Earth-abundant Transition Metals Research Articles

Analysis of the Active Species Responsible for Water Oxidation Using a Pentanuclear Fe Complex.

SummaryWater splitting with sunlight is today one of the most promising strategies that can be used to start the imperatively needed transition from fossil to solar fuels. To achieve this, one of the key reactions that need to be mastered is the electrocatalytic oxidation of water to dioxygen. Great developments have been achieved using transition metal complexes mainly based on Ru, but for technological applications it is highly desirable to be able to use earth-abundant transition metals. The intrinsic chemistry of first row transition metals and in particular the lability of their M-L bonds in water imposes serious challenges for the latter to work as real molecular catalysts. The present work addresses this issue based on a molecular pentanuclear Fe5 complex and describes the different protocols and tests that need to be carried out in order to identify the real active species, responsible for the generation of dioxygen.

Frontispiece: Earth‐Abundant Transition‐Metal‐Based Bifunctional Electrocatalysts for Overall Water Splitting in Alkaline Media

Highly efficient electrocatalysts are critical for high efficiency hydrogen production. Nevertheless, due to the scarcity and limited availability of the current commercial noble-metal-based electrode materials, the research direction has turned to seek alternative candidates by employing the earth-abundant transition metals (TMs). This Minireview provides a comprehensive survey of recent advanced achievement in developing strategies for TMs-based electrocatalysts toward hydrogen evolution reaction and oxygen evolution reaction under alkaline conditions. For more details see the Minireview by H. Lee et al. on page 6423 ff.

Study of Mn(III) Disproportionation Reaction Using Vanadium and Titanium Additives: Application to Redox Flow Batteries

Manganese, an earth-abundant transition metal, exists as Mn(III)/Mn(II) redox couple at a standard potential of +1.51V vs. SHE. It has drawn interest for implementation as positive side in V/Mn redox flow battery (RFB) due to higher achievable energy density than all-vanadium RFB. Unfortunately, Mn(III) disproportionation into Mn(II) and MnO2(s) led to loss of capacity, pressure drop increase and passivation of electrode surface during RFB cycling. We studied the influence of Ti(IV) or V(V) additives on Mn(III) stability in high acidic medium, by formulating 4 different electrolytes at equimolar ratio (Mn, Mn:Ti, Mn:V:Ti, Mn:V). Electrochemical characterizations reveals Mn(III) involvement in a complex nucleation/growth process at the electrode surface, along with its disproportionation. SEM combined to XPS demonstrates different oxide layers morphologies and chemical nature: MnO2 was formed in the presence of Mn and Mn:Ti, and MnO in the presence of V(V). Spectroelectrochemical study of Mn electrolytes highlights the displacement of the disproportionation reaction equilibrium in the presence of Ti(IV) or V(V) additives. Below 10% of electrolyzed Mn(III) was involved in the disproportionation reaction in the presence of Ti(IV) and V(V) (e.g., 25% without additives). V(V) was observed to enhance the stability of Mn(III) as compared to Ti(IV), which is of high interest for redox flow battery applications. In fact, Mn:V electrolyte could be used instead of Mn:Ti as the positive side of a Mn-V RFB will considerably reduce cross-contamination issues. As observed in the spectroelectrochemical study, the battery might be charged up to 90% SOC without loss of capacity. Moreover, in case of MnO particles formation, the strong acidic conditions will directly dissolve the particles preventing the battery from electrode passivation or/and pressure drop concerns. Figure 1

(Invited) Water-Splitting Electrocatalysts Made By Pulsed-Laser in Liquids Synthesis

Conversion of solar energy into storable fuels is urgently needed to combat climate change. Efficient, robust materials that are exclusively made of non-precious elements are imperative for tomorrow's global sustainable energy economy and necessitate the development of new nanostructured multi-metallic electrocatalysts that turn our most abundant feedstocks, water or carbon dioxide, into carbon-neutral fuels. The sheer number of possible elemental compositions in multi-metallic nanostructures requires a deep understanding of electrocatalytic mechanisms, including identification of reactive intermediates, to rationally design new materials and guide their optimization.We used pulsed-laser in liquids synthesis to realize series of earth-abundant multi-metallic first-row transition metal oxide and hydroxide nanostructures. The pulsed-laser in liquids technique is a flexible synthetic strategy for surfactant-free nanomaterials with independently controlled compositional, morphological, and structural properties, and defect densities. It permits rapid preparation or modification of tailored, complex nanostructures in sufficiently large quantities to study them in bulk. Importantly, metastable materials become accessible as nanoparticles form from a laser-induced plasma that is confined by the liquid and characterized by very high temperatures and pressures, thus enabling access to extreme regions of the material's phase diagram.Our approach enabled unprecedented atomistic-level structural and mechanistic insights into highly active and robust nickel–iron layered double hydroxide nanocatalysts for water oxidation in base; this understanding allowed rapid optimization. Operando spectroscopy data allowed us to identify a cis-dioxo iron(VI) reactive intermediate as the lowest-energy species before O–O bond formation during water oxidation electrocatalysis. Further, we capitalized on the unique advantages of the laser process for integration of our water-splitting electrocatalysts with photoanodes.

