Biosensors and Bioelectronics 53 (2014) 428–439

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Electrocatalysis and electroanalysis of nickel, its oxides, hydroxides and oxyhydroxides toward small molecules Yuqing Miao n,1, Lei Ouyang 1, Shilin Zhou, Lina Xu, Zhuoyuan Yang, Mingshu Xiao, Ruizhuo Ouyang n University of Shanghai for Science and Technology, Shanghai 200093, China

art ic l e i nf o

a b s t r a c t

Article history: Received 15 August 2013 Received in revised form 4 October 2013 Accepted 7 October 2013 Available online 17 October 2013

The electrocatalysis toward small molecules, especially small organic compounds, is of importance in a variety of areas. Nickel based materials such as nickel, its oxides, hydroxides as well as oxyhydroxides exhibit excellent electrocatalysis performances toward many small molecules, which are widely used for fuel cells, energy storage, organic synthesis, wastewater treatment, and electrochemical sensors for pharmaceutical, medical, food or environmental analysis. Their electrocatalytic mechanisms are proposed from three aspects such as Ni(OH)2/NiOOH mediated electrolysis, direct electrocatalysis of Ni(OH)2 or NiOOH. Under exposure to air or aqueous solution, two distinct layers form on the Ni surface with a Ni hydroxide layer at the air–oxide interface and an oxide layer between the metal substrate and the outer hydroxide layer. The transformation from nickel or its oxides to hydroxides or oxyhydroxides could be further speeded up in the strong alkaline solution under the cyclic scanning at relatively high positive potential. The redox transition between Ni(OH)2 and NiOOH is also contributed to the electrocatalytic oxidation of Ni and its oxides toward small molecules in alkaline media. In addition, nickel based materials or nanomaterials, their preparations and applications are also overviewed here. & 2013 Elsevier B.V. All rights reserved.

Keywords: Nickel oxide Nickel hydroxide Nickel oxyhydroxides Electrocatalysis Electroanalysis

Contents 1. 2.

3.

4.

5.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 429 Preparation of nickel, its oxides and hydroxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 429 2.1. Electrochemical deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 429 2.2. Chemical methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 430 2.3. Physical methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 430 Electrocatalytic mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 430 3.1. Ni(OH)2/NiOOH mediated electrocatalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 430 3.2. Ni and NiO based electrocatalysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 431 3.3. Ni2 þ based electrocatalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 432 3.4. Direct electrocatalysis of Ni(OH)2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 432 3.5. Direct electrocatalysis of NiOOH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 432 Electrocatalysis toward small molecules and its applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 433 4.1. Fuel cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 433 4.1.1. Electrochemical oxidation of water or electrochemical evolution of oxygen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 433 4.1.2. Inhibition of oxygen evolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 433 4.1.3. Electrochemical reduction of O2 or H2O2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 434 4.1.4. Electrochemical oxidation of hydrogen-containing fuel molecules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 434 Analytical applications. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 434 5.1. Clinic and food assay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 434 5.2. Environmental analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 436

n

Corresponding authors. Tel.: 86 21 65712622. E-mail addresses: [email protected], [email protected] (Y. Miao), [email protected] (R. Ouyang). 1 The authors contributed equally to this work and should be considered as co-first authors. 0956-5663/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.bios.2013.10.008

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5.3.

Electrochemical degradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 436 5.3.1. Electrocatalytic hydrodechlorination. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 436 5.3.2. Electrochemical oxidation of ammonia. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 436 5.3.3. Electrochemical oxidation of urea. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 436 5.4. Electrosynthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 437 5.5. Electrocatalysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 437 6. Ni based nanomaterials and their electrocatalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 437 6.1. Nanostructures with different morphologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 437 6.2. Nanocomposites. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 437 6.2.1. To improve the stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 437 6.2.2. To improve the conductivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 437 6.2.3. To improve the electrocatalytic activity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 438 6.2.4. Other nanocomposites. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 438 7. Outlooks. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 438 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 438 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 438

1. Introduction

Ni(OH)2 þOH   e  ⇄NiOOHþ H2O

Electrocatalysis toward small molecules, especially organic small compounds, is of importance in a variety of areas including fuel cells, energy storage, organic synthesis, wastewater treatment, and electrochemical sensors for medical, food or environmental analysis (Hutton et al., 2010; Wang et al., 2011). Electrocatalysis toward small molecules for various materials or substrates has been the extensively studied focus where a large number of metals (Au, Ni, Pt, Ru, Co, In, Ir, Cu etc.) and their oxides or hydroxides have been developed as electrochemical catalysts (Vedharathinam and Botte, 2012). Compared to other metals, Ni with higher natural abundance is economically feasible, suitable for numerous applications in batch. Also, it has low toxicity. In 1966, Nesterov and Korovin pioneeringly studied the anodic oxidation of hydrazine on nickel in alkaline solution (Nesterov and Korovin 1966). In 1970s, Fleischmann et al. reported the electrochemical oxidation of some organic compounds at a Ni anode in alkaline solution (Fleischmann et al., 1971, 1972a, 1972b). After then, many Ni based materials such as Ni2 þ (Raoof et al., 2009); Ni (Danaee et al., 2010); NiO (Sattarahmady et al., 2010), Ni(OH)2 (Wang et al., 2011) and NiOOH (Hutton et al., 2010) had been reported, exhibiting excellent electrocatalytic activity toward the oxidation of a wide range of small compounds, such as glucose, glycine, methanol, ethanol, cyclohexanol, insulin, ammonia, acetylcholine etc. in alkaline media (Jafarian et al., 2003; Vedharathinam and Botte, 2012). However, there have been various or even divergent expressions about their electrocatalytic mechanism. Usually, the Ni(OH)2/ NiOOH shift reaction is used to describe the redox couple mediated electrocatalysis for each of Ni, NiO and Ni(OH)2 (Fleischmann et al., 1971):

Cheek and Grady reported their research with the title of “Redox behavior of the nickel oxide electrode system: quartz crystal microbalance studies” (Cheek and O’Grady, 1997). However, what the authors really studied was Ni(OH)2 deposited onto the platinum films by cathodic reduction of nickel sulfate. Completely different from the Ni(II)/Ni(III) or Ni(OH)2/NiOOH mediated electrocatalytic mechanism, such a couple of redox peaks at NiO/carbon nanotubes electrode was even attributed to the redox couple of Ni(II)/Ni(0) and the electrocatalytic signal of glucose came from the shift of Ni(II)/Ni(0) (Zhang et al., 2010). In addition, not all electrocatalysis toward organic small compounds is based on the redox couple mediated electrocatalytic mechanism. Sometimes, the direct electrocatalysis of some organic small compounds occurred at Ni(OH)2 or NiOOH (Hutton et al., 2010; Jia et al., 2011). In this review, we comprehensively summarize the recent progress in the electrocatalysis and electroanalysis of nickel, its oxides, hydroxides and oxyhydroxides toward small molecules, especially organic small compounds. Their electrocatalytical mechanisms are well discussed. Their preparations and applications are described as well. Most of of the relevant literatures appeared in recent decade. One of the earliest literatures could be tracked to 1966.

