CeOx-Decorated Hierarchical NiCo2S4 Hollow Nanotubes Arrays for Enhanced Oxygen Evolution Reaction Electrocatalysis

Simultaneous Interface Engineering and Phase Tuning of CeO2-Decorated Catalysts for Boosted Oxygen Evolution Reaction

Heterogeneous catalysts have attracted extensive attention among various emerging catalysts for their exceptional oxygen evolution reaction (OER) capabilities, outperforming their single-component counterparts. Nonetheless, the synthesis of heterogeneous materials with predictable, precise, and facile control remains a formidable challenge. Herein, a novel strategy involving the decoration of catalysts with CeO2 is introduced to concurrently engineer heterogeneous interfaces and adjust phase composition, thereby enhancing OER performance. Theoretical calculations suggest that the presence of ceria reduces the free energy barrier for the conversion of nitrides into metals. Supporting this, the experimental findings reveal that the incorporation of rare earth oxides enables the controlled phase transition from nitride into metal, with the proportion adjustable by varying the amount of added rare earth. Thanks to the role of CeO2 decoration in promoting the reaction kinetics and fostering the formation of the genuine active phase, the optimized Ni3FeN/Ni3Fe/CeO2-5% nanoparticles heterostructure catalyst exhibits outstanding OER activity, achieving an overpotential of just 249 mV at 10 mA cm-2. This approach offers fresh perspectives for the conception of highly efficient heterogeneous OER catalysts, contributing a strategic avenue for advanced catalytic design in the field of energy conversion.

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.

Effect of urea and ammonium fluoride ratio on CuCo2S4/NF as a highly efficient HER catalyst

CuCo2S4 as a spinel-structured transition metal sulfide is a highly effective HER catalyst due to its excellent endurance, low overpotential, and low Tafel slope. In this work, the CuCo2S4/Ni foam (NF) catalysts with various morphologies have been successfully synthesized by controlling the ratio of urea and ammonium fluoride (NH4F) based on the hydrothermal method. Urea and NH4F ratio exhibit a great influence on the microstructure and the HER catalytic performance of CuCo2S4/NF catalysts is discussed in detail.

Sea urchin-like amorphous MgNiCo mixed metal hydroxide nanoarrays for efficient overall water splitting under industrial electrolytic conditions

Exploring amorphous mixed transition metal hydroxide electrocatalysts with high performance and stability for overall water splitting is a difficult challenge under industrial electrolytic conditions. Herein, a sea urchin-like amorphous MgNiCo hydroxide (Mg_xNi_1-xCo-OH, 0 < x < 1), self-assembled from nanowire arrays, is synthesized by the hydrothermal process. The synergistic effect between Mg and Ni/Co adjusts their crystal structure and morphology, which can improve the inherent activity and provide more active sites. Benefiting from the favorable structural features, Mg_0.5Ni_0.5Co-OH exhibits superior electrocatalytic oxygen and hydrogen evolution reaction (OER and HER) activity with a low overpotential of 277 and 110 mV (10 mA cm^-2) in 1 M KOH at 25 °C. Furthermore, overpotentials of 239 and 197 mV are required to achieve a current density of 50 mA cm^-2 for the OER and HER under simulated industrial electrolysis conditions (5 M KOH at 65 °C). Notably, Mg_0.5Ni_0.5Co-OH remarkably accelerates water splitting with a low voltage of 1.938 and 1.699 V for 50 mA cm^-2 in 1 M KOH at 25 °C and 5 M KOH at 65 °C, respectively. This work presents a novel amorphous strategy to design and construct sea urchin-like mixed metal hydroxide bifunctional efficient electrocatalysts for industrial applications.

