Gallic acid-assisted synthesis of Pd uniformly anchored on porous N-rGO as efficient electrocatalyst for microbial fuel cells

Article abstract of DOI:10.1039/c7dt04063f

The sluggish kinetic rate-limiting oxygen reduction reaction (ORR) at the cathode remains the foremost issue hindering the commercialization of microbial fuel cells (MFCs). Utilization of the effect of micromolecule conjugation and the synergistic effect between Pd nanoparticles and N-rGO (nitrogen-doped reduced graphene oxide) to stabilize a precious metal onto carbon materials is a promising strategy to design and synthesize highly efficient cathode catalysts. In this study, gallic acid is used to facilitate the coupling of palladium (Pd) with N-rGO to form GN@Pd-GA via a simple hydrothermal process. Notably, the as-synthesized GN@Pd-GA as a cathode catalyst shows an approximately direct four-electron feature and demonstrates a high ORR performance in 0.1 M KOH. Furthermore, the stability and methanol tolerance of GN@Pd-GA are superior to those of the commercial Pt/C catalysts. In addition, a maximum power density of 391.06 ± 0.2 mW m^-2 of MFCs equipped with GN@Pd-GA was obtained, which was 96.2% of the power density of MFCs equipped with a commercial Pt/C catalyst.

Full text of DOI:10.1039/c7dt04063f

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Gallic acid-assisted synthesis of Pd uniformly
anchored on porous N-rGO as efficient
electrocatalyst for microbial fuel cells†
Cite this: DOI: 10.1039/c7dt04063f
Xiao-Tong Wu, Jie-Cheng Li, Qiu-Ren Pan, Nan Li * and Zhao-Qing Liu
*
The sluggish kinetic rate-limiting oxygen reduction reaction (ORR) at the cathode remains the foremost
issue hindering the commercialization of microbial fuel cells (MFCs). Utilization of the effect of micro-
molecule conjugation and the synergistic effect between Pd nanoparticles and N-rGO (nitrogen-doped
reduced graphene oxide) to stabilize a precious metal onto carbon materials is a promising strategy to
design and synthesize highly efficient cathode catalysts. In this study, gallic acid is used to facilitate the
coupling of palladium (Pd) with N-rGO to form GN@Pd-GA via a simple hydrothermal process. Notably,
the as-synthesized GN@Pd-GA as a cathode catalyst shows an approximately direct four-electron feature
and demonstrates a high ORR performance in 0.1 M KOH. Furthermore, the stability and methanol toler-
ance of GN@Pd-GA are superior to those of the commercial Pt/C catalysts. In addition, a maximum
power density of 391.06 ± 0.2 mW m−2 of MFCs equipped with GN@Pd-GA was obtained, which was
96.2% of the power density of MFCs equipped with a commercial Pt/C catalyst.
Received 28th October 2017,
Accepted 18th December 2017
DOI: 10.1039/c7dt04063f
rsc.li/dalton
1
5,16
them, PtM
alloys can minimize the overpotential and
1
Introduction
1
7–19
promote fast reaction kinetics of ORR for MFCs.
Excessive consumption of fossil fuels has exacerbated the However, the formation of disorder PtM cores and low disper-
current energy crisis and environmental pollution, and there sity of active tensile strain on Pt (111) facet hinder long-term
is an urgent need to explore renewable and sustainable stability of Pt-based catalysts after exposing to acidic electro-
1
–3
20
energy technologies.
As ideal renewable energy devices, lyte solution. Alternatively, Pd and its derivatives, which are
microbial fuel cells (MFCs) connect energy and wastewater relatively affordable due to their earth-abundance and have
since they can harvest electric energy from metabolic pro- comparatively high catalytic activities and stabilities, have
cesses of bacteria and further solve the problem of environ- recently attracted significant attention as cathode cata-
2
–4
21,22
ment pollution from sewage.
However, the relatively low lysts.
For example, Wang and co-workers successfully
power density of MFCs greatly limit their practical appli- inlayed Pd nanoparticles within a Pt icosahedron, thus enhan-
5
cations. Specifically, a major barrier to increasing the power cing the ORR performance significantly by designing a defective
1
2,23
density of MFCs is the ORR efficiency (the reductive half reac- structure for cost-effective catalysts.
