Redefining Clean Hydrogen: From Colors to Emissions

Use the phrase “green hydrogen” in a room full of energy researchers, and you might spark confusion. 

To a scientist who uses the unofficial color spectrum that classifies hydrogen by its production method, “green hydrogen” is a very specific term – hydrogen produced via electrolysis using renewable electricity. 

To a scientist who does not use the color system, green hydrogen might sound like shorthand for emission-free hydrogen produced from any number of sources and pathways – electrolytic, direct-solar and more. 

Clarity of communication is just one of the reasons that researcher Ahmet Kusoglu advocates for a transition from hydrogen color nomenclature to an emissions-based definition in his recent publication in the Electrochemical Society Interface. Kusoglu is an expert on clean hydrogen production and staff scientist in the Energy Conversion Group at the Department of Energy’s (DOE’s) Lawrence Berkeley National Laboratory (Berkeley Lab). 

When researchers are not on the same page about key terminology, it can create barriers for the whole field. Right now, hydrogen science is at a pivotal moment. 

“With increasing emphasis on decarbonization efforts and clean energy transition, clean hydrogen’s role has been recognized more than ever, from clean energy bills and roadmaps to net-zero climate scenarios” said Kusoglu.

As it becomes clear that next-gen clean hydrogen technology will play a key role in global decarbonization policy, Kusoglu’s analysis offers an unambiguous nomenclature that can also accommodate new and emerging hydrogen production pathways in a way that a system based on a rainbow of colors cannot.

“The concept of a color wheel is reaching a saturation point where it no longer can assign any more primary colors to new pathways unless the original encoding and definitions are modified accordingly.” said Kusoglu. 

Kusoglu proposes an emissions-based classification that trades a color wheel visualization for an “intensity wedge” based on carbon emissions intensity. The intensity wedge plots a given hydrogen system on a sliding gradient from high-carbon to zero, or even negative, emissions.

An emissions-first mindset also aligns with important modern policies like the U.S. Inflation Reduction Act’s Clean Hydrogen Production Credit, which provides tax credit for clean hydrogen based on emissions levels. Emission thresholds created by international policies could easily be added to a technology-agnostic “intensity wedge” graph for common reference.

With the publication gaining broad readership, Kusoglu’s suggestion stands to energize a wider discussion within the field about how to discuss complex hydrogen systems with a range of life cycle emissions, as discussed in DOE’s recent Clean Hydrogen Production Standard Guidance.

“As clean hydrogen plays an increasingly significant role in the energy transition, clear communication of hydrogen’s fingerprint from resource and production method to associated emissions will be more important,” said Kusoglu. “This type of emission-based color scale also provides accessible contrast in grayscale, unlike the traditional color hue which cannot distinguish colors when examined in grayscale. Such nuances will be important for conveying information on hydrogen more clearly.