Temperature Effect on the Activity and Stability of Ir-Based Oxygen Evolution Catalysts

The proton exchange membrane water electrolysis (PEMWE) powered by renewable electricity offers a facile route for clean hydrogen production. The oxygen evolution reaction (OER) at the anode plays a major role in affecting the overall device efficiency due to its sluggish OER kinetics. Thus, it remains a challenge to develop robust and active catalysts for OER in acid media for efficient PEMWE. Currently, iridium (Ir)-based materials, such as mono- and multimetallic Ir, Ir-based oxides, pyrochlore iridate oxides, and Ir-based perovskites, are the most promising OER catalysts in acid media for PEMWEs. Extensive research has been conducted to enhance the specific activity of Ir species to make cost-effective. The present review aims to provide the recent progress on addressing the long-term durability issue of Ir-based catalyst for OER in acidic conditions, aspiring to inspire the researchers to design highly efficient and stable Ir-based catalysts. Moreover, the detailed OER mechanism along with the dissolution nature of Ir species is discussed and summarized. Finally, the status, challenges, and prospects for the development of Ir-based OER catalysts are discussed.

Mathematical modeling of an integrated photovoltaic-assisted PEM water electrolyzer system for hydrogen production

Iridium-based electrocatalysts remain the only practical anode catalysts for proton exchange membrane (PEM) water electrolysis, due to their excellent stability under acidic oxygen evolution reaction (OER), but are greatly limited by their high cost and low reserves. Here, we report a nickel-stabilized, ruthenium dioxide (Ni-RuO2) catalyst, a promising alternative to iridium, with high activity and durability in acidic OER for PEM water electrolysis. While pristine RuO2 showed poor acidic OER stability and degraded within a short period of continuous operation, the incorporation of Ni greatly stabilized the RuO2 lattice and extended its durability by more than one order of magnitude. When applied to the anode of a PEM water electrolyser, our Ni-RuO2 catalyst demonstrated >1,000 h stability under a water-splitting current of 200 mA cm-2, suggesting potential for practical applications. Density functional theory studies, coupled with operando differential electrochemical mass spectroscopy analysis, confirmed the adsorbate-evolving mechanism on Ni-RuO2, as well as the critical role of Ni dopants in stabilization of surface Ru and subsurface oxygen for improved OER durability.

