Ultrafast Formation of Amorphous Bimetallic Hydroxide Films on 3D Conductive Sulfide Nanoarrays for Large-Current-Density Oxygen Evolution Electrocatalysis

Porous Fe-Doped β-Ni(OH)2 Nanopyramid Array Electrodes for Water Splitting

We report a highly efficient and stable electrode composed of a porous Fe-doped β-nickel hydroxide nanopyramid array supported on nickel foam (U-Fe-β-Ni(OH)₂/NF) for overall water splitting. The unique structure is assembled via a self-templated strategy by utilizing the FeNi oxalate (FeNi-C₂O₄/NF) nanopyramid as the templates, followed by an anion-exchange reaction at room temperature. Due to the intrinsic activity of Fe-doped β-Ni(OH)₂ along with unique porous array structures consisting of two-dimensional (2D) active materials on three-dimensional (3D) conductive substrates, the developed electrode exhibited outstanding electrocatalytic activity for both the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in an alkaline medium. The introduced amount of Fe plays a significant role in promoting OER and HER activity compared to the β-Ni(OH)₂ electrode. The optimal electrode (U-Fe-β-Ni(OH)₂/NF-2) generated a current density of 10 mA cm^-2 at low overpotentials of 218 mV for the OER and 121 mV for the HER. The electrode also demonstrated considerably stable performance during the continuous water splitting process. Furthermore, we elucidated the promotion mechanisms of the active Fe-doped β-Ni(OH)₂ compound for the OER and HER based on extensive characterization and electrochemical measurements. Hence, this work provides a facile approach to developing low-cost, efficient, and stable hydroxide-based electrodes for bifunctional OER and HER in water splitting.

2D CoOOH Sheet-Encapsulated Ni2P into Tubular Arrays Realizing 1000 mA cm-2-Level-Current-Density Hydrogen Evolution Over 100 h in Neutral Water

Water electrolysis at high current density (1000 mA cm-2 level) with excellent durability especially in neutral electrolyte is the pivotal issue for green hydrogen from experiment to industrialization. In addition to the high intrinsic activity determined by the electronic structure, electrocatalysts are also required to be capable of fast mass transfer (electrolyte recharge and bubble overflow) and high mechanical stability. Herein, the 2D CoOOH sheet-encapsulated Ni2P into tubular arrays electrocatalytic system was proposed and realized 1000 mA cm-2-level-current-density hydrogen evolution over 100 h in neutral water. In designed catalysts, 2D stack structure as an adaptive material can buffer the shock of electrolyte convection, hydrogen bubble rupture, and evolution through the release of stress, which insure the long cycle stability. Meanwhile, the rich porosity between stacked units contributed the good infiltration of electrolyte and slippage of hydrogen bubbles, guaranteeing electrolyte fast recharge and bubble evolution at the high-current catalysis. Beyond that, the electron structure modulation induced by interfacial charge transfer is also beneficial to enhance the intrinsic activity. Profoundly, the multiscale coordinated regulation will provide a guide to design high-efficiency industrial electrocatalysts.

Bismuth iron molybdenum oxide solid solution: a novel and durable electrocatalyst for overall water splitting

The drive for finding active bifunctional electrocatalysts for efficient overall water splitting continues in order to extract energy in the form of hydrogen as a clean fuel. Bismuth iron molybdenum oxide solid solution, composed of orthorhombic Bi2MoO6 as the major component and monoclinic Bi3(FeO4)(MoO4)2 as the minor component, has been identified as a potential electrocatalyst for the first time.

2D Boron Imidazolate Framework Nanosheets with Electrocatalytic Applications for Oxygen Evolution and Carbon Dioxide Reduction Reaction

Ultrathin 2D materials possess unique properties that translate to enhanced efficiency as electrocatalysts, stimulating research toward methodologies that support their preparation. Herein, a two-step strategy is reported that involves the preparation of the new boron imidazolate framework (BIF-73) which is subsequently utilized as a precursor to yield the crystalline 2D nanosheet material (Fe@BIF-73-NS) via post-synthetic modification. This new electrocatalytic material stabilizes ultra-small (Fe₂ O₃ ) fragments resulting in an excellent electrocatalytic performance for the oxygen evolution reaction (OER: lower overpotential with 291 mV at the current density of 10 mA cm^-2 ) and carbon dioxide reduction reaction (faradaic efficiency of CO reaching 88.6% at -1.8 V vs Ag/AgCl) without the need for noble metals. Additionally, theoretical calculations and microscopy reveal that the superior OER performance can be attributed to the increased exposure of binding sites within the material to which the catalytically active Fe³⁺ centers are bound through a post-synthetic modification procedure. A red-shift of the Fermi level around the valence band is observed and is proposed to be a result of the aforementioned interactions. This work opens an avenue toward the development of 2D functional metal organic framework nanosheets for energy conversion applications.

