Reversible Hydrogen Potential Research Articles

Porous biomass-derived carbon modified by Cu, N co-doping and Cu nanoparticles as high-efficient electrocatalyst for oxygen reduction reaction and zinc-air battery

Exploration of new non-precious metal catalysts for highly efficient oxygen reduction reaction (ORR) is essential for sustainable energy utilization technologies. Herein, porous biomass-derived carbon modified by Cu, N co-doping and Cu nanoparticles was synthesized by hydrothermal followed pyrolysis strategy, and used for electrocatalytic ORR. The optimized Cu/Cu-N-C catalyst exhibits large surface area (235.91 cm3 g−1) and hierarchical pore structure. Benefiting from the above factors, the Cu/Cu-N-C catalyst performs a high onset potential (Eonset, 0.947 V vs. reversible hydrogen potential (RHE)) and a half-wave potential (E1/2, 0.875 V vs. RHE) in 0.1 M KOH solution. The calculated electron transfer number of 3.92 suggests the near-four-electron ORR process. Besides, the Cu/Cu-N-C catalyst also performs extraordinary stability and higher methanol tolerance than Pt/C. In addition, the assembled Zn-air battery based on the optimized Cu/Cu-N-C catalyst exhibits a higher power density of 150.7 mW cm−2 than the that of Pt/C-based battery. This work put forward a newfangled method to fabricate efficient ORR catalyst by loading active component on biomass-derived carbon, which follows the sustainable development concept, and provides a profitable reference for designing nonprecious metal carbon-based ORR electrocatalyst.

Fe/N-doped hollow porous carbon spheres for oxygen reduction reaction

Herein, we design a dual-template-assisted pyrolysis method to prepare ultra-small Fe3O4 nanoparticles anchored on Fe/N-doped hollow porous carbon spheres (0.010-Fe/NHPCS-800) for oxygen reduction reaction (ORR). The synthesized SiO2 nanospheres, which are selected as the hard template, contribute to forming macroporous structure. Pluronic ® F127 is employed to fabricate mesopores through high-temperature pyrolysis as a soft template. In this way, the 0.010-Fe/NHPCS-800 architecture represents an ordered hierarchically porous property with a large BET surface area (1812 m2 g−1), which can facilitate the mass transport of reactants and increase the electrochemically active area. The Fe3O4 nanoparticles wrapped by graphitic carbon layers provide more active sites, and the synergistic interaction between Fe3O4 nanoparticles and doping N has a positive effect on ORR performance. The 0.010-Fe/NHPCS-800 catalyst outperforms the most effective ORR activities among a series of Fe/NHPCS samples with onset potential of 0.95 V (versus reversible hydrogen potential) and half-wave potential of 0.81 V, which is almost the same as the commercial Pt/C (0.96 and 0.81 V, correspondingly) in 0.10 M KOH. However, both the stability and durability of 0.010-Fe/NHPCS-800 surpass those of commercial Pt/C. Given all these advantages, 0.010-Fe/NHPCS-800 is a promising candidate to take the place of Pt-based electrocatalysts for ORR in the future.

Al-ZnO/CdS Photoanode Modified with a Triple Functions Conformal TiO2 Film for Enhanced Photoelectrochemical Efficiency and Stability

ZnO/ CdS based nanorod arrays is an excellent class of photoanode materials with high photoelectric response. However, the poor photostability of CdS in aqueous solution seriously prevents its practical application. Although efforts have been made, it still challengeable to improve stability while maintaining initial photocurrent. Herein, a highly efficient photoanode system consisting of Al-doped ZnO nanorod arrays (NRs) served as effective electron-transfer layer and CdS as a light harvesting layer was designed. At last, TiO2 protective layer was deposited onto Al-ZnO/CdS photoanodes through atomic layer deposition (ALD) to enhance the photostability. The integrated Al-ZnO/CdS/TiO2 photoanode exhibits a photocurrent of 11.7 mA/cm2 at 1.23 V versus reversible hydrogen potential (RHE) and conversion efficiency of 5.9 % at 0.48 V versus RHE for 60 ALD TiO2 cycles. More importantly, the photostability of Al-ZnO/CdS/TiO2 photoanode was significantly prolonged comparing with the untreated Al-ZnO/CdS photoanode. The strengthen stability is benefited from the triple functions of ALD TiO2 layer, that is, isolating the direct contact between the photoanode with the surrounding liquid environment, passivating the surface state of the CdS, and capturing as well as storing the photogenerated hole.

