Materials Today Chemistry
Zeolitic imidazolate framework-67 derived cobalt-based catalysts for water splitting
Introduction
The energy shortage and environmental pollution caused by fossil fuel combustion are the major threats to the sustainable development of modern society. An ongoing effort was there to search for sustainable, clean, and high-efficiency energy sources to reduce the consumption of traditional fossil fuels. The development becomes urgent for the clean energy conversion and storage devices with high efficiencies, such as fuel cells [[1], [2], [3]], solar energy cells [[4], [5], [6], [7], [8], [9], [10], [11], [12]], water splitting apparatus [[13], [14], [15], [16], [17], [18], [19], [20], [21]], and batteries [[22], [23], [24], [25]], to fulfill the requirements of the sustainable development, energy conversion efficiency, and intermittent energy application. Tidal, wind, and solar energy are often considered good choices for clean energy in the past decades, but their intermittent nature has limited their practical application and further development [26,27]. Among the various selectivity, water splitting hydrogen production as the core medium infrastructure to connect clean energy and consumption terminals can achieve a clean energy consumption society. The high-efficiency production of hydrogen in a green way is one of the key factors of the hydrogen economy, which is mainly limited by the slow kinetics of the electrocatalytic processes constituted by hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) [[28], [29], [30], [31], [32], [33], [34], [35], [36], [37]]. The HER and OER reaction processes are strongly dependent on the inherent (electro)chemical (potential) and electronic properties of the electrode surface. Proper bonding forces of the catalyst with water, intermediates, hydrogen, and oxygen determine the catalytic activity and efficiency to improve sluggish reaction processes [[38], [39], [40], [41]]. Therefore, it is necessary to design and produce advanced electrocatalytic materials with a high performance to accelerate the kinetic process.
Currently, the main research focus of electrochemical catalysts aims to clarify the influences of the following factors on water electrolysis [[42], [43], [44], [45], [46], [47], [48], [49], [50], [51], [52]]: (1) the intrinsic activity of the active sites. The improved intrinsic activity is usually achieved by optimizing intermediates adsorption via tuning the coordination environment around the active sites by the heteroatom doping effect, the introduced defects, interface engineering, and so on. (2) High surface area. Through the design and regulation of the final structure of catalysts to achieve larger surface areas to carry more active sites for improved performance. Nanowires, nanosheets, and hollow porous three-dimensional (3D) structures always provide more surface areas to expose active sites. (3) Particle size is also a critical factor for the electrochemical performance. Small-sized nanoparticles, nano-clusters, and even single atoms have been obtained to fully utilize active atoms through the control of particle size. (4) The efficiency of charge transfer in the electrocatalytic process will affect the reaction kinetics. Coupling with functional materials, such as graphene, C3N4, reduced graphene oxide (rGO), and so on, will facilitate the transfer of interface electrons, thereby promoting the electrocatalytic process. (5) Long-term stability of electrocatalysts during water splitting is also an essential factor for practical applications.
Precious metal-based catalysts fulfill most of the requirements as cathode and anode of water splitting devices like RuO2 [53], RhO2 [54], IrO2 [53], and Pt [[55], [56], [57]] with promising electrocatalysis activity. However, their further development has been limited due to the high price and low reserves. Transition group metal like Fe, Co, Ni, and Cu, with platinum-like d-orbital electrons and the high content in the earth's crust, makes the development of highly efficient transition metal-based catalysts as alternatives to precious metal catalysts be the most effective way. A series of transition metal-based materials have made a great progress in electrochemical energy conversion [18,[58], [59], [60], [61], [62], [63], [64], [65], [66]].
As a kind of transition metal material, metal organic frameworks (MOFs)-derived materials have received much attention and have been developed in many kinds of electrocatalysis, including HER and OER, due to their excellent structure, large specific surface area, and porosity [[67], [68], [69], [70], [71], [72], [73], [74], [75], [76]]. The regular arrangement of metallic and organic components presented within the crystalline structures of MOFs leads to a uniform distribution of different components in the subsequently obtained MOF-derived hybrid structures. Zeolitic imidazolate frameworks (ZIFs), a sub-family of MOFs consisting of metal-imidazole-metal moieties with various transition metals (Co, Zn, Ni, etc.), have 3D topological structures similar to aluminosilicate zeolites [77]. ZIFs can combine the advantages associated with MOFs and zeolites by providing high crystallinity, ultrahigh surface area, exceptional mechanical stability, unimodal micropores, and designed functionalities. Whereas the low electron conductivity and poor chemical stability in the reaction medium limited the direct use of ZIFs in electrochemical processes [78]. However, beneficial from the abundant carbon and nitrogen ligands and high metal ion contents, ZIFs are expected to be ideal candidates as the precursor templates to design various metal-carbon hybrid materials to achieve a high active, electron conductivity, and stability for electrochemical water splitting [79].
