Selective electrooxidation of glucose towards gluconic acid on Ni@Au foam electrodes

Ni@Au electrodes are prepared by galvanic replacement of Ni atoms of a commercial Ni foam by Au atoms. The physicochemical characterizations indicate gold atomic ratios of ca. 6 %, independently on the galvanic replacement time (1, 2 and 3 min), but differences in the structure of the deposited gold layers. The shapes of the cyclic voltammograms recorded in a 0.1 M NaOH aqueous electrolyte indicate that both Au and Ni sites are accessible. In the presence of 0.1 M glucose, the same oxidation onset potential of ca. 0.3 V vs RHE and a comparable activity in terms of achieved geometric current densities were recorded for all the Ni@Au electrodes. The long term electrolyses of 0.1 M glucose in 0.1 M aqueous KOH electrolyte on the Ni@Au electrodes performed at cell voltages corresponding to anode potentials of 0.575 V, 0.675 V and 0.775 V vs RHE show a surprising excellent stability over 5 h, which is explained by the presence of a Ni(OH)₂ layer on the surface of the Ni foam in contact with the deposited gold layers. Conversions up to 60 % are obtained after 5 h electrolyses with the Ni-Au electrode obtained after 3 min deposition, with 100 % selectivity and faradaic efficiency towards gluconic acid for all the electrodes and for the lower potential of 0.575 V vs RHE. Increasing the glucose and KOH initial concentrations decreases the conversion rate, selectivity and faradaic efficiency.