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.     

Porous NiFe alloys synthesized via freeze casting as bifunctional electrocatalysts for oxygen and hydrogen evolution reaction

Binder-free NiFe-based electrocatalyst with aligned pore channels has been prepared by freeze casting and served as a bifunctional catalytic electrode for oxygen and hydrogen evolution reaction (OER and HER). The synergistic effects between Ni and Fe result in the high electrocatalytic performance of porous NiFe electrodes. In 1.0 M KOH, porous Ni₇Fe₃ attains 100 mA cm⁻² at an overpotential of 388 mV with a Tafel slope of 35.8 mV dec⁻¹ for OER, and porous Ni₉Fe₁ exhibits a low overpotential of 347 mV at 100 mA cm⁻² with a Tafel slope of 121.0 mV dec⁻¹ for HER. The Ni₉Fe₁//Ni₉Fe₁ requires a low cell voltage of 1.69 V to deliver 10 mA cm⁻² current density for overall water splitting. The excellent durability at a high current density of porous NiFe electrodes has been confirmed during OER, HER and overall water splitting. The fine electrocatalytic performances of the porous NiFe-based electrodes owing to the three-dimensionally well-connected scaffolds, aligned pore channels, and bimetallic synergy, offering excellent charge/ion transfer efficiency and sizeable active surface area. Freeze casting can be applied to design and synthesize various three-dimensionally porous non-precious metal-based electrocatalysts with controllable multiphase for energy conversion and storage.     

Novel 13X Zeolite/PANI electrocatalyst for hydrogen and oxygen evolution reaction

In this work, first 13X zeolite was prepared by the hydrothermal method. Then, the composite electrode was fabricated by using 13X zeolite and aniline monomer in nickel foam by electropolymerization technique in an acidic medium (13X/PANI). The synthesized 13X zeolite was characterized by physicochemical characterization techniques such as Fourier transform infra-red (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HR-TEM), X-ray diffraction (XRD) pattern and nitrogen sorption isotherm. 13X/PANI composite was further analyzed by XRD, XPS and FE-SEM techniques. Furthermore, the catalyst activity of the synthesized 13X, PANI and 13X/PANI composite electrodes was evaluated in hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) by using linear square voltammetry (LSV) and Tafel slope method. The Tafel slopes of HER were found to be 203 mV dec⁻¹, 440 mV dec⁻¹ and 282 mV dec⁻¹ for 13X, PANI and 13X/PANI-15 electrodes respectively. While the OER Tafel slopes were found to be 423 mV dec⁻¹, 310 mV dec⁻¹ and 168 mV dec⁻¹ for 13X, PANI and 13X/PANI-15, respectively. 13X/PANI-15 electrodes show excellent catalytic performance about the overpotential at 10 mA cm⁻² for HER and the overpotential at 20 mA cm⁻² for OER. The obtained results suggest fabricated novel electrodes are a potential candidate for HER and OER reaction and can be open new avenue for other electrochemical reactions.     

Facile one-step in-situ encapsulation of non-noble metal Co2P nanoparticles embedded into B,N, P tri-doped carbon nanotubes for efficient hydrogen evolution reaction

Developing high performance, good stability and noble-metal-free electrocatalysts for renewable hydrogen evolution reaction (HER) remain a substantial challenge. Herein, we introduce a novel facile one-step in-situ strategy through pyrolysis for the synthesis of Co₂P nanoparticles encapsulated Boron, Nitrogen, and Phosphorous tri-doped carbon nanotubes (Co₂P/BNP-CNTs). The synergetic effect between Co₂P nanoparticles and heteroatom doped CNTs contributes to the remarkable HER performance. The Co₂P/BNP-CNT-900 electrocatalyst shows a low overpotential of 133 mV at a current density of 10 mA cm⁻² and a small Tafel slope of 90 mV dec⁻¹ in 0.1 M KOH media. More importantly, the Co₂P/BNP-CNT-900 electrocatalyst exhibits superior long-term stability in alkaline solution at −0.25 V versus Reversible Hydrogen Electrode (RHE) for 15 h and up to 1000 cycles with negligible performance loss. Overall, our works suggest a one-pot facile synthesis strategy for rational designing high-performance electrocatalysts with enhanced HER performance.     

