Sub-5 nm Graphene Oxide Nanofilm with Exceptionally High H⁺/V Selectivity for Vanadium Redox Flow Battery

Abstract

Highly H+/V selective membranes are desired in high-performance vanadium redox flow batteries (VFRBs) to overcome the crossover phenomena of vanadium species. Herein, we demonstrate the molecular-sieving nanochannels (∼0.84 nm) inside a graphene oxide (GO) laminate efficiently blocked the transport of vanadium ions, while allowing the transport of H+. Furthermore, an ultrathin (sub-5 nm) and highly selective GO nanofilm was successfully coated on a porous substrate to improve the H+ flux using a facile spin-coating method. The GO-coated thin-film composite (TFC) membrane showed much higher H+ flux with an exceptionally high H+/V selectivity (H+ permeation rate/VO2+ permeation rate, up to 850) due to the molecular-sieving nanochannels inside the GO nanofilm, leading to a much more enhanced VRFB performance in terms of energy efficiency (EE, 84.7%) compared to the benchmark Nafion membrane (EE, 69.2%), at 20 mA cm–2.

Recent Development in Composite Membranes for Flow Batteries

“…Ex situ methods usually mean that the top layer is fabricated on the support layer with as imple coating process, such as casting, [47] spin-coating, [22] spraying, [21] or even simple overlapping. [60] Ex situ methods are facile and require only relatively simplee quipment, thus they are preferred for large-scale preparation.…”

“…In contrast, the top layer can separate the redox couples, achieving high selectivity.M ore importantly,acomposite membrane can be rationally designed by tuning as elective layer,r ealizing high ion selectivity without sacrificingi on conductivity. [22]…”

“…Another type of the composite membranes is based on as electivel ayer with more precise, consistent, and well-defined channels. Owing to intrinsic ordered structure or inherentp orosity,t he studyo fs everalm aterials has flourished, including graphene oxide (GO), [22,52] MOFs, [34] COFs, [35] zeolites, [21] PIMs, [36] PAFs, [37] and POCs. [38] In this type, the aforementioned materials are fabricated on the support layer,a sd emonstrated in Figure 4c.A dvantages are prominent when the ordereds tructure materials are constructed as selectivel ayers.…”

“…GO composite membranes possess flexibility and specific tunable lamella structure. [20,22,54] As materials can be intercalated between the lamella of GO, GO can provide variable functions and ah igh number of mass-transport channels without damaging the compact structure of membranes. [52, 54a, 55] MOF composite membranes with high crystallinity and regular pores structure can be functionalized more flexibly via introducing functional organic linkers or by post-synthetic modification (PSM).…”

“…Composite membranes with thin ordereds electivel ayers have recently been at the centero fr esearch of FBs. These ordered materials (such as GO and zeolite)h ave an affinity to water; [20,22] hence, when they are fabricated as the selective layers, it helps to generate the interconnected water channels, which are suitable for ion transport. [53] Even hydrophobic regular structurem aterials, such as PIMs, have been modified utilizing their distinct pore sized to endow hydrophilicity.H ydrophilic ordered channels and well-controlled thickness of the selective layer all can induce ah ighi onic conductivity.…”