Functionalized Ni-Cu NMFs with Cobalt

Oxides/hydroxides for Redox supercapacitors

Jerónimo, Guilherme; Eugénio, Sónia; Montemor, Maria de Fátima

CQE – DEQ, Instituto Superior Técnico, Avenida Rovisco Pais, 1, 1049-001, Lisbon, Portugal

Abstract

In this work, α-Co(OH)2 was deposited on Nickel-Copper NMF by cathodic pulsed

electrodeposition. The optimal conditions obtained for the deposition are 1650 cycles under

-1500 mV of potential. Afterwards, a thermal treatment was performed to oxidize Co(OH)2 into

Co3O4. The supercapacitor electrodes were characterized by electrochemical tests such as

Cyclic Voltammetry and Charge-Discharge Curves. The electrodes Co(OH)2 and Co3O4 showed

a capacitance of 987 F/g and 785 F/g, respectively. The stability of the electrodes was also

studied, by performing 3000 charge-discharge cycles. Co(OH)2 showed a capacitance retention

of 81% and Co3O4 a retention of 66%, after 3000 cycles. Finally, physic-chemical

characterization tests were performed using SEM/EDX, XRD and Raman techniques. They

allow surface morphology knowledge of the samples, as well as, their crystalline structure and

chemical composition.

Key words: Redox Supercapacitors; Electrodeposition; Ni-Cu Nanofoams; Cobalt; NMF

(Nanometallic Foam)

Introduction

The increasing of energy consumption, the lack of petrol resources and the irregular energy

production from renewable sources makes energy storage an important issue, in order to,

create a sustainable future. Electric energy storage systems appear as one of the best answers

to solve these problems. Electric storage systems are divided in 2 categories: Non-

electrochemical systems and electrochemical systems. The most important electrochemical

systems batteries, fuel cells and supercapacitors. Supercapacitors are electrochemical storage

systems that show high power capability (60-120s), excellent reversibility (90-95% or higher),

and long cycle life (>105).

Electrodes can be produced by a wide range of techniques; sol-gel, electrostatic spray

deposition or electrodeposition. Electrodeposition was the production method selected to

assemble the electrodes. One can stand out due to its low cost, no need for binders and can

easily be scaled-up. This deposition technique is suitable to produce porous materials like NMF

(Nanostructured metallic foams) and oxide/hydroxide deposits with a simple single-step

process. Besides that, structural and morphological properties of the deposits can be defined by

changing deposition parameters, electrolyte concentration or temperature.

Supercapacitors still have some drawbacks compared to other electrochemical

systems. Their low energy densities (<10 Wh/kg) are the main disadvantage compared to

batteries (>100Wh/kg). To, improve this property, without damaging others like stability or high

power density, an innovative electrode composite was tested in this project. The use of a Ni-Cu

NMF act as a support for the deposition of Co(OH)2 providing a high surface allowing the fast

diffusion of active species making it the perfect environment for electrochemical reactions to

happen, increasing the energy density of the electrode. Typically, these foams offer high

surface area 3D nanostructures, with 50% porosity or more, and a wide porous size distribution.

They combine the properties of metals with good thermal, electric conduction and low weight,

very important in new technology applications. Electrode materials have a major role on

improving redox supercapacitors storage properties.

The redox active material and electrolyte can improve significantly the energy density of

supercapacitor electrodes. Active material RuO2 is frequently reported has the highest

capacitance material, ranging between 720 and 1300 F/g, but low Ru reserves, high cost and

toxicity make impossible to use it at a large scale. However, electrode materials, such as, MnO2,

NiO, V2O5, Co(OH)2 and Co3O4 have been studied and tested. Normally, oxides/hydroxides can

give higher energy densities compared to conventional carbon materials and higher stability

than polymers. Co3O4 and Co(OH)2 are considered active redox pseudocapacitive materials for

the electrodes produced because they display good reversibility, great superficial area, good

conductivity, good stability against corrosion and they are relatively cheap and safe to work

with. Co(OH)2 is an interesting material due to his layered structure and big interlayer space,

ensuring a great surface area and a high ion insertion/deinsertion rate. The cobalt oxide, Co3O4

shows a spinel form as crystalline structure [AB2O4], these kinds of structures are very compact,

stable and offer a high superficial area, these properties fit the required ones for a good redox

supercapacitor.