2. Preparation of nickel, its oxides and hydroxides Ni, its oxides, hydroxides and oxyhydroxides can be prepared in a large number of ways, including electrochemical growth, chemical precipitation or physical techniques, etc.

Ni(OH)2 þ OH  e  ⇄NiOOH þH2O 2.1. Electrochemical deposition NiOOH þorganic compound ) Ni(OH)2 þ product Sometimes, the authors simply used Ni2 þ /Ni3 þ rather than Ni (OH)2/NiOOH to explain such a machanism (Danaee et al,. 2010). In the study of Liu et al. and Shamsipur et al., a pair of redox peaks and electrochemical response to glucose at Ni nanoparticle or NiO/carbon nanotubes modified electrodes in NaOH solution were attributed to the shift of Ni(II)/Ni(III) redox couple (Liu et al., 2009a, 2009b; Shamsipur et al., 2010). Mahshid et al. contributed the electrochemical signals of the Ni(II)/Ni(III) peaks to the following two possible electrochemical reactions (Mahshid et al., 2011): NiO þOH   e  ⇄NiOOH

Electrochemical approaches are often employed to deposit Ni, Ni(OH)2 or NiOOH. Usually, nickel is catholically electrodeposited from a nickel salt containing solution: Ni2 þ þ2e  ) Ni Electrochemical preparation of Ni(OH)2 was achieved by applying a suitable anodic potential on Ni or Ni deposited electrodes in alkali solution (Hutton, Vidotti et al., 2010): Niþ2OH   2e  ) Ni(OH)2 In some cases, it is also possible to form mixtures of Ni(OH)2 and metallic Ni due to both of the above-mentioned processes

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occurring during the electrodeposition. NiOOH is also yielded if Ni or Ni(OH)2 is fully oxidized by potential cycling in basic solution. The electrochemical conversion from nickel hexacyanoferrate (Ni-HCF) to its oxides or hydroxides has been reported where a typical strategy is often involved by potential cycling the nickel hexacyanoferrate precursor modified electrodes in NaOH or KOH alkali solution, resulting in the breaking of the M–CN–Fe bond and the formation of Ni(OH)2 due to the strong interaction between metal ions and OH  (Joseph et al., 1991; Berchmans et al., 1995). Interestingly, metal oxides or hydroxides could be electrochemically converted back to metal hexacyanoferrates (Wang et al., 2012). 2.2. Chemical methods Shape Control of inorganic nanomaterials has received increasing attention due to its important role in influencing magnetic, electrical, optical, and other properties (Xu et al., 2007). There are many factors in chemical synthesis such as solvents used, temperature of preparation, concentration of the reactants, the use of surfactant templates, etc. which might affect the shape of products. To obtain Ni(OH)2, NiO and Ni individually by heating control. In a typical preparation of hollow spheres of β-Ni(OH)2, a certain amount of 1,2-ethanediamine (EDA) and Ni(NO3)2 aqueous solutions were sequentially added to NaOH aqueous solution, followed by a thorough mixing of all reagents (Zhang and Zeng, 2009). Afterwards, the bottles were placed in an electric oven at 100 1C for 30 min; light green β-Ni(OH)2 solid products were harvested after three centrifugation-redispersion cycles with deionized water and then dried at 60 1C overnight. Finally, the hollow spheres of β-Ni(OH)2 were obtained. Here, EDA plays a role of forming 6-coordinated nickel ion complex [Ni(EDA)3]2 þ as a nickel precursor to slow down the precipitation rate. Comparatively, NiO was usually prepared by thermal decomposition of Ni(OH)2 at higher temperature. For instance, hollow NiO nanostructures were produced by thermal decomposition of the above as-synthesized β-Ni(OH)2 at 400 1C for 2 h under laboratory air in a tubular electric furnace (Zhang and Zeng, 2009). Similarly, Ni can be obtained by thermal decomposition of Ni(OH)2 or NiO at higher temperature in the oxygen-free environment. Here, by heating the above as-synthesized β-Ni(OH)2 in a H2 gas flow at 450 1C for 1 h, the hollow Ni nanostructures were prepared in a horizontal quartz-tube reactor (Zhang and Zeng, 2009). 2.3. Physical methods A variety of physical methods have been reported to prepare Ni or NiO thin films, such as sputtering, electron beam evaporation (Caffio et al. 2003), chemical vapor deposition, pulsed laser deposition, spray pyrolysis and electrospinning. In a typical electrospinning process, Ag/NiO fibers in nanoscale were formed through the following chemical reactions (Wu et al., 2007):

et al., 2001): NiðOHÞ2 þ OH   e 

charge

$

discharge

NiOOH þ H2 O

Ni(OH)2 exists in two polymorphous crystal structures of αand β-form. Usually, β-Ni(OH)2 is chosen as the active electrode material due to its high stability in strong alkaline electrolyte and good reversibility when charged to β-NiOOH (Liang et al., 2004). βNi(OH)2 and α-Ni(OH)2 are transformed into β-NiOOH and γNiOOH, respectively while charging. Once overcharge happens, βNi(OH)2 is transformed back into γ-NiOOH. α-Ni(OH)2 dehydrates in the concentrated alkali, forming β-Ni(OH)2. The overall reaction pathway is illustrated as follows (French et al., 2001):

α  NiðOHÞ2

charge

$

discharge

↑overcharge

dehydration in alkali↓

β  NiðOHÞ2

γ  NiOOH

charge

$

discharge

β  NiOOH

Such a redox transition between Ni(OH)2 and NiOOH has been also used to describe the electrocatalytic oxidation mechanism of Ni(OH)2 toward many organic small compounds in alkaline electrolyte (Fleischmann et al., 1971): Ni(OH)2 þOH   e  ⇄NiOOHþ H2O NiOOHþorganic compound ) Ni(OH)2 þproduct In Fig. 1, a typical Ni(OH)2/NiOOH mediated electrocatalysis toward organic small compound was clearly observed at Ni/indium tin oxide (ITO) electrode (Tian et al., 2013). In the absence of glucose, one couple of well-defined Ni(OH)2/NiOOH redox peaks was observed. Upon the addition of glucose, the anodic peak current was dramatically enhanced accompanied with a decreasing cathodic peak current, which was attributed to the production of Ni(OH)2 through the reaction between NiOOH and glucose. The produced Ni(OH)2 was further oxidized to NiOOH at electrode surface. The oxidation of glucose was electrocatalyzed by Ni(OH)2/NiOOH redox couple according to the following reactions (Tian et al., 2013): Ni(OH)2 þOH   e  ⇄NiOOHþ H2O NiOOHþglucose ) Ni(OH)2 þ glucolactone Ni(OH)2/NiOOH redox system possesses excellent reversibility and also exhibits high electrocatalysis activity toward many small molecules in alkaline electrolyte.

