Oxygen-Doped Nickel Iron Phosphide Nanocube Arrays Grown on Ni Foam for Oxygen Evolution Electrocatalysis

Co-Adjusting d-Band Center of Fe to Accelerate Proton Coupling for Efficient Oxygen Electrocatalysis

The problem in d-band center modulation of transition metal-based catalysts for the rate-determining steps of oxygen conversion is an obstacle to boost the electrocatalytic activity by accelerating proton coupling. Herein, the Co doping to FeP is adopted to modify the d-band center of Fe. Optimized Fe sites accelerate the proton coupling of oxygen reduction reaction (ORR) on N-doped wood-derived carbon through promoting water dissociation. In situ generated Fe sites optimize the adsorption of oxygen-related intermediates of oxygen evolution reaction (OER) on CoFeP NPs. Superior catalytic activity toward ORR (half-wave potential of 0.88 V) and OER (overpotential of 300 mV at 10 mA cm-2) express an unprecedented level in carbon-based transition metal-phosphide catalysts. The liquid zinc-air battery presents an outstanding cycling stability of 800 h (2400 cycles). This research offers a newfangled perception on designing highly efficient carbon-based bifunctional catalysts for ORR and OER.

Unlocking the Potential of Amorphous Prussian Blue with Highly Active Mn Sites at Room Temperature for Impressive Oxygen Evolution Reaction and Super Capacitor Electrochemical Performance

The key to increasing the rate of oxygen evolution reaction (OER) lies in accelerated four-electron dynamics, while the key to facilitating the development of supercapacitors lies in the design of electrode materials. This paper synthesized manganese-iron Prussian blue (MnFe-PBA@IF) at room temperature, and hexagonal concave structures w ere prepared using a fast-reducing matrix. Interestingly, MnFe-PBA@IF has an amorphous structure favorable to exposing more active surfaces. According to Gibbs free energy calculations on MnFe-PBA, charge depletion of manganese atoms can greatly enhance the adsorption of electron-rich oxygen-containing groups on the surface. Furthermore, the overpotential in 1 m KOH is 280 mV. Also, it can be used as a supercapacitor with a stable operating voltage range of -0.9-0 V and a specific capacity of 1260 F g-1 . This work provides new insights into the synthesis of OER catalysts for Prussian blue ferromanganese at room temperature. Non-gold-bonded adsorption, highly active metal centers and active surfaces are the underlying reasons for the superior performance of supercapacitors. Therefore, Prussian blue with good energy storage performance and high active surface can be used as multifunctional energy storage and conversion electrodes.

Single Atom Ru Doped Ni2 P/Fe3 P Heterostructure for Boosting Hydrogen Evolution for Water Splitting

Modulating the chemical composition and structure has been considered as one of the most promising strategies for developing high-efficient water splitting catalysts. Here, a single-atom Ru doped Ni2 P/Fe3 P catalyst is synthesized by introducing the dispersed Ru atoms to adjust Ni2 P/Fe3 P heterostructure. Single atom Ru provides effective hydrogen evolution reaction (HER) active sites for boosting catalytic activities. The catalyst with only 0.2 wt.% content of Ru exhibits an overpotential of 19.3 mV at 10 mA cm-2 , which is obviously lower than 146.1 mV of Ni2 P/Fe3 P. Notably, an alkaline overall water electrolyzer based on Ru-Ni2 P/Fe3 P catalysts achieves a cell voltage of 1.47 V and operates over 600 h at 10 mA cm-2 , which is superior to that of benchmark RuO2 //Pt/C (1.61 V). The theoretical calculations further confirm that Ru single atom doping can effectively optimize the hydrogen/water adsorption free energy of the active site and therefore improve the HER activity of heterostructure. This work provides a valuable reference to design high-activity and durability catalyst for water splitting through the double modulation of interface-effect and atomic doping.

Electrochemical Oxidation of Furfural on NiMoP/NF: Boosting Current Density with Enhanced Adsorption of Oxygenates

Substituting the low-value oxygen evolution reaction (OER) with thermodynamically more favored organic oxidation such as furfural oxidation reaction (FOR) is regarded as a perspective approach to decrease energy cost of hydrogen evolution from water splitting. However, the kinetic of FOR can be even more sluggish than OER under large current density. In this work, a strategy is proposed to accelerate FOR by enhancing the adsorption of oxygenates on active sites. Over the prepared NiMoP/NF anode, only 1.46 V versus RHE is required in furfural solution to achieve 500 mA cm-2 , significantly better than the OER activity over commercial RuO2 /NF under the same current density (1.57 V vs RHE).

Multifunctional Catalytic Hierarchical Interfaces of Ni12 P5 -Ni2 P Porous Nanosheets Enabled Both Sulfides Reaction Promotion and Li-Dendrite Suppression for High-Performance Li-S Full Batteries

The development of lithium-sulfur (Li-S) batteries is very promising and yet faces the issues of hindered polysulfides conversion and Li dendrite growth. Different from using different materials strategies to overcome these two types of problems, here multifunctional catalytic hierarchical interfaces of Ni12 P5 -Ni2 P porous nanosheets formed by Ni2 P partially in situ converted from Ni12 P5 are proposed. The unique electronic structure in the interface endows Ni12 P5 -Ni2 P effective electrocatalysis effect toward both sulfides' reduction and oxidation through reducing Gibbs free energies, indicating a bidirectional conversion acceleration. Importantly, Ni12 P5 -Ni2 P porous nanosheets with hierarchical interfaces also reduced the Li nucleation energy barrier, and a dendrite-free Li deposition is realized during the overall Li deposition and stripping steps. To this end, Ni12 P5 -Ni2 P decorated carbon nanotube/S cathode showing a high capacity of over 1500 mAh g-1 , and a high rate capability of 8 C. Moreover, the coin full cell delivered a high capacity of 1345 mAh g-1 at 0.2 C and the pouch full cell delivered a high capacity of 1114 mAh g-1 at 0.2 C with high electrochemical stability during 180° bending. This work inspires the exploration of hierarchical structures of 2D materials with catalytically active interfaces to improve the electrochemistry of Li-S full battery.

