English

• 0.   Introduction

  Hydrogen is a clean energy source and feedstock used in numerous industries^[1-2]. Its calorific is as high as 22 MJ·kg^-1, nearly 2.75 times higher than hydrocarbon fuels^[3], which is considered the most important energy carrier and fuel of the future. However, the commonly used methods of hydrogen production, such as hydrogen production from fossil energy sources, photocatalytic hydrogen production^[4-5], and hydrogen production from water electrolysis^[6], inevitably suffer from high energy consumption, low efficiency, and environmental pollution. Microbial electrolysis cell (MEC) is a new technology developed in recent years, which can simultaneously treat organic wastewater and produce hydrogen to convert biomass energy into hydrogen energy. The concept of MEC originated in 2005 to explore the waste-to-energy generation route based on sustainability and renewability. MEC is based upon bio-catalyzed reactions and is also named a bio-catalyzed electrolysis cell (BEC) or bio-electrochemically assisted microbial reactor (BEAMR)^[7]. MECs are capable of chemical conversion of organic matter to hydrogen in the complete absence of oxygen with a small voltage supply. In MEC, microorganisms at the anode decompose organic matter to produce electrons and protons, the electrons produced at the anode reach the cathode through an external circuit, and the protons reach the cathode through the solution, where they combine to produce hydrogen at an applied auxiliary voltage^[8].

  The hydrogen evolution reaction (HER) takes place at the cathode. In an MEC system, the cathode is the main electron donor^[9]. The cathode base materials and the catalyst used on it have a major impact on the performance of the MECs^[10]. To date, platinum (Pt)-based materials have exhibited the most effective catalytic performance toward hydrogen production in MECs. However, its practical application is limited by its high cost, rapid poisoning by cations, and short lifetime^[11]. Therefore, it is critical to find catalysts that can replace Pt for the commercialization of MEC systems.

  Recently, many catalysts have been explored. These novel catalysts include non-precious metal bifunctional electrocatalysts—two-dimensional cobalt-doped MnPSe₃ nanosheets (CMPS) for alkaline seawater splitting HER and oxygen evolution reaction (OER) bifunctional catalysts^[12]; advanced electrocatalysts for energy-efficient hydrogen production systems such as methanol oxidization-assisted hydrogen production (MOAHP), HzOR-assisted hydrogen production (HOAHP), and urea oxidization-assisted hydrogen production (UOAHP)^[13]; and nanoporous gold supported multi-functional layered double hydroxides for oxygen/hydrogen evolution reactions, hydrazine oxidation reaction, and overall hydrazine splitting^[14]. Molybdenum disulfide (MoS₂), a transition metal sulfide with a layered inorganic compound structure, has been explored for use as a replaceable catalyst for platinum-based materials due to its large specific surface area, low price, earth abundance, and good stability^[15-16]. To the best of our knowledge, there are several examples of the use of molybdenum disulfide-based catalysts as cathode catalysts for hydrogen production in MEC (Table S1, Supporting information): Hwang et al.^[17] developed a novel MoS₂ nano-carbon (NC) coated cathode for hydrogen production in a MEC. The MoS₂-NC₂₀₀ cathode, electrodeposited at -200 μA·cm^-2, showed the maximum hydrogen production rate (HPR) of (0.152±0.002) m³·m^-2·d^-1 at an applied voltage of 0.9 V which is comparable to the previously reported Pt electrodes. However, the vertical stacking of multiple S-Mo-S layers of MoS₂ results in poor conductivity and insufficient exposed active sites, leading to its poor HER^[8].

  Copper sulfide (CuS) is also a transition metal sulfide with low cost, good stability, low cytotoxicity^[18], excellent optical^[19], electronic^[20], catalytic, and other physicochemical properties. Due to these excellent properties, CuS has been widely studied for a variety of applications, such as biosensors^[21], electro-catalytic hydrogen evolution^[22], and batteries^[23]. It might be an effective approach if CuS and MoS₂ could be combined to form a composite catalyst. The excellent ability of CuS to regulate electrons and to separate reactive species^{[21, 24]} could just compensate for the inadequacy of MoS₂ alone as a catalyst for electrochemical reactions.

  Inspired by this, we used a one-step hydrothermal method to complex two transition metal sulfides, MoS₂ and CuS. After analyzing the electrochemical hydrogen evolution reaction of MoS₂/CuS, it was applied to the hydrogen production system of a two-chamber MEC and further evaluated in terms of HPR, Coulombic efficiency (CE), cathodic hydrogen recovery rate (CHR), and energy recovery. It is expected that the addition of CuS can improve the conductivity of MoS₂ as well as increase its edge active sites, thus enhancing the hydrogen production performance.

