Power to Hydrogen ($6M for reversible fuel cell technology to maximize the usefulness of intermittent renewable electricity)

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.

The structure of the AEM reversible fuel cell of Power to Hydrogen (ref. US11228051B2)
The structure of the AEM reversible fuel cell of Power to Hydrogen (ref. US11228051B2).

The AEM reversible fuel cell comprises the following components:

  • Hydrogen electrode end plate

The hydrogen end plate includes inlet and outlet ports for hydrogen as well as a tab for current collection. It is made of stainless steel or nickel alloy for conducting electricity and transferring compressive forces.

  • Hydrogen electrode current collector

The hydrogen electrode current collector is sealed by a seal frame which is made of epoxy, glue, sealants, other polymers, coated metal gaskets, or ceramic gasket materials. The hydrogen electrode current collector is made of stainless steel mesh or nickel mesh that conducts electricity and allows for passage of hydrogen.

  • Hydrogen electrode

The hydrogen electrode is sealed by a seal frame. The hydrogen electrode may be porous carbon paper coated with a mixture of catalyst and anion-conducting ionomer, thereby forming a Gas Diffusion Electrode (GDE). The catalyst may also be coated on the AEM. 50 wt% ruthenium supported by carbon is an example of a catalyst for the hydrogen electrode that is active for hydrogen evolution or hydrogen oxidation.

  • Hydrogen-side AEM layer

The hydrogen-side AEM layer is placed on top of the seal of the hydrogen electrode layer. To facilitate handling and processing prior to cell assembly, a thin, anion-conducting, gas-impermeable AEM is cast on a porous support. The porous support material can be polymer, carbon, metal, or ceramic that could provide the necessary structural support for the AEM to withstand pressure differentials. The side of such an AEM layer that is impermeable is in contact with the hydrogen electrode. During operation, the porous side of the AEM layer is permeated with aqueous electrolyte.

  • Electrolyte layer

The electrolyte layer consists of a porous matrix permeated with an aqueous KOH electrolyte. A thin separator seal frame around the electrolyte layer has inlet and outlet ports for the flow of KOH electrolyte.

The porous matrix material can be either conductive or non-conductive, such as porous polypropylene, porous polyethylene, asbestos, porous PTFE, metal foam, ceramic foam, nickel metal foam, carbon paper, carbon cloth, carbon sponge, carbon fabric, metal cloth, ceramic cloth, metal sponge, polymer sponge, ceramic sponge, natural sponge, ceramic fabric, metal fabric, polymer fabric, multi-layer etched polymer membrane with flow-through channels, etched or cut channels in a thin sheet, woven mesh, and non-woven mesh.

The porous matrix is infiltrated with catalysts. The catalyst could accelerate decomposition of free radicals, thereby increasing the lifespan of the AEM reversible fuel cell. The catalyst can be Pt, Co, Ni, Fe, active carbon, or functionalized polymers.

  • Oxygen-side AEM layer

The oxygen-side AEM layer is a porous separator, such as porous polypropylene.

  • Oxygen electrode

The oxygen electrode is sealed by a sela frame. The oxygen electrode consists of catalysts coated on the surface of porous material, such as carbon paper, fluorinated ionomer and fluorinated binder, porous non-woven stainless steel fabric, or porous nickel foil.

Preferred catalysts for the oxygen electrode may be a mixture of nitrogen-doped carbon and Fe/Co metal particles or a mixture of Pt, Ni, Co, and/or Fe metal particles. The fluorinated ionomer includes NAFION®, polymers with N+H3R functional group, polymers with N+H2R2 functional group, polymers with N+HR3 functional group, polymers with N+R4 functional group, polymers with P+ functional group, and fluorinated anionic polysiloxanes. The fluorinated binder includes PTFE dispersions, PTFE particles, and PTFE-coated particles.

  • Oxygen electrode current collector

The oxygen electrode current collector is made of stainless steel mesh or nickel mesh that allows gas to enter and exit the electrode.

  • Oxygen electrode end plate

The oxygen end plate includes an oxygen inlet and outlet, as well as a current collection tab.

The operation of the AEM reversible fuel cell

The diagram below schematically depicts the water electrolysis in the reversible fuel cell to produce hydrogen and oxygen. Since the electrodes are free of water, the step of gas/water separation step required to to recover the gas product is avoided.

The electrolysis operation of the AEM reversible fuel cell of Power to Hydrogen.
The electrolysis operation of the AEM reversible fuel cell of Power to Hydrogen.

