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Title

Palladium Nanoparticles Combined with Reduced
Graphene Oxide and Multiwall Carbon Nanotubes
for Alkaline Ascorbic Acid Oxidation

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

Citation

Japanese Journal of Applied Physics, 62: 027003

Issue Date

2023-02-17

Type

Journal Article

Text version

author

URL

http://hdl.handle.net/10119/18809

RightsThis is the author's version of the work. It is posted
here by permission of The Japan Society of Applied
Physics. Copyright (C) 2023 The Japan Society ofApplied Physics. Md. Mahmudul Hasan, Zhongping
Li, Yuki Nagao, Japanese Journal of Applied
Physics, 62, 2023, 027003.
https://doi.org/10.35848/1347-4065/acb897

Description

Japan Advanced Institute of Science and Technology

Template for JJAP Regular Papers, STAP Articles, and reviews (Oct. 2022)

Palladium Nanoparticles Combined with Reduced Graphene Oxide
and Multiwall Carbon Nanotubes for Alkaline Ascorbic Acid
Oxidation
Md. Mahmudul Hasan,1 Zhongping Li,1 Yuki Nagao,1,*
1School of Materials Science, Japan Advanced Institute of Science and Technology, 1-1

Asahidai, Nomi, Ishikawa 923-1292, Japan.
E-mail: ynagao@jaist.ac.jp

Ascorbic acid (vitamin C) is recognized as a viable alternative fuel for alkaline direct liquid
fuel cells (DLFCs). After the potential anode catalyst was prepared by incorporating
palladium nanoparticles (Pd NPs) into reduced graphene oxide (rGO) and multiwall carbon
nanotube (MWCNT) hybrid nanocomposite (Pd/rGO/MWCNT) through a chemical
reduction method, it was applied for electrooxidation of ascorbic acid (AA) in the alkalinecondition. For AA electrooxidation, the Pd/rGO/MWCNT modified glassy carbon electrode(Pd/rGO/MWCNT/GCE) exhibited the highest current density of 5.18 mA cm-2: muchhigher than a bare glassy carbon electrode (0.6 mA cm-2). The Pd/rGO/MWCNT/GCE also
demonstrated excellent stability for AA oxidation in the alkaline condition.

1. Introduction
The increased demand for energy along with the widespread use of fossil fuels has put
enormous strain on the world's natural resources and caused global climate change.
Scientists have recently shown interest in electrochemical energy storage and conversion
devices to address climate change.1)–6) Fuel cells have been recognized as a possible
alternative zero-emission energy conversion device.7)–10) Safety, environmental, andstorage issues remain challenging for fuel cells such as borohydride fuel cells, hydrazine
fuel cells, alcohol-based fuel cells, and hydrogen fuel cells.11)–14) Ascorbic acid (AA), a
potential environmentally benign fuel for direct liquid fuel cells (DLFCs), is naturally
abundant and produces nontoxic byproducts.15)–19) During electrochemical oxidation, AA
releases two electrons and two protons.20),21) One can reasonably expect that DLFC

