Tuning Oxygen Reduction Reaction(ORR) Performance of Palladium-based Electrocatalysts Via Support

Author:Li Jing

Supervisor:wang jian guo


Degree Year:2019





The oxygen reduction reaction(ORR)as cathode reaction of fuel cells is a kinetic sluggish process.To date Pt-based catalysts are taken as the benchmark ORR catalysts,but the high cost and low stability disadvantages of the catalysts limit the wide application of fuel cells.Therefore,how to prepare the ORR catalysts with high activity,excellent stability and low cost has become one of the significant research topics in the field of fuel cells.Aiming at this problem,this paper focused on the application of supported Pd catalysts in electrocatalytic oxygen reduction reaction.The support tuning effect on Pd nanoparticles was studied,and a series of efficient Pd-based ORR catalysts were rationally designed and prepared.It’s expected that the tuning effect of support on metal catalysts would be deeply understood,and to provide a theoretical reference for further application of supported Pd-based catalysts in the fuel cell.The main results are as follows:(1)Supported Pd(3.6 nm in average)catalysts on TiO2 with oxygen vacancies(VO)have been prepared via facile pyrolysis.Compared with commercial Pt/C(20 wt%)catalyst,Pd/TiO2-VO(10 wt%)demonstrated superior oxygen reduction activity with30 mV positive shift of half-wave potentials,better durability with less loss(3.8%vs.34.9%)in the current density after 10 h,and higher methanol tolerance capability in alkaline solution.The mechanism of the enhanced ORR performance on Pd/TiO2-VO catalyst was studied.The electron transfer from TiO2-VO to Pd nanoparticles(NPs)led to the electron-rich Pd surface and strong metal-support interactions(SMSIs).The electron-rich Pd NPs enhanced the adsorptions of key intermediates,lowered the Gibbs free energy of ORR,and improved the ORR activity.SMSIs between Pd NPs and TiO2-VO endowed the catalyst with excellent stability and immunity to methanol poisoning.Therefore,the electron transfer from TiO2-VO to Pd NPs plays a crucial role in promoting ORR performance of Pd-based electrocatalysts.(2)Pd nanoparticles were supported on TiN partially oxidized by nitric acid.As the oxygen content in TiN support increased,the particle size of Pd nanoparticles decreased(Pd/O-TiN-1(2.7 nm)<Pd/O-TiN-2(4.3 nm)<Pd/TiN(8.5 nm))and the distribution of Pd nanoparticles on supports became more uniformly.The partially oxidized TiN support not only could tune the Pd nanoparticles size,but also impact the electronic structure of the Pd nanoparticles to stabilize the metal Pd,and made it less susceptible to oxidation.The order of ORR activity was Pd/O-TiN-1>Pd/O-TiN-2>Pd/TiN.The ORR activity of Pd/O-TiN-1(onset potential 0.97 V,half-wave potential0.82 V)was even superior to that of commercial Pt/C,which was much better than Pd/TiN(onset potential 0.92 V,half-wave potential 0.77 V).The Pd nanoparticles supported on TiN partially oxidized by nitric acid had higher ORR activity,indicating that the metal Pd with small size,uniform dispersion was beneficial to improve the ORR activity.Pd/O-TiN-1 not only had high ORR activity,but also had excellent kinetic performance,methanol tolerance capability and stability(a high current retention of 96%after 10 h),which was due to the slight oxidation of TiN.The partial oxidation of TiN retained the excellent conductivity of TiN,meanwhile a small amount of TiO2 was generated on the surface,which led to strong metal-support interaction between Pd and TiO2.(3)The Pd nanoparticles with an average diameter of 5.2 nm were uniformly supported on the partially oxidized SiC(O-SiC)by deposition-precipitation method.Pd/O-SiC catalyst had excellent ORR activity(onset potential 0.95 V,half-wave potential 0.78 V),resistance to methanol toxicity and stability(a slight current loss of3.5%after 10 h),compared with the commercial Pd/C(onset potential 0.97 V,half-wave potential 0.86 V),and even better than commercial Pt/C in oxygen saturated 0.1M KOH solution.This was mainly due to the charge transfer from O-SiC support to Pd nanoparticles,so that Pd was negatively charged.Electron-rich Pd enhanced the adsorption of key intermediate OOH in ORR,and reduced the Gibbs free energy of the reaction,thereby increased the ORR activity.In addition,the stability of the SiC and the large binding energy of the O-SiC to the Pd nanoparticle enhanced the methanol toxicity and stability of the Pd/O-SiC catalyst.According to the DFT calculation,the effect of oxygen on the surface of SiC was analyzed.After partial oxidation of SiC surface,the binding energy of support to Pd was nearly maintained,the adsorption of key intermediate was greatly enhanced,and the Gibbs free energy of the ORR key electrochemical step was reduced,then improved the ORR performance.(4)N-doped TiO2(N-TiO2)was prepared by high-temperature calcination of TiO2(B)in an ammonia atmosphere,and Pd nanoparticles with an average particle diameter of 2.8 nm were supported onto N-TiO2 by direct pyrolysis.The characterization results of Pd/N-TiO2 catalysts showed that with the increase of calcination temperature(700,800 and 900°C),the ratio of Ti-O bond in N-TiO2decreased gradually from 55.1%to43.1%,while those of Ti-N-O and Ti-N bonds increased gradually from 25.9%to 31.9%and from 19.0%to 25.0%,respectively,which indicated that N partially replaced O in TiO2.TiOxNy with ORR activity and TiN with conductive property were generated on the surface of TiO2 by high temperature calcination of TiO2(B)in ammonia atmosphere.In addition,charge was transferred from support to Pd nanoparticles in the Pd/N-TiO2catalysts,then promoted the ORR activity,and the strong interactions between Pd and the supports improved stability of the Pd-based catalysts.