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Fabrication and characterization of nanostructuredNieIrO2 electrodes for water electrolysis

Mirko Battaglia, Rosalinda Inguanta*, Salvatore Piazza, Carmelo Sunseri

Laboratorio di Chimica Fisica Applicata, Dipartimento di Ingegneria Chimica Gestionale Informatica Meccanica,

Universit�a di Palermo, Viale delle Scienze, Ed.6, 90128 Palermo, Italy

a r t i c l e i n f o

Article history:

Received 9 June 2014

Received in revised form

11 August 2014

Accepted 14 August 2014

Available online 10 September 2014

Keywords:

Ni nanowires

Template electrosynthesis

Iridium oxide

Alkaline water electrolyser

Oxygen evolution

* Corresponding author.E-mail address: rosalinda.inguanta@unip

http://dx.doi.org/10.1016/j.ijhydene.2014.08.00360-3199/Copyright © 2014, Hydrogen Ener

a b s t r a c t

Nanostructured NieIrO2 electrodes were fabricated by electrodeposition in a two-step

procedure: first arrays of nickel nanowires (NWs) were electrodeposited within pores of

polycarbonate (PC) membranes, then iridium oxide nanoparticles were deposited on the Ni

metal after membrane dissolution, for improving the catalytic activity. The aim was to

compare performance of these electrodes with traditional ones consisting of Ni film.

Different methods of deposition of the IrO2 electrocatalyst were investigated and the effect

on electrodes stability and activity is discussed. Despite a low coverage of Ni NWs by the

electrocatalyst, results indicate a faster kinetics of O2 evolution in 1 M KOH solution and an

improvement of performances for electrolysers having a nanostructured anode.

Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

Introduction

Hydrogen production by water electrolysis has a remarkable

industrial importance owing to the high purity of the product

obtained by this method [1]. Up to date this is the only way of

obtaining a pure enough fuel for low and medium tempera-

ture fuel cells [2,3]. On the other hand, the continuous in-

crease of energy consumption in the world and the pressing

necessity to limit emissions of greenhouse gas push for

alternative routes of energy generation and storage, in order

to have a transition toward a more sustainable system. The

possibility of using as energy carrier hydrogen gas generated

from electrolyzers coupled to renewable energy sources is one

of the most attractive solutions for this problem [4,5].

However, in this perspective there is still much to improve

in terms of performance increase and costs reduction [6]. In

a.it (R. Inguanta).65gy Publications, LLC. Publ

fact, the intrinsically low kinetic of the oxygen evolution re-

action (OER) affects strongly cost of the product and stimu-

lates research for new anodes having better performances

[7e9]. Usually, water electrolyzers in alkaline medium utilize

for both electrodes nickel-basedmaterials, which couple good

catalytic activity, chemical and mechanical stability and

reasonably low cost compared to other possiblematerials, like

DSA anodes [10e12]. Different methods for improving per-

formances of these electrodes have been proposed [13 and

reference therein] after comparing different types of cathode

in a single cell with zero-gap configuration. Pletcher et al. [12]

found that catalysts based on NieMo or Ru oxide give good

performances stable over 10 days. Solmaz [14] observed that

electrochemical deposition of NieIr over Ni/C cathodes im-

proves electrode activity toward hydrogen evolution.

Different Ni/Fe composite anodes were tested in alkaline

electrolyzers by P�erez-Alonso et al. [15]. Electrodes were

ished by Elsevier Ltd. All rights reserved.

Fig. 1 e Applied potential waveform (blue line) for the

fabrication of Ni nanowires and current transients (red

line) of the first three cycles. (For interpretation of the

references to colour in this figure legend, the reader is

referred to the web version of this article.)

Table 1 e List of different electrodes and principalfabrication conditions.

