Co-based MOF-derived Co/CoN/Co2P ternary composite embedded in N- and P-doped carbon as bifunctional nanocatalysts for efficient overall water splitting

In this work, we developed ternary metallic cobalt-cobalt nitride-dicobalt phosphide composite embedded in nitrogen and phosphorus co-doped carbon (Co/CoN/Co₂P-NPC) as bifunctional catalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). The as-prepared Co/CoN/Co₂P-NPC is achieved by simultaneous annealing and phosphating of a Co–N rich metal-organic frameworks (MOFs) precursor. Compare with the phosphorus-free Co/CoN embedded nitrogen-doped carbon electrocatalyst (Co/CoN-NC), the as-prepared Co/CoN/Co₂P-NPC display superior HER and OER low overpotential of 99 mV and 272 mV at current density of 10 mA cm⁻². When Co/CoN/Co₂P-NPC electrocatalyst is use as bifunctional catalysts in overall alkaline water splitting, it exhibit excellent behaviour with 10 mA cm⁻² current at overall cell potential of 1.60 V. The excellent performance of Co/CoN/Co₂P-NPC electrocatalyst is attributed to the phosphating process that could further enhance synergistic effect, create stronger electronic interactions, and form efficient dual heteroatom doping to optimize the interfacial adhesion within the electrocatalyst. This present work will create more opportunities for the development of new, promising and more active sites electrocatalysts for alkaline electrolysis.     

Co3O4 nanoparticles decorated Polypyrrole/carbon nanocomposite as efficient bi-functional electrocatalyst for electrochemical water splitting

An effective bi-functional electrocatalyst of Co₃O₄/Polypyrrole/Carbon (Co₃O₄/Ppy/C) nanocomposite was prepared through a simple dry chemical method and used to catalyze the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Three types of carbon support as Vulcan carbon, reduced graphite oxide (RGO) and multi-walled carbon nanotubes (MCNTs) were used to study the influence on electrochemical reactions. Spherical shaped Co₃O₄ nanoparticles with 8–10 nm was found uniformly distributed on Ppy/C composite, which were analyzed by X-ray diffraction and transmission electron microscopy techniques. Amongst, Co₃O₄/Ppy/MWCNT shows improved bifunctional electrocatalytic activity towards both OER and HER with relatively low over potential (340 mV vs. 490 mV at 10 mA cm⁻²) and Tafel slope (87 vs. 110 mV dec⁻¹). In addition to that, MWCNT supported Co₃O₄/Ppy nanocomposite exhibits good electronic conductivity and electrochemical stability up to 2000 potential cycles. The results clearly indicate that the Co₃O₄/Ppy/MWCNT nanocomposite could be the promising bi-functional electrocatalyst for efficient water electrolysis.     

Mo/P doped NiFeSe as bifunctional electrocatalysts for overall water splitting

Designing high-efficiency catalysts for overall water splitting is critical to reduce the cost of hydrogen fuel as a clean and renewable energy source in future society. In this work, a Mo-, P-codoped NiFeSe was successfully synthesized on nickel foam (NF) by one-step electrodeposition. Through the doping strategy, the conductivity can be well promoted, and the production of nanosheets on the catalyst surface and active phases during hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) provided much more active sites, which leaded to efficient HER/OER performances of as-synthesized Mo-, P-codoped NiFeSe catalysts, i.e., a low overpotential of 100 mV/200 mV at current density of 10 mA cm⁻² in 1.0 M KOH with stability of 95 h/60 h, respectively. It only required 1.53 V to deliver a current density of 10 mA cm⁻² in overall water splitting and maintained outstanding durability for 100 h. This work is beneficial to future design of high efficient and low-cost bifunctional catalysts for overall water splitting.     

