Electroactive Co(iii) salen metal complexes and the electrophoretic deposition of their porous organic polymers onto glassy carbon.

Marcello B Solomon,Clifford P Kubiak,Deanna M D'Alessandro,Aditya Rawal,Seth M Cohen,Katrina A Jolliffe,James M Hook

doi:10.1039/c8ra04385j

Abstract

This paper reports the CO2 electroreduction properties of three bis-bromo Co(iii) salen metal complexes and their Porous Organic Polymers (POPs) as a platform for using the salen core as a multi-electron reducing agent. Although Co(iii) salen metal complexes have been studied extensively for their chemical catalysis with CO2, their electrochemical behaviour, particularly their reduction, in the presence of CO2 is much less explored. The discrete Co(iii) complexes enabled the reduction of CO2 to CO in faradaic efficiencies of up to 20%. The reductive electrochemical processes of Co(iii) salen complexes are relatively unknown; therefore, the mechanism of reduction for the complexes was investigated using IR and UV-Vis-NIR spectroelectrochemical (SEC) techniques. The discrete bis-bromo salen complexes were incorporated into POPs with tris-(p-ethynyl)-triphenylamine as a co-ligand and were characterised using solid state NMR, IR, UV-Vis-NIR and Field Emission Scanning Electron Microscopy (FE-SEM). The POP materials were electrophoretically deposited onto glassy carbon under milder conditions than those previously reported in the literature. Direct attachment of the POP materials to glassy carbon enabled improved solid state electrochemical analysis of the samples. The POP materials were also analysed via SEC techniques, where a Co(ii/i) process could be observed, but further reductions associated with the imine reduction compromised the stability of the POPs.

Highlights

Since the industrial revolution, the consumption of fossil fuels has been rapidly increasing to cater for the needs of an ever growing world population
We further report a method for the surface attachment of Porous Organic Polymers (POPs) onto glassy carbon via Electrophoretic deposition (EPD) which uses milder potentials than those previously reported for EPD onto an Fluorine-doped Tin Oxide (FTO) substrate, providing a benign pathway for attaching solid-state materials to glassy carbon
The synthesis of salen ligands was achieved by the Schiff-base condensation of 5-bromosalicylaldehyde with varying bridging diamines to afford the free-base salen in good yield

Summary

Introduction

The consumption of fossil fuels has been rapidly increasing to cater for the needs of an ever growing world population. Industrialisation has been heavily dependent on the use of coal, which today is responsible for over 40% of the production of electricity worldwide.[1] In excess of 12 million tonnes of oil and 8 million cubic tonnes of natural gas are consumed daily to provide energy, which has resulted in excess carbon dioxide (CO2) emissions into the atmosphere at a rate faster than the current carbon cycle can mitigate.[2,3,4,5,6,7,8,9,10] Carbon capture and sequestration (CCS) has long been seen as a viable option as it allows for the retro tting of existing power plants to separate CO2 from the ue stream prior to its release into the atmosphere.[11] A concern arising from the storage of CO2 is its effective transport, which poses nancial, logistical and environmental challenges. Processes that can combine the capture of CO2 with its use as a feedstock may assist in the handling of emissions

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Results

Conclusion