Highly Efficient Vitamin-B12 Pyrolyzed N-Co-C Electrocatalyst for Hydrogen Evolution Reaction

Abstract

Water splitting by electrocatalysis into hydrogen via the hydrogen evolution reaction (HER) offers a promising solution for green renewable energy generation1. In past several years, Pt and Pt based nanomaterials are the mostly served as an efficient electrocatalyst for HER; however, due to high-cost and low abundancy of Pt limits its vast utility for the commercial HER application. Therefore, it's highly needed to develop nonprecious metal based electrocatalyst for HER towards cost effective commercial application2. Over the past decades, tremendous efforts have been made for replacing the Pt based electrocatalysts for HER. Recent advances in carbon nanomaterials (RGO, CNT, etc.,) have shown their promising future in energy-related electrocatalytic reactions, such as ORR, OER, and HER. After heteroatom (such as N, B, P, and S) doping, the enhanced catalysis activity of carbon nanomaterials has been widely illustrated for ORR. Among all the reported electrocatalysts, the co-doping of trace transition metals on heteroatoms doped carbon materials leads to form metal complexes (Ex. Co-Nx), showing promising HER activity in the aqueous electrolyte. In addition, the reported density functional theory (DFT) simulation shows that when combining both coordinations into one complex, the optimized charge distribution results in an ideal value of ΔGH in Metal-Carbon-Nitrogen (M-C-N) hybrid system and which is much better than the single or mixture system of M-C and M-N3. In this presentation, we will demonstrate a new HER electrocatalyst based on N-Co-C system. The N-Co-C catalysts are prepared by a one-step pyrolysis process at the high temperature in which vitamin-B12 used as a metal precursor together with reduced graphene oxide (RGO). We observed that the electrocatalytic activities of N-Co-C catalysts are strongly related with the pyrolysis temperature and metal loading. The N-Co-C catalyst pyrolyzed at 800oC showed the highest HER activity with lowest overpotential of 110 mV at a current density of 10 mA cm-2. It shows the optimum Tafel slope of 65 mV decade-1 and excellent stability over 20 h in acidic condition. The as synthesized catalyst was characterized by XRD, XPS and XAS. The results indicate that Co-corrin complexes (Vitamin B12) together with RGO have been decomposed at the high temperature to form N-Co-C structure. This conjugation induces that, downshift of the d-band center of cobalt, and thereby decreases its hydrogen binding energy. This, in turn, favors the electrochemical desorption of Hads and leads to a relatively moderate Co−H binding strength, resulting in the enhancement of the hydrogen evolution reaction. The comparison of HER activity and stability of N-Co-C electrocatalyst in alkaline electrolytes will be discussed at the meeting.