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Anion exchange membranes (AEMs) are membranes with positively charged functional groups that enable anion transport. AEMs can be used as a solid ion conductor in hydrogen fuel cells, which have...

Despite the recent progress in increasing the power generation of Anion‐exchange membrane fuel cells (AEMFCs), their durability is still far lower than that of Proton exchange membrane fuel cells (PEMFCs). Using the complementary techniques of X‐ray micro‐computed tomography (CT), Scanning Electron Microscopy (SEM) and Energy Dispersive X‐ray (EDX) spectroscopy, we have identified Pt ion migration as an important factor to explain the decay in performance of AEMFCs. In alkaline media Pt+2 ions are easily formed which then either undergo dissolution into the carbon support or migrate to the membrane. In contrast to PEMFCs, where hydrogen cross over reduces the ions forming a vertical “Pt line” within the membrane, the ions in the AEM are trapped by charged groups within the membrane, leading to disintegration of the membrane and failure. Diffusion of the metal components is still observed when the Pt/C of the cathode is substituted with a FeCo−N−C catalyst, but in this case the Fe and Co ions are not trapped within the membrane, but rather migrate into the anode, thereby increasing the stability of the membrane.

Aluminum nitride (AlN) single crystals are desired as substrates for nitride based electronic and optoelectronic applications[1]. A low density of dislocations and other structural defects in bulk AlN substrates is in demand to grow high-quality heteroepitaxial epilayers. Physical vapor transport (PVT) grown AlN crystals possess low density dislocations and other structural defects[2]. In AlN crystals grown by PVT, basal plane slip is the most frequently observed deformation mechanism. However, prismatic slip takes place as well in such crystals [3, 4].When the diameter of AlN wafers expands to 50 mm and larger, managing the thermal gradients in the PVT growth chamber becomes critical for reducing thermal stresses that cause deformation. Recent synchrotron X-ray topography [5] studies of 50 mm diameter AlN wafers showed that while most wafers contained few to no basal plane dislocations (BPDs), some wafers possessed a 6-fold BPD dislocation pattern [2] which was aligned along the <11-20> directions. This configuration indicated that prismatic slip had likely occurred. Screw typed prismatic dislocations cross slipped from the prismatic plane onto the basal plane, and underwent basal plane slip, leading to dislocation multiplication. To investigate the origins of prismatic slip, a radial thermal gradient model was established showing[6] the resolved shear stress across the entire area of the crystal boule during growth. The results from the model were compared with experimental observations to investigate the role of radial thermal gradients in the nucleation of prismatic slip in AlN during bulk growth. This insight will help guide efforts to minimize structural defects in PVT AlN.References: R. T. Bondokov, S. G. Mueller, K. E. Morgan, G. A. Slack, S. Schujman, M. C. Wood, J. A. Smart,L. J. Schowalter, Large-area AlN substrates for electronic applications: An industrial perspective, Journal of Crystal Growth, 310, (2008).R. Dalmau, J. Britt, H. Y. Fang, B. Raghothamachar, M. Dudley,R. Schlesser, Mater. Sci. Forum, 1004, 63 (2020).Q. Wang, D. Lei, G. He, J. Gong, J. Huang,J. Wu, Characterization of 60 mm AlN Single Crystal Wafers Grown by the Physical Vapor Transport Method, physica status solidi (a), 216, 1900118 (2019).B. Raghothamachar, M. Dudley, J. Rojo, K. Morgan,L. Schowalter, X-ray characterization of bulk AIN single crystals grown by the sublimation technique, Journal of Crystal Growth, 250, 244 (2003).B. Raghothamachar, M. Dudley,G. Dhanaraj, in Springer Handbook of Crystal Growth, ed. by G. Dhanaraj, K. Byrappa, V. Prasa and M. Dudley (Springer, 2010), p. 1425J. Guo, Y. Yang, B. Raghothamachar, J. Kim, M. Dudley, G. Chung, E. Sanchez, J. Quast,I. Manning, Prismatic Slip in PVT-Grown 4H-SiC Crystals, Journal of Electronic Materials, 46, (2017). Figure 1