Iridium Oxide Research Articles

Enhanced electrochemical dissolution of iridium oxide in acidic electrolytes through presence of metal ions: shortened lifetime and hope for recovery

Cyclic deposition and stripping of metal ions (e.g. Cu2+) on Ir-oxides enhance electrochemical dissolution of Ir-oxides, likely due to weakening of the Ir–O–Ir bonds. The phenomenon is influential to IrOx catalyst stability and recovery perspective.

Functionalized orthopaedic implant as pH electrochemical sensing tool for smart diagnosis of hardware infection.

In the orthopaedic surgery field, the use of medical implants to treat a patient's bone fracture is nowadays a common practice, nevertheless, it is associated with possible cases of infection. The consequent hardware infection can lead to implant failure and systemic infections, with prolonged hospitalization, time-consuming rehabilitation treatments, and extended antibiotic therapy. Hardware infections are strictly related to bacterial adhesion to the implant, leading to infection occurrence and consequent pH decreasing from physiological level to acid pH. Here, we demonstrate the new strategy to use an orthopaedic implant functionalized with iridium oxide film as the working electrode for the potentiometric monitoring of pH in hardware infection diagnosis. A functional investigation was focused on selecting the implant material, namely titanium, titanium alloy, and stainless steel, and the component, namely screws and implants. After selecting the titanium-based implant as the working electrode and a silver wire as the reference electrode in the final configuration of the smart sensing orthopaedic implant, a calibration curve was performed in standard solutions. An equation equal to y = (0.76 ± 0.02) - (0.068 ± 0.002) x, R2 = 0.996, was obtained in the pH range of 4-8. Subsequently, hysteresis, interference, matrix effect, recovery study, and storage stability were investigated to test the overall performance of the sensing device, demonstrating the tremendous potential of electrochemical sensors to deliver the next generation of smart orthopaedic implants.

Benchmarking Stability of Iridium Oxide in Acidic Media under Oxygen Evolution Conditions: A Review: Part II

Part I () introduced state-of-the-art proton exchange membrane (PEM) electrolysers with iridium-based catalysts for oxygen evolution at the anode in green hydrogen applications. Aqueous model systems and full cell testing were discussed along with proton exchange membrane water electrolyser (PEMWE) catalyst degradation mechanisms, types of iridium oxide, mechanisms of iridium dissolution and stability studies. In Part II, we highlight considerations and best practices for the investigation of activity and stability of oxygen evolution catalysts via short term testing.

A multi-functional nano-platform based on LiGa4.99O8:Cr0.01/IrO2 with near infrared-persistent luminescence, "afterglow" photodynamic and photo-thermal functions.

Multi-functionalised nano-platforms based on persistent-luminescence nanoparticles (PLNPs) have attracted considerable attention for biomedical applications owing to their lack of background noise and suitability for in vivo imaging without the need for in situ excitation. However, nano-platforms based on PLNPs for continuous photodynamic therapy (PDT) are currently lacking. Herein, we report a nano-platform (LiGa4.99O8:Cr0.01/IrO2, LGO:Cr/IrO2) prepared using PLNPs (LiGa4.99O8:Cr0.01, LGO:Cr) covalently bonded with iridium oxide nanoparticles (IrO2 NPs), producing near-infrared (NIR) persistent luminescence, "afterglow" PDT and photo-thermal therapy (PTT) effects. The LGO:Cr/IrO2 not only exhibits NIR-persistent luminescence at 719 nm and a PTT effect under 808 nm irradiation but also a continuous "afterglow" PDT effect without the need for in situ excitation owing to persistent energy transfer from LGO:Cr to the IrO2 NPs, in turn generating reactive oxygen species (ROS). This multi-functional nano-platform is expected to further promote the application of PLNPs in tumour treatment.

A Comparative Study on the Effect of Substrate Structure on Electrochemical Performance and Stability of Electrodeposited Platinum and Iridium Oxide Coatings for Neural Electrodes.

Implantable electrodes are crucial for stimulation safety and recording quality of neuronal activity. To enhance their electrochemical performance, electrodeposited nanostructured platinum (nanoPt) and iridium oxide (IrOx) have been proposed due to their advantages of in situ deposition and ease of processing. However, their unstable adhesion has been a challenge in practical applications. This study investigated the electrochemical performance and stability of nanoPt and IrOx coatings on hierarchical platinum-iridium (Pt-Ir) substrates prepared by femtosecond laser, compared with the coatings on smooth Pt-Ir substrates. Ultrasonic testing, agarose gel testing, and cyclic voltammetry (CV) testing were used to evaluate the coatings' stability. Results showed that the hierarchical Pt-Ir substrate significantly enhanced the charge-storage capacity of electrodes with both coatings to more than 330 mC/cm2, which was over 75 times that of the smooth Pt-Ir electrode. The hierarchical substrate could also reduce the cracking of nanoPt coatings after ultrasonic, agarose gel and CV testing. Although some shedding was observed in the IrOx coating on the hierarchical substrate after one hour of sonication, it showed good stability in the agarose gel and CV tests. Stable nanoPt and IrOx coatings may not only improve the electrochemical performance but also benefit the function of neurobiochemical detection.

