Versogen, an American cleantech startup founded in 2018, produces low-cost green hydrogen at scale using anion exchange membranes and earth-abundant materials. The company has received $14.5 million in Series A funding from investors such as Doosan Corporation and HyAxiom, Inc.. The company supplies partners with its membrane for use in the research and production of fuel-cell engines, but it is fully turning its focus to hydrogen production.
Challenges: hydrogen fuel
Electrolyzing water is a promising way to make hydrogen, which is a great alternative energy carrier that can be used to generate power for the grid, industry, and transportation. Alkaline electrolyzers have come a long way since their first commercial installation by NEL Hydrogen in 1927. They are now the most common way of water electrolysis to make hydrogen, which is used in a wide range of industrial processes.
However, alkaline electrolyzers have certain critical disadvantages.
(1) They are inefficient because of their high internal resistance, which is caused by gas bubbles that form within the liquid electrolyte and adsorb onto the electrode surface, and their thick diaphragms, especially at high current densities. This severely limits their performance at high current densities. Consequently, they operate at a relatively low current density of 250–450 mA/cm².
(2) The concentrated liquid electrolyte (25−40 wt% KOH or NaOH) also causes shunt currents, which lead to efficiency losses and hardware corrosion problems.
(3) Due to the slow ion transport through liquid electrolytes, the transient response of alkaline electrolyzers is slow. Alkaline electrolyzers require a stable power source and are typically connected to the grid, making it difficult to use intermittent renewable energy.
(4) Alkaline electrolyzers typically produce hydrogen at lower than desired pressures, necessitating the use of additional compressors for the hydrogen storage or transportation, which increases the cost and complexity of existing alkaline electrolyzer systems.
General Electric invented a proton exchange membrane (PEM) electrolyzer in 1973 to address the drawbacks of alkaline electrolyzers. PEM electrolyzers can operate at much higher current densities than alkaline electrolyzers while maintaining high voltage efficiencies, Â as a result of a solid electrolyte membrane and zero-gap configuration that reduces internal resistance. They have a quick dynamic response because proton transport across the solid polymer electrolyte membrane responds swiftly to any change in the power input. The high pressure operation of a PEM electrolyzer has the advantage of delivering hydrogen to the user at a high pressure, requiring less energy and cost for further compression and storage.
However, the PEM electrolyzers also have several disadvantages.
(1) PEM provides a highly corrosive acidic environment, necessitating the selection of specific electrocatalyst and construction materials. The harsh acidic environment restricts the choice of suitable electrocatalyst material to costly platinum group metal electrocatalysts (e.g., Pt as the cathode electrocatalyst and IrOâ‚‚Â as the anode electrocatalyst).
(2) Titanium, which is more expensive than stainless steel, is used to construct the stack components of PEM electrolyzers. Stainless steel is typically used to construct the stack components of alkaline electrolyzers.
(3) The widely used Nafion-based membranes in PEM electrolyzers are expensive, and it has been difficult to find cheaper alternatives.
(4) The system lifetime reported for state-of-the-art alkaline electrolyzers is between 30 and 40 years, whereas the system lifetime for state-of-the-art commercial PEM electrolyzers is between 5 and 20 years.
The high cost and inadequate durability significantly impede the commercialization of PEM electrolyzers at a large scale. PEM electrolyzers are currently being developed and have limited applications.
Hydroxide exchange membrane (HEM) electrolyzers are an emerging technology that offers an alternative solution that preserves the low-cost benefits of alkaline electrolyzer and utilizes the advanced design of PEM electrolyzers.
Using this configuration with a hydroxide-conducting polymer membrane instead of the harsh acidic proton-conducting membrane of PEM electrolyzers, HEM electrolyzers could work with non-precious metal catalysts and remove precious metal-coated titanium-based stack materials. The zero-gap solid electrolyte assembly also allows for high-voltage efficiency, large current density, fast dynamic response (on the order of milliseconds instead of seconds, like slower alkaline electrolyzer), and the ability to operate at differential pressures. HEM electrolyzers have potential to achieve performance parity with PEM electrolyzers while having low capital cost.
However, there are several challenges for HEMs and hydroxide exchange ionomers (HEIs) in HEM electrolyzers.
(1) The biggest challenge for HEMs/HEIs is high chemical stability at 80 °C or higher, ideally 95 °C (e.g., in the presence of nucleophilic hydroxide ions). In the presence of hydroxide ion nucleophiles, cationic functional groups in HEMs/HEIs can degrade via direct nucleophilic substitution and Hofmann elimination. The majority of base polymers for HEM/HEI applications (e.g., polysulfone and poly(phenylene oxide)) contain unavoidable ether linkages along the backbone, which makes HEMs/HEIs unstable at high pH. Strongly nucleophilic hydroxyl ions attack weak bonds and degrade polymer backbone.
(2) HEMs have lower hydroxide ionic conductivities than Nafion because hydroxide ion is less mobile than proton. Hydroxide conductivity increases with HEM/HEI ion-exchange capacity (IEC). High IEC leads to a membrane with high water uptake (i.e., a high swelling ratio), decreasing its morphological stability and mechanical strength, especially after repeated wet-dry cycles. This highly swollen state when wet makes HEMs brittle when dry.
(3) Mechanical flexibility and strength in a dry state is another HEM challenge. Most HEMs are brittle when dry, especially after swelling. It is difficult to obtain and handle large thin membranes for commercial use of HEMs. The ionomers must have good mechanical properties to form and maintain a triple phase structure in the fuel cell electrode at temperatures above 80 °C.
(4) An HEI's polymer should be soluble in a mixture of lower boiling alcohol and water but insoluble in pure alcohol or water so it can be easily incorporated into an electrode catalyst layer without dissolving.
Versogen Technology
In 2018, Versogen, founded by Yushan Yan at the University of Delaware, aims to produce green hydrogen with inexpensive catalysts and high performing anion exchange membrane.
Versogen currently offers a family of hydroxide exchange membrane/hydroxide exchange ionomers consisting of a high-molecular-weight ether-bond-free, rigid and hydrophobic aryl backbone incorporating alkaline-stable piperidinium cations.