Metallic Co 4 N Porous Nanowire Arrays Activated by Surface Oxidation as Electrocatalysts for the Oxygen Evolution Reaction

Oxygen vacancies confined in Co3O4 quantum dots for promoting oxygen evolution electrocatalysis

Abundant oxygen vacancies confined in Co₃O₄ quantum dots provide more efficient Co(ii), more active sites and improved conductivity for superior OER performance.

Surface reconstruction of cobalt phosphide nanosheets by electrochemical activation for enhanced hydrogen evolution in alkaline solution

The surface reconstruction of cobalt phosphide nanosheets is investigated by an in situ electrochemical strategy for enhanced hydrogen evolution.

Transition metal phosphides exhibit promising catalytic performance for the hydrogen evolution reaction (HER); however their surface structure evolution during electrochemical operation has rarely been studied. In this work, we investigate the surface reconstruction of CoP nanosheets by an in situ electrochemical activation method. After remodeling, CoP nanosheets experience an irreversible and significant evolution of the morphology and composition, and low-valence Co complexes consisting of Co(OH)_x species are formed on the surface of CoP nanosheets, and they largely accelerate the dissociation of water. Benefiting from the synergistic effect of CoP and Co(OH)_x, the working electrode shows a remarkably enhanced HER activity of 100 mV at 10 mA cm^–2 with a Tafel slope of 76 mV dec^–1, which is better than that of most transition metal phosphide catalysts. This work would provide a deep understanding of surface reconstruction and a novel perspective for rational design of high performance transition metal phosphide electrocatalysts for water related electrolysis.

Pyrite-Type CoS2 Nanoparticles Supported on Nitrogen-Doped Graphene for Enhanced Water Splitting

It is extremely meaningful to develop cheap, highly efficient, and stable bifunctional electrocatalysts for both hydrogen and oxygen evolution reactions (HER and OER) to promote large-scale application of water splitting technology. Herein, we reported the preparation of CoS₂ nanoparticles supported on nitrogen-doped graphene (CoS₂@N-GN) by one-step hydrothermal method and the enhanced electrochemical efficacy for catalyzing hydrogen and oxygen in water electrolysis. The CoS₂@N-GN composites are composed of nitrogen-doped graphene and CoS₂ nanocrystals with the average size of 73.5 nm. Benefitting from the improved electronic transfer and synergistic effect, the as-prepared CoS₂@N-GN exhibits remarkable OER and HER performance in 1.0 M KOH, with overpotentials of 243 mV for OER and 204 mV for HER at 10 mA cm^-2, and the corresponding Tafel slopes of 51.8 and 108 mV dec^-1, respectively. Otherwise, the CoS₂@N-GN hybrid also presents superior long-term catalytic durability. Moreover, an alkaline water splitting device assembled by CoS₂@N-GN as both anode and cathode can achieve a low cell voltage of 1.53 V at 60 °C with a high faraday efficiency of 100% for overall water splitting. The tremendously enhanced electrochemical behaviors arise from favorable factors including small sized, homogenously dispersed novel CoS₂ nanocrystals and coupling interaction with the underlying conductive nitrogen-doped graphene, which would provide insight into the rational design of transition metal chalcogenides for highly efficient and durable hydrogen and oxygen-involved electrocatalysis.

Surface/Interfacial Engineering of Inorganic Low-Dimensional Electrode Materials for Electrocatalysis

