Ir Precursor Research Articles

Enhancing Electrochemical Seawater Oxidation with Noble Metal-Decorated Co3O4 Nanocubes

The pressing global challenge of overwhelming reliance on nonrenewable fossil fuels demands innovative solutions to decarbonize energy and industrial parts. Green hydrogen is produced through water electrolysis using renewable energy sources such as solar and wind and increasingly recognized as a promising alternative. This method leads to significant technological advancements and cost reductions in renewable electricity. Given that seawater accounts for 96.5% of Earth’s total water reserves, it presents a nearly inexhaustible resource for hydrogen production. However, the seawater electrolysis suffers from some challenges due to the complexity of seawater including various impurities that lead to corrosive and toxic byproducts. The experimental focus of seawater electrolysis encompasses the hydrogen evolution reaction (HER) at the cathode and the oxygen evolution reaction (OER) at the anode. Additionally, the anodic reaction is accompanied by competing with the chlorine evolution reaction (CER). The CER involving a two-electron reaction demonstrated faster kinetics than the OER (four-electron reaction) along with a decrease in the potential thermodynamic difference in acidic and neutral conditions. Though some research focuses on enhancing OER selectivity over CER, certain industries may prefer chlorine (Cl2) generation for wastewater treatment purposes. Noble metals for seawater electrolysis have been used with observed stability throughout overall pH values. However, they are a scarce element and quite expensive, so that lead to problems for industrial scale. Therefore, many transition metal-based catalysts have been introduced to reduce contents of noble metals for a long maintenance. Among these, cobalt is cost-effectiveness and showed better synergistic behavior on coupling with precious metals. In this regard, we synthesized spinel type of cobalt oxide, and then performed a simple deposition method with Ir and/or Pd precursor solution. The Co3O4 nanocubes (denoted as Co3O4NCs) was synthesized by hydrothermal reaction and they exhibited the size 145 ± 30 nm. The photodeposition of noble metals was subsequently conducted for enhancing catalytic activity. This approach aimed to reduce the usage of noble metals and significantly boost the electrochemical catalytic activity for both OER and the chloride oxidation reaction (COR). The 2wt% Ir decorated catalysts showed a considerable improvement in overall catalytic activity, while the catalysts with 6wt% Pd excelled the COR activity under simulated seawater conditions. Especially, metal palladium has been known as an excellent selectivity and activity for chlorine species, but it is too reactive to be a stable. However, the combined deposition of Pd onto Co3O4NCs@IrO2 surface not only demonstrated a synergistic effect, significantly outperforming other metal electrodes under 0.5 M NaCl conditions (FEHOCl ~100%), but also contributed to a notable improvement in catalyst stability at 40 mA/cm2 over 8 hours. This synergistic interaction between Ir and Pd on the Co3O4NCs surface, and the resultant enhanced stability, indicate the significance of our findings. Furthermore, the effective employment of noble metals would benefit from a design of catalysts based on precious metals. Our research paves the way for advancing hydrogen production technologies and propelling the hydrogen economy forward, illustrating the viability of seawater.

Formation Mechanism and Hydrothermal Synthesis of Highly Active Ir1-xRuxO2 Nanoparticles for the Oxygen Evolution Reaction.

Iridium dioxide (IrO2), ruthenium dioxide (RuO2), and their solid solutions (Ir1-xRuxO2) are very active electrocatalysts for the oxygen evolution reaction (OER). Efficient and facile synthesis of nanosized crystallites of these materials is of high significance for electrocatalytic applications for converting green energy to fuels (power-to-X). Here, we use in situ X-ray scattering to examine reaction conditions for different Ir and Ru precursors resulting in the development of a simple hydrothermal synthesis route using IrCl3 and KRuO4 to obtain homogeneous phase-pure Ir1-xRuxO2 nanocrystals. The solid solution nanocrystals can be obtained with a tunable composition of 0.2 < x < 1.0 and with ultra-small coherently scattering crystalline domains estimated from 1.3 to 2.6 nm in diameter based on PDF refinements. The in situ X-ray scattering data reveal a two-step formation mechanism, which involves the initial loss of chloride ligands followed by the formation of metal-oxygen octahedra clusters containing both Ir and Ru. These octahedra assemble with time resulting in long-range order resembling the rutile structure. The mixing of the metals on the atomic scale during the crystal formation presumably allows the formation of the solid solution rather than heterogeneous mixtures. The size of the final nanocrystals can be controlled by tuning the synthesis temperature. The facile hydrothermal synthesis route provides ultra-small nanoparticles with activity toward the OER in acidic electrolytes comparable to the best in the literature, and the optimal material composition very favorably combines low overpotential, high mass activity, and increased stability.

