Metal oxide/carbon nanosheet arrays derivative of stacked metal organic frameworks for triggering oxygen evolution reaction

Numerous clean energy systems rely on the oxygen evolution process (OER), which takes place during water splitting reaction. For this purpose, transition-metal oxides have garnered considerable attentions as a prominent OER electrocatalysts. In present study, we fabricate the nanosheet arrays of metal oxide/carbon (MOx/C; M = Fe, Ag, and Mn) fabricated via hydrothermal route. As templates, this approach employs the covered 2-dimensional (2D) metal-organic frameworks (2D-MOFs), and these MOx/C arrays made from 2D MOFs exhibit significant electrocatalytic activity and durability. Among all, Ag₂O/C showed the overpotentials of 270 mV at a current density (j) of 10 mA cm^?2, while the tafel slope is 45 mV dec^?1, that is lower than other metal oxide-based catalysts like MnO/C, and Fe₂O₃/C. It also shows 48 h high stability due to the conductive nature, larger surface area and the presence of carbon cage for easy transfer of electrons. The conceptual framework and synthetic strategy employed in this study can be applied to create more multi-metal oxide anchoring Ag₂O carbon matrix-based electrocatalysts that are extremely efficient, affordable, and perform significantly better in OER and other future applications.     

Hierarchical FeC/MnO2 composite with in-situ grown CNTs as an advanced trifunctional catalyst for water splitting and Metal−Air batteries

In high demand is developing trifunctional electrocatalysts to simultaneously drive hydrogen evolution reaction (HER) and oxygen evolution/reduction reaction (OER/ORR) for metal-air batteries and water splitting. Here we develop the carbon nanotubes (CNTs)-grafted FeC/MnO₂ nanocomposite catalyst by carbonizing FeMn metal-organic frameworks. The synergistic effect between FeC and MnO₂ dominantly contributes the ORR, OER, and HER. The transition metal-mediated growth of CNTs by an in-situ catalysis mechanism enables high electrical conductivity, abundant active sites, as well as efficient reaction pathways. The optimized chemical composite and unique hierarchical structure endow the FeC/MnO₂ with low overpotentials for multiply electrochemical reactions. Consequently, the composite catalyst successfully serves as the bifunctional electrode for water splitting with a voltage of 1.66 V at 10 mA cm^?2 as well as the cathode for all-solid-state metal-air battery with Pt/C-comparable performance. The advanced transition metal composite presented in this work provides the guidance for rationally developing trifunctional electrocatalysts for efficient integrated energy conversion systems.     

Heterostructure interface effect on the ORR/OER kinetics of Ag–PrBa0.5Sr0.5Co2O5+δ for high-efficiency Li–O2 battery

Developing high-efficiency and cost-effective bifunctional electrocatalysts toward the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is an urgent issue for the oxygen-based electrochemical devices. Herein, an interface engineering concept has been proposed to achieve high-performance Ag–PBSC (Ag–PrBa_0.5Sr_0.5Co₂O_5+δ) heterostructure nanofibers catalyst. Benefiting from the significant ligand action and interparticle cooperation of exsolved Ag NPs and PBSC double perovskite, ORR/OER catalytic kinetics have been successfully boosted. In details, the PBSC double perovskite possessing abundant oxygen vacancies can provide oxygen channels and facilitate the transfer of electrons and oxygen. The embedded Ag NPs can deliver superior catalytic durability for the heterostructure interface. As expected, the as-synthesized Ag-PBSC heterostructure catalyst performs a favorable electrochemical performance in the oxygen-based applications. In alkaline media, the catalyst exhibits an excellent activity for ORR (E_onset: 0.88 V vs. RHE and E_1/2: 0.72 V vs. RHE) and OER (1.67 V at 10 mA cm^–2). When adopting the Ag–PBSC heterostructure catalyst in LOBs, the corresponding battery provides an outperforming capacity performance (13 000 mAh g^–1), low discharge–charge polarization (1.37 V), and considerable cycling performance (128 cycles at the restricted capacity of 3 000 mAh g^–1 and 400 mA g^–1). Apparently, the work described here confirms that the interface engineering of perovskites can open up opportunities to develop highly active and durable heterostructure electrocatalysts for multitudinous oxygen-based electrochemical applications.     