Apparent activity and specific activity of lanthanides (La, Ce, Nd) decorated Co-MOF derivatives for electrocatalytic water splitting

Lanthanide (Ln) rare Earth (RE) elements are often used to incorporate and regulate the local coordination environment and electronic configuration of transition metal based electrocatalysts for acquiring improved electrocatalytic performance. But for a given pristine electrode, is a Ln element concentrated more on promoting the apparent activity of original electrode or on enhancing its specific activity? To address this issue, Ln (La, Ce and Nd) decorated ZIF-67 derivative electrodes (Ln/Co/NC) were fabricated following with the detailed experimental testing of apparent activity and specific activity of assembled electrodes. X-ray photoelectron spectroscopy data confirmed that Ce, Nd and La have played their own role in regulating the coordination electronic structure of the surface atoms of the derived Co/NC by forming different types of chemical bonds. Electrochemical (EC) results confirmed that Ce is concentrated more on the apparent activity of derived Co/NC electrode with the smallest overpotential at 50 mA cm^-2(η₅₀), while Nd contributes more to its reaction kinetic property with the smallest value of Tafel slope in alkaline hydrogen evolution reaction process. But for oxygen evolution reaction, all of La, Ce and Nd deteriorate the apparent activity of the pristine Co/NC electrode. Comparatively, La shows a greater ability to modulate the specific activity of Co/NC with a larger electrochemical active surface area normalized current density, while Nd exhibits the best ability to re-establish the properties of reaction centers. This work illustrates the difference influence of La, Ce and Nd on the apparent activity and specific activity of the ZIF-67 derivative Co/NC electrode. It will do some favors in engineering RE elements modified composite electrodes for EC applications.

Iron-Cobalt-Cerium Multimetallic Oxides Derived from Prussian Blue Precursors: Enhanced Oxygen Evolution Electrocatalysis

Exploring non-precious metal-based electrocatalysts is still challenging in 21^st century. In this work, a series of hexagonal bipyramidal Ce-based PBA materials as precursors with different Fe/Co metal ratios, namely as CeFe_x Co_1-x -PBA, are successfully constructed via co-precipitation method and converted into corresponding metal oxides (denoted as Fe_x Co_1-x CeO_y ) via thermal treatment. Then, they as electrocatalysts realize highly efficient oxygen evolution reaction (OER). Especially, as-synthesized Fe_0.7 Co_0.3 CeO_y electrocatalyst shows very low overpotentials of 320 mV at the current density of 10 mA cm^-2 and the Tafel slop of 98.4 mV dec^-1 in 1 M KOH with remarkable durability for 24 h, which was due to the synergistic effect of multi-metal FeCoCe centers. Furthermore, a two-electrode cell of Fe_0.7 Co_0.3 CeO_y /NF||Pt/C/NF realizes outstanding overall water splitting with a voltage of only 1.71 V at 10 mA cm^-2 and remarkable long-term durability, that is even superior to benchmark IrO₂ /NF||Pt/C/NF counterpart.

Recent development and challenges in fuel cells and water electrolyzer reactions: an overview

Water electrolysis is the most promising method for the production of large scalable hydrogen (H2), which can fulfill the global energy demand of modern society. H2-based fuel cell transportation has been operating with zero greenhouse emission to improve both indoor and outdoor air quality, in addition to the development of economically viable sustainable green energy for widespread electrochemical applications. Many countries have been eagerly focusing on the development of renewable as well as H2-based energy storage infrastructure to fulfill their growing energy demands and sustainable goals. This review article mainly discusses the development of different kinds of fuel cell electrocatalysts, and their application in H2 production through various processes (chemical, refining, and electrochemical). The fuel cell parameters such as redox properties, cost-effectiveness, ecofriendlyness, conductivity, and better electrode stability have also been highlighted. In particular, a detailed discussion has been carried out with sufficient insights into the sustainable development of future green energy economy.

Surface Reconstruction of Co4N Coupled with CeO2 toward Enhanced Alkaline Oxygen Evolution Reaction

Constructing the active interface in a heterojunction electrocatalyst is critical for the electron transfer and intermediate adsorption (O*, OH*, and HOO*) in alkaline oxygen evolution reaction (OER) but still remains challenging. Herein, a CeO₂/Co₄N heterostructure is rationally synthesized through the direct calcination of Ce[Co(CN)₆], followed by thermal nitridation. The in situ electrochemically generated CoOOH on the surface of Co₄N serves as the active site for the OER, and the coupled CeO₂ with oxygen vacancy can optimize the energy barrier of intermediate reactions of the OER, which simultaneously boosts the OER performance. Besides, electrochemical measurement results demonstrate that oxygen vacancies in CeO₂ and optimized absorption free energy originating from the electron transfer between CeO₂ and CoOOH contribute to enhanced OER kinetics. This work provides new insight into regulating the interface heterostructure to rationally design efficient OER electrocatalysts under alkaline conditions.

