PEM

PEM is an acronym with several distinct meanings in technology and science. In cryptography and networking, it is commonly associated with Privacy-Enhanced Mail and, more broadly, with the PEM file format used to store cryptographic materials. The PEM file format is an ASCII representation of binary data, typically DER-encoded, wrapped in header and footer lines such as "BEGIN CERTIFICATE" and "END CERTIFICATE." The content is base64-encoded and may include certificates, private keys, or public keys. PEM files are widely used in TLS and SSL deployments and in software tools like OpenSSL. File extensions such as .pem, .crt, .cer, and .key are often used to denote PEM-encoded data. Although the term originated with Privacy-Enhanced Mail, the format is now employed more broadly and independently of that original standard.

In energy technology, PEM stands for Proton Exchange Membrane, a key component of polymer electrolyte membrane fuel cells and electrolyzers. The membrane is a solid polymer that conducts protons while acting as an electronic insulator and separating the reactant streams. Typical operating temperatures are in the low to mid range, around 60–100°C, which allows relatively quick startup and straightforward water management. Nafion is a well-known example of a PEM material, though many other polymers are used. PEM systems enable applications in transportation, backup power, and distributed generation, but face challenges such as cost, long-term durability, and sensitivity to fuel impurities.