Alchemr, an American clean tech company founded in 2018, develops low-cost and efficient anion exchange membrane water electrolyzers based on stable anion exchange membrane and a simple electrolyzer stack design that reduces shunt current. The company builds the world’s first >500 kW single-stack AEM electrolyzers.
Challenges: hydrogen fuel
As more and more renewable energy sources, like solar and wind, are added to the grid, energy storage systems are required to store excess renewable energy. Water electrolysis is one of the energy storage technologies that uses excess renewable electricity to produce hydrogen (H₂) fuel by electrolyzing water (H₂O). The produced green hydrogen can be used in fuel cells or by powering gas turbines to generate electricity without emitting CO₂.
This book Hydrogen Production: by Electrolysis is written by an internationally renowned team from academia and industry.
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There are currently a number of water electrolysis technologies for producing hydrogen.
Alkaline electrolyzers are the most prevalent commercial water electrolysis technology in the multi-megawatt range. The alkaline electrolyzer is fed with a concentrated KOH solution (about 10 M). The electrolyte-filled electrode chambers are separated by a thick porous diaphragm, such as the 500 µm thick AGFA’s Zirfon membrane. By absorbing feed solution in its pores, the porous diaphragm becomes conductive.
The alkaline electrolyzer can use low-cost materials, such as inexpensive diaphragm separators, non-precious catalysts for hydrogen and oxygen evolution reactions, and steel bipolar plates. Alkaline water electrolyzers have several disadvantages due to the permeable nature of the thick porous diaphragm to hydrogen and oxygen:
- They have limited operational capacity with current densities between 0.2 and 0.4 A cm⁻² to produce at very slow rates so that the hydrogen concentration can reach the lower explosion limit of 4%. Because they cannot electrochemically compress the hydrogen to a high pressure, a costly mechanical compression system is required to pressurize hydrogen for storage;
- The produced gas contains alkaline liquid and water vapor, which must be removed by supplementary equipment; and
- Due to their slow response, they are poorly adapted to renewable energy generation systems, such as solar cells and wind turbines.
General Electric introduced proton exchange membrane (PEM) electrolyzers in the 1960s. Now, PEM electrolyzers are a market-ready technology. The PEM electrolyzer uses a gas-impermeable PEM and a pure water feed. The electrode chambers are separated by a thin PEM with zero gap configuration. PEM electrolyzers show several advantages over conventional alkaline electrolyzers:
- They produce highly pure H₂ at a high current density (over 2 A cm⁻²) during operation. A higher operation current density correlates to an increased production rate;
- Hydrogen can be electrochemically compressed to a pressure of 30 bar. The gas pressure difference between the electrode chambers can be over 100 bar; and
- They can be adapted to the intermittent discontinuous renewable energy sources such as wind power and photovoltaic power due to their fast response.
However, the PEM electrolyzer has several drawbacks that limit its widespread application. PEM’s highly corrosive acidic environment requires the use of expensive and rare materials, such as carbon-supported platinum cathode catalyst, IrO₂ anode catalyst, and titanium-based bipolar plates, in order to achieve optimal performance.
Anion exchange membrane (AEM) electrolyzers can use significantly less expensive materials than PEM electrolyzers while maintaining the same level of performance. The AEM electrolyzer is a combination of the PEM electrolyzer and the alkaline electrolyzer:
- As in the PEM technology, a thin AEM separator between both electrodes with a zero-gap cell configuration is used; and
- Similar to the alkaline electrolyzer, the AEM allows the exchange of hydroxide ions (OH⁻) thus creating an alkaline environment.
The high internal pH of the AEM and ionomer dispersions enables the use of cheap and abundant electrode materials (such as nickel) instead of scarce platinum group metals and low-cost steel bipolar plates instead of titanium bipolar plates.
However, the main challenge of AEM electrolyzers is their lower operation current density than PEM electrolyzers. A low operation current density corresponds to a low hydrogen production rate. It would be difficult to compete with a PEM electrolyzer if a stable AEM water electrolysis system could save 90% of the catalyst costs while operating at a lower current density. Thus, an increase in current density of AEM electrolyzer will reduce the stack costs proportionally.
The operation current density of an AEM electrolyzer is dependent on numerous variables, including electrode catalyst, AEM conductivity, operation temperature, and alkaline electrolyte concentration. A concentrated KOH electrolyte allows for a higher operation current density due to an increase in the availability of OH⁻ ions, the mobility of ions, and reduced charge transfer resistance.
However, as the KOH concentration increases, the electrolyte itself becomes a good electrical conductor. A portion of the electrical current applied to the electrolyzer stack can follow a path through the fluid in the manifold rather than through the electrolytic cells. These currents are usually called “shunt current” and are considered parasitic because they are not used in the cell reactions and consequently reduce the efficiency of the electrolyzer stack.
