Water Oxidation Catalysts Research Articles

Layered double hydroxides as a robust catalyst for water oxidation through strong substrate-catalytic layer interaction

Electrochemical preparation of hydrogen permits a way to meet our energy and environmental issues, but the anodic reaction is kinetically slow and thus is the bottleneck in the overall water splitting. Achieving cost-effective electrocatalysts represents a big challenge for practical water oxidation. Herein, We report the tuning of the local electron structure of nickel-iron layered double hydroxides (NiFe LDH) by achieving a highly coupled interface between active sites and the current collector. Our developed NiFe-LDH/Fe catalysts as the anode can provide 10 mA cm−2 at 1.455 V vs. RHE with a Tafel slope of only 32.7 mV dec−1. Simultaneously, the catalyst has high catalytic stability. Combined with the X-ray photoelectron spectroscopy and density functional theory calculation, the coupling interface between the substrate and catalytic layer can effectively regulate the electron density of the Fe sites, which is the main reason for the high catalytic ability of the layered double hydroxide catalyst.

Size‐Controlled Synthesis of IrO2 Nanoparticles at High Temperatures for the Oxygen Evolution Reaction

AbstractIridium oxide is the state‐of‐the‐art catalyst for electrochemical water oxidation in an acidic medium. Despite being one of the rarest elements in the Earth's crust, there is a pressing need to maximize the utilization and longevity of active iridium centers. While conventional low‐temperature synthesis can yield nanostructures with high mass‐specific activity, they are often insufficiently stable during water oxidation. Structurally ordered iridium oxide is one of the most stable electrocatalysts utilized in polymer electrolyte membrane water electrolysis that benefits from the chemically ordered structure. However, its preparation requires thermal treatment at high temperatures, which improves its durability but can also result in reduced surface area and altered particle morphology. In this study, the challenge of controlling nanoparticle size during the preparation of structurally ordered iridium oxide is successfully addressed, which typically requires high‐temperature thermal treatment. By utilizing a silica nanoreactor as a hard template, a precise control is achieved over the nanoparticle size during high‐temperature thermal treatment. This approach maintains high durability while avoiding the common problem of reduced surface area and altered particle morphology. Specifically, this study is able to synthesize iridium oxide nanoparticles at temperatures up to 800 °C, while keeping their dimensions below 10 nm.

Homogeneous NiMoO4–Co(OH)2 bifunctional heterostructures for electrocatalytic oxygen evolution and urea oxidation reaction

Developing bifunctional electrocatalysts for catalyzing water oxidation and urea oxidation reactions is of vital importance for alleviating the energy crisis and environmental damage, but it is still formidably challenging. Here, the potential of a heterostructure of NiMoO4 nanorods conformally covered by metal-organic framework (MOF) derived Co(OH)2 nanosheets is demonstrated. Under the assistance of a series of advanced characterizations and electrochemical measurements, the rich heterointerfaces in the NiMoO4–Co(OH)2 are found to be beneficial for promoting the electrocatalytic performance. As a bifunctional catalyst for the electrooxidation of water and urea, the optimized NiMoO4–Co(OH)2-100 demonstrates higher catalytic activity, kinetics and durability than individual NiMoO4 and Co(OH)2. Specifically, an overpotential as low as 285 mV can drive 10 mA cm−2 for oxygen evolution reaction (OER), and a voltage of 1.32 V is demanded for achieving 10 mA cm−2 towards urea oxidation reaction (UOR), significantly lower than the benchmarked catalysts. Presented outcomes can greatly contribute to the industrial application of urea-containing wastewater purifying, by offering a deep understanding on the interface engineering of nanocatalysts.

