Low-Dimensional Hyperthin FeS2 Nanostructures for Efficient and Stable Hydrogen Evolution Electrocatalysis

CuS-NiTe2 embedded phosphorus-doped graphene oxide catalyst for evaluating hydrogen evolution reaction

The hydrogen evolution reaction (HER), a crucial half-reaction in the water-splitting process, is hindered by slow kinetics, necessitating efficient electrocatalysts to lower overpotential and enhance energy conversion efficiency. Transition-metal electrode materials, renowned for their robustness and effectiveness, have risen to prominence as primary contenders in the field of energy conversion and storage research. In this investigation, we delve into the capabilities of transition metals when employed as catalysts for the HER. Furthermore, we turn our attention to carbon nanomaterials like graphene, which have exhibited tremendous potential as top-performing electrocatalysts. Nevertheless, advancements are indispensable to expand their utility and versatility. One such enhancement involves the integration of phosphorus-doped graphene. Our research focuses on the synthesis of CuS-NiTe2/PrGO, a nanocomposite with a crystalline structure, through a straightforward method. This nanocomposite exhibits enhanced catalytic activity for the HER, boasting a Tafel slope of 57 mV dec-1 in an acidic environment. Consequently, our findings present a straightforward and efficient approach to developing high-performance electrocatalysts for HER.

Highthroughput Screening of CuBi Bimetallic Catalyst Array for Electrocatalytic CO2 Reduction Reaction by Scanning Electrochemical Microscope

The testing and evaluation of catalysts in CO2 electroreduction is a very tedious process. To study the catalytic system of CO2 reduction more quickly and efficiently, it is necessary to establish a method that can detect multiple catalysts at the same time. Herein, a series of CuBi bimetallic catalysts have been successfully prepared on a single glass carbon electrode by a scanning micropieptte contact method. The application of scanning electrochemical microscopy (SECM) enabled the visualization of the CO2 reduction activity in diverse catalyst micro-points. The SECM imaging with Substrate generation/tip collection (SG/TC) mode was conducted on CuBi bimetallic micro-points, revealing that HER reaction emerged as the prevailing reaction when a low overpotential was employed. While the applied potential was lower than -1.5 V vs. Ag/AgCl, the reduction of CO2 to formic acid became dominant. Increasing the bismuth proportion in the bimetallic catalyst can inhibit the hydrogen evolution reaction at low potential and enhances the selectivity of the CO product at high cathode overpotential. This research offers a novel approach to examining arrays of catalysts for CO2 reduction.

Augmenting the band gap of iron diselenide pyrite via ruthenium alloy integration

The study aimed to enhance the properties of thin F e S e 2 films by incorporating ruthenium through spray pyrolysis. Films were deposited on pre-heated glass substrates and subjected to controlled heating in a selenium-rich environment. X-ray diffraction analysis confirmed the presence of F e S e 2 phase. Films with specific ruthenium ratios showed notable improvements in optical attributes, including increased absorption coefficient and a higher direct band gap, aligning with desired values for photovoltaic applications. Hall Effect measurements revealed N-type conductivity with varying concentrations and temperature-dependent electrical properties. The results highlight the efficacy of ruthenium as a promising alloying candidate for developing photovoltaic materials, emphasizing the versatility of the produced films across multiple domains.

2D FeSx Nanosheets by Atomic Layer Deposition: Electrocatalytic Properties for the Hydrogen Evolution Reaction

2-dimensional FeS_x nanosheets of different sizes are synthesized by applying different numbers of atomic layer deposition (ALD) cycles on TiO₂ nanotube layers and graphite sheets as supporting materials and used as an electrocatalyst for the hydrogen evolution reaction (HER). The electrochemical results confirm electrocatalytic activity in alkaline media with outstanding long-term stability (>65 h) and enhanced catalytic activity, reflected by a notable drop in the initial HER overpotential value (up to 26 %). By using a range of characterization techniques, the origin of the enhanced catalytic activity was found to be caused by the synergistic interplay between in situ morphological and compositional changes in the 2D FeS_x nanosheets during HER. Under the application of a cathodic potential in alkaline media, the as-synthesized 2D FeS_x nanosheets transformed into iron oxyhydroxide-iron oxysulfide core-shell nanoparticles, which exhibited a higher active catalytic surface and newly created Fe-based HER catalytic sites.