Low-temperature synthesized amorphous quasi-high-entropy carbonate electrocatalyst with superior surface self-optimization for efficient water oxidation

Anionic High-entropy materials have seldom been reported as a new library of water oxidation electrocatalysts owing to great difficulty in uniformly distributing multiple elements with different physicochemical properties and harsh synthesis conditions. Herein, a series of amorphous quasi-high-entropy carbonates for the first time is prepared via a facile low-temperature hydrothermal route. The optimized CoCrFeMnMoCO₃ with hydrothermal treatment of 6 h can serve as a promising oxygen evolution reaction (OER) electrocatalyst for water splitting on account of amorphous structure rendering more exposed active sites, superior synergistic effect realizing surface component self-optimization, high-valence ferritic species (Fe^(3+δ)+) providing high catalytic activity and high-entropy stabilization guaranteeing long-term OER performance, thus exhibiting the low overpotentials of 302 and 355 mV at the current densities of 10 and 100 mA cm⁻², respectively, the small Tafel slope of 36.7 mV dec⁻¹, and excellent durability longer than 38 h, dramatically exceeding its corresponding crystalline counterpart and benchmark RuO₂ catalyst, as well as yielding the current density of 10 mA cm⁻² with impressive low voltage of 1.56 V while used as bifunctional electrocatalyst for overall water splitting. This study lights a broad avenue to design other anionic high-entropy materials as promising OER catalysts.     

Enhanced oxygen evolution reaction performance of synergistic effect of TiO2/Ti3C2/FeNi LDH

The introduction of high-efficiency electrocatalysts can improve the efficiency of oxygen evolution reaction (OER). However, the synergistic effect of elcetrocatalyst and cocatalyst is rarely studied. In this paper, by combining FeNi layered double hydroxide (LDH) electrocatalyst with a two-dimensional (2D) Ti₃C₂ co-catalyst on TiO₂ photocatalyst, the OER performance of TiO₂/Ti₃C₂/FeNi LDH is greatly improved due to the synergistic coupling effect. The microstructure, electrochemical performance and oxygen generation mechanism of TiO₂/Ti₃C₂/FeNi LDH are investigated in this paper. The results showed that the vertical array of FeNi LDH nanosheets created many nanoscale channels for reaction intermediates, enabling them to enter the most active sites on the surface. More importantly, the addition of Ti₃C₂ with high conductivity greatly effectively improved the charge separation and transfer between TiO₂ and FeNi LDH. Therefore, the required over-potential for current density 100 mA/cm² was only 633 mV for TiO₂/2 Ti₃C₂/FeNi LDH. Meanwhile, Tafel slope was as low as 30 mV/dec with good stability. This work provides a reference for using Ti₃C₂ as a new type of co-catalyst material to obtain an efficient, stable and economical OER reaction catalyst.     

Fabrication of efficient nanostructured Co3O4-Graphene bifunctional catalysts: Oxygen evolution,hydrogen evolution,and H2O2 sensing

Nanostructured Co₃O₄-graphene hybrid catalysts are fabricated by a one-step vacuum kinetic spray technique from microparticles of Co₃O₄ and graphite powders. The Co₃O₄-graphene hybrid catalysts with various Co₃O₄ contents are studied concerning the oxygen evolution reaction (OER), and hydrogen evolution reaction (HER) in 1.0 M KOH, as well as, H₂O₂ sensing in 0.1 M NaOH. We find that increasing graphene content in the hybrid catalysts results in an overall improvement of the OER electrocatalytic activity due to the enhancement in the charge transfer kinetics. The hybrid catalyst with 25 wt% Co₃O₄ reveals the optimum electrocatalytic activity toward the OER with the lowest overpotential (η) of 283 mV@ 10 mA cm⁻² and superior reaction kinetics with a low Tafel slope of 25 mV dec⁻¹. Besides, the OER stability at 50 mA cm⁻² for 50 h in 1.0 M KOH was verified. The hybrid catalyst with 50 wt% Co₃O₄ revealed the highest activity toward the HER with η of 108 mV@ 10 mA cm⁻², Tafel slope of 90 mV.dec⁻¹, and stability at 50 mA cm⁻² for nearly 30 h. Moreover, it reveals ultrahigh H₂O₂ amperometric detection with superior sensitivity of 18,110 μA mM⁻¹ cm⁻², linear detection range from 20 μM to 1 mM, and a limit of detection of 0.14 μM.     

