Facile preparation of Ni,Co - Alloys supported on porous carbon spheres for supercapacitors and hydrogen evolution reaction application

Carbon nanomaterials that are electrochemically bifunctional and active in supercapacitor and hydrogen-evolution-reaction (HER) applications have attracted recent research attention. We have prepared porous carbon spheres - doped Ni, Co - alloys by using a hydrothermal method with melamine and pectin as precursors and with the addition of nickel nitrate and cobalt nitrate. The corresponding materials exhibit good electrochemical performance and excellent electrocatalytic activity for HER. The overpotential (OP) of the as-prepared materials can achieve 240 mV keeping a Tafel slope (TS) of 55 mV dec⁻¹ at 10 mA cm⁻² in an acid (0.5 M H₂SO₄) solution. The corresponding samples maintain stability after 24 h and 5000 cycles in the chronovoltage and cyclic voltammetry methods, respectively. When the as-prepared materials are oxidized by 30% H₂O₂ for 12 h, the corresponding as-prepared oxidation samples exhibit excellent electrochemical performance in a supercapacitor application. The specific capacitance (SC) of the as-obtained materials reaches 312 F g⁻¹ at 1 A g⁻¹ with decent rate capability and cyclic stability. This work provides new applications for bifunctional electrochemically active materials.     

Facile galvanic replacement method for porous Pd@Pt nanoparticles as an efficient HER electrocatalyst

In realm of renewable energy, development of an efficient and durable electrocatalyst for H₂ production through electrochemical hydrogen evolution reaction (HER) is indispensable. Herein, we demonstrate a simple preparation of carbon-supported nanoporous Pd with surface coated Pt (CS–PdPt) by a simple galvanic replacement reaction (GRR). The phase purity and porosity have been confirmed by XRD, HRTEM, and N₂ sorption techniques. As HER electrocatalyst, CS-PdPt showed a low overpotential of 26 mV in 0.5 M H₂SO₄ at current density of 10 mA cm⁻², which is lower than the commercial Pt/C electrode. The CS-PdPt catalyst exhibits an overpotential of 46 mV in 1 M KOH, and 50 mV in neutral buffer (1 M PBS) at 10 mA cm⁻². The CS-PdPt furnished with small Tafel values of 33, 88, and 107 mV dec⁻¹ in acidic, alkaline, and neutral medium, respectively. Accelerated durability test at 100 mV s⁻¹ for 1000 cycles demonstrated a negligible change in HER activity.     

In-situ synthesis of porous Ni2P nanosheets for efficient and stable hydrogen evolution reaction

The design and development of highly efficient and stable non-noble metal electrocatalysts for hydrogen evolution reaction (HER) have attracted increasing attention. However, some key issues related to large overpotential, high cost and poor stability at high current density still remains challenging. In this work, we report a facile in-situ integration strategy of porous Ni₂P nanosheet catalysts on 3D Ni foam framework (PNi₂P/NF) for efficient and stable HER in alkaline medium. The two-step method can creates high density of ultra-thin porous Ni₂P nanosheets firmly rooted into Ni foam substrate which can guarantee excellent electrical contacts, strong substrate adherence and large amount of active sites. Such a binder-free flexible HER cathode exhibits superior electrocatalytic performance with an overpotential of 134 mV at current density of 10 mA cm⁻². It also shows superior stability at higher current densities of 100 and 500 mA cm⁻² for at least 48 h and negligible performance degradation is observed.     

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.     

Cellulose nanocrystals (CNC) derived Mo2C@sulfur-doped carbon aerogels for hydrogen evolution

Hydrogen evolution reaction (HER), is considered as an ideal alternative approachs to settle the energy crisis. Therefore, we need to explore efficient and stable non-Pt-based electrocatalysts for hydrogen production from water electrolysis. In this work, S-doped ultrafine molybdenum carbide anchored on cellulose nanocrystals (CNC) derived carbon composite aerogels (Mo₂C@S-CA) were synthesized for HER by a simple one-step carbonization method, utilizing inorganic-organic hybrid ammonium molybdate/CNC (AMM/CNC) as precursor. The obtained Mo₂C@S-CA aerogels can not only provide plenty of active sites, but also accelerate the hydrogen release from the reaction surface of the electrocatalysts. The as-synthesized catalysts exhibit superior HER activity with a small overpotential value of 176 mV vs. RHE at 10 mA cm^?2 and excellent long-term stability after 10,000 cycles in 0.5 M H₂SO₄. These superb properties make the catalyst be a promising electrocatalyst for the HER. This work highlights the importance of biomass-derived multifunctional value-added composite aerogels in enhancing the electrolysis of water.     

