RECENT ADVANCEMENTS IN ELECTROCATALYTIC CO₂ REDUCTION USING DIFFERENT PORPHYRINOIDS

Abstract

The sharp, sustained rise in the concentration of carbon dioxide (CO₂) in the atmosphere, predominantly as a result of fossil fuel combustion, industrialization, deforestation and increased agricultural production, is one of the most pressing environmental challenges of the twenty-first century. The levels of atmospheric CO₂ have recently reached over 420 parts per million (ppm) compared to pre-industrial levels of around 280 ppm. This dramatic increase is the main contributor to anthropogenic climate change, causing global warming, sea level rise, ocean acidification and many natural ecosystems around the globe being affected. The electrocatalytic CO₂ reduction reaction (CO₂RR) is one of the various methods being investigated to tackle the issues of CO₂ accumulation. CO₂RR not only reduces CO₂ emissions but also generates valuable chemical feedstocks and fuels such as carbon monoxide (CO), formic acid (HCOOH), formaldehyde (HCHO), methanol (CH₃OH), methane (CH₄), ethylene (C₂H₄), and ethanol (C₂H₅OH), all of which can be used in various commercial applications instead of fossil fuels. If powered by electricity from renewable energies like solar, wind or hydro, CO₂RR is a carbon-neutral or even carbon-negative process. Porphyrinoids are a structurally very diverse and wide-spread group of macrocycles that has sparked special research interest due to its potential as catalytic materials in the reaction of CO₂RR. The porphyrinoids include porphyrins, metalloporphyrins, phthalocyanines, metallophthalocyanines, corroles, chlorins, bacteriochlorins and many derivatives. They have an important common structure feature: a highly conjugated macrocyclic aromatic structure with a central cavity to coordinate transition metal ions, giving well-defined and highly tunable metal coordination sites that act as the active centers for catalysis. The unique features of porphyrinoid-based electrocatalysts are their highly tuneable electronic structure, ease of synthetic modification at peripheral positions, excellent light-harvesting properties, precise single-atom metal centres as a basis for detailed mechanistic studies, and high selectivity for certain CO₂ reduction products. A number of recent studies have shown that these materials can be very efficient, have low overpotentials and are very stable for catalytic activity, providing significant promise for their use on a large scale. This dissertation provides a thorough overview of the recent developments on electrocatalytic CO₂ reduction with various porphyrinoid catalyst structures. The review systematically covers the basic principles of CO₂ chemistry and electrochemistry, structural and electronic properties of porphyrinoids, mechanisms of CO₂RR at porphyrinoid metal centres, performance of specific subclasses of porphyrinoids (porphyrins, phthalocyanines, corroles, and chlorins), effects of structural modification and the identity of the metal centre, immobilization strategies for the fabrication of electrodes, and challenges and future directions in this rapidly evolving field. The results presented in this work show that the iron and cobalt porphyrins have very high selectivity for CO production (Faradaic efficiencies > 95% at relatively low overpotentials). Cobalt phthalocyanines have been proven to be very active when immobilized on carbon supports, which catalyse the conversion of CO₂ to CO. Inspired by recent progress in corrole chemistry, a high CO selectivity with minimum competing hydrogen evolution can be realized. It has been demonstrated that the catalytic activity can be significantly improved by axial ligand modifications, peripheral functionalization and surface immobilization. The addition of porphyrinoids to metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) is an intriguing area that has produced highly active and robust catalysts. in The goal of this dissertation is to deliver a well-rounded and current overview of the state-of-the art porphyrinoid-based CO₂RR catalysis for graduate students, researchers, and electrocatalysis and materials practitioners, with an emphasis on the knowledge gaps and potential research directions for the field.