Free-standing high-entropy alloy plate for efficient water oxidation catalysis: structure/composition evolution and implication of high-valence metals

We herein report free-standing FeCoNiCrMo high-entropy alloy (HEA) plate as an excellent oxygen evolution reaction (OER) catalyst with activity and durability superior to benchmarking NiFe layered double hydroxide (LDH). It requires overpotentials of 303 and 372 mV to reach current densities of 100 and 700 mA cm−2, respectively, and maintains its activity for at least 120 h in alkaline media. Its dynamic structural/compositional evolution is uncovered, in which Mo metals are rapidly oxidized and dissolved into electrolyte, provoking the concomitant Cr leakage and leaving behind reconstructed active NiFeCo oxyhydroxide nanostructures on the surface. Its enhanced OER (intrinsic) activity is found to be associated with the absorption of resultant Mo oxo-anion (MoO42-) and the persistence of trace Cr atoms in surficial NiFeCo oxyhydroxide phase. This work offers useful insights into the origin of high OER activity of HEA catalysts and also provides a feasible way for mass-production of active OER catalysts.

Activation stainless steel with electrochemical etching-hydrothermal as an efficient and stable alkaline water oxidation electrode

Stainless steel (SS) has great potential to become an efficient oxygen evolution reaction (OER) catalytic material, but an effective activation method is still lacking. Here, an electrochemical etching-hydrothermal activation strategy is proposed to turn SS into an efficient and stable OER electrode, which can deliver high current density of 300 mA cm−2 with only 314 mV, as well as offers a low Tafel slope of 30 mV dec−1. The catalytic phase formed by the activation has a low surface Ni/Fe ratio and completely disobeys the traditional “Fe-effect” activity trend. The performed O–K edge X-ray adsorption spectra revealed that the high valence states of Ni sites with modified electronic structure may be responsible for the enhanced OER performance. More importantly, the assembled overall water-splitting cell showed that it can withstand long-term durability of 100 h at an ultra-high current density of 2 A cm−2. The strategy presented here could yield an opportunity to convert SS into an efficient and robust catalyst for alkaline water oxidation and is expected to lead to more promising ones due to the diverse composition of SS.

Ligand Tuning in Cu(pyalk)2 Water Oxidation Electrocatalysis

Molecular copper water oxidation electrocatalysts have been extensively studied in recent years for their potential use in artificial photosynthetic systems for solar energy conversion. Although ligand modification and its ability to influence catalytic properties is a key advantage of molecular systems, there are, as yet, few examples of systematic studies of these effects. Our oxidatively resistant pyalk (2-pyridyl-2-propanoate) ligand forms a complex with copper(II) that catalyzes water oxidation and provides an attractive scaffold for systematic ligand tuning. Here, we report a series of analogous copper complexes with electron-donating (methoxy-) and -withdrawing (methoxycarbonyl-) groups at the para-position of the pyalk ligand. Trends in the pKa and redox potential align with first-principles predictions for the electron-withdrawing and electron-donating groups. While the modified complexes show good activity for water oxidation, lowered faradaic efficiency in comparison to the parent complex highlights the importance of stability considerations for catalyst tuning.

Hydrophilic MnO2 nanowires coating with o-fluoroaniline for electrocatalytic water oxidation

In the active Mn4CaO5 cluster of the natural water oxidation system, one of the dangling Mn atoms is surrounded by water molecules through coordination and hydrogen bonding. It means that the enrichment of water molecules around the Mn catalysts has a positive effect on the water oxidation catalysis. Inspired by nature photosynthesis, the hydrophilic surfaces are believed to be able to capture water molecules to assist water oxidation. Herein, we report MnO2@PFANI nanowires with improved mass transfer efficiency and conductivity for electrocatalytic water oxidation. The as-prepared nanowires are synthesized by coating hydrophilic polymers on MnO2 nanowires. The hydrophilic surface captures water molecule and promotes mass transfer processes, and the conductivity of the polymer can also increase charge transfer processes. To reach a current density of 10 mA cm−2, a low overpotential of 440 mV is required in a 1.0 M KOH solution for the MnO2@PFANI nanowires. The hydrophilicity and conductivity are systematically investigated by both physical and electrochemical measurements. This study provides a surface engineering idea for the fabrication of efficient electrocatalysts.