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

Hydrous Iridium Oxide Thin Films: Revealing the Structure of a Highly Active Model Catalyst By in-Situ Ir L 3-Edge XANES and EXAFS

Currently, only iridium oxide shows favourable activity and stability under the harsh oxygen evolution reaction (OER) conditions for application in proton exchange membrane water electrolysis (PEM-WE) [1-3]. There is a well established inverse relationship between OER activity and stability of iridium oxide-based OER catalysts given its degree of crystallinity and hydration [4-6]. Crystalline, anhydrous iridium oxides are more stable but have diminished OER activity compared to amorphous, hydrous iridium oxides, which are more active. Different OER mechanisms have been theorized to occur on these materials, explaining their difference in performance [6]. Several spectroscopic investigations have been performed to explore the properties of such different iridium oxides, in particular to identify the chemical and electronic state of their active sites. However, a consensus regarding the active site structure and OER mechanism has yet to be reached [7-10].Using scanning electron microscopy, x-ray photoelectron spectroscopy, and combining in-situ Ir L 3-edge x-ray absorption spectroscopy with density functional theory (DFT) calculations and ab initio thermodynamics, we have investigated in-situ electrochemically grown porous hydrous iridium oxide thin films (HIROF) as a model system to examine the chemical and electronic structure of the highly active, hydrous iridium oxide species. In-situ extended x-ray absorption fine structure(EXAFS) results show that HIROF grows preferentially in a form most often associated with the OER catalytically active site of hydrous iridium oxides. Calculations over different possible structures allowed us to identify a unique structural group with enhanced hydrogenation that best fits the EXAFS data. In-situ x-ray absorption near edge structure (XANES) results reveal a lower onset potential for the redox behaviour of HIROF compared to rutile IrO2. Based on this study, we propose a new structural model explaining the high activity and poorer stability of hydrous iridium oxides compared to crystalline IrO2.

Understanding the Role of Electronic Interactions in Oxide–Supported Iridium Oxide Catalysts for the Oxygen Evolution Reaction

As our society seeks increased energy security and resilience, the need for efficient, cost-effective, carbon-free hydrogen generation will continue to grow. Proton exchange membrane (PEM) water electrolysers are a promising technology to produce “green” hydrogen, however, at present, their large-scale commercialization is hindered by the high capital costs of critical components. The anodic membrane electrode assembly requires large quantities of iridium (Ir) to facilitate the kinetically demanding oxygen evolution reaction (OER). Owing to its scarcity, iridium is a severe bottleneck and expense in the widespread implementation of these technologies. The iridium content in electrolyser anodes can be lowered substantially by designing catalysts which consist of finely dispersed nanoparticles of iridium dioxide affixed onto suitable support materials. This approach enhances the active surface area and operational lifetime of these electrocatalysts towards the OER while utilising a fraction of the otherwise required iridium. To reap these benefits, the support needs to be a high surface area, corrosion-resistant material that is a good electronic conductor1,2. Metal oxides have dominated as oxygen evolution support materials for PEM electrolysis; however, they typically need to be doped with an impurity to demonstrate sufficient electronic conductivity. Iridium oxides supported on antimony−doped tin oxide (ATO) support have been found to behave as high-performing OER electrocatalysts in acidic conditions3,4. While there is no doubt that existing studies prove a link between the presence of ATO support and the improved performance of ATO-supported iridium oxides versus unsupported catalysts, an in-depth characterisation of the charge transfer between the ATO support and deposited IrO2 nanoparticles could prove useful in understanding the reason for these performance enhancements on the basis of the electronic properties of the catalyst and the support.In this study, we investigated the performance of highly dispersed IrO2 nanoparticles, supported on doped tin oxide supports, consisting of various metallic dopants and iridium loadings. The electronic interactions between the Ir-phase and the support were studied by measuring the valence band spectra of the prepared materials with X-ray and ultraviolet photoelectron spectroscopy, and Hall measurements were utilised to understand the nature of electronic conductivity in the prepared materials. The uncovered physicochemical and electronic characteristics are related to the electrochemical performance of these materials towards the OER. These novel findings contribute insights which can be used as the basis for the rational design of next-generation oxide-supported Ir-based catalysts with tunable electronic properties for sustainable PEM electrolysis applications.

Effect of Electrolyte Anions on the Activity of Iridium Oxide Catalysts for Water Electrolysis

