What Are Fuel Cells?

Fuel cells are electrochemical devices that convert the chemical energy of a fuel directly into electricity without combustion. The operating principle resembles that of a battery, with an anode, a cathode, and an electrolyte, but a fuel cell is not depleted because fuel and oxidant are supplied continuously from external sources. Hydrogen is the most common fuel; oxygen from air is the oxidant. At the anode, hydrogen is oxidized to release protons and electrons; the electrons travel through an external circuit, providing useful electrical work, while the protons migrate through the electrolyte to the cathode, where they combine with oxygen and returning electrons to produce water. The U.S. Department of Energy's fuel cell types overview classifies fuel cells by electrolyte type, which determines operating temperature, catalyst requirements, fuel flexibility, and suitability for different applications.

Fuel cells are distinguished from conventional heat engines by their high theoretical efficiency: because they bypass the Carnot cycle, efficiencies above 60 percent are achievable in electric-only mode, rising further in combined heat and power configurations. They produce no nitrogen oxide or particulate emissions during operation, and their only byproduct when fueled with pure hydrogen is water.

Polymer Electrolyte Membrane Fuel Cells

The polymer electrolyte membrane (PEM) fuel cell, also called the proton exchange membrane fuel cell, uses a thin solid fluoropolymer membrane as its electrolyte. The membrane conducts protons but blocks electrons and is impermeable to hydrogen and oxygen gas. Platinum or platinum-alloy catalysts deposited on porous carbon supports facilitate the electrode reactions. PEM cells operate at low temperatures, typically 60 to 110 degrees Celsius, enabling rapid startup and high power density, which makes them the standard choice for fuel cell electric vehicles and portable power. The main challenges for PEM cells are the cost and scarcity of platinum catalysts and the requirement for high-purity hydrogen, since carbon monoxide poisons the anode catalyst at concentrations above a few parts per million.

Solid Oxide and High-Temperature Fuel Cells

Solid oxide fuel cells (SOFCs) use a hard ceramic electrolyte, typically yttria-stabilized zirconia, and operate at temperatures of 700 to 1,000 degrees Celsius. At these temperatures, oxygen ions rather than protons carry charge through the electrolyte, moving from cathode to anode. The high operating temperature eliminates the need for precious-metal catalysts, allows internal reforming of hydrocarbons, and produces high-grade waste heat suitable for recovery in combined heat and power systems. SOFCs achieve electrical efficiencies above 50 percent and combined efficiencies above 80 percent in stationary power applications. Molten carbonate fuel cells (MCFCs) operate at 650 degrees Celsius and also allow natural gas and biogas as fuels without external reforming, making them practical for large stationary plants.

Fuel Storage and System Integration

Hydrogen for fuel cell systems is stored as compressed gas at 350 to 700 bar in composite-overwrapped pressure vessels for vehicle applications, or as cryogenic liquid for high-volume stationary use. The storage subsystem is tightly coupled to system performance: storage pressure determines refueling time and onboard energy density, while hydrogen purity requirements constrain the acceptable source. DC-DC power converters and inverters condition the variable DC output of the stack for load or grid connection, and in hybrid architectures a battery or ultracapacitor bank works alongside the stack to handle transient demands that exceed the fuel cell's dynamic response capability.

Applications

Fuel cells have applications in a range of sectors, including:

• Stationary power generation for hospitals, data centers, and remote sites requiring reliable, low-emission electricity
• Fuel cell electric vehicles, where PEM stacks provide traction power with rapid refueling
• Combined heat and power systems for buildings and industrial facilities
• Backup and uninterruptible power systems for telecommunications and critical infrastructure
• Marine vessels and aerospace auxiliary power units, where efficiency and low emissions are priorities