Nanoengineered Zn-modified Nickel Sulfide (NiS) as a bifunctional electrocatalyst for overall water splitting

Developing an efficient and inexpensive electrocatalyst is of paramount importance for realizing the green hydrogen economy through electrocatalytic water splitting. Here, we demonstrated a facile large-scale, industrially viable binder-free synthesis of Zn-doped NiS electrocatalyst on bare nickel foam (NF) through a hydrothermal technique. The present catalyst, i.e., nickel sulfide (NiS) nanosheets on nickel foam with optimized doping of Zn atom (Zn–NiS-3), displays excellent catalytic efficacy for both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). It requires an overpotential of 320 mV for OER at a current density of 50 mA cm⁻² and an overpotential of 208 mV for HER at a current density of 10 mA cm⁻². The water electrolyser device having Zn–NiS-3 electrocatalyst as both cathode and anode show excellent performance, requiring a cell voltage of only 1.71 V to reach a current density of 10 mA cm⁻² in an alkaline media. The density functional theory (DFT) based calculations showed enhanced density of states near Fermi energy after Zn doping in NiS and attributed to the enhanced catalytic activities. Thus, the present study demonstrates that Zn–NiS-3@NF can be coined as a viable electrocatalyst for green hydrogen production.     

Hierarchical Co/MoO2@N-doped carbon nanosheets derived from waste lotus leaves for electrocatalytic water splitting

It is imperative for electrochemical water splitting to seek functional materials with high performance, competence, durability, economical, and eco-friendly. Herein, we develop a simplistic two-step annealing strategy to synthesize the hierarchical Co/MoO₂@nitrogen (N)-doped carbon nanosheets (CMO@NC) electrocatalysts derived from the low-cost and sustainable lotus leaves biomass for water-splitting. The optimum catalyst (CMO@NC/450) exhibits a notable low overpotential of 130 and 272 mV at a current density of 10 mA cm^?2 for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in 1.0 M KOH, owing to their unique surface features, large surface area, abundant meso/micropores, and high pyridinic N-contents. Also, the CMO@NC/450 catalyst-equipped with a two-electrode configuration exhibits notable water splitting activity only requires a cell-potential of 1.629 V@10 mA cm^?2 in 1.0 M KOH. The results reveal that hierarchical flower-like morphology increases the contact area, prevents aggregation, and enables massive active-sites for HER and OER. Additionally, the synergistic effects between Co/MoO₂ and the N-doped-carbon heterostructure enhance charge-delocalization, ultimately improving electrocatalytic performance and stability. This work is aimed to promote the exploration and design of suitable doping structures and compositions for the development of highly effective and sustainable biomass-derived catalysts in a wide-range of electrochemical applications.     

Petal-like NiCoP sheets on 3D nitrogen-doped carbon nanofiber network as a robust bifunctional electrocatalyst for water splitting

Searching high-active, stable and abundant bifunctional catalysts to replace noble metals for hydrogen and oxygen evolution reactions (HER and OER) is desired. Herein, petal-like NiCoP sheets were synthesized on carbon paper covered with a 3D nitrogen-doped carbon nanofiber network (NiCoP/CNNCP) by a simple hydrothermal process followed by phosphorization. The HER overpotential in 0.5 M H₂SO₄ and OER overpotential in 1 M KOH of the NiCoP/CNNCP electrode only required 55 mV and 260 mV to drive a current density of 10 mA cm^?2, respectively, which was comparable or even better than most nickel-and cobalt-based phosphide catalysts. The overall water-splitting electrolyzer with an asymmetric electrolyte system assembled using NiCoP/CNNCP as bifunctional electrodes required an extremely low cell voltage of 1.04 V to achieve a current density of 10 mA cm^?2, which was much lower than almost all alkaline electrolysis systems.     

Efficient CoMoRu0.25Ox/NF nanoplate architectures for overall electrochemical water splitting

The development of inexpensive electrocatalysts with excellent electrocatalytic activity for the hydrogen and oxygen evolution reactions (HER and OER, respectively) has been challenging. In this study, we synthesized cobalt molybdenum ruthenium oxide with porous, loosely-assembled nanoplate morphology. The CoMoRu_0.25O_x/NF electrocatalyst exhibited the highest electrocatalytic activity, requiring overpotentials of 230 and 78 mV for the OER and HER, respectively, to attain a current density of 10 mA cm^?2; moreover, its long-term stability was outstanding. The electrocatalyst required a cell voltage of only 1.51 V for overall water splitting in an alkaline medium, which was lower than that required by many CoMo-based catalysts.     