Despite the fact that Pd-
−
tion O /OH of the cathodic reaction, e.g., in alkaline media) based compounds exhibit high performance in electrocatalytic
on the air-cathode. As such, sluggish kinetics of the ORR processes, the scarcity and high dosage requirements of Pd
2
6
,7
8
24,25
largely restricts the rate of energy conversion in the MFCs.
To date, the best ORR performance of cathode catalysts is
generally demonstrated by Pt-based catalysts such as Pt–Ni efficiency and stability is to introduce carbon materials (e.g.,
severely limit its practical applications.
Encouragingly, an effective strategy to enhance the catalytic
9
,10
11,12
13,14
alloys,
PtPd alloys,
and PtFe nanoparticles.
Among carbon nanotubes, carbon nanowires, and carbon plates),
which have been identified as potential conductive
2
6–28
materials.
Among them, graphene and its derivatives (N or
School of Chemistry and Chemical Engineering/Guangzhou Key Laboratory for
Environmentally Functional Materials and Technology/Key Laboratory for Water
Quality and Conservation of the Pearl River Delta, Ministry of Education, Guangzhou
University, Guangzhou 510006, PR China. E-mail: nanli@gzhu.edu.cn,
lzqgzu@gzhu.edu.cn; Tel: +86-20-39366908; Fax: +86-20-39366908
S-doped graphene) have been widely applied in energy conversion
and storage owing to their low internal resistance and high activity
29,30
and conductivity.
Obviously, heteroatom (N, P, or S)-doped
carbon-based materials can serve as alternatives with highly
31
efficiency and low-cost to Pd-based catalysts for the ORR.
†
Electronic supplementary information (ESI) available: Tables S1–S3. See DOI:
10.1039/c7dt04063f
However, the complicated synthetic process and harsh reaction
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conditions greatly limit their further applications. Therefore, it is 2.3 Synthesis of porous nitrogen-doped graphene
imperative to design and develop efficient and stable electrocata-
lysts for MFCs by a simple, easy, and scalable strategy.
Porous nitrogen-doped graphene was prepared by a hard tem-
32
plate method. To dope the graphitic framework with N, 25 mL
Recently, a biomolecule-assisted strategy to construct metal
nanoparticles with high dispersities has become a hot topic of
−1
of the as-prepared GO aqueous dispersion (4 mg mL ) was
mixed with 75 mL distilled water by sonication for 2 h to form
3
3
research. Accordingly, in this study, gallic acid-assisted Pd
nanoparticles decorated on N-doped porous graphene have
been successfully developed by a simple process. Impressively,
the as-synthesized catalyst exhibits excellent ORR performance
with a large limited current density (−4.03 mA cm− ) and a
positive onset potential (−0.101 V vs. SCE), close to that of the
bench mark noble metal catalyst Pt/C. An in-depth study
shows that the high ORR performance of Pd/N-rGo is mainly
ascribed to the synergistic effect between Pd nanoparticles and
N-rGo, in which the porous N-doped graphene provides a con-
ductive surface area and abundant active sites to facilitate the
charge transfer efficiency, and the channels for electrolyte
diffusion to the Pd nanoparticles are retained. This finding
will provide a new path for the development of a highly
efficient and stable ORR catalyst for MFCs.
−1
a GO aqueous solution of low concentration (1 mg mL ).
A solution containing both 1.0 g SiO2 spheres and 2.0 g
cetyltrimethylammonium bromide (CTAB) was stirred for 4 h
at room temperature. Then, the filter residue was dried at
1
50 °C for 12 h. Afterward, urea with mass ratio of 1 : 2 was
added in the powder (GO@SiO ), and the mixture was ground
2
and transferred into a porcelain boat and annealed at 800 °C
for 4 h under a N atmosphere. Finally, the powder was further
2
treated by alkali etching (6 M KOH) to obtain porous structural
nitrogen-doped reduced graphene oxide (GN). In addition,
porous graphene (G) was obtained via the abovementioned
method without adding urea.