Thus, this project aimed at searching and developing new supercapacitor electrode

materials that can improve energy density. In this study both Co(OH)2 and Co3O4 (obtained by

oxidation of the hydroxide). The novelty is that these materials were grown over Ni-Cu NMFs

and the system was evaluated as possible electrodes for redox supercapacitor. The electrode

performance was assessed by physic-chemical and electrochemical characterization.

The work highlights the effect of the applied potential and cycle number on the mechanism of

formation 3D floccule of Co(OH)2 structures, before and after thermal treatment, and gives

relevant insights concerning the composition and morphology of Co(OH)2 and oxidized Co(OH)2

(after thermal treatment) over Ni-Cu NMFs with ability to be used electrodes for redox

supercapacitors.

Experimental

Co(OH)2 Electrodeposition plus Thermal Treatment

The electrode composite was prepared over a stainless steel collector. A Ni-Cu foam was

produced and was further functionalized with electrodeposited Co(OH)2, [Figure 1]. The NMF

increases the electrode surface area, while stainless steel support acts as a current collector of

improved corrosion resistance. The NMFs were electrodeposited over the steel collector for 3

minutes applying a current density of 1.8A/cm2 using a Sorensen LH110 power supply Co(OH)2

was electrodeposited under a cathodic pulsed regime using a Voltalab PGZ 100 at room

temperature. The applied cathodic potential varied between -1500 and -2000 mV and the

number of cycles was ranged from 1100 to 1650. The electrolyte used for Co(OH)2

electrodeposition had the following composition: 0.05M Co(NO3)2.6H2O + 0.05M KNO3

(pH=6.3). A conventional three electrode assembly was used to deposit the active material. The

bath pH was set to 6.3 by adding dilute KOH. The nitrate based electrolyte leads to specific

nanostructured 3D morphologies. Analytical grade chemicals were used to prepare all solutions.

Afterwards, the samples with optimal deposition parameters (higher capacitance) were

submitted to a thermal treatment at 250ºC for 2 hours on a Memert oven model Universal UN30,

expecting to oxidize Co(OH)2 into Co3O4.

Electrode characterization

The electrochemical characterization was performed by cyclic voltammetry and charge-

discharge tests. Charge-discharge and cyclic voltammetry characterize the electrochemical

behavior of the surface materials. They are a function of the current density applied. A stability

test was also performed on the electrodes applying 3000 charge-discharge cycles with 20 A/g

current density. The voltammograms were performed with 50mV/s scan rate, within a potential

window of -1100 mV to 750 mV in 1M KOH electrolyte.

The capacitance is related to the storage capacity of the electrode and was calculated by:

C=It/mΔE, where C is the capacitance in F/g, I the current (A), t, discharge time in seconds, m

mass of the active material in grams and ΔE, potential window obtained by cyclic voltammetry.

The physic-chemical characterization was studied through SEM/EDX, Raman and XRD. The

morphology and chemical composition were defined by SEM/EDX using a FEG-SEM, Analytical

EOL 7001F. The Raman was performed using a LabRAM HR Evolution with a BX41 (Olympus,

Tokyo, Japan) microscope. A solid state laser operating at 532nm, using a magnification 50x

lens. The Raman band of a silicon wafer was used to calibrate the spectrometer. The spectra

were collected between 100 and 1250 cm-1

with an acquisition time of 10 – 25 s and 6

accumulations. A spectra with 600 lines/mm provides 2 cm-1

resolution; the confocal aperture

was established at 100 nm.

