IrO2 Catalyst Research Articles

In‐situ Generation of Hydroxyl Layers in CoO@FeSe2 Catalyst for High Selectivity Seawater Electrolysis

Comprehensive SummarySeawater electrolysis holds great promise for hydrogen production in the future, while the development of anodic catalysts has been severely hampered by the side‐reaction, chloride evolution reaction. In this work, nano‐flower‐cluster structured CoO@FeSe2/CF catalysts are synthesized via a scalable electrodeposition technique, and the performance is systematically studied. The oxygen evolution reaction (OER) overpotential of CoO@FeSe2/CF is 267 mV at 100 mA·cm−2, which is significantly lower than that of the IrO2 catalyst (435 mV). Additionally, the catalyst shows high selectivity for OER (97.9%) and almost no loss of activity after a durability test for 1100 h. The impressive performance is attributed to the dense rod‐like structure with abundant active centers after electrochemical activation and the in‐situ generated CoOOH and FeOOH that improve the catalytic activity of the catalyst. The synergistic effect induced by the non‐uniform structure endows the catalyst with excellent stability.

IrO2 deposited on RuO2 as core-shell structured RuO2@IrO2 for oxygen evolution reaction in electrochemical water electrolyzer

The ever-rising energy demand and environmental concerns require the application of electrochemical water splitting to produce hydrogen as a substitute energy to fossil fuel. Proton exchange membrane water electrolyzers split water into oxygen and hydrogen at the anode and cathode, respectively. Anode catalysts necessitate extensive study, because significant overpotential is required to accelerate the 4-e sluggish oxygen evolution reaction in acidic media. RuO2@IrO2 was synthesized in this work, which aims to achieve high activity and stability. In doing so, RuO2·xH2O was first prepared using the Adams method, followed by the subsequent precipitation of Ir(OH)3 on its surface via the hydrolysis of iridium precursor in a mix of NH3·H2O in ethanol. Upon heat treatment, RuO2@IrO2 with an appropriate Ru/Ir molar ratio was obtained and applied for oxygen evolution reaction. Electrochemical evaluation results show that the optimized RuO2@IrO2-20 performed the best in terms of enhanced mass activity (1.62 A mg−1oxide @1.6 V) and lower overpotential (275 mV@10 mA cm−2) compared with single RuO2 and IrO2 catalysts. In addition, both chronopotentiometry and chronoamperometry test results show that the stability of RuO2@IrO2-20 is highly competitive. The intimate contact between RuO2 and IrO2 combines both the most active RuO2 and stable IrO2 as an integrated catalyst.

IrOx-MoO3 nano-heterostructure electrocatalysts for efficient acidic water oxidation

Rational design of Ir-based electrocatalyst with high performance and low cost remains as an urgent requirement to speed up the sluggish acidic water oxidation to produce hydrogen in renewable energy conversion and storage systems. In this work, the highly dispersed IrOx nanoparticles are fabricated on MoO3 sheets coated Ti meshes (c-IrOx-MoO3/Ti) as efficient electrocatalytic electrodes for acidic water oxidation. The heterogeneous c-IrOx-MoO3/Ti catalysts show much higher acidic oxygen evolution reaction activity (∼200 mV overpotentials at 10 mA cm−2) and long-term stability (longer than 130 h at 100 mA cm−2) with extremely low Ir content (∼28.35 μgIr cm−2) as compared to the IrOx/Ti and commercial IrO2 catalysts. The special intertwined formation of MoO3 sheets through electrodeposition effectively promotes the uniform dispersion of IrOx nanoparticles and the electrocatalytic performance. Theoretical calculations reveal that the electronic modulation on the MoO3-IrOx heterogeneous interface optimized the adsorbed energies for oxygen intermediates on the active Ir sites of the catalysts. The self-assembled proton exchange membrane single-cell electrolyzer test further verifies that the fabricated c-IrOx-MoO3/Ti catalysts demonstrate a greatly lower voltage and higher stability for water electrolysis than the commercial IrO2 catalysts. This work provides a new pathway to designing highly active and acid-resistant Ti-supported water oxidation catalysts with ultra-low Ir content.

Kinetics of the Oxygen Evolution Reaction (OER) on Amorphous and Crystalline Iridium Oxide Surfaces in Acidic Medium.