In-situ growth of Ni–CoSe2 on biomass-derived carbon tubes as an efficient electrocatalyst for overall water splitting

A Ni–CoSe₂/BCT composite composed of biomass-derived carbon tubes and transition metal selenides was successfully constructed and explored as a highly efficient bifunctional electrocatalyst for overall water splitting.

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Efficient overall water splitting over a Mo(IV)-doped Co3O4/NC electrocatalyst

To meet the demand of producing hydrogen at low cost, a molybdenum (Mo)-doped cobalt oxide (Co₃O₄) supported on nitrogen (N)-doped carbon (x%Mo–Co₃O₄/NC, where x% represents Mo/Co molar ratio) is developed as an efficient bifunctional electrocatalyst for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). This defect engineering strategy is realized by a facile urea oxidation method in nitrogen atmosphere. Through X-ray diffraction (XRD) refinement and other detailed characterizations, molybdenum ion (Mo⁴⁺) is found to be doped into Co₃O₄ by substituting cobalt ion (Co²⁺) at tetrahedron site, while N is doped into carbon matrix simultaneously. 4%Mo–Co₃O₄/NC is the optimized sample to show the lowest overpotentials of 91 and 276 mV to deliver 10 mA cm^?2 for HER and OER in 1 M potassium hydroxide solution (KOH), respectively. The overall water splitting cell 4%Mo–Co₃O₄/NC||4%Mo–Co₃O₄/NC displays a voltage of 1.62 V to deliver 10 mA cm^?2 in 1 M KOH. The Mo⁴⁺ dopant modulates the electronic structure of active cobalt ion (Co³⁺) and boosts the water dissociation process during HER, while the increased amount of lattice oxygen and formation of pyridinic nitrogen due to Mo doping benefits the OER activity. Besides, the smaller grain size owing to Mo doping leads to higher electrochemically active surface area (ECSA) on 4%Mo–Co₃O₄/NC, resulting in its superior bifunctional catalytic activity.     

Construction of P,N-codoped carbon shell coated CoP nanoneedle array with enhanced OER performance for overall water splitting

Water electrolysis to generate hydrogen (H₂) and oxygen (O₂) was a sustainable alternative for clean energy in the future but remained challenging. Herein, we fabricated a nanoneedle-like CoP core coated by a P,N-codoped carbon shell (CoP@PNC@NF). The hierarchical structure, unique nanoneedle-like morphology, CoP core, and P,N-codoped carbon shell were responsible for the high electrocatalytic activity. Electrocatalytic tests demonstrated that CoP@PNC@NF displayed low overpotentials of 137.6 and 148.4 mV, as well as Tafel slopes of 59.89 and 56.40 mV dec⁻¹ for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively, at 10 mA cm⁻² in 1.0 M KOH. The bifunctional electrocatalyst CoP@PNC@NF also exhibited a low cell voltage of 1.458 V to yield 10 mA cm⁻² in the two-electrode system and could maintain the activity for 50 h. The Faradaic efficiencies of CoP@PNC@NF for both HER and OER were nearly 100%. The result outperformed the precious-metal-based electrocatalyst apparatus (RuO₂||Pt/C) and other carbon-coated transition-metal phosphides (TMPs). This work paved the way for the rational design of carbon shell-coated TMPs with low energy barriers for converting and storing electrochemical energy.     