Stepwise Understanding on Hydrolysis Formation of the IrOx Nanoparticles as Highly Active Electrocatalyst for Oxygen Evolution Reaction

In this study, we have investigated the synthesis of supported iridium oxide (IrOx) nanoparticles (NPs) through hydrolysis in a surfactant-free aqueous bath as a possible route for the large-scale production of highly active electrocatalyst for oxygen evolution reaction (OER) in acidic water electrolyzers. The process involves (i) formation of Ir-hydroxides complex from an Ir precursor in basic media followed by (ii) protonation in acidic media to form colloidal hydrated IrOx NPs and (iii) conversion and deposition of IrOx NPs on the surface of carbon or TiN support by probe sonication. The IrOx NPs produced through hydrolysis route form highly stable colloidal solution. Since it is essential to precipitate the catalyst NPs from the colloidal solution for their use in water electrolyzer electrode development, here, we investigate the optimal reaction conditions, e.g., pH, temperature, time, and presence of support, for efficient synthesis of the catalyst NPs. The reaction intermediates formed at different reaction steps are explored to get insights into the chemistry of the process. Under the optimal synthesis conditions, 100% precipitation of IrOx NPs was achieved. Further, the precipitated TiN supported IrOx NPs exhibited high OER activity, superior to that of the commercial benchmark IrO2 electrocatalyst. The study provides a scalable synthesis route for highly active, low Ir-content OER electrocatalysts for acidic water electrolyzers. Graphical

Ir-Doped CuPd Single-Crystalline Mesoporous Nanotetrahedrons for Ethylene Glycol Oxidation Electrocatalysis: Enhanced Selective Cleavage of C-C Bond.

The development of highly efficient electrocatalysts for complete oxidation of ethylene glycol (EG) in direct EG fuel cells is of decisive importance to hold higher energy efficiency. Despite some achievements, their progress, especially electrocatalytic selectivity to complete oxidated C1 products, is remarkably slower than expected. In this work, we developed a facile aqueous synthesis of Ir-doped CuPd single-crystalline mesoporous nanotetrahedrons (Ir-CuPd SMTs) as high-performance electrocatalyst for promoting oxidation cleavage of C-C bond in alkaline EG oxidation reaction (EGOR) electrocatalysis. The synthesis relied on precise reduction/co-nucleation and epitaxial growth of Ir, Cu and Pd precursors with cetyltrimethylammonium chloride as the mesopore-forming surfactant and extra Br- as the facet-selective agent under ambient conditions. The products featured concave nanotetrahedron morphology enclosed by well-defined (111) facets, single-crystalline and mesoporous structure radiated from the center, and uniform elemental composition without any phase separation. Ir-CuPd SMTs disclosed remarkably enhanced electrocatalytic activity and excellent stability as well as superior selectivity of C1 products for alkaline EGOR electrocatalysis. Detailed mechanism studies demonstrated that performance improvement came from structural and compositional synergies, which kinetically accelerated transports of electrons/reactants within active sites of penetrated mesopores and facilitated oxidation cleavage of high-energy-barrier C-C bond of EG for desired C1 products. More interestingly, Ir-CuPd SMTs performed well in coupled electrocatalysis of anode EGOR and cathode nitrate reduction, highlighting its high potential as bifunctional electrocatalyst in various applications.

