Thursday, January 16, 2014

Toilet purification system doubles as hydrogen fuel cell

24 December 2013

An electrolysis cell that couples energy storage with water purification and reuse has been developed as part of a wider project to make a self-sustaining toilet.
Electrochemical approaches to water purification are not unknown but often focus on wastewater in industrially developed areas. Michael Hoffmann, and colleagues at the California Institute of Technology in the US, hope their electrochemical water splitting method for purifying human waste, at the same time as generating hydrogen gas, will eventually be introduced into areas with underdeveloped infrastructure.
Hoffmann says the inspiration for the work stemmed from their research into solar fuel production. ‘It did not make sense to simply discharge the oxygen generated during water splitting. We wanted to use the reactive oxygen species generated to oxidise organic chemical contaminates in wastewater.’
The electrochemical system reduces the yellow colour of wastewater at the same time as producing hydrogen

A bismuth-doped titanium dioxide electrode is central to the electrochemical reaction. In addition to hydrogen, the reaction also produces a series of reactive oxygen species. These reactive oxygen species react with chloride ions in urine to generate reactive chlorine species that disinfect the wastewater by killing any bacteria or viruses present in it, to leave water that is suitable for toilet flushing or crop irrigation.
Effluent gases – primarily hydrogen and nitrogen – produced by the electrochemical cell are run through a high temperature solid state fuel cell via a proton transfer membrane. These react with water vapour and oxygen at around 200 °C to produce an electrical current, which can be stored in batteries and the exothermic reaction is used maintain the temperature needed for this reaction.
Diogo Santos, an electrochemical water splitting expert at the University of Lisbon in Portugal says the study ‘brings important advances to the use of electrochemical treatment for the conversion of waste to energy in a simple and low-cost way.’
Hoffmann and his team are working with the Bill and Melinda Gates Foundation and the Koehler Company to create a working prototype of their toilet system.
Source: Royal Society of Chemistry

Ammonia freezes up under pressure

13 January 2014

Researchers in France have shown that when molecular ammonia is put under enormous pressure, it becomes unstable and then forms a solid ionic ice. The change was first predicted in 2008 by theoretical calculations, but until now it had never been shown experimentally.

ammonia_phase
At high temperatures and pressures ammonia takes on a superionic structure never before seen

At room temperature and pressure ammonia (NH3) is a gas. Its low temperature ice form is a typical molecular crystal, with weak hydrogen bonds between molecules, much like water ice. But ammonia behaves differently elsewhere in the universe where temperatures and pressures are extremely high, such as in the cores of the gas giants Neptune and Uranus. A team led by Sandra Ninet and Frederic Datchi at the Institute of Mineralogy and Condensed Matter Physics recently showed that at temperatures above 750 K and pressures beyond 60 GPa, the molecules dissociate to form ‘hot ice’ – a superionic phase composed of NH3, NH4+ and NH2- that behaves simultaneously as a solid crystal and a liquid.

In 2008, theoretical simulations predicted that low temperature ammonia could also enter an ice phase containing alternating layers of NH4+ and NH2- ions, at very high pressures. Now, Ninet and Datchi have found strong evidence that supports this theory. They used a diamond anvil cell to squash room temperature ammonia at pressures up to 194 GPa, over 1.5 million times atmospheric pressure.

‘Above 150 GPa, strong changes in the experimental IR and Raman spectra are observed, indicating a transition to another high-pressure phase,’ they write in a paper published on the preprint server arXiv. ‘The IR band around 2500 cm-1 cannot be due to molecular NH3, which therefore implies the presence of NH4+ ions.’

The spectra agree only partly with previous theoretical predictions, however, as they confirm ammonia’s ionicity but do not match up with the crystal structures suggested in the structural model. Further calculations, guided by the experiments, have helped the team refine the model and suggest that the high pressure ionic phase contains two different crystalline forms.

‘It is pleasant that the prediction has been at least partly verified by experiment,’ says Artem Oganov from the State University of New York in the US, whose group also study materials at high pressure. ‘Spontaneous ionisation seems to be a rather common phenomenon under pressure – it probably happens to soften interatomic repulsions and reduce the average atomic volume.’ He adds that these findings could help improve understanding of the conditions inside giant gas planets made of ammonia, methane and water.

Ninet, Datchi and colleagues now plan to refine ammonia’s phase diagram by exploring the boundary between its ionic and superionic phases.

Source: Royal Society of Chemistry