Highly-interconnected porous structures have many applications in the fields of catalysis, sensors, electrochemical energy storage (batteries), dye sensitized solar cells, and biomedical devices such as hip implants. For example, porous conductive current collectors are used in commercially available rechargeable batteries, such as nickel metal hydride (NiMH) batteries. Many commercial NiMH battery cathodes employ sintered plaque or nickel foam current collectors; however, the performance of nickel foam is limited by large pore sizes and a broad pore size distribution. When the foam is impregnated with nickel hydroxide, the charge storage media, protons and electrons, have to travel a long distance between the nickel metal and the Ni(OH)2/electrolyte interface.
Self-assembled colloids may be employed as a template for the formation of a highly porous battery electrode, as described in U.S. Patent Application Publication 2010/0068623A1, “Porous Battery Electrode for a Rechargeable Battery and Method of Making the Electrode,” published on Mar. 18, 2010, and hereby incorporated by reference in its entirety. Once formed, a lattice of microparticles (the “colloidal template”) may be infiltrated or coated with a suitable material (e.g., a conductive metal), and then removal of the microparticles leads to a porous structure which is an inverse of the template.
Such a porous structure may be characterized by its pore (or void) volume fraction (porosity), pore size, interconnectivity, neck openness, tortuosity, and pore hierarchy. Typically, most attention is paid to controlling the porosity and the pore size distribution. However, the remaining parameters may play even more important roles in the transport-related physiochemical properties of functional porous materials. Since closed pores are not accessible for chemical reactants, the presence of passageways (“necks”) between voids and the extent of interconnectivity may be critically important. If the neck size between two adjacent voids is small, what may be referred to as the “ink bottle effect” limits the species exchanges between them.
Because the contact area between spherical microparticles of a colloidal template is very small, there may be only a small passageway or neck between adjacent voids in the resulting inverse structure. As explained, the small size of the necks may inhibit mass transport of various chemical species. Sluggish mass transport may be very unfavorable for chemical reactions that rely on the porous foam.