Novel side chain functionalized polystyrene/O-PBI blends with high alkaline stability for anion exchange membrane water electrolysis (AEMWE)

We report the synthesis of a polystyrene-based anion exchange polymer bearing the cationic charge at a C6-spacer. The polymer is prepared by a functionalized monomer strategy. First, a copper halide catalyzed C–C coupling reaction between a styryl Grignard and 1,6-dibromohexane is applied, followed by quaternization with N-methylpiperidine and free radical polymerization. The novel polymer is blended with the polybenzimidazole O-PBI to yield mechanically stable blend membranes representing a new class of anion exchange membranes. In this regard, the ratio of the novel anion exchange polymer to O-PBI is varied to study the influence on water uptake and ionic conductivity. Blend membranes with IECs between 1.58 meq. OH− g−1 and 2.20 meq. OH− g−1 are prepared. The latter shows excellent performance in AEMWE, reaching 2.0 A cm−2 below 1.8 V in 1 M KOH at 70 °C, with a minor degradation rate from the start. The blend membranes show no conductivity loss after immersion in 1 M KOH at 85 °C for six weeks indicating high alkaline stability.


Polymer and membrane characterization:
Nuclear Magnetic Resonance: NMR spectra were measured at room temperature with a JEOL JNM-ECZ-500R with a proton resonance frequency of 500 MHz.For the different NMR measurements, samples were dissolved in deuterated chloroform (CHCl3-d), deuterated dimethyl sulfoxide (DMSO-d6) or deuterium oxide (D2O).The signal of the residual protons in the deuterated solvent was selected as the internal standard with a shift of 7.26 ppm for CHCl3-d, 2.50 ppm for DMSO-d6, and 4.79 ppm for D2O.
Gel permeation chromatography (GPC): GPC measurements were performed using a SECcurity 2 1260 from PSS.A PSS GRAM GUARD was used as a guard column, and three separation columns (1x PSS GRAM 10 µm 100 Å and 2x PSS GRAM 10 µm 3000 Å) were applied for sample analysis.The eluent was DMSO containing 0.1 M LiCl with a flow rate of 1.0 mL min -1 at 70 °C.A dual variable wavelength UV-Vis (P/N 404-2107, PSS) and a refractive index detector (P/N 404-2106, PSS) were used as detectors.The relative molecular weight was obtained by calibration with narrowly distributed poly(methyl methacrylate) standards from PSS.

Thermogravimetric analysis (TGA):
The thermal stability of the polymers and membranes was analyzed using a TGA 8000 from PerkinElmer with a heating rate of 20 K min -1 from 30 °C to 800 °C under a synthetic air atmosphere.
Differential Scanning calorimetry (DSC): DSC curves were obtained using a Mettler Toledo DSC 3+.Analyses were performed under nitrogen flow (50 mL min -1 ) at a heating rate of 10 K min -1 .The method consisted of three measurement steps with stationary phases of 5 min in between: (1) heating from -50 °C until 250 °C; (2) cooling down to -50 °C and (3) repeating step 1.The glass transition temperature (Tg) was calculated according to the ISO standard method on the last heating curve.

Dynamic mechanical analysis (DMA):
Tensile tests were conducted in a DMA 1 from Mettler Toledo.Three samples of 6x25 mm in Cl -form for each tested membrane were prepared.The membranes were measured at 25 °C and 70 °C under dry conditions and immersed in a water bath at the same temperatures.

Electrochemical Impedance Spectroscopy (EIS):
A Zennium X from Zahner was used for electrochemical impedance spectroscopy.The potentiostat was connected to an external cell having two gold electrodes with an area of 0.25 cm 2 .A 1.5 x 1.5 cm membrane sample was immersed in 1 M aqueous NaCl solution at 85 °C for 24 h.After cooling to room temperature, the membrane was sandwiched by two pieces of AEMION® in a stack, and the membrane sandwich was placed flat between the two gold electrodes.Before and after the measurement, the reference membranes were measured without the sample.Single drops of 1 M NaCl solution were applied as electrolytes between every electrode or membrane interphase.Three pieces of a membrane sample were measured twice each.The resulting resistance of the reference membranes was subtracted from the stack resistance to obtain the sample resistance.The conductivity was calculated according to equation (1) whereby d represents the membrane thickness, R the measured resistance, and A the electrode area: Hydroxide Conductivity: Membrane pieces with a thickness (d) of about 44 µm and a width (w) of about 0.5 cm were immersed in degassed 1 M KOH for 24 h to convert the membranes into their mixed hydroxide form, afterwards the membranes were washed three times with degassed DI water to remove excess KOH.The membranes were loaded into an MTS 740 (Scribner Associates) four-point probe conductivity cell.The humidity was controlled at 95 % RH, and the flow of nitrogen was at 500 sccm min -1 .For temperature-dependent measurements, the cell was first equilibrated at 30 °C for 1 h, and then the resistance of the membrane was measured.Then the temperature was raised by 10 °C, and the cell was equilibrated at the respective temperature for 1 h.After equilibration, the resistance was measured again.All measurements were repeated three times.The conductivity was calculated with equation (2), whereby L is the distance between the two sensing electrodes (l = 0.425 cm), R is the measured resistance, w is the width, and d is the thickness of the membrane: (2) radical polymerization.The benzylic protons' (Figure S1, H-6) and the CH2-Br group (Figure S1, H-5) match perfectly within NMR spectroscopy's accuracy.Furthermore, all relevant signals of the target compound are also present in the 13 C NMR spectrum (Figure S2).Thus, the styryl Grignard was coupled to only one equivalent 1,6dibromohexane because of the high excess of 1,6-dibromohexane.

Figure S 1: 1 H NMR spectra of 4-(6-bromohexyl) styrene (b) 1-methyl-(6-(4-vinylphenyl) hexyl) piperidin-1-ium bromide (c)
poly (1-methyl-(6-(4-vinylphenyl) hexyl) piperidin-1-ium bromide).All DMA display a similar shape except those measured at 70 °C and 0 % RH.This probably can be attributed to the plasticizing effect of adsorbed water.In the case of 70 °C and 0 % RH, this water is expected to be removed, leading to a stiffer material.This is also reflected by the Youngs Moduli summarized in the following table : In contrast, we observed increased water uptake for the blend membranes accompanied by a significant plasticizing effect under humidified conditions (90 % RH, 25°C and water bath, 70°C).This is due to the water absorption of the quaternary ammonium groups.Unfortunately, we could not measure P4HexPipSt as a pure sample due to its water solubility and brittleness.

Figure S 6 :
Figure S 6: Relative mass changes of blend membranes with different IECs indicating no dissolution of the water-soluble P4HexPipSt.

Figure S 7 :
Figure S 7: Swelling ratio in longitudinal (SRL) direction and transversal (SRT) direction in dependency of the IEC measured at 85°C in DI water.

Figure
Figure S HAADF-STEM images of membrane polymer after full-cell testing, b) STEM-EDX spectrum images showcasing the homogeneous distribution after testing of C, N, and O, as well as W utilized for staining, and c) spectrum images of NiFe LDH catalyst residuals.Structure size within the polymer were evaluated via the radial profiles of the fast Fourier transformed micrographs (FFT) shown in a).

Table S 1
: Thicknesses of blend membranes with different weight ratios of P4HexPipSt and O-PBI.
*In DI water

Table S 2
: DMA analysis of pure O-PBI with the respective Youngs Moduli.