Nonprecious Nanoalloys Embedded in N‐Enriched Mesoporous Carbons Derived from a Dual‐MOF as Highly Active Catalyst towards Oxygen Reduction Reaction

AbstractIn this work, we reported the synthesis of a novel electro‐catalyst composed of Fe and Co bimetallic nanoalloys embedded in N‐enriched carbon framework (FeCo@NC) for oxygen reduction reaction. The FeCo@NC was synthesized through pyrolyzing a dual metal‐organic framework (MOF) under argon atmosphere. This FeCo@NC electrocatalyst showed remarkable performance towards ORR with a high half‐wave potential of 0.827 V versus reversible hydrogen potential (RHE), which was comparable to the state‐of‐the‐art commercial Pt/C catalyst. The enhanced activity could be ascribed to the large specific surface area, the abundant active sites and the synergistic effect of bimetallic alloy.

Cu, Co‐Embedded N‐Enriched Mesoporous Carbon for Efficient Oxygen Reduction and Hydrogen Evolution Reactions

Rational synthesis of hybrid, earth‐abundant materials with efficient electrocatalytic functionalities are critical for sustainable energy applications. Copper is theoretically proposed to exhibit high reduction capability close to Pt, but its high diffusion behavior at elevated fabrication temperatures limits its homogeneous incorporation with carbon. Here, a Cu, Co‐embedded nitrogen‐enriched mesoporous carbon framework (CuCo@NC) is developed using, a facile Cu‐confined thermal conversion strategy of zeolitic imidazolate frameworks (ZIF‐67) pre‐grown on Cu(OH)2 nanowires. Cu ions formed below 450 °C are homogeneously confined within the pores of ZIF‐67 to avoid self‐aggregation, while the existence of CuN bonds further increases the nitrogen content in carbon frameworks derived from ZIF‐67 at higher pyrolysis temperatures. This CuCo@NC electrocatalyst provides abundant active sites, high nitrogen doping, strong synergetic coupling, and improved mass transfer, thus significantly boosting electrocatalytic performances in oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER). A high half‐wave potential (0.884 V vs reversible hydrogen potential, RHE) and a large diffusion‐limited current density are achieved for ORR, comparable to or exceeding the best reported earth‐abundant ORR electrocatalysts. In addition, a low overpotential (145 mV vs RHE) at 10 mA cm−2 is demonstrated for HER, further suggesting its great potential as an efficient electrocatalyst for sustainable energy applications.

Enhanced InGaN/GaN photoelectrodes for visible‐light‐driven hydrogen generation by surface roughening

III‐Nitride semiconductor materials are considered as promising candidates for photoelectrodes (PEs) due to their adjustable direct band gap covering a very broad spectral range. In this study, InGaN/GaN based p–i–n photoelectrodes have been fabricated and nano‐sized surface roughening process has been applied. The photocurrent gets 2.5 times enhancement and the incident photon conversion efficiency (IPCE) is improved to be ∼30% at the wavelength of 400 nm. In addition, the turn‐on voltages of InGaN/GaN‐based PEs have been effectively reduced from +0.8 V down to about −0.4 V (vs. reversible hydrogen potential, RHE). Such improvements are mainly attributed to reduced dislocation density, increased surface area, and optimized build‐in electrical field. These findings give innovative insight to improve photoelectrochemical devices for hydrogen generation.

Solar Hydrogen Production by Amorphous Silicon Photocathodes Coated with a Magnetron Sputter Deposited Mo2C Catalyst.

Coupling of Earth-abundant hydrogen evolution catalysts to photoabsorbers is crucial for the production of hydrogen fuel using sunlight. In this work, we demonstrate the use of magnetron sputtering to deposit Mo2C as an efficient hydrogen evolution reaction catalyst onto surface-protected amorphous silicon (a-Si) photoabsorbers. The a-Si/Mo2C photocathode evolves hydrogen under simulated solar illumination in strongly acidic and alkaline electrolytes. Onsets of photocurrents are observed at potentials as positive as 0.85 V vs RHE. Under AM 1.5G (1 sun) illumination, the photocathodes reach current densities of -11.2 mA cm(-2) at the reversible hydrogen potential in 0.1 M H2SO4 and 1.0 M KOH. The high photovoltage and low-cost of the Mo2C/a-Si assembly make it a promising photocathode for solar hydrogen production.

