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.     

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.     

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.     

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.     

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.     

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.     

Free-standing carbon nanotubes as non-metal electrocatalyst for oxygen evolution reaction in water splitting

Oxygen evolution reaction (OER) has significant impact on the overall electrochemical water splitting. We introduce, for the first time, a facile approach towards the fabrication of versatile electrode composed of free-standing multiwalled carbon nanotubes (MWCNTs) as electrocatalyst for the water splitting reaction. Directly extracted MWCNTs as sheets from vertically grown arrays transferred over the glass substrate, are used without any post treatment as a working electrode for OER. Onset potential of 1.60 V was achieved for MWCNTs which is significantly reduced as compared to platinum based metal electrode (1.72 V) with excellent current density. No surface modification, metal-free nature, flexibility and low cost with excellent catalytic activity proved this material as a promising candidate for the replacement of metal based electrodes in electrochemical water splitting.     

An efficient way to improve water splitting electrocatalysis by electrodepositing cobalt phosphide nanosheets onto copper nanowires

Herein, catalysts with improved electrocatalysis of water splitting in alkaline media are accomplished by constructing bushy Cu nanowires on Cu foam to support cobalt phosphide nanosheets (Co–P/Cu NWs/CF). Results demonstrate that the highly efficient electroactivity of Co–P/Cu NWs/CF originates from the high intrinsic activity of Co–P species, Cu nanowires which possess accessible active sites, plentiful bubbles releasing channels, and honored electric conductivity, as well as the interaction between Cu NWs and Co–P species. To achieve a current density of 200 mA cm^?2, Co–P/Cu NWs/CF require an overpotential of only 155 mV for hydrogen evolution and 349 mV for oxygen evolution, which are less than that of Co–P/Cu(OH)₂ NWs/CF and Co–P/CF. This work provides a simple and efficient approach to designing water splitting electrocatalysts by manufacturing nanostructure-rich conductive substrates to support active species.     

Facile synthesis of binary NiCoS nanorods supported on nickel foam as efficient electrocatalysts for oxygen evolution reaction

A facile two-step method has been applied to synthesize novel binary metal NiCoS nanorods supported on nickel foam (NF) as electrocatalysts for oxygen evolution reaction (OER). Firstly, electrodeposition process is conducted to fabricate binary Ni-Co hydroxides on NF (NiCo/NF). Then, a hydrothermal sulfuration of NiCo/NF has been adopted to prepare NiCoS nanorods arrays uniformly grown on the surface of NF (NiCoS/NF). XRD indicates that NiCoS/NF has mixed crystal phases of Ni₃S₂, CoS and Co₉S₈. SEM images display the uniform NiCoS nanorods composed of many vertical nanosheets on the surface, implying more exposed active sites. OER measurements demonstrate that NiCoS/NF has better activity with an overpotential of 370 mV to reach 100 mA cm^?2 than NiCo/NF and CoS_x/NF. Electrochemical impedance spectroscopy (EIS) tests confirm the faster charge-transfer rate of NiCoS/NF and smaller Tafel slope derived from binary NiCoS, implying the excellent electrocatalytic performances of binary metal sulfides.     

Vertically grown CoS nanosheets on carbon cloth as efficient hydrogen evolution electrocatalysts

The development of efficient and inexpensive water splitting electrocatalysts is essential for the large-scale production of hydrogen. Herein, we show that a novel nanohybrid with CoS nanosheets vertically grown on carbon cloth (CoS/CC) can be used as an efficient self-supported hydrogen-evolving cathode for water splitting over a wide pH range. This material affords a current density of 10 mA/cm² at a small overpotential of 192 mV and 212 mV in basic and acidic media, respectively, along with a long-term stability for over 50 h. The unique 3D structure constructed by the vertically arranged nanosheets and the intimate contact between the CoS nanosheets and the underlying conductive carbon are believed to be responsible for the excellent catalytic performance.     

Cobalt phosphide nanoparticles film growth on carbon cloth: A high-performance cathode for electrochemical hydrogen evolution

In this communication, cobalt phosphide nanoparticles film was developed on carbon cloth (CoP NPs/CC) through low-temperature phosphidation of its corresponding Co NPs/CC precursor. When directly used as a cathode for electrochemical hydrogen evolution in strongly acidic solutions, the CoP NPs/CC electrode exhibits high performance with a low onset overpotential of 33 mV, a Tafel slope of 70 mV dec⁻¹ and a Faradaic efficiency of nearly 100%. This catalyst maintain its catalytic activity for at least 30 h and only needs overpotentials of 48 and 190 mV to attain current densities of 10 and 100 mA cm⁻², respectively.     