2.4 Synthesis of Pd-anchored porous nitrogen-doped
graphene
The as-synthesized 20 mg GN and G were respectively dis-
persed in 20 mL distilled water to form two suspension solu-
tions, which were mixed with 0.96 mL Pd(NH ) Cl (11.3 mg
3
4
2
2
Experimental
−
1
mL ) and kept at 60 °C for 15 min. Moreover, Pd(NH ) Cl
2
3
4
2
.1 Synthesis of silicon dioxide (SiO ) spheres
2
solution was prepared using PdCl2 (0.82 g), which was dis-
solved in 5 mL HCl (5%), and then, NH ·H O (25%–30%) was
Regular silicon spheres were synthesized by the polymerization
method. Typically, an aqueous-alcoholic solution was prepared
by mixing 150 mL of ethanol with 20 mL of distilled water.
Subsequently, 8 mL of ammonium hydroxide was added under
vigorous stirring (bottle A). Then, 10 mL of tetraethoxysilane
3
2
added dropwise to obtain the pink turbid liquid. The 11.30 mg
−
1
mL
water in a 100 mL volumetric flask while steadily adding
NH ·H O (observation: the pink liquid transformed into a
3 4 2
Pd(NH ) Cl solution was then mixed with distilled
3
2
transparent solution). Next, 10 mL gallic acid (GA) (1.43 mg
(TEOS) was added to 60 mL of ethanol to form the precursor
−
1
mL ) was added dropwise to the suspension that was then
kept at 60 °C for 15 min; 5 mL hydrazine hydrate (80%) was
added to in situ restored Pd at 60 °C for 6 h. The powders
formed were separated out of the solution by vacuum suction
filtration and dried at 50 °C for 12 h, which were denoted as
GN@Pd-GA and G@Pd-GA. Moreover, GN-doped Pd was pre-
pared without adding gallic acid and denoted as GN@Pd.
solution (bottle B). After stirring the contents of both bottles
for 15 min, the contents of bottle B were added rapidly to
bottle A under slow stirring until flocculent materials appeared
at 30 °C and kept for 6 h.
2
.2 Synthesis of graphene oxide (GO)
Graphene oxide was synthesized according to the modified
3
4
2.5 Construction and operation of the MFCs
Hummers methods reported in the literature. Briefly, graph-
ite carbon (1.0 g) and NaNO
3
(500 mg) were added con- In this study, a single-chamber air-cathode MFC device was
3
5
secutively to a round-bottom flask containing 23 mL con- used, with a liquid volume of 28 mL. The anode was pre-
centrated sulfuric acid (H SO ). The mixture was then treated by dipping a bare carbon cloth in an acetone solution
2
4
poured into an ice bath water and stirred evenly. KMnO
4
before annealing it at 500 °C for 30 min (after drying). The air-
(
3.0 g) was then slowly added to the mixture. As the solution cathode was also fabricated by covering a certain amount of
−
2
turned dark green, the mixture was added to 45 mL the as-prepared catalyst materials (3 mg cm ) or Pt/C catalyst
distilled water before being heated to 35 °C under constant (wt 20%, 2.5 mg cm− ) on the carbon cloth, and the effective
stirring for 30 min; subsequently, the container was trans- area of both the anode and the cathode was measured to be
ferred to a water bath at 70 °C, and the temperature was approximately 7 cm− . A Ti (titanium) wire was used to
increased to 98 °C over 15 min. The solution was then added connect both ends of the electrode with an external resistance
2
2
2 2 2
to a mixture of H O (140 mL) and 30% H O (10 mL). The (1000 Ω). The activate matter obtained from the downstream
2
50 mL graphene oxide (4 mg mL−1) aqueous dispersion was Pearl River ecosystem. Importantly, the culture medium con-
obtained by means of stratification, separation and abstersion sists of a mixture of glucose (1 g L− ), trace elements
1
until the value of pH equalled to 7. The utilized GO solution (12.5 mL), vitamin solution (5 mL) and phosphate-buffered
−
1
should undergo drastic ultrasonic dispersion each time.
nutrient medium (pH = 7.0) containing NH Cl (0.31 g L ),
4
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4 2 2 4 2
·H O (1.30 g L ), Na HPO ·3H
O (9.4 g L−1), and KCl (CV) were obtained at the rates ranging from 50 mV s to
−
1
−1
NaH
(
2
PO
0.13 g L− ).
1
32,36
A 16-channel data acquisition system (MPS 2 mV s . The electrochemically active surface area (ECSA) of
−1
0
10602, Beijing) was used to obtain the output voltage of the the catalyst was determined using an electrical double-layer
3
9,40
MFCs for different cathode materials (per minute). Moreover, capacitor (CDL).
