Water Oxidation Research Articles

Regulating Reconstruction Activity of Cobalt Electrode for Optimized Water Oxidation

Regulating Reconstruction Activity of Cobalt Electrode for Optimized Water Oxidation

Unveiling the Aggregation of M−N−C Single Atoms into Highly Efficient MOOH Nanoclusters during Alkaline Water Oxidation

AbstractM−N−C‐type single‐atom catalysts (SACs) are highly efficient for the electrocatalytic oxygen evolution reaction (OER). And the isolated metal atoms are usually considered real active sites. However, the oxidative structural evolution of coordinated N during the OER will probably damage the structure of M−N−C, hence resulting in a completely different reaction mechanism. Here, we reveal the aggregation of M−N−C materials during the alkaline OER. Taking Ni−N−C as an example, multiple characterizations show that the coordinated N on the surface of Ni−N−C is almost completely dissolved in the form of NO3−, accompanied by the generation of abundant O functional groups on the surface of the carbon support. Accordingly, the Ni−N bonds are broken. Through a dissolution‐redeposition mechanism and further oxidation, the isolated Ni atoms are finally converted to NiOOH nanoclusters supported by carbon as the real active sites for the enhanced OER. Fe−N−C and Co−N−C also have similar aggregation mechanism. Our findings provide unique insight into the structural evolution and activity origin of M−N−C‐based catalysts under electrooxidative conditions.

Facet Engineering of Cobalt Manganese Oxide for Highly Stable Acidic Oxygen Evolution Reaction

AbstractDeveloping cost‐effective, robust, and durable catalysts for water oxidation in acidic conditions remains a significant challenge for the practical application of proton exchange membrane electrolyzers. Cobalt‐based spinel catalysts, known for their effective oxygen evolution reaction (OER) activity in acidic environments, are promising alternatives to the costly iridium‐based catalysts. However, their application is often limited by poor stability under corrosive acidic conditions and oxidative potential. Here it is demonstrated that doping Co2MnO4 with Ni effectively regulates the structural and electronic properties of the catalysts, enabling stable operation for over 285 h at 100 mA cm−2 in 0.5 m H2SO4. This stability enhancement is primarily due to the increased exposure of the (400) facet, a result supported by theoretical studies. The proton exchange membrane water electrolysis (PEMWE) performance of Ni(5%)Co2MnO4 further demonstrates its potential, achieving 1 A cm−2 at a cell voltage of 2.3 V, with minimal degradation over 100 h at 500 mA cm−2. This study not only provides insights into the design of advanced OER catalysts through the doping of heteroatoms but also offers a pathway to enhance the sustainability and economic viability of acidic OER applications.

Two Co(II) Isostructural Bifunctional MOFs via Mixed-Ligand Strategy: Syntheses, Crystal Structure, Photocatalytic Degradation of Dyes, and Electrocatalytic Water Oxidation.

The solvothermal reactions involving cobalt ions with 5-methylisophthalic acid (H2MIP) and 1,3-bis(2-methylimidazol)propane (BMIP) yielded two cobalt(II) organic frameworks: {[Co4(MIP)4(BMIP)3]·1/2DMA}n (SNUT-31) and {[Co4(MIP)4(BMIP)3]·(EtOH)2·H2O]}n (SNUT-32) where DMA represents N,N-dimethylacetamide and EtOH signifies ethyl alcohol. Single-crystal X-ray diffraction analyses reveal that SNUT-31 and SNUT-32 possess an isomorphic structure, featuring a unique 2-fold interpenetration of 3D frameworks in a parallel manner. Notably, both SNUT-31 and SNUT-32 demonstrate remarkable performance in electrocatalytic oxygen evolution reactions and exhibit exceptional photocatalytic degradation capabilities against a model comprising three distinct dyes: rhodamine B, methyl orange, and methyl blue.

