NiCo-Based Electrocatalysts for the Alkaline Oxygen Evolution Reaction: A Review

Activating active motifs in Ni-Fe oxide by introducing dual-defect for oxygen evolution reaction in alkaline seawater

Developing simple and energy-saving pathways to prepare high-efficient and robust non-noble metal based electrocatalysts remains a huge challenge to hydrogen production from seawater electrolysis. Here we demonstrate a facile hydrothermal-calcination-etching approach that simultaneously achieves the required surface N doping and Fe vacancies generation to activate the Ni-O-Fe active motifs in N-vFe-NiFe2O4/NF. The unique localized environments (Ni-N-Fe structures and unsaturated O- and N-coordination) due to dual-defect strategy can effectively regulate the electronic structure of the Ni-O-Fe motif to make the motif more reactive. As a result, the N-vFe-NiFe2O4/NF catalyst exhibits overpotentials of 210, 213 and 222 mV to deliver 100 mA cm-2 in 1.0 M KOH, simulated seawater and alkaline seawater environments, respectively. Theoretical calculations prove that the Ni-O-Fe structure is the active motif and that the presence of special localized environments can optimize the adsorption of key intermediates on the activated active motifs.

Unraveling the Role of the Stoichiometry of Atomic Layer Deposited Nickel Cobalt Oxides on the Oxygen Evolution Reaction

Nickel cobalt oxides (NCOs) are promising, non-precious oxygen evolution reaction (OER) electrocatalysts. However, the stoichiometry-dependent electrochemical behavior makes it crucial to understand the structure-OER relationship. In this work, NCO thin film model systems are prepared using atomic layer deposition. In-depth film characterization shows the phase transition from Ni-rich rock-salt films to Co-rich spinel films. Electrochemical analysis in 1 m KOH reveals a synergistic effect between Co and Ni with optimal performance for the 30 at.% Co film after 500 CV cycles. Electrochemical activation correlates with film composition, specifically increasing activation is observed for more Ni-rich films as its bulk transitions to the active (oxy)hydroxide phase. In parallel to this transition, the electrochemical surface area (ECSA) increases up to a factor 8. Using an original approach, the changes in ECSA are decoupled from intrinsic OER activity, leading to the conclusion that 70 at.% Co spinel phase NCO films are intrinsically the most active. The studies point to a chemical composition dependent OER mechanism: Co-rich spinel films show instantly high activities, while the more sustainable Ni-rich rock-salt films require extended activation to increase the ECSA and OER performance. The results highlight the added value of working with model systems to disclose structure-performance mechanisms.

Metal-insulator-semiconductor photoelectrodes for enhanced photoelectrochemical water splitting

Photoelectrochemical (PEC) water splitting provides a scalable and integrated platform to harness renewable solar energy for green hydrogen production. The practical implementation of PEC systems hinges on addressing three critical challenges: enhancing energy conversion efficiency, ensuring long-term stability, and achieving economic viability. Metal-insulator-semiconductor (MIS) heterojunction photoelectrodes have gained significant attention over the last decade for their ability to efficiently segregate photogenerated carriers and mitigate corrosion-induced semiconductor degradation. This review discusses the structural composition and interfacial intricacies of MIS photoelectrodes tailored for PEC water splitting. The application of MIS heterostructures across various semiconductor light-absorbing layers, including traditional photovoltaic-grade semiconductors, metal oxides, and emerging materials, is presented first. Subsequently, this review elucidates the reaction mechanisms and respective merits of vacuum and non-vacuum deposition techniques in the fabrication of the insulator layers. In the context of the metal layers, this review extends beyond the conventional scope, not only by introducing metal-based cocatalysts, but also by exploring the latest advancements in molecular and single-atom catalysts integrated within MIS photoelectrodes. Furthermore, a systematic summary of carrier transfer mechanisms and interface design principles of MIS photoelectrodes is presented, which are pivotal for optimizing energy band alignment and enhancing solar-to-chemical conversion efficiency within the PEC system. Finally, this review explores innovative derivative configurations of MIS photoelectrodes, including back-illuminated MIS photoelectrodes, inverted MIS photoelectrodes, tandem MIS photoelectrodes, and monolithically integrated wireless MIS photoelectrodes. These novel architectures address the limitations of traditional MIS structures by effectively coupling different functional modules, minimizing optical and ohmic losses, and mitigating recombination losses.

Stabilizing atomic Ru species in conjugated sp2 carbon-linked covalent organic framework for acidic water oxidation

Suppressing the kinetically favorable lattice oxygen oxidation mechanism pathway and triggering the adsorbate evolution mechanism pathway at the expense of activity are the state-of-the-art strategies for Ru-based electrocatalysts toward acidic water oxidation. Herein, atomically dispersed Ru species are anchored into an acidic stable vinyl-linked 2D covalent organic framework with unique crossed π-conjugation, termed as COF-205-Ru. The crossed π-conjugated structure of COF-205-Ru not only suppresses the dissolution of Ru through strong Ru-N motifs, but also reduces the oxidation state of Ru by multiple π-conjugations, thereby activating the oxygen coordinated to Ru and stabilizing the oxygen vacancies during oxygen evolution process. Experimental results including X-ray absorption spectroscopy, in situ Raman spectroscopy, in situ powder X-ray diffraction patterns, and theoretical calculations unveil the activated oxygen with elevated energy level of O 2p band, decreased oxygen vacancy formation energy, promoted electrochemical stability, and significantly reduced energy barrier of potential determining step for acidic water oxidation. Consequently, the obtained COF-205-Ru displays a high mass activity with 2659.3 A g-1, which is 32-fold higher than the commercial RuO2, and retains long-term durability of over 280 h. This work provides a strategy to simultaneously promote the stability and activity of Ru-based catalysts for acidic water oxidation.

Engineering Internal and External Low-Coordination Atoms in Nickel-Organic Framework Nanoarrays to Promote the Electrochemical Oxygen Evolution Reaction

Monometallic nickel-organic frameworks based on a carboxylated ligand [2,6-naphthalenedicarboxylic acid (Ni-NDC)] have abundant and uniformly distributed single-atom Ni sites, enabling superior oxygen evolution reaction (OER) activity. In theory, most of the Ni atoms inside Ni-NDC microcrystals are coordinatively saturated except for the surface. Therefore, there are no accessible low-coordination atoms (LCAs) as electrocatalytic sites for the OER. One effective way is to expose more LCAs by preparing self-supporting Ni-NDC nanoarrays (Ni-NDC NAs) with hierarchical secondary structural units. Another effective method is to create more internal LCAs by removing partial ligands or coordination atoms attached to the Ni atoms. Herein, by combining the two strategies, we engineered LCAs in the interior and exterior of Ni-NDC to synergistically accelerate the OER. In brief, ultrathick "brick-like" Ni-NDC NAs were first prepared with dissolution and coordination effects of NDC on self-sacrificial templates of "agaric-like" nickel hydroxide nanoarrays [Ni(OH)2 NAs]. Subsequently, dual-coordinated NDC was partially replaced by monocoordinated 2-naphthoic acid (NA). The Ni-NDC NAs were further tailed into ultrathin "liner leaf-like" nanoneedle arrays (LCAs-Ni-NDC NAs). As a consequence, the LCAs-Ni-NDC NAs have more internal and external LCAs, which can deliver an OER performance that is superior to that of Ni-NDC NAs.

