High-efficiency Co6W6C catalyst with three-dimensional ginger-like morphology for promoting the hydrogen and oxygen evolution reactions

Development of electrocatalysts composed of low cost and abundant elements that exhibit catalytic activity comparable to noble metals is important for water splitting. As such, in this study, a catalyst material with a ginger-like morphology consisting of Co₆W₆C is synthesized via a hydrothermal reaction and pyrolysis treatment. The Co₆W₆C catalyst exhibits satisfactory electrochemical properties towards both hydrogen and oxygen evolution reactions in an alkaline electrolyte, with a low overpotential, low Tafel slope, and durable stability. Co₆W₆C possesses a high activity for the hydrogen evolution reaction in alkaline conditions, with an onset potential and overpotential of −0.024 V and 101 mV, respectively, and low Tafel slope of 80.5 mV dec⁻¹ at a current density of 10 mA cm⁻². In addition, Co₆W₆C achieves a current density of 10 mA cm⁻² for the oxygen evolution reaction at an overpotential of only 343 mV. Furthermore, electrochemical stability tests indicate that the Co₆W₆C catalyst maintains 91% of the original current after 60,000 s for the hydrogen evolution reaction and 95% of the original current after 45,000 s for the oxygen evolution reaction. Moreover, electrochemical splitting of water via a two-electrode system employing this catalyst can hold 89% of the initial current after 40,000 s in 1 M KOH.     

Carbon supported nickel phosphide as efficient electrocatalyst for hydrogen and oxygen evolution reactions

Hydrogen production through water splitting is an efficient and green technology for fulfilling future energy demands. Carbon nanotubes (CNT) supported Ni₂P has been synthesized through a simpler hydrothermal method. Ni₂P/CNT has been employed as efficient electrocatalysts for hydrogen and oxygen evolution reactions in acidic and alkaline media respectively. The electrocatalyst has exhibited low overpotential of 137 and 360 mV for hydrogen and oxygen evolution reactions respectively at 10 mA cm^?2. Lower Tafel slopes, improved electrochemical active surface area, enhanced stability have also been observed. Advantages of carbon support in terms of activity and stability have been described by comparing with unsupported electrocatalyst.     

Synthesis of Co2FeAl alloys as highly efficient electrocatalysts for alkaline hydrogen evolution reaction

The development of cost-effective non-precious metal electrocatalysts is a major challenge for water splitting applications, but it is important for the realization of renewable energy systems. Alloying has proved an effective way to design metal-based electrocatalysts, and by controlling the annealing temperature, the surface morphology and crystallinity of the alloy can be tuned to control the hydrogen evolution reaction (HER) performance. In this work, with a simple coprecipitation method, we have prepared Co₂FeAl alloys at different annealing temperatures (550 °C–670 °C), which exhibit excellent crystallinity and electrocatalytic performance for HER in alkaline solution. Among all conditions, the Co₂FeAl alloys prepared at 620 °C shows the better crystallinity and the higher purity, and it could achieve a low overpotential of 149 mV at 10 mA cm^?2 in alkaline solution. The overpotential demonstrates persistent stability with only 3 mV change after over 1000 cycles. Both density functional theory (DFT) calculations and experimental results revealed that alloying optimizes the electronic structure near the Fermi surface of the system, improving the electron transport efficiency and enhancing the catalytic activity. These Co₂FeAl alloys are appealing candidates for high-performance alkaline HER electrocatalytic electrodes in water electrolysis due to their outstanding electrocatalytic properties.     

