The Importance of Water Transport in High Conductivity and High-Power Alkaline Fuel Cells (original) (raw)

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 cross-linked 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. Anion-exchange membranes (AEMs) are a key component in alkaline exchange membrane fuel cells (AEMFCs), flow batteries and electrolyzers. 1 Alkaline conditions are attractive because of the facile electrochemical reaction kinetics at high pH for oxygen reduction and water oxidation. 2-7 Device operation at high pH allows for the use of non-precious metal catalysts, simpler design for the balance of plant, and reduced fuel crossover. 9-12 However, it is imperative that the AEMs are thin, have long-term alkaline stability, and high hy-droxide ion conductivity. 8 There have been issues in the past with AEMs showing low ionic conductivity, poor stability at high pH, and high water uptake (leading to dimensional change); however, these issues are being systematically addressed. 13-15 The formation of multi-block copolymers (BCP) are a means to achieve phase segregation within polymers in order to create high-mobility ion conduction channels within the hydrophilic phase of the polymer membrane. 16-20 Previously, we have reported AEMs consisting of poly(norbornene) BCPs with record high hydroxide conductivity , 198 mS/cm, and very high peak power density in a hy-drogen/oxygen fuel cell, 3.4 W/cm 2 at 80°C. 21-24 In addition, the poly(norbornene) polymer, as well as the membranes made from the polymer, were shown to have excellent thermal and mechanical properties. The S N 2 substitution and Hoffmann elimination degradation routes were suppressed by tethering the quaternary ammonium head-group to the all-hydrocarbon poly(norbornene) backbone via a long alkyl hydrocarbon chain. 25-28 Trimethyl quaternary ammonium cation head-groups have been shown to be stable cations in AEMs, and their low molecular weight enables high ion exchange capacity (IEC). 29 Water management is a key factor in achieving high AEMFC performance. The AEM plays a key role in balancing the water content and distribution during device operation. 30,31 It has been shown that a significant majority of the reacting water at the AEMFC cathode is provided by back-diffusion of water produced at the AEMFC anode. 32 This suggests that high AEM water permeability is bene-* Electrochemical Society Student Member. ficial in AEMFCs. However, excessive AEM water uptake can flood the ion conducting channels within the polymer and lead to membrane softening and mechanical failure. 33,34 Thus, high water permeability without high water solubility appears to be a critical feature for AEM-FCs. Waters of hydration are necessary for hydroxide ion conduction; however, excessive unbound (i.e. free) water leads to low hydroxide mobility and membrane distortion. 23 Hence, it is necessary to balance the amount of free and bound water inside the membranes to yield the maximum hydroxide mobility and water transport. 21-23 Cross-linking is an effective way to reduce water uptake and swelling. 24 However, AEMs with high cross-linking density can become too rigid, leading to poor ion mobility, mechanical properties and water diffusivity (i.e. high water solubility without high diffusivity). 35-37 In the case of polymers with high IEC, light cross-linking is an effective strategy to balance the high conductivity and water uptake (WU) without sacrificing IEC. 22,23 In addition, thinner membranes can enable rapid water transport without high water uptake, and enable high current density AEMFCs. Conveniently, light cross-linking also helps in the production of thin membranes with good mechanical properties. AEM carbonation upon exposure to CO 2 is another important factor in AEMFC performance. When CO 2-containing air is fed to an operating AEMFC, the hydroxide anions produced by the reduction of oxygen at the positive electrode react with carbon dioxide to produce carbonated anions with lower mobility than hydroxide (i.e. carbonate or bicarbonate), increasing ohmic-related losses. 38 Additionally, these carbonated anions can rapidly populate the AEM and AEMFC anode, leading to significant thermodynamic and kinetic-related losses. 32,39 It has been stated that the adverse effects of membrane carbonation could be minimized by using AEMs with very high ionic conductivity so that the decrease in mobility upon carbonation can be mitigated and cell decarbonation during operation through the so-called "self-purging" mechanism can occur more rapidly. Hence, AEMs with very high conductivity are most desirable. 23 In this study, the synthesis of chemically stable AEMs with record high conductivity, 212 mS/cm at 80°C, and their implementation into AEMFCs are described. This new AEM enables record performance in a hydrogen/oxygen AEMFC with a peak power density of 3.5 W/cm 2 and maximum current density of 9.7 A/cm 2 at 0.15 V at 80°C when) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 130.207.74.202 Downloaded on 2019-11-04 to IP