Chemical Structure of Nitrogen-Doped Graphene With
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Article pubs.acs.org/JPCC Chemical Structure of Nitrogen-Doped Graphene with Single Platinum Atoms and Atomic Clusters as a Platform for the PEMFC Electrode † † † † § † ∥ Samantha Stambula, Nicolas Gauquelin, Matthieu Bugnet, Sandeep Gorantla, , Stuart Turner, , ‡ ⊥ ‡ ‡ ⊥ ‡ † Shuhui Sun, , Jian Liu, Gaixia Zhang, , Xueliang Sun, and Gianluigi A. Botton*, † Department of Materials Science and Engineering, McMaster University, 1280 Main Street West, Hamilton, Ontario, L8S 4L8, Canada ‡ Department of Materials and Mechanical Engineering, Western University, 1151 Richmond Street North, London, Ontario, N6A 5B9, Canada *S Supporting Information ABSTRACT: A platform for producing stabilized Pt atoms and clusters through the combination of an N-doped graphene support and atomic layer deposition (ALD) for the Pt catalysts was investigated using transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). It was determined, using imaging and spectroscopy techniques, that a wide range of N-dopant types entered the graphene lattice through covalent bonds without largely damaging its structure. Additionally and most notably, Pt atoms and atomic clusters formed in the absence of nanoparticles. This work provides a new strategy for experimentally producing stable atomic and subnanometer cluster catalysts, which can greatly assist the proton exchange membrane fuel cell (PEMFC) development by producing the ultimate surface area to volume ratio catalyst. 1. INTRODUCTION thus the Pt atoms migrate to edge locations where free dangling 9 Alternative energy sources have become a key research interest bonds are available; unfortunately, this high mobility results in a poor catalyst distribution and possibly enhanced particle in the quest to address environmental degradation and the 6 depletion of nonrenewable fuel sources. As a promising agglomeration. candidate for environmentally benign electric power generation Functionalization is a common method used to create binding sites on the graphene lattice through the introduction technology, proton-exchange membrane fuel cells (PEMFCs) 10,11 have drawn a great deal of attention thanks to their fast start- of dopant atoms. A wealth of knowledge is available in the up,1 high power density,1 and low or zero emissions.2 literature on various types of nitrogen-dopants (N-dopants) in N-doped graphene, consisting of amino-, pyridinic-, pyrrolic-, Currently, platinum (Pt) has proven to perform the oxygen − and graphitic-dopants.12 14 It has been experimentally reported reduction reaction (ORR) as a homocatalyst at the cathode of ffi the PEMFC most efficiently;3 however, Pt-based electro- that the N-doping increases the e ciency of the PEMFC in comparison to conventional supports, as it increases the catalysts and their associated electrodes can be the largest 12 15 4 dispersion of the Pt catalyst, prevents Pt agglomeration, expense during the production of the PEMFC. Downsizing Pt 16−18 catalysts to subnanometer clusters could significantly increase and participates in the ORR as active sites for catalysis. The significance of each type of N-dopant on the ORR is catalytic activity by enhancing the surface area to volume ratio, fi resulting in additional active sites and a decreased overall cost controversial in the scienti c literature, as direct observation of the ORR has not been completed with current experimental by reducing the total mass of Pt per electrode. Other limitations 19 plaguing the efficiency and mass-production of the PEMFC techniques. Kim et al. reference a series of publications include catalyst degradation through operation,5 nonuniform Pt illustrating the disagreement in the literature regarding the distribution,6 and low-surface area electrode supports.5 origin of the active source for ORR between graphitic-N and Conventionally, C black supports with Pt nanoparticles are pyridinic-N. Through their own calculations and ORR reaction used in PEMFCs. Recently, graphene supports have been scheme, they determined that the debate on the active site may investigated, as this remarkable two-dimensional material be due to the transformation of a graphitic-type dopant into a exhibits high electrical conductivity,7 mechanical robustness,8 and a high surface area.7 The intrinsic nature of the strong sp2 Received: September 7, 2013 chemical bonds composing the graphene lattice does not Revised: January 16, 2014 readily permit the chemisorption of Pt on the graphene surface, Published: February 12, 2014 © 2014 American Chemical Society 3890 dx.doi.org/10.1021/jp408979h | J. Phys. Chem. C 2014, 118, 3890−3900 The Journal of Physical Chemistry C Article pyridinic-type dopant during the ORR process. It has also been oxidation, thermal exfoliation, and chemical reduction. In proposed, through simulations, that graphitic-type15 and detail, natural graphite powder (1 g, 99.99%, Sigma-Aldrich) 20 fi pyrrolic-type doping induces a positive charge on the adjacent was rst stirred in concentrated sulphuric acid (H2SO4, 23 mL), C atom due to the increased electronegativity of the N atom, followed by an addition of sodium nitrate (NaNO3, 0.5 g) at resulting in the C atom acting as a potential molecular room temperature. The stirring persisted for 16 h, where the adsorption site. Through DFT calculations, Ni et al.21 mixture was then cooled to 0 °C. Potassium permanganate ff demonstrated that graphitic-type doping o ers the lowest (K2MnO4, 3 g) was then added to the solution. After 2 h, the ° overall energy barrier for the dissociation of O2, thus suggesting solution formed a green slurry around a temperature of 35 C, that the graphitic-dopant is the most active for the ORR. which was stirred for another 3 h. Subsequently, 46 mL of A method to increase the surface area to volume ratio of the deionized water (H2O) was slowly added into the slurry at a Pt catalyst is through tuning the deposition process. Pt is temperature of approximately 98 °C. The suspension temper- commonly deposited through solution synthesis, electro- ature was maintained for 30 min before it was further diluted 22 deposition, and physical sputtering; however, the use of with another addition of H2O and hydrogen peroxide (H2O2, atomic layer deposition (ALD) has been shown to create fi − 140 mL). The suspension was subsequently ltered and washed particles with an average size of 2 to 5 nm,23 25 and in until it reached a neutral pH, where it was then dried in a particular cases ultrasmall nanoparticles (<1 nm)11 and single vacuum oven at 60 °C to obtain graphite oxide (GO). To atoms25 have been produced. The effect of Pt atoms and transform the GO to graphene, the as-synthesized GO was clusters on ORR is also debated in scientific literature, as the heated around 1050 °C in an Ar atmosphere for 30 s in a tube consistent production of Pt clusters is hardly achievable, and furnace. N-doped graphene was further obtained by heating the substrate effects need to be considered. Toyoda et al.26 graphene powder under high purity ammonia/Ar at 900 °C.30 demonstrated through calculations that chemisorbed small Pt For the purpose of being used as a reference material, a clusters (less than 4 atoms) on defective graphene results in a commercially available TEM grid containing graphene was larger d-band separation from the Fermi energy and can lead to purchased from Graphene Supermarket (product information greater ORR efficiency. Further, additional calculations by available at https://graphene-supermarket.com/). The gra- 27 Zhou et al. showed that small Pt clusters (Pt2) have the phene was produced through the chemical vapor deposition 31 largest average H2 dissociative chemisorption energy and H (CVD) method and consisted of 1 to 6 layers. The supplier atom desorption energy, which suggests that the formation of provided directly transferred graphene on a 300 mesh Cu TEM very small clusters, consisting of few Pt atoms, will drastically grid with an amorphous lacey C support.31 increase the catalytic activity of the anode. As suggested 2.1.2. ALD of Pt on N-Doped Graphene. Pt was deposited 27,26 through calculations, the use of Pt atoms and clusters on N-doped graphene by ALD (Savannah 100, Cambridge should exhibit an enhanced catalytic efficiency compared to Nanotechnology Inc., USA) using (methylcyclopentadienyl)- conventional Pt nanoparticles on C black, due to the increased trimethylplatinum (MeCpPtMe3, Aldrich, purity 98%) and O2 rate of the H oxidation and the ORR. (99.999%) as precursors, and N2 (99.9995%) as the purge gas. Although studies have shown successful deposition of the Pt The apparatus used for ALD consists of a wall reactor with an atoms and clusters, they lack experimental evidence of the outlet leading to a vacuum pump, which also prevents air from correlation of the Pt structure and location with respect to the entering the system. The inlet was connected to a system of structure of the graphene nanosheets and N-dopant sites. This valves that controlled the delivery of the purge gas and is crucial for understanding and optimizing the production of Pt precursors. To increase the heat transfer from the stage to the atoms and clusters in the absence of nanoparticles. The effect of N-doped graphene, hot wall conditions were replicated by the graphene structure, specific N-dopants, and the Pt size placing the N-doped graphene powder inside a machined (atomic to subnanometer clusters) on the catalytic activity can stainless steel container with a perforated Al lid. The container only be understood and optimized through detailed character- for the N-doped graphene was then placed inside the reactor ization of the material’s structure at the atomic scale. Through directly on the heated stage. The wall and stage temperatures of the use of transmission electron microscopy (TEM), the N- the reactor were kept at 150 and 250 °C, respectively. The ° doped graphene and Pt catalyst are here fully characterized. We container for MeCpPtMe3 was kept at 65 C, providing a fl 24 show that the combination of N-doped graphene and ALD Pt steady state ux of MeCpPtMe3 to the reactor.