Oxygen Evolution Reaction Catalysts Research Articles

Unveiling active sites in FeOOH nanorods@NiOOH nanosheets heterojunction for superior OER and HER electrocatalysis in water splitting

The development of cost-effective, highly active, and stable electrocatalysts for water splitting to produce green hydrogen is crucial for advancing clean and sustainable energy technologies. Herein, we present an innovative in-situ synthesis of FeOOH nanorods@NiOOH nanosheets on nickel foam (FeOOH@NiOOH/NF) at an unprecedentedly low temperature, resulting in a highly efficient electrocatalyst for overall water splitting. The optimized FeOOH@NiOOH/NF sample, evaluated through time-dependent studies, exhibits exceptional oxygen evolution reaction (OER) performance with a low overpotential of 261 mV at a current density of 20 mA cm−2, alongside outstanding hydrogen evolution reaction (HER) activity with an overpotential of 150 mV at a current density of 10 mA cm−2, demonstrating excellent stability in alkaline solution. The water-splitting device featuring FeOOH@NiOOH/NF-2 electrodes achieves a voltage of 1.59 V at a current density of 10 mA cm−2, rivalling the state-of-the-art RuO2/NF||PtC/NF electrode system. Density functional theory (DFT) calculations unveil the efficient functionality of the Fe sites within the FeOOH@NiOOH heterojunction as the active OER catalyst, while the Ni centres are identified as the active HER sites. The enhanced performance of OER and HER is attributed to the tailored electronic structure at the heterojunction, modified magnetic moments of active sites, and increased electron density in the dx2-y2 orbital of Fe. This work provides critical insights into the rational design of advanced electrocatalysts for efficient water splitting.

Reducing the overpotential of overall water spitting by micro-pump-like electrode engineering

Electrolyzing water for hydrogen generation consumes a significant amount of electricity. Minimizing bubble accumulation on the electrodes can reduce the overpotential, thereby enhancing the efficiency of water electrolysis. In this work, Co1.8Mn1.2S4 as the catalyst for oxygen evolution reaction (OER) was optimized from 54 compounds of oxides and sulfides of Ni, Co, and Mn, then designed a three-dimensional electrode with a pump-like function to expel bubbles from the electrodes. Using laser-assisted curing 3D printing technology, a capillary array with side holes was fabricated and utilized as a support structure for the Co1.8Mn1.2S catalyst. During water electrolysis, the electrolyte enters the interior of the capillary through the side holes, and the bubbles inside the capillary are released from both ends of the capillary. The optimized electrode achieved 10 mA cm−2 at an overpotential of 345 mV during the OER, and reached 100 mA cm−2 at an overpotential of 435 mV. In-situ optical microscopy observations and fluid dynamics simulations demonstrated that bubbles were effectively released through the main outlet of the capillary. In overall water splitting, the current density reached 10 mA cm−2 at 1.646 V, without significant current decay after 60 h continuous operation. The electrode design presented holds promise for broader applications in catalytic reactions, such as CO2 reduction, electrochemical ammonia synthesis, and electrochemical treatment of water pollutants.

Self-supported FeCoNi(OHy)Ox oxy-hydroxide doped with Cr and Cu as robust low-loading catalyst for the alkaline oxygen evolution reaction

Self-supported FeCoNi(OHy)Ox oxy-hydroxides co-doped with Cr and Cu were obtained from a CrFeCoNiCu low ordered high entropy precursor synthesised by electrodeposition onto carbon cloth fibres (act-CrFeCoNiCu/CC). For comparison a dopant-free FeCoNi(OHy)Ox catalyst (act-FeCoNi/CC) was also synthesised following the same route. No heat treatment was performed to retain their low ordered structure. When assessed towards the oxygen evolution reaction (OER) in alkaline electrolyte, act-CrFeCoNiCu/CC and act-FeCoNi/CC catalyst reached a comparable current density of 10 mA cm−2 at ca. 1.6 VRHE (η10 of 380–390 mV). Nevertheless, Cr and Cu co-doping translated into improved stability. Evident degradation was observed during OER for act-FeCoNi/CC after 12 h operating at 100 mA cm−2, whereas no signs of degradation were detected for act-CrFeCoNiCu/CC under the same reaction conditions. The latter catalyst even delivered a constant potential for 160 h at 50 mA cm−2 aided to the stabilizing effect of Cr and Cu co-doping.

