Supercapacitors

Electrochemical supercapacitor slectrodes from sponge-like graphene nanoarchitectures with ultrahigh power density

Abstract:

supercapacitor

We employed a microwave synthesis process of cobalt phthalocyanine molecules templated by acid-functionalized multiwalled carbon nanotubes to create three-dimensional sponge-like graphene nanoarchitectures suited for ionic liquid-based electrochemical capacitor electrodes that operate at very high scan rates. The sequential “bottom-up” molecular synthesis and subsequent carbonization process took less than 20 min to complete. The 3D nanoarchitectures are able to deliver an energy density of 7.1 W·h kg–1 even at an extra high power density of 48 000 W kg–1. In addition, the ionic liquid supercapacitor based on this material works very well at room temperature due to its fully opened structures, which is ideal for the high-power energy application requiring more tolerance to temperature variation. Moreover, the structures are stable in both ionic liquids and 1 M H2SO4, retaining 90 and 98% capacitance after 10 000 cycles, respectively.

Z Xu, Z Li, CMB Holt, X Tan, H Wang, B Shalchi-Amirkhiz, T Stephenson and D Mitlin Journal of Physical Chemistry Letters, 3 (2012) 2928-2933
doi:10.1021/jz301207g

Batteries

Silicon nanowire core aluminum shell coaxial nanocomposites for lithium ion battery anodes grown with and without a TiN interlayer

Abstract:

Silicon Nanowires

We investigated the effect of aluminum coating layers and of the support growth substrates on the electrochemical performance of silicon nanowires (SiNWs) used as negative electrodes in lithium ion battery half-cells. Extensive TEM and SEM analysis was utilized to detail the cycling induced morphology changes in both the Al-SiNW nanocomposites and in the baseline SiNWs. We observed an improved cycling performance in the Si nanowires that were coated with 3 and 8 wt.% aluminum. After 50 cycles, both the bare and the 3 wt.% Al coated nanowires retained 2600 mAh/g capacity. However beyond 50 cycles, the coated nanowires showed higher capacity as well as better capacity retention with respect to the first cycle. Our hypothesis is that the nanoscale yet continuous electrochemically active aluminum shell places the Si nanowires in compression, reducing the magnitude of their cracking/disintegration and the subsequent loss of electrical contact with the electrode. We combined impedance spectroscopy with microscopy analysis to demonstrate how the Al coating affects the solid electrolyte interface (SEI). A similar thickness alumina (Al2O3) coating, grown via atomic layer deposition (ALD), was shown not to be as effective in reducing the long-term capacity loss. We demonstrate that an electrically conducting TiN barrier layer present between the nanowires and the underlying stainless steel current collector leads to a higher specific capacity during cycling and a significantly improved coulombic efficiency. Using TiN the irreversible capacity loss was only 6.9% from the initial 3581 mAh/g, while the first discharge (lithiation) capacity loss was only 4%. This is one of the best combinations reported in literature.

EL Memarzadeh, WP Kalisvaart, A Kohandehghan, B Zahiri, CMB Holt and D Mitlin Journal of Materials Chemistry, 22 (2012) 6655-6668
doi:10.1039/c2jm16167b

Oxygen Reduction Catalysis

Highly corrosion resistant platinum-niobium oxide-carbon nanotube electrodes for the oxygen reduction in PEM fuel cells

Abstract:

ORR

Nanocomposite materials consisting of platinum deposited on carbon nanotubes are emerging electrocatalysts for the oxygen reduction reaction in PEM fuel cells. However, these materials albeit showing promising electrocatalytic activities suffer from unacceptable rates of corrosion during service. This study demonstrates an effective strategy for creating highly corrosion-resistant electrocatalysts utilizing metal oxide coated carbon nanotubes as a support for Pt. The electrode geometry consisted of a three-dimensional array of multi-walled carbon nanotubes grown directly on Inconel and conformally covered by a bilayer of Pt/niobium oxide. The activities of these hybrid carbon-metal oxide materials are on par with commercially available carbon-supported Pt catalysts. We show that a sub-nanometre interlayer of NbO2 provides effective protection from electrode corrosion. After 10,000 cyclic voltammetry cycles from 0.5 V to 1.4 V, the loss of electrochemical surface area, reduction of the half-wave potential, and the loss of specific activity of the NbO2 supported Pt were 10.8%, 8 mV and 10.3%, respectively. Under the same conditions, the catalytic layers with Pt directly deposited onto carbon nanotubes had a loss of electrochemical area, reduction of half-wave potential and loss of specific activity of 47.3%, 65 mV and 65.8%, respectively. The improved corrosion resistance is supported by microstructural observations of both electrodes in their post-cycled state. First principles calculations at the density functional theory level were performed to gain further insight into changes in wetting properties, stability and electronic structure introduced by the insertion of the thin NbO2 film.