A high active hydrogen evolution reaction electrocatalyst from ionic liquids-originated cobalt phosphide/carbon nanotubes

Ionic liquid/carbon nanotubes (IL/CNTs) composite was applied as the precursor to prepare CNTs-supported cobalt phosphide via low-temperature phosphidation. CoP_(MBMG)/CNTs, generated from N,N-bis(4-(methoxycarbonyl)benzyl)-N-methyl-d-glucaminium dibromodichlorocobaltate(II) (MBMG)₂-CoCl₂Br₂), exhibits the best catalytic activity toward hydrogen evolution reaction with an onset overpotential of 55 mV, a Tafel slope of 58 mV dec^?1, 95% Faradaic efficiency (FE), current densities of 10 and 20 mA cm^?2 at overpotentials of 135 and 160 mV, and it can maintain the catalytic activity for at least 27 h. FT-IR, Raman spectroscopy, XPS and XRD were utilized to investigate the phosphidation process. All experimental results confirmed that anion from (MBMG)₂-CoCl₂Br₂ can form CoP and glucaminium-based cation can become amorphous carbon after phosphidation to obtain the high HER activity of CoP_(MBMG)/CNTs.     

Manganese oxide with different morphology as efficient electrocatalyst for oxygen evolution reaction

Three-dimensional (3D) manganese oxides consisted of tetragonal phase Mn₃O₄ and α-MnO₂ with different morphology have been directly grown vertically on Ti foil by a simple electrochemical method without any template and used as the catalysts for oxygen evolution reaction (OER). The results show that manganese oxides with different morphology show high activity and good stability for OER and the manganese oxide (MnO_x) nanowire arrays obtained at 70 °C show higher activity and better stability than MnO_x with cotton wool structure and MnO_x nanosheet arrays.     

CoP@NC electrocatalyst promotes hydrogen and oxygen productions for overall water splitting in alkaline media

Design and synthesis of high-performance bi-functional electrocatalysts can play a crucial role for electrolytic water splitting. Herein, we develop a simple phosphating process to construct cobalt phosphide@nitrogen-doped carbon (CoP@NC) using metal organic frame (MOF) as a precursor and a template. In alkaline solution, CoP@NC-350 exhibits exceptional hydrogen and oxygen evolution reaction performances with over potentials of 75 mV and 268 mV at 10 mA cm^?2, respectively. For a symmetric CoP@NC-350 two-electrode water splitting setup, the potential can be low as 1.69 V to obtain 10 mA cm^?2. Therefore, low-temperature phosphating treatment can be a simple and promising method to produce electrocatalysts for water splitting.     

Large-area manganese oxide nanorod arrays as efficient electrocatalyst for oxygen evolution reaction

Large-area manganese oxide nanorod arrays (MnO₂ NRAs) have been directly grown vertically on Ti foil with a uniform length and diameter by a simple electrochemical method without any templates. The deposition temperature is one of the most important parameters for formation MnO₂ NRAs and at 25 °C no MnO₂ NRAs can be obtained. The results show that MnO₂ has high activity and good stability for oxygen evolution reaction (OER) and the structure of nanorod arrays pronounced enhances MnO₂ activity. The onset potential of MnO₂ NRAs is lower than that of Pt foil and lower 401 mV than that of MnO₂ film, indicating that the structure of MnO₂ NRAs shows an easy OER for water split. The MnO₂ NRAs may be of great potential in electrochemical water split.     

One-step synthesis of carbon-encapsulated nickel phosphide nanoparticles with efficient bifunctional catalysis on oxygen evolution and reduction

As a promising and cost-efficient alternative to noble metal catalysts, transition metal phosphides (TMPs) show highly catalytic performance toward oxygen reduction and evolution reactions (ORR and OER). Mesoporous carbon-coated nickel phosphide (NiP) nanoparticles were successfully synthesized by thermal decomposition at 500 °C under N₂/H₂ (95:5) atmosphere. The NiP/C hybrid exhibits excellent OER/ORR activity. It can generate an OER current density of 10 mA cm^?2 at the overpotential of 0.26 V with a low Tafel slope of 43 mV dec^?1, and produce a limited ORR current density of 5.10 mA cm^?2 at 1600 rpm with a half-wave potential of 0.82 V via a 4-electron pathway. In addition, the OER/ORR catalytic currents remain considerable stable without significant loss for more than 25 h polarization. This work will open up a new avenue to design a bifunctional catalyst with a superior OER/ORR activity and stability, and this cost-efficient strategy will pave the way for the industrial application of the renewable energy technologies.     

