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New underwater power system can switch autonomously between modes

A new power generator underwater which provides an energy supply

29 May 2019

A new power generator inspired by marine organisms that switches from aerobic and anaerobic modes can provide an energy supply to underwater devices while also providing energy to meet short-term power demands.
Tiffany Hionas

Researchers have designed an underwater device that works autonomously within the depths of the ocean are tasked to carry out operations, including charting submarine landforms, currents, temperatures, and inspecting and repairing pipelines and deep-sea cables.

However, these tasks involve complications for power generations which have to switch between two different modes of operation autonomously – high power density (short-term high current flow) and a high energy density (long-run time with basic power use).

Researchers from East China Normal University in Shanghai, China, and the Chinese Research Academy of Environmental Sciences have developed a method which directs electrochemical extraction of energy from seawater, allowing underwater devices to adapt to numerous mode switches.

The team, having noticed the ability of marine organisms to switch their cell respiration between aerobic and anaerobic modes when using different materials as electron acceptors, have applied the same principles to design a new power generator.

A cathode made of Prussian blue with cyanide and iron ions can accept and release electrons easily. This open framework structure can generate electricity from seawater when combined with a metal anode. Electrons will flow into the cathode to be transferred to dissolved oxygen when the power demand is small. Theoretically, there is inexhaustible dissolved oxygen in seawater which means power at low current can be provided for an unlimited time. The issue is that there is low dissolved oxygen concentration.

When power demand is increased, there is not enough oxygen at the cathode to take up all incoming electrons. The solution is to reduce the oxidation state of the iron atoms so that the Prussian blue can store the electrons.

Sodium ions maintain a charge balance, and as they are high in concentration in seawater, they can be absorbed in a short time. Electrons are transferred to oxygen once again when the current demand slows down, and the sodium ions depart. 

This system works in corrosive seawater and can operate throughout mode switches. It ran continuously for four days in its high energy mode without losing power. Overall, it seems these researchers have designed a new power generator, demonstrating how clean and low-cost energy can successfully be produced from seawater.

DOI: 10.1002/anie.201901759  

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