Ru/Ir-Based Electrocatalysts for Oxygen Evolution Reaction in Acidic Conditions: From Mechanisms, Optimizations to Challenges

The generation of green hydrogen by water splitting is identified as a key strategic energy technology, and proton exchange membrane water electrolysis (PEMWE) is one of the desirable technologies for converting renewable energy sources into hydrogen. However, the harsh anode environment of PEMWE and the oxygen evolution reaction (OER) involving four-electron transfer result in a large overpotential, which limits the overall efficiency of hydrogen production, and thus efficient electrocatalysts are needed to overcome the high overpotential and slow kinetic process. In recent years, noble metal-based electrocatalysts (e.g., Ru/Ir-based metal/oxide electrocatalysts) have received much attention due to their unique catalytic properties, and have already become the dominant electrocatalysts for the acidic OER process and are applied in commercial PEMWE devices. However, these noble metal-based electrocatalysts still face the thorny problem of conflicting performance and cost. In this review, first, noble metal Ru/Ir-based OER electrocatalysts are briefly classified according to their forms of existence, and the OER catalytic mechanisms are outlined. Then, the focus is on summarizing the improvement strategies of Ru/Ir-based OER electrocatalysts with respect to their activity and stability over recent years. Finally, the challenges and development prospects of noble metal-based OER electrocatalysts are discussed.

The oxygen evolution reaction on cobalt atom embedded nitrogen doped graphene electrocatalysts: a density functional theory study

The oxygen evolution reaction (OER) is essential for the development of renewable energy conversion and storage technologies. Eight N-doped graphenes containing variable numbers of embedded cobalt atoms (Coxy-NG, x = 1-4, y = 1-3, where x represents the number of embedded Co atoms and y represents different configurations) were designed and their OER electrocatalytic activities were systematically studied through density functional theory calculations. The significant roles of the number of Co atoms and their configuration in their OER performance were discussed in detail. Co31-NG occupies the peak of the activity volcano plot with a low overpotential of 0.31 V, which is smaller than Co11-NG with only one Co atom and even superior to the widely used IrO2 (0.56 V). The electronic structure and electron density analysis reveal that the outstanding electrocatalytic performance is due to the orbital hybridization between Co and N atoms and the increased positive charge on in-plane Co due to the out-of-plane Co atoms/clusters. This work clarifies the important role of transition atoms and provides excellent examples for reducing the overpotential through embedding several transition metal atoms onto single-atom electrocatalysts.

Construction of amorphous/crystalline Fe doped CoSe for effective electrocatalytic oxygen evolution

Herein, amorphous/crystalline Fe-doped CoSe was synthesized (Fe-CoSe/NF), and it exhibited high oxygen evolution reaction (OER) performance. The synergistic effect of the Fe dopant and the amorphous/crystalline structure is conducive to the formation of high valence Co3+ and Fe3+ active sites. Fe-CoSe/NF shows low overpotentials of 269 mV@50 mA cm-2 and 280 mV@100 mA cm-2.

Phosphorus-rich CoP4@N-C nanoarrays for efficient nitrate-to-ammonia electroreduction

The electrochemical nitrate reduction reaction (NO3-RR) is a novel green method for ammonia synthesis. However, the lack of sufficient catalysts has hindered the development of the NO3-RR. This research develops a transformation of porous CoP@N-C/CC into porous phosphorus-rich CoP4@N-C/CC through high-temperature calcination. Due to its unique phosphating-rich structure, CoP4@N-C/CC exhibits an excellent Faraday efficiency (FE: 92.3%) and NH3 yield (610.2 μmol h-1 cm-2). Such a catalyst with more P-P bonds can provide more active sites, effectively enhancing the adsorption and reaction processes of reactant molecules. In addition, the catalyst has good durability and catalytic stability, which provides a possibility for the future application of electrocatalytic ammonia production.

Research progress of dual-atom site catalysts for photocatalysis

Dual-atom site catalysts (DASCs) have sparked considerable interest in heterogeneous photocatalysis as they possess the advantages of excellent photoelectronic activity, photostability, and high carrier separation efficiency and mobility. The DASCs involved in these important photocatalytic processes, especially in the photocatalytic hydrogen evolution reaction (HER), CO2 reduction reaction (CO2RR), N2/nitrate reduction, etc., have been extensively investigated in the past few years. In this review, we highlight the recent progress in DASCs that provides fundamental insights into the photocatalytic conversion of small molecules. The controllable preparation and characterization methods of various DASCs are discussed. Subsequently, the reaction mechanisms of the formation of several important molecules (hydrogen, hydrocarbons and ammonia) on DASCs are introduced in detail, in order to probe the relationship between DASCs's structure and photocatalytic activity. Finally, some challenges and outlooks of DASCs in the photocatalytic conversion of small molecules are summarized and prospected. We hope that this review can provide guidance for in-depth understanding and aid in the design of efficient DASCs for photocatalysis.

Easy conversion perovskite fluorides KCo1-xFexF3 for efficient oxygen evolution reaction

Herein, we report an easily oxidized Co-Fe perovskite fluoride as an efficient catalyst for the oxygen evolution reaction (OER). In situ Raman spectroscopy showed that the presence of F promotes reconstruction to form highly active (Co3+Fe3+)OOH, and the current density of 10 mA cm-2 can be achieved at the overpotential of only 118 mV in 1 M KOH aqueous solution. This work helps to understand the role of fluoride during the OER.

Computational chemistry for water-splitting electrocatalysis

Electrocatalytic water splitting driven by renewable electricity has attracted great interest in recent years for producing hydrogen with high-purity. However, the practical applications of this technology are limited by the development of electrocatalysts with high activity, low cost, and long durability. In the search for new electrocatalysts, computational chemistry has made outstanding contributions by providing fundamental laws that govern the electron behavior and enabling predictions of electrocatalyst performance. This review delves into theoretical studies on electrochemical water-splitting processes. Firstly, we introduce the fundamentals of electrochemical water electrolysis and subsequently discuss the current advancements in computational methods and models for electrocatalytic water splitting. Additionally, a comprehensive overview of benchmark descriptors is provided to aid in understanding intrinsic catalytic performance for water-splitting electrocatalysts. Finally, we critically evaluate the remaining challenges within this field.

Spatial configuration of Fe-Co dual-sites boosting catalytic intermediates coupling toward oxygen evolution reaction

Oxygen evolution reaction (OER) is the pivotal obstacle of water splitting for hydrogen production. Dual-sites catalysts (DSCs) are considered exceeding single-site catalysts due to the preternatural synergetic effects of two metals in OER. However, appointing the specific spatial configuration of dual-sites toward more efficient catalysis still remains a challenge. Herein, we constructed two configurations of Fe-Co dual-sites: stereo Fe-Co sites (stereo-Fe-Co DSC) and planar Fe-Co sites (planar-Fe-Co DSC). Remarkably, the planar-Fe-Co DSC has excellent OER performance superior to stereo-Fe-Co DSC. DFT calculations and experiments including isotope differential electrochemical mass spectrometry, in situ infrared spectroscopy, and in situ Raman reveal the *O intermediates can be directly coupled to form *O-O* rather than *OOH by both the DSCs, which could overcome the limitation of four electron transfer steps in OER. Especially, the proper Fe-Co distance and steric direction of the planar-Fe-Co benefit the cooperation of dual sites to dehydrogenate intermediates into *O-O* than stereo-Fe-Co in the rate-determining step. This work provides valuable insights and support for further research and development of OER dual-site catalysts.

