Water Oxidation In Alkaline Media Research Articles

A facile synthesis of N-doped carbon encapsulated multimetallic carbonitride as a robust electrocatalyst for oxygen evolution reaction

Electrocatalytic water splitting is a promising solution for generating clean hydrogen. Transition metal compounds are among the most extensively investigated catalysts developed to date for water oxidation in alkaline media, a process also known as the oxygen evolution reaction (OER). However, the application of these catalysts was constrained by insufficient stability arising from surface oxidation and metal dissolution under high OER potential. In this work, we developed a facile approach using urea-based gel as the precursor of preparing a series of multimetallic carbonitride particles which were encapsulated by N-doped carbon (NC). In particular, (MoCoFeNiZr)CN@NC core–shell structure delivered a low overpotential of 246 mV at a current density of 10 mA cm−2 in 1 M KOH during OER. Importantly, operando differential electrochemical mass spectrometry (DEMS), together with multiple microscopic and spectroscopic analyses, indicated that the NC shells effectively maintained the crystalline stability of carbonitride via suppressing the surface reconstruction during catalysis. The highly graphitic NC also demonstrates excellent stability against oxidation. This work shows a promising strategy of stabilizing electrocatalyst at high anodic potential, paving the way for the development of robust electrode materials for energy conversion.

ZnAl2O4 decorated on graphitic carbon nitride (g-CN) composite as efficient electrocatalyst for water oxidation in alkaline medium

The main purpose of this research is to form highly effective and novel electrocatalysts for water splitting procedure, a pivotal step in the generation of sustainable energy. Cost-effective metal oxides exhibited superior benefits compared to alternative materials when used for oxygen evolution reactions (OER) in alkaline electrolyte. Our study focuses on the preparation of ZnAl2O4@g-CN composite by utilizing hydrothermal approach for electrocatalytic water splitting. The electrocatalytic potential of prepared electrocatalyst was observed by a comprehensive series of tests. Numerous electrochemical approaches such as chronoamperometry and electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) were implemented to analyze catalytic procedure and durability in basic medium. The electrocatalytic tests of ZnAl2O4@g-CN composite reveal a remarkable overpotential of 209 mV, enabling the attainment of current density (Cd) 10 mA/cm2. Additionally, the composite shows significantly lower Tafel value (36 mV/dec) and has remarkable durability for 30 h. These characteristics mutually designate that electrocatalyst is well-suited for OER procedure. Therefore, the fabricated composite exhibited excellent efficacy in OER procedure as well as for several other upcoming applications.

Tuning Oxygen Evolving Catalytic Activity of a Precious Metal-Free Nickel Boride by a Cost-Effective Method