Understanding the Role of Spin State in Cobalt Oxyhydroxides for Water Oxidation

AbstractAlthough the electronic state of catalysts is strongly corrected with their oxygen evolution reaction (OER) performances, understanding the role of spin state in dynamic electronic structure evolution during OER process is still challenging. Herein, we developed a spin state regulation strategy to boost the OER performance of CoOOH through elemental doping (CoMOOH, M=V, Cr, Mn, Co and Cu). Experimental results including magnetic characterization, in situ X‐ray absorption spectroscopy, in situ Raman and density functional theory calculations unveil that Mn doping could successfully increase the Co sites from low spin state to intermediate spin state, leading to the largest lattice distortion and smallest energy gap between dxy and dz2 orbitals among the obtained CoMOOH electrocatalysts. Benefiting from the promoted electron transfer from dxy to dz2 orbital, facilitated formation of active high‐valent *O−Co(IV) species at applied potential, and reduced energy barrier of rate‐determining step, the CoMnOOH exhibits the highest OER performance. Our work provides significant insight into the correction between dynamic electronic structure evolution and OER performance by understanding the role of spin state regulation in metal oxyhydroxides, paving a new avenue for rational design of high‐activity electrocatalysts.

Improving the Electrocatalytic Activity of a Core/Shell NiCo-ZIF@PBA Catalyst by Co-O-Fe Bridge Bonds for Water Oxidation.

In response to the limitations of slow reaction kinetics and elevated overpotentials in the OER, a novel core-shell structured electrocatalyst, termed NiCo-ZIF-67@Fe-Co-Ni-PBA or NiCo-ZIF@PBA, has been developed. This material demonstrates exceptional catalytic performance, exhibiting a minimal overpotential of approximately 188 mV at 10 mA cm-2, alongside a Tafel slope of 109 mV dec-1. Its robust stability in a 1 M KOH solution during the OER operations is noteworthy. Theoretical insights from DFT calculations reveal that a Co-O-Fe bridging configuration within NiCo-ZIF@PBA lowers the energy barrier for the reaction to 1.79 eV, a significant reduction from 2.67 eV observed with NiCo-ZIF-67. The improvement in electrochemical performance is primarily due to the emergence of Co3+ ions, which results from the efficient charge transfer occurring at the interface of the PBA and NiCo-ZIF core-shell structure. These findings suggest a promising strategy for designing advanced core-shell materials for electrocatalytic applications.

Dual Cocatalysts on Covalent Organic Framework as Charge Transfer Mediator Facilitating Photocatalytic Water Oxidation

AbstractCovalent organic frameworks (COFs) have been regarded as promising photocatalytic materials. However, the sluggish diffusion of photoinduced charges in a single‐component COF remains the obstacles that have to be overcome. Here, we demonstrate a synthesis of a COF bearing dual cocatalysts Co3O4 and Re complex as photocatalyst for water oxidation. The dual cocatalysts are coordinated on the COF, and enable the spatial separation of photoinduced charges. The generated electrons were attracted to Re moiety, while the holes were localized on Co3O4 cocatalyst for water oxidation to produce O2 product. The synergy of dual cocatalysts of COF promote the charge separation and redox reaction. Moreover, hydroxyl radicals generated from the hydration of COF contribute to O−O bond formation, which is the key step of O2 formation. Accordingly, an excellent O2 production rate of 400 μmol g−1 h−1 under visible light irradiation was obtained. This work may pave the way to the development of highly efficient COF photocatalysts for solar energy conversion.

Mo-Doped α-MnO2 for Enhanced Electrocatalytic Water Oxidation.

Manganese is a key metal involved in the catalysis of natural photosynthesis. Thus, the investigation of Mn-based electrocatalysts for water oxidation is of high importance. This work reports the doping of Mo into α-MnO2 nanorods to improve the water oxidation performance. The doping of Mo can transform the microstructure of α-MnO2 from nanorods into nanosphere superstructures. As a dopant, Mo expands the α-MnO2 lattice to result in a decrease in the average oxidation state of Mn and the generation of oxygen vacancies, which are beneficial to water oxidation catalysis. Under optimized doping, the overpotential of 2.34 wt.% Mo/α-MnO2 is reduced by 80 mV (at 10 mA/cm2) compared with pure α-MnO2.