Synthesis of Ni2P/Ni5P4 on porous N decorated rGO foam for efficiently electrocatalytic hydrogen evolution reaction

As a new generation of non-precious metal catalysts, nickel phosphide is regarded as an ideal substitute for precious metal platinum in electrochemical hydrogen evolution. Here, a hydrogen evolution reaction (HER) electrocatalyst is developed by in situ growth of Ni₂P/Ni₅P₄ heterostructures on porous N decorated rGO foam (named Ni₂P/Ni₅P₄/N-rGO). The porous rGO foam structure provides a larger surface area and abundant active sites. The Ni₂P/Ni₅P₄ nanoparticles with heterostructures are uniformly distributed on the rGO sheet, which enhance the charge transfer ability. The decorating of N element also correspondingly improves the HER performance. The as-prepared Ni₂P/Ni₅P₄/N-rGO exhibits excellent HER performance in alkaline medium. When the current density is 10 mA cm^?2, the overpotential is only 22 mV. No obvious loss of HER activity after 2000 cyclic voltammetry indicates that the composite has excellent stability. This work presents a valuable route for fabricating inexpensive and high-performance catalysts for electrocatalysis.     

Ni(OH)2/ NiSe2 hybrid nanosheet arrays for enhanced alkaline hydrogen evolution reaction

Developing efficient, non-noble electrocatalysts toward hydrogen evolution reaction (HER) in alkaline electrolytes is of important significance for future energy supplement but still a challenge. Recently, pyrite-type NiSe₂ nanomaterial has been considered as an idea HER electrocatalyst due to its high conductivity, strong corrosion resistance and low cost. However, the HER performance of NiSe₂ in alkaline electrolytes is still unsatisfactory, which is possibly limited to the activate water dissociation in alkaline media. Herein, a novel hybrid electrocatalyst of Ni(OH)₂/NiSe₂ nanosheet arrays on carbon cloth (Ni(OH)₂/NiSe₂/CC) is fabricated, exhibiting excellent HER catalytic activity with a low overpotential of 82 mV to drive a current density of 10 mAcm⁻² as well as maintaining a long-term durability for 12 h in 1.0 M KOH, which is 77 mV less than that of NiSe₂/CC and superior to most recently reported non-noble HER electrocatalysts. In addition, the Tafel slope of Ni(OH)₂/NiSe₂/CC (60 mV dec ⁻¹) is also much smaller than that of NiSe₂/CC (112 mV dec ⁻¹), suggesting a promotion kinetics of HER process for Ni(OH)₂/NiSe₂/CC. Our further experimental results show that the significantly improved activity of Ni(OH)₂/NiSe₂/CC electrode should ascribe to its enlarged active surface, good conductivity and interfacial synergy between Ni(OH)₂ and NiSe₂. The synergetic strategy may provide an efficient way to promote the HER activity of other non-noble transition metal selenides in alkaline electrolyte.     

Ni(II) supramolecular gel-derived Ni(0) nanoclusters decorated with optimal N,O-doped graphitized carbon as bifunctional electrocatalysts for oxygen and hydrogen evolution reactions

Developing bifunctional, inexpensive and scalable electrocatalyst for both oxygen and hydrogen evolution reactions (OER and HER) is of essence, considering the thrust for clean fuel hydrogen, and the association of OER with several renewable energy systems, including metal-air batteries. A systematic understanding of electrocatalysts based on the amount and speciation of heteroatom doping on the carbon matrix is fundamental to catalyst design, but remains rarely investigated. This work presents the controlled synthesis of a series of homogeneously dispersed Ni nanoclusters confined in multiple layers of heteroatom-doped graphitized carbon, from the pyrolysis of a readily preparable Ni(II)-triazole gel. The best catalyst showed superior activity requiring low overpotentials of 360 mV & 250 mV and Tafel slopes of 69 mV dec^?1 & 115 mV dec^?1 for OER and HER respectively, with prolonged stability under challenging electrocatalytic conditions. Judicious modulation of the type of heteroatom dopants on Ni@N,O-doped carbon redistributed the electron-density and provided additional active sites, which assisted the adsorption/desorption of OER and HER intermediates during electrocatalysis and improved electron conductivity, benefitting both OER and HER. Our results highlight a simplistic approach for the meticulous synthesis of bifunctional electrocatalysts from supramolecular metallogels, opening new horizons for designing materials for energy applications.     

