(Invited) Advanced Atomically Dispersed Metal Cathode Catalysts for Hydrogen Fuel Cells

Abstract

Development of clean and sustainable renewable energy is able to address the challenges associated with the rapidly increasing global energy demand and possible climate changes caused by carbon dioxide emission. However, people argue that we can’t effectively utilize renewable energy such as sunlight and wind due to their intermittent problems, until appropriate energy conversion and storage technologies are developed. Among available energy conversion technologies, proton exchange membrane fuel cells (PEMFCs), which directly convert chemical energy of fuels to electricity, have been widely considered a clean energy technology due to their high efficiency, high power density, environmental friendliness, and high reliability. PEMFCs have demonstrated potential applications for stationary power sources, portable electronic devices, and electric vehicles. However, several challenges must be overcome before large-scale commercialization of PEMFCs can be realized, such as high costs, insufficient durability, and hydrogen fuel related technologies. The high cost problem is due to the use of large amount of Pt catalysts for the sluggish oxygen reduction reaction (ORR) in PEMFC cathodes. It has been reported that platinum group metal (PGM) electrocatalysts represent about 45% of the entire cost in the state-of-the-art PEMFCs. To address the cost issues and significantly reduce the economic and environmental costs of fuel cell technologies, Pt catalysts must be replaced by PGM-free catalysts derived from earth-abundant elements. Therefore, it is scientifically and technically important to develop high-performance PGM-free cathode catalysts with significant improved activity, stability, and MEA performance in near future[1-5]. Compared to other studied materials (oxides, sulfides, carbides), carbon-based catalysts often possess many advantages because of their excellent electrical conductivity, high surface areas, low-cost, and easiness of functionality. However, proper doping with heteroatoms (e.g., N, S, P) and transition metals (e.g., Fe, Co, or Mn) to modify the electronic and geometric structures of carbon is the key to enhancing catalytic performance. Herein, we introduce a new class of high-performance atomically dispersed and nitrogen coordinated metal catalyst through controlled chemical dopings of metals ions (Fe, Co, or Mn) into zinc-zeolitic imidazolate framework (ZIF), a type of metal-organic framework (MOF). The novel synthetic chemistry enables accurate controls of metal doping levels, particle sizes, and nanostructures of catalysts, which allows use to establish synthesis-structure-activity correlations. The best performing catalyst with optimal morphology and structure has achieved a new performance milestone for the ORR in challenging acidic media comparable with state-of-the-art Pt/C. The high-performance atomic metal-rich MOF catalysts hold great promise to completely replace Pt for future fuel cells as well as for other sustainable electrochemical energy storage and conversion applications.