Combination of Highly Efficient Electrocatalytic Water Oxidation with Selective Oxygenation of Organic Substrates using Manganese Borophosphates

Unlocking Efficiency: Minimizing Energy Loss in Electrocatalysts for Water Splitting

Catalysts play a crucial role in water electrolysis by reducing the energy barriers for hydrogen and oxygen evolution reactions (HER and OER). Research aims to enhance the intrinsic activities of potential catalysts through material selection, microstructure design, and various engineering techniques. However, the energy consumption of catalysts has often been overlooked due to the intricate interplay among catalyst microstructure, dimensionality, catalyst-electrolyte-gas dynamics, surface chemistry, electron transport within electrodes, and electron transfer among electrode components. Efficient catalyst development for high-current-density applications is essential to meet the increasing demand for green hydrogen. This involves transforming catalysts with high intrinsic activities into electrodes capable of sustaining high current densities. This review focuses on current improvement strategies of mass exchange, charge transfer, and reducing electrode resistance to decrease energy consumption. It aims to bridge the gap between laboratory-developed, highly efficient catalysts and industrial applications regarding catalyst structural design, surface chemistry, and catalyst-electrode interplay, outlining the development roadmap of hierarchically structured electrode-based water electrolysis for minimizing energy loss in electrocatalysts for water splitting.

Structural and Electronic Factors behind the Electrochemical Stability of 3D-Metal Tungstates under Oxygen Evolution Reaction Conditions

Transition metal tungstates (TMTs) possess a wolframite-like lattice structure and preferably form via an electrostatic interaction between a divalent transition metal cation (MII) and an oxyanion of tungsten ([WO4]2-). A unit cell of a TMT is primarily composed of two repeating units, [MO6]oh and [WO6]oh, which are held together via several M-μ2-O-W bridging links. The bond character (ionic or covalent) of this bridging unit determines the stability of the lattice and influences the electronic structure of the bulk TMT materials. Recently, TMTs have been successfully employed as an electrode material for various applications, including electrochemical water splitting. Despite the wide electrocatalytic applications of TMTs, the study of the structure-activity correlation and electronic factors responsible for in situ structural evolution to electroactive species during electrochemical reactions is still in its infancy. Herein, a series of TMTs, MIIWVIO4 (M = Mn/Fe/Co/Ni), have been prepared and employed as electrocatalysts to study the oxygen evolution reaction (OER) under alkaline conditions and to scrutinize the role of transition metals in controlling the energetics of the formation of electroactive species. Since the [WO6]oh unit is common in the TMTs considered, the variation of the central atom of the corresponding [MO6]oh unit plays an intriguing role in controlling the electronic structure and stability of the lattice under anodic potential. Under the OER conditions, a potential-dependent structural transformation of MWO4 is noticed, where MnWO4 appears to be the most labile, whereas NiWO4 is stable up to a high anodic potential of ∼1.68 V (vs RHE). Potential-dependent hydrolytic [WO4]2- dissolution to form MOx active species, traced by in situ Raman and various spectro-/microscopic analyses, can directly be related to the electronic factors of the lattice, viz., crystal field splitting energy (CFSE) of MII in [MO6]oh, formation enthalpy (ΔHf), decomposition enthalpy (ΔHd), and Madelung factor associated with the MWO4 ionic lattice. Additionally, the magnitude of the Löwdin and Bader charges on M of the M-μ2-O-W bond is directly related to the degree of ionicity or covalency in the MWO4 lattice, which indirectly influences the electronic structure and activity. The experimental results substantiated by the computational study explain the electrochemical activity of the TMTs with the help of various structural and electronic factors and bonding interactions in the lattice, which has never been realized. Therefore, the study presented here can be taken as a general guideline to correlate the reactivity to the structure of the inorganic materials.