A Review of Transition Metal Boride, Carbide, Pnictide, and Chalcogenide Water Oxidation Electrocatalysts

Transition metal borides, carbides, pnictides, and chalcogenides (X-ides) have emerged as a class of materials for the oxygen evolution reaction (OER). Because of their high earth abundance, electrical conductivity, and OER performance, these electrocatalysts have the potential to enable the practical application of green energy conversion and storage. Under OER potentials, X-ide electrocatalysts demonstrate various degrees of oxidation resistance due to their differences in chemical composition, crystal structure, and morphology. Depending on their resistance to oxidation, these catalysts will fall into one of three post-OER electrocatalyst categories: fully oxidized oxide/(oxy)hydroxide material, partially oxidized core@shell structure, and unoxidized material. In the past ten years (from 2013 to 2022), over 890 peer-reviewed research papers have focused on X-ide OER electrocatalysts. Previous review papers have provided limited conclusions and have omitted the significance of "catalytically active sites/species/phases" in X-ide OER electrocatalysts. In this review, a comprehensive summary of (i) experimental parameters (e.g., substrates, electrocatalyst loading amounts, geometric overpotentials, Tafel slopes, etc.) and (ii) electrochemical stability tests and post-analyses in X-ide OER electrocatalyst publications from 2013 to 2022 is provided. Both mono and polyanion X-ides are discussed and classified with respect to their material transformation during the OER. Special analytical techniques employed to study X-ide reconstruction are also evaluated. Additionally, future challenges and questions yet to be answered are provided in each section. This review aims to provide researchers with a toolkit to approach X-ide OER electrocatalyst research and to showcase necessary avenues for future investigation.

Strain Engineering of the NiTe/Ni2P Heterostructure to Boost the Oxygen Evolution Reaction

Discovering highly efficient and stable non-precious metal catalysts for the oxygen evolution reaction (OER) is crucial for energy conversion in water splitting. However, preparing high-performance OER catalysts and elucidating the structural changes in the process are still challenging. Herein, we synthesize the NiTe/Ni2P heterostructure and demonstrate the strain engineering of NiTe/Ni2P via the lattice incompatibility between the phosphide and the telluride. The strain engineering of the NiTe/Ni2P heterostructure not only significantly boosts the OER activity but also effectively stabilizes the intrinsic structure of the catalyst after the OER process by using the in situ-produced metal salt as a protection layer. After the OER stability test, no oxyhydroxide phase is observed, and in situ Raman spectroscopy reveals that a voltage-dependent phase transition appears during the OER, which is different from most previously reported Ni-based catalysts, for which the generation of irreversible NiOOH occurs after the OER. Density functional theory calculations further reveal that the tensile strain of Ni2P will inhibit the presence of irreversible phase transitions of Ni2P into NiOOH due to the weak adsorption ability of the oxygen species caused by strain engineering. In short, this work opens a new gate for using strain nanotechnology to design high-performance OER catalysts.

Balance between Activity and Stability of Single Metal and Intermetallic Compounds for Electrocatalytic Hydrogen Evolution Reaction

The higher population of the antibonding state around the Fermi level will result in better activity yet lower stability of HER (Re vs Ru metal). There seems to be a limitation or balance for using a single metal since the bonding scheme of a single metal is relatively simple. Combining Re (strong bonding), Ru (HER active), and Zr metal (corrosion-resistant) grants ternary intermetallic compound ZrRe_1.75Ru₀₂₅, exhibiting excellent HER activity and stability in acidic and alkaline electrolytes. The overpotential at a current density of 10 mA/cm² (η₁₀) for ZrRe_1.75Ru₀₂₅ is much lower compared to that of ZrRe₂. Although the HER activity of ZrRe_1.75Ru₀₂₅ is not comparable to that of ZrRu₂, it demonstrates outstanding HER stability, while the current density of ZrRu₂ is over ca. 16% after 6 h. This suggests that intermetallic compounds can break the constraint between activity and stability in a single metal for HER, which may be applied in other fields as well.

Magnetic Field-Enhanced Electrocatalytic Oxygen Evolution on a Mixed-Valent Cobalt-Modulated LaCoO3 Catalyst

Extensive efforts to enhance the oxygen evolution reaction (OER) catalytic performance of transition metal oxides mainly concentrate on the extrinsic morphology tailoring, lattice doping, and electrode interface optimizing. Nevertheless, little room is left for performance improvement using these methods and an obvious gap still exists compared to the precious metal catalysts. In this work, a novel "mixed-valent cobalt modulation" strategy is presented to enhance the electrocatalytic OER of perovskite LaCoO₃ (LCO) oxide. The valence transition of cobalt is realized by ethylenediamine post reduction procedure at room temperature, which further induces the variation of magnetic properties for LCO catalyst. The optimized LCO catalyst with Co²⁺ /Co³⁺ of 1.98 % exhibits the best OER activity, and the overpotential at 10 mA cm^-2 current density is decreased by 170 mV compared pristine LCO. Impressively, the ferromagnetic LCO catalyst can perform magnetic OER enhancement. By application of an external magnetic field, the overpotential of LCO at 10 mA cm^-2 can be further decreased by 20 mV compared to that of under zero magnetic field, which arises from the enhanced energy states of electrons and accelerated electron transfer process driven by magnetic field. Our findings may provide a promising strategy to break the bottleneck for further enhancement of OER performance.