Molecular Nature of Electrodeposits in Electrochemical Water Oxidation

AbstractThe molecular nature and the structure of the real catalyst are matters of serious concern in homogeneous catalysis. Catalyst transformation in molecular water oxidation catalysis (WOC) is usually linked to catalyst decomposition with the generation of nanostructured materials. This is mainly due to the operational conditions that are conducive for the formation of metal oxides (MOx), which have a strong affinity for adsorbing onto the electrodes’ surfaces. Recently, the exhibition of the molecular nature of the electrodeposits rather than MOx is a breakthrough in the field of WOC, suggesting that deposits in electrocatalysis are not constrained to metal oxides/hydroxides after molecular catalyst decomposition. Therefore, several points should be considered before deciding whether the deposits are aggregates of a molecular structure or simple metal oxides formed due to the decomposition of the original structure. Thus, the aim of this perspective is to showcase the examples of homogeneous WOCs where metal–organic species are electrodeposited instead of metal oxide‐based catalysts. The transformation of heterogenized molecular catalysts for WO into a more electroactive form while preserving their molecular identity, is also discussed. This summary provides an overview of the molecular nature of electrodeposits, how to identify their presence and decide about their nature.

Bioinspired Active Site with a Coordination-Adaptive Organosulfonate Ligand for Catalytic Water Oxidation at Neutral pH

Many enzymes use adaptive frameworks to preorganize substrates, accommodate various structural and electronic demands of intermediates, and accelerate related catalysis. Inspired by biological systems, a Ru-based molecular water oxidation catalyst containing a configurationally labile ligand [2,2':6',2″-terpyridine]-6,6″-disulfonate was designed to mimic enzymatic framework, in which the sulfonate coordination is highly flexible and functions as both an electron donor to stabilize high-valent Ru and a proton acceptor to accelerate water dissociation, thus boosting the catalytic water oxidation performance thermodynamically and kinetically. The combination of single-crystal X-ray analysis, various temperature NMR, electrochemical techniques, and DFT calculations was utilized to investigate the fundamental role of the self-adaptive ligand, demonstrating that the on-demand configurational changes give rise to fast catalytic kinetics with a turnover frequency (TOF) over 2000 s-1, which is compared to oxygen-evolving complex (OEC) in natural photosynthesis.

CataLight: The Collaborative Research Center for Light-Driven Catalysis in Soft Matter.

This invited Team Profile was created by the Collaborative Research Center for Light-Driven Catalysis in Soft Matter, CataLight, located at Friedrich Schiller University Jena, Ulm University, Max Planck Institute of Polymer Research (MPI) and Johannes Gutenberg University Mainz, University of Vienna, and the Center of Electron Microscopy, Ulm University. The authors, who include members of the Kranz and Leopold Groups, the Schacher Group, and the Streb Group, recently published an article on local measurements of light-driven activity at heterogenized water oxidation catalysts using nanoporous block copolymers: "Multimodal Analysis of Light-Driven Water Oxidation in Nanoporous Block Copolymer Membranes", J. Kund, J.-H. Kruse, A. Gruber, I. Trentin, M. Langer, C. Read, G. Neusser, D. Blaimer, U. Rupp, C. Streb, K. Leopold, F. H. Schacher, C. Kranz, Angew. Chem. Int. Ed. 2023, e202217196.

CataLight: The Collaborative Research Center for Light‐Driven Catalysis in Soft Matter

AbstractThis invited Team Profile was created by the Collaborative Research Center for Light‐Driven Catalysis in Soft Matter, CataLight, located at Friedrich Schiller University Jena, Ulm University, Max Planck Institute of Polymer Research (MPI) and Johannes Gutenberg University Mainz, University of Vienna, and the Center of Electron Microscopy, Ulm University. The authors, who include members of the Kranz and Leopold Groups, the Schacher Group, and the Streb Group, recently published an article on local measurements of light‐driven activity at heterogenized water oxidation catalysts using nanoporous block copolymers: “Multimodal Analysis of Light‐Driven Water Oxidation in Nanoporous Block Copolymer Membranes”, J. Kund, J.‐H. Kruse, A. Gruber, I. Trentin, M. Langer, C. Read, G. Neusser, D. Blaimer, U. Rupp, C. Streb, K. Leopold, F. H. Schacher, C. Kranz, Angew. Chem. Int. Ed. 2023, e202217196.