Understanding Advanced Transition Metal-Based Two Electron Oxygen Reduction Electrocatalysts from the Perspective of Phase Engineering

Non-noble transition metal (TM)-based compounds have recently become a focal point of extensive research interest as electrocatalysts for the two electron oxygen reduction (2e- ORR) process. To efficiently drive this reaction, these TM-based electrocatalysts must bear unique physiochemical properties, which are strongly dependent on their phase structures. Consequently, adopting engineering strategies toward the phase structure has emerged as a cutting-edge scientific pursuit, crucial for achieving high activity, selectivity, and stability in the electrocatalytic process. This comprehensive review addresses the intricate field of phase engineering applied to non-noble TM-based compounds for 2e- ORR. First, the connotation of phase engineering and fundamental concepts related to oxygen reduction kinetics and thermodynamics are succinctly elucidated. Subsequently, the focus shifts to a detailed discussion of various phase engineering approaches, including elemental doping, defect creation, heterostructure construction, coordination tuning, crystalline design, and polymorphic transformation to boost or revive the 2e- ORR performance (selectivity, activity, and stability) of TM-based catalysts, accompanied by an insightful exploration of the phase-performance correlation. Finally, the review proposes fresh perspectives on the current challenges and opportunities in this burgeoning field, together with several critical research directions for the future development of non-noble TM-based electrocatalysts.

Evolution of Mn-Bi2O3 from the Mn-doped Bi3O4Br electro(pre)catalyst during the oxygen evolution reaction

Mn-doped Bi3O4Br has been synthesized using a solvothermal route. The undoped Bi3O4Br and Mn-Bi3O4Br materials possess orthorhombic unit cells with two distinct Bi sites forming a layered atomic arrangement. The shift in the (020) plane in the powder X-ray diffraction (PXRD) pattern confirms Mn-doping in the Bi3O4Br lattice. Elemental mapping indicated 7% Mn doping in the Bi3O4Br lattice structure. A core-level X-ray photoelectron study (XPS) indicates the presence of BiIII and MnII valence-states in Mn-Bi3O4Br. Doping with a cation (MnII) containing a different charge and ionic radius resulted in vacancy/defects in Mn-Bi3O4Br which further altered its electronic structure by reducing the indirect band gap, beneficial for electron conduction and electrocatalysis. The irreversible MnII to MnIII transformation at a potential of 1.48 V (vs. RHE) precedes the electrochemical oxygen evolution reaction (OER). The Mn-doped electrocatalyst achieved 10 mA cm-2 current density at 337 mV overpotential, while the pristine Bi3O4Br required 385 mV overpotential to reach the same activity. The pronounced OER activity of the Mn-Bi3O4Br sample over the pristine Bi3O4Br highlights the necessity of MnII doping. The superior activity of the Mn-Bi3O4Br catalyst over that of Bi3O4Br is due to a low Tafel slope, better double-layer capacitance (Cdl), and small charge-transfer resistance (Rct). The chronoamperometry (CA) study depicts long-term stability for 12 h at 20 mA cm-2. An electrolyzer fabricated as Pt(-)/(+)Mn-Bi3O4Br can deliver 10 mA cm-2 at a cell potential of 2.05 V. The post-CA-OER analyses of the anode confirmed the leaching of [Br-] followed by in situ formation of Mn-doped Bi2O3 as the electrocatalytically active species. Herein, an ultra-low Mn-doping into Bi3O4Br leads to an improvement in the electrocatalytic performance of the inactive Bi3O4Br material.

Operando studies of Mn oxide based electrocatalysts for the oxygen evolution reaction

Inspired by photosystem II (PS II), Mn oxide based electrocatalysts have been repeatedly investigated as catalysts for the electrochemical oxygen evolution reaction (OER), the anodic reaction in water electrolysis. However, a comparison of the conditions in biological OER catalysed by the water splitting complex CaMn4Ox with the requirements for an electrocatalyst for industrially relevant applications reveals fundamental differences. Thus, a systematic development of artificial Mn-based OER catalysts requires both a fundamental understanding of the catalytic mechanisms as well as an evaluation of the practicality of the system for industrial scale applications. Experimentally, both aspects can be approached using in situ and operando methods including spectroscopy. This paper highlights some of the major challenges common to different operando investigation methods and recent insights gained with them. To this end, vibrational spectroscopy, especially Raman spectroscopy, absorption techniques in the bandgap region and operando X-ray spectroelectrochemistry (SEC), both in the hard and soft X-ray regime are particularly focused on here. Technical challenges specific to each method are discussed first, followed by challenges that are specific to Mn oxide based systems. Finally, recent in situ and operando studies are reviewed. This analysis shows that despite the technical and Mn specific challenges, three specific key features are common to most of the studied systems with significant OER activity: structural disorder, Mn oxidation states between III and IV, and the appearance of layered birnessite phases in the active regime.

