Ir Oxidation State Research Articles

(Invited) Dissolution of Iridium: Aqueous vs. Solid Polymer Electrolytes

Iridium oxide possesses unique physicochemical properties, making it the only oxygen evolution reaction (OER) catalyst used in modern proton exchange membrane water electrolysis (PEMWE). It is electronically conductive and relatively active towards the OER. Moreover, it can maintain these properties for a long time, guaranteeing the required long-term operation of PEMWE. Nevertheless, iridium oxide is not entirely immune during the OER. In the currently used PEMWE, the dissolution of Ir is masked by its high loading. This way-around solution will likely not work in low Ir loading anodes. To overcome this challenge, other material or operational solutions will be required. Both should be based on a more advanced understanding of the Ir dissolution mechanisms.In this presentation, we overview the current status of the iridium dissolution research. Aqueous and solid polymer electrolyte systems are considered as the former suits best model fundamental studies, and the latter represents real devices. As the first step in understanding Ir dissolution stability, the thermodynamics of Ir in broad potential and pH ranges, as visualized by Pourbaix diagrams, is discussed. Experimental data on Ir dissolution kinetics obtained in on-line inductively coupled plasma mass spectrometry (ICP-MS) studies is presented next. It is shown that changes in the Ir oxidation state during oxidation/reduction and OER are responsible for Ir losses.1 The OER-triggered Ir dissolution is explained by suggesting the existence of common intermediates in water oxidation and Ir leaching.2, 3 Besides mechanistic insights, Ir dissolution rates and amounts for different Ir oxides are quantified. Such quantification is more challenging in polymer electrolyte systems. Hence, a detailed analysis of various cell components to monitor Ir is performed. It is shown that in a full-cell setup, the main sink of Ir is the membrane and cathode catalyst layer.4 While providing necessary information, experiments on the full-cell level are tedious and slow. We show that the same information can be obtained using half-cell gas diffusion electrode (GDE) setups.5 The talk will be finalized by comparing and contrasting both electrolyte systems and discussing the next steps required to advance our understanding of Ir dissolution to provide mitigating strategies in stabilizing low Ir loading PEMWE anodes.

Key Role of Oxidising Species for Water Oxidation on Iridium Oxides Revealed By Time-Resolved Optical and X-Ray Spectroscopies

Iridium oxide is the state-of-the-art electrocatalyst for water oxidation in polymer electrolyte membrane (PEM) electrolysers, crucial for green hydrogen production. Despite its extensive industrial use, the water oxidation mechanism on this metal oxide and the key factors controlling the reaction rate remain unclear. Understanding these controls at a fundamental level on such benchmark metal oxides is vital for designing more active and stable electrocatalysts for water oxidation in PEM electrolysers.In this talk, I will present our research on probing oxygen evolution reaction (OER) relevant species on iridium-based catalysts. This involves a combination of time-resolved operando optical spectroscopy, soft and hard X-ray absorption spectroscopy (XAS), and electrochemical mass spectrometry. Initially, I will discuss the intermediate and catalytically active states of iridium oxides. This is based on correlating optically detected states with oxygen molecular products identified through on-chip electrochemical mass spectrometry. Subsequently, I will demonstrate how we used optical spectroscopy to quantify these states as a function of potential. The nature of these states, including the Ir oxidation state and surface adsorbates on iridium sites, will be elucidated using a combination of time-resolved Ir-L edge XAS, O-K edge XAS, and supported by Density Functional Theory (DFT).With these quantification results of active species for OER, I will compare the intrinsic kinetics of two state-of-the-art iridium oxide structures - amorphous IrOx versus crystalline rutile IrO2.[1] The effect of the electrolyte on the intrinsic activity of iridium oxides will also be discussed.[2] Finally, based on this molecular-level understanding, I will present a modified volcano model for OER catalyst design. This model considers the impact of adsorbate-adsorbate interactions on binding energetics, demonstrating critical design principles for high intrinsic activity OER catalysts.The authors acknowledge the funding support from the Imperial College-Chinese Scholarship Council Studentship and bp-ICAM 92, which made this research possible.[1] Caiwu Liang, Reshma Rao, Karine Svane et al. Unravelling the effects of active site densities and energetics on the water oxidation activity of iridium oxides, 07 March 2023, PREPRINT (Version 1) available at Research Square [https://doi.org/10.21203/rs.3.rs-2605628/v1][2] Caiwu Liang, Yu Katayama, Yemin Tao, Asuka Morinaga, Benjamin Moss, Verónica Celorrio, et al. Role of electrolyte pH on water oxidation for iridium oxides. ChemRxiv. Cambridge: Cambridge Open Engage; 2023

