Boosting oxygen evolution reaction rates with mesoporous Fe-doped MoCo-phosphide nanosheets.

Transition metal-based catalysts are commonly used for water electrolysis and cost-effective hydrogen fuel production due to their exceptional electrochemical performance, particularly in enhancing the efficiency of the oxygen evolution reaction (OER) at the anode. In this study, a novel approach was developed for the preparation of catalysts with abundant active sites and defects. The MoCoFe-phosphide catalyst nanosheets were synthesized using a simple one-step hydrothermal reaction and chemical vapor deposition-based phosphorization. The resulting MoCoFe-phosphide catalyst nanosheets displayed excellent electrical conductivity and a high number of electrochemically active sites, leading to high electrocatalytic activities and efficient kinetics for the OER. The MoCoFe-phosphide catalyst nanosheets demonstrated remarkable catalytic activity, achieving a low overpotential of only 250 mV to achieve the OER at a current density of 10 mA cm-2. The catalyst also exhibited a low Tafel slope of 43.38 mV dec-1 and maintained high stability for OER in alkaline media, surpassing the performance of most other transition metal-based electrocatalysts. The outstanding OER performance can be attributed to the effects of Mo and Fe, which modulate the electronic properties and structures of CoP. The results showed a surface with abundant defects and active sites with a higher proportion of Co2+ active sites, a larger specific surface area, and improved interfacial charge transfer. X-ray photoelectron spectroscopy (XPS) analysis revealed that the catalyst's high activity originates from the presence of Mo6+/Mo4+ and Co2+/Co3+ redox couples, as well as the formation of active metal (oxy)hydroxide species on its surface.

Surface Reconstruction Facilitated by Fluorine Migration and Bimetallic Center in NiCo Bimetallic Fluoride Toward Oxygen Evolution Reaction.

Oxygen evolution reaction (OER) is a critical anodic reaction of electrochemical water splitting, developing a high-efficiency electrocatalyst is essential. Transition metal-based catalysts are much more cost-effective if comparable activities can be achieved. Among them, fluorides are rarely reported due to their low aqueous stability of coordination and low electric conductivity. Herein, a NiCo bimetallic fluoride with good crystallinity is designed and constructed, and significantly enhanced catalytic activity and conductivity are observed. The inevitable oxidation of transition metal ions at high potential and the dissociation of F- are attributed to the low aqueous stability of coordination. The theoretical researches predicte that transition metal fluorides should have a strong tendency to electrochemical reconstruction. Therefore, based on the observations on their electrochemical behavior, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, and bode plots, it is further demonstrated that surface reconstruction occurred during the electrochemical process, meanwhile a significant increase of electrochemically active area, which is created by F migration, are also directly observed. Additionally, DFT calculation results show that the electronic structure of the catalysts is modulated by the bimetallic centers, and this reconstruction helps optimizing the adsorption energy of oxygen-containing species and improves OER activity.

A Highly-Efficient Boron Interstitially Inserted Ru Anode Catalyst for Anion Exchange Membrane Fuel Cells.

Developing high-performance electrocatalysts for alkaline hydrogen oxidation reaction (HOR) is crucial for the commercialization of anion exchange membrane fuel cells (AEMFCs). Here, boron interstitially inserted ruthenium (B-Ru/C) is synthesized and used as an anode catalyst for AEMFC, achieving a peak power density of 1.37 W cm-2 , close to the state-of-the-art commercial PtRu catalyst. Unexpectedly, instead of the monotonous decline of HOR kinetics with pH as generally believed, an inflection point behavior in the pH-dependent HOR kinetics on B-Ru/C is observed, showing an anomalous behavior that the HOR activity under alkaline electrolyte surpasses acidic electrolyte. Experimental results and density functional theory calculations reveal that the upshifted d-band center of Ru after the intervention of interstitial boron can lead to enhanced adsorption ability of OH and H2 O, which together with the reduced energy barrier of water formation, contributes to the outstanding alkaline HOR performance with a mass activity of 1.716 mA µgPGM -1 , which is 13.4-fold and 5.2-fold higher than that of Ru/C and commercial Pt/C, respectively.

Understanding Alkaline Hydrogen Oxidation Reaction on PdNiRuIrRh High-Entropy-Alloy by Machine Learning Potential.

