Synthesis and Characterization of Bismuth-based Catalyst for Electrochemical Reduction of CO2

Over the past two centuries, global annual CO2 emissions have increased by approximately 40 billion tons, becoming the main cause of climate change. To address this pressing issue, many strategies have been developed to reduce atmospheric CO2 concentration, collectively known as Carbon Capture, Utilization, and Storage (CCUS). [1] Among these, the electrochemical reduction of CO2 (ECO2RR) stands out as a promising approach, converting CO2 into reusable carbon-based products with oxygen as the only by-product. However, a significant challenge remains in developing catalysts that exhibit both high activity and selectivity. This thesis investigates the synthesis and characterization of bismuth-based (Bi-based) catalysts for the electrochemical production of formate. The catalysts were characterized by different techniques such as XRD, SEM, TEM, and TGA to study their physico-chemical properties, their ECO2RR performance was evaluated through chronoamperometry testing. The initial phase of the study aimed to replicate the BiSub@AC-400 catalyst, following previously established methods.[2] Despite modifications to the synthesis parameters, the original material could not be reproduced, with a main difference being the bismuth crystalline phase. While the reference catalyst predominantly contained Bismutite, during this work metallic Bi and Bi2O3 were obtained. This might be the reason why the resulting BiSub@AC-400 exhibited superior performance, achieving a current density of -8.8 mA/cm2 at -1.07 V vs. RHE, compared to the reported -6.2 mA/cm2. Subsequently, the research focus shifted to optimizing the synthesis process to improve the distribution of bismuth on the activated carbon (AC) and reduce the nanoparticle (NPs) size, a feature critical for enhancing catalytic activity due to the increased surface-to-volume ratio. To achieve this, bismuth subsalicylate (BiSub) was solubilized in two different environments: ethanol with 1.8M salicylic acid (SA) and in a mixture of ethanol and acetic acid (AcOH), producing BiNPs@AC-SA and BiNPs@AC-AcOH catalysts, respectively. At a catalyst loading of 0.298 mg/cm2 and -1.07 V vs. RHE, the BiSub@AC-400, BiNPs@AC-SA and BiNPs@AC-AcOH achieved -8.7 mA/cm2, -7.7 mA/cm2 and -7.8 mA/cm2, with faradaic efficiencies (FE) of 83 ± 5%, 75 ± 6% and 82 ± 3% respectively. The reduced performance of BiNPs@AC-SA and BiNPs@AC-AcOH may result from the presence of amorphous bismuth or diminished wettability of the materials. For BiNPs@AC-AcOH, the effect of lowering the BiSub:AC ratio by 90% was also investigated. This significant reduction in bismuth content resulted in a decrease in both current density (-4.1 mA/cm2) and FE (70 ± 3%). However, it improved formate production per unit bismuth weight, likely due to a reduction in NPs size. This study highlights the effect of synthesis parameters on material properties and catalytic performance for ECO2RR, offering valuable insights for the design of more efficient catalysts. Further research on BiNPs@AC-SA and BiNPs@AC-AcOH performance over a wider range of applied potentials is encouraged to optimize their catalytic efficiency.