DEVELOPMENT OF IRIDIUM OXIDE CATALYSTS FOR ACIDIC WATER ELECTROLYSIS
Creators: Dhawan, Himanshi
Publisher: name: University of Alberta Library
Year: 2024

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

<ul>
<li>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.</li>
</ul>