Structural Regulation Of NiCo Hierarchical Alloy Doped By Group-VIB Elements And Their Compounds And Studies On Electrocatalytic Performance

Water splitting has become an important approach to get access to hydrogen（H₂）, which is regarded as an ideal alternative to energy carrier for future energy infrastructure with dense energy property of 140 MJ/kg,higher than petrol（44 MJ/kg）and coal（24MJ/kg）.However,conducting the half reaction of hydrogen evolution（HER）demands a dynamic overpotential and excessive energy consumption due to the sluggish proton-coupled electron transfer.First row（3d）transition metals,including Ni,Co,and Fe based alloys,are particularly promising HER catalysts since they can accelerate the reaction rate,reduce the overpotential and the energy loss efficiently.In the rate-determining step of HER,binding the reaction intermediates(H_adsspecies)to the active sites is critical,and the Gibbs energy(（35）G_H*)could be modulated by incorporating metal element which in turn regulates the HER performance.Currently,doping Mo（non-3d high-valence metal）into inert materials has attracted substantial interest due to its unique“noble-metal-like”surface chemistry.The outstanding intrinsic activity originated from the unique filled states of Mo 4d orbital and strong electron-donating ability.Chromium（Cr）,molybdenum（Mo）,and tungsten（W）elements belong to the same group（VIB）,they may exhibit similar e_g orbitals which could bind with the surface-atom adsorbates and affect the binding of reaction intermediates(metal-H_ads).But the different outermost orbital structures may make them exhibit different modulation effect.However,the series studies on the VIB group elements doping effect on HER performance of transition metal alloys are relative deficient.Based on the above issues,the main findings of this thesis are as follows:1.We present a one-step electrdeposition approach（constant current density mode）to fabricate the hierarchical W-NiCo alloy.PXRD results show that W atom crystallizes into fcc NiCo alloy lattices,inducing the cubic unit cell dimension to be 3.598（?）,which is0.95%higher than that of the reference（（?）=3.564（?））.The（111）crystal planes are preferentially oriented.The E_2g-like Raman frequency of W-NiCo shows an obvious blue shift,attributed to the slight change of the chemical bond caused by appropriate W doping into the lattice of NiCo.The contact angle（θ）of W-NiCo is measured to be close to 8.64（?）on average,which reveals complete wettability.Based on the law derived by Barton and Bockris,we found that the large current density(-0.6 A·cm^-2（?）-1.0 A·cm^-2)and strong electric feld applied to the cathode may promote the concentration polarization,which contributes to a much smaller induction time（τ）of protrusion formation of W-NiCo,reduces the thickness of diffusion layer,and facilitates the formation of hierarchical structure.2.The ternary NiCoMo catalyst was readily obtained by a one-step electrodeposition process of Ni,Co,and Mo precursors on titanium（Ti）support via a constant current mode.By tailing the deposition conditions,we fabricated NiCoMo catalysts with three-dimensional dendritic structures,exhibiting large amounts of electrochemically active sites.The NiCo catalyst was doped by non 3d metals molybdenum（Mo）for improvement in catalyzing the hydrogen evolution reaction（HER）.To attain the benchmark HER current density of-10 m A·cm^-2,an overpotential of～132 m V is required in 0.1 M KOH for the NiCoMo,and they produced the decreasing in Tafel slope of～108 m V·decade^-1 exceeding those of binary NiCo alloy catalysts and other contents of Mo doping.In a synergistic effect,dopant incorporation of Mo element may provide near-optimal adsorption energies for HER intermediates,promoting the process of water dissociation and hydrogen intermediates production and binding into molecular hydrogen.3.We present a facile one-step electrodeposition approach to incorporate fcc NiCo alloy with VIB group transition metals（M-NiCo,M=chromium（Cr）,molybdenum（Mo）,or tungsten（W））systematically.Herein,W-NiCo holey nanotower arrays show tremendously enhanced HER activity compared with pristine NiCo alloy,which provides an overpotential of 109.2 m V（reduced by 58.7%）to achieve a current density of-10 m A·cm^-2,with a Tafel slope of 110.3 m V·dec^-1 in alkaline medium.The same law applies to the acid electrolyte with an overpotential of 78.5 m V（reduced by 66.1%）and low Tafel slop of 56.6 m V·dec^-1.Based on the experimental and theory analysis,We find W-doping can promote more active sites exposed,tune the electron configuration,moderate the gibbs free energy of the H intermediate,and accelerate the desorption of H₂.It makes the rate-determining step change from Volmer to Heyrovsky reaction and finally surmount the limit of sluggish dynamics.4.We present the transformation of beta molybdenum carbide（（?）-Mo₂C）to alpha molybdenum carbide（（?）-Mo C）using a near room temperature solvent treatment followed by mild thermal annealing.The choice of solvent（ethanol）and the size of the（?）-Mo₂C nanoparticles（sub-3nm NPs）proved critical towards the complete transition from the hcp phase to the fcc phase.The resultingα-Mo C nanocrystals exhibited a 20-fold enhancement in mass activity over（?）-Mo₂C towards the electrochemical oxidation of formic acid（FAOR）.Cyclic voltammetry（CV）curves reveal thatα-Mo C/C reached a specific area activity of 13 m A/cm² and a specific mass activity of 390 m A/mg Mo.Conversely,（?）-Mo₂C/C had values of 1.1 m A/cm² and 20 m A/mg Mo.Moreover,the electrocatalytic activity for FAOR was shown to be directly related to the relative abundance ofα-Mo C.This study provides a new approach toward the complete phase transition of nanostructured TMCs under mild conditions which has relevance in various applications such as catalysis.