Metal-organic frameworks (MOFs) are porous crystalline materials of organic molecules (‘linkers’), that interconnect with metal ion clusters to form three dimensional structures. These small structures, up to around 100 micrometres, are ordered and highly modular, and form materials of vastly different properties. They have been used to improve the efficiency of hydrogen storage in gas cylinders and are being explored for their potential as a mechanism to load and release therapeutic drugs for medical treatments. Recently, MOFs have been developed to act as electrocatalysts with catalytic units (an internationally recognised unit of measurement for quantifying the catalytic activity of enzymes) included in the metal nodes, in the linker molecules or even trapped in the MOF pores. The European Research Council-supported MOFcat (Fundamental and Applied Science on Molecular Redox-Catalysts of Energy Relevance in Metal-Organic Frameworks) project explored MOF-based electrolysers which oxidise water, generating the electrons and protons necessary to produce hydrogen and reduce carbon dioxide (CO2). In the MOFs developed, the metal nodes provide scaffolding, ensuring the MOF’s structural integrity and longevity, while the linkers provide the sites for the catalysts that drive the desired chemical reactions. “To enable the most efficient electrocatalysis, we developed new methods to analyse charge transport in these crystalline materials,” says Sascha Ott, the project’s leader, from Uppsala University. “We found that limitations were often not caused by the turnover frequency of the catalytic units, but by limitations to the speed of electrical charge transportation through the MOFs.”
The advantages of MOFs
As MOFs are highly ordered materials, they hold a significant advantage over solution-based approaches where the catalyst isn’t controllable. Precisely placing the catalyst within the MOF offers possibilities for controlled and site-specific interactions that improve its performance. MOFcat developed a photoelectrochemistry platform which harnesses light energy to speed up chemical transformations. This new methodology can be applied to both fuel production and for organic photoredox catalysis. To demonstrate the latter, MOFs were used to coat semiconductors which, when illuminated, provided electrical energy to carry out chemical transformations. The team then made several MOF-based electrode materials. One, dubbed ‘UU-100’, was synthesised using the relatively abundant metal cobalt. UU-100 generated hydrogen for 18 hours without decreasing its catalytic activity. “Incorporating molecular cobalt catalysts into the MOF increased hydrogen turnover by a factor of over 1 000, compared to catalysts in homogenous solutions,” explains Ott. Most of the materials’ characterisation was done using electrochemistry techniques, sometimes coupled to ultraviolet-visible spectroscopy. For example, a MOF was grown onto a transparent electrode where the linkers change colour when electrochemically reduced, making it possible to observe charge transport through the film over time.
‘Door opening’ technology
MOFcat’s platform offers a technology that is both more efficient and cleaner than many alternatives. For example, in photoredox catalysis, the so-called sacrificial chemical reagents that generate the electrons and oxidising equivalents necessary for catalysis remain in the mixture as waste chemicals. This can be avoided using the MOFcat platform. The precise molecular chemistry of MOFcat, can also fine-tune catalyst reactivity. This reduces CO2 while giving higher yields with fewer by-products that would otherwise be difficult and expensive to remove. Another example could be the electrochemical reduction of CO2, for carbon capture or the generation of products such as methanol or ethylene, which is currently not very selective. This is especially true when materials such as metal oxides are used as catalysts. “Our electrochemical MOF-based electrocatalysis technology could, in principle, be placed at CO2 emitting sources such as fossil fuel power plants or oil refineries, to reduce their CO2 emissions,” adds Ott. The team is now seeking further support to test more MOFs in different electrolyser designs and to develop a prototype for their catalytic MOF materials.
MOFcat, Metal-organic framework, MOF, electrocatalysis, catalytic, hydrogen, carbon dioxide, CO2, photoredox, electrode, light