Immobilized Molecular Catalysts for Heterogeneous Electrochemical H 2 Evolution (HER) and CO 2 reduction (CO 2 RR)

The global use of fossil fuels results in unsustainable levels of CO₂ gas being released into the atmosphere, furthering the impacts of global warming.¹ Possible capture of CO₂ from high emission processes and subsequent selective reduction (CO₂ reduction reaction, CO₂RR) to convert it into renewable fuels could move those processes towards carbon neutrality. To move towards carbon zero, green hydrogen is a desirable alternative to fossil fuels. Hydrogen has a higher energy density and only produces water as a waste product. ‘Green’ hydrogen is produced from water using renewable energy (hydrogen evolution reaction, HER). Most hydrogen produced currently, for use in industry, is ‘grey’ hydrogen made from fossil fuels in energy intensive and carbon emitting processes, so this must also be replaced by green hydrogen. The HER and CO 2 RR are energy intensive processes so catalysts are needed to reduce the energy of these chemical processes, to make them more energy and cost efficient.² The CO₂RR is a process with many possible products, highlighting the need for catalysts that can not only reduce the energy required for reduction but also produce the desired useful products selectively. We have prepared a range of porphyrin-like dimetallic macrocyclic complexes³,⁴ and these are now being tested for electrocatalytic activity for HER and CO₂RR. This testing is being carried out under heterogenous conditions, through immobilisation of the catalysts on solid supports. This presentation will cover the synthetic steps involved in producing these macrocyclic complexes and the results of the heterogeneous electrochemical HER and CO₂RR testing on selected complexes. 1. Tollefson, J., The hard truths of climate change - by the numbers. Nature 2019, 573 (7774), 324-327. 2. Dalle, K. E.; Warnan, J.; Leung, J. J.; Reuillard, B.; Karmel, I. S.; Reisner, E., Electro- and solar-driven fuel synthesis with first row transition metal complexes. Chem. Rev. 2019, 2752-2875. 3. Li, R.; Mulder, T. A.; Beckmann, U.; Boyd, P. D. W.; Brooker, S., Dicopper(II) and dinickel(II) complexes of Schiff-base macrocycles derived from 5,5-dimethyl-1,9-diformyldipyrromethane. Inorganica Chimica Acta 2004, 357, 3360–3368. 4. Malthus, S. J.; Cameron, S. A.; Brooker, S., Improved access to 1,8-diformyl-carbazoles leads to metal-free carbazole-based [2+2] Schiff base macrocycles with strong turn-on fluorescence sensing of zinc(II) ions. Inorg. Chem. 2018, 57, 2480–2488.