Water splitting is a promising pathway for hydrogen production, providing an environmentally friendly fuel source. More recently, great attention has been given to transition metal dichalcogenides (TMDCs) because of their interesting chemical and physical properties. In particular, tungsten disulfide (WS2) has garnered significant attention as a catalyst for this application due to its unique layered 2D structure. In this study, few-layered WS2 and phosphorus-doped WS2 (WS2/P) nanoflakes are synthesized on SiO2/Si substrates as electrocatalysts for hydrogen evolution reactions (HER) in acidic conditions. Analyses of the synthesized WS2 and WS2/P films reveal that the few-layered WS2 is of high quality, exhibiting continuity and uniformity. The presence of a strong peak in the photoluminescence spectrum confirms the mono/few layer nature of the synthesized samples. In additionally, scanning force microscopy in quantitative imaging mode reveals that the thinnest layers observed on the substrate have a height of 1.35 nm, indicating the presence of double-layer WS2. The WS2/P electrocatalyst demonstrates superior HER performance compared to pristine WS2, showing a low overpotential of 245 mV at 10 mA.cm−2 and a small Tafel slope of 123 mV.dec−1. Furthermore, WS2/P exhibits a greater electrochemical surface area and excellent catalytic stability under acidic conditions. Consequently, few layer phosphorus-doped WS2 proves to be a highly suitable electrocatalyst for hydrogen production compared to the WS2.

Water oxidation, or the oxygen evolution reaction (OER), is the half-reaction that limits the efficiency of overall water splitting and represents a major bottleneck in the production of sustainable hydrogen fuel. Developing efficient, cost-effective, and environmentally benign OER catalysts is therefore critical for advancing green energy technologies. In this study, magnetite and cobalt–iron oxide nanoparticles with varying cobalt contents were synthesized via a green co-precipitation method using Seidlitzia rosmarinus plant extract, and their properties were compared with those of counterparts prepared by a conventional chemical route. X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FTIR) revealed that green-synthesized samples exhibited a more amorphous character and contained hematite phases, while chemically synthesized materials were more crystalline and magnetite-rich. Field emission scanning electron microscopy (FESEM) and elemental mapping confirmed homogeneous cobalt distribution across all samples. Magnetic measurements via vibrating sample magnetometry (VSM) showed significantly reduced saturation magnetization in green-synthesized samples, attributed to cobalt substitution in the hematite structure, which induced paramagnetic behavior. Electrochemical characterization revealed superior OER performance for the green-synthesized cobalt–iron oxide nanoparticles, with the best sample (20 wt

Recently, Ni molecular catalysis has been extensively applied in oxygenation reactions. This work is underpinned by the characterization techniques and the discovered instability of the Ni-bipyridine/phenanthroline system, which results in Ni (hydr)oxide production under oxidative conditions. The practical applications of this mechanism by employing a prepared Ni (hydr)oxide-based electrode specifically in the oxygenation of sulfides, achieving noteworthy yields in contrast to noncatalyst control experiments, are explored. Thus, a Ni (hydr)oxide-based material is proposed as a candidate for the true catalyst for sulfide oxidation in the presence of the Ni-bipyridine/phenanthroline system. The findings of this study are expected to stimulate discussion and encourage new viewpoints within the chemical community regarding the potential applications and mechanisms of molecular catalysts in oxidation reactions.

