Oxygen Evolution Reaction Onset Research Articles

Three dimensional NiS2–Ni(OH)2/CNT nanostructured assembly for supercapacitor and oxygen evolution reaction

In this work, three-dimensional hetero-nanostructures comprising nickel sulfide/nickel hydroxide and multiwalled carbon nanotubes have been synthesized using hydrothermal method for energy conversion and storage application. As-prepared NiS2–Ni(OH)2/CNT composite shows excellent storage properties with a specific capacitance of 1250 F/g with pseudocapacitive behavior. Interestingly, NiS2–Ni(OH)2/10% CNT electrode exhibited better cycling stability with 80% retention of specific capacitance after 3000 cycles at a current density of 5 A/g. Further, when employed as oxygen evolution reaction (OER) catalyst, NiS2–Ni(OH)2/10%CNT composite electrode exhibits an OER onset potential of 1.45 V vs reversible hydrogen electrode (RHE) and it can catalyze OER efficiently with the overpotential of 290 mV to generate 10 mA/cm2 current density (with a lower Tafel slope 110 mV/decade) in an alkaline solution. The stability studies also confirm an excellent performance of NiS2–Ni(OH)2/CNT even after 2000 cycles without obvious degradation, due to the formation of three-dimensional network of layered sulphide and one-dimensional CNTs that plays a main role in the energy conversion and storage.

Carbon materials as additives to the OER catalysts: RRDE study of carbon corrosion at high anodic potentials

Carbon materials are widely applied as conductive additives in studies of the oxygen evolution reaction (OER) in alkaline media catalyzed by transition metal oxides. In this work we investigate the anodic behavior of three representative carbon materials: furnace black (Vulcan XC-72R), acetylene black, and pyrolytic carbon of the Sibunit family (Sibunit-152), in 1 M NaOH at high potentials (ranging from the OER onset and up to ca. 2 V vs. RHE) in the time span from several minutes to several hours. We apply the rotating ring-disk electrode (RRDE) to separate the OER current from the carbon corrosion current. We then use transmission electron microscopy (TEM) to visualize changes in the carbon morphology resulting from corrosion. Finally, we study the OER performance of composite electrodes comprising carbon materials mixed with a La0.5Sr0.5Mn0.5Co0.5O3−δ perovskite OER catalyst, and discuss possible influence of the oxide on the carbon corrosion.

Investigating the oxygen evolution reaction on Ir(111) electrode in acidic medium using conventional and dynamic electrochemical impedance spectroscopy

In this study, conventional and dynamic electrochemical impedance spectroscopy measurements (EIS and DEIS, respectively) were performed on Ir(111) single crystal electrode and Ir nanoparticles supported onto high-surface area carbon in acidic electrolyte and different potential ranges. We found that the EIS and DEIS results compare fairly well below the onset of oxygen evolution reaction (OER). However, in the OER potential region, some differences were observed between the information derived from these measurements. Cyclic voltammetry and inductively-coupled plasma mass spectrometry experiments revealed that the long time required by EIS cause chemical and structural changes of the Ir(111) surface and oxygen evolution, thus adversely affecting the reliability of the impedance measurement. On the contrary, the short time required for DEIS measurements help mitigating these negative effects, thus highlighting the benefits of such technique to investigate the early stages of surface oxide formation on Ir single crystals.

Metal Silicide Nanosphere Decorated Carbon‐Rich Polymer‐Derived Ceramics: Bifunctional Electrocatalysts towards Oxygen and their Application in Anion Exchange Membrane Fuel Cells

