Green, yellow, blue, gray and now even gold: for a colorless gas, hydrogen is given the whole rainbow of colors. What is green hydrogen and what is an electrolyzer?

Green Hydrogen is a type of hydrogen produced using renewable energy sources, such as wind or solar power. This renewable energy is used to power an electrolyzer that separates water into its component elements, hydrogen and oxygen. The process of producing green hydrogen is referred to as "water electrolysis," and it is considered by many as a key technology to reduce carbon emissions and combat climate change. A lot of people incorrectly equate hydrogen produced with electrolyzers as green hydrogen, but the key definition is that the electrolyzer needs to consume only (well, the vast majority of) renewable energy resources to be called green.

So what’s the buzz around Green Hydrogen?

Green hydrogen is a versatile green molecule that has the potential to decarbonize several industries:

• Natural Gas Sector: The natural gas sector can utilize hydrogen in many ways; first, through blending into the natural gas system to provide low carbon energy, and second, as a dedicated hydrogen service for industrial heating, commercial / residential space heating applications and even for transportation uses.
• Synthetic Fuels: It can be used to create synthetic natural gas or other synthetic fuels, by combining it with sequestered CO2 and turning it into a gaseous or liquid hydrocarbon, or through a Haber-Bosch process to make green ammonia for fertilizer use.
• Energy Carrier: It can also be used as a storage solution for renewable energy, as excess energy from wind and solar sources can be used to produce hydrogen for later use. In that capacity, given that green molecules can be economically transported for long distances and stored indefinitely (in the right conditions), green hydrogen can provide long duration energy storage needed to stabilize the electric grid while enabling additional renewable energy penetration, for example by creating demand in geographic areas that have high potential for renewable energy production but that are isolated from load centers.

Hydrogen from electrolysis (remember, that doesn’t necessarily mean green!) has been a relatively niche market, representing only around 4% of worldwide hydrogen production (Source: Irena), but ask anyone that knows anything (or nothing) about hydrogen and you’ll hear that most of the “new” hydrogen production will be of the green kind.

This is because of the combination of two things:

1. Renewable Energy wholesale costs keep coming down, and electricity is the main driver of green hydrogen production cost. Hence, low electricity prices = low hydrogen prices (at least directionally).
2. Electrolyzer technologies are improving - fast! This is the “capex” side of the hydrogen equation, and technical developments by startups are increasing reliability and efficiency of those machines, while industrial conglomerates and fuel cell manufacturers like Plug Power are spinning up “electrolyzer gigafactories” to meet the government-induced hydrogen demand.

Below are the leading electrolyzer technologies and their advantages and disadvantages.

Alkaline Electrolyzers

Alkaline electrolyzers are commercially available today even in large capacity installations (>100MW) and operate via transport of hydroxide ions (OH-) through an aqueous electrolyte (typically a solution of sodium or potassium hydroxide) from the cathode to the anode.

Advantages:

• Commercially available, most developed technology today
• Low CapEX ($1,000 - 5,000 /kW depending on scale)
• Operate at less than 100°C
• Long lifetime (60,000 - 90,000 hrs or 7-10 years in continuous operation)

Disadvantages:

• Optimized for high efficiency under continuous operation - difficult to marry with intermittent renewable energy, although they can be regulated from 25% to 100% capacity
• Low current density at 400 mA/cm2 (only 20% that of PEM or SOEC) - needs a large area / volume to operate
• Good efficiency (4.5 - 6.6 kWh/Nm3 but not as good as PEM or SOEC)

NEL Hydrogen, a Norwegian company established in 1927, is a well known electrolyzer manufacturer that commissioned the largest water electrolysis plant in the world using hydropower in 1940 - therefore making Green Hydrogen as early as the 20th century! Decarbonization pressures are making the deployment of Green Hydrogen more urgent and timely than ever, but producing hydrogen from renewable sources is not a novel concept.

