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

CuCo2S4 nanoparticles synthesized via a thermal decomposition approach: evaluation of their potential as peroxidase mimics

The current study demonstrates the synthesis of CuCo2S4 nanoparticles using a novel thermal decomposition approach. The CuCo2S4 nanoparticles were synthesized under various conditions by changing the source of sulfur and the solvent. The CuCo2S4 nanoparticles were characterized using an array of analytical techniques. Powder XRD results indicate the successful formation of CuCo2S4 nanoparticles. TEM results show agglomerated nanoparticles with close to spherical morphology and XPS measurements indicate the presence of Cu2+, Cu+, Co3+, Co2+, and S2- in the samples. The CuCo2S4 nanoparticles exhibit weak ferromagnetic and paramagnetic behaviour at 5 K and 300 K, respectively. The CuCo2S4 nanoparticles were explored for their enzyme mimetic activity using 3,3',5,5' tetramethylbenzidine (TMB) as a substrate. They exhibit better catalytic activity compared to that of a natural enzyme (horseradish peroxidase) and other metal sulfide nanoparticles reported in the literature.

Design Principles and Mechanistic Understandings of Non-Noble-Metal Bifunctional Electrocatalysts for Zinc-Air Batteries

Zinc-air batteries (ZABs) are promising energy storage systems because of high theoretical energy density, safety, low cost, and abundance of zinc. However, the slow multi-step reaction of oxygen and heavy reliance on noble-metal catalysts hinder the practical applications of ZABs. Therefore, feasible and advanced non-noble-metal electrocatalysts for air cathodes need to be identified to promote the oxygen catalytic reaction. In this review, we initially introduced the advancement of ZABs in the past two decades and provided an overview of key developments in this field. Then, we discussed the working mechanism and the design of bifunctional electrocatalysts from the perspective of morphology design, crystal structure tuning, interface strategy, and atomic engineering. We also included theoretical studies, machine learning, and advanced characterization technologies to provide a comprehensive understanding of the structure-performance relationship of electrocatalysts and the reaction pathways of the oxygen redox reactions. Finally, we discussed the challenges and prospects related to designing advanced non-noble-metal bifunctional electrocatalysts for ZABs.

Emerging 2D Copper-Based Materials for Energy Storage and Conversion: A Review and Perspective

2D materials have shown great potential as electrode materials that determine the performance of a range of electrochemical energy technologies. Among these, 2D copper-based materials, such as Cu-O, Cu-S, Cu-Se, Cu-N, and Cu-P, have attracted tremendous research interest, because of the combination of remarkable properties, such as low cost, excellent chemical stability, facile fabrication, and significant electrochemical properties. Herein, the recent advances in the emerging 2D copper-based materials are summarized. A brief summary of the crystal structures and synthetic methods is started, and innovative strategies for improving electrochemical performances of 2D copper-based materials are described in detail through defect engineering, heterostructure construction, and surface functionalization. Furthermore, their state-of-the-art applications in electrochemical energy storage including supercapacitors (SCs), alkali (Li, Na, and K)-ion batteries, multivalent metal (Mg and Al)-ion batteries, and hybrid Mg/Li-ion batteries are described. In addition, the electrocatalysis applications of 2D copper-based materials in metal-air batteries, water-splitting, and CO₂ reduction reaction (CO₂ RR) are also discussed. This review also discusses the charge storage mechanisms of 2D copper-based materials by various advanced characterization techniques. The review with a perspective of the current challenges and research outlook of such 2D copper-based materials for high-performance energy storage and conversion applications is concluded.

CuCo2S4@B,N-Doped Reduced Graphene Oxide Hybrid as a Bifunctional Electrocatalyst for Oxygen Reduction and Evolution Reactions