CoNiSe2 Nanostructures for Clean Energy Production

Comparative investigation of the electrochemical oxygen evolution reaction (OER) activity for clean energy production was performed by fabricating three different electrodes, namely, NiSe2, CoSe2, and CoNiSe2, synthesized by hydrothermal treatment. Cubic, orthorhombic, and hexagonal structures of NiSe2, CoSe2, and CoNiSe2 were confirmed by X-ray diffraction (XRD) and also by other characterization studies. Perfect nanospheres, combination of distorted nanospheres and tiny nanoparticles, and sharp-edge nanostructures of NiSe2, CoSe2, and CoNiSe2 were explored by surface morphological images. Higher OER activity of the binary CoNiSe2 electrode was achieved as 188 mA/g current density with a comparatively low overpotential of 234 mV along with higher conductivity and low charge transfer resistance when compared to its unary NiSe2 and CoSe2 electrodes. A low Tafel slope value of 82 mV/dec was also achieved for the same binary CoNiSe2 electrode in a half-cell configuration. The overall 100% retention achieved for all of the fabricated electrodes in a stability test of OER activity suggested that the excellent optimum condition was obtained during the synthesis. This could definitely be a revelation in the synthesis of novel binary combinations of affordable metal selenides for clean energy production.

Controlled Synthesis of Cr-Co0.85 Se Nanoarrays for Water Splitting at an Ultralow Cell Voltage of 1.43 V

Water splitting has attracted more and more attention as a promising strategy for the production of clean hydrogen fuel. In this work, a new synthesis strategy was proposed, and Co_0.85 Se was synthesized on nickel foam as the main matrix. The doping of appropriate Cr amount into the target of Co_0.85 Se and the Cr-Co_0.85 Se resulted in an excellent electrochemical performance. The doping of Cr introduces Cr³⁺ ions which substitute Co²⁺ and Co³⁺ ions in Co_0.85 Se, so that the lattice parameters of the main matrix were changed. It is worth noting that the Cr0.15-Co_0.85 Se/NF material exhibits an excellent performance in the oxygen evolution reaction (OER) test. When the current density reaches 50 mA cm^-2 for OER, the overpotential is only 240 mV. For the hydrogen evolution reaction (HER) tests, the overpotential is only 117 mV to drive 10 mA cm^-2 of current density. Moreover, when the Cr0.15-Co_0.85 Se/NF material is used as a two-electrode device for whole water splitting, the required cell voltage is only 1.43 V to reach a current density of 10 mA cm^-2 , which is among the lowest values of the published catalysts up to now. In addition, the Cr0.15-Co_0.85 Se/NF catalyst also exhibits excellent stability during a long period of water splitting. The experimental result demonstrates that the change of the lattice structure has an obvious influence on the electrocatalytic activity of the material. When an external electric field is applied, it facilitates the rapid electron transfer rate and enhances the electrocatalytic performance and stability of the material.

Amorphous Ni-Fe-Mo Suboxides Coupled with Ni Network as Porous Nanoplate Array on Nickel Foam: A Highly Efficient and Durable Bifunctional Electrode for Overall Water Splitting

It is a great challenge to fabricate electrode with simultaneous high activity for the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). Herein, a high-performance bifunctional electrode formed by vertically depositing a porous nanoplate array on the surface of nickel foam is provided, where the nanoplate is made up by the interconnection of trinary Ni-Fe-Mo suboxides and Ni nanoparticles. The amorphous Ni-Fe-Mo suboxide and its in situ transformed amorphous Ni-Fe-Mo (oxy)hydroxide acts as the main active species for HER and OER, respectively. The conductive network built by Ni nanoparticles provides rapid electron transfer to active sites. Moreover, the hydrophilic and aerophobic electrode surface together with the hierarchical pore structure facilitate mass transfer. The corresponding water electrolyzer demonstrates low cell voltage (1.50 V @ 10 mA cm-2 and 1.63 V @ 100 mA cm-2) with high durability at 500 mA cm-2 for at least 100 h in 1 m KOH.

