The electro-oxidation of small organic molecules on platinum occurs via a mechanism that includes parallel reaction pathways and steps of adsorption/desorption and oxidation. The oxidation of adsorbed carbonaceous residues usually involves the participation of adsorbed oxygenated species. In this work, the impact of water concentration on the electro-oxidation of methanol on polycrystalline platinum was investigated, using phosphoric acid as supporting electrolyte. The bulk water concentration was varied from 14 to 50 mol L−1, and the system studied under both conventional and oscillatory regimes. Changing the [H2O]/[H3PO4] ratio implies changes in both water concentration and nature and population of electrosorbing anions. The developed strategy allowed at discriminating both effects and study in-depth the contribution of water concentration itself. Bulk water concentration was shown to play a major role on the electro-oxidation of methanol, and the lower the water concentration, the higher the potential at which the reaction sets in. Chronoamperometric data evidenced that water has a stronger impact on the reaction kinetics. The reaction order with respect to the water was estimated at different applied potentials and found to decrease as the potential increases. Concerning the potential oscillations, water concentration was found to affect the oscillatory frequency, waveform and amplitude. In summary, an increase in water concentration led to an increase in the oscillatory frequency; and the amplitude reaches a maximum when [H2O] = 30 mol L−1.

There is an increasing interest in the use of small organic molecules in the interconversion between chemical and electrical energies. Among the strategies to improve the processes of yielding electrical energy in fuel cells and the production of cleaner hydrogen in electrochemical reform, there is the use of kinetic instabilities to improve the conversion and selectivity. Herein, we report on the electrocatalytic efficiency of the oxidation of ethylene glycol, glycerol, and glucose, under regular and oscillatory regimes, on polycrystalline platinum, in sulfuric acid aqueous solution, and at 25 °C. Despite the high overpotentials for the electro-oxidation of these molecules, the electrochemical activity along quasi-stationary potentio-/galvanostatic experiments evidenced that, in all cases, relatively lower potential values, and thus higher activity, are reached during oscillations. Noticeably, higher power densities for the electro-oxidation of ethylene glycol and glycerol under the oscillatory regime were found in a hypothetical direct liquid fuel cell. The use of identical experimental conditions of that of our previous study [J. Phys. Chem. C, 2016, 120, 22365] allowed to discuss some universal trends for seven small organic molecules. We compile the results in terms of the peak current, the maximum poisoning rate found along the oscillations, and the oscillation frequency. The three parameters were found to decrease in the order: formaldehyde > formic acid > methanol > ethanol > ethylene glycol > glycerol > glucose. In addition, we discussed the increase of the voltammetric current due to the self-organized poisoning rate and reinforced the trend that high electrocatalytic activity implies high susceptibility to surface poisoning for this set of species. Finally, the analysis done for all species (formic acid, formaldehyde, methanol, ethylene glycol, ethanol, glycerol, and glucose) adds to the available thermodynamic data and is a benchmark against which the activities under the oscillatory regime at 25 °C may be compared or assessed. This point of reference permits to explore further experimental conditions that are relevant for energy-related devices, including the conversion of chemical into electrical energy and the electrochemical reform to produce clean hydrogen in electrolyzers.

There is an increasingly interest in the use of small organic molecules in the interconversion between chemical and electrical energies. Among the strategies to improve the processes of yielding electrical energy in fuel cells and the production of clear hydrogen in electrochemical reform is the use of kinetic instabilities to improve the conversion and selectivity. Herein we report on the electrocatalytic efficiency of the oxidation of ethylene glycol, glycerol, and glucose, under regular and oscillatory regimes, on polycrystalline platinum, in sulfuric acid aqueous solution, and at 25 oC. Despite the high overpotentials for the electro-oxidation of these molecules, the electrochemical activity along quasi-stationary potentio/gavanostatic experiments evidenced that, in all cases, relatively lower potential values, and thus higher activity, are reached during oscillations. Noticeably higher power densities for the electrooxidation of ethylene glycol and glycerol under oscillatory regime in a hypothetical direct liquid fuel cell. The use of identical experimental conditions of that of our previous study[J. Phys. Chem. C 120 (2016) 22365] allowed at discussing some universal trends for seven small organic molecules. We compile the results in terms of the peak current, the maximum poisoning rate found along the oscillations, and the oscillation frequency. The three parameters were found to decrease in the order: formaldehyde > formic acid > methanol > ethanol > ethylene glycol > glycerol > glucose. In addition, we discussed the increase of the voltammetric current with the self-organized poisoning rate and reinforce the trend that high electrocatalytic activity implies high susceptibility to surface poisoning for this set of species. Finally, the analysis done for all species (formic acid, formaldehyde, methanol, ethylene glycol, ethanol, glycerol, and glucose) adds to the available thermodynamic data and is a benchmark against which the activities under oscillatory regime at 25 oC may be compared or assessed. This point of reference permits to explore further experimental conditions that are relevant for energy-related devices, including the conversion of chemical into electrical energy and the electrochemical reform to produce clean hydrogen in electrolyzers.

Water plays a pivotal role in several electrocatalytic reactions. In the electro-oxidation of small organic molecules, water can for instance assist the oxidation of adsorbed species or inhibit the reaction through the oxidation of surface sites. This paper is the third in a series of studies aiming at evaluating the impact of water concentration on the electro-oxidation of small organic molecules on polycrystalline platinum. The study was performed in phosphoric acid electrolyte, and the water concentration was varied from 14 to 50 mol L-1. Voltammetric profiles and potential oscillations were studied at distinct [H2O]/[H3PO4] ratios and constant concentration of formic acid. There is a diminution in the catalytic activity accompanying the decrease in the water concentration. The removal of carbon monoxide demands oxygenated species and can proceed in the absence of water for ethanol and methanol, but not for formic acid. Under oscillatory regime, the impact of water concentration, it is seen that higher water concentration implies higher oscillation frequencies, shorter and less stable time-series, and sharper transitions from low to high potential, where the adsorbed carbon monoxide is oxidized by adsorbed oxygenated species. Results are discussed in connection with the voltammetric study and also compared with other parent systems.