*3.4. Operando Flow Cell Study*

The homemade operando XAS electrochemical flow cell used in this study was extensively described previously [59]. As explained in Section 3.2, electrode materials were spray coated at the center of Kapton films. Black pearl-2000 (Cabot Corp., Boston, MA, USA) was used as the counter electrode material. A silver chloride electrode (Ag+/AgCl) (Hugo Sachs Elektronik, March, Germany) was used as the reference electrode. During electrochemical testing, the electrolyte was drawn into the cell and collected in a syringe at the flow rate of 0.1 mL min<sup>−</sup>1. The chronoamperometry measurement was carried out holding for 120 sec at each anodic potential step from 1.2 VRHE to 1.54 VRHE, and again during the reverse sequence back to 1.2 VRHE. At each potential step, both transmission XAS spectra at

the Co and Fe K-edges were collected simultaneously for 60 seconds. The energy position of Co and Fe K-edge were read with respect to the half of the edge-step.