Ba0.95La0.05FeO3−δ–multi-layer graphene as a low-cost and synergistic catalyst for oxygen evolution reaction

Oxygen evolution reaction (OER) is critical to many important energy conversion and storage processes. However, the kinetics of OER is typically sluggish, and conventional OER catalysts are scarce and expensive. Hence, inexpensive and effective catalysts that accelerate the OER kinetics and reduce the overpotential are urgently needed. In this paper, we reported multi-layer graphene-supported perovskite Ba_0.95La_0.05FeO_3−δ (BLF–MLG) as a highly active OER catalyst. Electrochemical results showed that the BLF–MLG hybrid exhibited a high current density (∼23 mA/cm² at 0.60 V vs. mercury/mercury oxide electrode (MMO)) and a more negative onset potential (∼0.43 V vs. MMO). Furthermore, the hybrid showed a smaller Tafel slope (77 mV/decade) and improved long-term stability. The remarkable OER performance is mainly attributed to the synergistic effect between BLF and graphene.     

Methane decomposition over ceria modified iron catalysts

The catalytic behaviour of ceria supported iron catalysts (Fe–CeO₂) was investigated for methane decomposition. The Fe–CeO₂ catalysts were found to be more active than catalysts based on iron alone. A catalyst composed of 60 wt.% Fe₂O₃ and 40 wt.% CeO₂ gave optimal catalytic activity, and the highest iron metal surface area. The well-dispersed Fe state helped to maintain the active surface area for the reaction. Methane conversion increased when the reaction temperature was increased from 600 to 650 °C. Continuous formation of trace amounts of carbon monoxide was observed during the reaction due to the oxidation of carbonaceous species by high mobility lattice oxygen in the solid solution formed within the catalyst. This could minimise catalyst deactivation caused by carbon deposits and maintain catalyst activity over a longer period of time. The catalyst also produced filamentous carbon that helped to extend the catalyst life.     

Spinel CoFe2O4 /carbon nanotube composites as efficient bifunctional electrocatalysts for oxygen reduction and oxygen evolution reaction

In the development of fuel cells, it is the key to large-scale commercialization of fuel cells to rationally design and synthesize efficient and non-noble metals-based bifunctional electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). In this paper, spinel CoFe₂O₄/carbon nanotube composites (CoFe₂O₄/CNTs/FA) were synthesized by solvothermal and calcination method. XRD, TEM, XPS and BET characterizations indicate that the addition of complexing agent fumaric acid can improve the crystal growth kinetics and morphology of CoFe₂O₄/CNTs nanohybirds. The as-synthesized CoFe₂O₄/CNTs/FA pyrolyzed at 500 °C have an outstanding bifunctional catalytic activity for ORR and OER with the potential of 1.62V (vs. RHE) at a current density of 10 mA/cm² and half-wave potential E_1/2 = 0.808V (vs. RHE) in alkaline electrolyte, respectively. It is obviously better than unloaded CoFe₂O₄ nanoparticles and commercial CNTs. CoFe₂O₄/CNTs/FA also exhibit better methanol tolerance ability and durability than commercial Pt/C and RuO₂ catalyst. This investigation broadens an idea of simple compounding of spinel with carbon-based materials to improve electrochemical properties.     

Simple preparation of nanoporous few-layer nitrogen-doped graphene for use as an efficient electrocatalyst for oxygen reduction and oxygen evolution reactions

We prepared nitrogen-doped graphene (NG) by simple pyrolysis of graphene oxide and polyaniline, which was selected as the N source. The resulting NG contains 2.4 at.% N, of which as high as 1.2 at.% is quaternary N. Electrochemical characterizations reveal that the NG has excellent catalytic activity toward oxygen reduction reaction (ORR) in an alkaline electrolyte, including a desirable four-electron pathway for the formation of water, large kinetic-limiting current density, long-term stability and good tolerance to methanol crossover. In addition, we demonstrate that the NG also has high catalytic activity toward oxygen evolution reaction (OER), rendering its potential application as a bi-functional catalyst for both ORR and OER.     