Developing Mn-based water-oxidation reaction (WOR) catalysts is key for renewable energy storage, utilizing Mn's abundance, cost-effectiveness, and natural role. Cerium(IV) ammonium nitrate (CAN) has been widely utilized as a sacrificial oxidant in the exploration of WOR catalysts. In this study, advanced techniques, such as X-ray absorption spectroscopy (XAS), in situ Raman spectroscopy, and in situ electron paramagnetic resonance (EPR), to delve into the WOR facilitated by CAN and birnessite were employed. XANES analysis has demonstrated that the average oxidation states (AOSs) of Mn in birnessite, a birnessite/CAN mixture, and in the birnessite/CAN mixture postwater addition are 3.7, 3.8, and 3.9, respectively. In situ Raman spectroscopy performed in the presence of birnessite and CAN revealed a distinct peak at 784 cm-1, which is attributed to Mn(IV)=O. A shift of this peak to 769 cm-1 in H2 18O confirms its association with Mn(IV)=O. No change in this peak was observed in D2O, further supporting the notion that it is linked to Mn(IV)=O rather than Mn-OH (D). Furthermore, EPR spectroscopy shows the presence of Mn(IV). It is suggested that the WOR mechanism initiates with the oxidation of birnessite by CAN, which enhances the concentration of Mn(IV) sites in the birnessite structure. Under acidic conditions, birnessite, enriched in Mn(IV), facilitates oxygen evolution and subsequently transitions into a form with reduced Mn(IV) levels. This process highlights the critical function of the Mn (hydr)oxide structure, similar to its role in the water-oxidizing complex of Photosystem II, where it serves as charge storage for oxidizing equivalents from CAN, paving the way for a four-electron reaction that drives the WOR.

Large-scale solar-driven water splitting necessitates the advancement of practical and robust water-oxidation reaction (WOR) catalysts. The deposited NiFe (hydr)oxide on an indium tin oxide-coated polyethylene terephthalate (ITOP) substrate resulted in a transparent, flexible, stable electrode for WOR. The synthetic method used adsorbs minimal Ni/Fe on the ITOP surface, reducing costs and environmental impact. These trace amounts of ions are effective in WOR and maintain the ITOP's transparency, preserving its visual clarity. The electrode was applied for WOR during the electrolysis of the baking soda solution. The Tafel plot reveals a linear correlation between the logarithm of current density (log(j)) and overpotential, with a Tafel slope of 59.2 mV/decade. Notably, the onset of WOR, depicted in the Tafel plot, occurs at 1.60 V (overpotential: 370 mV).

Metal complexes are extensively explored as catalysts for oxidation reactions; molecular-based mechanisms are usually proposed for such reactions. However, the roles of the decomposition products of these materials in the catalytic process have yet to be considered for these reactions. Herein, the cyclohexene oxidation in the presence of manganese(III) 5,10,15,20-tetra(4-pyridyl)-21H,23H-porphine chloride tetrakis(methochloride) (1) in a heterogeneous system via loading the complex on an SBA-15 substrate is performed as a study case. A molecular-based mechanism is usually suggested for such a metal complex. Herein, 1 was selected and investigated under the oxidation reaction by iodosylbenzene or (diacetoxyiodo)benzene (PhI(OAc)2). In addition to 1, at least one of the decomposition products of 1 formed during the oxidation reaction could be considered a candidate to catalyze the reaction. First-principles calculations show that Mn dissolution is energetically feasible in the presence of iodosylbenzene and trace amounts of water.

Oxygen evolution reaction (OER) through water oxidation under acidic conditions catalyzed by first-row transition -metal-based compounds remains a challenge in artificial photosyn-thesis. A critical issue for OER under acidic conditions is the solubility of first-row transition-metal compounds during the reaction. This study investigates the silicate-stabilized Mn oxide for OER under acidic conditions (H2SO4 (0.10 M)). Compared to Mn oxide, the silicate-stabilized Mn oxide is significantly more stable under acidic conditions , with an overpotential of 457 mV for the onset of OER. The Mn oxides forming in the absence and presence of silicate groups are alpha-Mn2O3 and alpha-MnO2, respectively. For the Mn oxides forming in the presence of silicate groups, the corresponding current densities of 1 and 10 mA/cm2 are recorded at 527 and 640 mV, respectively. Silicate-stabilized Mn oxide was characterized by several methods before and after OER. The 29Si NMR spectrum for silicate-stabilized Mn oxide shows that the Si- O groups chemically bonded to Mn ions. The scanning transmission electron microscopy shows small 2-10 nm particles of Si-O compound in silicate-stabilized Mn oxide, especially Si-O, to stabilize the higher-indices facets of the Mn oxide crystallites. X-ray absorption spectroscopy confirms that the predominant structure for silicate-stabilized manganese oxide is alpha-MnO2, with di-and mono-mu-oxo-bridged Mn atoms. After prolonged oxygen evolution, a certain fraction of the mono-mu-oxo bridges disappears for silicate-stabilized manganese oxide. Adding silicate to Mn oxide is a low-cost and environmentally friendly procedure to increase the stability of Mn oxide toward OER under acidic conditions. Thus, our procedure is a clear improvement on current methods to stabilize Mn oxide for OER under acidic conditions.