Bottom-up Design of Bimetallic Cobalt-Molybdenum Carbides/Oxides for Overall Water Splitting

Earth-abundant transition-metal-based catalysts for electrochemical water splitting are critical for sustainable energy schemes. In this work, we use a rational design method for the synthesis of ultrasmall and highly dispersed bimetallic CoMo carbide/oxide particles deposited on graphene oxide. Thermal conversion of the molecular precursors [H3 PMo12 O40 ], Co(OAc)2 ⋅4 H2 O and melamine in the presence of graphene oxide gives the mixed carbide/oxide (Co6 Mo6 C2 /Co2 Mo3 O8 ) nanoparticle composite deposited on highly dispersed, N,P-doped carbon. The resulting composite shows outstanding electrocatalytic water-splitting activity for both the oxygen evolution and hydrogen evolution reaction, and superior performance to reference samples including commercial 20 % Pt/C & IrO2 . Electrochemical and other materials analyses indicate that Co6 Mo6 C2 is the main active phase in the composite, and the N,P-doping of the carbon matrix increases the catalytic activity. The facile design could in principle be extended to multiple bimetallic catalyst classes by tuning of the molecular metal oxide precursor.

In Situ Growth of Amorphous Fe(OH)3 on Nickel Nitrate Hydroxide Nanoarrays for Enhanced Electrocatalytic Oxygen Evolution

Development of highly efficient electrocatalyst for the oxygen evolution reaction (OER) is urgently demanded by the clean hydrogen energy. Herein, in order to further boost the OER activity of metal nitrate hydroxide materials, amorphous Fe(OH)₃ layer is in situ grown on nickel nitrate hydroxide (NiNH) nanoarrays supported on nickel foam (NF) through an interfacial hydrolysis approach, where the loading amount of the Fe(OH)₃ can be simply manipulated by the hydrolysis time. Taking advantage of the synergy of Fe(OH)₃ and NiNH, the optimized Fe(OH)₃@NiNH/NF sample shows a very promising electrocatalytic OER activity in 1 M KOH solution, requiring a very low overpotential of 212 mV vs. reversible hydrogen electrode (RHE) to deliver a geometrical catalytic current density of 100 mA cm^-2 and a low Tafel slope of 49 mV dec^-1. This work provides a new strategy for boosting the electrocatalytic activity of metal hydroxide nitrates through the interface engineering.

Ligand-assisted capping growth of self-supporting ultrathin FeNi-LDH nanosheet arrays with atomically dispersed chromium atoms for efficient electrocatalytic water oxidation

Self-supporting ultrathin FeNi-layered double hydroxide nanosheet arrays with atomically dispersed Cr atoms were firstly fabricated from stainless steel mesh by a facile ligand-assisted capping growth approach. Their unique nanostructure and a strong synergetic effect between the atomically dispersed Cr dopants and the active sites afford an exceptional OER activity.

Controlled phosphating: a novel strategy toward NiP3@CeO2 interface engineering for efficient oxygen evolution electrocatalysis

Although Ni phosphides are efficient for hydrogen evolution reactions, they are unfavorable for oxygen evolution reactions, so their application in alkaline water electrolysis is limited. It is a feasible method for creating a novel Ni phosphide/oxide heterogeneous interface to promote the oxygen evolution kinetics of Ni phosphide materials in an alkaline medium, yet it has been an unprecedented challenge for researchers. In this work, NiP3@CeO2 hybrid nanoparticles are firstly in situ grown on Ni foam (NiP3@CeO2/NF) via a novel controlled phosphating strategy. The NiP3@CeO2/NF catalysts display a fairly small overpotential of 200 mV to achieve a current density of 25 mA cm-2 for the oxygen evolution reaction (OER) under alkaline conditions, 110 mV smaller than that of NiO@CeO2/NF. It is noteworthy that the improved electrocatalytic performance of NiP3@CeO2/NF can be attributed to rapid electron transfer and the synergistic catalytic effect of the hybrid material. Density functional theory results demonstrate that NiP3 shows a stronger water adsorption energy than CeO2. The novel strategy of controlled phosphating to construct transition metal phosphide/oxide interfaces provides new ideas and methods for the development of efficient and practical water splitting catalysts.

