Electrocatalysts in nanotubes

Resumo

In the context of limited availability of fossil fuels and the impact of the current fossil-based energy utilisation, the synthetic development of novel electrocatalysts materials for applications in environmentally friendly energy conversion have been extensively investigated. This thesis is focused on the development of new hybrid metal-carbon nanostructures as efficient electrocatalyst materials for hydrogen fuel cell applications with enhanced performance and/or durability. The nanoscale confinement and graphitic step–edge stabilization of precious metal nanoparticle based electrocatalysts with in hollow graphitized carbon nanofibers were performed. These metal-carbon nanostructures were then investigated using high resolution transmission electron microscopy (HRTEM) for their structures, and by electrochemical method in order to determine their electrocatalytic performance and durability in the redox reaction of oxygen and hydrogen. Properties of supported metal nanoparticles are significantly influenced by the nature of the carbon surface. Careful design of platinum-based electrocatalyst involving confinement of pre-formed platinum nanoparticles (PtNP) into the internal cavities of hollow shortened graphitized carbon nanofibers (S-GNF) are an excellent approach for creating highly durable nanoreactors (PtNP@S-GNF) towards oxygen reduction reaction. Systematic structural-electrochemical correlations of PtNP on carbon black and on the surface, and inside GNF possessing two qualitatively different surfaces (external continuous graphitic layers and internal stepped layers of grapheme) demonstrate the importance of metal-carbon interactions. Once PtNP@S-GNF nanoreactor is assembled, surfactant molecules, which are necessary to control the PtNP size during their formation in solution, are effectively removed by heating, while PtNP remain immobilised within the S-GNF cavity due to stabilising effects of the graphitic step-edges that inhibit ripening and coalescence of the nanoparticles allowing the retention of their electrocatalytic properties. Catalyst confinement in PtNP@S-GNF creates a nanoscale environment in which at potentials relevant to fuel cell cathodes, the reduction of O2 proceeds exclusively via a four-electron pathway, and in contrast to commercial Pt/C or PtNP deposited on GNF surface, the specific activity and the electrochemical active surface area remain largely unchanged after durability test. This heralds a new methodology for construction of hybrid electrocatalyst nanomaterial, PtNP@S-GNF where metal NPs are confined and simultaneously electrically connected to the electrode after 50,000 cycles retaining 80% of their activity which can enable the sustainable use of platinum for fuel cell applications. The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalytic performances of bifunctional electrocatalyst, Mn4O3NP supported on graphitised nanofiber (Mn3O4/GNF) and encapsulated into shortened GNF (Mn3O4@S-GNF) were carried out. We observed that Mn3O4NP on exterior surface of GNF exhibited lower activity than that on interior surface of S-GNF which was explained by arguing that a higher surface of interaction of Mn3O4NP with the step-edge inside GNF than with the convex surface of the outside of GNF, hence a better connectivity of catalytic centers in Mn3O4@S-GNF leading to a higher E1/2 potential as compared to Mn3O4/GNF. We also noticed that both GNF and S-GNF show better durability behaviour than rest of carbon nanostructures because of stronger graphitic atomic structure. The PdS2NP supported on carbon nanostructures were investigated for HER/HOR as a bifunctional electrocatalyst and it was tested in the same test-bed of GNF and S-GNF just as in the case of platinum nanoparticle electrocatalyst. Electrochemical durability test on the catalytic performance of PdS2@S-GNF(PR24) shows that Pd nanoparticles remain immobilised on S-GNF surface. Which indicate that Pd has the possibility of replacing more expensive platinum as potential electrocatalyst for the use in fuel cell and water splitting devices. These surprising nanoscale confinement properties of metal nanoparticle based hybrid nanostructure will open up a new strategy for their use in fuel cell for redox reaction platform. We also believe that the electrocatalysis in carbon nanoreactors can be extended to other processes of high technological value.