Metal and phosphonium-based ionic liquid: A new Co and P dual-source for synthesis of cobalt phosphide toward hydrogen evolution reaction

Phosphonium-based ionic liquid, containing cobalt ion, was applied as novel phosphorus and metal dual-source for fabrication of cobalt phosphide. Trihexyl(tetradecyl)phosphonium tetrachlorocobaltate(II) ([P_6,6,6,14]₂[CoCl₄]) with carbon nanotubes (CNTs) was utilized to obtain the Co₂P/CNTs via one step phosphidation. This material exhibits a good catalytic activity toward hydrogen evolution reaction (HER) including an onset overpotential of 85 mV, a Tafel slope of 47 mV dec⁻¹, current densities of 10 and 20 mA cm⁻² at overpotentials of 150 and 178 mV, and it has a good stability to keep the HER activity. All experimental results confirmed that [P_6,6,6,14]₂[CoCl₄] can form Co₂P without adding other reagent and CNTs can improve the electrical conductivity and contribute to the formation of cobalt phosphide.     

Self-supported three-dimensional WP2 (WP) nanosheet arrays for efficient electrocatalytic hydrogen evolution

The development of highly efficient, stable, eco-friendly and low-cost noble-metal-free electrocatalysts is still a great challenge to generate large scale hydrogen fuel from water. In this concern, self-supported WP₂ and WP nanosheet (NS) arrays were prepared through an in-situ solid-phase phosphidation of WO₃ nanosheet arrays on carbon cloth (CC), whereas, different phosphating temperatures of 650 °C, 800 °C for 2 h, has been utilized to attain different WP₂ NS/CC, WP NS/CC catalysts. Remarkably, the electrocatalysts of WP₂ and WP NS arrays exhibit an outstanding hydrogen evolution (HER) performance in acidic environment, with a low overpotential of 140 mV and 175 mV at 10 mA cm⁻², a Tafel slope of 85 mV dec⁻¹ and 103 mV dec⁻¹, respectively. Furthermore, Density Functional Theory (DFT) calculations reveal that the enhanced HER activity of WP₂ catalyst is attributed to the lowered hydrogen adsorption free energy on WP₂ surface, which is much lesser than that on the WP catalyst surface. As a result, WP₂ exhibit superior intrinsic catalytic activity than WP. This study offers a valuable way for the synthesis of highly efficient three-dimensional self-supporting catalytic electrodes, and beneficial for realizing the intrinsic electrocatalytic properties of tungsten phosphide for improved water splitting reactions.     

Co0.85Se on three-dimensional hierarchical porous graphene-like carbon for highly effective oxygen evolution reaction

Designing an efficient, cheap and abundant catalyst for oxygen evolution reaction (OER) is crucial for the development of sustainable energy sources. A novel catalyst which could be a promising candidate for such electrocatalysts is described. Co_0.85Se supported on three-dimensional hierarchical porous graphene-like carbon (HPG) exhibits outstanding catalytic performances for OER in alkaline medium. It is found that the onset overpotential is 311 mV on the Co_0.85Se/HPG electrode, which is more 28 and 41 mV negative than that on the Co/HPG and Co₃O₄/HPG electrodes. What's more, the value of Tafel slope is 61.7 mV dec⁻¹ and the overpotential at the current density of 10 mA cm⁻² is 385 mV on this electrode. The Co_0.85Se/HPG of this work is an appealing electrocatalyst for OER in basic electrolyte.     

Multi-shelled CoS2–MoS2 hollow spheres as efficient bifunctional electrocatalysts for overall water splitting

The exploration of highly efficient non-precious electrocatalysts is essential for water splitting devices. Herein, we synthesized CoS₂–MoS₂ multi-shelled hollow spheres (MSHSs) as efficient electrocatalysts both for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) using a Schiff base coordination polymer (CP). Co-CP solid spheres were converted to Co₃O₄ MSHSs by sintering in air. CoS₂–MoS₂ MSHSs were obtained by a solvothermal reaction of Co₃O₄ MSHSs and MoS₄²⁻ anions. CoS₂–MoS₂ MSHSs have a high specific surface area of 73.5 m²g^-1. Due to the synergistic effect between the CoS₂ and MoS₂, the electrode of CoS₂–MoS₂ MSHSs shows low overpotential of 109 mV with Tafel slope of 52.0 mV dec⁻¹ for HER, as well as a low overpotential of 288 mV with Tafel slope of 62.1 mV dec⁻¹ for OER at a current density of 10 mA cm⁻² in alkaline solution. The corresponding two-electrode system needs a potential of 1.61 V (vs. RHE) to obtain anodic current density of 10 mA cm⁻² for OER and maintains excellent stability for 10 h.     