Δ

2AgNO3 ⟹2Ag þ 2NO2 þ O2 2NiðNO3 Þ2 ⟹Δ 2NiO þ 4NO2 þ O2 3. Electrocatalytic mechanisms 3.1. Ni(OH)2/NiOOH mediated electrocatalysis Ni(OH)2 has been widely used as a very important active electrode material in many alkaline rechargeable batteries (Ni/ Cd, Ni/H2, Ni/Fe, Ni/MH, Ni/Zn etc.) and high-performance supercapacitors (Wu and Wu 2013). The battery or capacitor reaction involves the oxidation of Ni(OH)2 during charging and the reduction of NiOOH during discharging in alkaline electrolyte, which may be described briefly in the below reverse equation (French

Fig. 1. CVs of Ni/ITO in 0.1 M NaOH with (b) and without (a) 6 mM glucose (Tian et al., 2013).

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3.2. Ni and NiO based electrocatalysis The redox transition of Ni(OH)2/NiOOH has been often used to describe the mechanism for the electrocatalytic oxidation of Ni and its oxides toward organic small molecules in alkaline electrolyte. Under exposure to air or aqueous solution, several-atomiclayers thick passive film comprised of two distinct layers forms on the Ni surface: a Ni hydroxide layer at the air–oxide interface and an oxide layer between the metal substrate and the outer hydroxide layer (Medway et al., 2006). The XPS results revealed the formation of a core–shell structure where a pure Ni core was surrounded by a NiO þNi(OH)2 shell (You et al., 2003; Prieto et al., 2012). Similarly, a thin hydroxide film formed on the NiO surface in atmosphere or aqueous solution (Nakamura et al., 2005). Water molecules in air are strongly adsorbed on the surface of Ni or NiO where oxygen exerts a proton-acceptor site to the water molecule through hydrogen bonding (Nakamura, Tanaka et al., 2005; Medway, Lucas et al., 2006), leading to the conversion of surface NiO to the hydroxide phase Ni(OH)2 (Giovanelli et al., 2003): þ

Ni(metal) þH2O ) (NiOH)ad þH þ e



(NiOH)ad þH2O ) (NiOH  H2O)ad (NiOH  H2O)ad ) Ni(OH)2 þ H þ þe  The hydroxyl progress of NiO is simply expressed as follows: NiO þphysisorbed water ) NiO  H2O (strongly adsorbed) ) Ni (OH)2 Such an oxidation or hydroxyl phenomenon is also often found on the surface of various metals or their oxides such as Au, Cu, Ni, Pt, Ru, CoO, Fe2O3, MnO2, ZnO, etc. The detailed models for Ni electrodes passivated in acidic electrolytes have been introduced (Medway et al., 2006). A number of ex situ spectroscopy techniques including XPS, UPS and in situ Raman spectroscopy have verified that the passive film is a duplex structure, consisting of a NiO layer with a Ni(OH)2 layer overlayed However, in the study of Mahshid et al., Ni was oxidized and then dissolved without forming NiOOH on the electrode in acidic and neutral solutions, indicative of no catalytic activity toward the oxidation of glucose (Mahshid et al., 2011). The transformation from Ni or NiO to Ni(OH)2 could be accelerated in alkaline condition. There is a general agreement that upon the immersion of metallic Ni into alkaline electrolyte (i.e. 1 M KOH) a layer of Ni hydroxide instantaneously forms on the Ni surface with an initial thickness of ca. 10 atomic layer equivalent to 40–55 Å (Fleischmann et al., 1971; Giovanelli et al., 2003). The process is given as follows: Niþ 2OH   2e  ) Ni(OH)2 As shown in Fig. 2, modeling of X-ray reflectivity data from the air-formed oxide at Ni electrode in 1 M KOH electrolyte leads to a four-layer model: a electrolyte layer, a Ni hydroxide layer, Ni oxide layer and a Ni substrate (Medway et al., 2006). The transformation process from NiO to Ni(OH)2 could be summarized as follows: NiO þOH  ) Ni(OH)2 It was observed that as the oxidation potential increased, Ni became less susceptible to dissolution due to the formation of an oxide based passive film (Pulvirenti et al., 2009). The growth of NiO and Ni(OH)2 on Ni surface is self-limited as their structures are extremely dense and the conductivity very poor. So it is imaginable that the amount of NiO and Ni(OH)2 spontaneously formed at the surface of Ni is quite limited.

Fig. 2. A schematic diagram of the density model used to fit the X-ray reflectivity data from the air-formed oxide at Ni electrode in 1 M KOH electrolyte (Medway et al., 2006).

The transformation from Ni or NiO to Ni(OH)2 or NiOOH could be further speeded up in the strong alkaline solution under the cyclic scanning at relatively high positive potential. As a result, the thickness of the Ni hydroxide layer increases: Niþ2OH   2e  ) Ni(OH)2 At higher potentials, Ni(OH)2 is transformed to NiOOH and it is also possible to directly yield NiOOH from Ni or NiO (Medway et al., 2006): Ni(OH)2 þOH   e  ) NiOOHþ H2O NiO þOH   e  ) NiOOH Niþ3OH   3e  ) NiOOH þH2O The application of higher potentials referred as the “oxyhydroxide region” leads to the conversion from more Ni or NiO to Ni (OH)2/NiOOH. On the one hand, oxygen already present between the layers of the air-formed oxide is able to diffuse into the Ni lattice, which means that the NiO interface moves inside the Ni substrate (Medway et al., 2006). On the other hand, the electrode at more positive potential prefers to adsorb negatively charged OH  onto the NiO surface. The redox reaction of Ni(OH)2/NiOOH is accompanied with the repeated transfer of OH  and H2O between the interfaces of NiO, Ni(OH)2 and NiOOH. Because of the formation of hydroxide and oxyhydroxide, OH  ions from the oxyhydroxide layer continue to permeate into the Ni substrate. This could be supported by improved conducting properties of the oxyhydroxide over the hydroxide, as well as the increased driving potential (Medway et al., 2006). In the study of Mu et al., nano-NiO modified carbon paste electrode did not give obvious redox peaks of Ni(OH)2/NiOOH. Its electrochemical response to glucose was low when the amount of the employed NiO nanoparticles was very small and the amount of Ni(OH)2 spontaneously forming at the surface was still extremely limited (Mu et al., 2011). Scanning the electrode up to a high potential of 1.2 V vs Ag/AgCl led to the conversion from more NiO to Ni(OH)2 and NiOOH successively. As a result, enough redox couple of Ni(OH)2/NiOOH was produced, exhibiting an highly increased electrocatalysis activity toward glucose. Prior to the electroanalytical use, Ni electrodes were conditioned by potential cycling over 300 times between approximately 0 and 0.5 V vs SCE (saturated calomel electrode) at 50 mV/s in 1 M KOH (Toghill et al., 2010). This was to allow the enrichment of the Ni(OH)2 species at the surface of the nickel, resulting in the thickening of the electrocatalytic layers. If the working potential is increased above 0.45 V vs Hg/HgO, α-Ni(OH)2 and β-Ni(OH)2 are oxidized to γ-NiOOH and α-NiOOH, respectively. β-NiOOH can also be converted to γ-NiOOH above 0.6 V (Yeo and Bell, 2012).

