Sunlight is the ultimate renewable energy resource. Already a variety of solar-powered energy-harvesting systems exist to exploit it, but one of the most popular recent topics is the production of hydrogen through water splitting (WS) for sustainable energy storage. WS consists of two half-reactions: hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Currently, the state-of-the-art electrocatalysts for WS are precious metals, including IrO2 or RuO2 for the OER and Pt for the HER. The HER rate is often limited by the OER due to the more sluggish kinetics, which lowers the overall energy conversion efficiency. In this thesis, a few synthesis methods have been developed to introduce novel and efficient electrocatalysts based on the earth abundant carbon nanostructures for both the HER and OER. The first synthesis protocol is the development of bifunctional catalysts that are active for both the HER and OER is a key factor in enhancing electrochemical WS activity and simplifying the overall system design. Hence, we have developed a metal-free electrocatalyst based on N-dope multiwalled carbon nanotubes (CNTs). N.MWNT efficiently catalyze water splitting to produce both hydrogen and oxygen. Metal-free N.MWNT exhibits activity comparable to or higher than, non-precious metal electrocatalyst. We further introduce a new class of catalyst support based on the polymer-CNT (ES-MWNT) composite for decorating magnetic core-shell nanoparticles (NiFe@γ-Fe2O3 NPs). ES-MWNT catalyst support induces synergistic effect for the NiFe@γ-Fe2O3 NPs resulting in the promising bifunctional electrocatalyst for both the HER and OER. We have also demonstrated the design of Ni and Fe encapsulated in an ultra-thin graphene layer (NiFe@UTG) via pulsed laser ablation in liquid (PLAL) with tuneable structure. NiFe@UTG has the optimal structure of the metal@C materials for efficient hydrogen production in both acidic and alkaline media. The thin carbon-shell prevents metal dissolution in the harsh media and also prevents the agglomeration of the NPs during the long-term electrochemical measurements. The last material synthesis strategy was implemented to show the critical role of catalyst support for immobilizing atomic-scale catalysts to reduce the utilization of the noble metals in energy applications. In this work, ultra-low amount of the Pt atoms (0.02 at%) decorated on the surface of the NiFe@UTG materials show the catalytic activity same as that of commercial Pt/C catalyst. Experimental results combined with DFT calculations reveal the critical role of both metal-core and carbon-shell to achieve this promising activity in Ptat/NiFe@UTG.
|Tila||Julkaistu - 2019|
|OKM-julkaisutyyppi||G5 Tohtorinväitöskirja (artikkeli)|