Effect of cross‐linking degree on proton conductivity of a Schiff‐Base network impregnated with Brønsted acids

Abstract The cross‐linking degree (crystallinity) and proton sources of conductors strongly impacts proton‐conducting properties, but the attention is still limited, especially for covalent‐organic frameworks. Here, we prepared a couple of cross‐linking degrees of Schiff‐Based Network by solvothermal (high surface specific area, SNW‐S) and microwave‐assisted (low cross‐linking structure, SNW‐M) strategies, respectively; subsequently, Brønsted acids (BAs)‐entrapped Schiff‐Based Network (SNW) composites as fillers are blended into polyvinyl pyrrolidone/poly(vinylidene fluoride) (PP) blends to form hybrid membranes. Varied crystalline degrees and microstructures of the SNWs are monitored by XRD, FTIR, and nitrogen adsorption, coupled with relevant conduction measurements like water uptake and proton conductivity (PC) for the composites and hybrid membranes. Structurally, under the aid of “acid–base pairs” between amine groups in Schiff‐Based Network and Brønsted acids as proton carriers, the resulting H 2 SO 4 ‐incorporated SNW‐M with poor cross‐linking displays the high proton conductivity (4.96 × 10 −3 S/cm) with an activation energy of 0.13 eV, whereas H 2 SO 4 and H 3 PO 4 ‐incorporated SNW‐S appear the relatively lower conductivities, imputable to the differences in the cross‐linking degree of Schiff‐Based Network and interactions between proton carriers and active sites in Schiff‐Based Network. Additionally, the hybrid membranes also show σ values in the identical level of the composites, reaching to 8.02 × 10 −3 S/cm at 323 K.