A nanoelectrode-based study of water splitting electrocatalysts

The development of low-cost and efficient catalytic materials for key reactions like water splitting, CO₂ reduction and N₂ reduction is crucial for fulfilling the growing energy consumption demands and the pursuit of renewable and sustainable energy. Conventional electrochemical measurements at the macroscale lack the potential to characterize single catalytic entities and nanoscale surface features on the surface of a catalytic material. Recently, promising results have been obtained using nanoelectrodes as ultra-small platforms for the study of the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) on innovative catalytic materials at the nanoscale. In this minireview, we summarize the recent progress in the nanoelectrode-based studies on the HER and OER on various nanostructured catalytic materials. These electrocatalysts can be generally categorized into two groups: 0-dimensional (0D) single atom/molecule/cluster/nanoparticles and 2-dimensional (2D) nanomaterials. Controlled growth as well as the electrochemical characterization of single isolated atoms, molecules, clusters and nanoparticles has been achieved on nanoelectrodes. Moreover, nanoelectrodes greatly enhanced the spatial resolution of scanning probe techniques, which enable studies at the surface features of 2D nanomaterials, including surface defects, edges and nanofacets at the boundary of a phase. Nanoelectrode-based studies on the catalytic materials can provide new insights into the reaction mechanisms and catalytic properties, which will facilitate the pursuit of sustainable energy and help to solve CO₂ release issues.

Fabrication and Characterization of Nanostructured Rock Wool as a Novel Material for Efficient Water-Splitting Application

Rock wool (RW) nanostructures of various sizes and morphologies were prepared using a combination of ball-mill and hydrothermal techniques, followed by an annealing process. Different tools were used to explore the morphologies, structures, chemical compositions and optical characteristics of the samples. The effect of initial particle size on the characteristics and photoelectrochemical performance of RW samples generated hydrothermally was investigated. As the starting particle size of ball-milled natural RW rises, the crystallite size of hydrothermally formed samples drops from 70.1 to 31.7 nm. Starting with larger ball-milled particle sizes, the nanoparticles consolidate and seamlessly combine to form a continuous surface with scattered spherical nanopores. Water splitting was used to generate photoelectrochemical hydrogen using the samples as photocatalysts. The number of hydrogen moles and conversion efficiencies were determined using amperometry and voltammetry experiments. When the monochromatic wavelength of light was increased from 307 to 460 nm for the manufactured RW>0.3 photocatalyst, the photocurrent density values decreased from 0.25 to 0.20 mA/mg. At 307 nm and +1 V, the value of the incoming photon-to-current efficiency was ~9.77%. Due to the stimulation of the H+ ion rate under the temperature impact, the Jph value increased by a factor of 5 when the temperature rose from 40 to 75 °C. As a result of this research, for the first time, a low-cost photoelectrochemical catalytic material is highlighted for effective hydrogen production from water splitting.

γ-FeO(OH) with Multi-surface Terminations Intrinsically Active for Electrocatalytic Oxygen Evolution Reaction

Due to poor conductivity, electrocatalytic performance of independently prepared iron oxy-hydroxide (FeO(OH)) is inferior whereas in-situ derived FeO(OH) from the iron based electro(pre)catalyst shows superior oxygen evolution reaction (OER). Use...

Anchoring stable FeS2 nanoparticles on MXene nanosheets via interface engineering for efficient water splitting

Exploring highly-efficient, economical and environment friendly electrocatalysts towards hydrogen and oxygen evolution reaction (HER and OER) is necessary but challenging for water splitting. Herein, FeS2 nanoparticles were anchored on the...

In situ produced Co9S8 nanoclusters/Co/Mn-S, N multi-doped 3D porous carbon derived from eriochrome black T as an effective bifunctional oxygen electrocatalyst for rechargeable Zn-air batteries

Construction of high-efficiency, low cost and stable non-noble metal catalyst on air cathode is of great importance for design and assembly of rechargeable Zn-air battery. Eriochrome black T (EBT) has phenolic hydroxyl and -N=Ν- groups, which provides multiple coordination sites for metal ions. Herein, Co₉S₈ nanoclusters implanted in Co/Mn-S,N multi-doped porous carbon (Co₉S₈@Co/Mn-S,N-PC) are fabricated with the mixture (i.e. EBT, metal precursors and dicyandiamide) by a coordination regulated pyrolysis strategy. Specifically, EBT effectively chelates with the Co and Mn ions, resulting in multiple incorporation and fine modulation of the carbon electronic structures. Meanwhile, its sulfonic acid groups are reduced at such high temperature, accompanied by simultaneously embedding S element in the carbon, ultimately in situ forming Co₉S₈ nanoclusters. The Co₉S₈@Co/Mn-S,N-PC performs as an effective bifunctional oxygen catalyst, displaying a positive half-wave potential of 0.85 V and a large limiting current density of 5.89 mA cm^-2 for oxygen reduction reaction (ORR) in alkaline media, coupled with a small overpotential of 320 mV at 10 mA cm^-2 towards oxygen evolution reaction (OER), outperforming commercial Pt/C and RuO₂ catalysts, respectively. Furthermore, the assembled rechargeable Zn-air battery with Co₉S₈@Co/Mn-S,N-PC exhibits the much better charge/discharge performance and long-term durability (210 h, 630 cycles). This research opens an instructive avenue to develop high-efficient and stable bifunctional oxygen electrocatalysts in energy transformation and storage devices.

