Electrocatalytic Oxidation Of Methanol On Amorphous Ni-B-M Nanoparticles In Alkaline Media

The alkaline direct methanol fuel cell（AMDMFC）is an ideal mobile and portable power source,which is of great significance to the national new energy strategy.In view of the problem of low activity,low stability and high cost of precious metal Pt–based electrocatalytic materials for the methanol oxidation,three kinds of elements Co,Cu and Cr with different d electron,together with Ni in the fourth period,are used to adjust nano–alloy electronic state.In this paper a series of amorphous Ni–B–M（Co,Cu,Cr）nanoparticles were prepared by chemical reduction to investigate the possible influencing factors and action mechanism of different electronic structural elements M toward the methanol electrocatalytic oxidation on Ni–B–M nanoparticles.At the same time the effect of different M content toward methanol electrocatalytic oxidation also was studied.Our research will have theoretical significance concerning the methanol electro–oxidation on amorphous Ni–B particles in an alkaline media.The results are as follows:（1）Five amorphous Ni–B nanoparticle samples with different molar ratios of B and Ni precursors were prepared.The TEM images show that as the molar ratio of B/Ni precursors increased,the particle sizes of Ni–B nanoparticles decreased from 50 nm to 15 nm,but the B/Ni atomic ratio did not change significantly.XPS results indicate that the binding energy（BE）of B 1s in Ni–B nanoparticles significantly increased comparing with the B⁰ standard BE,implying that electrons of B transfered to Ni.The electrochemical results show that the other four samples are slightly different except that the methanol elctro–oxidation peak current was relatively low on Ni–B-2 sample,and the particle size reduction was beneficial to the catalytic activity.Among them,the B/Ni molar ratio is 4,that is,the activity of Ni–B-4 is relatively highest.（2）Four amorphous Ni–B–Co_x nanoparticles of different Co doping contents were prepared.TEM images indicate that the particle size was about 15nm.XPS results show that the BE of Ni and B decreased as the Co content increased,and the negative charges of Ni atom slightly increased with respect to Ni–B;DSC results reveal that the crystallization temperatures of Ni–B and Ni–B–Co nanoparticles increased as the Co contents increased.The electrochemical results show that the methanol electro–oxidation peak current initially increased and decreased as the Co content increased.The electrocatalytic activity of Ni–B–Co_0.05 nanoparticles increased by 51.50%comparing with Ni–B nanoparticles,but the long–term poisoning rate of Ni–B–Co_x nanoparticles slightly increased,which was not conducive to the improvement of activity stability.The reaction order of methanol is about 0.5,which initially increased and decreased as the Co content increased;the reaction order of hydroxide ions was close to 1,and Co doping reduced the reaction order of hydroxide ions.Based on overall experimental results,Ni is the main catalytic active material,and Co has auxiliary catalysis.It is mainly embodied in two aspects.One is the electron effect,the 3d electron transfer from Ni to Co increased the vacancy of Ni3d bands,which can enhance the adsorption ability of Ni 3d orbits toward the methanol and intermediates.This maybe accelerate the rate of methanol dehydrogenation.The second is to stimulate the formation of Ni OOH actives at low potential.In addition,it is proposed a formation of electrochemical catalysis layer on the surface of amorphous Ni–B and Ni–B–Co nanoparticles that acted as an interface between the bulk Ni–B（or Ni–B–Co）nanoparticles and the solution.（3）Amorphous Ni–B–Cu_x nanoparticles with different Cu doping contents were prepared.TEM images show that the particle size of Ni–B–Cu_0.02nanoparticles was about 8 nm,the particle size of Ni–B–Cu_0.05 nanoparticles was about 6 nm,and Cu doping reduced the Ni–B–Cu nanoparticles.XPS results show that the BE of Ni and B reduced as the Cu content increased,and the d electron of Cu transfered to Ni.The negative charge of Ni slightly increased relative to Ni–B.The electrochemical performance results show that the methanol electro–oxidation peak current on Ni–B–Cu_x nanoparticles initially increased and decreased as the Co content increased.The electrocatalytic activity of Ni–B–Cu_0.02 was the highest,which was about 45.09%higher than Ni–B.But when the Cu content was high（≥0.02）,the poisoning rate of Ni–B–Cu_xnanoparticles increased.The reaction order of both methanol and hydroxide ions initially increased and decreased as the Co content increased.According to theoretical analysis,the influence factor of Cu toward the catalytic activity of Ni–B–Cu_x nanoparticles are mainly reflected in three aspects.One is the electronic effect,that is,the transfer of Cu 3d electrons to the Ni 3d band will weaken the adsorption ability of Ni 3d orbits toward the methanol and intermediates.The second is that Cu can promote the generation of Ni OOH.Thirdly,Cu（III）can act as a new catalytic active site to adsorb,dehydrogenate,and oxidize intermediates together with the Ni active site.（4）Ni–B–Cr_x nanoparticles with different Cr doping were prepared.TEM results show that the particle size of Ni–B–Cr significantly reduced as the Cr content increased,even up to about 5nm.XPS results show that the BE of B1s of Ni–B–Cr_x nanoparticles was consistent with that of Ni–B nanoparticles.The BE of Ni gradually decreased as the Cr content increased,and the negative charge of Ni atoms slightly increased comparing with Ni–B nanoparticles.According to the results of electrochemical experiments,the methanol electro–oxidation peak current on the Ni–B–Cr_x nanoparticles with low Cr content was comparable to that of Ni–B nanoparticles.But the peak current of methanol electro–oxidation on Ni–B–Cr_0.05 nanoparticles with high Cr content significantly reduced,and Cr doping reduced the electrocatalytic activity of Ni–B–Cr_x nanoparticles,however,improved the activity stability of Ni–B–Cr_x nanoparticles.The influence of Cr element toward the electrocatalytic activity of Ni–B nanoparticles is mainly reflected in three aspects:the excitation effect of Ni OOH active species formation at low potential,chromate passivation,and electron effect.（5）Comparing the electron effects of Co,Cu and Cr doping shows such a rule:when d electrons of Ni is slightly lost,which would enhance the adsorption ability of Ni 3d orbital toward methanol and intermediates,and improve the adsorption and dehydrogenation of methanol,thus enhance the catalytic activity of methanol oxidation.However,when d electrons of Ni lose too much,the adsorption ability of Ni 3d orbital toward methanol and intermediates is too large,which is not conducive to the oxidation of methanol.When the Ni 3d orbital acquirs electrons,the adsorption ability of Ni 3d orbital toward methanol and intermediates would reduce,which is detrimental to the adsorption and dehydrogenation of methanol.