Self‐Assemble and In Situ Formation of Ni1−xFexPS3 Nanomosaic‐Decorated MXene Hybrids for Overall Water Splitting

Herein, the authors present the development of novel 0D–2D nanohybrids consisting of a nickel‐based bimetal phosphorus trisulfide (Ni_1?xFe_xPS₃) nanomosaic that decorates on the surface of MXene nanosheets (denoted as NFPS@MXene). The nanohybrids are obtained through a facile self‐assemble process of transition metal layered double hydroxide (TMLDH) on MXene surface; followed by a low temperature in situ solid‐state reaction step. By tuning the Ni:Fe ratio, the as‐synthesized NFPS@MXene nanohybrids exhibit excellent activities when tested as electrocatalysts for overall water splitting. Particularly, with the initial Ni:Fe ratio of 7:3, the obtained Ni_0.7Fe_0.3PS₃@MXene nanohybrid reveals low overpotential (282 mV) and Tafel slope (36.5 mV dec^?1) for oxygen evolution reaction (OER) in 1 m KOH solution. Meanwhile, the Ni_0.9Fe_0.1PS₃@MXene shows low overpotential (196 mV) for the hydrogen evolution reaction (HER) in 1 m KOH solution. When integrated for overall water splitting, the Ni_0.7Fe_0.3PS₃@MXene || Ni_0.9Fe_0.1PS₃@MXene couple shows a low onset potential of 1.42 V and needs only 1.65 V to reach a current density of 10 mA cm^?2, which is better than the all noble metal IrO₂ || Pt/C electrocatalyst (1.71 mV@10 mA cm^?2). Given the chemical versatility of Ni_1?xFe_xPS₃ and the convenient self‐assemble process, the nanohybrids demonstrated in this work are promising for energy conversion applications.     

Interface Engineering of Hierarchical Branched Mo‐Doped Ni3S2/NixPy Hollow Heterostructure Nanorods for Efficient Overall Water Splitting

Rational design and construction of bifunctional electrocatalysts with excellent activity and durability is imperative for water splitting. Herein, a novel top‐down strategy to realize a hierarchical branched Mo‐doped sulfide/phosphide heterostructure (Mo‐Ni₃S₂/Ni_xP_y hollow nanorods), by partially phosphating Mo‐Ni₃S₂/NF flower clusters, is proposed. Benefitting from the optimized electronic structure configuration, hierarchical branched hollow nanorod structure, and abundant heterogeneous interfaces, the as‐obtained multisite Mo‐Ni₃S₂/Ni_xP_y/NF electrode has remarkable stability and bifunctional electrocatalytic activity in the hydrogen evolution reaction (HER)/oxygen evolution reaction (OER) in 1 m KOH solutions. It possesses an extremely low overpotential of 238 mV at the current density of 50 mA cm^?2 for OER. Importantly, when assembled as anode and cathode simultaneously, it merely requires an ultralow cell voltage of 1.46 V to achieve the current density of 10 mA cm^?2, with excellent durability for over 72 h, outperforming most of the reported Ni‐based bifunctional materials. Density functional theory results further confirm that the doped heterostructure can synergistically optimize Gibbs free energies of H and O‐containing intermediates (OH*, O*, and OOH*) during HER and OER processes, thus accelerating the catalytic kinetics of electrochemical water splitting. This work demonstrates the importance of the rational combination of metal doping and interface engineering for advanced catalytic materials.     

Ultrathin High Surface Area Nickel Boride (NixB) Nanosheets as Highly Efficient Electrocatalyst for Oxygen Evolution

The overriding obstacle to mass production of hydrogen from water as the premium fuel for powering our planet is the frustratingly slow kinetics of the oxygen evolution reaction (OER). Additionally, inadequate understanding of the key barriers of the OER is a hindrance to insightful design of advanced OER catalysts. This study presents ultrathin amorphous high‐surface area nickel boride (Ni_x B) nanosheets as a low‐cost, very efficient and stable catalyst for the OER for electrochemical water splitting. The catalyst affords 10 mA cm^?2 at 0.38 V overpotential during OER in 1.0 m KOH, reducing to only 0.28 V at 20 mA cm^?2 when supported on nickel foam, which ranks it among the best reported nonprecious catalysts for oxygen evolution. Operando X‐ray absorption fine‐structure spectroscopy measurements reveal prevalence of NiOOH, as well as Ni‐B under OER conditions, owing to a Ni‐B core@nickel oxyhydroxide shell (Ni‐B@NiO_x H) structure, and increase in disorder of the NiO_x H layer, thus revealing important insight into the transient states of the catalyst during oxygen evolution.     

