Efficient Electrochemical Water Oxidation Research Articles

Efficient electrochemical water oxidation catalyzed by a novel nickel complex: Critical effect of the flexibility of pyridine-amine based pentadentate ligand on catalytic property

Efficient electrochemical water oxidation can be realized by reasonable modulation of the ligand structure of metal complex-based molecular water oxidation catalysts (WOCs). Herein, the synthesis and prowess in electrocatalytic water oxidation of a mononuclear water-soluble nickel complex, [Ni(tmbptu)(OH2)](ClO4)2 (1, tmbptu = 2,6,10-trimethyl-1,11-bis(2-pyridyl)-2,6,10-triazaundecanone), is reported for the first time. Complex 1 is found to be an efficient homogeneous WOCs under near neutral condition with long-term catalytic stability by a series of characterizations, including dynamic light scattering (DLS) measurement, scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), energy dispersive X-ray (EDX) analysis, chelation experiment, and Fe3+ test. Electrochemical tests reveal that water oxidation is accomplished via the generation of NiIV species generated by two successive e−/H+ proton coupled electron transfer (PCET) processes. Astonishingly, compared to the homologous complex [Ni(tmbptn)(OH2)](ClO4)2 (2, tmbptn = 2,5,8-trimethyl-1,9-bis(2-pyridyl)-2,5,8-triazanonane), which is a well-defined molecular WOC with high catalytic onset overpotential, 1 displays lower onset overpotential and much higher catalytic activity. This work suggesting that though the tertiary amine coordination environment have gain effect on the redox potential of metal center due to its poor sigma-donor effect, resulting in high onset overpotential of metal complex for water oxidation, this negative impact of the pyridine-amine ligand on the catalytic properties of complex can be weaken by adjusting the flexibility of backbone of those ligands.

Stabilizing Sulfur Sites in Tetraoxygen Tetrahedral Coordination Structure for Efficient Electrochemical Water Oxidation.

Ion regulation strategy is regarded as a promising pathway for designing transition metal oxide-based electrocatalysts for oxygen evolution reaction (OER) with improved activity and stability. Precise anion conditioning can accurately change the anionic environment so that the acid radical ions (SO4 2- , PO3 2- , SeO4 2- , etc.), regardless of their state (inside the catalyst, on the catalyst surface, or in the electrolyte), can optimize the electronic structure of the cationic active site and further increase the catalytic activity. Herein, we report a new approach to encapsulate S atoms at the tetrahedral sites of the NaCl-type oxide NiO to form a tetraoxo-tetrahedral coordination structure (S-O4 ) inside the NiO (S-NiO -I). Density functional theory (DFT) calculations and operando vibrational spectroscopy proves that this kind of unique structure could achieve the S-O4 and Ni-S stable structure in S-NiO-I. Combining mass spectroscopy characterization, it could be confirmed that the S-O4 structure is the key factor for triggering the lattice oxygen exchange to participate in the OER process. This work demonstrates that the formation of tetraoxygen tetrahedral structure is a generalized key for boosting the OER performances of transition metal oxides.

Stabilizing Sulfur Sites in Tetraoxygen Tetrahedral Coordination Structure for Efficient Electrochemical Water Oxidation

AbstractIon regulation strategy is regarded as a promising pathway for designing transition metal oxide‐based electrocatalysts for oxygen evolution reaction (OER) with improved activity and stability. Precise anion conditioning can accurately change the anionic environment so that the acid radical ions (SO42−, PO32−, SeO42−, etc.), regardless of their state (inside the catalyst, on the catalyst surface, or in the electrolyte), can optimize the electronic structure of the cationic active site and further increase the catalytic activity. Herein, we report a new approach to encapsulate S atoms at the tetrahedral sites of the NaCl‐type oxide NiO to form a tetraoxo‐tetrahedral coordination structure (S‐O4) inside the NiO (S‐NiO ‐I). Density functional theory (DFT) calculations and operando vibrational spectroscopy proves that this kind of unique structure could achieve the S‐O4 and Ni‐S stable structure in S‐NiO‐I. Combining mass spectroscopy characterization, it could be confirmed that the S‐O4 structure is the key factor for triggering the lattice oxygen exchange to participate in the OER process. This work demonstrates that the formation of tetraoxygen tetrahedral structure is a generalized key for boosting the OER performances of transition metal oxides.

