Electrodeposited Iridium Oxide Film Research Articles

Core-Shell Structured Water Oxidation Anode for Minimum Platinum Group Metal Usage in Proton Exchange Membrane Water Electrolyzer

The development of enhanced electrocatalysts and optimization of electrode structures are of great importance for the practical and widespread deployment of proton exchange membrane water electrolyzers (PEMWEs). In particular, the scarcity and high price of Ir are considered to be bottlenecks inhibiting the large-scale commercialization of PEMWEs. Even compared with Pt, a rare-earth metal with a low production rate of 150~200 tons per year, Ir availability is extremely limited, as only a few tons per year are produced [1]. Therefore, It is envisioned that the Ir loading on the anode should be lowered (e.g., to 0.05 mg/cm2) to meet the annual demand for the PEMWE system installation on the scale of ≈5 GW by 2040 [2].To reduce the usage of rare-earth metals in PEMWEs, a highly active water-oxidizing anode was fabricated by depositing a core–shell structured catalyst on a titanium porous transport layer (Ti PTL). Earth-abundant metal-based iron-nitride (Fe2N) nanostructure was deposited on a Ti PTL by sequential hydrothermal and nitridation reaction. The prepared Fe2N nanostructure was then passivated by thin, electrodeposited iridium-oxide films (EIROF). Fe2N@EIROF core-shell structured anodes exhibited substantially low ohmic and mass-transfer resistances, leading to a high current density of 4.50 A/cm2 (@1.9 Vcell). The unique Fe2N@EIROF core-shell structure enhanced PEMWE performance in terms of Ir mass activity by substituting inner Ir atoms with earth-abundant metal-based iron nitride. A high Ir mass activity of 103 A/mgIr (@1.9 Vcell) was achieved by reducing Ir loading amount to 0.064 mg/cm2 on the anode. The chemical instability of transition metal-based supports in a PEMWE operating condition was overcome by the passivation with Ir catalyst layers. As a result, the prepared core–shell-structured catalysts were stable under the PEMWE operating condition achieving high Ir mass activities.[1] K. Nose, T.H. Okabe, Chapter 2.10 - Platinum Group Metals Production, in: S. Seetharaman (Ed.) Treatise on Process Metallurgy, Elsevier, Boston, 2014, pp. 1071-1097,[2] C. Minke, M. Suermann, B. Bensmann, R. Hanke-Rauschenbach, Is iridium demand a potential bottleneck in the realization of large-scale PEM water electrolysis?, Int. J. Hydrogen Energy, 46 (2021) 23581-23590

High–performance water electrolyzer with minimum platinum group metal usage: Iron nitride–iridium oxide core–shell nanostructures for stable and efficient oxygen evolution reaction

To reduce the usage of rare-earth metals in a proton-exchange-membrane water electrolyzer (PEMWE), a highly active water-oxidizing anode based on a core–shell catalyst structure was developed. Earth-abundant metal-based iron-nitride nanostructure was adopted to support thin, electrodeposited iridium-oxide films. PEMWEs with core–shell nanostructure has substantially low ohmic and mass-transfer resistances, suggesting that the introduction of Fe2N nanostructure on Ti PTL enhances the transfer of protons, water, and oxygen on the catalyst layer. Furthermore, a high Ir mass activity of 103 A/mgIr was achieved with reduced Ir loading of 0.036 mg/cm2 on the Ti PTL. The well-known weakness of transition-metal nitrides (TMNs) for use in PEMWEs, that is, their chemical instability in corrosive acidic environments, was overcome by carefully passivating the surfaces of the TMNs with chemically stable Ir catalyst layers. As a result, the prepared core–shell-structured catalysts were stable under the PEMWE operating condition.