Low-temperature total oxidation of methane by pore- and vacancy-engineered NiO catalysts

Designing methane combustion catalysts operated under low temperature (<400°C) remains a huge challenge, especially for noble-metal–free catalytic systems. With NaCl as a crystalline scaffold, NiO catalyst with abundant oxygen vacancies and an ultra-high–specific surface area of 181 m² g⁻¹ is obtained. The mesoporous NiO exhibits outstanding CH₄ combustion performance (T₉₀ = 370°C at the weight hourly space velocity (WHSV) = 20,000 mL g⁻¹ h⁻¹). X-ray photoelectron spectroscopy (XPS), H₂-temperature-programmed reduction (TPR), kinetic measurements, and O¹⁸ isotope-labeling experiments together disclose the key role of surface lattice oxygen and reaction mechanism by NiO catalysts. More importantly, the excellent stability of NiO by doping La was obtained (low-temperature thermal stability: 385°C, 400 h, 4 vol% H₂O).     

Highly porous metal organic framework derived NiO hollow spheres and flowers for oxygen evolution reaction and supercapacitors

In this study, metal-organic-framework (MOF) derived porous NiO hollow spheres and flowers were obtained using facile solvothermal synthesis and heat treatment. After pyrolyzing, the flower like and hollow spherical like morphology of NiO nanoparticles was successfully inherited from the initial MOF-based templates. The electrochemical studies demonstrated that the porous NiO hallow spheres unveiled a better supercapacitive performance (specific capacitance (C_s) = 1058 F g^?1 at current density (j) = 2 A g^?1) and oxygen evolution reaction (OER) catalytic activity (overpotential (?) = 323 mV) compared to porous NiO flowers (C_s = 857 F g^?1 at j = 2 A g^?1 and ? = 346 mV). Moreover, excellent capacity retention of over 93% was obtained in porous NiO-hs nanoparticles even after 5000 cycles. The fabricated NiO//Fe₂O₃ asymmetric supercapacitor delivered an energy density (E) of 35.75 W h Kg^?1 under power density (P) of 780 W kg^?1 and showed promising stability over 3000 cycles. Considering the ease of preparation and high catalytic activity and supercapacitive performance, these prous NiO hallow structures can be considered as a potential electrode material for next generation energy storage devices and OER catalysts.     

Highly efficient complete oxidation of dilute benzene over ultrafine Cu0.1Ce0.5Zr0.4O2−δ catalyst in a fluidized bed reactor

Ultrafine Cu_0.1Ce_0.5Zr_0.4O_2−δ catalyst operated in a fluidized bed reactor was found to be very effective for complete oxidation of dilute benzene in air. The complete conversion of benzene could be achieved at reaction temperature as low as 220 °C. The mechanism of benzene oxidation over the Cu_0.1Ce_0.5Zr_0.4O_2−δ catalyst was investigated by conducting pulse reaction of pure benzene in the absence of O₂ over the catalyst and the results indicated the involvement of lattice oxygen from the catalyst in benzene oxidation.     

Electrical properties of V2O5–WO3/TiO2 EUROCAT catalysts evidence for redox process in selective catalytic reduction (SCR) deNOx reaction

