Structure and stability of an iron-based catalyst for the oxygen reduction reaction, prepared by heat treatment
of carbon-supported iron(III) tetramethoxyphenylporphyrin chloride (FeTMPP−Cl), were investigated. The
oxygen reduction in acid electrolyte was examined with the rotating (ring) disk electrode. The measurements
confirmed that H2O2 is generated as a byproduct of the oxygen reduction. The structural elucidation of the
catalyst showed that the porphyrin decomposes during heat treatment. Nitrogen atoms of the heat-treated
porphyrin become bonded at the edge of graphene layers as pyridine- and pyrrole-type nitrogen. Two Fe3+
components as well as metallic, carbidic and oxidic iron were detected by Mössbauer spectroscopy. An
electrochemical longevity test and two degradation experiments with sulfuric acid and H2O2 showed that
H2O2 causes the degradation of active sites. A 6-fold coordinated Fe3+ compound seems to be responsible for
the catalytic activity. Only 8% of the primary iron content is present in the active iron component.
During the lifetime of a polymer electrolyte fuel cell, the pore structure of the Pt/C catalyst layer may change as a result of carbon corrosion. Three-dimensional visualization of porosity changes is important to understand the origin of fuel cell performance deterioration. A focused ion beam/scanning electron microscopy (FIB/SEM) approach was adopted together with electron tomographic studies to visualize the three-dimensional pore structure of a Pt/C catalyst. In the case of pristine catalyst layers, the pores form an interconnected network. After 1000 start-up/shut-down cycles, severe carbon corrosion leads to a collapse of the support structure. The porosity of the degraded catalyst layer shrinks drastically, resulting in a structure of predominantly isolated pores. These porosity changes hinder the mass transport in the catalyst layer, consequently leading to a substantial loss of fuel cell performance. FIB/SEM serial sectioning and electron tomography allows three-dimensional imaging of the catalyst pore structure, which is a prerequisite for modeling and optimizing mass transport in catalyst layers.
PtCo3/C catalysts for oxygen reduction were prepared and heat-treated between 350 and 1000 °C under reductive conditions. The catalysts were activated by cyclic voltammetry, which resulted in the partial oxidative leaching of cobalt. Subsequent rotating disk electrode measurements showed a maximum mass activity for samples tempered at 800 °C, which yielded a 2.4 times higher activity than commercial PtCo
x
/C. TEM images revealed that the particle size of the PtCo3/C catalysts remains almost constant until a temperature of 600 °C is reached; a further temperature increase leads to a noticeable particle growth and a broadening of the particle size distribution. XRD measurements showed that heat treatment leads to the formation of fcc-Pt, Pt3Co, fct-PtCo, fcc-PtCo, and cobalt. The fct- and fcc-PtCo phases are present in samples with the highest catalytic activities.
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