Heyrovsky Mechanism Research Articles

The Chemical Properties of Hydrogen Atoms Adsorbed on M0‐Nanoparticles Suspended in Aqueous Solutions: The Case of Ag0‐NPs and Au0‐NPs Reduced by BD4−

AbstractThe nature of H‐atoms adsorbed on M0‐nanoparticles is of major importance in many catalyzed reduction processes. Using isotope labeling, we determined that hydrogen evolution from transient {(M0‐NP)‐Hn}n− proceeds mainly via the Heyrovsky mechanism when n is large (i.e., the hydrogens behave as hydrides) but mainly via the Tafel mechanism when n is small (i.e., the hydrogens behave as atoms). Additionally, the relative contributions of the two mechanisms differ considerably for M=Au and Ag. The results are analogous to those recently reported for the M0‐NP‐catalyzed de‐halogenation processes.

The Chemical Properties of Hydrogen Atoms Adsorbed on M0 -Nanoparticles Suspended in Aqueous Solutions: The Case of Ag0 -NPs and Au0 -NPs Reduced by BD4.

The nature of H-atoms adsorbed on M0 -nanoparticles is of major importance in many catalyzed reduction processes. Using isotope labeling, we determined that hydrogen evolution from transient {(M0 -NP)-Hn }n- proceeds mainly via the Heyrovsky mechanism when n is large (i.e., the hydrogens behave as hydrides) but mainly via the Tafel mechanism when n is small (i.e., the hydrogens behave as atoms). Additionally, the relative contributions of the two mechanisms differ considerably for M=Au and Ag. The results are analogous to those recently reported for the M0 -NP-catalyzed de-halogenation processes.

An electrochemical study of the hydrogen evolution reaction at YNi2Ge2 and LaNi2Ge2 electrodes in alkaline solution

The kinetics of the electrochemical hydrogen evolution on YNi2Ge2 and LaNi2Ge2 in 1 М KОН at a temperature of 22–48 °С was studied by polarization measurements and impedance spectroscopy. Polarization curves are the Tafel lines with a slope of 79–90 mV dec−1. It was shown that the Tafel slope of 80–90 mV dec−1 can be obtained within the framework of the Volmer–Heyrovsky mechanism and simultaneous occurrence of hydrogen absorption reaction with both Langmuir isotherm of hydrogen adsorption and logarithmic Temkin isotherm. The polarization and impedance data for the studied compounds were explained on the basis of the Volmer–Heyrovsky mechanism with parallel proceeding hydrogen absorption. The values of some rate constants (including the rate constants per unit real surface area) were estimated.

Porous magnetic iron- manganese oxide nanocubes derived from metal organic framework deposited on reduced graphene oxide nanoflake as a bi-functional electrocatalyst for hydrogen evolution and oxygen reduction reaction

In this work, the porous spinel-type of magnetic iron-manganese oxide nanocubes with a hollow structure deposited on the reduced graphene oxide nanoflakes (PMFMO@RGONF nanocomposite) is synthesized and examined as a non-noble electrocatalyst for hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR). The Tafel slope of PMFMO@RGONF nanocomposite is about 107 mV dec−1 in the 0.50 mol L−1 H2SO4 solution and suggests the Volmer–Heyrovsky mechanism for HER. The results show that this electrocatalyst has an excellent stability (at least 10000 cycles) in the sulfuric acid solution. Hence, The LSV curve shows that the Pt(20%)/C has a better onset-potential than present electrocatalyst for HER but PMFMO@RGONF nanocomposite produces current density (j) of 300 mA cm−2 at overpotential (ɳ) of −721 mV (ɳ300 = −721 mV(vs. Ag/AgCl)) which is smaller than Pt(20%)/C (ɳ300 = −802 mV). The PMFMO@RGONF nanocomposite as an electrocatalyst follows four electrons mechanism for ORR in 0.10 mol L−1 KOH. As an important result, after addition of methanol into the electrolyte, this electrocatalyst is stable for a long time (at least 10000s) while the catalytic activity of Pt (20%)/C is quickly lost in the presence of methanol.

