Activating Inert,Nonprecious Perovskites with Iridium Dopants for Efficient Oxygen Evolution Reaction under Acidic Conditions

Simultaneous realization of improved activity, enhanced stability, and reduced cost remains a desirable yet challenging goal in the search of oxygen evolution electrocatalysts in acid. Herein we report iridium‐containing strontium titanates (Ir‐STO) as active and stable, low‐iridium perovskite electrocatalysts for the oxygen evolution reaction (OER) in acid. The Ir‐STO contains 57 wt % less iridium relative to the benchmark catalyst IrO₂, but it exhibits more than 10 times higher catalytic activity for OER. It is shown to be among the most efficient iridium‐based oxide electrocatalysts for OER in acid. Theoretical results reveal that the incorporation of iridium dopants in the STO matrix activates the intrinsically inert titanium sites, strengthening the surface oxygen adsorption on titanium sites and thereby giving nonprecious titanium catalytic sites that have activities close to or even better than iridium sites.     

Activating Inert,Nonprecious Perovskites with Iridium Dopants for Efficient Oxygen Evolution Reaction under Acidic Conditions

Simultaneous realization of improved activity, enhanced stability, and reduced cost remains a desirable yet challenging goal in the search of oxygen evolution electrocatalysts in acid. Herein we report iridium‐containing strontium titanates (Ir‐STO) as active and stable, low‐iridium perovskite electrocatalysts for the oxygen evolution reaction (OER) in acid. The Ir‐STO contains 57 wt % less iridium relative to the benchmark catalyst IrO₂, but it exhibits more than 10 times higher catalytic activity for OER. It is shown to be among the most efficient iridium‐based oxide electrocatalysts for OER in acid. Theoretical results reveal that the incorporation of iridium dopants in the STO matrix activates the intrinsically inert titanium sites, strengthening the surface oxygen adsorption on titanium sites and thereby giving nonprecious titanium catalytic sites that have activities close to or even better than iridium sites.     

Electrochemical comparison between IrO2 prepared by thermal treatment of iridium metal and IrO2 prepared by thermal decomposition of H2IrCl6 solution

The surface redox activities, the oxygen evolution reaction (OER), the oxidation of formic acid (FA) and the anodic stability have been investigated and compared on IrO₂ electrodes prepared by two techniques: the thermal decomposition of H₂IrCl₆ precursor (TDIROF) and the thermal treatment of metallic iridium (TOIROF). It was found, that the surface redox activities involved on both IrO₂-based electrodes are similar. Concerning the oxygen evolution reaction and the oxidation of formic acid, both films show similar mechanism.The electrode stability measurements have shown that both films are not corroded under strong OER or organics oxidation conditions and therefore, to summarize, both IrO₂-based films exhibit similar electrochemical behaviours.     

A Dissolution/Precipitation Equilibrium on the Surface of Iridium‐Based Perovskites Controls Their Activity as Oxygen Evolution Reaction Catalysts in Acidic Media

Recently, Ir^V‐based perovskite‐like materials were proposed as oxygen evolution reaction (OER) catalysts in acidic media with promising performance. However, iridium dissolution and surface reconstruction were observed, questioning the real active sites on the surface of these catalysts. In this work, Sr₂MIr^(V)O₆ (M=Fe, Co) and Sr₂Fe_0.5Ir_0.5^(V)O₄ were explored as OER catalysts in acidic media. Their activities were observed to be roughly equal to those previously reported for La₂LiIrO₆ or Ba₂PrIrO₆. Coupling electrochemical measurements with iridium dissolution studies under chemical or electrochemical conditions, we show that the deposition of an IrO_x layer on the surface of these perovskites is responsible for their OER activity. Furthermore, we experimentally reconstruct the iridium Pourbaix diagram, which will help guide future research in controlling the dissolution/precipitation equilibrium of iridium species for the design of better Ir‐based OER catalysts.     