Tin Oxide as a Protective Heterojunction with Silicon for Efficient Photoelectrochemical Water Oxidation in Strongly Acidic or Alkaline Electrolytes

Photoelectrodes without a p–n junction are often limited in efficiency by charge recombination at semiconductor surfaces and slow charge transfer to electrocatalysts. This study reports that tin oxide (SnO_x) layers applied to n‐Si wafers after forming a thin chemically oxidized SiO_x layer can passivate the Si surface while producing ≈620 mV photovoltage under 100 mW cm^?2 of simulated sunlight. The SnO_x layer makes ohmic contacts to Ni, Ir, or Pt films that act as precatalysts for the oxygen‐evolution reaction (OER) in 1.0 m KOH(aq) or 1.0 m H₂SO₄(aq). Ideal regenerative solar‐to‐O₂(g) efficiencies of 4.1% and 3.7%, respectively, are obtained in 1.0 m KOH(aq) with Ni or in 1.0 m H₂SO₄(aq) with Pt/IrO_x layers as OER catalysts. Stable photocurrents for >100 h are obtained for electrodes with patterned catalyst layers in both 1.0 m KOH(aq) and 1.0 m H₂SO₄(aq).     

One‐Step Controllable Synthesis of Catalytic Ni4Mo/MoOx/Cu Nanointerfaces for Highly Efficient Water Reduction

Currently, in addition to the electroactive non‐noble metal water‐splitting electrocatalysts, a scalable synthetic route and simple activity enhancement strategy is also urgently needed. In particular, the well‐controlled synthesis of the well‐recognized metal–metal nanointer face in a single step remains a key challenge. Here, the synthesis of Cu‐supported Ni₄Mo nanodots on MoO_x nanosheets (Ni₄Mo/MoO_x) with controllable Ni₄Mo particle size and d‐band structure is reported via a facile one‐step electrodeposition process. Density functional theory (DFT) calculations reveal that the active open‐shell effect from Ni‐3d‐band optimizes the electronic configuration. The Cu‐substrate enables the surface Ni–Mo alloy dots to be more electron‐rich, forming a local connected electron‐rich network, which boosts the charge transfer for effective binding of O‐related species and proton–electron charge exchange in the hydrogen evolution reaction. The Cu‐supported Ni₄Mo/MoO_x shows an ultralow overpotential of 16 mV at a current density of 10 mA cm^?2 in 1 m KOH, demonstrating the smallest overpotential, at loadings as low as 0.27 mg cm^?2, among all non‐noble metal catalysts reported to date. Moreover, an overpotential of 105 mV allows it to achieve a current density of 250 mA cm^?2 in 70 °C 30% KOH, a remarkable performance for alkaline hydrogen evolution with competitive potential for applications.     

Selectively Etching Vanadium Oxide to Modulate Surface Vacancies of Unary Metal–Based Electrocatalysts for High‐Performance Water Oxidation

Electrocatalytic water splitting for hydrogen generation is hindered by the sluggish kinetics of water oxidation, and highly efficient electrocatalysts for the oxygen evolution reaction (OER) are urgently required. Numerous bi‐ and multimetal‐based, low‐cost, high‐performance OER electrocatalysts have been developed. However, unary metal–based high‐performance electrocatalysts are seldom reported. In the present study, Co₂(OH)₃Cl/vanadium oxide (VO_y) composites are synthesized, from which VO_y is completely etched out by a simple cyclic voltammetry treatment, which simultaneously transforms Co₂(OH)₃Cl in situ to ultrafine CoOOH. The selective removal of VO_y modulates the nature of the surface in the obtained CoOOH by creating surface oxygen vacancies (V_o), along with disordered grain boundaries. The best‐performing CoOOH with optimum V_o is found to be associated with a low overpotential of 282 mV at 10 mA cm^?2 catalytic current density on a simple glassy carbon electrode for OER. This facile protocol of selectively etching VO_y to modulate the nature of the surface is successfully applied to synthesize another Fe‐based electrocatalyst with high OER performance, thus establishing its utility for unary metal–based electrocatalyst synthesis.     

