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

Abstract

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 example, a method followed by annealing was used to prepare NiO catalysts by and co-workers (Figure It is worth noting that by the addition of cerium two of catalysts can be one with CeO2 in NiO matrix and the other CeO2 NPs and co-workers a simple two-step process to prepare on the annealing step, the Ni from the Ni substrate into the deposited CeOx which can abundant oxygen into Similarly, by a facile postannealing treatment, with ions was converted into CeO2 with as catalytic Li and prepared a novel with abundant oxygen vacancies via a facile two-step followed by a process, and synthesized a with abundant surface oxygen vacancies and It is that Ni and co-workers a novel CeO2/LaFeO3 hybrid via a facile chemical bath method. to the strong interaction between the two from LaFeO3 to CeO2 was which can promote catalytic TMCs TMCs mainly transition metal and transition metal These materials are of interest of their excellent electrical conductivity and chemical resistance However, the of the intermediates (M–OH and M–OOH) in OER is to low of is of to improve their intrinsic catalytic MOF-derived and electrodeposition are and common synthetic MOF-derived method is for the synthesis of cobalt using CoS as a for the in situ generation of CeOx Tang and obtained a hollow CeOx/CoS hybrid as shown in Figure the the were dispersed in ethanol of followed by refluxing at temperature to form amorphous CoS hollow CeOx NPs were in situ and anchored on the surface of CoS hollow via an in situ surface process, in which the CoS and was heated at 180 °C. they prepared a with a structure to through a process, that long CeO2 were as templates to induce the and of In addition, and a novel 2D "senbei"-like carbon nanosheets with a 2D as precursor (Figure After heating the at high precursors were obtained. Then precursors were in ethanol with the addition of in an bath to form "senbei"-like which can provide a large specific surface area and the electron transmission By a hybrid as a Huang and a mesoporous which abundant active sites and oxygen Moreover, for the synthesis of Ce-based cobalt a reduction method was applied by and co-workers to obtain CeO2/CoSe2 using as a and a as the site for CeO2 Figure 3 | (a) Illustration of the fabrication process of the hybrid nanostructure Reprinted with permission from ref Copyright 2018 Wiley-VCH. (b) The synthesis route of Reprinted with permission from ref Copyright 2020 Royal Society of Chemistry. (c) The formation process of Co/Ce–Ni3S2/NF Reprinted with permission from ref Copyright 2020 University and of (d) Synthesis process of the Co4N–CeO2/GP Reprinted with permission from ref Copyright 2020 Wiley-VCH. (e) Schematic illustration of the of CoP/CeO2 Reprinted with permission from ref Copyright 2020 Download figure Download PowerPoint Unlike the above Ce-based cobalt the Ce-based nickel are synthesized by an electrodeposition and hydrothermal For instance, and synthesized a Ce-doped Ni3S2 electrode with NF as the substrate by a facile one-step electrodeposition method, in which the Ce could be by the of Tang and co-workers a facile one-step hydrothermal method to synthesize Co and Ce Ni3S2 nanosheets on NF (Figure which can more sites and enhance electrical conductivity over pure Ni3S2 Similarly, through a facile sulfidation and in situ generation process, CeOx hollow nanotubes were synthesized on the to form TMPs TMPs and have been for OER owing to their in the presence of the of TMPs is to promote its OER and is an efficient In the synthesis of the nitridation and process of precursors is The precursors can be synthesized by various methods, and then by to form metal treatment is the necessary in the fabrication of metal Du and obtained a novel hybrid nanosheet array on a via a two-step route (Figure First, through the facile anion electrodeposition method, a nanosheet precursor was obtained. followed by nitridation of Co(OH)2 a high the hybrid was converted into with CeO2 Additionally, and synthesized hollow nanosheets through a solvothermal process followed by subsequent nitridation For the synthesis of metal is used the For example, a hybrid structure of [email protected]2 into protected]2 was synthesized via a process, including the hydrothermal reaction, phosphorization, and and prepared a CoP/CeO2 through a simple two-step route (Figure facile solvothermal reaction was used to obtain the and then followed by a treatment at low the was into CoP/CeO2 in which abundant oxygen vacancies and more active sites were introduced. Similarly, using precursors with a process has been to be efficient method to synthesize Ce-based Additionally, Pan and prepared a hybrid electrode by a simple electrodeposition and method. The a unique which the and more active sites. a strategy was

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