Engineering the orbital splitting and electronic reoccupation at alkali-metal-doped perovskites with face-shared IrO6 octahedral dimers

The intriguing catalysts with excellent catalytic activity and durable stability in acidic oxygen evolution reaction (OER) are extremely desirable for efficient energy conversion. Herein, engineering the orbital splitting and electronic reoccupation at metal sites of alkali-metal-doped perovskites with face-shared IrO6 octahedral dimers is realized through structural distortion and symmetry breaking in an effort to boost the sluggish reaction kinetics and mitigate the degradation of the Ir-based catalysts in acidic electrolyte as well. Doping Na or K element can increase the oxidation state of Ir at IrO6 octahedral dimers, which not only displaces the Ir d-band centers for higher catalytic activity but also leads to a good stability of Ir centers with higher dissolution potentials. This work provides a new insight into enhancing the reactive activity by doping alkali metals.

Deciphering the Roles of Chemistry and Structure in Iridium Oxide Oxygen Evolution Catalysts through the Study of Annealed Iridium Oxide Nanoparticles

The development of proton exchange membrane (PEM) water electrolysis, which enables the production of de-carbonized hydrogen, is hindered by the low efficiency of the oxygen evolution reaction (OER). This is especially true in acidic environment where only noble metals (mainly Ru and Ir) can be used as electrocatalysts. Iridium oxide nanocatalysts (IrOx) are promising due to their high OER activity and good resistance to acidic and oxidative conditions. However, finding a compromise between their electrocatalytic activity and stability remains challenging. Moreover, it is not yet clear which parameter (the material structure1 or the initial iridium oxidation state2) controls these two properties.To address these challenges, we synthesized IrOx nanoparticles (NPs) supported on carbon (IrOx/C), through a modified polyol process, and annealed them under air, at temperatures ranging from 200◦C to 850◦C. This allows us to deconvolute the effects of the oxidation state, the structure and the size of IrOx NPs and to tune their activity and stability3. To analyze the structure, morphology, and oxidation state of Ir in these catalysts, we used transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and both laboratory-based and synchrotron-based X-ray diffraction (XRD) techniques.We observed in situ the crystallization of IrOx NPs, leading to crystallite size growth and change in Ir electronic state (see Figure 1a). Additionally, we investigated the morphological changes of IrOx NPs with respect to annealing temperature (refer to Figure 1b-f). We assessed their electrocatalytic activity towards the OER and determined their stability number4 (S-number) using inductively coupled plasma mass spectrometry and electrochemistry. We observed that increasing the annealing temperature led to an increase in particle size and degree of crystallinity, but did not result in significant changes in the iridium oxidation state (refer to Figure 1g). We observed the evolution of the electrochemical activity of the nanoparticles with the annealing temperature (see Figure 1h). Our results show a correlation between the changes in electronic structure, morphology, and size, and the evolution of OER activity and stability of the IrOx NPs. Specifically, we identified certain conditions that achieved an optimal balance between activity and stability.(1) Elmaalouf, M.; Odziomek, M.; Duran, S.; Gayrard, M.; Bahri, M.; Tard, C.; Zitolo, A.; Lassalle-Kaiser, B.; Piquemal, J.-Y.; Ersen, O.; Boissière, C.; Sanchez, C.; Giraud, M.; Faustini, M.; Peron, J. Nat. Commun. 2021, 12 (1), 3935. https://doi.org/10.1038/s41467-021-24181-x.(2) Claudel, F.; Dubau, L.; Berthomé, G.; Sola-Hernandez, L.; Beauger, C.; Piccolo, L.; Maillard, F. ACS Catal. 2019, 9 (5), 4688–4698. https://doi.org/10.1021/acscatal.9b00280.(3) Geiger, S.; Kasian, O.; Shrestha, B. R.; Mingers, A. M.; Mayrhofer, K. J. J.; Cherevko, S. J. Electrochem. Soc. 2016, 163 (11), F3132. https://doi.org/10.1149/2.0181611jes.(4) Geiger, S.; Kasian, O.; Ledendecker, M.; Pizzutilo, E.; Mingers, A. M.; Fu, W. T.; Diaz-Morales, O.; Li, Z.; Oellers, T.; Fruchter, L.; Ludwig, A.; Mayrhofer, K. J. J.; Koper, M. T. M.; Cherevko, S. Nat. Catal. 2018, 1 (7), 508–515. https://doi.org/10.1038/s41929-018-0085-6.Acknowledgments: This work was supported by the French National Research Agency in the frame of the MATHYLDE project (grant number n° ANR-22-PEHY-0001).Figure 1 – a. X-ray diffractograms obtained on IrOx/C annealed at 290, 340, 380, 420, 470, 520, 570, 620, 670 and 850°C under air. Each measurement lasted 13 minutes. The hashes correspond to tetragonal IrO2 (PDF card 00-043-1019) and the stars to metallic Ir (PDF card 00-006-0598). b-f. TEM images of IrOx/C NPs as-synthesized and calcined at 290, 380, 420, 570 and 850 °C. g. X-ray photoelectron spectra showing the Ir 4f levels of IrOx/C NPs as-synthesized and calcined at 380, 420, 520 and 850 °C. The dotted lines indicates the theoretical position of the peaks for Ir(IV) and Ir(III). h. Cyclic voltammograms measured in 0.1 M HClO4 at 50 mV s−1 indicating the change in electrochemical properties of IrOx/C NPs with annealing temperature. Figure 1

