Anion exchange membrane fuel cells (AEMFCs) have attracted extensive attention in the recent years, primarily due to the distinct advantage potentials they have over the mainstream proton exchange membrane fuel cells. The anion exchange membrane (AEM) is the key component of AEMFC systems. Due to the unique characteristics of water management in AEMFCs, understanding the water mobility through AEMs is key for this technology, as it significantly affects (and limits) overall cell performances. This work presents a study of the equilibrium state and kinetics of water uptake (WU) for AEMs exposed to vapor source H2O. We investigate different AEMs that exhibit diverse water uptake behaviors. AEMs containing different backbones (fluorinated and hydrocarbon-based backbones) and different functional groups (various cations as part of the backbone or as pendant groups) were studied. Equilibrium WU isotherms are measured and fitted by the Park model. The influence of relative humidity and temperature is also studied for both equilibrium and dynamic WU. A characteristic time constant is used to describe WU kinetics during the H2O sorption process. To the best of our knowledge, this is the first time that WU kinetics has been thoroughly investigated on AEMs containing different backbones and cationic functional groups. The method and analysis described in this work provides critical insights to assist with the design of the next generation anion conducting polymer electrolytes and membranes for use in advanced, high-performance AEMFCs.

Abstract:We have developed ah ighly active nanostructured iridium catalyst for anodes of proton exchange membrane (PEM) electrolysis.C lusters of nanosized crystallites are obtained by reducing surfactant-stabilized IrCl 3 in water-free conditions.T he catalyst shows af ive-fold higher activity towards oxygen evolution reaction (OER) than commercial Ir-black. The improved kinetics of the catalyst are reflected in the high performance of the PEM electrolyzer (1 mg Ir cm À2 ), showing an unparalleled low overpotential and negligible degradation. Our results demonstrate that this enhancement cannot be only attributed to increased surface area, but rather to the ligand effect and low coordinate sites resulting in ahigh turnover frequency (TOF). The catalyst developed herein sets ab enchmark and as trategy for the development of ultra-low loading catalyst layers for PEM electrolysis.

A quantitative in situ investigation of the structure of the catalytic layer of polymer electrolyte membrane fuel cells using material-sensitive and conductive atomic force microscopy is reported. The distribution and size of the ionomer phase at the surface of the catalytic layer is retrieved from adhesion force mappings, measured at high humidity and up to 75 °C. The average ionomer layer thickness varies between 7 and 13 nm for three differently prepared samples, as concluded from the histograms. Evidence of a lamellar structure of the thinner ionomer layers is presented. A significant thinning of the ionomer layers after long-term fuel cell operation is observed.

The conductivity of fuel cell membranes as well as their mechanical properties at the nanometer scale were characterized using advanced tapping mode atomic force microscopy (AFM) techniques. AFM produces high-resolution images under continuous current flow of the conductive structure at the membrane surface and provides some insight into the bulk conducting network in Nafion membranes. The correlation of conductivity with other mechanical properties, such as adhesion force, deformation and stiffness, were simultaneously measured with the current and provided an indication of subsurface phase separations and phase distribution at the surface of the membrane. The distribution of conductive pores at the surface was identified by the formation of water droplets. A comparison of nanostructure models with high-resolution current images is discussed in detail.

Abstract:We have developed ah ighly active nanostructured iridium catalyst for anodes of proton exchange membrane (PEM) electrolysis.C lusters of nanosized crystallites are obtained by reducing surfactant-stabilized IrCl 3 in water-free conditions.T he catalyst shows af ive-fold higher activity towards oxygen evolution reaction (OER) than commercial Ir-black. The improved kinetics of the catalyst are reflected in the high performance of the PEM electrolyzer (1 mg Ir cm À2 ), showing an unparalleled low overpotential and negligible degradation. Our results demonstrate that this enhancement cannot be only attributed to increased surface area, but rather to the ligand effect and low coordinate sites resulting in ahigh turnover frequency (TOF). The catalyst developed herein sets ab enchmark and as trategy for the development of ultra-low loading catalyst layers for PEM electrolysis.

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. Perfluorinated sulfonated ionomer membranes (PFSA) have been established as proton-conducting electrolytes in many technical applications, most notably in brine electrolysis but also with increasing importance as electrolytes for fuel cell applications. For highperformance operation of fuel cells, the water management of the membrane is of great importance. Therefore, the nature of water transport and the conductive structure of the ionomer membrane are still under investigation. In particular, the nanoscopic ionomer properties at non-equilibrium conditions under steady-state current flow close to operation conditions are expected to differ from equilibrium state and have not been characterized in detail.During solidification of perfluorinated polymers (PFSA) casted from dispersions, a phase separation occurs that is the key factor in providing ionic conductivity. The hydrophilic sulfonate-terminated end groups of the side chains cluster together to form an ionic phase. The perfluorinated molecular backbone is assumed to form bundles due to the hydrophobic interaction and to undergo partial crystallization that is considered to enhance mechanical strength.1 The size of these crystalline areas, as determined from scattering data, is reported to be within 0.5 to 10 nm.2,3 The spacing of the average hydrophobic/hydrophilic separations was obtained from the position of the ionomer peaks in scattering data and determined to approximately 3-7 nm depending on the water content.4 While the hydrophobic phase provides mechanical stability, the ionic phase provides a waterbased proton-conducting network. It is generally assumed that this i...

Aerogel is introduced as an OER catalyst substrate for PEM electrolyzer and shows superior activity.

Dedicated to Prof. Jacek Lipkowski, in recognition of his achievements and on the occasion of his 70th birthday.Keywords: AFM PFSA conductive ionic network fuel cell catalytic layer ionomer content A B S T R A C T Using material-sensitive and conductive atomic force microscopy (AFM) on cross sections of perfluorinated and sulfonated membranes at low humidity, crystalline polymer lamellae were imaged and their thickness determined to approximately 6 nm. In the capacitive current, water-rich and waterpoor areas with different phase structures were investigated. The formation of a local electrochemical double layer within the water-rich ionically conductive areas at the contact of the AFM tip with the electrolyte enabled their visibility. The large water-filled ionically conductive areas include numerous ionic domains. Under equilibrium conditions, these areas are spherical (appearing circular in the images) and with distinct size distribution. Forcing a current through the membranes (current-induced activation) led to merging of the water-filled ionically conductive areas in the voltage direction and resulted in an anisotropic ionically conducting network with flat channels. The distribution of the current in the membrane and catalytic layers of a pristine membrane electrode assembly (MEA) was analyzed. From the adhesion force mappings, an inhomogeneous distribution of ionomer in the catalytic layer was detected. Cross currents between Pt/C particles through large ionomer particles within the catalytic layer were detected and the ionomer content across an electrode was evaluated.