No abstract

The effective mobility of hydroxide, chloride and fluoride ions is reported in various anion exchange membranes (AEM) with a backbone of polysulfone (PSU) or poly(2,6-dimethyl-1,4-phenylene)oxide (PPO). The concentration dependence of the effective mobility is used to derive the porosity (π), tortuosity (τ), and percolation thresholds and to plot the ionic conductivity vs the hydration number. Semi-logarithmic plots of the effective ion mobility u(i) vs the square root of concentration √c(i) for hydroxide, fluoride and chloride ions in various PSU-and PPO-based ionomers at 25 and 60 °C show linear relations, from which the ratio π/τ can be determined. This existence of linear u(i)=f(√c(i)) plots is related to the very particular boundary conditions experienced by mobile ions, migrating in close vicinity to the immobile grafted counter-ions placed at the interfaces between polymer domains and electrolyte solution. The π/τ values for PSU-QA (0.29) and PPO-QA (0.38) are consistent with a relatively low hydrophilic-hydrophobic nanophase separation, which leads to channels with low diameter and high tortuosity. The tortuosity determined from a Bruggeman-type relation is 1.9 for PSU-QA and 1.6 for PPO-QA. The percolation thresholds , determined from the universal percolation equation near and above , are at a water volume fraction of 0.07 for PSU-TMA and 0.03 for PPO-QA indicating that these AEM have a two-dimensional structure of the hydrated domains. The prefactor, which should represent a good indication as to the maximum achievable ionic conductivity, is slightly below 100 mS/cm for both PSU-TMA and PPO-based ionomers. Plots of experimental and computed hydroxide and chloride ion conductivities as function of the hydration number () show a maximum ionic conductivity for a value of the hydration number around 60, corresponding to optimal hydration conditions. At λ = 100, the ratio of conductivity between PSU-QA (OH form) and PPO-QA (Cl form) indicates that the degree of dissociation of ion pairs is about 30% lower for hydroxide than for chloride ions, which is consistent with the effective ionic radii of Cl and OH -.

The stability of anion exchange membranes is paramount for the use in alkaline fuel cells. Long chain ionomers are supposed to be more alkaline-resistant with respect to short chain isomers. In this paper the synthesis, properties and stability of ionomers with a long side chain are investigated. Poly(2,6-dimethyl-1,4-phenylene)oxide (PPO) is chosen as backbone, due to its reported stability in alkaline conditions. The functional group is pentyl-ammonium with trimethylamine (TMA) or 1,4-diazabicyclo[2.2.2]octane (DABCO) as model amines. The synthesis is carried out via metalation reaction and is optimized as a function of temperature and time. The water uptake is relatively low, in accordance with the large hydrophobicity of the PPO backbone. The through-plane ionic conductivity is consistent with literature data; it amounts to 15.3 mS/cm at 80 °C for the TMA derivative. The mechanical properties are typical of ionomers below the glass transition temperature (for the TMA derivative at ambient humidity: Young Modulus = 1310 ± 30 MPa). The stability in alkaline conditions, studied by thermogravimetric analysis and measurements of ionic conductivity and ion exchange capacity, is higher than that of short-side chain ionomers with the same basic group. The decrease of ionic conductivity (57 vs 22% residual conductivity after 72 h in 2 M NaOH at 80 °C) and IEC is monitored showing that the degradation is fast in the first hours and may by described by second order kinetics. These results help in selecting high performance anion exchange membranes for electrochemical energy technologies.