Atomically dispersed silver atoms incorporated in spinel cobalt oxide (Co3O4) for boosting oxygen evolution reaction.

Incorporating noble metal single atoms into lattice of spinel cobalt oxide (Co3O4) is an attractive way to fabricate oxygen evolution reaction (OER) electrocatalysts because of the high activity and economic benefit. The commonly used high valence noble metal dopants such as ruthenium, iridium and rhodium tend to supersede Co3+ at octahedral site of Co3O4 and result in great activity, the origins of admirable activity were also wildly investigated. However, bare explorations on doping noble metal single atom into tetrahedral site of Co3O4 to construct OER catalyst have been reported, corresponding catalytic activity and mechanism remain mystery. Here, a promising structure that tetrahedrally substituent Ag single atom embedded in Co3O4 nanoparticles on the surface of carbon nanotube (Ag-Co3O4/CNT) was presented, and its performance in OER was probed. The high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) and X-ray absorption spectroscopy (XAS) demonstrate the successful embeddedness of atomical Ag atom in Co3O4 lattice, the resultant electronic interaction is conducive to promote charge transfer for OER. Theoretical calculations further disclose that atomical Ag dopant prefers to replace tetrahedral Co2+ rather than octahedral Co3+. The substitution Ag acts as the active site through Ag-Co bridge and facilitates the desorption process, which improves the turnover frequency (TOF) and boosts the intrinsic activity of Ag-Co3O4/CNT. Benefiting from the essentials above, Ag-Co3O4/CNT displays remarkable activity (236 mV@10 mA cm-2) and robust stability for alkaline OER. This finding offers a potential direction for the design of noble metal single atom involved Co3O4 based OER electrocatalysts.

A hybrid of MIL-53(Fe) rhombus and conductive CoNi2S4 nanosheets as a synergistic electrocatalyst for the oxygen evolution reaction.

Hydrogen energy is considered to be a zero-carbon chemical energy alternative to traditional fossil energy, and electrolysis of water, as one of the most effective methods of producing hydrogen, can produce high-purity hydrogen under the premise of zero pollution. The oxygen evolution reaction (OER) is a slow and energy-intensive four-electron process that limits the rate of decomposition of electrolyzed water and is considered as the bottleneck for overall water splitting. In this paper, CoNi2S4 nanosheets were assembled on blank nickel foam with a conventional two-step hydrothermal method, which then was continued with a hydrothermal method to load the diamond-block structure of MIL-53(Fe) on top of CoNi2S4 nanosheets, denoted as MIL-53(Fe)@CoNi2S4/NF. The MIL-53(Fe)@CoNi2S4/NF catalyst exhibited excellent electrochemical performance in 1 M KOH aqueous solution, which required an overpotential of only 201 mV when the current density reached 20 mA cm-2. In addition, after long-term stability testing, the MIL-53(Fe)@CoNi2S4/NF catalyst maintained its favourable OER activity due to the lattice structure of the rhombic blocks which enhanced both the stability of the catalyst structure and the internal ion transport channels.

In situ generated Cu-Co-Zn trimetallic sulfide nanoflowers on copper foam: a highly efficient OER electrocatalyst.

The electrocatalytic oxygen evolution reaction (OER) is an integral part and a stepping stone to various electrochemical technologies in the field of electrochemical energy conversion. The development of OER catalysts with low-cost materials, industry-related activity and long-term durability is highly needed, but remains challenging at this stage. In this paper, Cu ions in a copper foam (CF) substrate were replaced with Cu(OH)2 grown on CF to participate in the subsequent reaction, and then a subsequent two-step hydrothermal method was used to obtain the nanoflower-like Cu-Co-Zn trimetallic sulfide (named CuCoZn-S-3) catalyst, whose unique flower structure ensures that the catalyst surface exhibits a larger electrochemical active area, so as to expose plentiful active sites. The synergism between metals regulates the electron environment and accelerates the charge transfer rate, greatly improving the electrocatalytic activity of the catalyst. The prepared CuCoZn-S-3 exhibits excellent OER performance under alkaline conditions. It requires overpotentials of only 175 mV and 242 mV to drive current densities of 10 mA cm-2 and 100 mA cm-2, respectively. The Tafel slope of CuCoZn-S-3 is 62.3 mV dec-1. This study may provide a viable strategy for the rational preparation of low-cost and efficient OER electrocatalysts in alkaline medium.