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performance can be enhanced by the development of a more efficient electrocatalyst for
AA electrooxidation.
Carbon-based materials have attracted great interest in the field of electrocatalysis
because of their unique physical and chemical characteristics.22),23) Carbon electrodes arereadily renewable and provide several benefits including ease of preparation, high stability,
repeatability, and strong conductivity.24) Conductive materials are helpful as transductive
materials for rapid electron transfer at the electrocatalyst–electrode interface.25) Graphene,
a two-dimensional sp2 bonded carbon substance with a honeycomb structure, is usedwidely as a catalyst because of its good features, including its large surface area, highconductivity, and biocompatibility.26) The oxidation of graphite powder produces graphene
oxide (GO). The reduction of GO is called reduced graphene oxide (rGO).27),28) Actually,
GO contains oxygen-containing functional groups, making it less conductive than rGO andpotentially increasing resistance.22),29) Its superior conductivity along with a large surface
area and high electrocatalytic activity makes rGO a potential candidate for AA
electrooxidation.30) Earlier studies have demonstrated that rGO has outstanding catalytic
activity for AA electrooxidation with low oxidation potential and higher current
response.30),31) Previous studies showed different types of carbon materials showed good
performance both in acidic and alkaline membrane based AA based fuel cells.15),17),19)
Vulcan carbon exhibited the best performance (158 mW cm−2) using a split pH (cationexchange) based AA fuel cell. 32) Therefore, developing alternative carbon catalysts likerGO could open a new opportunity to enhance AA catalytic performance.
Several methods for synthesizing rGO from GO, including chemical, thermal, andhydrothermal procedures have been reported.30),33),34) The chemical reduction processprovides several benefits including excellent stability, economic effectiveness, and ashorter manufacturing time.35) However, agglomeration of rGO results in decreasing
electrical performance and exhibits low surface area.36) For catalytic performance, thestacking effect of graphene sheets has proved crucially important. Multiwall carbon
nanotubes (MWCNTs), which are often used to enhance electrocatalytic performance, haveexcellent mechanical properties, high surface area, and chemical stability.37) Moreover,
MWCNTs are widely recognized for their ability to bond with rGO through van der Waals
forces and hydrogen bonds. Consequently, MWCNTs might be used as a spacer between

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interfacial graphene layers during the reduction of rGO, thereby preventing agglomerationand increasing the specific surface area for electrocatalysis.36),38) The combination of rGO
with MWCNT catalyst demonstrated excellent electrocatalytic activity for AA oxidation.25)
So the rGO and the MWCNT could be used as alternative carbon materials for the AA
oxidation.
To enhance the catalytic efficiencies of rGO and MWCNT nanocomposites further,
researchers have added metal nanoparticles (NPs).33),36),39)–41) Of them, Pt and Pd metals are
the most promising candidates for anode catalysts in energy conversion and storage
devices because of their superior electrocatalytic efficacy.42)–44) However, Pd-based anodecatalyst shows higher performance than Pt in the electrocatalytic oxidation of AA.45)–47)
The Pd catalyst exhibits best AA-based fuel cell performance than the Pt catalyst.48)
Therefore, the combination of Pd NPs with rGO and MWCNTs hybrid nanocomposite
might enhance AA electrooxidation. This report is the first to describe a study of the
combination of Pd NPs with rGO and MWCNT for AA electrooxidation.
Numerous studies have examined the inclusion of Pd NPs in a hybrid
nanocomposite of rGO and MWCNT.33),39),40),45),49),50) Furthermore, one-pot synthesis of thechemical reduction method is preferable for a simple and shorter preparation time. For thisstudy, a one-pot chemical reduction process was developed to prepare Pd NPs incorporatedrGO and MWCNT hybrid nanocomposite (Pd/rGO/MWCNT) for AA electrooxidation,with the application as an anode catalyst for the AA-based DLFCs system. An earlier study
demonstrated that the direct alkaline AA-based DLFCs (DAAFC) performed better than inacidic conditions.32) Recent studies showed the high performance of the DAAFC by using
Pd and carbon-based catalysts.15),32),51) The excellent performance of AA-based fuel cells is
comparable to that of alcohol-based fuel cells.32) This will help to construct anenvironmentally beneficial energy conversion system which is a barrier for alcohol-based
fuel cells due to CO2 emissions. Therefore, we concentrated on the improvement of the AA
oxidation by introducing Pd NPs with rGO and MWCNT carbon materials. We havestudied the electrochemical performance of a hybrid nanocomposite for AA
electrooxidation in alkaline condition.

