enhanced electrocatalytic performance of interconnected rh nano-chains towards formic acid oxidation

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Cite this: Energy Environ. Sci., 2011, 4, 1029

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Enhanced electrocatalytic performance of interconnected Rh nano-chainstowards formic acid oxidation†

Bhaskar R. Sathe,a Beena K. Balana and Vijayamohanan K. Pillai*ab

Received 4th July 2010, Accepted 26th November 2010

DOI: 10.1039/c0ee00219d

A chain-like assembly of rhodium nanoparticles (5–7 nm mean diameter) has been synthesized from

rhodium chloride with the help of polydentate molecules like tartaric and ascorbic acids (1 : 3 in mM

scale) as capping agents at room temperature. Subsequent characterization using transmission electron

microscopy, X-ray diffraction and X-ray photoelectron spectroscopy reveals a unique inter-connected

network like features, while their electrochemical behavior using cyclic voltammetry and current–time

transient suggests potential applications as electrocatalysts in fuel cells. A significant negative shift in

the onset potential as well as higher anodic peak current density for formic acid oxidation on Rh-

tartaric acid (Rh-TA) as compared to that of bulk Rh metal confirms their higher electrocatalytic

activity. Interestingly, the enhancement factor (R) with respect to that of bulk metallic Rh towards

formic acid oxidation ranges up to 2000% for Rh-TA and 1200% for Rh-AA (Rh-ascorbic acid)

respectively. The composition of Rh nano-chains has been further analyzed with thermogravimetry and

Fourier transform infra-red spectroscopy to demonstrate the importance of controlling the chain

topology using polyfunctional organic molecules. These findings open up new possibilities for tailoring

nanostructured electrodes with potential benefits since the development of a better electrocatalysts for

many fuel cell reactions continues to be an important challenge.

aPhysical and Materials Chemistry Division, National ChemicalLaboratory, Pune, 411008, India. E-mail: [email protected]; Fax: +91-20-25902636; Tel: +91-20-25902588bCentral Electrochemical Research Institute (CECRI), Chennai,Karaikudi, India

† Electronic supplementary information (ESI) available: Energydispersive spectra (EDS); cyclic voltammetry (CV); and transmissionelectron images of both (Rh-TA) and (Rh-AA) nano-chains afterelectrochemical studies. See DOI: 10.1039/c0ee00219d

Broader context

Rhodium nanostructures are attractive materials which need to be ex

tolerance for CO with respect to Pt and their better size-dependant

have several unique capabilities like chemical inertness and their sp

remains yet to be tapped. Although there has been an enormous ad

Rh nanostructures, most of these approaches have several limitatio

boiling solvents), and toxic (hydrazine derivatives, thiols and amines

is also a need for high power consumption and, hence, developing e

at room temperature is important for many technological applic

nanoparticles in the beginning using specific capping agents followed

along with their potential applications in the area of fuel cells. Inte

catalytic activity for formic acid oxidation depending on their size

This journal is ª The Royal Society of Chemistry 2011

I. Introduction

Preparation and characterization of monodispersed nano-

particles has been a central issue of surface and interfacial elec-

trochemistry with many technologically important applications.

Although high yield synthesis of many of these particles has been

achieved in the past, simultaneous control of their shape/surface

structure and structural anisotropy is still a challenge for many

materials.1 Interestingly, as compared to homogeneously struc-

tured nanoparticles, interconnected (inter-assembled) nano-

particles promise further degrees of freedom towards tuning the

ploited properly for fuel cell applications in view of their higher

electrocatalytic properties despite higher cost. Moreover, they

ecific catalytic activity for a variety of organic transformations

vancement in the development of synthetic methods for making

ns such as the use of highly expensive (transfer agents or high

) chemicals. In some cases, like chemical vapor deposition, there

nvironmentally friendly, strategies for stable Rh nanostructures

ations. Accordingly, this work describes the synthesis of Rh

by their assembly as high aspect ratio structures, such as chains,

restingly, these nanochains show a unique variation in electro-

and capping agent.