Enriched Oxygen Coverage Localized within Ir Atomic Grids for Enhanced Oxygen Evolution Electrocatalysis.

Inefficient active site utilization of oxygen evolution reaction (OER) catalysts have limited the energy efficiency of proton exchange membrane (PEM) water electrolysis. Here, an atomic grid structure is demonstrated composed of high-density Ir sites (≈10atoms per nm2) on reactive MnO2-x support which mediates oxygen coverage-enhanced OER process. Experimental characterizations verify the low-valent Mn species with decreased oxygen coordination in MnO2-x exert a pivotal impact in the enriched oxygen coverage on the surface during OER process, and the distributed Ir atomic grids, where highly electrophilic Ir─O(II-δ)- bonds proceed rapidly, render intense nucleophilic attack of oxygen radicals. Thereby, this metal-support cooperation achieves ultra-low overpotentials of 166mV at 10mA cm-2 and 283mV at 500mA cm-2, together with a striking mass activity which is 380 times higher than commercial IrO2 at 1.53V. Moreover, its high OER performance also markedly surpasses the commercial Ir black catalyst in PEM electrolyzers with long-term stability.

Oxygen evolution reaction of tellurium-incorporated nickel-iron layered double hydroxide microcrystals

Transition-metal-based layered double hydroxide (LDH) crystals have generated substantial interest as highly efficient oxygen evolution reaction (OER) catalysts because of their abundance, low cost, environmental friendliness, and favourable adsorption/desorption energies for intermittent reactants. However, the application of LDH crystals as high-performance electrocatalysts is frequently hindered by their sluggish electronic transport kinetics resulting from their intrinsically low conductivity. Herein, we report the effects of incorporating metalloids into transition metal LDH crystals on their electrocatalytic activity. In this study, tellurium (Te) incorporated NiFe (xTe-NiFe) LDH microcrystal, where x = 0.2, 0.4, 0.6 and 0.8, were grown on three-dimensional porous nickel foam using a facile solvothermal method. The electrocatalytic OER properties were analyzed using linear sweep voltammetry and electrochemical impedance spectroscopy. The amount of Te was found to play a crucial role in improving the catalytic activity of NiFe LDH. The optimum amount of Te (x) introduced into NiFe LDH was examined with respect to the OER performance.

Dissociative electrochemical analysis of materials degradation of an anion exchange membrane electrolyser

For anion exchange membrane (AEM) electrolysers, degradation studies provide valuable insights into how individual components degrade. Often, this presents a formidable challenge as each component significantly contributes to overall degradation. Here, we propose a methodology to individually compare the degradation of key components in AEM electrolysers: hydrogen evolution reaction (HER) catalyst, oxygen evolution reaction (OER) catalyst, and the AEM itself. First, to measure the degradation of catalyst-coated substrates (CCS) commonly used in AEM electrolysers, the AEM was replaced with a diaphragm (Zirfon Agfa UTP 500) permeable to OH− ions and stable in alkaline media. Subsequently, a cell with a stable catalytic layer (Nafion-coated Ni-felt substrates) was assembled to focus on membrane degradation with an AEM Sustainion X37-50 Grade 60. Chronopotentiometry and electrochemical impedance spectroscopy (EIS) revealed that the cell with the AEM experienced a threefold increase in ohmic resistance, attributed to the loss of functional groups. Additionally, an ion exchange capacity (IEC) analysis indicated that Sustainion lost approximately 20% of its ion exchange capacity. In contrast, the cell composed of CCS and Zirfon showed lower degradation. EIS data fitting demonstrated increased kinetic-related resistances while the ohmic resistance remained unchanged. This work presents a practical approach for comparing component degradation in AEM electrolysers.