Determining the proton diffusion coefficient in highly hydrated iridium oxide films by energy dispersive X-ray absorption spectroscopy

Charge transfer reactions in electrodeposited iridium oxide films (EIROF) are investigated by means of operando energy dispersive X-ray absorption spectroscopy (EDXAS), where oxidation and reduction conditions are selected to drive the Ir(III)/Ir(IV) and Ir(IV)/Ir(V) reactions in acidic solutions. The Ir(III)/Ir(IV) couple is related to a well-known electrochromic phenomenon, while the Ir(IV)/Ir(V) couple might play an important role in the catalysis of the oxygen evolution reactions (OER). In the experiments, current intensity and time-resolved X-ray absorption spectroscopy (XAS) are simultaneously recorded upon application of appropriate potential steps, leading to the independent determination of both the relevant reaction rates and the rate-determining steps. This is allowed by the fast acquisition time (∼10−2 s) at the ESRF Energy Dispersive XAS (EDXAS) ID24 beam-line, in combination with the highly hydrated amorphous iridium oxide electrode material, which in turn allows to maximize the fraction of Ir sites participating in the electrochemical processes. If the experimental conditions exclude the possibility of having either oxygen evolution (or reduction), the Degree of Reaction (DoR), determined by both electrochemistry and XAS, exhibits exponential time dependence, clearly pointing to diffusion-controlled processes. Vice versa, under concomitant OER + oxidation of iridium centers or ORR + iridium reduction, the electrochemical and XAS DoRs highlight different phenomena, providing fully complementary information of the ongoing electrode reactions. In all cases, data elaboration allows to determine the diffusion coefficient of H+ ions within the catalyst layer, that is compared and confirmed by data obtained by electrochemical impedance spectroscopy (EIS). The high values of D obtained for EIROF is compared to values obtained on other IrO2 materials can help in explaining the relevant high electrocatalytic activity.

Electrochemical deposition and characterization of iridium oxide films on Ti2AlC support for oxygen evolution reaction

Two types of iridium oxide films formed at the Ti2AlC substrate were investigated: (1) anodically electrodeposited iridium oxide film from the solution based on IrCl3xH2O; (2) iridium oxide film prepared by cycling thin layer of electrodeposited Ir in the 0.5 M H2SO4 from − 0.25 to 1.20 V. It was shown that during anodic electrodeposition of iridium oxide film (1) coulombic efficiency decreases with increasing anodic potential, being only 3% at E = 0.7 V vs. SCE and 26% at E = 0.62 V vs. SCE. A pair of peaks corresponding to the transition Ir(III)-oxide/Ir(IV)-oxide was present on the CVs recorded in 0.5 M H2SO4. While cycling pure Ir thin layer in the solution of 0.5 M H2SO4 from − 0.25 to 1.20 V (2) up to 100 cycles, typical CV response was characterized with the prepeak and a pair of peaks corresponding to the Ir(III)/Ir(IV)-oxide transition. With the increase of cycles number to 150, additional peak at potential of 1.0 V appeared. This peak was formed on the account of pair of peaks corresponding to the Ir(III)/Ir(IV)-oxide transition. The oxygen evolution reaction (OER) was investigated at both iridium oxide films. It was shown that the Tafel slope for the OER was ~ 40 mV dec−1 for the first polarization curve, confirming that the rds was a reaction S–OH → S–Oads + H+ + e−. As the number of recorded polarization curves increased, the activity of both types of iridium oxide films decreased, due to dissolution of iridium oxide films at the potentials of the OER. It is shown that anodically electrodeposited iridium oxide film is more active for the OER than that obtained by cycling electrodeposited iridium layer. However, both iridium oxide films exhibited insufficient stability.

Iridium and Ruthenium oxide miniature pH sensors: Long-term performance

The characteristics of electrodeposited Iridium oxide films (EIROF), and Ruthenium oxide films (RUOF) under normal and annealed conditions have been investigated. RUOFs were sputter deposited onto a substrate whilst EIROF sensors were made by electrodeposition. The adhesion of the RUOF sensors was investigated on Platinum and Titanium surfaces. The electrical impedance of the two metal oxides was measured in PBS demonstrating that both films have up to a 100 fold higher electrical double layer capacitance than bright Platinum electrodes. The EIROF films show super-Nernstian response, but the formal potential shows a continuous drift with time of approximately 2 mV day−1, measured over 40 days. Both RUOF sensors show approximately Nernstian behaviour. Short term pH precision of the annealed RUOF falls within a confidence interval of ± 0.1 pH, and stability is maintained within a confidence of ± 0.24 pH for up to 147 days. The un-annealed RUOF maintains the same precision but only for 20 days. Compared to EIROF sensors, no drift correction is required for the annealed RUOF making this metal oxide suitable for long term measurements where re-calibration is not possible.

