Tributsch, H.; Koslowski, U.I.; Dorbandt, I.: Experimental and theoretical modeling of Fe-, Co-, Cu-, Mn-based electrocatalysts for oxygen reduction. Electrochimica Acta 53 (2008), p. 2198-2209
10.1016/j.electacta.2007.09.027
Open Access Version
Abstract:
Abstract: Experience gained during efforts towards optimization of noble-metal-free electrocatalysts for oxygen reduction is simultaneously used to understand the chemical and morphological necessities for inducing efficient multielectron transfer catalysis. The analysis of many preparative experimental steps between the moderately performing metal porphyrines and the highly efficient transition metal- and sulfur- containing pyrolised catalyst material contributes to the following model of the catalyst: 1. The metals function enclosed in nitrogen or graphitic environment where they are shielded against oxidation. 2. The metals can be exchanged but are not identical in their efficiency. 3. Higher efficiency is only achieved, when the function of a binary reaction center is warranted. 4. The carbonization of the environment is critical and provides intercalated metal centers and attached metal complexes in graphite environment for interaction with the nitrogen-chelated partner center in the simultaneously obtained graphene layers. Experimental support for these models from EXAFS, RAMAN, Mössbauer and X-ray spectroscopy is given and a parallel is drawn with the cytochrome oxidase oxygen reduction catalysis, which is proposed to proceed according to roughly the same mechanism. A special effort is made to discuss strategies for shielding and protecting catalytically active abundant transition metals against chemical reaction with oxygen or hydrogen peroxide.