Heterostructured Co3O4/VO2 nanosheet array catalysts on carbon cloth for hydrogen evolution reaction

Developing highly efficient, low-cost, and robust water splitting hydrogen production catalysts is critical for hydrogen energy applications. This study presents the synthesis of Co₃O₄/VO₂ heterogeneous nanosheet structures on carbon cloth (Co₃O₄/VO₂/CC). The obtained Co₃O₄/VO₂/CC hybrid catalyst has a low overpotential of 108 mV at a current density of 10 mA cm^?2, a Tafel slope of 98 mV dec^?1, and high stability in 1.0 M KOH for 10 h. The experimental results and density functional theory (DFT) calculations results also show that Co₃O₄ coupled with VO₂ in Co₃O₄/VO₂/CC can optimize hydrogen adsorption energy and facilitate electron transport, thereby accelerating the catalytic kinetics for hydrogen evolution reaction (HER). This work also provided an alternative method to design and construct non-noble metal oxide-based catalysts for alkaline hydrogen production.     

Interface regulation of Pt quantum dots doped nickel phosphide and cobalt hydroxide to promote electrocatalytic overall water splitting

The transition metal phosphates are earth-abundant minerals that have been shown to perform well in electrocatalytic water splitting, whereas these catalysts still tend to have excessively high overpotentials and slow kinetics in HER and OER processes. In the present work, hybrid catalysts consisting of Pt quantum dots doped NiP (NiP-Pt) nano-embroidery spheres and Co(OH)₂ nanosheets were successfully prepared by two-step electrodeposition method. The excellent catalytic performance of the catalyst relies principally on the synergistic interaction between NiP and Pt quantum dots. Additionally, the NiP-Pt exhibits strong electronic interactions at the interface with Co(OH)₂. Consequently, the catalyst has a strong catalytic performance in terms of HER and OER catalytic performance. In terms of HER, an overpotential of only 40 mV is required when the current density reaches 10 mA cm^?2, corresponding to a Tafel slope of 49.85 mV·dec^?1. At the same time, the catalyst also performs well at OER, with a current density of 10 mA cm^?2 at an overpotential of 186 mV and a Tafel slope of 53.049 mV·dec^?1 much less than most electrocatalysts. This study involving electrodeposition and doping of quantum dots provides a new idea for the efficient synthesis of fundamental HER and OER bifunctional catalysts.     

Ni–Zn nanosheet anchored on rGO as bifunctional electrocatalyst for efficient alkaline water-to-hydrogen conversion via hydrazine electrolysis

Bifunctional Ni-15 at.% Zn/rGO catalyst was fabricated by a two-step electrodeposition to be used for efficient alkaline water-to-hydrogen conversion via hydrazine electrolysis. Experiments show that the as-deposited Ni-15 at.% Zn/rGO nanosheet arrays with porous structure possesses excellent catalytic activity and stability towards both hydrogen evolution reaction (HER) and hydrazine oxidation reaction (HzOR). A small overpotential of 49 mV at 10 mA cm⁻² with a low Tafel slope of 26.3 mV dec⁻¹, and a retention rate of 91.4% after 12 h at 10 mA cm⁻² are observed for Ni-15 at.% Zn/rGO towards HER. Moreover, Ni-15 at.% Zn/rGO also shows an extra-high current density of 1097 mA cm⁻² at 0.6 V vs RHE with a low Tafel slope of 33.5 mV dec⁻¹, and a high durability of 90.5% after 5000 s towards HzOR. Moreover, two-electrode cell was constructed using Ni-15 at.% Zn/rGO as both cathode and anode for HER and HzOR, achieving 100 mA cm⁻² at an ultralow cell voltage of 0.418 V. The above outstanding bifunctional catalytic performance should be attributed to its large ECSA, high electrical conductivity and most importantly, its superaerophobic surface induced by the porous structure with nanosheet arrays.     

