Porphyrin and Phthalocyanine Graphene Assemblies As Electrocatalysts for CO2 Reduction
Authors: Officer David L 
Published: February 2020

Electrochemical CO2 conversion is in the global spotlight as one of the most promising ‘clean’ approach for the conversion of atmospheric and marine CO2 into clean and renewable liquid fuels as well as offering a potential approach to the amelioration of a major greenhouse gas. In particular, carbon monoxide (CO) produced by electrochemical CO2 reduction is a principal industrial feedstock for the production of hydrocarbons and bulk oxygenated products. Consequently, the development of a highly energy efficient and selective heterogeneous catalyst for the conversion of CO2 to CO is of major interest in energy and environmental science research and industry.

It is challenging to split CO2 due to the high activation energy required in the reduction process. In order to resolve this, a variety of catalysts have been investigated to reduce the activation energy including precious metals, non-precious metals, transition metal oxides, transition metal chalcogenides and metal complexes. In the presence of molecular catalysts, specifically, CO2 reduction typically proceeds by a proton-coupled electron transfer reaction, resulting in lower overpotentials due to the stabilization of the metal-carbon dioxide complex. Depending on the process under the CO2 atmosphere, a variety of gaseous or liquid products can be generated at various potentials. However, the competitive hydrogen evolution reaction (HER) can concomitantly occur at a similar potential to the electrochemical CO2 reduction reaction.Therefore, selectivity is a vital property in a practical electrocatalyst for this process.

Metal complexes (e.g. metalloporphyrins and metallophthalocyanines) have been widely studied as homogeneous catalysts for electrochemical CO2 reduction due to their high selectivity, low cost, and ease of preparation on a large scale. However, the use of the metal complexes as homogeneous catalysts is not ideal for practical applications since the catalytic activities are influenced by a number of factors including (i) diffusion of the catalyst into the diffusion layer adjacent tothe electrode surface, limiting the number of active catalytic species, (ii) catalyst deactivation processes such as dimerization and aggregation and (iii) the use of volatile and flammable organic solvents like dimethylformamide and acetonitrile, which is not ideal.[1,2] In addition, unlike heterogeneous catalysts, it is not easy to separate or reuse the homogeneous catalysts following the reaction, one of the most important requirements in commercial use.

For these reasons, there have been a number of studies in which the molecular complexes have been immobilised and used as heterogeneous catalysts for the electrochemical CO2 reduction.[3-5] Although the majority of metal complexes have demonstrated improved catalytic performance through immobilisation, most of the catalysts for the production of CO operate at high overpotentials and show poor catalytic durability of less than 5 h, resulting in a low energy efficiency for the electrochemical CO2 reduction catalysis. Therefore, a stable, efficient metal complex-based heterogeneous catalyst for the electrocatalytic reduction of CO2 at low overpotentials has yet to be realised.

We have developed simple and facile self-assembly methods for the fabrication of heterogeneous composite electrocatalysts for CO2 reduction to CO using reduced graphene oxide with both metalloporphyrins and metallophthalocyanines by π-π stacking and electrostatic interactions.[6,7] The resulting electrocatalysts possess a number of advantages including (1) facile and rapid electron transfer, (2) easy electrolyte and CO2 accessibility, (3) structural robustness, (4) excellent faradaic efficiencies (>96%) at overpotentials as low as 280 mV, and (5) superior long-term stability (up to 50 h) under the electrochemical reduction conditions. In contrast to previous work, these catalysts exhibited excellent electrocatalytic CO2 reduction performance and stability.

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