Energy Environ. Sci., 2011, 4, 1029–1036 | 1029

http://dx.doi.org/10.1039/c0ee00219d





Scheme 1 Diagrammatic representation of the mechanism of Rh

nanoparticles via nucleation and growth through aggregation followed

by the self-assembly into aligned nano-chains.

structure–function relationship related to their size and shape of

each segment as well as the coupling between them.2 Accord-

ingly, the literature is replete with various attempts for the

coupling of ‘‘as-synthesized’’ metal nanoparticles, often with

in situ assembly of nanostructured colloids, nanoparticle

agglomerates, submicron hollow spheres and nanoparticle rings

using, bi-functional surfactants and analogous polymeric

systems.3–7 Properties exhibited by the chain-like one-dimen-

sional (1D) nanostructures include distinct collective behavior as

a function of their spacing, in sharp contrast to similar properties

of individual particles dictating their potential applications.8–10

For example, cooperative behavior of organized assembly of

transition metals like Pd, Pt, Rh, Ir and Ru are of special

importance in areas like oxidation catalysts, fuel cells, solar

energy conversion, environmental remediation etc., owing to

their excellent activity, tenability and stability.11–13

Despite its high cost, Rh is an attractive material which needs to

be exploited properly for fuel cell applications in view of its higher

tolerance for CO (present inadvertently in the fuel stream or

formed as intermediates in some cases, such as when methanol is

used) with respect to Pt and more significantly due to its better size-

dependant electrocatalytic properties.14,15 Moreover, it has several

unique capabilities like chemical inertness towards mineral acids

and its specific catalytic activity for a variety of organic trans-

formations13 on single crystals,16 multivalent redox capability17

and ability to act as a supported catalyst on a variety of flat oxide

substrates.18,19 However, the dependence of the rate of formic acid

oxidation on the Rh nanoparticles is not entirely understood. This

is partly due to the fact that many of these studies have been per-

formed under widely varying catalytic conditions and with

samples using different synthetic methods. Even though, there has

been an enormous advancement in the development of synthetic

methods by solution chemistry, these approaches have some

limitations, such as the use of highly expensive (transfer agents or

high boiling solvents), or toxic (hydrazine derivatives, thiols and

amines) chemicals, the need for the excess use of solvent due to very

low concentrations of metal precursor (<0.01 M), or high electrical

power consumption due to many high temperature reactions

(especially during chemical vapor deposition). Hence, room

temperature methods using environmentally friendly and readily

available chemicals with safe processing steps are desired to realise

completely the potential applications of these nanostructured

materials and their assemblies.

Herein, we propose an environmentally friendly and facile

strategy for the synthesis of water soluble Rh nano-chains

without using harmful chemicals or solvents, or any other metal

catalyst for nucleation via polydentate molecules like tartaric

(TA) and ascorbic (AA) acids. More specifically, this involves an

one-pot synthesis combining both the in situ formation of Rh

nanoparticles and their self-assembly into a chain like

morphology due to the presence of these capping agents, with

several novel features as shown in Scheme 1. To the best of our

knowledge, there is no report on the synthesis of Rh nano-chains

at room temperature using such green/nontoxic capping agents,

although this mode of confined growth has been reported for Au,

Ag and Pt. More interestingly, these nano-chains show a unique

variation in electrocatalytic activity for formic acid oxidation,

which is important for applications like micro-fuel cells in

comparison with that of bulk Rh.

1030 | Energy Environ. Sci., 2011, 4, 1029–1036

II. Experimental aspects

II.1 Materials

Rhodium chloride (99.9%), ascorbic acid (AA), tartaric acid

(TA) and sodium borohydride (NaBH4) were purchased from

Aldrich Chemicals, while AR grade sulfuric acid (H2SO4), formic

acid (HCOOH), perchloric acid (HClO4), ethanol and acetone

from Merck. All reagents were used without further purification

and de-ionized water (18MU) from Milli-Q system was used in all

experiments.

II.2 Synthesis

Rh nano-chains were synthesised by a monophasic (aqueous)

mixture using 1 : 2 molar ratio of rhodium chloride to TA, stirred

in an ice bath for 30 min, followed by reduction using drop-wise

addition of 0.1 M aq. NaBH4 solution. The orange red colour of

Rh solution turned blackish gray indicating the formation of Rh

nanoparticles within 30 min. Further, the same reaction was

extended for next 3 h (Ostwald ripening) for the synthesis of Rh

nano-chains (Rh-TA). Subsequently, these were precipitated

repeatedly using ethanol to remove any unbound capping agent.

