Grain boundary conductivity limitations are ubiquitous in material science. We show that illumination with above-bandgap light can decrease the grain boundary resistance in solid ionic conductors. Specifically, we demonstrate the increase of the grain boundary conductance of a 3 mol% Gd-doped ceria thin film by a factor of approximately 3.5 at 250 °C and the reduction of its activation energy from 1.12 to 0.68 eV under illumination, while light-induced heating and electronic conductivity could be excluded as potential sources for the observed opto-ionic effect. The presented model predicts that photo-generated electrons decrease the potential barrier heights associated with space charge zones depleted in charge carriers between adjacent grains. The discovered opto-ionic effect could pave the way for the development of new electrochemical storage and conversion technologies operating at lower temperatures and/or higher efficiencies and could be further used for fast and contactless control or diagnosis of ionic conduction in polycrystalline solids.
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Source data are provided with this paper. Source data for Figs. 1–4 and Supplementary Figs. 9–11 are available from the online data repository Figshare with identifier https://doi.org/10.6084/m9.figshare.c.5707109.v4. Remaining data that support the findings of this study are available from the corresponding authors upon reasonable request.
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We thank G. F. Harrington for the valuable input on sample preparation and characterization and K. May for help with the atomic force microscopy measurements. T.D. and H.L.T. acknowledge support for their research from the US Department of Energy, Basic Energy Sciences under award number DE-SC0002633 (Chemomechanics of Far-From-Equilibrium Interfaces). D.K. acknowledges support by the Japan Society for the Promotion of Science Core-to-Core Program, A. Advanced Research Networks: ‘Solid Oxide Interfaces for Faster Ion Transport’, as well as funding from the Kakenhi Grant-In-Aid for young scientists, grant numbers 18K13993 and 20K15028. D.K. and H.L.T. appreciate preliminary discussions with H. Matsumoto of Kyushu University and T. Lippert and D. Pergolesi of the Paul Scherrer Institute. J.L.M.R., J.C.G.-R. and T.D. acknowledge the Swiss National Science Foundation for grant number BSSGI0_155986/1, and Equinor for grant Agr no. 4502981450. This work made use of the Materials Research Science and Engineering Center Shared Experimental Facilities at the Massachusetts Institute of Technology, supported by the National Science Foundation under award number DMR-14-19807. A portion of this work was performed at the Center for Nanoscale Systems, a member of the National Nanotechnology Coordinated Infrastructure Network, which was supported by the National Science Foundation under National Science Foundation award no. 1541959.
The authors declare no conflict of interest.
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Unprocessed impedance data.
Temperature-dependent conductivity (fits of data from Fig. 1).
SFITs (unprocessed raw data over time).
IMPS (measured raw data and fitted time constant).
Unprocessed impedance data gathered from additional sample A1.
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Defferriere, T., Klotz, D., Gonzalez-Rosillo, J.C. et al. Photo-enhanced ionic conductivity across grain boundaries in polycrystalline ceramics. Nat. Mater. 21, 438–444 (2022). https://doi.org/10.1038/s41563-021-01181-2