(217f) Kinetic Investigation of Nickel-Iron Layered Double Hydroxide for Hydrogen Evolution in an Alkaline Electrolyte

Nickel-iron layered double hydroxide (NiFe LDH) and other transition metal layered materials have recently gained attention for their simple preparation methods, earth abundance, low cost, and most importantly, high activity toward hydrogen evolution (HER) and oxygen evolution (OER) reactions [1],[2]. In this work, we investigate the kinetics of an active NiFe LDH toward HER in an alkaline electrolyte. The mechanism of HER starts with electrochemical adsorption of a water molecule onto an active site on the cathode to produce the adsorbed species H (a step that is typically referred to as the Volmer step). The hydrogen gas can then be produced from two competing reaction paths: a chemical recombination of the adsorbed species H (Tafel step), and an electrochemical desorption step (Heyrovsky step).

Our NiFe LDH films were deposited on Ni foam substrates via a simple one-step hydrothermal synthesis route in an autoclave reactor maintained at a temperature of 155 C for 12 hr [1]. The films were then used as both OER and HER electrodes in a three-electrode electrochemical cell set up with 1 M KOH solution as an electrolyte.

The deposited film demonstrated an overpotential of 247 mV at a current density of 10 mA/cm² toward the HER. Linear sweep voltammetry (LSV) data were obtained at a scan rate of 1 mV/s between the cathodic potential range of -0.9 to -1.7 V vs Hg/HgO reference electrode. The calculated cathodic overpotential measurements were corrected for the voltage drop corresponding to the uncompensated solution resistance. A Tafel plot constructed from the data shows a Tafel slope of -38.4 mV/dec, which indicates that the HER mechanism is controlled by either the Herovsky or Tafel steps [3]. In addition, electrochemical impedance spectroscopy (EIS) measurements were taken at different potentials within the LSV potential range. The EIS data were then fitted to an Armstrong and Henderson equivalent electric circuit using both Z-view software and EIS Spectrum Analyser to obtain the equivalent circuit components.

Using a least squares method, a theoretical model relating current density to the electrochemical reaction rates [4] was fitted with the LSV experimental results (Figure 1 attached). The chemical reaction rate constants for the individual HER mechanism steps - Tafel, Volmer, and Herovsky - were obtained as a result of the fitting procedure. The resulting kinetic model shows a good agreement between theoretically calculated Faradaic resistance and experimental EIS results (Figure 2 attached).

The kinetics data revealed the Herovsky step as the rate determining step, which is in agreement of the calculated Tafel slope. Furthermore, the kinetics data show a dependence of the HER mechanism on the applied potential. For overpotential values between 0 and -0.4 V (corresponding to an applied potential between -0.9 to -1.6 V vs Hg/HgO), the HER mechanism is a sequence starting with the Volmer step, followed by the Tafel step. At higher cathodic overpotential values, the mechanism starts with a Volmer step followed by a rate controlling Herovsky step, with negligible contribution of the Tafel step. It was also observed that in higher cathodic overpotential region, the Tafel step reaction rate is independent of applied potential. This can be attributed to the surface coverage of the adsorbed intermediate reaching a constant high coverage value in this potential region.

Figure 1: Experimental and simulated results of current vs overpotential

Figure 2: Experimental and simulated results of Faradaic resistance vs overpotential

References:

[1] Lu, Z. et al. (2014). Three-dimensional NiFe layered double hydroxide film for high-efficiency oxygen evolution reaction. Chemical Communications (Cambridge, England), 50(49), 6479-82. doi:10.1039/c4cc01625d

[2] Luo, J. et al. (2014). Water photolysis at 12.3% efficiency via perovskite photovoltaics and Earth-abundant catalysts. Science (New York, N.Y.), 345(6204), 1593-6. doi:10.1126/science.1258307

[3] Zhu, Y., Liu, T., Li, L., Song, S., & Ding, R. (2018). Nickel-based electrodes as catalysts for hydrogen evolution reaction in alkaline media. Ionics : International Journal of Ionicsthe Science and Technology of Ionic Motion, 24(4), 1121-1127. doi:10.1007/s11581-017-2270-z

[4] Krstaji, N., Popovi, M., Grgur, B., Vojnovi, M., & Sepa, D. (2001). On the kinetics of the hydrogen evolution reaction on nickel in alkaline solution Part I. The mechanism. J. of Electroanalytical Chemistry, 512(1-2), 16-26.