AbstractThe current hindrance in the commercialization of electrochemical devices such as fuel cells and metal–air batteries is the high cost and poor stability of widely employed noble‐metal‐based electrocatalysts. In this report, a new intermetallic nickel/cobalt silicide‐decorated carbon‐rich silica‐based ceramic composites (Co/SiOC or Ni/SiOC) were successfully synthesized as alternative bifunctional electrocatalysts. The oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) kinetics of the prepared composite materials were evaluated by electrochemical characterization. Co/SiOC exhibited more positive ORR kinetics with the onset potential of 0.87 V vs. RHE compared to that of Ni/SiOC, which is slightly lower compared to Pt/C. Conversely, Ni/SiOC exhibits a 50 mV more positive OER onset potential compared to Co/SiOC with almost similar OER kinetics to RuO2. The bifunctional ability concluded that Ni/SiOC was a good bifunctional catalyst with an oxygen electrode potential 0.89 V. As Co/SiOC performs as a better ORR catalyst, it was utilized as a cathode catalyst for the construction of anion exchange membrane fuel cell and its fuel cell performance was evaluated. The Co/SiOC composites produce a peak power density of 54 mW/cm2, representing a satisfactory result when compared with Pt/C. The results reveal that the intermetallic nickel/cobalt silicide nanosphere decorated ceramic materials can have a promising future as efficient alternative electrocatalysts in electrochemical devices.

Protonated supramolecular complex-induced porous graphitic carbon nitride nanosheets as bifunctional catalyst for water oxidation and organic pollutant degradation

Polymeric graphitic carbon nitride (g-C3N4) materials are promising catalysts to compete with precious metals, but they mostly only show moderate catalytic activity, particularly low oxidation power for oxygen evolution reaction (OER). Although activating strategies, such as nanostructure designing and band structure engineering, have led to remarkable activity gains, there remains considerable room to improve performance that aims for real applications. We report here a novel strategy—protonating supramolecular aggregates followed by thermal polycondensation—to prepare porous g-C3N4 nanosheets (SH-g-C3N4) with high oxidation power, which employ as bifunctional OER catalyst and highly efficient photocatalyst material. The OER onset potentials of 1.47 V and Tafel slope of 128 mV dec−1 over SH-g-C3N4 are superior to the g-C3N4 obtained from supramolecular aggregates without protonation (S-g-C3N4). Furthermore, its Tafel slope would be reduced to 63.6 mV dec−1 under visible light irradiation, which is comparable to that of the active commercial RuO2 and IrO2 catalysts. Interestingly, the achieved nanosheets display enhanced visible light photocatalytic activity. The striking catalysis performance of SH-g-C3N4 can be ascribed to the unique advantage of its highly porous structure and favorable electronic structure and the generation of lots of ·O 2 − and ·OH radicals promoting the kinetic process. Our theoretical calculations reveal the mechanism of photoelectric catalyst for oxygen evolution reaction. This work provides a facile and scalable method to design highly efficient polymeric catalysts.

NiFe Hydroxide Lattice Tensile Strain: Enhancement of Adsorption of Oxygenated Intermediates for Efficient Water Oxidation Catalysis

AbstractThe binding strength of reactive intermediates with catalytically active sites plays a crucial role in governing catalytic performance of electrocatalysts. NiFe hydroxide offers efficient oxygen evolution reaction (OER) catalysis in alkaline electrolyte, however weak binding of oxygenated intermediates on NiFe hydroxide still badly limits its catalytic activity. Now, a facile ball‐milling method was developed to enhance binding strength of NiFe hydroxide to oxygenated intermediates via generating tensile strain, which reduced the anti‐bonding filling states in the d orbital and thus facilitated oxygenated intermediates adsorption. The NiFe hydroxide with tensile strain increasing after ball‐milling exhibits an OER onset potential as low as 1.44 V (vs. reversible hydrogen electrode) and requires only a 270 mV overpotential to reach a water oxidation current density of 10 mA cm−2.

NiFe Hydroxide Lattice Tensile Strain: Enhancement of Adsorption of Oxygenated Intermediates for Efficient Water Oxidation Catalysis.

The binding strength of reactive intermediates with catalytically active sites plays a crucial role in governing catalytic performance of electrocatalysts. NiFe hydroxide offers efficient oxygen evolution reaction (OER) catalysis in alkaline electrolyte, however weak binding of oxygenated intermediates on NiFe hydroxide still badly limits its catalytic activity. Now, a facile ball-milling method was developed to enhance binding strength of NiFe hydroxide to oxygenated intermediates via generating tensile strain, which reduced the anti-bonding filling states in the d orbital and thus facilitated oxygenated intermediates adsorption. The NiFe hydroxide with tensile strain increasing after ball-milling exhibits an OER onset potential as low as 1.44 V (vs. reversible hydrogen electrode) and requires only a 270 mV overpotential to reach a water oxidation current density of 10 mA cm-2 .