  1.   Experimental

  1.1   Chemicals

  All reagents were analytical grade and used without further purification. Sodium molybdate dehydrate (Na₂MoO₄·2H₂O) and ammonium chloride (NH₄Cl) were bought from Tianjin Guangfu Technology Development Co., Ltd. Thiourea (NH₂CSNH₂) and sodium acetate (CH₃COONa) were bought from Tianjin Tianli Chemical Reagent Co., Ltd. Oxalic acid (C₂H₂O₄) and anhydrous calcium chloride (CaCl₂) were bought from Tianjin Komeo Chemical Reagent Co., Ltd. Copper nitrate trihydrate (Cu(NO₃)₂·3H₂O) was bought from Tianjin Beichen Fangzheng Reagent Factory. Anhydrous disodium hydrogen phosphate (Na₂HPO₄) and anhydrous sodium dihydrogen phosphate (NaH₂PO₄) were bought from Shanghai Ball Chemical Reagent Co., Ltd. Potassium bicarbonate (KHCO₃), magnesium sulfate heptahydrate (MgSO₄·7H₂O), and potassium ferricyanide (K₃[Fe(CN)₆]) were bought from Shanghai McLean Biochemical Technology Co., Ltd. Deionized water was used for the preparation of all solutions and rinsing.

  1.2   Catalyst synthesis

  The catalyst MoS₂/CuS was prepared by a step hydrothermal method. The precursors Na₂MoO₄·2H₂O and NH₂CSNH₂ in a molar ratio of 1∶2 (5 mmol Na₂MoO₄·2H₂O and 10 mmol NH₂CSNH₂) were added to 60 mL of distilled water. This mixture was mildly stirred for 30 min to get a homogeneous solution and labeled as solution-1. Then, 0.6 g C₂H₂O₄ and 1 mmol Cu(NO₃)₂·3H₂O were dissolved in 20 mL of distilled water and added into solution-1 dropwise at constant stirring for 1 h. Further, the final solution was transferred into the Teflon-lined stainless-steel autoclave and treated thermally at 180 ℃ for 24 h. The naturally cooled resulting colloidal solution was centrifuged with distilled water and anhydrous ethanol and the product was vacuum-dried overnight. The synthesized catalyst was designated as MoS₂/CuS-20%. Then, we adjusted the amount of added Cu(NO₃)₂·3H₂O substances to 0.25, 0.5, and 1.5 mmol to prepare MoS₂/CuS-5%, MoS₂/CuS-10%, and MoS₂/CuS-30%, respectively.

  1.3   Cathode preparation

  The carbon cloth electrodes were modified with MoS₂/CuS-20% composites. An unmodified MoS₂ electrode and a Pt mesh (Gaoss Union PTX, Wuhan Gaoshi Ruilian Technology Co. Ltd., Wuhan, China) were used as controls. Each electrode had a projected surface area of 4 cm² (2 cm×2 cm). For modification, 40 mg MoS₂/CuS-20% composite was mixed with 1 mL 0.5% Nafion solution and dropped on a carbon cloth. The mixture was sonicated for 0.5 h to obtain a homogeneous suspension. The suspension was applied to the carbon cloth uniformly using a pipette (both sides were coated). Finally, the modified carbon cloth was dried in the air. The final loading quantity of the modifying material was 10 mg·cm^-2. The effect of catalyst loading on catalytic performance is shown in Fig.S1 (Supporting information). Other catalyst cathodes were also prepared by the same procedure.