In this report, a facile synthetic route is adopted for typically designing a hybrid electrocatalyst containing boron, nitrogen dual-doped reduced graphene oxide (B,N-rGO) and thiospinel CuCo₂S₄ (CuCo₂S₄@B,N-rGO). The electrocatalytic activity of the hybrid catalyst is tested with respect to oxygen evolution (OER) and oxygen reduction (ORR) reactions in alkali. Physicochemical characterizations confirm the unique formation of a reduced graphene oxide-non-noble-metal sulfide hybrid. Electrochemical evaluation by cyclic voltammetry (CV) and linear-sweep voltammetry (LSV) reveals that the CuCo₂S₄@B,N-rGO hybrid possesses enhanced ORR and OER activity compared to the B,N-rGO-free CuCo₂S₄ catalyst. The synthesized CuCo₂S₄@B,N-rGO hybrid demonstrates remarkable enhancement in catalytic performance with an improved onset potential (1.50 and 0.88 V) and low Tafel slope (112 and 73 mV dec^-1) for both OER and ORR processes, respectively. In addition, the catalyst exhibits a diminutive potential difference (0.81 V) between the potential corresponding to the 10 mA cm^-2 current density for OER and the half-wave potential for ORR. The superior catalytic activity and high durability of the hybrid material may be attributed to the synergistic effect arising from the metal sulfide and dual-doped reduced graphene oxide. The present study illuminates the possibility of using the dual-doped graphene oxide and metal sulfide hybrid as a competent bifunctional cathode catalyst for renewable energy application.

Realizing Superior Redox Kinetics of Hollow Bimetallic Sulfide Nanoarchitectures by Defect-Induced Manipulation toward Flexible Solid-State Supercapacitors

As a typical battery-type material, CuCo₂ S₄ is a promising candidate for supercapacitors due to the high theoretical specific capacity. However, its practical application is plagued by inherently sluggish ion diffusion kinetics and inferior electrical transport properties. Herein, sulfur vacancies are incorporated in CuCo₂ S₄ hollow nanoarchitectures (HNs) to accelerate redox reactivity. Experimental analyses and theoretical investigations uncover that the generated sulfur vacancies increase the active electron states, reduce the adsorption barriers of electrolyte ions, and enrich reactive redox species, thus achieving enhanced electrochemical performance. Consequently, the deficient CuCo₂ S₄ with optimized vacancy concentration presents a high specific capacity of 231 mAh g^-1 at 1 A g^-1 , a ≈1.78 times increase compared to that of pristine CuCo₂ S₄ , and exhibits a superior rate capability (73.8% capacity retention at 20 A g^-1 ). Furthermore, flexible solid-state asymmetric supercapacitor devices assembled with the deficient CuCo₂ S₄ HNs and VN nanosheets deliver a high energy density of 61.4 W h kg^-1 at 750 W kg^-1 . Under different bending states, the devices display exceptional mechanical flexibility with no obvious change in CV curves at 50 mV s^-1 . These findings provide insights for regulating electrode reactivity of battery-type materials through intentional nanoarchitectonics and vacancy engineering.

Nitrogen doped CuCo2O4 nanoparticles anchored on beaded-like carbon nanofibers as an efficient bifunctional oxygen catalyst toward zinc-air battery

The elaborative design and construction of first-rank bifunctional oxygen electrocatalysts featuring low price, high activity and strong stability is critical for the large-scale applications of rechargeable Zn-air batteries. Here, a resultful strategy is proposed for fabricating nitrogen-doped 1D beaded-like structure carbon nanofibers uniformly decorated with nitrogen-doped CuCo₂O₄ nanoparticles (N-CuCo₂O₄@CNFs) toward boosting oxygen evolution reaction/oxygen reduction reaction (OER/ORR) catalysis. Taking advantage of the synergistic effect between interconnected 1D hierarchical porous carbon nanofiber structure and high catalytic activity of N-doped CuCo₂O₄ nanoparticles derived from bimetallic MOFs, the N-CuCo₂O₄@CNFs catalysts possess enhanced reaction kinetics and preferable charge transfer ability. Impressively, the obtained catalysts exhibit prominent electrocatalytic ability and superior stability for OER/ORR, even surpass the commercial RuO₂ and Pt/C. More significantly, the Zn-air batteries employing the N-CuCo₂O₄@CNFs-800 as cathode display a higher power density of 175.6 mW cm^-2, a lower charge-discharge voltage gap of 0.82 V at 10 mA cm^-2, as well as a better cycling stability with respect to those of Pt/C + RuO₂ mixture, demonstrating the great potential of N-CuCo₂O₄@CNF as a high-efficiency catalyst for clean energy devices.