Achieving High-Performance Electrocatalytic Water Oxidation on Ni(OH)2 with Optimized Intermediate Binding Energy Enabled by S-Doping and CeO2 -Interfacing

The adsorption energy of the reaction intermediates has a crucial influence on the electrocatalytic activity. Ni-based materials possess high oxygen evolution reaction (OER) performance in alkaline, however too strong binding of *OH and high energy barrier of the rate-determining step (RDS) severely limit their OER activity. Herein, a facile strategy is shown to fabricate novel vertical nanorod-like arrays hybrid structure with the interface contact of S-doped Ni(OH)2 and CeO2 in situ grown on Ni foam (S-Ni(OH)2 /CeO2 /NF) through a one-pot route. The alcohol molecules oxidation reaction experiments and theoretical calculations demonstrate that S-doping and CeO2 -interfacing significantly modulate the binding energies of OER intermediates toward optimal value and reduce the energy barrier of the RDS, contributing to remarkable OER activity for S-Ni(OH)2 /CeO2 /NF with an ultralow overpotential of 196 mV at 10 mA cm-2 and long-term durability over 150 h for the OER. This work offers an efficient doping and interfacing strategy to tune the binding energy of the OER intermediates for obtaining high-performance electrocatalysts.

Modulation of Phase Transition in Cobalt Selenide with Simultaneous Construction of Heterojunctions for Highly-Efficient Oxygen Electrocatalysis in Zinc-Air Battery

Phase transformation of cobalt selenide (CoSe2 ) can effectively modulate its intrinsic electrocatalytic activity. However, enhancing electroconductivity and catalytic activity/stability of CoSe2 still remains challenging. Heterostructure engineering may be feasible to optimize interfacial properties to promote the kinetics of oxygen electrocatalysis on a CoSe2 -based catalyst. Herein, a heterostructure consisting of CoSe2 and cobalt nitride (CoN) embedded in a hollow carbon cage is designed via a simultaneous phase/interface engineering strategy. Notably, the phase transition of orthorhombic-CoSe2 to cubic-CoSe2 (c-CoSe2 ) accompanied by in situ CoN formation is realized to build the c-CoSe2 /CoN heterointerface, which exhibits excellent/highly stable activities for oxygen reduction/evolution reactions (ORR/OER). Notably, heterostructure can modulate the local coordination environment and increase Co-Se/N bond lengths. Theoretical calculations show that Co-site (c-CoSe2 ) with an electronic state near Fermi energy level is the main active site for ORR/OER.Energetical tailoring of the d-orbital electronic structure of the Co atom of c-CoSe2 in heterostructure by in situ CoN incorporation lowers thermodynamic barriers for ORR/OER. Attractively, a zinc-air battery with a c-CoSe2 -CoN cathode displays excellent cycling stability (250 h) and charge/discharge voltage loss (0.953/0.96 V). It highlights that heterointerface engineering provides an option for modulating the bifunctional activity of metal selenides with controlled phase transformation.

An oxygen vacancy-modulated bifunctional S-NiMoO4 electrocatalyst for efficient alkaline overall water splitting

S-doped nickel molybdate nanorods grown on nickel foam (S-NiMoO4/NF) were fabricated by a two-step hydrothermal method. The resultant S-NiMoO4/NF exhibited remarkable bifunctional electrocatalytic activity, with overpotentials of 235 mV for the hydrogen evolution reaction and 150 mV for the oxygen evolution reaction at a current density of 50 mA cm-2. Assembled into the two-electrode S-NiMoO4/NF electrolyzer in alkaline electrolytes for overall water splitting, it required only low cell voltages of 1.55 V and 1.63 V to drive 50 mA cm-2 and 100 mA cm-2, respectively. No significant performance degradation occurred during the water electrolysis process. The experimental results confirmed that S-doping induced the increase of the oxygen vacancies, accelerating the reaction kinetics and thus improving the electrocatalytic performance. Meanwhile, more active sites exposure on the surface of S-NiMoO4/NF enhanced the reactivity. This work may guide the development of efficient bifunctional catalysts in alkaline electrolysis through oxygen vacancy regulation.

Highly efficient sulfur-doped Ni3Fe electrocatalysts for overall water splitting: Rapid synthesis, mechanism and driven by sustainable energy

Designing efficient electrocatalysts and insight into their electrocatalytic mechanisms are significantly important for storing and converting the intermittent sustainable energy sources into clean hydrogen. In this study, we synthesize the bifunctional sulfur-doped Ni3Fe (NiFeS) electrocatalysts by a simple electrodeposition method only taking 30 s. After optimizing the components, it was found that the synthesized NiFeS electrocatalysts exhibit the excellent hydrogen and oxygen evolution reaction performances in 1.0 M potassium hydroxide solution. The results of experimental and theoretical calculations reveal that the introduced sulfur could optimize the electronic distribution, which make Ni electron-rich and Fe electron-deficient, thereby weakening the energy barriers of potential-determining steps, i.e. the absorption of H2O molecule on Ni sites for HER and formation of *OOH on Fe sites for OER, respectively. Besides, the NiFeS electrocatalysts are used as the bifunctional electrodes to water splitting, which only need 1.51 V to reach 10 mA·cm-2, and exhibits excellent durability and a >95% Faraday efficiency. Furthermore, the intermittent kinetic, wind and solar energies are used to power the assembled electrolyzer with NiFeS bi-electrodes to verify their great application potential. This work not only proved a deep insight into mechanism of the boosted electrocatalytic activities of NiFeS, but also the synthesized NiFeS electrocatalysts have great application prospect in the conversion of intermittent and sustainable energy sources into hydrogen by water electrocatalysis.

Ni-modified FeOOH integrated electrode by self-source corrosion of nickel foam for high-efficiency electrochemical water oxidation

The construction and application of efficient iron oxyhydroxide (FeOOH) is still a challenge in the field of energy conversion. Here, a facile preparation method is developed by directly utilizing commercialized nickel foams (NF) as the nickel source and the supporting framework, as well as the ingenious use of etching effect originating from acidic medium in the process of iron salt hydrolysis. As a result, a Ni-modulated FeOOH integrated electrode (Ni-FeOOH/NF) is obtained. Unexpectedly, the implementation of our scheme effectively activates the catalytic intrinsic activity of FeOOH, successfully transforming the inert NF into an integrated electrode with high oxygen evolution reaction (OER) performance. Specifically, the Ni-FeOOH/NF exhibits the overpotential of 277 mV (@100 mA cm-2) and superior stability for OER. Additionally, the as-prepared Ni-FeOOH/NF electrode could also operate steadily for OER in alkaline adjusted saline water. Our research provides a new idea for the preparation of satisfactory Fe-based metal materials as OER electrocatalysts.

NiFe-layered double hydroxide/CoP2@MnP heterostructures of clustered flower nanowires on MXene-modified nickel foam for overall water-splitting

Exploiting efficient and economical electrocatalysts is indispensable to promoting the sluggish kinetics of overall water-splitting. Herein, we designed a phosphate reaction and two-step hydrothermal method to construct a 3D porous clustered flower-like heterogeneous structure of NiFe-layered double hydroxide (NiFe) and CoP2@MnP (CMP) grown in-situ on MXene-modified nickel foam (NF) substrate (denoted as NiFe/CMP/MX), with favorable kinetics. Density functional theory calculations (DFT) demonstrate that the self-driven transfer of heterojunction charges causes electron redistribution of the catalyst, and optimizes the electron transfer rate of the active site and the d-band center near the Fermi level, thereby reducing the adsorption energy of H and O reaction intermediates (H*, OH*, OOH*). As expected, the combination of CMP and NiFe with naturally conductive MXene forms a strong chemical and electron synergistic effect, which enables the synthesized NiFe/CMP/MX heterogeneous structure exhibits good activity for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) with a low overpotential of 200 mV and 126 mV at 10 mA cm-2, respectively. Furthermore, the overpotential of 1.58 V is enough to drive a current density of 10 mA cm-2 in a two-electrode configuration, which is better than noble metals (RuO2(+)//Pt/C(-)) (1.68 V).

