Power to Hydrogen, an American cleantech startup founded in 2019, develops a new low-cost AEM reversible fuel cell that can run stable cycles between electrolyzing water and generating electricity. The reversible fuel cell can electrochemically produce highly pressurized hydrogen and oxygen gasses and convert the stored hydrogen and oxygen back into electricity and water.
Challenges: hydrogen fuel
More and more renewable energy sources, such as solar and wind, are being added to the grid. To maximize the usefulness of  intermittent renewable electricity, energy storage is required to maintain the reliability of customer delivery of renewable electricity.
reversible fuel cells are a unique technology that combines both energy storage and fuel cell technologies. Typically, reversible fuel cells  use hydrogen (H₂) as the fuel. They can produce hydrogen fuel by electrolyzing water. The produced hydrogen can be stored in large gas cylinders for less than $20/kW-hr, significantly less than the cost of batteries. Through an electrochemical process, reversible fuel cells are able to convert the chemical energy stored in hydrogen into electrical energy. They offer high energy conversion efficiency, long-term reliability, and the ability to store energy, making them a promising technology for transportation and grid energy storage applications.
Unfortunately, existing water electrolysis technologies for reversible fuel cells have several limitations.
Alkaline electrolyzers are a well-established method of water electrolysis. The two electrodes are immersed in a liquid alkane electrolyte and separated by an insulating porous separator. When a voltage is applied, hydrogen and oxygen (O₂) are evolved from the cathode and anode, respectively. Due to the separator’s permeability, hydrogen gas cannot be substantially pressurized via electrochemical means. A mechanical compressor is typically used to compress hydrogen, requiring an additional system component that is exceedingly expensive for many scales and applications. In addition, small pressure differences between the two sides of the electrolyzer can cause catastrophic failures. Therefore, alkaline electrolyzers are not suitable for reversible fuel cells.
Proton exchange membrane (PEM) electrolyzer uses a gas-impermeable polymer membrane as the electrolyte. Water vapor or liquid water is fed to at least one of the electrodes. The PEM electrolyzer can produce electrochemically compressed gas and can operate with pressure differences exceeding 100 bar. PEM electrolyzers can be made to operate reversibly to function as fuel cells.
However, PEM electrolyzers operate at limited hydrogen pressure, typically about 30 bar, because the membrane is quite permeable to hydrogen. At a higher hydrogen pressure, more hydrogen permeates through the membrane, which raises safety concerns. A thicker membrane may be used to combat hydrogen permeability losses. However, this results in ohmic efficiency losses due to an increase in ionic resistance. In addition, PEM electrolysis systems are too expensive for widespread commercial adoption for many grid-scale energy storage applications. The acidic electrolyte necessitates the use of expensive components for long term stability. Platinum and iridium are used as electrode catalysts. Electrode current collectors must be fabricated of corrosion-resistant materials.
With the development of anion exchange membranes (AEMs) that conduct hydroxide ions and other anions, alkaline electrolyte based AEM electrolyzers use much less expensive materials than PEM electrolyzers to produce pressurized hydrogen. Such electrolyzers can theoretically achieve 38 times lower permeability than a PEM electrolyzer at equivalent conductivity. Consequently, the AEM electrolyzer could operate as efficiently as a PEM electrolyzer at 38 times higher pressure, or > 900 bar as opposed to 30 bar. High pressure reduces mass and volume in storage and increases efficiency.
A common design for AEM electrolysis cells is the combination of a gas-impermeable membrane separator with electrodes flooded by electrolyte. This is impractical for fuel cell operation because gas cannot be fed to catalysts in the flooded electrodes at a sufficient rate to generate high current density. In addition, this cell design requires additional water and gas separation steps to recover the gas product.
In the absence of liquid electrolyte, hydrocarbon-based AEMs have challenges with remaining conductive. AEM can serve as an electrolyte in the absence of liquid electrolyte. This cell design requires ionomers in the electrode layer for ion conduction to permeate the electrode and operate at substantial current density. However, it is extremely difficult to keep an AEM hydrated and active for more than a few hours in the absence of liquid electrolyte.
Degradation of hydrocarbon ionomers and the AEM presents a further obstacle for reversible fuel cells based on AEM. Specifically, the high voltage oxygen electrode degrades ionomer electrolysis rapidly. Highly active oxygen- and hydrogen-oxygen-containing intermediate species, such as free radical species, can attack and degrade polymeric hydrocarbon AEMs adjacent to the oxygen electrode.
Power to Hydrogen Technology
Power to Hydrogen has developed a novel AEM reversible fuel cell that overcomes the limitations of pressure and stability as described above. The novel AEM reversible fuel cell uses two AEMs separated by a porous matrix layer that is permeated with aqueous KOH electrolyte. The AEMs and porous matrix are used to separate the electrodes. The electrodes are free of electrolyte, which eliminates the gas/electrolyte separation process to recover the product as happened in conventional electrolyzers.
The AEM reversible fuel cell uses fluorinated ionomer in the oxygen electrode, which improves operation stability between cycles of water electrolysis and power generation. It produces highly pressurized hydrogen and oxygen gasses over 200 bar for storage, then later converts the stored hydrogen and oxygen back into electricity and water.
Power to Hydrogen reversible fuel cell
The structure of the novel AEM reversible fuel cell of Power to Hydrogen is schematically depicted below.
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