Structures of Boron Loaded Turbostratic Graphene with Enhanced Performance in Solid Acid Fuel Cell
Authors: Tian Mengkun , Tennyson Wesley , Papandrew Alexander Blair , Elgammal Ramez Ahmed , Eres Gyula , Duscher Gerd , Puretzky Alexander A. , Geohegan David B. , Zawodzinski Thomas Anthony 
Published: February 2020

Solid acid fuel cells (SAFC) working at intermediate temperature (250°C) have many advantages compared to the lower temperature proton exchange membrane fuel cell (PEMFC), such as increased catalyst activity and are more resistant to CO poisoning.1, 2 The state-of-the-art cathode is comprised of porous protonic conductor coated by Pt.3, 4 There are two major roles of Pt in the cathode: (1) the oxygen reduction reaction (ORR) catalyst and (2) the electron conductor. To maintain a highly electron conductive network, a high fraction of Pt has to be loaded in ‘conventional’ SAFC electrodes. However, to generate wider application, the cost of the cathode must be reduced significantly.

The function of Pt as electron conductor can be replaced by the catalyst support to achieve this reduction in loading. The catalyst support must have high electron conductivity, corrosion-resistance and high surface area. The carbon nanostructures are the common catalyst supports used in the fuel cells. In our previous studies, we demonstrated that multi-wall carbon nanotubes are much more stable during fuel cell testing compared to the single wall carbon structures.5 Indeed, pure single layer or few-layer carbon supports are well known to be particularly vulnerable to corrosion in the present of water.

However, previous research showed that substitutional boron can lower the electron density of the reactive carbon, leading to a reduction in the rate of O2 chemisorption.6 Meanwhile, very small amounts of boron (~1%) have been demonstrated to enhance the conductivity of the single carbon structure.7In our recent studies, we synthesize the pure single wall carbon, carbon structures in hydrogen gas and with boron loading. Surprisingly, the boron loading induces the growth of few-layer graphenes (FLGs), which exhibit much better corrosion resistance than the single wall carbon and carbon synthesized in hydrogen gas which consists of multi-wall and single-wall carbon structure. In this research, we focus on the structural investigation of those FLGs. Characterization of the FLGs are performed by high resolution transmission electron microscopy (HRTEM) imaging, monochromated electron energy-loss spectroscopy (EELS), nano-beam electron diffraction (NBED), aberration corrected scanning transmission electron microscopy (STEM) imaging, X-ray photoelectron spectroscopy (XPS), nuclear magnetic resonance (NMR), Raman spectroscopy and X-ray diffraction (XRD). Representative STEM images in a low and high magnification are shown in Figure 1 (a)-(b). A significantly structure deviation compared to pure graphene/graphite is evident due to 1-3% boron loading. Those results indicate the few-layer graphene does not stack in a normal graphitic way in the c direction. The EELS, XPS and NMR show complicated electronic states of the boron in the carbon, implying boron does not have to be in the substitutional site. For example, Figure 1(c) show a representative EELS spectrum taken from a FLG. The featureless boron K-edge looks like to be taken from amorphous boron instead of substitutional boron. This result is also supported by the NBED and XRD, showing a non-flat features of the FLGs. Finally, we used the density functional theory (DFT) to explain the excellent oxygen resistance of FLG with this unique structure.

Acknowledgments

This work is supported by ARPA-E via cooperative agreement DE-AR0000499. The synthesis science is supported by BES-MSED. We thank JIAM and CNMS for microscopy use.

Reference

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Figure 1