Cross-Linkable Polymeric Ionic Liquid Improve Phosphoric Acid Retention and Long-Term Conductivity Stability in Polybenzimidazole Based PEMs

Abstract

A series of composite cross-linked membrane based on fluorine-containing polybenzimidazole (6FPBI) and a cross-linkable polymeric ionic liquid (cPIL) have been prepared for high temperature proton exchange membrane (HT-PEM) applications. Particularly, the obtained composite cross-linked membranes showed excellent phosphoric acid doping ability and proton conductivity. On the basis of the trade-off between mechanical strength and proton conductivity of composite membranes, the optimal content of cPIL is 20 wt % (6FPBI-cPIL 20 membrane). For instance, the 6FPBI-cPIL 20 membrane with a PA doping level of 27.8 exhibited a proton conductivity of 0.106 S cm −1 at 170 °C, which is much higher than that of pristine 6FPBI membrane. The most outstanding contribution of this work is that the 6FPBI-cPIL membranes showed improved phosphoric acid retention and long-term conductivity stability under harsh conditions (80 °C/40% RH) for 96 h. In particular, the proton conductivity and PA doping level of the 6FPBI-cPIL 20 membrane remained at a high level of 0.064 S cm −1 and 8.5 after 96 h of the test, respectively.

Enhancing proton conductivity of phosphoric acid‐doped Kevlar nanofibers membranes by incorporating polyacrylamide and 1‐butyl‐3‐methylimidazolium chloride

“…48 Wang has reported that the anhydrous σ value was 1.16 × 10 −1 S/cm at 170 C and it decreased to 0.62 × 10 −1 S/cm after 96 hours for PA-doped fluorinecontaining polybenzimidazole(6FPBI)/cross-linkable polymeric ionic liquid(cPIL) membranes. 49 In PA-doped membranes, the formed free PA molecule chains and further the interconnected molecule network dominated proton conduction behaviors. 50 The improvement on σ was benefited from more carriers participating proton transfer process and the reduction of proton conduction resistance.…”

“…Specifically, the tensile strength of 6FPBI membrane could reach 116 ± 6 MPa, which was higher than (118 ± 4-66 ± 2) MPa for 6FPBI/(10-40)%cPIL membranes. 49 Furthermore, PA molecules could expand the accumulation volume of Kevlar nanofibers and therefore the nanofibers separation unavoidably deteriorated the stiffness of PA-doped Kevlar-based membranes. From Figure 10 membranes and 0.59 MPa for Kevlar/PAM/bmimCl(fd)/ PA membranes.…”

Synthesis and Properties of Phosphoric-Acid-Doped Polybenzimidazole with Hyperbranched Cross-Linkers Decorated with Imidazolium Groups as High-Temperature Proton Exchange Membranes

“…During the operation of HT-PEMFCs, water is continuously generated in the cathode, which will remove the phosphoric acid doped by the electrolyte membrane, leading to a decreased performance after long-term operation. To simulate the phosphoric acid loss process of the membranes, the PA-doped membranes were placed at 80 • C and 40% relative humidity (RH), through which the PA loss by water condensation could be observed [24]. Phosphoric acid retention of the membranes is shown in Figure 3.…”

“…Compared to branching, cross-linking is an effective way to improve mechanical strength because it strengthens the interactions among the polymer chains. Recently, cross-linking has been widely explored in PA/PBI membranes for HT-PEMFCs, and improved properties have been achieved compared to those of linear PBI membranes [12,[24][25][26][27][28]. Based on the advantages of cross-linking and branching, applying both methods to one membrane can enhance performance.…”

“…Compared to QA, the imidazolium (Im) group has a conjugated ring structure with a larger space, which can accommodate more PA molecules and stabilize them by delocalization and hydrogen bond formation [17]. A series of polymers containing Im groups have been developed as high-temperature PEMs (HT-PEMs), and the proton conductivity of these membranes was obviously enhanced [16,17,24,32,33]. To further improve performance, this work introduced Ims to the hyperbranched cross-linked OPBI structure (ImOPBI, Scheme 1).…”