Reverse micelle-based preparation of carbon-supported La _1-xSr_xMn_1-yFe_yO_3+δ for oxygen reduction electrode

La_1-xSr_xMn_0.8Fe_0.2O _3+δ (x = 0-0.4) and La_0.8Sr_0.2Mn _1-yFe_yO_3+δ (y = 0-0.8) supported on carbon were successfully prepared by a reverse micelle method. Aqueous solutions dissolving nitrates of constituent metals of the intended oxides and tetramethylammonium hydroxide (precipitant) were separately transformed into reverse micelle dispersions by using poly(oxyethylene)₅-lauryl ether (surfactant) and cyclohexane (oil). These dispersions were mixed together to derive a reverse micelle dispersion containing mixed hydroxides as precursors of the oxides, into which carbon powder suspended in cyclohexane was put under agitation. The suspension was destabilized with ethanol, and the resulting precipitate (carbon-supported precursors) was calcined in N₂ atmosphere to prevent the carbon matrix from being combusted. Single-phase oxides supported on carbon were obtained by calcination at 700°C unless the oxides were free of Fe. Oxygen reduction activity of the gas-diffusion-type electrodes fabricated with thus prepared carbon-supported oxides increased sharply and decreased gradually with increasing x and y, respectively. Among the prepared oxides, the greatest activity, i.e., 500 mA/cm₂ at -67 mV (vs. Hg/HgO electrode) in 9 M NaOH at 85°C under O₂ flow, was achieved by the oxide with x = 0.4 and y = 0.2. Optimal loading on carbon as well as durability under oxygen reduction conditions were tested for selected oxides. 17 wt % La_0.6Sr_0.4Mn_0.8Fe _0.2O_3+δ loading electrode was compared with 27.7 wt % Pt-loading electrode in an oxygen reduction activity, and it was found that the former electrode was better than the Pt loading electrode.