Critical thinking on baseline corrections for electrochemical surface area (ECSA) determination of Pt/C through H-adsorption/H-desorption regions of a cyclic voltammogram

Abstract

Determination of the electrochemical surface area (ECSA) of platinum supported on carbon (Pt/C) electrocatalysts through the hydrogen adsorption/desorption region has been revisited. Conventionally, ECSA is calculated by using the area of the H-adsorption (Had), H-desorption (Hde) or both of the regions to calculate the corresponding charge. Use of an appropriate baseline for the double-layer region is highly important for accurate determination of the associated charge. Unlike pure platinum, where a horizontal baseline for double-layer region is appropriate, the Pt/C catalysts have a skewed baseline for the double-layer region due to presence of surface functional groups from the support carbon. The skewness is not similar for the negative-going (cathodic) and the positive-going (anodic)scans, leading to significantly different Had and Hde charges. Again, the baselines from carbon may vary significantly during the catalyst durability study through an accelerated stress test (AST), due to oxidation of the support carbon. Present study investigates the effects of the baseline contribution on the ECSA calculations of Pt/C. It is observed that the baseline contribution for charge calculation (i) depends on the nature of the carbon support with significantly higher values (~10 fold) for Hde as compared to that for Had and (ii) increases for both the regions with the number of stress cycles during an AST. Hence, for Pt/C catalysts, while using a constant current baseline as a close approximation of the true baseline, charge associated to Had should be used for calculation of ECSA to access the catalyst activity as well as the durability in terms of %ECSA loss during AST.