Spontaneous synthesis of silver nanoparticles on cobalt-molybdenum layer double hydroxide nanocages for improved oxygen evolution reaction.

Modulating electronic resistance properties and enhancing both active site populations and per-site activity are highly desirable for the application of layered double hydroxides (LDHs) in the electrocatalytic oxygen evolution reaction (OER). Herein, a metal-support structure consisting of silver (Ag) nanoparticles supported by MoO42- intercalated Co-LDH (CoMo-LDH) nanocages (Ag@CoMo-LDH) was developed using a sacrificial template method and a subsequent spontaneous strategy. The resultant hybrid was shown to be a highly efficient OER electrocatalyst in alkaline media. The required overpotential of Ag@CoMo-LDH for affording a geometric current density of 10 mA cm-2 is as low as 205 mV, which is not only significantly lower than that of separate CoMo-LDH or Ag nanoparticles but also superior to that of most developed OER electrocatalysts reported recently. The constituents and respective work mechanism of Ag@CoMo-LDH are discussed in detail. The superior performance of Ag@CoMo-LDH is related to the unique construction and the effective and stable heterointerfaces between Ag nanoparticles and CoMo-LDH, which accelerate the electron and mass transfer, provide a large number of new active sites and optimize the activity of the original sites. Impressively, Ag@CoMo-LDH also exhibited promising practical prospect on account of the remarkable cyclic and long-term stability. This finding demonstrates that pointedly integrating multiple strategies into one system is a promising way to construct new LDH-based OER electrocatalysts with synthetically improved performance, providing a promising model for developing advanced electrocatalysts in energy conversion devices.

Ir-Doped Co(OH)2 nanosheets as an efficient electrocatalyst for the oxygen evolution reaction.

In recent years, Co-based metal-organic frameworks (Co-MOFs) have received significant research interest because of their large specific surface area, high porosity, tunable structure and topological flexibility. However, their comparatively weak electrical conductivity and inferior stability drastically restrict the application of Co-MOFs in the synthesis of electrocatalysts. In this study, ZIF-67 was grown on nickel foam by a room temperature soaking method, and then Ir-Co(OH)2@ZIF-67/NF was assembled by a hydrothermal method. The prepared Ir-Co(OH)2@ZIF-67/NF nanosheets exhibit remarkable conductivity, larger electrochemical active surface area and wider electron transport channels. Only ultralow overpotentials of 198 mV, 263 mV, and 300 mV were needed for Ir-Co(OH)2@ZIF-67/NF to reach the current densities of 10 mA cm-2, 50 mA cm-2, 100 mA cm-2, meanwhile, no obvious degradation of the current density at 10 mA cm-2 was observed for about 16 h. This work may provide a promising strategy for developing high-performance MOF-derived materials as electrocatalysts for the OER under alkaline conditions.

Spherical V-doped nickel-iron LDH decorated on Ni3S2 as a high-efficiency electrocatalyst for the oxygen evolution reaction.

Due to the slow reaction kinetics of the oxygen evolution reaction (OER), the electrolysis rate of water is greatly limited. Therefore, it is of great significance to study stable and efficient non-noble metal based electrocatalysts. In this paper, three-dimensional (3D) spherical V-NiFe LDH@Ni3S2 was developed by exquisitely decorating ultra-thin V-doped NiFe layered dihydroxide (NiFe-LDH) on Ni3S2 nanosheets supported by nickel foam (NF). It is worth mentioning that V-NiFe LDH@Ni3S2 exhibits an excellent electrocatalytic performance and only 178 mV overpotential is required in 1 M KOH to achieve a current density of 10 mA cm-2. Long-term chronoamperometry manifests its superior electrochemical stability. The combination of NiFe LDH and conductive substrate coupling can drastically afford abundant active sites and accelerate charge transfer, and V doping can markedly regulate the electronic structure. Therefore, the activity and durability of the electrocatalysts are greatly improved. This study may provide a new strategy for the preparation of efficient OER electrocatalysts.