Amorphous and crystalline IrO2 catalysts are synthesized by the Adams method and characterized with X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The oxygen evolution reaction (OER) is investigated on both the catalyst surfaces in 0.5 M H2SO4 electrolyte. The Tafel slope estimated in the temperature range of 293-333 K on the two surfaces indicates a change in the rate-limiting steps. The data are also analyzed in terms of the Eyring equation to estimate the activation enthalpy (ΔH#) and pre-exponential factor (Af) as a function of overpotential and therefore the charge-transfer coefficient (α). The estimated α values suggest strong electrocatalysis on both the surfaces. While the ΔH# plays a decisive role in the electrocatalysis on the amorphous sample, the trend of Af indicates that an increase in the entropy on the crystalline surface is pivotal in reducing the reaction barrier.

Novel two-dimensional square-AlAs: Direct semiconductor coupled with high electron-hole mobility ratio toward sunlight-driven water-splitting

The design of new monolayered atomic motifs always attracts enormous attention due to their interesting properties induced by unique crystal phases and promising catalytic applications in low dimensional devices. Herein, we propose a novel square-AlAs monolayer based on ab initio density functional theory calculations, whose good stability is confirmed from the perspectives of energy, dynamics, thermodynamics, and mechanics. Electronic structure calculations reveal that the newly predicted square-AlAs monolayer is a direct-gap semiconductor with a bandgap of 1.39 eV, which is sensitive to in-plane strain, meanwhile the strain induced changes in bandgap allow advantageous modulations of optical absorption in visible-light region. Unfortunately, the unfavorable hydrogen evolution reaction (HER) band edge levels make square-AlAs not suitable for photocatalytic water-splitting in spite of the appropriate bandgap, high electron-hole mobility ratio and strong visible light absorption. However, a favorable effect caused by strain engineering is revealed by the fact that the square-AlAs can meet the requirement of photocatalytic water-splitting under the suitable biaxial loading. Particularly, compared to pure AlAs, the Gibbs free energy of hydrogen adsorption indicate that Boron-doping can effectively decrease the HER overpotential. Moreover, the external potential provided by photoexcited electrons for oxygen evolution reaction (OER) is quite positive, which can serve as a free and pollution-free driving force to drive OER to proceed spontaneously for pure AlAs or reduce the OER overpotential to a very low value (0.15 V) for Boron-doped AlAs, overmatching widely used IrO2 catalyst.

Investigation of Surface Chemistry of Pemwes Iridium Oxide Catalyst Layers as a Function of Ink Aging Time

Proton exchange membrane water electrolyzers (PEMWEs) are an advanced and effective solution to producing hydrogen from water for green hydrogen production. To improve PEMWEs operation, it is necessary to analyze the catalyst layer (CL) to better understand relationships between surface properties and processing, performance, and durability. However, understanding the CL composition, structure, and morphology at all relevant scales is difficult. X-ray photoelectron spectroscopy (XPS) is a surface-sensitive technique that is capable of measuring subtle differences in chemistry of a sample’s surface within top 3-10 nms. However, there are several challenges associated with limited information that has been reported in the literature thus far.This study focuses on investigating the CLs made with IrO2 catalysts using Mayer Bar coating method, one of the scalable fabrication routes. XPS analysis was conducted using a recently developed approach that mitigates issues with degradation of ionomer induced during X-ray exposure. First, XPS was used to probe variations between different areas of the CLs to assess spatial homogeneity to confirm good quality of the coatings. XPS was also employed to investigate CLs made with inks aged for different amount of time (0, 3, and 14 days), noting some differences in the surface chemistry. Results from this study will discuss the effect of ink age on surface composition at the catalyst-ionomer interface, as well as address specific challenges in fitting the O 1s and Ir 4f core level spectra which are inherently challenging to curve fit. This work motivates further studies investigating other aspects related to processing and fabrication as well as investigations of CLs stability.

Investigation of Ni Foam and Stainless-Steel Mesh Substrates Toward Oxygen Evolution Reaction in Alkaline Seawater Electrolysis

Electrolysis of alkaline seawater is a promising approach for the large-scale production of hydrogen, however, little effort has been devoted in studying the substrates for oxygen evolution reaction (OER) electrocatalysts. Stainless-steel mesh (SS mesh) and Ni foam were investigated systematically for OER in alkaline seawater electrolysis in this study. The overpotentials and Tafel slopes with SS meshes are smaller than Ni foams. Interestingly, the performance of the SS mesh even outperforms various non-noble metal electrocatalysts and is comparable to commercial RuO2 and IrO2 catalysts. Additionally, the Ni foam exhibits much poorer resistance to corrosion compared with SS meshes. This work suggests inexpensive and commercially available SS mesh is an outstanding substrate for OER in alkaline seawater electrolysis.

