Hydrogen-etched CoS2 to produce a Co9S8@CoS2 heterostructure electrocatalyst for highly efficient oxygen evolution reaction

There is a pressing requirement for developing high-efficiency non-noble metal electrocatalysts in oxygen evolution reactions (OER), where transition metal sulfides are considered to be promising electrocatalysts for the OER in alkaline medium. Herein, we report the outstanding OER performance of Co₉S₈@CoS₂ heterojunctions synthesized by hydrogen etched CoS₂, where the optimized heterojunction shows a low η₅₀ of 396 mV and a small Tafel slope of 181.61 mV dec⁻¹. The excellent electrocatalytic performance of this heterostructure is attributed to the interface electronic effect. Importantly, the post-stage characterization results indicate that the Co₉S₈@CoS₂ heterostructure exhibits a dynamic reconfiguration during the OER with the formation of CoOOH in situ, and thus exhibits a superior electrocatalytic performance.

Herein, we report the outstanding OER performance of Co₉S₈@CoS₂ heterojunctions synthesized by hydrogen etched CoS₂, where the optimized heterojunction shows a low η₅₀ of 396 mV and a small Tafel slope of 181.61 mV dec⁻¹.     

Self-supported Cu3P nanowire electrode as an efficient electrocatalyst for the oxygen evolution reaction

Hydrogen is an ideal energy carrier due to its abundant reserves and high energy density. Electrolyzing water is one of the carbon free technologies for hydrogen production, which is limited by the sluggish kinetics of the half reaction of the anode – the oxygen evolution reaction (OER). In this study, a self-supported Cu₃P nanowire (Cu₃P NWs/CF) electrode is prepared by electrodeposition of a Cu(OH)₂ nanowire precursor on conductive Cu foam (Cu(OH)₂ NWs/CF) with a subsequent phosphating procedure under a N₂ atmosphere. When used as an OER working electrode in 1.0 M KOH solution at room temperature, Cu₃P NWs/CF exhibits excellent catalytic performance with an overpotential of 327 mV that delivers a current density of 20 mA cm⁻². Notably, it can run stably for 22 h at a current density of 20 mA cm⁻² without obvious performance degradation. This highly efficient and stable OER catalytic performance is mainly attributed to the unique nanostructure and stable electrode construction. Interestingly, this synthesis strategy has been proved to be feasible to prepare large-area working electrodes (e.g. 40 cm⁻²) with unique nanowire structure. Therefore, this work has provided a good paradigm for the mass fabrication of self-supporting non-noble metal OER catalysts and effectively promoted the reaction kinetics of the anode of the electrolyzing water reaction.

We prepared Cu₃P nanowires via a simple two-step method and Cu(OH)₂ NWs/CF was converted to Cu₃P/NWs after a phosphating process. The prepared Cu₃P NWs/CF electrode shows high efficiency and excellent stability to OER in alkaline medium.     

Genomic DNA-mediated formation of a porous Cu2(OH)PO4/Co3(PO4)2·8H2O rolling pin shape bifunctional electrocatalyst for water splitting reactions

Among the accessible techniques, the production of hydrogen by electrocatalytic water oxidation is the most established process, which comprises oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Here, we synthesized a genomic DNA-guided porous Cu₂(OH)PO₄/Co₃(PO₄)₂·8H₂O rolling pin shape composite structure in one pot. The nucleation and development of the porous rolling pin shape Cu₂(OH)PO₄/Co₃(PO₄)₂·8H₂O composite was controlled and stabilized by the DNA biomolecules. This porous rolling pin shape composite was explored towards electrocatalytic water oxidation for both OER and HER as a bi-functional catalyst. The as-prepared catalyst exhibited a very high OER and HER activity compared to its various counterparts in the absence of an external binder (such as Nafion). The synergistic effects between Cu and Co metals together with the porous structure of the composite greatly helped in enhancing the catalytic activity. These outcomes undoubtedly demonstrated the beneficial utilization of the genomic DNA-stabilised porous electrocatalyst for OER and HER, which has never been observed.

Among the accessible techniques, the production of hydrogen by electrocatalytic water oxidation is the most established process, which comprises oxygen evolution reaction (OER) and hydrogen evolution reaction (HER).     