Polymer electrolyte membrane (PEM) electrolysis is widely considered a promising technology for energy storage and clean hydrogen production, due to the advantages it has over state-of-the-art alkaline water electrolysis.1 Typically, PEM electrolysers can operate at higher current densities than their alkaline counterparts, have a more robust design, and due to the zero-gap design, exhibit smaller Ohmic losses. Furthermore, PEM electrolysers can yield hydrogen at elevated pressures, eliminating the need for compression. Nevertheless, a series of challenges hinder the large-scale commercial implementation of PEM electrolysers. One of the most critical drawbacks is the need for noble metal electrocatalysts, especially at the anode of the cell, such as iridium or ruthenium, to catalyze the oxygen evolution reaction (OER) (1).2H2O → O2 + 4H+ + 4e- E° = 1.23 V vs NHE (1)These metals are used due to their high catalytic activity and long-term stability in the corrosive acidic environment of the PEM cells and their replacement by more available materials is difficult. Over the years, the main efforts of the scientific community towards solving this challenge were focused on increasing the noble metal dispersion, by using various high surface area catalyst supports, using core-shell catalysts, with the aim to minimize the amount of noble metal, modifying the morphology of the electrode, to improve mass transport limitations and electric contact between components and maximize noble metal utilization.2-4 Antimony-doped tin oxide has been reported as a stable OER catalyst support, offering sufficient electronic conductivity at the same, in order to minimize Ohmic losses.5 Furthermore, recent reports indicate that SnO2 does not only act as a catalyst support but takes part in catalysis.6 Building upon literature reports, we prepared a series of Sn/Ir OER catalysts by a two-step sodium borohydride reduction method, in which Sn nanoparticles were deposited over the surface of iridium black particles, decorating it. Catalysts with varying Sn content were incorporated in membrane electrode assemblies (MEAs) and their cell performance was evaluated in a single cell PEM electrolyser. We found that catalysts with up to 20 wt% tin content exhibited an improved cell performance compared to MEAs with similar iridium black loading. We modeled Ohmic, kinetic and mass transport overpotential contributions to experimentally acquired polarization curves and identified that the better cell performance arises mainly from the higher intrinsic catalytic activity, reflected by a higher apparent exchange current density of Sn/Ir OER catalysts compared to Ir black catalysts. The Sn/Ir MEAs also displayed lower mass transport overpotentials, which indicate improved transport in the catalyst layer. This can be due to a more porous structure and less agglomeration of iridium particles induced by the presence of tin over their surface.Interestingly, this performance improvement was only observed for iridium black catalysts. We prepared a series of IrO2-SnO2 catalysts by solution combustion synthesis method, which is known to yield high surface area disperse oxide materials. Comparing the performance of these catalysts to commercial iridium oxide catalysts in the PEM electrolysis cell, the MEAs with SnO2-containing catalysts displayed worse cell performance than MEAs with commercial iridium oxide. Modelling the overpotential contributions showed a lower apparent exchange current density, indicating a lower intrinsic catalytic activity of IrO2-SnO2 catalysts, as well as a higher Ohmic resistance, due to SnO2 addition. These observations indicate the importance of iridium oxidation state for harnessing the promotion effect of tin addition and give important insights into designing more efficient OER catalysts for PEM electrolysis cells.

To achieve carbon neutrality, extensive research is ongoing to produce green hydrogen via water electrolysis combined with renewable energy sources. Understanding and developing catalysts for membrane electrode assembly (MEA) type water electrolysis systems, such as proton exchange membrane water electrolysis (PEMWE) and anion exchange membrane water electrolysis (AEMWE), are critical steps toward this goal. Therefore, this presentation will focus on catalyst development for PEMWE and AEMWE, with emphasis on cost-effective catalysts.For oxygen evolution reaction (OER) catalysis in PEMWE, the primary challenge is to minimize iridium (Ir) usage for sustainable green hydrogen production. Layered monoclinic iridium nickel oxide (IrNiOx) platelets were synthesized using a molten salt method and employed for the OER. These platelets exhibited high OER activity with reduced Ni dissolution in acidic conditions. When applied in MEA, they demonstrated enhanced interconnectivity within the catalyst layer, promoting electron transfer. Even at low Ir loading (0.2 mgIr cm-2), the platelets showed good performance with an initial cell voltage of 1.70 V at 1 A cm-2 and minimal degradation over 100 hours. This highlights the effectiveness of incorporating transition metals into Ir oxide to reduce Ir usage in PEMWE.For oxygen evolution reaction (OER) catalysis in AEMWE, attention is focused on nickel iron (NiFe) hydroxide catalysts for their high activity and stability in alkaline conditions. However, the lack of initial electrical conductivity of as-prepared NiFe LDH limits its potential as an electrocatalyst. To address this issue, a monolayer structuring approach was proposed. This method improved mass transport to allow a high energy conversion efficiency of 72.6% with exceptional stability over 50 hours at 1 A cm-2. Additionally, lack of initial electrical conductivity hinders determination of electrochemically active surface area (ECSA) of NiFe LDH using conventional double layer capacitance method. Use of electrochemical impedance spectroscopy (EIS) at reactive OER potentials to extract the capacitance that is hypothesized to arise due to reactive OER intermediates (O*, OH*, OOH*) adsorbed on the catalyst surface was thus investigated. This allowed the estimation of ECSA and intrinsic activity of NiFe LDH, validating the methodology through rigorous catalyst loading and support studies.For hydrogen evolution reaction (HER) catalysis in AEMWE, replacing platinum (Pt) with Ni-based alloys containing molybdenum (Mo) species has shown promise. Dynamic active sites of Ni catalysts with Mo species require innovative approaches to maintain initial performance, warranting material innovation or careful manipulation of operating protocol. The approaches in which this is made possible is shortly presented, together with the innovations that allows probing of such phenomenon.

