Self-Templated Formation of Hollow Structures for Electrochemical Energy Applications.

The rational design and synthesis of hollow structured functional materials are of great significance as both fundamental challenges in materials science and practical solutions for efficient energy utilization in modern society. The unique structural features of hollow functional materials bring outstanding electrochemical properties for both energy storage and electrocatalysis. However, conventional templating methods are normally less efficient in constructing hollow structures with desirable compositions and architectures. In the past decade, many novel synthetic approaches directly converting templates into hollow structures have been developed. Collectively termed as the "self-templated" strategy, it makes use of various physical/chemical processes to transform solid templates into hollow structures of target materials. Of particular note is the outstanding capability to construct complex hollow architectures of a wide variety of inorganic or hybrid functional materials, thus providing effective solutions for various electrochemical energy applications. In this Account, we present the recent progress in self-templated formation of hollow structures especially with complex architectures, and their remarkable performance in electrochemical energy-related technologies. These advanced self-templated methods are summarized as three categories. "Selective etching" creates hollow structures from solid templates of same materials by removing some of the internal parts, forming multishelled or unusual hollow architectures. "Outward diffusion" utilizes the relocation of mass in templates from inner region to outer region driven by various mechanisms, to construct hollow structures with multiple or hierarchical shells. "Heterogeneous contraction" typically applies to thermally decomposable templates and generates various hollow structures under nonequilibrium heating conditions. We further demonstrate some remarkable electrochemical properties of such hollow structures in virtue of their exceptional composition-/structure-induced merits. As electrode materials for lithium-ion batteries, hybrid or multishelled metal oxides exhibit high cyclability because of their capability to well accommodate the lithium insertion strain. Also the rate capability is effectively improved by the fast lithium insertion/deinsertion in multishelled or hierarchical hollow structures. These exceptional structural merits also significantly enhance the reaction kinetics and prolong the cycling lifetime of metal-sulfides-based electrodes, which enables the assembly of hybrid supercapacitors with high energy and power densities. On the other hand, multicompositional hollow structures with large exposed surface area and rich open pore channels offer abundant robust active sites and fast charge/mass transport for electrocatalytic reactions. These studies demonstrate that the versatility and superiority of self-templated methods for hollow structured functional materials have greatly promoted their applications for electrochemical energy storage and conversion. With continued research efforts, we are expecting greater and broader impacts brought by the rapidly growing family of hollow structures formed by self-templated methods.