Edwin C. Thomsen

Capacity Decay Mechanism of Microporous Separator‐Based All‐Vanadium Redox Flow Batteries and its Recovery

The results of the investigation of the capacity decay mechanism of vanadium redox flow batteries with microporous separators as membranes are reported. The investigation focuses on the relationship between the electrochemical performance and electrolyte compositions at both the positive and negative half-cells. Although the concentration of total vanadium ions remains nearly constant at both sides over cycling, the net transfer of solution from one side to the other and thus the asymmetrical valance of vanadium ions caused by the subsequent disproportionate self-discharge reactions at both sides lead to capacity fading. Through in situ monitoring of the hydraulic pressure of the electrolyte during cycling at both sides, the convection was found to arise from differential hydraulic pressures at both sides of the separators and plays a dominant role in capacity decay. A capacity-stabilizing method is developed and was successfully demonstrated through the regulation of gas pressures in both electrolyte tanks.

Formation of Interfacial Layer and Long-Term Cyclability of Li–O2 Batteries

The long-term operation of Li-O2 batteries under full discharge/charge conditions is investigated in a glyme-based electrolyte. The formation of stable interfacial layer on the electrode surface during the initial cycling stabilizes reaction products at subsequent cycling stages as demonstrated by quantitative analyses of the discharge products and the gases released during charging. There is a quick switch from the predominant formation of Li2O2 to the predominant formation of side products during the first few cycles. However, after the formation of the stable interfacial layer, the yield of Li2O2 in the reaction products is stabilized at about 33-40%. Extended cycling under full discharge/charge conditions is achievable upon selection of appropriate electrode materials (carbon source and catalyst) and cycling protocol. Further investigation on the interfacial layer, which in situ forms on air electrode, may increase the long-term yield of Li2O2 during the cycling and enable highly reversible Li-O2 batteries required for practical applications.

Electrochemical properties of lanthanum strontium aluminum ferrites for the oxygen reduction reaction

Abstract The oxygen reduction reaction was studied on the La 1− x Sr x Al y Fe 1− y O 3 ( x =0.2, y =0.1, 0.2, 0.3, 0.4) system by cyclic voltametry and electronic conductivity. Activation energies for the bulk and film conductivities were determined. Tafel analysis afforded the activation energies “from the temperature dependence of the exchange current densities” as well as the charge transfer coefficient. The electrical conductivity of bulk material was found to decrease with aluminum content. Formation of the materials into thin porous films further decreased the conductivity after correcting for porosity. Aluminum substitution substantially decreased the performance through influence of the pre-exponential factor in the Butler–Volmer formulation. Neither the activation energies nor the charge transfer coefficient for these materials varied significantly. Aluminum does not adversely influence the basic mechanism of oxygen reduction. It may occupy and block electrochemically active sites on the electrode surface, but it does not appear to decrease the intrinsic activity of available surface sites.

Oxygen Reduction Activity of Lanthanum Strontium Nickel Ferrite

The reduction of oxygen on nickel-doped lanthanum strontium ferrite was studied by current interrupt cyclic voltammetry. Nickel doped on the B site of the perovskite ranged from 0 to 40%. Nickel strongly influenced the sintering of the films. The minimum temperature at which a stable adherent robust film could be formed increased with nickel content. The electrochemical performance for reduction of oxygen was compared with nickel content. Undoped lanthanum strontium ferrite consistently showed greater activity than the doped materials. The data were further analyzed to obtain the exchange current density as a function of temperature and then further analyzed to obtain the activation energy and pre-exponential factor. These values did not correlate with nickel composition but did correlate with one another. The variation in performance was tentatively attributed to subtle variations in microstructure.

Electrical, Thermoelectric, and Structural Properties of La ( M x Fe1 − x ) O3 ( M = Mn , Ni , Cu )

Electrical, thermoelectric, and structural properties were studied in transition metal ion-substituted LaFeO 3 : La(Mn x Fe 1-x )O 3 , La(Ni x Fe 1-x )O 3 , and La(Cu x Fe 1-x )O 3 . Structural analysis showed that a continuous series of solid solutions with no intermediate phases are forming over a wide range (0 < x < 1) with substitutions of Mn and Ni, whereas the maximum Cu content is 30% from this study. The Ni-substituted LaFeO 3 specimens have substantially higher conductivity than those substituted with either Mn or Cu, measured in air from 100 to 1000°C. The Seebeck coefficient of La(Mn x Fe 1-x )O 3 and La(Cu x Fe 1-x )O 3 has a strong temperature dependence, indicating a thermally activated carrier formation. The activation energy for carrier formation in La(Cu x Fe 1-x )O 3 is greater than that in La(Mn x Fe 1-x )O 3 . Thermoelectric and electrical properties evidence conduction through polaron hopping in both Mn- and Cu-substituted LaFeO 3 , whereas the Ni-substituted LaFeO 3 shows metallic conductivity.