Ni‐Metalloid (B,Si, P,As, and Te) Alloys as Water Oxidation Electrocatalysts

Breakthroughs toward effective water‐splitting electrocatalysts for mass hydrogen production will necessitate material design strategies based on unexplored material chemistries. Herein, Ni‐metalloid (B, Si, P, As, Te) alloys are reported as an emergent class of highly promising electrocatalysts for the oxygen evolution reaction (OER) and insight is offered into the origin of activity enhancement on the premise of the surface electronic structure, the OER activation energy, influence of the guest metalloid elements on the lattice structure of the host metal (Ni), and surface‐oxidized metalloid oxoanions. The metalloids modify the lattice structure of Ni, causing changes in the nearest Ni–Ni interatomic distance (d_Ni–Ni). The activation energy E_a scales with d_Ni–Ni indicating an apparent dependence of the OER activity on lattice properties. During the OER, surface Ni atoms are oxidized to nickel oxyhydroxide, which is the active state of the catalyst, meanwhile, the surface metalloids are oxidized to the corresponding oxoanions that affect the interfacial electrode/electrolyte properties and hence the adsorption/desorption interaction energies of the reacting species.     

All‐In‐One Perovskite Catalyst: Smart Controls of Architecture and Composition toward Enhanced Oxygen/Hydrogen Evolution Reactions

A conventional water electrolyzer consists of two electrodes, each of which is embedded with a costly and rare electrocatalyst, typically IrO₂/C for oxygen evolution reaction (OER) and Pt/C for hydrogen evolution reaction (HER), respectively. HER and OER electrocatalysts usually require very different pH values to keep them stable and active. Thus, the development of earth‐abundant nonprecious metal catalysts for both HER and OER is of great importance to practical applications. This work reports the results of integrated water electrolysis using the hybrids of electrospun La_0.5(Ba_0.4Sr_0.4Ca_0.2)_0.5Co_0.8Fe_0.2O_3–δ (L‐0.5) perovskite nanorods attached to reduced graphene oxide (rGO) nanosheets as bifunctional electrodes. Via rationalizing the composition and morphology of L‐0.5/rGO nanohybrids, excellent catalytic performance and stability toward OER and HER are achieved in alkaline media. The operating voltage of integrated L‐0.5/rGO electrolyzer is tested to be 1.76 V at 50 mA cm^–2, which is close to that of the commercially available IrO₂/C‐Pt/C couple (1.76 V @ 50 mA cm^–2). Such a bifunctional electrocatalyst could be extended toward practical electrolysis use with low expanse and high efficiency. More generally, the protocol described here broadens our horizons in terms of the designs and the diverse functionalities of catalysts for use in various applications.     

Bi‐Based Metal‐Organic Framework Derived Leafy Bismuth Nanosheets for Carbon Dioxide Electroreduction

Electroreduction of carbon dioxide (CO₂) into high‐value and readily collectable liquid products is vital but remains a substantial challenge due to the lack of highly efficient and robust electrocatalysts. Herein, Bi‐based metal‐organic framework (CAU‐17) derived leafy bismuth nanosheets with a hybrid Bi/Bi? O interface (Bi NSs) is developed, which enables CO₂ reduction to formic acid (HCOOH) with high activity, selectivity, and stability. Specially, the flow cell configuration is employed to eliminate the diffusion effect of CO₂ molecules and simultaneously achieve considerable current density (200 mA cm^?2) for industrial application. The faradaic efficiency for transforming CO₂ to HCOOH can achieve over 85 or 90% in 1 m KHCO₃ or KOH for at least 10 h despite a current density that exceeds 200 mA cm^?2, outperforming most of the reported CO₂ electroreduction catalysts. The hybrid Bi/Bi? O surface of leafy bismuth nanosheets boosts the adsorption of CO₂ and protects the surface structure of the as‐prepared leafy bismuth nanosheets, which benefits its activity and stability for CO₂ electroreduction. This work shows that modifying electrocatalysts by surface oxygen groups is a promising pathway to regulate the activity and stability for selective CO₂ reduction to HCOOH.     

