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

Abstract

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 in Sulfate-Loaded Lanthanide Triphosphonates. Proton Conductivity and Application as Fillers in PEMFCs

“…This percentage was previously shown as the optimal composition. 30 The powder diffraction patterns of all composite membranes display the characteristic diffraction peaks of the different crystalline lanthanide phosphonates together with the broad peak, between 12 and 20° (2θ), typical of the pristine Nafion membrane ( Figure S25 ). In addition, the SEM micrographs of the composite membranes in combination with EDX analysis ( Figure 5 a and Figure S26 ), confirm a uniform distribution of the metal phosphonate particles inside the polymeric matrix, especially for N/Tb-II .…”

“…In addition, the obtained values are compared with those observed for Nafion 117 membranes 61 , 62 and are among the highest values reported for phosphonate-based lanthanide CPs. 16 , 30 , 60 , 63 − 65 …”

“…Smaller particles are thought to decrease proton conductivity, as observed for other compounds. 30 , 65 , 66 …”

“…Recently, results from our collaborative effort have been published on the use of layered lanthanide CPs utilizing a phosphonate derivative of taurine (namely, 2-[bis(phosphonomethyl)amino]ethanosulfonic acid), which assists in maintaining the proton conductivity of Nafion membranes at least up to 90 °C and perform satisfactorily in PEMFC single cells. 30 …”

Porphyrinylphosphonate‐Based Metal–Organic Framework: Tuning Proton Conductivity by Ligand Design

“…MOFs with phosphonate groups as coordinating components are among the most suitable materials for solving these tasks owing to the presence of three oxygen atoms capable of both strongly binding different metal cations to generate robust frameworks and forming multiple ordered hydrogen bonds facilitating proton transfer. To date, plenty of phosphonate‐based MOFs possessing high proton conductivity and chemical stability have been investigated [7–12] . However, only one work has been dedicated to testing the zirconium‐phosphate‐based conducting coordination polymer (NH 4 ) 3 [Zr(H 2/3 PO 4 ) 3 ], which showed a record‐high electrical power density of 12 mW cm −2 at 180 °C, as a solid‐state electrolyte assembled into an H 2 /O 2 fuel cell [13] .…”