"B" site-modulated perovskite oxide materials for efficient electrochemical water oxidation to hydrogen peroxide

Two-electron water oxidation (2e− WOR) has emerged as a promising method for H2O2 synthesis. However, achieving catalyst’ stability for hydrogen peroxide (H2O2) production through 2e− WOR under harsh oxidative conditions remains a significant challenge. Perovskite oxide catalysts have shown potential at low currents, but their electrocatalytic activity diminishes at high current densities. In this study, we successfully fabricated highly efficient 2e− WOR electrocatalysts by carefully selecting suitable B site metals with optimal O* and OH* adsorption characteristics. Our results demonstrate that the LaCoO3 catalyst exhibit exceptional H2O2 production rates, reaching 2977.29 μmol min−1 g−1. Furthermore, the LaCoO3 catalysts demonstrate remarkable durability and stable electrochemical performance even after an ultra-long 120-hour stability test. Density Functional Theory calculations (DFT) further demonstrate that we successfully screened catalysts that are thermodynamically more favorable at a 2e− WOR pathway and accelerate the rate-determining step of OH* to H2O2 by B-site modulation. This work presents a simple and effective strategy for designing perovskite catalysts tailored for efficient 2e− WOR.

Efficient electrochemical water oxidation catalyzed by N4-coordinated nickel complexes under neutral conditions

Two novel nickel complexes are reported for the first time. Both complexes can catalyze water oxidation under neutral conditions with low onset overpotential. Combined experiments confirm that they are genuine molecular water oxidation catalysts.

Cation etching-induced deep self-reconstruction to form a polycrystalline structure for efficient electrochemical water oxidation.

In this work, we report a straightforward in situ leaching strategy to achieve rapid structure self-reconstruction of NiRu-OH/NF. The as-prepared electrode shows excellent OER performance with a low overpotential of 321 mV at 100 mA cm-2. It can deliver 10 mA cm-2 at 1.53 V in a water-alkali electrolyzer as the anode and operates steadily at 100 mA cm-2 with a small cell voltage for 50 h.

Activating lattice oxygen by a defect-engineered Fe2O3–CeO2 nano-heterojunction for efficient electrochemical water oxidation

Self-supporting Fe2O3–CeO2 nano-heterojunction electrodes with rich oxygen vacancies present high catalytic performance for oxygen evolution reaction, where defect-engineering promotes the interfacial interaction and activates the lattice oxygens.

Intrabasal Plane Defect Formation in NiFe Layered Double Hydroxides Enabling Efficient Electrochemical Water Oxidation.

Defect engineering has proven to be one of the most effective approaches for the design of high-performance electrocatalysts. Current methods to create defects typically follow a top-down strategy, cutting down the pristine materials into fragmented pieces with surface defects yet also heavily destroying the framework of materials that imposes restrictions on the further improvements in catalytic activity. Herein, we describe a bottom-up strategy to prepare free-standing NiFe layered double hydroxide (LDH) nanoplatelets with abundant internal defects by controlling their growth behavior in acidic conditions. Our best-performing nanoplatelets exhibited the lowest overpotential of 241 mV and the lowest Tafel slope of 43 mV/dec for the oxygen evolution reaction (OER) process, superior to the pristine LDHs and other reference cation-defective LDHs obtained by traditional etching methods. Using both material characterization and density functional theory (DFT) simulation has enabled us to develop relationships between the structure and electrochemical properties of these catalysts, suggesting that the enhanced electrocatalytic activity of nanoplatelets mainly results from their defect-abundant structure and stable layered framework with enhanced exposure of the (001) surface.

