The hydrogen evolution reaction on nickel-polyaniline composite electrodes

The electrolytic codeposition of nickel composite coatings with different amounts of polyaniline particles (PAni) was used to produce electrodes for the catalytic hydrogen evolution reaction (HER). The electrodes produced were imaged by scanning electron microscopy, and their catalytic activities for HER were determined by linear Tafel polarization and electrochemical impedance spectroscopy (EIS) in a 0.5 M H₂SO₄ solution. The electron micrographs showed that the amount of exposed polyaniline surface was directly related to the polyaniline concentration in the solution used for the electrolytic codeposition. The linear Tafel polarizations indicated that the HER was limited by the Volmer step, and the EIS measurements showed that the presence of PAni on the electrode surface affected the HER. Electrodes with higher composite contents exhibited enhanced catalytic activity.     

Preparation and study of electrocatalytic activity of Ni-Pd(OH)2/C nanocomposite for hydrogen evolution reaction in alkaline solution

A new catalyst (Ni-Pd(OH)₂/C) for hydrogen evolution reaction (HER) was prepared by coelectrodeposition of Pd(OH)₂/C nanoparticles and Ni on a Cu substrate in two steps. Furthermore, the effect of Mo ions in alkaline solution (1 M NaOH) on the electrocatalytic activity of Ni-Pd(OH)₂/C nanocomposite was studied as an in-situ activator for the HER. The various electrochemical methods were employed to study the HER activity of the investigated new catalyst, including linear sweep voltammetry (LSV), the steady-state polarization Tafel curves, electrochemical impedance spectroscopy (EIS) and chronoamperometry (CA). The electrochemical measurements showed that the Ni-Pd(OH)₂/C nanocomposite as a catalyst for the HER has an excellent catalytic activity with good stability in alkaline solution. Furthermore, the rate constants of the forward and backward reactions of Volmer and Heyrovský steps were estimated using Tafel-impedance data and revealed that the proton discharge electrosorption or Volmer reaction (k₁= (6.8 ± 0.7) × 10⁻⁸ mol cm⁻² s⁻¹) was the rate determining step (RDS) of the HER on the surface of Ni-Pd(OH)₂/C nanocomposite. Also, it was observed that the presence of Mo ions in alkaline solution could significantly increase the HER activity of Ni-Pd(OH)₂/C nanocomposite. The comparison of RDS rate constant value with surface roughness (R_f) of Ni-Pd(OH)₂/C catalyst showed that its high activity toward the HER originated from both increase in the surface roughness (∼20%) and increase in synergistic effect (∼80%).     

Kinetics and electrocatalytic behavior of nanocrystalline CoNiFe alloy in hydrogen evolution reaction

Electrocatalytic activity of electrodeposited nanocrystalline CoNiFe alloy in hydrogen evolution reaction (HER) was studied in 1 M NaOH solution. The steady-state polarization measurement and electrochemical impedance spectroscopy were used to determine the Tafel slope, the exchange current density and the activation energy. The simulation of the data obtained from these two methods, using a nonlinear fitting procedure, allowed us to determine the surface coverage and the rate constants associated with the mentioned reaction. The ac-impedance spectra measured at various overpotentials in the HER region exhibited three semicircles in the complex plane plots. The high-frequency semicircle was attributed to the electrode geometry, whereas the intermediate and low frequency semicircles were due to the electrode kinetics. The results also demonstrated that CoNiFe alloy possessed slightly higher specific electrocatalytic activity than Ni itself. Nevertheless, the contribution of high specific surface area of nanocrystalline alloy in hydrogen evolution was by far more significant. The effects of temperature and poisoning on the electrochemical parameters were also studied.     