Chlorine-assisted synthesis of CuCo2S4@(Cu,Co)2Cl(OH)3 heterostructures with an efficient nanointerface for electrocatalytic oxygen evolution

The demand for sustainable energy sources urges the development of efficient and earth-abundant electrocatalysts. Herein, chlorine assisted ion-exchange and in-situ sulfurization processes were combined to construct CuCo₂S₄@(Cu,Co)₂Cl(OH)₃ heterostructures from Cu(OH)₂ nanoarrays. Chlorine element in the cobalt source stimulated the formation of (Cu,Co)₂Cl(OH)₃ precursor, and further facilitated partial transformation of the precursor to CuCo₂S₄ on the surface to achieve composite structure. The mixed valences of Co element (Co³⁺ in CuCo₂S₄ and Co²⁺ in (Cu,Co)₂Cl(OH)₃) and OS interpenetrated nanointerface in the composite catalysts provided low electron transfer resistance for good alkaline oxygen evolution reaction (OER) activities. In 1 mol L^-1 KOH electrolyte, the overpotentials of the optimal composite catalyst reached 253 and 290 mV respectively at the current density of 20 and 50 mA cm^-2, which is comparable to the activity of commercial Ir/C (281 mV@20 mA cm^-2). These findings could provide opportunities for designing effective and inexpensive composite electrocatalysts through nanointerface engineering strategy.

Origin of the electrocatalytic oxygen evolution activity of nickel phosphides: in-situ electrochemical oxidation and Cr doping to achieve high performance

Zinc-air batteries (ZnABs) with high theoretical capacity and environmental benignity are the most promising candidates for next-generation electronics. However, their large-scale applications are greatly hindered due to the lack of high-efficient and cost-effective electrocatalysts. Transition metal phosphides (TMPs) have been reported as promising electrocatalysts. Notably, (Ni_1-xCr_x)₂P (0 ≤ x ≤ 0.15) is an unstable electrocatalyst, which undergoes in-situ electrochemical oxidation during the initial oxygen evolution reaction (OER) and even in the activation cycles, and is eventually converted to Cr-NiOOH serving as the actual OER active sites with high efficiency. Density functional theory (DFT) simulations and experimental results elucidate that the OER performance could be significantly promoted by the synergistic effect of surface engineering and electronic modulations by Cr doping and in-situ phase transformation. The constructed rechargeable ZnABs could stably cycle for more than 208 h at 5 mA cm^-2, while the voltage degradation is negligible. Furthermore, the developed catalytic materials could be assembled into flexible and all-solid-state ZnABs to power wearable electronics with high performance.

Recent advances of noble-metal-free bifunctional oxygen reduction and evolution electrocatalysts

Bifunctional oxygen reduction and evolution constitute the core processes for sustainable energy storage. The advances on noble-metal-free bifunctional oxygen electrocatalysts are reviewed.

Cobalt-based metal–organic frameworks as functional materials for battery applications

Research progress on cobalt-based metal–organic frameworks as functional materials for battery applications has been presented.

Engineering of Amorphous Structures and Sulfur Defects into Ultrathin FeS Nanosheets to Achieve Superior Electrocatalytic Alkaline Oxygen Evolution

Integration of amorphous structures and anion defects into ultrathin 2D materials has been identified as an effective strategy for boosting the electrocatalytic performance. However, the in-depth understanding of the relationship among the amorphous structure, vacancy defect, and catalytic activity is still obscure. Herein, a facile strategy was proposed to prepare ultrathin and amorphous Mo-FeS nanosheets (NSs) with abundant sulfur defects. Benefited from the ultrathin, amorphous nanostructure, and synergy effect of Mo-doping and sulfur defect, the Mo-FeS NSs manifested excellent electrocatalytic activity toward oxygen evolution reaction (OER) in alkaline medium, as shown by an ultralow overpotential of 210 mV at 10 mA cm^-2, a Tafel slope of 50 mV dec^-1, and retaining such good catalytic stability over 30 h. The efficient catalytic performance for Mo-FeS NSs is superior to the commercial IrO₂ and most reported top-performing electrocatalysts. Density functional theory calculations revealed that the accelerated electron/mass transfer over the oxygen-containing intermediates can be attributed to the amorphous structure and sulfur-rich defects caused by structural reconfiguration. Furthermore, the S vacancies could enhance the activity of its neighboring Fe-active sites, which was also beneficial to their OER kinetics. This work integrated both amorphous structures and sulfur vacancies into ultrathin 2D NSs and further systematically evaluated the OER performance, providing new insights for the design of amorphous-layered electrocatalysts.

