Proton Conductive Zr-Phosphonate UPG-1—Aminoacid Insertion as Proton Carrier Stabilizer

Abstract

Proton exchange membrane fuel cells (PEMFCs) are an attractive green technology for energy generation. The poor stability and performances under working conditions of the current electrolytes are their major drawbacks. Metal-Organic Frameworks (MOFs) have recently emerged as an alternative to overcome these issues. Here, we propose a robust Zr-phosphonate MOF (UPG-1) bearing labile protons able to act a priori as an efficient electrolyte in PEMFCs. Further, in an attempt to further enhance the stability and conductivity of UPG-1, a proton carrier (the amino acid Lysine, Lys) was successfully encapsulated within its porosity. The behaviors of both solids as an electrolyte were investigated by a complete experimental (impedance spectroscopy, water sorption) and computational approach (MonteCarlo, water sorption). Compared with the pristine UPG-1, the newly prepared Lys@UPG-1 composite showed similar proton conductivity but a higher stability, which allows a better cyclability. This improved cyclability is mainly related to the different hydrophobic-hydrophilic balance of the Lys@UPG-1 and UPG-1 and the steric protection of the reactive sites of the MOF by the Lys.

Investigation of Sulfonated Graphene Oxide as the Base Material for Novel Proton Exchange Membranes

“…Moreover, the restrained values’ variability could hint the good homogeneity of this sample. It is necessary to specify that the sulfonated membranes exhibited substantially higher outcomes than the ones reported in the literature for innovative proton conductors [ 13 , 51 , 52 , 54 , 55 ]. A possible explanation is the presence of small traces of sulfuric acid trapped in between the stacked GO layers, which could have led to an overestimation of the real conductivity value.…”

Designable Guest‐Molecule Encapsulation in Metal–Organic Frameworks for Proton Conductivity

“…[42] Recently, Vilela and co-workers have synthesized a new hybrid material, Lys@UPG-1, by encapsulating the amino acid lysine in a Zr-based MOF (UPG-1). [43] The Lys@UPG-1 composite shows similar proton conductivity but a high stability comparing with the pristine UPG-1. The activation energy of Lys@UPG-1 is 0.13 eV and UPG-1 is 0.02 eV, indicating that proton transfer follows the Grotthuss mechanism.…”

“…When amino acids molecules are introduced into the host molecule as guests, they can also interact with the host molecule and adsorbed water molecules to construct a proton conductivity pathway [42] . Recently, Vilela and co‐workers have synthesized a new hybrid material, Lys@UPG‐1, by encapsulating the amino acid lysine in a Zr‐based MOF (UPG‐1) [43] . The Lys@UPG‐1 composite shows similar proton conductivity but a high stability comparing with the pristine UPG‐1.…”

Ion-Exchanged UPG-1 as Potential Electrolyte for Fuel Cells

“…Among the large variety of electrolyte materials (e.g., polymers such as the benchmarked Nafion, zeolites, perovskites, polyoxometalates, or covalent organic frameworks), metal–organic frameworks (MOFs) have recently been proposed as promising electrolytes in PEMFCs. ,,− MOFs are porous crystalline hybrid materials built up from inorganic subunits (atoms, clusters, chains, etc.) and organic polydentate linkers (carboxylates, azolates, phosphonates, etc.)…”

“…and organic polydentate linkers (carboxylates, azolates, phosphonates, etc.) that combine an exceptional regular porosity (up to S BET = 7800 m 2 ·g –1 ) with a large chemical tunability and structural versatility. , Three different strategies have been reported to synthesize either MOFs or MOF composites with high proton conductivity, , including (i) intrinsically conductive MOF structures based on organic polycomplexant ligands bearing labile protons (phosphonate, sulfonate, and carboxylate) functionalized or not with additional groups (e.g., −SO 3 H, −NH 2 , −OH, and −CO 2 H) able to modify the p K a values of the material, − (ii) the insertion within the MOF porosity of proton carrier species (e.g., H 2 SO 4 , H 3 PO 4 , imidazole, 1 H -1,2,4-triazole, lysine, and ionic liquids), ,− and (iii) the exchange of labile protons present in the structure by other cationic species (organic or inorganic), with the aim of enhancing the proton conductivity . Among the large panel of MOFs (first strategy), phosphonate-based structures appear to be excellent electrolyte candidates because they combine the presence of a larger number of labile protons (vs carboxylate- or sulfonate-MOFs), associated with good conductivity and with a higher stability .…”