The Many Colors of Hydrogen

Hydrogen is the most abundant element in the universe. It is estimated that hydrogen makes up ~75% of the mass in the universe. It is theorized that almost all of the hydrogen (along with some helium) was formed within the first three minutes after the Big Bang. In space, hydrogen generally exists as a neutral atomic gas, H. Hydrogen is the lightest and simplest of all the elements, containing one electron orbiting one proton. Hydrogen is the fuel that powers stars through nuclear fusion to produce helium and energy. Hydrogen and helium are the building blocks from which all other elements in the universe were formed via nuclear reactions in stars.

The form that we are interested in using here on earth is diatomic hydrogen, H2. While hydrogen is very abundant in space, on Earth hydrogen is relatively rare. The Earth’s atmosphere contains 0.5–1.0 parts per million (ppm) of hydrogen while hydrogen makes up 0.75 weight % of Earth’s crust. We are most familiar with hydrogen combined with oxygen in the form of water, H2O. Besides water, on earth hydrogen is plentiful in hydrocarbons (such as methane, CH4), and in other organic matter. One challenge of using hydrogen is efficiently isolating it from these compounds.

Why are we interested in obtaining hydrogen to use as a fuel? It has a high energy content, one (1) Kilogram of hydrogen has the same energy content as 1 gallon of gasoline. Hydrogen has ~3 times more energy by weight than gasoline. The byproduct of hydrogen combustion is water making it the perfect green and renewable fuel.

With the amount of water on the Earth’s surface, one would think that hydrogen extraction from the water would be an obvious and easy way to obtain hydrogen to use as a fuel. The hydrogen oxygen bond in water is very stable. It requires 467 kilo joules per mole of water to break the bond. Due to the stability of this bond, we must use a fair amount of energy to break this bond to obtain hydrogen from water. An electrolysis system with no inefficiencies would require 39.4 kilowatt hours (kWh) of electricity to produce a kilogram of hydrogen. Most commercial devices have some inefficiencies, so a standard operating amount is about 50 kWh per kilogram.

Another source of hydrogen is to separate it from hydrogen-rich hydrocarbons such as methane (CH4), coal, biomass, and organic waste. As with water, the C-H bond in these materials is very stable, requiring significant energy to break the bond.

Hydrogen is used in petroleum refining, ammonia production for fertilizers, and methanol synthesis. It powers rockets for space exploration and acts as clean fuel in hydrogen fuel cells for transportation and electricity generation. In addition hydrogen can be used as a fuel directly to power transportation vehicles (cars, trucks). The use of hydrogen fuel cells will generate electricity for transportation and portable power, with water and heat as the byproducts. Hydrogen can also be used to store energy from renewable sources like solar and wind.

There are numerous methods of obtaining hydrogen from water and organic sources. Each of these methods have been given a color designation (Black/Brown, Gray, Blue, and Green) based on the environmental impact of the method. Hydrogen itself has no color or odor. The remainder of this paper will discuss each of these methods, their advantages and disadvantages, and the relative cost of each.

Black/Brown Hydrogen Production

Black/Brown hydrogen are related, the difference being the feedstock to produce hydrogen. Black hydrogen refers to anthracite coal as the feedstock while brown hydrogen refers to lignite coal as the feedstock. Neither Black nor Brown hydrogen utilize carbon capture and storage (CCS), a process where the CO2 is capture and stored in underground reservoirs rather than being released to the atmosphere. This type of hydrogen production is not often discussed because it is the method of producing hydrogen that has the highest environmental impact. To obtain hydrogen, coal is gasified (heating the coal to high temperatures, 800°C – 1,500°C, at pressures of typically 30 KPa – 70 KPa) producing carbon dioxide (CO2), carbon monoxide (CO), and hydrogen (H2). The CO can be further processed with H2O to produce additional H2 and CO2.

Gasification is a highly carbon-intensive process that releases significant amounts of CO2 both from the coal that is gasified and from the consumption of fossil fuels used to generate the heat needed for the process. The process involves heating coal with steam and air, often serving as a low-cost, high-emission industrial fuel source, releasing up to 19 Kg of CO2 per kilogram of hydrogen produced.

The advantages of Black/Brown hydrogen are that they are a well-established method and technology and in coal-rich regions, where there is an abundant feedstock. It is the least expensive method for producing hydrogen. The disadvantages are Black/Brown hydrogen has the highest carbon emissions of all hydrogen production methods, and it has a negative environmental and health impact due to the coal and fossil fuel use.