Single-unit-cell-thick layered electrocatalysts: from synthesis to application

Electrocatalysts are critical for water splitting, carbon dioxide reduction, and zinc-air battery. However, the low-exposed surface areas of bulk electrocatalysts usually limit the complete utilization of active sites. Ultrathin electrocatalysts have noteworthy advantages in maximizing the use of active sites. Among the pioneering works on such performing catalysts, the development of single-unit-cell-thick layered electrocatalysts (STLEs) has attracted extensive attention owing to their superior specific surface area and large number of vacancies, which can provide abundant available surface active sites. Therefore, this minireview provides recent advances in STLE synthesis and applications, which are helpful for electrocatalysis-oriented researchers. Finally, the future perspectives and challenges for developing high-performance STLEs are proposed.

Recent advances in the template-confined synthesis of two-dimensional materials for aqueous energy storage devices

The template-confined synthesis strategy is a simple and effective methodology to prepare two-dimensional nanomaterials. It has multiple advantages including green process, controllable morphology and adjustable crystal structure, and therefore, it is promising in the energy storage realm to synthesize high-performance electrode materials. In this review, we summarize the recent advances in the template-confined synthesis of two-dimensional nanostructures for aqueous energy storage applications. The material design is discussed in detail to accommodate target usage in aqueous supercapacitors and zinc metal batteries. The remaining challenges and future prospective are also covered.

Optimized Metal Chalcogenides for Boosting Water Splitting

Electrocatalytic water splitting (2H2O → 2H2 + O2) is a very promising avenue to effectively and environmentally friendly produce highly pure hydrogen (H2) and oxygen (O2) at a large scale. Different materials have been developed to enhance the efficiency for water splitting. Among them, chalcogenides with unique atomic arrangement and high electronic transport show interesting catalytic properties in various electrochemical reactions, such as the hydrogen evolution reaction, oxygen evolution reaction, and overall water splitting, while the control of their morphology and structure is of vital importance to their catalytic performance. Herein, the general synthetic methods are summarized to prepare metal chalcogenides and different strategies are designed to improve their catalytic performance for water splitting. The remaining challenges in the research and development of metal chalcogenides and possible directions for future research are also summarized.

CuCo2S4 Nanosheets@N-Doped Carbon Nanofibers by Sulfurization at Room Temperature as Bifunctional Electrocatalysts in Flexible Quasi-Solid-State Zn-Air Batteries

The performance of quasi-solid-state flexible zinc-air batteries (ZABs) is critically dependent on the advancement of air electrodes with outstanding bifunctional electrocatalysis for both the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), together with the desired mechanical flexibility and robustness. The currently available synthesis processes for high-efficiency bifunctional bimetallic sulfide electrodes typically require high-temperature hydrothermal or chemical vapor deposition, which is undesirable in terms of the complexity in experimental procedure and the damage of flexibility in the resultant electrode. Herein, a scalable fabrication process is reported by combining electrospinning with in situ sulfurization at room temperature to successfully obtain CuCo2S4 nanosheets@N-doped carbon nanofiber (CuCo2S4 NSs@N-CNFs) films, which show remarkable bifunctional catalytic performance (Ej = 10 (OER) - E 1/2 (ORR) = 0.751 V) with excellent mechanical flexibility. Furthermore, the CuCo2S4 NSs@N-CNFs cathode delivers a high open-circuit potential of 1.46 V, an outstanding specific capacity of 896 mA h g-1, when assembled into a quasi-solid-state flexible ZAB together with Zn NSs@carbon nanotubes (CNTs) film (electrodeposited Zn nanosheets on CNTs film) as the anode. The ZAB also shows a good flexibility and capacity stability with 93.62% capacity retention (bending 1000 cycles from 0° to 180°), making it an excellent power source for portable and wearable electronic devices.

