Efficient and stable Ni–Co–Fe–P nanosheet arrays on Ni foam for alkaline and neutral hydrogen evolution

The development of highly efficient and superior durability electrocatalysts is vital to expedite hydrogen evolution reaction (HER). Herein, a mixed amorphous and nano-crystalline Ni–Co–Fe–P alloy on Ni foam after 75 s dealloying in 3 M HCl (Ni–Co–Fe–P/NF-3-75) is synthesized by the preparation strategy of two-step method consisting of electroless deposition and dealloying process. Ni–Co–Fe–P/NF-3-75 shows an excellent HER performance and high durability in both alkaline and neutral conditions by optimizing the composition of the catalysts, acid concentration, and the time of dealloying. Benefitting from the high conductivity of Ni foam carrier, coordination between polymetallic phases, and the large exposure of defects, the as-prepared Ni–Co–Fe–P/NF-3-75 requires only a low overpotential of 56 mV and 104 mV to reach the current density of 10 mA cm⁻² in 1.0 M KOH and 1.0 M phosphate buffer (PBS), respectively. Remarkably, the Ni–Co–Fe–P/NF-3-75 electrode exhibits superior cycling stability and long-term robust durability without obvious overpotential decline. The successful preparation of the Ni–Co–Fe–P/NF-3-75 catalyst indicates that this method provides an efficient way to synthesize polymetallic phosphides for hydrogen evolution reaction.     

Novel Ni–Cr-based alloys as hydrogen fuel sources through alkaline water electrolytes

The hydrogen evolution reaction (HER) from water splitting is particularly attractive for green and clean power sources, but it remains a significant challenging issue. For the successful application of hydrogen-based renewable energy such as fuel cells, highly active and cost-saving metallic electro-catalysts for H₂ fuel production are vital. The goal of this study is to look into the effects of alloyed molybdenum, chromium, and iron on the electrocatalytic activity of Ni-based alloys in an alkaline electrolyte for HER. SEM with an energy dispersive spectroscopy (EDX) unit was used to determine the chemical composition of alloys. The electrocatalytic effectiveness of the explored cathodes on HER was evaluated in alkaline solutions employing open circuit potential measurements, linear polarization and electrochemical impedance spectroscopy (EIS). The impact of KOH concentrations on HER rate was also investigated. Tafel curves were used to calculate the HER's kinetic parameters, and the mechanisms of the HER were discovered. EIS observations at HER's cathodic potential have been explored and compared to a theoretical model. The Ni–Cr–Mo–Fe alloy has a low over-potential of 232 mV @ 10 mA cm⁻² and a Tafel slope of 57.7 mV dec⁻¹ in 1.0 M KOH media, resulting in an efficient HER. These findings indicate that the addition of Mo and Fe to Ni–Cr improves the catalytic efficiency of HER significantly. The Ni–Cr–Mo–Fe cathode is an economical and practical material for alkaline HER production.     

Cyclic voltammetry electrodeposition of self-standing Ni–Fe–Sn ternary alloys for accelerating alkaline hydrogen evolution

Designing cost-effective and high-efficiency non-noble electrocatalysts for the hydrogen evolution reaction (HER) remains a significant challenge for electrochemical water splitting to store clean and renewable energy. Herein, we have developed Ni–Fe–Sn electrocatalysts grown on Ni foam (Ni–Fe–Sn@NF) for the HER through a simple and facile synthetic route of cyclic voltammetry electrodeposition. The optimized Ni–Fe–Sn electrocatalysts possess excellent electrocatalysis toward hydrogen evolution with a low overpotential of 103 mV at a current density of 10 mA cm⁻², together with a small Tafel slope value of 97.4 mV·dec⁻¹ in 1 M KOH. The electrocatalyst also features excellent stability even after 2000 cycles and strong durability after 50 h under alkaline condition. The high HER performance can be attributed to the moderately optimized electronic structure of the Ni, Fe, and Sn center and bind-free nature of the electrode, which facilitates electron transfer and speeds up reaction kinetics. Moreover, the improved electrochemical surface area also enhances the performance on hydrogen production. This strategy presents a facile and simple tactic for the synthesis of noble-metal-free electrocatalysts with excellent HER performance.     