Mechanistic insights into multimetal synergistic and electronic effects in a hexanuclear iron catalyst with a [Fe3(μ3-O)(μ2-OH)]2 core for enhanced water oxidation.

Multinuclear molecular catalysts mimicking natural photosynthesis have been shown to facilitate water oxidation; however, such catalysts typically operate in organic solutions, require high overpotentials and have unclear catalytic mechanisms. Herein, a bio-inspired hexanuclear iron(III) complex I, Fe6(μ3-O)2(μ2-OH)2(bipyalk)2(OAc)8 (H2bipyalk = 2,2'-([2,2'-bipyridine]-6,6'-diyl)bis(propan-2-ol); OAc = acetate) with desirable water solubility and stability was designed and used for water oxidation. Our results showed that I has high efficiency for water oxidation via the water nucleophilic attack (WNA) pathway with an overpotential of only ca. 290 mV in a phosphate buffer of pH 2. Importantly, key high-oxidation-state metal-oxo intermediates formed during water oxidation were identified by in situ spectroelectrochemistry and oxygen atom transfer reactions. Theoretical calculations further supported the above identification. Reversible proton transfer and charge redistribution during water oxidation enhanced the electron and proton transfer ability and improved the reactivity of I. Here, we have shown the multimetal synergistic and electronic effects of catalysts in water oxidation reactions, which may contribute to the understanding and design of more advanced molecular catalysts.

Structurally Locked High-Crystalline Covalent Triazine Frameworks Enable Remarkable Overall Photosynthesis of Hydrogen Peroxide.

The development of green and efficient hydrogen peroxide (H2O2) production is of great interest but remains challenging. Herein, we develop a new and simple strategy via locking the coplanarity in highly crystalline covalent triazine frameworks (CTFs) to remarkably boost direct photosynthesis of H2O2 from oxygen and water. The exfoliated ultrathin 2D-CTF nanosheets exhibit excellent photocatalytic H2O2 evolution with an ultrahigh solar-to-chemical efficiency of 0.91% and a superb apparent quantum yield of 16.8% at 420 nm, surpassing all previous CTFs and most of the metal-free photocatalysts ever reported. Our detailed experimental and theoretical studies reveal that the spatially locked structure in the crystalline CTF photocatalyst can not only greatly enhance the separation and transfer of photoexcited charge-carriers for promoting H2O2 photogeneration but also alter the local electronic structures that unexpectedly turn water oxidation from a four-electron route to a two-electron pathway, resulting in a 100% atom utilization efficiency. This work provides valuable insights into the designed synthesis of highly efficient metal-free photocatalysts and precise control over photocatalytic reaction pathways in organic materials.

Heat transfer enhancement using ternary hybrid nanofluid for cross-viscosity model with intelligent Levenberg-Marquardt neural networks approach incorporating entropy generation

Thermal heat transfer analysis of trihybrid nanofluids using an intelligent Levenberg-Marquardt neural network (ANN-LMA) approach, with a focus on entropy generation, has been conducted. The flow equations were modeled in Cartesian coordinates and simplified using dimensionless variables. Partial differential equations were converted into ordinary differential equations through appropriate similarity transformations. These ordinary differential equations were then solved using the finite element method applied to a data set evaluated from (ANN-LMA) approach. This dataset can be input into MATLAB to generate predicted solutions for flow patterns. The ANN-LMA technique was employed to evaluate the efficiency of heat transfer characteristics for nanofluids in various scenarios. Incorporating carbon nanotubes (both single-wall (SWCNT) and multi-wall (MWCNT)) along with iron oxide in water, the study demonstrates their effectiveness in enhancing heat transfer. These nanofluids have broad industrial applications, such as in coolant enhancement, cancer therapy, and solar radiation management, and show promising results. This study specifically examines the flow properties of water-based CNT cross-trihybrid nanofluids over a convectively heated surface, leveraging their unique characteristics. Improvements in heat transfer are achieved through the introduction of dissipative heat, thermal radiation, and external heat sources or sinks. The performance of the computational solver was assessed using error histograms, regression analyses, and Mean Squared Error (MSE) results. The physical significance of the designed factors is graphically depicted and discussed in detail. It was found that radiative heat increases surface heat energy through substantial accumulation, thereby enhancing heat transfer properties, while dissipative heat, due to Joule dissipation and other external sources, significantly raises the fluid temperature.