Solid Oxide Iron-Air Battery for Long-Duration Energy Storage: A Study on Reduction Kinetics of Energy Storage Material Fe-ZrO2 Catalyzed by Ir Particles

Long duration electricity storage (LDES) with 10+ hour cycle duration is an economically competitive option to accelerate the penetration of renewable energy into the utility market. Unfortunately, none of the available energy storage technologies can meet the LDES’ requirements for duration and cost. The benchmark Li-ion technology can only store and discharge up to 4-hour energy, beyond which it would be cost prohibitive. In this presentation, a new solid-oxide iron-air batteries (SOIABs) with energy-dense solid iron as the energy storage material is shown to have inherent advantages for LDES applications. The presentation will start with the working principle of the SOIAB, baseline performance and bottlenecks of this new technology. It will then show that with a small amount of IrO2 (or Ir during operation) catalyst added into the energy store “Fe-bed”, a lab-size (f1′′) SOIAB can achieve an energy density of 625 Wh/kg, 12.5-hour cycle duration and 90% of round-trip efficiency under LDES-related working conditions. Given the excellent low-rate performance and the use of earth-abundant, low-cost Fe as the energy storage material, we finally conclude that SOIAB is a well-suited battery technology for LDES applications.

Membrane Electrolyzers for Hydrogen Production: From Catalyst Fundamentals to Devices

Water electrolysis powered by renewable sources such as wind or solar produces clean hydrogen gas, which is used for many industrial processes and will be essential in a future clean energy economy. Commercialized membrane electrolyzers use acidic proton exchange membranes (PEMs), which offer high performance but require the use of expensive precious-metal catalysts such as IrO2 and Pt that are nominally stable under the locally acidic conditions. Anion exchange membrane (AEM) electrolyzers in principle offer the performance of PEM electrolyzers with the ability to use earth-abundant catalysts and inexpensive bipolar plate materials. I will present our fundamental work in understanding the chemical and electrochemical processes in earth-abundant water-oxidation catalysts, including the use of integrated reference-electrode device architectures and cross-sectional materials analysis, as well as progress in building high-performance AEM electrolyzers. Baseline systems operate at 1 A·cm-2 in pure water feed at < 2 V at a moderate temperature of ~70 °C using either IrO2 or Co3O4 anode catalyst layers, Versogen TP-85 alkaline ionomers, and stainless-steel porous transport layers. These devices, however, degrade rapidly compared to PEM electrolyzers. The voltage profile corresponding to degradation has initial fast (~10 mV/h) and steady-state slow components (~1 mV/h), which we link to chemical and structural changes in the ionomer catalyst reactive zone. We further discover that dynamic Fe-based OER catalysts – that have world record performance in traditional liquid alkaline electrolyzer systems – perform poorly with enhanced degradation rates in alkaline membrane electrolysis, illustrating fundamentally different chemical design principles for OER catalysts. Lastly, I will discuss these processes in electrolyzer devices and in model systems, as well as present promising new chemical strategies we have developed to mitigate degradation and enhance performance using novel ionomers and electrode catalyst compositions and architectures.

Autonomous Materials Synthesis Strategies Towards Accelerated Discovery of Novel Functional Nanomaterials for Green Energy Production