Substrate‐Driven Catalyst Reducibility for Oxygen Evolution and Its Effect on the Operation of Proton Exchange Membrane Water Electrolyzers

To increase the efficiency of hydrogen production by proton exchange membrane water electrolyzers (PEMWEs), the relationships between the specific activity and stability of the membrane–electrode assembly (MEA) must be clarified. Ir oxide electrodeposited on Ti substrate is used as an oxygen electrode, and its electronic properties and electrochemical behavior in PEMWE operation are observed. The electrode, fabricated through a facile strategy based on annealing in the air atmosphere, enhances the specific oxygen evolution reaction (OER) activity and stability in PEMWE operation. Furthermore, the electronic catalyst–substrate interactions associated with different reducibilities in the heteroatom system are studied. The morphology, electronic properties, and chemical state of the oxygen electrode are investigated through X‐ray spectroscopy, microscopy techniques, and computational analyses. Thermal treatment of the catalyst‐coated substrate decreases the bulk oxidation state of Ir and increases the surface oxidation state. According to the electrochemical and physical behavior analyses of PEMWEs, the oxygen content in the Ir oxide structure, which defines the OER activity and its stability, is influenced by the crystalline structure and formation of a stable interface between the catalyst and substrate. The outcomes can facilitate the development of strategies for enhancing the performance of PEMWEs and designing rational MEAs.

Efficient and sustainable water electrolysis achieved by excess electron reservoir enabling charge replenishment to catalysts

Suppressing the oxidation of active-Ir(III) in IrOx catalysts is highly desirable to realize an efficient and durable oxygen evolution reaction in water electrolysis. Although charge replenishment from supports can be effective in preventing the oxidation of IrOx catalysts, most supports have inherently limited charge transfer capability. Here, we demonstrate that an excess electron reservoir, which is a charged oxygen species, incorporated in antimony-doped tin oxide supports can effectively control the Ir oxidation states by boosting the charge donations to IrOx catalysts. Both computational and experimental analyses reveal that the promoted charge transfer driven by excess electron reservoir is the key parameter for stabilizing the active-Ir(III) in IrOx catalysts. When used in a polymer electrolyte membrane water electrolyzer, Ir catalyst on excess electron reservoir incorporated support exhibited 75 times higher mass activity than commercial nanoparticle-based catalysts and outstanding long-term stability for 250 h with a marginal degradation under a water-splitting current of 1 A cm−2. Moreover, Ir-specific power (74.8 kW g−1) indicates its remarkable potential for realizing gigawatt-scale H2 production for the first time.

IrO2/Co3O4 supported mesoporous SBA-16: An efficient electro-catalyst for oxygen evolution reaction