Catalytic supercritical water oxidation of o-chloroaniline over Ru/rGO: Reaction variables, conversion pathways and nitrogen distribution

To ascertain the reaction variables on o-chloroaniline (o-ClA) mineralization, total nitrogen (TN) removal rate, and N-species distribution, o-ClA was subjected to catalytic supercritical water oxidation (CSCWO) in a fused quartz tube reactor (FQTR). The findings demonstrated that when the temperature, reaction time, and excess oxidant were 400 °C, 90 min, and 150%, respectively, the mineralization rate of o-ClA could reach more than 95%. Moreover, potential degradation pathways of o-ClA in supercritical water oxidation (SCWO) was proposed according to the GC-MS results. TN removal rate is significantly impacted by Ru/rGO, despite the fact that its catalytic effect on the mineralization of o-ClA was not particularly noteworthy. Compared with no catalyst, the TN removal rate of o-ClA obviously increased from 44.1% to 90.3% at 400 °C, 10 wt% Ru loading, 90 min and 200% excess oxidant. In addition, N-species distribution in SCWO and CSCWO were also investigated. Results indicated that the Ru/rGO catalyst could accelerate the oxidation of ammonia-N and convert it to nitrate-N, promoting N2 generation. Finally, the possible N transformation pathway in CSCWO of o-ClA was proposed. As a result, this work offers fundamental information about o-ClA catalytic oxidation removal in the SCWO process.

Construction of nickel selenides heterointerfaces with electron redistribution for solar-driven water splitting

Developing heterostructures electrocatalysts was a promising method to improve the water splitting efficiency. However, due to the difficulty of synthesis, such low-cost and high-activity heterostructures electrocatalysts have not been developed. In this work, Ni3Se2/NiSe heterostructure nanosheets with rich-phase boundaries were synthesized by soaking at room temperature and annealing treatment using nickel foam as both Ni source and substrate. Such a nanosheet-like Ni3Se2/NiSe heterostructure could expose more active sites and efficient mass transfer at solid-liquid-gas three-phase interfaces, which exhibits a lower overpotential of 336 mV at 100 mA cm−2 and exceptional stability over 80 h at the current density of 300 mA cm−2 for OER in alkaline solution. Furthermore, the heterostructures electrode implements solar-driven water with a high solar-to-hydrogen efficiency of 18.3%. This heterostructure strategy might be a major breakthrough for improving the transition metal selenides and designing high-active and stable catalysts for electrochemical water oxidation.

Effective CuO/CuS heterostructures catalyst for OER performances

Although the generation of hydrogen by electrolysis of water is cheapest from all other available sources but splitting of water still takes place with sluggish kinetics and is a challenging barrier for H2 production on large scale. Moreover, research work is still in progress to understand the oxygen evolution reaction (OER) and design the catalysts with improved OER performance. Herein, we report synthesis, characterization, and OER performance of copper oxide/copper sulfide (CuO/CuS) heterostructures and its individual components copper oxide (CuO) and copper sulfide (CuS) as low-cost catalysts for water oxidation. CuO/CuS has been demonstrated to be a stable and effective catalyst that initiates the OER at a potential of 1.49 V. (vs. RHE). The CuO/CuS catalyst drives the OER at only 270 mV overpotential and Tafel slope of 67 mV dec−1 with a current density of 10 mA cm−2 in 1.0 M KOH. CuO/CuS heterostructures outperforms OER performance of previously reported Cu-based catalysts. Long-term stability and performance toward OER are demonstrated by the oxygen evolution at an applied potential of 1.65 V (vs. RHE) over an extended period. This feature brings a favourable, non-noble, highly efficient, and robust OER catalyst.

