Iridium Oxide Research Articles

Quantification of electrochemically accessible iridium oxide surface area with mercury underpotential deposition.

Research drives development of sustainable electrocatalytic technologies, but efforts are hindered by inconsistent reporting of advances in catalytic performance. Iridium-based oxide catalysts are widely studied for electrocatalytic technologies, particularly for the oxygen evolution reaction (OER) for proton exchange membrane water electrolysis, but insufficient techniques for quantifying electrochemically accessible iridium active sites impede accurate assessment of intrinsic activity improvements. We develop mercury underpotential deposition and stripping as a reversible electrochemical adsorption process to robustly quantify iridium sites and consistently normalize OER performance of benchmark IrOx electrodes to a single intrinsic activity curve, where other commonly used normalization methods cannot. Through rigorous deconvolution of mercury redox and reproportionation reactions, we extract net monolayer deposition and stripping of mercury on iridium sites throughout testing using a rotating ring disk electrode. This technique is a transformative method to standardize OER performance across a wide range of iridium-based materials and quantify electrochemical iridium active sites.

Cobalt Phosphide Decorating Metallic Cobalt With a Nitrogen‐Doped Carbon Nano‐Shell Surpasses Platinum Group Metals for Oxygen Electrocatalysis Applications

ABSTRACTIt has been a long‐standing challenge to cultivate capable and resilient oxygen electrocatalysts with higher activity, low price, and long lifetime to replace the commonly used platinum group metals, i.e., Pt for oxygen reduction reaction (ORR) and RuO2/IrO2 for oxygen evolution reaction (OER). This work presents a promising approach to address the challenges associated with oxygen electrocatalysis by introducing a cobalt phosphide/metallic cobalt (Co2P/Co) core wrapped in a nitrogen‐doped conductive carbon (CN) nano‐shell, demonstrated as Co2P/Co@NC. The strong chemical bonding between metallic cobalt and phosphorus, nitrogen and conductive carbon contributes to the enhanced conductivity and stability of the electrocatalyst. The nitrogen doping in the carbon shell provides additional Co–N active sites, which are crucial for ORR activity. Co2P/Co@NC demonstrates promising activity and stability compared to noble metals such as Pt for ORR in an alkaline medium. This suggests its potential as a cost‐effective alternative to Pt‐based catalysts. Further, due to factors such as strong cobalt‐phosphide bonding, high cobalt oxidation states and excellent conductivity of the nitrogen‐doped carbon shell, the Co2P/Co@NC outperforms noble metal oxides like iridium dioxide (IrO2) and ruthenium dioxide (RuO2) for OER. Co2P/Co@NC exhibits a low potential difference of 0.63 V, which is among the lowest reported for bifunctional electrocatalysts capable of both ORR and OER. Overall, the described strategy offers a promising avenue for developing efficient, low‐cost and stable electrocatalysts for oxygen reactions, which are crucial for various electrochemical energy conversion and storage technologies, such as fuel cells and metal–air batteries.

Amorphous IrOx nanoparticles on N-doped carbon matrix islands for enhancing proton exchange membrane water electrolysis

The development of an electrocatalyst and electrode that maintain their integrity under a high current density is vital for the advancement of PEMWE. In this context, we develop amorphous iridium oxide (IrOx) anchored on an N-doped carbon matrix island, directly formed on titanium porous transport layer through a simple method consisting of spray coating and pyrolysis. Remarkably, this configuration achieves 10 mA/cm2 current density for oxygen evolution reaction (OER) in acidic electrolyte, with only a 240 mV overpotential required, and the durability of this structure is confirmed by long-term and accelerated stress tests. These merits enable the unit cell to achieve over 6 A/cm2 at 2 V cell voltage without considerable degradation. This catalyst fulfills the requirement of the US Department of Energy’s 2026 target in performance (> 3 A/cm2 at 1.8 V) and in energy efficiency (> 65 %), as well as in the electricity cost for H2 production (US $2.0/kg H2).