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3.3. Ni2 þ based electrocatalysis Raoof et al. reported a poly(N-methylaniline)/Ni2 þ modified electrode for electrocatalytic oxidation of formaldehyde in alkaline medium where Ni2 þ was incorporated into the polymeric film by immersing the modified electrode in nickel sulfate solution (Raoof et al., 2009). This modified electrode was found to be capable of catalyzing the electrochemical oxidation of formaldehyde. In fact, the redox couple of Ni(OH)2/NiOOH contributed to such an electrochemical oxidation indeed since Ni2þ was transformed to Ni(OH)2 once poly(N-methylaniline)/Ni2 þ modified electrode touched 0.1 M NaOH solution. 3.4. Direct electrocatalysis of Ni(OH)2 In the study of Jia et al., both the direct and indirect electrocatalysis toward cysteine (CySH) in 0.1 M NaOH were observed in Fig. 3A (Jia et al., 2011). The increase of peak current at around 0.42 V exhibits the Ni(OH)2/NiOOH mediated indirect electrocatalytical oxidation of CySH, which was attributed to the production of Ni(OH)2 through the reaction between NiOOH and CySH where Cys was oxidized to the corresponding disulfide by NiOOH. Then, the produced Ni(OH)2 was further oxidized back to NiOOH at electrode surface (Jia et al., 2011): Ni(OH)2 þ OH  e  ⇄NiOOH þH2O NiOOHþ 2CySH ) Ni(OH)2 þ CyS–SyC The direct electrocatalysis of Ni(OH)2 toward CySH occurred at about 0.1 V with a plain peak at about 0.21 V: 2CySH ) CyS–SyC þ2e  þ2H þ The possible mechanism for the direct electrocatalysis is tentatively given in three steps (Spãtaru et al., 2001; Jia et al., 2011): (i) firstly, CySH is adsorbed onto the Ni(OH)2 surface through hydrogen bond, coordination bond and thiol–metal bond which decreases the activation energy for oxidation; (ii) then the electron transfer happens from CySHads to electrode, resulting in the formation of the sulfhydryl radical (CySdads); (iii) immediately, the formed radicals dimerize to form the corresponding disulfide (Spãtaru et al., 2001; Jia et al., 2011): CySH⇄CySHads e

CySHads ⟹CySd ads þ H þ CySd ads ) CyS  SyC

Although Ni(OH)2 displays excellent electrocatalytic oxidation toward CySH in alkaline solution, the determination of CySH in nearly neutral solution has been highly preferred because of the poor stability of CySH in alkaline solution. Also, the real sample of CySH often shows a pH close to neutral. In pH 7.5 PBS, a direct electrochemical oxidation of CySH at Ni(OH)2 ultrathin film electrode exhibits a broad and highly irreversible oxidation peak with a low onset potential at approximately 0.03 V vs SCE and a peak potential at about 0.25 V in Fig. 3B (Jia et al., 2011). At pH 7.5, the blank signal was fairly low and the response signal much higher than the blank. The prepared Ni(OH)2 ultrathin film shows good potential to develop amperometric assay of important biological thiols in nearly neutral solution with high sensitivity. Giovanelli et al. also reported a direct electrocatalysis of Ni (OH)2 toward sulfide (Giovanelli et al., 2003). The generation of Ni(OH)2 layer was first carried out by continuously scanning a Ni electrode in 0.1 M NaOH using cyclic voltammetry. Finally, an oxidation peak of Ni(OH)2/NiOOH showed up at 0.42 V. However, upon the introduction of sulfide to the electrolyte solution, a preoxidative wave at 0.25 V emerged and continued to grow up with the increase of sulfide concentration. The authors tentatively ascribed the emergence of the pre-wave to the direct oxidation of the sulfide species at Ni(OH)2 layer.

3.5. Direct electrocatalysis of NiOOH Direct electrocatalysis of NiOOH was reported by Hutton et al. where Ni(OH)2 nanoparticles were electrodeposited on borondoped diamond electrodes (BDDs) (Hutton et al., 2010). As shown in Fig. 4, in the presence of the alcohols, the current signal rose dramatically above 0.23 V where most of Ni(OH)2 were transformed to NiOOH. The electrocatalytic oxidation of NiOOH toward ethanol and methanol was considered to arise from the unpaired d-electrons or empty d-orbitals associated with NiOOH, which were available for bond formation with adsorbed species or redox intermediates (Vidotti et al., 2009; Hutton et al., 2010). Direct electrocatalytic oxidation of NiOOH toward ammonia was found at Ni/Ni(OH)2 electrode prepared by cyclic scanning a Ni electrode in 1 M NaClO4 (Kapałka et al. 2010). As shown in Fig. 5, the A2 oxidation peak could be attributed to the direct electrocatalysis of NiOOH toward ammonia. The ammonia oxidation at peak A1 seems not to involve NiOOH reduction to Ni(OH)2 because the C1 reduction peak did not increase as the concentration of ammonia increased. Herein, the authors did not give an explanation about the current increase of the reduction peak C1. The authors consider a direct electron transfer from ammonia to the

Fig. 3. (A): CVs of Ni(OH)2-GCE in the absence (a) or presence (b) of 1 mM Cys in 0.1 M NaOH solution (Jia et al., 2011). (B): CVs of GCE (a and b) and Ni(OH)2-GCE (c and d) in the absence (a and c) or presence (b and d) of 1 mM Cys in PBS of pH 7.5 (Jia et al., 2011).

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In the study of Yan et al., the direct electrocatalysis of NiOOH toward urea was observed with the onset potential of urea oxidation at 0.39 V in Fig. 6D where the sample D of nickel and cobalt bimetallic hydroxide catalysts was prepared in the deposition solution containing 0.1 M KNO3 þ 0.005 M Ni(NO3)2 þ0.005 M Co(NO3)2 (Yan et al., 2012). For sample C prepared in the deposition solution of 0.1 M KNO3 þ0.008 M Ni(NO3)2 þ 0.002 M Co (NO3)2, both Ni(OH)2/NiOOH mediated indirect electrocatalysis and direct electrocatalysis by NiOOH took place.

4. Electrocatalysis toward small molecules and its applications Nickel based materials exhibit excellent electrocatalytic activity toward many small molecules with potential applications in fuel cells, electroanalysis, electrosynthesis, electrodegradation etc. 4.1. Fuel cells

Fig. 4. CVs performed at 5 mV/s at a Ni(OH)2 nanoparticle modified BDD electrode in 0.1 M KOH and (a) 0.5 M ethanol and (b) 0.47 M methanol. Inset: CV of bare BDD in a 0.1 M KOH solution containing both 1 M ethanol and 1 M methanol (Hutton et al., 2010).