These freshly prepared samples were used for further studies; Rh-

AA nano-chains were also synthesised by a similar procedure,

but with extended time. Bulk Rh was prepared by the reduction

of Rh3+ in the absence of capping molecules.

II.3 Characterization

Structural and morphological studies were carried out by

transmission electron microscopy (TEM) attached with selected

area electron diffraction (SAED) on a JEOL model 1200 EX

instrument operated at an accelerating voltage of 120 kV. TEM

samples were prepared by placing a drop of the Rh-TA/Rh-AA

samples in water onto a carbon-coated Cu grid (3 nm thick),

dried in air and loaded into the electron microscopic chamber.

X-Ray diffraction (XRD) was recorded on a Philip 1730 instru-

ment using Cu-Ka radiation at a step of 0.02� (2q). Thermog-

ravemetric analysis (TGA) were carried out using a Perkin-Elmer


TGA 7 thermal analyzer from 50 �C to 900 �C at a rate of

10 �C min�1 in air. X-Ray photoelectron spectroscopic (XPS)

measurements were carried out on a VG Micro Tech ESCA 3000

instrument at a pressure of >1� 10�9 Torr (pass energy of 50 eV,

electron take off angle 60� and the overall resolution was

�0.1 eV). The IR spectrum was recorded at room temperature on

a Perkin Elmer spectrum GX 69229/30 FTIR spectrometer in

a KBr matrix.

Fig. 1 (a) Transmission Electron Micrograph (TEM) of Rh-TA nano-

particles after 30 min; (b) Rh nano-chains by the interlinking of nano-

particles after 3 h on the micron scale; along with (c) their particle size

distribution with an average particle diameter of 7.0 � 0.5 nm; (d) SAED

pattern reveals rings corresponding to crystalline Rh (fcc).

II.4 Electrochemical studies

All the electrochemical measurements were performed on an

Autolab PGSTAT30 (ECO CHEMIE) instrument using a stan-

dard three electrode cell comprising of an Rh nano-chains

modified glassy carbon disc as working electrode, Pt foil as

counter electrode and Hg/Hg2SO4 as reference electrode in 0.5 M

H2SO4 at room temperature and the electrochemically active

area (ARh) was determined from the adsorption/desorption

charge of hydrogen measured from cyclic voltammetry

(ca. 230 mC cm�2 for a polycrystalline surface) (see the ESI, SI–

I†). The glassy carbon electrode (GCE; 3 mm diameter from

ECO CHEMIE) was polished before each experiment with 1.0,

0.05 and 0.03 mm alumina slurry and then sonicated in de-ionized

water for 10 min. The calculated amount of Rh nano-chains were

sonicated in water for 10 min and drop-casted on an active tip of

the glassy carbon electrode maintaining the same Rh loading on

the electrode surface for both. After evaporation under a partial

vacuum for 30 min, 10 mL of 0.1 wt % Nafion solution was

dropcasted on the electrode surface to cover and stabilize the

nano-chains. Prior to the analysis, the electrolyte was saturated

by passing ultra-high purity nitrogen. Further, in order to

measure the electrocatalytic activity of these nano-chains

towards formic acid oxidation, both CV and current-time (I–t)

transients were recorded in a mixture of 0.5 M HCOOH and

0.5 M H2SO4. I–t transients were recorded at a respective onset

potentials versus Hg/Hg2SO4 at an interval of 0.05 V for a period

of 100 s after ensuring sufficient care to oxidize any adsorbate on

the electrode surface so as to obtain a clean, virgin surface.

Accordingly, in order to see the effect of adsorbed capping

molecules towards the electrooxidation of formic acid, voltam-

mograms were recorded under different scan rates for both nano-

chains after dipping (for 1 min) in their respective capping

molecule (1 mM TA and 1 mM AA) solutions. We have also used

HClO4 which is less strongly specifically adsorbed than H2SO4 as

an alternative electrolyte to confirm these findings.