NiFeRu/C and Ru, Fe-Ni5P4/C as complementary electrocatalyst for highly efficient overall water splitting

Designing and fabricating highly competent and inexpensive electrocatalysts are highly desirable for application in electrocatalytic water splitting. In this study, we synthesized NiFeRu/C nanofibers and Ru, Fe dual-doped Ni5P4 (Ru, Fe-Ni5P4)/C nanofibers as complementary electrocatalysts for overall water splitting through electrospinning, carbonization, and phosphorization treatment, respectively. The NiFeRu/C nanofibers and Ru, Fe-Ni5P4/C nanofibers showed high hydrogen evolution reaction and oxygen evolution reaction activity, respectively, due to the presence of numerous exposed active sites and optimized adsorption capacity for the reaction intermediates contributed by the synergistic interaction among different metal components in the electrocatalysts. Hence, the assembled asymmetrical electrolytic cell effectively promoted overall water splitting, requiring a voltage of 1.569, 1.744, and 1.872 V to achieve a current density of 100, 500, and 1,000 mA cm-2, respectively, and it was better than Pt/C||IrO2. Additionally, the electrolytic cell could work at 500 mA cm-2 for 100 h without any noticeable deterioration in activity, which indicated that it was durable at high current density. In this study, we described a novel method for designing highly efficient electrocatalysts for overall water splitting.

Accelerating structure reconstruction to form NiOOH in metal-organic frameworks (MOFs) for boosting the oxygen evolution reaction

Structural reconstruction of electrocatalysts to generate metal hydroxide/oxyhydroxide species is critical for an efficient oxygen evolution reaction (OER), but the controllable regulation of the reconstruction process still remains a challenge. Given the designable nature of metal-organic frameworks (MOFs), herein, we have reported a localized structure disordering strategy to accelerate the structural reconstruction of Ni-BDC to generate NiOOH for boosting the OER. The Ni-BDC nanosheets were modified by Fe3+ and urea to form cracks, which could promote the accessibility of the Ni sites by the electrolyte and thus promote the reconstruction to form NiOOH. In addition, the interaction between Ni2+ and Fe3+ allows the electron flow from Ni2+ to Fe3+, further enhancing the NiOOH generation. As a result, the optimized sample exhibits excellent OER activity with a small overpotential of 251 mV at 10 mA cm-2, which is superior to most of the MOF-based OER catalysts reported previously. This work provides a controllable strategy to regulate the structural reconstruction for promoting the OER, which could provide important guidance for the development of more efficient OER electrocatalysts.

Ni(OH)2-derived lamellar MoS2/Ni3S2/NF with Fe-doped heterojunction catalysts for efficient overall water splitting

Heterostructures formed by combining semiconductor materials with different band structures can provide work functions, d-band positions and electronic properties different from bulk materials and are considered as an effective strategy to improve the catalytic activity through electronic modification. In this study, an efficient MoS2/Fe-Ni3S2/NF heterojunction material was prepared by a two-step hydrothermal method. With the help of flake Ni(OH)2 synthesized in the first step, growth sites were provided for flake Ni3S2. The electronic structure of Ni3S2 was optimized by Fe doping, while the construction of the MoS2/Fe-Ni3S2 heterostructure allowed the catalyst to expose more active sites. MoS2/Fe-Ni3S2/NF exhibited a small charge transfer resistance and excellent electrocatalytic performance. At a current density of 10 mA cm-2, only low overpotentials of 148 mV and 118 mV were required for the oxygen precipitation reaction (OER) and hydrogen precipitation reaction (HER), respectively. Notably, when MoS2/Fe-Ni3S2/NF is used as the anode and cathode for overall hydrolysis, only 1.51 V is required to reach a current density of 10 mA cm-2, demonstrating its great potential for application in hydrolysis. This work provides a feasible idea for the rational construction of non-precious metal bifunctional electrocatalysts with excellent performance.

A low-cost and efficient route for large-scale synthesis of NiCoSx nanosheets with abundant sulfur vacancies towards quasi-industrial electrocatalytic oxygen evolution

Transition-metal sulfides (TMS) have piqued a great deal of interest due to their unprecious nature and high intrinsic catalytic activity for water splitting. In this work, a low-cost and efficient route was developed, which included electrodeposition to prepare Ni-Co layered double hydroxide (NiCo-LDH) followed by ion exchange to form nickel cobalt sulfide (NiCoSx). Electrochemical reduction was used to modulate sulfur vacancies in order to produce sulfur vacancies-rich NiCoSx with nanosheet arrays on -three-dimensional nickel foam (NiCoSx-0.4/NF) with a large area of more than 250 cm2. Combining data from experiments and density functional theoretical (DFT) calculations reveals that engineered sulfur vacancies change the electronic structure, electron transfer property, and surface electron density of NiCoSx, significantly improving the free energy of water adsorption and boosting electrocatalytic activity. The developed NiCoSx-0.4/NF has long-term stability of more than 300 h at 500 mA cm-2 in 1 M KOH at ambient temperature and only needs a 289 mV overpotential at 100 mA cm-2. Remarkably, the synthesized electrocatalyst rich in sulfur vacancies, exhibits exceptional performance with a high current density of up to 1.9 A cm-2 and 1 A cm-2 in 6 M KOH and leads to overpotentials of 286 mV at 80 °C and 358 mV at 60 °C, respectively. The catalyst's practicability under quasi-industrial conditions (60 °C, 6 M KOH) is further demonstrated by its long-term stability for 220 h with only a 3.9 % potential increase at 500 mA cm-2.

Carbon nanotubes encapsulating Pt/MoN heterostructures for superior hydrogen evolution

Achieving efficient hydrogen evolution reaction (HER) catalysts to scale up electrochemical water splitting is desirable but remains a major challenge. Here, nitrogen-doped carbon nanotubes (NCNTs) loaded with PtNi/MoN electrocatalyst (PtNi/MoN@C) is synthesized by a simple strategy to obtain stronger interphase effects and significantly improve HER activity. The surface morphology of the materials is altered by Pt doping and the electronic structure of MoN is changed, which optimizing the electronic environment of the materials, shifting the binding energy and giving the materials a higher electrical conductivity, this ultimately leads to faster proton and electron transfer processes. The synergistic effect of Pt nanoparticles, MoN and the good combination with carbon leads to a high HER activity of 18 mV to reach 10 mA cm-2 in alkaline solution, outperforming that of the commercial Pt/C. Theoretical studies show that the heterostructures can efficiently enhance the electron transport and reduce the △GH*.

Lattice tensile strain cobalt phosphate with modulated hydroxide adsorption and structure transformation towards improved oxygen evolution reaction

The adsorption energy of oxygen-containing intermediates for the oxygen evolution reaction (OER) electrocatalysts plays a key role on their electrocatalytic performances. Rational optimization and regulation of the binding energy of intermediates can effectively improve the catalytic activities. Herein, the binding strength of Co phosphate to *OH was weakened by generating lattice tensile strain via Mn replacement, which modulated the electronic structure and optimized the reactive intermediates adsorption with active sites. The tensile-strained lattice structure and stretched interatomic distance were confirmed by X-ray diffraction and extended X-ray absorption fine structure (EXAFS) spectra measurements. The as-obtained Mn-doped Co phosphate exhibits excellent OER activity with an overpotential of 335 mV at 10 mA cm-2, which is much higher than pristine Co phosphate. In-situ Raman spectra and methanol oxidation reaction experiments demonstrated that Mn-doped Co phosphate with lattice tensile strain shows optimized *OH adsorption strength, and is favorable to structure reconstruction and form highly active Co oxyhydroxide intermediate during OER process. Our work provides insight into the effects of the lattice strain on the OER activity from the standpoint of intermediate adsorption and structure transformation.

