H2 Production System Research Articles

Trichloroethene Dechlorination and H2 Evolution Are Alternative Biological Pathways of Electric Charge Utilization by a Dechlorinating Culture in a Bioelectrochemical System

Recently,the proof-of-principle of an innovative bioelectrochemical process fortrichloroethene (TCE) bioremediation was presented. In this newly developed process, a solid-state electrode polarized to -450 mV (vs SHE), in the presence of a low-potential redox mediator (methyl viologen), is employed as an electron donor for the microbial reductive dechlorination of TCE to lower or nonchlorinated end products. In the present study, we investigated the influence of methyl viologen and TCE concentrations on process performance. Using a highly enriched hydrogenotrophic dechlorinating culture, as a source culture in batch experiments, we found that TCE dechlorination and H2 evolution were the two main biological reactions which were stimulated. The relative contribution of the two reactions appeared to be strongly dependent on the mediator concentration. At the lowest methyl viologen (MV) concentrations (25-750 microM), only TCE dechlorination was stimulated, and no H2 was produced; at higher MV concentrations, both reactions occurred simultaneously, although they showed distinct kinetic features. In batch experiments in which TCE was omitted from the system, the rate of H2 production was remarkably increased (up to 80 times), suggesting that protons represented an alternative electron sink in the absence of the more energetically favorable TCE. Clearly, optimization of the process for TCE dechlorination requires H2 evolution to be minimized by, for instance, operating the system at low mediator concentrations, and this can be possibly achieved through proper physical immobilization of the mediator at the electrode surface. On the other hand, the observed bioelectrocatalytic H2 production occurred at virtually no overpotentials with respect to the thermodynamic 2H+/H2 potential. This finding revealed that the dechlorinating culture employed represented quite an exceptional and previously unrecognized biocatalytic system for H2 production.

Natural Gas Decarbonization Technologies for Advanced Power Plants

In this paper two options for H2 production, by means of natural gas, are presented and their performances are evaluated when they are integrated with advanced H2/air cycles. In this investigation two different schemes have been analyzed: an advanced combined cycle power plant (CC) and a new advanced mixed cycle power plant (AMC). The two methods for producing H2 are as follows: (1) steam methane reforming: it is the simplest and potentially the most economic method for producing hydrogen in the foreseeable future; and (2) partial oxidation of methane: it could offer an energy advantage because this method reduces the energy requirement of the reforming process. These hydrogen production plants require material and energetic integrations with power section and the best interconnections must be investigated in order to obtain good overall performance. With reference to thermodynamic and economic performance, significant comparisons have been made between the above introduced reference plants. An efficiency decrease and an increase in the cost of electricity has been obtained when power plants are equipped with a natural gas decarbonization section. The main results of the performed investigation are quite variable among the different H2 production technologies here considered: the efficiency decreases in a range of 5.5 percentage points to nearly 10 for the partial oxidation of the natural gas and in a range of about 9 percentage points to over 12 for the steam methane reforming. The electricity production cost increases in a range of about 41–42% for the first option and in a range of about 34–38% for the second one. The AMC, coupled with partial oxidation, stands out among the other power plant solutions here analyzed because it exhibits the highest net efficiency and the lowest final specific CO2 emission. In addition to this, economic impact is favorable when AMC is equipped with systems for H2 production based on partial oxidation of natural gas.

Photocatalytic Hydrogen Production from Gas-phase Methanol and Water with Nanocrystalline TiO2 Thin Films in High Vacuum

ABSTRACTPhotocatalytic hydrogen production with gas-phase reactions in high vacuum was examined for nanocrystalline anatase-type titanium dioxide (TiO2) thin films. The hydrogen generation process on platinized TiO2 specimens was investigated using a quadrupole mass spectrometer at a real-time scale under various partial pressures of gaseous methanol and water. As a result, hydrogen generation was successfully detected under ultraviolet ray (UV) illumination even in high vacuum (∼ 10−7 Torr). And the amount of produced H2 largely depends on the temperature of TiO2 samples, probably due to different surface states of TiO2. This study suggests the possibility of new high-speed H2 production system with gas-phase photocatalytic reactions.

