Tantalum-stabilized ruthenium oxide electrocatalysts for industrial water electrolysis

Enhancing Heterointerface Coupling for Durable Industrial-Level Proton Exchange Membrane Water Electrolysis

The industrial-level application of proton exchange membrane water electrolysis (PEMWE) lies in the capacity of operating at high current density in order for higher power density and lower operational cost. However, it poses a significant challenge to the overall performance of catalysts. Heterointerface engineering has emerged as an ideal strategy for addressing the anodic intrinsic activity limitations. Nevertheless, due to the fragile interface structure with weak interactions between different components, it is difficult to maintain the high activity and long-term stability of heterostructured catalysts. Herein, we report a ternary heterostructured catalyst, RuIrOx-CeO2, featuring a strong-coupled interface between RuIrOx phase and CeO2 phase. This strong-coupled interface exhibits both electronic and oxygen interaction, which effectively inhibits the active phase separation. When applied in PEMWE (0.8 mgIr cm-2 for the anode and 0.4 mgPt cm-2 for the cathode), the resultant catalyst expresses impressive activity, achieving a current density of 3.0 A cm-2 at a cell voltage of 1.75 V in PEMWE and demonstrates a stable 2000-h operation at 5.0 A cm-2 with an imperceptible voltage degradation of <1 µV h-1.

Oxygen Evolution Enhancement of Bulk FeCoNiAlMo High-Entropy Alloy through Electrochemical Dealloying in ChCl-EG Deep Eutectic Solvent

High-entropy alloys (HEAs), which represent a new type of multielement alloys, have received growing research as potential electrocatalytic oxygen evolution reaction (OER) materials because of their excellent catalytic performances. In this study, the FeCoNiAlMo HEA with an equal atomic ratio was used as the precursor to form a three-dimensional porous structure through electrochemical dealloying in choline chloride ethylene glycol (ChCl-EG). At a current density of 10 mA cm-2, the overpotential of the porous alloy reached as low as 274 mV, which is lower than that of the commercial RuO2-IrO2. Furthermore, after long-term electrolysis, the alloy exhibited an excellent oxygen evolution performance and a good stability. Moreover, it was deduced that the dual-phase Fe20Co20Ni20Al20Mo20 HEA was mainly composed of face-centered cubic (FCC) and body-centered cubic (BCC) phases, and its phase distribution morphology was similar to that of the Turing structure. During the dealloying process, owing to the different corrosion resistances of the elements and the different elemental distributions within the two phases, the FCC phase underwent preferential corrosion. This unique structure synergistically reduces the energy barrier during water dissociation, imparting the material with a significant advantage in the OER.

The Regulation Mechanism of Oxygen Vacancies in Ruddlesden-Popper Perovskite Ln2NiO4 (Ln = La, Pr, Nd) Air Electrode for Reversible Protonic Solid Oxide Cells

Reversible protonic solid oxide cells (R-PSOCs) are promising green energy storage devices for efficient hydrogen/electricity conversion. Due to the complex environment of the air electrode, the microscopic influence mechanism of oxygen vacancies in perovskites on oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is unclear. In this study, the layered Ruddlesden-Popper perovskite Ln2NiO4 (Ln = La, Pr, Nd) air electrodes are constructed to investigate the effect of oxygen vacancies on the water/oxygen coupling in dual mode. The Pr2NiO4+δ full cell exhibits the highest peak power density of 0.692 W cm-2 in fuel cell mode and a maximum current density of -1.2 A cm-2 in electrolysis cell mode at 700 °C. The changes in electrochemical impedance spectroscopy show that Pr2NiO4+δ can absorb a small amount of interfacial water in SOFC mode to promote triple-conductivity. Meanwhile, it can have good electrolytic performance in an atmosphere of 10% H2O in the SOEC mode. The enriched oxygen vacancies of Pr₂NiO4+δ can provide a broad platform for both the ORR and OER, while the appropriate hydrophilicity can achieve a better balance state by the competitive adsorption of water/oxygen. These comprehensive characteristics make Pr2NiO4+δ suitable to be a potential Ruddlesden-Popper perovskite air electrode material for RSOCs.

