Stable Water Oxidation Research Articles

Stable acidic water oxidation by tunable electronic states in β-PbO2 anode electrocatalyst aided by incorporation of various anions and cations

Developing stable and cost-effective electrodes is one of the main challenges of clean hydrogen fuel production. However, the investigation of non-precious materials for this application, from both experimental and theoretical aspects, has been limited in scope. In the present research, the lead dioxide (β-PbO2) electrode was prepared by an anodic electrodeposition method on Ti substrate to use as the anode in electrochemical water splitting cells, where the oxygen evolution half-reaction occurs. The substitution of F– anion and Fe3+ and Co2+ cations into the structure of lead dioxide, to improve the properties and performance of these anodes, was investigated through a combination of experimental studies and quantum mechanical (QM) calculations. The β-PbO2, β-PbO2-F, β-PbO2-Fe, and β-PbO2-Co electrodes were characterized by the XRD, SEM, EDS, and electrochemical analyses, including CV, LSV, CP, and EIS. Furthermore, the electrode structures were investigated in theoretical studies by the density functional theory (DFT) method. The Gibbs energy change of intermediates formation and electronic properties, including band structure, the density of states, and charge density difference, were calculated and studied. The results showed that the substitutional elements improved the PbO2 activity toward oxygen evolution reaction (OER) by reducing the intermediates' adsorption resistance from 84.2 in pure PbO2 to 11.7 Ω.cm2 in PbO2-Fe. Fe3+ substitution had the most effect on durability and increased the efficient lifetime of the anode about 4 times relative to pure PbO2. The electrochemically active surface area (ECSA) was also increased intensively by doping Fe3+. It is concluded from the DFT calculations that the reaction energy barriers are reduced by ion incorporation. The OER overpotential decreases significantly from 920 mV on pure PbO2 surface to 350 mV by Co2+ substitution. The enhancement of the intrinsic activity of cation-substituted anodes was evidenced by the remarkable reduction of band gap energy at the Fermi level.

Stable tetravalent Ni species generated by reconstruction of FeB-wrapped NiMoO pre-catalysts enable efficient water oxidation at large current densities

High valence transition metals offer highly active electrocatalytic sites for oxygen evolution reaction (OER) but suffer from high formation energy and are thermodynamically unstable in the resting state. Herein, we report the generation of NiFeOOH with highly oxidized and stable tetravalent Ni species through reconstruction and electronic structure modulation. The resulted NiFeOOH electrocatalyst catalyzes OER with ultralow overpotentials of 166 and 213 mV at 10 and 100 mA cm-2, respectively, and high stability (> 100 h) at an ultrahigh current density of 1000 mA cm-2. The formation of stable tetravalent Ni species in NiFeOOH nanosheets/rods structure results in higher Ni-O covalency and enhanced the reactivity of lattice oxygen, thus transferring the reaction path and reducing the reaction energy barrier for OER. This work paves the way for the rational design of electrocatalysts with high-valence metal species and activated lattice oxygen for stable water oxidation at large current densities.

Ferricyanide Armed Anodes Enable Stable Water Oxidation in Saturated Saline Water at 2 A/cm2.

The direct seawater electrolysis at high current density and low overpotential affords an effective strategy toward clean and renewable hydrogen fuel production. However, the severe corrosion of anode as a result of the saturation of Cl- upon continuous seawater feeding seriously hamper the electrolytic process. Herein, cobalt ferricyanide / cobalt phosphide (CoFePBA/Co2 P) anodes with Cap/Pin structure are synthesized, which stably catalyze alkaline saturated saline water oxidation at 200-2000 mA cm-2 over hundreds of hours without corrosion. Together with the experimental findings, the molecular dynamics simulations reveal that PO4 3- and Fe(CN)6 3- generated by the electrode play synergistic role in repelling Cl- via electrostatic repulsion and dense coverage, which reduced Cl- adsorption by nearly 5-fold. The novel anionic synergy endow superior corrosion protection for the electrode, and is expected to promote the practical application of saline water electrolysis.