Electrocatalytic N2 Reduction on FeS2 Nanoparticles Embedded in Graphene Oxide in Acid and Neutral Conditions

The development of stable, low-cost, and highly efficient electrocatalysts for the N₂ reduction reaction (NRR) process is challenging but crucial for ammonia production. Herein, we demonstrate the synthesis of pyrite nanoparticles wrapped by graphene oxide (FeS₂@GO) acting as a highly efficient NRR catalyst in a wide pH range. The FeS₂ nanoparticles are uniformly dispersed across the GO nanosheet, thus leading to the fine exposure of active sites, the promotion of charge transfer, and the increment of a contact surface area, which are all beneficial for a desired catalyst. In the meantime, the low-coordinated Fe atoms are activated as highly active sites, which is in favor of the enhanced electrochemical performance for the NRR. Furthermore, density functional theory (DFT) calculations illustrated that the high activity of N₂ reduction over the FeS₂@GO catalyst arises from the well-exposed Fe active sites and the increment of charge density at the valence band edge. Benefiting from the well-optimized interface, the barrier of the addition of the first hydrogen atom to N₂ forming *NNH species as the potential-determining step is as low as 0.93 eV in N₂ electroreduction. The electrochemical test results reveal that, as expected, FeS₂@GO exhibits high Faradaic efficiencies (4.7% in 0.1 M HCl solution and 6.8% in 0.1 M Na₂SO₄ solution) and advanced NH₃ yields (78.6 and 27.9 μg h^-1 mg_cat.^-1 in 0.1 M HCl and 0.1 M Na₂SO₄ solutions, respectively) in both acid and neutral conditions. This work offers a new avenue for exploring novel electrocatalysts, which has great promise to accelerate the practical application of the NRR.

Synthetic control over polymorph formation in the d-band semiconductor system FeS2

Pyrite, also known as fool's gold is the thermodynamic stable polymorph of FeS2. It is widely considered as a promising d-band semiconductor for various applications due to its intriguing physical properties. Marcasite is the other naturally occurring polymorph of FeS2. Measurements on natural crystals have shown that it has similarly promising electronic, mechanical, and optical properties as pyrite. However, it has been only scarcely investigated so far, because the laboratory-based synthesis of phase-pure samples or high quality marcasite single crystal has been a challenge until now. Here, we report the targeted phase formation via hydrothermal synthesis of marcasite and pyrite. The formation condition and phase purity of the FeS2 polymorphs are systematically studied in the form of a comprehensive synthesis map. We, furthermore, report on a detailed analysis of marcasite single crystal growth by a space-separated hydrothermal synthesis. We observe that single phase product of marcasite forms only on the surface under the involvement of H2S and sulphur vapor. The availability of high-quality crystals of marcasite allows us to measure the fundamental physical properties, including an allowed direct optical bandgap of 0.76 eV, temperature independent diamagnetism, an electronic transport gap of 0.11 eV, and a room-temperature carrier concentration of 4.14 × 1018 cm-3. X-ray absorption/emission spectroscopy are employed to measure the band gap of the two FeS2 phases. We find marcasite has a band gap of 0.73 eV, while pyrite has a band gap of 0.87 eV. Our results indicate that marcasite - that is now synthetically available in a straightforward fashion - is as equally promising as pyrite as candidate for various semiconductor applications based on earth abundant elements.

Bimetallic copper nickel sulfide electrocatalyst by one step chemical bath deposition for efficient and stable overall water splitting applications

Transition metal sulfides have been intensively investigated as an effective catalyst for overall water splitting application due to their inexorable bifunctional oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) activity. However, the chalcogenides are oxidised during the OER process and hence limit the stability of the electrocatalyst. The synthesized materials should have a higher oxidation state corresponding to the active species in order to improve the stability. In this study, we have employed a one-step chemical bath deposition (CBD) route to synthesis bimetallic copper nickel sulfide (CuNiS) electrocatalyst. We have accomplished a superior OER electrocatalytic activity with a lower overpotential of 337 mV at 10 mA/cm² current density and a small Tafel slope of 43 mV/dec. Also, we have achieved an excellent HER activity with a very low overpotential of 99 mV at 10 mA/cm² and a Tafel slope of 63 mV/dec. The constructed electrolyzer attained a lower cell voltage of only 1.55 V to reach the current density of 10 mA/cm²_. The stability test carried at a high current density of 200 mA/cm² for 50 h showed less than 5% increase in Ni³⁺ active species at the surface ensure the stable performance nature. Thus, this work provides a promising methodology for the synthesis of bimetallic sulfides for enhanced electrocatalytic water splitting with exceptional reliability.