4.1.1. Electrochemical oxidation of water or electrochemical evolution of oxygen Hydrogen is the most abundant, renewable element in the universe. As energy carrier, it has excellent electrochemical reactivity with the advantages of cleanness and efficiency. A sustainable supply of hydrogen for fuel cells can be achieved by the electrolysis of water (Yeo and Bell, 2012; Hall et al., 2013). This process consists of two parts: the cathodic hydrogen evolution reaction 2H þ þ 2e  ) H2 and the anodic oxygen evolution reaction (OER) 4OH  ) 2H2O þ4e  þO2 Electrocatalysis toward oxygen evolution: The potential required to split water is higher than its thermodynamic value of 1.23 V (vs RHE) due to the high overpotential of the oxygen evolution reaction (OER). Ni(OH)2 can catalyze the above OER. It is generally considered that the mechanism of oxygen evolution at Ni(OH)2 electrode includes a chemical association step between OHads and  OH  and two electrochemical steps, OH  discharge and Oads discharge (Armstrong et al., 1988; Wang et al., 2004): OH  ⇄OHads þe  þ H2 O OHads þOH  ⇄Oads  Oads ⇄Oads þe

Fig. 5. Cyclic voltammograms of (1) 0 mM, (2) 10 mM, (3) 20 mM, (4) 40 mM, (5) 60 mM, (6) 80 mM, (7) 100 mM, and (8) 150 mM NH4ClO4 in 1 M NaClO4 þ NaOH at pH 9 on Ni/Ni(OH)2. The insets show the current peak density as a function of the total ammonia concentration (NH4 and NH3) as well as the NH3 fraction (Kapałka et al., 2010).

anode: NiOOH þ NH3 ) NiOOHðNH3 Þads ) NiOOH þ 1=2N2 þ 3H þ þ 3e  The direct electron transfer reaction proposed above produces N2 as a result of ammonia oxidation. However, a non-negligible amount of nitrate is formed as well. The nitrate might be formed during the oxygen transfer reaction in which water is activated, resulting in a transfer of an oxygen atom to the ammonia molecule:

2Oads⇄O2 The evolution of oxygen is concluded as 4OH  ) O2 þ2H2O þ4e  According to an in situ Raman study of nickel oxide and goldsupported nickel oxide catalysts for the electrochemical evolution of oxygen, a very thin layer of Ni hydroxide deposited on Au has a significantly higher OER activity relative to a thick layer of Ni hydroxide formed on bulk Ni or electrodeposited on Au (Figs. 7 and 8) (Yeo and Bell, 2012). It is proposed that the high activity of Ni oxide submonolayer on Au is due to the charge transfer from the oxide to the highly electronegative Au, leading to the possible formation of a mixed Ni/Au surface oxide.

NiOOH(NH3)ads þ3H2O ) NO3  þ9H þ þ8e  The oxidation strongly depends on pH, showing that NH3 rather than NH4 þ is oxidized on nickel electrode. The main products of ammonia oxidation are gaseous nitrogen compounds and nitrate.

4.1.2. Inhibition of oxygen evolution Interestingly, catalytic oxygen evolution is also a parasitic reaction parallelly occurring with the Ni(OH)2/NiOOH redox reaction during charging in rechargeable alkaline batteries. The produced

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Fig. 6. Cyclic voltammograms of samples C and D based Ni–Co hydroxide electrodes in 5 M KOH solution in the absence (♯1) and presence (♯2) of 0.33 M urea at scan rate of 10 mV/s (Yan et al., 2012).

the oxygen evolution overpotential, promoting the full charge of the electrode (Armstrong et al., 1988). 4.1.3. Electrochemical reduction of O2 or H2O2 Contrary to the oxygen evolution electrocatalysized by Ni(OH)2, the electrochemical reduction of O2 can be enhanced by a 3D flowerlike α-Ni(OH)2 (Xu et al., 2007). Oxygen reduction potentials and peak currents were obtained in the electrolyte saturated with O2 at  623 mV vs SCE which was in the region expected for a four-electron reduction of O2: 2H2O þO2 þ4e  ) 4OH  The CV experiments also showed that H2O2 could be reduced to OH  on the α-Ni(OH)2 electrode.

Fig. 7. (a) Cyclic voltammograms of Ni α/γ (red trace) and Ni β/β (black trace) electrodes in 0.1 M KOH where oxygen evolution occurs on both surfaces above 650 mV (Yeo and Bell, 2012). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

4.1.4. Electrochemical oxidation of hydrogen-containing fuel molecules In fuel cells, many hydrogen-containing small molecules such as methane (Jafarian et al., 2003), formaldehyde (Raoof et al., 2009), methanol (Danaee et al., 2008; Shamsipur et al., 2013), ethanol (Hutton et al., 2010), glucose or carbohydrate (Hutton et al., 2010), formic acid etc. (Du et al., 2010; Lu et al., 2010), are often used as the fuels which can be well electrocatalyzed by nickel, its oxides, hydroxides and oxyhydroxides. Carbohydrates represent roughly 75% of the annually renewable biomass for energy. D-Mannose is a freely available sugar which can be obtained from the hemicellulosic part of the biomass (Parpot et al., 2007). The oxidation of D-mannose was studied on platinum, gold and nickel in alkaline media. Nickel electrode provides the highest current densities compared with platinum and gold electrodes. The mechanism was given as follows (Parpot et al., 2007): NiOOHþR1R2CHOH ) Ni(OH)2 þR1R2COH

Fig. 8. Cyclic voltammograms of α-Ni(OH)2 obtained at a rate of 50 mV/s: in (a) N2 and (b) O2 (Xu et al., 2007).

R1R2COH ) products

5. Analytical applications oxygen gas bubble may generate internal stresses within the electrode pores, leading to electrode degradation (Wang et al., 2004). OER decreases charge efficiency, and thus causes relatively low discharge capacity. To improve the electrode performance, a number of additives have been introduced into the Ni(OH)2 lattice (Wang et al., 2004). The research of Armstrong confirmed that the presence of cobalt coating at the surface of Ni(OH)2 can increase

5.1. Clinic and food assay Glucose: Glucose sensing is of great importance in various areas including biological and chemical analyses, clinical detection, environmental monitoring, and food processing industries. Many previous studies on glucose detection were usually based

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Fig. 9. Reaction of Purpald with aldehydes to give a purple (Kerr and Mike, 1998).

on the use of glucose oxidase catalyzing the oxidation of glucose. However, due to the intrinsic instability of enzymes and loss of enzyme activity during the immobilization process, such enzyme-based sensors usually suffer from the low reproducibility and stability. Therefore, the development of electrochemical non-enzymatic glucose biosensors has received continuous interest (Park et al., 2006; Toghill and Compton, 2010). A lot of studies have been done to detect glucose based on its direct electrochemical oxidation at Ni (Liu et al., 2009), its oxides (Luo et al., 2013) and hydroxides (Ganesh et al., 2011). Carbohydrates: Electrocatalytic oxidation of carbohydrates is of great interest in many areas, ranging from analytical applications in medical diagnosis and food industry to wastewater treatment. Carbon nanotube–NiCo–oxide composites were employed to fabricate electrodes for the electrochemical detection of carbohydrates (Arvinte et al., 2011). The modified electrode shows excellent electrocatalytic activity toward monosaccharides oxidation by reducing overpotential in alkaline solutions. Six monosaccharides such as glucose, mannose, galactose, fructose, arabinose and xylose, were determined amperometrically at the surface of this modified electrode with high sensitivity over a wide range of concentrations from 0.02 up to 12.12 mM. Low detection limit of 5 μM for glucose was obtained. Although the Ni(OH)2 exhibits high electrocatalytical capability to oxidize many organic molecules, the selectivity is poor. Highperformance ion chromatography (HPIC) offers many advantages for the separation of multi-component samples. In order to overcome the disadvantage of poor selectivity, Ni(OH)2 modified electrodes were coupled with HPIC for the electrocatalytic detection of carbohydrates (Vidotti et al., 2009). The carbohydrates were successfully separated due to their different retention times and detected according to their individual chemical identity in such sequence as glucose, fructose, lactose and sucrose with detection limits of 30, 75, 90 and 160 ppb, respectively.