Carbon Nanocage with Maximum Utilization of Atomically Dispersed Iron as Efficient Oxygen Electroreduction Nanoreactor

As key parameters of electrocatalysts, the density and utilization of active sites determine the electrocatalytic performance toward oxygen reduction reaction. Unfortunately, prevalent oxygen electrocatalysts fail to maximize the utilization of active sites due to inappropriate nanostructural design. Herein, a nano-emulsion induced polymerization self-assembly strategy is employed to prepare hierarchical meso-/microporous N/S co-doped carbon nanocage with atomically dispersed FeN₄ (denoted as Meso/Micro-FeNSC). In situ scanning electrochemical microscopy technology reveals the density of available active sites for Meso/Micro-FeNSC reach to 3.57 × 10¹⁴ sites cm^-2 , representing more than threefold improvement compared to micropore-dominant Micro-FeNSC counterpart (1.07 × 10¹⁴ sites cm^-2 ). Additionally, the turnover frequency of Meso/Micro-FeNSC is also improved to 0.69 from 0.50 e^- site^-1 s^-1 for Micro-FeNSC. These properties motivate Meso/Micro-FeNSC as efficient oxygen electroreduction electrocatalyst, in terms of outstanding half-wave potential (0.91 V), remarkable kinetic mass specific activity (68.65 A g^-1 ), and excellent robustness. The assembled Zn-air batteries with Meso/Micro-FeNSC deliver high peak power density (264.34 mW cm^-2 ), large specific capacity (814.09 mA h g^-1 ), and long cycle life (>200 h). This work sheds lights on quantifying active site density and the significance of maximum utilization of active sites for rational design of advanced catalysts.

MOF-derived nanoarrays as advanced electrocatalysts for water splitting

Developing efficient, nanostructured electrocatalysts with the desired compositions and structures is of great significance for improving the efficiency of water splitting toward hydrogen production. In this regard, metal-organic framework (MOF) derived nanoarrays have attracted great attention as promising electrocatalysts because of their diverse compositions and adjustable structures. In this review, the recent progress in MOF-derived nanoarrays for electrochemical water splitting is summarized, highlighting the structural design of the MOF-derived nanoarrays and the electrocatalytic performance of the derived composite carbon materials, oxides, hydroxides, sulfides, and phosphides. In particular, the structure-performance relationships of the MOF-derived nanoarrays and the modulation strategies toward enhanced catalytic activity for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are discussed, providing insights into the development of advanced catalysts for the HER and OER. The challenges and prospects in this promising field for future industrial applications are also addressed.

Highly efficient oxygen evolution reaction enabled by phosphorus-boron facilitating surface reconstruction of amorphous high-entropy materials

Efficient, cost-effective and durable electrocatalysts are highly required to overcome the slow kinetics and high overpotential of oxygen evolution reaction (OER). Here we report a series of novel amorphous high-entropy borophosphate catalysts FeCoNiMBPO_x (M = Mg, Al, Cr, Mn) prepared by a low-temperature reduction method. The leaching of boron and phosphorus accelerates the surface self-reconstruction of FeCoNiMnBPO_x, and the subsequently formed high-oxidation-state metal-OOH species is beneficial to improve the catalyst performance. Moreover, the unique amorphous structure with abundant defects provides more active sites for OER. As a return, all the samples exhibit excellent OER activity and stability. Among them, FeCoNiMnBPO_x with the highest conductivity and the largest electrochemical active surface area (ECSA) exhibits the best electrocatalytic performance, requiring only low overpotentials of 248 mV and 294 mV to reach current densities of 10 mA cm^-2 and 100 mA cm^-2, respectively. This sample also shows an exceptional durability for 50 h without a significant increase in potential, which is superior to that of the benchmark RuO₂ electrocatalyst. The combination of the adsorbate evolution mechanism (AEM) and the lattice oxygen-mediated mechanism (LOM) are responsible for the excellent catalyst performance. This work provides new ideas for designing high-activity multiple-element catalysts.

Function of Internal and External Fe in a Ni-Based Precatalyst System Toward Oxygen Evolution Reaction

It is well known that the "iron" impurity will influence the oxygen evolution reaction (OER) in an alkaline electrolyte, especially for the Ni-based electrocatalyst. Many research studies have investigated the function of Fe in the OER active phase, such as M(OH)₂/MOOH (M = Ni and/or Fe), while, surprisingly, very few studies have examined the function of Fe in the "precatalyst" system. Accordingly, in this work, the Ni_3-xFe_xP (x = 0, 0.5, 1) series as an Ni-based precatalyst was employed to inspect the function of internal and external Fe in the Ni-based precatalyst system. It was realized that the sample with internal Fe (i.e., Ni_2.5Fe_0.5P and Ni₂FeP) exhibits efficient OER activity compared to that of the Fe-free one (i.e., Ni₃P) owing to the large amount of active M(OH)₂/MOOH formed on the surface. This indicates that the internal Fe in the present system may have the ability to facilitate the phase transformation; it was later rationalized from electronic structural calculations that the d band center of the internal Fe (middle transition metal) and Ni (late transition metal) holds the key for this observation. Adding excessive ferrous chloride tetrahydrate (FeCl₂·4H₂O) as the external Fe in the electrolyte will greatly improve the OER performances for Ni₃P; nevertheless, that the OER activity of Ni₂FeP is still much superior than that of Ni₃P corroborates the fact that the Fe impurity is not the only reason for the elevated OER activity of Ni₂FeP and that internal Fe is also critical to the phase transformation as well as OER performance.