Combinatorial search for hydrogen storage alloys: Mg–Ni and Mg–Ni–Ti

A combinatorial study was carried out for hydrogen storage alloys involving processes similar to those normally used in their fabrication. The study utilized a single sample of combined elemental (or compound) powders which were milled and consolidated into a bulk form and subsequently deformed to heavy strains. The mixture was then subjected to a post annealing treatment, which brings about solid state reactions between the powders, yielding equilibrium phases in the respective alloy system. A sample, comprising the equilibrium phases, was then pulverized and screened for hydrogen storage compositions. X-ray diffraction was used as a screening tool, the sample having been examined both in the as processed and the hydrogenated state. The method was successfully applied to Mg–Ni and Mg–Ni–Ti yielding the well known Mg₂Ni as the storage composition. It is concluded that a partitioning of the alloy system into regions of similar solidus temperature would be required to encompass the full spectrum of equilibrium phases.     

Effect of microstructure on the electrocatalytic activity for hydrogen evolution of amorphous and nanocrystalline Zr–Ni alloys

Rapidly quenched Zr₂Ni amorphous and nanocrystalline ribbons were studied as electrocatalysts for hydrogen evolution in 6 M KOH. Linear polarization, potentiostatic hydrogen charge/discharge and EIS measurements at various potentials were carried out for the Zr alloys with different microstructure with the aim to extract information about the mechanism of hydrogen evolution and absorption and estimate the kinetic parameters of the hydrogen evolution reaction (HER). Though the melt-spun Zr₆₇Ni₃₃ alloys with varying microstructure do not show substantially different catalytic activity for HER, it could be clearly demonstrated that the nanocrystalline material reveals better catalytic performance than the entirely amorphous and nano-/amorphous alloys with the same chemical composition. It was found that all studied Zr–Ni alloys absorb hydrogen under the conditions of the hydrogen evolution experiments, as the amount of the absorbed hydrogen depends to a large degree on the alloys microstructure as well as on the applied potential during the HER experiment. The diffusion coefficient of hydrogen into the amorphous Zr₆₇Ni₃₃ alloy, as well as the thickness of the hydrided layer were found to be noticeably larger than those of the nanocrystalline alloy at the same conditions of hydrogen charging. Therefore the improved electrocatalytic properties of the nanocrystalline alloy could only be explained by its favorable microstructure (e.g. higher density of defects) and weaker hydrogen absorption into the nanostructured material under the conditions of the HER.     

Effect of carbon content on Ni–Fe–C electrodes for hydrogen evolution reaction in seawater

Hydrogen evolution reaction (HER) on the Ni–Fe–C electrodes electrodeposited at current density ranging from 100 to 300 A/m², as well as their electrochemical properties in 3.5% NaCl solution at 90 °C and pH = 12, had been investigated by polarization measurements, EIS technique. It was shown that the carbon content and grain size of Ni–Fe–C coatings are affected by current density. In addition, the hydrogen evolution overpotential of Ni–Fe–C electrodes was related with carbon content and grain size. The Ni–Fe–C electrodes with optimum catalytic activity for the HER were found to contain the maximum carbon content 1.59% and the minimum grain size 3.4 nm. The results of a comparative analysis between carbon content and intrinsic activity are that carbon content plays an important role in intrinsic activity of Ni–Fe–C electrodes.     

Optimizing Ti–Zr–Cr–Mn–Ni–V alloys for hybrid hydrogen storage tank of fuel cell bicycle

AB₂-type Ti-based alloys with Laves phase have advantages over other kinds of hydrogen storage intermetallics in terms of hydrogen sorption kinetics, capacity, and reversibility. In this work, Ti–Zr–Cr-based alloys with progressive Mn, Ni, and V substitutions are developed for reversible hydrogen storage under ambient conditions (1–40 atm, 273–333 K). The optimized alloy (Ti_0.8Zr_0.2)_1.1Mn_1.2Cr_0.55Ni_0.2V_0.05 delivers a hydrogen storage capacity of 1.82 wt%, the hydrogenation pressure of 10.88 atm, and hydrogen dissociation pressure of 4.31 atm at 298 K. In addition, fast hydrogen sorption kinetics and low hydriding-dehydriding plateau slope render this alloy suitable for use in hybrid hydrogen tank of fuel cell bicycles.     

