Strategies for the enhanced water splitting activity over metal–organic frameworks-based electrocatalysts and photocatalysts
Graphical abstract
Introduction
With the quick expansion of human population and industrialization, the energy demand is continuously increasing, and thus the increased consumption of fossil fuels, which consequently causes severely environmental issues, including greenhouse effect, air and water pollution. Owing to the limited reserves of traditional fossil fuels, it is significant to explore clean and sustainable energy sources. Among the variety of energy carriers, hydrogen comes into play as a promising one, with the merits of high energy density as well as renewable and carbon-free features. Therefore, many efforts have been devoted to hydrogen production by researchers. Currently, coal gasification, steam reforming, and water splitting are the main three strategies for industrial hydrogen generation [1,2]. Majority of the hydrogen production depends on coal gasification and steam reforming through the reaction between non-renewable fossil fuels and water. However, the reduction products of these two methods always contain CO or CO2, which not only require further separation for the high-purity hydrogen but also make the greenhouse effect worse. Moreover, both the reactions are energy extensive that always need high temperature and pressure [3]. In comparison, water splitting can be the much-anticipated strategy to produce hydrogen, which can be conducted at ambient conditions using water as an abundant and renewable source to produce environmentally preferred productions (H2 and O2). Despite the above advantages, the practical application for massive hydrogen generation through water splitting is severely hindered by its strong uphill that requires a large power inputting to overcome [4]. Moreover, the two half reactions of water splitting, known as hydrogen evolution reactions (HERs) and oxygen evolution reactions (OERs), are sluggish in kinetics that always require catalysts to boost them. Nowadays, water splitting for hydrogen production over the catalysts by inputting solar or electrical energy is regarded as a sustainable route, and much attention has been given to this field [[5], [6], [7], [8], [9], [10], [11], [12]].
In the case of electrocatalytic water splitting, the aim of the electrocatalysts utilization is to lower the reaction overpotential and accelerate the reaction rate as much as possible, which will then reduce the overall electric power consumption. In particular, Pt-based materials display superior HER activity and Ir- or Ru-based compounds can trigger the OER process at a relatively low overpotential, but their high cost, scarcity, and low durability severely limit their widely practical application [13,14]. Therefore, it is of great significance to design and synthesize highly active and durable alternatives. Fortunately, great progresses have been made. Until now, a variety of electrocatalysts based on earth-abundant metals, including phosphides, oxides, (oxy)hydroxides, nitrides, borides, carbides, and chalcogenides, have obtained appealing performance for HER and OER [3,15,16]. Solar-driven water splitting is another promising route to generate hydrogen, because sunlight is a kind of ‘free’ natural energy source and practically inexhaustible. Consequently, tremendous photocatalysts have been fabricated and demonstrated to exhibit efficient water splitting performance under light irradiation [[17], [18], [19]]. Among them, TiO2 is the most popular photocatalytic material for water splitting and has been extensively studied with the merits of remarkable stability, low cost, and environmental friendliness [20]. Nevertheless, the conversion of solar energy to chemical energy occurs only under the ultraviolet (UV) irradiation for TiO2 because of its wide bandgap of 3.2 eV. The fraction of UV irradiation in the entire solar energy only accounts for about 5%, whereas which is some 45% for visible light, and 50% for near-infrared (NIR) light. Therefore, developing new visible-light-driven photocatalysts or broadening the light absorption range of the existing semiconductor is necessary and meaningful. As HER and OER processes commonly take place on the exposed surface of the catalysts, the electronic structure of the facial elements and geometric structure of the catalysts are of great importance to their performance [[21], [22], [23], [24]]. As a result, materials with the porous structure are extremely appealing toward water splitting. On this occasion, metal–organic frameworks (MOFs) stand out as distinguished candidates for electrocatalytic and photocatalytic water splitting.