Ir‐Doped CuPd Single‐Crystalline Mesoporous Nanotetrahedrons for Ethylene Glycol Oxidation Electrocatalysis: Enhanced Selective Cleavage of C−C Bond

AbstractThe development of highly efficient electrocatalysts for complete oxidation of ethylene glycol (EG) in direct EG fuel cells is of decisive importance to hold higher energy efficiency. Despite some achievements, their progress, especially electrocatalytic selectivity to complete oxidated C1 products, is remarkably slower than expected. In this work, we developed a facile aqueous synthesis of Ir‐doped CuPd single‐crystalline mesoporous nanotetrahedrons (Ir‐CuPd SMTs) as high‐performance electrocatalyst for promoting oxidation cleavage of C−C bond in alkaline EG oxidation reaction (EGOR) electrocatalysis. The synthesis relied on precise reduction/co‐nucleation and epitaxial growth of Ir, Cu and Pd precursors with cetyltrimethylammonium chloride as the mesopore‐forming surfactant and extra Br− as the facet‐selective agent under ambient conditions. The products featured concave nanotetrahedron morphology enclosed by well‐defined (111) facets, single‐crystalline and mesoporous structure radiated from the center, and uniform elemental composition without any phase separation. Ir‐CuPd SMTs disclosed remarkably enhanced electrocatalytic activity and excellent stability as well as superior selectivity of C1 products for alkaline EGOR electrocatalysis. Detailed mechanism studies demonstrated that performance improvement came from structural and compositional synergies, which kinetically accelerated transports of electrons/reactants within active sites of penetrated mesopores and facilitated oxidation cleavage of high‐energy‐barrier C−C bond of EG for desired C1 products. More interestingly, Ir‐CuPd SMTs performed well in coupled electrocatalysis of anode EGOR and cathode nitrate reduction, highlighting its high potential as bifunctional electrocatalyst in various applications.

Iridium Single Atom Catalysts for Oxygen Evolution Reaction in Acidic Medium

In order to reduce CO2 emissions and produce clean hydrogen one solution is to use sustainable energies (wind, solar or hydraulic) coupled with water electrolyser to produce H2 and store it before use it in fuel cell. The electrolyzer based on proton exchange membranes (PEM) is a promising technology because it can reach high current density (2 A.cm-2) [1] and can be easily coupled with intermittent energy sources [2]. However, the acidic conditions and working temperatures (80 °C) require the use of noble metals. On the anode side the oxidative condition prevents the use of carbon supported catalyst and the best catalyst for the oxygen evolution reaction (OER) so far is IrOx.The use of Ir single atom catalyst (SAC) supported on conducting oxide is a promising way to reduce the quantity of the scarce and expensive noble metal, increasing the electroactive surface and durability. SACs present an excellent dispersion and activity [3] and some conducting oxides are very promising regarding electrical conductivity and stability under oxidative conditions [4]. There are different approaches to prepare Ir SAC (wet impregnation, co-precipitation, photoelectrochemical reduction...) [3], whose success mainly depends on the nature of the Ir precursor, its interaction with the support and the thermal treatment temperature/duration. Some research groups have prepared and characterized Ir SAC on indium-tin oxide [5] but the activity and stability still need to be improved.The objective of this communication is to present the synthesis and the characterization of IrSAC on different conducting oxide supports. Ir precursor was synthesized and deposed on various supports. The physical characterization entails XRD, TEM, MEB, EDX to assess the morphology and composition of the catalyst. Spectroscopic analysis like XPS and XAS brings information on the chemical environment of Ir sites, while electrochemical measurements give information on the OER activity and stability of the catalyst.

(Keynote) Iridium Deposition By Galvanic Displacement of Cu on a One-Pot Configuration

