关键词: MoS₂/GQDs, 碳基电极, 微生物电解池, 产氢

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 SO_x. 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.

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