Phosphorus-doped nickel selenides nanosheet arrays as highly efficient electrocatalysts for alkaline hydrogen evolution

Earth-abundant transition-metal dichalcogenides are considered as promising electrocatalysts to accelerate the hydrogen evolution reaction （HER）. Among them, the pyrite nickel diselenide (NiSe₂) has been received special attention due to its low cost and high conductivity, but it suffers a poor HER performance in alkaline media possibly attributed to its inadequate hydrogen adsorption free energies. Here, we report a novel P-doped NiSe₂ nanosheet arrays anchored on the carbon cloth with an obviously optimized HER performance. The catalyst only needs a low overpotential of 86 mV at a current density of 10 mA cm^?2 and a Tafel slop of 61.3 mV dec^?1,as well as maintains a long-term durability for 55 h in 1.0 M KOH, which is superior to the pristine NiSe₂ (135 mV@10 mA cm^?2) and most recently reported non-noble metal electrocatalysts. The XRD, EDS, TEM and XPS results validated the successful doping of P element into NiSe₂ nanosheet, while the density functional theory (DFT) calculation demonstrated the P doping can optimize the electronic structures and the hydrogen adsorption free energy of NiSe₂. This work thus opens up new ways for rationally designing high-efficient HER electrocatalysts and beyond.     

Defect-induced FexNi1-xSe2 nanoparticles based electrocatalysts towards enhanced hydrogen and oxygen evolution reactions

Transition metal selenides are regarded as promising materials for the production of clean energy through electrocatalytic water splitting. Creation of defects in these metal selenides is one of the prudent strategies to enrich the active sites which in turn enhances the electrocatalytic activity of these materials and makes them viable for broader applications. Herein, defect-induced, iron-doped nickel selenide nanoparticles were prepared for the first time and their electrocatalytic efficacy towards hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) has been demonstrated. Fe_xNi_1-xSe₂ nanoparticles (x = 0.25, 0.50, 0.75) were prepared using a facile hydrothermal method, in which defects were induced by annealing at 300 °C to obtain DI-Fe_xNi_1-xSe₂. The structural and morphological investigations confirmed the size reduction and creation of defects after annealing, without any significant change in the crystal structure, which in turn is expected to promote the electrocatalytic activity. Accordingly, among all the materials investigated, DI-Fe_0.25Ni_0.75Se₂ has shown the highest HER activity in 0.5 M H₂SO₄ at a lesser overpotential of 128 mV at 10 mA cm^?2 and the Tafel slope was calculated to be 37.9 mV dec^?1. Interestingly, the same material has displayed high performance towards OER in 1 M KOH with a lesser overpotential at 205 mV and a Tafel slope of 55.5 mV dec^?1. Thus obtained electrocatalytic activity was much better than the reported nickel selenide based electrocatalysts. Further, the DI-Fe_0.25Ni_0.75Se₂ electrocatalyst has demonstrated impressive stability in the acidic and alkaline medium during continuous electrolysis even up to 12 h.     

Nickel phosphide decorated Pt nanocatalyst with enhanced electrocatalytic properties toward common small organic molecule oxidation and hydrogen evolution reaction: A strengthened composite supporting effect

In this paper, well-dispersed Ni₂P-NiP₂-Pt/CNTs catalyst promoted by nickel-phosphorus compounds was readily synthesized by a two-step hydrothermal process. The as-synthesized Ni₂P-NiP₂-Pt/CNTs displayed improved electrocatalytic properties towards electro-oxidation of common small organic fuels such as methanol, ethanol and formic acid in contrast with Pt/CNTs and Pt/CNPs in acidic electrolytes. Meanwhile, the Ni₂P-NiP₂-Pt/CNTs catalyst also exhibited the excellent performance toward hydrogen evolution reaction with a more negative onset potential (?15 mV) and a smaller Tafel slope (29.8 mV dec^?1) when compared with Pt/CNTs (?29 mV, 30.6 mV dec^?1) and Pt/CNPs (?32 mV, 31.3 mV dec^?1) in 1.0 M H₂SO₄ solution. The catalytic activity enhancement possibly derives from the induced large specific surface area of carbon nanotubes as well as the strengthened synergistic effect between multiple supporting interactions.     

Maize-like CoP nanorod arrays as an efficient and robust electrocatalyst for superior hydrogen generation

High-performance, low-cost and robust electrocatalysts for the hydrogen evolution reaction (HER) play a critical role in large-scale hydrogen production via water splitting. Herein, we proposed a synthesis strategy for the self-assembly of maize-like CoP nanorod arrays with abundant active sites via a combination of conventional hydrothermal reaction and low-temperature phosphorization. This unique architecture exhibited remarkable catalytic performance for the HER, with a low overpotential of 130 mV at a current density of 10 mA cm^?2 and a small Tafel slope of 59 mV dec^?1 in 1.0 M KOH electrolyte, as well as good stability as verified by chronoamperometry measurement for 10 h. Density functional theory calculations further revealed that these maize-like CoP nanorod arrays with dense active sites and a high phosphorization degree could boost the HER performance in terms of low adsorption energy and free energy. This work provided a facile strategy towards manipulating morphology engineering to enhance the HER activity of CoP-based catalysts.