Distribution of Oxygen Vacancies in RuO2 Catalysts and Their Roles in Activity and Stability in Acidic Oxygen Evolution Reaction

By combining density functional theory (DFT) calculations and the cluster expansion (CE) model in an active-learning framework, we comprehensively studied the distribution features of oxygen vacancies (OV's) as well as their contributions to the stability and activity of the RuO2 catalyst in acidic oxygen evolution reaction (OER). The results show that OV's prefer to be located at bridge oxygen sites on the RuO2(110) surface and the next-nearest-neighbor trans positions of surface RuO6 octahedra in pairs due to interactions between two OV's, and high concentrations of OV's exhibit a continuous zigzag distribution in the (110) plane of RuO2. The oxygen vacancy distribution can be explained by the charge repulsion between the low-valent Ru and O, which is referred to as the "heterovalent ion-oxygen exclusion principle". In addition, the DFT results show that the presence of OV's cannot improve the inherent OER activity of specific Ru sites since low-valent Ru sites hinder deprotonation of the second water molecule. Nevertheless, OV's can improve the stability of RuO2 by suppressing the lattice oxygen mechanism (LOM) path. In summary, this work provides deeper insights into the mechanism of the OER of RuO2 with OV's in acidic media and a possible way to improve catalyst performance by using oxygen vacancy engineering.

Measurements of Local pH Gradients for Electrocatalysts in the Oxygen Evolution Reaction by Electrochemiluminescence

An accurate understanding of the mechanism of the oxygen evolution reaction (OER) is crucial for catalyst design in the hydrogen energy industry. Despite significant advancements in microscopic pH detection, selective, sensitive, speedy, and reliable detection of local pH gradients near the catalysts during the OER remains elusive. Here, we pioneer an electrochemiluminescence (ECL) method for local pH detection during the OER. For this purpose, a new class of ECL emitters based on ECL resonance energy transfer was theoretically predicted and facilely synthesized by grafting functional fluorescent dyes onto noble 2D carbon nitride. By positioning one of the as-prepared ECL emitters with pH-responsibility neighboring the OER catalysts, local pH gradient generation near the catalysts could be qualitatively measured in real-time with a subsecond resolution. It provided details of the reaction mechanism of the OER and unveiled the catalyst degrading pathway caused by proton accumulation. Besides, the average proton generation rate on the catalyst was also extractable from the local pH measurement as a quantitative descriptor of the OER reaction rate. Owing to the high designability of the grafting method, this study opens up new strategies for studying reaction mechanisms and detecting intermediates.

Abundant Amorphous/Crystalline Interfaces of C/A-NixP/NiOH Heterojunction Catalyst for Efficient Urea Oxidation Reaction

Replacing the kinetically slow oxygen evolution reaction (OER) with urea electro-oxidation significantly reduces the energy requirement for electrolysis of water. However, designing and optimizing efficient electrocatalysts for the industrial application of urea oxidation coupled to hydrogen production remains a challenge. Herein, we construct a C/A-NixP/NiOH heterojunction catalyst with actually abundant amorphous/crystalline interfaces for the urea oxidation reaction (UOR) by an interfacial-sequential treatment method of electrodeposition and low-temperature gas-phase phosphatization on carbon cloth (CC). Remarkably, in UOR, the C/A-NixP/NiOH catalyst required only 1.332 V to reach a current density of 10 mA cm-2 with negligible potential decay over 12 h. The excellent performance is attributed to the synergistic interaction between the inner amorphous NiOH layer and the outer crystalline NixP layer, as well as the abundant amorphous/crystalline interface, an interfacial structure that can expose more active sites as well as enhance the intrinsic activity, thus improving the reaction kinetics and stability of UOR. This work paves the way for the development of low-cost and high-efficiency catalysts for urea oxidation.