The production of hydrogen (H2) by electrochemical water splitting [Green Hydrogen (Green H2)], if driven by green electricity, has attracted considerable interest as an alternative sustainable energy carrier, e.g., in fuel cells for electricity or power and heat generation [1]. However, the lack of satisfactory and cheap electrocatalysts (ECs) to drive the vital electrochemical reactions of hydrogen evolution (HER) and oxygen evolution (OER) in water splitting is the biggest challenge for this technology.Currently, the most advanced electrocatalysts for the two half-reactions of water electrolysis (HER and OER) are noble metal-based materials [2]. Despite the excellent catalytic properties of these noble metal catalysts, their high cost, and scarcity make their commercial use both uneconomical and impractical in low temperature water electrolysis (LT-WE) systems at industrial scale for green H2 as a clean energy alternative to cheaper fossil fuels[3]. In addition to addressing this issue by the development of precious-material-based with a low-loading of precious metal elements, the development of desirable and high-efficiency electrocatalysts (ECs) based on earth-abundant metal elements materials is paramount. However, their catalytic activity is often limited by poor electron transfer, low conductivity, low stability, and small surface area of these materials. Thus, it is crucial to develop earth-abundant metal elements-based materials that are more cost-effective to reduce the high cost of active catalysts for water electrolysis. Although several low-cost materials have already been designed, the potential required for the practical application of water splitting is still very high compared to the theoretical potential (1.23 V), which means that much energy is consumed, which theoretically could be reduced. Due to their remarkable advanced charge transfer and durability for water oxidation in alkaline media, electrocatalysts derived from metal borides-based materials have received a lot of attention as new high-performance catalysts for water oxidation.[4] These properties stem from the multi-dimensional covalent bonding of the metalloid with the surrounding metal atoms.We present here an approach to tune the OER of transition metal borides (TMBs) using the cost-effective and less energy-consuming method that is nevertheless a highly efficient material for OER (Fig. 1). We have developed a low-cost, rational and easily scalable method to enhance the catalytic activity of nickel boride (Ni–B), which requires an annealing process to optimize the catalytic activity. However, this method may have limited applicability on a large scale as it requires a highly inert environment and high-temperature treatment of Ni–B catalysts. We also elucidate a structure-activity relationship using scale-bridging techniques such as TEM and electrochemical characterization. The developed based on non-noble electrocatalysts are prepared by a simple method of chemical reduction of transition metal elements with boron in aqueous solution. The production is a one-step process and the material does not require any further treatment, yet this material shows comparable activity to the state of the art (precious metal-based materials) for OER in alkaline solution at a selected current density. This improved catalytic performance is further corroborated by the microstructural investigations in the TEM. The nanoparticles obtained have an improved porous structure compared to Ni–B and thus provide more available sites for the surface reactions and hence for the catalytic performance of the material. Furthermore, it is shown that activation enables the morphological and structural changes, while some transition metal elements act as sacrificial elements to provide more accessible and stable sites for oxygen-forming centers. This paves the way for a better understanding of metal boride-derived electrocatalysts for water oxidation, a crucial chemical reaction in water electrolysis and other electrochemical energy technologies. Acknowledgements: This project has received funding from the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement No 945422. We acknowledge the use of the DFG-funded Micro-and Nanoanalytics Facility (MNaF) at the University of Siegen (INST 221/131-1).

In situ transformation from metal-organic framework into iridium-iron hollow hexagonal rod to promote oxygen evolution reaction

Developing highly efficient and durable electrocatalysts for water oxidation in alkaline media is crucial for hydrogen production in industrialization. IrO2 is a highly promising electrocatalyst for oxygen evolution reaction (OER), but its large-scale is still limited to the sluggish OER kinetics in alkaline solution due to the multi-steps adsorption of -OH and H2O. Herein, we reported a novel in-situ generation from metal-organic framework strategy for preparing hollow hexagonal rod IrFex electrocatalysts (IrFex HHRs). Specifically, the optimized IrFe3 HHRs shows an excellent OER electrocatalytic activity, including low overpotential of 265, 295 and 307 mV at the current density of 10, 50 and 100 mA cm−2, and high stability (100 h). The experimental results revealed that the existent interfacial Fe-O-Ir bonds between Ir and MIL-88A (Fe) can promote electrons transport to improve the electrocatalyst activity. This work represents a new strategy for designing an efficiency electrocatalysts with low iridium content derived from Ir doped-bimetal MOF material.

Multiwall carbon nanotubes supported rhenium doped Co3O4 as an efficient electrocatalyst for water oxidation in alkaline medium

The recent trends for the development of efficient and low cost transition metal (TMs) based anode catalysts for water splitting is crucial and important. Even though, research is continuously progressing in different directions to design catalyst; however, catalytic OER (oxygen evolution reaction) activity has remained insufficient. Till now, various transition metal oxides (TMOs) have been used as efficient and cost-effective electrocatalyst for the oxygen evolution reaction. The catalytic performance of TMOs could be enhanced by doping with the metal, which increases the catalyst's charge transfer and electronic conductivity. Decoration of TMOs with the multi-wall carbon nanotube (MWCNT) further improves catalytic activity by providing numerous active sites. Here, Co3O4/MWCNT and Rhenium (Re) doped Co3O4/MWCNT composite were prepared by simple hydrothermal process and used as electrocatalyst for OER in alkaline medium. Different concentrations of Re (0.05 mmol, 0.1 mmol, 0.2 mmol, 0.3 mmol) were doped and it was observed that the electrochemical results of the 0.2 mmol Re doped Co3O4/ MWCNT (R0.2CO) was most efficient electrocatalyst which offers the OER overpotential of 298 mV/dec to attain the 10 mA cm−2 current density, lower Tafel slope (70 mV/dec), higher value of turn over frequency (0.95), lower value of the charge transfer resistance (3.10Ω) and good stability, demonstrating the better OER catalytic activity as compared to un-doped Co3O4/MWCNT (R0.0CO) catalyst. The investigated catalyst R0.2CO is more efficient catalyst than most of the reported similar type of catalysts and offers comparable electrocatalytic activity with benchmark electrocatalyst RuO2.

