Synthesis and proton conduction properties of lanthanide amino-sulfophosphonates

Crystalline acid-functionalized metal phosphonates are potential candidates as proton conducting electrolytes. Their frameworks can be chemically modified to contain proton carriers such as acidic groups (P-OH; -SO3H, -COOH,…) and guest molecules (H2O, NH3,…) that generates hydrogen bond networks stable in a wide range of temperature [1,2]. In this work, focus is laid on properties derived from the combination of lanthanide ions with the amino-sulfophosphonate ligand (H2O3PCH2)2-N-(CH2)2-SO3H. Hightrough-put screening was followed to reach the optimal synthesis conditions under solvothermal conditions at 140 ºC. Isolated polycrystalline solids, Ln[(O3PCH2)2-NH-(CH2)2-SO3H].2H2O (Ln= La, Pr and Sm), crystallize in the monoclinic (La) and orthorhombic (Pr and Sm) systems with unit cell volume of ~2548 Å3. Preliminary proton conductivity measurements for Sm derivative have been carried out between 25º and 80 ºC at relative humidity (RH) values of 70 % and 95 %. The sample exhibits enhanced conductivity at high RH and T (Figure 1) and constant activation energies of 0.4 eV, typical of a Grothuss mechanism of proton.

Structure-properties correlations in divalent metal phosphonates

Crystalline metal phosphonates may offer acidic sites, structural flexibility and guest molecules (H2O, heterocyclics, etc.) which can act as proton carriers. In addition, some frameworks are also amenable for post‐synthesis modifications in order to enhance desired properties [1,2]. In this work, we present the synthesis and structural characterization of two hydroxyphosphonoacetates hybrids based on magnesium, [Mg5(O3PCHOHCOO)2(HO3PCHOHCOO)2·8H2O] [Mg5(HPAA)2(H1HPAA)2·8H2O], and zinc, [Zn6K(O3PCHOHCOO)4(OH)·6.5H2O] [Zn6K(HPAA)4(OH)·6.5H2O]. Both solids present three-dimensional frameworks and their crystal structures were solved ab initio from X-ray powder diffraction. The proton conductivity of [Zn6K(HPAA)4(OH)·6.5H2O] as well as ammonia derivatives of M(II)(HO3PCHOHCOO)·2H2O [M(II)=Zn, Mg] will be reported and discussed.

Ammonia effects on proton conductivity properties of coordination polymers

Crystalline metal phosphonates are referred to as a type of structurally versatile coordination polymers [1]. Many of them contain guest molecules (H2O, heterocyclics, etc.), acidic sites and, furthermore, their structure can be also amenable for post‐synthesis modifications in order to enhance desired properties [2]. In the present work, we examine the relationships between crystal structure and proton conductivity for several metal phosphonates derive from multifunctional ligands, such as 5-(dihydroxyphosphoryl)isophthalic acid (PiPhtA) [3] and 2-hydroxyphosphonoacetic acid (H3HPAA). Crystalline divalent metal derivatives show a great structural diversity, from 1D to 3D open-frameworks, possessing hydrogen-bonded water molecules and acid groups. These solids present a proton conductivity range between 7.2·10-6 and 1.3·10−3 S·cm-1. Upon exposure to ammonia vapor, from an aqueous solution, solid state transformations are observed accompanied of enhance proton conductivities. The stability of these solids under different environment conditions (temperature and relative humidities) as well as the influence of the ammonia adsorption on the proton conduction properties of the resulting solids will be discussed.