Self-Supported Transition-Metal-Based Electrocatalysts for Hydrogen and Oxygen Evolution

Electrochemical water splitting is a promising technology for sustainable conversion, storage, and transport of hydrogen energy. Searching for earth-abundant hydrogen/oxygen evolution reaction (HER/OER) electrocatalysts with high activity and durability to replace noble-metal-based catalysts plays paramount importance in the scalable application of water electrolysis. A freestanding electrode architecture is highly attractive as compared to the conventional coated powdery form because of enhanced kinetics and stability. Herein, recent progress in developing transition-metal-based HER/OER electrocatalytic materials is reviewed with selected examples of chalcogenides, phosphides, carbides, nitrides, alloys, phosphates, oxides, hydroxides, and oxyhydroxides. Focusing on self-supported electrodes, the latest advances in their structural design, controllable synthesis, mechanistic understanding, and strategies for performance enhancement are presented. Remaining challenges and future perspectives for the further development of self-supported electrocatalysts are also discussed.

Amorphous Catalysts and Electrochemical Water Splitting: An Untold Story of Harmony

In the near future, sustainable energy conversion and storage will largely depend on the electrochemical splitting of water into hydrogen and oxygen. Perceiving this, countless research works focussing on the fundamentals of electrocatalysis of water splitting and on performance improvements are being reported everyday around the globe. Electrocatalysts of high activity, selectivity, and stability are anticipated as they directly determine energy- and cost efficiency of water electrolyzers. Amorphous electrocatalysts with several advantages over crystalline counterparts are found to perform better in electrocatalytic water splitting. There are plenty of studies witnessing performance enhancements in electrocatalysis of water splitting while employing amorphous materials as catalysts. The harmony between the flexibility of amorphous electrocatalysts and electrocatalysis of water splitting (both the oxygen evolution reaction [OER] and the hydrogen evolution reaction [HER]) is one of the untold and unsummarized stories in the field of electrocatalytic water splitting. This Review is devoted to comprehensively discussing the upsurge of amorphous electrocatalysts in electrochemical water splitting. In addition to that, the basics of electrocatalysis of water splitting are also elaborately introduced and the characteristics of a good electrocatalyst for OER and HER are discussed.

Three-Dimensional Hierarchical Porous Nanotubes Derived from Metal-Organic Frameworks for Highly Efficient Overall Water Splitting

Effective design of bifunctional catalysts for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is important but remains challenging. Herein, we report a three-dimensional (3D) hierarchical structure composed of homogeneously distributed Ni-Fe-P nanoparticles embedded in N-doped carbons on nickel foams (denoted as Ni-Fe-P@NC/NF) as an excellent bifunctional catalyst. This catalyst was fabricated by an anion exchange method and a low-temperature phosphidation of nanotubular Prussian blue analogue (PBA). The Ni-Fe-P@NC/NF displayed exceptional catalytic activity toward both HER and OER and delivered an ultralow cell voltage of 1.47 V to obtain 10 mA cm⁻² with extremely excellent durability for 100 h when assembled as a practical electrolyser. The extraordinary performance of Ni-Fe-P@NC/NF is attributed to the abundance of unsaturated active sites, the well-defined hierarchical porous structure, and the synergistic effect between multiple components. Our work will inspire more rational designs of highly active non-noble electrocatalysts for industrial energy applications.

•

Nanotubular Prussian blue analogue as a precursor is synthesized by anion exchange

•

The catalyst exhibits excellent catalytic activity for hydrogen and oxygen production

•

The catalyst-based electrolyser has a low cell voltage of 1.47 V to obtain 10 mA cm⁻²

•

The electrolyser shows an extremely excellent durability for 100 h at 50 mA cm⁻²

Mixed-Metal-Cluster Strategy for Boosting Electrocatalytic Oxygen Evolution Reaction of Robust Metal-Organic Frameworks