A Vanadium(III) Complex with Blue and NIR-II Spin-Flip Luminescence in Solution.

Luminescence from Earth-abundant metal ions in solution at room temperature is a very challenging objective due to the intrinsically weak ligand field splitting of first-row transition metal ions, which leads to efficient nonradiative deactivation via metal-centered states. Only a handful of 3dn metal complexes (n ≠ 10) show sizable luminescence at room temperature. Luminescence in the near-infrared spectral region is even more difficult to achieve as further nonradiative pathways come into play. No Earth-abundant first-row transition metal complexes have displayed emission >1000 nm at room temperature in solution up to now. Here, we report the vanadium(III) complex mer-[V(ddpd)2][PF6]3 yielding phosphorescence around 1100 nm in valeronitrile glass at 77 K as well as at room temperature in acetonitrile with 1.8 × 10-4% quantum yield (ddpd = N,N'-dimethyl-N,N'-dipyridine-2-ylpyridine-2,6-diamine). In addition, mer-[V(ddpd)2][PF6]3 shows very strong blue fluorescence with 2% quantum yield in acetonitrile at room temperature. Our comprehensive study demonstrates that vanadium(III) complexes with d2 electron configuration constitute a new class of blue and NIR-II luminophores, which complement the classical established complexes of expensive precious metals and rare-earth elements.

Two-Dimensional Earth-Abundant Transition Metal Oxides Nanomaterials: Synthesis and Application in Electrochemical Oxygen Evolution Reaction.

Development of a universal synthetic strategy for two-dimensional (2D) Earth-abundant transition metal oxides nanomaterials is highly vital toward numerous electrochemical applications. Herein, a facile and general synthesis of highly ordered two-dimensional metal oxides nanomaterials includes Co3O4, NiO, CuO, and Fe3O4 nanosheets as an electrocatalyst for oxygen evolution reaction (OER) is demonstrated. Among the synthesized 2D transition metal oxides, the Co3O4 nanosheet exhibits smallest overpotential (η) of ∼384.0 mV at a current density of 10.0 mA cm-2 and Tafel slope of ∼52.0 mV dec-1, highest mass activity of ∼112.3 A g-1 at the overpotential of ∼384.0 mV, and high turn over frequency (TOF) of 0.099 s-1, which is relatively favorable with state-of-the-art RuO2 catalyst. The present synthetic approach may unlock a brand new pathway to prepare shape-controlled Earth-abundant transition metal oxides nanomaterials for electrocatalytic OER.

CuIn-ethylxanthate, a “Versatile Precursor” for Photosensitization of Graphene-Quantum Dots and Nanocatalyzed Synthesis of Imidazopyridines with Ideal Green Chemistry Metrics

Recently, the development of hybrid nano catalyst involving earth abundant transition metals for photosensitization and multi-component reaction in industry and academia has been a matter of intens...

Photophysics and Photochemistry of Iron Carbene Complexes for Solar Energy Conversion and Photocatalysis

Earth-abundant first row transition metal complexes are important for the development of large-scale photocatalytic and solar energy conversion applications. Coordination compounds based on iron are especially interesting, as iron is the most common transition metal element in the Earth’s crust. Unfortunately, iron-polypyridyl and related traditional iron-based complexes generally suffer from poor excited state properties, including short excited-state lifetimes, that make them unsuitable for most light-driven applications. Iron carbene complexes have emerged in the last decade as a new class of coordination compounds with significantly improved photophysical and photochemical properties, that make them attractive candidates for a range of light-driven applications. Specific aspects of the photophysics and photochemistry of these iron carbenes discussed here include long-lived excited state lifetimes of charge transfer excited states, capabilities to act as photosensitizers in solar energy conversion applications like dye-sensitized solar cells, as well as recent demonstrations of promising progress towards driving photoredox and photocatalytic processes. Complementary advances towards photofunctional systems with both Fe(II) complexes featuring metal-to-ligand charge transfer excited states, and Fe(III) complexes displaying ligand-to-metal charge transfer excited states are discussed. Finally, we outline emerging opportunities to utilize the improved photochemical properties of iron carbenes and related complexes for photovoltaic, photoelectrochemical and photocatalytic applications.