Solution-phase phosphorus substitution for enhanced oxygen evolution reaction in Cu2WS4

Transition metal phosphides are among the most promising materials for achieving efficient electrocatalytic performance without the use of rare or expensive noble metals. However, previous research into phosphides for the hydrogen evolution reaction (HER) or oxygen evolution reaction (OER) has focused on high-temperature vapor-phase processes, which are not practical for large-scale applications. Here, we introduce a simple, one-step solution-phase method of phosphide synthesis by modifying Cu₂WS₄ using triphenylphosphine (TPP), which serves to substitute S with P and transform the normally inactive basal plane of Cu₂WS₄ into a defect-rich, activated basal plane. The OER activity was significantly enhanced by phosphorus substitution, with the resulting Tafel slope of the sample with ∼8 at% phosphorus reaching 194 mV dec⁻¹, a result close to that of the best OER catalyst (RuO₂, 151 mV dec⁻¹). The sample possessed stable OER performance, showing no degradation in current density over ∼24 hours (500 cycles), proving the robust and stable nature of the phosphorus substitution. These results open the possibility for further phosphide catalyst development using this low-cost, solution-phase method.

Solution-phase synthesis of a transition metal phosphide for use as a highly efficient electrocatalyst.     

A Co-MOF-derived Co9S8@NS-C electrocatalyst for efficient hydrogen evolution reaction

The exploitation of efficient hydrogen evolution reaction (HER) electrocatalysts has become increasingly urgent and imperative; however, it is also challenging for high-performance sustainable clean energy applications. Herein, novel Co₉S₈ nanoparticles embedded in a porous N,S-dual doped carbon composite (abbr. Co₉S₈@NS-C-900) were fabricated by the pyrolysis of a single crystal Co-MOF assisted with thiourea. Due to the synergistic benefit of combining Co₉S₈ nanoparticles with N,S-dual doped carbon, the composite showed efficient HER electrocatalytic activities and long-term durability in an alkaline solution. It shows a small overpotential of −86.4 mV at a current density of 10.0 mA cm⁻², a small Tafel slope of 81.1 mV dec⁻¹, and a large exchange current density (J₀) of 0.40 mA cm⁻², which are comparable to those of Pt/C. More importantly, due to the protection of Co₉S₈ nanoparticles by the N,S-dual doped carbon shell, the Co₉S₈@NS-C-900 catalyst displays excellent long-term durability. There is almost no decay in HER activities after 1000 potential cycles or it retains 99.5% of the initial current after 48 h.

A porous Co₉S₈@NS-C-900 composite was fabricated by the pyrolysis of crystal Co-MOF involving thiourea. The composite exhibits efficient electrocatalytic activities and long-term durabilities towards HER in alkaline electrolytes.     

Novel heterostructure Cu2S/Ni3S2 coral-like nanoarrays on Ni foam to enhance hydrogen evolution reaction in alkaline media

Exploring efficient alternatives to precious noble metal catalysts is a challenge. Here, a new type of non-noble metal Cu₂S/Ni₃S₂ heterostructure nanosheet array is fabricated on 3D Ni foam. This electrocatalyst has excellent activity and durability to Hydrogen Evolution Reaction (HER) under alkaline conditions. The synergistic catalysis produced by the {$\bar{2}$10} and (034) crystal planes and the increase in charge transfer and the number of active sites caused by lattice defects greatly improve the electrocatalytic activity of Ni₃S₂. In the HER process, the Cu₂S/Ni₃S₂ interface increases the formation of S–H bonds, and Cu₂S promotes the transformation during the HER process into S-doped CuO, optimizing the adsorption capacity of S-doped sites for H. Among electrocatalysts made with different feed ratios, Cu₂S/Ni₃S₂/NF-3, for HER, only needs an overpotential of 50 mV to deliver a current density of 10 mA cm⁻². This work provides a promising non-noble metal electrocatalyst for water splitting under alkaline conditions.

We fabricated a heterostructure Cu₂S/Ni₃S₂ nanosheet array, which can accelerate charge transfer and provide more active sites. This work provides a promising non-noble metal electrocatalyst for water splitting under alkaline conditions.     

