Pseudocapacitive Contributions to Charge Storage in Highly Ordered Mesoporous Group V Transition Metal Oxides with Iso-Oriented Layered Nanocrystalline Domains

Amphiphilic block copolymers are very attractive as templates to produce inorganic architectures with nanoscale periodicity because of their ability to form soft superstructures and to interact with inorganic materials. In this paper, we report the synthesis and electrochemical properties of highly ordered mesoporous T-Nb2O5, L-Ta2O5, and TaNbO5 solid solution thin films with iso-oriented layered nanocrystalline domains. These oxide materials were fabricated by coassembly of inorganic sol-gel reagents with a poly(ethylene-co-butylene)-b-poly(ethylene oxide) diblock copolymer, referred to as KLE. We establish that all materials employed here are highly crystalline and have an ordered cubic pore-solid architecture after thermal treatment. We also demonstrate that these group V transition metal oxides can be readily produced with a high degree of crystallographic alignment on virtually any substrate in contrast to classical solution-phase epitaxy which requires the use of a single-crystalline substrate to achieve oriented crystal growth. Moreover, we show the benefits of producing a material with both a mesoporous morphology and crystallographically oriented domains. Mesoporous T-Nb2O5 films exhibit high levels of pseudocapacitive charge storage and much higher capacities than mesoporous amorphous films of the same initial Nb2O5 composition. Part of this high capacity stems from very facile intercalation pseudocapacitance. This process occurs at rates comparable to traditional redox pseudocapacitance in high surface area Nb2O5 because of the periodic nanoscale porosity, the iso-orientation of the layered nanocrystalline pore walls, and the mechanical flexibility of periodic porous materials.