Mechanism investigation of α-Ni(OH)2 electrodeposition from a NiCl2 solution

https://doi.org/10.1016/j.ijhydene.2020.09.253Get rights and content

Highlights

  • NiCl2 solution was used for α-Ni(OH)2 electrodeposition.

  • The mechanism of by-product formation and method to prevent were studied.

  • The mechanism of β-Ni(OH)2 formation and method to prevent were analyzed.

Abstract

α-Ni(OH)2 was electrodeposited from NiCl2 solution by membrane electrolysis method. Various ethanol contents and concentration electrolytes were used to investigate the relevant mechanism. XRD and CV results demonstrated a preferentially conducted side reaction of Ni2+ to Ni. Ethanol was proved to be effective to poison this reaction. Linear relationships of the product mass and electrodeposition time for electrodepositions in 0.1 M NiCl2 solutions indicated an ignored output of Ni. A four-step mechanism for Ni(OH)2 electrodeposition from the side reaction priority system was proposed. XRD and FT-IR results suggested β-Ni(OH)2 and α-Ni(OH)2 were certain to be synthesized in 0.05–0.1 and 0.2–1.0 M NiCl2 solution. β-Ni(OH)2 formed in these low concentration electrolytes was proved to be prevented by circulating the cathodic electrolyte. Analyses suggested β-Ni(OH)2 formed in the present cases might be either converted from α-Ni(OH)2 under high alkaline digestion or synthesized originally due to the proper ratio of Ni2+ and OH.

Introduction

Due to its excellent electrochemical properties [1], nickel hydroxide is widely used in Ni-based alkaline rechargeable batteries (Ni-MH, Ni–Fe, Ni–Zn, and Ni–Cd) [[2], [3], [4]], catalyst for hydrogen evolution reaction [5,6], supercapacitors [7], and electro-chromic devices [8]. In particular, it is worthwhile to investigate nickel hydroxide as the active material of Ni-based batteries because electric vehicles are becoming increasingly popular and the widely recognized development models are environmentally friendly.

Nickel hydroxide can adopt two different crystal forms, β-Ni(OH)2 or α-Ni(OH)2 [[9], [10], [11]]. Currently, β-Ni(OH)2 finds widespread use in commercial nickel-based batteries [[12], [13], [14]]. It can be reversibly oxidized to β-NiOOH by exchanging a single electron for each Ni atom [1]. β-NiOOH can be irreversibly over-oxidized to γ-NiOOH, which shares a larger interspace [15]. A volume expansion as well as a loose contact between the active material and the current collector can be resulted in this process [[16], [17], [18]]. α-Ni(OH)2 can be directly oxidized to γ-NiOOH in a reversible route [13,19]. A greater number of electrons are transferred for each Ni atom in this process. The theoretical capacity of α-Ni(OH)2/γ-NiOOH pair is therefore higher than that of β-Ni(OH)2/β-NiOOH pair [19,20]. In addition, no evident volume swelling is detected during the oxidizing process of α-Ni(OH)2, due to the similar lattice parameters of α-Ni(OH)2 and γ-NiOOH [15,20]. Thus, investigating α-Ni(OH)2 is more practically relevant. Unfortunately, α-Ni(OH)2 is metastable in strongly basic solutions and can convert into β-Ni(OH)2 gradually [21]. It complicates its preparation [22].

Numerous literatures have been devoted to prepare α-Ni(OH)2 by various methods [4,19,23]. Electrodeposition is a relatively simple and economical method, and the resulting α-Ni(OH)2 shows high power density and high volumetric energy density [[24], [25], [26]]. This method is therefore often used to prepare α-Ni(OH)2, most commonly from a Ni(NO3)2 solution [27]. In a typical electrodeposition process from a Ni(NO3)2 solution, the main cathodic reaction is the reduction of NO3 to NH3 and OH (Eq. (1)); and NH3 further reacts with water to form NH4+ and OH (Eq. (2)) [28,29]. OH ions generated from Eqs. (1), (2)) react with Ni2+ to form a Ni(OH)2 precipitate (Eq. (3)).NO3+6H2O+8eNH3+9OHNH3+H2ONH4++OHNi2++2OHNi(OH)2

According to Eqs. (1), (2)), the ultimate stoichiometry of the electrons and OH ions is 1:1.25 (8: 10). The high OH yield improves the power utilization and is good for energy conservation. However, due to the high rate of OH formation, the pH near the electrode is maintained at a high level, which is beneficial for synthesizing β-Ni(OH)2. The formed ammonia degrades the properties of the electrolyte, thus having a detrimental effect on the subsequent electrodeposition; and the NO3 ions intercalated in the active materials increase the self-discharge effect of the batteries [30]. Considering the existing problems, α-Ni(OH)2 electrodeposition from a NiCl2 solution is proposed in the present work. The reactions involved in a NiCl2 bath can be described as follows.

