Expansion of the urea electrocatalytic oxidation window by adsorbed nickel ions

Abstract

The electrocatalytic oxidation of urea using Ni-based catalysts is able to oxidize urea at high rates, reaching hundreds of mA cm−2 depending on electrode and solution properties. Most of the demonstrated systems, however, report a drop-off in current ~ 0.3 V positive of the Ni2+/Ni3+ redox transition resulting in a dramatic decrease of activity. The drop-off limits the electrochemical window available for urea decomposition. To overcome this narrow window, Ni2+ ions are herein adsorbed onto carbon black electrodes cohesively bound with Nafion. The resulting electrodes are electrochemically tested for urea electro-oxidation activity whereby their cyclic voltammograms display no drop-off in activity up to 1.5 V positive of the Ni2+/Ni3+ transition. The oxidation currents of the adsorbed Ni2+ electrodes demonstrate urea decomposition rates nearly identical to Ni-metal films, and impedance measurements show similar charge transfer limitations. Electrodes are monitored after electrochemical cycling with atomic force microscopy to reveal that adsorbed species coalesce into surface particles, which behave like bulk Ni(OH)2, implying that the commonly observed drop-off in current may be due to structural characteristics of Ni(OH)2 and NiOOH. The simplicity of the adsorption process and its ability to widen the electrochemical window for urea oxidation allows new substrates to be incorporated into urea degradation systems without sacrificing high urea oxidation rates.