Exploitation of highly active and cost-effective electrode materials for the design of new types of renewable energy storage and conversion systems has been tremendously stimulated by the higher attention being paid to global energy security and invention of alternative clean sustainable energy technologies. Low-dimensional solid materials with special atomic and electronic structures are deemed desirable platforms for establishing clear relationships between surface/interface structure characteristics and electrocatalytic activity, representing enormous potential in the pursuit of high-performance electrocatalysts. Recent achievements revealed that surface and interfacial atomic engineering is capable of achieving novel physical and chemical properties as well as superior synergistic effects in inorganic low-dimensional nanomaterials for electrocatalysis. Compared to bulk counterparts, the electronic structure in the surface of inorganic low-dimensional nanomaterials is more sensitive to and can thus be regulated more easily by surface and interfacial modification strategies, resulting in greatly optimized electrocatalytic performance. In this Account, we focus on recent progress in surface and interfacial modification strategies to efficaciously engineer the electrocatalytic performance of inorganic low-dimensional electrode materials. We summarize several important regulation strategies of dimensional confinement, incorporation, surface reconstruction, interface modulation, and defect engineering, which immensely optimize the spin configuration, electrical conductivity, catalytic active site exposure, and reaction energy barrier of inorganic electrode material. At dimensionally confined atomic-scale thickness, more surface-facet atoms are exposed as active sites, which provide an ideal platform for applying surface incorporation and defect engineering, subsequently producing more catalytic active sites and better adsorption free energy for the improvement of catalytic activity. Moreover, regulation of the interfacial character of electrode materials, such as the surface strain, contact area, and bridged bonds, can optimize the electron transfer capacity and reaction kinetics process. On the other hand, once exposed to a strong alkaline solution under oxidizing potentials, the real active layer of electrode materials (such as transition-metal sulfides, nitrides, and phosphides) could be activated by a surface reconstruction strategy, realizing a unique core-shell structure with a highly conductive electron transfer channel inside and highly active catalytic sites outside for electrocatalysis. Based on these points of view, focusing on inorganic low-dimensional electrode materials, the proper choice of surface and interfacial modification strategies would effectively modulate their electrocatalytic activity, realizing unlimited potential applications in promising areas of electrocatalytic water splitting, rechargeable metal batteries, and fuel cells. Overall, we anticipate that surface and interfacial regulation approaches can provide a new understanding of the design of inorganic electrode materials, facilitating the rapid promotion of electrocatalytic performance in electrode materials for electrocatalysis.

Ultrathin cobalt oxide nanostructures with morphology-dependent electrocatalytic oxygen evolution activity

Engineering compositions, structures, and defects can endow nanomaterials with optimized catalytic properties. Here, we report that cobalt oxide (CoOx) ultrathin nanosheets (UTNS, ∼1.6 nm thick) with a large number of oxygen defects and mixed cobalt valences can be obtained through a facile one-step hydrothermal protocol. The large number of oxygen defects make the ultrathin CoOx nanosheet a superior OER catalyst with low overpotentials of 315 and 365 mV at current densities of 50 and 200 mA cm-2, respectively. The stable framework-like architectures of the UTNS further ensure their high OER activity and durability. Our method represents a facile one-step preparation of CoOx nanostructures with tunable compositions, morphologies, and defects, and thus promotes OER properties. This strategy may find its wider applicability in designing active, robust, and easy-to-obtain catalysts for OER and other electrocatalytic systems.

Emerging Materials in Heterogeneous Electrocatalysis Involving Oxygen for Energy Harvesting

Water-based renewable energy cycle involved in water splitting, fuel cells, and metal-air batteries has been gaining increasing attention for sustainable generation and storage of energy. The major challenges in these technologies arise due to the poor kinetics of the oxygen reduction reaction (ORR) and the oxygen evolution reactions (OER), besides the high cost of the catalysts. Attempts to address these issues have led to the development of many novel and inexpensive catalysts as well as newer mechanistic insights, particularly so in the last three-four years when more catalysts have been investigated than ever before. With the growing emphasis on bifunctionality, that is, materials that can facilitate both reduction and evolution of oxygen, this review is intended to discuss all major families of ORR, OER, and bifunctional catalysts such as metals, alloys, oxides, other chalcogenides, pnictides, and metal-free materials developed during this period in a single platform, while also directing the readers to specific and detailed review articles dealing with each family. In addition, each section highlights the latest theoretical and experimental insights that may further improve ORR/OER performances. The bifunctional catalysts being sufficiently new, no consensus appears to have emerged about the efficiencies. Therefore, a statistical analysis of their performances by considering nearly all literature reports that have appeared in this period is presented. The current challenges in rational design of these catalysts as well as probable strategies to improve their performances are presented.