At the anode, Cl ions are oxidized at the inert matrix surface (Eq. (4)):2Cl- → Cl2 + 2e, E0 = 1.358 V vs. SHE

At the cathode, H2O molecules are reduced to hydrogen and OH (Eq. (5)):2H2O + 2e → H2 + 2OH, E0 = −0.059 pH V vs. SHE

Then, OH ions assembled on the cathode surface react with Ni2+ to form a Ni(OH)2 precipitate (Eq. (3)).

The following side reaction may be related at the cathode:Ni2+ + 2e → Ni, E0 = −0.257 V vs. SHE

H2 and Cl2 produced in the NiCl2 electrolysis process can be used to synthesize hydrochloric acid and reused in the NiCl2 preparation plant [31], ensuring an environmentally friendly procedure for α-Ni(OH)2 preparation. The ratio of electrons and OH ions is 1:1, which is smaller than that in the Ni(NO3)2 bath. The Cl ions intercalated in α-Ni(OH)2 can keep electrochemically stable during oxidizing/reducing process (ECl/Cl20=1.358Vvs.SHE). The strongly oxidizing Cl2 and ClO in the anodic electrolyte are prevented to access into the cathodic electrolyte by the cation exchange membranes (CEM) between chambers.

Currently, there are few studies about α-Ni(OH)2 electrodeposition from a NiCl2 solution. In 2019, Yao et al. demonstrated they electrodeposited α-Ni(OH)2 using a pure water solution of NiCl2 for the first time; and they reported that metal can be deposited when the electrode potential of Eq. (6) is higher than that of Eq. (5) [32]. Beyond that word, they offered no explanation of the relevant mechanism. A comprehensive understanding of α-Ni(OH)2 electrodeposition from a NiCl2 solution is therefore still not available. However, in the membrane electrolysis process of NiCl2, the slight separation of the standard potential for H2O to H2+2OH and Ni2+ to Ni (Eqs. (5), (6))) may contribute to a metallic by-product; and both crystallographic forms of Ni(OH)2 may be electrodeposited. If the undesired results do take place in the experiment, and they are not disposed of properly, the experiment will serve no practical relevance. The present work aims at offering an insight into α-Ni(OH)2 electrodeposition from a NiCl2 solution. Thus, the mechanism of the by-product formation as well as the method to prevent and the mechanism of β-Ni(OH)2 formation as well as the method to prevent were investigated.

Section snippets

Reagents and electrolyte system preparation

NiCl2·6H2O (analytical grade, ≥ 98%) and a mixture of ethanol and water at various ratios (ethanol: analytical grade, ≥ 99.7%) acted as the solute and the solvent. A typical experiment set-up (Fig. 1) was consisted of a DC power supply (L3055PL, eTM) and an electrolytic cell. In some electrodeposition processes of the present work, a digital multimeter (Agilent, 34470 A) was used to record the cathode potential vs. Ag/AgCl every 0.5 s. The electrolytic cell was assembled with two anode sheets

By-product formation and the method to prevent

From the image of the cathode sheet obtained under 1.0 h of electrodeposition in the pure water solution of 0.1 M NiCl2 (Fig.2b), a green substance can be seen attach to the electrode. Washed away the green substance and dried the electrode thoroughly in oven, a new electrode sheet was obtained. To its surface, a thin film of black material is adhered (Fig. 2c). Elevating the electrolyte concentration to 0.5 M, more black material was produced (Fig. 2d). While, when ethanol and water was used

Conclusions

In the present work, α-Ni(OH)2 was attempted to synthesize from a NiCl2 solution. A series of ethanol content solutions of 0.1 M NiCl2 and various concentration electrolytes with a solvent containing 50% (v/v) ethanol were used to investigate the relevant electrodeposition mechanism. The results of XRD patterns and CVs demonstrated a side reaction of Ni2+ to metallic Ni. This reaction was found firstly conducted in NiCl2 electrolysis process. Ethanol was proved to be effective to poison this

Declaration of competing interest

The authors declare that they have no known competing financial interest or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This study was financially supported by the National Natural Science Foundation of China (No. U1710257).

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