Multimetallic transition metal phosphide nanostructures for supercapacitors and electrochemical water splitting

Transition metal phosphides (TMPs) have recently emerged as an important class of functional materials and been demonstrated to be outstanding supercapacitor electrode materials and catalysts for electrochemical water splitting. While extensive investigations have been devoted to monometallic TMPs, multimetallic TMPs have lately proved to show enhanced electrochemical performance compared to their monometallic counterparts, thanks to the synergistic effect between different transition metal species. This topical review summarizes recent advance in the synthesis of new multimetallic TMP nanostructures, with particular focus on their applications in supercapacitors and electrochemical water splitting. Both experimental reports and theoretical understanding of the synergy between transition metal species are comprehensively reviewed, and perspectives of future research on TMP-based materials for these specific applications are outlined.

Wood-Derived Monolithic Catalysts with the Ability of Activating Water Molecules for Oxygen Electrocatalysis

Oxygen reduction reaction (ORR) is the key reaction on cathode of rechargeable zinc-air batteries (ZABs). However, the lack of protons in alkaline conditions limits the rate of ORR. Herein, an activating water strategy is proposed to promote oxygen electrocatalytic activity by enhancing the proton production from water dissociation. FeP nanoparticles (NPs) are coupled on N-doped wood-derived catalytically active carbon (FeP-NWCC) to associate bifunctional active sites. In alkaline, FeP-NWCC possesses outstanding catalytic activities toward ORR (E_1/2  = 0.86 V) and Oxygen evolution reaction (OER) (overpotential is 310 mV at 10 mA cm^-2 ). The liquid ZABs assembled by FeP-NWCC deliver superior peak power density (144 mW cm^-2 ) and cycle stability (over 450 h). The quasi-solid-state ZABs based on FeP-NWCC also display excellent performances. Theoretical calculation illustrates that the superb bifunctional performance of FeP-NWCC results from the elevated dissociation efficiency of water via FeP NPs to assist the oxygen catalytic process. The strategy of activating water provides a new perspective for the design of ORR/OER bifunctional catalysts. This work is a model for the application of forest biomass.

Multistep Sulfur Leaching for the Development of a Highly Efficient and Stable NiSx/Ni(OH)2/NiOOH Electrocatalyst for Anion Exchange Membrane Water Electrolysis

Nickel (poly)sulfides have been widely studied as anodic catalysts for alkaline water electrolysis owing to their diverse morphologies, high catalytic activities in the oxygen evolution reaction (OER), and low cost. To utilize low-cost and high-efficiency polysulfides with industry-relevant cycling stability, we develop a Ni-rich NiS_x/Ni(OH)₂/NiOOH catalyst derived from NiS₂/Ni₃S₄ nanocubes. Ni-rich NiS_x/Ni(OH)₂/NiOOH shows improved OER catalytic activity (η = 374 mV@50 mA cm^-2) and stability (0.1% voltage increase) after 65 h of a galvanostatic test at 10 mA cm^-2 compared with commercial Ni/NiO and hydrothermally synthesized Ni(OH)₂ (both show η > 460 mV@50 mA cm^-2 along with 4.40 and 1.92% voltage increase, respectively). A water-splitting electrolyzer based on Pt/C||AF1-HNN8-50||NiS_x/Ni(OH)₂/NiOOH exhibits a current density of 1800 mA cm^-2 at 2.0 V and 500 h high-rate stability at 1000 mA cm^-2 with negligible attenuation of only 0.12 mV h^-1. This work provides an understanding of truly stable species, intrinsic active phases of Ni polysulfides, their high-rate stability in a real cell, and sheds light on the development of stable chalcogenide-based anodic electrocatalysts for anion exchange membrane water electrolysis (AEMWE).

Freestanding Metal-Organic Frameworks and Their Derivatives: An Emerging Platform for Electrochemical Energy Storage and Conversion

Metal–organic frameworks (MOFs) have recently emerged as ideal electrode materials and precursors for electrochemical energy storage and conversion (EESC) owing to their large specific surface areas, highly tunable porosities, abundant active sites, and diversified choices of metal nodes and organic linkers. Both MOF-based and MOF-derived materials in powder form have been widely investigated in relation to their synthesis methods, structure and morphology controls, and performance advantages in targeted applications. However, to engage them for energy applications, both binders and additives would be required to form postprocessed electrodes, fundamentally eliminating some of the active sites and thus degrading the superior effects of the MOF-based/derived materials. The advancement of freestanding electrodes provides a new promising platform for MOF-based/derived materials in EESC thanks to their apparent merits, including fast electron/charge transmission and seamless contact between active materials and current collectors. Benefiting from the synergistic effect of freestanding structures and MOF-based/derived materials, outstanding electrochemical performance in EESC can be achieved, stimulating the increasing enthusiasm in recent years. This review provides a timely and comprehensive overview on the structural features and fabrication techniques of freestanding MOF-based/derived electrodes. Then, the latest advances in freestanding MOF-based/derived electrodes are summarized from electrochemical energy storage devices to electrocatalysis. Finally, insights into the currently faced challenges and further perspectives on these feasible solutions of freestanding MOF-based/derived electrodes for EESC are discussed, aiming at providing a new set of guidance to promote their further development in scale-up production and commercial applications.

Designing bimetallic zeolitic imidazolate frameworks (ZIFs) for aqueous catalysis: Co/Zn-ZIF-8 as a cyclic-durable catalyst for hydrogen peroxide oxidative decomposition of organic dyes in water

ZIF-8 is well known hybrid material that is self-assembled from inorganic and organic moieties. It has several potential applications due to its unique structure. One of these potential applications is in advanced oxidation processes (AOP) via a heterogeneous catalysis system. The use of modified ZIF-8/H2O2 for the destruction of the azo dye methyl orange (MO) is presented in this work to explore its efficacy. This work presents the bimetallic Co/Zn-ZIF-8 as an efficient catalyst to promote H2O2 oxidation of the MO dye. Co/Zn-ZIF-8 was synthesized through a hydrothermal process, and the pristine structure was confirmed using XRD, FTIR, and XPS. The Co/Zn-ZIF-8/H2O2 system successfully decolorized MO at the selected pH 6.5. It was found that more than 90% of MO (10 ppm) was degraded within only about 50 minutes. Proposed radical and redox mechanisms are presented for H2O2 decomposition where the redox mechanism is suggested to predominate via a Co(ii)/Co(iii) redox consecutive cyclic process.