Water electrolysis with proton exchange membranes (PEMWE) is expected to play a vital role in the future hydrogen infrastructure. The hydrogen evolution is effectively catalyzed by platinum, and iridium oxide is a highly active and reasonably stable catalyst for the oxygen-evolution reation (OER) [1]. However, iridium is costly and scarce, and efficient utilization of it is necessary for large-scale deployment of PEMWE technology. This can be achieved in various ways, such as dilution with other and cheaper compounds such as Ta2O5 [2] and alloying with more abundant metals followed by post-synthesis-treatment such as de-alloying or surface enrichment [3]. A possible solution is the application of ultra-thin layers of iridium oxide on a less costly substrate. A suitable method for achieving such layers is that of surface-limited galvanic displacement [4], i.e. the underpotential deposition of a metal M followed by its oxidation by another and more noble metal N.In this work we have investigated Ir plating onto a polycrystalline gold electrode by successive galvanic displacement of Cu monolayers in a one-pot method, i.e by performing the Cu underpotential-deposition step in a solution also containing iridium precursors. The process is indicated in the figure. By using low concentrations of the precursors H2IrCl6 and IrCl3 in H2SO4, Cu submonolayers could be formed on a Au(poly) rotating disc electrode by underpotential deposition without any significant direct electrochemical formation of Ir or IrO2. The Cu submonolayer formed was then available for reaction with the Ir precursor before a new Cu submonolayer was formed. By evaluating the amount of Cu deposited in each submonolayer and the resulting Ir deposit, our results indicate that H2IrCl6 is reduced onto the Au electrode as metallic Ir via the formation of Ir(III). Due to transport of Ir(III) away from the reaction surface, this resulted in a low yield. No Ir deposited was achieved from IrCl3 solutions, possibly due to adsorption of irreducible species onto the Au surface.The results will be discussed in terms of possible reaction paths and the relative stability of iridium complexes in chloride-containing solutions. Acknowledgements This work was performed within the project "Metal(-oxide) catalyst-monolayer as cost-effective electrocatalysts for PEM water electrolysis" funded by the Research Council of Norway, contract no. 254976/E20. References Carmo, D. L. Fritz, J. Mergel, and D. Stolten, “A comprehensive review on PEM water electrolysis,” Int. J. Hydrogen Energy, 38, pp. 4901–4934, 2013.Comninellis and G. P. Vercesi, “Characterization of DSA®-type oxygen evolving electrodes: Choice of a coating,” J Appl Electrochem, 21, pp. 335–345, 1991.N. Nong, L. Gan, E. Willinger, D. Teschner, and P. Strasser, “IrOx core-shell nanocatalysts for cost- and energy-efficient electrochemical water splitting,” Chem. Sci., 5, pp. 2955–2963, 2014.Brankovic, J. X. Wang, and R. R. Adžic, “Metal monolayer deposition by replacement of metal adlayers on electrode surfaces,” Surface Science, vol. 474, L173–L179, 2001. Figure 1

Study on the effect of IrO2/TiO2 catalyst composition coated on porous transport layer on the performance and durability of polymer electrolyte membrane water electrolysis

The porous transport layer (PTL) is an essential component of the polymer electrolyte membrane water electrolysis (PEMWE). With the multi-function of electrical and mass transport, PTL must deal with surface passivation during long-term operation, acidic environment, and high operating temperature. A thin IrO2/TiO2 catalyst layer is coated on the PTL surface by combining the spray-coating and thermal treatment methods. This catalyst layer not only prevents the PTL passivation but also enhances the performance of PEMWE. In this study, by controlling the ratio of Ir and Ti precursors in the catalyst ink, 4 samples with different Ir:Ti ratios (PTL3.1, PTL4.1, PTL6.1, PTL9.1) are evaluated to assess the effect of varying Ir:Ti ratios on the water electrolysis performance. The results indicate that the PEMWE performance of IrO2/TiO2 catalyst coated PTLs increases with the Ir:Ti ratio. In our study, the Ir:Ti ratio of 6:1 is expected as the optimum for the PEMWE. The durability test results also show that the IrO2/TiO2 catalyst coated PTL helps to limit the degradation of PTL after the accelerated stress test.

A highly dispersed amorphous iridium nanoclusters electrocatalyst synthesized at a quasi-room temperature for efficient oxygen evolution reaction

The water-splitting by proton-exchange-membrane (PEM) electrolyser is a highly appealing technology for hydrogen (H2) production. However, oxygen evolution reaction (OER) at anode of PEM electrolyser requires expensive and scarce iridium (Ir) catalysts to overcome its sluggish kinetics. Therefore, minimizing the amount of Ir used in OER electrocatalysts is critical for commercial application of PEM electrolyser. Herein, we report a flexible one-pot pathway to prepare carbon supported amorphous Ir nanoclusters (NCs) with low-Ir-loading (5.1 wt% Ir) via high-pressure hydrogen (1 MPa) reducing Ir(IV) precursors at a quasi-room temperature (50℃). The obtained amorphous Ir NCs catalyst shows a high performance for OER in an acidic electrolyte with a overpotential of 290 mV and Tafel slope of 55 mV/dec, superior to those of crystalline Ir nanoparticles (NPs) and commercial Ir catalysts (340 mV; 77 mV/dec). We find that the high OER activity is related to amorphous IrOx formed on the Ir NCs surface. When Ir NCs was used as the anodic catalyst, the current density of the electrolyzer reached 30 mA cm−2 only requiring a cell voltage of 1.56 V. This work demonstrates that Ir NCs catalyst exhibits a promising application in practical overall water splitting for H2 production.

