Hydrogen evolution in enzymatic photoelectrochemical cell using modified seawater electrolytes produced by membrane desalination process

In the near future, potential water shortages are expected to occur all over the world and this problem will have a significant influence on the availability of water for water-splitting processes, such as photocatalysis and electrolysis, as well as for drinking water. For this reason, it has been suggested that seawater could be used as an alternative for the various water industries including hydrogen production. Seawater contains a large amount of dissolved ion components, thus allowing it to be used as an electrolyte in photoelectrochemical (PEC) systems for producing hydrogen. Especially, the concentrate (retentate) stream shows higher salinity than the seawater fed to the membrane desalination process, because purified water (fresh water) is produced as the permeate stream and the waste brine is more concentrated than the original seawater. In this study, we investigated the hydrogen evolution rate in a photoelectrochemical system, including the preparation and characterization of an anodized tubular TiO₂ electrode (ATTE) as both the photoanode and the cathode with the assistance of an immobilized hydrogenase enzyme and an external bias (solar cell), and the use of various qualities of seawater produced by membrane desalination processes as the electrolyte. The results showed that the rate of hydrogen evolution obtained using the nanofiltration (NF) retentate in the PEC system is ca. 105 μmol/cm² h, showing that this is an effective seawater electrolyte for hydrogen production, the optimum amount of enzyme immobilized on the cathode is ca. 3.66 units per geometrical unit area (1 cm×1 cm), and the optimum external external bias supplied by the solar cell is 2.0 V.     

Hydrogen production using alkaline electrolyzer and photovoltaic (PV) module

In this paper it is presented hydrogen production using alkaline water electrolysis where a 30 W photovoltaic (PV) module was involved as a source of electric energy. Therefore, the process is without emitting CO₂. There is constructed and tested an alkaline electrolyzer with 50 × 50 × 2 mm Ni metal foam electrodes, 50 × 50 × 0.4 mm Zirphon^® membrane and 25% alkaline (KOH) solution electrolyte. Electrolyzer UI characteristics for natural and forced flow of electrolyte with PV module UI characteristics are presented. The results are in favor of forced flow circulation, and these are better if the flow velocities are higher. Calculated Energy efficiency (based on hydrogen high heating value) for both types of circulation is above 55%. There are much evidence for further improvement of the system components and consequently electrolyzer and system efficiency.     

Photovoltaic-powered desalination system for remote Australian communities

This paper reports on the design and successful field testing of a photovoltaic (PV)-powered desalination system. The system described here is intended for use in remote areas of the Australian outback, where fresh water is extremely limited and it is often necessary to drink high salinity bore water. A hybrid membrane configuration is implemented, whereby an ultrafiltration (UF) module is used for removing particulates, bacteria and viruses, while a reverse osmosis (RO) or nanofiltration (NF) membrane retains the salts. The concepts of water and energy recovery are implemented in the design. Field trials, performed in White Cliffs (New South Wales), demonstrated that clean drinking water was able to be produced from a variety of feed waters, including high salinity (3500 mg/l) bore water and high turbidity (200 NTU) dam water. The specific energy consumption ranged from 2 to 8 kW h/m³ of disinfected and desalinated drinking water, depending on the salinity of the feed water and the system operating conditions. The optimum operating pressure when filtering bore water was determined to be in the range 6–7 bar.     

Treatment of geothermal waters for production of industrial,agricultural or drinking water

A conceptual study has been carried out to convert geothermal water and condensate into a valuable industrial, agricultural or drinking water resource. Laboratory and field pilot test studies were used for the conceptual designs and preliminary cost estimates, referred to treatment facilities handling 750 kg/s of geothermal water and 350 kg/s of steam condensate. The experiments demonstrated that industrial, agricultural and drinking water standards could probably be met by adopting certain operating conditions. Six different treatments were examined. Unit processes for geothermal water/condensate treatment include desilication of the waters to produce marketable minerals, removal of dissolved solids by reverse osmosis or evaporation, removal of arsenic by oxidation/precipitation, and removal of boron by various methods including ion exchange. The total project cost estimates, with an accuracy of approximately ±25%, ranged from US$ 10 to 78 million in capital cost, with an operation and maintenance (or product) cost ranging from US$ 0.15 to 2.73 m⁻³ of treated water.     