An Operando-Raman study on oxygen evolution of manganese oxides: Roles of phase composition and amorphization

Manganese oxide catalysts exist in various forms of polymorphs, which have direct influence on their oxygen evolution reaction (OER) performance. However, their structure flexibility under working conditions remains a great challenge for identifying their active structures. In this work, OER performances of manganese oxide electrocatalysts with representative phase compositions (birnessite, bixbyite) were examined using operando-Raman spectroscopy. The results indicate that catalysts containing birnessite composition do not exhibit apparent OER activity before an charging activation process. Shifting of Raman peaks around ∼640 cm−1 can serve as an indicator for predicting their OER onset potentials. The amorphization of mixed-phase catalyst is in favor of this activation and reduces the OER onset potential by 120 mV. Moreover, the coexistence of bixbyite-like Mn(III) center and amorphized birnessite composition combines their abilities of catalyzing both electrochemical and chemical steps, creating a more bias-efficient reaction channel. This leads to the lowest Tafel slope and improved activity in the high overpotential region, suggesting that amorphous OER catalysts with structural flexibility can facilitate the OER kinetics.

Electrochemical and Electrocatalytic Activity in Two Phosphate-Based Systems: Pyrophosphate and Metaphosphate for Sodium-Based Batteries

Keywords: Energy storage devices, polyanionic materials, pyrophosphate, metaphosphate, cathodes, electrocatalysts. Present world needs to harvest energy in the form of electricity to power myriads of consumer electronic devices to hybrid vehicles. Electrical energy can be efficiently stored in electrochemical storage devices such as supercapacitors, batteries, and fuel cells. Among them, rechargeable (metal-ion/ metal-air) batteries form the most promising and commercially viable solution. Owing to the light weight and high energy density, Li-ion battery has seen unprecedented commercial success. For next generation Li-ion batteries, various phosphate-based polyanionic materials (e.g. LiFePO4) have been proposed as robust and safe cathode materials [1]. In addition to efficient intercalation activity, various phosphate-based compounds have been reported to work as efficient bi-functional catalysts for water splitting showing good oxygen evolution reaction (OER) with appreciable oxygen reduction reaction (ORR) activity [2]. So, these phosphate compounds can be used to develop metal-air batteries. Off-late, sodium-ion batteries (SIBs) are being extensively explored as economic alternatives to Li-ion batteries. Here, the electrochemical and electrocatalytic performance of two distinct class of PO4-based compounds, pyrophosphates [Na2MP2O7] and metaphosphates [NaM(PO3)3], have been explored to develop Na-ion and Na-air batteries (Figure 1). They can be easily synthesized by solution combustion synthesis as well as conventional solid-state synthesis involving moderate annealing at 600 ºC for 6 h. Pyrophosphates: Among pyrophosphates, Na2FeP2O7 has been reported as a low cost cathode with promising electrochemical performance and thermal stability. Binary Na2(Fe1-yMny)P2O7 (0 ≤ y ≤ 1) pyrophosphate family has been extensively studied as well as the best composition with highest stability has been investigated. We have investigated the binary Na2(Fe1-xZnx)P2O7 (0 ≤ x ≤ 1) pyrophosphate family, where Na2FeP2O7 (P-1, #2) and Na2ZnP2O7 (P42/n, #86) end members are anisostructural. We will report the structural and electrochemical characterization of novel Na2(Fe1-xZnx)P2O7 (0 ≤ x ≤ 1) binary pyrophosphates. Further, we will describe the Na-insertion mechanism in Na2(Fe1-xZnx)P2O7 (x = 0, 0.25) electrodes obtained by a small-amplitude (incremental) techniques such as galvanostatic intermittent titration (GITT) and potentiostatic intermittent titration (PITT). Pursuing pyrophosphate chemistry, the superior bifunctional electrocatalytic activity of Na2CoP2O7 will be demonstrated [3]. Na2CoP2O7 form a potential catalyst to implement in Na-air batteries. Metaphosphates: Metaphosphates have been recently reported by our group as electrochemically active compounds for reversible Na+ (de)intercalation. We will showcase (i) a 2.8 V Fe3+/Fe2+ redox activity in orthorhombic Na(Fe1-xMnx)(PO3)3 (x = 0-1) binary solid-solutions developed by combustion synthesis [4] and (ii) a 3.2 V Co3+/Co2+ redox activity in cubic polymorph of NaCo(PO3)3 [5]. However, their high molecular weight leads to limited reversible capacity. Nonetheless, the presence of metal redox centre can be exploited to harness electrocatalytic performance. We will demonstrate the electrocatalytic properties (e.g. oxygen evolution reaction, OER) of the Fe, Mn, and Co analogues of metaphosphate [NaM(PO3)3] family for Na-air batteries application. OER onset potential of NaCo(PO3)3 system was found to be 1.45 V, which is close to the conventional RuO2 electrode [6]. Synergising experiments with computational modelling, we will showcase the electrochemical and electrocatalytic activity of NaFe(PO3)3 and NaCo(PO3)3 for the first time. [1] A. K. Padhi, K. S. Nanjundaswamy, and J. B. Goodenough, J. Electrochem. Soc., 144 (1997), 1188. [2] H. Kim et al, and K. Kang, Nat. Commun., 6 (2015), 8253. [3] R. Gond et al, and P. Barpanda, ChemElectroChem., 5 (2018), 153. [4] R. Gond et al, and P. Barpanda, Inorg. Chem., 56 (2017), 5918. [5] R. Gond et al, and P. Barpanda, Inorg. Chem., (2018), in press. [6] R. Gond et al, and P. Barpanda, manuscript submitted. Figure 1