Developing environmentally friendly and renewable energy sources is essential due to the environmental and energy security issues caused by traditional combustion of fossil fuel-based energy sources. Therefore, as a clean and sustainable energy carrier, hydrogen has attracted great attention in recent years. In this regard, water electrolysis utilizing renewable energy sources to produce hydrogen is considered one of the most promising technologies. It is believed elucidation of the reaction mechanism of highly active iridium oxides is promising for the further development of cost-efficient PEM water electrolysis catalysts. The properties of the MEA could be affected by many parameters such as the catalysts, operating temperature, and pressure1,2. Another important factor is the cation impurities that can be found in the circulating water, which could severely degrade the MEA performance during the operation of a PEM water electrolyze. However, there are less reports on anion impurities such as SO4 2- and PO4 3-. Therefore, in this study, the relationship between electrolyte anions and OER activity was investigated for IrOx catalysts with different crystal structure. By using operando X-ray absorption spectroscopy (XAS), we investigated the electronic state of Ir and O under operation potential.IrOx with different degree of crystallinity (SA3.5 and SA58) were provided by Tanaka Kikinzoku Kogyo K.K., Chemical & Refining Company and named after their BET surface areas. These two samples were then tested for electrochemical properties in different concentrations of phosphoric acid and perchloric acid, respectively. The electrochemical measurements were performed using a standard three-electrode cell connected to an MPG-205-NUC system (Bio-Logic). The cyclic voltammograms (CV) were scanned from 0.1 V to 1.2 V vs. RHE at 50 mV s-1. The electrochemical activities of the catalysts were subsequently investigated using linear sweep voltammetry (LSV) in the range of 1.1 to 1.7 V vs RHE at 10 mV s−1. operando XAS were collected using a home-made flow-type cell, where Pt wire and RHE worked as counter and reference electrodes, respectively. The catalyst ink was coated onto an Nafion membrane to prepare both the X-ray window and working electrode. The electrolyte was circulated at the flow rate of 100 mL min-1.The catalytic performance of IrOx was investigated at different types and content of electrolyte under H3PO4 and HClO4. Figure 1 shows the CVs of SA3.5 and SA58 under different types and content of electrolyte. In the presence of phosphate anions, SA3.5 showed a tendency to decrease Ir redox between 0.6 and 1.1 V with increasing electrolyte concentration. On the other hand, there were no obvious changes in SA58. It was found that IrOx catalysts have an great effect of specific adsorption of anions and low symmetry of monoclinic phase catalysts (SA 3.5) are more susceptible to anion poisoning. Figure 2 shows the result of examining the poisoning mechanism using soft X-ray XAS. O K-edge results present the formation of µ2-O (O-O) bond at around 528.7 eV, which was previously observed by Nong, H. N. et. al. through operando O-K edge measurements during OER process3. The peak intensity at 528.7 eV of the amorphous samples was in the same order as the OER activities. According to potential dependence change of the pre-edge peak, under the presence of phosphate anions, the hole formation to Ir-O hybrid orbitals was suppressed in SA3.5. This result indicates that the anion adsorbs to the active site, poisoning the catalyst surface and inhibiting the reaction. Acknowledgements This work is based on results obtained from a project (JPNP14021) commissioned by the New Energy and Industrial Technology Development Organization (NEDO) of Japan. References Zhang, Y.; Zhu, X.; Zhang, G.; Shi, P.; Wang, A.-L.; et al. J. Mater. Chem. A: 2021, 9, 5890-5914.Chen, Z.; Duan, X.; Wei, W.; Wang, S.; Ni, B.-J.; et al. Nano Energy: 2020, 78, 105270.Nong, H. N.; Falling, L. J.; Bergmann, A.; et al. Nature 2020, 587 (7834), 408-413. Figure 1

ALD-Based IrOx/Ir Supported Electrocatalysts for Water Electrolysis Technology

Hydrogen (H2) is an alternative energy vector to replace fossil fuels, with potential applications like hydrogen fuel cell-powered vehicles and in the chemical industry. H2 production through polymer electrolyte membrane (PEM) water electrolysis (WE) appears to be a highly promising technology. While iridium oxide is the most commonly used anode oxygen evolution reaction (OER) catalyst for PEMWE, its scarcity makes it an extremely critical raw material. In order to obtain active and stable anodes with an ultra-low iridium loading, we are developing a strategy based on doped metal oxides as conductive supports, on which iridium-based thin films and/or nanostructures are deposited using atomic layer deposition (ALD). ALD can indeed allow the control of the amount of deposited species at sub-nanometric scale by changing the number of ALD cycles, thus ensuring a minimal iridium loading for efficient OER catalysis. In this study, the influence of the temperature of deposition was investigated to obtain either IrOx ultrathin films or Ir nanoparticles. The morphology and composition of these deposits were characterized by scanning and transmission electron microscopy, X-ray photoelectron spectroscopy and X-ray diffraction. The nature and crystallinity of these electrocatalysts will indeed have a huge impact on their catalytic activity and the subsequent electrode’s efficiency as well as in their long-term durability. In brief, we will present the production of nano-structured OER electrocatalysts, in which the bulk consists of a sufficiently conductive metal oxide support with the surface uniformly coated with an ultra-low loading of iridium deposited via conformal ALD.

Activity and Durability of Mn Binary Oxide-Based Electrocatalyst for an Alternative Anode