Template free-synthesis of cobalt–iron chalcogenides [Co0.8Fe0.2L2, L = S,Se] and their robust bifunctional electrocatalysis for the water splitting reaction and Cr(vi) reduction

The ease of production of materials and showing multiple applications are appealing in this modern era of advanced technology. This paper reports the synthesis of a pair of novel cobalt–iron chalcogenides [Co_0.8Fe_0.2S₂ and Co_0.8Fe_0.2Se₂] with enhanced electro catalytic activities. These ternary metal chalcogenides were synthesized by a one-step template-free approach via a hexamethyldisilazane (HMDS)-assisted synthetic method. Transient photocurrent (TPC) studies and electrochemical impedance spectra (EIS) of these materials showed free electron mobility. Their bifunctional activities were verified in both the electrochemical oxygen evolution reaction (OER) and in the electrochemical reduction of toxic inorganic heavy metal ions [Cr(vi)] in polluted water. The materials showed robust catalytic ability in the oxygen evolution reaction with minimum possible over potential (345 and 350 mV @ η10) as determined by linear sweep voltammetry and the lower Tafel values (52.4 and 84.5 mV dec⁻¹) for Co_0.8Fe_0.2Se₂ and Co_0.8Fe_0.2S₂ respectively. Surprisingly, both the materials also showed an excellent activity towards electrochemical Cr(vi) reduction to Cr(iii). Besides the maximum current achieved for Co_0.8Fe_0.2S₂, a minimum value for the Limit of detection (LOD) was obtained for Co_0.8Fe_0.2S₂ (0.159 μg L⁻¹) compared to Co_0.8Fe_0.2Se₂ (0.196 μg L⁻¹). We tested the durability of catalysts, the critical factor for the prolonged use of catalysts, through the recyclability measurements of these materials as catalysts. Both the catalysts presented outstanding durability and balanced electro catalytic activities for up to 1500 CV cycles, and chronoamperometry studies also confirmed exceptional stability. The enhanced catalytic activities of these materials are ascribed to the free electron movement, evidenced by the increased TPC measured and EIS. Therefore, the template-free synthesis of these electro catalysts containing non-noble metal illustrates the practical approach to develop such types of catalysts for multiple functions.

The ease of production of materials and showing multiple applications are appealing in this modern era of advanced technology. Cobalt–iron chalcogenides showing multiple application is reported.     

The identification of active N species in N-doped carbon carriers that improve the activity of Fe electrocatalysts towards the oxygen evolution reaction

Nitrogen-doped carbon nanomaterials have become some of the most effective carriers for transition metal-based electrocatalysts towards the oxygen evolution reaction. However, the specific active nitrogen species in nitrogen-doped carriers remains unclear up to now. To identify the active nitrogen species, herein, we prepare nitrogen-doped carbon nanospheres containing different types of nitrogen species and a small amount of Fe atoms. Electrochemical tests demonstrate that the Fe/nitrogen-doped carbon nanospheres with more graphitic nitrogen exhibit much higher activity for the oxygen evolution reaction than those with more pyridinic nitrogens and pyrrolic nitrogens in alkaline media, revealing that the graphitic nitrogen is the active species that greatly improves the activity of Fe catalysts. Density functional theory calculations further reveal that the graphitic nitrogen enhances the activity and stability of Fe-based catalysts mainly through increasing the adsorption energy, charge and spin densities of the Fe atoms loaded around it. These findings provide a brand-new perspective for rationally designing more effective transition metal-based electrocatalysts for the oxygen evolution reaction through controlling the active graphitic nitrogen distribution in carbon carriers.

Experiments and DFT calculations demonstrate that graphitic N is the active species which improves the OER activity of Fe catalyst.     

Electrochemical deposition of amorphous cobalt oxides for oxygen evolution catalysis

The oxygen evolution reaction (OER) is crucial in water splitting for hydrogen production. However, its high over-potential and sluggish kinetics cause an additional energy loss and hinder its practical application. The cobalt spinel oxide Co₃O₄ exhibits a high catalytic activity for the OER in alkaline solutions. However, the activity requires further enhancement to meet the industrial demand for hydrogen production. This paper presents an electrochemical deposition method to obtain cobalt oxides with a controllable crystallinity on carbon paper (CP). Usually, cobalt oxides grown on CP have a Co₃O₄ spinel oxide structure. The self-supported Co₃O₄/CP exhibited a considerable catalytic activity for the OER. When a VS₂ layer grown on the CP beforehand by a hydrothermal method was used as substrate, the deposited cobalt oxides were in an amorphous state, denoted as CoO_x/VS₂/CP, which exhibited a higher OER activity and better stability than those of Co₃O₄/CP. The enhancement in the catalytic activity was attributed to the mixture formation of different types of cobalt species, including Co₃O₄, CoO, Co(OH)₂, and metallic Co, because of the reduction by VS₂. We also clarify the significance of the crystallinity of cobalt oxides in the improvement in the OER activity. This process can also be applied to the direct formation of other types of self-supported oxide electrodes for OER catalysis.