Honeycomb‐Structured IrOx Foam Platelets as the Building Block of Anode Catalyst Layer in PEM Water Electrolyzer

AbstractAchieving robust long‐term durability with high catalytic activity at low iridium loading remains one of great challenges for proton exchange membrane water electrolyzer (PEMWE). Herein, we report the low‐temperature synthesis of iridium oxide foam platelets comprising edge‐sharing IrO6 octahedral honeycomb framework, and demonstrate the structural advantages of this material for multilevel tuning of anodic catalyst layer across atomic‐to‐microscopic scales for PEMWE. The integration of IrO6 octahedral honeycomb framework, foam‐like texture and platelet morphology into a single material system assures the generation and exposure of highly active and stable iridium catalytic sites for the oxygen evolution reaction (OER), while facilitating the reduction of both mass transport loss and electronic resistance of catalyst layer. As a proof of concept, the membrane electrode assembly in single‐cell PEMWE based on honeycomb‐structured IrOx foam platelets, with a low iridium loading (~0.3 mgIr/cm2), is demonstrated to exhibit high catalytic activity at ampere‐level current densities and to remain stable for more than 2000 hours.

Honeycomb-Structured IrOx Foam Platelets as the Building Block of Anode Catalyst Layer in PEM Water Electrolyzer.

Achieving robust long-term durability with high catalytic activity at low iridium loading remains one of great challenges for proton exchange membrane water electrolyzer (PEMWE). Herein, we report the low-temperature synthesis of iridium oxide foam platelets comprising edge-sharing IrO6 octahedral honeycomb framework, and demonstrate the structural advantages of this material for multilevel tuning of anodic catalyst layer across atomic-to-microscopic scales for PEMWE. The integration of IrO6 octahedral honeycomb framework, foam-like texture and platelet morphology into a single material system assures the generation and exposure of highly active and stable iridium catalytic sites for the oxygen evolution reaction (OER), while facilitating the reduction of both mass transport loss and electronic resistance of catalyst layer. As a proof of concept, the membrane electrode assembly in single-cell PEMWE based on honeycomb-structured IrOx foam platelets, with a low iridium loading (~0.3 mgIr/cm2), is demonstrated to exhibit high catalytic activity at ampere-level current densities and to remain stable for more than 2000 hours.

Oxygen 1s X-ray Photoelectron Spectra of Iridium Oxides as a Descriptor of the Amorphous-Rutile Character of the Surface.

Characterization of the surface of iridium oxide (IrOx) materials is of crucial importance to understand catalysts for the oxygen evolution reaction (OER) in low-temperature water electrolysis. While much of our current knowledge is based on well-defined single-crystal surfaces, surface-sensitive techniques like X-ray photoelectronic spectroscopy (XPS) are relevant to characterize the nanostructures considered. In this work, we describe a simple approach to use oxygen 1s spectra as an identifier of the amorphous/crystalline characteristics of iridium oxide structures from purely amorphous to purely crystalline. This conceptual approach was validated on seven commercially available materials. The presence of oxygen-associated defects in the surface moieties/species is shown even for purely crystalline materials with defect concentration increasing with greater amorphous character. This methodology provides us with an accessible ex situ descriptor of the catalyst surface as a baseline for further studies of the impact on catalytic properties.

Alkali Metal Iridates as Oxygen Evolution Catalysts Via Thermal Transformation of Amorphous Iridium (oxy)hydroxides.

AbstractEfficient water‐splitting is severely limited by the anodic oxygen evolution reaction (OER). Iridium oxides remain one of the only viable catalysts under acidic conditions due to their corrosion resistance. We have previously shown that heat‐treating high‐activity amorphous iridium oxyhydroxide in the presence of residual lithium carbonate leads to the formation of lithium‐layered iridium oxide, suppressing the formation of low‐activity crystalline rutile IrO2. We now report the synthesis of Na‐IrOx and K‐IrOx featuring similarly layered crystalline structures. Electrocatalytic tests confirm Li‐IrOx retains similar electrocatalytic activity to commercial amorphous IrO2 ⋅ 2H2O and with increasing size of the intercalated cation, the activity towards the OER decreases. However, the synthesised electrocatalysts that contain layers show greater stability than crystalline rutile IrO2 and amorphous IrO2 ⋅ 2H2O, suggesting these compounds could be viable alternatives for industrial PEM electrolysers where durability is a key performance measure.

Synthesis and Electrochemical Characterization of Ru-Modified Iridium Oxide Catalysts for PEM Electrolysis

Synthesis and Electrochemical Characterization of Ru-Modified Iridium Oxide Catalysts for PEM Electrolysis

Size-Dependent Core-Shell Fine Structures and Oxygen Evolution Activity of Electrochemical IrOx Nanoparticles Revealed by Cryogenic Electron Microscopy.

