In this report, we demonstrate how a uniform angstrom-level oxide overcoat (0.7–1.5 Å) by atomic layer deposition is highly effective not only in enhancing the thermal stability of underlying infiltrated ceria nanoparticles but also in facilitating electrode kinetics.

Ionic transport in solid oxides lays the fundamentals of solid-state electrochemical energy and sensor devices. While ionic transport in polycrystalline oxides has been studied for decades, most studies are reliant upon macro-scale observations where direct revelation of localized transport behavior is not feasible. In this presentation, we report a direct quasi in situ observation of oxygen ion transport in SrTiO3 (STO) and Y2O3-stabilized ZrO2 (YSZ) by leveraging scanning probe microscopy variants. First, a cluster of oxygen vacancies formed by an electrical bias at a tip-sample contact. Then, we exposed the sample to an elevated temperature to allow a facilitated ionic diffusion. The resultant lateral distribution of oxygen vacancies was obtained by Kelvin probe force microscopy (KPFM). Depth-wise distribution of oxygen vacancies were acquired by a combination of in situ etching with a diamond-coated probe and KPFM scans, which enabled an observation of ionic charge transport in a highly localized fashion in both lateral and depth-wise directions. From 3D spatial charge imaging, the activation energies of surface ionic diffusion in the lateral direction (Esurf) and those of bulk diffusion in the depth-wise direction (Ebulk) were quantified; for YSZ, Esurf = 0.34 eV and Ebulk = 1.13 eV, and for STO, Esurf = 0.50 eV and Ebulk = 1.30 eV. It was also found that the surface ion diffusivity of YSZ (1.68 × 10-8 cm2/s) is more than 3 orders of magnitude higher than that of STO (4.86 × 10-12 cm2/s), and that bulk diffusivity of YSZ (1.70 × 10-11 cm2/s) is more than 7 orders of magnitude higher than that of STO (7.61 × 10-19 cm2/s). To the best of our knowledge, this is the first study where the ionic diffusivities along the surface and into the bulk are separately quantified by direct localized observations of 3D distribution of ionic species, as opposed to a bulk measurement from which diffusivities were indirectly deduced. The authors acknowledge the support by National Science Foundation (DMR 1753383).

Recently, transition metal based layered double hydroxides (LDHs) based materials have attracted attention for the enhancement of oxygen evolution reaction (OER) performances due to their unique structural property, high specific capacitance, facile synthesis process and low cost. [1,2] Atomic layer deposition (ALD) is a powerful technique for realizing highly dispersed nano-species at the atomic level with a precise control over the size, shape, and composition by leveraging its self-limiting nature of surface reactions. ALD precursors could be readily combined with LDH structures due to the abundant hydroxy group on the surface of LDH structure. The additional ALD-derived doping can regulate the electronic structures of original LDH and further strength the synergetic effects between the host and doped metals for enhancing electrocatalytic activities.[3] In this study, a simple and effective method of introducing Ce as a third metal was used to prepare high-performance Ni/Co-LDH based electrodes: hydrothermal approach followed by an ALD process, using Ni foam as the substrate to achieve the growth of Ni/Co nanostructures. Smooth 2D LDH plates of a uniform structure with high surface area and abundant active sites were synthesized. The resultant CoNi-LDH@5Ce and CoNi-LDH@5Ti exhibit optimal activity and stability toward OER. In particular, CoNi-LDH@5Ce exhibits excellent OER performance with a small overpotential of 270 mV to reach 50 mA cm-2. ALD-treated samples exhibit a decreased charge transfer resistance, indicating a more efficient OER. Furthermore, both CoNi@5Ti and CoNi@5Ce show lower ohmic resistance of 1.5 Ω, which is additionally beneficial to OER performance. The enhanced performance could be attributed to a synergic effect between the doped elements (Ce, Ti) and base transitional metals (Co, Ni). Furthermore, metal oxides – especially cobalt oxide – generated on the surface during ALD process is conjectured to have contributed to an acceleration of O-O bond formation during OER process. References [1] Z. Jiang, Z. Li, Z. Qin, H. Sun, X. Jiao, D. Chen, . [2] T. M. Masikhwa, M. J. Madito, D. Y. Momodu, J. K. Dangbegnon, O. Guellati, A. Harat, M. Guerioune, F. Barzegar, N. Manyala, . [3] L. Lv, Z. Yang, K. Chen, C. Wang, Y. Xiong, Adv. Energy Mater. 2019, 9, 1803358.