  1.4   MEC construction and operation

  The constructed MEC is shown in Fig.S2. The MEC consisted of a dual chamber (150 mL, Zhonghua Instrument No.1 Store, Beijing, China) separated by a proton exchange membrane (N-117, Dupont). The liquid volume of the anode chamber consisted of 90 mL anolyte and 20 mL bacterial solution. The liquid volume of the cathode chamber consisted of 110 mL of phosphate buffer of K₃[Fe(CN)₆]. The anolyte contained NH₄Cl (0.1 g·L^-1), KHCO₃ (0.5 g·L^-1), MgSO₄·7H₂O (0.2 g·L^-1), CaCl₂ (0.075 5 g·L^-1), NaH₂PO₄ (4.536 0 g·L^-1), Na₂HPO₄ (8.8 g·L^-1), acid trace element (1 mL·L^-1, Table S2), and basic trace element (1 mL·L^-1, Table S3). The catholyte contained: K₃[Fe(CN)₆] (6.6 g·L^-1), NaH₂PO₄ (4.536 0 g·L^-1), and Na₂HPO₄ (8.8 g·L^-1). The anode was a carbon felt (Carbon Graphite Products Factory Co., Ltd., Tianjin, China) that had been cultivated under an MEC mode for three months. An external voltage of 0.8 V was applied to the MEC by connecting the cathode to the negative pole of a power supply (RYI3005-D, Jinqunhao Instrument Co., Ltd., Shenzhen, China), and the anode to the positive pole. The current was monitored with a multimeter (UNI-T 803, Unido Smart Test Store, Shanghai, China).

  1.5   Characterization of MoS₂/CuS

  It is necessary to evaluate various chemical properties of composite catalyst-coated electrodes, which are directly related to their performance. To confirm the chemical composition and the crystalline nature of the nanocomposites, the prepared catalysts were characterized using an X-ray diffractometer (XRD, Bruker D8 Advance Powder Diffractometer, operating voltage of 40 kV, tube current of 15 mA, λ=0.154 06 nm, in Cu Kα radiation mode at a 2θ range from 10° to 80°; scan rate: 5 (°)·min^-1). To further analyze the chemical valence of each element of MoS₂/CuS, X-ray photoelectron spectroscopy (XPS, Thermo Fischer, ESCALAB 250Xi, UK) was used. The morphologies and chemical composition of MoS₂/CuS were investigated with a field emission scanning electron microscope (FE-SEM, JSM-7900F, acceleration voltage of 10 kV) equipped with an energy-dispersive X-ray spectroscopy (EDX) instrument. The high-resolution transmission electron microscopy (TEM, JEM-F200, acceleration voltage of 200 kV) was employed to determine the crystal plane orientation of the catalysts.

  The hydrogen production was determined by gas chromatography (GC, Changzhou PANNA Instrument Co.) using a thermal conductivity detector (TCD) at 160 ℃ under argon as a gas carrier. The content of sodium acetate as a substrate was determined by UV-visible spectrophotometry (Flash Version: 3.00). When light passed through the sodium acetate solution, the sodium acetate solution selectively absorbed the light, and the absorbance of the sodium acetate solution varied with the wavelength of the light. The sodium acetate solution was scanned using a UV spectrophotometer in the wavelength range of 190 to 250 nm. 100 mg·L^-1 sodium acetate had the maximum absorption peak at 191 nm, and 191 nm was selected as the measured wavelength^[25].

  1.6   Electrochemical test

  The electrochemical tests were carried out using an electrochemical station (Admiral, Beijing Xinlifang Technology Development Co., Ltd., Beijing, China) in a three-electrode system with anolyte (pH=7.0) as electrolyte. For these tests, prepared cathodes were applied as the working electrode equipped with an Ag/AgCl electrode (GB/T 59977—2010) as the reference electrode and platinum wire (1 mm×37 mm) as the counter electrode. Linear sweep voltammetry (LSV) tests were carried out to investigate the activity of hydrogen evolution on different cathodes with a range of-0.9 to 0 V (vs RHE) (the same as below) at a rate of 5 mV·s^-1. The onset overpotential (η_onset) at a current density of 1 mA·cm^-2 was used to compare the rates of hydrogen evolution at different cathodes. Tafel plots were calculated to evaluate the electrode kinetics, where the linear portions of the Tafel plots were fitted to the Tafel equation (η=blg|j|+a, where j is the current density, a is the cathodic intercept related to the exchange current density, and b is the Tafel slope). To investigate the charge transfer resistance of cathode materials, electrochemical impedance spectroscopy (EIS) was performed in the frequency range of 100 kHz to 10 MHz. Cyclic voltammetry (CV) tests performed at different scan rates (10, 20, 30, 40, 50, 60, 70, 80, 90, and 100 mV·s^-1) were used to calculate the electrochemically active surface area (ECSA). Chronoamperometry (current-time) curves were established to evaluate the stability of cathode materials at a fixed current density of-1 mA·cm^-2.