Insulin: The immunoassay of insulin has been used for the determination of insulin in biological sample with a complex analytical process. Electrochemical detection of insulin is of great interest as it can provide good sensitivity and shorter analysis time. A very sensitive and stable oxidative response to insulin was achieved with a carbon nanotube–nickel–cobalt oxide modified electrode which enabled the detection of insulin at concentration as low as 0.22 μg/mL with a good sensitivity of 22.57 μA/mg mL at physiological pH (Arvinte et al., 2010). This work demonstrated greatly the promising applications of these electrodes in biological samples of clinical interest. Aminoacids: Aminoacids play key roles in many biophysical and biochemical processes (Vidotti et al., 2008). The abnormal regulation of amino acid is linked to some disorders such as Huntington, Alzheimer and Parkinson diseases. Glycine often acts as biosynthetic intermediate of all purines and porphyrins, as well as neurotransmission inhibitor of central nervous system. For most amino acids, including glycine, direct electrochemical detection is a subject of great interest because the underivatized amino acids are nonelectroactive at the conventional carbon electrodes leading to large difficulties to detect them. The doped Ni(OH)2 modified electrode was reported for the determination of glycine (Vidotti et al., 2008). The modified electrode presented NiOOH mediated electrocatalytical activity toward glycine oxidation. A sensitivity of 0.92 μA/mM and a linear response range from 0.1 up to 1.2 mM were achieved with a detection limit of 30 μM. NiOOH mediated electrooxidation was also found for L-arginine but not for L-alanine, L-glutamic acid and L-leucine. Acetylcholine: Acetylcholine is an important neurotransmitter, which affects learning, memory and muscle tone. The change of acetylcholine concentration may cause neuropsychiatric disorders like Alzheimer disease, Parkinson disease, etc. A electrochemical sensor for acetylcholine was developed by electroplating nano-NiO reinforced nickel on graphite substrate (Shibli et al., 2006) which

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showed good sensing performance with a response time as low as 8 s and a sensitivity of 34.88 μA/μM cm2. Phosphate anions: Quantitative analysis of phosphate anions (PO43  ) is important in biological diagnosis, environmental monitoring, and biomedical research. Cheng et al. reported the Ni(OH)2/NiOOH mediated electrocatalysis for the determination of phosphate with good selectivity (Cheng et al., 2009). In this system, the Ni(OH)2/NiOOH film formed at the plated nickel electrode was found to selectively adsorb phosphate in alkaline media, leading to a suppressed current of the electrocatalytic oxidation of glucose. Such suppressed current was used for the determination of phosphate in the presence of some possible interference from the coexisting ions such as nitrate, chloride, sulfate, acetate, oxalate, carbonate, and some anionic species of toxicological and environmental interest like chlorate, chromate, and arsenate ions. The electrode could be regenerated without extra treatment under the hydrodynamic condition. A good linearity ranging from 40 μM to 1 mM was obtained with the detection limit (S/N ¼3) of 0.3 μM. NADH: The NiO nanoparticle modified glassy carbon electrode was proposed for the determination of NADH (Sharifi et al., 2013), which showed excellent electrocatalytic activity toward oxidation of NADH due to the reduced overpotential. It was mentioned that the interactions between NADH and the NiO nanoparticles could promote the C–H bond rupture necessary for NADH oxidation. Ethanol: Ethanol determination is important in the control and management of food fermentation. Vidotti et al. synthesized Ni(OH)2-ordered mesoporous carbons nanocomposites and investigated their enhanced electrochemical properties toward ethanol oxidation (Lu et al., 2010). Excellent analytical advantages were provided by the nonenzymatic amperometric sensor of ethanol with a linear response up to about 80 mM, a sensitivity of 0.65 μA/mM and a detection limit of 4.77 μM (S/N¼ 3).

5.2. Environmental analysis Sulfide: The monitoring of sulfide is required in a variety of environmental and industrial applications, including odor assessments, health and safety investigations, and routine industrial/offsite monitoring programs. Giovnelli et al. reported electrochemical determination of sulfide at nickel electrode in alkaline media (Giovanelli et al., 2003). With sulfide present, a new oxidative wave was observed and the linear response to sulfide was found in a range from 20 to 200 μM. Formaldehyde or aldehyde: The Ni(OH)2/NiOOH redox couple mediated electrocatalysis toward formaldehyde in alkaline medium was discussed at a Ni/poly(o-toluidine)/Triton X-100 film modified carbon nanotube paste electrode (Raoof et al., 2012). The simple electrocatalytic oxidation mechanism was described as equations below: Ni(OH)2 þ OH  e  ⇄NiOOH þH2O NiOOHþ HCHOþ 2OH  ) Ni(OH)2 þCH2(O)O  þH2O Purpald has been used for many years to detect aldehyde for which Purpald condensed to form an unstable, colorless intermediate (Kerr and Mike, 1998). The oxidation of this intermediate by air or other oxidants yields a deep purple, highly colored compound with an absorption maximum at 550 nm. Usually, it took long time to allow O2 in air freely diffusing into the reaction system to form colored products. Especially, O2 cannot freely enter the closed system under the chromatographic conditions. An