Construction of Mo2C/W2C heterogeneous electrocatalyst for efficient hydrogen evolution reaction

Constructing high-performance catalyst for hydrogen evolution reaction (HER) is the effective way to eliminate energy crisis. Reasonable engineering of heterointerfaces can effectively create more active sites and promote electron transfer resulting in improvement in the catalytic activity. In this work, we synthesize the well-defined molybdenum carbides and tungsten carbides nano-heterostructure (Mo₂C/W₂C) by carbonization with CH₄/H₂ at 800 °C showing excellent HER activity, fast kinetics and electrochemical stability in both alkaline and acidic electrolytes. Mo₂C/W₂C requires only 140 and 132 mV overpotentials to reach catalytic current density of 10 mA cm^?2 in 0.5 M H₂SO₄ and 1 M KOH electrolyte, respectively. Tafel slope is as low as 51 and 76 mV dec^?1 in 0.5 M H₂SO₄ and 1 M KOH comparable to the benchmarked Pt/C. Moreover, Mo₂C/W₂C exhibits a superior stability with slight deterioration in HER performance after 5000 potential cycles. This work elucidates that the rational construction of heterointerfaces is favorable for design of efficient non-noble metal electrocatalyst for HER catalysis.     

Metal organic frameworks: Mastery in electroactivity for hydrogen and oxygen evolution reactions

Electrochemical water splitting is recognized as a conspicuous technique for sustainable and an alternative energy storage systems. Fabricating different catalysts for electrocatalysis is highly desirable to decrease the overpotential and ease practical applications. Metal-organic-frameworks (MOFs) have obtained significant consideration recently due to tunable porous structure, superior catalytic activity, and high surface area. Owing to the properties of MOF, these materials can be employed as catalysts for overall water splitting applications. Herein, the most recent advancement in MOFs for an efficient electrochemical water splitting are demonstrated. Primarily, the basics and reaction mechanisms of water splitting were summarized which is followed by the recent improvements in electrocatalytic properties of pristine MOFs, guest@MOFs, MOF derived different metallic compounds and carbon-based catalytic materials. The fast growing innovations in the electrocatalytic activities and their fundamental mechanisms are comprehensively summarized. Finally, a thorough discussion on the current challenges and future outlooks in water splitting is provided.     

Surfactant-assisted hydrothermal synthesis of nitrogen doped Mo2C@C composites as highly efficient electrocatalysts for hydrogen evolution reaction

We describe a facile surfactant-assisted hydrothermal route to synthesize nitrogen doped Mo₂C@C composites in the presence of cetyltrimethylammonium bromide (CTAB) as carbon source and structure guiding agent. The resulting Mo₂C@C composites consist of Mo₂C nanocrystals with sheet-like morphology and well-dispersed nitrogen element doping. Controllable experiments indicate that the additive amount of CTAB can efficiently tune porous structure and electrochemical activity of the as-prepared Mo₂C@C materials. This unique nitrogen doped Mo₂C@C composite provides several advantages for electrocatalytic applications: (1) nitrogen doped carbons can prevent the aggregation of Mo₂C nanocrystals and render it high conductivity; (2) the homogeneous dispersion of Mo₂C nanocrystals provides abundant active sites; (3) 2D morphology, the hierarchical porosity, and high surface areas allow large exposed field of active sites and facilitate mass transfer. As a result, the nitrogen doped Mo₂C@C composites deliver superior HER electrocatalytic activities with a low overpotential of only 100 mV and also a low Tafel slope of 53 mV/dec in alkaline condition. Such CTAB-assisted strategy may open up an opportunity towards synthesis of low cost and high performance Mo-based electrocatalysts for various applications, such as water splitting.     

Tunably fabricated nanotremella-like Bi2S3/MoS2: An excellent and highly stable electrocatalyst for alkaline hydrogen evolution reaction

Reasonable design of efficient and stable catalysts with low cost and abundant natural reserves is vital for electrocatalytic water splitting. Herein, novel nanotremella-like Bi₂S₃/MoS₂ composites with different mass ratios between Bi₂S₃ and MoS₂ have been successfully prepared through a hydrothermal approach and further applied to hydrogen evolution reaction (HER) in 1.0 M KOH electrolyte for the first time. When the mass ratio of Bi₂S₃ and MoS₂ is 5:5, as-prepared nanotremella-like Bi₂S₃/MoS₂ (marked as BMS-5) manifests favorable HER catalytic activity with overpotential of 124 mV at current density of 10 mA cm⁻² and relatively low Tafel slope of 123 mV dec⁻¹. Moreover, it exhibits an extraordinary durability for uninterrupted hydrogen generation. The enhanced HER performances are ascribed to the synergistic effects between Bi₂S₃ and MoS₂, giving rise to large electrocatalytic active area and fast HER kinetics. The results pave a new path to design and construct excellent Bi₂S₃/MoS₂ nanomaterials for electrocatalytic hydrogen generation.     