Well-patterned Au nanodots on MoS2/TiO2 hybrids for enhanced hydrogen evolution activity

It was realized that two-dimensional layered MoS2 holds great promise as a replacement for Pt-based catalysts for hydrogen evolution reaction (HER) owing to its optimal affinity to H2. However, the synthesis and assembly of MoS2 with remarkably effective HER is still a challenge. In this work, we fabricated layered MoS2 nanosheets (NSs) on highly ordered TiO2 nanotube arrays to form MoS2/TiO2 heterostructures. The disordered MoS2 NSs expose more active sites with better HER performance. The Au deposition results in nicely patterned nanodots (NDs) on in-plane MoS2 NSs which further improves the catalytic activity due to the increased conductivity, fast electron transport and efficient charge separation. In the end, the energy required to initialize HER decreases distinctly by 170 mV and the exchange current density is as high as 71.6 μA/cm2. The Tafel slop of 72 mV/dec suggests a Volmer–Heyrovsky mechanism. Furthermore, our results demonstrate the feasibility of using Au NDs decoration to map the in-plane atomic-level defect patterns on MoS2 NSs.

Electro-Reduction of Nitrogen on Molybdenum Carbides: A Density Functional Theory Study

There is an enormous interest in the commercialization of the electrochemical synthesis of ammonia, which critically depends on the development of new efficient electro-catalysts and suitable electrolytes for the electrolyzers. Metal nitrides and metal carbides have shown a great promise as electro-catalysts for a wide range of reactions; however, little is known about the structure of potential active sites that promote a specific electro-chemical process [1]. We report herein a density functional theory (DFT) study of the electro-reduction of nitrogen to ammonia on different surfaces of molybdenum carbides, Mo2C and MoC, in order to investigate the viability of using these materials as catalysts for electro-reduction of nitrogen. We first systematically scanned non-equivalent adsorption sites and studied the energetics of adsorption of molecular nitrogen, atomic nitrogen, and ammonia on various crystallographic surfaces of Mo2C and MoC as a means to identify the most favorable binding sites/surfaces that i) activate nitrogen by binding the nitrogen molecule in side-on position, which activates N-N bond, ii) have high affinity for N-adatoms, which prevents accumulation of H adatoms on the catalytic surface at low cell potentials and iii) do not have strong interaction with ammonia, which promotes the release of the product from the catalytic surface. We further studied the electro-reduction of nitrogen on the most active Mo2C and MoC surfaces by calculating the free energy diagrams for nitrogen electro-reduction to ammonia via associative and dissociative Heyrovsky mechanism. The free energy diagrams are helpful in determining potential-limiting step and predicting the potentials necessary to drive the reduction of nitrogen towards formation of ammonia. We further studied the interaction of various crystallographic surfaces of Mo2C and MoC with hydrogen adatoms. Namely, formation of hydrogen gas competes with formation of ammonia and can severely reduce the efficiency of the electrochemical reduction of nitrogen if the catalytic surface is covered with H-adatoms rather than N-adatoms. Kinetic volcano plots for hydrogen evolution on MoC and Mo2C were used to correlate experimentally measured exchange currents for hydrogen evolution over different metal surfaces with DFT calculated hydrogen adsorption free energies [2,3]. The kinetic volcano plots are then used to identify MoC and Mo2C surfaces on which hydrogen production is suppressed and that have the highest selectivity for nitrogen electro-reduction. [1] I. Matanovic, F. H. Garzon, and N. J. Henson, Electro-reduction of Nitrogen on Molybdenum Nitride: Structure, Energetics, and Vibrational Spectra from DFT, Phys. Chem. Chem. Phys. 2014, 16: 3014-3026. [2] S. Trasatti, Work function, electronegativity, and electrochemical behaviour of metal: III. Electrolytic hydrogen evolution in acid solutions, J. Electroanal. Chem. 1972, 39, 163. [3] J. K. Nørskov, T. Bligaard, A. Logadottir, J. R. Kitchin, J. G. Chen, S. Pandelov, U. Stimming, Trends in the Exchange Current for Hydrogen Evolution, J. Electrochem. Soc. 2005, 152(3):J23-J26. Figure 1