Cobalt Covalent Doping in MoS2 to Induce Bifunctionality of Overall Water Splitting.

The layer-structured MoS2 is a typical hydrogen evolution reaction (HER) electrocatalyst but it possesses poor activity for the oxygen evolution reaction (OER). In this work, a cobalt covalent doping approach capable of inducing HER and OER bifunctionality into MoS2 for efficient overall water splitting is reported. The results demonstrate that covalently doping cobalt into MoS2 can lead to dramatically enhanced HER activity while simultaneously inducing remarkable OER activity. The catalyst with optimal cobalt doping density can readily achieve HER and OER onset potentials of -0.02 and 1.45 V (vs reversible hydrogen electrode (RHE)) in 1.0 m KOH. Importantly, it can deliver high current densities of 10, 100, and 200 mA cm-2 at low HER and OER overpotentials of 48, 132, 165 mV and 260, 350, 390 mV, respectively. The reported catalyst activation approach can be adapted for bifunctionalization of other transition metal dichalcogenides.

Carbon-Free Perovskite Oxide Oxygen Evolution Reaction Catalysts for AEM Electrolyzer

In alkaline electrolysis of water, kinetic limitations account for up to 85% of the total efficiency losses [1]. In particular, the efficiency of water electrolysis is largely limited by the high overpotential of the oxygen evolution reaction (OER). Therefore, reducing the reaction overpotential at the OER catalysts is critical for the overall efficiency improvement of alkaline water electrolysis. Perovskite oxides are promising non-precious metal OER catalysts showing high activity in oxygen evolution in alkaline electrolytes, comparable to that of the state-of-the-art IrO2 catalysts [2], but their stability is in need of improvement. As these oxides exhibit relatively low electronic conductivity carbon, typically acetylene black (AB), is added to enhance their conductivity. However, the thermodynamic potential of carbon oxidation (C + 4OH- → CO2 + 2H2O + 4e-) is 1.03 V vs. RHE at pH 14, much below the OER onset potential. Carbon oxidation causes an increase in the electrode resistance and weakens the carbon-catalyst interaction, eventually limiting the electrolyzer lifespan. Consequently, improving the stability and electronic conductivity of the perovskite-oxide OER catalysts with further enhancement of the OER activity is critical to the application of perovskites as OER catalysts for anion exchnange membrane (AEM) electrolyzer. In this work, perovskite (La1-xSrx)CoyFe1-yO3-δ (LSCF) was chosen to study the OER activity, stability, and electronic conductivity. In typical ABO3 perovskites, the valance state of cations, i.e., the sum of A and B, should be +6 to balance the overall oxygen valence of -6. The non-stoichiometry caused by the change in the ratio of La/Sr and Co/Fe in the LSCF will affect the OER activity, stability and the electronic conductivity. The effect of this non-stoichiometry of LSCF on the OER performance in alkaline electrochemical cells and AEM electrolyzer, in conjunction with DFT calculations, will be presented in this talk. Acknowledgements This work was supported by the U.S. Department of Energy, Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office under the auspices of the HydroGEN Consortium.