Compared with alkaline water electrolysis (AWE), proton exchange membrane water electrolysis (PEMWE) has several advantages such as high current density, compact system and high purity of hydrogen gas. However, a conventional anode of PEMWE utilizes precious metal oxide such as iridium oxide (IrO2) with high cost and low resources. Recently, it has been reported that the performance of low-loading IrO2 anodes deteriorates due to the potential fluctuations simulated variable renewable energies [1-2]. Thus, it is required to develop a low-cost catalyst with high durability against the potential fluctuations. We have focused on the tantalum oxide-based electrocatalyst because of its low cost and high chemical stability in acid. In this study, we have investigated on the catalytic activity and durability of Mn added Ta oxide-based thin film (Mn-TaOx) for the alternative anode of PEMWE. For the purpose of improvement of durability, Mn-TaOx coated with ZrO2 fabricated by the Atomic Layer Deposition (ALD) was also prepared to evaluate the durability of potential cycling.A Ti rod was used as the substrate, and Mn-TaOx was prepared by RF magnetron sputtering deposition. Mn disk was used as the sputtering target, and the Ta plate was placed on the Mn target to prepare Mn-TaOx. Mn addition was fixed at 50 at%, and it was controlled by the area ratio considering sputtering rate of Mn and Ta metal. Partial pressure of Ar and O2 gas were kept constant at 0.23 Pa for 20 min during the sputtering. The substrate heating temperature was constant at 673 K during the sputtering. The prepared Mn-TaOx electrode was coated with ZrO2 fabricated by ALD (ZrO2 coating Mn-TaOx). The thickness of coating ZrO2 was varied in the range of 0.5 to 2.0 nm. All electrochemical measurements such as oxygen evolution reaction (OER) was used by conventional three electrode cell in 1 M H2SO4 at 303 K. In addition, electrochemical impedance spectroscopy (EIS) was also performed to estimate several factors such as charge transfer resistance (R ct), film resistance (R film) and so on. The protocol of potential cycling as a durability test was set the potential in the range of 1.5 to 2.0 V with 50 mV s-1 for 10000 cycles, and the slow scan voltammetry was conducted every 2000 cycle in the range of 1.2 to 2.0 V with 5 mV s-1 to compare the OER activity before and after test.The OER current of ZrO2 coating Mn-TaOx was smaller than that of Mn-TaOx without coating (Mn50-TaOx (0 nm)). In particular, the OER current of Mn-TaOx coated with 2 nm of ZrO2 (“2 nm”) was greatly decreased because the film thickness of ZrO2 is too thick so that it has low electric conductivity. Furthermore, the APD-ZrO2 covered on the surface, and it is also hard to react with reactant at active site. From results of EIS, the R ct and R film of “2 nm” increased. On the other hand, the OER current of Mn-TaOx coated with 1 nm of ZrO2 (“1 nm”) did not drastically decrease compared to Mn-TaOx and it was suggested that “1 nm” relatively maintained its activity.Figure 1 shows the geometric current density (i geo) at 2.0 V of ALD-ZrO2 coated on Mn-TaOx after potential cycling. The i geo at 2.0 V of Mn-TaOx without coating also shows in Fig. 1. The i geo of Mn50-TaOx (0 nm) was decreased about half value after 2000 cycles of durability test, but that of Mn-TaOx coated with ZrO2 did not decrease significantly even after 2000 cycles of potential cycling. In particular, the i geo of “1 nm” was the highest current after the 2000 cycles of potential cycling in this study, and the degradation didn’t observe in this cycle range. According to the results from XPS spectrum, a small amount of ZrO2 was detected on the surface of “1 nm” after the potential cycling. The redox peak of Mn(Ⅲ)/(Ⅳ) [3] observed at the ALD-ZrO2 coated on Mn-TaOx was also detected after the potential cycling, so that the Mn species as an active site of Mn-TaOx for the OER was still remained. Therefore, the reason why the “1 nm” was relatively stable is that the amounts of both active and protect site have maintained during the durability test.AcknowledgementThis work is partially supported by Suzuki Foundation.References(1) Ayers, Curr. Opin. Electrochem., 18, 9 (2019).(2) M. Alia, and G. C. Anderson, J. Electrochem. Soc., 166, F282 (2019).(3) Morita, C. Iwakura, and H. Tamura, Electrochim. Acta, 24, 357 (1979). Figure 1

Oxygen Evolution Reaction Activity of Iridium Oxide with Strange Structure, Its Toward PEM Water Electrolysis

Water electrolysis is a promising approach of hydrogen mass production.PEM water electrolysis has advantages such as high current density operation and quick response against input electricity.From these advantages, PEM water electrolysis is adequate to the hydrogen mass production system adopted the renewable energy.Iridium oxide shows good oxygen evolution reaction(OER) activity and durability for PEM water electrolysis anode catalyst, but there are material supply and cost issues as it is precious metal.To solve these issues, a number of studies have been conducted to improve OER activity of Iridium oxide.In this work, the crystal structure of iridium oxide is focused in order to improve OER activity.As a result of several synthesis survey, we have synthesized iridium oxide with unique structure and it shows more than four times OER activity compared to commercial catalysts in RDE.The mechanism for the high OER activity would be discussed based on the results of electrochemical properties, spectroscopy, and structural analysis.

(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

Towards Low Iridium Loading in Proton-Exchange Membrane Water Electrolysis: Full-Cell Performance of IrOx@TiO2 Core-Shell Particles in Anode Catalyst Layers