Fabrication of NiSx/C with a tuned S/Ni molar ratio using Ni2+ ions and Amberlyst for hydrogen evolution reaction (HER)

Recently, NiS_x based catalysts are promising electrocatalysts for Hydrogen Evolution Reaction (HER) via water electrolysis. In particular, the S/Ni ratio is crucial to improve catalytic activity of NiS_x based catalysts. Herein, we synthesized NiS_x based catalysts (Ni/S/C) with a tuned S/Ni molar ratio using Ni²⁺ ions exchange resin. We succeeded in synthesizing Ni/S/C with different S/Ni molar ratio in the range of 0.33–1.72 by changing Ni²⁺ ions exchange degree. The combination of NiS_x and conductive carbon support contributes to high catalytic activity of Ni/S/C on HER. Additionally, Ni/S/C with the S/Ni molar ratio of 0.6 showed the highest onset potential; Ni₃S₂ is the most active catalyst for HER in NiS_x species. Our synthesis method can easily tune the ratio of S/transition metal. This work provides a new direction for the catalyst design of transition metal sulfides and expands their utilization in sustainable catalytic reaction processes.     

Well-dispersed Pt nanodots interfaced with Ni(OH)2 on anodized nickel foam for efficient hydrogen evolution reaction

Hindered by price and scarcity, the exploitation of supported Pt-based electrocatalysts with Pt single atoms or Pt nanoclusters is an alternative way to decrease the dosage of Pt and improve the electrocatalytic performance for hydrogen evolution reaction (HER) of water splitting. The anodization technology is used to modify the surface of nickel foam (NF) to form the porous NiF₂ network structure. Then Pt nanodots interfaced with Ni(OH)₂ (Pt/Ni(OH)₂) hybrid on the anodized NF has been in-situ synthesized by a simple hydrothermal decomposition method. Results show that Pt nanodots on the substrate have good dispersion with the average size of 3 nm, and the Pt loading is only 0.229 mg cm⁻². The prepared electrode exhibits the low overpotentials of 25.9 mV and 211 mV at the current densities of 10 and 100 mA cm⁻², respectively, a small Tafel slope of 37.6 mV dec⁻¹, and the excellent durability for HER. The porous network nanostructure of Pt/Ni(OH)₂ hybrid, the large electrochemical surface area, the fast facilitated electron transport capability, and the firm adhesion of Pt nanodots with the anodized NF substrate contribute to the remarkable performance towards HER.     

Constructing highly active interface between layered Ni(OH)2 and porous Mo2N for efficient electrocatalytic oxygen evolution reaction

Layered Ni(OH)₂ materials are cheap and efficient electrocatalyst for water splitting. However, pristine Ni(OH)₂ materials usually show poor activity due to the low activity sites and poor conductivity in electrochemical reactions. Herein, layered Ni(OH)₂ nanosheets are grown on the porous Mo₂N particles for improved interfacial active sites and enhanced conductivity in the oxygen evolution reaction (OER). The OER overpotential of the optimized Mo₂N/Ni(OH)₂ composite material is distinctly reduced compared with pristine Ni(OH)₂. In addition, the optimized Mo₂N/Ni(OH)₂ composite material exhibits favorable durability in alkaline electrolyte. Further electrochemical investigation reveals that the Mo₂N/Ni(OH)₂ composite materials produce increased charge transfer capability and electrochemical active surface area. Theoretical calculation study demonstrates that a redistribution of electron occurred at the interface of Ni(OH)₂ and Mo₂N, which results in the decrease of energy barrier for the adsorption of OER reactive intermediates at the interfacial atoms. The enhanced performance of OER is thus mainly come from the constructed interface between layered Ni(OH)₂ and porous Mo₂N. This work gives a feasible method to develop cheap and efficient electrocatalysts for water splitting.     

