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.

含酸性基团的金属膦酸盐具有丰富的质子传导特性，可提升膜电极组件（membrane electrode assemblies, MEAs）的性能。本研究聚焦于镧系离子与牛磺酸膦酸衍生物（2-[双(膦酸甲基)氨基]乙磺酸，H5SP）构建的配位聚合物的质子传导特性。采用140 ℃下的高通量水热筛选法以获取最优合成条件并得到纯晶相产物。本研究分离并表征了7种组成为Ln[H(O3PCH2)2-NH-(CH2)2-SO3]·2H2O的化合物，它们可归为两种不同的晶体结构：单斜晶系的m-LaH2SP，以及正交晶系的o-LnH2SP（其中Ln代表镨Pr、钕Nd、钐Sm、铕Eu、钆Gd以及铽Tb），二者的晶胞体积分别约为1200 ų和2500 ų。通过X射线粉末衍射数据从头解析得到的晶体结构表明，其层状骨架结构随Ln³+阳离子半径的不同而存在差异。在正交晶系的o-LnH2SP系列中，磺酸根基团未参与配位，指向层间空隙；而在单斜晶系的m-LaH2SP中，膦酸根与磺酸根基团均与Ln³+中心配位。由此形成了不同的氢键网络与质子传输通路。本研究在25~80 ℃、相对湿度70%~95%的条件下开展了质子传导率测试，其中钐离子（Sm³+）衍生物的质子传导率约为1×10⁻³ S·cm⁻¹，其活化能特征符合质子传输的格罗特斯（Grotthuss）机制。初步的膜电极组件测试结果表明，此类层状镧系磺酸盐膦酸盐可帮助维持Nafion膜的质子传导率至至少90 ℃，且在单质子交换膜燃料电池中表现优异。