CDL curves could be used to obtain every
the polarization curves of different cathodes were acquired by intermediate point of voltage by linear fitting under different
adjusting the external circuit resistance from 40 Ω to 5000 Ω, scanning speeds. Herein, the ECSA analysis was conducted by
and the anode electrode potentials were measured by inserting the following formula:
a saturated calomel electrode (SCE) electrode to compose a
two-electrode system. By means of the formula P = V × I, the
power density P could be calculated. Both I and P were normal-
ECSA ¼ CDL=Cs
ð2Þ
In this formula, CDL represents the double layer capacitance
ized to the effective area of the cathode surface, and the and C represents the specific capacitance. We have discussed
s
current density I was calculated using the formula: I = the ECSA through the values of C
due to the linearity
DL
4
1,42
V (cell voltage)/(R × A) (external resistance and effective area).
between them.
2
.6 Characterization
The surface morphology and crystal microstructure of the as- Results and discussion
prepared GN@Pd-GA composites were analyzed by field emis-
The synthesis process of GN@Pd-GA material is shown in
sion scanning electron microscopy (FE-SEM, JEOL JSM-7001F)
and transmission electron microscopy (TEM, JEM2010-HR).
The crystal structure of the materials was elucidated by Bruker,
D8 ADVANCE. The surface oxidation states were analyzed
using a Fourier transform infrared spectrometer (FTIR, Tensor
2
+
Scheme 1. Primarily, the Pd complex encircles the as-pre-
pared nitrogen-doped carbon spheres. The added gallic acid
contains acidic functional groups (–COO, –O) and a benzene
moiety, which absorbs metal ions and conjugate with gra-
phene to form π–π bonds, such that it can effectively increase
7, 400–4000 cm− ). The specific surface area and pore size
1
2
2
+ 43–45
the thermal stability and then couple with Pd .
The Pd
distribution of the samples were investigated by the Brunauer–
Emmett–Teller (BET) method.
nanoparticles are obtained in situ by adding hydrazine
hydrate, and the carboxyl group is also reduced to the hydroxyl
group. The FTIR spectra shows a characteristic diffraction peak
at 1552 cm− , corresponding to the stretching vibration of
2
.7 Electrochemical measurements
1
The ORR performance of the samples was evaluated by the
CHI 760D electrochemical workstation (Shanghai, Chenhua)
with a rotating disk electrode (RDE) system: the experiments
were performed in a standard three-electrode system, with a
saturated calomel electrode (SCE) as the reference electrode, a
glassy carbon electrode as the counter electrode, and the cata-
lyst film coated on a rotating disk electrode (∼0.1 mg cm−
46
−1
ν(–C–O–C–) (Fig. S1†). Moreover, the peak at near 3430 cm
has a wide and strong absorption, which can be attributed to
47
ν(–OH) stretching vibration. The ν(–OH) absorption peak
intensity decreases until its value is almost zero; this indicates
that oxygen in its elemental state (O
2
) is still present despite
the thermal treatment. Moreover, although there are two peaks
3
−1
−1
at 1203 cm and 1142 cm , which belong to the vibration of
loaded on the electrode surface) as the working electrode. A
−1
ν(–C–O), only the 1203 cm absorption peak experiences a red
1
0
.1 mol L− KOH electrolyte solution was used for the ORR
shift because the hydrogen ion of gallic acid is beneficial to
enhance the conjugation between benzene and the oxygen
tests (in an alkaline solution). The RDE system was used for
gaining insights into the electron transfer under oxygen
reduction, where the rotational speed was varied from 400 to
48
atom.
As evidenced by the X-ray diffraction (XRD) pattern shown
in Fig. 1a, the diffraction peaks for graphite within the
samples are well-indexed to the spinel structure (PDF No. 41-
−1
2
500 rpm at a scan rate of 10 mV s
Herein, Koutecky–Levich (K–L) curve analysis was used to
calculate the number of electrons transferred (n) using the fol-
.