Amorphous cobalt oxides electrodeposited on VS₂ grown on carbon paper show better catalytic activity for the oxygen evolution reaction than crystalline Co₃O₄ on carbon paper.     

Ni stabilized rock-salt structured CoO; Co1−xNixO: tuning of eg electrons to develop a novel OER catalyst

The oxygen evolution reaction (OER) is a key half-reaction in hydrogen–oxygen electrolysers that is very important for efficient electrochemical energy generation, storage and fuel production that offers a clean alternative to fissile fuel combustion based energy systems. Several transition metal containing perovskites were recently explored for the development of superior OER catalysts, and their activity was correlated with the applied potentials at a specific current density to e_g electron density present in the materials. The rock salt structure is envisaged here as a model host structure similar to perovskite to tune the e_g electrons to obtain superior electro-catalytic activity. Incorporation of Ni into CoO lattices helps to stabilize the rock salt structure and modulate the e_g electrons to develop superior OER and ORR electrocatalysts. Nickel doped rock salt structured CoO, Ni_xCo_1−xO (0 ≤ x ≤ 0.5), were synthesized by employing a solid state metathesis synthesis route. The compounds were characterised by powder X-ray diffraction (XRD), TGA, FT-IR and X-ray Photoelectron Spectroscopy (XPS). Ni_0.3Co_0.7O with 1.3 e_g electrons showed superior electrocatalytic activity for the oxygen evolution reaction. The overpotential for the Ni_0.3Co_0.7O sample was also found to be ∼0.450 V for 1 M and about ∼0.389 V at 5 M concentration of the KOH electrolyte.

Incorporation of Ni into CoO lattices helps to stabilize the rock salt structure and modulate the e_g electrons to develop superior OER and ORR electrocatalysts.     

One-step conversion of tannic acid-modified ZIF-67 into oxygen defect hollow Co3O4/nitrogen-doped carbon for efficient electrocatalytic oxygen evolution

Controllable structure and defect design are considered as efficient strategies to boost the electrochemical activity and stability of catalysts for the oxygen evolution reaction (OER). Herein, oxygen defect hollow Co₃O₄/nitrogen-doped carbon (O_V-HCo₃O₄@NC) composites were successfully synthesized using tannic acid-modified ZIF-67 (TAMZIF-67) as the precursor through a one-step pyrolysis. Tannic acid provides abundant oxygen during the pyrolysis process of the modified ZIF-67, which can contribute to the formation of oxygen defects and the construction of a hollow structure. The existence of oxygen defects is shown by X-ray photoelectron spectroscopy and electron paramagnetic resonance, whereas the hollow structure is confirmed by transmission electron microscopy. The optimized O_V-HCo₃O₄@NC shows good electrocatalytic activity and exhibits a low overpotential of 360 mV at a current density of 10 mA cm⁻² in 0.1 M KOH due to the hollow structure, abundant oxygen defects, and good electrical conductivity. This work provides valuable insights into the exploration of promising OER electrocatalysts with oxygen defects and special structures.

Oxygen defect hollow Co₃O₄/nitrogen-doped carbon (O_V-HCo₃O₄@NC) nanocomposites were successfully synthesized by simple one-step pyrolysis of tannic acid-modified ZIF-67 (TAMZIF-67). O_V-HCo₃O₄@NC shows good OER electrocatalytic activity and stability.     

Modifying Y zeolite with chloropropyl for improving Cu load on Y zeolite as a super Cu/Y catalyst for toluene oxidation