Disordering the Atomic Structure of Co(II) Oxide via B-Doping: An Efficient Oxygen Vacancy Introduction Approach for High Oxygen Evolution Reaction Electrocatalysts

The introduction of oxygen (O) vacancies has been considered to be an important but challenging way to enhance the activity of electrocatalysts for the oxygen evolution reaction (OER). Substitution by heteroatoms with high electron-donating ability may be a feasible strategy for triggering O-vacancies to maintain thermodynamic stability. Herein, density functional theory (DFT) calculations predict that the incorporation of boron (B) is favorable to the generation of O-vacancies in transition metal oxides. Then, CoO nanowires with O-vacancies are prepared via incorporation of B using a facile pyrolysis strategy. As evidenced by the combined results of electron paramagnetic resonance spectroscopy and X-ray absorption near edge structure, O-vacancies in CoO are mainly derived from the disordering of the local structure caused by B doping. DFT calculation results further reveal that the oxidation of *OOH is the rate-limiting step for O-vacancies enriched CoO in the OER and that the presence of O-vacancies can efficiently lower the reaction barrier for breaking CoO bond, contributing to the improvement of OER kinetics. As expected, the O-vacancies enriched CoO exhibits a low overpotential of 280 mV to reach the current density of 10 mA cm^-2 under basic conditions.

Bimetallic NiMoN Nanowires with a Preferential Reactive Facet: An Ultraefficient Bifunctional Electrocatalyst for Overall Water Splitting

Faceted nanomaterials with highly reactive exposed facets have been the target of intense researches owing to their significantly enhanced catalytic performance. NiMoN nanowires with the (100) facet preferentially exposed were prepared by an in situ N/O exchange and the morphology tuned by using a rationally designed NiMoO₄ precursor. The facet-tuned NiMoN nanowires exhibited excellent electrocatalytic activity for the hydrogen evolution reaction (HER) under both alkaline and acidic conditions that was comparable to that of noble metal platinum. DFT calculations further revealed that the catalytic activity of NiMoN nanowires towards HER on the (100) reactive facet is significantly greater than that on the (001) or (101) facets, owing to the low adsorption free energy of H* (ΔG_H* ) on the (100) facet. The NiMoN nanowires also demonstrated outstanding activity towards the alkaline oxygen evolution reaction and an excellent durable activity for overall water splitting, with a cell potential as low as 1.498 V at 20 mA cm^-2 . This work provides insights into improving electrocatalytic activity and developing advanced non-noble metal bifunctional electrocatalysts.

Activation of defective nickel molybdate nanowires for enhanced alkaline electrochemical hydrogen evolution

Designing highly-efficient and cost-effective electrocatalysts for the hydrogen evolution reaction (HER) in an alkaline solution is more complex and sluggish than for an acidic one. Herein, we report a controllable N-doping strategy to synthesize a series of N-doped porous metallic NiMoO4 nanowires with concomitant oxygen vacancy defects (N-Vo-NiMoO4 NWs) for promoting the alkaline HER ability and durability. Both experimental and theoretical results demonstrate that the doped-N at NiO6 octahedral sites and the abundant oxygen vacancy defects confined in N-Vo-NiMoO4 NWs with modified electronic arrangement could enhance the metallic conductivity, affect the surface areas, and lower the adsorption energy of hydrogen, resulting in an increased HER property. However, the excess doped-N leads to an opposite effect due to the reduced valence state of Ni centres. Therefore, alkaline HER ability of N-Vo-NiMoO4 NWs exhibits a volcano-like trend vs. the nitrogen content, with N3-Vo-NiMoO4 NWs being the best one. As a result, the N3-Vo-NiMoO4 NWs show nearly zero onset overpotential, an overpotential of 55 mV at 10 mA cm-2, and a Tafel slope of only 38 mV dec-1 in 1.0 M KOH, which are superior to those of state-of-the-art platinum-free electrocatalysts.