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2. Experimental methods
2.1 Chemicals and Instruments
Analytical grade MWCNT (20–30 nm), AA, sodium tetrachloropalladate (II) (Na2PdCl4),
5% Nafion solution, sodium borohydride (NaBH4), sulfuric acid (H2SO4), and potassiumhydroxide (KOH) were purchased and used as received from Fujifilm Wako Pure Chemical
Corp. The GO was purchased from NiSiNa Materials Co. Milli-Q water was used toprepare all the necessary solutions.
2.2 Synthesis of the Nanocomposite
Pd/rGO/MWCNT, Pd/rGO, and rGO/MWCNT catalysts were prepared using a chemical
reduction technique with NaBH4 as the reductant. To prepare Pd/rGO/MWCNT, 20 mg GO
and 20 mg MWCNT were dispersed ultrasonically in 20 mL of deionized water.
Subsequently, 5.0 mL of 15 mM Na2PdCl4 solution (dissolved in 0.1 M H2SO4) was added.
The solution was then treated ultrasonically for another 30 min. Gradually and dropwise,
7.6 mL of freshly prepared 0.5 M NaBH4 was added to the reactant to produce a solution
pH 10. The reactant was then agitated at room temperature for 4 h. The catalyst was thenfiltered using filter paper and was rinsed three times with deionized water and ethanol.
Finally, the catalyst was dried in a vacuum oven at 30 °C (12 h). The other two catalysts
were prepared in the same manner as described above, but only metal precursor and GO
were added to Pd/rGO. For rGO/MWCNT, no metal precursor was added.
2.3 Characterization
Field-emission scanning electron microscopy (FE-SEM, S4100; Hitachi Ltd.) and scanningelectron microscopy (SEM) combined with energy-dispersive X-ray spectroscopy (EDX,
TM3030Plus miniscope; Hitachi Ltd.) were used to describe the morphology of the
as-prepared catalysts. To ascertain the elemental composition, XPS measurement was doneusing a delay-line detector (DLD) spectrometer (Kratos Axis-Ultra; Kratos Analytical Ltd.)equipped with an Al K radiation source (1486.6 eV). The size of Pd NPs was ascertainedusing transmission electron microscopy (TEM) (H-7100; Hitachi Ltd.). Powder X-ray
diffractometer (XRD) patterns were acquired using a completely automated horizontal
multipurpose X-ray diffractometer (Rigaku Smartlab; Rigaku Corp.) with a 2θ range of 20°
to 90°. Brunauer–Emmett–Teller (BET) technique was used to determine the surface areaof the catalysts using BELSORP-mini II-BP. All electrochemical measurements were4

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performed using a CHI 701 potentiostat equipped with a three-electrode setup. The
working electrode, reference electrode, and counter electrode were GCE (3 mm diameter),
Ag/AgCl (sat. KCl), and Pt wire, respectively.
2.4 Preparation of Pd/rGO/MWCNT/GCE
The GCE surface was cleaned by rubbing alumina powder (0.05 µm diameter) and wasthen sonicated with ethanol and water for 10 min to remove contaminants. The GCE
surface was then cleaned electrochemically by potential cycling from 0 to +1.0 V at a scanrate of 100 mV s-1 in 0.1 M H2SO4 (Ar saturated) and was allowed to dry at ambient
temperature. Then the GCE was modified with a suspension of a catalyst to investigate AA
electrooxidation. To begin, the catalyst ink solution was prepared by dispersion of 2 mg
Pd/rGO/MWCNT in 200 µL of 0.25% Nafion–ethanol solution, followed byultrasonication for a homogeneous mixture. Then, a catalyst ink solution containing 0.04
mg Pd was dripped over the clean GCE surface and was allowed to dry at room
temperature. The modified electrode was designated as Pd/rGO/MWCNT/GCE, whereas
Pd/rGO/GCE and rGO/MWCNT/GCE were used respectively to refer to Pd/rGO andrGO/MWCNT. Fig.1 comprehensively presents the preparation of Pd/rGO/MWCNT/GCE.
All the electrochemical studies were conducted in an alkaline environment with deaeration
at room temperature (25 °C ± 0.5 °C).

Fig. 1. Preparation of Pd/rGO/MWCNT/GCE and deposition to the GCE.

