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.     

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.     

Highly Active Cobalt‐Based Electrocatalysts with Facile Incorporation of Dopants for the Oxygen Evolution Reaction

In situ formation of electroactive cobalt species for the oxygen evolution reaction is simply achieved by applying an anodic bias to a commercially available cobalt precursor and Nafion binder mixture coated on a glassy carbon electrode. This preparation does not require energy‐intensive materials preparation steps or noble metals, yet a low overpotential of 322 mV at 10.2 mA cm^?2 and a high current density of more than 300 mA cm^?2 at 1.7 V_NHE were obtained in 1 m KOH. An operando electrochemical Raman spectroscopy study confirmed the formation of cobalt oxyhydroxide species and the iron stimulated the equilibrium state between Co³⁺ and Co⁴⁺. The iron present in the alkali electrolyte or ink solution effectively activated the cobalt species, and most of the first row transition metals could also enhance the catalytic performance. The concept presented here is one of the simplest strategies for preparing highly active electrocatalysts and is very flexible for the replacement of cobalt by other transition metals.     

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.     

Sodium Cobalt Metaphosphate as an Efficient Oxygen Evolution Reaction Catalyst in Alkaline Solution

Sodium cobalt metaphosphate [NaCo(PO₃)₃] has CoO octahedra (CoO₆) and shows superior oxygen evolution reaction (OER) activity in alkaline solution, comparable with the state‐of‐the‐art precious‐metal RuO₂ catalyst. OER catalysts of this metaphosphate are prepared by combustion (Cb) and solid‐state (SS) methods. The combustion‐assisted method offers a facile synthesis and one‐step carbon composite formation. Unusually high catalytic activity was observed in NCoM‐Cb‐Ar and could be due to chemical coupling effects between NaCo(PO₃)₃ and partially graphitized carbon. This novel electrocatalyst exhibits very small overpotential of 340 mV with high mass activity of 532 A g^?1. Good charge transfer abilities and chemical coupling between NaCo(PO₃)₃ and amorphous carbon gives the OER activity in NCoM‐Cb‐Ar.     

In Situ Activating Ubiquitous Rust towards Low‐Cost,Efficient, Free‐Standing,and Recoverable Oxygen Evolution Electrodes

Developing effective ways to recycle rusted stainless steel and to promote the sluggish oxygen evolution reaction (OER), associated with water splitting and metal–air batteries, is important for a resource‐sustainable and environment‐friendly society. Herein, we propose a strategy to enable rusted stainless steel plate to be used as an abundant and low‐cost OER catalyst, wherein a hydrothermal combined in situ electrochemical oxidation–reduction cycle (EORC) method is developed to mimic and expedite the corrosion process, and thus activate stainless steel into free‐standing OER electrodes. Benefiting from the plentiful electrolyte‐accessible Fe/(Ni) oxyhydroxides, high conductivity and mechanical stability, this electrode exhibits remarkable OER performances including low overpotential, fast kinetics, and long‐term durability. The slight degradation in current after long‐term use can be repaired immediately in situ by an EORC.     

Oxygen Isotope Labeling Experiments Reveal Different Reaction Sites for the Oxygen Evolution Reaction on Nickel and Nickel Iron Oxides

Nickel iron oxide is considered a benchmark nonprecious catalyst for the oxygen evolution reaction (OER). However, the nature of the active site in nickel iron oxide is heavily debated. Here we report direct spectroscopic evidence for the different active sites in Fe‐free and Fe‐containing Ni oxides. Ultrathin layered double hydroxides (LDHs) were used as defined samples of metal oxide catalysts, and ¹⁸O‐labeling experiments in combination with in situ Raman spectroscopy were employed to probe the role of lattice oxygen as well as an active oxygen species, NiOO^?, in the catalysts. Our data show that lattice oxygen is involved in the OER for Ni and NiCo LDHs, but not for NiFe and NiCoFe LDHs. Moreover, NiOO^? is a precursor to oxygen for Ni and NiCo LDHs, but not for NiFe and NiCoFe LDHs. These data indicate that bulk Ni sites in Ni and NiCo oxides are active and evolve oxygen via a NiOO^? precursor. Fe incorporation not only dramatically increases the activity, but also changes the nature of the active sites.     

A Simple Method for Synthesizing Highly Active Amorphous Iridium Oxide for Oxygen Evolution under Acidic Conditions

Water splitting for hydrogen production has been recognized as a promising approach to store sustainable energy. The performance of this method is limited by the oxygen-evolution reaction. Herein, an approach for synthesizing a highly active oxygen-evolving catalyst by a one-step, low-cost, environmentally friendly, and easy-to-perform method is presented, which works by using iridium metal as the anode at a relatively high potential. The obtained IrO_x/Ir interface showed an overpotential of 250 mV at 10 mA cm⁻² in 0.1 m HClO₄ and remained stable under electrochemical conditions. The IrO_x that was mechanically separated from the surface of IrO_x/Ir metal after operation showed a threefold increase in activity compared to the current benchmark IrO₂ catalyst. Various characterization analyses were used to identify the structure and morphology of the catalyst, which suggested nanosized, porous, and amorphous IrO_x on the surface of metallic Ir. This synthetic approach can inspire a variety of opportunities to design and synthesize efficient metal oxide-based electrocatalysts for sustainable energy conversion and utilization.     