A facile method to synthesize 3D nanosheets of Fe/S dopedα-Ni(OH)2 as an electrocatalyst for improved oxygen evolution reaction

S-doped Fe/Ni oxide and Fe/Ni hydride oxide catalysts exhibit good oxygen evolution reaction (OER) performance. Nevertheless, the over-doping of S and the agglomeration of active sites still hinder the improvement of the performance of these catalysts. The S/O ratio regulation can optimize the electronic structure effectively so as to improve the OER performance of the catalysts, but few studies have focused on this study. Here, we find a facile room-temperature method to synthesize the unique 3D ultra-thin FeNiOS nanosheets with an adjustable S/O ratio for OER. The FeNiOS-NS catalysts exhibit excellent OER performance with an overpotential of 235 mV at 10 mA cm^-2and a small Tafel slope of 64.2 mV dec^-1in 0.1 M KOH, which originated from the sufficient exposure of the active Fe-Ni component and the optimized electronic structure due to the tunable S/O ratio. This study demonstrates a novel strategy to optimize the OER performance of Ni-based catalysts.

CeO2 decorated bimetallic phosphide nanowire arrays for enhanced oxygen evolution reaction electrocatalysis via interface engineering

To realize electrocatalytic water splitting for hydrogen production, the development of efficient and durable anode materials containing earth-abundant elements is of great significance. In this work, we demonstrate a novel heterostructure with easily depositing CeO₂ nanoparticles on the surface of the metal-organic framework (MOF)-derived Co_0.4Ni_1.6P nanowire arrays. Such an interface engineering strategy triggers the formation of abundant oxygen vacancies and provides more electrocatalytically active sites. Besides, the synergistic effect in this composite can regulate the electronic structure, and lead to an enhanced charge-transfer ability. Benefiting from the above superiorities, this heterostructure exhibits remarkable electrocatalytic performance towards the oxygen evolution reaction (OER) in 1 M KOH electrolyte, requiring overpotentials (η) of 268 and 343 mV to yield current densities of 10 and 100 mA cm^-2, respectively, accompanied by a low Tafel slope of 79.3 mV dec^-1. Furthermore, the electrocatalytic performance of this heterostructure for the OER in simulated alkaline seawater (1 M KOH + 0.5 M NaCl) was also studied, and it achieved low η values of 345 and 394 mV to drive 100 and 200 mA cm^-2, respectively. This work presents a simple approach to fabricate heterostructural electrocatalysts with CeO₂ nanoparticles for high-performance water/seawater electrolysis.

Successive Anion/Cation Exchange Enables the Fabrication of Hollow CuCo2S4 Nanorods for Advanced Oxygen Evolution Reaction Electrocatalysis

Hollow CuCo₂S₄ nanorods (H-CCS-Ns) have been successfully developed via a facile successive anion/cation-exchange method. The outstanding electrocatalytic performance of H-CCS-Ns is mainly attributed to its distinctive hollow structure, which accelerates the electron transfer rate and provides abundant active sites. Moreover, a mechanism study indicates that H-CCS-Ns has highly active octahedral Co³⁺, and the existence of Co³⁺ cations optimizes the adsorption of oxygen-involved intermediates, making H-CCS-Ns a promising OER electrocatalyst. Optimized H-CCS-Ns only need an ultralow overpotential of 220 mV to drive a current density of 10 mA·cm^-2 and exhibit distinguished cycling stability with a negligible fluctuation for 30 h. More impressively, when H-CCS-Ns are assembled with Pt/C for overall water splitting, a voltage as low as 1.545 V is required at a current density of 10 mA·cm^-2, and the catalyst shows outstanding stability for as long as 38 h. This study offers a feasible strategy to design hollow spinel catalysts for efficient OER catalysis.

Recent Advances in Seawater Electrolysis

Hydrogen energy, as a clean and renewable energy, has attracted much attention in recent years. Water electrolysis via the hydrogen evolution reaction at the cathode coupled with the oxygen evolution reaction at the anode is a promising method to produce hydrogen. Given the shortage of freshwater resources on the planet, the direct use of seawater as an electrolyte for hydrogen production has become a hot research topic. Direct use of seawater as the electrolyte for water electrolysis can reduce the cost of hydrogen production due to the great abundance and wide availability. In recent years, various high-efficiency electrocatalysts have made great progress in seawater splitting and have shown great potential. This review introduces the mechanisms and challenges of seawater splitting and summarizes the recent progress of various electrocatalysts used for hydrogen and oxygen evolution reaction in seawater electrolysis in recent years. Finally, the challenges and future opportunities of seawater electrolysis for hydrogen and oxygen production are presented.

