Preparation And Electrochemical Performance Of One-dimensional Platinum Group Nanocatalysts For Fuel Cells

Fuel cell is an electrical system that can directly convert the chemical energy of fuel into electrical energy.A certain amount of catalyst is needed to reduce the requirements of reaction environment and increase the reaction rate during the fuel cell operation.Therefore,the research and exploration of catalysts are significance to the development and application of fuel cells.At present,Pt is recognized as the most effective catalyst in the field of fuel cell.Traditional platinum catalysts have many problems such as short cycle life,low catalytic efficiency,and unsatisfactory stability.In addition,due to the extremely low content of platinum on the earth and the high price,the price of fuel cell is generally high at the present stage and it is in the market introduction stage.To address key issues that hinder the commercialization of fuel cell technology,this thesis begins on non-Pt,low Pt and high Pt atomic utilization to solve the problems of expensive catalysts and scarce resources.At the same time,the traditional catalysts have the problems of low activity and poor stability,which can be solved by doping,precisely controlling the elemental composition of materials,and exposing active crystal surface.Based on applied electrochemistry,nanomaterials,catalytic chemistry,physical chemistry and other disciplines,this thesis aims to develop cost-effective,high-activity,high-stability catalysts and reveal their catalytic mechanism by combining theoretical calculations with experiments.It mainly includes the following three aspects of research work and results:Firstly,based on the idea of developing efficient non-Pt materials with properties comparable to Pt,Pd,which is also a platinum group,is expected to become a substitute for platinum due to its similar chemical properties to platinum.One-dimensional nanomaterials have become an important branch of research on high-performance catalyst materials due to their excellent flexible structures,fast electron transport,and anisotropy.Herein,a series of palladium-based（Pd Fe,Pd Co,Pd Ni,and Pd）nanowires with good oxygen reduction activity in acidic media were fabricated by doping modification.The Pd M nanowires with diameter about 10 nm and length above microns.Benefiting from the combination of morphological advantages and bifunctional group effects,the as-synthesized Pd M nanowires is extraordinarily improved activity for ORR performance,the optimal half-wave of Pd Ni nanowires/C（839 m V）is positive shift 138 and 25 m V compared with pure Pd nanowires/C（701 m V）and commercial Pt/C（814 m V）,respectively.The mass activity of Pd Ni nanowires/C only dropped 28%after 10000 potential cycles between 0.6-1.1 V versus RHE in acid medium.This work provides a promising and reliable highly active electrocatalysts,which can comparable to Pt-based fuel cell electrocatalysts.Secondly,based on the above research methods,based on the preparation of a high Pt atom efficiency,the goal of reducing the catalyst cost and improving the catalyst activity was achieved.Pt-based nanotubes are widely studied as a promising class of ORR electrocatalysts.A rational porosity for wall of nanotubes could lead to higher atomic utilization and better mass transportation.To confirm this speculation,we prepare a novel class of Pd Pt porous nanotubes with sub-nano Pt-shell.We first synthesized Pd Pt porous nanowires using the nanoscale Kirkendall effect using Pd nanowires as a template,and then used acidic Fe Cl₃ as an etchant to prepare Pd Pt porous nanotubes with sub-nanometer Pt shells by heating and etching.The diameter of nanotubes is about 10 nm,the length is about several micrometers,0.5±0.3 nm thick Pt walls with a hollow porous morphology.Their ESCA of 95 m²/g and mass activity of 2.29 A/mg _Pt,whica are 14.3 and 9.6 times higher than those of commercial benchmark Pt/C catalyst,respectively.The electrocatalytic durability is also shown.The improved performance can be ascribed to unique geometry（one dimension and nanotubes with porous walls）and Sabatier principle:alloying a second metal with Pt has been extensively illustrated to optimize the adsorption energies for oxygenate intermediates in ORR（such as*O,*OOH and*OH）,thus leading to enhanced activities and decreased Pt catalyst usage.The sub-nano Pt-skin Pd Pt porous nanotubes designed in this thesis provide a new way to meet the needs of fuel cell commercialization with high performance,low Pt,high stability Pt-based catalyst.Finally,based on previous research methods,four Pd Pt Ir（Pd Pt₁Ir₅,Pd Pt₂Ir₁,Pd Pt₃Ir₁,Pd Pt₅Ir₁）porous nanotubes with different element contents were synthesized by precisely controlling the feeding ratio of Pt⁴⁺and Ir³⁺.Electrochemical tests show that Pd Pt₃Ir₁exhibits the best oxygen reduction activity.Then,we annealed the Pd Pt₃Ir₁ PNTs at 350°C,400°C,450°C,and 500°C.Electrochemical tests show that Pd Pt Ir porous nanotubes exhibit the best oxygen reduction mass activity after annealing at 400°C of 1.26 A/mg _Pd+Pt+Ir in acidic media,respectively.The Pd Pt₃Ir₁ porous nanotubes-400 are also stable under acidic electrolyte,having almost no change after 30 000 cycles.The DFT calculations demonstrate that the introduction of Ir atoms into Pt and Pd optimize the oxygen adsorption energy E₀ of the（211）facet.Specifically,the ORR-OER overpotential gap of 642 m V of the Pd Pt₃Ir₁porous nanotubes-400.Such an ultra-low overpotential gap reveals their superior oxygen reduction and evolution reactions activity.This study provides a new idea and method for the rational design of highly active one-dimensional nanomaterials.