Preparation Of Platinum-based Alloys And Study On Their Hydrogen Electrode Kinetics

Hydrogen energy,as renewable energy sources,is the most likely fuel to solve global warming and energy security.The hydrogen oxidation reaction(HOR)and the hydrogen evolution reaction(HER)will inevitably reign over the hydrogen economy.The HOR primarily finds application in Polymer electrolyte membrane fuel cell(PEMFC),while the HER features in various electrolysers such as water splitting.Many materials are more stable in alkaline compared to in acid.Unfortunately,the HER/HOR kinetics on Pt is much slower in alkaline than in acid electrolytes.Although previous studies,mainly focusing on the various effects of Pt-Had in combination with HER/HOR activity,have helped to improve alkaline HER/HOR activities of pure Pt,several key questions remain unanswered in the bimetallic system.First,the controversy between hydrogen b:inding energy(HBE)and bifunctional mechanism:these two lines of research-studying the strength of the bond between adsorbed hydrogen(Had)and the metal to pinpoint the influence of electronic effect,and comparing Pt with more oxophilic metal or alloy that combination of Pt with more oxophilic components to measure the role of the surface hydroxyl species(OHad)-have been crucial to understanding the reasons for the poor HER/HOR activity of Pt in alkaline electrolytes.More importantly,HER/HOR are generally considered as a reaction determined by the Volmer step on Pt with the value of symmetry factor is 0.5 in HER/HOR in alkaline media.The opinion that HER and HOR on the Pt-based surface have the same active sites,which is available on Pt in acid media due to their very fast kinetic and high reversibility,should be strongly doubted in alkaline media where the kinetic of HER/HOR decrease 2 to 3 orders of magnitude.Notably,the addition of foreign metals may also influence the reversibility of HER/HOR.In view of the above problems,the main research contents of this paper are as follows:(1)We have used a rational design approach to isolate the contributions from Had and OHad for the HER/HOR in alkaline environments,by successfully transferring the knowledge gained from polycrystalline Pt surfaces to Pt-based nanoparticles catalysts.First,by using bulk Pt decorating with different coverage Ni(OH)2 as model catalysts and the subsequence HER/HOR activity trends,we found the rate of the HER is controlled by both theHad and OHad while HOR is solely determined by Had.And theobserved enhancement HOR activity should be attributed to the tuning Pt-Had.rather than the bifunctional mechanism.A further and deeper understanding of the catalyst surface was received by in situ FTIR spectroscopy using CO as a probe molecule,showing a correlation between substrate OHad energetics and HER activity,together with a correlation between substrate H2/Had and HOR activity.Finally,the causes of this imbalance were determined by Zeta potential and DFT calculations,and the reaction mechanism was proposed.(2)To explain the orders of magnitude difference in activity on going from acid to base,wesynthesized a electronic effect and ligand effect co-engineered Pt-Ni NPs through modifying a previous method.The compositions of Pt-Ni NPs were readily tuned by controlling the amount of Ni(acac)2 supplied,then we used acid to remove surface Ni.We evaluated their HER performance in 0.1 M HClO4 and 0.1 M NaOH.Besides the practical significance of the PtNi1.5 activities being higher than other compositions of Pt-Ni NPs and Acid-Pt-Ni NPs,the PtNi1.5 also decreased the HER activity difference between acid and base from 2?3 orders to 2.5 times.From acid to alkaline,the activation free energies increase the most for the Volmer reaction,resulting in a switch of the rate-determining step from the Heyrovsky to the Volmer reaction,and a shift to a weaker optimal hydrogen binding energy.The much higher activation barrier for the Volmer reaction in base than in acid is ascribed as follows:slower transport of OH-than H+ in water and a stronger Pt-Had bond in water molecules(HO-H)than in hydrated protons(H2O-H+),which has been confirmed by in situ Raman spectroscopy and in situ FTIR spectroscopy.