Heterogeneous and homogenous catalysts for hydrogen generation by hydrolysis of aqueous sodium borohydride (NaBH4) solutions

It is clear that in order to satisfy global energy demands whilst maintaining sustainable levels of atmospheric greenhouse gases, alternative energy sources are required. Due to its high chemical energy density and the benign by-product of its combustion reactions, hydrogen is one of the most promising of these. However, methods of hydrogen storage such as gas compression or liquefaction are not suitable for portable or automotive applications due to their low hydrogen storage densities. Accordingly, much research activity has been focused on finding higher density hydrogen storage methods. One such method is to generate hydrogen via the hydrolysis of aqueous sodium borohydride (NaBH4) solutions, and this has been heavily studied since the turn of the century due to its high theoretical hydrogen storage capacity (10.8 wt%) and relatively safe operation in comparison to other chemical hydrides. This makes it very attractive for use as a hydrogen generator, in particular for portable applications. Major factors affecting the hydrolysis reaction of aqueous NaBH4 include the performance of the catalyst, reaction temperature, NaBH4 concentration, stabilizer concentration, and the volume of the reaction solution. Catalysts based on noble metals, in particular ruthenium (Ru) and platinum (Pt), have been shown to be particularly efficient at rapid generation of hydrogen from aqueous NaBH4 solutions. However, given the scarcity and expense of such metals, a transition metal-based catalyst would be a desirable alternative, and thus much work has been conducted using cobalt (Co) and nickel (Ni)-based materials to attempt to source a practical option. “Metal free” NaBH4 hydrolysis can also be achieved by the addition of aqueous acids such as hydrochloric acid (HCl) to solid NaBH4. This review summarizes the various catalysts which have been reported in the literature for the hydrolysis of NaBH4

A simple, low-cost, and robust system to measure the volume of hydrogen evolved by chemical reactions with aqueous solutions

There is a growing research interest in the development of portable systems which can deliver hydrogen on-demand to proton exchange membrane (PEM) hydrogen fuel cells. Researchers seeking to develop such systems require a method of measuring the generated hydrogen. Herein, we describe a simple, low-cost, and robust method to measure the hydrogen generated from the reaction of solids with aqueous solutions. The reactions are conducted in a conventional one-necked round-bottomed flask placed in a temperature controlled water bath. The hydrogen generated from the reaction in the flask is channeled through tubing into a water-filled inverted measuring cylinder. The water displaced from the measuring cylinder by the incoming gas is diverted into a beaker on a balance. The balance is connected to a computer, and the change in the mass reading of the balance over time is recorded using data collection and spreadsheet software programs. The data can then be approximately corrected for water vapor using the method described herein, and parameters such as the total hydrogen yield, the hydrogen generation rate, and the induction period can also be deduced. The size of the measuring cylinder and the resolution of the balance can be changed to adapt the setup to different hydrogen volumes and flow rates

An assessment of the viability of hydrogen generation from the reaction of silicon powder and sodium hydroxide solution for portable applications

The gravimetric hydrogen storage efficiency of silicon has been widely reported as 14wt.%, suggesting that this material should be an excellent hydrogen generation source for portable applications. However, in the case of the reaction of silicon powder with 20wt.% sodium hydroxide solution at 50°C, the observed production of hydrogen fails to realize these high expectations unless a large excess of basic solution is used during the reaction, rendering the use of silicon in such systems uncompetitive compared with chemical hydride based technologies. By investigating the molar ratio of water:silicon from a large excess of water towards the stoichiometric 2:1 ratio dictated by the reaction equation, this study shows that for the reaction of silicon in 20wt.% sodium hydroxide solution, the quantity of hydrogen produced decreases as the 2:1 ratio expected from the equation for the reaction is approached. Furthermore, in order to reach 80% of the theoretical efficacy, a molar ratio of 20:1, or 12mL of 20wt.% sodium hydroxide solution per gram of silicon, would be required. These results suggest that the actual gravimetric hydrogen storage capacity is less than 1%, casting doubts as to whether the use of silicon for hydrogen generation in real systems would be possible

An old solution to a new problem? Hydrogen generation by the reaction of ferrosilicon with aqueous sodium hydroxide solutions