Hydrogen as a promising energy carrier to bridge the gap between supply and demand of renewable energies can be produced by proton-exchange membrane water electrolysis (PEMWE). However, the scarcity of iridium limits the widespread and large-scale implementation of PEMWE. Thus, the state-of-the-art iridium loading for the oxygen evolution reaction (OER) of ~1-2 mgIr·cm-2 must be distinctly reduced.1, 2 A decrease in loading involves a reduction of the catalyst layer (CL) thickness. The reduction is, however, only possible down to a certain threshold (≤ 0.5 mgIr·cm-2), below which the CL shows cracks leading to incomplete connection of the catalyst particles.1, 3 To overcome this issue, we developed a novel catalyst structure with reduced iridium content (< 50 wt% Ir). The synthesized catalyst exhibits a core-shell structure consisting of an insulating TiO2 core (d ≈ 1 µm) coated with iridium oxide (IrOx@TiO2). This optimized structure enables a reduction of the loading while maintaining the thickness and, thus, the electrical connection within the CL.In two related contributions, we demonstrate the suitability of the catalyst by a comprehensive analysis: The first contribution mainly focuses on structural optimization, activity, and stability investigations of the catalyst powder. In this contribution, we optimize the catalyst layer aiming towards high catalyst utilization and in-plane conductivity at iridium loadings below 0.5 mgIr·cm-2. PEMWE full-cell performance is investigated in catalyst-coated membrane (CCM) configuration and the structure and thickness of the CL are characterized by SEM cross-sections. The results of the full-cell tests with our catalyst in comparison to a state-of-the-art IrO2/TiO2 catalyst are shown in Fig. 1. The polarization curve of our catalyst exhibits high OER activity and performance at low loadings (Graph A: 1.76 V @ 1.5 A·cm-2). In comparison to the state-of-the-art catalyst (Graph B), our catalyst outperforms the commercial catalyst in terms of activity. This becomes evident when comparing the HFR-free polarization curves in the low current density region. It could be assigned to a higher catalyst utilization due to the maintained CL thickness or, more likely, to a higher activity of the catalyst itself.1 The performance difference between our and the state-of-the-art catalyst in higher current density regions mainly stems from ohmic losses in the electrode as can be seen from the high frequency resistance (HFR) in Fig. 1. These ohmic losses, however, can supposedly be assigned to the material itself and not to a decreased in-plane conductivity due to a disconnection of parts of the CL.In conclusion, we successfully synthesized an IrOx@TiO2 catalyst with a core-shell structure. This catalyst enables the fabrication of catalyst layers with decreased loadings below 0.5 mgIr·cm-2 while maintaining the catalyst utilization demonstrated in full-cell testing. Therefore, our newly developed catalyst contributes to a scale-up of the PEMWE technology by drastically reducing the demand for the scarce element iridium.Figure 1: Polarization curves, HFR-free polarization curves and HFR values at 80°C (ambient pressure, 100 mlH2O·min-1 on anode) of CCMs fabricated with different anode catalyst types and loading. A: in-house core-shell catalyst with 0.2 mgIr·cm-2 and B: commercial catalyst with 0.3 mgIr·cm-2 (IrO2/TiO2 with 75 wt% Ir, Umicore).References M. Möckl, M. F. Ernst, M. Kornherr, F. Allebrod, M. Bernt, J. Byrknes, C. Eickes, C. Gebauer, A. Moskovtseva and H. A. Gasteiger, J. Electrochem. Soc., 169(6), 64505 (2022).C. Minke, M. Suermann, B. Bensmann and R. Hanke-Rauschenbach, Int. J. of Hydrog. Energy, 46(46), 23581–23590 (2021).M. Bernt, A. Siebel and H. A. Gasteiger, J. Electrochem. Soc., 165(5), F305-F314 (2018). Figure 1

Investigating the Reaction Mechanism of Iridium Oxides for PEM Water Electrolysis by Operando Soft X-Ray Absorption Spectroscopy

With the increase in fossil energy consumption and the attendant pronounced climate changes, the call for sustainable energy forms has become urgent and has attracted more and more attention.1 Hydrogen (H2), generated by the electricity-derived water electrolysis, is now considered the promising energy carrier for conversion between electricity and chemical energy.2 The most developed water electrolysis technology up to date is the proton-exchange membrane (PEM) water electrolysis, where commercial Pt/C works as the cathodic catalyst for hydrogen evolution reaction (HER) and noble metal-based materials serve as the anodic catalyst for oxygen evolution reaction (OER).3 Considering the sluggish four-electron reaction kinetics of OER and the harsh acidic environment of PEM water electrolysis, the selectivity of high-efficient OER catalysts is narrowed down to Ir-based catalysts because of their superior activity, long-term stability and corrosion resistance against acidic environment.4 However, the scarcity of Ir on earth has impeded the wide application of PEM water electrolysis and has driven the research of cost-efficient OER catalysts. In this study, we thoroughly investigated several amorphous IrOx catalysts with different degree of crystallinity (SA103, SA58, SA14.6 and SA58) and revealed the relation between their structures and OER activities. Besides, BET surface areas, X-ray diffraction (XRD), Transmission electron microscopy (TEM), scanning TEM (STEM), PDF, and XAS were measured to thoroughly understand the catalytic properties.XRD patterns in Figure 1(a) showed that compared with the well-crystallized IrO2, SA58 sample showed slightly broad peak, while the diffraction peaks were much broad of other samples, indicating their amorphous structures. The amorphous structure of IrOx was characterized through atomic pair distribution function (PDF) analyses as shown in Figure 1(b). Different from the typical tetragonal symmetry of crystal IrO2, the combination of orthorhombic symmetry and monoclinic symmetry was found in the amorphous IrOx. Further, OER activities of all samples were investigated in 0.1 M HClO4 and the current density was calculated against the BET surface areas to examine the intrinsic catalytic activity (Figure 1(c)). The crystallized SA58 sample showed the lowest activity, while SA3.5 sample outperformed all other amorphous oxides. From PDF, it was found that the highest OER active SA3.5 sample exhibited the highest monoclinic phase ratio content. The catalytic structure properties were further evidenced by the XAS results. EXAFS results showed different electronic environment of monoclinic phase samples and further fitting results were in good agreement with the PDF analyses. Moreover, O K-edge results showed the formation of µ2-O (O-O) bond at around 528.7 eV, which was previously observed by Nong, H. N. et. al. through operando O-K edge measurements during OER process.5 It was concluded that that the low symmetry of monoclinic phase in amorphous samples resulted in the structure defection, and thus lead to large amount of active electrophilic OI - species, which boosting the OER activity. This finding provided fundamental understanding of the amorphous iridium oxides and could promisingly shed light on the future OER catalyst design. Acknowledgements This work is based on results obtained from a project (JPNP14021) commissioned by the New Energy and Industrial Technology Development Organization (NEDO) of Japan. References Götz, M.; Lefebvre, J.; Mörs, F.; McDaniel Koch, A.; Graf, F.; Bajohr, S.; Reimert, R.; Kolb, T., Renewable Power-to-Gas: A technological and economic review. Renew. Energy 2016, 85, 1371-1390.Dincer, I.; Acar, C., Review and evaluation of hydrogen production methods for better sustainability. Int. J. Hydrogen Energy 2015, 40 (34), 11094-11111.Carmo, M.; Fritz, D. L.; Merge, J.; Stolten, D., A comprehensive review on PEM water electrolysis. Int. J. Hydrogen Energy 2013, 38 (12), 4901-4934.An, L.; Wei, C.; Lu, M.; Liu, H.; Chen, Y.; Scherer, G. G.; Fisher, A. C.; Xi, P.; Xu, Z. J.; Yan, C.-H., Recent Development of Oxygen Evolution Electrocatalysts in Acidic Environment. Adv. Mater. 2021, 33 (20), 2006328.Nong, H. N.; Falling, L. J.; Bergmann, A.; Klingenhof, M.; Tran, H. P.; Spöri, C.; Mom, R.; Timoshenko, J.; Zichittella, G.; Knop-Gericke, A.; Piccinin, S.; Pérez-Ramírez, J.; Cuenya, B. R.; Schlögl, R.; Strasser, P.; Teschner, D.; Jones, T. E., Key role of chemistry versus bias in electrocatalytic oxygen evolution. Nature 2020, 587 (7834), 408-413. Figure 1