H 2 ∕ O 2 Cycles: Thermodynamic Potentialities and Limits

In this paper, the thermodynamic potentialities and limits of the H2∕O2 turbine cycles (afterward named only H2∕O2 cycles) are investigated. Starting from the conventional gas turbine and steam turbine technology, the paper qualitatively tackles problems related to a change of oxidizer and fuel: from these considerations, an internal combustion steam cycle is analyzed where steam, injected into the combustion chamber together with oxygen and hydrogen, is produced in a regenerative way and plays the important role of inert. A proper parametric analysis is then performed in order to evaluate the influence of the main working parameters on the overall performance of H2∕O2 cycles. All the results are carried out by neglecting the energy requirements for O2 and H2 production systems, but taking into account the work required by the O2 and H2 compression. This choice permits a great freedom in the definition of these thermodynamic cycles; moreover, it is possible to come to some general conclusions because the H2 and/or O2 production systems and their integrations with thermodynamic cycles do not have to be specified. Therefore, this paper can be framed in a context of centralized production of oxygen and hydrogen (by nuclear or renewable energy sources, for example) and their distribution as pure gases in the utilization place. By adopting some realistic assumptions, for example, a top temperature of 1350°C, the potentialities of H2∕O2 cycles are very limited: the net efficiency attains a value of about 50%. Instead, by adopting futurist assumptions, for example, a top temperature of 1700°C, a different H2∕O2 cycle scheme can be proposed and its performance becomes more interesting (the net efficiency is over 60%). The paper tackles the main thermodynamic and technological subjects of the H2∕O2 cycles: for example, it is underlined how the choice of the working parameters of these cycles strongly influences the attainable performance.

A new oxygen sensitivity and its potential application in photosynthetic H2 production.

We have discovered a new competitive pathway for O2 sensitivity in algal H2 production that is distinct from the O2 sensitivity of hydrogenase per se. This O2 sensitivity is apparently linked to the photosynthetic H2 production pathway that is coupled to proton translocation across the thylakoid membrane. Addition of the proton uncoupler carbonyl cyanide-p-trifluoromethoxy- phenylhydrazone eliminates this mode of O2 inhibition on H2 photoevolution. This newly discovered inhibition is most likely owing to background O2 that apparently serves as a terminal electron acceptor in competition with the H2 production pathway for photosynthetically generated electrons from water splitting. This O2-sensitive H2 production electron transport pathway was inhibited by 3[3,4-dichlorophenyl]1,1-dimethylurea. Our experiments demonstrated that this new pathway is more sensitive to O2 than the traditionally known O2 sensitivity of hydrogenase. This discovery provides new insight into the mechanism of O2 inactivation of hydrogenase and may contribute to the development of a more-efficient and robust system for photosynthetic H2 production.

H2 production from maltose derived from starch using the visible light, photosensitization of Mg chlorophyll-a from Spirulina

A photoinduced H2 production system, coupling d-maltose degradation by glucoamylase and glucose dehydrogenase (GDH) and H2 production with colloidal platinum as a catalyst using the visible light-induced photosensitization of Mg chlorophyll-a (Mg Chl-a), has been developed. H2 production was continuous when the reaction mixture containing d-maltose, glucoamylase, GDH, NAD+, Mg Chl-a, Methyl Viologen (MV2+, an electron relay reagent) and colloidal platinum was irradiated by visible light. The amount of H2 production was estimated to be 5 ± 0.5 μmol after 4 h irradiation. The yield of the conversion of d-maltose to H2 in this system was ca. 1.8 %.