NiMo-based materials have been identified as potential candidates of Pt/C electrocatalysts for hydrogen evolution reaction (HER) due to appropriate binding energy to hydrogen, and good resistance to corrosive environments. However, little work has been carried out to enhance the catalytic performance in large-scale water-alkali electrolysis. The NiMo amorphous coating, as a high-efficient and cost-effective catalyst toward HER, was synthesized by a facile electrodeposition strategy in this study. The effects of the pH value of electrolyte on the structure and HER activity of NiMo coating were investigated. The as-prepared NiMo(pH10) exhibited the highest HER activity with overpotentials of 63.9 and 157.1 mV (vs. RHE, with 80% potential drop due to electrical resistance (iR) compensation) at the current density of − 10 mA·cm−2 and − 100 mA·cm−2. This NiMo(pH10) coating also had excellent long-term durability of up to 100 h stable operation under the constant current density of − 100 mA·cm−2. The rapid HER kinetics and outstanding endurance can be ascribed to the NiMo compact coating with amorphous structures as well as good contact between NiMo coating and Ni foam substrate, endowing it grand feasibility in practical industrial applications.

There has been a growing interest in water oxidation in recent two decades. Along with that, remarkable discovery of formation of a mysterious catalyst layer upon application of an anodic potential of 1.13 V vs. standard hydrogen electrode (SHE) to an inert indium tin oxide electrode immersed in phosphate buffer containing Co(II) ions by Nocera et.al, has greatly attracted researchers interest. These researches have oriented in two directions; one focuses on obtaining better understanding of the reported mysterious catalyst layer, further modification, and improved performance, and the second approach is about designing coordination complexes of cobalt and investigating their properties toward the application in water splitting. Although there have been critical debates on true catalysts that are responsible for water oxidation in homogeneous systems of coordination complexes of cobalt, and the case is not totally closed, in this short review, our focus will be mainly on recent major progress and developments in the design and the application of cobalt oxide-based materials in catalytic, electrocatalytic, photocatalytic, and photoelectrocatalytic water oxidation reaction, which have been reported since pioneering report of Nocera in 2008 (Kanan Matthew and Nocera Daniel in Science 321:1072–1075, 2008).

The development of new procedures for synthesizing new, low-cost, and stable metal oxides for the oxygen-evolution reaction (OER) by water oxidation is critical. Since the 2000s, there has been a rapid rise in the use of first-row transition metal (hydr)oxides for the OER. However, there is still a need to design and synthesize an efficient and stable catalyst for the OER. The present paper aims to design and synthesize an OER catalyst based on "a look at nature" strategy. In a simple and chemical evolution-like experiment, for the first time, a solution composed of various perchlorate cations, namely, Li(I), Mg(II), Ca(II), Al(III), Mn(II), Fe(II), Co(II), Ni(II), Cu(II), Zn(II), Cd(II), Ba(II), and Na2SiO3, at a concentration of 1.0 mM at pH=4 and a potential of 1.40 V vs. NHE, which was under stirring for 15 days, was investigated. The stable OER catalyst on the electrode was characterized by X-ray absorption spectroscopy, scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. Based on the extrapolation of the Tafel plot, the onset of the OER in the presence of this catalyst was 130 mV lower than a bare electrode without it. This approach could be a roadmap to design and synthesize new and stable catalysts.

All studies on oxygen-evolution reaction by Mn oxides in the presence of cerium(IV) ammonium nitrate (CAN) have been so far carried out by synthesizing Mn oxides in the first step. And then, followed by the investigation of the Mn oxides in the presence of oxidants for oxygen-evolution reaction (OER). This paper presents a case study of a new and promising strategy for in situ catalyst synthesis by the adding Mn-II to either CAN or KMnO4/CAN solution, resulting in the formation of Mn-based catalysts for OER. The catalysts were characterized by scanning electron microscopy, energy-dispersive spectroscopy, transmission electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, X-ray absorption spectroscopy, and X-ray photoelectron spectroscopy. Both compounds contained nano-sized particles that catalyzed OER in the presence of CAN. The turnover frequencies for both catalysts were 0.02 (mmolO2 /mol(Mn) center dot s).