Ferricyanide Armed Anodes Enable Stable Water Oxidation in Saturated Saline Water at 2 A/cm2

AbstractThe direct seawater electrolysis at high current density and low overpotential affords an effective strategy toward clean and renewable hydrogen fuel production. However, the severe corrosion of anode as a result of the saturation of Cl− upon continuous seawater feeding seriously hamper the electrolytic process. Herein, cobalt ferricyanide / cobalt phosphide (CoFePBA/Co2P) anodes with Cap/Pin structure are synthesized, which stably catalyze alkaline saturated saline water oxidation at 200–2000 mA cm−2 over hundreds of hours without corrosion. Together with the experimental findings, the molecular dynamics simulations reveal that PO43− and Fe(CN)63− generated by the electrode play synergistic role in repelling Cl− via electrostatic repulsion and dense coverage, which reduced Cl− adsorption by nearly 5‐fold. The novel anionic synergy endow superior corrosion protection for the electrode, and is expected to promote the practical application of saline water electrolysis.

Constructing the oxygen diffusion paths for promoting the stability of acidic water oxidation catalysts

Anchoring Ir onto metal-oxide support can effectively address the scarcity and insufficient activity problems of Ir-based acidic water oxidation catalysts by triggering the lattice-oxygen-mediated mechanism (LOM). However, the stability of these catalysts is usually sacrificed due to support overoxidation and reconstruction during the LOM pathway. Here, we propose constructing oxygen diffusion paths at a mixed-metal oxide support (ZrOx and TaOx) interface for a stable LOM. The new synthesized Ir-ZrTaOx catalyst exhibits two orders higher mass activity than IrO2 and retains 1,000-h electrolysis. It demonstrates low degradation (4.7 μV h−1) during the 200-h operation in a proton-exchange membrane (PEM) electrolyzer using only 1/4 Ir loading. Operando spectroscopic evidence combined with theory studies suggests that the oxygen diffusion path at the ZrOx/TaOx interface inhibits the overoxidation and detrimental reconstruction of TaOx. The strategy of stabilizing the LOM will benefit the future design of stable and efficient water oxidation catalysts.

Frontispiz: High‐Density Cationic Defects Coupling with Local Alkaline‐Enriched Environment for Efficient and Stable Water Oxidation

Electrocatalysis. In their Research Article (e202217815) Yangen Zhou, Zhongliang Tian, Yanqing Lai, Guihua Yu et al. report a nonequilibrium precipitation approach to construct a highly disordered crystal structure of layered double hydroxides as a model oxygen evolution catalyst.

Frontispiece: High‐Density Cationic Defects Coupling with Local Alkaline‐Enriched Environment for Efficient and Stable Water Oxidation

Electrocatalysis. In their Research Article (e202217815), Yangen Zhou, Zhongliang Tian, Yanqing Lai, Guihua Yu et al. report a nonequilibrium precipitation approach for the construction of a highly disordered crystal structure of layered double hydroxides as a model oxygen evolution catalyst.

Photoelectrochemical Water Oxidation Using Cobalt Phosphate‐Modified Nitrogen‐Doped Titania Nanotube Arrays

Water Splitting In article number 2200229, Kazuhiko Maeda, Hyunwoong Park, and co-workers accomplished stable photoelectrochemical water oxidation driven by sunlight to yield oxygen gas with cobalt phosphate-modified, nitrogen-doped titania nanotube arrays, which can absorb visible light with wavelengths as long as 580 nm.

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.

Construction of Zn-doped RuO2 nanowires for efficient and stable water oxidation in acidic media

Oxygen evolution reaction catalysts capable of working efficiently in acidic media are highly demanded for the commercialization of proton exchange membrane water electrolysis. Herein, we report a Zn-doped RuO2 nanowire array electrocatalyst with outstanding catalytic performance for the oxygen evolution reaction under acidic conditions. Overpotentials as low as 173, 304, and 373 mV are achieved at 10, 500, and 1000 mA cm−2, respectively, with robust stability reaching to 1000 h at 10 mA cm−2. Experimental and theoretical investigations establish a clear synergistic effect of Zn dopants and oxygen vacancies on regulating the binding configurations of oxygenated adsorbates on the active centers, which then enables an alternative Ru−Zn dual-site oxide path of the reaction. Due to the change of reaction pathways, the energy barrier of rate-determining step is reduced, and the over-oxidation of Ru active sites is alleviated. As a result, the catalytic activity and stability are significantly enhanced.