1487), whereas the shift in the (002) graphitic diffraction peak
3
7
lowing formula:
at 24.3° verifies the successful carbonization of N-doped gra-
4
9
j
¼ jkꢀ1 þ ð0:62nFAD2=5Vꢀ1=6CÞꢀ1
ꢀ1=2
ω
ð1Þ
phene. Moreover, the presence of Pd is confirmed by the
ꢀ1
peaks present at 2θ of 40.1°, 46.7°, and 68.1°, which originate
In this formula, j represents the measured current (A), F is from the (111), (200), and (220) diffraction of Pd, respectively,
−
1
the Faraday constant (96 485 C mol ), A represents the which can be ascribed to the face-centered cubic-phases of
2
50
effective area of the disk electrode (0.126 cm ), D is the Pd (fcc). The sharp diffraction peaks of Pd reveal the high
diffusion rate of oxygen, V represents the kinematic viscosity, degree of crystallinity of the Pd nanoparticles. The character-
C is the oxygen concentration in the electrolyte, and ω rep- istics of the N-doped carbon in the obtained product are
3
8
resents the rotation rate of the electrode. After data collec- further analyzed by Raman spectroscopy. Moreover, the pres-
tion, working electrodes were prepared to conduct a steady- ence of graphitic carbon was confirmed by the analysis of the
state potential sweep from open-circuit voltage in both a posi- Raman spectrum shown in Fig. 1b. Herein, two characteristic
−
1
tive direction (+0.05 V) and negative direction (−0.05 V) in an peaks of carbon materials are present, located at 1369 cm
2
O -saturated 0.1 M KOH solution. The cyclic voltammograms and 1589 cm− , corresponding to the D and G bands, respect-
1
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Scheme 1 Schematic of the synthesis process for GN@Pd-GA.
Fig. 1 (a) X-ray diffraction of the as-synthesized samples and (b) Raman pattern of GN@Pd-GA.
ively. It is well-documented that the D band is associated with
defects and induced disorder, whereas the G band is derived
2
51
from sp hybridized carbon. The peak intensity ratio of the D
band to the G band (I /I ) is related to the degree of disorder-
D
G
ing during the graphitization of carbon materials (Fig. S11†).
The value of ID/IG for the GN@Pd-GA material is calculated to
be 1.06, and the I
increase in I /I value further implies a major disorder in the
carbon lattice.
The microstructure is related to the type of pores for mass
transport of the graphite carbons. The N sorption isotherm
D G
/I value of rGO is calculated to 0.91: the
D
G
2
illustrated in Fig. 2 shows the presence of abundant mesopores
within GN@Pd-GA, which is created by doping N and sub-
sequent decomposition of the SiO
2
sacrificial template.
Fig. 2 The nitrogen adsorption–desorption isotherm for GN@Pd-GA
Mesopores with different diameters can be found in the BJH (the inset shows the corresponding pore size distribution spectrum).
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Fig. 3 (a, b) The SEM images of GN@Pd-GA; (c, d) TEM images of GN@Pd-GA; and (e) high-resolution TEM image; (f–h) SAED patterns of GN@Pd-
GA; and (i) X-ray photoelectron spectroscopy of GN@Pd-GA.
pore size distribution (the inset of Fig. 2) of GN@Pd-GA. The of nitrogen in the as-synthesized N-doped graphene with the
GN@Pd-GA also exhibited a high Brunauer–Emmett–Teller weight of 2.32%.