Immobilization of Oxyanions on the Reconstructed Heterostructure Evolved from a Bimetallic Oxysulfide for the Promotion of Oxygen Evolution Reaction

Efficient and durable oxygen evolution reaction (OER) requires the electrocatalyst to bear abundant active sites, optimized electronic structure as well as robust component and mechanical stability. Herein, a bimetallic lanthanum-nickel oxysulfide with rich oxygen vacancies based on the La2O2S prototype is fabricated as a binder-free precatalyst for alkaline OER. The combination of advanced in situ and ex situ characterizations with theoretical calculation uncovers the synergistic effect among La, Ni, O, and S species during OER, which assures the adsorption and stabilization of the oxyanion [Formula: see text] onto the surface of the deeply reconstructed porous heterostructure composed of confining NiOOH nanodomains by La(OH)3 barrier. Such coupling, confinement, porosity and immobilization enable notable improvement in active site accessibility, phase stability, mass diffusion capability and the intrinsic Gibbs free energy of oxygen-containing intermediates. The optimized electrocatalyst delivers exceptional alkaline OER activity and durability, outperforming most of the Ni-based benchmark OER electrocatalysts.

Challenges for Hybrid Water Electrolysis to Replace the Oxygen Evolution Reaction on an Industrial Scale

To enable a future society based on sun and wind energy, transforming electricity into chemical energy in the form of fuels is crucial. This transformation can be achieved in an electrolyzer performing water splitting, where at the anode, water is oxidized to oxygen-oxygen evolution reaction (OER)-to produce protons and electrons that can be combined at the cathode to form hydrogen-hydrogen evolution reaction (HER). While hydrogen is a desired fuel, the obtained oxygen has no economic value. A techno-economically more suitable alternative is hybrid water electrolysis, where value-added oxidation reactions of abundant organic feedstocks replace the OER. However, tremendous challenges remain for the industrial-scale application of hybrid water electrolysis. Herein, these challenges, including the higher kinetic overpotentials of organic oxidation reactions compared to the OER, the small feedstock availably and product demand of these processes compared to the HER (and carbon dioxide reduction), additional purifications costs, and electrocatalytic challenges to meet the industrially required activities, selectivities, and especially long-term stabilities are critically discussed. It is anticipated that this perspective helps the academic research community to identify industrially relevant research questions concerning hybrid water electrolysis.

Evolution of Carbonate-Intercalated γ-NiOOH from a Molecularly Derived Nickel Sulfide (Pre)Catalyst for Efficient Water and Selective Organic Oxidation

The development of a competent (pre)catalyst for the oxygen evolution reaction (OER) to produce green hydrogen is critical for a carbon-neutral economy. In this aspect, the low-temperature, single-source precursor (SSP) method allows the formation of highly efficient OER electrocatalysts, with better control over their structural and electronic properties. Herein, a transition metal (TM) based chalcogenide material, nickel sulfide (NiS), is prepared from a novel molecular complex [Ni^II (PyHS)₄ ][OTf]₂ (1) and utilized as a (pre)catalyst for OER. The NiS (pre)catalyst requires an overpotential of only 255 mV to reach the benchmark current density of 10 mA cm^-2 and shows 63 h of chronopotentiometry (CP) stability along with over 95% Faradaic efficiency in 1 m KOH. Several ex situ measurements and quasi in situ Raman spectroscopy uncover that NiS irreversibly transformed to a carbonate-intercalated γ-NiOOH phase under the alkaline OER conditions, which serves as the actual active structure for the OER. Additionally, this in situ formed active phase successfully catalyzes the selective oxidation of alcohol, aldehyde, and amine-based organic substrates to value-added chemicals, with high efficiencies.

Integrating Electrochemical CO2 Reduction on α-NiS with the Water or Organic Oxidations by Its Electro-Oxidized NiO(OH) Counterpart to an Artificial Photosynthetic Scheme