Among them, ZIF-67 is assembled Co2+ ligand and 2-methylimidazole with a cubic crystal symmetry. ZIF-67 and its derivatives have attracted intensive attention for water splitting application due to the exceptional flexibility in structural/morphological engineering and composition control and superior intrinsic activity of Co. However, the poor conductivity of ZIF-67 limits its direct application as an electrocatalyst in water splitting. It should be noted that the imidazole group in ZIFs would be transformed into Nitrogen-doped carbon (NC) after simple calcination, which provides a carrier with good connectivity and results in optimization of the mechanical strength, electron transfer rate, and the improvement of chemical stability [[80], [81], [82], [83]]. Researchers have developed various derivatives of ZIF-67 by subsequent treatment of carbonization, sulfurization, phosphating, and other processes to obtain cobalt-based carbides, sulfides, phosphides, and selenides as efficient electrocatalysts for HER and OER [84,85]. The processed ZIFs materials are prone to produce hollow structures and larger surface holes, which is conducive to mass transfer and exposure to more active sites. Meanwhile, the designed functionalities make ZIFs-derived materials have more potential for tunability and durability in many applications than traditional precious metal catalysts via combination with secondary metal-ligand (including precious metal) and diverse structures. For further development, several issues of ZIF derivatives should be resolved. One of them is the unsatisfied stability of ZIF derivatives in acid electrolytes, especially under high current conditions, which limits their wide range of applications. Another issue is maintaining high activity under high currents to meet the needs of industrial applications. ZIF-67-derived cobalt-based materials, such as oxides, phosphides, sulfides, and selenides are described comprehensively. Furthermore, the improvements in the catalytic performance, such as low overpotential, fast kinetics, and increasing stability, can be achieved via several material optimization strategies, which include structure design, doping, interface engineering, and composite conductive substrate, as summarized in Fig. 1.
In this review, we begin with a brief introduction of the structure and synthesis methods of ZIF-67 to understand the structural characteristics and advantages as a precursor for water splitting. Then, we summarized the application of ZIF-derived hollow, porous carbon materials, including oxides/sulfides/phosphides, as electrocatalysts in electrolyzed water in recent years. ZIF-67-derived cobalt-based materials exhibit an excellent catalytic efficiency and stability by modifying the structure and micro-composition. Based on the result discussed, we also conclude the combining performance of key parameters and influencing factors for the catalytic reaction to clarify its potential connection. Finally, we have addressed our perspective and future challenges in the development of ZIF-67-based electrocatalysts for water splitting. Avoiding the collapse of the catalyst structure during the preparation and water splitting process to maintain high activity and durability is the main bottleneck of the derivatives.
Section snippets
Structure and synthesis of ZIF-67
Generally, the ZIFs consist of transition-metal cations (M) and imidazole-based ligands (lm), in which the M components are bridged with lm by the bonding angle of 145° in the form of M–lm–M [86,87]. The structure makes it possible to synthesize various extended 3D open frameworks with topologies similar to what is found in aluminosilicate [77]. ZIFs can be synthesized in various strategies, such as microfluidic synthesis [88], electrospray [89], sol-gel [90], mechanochemical preparation [91],
Application of ZIFs derivatives for water splitting
ZIF-derived materials often have a hollow porous structure that will provide a large surface area to expose more active sites, while reducing the diffusion distance of the electrolyte and accelerating the mass transfer process. In addition, the carrier of the derived material is NC, which has a good electrical conductivity and can prevent the aggregation of active sites. ZIF-67 contains a large number of monodisperse cobalt cations in the matrix, and cobalt is widely used in electrocatalysis as
Key performance-related parameters
Both HER and OER processes of splitting water involve the adsorption of reactants on the catalyst surface and the desorption of reaction products. Some performance parameters can help us evaluate whether the prepared material is suitable for use as a catalytic material for water splitting. The main performance-related parameters, such as overpotential at a given current density, Tafel slope, and BET, were briefly summarized in Table 1, Table 2, respectively, to have a comparative view of the
Conclusion and outlook
In this review, we have summarized recent developments of ZIF-derived nanomaterials as electrocatalysts for water splitting. The bond strength between the active center and the reactants becomes an important parameter to evaluate the catalytic performance because it influences the activation and desorption of reactants at the active site. ZIFs are promising precursors or templates for the preparation of various electrocatalysts to reach an exposure of specific sites capable of low energetic
Notes
The manuscript was written through the contributions of all authors. All authors have approved the final version of the manuscript, and declare no competing financial interest.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work is financially supported by the National Natural Science Foundation of China (Grant No. 51572166 and 52172071), the Rare and Precious Metals Material Genetic Engineering Project of Yunnan Province (202002AB080001-1), and the Shanghai Key Laboratory of High Temperature Superconductors (No.14DZ2260700). Bin Liu and Wenxian Li acknowledge research support from the Program for Professors of Special Appointments (GZ2020012 and TP2014041) at Shanghai Institutions of Higher Learning.
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