(779c) MnO2-Functionalized Graphene Nanosheets Supported Pt Nanoparticles With Excellent Performance for Electrooxidation of Methanol

1. Introduction

Direct methanol fuel cells (DMFCs) as promising
alternative power sources have attracted considerable attention due to their
high theoretical energy conversion efficiency, low pollutant emission, and
convenient storage and transport of liquid fuel. Platinum is the best-known
precious metal catalyst that exhibits significant electrocatalytic activity for
methanol oxidation. However, given the limited resources and high cost of
platinum, it is very necessary to improve the utilization efficiency of noble
metals for the practicality and commercialization of DMFCs. In this regard,
hybrid electrode materials consisting of metal Pt incorporated into other
elements have been explored. Among them, MnO₂ is of particular
interest due to their excellent proton conductivity, low density, and good
permeation. On the other hand, graphene, a single layer of graphite, has been a
hot topic in this area because of its unique physicochemical properties, such
as extremely high specific surface area and superior electronic conductivity.
Therefore, the interaction of Pt nanoparticles (NPs), MnO₂ and
graphene sheets (GS) is promising to simultaneously
possess respective merits of
each component, which is highly desirable and technologically important for
improving the catalytic performance of Pt catalysts. Herein, we report a simple
and cost-effective strategy for the synthesis of a ternary Pt/MnO₂/GS
composite as an advanced electrocatalyst for high-performance DMFCs.

2. Methods

As shown in Figure 1, graphene oxide (GO) was prepared
from powdered flake graphite by a modified Hummer's method. With the
introduction of KMnO₄ aqueous solution into the GO dispersion
system, the carbon atoms of graphene framework would reduce MnO₄^-in situ to form MnO₂ in a water-containing system at room temperature. Thereafter, Pt(NO₃)₂ was added to the
MnO₂/GO nanosheets colloid suspension under stirring. During the
solvothermal reaction, both of the reduction of graphene oxide and Pt NPs
loading can be achieved, leading to the formation of Pt/MnO₂/GS
composite. Pt/GS, Pt/XC-72 (commercial carbon black, Vulcan XC-72) and Pt/MnO₂/XC-72
were also synthesized for comparison.

Figure 1 Illustrations of the synthesis of Pt/MnO₂/graphene
catalyst.

3. Results and discussion

X-ray photoelectron spectra (XPS) was used to
determine the composition of the as-prepared composites. As shown in Figure 2a-b, the intensity of oxygen-containing groups such as C-OH (286.1 eV), C-O-C (287.6 eV), H-O-C=O (288.9 eV) in Pt/MnO₂/GS was obviously reduced during the solvothermal
reaction, while the peak at 284.5 eV, C-C bond, became predominant. The
significantly lower O/C ratio in the composite can be linked to the much faster
rate of charge transfer, which is favorable for the oxidation of methanol. XPS
analysis also shows that there are different Pt oxidation states in the
catalyst. It can be seen from Figure 2c that the Pt 4f signal of Pt/MnO₂/GS represents the sum of two pairs of doublets: the most intense doublet (71.4 and 74.7 eV) is due to metallic Pt and the second doublet
(72.0 and 76.0 eV) can be ascribed to the +2 oxidation
state of Pt. In addition, the Mn 2p XPS spectrum of the Pt/MnO₂/GS
composite exhibits two characteristic peaks at 642.2 eV and 654.0 eV, corresponding
to the Mn 2p_3/2 and Mn 2p_1/2 spin-orbit peaks of MnO₂,
respectively (Figure 2d). Thus it can be concluded that MnO₂,
graphene and Pt coexist in the prepared nanohybrid.

Figure 2 C 1s core-level XPS spectra of (a) graphite oxide and (b) Pt/MnO₂/GS; (c) Pt 4f and (d) Mn 2p core-level XPS spectra of Pt/MnO₂/GS.

Figure 3 depicts transmission electron microscopy
(TEM) images of the MnO₂/GO, Pt/MnO₂/GS, Pt/GS and
Pt/XC-72 nanohybrids. As shown in Figure 3a, it was found that the almost transparent GO sheets were thinly covered with well-dispersed layered MnO₂. No obvious stacking of these MnO₂ lamellas was
observed, mainly due to the fact that the formed MnO₂ coating could
function as a barrier preventing further access of the MnO₄^-
ions to the functionalized carbon surface. Afterwards, MnO₂-functionalized
GO sheets were used to load Pt NPs for producing ternary Pt/MnO₂/GS
composite, as displayed in Figure 3b. The most striking feature is that the
ultrafine Pt metal NPs with diameters less than 3 nm were uniformly distributed
on the surface of MnO₂/GS sheets. Lattice fringes of both the (111)
plane in face-centered cubic Pt (0.22 nm) and the (400) plane in MnO₂
(0.24 nm) are shown in the high-resolution TEM (HRTEM) image (the inset of Figure
3b), which correlate well with the known data. In Figure 3c-d, it is clearly seen that Pt NPs were also deposited on GS and XC-72 supports, however, their distributions became much broader than those on MnO₂/GS surfaces
and the aggregation of some Pt NPs even took place. The statistical size
analysis of Pt NPs indicates that the average particle sizes are about 1.7 nm,
3.6 nm and 4.7 nm for Pt/MnO₂/GS, Pt/GS and Pt/XC-72, respectively (Figure
3e). The findings can be explained in terms of the reasons that the presence of
hydrous MnO₂ can create large hydrophilic regions on the surface of
GO, which can facilitate the diffusion of Pt²⁺ ions and effectively
prevent the metal NPs from agglomeration.