  1.7   Microbial community analysis

  To identify the microbial community of anode biofilm, total community genomic DNA extraction was performed using a Quant-iT PicoGreen dsDNA Assay Kit assay (Life, USA) following the manufacturer's instructions. The prokaryotic V3 and V4 regions of 16S rRNA genes were amplified using polymerase chain reaction (PCR; 2×Hieff^®Robust PCR Master Mix). Two general PCR primers for bacterial gene amplicon were used: forward primer 341FCCTACGGGNGGCWGCAG and reverse primer 805R-GACTACHVGGGTATCTAATCC. Amplification products were purified using an Illumina MiSeq system (Illumina MiSeq, USA) and subjected to base-pair sequencing. The sequencing results were further processed and the operational taxonomic units were assigned following the manufacturer's instructions^[26].

  1.8   Calculation

  1.8.1   Calculation of hydrogen production yield (HPY, mmol)

  According to the standard curve of H₂ (Fig.S3), the total moles of H₂ in the headspace volume (63 mL, volume of water quantity) were calculated based on the peak area and the following Eq.1:

  $ y=152\;845.79 x-9\;986.45 \quad\left(R^2=0.995\;91\right) $

  (1)

  where y is the peak area of gas chromatography and x (mmol) is the HPY of the reaction system.

  1.8.2   Calculation of HPR

  $ \mathrm{HPR}=\frac{10^{-3} V_{\mathrm{m}} \mathrm{HPY}}{V T} $

  (2)

  where V_m is the molar volume of gas (22.4 L·mol^-1), V is the cathode volume (L), and T is the reaction time (d).

  1.8.3   Calculation of sodium acetate content (n_s, g·L^-1) in the substrate

  The content of sodium acetate in the reaction system was calculated using the standard curve of sodium acetate (Fig.S4), as follows:

  $ y=5.828\;8 x+0.111\;73 \quad\left(R^2=0.998\;18\right) $

  (3)

  where y is the absorbance value measured using a UV-visible spectrophotometer and x represents the n_s.

  1.8.4   Calculation of CE (%) and CHR (%)

  $ \mathrm{CE}=\frac{M \int_0^t I \mathrm{d} t}{F b V n_{\mathrm{s}}} $

  (4)

  $ \mathrm{CHR}=\frac{n_{\mathrm{H}_2}}{n_{\mathrm{CE}}}=\frac{n_{\mathrm{H}_2}}{\int_0^t I \mathrm{d} t} $

  (5)

  where M is the molar mass of sodium acetate (82 g·mol^-1), I is the electric current generated in MECs (A), F is Faraday constant (96 485 C·mol^-1), t is the reaction time, and b is the number of electrons transferred (b=8), n_H2 is the amount of substance of hydrogen that could be theoretically produced, n_CE is the amount of substance of hydrogen that could be produced from the measured current.

  1.8.5   Calculation of energy recovery

  The energy efficiencies based on input electricity (η_E, %), energy efficiency related to the consumed substrate (η_S, %), and overall energy efficiency (η_E+S, %) were calculated with Eq.6-8:

  $ \eta=\frac{W_{\mathrm{H}_2}}{W_{\mathrm{E}}} $

  (6)

  $ \eta=\frac{W_{\mathrm{H}_2}}{W_{\mathrm{s}}} $

  (7)

  $ \eta_{\mathrm{E}+\mathrm{S}}=\frac{W_{\mathrm{H}_2}}{W_{\mathrm{E}}+W_{\mathrm{S}}} $

  (8)

  where W_H2 is the heat of combustion of the hydrogen produced, calculated with W_H2 = ΔH_H2n_H2 (ΔH_H2=285.83 kJ·mol^-1, is the hydrogen heat value); W_E is the energy added to the circuit by the power source, computed with $ W_{\mathrm{E}}=E_{\text {ap }} \int_0^t I \mathrm{d} t$ (E_ap is the applied voltage to MECs); W_S is the heat of combustion of the substrate (acetate), calculated as W_S=ΔH_SN_S (ΔH_S=870.28 kJ·mol^-1, is the substrate heat value). In this study, the volumetric current density (A·m^-2) was calculated by normalizing the current with the volume of the cathode chamber.

  2.   Results and discussion

  2.1   Chemical composition and morphology characterization

  The XRD patterns of the catalyst MoS₂/CuS are clearly shown in Fig. 1. The main characteristic peaks of 14.378°, 29.026°, 35.870°, and 44.151° in 2θ correspond to (002), (004), (102) and (006) crystal panes of MoS₂, respectively. The peaks of MoS₂ (PDF No.37-1492) and CuS (PDF No.06-0464) can also be found in the XRD patterns of MoS₂/CuS, which indicates the successful preparation. Moreover, it can be seen from the figure that the diffraction peaks of MoS₂/CuS were sharp and intense compared with pure MoS₂, indicating its highly crystalline nature.

