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The basics of hydrogen fuel cells

Explore scientific details, industry applications and key decarbonization benefits of this crucial clean energy technology.


What is a hydrogen fuel cell?

  • Hydrogen fuel cells are devices that convert the chemical energy of hydrogen into electricity. They function through an electrochemical process in which hydrogen combines with oxygen to form water, generating electricity as a byproduct.

    A highly promising clean energy technology, hydrogen fuel cells will be integral to industrial decarbonization efforts. Thus far, hydrogen fuel cells have been used in applications including heavy duty fuel cell vehicles, spacecraft, data centers, warehouses and more. Potential future uses range from portable electronic devices to hospitals to military applications.

How do they work?

  • Hydrogen fuel cell benefit background

    Hydrogen fuel cells work like electrolyzers in reverse. Whereas electrolyzers use electricity to break water into hydrogen and oxygen, fuel cells generate electricity by combining hydrogen with oxygen to form water.

    Hydrogen fuel is supplied at the anode side of the fuel cell from an external storage tank, while oxygen on the cathode side is drawn from the air. Hydrogen is oxidized (loses electrons, becomes positively charged) at the anode, while oxygen is reduced (gains electrons, becomes negatively charged) at the cathode. An electrolyte is used to transfer ions between the cathode and anode. Water is formed at either the cathode or anode depending on the fuel cell type.

    During oxidation, excess electrons travel through an external load circuit from the anode to the cathode. This electric current provides the power output of the fuel cell.

Types of hydrogen fuel cells

There are four main types of hydrogen fuel cell technologies, differing chiefly in the electrolyte used to initiate oxidation. Each has its own advantages.

PEM fuel cells

  • Diagram showing the functionality of a PEM fuel cell.
  • PEM fuel cells

    Proton-exchange membrane (PEM) fuel cells are also known as polymer electrolyte membrane fuel cells. Hydrogen is supplied at the anode and oxidized using a catalyst. Hydrogen ions pass through the PEM to reach the cathode, while electrons reach the cathode through an external load circuit. Oxygen is reduced at the cathode, where it combines with hydrogen ions to form water. The current created by the electron flow provides the power output of the PEM fuel cell.

    Hydrogen PEM fuel cells operate at lower temperatures (typically 50-100 °C) than other hydrogen fuel cells while providing high power densities and fast startup times. They are being developed primarily for transportation, including passenger and commercial vehicles as well as potential aerospace applications.

Solid oxide fuel cells

  • Diagram showing the functionality of a solid oxide fuel cell.
  • Solid oxide fuel cells

    Solid oxide fuel cells are highly durable and energy efficient, and are ideal for applications in which waste heat can be harvested. Unlike PEM fuel cells, their long startup times and high operating temperatures of up to 1,000 °C make them suboptimal for most transportation applications. Solid oxide fuel cells are being developed chiefly for stationary power generators in buildings such as hospitals and data centers.

    Because of their high operating temperatures, solid oxide fuel cells must be extremely durable. Fuel cell components made with 3M™ Nextel Ceramic Fibers and Textiles can withstand extreme temperatures and thermal cycling to help you protect stack integrity and extend cell life.

Alkaline fuel cells

  • Diagram showing the functionality of an alkaline fuel cell.
  • Alkaline fuel cells

    Alkaline fuel cells utilize an aqueous alkaline electrolyte (most commonly potassium hydroxide) to conduct hydroxide ions from the cathode to the anode. Solid-state alkaline fuel cells using an anion-exchange membrane have also seen experimentation. Water is formed at the anode, generating an electron flow in the load circuit.

    Alkaline fuel cells are the oldest type of hydrogen fuel cells. Invented in 1932, they were used by NASA in the Apollo missions from 1968 to 1972. While alkaline fuel cells have largely been superseded by PEM or solid oxide alternatives, they see usage in spacecraft applications due to their high energy efficiency and reliability. Relatively low material costs are another benefit of alkaline fuel cells.

Phosphoric acid fuel cells

  • Diagram showing the functionality of a phosphoric acid fuel cell.
  • Phosphoric acid fuel cells

    Phosphoric acid fuel cells use a liquid phosphoric acid electrolyte. Similar to PEM fuel cells, hydrogen ions pass through the electrolyte to the cathode, while electrons flow through the load circuit. Hydrogen ions combine with oxygen to form water at the cathode.

    Compared to other hydrogen fuel cells, phosphoric acid fuel cells exhibit greater tolerances to impurities such as CO₂ in the fuel stream. This enables the usage of hydrogen produced from steam reforming, which emits CO₂ as a byproduct. Operating at temperatures of 150-200 °C, they enable effective waste heat collection without the extreme temperatures of solid oxide fuel cells.

Opening the door to clean energy technology

  • Green hydrogen icon of a hydrogen with a leaf circling it.
    The importance of green hydrogen

    Hydrogen fuel cells are widely expected to play a critical role supporting the clean energy transition and helping meet global climate goals. During fuel cell operation, only three things are created: electricity, heat and water.

    However, this does not necessarily make hydrogen fuel cells a zero-emission technology. CO₂ can still be emitted during hydrogen production depending on the raw materials and energy sources used.

    Hydrogen produced via water electrolysis, using low emission, renewable energy sources such as wind or solar, is called green hydrogen. Fuel cells supplied with green hydrogen can provide substantial energy with a very small carbon footprint, helping reduce industry reliance on fossil fuels and pave the way for decarbonization.

  • An icon of a wind turbine turning for
    A renewable energy carrier

    It is important to understand that hydrogen is not an energy source. Unlike fossil fuels, which occur naturally underground and provide energy once extracted and refined, hydrogen production always consumes more energy than it provides in fuel.

    Instead, hydrogen is an energy carrier. It enables the storage, transport and utilization of energy originally provided by other sources.

    For intermittent energy sources such as wind or solar, hydrogen fuel production is a means of storing excess energy generated during peak output periods. Fuel cells allow this energy to be accessed during off-peak periods and in applications (such as vehicles) far removed from the energy source. This is the critical role hydrogen fuel cells play in clean energy technology and industry decarbonization.

  • Icon of an energy efficient battery with a leaf

    Advantages over batteries

    • Like lithium-ion electric batteries, hydrogen fuel cells enable renewable energy to be utilized in a wide variety of applications. Unlike batteries, fuel cells convert fuel from a hydrogen storage tank into energy rather than storing the energy themselves.
    • Hydrogen fuel cells provide several advantages over lithium-ion batteries.
    • First, hydrogen is much more energy dense than the typical battery. In automotive applications, a hydrogen fuel cell and storage tank can enable greater range, at less weight, than comparable battery-powered vehicle designs.
    • Second, fuel cell refueling is much faster on average than battery recharging. This is critical in commercial transportation and shipping (long haul semi-trailer trucks and last mile delivery vans), where vehicle downtimes must be minimized.
    • Finally, hydrogen fuel cells provide much longer lifespans than most lithium-ion batteries and enable better end-of-life recyclability. This decreases the carbon footprint incurred from fuel cell manufacture.

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