High-entropy alloy (HEA) catalysts have been widely studied in electrocatalysis. However, identifying atomic structure of HEA with complex atomic arrangement is challenging, which seriously hinders the fundamental understanding of catalytic mechanism. Here, we report a HEA-PdNiRuIrRh catalyst with remarkable mass activity of 3.25 mA µg-1 for alkaline hydrogen oxidation reaction (HOR), which is 8-fold enhancement compared to that of commercial Pt/C. Through machine learning potential-based Monte Carlo simulation, we reveal that the dominant Pd-Pd-Ni/Pd-Pd-Pd bonding environments and Ni/Ru oxophilic sites on HEA surface are beneficial to the optimized adsorption/desorption of *H and enhanced *OH adsorption, contributing to the excellent HOR activity and stability. This work provides significant insights into atomic structure and catalytic mechanism for HEA and offers novel prospects for developing advanced HOR electrocatalysts.

Electronic Modulation of Ru Nanosheet by d-d Orbital Coupling for Enhanced Hydrogen Oxidation Reaction in Alkaline Electrolytes.

The alkaline polymer electrolyte fuel cells (APEFCs) hold great promise for using nonnoble metal-based electrocatalysts toward the cathodic oxygen reduction reaction (ORR), but are hindered by the sluggish anodic hydrogen oxidation reaction (HOR) in alkaline electrolytes. Here, a strategy is reported to promote the alkaline HOR performance of Ru by incorporating 3d-transition metals (V, Fe, Co, and Ni), where the conduction band minimum (CBM) level of Ru can be rationally tailored through strong d-d orbital coupling. As expected, the obtained RuFe nanosheet exhibits outstanding HOR performance with the mass activity of 233.46 A gPGM -1 and 23-fold higher than the Ru catalyst, even threefold higher than the commercial Pt/C. APEFC employing this RuFe as anodic catalyst gives a peak power density of 1.2 W cm-2 , outperforming the documented Pt-free anodic catalyst-based APEFCs. Experimental results and density functional theory calculations suggest the enhanced OH-binding energy and reduced formation energy of water derived from the downshifted CBM level of Ru contribute to the enhanced HOR activity.

A yolk-shell structure construction for metal-organic frameworks toward an enhanced electrochemical water splitting catalysis.

NiFe-based transition metal catalysts are widely used in electrocatalysis, especially in the field of water splitting, due to their excellent electrochemical performance. Herein, a simple method was designed to synthesize a Ni MOF based on nickel foam and it was modified with Fe. After the introduction of Fe, the resulting material exhibits an obvious yolk-shell structure, which greatly increases the specific surface area and facilitates the construction of active sites. At the same time, the synergy between Ni and Fe is conducive to optimizing the electronic structure and effectively improving the poor stability of the MOF. As a result, the synthesized Ni MOF-Fe-2 only needs an overpotential of 229 mV to achieve the OER at a current density of 10 mA cm-2, which is better than most reported transition metal-based electrocatalysts. To our surprise, it showed extraordinary stability under the voltage used for water splitting.

Oxygen-Inserted Top-Surface Layers of Ni for Boosting Alkaline Hydrogen Oxidation Electrocatalysis.

Precisely tailoring the electronic structures of electrocatalysts to achieve an optimum hydroxide binding energy (OHBE) is vital to the alkaline hydrogen oxidation reaction (HOR). As a promising alternative to the Pt-group metals, considerable efforts have been devoted to exploring highly efficient Ni-based catalysts for alkaline HOR. However, their performances still lack practical competitiveness. Herein, based on insights from the molecular orbital theory and the Hammer-Nørskov d-band model, we propose an ingenious surface oxygen insertion strategy to precisely tailor the electronic structures of Ni electrocatalysts, simultaneously increasing the degree of energy-level alignment between the adsorbed hydroxide (*OH) states and surface Ni d-band and decreasing the degree of anti-bonding filling, which leads to an optimal OHBE. Through the pyrolysis procedure mediated by a metal-organic framework at a low temperature under a reducing atmosphere, the obtained oxygen-inserted two atomic-layer Ni shell-modified Ni metal core nanoparticle (Ni@Oi-Ni) exhibits a remarkable alkaline HOR performance with a record mass activity of 85.63 mA mg-1, which is 40-fold higher than that of the freshly synthesized Ni catalyst. Combining CO stripping experiments with ab initio calculations, we further reveal a linear relationship between the OHBE and the content of inserted oxygen, which thus results in a volcano-type correlation between the OH binding strength and alkaline HOR activity. This work indicates that the oxygen insertion into the top-surface layers is an efficient strategy to regulate the coordination environment and electronic structure of Ni catalysts and identifies the dominate role of OH binding strength in alkaline HOR.

An Fe-doped Co-oxide electrocatalyst synthesized through a post-modification method toward advanced water oxidation.

