Direct measurement of electrochemical reaction kinetics in flow-through porous electrodes

• A customized microfluidic cell is devised to study flow-through porous electrodes.
• Tafel analysis is shown for the first time to be feasible on porous carbon paper.
• The kinetics of VO2 +/VO2+ and V2 +/V3 + reactions are directly evaluated and compared.
• The V2 +/V3 + reaction is likely the limiting reaction in vanadium redox batteries.
• This technique has potential for understanding mass transport in porous electrodes.

This work demonstrates the feasibility of measuring electrochemical reaction rates on common flow-through porous electrodes by traditional Tafel analysis. A customized microfluidic channel electrode was designed and demonstrated by measuring the intrinsic kinetics of the V2 +/V3 + and VO2 +/VO2+ redox reactions in carbon paper electrodes under forced electrolyte flow. The exchange current density of the V2 +/V3 + reaction was found to be nearly two orders of magnitude slower than the VO2 +/VO2+ reaction, indicating that this may be the limiting reaction in vanadium redox flow batteries. The forced convection in this technique is found to generate reproducible exchange current densities which are consistently higher than for conventional electrochemical methods due to improved mass transport.

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