Electrocatalysts designed by alloying two or more elements (e.g., transition metals) showed promising enhancement in catalytic activity and durability due to synergistic effects from alloying, such as strain engineering or electron exchange. The potential of the alloying approach is constrained by standard synthesis methods (e.g., wet chemistry and thermal decomposition techniques) due to the governing thermodynamics rules (e.g., miscibility of the different alloying elements). That renders the synthesis of high entropy alloys (HEA), with more than 3 elements, to be complicated and tedious to achieve by those conventional methods.In the presented talk we are presenting different strategies to synthesize metastable high entropy alloys (HEA) and their outstanding catalytic performance towards OER, in correlation to their unique chemistries and structures. Rapid cycling of temperature from room 25 °C to highly elevated levels (1700 °C), in conjunction with ultra-rapid cooling, within milli-second range have been demonstrated by different methods including rapid Joule heating, Intense light flashing, and microwave plasma shock. Our results show successful synthesis of non-noble metals HEA NPs, possessing higher catalytic activity than IrO2 catalyst for OER. The nature of the alloying elements dictates OER activity by promoting different oxidation states of the catalytically active transition metals (e.g., Fe, Ni and Co). In addition, a phenomenal in-situ chemical welding of HEA NP to carbon support, yielded HEA catalysts with two orders of magnitude longer durability than IrO2. Our developed techniques resulted in formation of super-strong metal-carbide bonds “chemically welding” HEA NP to the catalyst support.Synthesis of various HEA catalyst and their electrocatalytic performance screening has been carried out by using liquid handler robots and multi-axes robotic arm systems. Moreover, to optimize the design of materials (structure and chemistry), we are using active learning techniques to evaluate the achieved performance results by applying Gaussian process, followed by applying Bayesian Optimization techniques to propose novel chemistries and structures with a higher probability to achieve further improved performances. The presented the study does not open the door only towards developing novel catalysts for energy production. However, it presents a strategy with multiple successfully demonstrated platforms towards accelerated discovery of novel materials. The high-throughput nature of the synthesis protocols presented in this talk can produce tens of thousands of different novel chemistries, at the same time taken by conventional methods to synthesize just a few, at a minimal energy consumption, cost, and manpower.

Hybrid Anode Design of Polymer Electrolyte Membrane Water Electrolysis for the Reduction of Total Platinum Group Metal Loading

Introduction Reducing capital expenditure of polymer electrolyte membrane water electrolysis (PEMWE) is essential for reducing the production cost of green hydrogen. The amount of platinum group metal (PGM) loading significantly impacts the capital expenditure. Especially iridium catalyst and platinum coating on the porous transport layer of the anode have to be reduced. Our research group has developed the catalyst-integrated porous transport electrode (PTE), where iridium catalysts were directly deposited on porous transport layer (PTL) acting as an electronic conductive coating on the PTL1 , 2. This structure could reduce total PGM loading of PEMWE cells due to being virtually free of Pt coating on the PTL, typically 1 to 5 mg cm-2 3. However, the catalyst-integrated PTE has an issue of the reduction of activation overvoltage in increasing iridium catalyst loading because the mass activity of iridium catalysts in the catalyst-integrated PTE decreases with increasing iridium loading. The hybrid anode consisting of both the conventional catalyst layer and the catalyst-integrated PTE as the PTL could reduce activation overvoltage effectively, and consequently the capital expenditure significantly with less Ir and Pt loadings. Experimental Titanium microfiber sheets with nominal porosity of ca. 70% (Nikko Techno, Ltd., Osaka, Japan) were used as the substrates acting as catalyst supports and PTLs. To increase the surface area of the titanium PTL, chemical etching with NaOH solution was performed. First, titanium sheets were etched in an aqueous 1 M NaOH solution at 60 °C for 1 h. After that, the etched titanium sheets were washed under ultra-sonication in 0.01M HNO3 solution for 30 min and then washed in deionized water at room temperature for 10 min. Heat treatment was then performed at 400 °C in 5% H2-N2 gas for 30 min. The catalyst-integrated PTE was prepared by depositing iridium onto the NaOH-etched titanium sheets via arc plasma deposition. Electrocatalyst paste for the PEMWE cathodes was prepared by dispersing the Pt/C (Pt 46.5 wt.%, TEC10E50E, Tanaka Kikinzoku Kogyo Co., Tokyo, Japan), 99.5% ethanol, deionized water, and 5% Nafion solution. IrO2 electrocatalyst paste for the PEMWE anodes was prepared by dispersing the IrO2 catalyst (IrO2 (IV) TYPE II, Tokuriki Honten Co. Ltd., Tokyo, Japan), 99.5% ethanol, deionized water, and 5% Nafion solution. Using a spray printing system (Nordson, USA), these anode and cathode electrocatalyst pastes were spray-printed onto the electrolyte membrane (Nafion 212, E. I. du Pont de Nemours and Co., Wilmington, USA). The hybrid anodes were prepared by using the catalyst-integrated PTEs instead of Pt-coated PTLs. Results and discussion The conventional anode with 0.1 mg cm-2 of IrO2, the catalyst-integrated PTE with 0.344 mg cm-2 of Ir loading, and the hybrid anode consisting of this conventional IrO2-based catalyst and the catalyst-integrated PTE were prepared and evaluated. Figure 1 shows (a) I-V characteristics, (b) activation overvoltage, and (c) ohmic overvoltage of cells with three types of anodes. Figure 1(a) shows that the hybrid design is effective for significant performance improvement over the individual electrode design with the IrO2 layer or the PTE. Electrolysis voltage of the cell with the hybrid anode at 10 A cm-2 was around 2.2 V. The activation overvoltage with the hybrid anode was lower than that with the conventional IrO2-based anode only and the catalyst-integrated PTE only, indicating that the iridium catalysts in the catalyst-integrated PTE are active even in the hybrid anode, as shown in Fig. 1(b). The ohmic overvoltage with the catalyst-integrated PTE was higher than that with the conventional anode, but the hybrid anode exhibited a comparable value to the conventional one. This may be because of the introduction of the intermediate catalyst layer consisting of ionomers and IrO2 between the catalyst-integrated PTE and the electrolyte membrane, which is a mixed conductor of ions and electrons, especially reducing the proton transport resistance between the membrane and the PTE. Thus, hybrid anodes with catalyst-integrated electrodes instead of Pt-coated PTLs can potentially improve the performance of PEM electrolyzers and reduce the total PGM loading. References M. Yasutake, D. Kawachino, Z. Noda, J. Matsuda, K. Ito, A. Hayashi, and K. Sasaki, ECS Trans., 92(8), 833 (2019).M. Yasutake, D. Kawachino, Z. Noda, J. Matsuda, S. M. Lyth, K. Ito, A. Hayashi, and K. Sasaki, J. Electrochem. Soc., 167, 124523 (2020).The International Renewable Energy Agency, Green Hydrogen Cost Reduction, (2020). Figure 1