L Zhang, L Wang, CMB Holt, B Zahiri, K Malek, T Navessin, MH Eikerling and D Mitlin Energy & Environmental Science, 5 (2012) 6156-6172
doi:10.1039/c2ee02689a

Chemical Reactor Corrosion-Fouling

Corrosion-fouling of 316 stainless steel and pure iron by hot oil

Abstract:

Reactor Fouling

We examined the fouling and corrosion that took place when 316 stainless steel and pure iron wires were electrically heated to 540–680 °C in a liquid bath of the atmospheric bottoms fraction of a crude oil. The foulant was determined to be heterogeneous, with a thick macroscale outer layer of pitch, covering a microscale sheath of coke, which was in turn both covering and interspersed with a microscale layer of iron sulfide. This foulant was observed to delaminate from the wire surface, presumably as a result of both the generation of growth stresses and the action of gas bubbles that were evolved during the fouling process. Unexpectedly but conclusively, we observed that the underlying wire surface was heavily corroded. In the case of the stainless steel, we observed a microscale chromium oxide layer that separated the foulant from the underlying metal. This layer presumably reduced the rate of metal dissolution. The degree of corrosion was much higher in the pure iron samples, where such a layer did not exist. Our hypothesis is that there is a synergy between the measured macroscopic fouling and the underlying microscopic corrosion, where the iron from the wire reacts with the sulfur in the oil to build up the thick sulfide.

T Stephenson, A Kubis, M Derakhshesh, CMB Holt, P Eaton, B Newman, A Hoff, M Gray and D Mitlin Energy & Fuels, 25 (2011) 4540-4551
doi:10.1021/ef200823j

Hydrogen Storage in Metals

Stable hydrogen storage cycling in magnesium hydride, in the range of room temperature to 300°C, achieved using a new bimetallic Cr-V nanoscale catalyst

Abstract:

H2

We created a bimetallic chromium vanadium hydrogen sorption catalyst for magnesium hydride (MgH2). The catalyst allows for significant room-temperature hydrogen uptake, over 10 cycles, at absorption pressures as low as 2 bar. This is something that has never been previously achieved. The catalyst also allowed for ultrarapid and kinetically stable hydrogenation cycling (over 225 cycles) at 200 and at 300 °C. Transmission electron microscopy analysis of the postcycled samples revealed a nanoscale dispersion of Cr-V nanocrystallites within the Mg or MgH2 matrix. TEM analysis of the partially absorbed specimens revealed that even at a high absorption pressure, that is, a high driving force, relatively few hydride nuclei are formed at the surface of the pre-existing magnesium, ruling out the presence of any contracting volume (also termed contracting envelope or core shell) type growth. HRTEM of the cycled and desorbed powder sample demonstrated that the bcc Cr-V phase is crystalline and nanoscale. We experimentally demonstrated that the activation energy for hydrogen absorption is not constant but rather evolves with the driving force. This finding sheds new insight regarding the origins of the wide discrepancy in the literature – reported values of the hydrogenation activation energy in magnesium hydride and in related metal hydride systems.

B Zahiri, M Danaie, X Tan, B Shalchi-Amirkhiz, G Botton and D Mitlin Journal of Physical Chemistry C, 116 (2012) 3188-3199
doi:10.1021/jp211254k

Electron Microscopy

EM

Most material characterization methodologies can analyze only statistically averaged information of large amounts of material. TEM however offers the possibility of site-specific information about morphological, chemical and electronic properties of nearly any material.