Developing an efficient catalyst is desirable when for example moving from a noble metal-based catalyst to a transition metal-based one for VOC removal. In this work, the chloropropyl-modified NaY zeolite (NaY-CPT) was first synthesized in an extremely dense system through introducing 3-chloropropyl-trimethoxysilane (CPT) in the aluminosilicate sol. Then the Cu/Y-CPT catalyst was fabricated by impregnating Cu species on the NaY-CPT zeolite and the highly effective Cu/Y based catalyst has been achieved for catalytic toluene oxidation. The structure evolution of CPT modified sol and its effect on texture properties of NaY-CPT and thereby reduction ability of Cu/Y catalyst were systematically investigated by synchrotron radiation small angle X-ray scattering (SR-SAXS), EXAFS and other characterization. The CPT modified sol can promote the formation of more active aluminosilicate species, greatly accelerating crystal growth and improving framework Si/Al ratio of NaY zeolite. Due to the presence of the CPT group, the Cu/Y-CPT catalyst enhanced the interaction between Cu species and the zeolite matrix, resulting in small sized CuO nanoparticles (2.0–4.0 nm) anchoring to NaY-CPT. The Cu/Y-CPT catalyst renders more isolated Cu²⁺ species and adsorbed oxygen species, which are reactive in the oxidation reaction due to their high reducibility and mobility. Finally, the Cu/Y-CPT catalyst exhibits 90% toluene conversion at 296 °C (T₉₀), lower than the value of 375 °C on the conventional Cu/Y-con catalyst. Meanwhile, the optimal Cu/Y-CPT catalyst also gives higher toluene conversion and stability in moisture conditions.

The specific Cu/Y-CPT catalysts having high CuO dispersibility can expose more active sites and exhibit remarkable toluene oxidation activity.     

A simple method for the preparation of a nickel selenide and cobalt selenide mixed catalyst to enhance bifunctional oxygen activity for Zn–air batteries

Developing a low-cost, simple, and efficient method to prepare excellent bifunctional electrocatalysts toward the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is critical in rechargeable zinc–air batteries. Non-stoichiometric M_0.85Se (M = Ni or Co) nanoparticles are synthesized and modified on nitrogen-doped hollow carbon sphere (NHCS). The NHCS loaded Ni_0.85Se (Ni_0.85Se-NHCS) with rich Ni³⁺ presents higher OER activity, whereas the NHCS-loaded Co_0.85Se (Co_0.85Se-NHCS) with abundant Co²⁺ displays better ORR activity, respectively. When Co_0.85Se-NHCS is mixed with Ni_0.85Se-NHCS in a mass ratio of 1 : 1, the resulting mixture (Ni_0.85Se/Co_0.85Se-NHCS-2) shows better ORR and OER dual catalytic functions than a single selenide. Moreover, zinc–air batteries equipped with Ni_0.85Se/Co_0.85Se-NHCS-2 as the oxygen electrode catalyst exhibit excellent charge and discharge performance as well as improved stability over precious metals. This work has developed a simple and effective method to prepare excellent bifunctional electrocatalysts for ORR and OER, which is beneficial for the practical large-scale application of zinc–air batteries.

The mixture Ni_0.85Se/Co_0.85Se-NHCS-2 displayed superior electrocatalytic performance to that of Ni_0.85Se-NHCS or Co_0.85Se-NHCS alone. This provided a simple approach to develop ORR/OER bifunctional electrocatalysts for zinc–air batteries.     

Co9S8 nanoparticle-decorated carbon nanofibers as high-performance supercapacitor electrodes

This work reported Co₉S₈ nanoparticle-decorated carbon nanofibers (CNF) as a supercapacitor electrode. By using a mild ion-exchange method, the cobalt oxide-based precursor nanoparticles were transformed to Co₉S₈ nanoparticles in a microwave hydrothermal process, and these nanoparticles were decorated onto a carbon nanofiber backbone. The composition of the nanofibers can be readily tuned by varying the Co acetate content in the precursor. The porous carbon nanofibers offered a fast electron transfer pathway while the well dispersed Co₉S₈ nanoparticles acted as the redox center for energy storage. As a result, high specific capacitance of 718 F g⁻¹ at 1 A g⁻¹ can be achieved with optimized Co₉S₈ loading. The assembled asymmetric supercapacitor with Co₉S₈/CNF as the cathode showed a high energy density of 23.8 W h kg⁻¹ at a power density of 0.75 kW kg⁻¹ and good cycling stability (16.9% loss over 10 000 cycles).

Through electrospinning and the ion-exchange method, Co₉S₈ nanoparticle-decorated carbon nanofibers (Co₉S₈/CNF) have been fabricated, and exhibit good supercapacitor performance.     

