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Title

Christmas-Tree-Shaped Palladium Nanostructures
Decorated on Glassy Carbon Electrode for Ascorbic
Acid Oxidation in Alkaline Condition

Author(s)Hasan, Md. Mahmudul; Nagao, Yuki

Citation

ChemistrySelect, 6(24): 5885-5892

Issue Date

2021-06-22

Type

Journal Article

Text version

author

URL

http://hdl.handle.net/10119/18094

RightsThis is the peer reviewed version of the following
article: Copyright (C) 2021 Wiley-VCH.
Md.Mahmudul Hasan, Yuki Nagao, ChemistrySelect,
6(24), 2021, pp.5885-5892, which has been
published in final form at
https://doi.org/10.1002/slct.202100974. This
article may be used for non-commercial purposes inaccordance with Wiley Terms and Conditions for
Use of Self-Archived Versions. This article may not
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Description

Japan Advanced Institute of Science and Technology

FULL PAPER
Christmas-Tree-Shaped Palladium Nanostructures Decorated on
Glassy Carbon Electrode for Ascorbic Acid Oxidation in Alkaline
Condition
Md. Mahmudul Hasan, Yuki Nagao*
Md. M. Hasan, Dr. Y. Nagao
School of Materials Science,
Japan Advanced Institute of Science and Technology,
1-1 Asahidai, Nomi, Ishikawa 923-1292, Japan
E-mail: ynagao@jaist.ac.jp
Supporting information for this article is given via a link at the end of the document.

Abstract:

Christmas-tree-shaped

Pdnanostructures

wereElectrochemical deposition is a process that offers an

synthesized using a simple one-step electrodeposition method

alternativewith no additives on a glassy carbon electrode (GCE) surface.

nanostructure surfaces.[11] To form different metal nanostructures,

Growth of the hierarchical nanostructures was optimized throughelectrochemical deposition techniques had been used on different

theappliedpotential,

deposition

time,

and

pathwayforthefabrication

of

clean

metal

precursor

substrates, particularly on a glassy carbon electrode (GCE)

concentration. Comprehensive characterization techniques such

surface.[12,13] The GCE is widely employed for electrochemical

as scanning electron microscopy (SEM), energy-dispersive X-ray

deposition of metal nanostructures because of its chemical

spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), X-

inertness, impermeability to gases and liquids, and high-ray powder diffraction (XRD), and cyclic voltammetry (CV) weretemperatureused to characterize structural features of the Christmas-tree-electrodes or additives in the electrolytes is necessary in certain

resistance.[14]Pre-modification

of

the

GCE

shaped Pd nanostructures. Our Christmas-tree-shaped Pd

cases for preparing metal nanostructures through electrochemicalnanostructures showed excellent catalytic activity for ascorbic

deposition.[15] To prevent the effects of additives or surfactants inacid (AA) electro-oxidation in the alkaline condition. The modified

electrochemical and analytical applications, a simple one-pot

electrode exhibited current density of 4.5 mA cm : much higher

electrodeposition

than that of unmodified GCE (0.6 mA cm-2). This simple

nanostructured surfaces is receiving much attention.[16]-2technique

for

the

fabrication

of

metal

electrodeposition technique with well-defined hierarchical Pd

Palladium (Pd) has attracted much interest in the area ofnanostructures is expected to offer new perspectives using Pd-

heterogeneous catalysis because of its high catalytic activity.

based nanostructured surfaces in different research areas.

SeveralstudieshaveassessedtheformationofPdnanostructures on different substrates using electrodeposition.
Flower-shaped

Introduction

heterogeneous

catalysis,

biosensors,

synthesizedbyadditive.[18] Recently, electrodeposition with no additives wasandapplied to prepare different Pd nanostructures. Pd nanoflake[1]electrocatalysis.[1–3] The fabrication process can control theand triangular Pd Rod nanostructures[7] on the Au substrate, Pd

shapes and sizes of metal nanostructures, which provide unique

nanodendrites[19] and nanourchins[20] on the ITO surface, and Pd

physical and chemical properties. Therefore, well-defined

nanothorns[21,22] on the carbon surface have been reported. Suchnanostructure fabrication has garnered much attention recently.[4–
8]

were

surface by electrodeposition using ethylenediamine as an

chemical properties, eliciting intensive research interest for theirtonanoparticles

polymer layer.[17] Pd nanodendrites were developed on the GCE

Nanostructured metal surfaces exhibit unique physical andapplication

Pdelectrodeposition on an indium tin oxide (ITO) electrode with a thin

Pd nanostructures showed good electrocatalytic performance.