High Entropy Oxides Synthesis by Rapid Plasma Generation with Applications Towards Electrocatalytic Hydrogen Generation

It has been challenging over the past three decades to find viable energy alternatives with higher efficiency and lower cost to beat the non-friendly and commercially dominant hydrocarbon energy sources. Hydrogen is deemed to be one of the most prominent candidates, as a green fuel. Hydrogen can be generated in abundance from water splitting. Utilization of hydrogen in fuel cells, with no doubt, produces no harmful gases. Nevertheless, the two parts of the technology: hydrogen production and reduction are cumbersome. Both processes rely on kinetically sluggish electrocatalytic reactions. Enormous research efforts have been spent, with thousands of studies presented in literature, despite the technology left with a dim light shed on from commercial and practical point of views. Many materials-related challenges are hindering the flourishing of green hydrogen technology, where poor catalyst durability is a major challenge, despite being made from very expensive elements (e.g., Ir, ru or Pt). Therefore, gigantic effort of research has been devoted to find inexpensive alternatives. Transition metals showed interesting performance towards water splitting (e.g., oxygen evolution (OER)).Recent research showed that electrocatalysts designed by alloying two or more elements (e.g., transition metals) showed promising enhancement in catalytic activity and durability due to synergistic effects from alloying, such as strain engineering or electron exchange. The potential of the alloying approach is constrained by standard synthesis methods (e.g., wet chemistry and thermal decomposition techniques) due to the governing thermodynamics rules (e.g., miscibility of the different alloying elements). That renders the synthesis of high entropy alloys (HEA), with more than 3 elements, to be complicated and tedious to achieve by those conventional methods.In the presented talk we are presenting the utilization of microwaves to generate plasma to synthesize metastable high entropy oxides (HEA) and their outstanding catalytic performance towards OER, in correlation to their unique chemistries and structures. Plasma generated enable temperature increase of the precursor substrate to elevated levels (~1000s °C), in conjunction with rapid cooling, within seconds range. Our results show successful synthesis of non-noble metals HEA NPs, possessing higher catalytic activity than IrO2 catalyst for OER. The nature of the alloying elements dictates OER activity by promoting different oxidation states of the catalytically active transition metals (e.g., Fe, Ni and Co). Our developed techniques resulted in formation of super-strong metal-carbide bonds “chemically welding” HEA NP to the catalyst support.