In situ confined vertical growth of a 1D-CuCo2S4 nanoarray on Ni foam covered by a 3D-PANI mesh layer to form a self-supporting hierarchical structure for high-efficiency oxygen evolution catalysis

Inspired by the patchwork of artificial turf, where planting in a smaller area can result in a more uniform lawn that grows in one direction, here, we defined the growth position and orientation of a CuCo₂S₄ nanoarray for the first time by electroplating a PANI mesh layer onto a Ni foam to obtain a self-supporting hierarchical electrode material. The nitrogen species derived from the PANI building blocks act as bridging sites to bind with metal ions, which provides a strong coupling effect for the in situ growth of CuCo₂S₄. At the same time, the mesh structure of PANI divides the growable location into smaller blocks. Compared with a mesh plane with uniformly distributed nitrogen sites, only a small portion of the nitrogen sites are located on the narrow-width fence structure, which may make it difficult for CuCo₂S₄ to grow onto the fence structure, thereby limiting the self-growth space and confining CuCo₂S₄. The uniformly distributed growth sites direct CuCo₂S₄ to grow perpendicular to the plane while limiting their growth size. The excellent structural features further enhance the electrochemical oxygen evolution activity, and the oxygen evolution overpotential at a current density of 100 mA cm^-2 is only 291 mV, which is superior to that of the currently known cobalt-copper-based catalyst materials. In addition, the stable structure provides excellent electrode cyclic stability. The preparation of hierarchical self-supporting cobalt-copper bimetallic sulfide nanoarrays provided a reference direction for other transition metal catalytic materials and provided a basis for industrial applications.

Bimetallic Nickel Cobalt Sulfide as Efficient Electrocatalyst for Zn-Air Battery and Water Splitting

The development of efficient earth-abundant electrocatalysts for oxygen reduction, oxygen evolution, and hydrogen evolution reactions (ORR, OER, and HER) is important for future energy conversion and energy storage devices, for which both rechargeable Zn-air batteries and water splitting have raised great expectations. Herein, we report a single-phase bimetallic nickel cobalt sulfide ((Ni,Co)S₂) as an efficient electrocatalyst for both OER and ORR. Owing to the synergistic combination of Ni and Co, the (Ni,Co)S₂ exhibits superior electrocatalytic performance for ORR, OER, and HER in an alkaline electrolyte, and the first principle calculation results indicate that the reaction of an adsorbed O atom with a H₂O molecule to form a *OOH is the potential limiting step in the OER. Importantly, it could be utilized as an advanced air electrode material in Zn-air batteries, which shows an enhanced charge-discharge performance (charging voltage of 1.71 V and discharge voltage of 1.26 V at 2 mA cm^-2), large specific capacity (842 mAh g_Zn^-1 at 5 mA cm^-2), and excellent cycling stability (480 h). Interestingly, the (Ni,Co)S₂-based Zn-air battery can efficiently power an electrochemical water-splitting unit with (Ni,Co)S₂ serving as both the electrodes. This reveals that the prepared (Ni,Co)S₂ has promising applications in future energy conversion and energy storage devices.

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.

High Areal Capacity Li-Ion Storage of Binder-Free Metal Vanadate/Carbon Hybrid Anode by Ion-Exchange Reaction

Storing more energy in a limited device area is very challenging but crucial for the applications of flexible and wearable electronics. Metal vanadates have been regarded as a fascinating group of materials in many areas, especially in lithium-ion storage. However, there has not been a versatile strategy to synthesize flexible metal vanadate hybrid nanostructures as binder-free anodes for Li-ion batteries so far. A convenient and versatile synthesis of M_x V_y O_x+2.5y @carbon cloth (M = Mn, Co, Ni, Cu) composites is proposed here based on a two-step hydrothermal route. As-synthesized products demonstrate hierarchical proliferous structure, ranging from nanoparticles (0D), and nanobelts (1D) to a 3D interconnected network. The metal vanadate/carbon hybrid nanostructures exhibit excellent lithium storage capability, with a high areal specific capacity up to 5.9 mAh cm^-2 (which equals to 1676.8 mAh g^-1 ) at a current density of 200 mA g^-1 . Moreover, the nature of good flexibility, mixed valence states, and ultrahigh mass loading density (over 3.5 mg cm^-2 ) all guarantee their great potential in compact energy storage for future wearable devices and other related applications.