Our Chemistry

Our patent-pending cell chemistry builds on decades of prior research on aluminum-water and aluminum-air batteries[1].  In the last few years we have cleared the remaining technical hurdles that had kept the full potential of this chemistry unrealized in the past.
As shown in the cell diagram below, our system consists of three main components: an activated aluminum anode, an aqueous alkaline electrolyte, and a hydrogen evolving cathode. Note the injection of water as an oxidizer into the system, and the removal of the non-toxic aluminum hydroxide and hydrogen gas by-products. This mass transfer can be done continuously or intermittently. The three basic components of this cell are described in greater detail below.
SimplifiedChemistryDiagram

Anode

Our anodes are composed of an aluminum alloy specially formulated to serve as an electrochemical fuel.  Primarily made of aluminum, this alloy contains small amounts of other non-toxic metals that activate the aluminum for reaction with water while simultaneously inhibiting corrosion when low amounts of current are being drawn from the system.  On the spectrum of activation our alloys are on the low end[1]; they do not react vigorously with plain seawater. This low reactivity contributes to the safety and stability of the system. Our alloys are formulated such that they only react significantly in the presence of our electrolyte, and even then the reaction progresses moderately and without the risk of a run-away even under the most severe conditions, including system puncture or internal short circuit resulting from mechanical damage. The alloy contents can be tailored to fit the specific power and lifetime requirements of a given application.
Optical microscope image of a partially-consumed anode.
SEMGrainBoundaries
Scanning Electron Microscope of a partially-consumed anode.

 

Electrolyte

Our electrolyte is moderately alkaline and composed primarily of water. It accepts both seawater and freshwater sources. In addition to serving as fuel for the aluminum oxidation and a path for ions to travel between electrodes, the flow of electrolyte through the cell also facilitates waste removal and thermal management. Furthermore, the non-toxic chemical additives present in the electrolyte serve to activate the anode material and speed the cathode reaction, while hindering unwanted aluminum corrosion. The strength of the electrolyte can be tailored to meet specific power and lifetime requirements, and at normal temperatures and concentrations it can be handled safely with bare hands.
Electrolyte-ColorBalanced
Image of electrolyte with hydrogen rising through it.

Cathode

The role of the cathode in our system is similar to the role of the positive electrode in other proposed metal-water batteries[2]: to efficiently split water into hydroxide ions and hydrogen gas. We draw on the large body of research and industrial engineering that has been done to optimize hydrogen-evolving electrodes in related fields[3]. This type of electrode finds use in many industrial processes, and is also a principal component in the electrolyzers on submarines that generate breathable air. We use well-understood processes to custom fabricate our cathodes, tailored and optimized for our specific application. The SEM images below illustrate some of the morphologies that we use to achieve the high microscopic surface area necessary for optimally driving this reaction.
SEM-Cathode-1
Scanning electron microscope (SEM) image of the surface morphology of one of Open Water’s cathodes, made by plating platinum on a titanium substrate.
SEM-Cathode-2
Scanning electron microscope (SEM) image of the surface morphology of one of Open Water’s cathodes, made by plating nickel on a carbon substrate.

 

 

 

 

 

 

 

 

 

 

 

Mass Balance and Power

Our chemistry achieves an optimal total efficiency of about 33%. When our cells are operating near this point they consume a predictable amount of aluminum and water and convert them to a predictable amount of byproducts. The figure below depicts these mass consumption and production rates per Watt of electrical power generated. In most instances the hydrogen gas is continuously bubbled away. The solid waste is collected by the waste management subsystem and either periodically ejected to maintain a desired level of buoyancy (e.g. in the case of UUVs), or retained for the life of the system for simplicity and/or anchoring mass (e.g. for ocean-floor power systems).

Rates of reactant consumption and product generation per Watt of electrical power output.

 

Comparison to Other Aluminum Technologies

A comparison of our system to other aluminum-based power systems like the ARL/Penn State combustor[4] and General Atomics’ “ALPS” hydrogen fuel cell system[5] can be found on our “How We’re Different” page.

 


[1] Li, Qingfeng, and Niels J. Bjerrum. “Aluminum as anode for energy storage and conversion: a review.” Journal of Power Sources 110.1 (2002): 1-10.
[2] Linden, David. “Handbook of batteries and fuel cells.” New York, McGraw-Hill Book Co., 1984, 1075 p. (1984).
[3] Lasia, Andrzej. “Hydrogen evolution reaction.” Handbook of Fuel Cells (2010).
[4] Miller, T.F., Walter, J.L., Kiely, D.H., “A next generation AUV energy system based on aluminum-seawater combustion,” Workshop on Underwater Vehicles, June 20-21, 2002.
[5] http://fuelcellseminar.com/wp-content/uploads/b2b32-2-1.pdf Accessed April 5th 2015