Alcohol to aldehyde conversion is a critical reaction in the industry. Herein, a new electrochemical method is introduced that converts 1 mmol of alcohols to aldehydes and ketones in the presence of N-hydroxyphthalimide (NHPI, 20 mol%) as a mediator; this conversion is achieved after 8.5 h at room temperature using a piece of Ni foam (1.0 cm2) and without adding an extra-base or a need for high temperature. Using this method, 10 mmol (1.08 g) of benzyl alcohol was also successfully oxidized to benzaldehyde (91%) without any by-products. This method was also used to oxidize other alcohols with high yield and selectivity. In the absence of a mediator, the surface of the nickel foam provided oxidation products at the lower yield. After the reaction was complete, nickel foam (anode) was characterized by a combination of scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), and spectroelectrochemistry, which pointed to the formation of nickel oxide on the surface of the electrode. On the other hand, using other electrodes such as Pt, Cu, Fe, and graphite resulted in a low yield for the alcohol to aldehyde conversion.

Co encapsulated in N-doped carbon nanotubes (Co-NCNT) catalysts are of high interest as bifunctional electrocatalyst material for both efficient oxygen evolution and reduction (OER/ORR) in applications of rechargeable metal-air batteries. Up to now, the role played by the functional metal species in OER/ORR is still insufficiently understood. The main focus of our research is to shed light on the mechanistic role of the Co species that serve as active sites in the bi-functional Co-NCNT catalysts. It is found that 5700 exhibits an outstanding OER/ORR activity. We thus hypothesize that Co-II and Co-III clusters predominately function as active sites in the OER and ORR processes, respectively. Furthermore, OER/ORR activity for Co-NCNT catalyst primarily correlates to e(g) occupation. A near-unity occupancy of the e(g) orbital of S700 is revealed to be the cause for the maximum intrinsic OER/ORR activity, which provides guidelines for the design of highly active catalysts. (C) 2020 Elsevier Inc. All rights reserved.

Water splitting is a promising reaction for storing sustainable but intermittent energies. In water splitting, water oxidation is a bottleneck, and thus different catalysts have been synthesized for water oxidation. Metal-organic frameworks (MOFs) are among the highly efficient catalysts for water oxidation, and so far, MOF-based catalysts have been divided into two categories: MOF-derived catalysts and direct MOF catalysts. In particular, a nickel/cobalt MOF is reported to be one of the best direct catalysts for water oxidation. For the first-row transition MOF structures in general, a hypothesis is that the harsh conditions of OER could cause the decomposition of organic ligands and the formation of water-oxidizing oxide-based structures. By electrochemical methods, scanning electron microscopy, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy, and X-ray absorption spectroscopy, a nickel/cobalt MOF known to be a highly efficient catalyst for water oxidation is shown to form Ni/Co oxide, making it a candidate catalyst for oxygen evolution. MOFs are interesting precatalysts for metal oxide water-oxidizing catalysts, but control experiments are necessary for determining whether a certain MOF or other MOFs are true catalysts for OER. Thus, finding a true and direct MOF electrocatalyst for OER is a challenge.

So far, all studies on oxygen-evolution reactions using Mn oxides in the presence of cerium(IV) ammonium nitrate (CAN) have been carried out by synthesizing Mn oxides in the first step, followed by the investigation of the Mn oxides in the presence of oxidants for the oxygen-evolution reaction (OER). This work presents a case study of a new and promising strategy for in situ catalyst synthesis by adding MnII to a CAN solution, resulting in the formation of Mn-based catalysts for OER. More information can be found in the Full Paper by M. Görlin, M. M. Najafpour, et al. on page 1330.