Tailoring cation vacancies in Co,Ni phosphides for efficient overall water splitting

High-efficiency water splitting catalysts are competitive in energy conversion and clean energy production. Herein, a bifunctional water splitting catalyst CoNiP with cation vacancy defects (CoNiP–V) is constructed through defect engineering. The results show that abundant cation vacancy defects in CoNiP–V are bifunctional active centers in the process of water electrolysis, which enhance the activity of oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). In 1.0 mol L^?1 potassium hydroxide, CoNiP–V requires a pretty low overpotential of 58 mV to reach a geometrical current density of 10 mA cm^?2 for HER. To deliver a current density of 100 mA cm^?2, only 137 mV and 340 mV of overpotential are needed for HER and OER, respectively. Moreover, the cell with CoNiP–V as both cathode and anode exhibits good stability, which only needs 1.61 V to achieve a current density of 100 mA cm^?2, and the cell voltage barely rises 10% after 100 h’ test under 100 mA cm^?2. Therefore, CoNiP–V is promising for the development of efficient water splitting catalysts.     

Mo2C regulated by cobalt components doping in N-doped hollow carbon nanofibers as an efficient electrocatalyst for hydrogen evolution reaction

Hydrogen is a viable substitute to fossil fuels and electrochemically catalyzed hydrogen evolution has attracted wide attention due to its stability and effectiveness. Nevertheless it is still a major challenge to design and prepare highly active noble metal-free electrocatalysts with controllable structure and composition for efficient hydrogen evolution reaction (HER). Herein, Mo₂C regulated by cobalt components (Co and CoO) doping in N-doped hollow carbon nanofibers (marked as Mo₂C/Co/CoO-NHCNFs) are firstly designed and prepared via a facile coaxial electrospinning followed by calcination process. The one-dimensional conductive carbon host, hollow structure and synergistic effect among CoO, Co and Mo₂C can jointly promote electron transfer, augment exposure of active sites and adjust the electronic structure of the active sites, resulting in the excellent of HER performances. The optimized catalyst has a high specific surface area of 101.27 m² g^?1. Meanwhile, it has a low overpotential of 143 mV at a current density of 10 mA cm^?2 and a small Tafel slope of 74 mV dec^?1 in 1.0 M KOH.Satisfactorily, the overpotential is reduced by 231 mV at the same current density compared with Mo₂C doped in N-doped carbon nanofibers (named as Mo₂C-NCNFs). Moreover, the Mo₂C/Co/CoO-NHCNFs also demonstrate superior long-term stability. The formative mechanism of Mo₂C/Co/CoO-NHCNFs is expounded, and the construction technique is established. The design philosophy and the simple and economical method are of significance for development of HER electrocatalysts.     

Facile synthesis of three-dimensional spherical Ni(OH)2/NiCo2O4 heterojunctions as efficient bifunctional electrocatalysts for water splitting

Synthesis of highly efficient, non-noble and bi-functional electrocatalysts is exceedingly challenging and necessary for water splitting devices. In this work, three-dimensional spherical Ni(OH)₂/NiCo₂O₄ heterojunctions are prepared by a one-step hydrothermal method and the hybrids are explored as efficient electrocatalysts for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in an alkaline electrolyte via tuning different Ni/Co atomic ratios of heterojunctions. The optimized Ni(OH)₂/NiCo₂O₄ (S (1:1)) exhibits high electrocatalytic activity with an ultralow over-potential of 189 mV at 10 mA cm⁻² for the HER. With regard to the OER, the over-potential of the as-synthesized S (1:1) heterojunction is only 224 mV at the current density of 10 mA cm⁻². The improved catalytic performance of the Ni(OH)₂/NiCo₂O₄ heterojunctions is attributed to the chemical synergic combining of Ni(OH)₂ and NiCo₂O₄, large specific surface area for exposing more accessible active sites, and heterointerface for activating the intermediates that facilitates electron/electrolyte transport. The prepared catalyst exhibits good durability and stability in HER and OER catalyzing conditions. This study provides a feasible approach for the building of highly efficient bifunctional water splitting electrocatalysts and stimulates the development of renewable energy conversion and storage devices.     

