Study on Electrochemical Carbon Dioxide Conversion System Based on Metal Nanomaterials

Author:Han Na

Supervisor:li yan guang


Degree Year:2018





With the continuous expansion of the global population and ever-increasing level of the world economy,the increasingly serious global energy shortages and environmental pollution have been highly concerned by the international community and governments.High-efficiency energy storage and conversion technologies can not only achieve efficient utilization of intermittent renewable energy,but also provide fundamental raw materials and fuels for the chemical industry that are related to the national economy and people’s livelihood.Therefore,it is considered as one of the most promising technologies for productive use of clean energy.Among them,water electrolysis and CO2 reduction appear to be promising for solving energy shortage problems and enabling a pathway for sustainable chemicals.Thus,the development of high-efficiency,stable,and low-cost electrocatalysts has become the first priority of energy conversion and storage technologies,which plays a key role in improving the reaction conversion efficiency and product selectivity.However,before the practical application of these technologies are a series of obstacles,such as high overpotential,low conversion efficiency and slow kinetics.For this,a lot of efforts have been made by scientists worldwide and some breakthroughs have been achieved,though there is still a long way to go before large-scale deployment of these technologies in industries hitherto.The focus of this dissertation is to design high-activity,high-stability electrocatalysts for oxygen evolution reaction and CO2 reduction reaction.In this study,hydrothermal synthesis,microwave reaction and other synthetic methods were used to realize the controllable preparation of functional nanomaterials.In combination with a series of characterization techniques,we explored the morphology,crystalline state and phase characteristics of these nanomaterials.By the effectively controlling of the surface interface structure and the microscopic electronic states,we investigated the structure-activity relationships.To gain further insights into the electrocatalytic mechanism in oxygen evolution and CO2 reduction reactions of these materials,we carried out DFT calculations to simulate the electronic structure and catalytic process of the catalysts in atomic scale,which provided both experimental and theoretical foundations for future study on further optimization of catalytic performance and development of new efficient electrocatalysts.The main research contents and results of this dissertation are as follows:1.In order to seek a highly active and stable electrocatalyt for oxygen evolution reaction,we used a simple one-step hydrothermal synthesis method to prepare molybdate ion intercalated ultrathin nickel-iron layered double hydroxides(Ni Fe Mo).The catalyst showed excellent electrocatalytic oxygen evolution performance in 1 M KOH solution: the onset overpotential was about 230 m V,the Tafel slope was 40 m V/dec,and the current density was 10 m A/cm2 with overpotential of 280 m V.As a result,the activity of Ni Fe Mo catalyst is much higher than that of pure nickel-iron layered double hydroxides and commercial ruthenium oxide catalysts.Notably,this method can also realize the in-situ growth of the Ni Fe Mo material on the surface of the three-dimensional nickel foam electrode,thereby obtaining an OER catalytic electrode with ultra-high specific area,abundant active sites,good conductivity and robust stability.As a consequence,this strategy would greatly reduce the cost for OER.2.We developed a facile microwave heating method to prepare a hybrid material consisting of the CNT core and cobalt polyphthalocyanine sheath for high-performance CO2 RR electrocatalysis.In this system,by virtue of the synergistic effect produced by the compounding of the cobalt phthalocyanine polymer and the carbon nanotubes,it not only increased the conductivity and electrochemically active surface area,but also accelerated the kinetic process of CO2 reduction,thus enhancing the activity and stability of the catalyst.The results showed that the onset overpotential of Co PPc/CNT catalyst for CO2 reduction was comparable to those of Au and Ag based catalysts,the CO Faradaic efficiency was as high as 90%,and its outstanding turnover frequency had exceeds most organic or inorganic competitors.In addition,the Co PPc/CNT catalyst exhibited excellent stability for at least 24 h.3.In this work,we developed the synthesis of ultrathin bismuth nanosheets through in situ topotactic reduction from Bi OI nanosheet templates under cathodic electrochemical environments.We further explored into the mechanism of in-situ topological transformation from layered tetragonal Bi OI to layered rhombohedral Bi,and elucidated the changing laws of crystal structure in the process of topological transformation.The results showed that in this ultrathin two-dimensional structure,the number of active Bi sites exposed on the surface of the Bi NS far exceeds the number of Bi atoms in bulk Bi powders.As a result,the Bi NS catalyst exhibited lower overpotential and higher formate selectivity.DFT studies suggested that the high reaction selectivity toward formate was due to its stabilized intermediate(OCHO*)on Bi(001)surface relative to COOH* or H*,providing a clue as to bismuth-based material’s superior carbon dioxide reduction performance.Furthermore,Bi NS was coupled with an Ir-based oxygen evolution electrocatalyst to achieve efficient full-cell electrolysis.When powered by two AA-size alkaline batteries,the full cell exhibited impressive Faradaic efficiency of 95% and electricity-to-formate conversion efficiency of 47%.