FEATURE

December 2011 Membrane Technology9

‘‘Inexhaustible’’ source of hydrogen may be unlocked by salt water

‘This system could produce hydrogen any-place that there is wastewater near sea water,’ explained Bruce Logan, Kappe Professor of Environmental Engineering, The Pennsylvania State University.

‘It uses no grid electricity and is completely carbon neutral. It is an ‘‘inexhaustible’’ source of energy.’

“People have proposed making electricity out of RED stacks. But you need so many membrane pairs and are trying to drive an unfavourable reaction.”

Microbial electrolysis cells that produce hydrogen are the basis of this recent work, but previously, in order to produce hydrogen, the fuel cells required some electrical input. Now, Professor Logan, working with post-doctoral fellow Younggy Kim, is using the difference between river water and sea water to add the extra energy needed to produce hydrogen.

Produced efficientlyTheir results, published in a paper entitled ‘Hydrogen production from inexhaustible supplies of fresh and salt water using micro-bial reverse-electrodialysis’, which appears in a recent issue of Proceedings of the National Academy of Sciences (2011 108(39) 16176–16181), ‘show that pure hydrogen gas can be produced efficiently from virtually limitless supplies of sea water and river water and bio-degradable organic matter’.

Professor Logan’s cells are between 58% and 64% efficient and produced between 0.8 m and 1.6 m of hydrogen for every cubic metre of liquid running through the cell each day. The researchers estimated that about 1% of

the energy produced in the cell was needed to pump water through the system.

Reverse electrodialysisThe key to these microbial electrolysis cells is reverse electrodialysis (RED), which extracts energy from the ionic differences between salt water and fresh water. A RED stack consists of alternating ion-exchange membranes – posi-tive and negative.

‘People have proposed making electricity out of RED stacks. But you need so many mem-brane pairs and are trying to drive an unfavour-able reaction,’ commented Professor Logan.

For RED technology to hydrolyse water – split it into hydrogen and oxygen – 1.8 V are needed which, in practice, would require about 25 pairs of membranes and increase pumping resistance. However, combining RED technology with exoelectrogenic bacteria – bacteria that consume organic material and produce an electric current – reduces the number of RED stacks to five membrane pairs.

Inexhaustible source of energyPrevious work with microbial electrolysis cells showed that they could, by themselves, pro-duce about 0.3 V of electricity, but not the 0.414 V needed to generate hydrogen in these fuel cells. Adding less than 0.2 V of outside electricity released the hydrogen. Now, by incorporating 11 membranes – five membrane pairs that produce about 0.5 V – the cells pro-duce hydrogen.

‘The added voltage that we need is a lot less than the 1.8 V necessary to hydrolyse water,’ explained Professor Logan.

‘Biodegradable liquids and cellulose waste are abundant and with no energy in and hydro-gen out we can get rid of wastewater and by-products. This could be an inexhaustible source of energy.’

Professor Logan and Kim’s research used platinum as a catalyst on the cathode, but sub-sequent experimentation showed that a non-precious metal catalyst, molybdenum sulfide, had a 51% energy efficiency.

(The King Abdullah University of Science and Technology in Saudi Arabia supported this work.)

Contact:

Bruce Logan, Kappe Professor of Environmental

Engineering, Department of Civil and Environmental

Engineering, 231Q Sackett Building,

The Pennsylvania State University,

University Park, PA 16802, USA.

Tel: +1 814 863 7908,

Email: blogan@psu.edu,

www.engr.psu.edu/ce/enve/logan

A grain of salt or two may be all that microbial electrolysis cells need to produce hydrogen from wastewater or organic by-products, without adding carbon dioxide to the atmosphere or using grid electricity, according to engineers in the USA at The Pennsylvania State University.

Professor Bruce Logan’s bacterial hydrolysis cell showing the reverse electrodialysis stack (photograph credit: Bruce Logan, Kappe Professor of Environmental Engineering, Department of Civil and Environmental Engineering, The Pennsylvania State University, USA).