SEM, EDS and Electrical Capacitance Study on Electrodeposited Ni-Cu Layers

Nickel-Copper metallic layers were deposited onto a steel substrate by using the electrochemical method. The morphology and the chemical composition of the deposited layers were studied by scanning electron microscopy. The electrical capacitance was measured on a functional supercapacitor made of two Ni-Cu deposited layers and a Nafion 117® membrane hydrated with distilled water, which served as a dielectric separator.

Supercapacitors are devices used for energy storage and release at demand intended to replace the conventional batteries [1][2][3][4][5]. The research in energy storage field is focused on improving the existing energy devices in terms of storage capacity, easy charging and extended lifetime [6].
Electrodeposition is a technique used to obtain materials with controlled structure and particular properties suitable to be used in the supercapacitors manufacture [7,8].
Several studies have been carried out to investigate the electrochemical behaviour of Ni-Cu alloys in acidic, neutral or slightly alkaline solutions, but few have been reported on this subject [9][10][11].
The high interest shown in the study of Ni-Cu alloys is related to their corrosion resistance, mechanical and magnetic properties, and optical properties. This type of alloy can be used to make optical mirrors due to its high reflection and gloss [12][13][14]. In addition to these properties, (such as: electrical and thermal conductivity, ductility and malleability are added [13][14][15][16].
The Ni-Cu electrochemical deposition process is carried out in two steps. In the first stage, a dendritic Ni-Cu deposition is formed by applying a controlled current density, and hydrogen bubbles are generated at the surface of the electrode. During Ni-Cu electrochemical deposition the hydrogen bubbles disrupt the continuous growth of metal layer generating dendritic morphologies. In the second stage, the production of the pores in the Ni-Cu layer occurs due to the selective Cu dealloying from the Ni-Cu dendritic structure. The convection created by detachment of hydrogen bubbles leads to the reduction of the diffusion layer from the surface of the electrode [4,17].
The aim of this paper is to characterize some Ni-Cu deposits on steel substrates obtained at different deposition parameters that may be used as supercapacitor components.

Materials and methods
A steel plate of 1.6 cm 2 was used as metallic substrate for the deposition. The plates were mechanically polished with emery paper, rinsed with water and ethanol. They are electrically isolated on one side, so that the layer deposition takes place only on one side of the steel plate. * email: di_vaireanu@yahoo.co.uk, Phone: 0214023939 The electrochemical deposition cell contained a platinum mesh as anode (active surface area 5 cm 2 ), the steel plate as a cathode, the electrolyte solution containing NiSO 4 ·7H 2 O, CuSO 4 ·5H 2 O, H 2 SO 4 , HCl (Sigma-Aldrich, ChimReactiv) similar to that reported in [11]. The electrochemical installation is made of the cell, a variable resistance, a power source, an ammeter and a thermostatic water bath to maintain the temperature at constant values.

Methods of investigation
The morphological and compositional properties of the Ni-Cu deposited layers were studied by Scanning Electron Microscopy (SEM, Zeiss Merlin Gemini II) and Dispersive Energy Spectrometry (using SEM, OXFORD Instrument with X-Max 50 detector).
The electrical capacity performance was assessed by constructing a symmetrical capacitor supercapacitor made of two steel plates covered at identical parameters, with Ni-Cu layers containing an activated Nafion 117® membrane as a dielectric separator. The membrane was used in hydrated state in distilled water. The measurements were made with a dedicated professional LCR-Multimeter 4095 (Germany), the system configuration is similar as in [18].