Electrode type Substrate Fabrication conditions

NiFilm-PCM Surface of

Polycarbonate

Membrane

Potentiostatic deposition

�1.5 V(SCE)

NiFilm-CP Carbon Paper Potentiostatic deposition

�1.5 V(SCE)

NiNws Pores of

Polycarbonate

Membrane

Pulsed Potential

Deposition

From OCP to �1.5

V(SCE) at 1 V s�1;

Stop at �1.5 V(SCE)

for 0.1 s;

From �1.5 V(SCE) to

OCP at 1 V s�1;

Stop at OCP for 10 s

IrO2-NiFilm-CP NiFilm-CP Potentiostatic deposition

0.6 V(SCE);

IrO2-NiFilm-PCM NiFilm-PCM Galvanostatic deposition

1 mA cm�2;

IrO2-NiNws NiNws Cyclic voltammetry

deposition

From �0.5 V(Ag/AgCl)

to 0.65 V(Ag/AgCl) at 1 V s�1

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 6 7 9 7e1 6 8 0 516798

obtained through electrodeposition from a salt solution and

their performance was observed to depend on composition

and morphology; in particular, the electrode Ni50-Fe50

deposited on nickel foam gave a stable cell potential of 2.2 V

under a c.d. of 300 mA cm�2 in a 30%wt KOH solution at 80 �C.Rosalbino et al. [16] have showed good catalytic activity to-

ward OER of Ni-based alloys (with Co, Cr, Mn, and Cu) and in

particular the best electrocatalyst was the Ni60Co30Cr10 alloy.

Very interesting results are also obtained by Chanda et al. [17],

who fabricated spinel-type ternary transition metal oxides of

Ni, Co and Fe by hydroxide precipitation method. These au-

thors have shown that the insertion of Fe causes a significant

improvement of the electrocatalytic properties towards OER

with respect to classical NiCo2O4 electrodes. Also Ni based

alloys (NieMo, NieCoeMo) cathodes exhibit a very high cat-

alytic towards the hydrogen evolution reaction (HER) and

allow a sensible reduction of energy consumption [18e20]. In a

recent work [21], a new type of electrode for alkaline water

electrolyses was produced by physical vapour deposition of Al

onto a Ni substrate and it was tested both for HER and OER

with the aim of their use in bipolar plates. These results are

remarkable because in order to minimize complexity and cost

of bipolar electrodes, it is necessary to develop an electro-

catalytic surface that is efficient and durable for both the

anodic and cathodic reaction.

In order to increase performance of Ni-based electrodes,

one possible approach is to develop low cost nanostructured

Ni electrodes, having very high specific area and good catalytic

efficiency [22]. In the last years, we obtained metal nickel

nanostructures with different geometries, by template elec-

trodeposition [23e25].

Knowing the good electrocatalytic properties of iridium

oxide for the OER [26,27], in this paper we have fabricated

nanostructured NieIrO2 composite electrodes depositing

iridium oxide particles onto nickel NWs. Electrodes were

tested by cyclic voltammetry (CV) and their performances

with regard to oxygen evolution were measured in 1 M KOH

solution and compared with those of bulk Ni films with or

without IrO2 deposition. Also the different types of anodes

were assembled vs. a Ni sheet cathode in an alkaline elec-

trolyser having 1 cm gap configuration, whose performance

was measured.

Experimental

Different nickel substrates were prepared for comparison: a)

Ni films supported on carbon paper (CP), b) Ni films supported

on polycarbonate membranes (PCM), c) Ni nanowires (NWs).

Carbon paper and polycarbonate membrane were selected as

substrate in order to fabricate weightless electrodes. On these

supports, nickel deposition was performed potentiostatically

at �1.5 V(SCE) from aWatt's bath (300 g/l di NiSO4$6H2O, 45 g/l

NiCl2$6H2O, 45 g/L di H3BO3) at room temperature for 90 min.

For the growth of Ni nanowires commercial membranes

(Whatman, Cyclopore, thickness: 20 mm) were used, after

sputtering a thin gold film onto one side, in order to render

this surface conductive.