Selenide-based 3D folded polymetallic nanosheets for a highly efficient oxygen evolution reaction.

Electrochemical water splitting, which is considered to be one of the fruitful strategies to achieve efficient and pollution-free hydrogen production, has been deemed as a key technology to achieve renewable energy conversion. Oxygen evolution reaction (OER) is a decisive step in water splitting. Slow kinetics seriously limits the effective utilization of energy thus it is extremely urgent to develop electrocatalysts that can effectively reduce the reaction energy barrier thus accelerate OER kinetics. Here, Mn-Co0.85Se/NiSe2/NF nanosheets with 3D folded structure was assembled on Ni foam by electrodeposition and vapor-deposition method. Mn-Co0.85Se/NiSe2/NF can achieve a current density of 10 mA cm-2 with only 175 mV overpotential in an alkaline environment of 1 M KOH, which is much lower than other reported catalysts. In addition, catalyst Mn-Co0.85Se/NiSe2/NF also performed well in long-term stability tests. Through the synergy of polymetallic, the improvement of catalyst surface energy together with the tuning of electronic structure and the optimization of conductivity can be realized. This work may provide a feasible strategy for the design of efficient selenide-based oxygen evolution reaction catalysts.

Ru doped molybdenum-based nanowire arrays for efficient hydrogen evolution over a broad pH range.

Searching and developing earth-abundant electrocatalysts with predominant activity and favorable stability are significant to resolve increasing environmental pollution and serious energy crisis. In this paper, Mo-based nanowire arrays (NWAs) were synthesized on carbon fiber paper (CFP), and then Ru-MoP NWAs/CFP was successfully assembled through impregnation of Ru and phosphorization. The as-obtained Ru-MoP NWAs/CFP exhibited excellent performance over a broad pH range. As a result, overpotentials of 39.0, 49.9, and 67.1 mV were required to reach the current density of 10 mA cm-2 in 0.5 M H2SO4, 1 M KOH, and 1 M PBS, respectively. The outstanding performance of Ru-MoP NWAs/CFP is mainly related to the introduction of Ru and P atoms, which may enhance the conductivity of the catalyst and develop additional HER active sites. Our work may afford a novel designing approach to develop high performance HER electrocatalysts.

Ru doping induces the construction of a unique core-shell microflower self-supporting electrocatalyst for highly efficient overall water splitting.

Since the large reaction energy barrier caused by multi-step electron transfer processes of the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) gravely restricts the practical application of electrocatalytic water splitting, it is urgent to develop a dual-functional electrocatalyst which can effectively reduce the reaction energy barrier and actually speed up the reaction. Herein, the Ru species are doped into the complex of magnetite and FeNi-layered double hydroxide by a one-step oil bath method, and a self-supporting binder-free bifunctional electrocatalyst was synthesized on the surface of iron foam (named Ru-Fe3O4@FeNi-LDH/IF). The unique 3D core-shell microflower structure of Ru-Fe3O4@FeNi-LDH/IF, the combination of active ingredient and conductive substrate, together with the doping of Ru may immensely provide a large number of active sites, adjust the electronic structure, accelerate electron transfer, and thus greatly improve the electrocatalytic activity and durability. It is worth mentioning that when Ru-Fe3O4@FeNi-LDH/IF is used as the anode and cathode for overall water splitting, only 1.52 V battery voltage can generate a current density of 10 mA cm-2, and also maintain a prominent stability for at least 36 hours. This work provides a feasible strategy for heteroatom-doping LDH as a bifunctional electrocatalyst.

An ingeniously assembled metal-organic framework on the surface of FeMn co-doped Ni(OH)2 as a high-efficiency electrocatalyst for the oxygen evolution reaction.