Flexible bilayer Ti3C2Tx MXene/cellulose nanocrystals/waterborne polyurethane composite film with excellent mechanical properties for electromagnetic interference shielding

To address electromagnetic wave (EMW) pollution, the development of ultrathin and flexible electromagnetic shielding materials is essential. High electromagnetic interference (EMI) shielding performance while retaining robust mechanical properties remains a challenging hurdle. In this work, bilayer structured MXene/CNC/WPU (MCW-X) composite films were fabricated via alternating filtration assisted by vacuum. On the one hand, a conductive network is formed between CNC and MXene nanosheets, which enhances the electrical conductivity of the composite film as well as the stable water-oxidation stability. On the other hand, the interaction between WPU and MXene nanosheets increases the sliding between the layers of MXene nanosheets, hence enhancing the tensile strength and toughness. Meanwhile, the incorporation of CNC and WPU into MXene nanosheets increases the interlayer spaces of the composite films, optimizes impedance matching, and boosts the EMI shielding performance. MCW-4.5 sample composite (CNC 20 wt%) exhibits excellent electrical conductivity of 606.1 S/cm, resulting in a high-performance EMI shielding of 54.2 dB in X-band with a thickness of 35 µm. Meanwhile, it exhibits exceptional mechanical properties (elastic modulus of 3652.70 MPa, tensile strength of 52.2 MPa). Consequently, MCW-X composite films have a significant utilization potential in the field of green, ultrathin, and flexible EMI shielding.

One-step cladding metal oxide nanowires with a carbon-dots-embedded ZnO amorphous layer toward boosted photoelectrochemical water oxidation

An effective method was proposed for constructing carbon dots (CDs)-sensitized multijunction composite photoelectrodes via one-step cladding a CDs-embedded ZnO amorphous overlayer on vertically aligned metal oxide nanowires. This strategy involved the double role of hexamethylenetetramine (HMTA) in the ethylene glycol (EG) solvent mixed with a controllable trace amount of water. In the water-deficient synthetic system, a limited portion of HMTA served as the pH buffer and hydroxyl source to force the hydrolytic process of zinc ions for the production of ZnO. The precipitated ZnO clusters were instantly capped by EG molecules through the activated alkoxidation reaction, and further crosslinked into an amorphous network surrounding the individual nanowires. Meanwhile, the excess HMTA was simultaneously depleted as the precursor for producing CDs in the EG solution through thermal condensation, which were packed in the gradually formed aggregates. We revealed that a CDs-embedded amorphous ZnO overlayer with an appropriate proportion of ingredient could be tailored through an optimal tradeoff between hydrolysis and condensation of HMTA. Benefiting from the synergy of the amorphous ZnO layer and the embedded CDs, the multijunction composite photoanodes exhibited significantly improved PEC performance and stability for water oxidation.

Light-Mediated Electrochemical Synthesis of Manganese Oxide Enhances Its Stability for Water Oxidation.