Frustrated Lewis Pair-Promoted Organocatalytic Transformation of Hydrosilanes into Silanols with Water Oxidant.

Owing to their unique properties, the silanols have attracted intense attention but remain challenging to prepare from the organocatalytic oxidation of hydrosilanes using H2O as a green oxidant. Herein, we employ a frustrated Lewis pair (FLP) to successfully suppress the formation of undesired siloxanes and produce silanols in high to excellent yields in the presence of H2O. Mechanistic studies suggest that the reaction is initiated with the activation of FLP by H2O rather than by silanes and goes through a concerted SN2 mechanism. More importantly, the combination of the FLP-catalyzed oxidation of hydrosilanes with B(C6F5)3-catalyzed dehydrogenation enables us to realize the precise synthesis of sequence-controlled oligosiloxanes. This method exhibits a broad substrate scope and can be easily scaled up, thus exhibiting promising application potentials in the precision synthesis of silicon-containing polymer materials.

Interface Synergistic Effect of NiFe-LDH/3D GA Composites on Efficient Electrocatalytic Water Oxidation.

Currently, NiFe-LDH exhibits an excellent oxygen evolution reaction (OER) due to the interaction of the two metal elements on the layered double hydroxide (LDH) platform. However, such interaction is still insufficient to compensate for its poor electrical conductivity, limited number of active sites and sluggish dynamics. Herein, a feasible two-step hydrothermal strategy that involves coupling low-conductivity NiFe-LDH with 3D porous graphene aerogel (GA) is proposed. The optimized NiFe-LDH/GA (1:1) produced possesses a 257 mV (10 mA cm-2) overpotential and could operate stably for 56 h in an OER. Our investigation demonstrates that the NiFe-LDH/GA has a three-dimensional mesoporous structure, and that there is synergistic interaction between LDH and GA and interfacial reconstruction of NiOOH. Such an interface synergistic coupling effect promotes fast mass transfer and facilitates OER kinetics, and this work offers new insights into designing efficient and stable GA-based electrocatalysts.

Functionalized Modification of Conjugated Porous Polymers for Full Reaction Photosynthesis of H2O2

AbstractModulating the molecular structure to achieve the full reaction including oxygen reduction reaction and water oxidation reaction is a promising strategy for efficient photosynthesis of hydrogen peroxide (H2O2) but remains a challenge. Herein, a triphenylamine and naphthalimide‐based conjugated porous polymers are synthesized with photo oxidation‐reduction structures, then sulfonate (─SO3H) and quaternary ammonium groups are introduced via a post‐modification strategy to produce two photocatalysts named NI‐TPA‐NI‐SO3H and NI‐TPA‐NI‐N, respectively. Introducing charged functional groups has improved the hydrophilicity and oxygen (O2) adsorption, beyond that, the ─SO3H further stabilizes the adsorbed O2 via hydrogen bonding as well as accelerates the photogenerated carrier separation and electron/proton transport that enables full reaction photosynthesis of H2O2. Therefore, motivated by efficient charge separation, stabilized O2 adsorption, and boosted proton‐coupled electron transfer, NI‐TPA‐NI‐SO3H exhibits the highest light‐driven H2O2 production rate among the three photocatalysts, reaching 3.40 mmol g−1 h−1, which is 4.9‐fold of NI‐TPA‐NI. Remarkably, in the presence of ethylenediaminetetraacetic acid disodium salt, its rate significantly enhances to 14.5 mmol g−1 h−1, superior to most reported organic photocatalysts to the best of the knowledge.

Solar Fuel Synthesis Using a Semiartificial Colloidal Z-Scheme.