Ageing of Mg–Ni–H hydrogen storage alloys

In this work, ageing of Mg/Mg₂Ni mixtures was investigated. It was observed that hydrogen desorption kinetics from hydrided Mg/Mg₂Ni was improved considerably after ageing at room temperature for several days. The ageing was interpreted in terms of phase changes. Even after almost complete hydridation, besides two main phases – MgH₂ and Mg₂NiH₄ – a certain amount of Mg₂NiH_0.3 was always present. Similar as Mg₂NiH₄ phase, Mg₂NiH_0.3 islands were located on the surface of MgH₂ grains. Mg₂NiH_0.3 transformed into Mg₂NiH₄ at the expense of hydrogen from an adjoining MgH₂ grain. In such a way, a clean double layer (Mg)–Mg₂NiH₄ was formed, acting as a gate for easy hydrogen desorption from MgH₂. It was found that the Mg₂NiH₄ phase was slightly enriched on non-twinned modification LT1 during the ageing. As a result, both the creation of (Mg)–Mg₂NiH₄ desorption bridges and enrichment of Mg₂NiH₄ on LT1 during the ageing facilitated onset of rapid hydrogen desorption.     

Study on Ni–Fe–C cathode for hydrogen evolution from seawater electrolysis

Hydrogen evolution reaction in 3.5 wt% NaCl (simulated seawater) was investigated using Ni–Fe–C cathode, prepared by cathode electrodeposits method on the matrix of A3 steel. The as-prepared Ni–Fe–C cathode coating materials has reached nanometer grade, what is more, the limit of average grain size was about 4.3 nm. As decreasing of the average coating grain sizes, hydrogen evolution overpotential was not decreasing linearly. There was a boundary average coating grain sizes of about 4.3–6.4 nm. The optimal preparation process of Ni–Fe–C cathode was listed as electroplating current density 200 A/m², temperature 30 °C, pH 1.5 and 60 min. The hydrogen overpotential was only about 65 mV, which was tested in the 3.5 wt% NaCl of 90 °C at pH 12.     

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.     

Synthesis and hydrogen storage behavior of Mg–V–Al–Cr–Ni high entropy alloys

The ternary MgVAl, MgVCr, MgVNi, quaternary MgVAlCr, MgVAlNi, MgVCrNi and quinary MgVAlCrNi alloys were produced by high energy ball milling (HEBM) under hydrogen pressure (3.0 MPa) as a strategy to find lightweight alloys for hydrogen storage applications. Most of the ternary and quaternary alloys presented multiphase structure, composed mainly of body-centered cubic (BCC) solid solutions and Mg-based hydrides. Only the quinary MgVAlCrNi high entropy alloy (HEA) formed a single-phase structure (BCC solid solution), which is a novel lightweight (ρ = 5.48 g/cm³) single-phase HEA. The hydrogen storage capacity of this alloy was found to be very low (approximately 0.3 wt% of H). Two non-equiatomic alloys with higher fraction of Mg and V (strong hydride former elements), namely Mg₂₈V₂₈Al₁₉Cr₁₉Ni₆ and Mg₂₆V₃₁Al₃₁Cr₆Ni₆, were then designed, aiming at higher storage capacity. Both alloys were produced by HEBM. The results show that the non-stoichiometric alloys also presented low hydrogen storage capacity. The low affinity of these alloys with hydrogen was discussed in terms of enthalpy of hydrogen solution and enthalpy of hydride formation of the single components. This study brought to light the importance of considering both enthalpy of hydrogen solution and enthalpy of hydride formation of the alloying elements for designing Mg-containing HEA for hydrogen storage. Once Mg has a positive enthalpy of hydrogen solution, the alloys composition must be balanced with alloying elements with higher hydrogen affinity, i.e., negative values of enthalpy of solution and hydride formation.     

Nanostructured Ni–Co alloy electrodes for both hydrogen and oxygen evolution reaction in alkaline electrolyzer

Ni–Co alloy nanostructured electrodes with high surface area were investigated both as a cathode and anode for an alkaline electrolyzer. Electrodes were obtained by template electrosynthesis at room temperature. The electrolyte composition was tuned in order to obtain different NiCo alloys. The chemical and morphological features of nanostructured electrodes were evaluated by EDS, XRD and SEM analyses. Results show that electrodes with different composition of Ni and Co, made of nanowires well anchored to the substrate, were obtained. For both hydrogen and oxygen evolution reactions, electrochemical and electrocatalytic tests, performed in 30% w/w KOH aqueous solution, were carried out to establishing the best alloy composition. Mid-term tests conducted at a constant current density were also reported. Nanostructured electrodes with a Co atomic composition of 94.73% have the best performances for both hydrogen and oxygen evolution reactions. In particular, with this alloy, a potential of ?0.43 V (RHE) and of 1.615 V (RHE) was measured for hydrogen and oxygen evolution reaction at ?50 mA cm^?2 and at 50 mA cm^?2, respectively, after 6 h of electrolysis. The calculated Tafel's slopes for HER and OER were ?0.105 and 0.088 V/dec, respectively. Furthermore, HER and OER η₁₀ potential values were measured founding ?0.231 V (RHE) and 1.494 V (RHE) respectively.     