(Invited) Fundamental Structure-Function Relationships for Iridium Oxide Catalysts in PEM Water Electrolyzers

The intermittency of renewable energy poses significant challenges to developing a grid system that utilizes primarily renewable-based sources. Proton exchange membrane water electrolyzers (PEMWEs) can convert surplus energy from the grid into chemical energy in the form of green hydrogen, which can then be used to generate electricity when the renewable supply is low. Despite much technological advancement over the last two decades, PEMWEs still face many barriers to widespread commercialization. Around a quarter of the cost of the PEMWE system is attributed to iridium, a highly scarce material used as the oxygen evolution reaction (OER) catalyst on the anode side. To enable lower loadings/higher utilization of these catalysts, fundamental studies on iridium activity and degradation are necessary. As it stands in the literature, studies on how physical properties of the catalyst (i.e. crystallinities, valance states, etc.) affect PEMWE performance are scarce. This work aims to elucidate the relationship between structural properties and efficiency/performance for iridium oxide (IrOx ) catalysts in PEMWEs. Five commercially available iridium oxide catalysts were characterized (47491 Alfa Aesar, TEC77100, TEC77110, FIO-01, FIO-11) using transmission electron microscopy (TEM), scanning electron microscopy (SEM), x-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), Brunauer–Emmett–Teller (BET) analysis, and x-ray diffraction (XRD) to obtain crystallinities, particle size distributions, specific surface areas, thermal stabilities, elemental compositions, and valance states. Afterwards, kinetic performance was baselined by taking cyclic voltammograms (CVs) via rotating disk electrode (RDE) experiments and electrochemical surface areas (ECSA) were approximated by using a novel analysis developed in this work that can relate the hysteresis in OER potentials to a double layer capacity. Results show that greater amorphous character of IrOx was correlated with increased kinetic performance at the expense of electrochemical stability across redox potentials between 0-1.6V. From the cyclic voltammograms, the catalyst with the most amorphous character underwent multiple (two, possible three) redox transitions, while the one with the most crystalline character showed no evidence of any redox transitions. It was also found that increased ECSA was correlated with greater physical surface areas obtained from BET analysis and that greater amorphous character resulted in a higher Ir III :Ir IV ratio. For future work, we aim to incorporate these fundamental structure-function insights to create standardized accelerated stress tests for PEMWEs, as standards for durability testing have yet to be established.

