Why shouldn't double-layer capacitance (C_dl) be always trusted to justify Faradaic electrocatalytic activity differences?

Electrocatalytic water splitting activity trend is mainly justified using two activity markers namely the electrochemical surface area measured in terms of double-layer capacitance (C_dl) and the charge transfer resistance (R_ct). In general, a better active catalyst always tends to have higher C_dl and lower R_ct. The non-Faradaic C_dl has so far been shown to match with the Faradaic catalytic activity trend. Nonetheless, emphasizing its non-Faradaic origin and the fact that all ion adsorption and desorption sites should not necessarily be electrocatalytically active sites, we show here a strong disagreement between non-Faradaic C_dl and Faradaic oxygen evolution reaction (OER) activity trend between stainless steel (SS) and bleached stainless steel (BSS). BSS apparently exhibited higher C_dl due to increased OER inactive iron oxide contents on the surface while significantly losing its OER activity. This study, therefore, questions the credibility of using non-Faradaic C_dl to justify the purely Faradaic electrocatalytic activity trend.