During electrolysis operation, water molecules traverse the hydrogen-side AEM layer and are reduced at the hydrogen electrode to produce hydrogen gas and hydroxide ions. The hydrogen gas is collected and stored after diffusing through the porous hydrogen electrode. The hydroxide ions diffuse across the hydrogen-side AME layer to the KOH electrolyte.

At the oxygen electrode, hydroxide ions that diffuse across the oxygen-side AME layer to the oxygen electrode are oxidized to oxygen gas and water molecules. Oxygen diffuses through the porous oxygen electrode and is collected and stored. Water molecules diffuse across the oxygen-side AEM layer to the KOH electrolyte.

When operating as a fuel cell, the AEM reversible fuel cell consumes stored gasses and produces electricity, as schematically depicted in the diagram below.

The fuel cell operation of the AEM reversible fuel cell of Power to Hydrogen.
The fuel cell operation of the AEM reversible fuel cell of Power to Hydrogen.

Hydrogen gas diffuses through the porous hydrogen electrode to the interface between the hydrogen electrode and hydrogen-side AEM layer. Hydroxide ions diffuse from the KOH electrolyte to the hydrogen electrode across the hydrogen-side AEM layer. At the interface, hydrogen gas and hydroxide ions react to produce water molecules, which diffuse across the AEM layer to the electrolyte. In this reaction, electrons are released and flow through an external load to the oxygen electrode.

At the oxygen electrode, oxygen gas diffuses over the porous oxygen electrode to the oxygen electrode/oxygen-side AEM layer interface. Water molecules from the KOH electrolyte diffuse across the oxygen-side AEM layer to the oxygen electrode. Oxygen and water molecules react to produce hydroxide ions, which diffuse across the oxygen-side AEM layer to the electrolyte.

During a practical application,  cycles between fuel cell and electrolysis operation are performed. For example, these cycles involved about 5 hours of fuel cell current load, followed by 10 minutes of open circuit, followed by about 15 hours of electrolysis load, followed by another 10 minutes of open circuit. The fuel cell load was 150 mA/cm². The electrolysis load was 50 mA/cm². The AEM reversible fuel cell of Power to Hydrogen shows good stability under these operating conditions.

Power to Hydrogen Patent

  • US11228051B2 Electrochemical cell and method of using same
  • WO2022086845A1 Electrochemical cell and method of using same
  • US10844497B2 Electrochemical cell and method of using same
  • JP7152032B2 電気化学電池及びその使用方法

Power to Hydrogen Technology Applications

Renewable energy storage

Power to Hydrogen reversible fuel cell technology can store renewable energy via water electrolysis to produce green hydrogen and oxygen which are stored. When electricity is demanded, the fuel cell consumes hydrogen and oxygen and outputs electricity.

Moon energy storage

Power to Hydrogen is working with NASA on in-situ resource utilization to provide hydrogen fuel and oxygen for life support on the moon, as well as lunar energy storage.

Power to Hydrogen Products

Clean Energy Bridge

Clean Energy Bridge™ uses cost-effective alkaline-compatible materials and platinum group metal (PGM) catalysts containing no extremely rare Iridium. It electrochemically compresses hydrogen gas at pressures over 200 bar, eliminating the need for compression to achieve storage pressure. Clean Energy Bridge™ is compatible with intermittent renewable energy sources, such as wind turbines and photovoltaics.

Power to Hydrogen is working with NASA on in-situ resource utilization to provide hydrogen fuel and oxygen for life support on the moon, as well as lunar energy storage.

Power to Hydrogen Funding

Power to Hydrogen has raised a total of $6M in funding over 4 rounds:

Their latest funding was raised on July 30, 2022 from a Grant round.

The funding types of Power to Hydrogen by 2023.
The funding types of Power to Hydrogen.
The cumulative raised funding of Power to Hydrogen by 2023.
The cumulative raised funding of Power to Hydrogen.

Power to Hydrogen Investors

Power to Hydrogen is funded by 3 investors:

ARPA-E and Third Derivative are the most recent investors.

The funding rounds by investors of Power to Hydrogen by 2023.
The funding rounds by investors of Power to Hydrogen.

Power to Hydrogen Founder

Paul Matter and Chris Holt are Co-Founders.

Power to Hydrogen Board Member and Advisor

Angie Ackroyd, Tom Fletcher, Peter Fusaro, Robert Schuetzle, Courtney Reich, and Shyam Kocha are advisors.

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