Aqueous redox flow battery (RFB) technology is limited by relatively low energy and power densities; thus positive redox couples with higher thermodynamic potentials are of interest. However, higher voltage couples introduce additional complications related to electrode corrosion and the possibility of oxygen evolution as an un-wanted side reaction. In this effort, the stability and catalytic activity for typical carbon felt electrodes were determined for the positive reaction in common (Br^-/Br₂, V^4+/5+) and higher voltage (Mn^2+/3+, Ce^3+/4+) RFB chemistries. Weight loss measurements were performed as a test of carbon oxidation and/or loss of material due to mechanical breakdown. In addition to baselining existing carbon electrodes, mixed metal oxide (MMO) electrodes provided by De Nora Tech were evaluated in similar tests to provide a comparison of performance.

Carbon felt electrodes were tested in each electrolyte. The felt was Morgan Advanced Materials’ VDG grade, 1/8” (3.2 mm) nominal thickness. This is a ‘graphitic’ felt, which is produced by pyrolysis of a rayon precursor at 1900°C. The felts were heat treated before use to improve their wettability; the heat treatment consisted of being held at 400°C in air for 6 hours. For each redox couple the following electrochemical measurements were made:

• Polarization curves (initial, final and at intermediate times)
• AC impedance (initial, final and intermediate)
• Polarization at a fixed current density for at least one week of continuous operation (typically at 100 mA/cm²) in a symmetric cell arrangement where one electrode was continually performing the oxidation reaction, and one electrode was continually performing the reduction reaction)

Each felt electrode was weighed prior to, and after, being tested. Oxygen evolution rates were estimated from the difference between polarization curves obtained with and without the redox couple in solution. Similar measurements were made with six different MMO formulations which were deposited on titanium foam substrates. All six formulations are commercially available from De Nora Tech.

With each of the four redox couples (which are all at +1V vs NHE or higher) noticeable decay in the electrochemical performance of the felt electrodes was observed over one week on test. With the bromine and manganese electrolytes, significant weight loss of both the felt electrode and the graphite current collector was found, particularly for the positive (oxidizing) electrode in each cell. In the vanadium electrolyte, a distinct performance loss was observed; this is likely due to oxidation of the carbon surface as reported by Bourke et al [1,2]. The positive electrodes in the Vanadium cells lost between 2-10% of their weight. Several trials were conducted in this system as the weight loss results were less consistent than with the other chemistries. With the Ce couple, which has the highest potential, significant amounts of oxygen evolution (>50% of the total current) were observed with carbon felt, as well as the desired Ce³⁺ to Ce⁴⁺ reaction.

All of the MMO formulations were highly active for each of the positive electrode chemistries, although there were differences in activity observed between the different electrodes as shown in Figure 1 below. For the vanadium and bromine electrolytes, the redox potential is sufficiently low such that oxygen evolution was minimal. For the manganese electrolyte, two of the MMO formulations yielded greater than 90% selectivity for manganese oxidation. For all of the MMO electrodes tested with the cerium couple, oxygen evolution is pre-dominate (>90% of the total current).

References:

1. A. Bourke, M. A. Miller,R. P. Lynch, X. Gao, J. Landon, J. S.Wainright, R. F. Savinell, and D. N. Buckley, J. Electrochem Soc., 163(1) A1, (2016)
2. A. Bourke, M. A. Miller, R. P. Lynch, J. S. Wainright, R. F. Savinell, and D. N. Buckley, J. Electrochem. Soc. 162(8): A1547, (2015)

Figure 1: Kinetic activity for graphite and De Nora MMO electrodes as measured by the apparent exchange current density for the vanadium, bromine and manganese redox couples. The exchange current density was calculated based on the geometric area of each electrode, not the total electrochemically active area.