Fig. 2 (a) Transmission Electron Micrographs (TEM) of mono-

dispersed Rh-AA nanoparticles after 30 min; (b) Rh-AA nano-chains

after 3 h assembled on the micron scale; along with (c) their particle size

distribution with an average particle diameter of 5.0 � 0.5 nm; and (d)

SAED pattern revealing rings corresponding to crystalline Rh (fcc).

III. Results and discussion

Fig. 1 shows a comparison of the TEM images of freshly

prepared Rh-TA nanoparticles having a particle size of 7 �0.5 nm (a) stable for 30 min., while (b) nano-chains of similar size

are seen to be self-assembled in the image taken after 3 h.

Further variation in time does not affect the morphology

significantly, although more particles with poorly defined shapes

having considerable anisotropies are formed. However, coales-

cence to larger particles is not observed, perhaps due to the

presence of polyfunctional capping agents. In comparison,

Fig. 2(a) and (b) shows similar TEM images of Rh-AA nano-

particles after 30 min and Rh-AA nano-chains after 3 h at the


same magnification, respectively, revealing nearly mono-

dispersed chains of 5.0 � 0.5 nm (see Fig. 2c) having lengths of

several microns. These aggregates however, show a close-packed

assembly of smaller Rh nanoparticles compared to those in

Fig. 1(a) and (b) respectively for Rh-TA.

We have also monitored the chain formation quantitatively by

proper size and shape analysis from TEM images at different

intervals of the growth process. In the first step, the primary Rh

nanocrystallites nucleate and grow into nearly monodispersed

nanoparticles as shown in Fig. 1(a) and 2(a) respectively. Finally,

these nanoparticles reach their equilibrium size and further self-

assemble into nano-chains by virtue of the polyfunctional nature

of the organic molecules (TA and AA). However, at longer times

(i.e. 3 h), there is no further growth into bigger nanoparticles,


except perhaps their interlinking in nano-chain formations with

a similar size distribution (Fig. 1c and 2c). Hence, it is clear that

poly-functional molecules play an important role in improving

their connectivity and stability.

However, they have same crystal structure as shown in

Fig. 1(d) and 2(d) by representative SAED patterns of Rh-TA

and Rh-AA where, crystalline nature of the Rh, exhibiting

dominant reflections corresponding to (111), (200), (220) and

(311) planes of the fcc Rh, is in complete agreement with XRD

results.20 SAED rings are not continuous but composed of

discrete spots, which suggest preferential orientation of the Rh

nanoparticles in the interconnected network.

Salient features of the XRD pattern of these nano-chains

along with bulk Rh exhibit predominant reflections in the range

of 0–80�, which confirms the formation of fcc structure as shown

in Fig. 3(a). This is in good agreement with corresponding SAED

results and also with earlier reports.20 The crystallite size of our

nano-chains (i.e. �7 nm and �5 nm for Rh-TA and Rh-AA,

respectively) as inferred from the FWHM (full width at half

maxima) of 43.54� peak (corresponding d values of 2.25 �A and

2.27 �A), is again in excellent agreement with the corresponding

size calculated from TEM images shown in Fig. 1 and 2.

Thermogravimetric analysis also indirectly provides a measure

of the strength of chemical bonding between the Rh surface

atoms and capping (TA/AA) molecules. Accordingly, Fig. 3(b)

shows a superimposed TGA curve obtained for Rh-TA and Rh-

AA nano-chains in the temperature range of 50–900 �C under

air. Initially, �12% weight loss corresponding to the TA takes

place in the range of 160–570 �C. Further, the TGA curve for Rh-

AA shows a �20% weight loss corresponding to the removal of

AA within the range of 200–850 �C, with slow exponential decay,

which could be due to polyfunctionality along with their unequal

bonding features to Rh throughout the network of nano-chains.