  Figure 1

  

  Figure 1.  XRD patterns of MoS₂ and MoS₂/CuS

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  XPS analysis was employed to investigate both elemental compositions of the electrode surface and their corresponding oxidation states ascribing the electronic configuration of respective atoms. Fig. 2a shows the XPS survey spectra of MoS₂ and MoS₂/CuS-20%, where element C (284.40 eV) was the background element used for measurement. It is easy to find that Mo, S, and Cu elements were distributed on the surface of MoS₂/CuS-20%. To further confirm the specific chemical states of Mo, S, and Cu, the fine spectra of these three elements were also analyzed. From Fig. 2b, the two peaks at 228.87 and 231.92 eV correspond to Mo⁴⁺3d_5/2 and Mo⁴⁺3d_3/2 respectively, and also to the 2H phase of MoS₂. The peaks at 229.97 and 233.32 eV correspond to Mo⁴⁺3d_5/2 and Mo⁴⁺3d_3/2 respectively, and also to the 1T phase of MoS₂. The peaks at 232.74 and 235.80 eV correspond to Mo⁶⁺3d_5/2 and Mo⁶⁺3d_3/2, which are bound to form Mo—O bonds due to the presence of oxygen in the air^[27]. The peak at 226.20 eV corresponds to the S2s^[28-29]. Cu was not detected in MoS₂, and the Cu2p spectrum of MoS₂/CuS-20% is shown in Fig. 2c. The peaks at 932.44 and 952.10 eV correspond to Cu⁺2p_3/2 and Cu⁺2p_1/2, respectively, and the presence of Cu⁺ suggests that a small amount of Cu₂S is generated. The peaks at 933.93 and 953.12 eV correspond to Cu²⁺2p_3/2 and Cu²⁺2p_1/2, but it can be seen that the intensity of the Cu²⁺ peak was higher than that of the Cu⁺ peak. This indicates that Cu²⁺ was dominant and most of the Cu was bound to form CuS. The residual binding energy of 956.54 eV corresponds to the satellite peak of Cu²⁺2p_1/2 and the peak at 945.80 eV as well as 949.96 eV corresponds to the satellite peaks of Cu⁺2p_1/2. Fig. 2d shows the fine XPS spectra of S2p in MoS₂ and MoS₂/CuS-20%. It can be seen that the binding energies of S^2-2p_3/2 and S^2-2p_1/2 are 161.70 and 162.90 eV in the 1T phase of MoS₂, 163.18 and 164.44 eV in the 2H phase of S^2-2p_3/2 and S^2-2p_1/2. This indicates that MoS₂ in both MoS₂ and MoS₂/CuS-20% existed as a mixture of 1T and 2H phases, and the addition of CuS did not change the existence morphology of MoS₂.

  Figure 2

  

  Figure 2.  XPS spectra of MoS₂ and MoS₂/CuS-20%: (a) survey; (b) Mo3d; (c) Cu2p; (d) S2p

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  Fig. 3a-3c shows the SEM images of MoS₂ and MoS₂/CuS-20%, it can be seen that pure MoS₂ was a nanospheres composed of nanosheets. After the composite of 20% CuS, the overall morphology of the nanospheres did not change significantly, but it was observed that the nanospheres made of nanosheets had more folds, which suggests that MoS₂/CuS-20% had more edge active sites, that is, the density of active sites increased. It is well known that the HER active site of MoS₂ is located at the edge, and its basal surface is catalytically inert with poor HER performance^[30].

  Figure 3

  

  Figure 3.  SEM image of (a) MoS₂ and (b, c) MoS₂/CuS-20%; TEM images of (d, e) MoS₂ and (f, g) MoS₂/CuS-20%; (h) High-resolution TEM image of MoS₂/CuS-20%; (i) EDS mappings of Mo, S, and Cu in MoS₂/CuS-20%; Lattice structure image of (j) CuS (108) plane and (k) MoS₂ (102) plane