Proton Exchange Membrane (PEM) Electrolyzers

A Proton Exchange Membrane (PEM) electrolyzer uses a solid polymer electrolyte (essentially a plastic material) to conduct hydrogen ions (protons) separated at the anode from water. The hydrogen reacts at the cathode with the free electrons (passed from anode to cathode through an external circuit) to form hydrogen gas. Today, PEM is half the installed capacity of Alkaline.

Advantages:

• High dynamic range, can quickly be partialized to below 10% capacity and ramped up to 100% capacity
• Higher electrical efficiency and higher current density (typically 1.5-2.5 A/cm2) than alkaline electrolyzers
• Operate at 70-90°C

Disadvantages:

• High Capex (high catalyst content - Platinum, Palladium, Iridium - with limited worldwide availability), about double that of Alkaline electrolyzers per KW
• R&D focused on understanding electrolyzer cell and stack degradation processes to increase operational life

Several startups are focused on developing PEM electrolyzer components, such as membranes, synthetic catalysts, or on developing entirely new electrolyzer stacks altogether. The main objectives are to achieve a CapEX reduction or, most importantly, better efficiencies and better operating parameters (such as the ability to operate the system both in a very low electricity input mode or very high electricity input mode). Companies in the PEM space are Ionomr Innovations and H2U Technologies, which are both developing membranes that do not require expensive Platinum Group Metals (PGM) catalysts to operate. H2U Technologies is also designing a new electrolyzer stack - not just the materials - and is beginning field testing. Companies like Celadyne Technologies are developing innovative membrane materials that increase durability and expand operating conditions of PEM systems. 1s1 Energy is developing a new PEM electrolyzer with the goal of dramatically improving operating parameters, particularly efficiency and current density (therefore enabling a broader set of operating conditions).

Anion Exchange Membrane (AEM) Electrolyzers

Anion Exchange Membranes transport anions (OH-) instead of protons and aim to combine the benefits of alkaline electrolysis (namely a stable, cheap catalyst) with the advantages of PEM electrolysis (particularly the fast dynamic response and higher efficiency and current density). This is a newer technology with promising potential, as it has CAPEX advantages against PEM and better operating parameters than AEM.

Advantages:

• No expensive catalysts
• Operates in a highly diluted alkaline environment and is therefore much safer to handle and can use steel instead of titanium (used in PEM) reducing costs
• Builds on advantages of traditional alkaline electrolysers, but avoids its weaknesses
• Fast dynamic range, can ramp up or scale down quickly and can be linked up with intermittent renewable energy sources.

Disadvantages:

• Not commercially available - yet

Several startups are innovating in the AEM space. Power 2 Hydrogen, Enapter, Arco Technologies and Versogen are all working on and testing AEM designs that can effectively utilize renewable energy with a lower capital cost than PEM. Ionomr, together with their PEM product, is also developing an AEM version of their membrane.

Solid Oxide Electrolyzer Cell (SOEC) Electrolyzers

Solid oxide electrolyzers are a different technology type that uses a solid ceramic material heated at high temperature (~500-800°C) to conduct negatively charged oxygen ions (O2-). Steam introduced with free electrons at the cathode generate the hydrogen gas and negatively charged ions. The oxygen ions pass through the solid ceramic membrane and react at the anode to form oxygen gas and generate electrons for the external circuit. The main advantage of SOECs is their electric efficiency,

Advantages:

• High efficiency (in excess of 80%)
• Can capture waste heat from industrial manufacturers or from nuclear plants to heat the ceramic electrolytes and increase efficiency
• Currently, about 6x the cost of Alkaline per KWe (source)

Disadvantages:

• Need constant energy source
• Needs high heat, typically 700-800°C, but can be lowered to 500-600°C

Bloom Energy, an established manufacturer of Fuel Cells, is one of the leaders in the development of SOEC designs. Pilot results from Bloom show production rates of 37.7 kWh /kg of Hydrogen produced, compared to 52-54 kWh/kg of typical PEM or Alkaline electrolyzers.

Hydrogen is the glue that holds all of renewable energy together, and Green Hydrogen in particular has the ability to link green electrons with green molecules, building an integrated energy system that is more efficient and more resilient than the sum of its parts.

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