Sensor Access to the Cellular Microenvironment Using the Sensing Cell Culture Flask

The Sensing Cell Culture Flask (SCCF) is a cell culture monitoring system accessing the cellular microenvironment in 2D cell culture using electrochemical microsensors. The system is based on microfabricated sensor chips embedded in standard cell culture flasks. Ideally, the sensor chips could be equipped with any electrochemical sensor. Its transparency allows optical inspection of the cells during measurement. The surface of the sensor chip is in-plane with the flask surface allowing undisturbed cell growth on the sensor chip. A custom developed rack system allows easy usage of multiple flasks in parallel within an incubator. The presented data demonstrates the application of the SCCF with brain tumor (T98G) and breast cancer (T-47D) cells. Amperometric oxygen sensors were used to monitor cellular respiration with different incubation conditions. Cellular acidification was accessed with potentiometric pH sensors using electrodeposited iridium oxide films. The system itself provides the foundation for electrochemical monitoring systems in 3D cell culture.

Potentiometric textile-based pH sensor

Determining the pH of sweat provides valuable information for athletes and patient monitoring. This work presents a textile-based, highly sensitive pH sensor for pH determination. Three conductive fabrics (Argenmesh, Ristop silver and stainless steel mesh (SSM)) were modified with a pH sensitive electrodeposited iridium oxide film (EIROF). The three electrodeposited fabrics were characterized by impedance measurements. The stainless steel mesh showed the best sensitivity to pH changes and therefore was selected for further experiments. Two configurations of this fabric were evaluated, looking for improvement in pH sensitivity and temperature dependence. The best result was obtained with the configuration that maximizes the contact surface between the stainless steel fibers, showing an error of 0.15% in the pH measurement of a buffer solution. This configuration was also used to perform in vivo measurements, obtaining an error of 4% when compared to the measurements performed with a commercial pH test strip. The implementation of sensor into textiles brings some advantages such as comfort, biocompatibility and washability, among others; making the future incorporation of a sensor into a garment very possible.

Electrodeposited Iridium Oxide on Carbon Fiber Ultramicroelectrodes for Neural Recording and Stimulation

Host encapsulation decreases the ability of chronically implanted microelectrodes to record or stimulate neural activity. The degree of foreign body response is thought to depend strongly on the cross-sectional dimensions of the electrode shaft penetrating neural tissue. Microelectrodes with cellular or sub-cellular scale shaft cross-sectional dimensions, such as carbon fiber ultramicroelectrodes have been previously demonstrated to elicit minimal tissue response, but their small geometric surface area results in high electrode impedances for neural recording, and reduced charge injection capacity during current pulsing for neural stimulation. We investigated electrodeposited iridium oxide films (EIROF) on carbon fiber ultramicroelectrodes as a means of enhancing the charge injection capacity and reducing electrode impedance. EIROF coatings reduced the electrode impedance measured at 1 kHz by a factor of 10 and improved charge storage and charge injection capacities. The maximum charge injection capacity was also strongly dependent on the interpulse bias and pulse width, and reflected a potential-dependent EIROF impedance. The charge injection capacity of the EIROF-coated carbon fiber ultramicroelectrodes measured in an inorganic buffered saline model of interstitial fluid exceeded 17 mC/cm2 with appropriate biasing, allowing charge-injection at levels well above reported charge/phase thresholds for intraneural microstimulation.

Self-Closed Parylene Cuff Electrode for Peripheral Nerve Recording

We have designed, fabricated, and characterized a self-closed parylene cuff electrode for peripheral nerve recording, whose curliness and closeness is realized by the difference in self-stress of different parylene layers due to the heat treatments. Compared with the conventional cuff electrodes, the self-closed structure without any additional mechanical locking structure minimizes the mechanical structure of the cuff electrode. Moreover, the self-closed structure made of thin and flexible parylene film minimizes mechanical damage to the nerve and the surrounding tissues. The zero insertion force interface facilitates the connection of 16 channels with external circuits through the integrated parylene bonding pads. To reduce the impedance and increase the charge storage capacity of the electrode, the electrodeposited iridium oxide film is electrodeposited on the electrode sites. Using the fabricated self-closed parylene cuff electrode, acute neural recoding of 16 channels was performed on the rat sciatic nerve to verify the capability of recording the neural activities. The present self-closed parylene cuff electrode is promising to be used as an neural interface for peripheral nerve recording.