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  Fig. 3d and 3e are the TEM images of the MoS₂ nanosheet. Fig. 3e illustrates the TEM image of the rectangular region from Fig. 3d. Fig. 3f-3h all show TEM images of MoS₂/CuS-20%, where Fig. 3g shows a localized enlargement of the rectangular region in Fig. 3f. By observing and comparing the TEM images of the two catalyst nanosheets, it can be seen that the crystal features of MoS₂/CuS-20% were more obvious, which is consistent with the results of XRD. Compared with pure MoS₂, the surface of MoS₂/CuS-20% nanosheets was rougher and more disordered, which weakened the adhesion of bubbles at the catalyst interface and promoted the timely release of H₂ bubbles^[31]. In addition, clear lattice fringes can be observed in the high-resolution TEM of Fig. 3h, in which 0.251 and 0.174 nm correspond to the (102) crystall plane of MoS₂ and the (108) crystal plane of CuS, respectively, which matches with the XRD results and also demonstrates the successful composite of MoS₂ and CuS. The lattice structure images of the CuS (108) crystal plane and MoS₂ (102) crystal plane are shown in Fig. 3j and 3k, respectively, and more detailed images of the lattice fringes are shown in Fig.S5. In addition, the EDS elemental distribution of MoS₂/CuS-20% was shown in Fig. 3i, and the uniform distribution of Mo, Cu, and S elements can be seen, which further indicates that we successfully synthesized MoS₂/CuS-20%.

  2.2   Electrochemical performance

  The LSV polarization curves of all catalysts measured are shown in Fig. 4a. Among them, the MoS₂/CuS-20% sample showed remarkable HER performance with an overpotential of 135 mV when the current density was increased to 1 mA·cm^-2 (Fig. 4b), which was much smaller than the other catalysts. The Tafel slopes of these samples were obtained by fitting the polarization curves, as shown in Fig. 4c. The Tafel slope of MoS₂/CuS-20% (100.34 mV·dec^-1) was lower than that of Pt mesh (110.89 mV·dec^-1), MoS₂ (206.43 mV·dec^-1), MoS₂/CuS-5% (186.22 mV·dec^-1), MoS₂/CuS-10% (157.25 mV·dec^-1), and MoS₂/CuS-30% (129.86 mV·dec^-1). By comparing these values, the specific path of HER can be confirmed as the Volmer-Heyrovsky mechanism^[32], and the smaller the Tafel value, the better the HER of the corresponding catalyst.

  Figure 4

  

  Figure 4.  (a) LSV, (b) overpotential, (c) Tafel slope curves, and (d) EIS of Pt mesh, MoS₂, and MoS₂/CuS

  Inset: the fitted equivalent circuit diagrams.

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  The EIS can reflect the charge transfer kinetics in HER (Fig. 4d). Comparing the semicircular diameter of different electrodes, it can be found that MoS₂/CuS-20% had the smallest radius in EIS, which had the smallest charge-transfer resistance, fast electron transfer, and high catalytic activity^[33]. The measured impedance data were fitted using ZView based on the circuit diagram shown in Fig. 4d. Detailed impedance fitting data is shown in Table S4. The fitting equivalent circuits include charge transfer resistance (R_ct), solution resistance (R_s), constant phase element (CPE), and solid phase diffusion Warburg (Z_W) resistance. The calculated R_ct of MoS₂/CuS-20% was minimized to 34.40 Ω, which was smaller than the charge transfer resistance of Pt mesh of 41.42 Ω, indicating that MoS₂/CuS-20% had a stronger charge transfer capability compared to Pt mesh. The double-layer capacitance (C_dl) values were used to evaluate the ECSA of the catalysts, and ECSA=C_dl/C_s, where C_s is the specific capacitance of the correspondent surface-smoothed sample under the same conditions, and the C_dl values were proportional to the ECSA. As shown in Fig. 5a-5c, the capacitive currents of Pt, MoS_2, and MoS₂/CuS-20% were measured in the potential range of non-Faraday intervals and the obtained capacitance currents were plotted as a function of different scanning speeds^[34]. The capacitance currents obtained were plotted as a function of different scan rates with a slope of C_dl^[35], as shown in Fig. 5d. Compared with MoS₂, Pt, and MoS₂/CuS-20% produced the highest C_dl (1.110 0 mF·cm^-2) with the largest ECSA, which implied that the addition of a certain amount of CuS gives MoS₂ more active reactive sites and a larger electrochemically active surface area to promote the electrocatalytic process effectively.

  Figure 5

  

  Figure 5.  (a-c) CV curves and (d) the fitted Cdl of Pt mesh, MoS₂, and MoS₂/CuS-20%

  Δj was the current density difference at 0.687 7 V (vs RHE) in CV.