Zn doped ZIF67-derived porous carbon framework as efficient bifunctional electrocatalyst for water splitting

The electrochemical splitting of water is considered to be an efficient and potential technique for producing clean hydrogen and oxygen. Although, there are lots of significant developments in composite of superior hydrogen evolution reaction (HER) or oxygen evolution reaction (OER) catalyst applied in water splitting currently, designing non-precious and low-cost bifunctional electrocatalysts with high performance is still an attractive challenging issue. In this article, we report a novel bifunctional electrocatalyst with cobalt-based nanoparticles (NPs) embedded in Zn-doped hierarchical porous three-dimension N-doped carbonization structure via an annealing process of metal organic frameworks (MOFs) connected by N-doped carbon nanotube (denoted as Co–Zn/PNC). This composite structure possesses the characteristics of more active sites, numerous mesopores and high conductivity. The resulting electrocatalyst (Co–Zn/PNC) can be used as both anode and cathode to roust the overall water splitting, getting a current density of 10 mA cm⁻² at a cell voltage of 1.63 V in 1.0 M KOH electrolyte.     

Ni activated Mo2C nanoparticles supported on stereotaxically-constructed graphene for efficient overall water splitting

Exploring cost-effective, high-efficiency and stable electrocatalysts for overall water splitting is greatly desirable and challenging for sustainable energy. Herein, a novel designed Ni activated molybdenum carbide nanoparticle loaded on stereotaxically-constructed graphene (SCG) using two steps facile strategy (hydrothermal and carbonization) as a bifunctional electrocatalyst for overall water splitting. The optimized Ni/Mo₂C(1:20)-SCG composites exhibit excellent performance with a low overpotential of 150 mV and 330 mV for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively to obtain a current density of 10 mA cm^?2 in 1.0 M KOH solution. In addition, when the optimized Ni/Mo₂C(1:20)-SCG composite is used as a bifunctional electrode for overall water splitting, the electrochemical cell required a low cell voltage of 1.68 V at a current density of 10 mA cm^?2 and long-term stability of 24 h. More significantly, the synergetic effects between Ni-activated Mo₂C nanoparticles and SCG are regarded as a significant contributor to accelerate charge transfer and promote electrocatalytic performance in hybrid electrocatalysts. Our works introduce a novel approach to design advanced bifunctional electrodes for overall water splitting.     

Ultrasmall Mo2C in N-doped carbon material from bimetallic ZnMo-MOF for efficient hydrogen evolution

The exploration and development of cost-effective and highly stable electrocatalysts with the highest possible energy efficiency remain a constant pursuit in the catalyst design and synthesis for electrocatalytic hydrogen evolution reaction (HER). In this work, a convenient approach is proposed to synthesize a type of ultrafine Mo₂C nanoparticles in average sizes of 3–4 nm embedded in hierarchically porous N-doped carbon material calcined from bimetallic ZnMo-MI (MI = 2-methylimidazole) is obtained at 1000 °C, denoted as ZnMo-MI-1000. First of all, the crystalline hybrid metal-organic framework of ZnMo-MI is fabricated from zeolitic imidazolate framework of Zn-MI precursors via solvothermal reaction, in which the conversion from Zn-MI to ZnMo-MI occurs gradually over time. After calcination, the as-obtained ZnMo-MI-1000 sample shows a satisfying HER performance with the small overpotential of 83.0 mV in 0.5 M H₂SO₄ and 100.1 mV in 1.0 M KOH to reach a current density of 10 mA cm^?2, which is attributed to ultrasmall Mo₂C, Mo and N-doped graphitic carbon matrix. The multiporous network of ZnMo-MI-1000 can provide continuous mass transportation with a minimal diffusion resistance that produce effective electrocatalytic kinetics in both acidic and alkaline media, which is utilized as a highly active and durable nonprecious metal electrocatalyst for HER.     

Facile construction of well-defined hierarchical NiFe2O4/NiFe layered double hydroxides with a built-in electric field for accelerating water splitting at the high current density

Interfacial charge redistribution induced by a strong built-in electric field can expertly optimize the adsorption energy of hydrogen and hydroxide for improving the catalytic activity. Herein, we develop a well-defined hierarchical NiFe₂O₄/NiFe layered double hydroxides (NFO/NiFe LDH) catalysts, exhibiting superior performance due to the strong interfacial electric field interaction between NiFe₂O₄ nanoparticle layers and NiFe LDH nanosheets. In 1 M KOH, NFO/NiFe LDH needs 251 mV and 130 to drive 50 and 10 mA cm^?2 for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Moreover, only 1.517 V cell voltage is needed to reach 10 mA cm^?2 towards overall water splitting. Notably, under simulated industrial electrolysis conditions, NFO/NiFe LDH only needs 289 mV to drive 1000 mA cm^?2. This work puts a deep insight into the role of the built-in electric field in transition metal-based catalysts for accelerating water splitting and scalable industrial electrolysis applications.     

