Hydrogen Production From Ammonia Electrolysis Cell(AEC) Coupled With Microbial Fuel Cells As Driven Power

Environmental pollution and energy crisis are the hot issues and challenges of the world today.In this context,effective control of the environmental pollution,efficient utilization of the waste resource and in-depth development of green energy has therefore become a world-wide priority to solve the above problems.From the viewpoint of sustainable development,the ammonia/urea-rich wastewater,such as high ammonia-nitrogen organic wastewater contains abundant energy and resource.In recent years,ammonia electrolysis technology has been widely investigated for hydrogen production using the simulated and actual ammonia/urea-rich wastewater as raw materials.Actually,the ammonia electrolysis cell(AEC)is an ideal installation for hydrogen production in alkaline media,which involves ammonia electro-oxidation reaction(AOR)on the anode and hydrogen evolution reaction(HER)on the cathode with an applied over-potential of only ca.0.06 V.Similar to the conventional water electrolysis technique,the AOR or AEC technology possesses the environmentally friendly advantages,and more importantly,it consumes less energy than the electrochemical water splitting.Consequently,the AOR or AEC technology has become a potential and highly-efficient strategy to treat ammonia/urea-rich alkaline wastewaters,which are difficult to achieve the complete denitrification through a biological degradation or a physical-chemical recovery process.In order to decrease the cost of hydrogen production,an AEC that operates under a low applied voltage is desired,and therefore,should be developed hereafter.Microbial fuel cells(MFCs)can successfully output electrical energy from wastewater to other devices,such as microbial electrolysis cells.Moreover,the recent researches demonstrated that the output voltage/power of MFC can be effectively improved through a series or a parallel connected MFCs unit.Under these circumstances,a series connected MFCs-driven AEC(MFCs-AEC)coupled system should be a feasible strategy to efficiently produce hydrogen in-situ from ammonia or urea-rich wastewater.Herein,the detailed studies have been carried out on the proposed MFCs-AEC coupled system for high-efficiency hydrogen production.Specific contents and conclusions are below:(1)N-doped reduced graphene oxide(N-rGO)substrate material was synthesized through a hydrothermal-calcination treatment with graphene oxide(GO)and melamine as precursors.Then,the cathodic catalyst Mo2C/N-rGO was prepared through a solid calcination method with the as-synthesized N-rGO and ammonium molybdate as raw materials.The physicochemical characterization of Mo2C/N-rGO composite suggested that nano M02C particles were uniformly anchored on the N-rGO surface.The electrochemical HER activity measurement showed that the Mo2C/N-rGO presented a lower over-potential(170 mV at 10 mA cm-2)and Tafel slop(66.0 mV dec-)in acidic media(0.5 M H2SO4),furthermore,it also exhibited better electrocatalytic HER activity(446.3 mV at 10 mA cm-2,Tafel slop:110.9 mV dec-1)in alkaline media(1 M KOH).In both acidic and alkaline media,the Mo2/N-rGO exhibited outstanding stability,with the potential differences between the used and the fresh electrodes of about 61 and 81 mV in acidic and alkaline media at 10 mA cm-2,respectively,which indicated that the N-rGO substrate can improve the affinity between the active component and substrate to increase the electrocatalytic stability.(2)Pt/N-rGO and Ptlr/N-rGO materials were synthesized through a potentiodynamic deposition method and applied as AOR anodes.The morphologies,structures,and chemical states of the Pt/N-rGO and PtIr/N-rGO composites were characterized by various physicochemical methods.The AOR activity of Pt/N-rGO,including various components of nitrogen-doping and substrates,was studied in-detail by cyclic voltammetry(CV)technology.The results showed that the existence of pyridine-N has a positive effect on AOR activity,because introducing pyridine-N significantly changed the surface charges,and then influenced the adsorption property of Pt/N-rGO towards the AOR.Also,the AOR activity of PtIr/N-rGO,including various preparation parameters,substrates and active components,was investigated in-detail by CV method.The results indicated that the introduction of N-rGO substrate and Ir component resulted the better catalytic activity(0.09 mA ?g-Pt and 0.25 mA cm ECSA)of the PtIr/N-rGO composite under the conditions of Pt/Ir ratio of 3,lower potential of-0.3 V vs.SCE,scan rate of 50 mVs-1 and loading cycle of 50.