Melamine-sponge-derived non-precious fuel cell electrocatalyst.
(Top left) SEM image of the Fe/MS catalyst, showing a 3D microfiber network with a large amount of macro-/meso-pores. (Top right) HAADF-STEM image of single Fe atoms (highlighted with orange circles) on the carbon support. (Bottom left) Current density versus potential for Fe/MS catalyst and conventional Pt/C catalyst. (Bottom right) Schematic of ORR electrocatalysis.At the heart of biological processes such as respiration and energy conversion systems such as fuel cells is the oxygen reduction reaction (ORR), which takes oxygen and produces water or hydrogen peroxide. But the reaction requires a catalyst, which in fuel cells is typically the expensive, precious metal platinum (Pt). Alternative catalysts for the ORR, could make fuel cells more economically viable and sustainable.
Now researchers from Tsinghua University in China believe that they have come up with a promising non-precious metal electrocatalyst in the form of a metal-nitrogen doped carbon catalyst derived from melamine sponges [Xia et al., Materials Today Energy 9 (2018) 271 https://doi.org/10.1016/j.mtener.2018.05.014].
“Developing economic and highly active metal electrocatalysts for ORR is critical for the development of low-temperature fuel cells,” explains Lin Gan.
The novel Fe-N doped C catalyst (or Fe-N/C) uses a low-cost melamine sponge with a foam-like three-dimensional macroporous structure as the starting point. The material, which is widely used for insulation, soundproofing, and as abrasive cleaning sponges, makes an ideal catalyst support precursor material because of its large surface area and extensive pore network. After impregnating the sponge with a Fe salt, the melamine sponge is annealed in ammonia to produce a hierarchical micro-/macro-/mesoporous carbon framework with N atoms, which create active Fe-N4 sites for catalysis to take place. The large surface area provides plenty of space for reactions to take place, while the porous network provides easy access for reactants.
“By controlling the annealing time, the porosity and the nitrogen chemical state can be fully adjusted,” says Gan. “The optimized melamine-sponge-derived electrocatalysts feature both high microporous surface area and hierarchical macro-/meso-pores favorable mass transport, which leads to exceptional ORR activity.”
The new catalyst shows long-term stability in alkaline solution, with only small losses in performance over 10 000 cycles, outperforming commercial Pt/C and most non-precious metal catalysts. Like other non-precious metal catalysts, Fe-N/C also shows impressive tolerance of methanol compared with commercial Pt/C catalysts.
The new catalyst represents a highly active alternative to precious metal Pt catalysts for ORR. The researchers believe that their new Fe-N/C electrocatalyst is among the best developed to date and is very promising for fuel cells applications given its low cost and potential for industrial scale-up.
“The melamine-sponge-derived Fe-N/C electrocatalyst is quite suitable for practical application because of its low cost and scalable synthesis,” says Gan. “We are now applying the catalyst in a realistic proton-exchange-membrane fuel cell, which is showing good initial cell performance.”
Further improvements in catalyst yield and avoidance of phosphorus impurities will be needed before practical applications, however, Gan cautions.