Phytic acid-coated titanium as electrocatalyst of hydrogen evolution reaction in alkaline electrolyte

The phytic acid-coated titanium (IP6/Ti) electrode was prepared through a simple drop-drying process, with an aim of improving electrocatalytic activity toward the hydrogen evolution reaction (HER). Scanning electron microscope and X-ray photoelectron spectroscopy showed that the IP6 coated the substrate surface uniformly and completely. Evaluation of the electrode activity was carried out in 1.0 M NaOH by linear polarization, electrochemical impedance spectroscopy (EIS) and chronopotentiometry. The kinetic parameters obtained from Tafel curves reveal that the IP6 coating can enhance the exchange current density of the HER by 489 times compared to the bare Ti, and reduce the HER activation energy by nearly 50%. The EIS data prove that the charge transfer resistance of the HER was considerably reduced due to the IP6 coating, with a decrease in real surface area of the electrode. The catalytic effect of IP6 is due to an improvement in the charge transfer kinetics of the HER. This work indicates that IP6 may be a potent candidate as a catalyst for hydrogen energy production.     

The hydrogen evolution reaction on single crystal gold electrode

As one of the important candidate of power sources for the future, the research and production of hydrogen gas has a significant importance. In this article, the emphasis is on the influence of impurities on hydrogen evolution reaction, i.e., the influence of an addition of decacyclene, C₁₂H₃₅C₆H₄SO₄Na, CH₃CH₂OH, chromanone, H₂SO₄, HNO₃, 4,4′-biphenediol and 1,2,3,4-tetraphenyl-1,3-cyclopentadiene was studied by electrochemical impedance technique. The adsorption structure for some organics was measured by scanning tunneling spectroscopy techniques. Superstructure of adsorbed decacyclene on Au(111) surface was captured. The ordered adsorption structure of 4,4′-biphenyldiol on Au(111) and (100) was also observed. The addition of decacyclene has shown an opposite effects on hydrogen evolution for Au(111) and (100) surface, i.e., it inhibits the reaction at Au(100) but enhances the one at Au(111). The results show that the addition of C₁₂H₃₅C₆H₄SO₄Na and HNO₃, especially the latter, can improve the hydrogen evolution. In the article the adsorption structure and hydrogen evolution reaction have been studied in order to give some useful information about the relation between the adsorption structure and the properties. The purpose of this article is to attempt to find the relation between electrochemical performance and the adsorption structure, and to explore the effect of some additives.     

Enhancement of hydrogen evolution reaction on platinum cathode by proton carriers

The catalytic effect of aniline and benzylamine has been investigated for hydrogen evolution reaction on platinum electrode in sulfuric acid aqueous solution using voltammetric methods and electrochemical impedance spectroscopy. Kinetic parameters which characterize the hydrogen evolution reaction (cathodic transfer coefficient, exchange current density) have been determined using Tafel plots. It has been found that the addition of amines leads to an increase of the exchange current density, explained by an increased concentration of protons available at the interface. The catalytic effect of benzylamine and aniline is strongly correlated to their molecular parameters, such as dipole moment and molecular surface coverage. Also, apparent activation energies for hydrogen evolution reaction from solutions with and without addition of aromatic amines have been calculated. Electrochemical impedance spectroscopy results confirm an overall enhancement of hydrogen evolution in the presence of amines, as indicated by the lower values of charge transfer and adsorption resistances and by the decrease of characteristic time constants.     

Electrochemical behaviour of single walled carbon nanotubes – Hydrogen storage and hydrogen evolution reaction

The electrochemical behaviour of single walled carbon nanotubes (SWCNT) related to the mechanism involved in the hydrogen electrode reaction applying electrochemical and spectroscopic techniques is studied. Cyclic voltammetry applied to electrodes containing different percentages of SWCNT demonstrates that this material can behave as efficient capacitor and that the hydrogen electrode reaction develops through the H-electrosorption followed by the formation of molecular H₂ and its evolution. Also, SWCNT are able to storage hydrogen within their porous structure. This is confirmed through the galvanostatic charge and discharge experiments. Electrochemical impedance spectroscopy allowed calculating the real area that takes part in the electrode reaction and the main and valuable conclusion is that the hydrogen electrode reaction consists of a simple charge transfer reaction and that the H adatom relaxation or diffusion processes can be disregarded. Furthermore, a model proposed for their structure which was validated through impedance experiments confirms those conclusions. Results of Raman spectra allowed identifying the nature of the electrodes confirming that after purification the material is composed of single walled carbon nanotubes.     