Electrical conductivity measurements on EUROCAT V₂O₅–WO₃/TiO₂ catalyst and on its precursor without vanadia were performed at 300°C under pure oxygen to characterize the samples, under NO and under NH₃ to determine the mode of reactivity of these reactants and under two reaction mixtures ((i) 2000 ppm NO + 2000 ppm NH₃ without O₂, and (ii) 2000 ppm NO + 2000 ppm NH₃ + 500 ppm O₂) to put in evidence redox processes in SCR deNO_x reaction.It was first demonstrated that titania support contains certain amounts of dissolved W⁶⁺ and V⁵⁺ ions, whose dissolution in the lattice of titania creates an n-type doping effect. Electrical conductivity revealed that the so-called reference pure titania monolith was highly doped by heterovalent cations whose valency was higher than +4. Subsequent chemical analyses revealed that so-called pure titania reference catalyst was actually the WO₃/TiO₂ precursor of V₂O₅–WO₃/TiO₂ EUROCAT catalyst. It contained an average amount of 0.37 at.% W⁶⁺dissolved in titania, i.e. 1.07 × 10²⁰ W⁶⁺ cations dissolved/cm³ of titania. For the fresh catalyst, the mean amounts of W⁶⁺ and V⁵⁺ ions dissolved in titania were found to be equal to 1.07 × 10²⁰ and 4.47 × 10²⁰ cm⁻³, respectively. For the used catalyst, the mean amounts of W⁶⁺ and V⁵⁺ ions dissolved were found to be equal to 1.07 × 10²⁰ and 7.42 × 10²⁰ cm⁻³, respectively. Since fresh and used catalysts have similar compositions and similar catalytic behaviours, the only manifestation of ageing was a supplementary progressive dissolution of 2.9 × 10²⁰ additional V⁵⁺ cations in titania.After a prompt removal of oxygen, it appeared that NO alone has an electron acceptor character, linked to its possible ionosorption as NO⁻ and to the filling of anionic vacancies, mostly present on vanadia. Ammonia had a strong reducing behaviour with the formation of singly ionized vacancies. A subsequent introduction of NO indicated a donor character of this molecule, in opposition to its first adsorption. This was ascribed to its reaction with previously adsorbed ammonia strongly bound to acidic sites. Under NO + NH₃ reaction mixture in the absence of oxygen, the increase of electrical conductivity was ascribed to the formation of anionic vacancies, mainly on vanadia, created by dehydroxylation and dehydration of the surface. These anionic vacancies were initially subsequently filled by the oxygen atom of NO. N^o atoms, resulting from the dissociation of NO and from ammonia dehydrogenation, recombined into dinitrogen molecules. The reaction corresponded to

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. In the presence of oxygen, NO did not exhibit anymore its electron acceptor character, since the filling of anionic vacancies was performed by oxygen from the gas phase. NO reacted directly with ammonia strongly bound on acidic sites. A tentative redox mechanism was proposed for both cases.     

Preparation of In,H-ZSM-5 for DeNOx reactions by solid-state ion exchange

A simple method is proposed to prepare In,H-ZSM-5 catalyst for DeNOx reactions. This consists of mechanically mixing the fine powders of In₂O₃ and H-ZSM-5 followed by heating in oxygen free inert gas flow to 580 °C where indium undergoes thermal auto-reduction and moves into exchange positions as In⁺ without destroying the crystalline structure of the zeolite.It was evidenced by IR, temperature-programmed reduction (TPR) and reoxidation that, once In⁺ was introduced into the lattice either by reductive solid-state ion exchange (RSSIE) or by thermal auto-reductive SSIE, it can be oxidized by O₂ or in the DeNOx reaction to (InO)⁺. The formed (InO)⁺ can easily be reduced to In⁺ suggesting that In,H-ZSM-5 might be a good catalyst for reactions where a redox cycle in the catalyst is involved in the reaction mechanism.Selective catalytic reduction (SCR) by methane proved that only a small fraction of In exchanged, together with some acid sites of the zeolite formed the active center for the catalytic reaction. XRD, XPS and FT-IR using pyridine proved that the structure of the zeolite and these centers are stable under reaction conditions and In is mainly in the form of (InO)⁺ in the used catalyst.     

Novel Pt/La1−xBixOF/SBA-16 catalysts for liquid-phase phenol oxidation

Novel catalysts of Pt/La_1−xBi_xOF/SBA-16 (SBA-16: Santa Barbara Amorphous No. 16) were synthesized, and their catalytic activities for phenol decomposition in the liquid phase were investigated. Lanthanum oxyfluoride (LaOF) was selected as the promoter due to its contribution to the smooth migration of oxygen species in the lattice and the acceleration of phenol adsorption on the catalyst surface. Reducible Bi³⁺ ions were introduced in the LaOF lattice to provide oxygen supply ability to LaOF. Among the prepared catalysts, the highest activity was obtained for the 7 wt% Pt/16 wt% La_0.99Bi_0.01OF/SBA-16 catalyst, which could remove 97% of phenol after 5 h of reaction at 80°C in an open-air atmosphere.     