Homologous NiCoP/CoP hetero-nanosheets supported on N-doped carbon nanotubes for high-rate hybrid supercapacitors

Rational design of highly active electrode materials made of low-cost and earth-abundant transition metal phosphides is essential to boost the energy density and overall performance of electrochemical capacitors. In this work, homologous 3D NiCoP/CoP hetero-nanosheets network supported on N-doped carbon nanotubes are synthesized through one-step phosphorization of nickel cobalt hydroxide. The synergistic effects between NiCoP and CoP in the heterointerface are studied systematically through tuning the molar ratio of original Ni/Co. As a result, the optimized N-CNTs@NiCoP/CoP (with the Ni/Co ratio of 1:2) reveals a remarkable specific capacity of 152 mAh g−1 at 1 A g−1 and a superior rate capability with a capacity retention of 61% even at 30 A g−1. A hybrid supercapacitor assembled with N-CNTs@NiCoP/CoP cathode and ZIF-67-derived porous carbon anode exhibits a high energy density of 45.5 Wh kg−1 at power density of 784 W kg−1 and excellent stability of 87% retention after 10,000 cycles at 12 A g−1. Such excellent electrochemical performance of N-CNTs@NiCoP/CoP is mainly ascribed to the strong synergistic effect between NiCoP and CoP in their heterojunctions, enlarged surface area and active sites due to the 3D nanosheets network and conductive N-CNTs support.

Tailoring the Mechanical Properties of Earth-Abundant Transition Metal Borides via Bonding Optimization

Borides containing 4d or 5d transition metals are among the most common types of high hardness materials because of the high valence electron density of the metals combined with short, covalent mai...

Rational Design of Spinel Cobalt Vanadate Oxide Co2 VO4 for Superior Electrocatalysis.

Electrochemical energy devices, such as fuel cells and metal-air batteries, convert chemical energy directly into electricity without adverse environmental impact. Attractive alternatives to expensive noble metals used in these renewable energy technologies are earth-abundant transition metal oxides. However, they are often limited by catalytic and conductive capabilities. Here reported is a spinel oxide, Co2 VO4 , by marrying metallic vanadium atomic chains with electroactive cobalt cations for superior oxygen reduction reaction (ORR)-a key process for fuel cells, metal-air batteries, etc. The experimental and simulated electron energy-loss spectroscopy analyses reveal that Co2+ cations at the octahedral sites take the low spin state with one eg electron , favoring advantageous ORR energetics. Measurement of actual electrical conductivity confirms that Co2 VO4 has several orders of magnitude increase when compared with benchmark cobalt oxides. As a result, a zinc-air battery with new spinel cobalt vanadate oxide as the ORR catalyst shows excellent performance, together with a record-high discharge peak power density of 380 mW cm-2 . Crucially, this is superior to state-of-the-art Pt/C-based device and is greatest among zinc-air batteries assembled with metal, metal oxide, and carbon catalysts. The findings present a new design strategy for highly active and conductive oxide materials for a wide range of electrocatalytic applications, including ORR, oxygen evolution, and hydrogen evolution reactions.

Ambidentate bonding and electrochemical implications of pincer-type pyridylidene amide ligands in complexes of nickel, cobalt and zinc

Pincer-type tridentate pyridyl bis(pyridylidene amide) (pyPYA2) ligand systems were coordinated to the Earth-abundant first row transition metals nickel, cobalt and zinc. A one-pot synthesis in water/ethanol afforded octahedral homoleptic bis-PYA complexes, [M(pyPYA2)2](PF6)2, whereas five-coordinate mono-PYA dichloride complexes, [M(pyPYA2)Cl2], were obtained upon slow addition of the ligand to the metal chlorides in DMF. Electrochemical measurements further revealed a facile oxidation of the metal centers from Ni2+ to Ni4+ and Co2+ to Co3+, respectively, while the Zn2+ system was redox inactive. These experiments further allowed for quantification of the much stronger electron donor properties of neutral N,N,N-tridentate pyPYA2 pincer ligands as compared to terpy. Remarkably, ortho-PYA pincer ligands feature amide coordination to the metal center via oxygen or nitrogen. This ambidentate ligand binding constitutes another mode of donor flexibility of the PYA ligand system, complementing the resonance structure dynamics established previously. NMR spectroscopic and MS analysis reveal that the meta-PYA ligand undergoes selective deuteration when coordinated to cobalt. This reactivity suggests the potential of this ligand as a transient proton reservoir for HX bond activation and, moreover, indicates the relevance of several resonance structures and therefore supports the notion that meta-PYA ligands are mesoionic.