3D graphene decorated with hexagonal micro-coin of Co(OH)2: A competent electrocatalyst for hydrogen and oxygen evolution reaction

Herein, 3D graphene is synthesized from the cation exchange resin by a cheap and efficient strategy, and then hexagonal micro-coin Co(OH)₂ particles are loaded by a simple double displacement reaction. Different analytical techniques confirm that the 3D graphene exhibit rose petal-like structure, which is decorated with Co(OH)₂ hexagonal micro coin structure. The hexagonal micro-coin Co(OH)₂ are the actual active sites for electrochemical reactions, while conductive graphene eases the transport of electrons which may further heighten their performance. The as-synthesized electrocatalysts are used to study different electrochemical measurement in an alkaline (1 M KOH) solution. The as-prepared Co(OH)₂-3 dimensional graphene-0.5 delivered the overpotential value of −0.367 and 1.599 V (vs RHE) (10 mA cm⁻²), the calculated Tafel slope values were 96 and 110 mV dec⁻¹ for hydrogen and oxygen evolution reactions correspondingly. Different concentrations of Co were used to study the effect of Co on electrochemical measurements. The data shows that Co(OH)₂-3DG-0.5 exhibited better performance than the other as-prepared Co(OH)₂ based electrocatalysts. The as-prepared electrocatalyst also shows low R_ct and reasonable stability for hydrogen and oxygen evolution reaction.     

Electrochemically induced in-situ generated Co(OH)2 nanoplates to promote the Volmer process toward efficient alkaline hydrogen evolution reaction

Hydrogen production via alkaline water electrolysis is of significant interest. However, the additional water dissociation step makes the Volmer step a relatively more sluggish kinetics and consequently leads to a slower reaction rate than that in acidic solution. Herein, we demonstrate an effective strategy that Co(OH)₂ can promote the Volmer process by accelerating water dissociation and enhance the electrocatalytic performance of CoP toward alkaline hydrogen evolution reaction. The Co(OH)₂ nanoplates are electrochemically induced in-situ generated to form a nanotree-like structure with porous CoP nanowires, endowing the hybrid electrocatalyst with superior charge transportation, more exposed active sites, and enhanced reaction kinetics. This strategy may be extended to other phosphides and chalcogenides and provide insight into the design and fabrication of efficient alkaline HER catalysts.     

CoP/Ni2P heteronanoparticles integrated with atomic Co/Ni dual sites for enhanced electrocatalytic performance toward hydrogen evolution

Transition metal phosphides (TMPs) have attracted considerable attention as an advanced electrocatalyst for hydrogen evolution reaction (HER). Nevertheless, the catalytic efficiency of single-component TMPs is still restricted that cannot endure long-term running and easy to be corroded especially under harsh conditions. In this work, a multicomponent electrocatalyst combined with CoP/Ni₂P heteronanoparticles and Co/Ni single-atom active sites (denoted as N–C@CoP/Ni₂P) is rational designed and prepared. The obtained N–C@CoP/Ni₂P electrode material exhibits enhanced performance with the overpotential of 153 mV at 10 mA cm^?2, and the small Tafel value of 53.01 mV dec^?1 in 0.5 M H₂SO₄, and a satisfied result is obtained in basic media as well. The outstanding HER performance is mainly benefiting from the synergistic effect between CoP and Ni₂P, and the highly catalytic faction of atomic Co/Ni dual sites. Furthermore, a powerful conductive network fabricated by N-doped carbon skeleton and in-situ grown CNTs improves the conductivity of catalyst. Such a stereoscopic 3D nanostructure is also facile to accelerate the shuttle of electrons and ions.     

Polyacrylonitrile-based highly porous carbon materials for exceptional hydrogen storage

In this paper, a common low-cost chemical material-polyacrylonitrile (PAN) is transformed into porous carbon with excellent specific surface area (2564.6–3048.8 m² g⁻¹) and highly concentrated micropore size distribution (0.7–2.0 nm). Benefit to the unique structure, the as-prepared materials show appealing hydrogen adsorption capacity (4.70–5.94 wt % at 20 bar, 7.15–10.14 wt % at 50 bar), demonstrating a promising prospect of practical application. This work also confirmed that the narrow and deep ultramicropore (<0.7 nm) could facilitate adsorption of hydrogen molecules significantly at atmospheric pressure, and the volume increase of supermicropore (0.7–2.0 nm) could lead to hydrogen capacity promotion at relative high pressure (>20 bar), which provides valuable guidance for the construction of ideal porous adsorbent for efficiency hydrogen storage.     