Effect of mixed-valence of manganese on water oxidation activity of La1-xCaxMnO3 (0 ≤ x ≤ 1) solid solutions

The water oxidation reaction is one of the crucial process involves 4e- and 4H+ transfer, which causes a high energy barrier and makes the water-splitting process a slow kinetic reaction. Herein, we have systematically observed the water oxidation activity of La1-xCaxMnO3 (0≤ x ≤ 1) solid solutions by tuning the valence of Mn ion through selective substitution of La3+ with Ca2+. With increasing Ca concentration in La1-xCaxMnO3 solid solutions, the water oxidation activity increases up to x = 0.5 and then decreases. Mixed-valence of Mn generated due to substitution of La3+ with Ca2+, which leads to the double exchange mechanism. This could results in high electrical conduction and a faster electron transfer rate because of the rapid charge exchange between Mn3+ and Mn4+ ions, which pinpoints the reason behind the enhanced oxygen evolution activity. Our findings will be a promising way to relate the Mn mixed-valence with its catalytic activity to design and development of efficient water oxidation catalysts.

Magnetic Interactions in a [Co(II)3Er(III)(OR)4] Model Cubane through Forefront Multiconfigurational Methods

Strong electron correlation effects are one of the major challenges in modern quantum chemistry. Polynuclear transition metal clusters are peculiar examples of systems featuring such forms of electron correlation. Multireference strategies, often based on but not limited to the concept of complete active space, are adopted to accurately account for strong electron correlation and to resolve their complex electronic structures. However, transition metal clusters already containing four magnetic centers with multiple unpaired electrons make conventional active space based strategies prohibitively expensive, due to their unfavorable scaling with the size of the active space. In this work, forefront techniques, such as density matrix renormalization group (DMRG), full configuration interaction quantum Monte Carlo (FCIQMC), and multiconfiguration pair-density functional theory (MCPDFT), are employed to overcome the computational limitation of conventional multireference approaches and to accurately investigate the magnetic interactions taking place in a [Co(II)3Er(III)(OR)4] (chemical formula [Co(II)3Er(III)(hmp)4(μ2-OAc)2(OH)3(H2O)], hmp = 2-(hydroxymethyl)-pyridine) model cubane water oxidation catalyst. Complete active spaces with up to 56 electrons in 56 orbitals have been constructed for the seven energetically lowest different spin states. Relative energies, local spin, and spin-spin correlation values are reported and provide crucial insights on the spin interactions for this model system, pivotal in the rationalization of the catalytic activity of this system in the water-splitting reaction. A ferromagnetic ground state is found with a very small, ∼50 cm-1, highest-to-lowest spin gap. Moreover, for the energetically lowest states, S = 3-6, the three Co(II) sites exhibit parallel aligned spins, and for the lower states, S = 0-2, two Co(II) sites retain strong parallel spin alignment.

Multimodale Analyse lichtinduzierter Wasseroxidation in nanoporösen Blockcopolymer‐Membranen**

AbstractDie heterogene lichtgetriebene Katalyse ist ein Eckpfeiler nachhaltiger Energieumwandlung. Die meisten katalytischen Studien konzentrieren sich auf die Analyse von Wasserstoff und Sauerstoff, wobei in der Regel über die Gesamtantwort gemittelt wird, und somit eine Korrelation von Heterogenitäten der Matrix mit unterschiedlichen molekularen Aktivitäten limitiert ist. Hier berichten wir von Untersuchungen eines heterogenisierten Katalysator‐Photosensibilisator‐Systems, bei dem ein Polyoxometalat‐Katalysator zur Wasseroxidation und ein molekularer Modell‐Photosensibilisator gemeinsam in einer nanoporösen Blockcopolymer‐Membran immobilisiert wurden. Mittels elektrochemischer Rastersondenmikroskopie (SECM) wurde die lichtinduzierte Sauerstoffentwicklung mit Natriumperoxodisulfat (Na2S2O8) als Opferelektronenakzeptor an den modifizierten Blockcopolymer‐Membranen lokal bestimmt. Ex situ elementspezifische Analysen erzielten räumlich Informationen über die lokale Konzentration und Verteilung der molekularen Komponenten. Untersuchungen der modifizierten Membranen mittels abgeschwächter Infrarot‐Totalreflexion (IR‐ATR) zeigten keinen Abbau des Wasseroxidationskatalysators unter den beschriebenen lichtinduzierten Bedingungen.

Multimodal Analysis of Light-Driven Water Oxidation in Nanoporous Block Copolymer Membranes.