Single‐Crystalline Ultrathin Co3O4 Nanosheets with Massive Vacancy Defects for Enhanced Electrocatalysis

The role of vacancy defects is demonstrated to be positive in various energy‐related processes. However, introducing vacancy defects into single‐crystalline nanostructures with given facets and studying their defect effect on electrocatalytic properties remains a great challenge. Here this study deliberately introduces oxygen defects into single‐crystalline ultrathin Co₃O₄ nanosheets with O‐terminated {111} facets by mild solvothermal reduction using ethylene glycol under alkaline condition. As‐prepared defect‐rich Co₃O₄ nanosheets show a low overpotential of 220 mV with a small Tafel slope of 49.1 mV dec^?1 for the oxygen evolution reaction (OER), which is among the best Co‐based OER catalysts to date and even more active than the state‐of‐the‐art IrO₂ catalyst. Such vacancy defects are formed by balancing with reducing environments under solvothermal conditions, but are surprisingly stable even after 1000 cycles of scanning under OER working conditions. Density functional theory plus U calculation attributes the enhanced performance to the oxygen vacancies and consequently exposed second‐layered Co metal sites, which leads to the lowered OER activation energy of 2.26 eV and improved electrical conductivity. This mild solvothermal reduction concept opens a new door for the understanding and future designing of advanced defect‐based electrocatalysts.     

A Simple Synthetic Strategy toward Defect‐Rich Porous Monolayer NiFe‐Layered Double Hydroxide Nanosheets for Efficient Electrocatalytic Water Oxidation

In this work, porous monolayer nickel‐iron layered double hydroxide (PM‐LDH) nanosheets with a lateral size of ≈30 nm and a thickness of ≈0.8 nm are successfully synthesized by a facile one‐step strategy. Briefly, an aqueous solution containing Ni²⁺ and Fe³⁺ is added dropwise to an aqueous formamide solution at 80 °C and pH 10, with the PM‐LDH product formed within only 10 min. This fast synthetic strategy introduces an abundance of pores in the monolayer NiFe‐LDH nanosheets, resulting in PM‐LDH containing high concentration of oxygen and cation vacancies, as is confirmed by extended X‐ray absorption fine structure and electron spin resonance measurements. The oxygen and cation vacancies in PM‐LDH act synergistically to increase the electropositivity of the LDH nanosheets, while also enhancing H₂O adsorption and bonding strength of the OH* intermediate formed during water electrooxidation, endowing PM‐LDH with outstanding performance for the oxygen evolution reaction (OER). PM‐LDH offers a very low overpotential (230 mV) for OER at a current density of 10 mA cm^?2, with a Tafel slope of only 47 mV dec^?1, representing one of the best OER performance yet reported for a NiFe‐LDH system. The results encourage the wider utilization of porous monolayer LDH nanosheets in electrocatalysis, catalysis, and solar cells.     