Although different solution-based metal nanoparticle syntheses

For

and their assemblies have been developed successfully and usedtheimprovementofelectrochemical

efficiency,

thepreparation of various morphological Pd nanostructures isin diverse applications, the fabrication process requires multiple

necessary.

steps and/or linker molecules.[9] These linker molecules andTocapping agents for the preparation of nanostructure could

address

particularly

potentially affect the surface morphology and electroanalyticalenvironmental

focusedon

clean

issues, researchers
energysources

have

such

electrochemical energy storage and conversion devices.

as[23–28]activities.[10]1FULL PAPER
Fuel cells are promising candidates for use as zero-emission

0.4V_175S, and Pd/GCE-0.4V_400S, respectively, for theenergy conversion devices.

deposition conditions of -0.1 V for 175 s, -0.1 V for 400 s, -0.1 V[29–32]

Although hydrogen fuel cells,

alcohol-based fuel cells, and hydrazine fuel cells have beenfor 600 s, -0.4 V for 175 s, and -0.4 V for 400 s. FE-SEM images

developed, their accessibility for commercial use remainsof Pd/GCE-0.1V_400S are displayed in Figure 1(a-d). FE-SEM

uncertain because of safety, environmental, and storage

reveals the formation of Christmas-tree-shaped nanostructures.

issues.

To overcome these obstacles, ascorbic acid (AA) can

These nanostructures are distributed homogeneously on the GCE

be an alternative source of fuel in direct liquid fuel cells (DLFC).

surface shown in Figure 1a. The high magnification images

AA is a natural fuel source that produces nontoxic by-products(Figure 1(b-d)) revealed the excellent form of the Christmas-tree-[33–36]during its oxidation, releases two electrons, and forms

shaped nanostructures of 200–300 nm in size. These Pd

dehydroascorbic acid (DHA), as shown in Scheme 1.[37–41] The

nanostructures, confirmed by EDX, are unique and markedly

performance of the AA-based DLFC is dependent on the catalytic

different from previously-reported results such as nanoparticles,

performance of AA electro-oxidation. The Pd metal is a promisingrod-like or dendritic structures.[7,19] The Christmas-tree-shaped Pd

catalyst for the enhancement of electro-oxidation of AA. Recent

nanostructures on the GCE surface were electrodeposited using

studies have shown that a Pd-based anode catalyst shows good

15 mM Na2PdCl4 solution at a constant applied potential of −0.1

catalytic performance for AA electro-oxidation.

In the alkaline

V for 400 s. We applied different potential and times to investigate

condition, the Pd catalyst exhibits higher AA electro-oxidation

the morphology of other modified electrodes, as shown inthan the acidic condition.[45] The different Pd morphological effectSupporting Information (Figure S1). Although we applied aon the catalytic activity of AA electro-oxidation in alkaline

potential of -0.1 V for 175 s (Figure S1a), the Christmas-tree-

conditions needs to be investigated.

shaped Pd nanostructures did not form. The Pd/GCE-0.1V_600S[42–44]has similar but not the same as Christmas-tree-shaped
nanostructures (Figure S1b). The number of sharp edges of thenanostructures is less than the nanostructures deposited on the
GCE for 400 s at -0.1 V (Figure 1d). The amount of metal
deposition increased with deposition time. This increaseengenders the formation of nanostructures having fewer sharpedges. The optimal electro-deposition time of 400 s provides the

Scheme 1. Oxidation of AA.
In

this

work,

we

reportuniquePdadequate formation of the Christmas-tree-shaped nanostructures

nanostructureswith many sharp edges on the GCE at -0.1 V. Therefore,

electrodeposited on the GCE by constant potential mode using no
additiveor

substratesurfacepre-modification.

Thedeposition

Pd

time

is

an

importantparameterfor

uniquenanostructure formation. To check the effect of applied potential,

nanostructures were defined as Christmas-tree-shaped Pd

we applied a different potential -0.4 V for 175 and 400 s. FE-SEM

nanostructures. The fabrication process of these uniqueimages (Figure S1(c-d)) revealed that Pd was deposited randomly

nanostructures was optimized. To our knowledge, this report ison

the first to demonstrate the formation of Christmas-tree-shaped

the

GCE

surfaceandnanostructures were obtained.