Novel Cobalt Germanium Hydroxide for Electrochemical Water Oxidation

Developing efficient and stable oxygen evolution catalyst (OEC) is a critical step to overcome the sluggish kinetics of water oxidation. Here, we hydrothermally synthesized a novel OEC, cobalt germanium hydroxide, CoGeO₂(OH)₂. The inherent Co-bonded hydroxyl groups facilitate the formation of active oxygen evolution reaction intermediates. Meanwhile, the facile leaching of Ge at the OEC-electrolyte interface contributes to surface reconstruction, generating Co-based (oxy)hydroxides, which would weaken its lattice constraint and suppress the excessive corrosion in the OEC bulk. As a result, CoGeO₂(OH)₂ reveals good catalytic activity and stability. This CoGe-based OEC achieves the overpotential at 10 mA cm^-2 (η_@10mA) of ∼340 mV, and the turnover frequency of ∼0.08 s^-1. And the electrolysis kept at ∼10 mA cm^-2 could be sustained for over 350 h. In addition, this p-type CoGeO₂(OH)₂ is demonstrated to be an effective electrocatalytic overlayer on n-type Ta₃N₅ photoanode, remarkably decreasing the onset for nearly 400 mV and increasing the photocurrent density at 1.23 V_RHE about 3.8 times.

Bigger is Surprisingly Better: Agglomerates of Larger RuP Nanoparticles Outperform Benchmark Pt Nanocatalysts for the Hydrogen Evolution Reaction

Although metallic ruthenium (Ru) is a potential electrocatalyst for the hydrogen evolution reaction (HER) to replace platinum (Pt) at a cost of only ≈4% of Pt, the persistent dissolution of Ru under operation conditions remains a challenge. Here, it is reported that agglomerates of large ruthenium phosphide (RuP) particles (L-RP, ≈32 nm) show outstanding HER performance in pH-universal electrolytes, which particularly demonstrates a surprisingly higher intrinsic activity and durability than small nanoparticles of RuP (S-RP, ≈3 nm) or metallic Ru on carbon supports. This is especially true in basic media, achieving electrocatalytic activity comparable to or even outperforming that of Pt/C, as reflected by lower overpotential at 10 mA cm^-2 , smaller Tafel slope, larger exchange current density, and higher turnover frequency while maintaining 200 h stable operation. Calculations suggest that ΔG_H* of RuP is much closer to zero than that of metallic Ru, and phosphorous doping is proven to enhance the rate of proton transfer in HER, contributing in part to the improved activity of RuP. The better performance of L-RP than that of S-RP is ascribed largely to the stabilization of the P species due to the lowered surface energy of large particles. Furthermore, the relatively low-cost materials and facile synthesis make L-RP/C a highly attractive next-generation HER electrocatalyst.

Synthesis of transition metal sulfide and reduced graphene oxide hybrids as efficient electrocatalysts for oxygen evolution reactions

The development of electrochemical devices for renewable energy depends to a large extent on fundamental improvements in catalysts for oxygen evolution reactions (OERs). OER activity of transition metal sulfides (TMSs) can be improved by compositing with highly conductive supports possessing a high surface-to-volume ratio, such as reduced graphene oxide (rGO). Herein we report on the relationship between synthetic conditions and the OER catalytic properties of TMSs and rGO (TMS-rGO) hybrids. Starting materials, reaction temperature and reaction time were controlled to synergistically boost the OER catalytic activity of TMS-rGO hybrids. Our results showed that (i) compared with sulfides, hydroxides are favourable as starting materials to produce the desired TMS-rGO hybrid nanostructure and (ii) high reaction temperatures and longer reaction times can increase physico-chemical interaction between TMSs and rGO supports, resulting in highly efficient OER catalytic activity.