Effect of Co Doping on Electrocatalytic Performance of Co-NiS2/CoS2 Heterostructures

There are abundant water resources in nature, and hydrogen production from electrolyzed water can be one of the main ways to obtain green and sustainable energy. Traditional water electrolysis uses precious metals as catalysts, but it is difficult to apply in massive volumes due to low reserves and high prices. It is still a challenge to develop hydrogen electrocatalysts with excellent performance but low cost to further improve the efficiency of hydrogen production. This article reported a potential candidate, the Co-NiS2/CoS2 (material is based on NiS2, and after Co doping, The NiS2/CoS2 heterostructure is formed) heterostructures, prepared by hydrothermal method with carbon paper as the substrate. In a 0.5 M sulfuric acid solution, the hydrogen evolution reaction with Co-NiS2/CoS2 as the electrode showed excellent catalytic performance. When the Co (Cobalt) doping concentration is increased to 27%, the overpotential is -133.3 mV, which is a drop of 81 mV compared with -214.3 mV when it is not doped. The heterostructure formed after doping also has good stability. After 800 CV cycles, the difference in overpotential is only 3 mV. The significant improvement of the catalytic performance can be attributed to the significant changes in the crystal structure and properties of the doped heterostructures, which provide an effective method for efficient electrocatalytic hydrogen production.

A Review: Scanning Electrochemical Microscopy (SECM) for Visualizing the Real-Time Local Catalytic Activity

Scanning electrochemical microscopy (SECM) is a powerful scanning probe technique for measuring the in situ electrochemical reactions occurring at various sample interfaces, such as the liquid-liquid, solid-liquid, and liquid-gas. The tip/probe of SECM is usually an ultramicroelectrode (UME) or a nanoelectrode that can move towards or over the sample of interest controlled by a precise motor positioning system. Remarkably, electrocatalysts play a crucial role in addressing the surge in global energy consumption by providing sustainable alternative energy sources. Therefore, the precise measurement of catalytic reactions offers profound insights for designing novel catalysts as well as for enhancing their performance. SECM proves to be an excellent tool for characterization and screening catalysts as the probe can rapidly scan along one direction over the sample array containing a large number of different compositions. These features make SECM more appealing than other conventional methodologies for assessing bulk solutions. SECM can be employed for investigating numerous catalytic reactions including the oxygen reduction reaction (ORR), oxygen evolution reaction (OER), hydrogen evolution reaction (HER), water oxidation, glucose oxidation reaction (GOR), and CO2 reduction reaction (CO2RR) with high spatial resolution. Moreover, for improving the catalyst design, several SECM modes can be applied based on the catalytic reactions under evaluation. This review aims to present a brief overview of the recent applications of electrocatalysts and their kinetics as well as catalytic sites in electrochemical reactions, such as oxygen reduction, water oxidation, and methanol oxidation.

Mapping surface morphology and phase evolution of iron sulfide nanoparticles

The size effect on the thermodynamic phase diagram of FexSy nanoparticles.

Local probe investigation of electrocatalytic activity

As the world energy crisis remains a long-term challenge, development and access to renewable energy sources are crucial for a sustainable modern society. Electrochemical energy conversion devices are a promising option for green energy supply, although the challenge associated with electrocatalysis have caused increasing complexity in the materials and systems, demanding further research and insights. In this field, scanning probe microscopy (SPM) represents a specific source of knowledge and understanding. Thus, our aim is to present recent findings on electrocatalysts for electrolysers and fuel cells, acquired mainly through scanning electrochemical microscopy (SECM) and other related scanning probe techniques. This review begins with an introduction to the principles of several SPM techniques and then proceeds to the research done on various energy-related reactions, by emphasizing the progress on non-noble electrocatalytic materials.

Production of Quasi-2D Platelets of Nonlayered Iron Pyrite (FeS2) by Liquid-Phase Exfoliation for High Performance Battery Electrodes

Over the past 15 years, two-dimensional (2D) materials have been studied and exploited for many applications. In many cases, 2D materials are formed by the exfoliation of layered crystals such as transition-metal disulfides. However, it has recently become clear that it is possible to exfoliate nonlayered materials so long as they have a nonisotropic bonding arrangement. Here, we report the synthesis of 2D-platelets from the earth-abundant, nonlayered metal sulfide, iron pyrite (FeS₂), using liquid-phase exfoliation. The resultant 2D platelets exhibit the same crystal structure as bulk pyrite but are surface passivated with a density of 14 × 10¹⁸ groups/m². They form stable suspensions in common solvents and can be size-selected and liquid processed. Although the platelets have relatively low aspect ratios (∼5), this is in line with the anisotropic cleavage energy of bulk FeS₂. We observe size-dependent changes to optical properties leading to spectroscopic metrics that can be used to estimate the dimensions of platelets. These platelets can be used to produce lithium ion battery anodes with capacities approaching 1000 mAh/g.

