Dynamic-Stacking of Reduced Graphene Oxide for Active Hydrogen Evolution
Authors: Ling Ning , Wang Zhen , Kim Sara , Oh Sang Ho , Park Jong Hyeok , Shin Hyunjung , Yang Heejun 
Published: February 2020

Despite remarkable electronic and mechanical properties of graphene, improving catalytic activity of graphene, an atomically-flat, inert and stable carbon network, remains a challenging issue in both fundamental and application studies. In particular, the adsorption of most molecules and ions, including hydrogen (H2 or H+), on graphene is not favorable, underlining the challenge for efficient electrochemical catalytic reaction on graphene electrode. Various defects, edges, and functionalization have been suggested to resolve the issue aiming to have catalytic graphene, but cost-effective, clean and active catalysis with graphene has not been realized yet. Here, we introduce dynamic-stacking of reduced graphene oxide (rGO) via an electrochemical way; conventional hydrogen evolution reaction (HER) was used as a dynamic process, generating hydrogen bubbles during electrochemical deposition of rGO on metallic WTe2, which creates a unique porous rGO structure on the atomically-distorted WTe2 surface with optimized edges and surface acidic groups. The hydrogen bubble template method enables an ideal, stable carbon structure for active HER with a Tafel slope of 39 mV·dec-1 and a double layer capacitance of 12.41 mF·cm-2. As our new dynamic synthesis method improves the catalytic activity of one of the most inert materials, graphene, new strategies for designing novel catalysts could be conceived for promising energy applications.

Figure 1. 1a) schematically description of the electrochemical synthesis of the rGO film on WTe2 by hydrogen bubble template method. Fig. 1b) shows the graphite and WTe2 shows poor HER activity (blue and black curve). Meanwhile, a striking HER activity is observed with our hybrid rGO-WTe2 (red curve) with a much smaller onset overpotential (vs reversible hydrogen electrode, RHE) and Tafel slope 39 mV·dec-1 (Fig. 1c). The unique HER performance of our rGO-WTe2 catalyst was investigated in terms of electrochemical surface area via double-layer capacitance (Cdl) measurements in Fig. 1d). The Cdl value was calculated by the slope of current density (at a voltage of 0.135 V vs. RHE) versus scan rate, and the value was 12.41 mF·cm-2, which is 5,000 times higher than that of state-of-the-art 3D graphene networks.[3] Fig. 1e) shows the optical microscopy image of rGO. The as-synthesized rGO on WTe2 substrate exhibits a highly porous structure in the SEM images in Fig. 1 (f-g). The characteristic dimension of the porous structure ranges from 100 nm - 2 μm in diameters with many edges exposed between rGO porous walls; the rGO flakes are stacked in a random way to have many edges as HER active sites. As HR-TEM images shown in Fig. 1 (h-i), the rGO flakes are mixed with crystal and amorphous graphene with an interlayer spacing of 0.43 nm.

Keywords: hydrogen evolution reaction, hydrogen bubble template, reduced graphene oxide, tungsten ditelluride (WTe2)
References:

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Kim, Jin‐Young, et al. Advanced Materials 25.16 (2013): 2308-2313.

H. Wang et al., Angew. Chemie - Int. Ed., vol. 57, no. 1, pp. 192–197, 2018.

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