Rational construction of mixed metal-organic frameworks (MOFs) has been proved to be an effective way to heighten the electrocatalytic performance for MOFs, while the function of mixed metal clusters in MOFs to the electrocatalytic activity has not been known. For the first time, we present a mixed-metal-cluster strategy to boost electrocatalytic oxygen evolution reaction (OER) performance for MOFs. Heterometal clusters (Fe₂M(μ₃-O) (CH₃COO)₆(H₂O)₃, denoted as Fe₂M (M: Co or Ni)), were chosen as the metal source to construct two bimetal PCN-250-Fe₂M. Then, we further mixed Fe₂Co and Fe₂Ni clusters to construct mixed-metal-cluster PCN-Fe₂Co-Fe₂Ni by the in situ solvothermal reaction. Consequently, the OER activity of PCN-Fe₂Co-Fe₂Ni shows a dramatic enhancement compared with that of the parent bimetal MOFs. The PCN-Fe₂Co-Fe₂Ni displays a lower overpotential of 271 mV (η₁₀), small Tafel slope (67.7 mV dec^-1), and good linear sweep voltammetry cycle stability for the OER. Combination of DFT calculation and experiment results show that the improved electrocatalytic activity of PCN-Fe₂Co-Fe₂Ni is ascribed to the increased electron density of Co and Ni active centers and electrochemically active areas.

Controlled Synthesis of CuCo2 S4 @Ni(OH)2 Hybrid Nanorod Arrays for Water Splitting at an Ultralow Cell Voltage of 1.47 V

Developing environmentally friendly and highly active water splitting catalysts would be of great significance for clean energy conversion and utilization processes. Heterogeneous CuCo₂ S₄ @Ni(OH)₂ nanorod arrays with abundant oxygen vacancy firstly have been designed through a controllable hydrothermal and electrodeposition method. The synergies and open structures of the particular hierarchical structure together with the abundant oxygen vacancies offer more surface reactive centers, which can promote the electron transfer rate and reduce the activation energy of intermediate species. The CuCo₂ S₄ @Ni(OH)₂ -20 min nanorod arrays are considered as an excellent and robust electrocatalyst for the proton reduction under an alkaline condition with an extraordinary low overpotential of 117 mV at 10 mA cm^-2 . The CuCo₂ S₄ @Ni(OH)₂ -20 min heterostructures electrode is also stable and robust for the water oxidation reaction, needing an overpotential of only 250 mV to obtain 100 mA cm^-2 . Therefore, an alkaline electrolyzer was designed using CuCo₂ S₄ @Ni(OH)₂ -20 min nanorod arrays as bifunctional electrocatalyst, which can complete overall water splitting at a cell voltage of 1.47 V with 10 mA cm^-2 , suggesting a promising combination of the same material for efficient overall water splitting device. The cell voltage of 1.47 V, to our knowledge, is among the lowest values of the published support catalysts for electrocatalytic water splitting up to now.

Electronic Structure and Crystalline Phase Dual Modulation via Anion-Cation Co-doping for Boosting Oxygen Evolution with Long-Term Stability Under Large Current Density

Designing a state-of-the-art nonprecious oxygen evolution reaction (OER) electrocatalyst with ultralong stability under high current density (≥100 h under 1000 mA cm^-2) is greatly desirable for the viable electrolysis of water. The synthesis of nanostructure catalysts is an effective method for improving the OER performance, but nanostructure-based catalysts are easily destroyed by mechanical force via the vigorous oxygen gas evolution process at a high current density. Herein, we present a facile strategy of N-anion and Fe-cation dual doping to construct a three-dimensional self-supported nickel selenide film-based catalyst via a one-step chemical vapor deposition process. The film exhibits outstanding OER activity with a small Tafel slope of 34.86 mV dec^-1 and an overpotential of 267 mV at 100 mA cm^-2 in 1 M KOH media. Impressively, the film-based catalyst can maintain this excellent catalytic activity over 100 h, even when operated at a high current density of 1 A cm^-2, thus exhibiting the best reported OER stability under high current density so far. Further studies reveal that anion-cation co-doping can simultaneously modulate the electronic state and phase structure of nickel selenide, thereby promoting the in situ formation and transformation of oxygen-vacancy-rich amorphous OER active species and resulting in the superior OER performance of the film-based catalyst.