A non-conducting mesoporous silica support (SBA-16) with active supported electroactive iridium and cobalt was synthesized by a simple hydrothermal and impregnation method. The synthesized catalysts were represented as 5% IrO2/SBA-16, 10% IrO2/SBA-16, and 5% Co3O4 / 10% IrO2, respectively. All catalysts were characterized by XRD, BET-Surface area, XPS, EDAX, and HR-TEM techniques to study their structural, morphological, chemical and micro structural properties. Small-angle XRD confirmed the mesoporous nature of the material formation, showing major faces at (100), (110), (200), and (220). High-angle XRD confirmed the formation of Co3O4 and IrO2 phase on the catalyst surface. Nitrogen sorption isotherm showed a ordered material with surface areas of the decreasing order of 5% IrO2/SBA-16, 10% IrO2/SBA-16, and 5% Co3O4/10% IrO2/SBA-16, respectively. EDAX analysis confirmed the presence of Ir and Co levels in the bimetallic catalysts. The oxidation state of Ir +4 and Co3+ were confirmed by XPS analysis. HR-TEM images showed that this material was highly porous and contained active metal particles dispersed throughout the surface. These metal particles play an important role in decreasing lower overpotential and current densities. In particular, the 5% Co3O4 /10% IrO2/SBA-16 showed a lower overpotential of 380 mV at a current density of 10 mA. Compared to the activities of all the catalysts, 5% Co3O4 /10% IrO2/SBA-16 showed higher activity due to the broad distribution of active sites with frequent electron transfers. Furthermore, during chronopotentiometry testing, a stable potential was maintained for over 12 hours at a constant current density of 10 mA.

Catalyst‐Support Interactions in Zr2ON2‐Supported IrOx Electrocatalysts to Break the Trade‐Off Relationship Between the Activity and Stability in the Acidic Oxygen Evolution Reaction

AbstractThe development of highly active and durable Ir‐based electrocatalysts for the acidic oxygen evolution reaction (OER) is challenging because of the corrosive anodic conditions. Herein, IrOx/Zr2ON2 electrocatalyst is demonstrated, employing Zr2ON2 as a support material, to overcome the trade‐off between the activity and stability in the OER. Zr2ON2 is selected due to its excellent electrical conductivity and chemical stability, and the fact that it induces strong interactions with IrOx catalysts. As a result, IrOx/Zr2ON2 electrocatalysts exhibit outstanding OER performances, reaching an overpotential of 255 mV at 10 mA cm−2 and a mass activity of 849 mA mgIr−1 at 1.55 V (vs the reversible hydrogen electrode). The activity of IrOx/Zr2ON2 is maintained at 10 mA cm−2 for 5 h, while in contrast, IrOx/ZrN and an unsupported IrOx catalyst undergo drastic degradation. Combined experimental X‐ray analyses and theoretical interpretations reveal that the reduced oxidation state of Ir and the extended IrO bond distance in IrOx/Zr2ON2 effectively increase the activity and stability of IrOx by altering reaction pathway from a conventional adsorbate evolution mechanism to a lattice oxygen‐participating mechanism. These results demonstrate that it is possible to effectively reduce the Ir content in OER catalysts through interface engineering without sacrificing the catalytic performance.

Elevating IrOx acidic oxygen evolution activity using SnO2-rGO hybrid support

• IrO x electrocatalysts on SnO 2 -rGO hybrid support for acidic OER were synthesized. • SnO 2 –rGO hybrid supports imparted active sites to the IrO x particles. • The OER activity of the catalysts was tuned by varying the Ir oxidation states. • IrO x on SnO 2 -rGO showed 10-fold higher mass activity than Ir black catalyst. Recently, Ir-based catalysts have been deposited onto high-surface-area and highly conductive materials to decrease noble metal loading in electrodes and thus increase the commercial viability of proton exchange membrane (PEM) water electrolyzers. carbon material supports impart active surface areas to the catalyst and metal-based supports enhance stability, thus improving the electrode performance for the energy-intensive and sluggish oxygen evolution reaction (OER). Herein, iridium oxide (IrO x ) electrocatalysts supported on a SnO 2 -reduced graphene oxide (rGO) composite (denoted as IrO x /SG) were used for the OER catalysis in acidic media. The effects of the SnO 2 –rGO hybrid support composition on the morphology, structure, surface composition, and performance of the IrO x catalyst were investigated. IrO x /SG catalysts were synthesized using ultrasonic spray pyrolysis and a modified polyol method. SnO 2 –rGO supports were employed to provide more anchoring sites for catalyst nanoparticle deposition and to modulate the catalytic activity and stability. Among the tested samples, the IrO x /S 80 G 20 catalyst (i.e., IrO x nanoparticles on 80 wt% SnO 2 and 20 wt% rGO support) showed the best OER performance. It exhibited a 10-fold higher mass activity than that of the commercial Ir black catalyst, which is possibly due to its high electrochemically active surface area (ECSA) and Ir 3+ ion and oxygen vacancy concentrations.