Realizing Ultrafast Oxygen Evolution by Introducing Proton Acceptor into Perovskites

The oxygen evolution reaction (OER) is of prime importance in multiple energy storage devices. Perovskite oxides involving lattice‐oxygen oxidation are generally regarded as highly active OER catalysts, but the deprotonation of surface‐bound intermediates limit the further activity improvement. Here, it is shown that this kinetic limitation can be removed by introducing Sr₃B₂O₆ (SB) which activates a proton‐acceptor functionality to boost OER activity. As a proof‐of‐concept example, an experimental validation is conducted on the extraordinary OER performance of a Sr(Co_0.8Fe_0.2)_0.7B_0.3O_3?δ (SCFB‐0.3) hybrid catalyst, made using Sr_0.8Co_0.8Fe_0.2O_3?δ as active component and SB as a proton acceptor. This smart hybrid exhibits an exceptionally ultrahigh OER activity with an extremely low overpotential of 340 mV in 0.1 m KOH and 240 mV in 1 m KOH required for 10 mA cm^?2 which is the top‐level catalytic activity among metal oxides reported so far, while maintaining excellent durability. The correlation of pH and activity study reveals that this enhanced activity mainly originates from the improved interfacial proton transfer. Such a strategy further demonstrated to be universal, which can be applied to enhance the OER activity of other high covalent oxides with close O 2p‐band centers relative to Fermi energy.     

High‐Performance Electrocatalytic Conversion of N2 to NH3 Using Oxygen‐Vacancy‐Rich TiO2 In Situ Grown on Ti3C2Tx MXene

To achieve the energy‐effective ammonia (NH₃) production via the ambient‐condition electrochemical N₂ reduction reaction (NRR), it is vital to ingeniously design an efficient electrocatalyst assembling the features of abundant surface deficiency, good dispersibility, high conductivity, and large surface specific area (SSA) via a simple way. Inspired by the fact that the MXene contains thermodynamically metastable marginal transition metal atoms, the oxygen‐vacancy‐rich TiO₂ nanoparticles (NPs) in situ grown on the Ti₃C₂T_x nanosheets (TiO₂/Ti₃C₂T_x) are prepared via a one‐step ethanol‐thermal treatment of the Ti₃C₂T_x MXene. The oxygen vacancies act as the main active sites for the NH₃ synthesis. The highly conductive interior untreated Ti₃C₂T_x nanosheets could not only facilitate the electron transport but also avoid the self‐aggregation of the TiO₂ NPs. Meanwhile, the TiO₂ NPs generation could enhance the SSA of the Ti₃C₂T_x in return. Accordingly, the as‐prepared electrocatalyst exhibits an NH₃ yield of 32.17 µg h^?1 mg^?1_cat. at ?0.55 V versus reversible hydrogen electrode (RHE) and a remarkable Faradaic efficiency of 16.07% at ?0.45 V versus RHE in 0.1 m HCl, placing it as one of the most promising NRR electrocatalysts. Moreover, the density functional theory calculations confirm the lowest NRR energy barrier (0.40 eV) of TiO₂ (101)/Ti₃C₂T_x compared with Ti₃C₂T_x or TiO₂ (101) alone.     

Enhancing Catalytic Activity of Titanium Oxide in Lithium–Sulfur Batteries by Band Engineering

The altering of electronic states of metal oxides offers a promising opportunity to realize high‐efficiency surface catalysis, which play a key role in regulating polysulfides (PS) redox in lithium–sulfur (Li–S) batteries. However, little effort has been devoted to understanding the relationship between the electronic state of metal oxides and a catalyst's properties in Li–S cells. Herein, defect‐rich heterojunction electrocatalysts composed of ultrathin TiO_2‐x nanosheets and carbon nanotubes (CNTs) for Li–S batteries are reported. Theoretical simulations indicate that oxygen vacancies and heterojunction can enhance electronic conductivity and chemical adsorption. Spectroscopy and electrochemical techniques further indicate that the rich surface vacancies in TiO_2‐x nanosheets result in highly activated trapping sites for LiPS and lower energy barriers for fast Li ion mobility. Meanwhile, the redistribution of electrons at the heterojunction interfaces realizes accelerated surface electron exchange. Coupled with a polyacrylate terpolymer (LA132) binder, the CNT@TiO_2‐x–S electrodes exhibit a long cycle life of more than 300 cycles at 1 C and a high area capacity of 5.4 mAh cm^?2. This work offers a new perspective on understanding catalyst design in energy storage devices through band engineering.     