Heterogeneous light-driven catalysis is a cornerstone of sustainable energy conversion. Most catalytic studies focus on bulk analyses of the hydrogen and oxygen evolved, which impede the correlation of matrix heterogeneities, molecular features, and bulk reactivity. Here, we report studies of a heterogenized catalyst/photosensitizer system using a polyoxometalate water oxidation catalyst and a model, molecular photosensitizer that were co-immobilized within a nanoporous block copolymer membrane. Via operando scanning electrochemical microscopy (SECM), light-induced oxygen evolution was determined using sodium peroxodisulfate (Na2 S2 O8 ) as sacrificial electron acceptor. Ex situ element analyses provided spatially resolved information on the local concentration and distribution of the molecular components. Infrared attenuated total reflection (IR-ATR) studies of the modified membranes showed no degradation of the water oxidation catalyst under the reported light-driven conditions.

O-O Bond Formation and Oxygen Release in Photosystem II Are Enhanced by Spin-Exchange and Synergetic Coordination Interactions.

The photosystem II (PSII)-catalyzed water oxidation is crucial for maintaining life on earth. Despite the extensive experimental and computational research that has been conducted over the past two decades, the mechanisms of O-O bond formation and oxygen release during the S3 ∼ S0 stage remain disputed. While the oxo-oxyl radical coupling mechanism in the "open-cubane" S4 state is widely proposed, recent studies have suggested that O-O bond formation may occur from either the high-spin water-unbound S4 state or the "closed-cubane" S4 state. To gauge the various mechanisms of O-O bond formation proposed recently, the comprehensive QM/MM and QM calculations have been performed. Our studies show that both the nucleophilic O-O coupling from the Mn4 site of the high-spin water-unbound S4 state and the O5-O6 or O5-OW2 coupling from the "closed-cubane" S4 state are unfavorable kinetically and thermodynamically. Instead, the QM/MM studies clearly favor the oxyl-oxo radical coupling mechanism in the "open-cubane" S4 state. Furthermore, our comparative research reveals that both the O-O bond formation and O2 release are dictated by (a) the exchange-enhanced reactivity and (b) the synergistic coordination interactions from the Mn1, Mn3, and Ca sites, which partially explains why nature has evolved the oxygen-evolving complex cluster for the water oxidation.

One-Step High-Temperature Electrodeposition of Fe-Based Films as Efficient Water Oxidation Catalysts

Electrolysis of water to produce hydrogen requires an efficient catalyst preferably made of cheap and abundant metal ions for the improved water oxidation reaction. An Fe-based film has been deposited in a single step by electrochemical deposition at temperatures higher than the room temperature. Until now, the electrodeposition of iron oxide has been carried out at 298 K or at lower temperatures under a controlled atmosphere to prohibit atmospheric oxidation of Fe2+ of the iron precursor. A metal inorganic complex, ferrocene, and non-aqueous electrolyte medium propylene carbonate have been used to achieve electrodeposition of iron oxide without the need of any inert or controlled atmosphere. At 298 K, the amorphous film was formed, whereas at 313 K and at higher temperatures, the hematite film was grown, as confirmed by X-ray diffraction. The transformation of iron of the ferrocene into a higher oxidation state under the experimental conditions used was further confirmed by X-ray photoelectron spectroscopy, ultraviolet-visible, and electron paramagnetic resonance spectroscopic methods. The films deposited at 313 K showed the best performance for water oxidation with remarkable long-term electrocatalytic stability and an impressive turnover frequency of 0.028 s-1 which was 4.5 times higher than that of films deposited at 298 K (0.006 s-1). The observed overpotential to achieve a current density of 10 mA cm-2 was found to be 100 mV less for the film deposited at 313 K compared to room-temperature-derived films under similar experimental conditions. Furthermore, electrochemical impedance data revealed that films obtained at 313 K have the least charge transfer resistance (114 Ω) among all, supporting the most efficient electron transport in the film. To the best of our knowledge, this is the first-ever report where the crystalline iron-based film has been shown to be electrodeposited without any post-deposition additional treatment for alkaline oxygen evolution reaction application.