2
−1
(
0
BET) surface area (113 m g ), with a total pore volume of
To understand the electrochemical activity of the as-syn-
3
−1
.384 cm
g
. The representative SEM and TEM images of thesized catalytic materials, linear polarization with a rotated
−
1
GN@Pd-GA are shown in Fig. 3. The panoramagram in Fig. 3 disc electrode (RDE) at a scan rate of 10 mV s in an oxygen
a and b) illustrates that the as-synthesized sample is a mul- saturated 0.1 mol L− KOH electrolyte was used to predict the
1
(
tiple-layer structure. In addition, the GN@Pd sample without ORR performance. The commercial Pt/C was also tested under
5
3
gallic acid only possesses a few Pd nanoparticles; this demon- the same conditions for comparison. Fig. 4a shows the CV
strates the importance of utilizing gallic acid to increase the (cyclic voltammetry) curves of the samples. Compared with
adhesion of the Pd nanoparticles (Fig. S2†). Detailed struc- G@Pd-GA and GN@Pd, GN@Pd-GA exhibits the approached
tural characteristics of GN@Pd-GA were further investigated electrochemical surface area comparable to that of commercial
by TEM. In addition to the above mentioned SEM images, the Pt/C. In addition, the CV curves obtained under N
TEM images displayed in Fig. 3(c–e) further confirmed that and O
2
-saturation
-saturation at a scan rate of 10 mV s− are shown in
2
1
thin layer graphene sheets were successfully synthesized, and Fig. S3† and provide evidence that GN@Pd-GA possesses high
Pd nanoparticles were uniformly loaded onto the N-doped activity under oxygen saturation. To gain further insights into
graphene. More importantly, a well-defined lattice fringe is the positive effects, the ORR performances of samples were
observed in Fig. 3f, where the lattice pattern values of evaluated by linear sweep voltammetry (LSV) curves at 1600
0
.22 nm and 0.19 nm corresponding to the (111) and (200) rpm (Fig. 4b). The G@Pd-GA exhibited an open circuit poten-
−2
crystal faces of Pd further verify the formation of Pd. The tial (0.59 V vs. SCE) with a current density of −4.03 mA cm at
corresponding electron diffraction patterns in Fig. 3(g and h) −0.6 V.
5
2
also indicate the crystal nature of the formed Pd. The EDS
GN@Pd showed an open circuit potential (0.57 V vs. SCE)
analysis of GN@Pd-GA in Fig. 3i shows that different with a current density of −3.10 mA cm− at −0.6 V, and
2
elements have different binding energies; combined with the GN@Pd-GA displayed a positive ORR open circuit potential
−2
calculated data (Table S1†), it can be seen that the Pd nano- (0.62 V vs. SCE) and a current density of −4.43 mA cm at
particles have been loaded with a weight percentage of −0.6 V, which was remarkably close to those of Pt/C (open
1
0.23% Pd, which conforms to the addition and the existence circuit potential of 0.65 V with a current density of −4.51 mA
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Fig. 4 (a) CV curves of the catalysts in an O
2
-saturated 0.1 M KOH solution at the scan rate of 10 mV s− ; (b) LSV curves of catalysts in a 0.1 M KOH
1
−1
solution at the scan rate of 5 mV s with 1600 rpm; (c) K–L curves, (d) electron transfer number, (e) Nyquist curves, and (f) the CDL curves of G@Pd-
GA, GN@Pd, GN@Pd-GA, and Pt/C.
cm− at −0.6 V) (Table S2†). Theoretically, the larger the by the chronoamperometric response at a constant cathodic
2
opening potential of the catalyst, the more difficult it is to be voltage of −0.3 V (vs. SCE) in an O -saturated 0.1 M KOH. As
2
corroded. More details are proposed to further evaluate the shown in Fig. 5a, the tolerance of methanol of GN@Pd-GA
complex ORR process. The Koutecky–Levich (K–L) curves exhibited a stronger resistance and better selectivity of metha-
−
1
−1/2
(
j
vs. ω
) of catalysts in Fig. 4c were used to calculate the nol as compared to that of Pt/C; this was likely due to the
transfer number (n) of kinetics under the current at −0.6 V in synergy effects between the Pd nanoparticles and rGO. To
the system of saturated oxygen, and the LSV curves of the cata- ensure sustained long-term performance from Fig. 5b,
lysts showed good linearity at various rotation speeds the current density of the as-prepared cathode showed a
3
5
(
Fig. S4†). As shown in Fig. 4d, the n value of the target reduction of 11.18% after 20 000 s, whereas Pt/C had lost
material for GN@Pd-GA was calculated to be 3.86, which was almost 18.9% of current, indicating the excellent stability of
close to the value of Pt/C (4.00). Obviously, the value of n of GN@Pd-GA.
GN@Pd-GA was much larger than those of GN@Pd (3.2) and
To compare the different cathodes (within MFCs), the per-
G@Pd-GA (3.77). It is well known that the oxygen reduction reac- formances of single-chamber MFCs equipped with the
−
tion is usually dependent on the transport pathway of 2 e or 4 different catalysts were examined. Fig. 6a shows the redox
−
54
e
reduction. Therefore, we can conclude that the layered process of the MFCs device assembled with a GN@Pd-GA
nitrogen-doped graphene provides an effective surface for cathode in a sewage medium. The maximum output voltage of
electron transport and increases the electron donation effect of the cathode catalysts is also shown in Fig. 6b and clearly
the nitrogen atom and the alkalinity of the ortho-carbon reveals that the GN@Pd-GA possesses the relatively high value
atoms to further greatly provide more active sites for the ORR of 0.4 ± 0.02 V. Furthermore, the power density of the GN@Pd-
GA cathode is 391.06 mW m− (Fig. 6c), which is slightly lower
2
process.
