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Structure and electro-catalytic properties of electrodematerials consisting of Pt nanorod decorated/RuO2

nanorod coated substrates

Michael Cross a,*, Walter Varhue a, Thomas Valdez b

aSchool of Engineering, University of Vermont, Burlington, VT 05405, USAbElectrochemical Technologies Group, NASA JPL, Pasadena, CA 91109, USA

a r t i c l e i n f o

Article history:

Received 9 February 2012

Received in revised form

28 June 2012

Accepted 1 July 2012

Available online 25 July 2012

Keywords:

Electrolysis

Hydrogen production

Nanorods

RuO2

Pt nanoclusters

* Corresponding author. Tel.: þ1 8026560734.E-mail address: mcross1@uvm.edu (M. Cr

0360-3199/$ e see front matter Copyright ªhttp://dx.doi.org/10.1016/j.ijhydene.2012.07.0

a b s t r a c t

Nanorod coated materials have the potential of being exceptional electro-catalysts. In this

investigation Pt nanorod decorated ruthenium dioxide square nanorods have been grown

on aluminized Si substrates and used as the cathode in a system to electrolyze a [2M]

aqueous solution of KOH. Gaseous hydrogen was produced at the cathode. The voltage

measured at the cathode relative to the solution, and that across both electrodes was found

to be dependent on the cathode material used. At a current density of 30 mA/cm2, the

potential drop from the cathode electrode to the liquid electrolyte was measured to be

�0.72 V for a solid Pt electrode and �0.69 V for the best Pt nanorod coated electrode.

Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction Nanostructuredmaterials have been shown to improve the

In this investigation Pt nanorod decorated/RuO2 square

nanorods have been grown on an aluminized Si substrate

surface and used as the cathode in a system to electrolyze

aqueous solutions of KOH. The production of H2 by the elec-

trolysis of water is a viable solution to the storage of energy

obtained from renewable sources such as solar, wind,

geothermal and/or tidal [1e6]. It can also alleviate the problem

of matching constant base generation with peak load demand

obtainedwith hydro and nuclear plant generation [7,8]. Finally

in terms of transportation, as the use of H2 powered vehicles

becomes widespread, H2 filling stations can be developed

which produce H2 by electrolysis from widely available elec-

trical power and water [9,10].

oss).2012, Hydrogen Energy P05

performance of electrodes used in the electrolysis process; e.g.

Ni nanowires [11], WO3 nanorods [12], and RuO2 nanorods

[13,14] as a result of their increased surface area. Electrodes

fabricated with planar Pt coatings exhibit excellent perfor-

mance as a result of the intrinsic electro-catalytic properties

of Pt. A worthy goal would be to fabricate electrodes with

nanostructured Pt components. To date this has been

accomplished only to a limited extent due in part to Pt’s

chemical and physical properties [15e23].

In this investigation the nanorodmaterials were grown via

a reactive co-sputtering process in an electron cyclotron

resonance (ECR) plasma reactor. The growth and character-

ization of RuO2 nanorod materials has been reported earlier

[24]. Previously the electro-catalytic properties of these RuO2

ublications, LLC. Published by Elsevier Ltd. All rights reserved.

Fig. 1 e SEM images of RuO2 NR film electrodes. All films were grown on an Al/Si substrate with a total deposition time of

30 min. Shown in (a) are 62 nm wide nanorods with no Pt, (b) 84 nm wide nanorods with Pt co-sputtered for the last 15 min

of the deposition time, (c) 81 nmwide nanorods with Pt co-sputtered for the last 20 min of the deposition time, and (d) 89 nm

wide nanorods with Pt co-sputtered for the last 25 min of the deposition time.

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 7 ( 2 0 1 2 ) 1 3 2 5 6e1 3 2 6 2 13257

nanorod materials was investigated and their ability to act as

the electro-catalyst in the oxidation reaction occurring at the

cathode in the electrolysis of an aqueous solution of KOH [13].

In the current investigation, the goal was to enhance the

Fig. 2 e TEM images of RuO2 film electrodes. All films were grow

(a) are 62 nm wide RuO2 nanorods with no Pt, (b) 84 nm wide Ru

deposition time, (c) 81 nmwide RuO2 nanorods with Pt co-sputte

wide RuO2 nanorods with Pt co-sputtered for the last 25 min of

catalytic properties of the RuO2 nanorods by incorporating Pt

into the RuO2 nanorod structure. The catalytic properties of Pt

in part are dependent on the ability of the Pt surface to remain

unfouled while participating in a variety of chemical

n on Al/Si with a total deposition time of 30 min. Shown in

O2 nanorods with Pt co-sputtered for the last 15 min of the

red for the last 20 min of the deposition time, and (d) 89 nm

the deposition time.

