Dependence of the oxygen reduction reaction at sol–gel derived Co-based catalysts on acidic solution pH and temperature

This work is focused on the oxygen reduction reaction (ORR) at non-precious metal catalysts formed using a modified sol–gel (SG) synthesis approach, in which carbon and nitrogen were added in the form of either ethylenediamine (en) or 1,2-phenylenediamine (pda) to a pre-formed Co oxide gel. It was confirmed that the Co-pda-derived material, when heat-treated at 900 °C, is a much better ORR catalyst than the analogous en-derived material, heat-treated to its optimum of 700 °C, giving an activity about 10–11 times higher and a lower H2O2 yield. The ORR was examined over a range of acidic solutions (all at room temperature and an oxygen pressure of 1 atmosphere). For both catalysts, the reaction rate was found to be independent of the H+ concentration at pH < 2.5, with the Co-pda and -en catalysts giving a transfer coefficient of ca. 1 and 0.5–0.7, respectively. At pH > 2.5, an unusual pH response was seen, suggestive of the development of local pH conditions inside the catalyst layer. While the catalysts were stable only to solution temperatures of ca. 55 °C, the ORR activation energy in the kinetic (Eact = 34 kJ/mol) and diffusion controlled (Eact = 8.8 kJ/mol) regions could still be determined. Based on all of these results, a possible mechanism for the ORR at below and above pH 2.5, for both the Co-pda and Co-en catalysts, was proposed.