New methods are needed to increase the activity and stability of earth-abundant catalysts for electrochemical water splitting to produce hydrogen fuel. Electrodeposition has been previously used to synthesize manganese oxide films with a high degree of disorder and a mixture of oxidation states for Mn, which has led to electrocatalysts with high activity but low stability for the oxygen evolution reaction (OER) at high current densities. In this study, we show that multipotential electrodeposition of manganese oxide under illumination produces nanostructured films with significantly higher stability for the OER compared to films grown under otherwise identical conditions in the dark. Manganese oxide films grown by multipotential deposition under illumination sustain a current density of 10 mA/cm2 at 2.2 V versus reversible hydrogen electrode for 18 h (pH 13). Illumination does not enhance the activity or stability of manganese oxide films grown using a constant potential, and films grown by multipotential deposition in the dark undergo a complete loss of activity within 1 h of electrolysis. Electrochemical and structural characterization indicate that photoexcitation of the films during growth reduces Mn ions and changes the content and structure of intercalated potassium ions and water molecules in between the disordered layers of birnessite-like sheets of MnOx, which stabilizes the nanostructured film during electrocatalysis. These results demonstrate that combining multiple external stimuli (i.e., light and an external potential) can induce structural changes not attainable by either stimulus alone to make earth-abundant catalysts more active and stable for important chemical transformations such as water oxidation.

High-Density Cationic Defects Coupling with Local Alkaline-Enriched Environment for Efficient and Stable Water Oxidation.

The inferior activity and stability of non-noble metal-based electrocatalysts for oxygen evolution reaction (OER) seriously limit their practical applications in various electrochemical energy conversion systems. Here we report, a drastic nonequilibrium precipitation approach to construct a highly disordered crystal structure of layered double hydroxides as a model OER catalyst. The unconventional crystal structure contains high-density cationic defects coupled with a local alkaline-enriched environment, enabling ultrafast diffusion of OH- ions and thus avoiding the formation of a local acidic environment and dissolution of active sites during OER. An integrated experimental and theoretical study reveals that high-density cationic defects, especially di-cationic and multi-cationic defects, serve as highly active and durable catalytic sites. This work showcases a promising strategy of crystal structure engineering to construct robust active sites for high-performance oxygen evolution in an alkaline solution.

High‐Density Cationic Defects Coupling with Local Alkaline‐Enriched Environment for Efficient and Stable Water Oxidation

AbstractThe inferior activity and stability of non‐noble metal‐based electrocatalysts for oxygen evolution reaction (OER) seriously limit their practical applications in various electrochemical energy conversion systems. Here we report, a drastic nonequilibrium precipitation approach to construct a highly disordered crystal structure of layered double hydroxides as a model OER catalyst. The unconventional crystal structure contains high‐density cationic defects coupled with a local alkaline‐enriched environment, enabling ultrafast diffusion of OH− ions and thus avoiding the formation of a local acidic environment and dissolution of active sites during OER. An integrated experimental and theoretical study reveals that high‐density cationic defects, especially di‐cationic and multi‐cationic defects, serve as highly active and durable catalytic sites. This work showcases a promising strategy of crystal structure engineering to construct robust active sites for high‐performance oxygen evolution in an alkaline solution.

Oxyanion Engineering Suppressed Iron Segregation in Nickel-Iron Catalysts Toward Stable Water Oxidation.

Nickel-iron catalysts represent an appealing platform for electrocatalytic oxygen evolution reaction (OER) in alkaline media because of their high adjustability in components and activity. However, their long-term stabilities under high current density still remain unsatisfactory due to undesirable Fe segregation. Herein, a nitrate ion (NO3 - ) tailored strategy is developed to mitigate Fe segregation, and thereby improve the OER stability of nickel-iron catalyst. X-ray absorption spectroscopy combined with theoretical calculations indicate that introducing Ni3 (NO3 )2 (OH)4 with stable NO3 - in the lattice is conducive to constructing the stable interface of FeOOH/Ni3 (NO3 )2 (OH)4 via the strong interaction between Fe and incorporated NO3 - . Time of flight secondary ion mass spectrometry and wavelet transformation analysis demonstrate that the NO3 - tailored nickel-iron catalyst greatly alleviates Fe segregation, exhibiting a considerably enhanced long-term stability with a six-fold improvement over FeOOH/Ni(OH)2 without NO3 - modification. This work represents a momentous step toward regulating Fe segregation for stabilizing the catalytic performances of nickel-iron catalysts.