To overcome the problem of the sluggish kinetics of the oxygen evolution reaction (OER), it is of great significance to develop an efficient and stable non-noble metal-based OER catalyst for electrocatalytic energy conversion and storage. Herein, a complex of a metal-organic framework and hydroxide is synthesized by performing a ligand etching strategy on FeMn co-doped Ni(OH)2 nanosheets in situ grown on nickel foam (FeMn-Ni(OH)2@MOF/NF). Benefiting from the unique sheet-on-sheet hierarchical structure, multi-metal active nodes and two active materials grown in situ, the resulting FeMn-Ni(OH)2@MOF/NF demonstrated brilliant OER activity with an overpotential of 199 mV to achieve a current density of 10 mA cm-2 and long-term stability. This research will provide a new strategy for the design of high-performance OER electrocatalysts.

Assembly of ZIF-67 nanoparticles and in situ grown Cu(OH)2 nanowires serves as an effective electrocatalyst for oxygen evolution.

Due to the slow kinetics of oxygen evolution at the anode, the efficiency of electrocatalytic water decomposition is critically reduced, and its large-scale application is severely restricted. Therefore, it is urgent to develop electrocatalysts with excellent performance and stability to accelerate the oxygen evolution reaction (OER) reaction kinetics. Herein, a self-supporting binder-free electrocatalyst was successfully prepared using in situ grown Cu(OH)2 nanowires on CF as the carrier to grow ZIF-67 via a room temperature immersion method. The combination of Cu(OH)2 nanowires and the unique structure of ZIF-67 forms a three-dimensional nanostructured catalyst, in which the unique structure and the existence of synergy may contribute to a larger electrochemical active surface area, expose more electrochemically active sites, adjust the electronic structure, and accelerate the rate of electron transfer, thus greatly improving the electrocatalytic activity and durability for OER. The as-prepared Cu(OH)2@ZIF-67/CF exhibited excellent OER performance under alkaline conditions and required overpotentials of 205 mV and 276 mV to drive current densities of 10 mA cm-2 and 100 mA cm-2, respectively, with a small Tafel slope of 70.5 mV dec-1 for OER. The stability test of Cu(OH)2@ZIF-67/CF at the current density of 10 mA cm-2 displayed excellent stability for 22 h. This study provides a feasible strategy for the rapid preparation of low-cost and efficient electrocatalysts in alkaline media.

Ruthenium-doped NiFe-based metal-organic framework nanoparticles as highly efficient catalysts for the oxygen evolution reaction.

Developing highly efficient and stable electrocatalysts toward the oxygen evolution reaction (OER) is essential for large-scale sustainable energy conversion and storage technologies. Herein, we design and synthesize a ruthenium (Ru) doped NiFe bimetallic metal-organic framework (MOF) deposited on the nickel foam (Ru-NiFe-MOF/NF) by a facile one-pot hydrothermal reaction. Ru-NiFe-MOF/NF exhibits favourable electrocatalytic OER activity in alkaline solution, and requires a low overpotential of 205 mV to achieve 10 mA cm-2, a small Tafel slope of 50 mV dec-1, and long-term electrochemical stability over 100 h. This work demonstrates the rational nano-architectural design and synthesis of predominantly efficient and robust cation-doped MOF-derived materials for energy catalysis and beyond.

In situ assembly of bimetallic MOF composites on IF as efficient electrocatalysts for the oxygen evolution reaction.

Since the complicated multiple electron transfer process and slow kinetics in the OER process seriously hinder the electrochemical decomposition of water, it is urgent to design and develop electrocatalysts with excellent performance and superior stability to reduce overpotential and accelerate the reaction dynamics of the OER. Herein, a unique ultra-thin nanosheet bimetal electrocatalyst NiFe-MOF/IF was synthesized by a one-step hydrothermal method, and characterized by SEM, XRD, TEM, and XPS. NiFe-MOF/IF shows superior OER electrocatalytic activity in 1 M KOH electrolyte solution, and an ultralow overpotential of only 230 and 262 mV was required to achieve a current density of 10 and 100 mA cm-2, respectively, with a relatively small Tafel slope of 30.46 mV dec-1 for the OER. No obvious degradation of the current density at 10 mA cm-2 was observed over about 16 h, which indicates the excellent stability of the catalyst. Favourable activity and benign durability of NiFe-MOF/IF can be attributed to the three-dimensional high porosity conductive substrates, in situ growth of MOF nanosheets, bimetallic synergy, and unique layering. This research provides a promising strategy for the application of MOF materials in the field of electrocatalysis.

