Synthesis of Noble Metal-Free and Transition Metal Nanomaterials for Electrocatalytic N2 and CO2 Reduction Under Ambient Conditions

Author:Qiu Wei Bin

Supervisor:liang ru ping


Degree Year:2019





With a rising global population,increasing energy demands,and impending climate change,major concerns have been raised over the security of our energy future.Developing sustainable and fossil-free pathways to produce global important chemicals and fuels could play a major role in reducing carbon dioxide emissions and providing the feedstocks needed to make the products we use on a daily basis.Electrocatalysis plays a central role in clean energy conversion technology because of its simple operation and high energy conversion efficiency.One prospective goal is to develop electrochemical conversion processes that can convert molecules in atmosphere(e.g.,nitrogen,and carbon dioxide)into higher-value products(e.g.,ammonia,hydrocarbons,and oxygenates)by coupling to renewable energy.Electrocatalysts play a key role in these energy conversion technologies because they increase the efficiency,selectivity,and rate of the chemical transformations involved.Current electrocatalysts,however,are inadequate.Development of electrocatalysts with the enhanced performance for N2 reduction reaction is inadequate and yet remains a grand challenge.Appropriate nanostructuring of the electrocatalyst can enhance the interaction between the catalyst and the reactant small molecule compound,promote adsorption and activation,reduce the energy barrier of the chemical reaction,and ultimately achieve more efficient clean energy conversion.Herein,we aim to design and synthesize high-efficiency and stable catalysts for electrocatalytic reduction of N2 and CO2.Based on the characteristics of boron nitride and iron oxide with good N2 reduction performance and high CO2 reduction activity of Ag,four new and high-efficiency electrocatalytic reduction of N2 and CO2catalysts were synthesized.The structure-activity relationship of the catalysts was systematically studied by various characterization methods.The catalytic mechanism of electrocatalytic N2 reduction by boron nitride and the electrocatalytic process of high-efficiency reduction of carbon dioxide by Ag nanowires were clarified.This research work is of great significance for nano-functional materials in electrocatalysis and energy application research.The main works and innovations of my graduation thesis will be written in details in the sections below:1.We report that a boron carbide nanosheet acts as a metal-free catalyst for high-performance electrochemical nitrogen-to-ammonia fixation at ambient conditions.Combined with theoretical calculations,the possible nitrogen fixation reaction pathway and energy distribution are given,and the step of determining the nitrogen fixation reaction on the catalyst is determined.The catalyst can achieve a high ammonia yield of 26.57μg h-1 mg-1cat.and a high Faradaic efficiency(FE)of15.95%at-0.75 V versus reversible hydrogen electrode,placing it among the most active aqueous-based nitrogen reduction reaction electrocatalysts.In 0.1 M sodium sulfate(Na2SO4),the catalyst still exhibits good activity and selectivity.Notably,it also shows high electrochemical stability and excellent selectivity.Density functional theory calculations suggest that the*NH2—*NH2→*NH2—*NH3 reaction is the rate-limiting step.This study not only provides an attractive metal-free electrocatalyst material for NH3 synthesis,but also opens up an exciting new avenue to the rational design of B4C-based nanocatalysts with enhanced performance for N2-fixation applications.2.The above studies indicate that the theoretical nitrogen reduction onset potential of B4C nanosheets is-0.34 V,but the experimental result is-0.65 V,so that better electrocatalytic nitrogen reduction performance can be obtained by improving the conductivity of B4C nanosheets.For this purpose,a novel catalyst has been fabricated by in-situ formation of boron-doped graphene(BG)on B4C nanosheets through an ultrasonic exfoliation,a 1600 oC thermal treatment process,and followed by cutting of BG into boron-doped graphene quantum dots(denoted as B4C-BGQDs).The BGQDs on the composite are small in size and can expose more active sites.Moreover,the B4C-BGQDs possess better conductivity and adsorption to nitrogen than B4C nanosheets,resulting better catalytic activity of electrocatalytic N2 fixation and reduction under ambient conditions.It can realize the high-efficiency electrocatalytic N2 reduction without the formation of by-product hydrazine hydrate.In 0.1 M HCl,an NH3 yield and FE as high as 28.6μg h–1 mg– V and 16.7%at-0.35 V can be achieved respectively.The catalyst has excellent long-term electrochemical durability.3.A layer of amorphous TiO2 was deposited on the surface ofα-Fe2O3nano-spindle by St?ber method,and then the mesoporous TiO2 coated Fe2O3nano-spindle was obtained by post-hydrolysis.Finally,the crystallinity of amorphous TiO2 can be controlled by annealing at different temperatures.The performance of electrocatalytic N2 reduction for the obtainedα-Fe2O3 nano-spindle composite coated with mesoporous TiO2 with different crystallinity were investigated.The porous nanostructures on TiO2 enhance the interaction between the catalyst and N2,promoting N2 adsorption and activation,providing excellent mass transfer performance.The experimental results show that the core-shell structured amorphous/crystalline hetero-phase mesoporous TiO2 coatedα-Fe2O3 nano-spindle composite prepared at 400°C achieved the best electrocatalytic activity for artificial N2 fixation and reduction.In 0.1 M Na2SO4,an NH3 yield and FE as high as 27.2μg h–1 mg–1cat.and 13.3%can be achieved at-0.5 V respectively.The catalyst has excellent long-term electrochemical durability.4.We demonstrate the in-situ development of a bromide-derived porous Ag nanowire film(BD-Ag)with adsorbed Br-anions via anodic oxidation of commercial Ag foil to coral-like porous AgBr(P-AgBr)and subsequent electrochemical reduction for efficient electrocatalytic conversion of CO2 to CO.The adsorbed Br-anions on the surface of BD-Ag can inhibit the electrolysis hydrogen evolution reaction,thereby improving the efficiency of electrocatalyzing CO2 to CO.The experimental results show that the BD-Ag adsorbed with a small amount of Br-anions is a highly efficient CO2 reduction electrocatalyst with high selectivity.When tested in 0.5 M KHCO3,such BD-Ag shows superior electrochemical CO2 reduction reaction activity with a low onset potential of only 296 mV and a CO faradaic efficiency as high as 96.2%.It also demonstrates excellent long-term electrochemical durability.This work offers an attractive catalyst material toward efficient and selective CO2 reduction for electrocatalytic applications.