Cu-doped GdFeO3 perovskites as electrocatalysts for the oxygen evolution reaction in alkaline media

Development of electrocatalysts for the oxygen evolution reaction (OER) plays a critical role in electrochemical water splitting systems. In this aim, iron based perovskite oxides with composition GdFe_1-x Cu_x O₃ (0 ≤ x ≤ 0.3) have been investigated. The effect of copper doping, calcination temperature on the OER performance in alkaline media was studied. The incorporation of Cu²⁺ (0 ≤ x ≤ 0.3) decreases the activity of calcined electrodes at 800 °C from 6.33 to 2.79 mA cm⁻², while that containing 0.2 mol of copper calcined at 600 °C, exhibits the higher activity (9.66 mA cm⁻²) at 0.66 V. The stability study during 8 h indicates that the undoped electrode calcined at 800 °C, exhibits relatively a better stability of the OER performance compared to that doped with 20% of copper calcined at 600 °C. The achieved results show promising potential for cost-effective hydrogen generation using earth-abundant materials and cheap fabrication processes.     

Edge-halogenated graphene nanosheets as an efficient metal-free electrocatalyst for hydrogen evolution reaction

Application of carbonic materials as catalysts has recently been considered due to some advantages like tunable molecular structures, easy synthesis methods, abundance, and high tolerance in acidic and alkaline media. Here, a new metal-free electrocatalyst of halogenated reduced graphene oxide was prepared using cyclic voltammetry X (F, Br, and I)-RGO electrodeposition method. The prepared electrocatalysts were studied as a novel metal-free electrocatalyst for the hydrogen evolution reaction, and the presence of several halogen and oxygen functional groups on the surface of nanosheets was verified by the furrier transform infra-red, FT-IR, spectroscopy, and the presence of doped halogens on the RGO surface was confirmed by energy-dispersive X-ray, EDX, spectroscopy. The structural features and surface morphology of electrocatalysts were investigated by scanning electron microscopy (SEM) analysis. The electrochemical treatment of the X (F, Br, I)-RGO electrode was studied by some techniques like electrochemical impedance spectroscopy, EIS, chronoamperometry, CA, and linear sweep voltammetry, LSV. The X (F, Br, I)-RGO catalyst showed a lower onset potential (?0.81 V. vs. SHE), higher exchange current density (3.1 × ×10^?1 mA cm^?2), and lower charge transfer resistance (1.09 Ω cm²) related to the RGO catalyst due to the high active sites by heteroatoms and graphene nanosheets.     

Size and strain dependent activity of Ni nano and micro particles for hydrogen evolution reaction

In the context of constant research for the improvement of alkaline water electrolysis process using advanced electrocatalytic materials for the hydrogen evolution reaction (HER), various nickel particle based electrode materials were prepared and characterized. The synthesis of nickel hydroxide nanoparticles was performed in water in presence of three different stabilizers (CTAB, PVP and KBr). A thermal treatment at 400 °C under 5% H₂/Ar atmosphere led to nickel nanoparticles. Mechanically milled commercial micrometric particles and nanoparticles synthesised by a polyol route completed a series of Ni powders showing broad ranges of size (5 nm–73 μm) and strain (6 ppm–0.7%). The electrocatalytic activity of the resulting electrode materials was evaluated versus powder morphology. Their apparent and intrinsic activity and the mechanism of the HER were studied by electrochemical impedance spectroscopy (EIS) and steady-state polarisation. A change in the HER mechanism is observed depending on particle size. This first systematic study demonstrates that the smaller the size and the more defective the particles, the greater the electrocatalytic activity. As a matter of fact, appreciable cathodic current densities of 100 mA cm⁻² at ∼ −300 mV of overpotential were obtained for nickel nanoparticles with 5 nm size and 0.7% strain.     

A direct method to determine transfer function parameters in case of hydrogen evolution reaction

Electrochemical impedance spectroscopy is one of the efficient methods for studying reaction mechanisms. In case of hydrogen evolution reaction, the transfer function involved is very complex. It is therefore usually obtained through indirect methods. First, a simple equivalent circuit is usually fitted to the experimental data using complex non-linear least square methods. The circuit parameters thus obtained are then transformed to transfer function parameters. In the present approach, the procedure for obtaining transfer function has been simplified through its piecewise definition in various frequency ranges. A direct method to obtain transfer function using asymptotic analysis has been presented. The method to reduce error involved in the analysis has also been discussed. It has been shown that the approach will also be applicable for other systems.     