In and ex situ characterization of an anion-exchange membrane for alkaline direct methanol fuel cell (ADMFC)

Anion exchange membrane fumasep^® FAA-2 was characterized with ex and in situ methods in order to estimate the membranes’ suitability as an electrolyte for an alkaline direct methanol fuel cell (ADMFC). The interactions of this membrane with water, hydroxyl ions and methanol were studied with both calorimetry and NMR and compared with the widely used proton exchange membrane Nafion^® 115. The results indicate that FAA-2 has a tighter structure and more homogeneous distribution of ionic groups in contrast to the clustered structure of Nafion, moreover, the diffusion of OH⁻ ions through this membrane is clearly slower compared to water molecules. The permeability of methanol through the FAA-2 membrane was found to be an order of magnitude lower than for Nafion. Fuel cell experiments in 1 mol dm⁻³ methanol with FAA-2 resulted in OCV of 0.58 V and maximum power density of 0.32 mW cm⁻². However, even higher current densities were obtained with highly concentrated fuels. This implies that less water is needed for fuel dilution, thereby decreasing the mass of the fuel cell system. In addition, electrochemical impedance spectroscopy for the ADMFC was used to determine ohmic resistance of the cell facilitating the further membrane development.     

Photoanodic and cathodic role of anodized tubular titania in light-sensitized enzymatic hydrogen production

An anodized tubular titania (TiO₂) electrode (ATTE) is prepared and utilized as both a photoanode and a cathode in a photoelectrochemical system designed to split water into hydrogen (for use in fuel cells) with the assistance of a hydrogenase enzyme and an external bias of 1.5 V. In particular, the cathodic ATTE acts as a substrate for the immobilization of the enzyme due to its large surface area that results from the tubular oxides. The optimum molar concentration of KOH in anode and cathode compartments is 1.0 M and the optimum amount of enzyme for the cathode is ca. 3.66 units per geometrical unit area (1 cm × 1 cm) of the cathodic ATTE. After exposure to air for three weeks, the enzyme shows a hydrogen evolution rate that is 85.8% of that of an argon-purged enzyme. The rate of hydrogen evolution is increased from ca. 65 (in a slurry system) to more than 140 μmol cm⁻² h⁻¹, even after eliminating the electron relay (methyl viologen) and costly platinum counter electrode.     

Electrochemical hydrogen pumping from high temperature plasma-chemical reactor involving H2O/SO2 gas mixture

We demonstrate that the energy efficiency of hydrogen production by electrochemical hydrogen pumping out of a plasma-energized mixture of water vapor and sulfur dioxide (SO₂) can be greatly enhanced by raising the rector temperature above 800 °C. The critical elements for this reactor design include the use of a microporous ceramic configuration for the discharge region, a bipolar electrode connecting the plasma reactor with the hydrogen pump, and a solid oxide membrane as the electrolyte of the pump. The amount of hydrogen produced per 100 joules of electrical energy consumed to operate the reactor at 850 °C is 16 mL, which is more than twice the volume produced from the same reactor, operating at 100 °C. The energy efficiency is almost 75% of that for the electrolysis of an H₂O/SO₂ mixture. This type of plasma-assisted hydrogen pump opens up the possibility of producing hydrogen gas from water using the thermal energy from a nuclear reactor.     

Influence of operation conditions on direct NaBH4/H2O2 fuel cell performance

Although hydrogen fuel cells have attracted so much attentions in these years because of the application prospect in electric vehicles, some obstacles have not been solved yet, among which hydrogen storage is one of the biggest. Direct borohydride fuel cell (DBFC) is another choice without hydrogen storage problem because borohydride is used as reactant directly in the fuel cell. In this paper, DBFC performance under different operation conditions was studied including electrolyte membrane type, operation temperature, borohydride concentration, supporting electrolyte and oxidant. Results showed that, with Pt/C and MnO₂ as anode and cathode electrocatalyst, respectively, Nafion^® 117 membrane as electrolyte, 1.0 M, 3.0 M and 6.0 M NaBH₄ and H₂O₂ solution in NaOH as reactant solution, 80 °C operation, the peak power density could reach 130 mW/cm².     