Ni NWs substrates (case c) were fabricated within pores of

the same PCM, by pulsed unipolar electrodeposition, starting

from the procedure reported in Ref. [23]. Here, a trapezoidal

wave was used as in Ref. [23], but potential was pulsed be-

tween open circuit potential (OCP) and �1.5 V(SCE) (see Fig. 1

and Table 1). This potential interval allows to produce Ni

nanowires with uniform length, avoiding formation of nano-

wires with different length, due to simultaneous HER, shown

in Ref. [23]. This finding represents a significant improvement

in the synthesis of Ni nanostructures, because it shows that it

is possible to control length uniformity of nanostructureswith

the same wave shape by changing only the potential interval.

After synthesis of the nanostructures, the template was

dissolved in CH2Cl2, at room temperature for 1min. In order to

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 6 7 9 7e1 6 8 0 5 16799

guarantee the complete removal of the template, the proce-

dure was repeated in fresh CH2Cl2 for three times.

Iridium oxide was deposited electrochemically onto the

different substrates in three different ways: potentiostatically

(at þ0.6 V/SCE for 30min [28]), galvanostatically (at 1 mA cm�2

for 3 h) or by cyclic voltammetry (600 cycles between �0.5 and

þ0.65 V vs. Ag/AgCl at 1 V/s [29,30]). A Dimensionally Stable

Anode (DSA) served as the counter-electrode. In all cases,

depositions were performed at room temperature from a de-

aerated solution of Ir2O3$xH2O prepared according the

method proposed in Ref. [31]: 0.13 g of IrCl3 (iridium precursor)

were dissolved in 10 ml of 0.1 M HCl solution, which was

heated up to 80 �C for 2 h, while stirring and keeping constant

its volume. At the end of this period, a change in colour from

olive to brown was observed, revealing formation of the

Ir(OH2)2Cl4� ion. After removing O2 by bubbling pure nitrogen

(99.9999%) for 1 h, solution was alkalinized with 0.33 ml of 6 M

NaOH in order to shift pH to about 9, colour changed again to

pale olive. Final solution was used for electrodepositions, and

it resulted stable over many weeks.

In Table 1 a list of different electrodes obtained in this work

is reported.

Experiments with controlled potential were performed

using a P.A.R PARSTAT mod.2273, whilst for galvanostatic

experiments a P.A.R mod.273a was employed. Data were

Fig. 2 e SEM images at different magnification of Ni film electro

paper (ced).

acquired by a desk computer through an analogic interface

using a LABVIEW™ 7 software. Cyclic voltammetry (CV) ex-

periments were monitored and processed through commer-

cial Power Suite software.

All electrodes were characterized by CV and quasi-steady-

state polarization (QSSP). These electrochemical tests were

carried-out in 1 M KOH solution at room temperature. CV

curves were measured in the potential range from 0 V (Ag/

AgCl) to 0.6 V(Ag/AgCl) with a scan rate of 10mV/s, while QSSP

curves were recorded between 1.4 and 2.2 V(Ag/AgCl) at a scan

rate of 1 V/s.

Performance of nanostructured electrodewith andwithout

deposited catalyst was tested assembling it in an alkaline

electrolyzer (1 M aqueous KOH electrolyte), having a Ni sheet

as cathode and 1 cm inter-electrode distance. Before electro-

chemical tests, electrodes were insulated in order to limit a

circular area of about 1.2 cm2 and tested at a constant current

of 100 mA cm�2 at room temperature.

Electrodes morphology was investigated using a FEG-ESEM

microscope (QUANTA 200), equipped with Energy Disperse

Spectroscopy (EDS) probe. Structure was analysed using a

Philips diffractometer (mod. APD 2000) having the Cu Ka ra-

diation (l ¼ 0.154 nm) as the source, with a step of 0.04� and a

measuring time of 2 s for each step. XRD peaks were identified

by comparison with the ICDD Database [32].

deposited on polycarbonate membrane (aeb) and carbon

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Results and discussion

Electrodes preparation

Fig. 2 reports SEM micrographs of the different substrates;

images 2a,b show Ni film deposited on PCM, from which a

compact and rough surface free from fissures or cracks ap-

pears. More structured deposit is obtained on CP (micrographs

2c,d), which reflects morphology of the carbon fibres, totally

covered by the metal; this results in a very large surface area

suitable for the subsequent IrO2 deposition.