Fluorite-related iridate Pr3IrO7: crystal growth, structure, magnetism, thermodynamic, and optical properties

Spin–orbit coupling in heavy 5d metal oxides, in particular, iridates have received tremendous interest in recent years due to the realization of exotic electronic and magnetic phases. Here, we report the synthesis, structural, magnetic, thermodynamic, and optical properties of the ternary iridate Pr3IrO7. Single crystals of Pr3IrO7 have been grown by the KF flux method. Structural analysis shows that Pr3IrO7 crystallizes in an orthorhombic phase with Cmcm symmetry. The electron energy loss spectroscopy study indicates that Pr is in a 3+ valence state, which implies a 5+ oxidation state of Ir. Magnetization data measured at high and low magnetic fields do not exhibit any bifurcation between M ZFC and M FC , however, a weak hump in M(T) is observed at 10.4 K. The specific heat data reveal two maxima at ∼253 and ∼4.8 K. The optical conductivity spectrum shows 24 infrared-active phonon modes and reveals an insulating behavior with an optical gap of size ∼500 meV. During cooling down, the temperature-dependent reflectivity spectrum reveals eight extra phonon modes below the structural phase transition (∼253 K). An anomaly is observed at around in the temperature evolution of infrared-active mode frequencies suggesting the presence of significant spin–phonon coupling in the system.

Heating up the OER: Investigation of IrO2 OER Catalysts as Function of Potential and Temperature**

AbstractDespite intensive investigations for unravelling the water splitting reaction, the catalyst behavior during the oxygen evolution reaction (OER) is still not fully understood. This is especially true under more demanding conditions like high potentials and high temperatures. Rotating disk electrode measurements show a gradual increase of OER current when increasing the temperature up to 80 °C. However, strong bubble formation at elevated temperatures makes in‐situ characterization of the catalyst challenging. Here we utilize an in‐situ electrochemical and heated flow cell, which aims at an efficient removal of bubbles from the catalyst surface and enables structural studies by X‐ray absorption spectroscopy (XAS) at temperatures up to 80 °C. Changes in the Ir L3‐edge X‐ray absorption near edge spectra (XANES) were observed with respect to the white line position and principal components related to structural changes were extracted. At temperatures of 60 °C and above, the white line position of XANES spectra reaches a steady state, which is possibly caused by an equilibrium of different Ir oxidation states. These findings provide first spectroscopic insights in the behavior of OER catalysts at elevated temperatures which are typical for industrial applications and rarely addressed until now.

Oxygen Evolution Reaction on Ir-Oxide Based Materials Studied By Modulation Excitation X-Ray Absorption Spectroscopy

The price of electricity produced from renewable sources has sharply sunk in the last decade, making these a cheaper choice than newly built thermal power plants in almost all parts of the world [1]. The missing link towards the full decarbonisation of the energy sector is a crossover vector that can account for the intrinsic intermittency of these renewable energy sources. Hydrogen seems to be the best candidate for this, particularly if produced using water electrolysis powered by excess energy from renewable sources. In particular, the load capability and high power density of polymer electrolyte water electrolysis (PEWE) renders this technology an excellent match to these renewables’ sporadic nature [2]. However, the need for scarce Ir to catalyze the sluggish oxygen evolution reaction (OER) taking place in PEWE anodes hinders the large scale application of this technology [3]. Therefore, a better understanding of the OER mechanism on Ir-based electrocatalysts is needed to guide catalyst design and decrease the efficiency losses related to this reaction.Over the last decades, the OER mechanism on Ir-based materials has been extensively investigated through operando/in-situ X-ray absorption as well as X-ray photoelectron spectroscopy (XAS, XPS) [4-7]. Despite enormous efforts, there is still a lack of consensus regarding the oxidation state of Ir under OER conditions and the nature of the OER-active sites. Therefore, in this work we employed operando modulation excitation XAS to study the OER mechanism on several calcined and uncalcined Ir-oxide catalysts. By combining this modulation excitation approach with phase sensitive analysis, we enhanced the sensitivity of XAS towards oxidation state changes that are otherwise undiscernible on low surface area materials, such as calcined IrO2. To derive more precise information from these phase resolved spectra, standard materials with Ir oxidation states ≥ +5 were also characterized. The comparison of the demodulated, operando spectra of a given material with the corresponding difference between its low-potential spectrum and the standard compounds’ spectra indicates that all oxides reach a maximum Ir oxidation state of +5 under OER conditions. In addition, multivariate curve resolution (MCR) analysis was employed to extract kinetic information from these period averaged spectra. This analysis unveiled that the oxidation of the surface iridium proceeds at the same rate as its reduction in the case of highly OER-active materials, while reduction is slower than oxidation for the less OER-active samples.In summary, this contribution presents novel insights on the Ir oxidation under potential control, and in doing so also features the capabilities of modulation excitation XAS and MCR to elucidate kinetics of electrochemical reactions.