In the context of the ever-increasing energy crisis, electrocatalytic water splitting has attracted widespread attention as an effective means to provide clean energy. However, the oxygen evolution reaction (OER), which is an important anodic half reaction, shows very slow kinetics due to the multi-step electron transfer process, which severely restricts the efficiency of energy conversion. Herein, we used a simple solvothermal method to dope iron into the cobalt-containing hydroxide precursor, and successfully prepared the Fe-doped Co-oxide electrocatalyst Co3-xFexO4-0.01. It only needs an overpotential of 294 mV to perform the OER at a current density of 10 mA cm-2, and has a low Tafel slope of 47.3 mV dec-1. Moreover, Co3-xFexO4-0.01 has excellent stability. There is no significant increase in the overpotential for oxygen evolution at a current density of 10 mA cm-2 after nearly 20 h. BET surface area test and XPS spectroscopy results show that Fe doping provides more mesopores and oxygen bridges, which is conducive to the construction of active sites and electronic regulation during the OER. This work can help design more bimetallic based highly active OER materials.

Inter-regulated d-band centers of the Ni3B/Ni heterostructure for boosting hydrogen electrooxidation in alkaline media.

Ni3B/Ni heterostructures have been constructed, which exhibit exceptional catalytic performance toward the hydrogen oxidation reaction (HOR) under alkaline media, with the mass activity being about 10 times greater than that of Ni3B and Ni, respectively, ranking among the most active platinum-group-metal-free electrocatalysts. Experimental results and theoretical calculations confirm electron transfer from Ni3B to Ni at the Ni3B/Ni interface, resulting in inter-regulated d-band centers of these two components. This inter-regulation gives rise to optimized binding energies of intermediates, which together contribute to enhanced alkaline HOR activity.

Boosting Hydrogen Oxidation Activity of Ni in Alkaline Media through Oxygen-Vacancy-Rich CeO2 /Ni Heterostructures.

The search for highly efficient platinum group metal (PGM)-free electrocatalysts for the hydrogen oxidation reaction (HOR) in alkaline electrolytes remains a great challenge in the development of alkaline exchange membrane fuel cells (AEMFCs). Here we report the synthesis of an oxygen-vacancy-rich CeO2 /Ni heterostructure and its remarkable HOR performance in alkaline media. Experimental results and density functional theory (DFT) calculations indicate the electron transfer between CeO2 and Ni could lead to thermoneutral adsorption free energies of H* (ΔGH* ). This, together with the promoted OH* adsorption strength derived from the abundance of oxygen vacancies in the CeO2 species, contributes to the excellent HOR performance with the exchange current density and mass activity of 0.038 mA cmNi -2 and 12.28 mA mgNi -1 , respectively. This presents a new benchmark for PGM-free alkaline HOR and opens a new avenue toward the rational design of high-performance PGM-free electrocatalysts for alkaline HOR.

Nitrogen Engineering on 3D Dandelion-Flower-Like CoS2 for High-Performance Overall Water Splitting.

Searching for highly efficient and stable bifunctional electrocatalysts toward hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is highly desirable for the practical application of water electrolysis under alkaline electrolyte. Although electrocatalysts based on transition metal sulfides (TMSs) are widely studied as efficient (pre)catalysts toward OER under alkaline media, their HER performances are far less than the state-of-the-art Pt catalyst. Herein, the synthesis of nitrogen doped 3D dandelion-flower-like CoS2 architecture directly grown on Ni foam (N-CoS2 /NF) is reported that possesses outstanding HER activity and durability, with an overpotential of 28 mV to obtain the current density of 10 mA cm-2 , exceeding almost all the documented TMS-based electrocatalysts. Density functional theory calculations and experimental results reveal that the d-band center of CoS2 could be efficiently tailored by N doping, resulting in optimized adsorption free energies of hydrogen (ΔG*H ) and water , which can accelerate the HER process in alkaline electrolyte. Besides, the resulting N-CoS2 /NF also displays excellent performance for OER, making it a high-performance bifunctional electrocatalyst toward overall water splitting, with a cell voltage of 1.50 V to achieve 10 mA cm-2 .

IrW nanobranches as an advanced electrocatalyst for pH-universal overall water splitting.