Low Ru doping induced interface and defects engineering in 2D square micro-mesoporous CoNiRuOx nanosieves for advanced oxygen evolution electrocatalysis

Efficient oxygen evolution reaction (OER) catalysts require the reasonable integration of geometric architecture, defects construction and interfacial electronic structure, which is difficult to combine multiple advantages into one low-cost catalysts. Herein, we designed a novel low Ru doping 2D square CoNiRuOx nanosieves (NSs) with abundant surface micropore and mesopore structure, rich oxygen defects and heterophase interfaces. Owing to the Ru incorporation, the electrons in Ni2+ could partially spontaneously transfer to the Ru4+ species by the bridge O2− with π donation effect according to the proposed “Ni-O-Co-O-Ru-O-Ni” electron interaction model. Benefitting from the porous surface with rich mass transfer channel, increased oxygen defects concentration, well-optimized electron redistribution, the CoNiRuOx nanosieves possessed a low overpotential of 261 mV to reach the current density of 10 mA cm−2, which is better than that of counterpart CoNiOx NSs and commercial RuO2 catalysts. The CoNiRuOx NSs also possessed the favorable durability with 50 h. Moreover, the CoNiRuOx//Pt/C electrode couple exhibited enhanced overall water splitting performance. This work provides offers insightful significance to design 2D micro-mesoporous materials for the robust electrocatalysis processes related to energy conversion technologies.

Oxygen-Deficient Ruthenium Oxide for Selective Oxygen Evolution in Additive-Free Brine Electrolysis

Here, low-crystalline ruthenium oxide (S-RuO x ) with abundant oxygen vacancies was synthesized, after which its activity and selectivity toward oxygen evolution reaction (OER) in additive-free brine solution were compared with those of commercial ruthenium(IV) dioxide (C-RuO2), a benchmark catalyst for OER in an alkaline electrolyte. S-RuO x delivered a current density of 10 mA cm−2 at a significantly low overpotential (465 mV) in a 0.5 M NaCl solution without requiring an alkali. The estimated Faradaic efficiency toward chloride oxidation reaction (COR), FE(COR), was 2%, and exceptional OER was achieved without generating chlorine oxide species. This sharply contrasts the fact that C-RuO2 required an overpotential of 525 mV to generate 10 mA cm−2, where the FE(COR) was 59%. The activity and selectivity toward OER decreased after reducing the oxygen vacancies by sintering S-RuO x at different temperatures. S-RuO x continued to generate 10 mA cm−2 in 0.5 M NaCl solution for ≥60 h while maintaining the increasing potential at <30 mV. However, FE(COR) increased from a few percent for 20 h to 34% probably because of an irreversible decrease in vacancies. Notably, the addition of an alkali or a buffer could only enhance OER.

Dual modification of cobalt silicate nanobelts by Co3O4 nanoparticles and phosphorization boosting oxygen evolution reaction properties

Oxygen evolution reaction (OER) process is the “bottleneck” of water splitting, and the low-cost and high-efficient OER catalysts are of great importance and attractive but they are still challenging. Herein, a dual modification strategy is developed to tune and enrich the structure of cobalt silicate (Co2SiO4) showing boosted OER properties. Cobalt oxide (Co3O4) decorated Co2SiO4 nanobelts, denoted as CS, is firstly prepared using a Co-based precursor by a facile hydrothermal reaction. Then, cobalt phosphide (CoP) nanoparticles are in-situ grown on CS (denoted as CS-P) by the phosphorization process, which provide many active sites and boost the surface reactivity. The experimental results and density function theory (DFT) calculations both reveal that the CoP on CS can improve the conductivity and ensure fast kinetics, thus leading to boost the OER properties of Co2SiO4. When the phosphorization temperature is at 400 °C (CS-P400), it gains the lowest overpotential of 297 mV, which is much lower than CS (340 mV) and Co2SiO4 (409 mV) at 10 mA·cm−2, and even superior to the state-of-the-art transition metal silicates. CS-P400 also achieves high electrochemical active surface area (ECSA) and small Tafel slope owing to its porous structures with large specific surface area and nanosheet-like structures which are good for exposing many active sites and favorable to the fast kinetics. This work not only provides a dual modification route to boost catalytic activity of Co2SiO4 (CS-P400), but also sheds light on a new avenue for developing highly dispersed CoP on silicates to boost OER performances.