Titanium Anodes with Active Coatings Based on Iridium Oxides: The Corrosion Resistance and Electrochemical Behavior of Anodes Coated by Mixed Iridium,Ruthenium, and Titanium Oxides

A study of the corrosion resistance and electrochemical behavior of titanium anodes with active coatings prepared from mixed oxides iridium, ruthenium, and titanium (OIRTA) is continued. The dependence of the catalytic activity, selectivity, and corrosion resistance of these anodes with x mol % RuO₂ + (30 ? x ) mol % IrO₂ + 70 mol % TiO₂ is studied in conditions of chlorine electrolysis on the ratio of concentrations of IrO₂ and RuO₂ in them at a constant loading of iridium in the coatings. It is established that the maximum corrosion resistance and selectivity is inherent in OIRTA with the RuO₂ concentration close to 4 mol %. Partial curves, which describe the dependence of the rates of dissolution of iridium out of OIRTA and the evolution of chlorine and oxygen in them on the electrode potential, are obtained. The dependence of the rates of these processes on the solution pH, the concentration of NaCl in it, and the thickness of the active layer is studied. It is shown that the rate of dissolution of iridium out of OIRTA and the concentration of oxygen in chlorine at a constant potential increase approximately proportionally to the coating thickness, from whence it follows that the said processes proceed over the entire depth of the coating. An assumption is put forth that the chlorine evolution on OIRTA of the optimum composition, with a loading of iridium equal to 2.5 g m^?2, at high anodic currents occurs in an outer-kinetics regime in the presence of diffusion limitations on the removal of chlorine out of the coating's depth.     

Selective-leaching method to fabricate an Ir surface-enriched Ir-Ni oxide electrocatalyst for water oxidation

Durable and precious metal-lean electrocatalyst for water oxidation in acidic media would be of great significance for the large-scale application of acidic water electrolysis. Here, we report an Ir-Ni binary oxide electrocatalyst for the oxygen evolution reaction (OER) fabricated by acid leaching of Ni from Ni-rich composite oxides prepared using pyrolysis method. This Ni-leached binary oxide possesses Ir-enriched surface, porous morphology, and rutile phase structure of IrO₂ with contracted lattice. In contrast, Ir-Ni binary oxide with the same composition prepared using simple pyrolysis method exhibits a rod-like aggregated morphology with Ni-enriched surface. Catalytic activity for OER of the Ni-leached binary oxide is higher than that of the pyrolyzed Ir-Ni oxide and pure IrO₂. More importantly, the Ni-leached binary oxide exhibits much superior durability during continuous oxygen evolution process under a constant potential of 1.6 V compared with the pyrolyzed binary oxide and pure IrO_2. Attributed to the Ir-rich surface and the anchor effect of inner Ni atoms to outer Ir atoms, the Ni-leached binary oxide shows a possibility of reducing the demand of the expensive and scarce Ir in OER electrocatalyst for acidic water splitting.     

Electrochemical oxidation of molecular nitrogen to nitric acid – towards a molecular level understanding of the challenges

Nitric acid is manufactured by oxidizing ammonia where the ammonia comes from an energy demanding and non-eco-friendly, Haber–Bosch process. Electrochemical oxidation of N₂ to nitric acid using renewable electricity could be a promising alternative to bypass the ammonia route. In this work, we discuss the plausible reaction mechanisms of electrochemical N₂ oxidation (N₂OR) at the molecular level and its competition with the parasitic oxygen evolution reaction (OER). We suggest the design strategies for N₂ oxidation electro-catalysts by first comparing the performance of two catalysts – TiO₂(110) (poor OER catalyst) and IrO₂(110) (good OER catalyst), towards dinitrogen oxidation and then establish trends/scaling relations to correlate OER and N₂OR activities. The challenges associated with electrochemical N₂OR are highlighted.

Electrochemical oxidation of N₂ to HNO₃ (N₂OR) is explored in conjunction with parasitic oxygen evolution reaction (OER) on a poor and a good OER catalyst, TiO₂ and IrO₂. We develop scaling relations to correlate OER and N₂OR activities on oxides.     