Fig. 3 e TEM images of an individual RuO2 nanorod, showing Pt clusters around the sidewall of the RuO2 nanorod surface.

The Pt was co-sputtered for the last 20 min of the total 30 min deposition period.

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 7 ( 2 0 1 2 ) 1 3 2 5 6e1 3 2 6 213258

reactions. The growth of RuO2 nanorods occurs via a self-

assembly process. The results presented herein, involving

the inclusion of Pt into the growth process of pure RuO2

nanorods, could not have been known prior to the actual

growth of the Pt doped RuO2 nanorods. The process and/or

growth environment was changed from the previous investi-

gation by supplying an atomic flux of Pt atoms via co-

sputtering from a second Pt target during the growth of the

RuO2 nanorods. The arrival, surface movement, and ultimate

inclusion of these Pt atoms into the growing thin filmmaterial

is complicated. The Pt adatoms are very non-reactive and

hence resist reaction and alloying with other thin film

constituents such as O2 or RuO2, and remain as a collection of

pure Pt metal islands in the RuO2 bulk. The structure and

performance of these Pt doped RuO2 nanorod electrodes is

described herein.

Fig. 4 e TEM image of an individual nanorod, showing Pt

nanorods bending as the bulk RuO2 nanorods continues to

grow. The Pt was co-sputtered for the final 20 min of the

30 min of total deposition.

2. Experimental

In the current investigation, the cathode electrodes were

fabricated from rectangular pieces of Al evaporation coated Si

wafer substrate pieces (Al/Si), covered with a layer of RuO2

nanorods (NRs), both including and not including Pt. Details of

the process to produce the nanorods has been reported earlier

[24]. The only change in the process required to obtain the Pt-

containing NR coating was to co-sputter from a second gun

using a pure Pt target, in the later portion of the process used

to obtain RuO2 NRs. The duration of the total sputter process

was 30 min in all cases, while the duration of Pt co-sputtering

was varied from 15 to 25 min at the end of the run. A SEM

image of the RuO2 NRs grown with the standard sputtering

process, without Pt, is shown in Fig. 1(a). The nanorods grown

Fig. 5 e TEM image of an individual nanorod, showing Pt

NRs terminating on the sidewall of the RuO2 nanorod. The

Pt was co-sputtered for 20 of the 30 min of total deposition.

Fig. 6 e Electrochemical characterization of various

cathodes constructed of Pt/RuO2 NRs on Al/Si. The

nanorods were grown with different periods of Pt co-

sputtering (0, 15, 20, and 25 min). A [2 M] KOH solution was

used as the electrolyte and a Pt wire was used as the

anode. The cathode voltage is reported relative to the

Saturated Calomel Electrode.

Table 1 e Pt and RuO2 nanorod data.

Pt depositiontime (min)

RuO2 NRWidth(nm)

RuO2

NR length(nm)

RuO2 NRdensity(mm�2)

Pt NRlength(nm)

Pt NRtop area(nm2)

15 84.3 824 10.2 10.9 19.9

20 81.1 720 11.7 11.8 18.1

25 89.4 746 10.2 13.5 51.5

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 7 ( 2 0 1 2 ) 1 3 2 5 6e1 3 2 6 2 13259

with the inclusion of Pt developed a surface coverage of Pt on

the RuO2 NR. A SEM image of these RuO2 NRs is shown in

Fig. 1(ced). At this magnification the only observable change

between these different samples is an increase in the girth of

the nanorods, from 62 � 11 nm for the nanorods with no Pt, to

89 � 7 nm for the nanorods with 25 min of co-sputtered Pt.

The nanorods shown in Fig. 1, grown with and without the

inclusion of Pt look similar with the exception of girth. At

higher magnification, the TEM image of the Pt film structure

becomes visible and it is observed that the Pt film is in the

form of Pt islands, see Fig. 2.

Plan view TEM images of a single RuO2 nanorod surface

under higher magnification are shown in Fig. 3, in these

images the top surfaces of the Pt nanorods are visible. More

Fig. 7 e Electrochemical characterization of various

cathodes constructed of Pt/RuO2 NRs on Al/Si. The RuO2

nanorods were grown with different periods of Pt co-

sputtering (0, 15, 20, and 25 min). A [2 M] aqueous KOH

solution was used as the electrolyte and a Pt wire was used

as the anode.

details regarding the structure of the Pt coating on the RuO2

NRs will be given in the results section of this document.

The apparatus used to perform the electrolysis experiment

included a BioAnalytical Systems Epsilon E2 potentiostat and

a three-electrode cell which was described more fully in

a previous publication [13]. A Solartron Analytical model 1286

was used to measure the cyclic voltammetry behavior of the

electrodes.