X-raydiffractogramsshow(Fig.S1) inbothcasespeaksrelative

to Ni metal and a sensible reduction of the C peak, testifying the

complete coveragebynickel; however, fromthe relativeheight of

Ni peaks we infer a polycrystalline structure with random

orientation for themetal deposited onto CP, whilst a preferential

growth along the (200) plane is apparent on PCM (Fig. S2).

Very different is morphology of the Ni NWs array (Fig. 3),

obtained depositing metal into PCM pores (see Experimental

section) and then dissolving template in CH2Cl2. In Fig. 3a a

large and compact population of nanowires is apparent.

Fig. 3b shows morphology of nanowires having a regular cy-

lindrical shape with diameters very close to 250 nm and

Fig. 3 e SEM images of Ni nanowires fabricated by electrodeposi

the template: (aeb) top-view, (c) cross-sectional view.

similar length (about 5 mm); their orientation is random,

reflecting geometry of template channels. Besides the array is

characterized by the presence of several voids between

nanowires that ensure the penetration of the electrolyte for

the subsequent process of IrO2 electrodeposition. The cross-

sectional view of Fig. 3c shows that nanowires are firmly

attached to the Ni support (some wires are broken due to

sample preparation for SEM analysis). Ni layer has a funda-

mental role because it acts as both current collector and me-

chanical support for nanowires. XRD analysis suggests a

polycrystalline structure of the NWs, without any preferential

orientation (Fig. S3).

As reported in the previous paragraph, deposition of IrO2

catalyst onto the substrates was carried out following three

different methods. However, potentiostatic deposition led to

degradation and breakdown of the electrodes, with conse-

quent loss of the electrocatalytic material, so that in the

following only results relative to cyclovoltammetric and gal-

vanostatic depositions will be discussed. Fig. 4a displays the

first cycles of IrO2 deposition on a nickel film substrate ob-

tained on polycarbonate support (see Fig. 2a,b): while elec-

trode potential was scanned between�0.5 and þ0.65 V vs. Ag/

AgCl at 1 V/s anodic c.d. raised up to about 10e12 mA cm�2,

with little hysteresis between different cycles. This hysteresis

tion into polycarbonate membrane after total dissolution of

Fig. 4 e (a) Growth of iridium oxide on polycarbonate membrane support by cyclicvoltammetry at 10 mV/s scan rate; (b) SEM

image of the iridium oxide deposit on PCM.

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disappeared totally with increasing the number of cycles and

the voltammogram did not changed anymore up to 600 cycles.

At the end, iridium oxide deposit appeared uniform and

compact with a typical dry-mudmorphology (Fig. 4b). Slightly

lower c.d. circulated during deposition of IrO2 onto a Ni-CP

film, but the shape of deposition voltammogram was quite

different (Fig. 5a); more importantly, in this case oxide

deposited also in form of dispersed spherical particles forming

aggregates (Fig. 5b). Finally, cyclovoltammetric deposition on

NWs array resulted in an uneven coverage of wires top by

spherical nanoparticles (Fig. 6b), sometimes forming aggre-

gates, despite a slightly higher deposition c.d.

In all cases, the absence of any peak relative to IrO2 in the

X-ray diffractograms indicates a disordered structure of the

deposited oxide catalyst, according to Ref. [33], whilst nature

of the deposited layer (or particles) was confirmed by EDS

analysis of the samples. In the case of deposition onto Ni NWs,

this latter showed peaks relative to Ir and O only when the

investigated area comprised the particles, as expected

(Fig. S4).