MOF nanosheet array-derived NiS with Co,N-doped carbon layer: A highly efficient oxygen evolution electrocatalyst

The development of the hydrogen industry from electrolytic water is confined in great measure to the slow kinetics of oxygen evolution reaction (OER), which means the rational design of a stabilized and efficient OER electrode is the key. The construction of easily accessible hydroxide ion (OH^?) adsorption sites can effectively accelerate the kinetics of the OER. Herein, the NiS/CoNC electrocatalysts, which can effectively adsorb OH^? and exhibited excellent OER performance in an alkaline environment, were obtained by pyrolysis and sulfidation of NiCo-MOF nanosheets on nickel foam (NF). In particular, the optimized NiS/CoNC electrocatalyst achieved an overpotential of 46/106 mV at 10/100 mA cm^?2 and Tafel slope of 57.41 mV dec^?1, which is superior to most reported materials. The outstanding function of OER results from the NiOOH/CoOOH active component generated by surface reconstruction after activation of the NiS/CoNC catalyst and the excellent charge transfer capability of the NiS component. This work offers a scheme to construct the electrocatalysts derived from MOF with excellent OER performance.     

Water gas shift reaction kinetics in Fischer–Tropsch synthesis over an industrial Fe–Mn catalyst

The kinetics of water gas shift (WGS) reaction over an Fe–Mn catalyst under Fischer–Tropsch synthesis (FTS) reaction conditions is studied in a spinning basket reactor. Experimental conditions are varied as follows: temperature of 533–573 K, reactor pressure of 10.0–26.5 bar, H₂/CO feed ratio of 0.66–2.0 and space velocity of 0.66–2.65×10⁻³ Nm³ kg_cat⁻¹ s⁻¹. By separately fitting WGS kinetics parameters with experimental data, which is possible in the spinning basket reactor with neglecting concentration and temperature gradients, different kinetics models of WGS are derived and discriminated on the basis of four sets of WGS elementary reactions. Kinetics experimental results show that the WGS reaction under FTS reaction conditions is far from equilibrium. Two types of WGS mechanisms are investigated. One is the formate mechanism, and the other is the direct oxidation mechanism. It is found that the formate mechanism is better in fitting experimental data than the direct oxidation mechanism over the Fe–Mn catalyst under the FTS reaction conditions. The optimized kinetics model with formate intermediate dissociation as the rate-determining step (RDS) can fit the WGS experimental results well. The simplified WGS kinetics model can easily be used for industrial modeling applications.     

Ir-based oxide electrodes: oxygen evolution reaction from mixed solvents

The influence of 30% (v/v) organic cosolvent in 1.0 mol dm^–3 HClO₄ on the OER electrode kinetics, surface properties and electrode stability of IrO₂-based electrodes was investigated by cyclic voltammetry and polarization curves. The Tafel coefficients in the presence of cosolvent are explained in terms of the change of the rate-determining step (r.d.s.) of the OER electrode mechanism, coating dissolution and/or cosolvent oxidation. Of the several cosolvents investigated t-BuOH and PC show less effects on the OER and electrode properties making them the best choice for organic eletrosynthesis applications, in contrast to AN, which causes coating dissolution, and DMF and DMSO which show an anticipation of the voltammetric current.     

Highly efficient complete oxidation of dilute benzene over ultrafine Cu0.1Ce0.5Zr0.4O2−δ catalyst in a fluidized bed reactor

Ultrafine Cu_0.1Ce_0.5Zr_0.4O_2−δ catalyst operated in a fluidized bed reactor was found to be very effective for complete oxidation of dilute benzene in air. The complete conversion of benzene could be achieved at reaction temperature as low as 220 °C. The mechanism of benzene oxidation over the Cu_0.1Ce_0.5Zr_0.4O_2−δ catalyst was investigated by conducting pulse reaction of pure benzene in the absence of O₂ over the catalyst and the results indicated the involvement of lattice oxygen from the catalyst in benzene oxidation.