Amorphous/crystalline Ni-Fe based electrodes with rich oxygen vacancies enable highly active oxygen evolution in seawater electrolysis

To realize large-scale production of hydrogen through seawater electrolysis, it is highly crucial to engineer high-activity and robustly stable catalytic materials for oxygen evolution reaction (OER). Here, a facile etching growth strategy based on Ni foam (NF) is employed to fabricate an amorphous/crystalline Ni-Fe based electrode with rich oxygen vacancies as a promising OER electrocatalyst (a/c-NiFeOxHy@NF). Of note, the introduction of Fe induces the generation of plentiful Ni(Fe)OOH species, which can contribute to the remarkable OER behavior. Profiting from the favorable geometric microstructure of ultrathin nanosheets coupled with 3D open-pore architecture and regulated electronic state by increased oxygen vacancies and abundant crystalline-amorphous boundaries, the resulting a/c-NiFeOxHy@NF displays prominent electrocatalytic OER activity in pure alkaline solution and seawater, achieving impressive overpotentials of only 219 and 233 mV to reach 100 mA cm−2, respectively. More significantly, the electrode can keep stable operation without obvious attenuation for over 1200 h at 100 mA cm−2, demonstrating its exceptional corrosion resistance. Such robustness of this electrode surpasses those of almost all reported OER electrocatalysts. Furthermore, in a self-assembled seawater electrolyzer with a/c-NiFeOxHy@NF as the anode and l-RuP@NF as the cathode, a large current density of 500 mA cm−2 is easily achieved at the voltage of 1.795 V at 65 °C. The work offers a novel paradigm for constructing low-cost, high-efficiency, and ultra-stable OER catalysts, which shows huge promise for industrial seawater electrolysis applications.

TiO2 nanotubes as enhanced electrocatalytic oxygen evolution reaction catalyst for water splitting in alkaline medium

The literature largely reports titania nanotubes (NTs), a standout component in nanomaterials with exceptional charge transport and carrier lifetime properties. We talked about how to make highly ordered, vertically oriented titanium dioxide (TiO2) nanotube arrays in a one-step potentiostatic anodization of titanium in an ethylene glycol (EG) electrolyte that also has water and sodium fluoride. This study analyzed the effect of anodization voltage on the formation of TiO2 nanotubes. We used field emission scanning electron microscopy (FESEM) and X-ray diffractometer (XRD) to characterize the morphology and structure of TiO2 NTs. Optical studies using UV-Vis diffuse reflectance and photoluminescence spectra also showed that changing the anodization voltage changes the band gap and the way electron-hole pairs recombine. The synthesis of uniform nanotube arrays and their crystal structure led us to the conclusion that 30 V was an ideal anodization potential exclusively for NT formation. The electrocatalytic activity increases as the sample is anodized at 50 V. Due to the crystal defect, this exhibits better oxygen evolution reaction (OER) activity in 1.0 M KOH electrolyte at 1.736 V vs. reversible hydrogen electrode (RHE) at a current density of less than 10 mA cm-2.

Study on Titanate perovskites with different morphologies for oxygen-evolution reaction catalysts

Metal exsolution from perovskite oxides has been used to produce well-designed electro-catalysts where metal nanoparticles are homogeneously distributed on the surface of perovskites. Our previous works revealed that the metal exsolution concept is effective not only for high-temperature system such as solid oxide fuel/electrolysis cells but also for alkaline water electrolysis at near-ambient temperatures. In this study, we prepare cuboidal and spherical Ni-doped CaTiO3 to investigate the effects of the different perovskite morphologies on Ni exsolution and catalytic activity for OER. Ni ions seem to be incorporated into the A-site of CaTiO3 with structural OH ions via solvothermal route, and the stoichiometry of spheres and cubes were confirmed as (Ca0.82Ni0.04)Ti1O2.63(OH)0.46 and (Ca0.90Ni0.04)Ti1O2.88(OH)0.12, respectively. The spheres show random distribution of Ni nanoparticles along grain boundaries, whereas Ni nanoparticles are ex-solved along the edges in the cubes. With the small amount of Ni contents (∼4 mol %), geometric current density of each cube and sphere exhibits comparable OER activities of ∼10 mA cm−2 and ∼7 mA cm−2 at 1.7 V to previously reported catalysts such as transition metals or perovskites. We suppose from Tafel slope that different A-site and oxygen stoichiometry of the CaTiO3 may affect deprotonation step during OER in terms of proton conduction.