The X-ray diffractometry (XRD) measurements were performed using CU Kα radiation (1.5418

angstroms) in a Bruker AXS D8 Advance instrument at 0.002º/sec scan rate. The 2ϴ angle

varied between 10 and 70º with 0.05 increment. The crystalline structures were identified by

XRD.

Figure 1 – Representation of Transversal cut from composite electrode

Results and discussion

Deposition Parameters Optimization

The deposition parameters of Co(OH)2 on Ni-Cu NMF were optimized to reach the proposed

goals, [Table 1]. A thin, homogenous and reproducible Co(OH)2 film with high storage capability

was intended. Cycle number and cathodic potential were changed to reach the required

properties. Variation of deposition parameters changes Co(OH)2 deposition. First of all, cycle

number was optimized - samples 1, 2, 3 and 4 [Table 1]. After that 1650 cycles was selected as

optimal cycle number due to quantity of deposited active material, specific capacitance and film

homogeneity.

The potential optimization was made on samples 5, 6 and 7 that were tested between -1500

and -2000 mV of potential [Table 1]. Pulsed electrodeposition can allow deposition of Co(OH)2

at more negative deposition potentials because NMFs are exposed for small periods of time

making hydrogen evolution reaction inefficient, hence the hydrogen bubbles cannot form

making the deposition process more homogeneous.

The optimal conditions selected were 1650 cycles and -1500 mV of potential. These are sample

3 deposition conditions because this sample showed the best deposition required

characteristics [Table 1].

Over deposition occurred for potentials lower than -1500 mV, where Co(OH)2 agglomerates

were easily formed, covering all the NMFs pores [Figure 2, sample 5] and decreasing all

electrode surface area. Pseudocapacitance depends on surface area and active material mass,

therefore such condition was not optimal. In the experiments, samples 6 and 7 showed higher

capacitances than samples 1, 2 and 4 [Table 1]. Thus, active material weight has a more

important effect on storage than surface area since samples 6 and 7 present higher Co(OH)2

mass and lower surface area due to the formation of Co(OH)2 agglomerates.

Table 1 - Electrode Optimization parameters

Sample Cycle

Number

Potential

(mV)

Co(OH)2

Weight

Specific

Capacitance

Sample 3

(mg/cm2) (F/g)

1 1500 -1500 0,284 885

2 1600 -1500 0,146 966

3 1650 -1500 0,179 1749

4 1100 -1500 0,382 785

5 1650 -1600 0,438 741

6 1650 -1700 0,459 1158

7 1650 -2000 0,562 968

Electrochemical characterization

Afterwards, electrochemical comparison was made for assessing the electrochemical

performance of the Co(OH)2 electrode before and after thermal treatment. The thermal

treatment was performed on samples with optimal deposition conditions, with the objective of

oxidizing Co(OH)2 into Co3O4.

Figures 3 a) and b) show the typical Co(OH)2 and Co3O4 over Ni-Cu NMFs voltammograms.

Both electrodes evidenced several peaks, corresponding to redox reactions, showing the

pseudocapacitive nature of the materials.

Figure 3 – Voltammograms obtained in 1M KOH electrolyte with 50mV/s scan rate a) Co(OH)2 over Ni-Cu NMF; b) Co3O4 over Ni-Cu NMF.

Table 2 – Anodic and cathodic peaks for the redox reactions for Co(OH)2 over Ni-Cu NMF,

Figure 3 a)

Sample 5

Figure 2 – Sample 3 and 5 – Images obtained using an Leica OM with 9.60x magnification

Table 3 – Anodic and cathodic peaks for the redox for Co3O4 over Ni-Cu NMF, Figure 3 b)

Tables 2 and 3 show the cathodic and anodic peaks for Co(OH)2 and Co3O4 electrodes. Co3O4

has one more pair of peaks (redox reaction (a1, c1)) due to Co(OH)2 - Co3O4 interaction.

