DEVELOPMENT OF CARBON-BASED AND NON-NOBLE METAL ELECTROCATALYSTS FOR ELECTROCHEMICAL ENERGY APPLICATIONS

• Main Author: Irmawati, Yuyun
• Format: Dissertations
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/86817
• Institution: Institut Teknologi Bandung

With the growing demand for electricity, it is crucial to develop energy conversion and storage technologies that are environmentally friendly, use abundant raw materials, and are cost-effective. Secondary Zn-air batteries stand out as a promising option because they utilize zinc metal anodes, which are widely available, and water-based electrolyte that is inherently safe and environmentally benign. Secondary Zn-air batteries operate via oxygen reduction reaction (ORR) during discharging and oxygen evolution reaction (OER) during charging. However, the commercialization of secondary Zn-air batteries remains hindered by the reliance on expensive and scarce noble metal-based electrocatalysts, such as Pt/C for ORR and Ir/C or RuO? for OER. As a result, the development of bifunctional electrocatalysts that can efficiently drive both ORR and OER, made from inexpensive and abundant materials, is a primary research goal. Non-noble metal coupled with nitrogen-doped carbon (M-N/C, M = Fe, Co, Ni, and Mn) is considered one of the primary candidates for replacing noble metal electrocatalysts. These materials are attractive due to their tunable morphology and electronic structure, abundant sources of raw material, and relatively simple fabrication process (i.e., by pyrolyzing a mixture of carbon, nitrogen, and metal salt precursors). Despite these advantages, the ORR-OER performance of most M-N/C electrocatalysts remains relatively lower than that of Pt/C and Ir/C. Several strategies have been proposed to improve the M-N/C electrocatalyst performance, including morphology modification, co-doping with non-metal atoms, and adding secondary metals to create metal alloys. These approaches aim to increase the number of accessible active sites and enhance their intrinsic activity, optimizing ORR-OER performances. In this study, the first strategy involves using reduced graphene oxide (rGO) aerogel doped with nitrogen and boron as the matrix for Fe nanoparticles (denoted as Fe-NB-rGO). Dual doping with nitrogen and boron boosts the ORR performance. Nitrogen doping, with its higher electronegativity, creates positively charged carbon atoms nearby that act as adsorption sites for O2. Meanwhile, with its lower electronegativity, boron doping induces charge polarization, making boron atoms more positively charged than the nearby nitrogen and carbon atoms, thus generating additional oxygen adsorption sites. Porous structures of rGO aerogel with hierarchical mesopores also improve reactant diffusion into active sites. The additional Fe nanoparticles evenly distributed on the nitrogen-boron-doped aerogel matrix can significantly improve the ORR-OER performance. As a result, Fe-NB-rGO has comparable ORR performance to Pt/C. When applied in Zn-air batteries, Fe-NB-rGO achieves a power density of 107 mW cm?² and stable performance for up to 58 hours. As the second strategy, through morphology modification, 1D-2D Co-N/C heterostructure electrocatalysts were successfully synthesized by pyrolysis process using g-C3N4 and 2D zeolitic imidazole frameworks-67 (2D ZIF-67) as the precursors. In-situ growth of 1D carbon nanofibers on the 2D Co-N/C flakes mitigates structural collapse, enhances the availability of active sites, and establishes conductive networks that facilitate efficient electron transport. Utilizing 2D ZIF-67, a metal-organic framework (MOF) precursor, facilitates a uniform distribution of active sites and yields Co nanoparticles wrapped in nitrogen-doped carbon layers (Co@N/C) with enhanced stability. As a result, 1D-2D Co-N/C shows bifunctional ORR-OER performance comparable to Pt/C and Ir/C. Employing 1D-2D Co-N/C in Zn-air batteries can deliver a high power density of 155 mW cm?² and stable performance for up to 180 hours. As the third strategy, a combination of morphology modification and metal alloy was employed to synthesize FeCo-N/C-based electrocatalysts with 1D-1D nanobrush heterostructures (FeCoNC-NB). Melamine was used as the precursor for growing carbon nanofibers on the surface of FeCo-N/C nanowires that formed from the decomposition of 1D FeCo-NTA MOF. The growth of nanofiber structures increases the number of accessible active sites while providing conductive pathways to accelerate electron transfer. Consistent with the previous result, the utilization of FeCo-NTA MOF precursor yields uniformly distributed active sites and FeCo nanoparticles encapsulated in nitrogen-doped carbon layers (FeCo@N/C), contributing to high stability. The formation of FeCo alloy can also significantly improve ORR-OER performance. As a result, FeCoNC-NB exhibits ORR performance comparable to Pt/C and superior OER performance relative to Ir/C. Employing FeCoNC-NB in Zn-air batteries can deliver a high power density of 195 mW cm?² with remarkable stability of up to 350 hours. The FeCoNC-NB also shows potential applications as an electrocatalyst for hydrogen evolution reactions, which expands its applications in water electrolyzers.