Fabrication and electrochemical comparison of SIROF-AIROF-EIROF microelectrodes for neural interfaces.

Iridium oxide has been widely used in neural recording and stimulation due to its good stability and large charge storage capacity (CSC). In general, the iridium oxide film used in the electrophysiological application can be grouped into three principal classifications: sputtering iridium oxide film (SIROF), activated iridium oxide film (AIROF) and electrodeposited iridium oxide film (EIROF). Although these kinds of iridium oxide all can remarkably reduce the impedance and increase the CSC of the microelectrode, they also exhibit markedly differences in electrochemical performances. After activation, the CSC of EIROF is 68.20 mC/cm(2), which is 88.7 % larger than that of the SIROF and 67.6 % larger than that of the AIROF. The impedance at 1 kHz of the three kinds of iridium oxide microelectrode is around 4000 ohm, it is acceptable for the neural interface application. The phase at 1 kHz of the AIROF microelectrode is the largest which is -6.1 degree, about 22.6 % of the SIROF and 44.5 % of the EIROF.

In Situ Dispersive EXAFS in Electrocatalysis: The Investigation of the Local Structure of IrOxin Chronoamperometric Conditions as a Case Study

An in situ study with dispersive EXAFS (Extended X-Ray Absorption Spectroscopy) at the Ir-LIIIedge is performed to characterize Electrodeposited Iridium Oxide Films (EIROF) under chronoamperometric conditions. The technique monitors the local chemical environment and electronic structure of iridium during the oxidation of Ir(III) to Ir(IV) with a time resolution of milliseconds. The study is performed in both acidic and basic media. The Fourier transforms of the time-resolved EXAFS signals clearly show that the short-range structure of Ir is similar to that of rutile-type IrO2and is maintained during the reaction, thus accounting for the flexibility of the structure of the electrode material in accommodating different oxidation states. From a more general point of view, the work demonstrates the capabilities of in situ experiments based on state-of-the-art dispersive EXAFS in clarifying the mechanistic aspects of electrochemical processes.

Reduce impedance of intracortical iridium oxide microelectrodes by hydrogel coatings

The study investigated the influence of poly(vinyl alcohol)/poly(acrylic acid) interpenetrating polymer networks (PVA/PAA IPNs) hydrogel film coatings on interface stability between electrodeposited iridium oxide films (EIROF) microelectrodes and neural tissue by monitoring changes in impedance. The impedance of EIROF and PtIr microelectrodes ( d = 75 μm) with and without the coatings was measured weekly, while the microelectrodes were implanted into the motor cortex of rats ( n = 48) for 28 days. The results show that without PVA/PAA coatings the impedance of EIROF microelectrodes was higher than that of PtIr microelectrodes in vivo, however, the impedance of EIROF microelectrodes with coatings resulted in the lowest impedance compared to the impedance of both PtIr microelectrodes with coatings and EIROF microelectrodes without coatings through the whole test period. Moreover, compared to the EIROF microelectrodes without coatings, the impedance of EIROF microelectrodes with coatings reduced by ∼40% at day 21 after implantation. The PVA/PAA IPNs hydrogel coating acted as a stable ions conductive layer which enabled the EIROF microelectrodes to maintain its electrochemical superiority in vivo and enhanced the mechanical stability of the EIROF microelectrodes. The study demonstrates a concept of neural microelectrode modification, in which the hydrophilic property is crucial.

Anodically electrodeposited iridium oxide films microelectrodes for neural microstimulation and recording

An efficient and reliable electrochemical method for preparing electrodeposited iridium oxide film (EIROF) microelectrodes by an anodic electrochemical process is reported. The EIROF microelectrodes exhibited remarkably high safe charge injection ( Q inj) limits of ∼2.6 mC/cm 2 for anodic-first pulses and ∼1.4 mC/cm 2 for cathodic-first pulses, and the electrode impedance at 1 kHz was significantly reduced by ∼92%. The EIROF microelectrodes also exhibited good mechanical and electrochemical stability, as well as an excellent super-Nernstian slope in a broad pH range (1–13). All of these characteristics are greatly desired for neural sensor applications including electrical neural microstimulation, neural signal recording as well as pH monitoring in vivo. Moreover, the electrodepositing method can be facilely incorporated into thin-film technology and Microelectromechanical Systems (MEMS).