Ultrafine CoRu alloy nanoparticles in situ embedded in Co4N porous nanosheets as high-efficient hydrogen evolution electrocatalysts.

The development of hydrogen evolution reaction (HER) electrocatalysts with outstanding efficiency and favorable stability at all pH values is of great significance but still a dominating challenge toward the development of electrochemical water-splitting technology. Herein, CoRu alloy nanoparticles assembled in Co4N porous nanosheets (named as CoRu@Co4N) have been successfully achieved from Ru(OH)3@Co(OH)2 through a one-step nitridation process. Benefiting from the unique structure, inherent alloy properties and strong alloy-support interaction derived from the in situ transformation, the resultant hybrids exhibit superior HER activities over a wide pH range, achieving very low overpotentials of 13 mV, 44 mV and 15 mV at 10 mA cm-2 under alkaline, neutral and acidic conditions, respectively. Such activities surpass most reported electrocatalysts and are comparable or even transcendent to commercial Ru/C and Pt/C. Furthermore, CoRu@Co4N also exhibits outstanding stability during the accelerated degradation test (ADT) and chronopotentiometry. Our work provides a new approach for designing pH-universal Ru-involved HER electrocatalysts with remarkable efficiency and prominent durability.

A highly efficient electrochemical oxygen evolution reaction catalyst constructed from a S-treated two-dimensional Prussian blue analogue.

It is highly desirable for porous coordination polymers (PCPs), including metal-organic frameworks (MOFs) and Prussian blue analogues (PBAs), to retain their intrinsic characteristics in electrocatalysis, instead of being used as precursors or templates for further total conversion to other compounds via high-temperature calcination. Here, a S-treated two-dimensional (2D) CoFe bimetallic PBA grown on carbon fiber paper (CFP) (named S-CoFe-PBA/CFP) is assembled and applied as a highly efficient oxygen evolution reaction (OER) electrocatalyst in 1 M KOH. The resultant S-CoFe-PBA/CFP demonstrates significantly improved OER catalytic activity; overpotentials of only 235, 259, and 272 mV are needed to drive current densities of 10, 50, and 100 mA cm-2, respectively, with a super low Tafel slope of 35.2 mV dec-1. Even more noteworthy, a current density of 90 mA cm-2 can be achieved when a potential of 1.5 V vs. RHE is applied, which is 6.4 times higher than that of commercial Ir/C in the same environment. The outstanding electrocatalytic performance can be ascribed to two reasons caused by the S-treatment process. On one hand, H+ from intermediates of *OH and *OOH can be captured by -SOx distributed on the surface of the catalyst, thus accelerating the breaking of O-H; on the other hand, partial phase transformation of CoFe-PBA leads to the in situ formation of amorphous CoSx nanogauze on the surface, and the resultant electronic interactions between the two phases contribute much to the improvement of charge transfer and adsorption for OER intermediates. This work provides a new avenue for the design of highly efficient PCP-based OER electrocatalysts.

Regulating the electronic structure of CoMoO4 microrod by phosphorus doping: an efficient electrocatalyst for the hydrogen evolution reaction.

It is of extreme importance to design efficient electrocatalysts for hydrogen evolution reaction (HER), which is considered as a promising approach to provide efficient and renewable clean fuel (hydrogen). Tuning the electronic structure through heteroatom doping demonstrates one of the most effective strategies to promote the electrocatalytic performance of HER. Herein, phosphorus-doping modulation is utilized to fabricate monoclinic P-CoMoO4 with optimized electron structure supported on nickel foam (P-CoMoO4/NF) for alkaline HER via a facile hydrothermal method, followed by low-temperature phosphidation. Notably, P-CoMoO4/NF shows outstanding electrocatalytic performance for HER in 1 M KOH with a low overpotential of 89 mV at 10 mA cm-2, a remarkable Tafel slope value of 59 mV dec-1, and excellent 24 h-long stability. The excellent catalyst activity and stability merits of P-CoMoO4/NF are comparable to the reported highly efficient non-precious metal HER electrocatalysts and could be applied as a powerful electrocatalyst in water electrolysis. This work provides a superior synthesis strategy for the effective design and rational fabrication of low-cost, highly active, and highly stable non-precious metal HER electrocatalysts for electricity-to-hydrogen applications.