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3. Results and discussion
3.1 Morphological and elemental analysesThefield-emission

scanning

electronmicroscopy(FE-SEM)

image

of

the

Pd/rGO/MWCNT composite is portrayed in Fig. 2a. Fig. 2b presents a high-magnification
image of one portion of the Pd/rGO/MWCNT composite. The wrinkles in the image
suggest that the chemical reduction process successfully produced rGO from GO.25) Fig.
S1 shows FE-SEM images of GO, Pd/rGO, and rGO/MWCNT. Compared to the GO, allsynthesized materials had completely different surface morphology. The change in themorphology further proved the successful reduction of GO by the chemical reduction
process. Fig. 2c shows a scanning electron microscopy (SEM) image of Pd/rGO/MWCNT
for energy-dispersive X-ray spectroscopy (EDX) analysis. Because of the presence of Pd
metal, bright spots appear. A similar observation was made in the case of Pd/rGO (Fig.
S1c). Fig. 2d–f respectively portray the elemental mapping of C, O, and Pd. Elemental
mapping demonstrated a uniform distribution of Pd metal across the rGO and MWCNT
systems. Fig. 2g shows elemental spectra of the Pd/rGO/MWCNT, which also revealed theexistence of Pd. A trace amount of oxygen is detected in the EDX mapping because of Pd
metal oxidation, which is also found in the XPS study. The rGO still contains a trace
amount of oxygen in addition to Pd metal oxidation.

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Fig. 2. Morphological and elemental characterization of Pd/rGO/MWCNT. (a) Low
magnification and (b) high magnification FE-SEM image; (c) SEM image for EDX
analysis, elemental mapping of (d) C, (e) O, (f) Pd, and (g) elemental spectra (inset table:
element weight ratio).
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Fig. 3a shows X-ray powder diffraction (XRD) patterns that confirm the Pd
diffraction peaks and the carbon peaks. The diffraction patterns of all Pd-containing
catalysts are comparable. The Pd (111) plane corresponds to the diffraction peak at
40.1°.52),53) Both Pd/rGO/MWCNT and rGO/MWCNT have peaks at 25.8° corresponding
to the C (002) plane to an interlayer spacing of 3.45 Å.54)–56) The broad diffraction peak of
the C (002) plane in Pd/rGO is attributable to the lesser degree of agglomeration of therGO sheets.57) Next, the oxidation state of carbon and Pd metals was determined via X-ray
photoelectron spectroscopy (XPS) analysis (Fig. 3b and c). XPS is an effective tool forverifying the successful reduction of GO because the oxo groups in the C 1s XPS spectra
will be reduced in the presence of rGO. Fig. S2a shows C 1s XPS data from the GO. For
GO, the deconvoluted C 1s showed the existence of C–C, C–O, and C=O, with therespective binding energies of 284.5 eV, 286.6 eV, and 288.2 eV.58),59) The deconvoluted C
1s peak of Pd/rGO/MWCNT is shown in Fig. 3b, where the C–O signal is markedly
weaker, and where the C=O peak is very low, showing effective GO reduction.58) Fig. S2band Fig. S2c show similar features in deconvoluted C 1s spectra of rGO/MWCNT and
Pd/rGO, respectively. This finding indicates that the GO was reduced by chemical
reduction in all cases. The peak attributable to π–π* interaction was discovered at binding
energy 291.1 eV in both Pd/rGO/MWCNT (Fig. 3b) and Pd/rGO (Fig. S2c).58) The Pd 3d
XPS spectra of Pd/rGO/MWCNT are portrayed in Fig. 3c. With a 5.2 eV gap, two peaks
were found with binding energies of 340.8 eV and 335.6 eV for Pd (3d3/2) and Pd(3d5/2).34),39),41),52),58) According to XPS measurements, Pd NPs were in a metallic state in
Pd/rGO/MWCNT. The chemical reduction process converts Pd precursor to metal Pd.
From XPS data, a small amount of PdO was discovered with higher binding energy thanmetal Pd. In Pd/rGO, the Pd 3d XPS spectra exhibited similar features to the
Pd/rGO/MWCNT (Fig. S2d). The Pd 3d XPS data of Pd/rGO also demonstrated Pd NPs
are in metallic states. The Pd NPs were examined next using transmission electron
microscopy (TEM). Fig. 3d presents a TEM picture of Pd/rGO/MWCNT. The Pd NPs are
dispersed on the rGO/MWCNT support. According to the inset histogram, the average sizeof the Pd NPs is roughly 9 nm. The TEM image shows the Pd NPs as spherical.