Recent Advances toward the Rational Design of Efficient Bifunctional Air Electrodes for Rechargeable Zn-Air Batteries

Large-scale application of renewable energy and rapid development of electric vehicles have brought unprecedented demand for advanced energy-storage/conversion technologies and equipment. Rechargeable zinc (Zn)-air batteries represent one of the most promising candidates because of their high energy density, safety, environmental friendliness, and low cost. The air electrode plays a key role in managing the many complex physical and chemical processes occurring on it to achieve high performance of Zn-air batteries. Herein, recent advances of air electrodes from bifunctional catalysts to architectures are summarized, and their advantages and disadvantages are discussed to underline the importance of progress in the evolution of bifunctional air electrodes. Finally, some challenges and the direction of future research are provided for the optimized design of bifunctional air electrodes to achieve high performance of rechargeable Zn-air batteries.

In-Situ Formed Hydroxide Accelerating Water Dissociation Kinetics on Co3N for Hydrogen Production in Alkaline Solution

Sluggish water dissociation kinetics on nonprecious metal electrocatalysts limits the development of economical hydrogen production from water-alkali electrolyzers. Here, using Co₃N electrocatalyst as a prototype, we find that during water splitting in alkaline electrolyte a cobalt-containing hydroxide formed on the surface of Co₃N, which greatly decreased the activation energy of water dissociation (Volmer step, a main rate-determining step for water splitting in alkaline electrolytes). Combining the cobalt ion poisoning test and theoretical calculations, the efficient hydrogen production on Co₃N electrocatalysts would benefit from favorable water dissociation on in-situ formed cobalt-containing hydroxide and low hydrogen production barrier on the nitrogen sites of Co₃N. As a result, the Co₃N catalyst exhibits a low water-splitting activation energy (26.57 kJ mol^-1) that approaches the value of platinum electrodes (11.69 kJ mol^-1). Our findings offer new insight into understanding the catalytic mechanism of nitride electrocatalysts, thus contributing to the development of economical hydrogen production in alkaline electrolytes.

NiFe (Oxy) Hydroxides Derived from NiFe Disulfides as an Efficient Oxygen Evolution Catalyst for Rechargeable Zn-Air Batteries: The Effect of Surface S Residues

A facile H₂ O₂ oxidation treatment to tune the properties of metal disulfides for oxygen evolution reaction (OER) activity enhancement is introduced. With this method, the degree of oxidation can be readily controlled and the effect of surface S residues in the resulted metal (oxy)hydroxides for the OER is revealed for the first time. The developed NiFe (oxy)hydroxide catalyst with residual S demonstrates an extraordinarily low OER overpotential of 190 mV at the current density of 10 mA cm^-2 after coupling with carbon nanotubes, and outstanding performance in Zn-air battery tests. Theoretical calculation suggests that the surface S residues can significantly reduce the adsorption free energy difference between O* and OH* intermediates on the Fe sites, which should account for the high OER activity of NiFe (oxy)hydroxide catalysts. This work provides significant insight regarding the effect of surface heteroatom residues in OER electrocatalysis and offers a new strategy to design high-performance and cost-efficient OER catalysts.

Trapping [PMo12O40]3- clusters into pre-synthesized ZIF-67 toward Mo x Co x C particles confined in uniform carbon polyhedrons for efficient overall water splitting