Substituent‐Induced Deformed Ni–Porphyrin as an Electrocatalyst for the Electrochemical Conversion of Water into Dioxygen

The electrochemical water oxidation ability of complexes 5,15‐diphenylporphinatonickel(II) (A) and 5,15‐bis(3,5‐di‐tert‐butylphenyl)porphinatonickel(II) (B) in the oxygen evolution reaction (OER) have been investigated in an alkaline medium. Complex B was found to be kinetically and thermodynamically more active than complex A. The overpotential and Tafel slope of complex B are lower than those of complex A by 30 mV and around 45 mV/decade, respectively, which supports the higher activity of B. Moreover, stability tests endorse the sustainability of both samples under alkaline conditions. It has been found that the 3,5‐di‐tert‐butylphenyl substituent in B plays a decisive role in achieving a better OER onset potential and current than that obtained with A, which is a result of the modulation of the structural parameters of B. Furthermore, the measured OER activities of A and B have been correlated with their molecular arrangement as well as differences in their bonding characteristics and dipole moments. For further insight, the obtained results have been confirmed by a theoretical study.

Bifunctional N-doped graphene Ti and Co nanocomposites for the oxygen reduction and evolution reactions

Abstract Developing efficient, durable, and low cost catalysts based on earth-abundant elements for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is essential for renewable energy conversion and energy storage devices. We report herein a simple one-pot procedure for the synthesis of non-precious metals N-doped graphene composites employing urea as nitrogen source, and their application as bifunctional catalysts for both the ORR and OER in alkaline environment. In this study, the effects of the addition of Ti and Co on the structure and performance of the N-doped graphene composites are investigated. The incorporation of Ti leads to a composite with both anatase and rutile TiO2 crystalline phases as well as Ti3+ species stabilized upon hybridization with N-doped reduced graphene oxide. The ORR onset potential for the Ti-based composite is 0.85 V (vs. RHE) and the number of electrons transferred is 3.5, showing superior stability than Pt/C after accelerated potential cycling in alkaline solution. However, this composite shows low activity and stability for the OER. On the other hand, the composite consisting of metallic Co and Co3O4 nanocrystals grown on N-doped reduced graphene oxide exhibits the best performance as bifunctional catalyst, with ORR and OER onset potentials of 0.95 V and 1.51 V (vs. RHE), respectively, and a number of electrons transferred of 3.6 (ORR). The results reveal the presence of important structural features such as metallic Co as the predominant crystalline component, amorphous Co3O4 phase and the coordination of Co-N-doped graphene. All of them seem to be fundamental for the high activity and stability towards ORR and OER.

Understanding the incorporating effect of Co2+/Co3+ in NiFe-layered double hydroxide for electrocatalytic oxygen evolution reaction

NiFe-layered double hydroxide (NiFe-LDH) has been widely accepted as promising catalyst candidates for the oxygen evolution reaction (OER). Recently, incorporating Co atoms in NiFe laminates has been recognized as an effective way to enhance the OER activity, but their roles have been rarely studied. Herein, density functional theory plus U (DFT + U) calculations are employed to evaluate the OER thermodynamics after introducing bivalent Co2+ or trivalent Co3+ and consequent sites sensitivity in NiFe-LDH catalyst. Generally, based on computational results, incorporation of Co2+/Co3+ into NiFe-LDH could modulate the electronic structure of metal sites and thus reducing their OER overpotential. Moreover, Co3+-doped NiFe-LDH has the lowest overpotential of η = 0.413 eV among the above proposed structure. This point was fully demonstrated by Co2+ and Co3+-incorporated NiFe-LDH nanosheets made by a co-precipitation method, by showing OER onset overpotential of 249 mV for Co3+-doped NiFe-LDH, 264 mV for Co2+-doped NiFe-LDH, which are 33 mV and 18 mV lower than that of pristine NiFe-LDH (282 mV), respectively. Such improved OER activity were attributed to the lowered overpotential at the *OOH formation step for Co2+-doped NiFe-LDH and the deprotonation step for Co3+-doped NiFe-LDH, which was the potential limiting step for their OER process. This work should be inspiring for future designing of more efficient NiFe-based oxygen evolution electrocatalysts.