Pd nanostructures on the GCE surface by electrodeposition. The

noChristmas-tree-shaped

The precursor (Na2PdCl4)

concentration controlled the morphology of the Pd deposits

catalytic activity of AA electro-oxidation was checked using the(Figure S2). Although applying deposition potential -0.1V for 400

Christmas-tree-shaped Pd nanostructures on the GCE in thes, lower Na2PdCl4 concentration (5 mM) resulted in Pd deposits

alkaline condition. The morphological effects and the mass

with less-defined structures. Higher Na2PdCl4 concentration (30activities of Pd on the catalytic activity of AA electro-oxidation inmM) resulted in random Pd formations. So 15 mM Na2PdCl4

the alkaline condition were also studied.

concentration is essential for the formation of Christmas-treeshaped Pd nanostructures. The supporting electrolyte also

Results and Discussion

performs a significant role in the development of vertically
expanding Pd nanostructures. It is well known that specific sulfuricMorphological and elemental analyses

acid anion adsorption, such as SO42- on Pd surfaces, can play a

We prepared five electrodes using constant potential mode (seekey role during electrodeposition in the vertical growth of Pd.[19]experimental section) and designated them as Pd/GCE-

The diffusion-limited current at -0.1 V[7] and the deposition time of

0.1V_175S, Pd/GCE-0.1V_400S, Pd/GCE-0.1V_600S, Pd/GCE-

2

FULL PAPER

Figure 1. Characterizations of Pd/GCE-0.1V_400S: (a) low magnification FE-SEM image; (b), (c), and (d) high magnification FE-SEM
image; (e) atomic ratio of elements by EDX analysis; (f) Pd 3d XPS spectrum; and (g) XRD pattern.
amount of O element may attribute to the presence of PdO, which

400 s as well as the concentration of precursor and supporting
electrolyte anion SO42- assist the formation of the Christmas-tree-

has been confirmed by XPS analysis. The XPS analysis wasshaped Pd nanostructures on the GCE.

conducted for further confirmation of the oxidation state of the Pd

Figure 1e shows the atomic ratio of the Pd/GCE-0.1V_400S.

in the Pd/GCE-0.1V_400S. The Pd 3d XPS spectrum showed two

The C, O, and Pd elements are present in the Pd/GCE-

major peaks (Figure 1f). Those two peaks were separated by 5.2

0.1V_400S. The amount of Pd is quite high compared to othereV having binding energies of 340.7 eV and 335.5 eV,elements. So the Pd is successfully deposited on the GCE by therespectively representing Pd (3d3/2) and Pd (3d5/2).[46–49] The XPS

constant potential mode of electrodeposition. In the EDX analysis,

data revealed that the deposited Pd was in both metallic and oxide

the C element comes from the presence of GCE and a small

states in the Pd/GCE-0.1V_400S. The amount of PdO, found at3

FULL PAPER

Figure 2. (a) Cyclic voltammograms of Pd/GCE-0.1V_400S in the absence (cyan line) and presence of 5.6 mM AA (red line) in 1 M
KOH solution. (b) Catalytic activity comparison of bare GCE (black line), and Pd/GCE-0.1V_400S (red line) for 5.6 mM AA in 1 M KOH
solution. (c) Response of AA electro-oxidation in 1 M KOH solution at Pd/GCE-0.1V_400S (red line), Pd/GCE-0.1V_175S (blue line),
Pd/GCE-0.1V_600S (green line), Pd/GCE-0.4V_175S (violet line), and Pd/GCE-0.4V_400S (orange line). (d) Mass activities of Pd/GCE0.1V_400S (red line), Pd/GCE-0.1V_175S (blue line), Pd/GCE-0.1V_600S (green line), Pd/GCE-0.4V_175S (violet line), and Pd/GCE0.4V_400S (orange line). All the cyclic voltammograms above were taken with a 50 mV s-1 scan rate.
higher binding energy, was about 15% compared to the metallic

other modified electrodes are shown in Figure S4. The Pd (111)

Pd. Two satellite peaks were apparent at 343.6 eV and 338.3

plane with a similar 2θ value corresponding to the Pd/GCE-

eV.

0.1V_400S was found among those electrodes.