The thermal treatment induces Co(OH)2 partial/total oxidation leading to current intensity

reduction, Figure 3 b). The hydrous nature of Co(OH)2 allows several phenomena to occur

which provide charge storage increase. Co(OH)2 uses the whole volume of the electrode, while

on the Co3O4 electrode only the surface is used for charge storage. Therefore, Co(OH)2 – based

materials showed much higher specific capacitance than Co3O4 - based materials.

Excellent capacitance exhibited by Co(OH)2 can be assigned to successive electron transfer on

redox active spots (Co2+

, Co3+

, Co4+

) which is counteracted by proton transfer leading to OH-

to

O2-

conversion on hydroxide structure. After thermal treatment redox process occurs mainly on

electrode surface, arising less intense redox peaks. Electrode oxidation also leads to surface

compaction diminishing ionic conductivity.

Figure 4 – a); b) Cyclic Voltammetry at different scan rates for a) Co(OH)2 e b) Co3O4,

respectively, in 1M KOH electrolyte;

Voltammograms are limited by water instability reactions, hydrogen and oxygen evolution. As

state before, for Co(OH)2 Figure 4 a) shows only 2 redox pair peaks while b) has 3 pairs. For

scan rates above 100mV/s redox reactions are dissipated because the metal offers greater

(a1, c1) (a2, c2)

Potential (V) 0.250, -0.15 0.50, 0.075

Redox Reaction Co(OH)2 + OH- ↔ CoOOH + H2O+e

- CoOOH + OH

- ↔ CoO2 +H2O + e

-

(a1, c1) (a2, c2) (a3, c3)

Potential (V) -0.075, -0.225 0.375, 0.075 0.50, 0.30

Redox Reaction

3Co(OH)2 + 2OH- ↔ Co3O4

+ 4H2O + 2e Co3O4 + H2O + OH

- ↔ 3

CoOOH + e-

CoOOH + OH- ↔

CoO2 + H2O + e-

resistance, although the voltammogram format is kept, meaning high reversibility for both

electrodes.

Figure 5 - a) and b) Charge-discharge curves at 1, 2, 4 and 10 A/g for Co(OH)2 and Co3O4 electrodes; c) Capacitance variation vs. applied current for both electrodes d) Capacitance vs.

cycle number;

The symmetry on charge-discharge curves for the different conditions evidenced good material

reversibility - Figure 5 a) and b). Both materials worked the same potential window. Co(OH)2

electrode showed higher charge-discharge times for all the conditions. Capacitances were

calculated based on these results. The big difference between electrode capacitance can be

due to the porous structure of the electrode composite allowing easy penetration of the

electrolyte shortening ion diffusion. On other side, the compaction of the surface caused by the

thermal treatment can slow the ion diffusion process. The capacitance reduction, with increased

current density, recorded for both materials can happen due to irreversible formation of Co

species by electrodeposition at high current densities, these ones can clog foam pores reducing

adsorption/desorption processes, resulting in lower capacitive performance. Capacitance

retention was evaluated under a consecutive 3000 cycles of charge discharge. The best

performance was observed for Co(OH)2 that revealed 81% retention.

Table 4 – Electrochemical results for Co(OH)2 and Co3O4 electrodes.

Physico-chemical characterization

SEM

Figure 6 a) shows the electrode composite after deposition of Co(OH)2 on the NMFs with

optimal conditions, (1650 cycles; -1500mV). The electrode presents a flocculated-like

morphology with features of approximately 100 nm. This morphology enhances the surface area

given by the foam. We can also see a homogeneous coverage of all surfaces, where the pores

are still open.

Figure 6 b) shows the Co(OH)2 agglomerates, marked by white circles, when potential of -1700

mV and 1650 cycles were used. These agglomerates can have 15 to 60 μm, covering foam

pores, reducing electrode storage performance. Figure 6 c) shows SEM image of Co(OH)2

sample after stability tests, significant changes were not visible on surface morphology. After

stability tests apparent morphological changes or material degradation were not detected,

confirming the excellent electrode stability after extensive use.