Developing highly efficient and durable electrocatalysts for overall water splitting over a wide pH range is of great interest for practical applications, but still remains a challenge. Specifically, to the best of our knowledge, a 3-in-1 electrocatalyst that can efficiently catalyze overall water splitting in acidic, alkaline, and neutral electrolytes has not been reported so far. Herein, we report the colloidal synthesis of well-dispersed IrW nanobranches with branch architectures and describe how the morphology varies with the amount of W doping. As expected, they exhibit outstanding catalytic performance and durability for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) at all pH values, which are much higher than those of Ir nanoparticles (NPs), and most reported state-of-the-art electrocatalysts. More importantly, when further used as both an anode and cathode for overall water splitting in 0.1 M HClO4, 0.1 M KOH, and 1.0 M PBS (phosphate buffer solution), cell voltages of 1.58, 1.60, and 1.73 V, respectively, were achieved at a current density of 10 mA cm-2. The present work opens up a new avenue for designing electrocatalysts for pH-universal overall water splitting, especially for application in highly corrosive acidic media and neutral media with limited ionic concentrations.

Self-Sacrificial Template-Directed Vapor-Phase Growth of MOF Assemblies and Surface Vulcanization for Efficient Water Splitting.

Direct use of metal-organic frameworks (MOFs) with robust pore structures, large surface areas, and high density of coordinatively unsaturated metal sites as electrochemical active materials is highly desirable (rather than using as templates and/or precursors for high-temperature calcination), but this is practically hindered by the poor conductivity and low accessibility of active sites in the bulk form. Herein, a universal vapor-phase method is reported to grow well-aligned MOFs on conductive carbon cloth (CC) by using metal hydroxyl fluorides with diverse morphologies as self-sacrificial templates. Specifically, by further partially on-site generating active Co3 S4 species from Co ions in the echinops-like Co-based MOF (EC-MOF) through a controlled vulcanization approach, the resulting Co3 S4 /EC-MOF hybrid exhibits much enhanced electrocatalytic performance toward the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), with overpotentials of 84 and 226 mV required to reach a current density of 10 mA cm-2 , respectively. Density functional theory (DFT) calculations and experimental results reveal that the electron transfer between Co3 S4 species and EC-MOF can decrease the electron density of the Co d-orbital, resulting in more electrocatalytically optimized adsorption properties for Co. This study will open up a new avenue for designing highly ordered MOF-based surface active materials for various electrochemical energy applications.

CoP-Doped MOF-Based Electrocatalyst for pH-Universal Hydrogen Evolution Reaction.

Although electrocatalysts based on transition metal phosphides (TMPs) with cationic/anionic doping have been widely studied for hydrogen evolution reaction (HER), the origin of performance enhancement still remains elusive mainly due to the random dispersion of dopants. Herein, we report a controllable partial phosphorization strategy to generate CoP species within the Co-based metal-organic framework (Co-MOF). Density functional theory calculations and experimental results reveal that the electron transfer from CoP to Co-MOF through N-P/N-Co bonds could lead to the optimized adsorption energy of H2 O (ΔG H 2 O * ) and hydrogen (ΔGH* ), which, together with the unique porous structure of Co-MOF, contributes to the remarkable HER performance with an overpotential of 49 mV at a current density of 10 mA cm-2 in 1 m phosphate buffer solution (PBS, pH 7.0). The excellent catalytic performance exceeds almost all the documented TMP-based and non-noble-metal-based electrocatalysts. In addition, the CoP/Co-MOF hybrid also displays Pt-like performance in 0.5 m H2 SO4 and 1 m KOH, with the overpotentials of 27 and 34 mV, respectively, at a current density of 10 mA cm-2 .

Carbon Encapsulated Hollow Co3O4 Composites Derived from Reduced Graphene Oxide Wrapped Metal-Organic Frameworks with Enhanced Lithium Storage and Water Oxidation Properties.

Transition metal oxides have received great attention for boosting the performances for lithium-ion batteries and oxygen evolution reaction (OER). Here, hollow Co3O4 nanoparticles encapsulated in reduced graphene oxide (rGO) ( h-Co3O4@rGO) were synthesized through a two-step annealing process of graphene oxide wrapped zeolitic imidazolate framework-67 (ZIF-67@GO) precursors. By taking advantage of the enhanced conductivity, high dispersity, high surface area, and unique hollow morphology derived from the GO-wrapped protecting annealing strategy, the as-synthesized h-Co3O4@rGO composite not only exhibits a reversible capacity as high as 1154.2 mAh g-1 at 500 mA g-1 after 100 cycles and high rate performance (746 mAh g-1 at 3000 mA g-1) but also displays superior OER performance with an overpotential of 300 mV to obtain 10 mA cm-2.

IrCo Nanodendrite as an Efficient Bifunctional Electrocatalyst for Overall Water Splitting under Acidic Conditions.

