Magnetic composites exhibit distinct flux properties due to gradient interfaces. The composites can be used to improve fuel cells and effect transport and separation of different species of materials. A variety of devices can be made utilizing the composites including a separator, a cell, an electrode for channeling flux of magnetic species, an electrode for effecting electrolysis of magnetic species, a system for channeling electrolyte species, a system for separating particles with different magnetic susceptibilities. Some composites can be used to make a dual sensor for distinguishing between two species of materials and a flux switch to regulate the flow of a redox species and a flux switch to regulate the flow of a chemical species. Some composites can control chemical species transport and distribution.