3. Results and discussion

3.1. Pt nanorod inclusions

The addition of the Pt sputtered atom flux produced NR

materials that can only be appreciated by observing the indi-

vidual RuO2 NRs under higher magnification, see Fig. 4. It was

determined in the earlier investigation that the RuO2 NRs

nucleate and grow to a nominal length of approximately 1 mm

in a period of time too short to be resolved [24]. Continued

growth and/or a longer deposition time results in the growthof

the existing nanorods in both the lateral and axial directions,

estimated to be 2.5 nm/min and 8.7 nm/min, respectively. The

TEM micrograph shown in Fig. 5 is visual confirmation of the

fact that nanorod growth proceeds both laterally and axially. It

can be imagined that the Pt adatoms arrive on the side surface

of the RuO2 NRs and produce Pt nanorods that developed such

that they bend in the direction of axial growth. It is assumed

that the Pt adatoms migrate about the RuO2 NR surface and

eventually nucleate into a Pt quantum dot and continue to

grow into a rod of atoms. During this time RuO2 constituent is

also adding to the sidewall of the NR aswell as to the top cap of

theNR. The Pt quantumdots continue to growbut their growth

is constrained laterally by the growth of the surrounding RuO2

material. The result is apillar orNRofPt growingperpendicular

from the RuO2 NR surface.

The tip of the RuO2 NR continues to grow axially and

therefore the Pt NRs appear to be growing with a bend toward

Table 2 e Pt nanorod data.

Pt depositiontime (min)

Pt NRdensity onthe RuO2

NR surface(nm�2)

Pt contentof the RuO2

NR surface(%)

Pt NRVolume(nm3)

Pt loading(mg/cm2)

15 0.017 34 217 0.021

20 0.016 29 213 0.020

25 0.007 36 696 0.025

Fig. 8 e Electrochemical characterization of various

cathodes constructed of Pt/RuO2 NRs on Al/Si, normalized

to Pt content. The RuO2 nanorods were grown with

different periods of Pt co-sputtering (15, 20, and 25 min).

A [2 M] aqueous KOH solution was used as the electrolyte

and a Pt wire was used as the anode.

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 7 ( 2 0 1 2 ) 1 3 2 5 6e1 3 2 6 213260

the end of the RuO2 NRs. The shape of the Pt NR is not perfectly

square as that found with the pure RuO2 NRs [24]. The Pt

nanorods on the surface of the RuO2NRs are irregularly shaped

and randomly positioned as can be observed in Fig. 5.

3.2. Electrochemical characterization

The Pt NR decorated RuO2 NR coated cathode electrodes were

characterized in an electrolysis cell discussed in a previous

publication [13]. The measured JeV curves were observed to

depend on the electrolyte concentration and material used to

construct the cathode electrodes. The work reported herein

used an electrolyte of a [2 M] solution of KOH in water. An

example of the current density plotted as a function of the

voltage applied between the cathode electrode and the Satu-

rated Calomel Electrode is shown in Fig. 6. The anode in all

cases was a piece of Pt wire.

In general the overvoltage is defined as the actual voltage

required to complete a chemical reaction above that which is

theoretically required under ideal conditions. The analysis and

Fig. 9 e Schematic representation of Pt NR growth on RuO2 NRs. S

at 5 min of the deposition time, (b) RuO2 nanorods with Pt co-spu

nanorods with Pt co-sputter initiation at 15 min of the depositio

assumptionsassociatedwith this analysishavebeendiscussed

in a previous publication [13]. A plot of the measured over-

voltage, as a function of the current density for five different

cathode materials: Pt wire, RuO2 NRs with no Pt, and three

versions of RuO2 NRs covered with Pt NRs is shown in Fig. 7.

It is interesting to note the order of performance; specifi-

cally the nanorod material with 20 min Pt co-sputtering per-

formed better than the nanorods with 15 and 25 min Pt co-

sputtering. It was expected that the performance of the

nanorod material electrode would increase with increasing

amounts of Pt. To understand the observed results it is

necessary to review the TEM images for the nanorod electrode

material shown in Fig. 2. The results of this analysis are

summarized in Tables 1 and 2, organized by increasing period

of time that the Pt sputter gun was powered: 15, 20, and

25 min. Here, Pt loading was determined by the product of the

total Pt NR coverage per RuO2 NR times the density of RuO2

NRs on the substrate surface.

The difference in performance for the three electrode

materials containing Pt shown in Figs. 6 and 7 must be

dependent on the Pt nanorod content and/or structure deco-

rating the RuO2 nanorods. Why does the electrode sample

produced after 20 min of Pt sputtering perform the best, even

better than electrodes containing more Pt? The Pt loading

values shown above in Table 2 were used to calculate the

performance of the electrodes in terms of specific current

(amps per gram of Pt). As shown in Fig. 8, the performance of

the 20 min Pt sputter sample was superior to the other elec-

trodes. It should be noted that the normalization to Pt loading

precludes the use of the Pt wire and the RuO2 NR sample data.

