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Corresponding Author

Hong-Yan Dai(daihongyan12@sina.com)

Abstract

Microbial electrolysis cell (MEC) is a relatively new bioelectrochemical technology that produces H2 and meanwhile treats organic wastewater. Cathode hydrogen evolution catalyst plays a key role in MEC. The doping of Graphene Quantum Dots (GQDs) into MoS2 nanosheets can improve the catalytic activity of MoS2 by creating abundant defect sites both in the edge plane and the basal plane, as well as enhancing the electrical conductivity. In this paper, using Na2MoO4 , cysteine and GQDs as raw materials, a series of MoS2/GQDs composites were firstly synthesized via hydrothermal method, and then loaded on the carbon-based electrode. The optimal electrode was selected by electrochemical testing methods and used as a cathode of MEC to research the hydrogen production capacity. SEM image showed that the MoS2/GQDs composite exhibited a popcorn-like nanosheet structure and the thickness of each nanosheet was about 10 nm. The specific surface area of MoS2/GQDS composite reached 67.155 m2·g-1, which was 16 times of the specific surface area of MoS2 (4.197 m2·g-1). TEM image showed that some lattice fringes representing GQDs were intermixed with lattice fringes representing MoS2, which indicated that GQDs were well embedded in MoS2 . EDS results showed that the MoS2/GQDS composite contained Mo, S, C and O, and the atomic ratio of Mo: S: C: O = 1:2.5:1.9:1.2, indicating that the majority of Mo and S in the composite existed in the form of MoS2, while a part of S existed in the form of SOx. The LSV data of MoS2/GQDs carbon paper electrode showed that the synthesized MoS2/GQDs composite (2#) had the best catalytic activity toward hydrogen evolution when the raw material ratio of Na2MoO4, cysteine and GQDs was 375:600:1 with the optimum load of 1.5 mg·cm-2. The Tafel slope of MoS2/GQDs electrode (2#, 1.5 mg·cm-2) was found to be 44.3 mV·dec-1, lower than that of pure MoS2 electrode, which indicated that the doping of GQDs into MoS2 nanosheets made the electron transport more efficiently and the interfacial resistance was significantly reduced. In the MEC tests, the maximum hydrogen current density of MoS2/GQDS cathode (2#, 1.5 mg·cm-2) MEC reached 14.70 ± 0.80 A·m-2, which was comparable to that of Pt/C cathode MEC (14.58 ± 0.92 A·m-2), indicating that MoS2/GQDS had a good catalytic activity for hydrogen evolution. The gas production, hydrogen production rate, coulombic efficiency, hydrogen recovery efficiency, cathodic hydrogen recovery efficiency, electrical and overall energy recovery efficiencies of MoS2/GQDs cathode (2#, 1.5 mg·cm-2) MEC were, respectively, 51.15±3.15 mL·cycle-1, 0.40±0.032 m3H2·m-3d-1, 91.16±0.054%, 66.64±5.39%, 72.44±2.60%, 217.26±7.42% and 77.37±1.50%, which were slightly higher than or comparable to those of Pt/C cathode MEC. In addition, MoS2/GQDs enjoyed good stability and price advantage, which might promote the practical application of MECs.

Graphical Abstract

Keywords

MoS2/GQDs, carbon-based electrode, microbial electrolysis cell, hydrogen production

Publication Date

2021-08-28

Online Available Date

2021-01-29

Revised Date

2020-12-10

Received Date

2020-07-24

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