High-Throughput Synthesis of Pillared-Layered Magnesium Tetraphosphonate Coordination Polymers: Framework Interconversions and Proton Conductivity Studies

Colodrero RM, Demadis KD. Exploiting the Multifunctionality of M2+/Imidazole-Etidronates for Proton Conductivity (Zn2+) and Electrocatalysis (Co2+, Ni2+) toward the HER, OER, and ORR

This work deals with the synthesis and characterization of one-dimensional (1D) imidazole-containing etidronates, [M2(ETID)(Im)3]·nH2O (M = Co2+ and Ni2+; n = 0, 1, 3) and [Zn2(ETID)2(H2O)2](Im)2, as well as the corresponding Co2+/Ni2+ solid solutions, to evaluate their properties as multipurpose materials for energy conversion processes. Depending on the water content, metal ions in the isostructural Co2+ and Ni2+ derivatives are octahedrally coordinated (n = 3) or consist of octahedral together with dimeric trigonal bipyramidal (n = 1) or square pyramidal (n = 0) environments. The imidazole molecule acts as a ligand (Co2+, Ni2+ derivatives) or charge-compensating protonated species (Zn2+ derivative). For the latter, the proton conductivity is determined to be ∼6 × 10-4 S·cm-1 at 80 °C and 95% relative humidity (RH). By pyrolyzing in 5%H2-Ar at 700-850 °C, core-shell electrocatalysts consisting of Co2+-, Ni2+-phosphides or Co2+/Ni2+-phosphide solid solution particles embedded in a N-doped carbon graphitic matrix are obtained, which exhibit improved catalytic performances compared to the non-N-doped carbon materials. Co2+ phosphides consist of CoP and Co2P in variable proportions according to the used precursor and pyrolytic conditions. However, the Ni2+ phosphide is composed of Ni2P exclusively at high temperatures. Exploration of the electrochemical activity of these metal phosphides toward the oxygen evolution reaction (OER), oxygen reduction reaction (ORR), and hydrogen evolution reaction (HER) reveals that the anhydrous Co2(ETID)(Im)3 pyrolyzed at 800 °C (CoP/Co2P = 80/20 wt %) is the most active trifunctional electrocatalyst, with good integrated capabilities as an anode for overall water splitting (cell voltage of 1.61 V) and potential application in Zn-air batteries. This solid also displays a moderate activity for the HER with an overpotential of 156 mV and a Tafel slope of 79.7 mV·dec-1 in 0.5 M H2SO4. Ni2+- and Co2+/Ni2+-phosphide solid solutions show lower electrochemical performances, which are correlated with the formation of less active crystalline phases.

Coordination Polymers Based on a Biphenyl Tetraphosphonate Linker: Synthesis Control and Photoluminescence

In this work, we used the rigid tetrapodal organic linker, [1,1'-biphenyl]-3,3',5,5'-tetrayltetrakis(phosphonic acid) (H₈btp), for the preparation of two lanthanide-organic framework families of compounds: layered [Ln₇(H₅btp)₄(H_5.5btp)₂(H₆btp)₂(H₂O)₁₂]∙23.5H₂O∙MeOH [where Ln³⁺ = Eu³⁺ (1Eu) and Gd³⁺ (1Gd)], prepared using microwave-irradiation followed by slow evaporation; 3D [Ln₄(H₃btp)(H₄btp)(H₅btp)(H₂O)₈]∙3H₂O [where Ln³⁺ = Ce³⁺ (2Ce), Pr³⁺ (2Pr), and Nd³⁺ (2Nd)], obtained from conventional hydro(solvo)thermal synthesis. It is shown that in this system, by carefully selecting the synthetic method and the metal centers, one can increase the dimensionality of the materials, also increasing structural robustness (particularly to the release of the various solvent molecules). Compound 1 is composed of 2D layers stacked on top of each other and maintained by weak π-π interactions, with each layer formed by discrete 1D organic cylinders stacked in a typical brick-wall-like fashion, with water molecules occupying the free space in-between cylinders. Compound 2, on the other hand, is a 3D structure with small channels filled with crystallization water molecules. A full solid-state characterization of 1 and 2 is presented (FT-IR spectroscopy, SEM microscopy, thermogravimetric studies, powder X-ray diffraction and thermodiffractometry). The photoluminescence of 1Eu was investigated.

Synthesis, Characterization, and Proton Conduction Behavior of Thorium and Cerium Phosphonates

Thorium (Th⁴⁺) and cerium (Ce⁴⁺) phosphonates have been synthesized by the sol-gel method using various phosphonic acids and analyzed using elemental and CHN analysis, spectral analysis (Fourier transform infrared spectroscopy and X-ray diffraction), thermogravimetric analysis, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The proton transport study was performed by measuring proton conductance at different temperatures using an impedance analyzer. The Grotthuss mechanism of the proton conduction has been discussed thoroughly on the basis of results obtained from the impedance measurement; the specific proton conduction (σ) and activation energy (E _a) have been evaluated and compared with the reported proton-conducting system having similar phosphonate moieties.