Trimetallic Oxyhydroxide Coralloids for Efficient Oxygen Evolution Electrocatalysis

Trimetallic oxyhydroxides are one of the most effective materials for oxygen evolution reaction (OER) catalysis, a key process for water splitting. Herein, we describe a facile wet‐chemical method to directly grow a series of coralloid trimetallic oxyhydroxides on arbitrary substrates such as nickel foam (NF) and carbon nanotubes (CNTs). The amount of iron in these oxyhydroxide sponges on NF and CNTs was precisely controlled, revealing that the electrocatalytic activity of the WCoFe trimetallic oxyhydroxides depends on the Fe amount in a volcano‐like fashion. The optimized W_0.5Co_0.4Fe_0.1/NF catalyst exhibited an overpotential of only 310 mV to deliver a large current density of 100 mA cm⁻² and a very low Tafel slope of 32 mV dec⁻¹. It also showed superior stability with negligible activity decay after use in the OER for 21 days (>500 h). X‐ray photoelectron spectroscopy revealed that the addition of Fe leads to an on average lower Co oxidation state, which contributes to the enhanced OER performance.     

HfN Nanoparticles: An Unexplored Catalyst for the Electrocatalytic Oxygen Evolution Reaction

Water electrolysis is one of the most promising methods to produce H₂ and O₂ as high potential fuels. Comparing the two half‐reactions, the oxygen evolution reaction (OER) is the more difficult to be optimized and still relies on expensive noble metal‐based catalysts such as Ru or Ir. In this paper, we prepared nanoparticles of HfN and Hf₂ON₂ and tested them for the OER for the first time. The HfN sample, in particular, showed the highest activity, requiring an overpotential of only 358 mV at 10 mA cm^?2 in Fe‐free electrolyte and, above all, exhibiting long‐term stability. This result places this system amongst one of the most promising catalysts for OER tested to date, in terms of sustainability, activity and stability. The prepared nanoparticles are small (less than 15 nm in diameter), well‐defined in shape and crystalline, and were characterised before and after electrochemical testing also via electron microscopy (EM), powder X‐ray diffraction (PXRD) and X‐ray photoelectron spectroscopy (XPS).     

NiFe Alloy Nanoparticles with hcp Crystal Structure Stimulate Superior Oxygen Evolution Reaction Electrocatalytic Activity

Tuning the crystal phase of metal alloy nanomaterials has been proved a significant way to alter their catalytic properties based on crystal structure and electronic property. Herein, we successfully developed a simple strategy to controllably synthesize a rare crystal structure of hexagonal close‐packed (hcp) NiFe nanoparticle (NP) encapsulated in a N‐doped carbon (NC) shell (hcp‐NiFe@NC). Then, we systemically investigated the oxygen evolution reaction (OER) performance of the samples under alkaline conditions, in which the hcp‐NiFe@NC exhibits superior OER activity compared to the conventional face‐centered cubic (fcc) NiFe encapsulated in a N‐doped carbon shell (fcc‐NiFe@NC). At the current densities of 10 and 100 mA cm^?2, the hcp‐NiFe@NC with Fe/Ni ratio of ≈5.4 % only needs ultralow overpotentials of 226 mV and 263 mV versus reversible hydrogen electrode in 1.0 m KOH electrolyte, respectively, which were extremely lower than those of fcc‐NiFe@NC and most of other reported NiFe‐based electrocatalysts. We proposed that hcp‐NiFe possesses favorable electronic property to expedite the reaction on the NC surface, resulting higher catalytic activity for OER. This research provides a new insight to design more efficient electrocatalysts by considering the crystal phase correlated electronic property.     

Activating CoOOH Porous Nanosheet Arrays by Partial Iron Substitution for Efficient Oxygen Evolution Reaction

Iron‐substituted CoOOH porous nanosheet arrays grown on carbon fiber cloth (denoted as Fe_xCo_1?xOOH PNSAs/CFC, 0≤x≤0.33) with 3D hierarchical structures are synthesized by in situ anodic oxidation of α‐Co(OH)₂ NSAs/CFC in solution of 0.01 m (NH₄)₂Fe(SO₄)₂. X‐ray absorption fine spectra (XAFS) demonstrate that CoO₆ octahedral structure in CoOOH can be partially substituted by FeO₆ octahedrons during the transformation from α‐Co(OH)₂ to Fe_xCo_1?xOOH, and this is confirmed for the first time in this study. The content of Fe in Fe_xCo_1?xOOH, no more than 1/3 of Co, can be controlled by adjusting the in situ anodic oxidation time. Fe_0.33Co_0.67OOH PNSAs/CFC shows superior OER electrocatalytic performance, with a low overpotential of 266 mV at 10 mA cm^?2, small Tafel slope of 30 mV dec^?1, and high durability.     