Ni3+‐Induced Formation of Active NiOOH on the Spinel Ni–Co Oxide Surface for Efficient Oxygen Evolution Reaction

Efficient and earth abundant electrocatalysts for high‐performance oxygen evolution reaction (OER) are essential for the development of sustainable energy conversion technologies. Here, a new hierarchical Ni–Co oxide nanostructure, composed of small secondary nanosheets grown on primary nanosheet arrays, is synthesized via a topotactic transformation of Ni–Co layered double hydroxide. The Ni³⁺‐rich surface benefits the formation of NiOOH, which is the main redox site as revealed via in situ X‐ray absorption near edge structure and extended X‐ray absorption fine structure spectroscopy. The Ni–Co oxide hierarchical nanosheets (NCO–HNSs) deliver a stable current density of 10 mA cm^?2 at an overpotential of ≈0.34 V for OER with a Tafel slope of as low as 51 mV dec^?1 in alkaline media. The improvement in the OER activity can be ascribed to the synergy of large surface area offered by the 3D hierarchical nanostructure and the facile formation of NiOOH as the main active sites on the surface of NCO–HNSs to decrease the overpotential and facilitate the catalytic reaction.     

Novel Route to Fe‐Based Cathode as an Efficient Bifunctional Catalysts for Rechargeable Zn–Air Battery

Efficient bifunctional oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalysts are of great importance for rechargeable metal–air batteries. Herein, FeN_x/C catalysts are synthesized by pyrolysis of thiourea and agarose containing α‐Fe₂O₃ nanoplate as Fe precursor, where α‐Fe₂O₃ nanoplate can prevent the aggregation of carbon sheets to effectively improve the specific surface area during the carbonization process. The FeN_x/C‐700‐20 catalyst displays excellent catalytic performance for both ORR and OER activity in alkaline conditions with more positive onset potential (1.1 V vs the reversible hydrogen electrode) and half‐wave potential, higher stability, and stronger methanol tolerance in alkaline solution, which are all superior to that of the commercial Pt/C catalyst. In this study, the detailed analyses demonstrate that the coexistence of Fe‐based species and high content of Fe‐N_x both play an important role for the catalytic activity. Furthermore, FeN_x/C‐700‐20 as cathode catalyst in Zn–air battery possesses higher charge–discharge stability and power density compared with that of commercial Pt/C catalyst, displaying great potential in practical implementation of for the rechargeable energy devices.     

Unraveling Geometrical Site Confinement in Highly Efficient Iron‐Doped Electrocatalysts toward Oxygen Evolution Reaction

Introduction of iron in various catalytic systems has served a crucial function to significantly enhance the catalytic activity toward oxygen evolution reaction (OER), but the relationship between material properties and catalysis is still elusive. In this study, by regulating the distinctive geometric sites in spinel, Fe occupies the octahedral sites (Fe³⁺_(Oh)) and confines Co to the tetrahedral site (Co²⁺_(Td)), resulting in a strikingly high activity (η_{j = 10 mA cm}^?2 = 229 mV and η_{j = 100 mA cm}^?2 = 281 mV). Further enrichment of Fe ions would occupy the tetrahedral sites to decline the amount of Co²⁺_(Td) and deteriorate the OER activity. It is also found that similar tafel slope and peak frequency in Bode plot of electrochemical impedance spectroscopy indicate that Co²⁺_(Td) ions are primarily in charge of water oxidation catalytic center. By means of electrochemical techniques and in situ X‐ray absorption spectroscopy, it is proposed that Fe³⁺_(Oh) ions mainly confine cobalt ions to the tetrahedral site to restrain the multipath transfer of cobalt ions during the dynamic structural transformation between spinel and oxyhydroxide, continuously activating the catalytic behavior of Co²⁺_(Td) ions. This material‐related insight provides an indication for the design of highly efficient OER electrocatalysts.     

