Bipolar Electrochemical Preparation Of Gradient Ni-Cu Alloys As Electrocatalyst For Hydrogen Evolution

The development of modern, industrial society causes more and more demand for new energy. Hydrogen energy as an efficient, clean, secondary energy needs to be developed. Water electrolysis is one of the best pathways for the hydrogen production.The biggest problem in electrolytic hydrogen production is the excessive energy consumption. More and more efforts have been directed to the study of high performance non-noble metal atalysts to decrease the overpotential required for hydrogen evolution reaction (HER). In this thesis, we prepared the copper-nickel alloys with various atomic ratios (NixCuy) on a metal substrate by using normal electrochemical method and the bipolar electrochemical method, respectively. The realstionship between the catalytic performance and the alloy composition was inverstigated.The nickel-copper nanoalloys with tunable composition and morphology were prepared by galvanostatic deposition on copper substrate. Both the composition and morphology of the NixCuy nanoalloys are highly dependent on the applied current density. The atomic ratio of Ni to Cu in the alloys changes from 1:9 to 3:1 with the increase of current density from 10 to 100 mA cm-2. The difference in electrocatalytic activity among these nanoalloys was evaluated through the HER in 1.0 M H2SO4 and 1.0 M KOH. The composition-dependence of the electrocatalytic activity of the alloys is more pronounced in 1.0 M H2SO4 than in 1.0 M KOH. By tuning the composition of NixCuy alloys, 13.5 and 5.7 times increase in exchange current density of the HER was achieved in 1.0 M H2SO4 and 1.0 M KOH, respectively. Meanwhile, 4.5 and 2.0 times decrease in charge transfer resistance was observed in the same two media. The best electrocatalytic activity to the HER was always achieved on the nanoalloy with a 1:1 atom ratio and a single crystal (111) plane. This favorable nanoalloy is composed of four-level dendritic nanochains. The results demonstrate that galvanostatic method can tune not only the composition but also the morphology of nanoalloys, both being important for nanoscale design of industrial electrocatalysts.We achieve the synthesis of gradient Ni-Cu alloys on copper substrate using bipolar electrodeposition, a technique based on the existence of a potential gradient at the interface of a bipolar electrode (BPE) and an electrolytic solution. The interfacial potential gradient causes the rates of electrodeposition of Ni and Cu to vary along the length of the BPE, leading to the electrodeposition of a chemical concentration gradient. The surface morphology of the electrodeposits was characterized using scanning electron microscopy (SEM), and their chemical composition was determined using energy dispersive X-ray spectroscopy (EDX). With the decrease of the potential difference, the percentage of Cu atoms is increased from 33 to 99. By tuning the composition of NixCuy alloys, 18.3 times increase in exchange current density of the HER was achieved in 1.0 M H2SO4, respectively. Meanwhile, 6.3 times decrease in charge transfer resistance was observed in the same media. It can be seen from SEM that the equiatomic NiCu alloy is also composed of dendritic nanochains. It is proved that the gradient alloys obtained by bipolar electrochemical method show a series of morphologies corresponding to the compositions of nano-alloys.In summary,the NiCu alloy in 1:1 atomic ratio is a high-performance hydrogen evolution electrocatalyst. Its catalytic activity can be attributed mostly to the intrinsic property of the material. The bipolar electrochemical technology can be used for the efficient preparation of gradient alloys, thus providing a new rapid screening method of high efficiency catalysts.