Metal phosphonates containing acidic groups exhibit a wide range of proton conduction properties, which may enhance the performance of membrane electrode assemblies (MEAs). In this work, focus is placed on proton conduction properties of coordination polymers derived from the combination of lanthanide ions with a phosphonate derivative of taurine (2-[bis(phosphonomethyl)amino]ethanesulfonic acid, H 5 SP). High-throughput hydrothermal screening (140°C) was used to reach optimal synthesis conditions and access pure crystalline phases. Seven compounds with the composition Ln[H(O 3 PCH 2) 2 −NH− (CH 2) 2 −SO 3 ]•2H 2 O were isolated and characterized, which crystallize in two different structures, monoclinic m-LaH 2 SP and orthorhombic o-LnH 2 SP (Ln = Pr, Nd, Sm, Eu, Gd, and Tb), with unit cell volumes of ∼1200 and ∼2500 Å 3 , respectively. Their crystal structures, solved ab initio from X-ray powder diffraction data, correspond to different layered frameworks depending on the Ln 3+ cation size. In the orthorhombic series, o-LnH 2 SP, the sulfonate group is noncoordinated and points toward the interlayer space, while for m-LaH 2 SP, both phosphonate and sulfonate groups coordinate to the Ln 3+ centers. As a consequence, different H-bonding networks and proton transfer pathways are generated. Proton conductivity measurements have been carried out between 25 and 80°C at 70−95% relative humidity. The Sm 3+ derivative exhibits a conductivity of ∼1 × 10 −3 S•cm −1 and activation energy characteristic of a Grotthuss-type mechanism for proton transfer. Preliminary MEA assays indicate that these layered lanthanide sulfophosphonates assist in maintaining the proton conductivity of Nafion membranes at least up to 90°C and perform satisfactorily in single proton-exchange membrane fuel cells.

Phase transformation dynamics and proton conduction properties are reported for cationic layer-featured coordination polymers derived from the combination of lanthanide ions (Ln 3+ ) with nitrilo-tris(methylenephosphonic acid) (H 6 NMP) in the presence of sulfate ions. Two families of materials are isolated and structurally characterized, i.e., [Ln 2 (H 4 NMP) 2 (H 2 O) 4 ](HSO 4 ) 2 · n H 2 O (Ln = Pr, Nd, Sm, Eu, Gd, Tb, Er, Yb; n = 4–5, Series I ) and [Ln(H 5 NMP)]SO 4 ·2H 2 O (Ln = Pr, Nd, Eu, Gd, Tb; Series II ). Eu/Tb bimetallic solid solutions are also prepared for photoluminescence studies. Members of families I and II display high proton conductivity (10 –3 and 10 –2 S·cm –1 at 80 °C and 95% relative humidity) and are studied as fillers for Nafion-based composite membranes in PEMFCs, under operating conditions. Composite membranes exhibit higher power and current densities than the pristine Nafion membrane working in the range of 70–90 °C and 100% relative humidity and with similar proton conductivity.

Phosphonate ligands demonstrate strong affinity for metal ions. However, there are several cases where the phosphonate is found non-coordinated to the metal ion. Such compounds could be characterized as salts, since the interactions involved are ionic and hydrogen bonding. In this paper we explore a number of such examples, using divalent metal ions (Mg2+, Ca2+, Sr2+ and Ni2+) and the phosphonic acids: p-aminobenzylphosphonic acid (H2PABPA), tetramethylenediamine-tetrakis(methylenephosphonic acid) (H8TDTMP), and 1,2-ethylenediphosphonic acid (H4EDPA). The compounds isolated and structurally characterized are [Mg(H2O)6]·[HPABPA]2·6H2O, [Ca(H2O)8]·[HPABPA]2, [Sr(H2O)8]·[HPABPA]2, [Mg(H2O)6]·[H6TDTMP], and [Ni(H2O)6]·[H2EDPA]·H2O. Also, the coordination polymer {[Ni(4,4’-bpy)(H2O)4]·[H2EDPA]·H2O}n was synthesized and characterized, which contains a bridging 4,4’-bipyridine (4,4’-bpy) ligand forming an infinite chain with the Ni2+ cations. All these compounds contain the phosphonate anion as the counterion to charge balance the cationic charge originating from the metal cation.

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