Transition metal nanoparticles embedded carbonaceous framework for electrocatalysis and zinc air batteries

Abstract

As the world’s energy demand grows and environmental challenges mount, there is a growing push to find sustainable energy sources and efficient solutions for energy conversion, such as water splitting, fuel cells, metal-air batteries, etc. Energy-efficient occurrence of the crucial electrochemical reactions, such as hydrogen evolution (HER), oxygen evolution (OER), and oxygen reduction (ORR), is at the core of these systems. The development of carbon-based electrocatalysts is the mainstay of this thesis, which are cost-effective, highly efficient, and endure for these key reactions. The synthesis and structural tuning of carbon nanostructures to maximize their efficiency by creating the active sites through N-doping and exposing them to the reactants through enhancing the surface area and pores. Furthermore, transition metal (TM) nanoparticles embedded in nitrogen-doped carbon frameworks offer a cost-effective and promising combination of catalytic parameters through reduction of activation barrier due to synergy effect, and stability in alkaline environments.

To demonstrate the synergy effect, several TM elements such as Fe, Co, Ni, and the combination of these elements are used. As a part of this work, a CVD-based pyrolysis method was used to synthesize nitrogen-doped carbon frameworks with Fe, Co, or Ni nanoparticles. Despite using a fixed nitrogen precursor, the resulting samples exhibited varying nitrogen concentrations, which is effective for ORR. Benefiting from the O2 adsorption property, these nanostructures are utilised as effective gas sensors. Moreover, Ni nanoparticles encapsulated porous carbon nanostructures with various concentrations of N-doping in carbon shells are explored for hydrogen generation in alkaline media (aq. 1 M KOH). The variation of nitrogen concentrations in carbon layers creates defects, which enhance the surface area and porosity, exposing extensive active sites for catalytic reactions.

Furthermore, these nanostructures are efficiently utilized as a cathode for energy storage devices, especially Zn-air batteries (ZAB). We have shown that the monometallic (Co) nanoparticles encapsulated CNTs or carbon nanostructures synergistically enhance the performance of ZAB. We demonstrated that, the effectiveness of the synergy effect increases as two metals are alloyed (CoRu). The variation in metal content (Co:Ru :: 3:1, 1:1, and 1:3) further affects the electronic structure and reduces the adsorption/desorption of oxygen intermediates, and improves ORR/OER kinetics, which enhances the ZAB performance. Further alloying as trimetallic alloy (i.e., Fe-Co-Cr) helps strengthening the synergy effect. Additionally, variation in the metallic content in the alloy adds value-added electronic modulation, which is beneficial for improving the performance of ZAB. Hence, in this thesis, we have designed and demonstrated the highly efficient metal particles embedded graphitic frameworks as electrocatalysts for HER, OER, and ORR in alkaline medium.