The Pt nanorod density was found to decrease with

increasing period of time that the Pt sputter gunwas powered.

An explanation of the variation of Pt nanorod density and size,

with the duration of time that the Pt sputter gunwas powered,

can best be accomplished by looking at the proposed growth

model sketched in Fig. 9 below.

The reason for the variation in Pt nanorod size and density

proposed here is due to the point at which the Pt nanorod

growth was initiated in the total growth process. The Pt

islands nucleate on the RuO2 NR surface that exists when the

Pt sputter target is first powered and growwith the RuO2 NR in

the lateral direction. When the Pt islands nucleate on

a smaller RuO2 NR, a larger diameter surface feature results,

with a lower density of Pt NRs.

hown in (a) are RuO2 nanorods with Pt co-sputter initiation

tter initiation at 10 min of the deposition time, and (c) RuO2

n time. The total deposition time was 30 min for all cases.

Fig. 12 e Operating efficiency of RuO2 nanorods grownwith

different periods of Pt co-sputtering (0, 15, 20, and 25 min).

Electrolysis was performed with a [2 M] aqueous solution

of KOH and a Pt wire was used as the anode.

Fig. 10 e CV plot of RuO2 nanorods grown with Pt co-

sputtered for 25 min. A [2 M] KOH aqueous solution was

used as the electrolyte.

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 7 ( 2 0 1 2 ) 1 3 2 5 6e1 3 2 6 2 13261

The electrochemically active surface area (ECSA) of plat-

inum decorated RuO2 nanorods were estimated with cyclic

voltammetry (CV). The samples where submersed in room-

temperature deaerated [2 M] KOH and allowed to equilibrate

for 1 h in the deaerated bath prior to analysis. CV scans were

performed at 10, 25, and 50 mV/s. The samples were cycled

between �0.891 V and 0.161 V versus the saturated calomel

electrode (SCE). A plot of the CV scans for the sample with Pt

deposited for 25 min is shown in Fig. 10. For this sample, the

hydrogen adsorption peaks at a scan rate of 25 mV/s were

used to estimate the ECSA, calculated to be approximately

43 m2/g at a Pt loading of 0.025 mg/cm2, which is comparable

to values listed in the literature [25e27]. With optimization of

the electrode fabrication, specifically the conductivity

between the NR film and the substrate, we expect this value to

be in the range of 80e100 m2/g.

To further assess the performance of the nanorod elec-

trolyzer system, the variation in activation potential for the

Fig. 11 e Activation potential of RuO2 nanorods grown with

different periods of Pt co-sputtering (0, 15, 20, and 25 min).

All nanorods were grown on Al/Si substrates. Electrolysis

was performed with a [2 M] aqueous solution of KOH and

a Pt wire was used as the anode.

different electrodes was considered. The activation potentials

plotted for current densities greater than 10mA/cm2 coincides

with the generation of bubbles due to water splitting and not

leakage, see Fig. 11.

Assuming an energy of 1.23 eV can be stored from the

electrolysis of one molecule of water, the above data has been

used to calculate the efficiency expected from the conversion

of electrical power to H2 as a fuel, see Fig. 12.

These results suggest that the use of the Pt NR decorated

RuO2 NRs as the electro-catalyst material on the cathode is

slightly more efficient than bulk Pt. Additionally, from a cost

standpoint, we calculate that the Pt material used in the

process described above results in an added cost of $10/m2 at

the current market price of $1500/oz [28].

4. Conclusions

The effect of the nanorod covered cathode surface on the

efficiency to produce hydrogen by electrolysis of an aqueous

KOH solution was found to be dependent on the electrode’s

surface coverage. The activity of the RuO2 NR coated elec-

trodes increased with the inclusion of Pt, which was observed

to form Pt NRs within the RuO2 NRs, with their top areas

exposed to the electrolyte. The activity of hydrogen produc-

tion increased with Pt incorporation, but did not increase

monotonically with Pt mass loading. For the electrode with

the highest Pt loading, the electrochemically active surface

area was measured to be 43 m2/g. The results were ultimately

used to calculate the efficiency expected from an electrolyzer

constructed of the different cathode materials.

Acknowledgments

We would like to thank Mr. Stephen Mongeon of IBM Micro-

electronics for assistance in TEM imaging. We are grateful to

the Navy, Office of Naval Research who sponsored the

research.

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 7 ( 2 0 1 2 ) 1 3 2 5 6e1 3 2 6 213262

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