Work Function Describes the Electrocatalytic Activity of Graphite for Vanadium Oxidation

Abstract

In many applications such as vanadium flow batteries, graphite acts as an electrocatalyst and its surface structure therefore determines the efficiency of energy conversion. Due to the heterogeneity of the material, activity descriptors cannot always be evaluated with certainty because the introduction of defects is accompanied by a change in surface chemistry. Moreover, surface defects occur in multiple dimensions, and their occurrence and influence on catalysis must be separated. In this work, we have studied the surface of graphite felt electrodes by different methods in terms of morphology and chemistry to understand the electrocatalytic activity. We then defined the interaction between the surface and the electronic structure with particular emphasis on the work function and valence band. Using model catalysts with different architectures, we established correlations between the electrocatalytic activity and the size of the conjugation and the orientation of the edges. Finally, it was possible to link the level of the work function to the electrocatalytic activity.

Embedding Atomically Dispersed Iron Sites in Nitrogen‐Doped Carbon Frameworks‐Wrapped Silicon Suboxide for Superior Lithium Storage

“…The lower value of the work function for SiO x /Fe-N-C was beneficial for promoting its electron transfer and catalyze the electrochemical reactions such as the dissociation/formation of amorphous Li x Si. [58,59] Moreover, the electron-density isosurface of SiO x /Fe-N-C also verified higher ability to transfer electrons compared with SiO x /N-C (Figure 6g,h). [60,61] The enhanced performances were attributed to the reasons: 1) Fe-N-C frameworks efficiently prevented the accumulation of SiO x NPs and improved the electrochemical activity of SiO x ; 2) Fe-N-C could both increase the conductivity and effectively alleviate the volume fluctuations caused by nano-SiO x upon cycling; 3) Fe single atoms with catalytic attribute promoted the dissociation of Li x Si phase and enhanced its electrochemical reversibility.…”

Edge Site Catalyzed Vanadyl Oxidation Elucidated byOperandoRaman Spectroscopy

“…To draw reliable conclusions about the processes that occur at the solid−liquid interface, not only must the dissolved electrolyte species but also the signals from the carbonaceous electrode be known. We selected HOPG as a model system to directly compare basal and edge planes of graphite, and, according to our previous study, zigzag edge modified graphene to specifically address the effect of highly active edge sites [35] . The basal plane of HOPG reveals an intense G and 2D band at 1581 and 2665 cm −1 , along with weaker defect‐related features at 1330 and 1620 cm −1 for the D and D’ bands (Figure 1c).…”

“…We selected HOPG as a model system to directly compare basal and edge planes of graphite, and, according to our previous study, zigzag edge modified graphene to specifically address the effect of highly active edge sites. [35] The basal plane of HOPG reveals an intense G and 2D band at 1581 and 2665 cm À 1 , along with weaker defectrelated features at 1330 and 1620 cm À 1 for the D and D' bands (Figure 1c). In contrast, for edge-plane exposed HOPG, the signals originating from the latter are very strong.…”

“…A detailed study of the surface chemistry and electronic structure of graphene modified by polycyclic aromatic hydrocarbons as well as pristine and damaged HOPG can be found elsewhere. [35] Briefly explained, increased activity was not related to a change in surface oxygen content, but additional edge sites reduce the work function of the electrode, which is beneficial for vanadyl oxidation. The electrochemical measurements identified two sample types diverging in electrocatalytic activity for each material class, which thus allows meaningful comparison of spectral differences under working conditions.…”

Diffusion-Mediated Morphological Transformation in Bifunctional Mn2O3/CuO–(VO)3(PO4)2·6H2O for Enhanced Electrochemical Water Splitting

“…The DFT calculation also supports the VOP overlayer to be efficient for charge carrier extraction from the semiconductor interface and a transfer to the electrolyte interface due to its lower work function (φ) value. 63,64 Additionally, VOP consists of the V atom at a higher oxidation state (+4), which is suitable for lowering the water adsorption energy at the surface, thereby providing more reactant species to undergo the oxidation and reduction reactions. 39,40 Therefore, the enhanced electrocatalytic performance of the bifunctional material Mn 2 O 3 /CuO−VOP is due to, first, the morphological transformation of Mn 2 O 3 /CuO into hollow spherical structures generating more electroactive sites and better access to electrolyte diffusion, second, the formation of the n−p heterojunction facilitating better charge accumulation at the semiconductor interface, and lastly, the faster charge transfer kinetics at the electrolyte interface due to the inherent high charge transfer coefficient and lower work function (φ) value of the overlying 2D vanadyl phosphate hexahydrate sheets.…”