Supplementary document for Non-reciprocal electromagnetic metasurface based on the nonlinearity of a liquid metamaterial - 6585161.pdf

Optimization descriptio

Electrochemical Reduction of Oxygen on Hydrophobic Ultramicroporous PolyHIPE Carbon

A new kind of polyHIPE (polymerized high internal phase emulsion)-based carbon derived from coreacted furfuryl alcohol and tannin was tested as an ORR catalyst. To understand the reduction process, the surface was extensively characterized from the point of view of texture and chemistry. The prepared materials show subtle differences in the chemistry but marked differences in the porosity. The best-performing sample had a very high volume of ultramicropores and the highest degree of defects on the surface. The oxygen was present on the surface mainly in epoxy and ether configurations. Those oxygen groups located in large pores promoted transfer of O₂ dissolved in water/electrolyte to small pores of the hydrophobic surface. There, a strong adsorption of oxygen was energetically favorable. This led to weakening of O–O bonds, subsequent dissociation of oxygen, and its reduction/protonation. The presented polyHIPE carbons show high electrochemical stability and better tolerance to methanol than Pt/C. High kinetic current density was measured on them

Conversion of Natural Tannin to Hydrothermal and Graphene-Like Carbons Studied by Wide-Angle X‑ray Scattering

The atomic structure of carbon materials prepared from natural tannin by two different techniques, high-temperature pyrolysis and low-temperature hydrothermal carbonization, was studied by wide-angle X-ray scattering. The obtained diffraction data were converted to the real space representation in the form of pair distribution functions. The X-ray photoelectron spectroscopy measurements provided information about the chemical state of carbon in tannin-based materials that was used to construct final structural models of the investigated samples. The results of the experimental data in both reciprocal and real spaces were compared with computer simulations based on the PM7 semiempirical quantum chemical method. Using the collected detailed information, structural models of the tannin-based carbons were proposed. The characteristics of the investigated materials at the atomic level were discussed in relation to their preparation method. The rearrangement of the tannin molecular structure and its transformation to graphene-like structure was described. The structure of tannin-based carbons pyrolyzed at 900 °C exhibited coherently scattering domains about 20 Å in size, consisting of two defected atomic layers and resembling a graphene-like arrangement

Combined Effect of Porosity and Surface Chemistry on the Electrochemical Reduction of Oxygen on Cellular Vitreous Carbon Foam Catalyst

A new mechanism of O₂ reduction, which follows principles different from those generally accepted for describing ORR reduction on heteroatom-doped carbons, is suggested. It is based on the ability of oxygen to strongly adsorb in narrow hydrophobic pores. In this respect, a cellular vitreous carbon foam–graphene oxide composite was synthesized. The materials were doped with sulfur and nitrogen and/or heat-treated at 950 °C in order to modify their surface chemistry. The resultant samples presented a macro-/microporous nature and were tested as ORR catalysts. To understand the reduction process, their surfaces were extensively characterized from texture and chemistry points of view. The treatment applied markedly changed the volumes of small micropores and the surface hydrophilicity/polarity character. The results showed that the electron transfer number was between 3.87 and 3.96 and the onset potential reached 0.879 V for the best-performing sample. It is noteworthy that the best-performing sample has the highest volume of pores smaller than 0.7 nm while there was no heteroatom doping. The hydrophobicity and the strong adsorption forces provided by these pores to pull oxygen inside are the possible reasons for the observed excellent performance. A decrease in the volume of these pores resulted in a decrease in the catalytic performance. When the surface was modified with heteroatoms, the performances worsened further because of the induced hydrophilicity