The integration of enzymes with semiconductor light absorbers in semiartificial photosynthetic assemblies offers an emerging strategy for solar fuel production. However, such colloidal biohybrid systems rely currently on sacrificial reagents, and semiconductor-enzyme powder systems that couple fuel production to water oxidation are therefore needed to mimic an overall photosynthetic reaction. Here, we present a Z-scheme colloidal enzyme system that produces fuel with electrons sourced from water. This "closed-cycle" semiartificial approach utilizes particulate SrTiO3:La,Rh and BiVO4:Mo (light absorbers), hydrogenase or formate dehydrogenase (cocatalyst), and a molecular cobalt complex (a redox mediator). Under simulated solar irradiation, this system continuously generates molecular hydrogen or formate, while co-producing molecular oxygen for 10 h using only sunlight, water, and carbon dioxide as inputs. In-depth analysis using quartz crystal microbalance, photoelectrochemical impedance spectroscopy, transient photocurrent spectroscopy, and intensity-modulated photovoltage spectroscopy provides mechanistic understanding and characterization of the semiconductor-enzyme hybrid interface. This study provides a rational platform to assemble functional semiartificial colloidal Z-scheme systems for solar fuel synthesis.

Magneto-electrochemical water splitting performance of graphene oxide/nickel aluminum-layered double hydroxide nanocomposites

Designing efficient, cheap catalysts for oxygen production is very important for water electrolysis and green hydrogen production. Here, synthesis and electrocatalytic performance of graphene oxide (GO)/nickel-aluminum layered double hydroxide (NiAl-LDH) composites were investigated for water-splitting applications. The composition, microstructure, and morphology of the nanocomposites were confirmed by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), and field emission scanning electron microscopy (FE-SEM), and their water oxidation performance was examined in 0.1 M potassium hydroxide solution in the presence and absence of magnetic field. The results show that optimized GO/NiAl-LDH nanocomposite (GO-1 %/NiAl-LDH) exhibits the lowest overpotential compared with other prepared nanocomposites. This performance demonstrates comparable electroactivity to well-developed electrocatalysts like the perovskite-based electrodes, it also shows an improved electrocatalytic activity compared to NiAl-LDHs due to the presence of graphene oxide in the composite. We interpreted this significant performance to improve electron transform and high active site at the synthesized GO/NiAl-LDH composites. The best electrocatalytic activity with an overpotential of 443 and 473 mV at the current density 10 mA.cm−2 were evidenced for GO-1 %/NiAl-LDH nanocomposite in the presence and absence of external magnetic field, respectively. Furthermore, the tafel slope were reported about 54 and 162 mV.dec−1 in the presence and absence of magnetic field, respectively. This improved water oxidation can be attributed to the magneto-hydrodynamic effect and the increased number of metal sites on LDHs under the magnetic field.

Self-Etching Synthesis of Superhydrophilic Iron-Rich Defect Heterostructure-Integrated Catalyst with Fast Oxygen Evolution Kinetics for Large-Current Water Splitting.

Developing catalysts with rich metal defects, strong hydrophilicit, and extensive grain boundaries is crucial for enhancing the kinetics of electrocatalytic water oxidation and facilitating large-current water splitting. In this study, we utilized pH-controlled etching and gas-phase phosphating to synthesize a flower-like Ni2P-FeP4-Cu3P modified nickel foam heterostructure catalyst. This catalyst features pronounced hydrophilicity and a high concentration of Fe defects. It exhibits low overpotentials of 156 mV and 210 mV at current densities of 10 and 100 mA cm-2 respectively, and maintains stability for up to 200 h at 100 mA cm-2 with only 7.3% degradation, showcasing outstanding electrocatalytic water oxidation performance. Furthermore, when integrated into a Ni2P-FeP4-Cu3P/NF||Pt-C/NF electrolyzer, it achieves excellent overall water splitting performance, reaching current densities of 10 and 400 mA cm-2 at just 1.47 V and 1.73 V, respectively, and operates stably for 60 h at 500 mA cm-2 with minimal degradation. Analysis indicates that high-valence oxyhydroxides/phosphides of Ni, Fe, and Cu act as the primary active species. The presence of abundant Fe defects enhances electron transfer, strong hydrophilicity improves electrolyte contact, and numerous grain boundaries synergistically modulate the activation energy between active sites and oxygen-containing intermediates, significantly improving the kinetics of water oxidation.