Evaluation of Perovskites as Electrocatalysts for the Oxygen Evolution Reaction

The oxygen evolution reaction (OER) is an enabling process for technologies in the area of energy conversion and storage, but its slow kinetics limits its efficiency. We performed an electrochemical evaluation of 14 different perovskites of variable composition and stoichiometry as OER electrocatalysts in alkaline media. We particularly focused on improved methods for a reliable comparison of catalyst activity. From initial electrochemical results we selected the most active samples for further optimization of electrode preparation and testing. An inverted cell configuration facilitated gas bubble detachment and thus minimized blockage of the active surface area. We describe parameters, such as the presence of specific cations, stoichiometry, and conductivity, that are important for obtaining electroactive perovskites for OER. Conductive additives enhanced the current and decreased the apparent overpotential of OER for one of the most active samples (La_0.58Sr_0.4Fe_0.8Co_0.2O₃).     

Construction of a sp3/sp2 Carbon Interface in 3D N‐Doped Nanocarbons for the Oxygen Reduction Reaction

The development of highly efficient metal‐free carbon electrocatalysts for the oxygen reduction reaction (ORR) is one very promising strategy for the exploitation and commercialization of renewable and clean energy, but this still remains a significant challenge. Herein, we demonstrate a facile approach to prepare three‐dimensional (3D) N‐doped carbon with a sp³/sp² carbon interface derived from ionic liquids via a simple pyrolysis process. The tunable hybrid sp³ and sp² carbon composition and pore structures stem from the transformation of ionic liquids to polymerized organics and introduction of a Co metal salt. Through tuning both composition and pores, the 3D N‐doped nanocarbon with a high sp³/sp² carbon ratio on the surface exhibits a superior electrocatalytic performance for the ORR compared to that of the commercial Pt/C in Zn–air batteries. Density functional theory calculations suggest that the improved ORR performance can be ascribed to the existence of N dopants at the sp³/sp² carbon interface, which can lower the theoretical overpotential of the ORR.     

A Dendritic Nanostructured Copper Oxide Electrocatalyst for the Oxygen Evolution Reaction

To use water as the source of electrons for proton or CO₂ reduction within electrocatalytic devices, catalysts are required for facilitating the proton‐coupled multi‐electron oxygen evolution reaction (OER, 2 H₂O→O₂+4 H⁺+4 e⁻). These catalysts, ideally based on cheap and earth abundant metals, have to display high activity at low overpotential and good stability and selectivity. While numerous examples of Co, Mn, and Ni catalysts were recently reported for water oxidation, only few examples were reported using copper, despite promising efficiencies. A rationally designed nanostructured copper/copper oxide electrocatalyst for OER is presented. This material derives from conductive copper foam passivated by a copper oxide layer and further nanostructured by electrodeposition of CuO nanoparticles. The generated electrodes are highly efficient for catalyzing selective water oxidation to dioxygen with an overpotential of 290 mV at 10 mA cm⁻² in 1 m NaOH solution.     

Stable Hierarchical Bimetal–Organic Nanostructures as HighPerformance Electrocatalysts for the Oxygen Evolution Reaction

The integration of heterometallic units and nanostructures into metal–organic frameworks (MOFs) used for the oxygen evolution reaction (OER) can enhance the electrocatalytic performance and help elucidate underlying mechanisms. We have synthesized a series of stable MOFs (CTGU‐10a1–d1) based on trinuclear metal carboxylate clusters and a hexadentate carboxylate ligand with a (6,6)‐connected nia net. We also present a strategy to synthesize hierarchical bimetallic MOF nanostructures (CTGU‐10a2–d2). Among these, CTGU‐10c2 is the best material for the OER, with an overpotential of 240 mV at a current density of 10 mA cm^?2 and a Tafel slope of 58 mV dec^?1. This is superior to RuO₂ and confirms CTGU‐10c2 as one of the few known high‐performing pure‐phase MOF‐OER electrocatalysts. Notably, bimetallic CTGU‐10b2 and c2 show an improved OER activity over monometallic CTGU‐10a2 and d2. Both DFT and experiments show that the remarkable OER performance of CTGU‐10c2 is due to the presence of unsaturated metal sites, a hierarchical nanobelt architecture, and the Ni–Co coupling effect.     

Salen-Based Conjugated Microporous Polymers for Efficient Oxygen Evolution Reaction

Exploring high-performance electrocatalysts, especially non-noble metal electrocatalysts, for the oxygen evolution reaction (OER) is critical to energy storage and conversion. Herein, we report for the first time that conjugated microporous polymers (CMPs) incorporating salen can be used as OER electrocatalysts with outstanding performances. The best OER electrocatalyst (salen-CMP-Fe-3) exhibits a low Tafel slope of 63 mV dec⁻¹ and an overpotential of 238 mV at 10 mA cm⁻². DFT and Grand Canonical Monte Carlo calculations confirmed that the significantly improved electrocatalytic properties can be attributed to the intrinsic catalytic activity of the salen moiety and the enrichment effect of the pore structures. This work demonstrates that salen-based conjugated polymers are a type of metal-coordinated porous polymer that show excellent catalyst performance.