Abstract
Ni and Ni-doped with transition metals (TM) such as Fe and Co represent the most suitable electrodes for hydrogen evolution reaction (HER) in alkaline media. Various compositions of co-precipitated Ni1 + xFe2 − xO4 and Ni1 + yCo2 − yO4 nanoparticles were investigated. The intrinsic HER catalytic activity is the same for all the catalysts, which we relate to similar values of the iso-electric point (IEP). However, the mass catalytic activity of the catalysts changes through a modification of the electrochemical surface area. Fractional reaction orders for hydrogen evolution revealed in all catalyst compositions are due to double layer effects and surface acid-base equilibria. Reaction order and Tafel slope of the catalysts are compatible with electrochemical adsorption as the rate-determining step for the HER. Tafel slopes were also evaluated independently from impedance spectroscopy, in good agreement with the polarization curves. Electrodes prepared from catalyst inks containing an anion-exchange ionomer displayed inferior catalytic activity for the HER as compared to electrodes prepared with Nafion in the ink. Chronoamperometry confirmed the sustained superior hydrogen kinetics over time of NiFe2O4 and NiCo2O4 composition over that of NiO.
Highlights
The overpotentials associated with the hydrogen evolution reaction (HER) at high pH have been reported to be exceptionally high compared those at low pH, and correspond to a hundred-fold decrease in activity at Pt, Ir, and Pd.[4]
Development of cheap transition metal-based alkaline HER catalysts will reduce the cost of the electrolysis operation, which is beneficial for overall water splitting application.[8]
There are two crucial drawbacks associated with Ni-based electrodes in alkaline electrolyzers; the first being a high operating cell voltage, and the second is the decrease in cathodic activity with time
Summary
The overpotentials associated with the HER at high pH have been reported to be exceptionally high compared those at low pH, and correspond to a hundred-fold decrease in activity at Pt, Ir, and Pd.[4]. Based on structural and composition investigation, NiFe2O4 and NiCo2O4 will represent NiCo and NiFe oxides with 33 mole-% Ni respectively in all results.
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