Rapidly catalysis of oxygen evolution through sequential engineering of vertically layered FeNi structure

For practical hydrogen (H2) production via water electrolysis, large-scale design and fabrication of catalysts for high-efficiency electrochemical transformation of hydroxyl ions to oxygen in alkaline environment is of paramount importance to reduce energy losses. Using anodization, electroplating, and sequential electrodeposition, we herein realized a universal approach to vertically assemble 2D Fe-Ni hydroxide nanosheets on nickel foam surface as catalyst for oxygen evolution reaction (OER). The electrode exhibited OER onset overpotentials (η) of ~ 190mV and a low η of ~ 270mV at a current density of 100mAcm−2 in KOH electrolyte, making it one of the most active OER catalysts reported so far. The FeNi films deposited via sequential deposition are highly ordered, and electrode is stable even under long term or large current density operation. Based on the results of theoretical calculation, dielectric force microscopy (DFM) analysis, and capacitance measurements, we found that 2D Fe-Ni hydroxide-loaded electrode with enhanced conductivity facilitates the sufficiently conversion of active sites and ensures optimal adsorption energies for intermediates of OER, as a result of structural effects as well as introduction of optimal amount of Fe on surface.

Ultrasonic synthesis, characterization and energy applications of Ni–B alloy nanorods

Abstract Alloy nanostructures had received much attention for their applications in energy, catalysis, and optics. This paper described a facile ultrasonic approach for the synthesis of Ni–B alloy nanorods (Ni–B NRs). The crystallinity, chemical state, surface area and surface morphology of Ni–B NRs were characterized using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), N 2 adsorption–desorption isotherms and high-resolution transmission electron microscopy (HR–TEM) analyses, respectively. The electrocatalytic behavior of Ni–B NRs toward the oxygen evolution reaction (OER) was studied by linear sweep voltammetry (LSV) and Tafel polarization methods in 0.1 M aqueous KOH. The LSV results of Ni–B NRs showed very low OER onset potential of 1.44 V RHE with an overpotential of 212 mV RHE . The Ni–B NRs attained 10 mA/cm 2 of current density at a very low potential of 1.61 V RHE than bare SS (1.87 V RHE ) and bare Pt (2.50 V RHE ). Further, Ni–B NRs showed a low Tafel slope of 45 mV/dec when compared to other standard materials, revealed the fast OER kinetics. The calculated specific capacitance of synthesized Ni–B NRs was 581 F/g, from the results of cyclic voltammetry at a scan rate of 2 mV/s. The electrocatalytic and supercapacitor studies exposed that the synthesized Ni–B NRs can be used as an electrocatalyst for OER as well as pseudocapacitor for commercial electric vehicles.

Enhanced Electrocatalytic Oxygen Evolution in Au-Fe Nanoalloys.

Oxygen evolution reaction (OER) is the most critical step in water splitting, still limiting the development of efficient alkaline water electrolyzers. Here we investigate the OER activity of Au-Fe nanoalloys obtained by laser-ablation synthesis in solution. This method allows a high amount of iron (up to 11 at %) to be incorporated into the gold lattice, which is not possible in Au-Fe alloys synthesized by other routes, due to thermodynamic constraints. The Au0.89 Fe0.11 nanoalloys exhibit strongly enhanced OER in comparison to the individual pure metal nanoparticles, lowering the onset of OER and increasing up to 20 times the current density in alkaline aqueous solutions. Such a remarkable electrocatalytic activity is associated to nanoalloying, as demonstrated by comparative examples with physical mixtures of gold and iron nanoparticles. These results open attractive scenarios to the use of kinetically stable nanoalloys for catalysis and energy conversion.

