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Synthesis and characterization of abiotic electrocatalysts based on reduced graphene oxide for oxygen reduction reaction
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Resumo(s)
The use of low-temperature fuel cells as power supplies of energy conversion devices is attracting considerable interest because of the direct electrochemical conversion of a fuel, e.g. hydrogen and glucose, and an oxidant, such as oxygen, producing electrical current. The sluggish kinetic of the oxygen reduction reaction (ORR) on the cathode half-reaction is particularly investigated since its acceleration relies on the development of efficient electrocatalysts. Unfortunately, the most promising catalysts for ORR are platinum-based materials that exhibit poor durability, limited resource and high cost. Under such circumstances, the development of non-noble, efficient and low-cost electrocatalysts has attracted a great deal of attention. The present dissertation focuses on the synthesis and physicochemical characterization of graphene-based materials doped with nitrogen and 2 and 10 wt % transition metals (Fe, Co, Mn, Cu, Ni and Rh), denoted as rGO/M 2 and 10 %, capable of reducing molecular oxygen. Firstly, nitrogen-doped reduced graphene oxide with atomically dispersed transition metal materials were synthesized using commercial graphene as precursor. A sequential extra-exfoliation and oxidation of the graphene increased the d-spacing between carbon layers and created porosity on the structure, which is essential for the diffusion of reactants on the material. Further simultaneous N doping and reduction of graphene oxide using thermal and low-temperature plasma treatment allowed the formation of M-Nx active sites that contribute greatly on the ORR activity. The obtained carbon structure exhibited a large specific surface area (c.a. 800 m2 g-1) doped with c.a. 1.98 wt % of nitrogen. The incorporation of atomically dispersed metal reached 21 % of 2 wt %, 3 % and 0.46 % of 10 wt % using different reduction methods. The engagement of aromatic macrocycle molecules, particularly iron and cobalt metalloporphyrins, in the graphene oxide structure was also studied. The synthesis of these hybrid materials was based on a procedure described previously, relying on the addition of the metalloporphyrin to the graphene structure, followed by its pyrolysis under N2 atmosphere. The ORR electrochemical characterization of the materials was performed using hydrodynamic convective systems: the rotating disk electrode (RDE) and the rotating ring-disk electrode (RRDE), in acidic, alkaline and neutral media. Among all the synthesized materials, iron- and cobalt-based materials showed the highest performance towards ORR. In particular rGO/Fe 2 % exhibited a remarkable activity in acidic (Eonset 0.76 V vs. RHE), alkaline (Eonset 0.91 V vs. RHE) and neutral (Eonset 0.78 V vs. RHE) media, comparable to Pt/C catalysts. A mixed 2- and 4-electron pathway was observed for rGO/Fe 2 % in acidic and alkaline media due to the contributions of several functional groups in the structure. The remaining materials displayed lower onset potential in acidic (0.44 to 0.71 V vs. RHE), alkaline (0.80 to 0.87 V vs. RHE) and neutral (0.64 to 0.74 V vs. RHE) media. The outstanding ORR performance of these materials is attributed to the presence of M-Nx actives sites dispersed in the carbon structure and intrinsic ORR activity of metalloporphyrins.
Descrição
Tese de mestrado, Química (Química) Universidade de Lisboa, Faculdade de Ciências, 2019
Palavras-chave
Grafeno oxidado Grafeno oxidado e reduzido Metal de transição Metais atomicamente dispersos Moléculas macrocíclicas aromáticas Reação de redução do oxigénio Teses de mestrado - 2019