*3.2. Material Characterizations*

Phase characterization of prepared materials was performed using powder X-ray diffraction (XRD, Bruker (Billerica, MA, USA) D8 system in Bragg–Brentano geometry, Cu K-α radiation (λ = 0.15418 nm)). Specific surface area was calculated by Brunauer–Emmett–Teller (BET) analysis of N2 adsorption/desorption isotherms (AUROSORB-1, Quantachrome, Boynton Beach, FL, USA). Transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX) (TECNAI F30 operated at 300 kV) were used to study the surface morphology and composition of the prepared materials.

For ex situ and operando X-ray absorption spectroscopy (XAS) measurements, catalyst powders were dispersed in a mixture of isopropanol and Milli-Q water in the equal ratio sonicated for 30 min. The ink was then spray coated on Kapton film. XAS spectra at the Co K-edge and Fe K-edge were recorded at the SuperXAS beamline of the Swiss Light Source (PSI, Villigen, Switzerland). The incident photon beam provided by a 2.9 T superbend magnet source was collimated by a Si-coated mirror at 2.5 mRad (which also served to reject higher harmonics) and subsequently monochromatized by a Si (111) channel-cut monochromator. A Rh-coated toroidal mirror at 2.5 mRad was used to focus the X-ray beam to a spot size of 1 mm by 0.2 mm maximal on the sample position. The SuperXAS beamline [55]

allowed for the rapid collection of 120 spectra during a measurement time of 60 sec (QEXAFS mode), which were then averaged. The spectra of samples were collected in transmission mode using N2 filled ionization chambers, where a Co foil (of Fe foil) was placed between the second and third ionization chamber as a reference to calibrate and align collected spectra. Extended X-ray absorption fine structure (EXAFS) spectra were analyzed using the Demeter software package (0.9.26, Bruce Ravel, Washington D.C., USA) [56], which included background subtraction, energy calibration (based on the simultaneously measured Co reference foil) and edge step normalization. The resulting spectra were converted to the photoelectron wave vector k (in units Å−1) by assigning the photoelectron energy origin, E0, corresponding to k = 0, to the first inflection point of the absorption edge. The resulting χ(k) functions were weighted with k<sup>3</sup> to compensate for the dampening of the EXAFS amplitude. These χ(k) functions were Fourier transformed over 3–12 Å−<sup>1</sup> and plotted. The scattering paths used for the EXAFS fittings of all catalysts were generated from the structure of CoOOH [57] for the first Co–O coordination shell using the FEFF6.2 library. Refer to Supplementary Information S6 for detailed description of FT-EXAFS fittings.

#### *3.3. Electrochemical Characterization*

The electrochemical activities of the prepared catalysts were evaluated in a standard three-electrode electrochemical cell with the thin-film rotating disk electrode (RDE) [58]. The setup for OER and cyclic voltammetry (CV) consists of a potentiostat (Biologic VMP-300, Cary, NC, USA) and a rotation speed controlled motor (Pine Instrument Co., AFMSRCE, Grove City, PA, USA). All the electrochemical measurements were performed at standard room temperature using a reversible hydrogen electrode (RHE) as the reference electrode in 0.1 M KOH. A gold mesh was used as the counter electrode. A Teflon cell was used to contain the electrolyte with the electrodes immersed under potential control. A porous thin film electrode was prepared by drop-casting a catalyst ink on a polished glassy carbon electrode (5 mm OD/0.196 cm2) [58]. The catalyst ink was prepared from a catalyst suspension made from sonicating (Bandelin, RM 16 UH, 300 Weff, 40 kHz, Berlin, Germany) 10 mg of catalyst in a solution mixture of 4 mL isopropyl alcohol (Sigma-Aldrich, 99.999%, Darmstadt, Germany) and 1 mL of Milli-Q water (ELGA, PURELAB Chorus, High Wycombe, UK), and 20 μL of Na+-exchanged Nafion [4]. The 0.1 M KOH electrolyte was prepared by dissolving KOH pellets (Sigma-Aldrich, 99.99%, Darmstadt, Germany) in Milli-Q water.

First, 25 reverse potentiostatic sweeps of CV were conducted in synthetic air-saturated electrolyte from 1.0 to 1.7 VRHE at a scan rate of 10 mV s−1. Then, chronoamperometric measurements were carried out holding each potential step for 30 seconds to obtain steady-state currents at each potential from 1.2 to 1.7 VRHE while rotating the working electrode at 900 rpm. The potential stability of catalyst materials was conducted using the same setup, by stepping potential between 1.0 and 1.6 VRHE 500 times holding 10 seconds at each potential to collect currents at steady-state. Five cycles of CV and electrochemical impedance spectroscopy (EIS) were carried out after every 100 cycles. All measured currents were normalized by the mass of catalyst loading, and potentials were corrected for the ohmic drop obtained from EIS.