Evaluation of carbon-supported Pt and Pd nanoparticles for the hydrogen evolution reaction in PEM water electrolysers

In the anodic catalyst layer of a proton-exchange membrane (PEM) water electrolyzer, the triple-phase boundary (TPB) is mainly distributed on the surface of ultrafine iridium-based catalysts encapsulated by the ionomer within the catalyst-ionomer agglomerate. It is found that the ionomer at the TPB acts as a barrier to mass transport and a buffer for the bubble coverage during the oxygen evolution reaction (OER). The barrier effect can decrease the OER performance of the catalysts inside the agglomerate by ≤23%, while the buffer effect can separate the bubble evolution sites from the OER sites, turning the instant deactivation caused by the bubble coverage into a gradual performance loss caused by local water starvation. However, this local water starvation still deteriorates the catalyst performance because of the affinity of the ionomer surface for bubbles. Introducing additional transport paths into the agglomerate can reduce the barrier effect and regulate the bubble behavior, reducing the overpotential by 0.308 V at 5 A cm-2.

Efficient and robust single-layer IrOx cluster electrocatalyst for oxygen evolution reaction

Oxygen evolution reaction (OER) plays a key role in proton exchange membrane water electrolysis (PEMWE), yet the electrocatalysts still suffer from the disadvantages of low activity and poor stability in acidic conditions. Here, a new class of CdRu2 IrOx nanoframes with distorted structure for acidic OER is successfully fabricated. Impressively, CdRu2 IrOx displays an ultralow overpotential of 189mV and an ultralong stability of 1500h at 10mA cm⁻2 toward OER in 0.5M H2 SO4 . Moreover, a PEMWE using the distorted CdRu2 IrOx can be steadily operated at 0.1 A cm⁻2 for 90h. Microstructural analyses and X-ray absorption spectroscopy (XAS) demonstrate that the synergy between Ru and Ir in CdRu2 IrOx induces the distortion of Ru-O, Ir-O, and Ru-M (M = Ru, Ir) bonds. In situ XAS indicates that the applied potential leads to the deformation octahedral structure of RuOx /IrOx and the formation of stable Ru5+ species for OER. Theoretical calculations also reveal that the distorted structures can reduce the energy barrier of rate-limiting step during OER. This work provides an efficient strategy for constructing structural distortion to achieve significant enhancement on the activity and stability of OER catalysts.

This article aims to develop a proton exchange membrane (PEM) electrolyzer emulator. This emulator is realized through an equivalent electrical scheme. It allows taking into consideration the dynamic operation of PEM electrolyzers, which is generally neglected in the literature. PEM electrolyzer dynamics are reproduced by the use of supercapacitors, due to the high value of the equivalent double-layer capacitance value. Steady-state and dynamics operations are investigated in this work. The design criteria are addressed. The PEM electrolyzer emulator is validated by using a 400-W commercial PEM electrolyzer. This emulator is conceived to test new DC-DC converters to supply the PEM ELs and their control as well, avoiding the risk to damage a real electrolyzer for experiment purposes. The proposed approach is valid both for a single cell and for the whole stack emulation.