The synergistically enhanced activity and stability of layered manganese oxide via engineering of defects and K+ ions for oxygen electrocatalysis

Structural defects and interlayered ions are two classic architectures that regulate the electrochemical activity of layered manganese oxides. However, the synergistic effect of defects and interlayered ions and how it...

Bifunctional Catalyst Derived from Sulfur-Doped VMoOx Nanolayer Shelled Co Nanosheets for Efficient Water Splitting

A novel sulfur-doped vanadium-molybdenum oxide nanolayer shelling over two-dimensional cobalt nanosheets (2D Co@S-VMoO_x NSs) was synthesized via a facile approach. The formation of such a unique 2D core@shell structure together with unusual sulfur doping effect increased the electrochemically active surface area and provided excellent electric conductivity, thereby boosting the activities for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). As a result, only low overpotentials of 73 and 274 mV were required to achieve a current response of 10 mA cm^-2 toward HER and OER, respectively. Using the 2D Co@S-VMoO_x NSs on nickel foam as both cathode and anode electrode, the fabricated electrolyzer showed superior performance with a small cell voltage of 1.55 V at 10 mA cm^-2 and excellent stability. These results suggested that the 2D Co@S-VMoO_x NSs material might be a potential bifunctional catalyst for green hydrogen production via electrochemical water splitting.

Promoting urea oxidation and water oxidation through interface construction on a CeO2@CoFe2O4 heterostructure

Spinel ferrites are considered practical and promising oxygen evolution reaction (OER) and urea oxidation reaction (UOR) electrocatalysts because of their advantages in the adsorption and activation of electrocatalytic substances. A CeO₂ functional metal oxide was used to modify a spinel oxide in order to further improve the electrocatalytic performance of the spinel oxide. In this work, a CeO₂@CoFe₂O₄/NF hybrid nanostructure was synthesized for the first time by typical hydrothermal and calcination methods. In an alkaline medium, CeO₂@CoFe₂O₄/NF displays superior OER activity and needs an overpotential of 213 mV to deliver a current density of 100 mA cm^-2, which makes it one of the most active catalysts reported so far. In addition, the as-prepared CeO₂@CoFe₂O₄/NF material needs a potential of 1.40 V at the same current density in 1.0 M KOH with 0.5 M urea, which displays superior UOR activity. The CeO₂@CoFe₂O₄/NF catalyst also displays good durability and the performance of the electrode is negligibly attenuated at a large current intensity of 125 mA cm^-2. Experimental results demonstrate that the activity of the CeO₂@CoFe₂O₄/NF catalyst is ascribed to the exposure of more active centers and a faster electron transfer rate. This work develops a novel method for exploiting Earth-abundant, robust and environmentally friendly OER and UOR electrocatalysts.

Plasma-Induced Defect Engineering and Cation Refilling of NiMoO4 Parallel Arrays for Overall Water Splitting

Developing highly active water splitting electrocatalysts with ordered micro/nanostructures and uniformly distributed active sites can meet the increasing requirement for sustainable energy storage/utilization technologies. However, the stability of complicated structures and active sites during a long-term process is also a challenge. Herein, we fabricate a novel approach to create sufficient atomic defects via N₂ plasma treatment onto parallel aligned NiMoO₄ nanosheets, followed by refilling of these defects via heterocation dopants and stabilizing them by annealing. The parallel aligned nanosheet arrays with an open structure and quasi-two-dimensional long-range diffusion channels can accelerate the mass transfer at the electrolyte/gas interface, while the incorporation of Fe/Pt atoms into defect sites can modulate the local electronic environment and facilitate the adsorption/reaction kinetics. The optimized Pt-NP-NMC/CC-5 and Fe-NP-NMC/CC-10 electrodes exhibit low overpotentials of 71 and 241 mV at 10 mA cm^-2 for the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER), respectively, and the assembled device reveals a low voltage of 1.55 V for overall water splitting. This plasma-induced high-efficiency defect engineering and coupled active site stabilization strategy can be extended to large-scale fabrication of high-end electrocatalysts.