Low‐Coordinate Iridium Oxide Confined on Graphitic Carbon Nitride for Highly Efficient Oxygen Evolution

Highly active and durable electrocatalysts for the oxygen evolution reaction (OER) is greatly desired. Iridium oxide/graphitic carbon nitride (IrO₂/GCN) heterostructures are designed with low‐coordinate IrO₂ nanoparticles (NPs) confined on superhydrophilic highly stable GCN nanosheets for efficient acidic OER. The GCN nanosheets not only ensure the homogeneous distribution and confinement of IrO₂ NPs but also endows the heterostructured catalyst system with a superhydrophilic surface, which can maximize the exposure of active sites and promotes mass diffusion. The coordination number of Ir atoms is decreased owing to the strong interaction between IrO₂ and GCN, leading to lattice strain and increment of electron density around Ir sites and hence modulating the attachment between the catalyst and reaction intermediates. The optimized IrO₂/GCN heterostructure delivers not only by far the highest mass activity among the reported IrO₂‐based catalysts but also decent durability.     

Recent advances in oxygen electrocatalysts based on tunable structural polymers

Due to their high energy density, great safety and eco-friendliness, zinc-air batteries (ZABs) attract much attention. During the process of charging and discharging, the two key processes viz. oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) limit their efficiency. In general, the noble metal-based electrocatalysts (ORR: platinum (Pt); OER: iridium (IV) oxide [IrO₂] and ruthenium oxide [RuO₂]) have long been used. Nonetheless, these noble metal electrocatalysts also have their limitations owing to high cost and poor stability. As alternatives, polymers are found to be most promising on account of their tunable structure, uniform network, high surface morphology and strong durability. Polymers are capable catalysts. In this review, recent advances as well as insight into the architecture of covalent organic polymers (COPs), metal coordination polymers (MCPs) and pyrolysis-free polymers (PFPs) are duly outlined.     

Photoactivity and phase stability of ZrO2-doped anatase-type TiO2 directly formed as nanometer-sized particles by hydrolysis under hydrothermal conditions

Anatase-type TiO₂ doped with 4.7 and 12.4 mol% ZrO₂ that were directly precipitated as nanometer-sized particles from acidic precursor solutions of TiOSO₄ and Zr(SO₄)₂ by simultaneous hydrolysis under hydrothermal conditions at 200°C, showed higher photocatalytic activity than pure anatase-type TiO₂ for the decomposition of methylene blue. The crystallite growth and the phase transformation from anatase-type to rutile-type structure caused by heating at high temperature were retarded by doping ZrO₂ into TiO₂. The anatase-type TiO₂ doped with ZrO₂ showed high phase stability and maintained anatase-type structure even after heating at 1000°C for 1 h.     

Regulating the Spin State of FeIII by Atomically Anchoring on Ultrathin Titanium Dioxide for Efficient Oxygen Evolution Electrocatalysis

Ferric oxides and (oxy)hydroxides, although plentiful and low‐cost, are rarely considered for oxygen evolution reaction (OER) owing to the too high spin state (e_g filling ca. 2.0) suppressing the bonding strength with reaction intermediates. Now, a facile adsorption–oxidation strategy is used to anchor Fe^III atomically on an ultrathin TiO₂ nanobelt to synergistically lower the spin state (e_g filling ca. 1.08) to enhance the adsorption with oxygen‐containing intermediates and improve the electro‐conductibility for lower ohmic loss. The electronic structure of the catalyst is predicted by DFT calculation and perfectly confirmed by experimental results. The catalyst exhibits superior performance for OER with overpotential 270 mV @10 mA cm^?2 and 376 mV @100 mA cm^?2 in alkaline solution, which is much better than IrO₂/C and RuO₂/C and is the best iron‐based OER catalyst free of active metals such as Ni, Co, or precious metals.     

Carbon/titanium oxide supported bimetallic platinum/iridium nanocomposites as bifunctional electrocatalysts for lithium-air batteries

Platinum (Pt) and iridium (Ir) catalysts are well known to strongly enhance the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) kinetics, respectively. Pt–Ir-based bimetallic compounds along with carbon-supported titanium oxides (C–TiO₂) have been synthesized for the application as electrocatalysts in lithium oxygen batteries. Transition metal oxide-based bimetallic nanocomposites (Pt–Ir/C–TiO₂) were prepared by an incipient wetness impregnation technique. The as-prepared electrocatalysts were composed of a well-dispersed homogenous alloy of nanoparticles as confirmed by X-ray diffraction patterns and Fourier transform scanning electron microscopy analyses. The electrochemical characterizations reveal that the Pt–Ir/C–TiO₂ electrocatalysts were bifunctional with high activity for both ORR and OER. When applied as an air cathode catalyst in lithium-air batteries, the electrocatalyst improved the battery performance in terms of capacity, reversibility, and cycle life compared to that of cathodes without any catalysts.     