(Invited) Benchmarking Oxygen Evolution Reaction Activity and Stability of Unsupported and Supported IrOx Nanoparticles

Proton-exchange membrane water electrolyzers (PEMWEs) electrodes use scarce and costly platinum group metals (PGMs), but only such devices are able to meet the requirements associated with renewable energies (large amplitude and frequent and rapid variations in the current applied to the cell). Catalysts represent only 8 % to the overall stack cost, 6 % being associated with the high iridium (Ir) loading used to electrocatalyze the anodic oxygen evolution reaction (OER). High surface area supported Ir oxide (IrOx) catalysts thus represent a promising strategy to reduce the cost of this technology and limit the geological pressure on Ir. However, the Gibbs-Thompson effect, which controls the electrochemical stability of nanomaterials casts a doubt on the viability of this approach. To shed light into the benefits and limitations of supported and unsupported IrOx catalysts, we benchmarked commercial materials (unsupported IrO2, Ir/C), unsupported porous IrOx microparticles, and IrOx nanoparticles (NPs) supported on carbon black or on doped tin oxide aerogels (AG)/nanofibers (NFs). Transmission electron microscopy (TEM) and identical-location transmission electron microscopy (IL-TEM) provided changes in morphology during accelerated stress testing. Complementarily, a flow cell connected to an inductively-coupled mass spectrometer (FC-ICP-MS) was used to assess their stability number (S-number, see Figure 1). The results show that supported IrOx nanocatalysts are extremely active toward the OER because they feature mixed Ir oxidation states and a high density of active sites (small crystallite/particle size). The lack of robustness of their supports, however, prevents the nanocatalysts from sustaining this high OER activity [1, 2]. Similar to what was observed on extended surfaces, we report that mild thermal annealing (450°C) leads to lower Ir atom dissolution rate. Overall, the best compromise between OER activity and stability was obtained for unsupported porous IrOx microparticles after mild thermal annealing under air at 450°C [2]. On the cathode side, IL-TEM measurements revealed mild changes in morphology for Pt/C nanoparticles [3]. Ackowledgements This work was supported by the French National Research Agency in the frame of the MOISE project (grant number ANR-17-CE05-0033). References S. Abbou, R. Chattot, V. Martin, F. Claudel, L. Solà-Hernández, C. Beauger, L. Dubau, F. Maillard, ACS Catal. 10 (2020) 7283-7284.C. Daiane Ferreira da Silva, F. Claudel, V. Martin, R. Chattot, S. Abbou, K. Kumar, I. Jiménez-Morales, S. Cavaliere, D. Jones, J. Rozière, L. Solà-Hernandez, C. Beauger, M. Faustini, J. Peron, B. Gilles, C. Beauger, L. Piccolo, F. H. Barros de Lima, L. Dubau, F. Maillard, ACS Catal. 11 (2021) 4107-4116. A. Viola, L. Dubau, F. Maillard, in preparation. Figure 1. S-number values calculated for supported and unsupported IrOx electrocatalysts during a galvanostatic accelerated stress test (j = 10 mA cm-2 geo, T = 80 °C, U cut-off = 2 V vs. RHE) performed in Ar-saturated 0.05 M H2SO4. Reprinted with permission from ref. [2]. Copyright 2021 American Chemical Society. Figure 1

Electrolyzer Performance Loss from Accelerated Stress Tests and Corresponding Changes to Catalyst Layers and Interfaces

Stress tests are developed for proton exchange membrane electrolyzers that utilize low catalyst loading, elevated potential, and frequent cycling with square- and triangle-waves to accelerate anode catalyst layer degradation during intermittent operation. Kinetics drive performance losses (ohmic/transport secondary) and are accompanied by decreasing exchange current density, decreasing cyclic voltammetric capacitance, and increasing polarization resistance. Decreased kinetics are likely due to a combination of iridium (Ir) migration into electrochemically inaccessible locations in the anode or membrane, Ir particle growth (supported by X-ray scattering), changes in the extent of the Ir oxidation state (supported by X-ray absorption spectroscopy), and anode catalyst layer reordering. Decreasing catalyst/transport layer contact and catalyst/membrane interfacial tearing may add contact resistances and account for increasing ohmic losses. Performance losses for low and moderate catalyst loading, as well as from accelerated and model wind/solar cycling protocols, were likewise dominated by kinetics but vary in severity. Accelerated cycling (1 cycle per minute) appears to reasonably accelerate relevant loss mechanisms and can be used to project electrolyzer lifetime from anode deterioration. Ongoing accelerated stress test development and studies into performance loss mechanisms will continue to be critical as electrolysis shifts to intermittent power and low-cost applications.