Within renewable energy systems, proton exchange membrane (PEM) electrolyzer relies on multiple proton-coupled electron transfer (PCET) reaction on heterogeneous catalyst surface. In PEM electrolyzer, the catalytic process of hydrogen adsorption through Volmer reaction, involving the combination of a proton and an electron on surface, holds significance. Recent attention has been focused on optimizing the proton and an electron coupling by lowering the activation barrier and driving force of the hydrogen adsorption reaction. This focus is primarily on the cathodic part, involving hydrogen evolution reaction (HER) and organic electro-hydrogenation systems, initiating the formation of a surface-adsorbed hydrogen atom (H*) as first step. Among them, hydrogen spillover has emerged as a new method to reduce the activation barrier in HER and hydrogenation systems. This mechanism contains the diffusion of adsorbed H+ from hydrogen-rich to deficient surface. An optimal catalyst for hydrogen spillover is characterized by strong dissociation abilities and weak binding to dissociated hydrogen atoms. Palladium (Pd) exhibits commendable equilibrium of these features, suggesting the potential for performance improvement through alloying with other metals. However, hydrogen spillover is specific to the cathodic part, where the reactant is the surface-adsorbed hydrogen atom, and do not extend to the anodic part, which involves reactants unrelated to the adsorbed hydrogen atom.Generally, in the anodic part of the electrolyzer system, water-based reactants (H2O in acid/ OH- in based) or organic-based reactants (such as hydrazine, ethanol, and formic acid) are utilized. Nowadays, organic-based reactants are preferred in low-voltage electrolysis due to the sluggish kinetics of electro-decomposition in water-based reactants. Among them, formic acid emerges as a promising organic molecule for achieving high efficiency. Formic acid electrooxidation reaction (FAOR) theoretically exhibits a negative oxidation potential compared to both hydrogen oxidation reaction (HOR) and oxygen evolution reaction (OER): Eo = -0.17 VSHE for HCOOH/CO2 < +0 VSHE for H2/H+ < +1.23 VSHE for H2O/O2. Platinum (Pt)-based catalysts are utilized in FAOR. Unfortunately, since HCOOH oxidation occurs under more positive potential than H* desorption (0.05 V < E < 0.35 VRHE), the FAOR encounters a limitation of overpotential that exceeds the potential of H* desorption. In other words, acceleration of H* desorption on active site could be necessary for the anodic reaction. To facilitate the movement of H* in both anodic and cathodic reactions, it is essential to consider accelerating both H* adsorption and desorption on the active site to ensure the efficiency. Despite the emphasis on activating the target reaction by controlling PCET, there are limited studies on dual mechanisms of H* adsorption and desorption in catalytic system.Biological energy conversion involves a diverse array of ion-coupled electron transfer reaction. Many organisms in electron transport chain facilitate the movement of electrons from electron donors to acceptors through redox reactions, linking this electron transfer with H+ ions. Here, we report Amphi-functional mediator (AFM) molecules, as a hydrogen atom modulator. Through the electrodeposition method onto Pt and Pd, we facilitate distinct movements of H*, resulting in low overpotential and enhanced catalytic efficiency. Depending on the migration energy of H*, AFM can function as both an acceptor and donor of H* to Pt and Pd, respectively. In a hydrogen-producing electrolyzer employing FAOR as the anodic process (FAOR||HER), AFC reduced the cell potential by 1.15 V at 10 mA cm-2, improving the faradaic efficiency from 87% to 100%. In an electrochemical hydrogen-carrier-producing reactor to hydrogenate toluene to methylcyclohexane (MCH), AFM successfully generated MCH, which cannot be produced without AFM. (Figure 1) Figure 1

The challenge of achieving high-performance iridium-based catalysts towards the oxygen evolution reaction in proton exchange membrane electrolyzers remains unresolved due to the highly acidic catalyst/PEM interface and oxidation conditions.

