Developing New Energy Material on improvement of Electrochemical activity for Energy Storage Devices

Abstract

We have systematically developed the new metal-free catalysts for hydrogen evolution reaction (HER), oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) in fuel cell which generate electricity by electrochemically reducing oxygen and oxidizing fuel into water as the byproduct. In this work, we have investigated metal-free materials as efficient catalyst such as the heteroatom-doped graphenes (GXs, where G and X mean graphene and heteroatom dopant) for HER and the heteroatom-doped C3N4 for OER and ORR. We compared doping effects on the electronic structure and HER activity with the second row elements (B and N) and third row elements (Si, P and S) as well as OER and ORR activity with P, S and PS. From the doping effect which shows better performance, we have firstly presented evidence that the structural deformation and periodic lattice defects play the fundamental role in the HER activity of GXs by adjusting the electronic property of graphene. We found that the third row elements-doped graphenes have higher HER activity with out-of-plane structural deformation compared to the second row elements-doped graphenes keeping planar structure. In addition, the third row elements-doped graphenes (GSi, GP and GS) show an interesting physical regularity described by a simple 3N rule: a doped graphene gives outstanding HER activity with sustained metallic property when its primitive cell size has 3N x 3N (N is integral) supercell size of pure graphene. Therefore, we address that a comprehensive understanding of the structure-activity relationship can pave the way to the development of new electrocatalytic materials. Secondly, the doping of C3N4 results in the best performance in terms of volcano plot between the activity of OER and ORR reactions and the adsorption free energy of intermediates on doped C3N4 materials. For P,S-doped C3N4, the lowest OER and ORR overpotentials were estimated to be 0.47 V and 0.36 V in alkaline media and 0.55 V and 0.37 V in acidic media, which are comparable to state-of-the-art RuO2 and Pt catalyst, respectively. OER and ORR with the minimum overpotentials arises from different nitrogen and carbon sites of the same structure in alkaline and acidic environment, respectively. These calculations suggest that substitution of P and S on C3N4 can increase the catalytic activity of OER nad ORR reactions as efficient electrocatalysts for key energy conversion processes including oxygen evolution reactions and oxygen reduction reactions.