Synthesis Of Highly Active Nickel-Based Hydrogen Evolution Electrodes And Their Hydrogen Evolution Bebavior In The Alkaline Medium

With the decrease of energy storage and the aggravation ofenvironmental pollution, much attention has been paid to thedevelopment of various new energy materials or renewable energyresource by many countries in recent years. As a secondary energy,hydrogen is considered as the most ideal energy carrier for its clean, highefficiency, storability and transportability. Among all of the approachesfor the synthesis of hydrogen, water electrolysis has been widely usedbecause of its environmental friendly, high efficitncy and considerableproduct purity. However, for this approach, numerous electric energy wasdemanded in the electrolysis process. It has become a common opinionthat reducing the hydrogen evolution over-potential during waterelectrolysis could reduce the energy consumption. Since the1970s, theinvestigation of various nickel-based hydrogen evolution electrodes withhigh activity provided broad prospects for the reducing of theover-potential for the hydrogen evolution process. nickel-based hydrogenevolution electrodes mainly have four major categories: nickel-based alloys, active porous nickel, nickel-based noble metal oxide electrode,and nickel-based dispersed composite electrode. But, the detailed study ofthe crystalline structure of nickel-based hydrogen evolution electrodesand the relationship between their hydrogen evolution behaviors are stilldevoid. For the preparation of porous nickel-based hydrogen evolutionelectrodes, few methods were reported to effectively control the porousstructure on the electrode surface. Besides, it is difficult to furtherimprove the hydrogen evolution reaction of the nickel-based noble metaloxide. Therefore, it is necessary to develop highly active nickel-basedelectrode for hydrogen evolution reaction.Based on the background mentioned above, herein, three main researchaspects were carried out.（1） The crystalline structure of Ni-S, Ni-Snbinary alloy, was detailedly investigated as well as its correspondinghydrogen evolution performance and mechanisms;（2） Active porousnickel electrodes with tunable pore size and density were successfullyobtained and also the main rules for the controllable preparation wereproposed.（3） Facile modification was made to further enhance theelectro-catalytic activity for the hydrogen evolution of the commercialnickel mesh-based noble metal oxide electrodes. In this article, we havesuccessfully prepared various hydrogen evolution electrodes including:Ni-S, Ni-Sn, porous nickel, and nickel-based noble metal oxide through electrodepositon method and thermal annealing method. The X-raydiffraction （XRD）, scanning electron microscopy （SEM）, energydispersive spectroscopy （EDS）, high-resolution transmission electronmicroscopy （HRTEM）, and X-ray photoelectron spectroscopy （XPS）were taken to characterize the morphologies and structures of thehydrogen evolution electrodes. Besides, the electrochemcialperformances of the as-prepared electrodes with typical structures andmorphologies for the hydrogen evolution reaction were detailedlyinvestigated by cathode polarization, electrochemical impadancespectroscopy, and cyclic voltammetry. Accordingly, we proposed theinternal relationship between the electrode structure and its hydrogenevolution behavior. These results are summarized as follows:（1）The Ni-S alloy with two kinds of structures, including: amorphous/Ni₃S₂mixed crystals, and Ni₃Sn₂were successfully prepared bygalvanostatic electrodeposition respectively. By using sulfo salicylicacid as surfactant, the residual stress could be effectively eliminatedduring the electro-deposition process. thus the micron-cracks on thedeposited electrode disappered The relationships between the crystalstructures of the obtained electrodes and their hydrogen evolutionbehaviors were studied. It is concluded that the presence of Ni₃S₂is themain factor which influenced the electrocatalytic hydrogen evolutionactivity. The mechanism of the hydrogen evolution process on the nickel-sulfur alloy electrode were controlled by Volmer, Heyrovsky,and diffusion processes In these processes, charge transfer is the ratecontrol step of the entire hydrogen evolution reaction. Besides, themetallic compound of nickel-sulfur alloy-Ni₃S₂performed the mostfast charge transfer process.（2）The nickel-tin alloy electrodes with three kinds of structures:amorphous, Ni₃Sn₂/Ni₃Sn₄mixed crystals, and Sn/Ni₃Sn₄wereprepared by galvanostatic electrodeposition. It is found that the crystalstructure of the amorphous alloy is constituted with nickel embryos andamorphous nickel-tin. The mixed crystals of Ni₃Sn₂/Ni₃Sn₄showedlayered self-assembly arrangement. By establishing the relationshipbetween the crystal structures and their hydrogen evolutionperformance, it showed that the amorphous Ni-Sn alloys has betterelectrocatalytic activity than pure nickel, the overpotential of hydrogenevolution reaction were reduced by nearly200mV. Its good activitywas attributed to the alloying of Ni and Sn, This effect coud decreasethe bonding ability between the d-orbit electrons in Ni and active Hatoms thus weakening the bond energy of the metal atom and H atom（M-H）. The desorption of active H atoms was enhanced and thehydrogen evolution was improved. The relationship between thecrystal structures and their hydrogen evolution mechanisms fo thenickel-tin alloys was detailedly discussed and it is found that the reaction procedure is controlled by two charge transfer processes ofVolmer and Heyrovsky. Besides, the non-faraday diffusion was alsoobserved. At the same polarization potential, the amorphous nickel-tinelectrode performed a much higher activity than that of the Ni₃Sn₄andNi₃Sn₂mixed electrode. This phenomenon is owing to the faster chargetransfer speed and faster diffusion rate.（3）Porous nickel hydrogen evolution electrode were obtained by aemplate-assisted composite electrodeposition method using modifiedpolystyrene （PS） microsphere as the template. The surface structuresuch as porous size and density of the electrode could be facilecontrolled by changing the content or size of the PS microspheres inthe plating bath as well as the depositing current density. The porosityof the prepared porous nickel electrode can reach10⁸/cm². From thepolarization curves of the prepared electrodes carried out in1M NaOHsolution, it is considered that with the increasing of the porous density,the electrocatalytic activity gradually increased. The apparentactivation energy of the porous nickel electrode in hydrogen evolutionreaction can reach to17.26kJ·mol^-1. The mechanism for the hydrogenevolution reaction of the porous nickle electrode was discussed and itis deduced that the process is controlled by both diffusion and chargetransfer. The excellent hydrogen evolution performance is attributed tothe large electrochemical specific surface area. （4）Nickel foam based noble metal oxides hydrogen evolution electrodewas prepared by thermal annealing using high surface area nickel foamas the conductive substrate. From the crystal structure and elementalanalysis, it showed that the active layer of the noble metal oxideelectrode was constituted by RuO₂, NiO, and CeO₂. The content ofRuO₂in the active layer was26.5g/m². Compared with the commercialelectrode, the hydrogen evolution over-potential of the obtained nickelfoam based noble oxides electrode is decreased by more than200mV,which showed potential prospects for industrial application. The asprepared nickel foam based precious oxides electrode performed a fastcharge transfer process during hydrogen evolution reaction. This ismainly owing to its high electrochemical active surface area whichcould provide more active reaction units. The hydrogen evolutionprocess is also controlled by two charge transfer processes of Volmerand Heyrovsky and a nonfaradaic diffusion process.