Composite Poly(norbornene) Anion Conducting Membranes for Achieving Durability, Water Management and High Power (3.4 W/cm 2 ) in Hydrogen/Oxygen Alkaline Fuel Cells

Abstract

Alkaline fuel cells and electrolyzers are of interest because they have potential advantages over their acid counterparts. High-conductivity anion conducting membranes were analyzed and used in alkaline hydrogen/oxygen fuel cells. The membranes were composed of reinforced block copolymers of poly(norbornenes) with pendant quaternary ammonium head-groups. It was found that membranes with light cross-linking provided excellent mechanical stability and allowed very high ion exchange capacity polymers to be used without penalty of excessive water uptake and swelling. The optimum membrane and fuel cell operating conditions were able to achieve a peak power density of 3.4 W/cm 2 using hydrogen and oxygen. The performance increase was greater than expected from minimizing ohmic losses. Mechanical deformations within the membrane due to excess water uptake can disrupt full cell operation. Cells were also run for over 500 h under load with no change in the membrane resistance and minimal loss of operating voltage. Energy conversion devices using solid polymer electrolytes such as fuel cells and electrolyzers are promising options for producing and storing clean energy because of their high thermodynamic efficiency and solid-state design. 1 These devices are also scalable and can be used for transportation, remote and distributed power, and large-scale facilities for electricity and hydrogen production. Polymer electrolyte membranes for fuel cells and electrolyzers are divided into two broad categories based on the dominant charge carrying ion: proton exchange membranes (PEMs) and anion exchange membranes (AEMs). There are already commercialized fuel cell electric vehicles and stationary power generators based on PEM membranes, however, there are significant costs associated with the platinum-based electrocatalysts and perfluorinated membranes. AEM-based devices have the potential to lower the cost of ownership compared to PEM-based devices because the high pH environment is advantageous for the oxygen reduction reaction (ORR, cathode reaction in AEM fuel cells) and oxygen evolution reaction (OER, an-ode reaction in AEM electrolyzers) kinetics, enabling the use of non-platinum catalysts. 2,3 Also, a variety of low-cost monomers can be used to synthesize hydrocarbon-based hydroxide ion conducting polymers that are stable in alkaline conditions, compared to the perfluorinated polymers needed for PEM-based electrochemical devices. 4,5 Perfluo-rinated polymers are expensive and present significant hazards due to monomer reactivity. The critical metrics for AEMs include (i) high anion (e.g. hydrox-ide) conductivity, (ii) long-term alkaline stability at the AEM fuel cell operating temperature, (iii) robust mechanical properties for withstanding in-use pressure differences and avoiding polymer creep under compression, and (iv) control over excessive water uptake, which can disrupt ion transport within the electrodes and membrane. 5,6 There have been several reports of AEMs with hydroxide conductivity of over 100 mS/cm (60°C to 80°C). 7-12 More recent reports of AEMs have shown conductivity at or near 200 mS/cm (at 80°C). 7,13,14 High conductivity AEMs have been paired with optimized electrodes (with either platinum or non-platinum catalysts) to give AEM-based fuel cells with peak power densities exceeding 1 W/cm 2. 15-19 The current record for peak power in a hydrogen/oxygen AEM fuel cell is 2 W/cm 2. AEM fuel cells are known to be sensitive to the relative humidity of the fuel and oxidant streams, as well as the water uptake in the AEM and ionomer. Proper water management in the membrane and electrodes is critical to achieve high power density. 16 Water is elec-trochemically generated at the anode during the hydrogen oxidation reaction (HOR) and is consumed at the cathode by the ORR in an AEM fuel cell. Water is transported from the cathode to the anode by electro-osmotic drag and accompanying anion transport. Water also back diffuses from the anode to cathode. Without adequate water content within the membrane and electrodes, ionic conductivity suffers and polymer degradation accelerates due to the higher reactivity of hydroxide at lower water concentration. On the other hand, if there is too much water, catalyst layers can be easily flooded, and the efficient flow of ions within the electrodes and membranes can be disrupted. Mechanical degradation in the membrane can also occur due to the higher internal stress and expansion within the AEM. The conductivity , pH, and mechanical properties of the AEM are also affected by the presence of carbon dioxide in air because it forms bicarbon-ate and carbonate ions within the membrane. Fuel cell measurements are often made with CO 2-free air to avoid the complications of car-bonate formation. 21 It is recognized that CO 2 uptake at the oxygen cathode can occur, which would lead to the formation of bicarbonate or carbonate in the membrane. The lower ion mobility for bicarbonate and carbonate compared to hydroxide results in higher ohmic losses within the anion conducting membrane and the possible accumulation of CO 2 in the hydrogen at the anode. Steps can be taken mitigate the negative effects of CO 2 at the air-cathode such as removing the CO 2 from air. Efficient ion channels are needed in the AEM to achieve high conductivity because the number of ions cannot be independently increased (i.e. higher ion exchange capacity (IEC)) because of the penalty due to excessive water uptake. It has been shown that high mobility ion channels can be formed through the phase segregation obtained by the use of block copolymers (BCP). 22-24 Nanochannels have been created through nanophase separation between hydropho-bic and hydrophilic blocks of a BCP. 25-27 It is important to note that not all BCP morphologies lead to high conductivity because the channels must also be interconnected for efficient ion conduction. 28 The nature of the polymer backbone and type/location of hy-drophilic groups within the polymer is important for long term AEM stability at high pH. It has been experimentally shown that polar moieties, such as ether, ketone or ester linkages, within the polymer or side-groups, are susceptible to nucleophilic attack and backbone) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 130.207.74.225 Downloaded on 2019-06-24 to IP