Mo,Fe-codoped metal phosphide nanosheets derived from Prussian blue analogues for efficient overall water splitting

Designing non-precious electrocatalysts with multiple active centers and durability toward overall water splitting is of great significance for storing renewable energy. This study reports a low-cost Mo, Fe codoped NiCoP_x electrocatalysts derived from Co-Fe Prussian blue analogue and following phosphorization process. Benefitted from the optimized electronic configuration, hierarchical structure and abundant active sites, the Mo,Fe-NiCoP_x/NF electrode has shown competitive oxygen evolution reaction (ƞ₁₀ = 197 mV) and hydrogen evolution reaction performance (ƞ₁₀ = 99 mV) when the current density is 10 mA cm^-2 in 1 M KOH solution. Moreover, the integrated water splitting device assembled by Mo,Fe-NiCoP_x/NF as both anode and cathode only needs a voltage of 1.545 V to reach 10 mA cm^-2. Density functional theory results further confirm that the Mo, Fe codoped heterostructure can synergistically optimize the d-band center and Gibbs free energy during electrocatalytic processes, thus accelerating the kinetics of electrochemical water splitting. This work demonstrates the importance of rational combination of metal doping and interface engineering for advanced catalytic materials.

Tailoring the Morphology of Cost-Effective Vanadium Diboride Through Cobalt Substitution for Highly Efficient Alkaline Water Oxidation

Design and development of efficient, economical, and durable electrocatalysts for oxygen evolution reaction (OER) are of key importance for the realization of electrocatalytic water splitting. To date, VB₂ and its derivatives have not been considered as electrocatalysts for water oxidation. Herein, we developed a series of electrocatalysts with a formal composition of V_1-xCo_xB₂ (x = 0, 0.05, 0.1, and 0.2) and employed them in an oxygen-evolving reaction. The incorporation of Co into the VB₂ structure caused a dramatic transformation in the morphology, resulting in a super low overpotential of 200 mV at 10 mA cm^-2 for V_0.9Co_0.1B₂ and displaying much greater performance compared to the noble-metal catalyst RuO₂ (290 mV). The longevity of the best-performing sample was assessed through the exposure to the current density of 10 mA cm^-2, showing relative durability after 12 h under 1 M KOH conditions. The Faradaic efficiency tests corroborated the initiation of OER at 1.45 V (vs RHE) and suggested a potential region of 1.50-1.55 V (vs RHE) as the practical OER region. The facile electron transfer between metal(s)-metalloid, high specific surface area, and availability of active oxy-hydroxy species on the surface were identified as the major contributors to this superior OER performance.

Iron and chromium co-doped cobalt phosphide porous nanosheets as robust bifunctional electrocatalyst for efficient water splitting

It is still a huge challenge to develop highly efficient and low-cost non-precious metal-based electrocatalysts for overall water splitting in alkaline electrolytes. Herein, Cr and Fe co-doped CoP porous mesh nanosheets (Mesh-CrFe-CoP NSs) were synthesized through hydrolysis reaction, ion exchange etching and subsequent low-temperature phosphating process. The Mesh-CrFe-CoP NSs provides overpotentials at a current density of 10 mA cm^-2under alkaline electrolyte of 103.7 mV and 256.4 mV for HER and OER, respectively. Furthermore, when using Mesh-CrFe-CoP NSs as anode and cathode, the water splitting system could afford a current density of 10 mA cm^-2at 1.55 V, which is better than an electrolytic cell composed of 20% Pt/C and RuO₂. The excellent electrocatalytic performance of Mesh-CrFe-CoP NSs is attributed to the co-doping and porous nanostructure. Specifically, the Cr and Fe co-doped porous CoP nanosheets electrocatalyst not only provided abundant exposure active sites, accelerated the entry of liquid and the diffusion of gas, but also regulated the electronic environment of active sites, and thus enhanced the electrochemical performance. This work proposes a strategy for the rational design of highly efficient and stable non-precious metal co-doped phosphide electrocatalysts in the of electrochemical water splitting.

Unraveling a Graphene Exfoliation Technique Analogy in the Making of Ultrathin Nickel-Iron Oxyhydroxides@Nickel Foam to Promote the OER

One of the major objectives of using the improved Hummers' method was to exfoliate the graphene layers by oxidizing and thereafter reducing them to obtain highly conductive reduced graphene layers, which can be used in the construction of electronic devices or as a part of catalyst composites in energy conversion reactions. Herein, we have employed a similar idea to exfoliate the layered double hydroxide (LDH), which is proposed as a promising material for the oxygen evolution reaction (OER) electrocatalysis. Usually, the efficiency of these materials is largely restricted due to their sheetlike morphology, which is susceptible to stacking. In this work, NiFe-LDH sheets were fabricated on nickel foam in a one-step co-precipitation technique and their ultrathin nanosheets (∼2 nm) are obtained by in situ oxygen-plasma-controlled exfoliation. In addition, the oxygen vacancies in exfoliated sheets were generated by a chemical reduction method that further improved the electronic conductivity and overall electrocatalytic performance of the catalyst. This approach can address the limitations of NiFe-LDH, such as poor conductivity and low stability, making it more efficient for electrocatalysis. It is also observed that the catalyst having 60 s O-plasma exposure after chemical reduction, i.e., NiFe-OOH_OV, outperformed remaining electrocatalysts and exhibited superior OER activity with a low overpotential of 330 mV to achieve a high current density of 50 mA cm^-2. The catalyst also displayed an ECSA-normalized OER overpotential of 288 mV at a current density of 10 mA cm^-2 and exhibited excellent long-term stability (120 h) in an alkaline electrolyte. Remarkably, ultrathin defect-rich catalyst continuously produced O₂, resulting in a high faradaic efficiency of 98.1% for the OER.