Similar features presented IrO2 deposited galvanostatically

(at 1 mA cm�2 for 3 h), but with some differences depending

on the substrate. While deposition on Ni-PCM gave an oxide

Fig. 5 e (a) Growth of iridium oxide on carbon paper support by c

iridium oxide deposit on CP.

film fully similar to that shown in Fig. 4b, deposition on Ni-CP

resulted in dispersed spherical nanoparticles of oxide. Galva-

nostatic deposition on Ni NWs produced larger and more

distributed agglomerates of nano-particles essentially located

at the wires top, giving a larger coverage of the front area by

the catalyst, as shown in the micrographs of Fig. 7. This

finding will explain the better performances of this kind of

electrode (see next paragraph). However, we remark that even

in this case, surface coverage by the oxide particles was not

complete. From the previous images it can be noted also that

metallic NWs are not damaged by the electrodeposition pro-

cess, but they retain the original morphology.

Electrodes characterization

The different electrodes were characterized electrochemically

through cyclic voltammetry and recording the quasi steady-

state polarization (QSSP) curves; in both cases experiments

were performed in 1 M KOH aqueous solution at room

temperature.

Presence of deposited catalyst originates a change in the

voltammograms: in particular a sensible increase (2e3 times

more, see Fig. 8) of the oxidation/reduction wave already

yclicvoltammetry at 10 mV/s scan rate; (b) SEM image of the

Fig. 6 e (a) Growth of iridium oxide on Ni nanowires by cyclicvoltammetry at 10 mV/s scan rate; (b) SEM image of the iridium

oxide deposit on Ni nanowires.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 6 7 9 7e1 6 8 0 516802

present, whose anodic peak was located at about þ0.39 V vs.

Ag/AgCl for depositions onto Ni films. This value suggests that

the observed wave is attributable to the redox couple Ni(OH)2/

NiOOH, according to Ref. [34]. In the case of IrO2 deposited

onto Ni NWs, the anodic peak potential was slightly shifted,

up to about þ0.43 V vs. Ag/AgCl (Fig. 8). We remark that all

curves shown in Fig. 8 were recorded at the same potential

scan rate of 10 mV/s. Such an increase of the area underlying

peaks after deposition of the oxide catalysts indicates a good

increment of the electroactive area due to the presence of IrO2.

This feature was observed evenwhen IrO2 was deposited onto

Ni NWs by cyclic voltammetry, despite the low coverage by

the oxide observed in the micrographs (see Fig. 3).

A rough estimation of the increment of the electroactive

area owing to the oxide catalyst deposition was obtained

comparing the area underlying the anodic peak in the vol-

tammogram in comparison with the corresponding area

recorded for a Ni film without any deposited IrO2 [35], (see

Table 2). These ratios suggest that the highest increments

occur when oxide is deposited onto a Ni-CP film, but also for

galvanostatic deposition of the oxide onto Ni NWs. This last

Fig. 7 e SEM images at different magnification of iridium oxide

deposition at 1 mA/cm2 for 3 h.

result is explainable with the very large surface area of the

nanostructures exposed to the deposition bath, even if

coverage by the oxide is far to be satisfying (see Fig. 7).

Our estimate was confirmed by the QSSP curves of Fig. 9,

where current density is recorded for the different electrodes

while potential is scanned very slowly (0.01 V/min) from 1.4 to

2.2 V(Ag/AgCl) at room temperature. It can be seen that the

highest current is recorded for the electrode obtained by gal-

vanostatic deposition of IrO2 onto Ni NWs, followed by the

electrode with the oxide deposited onto Ni-CP. We remark

that in the former case the c.d.measured at 2.1 V is close to the

value reported by Pletcher and co-workers at 55 �C [36], despite

we worked at room temperature. This suggests this kind of

electrode as possible high-performance anode for alkaline

water electrolysers.

