Chemical Tuning of Magnetic Properties through Ru/Rh Substitution in Th₇Fe₃‑type FeRh_6–<i>n</i>Ru_<i>n</i>B₃ (<i>n</i> = 1–5) Series

The new quaternary boride series FeRh_6–<i>n</i>Ru_<i>n</i>B₃ (<i>n</i> = 1–5) was synthesized by arc melting and characterized by powder and single-crystal X-ray diffraction (XRD), energy-dispersive X-ray analysis, and superconducting quantum interference device magnetometry. Single-crystal structure refinement showed the distribution of the iron atoms in two of three possible crystallographic 4d metal sites in the structure (Th₇Fe₃-type, space group <i>P</i>6₃<i>mc</i>). Rietveld refinements of the powder XRD data indicated single-phase synthesis of all the members. A linear decrease of the lattice parameters and the unit cell volume with increasing Ru content was found, indicating Vegard’s behavior. Susceptibility measurements show decreasing Curie temperature and magnetic moment (μ_a^5T) recorded at 5 T with increasing Ru content from <i>T</i>_C = 295 K and μ_a^5T = 3.35 μ_B (FeRh₅RuB₃) to <i>T</i>_C = 205 K and μ_a^5T = 0.70 μ_B (FeRhRu₅B₃). The measured coercivities lie between 1.0 and 2.2 kA/m indicating soft to semihard magnetic materials

Graphene- and Phosphorene-like Boron Layers with Contrasting Activities in Highly Active Mo₂B₄ for Hydrogen Evolution

Two different boron layers, flat (graphene-like) and puckered (phosphorene-like), found in the crystal structure of Mo₂B₄ show drastically different activities for hydrogen evolution, according to Gibbs free energy calculations of H-adsorption on Mo₂B₄. The graphene-like B layer is highly active, whereas the phosphorene-like B layer performs very poorly for hydrogen evolution. A new Sn-flux synthesis permits the rapid single-phase synthesis of Mo₂B₄, and electrochemical analyses show that it is one of the best hydrogen evolution reaction active bulk materials with good long-term cycle stability under acidic conditions. Mo₂B₄ compensates its smaller density of active sites if compared with highly active bulk MoB₂ (which contains only the more active graphene-like boron layers) by a 5-times increase of its surface area

Thermoelectric Properties of Ultralong Silver Telluride Hollow Nanofibers

Ultralong Ag_<i>x</i>Te_<i>y</i> nanofibers were synthesized for the first time by galvanically displacing electrospun Ni nanofibers. Control over the nanofiber morphology, composition, and crystal structure was obtained by tuning the Ag⁺ concentrations in the electrolytes. While Te-rich branched p-type Ag_<i>x</i>Te_<i>y</i> nanofibers were synthesized at low Ag⁺ concentrations, Ag-rich nodular Ag_<i>x</i>Te_<i>y</i> nanofibers were obtained at high Ag⁺ concentrations. The Te-rich nanofibers consist of coexisting Te and Ag₇Te₄ phases, and the Ag-rich fibers consist of coexisting Ag and Ag₂Te phases. The energy barrier height at the phase interface is found to be a key factor affecting the thermoelectric power factor of the fibers. A high barrier height increases the Seebeck coefficient, <i>S</i>, but reduces the electrical conductivity, σ, due to the energy filter effect. The Ag₇Te₄/Te system was not competitive with the Ag₂Te/Ag system due to its high barrier height where the increase in <i>S</i> could not overcome the severely diminished electrical conductivity. The highest power factor was found in the Ag₂Te/Ag-rich nanofibers with an energy barrier height of 0.054 eV

High Temperature Polymer Electrolyte Membrane Fuel Cells with High Phosphoric Acid Retention

Phosphoric acid loss poses immense hurdles for the durability of high-temperature polymer electrolyte membrane fuel cells (HT-PEMFCs). Here we report quaternary ammonium-biphosphate ion-pair HT-PEMFCs that do not lose phosphoric acids under normal and accelerated stress conditions. Our energetics study explains the acid loss behavior of the conventional phosphoric acid-polybenzimidazole (PA-PBI) system by two mechanisms. If PA loss occurs via acid evaporation, the acid loss is constant over time. On the other hand, when water activity in the PA-PBI system is high, exponential decay of PA loss occurs via the water replacement mechanism. Combined 31P NMR and computational studies show that the proposed ion-pair system has six times higher interaction energy, which allows for containing all PAs in the membrane electrode assemblies under a broad range of operating conditions. In addition, polar interactions between the phosphonic acid ionomer and phosphoric acid explain acid retention in the electrodes of the ion-pair HT-PEMFCs