Electrodeposited Porous Ni-Fe-Co Thin Films By Hydrogen Templating Onto an Inverted Recessed Disk Electrode

Abstract

Porous electrodes are advantageous for a variety of applications to enhance surface area per volume. Hydrogen templating is one well-known method to generate a porous metal, where metal deposits around generated gas bubbles from the same electrode surface. The technique has been demonstrated with noble (e.g., Au, Ag, Cu) and less noble (e.g., Ni) elemental deposits, but few alloy system.1 Ni-Fe-Co is of interest due to its electrocatalytic ability towards the oxygen evolution reaction for alkaline water splitting. Rafailović et al.2 galvanostatically electrodeposited an alloy at very high current density at pH 2 having 20 micron pore size with nano scale dendritic features, and Li and Podlaha3 potentiostatically electrodeposited Ni-Fe-Co alloys at pH < 0.5 into polycarbonate templates along with hydrogen templating to create pores in nanowires. There was a large distribution of pore size between 10 and 100 nm with the largest percentage between 20 and 40 nm. To better understand the control of pore size in the Ni-Fe-Co alloy system an inverted rotating disk electrode was used to control the limiting current of the proton reduction reaction during alloy electrodeposition. A boric acid, simple salt electrolyte was used with a three-electrode system to generate polarization curves and electrodeposits at different electrode rotation rates. The influence of mass transport and hydrogen bubble formation at different rotation rates were examined and affected porosity. Porosity was characterized by scanning electron microscopy (SEM) and the inspection of current during ferri/ferrocyanide polarization. Porous films were observed for deposits galvanostatically fabricated at current densities above the proton reduction limiting current density, but before water reduction, producing pore sizes on the order of 10s to 100s of nm, but below a micron. The ratio of the applied current density to proton reduction limiting current density was an important factor. J. Plowman, L. A. Jones, and S. K. Bhargava, Chemical Communications, 51, 4331-4346 (2015).D. Rafailović, C. Gammer, C. Rentenberger, T. Trišović, C. Kleber, and H. P. Karnthaler, Nano Energy, 2, 523-529 (2013).D. Li and E. J. Podlaha, Nano Letters, 19, 3569-3574 (2019).