Electrochemical water splitting is an appealing and promising approach for energy conversion and storage. As a key half-reaction of electricity-driven water splitting, the oxygen evolution reaction (OER) is a sluggish process due to the transfer of four protons and four electrons. Therefore, development of low-cost and robust OER electrocatalysts is of great importance for improving the efficiency of water splitting. Based on the merits of high surface area, rich pore structure, diverse composition and well-defined metal centers, metal–organic frameworks (MOFs) and their derivatives have been widely exploited as OER electrocatalysts.
Professors Li Fei of Dalian University of Technology and Academician Sun Licheng of Westlake University, among others, have reviewed the research progress of MOFs and their derivatives in the field of oxygen evolution reaction (OER) electrocatalysts. The review focuses on the design principles, synthesis methods, and catalytic performance of MOF-based materials. Additionally, it discusses the structure-performance relationships between MOFs, their derivatives, and OER, providing valuable insights for the rational development of efficient OER catalysts. Finally, it addresses current scientific and technological challenges as well as future prospects for sustainable industrial applications.
Graphic and Textual Overview
This work summarizes the recent progress of MOF-based OER catalysts in terms of their design principles, characterization, activity and catalytic mechanism. As depicted in Scheme 1, this review begins with the introduction of the categories and advantages of MOFs and their derivatives as OER electrocatalysts. The available methods for loading MOFs on electrodes and the factors affecting their catalytic performance are then presented. In the third part, the derivatives of MOFs and their electrochemical performance are systemically summarized. Finally, the challenges and opportunities towards water splitting with MOF-based materials are discussed.
MOF-based powdery electrocatalysts
MOF-based self-supported electrocatalysts
MOF-derivatives as electrocatalysts for the OER
Conclusions and outlook
1. the large surface area of the MOFs facilitates the exposure of more active sites to the electrolyte.
2. the highly versatile and well-defined porous structure of MOFs is favorable for electron transfer and mass transport.
3. the well-defined and tailorable structures for MOFs offer a promising possibility for designing functional materials with controlled morphologies.
4. the diversified compositions of MOFs allow heterometal doping, and the synergistic effects between multiple metal atoms are essential for high OER catalytic activity.
Benefiting from these merits, powdery MOFs and the integrated binder-free MOF electrodes have been successfully fabricated and showed high OER performance. In addition, MOFs are also suitable precursors/templates for the preparation of various derivatives, such as MOF-derived metal–carbons, metal oxides and phosphides/sulfides/selenides. These derivatives inherit the pristine microstructure of MOFs and simultaneously have the advantages of excellent chemical and mechanical stability. Although great advances have been made for MOF-based OER electrocatalysis, some key issues and challenges remain to be solved in order to develop efficient energy conversion materials in this field:
1. although thousands of MOFs have been reported, most of them suffer from poor stability under harsh reaction conditions, which severely restricts their scale-up applications. Therefore, novel design strategies to broaden the availability of MOFs is necessary.
2. The inherent conductivity is a major concern for practical applications of MOFs. Although coupling MOFs with 3D conductive metal substrates has proven to be an effective approach for the improvement of conductivity, it is not a general strategy applicable for the majority of MOFs. The fabrication of ultrathin MOFs is the other feasible strategy to enhance the conductivity of MOFs. While the current preparation
methods rely on top-down and bottom-up methods, other emerging methods in the efficient synthesis of ultrathin MOFs should be developed. Moreover, the functionalized modifications of MOFs by grafting conjugated organic linkers inside MOFs through coordination chemistry, or combining MOFs with conductive carbon materials such as carbon nanotubes, carbon nanosheets and graphene during the synthetic process are also means for improving the conductivity of MOF-based electrodes.
3. The catalytic mechanisms of MOFs for the OER are still poorly understood. In the recent work, only a few have suggested that the in situ formed metal hydroxides/oxyhydroxides were the real active speciesfor the OER. It is necessary to use the in situ/operando characterization techniques to study the main intermediates involved in the OER. Furthermore, theoretical modeling together with the well-established calculation methods are powerful tools to gain insights into the underlying catalytic mechanisms.
4. The electrocatalytic behaviors of MOFs for the OER are typically performed in alkaline media due to the fast reaction kinetics. However, the replacement of corrosive alkaline solution with neutral media is desirable in terms of cost and safety. Therefore, the development of efficient and stable MOFs for OER electrocatalysis under near neutral conditions is highly significant though still a big challenge.
As for the derivatives of MOFs, the following aspects should be addressed:
1. only limited precursors such as ZIF-67 and PBAs have been widely used for the OER, and exploring other efficient MOFs as precursors/templates through encapsulating active metal nanoparticles or organometallic complexes into MOFs would be useful to extend the diversity of MOF derivatives.
2. High temperature is often required for the preparation of MOF-derived metal–carbon materials. As we know, the high temperature is a harsh condition for the synthesis of materials. The loss of organic ligands caused by high-temperature calcination can lead to the easy sacrifice of active sites and the collapse of the original pore structures, thus adversely affecting the corresponding catalytic activity. Therefore, it is meaningful to replace this high-energy consumption procedure with more mild conditions in view of large-scale application.
3. Single atom catalysts with uniformly distributed metal sites at an atomic scale have emerged as a highly promising category to maximize the utilization of the individual metal atom. In spite of considerable progress made for electrocatalytic reduction reactions, the metal nodes in MOFs are inclined to aggregate into nanoparticles in MOF-derived OER electrocatalysts during the process of high-temperature pyrolysis. Therefore, establishing suitable synthetic strategies for MOF-derived OER single atom catalysts would be the next hotspot in the field of water splitting.