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.

Donor-Acceptor Couples of Metal and Metal Oxides with Enriched Ni3+ Active Sites for Oxygen Evolution

Exploiting precious-metal-free and high-activity oxygen evolution reaction (OER) electrocatalysts has been in great demands toward many energy storage and conversion processes, for example, carbon dioxide reduction, metal-air batteries, and water splitting. In this study, the simple solid-state method is employed for coupling Ni (electron donors) with lower-Fermi-level MoO₂ or WO_x (electron acceptors) into donor-acceptor ensembles with well-designed interfaces as robust electrocatalysts for OER. The resulting Ni/MoO₂ and Ni/WO_x electrocatalysts exhibit smaller overpotentials of 287 and 333 mV at 10 mA cm^-2 as well as smaller Tafel slopes of 51 and 65 mV/dec, respectively, with respect to the single Ni, MoO₂, WO_x, and even the benchmark RuO₂ in 1 M KOH. Specially, on account of a higher Fermi level of Ni in comparison with MoO₂ and WO_x, their strong electronic interaction results in directional interfacial electron transfer and increases the hole density over Ni, dramatically enriching the population of high-valence Ni³⁺ active sites and decreasing the Fermi level of Ni. The existence of Ni³⁺ can strengthen the chemisorption of OH^-, and the downshift of the Ni Fermi level can significantly expedite migration of electrons toward the surface of catalysts during OER, thus synergistically boosting the OER catalytic performance. Furthermore, the inner Ni/MoO₂ and Ni/WO_x heterostructures and the electrochemically induced surface layers of oxides/hydroxides collectively boost the OER kinetics. This study highlights the importance of designing highly efficient OER electrocatalysts with high-valence active species (Ni³⁺) and better matched energy levels induced by the work function difference through interfacial engineering.

Co/Sm-modified Ti/PbO2 anode for atrazine degradation: Effective electrocatalytic performance and degradation mechanism

In this work, Ti/PbO₂-Co-Sm electrode has been successfully prepared using electrodeposition and further applied for the electrocatalysis of atrazine (ATZ) herbicide wastewater. As expected, Ti/PbO₂-Co-Sm electrode displays highest oxygen evolution potential, lowest charge transfer resistance, longest service lifetime and most effective electrocatalytic activity compared with Ti/PbO₂, Ti/PbO₂-Sm and Ti/PbO₂-Co electrodes. Orthogonal and single factor experiments are designed to optimize the condition of ATZ degradation. The maximum degradation efficiency of 92.6% and COD removal efficiency of 84.5% are achieved in electrolysis time 3 h under the optimum condition (current density 20 mA cm^-2, Na₂SO₄ concentration 8.0 g L^-1, pH 5 and temperature 35 °C). In addition, Ti/PbO₂-Co-Sm electrode exhibits admirable recyclability in degradation progress. The degradation of ATZ is accomplished by indirect electrochemical oxidation and ∙OH is tested as the main active substance in ATZ oxidation. The possible degradation mechanism of ATZ has been proposed according to the degradation intermediates detected by LC-MS. This research suggests that Ti/PbO₂-Co-Sm is a promising electrode for ATZ degradation.

(Ni,Co)Se@Ni(OH)2 heterojunction nanosheets as an efficient electrocatalyst for the hydrogen evolution reaction

A heterogeneous structure formed by coupling two or more phases can reinforce the activity of active sites and expedite electron transfer, which is conducive to boosting its electrocatalytic activity. Herein, we designed nickel foam supported (NiCo₂)Se@Ni(OH)₂ (NCS@NH) heterojunction nanosheets by a two-step method. First of all, the NiCo₂S₄@Ni(OH)₂ (NiCo₂S₄@NH) nanosheets coated on nickel foam were acquired via a hydrothermal method. In the selenization treatment that followed, NiCo₂S₄@NH was converted into NCS@NH heterogeneous nanosheets in which the selenide nanoparticles decorated on the surface of the Ni(OH)₂ nanosheets formed heterojunction interfaces, and the heterogeneous structure could accelerate electron transfer, thus improving the catalytic activity. The Ni(OH)₂ nanosheets can adequately contact the electrolyte and promote the decomposition of water. Meanwhile, the thickness of the Ni(OH)₂ nanosheets gradually decreases with the increase of Co doping (1.5-2.5 mmol), consequently affecting the HER properties. Notably, when the amount of Co salt added is 2 mmol, NCS@NH exhibited superior HER properties (with a voltage of 253 mV at 100 mA cm^-2) and excellent stability for 24 h.

