Modification Of Graphene For Advanced Electrocatalysis

With the continuous growth of fossil fuels crisis,the development of green energy conversion and storage systems becomes very important for the sustainable human society.Among different novel green energy systems,fuel cells,metal-air batteries and water electrolyzers at present have been considered to be the optimum techniques for energy conversion and storage at high efficiency.However,their practical utilizations are hindered by the slow kinetics of the involved electrochemical reactions at both anode and cathode.Many research results have shown that although the precious metals,such as platinum and ruthenium can facilitate the electrochemical reactions of the mentioned energy systems,they suffer from insufficient activity,low stability and high cost.Therefore,it is of primary importance to design sustainable and active substitutions for precious electrocatalyts.Graphene is a two dimensional crystal made of single carbon layer.Due to the excellent ability of electron transport,large surface area and stable structure,graphene has been widely used in fields of(photo-)electric devices,sensors,energy conversion and storage,catalysis and environmental management,etc.Latest studies have shown that modification of graphene can modulate physical-chemical properties and structural distribution of graphene,for instance,heteroatoms doping can improve the electrocatalytic performance of graphene toward oxygen reduction reaction.Motivated by these interesting achievements,this thesis will center on structural and chemical modification of graphene to synthesize novel graphene based electrocatalysts and explore the mechanism of electrocatalysis.The research would likely to provide further fundamental insights into the rational design of high performance electrocatalysts and prompt the development of novel energy systems in commercial applications.This doctoral thesis mainly contains the following contents:(1)Ultra light and porous nitrogen doped graphene for electrocatalysis of oxygen reduction reactionUsing graphene oxides and melamine as the precursors,we proposed a novel approach to the synthesis of three dimensional nitrogen doped graphene(NG).This method avoids the stacking of graphene layers during nitrogen doping reaction.Thus,the resultant NG shows ultra light property with specific surface area increased by 30 times as compared to the one prepared without using ice template.The inner porosity is mostly at macroporous/mesoporous scale.At the same time,the doped nitrogen atoms modulate the Fermi level of graphene,improving the electrocatalytic activity of graphene toward oxygen reduction reaction.Therefore,both larger electrocatalytic current and lower overpotential of ORR on NG are observed.Mechanism study shows that the electrocatalytic activity of NG closely relates to the configurations of doped nitrogen.The conversion from pyrrolic-like to graphitic-like nitrogen can be used to explain the lowered overpotential of ORR.(2)Bioinspired copper-graphene nanocomposite for electrocatalysis of oxygen reduction and evolution reactionsWe proposed a novel approach to the synthesis of high performance electrocatalysts mimicking the copper(Ⅱ)active sites within natural laccase.Copper(Ⅱ)coordination structure was modified onto the basal plane of nitrogen doped graphene during pyrolysis,resulting in a bio-inspired copper-graphene nanocomposite.The conductive graphene as the supporting matrix can facilitate the electron transfer and transport rate occurred during electrocatalysis.In the meantime,pyrolysis of carbonaceous materials can reduce parts of copper(Ⅱ)ions into metallic copper under high temperature.The formed metallic copper and nitrogen doped graphene exhibit different electron donation effect toward copper(Ⅱ)ions,making the energy levels of copper(Ⅱ)ions approaching to that of oxygen substrates.Thus,the obtained copper-graphene nanocomposite greatly decreases the overpotential for electrochemical reduction and evolution of oxygen in both acidic and alkaline media.We find that the copper(Ⅱ)ions within copper-graphene nanocomposite can further be active sites to facilitate glucose oxidation reaction in alkaline solutions.The copper-graphene nanocomposite is thus promising as a building block to assemble glucose fuel cells.(3)Cobalt(Ⅱ)ions-graphene coordination structure for electrocatalysis of oxygen evolution reactionTaking advantage of the coordination interaction between cobalt(Ⅱ)ions and N or/and O dopants within graphene,we can obtain a type of electrocatalysts with tunable acti-vity.The high conductivity combined with the electron tuning ability of conjugated structure of graphene make the immobilized cobalt(Ⅱ)ions efficient have high electrocatalytic activity toward the evolution of oxygen with an overpotential of only 0.27 V in alkaline solutions.Further,the achieved electrocatalyst can be applied as a model platform at ionic level to explore different descriptors that might relate to the activity.The N/O doped graphene can interact with different transition metal ions,which enable us to systematically study the correlation between electron states of active sites and activities:1)the electrocatalytic activities as a function of d electron numbers of transition metal ions show a volcano-like trend;2)the counter ions of cobalt(Ⅱ)ions affect the redox potential of cobalt(Ⅱ)ions in the final electrocatalysts,which also have a volcano correlation with the electrocatalytic activities.This research reveals that the d electron numbers and electron densities of transition metals ions could be two general activity descriptors.(4)In situ fluorescence spectroelectrochemistryOxygen reduction and evolution reactions are complicated involving multiple electron and proton coupled transfer,and generation of transient oxygen intermediates.It is a key step to understand the electrocatalytic mechanism of ORR and OER for rationally designing high performance electrocatalysts.Herein,we proposed a technique of in situ fluorescence spectroelectrochemistry to unravel the stepwise electron transfer of oxygen electrocatalysis and the natures of oxygenate reactive species in real time using a fast radical reaction of specific oxygen probes.It is discovered that hydroxyl radicals are important transient states of both oxygen reduction and evolution reactions.Their chemisorption determines the overpotential of the reactions.This technique is sensitive,efficient,being applicable to different oxygen contained electrochemical reactions.