Synthesis and Characterization of Heterobimetallic (Pd/B) Nindigo Complexes and Comparisons to Their Homobimetallic (Pd₂, B₂) Analogues

Reactions of Nindigo-BF₂ complexes with Pd­(hfac)₂ produced mixed complexes with Nindigo binding to both a BF₂ and a Pd­(hfac) unit. These complexes are the first in which the Nindigo ligand binds two different substrates, and provide a conceptual link between previously reported bis­(BF₂) and bis­(Pd­(hfac)) complexes. The new Pd/B complexes have intense near IR absorption near 820 nm, and they undergo multiple reversible oxidations and reductions as probed by cyclic voltammetry experiments. The spectral, redox, and structural properties of these complexes are compared against those of the corresponding B₂ and Pd₂ complexes with the aid of time-dependent density functional calculations. In all cases the low-energy electronic transitions are ligand-centered π–π* transitions, but the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energiesand hence the absorption wavelength as well as the oxidation and reduction potentialsare significantly modulated by the moieties bound to the Nindigo ligand

Interplay between Composition, Structure, and Properties of New H₃PO₄‑Doped PBI₄N–HfO₂ Nanocomposite Membranes for High-Temperature Proton Exchange Membrane Fuel Cells

Polybenzimidazole (PBI) has become a popular polymer of choice for the preparation of membranes for potential use in high-temperature proton exchange membrane polymer fuel cells. Phosphoric acid-doped composite membranes of poly­[2,2′-(<i>m</i>-phenylene)-5,5′-bibenzimidazole] (PBI4N) impregnated with hafnium oxide nanofiller with varying content levels (0–18 wt %) have been prepared. The structure–property relationships of both the undoped and acid-doped composite membranes are studied using thermogravimetric analysis, modulated differential scanning calorimetry, dynamic mechanical analysis, wide-angle X-ray scattering, infrared spectroscopy, and broadband electrical spectroscopy. Results indicate that the presence of nanofiller improves the thermal and mechanical properties of the undoped membranes and facilitates a greater level of acid uptake. The degree of acid dissociation within the acid-doped membranes is found to increase with increasing nanofiller content. This results in a conductivity, at 215 °C and a nanofiller level <i>x</i> ≥ 0.04, of 9.0 × 10^–2 S cm^–1 for [PBI4N­(HfO₂)_<i>x</i>]­(H₃PO₄)_<i>y</i>. This renders nanocomposite membranes of this type as good candidates for use in high temperature proton exchange membrane fuel cells (HT-PEMFCs)

Toward Pt-Free Anion-Exchange Membrane Fuel Cells: Fe–Sn Carbon Nitride–Graphene Core–Shell Electrocatalysts for the Oxygen Reduction Reaction

We report on the development of two new <i>Pt-free</i> electrocatalysts (ECs) for the oxygen reduction reaction (ORR) process based on graphene nanoplatelets (GNPs). We designed the ECs with a <i>core–shell</i> morphology, where a GNP <i>core</i> support is covered by a carbon nitride (CN) <i>shell.</i> The proposed ECs present ORR active sites that are not associated with nanoparticles of metal/alloy/oxide but are instead based on Fe and Sn subnanometric clusters bound in <i>coordination nests</i> formed by carbon and nitrogen ligands of the CN <i>shell</i>. The performance and reaction mechanism of the ECs in the ORR are evaluated in an alkaline medium by cyclic voltammetry with the thin-film rotating ring-disk approach and confirmed by measurements on gas-diffusion electrodes. The proposed GNP-supported ECs present an ORR overpotential of only ca. 70 mV higher with respect to a conventional Pt/C reference EC including a XC-72R carbon black support. These results make the reported ECs very promising for application in anion-exchange membrane fuel cells. Moreover, our methodology provides an example of a general synthesis protocol for the development of new <i>Pt-free</i> ECs for the ORR having ample room for further performance improvement beyond the state of the art