Enriched Fe Doped on Amorphous Shell Enable Crystalline@Amorphous Core-Shell Nanorod Highly Efficient Electrochemical Water Oxidation.

The rational design of efficient and cost-effective electrocatalysts for oxygen evolution reaction (OER) with sluggish kinetics, is imperative to diverse clean energy technologies. The performance of electrocatalyst is usually governed by the number of active sites on the surface. Crystalline/amorphous heterostructure has exhibited unique properties and opens new paradigms toward designing electrocatalysts with abundant active sites for improved performance. Hence, Fe doped Ni-Co phosphite (Fe-NiCoHPi) electrocatalyst with cauliflower-like structure, comprising crystalline@amorphous core-shell nanorod, is reported. The experiments uncover that Fe is enriched in the amorphous shell due to the flexibility of the amorphous component. Further density functional theory calculations indicate that the strong electronic interaction between the enriched Fe in the amorphous shell and crystalline core host at the core-shell interface, leads to balanced binding energies of OER intermediates, which is the origin of the catalyst-activity. Eventually, the Fe-NiCoHPi exhibits remarkable activity, with low overpotentials of only 206 and 257mV at current density of 15 and 100mA cm-2 . Unceasing durability over 90h is achieved, which is superior to the effective phosphate electrocatalysts. Although the applications at high current remain challenges , this work provides an approach for designing advanced OER electrocatalysts for sustainable energy devices.

CoFe layered double hydroxides with adjustable composition and structure for enhanced oxygen evolution reaction

Gram-scale preparation of CoFe-LDH for highly efficient electrochemical water oxidation.

Modulating crystal and electronic structure of NiFe-MOFs by inorganic acid for highly efficient electrochemical water oxidation.

Seeking new methods to modulate the structure of metal-organic frameworks (MOFs) for diverse applications, particularly for water splitting, is intensively urgent but challenging. Herein, a simple hydrothermal method employing HCl as the modulator is developed to synthesize a series of NiFe-MOF-n/NF. The amount of HCl modulator not only changes the elemental composition and crystal structure but also modulates the electronic structure of NiFe-MOF-n/NF, thus improving intrinsic activity. Owing to the synergetic interactions between Ni and Fe atoms, free-standing feature, the optimized NiFe-MOF-2/NF yields excellent OER activity with overpotentials of 209 and 260 mV at 10 and 100 mA cm-2, respectively, a small Tafel slope of 36.4 mV dec-1 and excellent OER stability for 24 h at 100 mA cm-2 in 1 M KOH. This demonstrates that NiFe-MOF-2/NF are in situ converted into metal oxide/oxyhydroxide after OER, thereby serving as the real active sites. This study offers a feasible way to fabricate low-cost, efficient MOF-based electrocatalysts.

Efficient electrochemical water oxidation mediated by different substituted manganese-salophen complexes

• The electrocatalytic role of different substituted Mn II salophen complexes was investigated for water oxidation. • It was revealed that the studied complexes show excellent activity for OER. • The overpotential for the OER was in the range of 345–420 mV. • It was speculated that the O O bond formation occurs by oxidizing the active center of Mn complexes. • The Mn II complexes catalyzed the OER as real electrocatalysts. Developing robust and efficient water oxidation catalysts is an essential challenge in water splitting. In this work, the electrocatalytic behavior of the modified carbon paste electrodes with different substituted Mn II salophen complexes, MnL 1 -MnL 3 , (H 2 L 1 = N,N′-bis(salicylidene)-1,2-diaminobenzene, H 2 L 2 = N,N′-bis(salicylidene)-4-bromo-1,2-diaminobenzene and H 2 L 3 = N,N′-bis(2-hydroxy-3-methoxybenzilidine)-1,2-diaminobenzene) was evaluated for water oxidation by electrochemical methods. All three complexes were found highly effective, stable, and robust catalysts for water oxidation. Electrochemical experiments, Field emission-scanning electron microscope (SEM), energy-dispersive X-ray (EDX), and powder X-ray diffraction (PXRD) analyses indicated that the studied Mn II salophen complexes are molecular catalysts for water oxidation in origin. Cyclic voltammetry experiments in different pHs displayed that the O O bond formation probably occurs by oxidizing the active center of Mn II complexes through proton-coupled electron transfer (PCET). The results showed that the nature of the substituent on the salophen ligand could not affect the stability of the complexes under applied electrochemical conditions. It was found that Mn oxide nanostructures obtained by the thermal decomposition of MnL 1 can also effectively catalyze the reaction, but in comparison with MnL 1 , they have lower activity.