SrNb0.1Co0.7Fe0.2O3−δ Perovskite as a Next‐Generation Electrocatalyst for Oxygen Evolution in Alkaline Solution

The perovskite SrNb_0.1Co_0.7Fe_0.2O_3?δ (SNCF) is a promising OER electrocatalyst for the oxygen evolution reaction (OER), with remarkable activity and stability in alkaline solutions. This catalyst exhibits a higher intrinsic OER activity, a smaller Tafel slope and better stability than the state‐of‐the‐art precious‐metal IrO₂ catalyst and the well‐known BSCF perovskite. The mass activity and stability are further improved by ball milling. Several factors including the optimized e_g orbital filling, good ionic and charge transfer abilities, as well as high OH^? adsorption and O₂ desorption capabilities possibly contribute to the excellent OER activity.     

Formation,dissociation and expansion behavior of platinum group metal oxides (PdO,RuO2, IrO2)

The oxidation behavior of palladium, ruthenium and iridium powders of different grain sizes was investigated by TG, DTA and X-ray methods. The solid oxides formed during heating up (PdO, RuO₂, IrO₂) show different stability and decomposition temperatures depending on the oxygen pressure. The kinetics of the reaction MeO_x → Me+x/2 O₂ is discussed. High temperature X-ray studies confirmed the strong anisotropy of thermal expansion in the case of RuO₂ and IrO₂. The thermal expansion behavior of these oxides is compared to that of other rutile-type oxides.     

Oxygen evolution on SrFeO3 electrode

Oxygen evolution reactions on SrFeO₃ were investigated in alkaline and acidic solutions. It was found that the catalytic activity for the oxygen evolution reaction in the alkaline solution is high. The following reaction steps (V)+Fe+2H₂O→(O)+FeOH₂+2H⁺+2e^? in acidic solution and FeOH+OH^?→FeO^?+H₂O in alkaline solution are presumed to be rate-controlling in the anodic evolution of oxygen on SrFeO₃ electrode, where (V) denotes oxygen vacancy on the electrode surface. The reaction mechanism and the catalytic property are discussed in connection to the band structure of the oxide.     

Sol–gel processing of IrO<Subscript>2</Subscript>–TiO<Subscript>2</Subscript> mixed metal oxides based on an iridium acetate precursor

Mixed IrO₂–TiO₂ oxides were prepared by the sol–gel method by adding an aqueous solution of an iridium(III) acetate precursor [Ir₃O(OAc)₆ (HOAc)₃]OAc, to titanium tetraethoxide in ethanol. By using an acetylacetonate modifier to stabilize the hydrolysed titanium alkoxide and by omitting a catalyst, gels were produced in all cases. Transmission electron microscopy and EDX analysis confirmed the high dispersion of iridium in the dried gel material on the nanometre scale. The images also show spherical cage features up to 20 nm in diameter. High mass losses of the gels in the TGA scans suggested low degrees of hydrolysis of the acetate precursor, but calcination gave a crystalline, mixed oxide (Ti_xIr_1–xO₂) solid solution. The precursor is also soluble in ethanol, which provides a slightly modified route to similar materials.     

Cobalt Doped Titanium Dioxide Nanoparticles: Synthesis,Characterization and Electrocatalytic Study

In this work, green synthesis of cobalt doped titanium dioxide (Co‐TiO₂) has been carried out in aqueous medium using gelatin. The Co‐TiO₂ particles have been characterized using transmission electron microscopy (TEM), X‐ray diffraction (XRD), energy dispersive X‐ray (EDAX), FT‐IR spectroscopy and voltammetry techniques. XRD results show pure Co‐TiO₂ and TiO₂ powders with average crystallite size about 12 nm and 15 nm, respectively. Co loaded in TiO₂ hasn't influence crystalline structure. Moreover, efficient Co‐TiO₂‐based anode was fabricated by casting of the Co‐TiO₂ solution on glassy carbon electrode (Co‐TiO₂/GCE). The electrocatalysis of oxygen evolution reaction (OER) at Co‐TiO₂/GCE has been examined using linear scanning voltammetry (LSV) in alkaline media. The OER is significantly enhanced at Co‐TiO₂/GCE, as demonstrated by a negative shift in the LSV curve at the Co‐TiO₂/GCE compared to that obtained at the unmodified one. The value of energy saving of oxygen gas at a current density of 5 mA cm^?2 is 12.6 kW h kg^?1. The low cost as well as the marked stability of the modified electrode make it promising candidate in industrial water electrolysis process.     