Correction: Controlled synthesis of bifunctional particle-like Mo/Mn-NixSy/NF electrocatalyst for highly efficient overall water splitting.

Correction for 'Controlled synthesis of bifunctional particle-like Mo/Mn-NixSy/NF electrocatalyst for highly efficient overall water splitting' by Yaqiong Gong et al., Dalton Trans., 2019, DOI: 10.1039/c9dt00957d.

Controlled synthesis of bifunctional particle-like Mo/Mn-NixSy/NF electrocatalyst for highly efficient overall water splitting.

Heteroatom-doping engineering has been recognized as an effective strategy to improve the activity and stability of electrocatalytic materials. Herein, we fabricated a bimetallic Mo/Mn codoped three-phase nickel sulfide on Ni foam, with Mo/Mn-NixSy/NF successfully synthesized via hydrothermal synthesis and calcination. In order to better explore the codoping effect of Mo/Mn, we also synthesised Ni3S2/NF, NiS@Ni0.96S/NF, Mo-Ni3S2/NF and Mn-NiS@Ni0.96S/NF and their electrocatalytic activities (HER, OER, and overall water splitting) were systematically investigated. As expected, Mo/Mn-NixSy/NF catalysts exhibited excellent catalytic activities and long-term durability. High electrochemical performance of Mo/Mn-NixSy/NF exceeded that of most reported non-precious metal catalysts and also benchmark RuO2, IrO2 and Pt/C. Moreover, in order to better understand the catalytic process, three possible mechanisms were further proposed to rationalize the enhanced electrocatalytic performance. Our work might broaden the avenue to construct efficient non-precious bifunctional catalysts and further develop large-scale electricity-to-hydrogen applications.

Robust ultrafine ruthenium nanoparticles enabled by covalent organic gel precursor for selective reduction of nitrobenzene in water.

Metal nanoparticles (NPs) supported on nitrogen-doped porous carbon (NPC) are one type of promising heterogeneous catalysts. The tuning and understanding of metal-support interactions are crucial for the design and synthesis of highly durable and efficient heterogeneous catalytic systems. Here, we present an effective strategy to integrate ultrafine metal NPs into NPC via utilizing a covalent organic gel (COG) as the precursor for the first time. The ruthenium (Ru) NPs were uniformly dispersed in NPCs with the average size as low as 1.90 ± 0.4 nm. Irrespective of their ultrafine size, Ru NPs showed unprecedented stability and recyclability in Ru-catalyzed reduction of nitrobenzene and were greatly superior to commercial Ru/C and NPC-supported Ru NPs synthesized by the traditional post-loading method. This synthetic strategy can be extended to the synthesis of other metal or alloy NPs for a variety of advanced applications.

Experimental and theoretical study for CO2 activation and chemical fixation with epoxides.

The synthesis of five-membered cyclic carbonates via catalytic cycloaddition reaction of CO2 with epoxides is considered to be an effective technology for alleviation of the energy crisis and global warming. Various commercial organic bases and ionic salts were used as catalysts, while the relationship of catalytic activity and compound structure has been seldom explored. Herein, a facilely obtained binary catalytic system based on triethylamine/NBu4Br was developed for CO2 activation and chemical fixation. The highly efficient catalytic system showed outstanding conversion and above 99% selectivity under metal-free mild reaction conditions (100 °C, 1 atm) in one hour. The detailed process of CO2 activation and chemical fixation was investigated at the molecular level by a series of experiments and theoretical calculation, which provided a mode for the design and synthesis of a highly efficient catalytic system for conversion of CO2 under mild conditions.