Self-Assembled Monolayers of Vitamin B12 over N-Doped Graphene: A Promising Electro-Catalyst for Hydrogen Evolution and Electro-Oxidative Sensing of H2O2

Abstract

The redox, chemical and biochemical characteristics of molecular hydrogen (H2) and hydrogen peroxide (H2O2) promise sustainable, economically and environmentally friendly means to address some serious global issues related to energy, environment and human health. However, the non-availability of a cheap, efficient and durable electrocatalyst continues to hamper the practical exploitation of H2 and H2O2 as solution to these challenges. In the work to be presented, we shall present a simple strategy for the self-assembling of Vitamin B12 (Vit.B12) over nitrogen-doped graphene (NGr) for fabrication of Vit.B12-NGr nanohybrids; an electrochemically stable, reusable electrocatalyst with an excellent prospect for hydrogen evolution reaction (HER) and non-enzymatic electro-oxidative sensing of H2O2. Through the data recorded for the characterization of the Vit.B12-NGr nanohybrids through various techniques it shall be clearly proven that the conceived method ensures the self-assembling of Vitamin B12 as a monolayer over N-Gr. Results from our detailed voltammetric investigations over the Vit.B12-NGr modified electrodes besides establishing the redox accessibility, stability and enhanced activity of Co(II) centres in Vit.B12-NGr composites also suggest that the presence of NGr in Vit.B12-NGr composite not only assists the directed ET to Vit.B12, but it also improves the stability of the resulting composite by minimizing its loss to the solution phase. Results from our electrocatalytic investigations clearly demonstrate the excellent stability and electrocatalytic activity of Vit.B12-NGr nanohybrid towards HER (Figure 1). For HER, the Vit.B12-NGr nanohybrid displays onset potential of -0.096 V vs RHE, a low Tafel slope of ca. 68.3 mV/dec and an overpotential requirement of just -0.210 V for 10 mAcm-2. Moreover, we shall present results from our electro-sensing investigation that establish that the Vit.B12-NGr is a promising electrocatalytic surface for the electro-oxidative sensing of H2O2. In our investigations we observed that use of Vit.B12-NGr ensures the H2O2 sensing with a sensitivity of 430 µA/mM (8.775 µAmM-1cm-2) and a limit of detection as low as 0.02 µM. These HER activity and H2O2 sensing parameters displayed by Vit.B12-NGr are either superior or comparable to those observed for the various recently reported state-of-the-art HER electrocatalysts and H2O2 sensors. Figure 1. HER performance at various electrodes (A) LSV polarization curves (negative direction) of bare-GCE (black line), NGr/GCE (red line), Vit.B12-NGr/GCE (blue line) and Pt/C (green dotted line)in 0.5 M H2SO4 with scan rate 100 mV/s. (B) The corresponding HER Tafel plot of NGr/GCE, Vit.B12-NGr/GCE and Pt/C. (C) LSV curves on Vit.B12-NGr/GCE surface at the first cycle (black) and after 50 cycles (red dotted) (scan rate 100 mV/s). Figure 1