The US army portable fuel cell program

The US Army's programme for development of portable fuel cells is structured to exploit the potential of Proton Exchange Membrane (PEM) technology to meet the electric power demand of the individual soldier. The Army has identified the need for lightweight power sources to provide the individual soldier with continuous power for extended periods. The baseline power demand is defined at 10–20 W average, and an energy demand without resupply of 400 Wh per mission day. For missions that extend beyond one day, fuel cell systems provide very attractive weight and volume advantages over existing battery technologies, both primary and rechargeable. One of the key barriers to actual Army use of portable PEM fuel cells is the absence of an appropriate source of hydrogen. Programmes presently under way promise 1000 Wh per kilogram of H2 production system weight. These sources of H2 will integrate into a portable power system with high power and energy density that meets the power, energy and mission demands of the soldier.

Oxygen sensitivity of algal H2- production

Photoproduction of H2 by green algae utilizes electrons originating from the photosynthetic oxidation of water and does not require metabolic intermediates. However, algal hydrogenases are extremely sensitive to O(2), which limits their usefulness in future commercial H2-production systems. We designed an experimental technique for the selection of O2-tolerant, H2-producing variants of Chlamydomonas reinhardtii based on the ability of wild-type cells to survive a short (20 min) exposure to metronidazole in the presence of controlled concentrations of O2. The number of survivors depends on the metronidazole concentration, light intensity, preinduction of the hydrogenase, and the presence or absence of O2. Finally, we demonstrate that some of the selected survivors in fact exhibit H2-production capacity that is less sensitive to O2 than the original wild-type population.

Luminescence of mixed ZnS · CdS semiconductor catalysts in Nafion polymer films

The Luminescence of CdS and mixed ZnS · CdS semiconductor particles supported in Nafion polymer films has been studied to elucidate the previously reported synergism in the ZnS · CdS system for photoredox H2 production. (J. Phys. Chem. 89 (1985) 723). The luminescence of these systems is complex, depending on the history of the particular sample, the intensity and wavelength of the exciting light, and the presence of ions that can quench specific emitting states (e.g. S2- or methylviologen). For the mixed ZnS · CdS system the two semiconductors do not alloy but there is intimate contact between the ZnS and CdS phases, indicated by the complete quenching of ZnS luminescence. Prolonged irradiation produces a luminescent ZnS phase. The emission features agree reasonably well with sub-bandgap states deduced from earlier photoconductivity studies on CdS crystals. These results help explain why the reported emission features of CdS are as varied as the laboratories reporting them.

Proflavine-sensitized photoproduction of H2from water with electron-donors and a colloidal redox catalyst

Efficient photoproduction of H2 from water in the presence of electron donors such as ethylenediaminetetra-acetic acid or triethanolamine and a colloidal Pt-redox catalyst, sensitized by proflavine, is reported; this is a two-component redox system for H2 production as against the common three-component systems with an electron acceptor.

Reconstitution of a System for H2 Evolution with Chloroplasts, Ferredoxin, and Hydrogenase

Continuous light-dependent H2 production was studied in a reconstituted in vitro system using Spinacea oleracea chloroplasts, Clostridium pasteurianum hydrogenase and Spirulina maxima ferredoxin. PhotosystemII dependent production at 30 0C is 60 -70 μmol H2/mg chlorophyll. At 15 0C, this reaction proceeds for up to 20 h producing 1O μmol H2/mg chlorophyll. O2 (glucose, glucose oxidase) and peroxide (ethanol, catalase) traps do not extend the lifetime but enhance the rate of H2 production. Stoichiometry of the coupled system for H2 production in this system is 2 H2 formed : 1 glucose consumed. A conversion efficiency of water photolysis for H2 evolution of 70% was determined from the O2 produced, measured as the amount of glucose consumed, during the first 2 h of continuous illumination. Cessation of H2 production by the reconstituted system involves inactivation of photosystem II and a limitation in the coupling of low potential electrons to hydrogenase. Increasing ferredoxin leads to more rapid H2 evolution but longevity of the system remains unchanged. When H2 evolution ceases due to inactivation of water-splitting activity of photosystem II, about 40% of the hydro­genase and 25% of photosystem I activity are still present; inactivation is unclear when photo­system I is used to drive H2 production since when H2 production ceases, hydrogenase and photo­system I still retain activity. This may suggest that coupling between low potential reducing equivalents from photosystem I to hydrogenase is impaired.