Herein, we report the synthesis, characterization, crystal structure, density functional theory calculations, and water-oxidizing activity of a pivalate Mn-Ca cluster. All of the manganese atoms in the cluster are Mn(iv) ions and have a distorted MnO6 octahedral geometry. Three Mn(iv) ions together with a Ca(ii) ion and four-oxido groups form a cubic Mn3CaO4 unit which is similar to the Mn3CaO4 cluster in the water-oxidizing complex of Photosystem II. Using scanning electron microscopy, transmission electron microscopy, energy dispersive spectrometry, extended X-ray absorption spectroscopy, chronoamperometry, and electrochemical methods, a conversion into nano-sized Mn-oxide is observed for the cluster in the water-oxidation reaction.

We analyze the stability of anodically electrodeposited cobalt hydroxide films during operation as electrocatalysts for water oxidation and show that the stability considerations of these films are a complex issue concerning multiple distinct phenomena. We combine the electrochemical analysis with the quantitative analysis of cobalt content using X-ray fluorescence spectroscopy, transient behavior analysis using in operando XANES electrochemical analysis, and structural analysis using cryogenic X-ray absorption spectroscopy to show that three distinct behavioral regimes exist. Each behavioral regime suffers from a degree of corrosion, but both self-healing and self-repair mechanisms are capable of partially stabilizing the cobalt content within the catalyst film. However, even if the absolute cobalt content of the film is fully stabilized, the catalytic behavior of the film degrades over time. This behavior is attributed to observed structural evolution of the material, where mid- and long-range structural order increases during operation. The increased structural order appears independent of whether self-healing or self-repair mechanisms are activated and results in fundamental changes in the catalytic behavior, observable as decreased turnover frequency and increased Tafel slopes. These results reveal that the cobalt content or electrocatalytic performance of the catalyst films can be stabilized by controlling the applied voltage or solution composition, but simultaneous stabilization of both is not currently possible.

Direct (photo)electrochemical production of non-fossil fuels from water and CO2 requires water-oxidation catalysis at near-neutral pH in the presence of appropriate anions that serve as proton acceptors. We investigate the largely enigmatic structural role of anions in water oxidation for the prominent cobalt-phosphate catalyst (CoCat), an amorphous and hydrated oxide material. Co3([(P/As)O]4)2·8H2O served, in conjunction with phosphate-arsenate exchange, as a synthetic model system. Its structural transformation was induced by prolonged operation at catalytic potentials and probed by X-ray absorption spectroscopy not only at the metal (Co), but for the first time also at the anion (As) K-edge. For initially isostructural microcrystals, anion exchange determined the amorphization process and final structure. Comparison to amorphous electrodeposited Co oxide revealed that in CoCat, the arsenate binds not only at oxide-layer edges, but also arsenic substitutes cobalt positions within the layered-oxide structure in an unusual AsO6 coordination. Our results show that in water oxidation catalysis at near-neutral pH, anion type and exchange dynamics correlate with the catalyst structure and redox properties.

Nickel-vanadium layered double hydroxide has recently been considered as a highly active, low-cost electrocatalyst and as a benchmark non-noble metal-based electrocatalyst for water oxidation. The material showed a current density of 27 mA/cm(2) at an overpotential of 350 mV, which is comparable to the best-performing nickel-iron-layered double hydroxides for water oxidation in alkaline media. The enhanced conductivity and facile electron transfer were suggested among important factors for the high activity of nickel-vanadium layered double hydroxide. In the present study, the stability of an Ni-V catalyst was investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), X-ray absorption near edge structure (XANES), extended X-ray absorption fine structure (EXAFS), and electrochemical characterization methods. These methods show that the initial Ni-V catalyst during water oxidation in alkaline conditions is converted from an initial alpha-Ni(OH)(2) phase to a partially oxidized alpha-Ni(OH)(2/)NiOOH phase and VO(4)(3-)ions. We carefully evaluate the stability of the catalysts and analyze the compositional changes during prolonged water-oxidation conditions using inductively coupled plasma-optical emission spectroscopy (ICP-OES). The experiments using both Fe-free electrolyte and Fe-free nickel-vanadium layered double hydroxide reveal that vanadium do not affect the water-oxidizing activity of alpha-Ni(OH)(2).