Devices needing to operate remotely for long periods underwater, like subs and diving robots, are currently limited by their batteries. That could change with the announcement of a device that can produce electricity from seawater in two ways, one offering slow sustained power, the other can be used for short bursts where the need is higher, and can autonomously switch between the two when needed.
Humans rely on aerobic production of energy for light to medium exercise, using oxygen from the air to burn carbohydrates. For short sprints or power-lifting, our bodies turn to fast twitch muscles that metabolize anaerobically. Similarly, marine organisms run for long periods on the energy produced by reacting molecules with oxygen dissolved in water, but turn to different energy modes when more power is needed briefly.
Professor Ming Hu of East China Normal University thought this might provide a model for remote-controlled submersibles or diving robots that spend much of their time slowly cruising on low power, but occasionally need to repair pipelines or collect samples.
In the journal Angewandte Chemie, Hu describes a power source matching those needs using metal-organic frameworks (MOFs), a class of polymers useful for storing and purifying gasses, as a cathode. In this case, the MOF is made up of cyanide ions linking together iron ions.
Most of the time the electrons flow from a metal anode to the cathode, to be transferred to dissolved oxygen, converting it to O2-. Unless the submersible strays into an oceanic dead zone, where an environmental disaster has removed all the oxygen, it can operate like this indefinitely – after all fish have been powering themselves from dissolved oxygen for millions of years and more keeps getting stirred in.
However, even in well-mixed waters the amount of oxygen is too low to allow this approach to release large amounts of electricity quickly. To up the current something different is required. Hu's device responds to increases in power demand by sending electrons to the iron ions, changing their status from Fe3+ to Fe2+. This temporarily gives the MOF a net negative charge, attracting sodium Na+ ions from seawater, which restore the balance. The concentration of sodium in seawater is much higher than dissolved oxygen so electricity can be released quickly, without waiting for more water to flow past.
Since the MOF contains a finite number of iron ions, it can't run like this for long. However, the system is self-regenerating – once the need for extra power passes oxygen in fresh seawater returns the iron ions to Fe3+, removing the attraction to positive charges so the sodium gets washed out.
The energy produced is insufficient to make this approach a large scale source of clean energy, but a demonstration model was able to run 39 LEDs and a propeller for short periods, while resisting corrosion, which is a start.
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