Dynamic Hydrogen Bubble Templated NiCu Phosphide Electrodes for pH-Insensitive Hydrogen Evolution Reactions

A highly efficient porous NiCu-phosphide (NiCu-P) electrode is reported for a hydrogen evolution reaction (HER) in acidic, neutral, and basic electrolytes. The NiCu-P electrode was prepared via hydrogen bubbles dynamic templated electrodeposition of NiCu alloy onto nickel mesh, followed by phosphidation. Due to the synergistic interaction in NiCu-P, the material exhibits excellent HER activity in a wide pH range from 0.3–14. Current density of −10 mA cm–2 was achieved at low overpotentials of −226, −250, and −175 mV in 0.5 M H2SO4, 1 M phosphate buffer solution, and 1 M KOH, respectively. The fabricated NiCu-P electrode also shows an outstanding electrochemical stability, regardless of electrolyte pH. The NiCu-P has a slope value of −53 mV dec–1, indicating that HER at NiCu-P follows the Heyrovsky mechanism in 1 M KOH.

КИНЕТИКА РЕАКЦИИ ВЫДЕЛЕНИЯ ВОДОРОДА НА МОНОСИЛИЦИДЕ НИКЕЛЯ В КИСЛОМ И ЩЕЛОЧНОМ РАСТВОРАХ

Методами поляризационных измерений и импедансной спектроскопии изучена реакция выделения водорода на NiSi-электроде в растворах 0.5 M H2SO4 и 1 М КОН. Результаты электрохимических измерений позволяют предположить, что выделение водорода на NiSi в сернокислом растворе протекает по двухмаршрутному механизму Фольмера–Гейровского–Тафеля, а в щелочном растворе – по одномаршрутному механизму Фольмера–Гейровского.

The effects of ball milling on microstructures of graphene/Ni composites and their catalytic activity for hydrogen evolution reaction

The effects of the ball milling on microstructures of the graphene/Ni composites and subsequent hydrogen evolution reaction in acidic solution were investigated. Ni nanoparticles with average grain size of 4nm were uniformly dispersed on the graphene surface after short time ball milling. While amorphous characteristic was observed due to long time ball milling. The graphene/Ni composites with amorphous structure exhibited higher electrocatalytic performance than the graphene/Ni composites with nanocrystalline structure. The Volmer-Heyrovsky mechanism was found for the graphene/Ni composites with nanocrystalline structure, while the graphene/Ni composites with amorphous structure showed evident Heyrovsky mechanism. The excellent hydrogen evolution reaction and stability of the graphene/Ni composites could be attributed to superior interfacial contact, amorphous Ni, and synergistic effect between amorphous Ni and high conductive graphene.

In situ growth of MoS2 on carbon nanofibers with enhanced electrochemical catalytic activity for the hydrogen evolution

Developing an effective and facile method to achieve mass production of MoS2 nanostructures with abundant of edges may be the feasible way to meet the increasing demand for hydrogen evolution electrocatalysts. We developed a facile glucose-assisted hydrothermal method to in-situ grow MoS2 nanosheets on the commercial carbon nanofibers (CNFs). The controlled growth of MoS2 on CNFs (MoS2@CNFs) is leveraged to reveal mass ratio- and structure-dependent catalytic activity in the hydrogen evolution reaction (HER). Due to the unique shell structure, abundant edges of the MoS2 layer are exposed as active site, as well as the underlying CNFs effectively improves the conductivity, the resulting MoS2@CNFs hybrid exhibited high electrocatalytic activity in HER. The catalyst demonstrated the lowest overpotential of 52 mV, the highest current density of 101.49 mA cm−2 at ∼200 mV overpotential and the smallest Tafel slope of 49 mV/decade, suggesting the Volmer–Heyrovsky mechanism for the MoS2-catalyzed HER.

