Intrinsic proton relay in poly-phosphamide to bolster proton-exchange membrane fabrication and electrocatalytic proton reduction

Abstract

Complex synthetic routes and over-swelling of the perfluoro/sulfonated Nafion-based proton-exchange membrane (PEM) materials at high temperatures and strong acidic pH leads to a continuous quest for stable non-fluoro organic polymers with high proton conductivity. Herein, porous organic polymers of 300-700 nm hydrodynamic diameter containing tripodal polyamine (PPA-1a) and/or ethylenediamine (PPA-2) as the linker and possessing a phosphamide {P(O)-NH} moiety in the repeating unit, as confirmed by the 31P and 13C (CPMAS) NMR and other spectroscopic characterizations, are synthesized. A non-phosphamide tripodal polyamine (PPA-1b) is also prepared to establish the pivotal role of {P(O)-NH} moiety in proton-conductivity and electrocatalytic hydrogen evolution reaction (HER). Hierarchical mesoporosity with <10 nm average pore diameter and ~5 m2 g-1 of PPA-2 leads to proton conductivity (σ) of 4.7 x 10-2 S cm-1 in aqueous pH 4.5 at 358 K, superior to some commercial Nafions. The low activation barrier (Ea) of 0.12 eV indicates facile proton-hopping within the PPA-2 frame following a Grotthuss pathway. Conversely, the absence of phosphamide in PPA-1b and non-porosity results in low proton conduction. The density functional theory (DFT) study predicts that protonation at both “-P=O” and “–NH” sites of the phosphamide is energetically favorable to give stable tautomeric forms, which facilitate the proton-relay within the polymeric frame of PPA-2. The remarkably high proton conduction has led to the fabrication of PEMs using only 1 wt% PPA-2 with the poly(methyl methacrylate) (PMMA) and poly(vinyl alcohol) (PVA) supports and the optically transparent membranes show structural stability after a successful proton-exchange study with 0.5 M H2SO4. Owing to the proton adsorption ability of the {P(O)-NH} moiety, fast proton relay within the framework, and presence of redox-active PV center, PPA-2 behaves as an organo-electrocatalyst for hydrogen evolution reaction (HER) with a low overpotential of 311 mV at 10 mA cm-2. The pH dependency in the PV/IV redox-couple identified in the cyclic voltammetric study essentially indicates a proton-coupled-electron-transfer (PCET) mediated HER, while the proton adsorption on the {P(O)-NH} sites facilitates the Volmer step of the HER. In this study, phosphamide-based materials are exemplified as Nafion’s alternative for PEM design and metal-free energy material for HER.