[50]

These two satellite peaks are attributable to the highamount of Pd metallic state present in Pd/GCE-0.1V_400S. For
Electrocatalytic activity

the other modified electrodes, the Pd 3d XPS spectra showedsimilar features to those found for Pd/GCE-0.1V_400S (FigureS3). The Pd deposited on the GCE shows both metallic and oxide

The electrocatalytic performance of the Pd/GCE-0.1V_400S forstates. So different deposition conditions do not affect the

AA electro-oxidation in alkaline conditions was investigated underoxidation state of the Pd deposit.

different experimental conditions. The Christmas-tree-shaped Pd

Figure 1g shows the XRD pattern obtained for Pd/GCE-

nanostructures on the GCE (Pd/GCE-0.1V_400S) were used to

0.1V_400S. The broad diffraction peak at 25.1° corresponds toin Figure 2a. Cyclic voltammograms were recorded between -0.8

the C (002) plane of graphite. The diffraction peaks at 43.3°, 54.5°,

V and +0.7 V at a scan rate of 50 mV s-1 in the presence andand 78.8° respectively correspond to the C (100), C (004), and C

absence of AA (5.6 mM) in 1 M KOH solution. The Pd/GCE-(110) planes.

These peaks appeared because of the

0.1V_400S (red line) shows an oxidation wave of AA at -0.1 V. An

presence of GCE. The diffraction peak at 40.2° corresponds toabsence of AA in the 1 M KOH solution gives no oxidation peakthe Pd (111) plane.

Other diffraction peaks of Pd were found at(cyan line). A strong reduction peak at -0.4 V is attributed to the

46.7°, 68.1°, and 82.3°, respectively corresponding to the Pd

reduction of PdO to Pd.[55] We have also found a small oxidation(200), Pd (220), and Pd (311) planes.[54] The XRD patterns ofpeak around -0.2 V for the Pd/GCE-0.1V_400S in 1 M KOH[14,51,52]

[53]

4

FULL PAPER
having no AA (see Figure 2a cyan line). This oxidation peak is dueto the development of Pd2+ from Pd0 in alkaline conditions.[56,57]
Pd is first oxidized to form Pd2+ in an alkaline environment, whichfurther improved the oxidation of AA. To elucidate the catalytic
effects of the GCE and the Christmas-tree-shaped Pd
nanostructures on the AA electro-oxidation, we performed the CV
measurements. In Figure 2b, the voltammogram of the bare GCE(black line) shows a small AA oxidation peak compared with thatof the Pd/GCE-0.1V_400S (red line). The Pd/GCE-0.1V_400S

Figure 3. Cyclic voltammograms of Pd/GCE-0.1V_400S: (a)provides much higher current density (4.5 mA cm-2) for AA electro-

concentration effect of AA in 1 M KOH at 50 mV s-1 scan rate andoxidation than the bare GCE (0.6 mA cm ) in 1 M KOH solution.

(b) scan rate effect of 7.4 mM AA in 1 M KOH.

-2

The Pd plays a crucially important role in the excellent catalytic

nanostructures than Pd/GCE-0.1V_400S (Figure 1d). The sharp

activity of the AA electro-oxidation in the alkaline condition.

edges of the Christmas-tree-shaped nanostructures makeWe further evaluated the catalytic performance by

Pd/GCE-0.1V_400S superior for catalytic performance compared

comparing all the modified electrodes to investigate the influence

to other modified electrodes that have no such features. The ESAof the unique nanostructures of the Pd for AA electro-oxidation.

of the bare GCE is 0.06 cm2, even less than Pd/GCE-0.1V 400S.

Figure 2c displays the response of all modified electrodes in the

Therefore, as seen in Figure 2b, the current density of thepresence of 5.6 mM AA in KOH at a 50 mV s-1 scan rate. Among

Pd/GCE-0.1V 400S electrode is superior to the bare GCE. The

all electrodes, the Christmas-tree-shaped structure of theonset potential of AA electro-oxidation at the Pd/GCE-0.1V_400S

