On the Correlation between Mechanical Flexibility, Nanoscale Structure, and Charge Storage in Periodic Mesoporous CeO2 Thin Films

In this work, we report the synthesis and characterization of highly ordered mesoporous CeO2 thin films with crystalline walls. While this article focuses on electrochemical studies of CeO2 with periodic nanoscale porosity, we also examine the mechanical properties of these films and show how pore flexing can be used to facilitate intercalation of lithium ions. Mesoporous samples were prepared by dip-coating using the large diblock copolymer KLE as the organic template. We establish that the films have a mesoporous network with a biaxially distorted cubic pore structure and are highly crystalline at the atomic scale when heated to temperatures above 500 degrees C. Following a previously reported approach, we were able to use the voltammetric sweep rate dependence to determine quantitatively the capacitive contribution to electrochemical charge storage. The net result is that mesoporous CeO2 films exhibit reasonable levels of pseudocapacitive charge storage and much higher capacities than samples prepared without any polymer template. Part of this increased capacity stems from the fact that these films are able to expand normal to the substrate upon intercalation of lithium ions by flexing of the nanoscale pores. This flexing relieves stress from volume expansion that normally inhibits charge storage. Overall, the results described in this work provide fundamental insight into how nanoscale structure and mechanical flexibility can be used to increase charge storage capacity in metal oxides.