Study On Photo-electrocatalytic Water Oxidation Of Manganese Containing Polyoxometalates And Semiconductor Composites

In recent years,affected by global warming,environmental pollution and energy crisis,the international community's attention and demand for clean energy have been increasing.China is also vigorously promoting the realization of carbon balance and actively encouraging the research and development of green and low-carbon technologies.Clean energy includes solar energy,hydrogen energy,wind energy and tidal energy.Among them,hydrogen energy is the cleanest energy in the world.Hydrogen production methods include oil cracking and water decomposition.Oil cracking is the mainstream hydrogen production method at present.This method is accompanied by the generation of CO while producing hydrogen.The combustion product of CO is CO₂,which will cause secondary pollution.Hydrogen production by water decomposition can realize the circulation from water to hydrogen and then to water.Therefore,water is considered to be the largest potential source of hydrogen energy.However,due to the high energy consumption and high cost in the process of water decomposition,the wide application of artificial water decomposition to produce hydrogen and oxygen is limited.Therefore,it is urgent to develop efficient hydrogen and oxygen evolution catalysts to reduce the activation energy required for the reaction,so as to catalyze the decomposition of water to produce hydrogen quickly,economically and reliably.In order to develop a stable,efficient and low-cost water oxidation catalyst,based on the oxygen release center(OEC)[CaMn₄O₅]of natural photosynthesis system(PSII),this paper searched and found a series of compounds with similar composition,structure and Mn valence to[CaMn₄O₅]in manganese containing polyoxometalates(Mn_x-POMs).At the same time,in order to improve the disadvantages of narrow absorption range and low utilization rate of Mn_x-POMs to sunlight,the optimized Mn_x-POMs was assembled with four kinds of semiconductors with narrower band gap,and the film was assembled by layer by layer assembly method(Lb L).The photocatalytic water oxidation performance and stability of composite thin film electrode were compared.The research results are as follows:1.According to the composition and structural characteristics of[CaMn₄O₅],six kinds of Mn_x-POMs were selected from the reported literature,and their electrocatalytic water oxidation properties in homogeneous phase and assembled film were compared and analyzed.The manganese contents in the six POMs are 2,4,6(2species),14 and 19 respectively.It was found that when the amount of manganese is the same,Mn₆-POM-4 with Mn-O-Mn cubic structure diaplays better electrocatalytic water oxidation performance.In addition,because the oxidation state of Mn in Mn₁₄-POM is the same with that in[CaMn₄O₅],and there are two Mn-O-Mn cubic structures in Mn₁₄-POM,thus it shows the best electrocatalytic water oxidation performance among six compounds.Compared with six kinds of Mn_x-POMs,Mn₆-POM-4 and Mn₁₄-POM were selected for future study to be fabiricated with semiconductors in order to obtain efficient photoelectric water oxidation catalysts.2.The band gap(Eg)of Mn_x-POMs is about 3.7 e V,and the corresponding absorption wavelength is in the ultraviolet region.The Eg of graphite carbon nitride(g-C₃N₄)is 2.7 e V.The assembly of g-C₃N₄ nanosheets(NSs)and Mn₆-POM-4 effectively widens the absorption range of Mn₆-POM-4 to visible light.The photo-electrocatalytic water oxidation performance of[g-C₃N₄NSs/Mn₆-POM-4]_n composite membrane electrode was studied.It was found that the composite membrane electrode has high catalytic efficiency.The photocurrent density was 239?A/cm²(E=1.5 V vs.Ag/Ag Cl).Compared with[g-C₃N₄NSs]₂,the photocurrent density increased by 100%;Compared with[Mn₆-POM-4]₂,the photocurrent density increased by 49%.Mn₆-POM-4 was modified with low-cost g-C₃N₄NSs,which effectively improved the photo-electeocatalytic water oxidation performance of Mn₆-POM-4.3.Copper oxide(Cu O)is a narrow band gap semiconductor with Eg=1.7 e V,and the absorbance of Cu O is better than g-C₃N₄.Mn₁₄-POM was assembled with Cu O,and the photo-electrocatalytic water oxidation performance of[Mn₁₄-POM/Cu O]_ncomposite film electrode was studied.Under visible light,the composite membrane electrode showed good photo-electeocatalytic efficiency for photoelectric water oxidation,and its photocurrent density was 318.8?A/cm²(E=1.5 V vs.Ag/Ag Cl).Compared with[Mn₁₄-POM]₂ thin film electrode,the photocurrent density increased by 350%.Mn₁₄-POM was modified by the narrow band gap semiconductor Cu O,which effectively improved its photo-electrocatalytic water oxidation performance.4.Cuprous sulfide(Cu₂S)and copper sulfide(Cu S)are copper based chalcogenides.Because the electronegativity of element S is smaller than that of element O,the valence band is more negative,Eg is smaller and the light absorption range is wider.Mn₁₄-POM and Cu₂S or Cu S semiconductor composite film electrodes[Cu_xS/Mn₁₄-POM]_n)were prepared by Lb L technology,and their photo-electrocatalytic water oxidation properties were compared.The photo-electrocatalytic efficiency of[Cu₂S/Mn₁₄-POM]_n composite film electrode for water oxidation was excellent.The photocurrent density reached 1024?A/cm²(E=1.5 V vs.Ag/Ag Cl).Compared with[Mn₁₄-POM]₂ electrode,the photocurrent density was increased by nearly 600%.Mn₁₄-POM was modified by Cu₂S with Eg=1.1 e V,which further improved the photo-electrocatalytic water oxidation performance of Mn₁₄-POM.Mn₁₄-POM and Cu₂S had complementary advantages and played a synergistic catalytic role in the photo-electrocatalytic water oxidation process.