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Fig. 3. (a) XRD patterns of as-synthesized rGO/MWCNT, Pd/rGO, and Pd/rGO/MWCNT,
with XPS spectra of Pd/rGO/MWCNT (b) C 1s and (c) Pd 3d. (d) TEM image of
Pd/rGO/MWCNT (scale bar = 20 nm, inset: histogram of the Pd NPs). (e) Nitrogen
adsorption–desorption curves of Pd/rGO/MWCNT, Pd/rGO and rGO/MWCNT.
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As shown in Fig. 3e, the N2 adsorption and desorption test (BET analysis) was
conducted to analyze the surface area of all electrocatalysts. Table I presents the
BET-specific surface area and total pore volume of all catalysts. The specific surface areaof the Pd/rGO/MWCNT catalyst was higher than that of the Pd/rGO and rGO/MWCNT
catalysts. The adsorption research results demonstrated that the pore volume of the
Pd/rGO/MWCNT catalyst was similarly greater than that of the others. The inclusion of Pd
and MWCNT between graphene layers enhanced the catalyst pore volume. When Pd and
MWCNT are combined with rGO in Pd/rGO/MWCNT, the surface area, and pore volume
increase in comparison to Pd/rGO and rGO/MWCNT individually. The synergistic effectof combining two complementary materials might enhance catalytic performance.

Table I. BET surface area and pore volume of Pd/rGO/MWCNT, Pd/rGO, andrGO/MWCNTCatalyst

BET surface area (m2 g-1)

Pore volume (cm3 g-1)

Pd/rGO/MWCNT

153.7

0.61

Pd/rGO

130.1

0.26

rGO/MWCNT

130.7

0.29

3.2 Electrocatalytic performance
Cyclic voltammograms were obtained under various experimental conditions to investigate
the electrocatalytic performance of Pd/rGO/MWCNT/GCE for the electrooxidation of AA
molecules. In the presence of AA, the modified electrode response was evaluated, as
shown in Fig. 4a. Cyclic voltammograms between -0.8 V and +0.7 V were obtained in thepresence and absence of AA (5.6 mM) in 1 M KOH solution using Pd/rGO/MWCNT/GCE
at a scan rate of 50 mV s-1. A well-defined oxidation wave is generated by the oxidation of
AA at -0.04 V, although no oxidation peak exists without AA. We also found an oxidation
peak without the presence of AA at -0.15 V for the development of Pd2+ from Pd0 under
the alkaline condition.52),60) In our recent article we also observed the PdO peak in a similarpotential for the Pd catalyst in the 1 M KOH solution.52) The peak potential of AA
oxidation (-0.04 V) is more positive than the PdO peak (-0.15 V) as shown in Fig. 4a. The
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reduction of PdO to Pd was attributed to a reduction peak detected at -0.5 V in 1 M KOH
solution.52) A large capacitive current is observed in Fig. 4a, which is very common for
rGO/MWCNT systems.25) The AA oxidation peak is further evaluated by theconcentration-dependent study which is described in the later part.
We evaluate the electrocatalytic performance by comparing all catalyst electrodes
to the bare GCE. Cyclic voltammograms of AA electrooxidation in an alkaline condition
using the bare GCE and all modified electrodes are shown in Fig. 4b. We exclude the large
capacitance current during electrocatalytic performance evaluation. We choose the potential
range of AA oxidation for each catalyst and calculate the peak current density. The selected
potential range for Pd/rGO/MWCNT/GCE is -0.20 V to +0.20 V and for Pd/rGO/GCE is-0.25 V to +0.25 V. The peak potential for AA oxidation is found at -0.04 V for
Pd/rGO/MWCNT/GCE and Pd/rGO/GCE. The carbon catalysts also have catalytic activity
for the AA oxidation as discussed in the introduction. The rGO/MWCNT and unmodified
GCE showed low catalytic performance for AA oxidation. In Fig. 4b, rGO/MWCNT/GCE
showed the AA oxidation peak at more negative potential with lower current density thanother catalysts. In comparison to unmodified GCE and other catalysts, thePd/rGO/MWCNT/GCE exhibits much higher catalytic activity for AA electrooxidation. The
current density of the Pd/rGO/MWCNT/GCE (5.18 mA cm-2) is higher than Pd/rGO/GCE(4.02 mA cm-2), rGO/MWCNT/GCE (1.85 mA cm-2) and bare GCE (0.6 mA cm-2). This isdue to the synergetic effect of Pd NPs with the rGO and MWCNT hybrid nanocomposite.
The current increased significantly because of the increased surface area of the rGO and
MWCNT hybrid nanocomposite. When Pd NPs are introduced into this hybrid
nanocomposite, the electrocatalytic performance is improved considerably.