Ru nanoclusters coupling with hierarchical phosphorus and oxygen dual-doped carbon nanotube architectures for effective hydrogen evolution reaction

The construction of an effective catalyst for hydrogen evolution reaction (HER) is a top priority. Herein, we demonstrate ruthenium (Ru) nanoclusters coupled with phosphorus and oxygen dual-doped carbon nanotube (CNT) architecture (Ru-POCA). The increased hydrophilicity and negatively charged surface of CNTs can strongly trap Ru ions. The hierarchical structure is favorable of providing abundant pathways and exposing more active sites for HER. Due to the synergistic effect of the hierarchical structure and modified surface chemistry, Ru-POCA exhibits excellent catalytic HER activity. The overpotential is 22 and 40 mV with a Tafel slope of 28.0 and 27.1 mV dec⁻¹ in 1 M KOH and 0.5 M H₂SO₄ at 10 mA cm⁻². Moreover, Ru-POCA processes good catalytic stability in both acidic and alkaline electrolytes, while the boosted catalytic HER activity is fundamentally studied by density functional theory calculation. This work provides a rational approach to constructing hierarchically structured Ru-CNTs-based catalysts for hydrogen evolution reaction.     

Hydrothermally/electrochemically decorated FeSe on Ni-foam electrode: An efficient bifunctional electrocatalysts for overall water splitting in an alkaline medium

A new hybrid catalyst based on Ni foam (NF) and FeSe was prepared by a facial hydrothermal method, in which Se-decorated NF was subsequently electrochemically doped by Fe. Binder-free catalyst containing electrodes were directly tested for the hydrogen and oxygen evolution reaction (HER/OER). The FeSe/NF electrode displayed an OER current density of 100 mA cm⁻² at potential of 1.42 V, and a relatively small Tafel slope of 109 mV dec⁻¹ in a 1 M KOH solution. Also, FeSe/NF electrode exhibited reasonable HER overpotential of 200 mV at 10 mAcm⁻² current density with Tafel slope of 145 mV dec⁻¹. The XRD and TEM studies revealed that the formation of heterogeneous interfaces of NiSe₂ and FeSe₂,generated more active sites that can promote better ions and electron transport in the electrode/electrolyte interfaces. Furthermore, HRTEM analysis indicates that FeSe₂ rich in Se vacancy defects can be created with suitable M − O and M − H bond for better OER and HER performance, respectively. In a-two electrode alkaline water electrolyzer, current densities of 10 mA cm⁻² and 50 mA cm⁻² were obtained at cell voltages of 1.52 V and 1.85 V, respectively, using pure FeSe–NF as both the cathode and anode.     

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.     

Ni/NiO@rGO as an efficient bifunctional electrocatalyst for enhanced overall water splitting reactions

Herein, we fabricated bifunctional, noble metal-free, highly efficient nickel/nickel oxide on reduced graphene oxide (Ni/NiO@rGO) by chemical synthesis approach for electrochemical water splitting reaction. Its structural and morphological characterization using thermogravimetric analysis (TGA), transmission electron microscopy (TEM), field emission scanning electron microscope (FESEM), energy dispersive analysis of X-ray (EDAX) and X-ray diffraction (XRD) represents, Ni/NiO@rGO is having Ni/NiO NPs ∼10 nm (±2 nm) on graphene oxide with face-centered cubic (FCC) crystal structure. Moreover, the presence of Ni/NiO (2.26%), O (6.56%), N (0.74%) and C (90.44%) from EDAX analysis further confirms the formation of Ni/NiO@rGO and it also supported by FTIR studies. This nanocatalyst is examined further for electrocatalytic water splitting reactions (HER and OER). It demonstrated low overpotential 582 mV to achieve current density at 10 mA cm⁻² and smaller Tafel slope of 63 mV dec⁻¹ obtained in 0.5 M H₂SO₄ towards HER. Also, at the other end at onset potential of 1.6 V vs. RHE towards OER. It demonstrated low overpotential 480 mV to achieve current density at 10 mA cm⁻² and smaller Tafel slope of 41 mV dec⁻¹ in 0.5 M KOH towards OER observed. Hydrogen fuel is eco-friendly to the environment and noteworthy performance of earth-saving reactions.     

