Enhanced Proton Selectivity in Ionomer Nanocomposites Containing Hydrophobically Functionalized Silica Nanoparticles

Abstract

Perfluorosulfonic acid (PFSA) ionomers are ubiquitous as proton-exchange membranes (PEMs) in vanadium redox flow batteries (VRFBs), as they provide high proton conductivity and robust chemical stability. However, traditional PFSA ionomers suffer from high vanadium ion crossover, i.e., low ion selectivity, which reduces the efficiency and lifetime of the battery. Herein, a novel method to fabricate PFSA nanocomposites containing fluorocarbon-decorated silica nanoparticles is presented. These composite ionomers exhibit drastically reduced vanadium ion permeability and an almost two orders of magnitude increase in proton selectivity when compared to the current benchmark commercial ionomer. Small-angle neutron scattering data suggest that the nanostructures of these nanocomposites are drastically different from their pristine counterpart, where the periodic spacing of the hydrophobic domains is significantly reduced, while changes to the ionic structure were seen to be minimal. This work suggests that composite PEMs containing a secondary phase that alters the hydrophobic, nonion-conducting phase of the ionomer may prove to be a fruitful fabrication route to produce ionomer membranes with enhanced performance for use in VRFBs.

Role of nanoparticle size and surface chemistry on ion transport and nanostructure of perfluorosulfonic acid ionomer nanocomposites

“…These results are similar to those recently reported on Nafion membranes containing hydrophobically-functionalized SiNPs, where the periodic spacing of the hydrophobic domains was seen to be more significantly impacted than the hydrophilic (ionic) domains, though almost a two-orders of magnitude increase in ion selectivity was observed. 56 It is important to note that while changes in the tortuosity of the ionic network cannot be captured via neutron scattering, one would expect that both the SiNP size and surface chemistry may alter the tortuosity of the ionic domains. Changes to the tortuosity of the transport channels could help better explain the observed changes in both VO 2+ permeability and proton conductivity.…”

Side-Chain Grafting-Modified Sulfonated Poly(ether ether ketone) with Significantly Improved Selectivity for a Vanadium Redox Flow Battery

“…So far, because of its extraordinarily high conductivity and outstanding chemical stability, Nafion membranes are still one of the finest choices for VRFB ion-selective membranes . However, the drawbacks of a high expansion rate, substantial vanadium ion penetration, and high cost are the key issues for large-scale deployment of Nafion membranes in VRFBs. , Many efforts have been made to date to produce innovative ion-selective membranes with improved performance and cheaper costs, such as cross-linking-modified − and polymer-blended composite membranes. − Because of their low cost, stability, and ease of manufacture, sulfonated poly­(ether ether ketone) membranes (SPEEK) have received a lot of interest. ,,,− Amphoteric ion exchange membranes with fixed cations and anion exchange groups have received a lot of interest in recent years. , Due to electrostatic repulsion, the positive ion group can effectively prevent the positive diffusion of vanadium ions under battery operating conditions, while the negative ion group provides optimum proton conductivity. Sulfamic acid (SA) contains −SO 3 H (acidic) and −NH 2 (weak basic) groups, which can be used to replace the inherent conductivity/permeability balance of the proton exchange membrane …”

Selective Catalyst Surface Access through Atomic Layer Deposition

“…Ionomers are necessary to provide proton or hydroxide transport pathways from the catalyst surface to the membranes of these systems, through either anionic (for proton transport) or cationic (for hydroxide) groups on a polymer backbone . Alongside great conductivity for the required species, these functional groups provide an inherent selectivity because of their transport properties; however, they are far from being selective enough for systems where transport selectivity is critical. , …”

“…Regrettably, many MOFs suffer from stability issues in acidic and basic media, making them unsuitable for most electrocatalyst applications . Silica has seen use in many systems, as both a membrane additive and a catalyst coating, to impart ionic selectivity. In several works, nanoscale silica layers covering Pt nanoparticles were used to prevent Pt cation diffusion and hence agglomeration in PEMFC cathodes and anodes. − Silica overlayers were also utilized by one group to prevent detachment of electrodeposited Pt nanoparticles for photoelectrochemical water splitting, without inhibiting mass transport .…”

“…5 Alongside great conductivity for the required species, these functional groups provide an inherent selectivity because of their transport properties; however, they are far from being selective enough for systems where transport selectivity is critical. 6,7 Another methodology to impart selectivity on an electrocatalyst is through the use of nanoporous materials, which are selective through the size of their pores. Metal organic frameworks (MOFs) have been used to impart selective access to encapsulated nanoparticle catalysts due their highly tunable pore structure.…”