Browsing by Author "Birss, Viola I"

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    Development of Novel Nanoporous Carbon Scaffolds for PEMFC Applications
    (2021-03-26) Atwa, Marwa H; Birss, Viola I; Birss, Viola; Karan, Kunal; Roesler, Roland; Ponnurangam, Sathish; Gasteiger, Hubert
    The microstructure of conventional polymer electrolyte fuel cell (PEMFC) catalyst layers (CLs) consists of a mixture of high surface area carbon powder, Pt or Pt alloy nanoparticles (NPs), and an ionomer, which acts as a binder and a proton-conducting electrolyte. These CLs contain complex pathways and a poorly controlled distribution of Pt NPs and ionomer, which can cause kinetic and transport limitations for the oxygen reduction reaction (ORR). Therefore, we have developed a self-supported, robust, and binderless carbon membrane with an ordered, tunable pore structure, namely the nanoporous carbon scaffold (NCS). The NCS films have a high surface area (200-610 m2/g), ultra-low tortuosity (close to 1), high porosity (ca. 90%) and electrical conductivity (? 2 S/cm), and excellent 3-dimensional scalability (1-150+ cm2 demonstrated area, 1-1000 µm thickness). Herein, the effect of the microstructure of various NCS materials, loaded with both catalytic Pt NPs and ionomer, on the kinetics of the oxygen reduction reaction (ORR), which is the rate limiting reaction in PEMFCs, was explored. Two different methods of Pt NP loading (wet impregnation and atomic layer deposition (ALD)) were examined and two types of NCS films with two different structures (monodisperse and bimodal pore size distributions) were prepared and then integrated into a membrane electrode assembly (MEA) as a PEMFC cathode. For the Pt-loaded NCS films with monodisperse pore sizes (uniform pore size distribution with a standard deviation of ± 10-15%) of either 22, 50, or 85 nm, it was found that ALD Pt loading results in a much narrower NP size distribution than wet impregnation, and that the ALD-Pt/NCS85 catalysts out-perform the ALD-Pt/NCS22 materials, likely as Nafion can better penetrate the larger pores to provide proton conductivity. The much smaller neck diameter may be the real problem, where it was shown that larger neck diameters, keeping all other factors constant, can significantly lower the mass transport resistance. The bimodal NCS12 films possess a Craspedia (flower)-like structure, with the spheres (~900 nm) fully saturated with close-packed 12 nm primary pores and connected to neighboring spheres through non-porous carbon fibers, leaving ~ 0.25 µm secondary pores between the spheres. It was found that the 12 nm pores contain the vast majority of the highly dispersed ca. 2-3 nm Pt NPs, and yet contain no Nafion, thus indicating that water must be conducting protons in these pores during the ORR. These Pt NPs also show signs of surface strain, perhaps contributing to their high ORR activity. These factors, including the absence of blocking or poisoning of the Pt NPs by Nafion, have resulted in an extraordinary ORR mass activity (0.58 ± 0.1 A/mgPt), retained after 10k ADT cycles, and a very high limiting current of close to 2.7 A/cm2 in MEA cathode studies.
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    ItemOpen Access
    Nanostructured silicon for lithium ion batteries: from porous silicon to silicon nanowires
    (2020-09-10) Fulton, Alison J.; Shi, Yujun; Birss, Viola I; Thangadurai, Venkataraman; Cheng, Yufeng; Sun, Xueliang
    In this work, nanostructured Si materials have been fabricated for lithium ion battery (LIB) anode applications. These include Si nanowires (NWs) by chemical vapor deposition (CVD) and porous Si (PS) via electrochemical anodization. First, PS was fabricated by electrochemical anodization of n-type Si. The effect of the experimental parameters, including anodization time, applied potential and electrolyte concentration, on the morphology were systematically explored. A mesoporous transition layer was formed on top of a macroporous Si layer, with well-defined and separated pores. A mechanistic study of pore formation using chronoamperometry and dissolution valence studies demonstrated a competition between direct and indirect dissolution.Pulsed laser-induced dewetting (PLiD) of Au thin films was used to form Au nanoparticles (NPs) on both polished Si and PS substrates for use as the catalyst in the CVD growth of Si NWs. On polished Si, monodispersed, uniform NP distributions were obtained, with the NP diameter being linearly related to the initial thin film thickness. On PS substrates, the presence of pores and a unique ripple topography resulted in the formation of three distinct NP size distributions. It was demonstrated that the ripples on the PS substrates was of sufficient magnitude to influence dewetting only when the concentration was < 9.4 wt% HF. The laser irradiation time was found to significantly influence the formation of large (~ 200 nm) NPs, which formed in the valleys of the ripples, producing ordered arrays.The NP arrays prepared by PLiD on both polished Si and PS substrates were used for the first time as catalytic arrays in the vapor-liquid-solid synthesis of Si NWs by CVD using SiCl4. This proof-of-principle study followed the systematic exploration of the experimental parameters and how they influence Si NW morphology. By tuning the growth parameters, both verticallyarrayed and dense, entangled Si NWs could be produced. A diameter-dependent growth rate and activation energy were demonstrated, with direct proof of this phenomenon being obtained through use of PS substrates. In addition, it was discovered that Si NWs could be synthesized at 700 °C, which is 100 °C lower than previously reported in the literature for the use of SiCl4.Si NWs were also successfully synthesized on stainless steel substrates via CVD using a Au catalyst and SiCl4 for the first time. Si NWs were obtained under specific growth time windows, although Si deposition was confirmed on all samples despite the dominant presence of high-temperature oxidation products. These materials were characterized for LIB anode applications as a binder-free, carbon-free electrode design with promising results. The highest capacity obtained was 3373 mAh·g-1 and a high coulombic efficiency of approximately 99% was observed for most samples tested. Rate capability testing showed good reversibility while a capacity retention of 42% was found after 500 charge and discharge cycles. The electrochemical performance was superior to current graphite-based anode materials and showed the applicability of this novel synthetic approach towards the fabrication of high-performance LIB anode materials.