High-Valence Mo Doping and Oxygen Vacancy Engineering to Promote Morphological Evolution and Oxygen Evolution Reaction Activity.

The rational design of high-efficiency, low-cost electrocatalysts for electrochemical water oxidation in alkaline media remains a huge challenge. Herein, combined strategies of metal doping and vacancy engineering are employed to develop unique Mo-doped cobalt oxide nanosheet arrays. The Mo dopants exist in the form of high-valence Mo6+, and the doping amount has a significant effect on the structure morphology, which transforms from 1D nanowires/nanobelts to 2D nanosheets and finally 3D nanoflowers. In addition, the introduction of vast oxygen vacancies helps to modulate the electronic states and increase the electronic conductivity. The optimal catalyst MoCoO-3 exhibits greatly increased active sites and enhanced reaction kinetics. It gives a dramatically lower overpotential at 50 mA cm-2 (288 mV), much smaller than that of the undoped counterpart (418 mV) and comparable to those of the recently reported electrocatalysts. Density functional theory results further verify that the increased electronic conductivity and optimized adsorption energy toward oxygen evolution reaction intermediates are mainly responsible for the enhanced catalytic activity. Moreover, the assembled two-electrode electrolyzer (MoCoO-3||Pt/C) exhibits superior performance with the cell potential decreased by 233 mV to reach a current density of 50 mA cm-2 with respect to the benchmark counterpart catalysts (RuO2||Pt/C). This work might contribute to the rational design of effective, low-cost electrocatalyst materials by combining multiple strategies.

Chemically Etched Prussian Blue Analog-WS2 Composite as a Precatalyst for Enhanced Electrocatalytic Water Oxidation in Alkaline Media.

The electrochemical water-splitting reaction is a promising source of ecofriendly hydrogen fuel. However, the oxygen evolution reaction (OER) at the anode impedes the overall process due to its four-electron oxidation steps. To address this issue, we developed a highly efficient and cost-effective electrocatalyst by transforming Co-Fe Prussian blue analog nanocubes into hollow nanocages using dimethylformamide as a mild etchant and then anchoring tungsten disulfide (WS2) nanoflowers onto the cages to boost OER efficiency. The resulting hybrid catalyst-derived oxide demonstrated a low overpotential of 290 mV at a current density of 10 mA cm-2 with a Tafel slope of 75 mV dec-1 in 1.0 M KOH and a high faradaic efficiency of 89.4%. These results were achieved through the abundant electrocatalytically active sites, enhanced surface permeability, and high electronic conductivity provided by WS2 nanoflowers and the porous three-dimensional (3D) architecture of the nanocages. Our research work uniquely combines surface etching of Co-Fe PBA with WS2 growth to create a promising OER electrocatalyst. This study provides a potential solution to the challenge of the OER in electrochemical water-splitting, contributing to UN SDG 7: Affordable and clean energy.

