Cobalt phosphide nanoparticles embedded in 3D N-doped porous carbon for efficient hydrogen and oxygen evolution reactions

An electrocatalyst based on a unique three-dimensional (3D) N-doped porous carbon sheet networks embedded with CoP₂ nanoparticles (CoP₂@3D-NPC) was synthesized by a facile pyrolysis process as well as an in-situ phosphatization method. The improved CoP₂@3D-NPC hybrid materials show excellent electrocatalytic activity toward HER and OER. This material provides a low overpotential of 126 mV at 10 mA cm⁻² in 0.5 M H₂SO₄ and 167 mV at 20 mA cm⁻² in 1.0 M KOH for HER with a small Tafel slope value of 59 mV dec⁻¹, respectively. Besides, it is also active for the OER under alkaline conditions. Such a prominent property of the CoP₂@3D-NPC electrocatalyst could be attributed to its excellent electrical conductivity of 3D carbon substrate, strong synergistic effect between CoP₂ nanoparticles and carbon nanosheet as well as extra active sites created by the N-doped structure.     

Co/Mo2C composites for efficient hydrogen and oxygen evolution reaction

Non-precious transition metal electrocatalysts with high catalytic performance and low cost enable the scalable and sustainable production of hydrogen energy through water splitting. In this work, based on the polymerization of CoMoO₄ nanorods and pyrrole monomer, a heterointerface of carbon-wrapped and Co/Mo₂C composites are obtained by thermal pyrolysis method. Co/Mo₂C composites show considerable performance for both hydrogen and oxygen evolution in alkaline media. In alkaline media, Co/Mo₂C composites show a small overpotential, low Tafel slope, and excellent stability for water splitting. Co/Mo₂C exhibits a small overpotential of 157 mV for hydrogen evolution reaction and 366 mV for oxygen evolution reaction at current density of 10 mA cm⁻², as well as a low Tafel slope of 109.2 mV dec⁻¹ and 59.1 mV dec⁻¹ for hydrogen evolution reaction and oxygen evolution reaction, respectively. Co/Mo₂C composites also exhibit an excellent stability, retaining 94% and 93% of initial current value for hydrogen evolution reaction and oxygen evolution reaction after 45,000 s, respectively. Overall water splitting via two-electrode water indicates Co/Mo₂C can hold 91% of its initial current after 40,000 s in 1 M KOH.     

Needle-like CoP/rGO growth on nickel foam as an efficient electrocatalyst for hydrogen evolution reaction

In this work, CoP/NF is synthesized at different temperature (250 °C, 300 °C, 350 °C) (denoted as CoP/NF-T, T = 250, 300, 350). Then, CoP/NF-300 with the best performance towards hydrogen evolution reaction (HER), is used to synthesize compounds with different ratio of reduced graphene oxide (rGO) (CoP/rGO/NF-X, X (quality ratio of rGO/CoP) = 1,3,5). In terms of morphology, under the synergistic effect of rGO, uniform and dense CoP provides the possibility to increase the electrochemical area. While CoP/rGO/NF-3 shows the minimum overpotential of 136 mV to drive 50 mA/cm, and the smallest Tafel slope 135 mV/dec among as-synthesized materials. Furthermore, CoP/rGO/NF-3 has good stability during at least 25 h. These result can be construed as the large electrochemical active area, high conductivity and long-time stability.     

Novel one-step synthesis of nickel encapsulated carbon nanotubes as efficient electrocatalyst for hydrogen evolution reaction

In this work, we report the synthesis of Ni nanoparticles encapsulated in carbon nanotubes (CNTs) by a facile and novel one-step pyrolysis method which are obtained from fumaric acid and nickel acetate as carbon and nickel sources respectively. The synthesized Ni encapsulated CNTs were characterized by various methods and were confirmed to possess large surface areas and numerous mesopores, they were applied as non-precious metal electrocatalyst for HER in 1 M KOH solution. The results show that the Ni encapsulated carbon nanotubes synthesized at 650 °C exhibited the best catalytic activity and stability with the smallest Tafel slope of 102 mV dec⁻¹, an onset potential of 110 mV and overpotential of 266 mV to achieve a current density of −10 mA cm⁻².     