Iridium Oxide Film Electrodes for Anodic Stripping Voltammetry

AbstractThe new iridium oxide film electrode, applied for the determination of lead(II), cadmium(II) and copper(II) traces using differential pulse anodic stripping voltammetry (DP ASV) is presented. The electrode display an interesting stripping voltammetric performance which compares with electrodes commonly used in voltammetry. The deposited film is known as anodically electrodeposited iridium oxide film (AEIROF). The AEIROF electrode is characterized by long‐term stability (more than 40 days) and very good reproducibility of the analytical signals in this time (≤12% for 0.5 μM of lead). The regeneration of iridium film is very simple in a time shorter than 60 seconds. The effects of various factors such as: thickness of AEIROF film, preconcentration potential and time, supporting electrolyte composition, potential interferences are optimized. The detection limit for AEIROF film electrode based on glassy carbon for an accumulation time of 30 s is as low as 7 nM for lead(II). The repeatability of the method at a concentration level of the lead(II) as low as 0.5 μM, expressed as RSD is 2.5% (n=10). The proposed method was successfully applied and validated by studying certified reference material CTA‐OTL‐1. Such an attractive use of ‘mercury–free’ ‐ environmentally friendly electrodes offers great promise to measure trace metals.

Measurements of the capacitance and the response time of solid-state potentiometric sensors by an electrochemical time-of-flight method

A micro-electrochemical time-of-flight technique relying on galvanostatic ion generation and potentiometric sensing (P-ETOF) has been developed in order to characterize dynamic properties of solid-state potentiometric sensors and to measure their capacitance. Lithographically-fabricated generator-sensor devices featuring 10 μm-wide micro-electrodes spaced by 20–50 μm are used in either an open face mode, where hemi-cylindrical diffusion governs mass transport of generated ions, or in a narrow channel mode, where linear diffusion describes transport of ions between the generator and sensor micro-electrodes in a thin (~3 μm) layer of electrolyte. P-ETOF-open face mode is capable of generating silver ions or protons at a rate equivalent to a current density of 10 A/cm2. This induces a maximum, mass transport controlled rate of a sensor potential rise of ~80 V/s. P-ETOF was used to characterize anodically electrodeposited iridium oxide film pH micro-sensors. Their maximum rate of potential change was determined to depend inversely on the oxide film’s thickness and varied from 34 V/s for 40 nm-thick films to 9 V/s for the 650 nm-thick iridium oxide films. The capacitances of these pH micro-sensors were determined to depend linearly on their thicknesses, with a slope of 3 μF cm−2 nm−1 and a specific capacitance of 30 F/cm3. These results point to slow proton transport in the bulk of the oxide film as a mechanistic element of their pH response.

Improved electrodeposited iridium oxide pH sensor fabricated on etched titanium substrates.

In the present paper, the preparation and characterization of an improved solid-state pH sensor are described. The sensor is based on anodically electrodeposited iridium oxide film, as a pH-sensing layer. Merits of the present sensor include (i) excellent adhesion of the pH sensitive layer to the substrate, (ii) excellent reproducibility of sensor fabrication, (iii) faster preparation procedure, and (iv) low cost of the titanium substrate. These advantages are realized by combining acid-etched titanium as the electrode substrate with an optimized electrodeposition solution consisting of IrCl4 as an iridium source, hydrogen peroxide, potassium oxalate, and potassium carbonate. Heating the electrodeposition solution to 90 degrees C reduced the time required for solution development from approximately 3 days to 10 min. The pH-sensing layer is protected with a layer of Nafion and a microporous polyester membrane. The improved sensor showed a super-Nernstian response (-73.7 +/- 1.2 mV/pH unit) in the pH range of 1.5-11.5. The present pH sensor, fabricated in a tubular form, is used as a detector in a flow injection analysis (FIA) system for pH measurements. Optimization of the FIA experimental parameters resulted in a linear dependence of peak heights on the pH of the injected samples in the pH range of 2-11.

Fabrication of anodically electrodeposited iridium oxide film pH microelectrodes for microenvironmental studies.