Mn-Dopant Differentiating the Ru and Ir Oxidation States in Catalytic Oxides Toward Durable Oxygen Evolution Reaction in Acidic Electrolyte.

Designing an efficient and durable electrocatalyst for the sluggish oxygen evolution reaction (OER) at the anode remains the foremost challenge in developing proton exchange membrane (PEM) electrolyzers. Here, a highly active and durable cactus-like nanoparticle with an exposed heterointerface between the IrO2 and the low oxidation state Ru by introducing a trace amount of Mn dopant is reported. The heterostructure fabrication relies on initial mixing of the Ru and Ir phases before electrochemical oxidation to produce a conjoined Ru/IrO2 heterointerface. Benefitting from electron transfer at the heterointerface, the low oxidation state Ru species shows excellent initial activity, which is maintained even after 180 h of continuous OER test. In a half-cell test, the Mn-doped RuIr nanocactus (Mn-RuIr NCT) achieves a mass activity of 1.85 A mgIr+Ru -1 at 1.48 VRHE , which is 139-fold higher than that of commercial IrO2 . Moreover, the superior electrocatalytic performance of Mn-RuIr NCT in the PEM electrolysis system ensures its viability in practical uses. The results of the excellent catalytic performance for acidic OER indicate that the heterostructuring robust rutile IrO2 and the highly active Ru species with a low oxidation state on the catalyst surface drive a synergistic effect.

Amorphous p‐Type Conducting Zn–xIr Oxide (x > 0.13) Thin Films Deposited by Reactive Magnetron Cosputtering

Zinc–iridium oxide (Zn–Ir–O) thin films have been demonstrated as a p‐type conducting material. However, the stability of p‐type conductivity with respect to chemical composition or temperature is still unclear. In this study we discuss the local atomic structure and the electrical properties of Zn–Ir–O films in the large Ir concentration range. The films are deposited by reactive DC magnetron co‐sputtering at two different substrate temperatures—without intentional heating and at 300 °C. Extended X‐ray absorption fine structure (EXAFS) analysis reveals that strongly disordered ZnO4 tetrahedra are the main Zn complexes in Zn–Ir–O films with up to 67.4 at% Ir. As the Ir concentration increases, an effective increase of Ir oxidation state is observed. Reverse Monte Carlo analysis of EXAFS at Zn K‐edge shows that the average Zn–O interatomic distance and disorder factor increase with the Ir concentration. We observed that the nano‐crystalline w‐ZnO structure is preserved in a wider Ir concentration range if the substrate is heated during deposition. At low Ir concentration, the transition from n‐ to p‐type conductivity is observed regardless of the temperature of the substrates. Electrical resistivity decreases exponentially with the Ir concentration in the Zn–Ir–O films.

X-Ray Absorption Spectroscopy Studies of Ir-Oxide Based Oxygen Evolution Catalysts Revisited

Hydrogen plays a pivotal role in the future decarbonization of the energy sector [1], but its production in a sustainable way is crucial for it to have a positive environmental impact. Due to its high power density, quick start-up and response time as well as pressurized hydrogen delivery, polymer electrolyte membrane water electrolysis (PEMWE) is perfectly suited to facilitate H2 production from renewable electricity [2]. However, at large scale application, the use of scarce and expensive, Ir-based catalysts in PEM water electrolyser anodes limits the proliferation of this technology [3, 4]. For this reason, significant research activities are being conducted to better understand the oxygen evolution reaction (OER) mechanism on benchmark IrO2 based catalysts, as to further improve catalyst design and decrease the efficiency losses related to this reaction.When trying to elucidate a reaction mechanism, the use of operando and/or in situ techniques is often beneficial, and thus in last decades the OER mechanism on Ir-based materials has been extensively investigated through operando / in-situ X-ray absorption as well as X-ray photoelectron spectroscopy (XAS, XPS) [5-10]. Despite enormous efforts in this research area, there is still a lack of consensus regarding the Ir oxidation state under OER conditions and the nature of the OER-active sites. In line with this, in this study we employed operando XAS to investigate the Ir oxidation state in commercial IrO2 (Umicore®) catalyst under OER polarization curve conditions. These XAS measurements were performed both in transmission and fluorescence modes; for the former, electrodes with loadings ≈ 40 fold higher than those used in rotating disk electrode (RDE) measurements were required in order to achieve appropriate XAS data quality. In fluorescence mode, however, thinner electrodes with only four times higher loadings than in RDE could be investigated. Prior to the operando XAS measurement, the utilization of these IrO2 catalyst layers in the operando flow cell (FC) was assessed to ensure that the acquired spectroscopic data was representative of the processes occurring under real conditions. Achieving a good catalyst layer utilization was especially important in the case of the thick electrodes used in transmission XAS measurement, since their resistance is much higher compared to that in thin film RDE measurements. The so obtained results were additionally affected by beam damage of the Nafion ionomer used as the catalyst layer binder. All these factors can further lead to data misinterpretation. Regarding the beam damage, reducing the beam flux helped to prevent this effect and allowed us to obtain reliable results. Additionally, the use of thin catalyst layers in the operando FC and its combination of time-resolved XAS (so called “quickXAS”) in fluorescence mode led to novel insight on the effects of evolved O2 bubbles on the spectroscopic results and subsequent interpretation thereof.In summary, this contribution will provide an overview of common pitfalls and good practices encountered when conducting operando XAS studies of OER-electrocatalysts.

