Synthesis Of Electrocatalysts Of Fuel Cells Based On The Modulation Of Structure

Proton exchange membrane fuel cells（PEMFCs）,which directly convert chemical energy into electricity,is highly promising due to its sustainability and zero emission.Compared to hydrogen oxidation reaction（HOR）,oxygen reduction reaction on the cathode of PEMFCs is highly sluggish,then leading to a high loading of Pt to trigger slow kinetics.In addition,Pt/C are prone to be dissolved or corroded during the long-term operation in the harsh environment,resulting in a decreased ORR activity.At present,prohibitive cost and unsatisfied stability of electrocatalysts severely impede the commercialization of PEMFCs.Therefore,the design and synthesis of catalyst with high activity and stability towards ORR is helpful for the extensive deployment of PEMFCs.In this work,Pt based nanomaterials with high activity and stability was synthesized and the role of second metal was uncovered.Firstly,Pt NPs and Au NPs were separately synthesized,mixed and deposited on carbon.It was found that the electronic structure of Pt NPs can be modulated by adjacent Au NPs.In addition,the composite catalysts did not show the enhanced ORR activity,but also exhibited the improved activity towards ethanol oxidation reaction（EOR）and formic acid oxidation（FOR）.In order to understand the role of Au during electrochemical reaction,selective under potential deposition was proposed,where Cu atom can only be deposited on Au surface,rather than on Pt surface.It was found that the composite catalysts where only Au NPs was covered,still showed enhanced activity for ORR and EOR,while exhibited a poor performance for FOR.Therefore,it could be concluded that ligand effects accounted for the improved ORR and EOR activity,while the high FOR performance was attributed to the synergy effect.It was revealed that Pt NPs can promote the rupture of C-H bond of formic acid adsorbing on Au surface,accelerating the FOR kinetics through electrochemical in-situ IR spectroscopy.Furtherly,octahedral Pt-Pd-Ni nanocages was synthesized through multi-step synthetic method.The phenomenon of the diffusion and dissolution of inter Ni atoms at Pt-Ni octahedra could be eliminated in the nanocage structure due to its hollow nature.The ORR mass activity and specific activity of octahedral Pt-Pd-Ni nanocages achieved to 10-fold and 25-fold enhancement,respectively,compared to commercial Pt/C.The ligand effect and compressive strain,confirmed by the X-ray absorption fine structure（XAFS）analysis and density functional theory（DFT）calculations,accounted for the enhanced activity by weakening the adsorption energy of hydroxyl groups.Particularly,it showed robust stability with only 16%decay in the mass activity after accelerated degradation tests（ADTs）,rationalized by its structural stability and high vacancy formation energy.A surface atomically engineering strategy was employed to tune the near surface structure of shape-selective Pt-based nanocatalysts(octahedral Pt_1.5Ni)by modulating the composition of near-surface of catlaysts.Engineered nanocatalyst exhibited an ultrathin Pt-rich shell（～2 atomic layers）coated on the impervious interior.The optimized octahedral nanocatalyst achieves superior specific and mass activity(7.7 m A cm_Pt^-2and 1.9 A mg^-1 _Pt at 0.9 V vs.RHE)for ORR,～20 and～10times higher than commercial Pt/C.The remarkable activity was mainly due to the strengthened ligand and strain effects arising from the near surface engineering,which were carefully elucidated by the combination of atomic-resolution HAADF-STEM,X-ray absorption fine structure（XAFS）,synchrotron-based X-ray photoelectron spectroscopy（XPS）and density functional theory（DFT）calculations.More importantly,this catalyst showed a robust stability after accelerated degradation testing because its compositional nature prevents surface Pt atoms and interior Ni atoms from diffusion and dissolution.Lastly,one-dimensional Pt nanowires with modulated structural disorder were fabricated by the efficient dealloying of Pt Ni NWs in an oxygen saturated solution,denoted as D-O₂-Pt NWs.The D-O₂-Pt NWs with the similar electronic and crystal structure to the Pt NWs,achieved superior mass and specific activity(0.86 A mg_Pt^-1and 0.99 m A cm_Pt^-2 at 0.9 V vs.RHE)for ORR,～5 and～5.5 times higher than commercial Pt/C by RDE.Importantly,a real H₂/air PEMFC using D-O₂-Pt NWs at the cathode showed 30%and 50%enhancement in the peak power density and polarization current density at 0.5 V,respectively,compared to Pt/C.The remarkable activity of D-O₂-Pt NWs was due to the structural effects,carefully elucidated by the combination of X-ray absorption fine structure（XAFS）,high resolution–transmission electron microscopy（HR-TEM）and average adsorbed CO oxidation potential(μ₁^CO).In addition,in contrast to the 60%activity loss of Pt/C after 50 000potential cycles of accelerated degradation tests,D-O₂-Pt NWs showed an extremely stable behavior with only 17%performance loss(0.67 A mg_Pt^-1).