One-pot wet-chemical synthesis of uniform AuPtPd nanodendrites as efficient electrocatalyst for boosting hydrogen evolution and oxygen reduction reactions

Uniform trimetallic AuPtPd nanodendrites (NDs) were synthesized by a simple and quick method, using l-proline and ascorbic acid (AA) as eco-friendly structure-guiding agent and reducing agent, respectively. The obtained AuPtPd NDs displayed greatly enlarged electrochemically active surface area (27.65 m² g^?1_metal) relative to home-made AuPt nanocrystals (NCs, 21.76 m² g^?1_metal), AuPd NCs (3.91 m² g^?1_metal), Pt black (20.88 m² g^?1_metal) and Pd black (8.30 m² g^?1_metal). For hydrogen evolution and oxygen reduction reactions, AuPtPd NDs showed excellent catalytic performances relative to the referenced catalysts. These results reveal the practical applications of the as-obtained catalyst in energy storage and conversion.     

Straightforward preparation of nickel selenide nanosheets supported on nickel foam as a highly efficient electrocatalyst for oxygen evolution reaction

The development of economical, durable, and efficient oxygen evolution reaction (OER) electrocatalysts is essential for large-scale industrial water electrolysis. Here, a straightforward strategy is proposed to synthesize a series of nickel selenide nanosheets supported on nickel foam (NiSe₂/NF) materials by directly selenizing nickel foam substrates at different temperatures under an inert atmosphere. When evaluated as electrocatalysts in OER, the optimal self-supported NiSe₂/NF-350 shows an excellent performance in 1.0 M KOH medium with an overpotential of 458 mV at 100 mA cm^?2, a small Tafel slope of 45.8 mV dec^?1, and a long-term stability for 36 h. Furthermore, the structural and compositional preservation for NiSe₂/NF-350 after stability test was also verified by various characterizations.     

NiOx nanoparticles supported on polyethylenimine functionalized CNTs as efficient electrocatalysts for supercapacitor and oxygen evolution reaction

Ni oxide based nanoparticles (NPs) have been widely used as electrocatalysts in the electrochemical energy storage and conversion applications. In this paper, NiO_x NPs are successfully synthesized by the self-assembly of Ni precursor onto polyethylenimine functionalized carbon nanotubes (PEI-CNTs) assisted with microwave radiation. NiO_x NPs with size around 2–3 nm are homogenously dispersed on the PEI-CNTs supports with no aggregation. The electrochemical activity of NiO_x NPs on PEI-CNTs, NiO_x/PEI-CNTs, as effective electrocatalysts is studied for supercapacitor and oxygen evolution reaction in alkaline solutions. NiO_x/PEI-CNTs show a capacitance of 1728 and 1576 F g⁻¹ based on active material, and 221 and 394 F g⁻¹ based on total catalyst loading on 12.5% and 25% NiO_x/PEI-CNTs, respectively, which is substantially higher than 152 F g⁻¹ of unsupported NiO. The NiO_x/PEI-CNTs electrodes exhibit reversible and stale capacitance of ∼1200 F g⁻¹ based on active materials after 2000 cycles at a high current density of 10 A g⁻¹. NiO_x/PEI-CNTs also exhibit significantly higher activities for oxygen evolution reaction (OER) of water electrolysis, achieving a current density of 100 A g⁻¹ at an overpotential of 0.35 V for 25% NiO_x/PEI-CNTs. It is believed that the uniformly dispersed nano-sized NiO_x NPs and synergistic effect between the NiO_x NPs and PEI-CNTs is attributed to the high electrocatalytic performance of NiO_x/PEI-CNTs electrocatalysts. The results demonstrate that NiO_x NPs supported on PEI-CNTs are highly effective electrocatalysts for electrochemical energy storage and conversion applications.     

