High-resolution cross sections images of the sulfonated perfluorinated solid electrolytes Nafion and Aquivion were measured by atomic force microscopy. The structure and the phase separation of membranes differed significantly before and after current flow. In pristine samples, the water-rich ionic phase that appears dark in the adhesion images forms approximately 30-50 nm large isolated mostly circular areas. After forcing a current through the sample, the formation of a continuously connected ionic phase could be observed for the first time. It consisted of micrometer-long but only 5 nm thick and 30-60 nm wide water-rich ionic sheets preferentially stretched in the direction of applied bias voltage. The surface structure was highly ordered with micrometer-large areas of differently oriented stacked onion-like polymer sheets often enclosing a core.
Capacitive current images of pristine samples revealed a lamellar stacking of higher conductive ionic and non-ionic phase. The structure of this phase separation differs significantly for Nafion and Aquivion, a molecule with a shorter side chain that consists of much larger connected ionic phase. The direct comparison of step height at the cross section surface revealed the occurrence of several preferential step heights with shift of all peaks to smaller height at Aquivion reflecting its shorter side chain. The most probable step height of 1.7 nm and 1.5 nm, respectively, coincides with a stacking of two polymer side chains with the half-stretched sulfonic acid groups that point inwards to form an ionic layer.
Renate Hiesgen1, Tobias Morawietz1, Michael Handl1,Martina Corasaniti2, K. Andreas Friedrich3 . Journal of The Electrochemical Society, 161 (12) F1214-F1223 (2014).Show Affiliations
1.University of Applied Sciences Esslingen, Esslingen 73728, Germany.
2.Solvay Specialty Polymers Italy S.p.A., Bollate (Mi) 20021, Italy.
3.German Aerospace Center, Institute of Technical Thermodynamics, Stuttgart 70569, Germany .
A material-sensitive atomic force microscopic (AFM) tapping mode was combined with current measurements to investigate structure, phase separation, and conductive structure of surfaces and cross sections of long side chain Nafion and short side chain AQUIVION PFSA ionomer membranes. We found unexpected large-scale ordered structures consistent with a dominant lamellar polymer structure at the cross sections. The highly terraced areas of both ionomers have a wide distribution of layer thicknesses from sub-nanometer to a few nanometers. In both broad size distributions, preferential sizes were identified that reflect the different lengths of the molecular side chains, indicating a stacking in layers. The nanoscale phase separation of the ionomer was analyzed by using the capacitive current distribution. In AQUIVION PFSA, larger connected water-rich ionic areas were found than in Nafion with same total ionic area. A steady-state current at the cross sections evolved only after an activation period by enforcing current flow though the membrane. A comprehensive and heterogeneous current distribution was observed with highly conductive areas. In contrast, on outer membrane surfaces, only non-continuous spot-like currents were observed. In general, our measurements are consistent with conduction in water layers in-between polymer chains and a bi-continuous structure under faradaic current flow.