Efficient hydrogen production, biomass up-conversion, and CO₂-to-fuel generation are the key challenges of the present decade. Electrocatalysis in aqueous electrolytes by choosing suitable transition-metal-based electrode materials remains the green approach for the trio of sustainable developments. Given that, finding electrode materials with multifunctional capability would be beneficial. Herein, the nanocrystalline α-NiS, synthesized solvothermally, has been chosen as an electrode material. As the first step to construct an electrolyzer, α-NiS deposited on conducting nickel foam (NF) has been used as an anode, and under the anodic potential, the α-NiS particles have lost sulfides to the electrolyte and transform to amorphous electro-derived NiO(OH) (NiO(OH)_ED), confirmed by different spectroscopic and microscopic studies. In situ transformation of α-NiS to amorphous NiO(OH)_ED results in an enhancement of the electrochemical surface area and not only becomes active toward oxygen evolution reaction (OER) at a moderate overpotential of 264 mV (at 20 mA cm^-2) but also can convert a series of biomass-derived organic compounds, namely, 2-hydroxymethylfurfural (HMF), 2-furfural (FF), ethylene glycol (EG), and glycerol (Gly), to industrially relevant feedstocks with a high (∼96%) Faradaic efficiency. During these organic oxidations, NiO(OH)_ED/NF participate in the multiple-electron oxidation process (up to 8e^-) including C-C bond cleavages of EG and Gly. During the cathodic performance of the α-NiS/NF, the structural integrity has been retained and the unaltered α-NiS/NF electrode remains more effective cathode for alkaline hydrogen evolution reaction (HER) and CO₂ reduction (CO₂R) compared to its in situ-derived NiO(OH)_ED/NF. α-NiS/NF can reduce the CO₂ predominantly to CO even at a higher potential like -0.8 V (vs RHE). The fabricated cell with α-NiS and its electro-oxidized NiO(OH)_ED counterpart, α-NiS/NF(-)/(+)NiO(OH)_ED/NF, is able to show an artificial photosynthetic scheme in which the NiO(OH)_ED/NF anode oxidizes water to O₂ and the α-NiS cathode reduces CO₂ majorly to CO in a moderate cell potential. In this study, α-NiS has been utilized as a single electrode material to perform multiple sustainable transformations.

A Liquid-Liquid-Solid System to Manipulate the Cascade Reaction for Highly Selective Electrosynthesis of Aldehyde

Electrocatalytic synthesis of aldehydes from alcohols exhibits unique superiorities as a promising technology, in which cascade reactions are involved. However, the cascade reactions are severely limited by the low selectivity resulting from the peroxidation of aldehydes in a traditional liquid-solid system. Herein, we report a novel liquid-liquid-solid system to regulate the selectivity of benzyl alcohol electrooxidation. The selectivity of benzaldehyde increases 200-fold from 0.4 % to 80.4 % compared with the liquid-solid system at a high current density of 136 mA cm^-2 , which is the highest one up to date. In the tri-phase system, the benzaldehyde peroxidation is suppressed efficiently, with the conversion of benzaldehyde being decreased from 87.6 % to 3.8 %. The as-produced benzaldehyde can be in situ extracted to toluene phase and separated from the electrolyte to get purified benzaldehyde. This strategy provides an efficient way to efficiently enhance the selectivity of electrocatalytic cascade reactions.

In Situ Coupling of Carbon Dots with Co-ZIF Nanoarrays Enabling Highly Efficient Oxygen Evolution Electrocatalysis

Metal-organic frameworks (MOFs) are regarded as one promising class of precatalysts for electrocatalytic oxygen evolution reaction (OER), yet most of them suffer from poor conductivity and lack of coordinatively unsaturated metal sites, which hinders the fast electrochemical reconstruction and thus a poor OER activity. To address this issue, a unique heterocomposite has been constructed by in situ inserting carbon dots (CDs) into cobalt-based zeolitic imidazolate framework (Co-ZIF) nanosheet arrays (Co-ZIF/CDs/CC) in the presence of carbon cloth (CC) via one-pot coprecipitation for alkaline OER. Benefiting from the synergism between CDs and Co-ZIF subunits such as superior conductivity, strong charge interaction as well as abundant metal sites exposure, the Co-ZIF/CDs/CC exhibits an enhanced promotion effect for OER and contributes to the deep phase transformation from CDs-coupled Co-ZIF to CDs-coupled active CoOOH. As expected, the achieved Co-ZIF/CDs/CC only requires an overpotential of 226 mV to deliver 10 mA cm^-2 in 1.0 M KOH, which is lower than that of Co-ZIF/CC and superior to most previously reported CC-supported MOF precatalysts. Moreover, it can also maintain a large current density of 100 mA cm^-2 for 24 h with negligible activity decay.