Electrodeposition of Ternary Pt100-x-yCoxNiy Alloys

Electrodeposition of ternary Pt100-x-yCoxNiy alloy films and the respective Pt binary alloys were examined and compared to the characteristics of the constituent elements. The potential dependence of alloy composition, structure and deposition current efficiency was determined using in situ electrochemical quartz crystal microbalance (EQCM) measurements and metallurgical analysis. The recently documented self-terminating electrodeposition of elemental Pt is disrupted by competitive adsorption and underpotential co-deposition of the iron group metals, whose onset overlaps with the Hupd region. Excess alloying enthalpy between Pt and the iron group metals supports the underpotential deposition of the iron group metals during on-going Pt deposition. The total iron group alloy content increases monotonically as the deposition potential becomes more negative. Below the reversible hydrogen potential the current efficiency drops with the onset of the proton reduction reaction. At more negative potentials the efficiency increases as the partial current for the Pt deposition and proton reduction reactions operate near diffusion control while the iron group metal deposition current increases monotonically. For ternary Pt100-x-yCoxNiy alloys grown from an electrolyte with [Ni2+]/[Co2+] = 1, Ni underpotential co-deposition is favored over Co, while in the overpotential regime anomalous co-deposition occurs with the Co content significantly exceeding that of Ni.

Photocatalytic Water Splitting Using Oxynitride and Nitride Semiconductor Powders for Production of Solar Hydrogen

57 Water splitting using a photocatalyst for direct solar hydrogen production from sunlight and water, has been regarded as an important approach to artificial photosynthesis.1,2 Solar hydrogen, which is the simplest solar fuel, can then be converted to various energy carriers such as organic hydrides, methanol, methane, and ammonia for transportation and storage. Thus, energy systems based on solar hydrogen can lead to a stable, secure, and ecologically-friendly society. Although solar fuel production using electricity from photovoltaic cells and concentrated solar thermal power is an existing technology, direct synthesis of solar fuels by artificial photosynthesis has more scalability in terms of system cost. To split water into hydrogen and oxygen, 1.23 V of energy is required, corresponding to a 2 electron reaction. An energy of 1.23 V is equivalent to a photon energy of 1000 nm, so that the visible light (400-800 nm in wavelength) that represents half of the solar energy spectrum can be thermodynamically used for water splitting. The challenge is to obtain an efficient photocatalyst that has an absorption edge in longer wavelength and the ability to split water. Highly efficient oxide photocatalysts, such as La-doped NaTaO3, and Zn-doped Ga2O3, have been reported with quantum efficiencies of over 50%.3-5 However, these oxides only absorb ultraviolet (UV) light with wavelengths shorter than 300 nm. In the solar spectrum at the earth’s surface, such short UV light is not present, so that these oxides cannot operate efficiently under solar irradiation. Most oxides have band gaps wider than the UV energy because of the deeper O 2p potential of the valence band. The valence band of nitrides is composed of the shallower N 2p potential, and thus their band gaps are generally narrower than those of oxides. For photocatalytic water splitting, photoexcited electrons in the conduction band reduce water to evolve hydrogen, and photogenerated holes in the valence band oxidize water to evolve oxygen. Therefore, the potential of the conduction band should be shallower than the reversible hydrogen potential and the potential of the valence band should be deeper than the reversible oxygen potential. In this article, the properties of oxynitride and nitride photocatalysts for water splitting are described. Surface modification of semiconductor photocatalyst particles with hydrogenor oxygen-evolving cocatalysts is an indispensable technique in the study of photocatalytic water splitting. In fact, hydrogen-evolution cocatalysts are required Photocatalytic Water Splitting Using Oxynitride and Nitride Semiconductor Powders for Production of Solar Hydrogen

A mechanistic study of borohydride anodic oxidation

In direct borohydride fuel cells, the anodic oxidation of borohydride is often accompanied by hydrogen evolution which decreases the number of electrons released to the external circuit from each borohydride ion and thus lowers the Faradaic efficiency and causes safety problems. Based on the simultaneous measurements of polarization curve and hydrogen evolution for the borohydride oxidation on a number of different catalysts, a two-stage model is proposed for the anodic oxidation of borohydride, featuring a 4-electron oxidation of borohydride to form adsorbed hydrogen atoms H ads * which may then either be oxidized to water or form hydrogen molecules through water reduction, depending on the nature of catalysts and the electrode potentials. In the view of the high reducing ability of borohydride, the intermediate H ads * may have a higher energy state and therefore, it could be oxidized at potentials negative to reversible hydrogen potential (RHE). A possible approach to reach the goal of “8-electron borohydride oxidation at potentials negative to RHE” is also suggested and realized with a titanium-oxide supported Pt catalyst.

