Enhancing CO₂ reduction through novel electrocatalysts: synthesis and characterization of Schiff base ligands, Pd/Pt complexes, and metal phthalocyanine hybrids

Abstract

One of the most promising alternatives for tackling excessive carbon dioxide (CO₂) emissions into the atmosphere is the electrocatalytic CO₂ reduction reaction (CO₂RR). This technology offers versatility in reducing CO₂ emissions under mild conditions and converting them into valuable products. In addition, CO₂ is a C₁ feedstock that could be repurposed by converting it to value-added products using a promising technology like the electrocatalytic CO₂RR. Nonetheless, the conversion of CO₂ is a challenging task attributed to its high chemical stability, slow kinetics, and formation of multiple products. The CO₂RR offers an alternative for obtaining multi-electron products with high energy density and high market value, like carbon monoxide (CO), formic acid (HCOOH), ethanol (CH₃CH₂OH), and ethylene (C₂H₄). Reaching such complicated products involving more than two proton couple electron transfer (PCET), is challenging due to the several possible pathways leading to mixture of products and low selectivity. The efficiency of this electrochemical reaction is strongly dependent on the electrocatalyst used to drive the CO₂RR forward.<br /> <br /> Hence, the primary objective of this thesis is to develop novel electrocatalysts and methodologies to enhance CO₂RR performance by careful structure design of the synthesized materials. Therefore, knowledge of the different coordination modes of CO₂ is essential for fine-tuning product selectivity and selecting appropriate electrocatalyst materials. A wide range of materials have been explored as CO₂RR electrocatalysts, with coordination complexes (CC) like Schiff base (SB) complexes and metal phthalocyanines (MPc) standing out for their high selectivity, ease of synthesis, and tunability. Furthermore, translating theoretical concepts to the experimental domain using batch cell to investigate the potential of molecular electrocatalyst and then scaling up to flow cells to reach commercial relevant parameters is the natural course of action when working on this field.<br /> <br /> This thesis is divided into 5 chapters: the first chapter is an in-depth introduction including the consequences of anthropogenic greenhouse gas (GHGs) emissions and the use of the CO₂RR as an alternative to mitigate and convert CO₂ emissions. The second chapter prioritizes reporting the synthetic procedures of the materials prepared during this thesis including the SB ligands and their corresponding Pd(II) and Pt(II) complexes, and molecular catalysts hybrids. Additionally, in this chapter it will also describe the electrochemical setups (batch cell and flow cell) used to probe the electrocatalytic activity of the prepared materials. The third chapter is a characterization-based chapter where all the synthesized SB compounds and hybrid materials are identified using several spectroscopic and structural techniques.<br /> <br /> Chapter 4 focuses on the study and testing of the electrocatalytic properties of the characterized materials. The motivation behind the use SB compounds as electrocatalysts for the CO₂RR is due to the lack of reports using them for this application and previously in the Piñero laboratory a salen-like ligand with promising electrochemical properties had been synthesized. This inspired us to use salen-like ligand frameworks as next-generation CO₂RR electrocatalysts. Additionally, coordinating Pd(II) and Pt(II) to the salen-like ligands was further investigated, despite their typical preference for hydrogen production over CO₂RR. Results from CO₂RR experiments revealed that introduction of a hydrogen bonding group (hydroxy substituent) and modifying the orientation of the aromatic rings (vertical or horizontal) could adjust the product selectivity of the synthesized Pd(II) and Pt(II) complexes.<br /> <br /> In addition, in this chapter a novel strategy for controlling CO₂RR selectivity through non-covalent surface modification of a Cu foam electrode using molecular catalyst hybrid is reported. This approach involves preparing a molecular catalyst hybrid using MPc and multi-walled carbon nanotubes (MWCNTs), followed by coating the Cu foam electrode with the hybrid materials and testing their performance in a flow cell. This modification led to a significant reduction of the Hydrogen Evolution Reaction (HER), while significantly increasing CO₂RR product yields. The final chapter presents all the findings related to this work in a concise manner, as well as the future directions this work can take by optimizing the reported compounds. In summary, the research presented in this doctoral thesis represents a significant advancement in the field of CO₂RR and lays the foundation for further exploration of salen-like complexes as efficient electrocatalysts and the use of non-covalent surface modification of Cu foam electrodes to improve electrocatalytic activity. The development of electrocatalysts to facilitate the CO₂RR holds significant promise as a viable option for mitigation of GHGs emission and creating a more sustainable future for the next generations. Finally, the work presented in this thesis resulted in two high impact peer-review publications and a third one that is in the process of being submitted; two other articles were published resulting from collaborations.