Facile eggplant assisted mixed metal oxide nanostructures: A promising electrocatalyst for water oxidation in alkaline media

In terms of efficiency and availability for electrochemical water splitting (EWS), producing electrocatalytic material for hydrogen (H2) fuel remains a difficulty. Furthermore, developing and producing low-cost, non-precious metal-based electrocatalysts for practical use is a global priority for researchers. Significant overpotential, low conductivity, and increased density are still factors to consider for alkaline water oxidation. We present here coprecipitated alumina (Al2O3) and eggplant-based cobalt oxide (Co3O4) nanostructures as electrocatalysts for the oxygen evolution reaction (OER). The as-prepared electrocatalyst offers active sites, enhanced electrical conductivity, and durability in the direction of OER. The desired electrocatalyst show lower overpotential value of 245 mV vs. a reversible hydrogen electrode (RHE) at a current density of 20 mA/cm2, a Tafel slope of 64 mV/dec, and stable electrochemical performance for 45 h as a result of a longer electrochemical activity. These findings suggest that this mixed metal oxide (nanostructures)-based electrocatalyst is a viable, active, and stable element for use in energy storage and conversion applications.

Dark Current Water Splitting Employing Ni3TeO6 as a Photocharged Photoelectrocatalyst

Herein, the photoelectrocatalytic and photocharging activity of a Ni3TeO6 (NTO‐700) photoelectrocatalyst for water oxidation in alkaline medium is demonstrated and calcined at 700 °C. The photoelectrocatalytic (PEC) activity of the sample is increased upon an increase in the illumination time. With a twofold increase in photocurrent density at 1.8 V, the PEC 150 sample (having been illuminated for 150 min) attained the highest PEC activity. The synthesized material has displayed an excellent charge storage capacity in KOH and Na2SO4 electrolyte solutions (both 0.1 m). The chrono‐amperometry measurement, subsequent to light interruption, has sustained almost 44% higher current density (even after 200 min) compared to the pure electrocatalytic baseline in a Na2SO4 electrolyte. The charge transfer resistance, Rct, decreases from 633.40 to 170.40 Ω, while the charge transfer rate constant, kct, increases from 7.93 to 27.03 s−1, as a function of illumination time. This points to fast separation of electron–hole (e−–h+) pairs and a slower recombination rate. The lower values of the charge transfer resistance and the time constant recorded for the light interrupted samples, as compared to the electrochemical sample, are attributed to the stored charge that drives water oxidation at a higher rate.

CdS/TiO2 nano hybrid heterostructured materials for superior hydrogen production and gas sensor applications

In efforts to minimize environmental pollution and carbon-based gas emissions, photocatalytic hydrogen production and sensing applications at ambient temperature are important. This research reports on the development of new 0D/1D materials based on TiO2 nanoparticles grown onto CdS hetersturctured nanorods via two-stage facile synthesis. The titanate nanoparticles when loaded onto CdS surfaces at an optimized concentration (20 mM), exhibited superior photocatalytic hydrogen production (21.4 mmol/h/gcat). The optimized nanohybrid was recycled for 6 cycles up to 4 h, indicating its excellent stabity for a prolonged period. Also, the photoelectrochemical water oxidation in alkaline medium was investigated to offer the optimized CRT-2 composite with 1.91 mA/cm2@0.8 V vs. RHE (0 V vs. Ag/AgCl) that was used for effective room-temperature NO2 gas detection exhibiting a higher response (69.16%) to NO2 (100 ppm) at room temperature at the lowest detection limit of ∼118 ppb than the pristine counterparts. Further, NO2 gas sensing performance of CRT-2 sensor was increased using UV light (365 nm) activation energy. Under the UV light, the sensor exhibited a remarkable gas sensing response quick response/recovery times (68/74), excellent long-term cycling stability, and significant selectivity to NO2 gas. Due to high porosity and surface area values of CdS (5.3), TiO2 (35.5), and CRT-2 (71.5 m2/g), excellent photocatalytic H2 production and gas sensing of CRT-2 is ascribed to morphology, synergistic effect, improved charge generation, and separation. Overall, 1D/0D CdS@TiO2 is proved to be an efficient material for hydrogen production and gas detection.

