In this work, the PtNiRu/SnO₂ nanocatalyst demonstrates the highest specific activity toward ethanol oxidation, despite having the lowest electrochemically active surface area (ECSA) among the tested samples. This raises a concern regarding the reliability of ECSA-based normalization in assessing catalytic performance, especially when factors such as SnO₂ coverage or aggregation may significantly reduce the accessible platinum surface area. Wouldn’t this potentially lead to an overestimation of the intrinsic activity of the catalyst? In such cases, wouldn’t it be more balanced to complement ECSA-normalized values with mass activity or geometric current density to ensure a fairer and more accurate comparison across different catalyst formulations?
The previous comment rightly points out a critical inconsistency in the interpretation of catalytic activity. The PtNiRu/SnO2 nanocatalyst demonstrates the highest specific activity despite having the lowest ECSA, which indeed raises concerns about relying solely on ECSA-normalized metrics, especially when surface blockage by SnO2 or aggregation can suppress true electrochemical accessibility. This calls into question the intrinsic activity values reported and their comparability across samples with structurally distinct surface morphologies.
In addition to that valid concern, I’d like to raise a further issue regarding the lack of quantitative mass-normalized or geometric current density data, which is essential for evaluating practical applicability and catalyst efficiency in real fuel cell configurations. Specific activity (normalized to ECSA) may not reflect actual current output per mass of platinum or per unit electrode area, both of which are critical metrics for economic and technological viability. Moreover, aggregation and SnO2 overloading (as acknowledged in the text) may lead to highly non-uniform catalyst dispersion on the electrode, which further complicates ECSA-based normalization.
Could the authors clarify whether the mass activity (e.g., mA/mg Pt) or geometric current densities were also evaluated, and how these metrics compare across the tested catalysts? Given that SnO2 was added in excess and partial detachment or aggregation was observed (Figure 2 and discussion), relying only on ECSA-based comparisons may obscure actual electrocatalytic efficiency and introduce substantial uncertainty.