Intrinsic proton relay in poly-phosphamides 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 lead 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 characterization studies, are synthesized. A non-phosphamide tripodal polyamine (PPA-1b) is also prepared to establish the pivotal role of the {P(O)–NH} moiety in proton-conductivity and the electrocatalytic hydrogen evolution reaction (HER). Hierarchical mesoporosity with <10 nm average pore diameter and ∼11 m² g⁻¹ surface area of PPA-2 leads to a proton conductivity (σ) of 4.7 × 10⁻² S cm⁻¹ in aqueous solution at pH 4.5 and 358 K, superior to some commercial Nafions. The low activation barrier (E_a) 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 “–PO” 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 the presence of the redox-active P^V center, PPA-2 behaves as an organo-electrocatalyst for the hydrogen evolution reaction (HER) with a low overpotential of 311 mV at 10 mA cm⁻². The pH dependency in the P^V/IV redox-couple identified in the cyclic voltammetry study indicates a proton-coupled-electron-transfer (PCET) mediated HER. At the same time, 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 as metal-free energy materials for the HER.