Results and discussions Morphology
An alloy of Fe, C and O having a dendritic structure of Ni-Cu deposits was used as the substrate. Figure 1 A show the SEM images of Ni-Cu layers, deposited at a current density of 1.5A/cm 2 for 150 s, at a temperature of 30°C. One may see that the deposit is porous, cracked and very thick with the dendritic shape of the crystals same as the deposition obtain at 100s (B). As seen at the samples deposited at a high current and high density, the layer deposited is a thin film on the surface of the crystals formed by the hydrogen bubble convection process. The pore diameters are approximately 10µm and the connecting bridges thickness is approximately 18µm for the deposition at 150 s and for 100 s pore diameter is about 11µm and the connecting bridges thickness is about 14µm. For the same current density but a deposition time of 50 s (C), the deposition is one with defined but thin pores, that the steel substrate is visible between the bonding bridges. The pore diameter is about 10µm and the connecting bridges thickness is about 9µm.
At a deposition current density of 0.7A/cm 2 for 150 s (D) the Ni-Cu layer is very thick and porous. The pore diameter is approximately 14µm, while the bond bridge thickness is about 14µm. If the deposition time decreases at 100 s (E) the pore diameter drops down to approximately 9µm and the thickness of the connecting bridges is to 7µm. The deposition performed at 50 s (F) and at current density of 0.7A/cm 2 is very thin, the steel substrate can be easily seen, but still have a dendritic shape of crystals. The pore diameter is about 14µm and the connecting bridges thickness is about 7µm. Figure 2 A2 shows the SEM images of Ni-Cu layers, deposited at a current density of 1.5A/cm 2 for 150 s, at a temperature of 40°C. One may see that the deposit is porous, cracked and very thick with the dendritic shape of the crystals same as the deposition obtain at 100s (B2). The pore diameter is approximately 15µm and the connecting bridges thickness is approximately 16ìm for the deposition at 150 s and for 100 s, the pore diameter is about 13ìm and the connecting bridges thickness is about 21µm.
For the same current density but a deposition time of 50 s (C2), the deposition is one with defined but thin pores, that the steel substrate is visible between the bonding bridges. The pore diameter is about 13µm and the connecting bridges thickness is about 18µm. At a deposition current density of 0.7A/cm 2 for 150 s (D2) the Ni-Cu layer is thick and porous. The pore diameter is approximately 19µm, while the bond bridge thickness is about 15µm.
If the deposition time decreases at 100 s(E2) the pore diameter drops down to approximately 17µm and the thickness of the connecting bridges is to 13µm. The deposition performed at 50 s (F2) and a current density of 0.7A/cm 2 is very thin, the steel substrate can be easily seen, but still have a dendritic shape of crystals. The pore diameter is about 13 µm and the connecting bridges thickness is about 8µm.

Composition
The nickel concentration increases as the deposition time and current density decreases. At a deposition time of 50 s , a current density of 0.7A/cm 2 and a deposition temperature of 30°C, the Ni concentration is 16.13% and that of Cu is 57.7% while at a deposition time of 150 s and a current density of 1.5/cm 2 , Ni concentration is 8.35% and Cu is 42.72%.
At a current density of 1.1A/cm 2 regardless of the time of deposition, the Ni concentration is the highest. At a deposition time of 150 s , the Ni concentration is 15.94%, at 100 s, is 17.39% and at 50 s, is 30%.
At a deposition temperature of 40°C, the Ni concentration has the highest values at a deposition of 100 s and a current density of 1.5 A/cm 2 , it does not exceed 22.69% and the Cu concentration is 34.71%.
The lowest Ni concentration, 6.9%, is recorded for a deposition at a current density of 1.1 A/cm 2 for 150 s and the equivalent Cu concentration is 45.4%.

Electrical capacitance measurements
In figure 3 is shown that the values of the electrical capacitance varies between 16.9µF/cm 2 at a deposition temperature of 30 o C and 16.4µF/cm 2 at a deposition temperature of 40 o C for a current density of 0.7A/cm 2 and 50 s deposition time. If the time deposition increases up to 150 s, the electrical capacitance varies between 8.8µF/ cm 2 at a deposition temperature of 30 o C and 17.9µF/cm 2 at a deposition temperature of 40 o C.
If the deposition current density increases at 1.5 A/cm 2 , the electrical capacitance at a deposition time of 50 s increases up to 35.8µF/cm 2 at a deposition temperature of 30 o C and decreases to 9.3µF/cm 2 when the deposition temperature is 40 o C. If the time deposition increases up to 150 s, the electrical capacitance varies between 42.9µF/  The highest value of electrical capacitance, 79.4µF/cm 2 , is measured at a deposition performed at a current density of 1.1A/cm 2 at 50 s deposition time and 30 o C deposition temperature.

Conclusions
Electrodeposition of Ni-Cu alloys carried out in acidic electrolyte solutions was used to obtain a nanostructured metallic foam in order to be used as electrode plates for supercapacitors. The Ni-Cu deposits on steel plates have randomly distributed circular pores with dendritic crystal structure. The parameters applied to the deposition stage, have a strong influence on the thickness, morphology of the final deposits, affecting also the pore diameters and the connecting bridges and the Ni-Cu ratio.