Relationships between electrochemically active surface area, geometric surface area and exchange current density
The geometric surface area of an electrode Ageom as measured with simple mechanical instruments is generally the point of reference when calculating current densities by dividing the measured currents by this surface area according to j = I/Ageom. As soon as an electrode is not flat and smooth anymore, is moving away from more or less ideal 2-dimenionality [1] this surface area is hardly correct anymore. The true surface area Atrue, sometimes also called electrochemically active surface area EASA, is frequently large than the geometric surface area by almost an order of magnitude when the electrode is rough as prepared by cutting or slicing, when subject to a roughening procedure this difference may grow up to several orders of magnitude. The roughness factor R describing this relationship is given by R = Atrue/Ageom. For many technical applications electrodes having a porous or otherwise 3-dimensional structure are of major and still growing importance, typical examples are metal foams or sintered metal powders.
The question which surface area of such 3-D electrodes should be used when calculating j is not at all an academic one. Values of j, and even more values of the exchange current density j0, are the most important parameter when comparing electrodes with respect to their activity in electrochemical processes. Poorly defined values of A may result in vastly different values of j0 simply because the used values of A have not been properly defined, not because the catalytic activity is really different.
In the present study a platinum electrode with well-defined Ageom (a platinum rod sheathed with an insulating hull of heat-shrinkable tubing) will be characterized with respect to the EASA using the scan rate charging current in the electrochemical double layer region of the platinum electrode and the double layer capacity obtained using impedance measurements. The exchange current will be obtained from the charge transfer resistance Rct using impedance measurements. Starting with a mechanically rough electrode the smoothness will be increased stepwise by mechanical polishing. Changes of the three mentioned parameters as a function of the degree of polishing will be correlated in order to find out which roughness is reflected in j0 and to identify the limit below which further polishing and decrease in EASA will not be reflected in values of j0 anymore.
The results will have an impact on studies of electrocatalytic activities and may be helpful in comparing data.
REFERENCES:
[1] R. Holze, The modeling gap: What we are missing between molecular dynamics of electrode reactions and simulation of battery packs, Electrochem. Energy Technol. 2 (2016) 24