Alchemr develops low-cost and efficient AEM electrolyzer stacks based on stable AEMs and a simple electrolyzer stack design capable of reducing shunt current.
AEM electrolyzer cell
The diagram below depicts the structure of an Alchemr’s AEM electrolyzer cell.
The electrolyzer cell includes the following components:
- Anode and cathode bipolar plates
The bipolar plates include inlet and outlet ports for hydrogen or oxygen as well as a tab for current collection. It also has flow patterns to direct the flow of gasses and reactants in electrodes. It is made of stainless steel for conducting electricity and transferring compressive forces.
- Gas diffusion layers
Gas diffusion layers (GDLs) ensure a uniform distribution of reactive gasses on the surface of the electrodes and the transport of electrons to or from the external electrical circuit. Gas diffusion layers can be made of carbon fiber paper, nickel fiber paper, or other porous and electrically conductive substrates.
- Cathode catalyst layer
The cathode catalyst layer comprises ionomer with embedded hydrogen evolving catalyst nanoparticles, such as Pt, Ir, Pd, Ru, Ni, Co, and Fe, that are supported on carbon substrates.
- Anode catalyst layer
The anode catalyst layer also comprises ionomer with embedded oxygen evolving (generating) catalyst nanoparticles, such as Ag, Ag alloyed with Pt, Pd, Cu, Zr, Ni, Fe, Mn, Co, Pt, Ir, or Ru blended with metal oxides, such as cerium oxide and zirconium oxide, and/or metal hydroxide.
Anion exchange membranes generally comprise polymers with multiple positively charged groups covalently attached. AEM separator comprises a hydrocarbon backbone, such as a copolymer of styrene and chloromethyl styrene, and a positively charged amine group, such as imidazoliums, pyridiniums, and piperidiniums as shown in the diagram above, where R₁-R₁₃ are each independently selected from hydrogen, halides, linear alkyls, branched alkyls, cyclic alkyls, heteroalkyls, aryls, heteroaryls, alkylaryls, heteroalkylaryls, and polymers thereof, such as the vinyl benzyl copolymers. These AEMs are stable in highly concentrated KOH solutions, which is desirable for achieving a high operation current density. Details are available in patent US9580824.
During operation, KOH electrolyte (pH greater than 8 at 25 ºC) is fed into the anode, flows through the cell, and exits the outlet. The electrolyte typically has a pH above 12 so that a high operation current of 1 A cm⁻² can be achieved. The optimal performance occurs when the conductivity of the electrolyte is between 0.1 and 0.4 S/cm. Once the electrolyte is flowing, a voltage is applied between the anode and cathode to promote the desired electrochemical reaction as shown in the diagram below.
AEM electrolyzer stack
An electrolyzer stack comprises at least two electrolyzer cells connected in series. The voltage is applied between the anode of one of the electrolyzer cells and the cathode of a different electrolyzer cell. It is desirable that the entire current supplied to the cell should produce gasses, but in practice, a portion of the current, called the “shunt current”, bypasses the AEM.
As shown in the diagram below, if the electrolyte solution flowing into the inlet manifold is a good electrical conductor, a portion of operation current can bypassing the AEM in the cell by flowing from the anode through the pipe (or tube) to the manifold and back into the anode of an adjacent electrolytic cell. Current that bypasses the AEM is wasted because it does not produce a reaction product, so it is desirable to eliminate or reduce the shunt current.
The Alchemr’s AEM electrolyzer stack uses two different methods to reduce the shunt current:
- The connecting tubes (pipes) are all made of non-conducting materials, eliminating a path for shunt current through electron conduction in metal, and
- The ionic conduction path back through the manifold is long so that very few ions flow from one anode to an adjacent anode.
The shunt current can be lowered to below 5% of the cell current if the length of the shortest ion conduction path (𝐿) between anodes (or cathodes) of different cells in the stack is:
𝐿 > 10×𝑡×σsol/σmem
where 𝑡 is the membrane thickness, σsol is the conductivity of the solution being fed into the anode in S/cm, and σmem is the ion conductivity of the membrane measured in the solution at the operating temperature. Typically, σmem is measured in 1 M KOH at 60 ºC.
Similarly, the shunt current between adjacent anodes or cathodes will typically be less than 1% of the cell current if:
𝐿 > 50×𝑡×σsol/σmem
The shortest ion conduction path between the anode or cathode of any of the two cells in the stack is greater than 3 cm.
- US11339483B1 Water electrolyzers employing anion exchange membranes
- WO2022216728A1 Water electrolyzers employing anion exchange membranes
Alchemr is building the world’s first >500 kW single-stack AEM electrolyzers. Its product enables major cost reductions in CAPEX and OPEX, eliminates supply chain risks, and is compatible with intermittent renewable power sources.
Rich Masel is Co-Founder.
Rich Masel is CEO.
Alchemr Board Member and Advisor
Rich Masel is a board member.