Except MOFs, there are kinds of porous materials for efficient hydrogen generation from water splitting, including carbon-based [[25], [26], [27]] and zeolitic [28] materials, so why MOFs are more appealing, because all of the porous materials exhibit well-developed porosities with high specific surface area for efficient mass transport and active site exposure? The main reason can be described as follows: (1) The porous structure that is crucial to water splitting in MOFs is periodically assembled and can be precisely controlled [29,30]. (2) The versatility in the basic construction unit (various metal ions and organic linkers) of MOFs endows them with versatility in composition and geometric structure [[31], [32], [33]], providing multiple possibilities to design and synthesize MOFs with suitable composition and functionality as outstanding water splitting catalysts. Consequently, the bandgap of MOFs, as well as the electronic structure of active centers, can be tailored through the organic ligand modification or/and metal center tuning, which can readily achieve the maximized electrocatalytic and photocatalytic performance. Moreover, the metal nodes in MOFs are highly separated by the ligands, which can provide tremendous single atom-like reaction sites for intermediate adsorption. Recently, the application of MOFs-based materials for water splitting can be divided into three types, including pristine MOFs direct utilization, MOFs-based composites construction, and MOFs-derived compounds formation [34]. Compared to the former two types of MOFs-based materials, the compounds derived from the MOFs cannot inherit the ordered and porous structure from MOFs, and the aggregation of highly dispersed metal sites will occur in the case of calcination treatment at high temperature as well as the loss of intrinsic active sites of MOFs. Moreover, it also makes the synthetic process to be complex. In this regard, the interest in the application of pristine MOFs and MOFs-based composites as electrocatalysts and photocatalysts toward water splitting is continuously increasing. Accordingly, a comprehensive review is desirable to summarize the recent progresses and challenges of pristine MOFs-based composites for electrocatalytic and photocatalytic water splitting. Herein, this review delineates the fundamentals of water splitting and several decisive effects on their activity as well as the strategies to boost their performance. Then, particular attention is oriented toward the significant examples of the application of the proposed strategies. Finally, the current challenges are heightened and the possible developing direction is proposed.
Section snippets
Electrocatalytic water splitting
Electrocatalytic water splitting is composed of two basic half reactions of cathodic HER and anodic OER (Fig. 1). Theoretically, the cell voltage for water splitting is 1.23 V under standard conditions. However, a high potential that is higher than the theoretical value is always required to initialize the water splitting, because of the existence of serial resistance and activation barrier of the electrode [35]. The difference between the utilized potential and the theoretical value is
Advantages of MOFs for catalysis
MOFs are a kind of porous materials with infinite lattices assembled from metal centers and organic ligands (Fig. 4) [113,114]. Benefitting from their unique feature, MOFs have appeared in various energy conversion application, such as oxygen reduction reactions [[115], [116], [117], [118]], metal–air batteries [119], CO2 reductions [[120], [121], [122], [123]], N2 reductions [[124], [125], [126]], as well as water splitting. The following sections will detailedly elucidate the favorable
Electrocatalytic water splitting over MOF-based electrocatalysts
It is aforementioned that using MOFs as catalysts for water splitting application has triggered much interest because of their unique compositional and structural features. Up to now, MOFs have been considered as one of the most promising electrocatalysts for water splitting, because of their unique structural and compositional features, and great progresses have been made. Despite this, the electrocatalytic water splitting activity of MOFs is still insufficient and only a few pure MOFs or
Photocatalytic water splitting over pristine MOFs-based photocatalysts
Since the first example of MOFs as photocatalyst for HER, more and more efforts have been devoted to this field [265]. Despite this, pristine MOFs still suffer from the limited light harvesting and short lifetime of photogenerated carriers. Fortunately, compared with the conventional inorganic semiconductor, the nascent MOFs photocatalysts exhibit several merits that can make them with better photocatalytic properties, including high porosity, high designability, and high crystalline [34,266].
Conclusions and prospects
In this review, the strategies for the enhancement of electrocatalytic and photocatalytic water splitting over MOFs-based catalysts have been summarized. Thanks to the variety of basic construction units of MOFs, developing highly catalytic sites can be achieved easily through using suitable metal nodes and ligands to fabricate MOFs. Benefitting from their large specific surface area and optimal pore, not only favorable mass transport and evolved gas release can be guaranteed, but also numerous
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.
Acknowledgment
This work was supported by the National Natural Science Foundation of China (51632008, 61721005).
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