A novel design of solid-state NaBH4 composite as a hydrogen source for 2 W PEMFC applications

Hydrolysis of sodium borohydride (NaBH₄) is a promising method for on-board hydrogen supply to polymer electrolyte membrane fuel cells (PEMFC). This article presents an attempt to design a novel solid-state NaBH₄ composite, which is made up of NaBH₄ powder, Co²⁺/IR-120 catalyst and silicone rubber, for hydrogen generator. The silicone rubber can act as a stabilizer in the solid-state NaBH₄ composite because of its surface hydrophobicity leading to reduced diffusion rate of water into the composite. The solid-state NaBH₄ composite can produce hydrogen stably near 25 mL min⁻¹ for at least 2 h without employment of any mechanical control system. Using the hydrogen generated from the solid-state NaBH₄ composite, a 2 W PEMFC stack is successfully operated to power a cellular phone.     

Modelling and development of photoelectrochemical reactor for H2 production

Photoelectrolysis of aqueous solutions, using one or more semiconducting electrodes in a photoelectrochemical reactor, is a potentially attractive process for hydrogen production because of its prospectively high energy efficiency, simplicity and potentially low cost. The design requirements and preliminary results of modelling a photoelectrochemical (PEC) reactor are described. Potential and current density distributions, due to ohmic potential losses in thin (non-photo) anodes on poorly conducting fluoride-doped tin oxide coated glass substrates, were modelled. The predicted current densities decayed rapidly from the terminals at the edges, towards the centre of a 0.1 × 0.1 m² anode, so limiting scale-up with such substrates. Spatial distributions of dissolved oxygen concentrations were also modelled, aiming to define operating conditions that would avoid forming bubbles, which reflect light specularly decreasing photon absorption efficiencies of photoelectrodes. The implications for the future optimization of the reactor are discussed.     

Stand-alone PEM water electrolysis system for fail safe operation with a renewable energy source

A complete stand-alone electrolyser system has been constructed as a transportable unit for demonstration of a sustainable energy facility based on hydrogen and a renewable energy source. The stand-alone unit is designed to support a polymer electrolyte membrane (PEM) stack operating at up to ∼4 kW input power with a stack efficiency of about 80% based on HHV of hydrogen. It is self-pressurizing and intended for operation initially at a differential pressure of less than 6 bar across the membrane electrode assembly with the hydrogen generation side being at a higher pressure. With a slightly smaller stack, the system has been operated at an off-site facility where it was directly coupled to a 2.4 kW photovoltaic (PV) solar array. Because of its potential use in remote areas, the balance-of-plant operates entirely on 12 V DC power for all monitoring, control and safety requirements. It utilises a separate high-current supply as the main electrolyser input, typically 30–40 V at 100 A from a renewable source such as solar PV or wind. The system has multiple levels of built-in operator and stack safety redundancy. Control and safety systems monitor all flows, levels and temperatures of significance. All fault conditions are failsafe and are duplicated, triggering latching relays which shut the system down. Process indicators monitor several key variables and allow operating limits to be easily adjusted in response to experience of system performance gained in the field.     

Low methanol crossover and high performance of DMFCs achieved with a pore-filling polymer electrolyte membrane

We compared the performance of the membrane electrode assembly for direct methanol fuel cells (DMFCs) composed of a pore-filling polymer electrolyte membrane (PF membrane) with that composed of a commercial Nafion-117 membrane. In DMFC tests, the methanol crossover flux was 23% lower in the PF membrane than in the Nafion-117 membrane even though the thickness of the PF membrane was 43% that of Nafion-117. This led to a higher DMFC performance and the lower overpotential of the cathode of the PF membrane. Feeding an aqueous 10 M methanol solution at 50 °C produced a low cathode overpotential, as low as 0.40 V at 0.2 A in the PF membrane, whereas the potential was 0.65 V at 0.2 A in the Nafion-117 membrane. In contrast, the ohmic loss and anode overpotential were almost the same in the two membranes. We confirmed that a reduction in methanol crossover using the PF membrane results in lower cathode overpotential and higher DMFC performance. In addition, the electro-osmotic coefficient was estimated as 1.3 in the PF membrane and 2.6 in Nafion-117, based on a water mass-balance model and values showing that the PF membrane prevents the flooding of the cathode at a low gas flow rate using. A highly concentrated methanol solution can be applied as a fuel without decreasing DMFC performance using PF membranes.     