Intermediate hydroxide enforced electrodeposited platinum film for hydrogen evolution reaction

Competitive catalytic activity of platinum (Pt) makes it as a promising cathode material for hydrogen evolution reaction. But cost of Pt makes it impractical for its use in commercial applications. Unlike literature known methods, our study entails on a methodology of ambient temperature electrodeposition of Pt films, without the use of a complexing agent or pH adjustments or both. Pt films are deposited in an electrochemical bath, which is prepared by adding platinum chloride complex [H₂PtCl₆.x H₂O] in triple-distilled water. Pt films deposited at different potentials are analyzed for their morphological (SEM), structural (XRD), electrochemical study (Cyclic Voltammetry and Linear sweep measurements). The growth and catalytic activity of Pt film show strong dependence on applied deposition potential (−0.25 V to −0.40 V) and reduction kinetics of [PtCl₆]²⁻ or [Pt(OH)Cl₅]²⁻ intermediate hydroxide ions, that occurs during the process. Binding energy (BE) of Pt(4f_7/2) peak in a film increases to 72.4 eV (until −0.30 V), which slightly decreases at a deposition potential of −0.40 V. XRD data show changes along (111) and (200) planes, to which [PtCl₆]²⁻ and [Pt(OH)Cl₅]²⁻ intermediate hydroxide ions are found to be responsible. The average particle size with respect to applied potential, obtained from SEM data is found to be 25–40 nm. The catalytic activity (Peak current density in cyclic voltammetry) versus deposition potential data is correlated with Pt film formation by reduction of intermediate hydroxide ions.     

Electrochemical performance of Ni-RE (RE = rare earth) as electrode material for hydrogen evolution reaction in alkaline medium

In this work, Ni-RE (RE = rare earth, La, Ce) materials were obtained by the solid state reaction using as Ni source: a) metal acetylacetonates and b) metal powders while rare earth element was added from acetylacetonates. These materials were synthesized for 3 h at different temperatures (800 °C or 900 °C, 1000 °C and 1200 °C) in order to evaluate their electrochemical performance on the Hydrogen Evolution Reaction (HER). The effects of the sintering temperature and the Ni source on the morphology, structure and particle size were evaluated and correlated with the displayed catalytic activity. The results showed that depending of the added rare earth element and Ni source the formed compounds varied from a mixture of oxides (NiO, CeO₂) and intermetallic compounds (LaNiO₃) at low annealing temperatures up to the formation of the NiO-CeO₂, NiO-LaNiO₃ and NiO-La₄Ni₃O₁₀ compounds at 1200 °C. From Scanning Electron Microscopy (SEM) results, it was observed that the agglomerates of Ni-RE electrode materials presented a more uniform shape (semispherical) and lower crystal sizes (0.2-2.0 μm) using acetylacetonate precursors than that obtained with Ni powders (5-50 μm). It was found that the individual organization of the nickel particles and their electrocatalytic activity is affected by diverse factors: a) the type of precursor used in the synthesis, b) the reaction temperature and c) the synergetic effect caused by the addition of the rare earth metal, which seems to be better for lanthanum than for cerium. The Tafel parameters of the stabilized Ni-RE electrodes revealed that the formation of Ni-La intermetallic compounds at low temperature favors the current densities on the HER. Thus a clear dependence of the electrocatalytic activity on the source of these Ni-RE materials was observed.     

Efficient and stable electrocatalyst for hydrogen evolution reaction prepared by hybrid technique in surface engineering: Electrochemical and magnetron sputtering methods

Developing low-cost, stable, and robust electrocatalysts is significant for high effective hydrogen evolution reaction (HER). In this work, a coating system with Cu₂O/NiMoCu on stainless steel (SS) is employed as a highly active and stable catalyst for HER in acidic solutions. Electrochemical measurements for as-designed system on SS show a low onset overpotential, small Tafel slope of ～32 mV/decade and long-term durability over 7 days of HER operation. To further inspections of electrocatalytic behavior of as-prepared system in HER, the EIS measurements are performed at several overpotentials and temperatures. It is found that high hydrogen evolution activity and stability of Cu₂O/NiMoCu hybrid is likely due to special morphology of Cu₂O which result in large number of active sites for hydrogen adsorption, and a synergetic effect giving electronic structure suitable for the HER.     