Metal-oxygen bonding characteristics dictate activity and stability differences of RuO2 and IrO2 in the acidic oxygen evolution reaction

Ruthenium dioxide (RuO2) and iridium dioxide (IrO2) serve as benchmark electrocatalysts for the acidic oxygen evolution reaction (OER), yet their intrinsic activity-stability relationships remain elusive. Herein, we employ density functional theory (DFT) calculations to systematically investigate the origin of divergent OER catalytic behaviors between RuO2 and IrO2 in acidic media. Mechanistic analyses reveal that RuO2 follows the adsorbate evolution mechanism with superior activity (theoretical overpotential: 0.698 V vs. 0.909 V for IrO2), while IrO2 demonstrates enhanced stability due to a higher dissolution energy change (>2.9 eV vs. -0.306 eV for RuO2). Electronic structure analysis reveals that RuO2 exhibits ionic-dominated metal-oxygen bonds with delocalized electron distribution, facilitating intermediate desorption but promoting detrimental RuO42- dissolution. In contrast, IrO2 features covalent bonding characteristics with more electron filling in Ir-oxygen bonds (2.942 vs. 2.412 for RuO2), thereby stabilizing surface intermediates against dissolution at the expense of higher OER barriers. This work establishes a clear correlation between the bonding nature and electrocatalytic performance metrics, offering fundamental insights for the rational design of acid-stable OER electrocatalysts with optimized activity-stability relationships.

Doping Mo Triggers Charge Distribution Optimization and P Vacancy of Ni2P@Ni12P5 Heterojunction for Industrial Electrocatalytic Production of Adipic Acid and H2

Synchronous electrosynthesis of value-added adipic acid (AA) and H2 is extremely crucial for carbon neutrality. However, accomplishing the preparation of AA and H2 at large current density with high selectivity is still challenging. Herein, a robust Mo-doped Ni2P@Ni12P5 heterojunction with more P vacancies on Ni foam is proposed for accomplishing simultaneous electrooxidation of cyclohexanol (CHAOR) to AA and hydrogen evolution reaction (HER) at large current density. Combined X-ray photoelectron spectroscopy, X-ray absorption fine structure, and electron spin resonance confirm that Mo incorporation induces the charge redistribution of Ni2P@Ni12P5, where Mo adjusts electrons from Ni to P, and triggers more P vacancies. Further experimental and theoretical investigations reveal that the d-band center is upshifted, optimizing adsorption energies of water and hydrogen on electron-rich P site for boosting HER activity. Besides, more Ni3+ generated from electron-deficient Ni induced by Mo, alongside more OH* triggered from more P vacancies concurrently promote CHA dehydrogenation and C─C bond cleavage, decreasing energy barrier of CHAOR. Consequently, a two-electrode flow electrolyzer achieves industrial current density (>230 mA cm-2) with 85.7% AA yield, 100% Faradaic efficiency of H2 production. This study showcases an industrial bifunctional electrocatalyst for AA and H2 production with high productivity.

Tuning Ru-O Coordination for Switching Redox Centers in Acidic Oxygen Evolution Electrocatalysis

Avoiding lattice oxygen involvement (oxygen redox) while promoting the coupling of adjacent adsorbed oxygen (metal redox) during the acidic oxygen evolution reaction (OER) is essential for gaining high activity and robust stability in RuO2-based catalysts but remains elusive. Here, we present a precise strategy to selectively activate the metal redox process while suppressing the undesired oxygen redox pathway by fine-tuning the Ru-O coordination number in amorphous RuOx. The optimized catalyst exhibits outstanding acidic OER performance, achieving a low overpotential of 215 mV at 10 mA cm-2 and maintaining stability for 300 h with a negligible degradation rate of 100 µV h-1. X-ray absorption measurements and multiple operando spectra reveal that only Ru2-O11 moieties can selectively activate the metal redox process, whereas Ru2-O9 and Ru2-O8 moieties either trigger both redox pathways or bypass them. Theoretical calculations reveal that Ru2-O11 moiety reduces crystal field splitting energy at active Ru sites, disables lattice oxygen activation, and lowers the energy barrier for oxygen coupling. The strategy developed in this work offers new avenues for switching redox centers and refining OER mechanisms to enhance catalytic performance and long-term stability.