Electrodeposited nickel–iron–carbon–molybdenum film as efficient bifunctional electrocatalyst for overall water splitting in alkaline solution

Designing and synthesizing of efficient and inexpensive bifunctional electrocatalysts for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is one of the current research topics. In this study, NiFeCMo film in nickel mesh substrate is prepared by one-step direct-current electrodeposition method. The obtained NiFeCMo film shows the excellent electrocatalytic activity, which only requires overpotentials of 254 mV for HER and 256 mV for OER to drive current density of 10 mA cm⁻², with corresponding Tafel slopes of 163.9 and 60.3 mV·dec⁻¹ in 30% KOH medium, respectively. Moreover, NiFeCMo film only needs a low cell voltage of 1.61 V to drive current density of 10 mA cm⁻² in an alkaline electrolyzer. Such remarkably HER and OER properties of NiFeCMo alloy is attributed to the increased effective electrochemically active surface area and the synergy effect among Ni, Fe, C and Mo.     

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.     

Amorphous-crystalline cobalt-molybdenum bimetallic phosphide heterostructured nanosheets as Janus electrocatalyst for efficient water splitting

Efficient and sustainable Janus catalysts toward hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are highly desirable for future hydrogen production via water electrolysis. Herein we report an active Janus electrocatalyst of amorphous-crystalline cobalt-molybdenum bimetallic phosphide heterostructured nanosheets on nickel foam (CoMoP/CoP/NF) for efficient electrolysis of alkaline water. As-reported CoMoP/CoP/NF consists of amorphous bimetal phosphide nanosheets doped with crystalline CoMoP/CoP heterostructured nanoparticles on NF. It can efficiently catalyze both HER (η = 127 mV@100 mA cm^?2) and OER (η = 308 mV@100 mA cm^?2) in alkaline electrolyte with long-term durability. Serving as anode and cathode of water electrolyzer, CoMoP/CoP/NF generates electrolytic current of 10, 50 and 100 mA cm^?2 at low voltage of 1.50, 1.59, and 1.67 V, respectively.     

Ternary nickel iron phosphide supported on nickel foam as a high-efficiency electrocatalyst for overall water splitting

Electrochemical water splitting is a promising technology for mass hydrogen production. Efficient, stable, and cheap electrocatalysts are keys to realizing this strategy. However, high price and preciousness of commonly used noble metal based catalysts severely hinder this realization. Herein, we report nickel iron phosphide (Ni-Fe_xP) bifunctional electrocatalyst via the in-situ growth of NiFe(OH)_x on nickel foam (NiFe(OH)_x/NF) followed by low-temperature phosphidation. As a hydrogen evolution reaction (HER) catalyst, the Ni-Fe_xP/NF only needs an overpotential of 119 mV to drive a current density of ?10 mA/cm² in a base media. It also shows excellent activity toward oxygen evolution reaction (OER) with low overpotentials of 254 mV, 267 mV, and 282 mV at 50, 100 and 200 mA/cm², respectively. Moreover, when this bifunctional catalyst is used for overall water splitting, a low cell voltage of 1.62 V is needed to deliver a current density of 10 mA/cm², which is superior to commercial electrolyzer and it also shows remarkable stability.     

Tuning the d-band center enables nickel-iron phosphide nanoprisms as efficient electrocatalyst towards oxygen evolution

Highly active and low-cost catalytic electrodes for oxygen evolution reaction (OER) are always crucial for obtaining clean hydrogen energy via large-scale electrolytic water splitting. Herein, endowing the nickel-iron phosphide (NiFeP) nanoprisms with the tunable electronic structures have been carried out by tailoring the energy level of d-band in our study. The bimetallic synergistic effect efficiently accelerates the formation and cleavage rates of MO bonding, enabling the greatly improved OER catalytic performance after doping Fe into Ni₂P. The large surface area benefiting from the porous architecture also facilitates more contact between electrocatalyst and alkaline electrolytes, resulting in an advanced OER activity. Therefore, NiFeP can drive the OER process with a low potential of 258 mV at a current density of 10 mA cm⁻² and a Tafel slope of 46 mV dec⁻¹ in 1.0 M KOH solution. The present work provides the bimetallic phosphide nanoprism electrocatalyst with the tailored electronic structure for further application relevant to renewable energy exploration.     