Exfoliated MoS2 with porous graphene nanosheets for enhanced electrochemical hydrogen evolution

Porous graphene (P-rGO) was synthesized from graphene oxide (GO) via a one-pot calcination method with CO₂ as an activation agent at 800 °C. Due to the special porous structure, the surface area of P-rGO can be increased to ～759 m²/g. The P-rGO was then used as a support to incorporate with chemical exfoliated molybdenum disulfide (MoS₂) for the fabrication of MoS₂/P-rGO composite. Compared to bulk MoS₂, the exfoliated MoS₂ is in the 1T phase with a metallic property and smaller charge transfer resistance, thus has a better activity in electrochemical hydrogen evolution reaction (HER). The HER activity of 1T MoS₂ could be further increased after the combination with P-rGO. The overpotential of 1T MoS₂/P-rGO was only ～130 mV vs. RHE, and the corresponding Tafel slope was ～75 mV Dec^?1. The special porous structure and good electric conductivity of P-rGO decrease the charge transfer resistance of the composite without sheltering too many active sites of MoS₂, thus leading to the enhanced HER activity. As an efficient noble metal free HER catalyst, the 1T MoS₂/P-rGO has great potential for large-scale hydrogen production.     

Heterostructure based on exfoliated graphitic carbon nitride coated by porous carbon for photocatalytic H2 evolution

In this contribution, the heterostructure based on exfoliated graphitic carbon nitride (ex-gCN) coated by a porous carbon layer was fabricated by a simple approach and tested as a photocatalyst for hydrogen evolution under simulated solar light illumination. Bulk-gCN was firstly exfoliated and annealed under a hydrogen atmosphere in carefully selected conditions. The catalyst with the highest photoactivity was fabricated at 400 °C for 4 h. This material exhibited about a 23-fold higher amount of photogenerated hydrogen (18.2 μmol/g) compared to reference ex-gCN (0.8 μmol/g). Boosted photoactivity could be attributed to the (i) highly developed Specific Surface Area leading to more active sites on the surface due to the porous carbon layer, (ii) better transfer, and separation of photogenerated carriers, and (iii) sufficient suppression of the recombination process. Moreover, the mechanism of photocatalytic H₂ evolution from water splitting based on a full physicochemical characterization of the studied materials was proposed.     

One-step synthesis of Ni3S2 nanowires at low temperature as efficient electrocatalyst for hydrogen evolution reaction

Ni₃S₂ is an emerging cost-effective catalyst for hydrogen generation. However, a large amount of reported Ni₃S₂ was synthesized via multi-step approaches and few were fabricated based on the one-step strategies. Herein, we report a facile one-step low-temperature synthesis of Ni₃S₂ nanowires (NWs). In this strategy, a resin containing sulfur element is recommended as a sulfur resource to form Ni₃S₂ NWs. It presents a plausible explanation on the vapor–solid–solid (VSS) growth mechanism according to the results of this experiment and reported in literature that has been published. The Ni₃S₂ NW exhibits a potential ∼199 mV at 10 mA cm⁻² and the long-term durability over 30 h at 20 mA cm⁻² HER operation, better than other reported Ni₃S₂. More importantly, according to replace transition metal foam as the initial metal, other transition metal sulfide can be readily synthesized via this original approach.     

Self-standing Co decorated Cu2O/CuS-based porous electrocatalyst for effective hydrogen evolution reaction in basic media

The self-standing Co decorated Cu₂O/CuS-based porous electrocatalyst was prepared with the help of simple electrodeposition and hydrothermal method. The structural characterizations of fabricated samples were performed with X-Ray diffraction spectroscopy and X-Ray photoelectron spectroscopy, while the morphology of catalysts was studied with the help of Field-Emission Spectroscopy and Transmission Electron Spectroscopy. The electrochemical performance of the hydrogen evolution reaction was checked in a basic electrolyte. The gradual increment in the electrochemical performance of Cu₂O was observed when it underwent sulfurization without and with Co precursor respectively. The best electrochemical performance for hydrogen evolution reaction with an overpotential of 150.29 mV to achieve a geometric current density of 10 mA/cm² was observed for the Cu₂O sample sulfurized with Co precursor. The results of different characterizations suggested that the improved electrochemical performance could be attributed to the increased intrinsic activity and surface porosity of the electrocatalyst after sulfurization.