Emerging methanol-tolerant AlN nanowire oxygen reduction electrocatalyst for alkaline direct methanol fuel cell

Abstract

Replacing precious and nondurable Pt catalysts with cheap materials is a key issue for commercialization of fuel cells. In the case of oxygen reduction reaction (ORR) catalysts for direct methanol fuel cell (DMFC), the methanol tolerance is also an important concern. Here, we develop AlN nanowires with diameters of about 100-150 nm and the length up to 1 mm through crystal growth method. We find it is electrochemically stable in methanol-contained alkaline electrolyte. This novel material exhibits pronounced electrocatalytic activity with exchange current density of about 6.52 3 10 28 A/cm 2 . The single cell assembled with AlN nanowire cathodic electrode achieves a power density of 18.9 mW cm 22 . After being maintained at 100 mA cm 22 for 48 h, the AlN nanowire-based single cell keeps 92.1% of the initial performance, which is in comparison with 54.5% for that assembled with Pt/C cathode. This discovery reveals a new type of metal nitride ORR catalyst that can be cheaply produced from crystal growth method. D irect methanol fuel cell (DMFC) is one of the most attractive power sources for portable and vehicular applications due to the simplicity of the system and the adaptability of the liquid fuel 1,2 . At present, the commercialization of DMFCs is hindered by several issues, including crossover of methanol from anode to cathode 3,4 , as well as the dependence on precious Pt catalyst for methanol oxidation and oxygen reduction reactions 5,6 . Methanol transported through the membrane will be electrochemically oxidized at the cathode, thus reducing the cathodic reactant. And in intermediate reactions during this process, for instance the adsorption of carbon monoxide onto the catalyst surface, the cathode will also be poisoned, which further lowers its performance [7][8][9] . Thus, the methanol-tolerant oxygen reduction reaction (ORR) catalysts have attracted much attention in recent years. One of the effective approaches is the nanostructured Pt alloy since Pt is the most effective electrocatalyst for ORR reaction. Pt-Pd Alloy Nanoflowers 10 , Pt/CoSe 2 Nanobelt 11 , core-Shell Pt decorated PdCo 12 and numbers of Pt alloy nanoparticles [13][14][15][16] fall into this category. Although great advances have been achieved for the improvement of methanol tolerance, a high usage of Pt metal is still of great concern for further practical applications of DMFCs.In recent years, some promising non-precious catalysts have been developed towards oxygen reduction reaction, including Co or Fe-based macrocyclic compounds , and even nitrogen-doped graphene 21,22 . The transition metal nitride catalysts also displayed methanol-tolerance for ORR reactions. In comparison with a sharp ORR current decrease of ,80% for commercial Pt/C catalyst after the addition of a 3 M methanol at 90 s in the LSV test, it was reported that a sulfur-nitrogen Co-doped carbon foam catalyst has no noticeable decrease under the same test condition 23 . The similar behavior was also found for sulfur-nitrogen Co-doped graphene oxide 24 and Co-doping c...

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