(3)A self-driven MFCs-AEC(CF)coupled system was constructed for hydrogen production with the as-prepared Mo2C/N-rGO/carbon felt and Pt/N-rGO/carbon felt as cathode and anode,respectively.During 24 h operation,the maximal yield of hydrogen gas is 10.3 mL(the average hydrogen production is 6.3 mL)and net power is 13.6 kJ g-1 ammonia with voltage efficiency of over 98%,current efficiency of 90%,power consumption of 3.49 Wh g-1 ammenia.Moreover,the effect of various MFCs connected models on hydrogen production was also investigated.The variation of the MFCs' models exhibited obvious influence on cathode and anode potentials/current of AEC in the MFCs-AEC(CF)coupled system,which in turn affected hydrogen production significantly.Compared with other hydrogen production systems,this self-driven MFCs-AEC(CF)coupled system can realize high-purity hydrogen production in-situ.(4)The N-doped graphene aerogel(NGA)was synthesized through a hydrothermal method with GO and poly(oxypropylene)diamine D400(D400)as precursors.The as-prepared NGA was reacted with the active component of Mo2C and Pt to form three dimensional(3D)porous Mo2C/NGA cathode and Pt/NGA anode,respectively.Using the above cathode and anode,a self-driven series connected MFCs-AEC coupled system(MFCs-AEC(NGA))was construct for hydrogen production.Results suggested that the Mo2C was uniformly anchored on the NGA substrate and the Mo2C/NGA presented higher HER activity in 1 M KOH with over-potential of 365 mV at 10 mA cm-2 and Tafel slop of 112 mV dec-1.Similarly,the Pt/NGA material also exhibited a better dispersion of Pt nanoclusters on the NGA surface and higher AOR activity with the starting potential of 0.625 V vs RHE and the maximal current density of 27.46 mA cm-2.All the performances could be attributed to N doping,the 3D porous NGA framework and the uniformly dispersed Mo2C and Pt active components.The MFCs-AEC(NGA)coupled system presented higher hydrogen production and net power by a factor of 1.46 and 1.42 than that of the MFCs-AEC(CF)coupled system,respectively,which suggested that the 3D self-standing NGA substrate can facilitate charge transfer and improve the whole coupled system's performance.(5)The effects of the sludge digestion liquid ratio on hydrogen production from the MFCs-AEC(NGA)coupled system were investigated using the actual high ammonia-nitrogen organic wastewaters as a raw influent.As for single-operation MFCs unit,the output voltages and power densities decreased with the increasing of the sludge digestion liquid ratio,indicating that the electro-genesis bacteria activity is suppressed owing to the high concentration of ammonia-nitrogen in the sludge digestion liquid(the original concentration is as high as 1400 mg L-1).After the continuous 24 h operation,the removal rate of COD was negligible and the concentration of ammonia-nitrogen decreased with the production of nitrite and nitrate,which was mainly attributed to the fact that a majority of COD is not available by the electro-genesis bacteria in the MFCs unit,due to the low biodegradability of the sludge digestion liquid.On the other hand,the ammonia-nitrogen could be transformed into nitrite and nitrate under the aid of the nitrosobacteria and nitrobacterium,and simultaneously,the trace dissolved oxygen diffused from air-cathode in the MFCs.The effects of the various ratios of the sludge digestion liquid on the MFCs-AEC(NGA)coupled system showed that the maximum hydrogen production of 3.6 mL was achieved under the ratio of 1:1,suggesting a larger current was produced to improve the hydrogen production.Under this condition,the ammonia-nitrogen concentration was decreased from 446 to 297 mg L-1 with the removal rate of 33.4%,only 3.5%of which was ascribed to the non-electrochemical reaction process.