Electrochemically oxidized amorphous iridium oxides (IrOx) offer significantly improved electrocatalytic activities on the oxygen evolution reaction (OER) compared to crystalline IrO2, yet the origin of their decent activity and their size-dependent properties have not been fully understood. An important argument is the formation of deprotonated oxygen species not only at the topmost surface but also at the near surface, which creates an electrophilic character that activates the OER electrocatalysis. However, high spatial resolution identification of the electrophilic oxygen species remains unachieved. We address this hitherto-unresolved problem on size-selected electrochemical IrOx nanoparticles (NPs) by using cryogenic scanning transmission electron microscopy combined with electron energy loss spectroscopy, which enables simultaneous atomic detection of the near surface compositional and electronic structures with minimal damage that are further correlated with their size-dependent OER activities. Depending on the particle size, the electrochemical IrOx NPs showed distinctly different core-shell fine structures ranging from amorphous and hydrous IrOxHy NPs to a "metallic Ir core/sub-stoichiometric IrOx interlayer/amorphous IrOxHy shell" NP structure. Moreover, the formation of deprotonated, electrophilic oxygen is directly identified at the substoichiometric IrOx interface layer. These features account for a previously unestablished particle size effect of the electrochemical IrOx NPs, showing increasing water oxidation reactivity with an increasing nanoparticle size. Our results provide important insights into how subsurface oxygen chemistry controls the surface reactivity in the nanoscale Ir-based OER electrocatalysts.

Electronic structure engineering of NiFe hydroxide nanosheets via ion doping for efficient OER electrocatalysis

Nickel-iron layered double hydroxides (NiFe-LDH) with tunable catalytic properties have shown promise as an outstanding alternative to ruthenium iridium oxide for the oxygen evolution reaction (OER). However, the intrinsic activity of such electrocatalysts is suboptimal, and a considerable gap remains in understanding the working mechanism. To address this issue, we employ a convenient corrosion method to synthesize Mo-modified nickel–iron hydroxide (Mo-NiFeOxHy) ultrathin nanosheets. Mo-NiFeOxHy demonstrates excellent OER activity, requiring only 216 mV at a current density of 10 mA cm−2, with enhanced stability. Theoretical calculations reveal that the Mo doping induces material distortion, shifting the d-band center closer to the Fermi level, which accelerates the kinetic rate and catalytic activity. In situ Raman experiments show that doping with Mo promotes the rapid formation of high-oxidation-state transition metal hydroxide species, further enhancing the catalytic properties of Mo-NiFeOxHy in OER. This work provides a novel strategy to tailor the structure and composition of NiFe-based electrocatalysts, demonstrating a great potential to break barriers in OER.

Ruthenium Single-Atom Modulated Protonated Iridium Oxide for Acidic Water Oxidation in Proton Exchange Membrane Electrolysers.

Proton exchange membrane water electrolysers promise to usher in a new era of clean energy, but they remain a formidable obstacle in designing active and durable electrocatalysts for the acidic oxygen evolution reaction (OER). In this study, a protonated iridium oxide embedded with single-atom dispersed ruthenium atoms (H3.8Ir1- xRuxO4) that demonstrates exceptional activity and stability in acidic water oxidation is introduced. The single Ru dopants favorably induce localized oxygen vacancies in the Ir─O lattice, synergistically strengthening the adsorption of OOH* intermediates and enhancing the intrinsic OER activity. In addition, the preferential oxidation of Ru and the electronegativity of the oxygen vacancies significantly stabilize the Ir─O active sites, improving the OER stability. Consequently, the H3.8Ir1─ xRuxO4 catalyst shows an overpotential of 255mV at 10mA cm-2 and displays exceptional catalytic endurance in acidic electrolytes, surpassing 1100 h, representing a remarkable one-order-of-magnitude increase in stability compared to that of pristine H3.8IrO4. A proton exchange membrane electrolyser utilizing the H3.8Ir1- xRuxO4 catalyst as an anode exhibits stable performance for more than 1280 h under a high current density of 2 A cm-2.

Monitoring the Structural Changes in Iridium Nanoparticles during Oxygen Evolution Electrocatalysis with Operando X-ray Total Scattering.