Ni/YSZ Anode Interactions with Impurities in Coal Gas

Performance of solid oxide fuel cell (SOFC) with nickel/zirconia anodes on synthetic coal gas in the presence of low levels of phosphorus, arsenic, selenium, sulfur, hydrogen chloride, and antimony impurities were evaluated. The presence of phosphorus and arsenic led to the slow and irreversible SOFC degradation due to the formation of secondary phases with nickel, particularly close to the gas inlet. Phosphorus and antimony surface adsorption layers were identified as well. Hydrogen chloride and sulfur interactions with the nickel were limited to the surface adsorption only, whereas selenium exposure also led to the formation of nickel selenide for highly polarized cells.

Tunable Oxygen Functional Groups as Electrocatalysts on Graphite Felt Surfaces for All‐Vanadium Flow Batteries

A dual oxidative approach using O2 plasma followed by treatment with H2 O2 to impart oxygen functional groups onto the surface of a graphite felt electrode. When used as electrodes for an all-vanadium redox flow battery (VRB) system, the energy efficiency of the cell is enhanced by 8.2 % at a current density of 150 mA cm(-2) compared with one oxidized by thermal treatment in air. More importantly, by varying the oxidative techniques, the amount and type of oxygen groups was tailored and their effects were elucidated. It was found that O-C=O groups improve the cells performance whereas the C-O and C=O groups degrade it. The reason for the increased performance was found to be a reduction in the cell overpotential after functionalization of the graphite felt electrode. This work reveals a route for functionalizing carbon electrodes to improve the performance of VRB cells. This approach can lower the cost of VRB cells and pave the way for more commercially viable stationary energy storage systems that can be used for intermittent renewable energy storage.

SOFC Ohmic Resistance Reduction by HCl-Induced Removal of Manganese at the Anode/Electrolyte Interface

The ohmic resistance of anode-supported solid oxide fuel cells having a manganese-based cathode was lowered when operated in synthetic coal gas containing hydrogen chloride. This effect was not observed for cells with cathodes that did not contain manganese. Substantial amounts of Mn were found throughout the grain boundaries of the 8 mole% yttria-stabilized zirconia (8YSZ) electrolyte. Exposure to HCl partially removed Mn near the anode/electrolyte interface, presumably by volatilization as MnCl2(g). This work suggests that one of the underlying causes of higher than expected electrolyte resistance in anode-supported SOFCs is a lowering of the ionic conductivity of 8YSZ by incorporation of manganese.

Electrical Conductivity of 2D-SiCf/CVI-SiC

Abstract Electrical conductivity (EC) data for several plate forms of two-dimensional, silicon carbide composite made with chemical vapor infiltration matrix and with Hi NicalonTM type S fibers (2D-SiCf/CVI-SiC) were acquired. The composite fibers were coated with pyrocarbon (PyC) of various thicknesses (50 to 310 nm) and an outer thin (˜60 μm) SiC “seal coat” was applied by CVD to the infiltrated plates. The EC was highly anisotropic in the transverse and in-plane directions. In-plane EC ranged from ˜150 to 1600 S/m, increased slowly with increasing temperature, and depended primarily on the total PyC thickness. High in-plane EC-values occur because it is dominated by conduction along the numerous, continuous PyC fiber coating pathways. Transverse EC ranged from ˜1 to 60 S/m, and increased strongly with increasing temperature up to 800°C. The transverse EC is controlled by conduction through the interconnections of the carbon-coating network within and between fiber bundles, especially at moderate temperatures (˜300 to 700°C). Below ˜300°C, the electrical resistance of the pure SiC seal coat becomes increasingly more important as temperatures are further lowered. Importantly, a “3-layer series” model predicts that transverse EC-values for a standard seal-coated 2D-SiCf/CVI-SiC with a monolayer PyC fiber coating of ˜50-nm thickness will be <20 S/m for all temperatures up to 800°C, as desired for a flow channel insert in a fusion reactor blanket component.

Direct Observation of Li2O2 Nucleation and Growth with In-Situ Liquid ec-(S)TEM

1. Fundamental and Computational Sciences Directorate, Pacific Northwest National Laboratory, Richland, USA 2. Energy and Environment Directorate, Pacific Northwest National Laboratory, Richland, USA 3. Environmental and Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, USA 4. Department of Chemical and Biological Engineering, University of WisconsinMadison, Madison, USA