Strategies for Developing Transition Metal Phosphides in Electrochemical Water Splitting

Electrochemical water splitting involving hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is a greatly promising technology to generate sustainable and renewable energy resources, which relies on the exploration regarding the design of electrocatalysts with high efficiency, high stability, and low cost. Transition metal phosphides (TMPs), as nonprecious metallic electrocatalysts, have been extensively investigated and proved to be high-efficient electrocatalysts in both HER and OER. In this minireview, a general overview of recent progress in developing high-performance TMP electrocatalysts for electrochemical water splitting has been presented. Design strategies including composition engineering by element doping, hybridization, and tuning the molar ratio, structure engineering by porous structures, nanoarray structures, and amorphous structures, and surface/interface engineering by tuning surface wetting states, facet control, and novel substrate are summarized. Key scientific problems and prospective research directions are also briefly discussed.

Heterostructure of ultrafine FeOOH nanodots supported on CoAl-layered double hydroxide nanosheets as highly efficient electrocatalyst for water oxidation

The supported and dispersed ultrafine active species for electrocatalytic water oxidation are quite promising for the high intrinsic activity. A novel heterostructure of ultrafine FeOOH nanodots with an average size of 2.3 nm supported on CoAl-LDH nanosheets, is constructed by a facile method under ambient conditions. The as-prepared FeOOH@CoAl-LDH shows a strong interfacial interaction upon the formation of heterostructure, and is demonstrated as a highly efficient and stable electrocatalyst that demands 272 mV to attain 50 mA cm^-2 and exhibits a Tafel slope of 40 mV dec^-1. Moreover, density functional theory calculations manifest the coupling of FeOOH with CoAl-LDH can effectively decrease the energy barrier during the water oxidation process by optimizing the adsorption free energy of intermediates in the reaction pathway. The successful development of FeOOH@CoAl-LDH can shed light on the design of novel electrocatalysts that can fully take advantages of small size, heterostructure and synergistic effect.

Advanced Transition Metal-Based OER Electrocatalysts: Current Status, Opportunities, and Challenges

Oxygen evolution reaction (OER) is an important half-reaction involved in many electrochemical applications, such as water splitting and rechargeable metal-air batteries. However, the sluggish kinetics of its four-electron transfer process becomes a bottleneck to the performance enhancement. Thus, rational design of electrocatalysts for OER based on thorough understanding of mechanisms and structure-activity relationship is of vital significance. This review begins with the introduction of OER mechanisms which include conventional adsorbate evolution mechanism and lattice-oxygen-mediated mechanism. The reaction pathways and related intermediates are discussed in detail, and several descriptors which greatly assist in catalyst screen and optimization are summarized. Some important parameters suggested as measurement criteria for OER are also mentioned and discussed. Then, recent developments and breakthroughs in experimental achievements on transition metal-based OER electrocatalysts are reviewed to reveal the novel design principles. Finally, some perspectives and future directions are proposed for further catalytic performance enhancement and deeper understanding of catalyst design. It is believed that iterative improvements based on the understanding of mechanisms and fundamental design principles are essential to realize the applications of efficient transition metal-based OER electrocatalysts for electrochemical energy storage and conversion technologies.

Engineering Microdomains of Oxides in High-Entropy Alloy Electrodes toward Efficient Oxygen Evolution

One important goal of the current electrocatalysis is to develop integrated electrodes from the atomic level design to multilevel structural engineering in simple ways and low prices. Here, a series of oxygen micro-alloyed high-entropy alloys (O-HEAs) is developed via a metallurgy approach. A (CrFeCoNi)₉₇ O₃ bulk O-HEA shows exceptional electrocatalytic performance for the oxygen evolution reaction (OER), reaching an overpotential as low as 196 mV and a Tafel slope of 29 mV dec^-1 , and with stability longer than 120 h in 1 m KOH solution at a current density of 10 mA cm^-2 . It is shown that the enhanced OER performance can be attributed to the formation of island-like Cr₂ O₃ microdomains, the leaching of Cr³⁺ ions, and structural amorphization at the interfaces of the domains. These findings offer a technological-orientated strategy to integrated electrodes.

Oxygen-doped hollow, porous NiCoP nanocages derived from Ni-Co prussian blue analogs for oxygen evolution

An oxygen-doped, hollow, porous NiCoP nanocage (O-NiCoP Cages) electrocatalyst was synthesized derived from Ni-Co Prussian blue analogs. O-NiCoP Cages exhibited an overpotential of 310 mV at 10 mA cm^-2 and a Tafel slope of 84 mV dec^-1, significantly higher than that of undoped NiCoP nanocages, and also better than that of RuO₂ and several reported phosphide electrocatalysts. This work provides a new strategy for the design of highly efficient oxygen evolution reaction (OER) electrocatalysts based on hollow, nanostructured and heteroatom-doped metal phosphides.