A new method for fabrication of anodically electrodeposited iridium oxide film pH microelectrodes has been developed in this study. Novel for its tip size (3-10-microm tip diameter), the microelectrode is fabricated in a tapered glass micropipet filled with a low melting point alloy. The tapered end is recessed and platinized. Thereafter, iridium oxide is electrodeposited over the platinized end in the recessed part. The microelectrode has a very short response time (t80 < 5 s) in the pH range of 0-12 with an accuracy of 0.05 pH unit. The pH microelectrode is not affected by most ions and complexing agents of relevance in environmental and biological studies; it can be used in fluids over wide ranges of stirring speeds (0-55 rpm) and temperatures (approximately 5-40 degrees C). Redox agents such as dissolved oxygen and hydrogen peroxide have no effect on the pH response while quinhydrone, ferro- and ferricyanide, and sodium sulfide have marked effects. However, the microelectrode can still be used in any sample when calibration is done in standards having similar redox characteristics.

Electrodeposited iridium oxide for neural stimulation and recording electrodes.

Iridium oxide films formed by electrodeposition onto noniridium metal substrates are compared with activated iridium oxide films (AIROFs) as a low impedance, high charge capacity coating for neural stimulation and recording electrodes. The electrodeposited iridium oxide films (EIROFs) were deposited on Au, Pt, PtIr, and 316 LVM stainless steel substrates from a solution of IrCl4, oxalic acid, and K2CO3. A deposition protocol involving 50 potential sweeps at 50 mV/s between limits of 0.0 V and 0.55 V (versus Ag AgCl) followed by potential pulsing between the same limits produced adherent films with a charge storage capacity of >25 mC/cm2. Characterization by cyclic voltammetry and impedance spectroscopy revealed no differences in the electrochemical behavior of EIROF on non-Ir substrates and AIROF. The mechanical stability of the oxides was evaluated by ultrasonication in distilled water followed by dehydration and rehydration. Stability under charge injection was evaluated using 200 micros, 5.9 A/cm2 (1.2 mC/cm2) cathodal pulses. Loss of iridium oxide charge capacity was comparable for AIROFs and the EIROFs, ranging from 1% to 8% of the capacity immediately after activation or deposition. The EIROFs were deposited and evaluated on silicon microprobe electrodes and on metallized polyimide electrodes being developed for neural recording and stimulation applications.

Electrodeposited iridium oxide pH electrode for measurement of extracellular myocardial acidosis during acute ischemia.

In the present paper, fabrication, characterization, and physiological applications of a solid-state pH electrode are described. The pH sensing layer was based on an anodic electrodeposited iridium oxide film (AEIROF). Sputtered platinum electrodes (1 mm diameter) fabricated on flexible Kapton films or platinum wires were used as planar or cylindrical supports. Each electrode site was coated with Nafion to attenuate the interference of anionic redox species and to protect the electrode surface during in vivo measurements. Performance of the AEIROF was evaluated, for the first time, as a pH electrode and proved to have a slightly super-Nernstian response with slope of -63.5 +/- 2.2 mV/pH unit for both wire and planar sputtered platinum electrodes. Linear pH responses were obtained in the pH range 2-10. The electrodes have a working lifetime of at least 1 month with accuracy of about 0.02 pH unit and fast response time. The electrodes showed very low sensitivities for different species, such as Na+, K+, Li+, NH4+, Ca2+, Mg2+, dissolved oxygen, lactate, ascorbate, and urate, which are important for physiological applications. The electrodes were applied in extracellular pH measurements during brief regional ischemia in a swine heart and no-flow ischemia in an isolated rabbit papillary muscle. A first report on extracellular pH, K+, and lactate simultaneous measurements during no-flow ischemia using the AEIROF pH electrode and the previously described K+ and lactate electrodes is presented as well.

The Electrochemical Behavior of Anodically Electrodeposited Iridium Oxide Films and the Reliability of Transmittance Variable Cells

The electrochemical properties of anodically electrodeposited iridium oxide films are investigated for application to transmittance variable cells. The reactivity of electrochromic reaction of the film increases with the increase in water content in LiClO4 propylene carbonate electrolyte, and decreases with increasing temperature of the heat treatment after electrodeposition. The coloration potential E of the film depends on the proton activity in the electrolyte, and increases linearly with the hydrogen electrode potential E' of the electrolyte at a slope of 1.0. This means that the coloration is caused by the release of H+ from the film or by the injection of OH- into the film. The reliability of transmittance variable cells made with the films is tested.