Spinel-Anchored Iridium Single Atoms Enable Efficient Acidic Water Oxidation via Intermediate Stabilization Effect

Iridium oxide is considered the only practical catalyst for oxygen evolution reaction (OER) in commercial proton exchange membrane (PEM) electrolyzers. However, its low activity and high cost greatly hinder the large-scale development of PEM electrolyzers for hydrogen production. Herein, we report atomically dispersed Ir atoms incorporated into a spinel Co3O4 lattice as an acidic OER catalyst, which exhibits excellent activity and stability for water oxidation. The catalyst significantly lowers the overpotential down to 226 mV at 10 mA cm–2 with an ultrahigh turnover frequency value of 3.15 s–1 (η = 300 mV), 3 orders of magnitude higher than that of commercial IrO2. Meanwhile, the catalyst shows superior corrosion resistance in an acidic OER condition, reaching a lifespan of up to 500 h at 10 mA cm–2. First-principles calculations reveal that the key *OOH intermediate can be stabilized by the lattice oxygen coordinated to the Ir active site via hydrogen bond formation, which substantially regulates the rate-limiting step and lowers the activation free energy of the OER process. This work demonstrates a strategy for improving the OER activity of Ir-based catalysts and provides insights into the regulation of the reaction mechanism.

In situ rapid and deep self-reconstruction of Fe-doped hydrate NiMoO4 for stable water oxidation at high current densities

The rational design of efficient electrodes that can withstand high current densities is urgent for electrolytic hydrogen generation, but it is a significant challenge to develop high activity and stability electrocatalysts with simple methods. Herein, we report an electrocatalyst that is obtained by in situ rapid depth self-reconstruction of Fe-doped hydrate NiMoO4 (denoted as NiFeMoO). Experimental and theoretical calculations show that the co-leaching of crystal water and MoO42− from the original structure induces the structure crack and electrolyte penetration, as well as synergistic Fe incorporation, leading to the generation of active catalyst phase of Fe doped γ-NiOOH (denoted as SR-NiFeMoO) during electro-oxidation. In 1 M KOH, the resulting SR-NiFeMoO, with unique poly-/low-crystalline features, possesses ultralow overpotential of 168 mV at 10 mA cm−2 and 226 mV at 100 mA cm−2, and performs stable catalysis for over 106 h at a high current density of 1000 mA cm−2. In addition, the presence of dissolved Mo leads to the change of electrolyte properties over time, which leads to the continuous reduction of potential for water oxidation. The discovery of deep self-reconstruction may provide a new methodology for the development of robust and efficient electrocatalysts for electrolytic hydrogen generation.

Regeneration and Degradation in a Biomimetic Polyoxometalate Water Oxidation Catalyst.

Complete understanding of catalytic cycles is required to advance the design of water oxidation catalysts, but it is difficult to attain, due to the complex factors governing their reactivity and stability. In this study, we investigate the regeneration and degradation pathways of the highly active biomimetic water oxidation catalyst [Mn3+ 2Mn4+ 2V4O17(OAc)3]3-, thereby completing its catalytic cycle. Beginning with the deactivated species [Mn3+ 4V4O17(OAc)2]4- left over after O2 evolution, we scrutinize a network of reaction intermediates belonging to two alternative water oxidation cycles. We find that catalyst regeneration to the activated species [Mn4+ 4V4O17(OAc)2(OH)(H2O)]- proceeds via oxidation of each Mn center, with one water ligand being bound during the first oxidation step and a second water ligand being bound and deprotonated during the final oxidation step. ΔΔG values for this last oxidation are consistent with previous experimental results, while regeneration within an alternative catalytic cycle was found to be thermodynamically unfavorable. Extensive in silico sampling of catalyst structures also revealed two degradation processes: cubane opening and ligand dissociation, both of which have low barriers at highly reduced states of the catalyst due to the presence of Jahn-Teller effects. These mechanistic insights are expected to spur the development of more efficient and stable Mn cubane water oxidation catalysts.

CeO2 supported high-valence Fe oxide for highly active and stable water oxidation

The high-valence Fe ions are stabilized on a CeO2 support and achieved a record low overpotential of 219 mV to reach the current density of 50 mA cm−2.