Solution combustion synthesis of hierarchical porous CoNiFeCu0.1 medium-entropy alloy: A highly efficient and robust electrocatalyst for water oxidation

The development of efficient and stable oxygen evolution reaction (OER) electrocatalysts with nonprecious materials is still a great challenge for the hydrogen-based energy industry. Here, we report the rational design and effective synthesis of hierarchical porous medium-entropy alloys (MEAs) as promising electrocatalysts for OER via a solution combustion method. Owing to the compositional advantages and unique hierarchical porous structure, the as-prepared CoNiFeCu0.1 MEA on nickel foam (CoNiFeCu0.1 @NF) shows superior electrocatalytic performance for OER with an overpotential of about 255 mV at 10 mA cm−2 and a small Tafel slope of 42.7 mV dec−1 in 1.0 M KOH solution. Moreover, it is revealed that the alloying of element Cu in CoNiFe MEA can significantly enhance the charge transfer ability for OER electrocatalysis. The present work shows the hierarchical porous MEAs prepared by solution combustion method have great potential as efficient and robust electrocatalysts for water oxidation in alkaline medium.

PdO@CoSe2 composites: efficient electrocatalysts for water oxidation in alkaline media.

In this study, we have prepared cobalt selenide (CoSe2) due to its useful aspects from a catalysis point of view such as abundant active sites from Se edges, and significant stability in alkaline conditions. CoSe2, however, has yet to prove its functionality, so we doped palladium oxide (PdO) onto CoSe2 nanostructures using ultraviolet (UV) light, resulting in an efficient and stable water oxidation composite. The crystal arrays, morphology, and chemical composition of the surface were studied using a variety of characterization techniques, including X-ray diffraction (XRD), scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared (FTIR) spectroscopy. It was also demonstrated that the composite systems were heterogeneous in their morphology, undergoing a shift in their diffraction patterns, suffering from a variety of metal oxidation states and surface defects. The water oxidation was verified by a low overpotential of 260 mV at a current density of 20 mA cm-2 with a Tafel Slope value of 57 mV dec-1. The presence of multi metal oxidation states, rich surface edges of Se and favorable charge transport played a leading role towards water oxidation with a low energy demand. Furthermore, 48 h of durability is associated with the composite system. With the use of PdO and CoSe2, new, low efficiency, simple electrocatalysts for water catalysis have been developed, enabling the development of practical energy conversion and storage systems. This is an excellent alternative approach for fostering growth in the field.

Low temperature in situ immobilization of nanoscale fcc and hcp polymorphic nickel particles in polymer-derived Si-C-O-N(H) to promote electrocatalytic water oxidation in alkaline media.

We synthesized nickel (Ni) nanoparticles (NPs) in a high specific surface area (SSA) p-block element-containing inorganic compound prepared via the polymer-derived ceramics (PDC) route to dispatch the obtained nanocomposite towards oxygen evolution reaction (OER). The in situ formation of Ni NPs in an amorphous silicon carboxynitride (Si-C-O-N(H)) matrix is allowed by the reactive blending of a polysilazane, NiCl2 and DMF followed by the subsequent thermolysis of the Ni : organosilicon polymer coordination complex at a temperature as low as 500 °C in flowing argon. The final nanocomposite displays a BET SSA as high as 311 m2 g-1 while the structure of the NPs corresponds to face-centred cubic (fcc) Ni along with interstitial-atom free (IAF) hexagonal close-packed (hcp) Ni as revealed by XRD. A closer look into the compound through FEG-SEM microscopy confirms the formation of pure metallic Ni, while HR-TEM imaging reveals the occurrence of Ni particles featuring a fcc phase and surrounded by carbon layers; thus, forming core-shell structures, along with Ni NPs in an IAF hcp phase. By considering that this newly synthesized material contains only Ni without doping (e.g., Fe) with a low mass loading (0.15 mg cm-2), it shows promising OER performances with an overpotential as low as 360 mV at 10 mA cm-2 according to the high SSA matrix, the presence of the IAF hcp Ni NPs and the development of core-shell structures. Given the simplicity, the flexibility, and the low cost of the proposed synthesis approach, this work opens the doors towards a new family of very active and stable high SSA nanocomposites made by the PDC route containing well dispersed and accessible non-noble transition metals for electrocatalysis applications.

