Porous electrodes, with unique continuous pores and large surface area, have a wide application in electrochemical devices such as batteries and capacitors. While the catalytic effect of these electrodes is shown correlated to their structures and surface compositions, little has been done to quantify the correlation and understand the underlying mechanism, partly due to the lack of an accurate method to measure the kinetics of electron transfer. Routine measurements on planar electrodes do not necessarily reflect the kinetics on the porous electrodes given the distinct structures and surfaces.

In my dissertation, I successfully developed two methods to quantify the electron transfer rate on porous electrodes. Porous carbon electrodes, as an indispensable component in redox flow batteri...[ Read more ]

Porous electrodes, with unique continuous pores and large surface area, have a wide application in electrochemical devices such as batteries and capacitors. While the catalytic effect of these electrodes is shown correlated to their structures and surface compositions, little has been done to quantify the correlation and understand the underlying mechanism, partly due to the lack of an accurate method to measure the kinetics of electron transfer. Routine measurements on planar electrodes do not necessarily reflect the kinetics on the porous electrodes given the distinct structures and surfaces.

In my dissertation, I successfully developed two methods to quantify the electron transfer rate on porous electrodes. Porous carbon electrodes, as an indispensable component in redox flow batteries (RFBs), are chosen as the model electrode for the kinetics study. In the first method, we encapsulate the porous carbon electrode in epoxy and polish the cross-section to transform it into a planar electrode. Rate constants measured through cyclic voltammetry (CV) agree well with the results from RFB tests, but the method is limited to redox couples of rapid kinetics and the cross-section of porous electrodes.

The second method employs diffusion-less CV. We imbibe electrolytes in the porous carbon electrodes and scan CV slowly to remove the effect of diffusion. The results can be well simulated with the Butler-Volmer equation and Faraday’s law to derive rate constants. The rate constants can be further related to the peak separations of CV through the derivation of a dimension-less parameter. The method reveals kinetics of common RFB redox couples at the orders of 10^-8 to 10^-4 cm/s, much lower than those on planar glassy carbon but consistent with RFB tests. We also confirm that heat treatment, routinely used in RFB research, slows down the kinetics of V(II)/V(III) and V(VI)/V(V) due to the increase in surface O/C ratio. At last, we show that the method can extend to porous metals, including metal foam electrodes with micrometer-size pores and nanoporous metals with nanometer-size pores. The method can thus serve to guide the design of electrodes and understand the mechanism of electrochemical reactions.