Figura 6 – SEM Images: a) Co(OH)2 deposited over Ni-Cu NMF, 1650 cycles, -1500mV of potential; b) ) Co(OH)2 deposited over Ni-Cu NMF, 1650 cycles, -1700mV; c) Co(OH)2

deposited over Ni-Cu NMF after stability tests; d) Oxidized Co(OH)2 over Ni-Cu NMF; e) Oxidized Co(OH)2 over Ni-Cu NMF after stability test

Active Material Capacitance (F/g) at 1 A/g

Capacitance (F/g) at 10 A/g

Capacitance retention after 3000 cycles (%)

Co(OH)2 987 710 81

Co3O4 785 431 66

c

a

d

e

b c

d

Figure 6 d) and e) display the electrode morphology of Co(OH)2 oxidized to Co3O4, before and

after stability tests. The oxidized Co(OH)2 also has a floccule type of structure, although some

changes are noticed on electrode surface. We can see on both images a smoother electrode

structure where planar parallel nanosheets are formed. Thermal treatment leads to surface

compaction that can cover foam pores, having similar response like electrode morphology as

shown in Figure 6 b). The stability tests induce material degradation, visible in Figure 6 d) by the

white dots. The degradation reduces electrode performance.

Raman Spectroscopy

Raman spectroscopy is one of the most used techniques to characterize crystalline structures

and defects of metals and metal hydroxides. Raman spectra of Cobalt oxides and hydroxides

are detected at wavelengths of 100 to 1250 cm-1

. The characteristic bands of Ni-Cu NMF were

detected between, 1500 and 4000 cm-1

, these compounds will not be studied by this technique.

Figure 7 – Raman Spectra of Co(OH)2, Co(OH)2 after stability tests, Co3O4 before and after stability tests from 0 to 1250 cm

-1 gap.

Table 5- Raman spectroscopy peaks, including the most polarized peak

*Bands assigned for CoO(OH) by Chey et al. (2012), and Xu et al. (2010)

The Co(OH)2 polarized band before stability tests were assigned to the Co-O symmetric

extension vibration mode. The second most intense peak was assigned to O-Co-O - bend

Before Stability Tests After Stability Tests

Active Material

Co(OH)2 Co3O4 Co(OH)2 Co3O4

Assigned Peaks (cm

-1)

447, 517, 1030 149, 435, 500,

658 [495, 645, 1013]* [517, 635, 1025]*

Polarized Band

517 658 495 517

mode. The existence of a third wide band at 1030 cm-1

, normally less intense, could be

assigned to OH deformation modes. Raman spectra confirmed the deposition of Co(OH)2.

Co(OH)2 and Co3O4 bands were reported by Chey et al. (2012), and the ones reported by Xu et

al. (2010) are assigned as CoO(OH) bands. The evident difference before and after stability

tests can occur due to long term exposure of the electrode to 1M KOH electrolyte that may

result in active material – electrolyte interaction. Other consequence is the reduced band

intensity. Electrode long exposure to basic or acid solutions leads to sample defects. For the

Co3O4 sample long term exposure showed inverse effect since after stability test the intensity

increases.

The recorded peaks may not be exactly the same as reported in bibliography due to the

nanocrystalline nature of the film leading to broadening and intensity reduction of Raman bands.

Conclusions

Cobalt hydroxide was electrodeposited over Ni-Cu NMFs and the optimal deposition

parameters were -1500 mV and 1650 cycles. Under these conditions a thin and homogeneous

film with high capacitance was deposited. The cobalt hydroxide deposition was strongly affected

by the applied potential. More negative potentials (<-1500 mV) induced active material

overdeposition, creating agglomerates, which lowered electrode performance. Cycle number

lower than optimal (<1650) leads to deposition of less than one layer, not taking advantage of

high surface area supplied by NMFs, affecting the final electrode performance.