Nanostructure polyoxometalates containing Co, Ni, and Cu as powerful and stable catalysts for hydrogen evolution reaction in acidic and alkaline solutions

A new form of [PW11MO39]5−@Ru-reduced graphene oxide (rGO) (M: Co, Ni, and Cu) was prepared by depositing [PW11MO39]5− on the surface of Ru-rGO. The behaviour of the polyoxometalates (POMs) toward hydrogen evolution reaction (HER) was studied in acidic and alkaline media. Different techniques were used for the characterization of the nanocomposites. These included voltammetry and electrochemical impedance spectroscopy to determine the electrocatalytic properties of the nanocomposites toward HER, and Tafel plots to calculate the kinetic parameters and to identify the possible mechanisms involved in the HER process. Results showed that HER on [PW11MO39]5−@Ru-rGO electrodes obeyed the Volmer–Heyrovsky mechanism. The enhanced HER rates on the modified electrodes were attributed not only to the presence of POM nanoparticles on the electrode surface but also to the resulting enhanced surface areas. These results demonstrate the effective performance of the nanocomposites used as HER catalysts in acidic and alkaline media in conformance with green chemistry objectives. The catalysts are especially advantageous for their desirable capacity, durability, favourable kinetics, and low price.

PtNi and PtMo nanoparticles as efficient catalysts using TEA-PS.BF4 ionic liquid as electrolyte towards HER

Carbon-supported bimetallic nanoparticles, PtNi/C and PtMo/C, were synthesized using the borohydride reduction method and were tested as cathode in the hydrogen evolution reaction (HER) employing acid aqueous solution of 3-triethylammonium-propanesulfonic acid tetrafluoroborate (TEA-PS.BF4) ionic liquid as the electrolyte. For comparison a commercial carbon-supported Pt nanoparticles (Pt/C) has been tested. The morphology, composition and structure of the carbon-supported nanoparticles were characterized by EDX, XRD, and TEM. The results obtained by chronoamperometry, Tafel plots and electrochemical impedance spectroscopy (EIS) exhibited an excellent catalytic effect and the same kinetics mechanism for all carbon-supported nanoparticles in HER studied. Results of Tafel analysis showed that the HER rate determining step is the Volmer–Heyrovsky mechanism. In the temperature range 25–80 °C, the PtNi/C and PtMo/C cathodes presented lower activation energy and higher current density than system using the Pt/C cathode. The PtMo/C cathode improves charge transfer kinetics and hydrogen adsorption, while the PtNi/C cathode facilitates the desorption step. These results demonstrate that the carbon-supported bimetallic nanoparticles PtNi/C and PtMo/C are responsible for increasing the catalytic performances of the HER when TEA-PS.BF4 aqueous solution is employed as electrolyte.

Identifying the significance of proton-electron transfer in CH4 production on Cu (100) in CO2 electro-reduction

In electrocatalysis, there are two mechanisms for hydrogenation reactions: Heyrovsky mechanism via the proton-electron transfer and Tafel mechanism via surface coupling with adsorbed hydrogen. In CO/CO2 electroreduction over copper catalysts, CH4 is a final product from the subsequent hydrogenation of carbon. In this work, by performing the constrained ab initio molecular dynamics simulations, we firstly identify that CH4 production on Cu(100) via the proton-electron transfer is more kinetically favored than via surface coupling, due to the negatively charged C atoms in CHx*. Our findings highlight the significance of proton-electron transfer in CO2 electroreduction, and help understand the essential difference between electrocatalysis and heterogeneous catalysis.

Interfacial engineering of MoS2/TiO2 hybrids for enhanced electrocatalytic hydrogen evolution reaction

Herein, we show that the synergistic effect between MoS2 and TiO2 enhances the hydrogen evolution reaction (HER) performance of their hybrids, which is tunable via interface engineering. Among several interfaces, MoS2/TiO2–H complexes exhibit the best HER activity. The observed Tafel slope of 66.9 mV/dec is well in range of previous literature reports, suggesting a Volmer–Heyrovsky mechanism. Enhanced activities were attributed to abundant active sites at the interfaces, as well as improved charge transfer efficiency. Our results emphasize the roles that interfaces play in enhancing the HER activities of MoS2-based heterogeneous catalysts.

