Zusammenfassung
Fuel cells and electrolyzers will most probably be an important cornerstone in the future energy system, often addressed as “hydrogen economy”, if hydrogen is used as an energy carrier. Polymer electrolyte membrane (PEM) based systems work in the acidic environment of the polymer membrane, which restricts the scope of catalyst materials to noble metals. The efficiency of the overall reaction is mostly defined by the sluggish kinetics of the oxygen half reaction, i.e. oxygen reduction or oxygen evolution reaction, respectively (ORR/OER). Typically platinum or platinum alloys are used in PEM fuel cells, while iridium oxides are common catalysts for PEM electrolyzers. The scarcity of those materials calls for strategies to reduce the overall catalyst amounts. Support materials allow an improved dispersion of the catalyst and additionally might enhance their stability, durability and activity. Specifications of the support materials are sufficient electrical conductivity, stability in acidic conditions and durability in oxidizing environments. Commonly applied carbon supports suffer from severe carbon corrosion especially at higher potentials, preventing their use in OER. A number of possible support materials have been suggested both for OER and ORR catalysts. Among these, this study is mainly focused on antimony tin oxide (ATO).
A facile and versatile salt-melt, hard-templating method for high-surface area ATO was developed. This new method allows adjusting the morphology, particle and pore sizes as well as the doping concentration with a previously unknown degree of control. The method was used to study the segregation behavior of antimony in spherical mesoporous ATO as a function of the antimony dopant concentration and the synthesis temperature. Additionally the electrochemical stability in terms of antimony and tin dissolution of various ATO materials was tested and correlated to the preparation conditions. This allowed determining a stable potential window of ATO showing the crucial importance of preventing a reduction of the ATO support at potentials below 0.3 VRHE. Additionally, a pretreatment protocol was suggested to reduce chemical antimony dissolution from the samples.
The developed ATO materials were applied as support materials for IrOx catalysts. Extended characterization led to the conclusion that iridium forms mixed oxides with the ATO support. At the same time, probably due to the stabilization of Ir(IV) in the SnO2 lattice, this allows the preparation of highly oxidized catalysts at surprisingly low temperatures. The activity and stability of the catalyst materials was extensively studied, and trends as a function of the synthesis temperature and the respective oxidation state were derived. The application of the stability number allowed the estimation of the intrinsic stability and the lifetime of the catalyst. It was shown that catalyst materials with iridium oxidation states below Ir(IV) have low stability numbers and therefore suffer from significant degradation. This generally questions the applicability of any non-highly oxidized iridium catalysts in real applications. A number of high-surface area support materials with different antimony concentrations were also used to support platinum catalysts. Determination of the role and impact of antimony on the performance and stability of the ATO supported platinum catalysts was elucidated. Strong metal-support interactions lead to the coverage of platinum particles with antimony, especially in a reductive atmosphere. At high antimony dopant concentration also platinum/antimony alloys are formed at low temperatures. This interaction has, on the one hand, a major impact on the performance of the catalyst and significantly reduces the electrochemically active surface area (ECSA) and the mass activity. On the other hand it leads to a stabilization of the catalyst by protecting parts of the catalyst surface. Performance tests in a membrane electrode assembly reveal the dissolution and penetration of antimony into the polymer membrane, which may poison the membrane and alter its conductivity.
Besides ATO, high-surface area Ti4O7, TiN and SiC were tested as alternative support materials. In the case of Ti4O7, likewise to ATO, strong metal-support interactions lead to the coverage of a significant share of the catalyst surface. This decreases both the ECSA and the mass activity, while not influencing the specific activity of the remaining active sites. An application of a highsurface area Ti4O7 material is, at least if prepared with the here applied method, despite its high stability against dissolution, in doubt due to its rapid oxidation. Synthesis strategies for highsurface area TiN and SiC materials were developed, though the materials were not further investigated due to limited stability against oxidation and low conductivity.
In general the applicability of the here tested high-surface area metal oxides, nitrides and carbides is limited by their stability. An exception is antimony tin oxide. Especially as a support material for OER, ATO has a significant advantage over carbon, other metal oxides and nitrides, as it is stable at high potentials and does not lose its conductivity, if reduction of the material can be prevented. The interaction with most common catalyst materials for ORR and OER with antimony leads to significant changes in the properties and performance of the catalysts. Nevertheless, ATO, but also the related fluorine doped tin oxide and indium tin oxide, are promising candidates as support materials for OER catalysts in PEM electrolyzers.