C-CoP hollow microporous nanocages based on phosphating regulation: a high-performance bifunctional electrocatalyst for overall water splitting

Developing economic, effective and stable bifunctional electrocatalysts to achieve sustainable hydrogen production is highly desired. Herein, C-coated CoP hollow microporous nanocages (C-CoP-1/12) are synthesized by calcination of a Prussian blue analog precursor and subsequent phosphorization treatment. Under alkaline condition, the C-CoP-1/12 exhibit splendid electrocatalytic performance with a low overpotential of 173 mV for hydrogen evolution reaction (HER) and 333 mV for oxygen evolution reaction (OER) at a current density of 10 mA cm-2. The C-CoP-1/12 show high electrocatalytic performance for overall water splitting at a low potential of only 1.650 V for the driving current density of 10 mA cm-2, and they exhibit remarkable stability for at least 24 h. The engineering of phosphating is the critical step for the synthesis of pure-phase CoP with hollow nanoarchitecture. Compared with Co2P, CoP possesses lower water dissociation barrier and favorable ΔGH* value according to theoretical calculations, resulting in superior electrocatalytic performance. Such impressive water splitting performance is mainly attributed to the collective effects of metal phosphide with unique electronic structure, the shortened electron transport paths, and the conductive C coating. This strategy is believed to provide a basis for the development of electrode materials with highly efficient electrocatalytic water-splitting capability.

Amorphous Oxide Nanostructures for Advanced Electrocatalysis

Amorphous oxides have attracted special attention as advanced electrocatalysts owing to their unique local structural flexibility and attractive electrocatalytic properties. With abundant randomly oriented bonds and surface-exposed defects (e.g., oxygen vacancies) as active catalytic sites, the adsorption/desorption of reactants can be optimized, leading to superior catalytic activities. Amorphous oxide materials have found wide electrocatalytic applications ranging from hydrogen evolution and oxygen evolution to oxygen reduction, CO₂ electroreduction and nitrogen electroreduction. The amorphous oxide electrocatalysts even outperform their crystalline counterparts in terms of electrocatalytic activity and stability. Despite of the merits and achievements for amorphous oxide electrocatalysts, there are still issues and challenges existing for amorphous oxide electrocatalysts. There are rarely reviews specifically focusing on amorphous oxide electrocatalysts and therefore it is imperative to have a comprehensive overview of the research progress and to better understand the achievements and issues with amorphous oxide electrocatalysts. In this minireview, several general preparation methods for amorphous oxides are first introduced. Then, the achievements in amorphous oxides for several important electrocatalytic reactions are summarized. Finally, the challenges and perspectives for the development of amorphous oxide electrocatalysts are outlined.

In Situ Transformation of Prussian-Blue Analogue-Derived Bimetallic Carbide Nanocubes by Water Oxidation: Applications for Energy Storage and Conversion

Using bimetallic Prussian blue analogue (PBA) as a precursor is effective for preparing electrocatalysts for the oxygen evolution reaction (OER); however, the role of these PBA-derived catalysts in the OER is still ambiguous. Herein, by simply controlling synthesis temperature, a bimetallic PBA-derived O,N-codoped Ni-Fe carbide, can be well tuned to optimize structure and OER performance. Importantly, by a series of ex situ and in situ investigations, real active species of NiFeO_x H_y are in situ formed on the surface during the OER, which reveals a "pre-catalyst" role of O,N-codoped Ni-Fe carbides. Furthermore, it has been successfully applied to highly efficient Zn-air batteries and outplays its RuO₂ counterpart. When applied to photoelectrocatalytic water oxidation as the co-catalyst, it improves the performance of the BiVO₄ photoanode by enhancing hole collecting and transporting ability. We believe this research not only provides a highly efficient and low-cost electrocatalyst for the OER, but also unveils the "pre-catalyst" role of PBA-derived materials in energy-storage and conversion devices.

Utilizing the Space-Charge Region of the FeNi-LDH/CoP p-n Junction to Promote Performance in Oxygen Evolution Electrocatalysis

The modulation of electron density is an effective option for efficient alternative electrocatalysts. Here, p-n junctions are constructed in 3D free-standing FeNi-LDH/CoP/carbon cloth (CC) electrode (LDH=layered double hydroxide). The positively charged FeNi-LDH in the space-charge region can significantly boost oxygen evolution reaction. Therefore, the j at 1.485 V (vs. RHE) of FeNi-LDH/CoP/CC achieves ca. 10-fold and ca. 100-fold increases compared to those of FeNi-LDH/CC and CoP/CC, respectively. Density functional theory calculation reveals OH^- has a stronger trend to adsorb on the surface of FeNi-LDH side in the p-n junction compared to individual FeNi-LDH further verifying the synergistic effect in the p-n junction. Additionally, it represents excellent activity toward water splitting. The utilization of heterojunctions would open up an entirely new possibility to purposefully regulate the electronic structure of active sites and promote their catalytic activities.

