Porous Nickel–Iron Oxide as a Highly Efficient Electrocatalyst for Oxygen Evolution Reaction

A porous Ni–Fe oxide with improved crystallinity has been prepared as a highly efficient electrocatalytic water oxidation catalyst. It has a small overpotential, a low Tafel slope, and an outstanding stability. The remarkably improved electrocatalytic performance is due to the porous structure, high extent homogeneous iron incorporation, ameliorative crystallinity, and the low mass transfer resistance.


Synthesis of electrocatalysts
The materials were obtained through a co-precipitation strategy with the presence of Tween 85, a polysorbate surfactant with a boiling point above 100 °C.
The oily polysorbate of 10 mL was firstly dissolved in 50 mL of 1 M NaOH solution at 60 °C under vigorous stirring, then a mixed aqueous solution containing Ni(NO 3 ) 2 and Fe(NO 3 ) 3 at different ratios (the total metal concentration is 2.5 M, volume is 20 mL) was added into the hot basic solution dropwisely. The suspension was stirred under the given temperature for 4 h. The solids were collected and washed by centrifugation in a sequence of water (2 times), acetone (1 time) and water (2 times) thoroughly. The obtained solids were dried in an oven at 60 °C and were subjected to further thermal treatment under different temperatures as required. The yield based on Fe is 90%. The bulk samples for comparison were synthesized from the same procedure except the addition of Tween. Samples are denoted as Ni-X-Y, in which X stands for the starting Ni percentage and Y stands for the treatment temperature in 3 degrees Celsius.

Characterization
Powder X-ray diffraction (XRD) patterns of the solids were recorded on a X-ray diffractometer (Rigaku D/Max2550VB+/PC, Cu Kα, λ = 1.5406 Å, 40 kV and 100 mA). Infrared spectra (IR) were recorded on an IR spectrometer (Bruker, Tensor27) using a standard KBr pellet technique. Thermogravimetric analysis was carried out by heating the dry powder sample at a rate of 5 °C/min with nitrogen flow at 100 mL/min over 25 °C to 800 °C in a TA Instruments SDT Q600. The carbon amount of the sample discussed in the main text was analyzed on a CHNS elemental analyzer (Elementar, Vario EL III). The morphologies of the solids were observed with transmission electron microscopy (TEM, FEI, Tecnai G2 F20) using an accelerating voltage of 200 kV. Energy-dispersive X-ray analysis (EDX) was conducted on an AMETEK Materials Analysis EDX equipped on the TEM. Scanning electron microscopy (SEM, FEI, Quanta 200) was referred for the observation of the electrocatalyst morphology on ITO electrodes. The high voltage was kept at 20 kV and the base pressure in the analyzer chamber was about 5×10 −3 Pa. The Brunauer-Emmett-Teller (BET) surface areas and pore size distributions were measured in Micromeritics ASAP 2020 using the liquid nitrogen adsorption method. The X-ray photoelectron spectroscopy analysis of the samples was carried out on a Kratos AXIS ULTRA XPS. Monochromatic Al Kα X-ray (hν = 1486.6 eV) was employed for analysis with photoelectron take-off angle of 90° with respect to surface plane.

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Correction of the binding energy was carried out using C 1s peak at 284.6 eV arising from the adventitious hydrocarbon.

Electrochemical studies
All electrochemical experiments were carried out using a CH Instruments O 2 ). After recording the concentration of O 2 for 1 h in the absence of an applied 6 potential, CPE was performed at a potential of 1.62 V. O 2 signal was recorded for an additional 2 h after terminating the electrolysis.          Ni-85-500.