Iridium Nanosheet Catalysts Supported on Titanium Oxide for Oxygen Evolution Reaction in Polymer Electrolyte Membrane Water Electrolysis

In order to overcome the disadvantage of intermittent production of renewable energy, Polymer electrolyte membrane water electrolysis(PEMWE), which is one of the technologies of producing hydrogen by electrolyzing water, is attracting attention. For the commercialization of PEMWE, it is important to reduce usage of iridium(Ir) for oxygen evolution reaction(OER) in the anode electrode. Many studies have been reported to reduce the amount of iridium used by developing a catalyst with higher performance than the iridium nanoparticle catalyst used as a commercial catalyst. As one of the strategies, the introduction of support materials that can increase the active area of the catalyst is drawing attention. While carbon-based supports are mainly used in other applications, in OER environments, carbon is easily corroded due to high voltage and strong acid conditions. Therefore, it is necessary to use a metal oxide support with high corrosion resistance. In particular, titanium oxide(TiO2) is one of the suitable support candidates because it is a stable material in an acidic OER environment. However, catalysts loaded on TiO2 with nanoparticles cause an electrical contact problem due to the low electrical conductivity of TiO2, which causes a degradation in activity. Many studies have reported doping methods to increase the conductivity of TiO2, but there are also reports that these methods reduce the durability of support. Thus, a new approach to synthesizing a better structure of catalysts is needed to utilize metal oxide support.Herein, we present an approach to iridium nanosheet (NS) catalysts supported on TiO2(Ir NS/TiO2). Ir NSs were synthesized by an facile method by directly annealing a mixture of Ir precursor and alkali salt. Ir NSs were supported on TiO2 to be utilized as a reaction site and also acts as an electron path to compensate for the low electrical conductivity of TiO2. Ir NS/TiO2 showed superior OER activity and stability under acidic conditions. These results showed that Ir NS/TiO2 could be utilized to reduce the amount of precious metal catalyst in PEMWE and increase the stability of OER supported catalysts.

Enantioselective Synthesis of N−N Biaryl Atropisomers through Iridium(I)‐Catalyzed C−H Alkylation with Acrylates

AbstractEnantioselective synthesis of N−N biaryl atropisomers is an emerging area but remains underexplored. The development of efficient synthesis of N−N biaryl atropisomers is in great demand. Herein, the construction of N−N biaryl atropisomers through iridium‐catalyzed asymmetric C−H alkylation is reported for the first time. In the presence of readily available Ir precursor and Xyl‐BINAP, a variety of axially chiral molecules based on indole‐pyrrole skeleton were obtained in good yields (up to 98 %) with excellent enantioselectivity (up to 99 % ee). In addition, N−N bispyrrole atropisomers could also be synthesized in excellent yields and enantioselectivity. This method features perfect atom economy, wide substrate scope, and multifunctionalized products allowing diverse transformations.

Enantioselective Synthesis of N-N Biaryl Atropisomers through Iridium(I)-Catalyzed C-H Alkylation with Acrylates.

Enantioselective synthesis of N-N biaryl atropisomers is an emerging area but remains underexplored. The development of efficient synthesis of N-N biaryl atropisomers is in great demand. Herein, the construction of N-N biaryl atropisomers through iridium-catalyzed asymmetric C-H alkylation is reported for the first time. In the presence of readily available Ir precursor and Xyl-BINAP, a variety of axially chiral molecules based on indole-pyrrole skeleton were obtained in good yields (up to 98%) with excellent enantioselectivity (up to 99% ee). In addition, N-N bispyrrole atropisomers could also be synthesized in excellent yields and enantioselectivity. This method features perfect atom economy, wide substrate scope, and multifunctionalized products allowing diverse transformations.