Comparatively higher thermal stability (�300 �C) of Rh-AA

with respect to Rh-TA along with their slow decrease in weight

Fig. 3 (a) X-Ray diffraction pattern of as-synthesized (I) Rh-TA and

(II) Rh-AA nano-chains; (III) bulk Rh revealing reflections from (111),

(200) and (220) planes (fcc). (b) Thermogravimetric curve of Rh-TA and

Rh-AA revealing slow decomposition of AA molecules (20% weight loss)

than that of TA, having 12% weight loss in air. (c) EDX profile of (IV)

Rh-TA and (V) Rh-AA nano-chains.


loss in the wide range could be due to the fundamental structure

of AA having a cyclic furan ring with a relatively higher thermal

stability than that of aliphatic TA along with their own contri-

bution from inter-chain van der Waals interactions. If these

respective decomposition temperatures are indicative of

a measure of the strength of the capping molecules to Rh,

Fig. 4(b) suggests that the oxalate–Rh bond of Rh-AA is

stronger than the bond between Rh-TA. However, for TA cap-

ped Rh nano-chains, the overall weight loss (�12%) is much

smaller than that corresponding (�20%) to AA capped Rh nano-

chains. This fact is also supported by the appearance of intensity

variation of multiple de-convoluted XPS peaks corresponding to

Rh, C and O signals. This clearly indicates the intriguing

geometrical effects of TA/AA molecules towards their stability

and assembly formation.21 Further, EDX analysis has been

carried out to verify local chemical composition of both Rh-TA/

Rh-AA nano-chains as shown in Fig. 3(c) (IV and V). The signals

at 0.26 keV and 0.51 keV correspond to carbon and oxygen from

capping molecules and peaks in the range 2.6–3.2 keV could be

identified as Lb1 and La1 emission X-ray signals of Rh confirming

the presence of TA and AA capped with Rh nano-chains.

Moreover, no impurity signals such as chloride ions from

precursor and of reducing agent have been detected.

FTIR studies also reveal the relative role of capping molecules

towards the interconnected nano-chain formation. Accordingly,

Fig. 4(a–d) shows the FTIR spectra of TA and AA before and

after capping with Rh. Interestingly, a comparison of these

spectra reveals the disappearance of two/four bands in the range

of 3300–3600 cm�1 for the nano-chains (Rh-TA and Rh-AA),

respectively, which corresponds to the symmetric –O–H

stretching of both the TA and AA skeleton. A careful compar-

ison of the IR spectra thus clearly indicates that the capping

molecules are linked through the –O–H groups to the Rh

nanoparticles as can be seen from (a) and (d) respectively.

Fig. 4 Superimposed FTIR spectra of (a) Rh-TA, (b) TA, (c) AA and

(d) Rh-AA performed in KBr matrix indicating strong peaks at 950 cm�1

and 500 cm�1 corresponding to Rh–O linkage of Rh-TA and Rh-AA,

respectively.


Fig. 6 X-Ray photoelectron (XP) spectra of Rh-AA revealing core level

information on capping (linker) molecule comprising (a) Rh 3d, (b) O 1s

and (c) C 1s signals.

Surprisingly, the appearance of additional strong bands in the

range of 1350–1650 cm�1 in both the cases suggests the involve-

ment of carbonyl and lone pair of oxygen for their network

formation. Moreover, the appearance of an additional strong

band at 950 cm�1 in case of Rh-TA and at 500 cm�1 in case of Rh-

AA is attributed to the presence of an Rh–O bond.22 The rest of

the peaks remain common in both the cases corresponding to the

capping molecule skeleton. Therefore, it is likely that Rh nano-

particle surfaces in the nano-chain already had a complex-like

environment because of the polydentate nature of TA and AA

molecules. This is also supported by the TG analysis which

reveals a slow weight change due to the disparate utilization of

polyfunctional association of TA and AA towards the assembly

formation.

XPS analysis was carried out in order to understand the

molecular level interactions of the TA and AA on Rh nano-

particle chains of both the electrocatalysts and these results are in

good agreement with that of FTIR, EDS and TG analysis.

Accordingly, Fig. 5 and 6 show XPS analysis of core level

spectra of Rh 3d, O 1s and C 1s of Rh-TA and Rh-AA nano-

chains, respectively. The peak position, line shape and peak-to-

peak separation (�4.7 eV) are the standard measures of Rh

oxidation state and the binding energy for Rh 3d doublet is

consistent with the Rh� oxidation state [Fig. 5(a) and Fig. 6(a)].

