Nano-size boron carbide intercalated graphene as high performance catalyst supports and electrodes for PEM fuel cells
Introduction
Direct-hydrogen-fueled proton exchange membrane (PEM) fuel cells are promising to address the serious energy and environmental crises to human beings due to their high efficiency and eco-friendly product (water). Thus they are promising to be widely used as the vehicle, portable and stationary power systems [1] as well as the storage system for solar and wind energy [2], [3]. However, the lower electrochemical stability of Pt based catalysts predominantly caused by the degradation of electrochemical specific surface area (ECA) of metallic Pt radically hinders the commercialization of PEM fuel cells [4], [5], [6]. The ECA loss of Pt could be derived from the dissolution at high potentials, and the aggregation of Pt nanoparticles (NPs) caused by the Ostwald ripening of Pt species, motion or detachment of Pt NPs on/from supports [7], [8]. Among them, the motion and detachment of Pt NPs could be controllable in terms of improving the interaction between metal-support and employing the oxidation-resisted supports.
At present, the common used catalyst support is nanocarbon black of 40–50 nm size in diameter with good conductivity and rich micropores (e.g., valcan XC-72) which benefit the dispersion of Pt NPs [9]. However, some Pt NPs would be fallen into micropores grown in surfaces of nanocarbon black and lead to the low Pt usage due to the inaccessibility to electrolytes, which increases the loss of the triple phase boundary [10]. Moreover, it easily suffers from oxidation, resulting in the detachment and aggregation of Pt NPs [11]. These bring about the critical ECA decay of Pt catalysts. Graphitic carbon such as carbon nanotubes, graphene is attracted as the durable supports. Among them, graphene as a unique 2D material endows its excellent physical properties including high theoretical surface area of over 2000 m2/g, electrical conductivity and mechanical stability as well as good chemical stability, which is a promising support material to replace conventional nanocarbon black [12], [13], [14], [15]. However, its strong chemically inert and hydrophobic surfaces trigger the poor interaction between metal-support, leading to the aggregation of Pt NPs [16]. Thus graphene oxide (GO), with affinity surfaces, is often employed as the advanced support [17], [18]. But unfortunately, the structure and morphology of GO nanosheets are not stable, they are readily curled, restacked or wrinkled [19], [20], [21], greatly decreasing the geometry surface area. At the same time, Pt NPs would be covered or wrapped by the deformed nanosheet, which reduces the triple phase boundary on Pt and brings about the low usage and stability of Pt catalysts.
To confine the deformation of GO nanosheets, recently, GO/nano-pilar (e.g., nanocarbon, nanoceramics) intercalation composites have been developed [19], [20], [21], [22]. Herein, the graphene layer can be separated and blocked by conductive nanopilars, and thus 2D GO nanosheets can be effectively remained. As carbon building blocks, nanocarbon is prone to electrochemical oxidation because carbon belongs to an unstable thermodynamic matter. In contrast, most of the nanoceramics (e.g., ZrC, ZrB2, SiC, B4C, TiO2) have very excellent chemical and electrochemical stability either in acid or alkaline media, and comparable electrical conductivity to carbon [9], [23], [24], [25]. As expected, the prepared GO/nano-ceramics supported Pt catalyst could show high ECA value and superior stability than the pristine GO supported Pt and commercial Pt/C catalysts. However, as we know, the specific weight (SW) of graphene (∼1.0) [26], GO (∼2.2) [27] and reduced GO (RGO) (∼1.0 < SW<∼2.2) is significantly lower than the common ceramics (e.g., ∼3.0 for SiC), which would increase the resistance of insertion of nanoceramics into graphene interlayers. This means the satisfying grapehene intercalation composite cannot be obtained by simply blending the light graphene with the heavy nanocermaics.
In this work, a new and lighter nanoceramic, B4C (SW is ∼2.5), is applied to match the SW of graphene. Moreover, differently from the conventional method, prior to mixing the graphene and ceramics, the SW of graphene is modulated by depositing Pt NPs on graphene nanosheets in advance. This special step can further balance the SW between the nanoceramic and graphene, which benefits the acquirement of the almost perfect nanoceramic intercalated graphene composite. At the same time, B4C nanoceramics also can be as the conducting media between graphene due to its good electron conductivity comparable to the commercial conductive carbon black (e.g., vulcan XC 72 carbon) [25]. As results, the platinized intercalation architecture presents very high electrochemical activity area and outstanding stability.
Section snippets
Preparation of the Pt-RGO/B4C catalyst
Graphene oxide (GO) was fabricated by exfoliating the natural graphite using the modified hummer method [28], and then added into the ethylene glycol solution (81 mg of GO). After ultrasonicated for 1 h, the highly dispersed GO suspension solution can be obtained and then moved into a 500 ml round bottom flask. Subsequently, H2PtCl6·6H2O (7 mL, 1 mg mL−1 Pt) and ethylene glycol (100 mL) mixed solution was added dropwise into the GO suspension and stirred vigorously for 20 min at room
Results and discussion
Morphologies of the Pt-RGO/B4C and Pt/RGO catalysts were investigated by SEM observations. As shown in Fig. 1a and b, graphene nanosheets are almost fully exfoliated by inserting nano-B4C ceramics, and the typical sandwich-like layered structural of graphene is well remained even the interlamellar spacing of the intercalation greatly increases. As pillars, the insertion of nano-B4C NPs into graphehe interlayers blocks the π–π bonding of graphehe nanosheets, avoiding the radical structural
Conclusions
The near perfect nano-ceramic intercalated graphene structure was prepared by selecting the lower specific weight nano-B4C as the intercalation to match the graphene. Thus the 2D graphene structure can be maintained well under electrochemical operating environments. As supporting Pt NPs, the novel catalyst shows prominent electrochemical active surface area, mass activity and mass transfer capability over the pristine graphene supported Pt catalyst (Pt/RGO) and the commercial Pt/C catalyst. At
Acknowledgments
This work was supported by the National Natural Science Foundation of China (51372186), the National Basic Research Development Program of China (973Program, 2012CB215504), the Natural Science Foundation of Hubei Province of China (2013CFA082). The authors wish to thank Associate Prof. Xiaoqing Liu and Dr. Tingting Luo for HR-TEM measurement support (JEM-2100F), in the Materials Analysis Center of Wuhan University of Technology.
References (34)
- et al.
Appl. Energy
(2011) - et al.
Int. J. Hydrogen Energy
(2014) - et al.
Int. J. Hydrogen Energy
(2011) - et al.
J. Power Sources
(2015) - et al.
Nano Energy
(2014) - et al.
Carbon
(2013) - et al.
Carbon
(2014) - et al.
Carbon
(2011) - et al.
Int. J. Hydrogen Energy
(2010) - et al.
Carbon
(2006)