A Hybrid Electrocatalyst with a Coordinatively Unsaturated Metal-Organic Framework Shell and Hollow Ni3S2/NiS Core for Oxygen Evolution Reaction Applications

Metal-organic frameworks (MOFs) have emerged as a promising class of materials. However, their insulating nature has limited their application as electrocatalysts. Herein, we report a heterogeneous nanostructure of a Ni-based MOF-modified Ni₃S₂/NiS hollow nanoparticle. The Ni₃S₂/NiS hollow core is prepared by a sulfuration process from a colloidal nickel nanoparticle using dodecanethiol followed by a low-temperature heat treatment in air to remove the adsorbed organic ligands. The thin shell of the Ni-based MOF (Ni-BDC) is synthesized using an in situ method in which the nickel sulfides supply the metal source and the additional terephthalic acid serves as the linker. Serving as an oxygen evolution reaction catalyst, this hybrid nanocomposite shows superior electrocatalytic performance with a low overpotential of 298 mV at 10 mA·cm^-2 without carbon addition and a long-time endurability with no detectable activity deterioration, which can be attributed to the synergistic effect of the advantageous heterogeneous structure, combining the good hydrophilicity and coordinative unsaturation of the Ni-BDC shell and the high conductivity and porosity of the Ni₃S₂/NiS core as well as the strongly coupled interface between them.

Stainless Steel as A Bi-Functional Electrocatalyst-A Top-Down Approach

For a hydrogen economy to be viable, clean and economical hydrogen production methods are vital. Electrolysis of water is a promising hydrogen production technique with zero emissions, but suffer from relatively high production costs. In order to make electrolysis of water sustainable, abundant, and efficient materials has to replace expensive and scarce noble metals as electrocatalysts in the reaction cells. Herein, we study activated stainless steel as a bi-functional electrocatalyst for the full water splitting reaction by taking advantage of nickel and iron suppressed within the bulk. The final electrocatalyst consists of a stainless steel mesh with a modified surface of layered NiFe nanosheets. By using a top down approach, the nanosheets stay well anchored to the surface and maintain an excellent electrical connection to the bulk structure. At ambient temperature, the activated stainless steel electrodes produce 10 mA/cm² at a cell voltage of 1.78 V and display an onset for water splitting at 1.68 V in 1M KOH, which is close to benchmarking nanosized catalysts. Furthermore, we use a scalable activation method using no externally added electrocatalyst, which could be a practical and cheap alternative to traditionally catalyst-coated electrodes.

Prussian blue analogues-derived bimetallic iron-cobalt selenides for efficient overall water splitting

The efficient and durable catalysts toward electrochemical water-splitting plays significant role in clean and renewable energy storage applications. Herein, we design the Prussian blue analogues precursor by self-assembled strategy and converted it to (Fe-Co)Se₂ composite by post-selenization method. Benefiting of unique porous morphology, high electrochemically active surface area and fast electron transfer ability, the excellent oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) performance of (Fe-Co)Se₂ catalyst were obtained. It was revealed that only require overpotentials of 251 mV to arrive a current density of 10 mA cm^-2 in OER system, and 90 mV was accomplished at a current density of 10 mA cm^-2 toward the HER activity, along with low Tafel slope of 47.6 and 58.7 mV dec^-1 was achieved for OER and HER, respectively. The (Fe-Co)Se₂ catalysts with high electrochemical activity at the same time long-time durability may encourage more practical catalytic applications for renewable energy technologies.

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.