Moreover, Fig. 5(b) shows XP spectra of O1s where the decon-

volution of the peak with respect to Gaussian fitting shows two

distinct peaks at 532.2 eV and 533 eV, corresponding to oxygen

species of carbonyl (>C]O) and –O� of all the hydroxyl groups

of TA respectively. These perhaps reveal potently active sites for

interconnected organization. Fig. 5(c) shows C 1s core level XP

specta having three distinct peaks at 284.8 eV, 281.7 and

287.5 eV, respectively. The peak at 284.8 eV is attributed to the C

1s peak for –C–C– linkages corresponding to TA, while another

Fig. 5 X-Ray photoelectron (XP) spectra of Rh-TA revealing core level

information on the linker capping molecule, (a) Rh 3d, (b) O 1s and (c) C

1s signals (experimental data points are shown as circle, resultant fitting

curves as continuous lines and individual fitted curves as dashed lines

deconvoluted by using the Shirley fitting algorithm).


higher binding energy peak at 287.5 eV for –C–O linkages

corresponds to the carbonyl group;21,23 another lower binding

energy peak at 281.7 might be due to overcharged carbon of the

carbonyl after network formation.

In comparison, Fig. 6(b) shows XP spectra of O1s where the

three distinct peaks at 529.3 eV, 532.2 eV and 533 eV correspond

to oxygen species of –C–O–C–, (>C]O) and –O� of all the

hydroxyl groups of AA respectively, which are active sites for

stabilization. Similar intensity variation along with the binding

energy change corresponding to C 1s associated with the carbon

of >C]O, C–C and C–O–C is analogous to those observed in

the case of Rh-AA.

A plausible explanation for the growth of interconnected

nano-chains may be offered on the basis of a kinetically

controlled mechanism since the particle surface becomes more

stable via deactivation of larger facets by TA, necessitating

a comparatively longer time for growth of nano-chains from

nanoparticles. However, extendable assembly of these nano-

particles is formed upon stirring, especially in the presence of free

functional groups on capped TA/AA leading to

nanochain formation as illustrated (Oswalds ripening) in Scheme

1. Interestingly, even after the formation of Rh nano-chains

(Fig. 1 and 2), the average diameter does not change, suggesting

no predominant effect of stirring on the anisotropic growth

modes, perhaps due to qualitative conformational changes in

their capped polyfunctional molecules finally leading to their

assembly.24 This is a clear evidence for our assertion that nano-

chains are formed by the interlinking of nanoparticles on the

basis of polydentate nature of the capping molecule. Recently,

Zhou et al. reported a similar strategy for the synthesis of Ni/

Ni3C nano-chains (size of �25.4 nm) by using a mixture of tri-

octylphosphine oxide (TOPO) and poly vinyl pyrollidone (PVP)

with their bigger size and poly-dispersity compared to our Rh

chains perhaps due to the weaker binding nature of the capping

molecules (PVP and TOPO).25 Another supportive reason for the

network formation can be drawn from the relative metal-binding


Fig. 8 (a) Superimposed CVs of (I) Rh-TA, (II) Rh-AA and (III) bulk

Rh in 0.5 M HCOOH + 0.5 M H2SO4 using a Hg/Hg2SO4 as reference

electrode and Pt foil as a counter electrode at a sweep rate of 50 mV s�1.

(b) Superimposed I–t transients of HCOOH oxidation for (I) Rh-TA, (II)

Rh-AA and (III) bulk Rh at an applied respective onset potentials

(0.25 V, 0.19 V and 0.20 V) for 100 s.

ability of both –OH and >C]O groups. The adjacent Rh

nanoparticles are interconnected either by hydrogen bonding

through the direct coupling between –OH and >C]O groups or

by covalent bonding (see Scheme 1), as is evident from the

combined evidence of TEM, TG, FTIR and XPS analysis.

Further, the role of –OH and >C]O as surface passivating

agents as well as linker centers for the chain formation is inferred

from their highly branched interconnected nano-chain formation

(TEM shown in Fig.1 and 2).