Tailoring Ni-Fe-B Electronic Effects in Layered Double Hydroxides for Enhanced Oxygen Evolution Activity.

NiFe layered double hydroxides (LDHs) are state-of-the-art catalysts for the oxygen evolution reaction (OER) in alkaline media, yet they still face significant overpotentials. Here, quantitative boron (B) doping is introduced in NiFe LDHs (ranging from 0% to 20.3%) to effectively tailor the Ni-Fe-B electronic interactions for enhanced OER performance. The co-hydrolysis synthesis approach synchronizes the hydrolysis rates of Ni and Fe precursors with the formation rate of B─O─M (M: Ni, Fe) bonds, ensuring precise B doping into the NiFe LDHs. It is demonstrated that B, as an electron-deficient element, acts as an "electron sink" at doping levels from 0% to 13.5%, facilitating the transition of Ni2+ to the active Ni3+δ, thereby accelerating OER kinetics. However, excessive B doping (13.5-20.3%) effectively generates oxygen vacancies in the LDHs, which increases electron density at Ni2+ sites and hinders their transition to Ni3+δ, thereby reducing OER activity. Optimal OER performance is achieved at a B doping level of 13.5%, with an overpotential of only 208mV to reach a current density of 500mA cm-2, placing it among the most effective OER catalysts to date. This Ni-Fe-B electronic engineering opens new avenues for developing highly efficient anode catalysts for water-splitting hydrogen production.

Harnessing dysprosium telluride/GPE as an effective electrode for energy storage and conversion

Hydrogen stands out as a clean alternative to fossil fuels, and its production via water electrolysis is promising, although the slow kinetics of the oxygen evolution reaction (OER) pose a significant challenge. Alongside energy conversion, the quest for effective energy storage remains critical. This study explores the potential of dysprosium telluride (Dy2Te3) as a highly efficient catalyst for OER and a supercapacitor electrode. The hydrothermally synthesized Dy2Te3 exhibits exceptional OER activity, characterized by an overpotential of 294 mV at 1.48 V vs. RHE, a low Tafel slope of 48 mV dec−1, and minimal interfacial resistance of 13 Ω. In a 1.0 M KOH solution, Dy2Te3 nanoparticles demonstrate outstanding supercapacitor performance, achieving a power density of 283.5 W kg−1, specific capacitance of 645.33 F g−1, and energy density of 22.8 Wh kg−1 at 1 A g−1. The high performance is attributed to enhanced electrical conductivity and a low impedance of 0.352 Ω in a three-electrode system. In a practical two-electrode configuration, Dy2Te3 nanoparticles deliver a specific capacitance of 479 F g−1, energy density of 94 Wh kg−1, and power density of 0.32 kW kg−1, with a reduced interfacial resistance of 1.03 Ω. These results underscore the potential of Dy2Te3 as a viable electrode material for both supercapacitors and water splitting, offering valuable insights into the application of lanthanide-based metal chalcogenides in energy technologies.

Fe-S dually modulated adsorbate evolution and lattice oxygen compatible mechanism for water oxidation.

Simultaneously activating metal and lattice oxygen sites to construct a compatible multi-mechanism catalysis is expected for the oxygen evolution reaction (OER) by providing highly available active sites and mediate catalytic activity/stability, but significant challenges remain. Herein, Fe and S dually modulated NiFe oxyhydroxide (R-NiFeOOH@SO4) is conceived by complete reconstruction of NiMoO4·xH2O@Fe,S during OER, and achieves compatible adsorbate evolution mechanism and lattice oxygen oxidation mechanism with simultaneously optimized metal/oxygen sites, as substantiated by in situ spectroscopy/mass spectrometry and chemical probe. Further theoretical analyses reveal that Fe promotes the OER kinetics under adsorbate evolution mechanism, while S excites the lattice oxygen activity under lattice oxygen oxidation mechanism, featuring upshifted O 2p band centers, enlarged d-d Coulomb interaction, weakened metal-oxygen bond and optimized intermediate adsorption free energy. Benefiting from the compatible multi-mechanism, R-NiFeOOH@SO4 only requires overpotentials of 251 ± 5/291 ± 1 mV to drive current densities of 100/500 mA cm-2 in alkaline media, with robust stability for over 300 h. This work provides insights in understanding the OER mechanism to better design high-performance OER catalysts.

