Mechanism Study For Hydrogen Oxsidation Reaction On Ni-based Catalysts In Alkaline Media

Ni-based electrocatalysts are prospective non-platinum-group hydrogen oxidation catalysts in the future.However,how Ni-based catalysts enhancing the hydrogen oxidation reaction(HOR)activity is still in debate because of the insufficient knowledge about mechanism.Therefore,by employing the recent developed constant potential density functional theory(DFT)calculation methods with implicit solvation model and explicit water model,combined with micro-kinetic modeling,a complete kinetics simulation was carried out to clarify the mechanism of how HOR activity was enhanced by Ni-based alloy.First,the study for HOR mechanism on monometallic Ni was carried out.Polarization curves of alkaline HOR on Ni catalyst was simulated in consideration of all possible reaction step,and it agrees well with experimental observations.Tafel-Volmer path was identified the preponderant path in HOR.The rate determined step is Tafel step in lower overpotential and Volmer step in higher overpotential.The adsorptive oxyhydrogen OH* was found not participate in HOR.Based on the conclusions of study on monometallic Ni,the alkaline HOR activities of a series of Ni-based catalysts whose active site is Ni was investigated.The results showed that Ni Cu,Ni Mo,Ni W and Ru Ni alloy can not reach high HOR performance as experimental observations.Besides,the summarized linear relation between hydrogen binding free energy and Tafel,Volmer activation barriers shows that the HOR activity supremum of Ni-based alloy is as same order of magnitude as monometallic Ni,which proves the infeasibility of enhancing HOR activity by using alloying effect to optimize hydrogen binding strength of Ni site.Furthermore,the research on Ru Ni alloy also discovered Tafel-Volmer HOR mechanism on new reaction site.Higher activity is due to the lower Tafel and Volmer activation barriers,but adsorptive oxyhydrogen OH* has not participate into HOR.In general,the mechanism of HOR on Ni and Ni-based catalysts has been clarified.The high HOR performance is mainly due to the optimization to the relation between hydrogen binding energy to Tafel barrier which from new reaction site induced alloying metal,but not hydrogen binding energy adjustment of Ni site by alloying effect.And adsorptive oxyhydrogen OH*was found absent in alkaline HOR on Ni and Ni-based catalysts which means oxyhydrogen binding energy(OHBE)is not a discripter in this case.