Co(OH)2 samples presented best results comparing to the ones after thermal treatment

(intended Co3O4). The oxidation of Co(OH)2 by the thermal treatment leads to surface

compaction. Surface compaction leads to smaller active area and consequently lower

capacitance. Co(OH)2 shows 987 F/g of capacitance and 81% of capacitance retention after 3

thousand cycles. In the other hand, Co3O4 shows a capacitance of 785 F/g and retention of

66%. A 1 A/g current was used for capacitance calculation and 20 A/g for stability tests.

SEM images evidenced the electrodeposited film morphology: the Ni-Cu NMF shows a dendritic

structure, while Co(OH)2 and Co3O4 have flocular structures; both are typical for films deposited

by electrodeposition. Stability tests were performed during 3000 charge-discharge cycles and

the electrodes developed an answer similar to that of CoO(OH) as verified by the bands position

in the Raman spectra. XRD was performed to define and characterize electrode crystalline

structure; however the results were not fully conclusive fully conclusive.

Acknowledgments:

I would like to thank CQE - DEQ for all the help to accomplish this master thesis. I also want to

thank professors Maria de Fátima Montemor, Rodrigo Della Noce and Sónia Eugénio who

helped me through the whole thesis execution. I also want to thank all my family and friends

who gave me strengths to finish this important stage of my life.

Bibliography

Chey, J.P., Chen, X., Wu, J.S., Liu, F., Zhang, X.B., Vinayak, P.D., (2012), Porous cobalt oxides

with tunable hierarchical morphologies for supercapacitor electrodes, Cryst. Eng. Comm., 14,

6702 – 6709

Della Noce, R., Eugénio, S., Boudard, M., Rapenne, L., Silva, T.M., Carmezim, M.J., Donne,

S.W., Montemor, M.F., (2016), One step process to form a nickel-based/carbon nanofoam

composite supercapacitor electrode using Na2SO4 as an eco-friendly electrolyte; RSC Adv, 6,

15920-15928

Della Noce, R., Eugénio, S., Silva, T.M., Carmezim, M.J., Montemor, M.F., (2015), α-

Co(OH)2/Carbon nanofoams composite as electrochemical capacitor electrode operating at 2V

in aqueous medium; J. Power Sources, 288, 234-242

Fundamentals of Electrochemical deposition, 2nd Edition, Milan Paunovic, Mordechay

Schlesinger, John Wiley & Sons, Inc., 25-41, 199-209, 221-237

Paunovic M., Schlesinger M., (2006), Fundamentals of Electrochemical deposition, 2nd Edition,

, John Wiley & Sons, Inc., 25-41, 199-209, 221-237

S. Eugénio, T.M. Silva., M.J. Carmezim, R.G. Duarte, M. F. Montemor, (2014),

“Electrodeposition and characterization of nickel-copper metallic foams for application as

electrodes for supercapacitors” Journal of Applied Electrochemistry, Volume 44, Issue 4, 455-

465

Tang, C.-W., Wang, C.-B.; Chien, S.-H., (2008), Characterization of cobalt oxides studied by

FT-IR, Raman, TPR and TG-MS; Thermochimica Acta, Vol.473, 68-73

Wang, G., Zhang, L., Zhang, J., (2012), A review of electrode materials for electrochemical

supercapacitors; Chem.Soc.Rev., 41, 797-828

Xu, J., Gao, L., Cao, J., Wang, W., Chen, Z., (2010), Preparation and electrochemical

capacitance of Cobalt oxide (Co3O4) nanotubes as supercapacitor material; Electrochemical

Acta, 56, 732 – 736

Yang, J., Liu, H., Martens, W.N., Frost, R.L., (2010), Synthesis and characterization of cobalt

hydroxide, cobalt oxyhydroxide and cobalt nanodiscs; J. Phys. Chem. C, 114, 111-119