Electrolytic Decomposition of Aqueous Hydrogen Bromide

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  • 臭化水素酸の電気分解

Description

The electrolytic decomposition of aqueous hydrogen bromide was experimentally studied for the development of the bromine-based thermochemica 1 water decomposition cycles. The reversible potentials of both hydrogen and bromine electrodes (EH2, EBr2) were determined from the e. m. f. measurements and their overpotentials on a smoothed Pt electrode were also measured in concentrated aqueous solutions of HBr at 25, 35, and 50°C, respectively.<BR>EH2 shifted linearly to noble potentials with an increase of the molality of HBr (mHBr) due to the increase of the mean molal activity coefficients and was higher than 150 mV (vs. SHE)in the azeotropic solution of HBr (Table 1, Fig.3). As supposed from the Nernst equation for bromine electrode, EBr2 became less noble with increasing mHBr or with decreasing mi, (Table 2, Fig.5). The theoretical decomposition voltage of HBr was about 700 mV or less than that in its azeotropic solutions containing dilute Br2, at anode.<BR>The current-potential relationship of hydrogen evolution reaction at a Pt cathode clearly followed the Tafel equation with a slope of 30 mV/decade (Table 3, Fig.6). The hydrogen overvoltages were less than 100 mV at 100 mA cm-2, but, the additional increases of the overvoltages due to evolving hydrogen bubbles were observed at higher current densities.<BR>The bromine electrode reaction at a Pt anode proceeded almost reversibly and a linear polarization curve was obtained in the range of current density below 100 mA cm-2 (Fig.7). The limiting current density was not observed up to 1000 mA cm-2 in a stirred solution. The bromine overvoltage decreased with increasing mHBr and was considerably smaller than that of hydrogen in the azeotropic solutions of HBr containing Br2.

Journal

  • NIPPON KAGAKU KAISHI

    NIPPON KAGAKU KAISHI 1981 (2), 221-227, 1981-02-10

    The Chemical Society of Japan

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