The synergetic effect of N, S-codoped carbon and CoOx nanodots derived from ZIF-67 as a highly efficient cocatalyst over CdS nanorods

Earth-abundant transition metal cocatalysts are promising rare and noble metal alternatives for efficient photocatalytic hydrogen evolution.

Transferring photocatalytic CO2 reduction mediated by Cu(N^N)(P^P)+ complexes from organic solvents into ionic liquid media

Photocatalytic carbon dioxide reduction utilizing metal complexes based on the earth-abundant transition metals iron and copper was transferred from organic solvents into ionic liquids with high selectivity and moderate turn-over numbers.

Plasmonic-induced overgrowth of amorphous molybdenum sulfide on nanoporous gold: An ambient synthesis method of hybrid nanoparticles with enhanced electrocatalytic activity.

Hybrid materials of earth abundant transition metal dichalcogenides and noble metal nanoparticles, such as molybdenum sulfide (MoSx) and gold nanoparticles, exhibit synergistic effects that can enhance electrocatalytic reactions. However, most current hybrid MoSx-gold synthesis requires an energy intensive heat source of >500 °C or chemical plating to achieve deposition of MoSx on the gold surface. Herein, we demonstrate the direct overgrowth of MoSx over colloidal nanoporous gold (NPG), conducted feasibly under ambient conditions, to form hybrid particles with enhanced electrocatalytic performance toward hydrogen evolution reaction. Our strategy exploits the localized surface plasmon resonance-mediated photothermal heating of NPG to achieve >230 °C surface temperature, which induces the decomposition of the (NH4)2MoS4 precursor and direct overgrowth of MoSx over NPG. By tuning the concentration ratio between the precursor and NPG, the amount of MoSx particles deposited can be systematically controlled from 0.5% to 2% of the Mo/(Au + Mo) ratio. Importantly, we find that the hybrid particles exhibit higher bridging and an apical S to terminal S atomic ratio than pure molybdenum sulfide, which gives rise to their enhanced electrocatalytic performance for hydrogen evolution reaction. We demonstrate that hybrid MoSx-NPG exhibits >30 mV lower onset potential and a 1.7-fold lower Tafel slope as compared to pure MoSx. Our methodology provides an energy- and cost-efficient synthesis pathway, which can be extended to the synthesis of various functional hybrid structures with unique properties for catalysis and sensing applications.

Metal-organic frameworks-derived hollow-structured iron-cobalt bimetallic phosphide electrocatalysts for efficient oxygen evolution reaction

The oxygen evolution reaction (OER) is a cornerstone reaction for many renewable energy technologies. Developing low-cost and durable alternatives to precious-metal catalysts with high activity for OER are highly desirable to reduce the processing cost and complexity of renewable energy systems. Recently, metal phosphides based on earth-abundant transition metals, especially bimetallic phosphates have emerged as promising candidates for efficient OER catalysts. The rational design of nanostructured bimetallic phosphates is critical for the practical applications of these electrocatalysts, which requires a well understanding of the composition control and nanocrystal growth process. Herein, we report a facile strategy for the synthesis of novel iron-cobalt bimetallic phosphides (FeCoP) with hollow structures by phosphating Fe (Co)-centered metal-organic frameworks (MOFs). The resultant FeCoP hollow polyhedron, which have large specific surface area providing rich catalytic active sites, show excellent electrocatalytic activity towards OER with low overpotential, small Tafel slope, and remarkable stability. This work provides a new strategy for exploring efficient phosphide catalysts and opens up new avenues for the development of OER catalysts.

Engineering stable electrocatalysts by synergistic stabilization between carbide cores and Pt shells.

Core-shell particles with earth-abundant cores represent an effective design strategy for improving the performance of noble metal catalysts, while simultaneously reducing the content of expensive noble metals1-4. However, the structural and catalytic stabilities of these materials often suffer during the harsh conditions encountered in important reactions, such as the oxygen reduction reaction (ORR)3-5. Here, we demonstrate that atomically thin Pt shells stabilize titanium tungsten carbide cores, even at highly oxidizing potentials. In situ, time-resolved experiments showed how the Pt coating protects the normally labile core against oxidation and dissolution, and detailed microscopy studies revealed the dynamics of partially and fully coated core-shell nanoparticles during potential cycling. Particles with complete Pt coverage precisely maintained their core-shell structure and atomic composition during accelerated electrochemical ageing studies consisting of over 10,000 potential cycles. The exceptional durability of fully coated materials highlights the potential of core-shell architectures using earth-abundant transition metal carbide (TMC) and nitride (TMN) cores for future catalytic applications.