Pd/GCE-0.1V_400S showed the highest catalytic activity of AA

is lower than all the prepared electrodes (Figure 2c). The

electro-oxidation in 1 M KOH solution. The catalytic activity can

Pd/GCE-0.1V_600S has almost similar onset potential with otherbe enhanced by increasing the electrode surface area. The

electrodes except Pd/GCE-0.1V_400S. This behavior can be also

electrochemical surface area (ESA) of all the electrodes was

explained by the morphological effect. Although Pd/GCE-0.1V

obtained by application of CV at different scan rates in 3.0 mM

600S has a similar morphology to Pd/GCE-0.1V 400S, a fewer

K3Fe(CN)6 solution (Figure S5) and was calculated using

number of sharp edges make less catalytic reactive for AA

Randles–Sevcik equation as[58]𝐼𝐼𝑝𝑝 = (2.69 × 105 )𝑛𝑛3/2 𝐴𝐴 𝐶𝐶 𝐷𝐷1/2 𝜐𝜐1/2

electro-oxidation.
The morphological effects of Pd nanostructures to the AA(1)electro-oxidation in 1 M KOH solution were investigated further
through the mass activity of the modified electrodes. The mass

where Ip represents the peak current (A), n stands for the number

activity of each electrode was calculated by normalization of theof electron transfer during the redox process of K3Fe(CN)6, A

Pd mass loading at the electrode. For each electrode, therepresents the electrode area (cm2), C denotes the concentration

deposition quantity of Pd (in mg) is determined by the massof the K3Fe(CN)6 solution (mol cm-3), D is the diffusion coefficientdifference before and after the electrodeposition of Pd on the GCE.

of the K3Fe(CN)6 in units of cm2 s−1, and 𝜐𝜐 expresses the scan

The calculated amounts of the Pd deposition were 0.12, 0.14,

rate (V s-1). The ESA of the Pd/GCE-0.1V_400S was 0.07 cm-2,

0.17, 0.20, and 0.25 mg, respectively, for Pd/GCE-0.1V_175S,

which is higher than Pd/GCE-0.1V_175S (0.05 cm-2) and

Pd/GCE-0.1V_400S, Pd/GCE-0.4V_175S, Pd/GCE-0.4V_400S,

Pd/GCE-0.4V_175S (0.05 cm-2). The ESA of Pd/GCE-0.4V_400S

and Pd/GCE-0.1V_600S. Figure 2d presents the mass activity of(0.07 cm-2) is similar to that of Pd/GCE-0.1V_400S. Although

the Pd/GCE-0.1V_400S (red line), Pd/GCE-0.1V_175S (blue line),

these two electrodes have similar ESA, the morphological

Pd/GCE-0.1V_600S (green line), Pd/GCE-0.4V_175S (violet line),

differences might have a strong effect on catalytic activities. Fromand Pd/GCE-0.4V_400S (orange line) in 1 M KOH solution. The

Figure 1d, the Christmas-tree-shaped nanostructures (Pd/GCE-

calculated mass activity of the Pd/GCE-0.1V_400S is 32.1 mA

0.1V_400S) have many sharp edges that can enhance the AA

mgPd−1, which is 1.34, 1.91, 2.41 and 2.76 times as large,

electro-oxidation. As a result, the current density increased.

respectively, as those of Pd/GCE-0.1V_175S (23.9 mA mgPd−1),

However, the ESA of the Pd/GCE-0.1V_600S (0.09 cm-2) is

Pd/GCE-0.4V_175S (16.8 mA mgPd−1), Pd/GCE-0.1V_600S (13.3

higher than that of the Pd/GCE-0.1V_400S, but the catalyticmA mgPd−1), and Pd/GCE-0.4V_400S (11.6 mA mgPd−1). This

activity is lower. The FE-SEM of the Pd/GCE-0.1V_600S (Figure

result further proved that the unique Christmas-tree-shaped Pd

S1b) showed fewer sharp edges of Christmas-tree-shaped

5

FULL PAPER
nanostructures with many sharp edges have good catalytic
activity towards AA electro-oxidation in 1 M KOH solution.
We have further investigated the electrocatalytic behaviors ofthe modified electrodes in terms of electrochemically active sitesof the catalyst. Electrochemical active surface area (ECSA) is asignificant

parameterthatdescribes

thenumberofelectrochemically active sites concerning metal catalyst mass.[56]ECSA = Q/sl

(2)

Where, Q is the coulombic charge of the metal oxide reduction
peak, s is the proportionality constant =0.405 mC cm-2, and l is
the metal loading (g m-2). The ECSA of all the electrodes issummarized in Table 1. It can be seen from Table 1 that the

Figure 4. Chronoamperometry of 10.7 mM of AA at Pd/GCE-

Pd/GCE-0.1V_400S exhibits a maximum ECSA value than other

0.1V_400S (red line), Pd/GCE-0.1V_600S (green line), Pd/GCE-

catalysts. That means the Pd/GCE-0.1V_400S has the highest

0.1V_175S (blue line), Pd/GCE-0.4V_175S (violet line), andactive reaction center for electrocatalysis. The lower ECSA value

Pd/GCE-0.4V_400S (orange line) in 1 M KOH solution (inset:

of Pd/GCE-0.1V_600S is due to the presence of fewer catalytic

magnification of chronoamperometric results for all electrodes

active sites. This further proves that the Christmas-tree-shaped
Pd nanostructures with many sharp edges make Pd/GCE-0.1V

from 990 to 1000 s).

