Electrocatalysis

Introduction

The world energy demands have been rising at exponential rates at present. nowadays, fossil fuels produce around 80% of total world energy. However, fast depletion of fossil fuels and exponential rise in global warming is forcing scientific communities to look for alternative sources of energy. Electrocatalytic conversion is an attractive way, which provides sustainable energy provision by means of interconversion of chemical and electrical energy. This development has motivated the exploration of efficient alternatives to the conventionally used high cost and scarce noble metal electrocatalysts like Pt, Au, Ru, Ir and its oxides and complexes. The formation of composites of MOFs with other materials may provide additional advantages, such as electrochemical stability and activity. A recent class of porous materials, popularly known as metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) have been extensively used as electrocatalysts or as precursors to derive efficient heterostructures owing to their synthetic flexibility and low cost besides customizable electronic and chemical properties.

Application of MOFs and COFs in Electrocatalysis

Application and research in electrocatalysis currently focuses on the reactions related to H₂ evolution reaction (HER), O₂ evolution reaction (OER), CO₂ reduction reaction, as well as O₂ reduction reaction (ORR)^[1]. MOFs and COFs as electrocatalysts can be applied all the above mentioned reactions.

Fig.1 Potential electrocatalytic reactions driven by MOF/MOF-derived electrocatalysts

• O₂ Evolution: The OER specifically refers to the electrochemical oxidation of water to produce O₂ gas. The lack of earth-abundant OER catalysts is arguably the biggest challenge facing the implementation of an artificial fuel economy because the OER is the ideal counter reaction for fuel generating reduction reactions such as the HER and CO₂ reduction. For MOFs, a number of MOFs mostly composed of Ni, Fe, Mn, Co, and Cu are used as OER electrocatalysts, particularly cobalt-based MOFs display outstanding electrocatalytic properties for OER. And MOFs with flexible ligand linkers can be expected to easily adapt to the electronic demands of the high oxidation states of the metal centers, which could be productively used for catalyzing the OER. For COFs, many COFs with different existence form such as pristine metal-free COFs and derivatives, metallized COFs and nanohybrids, COFs-supported single-atom nanoparticles can be used in OER.
• H₂ Evolution: In the HER, protons are reduced to form H₂. Active HER electrocatalysts for the successful application of water-splitting to produce H₂ is an effective way for solving the energy crisis. Conductive organic frameworks including monometallic MOFs and COFs, multi-metallic conductive MOFs and COFs, and metal-free conductive COFs are all considered as an alternative electrocatalysts for HER. Thereinto, MOFs with Co, Ni, Fe, Cu single-atom nodes and MoS_x clusters are the most frequently used.

Fig.2 Zirconium-based MOFs electrodeposited with a layer of nickel sulfide for electrocatalytic hydrogen evolution

• O₂ Reduction: The ORR is the reverse reaction of the OER where O₂ is reduced to water or hydroxide species. ORR plays an important role in electrochemical energy conversion in fuel cells. The kinetics of ORR that involves four-electron transfer is usually very slow, therefore, efficient electrocatalysts are required^[2]. In 2012, the use of a composite material composed of an iron-based MOFs and graphene for electrochemical ORR was proved, and a number of electrons transferred of 3.82 was achieved in alkaline electrolytes. A range of copper-based, cobalt-based, iron-based, and manganese-based MOFs have been directly applied for electrochemical ORR and found to show remarkable activity. For COFs, pristine metal-free COFs such as covalent triazine-based frameworks (CTFs), metal-free COF-derived carbons, metallized-COF related electrocatalysts and COF-supported single-atom sites and nanohybrids show great ability in electrocatalysis of ORR.

Fig.3 The use of a porphyrinic Zr-MOFs for electrocatalytic ORR

• CO₂ Reduction:As a result of the large depletion of traditional fossil energy and increasing emission of CO₂, exploiting the CO₂ reduction for valuable carbon based fuel production and provides a super-ideal approach towards the sustainable development of the world and also can reduce greenhouse gases. MOFs constructed from copper-based nodes have been widely applied for electrocatalytic CO₂ reduction, and subsequently, aluminum-based MOFs, cobalt-based MOFs, zirconium-based MOFs and etc. as the electrocatalysts can catalyze the reduction of CO₂. According to the literature reported by Cui^[3], COF-366-Co and COF-367-Co are the first COF-based electrocatalyst for the CO₂ reduction in aqueous solution, and then many COFs like COF-366-Cu, COF-366-Zn, COF-366-(OMe)2-Co, COF-366-F-Co, and COF-366-(F)4-Co are applied in electrocatalytic CO₂ reduction. The CO₂ can be transformed into carbon monoxide, formic acid, methanol, methane, ethanol, ethylene, and ethane by the MOFs and COFs-based electrocatalysts.

What Can Alfa Chemistry Do

Alfa Chemistry offers all sorts of high-quality MOFs and COFs for the electrocatalysis fields of electrocatalytic O₂ evolution, electrocatalytic H₂ evolution, electrocatalytic O₂ reduction, electrocatalytic CO₂ reduction and etc. And our professional technology teams that can also provide customers with high-quality MOFs and COFs design and customization services, no matter what design ideas you have, we will implement them together with you. In addition, Alfa Chemistry is committed to supporting customers a series of solutions in electrocatalytic fields by using MOFs and COFs. Please contact us immediately to order or cooperate in research and development with high quality and reasonable price.