One-step electrodeposition of cauliflower-like CozNiySx@polypyrrole electrocatalysts on carbon fiber paper for hydrogen evolution reaction

Transition-metal chalcogenides as the promising alternatives to noble-metal-based electrocatalysts for hydrogen evolution reaction (HER) with high activity and durability in water splitting have attracted extensive attention in recent years. Herein, Co_zNi_yS_x@PPy composites with three-dimensional (3D) cauliflower-like were firstly prepared on carbon fiber paper (CFP) via a simple and efficient electrochemical reduction of elemental sulfur in the precursor of S@PPy composite coated on CFP to react with Co and Ni ions in the electrolyte. The optimum electrode, i.e., Co_zNi_yS_x@PPy/CFP-6 (A-6) prepared by using an electrolyte with a Co/Ni molar ratio of 0/6, showed excellent catalytic activity (with an overpotential of 185 mV@10 mA cm⁻² and a small Tafel slope of 78.13 mV dec⁻¹) as well as long-term stability (at least 100 h) in 1 M KOH solutions. This work provides a novel way to fabricate effective and non-noble-metal electrodes for HER in water splitting.     

High-performance alkaline hydrogen evolution of NiMoP2 nanowire boosted by bimetallic synergic effect

Hydrogen evolution reaction (HER) in alkaline conditions usually requires a higher overpotential compared with acidic conditions due to the extra energy barrier originating from the additional water dissociation step. In this work, the theoretical calculation has confirmed that this challenge can be solved by the bimetallic synergic effect on binary transition metal catalysts. We report a simple method to synthesize NiMoP₂ nanowires with (100) plane which can precisely expose Ni and Mo atom. The synthesized NiMoP₂ nanowires exhibit a small overpotential of 87 mV to reach 10 mA cm⁻² with a low Tafel slope of 66 mV dec⁻¹ and long-term stability in alkaline solutions.     

Three-dimensional Ni2P–MoP2 mesoporous nanorods array as self-standing electrocatalyst for highly efficient hydrogen evolution

Electrochemical hydrogen evolution reaction (HER) via the splitting of water has required electrocatalysts with cost-effectiveness, environmentally friendliness, high catalytic activity, and superior stability to meet the hydrogen economy in future. In this context, we report the successful synthesis of self-standing mesoporous Ni₂P–MoP₂ nanorod arrays on nickel foam (Ni₂P–MoP₂ NRs/3D-NF) through an effective phosphidization of the corresponding NiMoO₄ NRs/3D-NF. The as-synthesis Ni₂P–MoP₂ NRs/3D-NF, as an efficient HER electrocatalyst, exhibits small overpotential of 82.2 and 124.7 mV to reach current density of 10 and 50 mA cm⁻², a low Tafel slope of 52.9 mV dec⁻¹ and it retains its catalytic performance for at least 20 h in alkaline condition. Our work also offers a new strategy in designing and using transition metal phosphide-based 3D nanoarrays catalysts with enhanced catalytic efficiency for mass production of hydrogen fuels.     

Partial phosphorization of porous Co–Ni–B for efficient hydrogen evolution electrocatalysis

Design of cost-effective and high-efficient electrocatalysts for hydrogen evolution reaction (HER) is of vital significance for the current renewable energy devices — fuel cells. Herein, we report a facile strategy to prepare partial phosphorization of Co–Ni–B material with porous structure via a water-bath boronizing and subsequent phosphorization process at moderate temperature. The optimal atomic proportion of Co to Ni is investigated via physical and electrochemical characterization. As a result, Co₉–Ni₁–B–P exhibits the best HER activity, which require an lower overpotential of ~192 mV to deliver a current density value of 10 mA cm⁻² and a smaller Tafel slope of 94 mV dec⁻¹ in alkaline media, relative to P-free Co–Ni–B catalysts, Co₉–Ni₁–B–P with other Co: Ni proportion and mono metallic borides The excellent electrocatalytic performance of Co₉–Ni₁–B–P is mainly ascribed to the three-dimensional (3D) porous structure and the coordinate functionalization between the borides and phosphides. This work provides a promising strategy for the exploration of quaternary composites as efficient and cost-effective electrocatalysts for HER.     

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.