Tin/copper-based Catalysts Constructed From Solid Waste Recycling For Electrocatalytic CO₂ Reduction Performances And Mechanisms

The energy crisis and global warming are becoming increasingly serious due to the rapid development of human society and the excessive use of fossil fuels,further prompting all countries around the world to be currently confronted with the main task of achieving the“carbon neutrality”and“net zero carbon emissions”.The electrochemical CO₂reduction reaction（CO₂RR）is regarded as a promising strategy to address the problems about the global warming and energy crisis,which can effectively convert CO₂ into value-added chemicals,materials and fuels.However,there are still bottleneck to be urgently settled such as low selectivity,high overpotential and poor stability in the process of CO₂RR,caused by the fact that the excellent thermodynamic stability of non-polar CO₂ molecule makes the CO₂RR extremely challenging.Therefore,it is significantly important to design and construct novel electrocatalysts with remarkable catalytic efficiency.On the other hand,the vigorous development of industry has produced abundant solid waste like electroplating sludge,in which the release of metals would seriously endanger environmental problems and human health.Considering that electroplating sludge contains a large amount of metal resources,converting these metals into electrocatalytic materials for fully recovery and utilization can not only solve environmental problems,but also alleviate the scarcity of existing metal resources on the earth.Inspired by the mentioned above,this paper used the electroplating sludge as the source of metal materials innovatively to synthesize various Sn and Cu-based electrocatalysts,whose CO₂RR performance and catalytic mechanism were also systematically studied.Based on this,the main research contents and results of this paper as following:（1）Synthesizing the boron-doped tin oxide nanospheres and studying the CO₂RR performance.The Sn element was firstly extracted from electroplating sludge by acid leaching,which further was applied to prepare the boron-doped tin oxide（B-Sn O₂）catalytic material using hydrothermal reflux.And the morphology of as-synthesized B-Sn O₂was about 50 nm nanosphere,in which it had both the crystal and amorphous structures.The study of Raman,in situ FTIR spectroscopy and functional density theory calculations revealed that the introduction of B dopant could promote the formation of electron-rich Sn sites and increase the number of active sites in the structure of B-Sn O₂,thereby encouraging the degree of activation of CO₂molecular and then effectively reducing the formation energy barrier of*OCHO intermediates on the Sn O₂ surface.The test results of CO₂RR showed that the faradaic efficiency(FE_HCOOH)of the B-Sn O₂ for formic acid both exceeded 90%within the broad working potential window from-0.7 to-1.3 V vs.RHE（600 m V）,in which the maximum of 95.1%had been achieved at-1.0 V vs.RHE.Furthermore,the B-doped Sn O₂ catalyst had a higher partial current density for HCOOH achieving the j _HCOOH of 42.35 m A cm^-2 at-1.0 V vs.RHE(71.47 m A cm^-2 at-1.3V vs.RHE),and there was no obvious attenuation for its catalytic activity and stability after the long-term catalytic reaction of 60 hours.In conclusion,the engineering of electronic modification can effectively ameliorate the problems of low current density,unsatisfactory selectivity,and poor stability of Sn-based electrocatalysts for HCOOH product in the process of CO₂RR.（2）Preparing the skeleton-anchored Sn nanosphere composites with N-doped carbon and exploring their CO₂RR properties.With the use of Sn electroplating sludge as raw material and melamine sponge as the carrier,the Sn nanospheres anchored on N-doped carbon skeleton catalyst（Sn@NC）was successfully synthesized through simple physical adsorption and calcination process.Benefiting from the abundant nitrogen and 3D macroporous structure in melamine sponge,the as-prepared material had excellent electrical conductivity and fast charge transfer ability,which further enhanced the adsorption ability of CO₂ molecules on its structural surface.The results of CO₂RR test found that,among all the samples,Sn@NC-700 catalyst showed the best CO₂RR performance for producing the HCOOH with the maximum Faradaic efficiency(FE_HCOOH)up to 91.2%and the maximum j _HCOOH of-32.2 m A cm^-2.In addition,the Sn@NC-700 catalyst exhibited the reduction yield of 601μmol h^-1 cm^-2 for formic acid at-1.23V vs.RHE,which was about 1.9 times that of Sn@NC-600 and 1.5 times that of Sn@NC-800.Therefore,this work provides a new route for constructing the metal matrix/carbon material composites.（3）Synthesis of the N/P co-doped carbon-coated Sn nanoparticle composites and CO₂RR performance investigation.Using Sn electroplating sludge extract as raw material and bacillus subtilis as Sn carrier,the N/P co-doped carbon-supported Sn nanoparticle composite（Sn@NPC）was successfully prepared by the procedure of adsorption-lyophilization-calcination.The morphological characterization found that the targeted material of Sn@NPC basically maintained the rod-like structure of bacillus subtilis,in which the Sn nanoparticles were uniformly dispersed on the carbon support without agglomeration.And the adsorption capacity and reaction mechanism of Sn@NPC composites for CO₂ molecules were also analyzed through the CO₂ adsorption experiments and theoretical calculations,suggesting that the Sn@NPC electrode could significantly enhance CO₂ adsorption,reduce the energy barrier of*OCHO and*HCOOH intermediates and then facilitate the rapid transfer of electrons in the CO₂RR.Moreover,the test results of CO₂RR revealed that the Sn@NPC electrode possessed the Faradic efficiency of 87.93%to generate formic acid at 1.05 V vs.RHE,whose FE also exceeded the 82%within a wide potential window from-0.8 to-1.3 V vs.RHE（500 m V）.Besides,the Sn@NPC had the superior electrocatalyst stability in the process of CO₂RR that it could steadily react for 105 hours with the FE_HCOOH of～85%and the j _HCOOH of～8.0 m A cm^-2at-1.0 V vs.RHE.So it can be concluded that the multifunctional strategy mentioned in this work innovatively offers a simple route to meet the requirements of the carbon neutrality,environmental protection and sustainable economic development.（4）Synthesis of the Sn-doped Cu O catalyst rich in oxygen vacancies and CO₂RR performance investigation.With the use of Sn and Cu extracted from electroplating sludge as raw material,the electrocatalyst of Cu O nanosheets（V_o-Cu O（Sn））containing oxygen vacancies and Sn dopants was successfully synthesized by a one-step low-temperature alkaline etching method.The oxygen vacancies and Sn dopants in the structure of V_o-Cu O（Sn）catalyst were both verified by XPS,EPR and related elemental analysis techniques.Density functional theory calculations further confirmed that the synergistic effect of oxygen vacancies and Sn atoms greatly reduced the energy barrier for the formation of*COOH and*CO intermediates,thereby prompting the targeted material of V_o-Cu O（Sn）with the remarkable conversion performance for the reduction product of CO in CO₂RR.The experimental data of CO₂RR found that the V_o-Cu O（Sn）electrode exhibited the Faradaic efficiency up to 99.9%with the catalytic stability of180 hours to develop the CO product at a low overpotential of 420 m V,and its current density in the conversion for CO was up to 35.22 m A cm^-2 at-1.03 V vs.RHE.Meanwhile,the FE_COof the as-synthesized V_o-Cu O（Sn）was both above 95%in the potential window range of-0.48to-0.93 V vs.RHE,exceeding the performance of most of the Cu-based electrocatalysts reported so far.Therefore,this work provides a novel perspective for designing defect engineering or doping to regulate the surface electron distribution of electrocatalysts to further improve CO₂RR performance.