Nanostructured carbons containing FeNi/NiFe2O4 supported over N-doped carbon nanofibers for oxygen reduction and evolution reactions

Non-precious metal-based electrocatalysts on carbon materials with high durability and low cost have been developed to ameliorate the oxygen-reduction reaction (ORR) and oxygen-evolution reaction (OER) for electrochemical energy applications such as in fuel cells and water electrolysis. Herein, two different morphologies of FeNi/NiFe₂O₄ supported over hierarchical N-doped carbons were achieved via carbonization of the polymer nanofibers by controlling the ratio of metal salts to melamine: a mixture of carbon nanotubes (CNTs) and graphene nanotubes (GNTs) supported over carbon nanofibers (CNFs) with spherical FeNi encapsulated at the tips (G/CNT@NCNF, 1 : 3), and graphene sheets wrapped CNFs with embedded needle-like FeNi (GS@NCNF, 2 : 3). G/CNT@NCNF shows excellent ORR activity (on-set potential: 0.948 V vs. RHE) and methanol tolerance, whilst GS@NCNF exhibited significantly lower over-potential of only 230 mV at 10 mA cm⁻² for OER. Such high activities are due to the synergistic effects of bimetallic NPs encapsulated at CNT tips and N-doped carbons with unique hierarchical structures and the desired defects.

Non-precious metal-based electrocatalysts on carbon materials with high durability and low cost have been developed to ameliorate the oxygen-reduction reaction (ORR) and oxygen-evolution reaction (OER).     

Tuning composition of CuCo2S4–NiCo2S4 solid solutions via solvent-less pyrolysis of molecular precursors for efficient supercapacitance and water splitting

Mixed metal sulfides are increasingly being investigated because of their prospective applications for electrochemical energy storage and conversion. Their high electronic conductivity and high density of redox sites result in significant improvement of their electrochemical properties. Herein, the composition-dependent supercapacitive and water splitting performance of a series of Ni_(1−x)Cu_xCo₂S₄ (0.2 ≤ x ≤ 0.8) solid solutions prepared via solvent-less pyrolysis of a mixture of respective metal ethyl xanthate precursors is reported. The use of xanthate precursors resulted in the formation of surface clean nanomaterials at low-temperature. Their structural, compositional, and morphological features were examined by p-XRD, SEM, and EDX analyses. Both supercapacitive and electrocatalytic (HER, OER) properties of the synthesized materials significantly vary with composition (Ni/Cu molar content). However, the optimal composition depends on the application. The highest specific capacitance of 770 F g⁻¹ at a current density of 1 A g⁻¹ was achieved for Ni_0.6Cu_0.4Co₂S₄ (NCCS-2). This electrode exhibits capacitance retention (C_R) of 67% at 30 A g⁻¹, which is higher than that observed for pristine NiCo₂S₄ (838 F g⁻¹ at 1 A g⁻¹, 47% C_R at 30 A g⁻¹). On the contrary, Ni_0.4Cu_0.6Co₂S₄ (NCCS-3) exhibits the lowest overpotential of 124 mV to deliver a current density of 10 mA cm⁻². Finally, the best OER activity with an overpotential of 268 mV at 10 mA cm⁻² was displayed by Ni_0.8Cu_0.2Co₂S₄ (NCCS-1). The prepared electrodes exhibit high stability, as well as durability.

A multi-component CuCo₂S₄ and NiCo₂S₄ thiospinel solid solution is prepared over an entire range by a low-temperature solvent-less route. The synergistic effect from both thiospinels on water splitting and capacitance is studied.     

Nanostructured RuO2–Co3O4@RuCo-EO with low Ru loading as a high-efficiency electrochemical oxygen evolution catalyst

Electrochemical water splitting technology is considered to be the most reliable method for converting renewable energy such as wind and solar energy into hydrogen. Here, a nanostructured RuO₂/Co₃O₄–RuCo-EO electrode is designed via magnetron sputtering combined with electrochemical oxidation for the oxygen evolution reaction (OER) in an alkaline medium. The optimized RuO₂/Co₃O₄–RuCo-EO electrode with a Ru loading of 0.064 mg cm⁻² exhibits excellent electrocatalytic performance with a low overpotential of 220 mV at the current density of 10 mA cm⁻² and a low Tafel slope of 59.9 mV dec⁻¹ for the OER. Compared with RuO₂ prepared by thermal decomposition, its overpotential is reduced by 82 mV. Meanwhile, compared with RuO₂ prepared by magnetron sputtering, the overpotential is also reduced by 74 mV. Furthermore, compared with the RuO₂/Ru with core–shell structure (η = 244 mV), the overpotential is still decreased by 24 mV. Therefore, the RuO₂/Co₃O₄–RuCo-EO electrode has excellent OER activity. There are two reasons for the improvement of the OER activity. On the one hand, the core–shell structure is conducive to electron transport, and on the other hand, the addition of Co adjusts the electronic structure of Ru.