Dynamic active sites in NiFe oxyhydroxide upon Au nanoparticles decoration for highly efficient electrochemical water oxidation

Noble metal decoration is one of the most efficient strategies to boost the performance for water oxidation. However, identifying the role of noble metals and especially the active sites remain a persistent challenge. Here, we report a direct synthesis of Au nanoparticles (NPs) decorated NiFe oxyhydroxides via in-situ electrochemical conversion and a comprehensive study of the activity boost via a combination of systematic experimental and theoretical studies. We found that Fe doping greatly improved the electrical conductivity and the Fe sites were the dominant active centers, collectively responsible for the remarkably enhanced performance in NiFe oxyhydroxide. Upon Au NPs decoration, the content of high-valence Fe/Ni ions was substantially reduced and the formation of surface oxygen vacancy was greatly suppressed. As a result, the dominant active centers transfer to the interfacial Au sites that directly participate in catalyzing water oxidation. Our study presents a fundamental insight into the origin of activity enhancement and especially the active centers in noble metal decorated NiFe oxyhydroxide catalysts.

Interfacial Atom‐Substitution Engineered Transition‐Metal Hydroxide Nanofibers with High‐Valence Fe for Efficient Electrochemical Water Oxidation

Developing low‐cost electrocatalysts for efficient and robust oxygen evolution reaction (OER) is the key for scalable water electrolysis, for instance, NiFe‐based materials. Decorating NiFe catalysts with other transition metals offers a new path to boost their catalytic activities but often suffers from the low controllability of the electronic structures of the NiFe catalytic centers. Here, we report an interfacial atom‐substitution strategy to synthesize an electrocatalytic oxygen‐evolving NiFeV nanofiber to boost the activity of NiFe centers. The electronic structure analyses suggest that the NiFeV nanofiber exhibits abundant high‐valence Fe via a charge transfer from Fe to V. The NiFeV nanofiber supported on a carbon cloth shows a low overpotential of 181 mV at 10 mA cm−2, along with long‐term stability (>20 h) at 100 mA cm−2. The reported substitutional growth strategy offers an effective and new pathway for the design of efficient and durable non‐noble metal‐based OER catalysts.

Interfacial Atom‐Substitution Engineered Transition‐Metal Hydroxide Nanofibers with High‐Valence Fe for Efficient Electrochemical Water Oxidation

AbstractDeveloping low‐cost electrocatalysts for efficient and robust oxygen evolution reaction (OER) is the key for scalable water electrolysis, for instance, NiFe‐based materials. Decorating NiFe catalysts with other transition metals offers a new path to boost their catalytic activities but often suffers from the low controllability of the electronic structures of the NiFe catalytic centers. Here, we report an interfacial atom‐substitution strategy to synthesize an electrocatalytic oxygen‐evolving NiFeV nanofiber to boost the activity of NiFe centers. The electronic structure analyses suggest that the NiFeV nanofiber exhibits abundant high‐valence Fe via a charge transfer from Fe to V. The NiFeV nanofiber supported on a carbon cloth shows a low overpotential of 181 mV at 10 mA cm−2, along with long‐term stability (>20 h) at 100 mA cm−2. The reported substitutional growth strategy offers an effective and new pathway for the design of efficient and durable non‐noble metal‐based OER catalysts.

