Iron–Salen Complex and Co2+ Ion‐Derived Cobalt–Iron Hydroxide/Carbon Nanohybrid as an Efficient Oxygen Evolution Electrocatalyst

Abstract Metal–salen complexes are widely used as catalysts in numerous fundamental organic transformation reactions. Here, CoFe hydroxide/carbon nanohybrid is reported as an efficient oxygen evolution electrocatalyst derived from the in situ formed molecular Fe–salen complexes and Co2+ ions at a low temperature of 160 °C. It has been evidenced that Fe–salen as a molecular precursor facilitates the confined‐growth of metal hydroxides, while Co2+ plays a critical role in catalyzing the transformation of organic ligand into nanocarbons and constitutes an essential component for CoFe hydroxide. The resulting Co1.2Fe/C hybrid material requires an overpotential of 260 mV at a current density of 10 mA cm−2 with high durability. The high activity is contributed to uniform distribution of CoFe hydroxides on carbon layer and excellent electron conductivity caused by intimate contact between metal and nanocarbon. Given the diversity of molecular precursors, these results represent a promising approach to high‐performance carbon‐based water splitting catalysts.


Synthesis of other salen ligands (Salen-2 to 5)
These salen ligands were synthesized as described above except the difference in the feed material, and the corresponding EIS-MS spectra were presented in Figure S2. Other samples with different Co 2+ /Fe 3+ molar ratios were also prepared by the same method.
Their OER activity were determined by LSV curves showed in Figure S3, indicating that the salen ligands with four phenolic hydroxyl groups were more promising as the precursor for preparing metal-carbon hybrid electrocatalysts towards water oxidation reaction.
According to a previously reported literature, [2] Co 1.2 Fe layer double hydroxides were synthesized by a co-precipitation reaction between a metal salt aqueous solution and a Na 2 CO 3 and NaOH mixed solution.

Electrode preparation
The catalyst ink was prepared by dispersing 5 mg of the catalyst into 1ml of ethanol/water (450/500 μl) mixed solution containing 50 μl 5% Nafion, the mixture was then ultrasonicated for 30 min to become homogeneous. The rotating disk electrode (RDE) made of glassy carbon (5 mm diameter, 0.196 cm 2 ) was polished using alumina powder on felt polishing pads prior to use. Subsequently, 12 μl of the ink was dropped on the surface of the glassy carbon with an overall catalyst loading of ~ 0.174 mg cm -2 . Finally, the resulting catalyst film was dried at 60 ℃ for electrochemical measurements.

Electrochemical measurements
Electrochemical measurements were carried out in a three-electrode system on CHI 660E Electrochemical Analyzer (Shanghai Chenhua Instrument Co., LTD) at room temperature in 1 M KOH aqueous solution. The RDE decorated with catalyst film was employed as working electrode, Pt mesh and HgO/Hg electrode were used as counter and reference electrodes, respectively. All potentials reported here were converted to the reversible hydrogen electrode (RHE) scale with E RHE = E HgO/Hg + 0.0592 pH + 0.12 V.
Before electrochemical measurements, the working electrode was activated by a chronoamperometry scan until a stable I-t curve obtianed. Liner sweep voltammetry (LSV) curves corrected with iR-compensation were tested at a scan rate of 5 mV s -1 . Tafel slopes were determined by cyclic voltammetry with a scan rate of 1 mV s -1 to reduce the capacitance and were calculated by plotting overpotential against Log (current density).
Chronopotentiometry curve was obtained at the applied current density of 10 mA cm -2 . The electrochemical impedance spectroscopy (EIS) was performed at an overpotential of 300 mV at the amplitude of the sinusoidal voltage of 5 mV over a frequency range from 0.1 Hz to 10 5 Hz. The double-layer capacitance (C dl ) measured by cyclic voltammetry at different scan rates (10 to 60 mV s -1 with an interval of 10 mV s -1 ) in a voltage range of 1.125 ~1.225 V (nonfaradaic region) was used to estimate the electrochemical active surface area (ECSA). By plotting the capacitive current density (j anodic -j cathodic ) against the scan rate, the value of C dl can be determined as half of the slope.
The TOF values of catalysts deposited on glassy carbon were calculated by assuming that every metal atom is involved in the catalysis:

TOF =
Where j is the current density obtained at overpotential of 350 mV, A is the surface area of the glassy carbon (0.196 cm -2 ), F is the Faraday constant (96485 C mol -1 ) and n is the moles of the metal atom loaded on the electrode.
To determine the faradic efficiency, a home-made single compartment gas-tight cell was