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

Abstract

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 I 2D/I G 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.

Degradation of Carbon Electrodes in the All‐Vanadium Redox Flow Battery

“…The Raman spectral mapping of the fresh heat-treated carbon paper (Figure 5a), shows that a higher I D /I G ratio was present on the fibers compared to the graphitic regions (made up of the carbonized binder), due to the more amorphous structure of the carbon fibers. [3] This variation between the graphitic and fiber regions has also been observed for untreated carbon paper. [3] Raman spectral mapping for 100-cycle degraded positive electrode (Figure 5b, where red color in the spectrum of colors indicates high structural defect intensity and blue color indicates lower defect intensity) indicates an increase in the I D /I G ratio compared to the fresh heat-treated carbon paper, suggesting an increase in lattice defects due to chargeÀ discharge cycling operation at the positive side.…”

“…[3] This variation between the graphitic and fiber regions has also been observed for untreated carbon paper. [3] Raman spectral mapping for 100-cycle degraded positive electrode (Figure 5b, where red color in the spectrum of colors indicates high structural defect intensity and blue color indicates lower defect intensity) indicates an increase in the I D /I G ratio compared to the fresh heat-treated carbon paper, suggesting an increase in lattice defects due to chargeÀ discharge cycling operation at the positive side. The Raman spectral mapping of the 100-cycle degraded negative electrode (Figure 5c) showed a reduction in overall defect distribution of I D /I G compared to heat-treated and positive electrodes.…”

“…Currently, carbon-based materials such as carbon paper and carbon felt (typically prepared by carbonization of polyacrylonitrile (PAN) which hereafter will be referred as 'PAN-based carbon paper') are preferred materials for application in VRFB because of their low cost, high surface area, electrochemical activity, electronic conductivity, porosity, ease of handling and relatively good mechanical strength. [3] Other carbon nanomaterials (e. g. carbon nanotubes, [4] carbon nanofibers, [5] and nano-doped graphene-based materials [6] ) can be added to the electrodes to enhance the interfacial kinetics, as well as increase the active surface area. The different components in the complex, heterogeneous carbon electrode materials in contact with the electrolyte in the VRFB can undergo degradation processes under the electrochemical chargeÀ discharge conditions, affecting the long-duration performance of the battery.…”

Electrospinning as a route to advanced carbon fibre materials for selected low-temperature electrochemical devices: A review

“…The nitrogen atoms created more defect edges and promoted wettability of electrospun nanofibres, and hence improved its performance towards electron and mass transfer. A common method of improving the performance of carbon electrodes for VRFBs is to heat-treat them in air to add oxygen functionality; though this increases the wettable surface area, it has recently been suggested by Singh et al [137] that nitrogen doping of carbons can have a catalytic effect for the V redox couples, beyond that of simply increasing the surface area with oxygen treatment. They found that though their N-doped carbon papers had an electrochemical surface area (ECSA) 16 times less than that of oxygen treated papers, there was superior catalytic activity towards the VO 2+ /VO2 + couple for the former, meaning N-doping of materials for RFBs electrodes should be further pursued in order to advance the state-of-the-art from the simple air/heat treatments most widely applied in the literature (for example, as outlined in an early work from Sun and Skyllas-Kazacos [138]).…”

Photoelectrocatalytic biosynthesis fueled by microplastics

“…The rationale of selecting CFP was its high electrical conductivity, large surface area, high chemical stability and good mechanical strength. 8 The CFP electrode exhibited randomly oriented microfibers with carbon-based functional groups (e.g., conjugated C=C, C-C, C-OH, and COOH). In depth structural characterization via scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy are shown in Supplementary Information (Supplementary Fig.…”