on-line post-column electrochemical reactor has been introduced to HPLC as a possible alternative to air or chemical oxidation for the analysis of aldehydes (Kerr and Mike, 1998). The NiOOH formed at Ni electrode surface acted as an oxidant to oxidize the colorless intermediate. The yielded purple products were analyzed with the spectrophotometric method for the determination of formaldehyde, acetaldehyde, and propionaldehyde after HPLC separation (Fig. 9). 5.3. Electrochemical degradation 5.3.1. Electrocatalytic hydrodechlorination Chlorophenols (DCPs) are significant contaminants with great potential risk to human beings. The destruction of chlorophenols in wastewater is a very critical issue which has attracted wide interest of researchers. In the study of Li et al., the electrocatalytic activity of the Pd–polypyrrole–Ni composite electrode was evaluated by the electrocatalytic hydrodechlorination of dichlorophenols (Li et al., 2012). Results indicated that the electrocatalytic hydrodechlorination of dichlorophenols fitted well with pseudofirst-order kinetic formula and the following order: 2,4-DCP 42,3-DCP 43,4-DCP 4 2,5 DCP-2,6-DCP 43,5-DCP 5.3.2. Electrochemical oxidation of ammonia Ammonia removal is one of the main tasks for domestic wastewater treatment since ammonia containing wastewaters have detrimental effects on the environment, such as eutrophication and fish kills (Kapałka et al., 2010). The electrochemical oxidation of ammonia was investigated on a Ni/Ni(OH)2 electrode prepared by potential cycling a Ni electrode in 1 M NaClO4 þ NaOH at pH 9. It was found that oxidation of ammonia was strongly pH dependent and happened mainly at pH values above 7. A considerable fraction of the ammonia was oxidized to nitrate (11%), while the rest was gaseous nitrogen compounds. Electrochemical removal of ammonia has been approved to be a promising technique for the degradation of ammonia. 5.3.3. Electrochemical oxidation of urea Urea management has been a major enviromental and health issue. It is widely used as an animal feed additive and nitrogenrelease fertilizer in the agricultural industry. Both human/animal urine and the industrial synthesis process of urea also result in large amount of urea-rich wastewater (Wang et al., 2011; Wang et al., 2012; Yan et al., 2012; Ji et al., 2013). The wastewater containing urea puts dangers to the ground and drinking water. It can naturally hydrolyze to ammonia and emits to the atmosphere impairing the air quality. Electrocatalytic oxidation of urea has been an effective approach for urea removal during wastewater treatment with no use of sophisticated and bulky instruments, which also exhibits potential applications in the areas of hydrogen production, electrochemical sensors and fuel cells. To meet the increasing global demands for energy, the use of abundantly available wastewater has been attracting increasing attention as an alternative clean energy sources independent of fossil fules. Therefore, urea electrolysis is also an efficient way for hydrogen production as a valuable fuel. The Ni/Co bimetallic hydroxides and two-dimensional Ni(OH)2 nanosheets were reported as electrocatalysts for urea electro-oxidation in alkaline medium (Wang et al. 2011, 2012; Yan et al., 2012). At the anodic compartment of the electrolytic cell, Ni(OH)2 is oxidized to its active form of NiOOH that catalyzes the electrolysis of urea to benign nitrogen: Anode: Ni(OH)2 þOH   e  ⇄NiOOHþ H2O

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CO(NH2)þ6OH  ) N2 þ5H2Oþ CO2 þ 6e  Cathode: 6H2Oþ 6e  ) 3H2 þ6OH  5.4. Electrosynthesis Cyclohexanol: As an oxidation product of cyclohexanol, cyclohexene or cyclohexanone, adipic acid has been widely used not only as an important intermediate in the manufacture of nylon but also as a plasticizer and a food additive. Therefore, the development of simple and environmentally friendly synthesis methods has been extensively explored. Yi et al. investigated the electrochemical oxidation of cyclohexanol on NiOOH modified Ni electrode in alkaline solutions (Yi et al., 2007). It turned out that the oxidation of cyclohexanol on the electrode followed the Ni(OH)2/ NiOOH mediated electrocatalytic reaction mechanism. Arylation of 3-Amino-6-Chloropyridazines: 3-Amino-6-aryl- and 3-amino-6-heteroarylpyridazines have retained a major attention of the organic community due to diverse biological activities displayed by these 1,2-diazaheterocycles, ranging from e.g. anticancer or analgesic effects to potential treatment of urinary incontinence, inflammatory pain, obesity, and neurodegenerative diseases. An electrochemical nickel-catalyzed arylation of 3-amino-6-chloro pyridazines was reported to prepare 3-amino-6-aryl- and 3-amino6-heteroarylpyridazines (Sengmany et al., 2012). Comparative experiments involving classical palladium-catalyzed reactions, like Suzuki, Stille or Negishi cross-couplings, revealed that the electrochemical method could constitute a reliable alternative tool for biaryl formation. 5.5. Electrocatalysis Amines and alcohols: A series of amines and alcohols have been oxidized at Ni anodes in aqueous alkaline solutions (Fleischmann et al., 1972a, 1972b). The kinetics and mechanism of the Ni(OH)2/ NIOOH mediated oxidation have been studied. A comparison of the different substrates shows (a) primary amines were always oxidized more rapidly than the corresponding alcohol; (b) for a series of amines RNH2, R2NH, and R3N, kRNH2 4 kR2NH 4kR3N; (c) the cyclic amine, pyrrolidine, was always oxidized much more rapidly than the corresponding acylic amine diethylamine; (d) branching of the hydrocarbon skeleton at the carbon α to the heteroatom decreased the oxidation rate; and (e) the rate of oxidation decreased with the increasing number of carbon atoms in alcohol or amine structure.

6. Ni based nanomaterials and their electrocatalysis Ni based nanomaterials have been extensively explored for the fundamental scientific and technological interests in accessing new classes of functional materials with unprecedented properties and applications in energy storage devices, supercapacitors, electrochemical sensors etc. 6.1. Nanostructures with different morphologies Shape Control of nickel based nanomaterials has received increasing attention due to its important role in influencing their magnetic, optical, electrical, and electrochemical properties (Xu et al., 2007). Their shapes may be affected by many factors such as the solvents used, temperature, concentration of the reactants, surfactant templates, etc. during the synthesis. Various morphologies of Ni, NiO or Ni(OH)2 such as nanorod, nanowire, nanotube

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(Zhuo et al., 2008), nanoribbon (Yang et al., 2005), nanosheet (Liu et al., 2009a, 2009b), mesoporous structure (Zhu et al., 2007) and hollow sphere (Wang et al., 2005) have been synthesized via a variety of chemical routes. Two-dimensional Ni(OH)2 nanosheets were synthesized by exfoliating surfactant intercalated layered Ni(OH)2 and developed as electrocatalysts for urea electro-oxidation (Wang et al., 2011). Compared with the bulk Ni(OH)2 powder, a decrease in overpotential and an enhancement of current density were achieved for the electro-oxidation of urea with Ni(OH)2 nanosheet. 6.2. Nanocomposites Nanocomposites of Ni, NiO and Ni(OH)2 with various additives were prepared to improve their electrochemical performance including electrocatalysis, conductivity and stability. 6.2.1. To improve the stability Currently, β-Ni(OH)2 is usually used as a precursor material in alkaline batteries. However, β-Ni(OH)2 has a low theoretical capacity (Ash et al., 2006). Also, if overcharge happens, the β-Ni (OH)2 is easily transformed to γ-NiOOH, accompanied with large volumetric change leading to poor electric contact between the current collector and β-Ni(OH)2/γ-NiOOH, and accordingly causes rapid capacity decay during the electrochemical charge/discharge cycles (Li et al., 2010). On the other hand, α-phase nickel hydroxide has a large charge capacity, low charge and high discharge voltages. Volume expansion is also significantly reduced due to the similar lattice of α-Ni(OH)2 and γ-NiOOH. However, pure α-Ni (OH)2 is very unstable in strong alkaline media used in batteries and easily transformed to β-Ni(OH)2. Thus it is extremely important to enhance the stability of the α or β-phase of Ni(OH)2 in alkaline media. Transition metal atoms such as Al, Cd, Co, Fe, Mn, Zn, or rare earth elements have been incorporated into the lattice of nickel hydroxide for the further improvement of the performance of the secondary alkaline batteries (Yao et al., 2013). The principle is to anchor anions and enhance bonding by increasing the positive charge in NiO2 layer, so that the anions and water molecules are maintained and the lattice constants remain stable (Bao et al. 2013). Double hydroxides: The research of Li et al. revealed that the Ni (OH)2 tubes coated with Ca(OH)2, Co(OH)2, and Y(OH)3 possessed superior electrode properties including high discharge capacity, excellent high-temperature and high-rate discharge ability, and good cycling reversibility (Li et al., 2005). Interlayer anions: The effect of interlayer anions on the electrochemical performance of Ni(OH)2 was investigated by Li et al. (2010), indicating that the Al-substituted α-type Ni(OH)2 with interlayer NO3  had better electrochemical performance, such as better reaction reversibility, higher proton diffusion coefficient, lower electrochemical impedance, higher specific capacity, and better cyclic stability, than that of the Al-substituted α-type Ni (OH)2 with interlayer SO42  . 6.2.2. To improve the conductivity The reversibility of Ni(OH)2/NiOOH redox shift is an important factor in the performance of Ni(OH)2 as active material for alkaline batteries, fuel cell and electroanalysis applications. However, as p-type semiconductor, the poor electric conductivity of Ni(OH)2 limits its electrochemical performance, additives such as C, Co, Cu, Ca, Pt, Au and Ag are thus commonly used to increase its conductivity and improve the electrochemical performance (Ortiz et al., 2012). Among the additives used in the Ni(OH)2 based electrodes, cobalt is especially important because it has been used as conductive additives to provoke a shift of Ni(II)/Ni(III) redox

