Synthesis, Characterization and Electrochemical Testing of Salt-Templated Carbon-Carbon Composites for Application in Vanadium Redox Flow Batteries
Authors: Schnucklake Maike , Kücken Sophie , Tichter Tim , Thomas Arne , Roth Christina 
Published: February 2020

New, robust and low-cost concepts for energy storage will be of vital importance in the future. They are needed to stabilize the electric grid, when an increasing amount of only intermittently-available renewable energies is implemented. Vanadium redox flow batteries (VRFB) are a promising solution to this issue, as they are efficient and do not involve solid phase transformations, which should result in a long cycle life. In VRFBs, vanadium in its four oxidation states serves as redox couples in sulfuric acid solution and is pumped through the porous electrodes. These are usually carbon felts derived from polyacrylonitrile (PAN) or rayon, which has been heat-treated at temperatures >1200 °C for sufficiently high electron conductivity.
With respect to the highly acidic electrochemical environment, such rather macroporous carbon felt electrodes offer good stability, mechanical strength and catalytic activity at comparatively low cost. However, the electrochemical activity of the untreated carbon felts for the positive (V4+/5+) and negative (V2+/3+) reaction is rather poor. That is why a conditioning step (30 h, 400 °C in air) is required, which reduces the felt’s hydrophobicity and introduces oxygen-containing functional groups on the carbon surface enhancing the performance significantly, in particular at the negative side. For nitrogen-containing high surface area carbons[1] and heat-treated, electrospun carbon materials[2] an increased catalytic activity has also been reported.
In this paper, we report on the synthesis, structural and electrochemical characterization of carbon-carbon composites, which were obtained by a salt-templating approach. By this procedure, a 3D catalytic network with high surface area was integrated into the conducting, porous macro-felt with the intention to make the subsequent activation treatment in air obsolete[3]. A commercially-available carbon felt in its stabilized, but not graphitized “green” state was embedded in a mixture of a heteroatom-containing carbon source (e.g. a nitrogen-containing ionic liquid) and a eutectic salt and heat-treated under argon atmosphere to form the respective heteroatom-doped carbon. After the reaction, the felt was separated from visible bulk N-doped carbon particles, which were not embedded in the felt and thoroughly washed to remove the remaining salt phases. Thus, a porous high surface area carbon-carbon composite electrode was obtained.
BET analysis revealed a surface area of 126 m²g-1, which is more than a 100 times higher than for the pristine felt with a literature value of < 0.5 m²g-1. Scanning electron micrographs (SEM) were recorded to analyze the distribution of the N-containing catalytic carbon phase within the coarse felt, while a nitrogen content of up to 4.5 wt.% was determined by elemental analysis. Moreover, X-ray photoelectron spectroscopy (XPS) showed two dominant peak contributions with binding energies at 398.3 eV and 400.9 eV, corresponding to pyridinic and quaternary-graphitic nitrogen atoms. Cyclic voltammetry tests demonstrated promising activities for both the positive and the negative electrode reaction comparable to the state-of-the-art heat-treated carbon felts. In future studies, the kind and amount of the impregnated material will be varied and precise kinetic measurements performed in order to separate catalytic contributions from the salt-templated phase from the commercial felt.

Figure 1