtain CeO2/CoSe2 heterostructures using triethylene glycol (TREG) as a reducing agent and a CoSe2/Diethylenetriamine (DETA) complex as the nucleation site for CeO2 loading.39 Figure 3 | (a) Illustration of the fabrication process of the hybrid nanostructure CeOx/CoS. Reprinted with permission from ref 34. Copyright 2018 Wiley-VCH. (b) The synthesis route of Co9S8/CeO2/Co–NC. Reprinted with permission from ref 54. Copyright 2020 Royal Society of Chemistry. (c) The formation process of Co/Ce–Ni3S2/NF nanosheets. Reprinted with permission from ref 55. Copyright 2020 Tsinghua University Press and Springer-Verlag GmbH Germany, part of Springer Nature. (d) Synthesis process of the Co4N–CeO2/GP electrode. Reprinted with permission from ref 57. Copyright 2020 Wiley-VCH. (e) Schematic illustration of the preparation of CoP/CeO2 heterostructure. Reprinted with permission from ref 59. Copyright 2020 Elsevier B.V. Download figure Download PowerPoint Unlike the above Ce-based cobalt sulfides, the Ce-based nickel sulfides are usually synthesized by an electrodeposition and hydrothermal method.92 For instance, Wen and co-workers93 synthesized a Ce-doped Ni3S2 electrode with NF as the substrate by a facile one-step electrodeposition method, in which the Ce amounts could be controlled by changing the ratios of Ce/(Ce+Ni). Nevertheless, Tang and co-workers developed a facile one-step hydrothermal method to synthesize Co and Ce dual-doped Ni3S2 nanosheets on NF (Figure 3c), which can introduce more defective sites and enhance electrical conductivity over pure Ni3S2 nanosheets.55 Similarly, through a facile sulfidation and in situ generation process, CeOx NPs-decorated NiCo2S4 hollow nanotubes were synthesized on the CC to form CeOx/NiCo2S4/CC electrocatalysts.94 TMPs TMPs (nitrides and phosphides) have been explored for OER catalysis owing to their relative inertness in the presence of strongly alkaline electrolytes.95 Further increasing the electroconductivity of TMPs is expected to promote its OER performance, and CeBM-decoration is an efficient approach. In the synthesis of CeBM-TMPs, the nitridation and phosphorization process of precursors is very important. The precursors can be synthesized by various methods, and then assisted by high-temperature phosphorization/nitridation to form metal pnictides. Nitridation treatment is the necessary step in the fabrication of metal nitrides. Du and co-workers57 obtained a novel superhydrophilic Co4N–CeO2 hybrid nanosheet array on a graphite plate (Co4N–CeO2/GP) via a two-step route (Figure 3d). First, through the facile anion intercalation-enhanced electrodeposition method, a Co(OH)2–CeO2 nanosheet precursor was obtained. Then, followed by selective high-temperature nitridation of Co(OH)2 under a high purity NH3 atmosphere, the Co(OH)2–CeO2 hybrid was converted into Co4N–CeO2 with CeO2 unchanged. Additionally, Schuhmann and co-workers96 synthesized CeO2@Co2N hollow nanosheets through a solvothermal process followed by subsequent nitridation process. For the synthesis of metal phosphides, sodium hypophosphite (NaH2PO2) is typically used during the phosphorization. For example, a hybrid nanoarray structure of [email protected]2 integrated into CC ([email protected]2 NRA/CC) was synthesized via a three-step process, including the hydrothermal reaction, phosphorization, and electrodeposition.58 Fu and co-workers59 prepared a CoP/CeO2 heterostructure through a simple two-step route (Figure 3e). A facile solvothermal reaction was used to obtain the Co(OH)3/CeO2 precursor, and then followed by a selective phosphating treatment at low temperature, the Co(OH)3/CeO2 was transformed into CoP/CeO2 heterostructure, in which abundant oxygen vacancies and more catalytically active sites were introduced. Similarly, using Ce-containing ZIF-67 precursors coupled with a phosphorization process has been reported to be efficient method to synthesize Ce-based CoP.97–99 Additionally, Pan and co-workers100 prepared a FeP/CeO2–NF hybrid electrode by a simple electrodeposition and high-temperature phosphorization method. The as-obtained FeP/CeO2–NF possessed a unique uneven columnar structure, which increases the ECSA and provides more active sites. Also, a novel, controlled phosphating strategy was