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Fig. 4. (a) Cyclic voltammograms of Pd/rGO/MWCNT/GCE in the absence and presenceof 5.6 mM ascorbic acid. (b) Catalytic activity comparison of bare GCE,
rGO/MWCNT/GCE, Pd/rGO/GCE, and Pd/rGO/MWCNT/GCE for 5.6 mM ascorbic acidat 50 mV s-1. (c) Concentration effect of ascorbic acid at 50 mV s-1. (d) Scan rate effect of
12.7 mM ascorbic acid using Pd/rGO/MWCNT/GCE catalyst. All experiments describedabove were conducted in a 1 M KOH solution.
The electrochemically active surface area (ECSA) is investigated further to assess theelectrochemical behavior of catalysts. The number of electrochemically active sites is
described by the ECSA for metal catalyst mass. The ECSA of the two Pd catalysts(Pd/rGO/MWCNT/GCE and Pd/rGO/GCE) were calculated using the following
equation.52)
ECSA= Q/sl

(1)

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Here, Q represents the Coulombic charge of the PdO reduction peak (Fig. 4b); s denotes
the proportionality constant =0.405 mC cm-2. Also, l denotes the metal loading in g m-2.
Table II presents the ECSA values of prepared catalysts. The Pd/rGO/MWCNT/GCE
catalyst showed the highest ECSA value. Therefore, the combination of Pd NPs with rGO
and MWCNT enhanced the surface area for the electrocatalysis of AA oxidation. To
evaluate electrocatalyst performance, we further checked the specific activity (SA) andmass activity (MA) for Pd/rGO/GCE and Pd/rGO/MWCNT/GCE towards AA oxidation inalkaline condition. The SA explains the inherent activity of the catalyst. The MA can be
helpful in terms of commercial applications in which we can evaluate the activity of thecatalyst per unit mass.61) The SA is the current per unit ECSA of the catalyst. The SA is
calculable from ECSA normalized catalytic current. The MA, the current per unit mass ofthe catalyst, is determined by the mass-normalized catalytic current.61),62) The SA and MA
values for Pd/rGO/MWCNT/GCE and Pd/rGO/GCE are also presented in Table II. The
MA and the SA of Pd/rGO/MWCNT/GCE are around 1.6 and 1.1 times higher,
respectively than those of Pd/rGO/GCE. The Pd/rGO/MWCNT/GCE also exhibits
excellent electrocatalytic properties in terms of higher SA and MA for AA electrooxidationin alkaline condition.
Table II. Electrochemical characterizations for Pd/rGO/MWCNT/GCE and Pd/rGO
catalyst in 5.6 mM AA + 1 M KOH solution
Catalyst
Pd/rGO/MWCNT/GCE
Pd/rGO/GCE

Metal loading

ECSA

MA

SA

(g m-2)(m2g-1Pd)(mA mg-1Pd)(mA cm-2)

