The Importance of Water Transport in High Conductivity and High-Power Alkaline Fuel Cells

Abstract

High ionic conductivity membranes can be used to minimize ohmic losses in electrochemical devices such as fuel cells, flow batteries, and electrolyzers. Very high hydroxide conductivity was achieved through the synthesis of a norbornene-based tetrablock copolymer with an ion-exchange capacity of 3.88 meq/g. The membranes were cast with a thin polymer reinforcement layer and lightly crosslinked with N,N,N ,N -tetramethyl-1,6-hexanediamine. The norbornene polymer had a hydroxide conductivity of 212 mS/cm at 80°C. Light cross-linking helped to control the water uptake and provide mechanical stability while balancing the bound (i.e. waters of hydration) vs. free water in the films. The films showed excellent chemical stability with <1.5% conductivity loss after soaking in 1 M NaOH for 1000 h at 80°C. The aged films were analyzed by FT-IR before and after aging to confirm their chemical stability. A H 2 /O 2 alkaline polymer electrolyte fuel cell was fabricated and was able to achieve a peak power density of 3.5 W/cm 2 with a maximum current density of 9.7 A/cm 2 at 0.15 V at 80°C. The exceptionally high current and power densities were achieved by balancing and optimizing water removal and transport from the hydrogen negative electrode to the oxygen positive electrode. High water transport and thinness are critical aspects of the membrane in extending the power and current density of the cells to new record values.

Achieving High‐Performance and 2000 h Stability in Anion Exchange Membrane Fuel Cells by Manipulating Ionomer Properties and Electrode Optimization

“…[28] Mohr's titration method was used to measure the IEC of the membranes after cross-linking and quaternization to confirm the accuracy of the NMR results. [11,27] In a typical procedure, the polymer in Br − form was converted to Cl − form by soaking in 0.1 m NaCl solution for 24 h. The film was removed from the NaCl solution, thoroughly washed with deionized (DI) water, and dried in vacuum for >24 h. The dry film weight was recorded. Next, the membranes were immersed in a fixed volume of 0.05 m NaNO 3 for 24 h. Finally, the released Cl − was titrated with an aqueous AgNO 3 solution using K 2 CrO 4 (10 wt%) as the indicator.…”

“…[28]; e) From ref. [27]. Hence, GT64 may represent an undesirable combination of properties for AEMFC electrodes, despite performing very well as an AEM.…”

“…[7] Huang et al, used similar materials in AEMFCs, enabling very high peak power density to be achieved (3.4 W cm −2 ) under H 2 /O 2 reacting gases, [26] which was recently improved to 3.5 W cm −2 by using thinner membranes. [27] In this study, three poly(norbornene) tetrablock copolymer ionomers with different IEC, molecular weight, dispersity, and water uptake were synthesized and fully characterized for their physical and electrochemical properties. The ionomers were integrated into operating AEMFCs with symmetric (same ionomer in both catalyst layers) and asymmetric (different ionomers in the anode and cathode catalyst layers) electrode configurations.…”

“…For GT32, σ/IEC (at 80 °C) was 65.2, whereas for GT64, σ/IEC (at 80 °C) was 54.2. [27,28] The water uptake (WU) of the GT32, GT64, and GT78 polymers was measured by first casting a film as described in the Experimental section. The WU for the GT78, GT64, and GT32 ionomers at room temperature were 163%, 90%, and 63%, respectively.…”

“…The increasing trend in WU of a polymer with IEC is due to a higher concentration of ion pairs and their hydration. [27,29,30] Also, increased IEC typically translates to a polymer with increased hydrophilicity. Hence, it is expected that in an operating cell, the higher IEC ionomers will take-up more water and swell to a greater extent while the lower IEC ionomer (relatively more hydrophobic character) will uptake less water and may even allow the electrodes to more effectively reject water.…”