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  2.3   Hydrogen production of MEC

  The MoS₂, MoS₂/CuS-5%, MoS₂/CuS-10%, MoS₂/CuS-20%, MoS₂/CuS-30%, and Pt mesh cathodes were assembled in MECs to investigate their hydrogen evolution performance at an applied voltage of 0.8 V, 37 ℃ and 1.5 g·L^-1 sodium acetate were studied in MEC. The HPR was calculated based on the amount of hydrogen produced. The average HPR of the MECs with MoS₂/CuS-20% cathode was 0.203 1 m_H2³·m^-3·d^-1, which was even slightly higher than that of the MEC with Pt mesh cathode (0.188 6 m_H2³·m^-3·d^-1) (Fig. 6a). From the overall current trend of MEC, the prepared cathode catalysts with MoS₂/CuS-20% produced a higher current density (Fig. 6b). The MEC with MoS₂/CuS-20% as the cathode produced a maximum current density of 8.25 A·m^-2, which was also higher than the maximum current density produced with the Pt mesh as the cathode (5.50 A·m^-2). The decrease in current density during the experiment may be due to the gradual consumption of sodium acetate substrate and the decrease of available carbon source of microorganisms, which led to the slowdown of their metabolism and the decrease of HPR^[33].

  Figure 6

  

  Figure 6.  (a) HPY and HPR of Pt mesh, MoS₂, and MoS₂/CuS and (b) variation of current density with reaction time during the reaction of Pt mesh, MoS₂, and MoS₂/CuS as cathodic catalysts

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  2.4   Cyclic stability of MEC

  To investigate the stability of the catalyst, we performed 12 h-chronoamperometry tests (Fig. 7a), and a MEC hydrogen production cycling experiment was carried out using MoS₂/CuS-20% as the cathode (Fig. 7b). It is clear from Fig. 7a that the potential change was negligible after 12 h of chronoamperometry test, which indicates that MoS₂/CuS-20% had high stability. The results of the cycling experiments also showed good cycling ability of MoS₂/CuS-20% in MEC (Fig. 7b). After each cycle, anolyte and catholyte were replaced and 1.5 g·L^-1 sodium acetate was added. There was no significant decrease in current density over the three cycles, indicating that the MoS₂/CuS-20% cathode had high stability and reproducibility in MEC.

  Figure 7

  

  Figure 7.  (a) Current density of MoS₂/CuS-20% as the cathode for cyclic reaction three times in the MEC; (b) Chronopotentiometry curve of MoS₂/CuS-20% at-1 mA·cm^-2

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  2.5   Hydrogen production performance of MEC

  The performance of the MEC was evaluated by CE, CHR, and energy recovery. The CEs of MoS₂/CuS-20% and Pt mesh were calculated to be 16.07% and 10.33%, respectively, based on the current measured over time and the sodium acetate content, as shown in Fig. 8a. The recorded current data indicate that the MoS₂/CuS-20% catalyst had higher CE values, while the CHR was lower than that of the Pt mesh. The η_E of the MECs were all higher than 100%. The η_E of Pt as a cathode reached 587.63%, which was higher than that of MoS₂/CuS-20% (422.8%). As shown in Fig. 8b, the η_s and η_E+S of MoS₂/CuS-20% were higher than that of Pt mesh. The above data can also be used to illustrate that the HPR and hydrogen production performance of MoS₂/CuS-20% can be compared with that of Pt mesh.

  Figure 8

  

  Figure 8.  (a) CE and CHR of the MEC hydrogen production system of catalysts and (b) corresponding η_E, η_s, and η_E+S

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  2.6   Microbial community structures of the electrode biofilms

  Our microorganisms were taken from coking wastewater, and wastewater usually contains a large number of microorganisms, which are very suitable for biological treatment^[36-38]. It is well known that the electrochemically active microorganisms attached to the anode surface play a major role in the MEC hydrogen production system. Therefore, the microbial community of anode biofilm was analyzed by macrogenetic sequencing. The length distribution of high-quality sequences contained in all samples was statistically analyzed using R language scripts (Fig.S6a), laying the groundwork for subsequent diversity analyses of species composition. To evaluate the diversity of the reactor anode biofilm community, the Shannon diversity evaluation index of the microbial community in each group was evaluated (Fig.S6b). The gentleness of the curve indicates that the sequencing results can adequately reflect the diversity contained in the current samples^[39-40].

