Electrocatalysis at Electrodes for Vanadium Redox Flow Batteries

Abstract

Flow batteries (also: redox batteries or redox flow batteries RFB) are briefly introduced as systems for conversion and storage of electrical energy into chemical energy and back. Their place in the wide range of systems and processes for energy conversion and storage is outlined. Acceleration of electrochemical charge transfer for vanadium-based redox systems desired for improved performance efficiency of these systems is reviewed in detail; relevant data pertaining to other redox systems are added when possibly meriting attention. An attempt is made to separate effects simply caused by enlarged electrochemically active surface area and true (specific) electrocatalytic activity. Because this requires proper definition of the experimental setup and careful examination of experimental results, electrochemical methods employed in the reviewed studies are described first.

Determination of Rate Constants and Reaction Orders of Vanadium‐Ion Kinetics on Carbon Fiber Electrodes.

“…Another approach has been described by Parson [22] in the IUPAC guidelines, which is shown in Equations (9) and (10). We used this approach in our previous work, [35] with the simplification that n ox;an and n red;an are set to zero, which means that VO 2 + is not taking part in the cathodic PE reaction and VO 2 + is not taking part in the anodic PE reaction.…”

“…an þ a an � n red;cat (10) As can be easily seen, the equations describing the determination of reaction orders from the concentration dependence of the exchange current density of Vetter and Parsons are slightly different. For chemical reactions, the reaction order n i is clearly defined as the partial derivative of the logarithmic reaction rate r with respect to the logarithmic concentration c i [Eq.…”

“…First, it is difficult to estimate the real electrochemically active electrode surface area, which is additionally also varying due to changes in the manufacturing process. Wu and Holze [10] have outlined in their detailed review on catalysts for VFB how important the correct consideration of the active surface area is and introduced the term "true electrochemically active surface area" (EASA). In general, the EASA is calculated using the roughness of the electrode and its geometric surface area.…”

“…In addition, the carbon material itself shows strong irregularities and varies in structure and reactivity. [10] The real electrochemically active surface area should therefore be better determined by measuring the double-layer capacity with electrochemical impedance spectroscopy (EIS) or cyclic voltammetry (CV). [12] Friedl et al have shown the application of these methods on MWCNT-modified glassy carbon electrodes and the possible determination of correct kinetic data.…”

Towards Production of a Highly Catalytic and Stable Graphene-Wrapped Graphite Felt Electrode for Vanadium Redox Flow Batteries

“…(1) Low rate capability (charge/discharge at high current density) (2) Short lifespan caused by low reversibility as well as poor round-trip efficiency (3) Low electrolyte utilization ratio (EU%) (4) Poor power peak values (5) Low columbic efficiency due to the contribution of side reactions Another important issue regarding the electrodes is the low amount of the active sites (i.e., O-containing groups), which affects the penetration of the electrolyte (hydrophilicity) and facilitates the oxygen transfer step, crucial step for the positive half-cell reaction [8,9]. Finally, improved mass transport is beneficial for the battery since it directly affects the rate of electrochemical conversion.…”