Activity and Durability of ALD-ZrO2 Coated IrO2 Film in Sulfuric Acid

The technology of power-to-gas (P2G) is drawing attention in and outside Japan for its ability to store renewable energy as hydrogen [1]. For the aspect of P2G, the water electrolyzer plays important role to produce “Green Hydrogen” [2]. On Sep. 26, 2022, Hydrogen energy ministerial meeting was held as online special event. In the open session, the water electrolysis is one of the main topics of the session, and due to the demand for “Green Hydrogen”, installed capacity of water electrolyzer has been required for the huge size of its module [3].In Japan, proton exchange membrane water electrolysis (PEMWE) with small size produced by Kobelco Eco-Solutions Corp have released already installed over 200 units for 25 years. Recently, the PEMWE with MW-class as installed in Yamanashi Hydrogen Company has also applied for the demonstration project [3]. However, the conventional electrocatalysts for PEMWE uses precious metal materials, and especially, the iridium oxide (IrO2) is used as an anode. As the iridium is by-product of platinum, its resources is very poor. Moreover, it is reported that the low loading of IrO₂ easily causes the degradation under dynamic operation condition simulated variable renewable energies (VRE) [4]. From this point of view, the durability of anode against VRE should be required for “Green Hydrogen” production. We focused on ZrO2 which has high stability in acidic solution [5]. Moreover, we found that an oxide-based film which is formed by Atomic Layer Deposition (ALD) showed bi-functional effects on the electrocatalyst for the oxygen reduction reaction; it can reflect the electrochemical properties of the substrate and it can also protect the consumption of the substrate by potential scan. In order to apply for PEMWE, we have investigated the catalytic activity of IrO2 electrodes deposited with ZrO2 using ALD for the oxygen evolution reaction (OER).IrO2 film on Ti rod as substrate was fabricated by thermal decomposition under air atmosphere at 400°C for 10 min. The loading amount of IrO2 was 2.35 mg cm-2 at constant. ZrO2 was deposited on IrO2 film by using ALD at 225°C, the film thickness of ZrO2 was varied in the range of 2 to 10 nm that calculated from the number of ALD cycles. After coating, the heat treatment was performed under air atmosphere at 400oC for 10 min. We used conventional three electrode cell with each sample as working electrode while the reversible hydrogen electrode (RHE) and carbon plate were used as reference and counter electrode to demonstrate the electrochemical measurement in 1 M H2SO4 at 303 K. After the pretreatment, the cyclic voltammetry was demonstrated in the range of 0.05 to 1.2 V. In order to evaluate the OER activity of samples, the slow scan voltammetry (SSV) was performed from 1.2 to 2.0 V vs. RHE.Figure 1 shows the polarization curves of OER on the ALD-ZrO2 of 5 nm coated on IrO2 with and without heat treatment (5nm, 5nm(HT)). The result of IrO2 with and without heat treatment (0nm, 0nm(HT)) are also shown in the Fig. 1. These results are IR-free data. The vertical axis is based on geometric current density (i geo). In the case of IrO2, the i geo of 0nm was larger than 0nm(HT). On the other hand, the i geo of 5nm(HT) was obviously larger than that 5nm in the case of ALD-ZrO2 of 5 nm coated on IrO2, and the i geo of 5nm(HT) at 1.55 V was almost same as that of 0nm(HT). The redox peak of Ir(IV)/Ir(V) or Ir(IV)/Ir(VI) was observed in the cyclic voltammogram (CV) of 5nm(HT) around 1.2 V. This peak also obtained in the CVs of both 0nm and 0nm(HT). However, it is not clearly shown in the CV of 5nm. From the results of SEM-EDX, the coating of ZrO2 of 5nm(HT) was remained after electrochemical measurement while the that of ZrO2 of 5nm was removed after electrochemical measurement. Therefore, the heat treatment after ALD coating was effective for enhancing OER activity.Acknowledgement: This work is partially supported by the Suzuki Foundation and JFE 21st Century Foundation.Reference Fuel Cell RD & D in Japan 2022, Fuel Cell Development Information Center (FCDIC), pp.8. Fuel Cell Development Information Center (FCDIC), Tolyo (2023).K. Ota, A. Ishihara, K. Matsuzawa, and S. Mitsushima, Electrochemistry, 78, 970 (2010).https://www.meti.go.jp/english/press/2020/1015_001.html M. Alia, and G. C. Anderson, J. Electrochem. Soc., 166, F282 (2019).A. Ishihara, Y. Ohgi, K. Matsuzawa, S. Mitsushima, and K. Ota, Electrochim. Acta, 55, 8005 (2010). Figure 1

Ionomer-Dependent Oxygen Evolution Reaction in a Half-Cell and a Liquid Electrolyzer

Electrocatalytic oxygen evolution reaction (OER) is an important area of research due to its significance in water electrolysis, which contributes to the clean-energy storage and conversion techniques. OER exhibits irreversible and sluggish kinetics, which necessitates a high overpotential to achieve the desired reaction rate. In an alkaline medium, non-precious metals (such as Ni, Fe, Co) or metal-free materials (such as 3d-transition metal oxides and heteroatom-doped carbons) can replace expensive iridium and ruthenium oxides, which is otherwise required in an acidic medium to facilitate the reaction. However, developing new catalysts for alkaline OER is limited by the scaling relations among adsorption energetics of key intermediates (i.e., OOH*, OH*, and O*).The electrochemical reaction kinetics depends critically on the nature of the interface between catalyst and the electrolyte where an electrical double layer (EDL) forms during OER. The current understanding of the EDL structure involves the interfacial interactions between adsorbates and catalyst layer (CL) which can be modified to improve the reaction efficiency. Ionomers are essential components of the CL and are vital in uniformly dispersing catalyst particles in ink solution. The ionomers also act as binder for the catalyst particles in the CL, helping to improve the electrode’s durability and stability over time. In addition, despite the fact that ionomers affect the properties of EDL, the impact of ionomer on the reaction pathway is often overlooked in electrocatalytic research.In this study, we investigate the role of ionomer in the catalyst layer by comparing different polymer dispersions, including PTFE, Nafion, and commercial As-4. The results indicate that the Nafion-contained CL performs better than other polymer CLs in TF-RDE/RRDE, as it facilitates direct O*-O* coupling. In contrast, anionic ionomer demonstrates superior long-term stability in alkaline liquid electrolyzers, despite oxidization degradation.