  Based on the statistical map of the number of taxa of microorganisms at each level (Fig. 9a), we analyzed the distribution at the phylum (Fig. 9b) and genus (Fig. 9c) levels to investigate the microbial community of the anode biofilm. The phylum-level shows that Proteobacteria (34.5%), Actinobacteria (23.6%), and Chloroflexi (12.7%) were the dominant bacterium in anode biofilm. In the coking wastewater, 5.5% of Bacteroidotas and 1% of Firmicutes were also detected. Proteobacteria are well known for playing important roles in extracellular electron transfer^[41], and Firmicutes were commonly reported in dark fermentative hydrogen production systems^[42-44]. At the genus level, JG30-KF-CM45 accounts for 4%, followed by 3.6% Methylotenera, 2.7% Nitrospira, and 2.4% Blastococcus. At present, we have not studied these species deeply enough and there are few reports in the literature. The analysis of microbial phylum and genus in coking wastewater shows that the anode biofilm is composed of mixed microbial communities, and there are some unknown species, so the molecular mechanism of microorganisms in electron transport has not been studied in this paper.

  Figure 9

  

  Figure 9.  Microbial community of the anode biofilm: (a) statistical map of the number of microbial taxa at each level; Percentage of the average relative abundance of phylum (b) and genus (c) levels

  Dominant biofilm genera (relative abundance >2%) are shown individually, while the rest are grouped as others in b and c.

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  3.   Conclusions

  In summary, the MoS₂/CuS-20% composite catalyst was prepared by a one-step hydrothermal method. The addition of a certain amount of CuS made the structure of MoS₂ more dense and the surface more rough and disordered, which not only increased the edge active site of MoS₂ but also facilitated the release of hydrogen on the catalyst surface. Electrochemical tests showed that MoS₂/CuS-20% had a Tafel slope of 100.34 mV·dec^-1, and its conductivity and electroactive area are better than pure MoS₂ and Pt mesh. The HPR of MoS₂/CuS-20% was (0.203 1±0.023 7) m_H2³·m^-3·d^-1, which was slightly higher than that of the Pt mesh in a 72 h-MEC hydrogen production system. The feasibility of MoS₂/CuS-20% as an alternative catalyst for Pt mesh in MEC has been demonstrated, and it is expected to be an excellent low-cost catalyst for hydrogen production in MEC in the future.

  Supporting information is available at http://www.wjhxxb.cn

  
  
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• 

  Figure 1  XRD patterns of MoS₂ and MoS₂/CuS

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  Figure 2  XPS spectra of MoS₂ and MoS₂/CuS-20%: (a) survey; (b) Mo3d; (c) Cu2p; (d) S2p

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  Figure 3  SEM image of (a) MoS₂ and (b, c) MoS₂/CuS-20%; TEM images of (d, e) MoS₂ and (f, g) MoS₂/CuS-20%; (h) High-resolution TEM image of MoS₂/CuS-20%; (i) EDS mappings of Mo, S, and Cu in MoS₂/CuS-20%; Lattice structure image of (j) CuS (108) plane and (k) MoS₂ (102) plane

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  Figure 4  (a) LSV, (b) overpotential, (c) Tafel slope curves, and (d) EIS of Pt mesh, MoS₂, and MoS₂/CuS

  Inset: the fitted equivalent circuit diagrams.

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  Figure 5  (a-c) CV curves and (d) the fitted Cdl of Pt mesh, MoS₂, and MoS₂/CuS-20%

  Δj was the current density difference at 0.687 7 V (vs RHE) in CV.

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  Figure 6  (a) HPY and HPR of Pt mesh, MoS₂, and MoS₂/CuS and (b) variation of current density with reaction time during the reaction of Pt mesh, MoS₂, and MoS₂/CuS as cathodic catalysts

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  Figure 7  (a) Current density of MoS₂/CuS-20% as the cathode for cyclic reaction three times in the MEC; (b) Chronopotentiometry curve of MoS₂/CuS-20% at-1 mA·cm^-2

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  Figure 8  (a) CE and CHR of the MEC hydrogen production system of catalysts and (b) corresponding η_E, η_s, and η_E+S

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  Figure 9  Microbial community of the anode biofilm: (a) statistical map of the number of microbial taxa at each level; Percentage of the average relative abundance of phylum (b) and genus (c) levels

  Dominant biofilm genera (relative abundance >2%) are shown individually, while the rest are grouped as others in b and c.

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•