Figure 3 TEM images of (a) MnO₂/GO, (b)
Pt/MnO₂/GS, (c) Pt/GS and (d) Pt/XC-72. The inset of (b) is an HRTEM
image of Pt/MnO₂/GS. (e) The histograms of Pt particle size
distribution of different carbon-based samples.

To explore the potential applications, the as-obtained
Pt/MnO₂/GS hybrid was tested as a nanoelectrocatalyst for oxidation
of methanol to evaluate its catalytic activity. By measuring the charge
collected in the hydrogen adsorption/desorption region after double-layer
correction, the electrochemically active surface area (ECSA) of Pt/MnO₂/GS
was calculated to be as high as 103.2 m² g^-1, more than 2 times as large as that for Pt/GS (49.1 m² g^-1), nearly 3 times the Pt/XC-72 (38.5 m² g^-1) and 2 times the Pt/MnO₂/XC-72 (54.4 m² g^-1) catalyst (Figure 4a), demonstrating that the Pt/MnO₂/GS electrocatalyst not only possesses a larger number of catalytically active sites available, but also is electrochemically
more accessible, which are essential for the electrocatalytic reactions. This remarkable
improvement can be primarily attributed to the concerted effect of MnO₂
nanolamellas and graphene sheets and the high dispersion of small-sized Pt NPs
on modified carbon sheets. Figure 4b presents the cyclic voltammograms (CVs) of
the electrodes coated with the Pt/MnO₂/GS, Pt/GS, Pt/XC-72 and
Pt/MnO₂/XC-72 catalysts in a 1 M H₂SO₄ solution containing 2 M CH₃OH. It can be seen that the Pt/MnO₂/GS catalyst exhibits an ultrahigh forward peak current density of 1224 mA mg^-1, which is
significantly higher than the one obtained from Pt/GS (357 mA mg^-1),
Pt/XC-72 (225 mA mg^-1), Pt/MnO₂/XC-72 (435 mA mg^-1).

Figure 4 (a) CVs of Pt/MnO₂/GS, Pt/GS,
Pt/XC-72 and Pt/MnO₂/XC-72 in 1 M H₂SO₄ at 20 mV s^-1. (b) CVs of Pt/MnO₂/GS, Pt/GS, Pt/XC-72 and Pt/MnO₂/XC-72 in 1 M H₂SO₄ and 2 M methanol at 20 mV s^-1.

Based on the results above, it can be deduced that the
rational combination of Pt NPs, MnO₂ and graphene into a
sophisticated 2D architecture is conducive to the development of high Pt
utilization catalysts. First, the small size and good dispersion of Pt NPs on
the modified carbon sheets can offer higher surface-areas that host more
accessible active sites, thus achieving an extremely high electrocatalytic
activity; Second, hydrous MnO₂ coating on graphene surface can facilitate
the diffusion of electrolyte and also allow the transportation of the poisoning
species and protons (H⁺) from the Pt sites during the reaction;
Third, graphene sheets in the ternary composite act as not only the support for
the deposition of Pt NPs but also the electronic conductive channels, leading
to a significant decrease in charge-transfer resistance and effectively
improving the electrochemical kinetics.

4.
Conclusions

In summary, a facile and cost-effective approach has
been developed to disperse Pt NPs on MnO₂-functionalized graphene
sheets. Graphene could be amazingly used as both a green reductant in synthesis
of MnO₂ and an ideal substrate for growing and anchoring Pt NPs with
a very narrow size distribution. Most importantly, the rationally designed Pt/MnO₂/GS
ternary electrocatalyst exhibits ultrahigh electrocatalytic activity, significantly
outperforming the Pt/GS, Pt/XC-72 and Pt/MnO₂/XC-72 catalysts. We
believe this design concept can be extended to the fabrication of other novel
nanocomposites containing noble metals, such as Pd, Au and Ag, opening up a
brand new avenue for a large spectrum of device applications.