6.11 - Hydrogen Fuel Cell

Learning Objectives

Describe how hydrogen fuel cells generate power and describe their environmental effects. The skill is explaining an environmental concept or process.

I. Hydrogen Fuel Cell Basics

Definition: A hydrogen fuel cell is a potentially renewable alternative fuel source that could replace fossil fuels for both electricity generation and transportation.

Inputs: Hydrogen gas ( $H_2$ ) and oxygen gas ( $O_2$ ).

Output: The only output is water.

2H_2 + O_2 \rightarrow 2H_2O + \text{electricity} + \text{heat}

Benefits of use: Generating power this way does not release greenhouse gases (other than water vapor) or air pollutants like SOx or NOx the way fossil fuel combustion does.

II. Fuel Cell Mechanism

The fuel cell generates an electrical current through these steps:

Hydrogen entry: Hydrogen fuel ( $H_2$ gas) enters the fuel cell.

Separation: The $H_2$ reaches the anode (the negative side), where a platinum catalyst splits each $H_2$ molecule into two protons ( $H^+$ ) and two electrons.

Electron flow (circuit): The protons pass through a membrane, but the electrons have to take an alternate path called a circuit. The flow of electrons through that circuit creates an electrical current that can power a device like a car or a light bulb.

Water formation: The electrons return to the other side. Oxygen comes in, breaks apart, and one oxygen atom combines with two hydrogen protons ( $H^+$ ) to form water ( $H_2O$ ).

III. Creating Hydrogen Gas

Difficulty: Getting hydrogen gas is hard because it does not exist naturally in a pure form on its own.

Source molecules: Hydrogen has to be separated from molecules that already contain it, most commonly water ( $H_2O$ ) and methane ( $CH_4$ ).

Energy intensive: That separation process takes a lot of energy.

A. Steam Reforming (Non-Renewable Method)

Process: The most common method. Methane reacts with very hot steam (about 700 to 1,000°C) over a catalyst, which pulls the hydrogen out of both molecules. The heat comes from burning more natural gas. About 95% of the hydrogen made in the U.S. comes from this method.

Overall reaction (the carbon ends up as

CO_2

CH_4 + 2H_2O \rightarrow CO_2 + 4H_2

Drawback: This process releases a lot of carbon dioxide. So the hydrogen fuel ends up dependent on a non-renewable underlying energy source.

B. Electrolysis (Potentially Renewable Method)

Process: An electrical current is applied to a tank of water, which splits the water into hydrogen on one side and oxygen on the other.

Drawback: Electrolysis releases no $CO_2$ directly, but it requires electricity. The resulting hydrogen is only as renewable and sustainable as the electricity source used to make it.

Green hydrogen: If the electricity comes from a renewable source like wind or solar, the result is called green hydrogen, which is truly renewable. But it is still rare: in 2024, less than 1% of the almost 100 million tonnes of hydrogen made worldwide came from low-emission methods like this (IEA, Global Hydrogen Review 2025).

IV. Trade-offs and Utility

It seems strange to use one energy source (natural gas or electricity) to create hydrogen, only to use the hydrogen to create electricity again later.

Value as an energy carrier: Hydrogen gas is a valuable energy carrier: it stores energy that was made somewhere else. It can be compressed (or chilled into a liquid at about -253°C), stored for weeks or months, moved by pipeline or truck, and used later. Storing it isn't easy, though: hydrogen is so light that it takes up a lot of space unless it is squeezed to very high pressure.

This solves the problem of not being able to easily bottle up energy from intermittent renewables like solar or wind for later use.
Stored hydrogen can be used to generate electricity and heat homes later, or used for industrial purposes like fertilizer creation.

V. Drawbacks and Limitations

Non-renewable basis: Almost all hydrogen today is made from fossil fuels (mostly natural gas, plus coal in China), which releases $CO_2$ . In the U.S., about 95% comes from steam reforming of natural gas.

Lack of distribution system: There is no massive, efficient distribution and transport network for hydrogen the way there is for gasoline (pipelines, tankers, gas stations). Building that infrastructure is a major barrier to wider adoption.

Vehicle fuel tanks: Current vehicle fuel tanks would need to be much larger to hold enough hydrogen for ranges comparable to gasoline vehicles.

Cost: Fuel cells need expensive platinum catalysts, and green hydrogen costs several times more to make than hydrogen from natural gas. Fuel cell cars like the Toyota Mirai exist, but there are very few hydrogen fueling stations, and most U.S. ones are in California.

VI. Practice Calculation: Energy Lost Making Hydrogen

Question: An electrolyzer uses 55 kWh of electricity to split water and make 1 kg of hydrogen. A fuel cell later turns that 1 kg of hydrogen back into 18 kWh of electricity. What percent of the original electricity do you get back?

\frac{18\ \text{kWh out}}{55\ \text{kWh in}} \times 100 \approx 33\%

Only about a third comes back; the rest is lost as heat in the electrolyzer and the fuel cell. That is what the CED means by "energy is still needed to create the hydrogen gas": hydrogen stores energy, it doesn't create it.

Big picture: Hydrogen fuel cells are a promising alternative, generating electricity cleanly by combining hydrogen and oxygen to produce only water, with no

CO_2

or major air pollutants at the point of use. But hydrogen does not exist naturally and has to be separated from other molecules, a process currently dominated by steam reforming, which relies on non-renewable natural gas and releases significant

CO_2

. In the end, hydrogen’s sustainability, whether it comes from steam reforming or the cleaner electrolysis route, depends entirely on the sustainability of the underlying energy source used to produce it.

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