Green mediated sol-gel synthesis of MxCu1-xO (M = La,Ce x = 0.02–0.06) as an efficient catalyst for electrocatalytic oxygen evolution reaction

The imperative for advancing contemporary human society lies in the essential requirement to create energy generation systems that are sustainable, environmentally friendly, and exceptionally efficient. The development of efficient electrocatalysts, that are environmentally benign, economically viable and easily available, for alkaline oxygen evolution reaction is a significant research area in this regard. The present study focuses on a green mediated sol–gel auto-combustion process for the synthesis of lanthanum and cerium doped copper oxide nanoparticles with excellent electrocatalytic activity. Citric acid and polyols enriched lemon juice serves as the medium for the synthesis of various metal doped copper oxide nanoparticles. The extensive electrochemical studies using these catalysts reveals that, among the various doped samples, 5% lanthanum and 5% cerium doped catalysts exhibited the best performance in oxygen evolution reaction. Doping was found to enhance electrical conductivity and create oxygen vacancies, which enhanced the electrocatalytic activity of the catalysts. Specifically, the 5% lanthanum-doped copper oxide showed an overpotential of 340 mV, while the cerium-doped material exhibited an overpotential of 337 mV at a current density of 10 mA cm−2 in 1 M KOH. Moreover, the lanthanum-doped sample displayed a Tafel slope of 49 mV dec−1, whereas the cerium-doped sample had a Tafel slope of 60 mV dec−1 with better durability. Additionally, the chronopotentiometric studies revels the stability and long-term performance of LCO5 and CCO5, maintaining consistent potential of 1.58 V throughout 5 h at a current density of 10 mA cm−2. This study is conclusive evidence that the incorporation of rare earth elements improves the conductivity and overpotential of pristine copper oxide nanoparticles, making them highly effective in the oxygen evolution reactions.

Design and Optimization of Nanoporous Materials as Catalysts for Oxygen Evolution Reaction—A Review

With the growing demand for new energy sources, electrochemical water splitting for hydrogen production is a technology that must be vigorously promoted. Therefore, to improve the efficiency of the oxygen evolution reaction (OER) at the anode, high-performance OER catalysts are essential. Given their advantages in electrocatalysis, nanoporous materials have garnered considerable attention in previous studies for OER applications. This review provides a comprehensive overview of various strategies to optimize active site utilization in nanoporous materials. These strategies include regulating pore size and porosity, constructing hierarchical nanoporous structures, and enhancing material conductivity. Additionally, it covers approaches to boost the intrinsic OER activity of nanoporous materials, such as tuning the composition of anions and cations, creating vacancies, constructing interfaces, and forming boundary active sites. While nanoporous materials offer significant potential for advancing OER, challenges remain, including difficulties in quantifying activity within nanopores, the unclear impact of nanoporous material morphology, challenges in accessing nanopore interiors with in situ techniques, and a lack of theoretical calculations on pore structure. However, these challenges also present opportunities, and we hope this review provides a fresh perspective to inspire future research.

Atomic Insights into the Competitive Edge of Nanosheets Splitting Water.

The oxygen evolution reaction (OER) provides the protons for many electrocatalytic power-to-X processes, such as the production of green hydrogen from water or methanol from CO2. Iridium oxohydroxides (IOHs) are outstanding catalysts for this reaction because they strike a unique balance between activity and stability in acidic electrolytes. Within IOHs, this balance varies with the atomic structure. While amorphous IOHs perform best, they are least stable. The opposite is true for their crystalline counterparts. These rules-of-thumb are used to reduce the loading of scarce IOH catalysts and retain the performance. However, it is not fully understood how activity and stability are related at the atomic level, hampering rational design. Herein, we provide simple design rules (Figure 12) derived from the literature and various IOHs within this study. We chose crystalline IrOOH nanosheets as our lead material because they provide excellent catalyst utilization and a predictable structure. We found that IrOOH signals the chemical stability of crystalline IOHs while surpassing the activity of amorphous IOHs. Their dense bonding network of pyramidal trivalent oxygens (μ3Δ-O) provides structural integrity, while allowing reversible reduction to an electronically gapped state that diminishes the destructive effect of reductive potentials. The reactivity originates from coordinative unsaturated edge sites with radical character, i.e., μ1-O oxyls. By comparing to other IOHs and literature, we generalized our findings and synthesized a set of simple rules that allow prediction of stability and reactivity of IOHs from atomistic models. We hope that these rules will inspire atomic design strategies for future OER catalysts.