400S electrode superior for AA electro-oxidation.

increases concurrently with the increasing scan rate. Because ofheterogeneous kinetics, the peak current increased with the scanrate. The plots of log peak current (Ip) differed linearly with the log

Table 1. ECSA of all the modified electrodes.

(scan rate) is shown in Figure S6 with a slope value of 0.47. This

Name of the electrode

Metal loading(g m-2)ECSA(m2 gm-1)

Pd/GCE-0.1V_175S

24.0

0.75

Pd/GCE-0.1V_400S

20.0

0.81

diffusion-controlled reaction as described in the concentration-

Pd/GCE-0.1V_600S

18.9

0.74

effect analysis. The peaks at -0.8 V to -0.7 V are increasing when

Pd/GCE-0.4V_175S

40.0

0.20

Pd/GCE-0.4V_400S

35.7

0.13

observation suggests that the AA electro-oxidation follows adiffusion control process that has a theoretical 0.5 slope value.[14]This result also confirms that the AA electro-oxidation follows athe scan rate increased. This increase is attributable to thehydrogen adsorption and desorption behavior of the Pd
catalyst.[55,56]

Figure 3a presents the concentration influence of AA (2.0–

Stability of the modified electrodes

10.7 mM) at the Pd/GCE-0.1V_400S surface at a scan rate of 50
mV s-1. The oxidation current density increased concomitantly

The stability check of the Pd/GCE-0.1V_400S (red line), Pd/GCE-with increased AA concentration. The peak potential of AA

0.1V_175S (blue line), Pd/GCE-0.1V_600S (green line), Pd/GCE-

oxidation shifted positively with increased concentration, which

0.4V_175S (violet line), and Pd/GCE-0.4V_400S (orange line) for

can be denoted as a diffusion-controlled reaction [12]. This shift is

AA electro-oxidation in 1 M KOH by chronoamperometry, isattributable to a disturbance of the mass transport processes ofshown in Figure 4. We applied the peak potential of the AA

the analyte. The current can be explainable by the mass transport

electro-oxidation for all modified electrodes for 1000 s. As shownof analyte from the bulk of the solution to the electrode surface.

in Figure 4, all curves that began with the dropping current density

As the concentration of AA increases, the diffusion process iswere attributable to the initial electro-oxidation of AA at the startup.

disrupted, and the electrochemical system applies more potential

Subsequently, the curves became flat and stable. The inset

to the re-establishment of mass transport. Therefore, the peakmagnified figure in Figure 4 shows the current density from 990 topotential shifted positively. Figure 3b presents the scan rate effect1000 s for all electrodes. The Pd/GCE-0.1V_400S (red line)on the current potential behaviors of 7.4 mM AA electro-oxidationin 1 M KOH solution. The oxidation peak of the AA molecules

6

FULL PAPER
showed good stability and excellent electrocatalytic activity

Pd/GCE-0.1V_400S electrode for AA oxidation. The analyticalcompared with all other modified electrodes.

performance of the Pd/GCE-0.1V_400S was further investigated

Table 2 summarizes information for recently reported AA

by the real sample analysis in 1 M KOH solution. We used theelectro-oxidation for comparison with our results. In all cases, theVitamin C tablet and Vitamin C juice. The spiked samples had apeak current density was determined by the peak current from therecovery range ranging from 98.2% to 101.5% (Table S1),

voltammetry measurement. The Pd/GCE-0.1V_400S shows theindicating the high efficiency of the Pd/GCE-0.1V_400S for AA

highest peak current density. The amperometry measurement isoxidation.

