title: Electrochemical Reduction of Carbon Dioxide in Advanced Electrolyser Systems
Creators: name: Zelocualtecatl Montiel, Ivan
Publisher: name: Universität Bern
Year: 2022
Subjects: subject: 540 Chemistry
subject: 570 Life sciences; biology

description: The concentration of atmospheric CO2 has increased alarmingly in the last few decades, triggering the search for new technologies that utilize CO2 as a raw material. Electrochemical CO2 reduction (ec-CO2R) is one of the most promising approaches to mitigating the increase in atmospheric CO2 levels. In this process, CO2 is converted into valuable products such as carbon monoxide, methane, ethylene, methanol, ethanol, and formic acid by using renewable energy. Among these CO2 electroreduction products, formate or formic acid is one of the most economically viable because it is an important chemical intermediate for many industrial processes and has promising applications in hydrogen storage.
Since the pioneering work of Hori et al., it is well known that various material catalysts such as Pd, Cd, In, Sn, and Pb have been studied for use in the production of formate via ec-CO2R. However, these materials are too expensive and, in most cases, environmentally unfriendly. Recently, bismuth-based materials have received attention as catalysts for ec-CO2R because of their low toxicity, low cost, excellent stability, and high selectivity for formate production from CO2 in aqueous electrolytes.
In this work, I have focused on the synthesis of bismuth foam catalysts using the dynamic hydrogen bubble template (DHBT) electrodeposition approach, followed by thermal annealing in air at 300 °C for 12 h, which transforms the as-prepared metallic Bi foam into a completely oxidised Bi2O3 foam. The electrochemical performance of the Bi2O3 foam catalyst was carried out in a conventional H-type cell system and exhibited high faradaic efficiency (FEformate = ~100%) and high partial current density for formate (PCDformate = –84.1 mA cm-2). The catalyst was also found to be highly stable after 100 h of continuous electrolysis.
The high FEformate and PCDformate achieved in these studies have been due to the presence of two reaction pathways, leading to high FE values across a broad potential window. The first reaction pathway involves the bismuth subcarbonate species at low cathodic potentials, while the second reaction pathway involves metallic bismuth species at high cathodic potentials. In this study, novel operando Raman spectroscopy was used for the first time to obtain information about the Bi2O3–Bi subcarbonate–Bimetal transition as a function of the applied potential. In addition, the morphological changes of the catalyst before and after ec-CO2R were monitored using identical location scanning electron microscopy (IL-SEM).
As high current densities on the order of –200 mA cm-2 cannot be achieved in H-type cell systems because of CO2 mass transport limitations, the Bi2O3-GDE foam catalyst used in the H-type cell was transferred to a flow cell (fluidic electrolyser). An excellent electrochemical performance was achieved in the flow cell electrolyser over a wide range of cathodic potentials, with high faradaic efficiencies and high partial current densities for formate production (FEformate = ~100% at –0.8 V vs. RHE and PCDformate = –414.7 mA cm-2 at –2.5 V vs. RHE in 1 M KOH). Importantly, an increase in the concentration of the electrolyte solution (5 M KOH) boosted the PCDformate values beyond –1 A cm-2.
Ex situ XRD and Raman spectroscopy studies demonstrated that the subcarbonate reaction pathway for formate production was still active at high cathodic potentials (–1.5 V vs. RHE). This bismuth subcarbonate forms in situ and is highly stable due to the abundance of CO2 under operando conditions in the gas-fed electrolyser. While, the metallic bismuth reaction pathway is also active at high cathodic potentials (up to –1.6 vs. RHE).
Finally, the use of advanced X-ray diffraction tomography (XRD-CT) revealed deeper insights into the spatial distribution of bismuth subcarbonate and metallic bismuth in the catalyst layer after ec-CO2R.