To further investigate the conductivity and electrochemical than that of Pt/C (406.65 mW m− ) (Table S4†). Impressively,
surface active area (ECSA) of the catalysts, electrochemical the increased number of active sites and the well-dispersed Pd
independence spectrum (EIS) and CV spectra were obtained particles can contribute to greatly improve the electric power
for the ORR catalyst. Clearly, the GN@Pd-GA has a lower Rct density. Fig. 6d also shows polarization curves originating
than GN@Pd (which did not utilize gallic acid) (Fig. 4e and from the MFCs equipped with electrocatalysts, confirming the
Table S3†). The obtained results indicated that the great con- rate-determining step in the ORR process. Therefore, the excel-
ductivity of GN@Pd-GA catalyst was attributed to the use of lent ORR activity of the GN@Pd-GA cathode is mainly attribu-
gallic acid as the conjugated plane contributed to accelerating ted to three factors: (i) the small size and high dispersion of
the charge transfer rate. In addition, the defects of N-doped Pd nanoparticles decorated on the porous N-rGO can provide
graphene can increase Rct and slow down the electron transfer. excellent conductivity, large ECSA (depends on the values of
2
The double layer capacitance (CDL) curves in the CV curves
CDL), and abundant active sites by the assisted synthesis using
(Fig. S5†) obtained under various scan rates in Fig. 4f reveal gallic acid; (ii) the robust stability of GN@Pd-GA may be
that the elctrochemically active area increases with the mainly due to the porous N-rGO; and (iii) the synergistic effect
addition of N in graphene and in the distributed Pd particles. between Pd and N-rGO can also be beneficial to the improve-
The durability of the obtained GN@Pd-GA was also evaluated ments of the electrocatalytic activity for GN@Pd-GA.
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Fig. 5 (a) Chronoamperometric responses to the injection of methanol into an O
2
-saturated 0.1 M KOH solution at the GN@Pd-GA and Pt/C elec-
2
trodes in −0.3 V; (b) chronoamperometric responses of the GN@Pd-GA and Pt/C electrodes at −0.6 V (vs. SCE) in O -saturated 0.1 M KOH solutions
at a rotation rate of 1600 rpm.
Fig. 6 (a) Schematic of the ORR process in the MFCs; (b) maximum cell voltages; (c) power density curves; and (d) entire cells and anode (vs. Ag/
AgCl) polarization curves for different catalyst materials.
ing strategy to develop Pd with enhanced catalytic efficiency
for energy conversion reactions.
Conclusion
In summary, well-dispersed Pd nanoparticles anchored on
N-rGO have been successfully synthesized through a facile self-
assembly and solvothermal method with the assistance of
Conflicts of interest
gallic acid. The as-synthesized GN@Pd-GA exhibits a remark-
able ORR performance, nearly comparable to that of commer-
cial Pt/C. The excellent ORR performance may be mainly
There are no conflicts to declare.
attributed to the efficient synergistic effect between Pd and
porous N-rGO. Most importantly, the MFCs equipped with the
Acknowledgements
GN@Pd-GA cathode catalyst accordingly displays preferable This work was supported by the Natural Science Foundations
stability and power density. This study demonstrates a promis- of China (Grant No. 21576056, and 21576057), Guangdong
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Natural Science Foundation (Grant No. 2015A030313503 and 20 A. Esfandiari, M. Kazemeini and D. Bastani,
017A030311016), Science and Technology Research Project Int. J. Hydrogen Energy, 2016, 41, 20720–20730.
of Guangdong Province (Grant No. 2016A010103043), 21 L. Bu, N. Zhang, S. Guo, X. Zhang, J. Li, J. Yao, T. Wu,
2
Featured Innovation Project of Guangdong University (Grant
No. 2016KTSCX107), Science and Technology Research
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