Gas-phase Electrolysis of Hydrobromic Acid

Abstract

The gas phase electrolysis of hydrobromic acid (48 wt%) was experimentally studied for the development of the bromine-based thermochemical hydrogen production processes, by using a fuel-cell-type electrolyzer with graphite felt electrodes and phosphoric acid (85 wt%) electrolyte (Fig.1). The addition of noble metal catalysts to the graphite electrodes improved the electrode performance in the following order: Pt>Pd≈Rh>Ru (Fig.2). The electrode performance depended on both the way for platinum-loading and the amount of platinum loaded on graphite: the electrodes loaded with platinum more than about 0.1 mg/cm2 by an impregnation method exhibited excellent performance (Figs.3 and 4). Both electrolytic current density and the degree of HBr conversion increased with increasing partial pressure of HBr (P_HBr) ; at-P_HBr = 1 5 kPa and at a cell voltage of 0.80 V, the current density and the degree of HBr conversion was 90 mA/cm² and 17%, respectively (Fig.5). The electrode performance scarcely depended on temperature in the range from 398 K to 453 K and the current efficiency was maintained at about 100% (Fig.6). By introducing a reference platinum wire electrode (Fig.8), anodic and cathodic polarization curves were measured. These polarization curves obtained for graphite electrodes revealed that the addition of platinum was effective particularly to reduce the hydrogen overvoltage at the cathode (Fig.9). A continuous operation test at O.80 V proved that a stable current density was obtained for the initial period of about 15 h, and thereafter it tended to drop gradually due to an increase of the cell resistance (Fig.10).