Fe and Mo Co-Modulated Coral-like Nickel Pyrophosphate in situ Derived from Nickel-Foam for Oxygen Evolution

A highly active catalyst for the oxygen evolution reaction (OER) is critical to achieve high efficiency in hydrogen generation from water splitting. Direct conversion of nickel foam (NF) into nickel-based catalysts has attracted intensive interest due to the tight interaction of the catalysts to the substrate surface. However, the catalytic performances are still far below expectation because of the problems of low catalyst amount, thin catalyst layer, and small active area caused by the limitations of the synthesis method. Herein, we develop a Fe3+ -induced synthesis strategy to transform the NF surface into a thicker catalyst layer. In addition to the excellent conductivity and high stability, the as-prepared FeMo-Ni2 P2 O7 /NF catalysts expose more active sites and facilitate mass transfer due to their thicker catalyst layer and highly dense coral-like micro-nano structure. Furthermore, the Mo, Fe co-modulation optimizes the adsorption free energies of the OER intermediates, boosting catalytic activities. Its catalytic activity is among the highest, and it exhibits a small Tafel slope of 34.71 mV dec-1 and a low overpotential of 161 mV for delivering a current density of 100 mA cm-2 compared to reported Ni-based catalysts. The present strategy can be further used in the design of other catalysts for energy storage and conversion.

Lattice-Strain Engineering for Heterogenous Electrocatalytic Oxygen Evolution Reaction

The energy efficiency of metal-air batteries and water-splitting techniques is severely constrained by multiple electronic transfers in the heterogenous oxygen evolution reaction (OER), and the high overpotential induced by the sluggish kinetics has become an uppermost scientific challenge. Numerous attempts are devoted to enabling high activity, selectivity, and stability via tailoring the surface physicochemical properties of nanocatalysts. Lattice-strain engineering as a cutting-edge method for tuning the electronic and geometric configuration of metal sites plays a pivotal role in regulating the interaction of catalytic surfaces with adsorbate molecules. By defining the d-band center as a descriptor of the structure-activity relationship, the individual contribution of strain effects within state-of-the-art electrocatalysts can be systematically elucidated in the OER optimization mechanism. In this review, the fundamentals of the OER and the advancements of strain-catalysts are showcased and the innovative trigger strategies are enumerated, with particular emphasis on the feedback mechanism between the precise regulation of lattice-strain and optimal activity. Subsequently, the modulation of electrocatalysts with various attributes is categorized and the impediments encountered in the practicalization of strained effect are discussed, ending with an outlook on future research directions for this burgeoning field.

Enhanced Oxygen Evolution Reaction Performance on NiSx@Co3O4/Nickel Foam Electrocatalysts with Their Photothermal Property

Based on the principle of heterogeneous catalysis for water electrolysis, electrocatalysts with appropriate electronic structure and photothermal property are expected to drive the oxygen evolution reaction effectively. Herein, amorphous NiS_x-coupled nanourchin-like Co₃O₄ was prepared on nickel foam (NiS_x@Co₃O₄/NF) and investigated as a electrocatalyst for photothermal-assisted oxygen evolution reaction. The experimental investigations and simulant calculations jointly revealed NiS_x@Co₃O₄/NF to be of suitable electronic structure and high near-infrared photothermal conversion capability to achieve the oxygen evolution reaction advantageously both in thermodynamics and in kinetics. Relative to Co₃O₄/NF and NiS_x/NF, better oxygen evolution reaction activity, kinetics, and stability were achieved on NiS_x@Co₃O₄/NF in 1.0 M KOH owing to the NiS_x/Co₃O₄ synergetic effect. In addition, the oxygen evolution reaction performance of NiS_x@Co₃O₄/NF can be obviously enhanced under near-infrared light irradiation, since NiS_x@Co₃O₄ can absorb the near-infrared light to produce electric and thermal field. For the photothermal-mediated oxygen evolution reaction, the overpotential and Tafel slope of NiS_x@Co₃O₄/NF at 50 mA cm^-2 were reduced by 23 mV and 13 mV/dec, respectively. The present work provides an inspiring reference to design and develop photothermal-assisted water electrolysis using abundant solar energy.

Regulating the Spin State of Metal and Metal Carbide Heterojunctions for Efficient Oxygen Evolution

Developing high-performance electrocatalysts for oxygen evolution reaction (OER) is of importance for improving the overall efficiency of water splitting. Herein, the CoFe/(Co_xFe_1-x)₃Mo₃C heterojunction is purposely designed as an OER catalyst, which displays a low overpotential of 293 mV for affording a current density of 10 mA cm^-2 and a small Tafel slope of 48 mV/dec. Various characterization results demonstrate that the significant work-function difference between CoFe and (Co_xFe_1-x)₃Mo₃C can induce interfacial charge redistribution, which results in the formation of Co and Fe sites with a high-spin state, thus stimulating the surface phase reconstruction of CoFe/(Co_xFe_1-x)₃Mo₃C to corresponding active oxyhydroxide. Meanwhile, the electrochemical leaching of Mo ions from the initial structure can contribute to the formation of defective sites, further benefiting OH^- adsorption and surface oxidation. Moreover, the remaining CoFe can accelerate electron migration during the electrocatalytic process. This study presents new insights into constructing efficient OER electrocatalysts with an optimized spin-state configuration via interfacial engineering.

FeNiCrCoMn High-Entropy Alloy Nanoparticles Loaded on Carbon Nanotubes as Bifunctional Oxygen Catalysts for Rechargeable Zinc-Air Batteries

An efficient and stable bifunctional oxygen catalyst is necessary to complete the application of the rechargeable zinc-air battery. Herein, an economical and convenient process was adopted to successfully coat high-entropy alloy Fe₁₂Ni₂₃Cr₁₀Co_55-xMn_x nanoparticles on carbon nanotubes (CNTs). In 0.1 M KOH solution, with a bifunctional oxygen overpotential (ΔE) of only 0.7 V, the catalyst Fe₁₂Ni₂₃Cr₁₀Co₃₀Mn₂₅/CNT exhibits excellent bifunctional oxygen catalytic performance, exceeding most catalysts reported so far. In addition, the air electrode assembled with this catalyst exhibits high specific capacity (760 mA h g^-1) and energy density (865.5 W h kg^-1) in a liquid zinc-air battery, with a long-term cycle stability over 256 h. The density functional theory calculation points out that changing the atomic ratio of Co/Mn can change the adsorption energy of the oxygen intermediate (*OOH), which allows the ORR catalytic process to be accelerated in the alkaline environment, thereby increasing the ORR catalytic activity. This article has important implications for the progress of commercially available bifunctional oxygen catalysts and their applications in zinc-air batteries.

Ultrasonic-assisted preparation of two-dimensional materials for electrocatalysts

Developing green, environmental, sustainable new energy sources is an important problem to be solved in the world. Among the new energy technologies, water splitting system, fuel cell technology and metal-air battery technology are the main energy production and conversion methods, which involve three main electrocatalytic reactions, hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR). The efficiency of the electrocatalytic reaction and the power consumption are very dependent on the activity of the electrocatalysts. Among various electrocatalysts, the two-dimensional (2D) materials have received widespread attention due to multiple advantages, such as their easy availability and low price. What' important is that they have adjustable physical and chemical properties. It is possible to develop them as electrocatalysts to replace the noble metals. Therefore, the design of two-dimensional electrocatalysts is a focus in the research area. Some recent advances in ultrasound-assisted preparation of two-dimensional (2D) materials have been overviewed according to the kind of materials in this review. Firstly, the effect of the ultrasonic cavitation and its applications in the synthesis of inorganic materials are introduced. The ultrasonic-assisted synthesis of representative 2D materials for example transition metal dichalcogenides (TMDs), graphene, layered double metal hydroxide (LDH), and MXene, and their catalytic properties as electrocatalysts are discussed in detail. For example, the CoMoS₄ electrocatalysts have been synthesized through a facile ultrasound-assisted hydrothermal method. The obatined HER and OER overpotential of CoMoS₄ electrode is 141 and 250 mV, respectively. This review points out some problems that need to be solved urgently at present, and provides some ideas for designing and constructing two-dimensional materials with better electrocatalytic performance.