Novel synthesis of in situ CeOx nanoparticles decorated on CoP nanosheets for highly efficient electrocatalytic oxygen evolution

CoP nanosheets decorated by in-situ CeOx nanoparticles were designed through a novel two-step solvothermal-phosphating strategy, which act as an electrode exhibit excellent electrocatalytic performance toward OER in alkaline condition.

Fe-Doped Co-Mo-S microtube: a highly efficient bifunctional electrocatalyst for overall water splitting in alkaline solution

Fe-Doped Co-Mo-S microtubes were successfully synthesized through a multistep synthetic route, employing MoO3 microrods as the sacrificial template, Co(NO3)2·6H2O and Fe(SO4)2·7H2O as the metal sources, 2-methylimidazole (2-MI) as the ligand and thioacetamide (TAA) as the S2- ion source. The as-prepared products were characterized by X-ray powder diffraction (XRD), energy dispersive spectrometry (EDS), inductively coupled plasma mass spectrometry (ICP-MS), X-ray photoelectron spectroscopy (XPS), (high-resolution) transmission electron microscopy (TEM/HRTEM) and HAADF-STEM-EDS elemental mapping. Experiments showed that the as-obtained Fe-doped Co-Mo-S microtube catalyst demanded overpotentials of ∼105 and 268 mV to afford the current density of -10 mA cm-2 for hydrogen evolution reaction (HER) and 10 mA cm-2 for oxygen evolution reaction (OER) with a durability of 60 h in 1.0 M KOH solution, respectively. In a two-electrode water-splitting device, the as-prepared Fe-doped Co-Mo-S microtubes acted as both anode and cathode simultaneously. To deliver a current density of 10 mA cm-2, a cell voltage of 1.605 V was required in 1.0 M KOH solution. After continuously catalyzing the overall water splitting for 60 h, the overpotential hardly changed, implying remarkable long-term stability. Obviously, the present Fe-doped Co-Mo-S microtubes have potential applications as bifunctional catalysts for electrochemical water splitting.

Iron-doped NiCo-MOF hollow nanospheres for enhanced electrocatalytic oxygen evolution

The development of metal-organic frameworks (MOFs) as high-efficiency electrocatalysts for water splitting has attracted special attention due to their unique structural features including high porosity, large surface areas, high concentrations of active sites, uniform pore sizes and shapes, etc. Most of the related reports focus on the in situ generation of high-efficiency electrocatalysts by annealed MOFs. However, the pyrolysis process usually destroys the porous structure of MOFs and reduces the number of active sites due to the absence of organic ligands and agglomeration of metal centers. In this work, we prepared unique NiCo-MOF hollow nanospheres (NiCo-MOF HNSs) by a solvothermal method and further fabricated Fe-doped NiCo-MOF HNSs (Fe@NiCo-MOF HNSs) by a simple impregnation-drying method. Significant enhancement of electrocatalytic activity of Fe@NiCo-MOF HNSs was witnessed because of the doped Fe. Compared with the parent NiCo-MOF HNSs, the optimized Fe@NiCo-MOF HNSs exhibited a lower overpotential of 244 mV at 10 mA·cm^-2 with a smaller Tafel slope of 48.61 mV·dec^-1, which was lowered by ca. 90 mV due to the influence of Fe doping on the electronic structure of the active centers of Ni and Co. The above materials also displayed excellent stability without obvious activity decay for at least 16 hours. These findings present a new entry in the design and fabrication of high-efficiency MOF-based electrocatalysts for energy conversion.

Constructing a CeO2−x@CoFe-layered double hydroxide heterostructure as an improved electrocatalyst for highly efficient water oxidation

CeO_2−x@CoFe LDH/NF acts as a high-efficiency OER electrocatalyst and used in 1.0 M KOH and in simulated alkaline seawater (1.0 M KOH + 0.5 M NaCl).