Research On SO₂ Poisoning Mechanism And Anti-poisoning Strategy Of Pt-based Oxygen Reduction Electrocatalyst

Proton exchange membrane fuel cell（PEMFC）,as a clean and efficient energy conversion device,is a potential candidate for replacing traditional internal combustion engine.Electronic vehicles powered by H₂-O₂ PEMFC have advantages of pollutant–free,high-power and fast-refilling,making them the main directions for transformation and development of the global automobile industry.In the practical application of fuel cell vehicles,air instead of pure oxygen is supplied as cathode feedstock,where the pollutant,SO₂ adsorb strongly on the surface of Pt and poison the active site on Pt-based cathode catalysts for oxygen reduction reaction（ORR）.Therefore,improving the SO₂ tolerance of PEMFC is highly desirable for widening the application of fuel cell vehicles.Guided by the poisoning and regeneration mechanism of SO₂ on Pt surface,this thesis develop several efficient regeneration protocols for preparation of SO₂ poisoned Pt/C catalysts and anti-poisoning catalysts.The evolution of SO₂ adsorbed on Pt surface under different potentials were studied.Form of S-containing species is related to the applied potential on Pt.When potential below 1 V,the adsorbed S-containing species interacts strongly with Pt by occupying the active surface and does not desorb,leading to the degradation of performance.The regeneration of Pt surface can only be achieved at high potential when S-containing species are oxidized to soluble SO₄^2-and HSO₄^-.However,only part of adsorbed SO₂ can be oxidized and desorb from Pt after each potential scan to high potential region,and at least 8 cycles of cyclic voltammetry（CV）scan between 0.05 and 1.5 V are required to remove all SO₂ on the surface of Pt.This protocol accelarate the degradation of Pt/C and lower the operating life of PEMFC.The transformation of different SO₂adsorption configurations depends on the applied potentials,and this is the underlying mechanism for the regeneration of deactivated Pt catalysts.Guided by the mechanism,a square wave-based protocol was developed,to minimise the effects on the stability of Pt/C.Parallel-bonded SO₂is hard to be oxidized at high potential.After transformed to atop and bridge site,SO₂ can be oxidized and removed at high potential.Thus,multiple pulses to high potential are required to fully remove SO₂.The oxidation and desorption capacity of SO₂ in three adsorption modes on Pt surface follows the order of atop-bonded,bridge-bonded and parallel-bonded.Because most of SO₂ molecules are atop-bonded at 0.4 V,a regeneration protocol based on square wave-based（0.4 V＿5 s＿1.5 V＿5 s,3 times）was developed to regenerate the Pt surface after only three cycles.Compared with traditional protocol based on cyclic voltammetry（0.05-1.5 V,8 cycles）,our protocol is more efficient and affects less the stability of Pt/C.The effect of NO on the poisoning behavior of SO₂ on Pt was studied,based on which a non-oxidative regeneration protocol was proposed.It was found that the adsorbed NO can be reduced to NH₄⁺at low potential and fully removed after potentiostatic polarization at 0.2 V for 5 s（0.2 V＿5 s）.The more NO exists in the mixture of NO/SO₂,the less SO₂adsorbs on Pt surface;and the poisoning of Pt by SO₂ can be fully restrained when NO is three times as concentrated as SO₂ in the mixture.The adsorption of NO on Pt is stronger,so that they occupy Pt surface during the competitive adsorption with SO₂.Besides,the coupling effect of NO weakens the interaction between SO₂ and Pt,SO₂ thus can be easily replaced by NO.As a result,the ORR activity of SO₂-poisoned Pt/C catalysts can fully recover after replacement of SO₂ by NO and subsequently reductive process,thus avoiding the negative effect of oxidative protocols on the stability of Pt/C.The effect of Ru on the anti-poisoning properties of Pt/C was studied,the mechanism of Ru improving the tolerance toward SO₂ was revealed.A Pt₆Ru₁/C catalyst and a Ru-Pt Ni/C alloyed octahedron catalyst（10Ru-Pt Ni/C）with high tolerance to SO₂ was fabricated.It is shown that the ORR activity is related positively to the Pt:Ru ratio,while SO₂ tolerance is negatively related to Pt:Ru ratio.The introduction of Ru weakens the interaction between Pt and SO₂,and the oxygen-containing groups adsorbed on Ru surface accelerate the oxidation process of SO₂.The tolerance toward SO₂ of Pt₆Ru₁/C catalyst is 30%higher than commercial Pt/C.The ORR activity of 10Ru-Pt Ni/C is not effected by SO₂ poisoning.