A Proton-Conductive Co(II)-Polyoxometalate Acts as a Precatalyst for Efficient Electrocatalytic Water Oxidation.

A cobalt(II)-containing polyoxometalate, [H3O]5[{Co(H2O)4}3{Na(H2O)4}W12O42]·3H2O (Co-POM), has been isolated in a one-step facile aqueous synthesis and characterized unambiguously using single-crystal X-ray crystallography along with routine spectral analysis. The paratungstate cluster anion [W12O42]12- coordinates with {CoII(H2O)4}2+ and {Na(H2O)4}+ complex cations resulting in the formation of the water-insoluble Co-POM compound having three-dimensional (3-D) extended structure. Motivated by the protonated water molecules existing as the counter cations in Co-POM, herein, we demonstrate the detailed proton conductivity studies of the Co-POM, reaching a value of 1.04 × 10-2 S cm-1 at 80 °C and 98% relative humidity (RH). The temperature- and humidity-dependent proton conductivity in Co-POM is governed by Grotthus mechanism with Ea = 0.25 eV. In addition, we examined the electrochemical behavior of Co-POM, in an alkaline borate buffer where it is found to be electrochemically unstable and acts as a precatalyst (and not a true catalyst) for oxygen evolution reaction (OER). We also discuss the "post-mortem" analysis of the postelectrolysis sample to identify the active species which turns out to be a cobalt oxide material (Co3O4) incorporating small amounts of tungsten. Thus, in the present electrocatalysis work, the Co-POM molecule transforms into an efficient water oxidation catalyst (WOC).

Insights into the structural, dielectric, optical and electrochemical characteristics reveal the limiting parameters toward efficient OER catalysts in the perovskite solid solution Sr1-xLaxTi1-xFexO3 (0.2 ≤ x ≤ 0.8)

Due to their structural stability and flexibility, perovskite oxides have paved their way as oxygen evolution reaction (OER) electrocatalysts. The in-depth understanding of the limiting factors toward good activity and fast charge transfer are still considered as major challenges for highly active OER perovskite catalysts. In this work, we disclose the changes in the perovskite structure and properties that influence the activity as we introduce new criteria to be considered for perovskites electrocatalysts design. The activity test of the series Sr1-xLaxTi1-xFexO3 (0.2 ≤ x ≤ 0.8) reveals the overpotential of 300 mV @ 10 mA.cm−1, and Tafel slop of 57.6 mV dec−1, as the lowest values corresponding to the composition x = 0.4. As function of the composition, the dielectric constant is found to decrease from 3000 (x = 0.2) to 953 (x = 0.4), and increases up to 5497 (x = 0.8). This variation of the dielectric constant is found to influence the activity of the catalysts where lowest overpotential of 300 mV @ 10 mA.cm−1 is found for x = 0.4 with the smallest dielectric constant. The phase transition detected in the perovskite series upon substitution has induced a distortion in the unit cell, which is found to be in alignment with the evolution of the overpotential in the perovskites series. The distortion has induced a preferable octahedra orientation for efficient OER reaction, this orientation correspond to the interatomic distance <Ti/Fe-O2> (1) = 1.89 Å and <Ti/Fe-O2> (2) = 2.04 Å (x=0.4) with the condition of <Ti/Fe-O2> (1) remains shorter than <Ti/Fe-O2> (2). The band gap, thus the conduction and valence bands edges, is found to decrease with the increase of the interatomic distance (Fe/Ti-O) and with the deviation of the Bond angle (B)-O2-(B) from the ideal value of 180°. These findings put light on the limiting factors of the perovskites activity by linking the structural parameters and the physical properties with the objective of accurately designing highly-efficient perovskite OER electrocatalysts.