Atomic H-Induced Mo₂C Hybrid as an Active and Stable Bifunctional Electrocatalyst

Abstract

Mo 2 C nanocrystals (NCs) anchored on vertically aligned graphene nanoribbons (VA-GNR) as hybrid nanoelectrocatalysts (Mo 2 C-GNR) are synthesized through the direct carbonization of metallic Mo with atomic H treatment. The growth mechanism of Mo 2 C NCs with atomic H treatment is discussed. The Mo 2 C-GNR hybrid exhibits highly active and durable electrocatalytic performance for the hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR). For HER, in an acidic solution the Mo 2 C-GNR has an onset potential 1 Deceased March 17, 2016 Page 1 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 2 of 39 mV and a Tafel slope of 65 mV dec -1 , in a basic solution Mo 2 C-GNR has an onset potential of 53 mV, and Tafel slope of 54 mV dec -1 . It is stable in both acidic and basic media. Mo 2 C-GNR is a high activity ORR catalyst with a high peak current density of 2.01 mA cm -2 , an onset potential of 0.93 V that is more positive vs reversible hydrogen electrode (RHE), a high electron transfer number n (∼3.90) and long-term stability. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 The usual process to synthesize metal carbides involves high temperature (> 1000 °C) carburization of metals with graphitic carbon, which is not suitable to producing materials for catalytic applications because the products have low surface areas. 18 The temperature-programmed reduction (TPR) method was developed in the 1980s and extensively used to synthesize high surface area transition-metal nitrides and carbides. 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 4 growing high melt point carbide materials at relatively low substrate temperature, such as WC, 26 M 3 C (M:Fe,Co,Ni), 27 silicon carbide (SiC) 28 and so on. In the HF-CVD processing, the filament was heated to over 2000 o C for activation of the source gas. It is highly flexible and scalable, easy to scale up to large-scale growth and, no less important, utilizes metal rather than vaporized metal-containing reagents as precursors, which introduce no other impurity. From the applications point of view, although Mo 2 C-based materials have been extensively studied for HER in acidic media, it is still challenging to develop Mo 2 C-based HER catalysts that work efficiently over a range of pH values. Moreover, Mo 2 C materials draw much less attention in ORR. 29 Only recently have these carbides been shown to be efficient noble metal-free catalysts for ORR. 30 The dual use of the Mo 2 C-based materials for both HER and ORR...

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