A trinuclear cobalt-based coordination polymer as an efficient oxygen evolution electrocatalyst at neutral pH

The dearth of an efficient, robust, abundant and cost-effective water oxidation catalyst is debatably the major hurdle for the technological advancement of artificial photosynthesis devices. Herein, a three dimensional (3D) cobalt-based coordination polymer {[Co₃(pyz)(fa)₃(dmso)₂]·2H₂O}_n, (1) (pyz = pyrazine, fa = fumarate, dmso = dimethyl sulfoxide) has been synthesized and demonstrated to act as an efficient electrocatalyst towards water oxidation at neutral pH. Compound 1 displays a stair-like arrangement parallel to the b-axis, with the cobalt clusters arranged in a zigzag fashion, and contains small, honeycomb-like channels parallel to the c-axis. Compound 1 shows a remarkable activity for water oxidation and attains a current density of 1 mA.cm^-2 at low overpotential (η = 257 mV) with a Tafel slope value of 80.5 mV.dec^-1. This high performance of 1 in catalysing the water oxidation reaction is attributed to its unique 3-D architecture. The results of electrochemical investigations, including long-term and controlled potential electrolysis, are anticipated to guide the forthcoming advancement in creating efficient, cheap and noble metal (Pt/Ru/Ir) free catalysts for the water oxidation reaction.

Water Oxidation Catalysts: The Quest for New Oxide-Based Materials

The expected shortage of fossil fuels as well as the accompanying climate change are among the major challenges of the 21st century. A global shift to a sustainable energy landscape is, therefore, of utmost importance. Over the past few years, solar technologies have entered the energy market and have paved the way to replace fossil-based energy sources, in the long term. In particular, electrochemical solar-to-hydrogen technologies have attracted a lot of interest—not only in academia, but also in industry. Solar water splitting (artificial photosynthesis) is one of the most active areas in contemporary materials and catalysis research. The development of low-cost, efficient, and stable water oxidation catalysts (WOCs) remains crucial for artificial photosynthesis applications, because WOCs still represent a major economical and efficient bottleneck. In the following, we summarize recent advances in water oxidation catalysts development, with selected examples from 2016 onwards. This condensed survey demonstrates that the ongoing quest for new materials and informed catalyst design is a dynamic and rapidly developing research area.

Efficient Hydrogen Evolution Activity and Overall Water Splitting of Metallic Co4N Nanowires through Tunable d-Orbitals with Ultrafast Incorporation of FeOOH

Cobalt nitride electrocatalysts have been investigated and proven to show excellent oxygen evolution reaction activity owing to their excellent metallic properties, but their hydrogen evolution reaction (HER) properties are rarely reported because of their unsatisfactory molecular energy level, especially the d-orbital. Herein, taking Co₄N as a case study, we tune the d-orbital of metallic Co₄N nanowires via rapid formation of iron oxyhydroxide (FeOOH). Experimental analyses show that FeOOH@Co₄N/SSM exhibits excellent HER catalytic activity with considerable low onset overpotential (22 mV), small Tafel slope (34 mV dec^-1), and excellent stability at current densities ranging from 20 to 100 mA cm^-2. Additionally, theoretical assessments display that the hybridization of Co₄N with FeOOH is beneficiary for optimizing and promoting the free energy of H adsorption due to the tuning of d-orbital. An overall water-splitting device assembled based on bifunctional FeOOH@Co₄N/SSM delivers an onset potential of 1.48 V with excellent stability up to 4 days. This shows a new strategy for designing a high-performance water-splitting device based on cobalt-based electrocatalysts.

High-Index-Faceted Ni3S2 Branch Arrays as Bifunctional Electrocatalysts for Efficient Water Splitting

For efficient electrolysis of water for hydrogen generation or other value-added chemicals, it is highly relevant to develop low-temperature synthesis of low-cost and high-efficiency metal sulfide electrocatalysts on a large scale. Herein, we construct a new core-branch array and binder-free electrode by growing Ni₃S₂ nanoflake branches on an atomic-layer-deposited (ALD) TiO₂ skeleton. Through induced growth on the ALD-TiO₂ backbone, cross-linked Ni₃S₂ nanoflake branches with exposed {[Formula: see text]} high-index facets are uniformly anchored to the preformed TiO₂ core forming an integrated electrocatalyst. Such a core-branch array structure possesses large active surface area, uniform porous structure, and rich active sites of the exposed {[Formula: see text]} high-index facet in the Ni₃S₂ nanoflake. Accordingly, the TiO₂@Ni₃S₂ core/branch arrays exhibit remarkable electrocatalytic activities in an alkaline medium, with lower overpotentials for both oxygen evolution reaction (220 mV at 10 mA cm^-2) and hydrogen evolution reaction (112 mV at 10 mA cm^-2), which are better than those of other Ni₃S₂ counterparts. Stable overall water splitting based on this bifunctional electrolyzer is also demonstrated.