Investigation of high-efficiency electrocatalysts for acidic overall water splitting is of great significance toward fulfillment of proton exchange membrane (PEM) electrolyzers but still remains challenging. Herein, we report the colloidally synthesis of IrCo alloy nanodendrites with petal-like architecture (NDs). Benefiting from unique hierarchical architecture and strong electronic interaction arising from synergistic alloying effect of IrCo at the atomic level, the resultant IrCo0.65 NDs display remarkable hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) performances with overpotentials of 17 and 281 mV to achieve 10 mA cm-2 in 0.1 M HClO4, respectively. Moreover, when further used as bifunctional electrocatalyst toward acidic overall splitting, a low cell voltage of 1.593 V is achieved at 10 mA cm-2.

Colloidal Synthesis of NiWSe Nanosheets for Efficient Electrocatalytic Hydrogen Evolution Reaction in Alkaline Media.

Searching for efficient, low-cost, and durable electrocatalysts toward the hydrogen evolution reaction (HER) is extremely urgent for future energy conversion systems. Herein, the colloidal synthesis of 2D tungsten-doped nickel selenide nanosheets by using Ni(acac)2 (acac=aceylacetonate), [W(CO)6 ], and selenium powder as precursors in oleylamine is reported. The introduction of tungsten is essential for the formation of 2D nanosheets. As a result, by taking the advantage of the unique layered structure and strong synergistic electronic effect between nickel, tungsten, and selenium, the as-synthesized Ni0.54 W0.26 Se nanosheets exhibit superior catalytic activity toward the HER in alkaline media, with an overpotential of 162 mV (η10 ), which is much lower than those of NiSe2 (η10 =330 mV) and WSe2 (η10 =378 mV), and higher than that of most previously reported selenide-based electrocatalysts.

Ultrathin Ir nanowires as high-performance electrocatalysts for efficient water splitting in acidic media.

The search for active and stable bifunctional electrocatalysts toward acidic overall water splitting is under increasing demand for the development of polymer electrolyte membrane (PEM) electrolyzers. However, developing bifunctional electrocatalysts with Pt-like activity and superior stability under acidic media still remains a big challenge. Herein, we report a successful synthesis of Ir wavy nanowires with an ultrathin diameter of 1.7 nm through a simple wet-chemical approach. Benefiting from the unique morphology with high aspect ratios and a large specific surface area, the as-synthesized ultrathin Ir wavy nanowires exhibit enhanced activity and durability for both the oxygen evolution reaction and the hydrogen evolution reaction in acidic electrolytes. Moreover, when used for overall acidic water splitting, a current density of 10 mA cm-2 is achieved at only a cell voltage of 1.62 V in 0.1 M HClO4 electrolyte with long-term stability. In view of the excellent electrochemical water splitting performance and superior stability in acidic electrolytes, we believe that the obtained Ir wavy nanowires could be potential alternative catalysts toward PEM water electrolysis.

Reduced Graphene Oxide-Wrapped Co9-x Fex S8 /Co,Fe-N-C Composite as Bifunctional Electrocatalyst for Oxygen Reduction and Evolution.

Searching for highly efficient bifunctional electrocatalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) using nonnoble metal-based catalysts is essential for the development of many energy conversion systems, including rechargeable fuel cells and metal-air batteries. Here, Co9-x Fex S8 /Co,Fe-N-C hybrids wrapped by reduced graphene oxide (rGO) (abbreviated as S-Co9-x Fex S8 @rGO) are synthesized through a semivulcanization and calcination method using graphene oxide (GO) wrapped bimetallic zeolite imidazolate framework (ZIF) Co,Fe-ZIF (CoFe-ZIF@GO) as precursors. Benefiting from the synergistic effect of OER active CoFeS and ORR active Co,Fe-N-C in a single component, as well as high dispersity and enhanced conductivity derived from rGO coating and Fe-doping, the obtained S-Co9-x Fex S8 @rGO-10 catalyst shows an ultrasmall overpotential of ≈0.29 V at 10 mA cm-2 in OER and a half-wave potential of 0.84 V in ORR, combining a superior oxygen electrode activity of ≈0.68 V in 0.1 m KOH.

Monodisperse Palladium Sulfide as Efficient Electrocatalyst for Oxygen Reduction Reaction.

In this study, we report a colloidal synthesis of palladium sulfides (including Pd16S7, Pd4S, and PdS) via a facile one-pot hot-solution synthetic route and their promising application as electrocatalyst for the oxygen reduction reaction (ORR). Among the different palladium sulfides tested, monodisperse Pd4S nanoparticles exhibit the best electrocatalytic activity toward ORR in alkaline medium, with the half-wave potential ca. 47 mV more positive than that of the state-of-the-art Pt/C catalyst. Density functional theory calculations indicate the existence of oxygen absorption sites in Pd4S surface result in optimized oxygen-binding ability for the four-electron oxygen reduction.