Dynamic Active Sites in Nife Oxyhydroxide Upon AU Nanoparticles Decoration for Highly Efficient Electrochemical Water Oxidation

Dynamic Active Sites in Nife Oxyhydroxide Upon AU Nanoparticles Decoration for Highly Efficient Electrochemical Water Oxidation

Efficient electrochemical water oxidation mediated by a binuclear copper complex with a helical structure

A binuclear Cu complex [Cu2(MePy3P)2] is found to be capable of catalyzing electrochemical water oxidation under neutral conditions via the intramolecular interaction of its two Cu cores, achieving a remarkable turnover frequency of 18.04 s−1 and onset overpotential of 480 mV for oxygen evolution.

Efficient Electrochemical Water Oxidation Mediated by Pyridylpyrrole-Carboxylate Ruthenium Complexes.

Spurred by the rapid growth of Ru-based complexes as molecular water oxidation catalysts (WOCs), we propose novel ruthenium(II) complexes bearing pyridylpyrrole-carboxylate (H2ppc) ligands as members of the WOC family. The structure of these complexes has 4-picoline (pic)/dimethyl sulfoxide (DMSO) in [Ru(ppc)(pic)2(dmso)] and pic/pic in [Ru(ppc)(pic)3] as axial ligands. Another ppc2- ligand and one pic ligand are located at the equatorial positions. [Ru(ppc)(pic)2(dmso)] behaves as a WOC as determined by electrochemical measurement and has an ultrahigh electrocatalytic current density of 8.17 mA cm-2 at 1.55 V (vs NHE) with a low onset potential of 0.352 V (vs NHE), a turnover number of 241, a turnover frequency of 203.39 s-1, and kcat of 16.34 s-1 under neutral conditions. The H2O/pic exchange of the complexes accompanied by oxidation of a ruthenium center is the initial step in the catalytic cycle. The cyclic voltametric measurements of [Ru(ppc)(pic)2(dmso)] at various scan rates, Pourbaix diagrams (plots of E vs pH), and kinetic studies suggested a water nucleophilic attack mechanism. HPO42- in a phosphate buffer solution is invoked in water oxidation as the proton acceptor.

CoFe2O4 nanoparticles@N-doped carbon coupled with N-doped graphene toward efficient electrochemical water oxidation

Developing high-performance nonprecious-metal electrocatalysts for the oxygen evolution reaction (OER) is crucial for efficient water splitting. Therefore, the reasonable design principle of catalyst, leading to high activity catalytic center and improving the accessibility of active sites, is undoubtedly crucial. Here, we used a kind of Prussian blue analogue (CoFe-PBA) nanoparticles to anchor the nitrogen-carbon doped CoFe 2 O 4 (CoFe 2 O 4 @NC) species to reduced graphene oxide aerogels as OER catalysts. The strong interaction between nanosized CoFe 2 O 4 @NC and the graphitic carbon shell led to synergistic effects in the OER, and the protection of the carbon shell guaranteed stability of the catalyst. As a result, the aerogel electrocatalyst exhibits more excellent activity and stability than the most advanced RuO 2 catalyst in OER under the same mass load in alkaline medium. It shows smaller overpotential of 250 mV to afford a current density of 10 mA cm −2 in a continuous working time of more than 70 hours. • The catalyst has highly exposed active surface, multi-dimensional mass transfer pathway and good charge transfer ability. • The in-situ formation of nitrogen-carbon doped structure can greatly improve the catalytic activity and stability. • The binary active sites on the conductive GA network reduce the interfacial resistance and enhance catalytic activity. • The "Plum Pudding Model" structure integrates multiple requirements for catalyst design strategies.

Correction: Activating the lattice oxygen in (Bi0.5Co0.5)2O3 by vacancy modulation for efficient electrochemical water oxidation

Correction for ‘Activating the lattice oxygen in (Bi0.5Co0.5)2O3 by vacancy modulation for efficient electrochemical water oxidation’ by Huan Liu et al., J. Mater. Chem. A, 2020, 8, 13150–13159, DOI: 10.1039/D0TA03411H.