Bifunctional nano-sponges serving as non-precious metal catalysts and self-standing cathodes for high performance fuel cell applications

In electrochemical cells, oxygen has been generally regarded as the ideal cathode reactant due to its non-toxicity, sustainability, and low-cost. However, the intrinsically slow oxygen reduction reaction (ORR) calls for electrocatalysts such as Pt and its alloys, and their high prices hamper the wide deployment of various electrochemical systems relying on ORR. Previously reported non-precious metal catalysts often involve complicated and lengthy synthesis processes as well as require additional catalyst loading electrodes, increasing the production complexity and cost of cathodes. Here we developed a bifunctional non-precious metal based electrocatalyst, which can also act as a self-standing sponge-like cathode, eliminating the usage of a catalyst loading/supporting layer. Our 3-dimensional (3D) catalysts/cathodes were tested in microbial fuel cells, showing outstanding catalytic activity and long term stability comparable to commercial Pt-based catalysts. Our cathodes were composed of self-assembled carbon nanotubes whose carbon is coordinated with iron and nitrogen for high ORR performance. For maximum cell performance, we found that the pore volume in the 3D cathode needs to be larger to have better oxygen diffusion but overly porous cathodes have less effective electrical conductivity, resulting in lower power generation. Our findings regarding the dependency of power generation on oxygen diffusion/reaction, effective electrical conductivity, and active surface area (or mass) provide a guidance to the future development of porous 3D electrocatalysts/cathodes. The new way of designing electrocatalysts/cathodes from conventional two-dimensional films to macroscale 3D self-assembled nanomaterials, with only ~1% cost of commercial Pt-based catalyst powders, will eliminate one of major hurdles in deploying electrochemical energy conversion systems.