Plasma-Assisted Synthesis of Catalysts for CO2 Electrochemical Reduction

Abstract

The CO₂ reduction reaction (CO₂RR) stands at the forefront of sustainable technological innovations, offering a strategic route to curtail the carbon footprint and combat climate change. This process is integral to the global initiative to close the carbon loop by transforming CO₂, a byproduct of fossil fuel combustion, into a plethora of industrially significant compounds, thus facilitating a transition towards a circular economy. The significance of CO₂RR extends beyond environmental remediation; it also enables the storage of intermittent renewable energy in the form of stable chemical bonds, potentially revolutionizing the energy sector. Despite its potential, the practical realization of CO₂RR at an industrial scale is impeded by the inefficacies of current catalysts, which are plagued by poor selectivity, sluggish kinetics, and subpar stability. These impediments are further exacerbated by the use of powder-based electrodes and traditional synthetic routes, which pose significant hurdles for scalability and integration into real-world industrial settings. Plasma-assisted synthesis emerges as a powerful technique to overcome these barriers, offering unparalleled control over the creation of novel catalyst structures, surface modifications, and doping processes. This thesis focuses on the application of plasma technology in the fabrication of advanced CO₂RR catalysts, addressing the shortcomings of conventional catalysts through innovative design and engineering.

First part of this thesis, we utilized plasma-assisted surface etching followed by surface modification to fabricate a CoPc-modified oxygen-terminated diamond nanocone electrode (CoPc/OD_cone) for highly efficient CO₂ electrochemical reduction. The application of diamond nanostructures enabled the creation of a high-performance electrode, demonstrating an impressive Faradaic efficiency (FE) of 94.1% for CO at −0.97 V vs. the Reversible Hydrogen Electrode (RHE). Significantly, over a wide potential range of −0.67 V to −1.07 V vs. RHE, the FE for CO consistently exceeded 80%. The exceptional performance of the CoPc/OD_cone electrode can be attributed to the synergistic effects between the nanostructured diamond surface and the CoPc catalyst. The hydroxyl-rich nature of the diamond surface facilitated the anchoring of CoPc molecules and the bonding with Co atoms in CoPc. Meanwhile, the nanostructured diamond enhanced CO₂ adsorption, thereby improving the catalyst’s performance. These findings provide valuable insights into the application of non-metallic carbon materials, particularly diamond, as metal-free catalysts for electrochemical reduction of CO₂. Furthermore, this study addresses the inherent challenges associated with diamond electrodes, such as suboptimal current density and insufficient Faradaic efficiency, paving the way for the development of more efficient catalysts for CO₂ electroreduction.

Second part, we have developed a rapid plasma-assisted method to synthesize defect rich porous bismuthene arrays on copper foam (D-Bi-ene/CF), which serve as self-supporting electrodes. This technique shortens the synthesis time to 30 seconds and significantly enhances catalytic performance by increasing the exposure of active edge sites and preventing the aggregation of bismuthene layers. The D-Bi-ene/CF electrodes exhibit exceptional electrochemical properties, achieving a FE for formate of over 90% across a broad electrochemical potential range from −0.96 V to −1.46 V vs. RHE. At −1.26 V vs. RHE, they achieve an FE_formate of 94.2% with a high partial current density of 97.1 mA cm^-2, indicating a marked improvement over conventional electrodes. Density Functional Theory (DFT) calculations validate these results, indicating that the abundance of edge sites provided by the porous structure lowers reaction barriers and stabilizes intermediates. This rapid synthesis process, coupled with the impressive electrochemical performance of the D-Bi-ene/CF electrodes, makes this method highly promising for high-throughput applications and the large-scale production of efficient CO₂ electroreduction electrodes.

Third part, we introduce a plasma-assisted method for fabricating boron-doped bismuth nanosheets on copper foam (B-Bi/CuF) that serve as self-supporting electrodes for the CO₂RR. The B-Bi/CuF catalyst exhibits an exceptionally wide electrochemical window of 800 mV, ranging from −0.56 V to −1.36 V versus RHE, and achieves over 90% FE towards formate production. At the potential of −1.06 V vs. RHE, the catalyst demonstrates a high formate FE of 94.7%, with a remarkable current density of 66.0 mA cm⁻². DFT calculations further corroborate that boron doping results in the formation of electron-deficient bismuth sites stabilizes the CO₂^∗− and ^∗HCOO intermediates and lowers the overall energy barriers, greatly enhancing the formation of formate. This work not only paves the way for the development of efficient bismuth-based catalysts for CO₂RR but also provides a deeper understanding of the role of boron doping in tuning the electronic properties of bismuth to achieve high performance in electrochemical applications.

In conclusion, this research demonstrates the vital role of plasma-assisted synthesis in advancing CO₂ electrochemical reduction technologies. Plasma processing confers advantages in catalyst synthesis such as precise control over material characteristics, nano structure, surface functionalization, and the ability to induce desirable defects and dopants. These capabilities collectively lead to catalysts with superior selectivity, stability, and catalytic efficiency, enabling fine-tuning of structures and surfaces for optimum performance. Looking ahead, integrating plasma-assisted catalysts promises significant advancements in the large-scale application of CO₂ conversion technologies, with continuous innovation in plasma techniques poised to further boost catalyst performance and aid global sustainability efforts.

Date of Award	17 Jun 2024
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Wenjun ZHANG (Supervisor)