MoS2||CoP heterostructure loaded on N,P-doped carbon as an efficient trifunctional catalyst for oxygen reduction,oxygen evolution,and hydrogen evolution reaction

Exploring multifunctional electrocatalysts is crucial for the development of energy conversion and storage equipments, such as fuel cells, water splitting devices and zinc-air batteries. Herein, we provide a rational design whereby the cobalt phosphide particles are introduced into molybdenum sulfide nanosheets to form a heterostructure (MoS₂||CoP) through the ultrasonic method and calcination. Subsequently, N, P-doped carbon (NPC) is obtained synchronously. The as-prepared MoS₂||CoP/NPC demonstrates highly effective multifunctional catalytic performance for oxygen evolution and hydrogen evolution reaction at lower overpotential, as well as oxygen reduction reaction at high half-wave potential. What this reveals is higher power density and superior stability in zinc-air battery. The excellent electrocatalytic activity of MoS₂||CoP/NPC may be attributed to the presence of the MoS₂||CoP heterostructure, as well as N, P-doped carbon coupled with a high percentage of pyridinic-N. This work proposes a novel and facile strategy to prepare the heterostructure compound and serves as a good reference for constructing efficient and low-cost electrocatalysts.     

Recent advances of MoO3 based materials in energy catalysis: Applications in hydrogen evolution and oxygen evolution reactions

This work mainly focuses on the hydrogen evolution reaction and oxygen evolution reaction of nanostructured molybdenum trioxide-based materials for energy catalysis. MoO₃ is an n-type wide bandgap semiconductor and has the ability to replace noble metal catalysts. Here we summarize the crystal structure and properties of nanostructured MoO₃. The work also highlights the recent advancement in electrocatalytic hydrogen evolution reaction, photocatalytic hydrogen evolution reaction, photoelectrochemical hydrogen evolution reaction, electrocatalytic oxygen evolution reaction, and photoelectrochemical oxygen evolution reaction in MoO₃ based materials.     

The P/NiFe doped NiMoO4 micro-pillars arrays for highly active and durable hydrogen/oxygen evolution reaction towards overall water splitting

Water splitting is an efficient strategy to produce purity hydrogen and convert intermittent electricity from renewable wind and solar sources. In this work, dense NiMoO₄ micro-pillars arrays (MPAs) were in-situ grown on nickel foam (NF) through facile hydrothermal method, then the NiMoO₄/NF were converted into NiMoO₄–P/NF and NiFe/NiMoO₄/NF via phosphating and electrodeposition method, respectively. The NiMoO₄–P/NF electrode required small overpotentials of 34 mV@10 mA cm⁻² and 130 mV@100 mA cm⁻² for hydrogen evolution reaction (HER). The NiFe/NiMoO₄/NF electrode exhibited excellent oxygen evolution reaction (OER) activity with overpotentials of 210 mV@10 mA cm⁻² and 300 mV@100 mA cm⁻². The overall water splitting using the anode-cathode couple of NiFe/NiMoO₄/NF||NiMoO₄–P/NF only consumes low voltages of 1.47 V@10 mA cm⁻² for 100 h and 1.66 V@100 mA cm⁻² for 50 h in 1 M KOH. The electronic modification and the well-designed hierarchical structure contribute the high energy-efficient and stabile overall water splitting.     