Asymmetric Coordination of Iridium Single‐atom IrN3O Boosting Formic Acid Oxidation Catalysis

AbstractRational design of the proximal coordination of an active site to achieve its optimum catalytic activity is the ultimate goal in single‐atom catalysis, but still challenging. Here, we report theoretical prediction and experimental realization of an asymmetrically coordinated iridium single‐atom catalyst (IrN3O) for the formic acid oxidation reaction (FAOR). Theoretical calculations reveal that the substitution of one or two nitrogen with more electronegative oxygen in the symmetric IrN4 motif splits and downshifts the Ir 5d orbitals with respect to the Fermi level, moderating the binding strength of key intermediates on IrN4−xOx (x=1, 2) sites, especially that the IrN3O motif shows ideal activity for FAOR with a near‐zero overpotential. The as‐designed asymmetric Ir motifs were realized by pyrolyzing Ir precursor with oxygen‐rich glucose and nitrogen‐rich melamine, exhibiting a mass activity of 25 and 87 times greater than those of state‐of‐the‐art Pd/C and Pt/C, respectively.

Asymmetric Coordination of Iridium Single-atom IrN3 O Boosting Formic Acid Oxidation Catalysis.

Rational design of the proximal coordination of an active site to achieve its optimum catalytic activity is the ultimate goal in single-atom catalysis, but still challenging. Here, we report theoretical prediction and experimental realization of an asymmetrically coordinated iridium single-atom catalyst (IrN3 O) for the formic acid oxidation reaction (FAOR). Theoretical calculations reveal that the substitution of one or two nitrogen with more electronegative oxygen in the symmetric IrN4 motif splits and downshifts the Ir 5d orbitals with respect to the Fermi level, moderating the binding strength of key intermediates on IrN4-x Ox (x=1, 2) sites, especially that the IrN3 O motif shows ideal activity for FAOR with a near-zero overpotential. The as-designed asymmetric Ir motifs were realized by pyrolyzing Ir precursor with oxygen-rich glucose and nitrogen-rich melamine, exhibiting a mass activity of 25 and 87 times greater than those of state-of-the-art Pd/C and Pt/C, respectively.

High-Performance Iridium Thin Films for Water Splitting by CVD Using New Ir(I) Precursors.

Thin films of iridium can be utilized in a wide range of applications and are particularly interesting for catalytic transformations. For the scalable deposition of functional Ir thin films, metalorganic chemical vapor deposition (MOCVD) is the method of choice, for which organometallic precursors that embody a high volatility and thermal stability need to be specifically tailored. Herein, we report the synthesis, analysis, and evaluation of new volatile Ir(I)-1,5-cyclooctadiene complexes bearing all-nitrogen coordinating guanidinate (N,N'-diisopropyl-2-dimethylamido-guanidinate (DPDMG)), amidinate (N,N'-diisopropyl-amidinate (DPAMD)), and formamidinate (N,N'-diisopropyl-formamidinate (DPfAMD)) ligands. The amidinate-based Ir complex [Ir(COD)(DPAMD)] together with O2 was implemented in MOCVD experiments resulting in highly crystalline, dense, and conductive Ir films on a variety of substrate materials. The Ir deposits achieved outstanding electrochemical performance with overpotentials in the range of 50 mV at -10 mA·cm-2 for catalytic hydrogen evolution reaction (HER) in acidic solution. The ability to deposit Ir layers via MOCVD exhibiting promising functional properties is a significant step toward large-scale applications.

Preferential chemical vapor deposition for the synthesis of the catalysts for CO mediated NOx selective catalytic reduction

In previous studies IrRu/Al2O3 has exhibited good activity for NOx reduction by CO at low temperature. The Ir-Ru alloy structure was revealed to be the origin of the outstanding activity of IrRu/Al2O3 in the CO-induced deNOx reaction. In this work, preferential chemical vapor deposition (pCVD) was applied as an effective and selective Ir-Ru alloy catalyst synthesis method, which selectively deposits Ir precursor on the pre-existing Ru nanoparticles. The synthesized IrRu/Al2O3 catalysts were characterized by transmission electron microscopy, X-ray diffraction, X-ray absorption spectroscopy, temperature programmed reduction, X-ray photoelectron spectroscopy, and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTs) analyses. The results confirmed the formation of Ir-Ru alloy, and showed that Ir can be preferentially deposited on Ru surface via pCVD, rather than on the Al2O3 support. IrRu/Al2O3 synthesized by pCVD exhibited better performance in the NOx reduction by CO than catalysts prepared by the conventional impregnation method. Its catalytic activity varied with Ir content, which was precisely controllable using pCVD. The investigation of catalyst surface by DRIFT revealed that the accelerated NO dissociation is the primary reason for its excellent low-temperature activity of the IrRu/Al2O3 catalyst prepared by pCVD.