p-p Orbital Hybridization Stabilizing Lattice Oxygen in Two-Dimensional Amorphous RuOx for Efficient Acidic Oxygen Evolution

Developing efficient Ru-based catalysts is crucial in reducing reliance on costly Ir for the acidic oxygen evolution reaction (OER). However, these Ru-based catalysts face a fundamental stability challenge due to the highly reactive nature of lattice oxygen. In this work, we propose an effective strategy to stabilize lattice oxygen in 2D amorphous RuOx through p-p orbital hybridization by incorporating dopants such as Al, Ga, and In. Notably, Ga doping exhibits remarkable acidic OER performance, leading to a 137 mV reduction in overpotential at 10 mA cm-2 and a 125-fold improvement in stability compared to undoped RuOx. This also surpasses the performances of most reported Ru-based catalysts. In contrast, doping with other elements from the same period, such as Mn, Co, or Cu, shows negligible improvements in catalytic performance. In situ electrochemical spectroscopic analysis, couples with theoretical calculations, reveals that the p-p orbital hybridization in the Ga-O coordination within Ga-RuOx effectively reduces the reactivity of lattice oxygen, suppresses the overoxidation of Ru, and switches the reaction pathway from the lattice oxygen mechanism to the adsorbate evolution mechanism. This novel p-p orbital hybridization strategy holds great potential for the development of efficient and robust electrocatalysts for OER and beyond.

Nanostructured amorphous Ni-Co-Fe phosphide as a versatile electrocatalyst towards seawater splitting and aqueous zinc-air batteries

Electrocatalysis provides a desirable approach for moving toward a sustainable energy future. Herein, a rapid and facile potential pulse method was implemented for a one-pot electrosynthesis of the amorphous Ni-Co-Fe-P (NCFP) electrocatalyst. The 2 mg cm-2 loaded electrode displayed excellent trifunctional electrocatalytic activities toward the hydrogen evolution reaction (η HER j=10 = 102 mV), oxygen evolution reaction (η OER j=10 = 250 mV), and oxygen reduction reaction (E ORR 1/2 = 0.73 V) in alkaline solutions. Interestingly, even a lower overpotential of η HER j=10 = 86 mV was obtained at a super-high mass loading of 18.7 mg cm-2, demonstrating its feasibility for industrial-level applications. The NCFP electrocatalyst also offered superior catalytic activity in alkaline seawater electrolysis at industrially required current rates (500 mA cm-2). When implemented as an air cathode catalyst of an aqueous and quasi-solid state zinc-air battery, both devices delivered excellent performance. This study provides insights into a transformative technology towards a sustainable energy future.

Sub-4 nm Ru-RuO2 Schottky Nanojunction as a Catalyst for Durable Acidic Water Oxidation

RuO2 with high intrinsic activity for water oxidation is a promising alternative to IrO2 in proton exchange membrane (PEM) electrolyzer, but it suffers from long-term stability issues due to overoxidation. Here, we report a sub-4 nm Ru-RuO2 Schottky nanojunction (Ru-RuO2-SN) prepared by a microwave reaction that exhibits high activity and long-term stability in both three-electrode systems and PEM devices. The lattice strain and charge transfer induced by the metal-oxide SN increase the work function of the Ru-RuO2-SN, optimize the local electronic structure, and reduce the desorption energy of the metal site to the oxygen-containing intermediates; as a result, it leads to the oxide path mechanism (OPM) and inhibits the excessive oxidation of surface ruthenium. The Ru-RuO2-SN requires only 165 mV overpotential to obtain 10 mA·cm-2 with 1400 h stability without obvious activity degradation, achieving a stability number (6.7 × 106) matching iridium-based catalysts. In a PEM electrolyzer with Ru-RuO2-SN as an anode catalyst, only 1.6 V is needed to reach 1.0 A·cm-2 and it shows long-term stability at 100 mA·cm-2 for 1100 h and at 500 mA·cm-2 for 100 h. The reaction mechanism for the high stability of Ru-RuO2-SN was analyzed by density functional theory calculations. This work reports a durable, pure Ru-based water-oxidation catalyst and provides a new perspective for the development of efficient Ru-based catalysts.