In-situ crystalline infiltrated Mo2C@C for efficient hydrogen evolution reaction

Molybdenum carbide (Mo₂C) is a promising electrocatalyst for hydrogen evolution reaction (HER) due to the similar electron orbital structure to platinum. Preparing Mo₂C catalyst in nanoscale increases the exposure of active catalyst site and significantly enhances electrochemical reaction and improve HER performance. However, Mo₂C is not a good electron conductor which still requires a more sufficient contact between the catalyst and electrode to transfer electron current. Here, we proposed in-situ Mo₂C growth on carbon nanotubes (CNTs) by using wet impregnation of ammonium molybdate tetrahydrate as Mo source, which established a crystalline transition contact between Mo₂C and CNTs and significantly improved HER performance. Our results further showed the optimized HER performance of Mo₂C@C_amt and achieved a lower Tafel slopes and low onset potential (η_onset) of 46.7 mV dec⁻¹, 51.3 mV dec⁻¹ and 73 mV and 127 mV (vs. RHE) under alkaline and acidic condition, respectively. Furthermore, Mo₂C@C_amt also shows less attenuation after 1000 times of cyclic voltammograms (CV) cycling stability test and 25 h continuous operation under alkaline and acidic condition.     

NiCo2O4 nanowire arrays rich in oxygen deficiencies for hydrogen evolution reaction

The rational design of highly efficient electrocatalysts to generate hydrogen by catalyzing hydrogen evolution reaction still remains a challenge. Herein, we report a simple strategy to significantly enhance the catalytic activities of NiCo₂O₄ nanowire arrays by simply tuning the amount of oxygen vacancies. Remarkably, the oxygen-deficient NiCo₂O₄ catalysts obtained in Ar environment show significantly improved catalytic activities toward hydrogen evolution reaction with the requirement of 104 mV overpotential to afford 10 mA cm⁻², 122 mV less than that for air-sintered NiCo₂O₄ (226 mV). Moreover, such catalysts also exhibit superior long-term durability for 24 h at 100 mA cm⁻². The present study further promotes the application of NiCo₂O₄ in other energy storage and conversion system.     

Flower-like Co3O4 microstrips embedded in Co foam as a binder-free electrocatalyst for oxygen evolution reaction

Designing appropriate oxygen evolution reaction (OER) electrocatalysts to meet the requirements of high efficiency, long-term durability, and low cost remains the challenge for scientific community. Cobalt oxide (Co₃O₄) has been proven as a promising candidate for OER with attractive activity and stability in alkaline media. In this study, flower-like Co₃O₄ microstrips have been successfully prepared and directly embedded in Co foam (denoted as Co₃O₄@Co foam) by a green and facile two-step strategy including hydrothermal treatment and subsequent annealing process under relatively low temperatures. It demonstrates that the OER performance of the Co₃O₄@Co foam electrode can rival to the commercial RuO₂ on glassy carbon electrode. The Co₃O₄@Co foam electrode displays high OER activity with a low overpotential of 273 mV at a current density of 10 mA cm⁻², and a low Tafel slope of 61.8 mV dec⁻¹. The flower-like Co₃O₄ microstrips greatly increase the active surface area to expose more active sites, and the directly growth of Co₃O₄ microstrips on Co foam with intimate contact improves the electron transport and ensures the stability of the Co₃O₄@Co foam electrode.     

Facile one-pot synthesis of binder-free nano/micro structured dendritic cobalt activated nickel sulfide: a highly efficient electrocatalyst for oxygen evolution reaction

Reasonable design and preparation of non-noble metal electrocatalysts with predominant catalytic activity and long-term stability for oxygen evolution reaction (OER) are essential for electrocatalytic water splitting. Ni foam (NF) is highlighted for its 3D porous structure, impressive conductivity and large specific surface area. Herein, nano/micro structured dendritic cobalt activated nickel sulfide grown on 3D porous NF (Co–Ni₃S₂/NF) has been successfully synthesized by one-step hydrothermal method. Due to the ingenious incorporation of Co, Co–Ni₃S₂/NF electrode shows auspicious electrocatalytic performance for OER compared with Ni₃S₂/NF electrode. As a result, Co–Ni₃S₂/NF needs overpotential of only 274 and 459 mV at current density of 10 and 50 mA cm⁻², respectively, while Ni₃S₂/NF requires overpotential of 344 and 511 mV. At potential of 2.0 V (vs. RHE), Co–Ni₃S₂/NF displays current density of 191 mA cm⁻², while Ni₃S₂/NF just attains current density of only 135 mA cm⁻². Moreover, Co–Ni₃S₂/NF demonstrates excellent stability for uninterrupted OER in alkaline electrolyte. The strategy of designing and preparing cobalt activated nickel sulfide grown on NF renders a magnificent prospect for the development of metal-sulfide-based oxygen evolution catalysts with excellent electrocatalytic performances.     

