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8 Platform one April/May 2017 | Unmanned Systems Technology Open Water Power has developed an aluminium-water primary battery that promises ten times the energy density of rechargeable lithium-ion batteries for unmanned underwater vehicles (writes Peter Donaldson). Following tank testing, the company is preparing for the first tests of the complete system in the Riptide micro- UUV and a second, undisclosed, vehicle. It expects to make its first sales this year, said chief science officer Ian McKay. Aluminium’s theoretical energy density is 11.11 kWh per litre. “But when you talk about the extra volume required for electrolyte and waste separation, the cell- level energy density is from 2 to 3.5 kWh per litre,” McKay said. That compares with 0.55 kWh per litre for lithium-ion batteries. The aluminium reacts with water to produce aluminium hydroxide and hydrogen as waste products, plus electricity. Open Water’s design uses ambient water as the electrolyte. The water turns into hydrogen and hydroxide ions at the cathode, while the aluminium turns into aluminium hydroxide at the anode. Aluminium is typically non-reactive, McKay said, because it is normally covered in a very thin oxidised layer of alumina – Al 2 O 3 – that passivates it. In water, there is also aluminium hydroxide – Al(OH) 3 – the reaction product that forms an insoluble gel and reduces battery voltage. “It’s gunk – you have to remove it to get a proper energy density,” he said. A key element of the project was formulating an aluminium alloy that is relatively light on aluminium hydroxide passivation, he said. That involved designing the electrochemistry of the electrolyte and the aluminium anode, controlling the flow pattern between the electrodes and then designing a mechanical waste separation system to deal with the dissolved reaction product. McKay declined to describe the mechanical element, but was more forthcoming about the chemistry. “Our anode is formulated in such a way as to disrupt the polymerisation of the aluminium hydroxide waste. So instead of forming a thick gel on the surface of the electrode, it sloughs off easily into the electrolyte,” he said. While the design includes a gas trap for hydrogen, it is potentially more than a waste product as it can be used as a free buoyancy control medium in UUVs and buoyancy-driven ocean gliders, he said. For surface vehicles it can be fed into a fuel cell, roughly doubling both the energy density and the power density. Water power of aluminium Marine craft Open Water Power’s aluminium-water battery is being tested in the Riptide micro-UUV