Electrodeposition of nickel–copper alloys to use as a cathode for hydrogen evolution in an alkaline media

The electrocatalytic activity of nickel–copper (Ni–Cu) alloy coated electrodes for the hydrogen evolution reaction (HER) in an alkaline media was studied. The Ni–Cu alloys were electrodeposited on a Cu substrate by direct current (DC) and pulse current (PC) electrodeposition in a fixed plating bath. A wide alloy composition range (6–81 mol% Ni) was achieved by controlling the applied current density between 5 and 300 mA cm⁻². It was found that the electrocatalytic activity for the HER depended on the composition of the Ni–Cu alloys, where electrodes having low Ni content gave high electrocatalytic activities. DC electrodeposition resulted in better electrocatalytic performances than PC. Pulse plating parameters other than the magnitude of the applied current density did not substantially influence the electrocatalytic performance of the Ni–Cu alloy electrodes. Ni content was found to have a stronger effect on the electrocatalytic activity for the HER than the deposit morphology.     

One-step potentiostatic electrodeposition of Ni–Se–Mo film on Ni foam for alkaline hydrogen evolution reaction

Exploring efficient, abundant, low-cost and stable materials for hydrogen evolution reaction (HER) is highly desired but still a challenging task. Herein, Ni–Se–Mo electrocatalysts supported on nickel foam (NF) substrate were synthesized by a facile one-step electrodeposition method. The Ni–Se–Mo film presents high electrocatalytic activity and stability toward HER, with a low overpotential of 101 mV to afford a current density of 10 mA cm⁻² in 1.0 M KOH medium. Such excellent HER performance of Ni–Se–Mo film induced by the synergistic effects from Mo-doped Ni–Se film leads to the fast electron transfer. This work provides the validity of interface engineering strategy in preparing highly efficient transition metal chalcogenides based HER electrocatalysts.     

Ni–CeO2 composite cathode material for hydrogen evolution reaction in alkaline electrolyte

In this work, nickel-based electrodes were prepared using composite electrodeposition technique in a nickel sulphamate bath containing suspended micro- or nano-sized CeO₂ particles. The prepared Ni–CeO₂ composite electrodes exhibit an enhanced high catalytic activity toward hydrogen evolution reaction (HER) in alkaline solutions. X-ray diffraction patterns indicated that the CeO₂ particles have been successfully incorporated into the Ni matrix and altered the texture coefficient (TC) of the Ni layer. The morphology of the obtained coatings was characterized by Scanning Electron Microscopy, and the CeO₂ content was determined by coupled energy dispersive X-ray spectrometry. The thermal stability of the composite electrodes was analyzed by thermogravimetric and differential scanning calorimetry, showing a good thermal stability. The catalytic activity of the composite electrodes for HER was measured by steady-state polarization and electrochemical impedance spectroscopy techniques in 1.0 M NaOH solution at room temperature. The exchange current density of HER on the Ni–CeO₂ composite electrodes was much higher than that on Ni electrode. EIS results suggested that a synergetic effect on HER may exist between CeO₂ particles and Ni matrix. Compared to nano-CeO_2, the micro-CeO₂ derived composite electrodes showed higher electrochemical activity. The possible correlation among particle size, content and catalytic activity is discussed.     

Study on low-vanadium Ti–Zr–Mn–Cr–V based alloys for high-density hydrogen storage