Sulfur-Doped CoSe2 Porous Nanosheets as Efficient Electrocatalysts for the Hydrogen Evolution Reaction

The electrochemical hydrogen evolution reaction (HER), as a promising route for hydrogen production, demands efficient and robust noble-metal-free catalysts. Doping foreign atoms into an efficient catalyst such as CoSe₂ could further enhance its activity toward the HER. Herein, we developed a solvothermal ion exchange approach to doping S into CoSe₂ nanosheets (NSs). We provide a combined experimental and theoretical investigation to establish the obtained S-doped CoSe₂ (S-CoSe₂) nanoporous NSs as highly efficient and Earth-abundant catalysts for the HER. The optimal S-CoSe₂ catalyst delivers a catalytic current density of 10 mA·cm^-2 for the HER at an overpotential of only 88 mV, demonstrating that S-CoSe₂ is one of the most efficient CoSe- and CoS-based catalysts for the HER. We performed density functional theory (DFT) calculations to determine the stable structural configurations of S-CoSe₂, and on the basis of which, we calculated the hydrogen adsorption Gibbs free energy (ΔG_H) on CoSe₂, CoS₂, and the S-CoSe₂ and the barrier energies of the rate-determining step of the HER on S-CoSe₂. DFT calculations reveal that S-doping not only decreases the absolute value of ΔG_H (move toward zero) but also significantly lowers the kinetic barrier energy of the rate-determining step of the HER on S-CoSe₂, leading to a greatly improved HER performance.

Optimized Metal Chalcogenides for Boosting Water Splitting

Electrocatalytic water splitting (2H2O → 2H2 + O2) is a very promising avenue to effectively and environmentally friendly produce highly pure hydrogen (H2) and oxygen (O2) at a large scale. Different materials have been developed to enhance the efficiency for water splitting. Among them, chalcogenides with unique atomic arrangement and high electronic transport show interesting catalytic properties in various electrochemical reactions, such as the hydrogen evolution reaction, oxygen evolution reaction, and overall water splitting, while the control of their morphology and structure is of vital importance to their catalytic performance. Herein, the general synthetic methods are summarized to prepare metal chalcogenides and different strategies are designed to improve their catalytic performance for water splitting. The remaining challenges in the research and development of metal chalcogenides and possible directions for future research are also summarized.

Ferromagnetic FeSe2 from a mixed sulphur-selenium complex of iron [Fe{(SePPh2NPPh2S)2N}3] through pyrolysis

Iron (III) thioselenoimidodiphosphinate complex, Fe{(SePPh₂NPPh₂S)₂N}₃], was synthesized from the ligand [Ph₂P(S)HNP(Se)Ph₂], and the complex employed as the combined source of the targeted elements (Fe and Se) to generate orthorhombic FeSe₂. This was achieved by thermolysis using a quartz glass tube, under reduced pressure at 500 °C during 1 h 30 min. The crystalline product was revealed by X-ray diffraction (XRD), while the morphology consisted of polygonal crystallites according to the scanning electron microscopy (SEM) studies. Superconducting quantum interference device (SQUID) measurements on the material confirmed its ferromagnetism as observed from the magnetization curve, indicated by the field-cooled and zero field-cooled conditions under a magnetic field of 100 Oe. This ferromagnetic material, FeSe₂ finds useful application in producing electrical semiconductors.

Co-Doped FeS2 with a porous structure for efficient electrocatalytic overall water splitting

The development of earth-abundant and high-efficiency electrocatalysts for overall water splitting is highly fascinating and still presents a challenge caused by the low activity for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) at the same time.

Applying surface strain and coupling with pure or N/B-doped graphene to successfully achieve high HER catalytic activity in 2D layered SnP3-based nanomaterials: a first-principles investigation

Applying external strain and coupling with pure or N/B-doped graphene can be viewed as effective strategies to further improve the HER activity of 2D layered SnP₃ nanomaterials by optimizing the adsorption state of H* and electronic properties.