Stability of IrO2 Oxygen Evolution Reaction Anode Co-Catalysts Under Transient Operation and Its Effect on PEM Fuel Cell Durability

Among various degradation phenomena in polymer-electrolyte-membrane fuel cells (PEMFCs), cell reversal (CR) occurring due to an under-stoichiometric supply of H2 to the anode is known to cause the irreversible corrosion of the carbon support of carbon-supported platinum (Pt/C) anode catalysts via the carbon oxidation reaction (COR). Several strategies to mitigate the damages of cell reversal have been documented in the literature, from which the addition of an oxygen evolution reaction (OER) catalyst (e.g., IrO2) to the anode has attracted great attention. The lower OER onset potential in the presence of IrO2 impedes the increase of the anode potential during cell reversal events, thus reducing the COR rate.A recent study from our group showed that in spite of the high OER activity of an IrO2-based anode co-catalyst, its stability under transient PEMFC operation conditions, e.g., during unmitigated start-up/shut-down (SUSD) events, should be considered for long-term durability (1). The near-surface layer(s) of IrO2 can be completely reduced to metallic Ir upon exposure to H2 under typical operating conditions in a PEMFC anode, leading to Ir dissolution during the anode potential transients associated with SUSD cycles. The diffusion of dissolved Irn+ species through the membrane towards the cathode and its further deposition on the Pt/C catalyst reduces its oxygen reduction reaction (ORR) activity and results in a significant performance loss during normal PEMFC operation. Since the origin of such degradation mechanism is the chemical reduction of the IrO2 anode co-catalysts in the hydrogen environment of the anode, a reduction-resistant IrO2 would be required for long-term to stabilize such an anode co-catalyst (1). In a recent contribution from our group, we have introduced a novel approach to prepare IrO2 anode co-catalysts that are irreducible (proven by TGA analyses and observed after extended PEMFC operation). In contrast to typically employed IrO2 catalysts (usually prepared by heat treatment of an Ir precursor at T ≈ 400 °C), the stability of these irreducible IrO2 (irr-IrO2) catalysts is highlighted by the application of SUSD transients, where Ir dissolution is drastically reduced, so that no iridium poisoning of the Pt/C cathode catalyst is observed (2).In this work, we investigate the stability of a state-of-the-art IrO2 catalyst and this novel irr-IrO2 catalyst when used as anode co-catalysts, applying two different accelerated stress tests (ASTs): a) SUSD transients + CR cycles (relevant for PEMFC systems); and b) CR cycles only (commonly used as test to evaluate the effectiveness anode co-catalysts). The application of both ASTs allows the separation and comparison of trends in the activity of the OER catalyst (i.e., its ability to mitigate the COR at the anode) and in its stability (i.e., its stability towards Ir dissolution). The later is particularly important when considering the applicability of anode co-catalysts for actual PEMFC applications. References M. Fathi Tovini, A. M. Damjanovic, H. A. El-Sayed, J. Speder, C. Eickes, J.-P. Suchsland, A. Ghielmi, and H. A. Gasteiger, Journal of The Electrochemical Society, 168 (6), 064521 (2021). M. Fathi Tovini, A. M. Damjanović, H. A. El-Sayed, F. Friedrich, B. Strehle, J. Speder, A. Ghielmi and H. A. Gasteiger, : I03-1466 Irreducible IrO2 Anode Co-Catalysts for PEM Fuel Cell Voltage Reversal Mitigation and Their Stability Under Transient Operation Conditions, in 241st Electrochemical Society Meeting, Vancouver, BC, Canada (2022).

Reactivity of Iridium Complexes of a Triphosphorus-Pincer Ligand Based on a Secondary Phosphine. Catalytic Alkane Dehydrogenation and the Origin of Extremely High Activity.

