X-Ray Micro-Tomography As a Diagnostic Tool for the Electrode Degradation in Vanadium Redox Flow Batteries

The vanadium redox flow battery (VRFB) is an electrochemical device suitable for large-scale energy storage due to its fast response time, long cycle life, and high energy efficiency¹. In VRFBs, the electrodes provide the sites to facilitate the redox reactions between V³⁺ / V²⁺ and V⁴⁺ / V⁵⁺ ions².

Graphite felt is the most common electrode material employed in VRFBs due to its low cost, high stability and conductivity, and reasonable surface area³. Micro-tomograms of fresh graphite felt have already been obtained and used to extract detailed structural characteristics of electrodes⁴. Based on the acquired structural information, 3D pore scale models have been built to investigate the effect of electrolyte flow rate, vanadium ion concentration and electrode morphology on VRFB performance.

However, even though this model-based design approach has helped optimize the fresh felt material, pore scale models for VRFBs would be much more accurate and realistic if the structural information of used (voltage-cycled) felts were incorporated in the model to capture the changes in felt structure and properties during operation.

Using a combination of micro-tomography (CT) measurements of voltage-cycled graphite felts (over ca. 30 charge / discharge cycles), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS) measurements of the same felt electrode, the first description of micro-structural evolution processes occurring in VRFB felts during operation is provided.

Reconstructed 3D images of the VRFB electrodes reveal fiber agglomeration and carbon electrochemical oxidation during continuous battery operation (Fig. 1a and b). Key geometric characteristics of the graphite felt samples are extracted from these 3D images allowing the calculation of porosity, tortuosity and volume specific surface area for each sample. Scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) measurements have been also employed in order to verify the significant structural changes on voltage-cycled graphite felts observed by micro-CT.

References

1. A. Weber et al., J. Appl. Electrochem., 41, 1137 (2011).

2. A. Parasuraman et al., Electrochim. Acta, 101, 27 (2013).

3. B. Li et al., Nano Letters, 13, 1330 (2013).

4. G. Qiu et al., Electrochim. Acta, 64, 46 (2012).

Figure 1. 3D reconstructed image of the fresh (A) and voltage-cycled (B) graphite felt electrode respectively.