(Invited) Catalysts, Inks and Electrodes for PEM Water Electrolysis: Scalable Approach for Highly Performed Catalyst Layer Fabrication

The majority of commercially available anodic electrocatalysts for Low Temperature PEM Water Electrolyzers are based on unsupported iridium-based materials. These electrocatalysts can be amorphous or crystalline metallic, oxidic, and/or hydroxide-derived compounds. The planned worldwide installations of electrolyzers (on the order of tens of Gigawatts per year) will require manufacturing of Membrane Electrode Assemblies (MEAs) via highly scalable methods, one of which is Roll-2-Roll production (R2R) [1, 2]. In contrast to spraying techniques, R2R requires the utilization of inks with a higher content of solids, which in the case of extremely dense Ir-based materials may cause a sedimentation of suspended electrocatalysts resulting in overall ink destabilization.In the presented work a systematic study of ink stability with commercial IrO2 catalysts over a period of 14 days was performed. During this period of time the minimal redispersion of slurries was done to mimic realistic conditions of inks aging. The inks were comprehensively characterized by rheological measurements, zeta potential, particle size distribution (Dynamic Light Scattering) and inks density. A series of decal electrodes was fabricated using Baker bar coating approach (a model of R2R production) and coatings were characterized by contact angle, X-Ray Photoelectron Spectroscopy and Transmission Electron Microscopy.Detailed electrochemical performance analysis including the separation of voltage losses [3] via Electrochemical Impedance Spectroscopy was performed to compare MEA performance with aging time. Acknowledgements and Statements. The authors acknowledge financial support from M2FCT and H2NEW consortia. References. [1] J. Sharma, X. Lyu, T. Reshetenko, G. Polizos, K. Livingston, J. Li, D. L. Wood, A. Serov "Catalyst layer formulations for slot-die coating of PEM fuel cell electrodes" Int. Journal of Hydrogen Energy 47 (2022) 35838-3585.[2] E. B. Creel, K. Tjiptowidjojo, J. A. Lee, K. M. Livingston, P. R. Schunk, N. S. Bell, A. Serov, D. L. Wood III "Slot-Die-Coating Operability Windows for Polymer Electrolyte Membrane Fuel Cell Cathode Catalyst Layers" J. of Colloid and Interface Science 610 (2022) 474-485.[3] A. Dizon et. al “Advanced Voltage Break Down Analysis by Statistical Open-Source Tool Case Study Based on Polymer Electrolyte Water Electrolysis”, ECS 242 (2022)

Transport Properties of Iridium Oxide Catalyst Layers for Proton Exchange Membrane Water Electrolyzers

One of the current challenges of proton exchange membrane (PEM) water electrolyzers (PEMWE) is to reduce the high costs associated with iridium as the anode catalyst material. In commercial PEMWE membrane electrode assemblies (MEAs), loadings of iridium are as high as 2 to 3 mgIr/cm2. However, for PEMWE systems to match global energy demand, the Ir-specific power density must increase by ~ 50-fold [1, 2]. This can be achieved by decreasing iridium loadings and increasing the utilization of iridium. Previous work has determined that homogeneity of anode catalyst layers is generally favorable and even necessary for good electrochemical performance at lower iridium loadings [1-3]. However, gaining a better understanding of the important micro- and nano-scale features of iridium oxide (IrO2) catalyst layers is still needed for informing fabrication methods and achieving higher Ir-specific power density operation. Our previous work used nano-scale X-ray computed tomography (nano-CT) and plasma-focused ion beam cross sectioning with scanning-electron microscopy (pFIB-SEM) to characterize several IrO2 catalyst layers varying in coating methods and steps along the fabrication and testing lifetime, finding differences in morphological properties particularly of the pore size distributions. Our current work expands on this image-based characterization to elucidate transport properties between catalyst layers fabricated with different coating methods and iridium loadings. Using modeling and simulation techniques, effective transport properties for pore and solid phases such as oxygen diffusivity, conductance, and tortuosity will be examined using the high-resolution image-based data. The results presented in this work will help elucidate the transport phenomena through the IrO2 catalyst layer and the relationship between fabrication, morphology, and performance.This conference presentation was developed based upon funding from the Alliance for Sustainable Energy, LLC, Managing and Operating Contractor for the National Renewable Energy Laboratory for the U.S. Department of Energy.[1] Bernt, M., Siebel, A., & Gasteiger, H. A. (2018). Analysis of Voltage Losses in PEM Water Electrolyzers with Low Platinum Group Metal Loadings. Journal of The Electrochemical Society, 165(5), F305–F314. https://doi.org/10.1149/2.0641805jes[2] Kulkarni, D., Huynh, A., Satjaritanun, P., O’Brien, M., Shimpalee, S., Parkinson, D., Shevchenko, P., DeCarlo, F., Danilovic, N., Ayers, K. E., Capuano, C., & Zenyuk, I. v. (2022). Elucidating effects of catalyst loadings and porous transport layer morphologies on operation of proton exchange membrane water electrolyzers. Applied Catalysis B: Environmental, 308. https://doi.org/10.1016/j.apcatb.2022.121213[3] Taie, Z., Peng, X., Kulkarni, D., Zenyuk, I. v., Weber, A. Z., Hagen, C., & Danilovic, N. (2020). Pathway to Complete Energy Sector Decarbonization with Available Iridium Resources using Ultralow Loaded Water Electrolyzers. ACS Applied Materials and Interfaces, 12(47), 52701–52712. https://doi.org/10.1021/acsami.0c15687