6.7

10.5

7.8

0.074

8.0

7.2

5.0

0.069

Next, we evaluated the AA oxidation reaction process for the Pd/rGO/MWCNT/GCE.
At a scan rate of 50 mV s-1, the concentration effect of AA on the
Pd/rGO/MWCNT/GCE was explored further, with findings presented in Fig. 4c. The
oxidation current increased as the concentration of AA increased. This finding indicatesthat the oxidation wave at -0.04 V is coming from the AA oxidation and the concentrationof AA does not affect the Pd/rGO/MWCNT catalyst active site. The peak potential of AA
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oxidation

shiftedpositively

as

the

concentration

increased,

indicatingadiffusion-controlled process.63) Interruption of the analyte mass transportation mechanism
is responsible for this change. Current is explained by mass transfer from the bulk
(solution) to the electrode surface. The increase in AA concentrations interrupts the
diffusion process. The electrochemical system is more likely to restore mass transportation.
As a result, the peak potential has shifted in a positive direction. If this oxidation wave is
not coming from the AA oxidation, then the peak intensity as well as, peak shifting does
not occur.
Fig. 4d presents the effects of scan rates on AA oxidation. The anodic peak current of
Pd/rGO/MWCNT/GCE increased concomitantly with increasing scan rate, varying from25 to 500 mV s-1. To understand more about the scan rate influence on peak current
behavior, we have conducted further research. The log peak current (Ip) versus log (scanrate) curve is shown in Fig. S3. The plots show a linear association with a slope of 0.49
(close to 0.5). The slope value indicates that, at the Pd/rGO/MWCNT/GCE surface, thesolution to the electrode surface mass transfer step controls the AA oxidation.20),52)
Chronoamperometry was also used to elucidate the electrochemical stability of all
catalysts.64) Stability assessment by chronoamperometry of Pd/rGO/MWCNT/GCE,
Pd/rGO/GCE, and rGO/MWCNT/GCE electrodes is presented in Fig. 5, applying at -0.04
V potential of AA oxidation in 1 M KOH. After 1000 s, the current density of
Pd/rGO/MWCNT/GCE is still higher than those of the other two catalysts. The stability of
Pd/rGO/MWCNT/GCE for AA oxidation in the alkaline state is high. We inferred fromthese data that the Pd/rGO/MWCNT catalyst might be employed as an anode catalyst inalkaline AA fuel cells.

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Fig. 5. Chronoamperometry of 5.6 mM ascorbic acid at rGO/MWCNT/GCE, Pd/rGO/GCE,
and Pd/rGO/MWCNT/GCE.
The incorporation of Pd NPs into the rGO and MWCNT hybrid nanocomposite
increases the electrochemical performance for AA electrooxidation considerably. Also, thePd/rGO/MWCNT catalyst exhibits excellent stability for AA oxidation in the alkalinecondition, promising candidate for AA based energy harvesting system.

4. Conclusions
After synthesizing Pd NPs incorporated rGO and MWCNT hybrid nanocomposite through a
one-pot chemical reduction process, we used them for AA electrooxidation in the alkalinecondition. Three modified electrodes were prepared using three synthesized catalysts.
Pd/rGO/MWCNT/GCE showed better catalytic performance for AA electrooxidationcompared to the other two modified electrodes (Pd/rGO/GCE and rGO/MWCNT/GCE).
Additionally, the Pd/rGO/MWCNT/GCE demonstrated excellent stability to AA
electrooxidation in an alkaline environment. The outstanding catalytic performance andsuggest that Pd NPs with rGO and MWCNT, as an alternate carbon material, could be used
for environmentally friendly energy harvesting systems.

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Acknowledgments
Y. N. appreciated the support of JSPS KAKENHI grant numbers JP21H00020, The Murata
Science Foundation, and JST CREST Grant Number JPMJCR21B3, Japan. We are gratefulto Professor Tsuyohiko Fujigaya of Kyushu University in Japan for his helpful scientificdiscussion.

Appendix
Supplementary data
Surface area calculation; SEM, XPS study of GO, rGO/MWCNT, Pd/rGO; Log (Ip) vs. log(scan rate) plots of Pd/rGO/MWCNT.

ORCID
Md. M. Hasan: https://orcid.org/0000-0001-5617-100X
Y. Nagao: https://orcid.org/0000-0003-1249-440X
Z. Li: https://orcid.org/0000-0002-0146-5526

Conflicts of interest
There are no conflicts to declare. No author has any competing financial interest
related to this report or the study it describes.

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