Mechanism investigation of α-Ni(OH)2 electrodeposition from a NiCl2 solution
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)).
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 . 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).
References (42)
- et al.
Effect of cobalt electrodes deposition on nickel hydroxide electrodes
Int J Hydrogen Energy
(2014) - et al.
Physcio-chemical and electro-chemical properties of nickel hydroxide precipitated in the presence of metal additives
Hydrometallurgy
(2006) - et al.
Hybrid nickel-metal hydride/hydrogen battery
Int J Hydrogen Energ
(2019) - et al.
Y(OH)3-coated Ni(OH)2 tube as the positive-electrode materials of alkaline rechargeable batteries
J Power Sources
(2005) - et al.
Ni(OH)2/NiSe2 hybrid nanosheet arrays for enhanced alkaline hydrogen evolution reaction
Int J Hydrogen Energ
(2019) - et al.
Three-dimensional Ni(OH)2 nanoflakes/graphene/nickel foam electrode with high rate capability for supercapacitor applications
Int J Hydrogen Energ
(2014) - et al.
Effect of precipitating agents on the structural, morphological, and colorimetric characteristic of nickel hydroxide particles
Collid and Interf Sci Commun
(2018) - et al.
Structure and electrochemical performance of Cu singly doped and Cu/Al co-doped nano-nickel hydroxide
Trans Nonferrous Metals Soc China
(2013) - et al.
Effect of interlayer anions on the electrochemical performance of Al-substituted α-type nickel hydroxide electrodes
Int J Hydrogen Energy
(2010) - et al.
Enhanced cycling performance of Al-substituted α-nickel hydroxide by coating with β-nickel hydroxide
J Power Sources
(2013)
Synthesis, spectroscopic and electrochemical performance of pasted β-nickel hydroxide electrode in alkaline electrolyte
Spectrochim Acta Part A Molecular & Biomolecular Spectroscopy
High-rate discharge properties of nickel hydroxide/carbon composite as positive electrode for Ni/MH batteries
J Power Sources
Charge transfer resistance reduction by the interlayer distance expansion of Ni-Al layered double for nickel-metal hydride battery anode
Electrochim Acta
A comparative study of structural and electrochemical properties of high-density aluminum substituted α-nickel hydroxide containing different interlayer anions
J Power Sources
Synthesis and electrochemical performance of mixed phase α/β nickel hydroxide
J Power Sources
Characterization and supercapacitor application of coin-like β-nickel hydroxide nanoplates
Electrochim Acta
Characterization of a turbostratic α-nickel hydroxide quantitatively obtained from a NiSO4 solution
J Power Sources
An electrochemically impregnated sintered-nickel electrode
J Power Sources
Electrochemical studies on electrolytic preparation of battery grade nickel hydroxide-Effect of (OH-) to Ni2+ ratio
J Power Sources
Al-stabilized α-nickel hydroxide prepared by electrochemical impregnation
Mater Chem Phys
Effect of electrolytic conditions on the deposition of nickel hydroxide
Thin Solid Films
Cited by (12)
Simultaneous extraction and separation of Ni(OH)<inf>2</inf>, Ni powder and Ni plate from waste nickel-cobalt scrap in one spot: Control sequence of electrical reduction
2024, Journal of Environmental Chemical EngineeringAnisotropic In-Plane strain engineering Ni(OH)<inf>2</inf> to activate alkaline hydrogen evolution reaction
2023, Chemical Engineering JournalHydrolytic dehydrogenation of NH<inf>3</inf>BH<inf>3</inf> over Cu/CoO<inf>x</inf>(OH)<inf>y</inf> nanocomposite for H<inf>2</inf> evolution
2023, FuelCitation Excerpt :As demonstrated in Fig. 5a, the characteristic peaks of 781.18 eV & 796.48 eV and 779.58 eV & 794.78 eV had been perfectly attributed to Co (II) resp. Co(III) [73], further indicating that confirming. Co(OH)2 intermediate had subsequently converted into CoOx(OH)y colloidal in air.
Enhancing hydrogen evolution through urea electrolysis over Co-doped Ni-P-O film on nickel foam
2022, Journal of Alloys and CompoundsCitation Excerpt :respectively, reflecting the presence of α-Ni(OH)2 in the film[34]. It is the production of the reaction between Ni2+ in solution and OH- produced by violent hydrogen evolution during the electrodeposition process[35,36]. A similar phenomenon is observed in the electrodeposition of NiS/Ni2P[37] and Ni-S-P-O[7].
A nano-spherical structure Ni<inf>3</inf>S<inf>2</inf>/Ni(OH)<inf>2</inf> electrocatalyst prepared by one-step fast electrodeposition for efficient and durable water splitting
2022, International Journal of Hydrogen EnergyCitation Excerpt :The Ni2+ can easily combine with OH− which came from the reduce of H2O and S2O32− to form Ni(OH)2, that prevented it from being further reduced to Ni. So there almost no Ni metal formed [61]. Therefore, the synthesis mechanism of Ni3S2 and Ni(OH)2 can be expressed as follows [8,62,63]:S2O32−+3H2O+8e− → 6OH−+2S2-Ni2++S2− → Ni3S22H2O+2e− → 2OH−+ H2Ni2++OH− → Ni(OH)2