Comparison of the performances of dibenzofuran- and dibenzothiophene-based poly(terphenyl piperidinium) anion exchange membranes for water electrolysis

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Poly(aryl piperidinium) derived from super acid-catalyzed polymerization is a potential candidate to improve the chemical stability of anion exchange membranes (AEMs). Among them, polymers with ether bonds and no electron-drawing groups can be utilized as AEMs for the construction of AEM water electrolyzer (AEMWE) due to their superior chemical stability. Herein, poly(p-terphenyl-dibenzothiophene piperidine) (PPDBTP) and poly(p-terphenyl-dibenzofuran piperidine) (PPDFP) are synthesized using dibenzothiophene and dibenzofuran as monomers. Subsequently, these polymers are used in the preparation of AEMs. The quaternized PPDFP (QPPDFP60) membrane, prepared with a molar ratio of 60% p-terphenyl relative to the total of dibenzofuran and p-terphenyl, exhibits the highest hydroxide ion conductivity of 130.15 mS cm-1 at 80 °C and superior alkaline stability, with an ionic conductivity retention ratio of 73.52% after soaking in 2.0 M NaOH at 80 °C for 1080 h. In contrast, the quaternized PPDBTP (QPPDBTP80) membrane, with a molar ratio of 80% p-terphenyl to the total of dibenzothiophene and p-terphenyl, shows a hydroxide ion conductivity of 124.16 mS cm-1 at 80 °C and ionic conductivity retention ratio of 68.56% after alkaline stability test. The water electrolyzer assembled with QPPDFP60 and QPPDBTP80 achieved current densities of 1.34 and 0.84 A cm-2 in 1.0 M KOH solution at 2.0 V and 60 °C, respectively. As a result, both experimental and density functional theory calculations demonstrate that AEMs prepared with dibenzofuran exhibit superior chemical stability, higher OH- conductivity and better water electrolyzer performance compared to those derived from dibenzothiophene. This study provides a fabrication strategy of AEMs using monomers containing furan and thiophene. Furthermore, it gives deep insight into the mechanisms underlying the improvements in both stability and hydroxide ion conductivity of AEMs.