Process kinetics for the electrocatalytic hydrogen evolution reaction on carbon-based Ni/NiO nanocomposite in a single-chamber microbial electrolysis cell

To explore the process kinetics of hydrogen evolution reaction (HER) on carbon-based Ni/NiO nanocomposite in the microbial electrolysis cells (MECs), the performance was systematically studied by different time-course sampling of five parallel single-chamber MECs operated under identical operating conditions, which included the electrochemical performance of anodes and cathodes, and the mechanism and kinetics of HER. It was hypothesized that the decreased performance of the nickel cathodes was due to corrosion and Ni dissolution. These results provide valuable insights into the effects of long-term operation on MEC performance.     

A new method for determining kinetic parameters by simultaneously considering all the independent conditions at an overpotential in case of hydrogen evolution reaction following Volmer–Heyrovsky–Tafel mechanism

Hydrogen evolution reaction following Volmer–Heyrovsky–Tafel mechanism and not under diffusion control can be characterized using Tafel polarization and AC admittance data at various frequencies and at various overpotentials. Such reaction has four independent kinetic parameters. One empirical constant related to charge required for complete surface coverage is also involved. A new approach to determine kinetic parameters utilizing these data and neglecting Heyrovsky and Tafel backward reaction rates has been proposed. This involves determining all the four kinetic constants using experimental data at a single overpotential. The empirical constant, i.e., charge required for complete surface coverage is determined by validating obtained kinetic constants at other overpotentials. The Levenberg–Marquardt algorithm to solve coupled nonlinear equations has been utilized for such purpose. The approach has been validated using the literature data.     

Electrocatalytic hydrogen evolution reaction on sulfur-deficient MoS2 nanostructures

Molybdenum disulfide (MoS₂) is a 2D layered structured material with a Mo:S of 1:2 and is a great attention seeker for hydrogen production through water-splitting. In the present work, we prepared nanostructured MoS_x with different sulfur molar concentrations (x = 2, 1, 0.5) through a one-step hydrothermal method. The decrease in sulfur concentration resulted in a new phase that is MoO₃ with a Mo:S of 1:0.5. The structural, morphological, and optical properties of all the samples were studied through X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Fourier Transform Infrared spectroscopy (FTIR), and Ultraviolet–Visible (UV–Vis) spectroscopy, respectively. Moreover, the electrochemical behavior was studied using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), linear sweep voltammetry (LSV), and Tafel slope. Optimum properties were observed for Mo:S (1:1) with an onset potential of 96 mV, an overpotential of 130 mV for hydrogen evolution reaction (HER) coupled with a specific capacitance of 889 F/g and low charge transfer resistance of 43 Ω. Further, it was noted that the electrocatalytic activity of MoS₁ was better than that of the composite and bare MoO₃. It is proposed that the excellent electrochemical activity arises from sulfur vacancies which provide active sites for HER and a free path for ions to flow through the material.     

Palladium-coated polyaniline nanofiber electrode as an efficient electrocatalyst for hydrogen evolution reaction

In this study, polyaniline (PANI) with abundant protonated regions was used for the first time as a palladium (Pd) support for enhanced performance in hydrogen evolution reaction (HER). For this purpose, the hierarchical Pd@PANI nanofiber electrode was easily synthesized by electrochemical polymerization of aniline on Au followed by potential-controlled electrochemical deposition of Pd nanoclusters on the PANI. The reported catalyst was characterized by X-ray diffraction, X-ray photoelectron spectroscopy, and field-emission scanning electron microscopy. Linear sweep voltammetry analysis was performed to evaluate the HER performance. Ion transfer behavior was investigated using electrochemical impedance spectroscopy analysis. The electrochemical tests show that the Pd@PANI/Au electrode has a low overpotential of ～60 mV at 10 mA cm^?2 and a small Tafel slope of 35 mV dec^?1 for HER in acidic media, with high catalytic activity and stability. These features will make the Pd@PANI/Au a promising candidate as a high-performance electrocatalyst for HER applications.