Stabilization and activation of polyoxometalate over poly(vinyl butylimidazolium) cations towards electrocatalytic water oxidation in alkaline media.

Sandwich polyoxometalate [WCo3(H2O)2(CoW9O34)2]12- (Co-WCo3) heterogenized with poly(vinyl butyl imidazolium) cations (PVIM+) acts as a stable electrochemical water oxidation catalyst in 1 M KOH with 0.28 V@10 mA cm-2 overpotential and TOF of 6.16 s-1. Various in situ spectroelectrochemical studies and control experiments provide the details of the catalytic activities and long-term stability of the catalyst.

Advances in nonprecious metal catalysts for efficient water oxidation in alkaline media

Advances in nonprecious metal catalysts for efficient water oxidation in alkaline media

Recent Advances on Perovskite Electrocatalysts for Water Oxidation in Alkaline Medium

Alkaline water splitting is a technique utilizing renewable resource-derived electricity to generate hydrogen with high purity. The oxygen evolution reaction (OER) with sluggish dynamics is the crucial half-reaction toward electrocatalytic water splitting, which needs a non-noble metal catalyst with high performance to make the reaction process be more energy-efficient and economical. Perovskite, as a kind of price moderate, compositional adjustable, structurally stable, and highly active material, has been widely used as the catalyst for the OER under alkaline conditions. In this review, we systematically expound the OER-related catalytic mechanism, provide the evaluative criteria of catalytic performance, and then summarize the corresponding measurement methods of catalysts for the alkaline OER. Furthermore, our emphasis is given to the synthetic methods, the activity descriptors, and the performance optimization strategies of perovskite-based catalysts. Finally, we present a number of future directions on the development of more highly efficient perovskite-based catalysts for the alkaline OER.

Ultrafast detection of ammonia at room temperature and subsequent electrochemical water splitting via the ionic liquid templated nano nickel oxide

In this work, biopolymers like chitosan and Bovine Serum Albumin (BSA) templated and ionic liquid (IL)-driven synthesis of crystalline NiO nanoparticles for ultrafast room temperature detection of ammonia together with its semi-empirical mechanistic details are reported. The XRD results have revealed the crystalline nature of synthesized NiO nanoparticles; the contact angle measurements for the adhesive nature of the NiO nanoparticles have been carried out and the films have been subjected to gas sensing studies by chemo-resistive method. The as-prepared BSA templated ionic liquid driven NiO based thin film sensor displays an ultrafast response for the room temperature detection of ammonia with an excellent response and recovery times of 38 and 13 s, respectively, towards a lower detection limit up to 1 ppm. The ultrafast sensing mechanism of the ammonia has been deduced with the aid of X-ray Photoelectron Spectroscopy (XPS) analysis. Furthermore, as-synthesized BSA/BSA-IL templated NiO electrodes have displayed excellent electrocatalytic water oxidation in alkaline media, with a collective water-splitting stable current density of 10 mA∙cm−2. We have observed the HOMO-LUMO gap and the Valence Band/Conduction Band (VB/CB) position from the Bredas method calculations which clarifies the highest aptitude of NiO prepared with BSA-IL template for superfast ammonia gas sensing as the CB energy is significantly lower than that of NiO synthesized via the BSA template. The dual approach of multifarious applications for the ultrafast ammonia sensing and the corresponding electrochemical water oxidation through the bio-inspired NiO-nanoparticles is in fact one of the novel applications being cited in this paper. The results highlight the grave implications of as-synthesized NiO-nanoparticles as a brilliant cost-effective gas sensor and as an effective alternative electrode for sustainable energy harvesting.