Hydrogen electrosorption in Pd–Pt alloys

Hydrogen electrosorption has been investigated in limited volume (LV) Pd–Pt alloy electrodes. The amount of absorbed hydrogen decreases with increasing Pt content in the bulk of the alloy, but does not change with the Pt content on the alloy surface. A change in the rate of the hydrogen absorption reaction occurs at a potential close to the reversible hydrogen potential. This effect can probably be related to the existence of the overpotentially adsorbed hydrogen on the surface of the electrode.

In Situ X‐Ray Absorption Studies of a Pt‐Ru Electrocatalyst

X‐ray absorption studies (XAS) were done on a carbon supported Pt‐Ru electrocatalyst in 1 M . Results at the Pt and edges confirmed that the Pt was alloyed with Ru and that the Ru content was about 25 atomic percent. There was a large excess of unalloyed Ru, with only about 10% of the Ru alloyed with the Pt. The Pt XAS indicated that the Ru increased the Pt d band vacancies and decreased the Pt‐Pt bond distances from 2.77 Å to values between 2.71 and 2.73 Å. The bifunctional mechanism for methanol oxidation on Pt‐Ru electrocatalysts needs to be modified to account for the effect of these electronic changes on the adsorption of H and residues from methanol decomposition. There are significant changes in the Pt XAS in going from the reversible hydrogen potential to 0.24 V. This may be due to the onset of the formation of species on the alloy. Further fine tuning of the electronic structure and the electrocatalysis may be possible through the use of ternary alloys.

Electroreduction of carbon dioxide on palladium electrodes at potentials higher than the reversible hydrogen potential

The electroreduction of CO 2 was observed on dispersed electrolytic deposits of palladium at potentials more positive than the reversible hydrogen potential (pH 8–10) with the HCOO − current yield close to 100%. It was assumed that adsorbed hydrogen atoms take part in the slow stage of the electroreduction of HCO 3 − (CO 2) to HCOO −.

The Effect of Sputtered RuO2 Overlayers on the Photoelectrochemical Behavior of CdS Electrodes

The photoelectrochemical behavior of sputtered overlayers on electrodes has been investigated. Flatband potential and photocurrent onset potential are shifted to values positive of the reversible hydrogen potential. Various dark current‐potential characteristics depending on film preparation and starting potential have been obtained. The results are explained by assuming that a Schottky barrier is formed at the interface.

Effect of electrolyte environment and Pt crystallite size on hydrogen adsorption—V

The role of Pt crystallite surface morphology on hydrogen adsorption isotherms in H 2SO 4 and alkaline electrolytes was examined by a potentiodynamic sweep technique. By varying the crystallite size (40–280Å) of highly dispersed Pt electrocatalysts, the relative concentrations of edges, vertices and crystallite faces which contribute to the surface morphology are changed. The potentiodynamic i- V profiles for adsorbed hydrogen oxidation on highly dispersed Pt electrocatalysts in 0.05 and 1 M H 2SO 4 showed similar changes with Pt crystallite size. Only two states of adsorbed hydrogen on highly dispersed Pt were observed in 0.05 M H 2SO 4, compared to four states reported by Angerstein-Kozlowska et al on smooth polycrystalline Pt electrodes. In 1 M NaOH and 35 wt % KOH, less than a monolayer of adsorbed hydrogen was present on highly dispersed Pt electrocatalysts at the reversible hydrogen potential. Two states of chemisorbed hydrogen were observed at 23–91°, while at low temperature (−47°) in 35 wt % KOH, an additional adsorbed hydrogen species was evident in the potentiodynamic i- V curves. A Pt crystallite size effect on the adsorption of hydrogen on highly dispersed Pt in alkaline electrolytes was not deduced.