Phosphate-modified cobalt silicate hydroxide with improved oxygen evolution reaction

Oxygen Evolution Reaction (OER) has gained significant attention due to its crucial role in renewable energy systems. The quest for efficient and low-cost OER catalysts remains a challenge of significant interest and importance. In this work, phosphate-incorporated cobalt silicate hydroxide (denoted as CoSi-P) is reported as a potential electrocatalyst for OER. The researchers first synthesized hollow spheres of cobalt silicate hydroxide Co₃(Si₂O₅)₂(OH)₂ (denoted as CoSi) using SiO₂ spheres as a template through a facile hydrothermal method. Phosphate (PO₄^3-) was then introduced to layered CoSi, leading to the reconstruction of the hollow spheres into sheet-like architectures. As expected, the resulting CoSi-P electrocatalyst demonstrated low overpotential (309 mV at 10 mA·cm^-2), large electrochemical active surface area (ECSA), and low Tafel slope. These parameters outperform CoSi hollow spheres and cobaltous phosphate (denoted as CoPO). Moreover, the catalytic performance achieved at 10 mA cm^-2 is comparable or even better than that of most transition metal silicates/oxides/hydroxides. The findings indicate that the incorporation of phosphate into the structure of CoSi can enhance its OER performance. This study not only provides a non-noble metal catalyst CoSi-P but also demonstrates that the incorporation of phosphates into transition metal silicates (TMSs) offers a promising strategy for the design of robust, high-efficiency, and low-cost OER catalysts.

Molten salt synthesis of NiCo-NiCo2O4@C nanotubes as anode materials for Li-ion batteries

The construction of carbon-encapsulated transition metal nanotube structures is a preferred method that can effectively slow down volume expansion, improve cycling stability and enhance the electrical conductivity of the reactive sites of lithium-ion batteries. In this study, nanotubes of carbon-coated NiCo-NiCo₂O₄ nanoparticles (NC-NCO@C) were prepared by a one-step molten salt method at high temperature using Ni and Co as catalytic centers and sodium acetate as carbon source. We used NC-NCO@C-2 nanotubes as anode materials for lithium-ion batteries(LIBs), which exhibited excellent lithium storage performance and good stability, with a specific capacity of 616.26 mAh g^-1 after 1000 cycles at a high current density of 1 A g^-1. In addition, NC-NCO@C-2 were used as anodes in lithium-ion full cells and LiFePO₄ (LFP) was used as the cathode. The NC-NCO@C-2//LFP full-cell exhibits high capacity and good cycling stability, with a capacity of 100.7 mAh g^-1 after 100 cycles and a capacity retention rate of 92%. The construction of NC, NCO, and carbon ternary complexes was found to activate and promote the reversible conversion of certain inorganic components at the solid electrolyte interfaces (SEI), which effectively reduced the volume change during cycling, increased the electrical conductivity, and improved the cycling stability of the electrode. The proposed one-step molten salt synthesis of Carbon-coated metals complexes with excellent compatibility characteristics, is expected to solve the problem of volume change in transition metals, which is encountered in LIBs applications.

Interfacial engineering of Co5.47N/Mo5N6 nanosheets with rich active sites synergistically accelerates water dissociation kinetics for Pt-like hydrogen evolution

The development of highly efficient hydrogen evolution electrocatalysts with platinum-like activity requires precise control of active sites through interface engineering strategies. In this study, a heterostructured Co_5.47N/Mo₅N₆ catalyst (CoMoN_x) on carbon cloth (CC) was synthesized using a combination of dip-etching and vapor nitridation methods. The rough nanosheet surface of the catalyst with uniformly distributed elements exposes a large active surface area and provides abundant interface sites that serve as additional active sites. The CoMoN_x was found to exhibit exceptional hydrogen evolution reaction (HER) activity with a low overpotential of 44 mV at 10 mA cm^-2 and exceptional stability of 100 h in 1.0 M KOH. The CoMoN_x^(-)||RuO₂⁽⁺⁾ system requires only 1.81 V cell voltage to reach a current density of 200 mA cm^-2, surpassing the majority of previously reported electrolyzers. Density functional theory (DFT) calculations reveal that the strong synergy between Co_5.47N and Mo₅N₆ at the interface can significantly reduce the water dissociation energy barrier, thereby improving the kinetics of hydrogen evolution. Furthermore, the rough nanosheet architecture of the CoMoN_x catalyst with abundant interstitial spaces and multi-channels enhances charge transport and reaction intermediate transportation, synergistically improving the performance of the HER for water splitting.

Understanding the sulphur-oxygen exchange process of metal sulphides prior to oxygen evolution reaction

Dynamic reconstruction of metal sulphides during electrocatalytic oxygen evolution reaction (OER) has hampered the acquisition of legible evidence for comprehensively understanding the phase-transition mechanism and electrocatalytic activity origin. Herein, modelling on a series of cobalt-nickel bimetallic sulphides, we for the first time establish an explicit and comprehensive picture of their dynamic phase evaluation pathway at the pre-catalytic stage before OER process. By utilizing the in-situ electrochemical transmission electron microscopy and electron energy loss spectroscopy, the lattice sulphur atoms of (NiCo)S_1.33 particles are revealed to be partially substituted by oxygen from electrolyte to form a lattice oxygen-sulphur coexisting shell surface before the generation of reconstituted active species. Such S-O exchange process is benefitted from the subtle modulation of metal-sulphur coordination form caused by the specific Ni and Co occupation. This unique oxygen-substitution behaviour produces an (NiCo)O_xS_1.33-x surface to reduce the energy barrier of surface reconstruction for converting sulphides into active oxy/hydroxide derivative, therefore significantly increasing the proportion of lattice oxygen-mediated mechanism compared to the pure sulphide surface. We anticipate this direct observation can provide an explicit picture of catalysts' structural and compositional evolution during the electrocatalytic process.

Defective multi-element hydroxides nanosheets for rapid removal of anionic organic dyes from water and oxygen evolution reaction

Water pollution by dyes is one of the biggest environmental problems. Adsorption technology has been widely used in wastewater treatment. In this work, high-entropy concept is used to design surface defective hydroxides realizing the rapid removal of dyes from water. Multi-element hydroxides (MEHs) containing three (CoMnNi, MEH-Ternary), four (CoMnNiZn, MEH-Quaternary), and five (CoMnNiZnFe, MEH-Quinary) metal elements are successfully synthesized through a polyol process. These as-synthesized MEHs are composed of nanosheets with a brucite-like structure. Along with the increase in compositional complexity (i.e., configurational entropy), the thickness of the nanosheets in these MEHs decreases, while the degree of surface defects increase. These surface defects are probably the active sites for anionic dyes adsorption, suggesting rapid adsorption kinetics with shortened diffusion path length. For MEH-Quinary in 0.2 mM Congo red (CR) and MEH-Ternary in 0.4 mM methyl orange (MO) aqueous solutions, respectively, high removal efficiency > 99.0% is achieved in the first 30 s. Their pseudo-second-order rate constants are two orders of magnitude higher than that of activated carbon and hydrotalcite. MEH-Quinary has maximum CR and MO adsorption quantity of 546.4 and 404.9 mg g^-1, respectively, by Langmuir model. The MEH-Quinary is also a potential electrocatalyst for oxygen evolution reaction.