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potential to less positive values, avoiding the oxygen evolution reaction and increasing the charge efficiency and electrocatalytical performance. In the study of Ash et al., bimetallic additives, Cd–Zn, Cd–Mn, Cd–Co, Cd–Fe, and Cd–Al during chemical precipitation on discharge capacity of Ni(OH)2, were studied (Ash et al., 2006). The results showed that doping with metal ions like Mn2 þ , Co2 þ and Al3 þ along with Cd2 þ gave a substantial increase in the discharge capacity of Ni(OH)2, indicating a synergistic effect. It is well known that silver has excellent conductivity and exhibits great electrocatalytic activity toward the oxidation of many small molecules as well. The electrocatalysis of Ag/Ni bimetallic alloys toward glucose oxidation was studied (Miao et al., 2013). It was found that the presence of Ag and Ni in the alloys facilitated each other's electrocatalysis toward glucose oxidation, displaying an interesting synergistic effect. At about 0.45 V vs SCE, the Ni(OH)2/NiOOH mediated electrocatalysis of Ni toward glucose oxidation was enhanced by Ag. The Ag/Nideposited electrode responded rapidly to the changes of glucose concentration, producing stable signals within 4 s. Two linear ranges were identified with one at low glucose levels from 1 to 9.89 μM and the other at high glucose concentrations from 19.68 to 106.89 μM. The detection limit of the electrode was 0.49 μM. Similarly, the presence of Ni improved the electrocatalytic oxidation of Ag toward glucose at 0.7 V. 6.2.3. To improve the electrocatalytic activity Ni–Cu: NiCu alloy electrodes yield a significantly higher oxidation response to glucose, glycine and methanol than single Ni (Yeo and Johnson, 2001; Danaee et al., 2008). It was speculated that the introduction of Ni atoms with a lower d-orbital occupancy than Cu into the Cu-matrix would result in surface d-states at oxide-covered Ni/Cu alloys that could function as adsorption sites for Lewis bases with non-bonded electron pairs, e.g. the o-atoms of hydroxyl moieties in alcohols and carbohydrates, and N-atoms in amino acids. The increased signal intensity of NiCu alloy electrodes provided evidence for a synergistic contribution of the dissimilar metallic sites in the surface oxide. Ni–Au: The results of Yeo and Bell's study clearly demonstrated that a very thin layer of Ni(OH)2 deposited onto Au had a significantly higher OER activity relative to a thick layer of Ni (OH)2 formed on bulk Ni or electrodeposited on Au (Yeo and Bell, 2012). It was proposed that the high activity of submonolayer deposits of Ni oxide on Au was due to charge transfer from the oxide to the highly electronegative Au, leading to the possible formation of a mixed Ni/Au surface oxide. NiO–Pt: Pt/NiO microsphere compositions have stronger electrocatalytic activity than normal bulk Pt or Pt nanoparticles toward methanol oxidation and stronger dissociation of methanol (Min et al., 2008). Here, the high electrocatalytic activity can be attributed to the degree of dispersion of the deposited Pt nanoparticles. The uniform Pt nanoparticle loading on the surface of NiO microspheres could effectively enhance the electrocatalytic activity 6.2.4. Other nanocomposites Some nanomaterials such as carbon nanotubes, graphene (Wang, C. et al., 2010; Wang, H. et al., 2010) and Ti/TiO2 (Wang, C. et al., 2010; Wang, H. et al., 2010) were widely used to prepare various nanocomposites (Lee et al., 2011). Due to their high surface area, high chemical stability, excellent electrical conductivity and electrocatalytical activity, electrochemical oxidation of glucose on a glassy carbon disc electrode modified with multi-walled carbon nanotubes and nickel oxide (GC/MWCNT/NiO) was examined by Shamsipur et al. (2010). It was found that using MWCNTs as

catalyst support for oxidation of glucose improved noticeably the electrocatalytical activity of NiO toward glucose in alkaline medium. This improvement and the high electrocatalytic activity might be attributed to the unique structure and large surface area to volume ratio of MWCNTs, as well as the specific interaction between NiO and MWCNTs.

7. Outlooks Electrocatalysis and electroanalysis of Ni, its oxides, hydroxides and oxyhydroxides have been extremely investigated. Their electrocatalytic mechanisms include Ni(OH)2/NiOOH mediated electrolysis, direct electrocatalysis of Ni(OH)2 or NiOOH. Most of their electrocatalysis are based on the mediation of Ni(OH)2/NiOOH redox couple in alkaline solution. However, the direct electrocatalysis of Ni(OH)2 exhibits a very low background signal and high response current in nearly neutral solution. More researches are attracted to explore the electrocatalysis of Ni based materials toward small molecules in nearly neutral solution. In contrast to Ni materials which are unstable and easily oxidized in air and aqueous solutions, their hydroxides (or oxides) are relatively stable (Safavi et al., 2009). Also the electrocatalytic oxidation mechanisms of Ni and its oxides toward small molecules in alkaline electrolyte are related to the redox transition of Ni (OH)2/NiOOH. It seems that Ni(OH)2 is a better candidate for electrocatalysis or electroanalysis than Ni or NiO. As p-type semiconductor, the poor electric conductivity of Ni (OH)2 limits its electrocatalytic performance. So various nickel based alloys, double-layered hydroxides and other composite materials will play a great role to increase its conductivity and improve its electrocatalysis toward small molecules. Nanotechnology, which provides new opportunities for and plays an increasingly important role in the design of Ni based materials and their applications, is revolutionizing the development of material sciences. The preparation of Ni based nanomaterials with the improved electrochemical performance is being extremely explored. The application of Ni based materials in electrocatalysis and electroanalysis remains a great interest.

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Electrocatalysis and electroanalysis of nickel, its oxides, hydroxides and oxyhydroxides toward small molecules.

The electrocatalysis toward small molecules, especially small organic compounds, is of importance in a variety of areas. Nickel based materials such a...
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