The optimized RuO₂/Co₃O₄–RuCo-EO electrode with Ru loading of 0.064 mg cm⁻² exhibits the excellent oxygen evolution activity with an overpotential of 220 mV at the current density of 10 mA cm⁻² and a Tafel slope of 59.9 mV dec⁻¹.     

Ultrahigh oxygen evolution reaction activity in Au doped co-based nanosheets

Oxygen evolution reaction (OER) has attracted enormous interest as a key process for water electrolysis over the past years. The advance of this process relies on an effective catalyst. Herein, we employed single-atom Au doped Co-based nanosheets (NSs) to theoretically and experimentally evaluate the OER activity and also the interaction between Co and Au. We reveal that Au–Co(OH)₂ NSs achieved a low overpotential of 0.26 V at 10 mA cm⁻². This extraordinary phenomenon presents an overall superior performance greater than state-of-the-art Co-based catalysts in a sequence of α-Co(OH)₂ < Co₃O₄ < CoOOH < Au–Co(OH)₂. With ab initio calculations and analysis in the specific Au–Co(OH)₂ configuration, we reveal that OER on highly active Au–Co(OH)₂ originates from lattice oxygen, which is different from the conventional adsorbate evolution scheme. Explicitly, the configuration of Au–Co(OH)₂ gives rise to oxygen non-bonding (O_NB) states and oxygen holes, allowing direct O–O bond formation by a couple of oxidized oxygen with oxygen holes, offering a high OER activity. This study provides new insights for elucidating the origins of activity and synthesizing efficient OER electrocatalysts.

Manipulating Co oxidation by loading Au atoms can substantially enhance the OER activity of those activated Co ions.     

Co/N-Doped hierarchical porous carbon as an efficient oxygen electrocatalyst for rechargeable Zn–air battery

Developing efficient electrocatalysts for ORR/OER is the key issue for the large-scale application of rechargeable Zn–air batteries. The design of Co and N co-doped carbon matrices has become a promising strategy for the fabrication of bi-functional electrocatalysts. Herein, the surface-oxidized Co nanoparticles (NPs) encapsulated into N-doped hierarchically porous carbon materials (Co/NHPC) are designed as ORR/OER catalysts for rechargeable Zn–air batteries via dual-templating strategy and pyrolysis process containing Co²⁺. The fabricated electrocatalyst displays a core–shell structure with the surface-oxidized Co nanoparticles anchored on hierarchically porous carbon sheets. The carbon shells prevent Co NP cores from aggregating, ensuring excellent electrocatalytic properties for ORR with a half-wave potential of 0.82 V and a moderate OER performance. Notably, the obtained Co/NHPC as a cathode was further assembled in a zinc–air battery that delivered an open-circuit potential of 1.50 V, even superior to that of Pt/C (1.46 V vs. RHE), a low charge–discharge voltage gap, and long cycle life. All these results demonstrate that this study provides a simple, scalable, and efficient approach to fabricate cost-effective high-performance ORR/OER catalysts for rechargeable Zn–air batteries.

The surface-oxidized Co nanoparticles incorporated in N-doped hierarchically porous carbon materials are designed as ORR/OER catalyst for rechargeable Zn–air batteries via dual-templating strategy and pyrolysis process.     

Au@Co2P core/shell nanoparticles as a nano-electrocatalyst for enhancing the oxygen evolution reaction

Core/shell nanoparticles (NPs) of Au@Co₂P, each comprising a Au core with a Co₂P shell, were prepared, and shown to efficiently catalyze the oxygen evolution reaction (OER). In particular, Au@Co₂P has a small overpotential of 321 mV at 10 mA cm⁻² in 1 M KOH aqueous solution at room temperature, which is about 95 mV less than pure Co₂P. More importantly, the Tafel slope of Au@Co₂P, at 57 mV dec⁻¹, is 44 mV dec⁻¹ lower than that of Co₂P. Hence, Au@Co₂P outperforms Co₂P drastically in practical production when a high current density is required.

Au@Co₂P core/shell nanoparticles were designed and prepared to improve the oxygen evolution reaction performance.