Layered Lanthanide Sulfophosphonates and Their Proton Conduction Properties in Membrane Electrode Assemblies

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, H5SP). 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(O3PCH2)2–NH–(CH2)2–SO3]·2H2O were isolated
and characterized, which crystallize in two different structures,
monoclinic m-LaH2SP and orthorhombic o-LnH2SP (Ln = Pr, Nd, Sm, Eu, Gd, and Tb), with unit cell volumes of ∼1200
and ∼2500 Å3, respectively. Their crystal structures,
solved ab initio from X-ray powder diffraction data, correspond to
different layered frameworks depending on the Ln3+ cation
size. In the orthorhombic series, o-LnH2SP, the sulfonate group is noncoordinated and points
toward the interlayer space, while for m-LaH2SP, both phosphonate and sulfonate groups coordinate
to the Ln3+ 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 Sm3+ 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.