Electrolysis – Proton-exchange membrane electrolyzer | Proton exchange membrane water electrolysis: State-of-the-art technique and systems

The global push towards cleaner energy sources has heightened the demand for high-performing, durable proton exchange membrane water electrolyzer (PEMWE) electrocatalysts. Green hydrogen, generated through water electrolysis powered by renewable energy, stands as a prominent solution in this endeavor to transition away from fossil fuels. Among various electrolyzer technologies, PEMWE systems currently offer the most competitive pathway for green hydrogen production. These systems typically employ a platinum on carbon supported (Pt/C) cathodic electrocatalyst for the hydrogen evolution reaction (HER) and an iridium oxide (IrOx) anodic electrocatalyst for the oxygen evolution reaction (OER). Efforts to enhance PEMWE performance primarily focus on improving the kinetics of the OER process, which accounts for the majority of activation overpotential during operation. To address cost concerns associated with IrOx, researchers are exploring alternative formulations, including doping with materials like niobium (Nb), aiming for comparable performance at reduced Ir loading [1], [2], [3]. It is argued that higher surface area and lower crystalline IrOx catalysts lead to lower activation overpotentials and enhanced durability, respectively. In this study, we investigate the performance of high-surface-area IrOx electrocatalysts (>200 m2/g) and Nb-doped IrOx variants, comparing them with commercially available benchmarks. Through rigorous examination utilizing Rotating Disk Electrode (RDE), Gas Diffusion Electrode (GDE) half-cell, and Membrane Electrode Assemblies (MEAs), we report on the performance and stability of our in-house prepared materials. Preliminary results examining various loadings of IrOx suggest that lower loadings lead to durable and active electrocatalyst layers. Our findings underscore the potential of Nb-doped IrOx as a cost-effective alternative for PEMWE electrocatalysts, offering promising advancements in the pursuit of efficient and sustainable hydrogen production.

Presently, the most efficient catalysts chosen for proton exchange membrane water electrolyzers (PEMWE) are platinum group metal (PGM) based, and the required high mass loading of these metals makes the technology expensive at large scale.1 Thus, it is desirable to find alternative catalysts or structures that can lower PGM loading while maintaining efficient PEMWE operation. The anodic oxygen evolution reaction (OER) of a PEMWE suffers from a high catalyst loading as a result of lack of stable catalyst supports that can withstand the highly corrosive electrochemical conditions. These supports would help disperse the PGMs within the catalyst layer. The current practice is to use unsupported iridium based nanoparticles as catalysts since most supports degrade under such harsh conditions. However, only surface atoms participate in electrocatalysis, leaving the bulk of the iridium as an expensive PGM support that functions solely as a conductive matrix. Replacing the core with less expensive and resource constrained materials that can be conductive yet stable under the highly oxidizing OER conditions is very desirable. Unfortunately, doping or defecting oxides to make them conductive hinders their stability, so other tactics must be utilized.2 In this talk we will present our work on materials prepared by coating insulating titanium dioxide nanoparticles (< 100 nm) with small amounts of conductive platinum or gold nanoparticles to prepare supports for iridium catalysts. Under OER conditions, all three materials are stable. In order to synthesize these structures, we identified synthesis routes to prepare well dispersed layers of these precious metal nanoparticles onto the titanium dioxide nanoparticles; making them the optimal substitute metals for an iridium core. These nanostructures were then coated with iridium to prepare supported OER catalysts with the desired activity and stability. Three synthetic procedures were implemented and compared in order to determine the most efficient way to prepare the nanoparticle composites, i.e. wetness impregnation, citric acid reduction, and light catalyzed deposition. X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), and Energy Dispersive X-ray Spectroscopy (EDS) were utilized to characterize the nanoparticles. XRD suggested that the final catalyst material did indeed contain a combination of crystalline titanium dioxide, iridium, and metal gold or platinum nanoparticles. SEM and EDS indicated that the iridium, as well as the gold or platinum nanoparticles, were well distributed upon the larger titanium dioxide nanoparticles. It was determined that the best distribution was found when using platinum nanoparticles and the photo reduction synthesis technique. The complete synthesis methods, characterization of the materials, as well as electrocatalytic performance towards OER using rotating disk electrode in acidic electrolyte, will be presented in this poster.