ABSTRACT
In the process of developing carbon-supported metal catalysts, determining the catalyst particle-size distribution is an essential step, because this parameter is directly related to the catalytic activities. The particle-size distribution is most effectively determined by small-angle X-ray scattering (SAXS). When metal catalysts are supported by high-performance mesoporous carbon materials, however, their mesopores may lead to erroneous particle-size estimation if the sizes of the catalysts and mesopores are comparable. Here we propose a novel approach to particle-size determination by introducing contrast variation-SAXS (CV-SAXS). In CV-SAXS, a multi-component sample is immersed in an inert solvent with a density equal to that of one of the components, thereby rendering that particular component invisible to X-rays. We used a mixture of tetrabromoethane and dimethyl sulfoxide as a contrast-matching solvent for carbon. As a test sample, we prepared a mixture of a small amount of platinum (Pt) catalyst and a bulk of mesoporous carbon, and subjected it to SAXS measurement in the absence and presence of the solvent. In the absence of the solvent, the estimated Pt particle size was affected by the mesopores, but in the presence of the solvent, the Pt particle size was correctly estimated in spite of the low Pt content. The results demonstrate that the CV-SAXS technique is useful for correctly determining the particle-size distribution for low-Pt-content catalysts, for which demands are increasing to reduce the use of expensive Pt.
ABSTRACT
Acid-infiltrated block polymer electrolyte membranes adopting a spherical or lamellar nanophase-separated structure were prepared by infiltrating sulfuric acid (H2SO4) into polystyrene-b-poly(4-vinylpyridine)-b-polystyrene (S-P-S) triblock copolymers to investigate the effects of its nanophase-separated structure on mechanical properties and proton conductivities under non-humidification. Lamellae-forming S-P-S/H2SO4 membranes with a continuous hard phase generally exhibited higher tensile strength than sphere-forming S-P-S/H2SO4 membranes with a discontinuous hard phase even if the same amount of Sa was infiltrated into each neat S-P-S film. Meanwhile, the conductivities of lamellae-forming S-P-S/H2SO4 membranes under non-humidification were comparable or superior to those of sphere-forming S-P-S/H2SO4 membranes, even though they were infiltrated by the same weight fraction of H2SO4. This result is attributed to the conductivities of S-P-S/H2SO4 membranes being greatly influenced by the acid/base stoichiometry associated with acid-base complex formation rather than the nanophase-separated structure adopted in the membranes. Namely, there are more free H2SO4 moieties that can release free protons contributing to the conductivity in lamellae-forming S-P-S/H2SO4 membranes than sphere-forming S-P-S/H2SO4, even when the same amount of H2SO4 was infiltrated into the S-P-S.
ABSTRACT
Non-platinum group metal (non-PGM) catalysts for the oxygen reduction reaction (ORR) are set to reduce the cost of polymer electrolyte membrane fuel cells (PEFCs) by replacing platinum at the cathode. We previously developed unique nitrogen-doped carbon foams by template-free pyrolysis of alkoxide powders synthesized using a high temperature and high pressure solvothermal reaction. These were shown to be effective ORR electrocatalysts in alkaline media. Here, we present a new optimised synthesis protocol which is carried out at ambient temperature and pressure, enabling us to safely increase the batch size to 2 g, increase the yield by 60%, increase the specific surface area to 1866 m2 g-1, and control the nitrogen content (between 1.0 and 5.2 at%). These optimized nitrogen-doped carbon foams are then utilized as effective supports for Fe-N-C catalysts for the ORR in acid media, whilst multiphysics modelling is used to gain insight into the electrochemical performance. This work highlights the importance of the properties of the carbon support in the design of Pt-free electrocatalysts.