Platinum nanoparticles decorated Nb2CTx MXene as an efficient dual functional catalyst for hydrogen evolution and oxygen reduction reaction

Here, a dual functional Nb₂CT_x@Pt nanocomposite has been synthesized by in situ reduction method. The Pt loading in the composite has been optimized to get minimum overpotential (141 mV at 10 mA/cm²) for hydrogen evolution reaction (HER) along with a promising Tafel slope of 46.3 mV/dec, while Pt/C shows an overpotential and Tafel slope of 104 mV and 32.4 mV/dec, respectively. The Pt mass activity for Nb₂CT_x@Pt3.8 composite at 100 mV overpotential was 3.44 A g^?1 while the Pt mass activity for conventional Pt/C was 0.7 A g^?1, which shows that the activity of Nb₂CT_x@Pt3.8 composite is approximately 5 times higher than Pt/C. In addition, the catalyst was found to be stable for continuous 500 cycles without any binder molecules. The oxygen reduction reaction (ORR) capability of the material was also evaluated and found that the catalyst exhibited a current density of ?4.28 mA/cm² in the diffusion limiting region in comparison with the current density of ?5.82 mA/cm² for Pt/C at 2600 revolutions per minute (RPM). The Pt mass activity of Nb₂CT_x@Pt3.8 composite for ORR is approximately 10 times higher than Pt/C. The Nb₂CT_x@Pt3.8 composite was able to reduce O₂ completely using the 4-electron pathway with very little peroxide production. From these results, the dual functionality of the Nb₂CT_x@Pt3.8 composite for both HER and ORR has been established.     

Zinc oxide functionalized molybdenum disulfide heterostructures as efficient electrocatalysts for hydrogen evolution reaction

Technology urges to replace the state-of-the-art catalysts such as platinum with low cost, earth abundant and durable electrocatalysts for efficient hydrogen evolution (HER) reaction which is going to become the major sustainable production of energy in future. Herein, we present the heterostructure based MoS₂.ZnO (MZO) heterostructures for successful electrochemical water splitting process. For HER, the prepared MoS₂.ZnO nanocomposites show the over potential as low as 239 mV at cathodic current density 10 mAcm⁻² with an exchange current density of 3.2 μAcm⁻². A Tafel slope of about 62 mV per decade suggested to have the Volmer-Heyrovsky mechanism for the HER process with MoS₂.ZnO nanocomposite as the catalyst. The small Tafel slope indicates a promising electrocatalyst for HER in practical application. The strong interface formation at the MoS₂.ZnO heterostructure facilitates higher catalytic activity and excellent cycling stability. The heterostructure formation based on semiconductor two dimensional (2D) transition metal dichalcogenides (TMDC) open up new avenues for effective manipulation of HER catalysts.     

MnxCo3-xO4 spinel oxides as efficient oxygen evolution reaction catalysts in alkaline media

The design of efficient electrocatalysts for oxygen evolution reaction (OER) is an essential task in developing sustainable water splitting technology for the production of hydrogen. In this work, manganese cobalt spinel oxides with a general formula of Mn_xCo_3-xO₄ (x = 0, 0.5, 1, 1.5, 2) were synthesised via a soft chemistry method. Non-equilibrium mixed powder compositions were produced, resulting in high electrocatalytic activity. The oxygen evolution reaction was evaluated in an alkaline medium (1 M KOH). It was shown that the addition of Mn (up to x ≤ 1) to the cubic Co₃O₄ phase results in an increase of the electrocatalytic performance. The lowest overpotential was obtained for the composition designated as MnCo₂O₄, which exhibited a dual-phase structure (∼30% Co₃O₄ + 70% Mn_1.4Co_1.6O₄): the benchmark current density of 10 mA cm⁻² was achieved at the relatively low overpotential of 327 mV. The corresponding Tafel slope was determined to be ∼79 mV dec⁻¹. Stabilities of the electrodes were tested for 25 h, showing degradation of the MnCo₂O₄ powder, but no degradation, or even a slight activation for other spinels.     

