ABSTRACT
In vanadium redox flow batteries (VRFBs), simultaneously achieving high proton conductivity, low vanadium-ion permeability, and outstanding chemical stability using electrolyte membranes is a significant challenge. In this study, we report the fabrication of a tri-directional poly(2,5-benzimidazole) (T-ABPBI) membrane using a direct casting method. The direct-cast T-ABPBI (D-T-ABPBI) membrane was fabricated by modifying the microstructure of the membrane while retaining the chemical structure of ABPBI, having outstanding chemical stability. The D-T-ABPBI membrane exhibited lower crystallinity and an expanded free volume compared to the general solvent-cast T-ABPBI (S-T-ABPBI) membrane, resulting in enhanced hydrophilic absorption capabilities. Compared to the S-T-ABPBI membrane, the enhanced hydrophilic absorption capability of the D-T-ABPBI membrane resulted in a decrease in the specific resistance (the area-specific resistance of S-T-ABPBI and D-T-ABPBI membrane is 1.75 and 0.98 Ωcm2, respectively). Additionally, the D-T-ABPBI membrane showed lower vanadium permeability (3.40 × 10-7 cm2 min-1) compared to that of Nafion 115 (5.20 × 10-7 cm2 min-1) due to the Donnan exclusion effect. Owing to the synergistic effects of these properties, the VRFB assembled with D-T-ABPBI membrane had higher or equivalent coulomb efficiencies (>97%) and energy efficiencies (70-91%) than Nafion 115 at various current densities (200-40 mA cm-2). Furthermore, the D-T-ABPBI membrane exhibited stable performance for over 300 cycles at 100 mA cm-2, suggesting its outstanding chemical stability against the highly oxidizing VO2+ ions during practical VRFB operation. These results indicate that the newly fabricated D-T-ABPBI membranes are promising candidates for VRFB application.
ABSTRACT
AB-polybenzimidazole (ABPBI) dissolution kinetics in an eco-friendly complex acid-free solvent based on dimethyl sulfoxide (DMSO), methanol and KOH, and the rheological behavior of their solutions are investigated. The optimal component ratio of solvent providing the complete ABPBI dissolution is determined. Methanol containing dissolved KOH contributes to the creation of a single-phase superbasic medium, which accelerates and improves the polymer solubility in a mixture with DMSO, significantly reducing the viscoelasticity of the resulting solution. The optimum methanol content is up to 60 wt.% related to DMSO. The polymer dissolution rate increases by 5 times in this composition. It found the polymer concentration of 9% is close to the dissolution limit due to the strong solution structuring, which is probably associated with an increase in the amount of water released during the KOH-methanol-DMSO interactions. As a result, the conditions for obtaining high concentrated solutions in a complex, mainly organic solvent for fiber spinning are developed. The viscoelastic properties of solutions are measured in the concentration range of 1-9% at temperatures of 20-50 °C. The flow activation energy for 7 and 9% solutions decreases by 1.5 and 2.3 times, respectively, as the content of methanol in the complex solvent increases from 10 to 60%.
ABSTRACT
In recent times, high-temperature polymer electrolyte membranes (HTPEMs) have emerged as viable alternatives to the Nafion-based low-temperature-operated polymer electrolyte membrane fuel cells. This is owing to their higher tolerance to fuel impurities, efficient water management, and higher cathode kinetics. However, the most efficacious HTPEMs such as poly(benzimidazole) (PBI) or 2,5-poly(benzimidazole) (ABPBI), which rely on the extent of phosphoric acid (PA) doping level for fuel cell performance, suffer from poor mechanical properties at higher acid doping levels and dopant leaching during continuous operation. To overcome these issues, we report the synthesis of ABPBI membranes and fabrication of ABPBI-zirconium pyrophosphate (α-ZrP)-based nanocomposite membranes by an ex situ methodology using methane sulfonic acid as the solvent. The incorporation of hydrophilic α-ZrP into the membrane resulted in higher dopability of PA (6.5 mol) and proton conductivity (46 mS/cm) of the membranes (10 wt % of α-ZrP) as against the corresponding values of 3.6 mol and 27 mS/cm, respectively, for the pristine membrane. More remarkably, these property improvements could be achieved while simultaneously augmenting the thermomechanical properties and oxidative stability of the membranes. The unit-cell tests showed a marked improvement in the maximum power density for the nanocomposite membrane (335 mW/cm2 at 10 wt % α-ZrP content) over the pristine ABPBI membrane (200 mW/cm2). We also report for the first time the feasibility of a 100 W HTPEM fuel cell (HTPEMFC) stack operated with the nanocomposite membrane with an active area of 39 cm2. The HTPEMFC stack delivered a stable voltage and power output, with a voltage drop rate of 0.84 µV/h over a run time of 730 h.