Quaternary-metal phosphide as electrocatalyst for efficient hydrogen evolution reaction in alkaline solution

Hydrogen evolution reaction (HER) is a critical process in electrocatalytic water splitting for hydrogen production. However, the development of low-cost electrocatalysts for highly efficient HER is still a huge challenge. Hence, we fabricate a multi-metal phosphide on Ni foam, FeCoNiNb_xP, through a facile hydrothermal reaction followed by phosphorization. We find that Nb promotes the formation of metal phosphides, and the main phases of the catalysts with Nb are multiphase phosphides. Importantly, the Nb incorporation significantly improves the HER activity of FeCoNiP. We show that FeCoNiNb_0.3P has the best HER activity, which only requires an overpotential of 78 mV to achieve a current density of 10 mA cm^?2 in 1 M KOH, and demonstrates excellent stability under both constant potential and varied current densities. Our findings show that the multiple-metal compounds are beneficial to the improvement of catalytic activity and provide guidance on the design of novel catalysts for applications.     

Fabrication of 3D ordered mesoporous nickel phosphide for efficient hydrogen evolution reaction

This article successfully fabricates ordered mesoporous nickel phosphide (OM-Ni₂P) by a nanocasting method using KIT-6 as the template and nickel nitrate as the nickel precursor. The synthesized OM-Ni₂P catalyst perfectly repeats the highly symmetrical mesoporous structure of KIT-6 and exhibits a large amount of the active Ni substance Ni^δ+ and the active P substance P^δ?. The ordered mesoporous structure not only promotes electron transfer but also facilitates the diffusion of the reaction medium. OM-Ni₂P shows superior hydrogen evolution reaction (HER) electrocatalytic performance than the CA-Ni₂P catalyst with a disordered mesoporous structure. The OM-Ni₂P material is promising to be a substitute to Pt-based electrocatalysts for hydrolysis.     

3D self-standing grass-like cobalt phosphide vesicles-decorated nanocones grown on Ni-foam as an efficient electrocatalyst for hydrogen evolution reaction

The growing hydrogen consumption has greatly promoted the development of efficient, stable and low-cost electrocatalysts for the hydrogen evolution reaction (HER). Constructing functional nanostructures is an efficacious strategy to optimize catalytic performance. Herein, we present a feasible route to fabricate distinctive 3D grass-like cobalt phosphide nanocones clad with mini-vesicles on the hierarchically porous Ni foam, which can directly serve as a binder-free electrocatalyst with superior catalytic activity and durability in HER. Thanks to its distinctive 3D microstructure featured with favourable pore-size distribution, abundant active sites provided by mini-vesicles and rapid electron transfer with the assistance of Ni foam, the as-grown grass-like CoP/NF electrocatalyst has shown a favourable overpotential in an acidic solution with an onset overpotential of ∼35 mV, an overpotential of 71 mV at a current density of 10 mA cm⁻², reduced by 60 mV in comparison with that realized by urchin-like CoP/NF nanoprickles. Moreover, it has exhibited an excellent HER activity in the alkaline medium, with an overpotential of 117 mV at 10 mA cm⁻², a Tafel slope of 63.0 mV dec⁻¹ and a long-term electrochemical durability.     

Ru/MoO2 decorated on CNT networks as an efficient electrocatalyst for boosting hydrogen evolution reaction

Generally, electrochemical hydrogen evolution reaction (HER) is hampered by slow kinetics and low round-trip efficiency. Electrocatalysts with a hierarchical structure and large surface area are expected to overcome these problems. Herein, we prepared a Ru/MoO₂/carbon nanotubes (RMC) hybrid with a hierarchical structure by a convenient solid-phase reaction (SPR) method, and studied the electrochemical activity for HER. After annealing as-prepared RuO₂/MoO₂/carbon nanotubes (ROMC) precursor in a tubular furnace under Ar atmosphere, RuO₂ and MoS₂ were in-situ transformed into Ru metal and MoO₂ phase by the redox SPR. Through various tests, we have confirmed that the new formed Ru metal and MoO₂ phase are combined and uniformly coated on the outer surface of CNTs. Interestingly, the RMC-500 exhibits the best HER performance with a low overpotential of 16 mV at l0 mA cm^?2, small Tafel slope of 45 mV dec^?1, higher electrochemical active surface area, and long-time durability in alkaline electrolyte.     

