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Green or renewable hydrogen generally means using renewable power to split water into hydrogen and oxygen, called water electrolysis. Assuring the power source is not tainted with fossil fuels requires third-party verification and careful accounting. Most hydrogen today is derived from methane and coal gasification, known as grey hydrogen.

The most common technologies deployed to produce green hydrogen from water are alkaline or proton exchange membrane (PEM) electrolyzers. They require electrodes and catalysts to propel the reaction. For alkaline types, use of a liquid potassium or sodium electrolyte does not require a membrane. In the highly corrosive environment, however, the electrodes in alkaline electrolyzers commonly use nickel, nickel-plated steel, or titanium coated with a nickel-based alloy. Electrolysis at high pH levels can be performed with non-critical (non-noble) metals (e.g., Ni, Co, and Mo) as catalysts, making for cost-effective production. The drawbacks with these materials are that they don’t respond rapidly to power fluctuations, have low production rates, have purity weakness, and require regular maintenance.

PEM electrolyzers require an acidic membrane, usually a polymer made with PFAS chemicals, as an electrolyte to transfer hydrogen protons (H+) between the electrodes and the water molecules, producing hydrogen and oxygen gases (see figure below). The membrane is bonded between a catalyst-coated anode and cathode to increase the reaction rate. Given the highly acidic environment in PEM electrolyzers, rare or critical metals and their oxides (platinum, iridium, ruthenium) are necessary as electrocatalysts for both hydrogen and oxygen reactions (see figure below). The high costs, limited supply, and non-domestic availability of these rare minerals cause barriers to rapid deployment of these electrolyzer.

The most effective method of reducing use of critical metals is to increase the surface area of the catalyst-coated membrane (CCM) while maintaining durability and performance. Using conductive nanofibers, surface area can be magnified by the equivalent of shrinking the area of a tennis court onto a tennis ball. By embedding the electrocatalyst into the nanofiber material instead of bonding it, the degradation and agglomeration of the critical metal can be largely prevented. With an ultra-low iridium content, our breakthrough iridium-containing catalyst can be reduced by over 90 percent while improving efficiency.

Another alkaline technology has shown promise for reducing use of critical metals:  anion exchange membrane (AEM) electrolyzers.   Instead of a proton exchange membrane, an anion exchange membrane forms the electrolyte. While the original designs applied iridium and platinum in thick layers, research has developed advantages such as use of non-critical metals as catalysts, lower ohmic resistance, and reasonable gas separation performance of solid membrane electrolytes. Intermittent power supply can also integrate with its gas diffusion characteristics. Nevertheless, the hydroxide ion conducting polymer membrane requires improvement in conductivity and stability. While strides have been made using nickel deposited by sputtering on the membrane, generating commercially viable hydrogen with AEM electrolyzers still demands advancement in membrane stability, energy efficiency, robustness, stack feasibility, cost reduction, and ion conductivity.

A fourth electrolyzer design, solid-oxide electrolysis cells (SOEC), demonstrates exceptional efficiency but requires a heat source to produce steam and operate at 700-800 degrees Celsius. The higher efficiency reduces cost by lowering the power required per kilogram produced. Solid-oxide refers to the ceramic electrolyte (ytrium-stabilized zirconia) required to withstand high temperatures.  SOECs cannot withstand on-off cycling even more than alkaline electrolysers. “SOEC doesn’t like expansion and contraction as a result of turning on and off, and this may present problems for the life of the stacks,” said Rick Beuttel, vice president for hydrogen at US manufacturer Bloom Energy. The key to a long lifetime for a SOE cell is a “hot standby” mode – operating the stacks at not less than five percent load at all times. See figure below.

Research is ongoing to add heat from external heat sources such as concentrating solar thermal collectors and geothermal sources. The restricted location requirements for cost-effective operation of SOECs make them a less adaptive and useful green hydrogen technology than PEM electrolyzers.