Bi-transition metal carbides (BTMCs) are promising in catalytic fields, but the synthesis of small-sized BTMCs remains a challenge. Here, Mo x Co x C (mainly below 20 nm in size) confined in uniform carbon polyhedrons (Mo x Co x C@C) was synthesized based on trapping [PMo12O40]3- (PMo12) clusters into pre-synthesized, uniform ZIF-67 (PMo/ZIF-67). The opening of the windows (0.34 nm) of ZIF-67 cages through heating is essential to allow the trapping of PMo12 into the cages. This trapping route provides a new method to successfully combine POMs and MOFs that can not be effectively combined via traditional POMOF-based (simultaneous assembly) routes. It also has advantages in controlling the uniformity and components of the materials. The size matching of PMo12 (1 nm) and the cages (1.16 nm) of ZIF-67 enables effective contact of the Co and Mo sources, thus giving small-sized Mo x Co x C protected by carbon via calcination. The optimized catalysts showed good performance for water splitting with a low η10 of 83 mV (295 mV) for the hydrogen (oxygen) evolution reaction, which is superior to those derived from ZIF-67 and precursors from POMOF-based routes. Our results also indicated that the HER activity is determined by the kind of BTMC, and the activity for the OER is relative to the oxygen-containing species formed during the initial OER test.

Assembling Ultrasmall Copper-Doped Ruthenium Oxide Nanocrystals into Hollow Porous Polyhedra: Highly Robust Electrocatalysts for Oxygen Evolution in Acidic Media

Here, a facile and novel strategy for the preparation of Cu-doped RuO2 hollow porous polyhedra composed of ultrasmall nanocrystals through one-step annealing of a Ru-exchanged Cu-BTC derivative is reported. Owing to the optimized surface configuration and altered electronic structure, the prepared catalyst displays a remarkable oxygen evolution reaction (OER) performance with low overpotential of 188 mV at 10 mA cm-2 in acidic electrolyte, an ultralow Tafel slope of 43.96 mV dec-1 , and excellent stability in durability testing for 10 000 cycles, and continuous testing of 8 h at a current density of 10 mA cm-2 . Density functional theory calculations reveal that the highly unsaturated Ru sites on the high-index facets can be oxidized gradually and reduce the energy barrier of rate-determining steps. On the other hand, the Cu dopants can alter the electronic structures so as to further improve the intrinsic OER activity.

Fabrication of hierarchical CoP nanosheet@microwire arrays via space-confined phosphidation toward high-efficiency water oxidation electrocatalysis under alkaline conditions

In spite of recent advances in the synthesis of transition metal phosphide nanostructures, the simple fabrication of hierarchical arrays with more accessible active sites still remains a great challenge. In this Communication, we report a space-confined phosphidation strategy toward developing hierarchical CoP nanosheet@microwire arrays on nickel foam (CoP NS@MW/NF) using a Co(H2PO4)2·2H3PO4 microwire array as the precursor. The thermally stable nature of the anion in the precursor is key to hierarchical nanostructure formation. When used as a 3D electrode for water oxidation electrocatalysis, such CoP NS@MW/NF needs an overpotential as low as 296 mV to drive a geometrical catalytic current density of 100 mA cm-2 in 1.0 M KOH, outperforming all reported Co phosphide catalysts in alkaline media. This catalyst also shows superior long-term electrochemical durability, maintaining its activity for at least 65 h. This study offers us a general method for facile preparation of hierarchical arrays for applications.

Oxygen Doping to Optimize Atomic Hydrogen Binding Energy on NiCoP for Highly Efficient Hydrogen Evolution

An outstanding hydrogen evolution electrocatalyst should have a free energy of adsorbed atomic hydrogen of approximately zero, which enables not only a fast proton/electron-transfer step but also rapid hydrogen release. An economic and industrially viable alternative approach for the optimization of atomic hydrogen binding energy is urgently needed. Herein, guided by density functional theory (DFT) calculations, it is theoretically demonstrated that oxygen doping in NiCoP can indeed optimize the atomic hydrogen binding energy (e.g., |ΔG_H* | = 0.08, 0.12 eV on Co, P sites). To confirm this, NiCoP electrodes with controllable oxygen doping are designed and fabricated via alteration of the reducing atmosphere. Accordingly, an optimal oxygen-doped NiCoP (≈0.98% oxygen) nanowire array is found to exhibit the remarkably low hydrogen evolution reaction (HER) overpotential of 44 mV to drive 10 mA cm^-2 and a small Tafel slope of 38.6 mV dec^-1 , and long-term stability of 30 h in an alkaline medium. In neutral solution, only a 51 mV overpotential (@10 mA cm^-2 ) is required, and the Tafel slope is 79.2 mV dec^-1 . Meanwhile, in situ Raman spectra confirm the low formation overpotential (-30 mV) of NiCo-phosphate at the surface of ≈0.98% oxygen-doped NiCoP, which enables the material to show better HER performance.