Preparation of ruthenium electrode materials and their application to the bactericidal properties of acidic electrolyzed oxidizing water

The anode chlorine evolution electrode materials used for producing acidic electrolyzed oxidizing water (AEOW) typically requires platinum, iridium, ruthenium, and other expensive and non-renewable precious metals. This not only results in high production costs but also hinders the development of the industry. To reduce the economic cost of the electrode and obtain better chlorine evolution anode materials, the effects of ruthenium electrode materials doped with different elements, ruthenium-tin doping ratio, and electrolytic process parameters on the AEOW physicochemical parameter of the electrode production were studied. The findings indicated that the novel SnO2/RuO2 electrode exhibited better catalytic performance, especially the electrode with a 1 : 3 ruthenium-tin doping ratio (SnO2/RuO2-3), the active chlorine content (ACC) was 123 mg L-1, and the oxidation-reduction potential (ORP) was 1381 mV, exhibiting higher ACC and ORP values. In addition, when the current density was 50 mA cm-2, the chlorine evolution reaction potential of the SnO2/RuO2-3 electrode decreased to 55 mV, the oxygen evolution reaction potential increased to 155 mV, and the difference in potential between the CER and OER enhanced to 446 mV relative to the RuO2 electrode. The CER selectivity of the SnO2/RuO2 electrode was significantly improved, which was approximately twice that of the RuO2 electrode. Furthermore, the effects of electrolysis voltage, time, and concentration on AEOW were investigated. AEOW with an ACC content of 120 mg L-1 killed more than 99.9% of Escherichia coli within 60 seconds.

Proton-Conducting, Vacancy-Rich HxIrOy Nanosheets for the Fabrication of Low-Ionomer-Dependent Anode Catalyst Layer in PEM Water Electrolyzer

The anode catalyst layer is composed of catalytically functional IrOx and protonic conducting ionomer and largely dictates catalytic performance of proton exchange membrane water electrolyzer (PEMWE). Here, we report a new type of anode nanocatalyst that possesses both IrOx's catalytic function and high proton conductivity that traditional anode catalysts lack and demonstrate its ability to construct high-performance, low-ionomer-dependent anode catalyst layer, the interior of which-about 85% of total catalyst layer-is free of ionomers. The proton-conducting anode nanocatalyst is prepared via protonation of layered iridate K0.5(Na0.2Ir0.8)O2 and then exfoliation to produce cation vacancy-rich, 1 nm-thick iridium oxide nanosheets (labeled as □-HxIrOy). Besides being a proton conductor, the □-HxIrOy is found to have abundant catalytic active sites for the oxygen evolution reaction due to the optimization of both edge and in-plane iridium sites by multiple cation vacancies. The dual functionality of □-HxIrOy allows the fabrication of low-iridium-loading, low-ionomer-dependent anode catalyst layer with enhanced exposure of catalytic sites and reduced electronic contact resistance, in contrast to common fully mixed catalyst/ionomer layers in PEMWE. This work represents an example of realizing the structural innovation in anode catalyst layer through the bifunctionality of anode catalyst.