Synergistically integrated Co9S8@NiFe-layered double hydroxide core-branch hierarchical architectures as efficient bifunctional electrocatalyst for water splitting

Efficient, low-cost, and robust electrocatalysts development for overall water splitting is highly desirable for renewable energy production yet still remains challenging. In this work, Co₉S₈ nanoneedles arrays are synergistically integrated with NiFe-layered double hydroxide (NiFe-LDH) nanosheets to form Co₉S₈@NiFe-LDH core-branch hierarchical architectures supported on nickel foam (Co₉S₈@NiFe-LDH HAs/NF). The Co₉S₈@NiFe-LDH HAs/NF exhibits high catalytic performances for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) with overpotential of 190 and 145 mV at 10 mA cm^-2, respectively. The density functional theory calculations predict that the synergy between Co₉S₈ and NiFe-LDH contributes to the high catalytic performance by lowering the energy barrier of HER. When used as both anode and cathode electrocatalyst, it can deliver 10 mA cm^-2 at a low cell voltage of 1.585 V with excellent long-term durability. This work opens a new avenue toward the exploration of highly efficient and stable electrocatalyst for overall water splitting.

Efficient fish-scale CeO2/NiFeCo composite material as electrocatalyst for oxygen evolution reaction

An electrochemical catalyst with efficient, stable, inexpensive energy storage for oxygen evolution and hydrogen evolution has raised global concerns on energy, calling for high-performance materials for effective treatments. In this paper, novel amorphous polymetallic doped CeO₂particles were prepared for an electrochemical catalyst via homogeneous phase precipitation at room temperature. Metal ions can be easily embedded into the oxygen vacancies formed by CeO₂, and the the electron transport capacity of the CeO₂/NiFeCo electrocatalyst is improved owing to the increase in active sites. In addition, the amorphous CeO₂/NiFeCo composite material is in a metastable state and will transform into different active states in a reducing or oxidizing environment. Furthermore, the amorphous material drives oxygen evolution reaction (OER) through the lattice oxygen oxidation mechanism (LOM), while LOM can effectively bypass the adsorption of strongly related intermediates in the adsorbate release mechanism, thus promoting OER procedure in a timely manner. As a result, CeO₂/NiFeCo exhibits a lower oxygen evolution overpotential of 260 mV at 10 mA cm^-2current density, which shows a predatorily competitive advantage compared with commercially available RuO₂and the reported catalysts.

CoFeP hierarchical nanoarrays supported on nitrogen-doped carbon nanofiber as efficient electrocatalyst for water splitting

Developing high-efficient bifunctional electrocatalysts is significant for the overall water splitting. Bimetallic phosphides show great potential for the bifunctional hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) catalysts due to the excellent catalytic performance. Herein, the CoFeP two-dimensional nanoarrays successfully grown on nitrogen doped electrospun carbon nanofibers (CoFeP NS@NCNF) through template-directed growth and following phosphorization treatment. Benefiting from the hierarchical nanoarrays structure, synergistic effect of high electrical conductivity carbon nanofiber substrate and bimetallic phosphide, the CoFeP NS@NCNF exhibits efficient bifunctional electrocatalytic activities for OER and HER in 1 M KOH with overpotentials of 268 mV (η₂₀) and 113 mV (η₁₀), respectively. Moreover, the CoFeP NS@NCNF coupled two-electrode system needs a low voltage of 1.59 V at 10 mA cm^-2 for overall water splitting. This work provides a promising way for the preparation of transition metal-based electrocatalysts with hierarchical structure derived from Prussian blue analogues (PBAs) for OER and HER.

Surface Fluorination Engineering of NiFe Prussian Blue Analogue Derivatives for Highly Efficient Oxygen Evolution Reaction

Surface engineering is of importance to reduce the reaction barrier of oxygen evolution reaction (OER). Herein, the NiFe Prussian blue analogue (NiFe-PBA)-F catalyst with a multilevel structure was obtained from NiFe-PBAs via a fluorination strategy, which presents an ultralow OER overpotential of 190 mV at 10 mA cm^-2 in alkaline solution, with a small Tafel slope of 57 mV dec^-1 and excellent stability. Interestingly, surface fluorination engineering could achieve a controllable removal of ligands of the cyan group, contributing to keep the framework structure of NiFe-PBAs. Particularly, NiFe-PBAs-F undergoes a dramatic reconstruction with the dynamic migration of F ions, which creates more active sites of F-doped NiFeOOH and affords more favorable adsorption of oxygen intermediates. Density functional theory calculations suggest that F doping increases the state density of Ni 3d orbital around the Fermi level, thus improving the conductivity of NiFeOOH. Furthermore, based on our experimental results, the lattice oxygen oxidation mechanism for NiFe-PBAs-F was proposed. Our work not only provides a new pathway to realize the controllable pyrolysis of NiFe-PBAs but also gives more insights into the reconstruction and the mechanism for the OER process.

Synergistically coupling of Fe-doped CoP nanocubes with CoP nanosheet arrays towards enhanced and robust oxygen evolution electrocatalysis

The rational design of high-performance and low-cost oxygen evolution reaction (OER) electrocatalysts for water splitting is of vital importance for development of renewable hydrogen energy. Herein, we demonstrate an interfacial engineering strategy to prepare Fe-doped CoP nanocubes/CoP nanosheet arrays heterostructure supported on carbon cloth (denoted as CoFeP/CoP/CC). The resultant CoFeP/CoP/CC heterostructure catalyst possesses abundant heterogeneous interfaces, which enables the exposure of reaction active sites and possibly modulation of electronic structure of the catalyst. Furthermore, this strong interfacial coupling of CoFeP and CoP as well as the integration structure on the carbon cloth guarantee high electronic conductivity and enhanced mechanical stability. Benefiting from these advantages, the CoFeP/CoP/CC-heterostructure exhibits high electrocatalytic OER performance with a low overpotential of 240 mV for reaching a current density of 10 mA cm^-2, which outperforms the commercial noble metal RuO₂ (255 mV) and many reported TMPs-based electrocatalysts. Moreover, this CoFeP/CoP/CC catalyst shows a remarkable OER catalytic stability over 100 h. This work provides an effective avenue for the design of the high-performance OER catalyst by interfacial engineering strategy.