Electrocatalytic Water Oxidation Activity-Stability Maps for Perovskite Oxides Containing 3d, 4d and 5d Transition Metals

Improving catalytic activity without loss of catalytic stability is one of the core goals in search of low-iridium-content oxygen evolution electrocatalysts under acidic conditions. Here, we synthesize a family of 66 SrBO₃ perovskite oxides (B=Ti, Ru, Ir) with different Ti : Ru : Ir atomic ratios and construct catalytic activity-stability maps over composition variation. The maps classify the multicomponent perovskites into chemical groups with distinct catalytic activity and stability for acidic oxygen evolution reaction, and highlights a chemical region where high catalytic activity and stability are achieved simultaneously at a relatively low iridium level. By quantifying the extent of hybridization of mixed transition metal 3d-4d-5d and oxygen 2p orbitals for multicomponent perovskites, we demonstrate this complex interplay between 3d-4d-5d metals and oxygen atoms in governing the trends in both activity and stability as well as in determining the catalytic mechanism involving lattice oxygen or not.     

Thermolytic formation and microstructure of IrO2 + Ta2O5 mixed oxide anodes from chloride precursors

The thermolytic formation of IrO₂+Ta₂O₅ mixed oxides from chloride precursors is studied by thermogravimetry (TGA) and differential thermal analysis (DTA). The structure and morphologies of the corresponding oxide films coated on titanium bases are determined by X-ray diffraction analysis (XRD) and scanning electron microscopy (SEM), respectively. The experimental results showed that, as a result of the interaction between Ir and Ta components, especially, the formation of solid solution phases during the thermolysis processes, the oxidative dissociation of the H₂IrCl₆+TaCl₅ mixture is facilitated. The catalytic effect reached the maximum at a nominal IrO₂ content of 70 mol% in the expected product, i.e. IrO₂+Ta₂O₅ mixed oxides, accompanied by the highest solid solubility between the two oxides and the finest rutile-structured crystalline grains in the oxides. For the mixed precursors with a low iridium content (e.g. 10 mol% nominal IrO₂ in IrO₂+Ta₂O₅) or a low tantalum content (e.g. 80 mol% nominal IrO₂), however, the decomposition of the major component is inhibited by the minor one at high temperatures (610-800 °C). The results show that the solid solution at low Ir contents (<30 mol% IrO₂) is unstable since it decomposes at high temperatures (≥750 °C). Two or more IrO₂ based rutile-constructed solid solution phases are thermolytically formed from the mixed precursors with nominal IrO₂ contents ≥30 mol%. The rutile-structured phases stably exist only in the case of IrO₂ contents ≥60 mol%.     

3D Nitrogen-Doped Graphene Encapsulated Metallic Nickel–Iron Alloy Nanoparticles for Efficient Bifunctional Oxygen Electrocatalysis

It is extremely desirable to explore high-efficient, affordable and robust oxygen electrocatalysts toward rechargeable Zn–air batteries (ZABs). A 3D porous nitrogen-doped graphene encapsulated metallic Ni₃Fe alloy nanoparticles aerogel (Ni₃Fe-GA₁) was constructed through a facile hydrothermal assembly and calcination process. Benefiting from 3D porous configuration with great accessibility, high electrical conductivity, abundant active sites, optimal nitrogen content and strong electronic interactions at the Ni₃Fe/N-doped graphene heterointerface, the obtained aerogel showed outstanding catalytic performance toward the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). Specifically, it exhibited an overpotential of 239 mV to attain 10 mA cm⁻² for OER, simultaneously providing a positive onset potential of 0.93 V within a half-wave potential of 0.8 V for ORR. Accordingly, when employed in the aqueous ZABs, Ni₃Fe-GA₁ achieved higher power density and superior reversibility than Pt/C−IrO₂ catalyst, making it a potential candidate for rechargeable ZABs.