Sponge‐Like Microfiber Electrodes for High‐Performance Redox Flow Batteries

Abstract

stack size. [11] Within the cells, the power density is an overall result of activation, ohmic, and concentration loss, which are affected by cell components with varying degrees. [12][13][14][15] Electrodes, performance-determining components in RFBs, and their geometric and surface properties significantly impact cell performance by influencing the electrolyte transport and the redox reaction. [16,17] Conventionally, woven or nonwoven carbon fibrous materials, such as carbon felts, papers, and cloths, have been employed as electrodes for RFBs, primarily due to their high electrical conductivity, high mechanical strength, and good (electro)chemical stability. [18][19][20] However, RFBs with pristine carbon electrodes always exhibit poor battery performance due to the intrinsic low electrochemical activity and specific surface area. [21] Therefore, catalyst deposition, [22][23][24][25][26][27][28][29] surface etching, [30][31][32][33] and heteroatom doping [34][35][36] are widely adopted to modify electrodes, among which constructing porous fiber electrode through various etching methods are considered one of the most efficient methods to increase the specific surface area while decoupling the influence on the transport properties. [37] For example, Liu and Zhang et al. etched the carbon paper electrodes with CO 2 and achieved an energy efficiency (EE) of nearly 80% at 140 mA cm −2 in a vanadium redox flow battery (VRFB). [30] Liu and Xi et al. constructed evenly distributed nanosized holes on fibers through FeOOH-derived Fe 3 O 4 etching and achieved an EE of 57.3% at 300 mA cm −2 . [31] Multiscale porous electrodes through ZnO etching were proposed by Wu and Zhou et al., [38] enabling an EE of 81.9% at the current density of 320 mA cm −2 . Wang et al. fabricated a gradient porous electrode with K 2 FeO 4 activation and demonstrated a 79.74% EE at 200 mA cm −2 . [39] Mukhopadhyay introduced electrochemical exfoliation to modify the electrodes, rendering the redox flow battery to achieve an 86.41% EE at 100 mA cm −2 . [40] However, the top-down modifications have lots of constraints. For example, pore engineering on carbon fibers always consumes additional energy in the post thermal or electrochemical treatment. More importantly, the electrode modification is based on the fixed geometric structure of commercial carbon material, making the transport property of electrodes hard to be tuned. Hence, switching from top-down electrode modification to bottom-up electrode fabrication is imperative to pursue more possibilities for high-performance electrodes.Fabricating fiber-based electrodes with a large specific surface area while maintaining high flow permeability is a challenging issue in developing high-performance redox flow batteries. Here, a sponge-like microfiber carbon electrode is reported with a specific surface area of as large as 853.6 m 2 g −1 while maintaining a fiber diameter in the range of 5-7 µm and a macropore size of ≈26.8 µm. The electrode is developed by electrospinning cross-linked poly(vinyl alco...

Metal‐free Fabrication of Nitrogen‐doped Vertical Graphene on Graphite Felt Electrodes with Enhanced Reaction Kinetics and Mass Transport for High‐performance Redox Flow Batteries

“…Specifically, VG's growth contributes to the increased electrochemical surface area, while nitrogen doping allows highly hydrophilic electrodes to improve mass transfer characteristics. [ 7 ]…”

“…Specifically, VG's growth contributes to the increased electrochemical surface area, while nitrogen doping allows highly hydrophilic electrodes to improve mass transfer characteristics. [7] Figure 4e,f illustrates the results of the EIS investigation into the charge transfer capabilities of the surface/interface between the electrode and electrolyte. The diameter of the semicircle at high frequency, reflecting the charge transport resistance of the interface reactions, for NVG@GF is the smallest among the electrodes characterized, indicating the lowest charge transfer resistance as a result of abundant pores and active sites created by VG structure and N-doping process.…”

“…The most commonly used electrode in VRFB applications is the porous carbonaceous matrix such as carbon paper, carbon cloth, carbon felt, or graphite felt (GF). [ 4–6 ] However, due to low specific surface area and poor electrochemical activity [ 7 ] for vanadium redox reactions, VRFB equipped with pristine electrodes typically work at low‐current densities (<150 mA cm −2 ), resulting in bulk and costly power stack. Therefore, developing electrodes with increased specific surface area and enhanced electrochemical activity is essential to enhance the power density of the system.…”