Construction of self-supporting, hierarchically structured caterpillar-like NiCo2S4 arrays as an efficient trifunctional electrocatalyst for water and urea electrolysis

In this study, we have developed intriguing self-supporting caterpillar-like spinel NiCo2S4 arrays with a hierarchical structure of nanowires on a nanosheet skeleton, which can be used as a self-supporting trifunctional electrocatalyst for the oxygen evolution reaction (OER), hydrogen evolution reaction (HER) and urea oxidation reaction (UOR). The caterpillar-like NiCo precursor arrays are first in situ grown on carbon cloth (NiCo2O4/CC) by a facile hydrothermal reaction, which is followed by an anion exchange process (or sulfuration treatment) with Na2S to form self-supporting spinel NiCo2S4 arrays (NiCo2S4/CC) with a roughened nanostructure. Taking advantage of the bimetallic synergistic effect, the unique hierarchical nanostructure, and the self-supporting nature, the resultant NiCo2S4/CC electrode exhibits high activities toward the OER, HER and UOR, which are highly superior to the monometallic counterparts of NiS nanosheets and Co9S8 nanowires on a carbon cloth substrate. The comparison of the three electrodes also indicates that the hierarchically structured bimetallic electrode combines the morphological and structural characteristics of monometallic Ni-based nanosheets and Co-based nanowires. When assembling a two-electrode electrolytic cell with NiCo2S4/CC as both the anode and cathode, an applied cell voltage of only 1.66 V is required to deliver a current density of 10 mA cm-2 in water electrolysis. By using the same two-electrode setup, the applied voltage for urea electrolysis is further reduced to 1.45 V that produces hydrogen at the cathode with the same current density. This study paves the way for exploring the feasibility of future less energy-intensive and large-scale hydrogen production.

One-step synthesis of amorphous nickel iron phosphide hierarchical nanostructures for water electrolysis with superb stability at high current density

The development of noble-metal-free high-performance bifunctional catalysts for both the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is essential but challenging for hydrogen production from water electrolysis. Herein, amorphous bimetallic nickel-iron phosphide hierarchical nanostructures enrooted on nickel-iron alloy foam (NiFeP/NFF) are facilely fabricated via direct phosphidation of NFF at low temperature and developed as an efficient self-supporting bifunctional electrocatalyst to catalyze both the OER and HER with high activity, fast kinetics and excellent stability. Moreover, an alkaline water electrolyzer simultaneously utilizing NiFeP/NFF as the cathode and anode only needs a cell voltage of 1.58 V to afford a current density of 10 mA cm-2, overpassing most of the reported bifunctional electrocatalysts and comparable to noble metal-based ones. Impressively, the NiFeP/NFF-based symmetric electrolyzer can work well without appreciable performance degradation at a high current density of 500 mA cm-2 for over 1000 h for continuous hydrogen production with 100% faradaic efficiency, showing superb durability and great promise for industrial application.

Iron and nitrogen Co-doped CoSe2 nanosheet arrays for robust electrocatalytic water oxidation

Fe–N–CoSe₂ electrocatalysts with good OER performance and long-term durability were synthesized using an anion and cation co-doping strategy.

Nanostructured metal phosphides: from controllable synthesis to sustainable catalysis

Metal phosphides (MPs) with unique and desirable physicochemical properties provide promising potential in practical applications, such as the catalysis, gas/humidity sensor, environmental remediation, and energy storage fields, especially for transition metal phosphides (TMPs) and MPs consisting of group IIIA and IVA metal elements. Most studies, however, on the synthesis of MP nanomaterials still face intractable challenges, encompassing the need for a more thorough understanding of the growth mechanism, strategies for large-scale synthesis of targeted high-quality MPs, and practical achievement of functional applications. This review aims at providing a comprehensive update on the controllable synthetic strategies for MPs from various metal sources. Additionally, different passivation strategies for engineering the structural and electronic properties of MP nanostructures are scrutinized. Then, we showcase the implementable applications of MP-based materials in emerging sustainable catalytic fields including electrocatalysis, photocatalysis, mild thermocatalysis, and related hybrid systems. Finally, we offer a rational perspective on future opportunities and remaining challenges for the development of MPs in the materials science and sustainable catalysis fields.

Metal–organic frameworks and their derivatives as electrocatalysts for the oxygen evolution reaction

This review summarizes the recent progress on MOFs and their derivatives used for OER electrocatalysis in terms of their morphology, composition and structure–performance relationship.

Suppressed Jahn-Teller Distortion in MnCo2 O4 @Ni2 P Heterostructures to Promote the Overall Water Splitting

Jahn-Teller distortion in cobalt based spinel electrocatalysts causes poor activity and stability in potentially promising catalysts for water splitting. Here, a novel strategy to resolve this problem by interface engineering is reported, in which, Jahn-Teller distortion in MnCo₂ O₄ is significantly suppressed by in situ growth Ni₂ P nanosheets onto the MnCo₂ O₄ . The significance of interface engineering in suppressing Jahn-Teller distortion of Mn³⁺ is further investigated by X-ray photoelectron spectroscopy, the resulting increased catalytic activity and the effects of suppressed distortion demonstrated by density functional theory calculations. The resulting MnCo₂ O₄ @Ni₂ P heterostructures exhibit superior electrocatalytic activity for the both oxygen evolution reaction and hydrogen evolution reaction with small overpotentials of 240 and 57 mV at 10 mA cm^-2 , respectively. Furthermore, the heterogeneous composite electrode demonstrates a superior current density of 10 mA cm^-2 at a voltage of 1.63 V with excellent durability in a water splitting cell.