Effect of Operating Temperature and Pressure on the Limiting Current Density of PEM Electrolysis Cell Based on Theoretical Prediction Model and Experiments

Introduction The PEM electrolysis cell (PEMEC) exhibits great potential to produce hydrogen gas. However, the high cost associated with cell components, such as the iridium oxide catalyst and titanium porous transport layer, poses a challenge to the commercialization of PEMEC. So, continued research is needed to improve the cost-effectiveness of PEMEC. Operation at high current densities can minimize the electrode area and reduce capital expenses. However, high current density operation is limited by limiting current density (LCD), which causes a sharp increase in electrolysis voltage. At high current densities, the generation of oxygen gas bubbles creates additional flow restrictions to the water supply and increase overvoltage. Although previous studies have achieved relatively high current densities [1-2], the limit of operating current density and its mechanism still remain unknown.In this study, it reveals the impact of operating temperature and pressure on LCD of a lab-scale PEMEC. The experiments are conducted over a range of temperatures from 80 to 90 ℃ and pressures from 0.1 to 0.3 MPa. In conjunction with experiments, a mathematical model is developed to analyze the mechanism of LCD. The model includes mass and momentum equations for liquid water and oxygen gas through the porous transport layer. And the model also involves electrochemical equations to study overvoltages associated with mass transport. By combining experimental and theoretical analysis, this study provides valuable insights into the factors affecting LCD of PEMECs and facilitates the optimization of high-performance operation. Experimental apparatus The specifications of crucial components such as membrane (PEM), catalyst layer (CL), porous transport layer (PTL) is listed in Table.1 . Nafion NR212 is used as PEM, and the thickness is 51 μm. The catalyst is contained on CL, and its loading amount are 1.5 mg/cm2 IrO2 on anode and 0.5 mg/cm2 Pt on cathode, respectively. The anodic PTL is a Titanium mesh plated by Pt (Nikko Techno, NKT-1803-03), and the carbon paper (SGL 38BA) is used as cathodic GDL. For separators, the anode separator is fabricated from titanium, and the cathode separator is carbon. The anodic and cathodic flow pattern is designed as single-serpentine channels, and the channel depth, channel width, and rib width are 1 mm ×1 mm ×1 mm, respectively. Results and discussions Fig.1 shows the theoretical and experimental I-V curves. In Fig.1, the electrolysis voltage in both experiment and simulation raises abruptly, which is corresponding to LCD. The experimental LCD is 11.6 A/cm2 as indicated by the black circle, and the predicted LCD is 10.1 A/cm2. Although the prediction of LCD is qualitative, simulation could follow the IV characteristics obtained by experiment before LCD.Fig.2 and Fig.3 shows the effect of operating temperature and pressure on LCD. The electrolysis voltage “E” is experimentally determined, while the gas saturation at interface of PTL-CH “Sg” is obtained through simulation. As shown in Fig.2, the lower operation temperature enlarges the LCD, the experimental LCD increases 8% from 90 ℃ to 80 ℃. Theoretically, high operating temperature increases oxygen bubbles and then rises the gas saturation S g at the anode CL. So, the water supply flowing to CL is insufficient, and the electrochemical performance also decrease. Therefore, the overvoltages caused by mass transport becomes larger in higher operating temperature. As shown in Fig.3, higher operating pressure shrinks the volume of oxygen gas and enhance the water supply from CH to CL, leading to a larger LCD. Under 80 ℃, the experimental LCD rises by 17 % from 0.1 MPa to 0.3 MPa.Although the theoretical analysis reproduced the experimental results in a qualitative manner, the theoretical analysis suggests that, when electrolysis current density is quite close to the LCD, water mass transport at anode reaches a limitation, indicating that water saturation drops to zero at anodic catalyst layer. References Lee J K, et al. Cell Reports Physical Science, 2020:100147.A Zinser, et al. International Journal of Hydrogen Energy, 2019, 44(52): 28077-28087. Figure 1

Iridium-Cobalt Oxide from Modified Adam’s Fusion Synthesis Method as Electrocatalyst for Oxygen Evolution Reaction

Green hydrogen from water electrolysis is one of the leading energy carriers towards energy sustainability; however, cost reduction is necessary to make this technology more commercially viable. One of the most expensive components in an electrolysis cell are the iridium-based catalysts. To reduce cost, we propose to minimize iridium loading through alloying with a less-expensive cobalt (Co) metal. Here, we synthesized iridium-cobalt oxide (IrCo) using a surfactant-assisted Adam’s fusion synthesis technique as a scalable method. The in-house IrCo catalyst demonstrates improved performance than the commercial iridium oxide (IrOx_C) in both acidic and alkaline media. Further activity enhancement was achieved through acid etching (IrCo_ae), preferentially removing Co to generate more active sites. XPS and ICP-MS results reveal that the Ir:Co atomic ratio is increased from 1.0 to 5.5 after acid etching. The elemental distribution is also supported by EDX mapping. The effects of Ir/Co molar ratio, the use of surfactant and their interaction with the acid etching process were also investigated. The dependence of Ir valency on Ir/Co ratio and the possible influence of Ir3+ surface concentration to achieve high OER activity, especially in the acidic media, are highlighted in the study. In the absence of surfactant, the segregation of Ir and Co is much stronger forming a Co core and an Ir shell, and the OER activity is lower compared to its surfactant-assisted counterpart. The results confirm that surfactant improves the interaction of Ir and Co, leading to a better OER performance. This study proves an effective Ir utilization with the proposed synthesis method and could be tailored and enhanced to improve the catalyst stability for green hydrogen production.