What Are Cathodes?

Cathodes are electrodes from which conventional electric current exits a device or toward which positive ions migrate in an electrolytic medium. In electronic and electrochemical devices, the cathode is defined by its role as the site of reduction, where species in contact with the electrode surface gain electrons. This single functional definition encompasses a wide range of physical implementations: the heated filament in a vacuum tube, the graphite layers in a lithium-ion battery, the photoemissive surface in a photomultiplier, and the terminal of an electrolytic cell are all cathodes, distinguished only by the mechanism through which charge transfer occurs. The concept originates in the electrochemical work of Michael Faraday, who coined the term in the 1830s to describe the electrode connected to the negative terminal of an external voltage source in electrolysis.

The physics of the cathode depends fundamentally on the mechanism of electron emission or ion exchange at its surface. This mechanism determines operating temperature requirements, material selection, operating lifetime, and the types of devices in which a given cathode design is viable.

Thermionic Emission Cathodes

Thermionic cathodes release electrons when heated to temperatures high enough that the thermal energy of conduction-band electrons exceeds the material's work function, allowing them to escape into vacuum. Tungsten filaments, historically the simplest thermionic emitter, operate at approximately 2500 K but require substantial heating power. Oxide-coated cathodes deposit a layer of barium and strontium oxides on a nickel substrate; the barium oxide surface layer reduces the effective work function to approximately 1 eV, enabling efficient emission at temperatures below 1100 K and greatly reducing heater power. Lanthanum hexaboride (LaB6) cathodes offer high current density and long operating life, making them standard in scanning electron microscopes and electron beam lithography systems, where source brightness and stability are critical. Barium dispenser cathodes, used in traveling-wave tubes and klystrons, supply barium by impregnating a porous tungsten matrix with barium aluminate, sustaining the emitting surface over thousands of hours of operation.

Electrochemical Cathodes

In batteries and fuel cells, the cathode is the positive electrode where reduction reactions release energy during discharge. In lithium-ion cells, the cathode material is typically a layered transition metal oxide such as lithium cobalt oxide (LiCoO2), lithium manganese oxide (LiMn2O4), or lithium iron phosphate (LiFePO4). Lithium ions migrate from the anode through the electrolyte and intercalate into the cathode lattice during discharge, while electrons travel through the external circuit to perform electrical work. The specific capacity, voltage plateau, cycle life, and thermal stability of a lithium-ion cell are determined largely by the cathode chemistry, and research on high-nickel oxide cathode formulations and sulfur-based cathodes is directed at increasing energy density for electric vehicle applications. In alkaline electrolysis cells used for hydrogen production, the cathode is where protons are reduced to hydrogen gas, and electrode materials research at institutions such as NIST focuses on catalyst layers that minimize overpotential losses.

Cold Cathode and Photoemission Devices

Cold cathodes emit electrons at room temperature through field emission or photoemission rather than thermal excitation. In field emission, a strong local electric field at a sharp tip reduces the barrier through which electrons quantum-tunnel into vacuum; carbon nanotube arrays and molybdenum tip arrays have been studied as field emission cathodes for flat-panel displays and X-ray sources. Photoemissive cathodes, coated with alkali metal compounds, release electrons when illuminated by photons whose energy exceeds the cathode's work function, forming the basis of photomultiplier tubes, image intensifiers, and streak cameras. Photocathode quantum efficiency and spectral response determine the sensitivity of photomultiplier detectors used in particle physics experiments, medical positron emission tomography scanners, and astronomical photometers.

Applications

Cathodes have applications in a range of fields, including:

• Vacuum electronics, including traveling-wave tubes, klystrons, and magnetrons for microwave generation
• Lithium-ion and solid-state batteries for portable electronics and electric vehicles
• Electrolysis and electroplating in industrial chemical processing
• Electron microscopy and electron-beam lithography in semiconductor manufacturing
• Photomultiplier tubes and streak cameras in high-energy physics and biomedical imaging