Effect of Superacidic Side Chain Structures on High Conductivity Aromatic Polymer Fuel Cell Membranes

Abstract

Proton-conducting superacidic polymer membranes with different fluoroalkyl sulfonate pendants attached to aromatic polymer backbones were synthesized via C−H functionalization and Suzuki coupling reactions. Variation in the chemical structures of the pendant acidic sulfonate moieties and their effects on membrane properties including water uptake, ion exchange capacity, morphology, and proton conductivity were systemically investigated. Membranes containing the short −OCF 2 SO 3 H pendant (PSU-S 5 ) showed a smaller hydrophilic domain size and lower proton conductivity than those containing the longer pendants −OCF 2 CF 2 SO 3 H (PSU-S 1 ) and −SCF 2 CF 2 SO 3 H (PSU-S 4 ), likely due to the short chain's less favorable aggregation and lower acidity. Polymer electrolyte membranes with unique branched fluoroalkyl sulfonate pendants (PSU-S 6 ) gave larger ionic domain sizes, more uniform hydrophilic channels, and higher proton conductivity than samples with analogous linear pendant chains (PSU-S 1 ), indicating that branched sulfonate structures may be a key future direction in the field of fuel cell membrane. ■ INTRODUCTIONIncreasing concerns about the environmental impact of our overdependence on fossil fuels have motivated research on alternative clean energy technologies. Proton exchange membrane (PEM) fuel cells, which are composed of a cathode, an anode, and a PEM, generate electricity cleanly via electrochemical reactions of hydrogen and oxygen to yield water and heat as the only byproducts. 1−5 The development of perfluorosulfonic acid ionomers, such as Nafion, has greatly contributed to fuel cell technologies, and these materials are still widely used as the benchmark membrane in fuel cells. Because of its perfluorinated structure and superacidic pendant side chain, Nafion possesses high proton conductivity as well as good chemical stability. However, Nafion is still not an ideal PEM material, and its drawbacks (e.g., high cost, low operation temperature, and high methanol crossover) require development of alternative PEMs for successful adoption of fuel cells as reliable and inexpensive energy conversion devices. 6−9 Over the past decades, extensive efforts have been devoted to the development of hydrocarbon-based PEMs, and many aryl and alkyl sulfonated polymers have been described. 10−16 In general, these sulfonated aromatic polymer PEMs with high ion exchange capacity and high conductivity swell excessively under high hydration conditions and give much lower proton conductivity than Nafion when the relative humidity (RH) or water content of the membrane is reduced. PEMs with good proton conductivity at high temperature (above 100°C) and low RH can bring many advantages to the fuel cell system, such as fast electrode reaction kinetics, good tolerance toward carbon monoxide and other fuel and air impurities, and simplified water management strategies. 8,13,17−19 To achieve highly conductive materials under low hydration conditions, creation of well-connected hydrophilic channels within the membrane throu...

Impact of Nano- and Mesoscales on Macroscopic Cation Conductivity in Perfluorinated-Sulfonic-Acid Membranes

“…the number of fluorocarbon or fluoroether groups) influences the partial charge on molecular groups neighboring sulfonates 29 and, consequently, the overall membrane conductivity. 4,68 Charge delocalization decreases the fraction of cations associated with the sulfonate groups and increases membrane conductivity. 29,28,[68][69] Charge delocalization is The unit-cell model for PFSA membranes captures the essence of known behavior at the nanoscale.…”

“…4,68 Charge delocalization decreases the fraction of cations associated with the sulfonate groups and increases membrane conductivity. 29,28,[68][69] Charge delocalization is The unit-cell model for PFSA membranes captures the essence of known behavior at the nanoscale. 4 We now extend the aqueous-domain results to predict macroscopic transport properties in PFSAs.…”

Perfluorosulfonyl Imide versus Perfluorosulfonic Acid Ionomers in Proton‐Exchange Membrane Fuel Cells at Low Relative Humidity

“…Recently, increasing the acidity of the functional moiety has emerged as a plausible approach to favor proton transport, and significantly higher proton conductivities were reported as compared to those of directly sulfonated aromatic ionomers . The enhanced conductivity was attributed to (i) the higher acidity of the acidic side chains (p K a ≈−6 in perfluorosulfonic acid and p K a ≈−1 in aryl sulfonic acid) inducing higher ability to dissociate proton at low relative humidity (RH); and (ii) the presence of a flexible hydrophobic spacer between the backbone and the acidic function, favoring nano‐phase separated microstructures with well interconnected ionic clusters . As a consequence, aromatic ionomers obtained by combining multiblock structure with perfluorosulfonic acid sidechains with optimized chemical structure and membrane manufacturing condition showed improved fuel cell performance and durability as compared to those of Nafion …”

“…[11][12][13][14][15][16] The enhanced conductivity was attributed to (i)the highera cidity of the acidic side chains (pK a %À6i np erfluorosulfonica cid and pK a %À1i na ryl sulfonica cid) [17] inducing higher ability to dissociate protona tl ow relative humidity (RH);a nd (ii)the presence of af lexible hydrophobic spacer between the backbonea nd the acidic function, favoring nanophase separated microstructures with well interconnected ionic clusters. [18,19] As ac onsequence, aromatici onomers obtained by combining multiblock structure with perfluorosulfonic acid sidechains with optimized chemical structurea nd membrane manufacturing condition showedi mproved fuel cell performance andd urability as compared to those of Nafion. [20][21][22][23] Following thesef indings, ap romising approach to further improvet he protonc onductivity at low RH could be to replace the perfluorosulfonic function by as tronger acidic moiety.T he perfluorosulfonyl imide acid is knownt ob eastronger acid than the perfluorosulfonic moiety in the gas phase [24] and in a Designingh ighly conductive ionomers at high temperature and low relative humidity is challenging in proton-exchange membrane fuel cells.…”

Synthesis of Sulfonated Poly(arylene ether)s in a One‐Pot Polymerization Process and Their Nafion‐Blend Membranes for Proton Exchange Membrane Fuel Cell Applications

“…All the present membranes increased in the proton conductivity at an elevated temperature (Figure ). SPAE‐80 was highest in the proton conductivity among all the SPAE membranes and it could reach 0.232 S cm −1 at 80 °C, which was significantly higher than that of Nafion 212 (0.205 S cm −1 at 80 °C), and could be ascribed to the increased sulfonic acid content and the water uptake ability of the membrane . Also, increased sulfonic acid content formed more network of hydrogen bonds, which is beneficial to improve proton conductivity according to Grottthuss mechanism .…”