Oxygen Evolution Reaction on Perovskite Electrocatalysts with Localized Spins and Orbital Rotation Symmetry

AbstractWe have studied, by using ab initio calculations, the electronic properties of electro‐catalysts for the oxygen evolution reaction (OER) with polarised density of states caused by localised spins in the d shell. Oxygen is a molecule in the triplet state (i.e., the outer electrons have parallel spins), which means that the spins localised in the p shell (↑O=O↑), the d shell and the conduction band electrons (t2gnegm) will couple through exchange interactions, which we think will provide favourable conditions for the OER. We compare the perovskites CaCu3Fe4O12 (CCF) and Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) with RuO2. CCF and BSCF both have fluctuating electronic structures accessible at room temperature that are linked to conducting spin‐polarised density of states, equivalent to the paramagnetic state of covalent transition metal oxides with fine charge conductivity. CCF and BSCF both possess a considerable number of unpaired electrons localised in the inner d shell, high‐spin configurations and competing inter‐atomic exchange interactions. As a first approximation, the average fluctuation of the magnetisation in the metal atoms correlates linearly with the OER onset potential for the studied compositions. By linking the dynamics of the localised inner‐electron spins to the conduction spins through exchange interactions, we can predict that other perovskites, such as Sr2Fe0.75Co0.25MoO6, will be OER active at room temperature, as they have similar electronic properties to CCF and BSCF.

NiFe Layered‐Double‐Hydroxide‐Derived NiO‐NiFe2O4/Reduced Graphene Oxide Architectures for Enhanced Electrocatalysis of Alkaline Water Splitting

AbstractElectrochemical water splitting is an environmentally friendly technology to store renewable but intermittent energy into hydrogen fuels. Nowadays, exploiting low‐costing, high‐performance, and robust catalysts for the electrochemical oxygen evolution reaction (OER) is essential to improve the overall efficiency of water splitting. Herein, the synthesis, structural characterization, and electrocatalytic OER performance of NiO‐NiFe2O4 nanoparticles anchored on reduced graphite oxide frameworks (NiO‐NiFe2O4/rGO) were investigated. Facile thermal annealing of the NiFe layered double hydroxide (NiFe‐LDH) precursor led to the formation of highly dispersible NiO‐NiFe2O4 nanoparticles (20–30 nm in size) across the rGO substrate with a NiO/NiFe2O4 molar ratio up to 4.42. In contrast to the nanostructured NiFe‐LDH/rGO catalyst, the NiO‐NiFe2O4/rGO nanohybrid exhibits a lower OER onset potential (Eonset=1.436 V vs. RHE), affords a smaller overpotential of 296 mV, and achieves a current density of 10 mA cm−2 with a Tafel slope of about 43 mV dec−1; these values are comparable to those of the benchmark IrO2 catalyst. The synergy between the abundant catalytically active sites through good dispersion of NiO‐NiFe2O4 across the rGO substrate and fluent electron transport arising from the rGO and NiFe2O4 components results in the outstanding electrocatalytic activity. The extremely high catalytic activity, facile synthesis, and low‐cost of the NiO‐NiFe2O4/rGO nanohybrid make it a very promising catalyst for the OER.

Preparation and characterization of supported RuxIr(1-x)O2 nano-oxides using a modified polyol synthesis assisted by microwave activation for energy storage applications

A simple and soft synthesis method, namely the polyol route, is proposed to synthesize RuxIr(1-x)O2 mixed-oxide nanoparticles deposited on a high surface area carbon support, with controlled size and size distribution, and composition. The samples are characterized by TGA, XRD, TEM, XPS and Raman analyses. Their behaviors as supercapacitors and catalysts for oxygen evolution reaction (OER) have been investigated in 0.5M H2SO4 medium. It was demonstrated that pure RuO2 presented the highest supercapacitive property, and that addition of iridium leads to decrease this property. On the other hand, pure RuO2 presented the lower onset potential for OER, but tended to be passivated as the potential was increased. Addition of iridium led to limit or to avoid the catalyst passivation and to increase the activity at high potential, but also to shift positively the OER onset potential.