DEVELOPMENT OF IRIDIUM OXIDE CATALYSTS FOR ACIDIC WATER ELECTROLYSIS

• Author / Creator

  Dhawan, Himanshi

• Scarce and expensive iridium, known for its high activity and corrosion resistance, is
  the most used catalyst for acidic oxygen evolution reaction (OER); however, reduc-
  ing its use by enhancing catalyst performance and utilization is vital for advancing
  proton exchange membrane water electrolysis (PEM-WE) technology. This thesis
  aims to synthesize active and stable Ir-based catalysts using facile thermal decom-
  position methods. Rigorous physicochemical and electrochemical characterization is
  conducted, alongside testing in a three-electrode rotating disk electrode set-up.
  The use of supports is a traditional approach in heterogeneous catalysis to increase
  the atom efficiency of supported metal, through increased active metal dispersion. In
  seeking appropriate support for acidic OER, key requirements include stability in an
  acidic environment, high surface area, and electrical conductivity. This study pro-
  poses that active-supported Ir oxide catalysts could be synthesized without the use of
  conductive support and in instances where a well-connected Ir oxide network in the
  supported catalyst can potentially assume the role of an electron conductor. Mon-
  oclinic ZrO2 is presented as a support for acidic OER, capable of withstanding the
  corrosive conditions and influencing the intrinsic catalyst activity.
  Supported Ir oxide catalysts were synthesized using the incipient wetness impreg-
  nation method (IWI) using hydrogen hexachloroiridate(IV) (H2IrCl6) precursor. Two
  different particle sizes of ZrO2, small (S) and large (L) were used to study the ef-
  fect of surface coverage on the catalyst properties and performance. Transmission electron microscopy (TEM) results showed multiple layers of Ir oxide nanoparticles
  forming well-connected networks in IrxZr(1−x)O2/ZrO2(L), while partial coverage with
  short-range networks was observed in IrxZr(1−x)Oy/ZrO2(S). X-ray diffraction (XRD)
  results and X-ray photoelectron spectroscopy results (XPS) suggest metal-support
  interactions between Zr and Ir, and the formation of IrxZr(1−x)O2 alloy, that lead to a
  lowered Ir oxidation state and an abundance of active Ir(III)/Ir(IV) species, which are
  known to be highly active for OER. IrxZr(1−x)O2/ZrO2(L) showcased remarkable spe-
  cific activity, comparable to the state-of-the-art commercial IrOx supplied by Tanaka
  (IrOx TKK), and displayed a four-fold increase in intrinsic activity. On the other
  hand, IrxZr(1−x)Oy/ZrO2(S) displayed lower activity due to a less developed Ir oxide
  network, underscoring the importance of a conductive network for non-conductive
  supports. Despite possessing high resistance to Ir dissolution compared to IrOx TKK,
  ZrO2-supported catalysts experience deactivation due to O2 bubble accumulation and
  Ir(III)/Ir(IV) species transformation to anhydrous IrO2 or higher Ir oxidation state.
  During the synthesis of the aforementioned catalysts, scanning electron microscopy

  • energy dispersive X-ray (SEM-EDX) analysis consistently revealed the presence of chlorine in both supported and unsupported catalysts. The subsequent part of this thesis is dedicated to exploring the influence of residual Cl− from precursors on the electrochemical and physicochemical properties of Ir oxide catalysts prepared by the thermal decomposition of H2IrCl6. To test this, we produced a baseline catalyst IrO2 through the calcination of H2IrCl6, introduced HCl into the H2IrCl6 precursor fol- lowed by calcination to a chloride-rich catalyst, and incorporated NH4OH into the precursor prior to calcination to reduce the amount of chloride. TEM imaging of the catalysts showed that IrO2 existed as agglomerated nanonee- dles and agglomerates; the chloride-rich catalyst, IrO2-HCl predominantly appeared as large agglomerates with minor IrO2 needles, while the catalyst with less chloride. IrOx-NH4OH primarily manifested as Ir oxide nanoparticles with IrO2 agglomerates in the minority. Selected area electron diffraction (SAED) results revealed the pres- ence of rutile IrO2 across all samples, yet IrCl3 phase prevailed in IrO2-HCl, while amorphous IrOx was evident in IrOx-NH4OH. These findings were also corroborated by XRD and XPS analyses. Electrochemical testing demonstrated a substantial in- crease in ECSA and specific activity for IrOx-NH4OH, exceeding that of IrO2 by an order of magnitude. Conversely, IrO2-HCl displayed a three-fold decline in specific and intrinsic activity in comparison to IrO2. The stability outcomes for IrO2-HCl also fell considerably short of those for IrO2 and IrOx-NH4OH. Overall, our results underscore that residual chloride from the precursor exerts a detrimental effect on the catalyst. Notably, this study represents the first investigation into the impact of precursor-derived chloride on the OER catalysts, shedding light on how synthesis methods lacking chloride removal can inherently impede catalyst activity. This fur- ther extends to the incipient wetness impregnation method, where catalyst synthesis relies solely on calcination to eliminate of the chloride in the catalyst.

• Subjects / Keywords

  • chloride poisoning
  • incipient wetness impregnation
  • iridium oxide
  • oxygen evolution reaction
  • supported catalysts

• Graduation date

  Spring 2024

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-at4e-8t90

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.