To reduce the cost and modulate hydrogen storage performances of Ti-based Laves phase alloys for the application of inputting 3.2 MPa feed hydrogen and outputting 8 MPa hydrogen with water bath, three series of less-vanadium Ti–Zr–Mn–Cr–V based alloys were prepared by induction levitation melting, and their microstructure and hydrogen storage properties were systematically investigated. All alloys consist of a single C14-type Laves phase with well-distributed elements. With vanadium decreasing in Ti_0.95Zr_0.05Mn_0.9+xCr_0.9+xV_0.2-2x (x = 0–0.02) and Ti_0.93Zr_0.07Mn_1.1+yCr_0.7+zV_0.2-y-z (y = 0, 0.05, z = 0–0.05) stoichiometric alloys, the hydrogen equilibrium pressure increases and hydrogenation kinetics is slightly deteriorated. After introducing Ti hyper-stoichiometry, Ti_0.93+wZr_0.07Mn_1.15Cr_0.7V_0.15 (w = 0–0.04) alloys show decreased hydrogen equilibrium pressure, high hydrogen capacity and enhanced kinetics. Among alloys mentioned, Ti_0.95Zr_0.07Mn_1.15Cr_0.7V_0.15 has optimum performances including useable capacity of 1.07 wt% at working conditions, together with satisfactory cycling durability. This study guides for compositional design of high-density hydrogen storage multi-component alloys.     

Amorphous Ni–S–Mn alloy as hydrogen evolution reaction cathode in alkaline medium

Amorphous Ni–S–Mn alloy electrodes were obtained by electrodeposition. The microstructure, surface morphology and composition of the new Ni–S–Mn alloy on the Ni substrate were analyzed by X-ray diffraction (XRD), X-ray photoelectron spectrometry (XPS), scanning electron microscopy (SEM) and energy dispersive analysis of X-ray (EDAX). The electrochemical kinetics and mechanism of the hydrogen evolution reaction (HER) of formed electrodes were studied by measurement of the steady-state polarization. Owing to the larger exchange current densities, the lower standard reaction activity energy and a larger surface roughness, the amorphous Ni–S–Mn alloy electrode performs at a higher electrochemical activity with greater stability for the HER in 30 wt% KOH solution at various temperatures than the Ni–S alloy electrode.     

Platinum–rare earth electrodes for hydrogen evolution in alkaline water electrolysis

In the new “Hydrogen Economy” concept, water electrolysis is considered one of the most promising technologies for hydrogen production. Novel electrocatalytic materials for the hydrogen electrode are being actively investigated to improve the energy efficiency of current electrolysers. Platinum (Pt) alloys are known to possess good catalytic activities towards the hydrogen evolution reaction (HER). However, virtually nothing is known about the effects of rare earth (RE) elements on the electrocatalytic behaviour of Pt towards the HER. In this study, the hydrogen discharge is evaluated in three different Pt–RE intermetallic alloy electrodes, namely Pt–Ce, Pt–Sm and Pt–Ho, all having equiatomic composition. The electrodes are tested in 8 M KOH aqueous electrolytes at temperatures ranging from 25 °C to 85 °C. Measurements of the HER by linear scan voltammetry allow the determination of several kinetic parameters, namely the Tafel coefficients, charge-transfer coefficients, and exchange current densities. Activation energies of 46, 59, 39, and 60 kJ mol⁻¹ are calculated for Pt, Pt–Ce, Pt–Sm and Pt–Ho electrodes, respectively. Results show that the addition of REs improves the activity of the Pt electrocatalyst. Studies are in progress to correlate the microstructure of the studied alloys with their performance towards the HER.     

Properties of mechanically alloyed Mg–Ni–Ti ternary hydrogen storage alloys for Ni-MH batteries

MgNiTi_x, Mg_1−xTi_xNi and MgNi_1−xTi_x (with x varying from 0 to 0.5) alloys have been prepared by high energy ball milling and tested as hydrogen storage electrodes. The initial discharge capacities of the Mg–Ni–Ti ternary alloys are inferior to the MgNi electrode capacity. However, an exception is observed with MgNi_0.95Ti_0.05, which has an initial discharge capacity of 575 mAh/g compared to 522 mAh/g for the MgNi electrode. The Mg–Ni-Ti ternary alloys show improved cycle life compared to Mg–Ni binary alloys with the same Mg/Ni atomic ratio. The best cycle life is observed with Mg_0.5Ti_0.5Ni electrode which retains 75% of initial capacity after 10 cycles in comparison to 39% for MgNi electrodes, in addition to improved high-rate dischargeability (HRD). According to the XPS analysis, the cycle life improvement of the Mg_0.5Ti_0.5Ni electrode can be related to the formation of TiO₂ which limits Mg(OH)₂ formation. The anodic polarization curve of Mg_0.5Ti_0.5Ni electrode shows that the current related to the active/passive transition is much less important and that the passive region is more extended than for the MgNi electrode but the corrosion of the electrode is still significant. This suggests that the cycle life improvement would be also associated with a decrease of the particle pulverization upon cycling.