In this way, the surface bound TA and AA moieties provide

more stability and controlled electrochemical properties to these

nanoparticles. Moreover, the use of TA and AA render the Rh

more active, as the tendency for agglomeration is prevented by

modifying the Rh surface geometrically/electronically through

their assembly formation. In other words, TA and AA form self-

assembled mono/multilayers on the surface providing stability

for the small size distribution and these small particles of 5–7 nm

do reveal that quantum size effects as confirmed by some of our

earlier reports of excellent catalytic and single electron perfor-

mance as a function of size.17,20 As we know from colloidal

nanoparticles, their surface passivation by organic capping

molecules does not block all the active sites, especially when

polydentate molecules like TA and AA are used (Fig. 9). The

control over particle size and dispersity by surface functionali-

zation, intended to enhance particle stability and dictate surface

chemistry, solubility and the degree of particle interactions also

helps to increase their electrocatalytic performance.

Accordingly, Fig. 7 shows the superimposed CV for Rh elec-

trocatalysts in 0.5 M H2SO4 (I) and in a mixture of 0.5 M H2SO4

and 0.5 M HCOOH solution (II) at 50 mV s�1 to correlate the

potential range of surface Rh-oxidation and formic acid oxida-

tion. Accordingly, superimposed CVs in 0.5 M H2SO4 (I) exhibits

a prominent oxidation reaction with an anodic peak at 0.40 V,

0.20 V and 0.23 V corresponding to (a) Rh bulk, (b) Rh-TA and

(c) Rh-AA along with a reduction of the oxide species at�0.05 V,

�0.20 V and �0.22 V, respectively, during the reverse cathodic

sweep. Furthermore, voltammogram (II) compares the electro-

chemical features of formic acid oxidation using bulk Rh, Rh-TA

and Rh-AA in a mixture of 0.5 M HCOOH and 0.5 M H2SO4.

When the anodic potential increases during the forward scan,

peaks at an onset potentials 0.25 V, 0.19 V and 0.20 V are

observed for bulk Rh, Rh-TA and Rh-AA respectively, corre-

sponding to the oxidation of HCOOH. During the forward scan,

formic acid oxidation produces a prominent anodic peak with

a current density 490 and 280 mA cm�2 for Rh-TA, Rh-AA,

respectively; while only 30 mA cm�2 is seen for bulk Rh. It has

Fig. 7 Superimposed Cyclic Voltammetric (CV) response of (a) Rh bulk, (b) R

HCOOH and 0.5 M H2SO4 solution at sweep rate of 50 mV s�1.


been widely accepted that on Pt and Pd electrodes, HCOOH is

oxidized to CO2 via a dual path mechanism, which involves many

potential dependant reactive intermediate species.16,26–29

Fig. 8(a), shows the superimposed CVs response of (I) Rh-TA,

(II) Rh-AA and (III) bulk Rh towards HCOOH oxidation at

a typical scan rate of 50 mV s�1. As discussed earlier, the vol-

tammogram shows two oxidation peaks typical of CO oxidation.

Significant negative shift in the onset potential and high anodic

peak current for the formic acid oxidation on both the Rh-TA

and Rh-AA as compared to that of bulk Rh confirm that Rh-TA

exhibits higher electrocatalytic activity as compared to other two

Rh catalysts (see Table I). This improvement in nano-chain

activity is in good agreement with the smaller particle distribu-

tion throughout the nano-chains, which is in conformity with

TEM analysis.

Further this enhanced electrocatalytic activity, confirmed from

I–t transients for all three catalysts also supports the above

conclusion. Accordingly, Fig. 8(b) shows a comparison of tran-

sient current density at room temperature after normalizing with

respect to the electroactive Rh surface area for formic acid

oxidation at the corresponding potential selected from the CV on

Rh-TA, Rh-AA nano-chains and on bulk Rh. Interestingly, the

current density shows a decreasing trend, Rh-TA > Rh-AA >

bulk Rh. Moreover, Tian et al., explored the activity of electro-

catalyst based on the enhancement factor (R), which is defined as

the ratio of the current density on Rh nanochain versus that

acquired on bulk Rh.26 Accordingly, it ranges up to 2000% for

Rh-TA and 1200% for Rh-AA in the potential range of 0.18 V to

0.22 V, respectively.

h-TA and (c) Rh-AA in (I) 0.5 M H2SO4 and (II) in the mixture of 0.5 M


Table 1 Cyclic voltammetric data for electrooxidation of HCOOH (0.5M) in H2SO4 (0.5 M) at a sweep rate of 50 mV s�1 for Rh nano-chainsalong with bulk Rh