The activity of PAni-Chitosan composite film decorated with Pt nanoparticles for electrocatalytic hydrogen generation

To develop low-cost, highly efficient and stable electrocatalysts is one of the main challenges for hydrogen production from a water electrolysis system. Herein, we describe platinum nanoparticles supported on pencil graphite modified by a Polyaniline-Chitosan composite film that could be used for such a system. This composite electrocatalyst was found to be very active in catalysing hydrogen evolution reactions in acidic medium. The catalytic activity for the hydrogen evolution reaction (HER) was analysed by electrochemical techniques that promoted the kinetics of the reaction following the Volmer–Heyrovsky mechanism. The porous structure of the catalyst provides large current densities for Had formation and hydrogen evolution. Thus, this study implies that the pencil graphite coated with Polyaniline-Chitosan/Pt nanostructured material is a promising electrocatalyst for electrolytic hydrogen production due to its favourable kinetics and that it retains its performance after long-term electrolysis.

Electrocatalytic hydrogen production on a modified pencil graphite electrode

The development of Polyaniline (PAni)-Chitosan (Chi) and PAni-Chi/Au conducting composite films coated on pencil graphite and used as electrocatalysts for hydrogen production from water is discussed in this study for the first time. The obtained PAni-Chi composite films and PAni-Chi/Au nanostructured composite films were characterized by scanning electron microscopy (SEM), energy-dispersive spectrometer (EDS) and evaluated as hydrogen evolution reaction (HER) electrocatalysts in an acidic medium. The effect of the film thickness of the PAni-Chi composite film and the effect of the Au electrodeposition on this film were systematically studied by cyclic voltammetry (CV), Tafel polarization curves and electrochemical impedance spectroscopy (EIS). It has been established that the main pathway for the HER is via the Volmer–Heyrovsky mechanism with the Volmer reaction being the rate-determining step. This highly active, stable and cost-effective modified pencil graphite electrode is a promising electrocatalyst for hydrogen production from water.

Electrocatalytic activity of composite Fe/TiO2 electrodeposits for hydrogen evolution reaction in alkaline solutions

Fe/TiO2 composite coatings were obtained by electrodeposition using a methanesulfonate plating bath containing colloidal TiO2 particles. Iron/titania electrodeposited composites exhibited enhanced electrocatalytic activity towards hydrogen and oxygen evolution reactions (HER and OER) in an alkaline solution. The electrocatalytic properties of Fe/TiO2 composites towards the HER were studied by means of voltammetry, electrochemical impedance spectroscopy and chronoamperometry methods. An increase in TiO2 content in iron matrix resulted in an increase in the electrocatalytic activity. The kinetic parameters were calculated on the basis of Tafel plots and EIS measurements and the possible mechanisms of the HER and OER were considered. The hydrogen evolution reaction on Fe/TiO2 electrodes obeyed Volmer–Heyrovsky mechanism. It was concluded that the increase in the rate of the HER and OER on Fe/TiO2 composite electrodes may be associated with the emergence of the redox couples on the electrode surface and the growth of the concentration of the surface active sites. The Fe/TiO2 electrocatalysts showed a good catalytic stability in the HER and OER in 1 M NaOH solution.

Influence of hydrogen absorption on the potential dependence of the Faradaic impedance parameters of hydrogen evolution reaction

The influence of hydrogen absorption reaction (HAR) proceeding through Hads with kinetic or with diffusion control on the dependence of the elements R1, R2, C2 of the equivalent electrical circuit for hydrogen evolution reaction (HER) on the electrode potential E has been considered. It has been shown that if HER proceeds through the Volmer–Heyrovsky mechanism, then the presence of HAR reduces the slope dlogR2/dE at all Е and the slope dlogС2/dE at sufficiently high cathodic polarisations. In the case of the Volmer–Tafel mechanism the presence of HAR mainly affects the resistance R2. The character of the dependences of R1, R2, C2 on Е is the same for both kinetic and diffusion control of HAR. An interpretation of the experimental impedance data for HER on CoSi2 in 0.5M H2SO4 has been presented.