Confined Ir single sites with triggered lattice oxygen redox: Toward boosted and sustained water oxidation catalysis

Confined Ir single sites with triggered lattice oxygen redox: Toward boosted and sustained water oxidation catalysis

Glycine-induced ultrahigh-surface-area IrO2@IrOx catalyst with balanced activity and stability for efficient water splitting

Polymer electrolyte membrane water electrolysis (PEMWE) uses intermittent renewable energy and plays a key role in storing energy in the form of hydrogen. However, its widespread application is limited owing to issues associated with oxygen evolution reaction (OER) catalysts, including activity and stability. Thus, it is necessary to develop highly active and durable catalysts in this regard. Herein, an IrOx@IrO2 catalyst with an ultrahigh surface area (G-450) was synthesized by a simple Adams fusion method and calcined at 450 °C with glycine as an additive. Owing to its micro/mesoporous structure, this catalyst exhibited an ultrahigh specific surface area (SSA) of 403 m2 g-1 and an amorphous structure with average oxidation states of Ir(IV). A trade-off between its OER activity and stability was achieved by controlling its SSA and Ir oxidation states via the optimization of the calcination temperature. Surface-rich Ir(III) species and high SSA enhanced the OER activity of G-450 (309 mV overpotential at 10 mA cm-2) compared with the IrO2 catalyst prepared without glycine (A-450, 351 mV overpotential at 10 mA cm-2). Further, the G-450 exhibited an Ir dissolution rate that was 2.5-fold lower than that of the IrO2 catalyst prepared at 350 °C (G-350) after 6 h chronopotentiometry at 10 mA cm-2, implying a higher stability owing to the presence of Ir(IV) species. Additionally, G-450 was introduced into the anode of the polymer electrolyte membrane (PEM) water electrolyzer single cell and demonstrated higher performance than A-450. The balanced activity and stability of IrOx@IrO2 enhanced the PEM water electrolyzer performance. Moreover, this facile synthetic method is also applicable to the synthesis of binary oxide compounds and other transition metal oxides.

Constant Change: Exploring Dynamic Oxygen Evolution Reaction Catalysis and Material Transformations in Strontium Zinc Iridate Perovskite in Acid.

While iridium-based perovskites have been identified as promising candidates for the oxygen evolution reaction (OER) in proton exchange membrane (PEM) electrolyzer applications, an improved fundamental understanding of these highly dynamic materials under reaction conditions is needed to inform more robust future catalyst design. Herein, we study the highly active SrIr0.8Zn0.2O3 perovskite for the OER in acid by employing electrochemical experiments with in situ and ex situ characterization techniques to understand the dynamic nature of this material at both short and long time scales. We observe initial intrinsic OER activity improvement with electrochemical cycling as well as an initial increase of Ir oxidation state under OER conditions via in situ X-ray absorption spectroscopy. We discover that the SrIr0.8Zn0.2O3 perovskite experiences an OER-induced metal to insulator transition (MIT) with extensive electrochemical cycling, caused by surface reorganization and changes to the material crystallinity that occur with exposure to an acidic and oxidizing environment. Our novel identification of an OER-induced MIT for iridate perovskites reveals an additional stability concern for iridate catalysts which are known to experience material dissolution challenges; this work ultimately aims to inform future catalyst material design for PEM water electrolysis applications.