Sr. no. ElectrocatalystOnsetpotential/V

Currentdensity/mA cm�2

Enhancementfactor (R)

1 Bulk Rh 0.025 30 —2 Rh-AA 0.021 280 12003 Rh-TA 0.019 490 2000

Moreover, these network nano-chains are stable even after

electrochemical measurements (see TEM images SI–III) indi-

cating that the morphology is potential independent, which is

unaffected by intermediate reactive species generated during the

formic acid oxidation. Fig. 9 shows superimposed CVs of (a) Rh-

TA and (b) Rh-AA towards (I) formic acid oxidation in

a mixture of 0.5 M HCOOH and 0.5 M H2SO4 solution and (II)

of the same after dipping for 1 min in TA and AA solutions at

50 mV s�1, respectively. Interestingly, voltammograms (II) give

a decrease in current density suggesting the influence of the

capping molecules in blocking the surface active sites. Thus, the

presence of capping molecules on the nanoparticle surface alters

their electrocatalytic activity although oxide formation is pre-

vented along with a concomitant reduction in the formic acid

oxidation.

In order to study the effect of specifically adsorbed anions, we

have used HClO4 as a supporting electrolyte, which is less

strongly specifically adsorbed than H2SO4 as an alternative

electrolyte having similar voltammetric features. Fig. 10 shows

superimposed CVs of (a) Rh-TA and (b) Rh-AA towards formic

Fig. 10 Superimposed CV response of (a) Rh-TA and (b) Rh-AA

towards formic acid oxidation in a mixture of 0.5 M HCOOH with (I)

0.5 M H2SO4 and (II) 0.5 M HClO4 respectively as a supporting

electrolyte at a sweep rate of 50 mV s�1.

Fig. 9 Superimposed CV response of (a) Rh-TA and (b) Rh-AA

towards (I) formic acid oxidation in a mixture of 0.5 M HCOOH and 0.5

M H2SO4 solution and (II) after dipped in respective TA and AA solu-

tions at sweep rate of 50 mV s�1 respectively.


acid oxidation in a mixture of 0.5 M HCOOH with (I) 0.5 M

H2SO4 and (II) 0.5 M HClO4, respectively, as a supporting

electrolyte at a sweep rate of 50 mV s�1. The comparatively

higher onset potential and larger peak area in 0.5 M HClO4

compared to those in 0.5 M H2SO4 solution suggest more

penetration power and strong adsorption tendency with respect

to ClO4� ions for both the Rh-TA and Rh-AA electrocatalysts.

The potential range of Rh-oxidation clearly reveals how Rh

surface oxidation and formic acid oxidation relate to each other

in these solvents as also confirmed by superimposed CVs in the

supporting information (SI–II†).

IV. Conclusions

A self-assembled network of Rh nanoparticles has been

successfully prepared at room temperature by a simple solution

phase approach, where nanoparticles of 7.0 � 0.5 nm and 5.0 �0.5 nm for Rh-TA and Rh-AA respectively are oriented and

fused together to form interesting topologies. These distinct

nano-chains have been further studied using TEM, TGA, FTIR,

XRD and XPS results, illustrating their stability, uniform size

and electrochemical properties as a model system for funda-

mental investigations as well as for promising applications in fuel

cell technology. Cyclic voltammetric and I–t transients indicate

enhanced electrocatalytic activity towards formic acid oxidation,

revealing a negative shift in the onset potential and higher

current density than that corresponding to bulk Rh. The redox

behavior of both Rh-AA and Rh-TA nano-chains further open

up their unique ability to manipulate the surface properties.

Acknowledgements

Authors thank the council of scientific and Industrial Research

(CSIR) New Delhi, for financial support through a NMITLI

programme to carry out this work.

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enhanced electrocatalytic performance of interconnected rh nano-chains towards formic acid oxidation

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