Efficient Ternary Mn-Based Spinel Oxide with Multiple Active Sites for Oxygen Evolution Reaction Discovered via High-Throughput Screening Methods

The discovery of more efficient and stable catalysts for oxygen evolution reaction (OER) is vital in improving the efficiency of renewable energy generation devices. Given the large numbers of possible binary and ternary metal oxide OER catalysts, high-throughput methods are necessary to accelerate the rate of discovery. Herein, Mn-based spinel oxide, Fe₁₀ Co₄₀ Mn₅₀ O, is identified for the first time using high-throughput methods demonstrating remarkable catalytic activity (overpotential of 310 mV on fluorine-doped tin oxide (FTO) substrate and 237 mV on Ni foam at 10 mA cm^-2 ). Using a combination of soft X-ray absorption spectroscopy and electrochemical measurements, the high catalytic activity is attributed to 1) the formation of multiple active sites in different geometric sites, tetrahedral and octahedral sites; and 2) the formation of active oxyhydroxide phase due to the strong interaction of Co²⁺ and Fe³⁺ . Structural and surface characterizations after OER show preservation of Fe₁₀ Co₄₀ Mn₅₀ O surface structure highlighting its durability against irreversible redox damage on the catalytic surface. This work demonstrates the use of a high-throughput approach for the rapid identification of a new catalyst, provides a deeper understanding of catalyst design, and addresses the urgent need for a better and stable catalyst to target greener fuel.

Oxygen Evolution Activity of Amorphous Cobalt Oxyhydroxides: Interconnecting Precatalyst Reconstruction, Long-Range Order, Buffer-Binding, Morphology, Mass Transport, and Operation Temperature

Nanocrystalline or amorphous cobalt oxyhydroxides (CoCat) are promising electrocatalysts for the oxygen evolution reaction (OER). While having the same short-range order, CoCat phases possess different electrocatalytic properties. This phenomenon is not conclusively understood, as multiple interdependent parameters affect the OER activity simultaneously. Herein, a layered cobalt borophosphate precatalyst, Co(H₂ O)₂ [B₂ P₂ O₈ (OH)₂ ]·H₂ O, is fully reconstructed into two different CoCat phases. In contrast to previous reports, this reconstruction is not initiated at the surface but at the electrode substrate to catalyst interface. Ex situ and in situ investigations of the two borophosphate derived CoCats, as well as the prominent CoP_i and CoB_i identify differences in the Tafel slope/range, buffer binding and content, long-range order, number of accessible edge sites, redox activity, and morphology. Considering and interconnecting these aspects together with proton mass-transport limitations, a comprehensive picture is provided explaining the different OER activities. The most decisive factors are the buffers used for reconstruction, the number of edge sites that are not inhibited by irreversibly bonded buffers, and the morphology. With this acquired knowledge, an optimized OER system is realized operating in near-neutral potassium borate medium at 1.62 ± 0.03 V_RHE yielding 250 mA cm^-2 at 65 °C for 1 month without degrading performance.

Boosting Hydrogen Production by Selective Anodic Electrooxidation of Ethanol over Trimetallic PdSbBi Nanoparticles: Composition Matters

The conventional hydrogen evolution from water electrolysis is severely impeded by the sluggish kinetics of oxygen evolution reaction (OER). In this work, an integrated electrolysis system of replacing the anodic OER with a thermodynamically favorable ethanol oxidation reaction (EOR) has been developed by using PdSbBi/C as an electrocatalyst. To maximize the EOR performance, the composition of PdSbBi nanoparticles is tuned by varying the ratio of Sb and Bi precursors. Ternary PdSbBi-based electrocatalysts exhibit enhanced activity and stability toward EOR compared to commercial Pd/C and binary catalysts. In particular, the Pd₇₆Sb₁₇Bi₇/C catalyst delivers a very high specific activity up to 52.4 mA cm^-2 and mass activity of 2.66 A mg^-1_Pd. Besides, this EOR process is demonstrated to have high selectivity with acetic acid as the oxidation product in the electrolyte. When coupled with a cathodic platinum mash, the two-electrode electrolyzer cell requires a voltage input of merely 0.61 V to afford a current density of 10 mA cm^-2. Density functional theory calculations reveal that the presence of Sb and Bi can promote the adsorption of hydroxide ions and facilitate the removal of reaction intermediates in the EOR pathway. This work provides a novel catalyst for the energy-efficient coproduction of acetic acid and hydrogen fuel.