Structural optimization of the direct methanol fuel cell passively fed with a high-concentration methanol solution

In a high-concentration direct methanol fuel cell (HC-DMFC), the methanol crossover is typically decreased to an acceptable level by two main mechanisms: high methanol transport resistance between the anode reservoir and the membrane electrode assembly (MEA), and high water back flow from the cathode to the anode. Based on the semi-passive HC-DMFC fabricated in this work, the effects of methanol barrier layer (MBL) thickness and electrolyte membrane thickness on cell performance, methanol and water crossover, and fuel efficiency have been studied. The results showed that a thicker MBL could significantly decrease the methanol and water crossover by increasing the mass transport resistance between the anode reservoir and the MEA, while a thinner Nafion^® membrane could also significantly decrease the methanol and water crossover by enhancing the water back flow from the cathode through the electrolyte membrane to the anode. Using Nafion^® 212 as the electrolyte membrane, and a 6.4 mm porous PTFE plate as the MBL, a semi-passive HC-DMFC operating at 70 °C produced the maximum power density of 115.8 mW cm⁻² when 20 M methanol solution was fed as the fuel.     

Development of a novel portable-size PEMFC short stack with electrodeposited Pt hydrogen diffusion anodes

This paper presents, for the first time, a five-cell polymer electrolyte membrane fuel cell (PEMFC) short stack with electrodeposited hydrogen diffusion anodes. The anodes were manufactured by means of galvanostatic pulse electrodeposition and the cathodes by air-brushing. Nafion^® 212 was employed as a solid polymer electrolyte membrane in all cases. The short stack, whose cells had an active geometric area of 14 cm², was assembled and tested under different operating conditions. A peak power of about 11 W was obtained at 50 °C and atmospheric pressure using hydrogen and air feed, whereas a smaller value of 8.6 W was obtained from a five-cell short PEMFC stack with conventional hydrogen diffusion anodes under the same operating conditions. The better performance of the cells described in this paper has been assigned to the higher utilization of the platinum in the electrodeposited anodes compared to the conventional ones.     

SPE water electrolysis with SPEEK/PES blend membrane

Sulfonated poly(ether ether ketone) (SPEEK) was blended with poly(ether sulfone) (PES) to make solid polymer electrolyte (SPE) membranes for hydrogen production via water electrolysis. The blend membranes were characterized in terms of proton conductivity and the swelling degree in water. Membrane electrode assemblies (MEA), with Ir anode and Pt cathode at the two side of the blended membrane, were prepared by a decal method. The effect of hot pressing conditions in fabricating the MEA and the influence of ionomers in the catalyst layers were investigated. The MEA, with an effective area of 4 cm², were tested using a single cell water electrolysis test stand. An electrolytic current of 1655 mA/cm² were obtained at 2 V and 80 °C with the SPEEK based MEA and under suitable fabrication conditions. The experimental results suggest that SPEEK/PES blend membrane could be an alternative to costly perfluorosulfonate membranes in SPE water electrolysis for hydrogen production.     

Optimized hydrogen generation in a semicontinuous sodium borohydride hydrolysis reactor for a 60 W-scale fuel cell stack

Catalyzed hydrolysis of sodium borohydride (SBH) is a promising method for the hydrogen supply of fuel cells. In this study a system for controlled production of hydrogen from aqueous sodium borohydride (SBH) solutions has been designed and built. This simple and low cost system operates under controlled addition of stabilized SBH solutions (fuel solutions) to a supported CoB catalyst. The system works at constant temperature delivering hydrogen at 1 L min⁻¹ constant rate to match a 60-W polymer electrolyte membrane fuel cell (PEMFC). For optimization of the system, several experimental conditions were changed and their effect was investigated. A simple model based only on thermodynamic considerations was proposed to optimize system parameters at constant temperature and hydrogen evolution rate. It was found that, for a given SBH concentration, the use of the adequate fuel addition rate can maximize the total conversion and therefore the gravimetric storage capacity. The hydrogen storage capacity was as high as 3.5 wt% for 19 wt% SBH solution at 90% fuel conversion and an operation temperature of 60 °C. It has been demonstrated that these optimized values can also be achieved for a wide range of hydrogen generation rates. Studies on the durability of the catalyst showed that a regeneration step is needed to restore the catalytic activity before reusing.     