carried out for the measurement of lower detection limit (LOD) and

The Christmas-tree-shaped Pd nanostructures on the GCE

the linear range value of the AA oxidation at Pd/GCE-0.1V_400S

were optimized using constant potential and deposition time. The

in 1 M KOH solution (Figure S7). With increasing concentration ofmodified electrode exhibits excellent catalytic performance

the AA, the current response increased linearly. The LOD (1 µM)

towards AA electro-oxidation with good stability in 1 M KOH. The

and the linear range value (1-950 µM) of AA electro-oxidation at

Pd/GCE-0.1V_400S has a strong effect on the electrochemical

Pd/GCE-0.1V_400S are comparable with the other catalyst

performance due to many sharp edges. The excellent catalytic

mentioning in Table 2. This further confirms the excellent catalytic

performance of our modified electrode has an immeasurable

performance of the Pd/GCE-0.1V_400S catalyst for AA electro-

effect on the development of Pd-based catalysts for energy

oxidation.

conversion devices and creates more interest in broad researchaudiences.

Table 2. Comparison of the catalytic performance of AA electrooxidation.
Peakcurrent
density(µA cm-2)

Pd/GCE-0.1V_400S
Pd/N-3D
mesoporous C/GCE

Electrode

CoPd/carbon/GCE

Conclusion

LOD
(µM)

Linear
range
(µM)

Ref.

4500

1.0

1-950

This
work

2475

-

4004000[42]

0.1

0.13422[43][44]

the Christmas-tree-shaped Pd nanostructures with many sharp
catalytic performance for AA electro-oxidation in alkaline

2465

UniqueChristmas-tree-shaped

Pdnanostructures

weredeveloped on the GCE surface using one-step electrodepositionrMWCNTs/Pd NTs/graphite

42

0.17

5007500

Ag NPs/PVP/GCE

1650

0.47

2-150

[59]

CNO/NiMoO4/
MnWO4/GCE

-

0.33

1-100

[60]

PAP/ZrO2NPs/CNTs
/GCE

35

0.35

1-295

[61]

with a constant potential mode technique without additives andpremodification of the electrode surfaces. The nanostructure
morphology depended strongly on the deposition potential, time,precursor concentration, and supporting electrolyte during
electrodeposition. We prepared different modified Pd/GCE
electrodes by application of different deposition conditions, andedges on the GCE (Pd/GCE-0.1V_400S) exhibited excellent
condition. Results also show that the Pd/GCE-0.1V_400S exhibits
good electrochemical stability for AA electro-oxidation in 1 M KOH.
The simple electrodeposition method used for the preparation of
Christmas-tree-shaped Pd nanostructures on the GCE surface is

Note: rMWCNTs=reduced multi-walled carbon nanotubes;
NTs=nanotubes; PVP= Polyvinylpyrrolidone; NPs= nanoparticles;
CNO= carbon nanoonions; NiMoO4= Nickel molybdate; MnWO4=
Manganese tungstate; PAP= Poly(aminopyrazine); ZrO2NPs=
Zirconium oxide nanoparticles; CNTs= carbon nanotubes (CNTs).

expected to provide new opportunities for the future development

The selectivity of Pd/GCE-0.1V_400S for the AA oxidation

Supporting information includes chemicals and instruments,of Pd-based energy conversion catalysts.

Supporting Information Summary

was further evaluated by the amperometry measurements in 1 M

modification of GCE, characterization and measurements

KOH solution (Figure S8). Different interfering compounds suchincluding FE-SEM images, XPS spectra, XRD patterns, Cyclic

as KNO3, ZnSO4, FeCl2, MgCl2 were added to the AA-containing

voltammograms, log (Ip) vs log (scan rate) plots, Amperometricsolution. Each of these interfering compounds has a very low

measurement, interference effect of various compounds for AA

current response, whereas AA has a distinguishable current

electro-oxidation,

response. This finding confirmed the high selectivity of thetechnique.

7

real

sampleanalysisbyamperometric

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H. Meng, F. Xie, J. Chen, P. K. Shen, J. Mater. Chem. 2011, 21,11352.

The authors would like to thank the Japan Advanced Institute of
Science and Technology (JAIST) for an internal grant, which
partially supported this research work. This work was partially
supported by research funding from JSPS KAKENHI Grant
Number JP21H00020.

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Conflict of Interest

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The authors declare no conflict of interest.

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FULL PAPER
Entry for the Table of Contents

The unique Christmas-tree-shaped Pd nanostructures were generated through a simple electrodeposition technique on the glassy
carbon electrode (GCE). The modified electrode showed excellent catalytic activity than the unmodified GCE for ascorbic acid electrooxidation in alkaline condition. This high efficiency is derived from the many hierarchical edges of nanostructures.

10