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  1. Durable Thin‐Film Porous Transport Electrodes for High Current Density PEM Water Electrolysis

    Young, James L. ; Beltrán, Diana E. ; Blair, Sarah J. ; ... - Advanced Science
    Proton exchange membrane water electrolyzers rely on relatively expensive Ir-based catalysts for efficient and durable hydrogen production. To reduce system costs, Ir loadings can be reduced if performance and durability are maintained. Sputter deposition is a readily scalable method to synthesize uniform, low-loading catalyst layers with controlled composition. A catalyst applied directly to the porous transport layer can have advantages for performance, manufacturing simplicity, and catalyst recovery. Suitable porous transport layer porosity can minimize activity losses when reducing loadings. Here, methods are presented to deposit metallic Ir as well as amorphous and rutile Ir oxides. The activity and durability ofmore » these materials in the porous transport electrode architecture is evaluated. The metallic and amorphous forms have better initial activity, however, operation at 3 A cm−2 with 0.1 mg Ir cm−2 shows that only rutile IrO2 maintains performance beyond 100 h with a 50 mV improvement after 700 h. A >10x reduced dissolution rate is shown for rutile IrO2. With a low-porosity transport layer and 0.4 mg Ir cm−2, a steady-state voltage decay rate of 6 µV h−1 is achieved. The results demonstrate that sputter-deposited rutile IrO2 porous transport electrodes with low Ir loading can be operated at high current density to reduce hydrogen production costs.« less

  2. Molecular Beam Epitaxy Growth of IrO2 Using Plasma-Only Oxidation

    Sacksteder, D. ; Dahl, S. ; Dorsey, J. ; ... - APL Materials
    Growth of IrO2 films via molecular beam epitaxy (MBE) using an oxygen plasma as the sole oxidant is demonstrated for the first time. We investigate the oxidizing conditions required for IrO2 by characterizing the species present in the plasma using optical emission spectroscopy and comparing the thermodynamic equilibrium between Ir/IrO2 under atomic and molecular oxygen. Across the pressure range accessible, monatomic oxygen was the primary reactive species present in the plasma. IrO2 films were grown under varying Ir flux, oxygen pressure, and substrate temperature to understand growth thermodynamics and kinetics. Structural characterization assessed film phase and composition, crystallinity, strain, andmore » surface morphology. The optimized films from this study validate plasma-only reactive growth by MBE as a successful method for growing IrO2.« less

  3. Breaking $$\mathrm{OER}$$ and $$\mathrm{CER}$$ scaling relations via strain and its relaxation in $$\mathrm{RuO}$$2 (101)

    Adiga, Prajwal ; Nunn, William ; Wong, Cindy ; ... - Materials Today Energy
    Green hydrogen production from abundant water sources is an important component of renewable energy storage. Water oxidation catalysts are typically considered bound by adsorbate scaling relations, limiting their activity for the oxygen evolution reaction (OER) as well as selectivity between OER and the chlorine evolution reaction (CER) that compete in saline water streams. RuO2 is highly active for both reactions, and recent measurements have shown the OER activity is greater on undercoordinated, high index facets compared to the lowest-energy (110) facet often studied. The growth of such orientations as epitaxial films, however, can result in appreciable strain and potential surfacemore » faceting via its relaxation. Here, we find the activity and selectivity towards OER and CER vary with thickness in epitaxial (101) RuO2 thin films: OER activity decreases 4x as film thickness increases from 8 nm to 48 nm, while CER activity is comparable. Thus, strain and its relaxation can be used to break scaling relationships between OER and CER, highlighting the important role that defects play in selective oxidation processes on RuO2 in chloride-containing media.« less

  4. In situ EXAFS study of Sr adsorption on TiO2(110) under high ionic strength wastewater conditions

    van Veelen, Arjen ; Francisco, Paul C. ; Edwards, Nicholas P. ; ... - Minerals
    In order to provide important details concerning the adsorption reactions of Sr, batch reactions and a set of both ex situ and in situ Grazing Incidence X-ray Absorption Fine Structure (GIXAFS) adsorption experiments were completed on powdered TiO2 and on rutile(110), both reacted with either SrCl2 or SrCO3 solutions. TiO2 sorption capacity for strontium (Sr) ranges from 550 ppm (SrCl2 solutions, second order kinetics) to 1400 ppm (SrCO3 solutions, first order kinetics), respectively, and is rapid. Sr adsorption decreased as a function of chloride concentration but significantly increased as carbonate concentrations increased. In the presence of carbonate, the ability ofmore » TiO2 to remove Sr from the solution increases by a factor of ~4 due to rapid epitaxial surface precipitation of an SrCO3 thin film, which registers itself on the rutile(110) surface as a strontianite-like phase (d-spacing 2.8 Å). Extended X-ray Absorption Fine Structure (EXAFS) results suggest the initial attachment is via tetradental inner-sphere Sr adsorption. Moreover, adsorbates from concentrated SrCl2 solutions contain carbonate and hydroxyl species, which results in both inner- and outer-sphere adsorbates and explains the reduced Sr adsorption in these systems. These results not only provide new insights into Sr kinetics and adsorption on TiO2 but also provide valuable information concerning potential improvements in effluent water treatment models and are pertinent in developing treatment methods for rutile-coated structural materials within nuclear power plants.« less