Bimetallic Co-Fe sulfide and phosphide as efficient electrode materials for overall water splitting and supercapacitor

The major center of attraction in renewable energy technology is the designing of an efficient material for both electrocatalytic and supercapacitor (SC) applications. Herein, we report the simple hydrothermal method to synthesize cobalt-iron-based nanocomposites followed by sulfurization and phosphorization. The crystallinity of nanocomposites has been confirmed using X-ray diffraction, where crystalline nature improves from as-prepared to sulfurized to phosphorized. The as-synthesized CoFe-nanocomposite requires 263 mV overpotential for oxygen evolution reaction (OER) to reach a current density of 10 mA/cm2 whereas the phosphorized requires 240 mV to reach 10 mA/cm2. The hydrogen evolution reaction (HER) for CoFe-nanocomposite exhibits 208 mV overpotential at 10 mA/cm2. Moreover, the results improved after phosphorization showing 186 mV to reach 10 mA/cm2. The specific capacitance (Csp) of as-synthesized nanocomposite is 120 F/g at 1 A/g, along with a power density of 3752 W/kg and a maximum energy density of 4.3 Wh/kg. Furthermore, the phosphorized nanocomposite shows the best performance by exhibiting 252 F/g at 1 A/g and the highest power and energy density of 4.2 kW/kg and 10.1 Wh/kg. This shows that the results get improved more than twice. The 97% capacitance retention after 5000 cycles shows cyclic stability of phosphorized CoFe. Our research thus offers cost-effective and highly efficient material for energy production and storage applications.

Ambient Electrosynthesis toward Single-Atom Sites for Electrocatalytic Green Hydrogen Cycling

With the ultimate atomic utilization, well-defined configuration of active sites and unique electronic properties, catalysts with single-atom sites (SASs) exhibit appealing performance for electrocatalytic green hydrogen generation from water splitting and further utilization via hydrogen-oxygen fuel cells, such that a vast majority of synthetic strategies toward SAS-based catalysts (SASCs) are exploited. In particular, room-temperature electrosynthesis under atmospheric pressure offers a novel, safe, and effective route to access SASs. Herein, the recent progress in ambient electrosynthesis toward SASs for electrocatalytic sustainable hydrogen generation and utilization, and future opportunities are discussed. A systematic summary is started on three kinds of ambient electrochemically synthetic routes for SASs, including electrochemical etching (ECE), direct electrodeposition (DED), and electrochemical leaching-redeposition (ELR), associated with advanced characterization techniques. Next, their electrocatalytic applications for hydrogen energy conversion including hydrogen evolution reaction, oxygen evolution reaction, overall water splitting, and oxygen reduction reaction are reviewed. Finally, a brief conclusion and remarks on future challenges regarding further development of ambient electrosynthesis of high-performance and cost-effective SASCs for many other electrocatalytic applications are presented.

2D Metal-Organic Frameworks as Competent Electrocatalysts for Water Splitting

Hydrogen, a clean and flexible energy carrier, can be efficiently produced by electrocatalytic water splitting. To accelerate the sluggish hydrogen evolution reaction and oxygen evolution reaction kinetics in the splitting process, highly active electrocatalysts are essential for lowering the energy barriers, thereby improving the efficiency of overall water splitting. Combining the distinctive advantages of metal-organic frameworks (MOFs) with the physicochemical properties of 2D materials such as large surface area, tunable structure, accessible active sites, and enhanced conductivity, 2D MOFs have attracted intensive attention in the field of electrocatalysis. Different strategies, such as improving the conductivities of MOFs, reducing the thicknesses of MOF nanosheets, and integrating MOFs with conductive particles or substrates, are developed to promote the catalytic performances of pristine MOFs. This review summarizes the recent advances of pristine 2D MOF-based electrocatalysts for water electrolysis. In particular, their intrinsic electrocatalytic properties are detailly analyzed to reveal important roles of inherent MOF active centers, or other in situ generated active phases from MOFs responsible for the catalytic reactions. Finally, the challenges and development prospects of pristine 2D MOFs for the future applications in overall water splitting are discussed.

Spinel-Derived Formation and Amorphization of Bimetallic Oxyhydroxides for Efficient Electrocatalytic Biomass Oxidation

Replacing the oxygen evolution reaction (OER) with water-assisted oxidation of organic molecules represents a promising approach for achieving sustainable electrochemical biomass utilization. Among numerous OER catalysts, spinels have received substantial attention due to their manifold compositions and valence states, yet their application in biomass conversions remains rare. Herein, a series of spinels were investigated for the selective electrooxidation of furfural and 5-hydroxymethylfurfural, two model substrates for versatile value-added chemical products. Spinel sulfides universally exhibit superior catalytic performance compared to that of spinel oxides, and further investigations show that the replacement of oxygen with sulfur led to the complete phase transition of spinel sulfides into amorphous bimetallic oxyhydroxides during electrochemical activation, serving as the active species. Excellent values of conversion rate (100%), selectivity (100%), faradaic efficiency (>95%), and stability were achieved via sulfide-derived amorphous CuCo-oxyhydroxide. Furthermore, a volcano-like correlation was established between their BEOR and OER activities based on an OER-assisted organic oxidation mechanism.

Hollow Carbon Nanopillar Arrays Encapsulated with Pd-Cu Alloy Nanoparticles for the Oxygen Evolution Reaction

Delicate design and bottom-up synthesis of hollow nanostructures for oxygen evolution electrocatalysts is a promising way to accelerate the reaction kinetics of overall water splitting. Herein, an efficient and versatile strategy for the controllable preparation of Pd-Cu alloy nanoparticles encapsulated in carbon nanopillar arrays (PD-Cu@HPCN) is developed. Core-shell structured MOF@imidazolium-based ionic polymers (ImIPs) have been prepared and adopted as a template, along with the decomposition of the inner Cu-MOFs when an anion exchange occurs between sodium tetrachloropalladate in solution and bromides in the external ImIP shell. Pd nanoparticles will be highly dispersed in the resulting Pd-Cu@HO-ImIP array, and subsequent topotactic transformation generates Pd-Cu@HNPC. No hazardous reagents or tedious steps are used to remove the inner Cu-MOF templates in contrast to the traditional top-down methods. Remarkably, the Pd-Cu@HPCN catalyst possesses outstanding oxygen evolution reaction (OER) activity, including small overpotential with 10 mA cm^-2 at an overpotential of 188 mV, a large double layer capacitance (73.8 mF cm^-2), and high stability (20 h). This simple, green, and efficient synthesis methodology represents a new way to design metal alloys for OER electrocatalysts or other electrocatalytic devices.

Scalable Synthesis of Multi-Metal Electrocatalyst Powders and Electrodes and their Application for Oxygen Evolution and Water Splitting

Multi-metal electrocatalysts provide nearly unlimited catalytic possibilities arising from synergistic element interactions. We propose a polymer/metal precursor spraying technique that can easily be adapted to produce a large variety of compositional different multi-metal catalyst materials. To demonstrate this, 11 catalysts were synthesized, characterized, and investigated for the oxygen evolution reaction (OER). Further investigation of the most active OER catalyst, namely CoNiFeMoCr, revealed a polycrystalline structure, and operando Raman measurements indicate that multiple active sites are participating in the reaction. Moreover, Ni foam-supported CoNiFeMoCr electrodes were developed and applied for water splitting in flow-through electrolysis cells with electrolyte gaps and in zero-gap membrane electrode assembly (MEA) configurations. The proposed alkaline MEA-type electrolyzers reached up to 3 A cm^-2 , and 24 h measurements demonstrated no loss of current density of 1 A cm^-2 .

