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This feature article provides a progress review of atomic layer deposition (ALD) for fabrication of oxide-ion as well as proton conducting ceramic fuel cells. A comprehensive analysis of structural, chemical, surface kinetics, and electrochemical characterization results of ALD membranes is also presented. ALD is a surface reaction limited method of depositing conformal, high quality, pinhole-free, uniform thickness nanofilms onto planar or three-dimensional structures. Deposition by one atomic layer at a time also affords unprecedented opportunities to engineer surface termination, to form compositionally graded structures or graded doping, and to synthesize metastable phases that cannot be realized otherwise. Indeed, thin ceramic electrolyte membranes made by ALD exhibit enhanced surface exchange kinetics, reduced ohmic losses, and superior fuel cell performance as high as 1.34 W cm À2 at 500 C. More importantly, ALD offers the opportunity to design and engineer surface structures at the atomic scale targeting improved performance of not only ceramic fuel cells, but also electrochemical sensors, electrolysers and pumps. Joon Hyung Shim is an Assistant Professor of Mechanical Engineering at Korea University (KU) and a Project Manager of Green Manufacturing Research Center (GMRC) at KU. Before joining KU as a faculty, he worked as a postdoctoral researcher at National Renewable Energy Laboratory (NREL) and a fulltime lecturer of Mechanical Engineering at Stanford University. He received the B.S. degree in Mechanical and Aerospace Engineering from Seoul National University in 2002. He also received the M.S. and Ph.D. degree in Mechanical Engineering from Stanford University in 2004 and 2009 respectively. Sangkyun Kang is an executive managing director of Hanchang Industries, where he oversees researches in energy materials. Before joining Hanchang Industries, he worked as a research staff member of Samsung Advanced Institute of Technology. There, he conducted ALD research as the leader of the thin lm ceramic fuel cell project. He

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Nanoscale yttria-stabilized zirconia (YSZ) electrolyte film was deposited by plasma-enhanced atomic layer deposition (PEALD) on a porous anodic aluminum oxide supporting substrate for solid oxide fuel cells. The minimum thickness of PEALD-YSZ electrolyte required for a consistently high open circuit voltage of 1.17 V at 500 °C is 70 nm, which is much thinner than the reported thickness of 180 nm using nonplasmatic ALD and is also the thinnest attainable value reported in the literatures on a porous supporting substrate. By further reducing the electrolyte thickness, the grain size reduction resulted in high surface grain boundary density at the cathode/electrolyte interface.

A simple, yet effective approach of stabilizing the nanostructure of porous metal‐based electrodes and thus, extending the life of microsolid oxide fuel cells is demonstrated. In an effort to avoid thermal agglomeration of metal electrodes, an ultrathin yttria‐stabilized zirconia (YSZ) is coated on the porous metal (Pt) cathode by the atomic layer deposition, a scalable, and potentially high‐throughput deposition technique. A very thin YSZ coating is found to maintain the morphology of its underlying nanoporous Pt during high temperature operations (500 °C). More interestingly, the YSZ coating is also found to improve oxygen reduction reaction activity by ≈2.5 times. This improvement is attributed to an enhanced triple phase area, especially in the vicinity of the Pt–electrolyte interface; cross‐sectional electron microscopy images indicate that the initially uniform ultrathin YSZ layer becomes a partially agglomerated coating, a favorable structure for a maximized reaction area and fluent oxygen access to the Pt–electrolyte interface.