Electrocatalytic Activity of Functionalized Carbon Paper Electrodes and Their Correlation to the Fermi Level Derived from Raman Spectra

Carbon paper electrodes are employed for different electrochemical applications such as flow batteries and fuel cells. However, redox reactions such as VO2+/VO2+ in a vanadium redox flow battery have been found to possess relatively slow kinetics, resulting in significant activation losses during operation. In this work, we demonstrate a facile and scalable method for nitrogen doping of carbon paper electrodes, leading to superior electrocatalytic activity. The effects of pyrolytic pretreatments under different conditions on the performance of carbon paper were also studied to elucidate their electrocatalytic activity from a material physics perspective, using Raman spectroscopy. The 2D Raman signature, a specific feature of the carbon structures, was employed to understand the effect of different pretreatments on the Fermi level of the carbon papers, which could help us elucidate their intrinsic electron transfer kinetics. The full wave half-maximum of the 2D Raman band and the intensity ratio I2D/IG were used to indicate changes in the Fermi level relative to the untreated carbon paper, and hence the electrocatalytic properties, which were confirmed using voltammetric techniques. Although heating of carbon paper in air at around 500 °C (a widely used method for activating carbon paper electrodes) increases the surface area by about 16 times compared to untreated and nitrogen-doped carbon paper, the latter exhibits superior electrocatalytic property for VO2+/VO2+, [Fe(CN)6]3–/4–, and the oxygen reduction reaction. This study provides greater physical insights into different pretreatments in terms of the energy barrier at the interface, which will aid the pursuit for better carbon-based electrode materials and provide mechanistic details about charge transfer processes at the interface.