Metallic Glass Nanofoam Anode Catalysts for Anion-Exchange Membrane Water Electrolyzers

Alkaline anion-exchange membrane water electrolyzers (AEMWEs) for hydrogen production are now receiving intensive attention due to their feasibility to use sustainable, low-cost platinum group metal (PGM-free) catalysts. Although a variety of highly efficient PGM-free catalysts for the oxygen evolution reaction (OER) have been explored, few of them demonstrated satisfied performance in real AEMWEs due to the insufficient electrical conductivity and unfavorable interfaces with ionomers in 3D porous electrodes. Herein, we report a series of highly porous ternary NiFeM (M: Cu, Co, and Mn) metallic glassy catalysts featured with nanofoam network morphologies, which are composed of amorphous OER active metal oxide shells and highly electrically conductive metallic glass alloy cores. Due to these unique properties, these NiFeM nanofoam catalysts demonstrated promising OER activities and stabilities in the half-cell with aqueous alkaline electrolytes, especially at high potentials. We also examined their magnetic properties and found no direct correlation with measured OER activity. These ternary NiFeM catalysts are further integrated with unique ionomers and AEMs to fabricate AEMWEs, showing superior performance to binary NiFe and commercial IrO2 catalysts when utilizing diluted KOH electrolytes. A different trend was identified when directly using desirable but challenging pure water, and the NiFeCu catalyst performed the best comparable to IrO2 especially at high current densities. Although deep understanding on limiting factor of pure water AEMWEs is still required, these NiFeM catalysts with favorable catalytic and morphological properties representing a new class of highly efficient PGM-free anode catalysts for viable AEMWEs toward clean hydrogen generation.

On the Effect of the Nature of Carbon Nanostructures on the Activity of Bifunctional Catalysts Based on Manganese Oxide Nanowires

Manganese oxide nanowires (MONW) combined with carbon nanostructures were synthesized using three different carbon materials, and their effect on the activity towards Oxygen Reduction Reaction (ORR) and Oxygen Evolution Reaction (OER) was investigated in alkaline electrolytes. The carbon structures were carbon nanofibers (CNF), multiwall carbon nanotubes (CNT) and reduced graphene oxide (rGO). Both MONW and carbon nanostructures were characterized by X-ray diffraction, scanning and transmission electron microscopy, N2 physisorption and X-ray photoelectron spectroscopy. The electrochemical activity was assessed in a three-electrode cell. Composite MONW/CNF showed the best activity towards ORR, and MONW/rGO exhibited the highest activity towards OER of the series. The addition of the carbon nanostructures to MONW increased the number of electrons transferred in the ORR, indicating a synergistic effect between the carbon and manganese oxide structures due to changes in the reaction pathway. The analysis of Tafel slopes and electrochemical impedance spectroscopies showed that carbons and MONW catalyze different steps of the reactions, which explains the better activity of the composites. This led us to synthesize a MONW/rGO-CNF composite, where rGO-CNF is a hybrid carbon material. Composite MONW/rGO-CNF showed an improved activity towards ORR, close to the benchmark Pt/C catalyst, and activity towards OER, close to MONW/rGO, and better than the benchmark IrO2 catalyst. It also showed remarkable stability in challenging operation conditions.

Amorphous-Crystalline Interfaces on Hollow Nanocubes Derived from Ir-Doped Ni-Fe-Zn Prussian Blue Analog Enables High Capability of Alkaline/Acidic/Saline Water Oxidations.