Facile construction of heterostructural Ni3(NO3)2(OH)4/CeO2 bifunctional catalysts for boosted overall water splitting

Interface engineering has aroused vitally widespread concern since it could be an effective strategy for exploring high-performance and low-cost water oxidation electrocatalysts. Herein, we report a hetero-structured Ni₃(NO₃)₂(OH)₄/CeO₂/NF (NNO/CeO₂/NF) electrode, exhibiting superior performance owning to the NO₃^? anion substitution for the OH^? in nickel hydroxide to form Ni₃(NO₃)₂(OH)₄, together with its interface synergy with ceria. In alkaline solution, the NNO/CeO₂/NF electrocatalyst could catalyze the OER with an overpotential of 330 mV to approach 50 mA cm^?2. Also, it needs only an overpotential of 120 mV to reach 10 mA cm^?2 for HER. Additionally, when a standard two-electrode water electrolyzer is fabricated by employing NNO/CeO₂/NF as both the cathode and anode, it can generate 10 mA cm^?2 at 1.64 V and operate steadily without performance degradation after 25 h. This research provides a novel perspective for reasonable design of advanced catalytic materials with improvements in the field of electrocatalysis.     

Controllable synthesis of Co/MnO heterointerfaces embedded in graphitic carbon for rechargeable Zn–air battery

Owing to the unique geometric and electronic structure, design and synthesis of electrocatalysts with well defined heterointerfaces are essential for clean energy technologies, for instance water-splitting and Zn-air batteries. Herein, a bifunctional electrocatalyst assembled by Co/MnO nanoparticles and nitrogen doping double-sphere carbon (denoted as Co/MnO@N-DSC), was fabricated via a solvothermal and pyrolysis strategy. The Co/MnO@N-DSC catalysts exhibit an enhanced bifunctional oxygen electrocatalytic performance for Oxygen reduction reaction (ORR) (E_1/2, 0.84 V vs. RHE) and oxygen evolution reaction (OER) (E_onset, 1.54 V vs. RHE). As an air cathode catalyst, the Co/MnO@N-DSC-based Zn-air battery can afford prime performance, over the commercial noble-metal-based Zn-air battery. Theoretical calculation results indicate that the synergism of Co (111)/MnO (200) heterointerfaces can enhance charge transfer and provide extra electrons for the reaction processes. This work provides a promising manoeuvre to uplift bifunctional catalytic activity by increasing the synergy from heterointerfaces of transition-metal/metal oxide in oxygen electrocatalysis.     

1T/2H MoS2/MoO3 hybrid assembles with glycine as highly efficient and stable electrocatalyst for water splitting

Despite great efforts have been made during the past decade to improve the efficiency of hydrogen evolution reaction (HER) onto the MoS₂-based electrocatalysts via increasing the number of active sites, further improvements are crucial to avoid the detachment of 2D MoS₂ nanosheets from the substrate during the long-term water splitting under intense HER. In this study, we report on the formation of highly efficient and surprisingly stable layer composed of 2D nanoplatelets from the hybrid-type MoS₂ as a new prospective electrocatalyst for HER from acidic water solution. This layer was formed via one-pot hydrothermal synthesis in the solution containing ammonium heptamolybdate, thiourea and glycine (Gly). The products obtained were characterized by SEM, HRTEM, XRD, Raman, XPS, and potential cycling. Note that at the designed hybrid-type MoS₂/MoO₃-Gly nanoplatelets the HER rate can achieve a stable electrochemical performance for days with ~100 mA cm⁻² current density at −0.35 V potential vs RHE.