Grey Hydrogen

Grey Hydrogen production currently accounts for over 95% of current global H2 production. Grey Hydrogen production is very similar to Black/Brown hydrogen, the difference being the Grey Hydrogen utilizes natural gas (or more specifically, natural gas’s main component, methane (CH4)). Through a process known as steam reforming, methane and water are reacted at high temperatures (700°C – 1,000°C) and high pressures (3 – 25 KPa). Typically the reaction is conducted using a catalyst such as nickel to accelerate the reaction. As in Black/Brown hydrogen production, the CO produced is further reacted with water to produce additional H2. Grey Hydrogen releases up to 10 Kg of CO2 per kilogram of hydrogen produced.

The advantages of Grey hydrogen production are that it is currently the most mature, technologically established, and cheapest method for hydrogen production compared to Green or Blue alternatives.

The disadvantages are that it contributes significantly to greenhouse gas emissions, relies on fossil fuels, and can cause methane (a major greenhouse gas) leakage during extraction and transport.

Blue Hydrogen

Blue hydrogen can be produced from fossil fuels (coal via gasification or natural gas (methane using steam reforming), with the resulting CO2 emissions from the processes captured and stored (CCS).

Another form of Blue Hydrogen can be produced from biomass or a combination of biomass and plastic waste. The process (gasification with CCS) is identical to that used to produce blue hydrogen from fossil fuels. Blue hydrogen can be carbon-intensive if there is a methane leak and/or incomplete CCS.

The advantages of Blue hydrogen are a 90–95% reduction in CO₂ compared to conventional Grey hydrogen. It acts as a reduced-carbon bridging fuel to support energy transition to Green hydrogen. It is less expensive to produce than Green hydrogen and utilizes mature technology and existing infrastructure, allowing for rapid deployment. When compared to electrolysis, Blue hydrogen requires less energy to produce, which is ideal for areas with limited renewable power. The disadvantages of Blue hydrogen are the underperformance of carbon capture technology, methane emissions and the resultant difficulty meeting stringent global emissions standards, and opposition from end-users who reject fossil-based materials in their value chain.

Green Hydrogen

Green hydrogen is produced with electricity created from renewable sources like wind or solar power. The electricity splits water into its component atoms using a process called electrolysis. This reaction takes place in a unit called an electrolyzer. This method results in very low or zero carbon emissions. The electricity to power the electrolyzer can be generated by numerous methods including solar, wind, hydro, nuclear, and from fossil fuels. With the exception of fossil fuels, all the other sources of electricity emit no CO2 while operating. There is research underway for the direct splitting of water. These include, but are not limited to: photoelectrochemical (PEC) splitting (the use of sunlight-absorbing semiconductors immersed in water to directly produce hydrogen), laser & mechanical methods (the use of innovative techniques include using pulsed lasers to split water or vibrating piezoelectric fibers to convert mechanical energy into chemical bonds).

The advantages of Green hydrogen include having zero emissions thereby being a means of eliminating greenhouse emissions during production. It can be a renewable energy integration (hydrogen can used as a storage medium when there is excess renewable energy (wind/solar), helping to balance load on the grid. It is versatile, hydrogen can be used for electricity generation, industrial fuel, or converted back to electricity. The disadvantages include high production costs (currently 3 to 7 times more expensive than fossil fuel-based hydrogen), low energy efficiency (overall efficiency of ~30%), and significant water consumption (~10-20 liters of water per kilogram hydrogen produced).

Cost Comparison

Cost of producing hydrogen can vary by an order of magnitude. Black hydrogen from coal costs $1.00 – $2.00 per kilogram. Grey hydrogen, from natural gas, costs $0.98 – $2.93 per to produce.

Blue hydrogen, or hydrogen produced with fossil fuels but subject to carbon capture, costs $1.8 – $4.7 per kilogram. Blue hydrogen from biomass cost $3.15 – $3.60 per kilogram. Green hydrogen, which is produced by running an electric charge through water, costs $ 4.5- $12 per kilogram.

Fuel of the Future

Hydrogen can be the fuel of the future. It is relatively abundant and burns without the emission of greenhouse gases Hydrogen can be used to generate electricity and as a storage medium for store solar and wind energy. When used to power fuel cells the electricity generated can balance the load on the electric grid.

In order for it to be a major fuel source, the cost of production needs to come down. The U.S. Department of Energy (DOE) targets reducing clean hydrogen production costs (Blue and Green) to $1 per kilogram by 2031 (via the “Hydrogen Shot” program), with intermediate goals of $2/kg by 2026. If these goals can be met, hydrogen usage as a fuel will grow.

Charles E. Taylor