Oxalate Assisted Growth of Hybrid Nickel Cobalt-Based Nanostructures for Hydrogen Generation Via Urea Oxidation

Hybrid mixed transition metal oxides nanostructures as spinal act as promising electrode materials for energy-related applications. In the present work, a design and synthesis of mixed metal oxide nanostructures have been done by increasing the nickel content in the crystal structure of Co3O4 spinel by using template-assisted oxalate route via conzventional coprecipitation method for syntheses. Selectively controlling the Ni: Co ratio will result in a significant change in the morphology of the nanostructures. Further, kinetically controlled thermal decomposition of nickel and cobalt oxalate precursors results in the formation of anisotropically aligned ultrafine nanoparticle self-assembly to form nanocubes and nanorods. Whereas the mixture of nickel-cobalt oxalate precursor after decomposition led to the formation of nanoparticles assembly that is interconnected and hierarchically arranged to form nanosheets that align themselves in a particular orientation to give a three-dimensional micro flower structure. This hybrid structure with increased porosity and higher exposed active sites acts as an efficient electrocatalyst to promote water and urea oxidation in alkaline media. The hybrid catalyst shows overpotential of 300 mV and -190 mV towards water oxidation in alkaline media with excellent stability for 24 h. Similarly, during urea oxidation, the significant loss in the potential to 1.31 V can be seen to achieve the current density of 10 mA/cm2 compared to the potential required in urea-free electrolysis. So, this work provides better insights for tuning snd designing the hybrid structures and regulating their morphological characteristics for enhanced catalytic performance.

Investigation of electrocatalytic activity of a new mononuclear Mn(II) complex for water oxidation in alkaline media.

Water splitting is a promising way to alleviate the energy crisis. In nature, water oxidation is done by a tetranuclear manganese cluster in photosystem II. Therefore, the study of water oxidation by Mn complexes is attractive in water splitting systems. In this report, a new mononuclear Mn(II) complex, MnL2 (HL = (E)-3-hydroxy-N'-(pyridin-2-ylmethylene)-2-naphthohydrazide) was prepared and characterized by spectroscopic techniques and single-crystal X-ray diffraction. Crystallographic analysis indicated that the geometry around the Mn(II) ion is distorted octahedral. The MnN4O2 coordination moiety is achieved by bounding of oxygen and two nitrogen donor atoms of two hydrazone ligands. The synthesized complex was also investigated for electrochemical water oxidation using electrochemical techniques, scanning electron microscopy, energy dispersive spectrometry, and PXRD analysis. Linear sweep voltammetry experiment showed that the modified carbon paste electrode by the complex displays high activity for water oxidation reaction with an overpotential of 565mV at a current density of 10mAcm-2 and Tafel slope of 105mV dec-1 in an alkaline solution. It was found that the complex structure finally changes during the reaction and converts to Mn oxide nanoparticles which act as active catalytic species and oxidize the water.

Deep Eutectic Solvent Synthesis of Perovskite Electrocatalysts for Water Oxidation.

Oxide perovskites have attracted great interest as materials for energy conversion due to their stability and structural tunability. La-based perovskites of 3d-transition metals have demonstrated excellent activities as electrocatalysts in water oxidation. Herein, we report the synthesis route to La-based perovskites using an environmentally friendly deep eutectic solvent (DES) consisting of choline chloride and malonic acid. The DES route affords phase-pure crystalline materials on a gram scale and results in perovskites with high electrocatalytic activity for oxygen evolution reaction. A convenient, fast, and scalable synthesis proceeds via assisted metathesis at a lower temperature as compared to traditional solid-state methods. Among LaCoO3, LaMn0.5Ni0.5O3, and LaMnO3 perovskites prepared via the DES route, LaCoO3 was established to be the best-performing electrocatalyst for water oxidation in alkaline medium at 0.25 mg cm–2 mass loading. LaCoO3 exhibits current densities of 10, 50, and 100 mA cm–2 at respective overpotentials of approximately 390, 430, and 470 mV, respectively, and features a Tafel slope of 55.8 mV dec–1. The high activity of LaCoO3 as compared to the other prepared perovskites is attributed to the high concentration of oxygen vacancies in the LaCoO3 lattice, as observed by high-resolution transmission electron microscopy. An intrinsically high concentration of O vacancies in the LaCoO3 synthesized via the DES route is ascribed to the reducing atmosphere attained upon thermal decomposition of the DES components. These findings will contribute to the preparation of highly active perovskites for various energy applications.