Why shouldn’t double-layer capacitance (Cdl) 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 (Cdl) and the charge transfer resistance (Rct). In general, a better active catalyst always tends to have higher Cdl and lower Rct. The non-Faradaic Cdl 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 Cdl and Faradaic oxygen evolution reaction (OER) activity trend between stainless steel (SS) and bleached stainless steel (BSS). BSS apparently exhibited higher Cdl 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 Cdl to justify the purely Faradaic electrocatalytic activity trend.