Electron density modulation of NiCo2S4 nanowires by nitrogen incorporation for highly efficient hydrogen evolution catalysis

Metal sulfides for hydrogen evolution catalysis typically suffer from unfavorable hydrogen desorption properties due to the strong interaction between the adsorbed H and the intensely electronegative sulfur. Here, we demonstrate a general strategy to improve the hydrogen evolution catalysis of metal sulfides by modulating the surface electron densities. The N modulated NiCo2S4 nanowire arrays exhibit an overpotential of 41 mV at 10 mA cm-2 and a Tafel slope of 37 mV dec-1, which are very close to the performance of the benchmark Pt/C in alkaline condition. X-ray photoelectron spectroscopy, synchrotron-based X-ray absorption spectroscopy, and density functional theory studies consistently confirm the surface electron densities of NiCo2S4 have been effectively manipulated by N doping. The capability to modulate the electron densities of the catalytic sites could provide valuable insights for the rational design of highly efficient catalysts for hydrogen evolution and beyond.

Metallic CuCo2S4 nanosheets of atomic thickness as efficient bifunctional electrocatalysts for portable, flexible Zn-air batteries

Optimized catalysts show great potential for renewable energy storage and conversion. Herein, we report metallic CuCo2S4 nanosheets (NSs) of atomic thickness as efficient bifunctional electrocatalysts for use in portable, flexible Zn-air batteries. The metallic CuCo2S4 NSs of atomic thickness with 4-atom-thick to 6-atom-thick layers are confirmed by temperature-dependent electrical resistance measurements and atomic force microscopy. Furthermore, extended X-ray absorption fine structure spectroscopy confirms that CuCo2S4 NSs with sulfur vacancies can further increase the OER activity. Due to high electrical conductivity and ultrathin nanosheet structure with abundant defects, CuCo2S4 NSs exhibit excellent reversible oxygen catalytic performance with an overpotential of 287 mV (at j = 10 mA cm-2) for the oxygen evolution reaction (OER) and an onset potential of 0.90 V for the oxygen reduction reaction (ORR). Additionally, the portable, flexible Zn-air battery using CuCo2S4 NSs as the air-cathode displays a high open circuit voltage and strong rechargeable capacity for 18 h. The present study highlights the importance of designing metallic catalysts having atomic thickness with surface defects for highly efficient and stable renewable energy storage and conversion.

Phase and composition controlled synthesis of cobalt sulfide hollow nanospheres for electrocatalytic water splitting