Biomass-derived carbon nanosheets coupled with MoO2/Mo2C electrocatalyst for hydrogen evolution reaction

Transition metal carbide such as molybdenum carbide has been widely used in electrolytic water for hydrogen production due to its potential catalytic property. The synthesis of molybdenum carbide-based high-efficient catalysts by simple process remains great challenges. Herein, Mo oxide/carbide material with hybrid morphology was synthesized by carbonizing mixture of lotus roots and Mo salt. The as-obtained material consists of MoO₂/Mo₂C (MOMC) anchored on biomass-derived nitrogen-doped carbon (NC) matrix. The results show that as-prepared material displays leaf-like and belt-like nanosheets, and the MOMC/NC catalyst with optimal Mo contents exhibits an excellent activity with a low overpotential of 138 mV to drive 10 mA cm^?2 and Tafel slope is 56.7 mV dec^?1 in alkaline medium, indicating that as-prepared catalyst will have promising application in the field of catalysis.     

Enhanced oxygen evolution reaction kinetics through biochar-based nickel-iron phosphides nanocages in water electrolysis for hydrogen production

Highly-efficient and stable non-noble metal electrocatalysts for overcoming the sluggish kinetics of oxygen evolution reaction (OER) is urgent for water electrolysis. Biomass-derived biochar has been considered as promising carbon material because of its advantages such as low-cost, renewable, simple preparation, rich structure, and easy to obtain heteroatom by in-situ doping. Herein, Ni₂P–Fe₂P bimetallic phosphide spherical nanocages encapsulated in N/P-doped pine needles biochar is prepared via a simple two-step pyrolysis method. Benefiting from the maximum synergistic effects of bimetallic phosphide and biochar, high conductivity of biochar encapsulation, highly exposed active sites of Ni₂P–Fe₂P spherical nanocages, rapid mass transfer in porous channels with large specific surface area, and the promotion in adsorption of reaction intermediates by high-level heteroatom doping, the (Ni_0.75Fe_0.25)₂P@NP/C demonstrates excellent OER activity with an overpotential of 250 mV and a Tafel slope of 48 mV/dec at 10 mA/cm² in 1 M KOH. Also it exhibits a long-term durability in 10 h electrolysis and its activity even improves during the electrocatalytic process. The present work provides a favorable strategy for the inexpensive synthesis of biochar-based transition metal electrocatalysts toward OER, and improves the water electrolysis for hydrogen production.     

Transition metal carbides coupled with nitrogen-doped carbon as efficient and stable Bi-functional catalysts for oxygen reduction reaction and hydrogen evolution reaction

Developing non-precious metal catalysts for oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER) is crucial for proton exchange membrane fuel cell (PEMFC), metal-air batteries and water splitting. Here, we report a in-situ simple approach to synthesize ultra-small sized transition metal carbides (TMCs) nanoparticles coupled with nitrogen-doped carbon hybrids (TMCs/NC, including WC/NC, V₈C₇/NC and Mo₂C/NC). The TMCs/NC exhibit excellent ORR and HER performances in acidic electrolyte as bi-functional catalysts. The potential of WC/NC at the current density of 3.0 mA cm^?2 for ORR is 0.814 V (vs. reversible hydrogen electrode (RHE)), which is very close to Pt/C (0.827 V), making it one of the best TMCs based ORR catalysts in acidic electrolyte. Besides, the TMCs/NC exhibit excellent performances toward HER, the Mo₂C/NC only need an overpotential of 80 mV to drive the current density of 10 mA cm^?2, which is very close to Pt/C (37 mV), making it the competitive alternative candidate among the reported non-precious metal HER catalysts.