The chemical hydrogen storage properties of ferrosilicon were investigated. A hydrogen yield of ~4.75 wt.% (with respect to the mass of ferrosilicon) was estimated by the reaction of varying quantities of ferrosilicon with 5 mL of 40 wt.% sodium hydroxide solution. The reaction of ferrosilicon with aqueous sodium hydroxide solution to form hydrogen was found to have an activation energy of 90.5 kJ mol −1 by means of an Arrhenius plot. It was observed that the induction period of the hydrogen generation reaction varies exponentially with temperature. Although this combination of high activation energy and a lengthy induction period at low temperatures reduces the attractiveness of ferrosilicon for portable hydrogen storage applications unless methods can be developed to accelerate the onset and rate of hydrogen generation, its low cost and widespread availability make it attractive for further studies focused on higher temperature stationary applications

Elucidating the process of hydrogen generation from the reaction of sodium hydroxide solution and ferrosilicon

For the first time, the process of hydrogen evolution from ferrosilicon 75 using sodium hydroxide solution has been investigated as a function of temperature using a combination of X-ray photoelectron spectroscopy, X-ray diffraction and physical measurements. Ferrosilicon 75, a mixture of silicon (~50wt.%) and iron disilicide (~50wt.%), has been shown to produce hydrogen by the action of sodium hydroxide solution on the silicon only, with the iron disilicide acting in the role of spectator/protector species for the silicon. Neither iron disilicide alone nor ferrosilicon 45, which does not contain a pure metallic silicon phase, was found to generate hydrogen under similar reaction conditions, further indicating that the presence of a pure metallic silicon phase is essential for hydrogen generation. As the iron disilicide acts as a diluent for the active silicon, it is hypothesized that this would result in a slower release of hydrogen than that which would be obtained from the reaction of silicon alone, which may be useful for applications which require a long-term, sustained release of hydrogen. A hydrogen yield of 462.5mL/g and a maximum hydrogen generation rate of 83mL/min g were obtained within 10min of reaction with 40wt.% NaOH at 348K

Activation of silicon towards hydrogen generation by pelletisation

One of the barriers to the use of the silicon-water reaction to generate hydrogen for hydrogen fuel cells in portable devices is the lengthy induction period of the reaction caused by the presence of the native oxide layer on the surface of the silicon. Herein is presented a simple pelleting process which can be used to effectively eliminate the induction period in the reaction of pressed silicon powders with 2 wt% sodium hydroxide solution by means of disrupting the native oxide layer. The activation energy of the reaction was found to be 73 kJ/mol by means of an Arrhenius plot. It was also found that the rate of reaction of hydrogen generation could be enhanced by mixing sodium chloride and sodium polyacrylate with the silicon powder before pressing

Aerosol-assisted CVD of bismuth vanadate thin films and their photoelectrochemical properties

Thin film bismuth vanadate (BiVO4) photoelectrodes are prepared by aerosol-assisted (AA)CVD for the first time on fluorine-doped tin oxide (FTO) glass substrates. The BiVO4 photoelectrodes are characterised by X-ray diffraction (XRD), Raman spectroscopy (RS), and energy-dispersive X-ray (EDX) spectroscopy and are found to consist of phase-pure monoclinic BiVO4. Scanning electron microscopy (SEM) analysis shows that the thin film is uniform with a porous structure, and consists of particles approximately 75-125nm in diameter. The photoelectrochemical (PEC) properties of the BiVO4 photoelectrodes are studied in aqueous 1M Na2SO4 and show photocurrent densities of 0.4mAcm-2, and a maximum incident-photon-to-electron conversion efficiency (IPCE) of 19% at 1.23V vs. the reversible hydrogen electrode (RHE). BiVO4 photoelectrodes prepared by this method are thus highly promising for use in PEC water-splitting cells. Thin film bismuth vanadate (BiVO4) photoelectrodes are prepared by AACVD for the first time on fluorine-doped tin oxide (FTO) glass substrates. The photoelectrochemical (PEC) properties of the BiVO4 photoelectrodes are studied in aqueous 1M Na2SO4 and show photocurrent densities of 0.4mAcm-2 and a maximum incident-photon-to-electron conversion efficiency (IPCE) of 19% at 1.23V vs. RHE. BiVO4 photoelectrodes prepared by this method are highly promising for use in PEC water-splitting cells

A Simple, Low-cost, and Robust System to Measure the Volume of Hydrogen Evolved by Chemical Reactions with Aqueous Solutions - video

This is a video article published in the journal JOVE (Journal of Visualized Experiments) on 17th August 2016. The video and associated article can also be accessed on the JOVE website on open access.<div>The video is 19.3MB large and is in MP4 format.</div