Understanding the structure of nanoparticles under (electro)catalytic operating conditions is crucial for uncovering structure-property relationships. By combining operando X-ray total scattering and pair distribution function analysis with operando small-angle X-ray scattering (SAXS), we obtained comprehensive structural information on ultrasmall (<3 nm) iridium nanoparticles and tracked their changes during oxygen evolution reaction (OER) in acid. When subjected to electrochemical conditions at reducing potentials, the metallic Ir nanoparticles are found to be decahedral. The iridium oxide formed in the electrochemical oxidation contains small rutile-like clusters composed of edge- and corner-connected [IrO6] octahedra of a very confined range. These rutile domains are smaller than 1 nm. Combined with complementary SAXS data analysis to extract the particle size, we find that the OER-active iridium oxide phase lacks crystalline order. Additionally, we observe an iridium oxide contraction under OER conditions, which is confirmed by operando X-ray absorption spectroscopy. Our results highlight the need for multitechnique operando studies for a complete understanding of the electrochemically formed Ir oxide active in OER.

Influence of surface roughness on the response time of iridium oxide pH sensors

Iridium oxide (IrOx) formed by the carbonate melt oxidation method has been reported to exhibit excellent pH response characteristics. Although slope of the response and long-term response stability have been reported in relation to fabrication conditions and composition, the response kinetics of IrOx pH sensors have not been investigated in detail. The IrOx layers were shown to have a columnar structure with high aspect ratio, which resulted in deep recesses in the IrOx surface. In the present study, the influence of the surface roughness of IrOx on the response time of IrOx pH sensors is investigated. The 90 % response times of the sensors were found to be in the order of several hundreds of milliseconds and became longer as the IrOx layer became thicker and as the surface recesses became deeper. The results indicated that the diffusion of ions in the recessed structures strongly influenced the response time of the IrOx pH sensors. It is therefore important to lower the density of the recessed structures in the surface of the IrOx layers oxidized in lithium carbonate melts to achieve IrOx pH sensors that display rapid and reproducible responses.

Ensuring Stability of Anode Catalysts in PEMWE: From Material Design to Practical Application.

Proton Exchange Membrane Water Electrolysis (PEMWE) has emerged as a clean and effective approach for the conversion and storage of renewable electricity, particularly due to its compatibility with fluctuating photovoltaic and wind power. However, the high cost and limited performance of iridium oxide catalysts (i.e. IrO2) used as anode catalyst in industrial PEM electrolyzers remain significant obstacles to widespread application. Although numerous low-cost and efficient alternative catalysts have been developed in laboratory research, comprehensive stability studies critical for industrial use are often overlooked. This leads to the failure of performance transfer from catalysts tested in liquid half-cell systems to those employed in PEM electrolyzers. This concept presents a thorough overview for the stability issues of anode catalysts in PEMWE, and discuss their degradation mechanisms in both liquid half-cell systems and PEM electrolyzers. We summarize comprehensive protocol for the assessment and characterization, analyze the effective strategies for stability optimization, and explore the opportunities for designing viable anode catalysts for PEM electrolyzers.

Quantifying physical degradation alongside recording and stimulation performance of 980 intracortical microelectrodes chronically implanted in three humans for 956-2246 days.

The clinical success of brain-machine interfaces depends on overcoming both biological and material challenges to ensure a long-term stable connection for neural recording and stimulation. Therefore, there is a need to quantify any damage that microelectrodes sustain when they are chronically implanted in the human cortex. Using scanning electron microscopy (SEM), we imaged 980 microelectrodes from Neuroport arrays chronically implanted in the cortex of three people with tetraplegia for 956-2246 days. We analyzed eleven multi-electrode arrays in total: eight arrays with platinum (Pt) electrode tips and three with sputtered iridium oxide tips (SIROF); one Pt array was left in sterile packaging, serving as a control. The arrays were implanted/explanted across three different clinical sites surgeries (Caltech/UCLA, Caltech/USC and APL/Johns Hopkins) in the anterior intraparietal area, Brodmann's area 5, motor cortex, and somatosensory cortex.Human experts rated the electron micrographs of electrodes with respect to five damage metrics: the loss of metal at the electrode tip, the amount of separation between the silicon shank and tip metal, tissue adherence or bio-material to the electrode, damage to the shank insulation and silicone shaft. These metrics were compared to functional outcomes (recording quality, noise, impedance and stimulation ability). Despite higher levels of physical degradation, SIROF electrodes were twice as likely to record neural activity than Pt electrodes (measured by SNR), at the time of explant. Additionally, 1 kHz impedance (measured in vivo prior to explant) significantly correlated with all physical damage metrics, recording, and stimulation performance for SIROF electrodes (but not Pt), suggesting a reliable measurement of in vivo degradation.We observed a new degradation type, primarily occurring on stimulated electrodes ("pockmarked" vs "cracked") electrodes; however, tip metalization damage was not significantly higher due to stimulation or amount of charge. Physical damage was centralized to specific regions of an array often with differences between outer and inner electrodes. This is consistent with degradation due to contact with the biologic milieu, influenced by variations in initial manufactured state. From our data, we hypothesize that erosion of the silicon shank often precedes damage to the tip metal, accelerating damage to the electrode / tissue interface. These findings link quantitative measurements, such as impedance, to the physical condition of the microelectrodes and their capacity to record and stimulate. These data could lead to improved manufacturing or novel electrode designs to improve long-term performance of BMIs making them are vitally important as multi-year clinical trials of BMIs are becoming more common.