Membrane electrode assemblies for high-temperature polymer electrolyte fuel cells based on poly(2,5-benzimidazole) membranes with phosphoric acid impregnation via the catalyst layers

A novel strategy for introducing phosphoric acid as the electrolyte into high-temperature polymer electrolyte fuel cells by using acid impregnated catalyst layers instead of pre-doped membranes is presented in this paper. This experimental approach is used for the development of membrane electrode assemblies based on poly(2,5-benzimidazole) (ABPBI) as the membrane polymer. The acid uptake of free-standing ABPBI used for this work amounts to ABPBI × 3.1 H₃PO₄ which has a specific conductivity of ∼80 mS cm⁻¹ at 140 °C. Rather thick catalyst layers (20% Pt/C, 1 mg Pt cm⁻², 40% PTFE as binder, d = 100-150 μm) are prepared on gas diffusion layers with a dense hydrophobic microlayer. After impregnation of the catalyst layers with phosphoric acid and assembling them with a mechanically robust undoped ABPBI membrane a fast redistribution of the electrolyte occurs during cell start-up. Power densities of about 250 mW cm⁻² are achieved at 160 °C and ambient pressure with hydrogen and air as reactants. Details of membrane properties, preparation and optimization of gas diffusion electrodes and fuel cell characterization are discussed. We consider our novel approach to be especially suitable for an easy and reproducible fabrication of MEAs with large active areas.     

Pyrococcus furiosus-immobilized anodized tubular titania cathode in a hydrogen production system

Anodized tubular titania (TiO₂) electrodes (ATTEs) are prepared and used as both the photoanode and the cathode substrate in a photoelectrochemical system designed to split water into hydrogen with the assistance of an enzyme and an external bias of 1.5 V. In particular, the ATTE used as the cathode substrate for the immobilization of the enzyme is prepared by two methods—adsorption and crosslinking. Results show that the optimized amount of enzyme is 10.98 units for the slurried enzyme, 3.66 units for the adsorbed one and 7.32 units for the crosslinked one, and the corresponding hydrogen evolution rates are 33.04, 148.58 and 234.88 μmol h⁻¹, respectively. The immobilized enzyme, specifically the chemically crosslinked one, seems to be much superior to the slurried enzyme, due to the enhanced charge-transfer process that is caused by the lower electrical resistance between the enzyme and the ATTE. This results in a greater number of accepted electrons and a larger amount of enzymes able to deal with the electrons.     

Review of brackish water reverse osmosis (BWRO) system designs

Brackish water are any water sources with TDS between 1000 and 15 000 mg/L. Brackish water cannot be consumed by us directly due to its high salinity. According to World Health Organization (WHO), water with salinity below 500 mg/L is acceptable as drinking water. There are quite a large number of research that had been done on BWRO. Each of them has agreed with a common design on optimum BWRO design with a slight modification in order to improve more and make a better BWRO system. BWRO systems which have been tested in real situation agree that the single stage system with module connected to reject water is the most optimum system both economically and environmentally. There is some improvement done to the design by using SWRO membrane at the second stage. This improvement increases recovery rate to about 83% and reduces boron concentration at the same time. Another design is by using hybrid combination of ultra-low and conventional RO membranes. Hybrid improves permeate quality. It is also possible to create a hybrid array by mixing membrane element types within a pressure vessel itself. Co-operating an efficient module arrangement into a complete BWRO system will reduce energy consumption. Energy-recovery device is a component that must be included in any small or large-scale systems. A small-scale RO system, without energy recovery, would typically consume two to three times more energy. This will be more for large-scale systems. While single stage system with module connected to reject water is preferred by researchers who have done real environment testing, simulation prefers to add another membrane to the reject water of the second module. This system is yet to be tested in real environment to prove its standing.     

A modified suspension spray combined with particle gradation method for preparation of protonic ceramic membrane fuel cells

In order to prepare a dense proton-conductive Ba(Zr_0.1Ce_0.7)Y_0.2O_3−δ (BZCY7) electrolyte membrane, a proper anode composition with 65% Ni₂O₃ in weight ratio was determined after investigating the effects of anode compositions on anode shrinkages for co-sintering. The thermal expansion margins between sintered anodes and electrolytes, which were less than 1% below 750 °C, also showed good thermal expansion compatibility. A suspension spray combined with particle gradation method had been introduced to prepare dense electrolyte membrane on porous anode support. After a heat treatment at 1400 °C for 5 h, a cell with La_0.5Sr_0.5CoO_3−δ (LSCO) cathode was assembled and tested with hydrogen and ammonia as fuels. The outputs reached as high as 330 mW cm⁻² in hydrogen and 300 mW cm⁻² in ammonia at 700 °C, respectively. Comparing with the interface of another cell prepared by dry-pressing method, this one also showed a good interface contact between electrodes and electrolyte. To sum up, this combined technique can be considered as commercial fabrication technology candidate.