Development of highly efficient and robust electrocatalysts for oxygen evolution reaction (OER) under specific electrolyte is a key to actualize commercial low-temperature water electrolyzers. Herein, a rational catalyst design strategy is first reported based on amorphous-crystalline (a-c) interfacial engineering to achieve high catalytic activity and durability under diverse electrolytes that can be used for all types of low-temperature water electrolysis. Abundant a-c interface (ACI) is implemented into a hollow nanocubic (pre)-electrocatalyst which is derived from Ir-doped Ni-Fe-Zn Prussian blue analogues (PBA). The implemented c-a interface is well maintained during prolonged OER in alkaline, alkalized saline, and acidic electrolytes demonstrating its diverse functionality for water electrolysis. Notably, the final catalyst exhibits superior catalytic activity with excellent durability for OER compared to that of benchmark IrO2 catalyst, regardless of chemical environment of electrolytes. Hence, this work can be an instructive guidance for developing the ACI engineered electroctalyst which can be diversely used for different types of low-temperature electrolyzers.

Preparation of porous iridium-ruthenium-based acidic water oxidation catalyst by ascorbic acid reduction and evaporation

Green hydrogen is the ultimate direction for energy development technology. Hereon, the porous materials of promoted iridium-ruthenium-based catalyst were prepared by ascorbic acid reduction and evaporation drying methods. The unsupported iridium ruthenium-based catalysts were porous structures, characterized by scanning electron microscopes (SEM). In 0.5 mol·L−1 H2SO4, the electrochemical oxygen precipitation reaction overpotential was only 251 mV at 10 mA·cm2, compared to commercial 303 mV. The polarization overpotential was not significantly reduced, after 20 000 cycles of accelerated cyclic aging experiments. The porous structure of the iridium-ruthenium-based catalysts showed higher catalytic activity for oxygen precipitation than the commercial IrO2(ZrO2) catalysts. Porous materials could improve the activity of catalysts, which provides a good research idea for the synthesis of electrocatalysts.

Silver Compositing Boosts Water Electrolysis Activity and Durability of RuO2 in a Proton‐Exchange‐Membrane Water Electrolyzer

Proton‐exchange‐membrane water electrolyzers (PEMWEs) are of particular interest for green hydrogen production, where the oxygen evolution reaction (OER) at the anode largely determines the overall efficiency. Up to now, only ultrafine IrO2 catalyst gives desirable performance, while its scarcity and high cost inhibit the widespread application. RuO2 catalyst is the most promising alternative, while its practical application is greatly hindered by poor durability. Herein, the greatly boosted performance of conventional sub‐micrometer RuO2 by compositing with Ag is reported, and both the morphology of Ag and the compositing way significantly affect the electrolysis performance. The PEMWE fabricated with a two‐layer RuO2/Ag nanowire (NWs) composite anode achieves 1.77 A cm−2 at 2.00 V, due to a prominent 44.6 times increase of the electronic conductivity, which greatly improves the catalyst utilization. In addition, mass transportation at high‐current‐density region is enhanced due to the highly porous feature of Ag NW layer. Long‐term stability under high current density of 1 A cm−2 for 100 h is proved with the composite anode, due to the suppressed degradation of RuO2 by silver compositing. This work may accelerate the widespread commercialization of PEMWEs by providing a new way for developing IrO2‐free anode.

Performance of Polymer Electrolyte Membrane Water Electrolysis Systems: Configuration, Stack Materials, Turndown and Efficiency

A cell model is developed and validated to analyze the performance of polymer electrolyte membrane water electrolysis (PEMWE) stacks and systems. It is used to characterize the oxygen evolution reaction (OER) activity on a TiO2-supported IrO2 catalyst and an unsupported IrO2 powder catalyst. Electrochemical, stack, and system thermoneutral potentials are defined and determined for isothermal and non-isothermal stack operation. Conditions are determined under which the system thermoneutral potential or flammability of H2 in the O2 anode stream limits the stack turndown and operating temperature. Performance is analyzed of a complete PEMWE system with an electrolyzer stack containing an IrO2/TiO2 anode catalyst (2 mg/cm2 Ir loading) and N117-like membrane mitigated for H2 crossover, anode balance-of-plant (BOP) components, cathode BOP system with temperature swing adsorption for H2 purification, and electrical BOP system with transformer and rectifier. At the rated power condition, defined as 2 A/cm2 at 1.9 V, 80 °C, and 30 bar H2 pressure, the stack/system efficiency is 65.3%/60.3% at beginning of life (BOL), decreasing to 59.3%/53.9% at end of life (EOL). The peak stack/system efficiency is 76.3%/70.2% at BOL, decreasing to 71.2%/65.6% at EOL. Improvements in catalyst activity and membrane are identified for a 50% increase in current to 3 A/cm2 at 1.8 V.