The Pivotal Role of s-, p-, and f-Block Metals in Water Electrolysis: Status Quo and Perspectives

Transition metals, in particular noble metals, are the most common species in metal-mediated water electrolysis because they serve as highly active catalytic sites. In many cases, the presence of nontransition metals, that is, s-, p-, and f-block metals with high natural abundance in the earth-crust in the catalytic material is indispensable to boost efficiency and durability in water electrolysis. This is why alkali metals, alkaline-earth metals, rare-earth metals, lean metals, and metalloids receive growing interest in this research area. In spite of the pivotal role of these nontransition metals in tuning efficiency of water electrolysis, there is far more room for developments toward a knowledge-based catalyst design. In this review, five classes of nontransition metals species which are successfully utilized in water electrolysis, with special emphasis on electronic structure-catalytic activity relationships and phase stability, are discussed. Moreover, specific fundamental aspects on electrocatalysts for water electrolysis as well as a perspective on this research field are also addressed in this account. It is anticipated that this review can trigger a broader interest in using s-, p-, and f-block metals species toward the discovery of advanced polymetal-containing electrocatalysts for practical water splitting.

Electrochemically Derived Crystalline CuO from Covellite CuS Nanoplates: A Multifunctional Anode Material

In the present era, electrochemical water splitting has been showcased as a reliable solution for alternative and sustainable energy development. The development of a cheap, albeit active, catalyst to split water at a substantial overpotential with long durability is a perdurable challenge. Moreover, understanding the nature of surface-active species under electrochemical conditions remains fundamentally important. A facile hydrothermal approach is herein adapted to prepare covellite (hexagonal) phase CuS nanoplates. In the covellite CuS lattice, copper is present in a mixed-valent state, supported by two different binding energy values (932.10 eV for Cu^I and 933.65 eV for Cu^II) found in X-ray photoelectron spectroscopy analysis, and adopted two different geometries, that is, trigonal planar preferably for Cu^I and tetrahedral preferably for Cu^II. The as-synthesized covellite CuS behaves as an efficient electro(pre)catalyst for alkaline water oxidation while deposited on a glassy carbon and nickel foam (NF) electrodes. Under cyclic voltammetry cycles, covellite CuS electrochemically and irreversibly oxidized to CuO, indicated by a redox feature at 1.2 V (vs the reversible hydrogen electrode) and an ex situ Raman study. Electrochemically activated covellite CuS to the CuO phase (termed as CuS_EA) behaves as a pure copper-based catalyst showing an overpotential (η) of only 349 (±5) mV at a current density of 20 mA cm^-2, and the TOF value obtained at η₃₄₉ (at 349 mV) is 1.1 × 10^-3 s^-1. A low R_ct of 5.90 Ω and a moderate Tafel slope of 82 mV dec^-1 confirm the fair activity of the CuS_EA catalyst compared to the CuS precatalyst, reference CuO, and other reported copper catalysts. Notably, the CuS_EA/NF anode can deliver a constant current of ca. 15 mA cm^-2 over a period of 10 h and even a high current density of 100 mA cm^-2 for 1 h. Post-oxygen evolution reaction (OER)-chronoamperometric characterization of the anode via several spectroscopic and microscopic tools firmly establishes the formation of crystalline CuO as the active material along with some amorphous Cu(OH)₂ via bulk reconstruction of the covellite CuS under electrochemical conditions. Given the promising OER activity, the CuS_EA/NF anode can be fabricated as a water electrolyzer, Pt(-)//(+)CuS_EA/NF, that delivers a j of 10 mA cm^-2 at a cell potential of 1.58 V. The same electrolyzer can further be used for electrochemical transformation of organic feedstocks like ethanol, furfural, and 5-hydroxymethylfurfural to their respective acids. The present study showcases that a highly active CuO/Cu(OH)₂ heterostructure can be constructed in situ on NF from the covellite CuS nanoplate, which is not only a superior pure copper-based electrocatalyst active for OER and overall water splitting but also for the electro-oxidation of industrial feedstocks.

