Unmasking the Latent Passivating Roles of Ni(OH)₂on the Performance of Pd-Ni Electrocatalysts for Alkaline Ethanol Fuel Cells

Nicked-based metal-organic framework-derived carbon (Ni/MOFDC) and its acid-treated counterpart (AT-Ni/MOFDC) have been prepared as supports for palladium nanoparticle electrocatalysts (Pd/Ni/MOFDC and Pd/AT-Ni/MOFDC). These materials have been prepared using facile microwave-assisted techniques. Several spectroscopic and microscopic techniques (such as FTIR, Raman, PXRD, XPS, XANES, FT-EXAFS, and TEM) have been used to thoroughly characterize physicochemical properties of the materials. It is revealed that acid treatment successfully cleaned the metallic Ni surface of the passivating hydroxides (Ni(OH)2 and NiOOH) to generate a very low concentration of Ni nanoparticles on the carbon support. The Ni-deficient Pd/AT-Ni/MOFDC shows excellent electrocatalytic performance toward ethanol oxidation reaction (EOR) in the alkaline medium compared to the Ni-hydroxide-rich Pd/Ni/MOFDC counterpart. As a proof-of-concept, these electrocatalysts have been employed as anodes and demonstrated for membraneless direct ethanol microfuel cells (μ-DEFCs) with a micro-3D-printed cell, with FeCo/C as electrocatalyst for the oxygen reduction reaction at the cathode. The Pd/AT-Ni/MOFDC displays increased peak power density (Pm = 26.49 mW cm-2) with 68% voltage retention after a 24 h galvanostatic discharge test at 40 mA cm-2 and reduced impedance. The improved electrocatalytic properties of the Pd/AT-Ni/MOFDC underscore the need to clean the nickel surface of its passivating hydroxides to harness its full promotional activities toward alcohol oxidation reaction on precious metal electrocatalysts.