Synthesis And Electrochemical Applications Of Ruthenium-based Nanomaterials

In recent years,with the excessive consumption of fossil energy and the increasingly serious environmental pollution problems,the scientific community has set off a wave of research on renewable energy.Among all renewable energy sources,hydrogen has attracted people’s attention due to it has advantages of large calorific value,extensive source,and non-pollution.Currently,part of the electricity from intermittent renewable energy(e.g.solar,wind,and tidal energy)does not match the energy demand and will cause serious energy waste.In an ideal hydrogen society,the wasted electricity can be used to electrolyze water to produce hydrogen,so that this portion of electric energy is stored in the form of hydrogen.Then,using fuel cell technology,the chemical energy stored in hydrogen and oxygen is converted into electrical energy,thereby improving energy utilization.However,the polymer exchange membrane fuel cells and water electrolysis devices still require high-loading noble metal catalysts to ensure their stable operation,and the expensive price of noble metals hinders its large-scale commercial application.In the past decades,although non-precious metal catalysts(Fe,Co,Ni,etc.)has made great progress,their activity and stability are still far from precious metals.Compared with other precious metals,Ru has a lower price and higher element abundance.Therefore,the development of high-performance and low-cost Ru-based electrocatalysts is of great significance to promote the development of the hydrogen energy society.This thesis is guided by the theories of applied electrochemistry,nanomaterial synthesis and electrochemical in-situ characterization,and aims to understand the electrodes reaction mechanism of fuel cells and electrolyzed water and reduce the cost of electrocatalysts.A series of Ru-based nanomaterials were synthesized by using a precisely controlled synthesis method.By changing the morphology,structure,and composition of the catalyst,the key activity descriptors that affect catalytic reactions were explored.Using electrochemical testing technology and advanced material characterization methods,the active sites and reaction paths of the catalyst in the electrode reaction process are studied.Finally,the catalyst is assembled in an electrochemical device to test its activity and stability under real working conditions,which provide a theoretical and practical basis for reducing the cost of fuel cells and electrolyzed water catalysts.This paper includes the following four aspects:(1)The monometallic Ru nanoparticles were prepared using the polyol synthesis method,then the Ru/Ru(OxHy)catalyst we synthesized through surface modification.Ru/Ru(OxHy)and traditional Fe-N-C catalyst to assemble a completely non-platinum proton exchange membrane fuel cell(PEMFC).For the PEMFC anode hydrogen oxidation reaction(HOR),the apparent hydrogen binding energy(HBEapp)is its activity descriptor,which is the combination of hydrogen binding energy(HBE)and water binding energy(ΔGH2O).Since Ru has a strong HBE,it is necessary to enhance water adsorption to make its HBEapp more moderate.Electrochemical oxidation or thermo chemical oxidation of Ru nanoparticles can obtain Ru/Ru(OxHy)catalyst,which increases the HOR mass activity of Ru nanoparticles by 21 times.As far as know,Ru/Ru(OxHy)is the non-platinum catalyst with the highest HOR activity.Then Ru/Ru(OxHy)was assembled in a fuel cell to obtain a peak power density(PPD)of 1.53 W cm-2.According to material characterization results and electrochemical experiment results,it is proved that Ru(OxHy)layer is formed on the surface of the nanoparticles,which enhances the adsorption of H2O.Then the HBEapp of the catalyst is more moderate,leading to improves HOR activity of Ru nanoparticles.Using Ru/Ru(OxHy)as the anode and Fe-N-C as the cathode,a completely non-platinum proton exchange membrane fuel cell was assembled and obtained a PPD of 450 mW cm-2,which is the lowest cost per unit power density reported fuel cell in the current literature.(2)By hydrothermal synthesis method,a core-shell structure Ru7Ni3/C catalyst is obtained.The particle core is RuNi alloy,and the shell is composed of Ru and NiOx.The alloying effect is a common way to control metal HBE.The Ni inside the Ru7Ni3/C nanoparticles can reduce the HBE of Ru,while the NiOx on the surface can enhance the adsorption of H2O,which leads to a more moderate HBEapp of the catalyst.So,Ru7Ni3/C catalyst has good HOR catalytic performance in an alkaline medium.The mass activity and specific activity of the Ru7Ni3/C catalyst are 3 times and 5 times that of the current PtRu/C catalyst with the current highest HOR activity in the alkaline medium.Using Ru7Ni3/C as the anode catalyst for hydroxide exchange membrane fuel cell(HEMFC),a PPD of 2.03 W cm-2 can be obtained,and it can steady operation for 100 h under the current density of 0.5 A cm-2.Besides,the price of Ru is about one-third of that of Pt,using Ru7Ni3/C catalyst can reduce the cost of the current mainstream HEMFC anode by 85%.(3)The hollow RuCuOx catalyst is obtained by the hydrothermal synthesis method,which is used as ’the oxygen evolution reaction(OER)catalyst of proton exchange membrane electrolyzers(PEMWEs).Currently,RuO2-based catalysts with a rutile structure have higher OER activity in acidic media,but their stability is poor.At present,there are two ways to improve the stability of RuO2-based catalysts.First,reduce the overpotential of OER;Second,increase the oxidation potential of lattice oxygen.The hollow RuCuOx catalyst has high OER performance,and its 10 mA cm-2 corresponds to an overpotential of 212 mV,which is 59 mV less than the overpotential of RuO2,thereby improving the stability of the catalyst.The experimental results show that the improvement of the OER performance of the RuCuOx catalyst is partly due to its unique hollow structure,which is conducive to material transport and charge transfer.In addition,the doping of Cu causes the formation of unsaturated Ru cations,resulting in more active sites.Besides,doping Cu on RuO2,the p-band center position of O adjacent to Cu in RuCuOx is closer to the Fermi level than RuO2,which is conducive to the OER process.(4)By calcining the synthesized sulfide nanosheets in the air atmosphere,the sulfate-functionalized S-RuFeOx nanonet catalyst was obtained.The corresponding overpotential of S-RuFeOx at 10 mA cm-2 is 187 mV.Compared with commercial RuO2,the overpotential is reduced by nearly 90 mV.At the same time,the stability of the S-RuFeOx catalyst has been greatly improved.The experimental results show that the high OER performance of the S-RuFeOx catalyst mainly comes from three points.First,its unique qusi-twodimensional nanonet structure is beneficial to the mass transportation and charge transfer process in the OER process;Second,sulfate doping can reduce the binding energy of*00-H,so that the rate-determining-step(RDS)of the OER process changes from the deprotonation of*OO-H to the formation of*OOH;Third,Fe promotes the dissociation of H2O,which is conducive to the formation of*OOH.Besides,sulfate facilitates the adsorption of interfacial water on the lattice oxygen,thereby enhancing the stability of the catalyst.