Bioelectrocatalysis. Hydrogenase as catalyst of electrochemical hydrogen ionization

The bioelectrocatalysis (acceleration of electrochemical reactions in the presence of enzymes) is discussed. The central point of bioelectrocatalysis is given by an efficient electron transport mechanism from the active center of the enzyme to an electrode. In principle, this could be realized by direct electron exchange between the active center of the enzyme and the electrode and also by an intermediate electron transport to a labile mediator of low molecular weight. The mediator mechanism of electron transport between the active center of the enzyme and a macroelectrode is discussed in detail. The requirements to such mediators are summarized. As a model, the system hydrogen-hydrogenase-methylviologen-pyrographite electrode was investigated. In this system one may kinetically ideally organize the electrochemical hydrogen oxidation by transforming the chemical energy of the fuel into an electrochemical potential, which enables one to realize the anodic oxidation of hydrogen on carbon electrodes. The mechanism of bioelectrocatalysis involves enzymatic reduction of methylviologen with molecular hydrogen and electrochemical oxidation of the mediator. The kinetics of the enzymatic and of the electrochemical reaction steps were studied. On the pyrographite electrode the reversible electrochemical oxidation and reduction of the mediator were possible with the kinetics being limited by the diffusion of the reagent. In this case a reversible redox hydrogen potential was established on the electrode. The dependences of the limiting anodic currents of hydrogen oxidation on the enzyme and on the mediator concentrations are investigated. Kinetical models describing the effects observed are proposed.

Corrosion Inhibition of Iron and Copper in Chloride Solutions by Benzotriazole

Abstract Benzotriazole is shown to be a good corrosion inhibitor of iron in acidic solutions even at chloride ion concentrations of 3.5% NaCI. The corrosion potential of copper in deaerated 1N H2SO4 shifted about 300 mV in negative direction to the reversible hydrogen potential with addition of 3.5% chloride ions. In the presence of benzotriazole, addition of chloride ions did not shift the corrosion potential to the reversible hydrogen potential. It is surmised that trace amounts of oxygen cause the noble corrosion potential of copper.

Mechanism of Cracking of High Strength Martensltic Stainless Steels in Sodium Chloride Solutions

Abstract This paper reviews previously proposed mechanisms for the environmentally induced cracking of high strength steels in sodium chloride. The electrochemical basis for the differentiation between hydrogen embrittlement and active path corrosion is examined along with the consequences of the recently demonstrated acidification that occurs in areas of localized geometry such as precracks, pits, and crevices. From electrochemical studies on a modified 12% chromium martensitic stainless steel, data are presented which indicate that hydrogen absorption occurs under conditions of exposure previously thought to preclude this possibility; that is, at applied bulk potentials noble to the reversible hydrogen potential. Data are also presented to show that the activation energy for crack growth at the corrosion potential and at cathodic and anodic applied potential is 9.5 Kcal/mole (±1); a value close to that reported previously for the cracking of cathodically embrittled steel. The experimental data are explained on the basis that an embrittlement process associated with hydrogen absorption is responsible for crack growth under all conditions of applied potential. With polarization at potentials active to the hydrogen reversible potential, hydrogen absorption results directly from the bulk cathodic discharge of protons. At the corrosion potential and more noble potentials, hydrogen absorption takes place from the cathodic discharge of protons occurring in the low pH conditions resulting from the hydrolysis of anodic dissolution products in pits. In the noble potential region, although active path anodic dissolution appears not to be the mechanism of crack growth, anodic dissolution is a prerequisite to crack growth by a hydrogen embrittlement mechanism. It is suggested that a similar mechanism can explain the cracking behavior of other high strength steels in sodium chloride solutions where conditions suitable for hydrolysis arise either by pitting or by crevice corrosion occurring in the presence of a mechanical precrack.

Application of the method of time-domain reflectometry to the study of electrode processes

Summary o 1. A technique has been developed for the application of the methods of time-domian reflectometry to measurements of electrode impedance in electrochemical systems. 2. The impedance has been measured at a platinum electrode in aqueous HClO 4 and H 2 SO 4 solutions. The capacitance is normally independent of time in the range 1–100 nsec except at the reversible hydrogen potential where the effect of the electrode reaction results in a time-dependent decrease of the electrode impedance at times >30 nsec. No detectable relaxation of the double layer occurs within the time range of the experiments.

Application of the method of time-domain reflectometry to the study of electrode processes

Summary o 1. A technique has been developed for the application of the methods of time-domian reflectometry to measurements of electrode impedance in electrochemical systems. 2. The impedance has been measured at a platinum electrode in aqueous HClO 4 and H 2 SO 4 solutions. The capacitance is normally independent of time in the range 1–100 nsec except at the reversible hydrogen potential where the effect of the electrode reaction results in a time-dependent decrease of the electrode impedance at times >30 nsec. No detectable relaxation of the double layer occurs within the time range of the experiments.