Developing cheap, highly efficient and stable electrocatalysts for both oxygen and hydrogen evolution reactions (OER and HER) is extremely meaningful to realize large-scale implementation of water splitting technology. Herein, we report the phase and composition controlled synthesis of cobalt sulfide (CoS_x) hollow nanospheres (HNSs) and their catalytic efficiencies for hydrogen and oxygen evolution reactions in alkaline media. Three CoS_x compounds, i.e., Co₉S₈, Co₃S₄, and CoS₂ HNSs, were precisely synthesized by simply adjusting the molar ratio of carbon disulfide to cobalt acetate using a facile solution-based strategy. Electrochemical results reveal that the as-prepared CoS₂ HNSs exhibit superior OER and HER catalytic performance to Co₉S₈ and Co₃S₄ HNSs in 1.0 M KOH, with overpotentials of 290 mV for the OER and 193 mV for the HER at 10 mA cm^-2, and the corresponding Tafel slopes of 57 and 100 mV dec^-1, respectively. In addition, the CoS₂ HNSs exhibit remarkable long-term catalytic durability, which is even superior to precious metal catalysts of RuO₂ and Pt/C. Moreover, an alkaline electrolyzer assembled using CoS₂ HNSs as both anode and cathode materials can achieve 10 mA cm^-2 at a low cell voltage of 1.54 V at 60 °C with a faradaic efficiency of 100% for overall water splitting. Further analysis demonstrates that the surface morphology, crystallographic structure and coordination environment of Coⁿ⁺ active sites in combination determine the HER/OER activities in the synthesized binary CoS_x series, which would provide insight into the rational design of transition metal chalcogenides for highly efficient hydrogen and oxygen-involved electrocatalysis.

In Situ Formation of Cobalt Nitrides/Graphitic Carbon Composites as Efficient Bifunctional Electrocatalysts for Overall Water Splitting

Developing cost-effective and highly efficient bifunctional electrocatalysts for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is of great interest for overall water splitting but still remains a challenging issue. Herein, a self-template route is employed to fabricate a unique hybrid composite constructed by encapsulating cobalt nitride (Co_5.47N) nanoparticles within three-dimensional (3D) N-doped porous carbon (Co_5.47N NP@N-PC) polyhedra, which can be served as a highly active bifunctional electrocatalyst. To afford a current density of 10 mA cm^-2, the as-fabricated Co_5.47N NP@N-PC only requires overpotentials as low as 149 and 248 mV for HER and OER, respectively. Moreover, an electrolyzer with Co_5.47N NP@N-PC electrodes as both the cathode and anode catalyst in alkaline solutions can drive a current density of 10 mA cm^-2 at a cell voltage of only 1.62 V, superior to that of the Pt/IrO₂ couple. The excellent electrocatalytic activity of Co_5.47N NP@N-PC can be mainly ascribed to the high inherent conductivity and rich nitrogen vacancies of the Co_5.47N lattice, the electronic modulation of the N-doped carbon toward Co_5.47N, and the hierarchically porous structure design.

MOF-Templated Fabrication of Hollow Co4N@N-Doped Carbon Porous Nanocages with Superior Catalytic Activity

Metallic Co₄N catalysts have been considered as one of the most promising non-noble materials for heterogeneous catalysis because of their high electrical conductivity, great magnetic property, and high intrinsic activity. However, the metastable properties seriously limit their applications for heterogeneous water phase catalysis. In this work, a novel Co-metal-organic framework (MOF)-derived hollow porous nanocages (PNCs) composed of metallic Co₄N and N-doped carbon (NC) were synthesized for the first time. This hollow three-dimensional (3D) PNC catalyst was synthesized by taking advantage of Co-MOF as a precursor for fabricating 3D hollow Co₃O₄@C PNCs, along with the NH₃ treatment of Co-oxide frames to promote the in situ conversion of Co-MOF to Co₄N@NC PNCs, benefiting from the high intrinsic activity and electron conductivity of the metallic Co₄N phase and the good permeability of the hollow porous nanostructure as well as the efficient doping of N into the carbon layer. Besides, the covalent bridge between the active Co₄N surface and PNC shells also provides facile pathways for electron and mass transport. The obtained Co₄N@NC PNCs exhibit excellent catalytic activity and stability for 4-nitrophenol reduction in terms of low activation energy (E_a = 23.53 kJ mol^-1), high turnover frequency (52.01 × 10²⁰ molecule g^-1 min^-1), and high apparent rate constant (k_app = 2.106 min^-1). Furthermore, its magnetic property and stable configuration account for the excellent recyclability of the catalyst. It is hoped that our finding could pave the way for the construction of other hollow transition metal-based nitride@NC PNC catalysts for wide applications.