Design And Regulation Of Pd-based Catalysts With High Stability And Activity

Fuel cells have received extensive attention due to their high efficiency and cleanliness.However,the oxygen reduction reaction（ORR）occurring at the cathode of the fuel cell severely restricts the development of fuel cells due to the sluggish reaction rate,high catalyst cost and poor stability.Pd-based catalysts exhibit high catalytic activity toward ORR,and are expected to replace commercial Pt/C as new catalysts for ORR.Although the alloy catalyst has high catalytic activity,the loss of alloying elements causes a decrease in its catalytic activity.Elemental doping is a common method for improving the catalytic stability of alloy catalysts.In this work,Pd₂₀Ni_80-xM_x（M=Mo,Nb;X=0,0.5,1）was used as the precursor alloys to prepare unsupported catalysts with high stability and activity toward ORR by dealloying.Firstly,the Pd₂₀Ni_80-xM_x（M=Mo,Nb;X=0,0.5,1）precursor alloys with a single phase were prepared by arc melting followed by annealing.The metallograph results showed that the addition of Mo and Nb could effectively retard the the diffusion of Pd and Ni atoms during annealing.Secondly,the annealed precursor alloys were subjected to voltammetric dealloying in 0.5M H₂SO₄ solution,the results showed that the structure and chemical compositions of the as dealloyed samples were decided by the potential range and scan cycles,after scan in 0-1.2 V（vs RHE）for 500 cycles,Pd nanoparticles deposited on all of the samples,forming rough surfaces.All of the dealloyed samples showed electrocatalytic activity toward ORR,the half wave potential of ORR on the dealloyed samlpes followed the sequence as:PdNi（0.857 V,vs RHE）=commercial Pt/C>Nb0.5>Nb1>Mo0.5>Mo1>Commercial Pd/C.After 1000 cycles of durability test in O₂ saturated 0.1M HCLO₄ solution in 0.3-1.0 V（vs RHE）,the ORR half wave potential of all the samples shift toward the negative side,the shift on Mo0.5 and Nb0.5 was the smallest,demonsting that this tow samples had high catalytic stablity toward ORR.Thirdly,the chronopotentiometric dealloying was performed on the as-annealed precursor alloys in 0.5M H₂SO₄ solution,the structure and chemical compositions of the as dealloyed samples were sensitive to the applied corrosion potential and corrosion time.When dealloyed under 0.6 V（vs RHE）for 3000 s,all of the samples exhibited a dense and uniform multicomponent Pd-rich shell on the surfaces,the shell provided the samples with high electrocatalytic acative toword ORR,as can be evidenced from the positive shifted of the half wave potential toword ORR:Mo0.5（0.938V,vs RHE）>PdNi>Nb0.5>Mo1>Nb1>commercial Pt/C（0.857 V,vs RHE）>commercial P d/C.After 1000 cycles of durability test in O₂ saturated 0.1M HCLO₄ solution in 0.3-1.0 V（vs RHE）,the ORR half wave potential of all the samples shift toward the negctive side,Mo0.5 and PdNi dealloyed samples exhibited 8 mV degradation in the half-wave potential,showing the optimal electrocatalytic stablity toward ORR.Finally,the corrosion pathway for different dealloying methods were discussed,the voltammetric corrosion incloded dissolution and redeposition.However,the chronopotentiometric corrosion only invloved dissolution and passivation caused by the diffusion.The relationship between element doping and catalytic activity and stability was discussed.the calculations based on First-principles showed that the addition of Ni,Mo and Nb maked the d-band center of Pd atoms on the surface move negatively,and the addition of Mo and N atoms increased the diffusion activation energy of Pd atoms on on the surface.The diffusion of Pd atoms on the surface was suppressed.