Unveiling Chemically Robust Bimetallic Squarate-Based Metal–Organic Frameworks for Electrocatalytic Oxygen Evolution Reaction

Here, this work reports an innovative strategy for the synthesis of chemically robust metal–organic frameworks (MOFs), and applies them as catalysts for the electrocatalytic oxygen evolution reaction (OER). A bimetallic squarate-based MOF (Sq-MOF) with a zbr topology serves as an excellent platform for electrocatalytic OER owing to its open porous structure, high affinity toward water, and presence of catalytically active 1D metal hydroxide strips. By regulating the Ni²⁺ content in a bimetallic squarate MOF system, the electrochemical structural stability toward OER can be improved. The screening of various metal ratios demonstrates that Ni₃Fe₁ and Ni₂Fe₁ Sq- zbr -MOFs show the best performance for electrocatalytic OER in terms of catalytic activity and structural stability. Ni₂Fe₁ Sq- zbr -MOF shows a low overpotential of 230 mV (at 10 mA cm⁻²) and a small Tafel slope of 37.7 mV dec⁻¹, with an excellent long-term electrochemical stability for the OER. Remarkably, these overpotential values of Ni₂Fe₁ Sq- zbr -MOF are comparable with those of the best-performing layered double hydroxide (LDH) systems and outperforms the commercially available noble-metal-based RuO₂ catalyst for OER under identical operational conditions.     

Insights into Ionic Transport and Structural Changes in Magnetite during Multiple‐Electron Transfer Reactions

Metal oxides, such as Fe₃O₄, hold promise for future battery applications due to their abundance, low cost, and opportunity for high lithium storage capacity. In order to better understand the mechanisms of multiple‐electron transfer reactions leading to high capacity in Fe₃O₄, a comprehensive investigation on local ionic transport and ordering is made by probing site occupancies of anions (O^2?) and cations (Li⁺, Fe³⁺/Fe²⁺) using multiple synchrotron X‐ray and electron‐beam techniques, in combination with ab‐initio calculations. Results from this study provide the first experimental evidence that the cubic‐close‐packed (ccp) O‐anion array in Fe₃O₄ is sustained throughout the lithiation and delithiation processes, thereby enabling multiple lithium intercalation and conversion reactions. Cation displacement/reordering occurs within the ccp O‐anion framework, which leads to a series of phase transformations, starting from the inverse spinel phase and turning into intermediate rock‐salt‐like phases (Li_xFe₃O₄; 0 < x < 2), then into a cation‐segregated phase (Li₂O?FeO), and finally converting into metallic Fe and Li₂O. Subsequent delithiation and lithiation processes involve interconversion between metallic Fe and FeO‐like phases. These results may offer new insights into the structure‐determined ionic transport and electrochemical reactions in metal oxides, and those of other compounds sharing a ccp anion framework, reminiscent of magnetite.     

Multiphase Nanostructure of a Quinary Metal Oxide Electrocatalyst Reveals a New Direction for OER Electrocatalyst Design

Ce‐rich mixed metal oxides comprise a recently discovered class of ­electrocatalysts for the oxygen evolution reaction (OER). In particular, at current densities below 10 mA cm^?2, Ni_0.3Fe_0.07Co_0.2Ce_0.43O_x exhibits ­superior activity compared to the corresponding transition metal oxides, despite the relative inactivity of ceria. To elucidate the enhanced activity and underlying catalytic mechanism, detailed structural characterization of this quinary oxide electrocatalyst is reported. Transmission electron microscopy imaging of cross‐section films as‐prepared and after electrochemical testing reveals a stable two‐phase nanostructure composed of 3–5 nm diameter crystallites of fluorite CeO₂ intimately mixed with 3–5 nm crystallites of transition metal oxides alloyed in the rock salt NiO structure. Dosing experiments demonstrate that an electron flux greater than ≈1000 e Å^?2 s^?1 causes the inherently crystalline material to become amorphous. A very low dose rate of 130 e Å^?2 s^?1 is employed for atomic resolution imaging using inline holography techniques to reveal a nanostructure in which the transition metal oxide nanocrystals form atomically sharp boundaries with the ceria nanocrystals, and these results are corroborated with extensive synchrotron X‐ray absorption spectroscopy measurements. Ceria is a well‐studied cocatalyst for other heterogeneous and electrochemical reactions, and our discovery introduces biphasic cocatalysis as a design concept for improved OER electrocatalysts.