Preparation of Co-Fe oxides immobilized on carbon paper using water-dispersible Prussian-blue analog nanoparticles and their oxygen evolution reaction (OER) catalytic activities

Carbon paper (CP) is one of the most promising metal-free gas-diffusion electrodes for fabricating sustainable energy-conversion devices such as metal-air secondary batteries using the oxygen reduction reaction (ORR) and CO2-reforming systems via the electrochemical CO2 reduction reaction (CO2RR). The common counterpart reaction for ORR and CO2RR is the oxygen evolution reaction (OER) from water. The low-overpotential OER catalyzed by nanoparticles (NPs) immobilized on CP has attracted attention for integration in energy-conversion devices. Earth-abundant and low-cost 3d transition metals have been intensively explored as OER catalysts. In this study, we have successfully prepared Fe-Co Prussian blue analog (PBA) NPs with the formulae of Fe1-xCox[Fe(CN)6]0.67·nH2O and Fe1-yCoy[Co(CN)6]0.67·mH2O, and the Fe-Co PBA NPs are dispersed into water by surface modification using [Fe(CN)6]4−. By a simple drop-coating method, the Fe-Co PBA NPs are immobilized on CP and thermally decomposed into Fe-Co oxides at 400 °C. The OER overpotentials of the Fe-Co oxides are gradually decreased by increasing the Co composition ratios, and the lowest overpotential is 0.46 V vs. RHE at 10 mA/cm2 in y = 0.6.

α-C-H Functionalization of π-Bonds Using Iron Complexes: Catalytic Hydroxyalkylation of Alkynes and Alkenes.

The discovery of catalytic systems based on earth-abundant transition metals for the functionalization of C-H bonds enables streamlined and sustainable solutions to problems in synthetic organic chemistry. In this Communication, we disclose an iron-based catalytic system for the functionalization of propargylic and allylic C-H bonds. Inexpensive and readily available cyclopentadienyliron(II) dicarbonyl complexes were employedas catalysts for a novel deprotonative activation mode for C-H functionalization, an approach that allows for the direct union of unsaturated building blocks with aryl aldehydes and other carbonyl electrophiles to deliver a range of unsaturated alcohol coupling products under operationally simple and functional group tolerant reaction conditions.

Recent Trends in Synthesis and Investigation of Nickel Phosphide Compound/Hybrid-Based Electrocatalysts Towards Hydrogen Generation from Water Electrocatalysis.

Sustainable andhigh performance energy devices such as solar cells, fuel cells, metal-air batteries, as well as alternative energy conversion and storage systems have been considered as promising technologies to meet the ever-growing demands for clean energy. Hydrogen evolution reaction (HER) is a crucial process for cost-effective hydrogen production; however, functional electrocatalysts are potentially desirable to expedite reaction kinetics and supply high energy density. Thus, the development of inexpensive and catalytically active electrocatalysts is one of the most significant and challenging issues in the field of electrochemical energy storage and conversion. Realizing that advanced nanomaterials could engender many advantageous chemical and physical properties over a wide scale, tremendous efforts have been devoted to the preparation of earth-abundant transition metals as electrocatalysts for HER in both acidic and alkaline environments because of theirlow processing costs, reasonable catalytic activities, and chemical stability. Among all transition metal-based catalysts, nickel compounds are the most widely investigated, and have exhibited pioneering performances in various electrochemical reactions. Heterostructured nickel phosphide (NixPy) based compounds were introduced as promising candidates of a new category, which often display chemical and electronic characteristics that are distinct from those of non-precious metals counterparts, hence providing an opportunity to construct new catalysts with animproved activity and stability. As a result, the library of NixPy catalysts has been enriched very rapidly, with the possibility of fine-tuning their surface adsorption properties through synergistic coupling with nearby elements or dopants as the basis of future practical implementation. The current review distils recent advancements in NixPy compounds/hybrids and their application for HER, with a robust emphasis on breakthroughs in composition refinement. Future perspectives for modulating the HER activity of NixPy compounds/hybrids, and the challenges that need to be overcome before their practical use in sustainable hydrogen production are also discussed.