This work is dedicated to Prof. Monika Willert-Porada. We kindly would like to thank her for her contribution to the project and her support. Low cost and abundant catalysts demonstrating high activity and stability towards the oxygen reactions, i. e., the oxygen reduction (ORR) and oxygen evolution reaction (OER), are crucial for the development of electrically rechargeable zinc-air batteries. Herein, the facile synthesis and systematic characterisation of two highly active and stable oxygen electrocatalysts, i. e., high surface area α-MnO 2 microspheres and nanoparticulate Co 3 O 4 , are reported. α-MnO 2 exhibits low half-wave potential and potential of À 0.197 and À 0.226 V (vs. Ag/AgCl) at À 3 mA cm À 2 , respectively, that are only marginally higher compared to commercial Pt/C (E 1/2 = À 0.161 V, E j =-3 = À 0.171 V) for ORR. Meanwhile, Co 3 O 4 needs a potential of 0.601 V (vs. Ag/ AgCl) to drive 10 mA cm À 2 being competitive to commercial Ir/C (E j = 10 = 0.60 V) for OER. In order to create a bifunctional catalyst, two approaches were pursued: i) Co 3 O 4 nanoparticles were homogeneously grown on the surface of α-MnO 2 microspheres yielding a radial hybrid composite catalyst material in the form of a core (α-MnO 2) shell (Co 3 O 4) structure and ii), much simpler, individual α-MnO 2 microspheres and Co 3 O 4 nanoparticles were physically mixed in a powder blend. The powder blend demonstrates superior overall bifunctional catalytic properties such that the individual catalysts still dominate their respective oxygen reaction and, due to synergistic interactions between both catalysts, an improved ORR activity could be achieved.
One of the challenges of high-temperature polymer electrolyte
membrane
fuel cells is the poisoning of the Pt catalyst with H3PO4. H3PO4 is imbibed into the routinely
used polybenzimidazole-based membranes, which facilitate proton conductivity
in the temperature range of 120–200 °C. However, when
leached out of the membrane by water produced during operation, H3PO4 adsorbs on the Pt catalyst surface, blocking
the active sites and hindering the oxygen reduction reaction (ORR).
The reduction of H3PO4 to H3PO3, which occurs at the anode due to a combination of a low
potential and the presence of gaseous H2, has been investigated
as an additional important contributing factor to the observed poisoning
effect. H3PO3 has an affinity toward adsorption
on Pt surfaces even greater than that of H2PO4
–. In this work, we investigated the poisoning
effect of both H3PO3 and H3PO4 using a half-cell setup with a gas diffusion electrode under
ambient conditions. By means of in situ X-ray absorption spectroscopy,
it was possible to follow the signature of different species adsorbed
on the Pt nanoparticle catalyst (H, O, H2PO4
–, and H3PO3) at different
potentials under ORR conditions in various electrolytes (HClO4, H3PO4, and H3PO3). It was found that H3PO3 adsorbs in a pyramidal
configuration P(OH)3 through a Pt–P bond. The competition
between H3PO4 and H3PO3 adsorption was studied, which should allow for a better understanding
of the catalyst poisoning mechanism and thus assist in the development
of strategies to mitigate this phenomenon in the future by minimizing
H3PO3 generation by, for example, improved catalyst
design or adapted operation conditions or changes in the electrolyte
composition.
Cellulose acetate (CA) was partially acrylated, and the resulting cellulose acetate acrylate (acryl-substitution degree of 0.2) underwent quantitative thio-Michael click reactions with various thiols. A toolbox of functional CA polymers was obtained in this way, and their properties were studied. The modification with fatty alkyl thiols led to hydrophobic materials with large water drop contact angles. Octadecylthio-, butoxycarbonylpropylthio-, and furanylthio-modifications formed highly transparent materials. The new derivative CAASFur disintegrated completely under industrial composting conditions. Films of modified CA polymers were cast and investigated in terms of barrier properties. The nanocomposite of CAAS18 compounded with a synthetic layered silicate (hectorite) of a large aspect ratio showed permeabilities as low as 0.09 g mm m −2 day −1 for water vapor and 0.16 cm 3 mm m −2 day −1 atm −1 for oxygen. This portfolio of functional CA polymers opens the door to new applications.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.