Trimetallic NiFeCo selenides nanoparticles supported on carbon fiber cloth as efficient electrocatalyst for oxygen evolution reaction

Trimetallic NiFeCo selenides (NiFeCoSe_x) anchored on carbon fiber cloth (CFC) as efficient electrocatalyst for oxygen evolution reaction (OER) in alkaline medium have been synthesized via a facile two-step method. Firstly, trimetallic NiFeCo (oxy) hydroxides have been electrodeposited on CFC support (NiFeCo/CFC). Secondly, a solvothermal selenization process has been used to convert NiFeCo/CFC into NiFeCoSe_x/CFC using N, N-dimethylformamide (DMF) as solvent. The composition and homogeneous distribution of NiFeCoSe_x/CFC nanoparticles are determined by XRD, XPS, SEM elemental mapping and EDX images. Furthermore, SEM images reveal that NiFeCoSe_x/CFC has volcano-shaped morphology with rough surface and homogenously distributed on the surface of CFC, which may provide more active sites for OER. The electrochemical measurements show that trimetallic NiFeCoSe_x/CFC possesses the better electrocatalytic activity with the lower overpotential (150 mV at 10 mA cm^?2), Tafel slope (85 mV dec^?1), larger double-layer capacitance (200 mF cm^?2) and long-term stability than unary or binary metal selenides. The enhanced activity of NiFeCoSe_x/CFC may be attributed to the trimetallic NiFeCo selenides and selenides-CFC synergistic interaction. It may offer a promising way to design transition multimetallic selenides supported on conductive support as electrocatalysts for OER.     

Ruthenium quantum dots supported on carbon nanofibers as an efficient electrocatalyst for hydrogen evolution reaction

The hydrogen evolution reaction (HER) is a key step for producing hydrogen by water electrolysis, and an economical, facile and environment friendly method of fabricating catalysts for HER is urgent and essential. In this work, we design a high efficient and stable HER catalyst though a simple adsorption and pyrolysis method. The fabricated catalyst presents ruthenium (Ru) quantum dots (QDs) uniformly distributes on the carbon nanofibers (CNF) with a three dimensional (3D) networks structure (Ru@CNF). By means of quantum size effect of Ru QDs and the 3D networks structure of the carbon nanofibers, the former is beneficial to provide more catalytic active sites and the latter is in favour of electron transport. The sample Ru@CNF exhibits a low overpotential of 20 mV at a current density of 10 mA cm⁻² and Tafel slope of 31 mV dec⁻¹ in 1 M KOH, which is better than that of Pt/C (28 mV and 36 mV dec⁻¹), and most of reported Ru-based and transition metal catalysts. Furthermore, it exhibits robust stability when testing at an overpotential of 75 mV for 24 h. Therefore, this work provides a low-cost, simple and feasible method for fabricating HER catalyst, which possesses commercial application prospect in the field of producing hydrogen by water electrolysis.     

N-doped carbon wrapped CoFe alloy nanoparticles with MoS2 nanosheets as electrocatalyst for hydrogen and oxygen evolution reactions

Developing efficient and cost-effective transition metal-based electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is crucial to generate clean and renewable hydrogen energy. The construction of hybrid catalysts with multiple active sites is an effective approach to promote catalytic performance. Herein, a molybdenum disulfide (MoS₂)-based hybrid with N-doped carbon wrapped CoFe alloy (MoS₂/CoFe@NC) was synthesized through a typical hydrothermal method. The MoS₂/CoFe@NC exhibits excellent electrocatalytic performance with overpotentials of 172 mV for HER and 337 mV for OER at 10 mA cm⁻², and long-term stability of 24-h electrolytic reaction in 1 M KOH solution. The chemical coupling between MoS₂ and CoFe@NC provides improved electronic structures and more accessible active sites. The CoFe@NC substrate accelerates the charge transfer to MoS₂ through a synergistic effect. This work demonstrates that the CoFe@NC is a promising substrate for depositing MoS₂ nanosheets (NSs) to achieve excellent catalytic performance for both HER and OER.     

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.