The good performance of nanostructured electrode with

deposited catalyst was also tested assembling it in an alkaline

electrolyzer, having a Ni sheet as cathode, 1 M aqueous KOH

electrolyte and 1 cm of inter-electrode distance. Fig. 10 shows

the cell voltage vs. time under a constant current of

100 mA cm�2 (referred to the anodic area) at room

electrodeposited on Ni nanowires by amperostatic

Fig. 8 e Cyclicvoltammetric curves measured at room temperature in 1 M KOH at 10 mV/s for Ni nanowires before (a) and

after (b) electrodeposition of iridium oxide.

Table 2 e Values of electro-active area of the differentelectrodes obtained from cyclic voltammetry peak.

Electrode type Peak area sample/peak Ni film

IrO2 bygalvanostaticdeposition

IrO2 bycyclic-voltammetry

deposition

IrO2-NiFilm-PCM 2.5 7.8

IrO2-NiFilm-CP 85.8 82.6

IrO2-NiNWs 137.6 78.9

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 6 7 9 7e1 6 8 0 5 16803

temperature. A quite stable cell voltage was recorded, slightly

increasing from 2.0 up to about 2.1 V after almost 2 days. For

comparison, the same figure reports the performance of the

same cell with different anodes, consisting of a simple Ni-CP

film or Ni NWs without deposited catalyst, respectively. In

both cases, cell voltage is higher with respect to the previous

case under the same circulating c.d., in agreement with the

results of QSSP curves of Fig. 9. It is important to highlight that

these results have been obtained at room temperature and

with 1 cm of inter-electrode distance, so that they can be

Fig. 9 e Quasi-steady-state polarization curves (0.01 V/min)

measured for different electrodes in 1 M KOH at room

temperature.

improved performing test at higher temperature and with a

zero-gap configuration.

Fig. 11 shows the SEM images of the electrode of Fig. 7 after

two days of electrolysis. It can be see that most of the nano-

particle clusters of iridium oxide (clearly present in Fig. 7)

were removed, probably due to gas evolution. This explains

the cell voltage increase during electrolysis shown in Fig. 10.

However it is important to note that Ni nanowires maintain

their initial morphology and are still attached to the Ni

support.

Conclusions

Nanostructured nickel electrodes were prepared using tem-

plate synthesis in polycarbonatemembranes, and subsequent

electrodeposition of IrO2 catalyst. Deposition of the catalyst

was carried out both under constant current and by cyclo-

voltammetry. Performances of the different nanostructured

electrodes were comparedwith those of Ni films supported on

carbon paper or polycarbonate membranes.

Fig. 10 e Cell voltage vs. time at 100 mA cm¡2 for alkaline

water electrolysers assembled with different anodes and a

Ni sheet as cathode in 1 M KOH at room temperature.

Fig. 11 e SEM images of the nanostructured electrode of Fig. 7 after two days of electrolysis.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 6 7 9 7e1 6 8 0 516804

Results show an influence of the deposition methods both

on electrodes morphology and their catalytic activity toward

the OER. In particular, a higher coverage of metallic NWs by

the oxide and larger agglomerates of IrO2 nano-particles were

observed using a galvanostatic deposition; even in this case

oxide particles were essentially located at the wires top. Also

in the case of IrO2 deposition onto Ni-CP films morphology

changed with the deposition method. In all cases, deposited

IrO2 displayed a disordered structure.

The different morphology of the electrodes and the

varying coverage degree of their surface by the catalyst pro-

duced different performances, revealed by cyclic voltamme-

try and quasi-steady-state polarization curves. The latter

showed the best activity of the nanostructured electrodes

after galvanostatic deposition of the oxide catalyst. This

result was confirmed also by the performance of assembled

alkaline electrolyzers having the different electrodes as

anode.

These results points out for possible application of nano-

structured Ni electrodes in industrial electrolyzers and pushes

for deeper investigation aimed to improve surface coverage of

NWs by the oxide catalyst.

Appendix A. Supplementary data

Supplementary data related to this article can be found at

http://dx.doi.org/10.1016/j.ijhydene.2014.08.065.

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