  5. Decomposition of CoF3 during battery electrode processing

    Li, Wei ; Groult, Henri ; Borkiewicz, Olaf J. ; ... - Journal of Fluorine Chemistry
    Metal fluorides are potential candidates as electrode materials for rechargeable batteries. During the electrode fabrication, however, thermal treatments can cause decomposition of thermally instable compounds. For this study, we showed that during the electrode processing of CoF3, the carbon and PVDF additives act as a protective layer that prevents CoF3 to readily decompose into CoF2 (rutile). We found that instead, it decomposes into an intermediate phase with a corundum like structure featuring Co vacancies, i.e., Co1.26IICo0.16III$$\square$$0.58F3 where $$\square$$ represents the vacancies. The structural analysis was possible owing to the use of the pair distribution function.

  6. The mobility of Nb in rutile-saturated NaCl- and NaF-bearing aqueous fluids from 1–6.5 GPa and 300–800 °C

    Tanis, Elizabeth A. ; Simon, Adam ; Tschauner, Oliver ; ... - American Mineralogist
    Rutile (TiO2) is an important host phase for high field strength elements (HFSE) such as Nb in metamorphic and subduction zone environments. The observed depletion of Nb in arc rocks is often explained by the hypothesis that rutile sequesters HFSE in the subducted slab and overlying sediment, and is chemically inert with respect to aqueous fluids evolved during prograde metamorphism in the forearc to subarc environment. However, field observations of exhumed terranes, and experimental studies, indicate that HFSE may be soluble in complex aqueous fluids at high pressure (i.e., >0.5 GPa) and moderate to high temperature (i.e., >300 °C). Inmore » this study, we investigated experimentally the mobility of Nb in NaCl- and NaF-bearing aqueous fluids in equilibrium with Nb-bearing rutile at pressure-temperature conditions applicable to fluid evolution in arc environments. Niobium concentrations in aqueous fluid at rutile saturation were measured directly by using a hydrothermal diamond-anvil cell (HDAC) and synchrotron X-ray fluorescence (SXRF) at 2.1 to 6.5 GPa and 300-500 °C, and indirectly by performing mass loss experiments in a piston-cylinder (PC) apparatus at similar to 1 GPa and 700-800 °C. The concentration of Nb in a 10 wt% NaCl aqueous fluid increases from 6 to 11 mu g/g as temperature increases from 300 to 500 °C, over a pressure range from 2.1 to 2.8 GPa, consistent with a positive temperature dependence. The concentration of Nb in a 20 wt% NaCl aqueous fluid varies from 55 to 150 mu g/g at 300 to 500 °C, over a pressure range from 1.8 to 6.4 GPa; however, there is no discernible temperature or pressure dependence. Here, the Nb concentration in a 4 wt% NaF-bearing aqueous fluid increases from 180 to 910 mu g/g as temperature increases from 300 to 500 °C over the pressure range 2.1 to 6.5 GPa. The data for the F-bearing fluid indicate that the Nb content of the fluid exhibits a dependence on temperature between 300 and 500 °C at ≥ 2 GPa, but there is no observed dependence on pressure. Together, the data demonstrate that the hydrothermal mobility of Nb is strongly controlled by the composition of the fluid, consistent with published data for Ti. At all experimental conditions, however, the concentration of Nb in the fluid is always lower than coexisting rutile, consistent with a role for rutile in moderating the Nb budget of arc rocks.« less

  7. On the consistency of QCBED structure factor measurements for TiO2 (Rutile)

    Jiang, Bin ; Zuo, Jian -Min ; Friis, Jesper ; ... - Microscopy and Microanalysis
    The same Bragg reflection in TiO2 from twelve different CBED patterns (from different crystals, orientations and thicknesses) are analysed quantitatively in order to evaluate the consistency of the QCBED method for bond-charge mapping. The standard deviation in the resulting distribution of derived X-ray structure factors is found to be an order of magnitude smaller than that in conventional X-ray work, and the standard error (0.026% for FX(110)) is slightly better than obtained by the X-ray Pendellosung method applied to silicon. This is sufficiently accuracy to distinguish between atomic, covalent and ionic models of bonding. We describe the importance of extractingmore » experimental parameters from CCD camera characterization, and of surface oxidation and crystal shape. Thus, the current experiments show that the QCBED method is now a robust and powerful tool for low order structure factor measurement, which does not suffer from the large extinction (multiple scattering) errors which occur in inorganic X-ray crystallography, and may be applied to nanocrystals. Our results will be used to understand the role of d electrons in the chemical bonding of TiO2.« less
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