FeS2/C Nanowires as an Effective Catalyst for Oxygen Evolution Reaction by Electrolytic Water Splitting

Electrolytic water splitting with evolution of both hydrogen (HER) and oxygen (OER) is an attractive way to produce clean energy hydrogen. It is critical to explore effective, but low-cost electrocatalysts for the evolution of oxygen (OER) owing to its sluggish kinetics for practical applications. Fe-based catalysts have advantages over Ni- and Co-based materials because of low costs, abundance of raw materials, and environmental issues. However, their inefficiency as OER catalysts has caused them to receive little attention. Herein, the FeS₂/C catalyst with porous nanostructure was synthesized with rational design via the in situ electrochemical activation method, which serves as a good catalytic reaction in the OER process. The FeS₂/C catalyst delivers overpotential values of only 291 mV and 338 mV current densities of 10 mA/cm² and 50 mA/cm², respectively, after electrochemical activation, and exhibits staying power for 15 h.

Porous TiO2/Co9S8 core-branch nanosheet arrays with high electrocatalytic activity for a hydrogen evolution reaction

Exploring new non-noble metallic catalysts with highly efficient and stable catalytic performance toward a hydrogen evolution reaction (HER) is a major requirement in the field of electrochemical water splitting. In this study, we report a facile strategy to synthesize a three-dimensional (3D) porous TiO₂/Co₉S₈ core-branch nanosheet arrays on Ni foam with superior HER catalytic activity. The hierarchically 3D porous architecture not only provides rich active sites, but also enables fast electron transport and easy electrolyte permeation. Consequently, the TiO₂/Co₉S₈ core-branch nanosheet arrays achieve a remarkably low overpotential of 150 mV at 10 mA cm^-2, small Tafel slope of 71 mV Dec^-1 and outstanding long-term stability for the HER reaction. The elaborate design should provide a new strategy to controllably synthesize porous and rational nanostructured materials as a prominent electrocatalyst for water splitting applications.

Fabrication of C/Co-FeS2/CoS2 with Highly Efficient Hydrogen Evolution Reaction

The mainstream strategy for designing hydrogen electrocatalysts is to adjust their surface electronic structure; however, the conductivity of the electrocatalyst and the synergy with its substrate are still challenges to overcome. In this work, we report a carbon-doped Co-FeS2/CoS2 (C/Co-FeS2/CoS2) electrode, prepared via a hydrothermal process with carbon cloth (CC) as the substrate and carbon doping. The C/Co-FeS2/CoS2 electrode shows excellent catalytic activity in the hydrogen evolution reaction (HER) with an overpotential of 88 mV at a current density of −10 mA∙cm−2 in 0.5 M H2SO4 solution. The Tafel slope is 66 mV∙dec−1. Such superior performance is attributed to the high electrical conductivity of the electrocatalyst and its synergy with the substrate. Our study provides an efficient alternative in the field of electrocatalysis.

Iron Sulfide Materials: Catalysts for Electrochemical Hydrogen Evolution

The chemical challenge of economically splitting water into molecular hydrogen and oxygen requires continuous development of more efficient, less-toxic, and cheaper catalyst materials. This review article highlights the potential of iron sulfide-based nanomaterials as electrocatalysts for water-splitting and predominantly as catalysts for the hydrogen evolution reaction (HER). Besides new synthetic techniques leading to phase-pure iron sulfide nano objects and thin-films, the article reviews three new material classes: (a) FeS2-TiO2 hybrid structures; (b) iron sulfide-2D carbon support composites; and (c) metal-doped (e.g., cobalt and nickel) iron sulfide materials. In recent years, immense progress has been made in the development of these materials, which exhibit enormous potential as hydrogen evolution catalysts and may represent a genuine alternative to more traditional, noble metal-based catalysts. First developments in this comparably new research area are summarized in this article and discussed together with theoretical studies on hydrogen evolution reactions involving iron sulfide electrocatalysts.

Rapid sonochemical synthesis of silver nano-leaves encapsulated on iron pyrite nanocomposite: An excellent catalytic application in the electrochemical detection of herbicide (Acifluorfen)

Herein, we developed a silver nanoparticles decorated iron pyrite flowers (FeS₂@Ag NL) based nanocomposite was prepared by a sonochemical method. The formation of FeS₂@Ag NL nanocomposite was confirmed by XRD, XPS, HR-TEM and analytical techniques. The FeS₂@Ag NL/SPCE was potentially applied towards electrochemical detection of toxic herbicide (acifluorfen-AFF). This provided an efficient sensor platform anchoring FeS₂@Ag NL on its surface. Under optimized conditions of differential pulse voltammetric transduction, a linear relationship between the current and the concentration was obtained in the range of 0.05-1126.45 µM for Acifluorfen. The detection limit was observed to be 0.0025 µM. the modified sensor exhibits excellent electrochemical performance, including good linear range, nanomolar detection limit, high sensitivity, and desirable stability. Particularly, the practical applicability was revealed by quantifying the AFF concentration in biological samples.