Electrocatalytic Hydrogen Production

One of effective solar-hydrogen conversion approaches is indirect usage of solar energy in electrocatalytic water electrolysis reactions. Highly purified hydrogen can be produced from water electrolysis, but the biggest disadvantage is high-energy consumption of this method [1]. In order to reduce the production costs by lowering the overpotential of hydrogen evolution reaction (HER), various electrocatalytic materials have been frequently tested as the best solutions. Due to the versatile properties, 2-D graphene based functional hybrid nanomaterials have been used for many fields such as, sensors, transparent conductors, purification, and dye-sensitized solar cells. Now a days, graphene oxide (GO) and based materials have been tested as active photocatalysts and photoelectrocatalyst for water splitting reactions [2]. Although GO and reduced azido graphene oxide (RGO) were used as active photocatalysts and photoelectrocatalyst for HER and active electrocatalyst for oxygen reduction reactions [3], there is no a study on the usage of GO and RGO for the electrocatalytic HER from water electrolysis. Therefore in this study we have investigated electrocatalytic activity of GO and RGO based modified electrodes for the electrocatalytic HER. The complexes having metal-based reduction processes observed before HER processes generally showed good electrocatalytic activity. It was stated in our previous studies that MPcs having metal-based reduction processes showed higher activities for HER [4]. Therefore in order to increase electrocatalytic activity of the modified electrodes, GO and RGO were decorated with an active electrocatalyst, terminally alkyne substituted cobalt(II)phthalocyanine ( TA-CoPc ). For this purpose GO and RGO were functionalized with azide groups. Then TA-CoPc molecules were bonded to the azide groups of GO and RGO with click electrochemistry (CEC) technique and finally CoPc decorated GO and RGO based modified glassy carbon electrodes (GCE/GO-N3/TACoPc and GCE/RGO-N3/TACoPc) were developed. The modified GCE/RGO-N3/TA-CoPc and GCE/GO-N3/TA-CoPc electrodes were characterized with square wave voltammetry and electrochemical impedance spectroscopy (EIS), and then tested as heterogeneous electrocatalysts for HER. Characterizations illustrated that GCE/GO-N3/TA-CoPc and GCE/RGO-N3/TA-CoPc electrodes have redox activities at small potentials and sufficient conductivities, which are the basic requirements for the electrocatalytic application of a modified electrode. GCE/RGO-N3/TA-CoPc electrode illustrates well electrocatalytic activity by decreasing the over-potential of the bare electrode and increasing the current density of the electrode significantly with absolutely high stability and reproducibility. Analyses of Tafel responses of the electrodes show the proceeding of a Volmer–Heyrovsky mechanism during HER. Acknowledgement: This work is supported by the research fund of TUBİTAK (Project no: 113M991) and Marmara University (Project no: FEN-C-YLP-080415-0118).

Reduced Graphene Oxide/O-MWCNT Hybrids Functionalized with p-Phenylenediamine as High-Performance MoS2 Electrocatalyst Support for Hydrogen Evolution Reaction

Efficient hydrogen evolution through water splitting at low overpotentials is crucial to develop renewable energy technology, which depends on the design of efficient and durable electrocatalysts composed of earth-abundant elements. Herein, a highly and stable electrocatalyst for hydrogen evolution reaction (HER) has been developed on the basis of MoS2 on p-phenylenediamine (PPD)-functionalized reduced graphene oxide/O-containing carbon nanotubes (rGO/O-MWCNT) hybrids via facile and green hydrothermal process. Among the prepared catalysts, the optimized MoS2/rGO/PPD/O-MWCNT with nanosized and highly dispersed MoS2 sheets provides a large amount of available edge sites and the improved electron transfer in 3D conductive networks. It exhibits excellent HER activity with a low overpotential of 90 mV and large current density of 47.6 mA·cm–2 at 200 mV, as well as excellent stability in an acidic medium. The Tafel slope of 48 mV·dec–1 reveals the Volmer–Heyrovsky mechanism for HER. Thus, this work paves a potential pathway for designing efficient MoS2-based electrocatalysts for HER by functionalized conductive substrates.