Interfacial molecular structure of phosphazene-based polymer electrolyte at the air-aqueous interface using sum frequency generation vibrational spectroscopy

Abstract

The change induced in the physicochemical properties of polymer while hosting ions provides a platform for studying its potential applications in electrochemical devices, water treatment plants, and materials engineering science. The ability to host ions is limited in very few polymers, which lack a detailed molecular-level understanding for showcasing the polymer-ion linkage behavior at the interfacial region. In the present manuscript, we have employed sum frequency generation (SFG) vibrational spectroscopy to investigate the interfacial structure of a new class phosphazene-based methoxyethoxyethoxyphosphazene (MEEP) polymer in the presence of lithium chloride salt at the air-aqueous interface. The interfacial aspects of the molecular system collected through SFG spectral signatures reveal enhanced water ordering and relative hydrogen bonding strength at the air-aqueous interface. The careful observation of the study finds a synchronous contribution of van der Waals and electrostatic forces in facilitating changes in the interfacial water structure that are susceptible to MEEP concentration in the presence of ions. The observation indicates that dilute MEEP concentrations support the role of electrostatic interaction, leading to an ordered water structure in proximity to diffused ions at the interfacial region. Conversely, higher MEEP concentrations promote the dominance of van der Waals interactions at the air-aqueous interface. Our study highlights the establishment of polymer electrolyte (PE) characteristics mediated by intermolecular interactions, as observed through the spectral signatures witnessed at the air-aqueous interface. The investigation illustrates the polymer-ion linkage adsorption effects at the interfacial region, which explains the macroscopic changes observed from the cyclic voltammetry studies. The fundamental findings from our studies can be helpful in the design and fine-tuning of better PE systems that can offer improved hydrophobic membranes and interface stability for use in electrochemical-based power sources.

Molecular dynamics simulation of salt diffusion in constituting phosphazene-based polymer electrolyte

“…Therefore, experimentally measured behavior of MEEP film on SiO 2 substrate provides the evidence of hydrophobic tendency of MEEP chains. This supports the observed MEEP density profiles behavior that makes phosphazene-based polymer layer more hydrophobically active towards air-aqueous interface, prominent due to terminal methoxy group on a hydrophilic surface [13].…”

“…Surface wetting is significantly influenced by the distance factor σ p , which upon increase in σ p can be used to modulate the hydrophilic propensity of various surfaces with hydrophobic, even-handed, and hydrophilic walls, as verified in previous studies [44,55]. In the density profile along the z-direction, it is evident that MEEP chains exhibit a pronounced inclination towards the air-aqueous interface, highlighting the hydrophobic tendency of the polymer chains toward the air phase [13].…”

“…Spectral signature ∼2825 cm −1 in ssp and ∼2825 cm −1 , and ∼2880 cm −1 in ppp are prominently visible. The spectral signature showcased in figure 4(c)(ii)) qualitatively provides the evidence of terminal methoxy (symmetric stretch-OCH 3 ), along with presence of symmetric stretch-OCH 2 part of ethoxy (OCH 2 CH 2 ) group is prominent at the air-polymer interface [13]. By considering the prominent spectral signature, it can be concluded that the activity of alkyl ether chain/pendent group attached to polymer structure backbone get aligned towards the surface normal.…”

“…Weaker coordination between Li + ionpolymer provide more freedom to the migration of Li + ion allowing more hopping while exhibiting polymer electrolyte OH-oscillator strength of the water molecules at the air-aqueous interface varies significantly as a function of MEEP concentration added in the 10 mM concentrated LiCl salt aqueous electrolyte. The source of this data is extracted from our work published in JPCM 2024 [13], which contains the SFG intensity profile lying in the wavenumber range 3000-3600 cm −1 . behaviour.…”

“…These qualities make it a recognized potent alternative to pure liquid electrolytes for use in batteries [7,10]. This has excited many scientist and industrialist to conduct research on phosphazene-based polymer electrolyte to elucidate its electrochemical behaviour for its use in polymer Li-ion batteries and organic solar cells [2,3,7,13,14]. These studies suggested the use of liquid additives to enhance ionic conductivity in polymer-salt mixture [3,15].…”