Engineering Lattice Oxygen Activation of Iridium Clusters Stabilized on Amorphous Bimetal Borides Array for Oxygen Evolution Reaction

Developing robust oxygen evolution reaction (OER) catalysts requires significant advances in material design and in-depth understanding for water electrolysis. Herein, we report iridium clusters stabilized surface reconstructed oxyhydroxides on amorphous metal borides array, achieving an ultralow overpotential of 178 mV at 10 mA cm^-2 for OER in alkaline medium. The coupling of iridium clusters induced the formation of high valence cobalt species and Ir-O-Co bridge between iridium and oxyhydroxides at the atomic scale, engineering lattice oxygen activation and non-concerted proton-electron transfer to trigger multiple active sites for intrinsic pH-dependent OER activity. The lattice oxygen oxidation mechanism (LOM) was confirmed by in situ ¹⁸ O isotope labeling mass spectrometry and chemical recognition of negative peroxo-like species. Theoretical simulations reveal that the OER performance on this catalyst is intrinsically dominated by LOM pathway, facilitating the reaction kinetics. This work not only paves an avenue for the rational design of electrocatalysts, but also serves the fundamental insights into the lattice oxygen participation for promising OER application.

Perspective on intermetallics towards efficient electrocatalytic water-splitting

Intermetallic compounds exhibit attractive electronic, physical, and chemical properties, especially in terms of a high density of active sites and enhanced conductivity, making them an ideal class of materials for electrocatalytic applications. Nevertheless, widespread use of intermetallics for such applications is often limited by the complex energy-intensive processes yielding larger particles with decreased surface areas. In this regard, alternative synthetic strategies are now being explored to realize intermetallics with distinct crystal structures, morphology, and chemical composition to achieve high performance and as robust electrode materials. In this perspective, we focus on the recent advances and progress of intermetallics for the reaction of electrochemical water-splitting. We first introduce fundamental principles and the evaluation parameters of water-splitting. Then, we emphasize the various synthetic methodologies adapted for intermetallics and subsequently, discuss their catalytic activities for water-splitting. In particular, importance has been paid to the chemical stability and the structural transformation of the intermetallics as well as their active structure determination under operating water-splitting conditions. Finally, we describe the challenges and future opportunities to develop novel high-performance and stable intermetallic compounds that can hold the key to more green and sustainable economy and rise beyond the horizon of water-splitting application.

Evolving Highly Active Oxidic Iron(III) Phase from Corrosion of Intermetallic Iron Silicide to Master Efficient Electrocatalytic Water Oxidation and Selective Oxygenation of 5-Hydroxymethylfurfural

In a green energy economy, electrocatalysis is essential for chemical energy conversion and to produce value added chemicals from regenerative resources. To be widely applicable, an electrocatalyst should comprise the Earth's crust's most abundant elements. The most abundant 3d metal, iron, with its multiple accessible redox states has been manifold applied in chemocatalytic processes. However, due to the low conductivity of Fe^III O_x H_y phases, its applicability for targeted electrocatalytic oxidation reactions such as water oxidation is still limited. Herein, it is shown that iron incorporated in conductive intermetallic iron silicide (FeSi) can be employed to meet this challenge. In contrast to silicon-poor iron-silicon alloys, intermetallic FeSi possesses an ordered structure with a peculiar bonding situation including covalent and ionic contributions together with conducting electrons. Using in situ X-ray absorption and Raman spectroscopy, it could be demonstrated that, under the applied corrosive alkaline conditions, the FeSi partly forms a unique, oxidic iron(III) phase consisting of edge and corner sharing [FeO₆ ] octahedra together with oxidized silicon species. This phase is capable of driving the oxyge evolution reaction (OER) at high efficiency under ambient and industrially relevant conditions (500 mA cm^-2 at 1.50 ± 0.025 V_RHE and 65 °C) and to selectively oxygenate 5-hydroxymethylfurfural (HMF).

Towards atomic precision in HMF and methane oxidation electrocatalysts

With the increasing emphasis on transitioning to a sustainable society, electrosynthetic routes to generate fuels and chemicals are rapidly gaining traction. While the electrolysis of water and CO2 has been heavily investigated over the last decade, electrocatalysis of other abundant resources such as biomass and methane is now increasingly coming into focus. As this area is relatively less mature, much work remains to be done. In particular, efforts to decipher reaction mechanisms and extract the fundamental insights are necessary to develop economically competitive electrosynthetic routes using biomass and methane. Against this backdrop, this feature article focuses on the recent developments within the community using atomically precise catalysts, both homogeneous and heterogeneous, as model systems to understand these reactions.