Ultrafast and Stable Proton Conduction in Polybenzimidazole Covalent Organic Frameworks via Confinement and Activation

Abstract

Polybenzimidazoles are engineering plastics with superb thermal stability and this specificity has sparked awideranging research to explore proton-conducting materials. Nevertheless,s uch materials encounter challenging issues owingt op hosphoric acid proton carrier leakage and slow proton transport. We report as trategy for designing porous polybenzimidazole frameworks to address these key fundamental issues.T he built-in channels are designed to be onedimensionally extended, unidirectionally aligned, and fully occupied by neat phosphoric acid, while the benzimidazole walls trigger multipoint, multichain, and multitype interactions to spatially confine ap hosphoric acid network in pores and facilitate proton conduction via deprotonation. The materials exhibit ultrafast and stable proton conduction for lowp roton carrier content and activation energy-a set of features highly desired for proton transport. Our results offer adesign strategy for the fabrication of porous polybenzimidazoles for use in energy conversion applications.

Ultrafast and Stable Proton Conduction in Polybenzimidazole Covalent Organic Frameworks via Confinement and Activation

“…Thebinding energies for C1sspectra of TPB-DABI-COF and H 3 PO 4 @TPB-DABI-COF are observed at 284. 9,286.1,and 286.6 eV (Figure 4c,d). Thef irst and second peaks are attributed to the C = Cand C = Nbonds in the COF skeleton.…”

“…In order to identify the interactions between H 3 PO 4 and benzimidazole unit, we succeeded in preparing single crystals of the model complex of benzimidazole and H 3 PO 4 (Figure 1c;S upporting Information). From the single crystallography (Table S3, CCDC 194985 [9] ), we observed that one nitrogen atom of benzimidazole is protonated to form imidazolium cation while H 3 PO 4 is deprotonated into H 2 PO 4 À anion;t hese two ionic species are interacted via electrostatic interaction in the complex. This protonation-deprotonation process yields H 2 PO 4 À open sites for facilitated proton transport.…”

High-density sulfonic acid-grafted covalent organic frameworks with efficient anhydrous proton conduction

“…3c, d and Table S4 †). 16,17,19,33,[41][42][43][44][45][46] The parameter of activation energy (E a ) is used to investigate the proton transfer mechanisms. Depending on the value of activation energy, there are two commonly recognized mechanisms, the Grotthuss mechanism (E a < 0.4 eV) and the vehicular mechanism (E a > 0.4 eV).…”

“…The anhydrous proton conductivity measurements of the COFs were performed using COF pellets. 17,19 Within a glovebox lled with Ar, the COFs were ground into homogeneous powder, and then about 100 mg of the material were pressed into circular pellets (16 mm in diameter, 550-700 mm in thickness) under a pressure of 20 MPa for 10 min. A coin cell was assembled by sandwiching the pellets between two stainless steel electrodes in the glovebox under Ar.…”

“…[11][12][13][14] The proton conductivity is mainly determined by proton concentration. In fact, most COF materials are proton insulators, and in order to increase the number of free migrating protons in COFs, there are two distinct strategies to ne-tune the framework and channel space: (i) decorating the framework with suitable acidic units (such as -SO 3 H), because the ability of those acidic moieties to readily release protons permits high proton conductivity; 15 (ii) encapsulating guest molecules with low pK a values (such as H 3 PO 4 , triazole, and imidazole) into the channels, [16][17][18][19][20][21] so that the embedded molecules form a network between the molecules and specic sites on the framework to construct proton-conduction pathways. However, most reports on the application of COFs as proton exchange membranes require humid conditions and working temperatures below 80 C to ensure stable proton conductivity, which is far from the desired operating conditions of high-temperature PEMFCs.…”