The role of various components in Ru-NiCo alloys in boosting the performance of overall water splitting

Understanding the synergistic mechanism of multi-component alloys is crucial and challenging for overall water splitting. Herein, Ru-NiCo_0.5-600 °C and Ru-Ni_0.75Co with excellent electrocatalytic activity are designed and synthesized. The Ru-NiCo_0.5-600 °C alloy exhibits remarkable HER activity with an overpotential of 42, 77 and 93 mV at 10 mA cm^-2 in alkaline, acidic and neutral conditions, and the Ru-Ni_0.75Co electrocatalyst presents outstanding OER activity with an overpotential of 176 mV at 10 mA cm^-2 in 1.0 M KOH. The Ru-NiCo_0.5-600 °C ||Ru-Ni_0.75Co cell requires only 1.48 and 1.69 V to reach 10 and 100 mA cm^-2 towards overall water splitting. A series of experiments reveal that the strong electronic coupling among Ru, Ni and Co regulates the electronic structure and enhances the intrinsic catalytic activity and stability of the as-synthesized Ru-NiCo electrocatalysts. Systematic experimental and theoretical results prove that Ni atoms act as the active sites of dissociating water, while Ru and Co are respectively the active centers of proton and hydroxyl adsorption for HER and OER. Our work provides a new perspective for profoundly understanding the synergistic effect of multi-component alloys towards water splitting.

Regulating the thickness of the carbon coating layer in iron/carbon heterostructures to enhance the catalytic performance for oxygen evolution reaction

The exploration of high-performance electrocatalysts for the oxygen evolution reaction (OER) is crucial and urgent for the fast development of green and renewable hydrogen energy. Herein, an ultra-fast and energy-efficient preparation strategy (microwave-assisted rapid in-situ pyrolysis of organometallic compounds induced by carbon nanotube (CNT)) is developed to obtain iron/carbon (Fe/C) heterogeneous materials (Fe/Fe₃C particles wrapped by carbon coating layer). The thickness of the carbon coating layer can be adjusted by changing the content and form of carbon in the metal sources during the fast preparation process. Fe/Fe₃C-A@CNT using iron acetylacetonate as metal sources possesses unique Fe/C heterogeneous, small Fe/Fe₃C particles encapsulated by the thin carbon coating layer (1.77 nm), and obtains the optimal electron penetration effect. The electron penetration effect derives from the redistribution of charge between the surface carbon coating layer and inner Fe/Fe₃C nanoparticles efficiently improving both catalytic activity and stability. Therefore, Fe/Fe₃C-A@CNT shows efficient OER catalytic activity, just needing a low overpotential of 292 mV to reach a current density of 10 mA cm^-2, and long-lasting stability. More importantly, the unique control strategy for carbon thickness in this work provides more opportunity and perspective to prepare robust metal/carbon-based catalytic materials at the nanoscale.

Surface valence regulation of cobalt-nickel foams for glucose oxidation-assisted water electrolysis

Electrooxidation reactions of organic molecules that require a much lower overpotential are currently considered as promising alternatives to replace the oxygen evolution reaction (OER) in water electrolysis. Herein, an ultrafast oxygen plasma treatment is implemented to modify commercial cobalt-nickel foam (CNF) to regulate the high-valence Co³⁺ and Ni³⁺, rendering more active sites, faster reaction kinetics and enhanced response towards glucose. Compared to the OER, the overpotential of the plasma-treated CNF at 10 mA cm^-2 was reduced to 133 mV via glucose electro-oxidation coupled with water splitting.

Facet Engineering of Advanced Electrocatalysts Toward Hydrogen/Oxygen Evolution Reactions

The crystal facets featured with facet-dependent physical and chemical properties can exhibit varied electrocatalytic activity toward hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) attributed to their anisotropy. The highly active exposed crystal facets enable increased mass activity of active sites, lower reaction energy barriers, and enhanced catalytic reaction rates for HER and OER. The formation mechanism and control strategy of the crystal facet, significant contributions as well as challenges and perspectives of facet-engineered catalysts for HER and OER are provided.

The electrocatalytic water splitting technology can generate high-purity hydrogen without emitting carbon dioxide, which is in favor of relieving environmental pollution and energy crisis and achieving carbon neutrality. Electrocatalysts can effectively reduce the reaction energy barrier and increase the reaction efficiency. Facet engineering is considered as a promising strategy in controlling the ratio of desired crystal planes on the surface. Owing to the anisotropy, crystal planes with different orientations usually feature facet-dependent physical and chemical properties, leading to differences in the adsorption energies of oxygen or hydrogen intermediates, and thus exhibit varied electrocatalytic activity toward hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). In this review, a brief introduction of the basic concepts, fundamental understanding of the reaction mechanisms as well as key evaluating parameters for both HER and OER are provided. The formation mechanisms of the crystal facets are comprehensively overviewed aiming to give scientific theory guides to realize dominant crystal planes. Subsequently, three strategies of selective capping agent, selective etching agent, and coordination modulation to tune crystal planes are comprehensively summarized. Then, we present an overview of significant contributions of facet-engineered catalysts toward HER, OER, and overall water splitting. In particular, we highlight that density functional theory calculations play an indispensable role in unveiling the structure–activity correlation between the crystal plane and catalytic activity. Finally, the remaining challenges in facet-engineered catalysts for HER and OER are provided and future prospects for designing advanced facet-engineered electrocatalysts are discussed.

Self-standing 2D/2D Co3O4@FeOOH nanosheet arrays as promising catalysts for the oxygen evolution reaction

The rational design of a highly efficient oxygen evolution reaction (OER) is crucial for the practical applications of water electrolysis. Herein, a hybrid Co₃O₄@FeOOH/NF electrode was fabricated by loading FeOOH sheets on the surface of Co₃O₄ nanosheet arrays via a newly developed chemical deposition protocol. The decoration of FeOOH on Co₃O₄ nanosheet arrays not only endows a strong electronic interaction between the two components but also offers sufficient active sites for the OER process. Benefitting from these advantages, Co₃O₄@FeOOH/NF exhibited outstanding OER activity in terms of a low overpotential of 209 mV at 10 mA cm^-2 and a low Tafel slope of 48.9 mV dec^-1. Moreover, nearly steady state operation current and negligible change in the phase and morphology of the catalyst also indicate remarkable stability. This work may provide an important guide for the design of high-performance electrocatalysts for energy conversion applications.

Oriented Design of Transition-Metal-Oxide Hollow Multishelled Micropolyhedron Derived from Bimetal-Organic Frameworks for the Electrochemical Detection of Multipesticide Residues

Transition-metal oxides (TMOs) with a hollow multishelled structure have emerged as highly potential materials for high-performance electrochemical sensing, benefiting from their superior electronic conductivity, exceptionally large specific surface area, excellent stability, and electrochemistry properties. In particular, binary TMOs are expected to outperform unitary TMOs due to the synergistic effect of the different metals. Herein, MnCo₂O_4.5 hollow quadruple-shelled porous micropolyhedrons (MnCo₂O_4.5 HoQS-MPs) were prepared and employed to construct an ultrasensitive sensing platform for a multipesticide assay. Profiting from complex hollow interior structures and abundant active sites, the MnCo₂O_4.5 HoQS-MPs manifest outstanding electrochemical properties as electrode materials for the pesticide assay. The MnCo₂O_4.5 HoQS-MP-based biosensor demonstrated remarkable performance for monocrotophos, methamidophos, and carbaryl detection, with wide linear ranges, as well as low detection limits. This work unveils a new pathway for the ultrasensitive detection of pesticides and demonstrates tremendous potential for detecting other environmentally deleterious chemicals.