The selective functionalization of alkanes and alkyl groups is a major goal of chemical catalysis. Toward this end, a bulky triphosphine with a central secondary phosphino group, bis(2-di-t-butyl-phosphinophenyl)phosphine (tBuPHPP), has been synthesized. When complexed to iridium, it adopts a meridional ("pincer") configuration. The secondary phosphino H atom can undergo migration to iridium to give an anionic phosphido-based-pincer (tBuPPP) complex. Stoichiometric reactions of the (tBuPPP)Ir complexes reflect a distribution of steric bulk around the iridium center in which the coordination site trans to the phosphido group is quite crowded; one coordination site cis to the phosphido is even more crowded; and the remaining site is particularly open. The (tBuPPP)Ir precursors are the most active catalysts reported to date for dehydrogenation of n-alkanes, by about 2 orders of magnitude. The electronic properties of the iridium center are similar to that of well-known analogous (RPCP)Ir catalysts. Accordingly, DFT calculations predict that (tBuPPP)Ir and (tBuPCP)Ir are, intrinsically, comparably active for alkane dehydrogenation. While dehydrogenation by (RPCP)Ir proceeds through an intermediate trans-(PCP)IrH2(alkene), (tBuPPP)Ir follows a pathway proceeding via cis-(PPP)IrH2(alkene), thereby circumventing unfavorable placement of the alkene at the bulky site trans to phosphorus. (tBuPPP)Ir and (tBuPCP)Ir, however, have analogous resting states: square planar (pincer)Ir(alkene). Alkene coordination at the crowded trans site is therefore unavoidable in the resting states. Thus, the resting state of the (tBuPPP)Ir catalyst is destabilized by the architecture of the ligand, and this is largely responsible for its unusually high catalytic activity.

Growth behavior of Ir metal formed by atomic layer deposition in the nanopores of anodic aluminum oxide.

The conformal coating or surface modification in high aspect ratio nanostructures is a tough challenge using traditional physical/chemical vapor deposition, especially for metal deposition. In this work, the growth behavior of iridium (Ir) metal formed by atomic layer deposition (ALD) in anodic aluminum oxide (AAO) templates was explored deeply. It is found that the surface hydrophilicity is crucial for the nucleation of ALD Ir. An in situ ALD Al2O3 layer with an ultra-hydrophilic surface can greatly promote the nucleation of ALD Ir in AAO nanopores. The effect of the Ir precursor pulse time, diameter, and length of AAO nanopores on the infiltration depth of ALD Ir was investigated systematically. The results show that the infiltration depth of ALD Ir in AAO nanopores is in proportion to the pore diameter and the square root of the Ir precursor pulse time, which follows a diffusion-limited model. Furthermore, the Ir precursor pulse time to obtain conformal Ir coating throughout all the AAO channels is in proportion to the square of the aspect ratio of AAO templates. In addition, the conformal Ir deposition in AAO nanopores is also related to the Ir precursor purge time and the O2 partial pressure. Insufficient Ir purge time could cause a CVD-like reaction, leading to the reduction of the infiltration depth in AAO. Higher O2 partial pressure can facilitate Ir nucleation with more Ir precursor consumption at the entrance of nanopores, decreasing the infiltration depth in AAO nanopores, so appropriate O2 partial pressure should be chosen for ALD Ir in high aspect ratio materials. Above all, our research is valuable for surface modification or coating of metal by ALD in high aspect ratio nanostructures for 3D microelectronics, nano-fabrication, catalysis and energy fields.

The Roles of Precursor-Induced Metal–Support Interaction on the Selective Hydrogenation of Crotonaldehyde over Ir/TiO2 Catalysts

Various supported Ir/TiO2 catalysts were prepared using different Ir precursors (i.e., H2IrCl6, (NH4)2IrCl6 and Ir(acac)3) and tested for vapor phase selective hydrogenation of crotonaldehyde. The choice of Ir precursor significantly altered the Ir-TiOx interaction in the catalyst, which thus had essential influences on the geometric and electronic properties of the Ir species, reducibility, and surface acidity, and, consequently, their reaction behaviors. The Ir/TiO2-N catalyst using (NH4)2IrCl6 as the precursor gave the highest initial reaction rates and turnover frequencies of crotyl alcohol formation. Such high performance was ascribed to the high Ir dispersion and high surface concentration of Ir0 species, as well as a higher surface acidity, in the Ir/TiO2-N catalyst compared to its counterparts, indicating the synergistic roles of the Ir-TiOx interface in the reaction, as the interfacial sites were responsible for the adsorption/activation of H2 and the C=O bond in the crotonaldehyde molecule.