Recent Development of Ir- and Ru-Based Electrocatalysts for Acidic Oxygen Evolution Reaction

Proton exchange membrane (PEM) water electrolyzers are one type of the most promising technologies for efficient, nonpolluting and sustainable production of high-purity hydrogen. The anode catalysts account for a very large fraction of cost in PEM water electrolyzer and also determine the lifetime of the electrolyzer. To date, Ir- and Ru-based materials are types of promising catalysts for the acidic oxygen evolution reaction (OER), but they still face challenges of high cost or low stability. Hence, exploring low Ir and stable Ru-based electrocatalysts for acidic OER attracts extensive research interest in recent years. Owing to these great research efforts, significant developments have been achieved in this field. In this review, the developments in the field of Ir- and Ru-based electrocatalysts for acidic OER are comprehensively described. The possible OER mechanisms are first presented, followed by the introduction of the criteria for evaluation of the OER electrocatalysts. The development of Ir- and Ru-based OER electrocatalysts are then elucidated according to the strategies utilized to tune the catalytic performances. Lastly, possible future research in this burgeoning field is discussed.

Inhibiting Overoxidation of Dynamically Evolved RuO2 to Achieve a Win-Win in Activity-Stability for Acidic Water Electrolysis

Proton exchange membrane (PEM) water electrolysis offers an efficient route to large-scale green hydrogen production, in which the RuO2 catalyst exhibits superior activity but limited stability. Unveiling the atomic-scale structural evolution during operando reaction conditions is critical but remains a grand challenge for enhancing the durability of the RuO2 catalyst in the acidic oxygen evolution reaction (a-OER). This study proposes an adaptive machine learning workflow to elucidate the potential-dependent state-to-state global evolution of the RuO2(110) surface within a complex composition and configuration space, revealing the correlation between structural patterns and stability. We identify the active state with distorted RuO5 units that self-evolve at low potential, which exhibits minor Ru dissolution and an activity self-promotion phenomenon. However, this state exhibits a low potential resistance capacity (PRC) and evolves into inert RuO4 units at elevated potential. To enhance PRC and mitigate the overevolution of the active state, we explore the metal doping engineering and uncover an inverse volcano-type doping rule: the doped metal-oxygen bond strength should significantly differ from the Ru-O bond. This rule provides a theoretical framework for designing stable RuO2-based catalysts and clarifies current discrepancies regarding the roles of different metals in stabilizing RuO2. Applying this rule, we predict and confirm experimentally that Na can effectively stabilize RuO2 in its active state. The synthesized Na-RuO2 operates in a-OER for over 1800 h without any degradation and enables long-term durability in PEM electrolysis. This work enhances our understanding of the operando structural evolution of RuO2 and aids in designing durable catalysts for a-OER.

Phase-Engineered Bi-RuO2 Single-Atom Alloy Oxide Boosting Oxygen Evolution Electrocatalysis in Proton Exchange Membrane Water Electrolyzer

Engineering nanomaterials at single-atomic sites can enable unprecedented catalytic properties for broad applications, yet it remains challenging to do so on RuO2-based electrocatalysts for proton exchange membrane water electrolyzer (PEMWE). Herein, the rational design and construction of Bi-RuO2 single-atom alloy oxide (SAAO) are presented to boost acidic oxygen evolution reaction (OER), via phase engineering a novel hexagonal close packed (hcp) RuBi single-atom alloy. This Bi-RuO2 SAAO electrocatalyst exhibits a low overpotential of 192 mV and superb stability over 650 h at 10 mA cm-2, enabling a practical PEMWE that needs only 1.59 V to reach 1.0 A cm-2 under industrial conditions. Operando differential electrochemical mass spectroscopy analysis, coupled with density functional theory studies, confirmed the adsorbate-evolving mechanism on Bi-RuO2 SAAO and that the incorporation of Bi1 improves the activity by electronic density optimization and the stability by hindering surface Ru demetallation. This work not only introduces a new strategy to fabricate high-performance electrocatalysts at atomic-level, but also demonstrates their potential use in industrial electrolyzers.