Underwater vehicles, detectors, and diving robots need their own supply of energy to run for extended periods autonomous of ships. A new, economical system for the direct electrochemical extraction of energy from seawater delivers the benefit of also being able to endure short spikes in power demand, while preserving longer term steady power. To achieve that, the system can independently switch between two modes of operation, as scientists state in the journal Angewandte Chemie.
Charting submarine landforms, temperatures, and currents, and checking and repairing pipelines and deep-sea cables are some of the few examples of tasks performed autonomously by underwater devices deep in the ocean. Under these tough conditions, the challenge for power generators is to generate both a high energy density (long run time with standard power use) and high power density (short-term high current flow) for tasks such as fast movement or action of a gripper.
Liang Tang, Hu Jiang, and Ming Hu and their team from the East China Normal University in Shanghai, Shanghai University, and the Chinese Research Academy of Environmental Sciences in Beijing, China, have drawn inspiration from marine organisms that can change their cell respiration between anaerobic and aerobic modes by using various materials as electron acceptors. The scientists have engineered a new power generator that operates by the same principles.
The crucial factor to the discovery is a cathode made of Prussian blue, an open framework structure having cyanide ions as “struts” and iron ions as “nodes”, which can simply accept and discharge electrons. When integrated with a metal anode, this structure can be used to produce electricity from seawater.
If the power demand is small, the electrons flowing into the cathode are conveyed straight to dissolved oxygen. Since dissolved oxygen in seawater is inexhaustible, power at low current can supposedly be provided for unrestricted time. However, the concentration of dissolved oxygen is not high. When the power demand, and thus current, are suddenly increased, there is not sufficient oxygen at the cathode to instantly accommodate all of the incoming electrons. The Prussian blue must, thus, store these electrons by decreasing the oxidation state of the iron atoms from +3 to +2. To preserve a charge balance, positively charged sodium ions stay within the framework. Since these are present in high concentration in seawater, a number of sodium ions—and, thus, many electrons—can be absorbed in a short span of time. When the current demand slowly decelerates, electrons are shifted to oxygen once again, oxygen regenerates the framework, Fe2+ is oxidized to Fe3+, and the sodium ions leave.
This new system is very consistent in corrosive seawater and can endure frequent mode switches. It worked unceasingly for four days in its high-energy mode without any loss of power. The high-power mode could supply 39 light-emitting diodes and a propeller.