Nanoarchitectonics for Transition-Metal-Sulfide-Based Electrocatalysts for Water Splitting

Heterogenous electrocatalysts based on transition metal sulfides (TMS) are being actively explored in renewable energy research because nanostructured forms support high intrinsic activities for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Herein, it is described how researchers are working to improve the performance of TMS-based materials by manipulating their internal and external nanoarchitectures. A general introduction to the water-splitting reaction is initially provided to explain the most important parameters in accessing the catalytic performance of nanomaterials catalysts. Later, the general synthetic methods used to prepare TMS-based materials are explained in order to delve into the various strategies being used to achieve higher electrocatalytic performance in the HER. Complementary strategies can be used to increase the OER performance of TMS, resulting in bifunctional water-splitting electrocatalysts for both the HER and the OER. Finally, the current challenges and future opportunities of TMS materials in the context of water splitting are summarized. The aim herein is to provide insights gathered in the process of studying TMS, and describe valuable guidelines for engineering other kinds of nanomaterial catalysts for energy conversion and storage technologies.

Co(ii)Tetraphenyltetraphenanthroporphyrin@MWCNTs: enhanced π–π interaction for robust electrochemical catalysis

A cobalt(ii)tetraphenyltetraphenanthroporphyrin with phenanthrene-fused pyrrole rings was applied for robust HER and ORR.

Bifunctional iron disulfide nanoellipsoids for high energy density supercapacitor and electrocatalytic oxygen evolution applications

FeS₂, prepared using a rapid microwave assisted method, exhibits excellent electrochemical performance for supercapacitor and OER applications.

Earth-Abundant Electrocatalysts in Proton Exchange Membrane Electrolyzers

In order to adopt water electrolyzers as a main hydrogen production system, it is critical to develop inexpensive and earth-abundant catalysts. Currently, both half-reactions in water splitting depend heavily on noble metal catalysts. This review discusses the proton exchange membrane (PEM) water electrolysis (WE) and the progress in replacing the noble-metal catalysts with earth-abundant ones. The efforts within this field for the discovery of efficient and stable earth-abundant catalysts (EACs) have increased exponentially the last few years. The development of EACs for the oxygen evolution reaction (OER) in acidic media is particularly important, as the only stable and efficient catalysts until now are noble-metal oxides, such as IrOx and RuOx. On the hydrogen evolution reaction (HER) side, there is significant progress on EACs under acidic conditions, but there are very few reports of these EACs employed in full PEM WE cells. These two main issues are reviewed, and we conclude with prospects for innovation in EACs for the OER in acidic environments, as well as with a critical assessment of the few full PEM WE cells assembled with EACs.

Earth-Abundant Transition-Metal-Based Electrocatalysts for Water Electrolysis to Produce Renewable Hydrogen

Fundamentals of water electrolysis, and recent research progress and trends in the development of earth-abundant first-row transition-metal (Mn, Fe, Co, Ni, Cu)-based oxygen evolution reaction (OER) and hydrogen evolution (HER) electrocatalysts working in acidic, alkaline, or neutral conditions are reviewed. The HER catalysts include mainly metal chalcogenides, metal phosphides, metal nitrides, and metal carbides. As for the OER catalysts, the basic principles of the OER catalysts in alkaline, acidic, and neutral media are introduced, followed by the review and discussion of the Ni, Co, Fe, Mn, and perovskite-type OER catalysts developed so far. The different design principles of the OER catalysts in photoelectrocatalysis and photocatalysis systems are also presented. Finally, the future research directions of electrocatalysts for water splitting, and coupling of photovoltaic (PV) panel with a water electrolyzer, so called PV-E, are given as perspectives.

Synthesis of Thin-Film Metal Pyrites by an Atomic Layer Deposition Approach

Late 3d transition metal disulfides (MS₂ , M=Fe, Co, Ni, Cu, Zn) can crystallize in an interesting cubic-pyrite structure, in which all the metal cations are in a low-spin electronic configuration with progressive increase of the e_g electrons for M=Fe-Zn. These metal pyrite compounds exhibit very diverse and intriguing electrical and magnetic properties, which have stimulated considerable attention for various applications, especially in cutting-edge energy conversion and storage technologies. The synthesis of the metal pyrites is certainly very important, because highly controllable, reproducible, and reliable synthesis methods are virtually essential for both fundamental materials research and practical engineering. In this Concept, a new approach of (plasma-assisted) atomic layer deposition (ALD) to synthesize the thin-film metal pyrites (FeS₂ , CoS₂ , NiS₂ ) is introduced. The ALD synthesis approach allows for atomic-precision control over film composition and thickness, excellent film uniformity and conformality, and superior process reproducibility, and therefore it is of high promise for uniformly conformal metal pyrite thin-film coatings on complex 3D structures in general. Details and implications of this ALD approach are discussed in this Concept, mainly from a conceptual perspective, and it is envisioned that, with this new ALD synthesis approach, a significant amount of new studies will be enabled on both the fundamentals, and novel applications of the metal pyrite materials.