The breakthrough of oxide pathway mechanism in stability and scaling relationship for water oxidation

Abstract An in-depth understanding of electrocatalytic mechanisms is essential for advancing electrocatalysts for the oxygen evolution reaction (OER). The emerging oxide pathway mechanism (OPM) streamlines direct O–O radical coupling, circumventing the formation of oxygen vacancy defects characteristic of the lattice oxygen mechanism (LOM) and bypassing additional reaction intermediates (*OOH) inherent to the adsorbate evolution mechanism (AEM). With only *O and *OH as intermediates, OPM-driven electrocatalysts stand out for their ability to disrupt traditional scaling relationships while ensuring stability. This review compiles the latest significant advances in OPM-based electrocatalysis, detailing design principles, synthetic methods, and sophisticated techniques to identify active sites and pathways. We conclude with prospective challenges and opportunities for OPM-driven electrocatalysts, aiming to advance the field into a new era by overcoming traditional constraints.

Carbonate-carbonate coupling on platinum surface promotes electrochemical water oxidation to hydrogen peroxide

Water electro-oxidation to form H2O2 is an important way to produce H2O2 which is widely applied in industry. However, its mechanism is under debate and HO(ads), hydroxyl group adsorbed onto the surface of the electrode, is regarded as an important intermediate. Herein, we study the mechanism of water oxidation to H2O2 at Pt electrode using in-situ Raman spectroscopy and differential electrochemical mass spectroscopy and find peroxide bond mainly originated from the coupling of two CO32- via a C2O62- intermediate. By quantifying the 18O isotope in the product, we find that 93% of H2O2 was formed via the CO32- coupling route and 7% of H2O2 is from OH(ads)-CO3•− route. The OH(ads)-OH(ads) coupling route has a negligible contribution. The comparison of various electrodes shows that the strong adsorption of CO3(ads) at the electrode surface is essential. Combining with a commercial cathode catalyst to produce H2O2 during oxygen reduction, we assemble a flow cell in which the cathode and anode simultaneously produce H2O2. It shows a Faradaic efficiency of 150% of H2O2 at 1 A cm−2 with a cell voltage of 2.3 V.

Elucidating the dynamics and transfer pathways of photogenerated charge carriers in V2O5/BiVO4 heterojunction photoanodes: A transient absorption spectroscopy study

Pairing of bismuth vanadate (BiVO4) with vanadium pentoxide (V2O5) forms a Type II heterojunction photoanode, which has been proven to be a high-performance photoanode architecture for efficient photoelectrochemical (PEC) water oxidation. To further support for advanced rational design and improvement of the heterojunction photoanode, it is quintessential to understand the mechanics and properties of photogenerated charge carriers formed. This study aims to probe the dynamics of photogenerated electron-hole pairs formed in pristine photoanode as well as the heterojunction photoanodes using transient absorption spectroscopy (TAS). Relative to the BiVO4/V2O5 structure, modelling and quantification of the decay constant helps in explaining why the V2O5/BiVO4 heterojunction photoanode exhibited a longer lifetime of the photogenerated charge carriers with a lower decay rate constant that leads to a much-improved overall generation of photocurrent density. An oxygen evolving catalyst of nickel oxyhydroxide (NiOOH) was then anchored on the exterior surfaces of the heterojunction photoanode system to further investigate and modify the transfer pathways of the photogenerated charge carriers. Furthermore, the hole- and scavenger-assisted TAS measurements on the BiVO4/V2O5 heterojunction photoanode revealed that the majority of trapped holes species are accumulated within the BiVO4 layer. The findings suggested that the Type II heterojunction formation in V2O5/BiVO4 could effectively reduce its charge recombination process.