Advanced Catalysts Based on Metal-organic Frameworks (Part 2)

Fossil fuels are non-renewable energy sources and may cause environmental pollution. One of the appropriate solutions is to develop clean and renewable sources of energy as an alternative to fossil fuels. Environmental pollution and lack of renewable energy sources are two significant problems affecting the current life of human society and economic progress. Researchers have addressed semiconductor-oriented heterogeneous photo-electrocatalysis, photocatalysis, and electrocatalysis by the fuel cells to solve these crises. Photocatalytic water splitting is a promising approach in resolving the energy crisis. This process involves harvesting solar light, charge transfer and separation, and evaluation of catalytic reactions of H2 and O2. In this regard, the main challenge is to find an efficient, environmental-friendly, cost-effective, and easily fabricated photocatalyst with high stability and corrosion resistance in different media. Thanks to their tunable structure, structural flexibility, high specific surface area, tunable pores, and unsaturated metal sites, metal-organic frameworks (MOFs) could be an efficient photocatalyst for hydrogen production under UV, NIR, and visible radiation. Therefore, MOFs and MOFs-based compounds are widely utilized as alternatives for expensive commercial catalysts developed based on rare elements such as Pt and Au. They can also be employed as precursors for the synthesis of different types of materials with different structures, sizes, and morphologies. This chapter summarizes MOF-based photocatalysts for the splitting of water are MOFs modification strategies.

Owing to the undeniable emission of anthropogenic CO2 emission into the atmosphere, the world has witnessed a continuous upsurge in the issue of global warming and energy insecurity. Numerous efforts have been adopted to alleviate these crises, but the most viable method is converting CO2 into value-added chemicals or fuels. Considering the cleanness of solar energy and the corresponding renewable energy sources, various novel classes of metal-organic framework materials were developed for CO2 photoreduction to energy-rich chemicals. This has made the study into different MOFs and MOF-based photocatalysts remain the hottest topics to date. The uniqueness of MOF materials over other photocatalysts includes their exceptional compositions, tuneability, larger surface areas, higher porosities, homometallic/heterometallic cluster as a secondary building unit, and diverse structural uniqueness. The development of these MOFs and MOF-based photocatalysts is essential to overcome the large and intrinsic thermodynamic barriers. Based on the considerable interest in these photocatalysts for CO2 reduction (CO2R), this chapter began with a brief insight into fundamental principles of photocatalysis, the process of photocatalytic conversion of CO2, thermodynamics aspects of CO2 photoreduction, mechanisms, and kinetics behind the photocatalytic CO2R. We further highlight some progress and the associated challenges with the applicability of MOFs and MOF-based photocatalysts for CO2R into energy-rich chemicals. Despite some challenges and hitches with MOFs for CO2 reduction, their future in combating global warming and energy insecurity is promising.

Electrochemical water splitting has received extensive attention and research due to its ability to effectively produce and store clean energy. Water splitting includes hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). The complex reaction mechanism of the two half-reactions leads to slow kinetics and high overpotentials, which need to be mitigated and reduced by increasing effective active sites and accelerating electron transfer. Hence, the development of favorable prices and robust electrocatalysts has become a research hotspot. Owing to a large specific surface area, regulatable chemical composition, pore structure, controllable topological structure, and clear surface function, metal-organic framework-based materials (MOFs) have been widely studied. Herein, we summarize relevant references in MOF-based materials with outstanding performance in water splitting and report the design, structure, and activity of a large number of MOF-based materials. In addition, great expectations are placed on the future development and application prospects of MOFbased electrocatalytic materials.

Oxygen reduction reaction (ORR) plays an important role in clean energy storage and conversion devices, such as metal-air batteries and fuel cells. However, the slow kinetic has impeded their large-scale applications. Hence, it is necessary to develop highly efficient electrocatalysts to accelerate the reaction rate of ORR. Owing to their ordered structure, ultrahigh specific surface area, abundant channel and functional adjustability, metal-organic frameworks (MOFs) and their derivatives were explored to catalyze ORR. In this chapter, we present a timely review of pristine MOFs and MOF-derived porous carbon-based materials as advanced electrocatalysts for ORR. We start with the introduction of the fundamentals of electrochemical oxygen reduction reaction (ORR), followed by surveying various forms of MOFs and MOFderived nanomaterials as advanced electrocatalysts towards ORR, including metal-free heteroatom-doped carbon-based materials, transition metal and nitrogen co-doped carbon (M-Nx-C), carbon-supported single-atom catalysts (SACs). Additionally, we briefly outline the challenges and prospects of this research filed.

Multifunctional catalysis attracts great interest due to the opportunity to apply one compound in different types of reactions, and particularly its role in energy conversion reactions, such as hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR). The requirements of several catalysts are combined in one molecule, which allows utilizing these compounds at both cathode and anode sides for a variety of energy devices. However, seeking optimal catalysts with multifunctional applications and appropriate activity and durability is a difficult task. One of the promising candidates is metal-organic frameworks due to their unique structure and high-specific surface area. Utilization of MOFs and their derivatives as multifunctional catalysts for HER/OER, OER/ORR, HER/ORR/OER, and corresponding energy provision technologies, such as water splitting electrolyzers, metal-ion batteries, is the top area of modern research. Herein, the recent examples of MOF-based electrocatalysts for HER/OER/ORR activity in terms of their bifunctionality and trifunctionality and further application at both cathode and anode sides of water electrolyzer and metal-ion battery are summarized.

Metal-Organic Frameworks are innovative materials that display interesting redox properties with multiple applications in electroanalytical chemistry and storage purposes. MOFs metal nodes present a redox pair (M2+/M3+) in the presence of alkaline electrolytes, which catalyse the electro-oxidation or a reduction of diverse kinds of molecules. This behaviour is used as the basic principle in the design of electrochemical sensors (modified electrodes) for the smart recognition and quantification of biomolecules and hazardous compounds by using inexpensive techniques such as voltammetry or chronoamperometry. In this regard, MOFs are combined with high conductive nanomaterials to create hybrid composites that increase the electron conductivity to macroscopic levels, and enhance the electro-analytical signal in comparison with the use of pristine MOFs. MOFs are also used to produce other kinds of framework structures such as carbonaceous frameworks embedded with nanoparticles. These derived materials have extensive applications in glucose electrochemical sensors. Herein, the principle of electrocatalysts with MOFs and their derived materials, the elaboration of electrochemical sensors and the recent application of MOFs materials as a catalyst on electrochemical sensors will be presented in this section.

Numerous CO2 conversion strategies including thermochemical, photoelectrochemical, electrochemical have been adopted extensively in the last decades. However, the electrochemical CO2 reduction (CO2R) to energy-rich chemicals and fuels remains alternative promising technology owing to its ease of operations with an effective green approach. Compared with other energy conversion technologies, the electrochemical reaction conditions are comparatively mild with the ability to operate the reactions in a room temperature and pressure, thereby bringing better feasibility for alleviating anthropogenic atmospheric CO2 emission that threatens global peace. The reaction processes and directions involved can be controlled freely by tuning reductive potential and temperature. In addition, the process of electrochemical reaction is usually proceeded by reactants to gain or lose electron(s) at the surface of the electrode without the need for redox agents, through which the required electricity is derived from some renewable energy sources (solar, wind, geothermal, etc) which do not generate any additional CO2. This makes electrochemical CO2R a green approach with no generation of contaminants. This chapter, therefore, highlighted different metalorganic frameworks (MOFs) and MOF-based materials for electrocatalytic CO2R to energy-rich chemicals. Various strategies for designing MOFs, challenges, and prospects of MOF materials for better improvement of the CO2R were also discussed.