Preparation And Photoelectrochemical Properties Of Sandwich Semiconductor-based Catalytic Membranes

Energy crisis and environmental pollution are two challenges that limiting the sustainable development of human society and economy.The semiconductor photocatalytic technology can convert solar energy into chemical energy,which has been considered to be one of possible solutions to counter energy crisis and environmental pollution.In the dissertation,the Cu₂O,g-C₃N₄ and BiOCl photocatalysts served as sunlight-harvesting material were synthesized through ultrasonic treatment method,electrostatic attraction strategy and oxidation-reduction method.Several approaches have been employed to improve the photocatalytic performance,such as element doping,quantum dot sensitization,external voltage application,and combination with semiconductor or other materials.The prepared photocatalysts were added into BPM to form a sandwich structure which was used for water splitting and NaOH regenerating from spent caustic,respectively.The main contents are as follows:?1?Carbon quantum dots?CQDs?-Cu₂O composites were prepared by facile one-step ultrasonic treatment.The SEM,XRD,XPS,UV-vis and photocurrent response curve measurement results showed that CQDs were evenly distributed on the surface of Cu₂O,GQDs–Cu₂O have excellent visible light absorption property and photoelectro-chemical properties.Then,GQDs-Cu₂O were introduced to a bipolar membrane to form an compact GQDs–Cu₂O/BPM sandwich structure.The thickness of cation exchange membrane is 59 mm,and anion-exchange membrane is 54 mm.GQDs–Cu₂O/BPM has lower membrane impedances than commercial BMP-1 under sunlight irradiation.It can also minimize pH gradient formation,resulting in lower loss associated with E_pH.As a diaphragm,GQDs–Cu₂O/BPM was used for H₂ production from water splitting.When the current density reached 90 mA cm^-2,the efficiency of H₂ generation reached up to 88.6% and 14.5% more efficient than that of BPM,whereas the energy saved is about 22.6% with respect to that of BPM under sunlight irradiation.?2?The g-C₃N₄ was proton-functionalized to from a positively charged surface?the zeta potential was +14.0 mV?.On the other hand,graphene oxide?GO?demonstrated a markedly negatively charged surface?the zeta potential was-46.7 mV?.Therefore,it is clear that the spontaneous self-assembly between the positively charged P-g-C₃N₄ and the negatively charged GO was realized by the electrostatic attraction of oppositely charged and ?–? stacking interactions to construct GO/P-g-C₃N₄ composite.This was followed by the reduction of GO to rGO to obtain the rGO/P-g-C₃N₄ composite material.The measurements results showed that exfoliation of P-g-C₃N₄ successfully coupled with the negatively charged GO via electrostatic attraction,forming a more compact and well-dispersed sheet-on-sheet structure.The rGO/P-g-C₃N₄ has excellent visible light absorption property and photoelectro-chemical properties.Moreover,the rGO/P-g-C₃N₄ has high separating efficiency of photo-generated charge carriers due to excellent electronic conductivity of the rGO.In order to evaluate the photoelectrocatalytic active,the rGO/P-g-C₃N₄ composite was added to a bipolar membrane to promote water dissociation in the interlayer,the pH changes could reflect the photoelectrocatalytic active perfectly.The pH variations in compartments B and D were the most obvious by using rGO/P-g-C₃N₄ photocatalysts?When the experiment time was 1 h,the pH of compartments B and D equal to 2.5 and 12.5,respectively?,suggesting that rGO/P-g-C₃N₄ photocatalysts have stronger photoelectrocatalytic performance.This study provides a novel and universal method to evaluate the photoelectrocatalytic active for semiconductor photocatalyst.?3?I-doped BiOCl was synthesized through oxidation-reduction method.The existence of iodine can reduce its band gap energy from 3.19 eV to 2.30 eV,thereby the visible light absorption property was improved.The prepared I-Bi OCl photocatalysts were added into BPM to form an I-Bi OCl/BPM sandwich structure.The current density-voltage curves and AC impedance spectroscopy results indicate that I-BiOCl photocatalyst has higher catalytic activity than BiOCl for water splitting.The membrane impedance and cell voltage drop of I-BiOCl/BPM clearly decreased under sunlight irradiation.As a diaphragm,I-BiOCl/BPM was used for H₂ production from water splitting.The efficiency of H₂ generation using the I-BiOCl/BPM sandwich structure reached 92% and 13% more efficient than the BPM under solar irradiation,whereas the energy saved was 35.3% compared with the BPM.?4?In order to expand the scope of applications of sandwich catalytic membranes,a configuration consists of three compartments for NaOH recovery from spent caustic was designed.The 0.30 mol L^-1 Na₂SO₄ in the cathode chamber and 0.10 mol L^-1 initial base concentration in the central chamber were the optimum concentrations for regenerating NaOH.When the experiment time lasts for 1 h,the current efficiency using the BiOCl/BPM sandwich structure reached 98% and is 8% more efficient than the BPM under sunlight irradiation.We typically think that current efficiencies >80% are desirable in commercial stacks,therefore our finding is signicant.Moreover,we can observe that the BiOCl/BPM sandwich structure exhibits the lowest energy consumption among all the BPMs for NaOH recovery.When the experiment time lasts for 1h,the energy saved by using the BiOCl/BPM sandwich structure is about 9% compared with that of the BPM under solar irradiation.Therefore,the phenomena prove again that the BiOCl/BPM sandwich structure plays an important role in NaOH recovery from spent caustic.