Insight into the Nature of Iron Sulfide Surfaces During the Electrochemical Hydrogen Evolution and CO2 Reduction Reactions

Greigite and other iron sulfides are potential, cheap, earth-abundant electrocatalysts for the hydrogen evolution reaction (HER), yet little is known about the underlying surface chemistry. Structural and chemical changes to a greigite (Fe₃S₄)-modified electrode were determined at -0.6 V versus standard hydrogen electrode (SHE) at pH 7, under conditions of the HER. In situ X-ray absorption spectroscopy was employed at the Fe K-edge to show that iron-sulfur linkages were replaced by iron-oxygen units under these conditions. The resulting material was determined as 60% greigite and 40% iron hydroxide (goethite) with a proposed core-shell structure. A large increase in pH at the electrode surface (to pH 12) is caused by the generation of OH^- as a product of the HER. Under these conditions, iron sulfide materials are thermodynamically unstable with respect to the hydroxide. In situ infrared spectroscopy of the solution near the electrode interface confirmed changes in the phosphate ion speciation consistent with a change in pH from 7 to 12 when -0.6 V versus SHE is applied. Saturation of the solution with CO₂ resulted in the inhibition of the hydroxide formation, potentially due to surface adsorption of HCO₃^-. This study shows that the true nature of the greigite electrode under conditions of the HER is a core-shell greigite-hydroxide material and emphasizes the importance of in situ investigation of the catalyst under operation to develop true and accurate mechanistic models.

Synthesis of nanostructured powders and thin films of iron sulfide from molecular precursors

Iron(iii) xanthate single-source precursors [Fe(S2COR)3] (R = methyl, ethyl, isopropyl and 1-propyl) were used to deposit iron sulfide thin films and nanostructures by two simple, efficient and low-cost methods (spin coating and solid state deposition). The single-crystal X-ray structures of the iron(iii) n-propyl xanthate and iron(iii) iso-propyl xanthate have been determined. Thermogravimetric analysis (TGA) studies of the complexes shows that decomposition of the complexes produces iron sulfide, pyrite or trolite. The crystallinity of iron sulfide thin films and powder samples was studied using X-ray diffraction (XRD), and their morphology was studied by scanning electron microscopy (SEM).

Construction of Polarized Carbon-Nickel Catalytic Surfaces for Potent, Durable, and Economic Hydrogen Evolution Reactions

Electrocatalytic hydrogen evolution reaction (HER) in alkaline solution is hindered by its sluggish kinetics toward water dissociation. Nickel-based catalysts, as low-cost and effective candidates, show great potentials to replace platinum (Pt)-based materials in the alkaline media. The main challenge regarding this type of catalysts is their relatively poor durability. In this work, we conceive and construct a charge-polarized carbon layer derived from carbon quantum dots (CQDs) on Ni₃N nanostructure (Ni₃N@CQDs) surfaces, which simultaneously exhibit durable and enhanced catalytic activity. The Ni₃N@CQDs shows an overpotential of 69 mV at a current density of 10 mA cm^-2 in a 1 M KOH aqueous solution, lower than that of Pt electrode (116 mV) at the same conditions. Density functional theory (DFT) simulations reveal that Ni₃N and interfacial oxygen polarize charge distributions between originally equal C-C bonds in CQDs. The partially negatively charged C sites become effective catalytic centers for the key water dissociation step via the formation of new C-H bond (Volmer step) and thus boost the HER activity. Furthermore, the coated carbon is also found to protect interior Ni₃N from oxidization/hydroxylation and therefore guarantees its durability. This work provides a practical design of robust and durable HER electrocatalysts based on nonprecious metals.

Electron density modulation of NiCo2S4 nanowires by nitrogen incorporation for highly efficient hydrogen evolution catalysis

Metal sulfides for hydrogen evolution catalysis typically suffer from unfavorable hydrogen desorption properties due to the strong interaction between the adsorbed H and the intensely electronegative sulfur. Here, we demonstrate a general strategy to improve the hydrogen evolution catalysis of metal sulfides by modulating the surface electron densities. The N modulated NiCo2S4 nanowire arrays exhibit an overpotential of 41 mV at 10 mA cm-2 and a Tafel slope of 37 mV dec-1, which are very close to the performance of the benchmark Pt/C in alkaline condition. X-ray photoelectron spectroscopy, synchrotron-based X-ray absorption spectroscopy, and density functional theory studies consistently confirm the surface electron densities of NiCo2S4 have been effectively manipulated by N doping. The capability to modulate the electron densities of the catalytic sites could provide valuable insights for the rational design of highly efficient catalysts for hydrogen evolution and beyond.