Efficient Electrocatalysts for Alkaline Oxygen Evolution Reaction from Wolframite Derived Heteroatom Materials

AbstractNew materials for oxygen evolution reaction (OER) electrocatalysts in alkaline conditions have been highly investigated in recent years for the advancement of water splitting. However, questions such as the lack of stability and the use of noble metals as electrocatalysts still need to be addressed. In this study, we report the synthesis and electrocatalytic properties of a series of wolframite tungsten‐based materials (MWO4, M=Fe, Mn, Ni, Co) towards OER in basic conditions. Our results showed that Ni0.5Mn0.5WO4 is a promising electrocatalyst, exhibiting ease of preparation, stability over the course of our experiments, and OER activity comparable to the standard iridium oxide. This study highlights the potential of wolframite tungsten‐based materials as a new class of electrocatalysts for OER in alkaline conditions.

Cerium-doped construction of oxygen vacancies in IrO2 to promote acidic OER reaction

Proton exchange membrane electrolysis of water is currently recognized in the world as an up-and-coming method for the preparation of green hydrogen energy. Due to its sustainability and low pollution, it has attracted much attention from practitioners recently. The most advanced iridium oxide (IrO2) is the most promising catalyst.However, developing a highly efficient and stable IrO2 catalyst that can adapt to industrial conditions remains a terrific challenge. One of the main problems to be solved is that the slow oxygen evolution reaction(OER) at the anode limits the production of hydrogen. We propose a straightforward method for synthesizing Ce metal-doped IrO2 with a high oxygen vacancy concentration while simultaneously improving the IrO2 catalytic activity and stability.The Ce-doped IrO2 (Ce-IrO2) exhibits a microscopic nanoparticle morphology and a high density of oxygen vacancies, enabling a rapid oxygen evolution reaction (OER) process with a low overpotential of 240 mV at 10 mA·cm-2 and a remarkable stability of over 50 h under acidic conditions.A growing body of research shows a synergistic effect of oxygen vacancies and mental dopants on the adsorption and evolution of active intermediates in the active center so that the oxygen evolution reaction activity of the Ir-based catalyst can be improved. We hope that ourwork will provide a sample method for acquiring catalysts capable of adapting to a wide range of conditions via metal doping.

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

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

Identifying restructured motifs on iridium oxide catalyst surfaces for water electrolysis

Identifying restructured motifs on iridium oxide catalyst surfaces for water electrolysis

Iridium oxide-modified reference screen-printed electrodes for point-of-care portable electrochemical cortisol detection

Cortisol is a well-known stress biomarker; this study focuses on using electrochemical immuno-sensing to measure the concentration of cortisol selectively and sensitively in artificial samples. Anti-cortisol antibodies have been immobilised on polycrystalline Au electrodes via strong covalent thiol bonds, fabricating an electrochemical bio-immunosensor for cortisol detection. IrOx was then anodically electrodeposited as a reference electrode on a commercial screen-printed electrode and electrochemical impedance spectrometry (EIS) studies were used to correlate the electrochemical response to cortisol concentration and the induced changes in charge transfer resistance (Rct). A linear relationship between the Rct and the logarithm of cortisol concentration was found in concentrations ranging from 1 ng/mL to 1 mg/mL with limit of detection at 11.85 pg/mL (32.69 pM). The modification of the reference electrode with iridium oxide has greatly improved the reproducibility of the screen-printed electrode. The sensing system can provide a reliable and sensitive detection approach for cortisol measurements.